Title of Invention

PROCESS FOR THE SEPARATION OF C5 HYDROCARBONS BY THE SELECTIVE DIMERIZATION OF ISOBUTENE

Abstract

Abstract PROCESS FOR THE SEPARATION OF C5 HYDROCARBONS PRESENT IN STREAMS PREVALENTLY CONTAINING C4 PRODUCTS USED FOR THE PRODUCTION OF HIGH-OCTANE HYDROCARBON COMPOUNDS BY THE SELECTIVE DIMERIZATION OF ISOBUTENE A process is described for the separation of C5 hydrocarbons present, in a quantity ranging from 0.2 to 20% by weight, in streams prevalently containing C4 products used for the production of high-octane hydrocarbon compounds, by the selective dimerization of isobutene, characterized in that the dimerization reaction is carried out in the presence of linear and branched alcohols and alkyl ethers in a quantity which is such as to have a molar ratio alcohols/alkyl ethers/isobutene in the feeding higher than 0.01.

Full Text

PROCESS FOR THE SEPARATION OP C5 HYDROCARBONS PRESENT IN STREAMS PREVALENTLY CONTAINING C4 PRODUCTS USED FOR THE PRODUCTION OF HIGH-OCTANE HYDROCARBON COMPOUNDS BY THE SE¬LECTIVE DIMERIZATION OF ISOBUTENE The present invention relates to a process for the separation of C5 hydrocarbons present in the C4 charges used for the production of high-octane hydrocarbon com¬pounds by the selective dimerization reaction of isobutene and to a lesser extent of possible linear olefins, in the presence of linear and branched alcohols and alkyl ethers, which favour the production of higher selectivities on the part of the catalyst. The mixture obtained can then be hy-drogenated with conventional methods to obtain a product with further enhanced octane characteristics. For mainly environmental reasons, the composition of gasolines is being reformulated and the general tendency is towards the production of fuels which burn better and have lower evaporative emissions. The main measures for achiev¬ing this objective are listed below (D. Sanfilippo, F. An- cillotti, M.Marchionna, Chim.&Ind., 76,(1994), 32): - reduction in the content of aromatic compounds and elimi¬ nation of benzene; - reduction in the volatility of gasolines to minimize evaporative losses; - reduction in the content of light olefins, photochemi-cally extremely reactive; - reduction in the sulfur content and final boiling point of the gasolines. All these measures consequently create the necessity of projecting new production processes of purely hydrocar¬bon compounds capable of positively contributing to the above demands. Among these, alkylated products are extremely impor¬tant as they have a high octane number, a low volatility and-are practically free of olefins and aromatic compounds. The alkylation process in liquid phase is a reaction be¬tween isoparaffinic hydrocarbons, such as isobutane, and olefins, for example propylene, butenes, pentenes and rela¬tive mixtures thereof, in the presence of an acid catalyst for the production of C7-c9 hydrocarbons with a high octane number to be used in gasolines (A. Corma, A. Martinez, Catal.Rev.-Sci.Eng., 3_5, (1993) ,483) . The main problem of alkylation processes is due to the fact that, with growing environmental regulations, both of the traditional processes (with hydrofluoric and sulfuric acid) are encountering considerable difficulties, which create uncertainties for the future; the process with hy¬drofluoric acid due to the toxicity of this acid, espe¬cially in populated areas, and that using sulfuric acid, as a result of the large production of acid sludge as well as the considerably corrosive nature of the catalyst. Alternative processes with solid acid catalysts are being developed but their commercial applicability has yet to be demonstrated. A hydrocarbon product of this type, on the other hand, is becoming increasingly more requested due to its octane characteristics (both the Research Octane Number (RON) and the Motor Octane Number (MON) are high) and those relating to the boiling point (limited volatility but low end-point) which position it in the group of compositions of great in¬terest for obtaining gasolines which are more compatible with current environmental requirements. An alternative refinery process for obtaining products with characteristics similar to those of alkylated products can be offered by the hydrogenation of so-called "polymer" gasoline. Oligomerization processes (often inaccurately called polymerization in the refining industry) were widely used in the '30s' and '40s' for converting low-boiling C3-C4 olefins into gasolines. The process leads to the production of a gasoline with a high octane number (RON about 97) but with a high sensitivity (difference between RON and MON) due to the purely olefinic nature of the product (J.H.Gary, G.E. Handwerk, "Petroleum Refining : Technology and Econom¬ics", 3rd Ed., M. Dekker, New York, (1994), 250). Typical olefins which are oligomerized are mainly pro¬pylene, which gives dimers or slightly higher oligomers de¬pending on the process used, and isobutene which mainly gives dimers but is always accompanied by a considerable quantity of higher oligomers. With particular attention to the oligomerization of isobutene, it is known that this reaction can be carried out either batchwise, semi-batchwise or in continuous, ei¬ther in gas or liquid phase, generally at temperatures ranging from 50 to 3 00°C and at atmospheric pressure or such pressures as to maintain the reagents in liquid phase, if necessary. Typical catalysts for the industrial oligomerization process of isobutene are represented by phosphoric acid, generally supported on a solid (for example kieselguhr), or cation-exchange acid resins. The latter allow blander con¬ditions to be used compared with supported phosphoric acid both in terms of temperature and pressure (50-100°C and 0.2-3 MPa with respect to 200-220°C and 3-10 MPa). Other catalysts are also claimed in literature, both liquid acids such as H2S04 and derivatives of sulfonic ac¬ids, and solids such as silico-aluminas, mixed oxides, zeo¬lites, fluorinated or chlorinated aluminas, etc.; none of these catalysts however has so far enabled an industrial process to be set up, as in the case of supported phospho¬ric acid (F. Asinger, "Mono-olefins: Chemistry and Technol¬ogy", Pergamon Press, Oxford, pages 435-456) and that of cation resins (G. Scharfe, Hydrocarbon Proc., April 1973, 171} . From the product point of view, the main problem of this process lies in the fact that excessive percentages of heavy oligomers such as trimers (selectivity of 20-40%) and tetramers (selectivity of 1-5%) of isobutene, are produced in the oligomerization phase. Tetramers are completely out¬side the gasoline fraction as they are too high-boiling and therefore represent a net loss in yield to gasoline; as far as trimers are concerned, their concentration should be greatly reduced as they have a boiling point (170-180°C) at the limit of future specifications on the final point of reformulated gasolines. The problem of reducing the formation of oligomers higher than dimers to percentages lower than 15% is, on the other hand, a problem typical of the oligomerization of isobutene, as also indicated in literature (C.T.O'Connor, M. Kojima, K.W. Shcumann, Appl. Catal., jL6, (19B5) , 193) . This level of heavy compounds is slightly higher than that of an alkylated product and is still tolerated in the gasoline pool. From what is specified above, there is evidently great interest in obtaining a new dimerization process of isobu-tene which allows the synthesis of a higher-quality prod¬uct, through reaching greater selectivities. By carrying out the selective dimerization reaction of isobutene in the presence of moderate quantities of linear and branched alcohols and alkyl ethers, the production of a fraction of oligomers is selectively obtained, which is particular rich in dimers (>85%) and practically free of tetramers and higher oligomers ( The reaction product is then preferably hydrogenated to give a completely saturated end-product with a high oc¬tane number and low sensitivity. The hydrogenation can be carried out with conventional methods as described, for example, in F. Asinger, "Mono-olefins: Chemistry and Technology", Pergamon Press, Oxford, page 455. For illustrative purposes, Table 1 indicates the oc¬tane number and relative boiling points of some of the products obtained, by means of the process, object of the present invention. The process, object of the present invention, for the separation of C5 hydrocarbons present, in a quantity rang¬ing from 0.2 to 20% by weight, in streams prevalently con¬taining C4 products used for the production of high-octane hydrocarbons, by the selective dimerization reaction of isobutene, is characterized in that the reaction is carried out in the presence of linear and branched alcohols and ethers in a quantity which is such as to have a molar ratio alcohols + ethers/isobutene in the feeding higher than 0.01 and preferably lower than 0.7. It should also be pointed out that in the case of hy¬drocarbon streams also comprising C4 and C5 olefins, it has been observed that at least a part of the latter can be converted by reaction with isobutene into the hydrocarbon product without altering the octane value. It is therefore preferable to effect an enriching treatment, by means of pre-isomerization, of the internal linear olefins, in order to favour the overall octane number of the mixture. The process claimed herein can be applied to cuts