MX2008008393A - Improved process for the dehydrogenation of alkyl-aromatic hydrocarbons for the production of vinyl-aromatic monomers - Google Patents

Abstract

Process for the production of vinyl-aromatic monomers which comprises:a) feeding an aromatic stream and an olefinic stream to alkylation;b) feeding the reaction product coming from the alkylation section to a first separation section;c) recovering the mono-alkylated aromatic hydrocarbon from the first separation section;d) feeding the mono-alkylated aromatic product to a dehydrogenation section;e) cooling and condensing the reaction gases in the shell of one or more heat exchangers;f) feeding the reaction product coming from the dehydrogenation section to a second separation section;g) recovering the stream of vinyl-aromatic monomer.

Description

IMPROVED METHOD FOR THE DEHYDROGENATION OF ALKYL-AROMATIC HYDROCARBONS FOR THE PRODUCTION OF VINYL-AROMATIC MONOMERS DESCRIPTION OF THE INVENTION The present invention relates to an improved process for the dehydrogenation of alkyl aromatic hydrocarbons for the production of vinylaromatic monomers. More specifically, the present invention relates to an improved process for the production of vinyl aromatic monomers from the dehydrogenation of the corresponding alkylated products. Even more specifically, the present invention relates to an improved process for the dehydrogenation of ethylbenzene for the production of styrene. Vinyl-aromatic monomers, such as styrene, are particularly known for their use in the preparation of plastics materials, such as compact / crystalline polystyrene (homopolymer), high impact polystyrene (HIPS), expanded polystyrene and other products. These monomers are produced mainly starting from the alkyl aromatic product The first one is dehydrogenated, in gaseous phase, at a high temperature and at low pressure, in the presence of steam, on suitable catalysts based on iron and potassium oxides. The effluent that comes from the dehydrogenation reaction, which contains several hydrocarbon products formed under the reaction conditions, is first condensed and then sent to a purification section where the liquid effluents are treated to separate and purify the various components by means of a series of fractionation columns operating in series, with respect to the flow of the vinyl aromatic monomer. In the case of styrene, the process generally involves feeding ethylbenzene to a device, typically a heat exchanger, where ethylbenzene vaporizes in the presence of steam; the mixture is sent to a system of heat exchangers where the steam overheats and, after mixing with more steam at a high temperature, is immediately sent to the first reaction stage. At the outlet of the first reaction stage, the mixture is heated in a suitable exchanger system, and then sent to the next reaction stage, possibly followed by a third. The effluent that comes from the last reaction stage is cooled, by preheating the ethylbenzene and the steam that is fed to the reaction section itself. In some cases, heat recovery can be effected by generating and overheating the steam alone or by vaporizing and superheating an azeotropic mixture of ethylbenzene and water. At the outlet of the heat recovery section, the reaction mixture undergoes a subsequent cooling, by contact with water of vaporization, and then condenses in one or several heat exchangers, which transfer heat to the cooling water or, sometimes , air. The condensing system typically consists of 2 exchangers in series. The first is where most of the condensation occurs, usually inside tubes, and where the cooling fluid can be air or water. The second exchanger, which is smaller, can be installed either in a horizontal or vertical position and is where the cooling of the incondensable part occurs, normally inside pipes and where the cooling fluid is commonly water. A biphasic liquid mixture leaves the condenser, which is separated by denaturation in an appropriate drum, together with a gaseous phase, rich in hydrogen, which is sucked by suitable compressors and then sent, after purification in a specific section, for other uses. After separation by gravity, the two liquid phases are treated conveniently. The aqueous phase is purified by the residual hydrocarbons and then removed, while the hydrocarbon phase is sent to the purification section consisting of fractionation columns., which can be three or four, depending on the type of process. The liquid mixture of aromatic hydrocarbons from the dehydrogenation section is fed to the first column. In this column, a mixture of benzene and toluene is separated in the upper part, which forms a byproduct of the styrene production process, while a stream containing unreacted ethylbenzene, styrene and products at high boiling temperature is extracted from the bottom. The stream coming from the bottom of the first column is fed to a second column, where a stream containing unreacted ethylbenzene is separated at the top and recycled to the dehydrogenation section, while a stream containing styrene Along with products at high temperature boiling is extracted from the bottom. The flow coming from the bottom of the second column is fed to a third column, where the styrene that forms the final product is removed in the top, while a stream consisting of styrene and products at high boiling temperature is extracted from the bottom. As the concentration of styrene present in the stream leaving the lower part of the third purification column is still high, it is subsequently