US3225111A

US3225111A - Hydrodealkylation process and apparatus

Info

Publication number: US3225111A
Application number: US245846A
Authority: US
Inventors: William R Lehrian; Elwood E Nelson; Vernon J Yeakley
Original assignee: Gulf Research and Development Co
Current assignee: Gulf Research and Development Co
Priority date: 1962-12-19
Filing date: 1962-12-19
Publication date: 1965-12-21
Anticipated expiration: 1982-12-21

Description

Dec. 21, 1965 w, R. LEHRIAN ETAL 3,225,111

HYDRODEALKYLATION PROCESS AND APPARATUS Filed Dec. 19, 1962 2 Sheets-Sheet 1 INVEIgORS 1. HR/xi/V 225%? E. NELSON BY VERNON J. YEA/(LEY United States Patent 3,225,111 HYDRODEALKYLATIGN PROCESS AND APPARATUS William R. Lehrian, Verona, Elwood E. Nelson, Gibsonia, and Vernon J. Yeakley, Oalrmont, Pa., assignors to Gulf Research dz Development Company, Pittsburgh, Pa., a corporation of Delaware Filed Dec. 19, 1962, Ser. No. 245,846 4 Claims. (Cl. 260-672) This invention relates to a process and apparatus for the hydrodealkylation of alkyl aromatics, particularly to the thermal hydrodealkylation of alkyl aromatics.

Alkyl aromatics, such as toluene, can be dealkylated to lighter aromatics, such as benzene, by subjecting such alkyl aromatics in the presence of hydrogen to an elevated temperature and elevated pressure for a controlled length of time. As a result of such reaction conditions the alkyl group is cleaved from the alkyl aromatic and combines with the hydrogen present to form a saturated aliphatic hydrocarbon. The desired aromatic can be separated from the saturated aliphatic hydrocarbon and unreacted alkyl aromatic and hydrogen, if present, in any convenient manner.

Thermal hydrodealkylation of alkyl aromatic hydrocarbons, such as toluene, xylenes, trimetl'iylbenzene isomers, alkyl naphthalenes and mixtures thereof, is effected at a temperature in excess of about 1000 F., but preferably at a temperature in the range of about 1100 to about 1500" F. While the mixture of alkyl aromatic and hydrogen is being heated to the defined hydrodealkylation temperature little or no hydrodealkylation of the alkyl aromatic occurs. The small amount of hydrodealkylation which may occur during the preheating stage occurs at the elevated temperatures at the end thereof. The mixture can be preheated to the defined hydrodealkylation temperature in the matter of about to about 300 seconds.

Once the defined hydrodealkylation temperature is reached hydrodealkylation occurs swiftly and can be completed in a matter of about 10 to about 90 seconds. Since hydrodealkylation of alkyl aromatics is highly exothermic, at the onset of the reaction there is a tendency for the reaction mixture to absorb the heat of reaction and thereby rise to a temperature level in excess of the desired hydrodealkylation temperature. Near the end of the reaction period, only a small amount of alkyl aromatic remains, and accordingly only a small amount of heat is liberated when it is dealkylated. This small amount of heat is more than suificient, however, to maintain the desired hydrodealkylation temperature level above the desired optimum temperature level. In in stances wherein it is desired to maintain a substantially constant temperature level in the reactor to assure uniformity of product, therefore, the inherent characteristics of the hydrodealkyla-tion reaction make it extremely dithcult to achieve such result.

We have found that the above difficulties can be avoided and the hydrodealkylation of alkyl aromatics can be effected at a selected temperature level in an essentially isothermal operation in accordance with the process and apparatus described and claimed herein. The advantages of the present invention can be understood by reference to the accompanying drawings which form a part of this specification. FIGURE I is an isometric assembly drawing, with parts broken away, showing the construction and design of the preheater and reactor and the housings therefor. FIGURE II is a graphical representation of data illustrating the operation of our process using our apparatus therefor.

Referring to FIGURE I there is illustrated a preheater 3,225,111 Patented Dec. 21, 1965 furnace 2 and a soaker furnace 4. Preheater 2 comprises a housing 6 in which there is mounted an elongated coil 8 wherein the mixture of alkyl aromatic hydrocarbon and hydrogen are raised to hydrodealkylation temperature. Soaker furnace 4 comprises a housing 10 ing which there is mounted an elongated coil 12 wherein the preheated charge from coil 8 is subjected to the desired hydrodealkylation temperature.

