US3205166A - Continuous simultaneous separation of the normal aliphatic and aromatic components of hydrocarbon mixtures - Google Patents

Description

SePf- 7, 1965 D. M. LUDLow ETAL 3,205,166

CONTINUOUS, SIMULTANEOUS SEPARATION OF THE NORMAL ALIPHATIC AND AROMATIC COMPONENTS OF HYDROCARBON MIXTURES Filed June s, 1962 F/ sh 37 24/ //v VENTO/Ps d' u-L-v 23 Dana/d M. ud/ow 35 38 By Norman H. Scott n-A/iphaf/'c Desorbenf 3/ y-dzm.

27\ 28... M 26 29 50 A fr0/m5 Ys United States Patent O 3,205,166 CNTEIUUS, SEMULTANEOUS SEPARATION F THE NRMAL ALIPHAI'IC AND AROMATIC CGMPNENTS OF HYDROCARBON MIXTURES Donald M. Ludlow, Clarendon Hills, and Norman H.

Scott, Villa Park, ill., assignors to Universal Oil Products Company, Des Plaines, Ill., a corporation of Delaware Filed June 8, 1962, Ser. No. 201,036 Claims. (Cl. 208-310) This invention relates to a continuously operating, cyclic process for separating the components of a normally liquid hydrocarbon mixture to produce one fraction consisting essentially of pure normal aliphatic hydrocarbons, a second fraction consisting essentially of only the aromatic components of said mixture and a third fraction consisting essentially of one or more hydrocarbon types selected from `the branched chain and cycloparaiiinic hydrocarbons. More specifically, this invention concerns the foregoing separation process utilizing a mixture of sorbents, one of which is selective for the retention of the normal aliphatic components of the mixture and the other sorbent is selective for the retention of the aromatic components present in the feed stock, said process comprising a sorption zone in which both the aromatic and normal aliphatic components of the feed stock are removed from the feed stock by retention on the mixed sorbents, a relatively downstream desorption zone into which a desorbent capable of displacing normal aliphatic sorbate from the sorbent is charged at conditions favorable for the desorption of said normal aliphatic sorbate, a separate, downstream desorption zone into which a desorbent capable of displacing aromatic adsorbate from the adsorbent is charged at conditions favorable for the desorption of said aromatic adsorbate, a ra'inate withdrawal outlet at the end of the sorption zone, desorhate product outlets at the ends of each of the desorption zones, and rectification Zones into which a portion of said raffinate, said normal aliphatic desorbate and said aromatic desorbate eiiiuent streams are continuously reiiuxed, the process of separation being further characterized in that all of the inlets for the iniiuent streams and all of the outlets for the effluent streams are simultaneously advanced in equal increments in the direction of fluid flow.

One object of this invention is to provide a process for simultaneously separating both the normal aliphatic and the aromatic components of mixed hydrocarbon streams on a continuous basis. Another object of this invention is to provide a continuously operated process for the separation of mixtures containing at least two components, each being selectively retained by separate sorbents maintained in a plurality of serially adjacent, iixed beds. Another object of this invention is to provide a process for the production of a premium jet engine fuel free of smoke-producing aromatic components and also free of normal aliphatic hydrocarbons of high melting point which precipitate out of the liquid fuel and clog fuel lines in jetengines used at the low temperatures encountered at high altitudes.

In one of its embodiments this invention relates to a continuous process for simultaneously separating the aromatic and normal aliphatic components from a normally liquid mixture of hydrocarbons which also contains a raliinate-type hydrocarbon selected from the branched chain and naphthenic hydrocarbons, which comprises charging said mixture into the sorption zone of a fixed bed of solid sorbent comprising mixed, selective sorbents for normal aliphatic and aromatic hydrocarbons, withdrawing a rainate effluent from the downstream outlet of the sorption Zone, charging a desorbent comprising a normal aliphatic hydrocarbon boiling outside of the boil- 3,205,166 Patented Sept. 7, 1965 ing range of the normal aliphatic component of said mixture into a normal paraflin desorption zone downstream with respect to said desorption zone outlet, withdrawing normal paraffin desorbate from the downstream outlet of the normal paraffin desorption zone, charging a desorbent for said aromatic hydrocarbon into a separate aromatic hydrocarbon desorption zone also downstream with respect to the outlet of said sorption zone, withdrawing aromatic desorbate from the downstream outlet of the aromatic desorption zone, simultaneously reiiuxing portions of each of said raflinate, said normal aliphatic desorbate and said aromatic desorbate into rectification zones next adjacent downstream to the respective sorption, normal paratiin desorption and aromatic desorption zones, and periodically advancing in equal increments downstream the inlet points of said feed stock mixture and said normal paraflin and said aromatic hydrocarbon desorbents, while simultaneously advancing the points of withdrawing raffinate and said normal aliphatic desorbate and aromatic desorhate streams.

The separation and recovery of specic structural classes of hydrocarbons from naturally occurring sources in which these hydrocarbons exist in admixture with each other in various molecular weights and chain lengths has become one of the major problems of the petrochemical industries. These hydrocarbons provide the initial raw materials required in the manufacture of the wide variety of organic products and the vast quantities of specialty chemicals used in todays commerce. In most instances the effectiveness of these chemicals for their particular use depend upon a supply of hydrocarbon starting materials having a specific structure or isomeric conliguration. Thus, branched chain hydrocarbons boiling in the gasoline range are considered of greater value as fuels in internal combustion, high compression engines because of their higher antiknock value than their relatively straighter chain aliphatic isomers. On the other hand, the long, straight chain aliphatic hydrocarbons are preferred as starting materials in the production of detergents of the alkyl aromatic sulfonate and of the long chain alcohol ester type. Also, in the selection of liquid petroleum fractions for use as fuels for jet engines (fractions generally comprising the kerosene and light gas oil cuts of petroleum, including hydrocarbons of from C12 to C24 composition), these cuts are preferably substantially free of aromatic hydrocarbons which burn in jet engines with a smoky exhaust, a particularly disadvantageous characteristic of the fuel when used in military aircraft. Hydrocarbons containing from l2 to about 28 carbon atoms per molecule, and particularly, hydrocarbons of from 18 to 24 carbon atoms have high melting points and even though mixed with lower molecular weight hydrocarbons, the high melting components tend to precipitate out of the jet fuel at low temperatures, such as the temperatures commonly encountered by jet aircraft in high altitude flight. High melting points are especially characteristic of the normal or straight chain isomers of the aliphatic series of hydrocarbons and a method of removing these, the worst offenders, from jet fuels would be especially advantageous for the improvement of such fuels. Thus, the branched chain and naphthenic components of these fractions are the preferred structural types of hydrocarbons for use as jet fuels, not only because they burn in jet engines without smoke production but, in addition, they have sufficiently low melting points that they maintain their fluidity (i.e., they do not precipitate from the jet fuel mixtures) at low temperatures. The present process provides a method for simultaneously removing both the objectionable aromatic and normal aliphatic components from normally liquid hydrocarbon fractions and is especially adapted for the treatment of jet engine a fuels boiling in the kerosene and gas oil range. The present process is also characterized as a continuous method for effecting the foregoing separation with respect to a relatively wide variety of normally liquid feed stocks in which the individual hydrocarbon components contain six or more carbon atoms per molecule, thereby including gasoline boiling range fractions, the aforementioned kerosene and gas oil fractions, as well as higher boiling cuts having boiling points up to about 350 C.

In the usual method for separating both the normal aliphatic and aromatic constituents of mixed hydrocarbon fractions, it is necessary to provide a sequence of process stages, in the first stage of which the normal aliphatic components are removed from the feed stock mixture in a sorption-type contacting process, preferably utilizing a molecular sieve type separating agent, such as the so-called A molecular sieves of the metal aluminosilicate type containing pores of from 4 to about 6 Angstrom units in mean cross-sectional diameter. These sorbents accept the straight chain components of the mixture into the porous structure of the molecular sieve, but reject the branched chain and cyclic components which make up the desired raffinate product. The aromatic components present in the feed stock are separated by retention on a separate sorbent from either the initial feed stock (i.e., prior to the removal of the normal components) or from the raflinate product of the foregoing sor'ption stage in a second stage aromatic separation step. Hence, processes heretofore utilized for separating both the aromatic and normal aliphatic components of a hydrocarbon mixture required the use of combination processes involving at least two sequential stages. Not only was extensive processing equipment required, but also generally the consumption of large quantities of utilities to adapt the separate stages to specific temperatures and pressures whereby the selective conditions required for the separate stages of the process are realized. In the present method the process is operated on a continuous basis in a compact arrangement of equipment under conditions in which both components are separated simultaneously at essentially the same process conditions and involves much less than half the consumption of heating, pumping and cooling utilities which would otherwise be required for the two-stage separation method.

The aforementioned molecular sieve sorbents for recovery of the normal parainic and oleiinic components of the present hydrocarbon feed stocks are members of a certain group of dehydrated zeolites in which the metal component of their metal aluminosilicate,composition is at least in part an alkaline earth metal, such as calcium, magnesium, etc. These solid sorbents which act as molecular sieves in the present separation process are characterized as porous solids in which the pore openings are from about 4 to about 6 Angstrom units in cross-sectional diameter. They are prepared by co-precipitating or otherwise combining certain specific proportions of alumina, silica, water and an alkali metal hydroxide, such as sodium, lithium or potassium hydroxide, thereafter maintaining the aqueous mixture at or near the boiling point of water for a sufficient reaction time to form the hydrated crystals of the alkali metal aluminosilicate. The latter crystals are separated and ion exchanged with an alkaline earth metal salt, such as calcium chloride in an aqueous solution to replace at least a portion of the alkali metal from the hydrated crystals and form the corresponding alkaline earth metal aluminosilicate crystals. These zeolitic salts, which when dehydrated to develop their porous structure are sometimes referred to as 5A molecular sieve; they exist in the form of iinely divided crystals which are generally composited with a clay binder and pressed into larger pills of uniform size and shape which are generally more readily usable in the process. Prior to actual use the resulting pills are calcined at temperatures of from 350 to about 500 C. to dehydrate the crystals in the aggregate and develop the porous structure requisite to their use as molecular sieves. These sorbents are readily available articles of commerce; their properties and the methods utilized for their preparation are described in the literature, such as the book entitled Molecular Sieves by Charles K. Hersh, published by Reinhold Publishing Corp., New York, New York, and in the patent literature, such as US. Patent No. 2,882,243, issued to Robert M. Milton, as well as other patents.