treated in a fourth column, or in a different apparatus, such as an evaporator. A stream rich in products at high boiling temperature that forms the process residue is produced and removed from the final part of the purification section, either a distillation column or an evaporator. What was indicated above is one of the possible styrene production schemes. There is at least one second scheme, also widely used, that differs mainly in the purification phase. This has a dehydrogenation section completely similar to that previously described, but the purification is based on three columns. In this case, the liquid hydrocarbon mixture is fed to the first column, where a mixture of benzene, toluene and ethylbenzene is separated at the top, while a stream containing styrene and products of high boiling temperature is separates at the bottom. The product extracted from the part The upper one is fed to a second column, where a mixture of benzene and toluene, which forms a by-product of the styrene production process, is extracted from the upper part, while a stream containing unreacted ethylbenzene is obtained in the upper part. bottom part and it is sent to the dehydrogenation unit. The stream coming from the lower part of the first column is fed to the third column, where the purified styrene is extracted from the upper part, while a stream whose composition is similar to that described in the previous scheme, is obtained in the lower part, which, as is still rich in styrene, is subsequently treated to recover it with procedures similar to those described above. The waste contains significant amounts of heavy viscous materials, which consist of breasts and / or other polymeric products that are formed as a result of the processing process. Finally, there is a third scheme for the production of styrene, applied less frequently, which differs with respect to the reaction part. According to this scheme, after the first stage of the dehydrogenation reaction, there is a section where the effluent coming from the reactor is heated as a result of an oxidation reaction carried out in a suitable layer of catalyst. Thanks to the catalyst, the oxidation, carried out by the introduction of air or oxygen, does not influence, or only to a minimum degree, the hydrocarbons present but mainly the hydrogen formed after the passage in the first dehydrogenation reactor. The reaction mixture leaving the oxidation stage, depleted in hydrogen and having a higher temperature, is fed to the second stage of dehydrogenation where the ethylbenzene is converted to styrene, under conditions favored particularly by the lack of hydrogen formed in the previous stage. Also in this case, there may be a third dehydrogenation step preceded, however, by a second oxidation step. The effluent coming from the last dehydrogenation reactor is cooled and condensed exactly with the same procedure as described above. Similarly, both liquid and gaseous effluents are in turn treated as specified above. Regardless of the procedure with which dehydrogenation is carried out and the method used to recover heat from the gases leaving the reactors, the products of the various reactions comprise, in addition to styrene, other chemical substances, some low-boiling temperature, for example methane, ethane and ethylene, which follow the gaseous phase, different such as benzene, toluene, stilbene, divinyl-benzene and other products with high boiling temperature, which condense and form the liquid mixture that is sent to the purification section following. Substances with a high boiling temperature formed, some of which, such as stilbene, have high melting points, tend to form small amounts of solid or liquid particles but extremely viscous which have a tendency to settle inside the apparatus during the cooling and condensation phase of the effluent from the last dehydrogenation reactor. In order to overcome this problem, in the styrene production processes described above, before the condensation phase, there is a cooling phase by direct contact with water. This operation allows most of the stuck particles mentioned above to be removed, preventing them from interfering with exchangers and machines, located downstream. During this phase of the process, extremely viscous solid or liquid particles tend to condense on the surface of the water droplets sprayed on the effluent gases and therefore both have to be removed from the system along with these. A small but significant fraction of substances with high boiling temperature that are formed during the cooling phase of the extremely viscous solid or liquid substances, is not stopped by washing with water and therefore reaches the equipment downstream, where effect the condensation of the reaction effluent. Part of this fraction is dragged with the gaseous effluent that reaches the compressors. Experience has shown that the presence of small deposits of solid or extremely viscous substances, as described above, can cause serious problems in the operation of the styrene production plants. Once they are deposited, in fact, for example on the surface of the heat exchanger pipes, their dimensions tend to increase, absorbing styrene together with small amounts of divinyl-benzene directly from the gas phase containing them. Once absorbed, the styrene follows its natural tendency to polymerize, thereby forming an additional solid mass which consequently increases the small particles of the high-boiling substance originally deposited. The low temperature with respect to those