Membrane cascade with falling separation temperature

Abstract

The invention relates to a process for separating a composition of matter with the aid of a membrane cascade having at least two stages, in which a separation is effected in each stage at at least one membrane at a separation temperature set for the particular stage. The invention further relates to a corresponding membrane cascade, to the use of said membrane cascade for catalyst separation from homogeneously catalyzed mixtures, and to a process for hydroformylation, in which the catalyst is separated by means of a membrane cascade. The problem addressed thereby is that of specifying a membrane-based process for separating compositions of matter, which has a minimum membrane area requirement and nevertheless fulfills the separation task and separation performance required. This problem is solved by the use of a membrane cascade with falling separation temperature.

Claims

1. Process for separating a composition of matter with the aid of a membrane cascade having at least two stages, in which a separation is effected in each stage at at least one membrane at a separation temperature set for the particular stage, wherein the particular separation temperature falls from stage to stage in the direction of the incoming composition of matter, wherein each stage of the membrane cascade has a dedicated temperature control unit which sets the temperature of the feed to the particular stage to the particular temperature set for that stage, and wherein the separation temperature of the final stage is at least 10 degrees lower than the separation temperature of the initial stage.

2. Process according to claim 1, wherein the membrane cascade is an enriching cascade.

3. Process according to claim 2, wherein the permeate that results from the membrane cascade has permeated through all the stages of the membrane cascade.

4. Process according to claim 2, wherein at least the retentate of one stage is recycled into the feed to the same stage and/or into the feed to another stage positioned counter to the direction of the incoming composition of matter.

5. Process according to claim 1, wherein the membrane cascade has exactly two or exactly three or exactly four stages.

6. Process according to claim 1, wherein at least one temperature control unit is a cooler.

7. Process according to claim 1, wherein the composition of matter originates from a homogeneously catalyzed chemical reaction, and in that it comprises at least one product of the reaction, at least one reactant unconverted in the reaction, and the catalyst system present in the reaction and/or at least one constituent and/or a degradation product thereof, the catalyst system or constituent thereof or degradation product thereof being dissolved in the composition of matter.

8. Process according to claim 7, wherein the reaction is a hydroformylation, the product being an aldehyde or an alcohol, the reactant being an olefin or synthesis gas, and the catalyst system being rhodium complexed with an organophosphorus complex.

9. Membrane cascade for separating a composition of matter, comprising at least two stages arranged in series, wherein each stage of the membrane cascade has a dedicated temperature control unit configured to set a separation temperature such thatviewed in the direction of the incoming composition of matterthe separation temperature falls from stage to stage, and wherein the separation temperature of the final stage is at least 10 degrees lower than the separation temperature of the initial stage.

10. The membrane cascade according to claim 9, configured for separation of a dissolved catalyst complex and/or at least one constituent and/or degradation product thereof from a composition of matter originating from a chemical reaction homogeneously catalysed in the presence of the catalyst complex.

11. Process for hydroformylating ethylenically unsaturated compounds by reaction with carbon monoxide and hydrogen in the presence of a catalyst system comprising a dissolved metal complex of a metal of the eighth transition group of the Periodic Table of the Elements with at least one organophosphorus compound as ligands, in which a reaction mixture comprising not only hydroformylation products but also unconverted reactants and the catalyst system or at least constituents and/or degradation products thereof in dissolved form is obtained, wherein the reaction mixture is sent to a catalyst separation in which the catalyst system or constituents and/or degradation products thereof are separated from the reaction mixture at least partly by means of membrane technology for the purpose of recycling into the hydroformylation, wherein the catalyst separation comprises a membrane cascade having at least two stages, in which a separation is effected in each stage at at least one membrane at a separation temperature set for that particular stage, the particular separation temperature falling from stage to stageviewed in the direction of the incoming reaction mixture, and wherein the separation temperature of the final stage is at least 10 degrees lower than the separation temperature of the initial stage.

