PROCESS FOR THE RECOVERY OF SOLVENT FROM SOLVENT-CONTAINING CELLULOSIC PARTICLES

Abstract

A process (100) for the recovery of solvent (1) from solvent-containing cellulosic particles (2) is shown, the process comprising the steps: a) extracting the solvent (1) from the cellulosic particles (2) by means of a liquid extraction medium (3), thereby obtaining a solvent-enriched extraction medium (5), and b) obtaining the recovered solvent (6) from the solvent-enriched extraction medium (5). In order to improve the efficiency of the process, it is proposed that in step a) the solvent (1) is extracted from the cellulosic particles (2) in a continuous flow extraction reactor (4), wherein the extraction medium (3) continuously flows through the extraction reactor (4) to extract the solvent (1) from the cellulosic particles (2).

Claims

1. A process for recovering a solvent from solvent-containing cellulosic particles, comprising the steps: a) extracting the solvent from the solvent-containing cellulosic particles by means of a liquid extraction medium, thereby obtaining a solvent-enriched extraction medium, and b) obtaining a recovered solvent from the solvent-enriched extraction medium, wherein in step a) the solvent is extracted from the solvent-containing cellulosic particles in a continuous flow extraction reactor, wherein the liquid extraction medium continuously flows through the continuous flow extraction reactor to extract the solvent from the solvent-containing cellulosic particles.

2. The process according to claim 1, wherein the solvent-containing cellulosic particles are obtained from a production waste of processes for producing regenerated cellulosic molded bodies, optionally from a lyocell spinning dope waste.

3. The process according to claim 2, wherein the solvent is a direct dissolution solvent, optionally an amine oxide, or optionally NMMO.

4. The process according to claim 1, wherein the liquid extraction medium comprises water.

5. The process according to claim 1, wherein the empty continuous flow extraction reactor is filled with the solvent-containing cellulosic particles prior to step a).

6. The process according to claim 1, wherein the solvent-containing cellulosic particles are contained in an aqueous suspension when filling the empty continuous flow extraction reactor.

7. The process according to claim 6, wherein excess liquid is removed from the aqueous suspension prior to filling the continuous flow extraction reactor with the aqueous suspension.

8. The process according to claim 7, wherein the aqueous suspension is filled via a bow-shaped sieve into the continuous flow extraction reactor in order to remove the excess liquid from the aqueous suspension.

9. The process according to claim 1, wherein the continuous flow extraction reactor has a top inlet and a bottom sieve outlet for the solvent-enriched extraction medium and the liquid extraction medium flows from top to bottom in the continuous flow extraction reactor.

10. The process according to claim 9, wherein the solvent-enriched extraction medium is obtained from the liquid extraction medium exiting the bottom sieve outlet of the continuous flow extraction reactor.

11. The process according to claim 1, wherein in step a), the liquid extraction medium continuously flows through the continuous flow extraction reactor, until a content of the solvent in the solvent-enriched extraction medium below a predefined residual value is reached.

12. The process according to claim 11, wherein after the content of the solvent in the solvent-enriched extraction medium below the predefined residual value is reached, a continuous flow of the liquid extraction medium through the continuous flow extraction reactor is stopped and the continuous flow extraction reactor is emptied.

13. The process according to claim 12, wherein after emptying the continuous flow extraction reactor, essentially solvent-free cellulosic particles are pressed and/or dried.

14. The process according to claim 1, wherein, the recovered solvent is obtained from the solvent-enriched extraction medium.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In the following, the invention is exemplified based on embodiments with reference to the drawings. In particular

[0026] FIG. 1 shows a schematic view of the process according to a first embodiment in a first process step,

[0027] FIG. 2 shows a schematic view of the process from FIG. 1 in a second process step, and

[0028] FIG. 3 shows a schematic view of the process from FIG. 1 in a third process step.

MODES FOR CARRYING OUT THE INVENTION

[0029] In FIGS. 1 to 3, the process 100 for the recovery of solvent 1 from solvent-containing cellulosic particles 2, according to a first embodiment of the invention, is shown. The process 100 according to the first embodiment comprises the following steps:

[0030] a) Filling the empty extraction reactor 4 with an aqueous suspension 7 containing the solvent-containing cellulose particles 2 and water as aqueous medium 11,

[0031] b) extracting the solvent 1 from the cellulosic particles 2 by means of a liquid extraction medium 3, more particularly water, in the continuous flow extraction reactor 4, wherein the extraction medium 3 continuously flows through the extraction reactor 4 from top to bottom to extract the solvent 1 from the cellulosic particles 2 into the extraction medium 3, thereby obtaining a solvent-enriched extraction medium 5, and

[0032] c) obtaining the recovered solvent 6 from the solvent-enriched extraction medium 5 exiting the bottom sieve outlet 15 of the extraction reactor 4.