mainly containing isobutane, isobutene, n-butane, n-butenes and saturated and olefinic C5 hydrocarbons. Although a wide variety of sources are available for the supply of these streams, the most common are those de¬riving from Dehydrogenation processes of iso-paraffins, from FCC units, Steam Cracking or processes for the produc¬tion of pure isobutene such as the dehydration of tert-butyl alcohol (TBA) or the Cracking of MTBE and/or ETBE; these streams differ from each other in the content of iso¬butene and linear butenes, as shown in Table 2. Should streams from Steam Cracking contain diolefins in addition to the desired mono-olefins, they must be eliminated by means of typical removal treatment (for exam¬ple solvent extraction or selective hydrogenation) . Saturated and olefinic C5 hydrocarbons can be present in these streams, in various amounts (0.2 - 20% by weight), depending on the efficiency of the C4-Cs separation step. The C5 olefins possibly present can be involved in dimeri-zation reactions. The stream sent to the reaction steps can contain branched alcohols or a blend of alcohols and alkyl ethers, in addition to the hydrocarbon components. The alcohols used are linear, preferably containing a number of carbon atoms ranging from 1 to 6, preferably from 4 to 7 carbon atoms; preferred linear alcohols are methanol and/or ethanol, whereas preferred branched alcohols are tert-butyl alcohol (TBA) and/or tert-amyl alcohol (TAA). The alkyl ether used can be selected from those con¬taining a number of carbon atoms ranging from 5 to 10: MTBE (methyl tert-butyl ether), ETBE (ethyl tert-butyl ether), MSBE (methyl sec-butyl ether), ESBE (ethyl sec-butyl ether), TAME (methyl tert-amyl ether), TAEE (ethyl tert-amyl ether) or mixtures thereof are preferred. Isobutene, together with the hydrocarbon stream in which it is contained, is sent with the mixture of alcohols and alkyl ethers, in stoichiometric defect, into contact with the acid catalyst where the dimerization takes place. The linear primary alcohol, in addition to interacting with the catalyst, also helps to limit the possible cracking of the alkyl ether and can possibly react with the darners and linear C4 olefins, whereas the branched alcohol (tertiary) does not react with the olefins due to its steric hin- drance. In order to obtain the dimerization product with the desired selectivity to dimers, it is essential to maintain a constant level of oxygenated products in the reaction en¬vironment but above all the contemporaneous presence of the three oxygenated compounds (linear alcohol, branched alco¬hol and alkyl ether) which, due to a synergic effect are capable of forming the catalytic species with the correct activity and stability. The optimal level of the sum of al¬cohols and alkyl ethers which should be present in the re¬action environment to obtain selectivities to dimers close to 85% by weight, depends on the composition of the hydro¬carbon charge. The higher the olefin content in the charge, the lower the amount of oxygenated products to be used. A wide variety of acid catalysts can be used for this process, but those preferred are styrene-divinyl benzene polymeric resins having sulphonic groups as catalytic cen¬tres . A large range of operative conditions can be used to produce high-octane hydrocarbons from isobutene in the de¬sired selectivities. It is possible to operate in vapour or liquid-vapour phase, but operating conditions in liquid phase are preferred. The pressure is preferably higher than the atmospheric value, in order to maintain the reagents in liquid phase, generally below 5 MPa, more preferably between 0.2 - 2.5 MPa. The reaction temperature preferably ranges from 30 to 120°C. The feeding space velocities of the oxygenated-hydrocarbon stream are preferably lower than 3 0 h-1, more preferably ranging from 1 and 15 h-1. Isobutene is mainly converted in the reaction zone, however part of the C4-C5 olefins present can also be con¬verted to useful product; in principle, there are no limits to the concentration of iso-olefin in the hydrocarbon frac¬tion, even if concentrations ranging from 2 to 60% are pre¬ferred; in case of streams having a high isobutene concen¬tration (dehydration or cracking) it is therefore conven¬ient to dilute the charge with C^-Cj hydrocarbons. There are no limits, on the contrary, for the ratio between iso¬butene and linear olefins. The process, object of the present invention, can be effected batchwise or in continuous, bearing in mind how¬ever that the latter is much more advantageous in indus¬trial practice. The reactor configuration selected is gen¬erally a double reaction step comprising one or more fixed bed reactors which can be