US3393527A - Method of fractionating natural gas to remove heavy hydrocarbons therefrom - Google Patents

Description

United States Patent 3,393,527 METHOD OF FRACTIONATING NATURAL GAS TO REMOVE HEAVY HYDROCAR- BONS THEREFROM Leonard K. Swenson, Kansas City, and Roger W. Parrish, Independence, Mo., assignors to J. F. Pritchard and Company, Kansas City, Mo., a corporation of Missouri Filed Jan. 3, 1966, Ser. No. 518,212 4 Claims. (CI. 62-16) ABSTRACT OF THE DISCLOSURE A method of separating a heavy hydrocarbon liquid fraction from a hydrocarbon gas product by expanding the gas product to produce a first condensate and a remaining gas product. The remaining gas product is cooled to form a second condensate which is passed in countercurrent mass transferring relationship in a rectification zone with vapor formed from flashing the first condensate thus forming a third condensate which is lean in light hydrocarbons and a rectification zone overhead vapor which is rich in light hydrocarbons.

This invention relates to the treatment of hydrocarbon mixtures and, more particularly, relates to the separation of heavier hydrocarbons from lighter mixtures from wellhead natural gas containing various hydrocarbon constituents.

Natural gas taken from a wellhead consists primarily of methane, but generally, contains various higher molecular weight hydrocarbons as well and the relative quantity of higher molecular weight hydrocarbons in the natural gas varies considerably, depending on the location of the well. In many instances, the heavier hydrocarbons and, in particular, the fraction including the butanes through the hexanes, are more valuable after separation from the gas than the gas itself prior to the separation and, as a result, it is economically practical to remove these heavier hydrocarbons from the gas.

Present day conservation laws require that residual uncondensed gas consisting primarily of methane, be put to beneficial use and thus the recovery of the heavier hydrocarbons is limited only by the need for such methane rich residual gas rather than the need for the heavy fraction. Because of this, the relative availability of heavy hydrocarbons is decreased, thereby making it economically necessary and desirable to provide methods capable of obtaining the highest possible percentage of recovery of heavy hydrocarbons from the natural gas. Additionally, the methane rich residual gas must be transported to points of use through long distance pipe lines exposed to varying atmospheric conditions. If the gas contains moisture or heavy hydrocarbons, a lowering of the temperature of the gas can cause condensation of liquids and formation of hydrates which are detrimental to the transportation of the gas. Thus, it is desirable and frequently necessary because of the rulings of regulatory bodies to process the gas prior to introduction thereof into the pipe line by removing heavy hydrocarbons and moisture so that the dew point of the gas is maintained at a low level.

Depending upon local requirements for hydrocarbon products and local regulations, the methods used and results desired may vary. In some instances it may be necessary to process the natural gas to an extent to effect recovery of as much as 50 to 60% of the ethane, as well as the bulk of the propane and substantially all of the butanes and heavier hydrocarbons, thus producing a residual gas containing 98 to 99% methane. In other instances, recovery of the lighter hydrocarbons such as ethane, propane and butane may not be as desirable; thus, the present process is designed to recover the bulk of the pentanes and virtually all of the hexanes with the light hydrocarbons remaining uncondensed in the residual gas.

First, in those cases requiring a maximum recovery of light hydrocarbons, it was the practice to utilize a lean oil absorption system wherein the natural gas was passed in counter current absorbing relationship through a refrigerated liquid stream of lean oil composed primarily of butanes, pentanes and hexanes, whereby the desired hydrocarbons were absorbed from the natural gas into the lean oil. A large quantity of methane was also absorbed into the lean oil, a result that was obviously undesirable. Thus, measures had to be taken to strip the lean oil. These measures generally took the form of a de-ethanizer column wherein the bulk of the methane and part of the ethane present in the lean oil were re-vaporized and recycled into the inlet natural gas stream. The lean oil containing adsorbed hydrocarbons was then passed through a lean oil still where the recovered hydrocarbons were separated and the lean oil was prepared for recycling. The power requirements for refrigeration and heating necessary to operate an absorption system are high and, in fact, re-

