DESC:Field of Invention
The present disclosure relates to a process for purification of tertiary amine N-oxides from spin bath solution obtained in cellulose processing. Specifically, the present disclosure relates to a process for reducing the loss of tertiary amine N-oxides in regeneration effluents.

Background of the invention
Tertiary amine N-oxides, in particular, N-methylmorpholine-N-oxide (NMMO), find application in cellulose processing for production of lyocell fibre. A conventional method of producing lyocell fibre using tertiary amine N-oxides includes the following four steps: mixing, dissolving, spinning, and washing. Cellulose pulp is mixed with NMMO solvent so as to dissolve cellulose and obtain a spinning solution. This spinning solution is subjected to a spinning process. When the spinning process is conducted, the spinning solution is extruded through spinneret into a spin bath for formation of lyocell fibres. During the spinning process, tertiary amine N-oxide is also accumulated in the spin bath.

For economic reasons, it is crucial to recover the amine oxide as completely as possible and reuse it for the production of lyocell fibres. Therefore, used or spent spin bath must be purified and concentrated in order to recover and re-use the amine oxide it contains. Also, the lyocell fibres are washed with water to remove any residual NMMO. The solution obtained from the spin bath and washing step are collected, recovered and concentrated for recycling. However, during pulp dissolution and spinning steps, NMMO gets contaminated with various impurities. Thus, it is necessary to remove the impurities from the spin bath, before concentrating it for recovery of NMMO. It is known that such impurities could be removed by using one or both of anion and cation resins.

While anion resin removes coloring impurities, the cation resin removes N-methyl morpholine (NMM), Morpholine (M) and metal impurities from the solution. However, the cation resin also captures NMMO. The adsorbed NMMO cannot be removed by water and is removed with regeneration chemicals such as aqueous solution of HCl. Thus, NMMO adsorbed on resin is lost in regeneration effluents. This also contributes to higher Chemical Oxygen Demand (COD) in the effluent stream which has to be treated in effluent treatment plant. There is thus a need to reduce the loss of NMMO in the regeneration effluent.

Brief Description of Drawings
Figure 1 illustrates a schematic diagram of the process in accordance with an embodiment of the present disclosure.

Summary of the invention
According to an embodiment of the invention, there is provided a process for purification of tertiary amine N-oxides from a spin bath solution after the production of lyocell fibre therein, the process comprising filtering the spin bath solution and then passing the filtered solution first through an anion exchange resin followed by a cation exchange resin, wherein the volumetric ratio of the cation exchange resin to the anion exchange resin is 0.25 to 0.77.
Detailed Description of the invention
The present disclosure relates to a process for purification of tertiary amine N-oxides, in particular N-methylmorpholine-N-oxide (NMMO), from spin bath solution obtained in cellulose processing. Said process comprises of contacting said spin bath solution with an anion resin followed by a cation resin, characterized in that the cation resin and the anion resin are in a volumetric ratio of less than 0.77.

In accordance with a preferred embodiment, the cation resin and the anion resin are used in the volumetric ratio of less than 0.33, and most preferably 0.25. In accordance with an embodiment, the spin bath solution comprises spent spin bath. In accordance with a related embodiment, the spin bath solution comprises of NMMO in an amount ranging between 16-30%, and preferably 20-25%.

In accordance with an embodiment, the cation resin is selected from a group consisting of a strongly acidic cation resin and a weakly acidic cation resin, and is preferably a weakly acidic cation resin. In accordance with an embodiment, the strongly acidic cation resin used in the process is styrene-divinyl benzene resin with sulphonic acid as a functional group. In accordance with an embodiment, weakly acidic cation resin used in the process is polyacrylic based resin with carboxylic acid as functional group.

In accordance with an embodiment, the anion resin is a strongly basic anion resin. In accordance with an embodiment, the strongly basic anion resin used in the process is styrene-divinyl benzene based resin with quaternary amine as functional group.

The process of the disclosure is carried out by passing the spin bath solution to be purified through a series of columns packed with the above-defined anion resin and the cation resin. In doing so, the impurities are adsorbed on the resins such that a purified NMMO solution emerges from the final column. Once the resin beads are considered as exhausted, feed to the columns are stopped. The final resin bed volume, which remains in the column, is then washed off. After evaporative concentration of the aqueous NMMO solution, the concentrate obtained can be used directly to prepare new cellulose solutions.

In accordance with an embodiment, the specific flow rate of the spin bath solution through the anion resin and the cation resin is maintained in a range of 5-20 BV/h, and preferably 10-15 BV/h. BV is the volume of anionic resin in the column.

In accordance with an embodiment, the spin bath solution is contacted with the anion resin for a contact time in a range of 3-15 minutes, and preferably 5-10 minutes. In accordance with a related embodiment, the spin bath solution is contacted with the cation resin for a contact time in a range of 3-10 minutes, and preferably 3-5 minutes.

In accordance with an embodiment, the resin beads are considered exhausted when the solution from outlet has an eletrical conductivity in a range of 10-20 ?S/cm, and preferably 15 ?S/cm.

