WATER REMOVAL IN A PROCESS FOR HYDROLYTICALLY DEPOLYMERIZING A POLYAMIDE

Abstract

The present invention relates to a water-efficient process for hydrolytically depolymerizing a polyamide prepared from -caprolactam.

Claims

1.-17. (canceled)

18. A water-efficient process for hydrolytically depolymerizing a polyamide prepared from -caprolactam, said polyamide being comprised in a solid material M, the process comprising (i) preparing an aqueous liquid stream S.sub.WC containing -caprolactam dissolved in water, comprising (i.1) providing the solid material M containing the polyamide; (i.2) providing an aqueous liquid stream S.sub.W; (i.3) preparing an aqueous mixture of the solid material M provided according to (i.1) and the aqueous liquid stream S.sub.W provided according to (i.2); (i.4) subjecting the aqueous mixture prepared according to (i.3) to depolymerization conditions in a chemical reactor unit R.sub.U, obtaining the aqueous liquid stream S.sub.WC containing -caprolactam dissolved in water; (ii) separating water from the aqueous liquid stream S.sub.WC by evaporation in at least two evaporation units, obtaining at least one aqueous vapor stream S.sub.V, wherein at least a part of at least one aqueous vapor stream S.sub.V is recycled into step (i.2) as a component of the aqueous liquid stream S.sub.W.

19. The process of claim 18, wherein preparing an aqueous liquid stream S.sub.WC containing -caprolactam dissolved in water according to (i) comprises (i.1) providing the solid material M containing the polyamide, M having a temperature T.sub.M, wherein T.sub.M<T.sub.P, T.sub.P being the melting point of the polyamide; (i.2) providing the aqueous liquid stream S.sub.W, wherein from 50 weight-% to 100 weight-% of S.sub.W consist of water and wherein S.sub.W has a temperature T.sub.SW, wherein T.sub.SW>T.sub.P; (i.3) preparing an aqueous mixture of the solid material M provided according to (i.1) and the aqueous liquid stream S.sub.W provided according to (i.2), comprising feeding the solid material M provided according to (i.1) and the liquid aqueous stream S.sub.W provided according to (i.2) into a chemical reactor unit R.sub.U, obtaining said mixture; (i.4) subjecting the aqueous liquid mixture prepared according to (i.3) to depolymerization conditions in the chemical reactor unit R.sub.U, obtaining the aqueous liquid stream S.sub.WC containing -caprolactam dissolved in water, wherein the depolymerization conditions comprise a depolymerization temperature T.sub.D at a depolymerization pressure p.sub.D, wherein T.sub.M<T.sub.D<T.sub.SW.

20. The process of claim 18, wherein T=T.sub.SWT.sub.P and T is in the range of from 10 to 70 C.

21. The process of claim 18, wherein from 91 to 100 weight-% of S.sub.W provided according to (i.2) consist of water.

22. The process of claim 18, wherein (ii) comprises (ii.1) optionally feeding the aqueous liquid stream S.sub.WC as a feed stream to a first evaporation unit EU1, obtaining at least one aqueous vapor stream S.sub.V1, and an aqueous liquid stream S.sub.L1 comprising -caprolactam dissolved in water; (ii.2) optionally feeding the aqueous liquid stream S.sub.WC or the aqueous liquid stream S.sub.L1 to a solid-liquid separation unit, SLU, obtaining an aqueous liquid stream S.sub.SLU comprising -caprolactam dissolved in water; (ii.3) feeding the aqueous liquid stream S.sub.WC or the aqueous liquid stream S.sub.L1 or the aqueous liquid stream S.sub.SLU to evaporation in at least two evaporation units EU2 and EU3, wherein an aqueous vapor stream S.sub.V2 is obtained from EU2 and an aqueous vapor stream S.sub.V3 is obtained from EU3, and wherein from EU3, an aqueous liquid stream S.sub.L3 comprising -caprolactam dissolved in water is obtained; wherein preferably from 75 to 100 weight-% of the aqueous liquid stream which is fed to evaporation according to (ii.3) consist of water and -caprolactam, said stream exhibiting a water concentration c.sub.H2O; the process further comprising recycling at least a part of at least one of streams S.sub.V2 and S.sub.V3 into step (i.2) as a component of the aqueous liquid stream S.sub.W.