covery beyond a certain percentage becomes economically impractical because energy requirements rise exponentially with the amount of recovery, attributable primarily to the fact that the residual gas leaving the system is in direct contact with lean oil in the absorber and thus, is nearly saturated with lean oil at the temperatures existing therein. Additionally, equipment costs are high because three distinct separation operations are required, namely, absorption with lean oil, de-ethanization and lean oil distillation, and a myriad of vessels, pumps, exchangers, boilers, compressors, evaporators, etc. are required to supply the necessary heating and counter-purposeful refrigeration.

Secondly, where it was desired to recover primarily the heavier hydrocarbons, it was the practice to lower the temperature of the gas until the heavy fraction was liquefied followed by separation of the gas from the liquid in a knockout pot. The liquid phase was stabilized by stripping of light hydrocarbons therefrom which were then recycled. It is diflicult to control the dew point of the residual gas leaving the process plant since it is in direct contact with the condensed heavy hydrocarbons and, therefore, the temperature must be lower than that necessary for liquefaction to insure a proper dew point in the residual gas.

Wellhead natural gas is usually available at pressures greater than the critical pressure of the residual sales gas of proper composition. Thus, it is necessary to decrease the pressure of the gas before separation can be accomplished. Generally, the pressure in the sales gas pipe line is higher than the pressure where separation can occur and, therefore, after processing, the residual gas must be recompressed for introduction into the pipe line.

In order to obtain maximum condensation of the liquid hydrocarbon fraction which it is desired to recover, the natural gas must be cooled to a required low temperature level which is at least below the level where hydrocarbonhydrates form. Thus, it is necessary to provide means, or take measures, aimed at preventing such hydrocarbonhydrate formation. The use of solid adsorbent drying units for removal of moisture is a well-known art, but expensive equipment is necessary and, therefore, it has become customary to inject methanol or some other suitable antifreeze into the natural gas to prevent freezing and formation of hydrates rather than removing the moisture. The injected methanol forms a low freezing point solution with the water which is condensed along with the heavy hydrocarbons that are liquefied when the temperature is lowered, and a two-phase mixture results. Most of the methanol will be present along with the water in a sep arate phase; however, a significant portion of the methanol will remain in solution with the heavy hydrocarbons and, in the past, the recovery of this portion has been impractical.

It is, therefore, the primary object of this invention to provide a novel method for recovering heavy hydrocarbon liquids from a gas containing methane and heavier hydrocarbons by fractionating the gas stream to remove a maximum quantity of selected heavy hydrocarbons at a minimum cost of production per unit thereby recovered.

As a corollary to the foregoing object, it is an important aim of the invention to provide a method as described wherein the fractionation step is carried out under pressure so that the removal of heavy hydrocarbon liquid from the natural gas can be closely controlled and efiiciently accomplished.

Another important object of this invention is to provide a method for processing natural gas by fractionation to obtain a selected heavy hydrocarbon fraction wherein the residual gas leaving the process plant is brought into contact with a liquid of substantially identical composition so that slight variances in operating conditions have little effect on the composition of such residual gas and the dew point thereof is closely controlled.

A further important object of this invention is to provide a method for separating even a substantial portion of the ethane fraction from natural gas by fractionation of the gas rather than by absorption of the ethane and heavier hydrocarbons into refrigerated lean oil wherein it is unnecessary to provide lean oil processing steps so that the number of separate operations is decreased and, therefore, both operating costs and capital expenditures are minimized. As a result of the foregoing object it is a further important aim of the invention to provide such a method wherein the residual gas leaving the fractionation zone is in vapor-liquid equilibrium with a liquid of substantially the same composition rather than a refrigerated lean oil liquid of higher molecular weight wherein the separation of heavier hydrocarbons from the residual gas is more complete so that the relative quantity of hydrocarbons heavier than methane separated from the gas is maximized.