[0033] Thus, in the process 100, the solvent 1 is extracted from the solvent-containing cellulosic particles 2 by means of a liquid extraction medium 3 in a continuous flow extraction reactor 4. The liquid extraction medium 3 continuously flows through the extraction reactor 4 and is enriched with solvent 1 from the cellulosic particles 2 and thus forms a solvent-enriched extraction medium 5. Said solvent-enriched extraction medium 5 can then be collected upon leaving the extraction reactor 4 and the recovered solvent 6 may be obtained from it.

[0034] In FIG. 1 a first step of the process 100 is depicted in more detail. The empty extraction reactor 4 is initially filled with the solvent-containing cellulosic particles 2 before the extraction of the solvent 1 may be started. In the present embodiment, the cellulosic particles 2 are filled into the extraction reactor 4 by using an aqueous suspension 7, containing the cellulosic particles 2. The aqueous suspension 7 enables better handling of the cellulosic particles 2, which otherwise tend to exhibit very disadvantageous flowing properties. In the present embodiment, the cellulosic particles 2 are initially obtained from production waste 8 of a lyocell process for the production of regenerated cellulosic molded bodies. Such production waste 8, which contains solvent 1, is in particular spinning dope waste 9 from a lyocell plant, for example obtained from bleeding a filmtruder or spinning dope lines, or obtained during extrusion of the regenerated cellulosic molded bodies. Usually the spinning dope waste 9 is a collection and mixture of waste from different source as mentioned above, and thus, contains a varying concentration of solvent 1. In this case, the solvent 1 is a direct dissolution solvent for cellulose, in particular an amine oxide like NMMO or any ionic liquid, and the spinning dope waste 9 originates from different stages of a lyocell plant. Thus, in the present embodiment, water is used as an extraction medium 3 to reclaim the solvent 1 from the spinning dope waste 9. Since the spinning dope waste 9 receives no further treatment prior to the present process 100, it is an at least partly uncoagulated solution of cellulose, wherein the cellulose is dissolved in the solvent 1, exhibiting a very high viscosity. Thus, to be able to easily handle the spinning dope waste 9 and efficiently extract the solvent 1 from it, it is first comminuted in a shredder 10 to form the solvent-containing cellulosic particles 2. Thereby, the spinning dope waste 9 and an aqueous medium 11 are added to the shredder 10, in order to both comminute and coagulate the spinning dope waste 9. In the present case of a lyocell spinning dope waste 9, the aqueous medium 11 may be preferably water, but it is equally possible to use any other compatible medium. The aqueous medium 11 is in particular identical to the extraction medium 3, used to extract the solvent 1 from the cellulosic particles 2 in the extraction reactor 4, in order to ensure high compatibility with the further extraction process, thus, additional washing or cleaning steps can be omitted. The solvent-containing cellulosic particles 2 obtained from the shredder 10 are contained in a suspension 7, with the liquid being the aqueous medium 11.

[0035] In a further embodiment, the shredder 10 may thereby be an underwater granulator, wherein the spinning dope waste 9 is fed through a rotating knife into a stream of water to form the cellulosic particles 2, which is not depicted in the figures.

[0036] As shown in FIG. 1, the suspension 7 is further fed into the extraction reactor 4 via a bow sieve 12 to remove the excess liquid 13 from the suspension 7. The bow sieve 12 is shaped in a way, that the cellulosic particles 2 cannot pass through the bow sieve 12, but flow along the top surface into the extraction reactor 4, while the excess liquid 13 may pass the bow sieve 12 and is collected. The excess liquid 13 contains essentially the aqueous medium 11, used to comminute the spinning dope waste 9, but may further contain solvent 1 extracted from the cellulosic particles 2. Depending on the amount of solvent 1 in the excess liquid 13, it may be preferable to also use it for obtaining the recovered solvent 6.