12. Process according to claim 3, wherein at least the retentate of one stage is recycled into the feed to the same stage and/or into the feed to another stage positioned counter to the direction of the incoming composition of matter.

13. The process of claim 1, wherein the pressure is the same at each stage of the membrane cascade.

14. The process of claim 11, wherein the pressure is the same at each stage of the membrane cascade.

15. The process of claim 11, wherein the catalyst separation results in a retention of the catalyst system of at least 98.33 percent.

16. The process of claim 11, wherein the membrane cascade has three stages and the catalyst separation results in a retention of the catalyst system of about 99.24 percent.

17. The process of claim 11, wherein the membrane cascade has four stages and the catalyst separation results in a retention of the catalyst system of about 99.50 percent.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further features and advantages of the invention can be ascertained from the following detailed description that is provided in connection with the drawings described below:

(2) FIG. 1 shows a simple membrane process according to the prior art;

(3) FIG. 2 shows a two-stage enriching cascade according to the prior art;

(4) FIG. 3 shows a two-stage stripping cascade according to the prior art;

(5) FIG. 4 shows a two-stage membrane cascade according to one embodiment of the present invention;

(6) FIG. 5 shows a three-stage membrane cascade according to another embodiment of the present invention;

(7) FIG. 6 shows a four-stage membrane cascade according to still another embodiment of the present invention;

(8) FIG. 7 is a graph showing the dependence of retention on temperature;

(9) FIG. 8 is a graph showing the dependence of permeability on temperature; and

(10) FIG. 9 shows a membrane separation with a jet loop reactor according to one embodiment of the present invention.

This problem is solved by the use of a membrane cascade with falling separation temperature.

(11) The invention therefore provides a process for separating a composition of matter with the aid of a membrane cascade having at least two stages, in which a separation is effected in each stage at at least one membrane at a separation temperature set for the particular stage, in which the particular separation temperature falls from stage to stage in the direction of the incoming composition of matter.

(12) The invention is based on the surprising finding that the area of a membrane cascadeand hence the requirement thereof for separation-active membrane materialcan be optimized while retaining the required separation performance (retention and volume flow rate of the feed to be processed) when the separation temperature within the cascade is lowered from stage to stage.

(13) This finding is surprising in that the same separation temperature was always maintained within known membrane cascades:

(14) This is because, if the customary heat losses are neglected, a conventional membrane cascade works isothermally. With the setting of the temperature of the feed to the membrane cascade, therefore, the separation temperature has been preset identically for all the stages.

(15) In contrast, the invention teaches the setting of the separation temperature within the cascade in a controlled and separate manner for each individual stage, in such a way that the separation temperature falls downstream from stage to stage.

(16) The invention makes use of the observation that the retention of the individual membrane separation stage is dependent on the separation temperature of this stage: this is because it is the case in principle that retention of the membrane rises with falling temperature. With rising retention, the permeate flow rate also becomes smaller, which enables a smaller membrane area within this stage.

(17) This means that the total membrane area of a cascade can be optimized by setting the retention in the first stage at a comparatively low level, but correspondingly increasing it from stage to stage by lowering the temperature. The result of this is that the membranes become more impermeable from stage to stage. The resultant decrease in volume flow rates of the permeate can be utilized to lower the membrane areas overall.

(18) However, this does not mean that the membrane area falls from stage to stage; on the contrary, the membrane area can also increase again towards the end. The exact size of the particular membrane area within the stage is calculated by the membrane specialist on the basis of the particular separation task to be fulfilled. Corresponding calculation methods are described in the cited textbook by Melin/Rautenbach.

(19) A crucial boundary condition in the optimization of the total membrane area is the falling separation temperature. This finding applies in principle to the separation of any compositions of matter by means of a membrane cascade, irrespective of the system and phase composition. It is merely necessary to use a membrane material suitable for the separation of the desired component. The term membrane material refers to the material of the separation-active layer of a membrane.

