MXPA98006459A - Method for the removal and recovery of mona and polyuclear aromatic co-products prepared in a reaction zone for the dehydrogenation of hydrocarbon - Google Patents

Abstract

Polynuclear aromatic compounds (PNA) produced as a by-product in a dehydrogenation zone for hydrocarbons are removed and recovered by mixing the effluent or emanation from the dehydrogenation zone with a recycled stream containing mononuclear aromatic compounds (MNA) and the mixture The resultant is contacted with an adsorbent to reduce the concentration of MNA and PNA and to produce a flow comprising dehydrogenated hydrocarbons and hydrocarbons that can be dehydrogenated. The spent adsorbent recovered from the adsorption step, containing MNA and PNA, is regenerated by contacting it with a hot, hydrogen-rich gas to recover at least a portion of the MNA and the PNA and, therefore, regenerate spent elastomer. The hydrocarbon-containing effluent from the regeneration step is then separated in a flow containing MNA and in a flow containing PNA, and at least a portion of the flow containing MNA is recycled in the mixing step.

Description

METHOD FOR THE REMOVAL AND RECOVERY OF CO-PRODUCTS AROMATICS MONO AND POLI UCLEARESELABORATED IN A ZONE OF REACTION FOR THE DEHYDROGENATION OF HYDROCARBONS FIELD The field of the technique to which this invention pertains is the removal and recovery of co-products of polynuclear aromatic compounds from a vapor effluent from a normal reaction zone for the dehydrogenation of hydrocarbons. BACKGROUND Dehydrogenation of hydrocarbons is a commercial hydrocarbon conversion process, since there is a growing demand for dehydrogenated hydrocarbons for the production of various chemical products such as detergents, high octane gasoline, components mixed with oxygenated gasoline, pharmaceutical products, plastics , synthetic gums and other products that are well known by the connoisseurs of the material. An example of this process is the dehydrogenation of isobutane to produce isobutylene which can be polymerized to provide tackifying agents for adhesives, additives for the viscosity index for motor oils and impact resistant additives and antioxidants for plastics. Another example of the growing demand for isobutylene is the production of components mixed with gasoline containing oxygen, which is requested by the government in order to reduce the emissions of pollutants from automobiles. Those skilled in the art of the hydrocarbon conversion process are very familiar with the production of olefins by the catalytic dehydrogenation of paraffinic hydrocarbons. In addition, many patents have been issued in which the dehydrogenation of hydrocarbons in general is discussed and discussed. For example, the United States patent US-A-4430517 presents this process of dehydrogenation. Despite the fact that the dehydrogenation of paraffinic hydrocarbons is well known, the most common use of this technology and the more severe operation of existing commercial facilities have serious problems that occur in the product recovery section of the processes for the dehydrogenation of hydrocarbons. This problem results in the production of remaining amounts of mono- and polynuclear aromatic compounds. The polynuclear aromatic compounds are not only the undesirable impurity, but also present a severe operational problem as they condense and form a plaque on the cold surfaces of the plant with very damaging results. These deposits of polynuclear aromatic compounds are difficult to remove, reduce the efficiency of heat exchangers and can eventually lead to clogging. In the case where the dehydrogenated compounds are used in subsequent processes, a sudden emergence of the polynuclear aromatic compounds in the dehydrogenation effluent may contaminate the products resulting from the subsequent processes. The presence of polynuclear aromatic compounds changes the color quality of the products and significantly reduces the value or capacity to market the products. BRIEF SUMMARY OF THE INVENTION Thus, the present invention relates to a solution for the handling of polynuclear aromatic compounds (PNA for its