Method of Recovering Aqueous N-Methylmorpholine-N-Oxide Solution Used in Production of Lyocell Fiber

Abstract

A method of recovering aqueous N-Methylmorpholine-N-Oxide solution used in production of Lyocell fiber comprises following steps. Bleach means for decoloring coloration in aqueous NMMO solution via alternate blow-mixing adsorption mode and static suspending adsorption mode reiteration. Filtration means for purifying the activated carbon powder and impurities by two filtering stages of first coarse filtering stage and second fine filtering stage. Concentration means for intensifying aqueous NMMO solution to obtain a condensed aqueous solution without NMMO solvent and a concentrated aqueous solution with NMMO solvent respectively by a sequential multi-stage evaporating system. Refinement means for purifying aqueous NMMO solution with promoting purity of concentrated aqueous solution to obtain required recovered aqueous solution by adding suitable agents in the redox reactions involved. Owing to streamlining and simplicity, the method not only has better competitiveness from promoted recovery cost, efficiency and quality but also meets regulations of environmental protection.

Description

CROSS REFERENCE TO RELATED APPLICATION

This is a Continuation in part of Non provisional application, Ser. No. 12/818,912, field Jun. 18, 2010. entitled “Method of recovering aqueous n-methylmorpholine-n-oxide solution used in production of lyocell fiber”

FIELD OF THE PRESENT INVENTION

This invention relates to a recovering method of solvent used in production of fiber, more particularly to a recovering method of solvent used in production of Lyocell fiber.

BACKGROUND OF THE INVENTION

Lyocell fiber is made from natural cellulose. Consequently, waste products of Lyocell fiber are naturally biodegradable and are eco-friendly without incurring environmental issues. Lyocell fiber has mechanical strength and tenacity near to that of synthetic fiber. Moreover, Lyocell fiber has excellent draping property, sufficient thickness feeling, comfortable in touch feeling, nice hygroscopicity and easy in dyeing. Furthermore, Lyocell fiber is easily blend-spinning with other materials of natural or synthetic fibers so that the final products from Lyocell fiber become high performance characteristics and added-value fabrics due to good quality and easiness in process.

Please refer to FIG. 1. An existing production process of Lyocell fiber mainly comprises following four steps: blend, dissolution, spin and rinse, wherein the blend step means for mixing wood pulp and dissolving solvent of primary N-methylmorpholine N-oxide (NMMO) to form preliminary mixture; the dissolution step means for dissolving cellulose in the preliminary mixture to form spinning dope;

the spin step means for spinning and extruding dope out of spinnerets to form raw spinning filaments; and the rinse step means for removing residual primary NMMO dissolving solvent in the raw spinning filaments via washing and drying processes to obtain refined products in Lyocell fibril-filaments of natural cellulose fiber.

Since the primary NMMO dissolving solvent features nontoxic, odorless and high boiling point, the production process of Lyocell fiber via the primary NMMO dissolving solvent is more eco-friendly than a conventional production process of synthetic fiber. However, the primary NMMO dissolving solvent is relatively expensive. Therefore, the primary NMMO dissolving solvent is usually recovered and reused so as to reduce the overall cost in mass production of Lyocell fiber. Though the current recovery rate of primary NMMO dissolving solvent from NMMO aqueous solution for the existing technique is over 99.5% so that the pollution issues incurred can be effectively obviated, the cost in recovering primary NMMO dissolving solvent is not essentially reduced. Accordingly, how to further reduce the processing cost in recovering primary NMMO dissolving solvent from NMMO aqueous solution so as to indirectly low down the overall processing cost in production process of Lyocell fiber via the primary NMMO dissolving solvent becomes a urgent and critical issue.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a method of recovering aqueous N-Methylmorpholine-N-Oxide solution used in production of Lyocell fiber so that the recovered aqueous NMMO solution can be reused repetitively as well as the recovery efficiency of the NMMO solvent can essentially promoted by the high performance thereof.

The method of recovering aqueous N-Methylmorpholine-N-Oxide solution used in production of Lyocell fiber of the present invention comprises steps of bleach (step 1), filtration (step 2), condensation (step 3) and refinement (step 4), wherein:

The bleach in step 1 means for decoloring the aqueous NMMO solution:

Firstly, the aqueous NMMO solution to be recovered is loaded into a bleaching tank, and activated carbon powder featuring with good adsorbability and suspendability is added into the aqueous NMMO solution therein; secondly, the activated carbon powder and the aqueous NMMO solution are mixed together by using an agitation blower; and finally, the agitation blower is intermittently energized so that an alternate blow-mixing adsorption mode and static suspending adsorption mode reiterates to have activated carbon powder fully contacted with the aqueous NMMO solution thoroughly in an energy-efficient manner;

The filtration in step 2 means for purifying the aqueous NMMO solution:

Two filtering stages of first coarse filtering stage and successive second fine filtering stage (ultrafiltration UF) are orderly adopted so as to remove the activated carbon powder and impurities from the aqueous NMMO solution 1, which has been decolored in previous bleach process; for the first coarse filtering stage, the activated carbon powder and the impurities of large particle size appeared in previous bleach process can be removed; for the second fine filtering stage (ultrafiltration UF), the tiny impurities of small particle size can be removed;

The concentration in step 3 means for intensifying the aqueous NMMO solution:

