CA1078876A - Alkylaromatic hydrocarbon dehydrogenation process - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Styrene is removed from a recycle water stream of an ethylbenzene dehydrogenation process by liquid-liquid extrac tion to prevent polymer buildup on heat exchange tubes and to lower coke formation in boiler tubes. The water stream is then fed into a heater to form steam for use within the reaction zone of the process. The solvent used for the extraction is benzene or a normal paraffin and preferably is the overhead product of a benzene-toluene column which separates the products of the process. The use of extraction lowers utility costs compared to stripping the recycle water stream.

Description

.

FIELD OF_THE ~NVENTION

The invention xelates to a process for the cata-lytic dehydrogenation of alkylaromatic hydrocarbons where-in water is condensed from the reactor effluent.and re- .
cycled fox the production o superheated s~eam used with-in the reaction zone~ The invention more specifically relates to a method of removing substantially all of an alkenylaromatic hydrocarbon, such as styrene, from the water by liquid-liquid extraction with a solvent, such as benzene.
.
10 DESCRIPTION OF T~IE PRIOR ~RT
, The art of alkylaromatic hydrocarbon dehydrogena-tion is well developed as shown by the many commercial plants in operation and the wealth of literature in the field~ Exemplary processes are shown in United States Patents 3,515,765 and 3,515,766 (Cl. 260-699). Both of .
these references teach a pracess or the catalytic dehy-drogenation of an alkylaromatic hydrocarbon r such as ethylbenzene, in which steam is passea through a reaction .~
..: '. .
zone in admixture with the alkylaromatic hydrocarbon.

2~ The effluent of the reac~ion zone is cooled sufficiently to cause condensation of the water and heavier hydrocar-bonsO The effluent is then passed into a separator, and . a water phase is removed and passed into a water strip--per for the removal of hydrocarbons. .-:.
. .

1~78~3~6 In the former reference, the water purified in the stripper is heat-exchanged with the reaction zone effluent to form steam used for stripping in a fraction-ation column. The steàm is removed as an ovèrhead vapor containing undehydrogenated hydrocarbons and returned to the reaction zone. In the latter reference, the stripped water is passed through a filter to remove hydrocarbons not removed in the stripper. It is recognized that the 0.01 to 0~08 mole percent of non-aromatic hydrocarbons in solution and in suspension will eventually foul~ heat-exchangers and boilers used to generate steam. The use of the filter therefore facilitates the passage of the stripped water into a fired feèd heater.
United States Patent 3,492,222 (Cl. 208-321) pre-sents a solvent recovery method for use in a liquid-liquid extraction process. Non-aromatic hydrocarbons are removed from an aqueous wash stream by contactlng the wash stream with an aromatic hydrocarbon. This is per-formed to avoid contamination of an aromatic extract by the non-aromatic hydrocarbons when this water is utilized to form the stripping steam used in separating the aro-matic extract from a rich solvent.

BRIEF SUMMARY OF THE INVENTION

The invention provides a process for the de-hydrogenation of alkylaromatic hydrocarbons wherein the utilities demand is reduced by pass~ing the recyle water stream formed by condensing the effluent of a dehydrogena-tion zone through a liquid-liquid extraction zone to remove .. ' - , , ' .

.: :

7~387~
alkenylaromatic hydrocarbons. The water stream is then passed into a steam ~eneration z~ne to form steam which is used in the dehyrogenation zone. The extraction operation is advantageously combined with the product fractionation zone by using the overhead pxoduct stream of the benzene-toluene column as the solvent stream used in the extraction zone and by returning the extract stream to the column as intermediate reflux material.
DESCRIPTION OF THE DRAWING
The accompanying drawing depicts the flow of a commercial unit designed to produce styrene from an ethylbenzene containing charqe stream.
A stream of ethylbenzene and steam enters the process through line 6. It is then admixed with a first portion of superheated steam passing through line 3 to form a feed stream carrled in line 7 and passed into a first catalyst bed within the dehydrogenation zone 8. The feed stream passes through a first bed of catalyst wherein the endothermic dehyrogenation reaction lowers the temperature of the reactants. A second portion of superheated steam carried through line 5 is therefore admixed with the effluent of the first catalyst bed to raise its temperature. The new admixture passes through a second bed of catalyst, and the effluent of this bed is then admixed with a third portion of steam passing through line 4. The effluent of the third catalyst bed leaves the dehydrogenation zone through line 9 and is passed through a heat-exchange means 10 wherein heat is recovered. The effluent stream continues in line 9 and ':

