Title of Invention

A PROCESS FOR THE DEHYDROGENATION OF ALKYL-AROMATIC HYDROCARBONS FOR THE PRODUCTION OF VINYL-AROMATIC MONOMERS

Abstract

Abstract IMPROVED PROCESS FOR THE DEHYDROGENATION OF ALKYL-AROMATIC HYDROCARBONS FOR THE PRODUCTION OF VINYL-AROMATIC MONOMERS Process for the production of vinyl-aromatic monomers which comprises: a) feeding an aromatic stream and an olefmic 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.

Full Text

IMPROVED PROCESS FOR THE DEHYDROGENATION OF ALKYL-AROMATIC HYDROCARBONS FOR THE PRODUCTION OF VINYL-AROMATIC MONOMERS The present invention relates to an improved process for the dehydrogenation of alkyl-aromatic hydrocarbons for the production of vinyl-aromatic monomers. More specifically, the present invention relates to an improved process for the production of vinyl-aromatic mono¬mers coming from the dehydrogenation of the corresponding alkylated products. Even more specifically, the present invention relates to an improved process for the dehydrogenation of ethylben-zene for the production of styrene. Vinyl aromatic monomers, such as styrene, are particu¬larly known for their use in the preparation of plastic ma¬terials, such as compact/crystalline polystyrene (homo-polymer), compact high impact polystyrene (HIPS), expanded polystyrene and other products. These monomers are preva¬lently produced starting from the corresponding alkyl-aromatic product which is first dehydrogenated, in gaseous phase, at a high temperature and low pressure, in the pres¬ence of steam, on suitable catalysts based on iron and po¬tassium oxides. The effluent coming from the dehydrogena-tion reaction, which contains various hydrocarbon products formed under the reaction conditions, is first condensed and subsequently sent to a purification section where said effluents in the liquid state are treated to separate and purify the various components by means of a series of frac¬tionation columns operating in series, with respect to the flow of vinyl-aromatic monomer. In the case of styrene, the process generally envis¬ages feeding ethylbenzene to a device, normally a heat ex¬changer, where the ethylbenzene is vaporized in the pres¬ence of steam; the mixture is sent to a system of heat ex¬changers where the vapour is overheated and, after mixing with further steam at a high temperature, it is subse¬quently sent to the first reaction step. At the outlet of the first reaction step, the mixture is heated in a suit¬able exchanger system, and then sent to the subsequent re¬action step, possibly followed by a third. The effluent from the last reaction step is cooled, by preheating the ethylbenzene and the vapour which then feed the reaction section itself. In some cases, the heat recovery can be effected by generating and overheating the steam alone or vaporizing and overheating an azeotropic mixture of ethylbenzene and water. At the outlet of the heat recovery section, the reac¬tion mixture undergoes a further cooling, by contact with vaporizing water, and is then condensed in one or more heat exchangers, transferring heat to cooling water or, some¬times, air. The condensing system typically consists of 2 exchang¬ers in series. The first is that in which most of the con¬densation takes place, normally inside tubes, and wherein the cooling fluid can be either air or water. The second exchanger, which is smaller, can be installed either in a horizontal or vertical position and is that in which the cooling of the incondensable part takes place, normally in¬side tubes and in which the cooling fluid is usually water. A two phases liquid mixture leaves the condenser, which is separated by decanting in an appropriate drum, to¬gether with a gaseous phase, rich in hydrogen, which is sucked by suitable compressors and is then sent, after pu¬rification in a specific section, for other uses. After separation by gravity, the two liquid phases are suitably treated. The aqueous phase is purified by the residual hy¬drocarbons and then removed, whereas the hydrocarbon phase is sent to the purification section consisting of frac¬tionation columns, which can be three or four, depending on the type of process. The liquid mixture of aromatic hydrocarbons coming from the dehydrogenation section is fed into the first col¬umn. In this column, a mixture of benzene and toluene is separated at the top, which forms a by-product of the pro¬duction process of styrene, whereas a stream containing non-reacted ethylbenzene, styrene and high-boiling products 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 non-reacted ethylbenzene is separated at the top and recycled to the dehydrogenation section, whereas a stream containing styrene together with high-boiling products is extracted from the bottom. The flow coming from the bottom of the second column is fed to a third column, where the styrene forming the end-product is removed at the top, whereas a stream consisting of styrene and high-boiling products is extracted from the bottom. As the concentration of styrene present in the stream leaving the bottom of the third purification column is I still high, it is further treated in a fourth column, or different apparatus, such as an evaporator. A stream rich in high-boiling products which forms the process residue is produced and removed from the latter part of the purifica¬tion section, whether it be a distillation column or an 3 evaporator. What is indicated above is one of the possible produc¬tion schemes of styrene. There is at least a second scheme, also widely used, which mainly differs in the purification phase. This has a dehydrogenation section completely analo¬gous 