Yet another important aim of this invention is to provide an efficient interrelated method for processing natural gas which includes steps designed to minimize the amount of recompression horsepower needed and heat rejected to cooling Water, thereby minimizing operating costs.

Yet another object of this invention is to provide a novel method for recovering and subsequently recycling methanol or similar antifreeze injected into a hydrocarbon product containing water to prevent the formation of ice or hydrates in the product when the temperature thereof is lowered to a level suflicient to cause liquefaction of a substantial portion of the heavier hydrocarbons in the product, the antifreeze and the water, wherein the liquefied mixture of heavier hydrocarbons, antifreeze and water are passed in countercurrent mass transferring relationship with a stream of water in an extraction column so that the water and antifreeze are transferred from the mixture into the water stream, thus producing a relatively antifreeze and water-free liquid hydrocarbon and a fractionatable water-antifreeze solution from which the antifreeze can be seperated for recycling and reinjection into the hydrocarbon product, thereby decreasing the operat- I ing expenses of the process plant.

In the drawings:

FIGURE 1 is a schematic flow diagram of a plant for carrying out one process embodying the present invention for use where maximum recovery of hydrocarbons heavier than methane is required;

FIG. 2 illustrates alternate apparatus for the plant of FIG. 1 which can be used in lieu of the portion of the apparatus enclosed in the dotted outline of FIG. 1;- and FIG. 3 is a schematic flow diagram of other apparatus for carrying out a second process embodying the present invention and specifically adapted for production of a stabilized heavy hydrocarbon liquid consisting primarily of pentanes and heavier hydrocarbons.

Referring to FIG. 1, the plant shown schematically therein is adapted for processing a wellhead gas containing a mixture of hydrocarbons. For purposes of illustrating the present process, it is assumed that a typical gas composition as defined below is to be processed.

Typical gas composition Percent (by vol.)

Nitrogen 0.07 Carbon dioxide 0.06 Methane 93.5 Ethane 3.5 Propane 1.2 Total butanes 0.7 Total pentanes 0.2

Total hexanes and heavier hydrocarbons 0.23

However, it is to be appreciated that the gas composition set forth is exemplary only and the present process may be used to process other gas compositions with equal results.

In the description of the instant process with particular reference to components of the plant schematically shown in FIG. 1, certain temperature and pressure parameters are set forth with reference to the specific gas composition in the table above. It is to be fully understood however, that the temperature and pressure conditions may obviously be varied as desired depending upon the results sought and do necessairly vary with the gas composition being processed.

In processing of gas of the composition defined, it is also assumed for purposes of the description only that gas is directed to the plant via line 10 at a pressure of approximately 1100 p.s.i. and a temperature of 70 F. Methanol or some other suitable antifreeze is injected from line 12 into the gas in line 10 at juncture 14. The temperature of the inlet gas is lowered to approximately 60 F. by indirect heat exchange in an exchanger 16 with residual gas leaving a reflux drum 18 through line 28. This lowering of the temperature of the inlet gas results in the production of a substantial quantity of liquefied heavier hydrocarbons as Well as a liquid phase consisting of methanol and Water which are separated from the gas in a knockout pot 20 and introduced into the lower section 22 of gas fractionator 24 through line 30. The remaining gas is expanded in expander 26 with the production of work and directed into fractionator 24 through line 32 at a temperature of F. and a pressure of approximately 600 psi. The conditions and composition of the stream leaving expander 26 through line 32 are such that substantial quantities of both liquid and vapor are present and, after introduction into fractionator 24 the gaseous phase will rise into upper section 34 and the liquid will flow downwardly into the bottom section 22 of fractionator 24.