(20) For separation of homogeneously dissolved catalyst systems from reaction mixtures, preference is given to using membranes having a separation-active layer of a material selected from cellulose acetate, cellulose triacetate, cellulose nitrate, regenerated cellulose, polyimides, polyamides, polyether ether ketones, sulphonated polyether ether ketones, aromatic polyamides, polyamide imides, polybenzimidazoles, polybenzimidazolones, polyacrylonitrile, polyaryl ether sulphones, polyesters, polycarbonates, polytetrafluoroethylene, polyvinylidene fluoride, polypropylene, terminally or laterally organomodified siloxane, polydimethylsiloxane, silicones, polyphosphazenes, polyphenyl sulphides, Nylon 6,6, polysulphones, polyanilines, polyurethanes, acrylonitrile/glycidyl methacrylate (PANGMA), polytrimethylsilylpropyne, polymethylpentyne, polyvinyitrimethylsilane, polyphenylene oxide, aluminas having various crystal structures, titanium oxides, silicon oxides, zirconium oxides, ceramic membranes hydrophobized with silanes, as described in EP 1 603 663 81, polymers having intrinsic microporosity (PIM) such as PIM-1 and others, as described, for example, in EP 0 781 166 and in Membranes by Cabasso, Encyclopedia of Polymer Science and Technology, John Wiley and Sons, New York, 1987.

(21) The abovementioned substances may be present, especially in the separation-active layer, optionally in crosslinked form through addition of auxiliaries, or in the form of what are called mixed matrix membranes with fillers, for example carbon nanotubes, metal-organic frameworks or hollow spheres, and particles of inorganic oxides or inorganic fibres, for example ceramic fibres or glass fibres.

(22) Particular preference is given to using membranes having, as a separation-active layer, a polymer layer of terminally or laterally organomodified siloxane, polydimethylsiloxane or polyimide, formed from polymers having intrinsic microporosity (PIM) such as PIM-1, or wherein the separation-active layer has been formed by means of a hydrophobized ceramic membrane. A detailed description of such membranes for use in high boiler discharge can be found in EP2401078A1.

(23) Very particular preference is given to using membranes formed from terminally or laterally organomodified siloxanes or polydimethylsiloxanes. Membranes of this kind are commercially available.

(24) As well as the abovementioned materials, the membranes may also include further materials. More particularly, the membranes may include support or carrier materials to which the separation-active layer has been applied. In such composite membranes, a support material is present as well as the actual membrane. A selection of support materials is described by EP0781166, to which reference is made explicitly.

(25) A selection of commercially available solvents for stable membranes are the MPF and Selro series from Koch Membrane Systems, Inc., different types of Solsep BV, the Starmem series from Grace/UOP, the DuraMem and PuraMem series from Evonik Industries AG, the Nano-Pro series from AMS Technologies, the HITK-T1 from IKTS, and also oNF-1, oNF-2 and NC-1 from GMT Membrantechnik GmbH and the Inopor nano products from Inopor GmbH.

(26) As well as the membrane material, the design of the membrane modules is also of relevance for the separation performance of the individual stage. The membrane is preferably designed as a spiral-wound element. Alternatively, it is also possible to use membranes, for example, in the form of plate modules, cushion modules, tube modules, pipe modules, capillary modules, hollow fibre modules or membrane discs.

(27) Another crucial factor for the required membrane area to fulfil the separation task is the connection of the stages within the cascade:

(28) This is because the membrane cascade is preferably an enriching cascade, i.e. a connection arrangement in which the individual stages are connected in series in the direction of permeate flow. The inventive concept works particularly well in the case of an enriching cascade, since the connection is effected in the direction of falling permeate flow, which allows the optimization of the overall membrane area to be particularly successful.

(29) For the same reason, the enriching cascade is designed such that the resulting permeate has permeated through all the stages of the membrane cascade.