23. The process of claim 22, wherein (ii) comprises (ii.1) feeding the aqueous liquid stream S.sub.WC as a feed stream to a first evaporation unit, EU1, obtaining at least one aqueous vapor stream S.sub.V1, and an aqueous liquid stream S.sub.L1 comprising -caprolactam dissolved in water; (ii.2) optionally feeding the aqueous liquid stream S.sub.L1 to a solid-liquid separation unit SLU, obtaining an aqueous liquid stream S.sub.SLU comprising -caprolactam dissolved in water; (ii.3) feeding the aqueous liquid stream S.sub.L1 or the aqueous liquid stream S.sub.SLU to evaporation in at least two evaporation units EU2 and EU3, wherein an aqueous vapor stream S.sub.V2 is obtained from EU2 and an aqueous vapor stream S.sub.V3 is obtained from EU3, and wherein from EU3, an aqueous liquid stream S.sub.L3 comprising -caprolactam dissolved in water is obtained.

24. The process of claim 23, wherein (ii.1) comprises (ii.1.1) feeding the aqueous liquid stream S.sub.WC as a feed stream to a first evaporation sub-unit EU11, obtaining an aqueous vapor stream S.sub.V11, and an aqueous liquid stream S.sub.L11 comprising -caprolactam dissolved in water; (ii.1.2) feeding the aqueous liquid stream S.sub.L11 as a feed stream to a second evaporation sub-unit EU12, obtaining an aqueous vapor stream S.sub.V12, and the aqueous liquid stream S.sub.L1 comprising -caprolactam dissolved in water.

25. The process of claim 24, wherein (ii.1) comprises (ii.1.1) feeding the aqueous liquid stream S.sub.WC as a feed stream to a first sub-evaporation unit, EU11, obtaining an aqueous vapor stream S.sub.V11, and an aqueous liquid stream S.sub.L11 comprising -caprolactam dissolved in water, wherein prior to feeding to EU11, the aqueous liquid stream S.sub.WC is optionally passed through at least one solid-liquid separation unit F1; (ii.1.2) feeding the aqueous liquid stream S.sub.L11 as a feed stream to a second sub-evaporation unit, EU12, obtaining an aqueous vapor stream S.sub.V12 and the aqueous liquid stream S.sub.L1 comprising -caprolactam dissolved in water, wherein prior to feeding to EU12, the aqueous liquid stream S.sub.L11 is optionally passed through at least one solid-liquid separation unit F2; wherein (ii.1) comprises at least one of passing S.sub.WC through F1 and passing S.sub.L11 through F2, wherein (ii.1) preferably comprises passing S.sub.WC through F1 and passing S.sub.L11 through F2.

26. The process of claim 22, wherein (ii.3) comprises (ii.3.1) feeding the aqueous liquid stream S.sub.L1 or the aqueous liquid stream S.sub.SLU, to evaporation in a first evaporation unit EU2, obtaining an aqueous vapor stream S.sub.V2 and an aqueous liquid stream S.sub.L21, wherein the concentration of -caprolactam in the stream S.sub.L21 is c.sub.CPLL21 with c.sub.CPLL21>c.sub.CPL, and wherein the concentration of water in the stream S.sub.V2 is c.sub.H2OV2 with c.sub.H2OV2>c.sub.H2O; (ii.3.2) feeding at least a part of the aqueous liquid stream S.sub.L21 to evaporation in a second evaporation unit EU3, obtaining an aqueous vapor stream S.sub.V3 and an aqueous liquid stream S.sub.L31, wherein the concentration of -caprolactam in the stream S.sub.L31 is c.sub.CPLL31 with c.sub.CPLL31>c.sub.CPLL21, and wherein the concentration of water in the stream S.sub.V3 is c.sub.H2OV2 with c.sub.H2OV2>c.sub.H2OL21.

27. The process of claim 28, wherein the evaporation unit EU2 comprises a film evaporator.

28. The process of claim 27, the process comprising passing at least a part of at least one aqueous vapor stream S.sub.V through the heating means of the film evaporator comprised in EU3.

29. The process of claim 26, wherein (ii.3.2) comprises feeding at least a part of the aqueous liquid stream S.sub.L21 to evaporation in a second evaporation unit EU3, obtaining an aqueous vapor stream S.sub.V3, and obtaining an aqueous liquid stream S.sub.L31 and a liquid stream S.sub.L32, wherein the concentration of -caprolactam in the stream S.sub.L31 is c.sub.CPLL31 with c.sub.CPLL31>c.sub.CPLL21, wherein the concentration of -caprolactam in the stream S.sub.L32 is c.sub.CPLL32 with c.sub.CPLL32>c.sub.CPLL21, and wherein the concentration of water in the stream S.sub.V3 is c.sub.H2OV2 with c.sub.H2OV2>c.sub.H2OL21; wherein the stream S.sub.L32 is recycled as feed stream into EU3.