F ractionator 24 consisting of upper section 34 and lower section 22 operates on a step-wise vaporization and condensation basis in a manner familiar to those skilled in the art. The liquids flowing downwardly through fractionator 24 are continually in direct mass and heat transferring relationship in such manner that the vapors flowing upwardly therein and the vapor phase and the liquid phase continually change in composition so that high boiling temperature constituents accumulate at the lower end of bottom section 22 and low boiling temperature constituents accumulate at the higher end of upper section 34. As the higher boiling temperature heavy hydrocarbon fraction accumulates at the bottom of lower section 22, it is preferably withdrawn through line 36 at a temperature of approximately 150 F. A portion of that liquid stream is removed and passed through line 38 into reboiler 40 where it is vaporized for reintroduction into lower end 22 through line 42. Any source of heat can be used in the operation of reboiler 40; however, steam which undergoes condensation is preferred. The vapor produced in reboiler 40 and reintroduced into lower section 22 through line 42, rises upwardly through fractionator 24 and is thereby in contact with the liquid flowing downwardly therein.

It is well understood by those skilled in the art that the temperature varies throughout the length of fractionator 24 such that the higher the elevation therein the colder the operation of the same since the relative quantity of lower boiling temperature constituents increases-with elevation. Thus, a lower temperature liquid with proper boiling temperature properties is withdrawn from fractionator 24 at an appropriate elevation through line 44 and introduced into economizer 46 where it is vaporized by passing in indirect heat exchange relationship with a refrigerant with the refrigerant being thereby cooled. This vaporized material is reintroduced into fractionator 24 through line 48 to supplement the vapors created in reboiler 40.

The vapor leaving upper section 34 of fractionator 24 through line 50 is preferably at a temperature of approximately ll4 F. and consists primarily of methane. This vapor is passed through condenser 52 where it IS cooled to approximately 115 F. and partially condensed. The partially condensed mixture leaves condenser 52 through line 54 for introduction into reflux drum 18 where the liquid and vapor phases are separated. The cold liquid methane is withdrawn from drum 18 through line 56 and forced by pump 58 back into the uppermost part of upper section 34 through line 60 for passage downwardly through fractionator 24.

The unvaporized portion of the overhead vapors are withdrawn from drum 18 through line 28 and passed through exchanger 16 for extraction of heat from the inlet fed gas as previously described. Upon leaving exchanger 16 through line 62, this residual methane is divided into two portions with one portion leaving through line 64 for use as fuel. The other portion flows through line 66 into compressor 68 where it is compressed to approximately 850 p.s.i. for reintroduction into the natural gas pipe line through line 70. Compressor 68 is driven by the work produced in expander 26.

The cold refrigerant brought into contact with the overhead methane vapor in condenser 52 is produced in a cascade refrigeration system in a conventional manner. Various combinations of refrigerants may be used; however, for purposes of explanation only, this description is based on the use of ethane and propane. Ethane may be conveniently used as the low temperature refrigerant and thus is employed to extract heat from the overhead vapors being partially condensed in condenser 52 and is thereby vaporized. The ethane vapors leave the condenser through line 72 and are pressurized by compressor 74 for introduction through line 76 into ethane condenser 78 where heat is extracted and the ethane condensed by passage in indirect heat exchange relationship With boiling propane therein. The ethane liquid thus produced leaves condenser 78 through line 80 for storage and accumulation in ethane receiver 82. The liquefied ethane is withdrawn from receiver 82 through line 84 and is flashed across valve 86 to lower the temperature thereof to approximately 120" F. for reintroduction into the shell side of condenser 52 through line 88.

The propane cycle preferably includes two compression cycles. Propane is accumulated and stored at approximately 110" F. in propane receiver 90 at a pressure which corresponds to the vapor pressure of propane at that temperature. Propane is withdrawn from receiver 90 through line 92 and flashed across valve 94 for introduction into propane interstage flash drum 96 through line 98. Propane in drum 96 is at a temperature of about 45 F. and a pressure which corresponds to the vapor pressure of propane at that temperature. The vapor produced by flashing the propane across valve 94 is withdrawn from drum 96 through line 100. The 45 F. proipane liquid leaves drum 96 through line 102 and is passed in indirect heat exchange with cold liquid in economizer 46 to cool the propane and vaporize the liquid as previously described. The subcooled propane is expanded across valve 104 and introduced into ethane condenser 78 where the propane is boiled at a temperature of approximately 15 F. to thereby condense ethane for use in the lower refrigeration stage. Propane is withdrawn from ethane condenser 78 through line 106 and compressed by lower stage propane compressor 108. Upon leaving lower stage compressor 108, the partially compressed propane is remixed with propane vapor leaving the interstage drum 96 through line and the combined stream is compressed in upper propane compressor 110. After being com-pressed by compressor 110, the pressure of the propane is such that it will condense at a temperature of 110 F. and thus, after heat exchange with cooling water in exchanger 112, the propane is in the liquid state for reintroduction into receiver 90.