(30) In order to improve the retention of the membrane cascade overall, one option is to recycle the retentate from at least one stage for the purpose of a further separation step. The retentate is accordingly recycled into the feed to the same stage from which the retentate has been recycled and/or into the feed to another stage counter to the direction of the incoming composition of matter. The recycled retentate is always mixed with a stream having a lower workup level.

(31) A membrane cascade necessarily has at least two stages. However, a three- or four-stage membrane cascade may also be economically viable.

(32) As already mentioned above, a membrane cascade is to be regarded as isothermal if no further technical measures are provided for temperature control within the membrane. The heat losses that naturally occur within the cascade in the permeate direction will generally not be sufficient to attain optimal separation temperatures in the individual stages. For this reason, it is appropriate for each stage of the membrane cascade to have a dedicated temperature control unit which sets the temperature of the feed to the particular stage to the particular temperature set for that stage. The temperature control unit need not necessarily be arranged within the membrane cascade, but may also be outside the latter. Thus, the composition of matter to be separated may especially be provided already having the temperature required for the first stage.

(33) The temperature control unit in the simplest case is a cooler, since the temperature is lowered in each case in the direction of permeate flow. The arrangement of a cooler within a membrane cascade does of course mean that the membrane cascade has a thermal energy requirement in the form of cooling coolant. The operating costs for an inventive membrane cascade may therefore be higher than those of the conventional membrane cascade which merely consumes mechanical power. However, these higher operating costs can be balanced out again by lower capital costs or improved retention and hence better product purity, such that the inventive membrane cascade is more economically viable than conventional plants in spite of its coolant requirement.

(34) As already mentioned, the process according to the invention is suitable in principle for separation of any compositions of matter. More preferably, however, it is used to separate off homogeneously dissolved catalyst systems, since it has been found to be particularly cost-efficient here. In a particularly preferred development of the invention, the composition of matter accordingly originates from a homogeneously catalysed chemical reaction, and it comprises at least one product of the reaction, at least one reactant unconverted in the reaction, and the catalyst system present in the reaction and/or at least one constituent and/or a degradation product thereof, the catalyst system or constituent thereof or degradation product thereof being dissolved in the composition of matter.

(35) The reaction is preferably a hydroformylation. Accordingly, the product is an aldehyde or an alcohol, and the reactants are olefins and synthesis gas. The catalyst system is preferably an organometallic complex of rhodium (although complexes of the other transition metals from groups 7-9 of the Periodic Table of the Elements may also find use) which may contain, for example, an organophosphorus compound as ligands. In this specific field of use, the process according to the invention was usable in a particularly advantageous manner.

(36) The invention also provides a membrane cascade intended for the inventive separation of a composition of matter. The membrane cascade is especially designed as an enriching cascade comprising at least two stages arranged in succession, in which each stage has a dedicated temperature control unit, by means of which the temperature of the feed to the particular stage can be set to a particular separation temperature set for that stage, and in which the particular separation temperatures are set such that the particular separation temperatureviewed in the direction of the incoming gas mixturefalls from stage to stage.

(37) The invention also provides for the use of this membrane cascade for separation of a dissolved catalyst complex and/or at least one constituent and/or a degradation product thereof from a composition of matter originating from a chemical reaction homogeneously catalysed in the presence of a catalyst complex.

(38) Since the process according to the invention is preferably used to separate off the catalyst within homogeneously catalysed industrial hydroformylation, this invention also provides a process for hydroformylating ethylenically unsaturated compounds by reaction with carbon monoxide and hydrogen in the presence of a catalyst system comprising a dissolved metal complex of a metal of the eighth transition group of the Periodic Table of the Elements with at least one organophosphorus compound as ligands, in which a reaction mixture comprising not only hydroformylation products hut also unconverted reactants and the catalyst system or at least constituents and/or degradation products thereof in dissolved form is obtained, wherein the reaction mixture is sent to a catalyst separation in which the catalyst system or constituents and/or degradation products thereof are separated from the reaction mixture at least partly by means of membrane technology for the purpose of recycling into the hydroformylation, which the catalyst separation comprises a membrane cascade having at least two stages, in which a separation is effected in each stage at at least one membrane at a separation temperature set for that particular stage, the particular separation temperatureviewed in the direction of the incoming reaction mixturefalling from stage to stage.