in which styrene polymerization normally occurs, and the the presence of small amounts of divinyl benzene, a product that contributes to increasing the molecular weight of the crosslinking polymer, makes the polymer formed completely insoluble, with the result that it is extremely difficult to remove. The consequences of deposition and the phenomenon of growth over time of the solid masses within the condensation system and sometimes within the hydrogen compressors create, either totally or partially, a sequence of undesirable events as described below: 1 decrease in the efficiency of the thermal exchange, particularly in the condenser; 2. increase in the condensation pressure and consequently the pressure upstream of the condensation system, in particular within the dehydrogenation reactors, to compensate for the efficiency of the reduced heat exchange; 3. increase in the condensation pressure and consequently upstream of the condensation system, in particular within the dehydrogenation reactors, due to the increase in pressure drops, caused by the decrease in the passage section as a result of the growth of the deposits of solid material; 4. lower performance of the dehydrogenation reaction due to the increase in pressure within the reactors, caused both by a decrease in the conversion and by a decrease in the selectivity; 5. decrease in the production capacity of the plant caused by: a decrease in the yield, a decrease in the efficiency of thermal exchange and a decrease in the gas passage sections; 6. loss of production due to the need to stop to clean, when the phenomena described above make the continuation not economical; 7, increase in costs, in particular those due to the stoppage of the plant and those for cleaning the equipment; 8. Serious damage to the equipment with the consequent need to stop and replace parts of it, when cleaning is not possible. The Applicant has discovered, as described in the appended claims, a condensation system that prevents the occurrence of the undesirable phenomena listed above, since it is completely free of the phenomenon of deposition of solid materials, according to the mechanism described above. An object of the present invention therefore relates to an improved process for the production of vinyl aromatic monomers, comprising: a) feeding a stream consisting of an aromatic hydrocarbon together with a stream consisting essentially of an olefin of 2 to 3 carbon atoms to an alkylation section; b) feeding the reaction product from the alkylation section to a first separation section; c) discharging from the first separation section a first stream consisting of unreacted aromatic hydrocarbon, which is recycled to the alkylation section, a second stream consisting essentially of a mono-alkylated aromatic hydrocarbon, a third stream consisting essentially of dialkylated aromatic hydrocarbons, sent to a transalkylation section, and a fourth stream consisting essentially of a mixture of polyalkylated aromatic hydrocarbons; d) feeding the second stream of step (c) to a dehydrogenation section; e) send the current leaving the last dehydrogenation reactor, after a first cooling with heat recovery and a subsequent washing with water spray, to a section where the condensation of most of the current occurs thermal exchange in specific equipment; f) feeding the reaction product from the condensation section (e) to a second separation / purification section, comprising at least one distillation column; g) discharging a stream consisting of the vinyl aromatic monomer with a purity greater than 99.7% by weight of the head of at least one distillation column. According to the present invention, the aromatic hydrocarbon fed to the alkylation section can be selected from those with a number of carbon atoms ranging from 6 to 9, but this is preferably benzene. Other aromatic hydrocarbons may be used in the process, object of the present invention, which may be selected from, for example, toluene and ethyl-benzene. The preferred hydrocarbon is refinery grade benzene with a purity greater than or equal to 95% by weight. The olefinic stream of 2 to 3 carbon atoms, for example ethylene or propylene, also of refinery grade with a purity greater than or equal to 95% by weight, is fed to the alkylation reactor together with the new aromatic hydrocarbon and, optionally, recycled. The two streams, aromatic and olefinic are fed to the alkylation unit to have aromatic / olefin molar ratios that satisfy the requirements of current technologies, typically from 1.8 to 50, preferably from 2 to 10. The alkylation reaction is carried out with conventional catalyst systems, for example according to a method described in European patent 432,814. Any alkylation reactor can be used in the process object of the present invention. For example, fixed bed or fluid bed reactors, transport bed reactors, reactors operating with a slurry mixture and catalytic distillation reactors can be adopted. Preferred alkylation catalysts can be aluminum trichloride or those selected from synthetic porous crystalline solids and natural ones based on silicon and aluminum, such as acid zeolites wherein the atomic ratio of silicon / aluminum ranges from 5/1 to 200/1. In particular, Y, beta, omega, mordenite, porous solids A, X and L and crystalline MCM-22, MCM-36, MCM-49, MCM-56 and ERS-10 are preferred. Alternatively, it is possible to use synthetic zeolites in the ZSM group wherein the atomic ratio of silicon / aluminum ranges from 20/1 to 200/1, such as zeolite ZSM-5.