Of the various methods for separating aromatic components from mixed hydrocarbon feed stocks, solvent extraction and absorption techniques applied to the hydrocarbon stock either before or after separation of the normal aliphatic component therefrom are most frequently utilized in sequential process arrangements. However, other aromatic separation techniques are also available and the choice of process eventually utilized will depend primarily upon the particular feed stock. A sorptiontype process which also utilizes a variety of sorbent of the metal aluminosilicate (molecular sieve) type containing pores in which the critical mean cross-sectional diameter is less than 10, but greater than 6 Angstrom units is particularly useful for recovering the aromatic components when the separating agent for the normal aliphatic recovery stage of the process is also a metal aluminosilicate of the aforementioned variety containing pores having mean cross-sectional diameters of from about 4 to about 6 Angstrom units. The sorbents containing pores having mean cross-sectional diameters of from 6 to l0 Angstrom units are of a type commonly referred to as 13X molecular sieves, prepared by a method of synthesis similar to that hereinabove described for the manufacture of the so-called 5A molecular sieves, the latter 5A compositions being selective for normal aliphatic hydrocarbons, except that generally higher SO2/A1203 ratios are employed in the process of synthesizing the 5A sorbents. The characterization of sorbents of the so-called 13X type and a description of the process for preparing the same is presented in U.S. Patent No. 2,882,244, issued to Robert M. Milton, and in other patents, as well as other literature.

The process of this invention will be further described by reference to the accompanying drawing which illustrates a particular serial arrangement of multiple fixed beds of the solid sorbent particles through which the fluid stream ows in one direction, although it is to be understood that other arrangements of sorbent beds and the use of other types of sorbents are also contemplated within the scope of the present invention. Thus, the diagram illustrates a multiple, fixed bed separation unit in which the normal aliphatic desorption zone is the first desorption zone downstream from the sorption` zone and the aromatic desorption zone is downstream from the normal aliphatic desorption zone. The process of this invention, however, is not necessarily dependent upon the specific arrangement of zones illustrated in the accompanying diagram; under other conditions of operation, the aromatic desorption zone may be upstream with respect to the normal aliphatic desorption zone and the present invention is intended to include both arrangements of desorption zones.

The terms upstream and downstream as referred to herein are intended to be interpreted in accordance with their commonly accepted definitions applied in the chemical process arts; that is, upstream refers to a point in the line of flow which is restrospective relative to the point of reference, whereas downstream refers to an advanced point in the direction of fluid flow relative to the point of reference. Thus, the points of reference are in every instance determined with respect to the moving fluid stream and not with respect to the stationary particulate solid phase, even though the latter may be visualized as moving in simulated flow by virtue of the continuously cyclic shifting points of inlet and outlet for the feed and product streams, respectively. In applying the terms upstream and downstream to a vertically stacked arrangement of fixed beds as a means of designating the spacial relationship between various points of reference, it is evident that the continuous lluid phase may flow in either a downilow or upow direction, depending upon whether the recycle stream (i.e., the socalled pump-around stream) which makes the process continuously cyclic, is taken from the bottom of the last bed in a particular series and discharged into the top of the first bed of the series to provides downflow of the fluid stream, or vice-versa for upflow.

The present description of the process is based upon a sorption process at a particular stage of the process cycle or at a particular point of time in the entire cycle when certain inlet and outlet lines are involved in carrying influent and eil'luent streams into and out of the separation column, it being understood, however, that the instantaneous position of the feed and product lines involved in the following description of the process is adopted for the sake of convenience and for the purpose of describing the process flow generally; at any given instant thereafter the feed and product inlets and outlets will have shifted to other positions illustrated, but not specifically described in the diagram.

Although the solid sorbent will be described as being distributed in a plurality of fixed beds B1-B20, within a vertically disposed contacting column 1, it is evident that the series of interconnecting beds in reality constitutes a continuous vertical bed of sorbent particles having c-onduits of reduced cross-sectional area between the superadjacent and sub-adjacent` so-called beds The beds may also be distributed throughout a number of horizontally spaced tanks, the outlet of one connected by a fluid-flow connecting conduit with the inlet of the next adjacent downstream tank or column and the outlet of the last tank in series connected by a conduit with the inlet of the rst tank in the series, with lines carrying influent and eliluent streams connected to the interconnecting conduits. The contacting or separation column 1 may also be made up of a continuous, uninterrupted bed of mixed sorbent particles with the various functional zones comprising the continuous bed of sorbent particles delineated by the points of inlet of the several influent streams and the points of outlet for the several effluent streams charged to or withdrawn from the column. A fluid pump 2 is provided between at least one pair of adjacent beds or between the end and the beginning of a continuous bed to provide a positive, unidirectional (downstream) ow of uid through the sorption column and through the serially arranged beds. One of the essential characteristics of the process flow is that a continuously owing stream of uid is circulated through the series of beds from the rst to the last bed in the series, at least three inlet streams being added to the continuous fluid stream and at least three outlet streams being withdrawn from the continuously circulating fluid. A means of introducing influent streams into the continuously flowing internal stream and for withdrawing eliluent product streams from the internal iluid and for shifting the inlet and outlet points in equal increments along the cyclic line of flow at timed intervals or continuously along the line of flow at a uniform rate and in accordance with a predetermined program is provided in the form of a uid distribution center, illustrated in one form of programming device as a flat-plate rotary valve 3, hereinafter more fully described.

A component unit of apparatus essential to the realization of the type of flow provided by the present method of operation is a suitable programming device for changing the points of inlet and outlet for the various inuent and eluent streams involved in the process and for advancing each of the inlet and outlet points in equal increments in a downstream direction during the operation of the process. Any suitable form of fluid distribution center may be utilized in the present process which simultaneously shifts all of the inlet and outlet points while maintaining the identity of the individual streams. Thus, a manifold arrangement of pipes carrying the various inuent and effluent streams may alternatively be provided, each pipe containing appropriately placed valves in the lines to direct the streams in such manner as to provide the present continuous process ow. In such alternative arrangement electrically operated switches which operate the Valves in compliance with electrical signals following a prearranged program are incorporated into the design to provide a suitable means for effecting the continuous process ilow of this invention. However, in the aforementioned manifold system of conduits and valves, a large number of smaller valves is required and the holdup of uids in the supply lines and euent lines becomes a factor of significant proportions in determining the purity of the influent and euent streams. An alternative programming principle in the form of a valve is illustrated diagrammatically in accompanying FIG. 1 as a circular, flat-plate rotary valve 3 having a stationary plate A and a continuously or intermittently revolving plate B, rotatable in fluid sealed relationship with respect to plate A. The latter plate A contains a number of inlet and outlet ports corresponding to the number of beds B1 to B20 in column 1 with which the ports are interconnected by lines carrying the influent and eilluent uids into and from the xed beds through internal passageways inside of plate A, to inlet and outlet ports in the valve through which the various fluid streams enter and emerge from the process ow. Plate B is designed to provide at least six passageways to accommodate at least three separate inlet streams and at least three separate outlet streams, the inlet passageways alternating with the outlet passageways. The various inlet and outlet lines connected to the ports in plate A of valve 3 connect at the other end of the lines with downcomers between each of the beds and since the flow of fluid through contacting column 1 is unidirectional, the interconnecting conduit between each pair of adjacent beds becomes the inlet or outlet point for the various influent and eluent streams, respectively. A suitable design and arrangement of a fluid distribution center of the type that is diagrammatically represented by flat-plate rotary valve 3 is more fully described as to both function and design in co-pending application Serial No. 805,575, tiled April l0, 1959, now Patent No. 3,040,777, for Don B. Carson et al. Rotatable plate B of fluid distribution center 3 may be rotated either continuously or in intermittent impulses, albeit continuously in one direction. It is evident that as plate B rotates, the streams owing into and out of the column through the iluid distribution center are directed to and withdrawn from the beds which will accomplish the desired flow pattern as established by a prearranged program built into the design of the rotary valve. With the completion of each rotation of plate B through 360, one cycle of operation is completed for the process, the cycle repeating itself for each complete rotation of plate B.

A combination of a rotary valve and manifold is also feasible in the present process as a fluid distribution center, for example, when the number of inlet and outlet channels cut into the plates of the rotary valve become excessive, a factor which should be considered when the number of influent and effluent streams exceeds about six in number.

A feed stock containing one or more rainate type components selected from the branched chain aliphatic and naphthenic hydrocarbons and one or more species of normal aliphatic hydrocarbons as one of the sorbate products of the process and one or more species of aromatic hydrocarbons as the other sorbate component of the feed stock, such as a petroleum-derived mixture of the above classes of hydrocarbons, is charged into the process flow through line 4 at a rate controlled by valve 5. The hydrocarbon feed stock mixture thereafter flows through heat exchanger 6 which adjusts the temperature of the stream to the optimum level desired in the sorption zone of the process. The resulting heat exchanged feed stock ilows through line 7 into pump 8 which increases the pressure of the feed stock to the level maintained at the particular point of feed stock inlet into column l. The inlet stream of feed stock at the desired sorption temperature and pressure enters fluid distribution center 3 through line 9 and by virtue of the internal channels and passageways in the distribution center the charge stock flows into line l which conveys the feed stock mixture into contacting column 1. The ow of the internal fluid stream in the separation column illustrated in the accompanying FIG. l is shown as upiiow through the serially arranged beds, although, alternatively, the fluid stream from the bottom of the column may alsoV be pumped into the top bed (B1) to thereby provide downow through column 1.

The mixture flowing into column l through line lil enters the process flow through the bottom of bed B5, joining a continuously flowing fluid stream flowing upwardly into bed B5, the first of a series of five beds comprising the sorption zone wherein the feed stock and the upstream fluid from bed B6 contact a mixture of sorbents containing desorbent in the porous structure of the solid particles by virtue of the preceding desorption stage of the process cycle.

In accordance with the process of this invention one of the sorbents comprising the mixture of solid particles in each bed of the column is selective for normal aliphatic hydocarbons and the yother sorbent in the mixture is selective for aromatic hydrocarbons, hereinabove described. For each and every sorbent heretofore indicated both normal aliphatic hydrocarbons and aromatic hydrocarbons are retained by the sorbent at relatively low temperatures to a greater extent than at more elevated temperatures, although for reasons of operating efficiency, it may be preferred to operate the sorption zone of the process flow at relatively elevated temperatures corresponding to or approaching the temperature required for desorption, as hereinafter specified, to thereby provide isothermal operation in both the sorption and desorption stages of the process iiow. In general, however, the preferred operating temperatures for the sorption zone `of the process are within the range of from ambient or atmospheric temperatures (approximately 25-30" C.) to maximum temperatures not substantially exceeding about 250 C. Since, however, the sorptivity of each sorbent (i.e., the weight percent sorbate retained by each of the sorbents) is inversely proportional to the sorption temperature, the

preferred upper temperature limit is generally from about 50 to about 75 C. In the event that the functional Zones of the process iiow are to be operated isothermally (a preferred method of operation for certain feed stocks) the actual operating temperature in the sorption Zone is at least partially determined by the temperature requirements in other zones of the process flow.

The operating pressure throughout the contacting column is substantially uniform, since each of the adjacent zones are interconnected by liuid flow conduits which permit substantial equalization of pressures throughout the column, except for the pressure differential required to maintain the fluid iioW through the column unidirectional. lt is generally preferable to operate the present process in liquid phase in order to maintain the size of the apparatus at a minimum, although under certain conditions, especially in the desorption stage of the process, vapor phase operation (which may accompany the use of higher temperatures required for desorption) the fluid phase may be in the vapor state. Whether the fluid components in contact with the solid sorbent are in the vapor or liquid state will depend to a certain degree upon the boiling point of the charge stock, higher temperature operation being generally required as the boiling range of the feed stock increases. Thus, the pressure variable in contacting column l may be maintained within the range of from atmospheric to or more atmospheres, depending upon the temperature required for sorption. Generally, however, pressures ranging from atmospheric to moderately superatmospheric, up to about 5 atmospheres, are preferred.