dg/,i~

, passes through a condenser 11. This causes the separa-tion of the effluent stream into a vapor phase and a liquid phase. These two phases pass through line 9 into a phase separator 12 wherein the li~uid phase separates into a hydrocarbon phase and a water phase. The uncon-densed vapors are removed from the phase separator through line 13, and the hydrocarbon phase is removed through line 15 and passed into a fractionation column 20. The water phase, which contains dlssolved styrene and ethyl-benæene, is removed through line 14 as a recyle water stream and passed into the top of a liquid-liquid extrac-tor 19. The denser water phase passes downward through the extractor countercurrently to a solvent stream which enters the extractor through line 18.
Substantially all of the styrene which is dis-solved in the entering water stream transfers into the solvent stream and is removed from the extractor in the extract stream removed in line 16. This extract stream is passed into the fractionation column 20 via line 16.
The treated recycle water stream is removed from the bottom of the extractor through line 1 and now contains dissolved benzene but is free of styrene and ethylbenzene.
This recycle water stream is passed into a steam super-heater 2 to effect the formation of the steam carried in line 3.
In fractionation column 20, there is effected a separation of the hydrocarbon phase entering through line 15 into a hydrocarbon stream comprising a substan-, .. : .

.:

10~78B'7~;

tially pure mixture of toluene and benzene and a bottoms stream comprising styrene, ethylbenzene and tar. The heat for the separation is supplied through a reboiler 21, and the bottoms stream is removed through line 17.
An overhead vapor stream is removed in line 22 and passed through a condenser 23. The resulting mixed-phase stream is passed into an overhead receiver 24. The uncondensed portions of the overhead vapor strèam are removed through line 25 which is connected to a vacùum source. This source maintalns the column at a subatmospheric pressure which reduces styrene polymerization by lowering the temp-erature required for the separation. A hydrocarbon stream is removed from the receiver as the overhead product~ A
first portion of this stream is returned to the column in line 26 as a reflux stream. The remaining portion passes .
through line 27, from which the solvent stream is with-drawn in line 18, and the net overhead product is removed in line 28.

DETAILED DESCRIPTION .

Large amounts of styrene are produced commercially by the dehydrogenation of ethylbenzene. The dehydrogena-tion process is endothermic, and therefore the predominant processes for the production of styrene admix superheated steam with the ethylbenzene before it is fed into the re-action zone. The superheated steam acts as a heat source which allows a greater amount of dehydrogenation to be performed in the catalyst bed before the tempera-., . .

10'78876 ture becomes too low for the reaction to proceed. The e~fluent of the dehydrogenation zone is normally condensed to effect a separation of thè hydrocarbons from the wàter.
It is desirable to reuse the water which is separated by recycling the water to a steam generation zone. This latter term is intended to refer to any boiler or waste heat steam generator, etc., wherein liquid water is con-verted to steam. This recycling reduces the necessity of treating additional makeup water; and it also elimin-ates the problem of disposal of the condensed hydrocar-bon-containing water.
~he water which is removed from the separation zone will have dissolved in it a varying amount of the hydrocarbons present in the separator. It will there-fore contain a mixture of various aromatic hydrocarbons, such as ethylbenzene, styrene, benzene and toluene and various polymeric compounds normally referred to as tar.
It is recognized in the art that the styrene, ethyl-benzene and tar must be removed before this water stream can be reused for the generation of steam. Tf this is not done, these materials will cause a severe coking problem in the tubes of the superheater causing a rapid shut~down of the process. Furthermore, the styrene forms a polystyrene coating on the surface of feed-effluent heat exchanger tubes. This tends to plug the exchanger and to reduce its heat transfer efficiencye The prior art therefore firsk strips the recycled water stream to remove substantially all of the ligher dis-788~6 soived hydrocarbon materials. The stripped water is then often passed through a filtration system to remove the remaining hydrocarbons, especially the high-boiling tar which is not removed in the stripping operation. This filtration often comprises the passage of the water stream through a bed of activated charcoal. The stripping of the recycle water stream consumes a fair amount of energy and therefore increases the utility costs of the overall process.
It is an objective of this invention to provide a process for the dehydrogenation of ethylbenzene with re-duced utility costs and wherein it is not nècessary to strip this recycle water stream. The effluent of a de-hydrogenation unit contains a large amount of low pres-sure steam which can be used for this ~relatively low temperature) stripping operation, but cannot be used in the higher temperature hydrocarbon separations. For this reason, the invention is most useful in an integrated petrochemlcal complex wherein there exists a use for low pressure steam. One example is a benzene drying column on an alkylation unit producing the ethylbenzene fed to the dehydrogenation unit. Alternatively, the invention increases the amount of low pressure steam available for compression in steam conservation systems.
The present invention resides in the realization that the removal of all hydrocarbons from the recycle water stream is not necessary, and that the problems of polymer formation in heat exchangers and coke buildup in ::