The alkylation reaction can be carried out under conditions of temperature and pressure which depend, as is well known to those skilled in the art, not only on the selected catalyst but also on the type of reactor and the choice of reagents. In the case of alkylation of benzene with ethylene, the reaction temperature generally ranges from 100 to 450 ° C. More specifically, with zeolite catalysts, for some processes either fixed or mobile in the gas phase, the temperature preferably ranges from 300 to 450 ° C or from 180 to 250 ° C for liquid phase processes, while in the In the case of a catalytic distillation reactor, the mixed gas-liquid phase is operated, the reaction temperature varies along the catalytic bed, ranges from 140 to 350 ° C, preferably from 200 to 300 ° C. When using reactors that operate with a slurry mixture and an aluminum trichloride catalyst, the temperature ranges from 100 to 200 ° C. The pressure inside the alkylation reactor is maintained at values ranging from 0.3 to 6 MPa, preferably from 0.5 to 4.5 MPa. The aromatic stream leaving the alkylation reactor is treated with conventional means to recover the reaction product from the Unconverted reagents and reaction byproducts. In particular, the separation system preferably consists of a series of at least three distillation columns from which the unreacted aromatic compound is recovered from the first, and recycled to the alkylation reactor and / or to a transalkylation unit described below. The mono-alkyl-substituted aromatic compound, for example ethylbenzene, is recovered from the second distillation column and fed to the dehydrogenation unit, while the dialkylated aromatics are recovered from the head of the third column and sent to the transalkylation unit, and the heavy products, consisting essentially of polyalkylated products, tetralins and alkyl-substituted diphenylethanes, which can be fed as additives to the second separation / purification section of the product from the dehydrogenation section, are recovered from the bottom. Dialkylated aromatic compounds, for example diethylbenzenes, can be fed to a transalkylation reactor for transalkylation with aromatic hydrocarbons of 6 to 9 carbon atoms, for example benzene, to produce the corresponding mono-alkyl-substituted aromatic compounds, such as ethylbenzene, and increase the yield of alkylation production.

