FR2515172A1 - Catalytic alkyl aromatic dehydrogenation to vinyl aromatic - with flash evapn. of feed stream with superheated steam - Google Patents

Abstract

Catalytic dehydrogenation of alkylaromatics (I) to vinylaromatic monomers (II) comprises flash evapn. of a stream of (I), then passing it together with superheated steam to a dehydrogenation zone, then removing (II)-contg. effluent gases. Pref. flash evapn. is by injection of steam into (I) which is pref. preheated to 120-150 deg.C by indirect heat-exchange with steam. Some steam is also supplied direct to the dehydrogenation zone and vaporised (I) can be further heated by heat exchange with effluent gases. An appts. for the process is also claimed. Flash evapn. avoids formation of a liq. phase enriched in (II) (present in (I) as a minor component), so that fouling of heat exchangers etc. is minimised and the downbrine required for cleaning is reduced. The method is esp. used to make vinyltoluene from ethyltoluene.

Description

 The present invention relates to a process for the catalytic dehydrogenation of alkyl aromatic compounds to produce vinyl aromatic compounds. The present invention relates more particularly to a process and an apparatus according to which a charge of alkyl aromatic compounds is first vaporized, and then it is subjected to a heat exchange with the effluent gases from the dehydrogenation zone, before introducing it. in the dehydrogenation zone. In particular, the present invention relates to a process and an apparatus for producing vinyl aromatic monomers by catalytic dehydrogenation of alkyl aromatic compounds during which the charge of alkyl aromatic compounds undergoes flash vaporization.

 Vinylaromatic compounds such as styrene, vinyltoluene, alpha-methylstyrene, divinylbenzene and the like are very useful as monomers because very useful polymers can be produced therefrom. These monomers are generally! produced by catalytic dehydrogenation of alkyl aromatic compounds to corresponding vinyl aromatic compounds in the presence of steam and at elevated temperature. Before introducing the charge into the dehydrogenation zone, it is usually preheated by first subjecting it to an indirect heat exchange with steam and then another indirect heat exchange with the hot effluent gases which are withdrawn from the dehydrogenation.

Although vaporization of the feedstock of alkyl aromatics may occur during the initial heat exchange operation with the vapor, vaporization of the feedstock occurs primarily in the load / effluent heat exchanger. Generally, the filler comprises, in addition to the desired alkyl aromatic compound, a small proportion of vinyl aromatic compound. This creates a problem in the charge / effluent heat exchanger because the alkyl aromatic compound tends to vaporize first, thus leaving a liquid phase having an enriched content of vinyl aromatic compound. These vinyl aromatic compounds are highly reactive and tend to polymerize in the liquid phase, at the temperatures encountered in the charge / effluent exchange. The preferential evaporation of the aromatic compound, as well as the enrichment in vinyl aromatic compound of the remaining liquid phase, increase the possibility of encountering and polymerizing molecules of aromatic vinyl compounds in the liquid phase
It can also be seen that the vinyl aromatic polymer deposits are formed in the charge / effluent heat exchanger. Polymerization occurs when the compounds contained in the liquid phase, enriched in vinyl aromatic compounds, come into contact with the walls of the hot tube of the charge / effluent heat exchanger, which moreover have a temperature of between 500 and 6000C.

 The carbon deposits which gradually build up on the catalyst in the dehydrogenation zone can be removed by periodically treating the catalyst with steam. However, these treatments have no influence on reducing the formation of polymer or the coking of the charge / effluent heat exchanger. After a certain period of operation, it is necessary to put the exchanger out of service to clean it. As a result, the cost of producing vinyl aromatic monomers is increased not only by the cost of cleaning but also by the loss of productivity of the apparatus.

 The problem of formation and coking in the inlet system leading to the dehydrogenation reactor is particularly important in the case of the dehydrogenation of ethyltoluene to produce vinyltoluene.

In this specific case, it is necessary to clean the heat exchanger at intervals of 1 to 6 months.

 The subject of the present invention is an improved method as well as an apparatus for the catalytic dehydrogenation of alkyl aromatic compounds into vinyl aromatic compounds, according to which, before introducing the charge into the dehydrogenation reactor, it is passed through a heat exchanger to carry out a heat exchange with the effluent gases which are withdrawn from the dehydrogenation reactor.

 The present invention also relates to a method and an apparatus for the catalytic dehydrogenation of alkylaromatic compounds, according to which the formation of a liquid phase enriched in vinylaromatic compounds is prevented, during the vaporization of the charge of alkylaromatic compounds.