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 ethyl benzene is separated at the top, whereas a stream containing styrene and high-boiling products is separated at the bottom. The product extracted from the top is fed to a second column where a mixture of benzene and toluene, which forms a by-product of the production process of sty¬rene, is extracted from the top, whereas a stream contain¬ing non-reacted ethylbenzene is obtained at the bottom and is sent to the dehydrogenation unit. The stream coming from the bottom of the first column is fed to the third column, where purified styrene is ex¬tracted from the top, whereas a stream whose composition is I similar to that described in the previous scheme, is ob¬tained at the bottom, which, as it is still rich in sty¬rene, is further treated to recover this with procedures similar to those described above. The residue contains significant quantities of heavy, ) viscous materials consisting of pitches and/or other poly- meric products which are formed as a result of the process¬ing process. There is finally a third scheme for the production of styrene, less frequently applied, which differs with re¬spect to the reaction part. According to this scheme, after the first dehydrogenation reaction step, there is a section in which the effluent from the reactor is heated as a re¬sult of an oxidation reaction carried out on a suitable layer of catalyst. Thanks to the catalyst, the oxidation, effected by introducing air or oxygen, does not influence, or only to a minimum extent, the hydrocarbons present but mainly the hydrogen formed after passage in the first dehy¬drogenation reactor. The reaction mixture leaving said oxi¬dation step, impoverished in hydrogen and having a higher temperature, is fed to the second dehydrogenation step where the ethylbenzene is converted to styrene, under con¬ditions particularly favoured by the absence of the hydro¬gen formed in the previous step. Also in this case, there can be a third dehydrogena¬tion step preceded however by a second oxidation step. The effluent from the last dehydrogenation reactor is cooled and condensed with exactly the same procedure as de¬scribed above. Analogously, both the liquid and gaseous ef¬fluents are in turn treated as specified above. Regardless of the procedure with which the dehydroge- nation is carried out and the method used for the heat re¬covery of the gases leaving the reactors, the products of the various reactions comprise, in addition to styrene, other chemical substances, some low-boiling, for example methane, ethane and ethylene, which follow the gaseous phase, others such as benzene, toluene, stilbene, divinyl-benzene and other high-boiling products, which condense and form the liquid mixture which is sent to the subsequent pu¬rification section. The high-boiling substances formed, some of which, such as stilbene, have high melting points, tend to form small quantities of solid or liquid but ex¬tremely viscous particles which have the tendency to become deposited inside the apparatus during the cooling and con¬densation phase of the effluent from the last dehydrogena-tion reactor. In order to overcome this problem, in the production processes of styrene described above, before the condensa¬tion phase, there is a cooling phase by direct contact with water. This operation allows most of the fouling particles mentioned above to be removed, preventing them from inter¬fering with the exchangers and machines situated down¬stream. During this phase of the process, solid or ex¬tremely viscous liquid particles do in fact tend to con¬dense on the surface of the drops of water sprayed in the effluent gases and therefore to be removed from the system together with these. A small but significant fraction of high-boiling substances which are formed during the cooling phase of the solid or extremely viscous liquid substances, is not stopped by the water washing and therefore reaches the equipment downstream, where the condensation of the re¬action effluent is effected. Part of this fraction is en¬trained with the gaseous effluent reaching the compressors. Experience has shown that the presence of small depos¬its of solid or extremely viscous substances, as described above, can cause serious drawbacks in the functioning of styrene production plants. Once deposited, in fact, for ex¬ample on the surface of the heat exchanger pipes, their di¬mensions tend to increase, absorbing styrene together with small quantities of divinyl-benzene directly from the gase¬ous phase containing them. Once absorbed, the styrene fol¬lows its natural tendency to polymerize, thus forming an additional solid mass which consequently increases the small particles of high-boiling substance originally depos¬ited. The low temperature, with respect to those in which the polymerization of styrene normally takes place, and the presence of small quantities of divinyl-benzene, a product which contributes to increasing the molecular weight of the polymer forming reticulations, makes the polymer formed completely insoluble with the result that it is extremely difficult to remove. The consequences of the deposition and growth phenome¬non with time of solid masses inside the condensing system and at times inside the hydrogen compressors create, either totally or partially, a sequence of undesired events as de¬scribed below: 1. decrease in the thermal exchange efficiency, particu¬larly on the condenser; 2. increase in the condensation pressure and consequently the pressure upstream of the condensing system, in particular inside the dehydrogenation reactors, to compensate the reduced heat exchange efficiency; 3. increase in the condensation pressure and consequently upstream of the condensing system, in particular in¬side the dehydrogenation reactors, due to the increase in the pressure drops, caused by the decrease in the passage section as a result of the growth of the solid material deposits; 4. lower yield of the dehydrogenation reaction due to the increase in the pressure inside the reactors, caused by both a decrease in the conversion and a decrease in the selectivity. 