As the mixed hydrocarbon feed stock containing both normal and aromatic hydrocarbons flows upwardly through the sorption zone, the mixture of sorbents maintained in each bed of the sorption Zone selectively remove the normal and aromatic components from the feed stream, leaving a so-called raflinate residue consisting of hydrocarbons less selectively or not appreciably sorbed by either sorbent. Depending upon the composition of the feed stock, therefore, a ratiiuate stream comprising one or more components of the feed stock, selected from the branched chain aliphatic hydrocarbons and/ or naphthenes (cycloparaflins), which are not retained by either sorbent in the process iiow, occupies the void spaces between the solid particles of sorbent. As the sorbent removes aromatic and normal aliphatic components from the continuously flowing feed stream, the desorbent which occupies the solid particles by virtue of the preceding desorption stage, is displaced therefrom into the perfluent rafiinate stream; hence, thedesorbate stream is a mixture of desorbent and raliinate in the downstream portion of the sorption Zone. The proportion of raffinate to desorbent in the iiuid phase increases as the downstream raffinate outlet is approached, and the concentration of sorbate components in the interstitial fluid phase simultaneously decreases. Thus, as the feed stock flows successively through bed B5, B4 B1, and finally into line 11 which is the downstream outlet of the last bed in the series yof five beds comprising the sorption Zone, the fluid stream has been progressively enriched in raflinate components; if suiiicient sorptive capacity has been provided in the sorbents maintained in the sorption zone, the stream leaving the outlet conduit 11 of the sorption zone is substantially free of both aromatic and normal aliphatic components. Since the shift of inlet and outlet points progresses downstream in the same direction as the iiow of fluid stream, the approach to equilibrium between the sorbate on the solid phase and the sorbate in the fluid phase in the farthermost downstream bed of the sorption zone depends upon the rate of shifting the feed inlets and product outlet points. The raffinate product stream which in certain operations may be the product primarily desired as the end product of the present separation process is conveyed by means of pump 2 from line 11 into line 12 from which a portion is withdrawn into line 13 as effluent product or by-product of the present process and from which a remaining portion is permitted to iiow continuously in a downstream direction as the present tertiary reflux stream. At the stage of the process illustrated in the accompanying ligure, depicting a particular instant in the cyclic process flow in which beds B5 to Bl constitute the instantaneous sorption zone, beds B2@ and B19 comprise the present tertiary rectification Zone into which tertiary reflux enters to displace the interstitial iluid residing between the particles of sorbent in the latter zone and prepares bed B2@ for its function as the last downstream bed of the sorption Zone after the next succeeding shift in feed inlets and product outlets of the present continuous process. The volumetric rate of flow of tertiary reflux into bed B20 is one of the critical process variables of the present process, and will hereinafter be more specifically set forth.

It is to be emphasized that in designating certain beds in contacting column 1 as the component beds of a specific functional zone of the instantaneous cycle illustrated in the accompanying figure, the particular beds which make up a particular functional zone in the illustrated stage of the process are such only instaneously during the continuous shift of feed stream inlets and product outlets in a downstream direction. Furthermore, the number of beds assigned to the particular functional zones and the proportion of the total number of beds in the cycle assigned to each zone is not necessarily fixed in the arrangement illustrated in the figure, the arrangement shown therein being for the sake of illustration only. At a point of time thereafter, following one or more shifts in the inlet and outlet points into and from contacting column 1, the functional zones occupy a different series of beds, but each zone remains fixed with respect to the relationship of one zone to all other zones and when once the cycle is commenced, the same number of beds make up each zone. The zones progress in a downstream direction and in a continuously cyclic pattern, advancing one bed at a time as the feed stream inlets and product outlets shift during the course of each cycle.

The portion of the rafiinate product stream withdrawn from the continuously cyclic fluid stream in contacting column 1 and leaving the process cycle through line 13 flows through the internal channels and ports provided in fluid distribution center 3 into raffinate product outlet line 14 containing valve i5 which controls the flow of tertiary reflux into bed B20 by controlling the remainder of the fluid stream in line 13 entering the bottom of contacting column 1. As indicated previously, the raflinate product is substantially free of normal and aromatic hydrocarbons when the fluid stream flowing out of the downstream outlet of the sorption zone is in equilibrium with the solid sorbent maintained within the sorption zone of the process flow.

The flow of tertiary reflux into bed B20 of the tertiary rectification Zone is maintained at a rate sufficient to displace substantially completely the interstitial fluid remaining between the particles of sorbent in bed B20 and to replace the fluid with tertiary reflux of the same composition as the raflinate stream flowing out of the process flow into the raflinate outlet. By thus replacing the interstitial fluid in bed B20 with fluid of raffinate composition during the period of time that bed B5 is the first bed of the sorption zone and bed B1 is the last downstream bed of the sorption zone, the fluid next withdrawn as rallinate product through the outlet of bed B20 when the latter bed becomes the last downstream bed of the sorption zone will uniformly be of the same composition as the raffinate product, thereby maintaining a continuous, uniform rafiinate product quality throughout each cycle of the process flow. Since the raffinate component boils at substantially the same temperature as the normal aliphatic and aromatic components of the feed stock which are desorbed in the respective downstream normal aliphatic and aromatic desorption zones, any tertiary reflux which enters the downstream beds beyond the tertiary rectification zone will became mixed with the normal aliphatic and/or aromatic desorbate streams, recontaminating the latter products and defeating the objective of the present separation process. This result is particularly evident when the volume of tertiary reflux entering the tertiary rectification zone exceeds the Volume of interstitial fluid in the first bed of the tertiary rectication zone before the next shift of fluid inlets and outlets. When such critical flow rate is exceeded, the fluid stream leaving the tertiary rectification zone is joined by the influent desorbent stream which carries the tertiary reflux rapidly downstream into the desorbate product outlet and out of the process flow. For these reasons, the volume of tertiary reflux permitted to flow into bed B20 during the period of the cycle that bed B5 is the first bed in the sorption zone is preferably slightly less than the total volume of interstitial fluid removed from bed B20 before the feed inlets and product outlets are shifted to the next downstream beds; more preferably the flow rate of tertiary reflux is maintained at from about 60 to about 95 percent of the interstitial fluid volume.

The term, balanced reflux, is utilized in the present process description to indicate that volume of fluid required to displace 100 percent of (not more and not less) 10 the interstitial fluid occupying the avoid spaces between the particles of sorbent in each bed during the period of time that each bed is on stream receiving fluid of current composition (i.e., before the next downstream shift in feed inlets and product outlets). Stated otherwise, balanced reflux is the rate of flow required to replace interstitial fluid during the mean period of time that each bed is on stream between shifts of said inlets and outlets, or the total cycle time divided by the number of beds; a bed is said to be over-refluxed or under-refluxed by the amount which the actual reflux flow rate exceeds or falls short of, respectively, balanced reflux. Thus, the preferred flaw rate of tertiary reflux is maintained at from about 60 to about 95 percent of balanced reflux, as above defined.

The tertiary rectification zone (as well as the secondary rectification zone, as hereinafter shown) generally contains at least two beds in series to thereby provide for deviation from plug flow wherein the fluid front which represents a line of demarcation in the concentration of components in the interstitial fluid and the influent reflux fluid, becomes blurred into a narrow zone because of mutual solubility of the liquids at the front or interface. Thus, the so-called front may extend through more than a plane perpendicular to the line of flow and may even extend through a zone of from one-third to one-half the depth of the bed. Therefore if the reflux rate of flow is maintained at any rate approaching percent of balanced reflux, more than one bed per rectification zone would be required to contain the advancing front At the same instant the tertiary reflux enters the inlet of ber B20 which is the first of two beds comprising the tertiary rectification zone (entering bed B20 during the same period that feed stock enters the first bed of the sorption zone (bed B5)), a stream of continuously flowing aromatic desorbent flows into the inlet of bed B18 through the conduit interconnecting the outlet of bed B19 with the inlet of bed B18 where the influent desorbent stream joins a continuously flowing stream of fluid leaving bed B19. If the volume rate of tertiary reflux flow is maintained at less than balanced reflux the continuous, internal stream of fluid leaving bed B19 comprises interstitial fluid displaced from bed B18 as tertiary reflux flows into subadjacent bed B20 and is of the same composition as the fluid which last previously entered bed B19 prior to the shift in inlets and outlets when bed B20 became the first bed of the tertiary rectification zone. Since the influent fluid last entering bed B19 prior to said shift was the current stream of aromatic desorbent, the stream of interstitial fluid currently leaving bed B19 is composed substantially of desorbent which last previously entered the inlet of bed B19.

The inlet stream of desorbent is supplied to column 1 through line 14 which receives desorbent distributed to line 14 through the fluid distribution center 3 which is Supplied from an external aromatic desorbent supply source through line 16. The desorbent stream for this purpose enters the process flow from desorbent supply conveyed to the process through line 17. For reasons hereinafter specified the desorbent supplied to the desorption zone immediately downstream from the tertiary rectification zone is designated as aromatic desorbent and the desorption zone is indicated as an aromatic desorption zone. The flow of aromatic desorbent into the process is controlled at a rate determined by valve 18 in line 17, the influent rate of aromatic desorbent constituting one of the operating process variables upon which the successful operation of the process depends. The aromatic desorbent influent rate of flow is preferably maintained at from about 1.5 to 1 to about 30 to Land more preferably, at from about 5 to 1 to about 20 to 1 moles of desorbent per mole lof sorbate capacity per bed during the period between succeeding shifts of the desorbent inlet, as for example for the period between shifting the desorbent inlet from bed B14 to bed B15, the last bed entering the desorption zone during the stage of the cycle shown in the accompanying diagram.