optionally selected from a tubu¬lar and adiabatic reactor. The presence of Cs hydrocarbons in the feed, however, ' intermediate boiling temperatures between C4 and oxygenated products, and they- also form azeotropic mixtures with the branched alcohols as shown in Table 3, which indicates the boiling points of the most representative low-boiling com¬ponents present in the streams Table 3 The C5 products cannot therefore be removed from the plant together with the C4 products, as they would intro¬duce oxygenated products (branched alcohols and ethers) into the stream, which are difficult to remove by means of the traditional techniques used for removing methanol (wa¬ter washing) and which are poisonous for the subsequent treatment processes of the streams (polymerization, allevia¬tion and metathesis) . The C5 products, on the other hand, cannot be maintained in the oxygenated stream as they would rapidly accumulate. With respect to the schemes shown in literature (US 6,011,191), it is therefore necessary to in¬troduce a Cs/branched alcohol azeotropic separation step, which can be inserted in different positions of the plant in relation to the C5 content in the charge and also the relative concentration of the Cs products present. The separation of the C5/branched alcohol azeotropic product can be effected using traditional fractionation columns in which the azeotropic mixture can be recovered at the head, bottom or as side cut. The process, object of the present invention, can be carried out, in particular, by means of the following es¬sential steps: a) feeding a stream containing isobutene and Cs hydrocar¬bons, together with one or more streams containing oxy¬genated products (linear and branched alcohols, ethers and water), to one or more reaction steps (consisting of one or more reactors); b) separating the C4/linear alcohol azeotropic product and possibly the C4 products from the C5 hydrocarbons, from the remaining oxygenated products and from the hydrocar¬bon product, in one or more distillation columns; c) recovering the linear alcohol from the azeotropic mix¬ture with the C4 products by means of conventional proc¬esses such as washing with water or absorption on inor¬ganic solids; d) separating the Cs products (as an azeotropic compound with the branched alcohol) from the remaining oxygenated products and from the reaction product, in one or more distillation columns, in order to obtain three streams with the desired purity; e) recycling the stream containing the remaining oxygenated products and that containing the linear alcohol recov¬ ered, to the two reaction steps; f) feeding linear alcohol and water (which forms the branched alcohol in the reactors by reaction with the tertiary olefin) to the reaction steps to compensate the losses of linear alcohol, which can react with the di- mers and linear C4 olefins, and branched alcohol which, on the contrary, leaves the plant together with the C5 products ,- g) recycling part of the C4 products, with or without lin¬ ear alcohol, to the reaction steps in order to maximize the isobutene conversion. For the process comprising the essential steps indi- cated above, the Cs products are present in the streams prevalently containing C4 products in a quantity preferably ranging from 0.5 to 10% by weight. The separation of the Cs/branched alcohol azeotropic product is preferably carried out starting from blends of: a) C5 - oxygenated products (ethers and branched alcohols) - reaction product, wherein the C5 hydrocarbons are re¬ covered as an azeotropic compound with the branched al¬ cohol as head effluent using a scheme based on a column, with recovery of the remaining oxygenated products as side cut, or two fractionation columns; b) Cs - oxygenated products {ethers and branched alcohols) - dimers, wherein the Cs hydrocarbons are recovered as an azeotropic compound with the branched alcohol as head effluent of a fractionation column; c) C4-C5 - oxygenated products (ethers and branched alco¬hols) - reaction product, effluent from a reaction step, wherein the C5 hydrocarbons are recovered as an azeo¬tropic compound with the branched alcohol as side cut of a fractionation column from whose head the C4/linear al¬cohol azeotropic product and possibly the C4 products are recovered, whereas a mixture containing the oxygen¬ated products and the reaction product is recovered at the bottom; d) C4-Cs - oxygenated products (linear and branched alco- hols) wherein the C5 hydrocarbons are recovered as an azeotropic compound with the branched alcohol as bottom effluent of a fractionation column from whose head the C4/linear alcohol azeotropic product and possibly the C4 products are recovered. Six process schemes