Process for olefin production by metathesis and reactor system therefor

Abstract

A process for obtaining an olefin by metathesis including at least two reaction pathways. In at least one first reaction pathway at least one stream with at least one olefin as starting material is fed to at least one first pre-bed reactor with at least one pre-bed having at least one compound selected from the group of alkaline earth metal oxides. The stream leaving the at least one first pre-bed reactor is subsequently fed to at least one main catalyst bed reactor downstream of the at least one first pre-bed reactor including at least one main catalyst bed with at least one first catalyst component comprising a metathesis catalyst, and at least one second catalyst component comprising a catalyst for double bond isomerization, whereby the first and second catalyst are physically mixed with each other.

Claims

1. A process for obtaining propene from at least two olefins as starting materials by metathesis, wherein one olefin is ethene and the other olefin is n-butene, comprising at least two reaction pathways, wherein in at least one first reaction pathway at least one stream comprising ethene and n-butene as starting material is fed to at least one first pre-bed reactor with at least one pre-bed comprising magnesium oxide effecting isomerisation of n-butene, and the stream leaving the at least one first pre-bed reactor is subsequently fed to at least one main catalyst bed reactor downstream of the at least one first pre-bed reactor comprising at least one main catalyst bed comprising at least one first catalyst component comprising tungsten oxide supported on silica, and at least one second catalyst component comprising magnesium oxide, whereby the first and second catalyst are physically mixed with each other, wherein in at least one second reaction pathway the at least one stream comprising ethene and n-butene as starting material is re-directed from the at least one first pre-bed reactor to at least one second pre-bed reactor with at least one pre-bed comprising magnesium oxide effecting the isomerisation of the olefin, and the stream leaving the at least one second pre-bed reactor is subsequently fed to the at least one main catalyst bed reactor downstream of the at least one second pre-bed reactor, wherein an operational temperature of the first pre-bed reactor and an operational temperature of the second pre-bed reactor are in a range between 150 C. and 350 C. and are lower than an operational temperature of the main catalyst bed reactor, and wherein the mass ratio of the pre-bed in the first pre-bed reactor and the second pre-bed reactor to the main catalyst bed in the main catalyst bed reactor is between 1:5 and 1:2.

2. The process according to claim 1, wherein the at least one first pre-bed reactor is turned off during the second reaction pathway and is subjected to at least one regeneration cycle during turn-off.

3. The process according to claim 1, wherein the number of reaction pathways is more than two.

4. The process according to claim 1, wherein the time on stream for one reaction pathway is larger than 40 h.

5. The process according to claim 1, wherein the mass ratio of the pre-bed in the first pre-bed reactor and the second pre-bed reactor to the main catalyst bed in the main catalyst bed reactor is between 1:5 and 2.5:1.

6. The process according to claim 1, wherein the operational temperature of the first pre-bed reactor and the operational temperature of the second pre-bed reactor are in a range of 150 C. to 300 C., and the operational temperature of the main catalyst bed reactor is in a range between 250 C. and 350 C.

7. The process according to claim 1, wherein the operational temperature of the first pre-bed reactor and the operational temperature of the second pre-bed reactor are at least 20 C. lower than the operational temperature of the main catalyst bed reactor.

8. The process according to claim 1, wherein the regeneration cycle of the pre-bed of the first pre-bed reactor and the second pre-bed reactor comprises in each case a thermal treatment in an oxygen gas atmosphere at temperatures between 400 C. and 600 C.

9. The process according to claim 1, wherein the main catalyst bed comprises magnesium oxide and tungsten oxide in a mass ratio of between 5:1 and 1:1.

10. The process according to claim 1, wherein the at least one main catalyst bed further comprises oxides of metals of the 6th and 7th group of the PSE deposited on at least one inorganic carrier.

11. The process according to claim 1, wherein the at least one main catalyst bed further comprises Group 2 metal oxides.

12. The process according to claim 1, wherein the pre-bed further comprises an oxide selected from the group consisting of calcium oxide, strontium oxide, barium oxide or mixtures thereof.

13. The process according to claim 4, wherein the time on stream for one reaction pathway is larger than 60 h.

14. The process according to claim 5, wherein the mass ratio of the pre-bed in the first pre-bed reactor and the second pre-bed to the main catalyst bed reactor is between 1:3 and 1:2.