international abbreviations) that are produced in an area for the dehydrogenation of hydrocarbons. According to the method of the invention, the effluent from the zone for dehydrogenation of hydrocarbons is mixed with a recycling jet containing mononuclear aromatic compounds (MNA) and the resulting mixture is contacted with a selective adsorbent. for MNA and PNA to reduce the concentration of poly and mononuclear aromatics and to produce a vapor comprising dehydrogenated hydrocarbons and hydrocarbons that can be dehydrogenated. The worn adsorbent containing mono and polynuclear aromatic compounds, recuperator in the adsorption step, are contacted with a hot gas rich in hydrogen to recover at least a portion of the MNA and PNA to thereby regenerate the worn adsorbent. The resulting hydrogen-rich gas, containing MNA and PNA, is then separated to produce a jet or stream containing MNA and a jet or stream containing PNA. At least one flow or jet portion contains MNA and, therefore, is recycled in the mixing step, thus obtaining the desired solution to handle the PNA problem. An embodiment of the present invention can be characterized as a method for the handling of the byproducts of the poly and mono-nuclear aromatic compounds produced in the reaction zone for the dehydrogenation of hydrocarbons, this method comprises: (a) mixing an effluent from a reaction zone for the dehydrogenation of the hydrocarbons comprising dehydrogenated hydrocarbons, dehydrogenated hydrocarbons and amounts of aromatic hydrocarbon, mono and nuclear residues with a recycling jet comprising mononuclear aromatic compounds; (b) contacting the mixture resulting from step (a) with a selective adsorbent for poly and mononuclear aromatics which reduce the concentration of the mono- and polynuclear aromatic compounds in the mixture and to produce a jet or stream comprising dehydrogenated hydrocarbons and hydrocarbons that can be dehydrogenated; (c) contacting the spent adsorbent, which retains the mononuclear and poly aromatic compounds recovered in step (b) with a hydrogen-rich gas to expel at least a portion of the mononuclear and poly aromatic compounds and therefore, regenerate the adsorbent; (d) separating the flow or jet having hydrogen and the poly and mononuclear aromatic compounds recovered in step (c) to produce a flux comprising mononuclear aromatics and a flux having polynuclear aromatic compounds; and (e) recycling at least a portion of the flow comprising mononuclear aromatics separated in step (d) to provide the recycled stream in step (a). BRIEF DESCRIPTION OF THE DRAWING The drawings is a flowchart of the simplified process of the preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION The present invention is a process for the removal and recovery of remaining amounts of polynuclear aromatic compounds from a vapor effluent from a zone for the dehydrogenation of hydrocarbons. The dehydrogenation of paraffinic hydrocarbons is well known to those skilled in the hydrocarbon processing art. According to the present invention, the preferred dehydrogenated hydrocarbons are usually gaseous and are selected from a group consisting of ethylene, propylene and butylene. In the dehydrogenation process, the fresh hydrocarbon that is fed is combined with recycled hydrogen and with hydrocarbons, without conversion, recycled. This forms a reactant flow that passes through a catalyst bed suitable for dehydrogenation, which is maintained under the appropriate dehydrogenation conditions, such as temperature, pressure and separation rate, and additionally the effluent from the catalytic reaction zone. it is processed to produce a flow or jet containing olefinic hydrocarbons. According to the present invention, the effluent from the reaction zone for the catalytic dehydrogenation contains hydrocarbons, without recovering, saturated, olefinic hydrocarbons, mononuclear aromatics in an amount from 100 to 5000 parts by weight (ppm) and aromatics polynuclear in an amount from 50 to 500 wppm. For preferred dehydrogenated hydrocarbons, this effluent jet is in the vapor state when it is extracted from the dehydrogenation zone.

