Method for carrying out a heterogeneously catalysed reaction

Abstract

A process for performing a heterogeneously catalysed reaction in a three-phase reactor, where there is at least one liquid phase, at least one gaseous phase and at least one solid phase in the reactor and the reactor has at least two zones, with the reaction mixture being conveyed downward in zone 1, the reaction mixture being conveyed upward in zone 2, zones 1 and 2 being separated from one another by a dividing wall, and in that the ratio between the average catalyst concentrations in zone 2 and in zone 1 is greater than 2.

Claims

1. A process for performing a heterogeneously catalysed reaction in a three-phase reactor having at least one liquid phase, at least one gaseous phase and at least one solid phase, the process comprising: conveying a reaction mixture downward in a first zone of the reactor; and then conveying the reaction mixture upward in a second zone of the reactor, wherein the first and second zones are separated from one another by a dividing wall, the first zone is substantially free of undissolved gas, and a ratio of an average catalyst concentration in the second zone to an average catalyst concentration in the first zone is greater than 2.

2. The process according to claim 1, wherein the ratio of the average catalyst concentrations in the second to first zone is greater than 5.

3. The process according to claim 1, wherein the gas required for the process, in the course of operation, is metered in in the finely dispersed state from a lower portion of the reactor.

4. The process according to claim 1, wherein a ratio of a catalyst concentration in the second zone at 90% of a fill height of the reactor measured from a bottom of the reactor to a catalyst concentration at 20% of the fill height measured from the bottom is less than 0.3.

5. The process according to claim 1, wherein a ratio of an average vertical flow rate in the first zone to an average vertical flow rate in the second zone is between 5 and 50.

6. The process according to claim 1, wherein a ratio of a reactor diameter in an upper portion to a reactor diameter in a lower portion is between 1 and 2.

7. The process according to claim 1, wherein a ratio of a diameter in an upper portion of the first zone to a diameter in a lower portion of the first zone is between 1 and 5.

8. The process according to claim 1, wherein the reaction mixture is conveyed downward in the first zone by at least one pump or at least one stirrer.

9. The process according to claim 1, wherein at least one liquid feed stream is introduced into an upper portion of the first zone.

10. The process according to claim 1, wherein at least one continuously operable and back-washable filter is installed in an upper portion of the second zone.

11. The process according to claim 1, wherein the reaction mixture is discharged continuously from the reactor and filtered through at least one external filter and the catalyst is passed back into the reactor after the filtration.

12. The process according to claim 1, wherein the second zone is divided into at least two segments by dividing walls, and at least one gas is metered in and finely distributed in a lower portion of the second zone.

13. The process according to claim 1, wherein the heterogeneously catalysed reaction is an oxidation reaction with an oxygen-containing gas.

14. The process according to claim 1, wherein a ratio of an oxygen concentration in the gas phase of the second zone at 20% of a fill height of the reactor measured from a bottom of the reactor to an oxygen concentration in the gas phase of the second zone at 90% of the fill height measured from the bottom is greater than 2.

15. The process according to claim 1, wherein the heterogeneously catalysed reaction is a continuous oxidative esterification of methacrolein with oxygen and methanol for preparation of methyl methacrylate.

16. A process for performing a continuous oxidative esterification of methacrolein with oxygen and methanol for preparation of methyl methacrylate in a three-phase reactor having at least one liquid phase, at least one gaseous phase and at least one solid phase, the process comprising: conveying a reaction mixture downward in a first zone of the reactor; and conveying the reaction mixture upward in a second zone of the reactor, wherein the first and second zones are separated from one another by a dividing wall, and a ratio of an average catalyst concentration in the second zone to an average catalyst concentration in the first zone is greater than 2.

Description

DRAWINGS

(1) List of reference numerals for FIG. 1

(2) FIG. 1 is a specific configuration of a reactor usable in the process according to the invention. This constitutes an embodiment of the invention which is particularly suitable for oxidation reactions in particular. On the other hand, however, the drawing does not serve to restrict the scope of protection of the present application in any way. This drawing is simplified such that narrowings of the reactor or of zone 1, for example, are not depicted.

(3) TABLE-US-00001 1: Motor a: Feed 1 (reactant 1) 2: Reaction mixture level b: (optional) Feed 2 (reactant 2) 3: Sedimentation zone (zone 2) c: (optional) Feed 3 (auxiliary 1) 4: (optional) Sedimentation system d: Gas 5: Filter system (with backwashing) e: Catalyst slurry outlet 6: Mixing/saturation zone (zone 1) f: Catalyst slurry inlet 7: Draft tube g: Offgas outlet (to the condenser) 8: at least one propeller stirrer h: Product mixture outlet 9: Air distribution nozzles (spargers) i: Filter backwash 10: Baffles (swirl breakers) j: (optional) Inert gas purge 11: Reaction zone (zone 2, lower portion) 12: Segment sheet components (baffles, dividing walls)

EXAMPLES

Example 1

(4) The reactor (according to FIGURE) had the following ratios of the dimensions:

(5) Reactor height/reactor diameter=1.6

(6) Fill height with reaction mixture/reactor height=0.75

(7) Reactor diameter (D2)/draft tube diameter (D1)=5.3

(8) The ratio between the average vertical flow rates (in the downward direction) V1 in the draft tube (zone 1) and the the average vertical flow rates (in the upward direction) V2 in the zone 2 is: V1/V2=(D2/D1).sup.21=27.4