Transalkylation can occur in a specific reactor or in the same alkylation reactor. The transalkylation reactor, when present, preferably consists of a reactor operating in the suspension phase, when the catalyst is aluminum trichloride, or in a fixed bed reactor, operating in the liquid phase, wherein a catalyst is present. conventional zeolite transalkylation, such as Y zeolite, beta zeolite or mordenite, preferably Y or beta zeolite. The transalkylation reaction can be carried out according to what is described in European patent 847,802. In the case of transalkylation of diethylbenzene with benzene, the molar ratio of benzene / ethylene, calculated with respect to the total moles of benzene present as such and diethylbenzene and the total moles of ethylene present as a substituent in the diethyl groups -benzenes, ranges from 2/1 to 18/1, preferably from 2.5 / 1 to 10/1. The temperature in the reactor is maintained at a value from 50 to 350 ° C, preferably from 130 to 290 ° C, while the pressure is maintained from 0.02 to 6 MPa, preferably from 0.5 to 5 MPa. The mono-alkylated aromatic product is fed to the catalytic dehydrogenation section comprising one or several reactors that operate with a fixed bed or fluid bed. The dehydrogenation reaction with a fluid bed reactor occurs at a temperature ranging from 450 to 700 ° C and at a pressure ranging from 0.01 to 0.3 MPa, in the presence of a catalyst based on one or more metals selected from gallium, chromium , iron, tin, manganese supported on alumina modified with 0.05-5% by weight of silica. In addition to the above metals, the catalyst system may comprise platinum and / or one or more alkali or alkaline earth metals. Examples of dehydrogenation processes of alkyl aromatic hydrocarbons are described in Italian patent 1,295,071, in United States patents 5,994,258 and 6,031,143 or in international patent applications WO 01/23336 or WO 03/53567. The dehydrogenation reaction with a fixed-bed reactor occurs at a temperature ranging from 500 to 700 ° C, preferably from 520 to 650 ° C, at a pressure ranging from 0.02 to 0.15 MPa, in the presence of a catalyst based on iron oxide and potassium carbonate containing other metal compounds in small quantities, which have the function of promoters. In the case of the styrene production process, dehydrogenation can occur, for example, with a fixed bed catalyst by feeding a steam mixture of ethylbenzene and steam, in a molar ratio of water / ethylbenzene ranging from 5 to 15, preferably from 6 to 12, in a first reactor where a partial conversion of ethylbenzene occurs. The reacted mixture leaving the first reactor is fed to a second reactor, after the temperature has been brought to the required value by means of a heat exchanger. The reaction mixture, in which the ethylbenzene is converted by at least 50%, is cooled and condensed before being sent to the purification section. If required, at the outlet of the second reactor, it is possible to include a third reactor to increase the conversion of ethylbenzene to more than 70%. The gases coming from the dehydrogenation reactors, which come out at a temperature ranging from 450 to 650 ° C, preferably from 550 to 610 ° C, are cooled in a series of heat exchangers that recover heat by preheating the gases that are fed to the the reaction section, up to a temperature ranging from 100 to 300 ° C, preferably from 120 to 180 ° C; then they pass through ducts and / or equipment where, due to a series of water sprinklers, they are washed and cooled to 30-100 ° C, preferably 55-70 ° C; immediately condense in the shirt of an exchanger of horizontal heat, in whose tubes a cooling fluid flows, for example water, where condensation of a mixed type occurs at reflux and equilibrium. In particular, the gases enter through openings located in the lower part of the jacket, and move upwards, coming into contact with the exchanger tubes to which they transfer heat, as they begin to condense, the liquid formed by the condensation refluxes by the action of the force of gravity, coming into contact with the gas that rises, which has not yet condensed. Consequently, the following phenomena occur simultaneously in the exchanger jacket: cooling, condensation and washing of the gases. The washing of the gases and the exchanger tubes in the part of the liquid that naturally flows down by gravity, ensures that solid or liquid viscous particles are continuously washed and removed from both the condenser and the incondensable gas that reaches the top of the mantle, where, due to a configuration obtained by means of longitudinal and transverse deflectors, the gas is adequately cooled before being sucked by the compressors. The configuration described above has important advantages since it allows the condensation to be carried out in a simple device and avoids the need for any type of cleaning for the entire duration of the operation. The liquid mixture is sent to the second separation / purification section for the recovery of the vinyl aromatic monomer. In particular, the liquid mixture that flows down to the bottom of the condenser jacket, is collected in specific areas and sent to a horizontal tank, located below the condenser, where the two liquid phases, the water-rich phase and that which is rich in hydrocarbons, are separated by decantation. The aqueous phase is sent to treatment to remove traces of hydrocarbons and solid particles, while the hydrocarbon phase, suitably filtered, is fed to the so-called separation / purification section for the recovery of the vinyl aromatic monomer, for example, styrene . The separation / purification section comprises at least one distillation column, although it is preferable to operate with three or four distillation columns connected in series with respect to the flow of monomer to be purified.