In accordance with an exemplary embodiment, the present process of purification of NMMO from spin bath solution has been illustrated in Figure 1. Firstly, the spin bath solution is filtered with Whatman paper (1 micron) using Buchner funnel. The electrical conductivity of the spin bath solution is measured with conductivity meter. The solution is taken in a beaker 1. The spin bath solution is passed into column 3 containing anion resin using pre-calibrated peristaltic pump 2. The outlet of column 3 is passed through column 4 containing cation resin and then through outlet 5. The solution from outlet 5 is collected continuously at different time intervals. Once the outlet conductivity of the solution crosses 15 ?S/cm then resin beads are considered as exhausted and feed to the columns are stopped. The remaining solution is taken from the columns and water wash is given to resin beads to take out the remaining NMMO. The water wash is maintained around 5-6 BV on the basis of total resin volume.

Once the resin beads have been exhausted, the anion and the cation resin are regenerated. Both the anion and cation resin may be regenerated using any process known in the art. In accordance with an embodiment, the anion resin is regenerated by treatment with (i) an aqueous solution of a strong inorganic acid such as hydrochloric acid or sulphuric acid followed by, (ii) an aqueous solution of sodium hydroxide. In accordance with an exemplary embodiment, in order to regenerate anion resin, first, the anion resin beads are cleaned with 4-5% HCl solution, to remove color. 2-3 BV of HCl solution is taken for cleaning. Flow of HCl solution is controlled in such a way that the HCl solution is contacted with the anion resin for 15-20 minutes. The HCl solution from outlet is collected. The remaining HCl solution in the beads are taken out with water wash. The water wash is maintained at 4-5 BV. Once the beads have been cleaned, anion resin is regenerated with 4-5% NaOH. 2-3 BV of NaOH solution is used for regeneration. The NaOH solution is contacted with anion resin for about 15-20 minutes. Water wash of 3-4 BV is given to wash remaining NaOH solution from resin.

In accordance with an embodiment, the cation resin is regenerated by treatment with an aqueous solution of a strong acid, such as hydrochloric acid. In accordance with an exemplary embodiment, cation resin is regenerated with 4-5% HCl solution. HCl solution is contacted with the cation resin for about 15-20 minutes. Water wash of 4-5 BV is given to take out remaining HCl from resin beads.

Purification of NMMO was done using the process of present disclosure. Four experiments were conducted using different ratios of cation/ anion, and different cation resins. The spin bath solution comprised 20% NMMO, as was determined by titration with 0.1N perchloric acid using crystal violet indicator. The details of the four experiments have been stated below. The following experimental examples are illustrative of the invention but not limitative of the scope thereof:

Example 1: Recovery of NMMO using cation/ anion ratio of 0.77
30 ml of strongly basic styrene-divinyl benzene based resin with quaternary amine as functional group commercially available as Lewatit S6368 was used as the anion resin in this case and 23 ml of strongly acidic styrene-divinyl benzene resin with sulphonic acid as a functional group commercially available under the tradename Lewatit S2568 was used as the cation resin.

The above anion resin and cation resin were taken in glass columns. Spin bath solution (comprising 20% NMMO) was first passed through the anion resin and then over cation resin. The specific flow rate was maintained in the range of 10-20 BV/h. The resin beads were considered as exhausted when outlet conductivity was 15 ?S/cm. The resins were regenerated.

In order to regenerate anion resin, first, the anion resin beads were cleaned with 4-5% HCl solution, to remove color. 2-3 BV of HCl solution was taken for cleaning. Flow of HCl solution was controlled in such a way that the HCl solution was contacted with the anion resin for 15-20 minutes. The HCl solution from outlet was collected. The remaining HCl solution in the beads was taken out with water wash. The water wash was maintained at 4-5 BV. Once the beads were cleaned, anion resin was regenerated with 4-5% NaOH. 2-3 BV of NaOH solution was used for regeneration. The NaOH solution was contacted with anion resin for about 15-20 minutes. Water wash of 3-4 BV was given to wash remaining NaOH solution from resin. On the other hand, the cation resin was regenerated with 4-5% HCl solution. HCl solution was contacted with the cation resin for about 15-20 minutes. Water wash of 4-5 BV was given to take out remaining HCl from resin beads. After the resins were regenerated, the composite effluent was analysed for NMMO using HPLC.

Example 2: Recovery of NMMO using cation/ anion ratio of 0.33
In another run, the ratio of cation/anion was decreased to 0.33 by taking, in glass columns, 30 ml anion resin comprising strongly basic styrene-divinyl benzene based resin with quaternary amine as functional group commercially available as Lewatit S6368 and 10 ml of cation resin comprising strongly acidic styrene-divinyl benzene resin with sulphonic acid as a functional group commercially available under the tradename Lewatit S2568. Feed specific flow rate and exhaust conditions were same as Example 1. The resins were regenerated and composite effluent was analysed for NMMO also as per Example 1.