(9) Performance of an Oxidative Esterification Reaction of Methacrolein to Methyl Methacrylate

(10) The pH of a 42.5% by weight solution of methacrolein (MAL) in methanol was adjusted to pH=7 with stirring by the addition of a 1% by weight solution of NaOH in methanol. This solution was fed at a constant feed rate continuously into the upper portion of the draft tube (zone 1) of the reactor usable in accordance with the invention according to FIG. 1 at pressure 5 bara and internal temperature 80 C. At the same time, a sufficient amount of 1% by weight NaOH solution in methanol, together with 600 g of Au/NiO/SiO.sub.2Al.sub.2O.sub.3MgO powder catalyst (prepared according to Example 1 from the application EP 2 210 664 A1), was fed into this reactor (including in the upper portion of the draft tube) that the value pH=7 in the reactor remained constant. In the lower portion of the reactor, in zone 2, air was metered in via multiple gas distributors. The product mixture was separated from the majority of the heterogeneous catalyst by means of a continuously backwashable sedimentation system (inclined clarifier) present at the periphery of the upper portion of zone 2 and filtered through a filtration system and analysed by means of gas chromatography (GC).

(11) After operation for 1 h, 3 samples of the product mixture were respectively taken at 20%, 50% and 90% of the fill height of the draft tube (zone 1), and three samples at 20%, 50% and 90% of the fill height of zone 2. The positions of the measurement points are always reported from the bottom. The solids content [in g/l] of the samples was determined.

(12) The C90%/C20% ratio in zone 2 was less than 0.1.

(13) The average concentration in zone 1 <C1> was calculated as the mean of the three samples from the draft tube:
<C1>=(C1(20%)+C1(50%)+C1(90%))/3, with C1(20%)C1(50%)C1(90%);

(14) the average concentration in zone 2 <C2> was calculated as follows:
<C1>=(total mass of catalyst<C1>*V1)/V2

(15) <C2>/<C1> was greater than 10.

(16) It was determined visually that air introduced via air distributors in the lower portion of zone 2, in the course of operation, was observed almost exclusively in zone 2, whereas zone 1 remained free at least of the gas undissolved in the reaction mixture.

(17) Oxygen concentration in the gas phase of zone 2 at 20% of the fill height of the reactor measured from the bottom was C(O.sub.2)20%21 vol % O.sub.2; oxygen concentration in the gas phase of zone 2 at 90% of the fill height measured from the bottom was C(O.sub.2)90%5 vol % O.sub.2, and thus C(O.sub.2)20%/C(O.sub.2)90%=4.2.

(18) Thus, continuous undisrupted operation of the plant for several months was ensured.

Comparative Example 1

(19) The reactor was identical to that used in Example 1, except for the draft tube which was absent. The reaction regime was identical to that in Example 1.

(20) After 1 h, two samples of catalyst suspension were taken, respectively at sites at 20% and 90% of the fill height measured from the bottom. A solids content [in g/I] was determined therein. The ratio C90%/C20% was 0.31. <C1>=<C2>

Comparative Example 2

(21) As Example 1, except that feed solutions were not introduced into the draft tube but into zone 2.

(22) The particle size distribution (measured by the laser diffraction method) of the fresh catalyst, and of the catalyst after the test in Example 1 (after 1000 h) and after the test in Comparative Examples 1 and 2 (after 1000 h in each case) is summarized in the table below. Activity and selectivity of the catalyst after 200 h and 1000 h in each case is likewise reported for all examples:

(23) TABLE-US-00002 D.sub.50 STY S (MMA) [m] [mol MMA/kg-h] [%] Fresh catalyst 61.5 E1 (200 h) 8.5 95.7 E1 (1000 h) 60.4 8.5 95.6 CE1 (200 h) 8.5 95.7 CE1 (1000 h) 51.2 8.0 93.3 CE2 (200 h) 8.2 92.4 CE2 (1000 h) 60.3 8.0 90.1

(24) It was observed that the mechanical abrasion of the catalyst in a reactor according to the invention having a draft tube (Example 1; E1) was lower than the abrasion in the reactor without a draft tube (Comparative Example 1; CE1). It was also observed that the activity and selectivity of the catalyst used decreased after an operating time of 1000 hours.

(25) In the embodiment with the addition of feed into zone 2 (Comparative Example 2; CE2), selectivity for MMA was much smaller again.

Example 2

(26) Similar to Example 1, but with no internal filter in the reactor after the sedimentation system. The reaction mixture was alternately filtered through one of two external filters installed in parallel with porosity 10 micrometres, with one filter constantly in use and the other simultaneously being backwashed. The catalyst remaining on the filter was returned to the reactor. Thus, continuous undisrupted operation of the plant for several months was ensured.

Comparative Example 3

(27) As Example 2, except using no sedimentation system (no inclined clarifier). Much more catalyst arrived continuously at the external filter. The filter switching cycle (switching to backwashing mode) had to be distinctly shortened in order that it was possible to discharge a continuous reaction mixture output. After operation for 2 months, operation had to be stopped and the two filters used had to be washed intensively with NaOH solution before operation could continue.

(28) It was observed that an internal sedimentation system, through prior removal of the majority of the solids, distinctly reduces the burden on the filters used and hence promotes undisrupted operation.