The charge comprising alkyl aromatic hydrocarbon and hydrogen is introduced into the system by line 14 adjacent the top of housing 6 at a temperature of about 60 to about 500 F. and a pressure of about 25 to about 1000 pounds per square inch gauge. Coil 8 as shown is composed of a series of horizontal sections joined together by appropriate return bends 16. Mounted on both walls of housing 6 adjacent the horizontal sections of coil 8 are selected numbers of heat-producing elements, such as gas burners 18, for raising the temperature of the charge in coil 8 to dealkylation temperature. In order to facilitate heating of the charge in coil 8, the consecutive horizontal sections thereof are preferably not positioned immediately over each other nor in the same horizontal plane but at an angle from the vertical. Accordingly, as the charge passes downwardly through the coil the temperature of the charge becomes progressively higher and as it leaves housing 6 by way of line 20 the temperature of the charge is within the range of about 1l00 to about 1500 F. and the pressure about 25 to about 1000 pounds per square inch gauge. If desired hydrogen at a temperature of about to about F. and a pressure of about 25 to about 1000 pounds per square inch gauge can be introduced into coil 8 at one or more points by line 22 and one or more of

lines

24, 26 and 28 in order to help control, by quenching and dilution, the temperature of the mixture in coil 8 to minimize reaction and avoid or control a runaway reaction. When the mixture leaves the preheater furnace 2 by way of line 20, little or no dealkylation of alkyl aromatic has occurred. Depending on the severity of the heating and the temperature and pressure of the mixture in coil 8, the amount of alkyl aromatic which may be dealkylated in preheater furnace 2 may be from about 0 to about 20 percent by weight thereof. Gases resulting from com- 'bustion in burners 18 are removed overhead from preheater furnace 2 by stack 30. Although the coils have been shown in FIGURE I in the preferred. embodiment it is apparent that they can be mounted in any desirable fashion within housing 8 and the charge therein can be made to flow in any desired direction therein provided the temperature of the charge within the desired time of about 10 to about 300 seconds is raised to the defined hydrodealkylation temperature.

The heated charge in line 20, now at a temperature level suitable for hydrodealkylation, is then introduced adjacent the base of housing 10 into coil 12 which is positioned and mounted therein in a manner similar to coil 8 in housing 6. Upon entry of the heated mixture into coil 12, therefore, substantial dealkylation of the alkyl aromatic hydrocarbon begins. Since hydrodealkylation in the present instance is highly exothermic, the reaction occurs with great liberation of heat. Left uncontrolled the reaction mixture would have a tendency to absorb the heat of reaction and the temperature thereof would rise beyond the desired hydrodealkylation temperature level. In accordance with the teachings of this invention the lower portions of the walls of soaker furnace 4- adjacent the horizontal sections of coil 12, and if desired, the base of housing 10, are provided with openings 32 for permitting cold air to move into housing 10 in direct contact with coil 12. In this way the cold air absorbs an appreciable amount of the heat of reaction within the coil and helps maintain the temperature of the mixture in the coil at the desired temperature level. In order to control the amount of air entering housing 10 by way of openings 32, suitable devices, such as doors 34 pivotally mounted on the exterior of housing 10 can be provided.

As the mixture rises within coil 12 and hydrodealkylation of alkyl aromatic occurs, the amount of alkyl aromatic remaining is progressively reduced. The amount of reaction occurring in the upper part of coil 12 being small, only a small amount of heat is liberated. Accordingly the rate of removal of heat from this portion of the coil can be reduced in order to maintain the desired temperature level therein. This can be done by maintaining the temperature level of the air adjacent the coils at appropriate temperature levels. In accordance with the teaching of this invention each of the side walls of housing 10 is provided adjacent the upper level thereof with selected numbers of heat-producing elements, such as burners 36, for helping to raise the temperature level of the air in the upper portion of housing 10 to a level sufficient to control the rate of heat removal from the upper portion of coil 12 and thereby maintain the mixture therein at the desired hydrodealkylation temperature level. Gases resulting from combustion of gases in burners 36 are removed from soaker furnace 4 by stack 38. If desired, additional temperature control can be obtained in coil 12 by introducing therein at one or more points by line 40 and one or more lines 42, 44 and 46, hydrogen at a temperature of about 80 to about 120 F. and a pressure of about 25 to about 1000 pounds per square inch gauge in an amount of about to about percent by weight based on the fiow rate of mixture in coil 12.

