Process for the isomerisation of olefins

Abstract: Silicalite to be used as catalyst in the isomerisation of olefins is pretreated by passing steam thereon at a temperature of 300 to 750°C. Particularly, isobutene is selectively produced by isomerisation of n-butenes.

Description: The present invention relates to the isomerisation of olefins. More particularly, it relates to an improvement in a process for the isomerisation of olefins over silicalite. Silicalite as used herein is an unmodified crystalline silica polymorph as was disclosed in US-4061724-A; said patent discloses a method for preparing silicalite. US-4587375-A discloses a process for the isomerisation of n-butenes comprising providing a feed containing at least 10 vol % n-butenes; and contacting said feed with silicalite at a temperature of at least about 300°C in the presence of steam in a water/feed molar ratio of about 0.5-5. However that process provides reaction products containing water. Further, that process produces excessive amounts of liquid by-products. Also, it has been observed that deactivation of the silicalite is excessive for the process to be industrially applicable. There is thus a need in art for an improvement of the existing isomerisation processes of olefins over silicalite. The process of the invention comprises the steps of : passing steam, either pure or diluted in an inert gas, over silicalite at a temperature of from 300 to 750°C; providing a feed containing (i) at least 10 vol % of n-butene and (ii) isobutene if any in an isobutene:n-butene ratio of maximum 1:2; passing said feed over said silicalite at a temperature of at least 500°C; and recovering a stream containing an increased amount of isobutene. Silicalite is defined hereabove. Steam is passed over silicalite at a temperature of from 300 to 750°C, preferably 450 to 600°C, either pure or diluted in an inert gas (inert gas as used herein is a gas that will not react with silicalite at the relevant temperature, such as nitrogen). The dilution factor, if any, may be of 1:99 to 99:1. Silicalite must be treated with steam during at least 24 hours, preferably at least 72 hours. Atmospheric pressure is generally used, but higher or lower pressures can be used. The space velocity is adapted to the duration of that step, and is generally comprised between 1 and 100 ml water per ml silicalite and per hour, preferably around 10 ml/ml.h. All space velocities herein are space velocities wherein the volumes are measured at normal temperature and pressure. The feed must contain at least 10 vol % of n-butenes, and isobutene if any in an isobutene:n-butene ratio of maximum 1:2 (preferably <1:5, most preferably <1:10); other hydrocarbons are the main other constituents. Preferably, the feed is an isobutene-depleted mixture of C 4 hydrocarbons, as is e.g. recovered from a methyl t-butyl ether (MTBE) unit. Other feeds can be FCC offgases and thermal cracking offgases. The feed is passed over silicalite at a temperature of at least 500°C, preferably from 500 to 650°C. That step is carried out in the gaseous phase, preferably at about atmosphere pressure. Space velocities can range from 0.05 to 50 h -1 , preferably from 0.5 to 30 h -1 . It is particularly advantageous to separate unreacted hydrocarbons present in the effluent from the isobutene present in said effluent and then to recycle the unreacted hydrocarbons back into the feed. The silicalite is generally shaped by extrusion. It is generally also composited (preferably during said extrusion) with inorganic matrix materials (often named binders), because it tends to be subject to physical collapse and attrition during normal loading and unloading of the reaction zone as well as during the process. Where a matrix material is used, it is highly preferred