The liquid leaving lower section 22 of fractionator 24 is a Z-phase mixture wherein one phase consists primarily of heavy hydrocarbon liquid, and the other phase consists primarily of -a methanol and water solution. However, part of the methanol present at this point is dissolved in the hydrocarbon phase. The 2-phase mixture is passed through cooler 114 where it is subjected to indirect heat exchange relationship with cooling water and thereby cooled to approximately 100 F. The cooled mixture of liquids is depressurized across valve 116 to a pressure of approximately 500 p.s.i. and introduced into the bottom portion of methanol extraction 60111111111 120. Essentially pure water is introduced at a temperature of 100 F. through line 122 into the top portion of column 120.

Two immiscible liquid phases are present simultaneously within column with the water phase being heavier than the hydrocarbon phase. The water phase is passed downwardly through column 120 in direct mass transferring relationship with the hydrocarbon liquids passing upwardly therein and the methanol dissolved in the hydrocarbon liquid when it is introduced into column 120 is extracted therefrom and transferred into the water phase. The essentially water and methanol-free heavy hydrocarbon liquid product leaves the system through line 124 and the water and methanol solution is withdrawn from the bottom of column 120 and heated in exchanger 126. The heated water-methanol solution is depressured across valve 128 and introduced into a methanol still 130 at about 10 p.s.i. where it is fractionated to produce a relatively pure methanol vapor which leaves the still through line 132 and is condensed by heat exchange with cooling water in methanol overhead condenser 134.

The liquefied methanol flows into methanol make-up drum 136 where it is suupplemented by the addition of make-up methanol through valve 138. Methanol withdrawn from drum 136 is split into two portions, the first portion being injected by reflux pump 140 into still 130 to serve as reflux therein, and the other portion being pressurized by recycle pump 142 for passage through line 12 and reintroduction into the inlet natural gas at juncture 14. Relatively pure liquid water leaves the bottom of still 130 through line 146 where a portion of the same is split off and revaporized in still reboiler 144 for passage upwardly in still 130. The remaining portion of this water is partially discarded through line 148 if there is an excess and the remainder is passed through line 150 into indirect heat exchange relationship with cold liquid methanol solution in exchanger 126 for cooling the water and heating the water-methanol solution. The water is further cooled by passing in heat exchange relationship with cooling water in cooler 152 and is forced by water recycle pump 154 back into column 120 through line 122.

Should water accumulate in undesirable locations within the process plant, a portion of the methanol flowing through line 12 is isolated and passed through line 156 for introduction into the natural gas downstream from knockout pot 20 through valve 158, or for introduction into the upper section 34 of fractionator 24 through valve 160. However, this feature is used only for upset conditions and valves 158 and 160 are normally closed.

Referring to FIG. 2, alternate overhead vapor con densation and reflux apparatus is schematically illustrated. This apparatus is equivalent to and is alternatively used in lieu of the equipment enclosed within the dotted lines in FIG. 1. The expanded natural gas is introduced through line 32 into fractionator 24. Inside the fractionator vessel and above upper section 34 thereof, a condensing zone 162 is installed. The vapor rising upwardly in upper section 34 comes into contact with condensing zone 162 where the vapors are partially condensed with the liquid thus produced flowing back and downwardly through the fractionator 24.