(39) The invention will now be illustrated in detail by working examples. The figures show: FIG. 4: two-stage membrane cascade (inventive); FIG. 5: three-stage membrane cascade (inventive); FIG. 6: four-stage membrane cascade (inventive); FIG. 7: dependence of permeability on temperature; FIG. 8: dependence of retention on temperature.

(40) FIG. 4 shows a first embodiment of the invention in the form of a two-stage enriching cascade suitable for performing a process according to the invention. The feed F of the two-stage enriching cascade 30 is the output from a homogeneously catalysed hydroformylation. More specifically, C.sub.5 olefins (pentenes) are reacted here with hydrogen and carbon monoxide in the presence of a rhodium-phosphite catalyst system to give hexanals.

(41) The reaction output is a mixture of the two hexanals formed (n-hexanal and 2-methylpentanal) in which the homogeneously dissolved catalyst is present in a concentration of 37 ppm (based on mass). The conversion of pentenes is 99% and the regioselectivity for the linear product is 67%. This corresponds to a reaction at a temperature of 100 C., a pressure of 40 bar, a CO/H.sub.2 ratio of 1, and a residence time in the reactor of 2 hours. The reaction output comprising the hexanals formed, unconverted pentenes, dissolved synthesis gas and the catalyst system are applied as feed F to the enriching cascade 30. If required, the reaction output can be degassed beforehand, in which case a residual amount of synthesis gas has to be maintained in the feed, in order to stabilize the organophosphorus ligand against deactivation.

(42) The enriching cascade 30 has two stages 31 and 32. Each stage 31, 32 is formed by two series-connected membrane modules, but each should be regarded as a single membrane. The membrane material used was a silane-modified ceramic based on a ZrO.sub.2 support and a pore size of 3 nm; the module design selected was tube modules of the Inopor M07-19-41-L type having a length of 40 inches and an area of 2.54 m.sup.2.

(43) The special feature of the inventive enriching cascade 30 is that each membrane stage, in the feed, has a dedicated temperature control unit 33, 34 for the particular stage 31, 32. The temperature control units 33, 34 are thermostatically regulated coolers which set the feed to the particular stages 31, 32 to a separation temperature set for that particular stage. The separation temperatures of the stages are selected such that the separation temperature falls in the direction of the incoming feed F:

(44) Thus, the first temperature control unit 33 sets the separation temperature of the first stage 31 to 43.4 C., while the temperature control unit 34 fixes the separation temperature of the second stage 32 at 30 C.

(45) The transmembrane pressure set in each stage is 60 bar, for which a pump 35, 36 is provided in the feed to each stage.

(46) The pump in the first stage 35 conveys the feed F through the first temperature control unit 33, such that it arrives at the first stage 31. A separation takes place therein, to the effect that the products and reactants present in the reaction mixture pass through the membranes preferentially, i.e. more quickly, and accordingly accumulate in the permeate 37 from the first stage. The catalyst system, in contrast, is not able to pass through the membrane as quickly, and so it accumulates in the retentate from the first stage, which is drawn off as resulting retentate R from the membrane cascade 30 and conducted back into the hydroformylation reactor.

(47) No membrane is entirely impermeable to the catalyst system. Consequently, the permeate 37 from the first stage 31 also comprises rhodium and organophosphorus ligand or degradation products thereof. To separate off further catalyst complex, the permeate 37 from the first stage 31 is applied as feed to the second stage 32. To compensate for the pressure drop experienced, which is of the magnitude of the transmembrane pressure that acts over the first stage, the pressure of the permeate 37 is increased again with the aid of the pump 36. Then cooling takes place in the temperature control unit 34, in order that a membrane separation can again take place in the second stage 32 at the same pressure level but at a lower separation temperature.