71!3~&;

boiler tubes can be avoided in a less costly manner by simply displacing undesirable hydrocarbons in an extrac-tion zone instead of removing hydrocarbons by stripping followed by filtration. It is only necessary to remove the Cg-plus alkylaromatic and alkenylaromatic hydrocar-bons and tar. Benzene and toluene will normally pass through the boiler tubes unaffected, but can link up to form undesired biphenyls. Saturated cyclic compounds, paraffins and olefins will have a minimal detrimental effect as they tend to crack almost completely to methane and hydrogen in the presence of' water. The present in-vention therefore comprises passing the water stream into liquid-liquid extraction zone wherein substantially all of the undesired alkylaromatic and alkenylaromatic hydro-i5 carbons are removed from the water stream by contact witha solvent stream comprising hydrocarbons having little or no tendency to obstruct the boiler tubes by coke formation.
As used herein, the term "substantially all" indicates the removal of at least gO~ and pre~erably 95~ of the unde-sirable hydrocarbons. Likewise, the term "substantially free" is intended to indicate a molar percentage of less -than 5~ for the material referred to in the subject pro-cess stream~ The concentration of dissolved benzene and toluene in the treated water will be fairly low and can be regulated by adjusting the temperature of the extraction zone.
The liquid-liquid extraction zone may take many forms. It may be a vertical extraction tower as shown in ~,.
.

., . . , . . . .:

~L~78876 the drawing or a series of batch contactin~ operations comprised of mixing and settling zones. The extraction tower may use a rotatiny disc contactor or a pulsed mode of operation to promote extraction. The equipment and de-sign considerations needed for the construction and opera-tion of the zone are within the knowledge of those skilled in the art. Detailed information can be obtained from such references as section 14 of the fourth edition of The Chemical Engineers' Handbook; McGraw-Hill, 1963, or the series of articles on pages 50 to 104 of Chemical Engineering Progress, (Vol. 62, No. 9), Sept. 1966.
Specifically, the size of the extraction zone and the required rate of the solvent stream are set by the ¢om-position and flow rate of the recycle water stream, the desired composition of the product water stream, the efficiency of the contactor and the solubilities of the various components in the two contacted streams.
As an example, to reduce the styrene concentration of a 380,000 lb./hr. water stream from 580 ppm to 5.8 ppm. in an extraction zone equivalent to one theoretical stage and operated at 150F. requires a benzene solvent stream of about 21,820 lbs.jhr. The treated water s-tream would contain about 379,940 lbs./hr. of water, 960 lbs.Jhr. of benzene and 2 lbs./hr. of styrene. The exact conditions used in the extraction zone will be set after a consideration of the temperature effect on solubilities, the unadjusted temperature of the chosen input streams and the desired temperatures of the ef-fluent streams. Extraction zones are normally run in a ~ . . . .