30. The process of claim 29, wherein feeding at least the part of the aqueous liquid stream S.sub.L21 to evaporation in the second evaporation unit EU3 according to (ii.3.2) comprises admixing the stream S.sub.L21 with the stream S.sub.L32 and feeding the combined stream to evaporation in the second evaporation unit EU3.

31. The process of claim 22, wherein according to (ii.3), obtaining the aqueous vapor stream S.sub.V2 from EU2 comprises (a) removing an aqueous stream S.sub.VL2 from EU2, S.sub.VL2 comprising an aqueous liquid phase and an aqueous vapor phase; (b) subjecting the aqueous stream S.sub.VL2 to vapor-liquid separation, obtaining the aqueous vapor stream S.sub.V2, and obtaining an aqueous liquid stream S.sub.L22.

32. The process of claim 31, further comprising dividing the aqueous vapor stream S.sub.V2 into two aqueous vapor streams S.sub.V21 and S.sub.V22, S.sub.V21 and S.sub.V22 having the chemical composition of S.sub.V2, wherein S.sub.V21 is subjected to condensation in a condensation unit, wherein at least a part of the condensed stream S.sub.V21 is recycled into step (i.2) as a component of the aqueous liquid stream S.sub.W.

33. The process of claim 31, further comprising feeding the aqueous liquid stream S.sub.L22 to evaporation in the second evaporation unit EU3, optionally after admixing with the aqueous liquid stream S.sub.L21.

34. The process of claim 22, comprising (ii.2) feeding the aqueous liquid stream S.sub.WC or the aqueous liquid stream S.sub.L1, preferably the aqueous liquid stream S.sub.L1, to a solid-liquid separation unit SLU, obtaining an aqueous liquid stream S.sub.SLU comprising -caprolactam dissolved in water; (ii.3) feeding the liquid stream S.sub.SLU to evaporation in at least two, preferably in two evaporation units EU2 and EU3, more preferably in two serially coupled evaporation units EU2 and EU3, wherein an aqueous vapor stream S.sub.V2 is obtained from EU2 and an aqueous vapor stream S.sub.V3 is obtained from EU3, and wherein from EU3, an aqueous liquid stream S.sub.L3 comprising -caprolactam dissolved in water is obtained.

Description

EXAMPLE

[0175] 47.2 kg of filtered (depolymerization) reactor discharge (aqueous discharge) were metered continuously into a first water removal distillation. The dosing rate was 7.5 kg/h. The concentration of caprolactam in the starting material (aqueous discharge) was about 3.6 to 5.8 wt.-%. The pressure in the apparatus was 1 bar(abs). The temperature in the sump tank was 103 C. The top temperature of the apparatus was 100.1 C. The ratio of reflux to condensate removal was 6:2. The apparatus comprised a sump tank of approx. 6 L, a column structure approx. 50 cm high including unstructured packing. The apparatus further comprised a head condenser including a collection tank, and a natural circulation heat exchanger which circulates a quantity of raw material removed from the sump tank around the sump tank in order to heat it up. The sump tank is a jacketed tank. The obtained product weighed 8.5 kg, with a caprolactam content of 15 wt.-%. 38.7 kg of condensate were formed via the top condenser of the apparatus. In a second, downstream dewatering stage, this product was then metered into a container where further water was distilled off (batchwise) at 90 C. and 200 mbar(abs) pressure. An approx. 0.4 m high batch column was located on the container, and a top condenser with reflux divider was connected to the upper part. A circuit with a pump and a heat exchanger for energy input was located on the container. This was operated at 110 C. After the 2.sup.nd drainage, the product had a caprolactam content of 32.3 wt.-%. The residual water content of this product was determined to be 12 wt.-%.