[0037] FIG. 2 shows the process 100 in a second process step. After filling the extraction reactor 4 with the cellulosic particles 2, the continuous flow of extraction medium 3 is started. The extraction medium 3 thereby enters the extraction reactor 4 through a top inlet 14 and flows between the cellulosic particles 2 towards the bottom of the extraction reactor 4. In the present embodiment, said top inlet 14 is arranged at the top side of the extraction reactor 4, whereas in a further embodiment, the top inlet 14 is arranged at the top of the extraction reactor 4. In an even further embodiment, the extraction medium 3 is homogeneously distributed over the whole extraction reactor 4 when entering the top inlet 14.

[0038] By flowing between the cellulosic particles 2, the extraction medium 3 gets in contact with the cellulosic particles 2, whereby solvent 1 is extracted from the cellulosic particles 2 into the extraction medium 3, thereby forming the solvent-enriched extraction medium 5. Said solvent-enriched extraction medium 5 is then obtained from a bottom sieve outlet 15 of the extraction reactor 4. The bottom sieve outlet 15 therefore comprises a sieve 16, which is permeable for the solvent-enriched extraction medium 5 but holds back the cellulosic particles 2. It is thus possible to continuously supply fresh extraction medium 3 to the extraction reactor 4 via the top inlet 14 and continuously obtain solvent-enriched extraction medium 5 from the bottom sieve outlet 15. The solvent-enriched extraction medium 5 exiting the bottom sieve outlet 15 is then fed through a solvent recovery device 17, in which the recovered solvent 6 is reclaimed from the solvent-enriched extraction medium 5.

[0039] Such a solvent recovery device 17 may be any device that is able to recover solvent from the solvent-enriched extraction medium 5, such as any combination of flotation, ion exchange, evaporation, etc. In one embodiment, in the solvent recovery device 17, a number of pre-purification steps may be followed by an evaporation of the excess water.

[0040] In a further embodiment, the excess liquid 13 obtained from the bow sieve 12 while filling the extraction reactor 4, is also fed through the solvent recovery device 17 to obtain recovered solvent 6. This is indicated in FIG. 1 by the second, dashed feed line into the solvent recovery device 17.

[0041] In one preferred embodiment, extraction medium 3 is continuously supplied to the extraction reactor 4 via the top inlet 14 until the concentration of solvent 1 in the solvent-enriched extraction medium 5, obtained from the bottom sieve outlet 15, falls below a predefined concentration value. Said concentration value may be chosen in accordance with the requirements on maximum residual concentration of solvent 1 in the cellulosic particles 2 and the constraints on maximum amount of extraction medium 3 used in the process 100. The concentration of solvent 1 in the solvent-enriched extraction medium 5 can be measured and monitored permanently, e.g. by means of a conductivity measurement. When the solvent-enriched extraction medium 5 reaches a concentration of solvent 1 below the predefined values, the continuous flow of fresh extraction medium 3 is stopped.

[0042] In FIG. 3, the process 100 is depicted in a third process step, after the extraction and the continuous flow of extraction medium 3 has been stopped. The solvent-enriched extraction medium 5 has been drained from the extraction reactor 4, leaving only the solvent-extracted cellulosic particles 18 in the extraction reactor 4. The extraction reactor 4 is then emptied and the solvent-extracted cellulosic particles 18 are fed through a pump 19 connected to the bottom outlet 20 of the extraction reactor 4. The pump 19 further transports the solvent-extracted cellulosic particles 18 into a dewatering device 21, where the residual solvent-enriched extraction medium 5 contained in the solvent-extracted cellulosic particles 18 is removed. The dried cellulosic particles 22, leaving the dewatering device 21, are then essentially free from solvent 1 and substantially dry.

[0043] In another embodiment, which is not further depicted in the figures, a residual amount of solvent-enriched extraction medium 5 may be left in the extraction reactor 4 prior to emptying, in order to improve the flowing properties of the solvent-extracted cellulosic particles 18 through the pump 19 and the bottom outlet 20 of the extraction reactor 4.

[0044] In one embodiment, the dewatering device 21 comprises a FAN separator, where the solvent-extracted cellulosic particles 18 are pressed to remove the residual solvent-enriched extraction medium 5. In another embodiment, the dewatering device 21 comprises a centrifuge, where the cellulosic particles 18 are centrifuged to remove the residual solvent-enriched extraction medium 5. In yet another embodiment, the dewatering device 21 may also comprise a dryer, following a FAN separator, centrifuge or the like, to further remove excess liquid from the cellulosic particles in order to obtain dried cellulosic particles 22.

Examples

[0045] In the following, the herein described process is demonstrated according to a number of examples.

