Method for purifying waste solvent

Abstract

Provided is a method for purifying a waste solvent by removing carbon dioxide contained in a waste solvent derived from supercritical waste liquid generated after supercritical drying by a decompression process, and removing ammonia by a multi-stage distillation process to obtain a solvent of high purity, which can be reused in producing silica aerogel or a silica aerogel blanket.

Claims

1. A method for purifying a waste solvent, the method comprising: decompressing a waste solvent containing ammonia and carbon dioxide, wherein the decompressing is performed at a temperature of 30-50° C. and a pressure of 100-200 mbar to remove carbon dioxide; introducing the decompressed waste solvent into a distillation column; and distilling the introduced decompressed waste solvent to yield a purified waste solvent, wherein the distillation is performed by a multi-stage distillation method.

2. The method of claim 1, wherein an amount of ammonia contained in the decompressed waste solvent introduced into the distillation column is 400-1000 ppm, and an amount of ammonia contained in the purified waste solvent is 50 ppm or less.

3. The method of claim 1, wherein an amount of ammonia contained in the purified waste solvent is 30 ppm or less.

4. The method of claim 1, wherein the waste solvent is derived from supercritical waste liquid generated after a supercritical drying process.

5. The method of claim 1, wherein a pH of the waste solvent after the decompression is 9.0-10.0.

6. The method of claim 1, wherein the distillation is performed by a multi-stage distillation method involving 20-30 stages.

7. The method of claim 1, wherein a ratio of a discharge rate by weight of steam discharged from the distillation column with respect to an inflow rate by weight of the waste solvent introduced into the distillation column is 0.01-0.07.

8. The method of claim 1, wherein a reboiler output based on the inflow rate of 1 kg/hr of the waste solvent introduced into the distillation column is 200-700 W.

9. The method of claim 1, wherein the distilling has a reflux ratio of 3-10.

10. The method of claim 1, wherein a recovery rate of the purified solvent is 93% or greater.

11. The method of claim 1, wherein no salt is formed in a cooling column during the distilling.

Description

EXAMPLE 1

(1) A mixed solution prepared by mixing tetraethyl orthosilicate (TEOS) and ethanol at a weight ratio of 3:1 was added with a solution of hydrochloric acid diluted with water (concentration=0.15 wt %) such that the pH of the mixed solution was 1 and then mixed to prepare silica sol (silica content in silica sol=4 wt %). Next, the silica sol was added with an ammonia catalyst at a volume ratio of 100:0.5 (silica sol:ammonia catalyst), deposited in glass fiber, and gelled to prepare a silica wet gel composite.

(2) Thereafter, the silica wet gel composite was immersed in ethanol, and then aged for 2 hours in an oven of 70° C. The aged silica wet gel composite was surface modified for 5 hours at 70° C. using a surface modification solution which is a mixture of ethanol and HMDS (volume ratio of ethanol:HMDS: 1:19).

(3) Subsequently, the surface-modified silica wet gel composite was placed in an extractor in supercritical equipment to perform supercritical drying thereon using supercritical CO.sub.2, and was dried at 150° C. and atmospheric pressure for 1 hour to produce a silica aerogel blanket.

(4) After performing decompression at 40° C. and 170 mbar on waste ethanol derived from supercritical waste liquid generated after the supercritical drying, the waste ethanol was purified by performing multi-stage distillation under conditions that the reboiler output is 374 W and the reflux ratio is 4 based on the inflow rate of 1 kg/hr of the waste ethanol introduced into the distillation column.

EXAMPLE 2

(5) Waste ethanol was purified in the same manner as in Example 1 except that the distillation was performed under the conditions set forth in Table 1.

Comparative Examples 1 to 4

(6) Waste ethanol was purified in the same manner as in Example 1 except that the decompression process was not performed and the distillation was performed under the specific conditions set forth in Table 1.

Comparative Example 5

(7) Waste ethanol was purified in the same manner as in Example 1 except that the decompression process was not performed, a single stage distillation process was performed instead of the multi-stage distillation process, and the distillation was performed under the specific conditions set forth in Table 1.

Experimental Example

(8) Whether or not the decompression process was performed, the pH of the waste ethanol introduced into the distillation column, the concentration of residual ammonia after the distillation process, the rate of ethanol recovery, and the generation of salt in a cooling column were measured and the results are shown in Table 1.

(9) TABLE-US-00001 TABLE 1 Comparative Comparative Comparative Comparative Comparative Example 1 Example 2 Example 1 Example 2 Example 3 Example 4 Example 5 Number of 25 25 25 25 25 25 1 distillation stages (stage) Whether or not ◯ ◯ X X X X X decompression process was performed pH of waste ethanol 9.5 9.5 8.2 8.2 8.2 8.2 8.2 introduced into distillation column Concentration (ppm) 420 420 420 420 420 420 420 of ammonia contained in waste ethanol Reboiler output (A) 2.0 2.3 1.7 2.0 2.0 2.3 — Reflux ratio 4 4 4 4 8 5 — Discharge rate of 0.032 0.051 0.02 0.03 0.017 0.069 0.076 steam based on inflow rate of waste ethanol Concentration (ppm) <20 <20 <20 <20 <20 <20 108 of residual ammonia Recovery rate (%) 96.8 94.9 98 97 98.3 93.1 92.4 of ethanol Generation of salt No No Yes Yes Yes Yes Yes in cooling column

(10) As shown in Table 1, in Examples 1 and 2 of the present invention, the decompression process is performed to remove carbon dioxide in advance before the distillation, so that the pH of the waste ethanol introduced into the distillation column is high when compared with Comparative Examples in which the distillation is performed without performing the decompression process, and salt is not generated in the cooling column during the distillation process. Thereafter, the multi-state distillation process was performed to recover ethanol of high purity having a concentration of residual ammonia of less 20 ppm, and the recovery rate of the ethanol was 93% or greater, which confirms that ethanol loss in the distillation process was minimized.

(11) However, in Comparative Examples 1 to 4, although ethanol of high purity having a concentration of residual ammonia of less 20 ppm was obtained at a recovery rate of 93% or greater, the decompression process was not performed before the distillation process, so that salt was generated in the cooling column during the distillation process.

(12) Meanwhile, in Comparative Example 5, the waste ethanol was purified through single stage distillation, so that the recovered ethanol had a concentration of residual ammonia greater than 100 ppm, which is not suitable to be reused for producing silica aerogel or a silica aerogel blanket, and purification efficiency was not good since the recovery rate of the ethanol was not high. Also, since the decompression process was not performed, salt was generated in the cooling column during the distillation process.

(13) The foregoing description of the present invention has been presented for purposes of illustration. It will be understood by those skilled in the art that various changes in form and details can be made therein without departing from the spirit and scope of the invention. It is therefore to be understood that the above-described embodiments are illustrative in all aspects and not restrictive.