: ~
8876 :

temperature range of from about 60F. to about 200DF.
and with a positive pressure ranging from about at-mospheric to 200 psig. The pressure does not affect the extraction operation and is therefore chosen after a consideration of the pressure drop in the extractor, the cost of an extractor designed for a higher pressure and the volatility of the liquids.
From this example it may be seen that the benzene solvent stream required is smaller than the water stream.
Therefore, even if the extract stream is fractionated to recover the styrene, the utilities cost is reduced due to the smaller amount and the lower latent heat of the benzene stream. The capital costs of using the in-. . .
vention should be no more than using a stripper. The structure of the stripper is very similar to an extrac-tion column, but also includes a reboiler and possibly an overhead condenser. Depending on the ease of the ex-traction, it may possibly be performed in a number of low cost contacting and settling chambers. These reduced costs are two of the advantages of the invention.
The solvent stream used in the extraction zone may -be any suitable liquid possessing a good solubility for styrene or other undesired hydrocarbons and which does not cause excessive coking in the heating tubes. The solvent stream may therefore comprise low molecuiar weight parafinic hydrocarbons such as heptane, hexane, pentane or butane or a mixture of them. The solvent stream may also comprise benzene, and may therefore be : ' :~ ' 3l~78876 a mixture of benzene and paraffinic hydrocarbons having from four to six caxbon atoms per molecule. It is pre-ferred that the solvènt stream is a relatively pure ben-zene stream. In many instances, the ethylbenzene which is dehydrogenated in the styrene process is produced in an alkylation unit located in the same complex. The in-tegration of these two processes is described in detail in United ~tates Patent 3,525,776. In just about all of these alkylation units a dragstream~comprising benzene and non-aromatics is removed to prevent the buildup of the non-aromatics in the unit. This dragstream can be used advantageously as the solvent stream prior to being discharged from the process, It should first be treated as necessary for the removal of any water-soluble inor-ganic materials which would have an adverse effect if introduced into boiler tubes. In a boron trifluoride promoted benzene alkylation process, these materials are boron oxide hydrates which are typically removed by pass-ing the dragstream through a bed of alumina.
The effluent of an ethylbenzene dèhydrogenation process is typically separated in a fractionation zone such as described in United States Patent 3,52S,776.
The hydrocarbonaceous phase removed from the phase separa-tion or settling zone is passéd into a first column re~
ferred to as a benzene-toluene column~ This column is operated at a subatmospheric pressure to allow its operation at lower temperatures and hence reduce the rate of styrene polymerization. Various inhibitors ~ 78~76 such as elemental sulfur or 2,4-dinitrophenol are added for this same purpose. Sulfur is also introduced into the column by returning high molecular weight mater-ial separated from the bottoms stream of a styrene pur-ification column. A more detailed description is con-tained in United States Patents 3,476,656; 3,408,263;
and 3,398,063. There is effected within the benzene-toluene column a separation of benzene and toluene from the effluent to produce an overhead`stream which is sub-stantially free of styrene and ethylbenzene. This streamcontains preferably at least 95 mole percent ben~ene and toluene. It is an embodiment of this invention that this overhead stream is used as the solvent stream. ~t may also be further fractionated to produce a substantially pure benzene stream which can then be used as the solvent stream. The bottoms of the benzene-toluene column is passed into a second fractionation column from which ethylbenzene is removed as an overhead product and re-cycled. The bottoms stream of this column is purified to obtain the styrene.
The present invention may be applied to any process for the dehydrogenation of alkylaromatic hydrocarbons wherein the dehydrogenation zone effluent is condensed to form a liquid water phase and a portion of this water is to be recycled for the production of steam. The specific mode of operation of the reaction zone or the composition of the catalytic material is not determlna-tive of the usefulness of the invention. The examples - ~12-' ~L~71~376 and description herein which refer specifically to the dehydrogenation of ethylbenzene are not intended to so limit the invention. This process may be applied to the dehydrogenation of other alkylaromatic hydrocarbons such as diethylbenzene, ethyltoluene, propylbenzene and iso-propylbenzene and also to alkylaromatic hydrocarbons hav-ing other ring structures, including naphthalenes and anthracene compounds.
The reaction zone preferably comprises two or three beds of dehydrogenation catalyst with means for the inter-mediate addition and admixture of steam. Suitable systems are presented in United States Patents 3,498,755;

3,515,763; and 3,751,232, The catalyst beds may be contained in separate reaction vessels and may have either a cylindrical or an annular shape. Different catalysts may be used in different beds as described in United States Patent 3,223,743. Such catalysts generally con-sist of one or more metallic components selected from Groups VI and VIII of the Periodic Table. These metal-lic components are typically carried on a refractoryinorganic oxide material such as alumina, silica, boria or mixtures thereof. One typical catalyst comprises 85% by weight ferric oxide, 2% chromia, 12% potassium hydroxide and 1% sodium hydroxide. A second typical catalyst comprises 90% by welght iron oxide, 4% chromia and6% potassium carbonate. Methods for preparing suitable catalysts are well known in the art. This is demonstrated by the teachings of United States Patent , . ~