The remaining uncondensed overhead vapor leaves the top of fractionator 24 through line 28. Liquid ethane refrigerant is pumped into zone 162 by an ethane pump 164 where it is vaporized by passing in indirect heat exchange with methane vapors being partially condensed therein. Vaporized ethane leaves zone 16 2 through line 166 for introduction into ethane separation drum 168. The ethane vapors leaving drum 168 are accumulated in line 170 for recornpression in ethane compressor 74 and introduction into ethane condenser 78 through line 76. Ethane is condensed in condenser 78 by being passed in indirect heat exchange with vaporizing propane in the same manner as previously described. Liquefied ethane leaves condenser 78 through line 80 and is accumulated and stored in receiver 82 for subsequent passage through line 84 and flashing across valve 86 into separation drum 168. The vapors formed by flashing the ethane across valve 86 are combined with the vapors leaving the top of condensation zone 162 through line 166 for passage through line 170 and recompression.

The process described above with reference to the typical gas composition will operate to recover 55% of the ethane present in the inlet gas, 99.9% of the propane therein, and essentially 100% of the butanes and heavier hydrocarbons, these constituents being liquefied and eventually withdrawn from the top of extraction column 120 through line 124. Thus, the overhead residual methane stream includes essentially 100% of the inlet methane and nitrogen, and 45% of the inlet ethane therefore being approximately 98% methane in composition.

FIGURE 3 schematically illustrates a plant which may be employed to carry out a process forming a part of the present invention wherein the recovery of a stabilized condensate product consisting primarily of pentanes and heavier hydrocarbons is desired with the remainder of the lighter hydrocarbons and methane going overhead for reintroduction into the natural gas pipe line. For purposes of illustration, it is assumed that wellhead gas of the composition below is introduced into the system through line 172 at a pressure of approximately 1500 psi. and a temperature of F.

Typical composition of inlet gas Percent (by vol.) Nitrogen 1.3 Methane 75.0 Carbon dioxide 4.4 Ethane 9.2 Propane 3.9 Butanes 2.0 Pentanes 1.2

Natural gas of the above shown typical composition at the pressure and temperature indicated is within the retrograde condensation zone and, therefore, the inlet gas is depressurized across valve 174 to a pressure of 1,000 psi. with the production of a substantial quantity of heavy hydrocarbons consisting chiefly of hexanes and higher boiling hydrocarbons. A substantial quantity of water is likewise condensed by the expansion across valve 174. The liquefied phase and the remaining gaseous phase are passed through line 176 into condensate separator 178 where condensed water is accumulated in sump 180 of separator 178 for disposal therefrom through line 182. The vaporous hydrocarbons are separated from the liquefied hydrocarbons and transported through line 184, through an acid gas treater 186 where carbon dioxide, hydrogen sulfide, etc. are removed if initially present.

The treated gas is cooled in gas exchanger 188 to a temperature of 40 F., thereby producing another quantity of liquefied hydrocarbons and this mixture of residual vapors and heavy hydrocarbon liquids is introduced into the lower portion of gas fractionator 190 where the liquids immediately fall to the bottom of fractionator 190 for passage therefrom through line 192. The cooled vapors introduced into fractionator 190 rise upwardly therein in countercurrent direct mass and heat transferring relationship with a reflux liquid and the higher boiling point constituents are continually removed from the vapor which accordingly becomes richer in light hydrocarbon constituents. The light hydrocarbon rich overhead vapor leaves fractionator 190 through line 194 at a pressure of approximately 950 psi, and the temperature thereof is reduced to approximately 0 F. in reflux condenser 196, thereby causing partial condensation of the overhead vapors. The condensate formed in condenser 196 along with residual gas is conveyed through line 198 into reflux drum 200 where the vapor phase and liquid phase are separated. The vapor phase leaves drum 200 through line 202 at a temperature of approximately 0 F. and is brought into indirect heat exchanging relationship with the gas leaving treater 186 in exchanger 188 to extract heat from and partially condense the gas and warm the overhead vapor before it leaves the process plant through line 204.