5. decrease in the plant production capacity caused by: a decrease in the yield, a decrease in the thermal ex¬change efficiency and a decrease in the passage sec¬tions of the gases; 6. production loss due to the necessity of shut down for cleaning, when the phenomena described above make con¬tinuation uneconomical; 7. increase in costs, in particular those due to the shut down of the plant and those for the cleaning of the equipment; 8. serious damage to the equipment with the consequent necessity of shut down and substitution of parts thereof, when cleaning is not possible. The Applicant has now found, as described in the en¬closed claims, a condensing system which prevents the oc¬currence of the undesired phenomena listed above as it is completely free of deposition phenomena 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 which comprises: a) feeding a stream consisting of an aromatic hydrocarbon together with a stream essentially consisting of a C2-C3 olefin to an alkylation section; b) feeding the reaction product coming from the alkylation section to a first separation section; c) discharging from the first separation section a first stream consisting of non-reacted aromatic hydrocarbon, which is recycled to the alkylation section, a second stream essentially consisting of a mono-alkylated aromatic hydrocarbon, a third stream essentially consisting of dial-kylated aromatic hydrocarbons, sent to a transalkylation section, and a fourth stream essentially consisting of a mixture of polyalkylated aromatic hydrocarbons; d) feeding the second stream of step (c) to a dehydrogena-tion section; e) sending the stream leaving the last dehydrogenation re¬actor, after a first cooling with heat recovery and a sub¬sequent washing with sprayed water, to a section in which the condensation takes place of most of the stream by ther¬mal exchange in specific equipment; f) feeding the reaction product coming from the condensa¬tion section (e) to a second separation/purification sec¬tion, comprising at least one distillation column; g) discharging a stream consisting of the vinyl-aromatic monomer with a purity higher than 99.7% by weight from the head of said at least one distillation column. According to the present invention, the aromatic hy¬drocarbon fed to the alkylation section can be selected from those with a number of carbon atoms ranging from 6 to 9 but is preferably benzene. Other aromatic hydrocarbons which can be used in the process, object of the present in¬vention, can be selected from, for example, toluene and ethyl-benzene. The preferred hydrocarbon is refinery-grade benzene with a purity higher than or equal to 95% by weight. The C2-C3 olefinic stream, for example, ethylene or propylene, also refinery-grade with a purity higher than or equal to 95% by weight is fed to the alkylation reactor to¬gether with the aromatic hydrocarbon, fresh and, option¬ally, recycled. The two aromatic and olefinic streams are fed to the alkylation unit so as to have aroraatic/olefin molar ratios which satisfy the requirements of current technologies, typically from 1.8 to 50, preferably from 2 to 10. The alkylation reaction is carried out with conven¬tional catalytic systems, for example according to the method described in European patent 432,814. Any alkylation reactor can be used in the process ob¬ject of the present invention. For example, fixed bed or fluid bed reactors, transport bed reactors, reactors oper¬ating with a slurry mixture and catalytic distillation re¬actors, can be adopted. The preferred alkylation catalysts can be aluminum trichloride or those selected from synthetic and natural porous crystalline solids based on silicon and alumin\am, such as acid zeolites in which the silicon/aluminum atomic ratio ranges from 5/1 to 200/1. In particular, Y, beta, omega zeolites, mordenite. A, X and L and the crystalline porous solids MCM-22, MCM-36, MCM-49, MCM-56 and ERS-10 are preferred. Alternatively, it is possible to use synthetic zeolites of the ZSM group in which the silicon/aluminum atomic ratio ranges from 20/1 to 20Q/1, such as ZSM-5 zeo¬lite. The alkylation reaction can be carried out under tem¬perature and pressure conditions which depend, as is well known to experts in the field, not only on the catalyst se¬lected but also on the type of reactor and choice of rea¬gents. In the case of the alkylation of benzene with ethyl¬ene, the reaction temperature generally ranges from 100 to 450°C. More specifically, with zeolitic catalysts, for ei¬ther fixed or mobile bed processes in gas phase, the tem¬perature preferably ranges from 300 to 450°C or from 180 to 250°C for processes in liquid phase, whereas in the case of a catalytic distillation reactor, operating in mixed gas-liquid phase, the reaction temperature, varying along the catalytic bed, ranges from 140 to 350°C, preferably from 200 to SOO°C. When reactors operating with a slurry mixture and an aluminum trichloride catalyst, are used, the tem¬perature ranges from 100 to 200°C. The pressure inside the alkylation reactor is main¬tained 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 for recovering the reaction product from the