!376 3,387,053, wherein the manufacture is described of a catalytic composite of at least 35 wt. % iron oxide as an active catalytic agent, from about 1 to 8 wt. % zinc or copper oxiae, about 0.5 to 50 wt. % of an alkali pro-moter, and from about 1 to 5 wt. % chromic oxide as a stabilizer and a binding agent. Catalysts preferably em-ployed are available commercially and commonly referred to as "Shell 105 " or "Shell 205 ".
Dehydrogenation conditions in general include a temperature of about 1000F. to about 1800F. and prefera-bly about lQ50F. to about 1250F. The temperature re-quired for any specific unit will depend on the activity ~' of the catalyst employèd. The pressure maintained within the dehydrogenation zone is generally quite low and may range from about 0 to 100 psig., with a preferred pressure range being from about 2.0 to 10 psig. The fee'd stream is charged to the dehydrogenation zone at a liquid hourly ' space velocity, based on liquid hydrocarbon charge at 60F., of about 0.1 hr. 1 to about 1.0 hr.-l, and prefer-ably from 012 to 0.7 hr. 1.
As previously mentioned, the alkylaromatic to be ' dehydrogenated is admixed with steam to counteract the ;';
temperature lowering effect of the dehydrogenation re-action. Preferably, steam is admixed with the feed stream and also added at intermediate points within the reaction zone. Some processes utilize indirect heat ex-change of the reactants or heating elements within the catalyst bed. The steam and alkylaromatic hydroc`arbon .
' Trademark ~14 ~ - , .

~ :

-, ' - ~ . . ' ~ .

: L~78~ 6 can be separately heated and commingled prior to contact-ing the reactants with the catalyst, or the steam and alkylaromatic can be first commingled and then heated.
When ethylbenzene is being dehydrogenated, the space velocity, the rate of steam admixture and the inlet temperature are adjusted to result in the effluent of each catalyst bed having a temperature of about 1100F.
- Preferably, steam is admixed with the feed stream to the dehydrogenation zone at a rate of about 0.65 to about 1.0 pound of steam per pound of ethylbenzene. A second por-tion is added to the effluent of the first catalyst bed at a rate of about 1.0 to about 1.2 pounds of steàm per pound of effluent, and a third portion is added to the effluent of the second bed at a rate of about 0.8 to about 1.3 pounds per pound of effluent. These rates are adjusted such that the total effluent stream from the dehyrogenation zone will contain from about 3 to about 6 pounds of steam per pound of styrene.
The effluent stream removed from the dehydrogenation zone is often first heat exchanged for the dual purposes of lowering its temperature to prevent polymerization of the st~rene and for the recovery of heat. The efflu-ent stream may be heat exchanged against a make-up stream of steam, a reactant stream of this or another process or used as a heat source for fractionation. Commercially, the effluent stream is often passed through several heat exchangers for the heating of different streams. The reaction zone effluent may also be passed through a 1~788'7~

quench zone to rapialy cool it and lessen polymerization.
The quench zone may be located after a heat exchange means as shown in United States Patents 3,515,765 and 3,515~766, or the effluent stream may pass directly from S the reactor into the quench zone as shown in United States patent 3,515,764. The cooling media fed to the quench zone is preferably liquid water removed from the phase separation zone. This water is not treated in the liquid-liquid extraction zone. The temperature of the effluent stream is finally lowered sufficiently to cause the con-densation of essentially all of the hydrocarbons having 6 or more carbon atoms. The term "condensing zone" is therefore intended to refer to one or more of these operations, including at least one heat exchange, wherein the effluent stream of the reaction zone is cooled to a temperature at which a liquid phase is formed which con-tains at least 50 percent of the product material and water in the effluent stream. When large amounts of heat are recovered from the effluent stream, a trim cooler is sufficient to cool the effluent stream to the desired temperature of about 100F.
The effluent stream is then passed into a phase separation zone wherein the effluent divides into a hydrocarbonaceous liquid phase, an aqueous liquid phase and a gaseous phase. There will be some water dissolved in the hydrocarbonaceous phase, which comprises ethyl-benzene, styrene, benzene and toluene. There will also be some hydrocarbons dissolved in the aqueous liquid ' .