In accordance with the present invention, the effluent from the reaction zone for the dehydrogenation is mixed with a recycle stream containing mononuclear aromatics. The amount of recycle flow of mononuclear aromatics is selected to prevent deposition and accumulation of polynuclear aromatic compounds on the internal surfaces of the plant, downstream of the reaction zone for dehydrogenation and preferably in an amount of 0.01 to 1. percent by weight of the emanation of the reaction zone. Then, the resulting mixture of the effluent from the reaction zone for the dehydrogenation and the recycled jet of the mononuclear aromatics is contacted with a selective adsorbent for MNA and PNA, to reduce the concentration of mono- and polynuclear aromatic compounds and to produce a purified, resulting jet containing hydrogen, dehydrogenated hydrocarbons and hydrocarbons that can be dehydrogenated. The preferred jet is subsequently compressed and cooled to a temperature ranging from -50 ° F to -200 ° F to produce a hydrogen-rich gaseous stream or stream that is preferably partly recycled into the reaction zone for dehydrogenation and a stream containing dehydrogenated hydrocarbons and hydrocarbons that can be dehydrogenated. Any suitable adsorbent can be used with the present invention, however, the preferred adsorbents are selected from a group consisting of activated alumina and activated carbon. The spent adsorbent, containing poly and mononuclear aromatic compounds recovered in the adsorption step, is contacted with hydrogen-rich gas, which has a temperature ranging from 300 ° F (149 ° C) to 700 ° F (371). ° C), to desorb or expel at least a portion of the aromatic compounds contained therein. The resulting jet or stream of poly and mononuclear aromatic compounds is cooled to a temperature ranging from 60 ° F (16 ° C) to 120 ° F (49 ° C9) and is introduced into an area for the separation of liquid vapor to produce a hydrogen-rich gaseous stream or stream that can be recycled if desired.Preferably, the hydrogen-rich gas has a portion of the net, without hydrogen, generated in the reaction zone for dehydrogenation.The liquid stream containing poly aromatic compounds and mononuclear is also removed from the vapor-liquid separation zone and separated, preferably by fractionation, to produce a flow or jet of polynuclear aromatic compounds and a jet containing mononuclear aromatics, wherein at least one portion thereof, is used as a recycle stream or jet, as described above. Since the mononuclear aromatic compounds are produced as a by-product in the reaction zone for dehydrogenation, the net flow, comprising mononuclear aromatics, is produced and removed from the process. DETAILED DESCRIPTION OF THE DRAWING Referring now to the drawing, the hydrocarbon feedstock that can be dehydrogenated in the process is introduced via line 1 and mixed with a stream of recycled hydrogen that is provided via line 26 and the resulting mixture it is introduced through line 2 in the dehydrogenation zone 3. The effluent that results or leaves the dehydrogenation zone 3, contains dehydrogenated hydrocarbons, dehydrogenated hydrocarbons and remaining amounts of mono- and polynuclear aromatic hydrocarbons that are transported by a duct 4 and mixed with the recycled stream containing mononuclear aromatics, supplied through duct 35 and the resulting mixture is transported via duct 5, duct 6, valve 7 and duct 8, and then introduced into the zone adsorbent 9. The resulting hydrocarbon flux containing a reduced concentration of Mono and polynuclear aromatic compounds are removed from the adsorption zone 9 and transported via a conduit 10, the valve 11, the conduit 12 and the conduit 23, and are introduced into the liquid and vapor separation zone 24. A gaseous flow Hydrogen rich is removed from the vapor and liquid separation zone 24 via conduit 26 and recycled as described above. A flow of net hydrogen gas is removed from the vapor and liquid separation zone 24, via conduit 40. The flow of liquid hydrocarbon containing dehydrogenated hydrocarbons and dehydrogenated hydrocarbons is removed from the vapor separation zone. and liquid 24 via conduit 25 and recovered. A gaseous flow rich in hydrogen is introduced via conduits 16 and 17 to an adsorption zone 18 outside the line. The resulting hydrogen gas stream, which contains extruded poly and mononuclear aromatic compounds, is removed from the adsorption zone 18 via conduit 19, conduit 22 and introduced into the heat exchanger 36. The cooled effluent exiting the exchanger heat 36 is transported via a conduit 37 and is introduced into the vapor and liquid separation zone 27. The gaseous, hydrogen-rich flow is removed from the vapor and liquid separation zone 27 by a conduit 38. The flow liquid containing poly and mononuclear aromatic compounds is removed from the separation zone 27 through the conduit 28 and is introduced into the fractionation zone 29. The flow containing polynuclear aromatic compounds is removed from the fractional zone 29 through the conduit 30 and he recovers.