are shown in figures 1-6, in order to clearly illustrate the present invention. Figure 1 shows a process scheme wherein C5 hydrocar¬bons are not present in the charge and the oxygenated prod¬ucts are methanol (linear alcohol) , TBA (branched alcohol) and MTBE (alkyl ether). The stream (1) containing isobutene, together with the reintegration feeding of methanol and water (2) and the re¬cycled streams of oxygenated products (MTBE and TBA) (15) and methanol (18), is sent to a first reaction step Rl, which can consist of one or more reactors, in which the C4 iso-olefin is selectively converted to dimers. The effluent (4) from the first reaction step, is sent to a first separation column CI in which a stream (5) es¬sentially containing C4 hydrocarbons and methanol is re¬moved from the head, whereas a stream (6) essentially con¬taining the reaction product and remaining oxygenated com¬pounds, is collected at the bottom. The head stream (5) is then fed, together with the re¬cycled streams of oxygenated products (16) and methanol (17), to a second reaction step R2, which can consist of one or more reactors, wherein the isobutene present is se¬lectively converted to dimers. The effluent (8) from the second reaction step is separated in a column C2 from whose bottom a stream (10) essentially containing MTBE, TBA, the dimerization product and part of the C4 products, is removed and sent to the column CI for the recovery of the product and oxygenated products. The head stream (9), consisting of C4 products and the C, /methanol azeotropic mixture is, on the other hand, sent to an MR unit for the recovery of the alcohol which can consist, for example, of an absorption system on molecular sieves or a water washing column. In both cases, the alcohol recovered (14) can be sent back to the two re¬action steps (streams 17 and 18) whereas the hydrocarbon stream (13) can be used in subsequent operations. The bottom stream (6) of the column CI is sent to a further separation column C3 wherein a stream (11) essen¬tially containing MTBE, TBA and dimers is removed at the head and is recycled to the two reaction steps (streams 15 and 16), whereas the reaction product (12) essentially con¬sisting of dimers, trimers and small quantities of oli¬gomers and ethers of dimers, is recovered at the bottom. When Cs hydrocarbons are present in the charge, on the contrary, according to the present invention, different plant configurations can be used, schematised in the fol¬lowing figures 2-6, to recover the C5/TBA azeotropic prod¬uct, depending on the quantity of C5 products present and the required purity of the streams. Figure 2 therefore shows a possible process scheme which differs from the previous scheme in that the stream con¬taining oxygenated products to be recycled (11) (ethers and TBA) is removed from the column C3 as side cut, whereas the C5/TBA azeotropic product (19) is recovered from the head of the column and can optionally be joined with the reac¬tion product. The process scheme becomes more complex when a more efficient separation of the mixture of C5 prod¬ucts/oxygenated products/react ion product is to be ef¬fected, as a new fractionation column C4 must be inserted, as shown in figure 3. In this new scheme, the head stream of the column C3 (11) is sent to a new column C4 where the Cs/TBA azeotropic mixture (20) is separated at the head and the stream of oxygenated products (19) is separated at the bottom and recycled to the two reaction steps. Alternatively, the C5 hydrocarbons can be recovered using the two fractionation columns of the C4 products wherein the CS/TBA azeotropic mixture can be thus recovered as side cut (19) both in the column CI (figure 4) and in the column C2 (figure 5) . A further option however consists in effecting the separation of the C4-C5 products in a last new column C5, as shown in figure 6, in which the C5/TBA azeotropic product is recovered as bottom stream (19) . CLAIMS 1) A process for the separation of C5 hydrocarbons present, in a quantity ranging from 0.2 to 20% by weight, in hy¬drocarbon streams prevalently containing C4 products used for the production of high-octane hydrocarbon com¬pounds, by the selective dimerization of isobutene, characterized in that the dimerization reaction is car¬ried out in the presence of linear and branched alcohols and alkyl ethers in such a quantity as to have a molar ratio alcohols + alkyl ethers/isobutene in the feeding higher than 0.01. 2) The process according to claim 1, wherein the molar ra¬tio alcohols + alkyl ethers/isobutene is lower than 0.7. 3} The process according to claim 1, wherein the reaction is carried out at a temperature ranging from 3 0 to 120°C, at a pressure lower than 5 MPa and at feeding space velocities lower than 30 h-1. 