The aromatic and normal aliphatic sorbate components of the feed stock retained by the mixture of solid sorbents in each of the xed beds of column l are each desorbed from their respective sorbents selective for each sorbate component by a fluid which is capable of replacing or at least displacing the sorbate components from the sorbent during the course of the desorption phase of the cycle. In the event that the objective of the present process is the recovery of a mixture of both the aromatic and normal aliphatic sorbate components, without further resolution between these classes of hydrocarbons, the contacting column may be supplied with a desorbent which has the ability to simultaneously or consecutively displace both classes of hydrocarbon components from the solid sorbent and in that event a single desorption zone and a single source of desorbent supply having a uniform composition may be utilized in the process flow; however, if each sorbate component is to be removed and recovered separately from their respective sorbents, whereby the identity of the structural types or classes of sorbate cornponents are to be individually maintained and the objective of the process is the recovery of the relatively pure sorbate products: (l) an essentially aromatic hydrocarbon product and (2) an essentially normal aliphatic hydrocarbon product, it is generally necessary to provide different desorbents for each of the sorbate types and to effect the selective desorption of each type in separate desorption zones. Thus, a desorbent effective for the displacement of the aromatic sorbate from its selective sorbent is generally not completely effective for the displacement of the normal aliphatic sorbate component from its selective sorbent. n the other hand, a substance which effectively desorbs or displaces aromatic hydrocarbons from its selective sorbent at the temperature at which such desorption of aromatic hydrocarbons occurs may also be effective for displacing a signicant proportion of the normal aliphatic sorbate component from its selective sorbent at the same or at a different temperature. Accordingly, the desorbent streams supplied to each of the separate aromatic and normal aliphatic desorption zone may be essentially of the same composition or the same material. For example, desorbents generally found effective for displacing normal aliphatic components from the pores of a molecular sieve sorbent are generally selected from the normal aliphatic hydrocarbons of higher or lower molecular weight than the normal aliphatic sorbate component of the feed stock. The sorbate Within the pores of the sorbent is displaced therefrom by virtue of the mass action effect of the molar excess of desorbent normal parafiin in the interstitial uid surrounding each particle of molecular sieve and the driving force necessary to accomplish the displacement is the number of moles of desorbent in the interstitial uid relative to the number of moles of normal aliphatic component within the porous structure of the molecular sieve. The absorptive mechanism involved in the retention of the aromatic component by the sorbent also permits desorption of the aromatic absorbate by the mass action elfect of a molar excess :of aromatic desorbent fluid in the interstitial spaces between the particles of sorbent containing the aromatic sorbate in its internal pores. In the event that the sorbent retains the aromatic sorbate via surface adsorption (such as activated carbon or silica gel) the normal paran desorbent will also displace the sorbate from the surface of the sorbent by mass action effect, especially if the resorbent is supplied to the aromatic desorption zone at a somewhat higher temperature than the temperature at which the aromatic sorption had taken place previously in the cycle. Thus, whether the common desorbent utilized for both normal aliphatic and aromatic desorption need be heated or whether it may be supplied at the same temperature to the separate desorption zones depends upon the type of sorbent utilized for separating the aromatic components. Since, however, both normal aliphatic and aromatic desorptions take place more readily and with lower volumetric flow rates of desorbent if the desorbent is heated prior to entry into the desorption zones, a common source of supply for the desorbent for each zone may be preferred.

In other instances the desorbent utilized for the desorption of the aromatic sorbate component of the feed stock may be another aromatic hydrocarbon (more preferably of lower molecular Weight) which generally provides the most efficient means of effecting desorption of the feed stock aromatic sorbate. Thus, in the treatment of a kerosene fraction for the recovery therefrom of bicyclic aromatics and/ or polyalkyl substituted or long chain alkyl substituted mono-nuclear aromatic hydrocarbons, or in the recovery of a monoor poly-alkyl substituted aromatic hydrocarbon from a lower boiling feed stock such as a naptha fraction of petroleum, the desorbent for the aromatic sorbate component may be an aromatic hydrocarbon such as benzene, which has the advantage over aromatic desorbents of being readily separated from the sorbate aromatic by fractional distillation overhead from the desorbate stream.

The aromatic desorption Zone is designated on the accompanying diagram as the first desorption zone downstream from the sorption zone to provide for one of the alternative methods of operating the present process in which a mixed aromatic-normal aliphatic sorbate product is to be recovered from the process and a molecular sieve type of sorbent (such as the above described 13X molecular sieves) is utilized as sorbent for the aromatic component of the feed stream. Since the pores in molecular sieve sorbents capable of accepting aromatic hydrocarbons are also of suflicient size to accept feed stock normal aliphatic hydrocarbons into their internal pore structure, the aromatic desorption zone must precede the normal aliphatic desorption zone in the line of How under such conditions, since in the reverse situation in which the normal aliphatic desorption zone precedes the aromatic desorption zone, the normal aliphatic desorbate would also act as a desorbent for the aromatic sorbate in the downstream aromatic desorption zone and such alternative process would thereby less selective, especially when utilizing a feed stock containing high molecular Weight aromatic components, such as the poly-nuclear aromatics and the desorbent is a mono-nuclear aromatic or normal aliphatic hydrocarbon. Inasmuch as the desorbed aromatic(s) flowing from the outlet of the aromatic desorption zone downstream into the normal aliphatic desorption zone have a mean cross-sectional diameter greater than the diameter of the pore openings of the normal aliphatic sorbent, these aromatics must occupy the interstitial void spaces between the particles of sorbent in the normal aliphatic desorption zone and therefore do not interfere with the desorption of the normal aliphatic components. In the event that the sorbent for the aromatic component(s) of the feed stock is an adsorbent, however, which retains the aromatic(s) by surface adsorption (such as silica gel, charcoal, etc.) it is feasible to operate the present process with the normal aliphatic desorption zone ahead of the aromatic desorption zone, immediately downstream from the tertiary rectification zone; that is, the first desorption zone downstream from the sorption zone, alternatively, may be for desorption of the normal aliphatic component(s) of the feed stock (not shown in the accompanying diagram). In this type of operation, the normal aliphatic desorbate of the flrst desorption zone flows downstream into the aromatic desorption zone and may be the actual desorbent instrumental in the desorption of the aromatic sorbate from the sorbent in the downstream aromatic desorption zone, particularly if the desorbate stream from the normal aliphatic desorption zone is Withdrawn from the outlet of the normal aliphatic desorption zone, heated in an external heater and reintroduced at a higher temperature into the inlet of the aromatic desorption zone.

The desorbate product withdrawn from the downstream aromatic desorption zone, however, will be a mixture of the aromatic and normal aliphatic feed stock components and not the individual sorbate components obtained when using separate sources of desorbent for each sorbate component. All of the foregoing alternative types of separation are contemplated Within the scope of the present invention, sincelin each instance the several alternatives constitute an extension of the basic novel concept provided in this disclosure.

One class of compounds utilizable as desorbent for the aromatic sorbate when the aromatic component of the feed stock is an aromatic hydrocarbon of intermediate or higher molecular weight, such as toluene, xylene or naphthalene, etc. is benzene or other aromatic hydrocarbon of lower molecular weight than the feed stock aromatic component, the aromatic desorbent being charged in molar excess suicient to effect desorption via the mass action effect. The aromatic desorbent charged into the process through line 17 may be supplied at the same temperature as the ambient temperature in the sorption zone if the molar proportion of aromatic desorbent to sorbate is greater than 1 to 1 in the desorption zone, thereby providing for an essentially isothermal sorption-desorption process. However, in the event that one or both of the desorption zones are to be operated at an elevated temperature (i.e., above the temperature of the sorption zone), heater 19 may be incorporated into aromatic supply line 17 to raise the temperature of the aromatic desorbent to the level required for desorption of aromatic sorbate. The desorbent at a flow rate determined by valve 1S thereafter llows from heat exchanger 19, through connecting line 20 into pump 21 which discharges the aromatic desorbent into line 16 at the pressure maintained at the inlet of bed B18. The stream thereafter flows into lluid distribution center 3 which directs the influent aromatic desorbent into line 14 connecting the inlet of bed B18, as aforesaid. Heat exchanger 19 in line 17 -operates as a heater when the aromatic desorption zone (beds B18 to B15, inclusive) is to be operated at a temperature higher than the sorption zone, as for example, when the aromatic desorbent is a paranic hydrocarbon, such as an isoparaffin, cycloparafn and/or normal paraffin boiling outside of the feed stock boiling range. Thus, when the aromatic sorbate is xylene, for example, a suitable parafiinic desorbent is a mixture of normal and isohexanes. The aromatic desorbent, usually recovered as an overhead distillate of the desorbate stream removed from the outlet of the aromatic desorption zone and recycled to line 17, enters the inlet of bed B18, and joins the continuous stream of displaced interstitial fluid flowing upwardly through the outlet of bed B19, After the aromatic desorbent enters the inlet of such bed B18 which is the first bed of the aromatic desorption zone, it meets sorbent particles containing progressively higher content of aromatic sorbate component as it ilows upwardly through the succeeding downstream beds: B17, B16 and B15 of the aromatic desorption zone. Thus, the desorbent stream richest in desorbent meets sorbent particles containing the last traces (presumably the least readily desorbed aromatic sorbate) nearer the inlet of the aromatic desorption zone. Since the fluid inlet and outlet points of contacting column 1 are moved in the same direction as the ilow of fluid through the column, the particles of sorbent in effect move down- Wardly through the column, in simulated countercurrent relationship to the fluid stream continuously flowing upwardly through the column. This effectively countercurrent flow relationship between the solid and fluid phases which accounts for the high degree of separation obtainable by means of the present process and the high degree of purity of the sorbate and railinate product streams recoverable by means of the present process may also be` realized by moving the inlets and outlets into and from contacting column 1 downwardly through the column in the same direction as a downwardly flowing continuous stream of fluid flowing through the column. The effectively countercurrent flow obtained by shifting the points of inlet and outlet in equal increments in the same direction as the flow of continuous lluid through the column also prepares the next downstream bed adjacent to the fluid inlets and product outlets for receipt of fluid of the same composition as will next be withdrawn or charged into the next downstream bed following the next shift of inlets and outlets. Thus, as a result of such countercurrent flow, bed B18 has undergone substantially complete desorption prior to the shift of the aromatic desorbent inlet to the inlet of bed B19 and during the period of time that bed B18 is the first bed of the aromatic desorption zone, the small amount, if any, of the remaining aromatic sorbate residing on the sorbent in bed B18 is completely desorbed and the sorptive activity of the sorbent in bed B18 is restored. In addition to the complete removal of aromatic sorbate from bed B18, the interstitial fluid remaining between the particles of sorbent is essentially desorbent which will constitute the continuous fluid stream displaced from bed B18 when the latter becomes the second bed of the tertiary rectification zone following the next shift of product outlets and feed stock inlets upwardly through contacting column 1.

As heretofore indicated, as the aromatic desorbate stream flows upwardly through the aromatic desorption zone it meets progressively less regenerated sorbent, the concentration of aromatic desorbate in the stream increases and the proportion of desorbent in the stream decreases, the sorbent retaining desorbent as it releases sorbate to the continuously flowing stream. At the outlet of bed B15 the desorbate stream contains the highest pro-` portion of aromatic sorbate at any point in contacting column 1 and it is at this point that the desorbate stream is partially withdrawn from column 1 through line 22 and the remaining portion continues to flow downstream into bed B14 as secondary reflux, bed B14 constituting the first bed of two beds comprising a secondary rectification zone. The ilow rate of secondary reflux is also a process variable of significant effect on the operation of the process, and the purity of the products recoverable from the process. Its volumetric rate of llow is determined by the rate of withdrawing aromatic desorbate product from column 1 through line 22, which in turn is determined by the setting of valve Z3 in the aromatic sorbate product withdrawal line 24 on the downstream side of lluid distribution center 3.

The outlet of bed B15 is established as the aromatic desorbate product outlet by virtue of the internal design of fluid distribution center 3 which maintains the outlet ports and lines connecting the outlets of beds B16, B17 and B18 closed, while opening the outlet port of the fluid distribution center connecting with line 22 and thereby permitting the flow of desorbate from bed B15 into line 22.