A single stage propane refrigeration system is used to supply propane refrigerant to condenser 196 for cooling and partially condensing the overhead vapors therein. The propane refrigerant is vaporized at a temperature of 10 F. in condenser 196 and the vapor thus produced is compressed in a Z-stage compressor 206 to a pressure sufiicient to condense the propane when the temperature thereof is reduced to approximately 100 F. The pressurized propane refrigerant is exchanged with cooling water in cooler 208 where heat is rejected to the cooling water, thereby lowering the temperature of the propane to 100 F., thereby causing it to condense. Upon leaving cooler 208, the propane liquid is stored and accumulated in propane receiver 210. The propane liquid is then depressurized across valve 212 from where it is conveyed Hexanes and heavier hydrocarbons 9 l back into condenser 196 for refrigeration by a boiling refrigerant.

The heavy hydrocarbon liquids separated from the inlet g'as stream in separator 178 are depressurized across valve 214 and introduced into a stabilizer feed flash drum 216 operating at a pressure of approximately 300 p.s.i. and a temperature of approximately 85 F. A significant quantity of the liquid expanded across valve 214 is thereby flashed and rises upwardly in the rectifying zone 218 situated atop flash drum 216. The liquid produced ingas fractionator 190 is introduced into the top portion of rectifying zone 218 for passage downwardly therein in direct heat and mass transferring relationship with the vapors rising upwardly therein, thus producing a gas richer in lighter hydrocarbons which leaves rectifying zone 218 through line 220 and a liquid richer in heavier hydrocarbons which passes out of rectifying zone 218 into flash drum 216 where it is accumulated and withdrawn from drum 216 through line 222.

The unstabilized heavy hydrocarbon liquid leaving flash drum 216 through line 222 is heated in exchanger 224 and introduced through line 226 into stabilizer column 228 where it is fractionated to produce an overhead vapor richer in light hydrocarbons about the unstabilized liquid, and a liquid richer in heavier hydrocarbons than the stabilized liquid. The vapors leaving the top of stabilizer 228 are passed in indirect heat exchange with cooling water in exchanger 230 thus causing condensation of a portion thereof. The liquid phase produced in exchanger 230 is separated from the remaining gas in stabilizer reflux drum 232 and pumped by pump 234 back into the top of column 228 to serve as downwardly flowing reflux liquid therein.

The stabilized heavier hydrocarbon liquid leaving the bottom of column 228 is divided into two portions, the first of which is revaporized in stabilizer reboiler 236 for reintroduction into column 228. The heat necessary for such vaporization can be supplied in any manner but is usually provided by passing the liquid stream in indirect heat exchange relationship with steam. The other portion of the stabilized liquid product leaving column 228 is passed in indirect heat exchanging relationship with the unstabilized liquid in exchanger 224 to extract heat from the stabilized product in an economical manner. The stabilized product consisting primarily of pentanes and heavier hydrocarbons is then passed in heat.

exchange relationship with cooling water in product cooler 238 from where it is transported through line 240 to points of use. The vapor phase leaving stabilizer reflux drum 232 is mixed with the vapors leaving the top of rectifying zone 218 for recompression by recycle compressor 242 and reintroduction into the inlet gas stream at juncture 244. Fuel for operation of steam boilers and other plant accessories is withdrawn from the recycle system through line 246.

It is of particular interest to note that the dew point of the sales gas vapors leaving the process plant through line 202 can be closely controlled by making minor adjustments in the propane refrigeration system and the temperature at which condenser 196 is operated. The gas leaving condenser 196 is in equilibrium with a liquid of substantially the same composition and, therefore, minor fluctuations in the temperature at which condenser 196 is operated have no substantial effect on the dew point of the vapors in line 202.

Specific gas compositions and typical processing conditions for handling the same have been set forth herein for clarity in explaining the present process. However, it is to be understood that the conditions and compositions have been selected for purposes of the description only and the scope of the invention is limited only by the appended claims.