10788~6 phase The gaseous phase or vent gas stream will com-prise hydrogen, methane, ethane, ethylene, carbon monoxide, carbon dioxide and other light gases which are formed in the process. The yaseous phase will separate from the liquid phasè rather easily and is vented off.
The liquid material is passed through a quiescent portion of the phase separation zone and the resulting liquid phases are separated by decantation. The design and operation of phase separation zones is well understood by those sXilled in the art~ For instance, Unitea States Patent 3,702,346 teaches the beneficial higher selecti-vity derived in a similar process by maintaining the product settler at a subatmospheric pressure, preferably in the range of from about 200 mm. Hg to about 600 mm. Hg absolute.
As an example of the preferred embodiment, which is the combination of liquid-liquid extraction of the recycle water stream with the operation of the effluent fractionation zone, a detailed description of the flows through a commercial three reactor ethylbenzene dehy-drogenation unit are presented. The combined feed stream to the unit has a flow rate of 13,698 lbs./hr.
and comprises a 637 B,P.S.D, (barrel per stream day) stream of fresh ethylbenzene and a 439 B.P.S.D. recycle ethylbenzene stream. After the addition of condensate, passage through the combined feed heat exchanger, the addition of steam and passage through a heater there is formed a 29,051 lbs./hr. feed stream for the first re-.
-.: . ' ,, . ::
; .

~C~78~376 :

actor which has a temperature of about 1125~., a pressure of about 8 psig., and an average molecular weight of about 29.6. The effluent from the first re~
actor has an average molecular weight of about 28.8, a temperature of about 1020F~ and a pressure of about 6 psi~. To this effluent there is added a 13,698 lbs./hr.
stream of steam having a temperature of 1565~. This forms a feed stream to the second reactor having a temper-ature of about 1170F., a pressure of about 8 psig. and a ~ -flow rate of 42,749 lbs./hr. The effluent of the second reactor has a temperature of about 1080F. and a pressure of about 6 psig. ~t is admixed with an 11,013 lbs./hr.
stream of steam having a temperature of about 1530PF. to effect the formation of a 53,762 lbs./hr. feed stream for 15 the third reactor, which has a temperature of about 1180F. and an average molecular weight of about 22.2.
The effluent of the third reactor has an average molecular weight of about 22, a temperature of about 1140~F. and a pressure of about 6 psig. These conditions will vary through the length of the run. For instance, the inlet temperature of the first reactor will vary from about 1100F. to about 1125F., and the inlet temperature of the third reactor will vary from about 1155F. to about 1180F. The weight hourly space velocities are about 0.6 in the first reactor, about 1.4 in the second reactor and about 2~0 in the third reactor. The catalysts used are know commercially as "Shell 105 " and "CCI 97 "
catalysts.

* -18-Trademark '- ' ' ~ -~ . .. , . ,, .. .. , ,. . ~ .
. ; " ,, . : . . . .: . . : . , . , .. , . :

1~78876 The effluent of the dehydrogenation zone is heat exchanged with the combined feed stream and its tempera-ture is reduced to about 435F. A stream of about 5,500 lbs./hr. of 240F. water is added to the effluent and lowers the effluent temperature to about 225~. The quenched effluent is then admixed with a 100F. hydrocar-bon spillback stream derived from a settler and passed through a condenser which lowers its temperature to about 100F. The effluent stream leaving~the condenser has a flow rate of about 60,599 lbs./hr. and is passed into the settler, The vapor stream leaving the settler is passed through a chiller which reduces its temperature to about 40F. The resulting condensation allows the return of the heavier hydrocarbons to the settler and pro-duces an off-gas stream with an average molecular weight of about 6.2. A 14,705 lbs,/hr. stream of the hydrocar-bon phase is removed from the settler. Of this, about 1,377 lbs./hr. is used as the hydrocarbon spillback stream. This leaves a stream of about 13,368 lbs./hr.
which is fed into a fractionation column re~erred to as the benzene-toluene column. This stream has an average molecular weight of about 104 and contains about 9 lbs.
of dissolved gases and about 10 lbs. of dissolved water.
A recycle water stream of about 45,254 lbs./hr. is re-moved from the water phase formed in the settler atabout 100F.
The feed stream enters the benzene-toluene column at a temperature of about 100F. and a pressure of about ~

-, . 19 :, .~.. , . :
-: ' . :
. .