Desorbate outlet line 24 is generally connected to a suitable fractional distillation column (not shown on the accompanying diagram) for the separation of the aromatic desorbent from the desorbate product which is recovered from the fractionation unit as a distillation residue. The desorbent overhead is recycled directly into line 17 for reuse in the process.

The flow of secondary reflux into bed B14 is preferably maintained at a rat-e somewhat less than (generally from 60 to 100 percent) of balanced reflux which, as hereinabove described, is a llow rate which will provide (and thus replace) from 60 to 100 percent of the interstitial fluid or void space volume in each bed during the period of time between successive changes of inlets and outlets of contacting column 1. The preferred ow rate of secondary reflux is from about to about 95 percent of balanced reflux, thereby providing substantially complete replacement of the interstitial fluid in bed B14 before the next shift in product outlets and feed stream inlets. The flow of secondary reflux is maintained at less than balanced reflux for the same reason as the under-reflux of the tertiary rectification zone; that is, to prevent contamination of the normal aliphatic desorbate by aromatics in the secondary reflux. For similar reasons two beds (B14 and B13) are provided for the secondary rectification zone.

During the same period that feed stock flows into the inlet of bed B5, aromatic desorbent flows into the inlet of bed B18, aromatic desorbate is withdrawn from the outlet of bed B15, and secondary reflux enters bed B14, a stream of normal aliphatic desorbent is charged into the inlet of bed B12 from line 25 which connects with the downcomer conduit between beds B12 and B13. The normal aliphatic desorbent may be of the same composition as the aromatic desorbent, as described above, although for most feed stocks, normal paraflins charged at a flow rate which will provide a molar excess of desorbent surrounding the sorbent particles relative to the sorbate in the pores of the sorbent are preferred desorbents for the normal aliphatic desorption zone, the normal paraffin desorbent displacing the sorbate normal aliphatic (either Vof paraflinic or olefinic structure) from the pores of the sorbent by a mass action mechanism, dependent upon maintaining the aforesaid molar excess of desorbent in the interstitial fluid. The desorbent is generally recovered by distillation from the normal aliphatic desorbate stream, as hereinafter described, and recycled directly to the desorbent inlet from supply through line 26 at a rate controlled by valve 27 and may be heated or cooled, as desired, in heat exchanger 28 prior to entry into the process flow. The effluent stream of heat exchanger 28 flow through line 29 into pump 30 which discharges the normal aliphatic desorbent at-the pressure existing in bed B12 into line 31 which feeds the normal aliphatic desorbent stream into fluid distribution center 3. By means of the internal channels and ports in fluid distribution center 3 the desorbent for the normal aliphatic sorbate componentts) of the feed stock is directed into line which conveys the desorbent from fluid distribution center 3 into .bed B12, as aforesaid.

Because of the fact that the normal aliphatic desorbate is refluxed into the primary rectification zone at rates greater than balanced reflux and the latter rectification zone is directly upstream from the sorption Zone, the normal paraffin load in the sorption zone is reduced if the desorbent normal parafllns in the desorbent stream are diluted with a nonsorbed diluent, such as an isoparafiin or cycloparaflin boiling outside of the feed stock boiling range. When utilized in the -desorption zone, the influent desorbent mixture preferably contains from 10 percent to 50 percent by volume of the diluent.

The normal aliphatic desorbate stream flowing .as a

continuous stream through the downstream `beds B11, B10 and B9 of the normal aliphatic desorption Zone varies in composition at any given point along the line of flow, the stream containing the highest proportion of de- -sorbent in equilibrium with sorbent of the lowest content of normal aliphatic component in an upstream direction as the inlet to bed B12 (desorbent inlet to the desorption zone) is approached; conversely the proportion of desor'bent in the fluid stream decreases and the proportion of desorbed normal aliphatic desorbate in the fluid increases as the outlet of bed B9 is approached. Normal aliphatic desorbate containing the highest molar proportion of normal aliphatic component in column 1 is withdrawn from the outlet of bed B9, in part through line 32 which conveys the normal aliphatic desorbate to fluid distribution center 3 and the remaining portion continues to flow in a downstream direction into bed B8, the first of two beds: B8 and B7 comprising the primary rectification zone. The portion which provides a continuous downstream flow of fluid into bed B8 is referred to herein as primary reflux and its rate of flow is also one of the important variables of the present separation process.

The desorbate product stream flows into line 32 because the latter line is the only outlet conduit from the series of beds B12, B111, B10 and B9 which has an openport in the fluid distribution center through which the desorbate stream may find an outlet by pressure relief. The normal aliphatic desorbate product withdrawn from column 1 leaves flu-id distribution center 3 through line 33 at a rate controlled by valve 34, the resulting mixture of normal aliphatic sorbate and desorbent being fractionally distilled as it is removed from separation column 1 to recover the desorbate product from desorbent which may be recycled directly to lthe process through line 26.

The desorbent selected for the normal aliphatic com-y ponent is generally a normal aliphatic hydrocarbon of lower molecular weight than the lowest molecular weight normal aliphatic component of the feed stock, or, alternatively, a normal aliphatic hydrocarbon of higher moleoular Weight than the highest molecular weight component lof the feed stock, differing from the lowest or highest molecular weight sorbate normal aliphatic component olf the feed stock by at least one carbon atom, and more preferably, by at least two carbon atoms per molecule. The former which boils below the boiling point of the normal aliphatic component of the feed stock may be sepa-rated `from the desorbate stream as an overhead, whereas the higher molecular weight desorbents which :boil above the boiling point of the feed stock sorbate are recovered as a residue in the distillation of the desorbate stream.

The rate of primary reflux entering bed B8, the first of two beds comprising the primary rectification zone is determined by the effluent flow rate of desorbate flowing through valve 314 in line 33, the nonwithdrawn portion providing a stream continuously flowing into bed B8. The flow rate of this stream is also an important process variable in operating the process of this invention, determining the purity of the ultimate normal aliphatic hydrocarbon desorbate recovered from .the process. The flow rate of primary reflux is preferably controlled to povide a reflux flow rate greater than balanced reflux; that is, at a rate which will provide a Volume of fluid slightly greater than the interstitial fluid present in the first bed of the primary rectification zone during the period between shifts in inlet and outlet points of the influent and effluent streams of column `1. ln this manner the interstitial fluid in bed BS is replaced by a volume of primary reflux at least equal to th-e interstitial fluid volume into bed B7, and if the primary reflux rate is sufliciently greater than balanced reflux, the interstitial fluid in bed B7 and even in bed B6 may lbe replaced by primary reflux during the period that .bed B8 is the first bed ofthe primary rectification Zone. In the preferred manner of operating the process, the :primary reflux rate is controlled by setting the flow rate of normal aliphatic desorbate through valve 34, the remainder of the stream flowing continuously into bed B8 being controlled to flow at a -rate equal to lfrom about 1:10 to SOO'percent, and more preferably from 120 to percent of balanced reflux.

The interstitial fluid thereby replaced from bed B8 (and bed B7, as well, if the primary reflux rate of flow is sufficient) .flows into Ibed B6 into which a flush stream is introduced, as hereinafter described, carrying the interstitial fluid into bed B5, the first bed of the sorption zone. Since the interstitial fluid thereby flushed from the Void spaces between the particles of sorbent in beds B6 and B7 is largely composed of feed stock left in the void spaces when these beds were the initial recipient beds for feed stock charged during a lprior stage of the process cycle, the residual -feed stock comprising the interstitial fluid is thereby recovered, being continuously and most advantageously displaced into the sorption Zone to which fresh feed stock is continuously added during the course of the process. By virtue of the fact that the interstitial fluid in bed B8, fully replaced by primary reflux comprising the reflux portion of the normal aliphatic desorbate stream, the fluid stream withdrawn Ifrom bed B8 after the next succeeding shift in feed inlets and product outlets t0 thellextdownstream adjacent Zone will have the same composition as the normal desonbate stream currently removed from bed B9 as product. In this manner the purity of the normal aliphatic desorbate stream is maintained at a maximum, constant level, uncontaminated by .resid-ual feed stock Awhich would otherwise remain as interstitial flu-id in the void spaces between the particles of sorbent were it not replaced continuously by primary reflux.

As hereinabove indicated, the primary reflux entering bed yB8 displaces interstitial rfluid (feed stock) into the next downstream bed as the process continuesto operate. However, unless the displaced fluid enters the sorption zone during the period of time that sorption takes place in the latter zone, this quantity of the influent `feed stock continues to be left behind in the upstream beds without actually 'being recovered in the process.` Although the interstitial -iluid could be conveniently flushed into the Isorption zone lby increasing the primary reflux flow rate, this mehtod of flushing bed B6 of feed stock residue results in a portion of the normal aliphatic hydrocarbon of the desonbate stream entering the sonption zone where the normal aliphatic sonbate and desorbent would unnecessarily increase the load of normal aliphatics on the sorbent in the sorption zone. Furthermore, the lines, fluid distribution center channels, ports, and other equipment last carrying feed stock to lthe sorption Zone and 'which will provide the withdrawal or outlet route for the normal aliphatic desorbate product following a subsequent succession oif downstream shifts of product outlets and feed inlets still contain a residue of feed stock left in this equipment when the latter provided the route for influent feed stock, for example, when the inlet of bed B9 was lthe feed stock inlet. By flushing the feed stock from this equipment just after the lines and valve ports have ser-ved to conduct lfeed stock to the sorption zone, the nonnormal aliphatic components of the feed stock do not then contaminate the normal aliphatic product when the latter is subsequently withdrawn through this equipment after a succeeding series of shifts of the product -outlets and feed inlets. The residue of feed stock in this equipment is, in effect, replaced by the flush stream. Hence, it becomes more preferable to free the first upstream bed from the point of feed stock inlet of residual feed stock left in the void spa-ces between particles of sorbent (i.e., interstitial fluid) by introducing a separate flush stream into the inlet of bed B6, the flush stream being composed of material which is either readily separated from the desorbate and/or raffinate stream, for example by distillation therefrom, if it is to be recovered for recycling, or the next upstream desonbate effluent (lwhich in 4the accompanying diagram is normal aliphatic desorbate). The preferred materials for use as a flush stream is a ilfuid more volatile than the feed stock selected from rafllnate type components or from themateria-l -utilized asthe normal aliphatic hydrocarbon desonbent, or the desorbate itself. Thus, the flush stream may be a volatile low molecular Weight isoparaflln, such as isobutane, 2,3-dimethylbutane, 2-methyl-butane, etc., a 10W molecular weight normal aliphatic hydrocarbon of the type utilized as desonbent, such as nfbutane, n-pentane, etc., or normal aliphatic sor-bate, since the latter, in any event, 'will Ibe next withdrawn through the flushed lines.

The influent flush stream is charged into the process flow from storage through line at a rate determined by valve 36, throughy pump 37 and thenv into line 38, through the internal channels of fluid distribution center 3, through line 39 into the inlet of bed B6, the first bed upstream from bed B5 currently receiving fresh feed stock.l The flush stream rate of flow is sufficient to` provide a Volume of fluid at least equal to the volume of fluid in the lines leading up to the'bed receiving the flush stream, and more preferably, from 100 to 200 percent of the volume of fluid in the lines and valves, as well as the ports in the fluid distribution center between ther inlet to the bed beingllushed and the inlet to the fluid distribution center.- This volume of flush is sufficient to remove feed stock from the physical equipment involved in bringing feed stock from line 4 into bed B6 during the period that feed stock flows into bed B5.