that the matrix be substantially free of hydrocarbon conversion actively; most preferred are extruded silica polymorph silicalite catalysts consisting essentially of silicalite. Suitable matrix materials can be selected from alumina, silica, zirconia, silica-magnesia and clays; alumina is generally preferred. The steam treatment can be carried out either before extrusion, preferably on the silicalite as such, or after extrusion. The run life of the silicalite can be increased by periodically stopping the flow of feed and stripping the silicalite with a gas selected from inert gases, oxidising gases and mixtures thereof, preferably air, at a temperature of 300 to 750°C, preferably 450 to 675°C, most preferably about 600°C. Inert gas as used herein is a gas that will not react with silicalite itself at the relevant temperature, such as nitrogen; oxidising gas as used herein is a gas such as oxygen that will react at the relevant temperature with carbonaceous deposits on the silicalite to form carbon oxides. Stripping is carried out at a pressure from atmospheric to 1 MPa, preferably from above atmospheric to 0.5 MPa, most preferably of about 0.3 MPa. It is continued until the outlet gas composition is substantially equal to the inlet gas composition; when an oxidising gas is used, it is convenient to monitor carbon dioxide concentration in the outlet gas. The performances after regeneration are subtantially identical to those observed initially. The invention will now be illustrated by the following examples. Example 1 An isobutene-depleted mixture of C 4 hydrocarbons was recovered from a MTBE unit; it had the following composition (wt%) : C 3 hydrocarbons 0.2 n-butane 12.0 isobutane 31.2 1-butene 22.0 cis-2-butene 11.9 trans-2-butene 18.4 isobutene 3.2 C 5 and heavier 1.1 A commercial catalyst was obtained, which had the following characteristics: (a) silicalite 80 wt% SiO 2 /Al 2 O 3 molar ratio 240 crystal size .001-.01 mm surface area 357 m 2 /g delta d at 28° (2 theta) .0035 nm (Co K alpha X-ray diffraction) crystal structure monoclinic Na 2 O <500 ppmw K 2 O <500 ppmw (b) Al 2 O 3 binder 20 wt% 15 ml catalyst were steamed under the following conditions : flow of 90 l nitrogen and 190 ml water per hour; temperature rising from 25 to 550°C at 52.5°C/h, then of 550°C during 96 hours; atmospheric pressure. 10 ml of steamed catalyst were introduced in a reactor and preconditioned during 10 hours at 550°C by passing thereover 20 Nl/h of air under atmospheric pressure. The feed was then passed over the catalyst using the following operating conditions : atmospheric pressure 550°C (isothermal reactor) LHSV = 25 l/l.h downflow mode After 12 days, the feed flow was interrupted to strip the catalyst with 30 Nl air per hour at 580°C under 0.3 MPa gauge pressure until CO 2 concentration at the outlet was below 100 ppm. The feed was then passed over the regenerated catalyst under the same conditions as previously. The results are indicated in Table 1. Conversion and yield are calculated as follows : conversion % = (1-butene + 2-butenes) in - (1-butene + 2-butenes) out (1-butene + 2-butenes) in x 100 yield % = (isobutylene) out - (isobutylene) in (1-butene + 2-butenes) in x 100 In Table 2, yields have been calculated for various hydrocarbons in the first run, using the hereabove formula mutatis mutandis. results obtained in example 1 and comparative example A. day Example 1 Comparative example A conversion % yield % conversion % yield % 1 49 21.7 86 5.1 2 54 18.3 84 7.3 3 55 21.1 80 10.2 4 56 20.9 74 14.5 5 57 20.6 61 19.8 6 57 20.5 43 17.5 7 57 20.5 20 10.0 8 56 20.6 (stopped) 9 56 20.7 10 55 19.0 11 51 20.9 12 49 21.4 ------ ---------------------- 13 42 20.4 14 49 20.9 15 53 20.3 16 54 20.2 17 54 19.4 18 54 19.2 19 54 20.5 20 53 19.1 21 49 