4) The process according to claim 1, wherein the feeding space velocities range from 1 to 15 h"1. 5) The process according to claim 1, wherein the linear al¬cohol has a number of carbon atoms ranging from 1 to 6. 6) The process according to claim 5, wherein the linear al¬cohol is selected from methanol and/or ethanol. 7) The process according to claim 1, wherein the branched alcohol has a number of carbon atoms ranging from 4 to 8) The process according to claim 7, wherein the branched alcohol is selected from tert-butyl alcohol or tert-amyl alcohol. 9) The process according to claim 1, wherein the alkyl ether has a number of carbon atoms ranging from 5 to 10. 10) The process according to claim 9, wherein the alkyl ether is selected from MTBE, ETBE, MSBE, ESBE, TAME, TAEE or mixtures thereof. 11) The process according to claim 1, wherein other possi¬ ble olefins present in the charge react to form high- octane products. 12) The process according to claim 1, wherein isobutene content in the charge is modified by dilution with C4- C7 streams. 13) The process according to at least one of the previous claims comprising the following essential steps: a) feeding the C4-Cs hydrocarbon cut containing isobu¬tene, together with one or more streams containing oxy¬genated products (linear and branched alcohols, ethers and water) , to one or more reaction steps (consisting of one or more reactors); b) separating the C4/linear alcohol azeotropic product and possibly the C4 products from the C5 hydrocarbons, from the remaining oxygenated products and from the hy- drocarbon product, in one or more distillation columns; c) recovering the linear alcohol from the azeotropic mixture with the C4 products by means of conventional processes such as washing with water or absorption on inorganic solids; d) separating the C5 hydrocarbons, as an azeotropic compound with the branched alcohol, from the remaining oxygenated products and from the reaction product, in one or more fractionation columns, in order to obtain three streams with the desired purity; e) recycling the streams containing the oxygenated products (branched alcohol and ether) and the linear alcohol recovered, to the two reaction steps; f) feeding linear alcohol and water (which forms the branched alcohol in the reactors by reaction with the tertiary olefin) to the reaction steps to compensate the losses of linear alcohol, which can react with the dimers and linear C4 olefins, and branched alcohol which, on the contrary, leaves the plant together with the C5 products; g) recycling part of the C4 products, with or without linear alcohol, to the reaction steps in order to maxi¬mize the isobutene conversion. 14) The process according to claims 1 and 13, wherein the separation of the C5/branched alcohol azeotropic prod- uct can be carried out starting from blends of: a) C5 - oxygenated products (ethers and branched alco¬hols) - reaction product, wherein the C5 hydrocarbons are recovered as an azeotropic compound with the branched alcohol as head effluent using a scheme based on a column, with recovery of the remaining oxygenated products as side cut, or two fractionation columns; b) C5 - oxygenated products (ethers and branched alco¬hols) - dimers, wherein the C5 hydrocarbons are recov¬ered as an azeotropic compound with the branched alco¬hol as head effluent of a fractionation column; c) C4-C5 - oxygenated products (ethers and branched al¬cohols) - reaction product, effluent from a reaction step, wherein the C5 hydrocarbons are recovered as an azeotropic compound with the branched alcohol as side cut of a fractionation column from whose head the C4/linear alcohol azeotropic product and possibly the C4 products are recovered, whereas a mixture containing the oxygenated products and the reaction product is re¬covered at the bottom; d) C4-C5 - oxygenated products (linear and branched al¬cohols) wherein the C5 hydrocarbons are recovered as an azeotropic compound with the branched alcohol as bottom effluent of a fractionation column from whose head the C4/linear alcohol azeotropic product and possibly the C4 products are recovered. 15} The process according to claims 1, 13 and 14, wherein the C5/branched alcohol azeotropic mixture is joined to the reaction product. 16) The process according to claims 1 and 13, wherein the dimerization reaction is carried out in one or more fixed bed, tubular and/or adiabatic reactors. 17) The process according to claim 13, wherein the C5 hy¬ drocarbons are present in streams prevalently contain¬ ing C4 products in a quantity ranging from 0.5 to 10% by weight.

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