A sequential multi-stage evaporating system is adopted so as to intensify the aqueous NMMO solution, which has been purified in previous filtration process so that a condensed aqueous solution without NMMO solvent and a concentrated aqueous solution with NMMO solvent are respectively obtained; the sequential multi-stage evaporating system mainly comprises a first evaporating vessel with a first steam tank, a second evaporating vessel with a second steam tank and a third evaporating vessel with a third steam tank, wherein: the first evaporating vessel and the first steam tank are connected by a first steam inlet pipe while the first steam tank and the second evaporating vessel are connected by a first steam outlet pipe such that the first steam tank is connected to a first vacuum pump; by controlling the concentration of the recovered aqueous solution at the outlet of the first evaporating vessel in range of 10-20 wt % and the concentration of the recovered aqueous solution at the outlet of the second evaporating vessel in range of 22-38 wt % as well as feeding the steam evaporated by the recovered aqueous solution at the outlet of the third evaporating vessel back to the first evaporating vessel as supplementary steam source via the steam recovering pipe after it has been orderly processed by the third steam tank, separating tank and steam compressor, the overall recovered quantity of the concentrated aqueous NMMO solution under the same consumed quantity of the primary steam source can be substantially increased so that the goal of promoting recovery efficiency can be achieved; similarly, the condensed aqueous solution collected by the cold condensed water pipe from the first evaporating vessel, second evaporating vessel and third evaporating vessel can also be recovered for reusing in the rinse process of the Lyocell fiber production to remove the solvent and impurities attached on the raw filaments; and

The refinement in step 4 means for purifying the aqueous NMMO solution:

To perform the refinement step here, an oxidizer (namely oxidizing agent) is added into the concentrated aqueous NMMO solution processed by previous refinement process (step 3) so that the residual N-methylmorpholine (NMM) is oxidized into N-methylmorpholine-N-oxide (NMMO) via oxidation reaction by the oxidizer; after the oxidation reaction aforesaid, some residual oxidizer becomes redundant impurity, which should be completely removed anyhow; accordingly, a reducer (namely neutralizing agent) is added into the concentrated aqueous NMMO solution processed by previous oxidation reaction process aforesaid to neutralize the residual oxidizer via reduction reaction by the reducer so that a recovered aqueous NMMO solution of high purity is obtained; wherein, the final applied quantities for the oxidizer and the reducer are decided by the testing result of the concentrated aqueous NMMO solution processed by foregoing redox reaction (namely reduction reaction and oxidation reaction) via potentiometric titration.

In conclusion of disclosure heretofore, the method of recovering aqueous N-Methylmorpholine-N-Oxide solution used in production of Lyocell fiber of the present invention features novelties in following processing steps: bleach means for decoloring coloration in aqueous NMMO solution via alternate blow-mixing adsorption mode and static suspending adsorption mode reiteration; filtration means for purifying the activated carbon powder and impurities by two filtering stages of first coarse filtering stage and second fine filtering stage; concentration means for intensifying aqueous NMMO solution to obtain a condensed aqueous solution without NMMO solvent and a concentrated aqueous solution with NMMO solvent respectively by a sequential multi-stage evaporating system; and refinement means for purifying aqueous NMMO solution with promoting purity of concentrated aqueous solution to obtain required recovered aqueous solution by adding suitable agents in the redox reactions involved. Therefore, the recovering method of the present invention not only has completely recovered massive aqueous NMMO solution to substantially reduce wastes discharged to the environment but also has almost recovered the NMMO solvent in the aqueous NMMO solution to essentially reduce material cost in the production of Lyocell fiber. Thus, the features of the present invention not only save processing cost but also meet requirements of environment protection. Owing to streamlining and simplicity, the method not only has better competitiveness from promoted recovery cost, efficiency and quality but also meets regulations of environmental protection.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart illustrating a currently existing process for Lyocell fiber.

FIG. 2 is a flowchart illustrating a preferred exemplary embodiment in a recovering method of aqueous solution used in production of Lyocell fiber for compatible with the existing process of Lyocell fiber used in the present invention.

FIG. 3 is a schematic diagram of a preferred exemplary embodiment in a bleach step showing the addition of activated carbon powder in a bleaching tank in association with a disposition of an agitation blower for the present invention.

FIG. 4 is a schematic diagram of a preferred exemplary embodiment in a concentration step showing the application of a sequential multi-stage evaporating system, which is also known as serial stepwise pressure-descending multi-effect evaporator system, for the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Please refer to FIGS. 2 through 4, which are schematics illustrating a preferred exemplary embodiment in a recovering method of aqueous solution used in production of Lyocell fiber having massive aqueous NMMO solution with primary N-methylmorpholine-N-oxide (NMMO) dissolving solvent of low concentration for compatible with the existing process of Lyocell fiber used in the present invention. As shown in FIG. 1, the existing production process of Lyocell fiber mainly comprises four steps of blend, dissolution, spin and rinse. Wherein, other than the existing primary NMMO dissolving solvent used in dissolving cellulose, the aqueous NMMO solution also contains a bit of residual N-methylmorpholine (NMM), which is created by the decomposition of heating primary NMMO during chemical reaction in dissolving cellulose process by the primary NMMO dissolving solvent.

A preferred exemplary embodiment for the method of recovering aqueous N-methylmorpholine-N-oxide (NMMO) solution used in production of Lyocell fiber comprises following steps: bleach (step 101), filtration (step 102), condensation (step 103) and refinement (step 104), wherein:

The bleach in step 101 means for decoloring the aqueous NMMO solution 1.