 The present invention also relates to a method and an apparatus for the catalyzed de-hydroge nation; tick of alkylaromatic compounds, according to which a complete vaporization of the charge of alkylaromatic compound is ensured before introducing it into the dehydrogenation reactor.

 The present invention also relates to a method and an apparatus for the catalytic dehydrogenation of alkylaromatic compounds according to which the production apparatus can be kept in service for long periods before it is necessary to clean it.

 The present invention also relates to a method and an apparatus for the catalytic dehydrogenation of alkylaromatic compounds according to which the tendency to accumulation of carbon deposits in the charge / effluent heat exchanger is considerably reduced.

 The present invention also relates to an apparatus for the catalytic dehydrogenation of alkylaromatic compounds to produce vinylaromatic compounds according to which the heat exchanger is less prone to fouling.

 The present invention also relates to a method and an apparatus which allow a more economical production of vinyl aromatic compounds by catalytic dehydrogenation of alkyl aromatic compounds, I by reducing dead times and increasing the productivity of the equipment.

The present invention also relates to a method and an apparatus which are particularly applicable to the catalytic dehydrogenation of ethyl toluene to produce vinyltoluene
The process of the present invention for catalytic dehydrogenation of alkylaromatic compounds into vinylaromatic monomers is characterized in that it consists in carrying out a flash vaporization of a stream of alkylaromatic compounds, in introducing the vaporized alkylaromatic compound and superheated steam into a zone dehydrogenation system which contains a bed of alkyl aromatic compound dehydrogenation catalyst, at a temperature high enough to cause catalytic dehydrogenation and to extract the vinyl aromatic compound contained in the effluent gases from this dehydrogenation zone.

 The apparatus of the present invention for carrying out the catalytic dehydrogenation of alkylaromatic compounds to produce vinylaromatic monomers is characterized in that it comprises a reaction zone containing a bed of catalyst for the dehydrogenation of alkylaromatic compound, means for introducing a stream of alkylaromatic compounds in this reaction zone, means for introducing superheated steam into this reaction zone, means for withdrawing effluent gases from this reaction zone, and means for flash vaporizing the stream of alkylaromatic compounds before its introduction into this reaction zone.

 According to one embodiment of the process of the invention, the flash vaporization of the alkylaromatic compound is carried out by describing a portion of the fairy superheated steam circulating in a steam inlet pipe leading to the reaction zone and by injecting this vapor derived directly from the alkyl aromatic compound stream; the stream of alkylaromatic compound is preheated by making it undergo an indirect heat exchange with steam before vaporizing it; the vaporized alkyl aromatic compound is then subjected to an indirect heat exchange with the effluent hot gases withdrawn from the dehydrogenation reaction zone, while the steam coming from the preheating exchanger is sent to a boiler and is superheated to thereby provide the superheated steam to carry out flash vaporization and catalytic dehydrogenation.

 For the detailed description of the invention presented below, reference is made to the appended drawing which represents a schematic diagram of an installation for producing vinyl aromatic compounds by catalytic dehydrogenation of the corresponding alkyl aromatic compounds.

 In particular, the detailed description of the invention relates to a system for producing vinyltoluene by dehydrogenation of ethyltoluene. It should however be understood that the process and the apparatus of the present invention are also suitable for the production of other vinyl aromatic monomers such as styrene, alpha-methylstyrene, divinyl-benzene and the like. Those skilled in the art must be able to easily adjust the operating conditions in order to adapt the system to other vinyl aromatic monomers.

According to the appended drawing, which represents a system for producing vinyltoluene, toluene and ethylene are introduced via lines 2 and 4 respectively into an alkylation reactor 6 in which toluene is alkylated according to the well-known Friedel reaction
Crafts to produce ethyltoluene. The known Friedel-Crafts catalysts such as aluminum trichloride (AlCl3) activated with hydrochloric acid (HC1) or ethyl chloride (CH3CH2Cl) can be used. The compound thus formed is sent from the alkylation reactor 6,! via line 8 to a heat exchanger 10, in which it is preheated by indirect heat exchange with steam passing through line 12. Generally, the ethyltoluene charge is preheated to a temperature between approximately 1200C and approximately 1500C .

The charge of ethyltoluene is then passed through line 14 to the flash vaporization zone into which superheated steam is injected through line 18 into the stream of alkyl aromatic compound
The heat exchange by direct contact between the superheated steam and the alkylaromatic charge causes the charge to evaporate almost instantaneously.

All of the charge current is vaporized, including the vinyltoluene present therein. Consequently, a liquid phase enriched in vinyltoluene is not allowed to form, and the subsequent formation of vinyltoluene oligomers in the supply lines leading to the dehydrogenation reactor is thus greatly reduced.