The feed stock displaced from the external equipment by the flush stream provides a stream of fluid continuously flowing into bed B5, joined at the inlet of bed B5 with a continuous stream of feed stock entering the process flow va line 10, to thereby complete one cycle of operation.

It is apparent that each of the operations hereinabove described occur substantially simultaneously as plate B of valve 3 is continuously rotated; at a given instant of time thereafter, each bed becomes successively a more downstream bed with respect to the fluid stream continuously flowing upstream with respect thereto. Hence, if at a given instant bed B5 is the point which first contacts fresh charge stock, at a given point of time thereafter, following a sufficient interval to permit the rotation of plate B of valve 3 to complete l/nth of its complete cycle, where n is the number of beds per cycle, bed B4 becomes the point of first contact with the feed stock and bed B5 becomes the flush zone in the manner of operating the process illustrated in the accompanying diagram. It will also be noted that as the feed stock inlet shifts, the raffinate outlet, the aromatic and normal aliphatic desorbent inlets and the corresponding desorbate outlets, as well as the flush stream inlet also shift in the same aliquot portion of the total cycle, since the position of these inlets and outlets` relative to each other are fixed in the initial design of the fluid distribution center. In addition, as these points are shifted throughout the cycle, the composition of the continuously flowing fluid stream at different points along the line of flow and the composition of the fluid stream in the sorbent beds also change.

In order to realize maximum use of the sorbents in column 1, the flow rate of fluid is adjusted to provide the maximum flow of fluid consistent with the maintenance of fixed bed conditions, which is dependent upon whether the fluid phase in column 1 is gaseous or liquid, the size of the solid sorbent particles and the proportion of interstitial space to the physical volume that the particles occupy within the confines of each of the' beds.

The process of this invention' is operated at conditions of temperature, pressure and under other process conditions which depend upon the particular feed stock involved, the particular sorbents and the product purity required. In liquid phase operations when utilizing a normally liquid feed stock the pressure maintained within the contacting column is substantially less than gas phase operations and particularly when processing a normally gaseous feed stock in liquid phase. These considerations inherently depend upon` the molecular weight of the feed stock. For example, the feed stock will generally be suppliedto the column in vapor state when the com# ponents of the' feed stock contain fewer than about 4 carbon atoms per molecule (especially at operating temperatures above about 50 C.), but more preferably in liquid phase for' feed stocks of higher average' molecular weight and at process temperatures below the critical temperature. For gas phase operation the temperature may suitably vary from about 30 to about 300 C. andy pressures may be within the range of from atmospheric to 10 atmospheres or more. Typical liquid phase conditions are, for example, temperatures of from 0 to' about C. and pressures of froml atmospheric to 30 atmospheres, or` higher. For gas phase operation, the charging rate of feed stock into the column is maintained at less than the rate at which fluidiz'ation of the sorbent particles occur (for example, whenthe' contacting zones' are not completely filled with solid sorbent), and more preferably at a rate not in excess of about' 5 volumes of feedpervolum'e of sorbent perminute'. For liquid phase conditions,the charging' rate of feed stock is preferably not substantially greater than aboutv 1.5 volumes of fresh' feed stock per volume of sorbent per bed (since fresh, regenerated sorbent is added to the sorption zone one bed at a time), per minute, for relatively large granular particles of sorbent and from about 0.1 to about 1.0 volume of feed per volume of sorbent per bed per minute for smaller sorbent particles. These rates are, of course, dependent also upon the depth and width of the sorbent ,beds as Well as upon other factors involved in the design of the apparatus .and process conditions maintained in the column.

This invention is further illustrated with respect to several of its specic embodiments in the following examples which, however, are not intended to limit the scope of the invention necessarily in accordance with the specific process therein described.

EXAMPLE 1 The following run describes a process for simultaneously separating aromatic and normal aliphatic hydrocarbons from a substantially constant boiling mixture of hydrocarbons which is difficult to separate except when fractionated in the presence of a solvent or in a multiplate distillation column using high liquid reflux ratios. The feed stock is a synthetic blend of 25% by weight of mixed xylenes, 35% by weight of n-nonane and 40% by weight of 2,4-dimethylheptane. A synthetic feed stock of known composition was used in the following run to determine accurately the degree of separation and recovery .of the individual components, although naturally occurring mixtures of hydrocarbons, such as various petroleum fractions, would ordinarily constitute the usual charge stock to a separation process of the present type. The process flow and manner of separation is essentially similar to the iow pattern and arrangement shown in the accompanying diagram. The plant includes a separation column containing in the aggregate, 24 individual fixed beds of solid sorbent'housed within 4 separate tubular chambers (fabricated from 6 sections of tubular steel pipe interconnected in series by T-joint couplings) each chamber containing 6 individual beds, with the top of each bed connected with the bottom of the next downstream bed, providing upiiow through the series of beds. A pump-around-pump located in the line connecting the top ofthe last downstream bed of the 4th chamber with the first bed at the bottom of the first chamber in the series provides a continuously cyclic tiow arrangement. The feed inlet bed at the bottom of chamber No. l is considered the 1st bedin the series of *beds `for purposes of describing the process herein. AA fluid distribution center comprising a fiat-plate rotary valve, each of the plates containing internal channels ,and passageways which direct the incoming and outgoing'l streams into and from the proper beds of the separation column is an essential part of the equipment, each of the outlet ports around the periphery of the valve connecting with the T-joint coupling between each of the beds by means of a fluidy conduit or pipe. The flat-plate rotary valve which serves as the uid distribution center is fabricated in the design shown in copending application Serial No. 805,- 575, filed April 10, 1959, for Don B. Carson et al.

Each of the fixed beds of the separation column are packed with a mixture of activated carbon and 5A molecular sieves, the former adsorbent being selective for the xylene component of the feed mixture and the later sorbent being selective for the n-nonane component of the feed mixture. The activated carbon,.manufactured by Pittsburgh Chemical Company, type BTL, is screened to sizes within the range of 30-50 mesh. The 5A molecular sieves which make up 60% by weight of the solid sorbent packed into each of the beds are -50 mesh type 5A Microtraps manufactured by the Davison Chemical Company, and consisting of calcium-exchanged sodium aluminosilicate composited with a clay binder material and pelletted into spherical particles of the above size, 90 percent of which are from to 30 mesh in size.

After packing the mixture of activated carbon and 5A molecular sieve particles into each of the beds, the mixed sorbent is activated by charging nitrogen heated to a temperature of 60WI F. into the bottom bed and allowing the heated gas to iiow through the superadjacent beds for 10 hours, followed by filling the column and the remaining portions of the apparatus with dry n-butane at a pressure of 50 lbs/sq. in. throughout the apparatus.

The above specilied'feed stock mixture is charged at a pressure of 5G lbs/sq. in. and at substantially ambient temperature (approximately 25 C.) into the inlet of the iirst bed of the sorption zone `(the inlet of the 6th bed from the top of the first chamber) comprising 6 beds in serially arranged relationship to each other. The degree of separation of the components of the feed mixture is established by the influent feed stock rate of flow and the rate of shifting the feed inlets and product outlets along the line of flow, the combined factors in effect establishing a process variable referred to as the sorbate to sorbent ratio. The ability of the sorbents to recover sorbate from the feed stock is also determined by their capacities (aside from any rate factors) which for the charcoal adsorbent is designated as from 1 to 1.5 parts by weight lof charcoal per part by weight of vxylene in the feed stock and for the molecular sieve sorbent is from 1.6 to 1.8 parts by weight of 5A molecular sieves per unit weight of n-nonane in the feed stock. The feed stock thereafter flows in a downstream direction through a series of 6 beds comprising the sorption zone, the raflinate product (iso-octane) beingl withdrawn from the outlet of the sixth downstream bed (outlet of the bed at the top of chamber No. 1) at the same time that a portion of the raffinate, herein referred to as tertiary refiux, is permitted to flow into the inlet of the seventh downstream bed (the inlet of the lowermost bed No. 6 of chamber No. 2). The rate of raffinate stream withdrawal determines thel primary reiiux iiow vrate and is generally maintained at less than vbalanced reflux varying from`80% to 95% of balanced reflux, as specified hereinafter in Table I.

Raiiinate product, 2,4-dimethylheptane, flows from the outlet of the 6th downstream bedlinto a line connected with the fluid distribution center from which raiiinate is withdrawn as product. Since the feed stock xylenes replace benzene desorbent left on the particles of charcoal during a preceding cycle of operation and n-nonane replaces n-pentane desorbent occupyingr the pores of the 5A molecular sieves, the latter displaced desorbents mix with the raffinate as the latter feed stockgcomponents flow through the sorption zone. The raffinate efiiuent is therefore separately fractionated as it iows from the separation 'column to form residue consisting essentially of 2,4-dimethylheptane having a purity set forth inthe results reported in Table I. The desorbent overhead from the distillation column is refractionated to separate n-pentane from the benzene which are recycled to the respective desorbent inlets to the separation column.

The portion of the raffinate stream which continues to iiow from the outlet of the 6th downstream bed into the first bed of the tertiary rectification zone is referred to herein as tertiary reflux. A pump in this line raises the pressure on this stream before it enters the 7th downstream bed to 65 p.s.i.g.the resulting head being sufcient to circulate the iiuid stream through the series of beds at the required rateof iiow.

The feed inlet points and product withdrawal outlet points for the various fluid streams flowing into and from the contacting column are determined by valve settings in the feed inlet and product outlet lines attached to the fluid distribution center and the points of inlet and outlet for the various streams are determined by the position Vof the internal channels and ports designed into the flat plate rotary valve.

The seventh and eigth downstream beds from the feed inlet point at the particular stage ofthe process cycle here in described make up the tertiary rectification zone wherein,

21 the interstitial duid remaining between the solid sorbent particles is replaced with tertiary redux duid entering from the upstream beds during the period of time that each of the beds remains on stream for the particular stage of the cycle (that is, the period of time between shifts of inlets and outlets into and from the column).

At the same instant that feed stock dows into the first bed of the series of 24 beds (bed No. 6 at the bottom of chamber No. 1) benzene desorbent, at the pressure existing at the inlet of the aromatic desorbent in the contacting column is charged into the inlet lof the 9th bed downstream from the feed stock inlet (3rd bed from the bottom of chamber No. 2) at a flow rate equal to l0 Volumes of benzene per volume of xylene charged into the first bed per unit of time. The next succeeding 5 downstream beds (9th through 13th beds of the series) constitute the aromatic desorption zone wherein the benezene desorbent displaces xylene adsorbate fromthe activated carbon adsorbent. The desorption of xylene by benzene desorbent takes place at the same temperature as the sorption temperature (30 C.) to provide an isothermal process.