The temperatures in coil 12 are maintained at a suitable selected temperature within the range of about 1100 to about 1500 F. and a pressure of about to about 1000 pounds per square inch gauge. A gas space velocity (actual volume of mixture per hour per volume of reactor) of about 100 to about 300 is maintained within coil 12. The product leaving soaker furnace 4 by line 48 is cooled to a temperature below 1000 F. in order to forestall secondary reactions. The product from soaker furnace 4 can be cooled in any suitable manner but is desirably cooled to a temperature of about 1000 F. or less by admixture with cold quench from line 50. The cold quench from line 50 can be a hydrogen-rich gas, as used for temperature control in line 40, a portion of the dealkylated benzene or naphthalene product or other suitable process medium, and can be at a temperature of about 60 to about 400 F. After the reactor efiluent is cooled, hydrogen and other gases are vented from the reaction mixture and the remainder is separated into its component parts by any suitable means, preferably by distillation at a temperature of about 175 to about 250 F. and a pressure of about one to about 10 pounds per square inch gauge.

As shown, coil 12, like coil 8 previously described, is composed of a series of horizontal sections joined together by appropriate return bends, with the consecutive horizontal sections thereof preferably not being positioned immediately over each other nor in the same horizontal plane but at an angle from the vertical. An advantage of this arrangement is that the horizontal sections are adjacent to the openings or burners in the walls of the housing 10. In addition the preferred design permits the rise of hot air upwardly over the coils in the soaker furnace 4 and therefore reduces the amount of heat which has to be supplied thereto by burners 36. If desired, however, though not preferred, the coil in housing 10 could be mounted in a substantially horizontal plane within an appropriate housing with opening in the housing wall adjacent the first part of the coil and burners adjacent the latter part of the coil.

It can be seen that the temperature of the air adjacent the lower end of housing 10 must be cooler than the air adjacent the upper end thereof, for more heat has to be absorbed in the lower end of the housing than in the upper portion thereof to maintain substantially isothermal conditions within coil 12. While it is difficult to define specifically the temperatures which must be maintained at any single point within housing 10, in general the temperature of the air therein is maintained progressively at temperatures from a low at the base of housing 10 of about F. to a high of about 1500 F. adjacent the top thereof, preferably from about 500 to about 1300 F.

In order to withstand the high temperatures involved and to overcome hydrogen embrittlement, the coils in the defined apparatus can be made of stabilized high alloy austenitic steels such as the following:

Grade Nominal The coils themselves can have a thickness of about A; to about one inch. The interior of the housings is internally lined with a refractory material.

The invention can further be illustrated by the following. The reaction, similar to that illustrated in FIGURE 1, comprises preheater tubes having an inner diameter of 3.75 inches and an outer diameter of 4.50 inches and reactor tubes having an inner diameter of 5.625 inches and an outer diameter of 6.625 inches. The total coil length, comprising 20 tubes in the preheater and 18 in the soaker furnace, is 760 feet, of which 400 feet are in the preheater section and 360 feet in the soaker furnace. The total coil volume amounts to 92.81 cubic feet, of which 30.68 are in the preheater section and 62.13 are in the soaker section. The coils in the preheater and in the soaker furnace are made of Type 347 stainless steel. Toluene at the rate of 20,349 cubic feet per hour, calculated at atmospheric temperature and pressure, and hydrogen at the rate of 61,046 cubic feet per hour, calculated at atmospheric temperature and pressure, are introduced into the preheater coils, after heating by heat exchange means, at a temperature of 350 F. and a pressure of 375 pounds per square inch gauge.