20.5 22 49 19.7 23 47 19.9 24 46 20.9 (stopped) yield % for various hydrocarbons in run 1. Example 1 Comp. ex. A Example 2 Comp. ex. B methane + 0.1 + 1.0 + 0.1 + 0.8 ethane 0.0 + 0.9 0.0 + 0.9 ethylene + 0.8 + 13.3 + 0.5 + 15.4 propane + 0.1 + 6.5 + 0.3 + 6.2 propylene + 11.6 + 28.8 + 7.5 + 35.6 butanes + 1.5 0.0 0.0 + 0.3 isobutene + 21.7 + 5.1 + 22.8 + 2.4 pentanes 0.0 + 3.7 0.0 + 1.7 pentenes + 11.2 + 4.7 + 7.9 + 7.0 C >5 + 2.0 + 22.0 + 1.0 + 14.8 Comparative example A Example 1 was repeated, except that the catalyst steaming step was omitted. The results are also indicated in Tables 1 and 2. As can readily be seen, the prior steaming of the catalyst has the unexpected effects of dramatically increasing its useful life (Table 1) and the isobutene selectivity (Table 2) as well as reducing the amount of liquid by-products (C >5 ). Example 2 Example 1 was repeated with a commercial silicalite catalyst without binder. The commercial silicalite, as obtained, had the following properties : SiO 2 /Al 2 O 3 molar ratio 240 crystal size .001-.01 mm surface area 399 m 2 /g delta d at 28° (2 theta) .0035 nm (Co K alpha X-ray diffraction) crystal structure monoclinic Na 2 O <500 ppmw K 2 O <500 ppmw The results are indicated in Table 3. Conversion and yield are calculated as for example 1. In Table 2, yields have been calculated for various hydrocarbons in the first run, as has been done for example 1. results obtained in example 2 and comparative example B. day . . Example 2 Comparative example B conversion % yield % conversion % yield % 1 40.0 20.2 85.1 2.4 2 39.9 19.6 84.6 2.9 3 37.3 19.0 83.8 3.5 4 35.7 18.4 82.0 4.3 5 33.7 17.9 79.9 5.7 6 35.2 17.2 74.3 9.6 7 30.6 16.2 67.6 14.2 8 29.3 15.0 55.9 19.1 9 23.5 15.2 32.9 16.2 10 21.7 13.8 17.7 9.5 11 19.0 12.8 10.5 4.7 12 24.3 16.2 7.3 2.4 . . (stopped) . . (stopped) Comparative example B Example 2 was repeated, except that the silicalite steaming step was omitted. The results are also indicated in Tables 2 and 3. As can readily be seen, the prior steaming of the silicalite catalyst without binder has the effect of dramatically increasing the useful life of the catalyst (Table 3) and the isobutene yield (Table 2). It has to be noticed that the yield to liquid hydrocarbons (C>5) is reduced drastically. Example 3 Example 2 was repeated with a commercial silicalite catalyst without binder. An isobutene-depleted mixture of C4 hydrocarbons was recovered from a MTBE unit; it had the following comoposition (wt%). C3 hydrocarbons 0.2 n-butane 11.3 isobutane 35.7 1-butene 19.2 cis-2-butene 12.2 trans-2-butene 17.7 isobutene 1.8 C5 and heavier 1.5 A commercial catalyst was obtained, which had the following characteristics: SiO2/Al2O3 molar ratio 50.4 crystal size 0.001 - 0.01 mm surface area 441 m2/g delta d at 28° - crystal structure < 60 % monoclinic Na2O < 500 ppmw K2O < 500 ppmw The results are indicated in Table 4; Conversion and yield are calculated as for example 1. In table 5, yields have been calculated for various hydrocarbons in the first run, as has been done for example 1. results in example 3 and comparative example C. day Example 3 Comparative example C conversion % yield % conversion % yield % 1 51 17.6 93 1.1 2 50 18.2 86 5.5 3 46 17.5 73 13.7 4 44 17.2 36 19.7 5 41 16.7 8 4.6 6 42 17.6 3 1.0 7 43 16.1 (stopped) 8 43 16.1 9 38 16.3 10 40 16.3 11 33 14.9 12 29 13.2 (stopped) yield % for various hydrocarbons in example 3 and comparative example C for run 1. Example 3 Comparative example C methane + 0.0 4.6 ethane + 0.0 4.5 ethylene + 1.1 13.9 propane + 0.2 29.8 propylene + 14.3 21.0 butanes + 1.6 0.0 isobutene + 17.6 1.1 pentanes + 1.1 2.0 pentenes + 10.6 1.2 C >5 + 4.0 14.7 Comparative example C Example 3 was repeated, except that the silicalite steaming step was omitted. The results are