EXAMPLE An example is provided, based on a comparison of industrial data in a styrene production plant, which demonstrates the advantages that can be obtained with the improved condensation system, with the condensation in the jacket (called Enhanced Plant), compared with a traditional plant, where condensation occurs inside the tubes of two devices configured in series, the first cooled with air and the second with water (called Reference Plant). The conditions of the plant are listed below. The plant consists of a production section of ethylbenzene to which ethylene and benzene are fed in the presence of a catalyst based on A1C1, at a pressure of approximately 0.5 MPa and at a temperature of 150 ° C. The effluent from the reaction section is fed to a separation section, where the catalyst based on A1C13 is separated and then to a distillation section where there are columns of three platforms. In the first, which operates at approximately 0.6 MPa and at a head temperature of 150 ° C, unreacted benzene is separated and recycled to the reaction section; the product from the bottom is fed to a second column operating at 0.25 MPa and at a temperature of 170 ° C, where the product of the head consists of ethylbenzene which is sent to the next dehydrogenation section, with a speed of flow of 40 t / h. The bottom product is fed to a third column that operates at a pressure of 0.01 MPa and at a head temperature of approximately 140 ° C. The product of the head, which consists of a mixture of polyethylated benzene compounds, mainly diethylbenzene, is recycled to the reaction section, while the lower part forms a by-product consisting of products with high boiling temperature. The dehydrogenation section, to which partially renewed and partially recycled ethylbenzene is fed, consists of two adiabatic reactors placed in series, containing a catalyst based on iron and potassium salts. The first reactor, to which ethylbenzene is fed in gaseous form in the presence of water vapor, with flow rates of 60 t / h (ethylbenzene) and 100 t / h (water vapor) respectively, operates at a temperature of input of approximately 610 ° C and a pressure of approximately 0.085 MPa. The second reactor, on the other hand, operates at an inlet temperature of 630 ° C and an inlet pressure of 0.05 MPa. The mixture leaving the 2nd reactor at a temperature of 590 ° C and at a pressure of 0.04 MPa, is cooled to approximately 150 ° C in two exchangers placed in series, where the heat is transferred, in the first exchanger, to the ethylbenzene stream, in the second steamed, which are fed to the first reactor. At the outlet of the last of these exchangers, the reaction gases are washed with approximately 50 t / h of water sprayed through a series of nozzles; the water injected in this manner is partially vaporized, cooling the gases to approximately 60 ° C and remaining partially in the liquid phase and flowing back towards the lower part of the tube. In this way, the gases are washed, before they reach a multi-tube heat exchanger; called condenser, where most of these are condensed, while the incondensable part, rich in hydrogen, is cooled to approximately 30 ° C and compressed into a suitable section. The condensed part at a pressure of 0.03 MPa and 55 ° C flows by gravity to a horizontal tank located below the multi-tube condenser; two phases are separated in the tank: the phase consisting mainly of water, which is removed after removing the residual hydrocarbons, and the hydrocarbon phase, rich in styrene, but still containing unreacted ethylbenzene and lower concentrations of benzene, Toluene and products of high boiling temperature, called dehydrogenated mixture. The hydrocarbon stream, with a flow rate of about 58 t / h, is pumped to the next section of Purification consisting of four distillation columns. The benzene and the toluene, which form the by-products, are separated in the first column, where the dehydrogenated mixture is fed, at the head, at a pressure of 0.02 MPa and at a temperature of about 55 ° C. The product from the lower part is fed to the second column, at a pressure of 0.03 MPa and at a temperature of approximately 100 ° C. A stream rich in ethylbenzene is separated from the head of the second column, operating at 0.01 MPa and at 65 ° C, which, after condensation, is pumped to the dehydrogenation section. The product from the bottom of the second column, at a temperature of about 90 ° C, is fed to the third column from which head most of the styrene is obtained with a purity of more than 99.7%, while the product from the bottom is fed to the fourth column, where more high purity styrene is obtained in the head and a product stream with high boiling temperature, with a small amount of residual styrene, which forms a by-product of waste, you get in the bottom. In the plant described above, the effluent condenser from the dehydrogenation section was originally of the type with condensation inside the tube and cooling fluid on the outside, followed by a smaller post-condenser. This condition is referred to hereinafter as the "Reference Plant". The condenser was then modified, as described above, and the condensation was effected within the jacket of a simple exchanger where gases enter from the bottom of the jacket and where condensation occurs with a mixed reflux and equilibrium configuration . This situation is referred to hereinafter as "Improved Plant". In the following table, several operating parameters of the plant described above are compared before and after carrying out the modification, object of the present invention, with variations in the running time, after changing the dehydrogenation catalyst. The parameters compared are the following: 1. pressure at the inlet of the condenser, measured downstream of the dehydrogenation section of ethylbenzene, step (d) before entry to the condensation / separation section, step (e), expressed in a relative form with respect to the initial number, that is to say that it is measured with a completely clean plant and new catalyst; 2. plant capacity, measured by the flow velocity of the styrene stream produced, expressed in a relative number with respect to the initial shape, ie that which is measured with a completely clean plant and new catalyst; 3. variation in the specific consumption of raw material, expressed as kilogram of ethylbenzene necessary to produce 1 ton of styrene, obtained as a ratio between the measurement of the flow velocity of the stream leaving the ethyl production section -benzene, measured in kg / hour, and the flow velocity of the styrene stream, measured in t / h; the value in the table is expressed as the difference between the value per month of operation considered and the initial value, ie that measured with a completely clean plant and new catalyst. For a better understanding of this parameter, it should be taken into account that it is influenced not only by the pressure (the higher the pressure, the lower the selectivity and therefore the specific consumption of ethylbenzene is higher), but also by the aging of the catalyst that jeopardizes performance. Aging depends on the time the catalyst has spent under high temperature conditions and has been in contact with depletions that are derived from the reactant stream.