-, ' ' ' ' ' ' . . , . ~., '' ~ .' ' ' . . ~ ' . . ` . . , .

~8876 150 mm, Hg absolute. The column may contain about 24 trays above the feed point and about 61 below the feed point, About 28 lbs./hr. of fresh sulfur and about 1,147 lbs,/hr. of sulfur-containing tar derived from the styrene purification column are added with the feed stream to inhibit polymerization o~ the styrene. The operation of the column produces an overhead vapor stream of about 11,849 lbs./hr. at a temperature of about 115F. and a pressure of about 100 ~m. Hg absolute.
Liquid material enters the reboiler at about 217F. and a pressure of about 210 mm. Hg absolute. About 13,881 lbs./hr. of bottoms material having an average molecular weight of about 107.5 is removed and passed into an ethyl-benzene column for recovery of the unconverted ethylben-zene for recycling. The bottoms of the ethylbenzene column is passed into a styrene column, from which a 6,891 lbs./hr. product stream is recovered.
The overhead condenser of the benzene-toluene column is operated so to produce a condensate having a temperature of 100F. The effluent of the condenser includes about 9 lbs./hr. of gases which pass to the ejectors used to maintain the subatmospheric pressure within the column and about 10 lbs,/hr. o~ water which is decanted from the overhead receive~. Normally, an 11,187 lbs./hr. reflux stream would be returned to the column and a 643 lbs./hr. net overhead product stream would be removed. The net overhead product comprises a relatively pure mixture of benzene and toluene which ~. .
.
' ,' ' ~ ' ' .' `' ''" ' 8i~6 may be fractionated to provide pure benzene. By the method of the invention, a stream of about 2,700 lbs./hr.
of the overhead material is diverted to a liquid-liquid extraction zone operated at a temperature of 100F. and atmospheric pressure and used as the solvent stream used within the zone. Contact with the recycle water stream removed from the settler causes the transfer of about 24 lbs./hr. of styrene and ethylbenzene rom the recycle water stream to the solvent stream.` A hydrocarbon stream comprising essentially all of the original solvent strèam, since the water stream was in equilibrium with hydrocar-bons in the settler, is removed from the extraction zone as an extract stream. This stream is returned to the benzene-toluene column and used as reflux material fed to the column at an intermediate point due to its slightly lessened purity. This recovers the styrene and ethyl-benzene removed from the recycle water stream. A very small amount of the recycle water stream, less than about 5 lbs~/hr., transfers to the extract stream and the re-maining portion forms a second water stream which is re-moved as the product of the extraction zone. This second water stream is recirculated for use in the reaction zone.
In accordance with this description, the preferred embodiment of the invention may be characterized as a process for the dehydrogenation of ethylbenzene, which comprises in cooperative combination the steps of admix-ing a feed stream comprising ethylbenzene with steam and contacting the resulting admixture with a dehydrogenation ,, :' ' ~
: . ':
,:

catalyst within a reaction zone maintained at dehydrogena-tion conditions and effecting the formation of an effluent :
stream comprising styrene, ethylbenzene and steam, effect- .
ing a partial condensation of the effluent stream by pass-age through a condensing zone, passing the effluent stream into a phase separation zone and effecting the formation of a hydrocarbonaceous phase comprising styrene, ethyl- .
benzene, toluene and benzene and an aqueous phase com-prising styrene, passing a first water stream comprising at least a portion of the aqueous.phase into a liquid-liquid extraction zone and effecting a removal of sub-stantially all of the styrene from khe first water stream by contact with a solvent stream comprising ben-zene, and effecting the formation of a second water stream which is substantially free of styrene,passing at least a portion of the second water stream into a steam generation zone and effecting the formation of steam which is fed into the reaction zone, passing the hydrocarbonaceous phase into a fractionation zone and effecting a separation of the benzene and toluene from the styrene and ethylben~ene, and effecting therein the formation of a hydrocarbon stream which is substantially free of styrene and ethy.lbenzene, and passing at least a portion of the hydrocarbon stream into the liquid-liquid extraction zone as the solvent stream.