(48) The permeate from the second stage 32 is discharged as resulting permeate P from the two-stage enriching cascade 30. It is very substantially free of the catalyst system or any degradation products thereof. It can then be sent to a distillative product separation in which the actual target product (hexanal) can be separated from high boilers formed in side reactions.

(49) Meanwhile, the retentate 38 from the second stage 32 is recycled within the membrane cascade 30, once via a recycling loop within the second stage 32, for which a recycling pump is arranged within the recycling loop 38, and via a second recycling loop 39, which recycles the retentate from the second stage 32 in the feed to the first stage 31. The second recycling loop 39 does not need a dedicated pump, since the pump 35 upstream of the first stage sucks in the recycled retentate from the second stage 32 as well.

(50) The value RR.sub.int is set to 12.1 kmol/kmol in the second stage and to 10.5 kmol/kmol in the first stage. This means that (based on molar amounts) 12.1 and 10.5 times more, respectively, is recycled internally (e.g. stream 38) than is conveyed externally into the stage beneath (e.g. stream 39, or to the global retentate in the first stage).

(51) Because of the falling separation temperature from 43 C. in the first stage to 30 C. in the second stage, the retention of the first stage is lower than that of the second stage. In the first stage the retention is 88.7%, while the retention in the second stage is 91.3%.

(52) Because of the lower retention, the permeate flow in the first stage 31 (i.e. the volume flow rate of the permeate 37) is much greater than the permeate flow of the second stage 32 (i.e. the volume flow rate of the resulting permeate P). Accordingly, the membrane area in the first stage is much greater at 163 m.sup.2 than that in second stage at 81.4 m.sup.2.

(53) Overall, the two-stage membrane cascade 30 shown in FIG. 4 requires a total membrane area of 244.4 m.sup.2. Thus, an overall retention based on the catalyst complex of 98.33% is achieved, This means that 98.33% of the catalyst introduced with the feed can be recovered and leaves the membrane cascade 30 again via the resulting retentate R in the direction of the reactor. The remaining 1.67% of the catalyst system is lost with the permeate P.

(54) In order to reduce catalyst losses by the resulting permeate P, the catalyst retention has to be enhanced further. One option for this purpose is a three-stage enriching cascade as shown, for example, in FIG. 5.

(55) As its name suggests, the three-stage enriching cascade 40 has three stages 41, 42, 43, which are connected to one another analogously to the two-stage enriching cascade 30 shown in FIG. 4. The resulting retentate R from the cascade 40 is drawn off from the first stage 41. The resulting permeate P permeates through all the stages 41, 42 and 43. The retentates from the second and third stages are recycled, partly into the feed to the same stage and partly into the feed to the preceding stage. The value RR.sub.int is 0.5 kmol/kmol in the first stage, 5.1 kmol/kmol in the second stage and 16.6 kmol/kmol in the third stage.

(56) The membrane material used is homogeneous and is the same as in the two-stage enriching cascade 30 shown in FIG. 4. The same transmembrane pressure of 60 bar was also selected. The separation temperature decreases again in the direction of permeate flow from 88.9 C. in the first stage 41, through 73.1 C. in the second stage 42, down to 30.17 C. in the third stage 43. The membrane area selected in the first stage was 64.5 m.sup.2; provided by two series-connected Inopor M07-19-41-L modules having a length of 40 inches and an area of 2.54 m.sup.2 each. The second stage 42 has a membrane area of 61 m.sup.2, likewise realized via two series-connected membrane modules. In the third and last stage 43, however, the membrane area was raised to 81.2 m.sup.2, provided by a single membrane. Overall, the three-stage enriching cascade requires 206.7 m.sup.2 of costly membrane material. However, the retention thereof, based on the catalyst complex, is 99.24%. Compared to the two-stage enriching cascade 30, the three-stage enriching cascade therefore achieves better retention with a smaller membrane area. The three-stage embodiment shown in FIG. 5 is therefore much more effective than the two-stage membrane cascade shown in FIG. 4.