The ethyltoluene vapors then pass through line 20 to the charge / effluent heat exchanger 22; wherein the vapors are again heated by subjecting them to heat exchange with the effluent hot gases from the dehydrogenation reactor. The steam from the preheater exchanger 10 is sent through line 24 to a heating device or a boiler 26 in which it is overheated. t
The temperature of the superheated steam produced in the boiler 26 is preferably between approximately 7000C and approximately 7600C.

The hot ethyltoluene vapors from the charge / effluent dryer 22 are then sent via line 30 to the dehydrogenation reactor 28. The speed at which the ethyltoluene vapors are introduced into the dehydrogenation reactor, expressed by an hourly space velocity of liquid is generally between about 0.4 and about 0.8.

The superheated steam from the boil 26 is also passed through the dehydrogenation reactor 28 through the pipe 32. Part of the superheated steam from the pipe 32 is diverted through the pipe 8 to the flash vaporization zone 16. The valve 34 is placed on the pipe 18 in order to regulate the steam flow to the flash vaporization zone.

 The dehydrogenation reactor 28 contains a bed of catalyst for the dehydrogenation of alkyl aromatic compounds. Generally, the catalyst consists of a basic metal catalyst presented in a granular form. Usually these catalysts contain a promoter of the iron oxide type. Many suitable catalysts are commercially available, and by way of example, mention may be made of Shell-105, Shell 015, Unitd-G64A or even United-G64D. The temperatures prevailing in the dehydrogenation reactor are between approximately 5500C and approximately 7000C, and preferably between approximately 5600C and approximately 650 C.

The respective proportions of steam and ethyltoluene which are introduced into the dehydrogenation reactor can be between approximately 1 and approximately 10 parts by weight of vapor per part of ethyltoluene, which corresponds to a vapor / hydrocarbon molar ratio of between approximately 7 , 5 for 1 and 75 for the
Preferably, the vapor / ethyltoluene weight ratio is between approximately 2 and approximately 4 parts by weight of vapor per part of ethyltoluene, which corresponds to a vapor / hydrocarbon molar ratio of between approximately 15 to 1 and approximately 30 to 1. In particular, the vapor / ethyltoluene molar ratio is at least about 22.5 to 1.

The effluent vapors leaving the dehydrogenation reactor 28 consist of a mixture comprising steam, vinyltoluene, hydrogen gas, a little non-dehydrogenated ethyltoluene as well as small amounts of other alkylaromatics. The effluent vapors are sent via line 36 to the charge / effluent exchange 22 in which part of their heat is transferred to the charge of vaporized ethyltoluene.
The temperature prevailing in the charge / effluent exchanger depends on the temperature of the effluent gases which are withdrawn from the dehydrogenation zone. Preferably this, the temperature of the effluent gases will not be lower than approximately 700 ° C. and more particularly 6500 ° C., while the temperature on the hot side of the exchanger will be between approximately 5000 ° C. and approximately 6000 ° C. If the temperature of the effluent gases exceeds this level, this may indicate that the temperature prevailing in the dehydrogenation zone is too high.

Excessively high temperatures in the dehydrogenation zone should be avoided since they can be damaging to the catalyst. In addition, excessively high temperatures in the feed / effluent exchanger can increase the formation of unwanted coke deposits.

 From the heat exchanger 22, the product vapors are sent through line 38 to another heat exchanger 40, in which a greater quantity of their heat is transferred to steam entering via line 42.

The partially cooled vapors are sent via line 44 to the rapid cooling zone 46 in which they are quenched with a stream of water entering via line 48. The quenched product is sent from rapid cooling zone 46 via line 50 to a condenser, such as air cooler 52, in which the product stream is finally cooled to a temperature between about 150 and about 750 C, the condensation of water and hydrocarbon being thus complete
The condensate from the air cooler 52 is sent through line 54 to a separation tank 56. The hydrocarbon constituents of the product stream separate from the aqueous constituents in the chamber 58, and the organic phase flows through the top of the reservoir 60 in chamber 62. The non-condensed gases such as hydrogen are withdrawn from the separator 56 through line 64.

The aqueous phase of the condensates is drawn off from the bottom of the chamber 58 via the pipe 66. Part of the water passing through the pipe 66 can be diverted through the pipe 48 to be introduced into the rapid cooling zone 46. The rest water circulating in line 66 can advantageously be sent to the boilers which produce the steam which must be used in the system. The crude vinyltoluene is withdrawn from the chamber 62 of the separator 56 via the line 68 and it is directed to the purification unit, as if free from a conventional distillation train, which is not shown in the drawing.