The flow containing mononuclear aromatics is removed from the fractionation zone 29 through the conduit 31 and is introduced into the receiver 32. The flux containing mononuclear aromatics is removed from the receiver 32 through the conduits 32 and 33 and recycle as mentioned above. A net stream or stream containing mononuclear aromatics is removed from receiver 32 through conduits 33 and 34 and recovered. The conduits 13 and 6, together with the valve 14, and the conduits 20 and 39, together with the valve 21, are used when the adsorption zone 18 is placed in line to replace the adsorption zone 9 during its regeneration. ILLUSTRATIVE EMBODIMENT A flow of isobutane feed, in an amount of 100 mass units per hour, is introduced into a zone for dehydrogenation to convert 50 weight percent to isobutylene. The recycled hydrogen is also introduced into the dehydrogenation zone in an amount of 160 M3 STD / M3 gas to provide 900 SCFB. The resulting effluent from the dehydrogenation zone contains 100 ppm of mononuclear aromatic compounds and 200 ppm of polynuclear aromatic compounds based on hydrocarbons and is mixed with a recycled stream containing mononuclear aromatics in an amount of 0.05 mass units per hour. The resulting mixture of effluent from the dehydrogenation zone and the recycled stream or stream is introduced into the adsorption zone containing activated alumina to reduce the level of mononuclear aromatics to less than 500 ppm and the level of polynuclear aromatic compounds to less than 500 ppm. 1 ppm. The resulting effluent from the adsorption zone is compressed and cooled to a temperature of -130 ° F, which is subsequently introduced into a vapor and liquid separation zone to produce a hydrogen-rich gas stream, which is recycled in the dehydrogenation zone and the liquid hydrocarbon stream containing isobutane and isobutylene. The hot hydrogen-rich gas stream, having a temperature of 600 ° F (315 ° C), is introduced into an out-of-line adsorption zone, spent, containing alumina activated with poly and mononuclear aromatic compounds, with the aim of regenerate the adsorption zone. The resulting hydrogen flux containing poly and mononuclear aromatics is cooled to 100 ° F (38 ° C) to condense the aromatics and introduce them into the vapor and liquid separation zone to produce a flow of hydrogen and a liquid flow that it contains poly and mononuclear aromatic compounds, whose liquid flow is fractioned to produce a flow containing polynuclear aromatic compounds and a flux containing mononuclear aromatic compounds. A portion of the flow containing mononuclear aromatics is recycled as described above. A net flux containing a mononuclear aromatic compound is recovered and removed from the process.

Claims (6)

1. A method for the handling of mono and polynuclear aromatic compounds produced in a reaction zone for dehydrogenation, which comprises: (a) mixing an effluent from a reaction zone for the dehydrogenation of hydrocarbons comprising dehydrogenated hydrocarbons, hydrocarbons that can be dehydrogenate and trace amounts of aromatic polo and mono-nuclear hydrocarbons with a recycle jet comprising mononuclear aromatics; (b) contacting the mixture resulting from step (a) with a selective adsorbent for poly and mononuclear aromatics which reduce the concentration of the mono- and polynuclear aromatic compounds in the mixture and to produce a jet or stream comprising dehydrogenated hydrocarbons and hydrocarbons that can be dehydrogenated; (c) regenerating the spent adsorbent, which retains the poly and mononuclear aromatics upon contact with the spent adsorbent of step (b) with a hydrogen-rich gas to expel at least a portion of the mononuclear and poly aromatic compounds; (d) separating the flow or jet having hydrogen and the poly and mononuclear aromatic compounds recovered in step (c) to produce a flux comprising mononuclear aromatics and a flux having polynuclear aromatic compounds; and (e) recycling at least a portion of the flow comprising mononuclear aromatics separated in step (d) to provide the recycled stream in step (a).

2. The method according to claim 1, wherein the dehydrogenated hydrocarbons are selected from a group consisting of ethylene, propylene and butylene.

3. The method according to claim 1, wherein the polynuclear aromatic compounds are present in the effluent from the reaction zone for dehydrogenation of the hydrocarbon in an amount from 50 to 500 wppm.

4. The method according to claim 1, wherein step (b) is carried out at a temperature of 10 to 65 ° C (50 to 150 ° F). The method according to claim 1, wherein the adsorbent is selected from a group consisting of activated alumina and activated carbon. The method according to claim 1, wherein the recycled stream comprising mononuclear compounds is present in an amount from 0.01 to 1 percent by weight of the effluent from the reaction zone for dehydrogenation of the hydrocarbons.