The data obtained above have been plotted in FIGURE II. The data do not include heat loss from the tube surfaces by radiation and are based on movement of air around the reactor tubes at the rate of one foot per second. The data do not take into consideration the sur face or volume of the return bends of the coils nor of the small amount of line leading from the preheater to the soaker furnace. In the graph the horizontal section of tubes in the preheater are marked in inverse order, with the first horizontal section being considered the 20th tube and the last or outlet section of tubing being number one. The data in the graph of FIGURE II show that only about 11 percent conversion has occurred by the time the preheated charge is introduced into the soaker furnace. To maintain the desired temperature level of 1350 F. in the coils adjacent the entrance thereof requires a rate of cooling of about 7,000 B.t.u. per hour per square foot of coil surface. As the reactants progress through the coil and the conversion level increases, the reaction rate decreases. As the reactor exit is approached, the rate of heat liberation also decreases, and the required rate of cooling falls to about 300 B.t .u. per hour per square foot of coil. This decreased rate of cooling is maintained by virtue of the air being heated by reaction heat and by the burners located in the upper portion of the soaker furnace.

Obviously many modifications and variations of the invention, as hereinabove set forth, can be made without departing from the spirit and scope thereof, and therefore only such limitations should be imposed as are indicated in the appended claims.

We claim:

1. Apparatus comprising a first housing and a second housing, each provided with a stack therefor, a first elongated coil extending into said first housing, disposed therein and extending outwardly thereof, heat-producing elements within said first housing, a second elongated coil extending into said second housing, disposed therein and extending outwardly thereof, means communicating between said first coil and said second coil, means adjacent the entry of said second coil into said second housing for permitting entrance of air into said second housing, and heat-producing elements in said second housing adjacent the exit end of said second coil.

2. Apparatus comprising a first housing and a second housing, each provided with a stack therefor, a first elongated coil extending into said first housing, disposed therein and extending outwardly thereof, heat-producing elements within said first housing, a second elongated coil extending into said second housing adjacent the base thereof and extending outwardly of said second housing adjacent the top thereof, means communicating between said first coil and said second coil, means adjacent the base of said second housing for permitting entrance of air into said second housing and heat-producing elements in said second housing adjacent the top thereof.

3. A process for hydrodealkylating an alkyl aromatic which comprises heating a mixture comprising essentially an alkyl aromatic and hydrogen in an elongated zone to a temperature of about 1100 to 1500 F. and passing said heated mixture from a lower level to a higher level in an elongated reaction zone composed largely of a series of horizontal runs while maintaining the temperature of the contents of said reaction zone at a selected temperature level within a range of about l100 to about 1500 F. by passing air having a temperature of about 100 to about 15 00 F. progressively upwardly from said lower level to said higher level in indirect contact with the contents of said reaction zone and additionally heating said air adjacent said upper level.

4. A process for hydrodealkylating toluene which comprises heating a mixture comprising essentially toluene and hydrogen in an elongated zone to a temperature of about 1100 to about 1500 F. and passing said heated mixture from a lower level to a higher level in an elongated reaction zone composed largely of a series of horizontal runs While maintaining the temperature of the contents of said reaction zone at a selected temperature level within a range of about 1100 to about 1500 F. by passing air having a temperature of about 100 to about 1500 F. progressively upwardly from said lower level to said higher level in indirect contact with the contents of said reaction zone and additionally heating said air adjacent said upper level.

References Cited by the Examiner UNITED STATES PATENTS 6/1933 Black et a1. 1961 1O 8/1945 Stewart 260672

Claims

1. APPARATUS COMPRISING A FIRST HOUSING AND A SECOND HOUSING, EACH PROVIDED WITH A STACK THEREFOR, A FIRST ELONGATED COIL EXTENDING INTO SAID FIRST HOUSING, DISPOSED THEREIN AND EXTENDING OUTWARDLY THEREOF, HEAT-PRODUCING ELEMENTS WITHIN SAID FIRST HOUSING, A SECOND ELONGATED COIL EXTENDING INTO SAID SECOND HOUSING, DISPOSED THEREIN AND EXTENDING OUTWARDLY THEREOF, MEANS COMMUNICATING BETWEEN SAID FIRST COIL AND SAID SECOND COIL, MEANS ADJACENT THE ENTRY OF SAID SECOND COIL INTO SAID SECOND HOUSING FOR