TABLE (*) The Reference Plant, with a traditional condenser, makes a stop to clean half of the catalyst's useful life. The stoppage of the plant, which lasts approximately 10 days, implies a loss in production and additional costs, both for the operations of unemployment as for the operations of the subsequent reactivation, and maintenance.

Claims (15)

1. Improved process for the production and purification of vinyl aromatic monomers, comprising: a) feeding a stream consisting of an aromatic hydrocarbon together with a stream consisting essentially of an olefin of 2 to 3 carbon atoms to an alkylation section; b) feeding the reaction product from the alkylation section to a first separation section; c) discharging from the first separation section a first stream consisting of unreacted aromatic hydrocarbon, which is recycled to the alkylation section, a second stream consisting essentially of a mono-alkylated aromatic hydrocarbon, a third stream consisting essentially of dialkylated aromatic hydrocarbons, sent to a transalkylation section, and a fourth stream consisting essentially of a mixture of polyalkylated aromatic hydrocarbons; d) feeding the second stream of step (c) to a dehydrogenation section; e) feeding the reaction product from the dehydrogenation section to a second separation / purification section, comprising at least one distillation column; f) discharge a stream consisting of the vinyl aromatic monomer with a purity greater than 99.7% by weight, of the head of at least one distillation column, wherein: after a first cooling with heat recovery, the gas leaves the dehydrogenation stage, then washed with water sprayed, fed and it condenses in the jacket of a heat exchanger with multiple tubes maintained vertical or horizontal, in whose tubes a cooling fluid flows; the gas is fed from the bottom of the exchanger with the liquid derived from the condensation that flows back and leaves the exchanger, either totally or partially, still coming from the lower part of the jacket and sent to the second separation section / purification (e); The gas and possible non-condensed substances leave the jacket on the upper part of the exchanger.