[0046] In the Examples 1 to 3, a batch of solvent-containing cellulosic particles is treated with the present inventive process to obtain recovered solvent. The solvent-containing cellulosic particles are obtained from the spinning dope waste of a lyocell process. Said lyocell spinning dope waste, comprising cellulose and NMMO as a solvent, was comminuted with added water to form a suspension of solvent-containing cellulosic particles with a mean size of approximately 3 to 5 mm. The suspension containing the cellulosic particles was then filled into an extraction reactor via a bow sieve to properly dewater the cellulosic particles. The extraction reactor thereby had an active volume of 2 m.sup.3 and a total cellulosic particle filling mass of 184 kg. Water as extraction medium was then continuously flown through the extraction reactor to extract NMMO from the cellulosic particles until a desired residual concentration of NMMO in the cellulosic particles was reached.

[0047] In Table 1, the results of Examples 1 to 3 are summarized.

[0048] In Example 1, the extraction medium was continuously fed through the extraction reactor, until the residual concentration of NMMO in the cellulosic particles reached a value below 10000 mg per kg of cellulosic particles. After emptying the extraction reactor, the cellulosic particles were measured to contain a NMMO residual content of 6808 mg/kg.sub.Cell. Thereby, a total of 17.4 kg of water per kg of cellulosic particles were fed through the extraction reactor. The complete extraction cycle was finished in approximately 5 hours. The solvent-enriched extraction medium (NMMO—water solution) obtained at the end of the extraction had a NMMO concentration of 0.48%.

[0049] Example 2 was similarly conducted to Example 1, but the extraction medium was continuously fed through the extraction reactor, until a residual concentration of NMMO in the cellulosic particles below 500 mg/kg.sub.Cell was reached, whereby a total water consumption of 30.7 kg/kg.sub.Cell was needed and the total extraction time amounted to approx. 6 hours. After emptying the extraction reactor, the cellulosic particles contained 464 mg/kg.sub.Cell NMMO. The obtained solution of NMMO in water finally had a NMMO concentration of 0.05%.

[0050] Similarly, in Example 3 the extraction medium was continuously fed through the extraction reactor, until a residual concentration of NMMO in the cellulosic particles below 50 mg/kg.sub.Cell was reached, whereby a total water consumption of 41.9 kg/kg.sub.Cell was needed and the total extraction time amounted to 4 hours. After emptying the extraction reactor, the cellulosic particles contained less than 36 mg NMMO per kg.sub.Cell. The obtained solution of NMMO in water finally had a NMMO concentration of 0.02%.

[0051] Furthermore, in Comparative Examples 1 and 2, the cellulosic particles were extracted by means of a batch process in a stirred vessel according to the state of the art. The cellulosic particles were therefore obtained by comminuting a lyocell spinning dope as described above and subsequently filled in the stirred vessel. In several cycles, the stirred vessel was then filled with water, the cellulosic particles-water mixture stirred for a certain amount of time, and the water drained again from the vessel. This cycle is then repeated several times.

[0052] The results for Comparative Examples 1 and 2 are summarized in Table 1.

[0053] In Comparative Example 1, 2 extraction cycles (2 stages) as described above were performed. After finishing the extraction, the residual content of NMMO in the cellulosic particles was determined to be 35000 mg/kg.sub.Cell. The extraction took 2 hours, whereby a total amount of 101 kg/kg.sub.Cell of water was consumed.

[0054] In Comparative Example 2, 11 extraction cycles (2 stages) were similarly performed. A residual content of NMMO in the cellulosic particles was determined to be 500 mg/kg.sub.Cell after the extraction. The extraction took 11 hours, whereby a total amount of 288 kg/kg.sub.Cell of water was consumed.

TABLE-US-00001 TABLE 1 Examples 1 to 3 and Comparative Examples 1 and 2 Residual Water NMMO NMMO Extraction consumption content concentration time [kg/kg.sub.Cell] [mg/kg.sub.Cell] [%] [h] Example 1 17.4 6808 0.48 5 Example 2 30.7 464 0.05 6 Example 3 41.9 <36 0.02 4 Comparative 101 35000 — 2 Example 1 Comparative 288 <500 — 11 Example 2

[0055] Thus, the inventive process is able to reduce the amount of water consumed during the extraction significantly, while the extraction time is reduced. Furthermore, the resulting solution for the recovery of the solvent (NMMO) has a much higher concentration, enabling a more efficient recovery of said solvent.