 According to the present invention, a complete vaporization of the alkyl aromatic feed is carried out without any temporary formation of a liquid phase enriched in vinyl aromatic compound which can polymerize in the feed / effluent heat exchanger or in d 'other parts of the system. This avoids the fouling and coking of the system traversed by the load. This extends the operating time of the system and ultimately ensures more economical production of vinylaromastic monomers.

 The reduction of coking in the system traversed by the charge, can also come from the fact that part of the superheated steam can react with the carbon deposits formed, according to the reaction of gas to water, thus consuming the residues and thus avoiding their accumulation.

 On the other hand, other flash vaporization techniques of the alkylaromatic compounds can also be used. For example, the load can be subjected to flash vaporization by a high pressure treatment, by increasing the temperature of the load and then by rapidly introducing the load into a low pressure area.

The following examples are given to better illustrate the present invention, but without limiting its scope
Example 1
Ethyltoluene and superheated steam were continuously introduced into a dehydrogenation reactor I containing a bed of dehydrogenation catalyst of the type
Shell-105, and a gaseous effluent containing vinyltoluene will be drawn from this reactor. The charge was passed through a tubular jacketed exchanger to subject it to heat exchange with the effluent hot gases withdrawn from the reaction zone. This heat exchange was carried out before introducing the feed into the dehydrogenation reactor. The temperature of the superheated steam was 7250C. The ethyltoluene was preheated by indirect heat exchange with steam. The charge was then vaporized by injecting superheated steam into the stream of ethyltoluene, before sending it to the charge / effluent exchanger. The rate at which the dehydrogenation zone was fed with ethyltoluene was controlled so as to maintain an hourly space velocity of liquid of approximately 0.6. On the other hand, the speed with which the steam is introduced was controlled to maintain a steam / hydrocarbon molar ratio of approximately 25: 1. The temperature of the effluent gases withdrawn from the dehydrogenation zone was approximately 650 C.

 After operating in this manner for 150 days, the charge / effluent exchanger was taken out of service for inspection. Negligible coking was observed, indicating that continuation was possible.

On the other hand, after a similar period of operation according to a conventional method, according to which superheated steam is not injected into the ethyltoluene stream to carry out a flash vaporization of the ethyltoluene before it passes through the charge / effluent exchanger , this exchanger is so fouled with carbon deposits that cleaning is necessary before resuming operation.

 The above embodiments are given in order to illustrate the present invention, but without limiting it. However, modifications can be made by those skilled in the art without departing from the scope of the invention.

Claims (33)