non-converted reagents and reaction by¬products. In particular, the separation system preferably consists of a series of at least three distillation columns from which the non-reacted aromatic compound is recovered from the first, and recycled to the alkylation reactor and/or a transalkylation unit described below. The mono-alkyl-substituted aromatic compound, for example ethyl-benzene, is recovered from the second distillation column and fed to the dehydrogenation unit, whereas the dialkyl-ated aromatic products are recovered from the head of the third column and sent to the transalkylation unit, and the heavy products, essentially consisting of polyalkylated products, tetralines and alkyl-substituted diphenyl ethanes, which can be fed as additives to the second sepa¬ration/purification section of the product coming from the dehydrogenation section, are recovered from the bottom. The dialkylated aromatic compounds, for example di¬ethyl benzenes, can be fed to a transalkylation reactor for transalkylation with Ce-Cg aromatic hydrocarbons, for exam¬ple benzene, to produce the corresponding mono-alkyl sub¬stituted aromatic compounds, such as ethyl-benzene, and in¬crease the yield of the alkylation production. The transalkylation can take place in a specific reac¬tor or in the same alkylation reactor. The transalkylation reactor, when present, preferably consists of a reactor operating in slurry phase, when the catalyst is aluminum trichloride, or in a fixed bed reac¬tor, functioning in liquid phase, in which a conventional zeolitic transalkylation catalyst is present, 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 the transalkylation of diethyl-benzene with benzene, the benzene/ethylene molar ratio, calculated with respect to the total moles of benzene present as such and as diethyl-benzene and the total moles of ethylene pre¬sent as substituents in the diethyl-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 of 50 to 350°C, preferably from 130 to 290°C, whereas the pressure is main¬tained at 0.02 to 6 MPa, preferably from 0.5 to 5 MPa. The mono-alkylated aromatic product is fed to the catalytic dehydrogenation section which comprises one or more reactors operating with a fixed bed or fluid bed. The dehydrogenation reaction with a fluid bed reactor takes place 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 modi- fied with 0.05-5% by weight of silica. In addition to the above metals, the catalytic system can comprise platinum and/or one or more alkaline -or alkaline earth metals. Exam¬ples of dehydrogenation processes of alkyl-aromatic hydro¬carbons are described in Italian patent 1,295,071, in U.S. 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 takes place at a temperature ranging from 500 to 7 00'*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¬lic compounds in small quantities having the function of promoters. In the case of the production process of styrene, the dehydrogenation can take place, for example, with a fixed bed catalyst by feeding a mixture of ethyl-benzene vapour and water vapour, in a water/ethyl-benzene molar ratio ranging from 5 to 15, preferably from 6 to 12, onto a first reactor in which a partial conversion of ethyl-benzene takes place. 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 ethyl-benzene is con¬verted for at least 50%, is cooled and condensed before be- ing 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 ethyl-benzene up to and over 70%. The gases coming from the dehydrogenation reactors, leaving at a temperature ranging from 450 to 650"C, pref¬erably from 550 to 610°C, are cooled in a series of ex¬changers which recover heat by preheating the feeding gases to the reaction section, up to a temperature ranging from 100 to SOO^C, preferably from 120 to IBO^C; they then pass through ducts and/or equipment where, due to a series of water sprayers, they are washed and cooled to 30-100°C, preferably 55-70°C; they are then condensed in the shell of a horizontal heat exchanger, in whose tubes a cooling fluid flows, for example water, in which condensation of a mixed type at reflux and equilibrium, takes place. In particular, the gases enter through the openings situated in the lower part of the shell, 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 condensa¬tion is refluxed by the action of the force of gravity, coming into contact with the rising gas which has not yet condensed. The following phenomena consequently take place contemporaneously in the shell of the exchanger: cooling, condensation and washing of the gases. The washing of the gases and exchanger tubes on the part of the liquid which naturally downflows by gravity, ensures that the solid or viscous liquid particles are con¬tinuously washed out and removed from both the condenser and the incondensable gas which reaches the upper part of the mantle, where, due to a configuration obtained by means of longitudinal and transversal baffles, the gas is suita¬bly cooled before being sucked by the compressors. The con¬figuration described above has important advantages as it allows the condensation to be effected in a single appara¬tus and avoids the necessity of any type of cleaning for the whole operating duration. The liquid mixture is sent to the second separa¬tion/purification section for the recovery of the vinyl-aromatic monomer. In particular, the liquid mixture which downflows into the lower part of the shell of the condenser is collected in specific areas and sent to a horizontal tank, situated below the condenser, where the two liquid phases, the phase rich in water and that rich in hydrocar¬bons, is separated by decanting. The