DESCRIPTION OF THE FIGURES

[0176] FIG. 1 is a schematic representation of a production unit used for the process according to preferred embodiments of the invention

[0177] The production unit comprises a reactor unit R.sub.U, three evaporation units EU1, EU2 and EU3 and a solid-liquid separation unit SLU. The solid material M comprising the polyamide and an aqueous liquid stream S.sub.W are fed into the reactor unit R.sub.U and subjected to depolymerization conditions comprising a depolymerization temperature T.sub.D at a depolymerization pressure p.sub.D as detailed in the foregoing. An aqueous liquid stream S.sub.WC is removed from the bottom of R.sub.U, S.sub.WC comprising -caprolactam dissolved in water. The aqueous liquid stream S.sub.WC is fed into the evaporation unit EU1 obtaining an aqueous vapor stream S.sub.V1, and an aqueous liquid stream S.sub.L1 comprising -caprolactam dissolved in water. The aqueous vapor stream S.sub.V1 is recycled as a component of the aqueous liquid stream S.sub.W, preferably via condensation. The aqueous liquid stream S.sub.L1 is passed through the solid-liquid separation unit SLU obtaining an aqueous liquid stream S.sub.SLU comprising -caprolactam dissolved in water. The aqueous liquid stream S.sub.SLU is then fed to evaporation in two evaporation units EU2 and EU3, said two units are serially coupled as shown in FIG. 1. An aqueous vapor stream S.sub.V2 is obtained from EU2 and an aqueous vapor stream S.sub.V3 is obtained from EU3. The aqueous vapor streams S.sub.V2 and S.sub.V3 are recycled as a component of the aqueous liquid stream S.sub.W, preferably via condensation. Further, an aqueous liquid stream S.sub.L21 comprising -caprolactam dissolved in water is removed from EU2 and fed into EU3 and an aqueous liquid stream S.sub.L3 comprising -caprolactam dissolved in water is obtained and removed from EU3.

[0178] FIG. 2 is a schematic representation of a production unit used for the process according to preferred embodiments of the invention

[0179] The production unit comprises a reactor unit R.sub.U, three evaporation units EU1, EU2 and EU3 and a solid-liquid separation unit SLU, except that compared to FIG. 1 the evaporation unit EU1 comprises an evaporation sub-unit EU11 and an evaporation sub-unit EU12. The aqueous liquid stream S.sub.WC removed from the bottom of R.sub.U is passed through the sub-unit EU11 obtaining an aqueous vapor stream S.sub.V11, and an aqueous liquid stream S.sub.L11 comprising -caprolactam dissolved in water. The aqueous liquid stream S.sub.L11 is then fed into the second evaporation sub-unit EU12, as a feed stream, to obtain an aqueous vapor stream S.sub.V12, and the aqueous liquid stream S.sub.L1 comprising -caprolactam dissolved in water. The aqueous vapor streams S.sub.V11 and S.sub.V12 are recycled as a component of the aqueous liquid stream S.sub.W, preferably via condensation. Downstream of EU1, the process is carried out as the process in FIG. 1.

[0180] FIG. 3 is a schematic representation of a production unit used for the process according to preferred embodiments of the invention

[0181] The production unit comprises a reactor unit R.sub.U, three evaporation units EU1, EU2 and EU3 and a solid-liquid separation unit SLU, except that compared to FIG. 2 the evaporation unit EU1 comprises in addition to evaporation sub-units EU11 and EU12, two solid-liquid separation units F1 and F2. Hence, the aqueous liquid stream S.sub.WC removed from the bottom of R.sub.U is passed through the solid-liquid separation unit F1, preferably filtration unit F1, wherein F1 preferably has a mesh size in the range of from 0.5 to 5 mm, more preferably in the range of from 1 to 3 mm, prior to being fed into the sub-unit EU11 to obtain an aqueous vapor stream S.sub.V11, and an aqueous liquid stream S.sub.L11 comprising -caprolactam dissolved in water. The aqueous liquid stream S.sub.L11 is then passed through the solid-liquid separation unit F2, preferably filtration unit F2, wherein F2 preferably has a mesh size in the range of from 0.5 to 5 mm, more preferably in the range of from 1 to 3 mm, prior to being fed into the second evaporation sub-unit EU12, as a feed stream, to obtain an aqueous vapor stream S.sub.V12, and the aqueous liquid stream S.sub.L1 comprising -caprolactam dissolved in water. The aqueous vapor streams S.sub.V11 and S.sub.V12 are recycled as a component of the aqueous liquid stream S.sub.W, preferably via condensation. Downstream of EU1, the process is carried out as the process in FIG. 1 or 2.