15. The process according to claim 11, wherein the second catalyst component further comprises calcium oxide, barium oxide, strontium oxide, or mixtures thereof.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention is further explained in more detail by the means of the following examples with reference to the Figure. It shows:

(2) FIG. 1 an embodiment of the reactor system according to the invention, and

(3) FIG. 2 a Diagram showing the propene yield vs. time on stream for a prior art process and the present process.

DETAILED DESCRIPTION OF THE INVENTION

(4) FIG. 1 refers to a reactor system for conducting the present process comprising a first pre-bed reactor (10) with at least one pre-bed (11) made of magnesium oxide (0.3-0.7 mm) and a second pre-bed reactor (20) with at least one pre-bed (21) made of magnesium oxide (0.3-0.7 mm) as well. First and second pre-reactor (10, 20) are arranged parallel to each other.

(5) The main catalyst bed reactor (30) comprising the main catalyst bed (31) being a physical mixture of MgO (0.3-0.7 mm) and WOx/SiO.sub.2 (0.3-0.7 mm) with a mass ratio of 3:1 is arranged downstream of the at least one first pre-bed reactor (10) or the at least one second pre-bed reactor (20), respectively.

(6) In the first reaction pathway the olefin mixture (1a) of ethene and n-butene is now fed to the first pre-bed reactor (10), wherein isomerisation of the olefin such as n-butene is effected and simultaneously traces of contaminants from the olefin feed are removed or destroyed.

(7) The olefin stream (2a) leaving the first pre-bed reactor (10) is subsequently fed into the main metathesis reactor (30) where metathesis takes place. The product stream (3) leaving the main metathesis reactor (30) consists of propene and traces of non-reacted starting material.

(8) If the metathesis reaction in the metathesis reactor (30) is now slowing down as a result of the coking process, for instance after a period of about 50-60 hours on stream, the first pre-bed reactor (10) is turned off and the olefin stream of ethene and n-butene (1b) is re-directed or switched to the second pre-bed reactor (20) in the second pathway step.

(9) The effluent (2b) leaving the second pre-bed reactor (20) after isomerisation and removal of poison is subsequently fed to the main metathesis reactor (30) for the actual metathesis reaction.

(10) While the first pre-bed reactor (10) is turned off said first pre-bed reactor is subjected to a regeneration cycle.

(11) If now the olefin production decreases or slows down using the second pre-bed reactor (20) for instance after a time on stream of about 50 hours said second pre-bed reactor (20) is subsequently turned off and the olefin stream with the starting material is re-directed or switched again to the first pre-bed reactor (10) after regeneration of the first pre-bed reactor (10); a third pathway of olefin metathesis is then started using the first pre-bed reactor again (10). The time on stream in the third pathway using the first pre-bed reactor again is about 30 hours.

Example 1: Standard Process with One Reactor, State of the Art

(12) Catalytic tests were performed in a tubular (4 mm i.d.) continuous-flow fixed-bed reactor made of quartz at 1.4 bar and 300 C. using a C.sub.2H.sub.4:trans-2-C.sub.4H.sub.8:N.sub.2=64.3:25.7:10 feed. C.sub.2H.sub.4 (Linde, purity>99.95%), trans-2-C.sub.4H.sub.8 (Linde, purity>99.0%) were extra purified with molsieve 3A, while oxysorb (Resteck) and molsieve 3A were applied for purifying N.sub.2 (Air Liquide, purity>99.999%).

(13) The main catalyst is a physical mixture of MgO (0.3-0.7 mm) and WO.sub.x/SiO.sub.2 (0.3-0.7 mm) with a weight ratio of 3.0. The MgO (0.3-0.7 mm) was additionally used as a pre-bed arranged upstream. Both beds were placed within the isothermal zone of the reactor. The weight hourly space velocity (WHSV) was of 1.9 h.sup.1 related to trans-2-C.sub.4H.sub.8 and the main catalyst.

(14) Before catalytic testing, the following pre-treatment was performed. The reactor was heated in a flow of pure nitrogen up to 400 C. with a heating rate of 5 K.Math.min.sup.1. The temperature was held constant for 2 h. Hereafter, an air flow was fed to the reactor followed by temperature rising to 525 C. with a heating rate of 5 K.Math.min.sup.1. After 2 hours in this flow at the final temperature, the reactor was cooled to 400 C. (2 K.Math.min.sup.1) in a flow of pure nitrogen. The temperature was held constant for 0.5 h followed by feeding an H.sub.2:N.sub.2=30:70 (mol/mol) gas mixture for 0.5 h. Then, the reactor was flushed with a flow of pure nitrogen and heated in the same flow up to 550 C. with a heating rate of 5 K.Math.min.sup.1. The temperature was held constant for 16 h. Finally, the reactor was cooled down to 300 C., where the metathesis reaction was studied.