aqueous phase is sent to treatment to remove the traces of hydrocarbons and solid particles, whereas the hydrocarbon phase, suitably fil¬tered, is fed to the so-called separation/purification sec¬tion for the recovery of the vinyl-aromatic monomer, for example, styrene. The separation/purification section comprises at least one distillation column even if 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 in¬dustrial data in a production plant of styrene, which dem¬onstrates the advantages that can be obtained with the im¬proved condensing system, with condensation in the shell (called Improved Plant), compared with a traditional plant, in which the condensation takes place inside the tubes of two apparatuses arranged in series, the first cooled with air and the second with water (called Reference Plant). The plant conditions are listed below. The plant consists of a production section of ethyl-benzene to which ethylene and benzene are fed in the pres¬ence of a catalyst based on AICI3, at a pressure of about 0.5 MPa and a temperature of 150°C. The effluent from the reaction section is fed to a separation section, in which the catalyst based on AICI3 is separated and subsequently to a distillation section in which there are 3 tray col¬umns. In the first, operating at about 0.6 MPa and a tem¬perature at the head of 150 °C, the non-reacted benzene is separated and is recycled to the reaction section; the bot¬tom product is fed to a second column operating at 0.25 MPa and a temperature of 170°C, where the head product consists of ethyl-benzene which is sent to the subsequent dehydroge-nation section, with a flow-rate of 40 t/h. The bottom product is fed to a third column operating at a pressure of 0.01 MPa and a temperature at the head of about 140°C. The head product, consisting of a mixture of polyethylated ben¬zene compounds, prevalently diethyl-benzene, is recycled to the reaction section, whereas the bottom forms a by-product consisting of high-boiling products. The dehydrogenation section, to which partly fresh and partly recycled ethyl-benzene is fed, consists of two adia-batic reactors situated in series containing a catalyst based on iron and potassium salts. The first reactor, to which ethyl-benzene is fed in gaseous form in the presence of water vapour, with flow-rates of 60 t/h (ethyl-benzene) and 100 t/h (water vapour) respectively, operates at an in¬let temperature of about 610 °C and a pressure of about 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 tempera¬ture of 590°C and a pressure of 0.04 MPa is cooled to about 150°C in two exchangers situated in series, in which the rteat is transferred, in the first exchanger, to the ethyl-benzene stream, in the second to the vapour, which are fed to the first reactor. 0.03 MPa and a temperature of about 100°C. A stream rich in ethyl-benzene is separated from the head of the second column, operating at 0.01 MPa and 65°C, which, after condensation, is pumped to the dehydrogenation section. The bottom product of the second column, at a tem¬perature of about 90"C, is fed to the third column from whose head most of the styrene is obtained with a purity of over 99.7%, whereas the bottom product is fed to the fourth column, where further high-purity styrene is obtained at the head and a stream of high-boiling products, with a small amount of residual styrene, forming a waste by¬product, is obtained at the bottom. In the plant described above, the condenser of the ef¬fluent from the dehydrogenation section was originally of the type with condensation inside the tube and cooling fluid outside, followed by a smaller post-condenser. This condition is hereafter called "Reference Plant". The condenser was then modified, as described above, and the condensation was effected inside the shell of a single exchanger in which the gases enter from the lower part of the shell and where the condensation takes place with a mixed reflux and equilibrium configuration. This situation is referred to below as "Improved Plant". In the table below, various functioning parameters of the plant described above are compared, before and after effecting the modification, object of the present inven¬tion, with variations in the running time, after changing the dehydrogenation catalyst. The parameters compared are the following: 1. pressure at the condenser inlet, measured downstream of the dehydrogenation section of ethyl-benzene, step (d) before the inlet to the condensation/separation section, step (e), expressed in a relative form with respect to the initial figure, i.e. that which is measured with a completely clean plant and fresh cata¬lyst; 2. plant capacity, measured by the flow-rate of the stream of styrene produced, expressed in a relative figure with respect to the initial form, i.e. that which is measured with a completely clean plant and fresh catalyst; 3. variation in the specific consumption of raw material, expressed as kg of ethyl-benzene necessary for produc¬ing 1 ton of styrene, obtained as a ratio between the flow-rate measurement of the stream leaving the pro¬duction section of ethyl-benzene, measured in kg/h, and the flow-rate of the styrene stream, measured in t/h; the value in the table is expressed as the dif¬ference between the value per operating month consid¬ered and the initial value, i.e. that measured with a completely clean plant and fresh catalyst. For a bet¬ter understanding of this parameter, it should be taken into account that it is influenced not only by the pressure, (the greater the pressure, the lower the selectivity and consequently the greater the specific consumption of ethyl-benzene), but also