Having thus described the invention, what is claimed as new and desired to be secured by Letters Patent is:

1. In a method for condensing and separating a desirable heavy hydrocarbon liquid fraction from a hydrocarbon gas product, wherein said gas product is expanded to a lower pressure within the retrograde condensation range thereby producing a first condensate at a first temperature, and wherein the remaining gas product is cooled to produce a second condensate richer in undesirable light hydrocarbons than said first condensate at a second lower temperature, the steps of:

' flashing said first condensate to produce a flash vapor and a residual heavy hydrocarbon liquid, said vapor being leaner in undesirable light hydrocarbons than said second condensate; and

passing said flash vapor and said second condensate in countercurrent mass transferring relationship to one another in a rectification zone thereby producing a third condensate which has been at least partially stripped of undesirable light hydrocarbons and a rectification zone overhead vapor which is rich in said undesirable light hydrocarbons.

2. The method of treating :a pressurized natural gas product including methane and light hydrocarbons with heavy hydrocarbons for producing a liquid richer in heavy hydrocarbons than said gas product and a vapor richer in methane than said gas product, said method comprising:

expanding said gas product to a lower pressure within the retrograde condensation range to effect liquefaction of a substantial portion of the heavy hydrocarbons without liquefaction of a significant portion of the methane and producing a first gaseous phase and a first liquid phase;

separating the first gaseous phase from the first liquid phase; lowering the temperature of said first gaseous phase to effect liquefaction of an additional portion of the heavy hydrocarbons still without liquefaction of a significant portion of the methane and producing a second gaseous phase and a second liquid phase;

separating the second gaseous phase from the second liquid phase;

passing said second gaseous phase through a fractionating zone to produce said vapor richer in methane than said gas product and a third liquid phase, the vapor overhead from said fractionating zone being cooled to a level to effect liquefaction of at least a portion thereof and producing a reflux liquid and said reflux liquid being passed in countercurrent relationship to the gaseous phase in the fractionation zone;

admixing said second liquid phase and said third liquid phase to produce a flash drum reflux liquid;

flashing said first liquid phase into a flash drum zone thereby producing a flash drum vapor richer in light hydrocarbons than said first liquid phase and a first part of unstabilized liquid richer in heavy hydrocarbons than the first liquid phase;

passing said flash drurn reflux liquid and said flash drum vapor in countercurrent relationship to one another in a rectification zone to produce a flash drum overhead vapor and a second part of unstabilized liquid;

passing said unstabilized liquid through a stabilizing zone to produce said liquid richer in heavy hydrocarbons than said gas product also richer in heavy hydrocarbons than said unstabilized product and a stabilizer overhead gas richer in light hydrocarbons than said unstabilized liquid;

admixing at least a portion of said stabilizer overhead gas and at least a portion of said flash drum overhead to form a recycle gas;

recompressing said recycle gas to said lower pressure;

and

admixing said recompressed recycle gas with said gas product before the separation thereof into said first gaseous phase and said first liquid phase.

3. The invention defined in claim 2, including the step of passing the vapor richer in methane produced in said fractionating zone in indirect heat exchange relationship 1 1 with said first gaseous phase to effect at least a portion of said lowering of the temperature thereof.

4. The invention defined in claim 2, including the step of passing the liquid richer in heavy hydrocarbons than said gas product produced in the stabilizing zone in indirect heat exchange relationship with said unstabilized liquid prior to the passage thereof into said stabilizing zone, to cool said liquid richer in heavy hydrocarbons and heat said unstabilized liquid.

References Cited UNITED STATES PATENTS 1,850,529 3/1932 Bottoms 62-27 12 Deming. Miller. Miller.

Jackson 62--23 X De Lury 6240 X Murphy 260-643 X Cunningham 6227 Bucklin 6223 X Bludworth 6227 X 10 NORMAN YUDKOFF, Primary Examiner.

W. PRETKA, Assistant Examiner.

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