(15) The diagram of FIG. 2 shows the yield of propene (equation 3) obtained when the standard catalyst bed configuration (standard curve) is compared with a setup as proposed in this invention disclosure.

Example 2: First Embodiment According to the Invention

(16) The test was carried out under reaction conditions and catalyst bed pre-treatment as described in Example 1 but using 3 separated reactors: 2 for MgO pre-bed and 1 reactor for the main bed, consisting of MgO:(WO.sub.x/SiO.sub.2)=3:1. The ratio of MgO pre-bed to the main bed was 0.21 (150 mg MgO, 716 mg main catalyst bed).

(17) The results are summarized in the diagram of FIG. 2. The open and solid symbols in this plot distinguish between operation with 1.sup.st and 2.sup.nd MgO pre-beds without interrupting the flow through the main reactor. The reaction was initially run with the 1.sup.st MgO pre-bed until observing the beginning of decrease in propene yield (about 60 h on stream).

(18) Then, the reaction feed was redirected from this pre-bed to the fresh 2.sup.nd MgO pre-bed without interrupting the flow through the main reactor. After approximately further 60 hours on stream and diminishing value of propylene yield, the freshly regenerated MgO pre-bed in the first reactor was again used and the metathesis reaction continued using the main catalyst bed without regeneration.

Example 3: Second Embodiment According to the Invention

(19) The test was carried out as described in Example 2 but using ratio of MgO pre-bed to main catalyst bed MgO:(WO.sub.x/SiO.sub.2) of 0.42 (300 mg pre-bed, 716 mg main catalyst bed). The propene yield with time on stream is shown in FIG. 2.

Example 4: Third Embodiment According to the Invention

(20) The test was carried out as described in Example 2 but using ratio of MgO pre-bed to main catalyst bed MgO:(WO.sub.x/SiO.sub.2) of 0.84 (600 mg pre-bed, 716 mg main catalyst bed). The propene yield with time on stream is shown in FIG. 2.

(21) The conversion of t-2-C.sub.4H.sub.8 was calculated on the basis of inlet and outlet mole fractions (equation 1). The product selectivity was calculated on a molar basis (equation 2). 1-C.sub.4H.sub.8 and cis-2-C.sub.4H.sub.8 were considered as reaction products. The propene yield is a product of the propylene selectivity and the t-2-butene conversion (equation 3).

(22) $\begin{array}{cc}{X}_{t-2C_4{H}_8}=\left(1-\frac{x_{t-2C_4{H}_8}^{\mathrm{outlet}}}{x_{t-2C_4{H}_8}^{\mathrm{inlet}}}\right)100\%& \left(1\right)\end{array}$
where x is mole fraction of t-2-butene.

(23) $\begin{array}{cc}{S}_{C_3{H}_6}=\frac{n_{C_3{H}_6}^{\mathrm{outlet}}}{n_{C_3{H}_6}^{\mathrm{outlet}}+n_{1-C_4{H}_8}^{\mathrm{outlet}}+n_{\mathrm{cis}-2C_4{H}_8}^{\mathrm{outlet}}+.Math.n_{C_5}^{\mathrm{outlet}}+.Math.n_{C_6}^{\mathrm{outlet}}}100\%,& \left(2\right)\end{array}$
where n.sub.i.sup.outlet is number of moles of propene, 1-butene, cis-2-butene, pentenes (C.sub.5) and hexenes (C.sub.6) at the reactor outlet.
Y.sub.C.sub.3.sub.H.sub.6=S.sub.C.sub.3.sub.H.sub.6X.sub.t-2-C.sub.4.sub.H.sub.8100%(3),
where S.sub.C.sub.3.sub.H.sub.6 is propene selectivity and X.sub.t-2-C.sub.4.sub.H.sub.8 is conversion of t-2-butene calculated according to equations 1 and 2, respectively.

(24) The examples clearly demonstrate that the time on stream performance for the new 3-reactor process according to the invention in the conversion range of technical interest between two regeneration cycles is extended.