.

.
-22- . - -.
, .

Claims (9)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a process for the dehydrogenation of ethyl-benzene wherein:
(i) ethylbenzene is admixed with superheated steam and passed through a reaction zone operated at dehydrogen-ation conditions to effect the formation of an effluent stream comprising ethylbenzen, styrene and water;
(ii) the effluent stream is passed into a phase separation zone to effect the formation of a first liquid stream comprising water, ethylbenzene and styrene and a second liquid stream comprising ethylbenzene and styrene;
and, (iii) water contained in said first liquid stream is returned to said reaction zone as steam, the improve-ment which comprises passing said first liquid stream in-to a liquid-liquid extraction zone and effecting a remov-al of substantially all ethylbenzene and styrene from said first liquid stream by contact with a solvent stream.

2. A process for the dehydrogenation of alkylaro-matic hydrocarbons which comprises in cooperative combin-ation the steps of:
(a) admixing a feed stream comprising an alkylaro-matic hydrocarbon with steam and contacting the resulting admixture with a dehydrogenation catalyst within a reac-tion zone maintained at dehydrogenation conditions and effecting the formation of an effluent stream comprising an alkenylaromatic hydrocarbon and steam;
(b) effecting a partial condensation of the efflu-ent stream by passage through a condensing zone;
(c) passing the effluent stream into a phase sepa-ration zone and effecting the formation of a hydrocarbon-aceous liquid phase and an aqueous liquid phase comprising the alkenylaromatic hydrocarbon;
(d) passing a first water stream comprising at least a portion of the aqueous liquid phase into a liquid-liquid extraction zone and effecting a removal of substantially all of the alkenylaromatic hydrocarbon from the first water stream by contact with a solvent stream and effecting the formation of a second water stream which is substantially free of the alkenylaromatic hydrocarbon; and, (e) passing at least a portion of the second water stream into a steam generation zone and effecting the for-mation of steam which is fed into the reaction zone.

3. The process of Claim 2 further characterized in that the alkylaromatic hydrocarbon is ethylbenzene.

4. The process of Claim 2 further characterized in that the solvent stream comprises benzene.

5. The process of Claim 2 further characterized in that the solvent stream comprises a paraffinic hydro-carbon.

6. A process for the dehydrogenation of ethyl-benzene which comprises in cooperative combination the steps of:
(a) admixing a feed stream comprising ethylbenzene with steam and contacting the resulting admixture with a dehydrogenation catalyst within a reaction zone maintained at dehydrogenation conditions and effecting the formation of an effluent stream comprising styrene, ethylbenzene and steam;
(b) effecting a partial condensation of the efflu-ent stream by passage through a condensing zone;
(c) passing the effluent stream into a phase separ-ation. zone and effecting the formation of a hydrocarbona-eous phase comprising styrene, ethylbenzene, toluene and benzene and an aqueous phase comprising styrene;
(d) passing a first water stream comprising at least a portion of the aqueous phase into a liquid-liquid extraction zone and effecting a removal of substantially all of the styrene from the first water stream by contact with a solvent stream comprising benzene, and effecting the formation of a second water stream which is substan-tially free of styrene;
(e) passing at least a portion of the second wa-ter stream into a steam generation zone and effecting the formation of steam which is fed into the reaction zone;
(f) passing the hydrocarbonaceous phase into a fractionation zone and effecting a separation of the ben-zene and toluene from the styrene and ethylbenzene, and effecting therein the formation of a hydrocarbon stream which is substantially free of styrene and ethylbenzene;
and, (g) passing at least a portion of the hydrocarbon stream into the liquid-liquid extraction zone as the sol-vent stream.

7. The process of Claim 6 further characterized in that an extract stream comprising benzene, toluene and styrene and which is removed from the liquid-liquid extrac-tion zone is passed into a fractionation column from which the hydrocarbon stream is removed as an overhead product stream.

8. The process of Claim 6 further characterized in that the hydrocarbon stream contains at least 90 mole percent benzene and toluene.

9. The process of Claim 6 further characterized in that the hydrocarbon stream is substantially free of toluene.