Generally, the speed of chemical reaction is proportional to the temperature involved. Therefore, the dissolving efficiency of the dissolving process in production of Lyocell fiber is increased by heating. However, heating also incurs harmful coloration to contaminate pigment impurities into the aqueous NMMO solution 1. Accordingly, bleach process to the aqueous NMMO solution 1 becomes necessary in initial stage. Referring to FIG. 3, the aqueous NMMO solution 1 to be recovered is loaded into a bleaching tank 2, and activated carbon powder 3 featuring with good adsorbability and suspendability is added into the aqueous NMMO solution 1. The activated carbon powder 3 and the aqueous NMMO solution 1 are mixed together by using an agitation blower 4. The agitation blower 4 is intermittently energized so that an alternate blow-mixing adsorption mode and static suspending adsorption mode reiterates to have activated carbon powder 3 fully contacted with the aqueous NMMO solution 1 thoroughly in an energy-efficient manner. Thereby, the adsorption efficiency of the activated carbon powder 3 is essentially increased. Wherein, the blow-mixing adsorption mode means for blowing ambient air into the bleaching tank 2 by impeller rotation of the agitation blower 4 to have activated carbon powder 3 fully contacted with the aqueous NMMO solution 1 thoroughly to facilitate the speed of the bleach process while the static suspending adsorption mode means for keeping the aqueous NMMO solution 1 in stationary manner to let the aqueous NMMO solution 1 precipitate automatically to save related energy. By means of a timer switch 5 acting on the agitation blower 4, the blow-mixing adsorption mode and static suspending adsorption mode are intermittently alternated in reiterative fashion so that overall bleach efficiency is substantially increased. A duration ratio for the blow-mixing adsorption mode and static suspending adsorption mode is in range of 1:3-1:6. Moreover, total time in the bleach process of the aqueous NMMO solution 1 is not longer than 8 hours, while the total time in the bleach process of the aqueous NMMO solution 1 is set to 8 hours in this preferred exemplary embodiment. By means of blow-mixing adsorption mode, not only the activated carbon powder 3 can be fully contacted with the aqueous NMMO solution 1 thoroughly but also the speed of the bleach process can be facilitated while by means of static suspending adsorption mode, not only the processing energy can be essentially saved but also the efficiency of the bleach process can be enhanced. Preferably, the added dosage of the activated carbon powder 3 is in range of 0.05 wt %-0.10 wt % on the basis of total weight for the aqueous NMMO solution 1.

The filtration in step 102 means for purifying the aqueous NMMO solution 1.

Two filtering stages of first coarse filtering stage and successive second fine filtering stage (ultrafiltration UF) are orderly adopted so as to remove the activated carbon powder 3 and impurities from the aqueous NMMO solution 1, which has been decolored in previous bleach process, in this preferred exemplary embodiment. For the first coarse filtering stage, a filter cartridge having filtering material with pore size in range of 1 μm-100 μm (1 .mu.m-100 .mu.m) is used so that the activated carbon powder 3 and the impurities of large particle size appeared in previous bleach process can be removed. For the second fine filtering stage (ultrafiltration UF), a filter material with pore size in range of 0.01 μm-1 μm (0.01 .mu.m-1 .mu.m) is used so that the tiny impurities of small particle size can be removed. After orderly processes of foregoing first coarse filtering stage and successive second fine filtering stage (ultrafiltration UF), the cleanness of the aqueous NMMO solution 1 reaches that of a fresh NMMO solvent.

Wherein, a cartridge filter is used in the first coarse filtering stage. Preferably, in order to increase a speed of the coarse filtration, a filter aid is beforehand pre-coated over the surface of the cartridge filter, and the filter aid is also added into the bleached aqueous NMMO solution 1 with quantity in range of 0.03-0.05 wt %. Moreover, the filter aid is made from mixture of diatomaceous earth and cellulose with weight ratio of the diatomaceous earth to the cellulose is 4:1 preferably. By means of foregoing double uses of the filter aid, the first coarse filtering stage not only prevents the activated carbon powder 3 from accumulating on the surface thereof in hindering the filtering speed but also regularly maintains filtering effect of high performance without decay owing to valid filtering area is increased.

It should be noted that some filtering dregs resulting from the first coarse filtering stage could be centrifugally dehydrated after completion of the first coarse filtering stage, wherein the filtering dregs contain residual filter aid and a larger quantity of residual activated carbon powder 3, which is mostly accumulated on the surface portion. After the filtering dregs is scraped off, the residual filter aid therein can be recovered and reused in the first coarse filtering stage step.

The concentration in step 103 means for intensifying the aqueous NMMO solution 1. A sequential multi-stage evaporating system is adopted so as to intensify the aqueous NMMO solution 1, which has been purified in previous filtration process, in this preferred exemplary embodiment, so that a condensed aqueous solution without NMMO solvent and a concentrated aqueous solution with NMMO solvent are respectively obtained. Practically, the sequential multi-stage evaporating system is a serial stepwise pressure-descending multi-effect evaporator system. The evaporating effectiveness of the sequential multi-stage evaporating system depends on the number of stages. In the preferred exemplary embodiment, the number of stages is 3 to have optimal trade-off between the evaporating effectiveness thereof and facility cost thereof although the number of stages is arbitrarily selected, and the operating mode is in series instead of in parallel. Therefore, the serial stepwise pressure-descending multi-effect evaporator system in the preferred exemplary embodiment is actually a sequential tri-stage evaporating system.

The sequential multi-stage evaporating system mainly comprises a first evaporating vessel 10 with a first steam tank 11, a second evaporating vessel 20 with a second steam tank 21 and a third evaporating vessel 30 with a third steam tank 31, wherein:

The first evaporating vessel 10 and the first steam tank 11 are connected by a first steam inlet pipe C1 while the first steam tank 11 and the second evaporating vessel 20 are connected by a first steam outlet pipe 12 such that the first steam tank 11 is connected to a first vacuum pump 13; Moreover, the first evaporating vessel 10 and second evaporating vessel 20 are connected by a first solution recovering pipe 14, on which a first concentration meter 15 and a first suction pump 16 are disposed respectively;

The second evaporating vessel 20 and the second steam tank 21 are connected by a second steam inlet pipe C2 while the second steam tank 21 and the third evaporating vessel 30 are connected by a second steam outlet pipe 22 such that the second steam tank 21 is connected to a second vacuum pump 23; Moreover, the second evaporating vessel 20 and third evaporating vessel 30 are connected by a second solution recovering pip 24, on which a second concentration meter 25 and a second suction pump 26 are disposed respectively;

The third evaporating vessel 30 and the third steam tank 31 are connected by a third steam inlet pipe C3 while the third steam tank 31 is connected to a third vacuum pump 32 and a third steam outlet pipe 33, which is further connected to a separating tank 40 for steam and aqueous solution; Moreover, the third evaporating vessel 30 and a second suction pump 70 are connected by a third solution recovering pipe 34, on which a third concentration meter 35 and a third suction pump 36 are disposed respectively;

Wherein, the aqueous solution outlet of the separating tank 40 is connected to the third solution recovering pipe 34 while the steam outlet of the separating tank 40 is connected to a mechanical steam compressor 41, which is further connected to a first evaporating vessel 10 via a steam recovering pipe 42.