by the aging of the catalyst which jeopardizes the performances. The aging depends on the time the catalyst has spent under high temperature conditions and in contact with poisons deriving from the reagent stream. reactivation, and maintenance operations. CLAIMS 1. An improved process for the production and purifica¬tion of vinyl-aromatic monomers which comprises: a) feeding a stream consisting of an aromatic hydrocarbon together with a stream essentially consisting of a C2-C3 olefin to an alkylation section; b) feeding the reaction product coming from the alkylation section to a first separation section; c) discharging from the first separation section a first stream consisting of non-reacted aromatic hydrocarbon, which is recycled to the alkylation section, a second stream essentially consisting of a mono-alkylated aro¬matic hydrocarbon, a third stream essentially consisting of dialkylated aromatic hydrocarbons, sent to a transal-kylation section, and a fourth stream essentially con¬sisting of a mixture of polyalkylated aromatic hydrocar¬bons; d) feeding the second stream of step (c) to a dehydrogena-tion section; e) feeding the reaction product coming from the dehydroge-nation section to a second separation/purification sec¬tion, comprising at least one distillation column; f) discharging a stream consisting of the vinyl-aromatic monomer with a purity of over 99.7% by weight, from the head of said at least one distillation column, wherein: after a first cooling with heat recovery, the gas leav¬ing the dehydrogenation step, after washing with sprayed water, is fed and condensed in the shell of a tube bun¬dle heat exchanger maintained vertical or horizontal, in whose tubes a cooling fluid flows; the gas feeding is effected from the lower part of the exchanger with the liquid deriving from the condensation which refluxes and leaves the exchanger, either totally or partially, still from the lower part of the shell and is sent to the second separation/purification section (e); the possible gas and non-condensed substances leave the shell from the upper part of the exchanger. 2. The process according to claim 1, wherein the gas leaving the condenser is sent to a further cooling step in a further heat exchanger (post-condenser) , maintained ver¬tical or horizontal, where the gas comes into contact with the tube bundle in the lower part of the exchanger, whereas the liquid deriving from the condensation of part of the gas, i.e. introduced specifically in the upper part of the shell, either totally or partially leaves the lower part of the shell and is sent to (e) , the possible gas and non-condensed substances leaving the shell of the exchanger from its upper part. 3. The process according to claim 1 or 2, wherein the gas leaving the condenser, or post-condenser, is sucked by a compressor which increases its pressure and sends it for further cooling or condensation to the shell of one or more vertical or horizontal heat exchangers situated in series or in parallel, in whose tubes a cooling fluid flows, in which the gas comes into contact with the tube bundle in the lower part of the exchanger, whereas the liquid deriv¬ing from the condensation of part of the gas, i.e. intro¬duced specifically in the upper part of the shell, either totally or partially leaves the lower part of the shell and is sent to (e), the possible gas and non-condensed sub¬stances leaving the shell of the exchanger from its upper part. 4. The process according to claim 1, 2 or 3, wherein the aromatic hydrocarbon fed to the alkylation section consists of refinery grade benzene, whereas the olefinic stream con¬sists of refinery grade ethylene or propylene. 5. The process according to claim 4, wherein the olefinic stream consists of ethylene. 6. The process according to any of the previous claims, wherein the aromatic and olefinic streams are fed to the alkylation unit so as to have aromatic/olefinic molar ra¬tios ranging from 1.8 to 50. 7. The process according to any of the previous claims, I wherein the alkylation reaction takes place in the presence of catalysts selected from aluminum trichloride, synthetic and natural porous crystalline solids based on silicon and aluminum, in which the silicon/aluminum atomic ratio ranges from 5/1 to 200/1 and synthetic zeolites of the ZSM group in which the silicon/aluminum atomic ratio ranges from 20/1 to 200/1. 8. The process according to any of the previous claims, wherein the alkylation reaction is carried out at a tem¬perature ranging from 50 to 450°C. 9. The process according to claim 5, wherein the catalyst consists of aluminum trichloride and the temperature ranges from 100 to 200°C. 10. The process according to any of the previous claims, wherein the alkylation reaction is carried out at a pres¬sure ranging from 0.3 to 6 MPa. 11. The 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. The process according to any of the previous claims, wherein the catalytic dehydrogenation reaction takes place 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. The process according to any of the previous claims from 1 to 11, wherein the catalytic dehydrogenation reac¬ tion takes place in a fixed bed reactor, at a temperature ranging from 500 to 700°c and 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. The process according to any of the previous claims, wherein the second separation/purification section com¬ prises three or four distillation columns connected in se¬ ries with respect to the flow of monomer to be purified. 15. The process according to claim 1, 2 or 3, wherein the heat exchanger is maintained horizontal and the cooling fluid is water.