Claims:  1. Process for the catalytic dehydrogenation of compounds  alkylaromatics to vinylaromatic monomers characterized  in that it consists in carrying out a flash vaporization  a stream of alkylaromatic compound, to introduce the vaporized alkylaromatic compound as well as steam  overheated in a dehydrogenation zone which contains  a bed of compound dehydrogenation catalyst  alkylaromatic, at a temperature high enough to  cause catalytic dehydrogenation, and to draw off  the vinyl aromatic compound contained in the effluent gases  of this dehydrogenation zone.  2. Method according to claim 1, characterized in  what we do flash vaporization by injecting  superheated steam in the stream of alkyl compound  aromatic.  3. Method according to any one of the claims  1 and 2, characterized in that the current of  alkylaromatic compound at a temperature between  around 1200C and around 1500C before submitting it to the  flash spray.  4. Method according to claim 3, characterized in that  that we preheat by subjecting the current  of alkyl aromatic compound to indirect heat exchange  with steam.  5. Method according to claims 3 and 4, characterized;  in that we then send this vapor to a boiler  and overheat it.  6. Method according to claim 5, characterized in that j  that we inject some of this superheated steam  into the alkyl aromatic compound stream to make it  its flash vaporization.  7. Method according to claim 5, characterized in  what we introduce some of this vapor on  heated in the dehydrogenation zone.  8. Method according to any one of the claims  1 to 7, characterized in that the compound is subjected  alkylaromatic vaporized in indirect heat exchange  with the effluent gases withdrawn from the dehydrogenation zone, this exchange taking place before introducing the vaporized alkylaromatic compound into the dehydrogenation zone. 9. Method according to any one of claims 1 to 8 characterized in that the alkylaromatic compound is introduced into the dehydrogenation zone at an hourly space velocity of liquid of between approximately 0.4 and approximately 0.8. 10. The method of claim 2, characterized in that about 3 to about 9 parts by weight of superheated steam per part of alkylaromatic compound is injected into the stream of alkylaromatic compound. 11. Method according to any one of claims 1 to 10, characterized in that a stream of alkylaromatic compound containing a small proportion of vinylaromatic compound is used. 12. Method according to any one of claims 1 to 11, characterized in that the superheated steam is introduced into the dehydrogenation zone in a molar ratio of between approximately 15: 1 and approximately 30: 1 and preferably of at least 22.5: 1 relative to the alkylaromatic compound i. 13. Method according to any one of claims 1 to 12, characterized in that one derives a part of the superheated steam introduced into the dehydrogenation zone, to introduce it into the charge of alkylaromatic compound. 14. Method according to any one of claims 1 to 13, characterized in that it further consists in condensing the effluent gases withdrawn from the dehydrogenation zone, in separating the condensate obtained into an aqueous liquid phase: and an organic phase liquid containing the vinyl aromatic compound and recovering the vinyl aromatic compound contained! in the organic phase. 15. The method of claim 14, characterized in that the liquid aqueous phase is recovered and used to produce the steam used in the system. 16. Method according to claim 15, characterized in that this aqueous liquid phase is recovered and that part is injected into the gases withdrawn from the dehydrogenation zone to make them undergo rapid cooling. 17. Method according to any one of claims 1 to 16 characterized in that one carries out the dishydrogenation of alkylaromatic compounds chosen from the group comprising ethylbenzene, isopropylbenzene, 1 ethyltoluene, diethylbenzene to respectively form styrene , 1 u methyl-styrene, vinyltoluene or divinylbenzene as the corresponding vinyl aromatic compound. 18. Method according to any one of claims 1 to 17, characterized in that the temperature prevailing in the dehydrogenation zone is between approximately 550 and 7000C and preferably between approximately 560 and 650 C. 19. Method according to any one of claims 1 to 18, characterized in that the temperature of the superheated steam is between about 700 and 76O0C. 20. A method according to any one of claims 1 to 19, characterized in that a dehydrogenation catalyst containing iron oxide is used as a promoter. 21. Method according to any one of the claims! 1 to 20, characterized in that the gases are drawn off from the dehydrogenation zone at a temperature below 7000C and preferably below 650 C. 22. Method according to any one of claims 1 to 21, characterized in that the temperature in the charge / effluent exchanger is maintained between approximately 500 and 6000C. g 23. Apparatus for producing vinyl aromatic compounds by catalytic dehydrogenation of alkyl aromatic compounds, characterized in that it comprises:  a reaction zone containing a bed of catalyst for the dehydrogenation of an alkylaromatic compound,  means for introducing a current of this alkylaromatic compound into this reaction zone,  means for introducing superheated steam into this reaction zone,  - Means for withdrawing the effluent gases from this reaction zone; and  - Means for subjecting the stream of alkylaromatic compound to flash vaporization before introducing it into the reaction zone. 24. Apparatus according to claim 23, characterized in that it further comprises means for carrying out a heat exchange between the vaporized alkylaromatic compound and the effluent gases withdrawn from the reaction zone. 25. Apparatus according to claim 24 characterized in that the means for performing the heat exchange cmprendent a multitubular shell exchanger. 26. Apparatus according to any one of claims 23 to 25, characterized in that the means for carrying out flash vaporization comprise means for injecting superheated steam into the stream of alkylaromatic compound. 27. Apparatus according to any one of claims 23 to 26 characterized in that it further comprises, means for preheating the alkylaromatic compound before subjecting it to flash vaporization, these means comprising a multitubular shell exchanger for producing a indirect heat exchange between the alkylaromatic compound and the vapor. 28. Apparatus according to any one of claims 23 to 27, characterized in that it further comprises means for superheating the steam after it has passed through the heat exchanger. 29. Apparatus according to any one of claims 23 to 28, characterized in that the means for introducing the superheated steam into the reaction zone, comprise a steam inlet pipe communicating between the heating means and the reaction zone . 30. Apparatus according to any one of claims 23 to 29, characterized in that the means for carrying out flash vaporization comprise means for injecting superheated steam into the stream of alkylaromatic compound, as well as means for deriving from the superheated steam from the steam inlet pipe to the injection means. 31. Apparatus according to claim 30, characterized in that it further comprises, a valve placed on the pipe you bypass steam to adjust the steam flow between the inlet pipe and the injection means. 32. Apparatus according to any one of the claims 23 to 31, characterized in that it further comprises means for condensing the effluent gases withdrawn from the reaction zone, means for separating the liquid condensate obtained into an aqueous phase and an organic phase containing the vinylaromatic compound, and means for recovering the organic phase containing the vinylaromatic compound. 33. Apparatus according to claim 32, characterized in that it further comprises means for injecting a part of the aqueous phase which has been separated, in the effluent gases to cool them quickly.