The aromatic desorbate stream consisting of a mixture of benzene and xylene, removed from the outlet of the 13th downstream bed (i.e., from the outlet of the 6th bed at the bottom of chamber No. 3), is divided into two streams: one stream at a rate of dow equal to 80 to 95% of balanced redux continues to dow in .a downstream direction into the inlet of the 14th bed downstream from the feed stock inlet (5th bed from the top of chamber No. 3). This stream (secondary redux) displaces interstitial duid from the Void spaces between the particles of sorbent and adsorbent in the 14th downstream bed, the displaced duid dowing fulther downstream into the 15th downstream bed which is the 2nd bed of the secondary rectification zone. The interstitial duid in the latterbed is displaced into the inlet of the 16th downstream bed where it is joined by a stream of normal aliphatic desorbent entering the 16th downstream bed.` The aromatic desorbate product is withdrawn from the outlet of the 13th bed downstream from the feed stock inlet through a line connecting the outlet with the fluid distribution center and thereafter through'an outlet conduit into a distillation column in which xylene product is separated as bottoms from the benzene desorbent (overhead). The recovery of xylene and the purity of the product is specified in the following Table I.

Normal pentane which displaces n-nonane from the pores of the 5A molecular sieve sorbent is charged in admixture with isopentane (50% by weight of n-pentane and 50% by weight of isopentane) as desorbent. This mixture is charged into the normal aliphatic` desorption zone at the same temperature 30 C.) as the temperature in the sorption zone and at a dow rate sufficient to provide a molar ratio n-pentane to n-nonane of 11.5 to 1.

The normal paraffin desorption zone is made up of 6 beds in series (16th to 21st beds downstream with respect to the feed stock inlet) in which the n-pentane desorbent displaces the n-nonane sorbate from the pores of the molecular sieve particles by a desorption mechanism based upon the Mass Action effect. The countercurrent flow effect provided by the present process also enhances the Mass Action effect by providing the greatest molecular ratio of desorbent to sorbate in the direction of decreasing sorbate content of the molecular sieves.

At the indicated dow rate of secondary redux which joins the continuous stream of normal paraffin desorbent entering the 16th downstream bed, the interstitial duid displaced from the void spaces between the particles of sorbent from the upstream beds is made up of normal paraffin desorbent which was last previously charged into the inlet of first upstream bed (15th bed) during the preceding stage of the cycle when the 15th bed in the series received normal aliphatic desorbent. By means of the dow diagram provided by the present process, each of the beds in series receives duid of the same composition which will be withdrawn from the bed after the next succeeding shift in feed inlets and product outlets, since the redux streams dow at a rate which will substantially sweep the existing interstitial duid from the void spaces during the period of time that the first bed of each of the rectification zones receives the respective redux streams. Each of the beds, therefore, is prepared in advance of the actual shift of inlets and outlets during the current on stream period prior to the actual withdrawal of duid from the beds as product streams.

As the normal aliphatic desorbent dows into the inlet of the 16th bed downstream from the feed stock inlet, a portion of the normal paraffin desorbate stream is withdrawn from the outlet of the 21st downstream bed (the 6th bed in the series of beds comprising the normal paradin desorption zone or the 3rd bed from the bottom of chamber No. 4) and a remaining primary redux portion is permitted to dow into the inlet of the 22nd downstream bed. The portion of normal aliphatic desorbate stream directed into the duid distribution center through the line connecting the latter unit with the outlet of the 21st downstream bed is adjusted to a dow rate which will provide a primary redux dow rate of from percent to percent of balanced redux. The primary redux portion of the desorbate stream enters the primary rectification zone comprising the 22nd and 23rd downstream beds respectively at a dow rate in excess of balanced redux to thereby ensure complete removal of feed stock from the void spaces between the particles of solid in these beds which was left behind as interstitial duid in these beds when they were last previously they first and second beds, respectively, ofthe sorption zone, prior to the shift of the feed inlet to the current downstream feed inlet point. By maintaining the dow of primary redux greater than the rate equal to balanced redux, the feed stock residing in the void spaces between the particles of sorbent does not contaminate the normal paradin desorbate stream to be removed from these beds in a succeeding shift of feed inlets and product outlets. At the present primary redux rate of dow greater than balanced redux the displaced interstitial duid is greater than the total void volume in the drst bed of the primary rectification zone, causing displacement of all of the remaining feed stock in the void spaces of the first bed of the primary rectification zone into the downstream beds comprising the sorption zone, and, depending upon the excess of primary redux over 100 percent of balanced redux, a portion of the normal paradin desorbate also enters the inlet of the sorption zone with the feed stock.

The normal paraffin desorbate removed as product stream from the fluid distribution center dows into a fractional distillation column wherein a normal and isopentane overhead (desorbent recycle) is fractionated from a residue of purified n-nonane, recovered as normal aliphatic product. Ihe recovery and purity of this product is reported in Table I, which follows. The overhead mix` ture of normal and isopentane is recycled directly to the aliphatic desorbent inlet.

During the course of each cycle of operation in which the feed inlets and product outlets are shifted in a downstream direction 3/24 aliquot of the entire cycle per shift, each of the feed lines to each of the 24 beds in succession (that is the line betwen the inlet of each of the beds and the corresponding outlet of the duid distribution center) carries feed stock to the first bed of the series of beds comprising the sorption zone. Subsequently, when the feed inlet is shifted to a further downstream bed which then becomes the first bed of the sorption zone, the line connecting the duid distribution center and the bed which previously received feed stock (that is, prior to shift in inlets and outlets) now becomes the outlet for the normal aliphatic desorbate. Unless the residual feed stock remaining in the line between the separation column and the rotary valve and the residual duid remaining in the internal channels of the rotary valve is removed therefrom prior to the withdrawal of normal aliphatic desorbate through the line, the residue o-f feed stock in the line otherwise contaminates the normal paraffin desorbate With that amount of other non-normal components present in the residue of feed stock in the internal channels of the valve and the line connecting the rotary valve with the sorbent bed. A Hush stream comprising the SOT-50 mixture of normal and isopentane utilized as desorbent is provided in the present process to free the internal channels of the iluid'distribu'tion center and the connecting lines ofv feed stock, the mixture entering the process flow at a rate which w-ill provide a volume suicient to completely flush the internal channels of the rotary valve and the interconnecting line between the contacting column and the rotary valve outlet ports after each shift of feed inlets and product outlets and before the shift to the next downstream inlets and outlets takes place. The flush stream is charged through the fluid distribution center and enters the line connecting the latter distribution center with the inlet of the 24th downstream bed (that is, the first bed upstream from the feed inlet point), the feed stock thereby removed from the internal channels of the` iluid distribution center and the connecting line flowing into the 24th downstream bed where it joins the continuous stream of fluid from the 23rd downstream bed displaced therefrom by virtue of primary reflux entering the first bed of the primary rectification zone. The fluid now remaining in the line connecting the 24th downstream bed and the fluid distribution center is a mixture of normal and isopentane which is readily fractionated from the substantially less volatile normal aliphatic desorbate product removed from the column through this route after the next two succeeding shifts in inlets and outlets. The required flow rate of the flush stream for each particular apparatus will vary depending upon the length lof the lines connecting the outlet ports of the iluid distribution center with the inlet to each of the beds, the volume of fluid occupying the internal channels and ports of the fluid distribution center and the degree of back-mixing as the flush stream flows from the distribution center into the separation column. To ensure complete displacement of feed stock from the internal passageways and connecting lines and to compensate foi possible back-mixing, it is generally preferred to maintain the flow rate of the flush stream suicient to provide from 1.2 to 3.5 volumes of flush per volume of aggregate uid space in the internal channels of the rotary valve and the connecting line between the valve and the contacting bed per shift of inlets and outlets.

As the various inlet and outlet streams flow into and from the beds of the separation column, as described PRODUCT PURITIES AND RECVERIES CORRELATED WITH PROCESS VARIABLES aforesaid, the rotating plate of the fluid distribution center slowly and continuously or intermittently rotates and after an average period of 1.8 minutes on stream for each bed of sorbent, the feed stock begins to enter the 2nd bed from the bottom of the first chamber, the ow of feed stock -into the lst bed of the lst chamber begins to taper olf and thereafter is gradually reduced to nil. The time required for each bed to be on stream is, in general, determined by the sorptive capacities of the sorbents (i.e., the capacity of the charcoal for xylene and the capacity of the molecular sieves for nfnonane), as indicated by the recoveries of each of the sorbate products. These capacities are also determined by the proportion of charcoal to molecular sieves packed into each of the beds, a factor predetermined by the relative concentrations of aromatic and normal paraffin sorbate components in the particular feed stock.

Substantially at the same instant that feed stock begins to enter the 2nd bed from the bottom of the 1st chamber (concurrent with the reduction in the flow of feed stock into the 1st bed of the 1st chamber) raflinate product begins to flow from the top of the 7th downstream bed (i.e., from the top of the lowermost bed in the 2nd chamber), benzene desorbent begins to flow into the inlet (bottom) of the 10th downstream bed (the bottom of the 3rd bed from the top of the 2nd chamber), xylene containing desorbate begins to flow from the outlet at the top of the 14th downstream bed (from the top of the 2nd bed from the bottom of the 3rd chamber), the mixture of nheptane and iso-octane comprising the normal aliphatic desorbent begins to flow into the inlet of the 17th downstream bed (the bottom of the 2nd bed from the top. of the 3rd chamber) normal aliphatic desorbate begins to ow from the outlet of the 22nd downstream bed from the top of the 3rd bed from the top of chamber No. 4), the flush stream begins to llow into the bottom of the lst bed in series (the 6th bed from the top of chamber No. 1) as the ow of feed stocks into the latter bed tapers of. In this manner, the process ilow yis maintained continuously cyclic and the iluid effectively ows in simulated countercurrent ow relationship to the solid sorbents in the serially arranged, interconnecting; beds. The following Table I presents the yields and purities of the sorbate products after fractionation of desorbents from the respective desorbate streams. The data also indicates the relationship of the above factors.- with the rates of reflux in each of the tertiary, secondary, and primary rectification zones, as well as the relationship of product purities and recovery with the rate of shifting feed inlet and product outlet points along the line of flow.

Table l Run No 1 2 3 4 5 6 7 8 9 Flow rates, percent of balanced reflux:

Tertiary reflux 0.85 0.85 0.85 1.2 1.2 1.2 0 85 0.85 1.2 Secondary reux 0.85 0.85 0.85 0.85 0.85 0.85 1 2 0.85 0.85 Primary reflux 1.40 1.40 1.40 1.8 1.40 1.2 0 90- 0.90 0` 00 Xylene desorbent (ben- K zene) moles CH/mole xylene 5 10 12 10 10 10 10 10 10 N-nonane desorbent `(N-C) moles n-Cs/ moles n-Cn 10 11 4 11 11 11 11 11 11 Recoveries, percent by Weight of component in blend:

2,4-Dimethy1heptane 97.6 98.0 97.4 94.1 98.5 98.7 96 95 82 Xylene 92 93.5 95.8 95 95 95 90 93 93 N-nonane 99 98.8 89 91.8 93.2 92.6 98 97 98 Purity of Products, percent by Wt.:

2,4-Dimethy1heptane 99 98.9 98.5 96.4 97 97.1 93 92 88 Xylene 93.4 92.8 94.1 87.2 89 85.() 83 85 85 N-nonane 98.9 99.1 98.0 98 97.9 98.4 86 87 86 25 EXAMPLE 11 Synthetically prepared silica gel formed by precipitating silica from an aqueous solution of sodium silicate by the gradual addition of hydrochloric acid to the water glass, mixing, thoroughly washing the precipitate and drying the Iresulting gel, followed by heating the 20-25 mesh portion of the product to a temperature of 400 C. for 6 hours to activate the silica gel when substituted for the charcoal adsorbent utilized in Example I above, produces comparable results, except that the proportion of silica gel adsorbent to A molecular sieves s increased to account for the reduced capacity of silica gel and the on-stream time per shift of inlets and outlets of the co1- umn is increased from 1.8 to 2.4 minutes. The yields of sorbate products (n-nonane and xylene) and raffinate product per unit of time are also reduced, corresponding to the reduction in capacity of each bed for the sorbate components.