(57) In order to further enhance the retention of the membrane cascade, a four-stage system may be provided. FIG. 6 shows a corresponding four-stage enriching cascade 50. The connection thereof is analogous to that of the three-stage enriching cascade 40 and two-stage enriching cascade 30. The composition of the feed F and the membrane material and the module design correspond to the membrane cascades from FIGS. 4 and 5. The stages 51, 52, 53 and 54 were selected as follows: First stage 51: separation temperature 90 C., transmembrane pressure 60 bar, number of membranes 2, membrane area 53.2 m.sup.2, RR.sub.int 0.3 kmol/kmol. Second stage 52: separation temperature 85.5 C., transmembrane pressure 60 bar, number of membranes 2, membrane area 73.9 m.sup.2, RR.sub.int 7.8 kmol/kmol. Third stage 53: separation temperature 80.9 C., transmembrane pressure 60 bar, number of membranes 2, membrane area 66.5 m.sup.2, RR.sub.int 4.8 kmol/kmol. Fourth stage 54: separation temperature 63.9 C., transmembrane pressure 60 bar, number of membranes 2 having a total area of 53.3 m.sup.2, RR.sub.int 6.1 kmol/kmol.

(58) The four-stage membrane cascade 50 achieves an overall retention of 99.5% with a total membrane area of 245.9 m.sup.2. The membrane area is thus almost exactly as high as that in the two-stage enriching cascade 30 from FIG. 4, but the retention is much better.

(59) For comparison, the three inventive enriching cascades 30, 40 and 50 were operated unchanged, but at a homogeneous separation temperature within the cascade. The results are shown in Table 1.

(60) TABLE-US-00001 Catalyst retention Comparative catalyst Isothermal with isothermal retention value with Number of temperature set Equivalent total temperature control non-isothermal stages [ C.] membrane area [m.sup.2] [%] control [%] 2 38.64 244.4 97.91 98.33 3 73.88 206.7 98.08 99.24 4 80.44 245.9 99.41 99.50

(61) The comparison shows that the inventive membrane cascades 30, 40 and 50, with the same membrane area, give higher retention than the cascades operated isothermally, contrary to the teaching of the invention.

(62) This means, conversely, that the same retention can be achieved with a smaller membrane area.

(63) FIGS. 7 and 8 show the effect of the temperature dependence of the retention and permeability, utilized in accordance with the invention. FIGS. 7 and 8 show the results with respect to catalyst retention (Rh retention) and permeability. The lower the temperature, the higher the retention and hence the lower the permeability.

(64) In an experimental plant with a jet loop reactor (61), shown in FIG. 9, hydroformylation reactions of 1-pentene (68) with synthesis gas (62) were conducted to give the corresponding aldehyde isomers. In the liquid circulation system (63) driven by a peripheral wheel pump (64), the catalyst-ligand system was separated off and recycled by means of a membrane separation stage (65) for continuous reuse of the catalyst-ligand system in the hydroformylation reaction in the jet loop reactor (61).

(65) For the reaction, the reactor was supplied continuously with 1-pentene (68) with exclusion of oxygen, in accordance with the removal of reaction product via the permeate from the membrane separation stage. The catalyst precursor was rhodium acetylacetonato dicarbonyl (CAS No. 14847-82-9). The ligand used was Alkanox P-24 (CAS No. 26741-53-7). The rhodium concentration and the ligand concentration in the loop reactor were kept constant at 10 mg/kg and 1170 mg/kg respectively by continuous metered addition. The reaction was conducted under synthesis gas pressure of 50 bar (CO/H.sub.2, mass ratio 1:1), at 110 C.