[0182] FIG. 4 is a schematic representation of a production unit used for the process according to preferred embodiments of the invention

[0183] The production unit comprises a reactor unit R.sub.U, three evaporation units EU1, EU2 and EU3 and a solid-liquid separation unit SLU, wherein the evaporation unit EU1 comprises sub-units EU11 and EU12, two solid-liquid separation units F1 and F2. Further, compared to FIG. 3, EU3 of the production unit comprises heating means to provide heat for evaporation. The aqueous liquid stream S.sub.WC removed from the bottom of R.sub.U is passed through the solid-liquid separation unit F1, preferably filtration unit F1, wherein F1 preferably has a mesh size in the range of from 0.5 to 5 mm, more preferably in the range of from 1 to 3 mm, prior to being fed into the sub-unit EU11 to obtain an aqueous vapor stream S.sub.V11, and an aqueous liquid stream S.sub.L11 comprising -caprolactam dissolved in water. The aqueous liquid stream S.sub.L11 is then passed through the solid-liquid separation unit F2, preferably filtration unit F2, wherein F2 preferably has a mesh size in the range of from 0.5 to 5 mm, more preferably in the range of from 1 to 3 mm, prior to being fed into the second evaporation sub-unit EU12, as a feed stream, to obtain an aqueous vapor stream S.sub.V12, and the aqueous liquid stream S.sub.L1 comprising -caprolactam dissolved in water. The aqueous vapor stream S.sub.V12 is recycled as a component of the aqueous liquid stream S.sub.W, preferably via condensation, while the aqueous vapor stream S.sub.V11 is passed through the heating means of the film evaporator comprised in EU3. It is also conceivable that only a part of S.sub.V11 be passed through the heating means of the film evaporator comprised in EU3, the other part of S.sub.V11 being recycled as a component of the aqueous liquid stream S.sub.W (not shown in FIG. 4). Downstream of EU1, the process is carried out as the process in FIG. 3 with the exception that EU3 comprises heating means.

[0184] FIG. 5 is a schematic representation of a production unit used for the process according to preferred embodiments of the invention

[0185] As in FIG. 4, the production unit comprises a reactor unit R.sub.U, three evaporation units EU1, EU2 and EU3 and a solid-liquid separation unit SLU, wherein EU1 comprises sub-units EU11 and EU12, two solid-liquid separation units F1 and F2 and wherein EU3 comprises heating means to provide heat for evaporation. The process illustrated by FIG. 5 is carried out as the one illustrated by FIG. 4 except that the aqueous vapor stream S.sub.V3 is recycled as a feed stream into EU2.

[0186] FIG. 6 is a schematic representation of a production unit used for the process according to preferred embodiments of the invention

[0187] As in FIG. 5, the production unit comprises a reactor unit R.sub.U, three evaporation units EU1, EU2 and EU3 and a solid-liquid separation unit SLU, wherein EU1 comprises sub-units EU11 and EU12, two solid-liquid separation units F1 and F2 and wherein EU3 comprises heating means to provide heat for evaporation. The process illustrated by FIG. 6 is run as the one illustrated by FIG. 5 except that an aqueous liquid stream S.sub.L31 and a liquid stream S.sub.L32, wherein the concentration of -caprolactam in the stream S.sub.L31 is c.sub.CPLL31 With c.sub.CPLL31>c.sub.CPLL21, wherein the concentration of -caprolactam in the stream S.sub.L32 is c.sub.CPLL32 With c.sub.CPLL32>c.sub.CPLL21, preferably with c.sub.CPLL31=c.sub.CPLL32, and wherein the concentration of water in the stream S.sub.V3 is c.sub.H2OV2 with c.sub.H2OV2>c.sub.H2OL21, are removed from EU3. The liquid stream S.sub.L32 is recycled as feed stream into EU3. Further, the aqueous liquid stream S.sub.L21 removed from EU2 comprising -caprolactam dissolved in water is admixed with the liquid stream S.sub.L32. The combined stream is then fed to evaporation into EU3.