For the first evaporating vessel 10, other than the recovered steam being fed via the steam recovering pipe 42, a primary steam from a steam boiler (not shown) is supplied via an input pipe for steam 60, and an aqueous NMMO solution 1, which has been purified in previous filtration process, is also supplied by an input pipe for aqueous NMMO solution 50, which orderly passes through the third steam tank 31, second steam tank 21 and first evaporating vessel 11 as well as a heat exchanger 6 and a input pump 51 for aqueous NMMO solution 1.

By controlling the concentration of the recovered aqueous solution at the outlet of the first evaporating vessel 10 in range of 10-20 wt % and the concentration of the recovered aqueous solution at the outlet of the second evaporating vessel 20 in range of 22-38 wt % as well as feeding the steam evaporated by the recovered aqueous solution at the outlet of the third evaporating vessel 30 back to the first evaporating vessel 10 as supplementary steam source via the steam recovering pipe 42 after it has been orderly processed by the third steam tank 31, separating tank 40 and steam compressor 41, the overall recovered quantity of the concentrated aqueous NMMO solution under the same consumed quantity of the primary steam source can be substantially increased so that the goal of promoting recovery efficiency can be achieved; Similarly, the condensed aqueous solution collected by the cold condensed water pipe 80 from the first evaporating vessel 10, second evaporating vessel 20 and third evaporating vessel 30 can also be recovered for reusing in the rinse process of the Lyocell fiber production to remove the solvent and impurities attached on the raw filaments; Thus, the aqueous NMMO solution 1 generated by the whole Lyocell fiber production process can be completely recovered to meet the legislation requirements of the environmental protection.

In order to prove the enhancement of the recovery efficiency for the sequential multi-stage evaporating system aforesaid, following real embodiment examples and comparative examples are experimented to illustrate the resultant recovery efficiency. The recovery efficiency of the NMMO is calculated by the following formula.

$\underset{\_}{\mathrm{RVEff}}\mathrm{of}\mathrm{NMMO}=\frac{\mathrm{CnAR}\%\times \mathrm{OQAR}}{\mathrm{CnBR}\%\times \mathrm{IQBR}}\times 100\%$

Where, RVEff denotes to “recovery efficiency”;

CnBR denotes to “Concentration before Recovery”;

CnAR denotes to “Concentration after Recovery”;

IQBR denotes to “Inlet Quantity before Recovery”; and

OQAR denotes to “Outlet Quantity after Recovery”.

EMBODIMENT EXAMPLE 1

Please refer to FIG. 3, which is a schematic diagram of a preferred exemplary embodiment in a bleach step showing the addition of activated carbon powder therein in association with a disposition of an agitation blower for the present invention. After previous processes of bleach (step 101) and filtration (step 102), the collected aqueous NMMO solution 1 of the Lyocell fiber will contain NMMO in 3.92 wt % weight percentage. Then, the collected aqueous NMMO solution 1 is input into the first evaporating vessel 10 via input pipe 50 by orderly passing third steam tank 31, second steam tank 21 and first steam tank 11 as well as heat exchanger 6 and input pump 51. Because the aqueous NMMO solution 1 has orderly passed third steam tank 31, second steam tank 21 and first steam tank 11, the steam evaporated by the recovered aqueous solution at the outlets of the first evaporating vessel 10, second evaporating vessel 20 and third evaporating vessel 30 can be fed back to the first evaporating vessel 10 as supplementary steam source via the steam recovering pipe 42 after it has been orderly processed by the first steam tank 11, second steam tank 21 and third steam tank 31 as well as separating tank 40 and steam compressor 41 so that the aqueous NMMO solution 1 can be heated up to the desired temperature before it enters the first evaporating vessel 10.

For the first dehydration in the first evaporating vessel 10, the process is performed by following operating parameters.

Heating medium: By inputting steam generated by a steam boiler (not shown) to serve as primary steam source into the first evaporating vessel 10, it evaporates the aqueous NMMO solution 1 therein to operate first dehydration.

Outlet Concentration: 12 wt % for the recovered aqueous NMMO solution 1 of the first evaporating vessel 10 (measured by first concentration meter 15).

Degree of Vacuum: 600 mmHg (by acting first vacuum pump 13 on the first steam tank 11).

Operating Temperature: 70.0-73.0° C. (70.0-73.0 degree of Celsius).

For the second dehydration in the second evaporating vessel 20, the process is performed by following operating parameters.

Heating medium: By inputting steam evaporated by aqueous NMMO solution 1 of the first evaporating vessel 10 to serve as vaporized steam source into the second evaporating vessel 20 orderly via first steam inlet pipe C1, first steam tank 11 and first steam outlet pipe 12, it evaporates the aqueous NMMO solution 1 therein to operate second dehydration.

Outlet Concentration: 28 wt % for the recovered aqueous NMMO solution 1 of the second evaporating vessel 20 (measured by second concentration meter 25).

Degree of Vacuum: 630 mmHg (by acting second vacuum pump 23 on the second steam tank 21).

Operating Temperature: 61.0-62.5° C. (61.0-62.5 degree of Celsius).

For the third dehydration in the third evaporating vessel 30, the process is performed by following operating parameters.

Heating medium: By inputting steam evaporated by aqueous NMMO solution 1 of the second evaporating vessel 20 to serve as vaporized steam source into the third evaporating vessel 30 orderly via second steam inlet pipe C2, second steam tank 21 and second steam outlet pipe 22, it evaporates the aqueous NMMO solution 1 therein to operate third dehydration.

Outlet Concentration: 50.05 wt % for the recovered aqueous NMMO solution 1 of the third evaporating vessel 30 (measured by third concentration meter 35).

Degree of Vacuum: 650 mmHg (by acting third vacuum pump 33 on the third steam tank 31).

Operating Temperature: 51.8-52.2° C. (51.8-52.2 degree of Celsius).