also indicated in Tables 4 and 5. Example 4 Example 2 was repeated with a prepared silicalite catalyst without binder. The preparation is based on patent 4.061.724. An isobutene-depleted mixture of C4 hydrocarbons was recovered from a MTBE unit; it had the following comoposition (wt%). C3 hydrocarbons 0.1 n-butane 12.4 isobutane 19.3 1-butene 36.5 cis-2-butene 7.7 trans-2-butene 15.8 isobutene 1.2 C5 and heavier 7.1 The catalyst was obtained after preparation and had the following characteristics: SiO2/Al2O3 molar ratio 892 crystal size 0.001 - 0.01 mm surface area 394 m2/g delta d at 28° - crystal structure ..% monoclinic Na2O < 500 ppmw K2O < 500 ppmw The results are indicated in Table 6; Conversion and yield are calculated as for example 1. In table 7, yields have been calculated for various hydrocarbons in the first run, as has been done for example 1. results in example 4 and comparative example D. day Example 3 Comparative example D conversion % yield % conversion % yield % 1 64 18.0 82 9.7 2 62 17.8 80 10.7 3 53 22.1 77 13.3 4 55 20.8 75 14.6 5 52 22.5 72 16.3 6 50 23.3 70 17.9 7 49 23.5 67 18.4 8 48 24.0 61 21.8 9 48 23.3 56 23.1 10 48 20.8 58 21.3 11 47 22.8 59 19.9 12 49 19.2 47 17.4 13 43 20.5 41 11.2 (stopped) (stopped) yield % for various hydrocarbons in example 4 and comparative example D for run 1. Example 4 Comparative example D methane + 0.2 0.9 ethane + 0.0 0.3 ethylene + 2.3 9.3 propane + 0.3 1.8 propylene + 21.6 33.6 butanes + 4.5 4.6 isobutene + 18.0 9.7 pentanes + 0.0 2.1 pentenes + 15.4 9.0 C >5 + 1.7 10.7 Comparative example D Example 4 was repeated, except that the silicalite steaming step was omitted. The results are also indicated in Tables 6 and 7. The examples 2 to 4 show that the steaming stabilizes the butenes conversion and the iso-butene production. Whilst not wishing to be bound by a theory, it is believed that after steaming the pores are blocked by the dealumination of the silicalite, which stabilizes the iso-butene yield.

Claims: Process for the isomerisation of olefins, comprising the steps of : passing steam, either pure or diluted in an inert gas, over silicalite at a temperature of from 300 to 750°C; providing a feed containing (i) at least 10 vol % of n-butene and (ii) isobutene if any in an isobutene:n-butene ratio of maximum 1:2; passing said feed over said silicalite at a temperature of at least 500°C; and recovering a stream containing an increased amount of isobutene. Process according to claim 1, wherein steam is passed over silicalite at a temperature of 450 to 600°C. Process according to either claim 1 or 2, wherein steam is diluted in an inert gas in a ratio of 1:99 to 99:1. Process according to any one of claims 1 to 3, wherein steam is passed over silicalite during at least 24 hours, preferably at least 72 hours. Process according to any one of claims 1 to 4, wherein steam is passed over silicalite at a space velocity comprised between 1 to 100, preferably around 10 ml water per ml silicalite and per hour. Process according to any one of claims 1 to 5, comprising the additional steps of separating unreacted hydrocarbons present in the effluent from the isobutene present therein, and recycling the unreacted hydrocarbons back into the feed. Process according to any one of claims 1 to 6, wherein an extruded polymorph silicalite catalyst consisting essentially of silicalite is used. Process according to claim 7, wherein the steam is passed over the silicalite before extrusion. Process according to any one of claims 1 to 8, when used for the selective isomerisation of n-butenes into isobutylene.

Inventors: LAMOTTE CHRISTIAN, ROMERS ERIC, LAMOTTE, CHRISTIAN, ROMERS, ERIC
Applicants: FINA RESEARCH, FINA RESEARCH S.A.
Application year: 1996
Main classification: C07C5
IPC codes: B01J29, C07C5