(66) The reaction product was conducted continuously through a membrane separation stage (65) designed as a one-stage nanofiltration membrane. The transmembrane pressure required is built up through the reactor pressure and a regulated permeate-side pressure. The desired crossflow of 500 km/h over the high-pressure side of the membrane is set by means of the peripheral wheel pump (64).

(67) In the membrane module of the membrane separation stage (65) was installed a prototype of a membrane hydrophobized by silanization, in the form of a single-channel tube from the Fraunhofer Institute for Ceramic Technologies and Systems KIS. The carrier consisted of Al.sub.2O.sub.3 having a median pore size of 3 m and a hydrophobized membrane layer based on a ZrO.sub.2 layer having a median pore diameter of 3 nm.

(68) With a channel length of 500 mm and an internal diameter of 7 mm, the active membrane area is about 100 cm.sup.2. The membrane crossflow was 4.4 m/s. Membrane temperatures of 30 to 90 C. were set. To stabilize the catalyst-ligand complex, a synthesis gas pressure (CO/H.sub.2, mass ratio 1:1) of 10 bar was maintained on the permeate side, as a result of which a transmembrane pressure of 10 to 30 bar was set at a retentate-side pressure of 20 to 40 bar.

(69) In the membrane separation stage, permeate (66) consisting predominantly of reaction product was withdrawn from the system through the membrane. The catalyst and the Alkanox ligand were very substantially retained by the membrane and accumulated in the retentate (67). The retentate (67) was conducted continuously back into the jet loop reactor (61).

(70) The process sequence was evaluated using measurement and analysis data which were obtained by gas chromatography analysis, HPLC analysis, atomic absorption spectrometry and optical emission spectrometry with inductively coupled high-frequency plasma. The reaction was examined with regard to the conversion of 1-pentene and the yield of and selectivity for aldehyde. The membrane separation stage (65) was examined with regard to permeate flow and retention of rhodium. The composition of the reaction output was as follows:

(71) TABLE-US-00002 1-Penten 2-M.pentanal Hexanal Remainder 3.7% 53.3% 42.2% 0.8%

LIST OF REFERENCE NUMERALS

(72) 0 composition of matter

(73) 1 membrane

(74) F feed

(75) P (resulting) permeate

(76) R (resulting) retentate

(77) 10 enriching cascade (prior art)

(78) 11 first stage of the enriching cascade

(79) 12 second stage of the enriching cascade

(80) 13 conveying pump for the first stage

(81) 14 retentate from the first stage

(82) 15 permeate from the first stage

(83) 16 high-pressure pump

(84) 17 retentate from the second stage

(85) 18 permeate from the second stage

(86) 20 stripping cascade (prior art)

(87) 21 first stage of the stripping cascade

(88) 22 second stage of the stripping cascade

(89) 23 pressure-increasing pump

(90) 24 retentate from the first stage

(91) 25 permeate from the first stage

(92) 26 permeate from the second stage

(93) 27 retentate from the second stage

(94) 30 two-stage enriching cascade

(95) 31 first stage of the enriching cascade

(96) 32 second stage of the enriching cascade

(97) 33 first temperature control unit

(98) 34 second temperature control unit

(99) 35 pump for the first stage

(100) 36 pump for the second stage

(101) 37 permeate from the first stage

(102) 38 recycle loop upstream of the second stage

(103) 39 recycle loop upstream of the first stage

(104) 40 three-stage enriching cascade

(105) 41 first stage of the enriching cascade

(106) 42 second stage of the enriching cascade

(107) 43 third stage of the enriching cascade

(108) 50 four-stage enriching cascade

(109) 51 first stage of the enriching cascade

(110) 52 second stage of the enriching cascade

(111) 53 third stage of the enriching cascade

(112) 54 fourth stage of the enriching cascade

(113) 61 jet loop reactor

(114) 62 synthesis gas

(115) 63 liquid circulation system

(116) 64 peripheral wheel pump

(117) 65 membrane separation stage

(118) 66 permeate

(119) 67 retentate

(120) 68 1-pentene