Besides, because the aqueous NMMO solution 1 has orderly passed third steam tank 31, second steam tank 21 and first steam tank 11, the steam evaporated by the recovered aqueous solution at the outlets of the first evaporating vessel 10, second evaporating vessel 20 and third evaporating vessel 30 can be fed back to the first evaporating vessel 10 as supplementary steam source via the steam recovering pipe 42 after it has been orderly processed by the first steam tank 11, second steam tank 21 and third steam tank 31 as well as separating tank 40 and steam compressor 41 so that the aqueous NMMO solution 1 can be heated up to the desired temperature before it enters the first evaporating vessel 10. Thus, the supplementary steam source recovered from the third evaporating vessel 30 can achieve effect in preheating the aqueous NMMO solution 1 in the first evaporating vessel 10.

All foregoing operating parameters of the preferred exemplary embodiment example I are collected and tabulated in “Extracted embodiment example 1 from Table-1” shown as below.

Moreover, the outlet quantity after recovery from the third evaporating vessel 30 is 925.5 ton(s) while the inlet quantity before recovery to the first evaporating vessel 10 is 11835 ton(s).

In summary, the following key operating parameters are recapitulated.

The Inlet Quantity before Recovery is 11,835 ton(s).

The Outlet Quantity after Recovery is 925.5 ton(s).

The Concentration before Recovery is 3.92%.

The Concentration after Recovery is 50.05%.

According to foregoing key operating parameters, the recovery efficiency (RVEff) is calculated by the following formula predetermined.

$\underset{\_}{\mathrm{RVEff}}\mathrm{of}\mathrm{NMMO}=\frac{\mathrm{CnAR}\%\times \mathrm{OQAR}}{\mathrm{CnBR}\%\times \mathrm{IQBR}}\times 100\%$ $\frac{50.05\%\times 925.5=46321.3\%}{3.92\%\times 11835=46393.2\%}\times 100\%$ $\underset{\_}{\mathrm{RVEff}}\mathrm{of}\mathrm{NMMO}=99.8\%$

Basing on the values for related parameters in the “Extracted embodiment example 1 from Table-2” shown as below, the recovery efficiency (RVEff) of NMMO obtained is 99.8%.

[Extracted embodiment example 1 from Table-1]
	1st dehydration	2nd dehydration	3rd dehydration
	1st evaporating	2nd evaporating	3rd evaporating
	vessel	vessel	vessel
H.M.	primary steam	vaporized steam from	vaporized steam from
	from steam boiler	1st steam tank 11	2nd steam tank 21
E.E.	OC	DV	OT	OC	DV	OT	OC	DV	OT
	wt	mm	° C.	wt	mm	° C.	wt	mm	° C.
	%	Hg		%	Hg		%	Hg
Em.	12	600	70.0-	28	630	61.0-	50.05	650	51.8-
1			73.0			62.5			52.2
R.S.	supplementary	CtAS with	CdAS without
	steam	NMMO solvent	NMMO solvent
From	steam recovering	solution recovering	cold condensed
	pipe 42	pipes 14, 24 and 34	water pipe 80
To	1st evaporating	storage tank 70	Rinse step
	vessel 10
Denotation
H.M. denotes to “Heating Media”.
E.E. denotes to “Embodiment Example”.
Em denotes to “Example”.
OC denotes to “Outlet Concentration”.
DV denotes to “Degree of Vacuum”.
OT denotes to “Operating Temperature”.
R.S. denotes to “Recovered Substance”.
CtAS denotes to “Concentrated Aqueous Solution”.
CdAS denotes to “Condensed Aqueous Solution”.

[Extracted embodiment example 1 from Table-2]
Embodiment	IQBR	CnBR	OQAR	CnAR	RVEff
Example	ton(s)	wt %	ton(s)	wt %	%
Example 1	11,835	3.92	925.5	50.05	99.80
Denotation
IQBR denotes to “Inlet Quantity before Recovery”.
CnBR denotes to “Concentration before Recovery”.
OQAR denotes to “Outlet Quantity after Recovery”.
CnAR denotes to “Concentration after Recovery”.
RVEff denotes to “Recovery Efficiency”.

EMBODIMENT EXAMPLE 2-9

The processing procedure of bleach step for preferred exemplary embodiment example 2-9 in the present invention is the same as that for preferred exemplary embodiment example 1 but with following differences.

Outlet Concentration: In range of 10-20 wt % for the recovered aqueous NMMO solution 1 of the first evaporating vessel 10 (measured by first concentration meter 15).

Outlet Concentration: In range of 22-38 wt % for the recovered aqueous NMMO solution 1 of the second evaporating vessel 20 (measured by second concentration meter 25).

For other operating parameters such as outlet concentration (OC), degree of vacuum (DV) and operating temperature (OT), they are listed in the Table-1. Moreover, for other key operating parameters such as inlet quantity before recovery (IQBR), outlet quantity after recovery (OQAR), concentration before recovery (CnBR) and concentration after recovery (CnAR), they are also listed in the Table-2. Thus, the recovery efficiency (RVEff) is calculated by the same formula aforesaid in accordance with foregoing key operating parameters listed in the Table-2.

COMPARATIVE EXAMPLE 1-9

The processing procedure of bleach step for preferred exemplary comparative example 1-9 in the present invention is the same as that for preferred exemplary embodiment example 1 but with following differences.

Outlet Concentration: In range of 10-20 wt % for the recovered aqueous NMMO solution 1 of the first evaporating vessel 10 (measured by first concentration meter 15).

Outlet Concentration: In range of 22-38 wt % for the recovered aqueous NMMO solution 1 of the second evaporating vessel 20 (measured by second concentration meter 25).

For other operating parameters such as outlet concentration (OC), degree of vacuum (DV) and operating temperature (OT), they are listed in the Table-1. Moreover, for other key operating parameters such as inlet quantity before recovery (IQBR), outlet quantity after recovery (OQAR), concentration before recovery (CnBR) and concentration after recovery (CnAR), they are also listed in the Table-2. Thus, the recovery efficiency (RVEff) is calculated by the same formula aforesaid in accordance with foregoing key operating parameters listed in the Table-2.