[0188] FIG. 7 is a schematic representation of a production unit used for the process according to preferred embodiments of the invention

[0189] As in FIG. 6, the production unit comprises a reactor unit R.sub.U, three evaporation units EU1, EU2 and EU3 and a solid-liquid separation unit SLU, wherein EU1 comprises sub-units EU11 and EU12, two solid-liquid separation units F1 and F2 and wherein EU3 comprises heating means to provide heat for evaporation. The process illustrated by FIG. 7 is carried out as the one illustrated by FIG. 6 except that the aqueous liquid stream S.sub.L21 removed from EU2 comprising -caprolactam dissolved in water is divided in two aqueous liquid streams S.sub.L211 and S.sub.L212. S.sub.L211 and S.sub.L212 have the chemical composition of S.sub.L21. The aqueous liquid stream S.sub.L212 is recycled as feed stream in EU2 and the aqueous liquid stream S.sub.L211 is admixed with the liquid stream S.sub.L32. The combined stream is then fed to evaporation into EU3.

[0190] FIG. 8 is a schematic representation of a production unit used for the process according to preferred embodiments of the invention

[0191] As in FIG. 7, the production unit comprises a reactor unit R.sub.U, three evaporation units EU1, EU2 and EU3 and a solid-liquid separation unit SLU, wherein EU1 comprises sub-units EU11 and EU12, two solid-liquid separation units F1 and F2 and wherein EU3 comprises heating means to provide heat for evaporation. However, in FIG. 8, the production unit further comprises a vapor-liquid separation unit (circle in FIG. 8) downstream of EU2. A stream S.sub.VL2, comprising an aqueous liquid phase and an aqueous vapor phase, is removed from EU2 and passed through the vapor-liquid separation unit to vapor-liquid separation, obtaining the aqueous vapor stream S.sub.V2, and obtaining an aqueous liquid stream S.sub.L22. The aqueous vapor stream S.sub.V2 is recycled as a component of the aqueous liquid stream S.sub.W. Apart from said differences, the process illustrated by FIG. 8 is carried out as the one illustrated by FIG. 7.

[0192] FIG. 9 is a schematic representation of a production unit used for the process according to preferred embodiments of the invention

[0193] As in FIG. 8, the production unit comprises a reactor unit R.sub.U, three evaporation units EU1, EU2 and EU3, a solid-liquid separation unit SLU and a vapor-liquid separation unit downstream of EU2, wherein EU1 comprises sub-units EU11 and EU12, two solid-liquid separation units F1 and F2 and wherein EU3 comprises heating means to provide heat for evaporation. However, in FIG. 9, the production unit further comprises a condensation unit downstream of the vapor-liquid separation unit. The aqueous vapor stream S.sub.V2 is divided into two aqueous vapor streams S.sub.V21 and S.sub.V22, S.sub.V21 and S.sub.V22 having the chemical composition of S.sub.V2. The aqueous vapor stream S.sub.V21 is subjected to condensation in the condensation unit C. The condensed stream S.sub.V21 is recycled as a component of the aqueous liquid stream S.sub.W. Apart from said differences, the process illustrated by FIG. 9 is carried out as the one illustrated by FIG. 8.

[0194] FIG. 10 is a schematic representation of a production unit used for the process according to preferred embodiments of the invention

[0195] As in FIG. 9, the production unit comprises a reactor unit R.sub.U, three evaporation units EU1, EU2 and EU3, a solid-liquid separation unit SLU, a vapor-liquid separation unit downstream of EU2 and a condensation unit C, wherein EU1 comprises sub-units EU11 and EU12, two solid-liquid separation units F1 and F2 and wherein EU3 comprises heating means to provide heat for evaporation. The process illustrated by FIG. 10 is carried out as the one illustrated by FIG. 9 except that the aqueous vapor stream S.sub.V22 is used for providing heat to EU2. Said stream S.sub.V22 is passed through the heating means of EU2, film evaporator, to obtain a condensed stream S.sub.V22 which is then recycled as a component of the aqueous liquid stream S.sub.W.

[0196] FIG. 11 is a schematic representation of a production unit used for the process according to preferred embodiments of the invention

[0197] As in FIG. 10, the production unit comprises a reactor unit R.sub.U, three evaporation units EU1, EU2 and EU3, a solid-liquid separation unit SLU, a vapor-liquid separation unit downstream of EU2 and a condensation unit C, wherein EU1 comprises sub-units EU11 and EU12, two solid-liquid separation units F1 and F2 and wherein EU3 comprises heating means to provide heat for evaporation. The process illustrated by FIG. 11 is carried out as the one illustrated by FIG. 10 except that the aqueous liquid stream S.sub.L22 is admixed with the aqueous liquid stream S.sub.L21 and subjected to evaporation to EU3.