Example 3: Recovery of NMMO using cation/ anion ratio of 0.25
In another run, the ratio of cation/anion was decreased to 0.25 by taking, in glass columns, 30 ml anion resin comprising strongly basic styrene-divinyl benzene based resin with quaternary amine as functional group commercially available as Lewatit S6368 and 7.5 ml of cation resin comprising strongly acidic styrene-divinyl benzene resin with sulphonic acid as a functional group commercially available under the tradename Lewatit S2568. Feed specific flow rate and exhaust conditions were same as Example 1. The resins were regenerated and composite effluent was analysed for NMMO also as per Example 1.

Example 4: Recovery of NMMO using cation/ anion ratio of 0.25 (using weakly acidic cation resin)
In another run, the ratio of cation/anion was decreased to 0.25 by taking, in glass columns, 30 ml anion resin comprising strongly basic styrene-divinyl benzene based resin with quaternary amine as functional group commercially available as Lewatit S6368 and 7.5 ml of cation resin comprising weakly acidic polyacrylic based resin with carboxylic acid as functional group commercially available under the tradename Lewatit CNP 80 WS. Feed specific flow rate and exhaust conditions were kept same as that in Example 1. The resins were regenerated and composite effluent was analysed for NMMO also as per Example 1.

Analysis:
All the regeneration effluents i.e. HCl, NaOH and water washes from each example were collected separately and mixed to obtain a composite effluent for each example i.e. four different composite effluents. The composite effluents were analysed for NMMO content using HPLC method. The results of the experiments were tabulated below in Table 1:

Table 1:
Ex No. Sample No. Cation/Anion ratio

Type of cation resin Total spin bath solution feed (ml) Total spin bath solution feed (gm) Total feed in terms of Anion resin (BV) Total effluent volume
(ml) NMMO in total effluent(ppm) Avg. NMMO in Effluent (ppm) NMMO loss in effluent(mg) % decrease in NMMO loss in effluent*
1 1 0.77 Strongly acidic 2550 2664.8 85 650 4160 4426 2877 0
2 0.77 2490 2602.1 83 650 4692
2 1 0.33 2580 2696.1 86 500 2287 2270 1135 61%
2 0.33 2520 2633.4 84 500 2253
3 1 0.25 2520 2633.4 84 470 1887 1836 863 70%
2 0.25 2460 2570.7 82 470 1784
4 1 0.25 Weakly acidic 2520 2633.4 84 470 400 430 202 93%
2 2550 2664.8 85 470 460
*All values relative to Example 1
It was observed that when strongly acidic cation resin was used, NMMO loss in the effluent was about 2877 mg when cation/anion ratio was 0.77, whereas it was 863 mg when said ratio was 0.25. Thus, although the resin performance in terms of BV is similar, there is about 70% decrease in NMMO loss in the effluent compared to Example 1. Further, when in Example 4 weakly acidic cation resin was used with the cation/anion ratio of 0.25, there is about 93% decrease in NMMO loss compared to Example 1.
Industrial Applicability
The present process of recovery of tertiary amine N-oxides reduces the loss of said amine oxide in the regeneration effluents. Said process is efficient and inexpensive to carry out. The process results in reducing the loss of tertiary amine N-oxides, in particular NMMO, by up to 93 %. Further, the process not only reduces the loss of NMMO, but also requires lesser volume of cation resin and regeneration chemicals. Thus, the present process of recovery of NMMO is cheaper as compared to traditional processes.

The effluent generated in the process comprises reduced amount of NMMO. This reduces the COD of the effluent while also reducing the cost towards the downstream processing of the effluent.

The above examples are non-limiting. The invention is defined by the claims that follow and their full range of equivalents.
,CLAIMS:We claim:
1. A process for purification of tertiary amine N-oxides from a spin bath solution after the production of lyocell fibres therein, the process comprising filtering the spin bath solution and then passing the filtered solution first through an anion exchange resin followed by a cation exchange resin, wherein the volumetric ratio of the cation exchange resin to the anion exchange resin is 0.25 to 0.77.

2. The process as claimed in claim 1 wherein, the specific flow rate of the filtered solution through the resins is maintained at 10 to 20 BV/hr.

3. The process as claimed in claim 1 wherein, the passage of filtered solution through the resins is stopped when the electrical conductivity at the outlet of the cation exchange resin is in a range of 10-20 ?S/cm.

4. The process as claimed in claim 1 wherein, the tertiary amine N-oxide is N-methylmorpholine-N-oxide.

5. The process as claimed in claim 1 wherein, the filtered solution comprises 16 to 30% N-methylmorpholine-N-oxide.

6. The process as claimed in claim 1 wherein, the cation exchange resin is a strongly acidic or a weakly acidic cation exchange resin.

7. The process as claimed in claim 6 wherein, the strongly acidic cation exchange resin is styrene-divinyl benzene resin with sulphonic acid as a functional group.
8. The process as claimed in claim 6 wherein, the weakly acidic cation resin is polyacrylic based resin with carboxylic acid as a functional group.

9. The process as claimed in claim 1 wherein, the anion exchange resin is a strongly basic anion exchange resin.

10. The process as claimed in claim 9 wherein, the anion exchange resin is styrene-divinyl benzene resin with quaternary amine as a functional group.