[Result]:

Referring to Table-2, with processing condition of the Outlet Concentration being in range of 10-20 wt % for the recovered aqueous NMMO solution 1 of the first evaporating vessel 10 (measured by first concentration meter 15) and the Outlet Concentration: being in range of 22-38 wt % for the recovered aqueous NMMO solution 1 of the second evaporating vessel 20 (measured by second concentration meter 25), each recovery efficiency (RVEff) for all the preferred exemplary embodiment example 1-9 of the present invention listed in upper portion thereof is better than that corresponding in all the preferred exemplary comparative example 1-9 of the present invention listed in lower portion thereof.

For convenience, the Table-1 and Table-2 are listed in other sheet as attached.

Obviously, with processing condition of the Outlet Concentration being in range of 10-20 wt % for the recovered aqueous NMMO solution 1 of the first evaporating vessel 10 (measured by first concentration meter 15) and the Outlet Concentration: being in range of 22-38 wt % for the recovered aqueous NMMO solution 1 of the second evaporating vessel 20 (measured by second concentration meter 25) as well as feeding the steam evaporated by the recovered aqueous solution at the outlet of the third evaporating vessel 30 back to the first evaporating vessel 10 as supplementary steam source via the steam recovering pipe 42 after it has been orderly processed by the third steam tank 31, separating tank 40 and steam compressor 41, the overall recovered quantity of the concentrated aqueous NMMO solution under the same consumed quantity of the primary steam source can be substantially increased so that the goal of promoting recovery efficiency can be achieved.

The refinement in step 104 means for purifying the aqueous NMMO solution 1.

Before the concentrated aqueous NMMO solution is subjected to the refinement step, a bit of residual N-methylmorpholine (NMM) that arises from decomposition of NMMO, which is caused by heating during the dissolution step in the production of Lyocell fiber. In this preferred exemplary embodiment, the quantity of residual NMM in the concentrated aqueous NMMO solution is in range of 0.1-0.3 wt %. To perform the refinement step here, an oxidizer (namely oxidizing agent) is added into the concentrated aqueous NMMO solution processed by previous refinement process (step 103) so that the residual N-methylmorpholine (NMM) is oxidized into N-methylmorpholine-N-oxide (NMMO) via oxidation reaction by the oxidizer under reaction temperature being 80±2° C. (80.+/−0.2.degree of Celsius). After the oxidation reaction aforesaid, some residual oxidizer becomes redundant impurity, which should be completely removed anyhow. Accordingly, a reducer (namely neutralizing agent) is added into the concentrated aqueous NMMO solution processed by previous oxidation reaction process aforesaid to neutralize the residual oxidizer via reduction reaction by the reducer to a quantity in range less than 0.06 wt % so that a recovered aqueous NMMO solution of high purity is obtained. Wherein, the oxidizer applied is H₂O₂ (hydrogen peroxide), and the reducer applied is N₂H₄H₂O (hydrazine hydrate) in this preferred exemplary embodiment. Moreover, the final applied quantities for the oxidizer and the reducer are decided by the testing result of the concentrated aqueous NMMO solution processed by foregoing redox reaction (namely reduction reaction and oxidation reaction) via potentiometric titration in this preferred exemplary embodiment.

Though a small quantity of NMM with concentration less than 0.06 wt % already contained in a fresh NMMO solvent, some more NMM may be created due to heating decomposition from a small portion of fresh NMMO solvent during production of Lyocell fiber. If the concentrated aqueous NMMO solution is not subjected to the refinement step here for processing foregoing overall NMM, the concentrated aqueous NMMO solution has a bad ability in dissolving cellulose when it is recovered for reuse so that it may not only easily cause adverse affect for spinning efficiency such as obstruction in spinneret orifices and breakages of spinning filaments and the like during the spin step in production of Lyocell fiber but also incur deteriorating physical properties for the fabrics thereof such as declined tenacity. Accordingly, in order to avoid the quality of Lyocell fiber fabrics from being detrimentally affected by the recovered aqueous NMMO solution in the present invention so as to have a satisfactory resultant quality thereof, the refinement step becomes critically imperative to oxidize existing residual NMM contained in the concentrated aqueous NMMO solution into NMMO other than the achievement for the preset concentration of the concentrated aqueous NMMO solution so that not only the purity of the NMMO is enhanced but also the wastage of the NMMO is reduced.

Besides, in this preferred exemplary embodiment, the reason for presetting reaction temperature of the N-methylmorpholine (NMM) to 80±2° C. (80.+/−0.2.degree of Celsius) is on the basis of following considerations. If the reaction temperature is excessively higher than the presetting reaction temperature, NMM and oxidizer H₂O₂ (hydrogen peroxide) in the concentrated aqueous NMMO solution can be easily decomposed and volatilized so that energy is wasted incurred by the violent status of the redox reaction. Contrarily, if the reaction temperature is excessively lower than the presetting reaction temperature, the refinement efficiency is decrease incurred by the invalid status of the redox reaction.

Wherein, the chemical reaction equation for the oxidation reaction of NMM by the oxidizer H₂O₂ (hydrogen peroxide) in the concentrated aqueous NMMO solution is shown as follows:

Reductant	Oxidation	Product
C5H11NO + H2O2	→	C5H11NO2 + H2O
The oxidation reaction takes place under presetting reaction temperature of 80 ± 2° C. (80 .+/−. 2 .degree of Celsius) for 2 hours.

Where, the “C₅H₁₁NO” denotes the molecular formula for the N-methylmorpholine (NMM); the “C₅H₁₁NO₂” denotes the molecular formula for the N-methylmorpholine-N-oxide (NMMO); and the “H₂O₂” denotes the molecular formula for the oxidizer (hydrogen peroxide).