Documents:

3281-CHENP-2008 EXAMINATION REPORT REPLY RECEIVED 15-10-2012.pdf

3281-CHENP-2008 AMENDED PAGES OF SPECIFICATION 25-03-2013.pdf

3281-CHENP-2008 AMENDED CLAIMS 25-03-2013.pdf

3281-CHENP-2008 FORM.3 25-03-2013.pdf

3281-CHENP-2008 OTHER PATENT DOCUMENT 25-03-2013.pdf

3281-CHENP-2008 PCT NOTIFICATION 25-03-2013.pdf

3281-CHENP-2008 AMENDED CLAIMS 14-08-2013.pdf

3281-CHENP-2008 AMENDED PAGE OF SPECIFICATION 14-08-2013.pdf

3281-CHENP-2008 EXAMINATION REPORT REPLY RECEIVED 25-03-2013.pdf

3281-CHENP-2008 EXAMINATION REPORT REPLY RECEIVED 14-08-2013.pdf

3281-CHENP-2008 FORM-1 14-08-2013.pdf

3281-CHENP-2008 OTHERS 14-08-2013.pdf

3281-CHENP-2008 POWER OF ATTORNEY 25-03-2013.pdf

3281-CHENP-2008 POWER OF ATTORNEY 14-08-2013.pdf

3281-chenp-2008 abstract.pdf

3281-chenp-2008 claims.pdf

3281-chenp-2008 correspondence -others.pdf

3281-chenp-2008 description (complete).pdf

3281-chenp-2008 form-1.pdf

3281-chenp-2008 form-18.pdf

3281-chenp-2008 form-3.pdf

3281-chenp-2008 form-5.pdf

3281-chenp-2008 pct.pdf