2. Process according to claim 1, wherein the gas leaving the condenser is sent to an additional cooling stage in an additional heat exchanger (post-condenser), maintained vertical or horizontal, where the gas comes into contact with the group of pipes in the lower part of the exchanger, while the liquid that is derived from the condensation of part of the gas, that is to say introduced specifically in the upper part of the jacket, either totally or partially leaves the lower part of the jacket and is sent to (e), the gas and possible non-condensed substances leave the exchanger jacket from its top.

3. Process according to claim 1 or 2, wherein the gas leaving the condenser, or post-condenser, is sucked by a compressor that increases its pressure and sends it for cooling or condensation after the jacket of one or more vertical heat exchangers or horizontal, placed in series or in parallel, in whose tubes a cooling fluid flows, where the gas comes into contact with the group of tubes in the lower part of the exchanger, while the liquid derived from the condensation of part of the gas , that is to say introduced specifically in the upper part of the jacket, either totally or partially leaves the lower part of the jacket and is sent to (e), the gas and possible non-condensed substances leave the jacket of the exchanger from its top .

4. Process according to claim 1, 2 or 3, wherein the aromatic hydrocarbon fed to the alkylation section consists of refinery grade benzene, while the olefinic stream consists of refinery grade ethylene or propylene.

5. Process according to claim 4, wherein the olefinic stream consists of ethylene.

6. Process according to any of the previous claims, wherein the aromatic and olefinic streams are fed to the alkylation unit to have aromatic / olefinic molar ratios ranging from 1.8 to 50.

7. Process according to any of the previous claims, wherein the alkylation reaction occurs in the presence of catalysts selected from aluminum trichloride, synthetic and natural porous crystalline solids based on silicon and aluminum, wherein the silicon / aluminum atomic ratio ranges from 5 / 1 to 200/1 and synthetic zeolites from the group ZSM where the atomic ratio of silicon / aluminum ranges from 20/1 to 200/1.

8. Process according to any of the previous claims, wherein the alkylation reaction is carried out at a temperature ranging from 50 to 450 ° C.

9. Process according to claim 5, wherein The catalyst consists of aluminum trichloride and the temperature ranges from 100 to 200 ° C.

10. Process according to any of the previous claims, wherein the alkylation reaction is carried out at a pressure ranging from 0.3 to 6 MPa.

11. Process according to any of the previous claims, wherein the aromatic stream leaving the alkylation reactor is fed to a separation system consisting of a series of at least three distillation columns for the recovery of the monoalkyl-substituted aromatic compound, to be sent to the dehydrogenation unit.

12. Process according to any of the previous claims, wherein the catalytic dehydrogenation reaction occurs in a fluid bed reactor, at a temperature ranging from 450 to 700 ° C and a pressure ranging from 0.01 MPa to 0.3 MPa in the presence of a catalyst selected from one or more of the following metals: gallium, chromium, iron, tin, manganese, supported on alumina modified with 0.05-5% by weight of silica.

13. Process according to any of the previous claims 1 to 11, wherein the catalytic dehydrogenation reaction occurs in a fixed bed reactor, at a temperature ranging from 500 to 700 ° C and at a pressure ranging from 0.02 MPa to 0.15 MPa in the presence of a catalyst based on iron oxide and potassium carbonate.

14. Process according to any of the previous claims, wherein the second separation / purification section comprises three or four distillation columns connected in series with respect to the flow of monomer to be purified.

15. Process according to claims 1, 2 or 3, wherein the heat exchanger is kept horizontal and the cooling fluid is water. c