We claim as our invention:

1. A continuous process for simultaneously separating the aromatic and normal aliphatic components from a normally liquid feed stock mixture of hydrocarbons containing said components and which also contains a rainatetype hydrocarbon selected from the branched chain and naphthenic hydrocarbons which comprises charging said mixture into the sorption zone of a fixed bed of a mixture of solid particles which are mixed, selective sorbents for normal aliphatic hydrocarbons and different solid particles which are selective sorbents for aromatic hydrocarbons, the first-mentioned particles being a dehydrated metal aluminosilicate having pores of cross-sectional diameter of from about 4 to about 6 Angstrom units and said different solid particles being selected from the group consisting of activated carbon, activated silica gel and a dehydrated metal aluminosilicate having pores of cross-sectional diameter greater than 6 and less than 10 Angstrom units, sorbing both normal aliphatic and aromatic components of the feed stock mixture in the particle mixture in said sorption zone, withdrawing a rainate effluent from the downstream outlet of the sorption zone, charging a desorbent comprising a normal aliphatic hydrocarbon boiling outside of the boiling range of the normal aliphatic component of said feed stock mixture into a normal aliphatic hydrocarbon desorption zone downstream with respect to said sorption zone outlet, withdrawing normal aliphatic hydrocarbon desorbate from the downstream outlet of said desorption zone, charging a desorbent for said aromatic hydrocarbon into a separate aromatic hydrocarbon desorption zone also downstream with respect to the outlet of said sorption zone, withdrawing aromatic desorbate from the downstream outlet of the aromatic desorption zone, simultaneously and continuously refluxing portions of each of said raflinate, said normal aliphatic desorbate and said aromatic desorbate into rectification zones next adjacent downstream to the respective sorption, normal aliphatic hydrocarbon desorption and aromatic desorption zones, and periodically advancing in equal increments downstream the inlets of said feed stock mixture and said normal aliphatic hydrocarbon and said aromatic hydrocarbon desorbents, while simultaneously advancing an equal increment of the total cycle the outlets for withdrawing raflnate and said normal aliphatic desorbate and said aromatic desorbate streams.

2. The process of claim 1 further characterized in that said different solid particles of said xed bed are silica gel particles having a retentive adsorptive capacity for said aromatic hydrocarbon.

3. The process of claim 1 further characterized in that said different solid particles of said fixed bed are activated charcoal particles having a retentive adsorptive capacity for aromatic hydrocarbons.

4. The process of claim 1 further characterized in that a tertiary reflux comprising a portion of said ranate stream is simultaneously and continuously reliuxed into the bed of sorbent next adjacent downstream from the outlet of said raffinate stream, the flow rate of tertiary reflux being from about 60 toabout 95 percent of balanced reux.

5. The process of claim 1 further characterized in that the rate of flow of reflux from the first desorption zone downstream from the rainate product outlet is maintained at from about 60 to about 95 percent of balanced reflux.

6. The process of claim 1 further characterized in that the rate of ow of reflux from the farthermost downstream desorption zone relative to the feed stock inlet is maintained at from about 110 to about 300 percent of balanced reflux.

7. The process of claim 1 further characterized in that the desorbent in each of the aromatic and aliphatic hydrocarbon desorption zones is a lower molecular weight, more volatile hydrocarbon of the same type as the aromatic and normal aliphatic sorbates desorbed in each of said desorption zones.

8. The process of claim 1 further characterized in that the desorbent for the aromatic component of the feed stock and the desorbent for the normal aliphatic component of the feed stock are identical and are normal paran hydrocarbons of lower molecular Weight than the normal aliphatic component of the feed stock, the aromatic desorbent prior to its entry into the aromatic desorption zone being heated to a temperature higher than the temperature of the sorption zone.

9. The process of claim 1 further characterized in that the aromatic component of the feed stock is selected from the group consisting of the polynuclear aromatic hydrocarbons, the polyalkyl-substituted mononuclear aromatic hydrocarbons and the alkyl-substituted mononuclear aromatic hydrocarbons in which the alkyl group contains more than one carbon atom, said process being further characterized in that said aromatic desorbent is a mononuclear aromatic hydrocarbon of lower boiling point and fewer carbon atoms than said feed stock aromatic component.

10. The process of claim 9 further characterized in that said feed stock aromatic component is selected from the group consisting of toluene and xylene and said desorbent aromatic is benzene.

11. The process of claim 1 further characterized in that said desorbent normal aliphatic hydrocarbon contains at least 4 carbon atoms and at least two fewer carbon atoms than the feed stock normal aliphatic component.

12. The process of claim 1 further characterized in that said normal aliphatic desorption zone is downstream in the cyclic How pattern with respect to the aromatic desorption zone.

13. The process of claim 1 further characterized in that the ow rate of desorbent in each of the respective aromatic and normal aliphatic desorption zones is sufficient to provide from 1.5 to 1 to about 30 to 1 moles of desorbent per mole of aromatic and normal aliphatic sorbate in the respective aromatic and normal aliphatic desorption zones.

14. The process of claim 1 further characterized in that the ow rate of desorbent in each of the respective aromatic and normal aliphatic desorption zones is sufcient to provide from 5 to 1 to about 20 to 1 moles of desorbent per mole of aromatic and normal aliphatic sorbate in the respective aromatic and normal aliphatic desorption zones.

15. The process of claim 1 further characterized in that a ush stream comprising a iiuid selected from the group consisting of a hydrocarbon boiling below the initial boiling point of the feed stock, desorbate stream of the same composition as the next upstream eiuent fluid and sorbate of the next upstream desorption zone, in an amount equal to from to 300 volume percent of the volume of fluid in the lines and equipment external to the rst bed of sorbent upstream from the feed stock inlet is charged into the inlet of said first bed upstream from the feed stock n 27 f inlet during the vperiod of time that feed stoel; flowsY into the bed currently receiving `feed stock.

References Cited by the Examiner 28 1 Olsen 260-676 Milton 260-676 Hann 260.-,676A Broughton 260-676 5 ALPHoN-so D. sULLIvANJrimary Examiner.

PAUL M, COUGHLAN, JR Examiner.

Claims (1)

1. A CONTINUOUS PROCESS FOR SIMULTANEOUSLY SEPARATING THE AROMATIC AND NORMAL ALIPHATIC COMPONENTS FROM A NORMALLY LIQUID FEED STOCK MIXTURE OF HYDROCARBONS CONTAINING SAID COMPONENTS AND WHICH ALSO CONTAINS A RAFFINATETYPE HYDROCARBON SELECTED FROM THE BRANCHED CHAIN AND NAPHTHENIC HYDROCARBONS WHICH COMPRISES CHARGING SAID MIXTURE INTO THE SORPTION ZONE OF A FIXED BED OF A MIXTURE OF SOLID PARTICLES WHICH ARE MIXED, SELECTIVE SORBENTS FOR NORMAL ALIPHATIC HYDROCARBONS AND DIFFERENT SOLID PARTICLES WHICH ARE SELECTIVE SORBENTS FOR AROMATIC HYDROCARBONS, THE FIRST-MENTIONED PARTICLES BEING A DEHYDRATED METAL ALUMINOSILICATE HAVING PORES OF CROSS-SECTIONAL DIAMETER OF FROM ABOUT 4 TO ABOUT 6 ANGSTROM UNITS AND SAID DIFFERENT SOLID PARTICLES BEING SELECTED FROM THE GROUP CONSISTING OF ACTIVATED CARBON, ACTIVATED SILICA GEL AND A DEHYDRATED METAL ALUMINOSILICATE HAVING PORES OF CROSS-SECTIONAL DIAMETER GREATER THAN 6 AND LESS THAN 10 ANGSTROM UNITS, SORBING BOTH NORMAL ALIPHATIC AND AROMATIC COMPONENST OF THE FEED STOCK MIXTURE IN THE PARTICLE MIXTURE IN SAID SORPITION ZONE, WIGHDRAWING A RAFFINATE EFFLUENT FROM THE DOWNSTREAM OUTLET OF THE SORPTION ZONE, CHARGING A DESORBENT COMPRISING A NORMAL ALIPHATIC HYDROCARBON BOILING OUTSIDE OF THE BOILING RANGE OF THE NORMAL ALIPHATIC COMPONENT OF SAID FEED STOCK MIXTURE INTO A NORMAL ALIPHATIC HYDROCARBON DESORPTION ZONE DOWNSTREAM WITH RESPECT TO SAID SORPTION ZONE OUTLET, WITHDRAWING NORMAL ALIPHATIC HYDROCARBON DESORBATE FROM THE DOWNSTREAM OUTLET OF SAID DESORPTION ZONE, CHARGING A DESORBENT FOR SAID AROMATIC HYDROCARBON INTO A SEPARATE AROMATIC HYDROCARBON DESORPTION ZONE ALSO DOWNSTREAM WITH RESPECT TO THE OUTLET OF SAID SORPTION ZONE, WITHDRAWING AROMATIC DESORBATE FROM THE DOWNSTREAM OUTLET OF THE AROMATIC DESORPTION ZONE, SIMULTANEOUSLY AND CONTINUOUSLY REFLUXING PORTIONS OF EACH OF SAID RAFFINATE, SAID NORMAL ALIPHATIC DESORBATE AND SAID AROMATIC DESORBATE INTO RECTIFICATION ZONES NEXT ADJACENT DOWNSTREAM TO THE RESPECTIVE SORPTION, NORMAL ALIPHATIC HYDROCARBON DESORPTION AND AROMATIC DESORPTION ZONES, AND PERIDICALLY ADVANCING IN EQUAL INCREMENTS DOWNSTREAM THE INLETS OF SAID FEED STOCK MIXTURE AND SAID NORMAL ALIPHATIC HYDROCARBON AND SAID AROMATIC HYDROCAROBN DESORBENTS, WHILE SIMULTANEOUSLY ADVANCING AN EQUAL INCREMENT OF HE TOTAL CYCLE THE OUTLETS FOR WITHDRAWING RAFFINATE AND SAID NORMAL ALIPHATIC DESORBATE AND SAID AROMATIC DESORBATE STREAMS.

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