Whereas, the chemical reaction equation for the reduction reaction of residual oxidizer H₂O₂ (hydrogen peroxide) by the reducer N₂H₄H₂O (hydrazine hydrate) in the concentrated aqueous NMMO solution is shown as follows:

Oxidant	Reduction	Product
N2H4•H2O + 2H2O	→	5H2O + N2
The reduction reaction also takes place under presetting reaction temperature of 80 ± 2° C. (80 .+/−. 2 .degree of Celsius) for 2 hours.

Where, the “N₂H₄.H₂O” denotes the molecular formula for the reducer (hydrazine hydrate).

By means of foregoing chemical reaction equations, the content of NMM in the concentrated aqueous NMMO solution can be firstly measured, then the required adding quantity of oxidizer H₂O₂ (hydrogen peroxide) can be roughly calculated by the reactive molar ratio of NMM to oxidizer H₂O₂ (hydrogen peroxide) in the first chemical reaction equation of the oxidation reaction subsequently. However, in order to ensure that almost all of NMM can be completely oxidized, an actual adding quantity of oxidizer H₂O₂ (hydrogen peroxide) to the concentrated aqueous NMMO solution is more than the forgoing calculated adding quantity with a bit of extra residual oxidizer H₂O₂ (hydrogen peroxide) remained after the oxidation reaction. Successively, after the oxidation reaction of NMM, the reducer N₂H₄.H₂O (hydrazine hydrate) is added to neutralize the extra residual oxidizer H₂O₂ (hydrogen peroxide). With foregoing two chemical reaction equations, even though adequate quantities of the oxidizer H₂O₂ (hydrogen peroxide) and the reducer N₂H₄.H₂O (hydrazine hydrate) are added to the concentrated aqueous NMMO solution, the final products still include H₂O and N₂ other than the NMMO after having foregoing two chemical reaction equations finished. Wherein, N₂ can be directly dispersed into air, and H₂O becomes a useful portion of the recovered aqueous NMMO solution. Thus, no impurities and redundant side products are created in the recovered aqueous NMMO solution during the refinement step so that the concentrated aqueous NMMO solution becomes not only high purity of recovered aqueous NMMO solution and but also high purity of fabrics from Lyocell fiber without any bad effect.

In conclusion of disclosure heretofore, the method of recovering aqueous N-Methylmorpholine-N-Oxide solution used in production of Lyocell fiber of the present invention features following novelties.

• •  Bleach in step 101 means for decoloring harmful coloration with contaminate pigment impurities in the aqueous NMMO solution 1 via alternate blow-mixing adsorption mode and static suspending adsorption mode reiteration by activated carbon powder 3.
  •  Filtration in step 102 means for purifying the activated carbon powder 3 and impurities from the aqueous NMMO solution 1 by means of two filtering stages of first coarse filtering stage and successive second fine filtering stage (ultrafiltration UF) being orderly adopted.
  •  Concentration in step 103 means for intensifying the aqueous NMMO solution 1 to obtain a desired condensed aqueous solution without NMMO solvent and an expected concentrated aqueous solution with NMMO solvent respectively by a sequential multi-stage evaporating system of high-performance.
  •  Refinement in step 104 means for purifying the aqueous NMMO solution 1 with promoting purity of the concentrated aqueous solution to obtain required recovered aqueous solution by adding suitable agents in the redox reactions involved so that not only the aqueous NMMO solution 1 but also massive water can be recovered and reused as processing materials in the production of Lyocell fiber.

Therefore, the recovering method of the present invention not only has completely recovered massive aqueous NMMO solution 1 to substantially reduce wastes discharged to the environment but also has almost recovered the NMMO solvent in the aqueous NMMO solution 1 to essentially reduce material cost in the production of Lyocell fiber. Thus, the features of the present invention not only save processing cost but also meet requirements of environment protection.

Claims

1. A method of recovering aqueous N-Methylmorpholine-N-Oxide (NMMO) solution used in production of Lyocell fiber comprises steps of bleach (step 1), filtration (step 2), condensation (step 3) and refinement (step 4), wherein:

The bleach in step 1 means for decoloring the aqueous NMMO solution:

Firstly, the aqueous NMMO solution to be recovered is loaded into a bleaching tank, and activated carbon powder featuring with good adsorbability and suspendability is added into the aqueous NMMO solution therein; secondly, the activated carbon powder and the aqueous NMMO solution are mixed together by using an agitation blower; and finally, the agitation blower is intermittently energized so that an alternate blow-mixing adsorption mode and static suspending adsorption mode reiterates to have activated carbon powder fully contacted with the aqueous NMMO solution thoroughly in an energy-efficient manner; thereby, the adsorption efficiency of the activated carbon powder is essentially increased; preferably, the added dosage of the activated carbon powder is in range of 0.05 wt %-0.10 wt % on the basis of total weight for the aqueous NMMO solution; wherein, the blow-mixing adsorption mode means for blowing ambient air into the bleaching tank by impeller rotation of the agitation blower to have activated carbon powder fully contacted with the aqueous NMMO solution thoroughly to facilitate the speed of the bleach process while the static suspending adsorption mode means for keeping the aqueous NMMO solution in stationary manner to let the aqueous NMMO solution precipitate automatically to save related energy; moreover, by means of a timer switch acting on the agitation blower, the blow-mixing adsorption mode and static suspending adsorption mode are intermittently alternated in reiterative fashion so that overall bleach efficiency is substantially increased; a duration ratio for the blow-mixing adsorption mode and static suspending adsorption mode is in range of 1:3-1:6; besides, total time in the bleach process of the aqueous NMMO solution is not longer than 8 hours; thus, by means of blow-mixing adsorption mode, not only the activated carbon powder can be fully contacted with the aqueous NMMO solution thoroughly but also the speed of the bleach process can be facilitated while by means of static suspending adsorption mode, not only the processing energy can be essentially saved but also the efficiency of the bleach process can be enhanced;

The filtration in step 2 means for purifying the aqueous NMMO solution:

Two filtering stages of first coarse filtering stage and successive second fine filtering stage (ultrafiltration UF) are orderly adopted so as to remove the activated carbon powder and impurities from the aqueous NMMO solution 1, which has been decolored in previous bleach process; for the first coarse filtering stage, a filter cartridge having filtering material with pore size in range of 1 μm-100 μm (1.mu-100.mu.m) is used so that the activated carbon powder and the impurities of large particle size appeared in previous bleach process can be removed; for the second fine filtering stage (ultrafiltration UF), a filter material with pore size in range of 0.01 μm-1 μm (0.01.mu.m-1.mu.m) is used so that the tiny impurities of small particle size can be removed; wherein, the cartridge filter used in the first coarse filtering stage, over the surface thereof is beforehand pre-coated a filter aid, which is made from mixture of diatomaceous earth and cellulose with weight ratio of the diatomaceous earth to the cellulose is 4:1 preferably so that it prevents the activated carbon powder from accumulating on the surface thereof in hindering the filtering speed; besides, the filter aid is also added into the bleached aqueous NMMO solution with quantity in range of 0.03-0.05 wt % to increase the valid filtering area; wherein, some filtering dregs, which contain residual filter aid and a larger quantity of residual activated carbon powder mostly accumulated on the surface portion, are created after the first coarse filtering stage; after the filtering dregs is scraped off, the residual filter aid therein can be recovered and reused in the first coarse filtering stage step;

The concentration in step 3 means for intensifying the aqueous NMMO solution:

A sequential multi-stage evaporating system is adopted so as to intensify the aqueous NMMO solution, which has been purified in previous filtration process so that a condensed aqueous solution without NMMO solvent and a concentrated aqueous solution with NMMO solvent are respectively obtained; the sequential multi-stage evaporating system mainly comprises a first evaporating vessel with a first steam tank, a second evaporating vessel with a second steam tank and a third evaporating vessel with a third steam tank, wherein: the first evaporating vessel and the first steam tank are connected by a first steam inlet pipe while the first steam tank and the second evaporating vessel are connected by a first steam outlet pipe such that the first steam tank is connected to a first vacuum pump; moreover, the first evaporating vessel and second evaporating vessel are connected by a first solution recovering pipe, on which a first concentration meter and a first suction pump are disposed respectively; the second evaporating vessel and the second steam tank are connected by a second steam inlet pipe while the second steam tank and the third evaporating vessel are connected by a second steam outlet pipe such that the second steam tank is connected to a second vacuum pump; moreover, the second evaporating vessel and third evaporating vessel are connected by a second solution recovering pip, on which a second concentration meter and a second suction pump are disposed respectively; the third evaporating vessel and the third steam tank are connected by a third steam inlet pipe while the third steam tank is connected to a third vacuum pump and a third steam outlet pipe, which is further connected to a separating tank for steam and aqueous solution; moreover, the third evaporating vessel and a second suction pump are connected by a third solution recovering pipe, on which a third concentration meter and a third suction pump are disposed respectively; wherein, the aqueous solution outlet of the separating tank is connected to the third solution recovering pipe while the steam outlet of the separating tank is connected to a mechanical steam compressor, which is further connected to a first evaporating vessel via a steam recovering pipe; for the first evaporating vessel, other than the recovered steam being fed via the steam recovering pipe, a primary steam from a steam boiler is supplied via an input pipe for steam, and an aqueous NMMO solution, which has been purified in previous filtration process, is also supplied by an input pipe for aqueous NMMO solution, which orderly passes through the third steam tank, second steam tank and first evaporating vessel as well as a heat exchanger and a input pump for aqueous NMMO solution; by controlling the concentration of the recovered aqueous solution at the outlet of the first evaporating vessel in range of 10-20 wt % and the concentration of the recovered aqueous solution at the outlet of the second evaporating vessel in range of 22-38 wt % as well as feeding the steam evaporated by the recovered aqueous solution at the outlet of the third evaporating vessel back to the first evaporating vessel as supplementary steam source via the steam recovering pipe after it has been orderly processed by the third steam tank, separating tank and steam compressor, the overall recovered quantity of the concentrated aqueous NMMO solution under the same consumed quantity of the primary steam source can be substantially increased so that the goal of promoting recovery efficiency can be achieved; similarly, the condensed aqueous solution collected by the cold condensed water pipe from the first evaporating vessel, second evaporating vessel and third evaporating vessel can also be recovered for reusing in the rinse process of the Lyocell fiber production to remove the solvent and impurities attached on the raw filaments; and

The refinement in step 4 means for purifying the aqueous NMMO solution:

Before the concentrated aqueous NMMO solution is subjected to the refinement step, a bit of residual N-methylmorpholine (NMM) that arises from decomposition of NMMO, which is caused by heating during the dissolution step in the production of Lyocell fiber; the quantity of residual NMM in the concentrated aqueous NMMO solution is in range of 0.1-0.3 wt %; to perform the refinement step here, an oxidizer (namely oxidizing agent) is added into the concentrated aqueous NMMO solution processed by previous refinement process (step 3) so that the residual N-methylmorpholine (NMM) is oxidized into N-methylmorpholine-N-oxide (NMMO) via oxidation reaction by the oxidizer under reaction temperature being 80±2° C. (80.+/−0.2.degree of Celsius); after the oxidation reaction aforesaid, some residual oxidizer becomes redundant impurity, which should be completely removed anyhow; accordingly, a reducer (namely neutralizing agent) is added into the concentrated aqueous NMMO solution processed by previous oxidation reaction process aforesaid to neutralize the residual oxidizer via reduction reaction by the reducer to an quantity in range less than 0.06 wt % so that a recovered aqueous NMMO solution of high purity is obtained; wherein, the oxidizer applied is H2O2 (hydrogen peroxide), and the reducer applied is N2H4H2O (hydrazine hydrate); moreover, the final applied quantities for the oxidizer and the reducer are decided by the testing result of the concentrated aqueous NMMO solution processed by foregoing redox reaction (namely reduction reaction and oxidation reaction) via potentiometric titration.