阅读说明：本技术 一种三氯蔗糖生产中废dmf的处理方法 (Method for treating waste DMF in sucralose production ) 是由 张正颂 沈东东 仇鑫 慈昊 于 2021-09-22 设计创作，主要内容包括：本发明涉及一种三氯蔗糖生产中废DMF的处理方法,其特征在于：(1)将脱酸塔底排出的废DMF送入减压精馏塔进行减压精馏,精馏塔顶部采出的不凝气送入甲醇精馏塔再次精馏,精馏回收的甲醇送入反应精馏塔进行反应；(2)减压精馏塔下部采出的酸性共沸物送入反应精馏塔；控制酸性共沸物与甲醇的体积比为1:0.9-1.5,反应精馏塔塔底反应液返回减压精馏塔,反应精馏塔塔顶馏出甲醇和乙酸甲酯的混合物返回甲醇精馏塔。本发明优点：通过反应精馏将生成的乙酸甲酯移走,促进反应的正向进行,提高了产率；使用原料中含有的乙酸作为催化剂,无需担心失活问题,且不会导致DMF过多分解；制得的乙酸甲酯的纯度高,产品DMF纯度高,降低了三氯蔗糖的生产成本。(The invention relates to a method for treating waste DMF in sucralose production, which is characterized by comprising the following steps: (1) feeding waste DMF discharged from the bottom of the deacidification tower into a vacuum rectification tower for vacuum rectification, feeding non-condensable gas extracted from the top of the rectification tower into a methanol rectification tower for rectification again, and feeding methanol recovered by rectification into a reaction rectification tower for reaction; (2) sending the acid azeotrope extracted from the lower part of the vacuum distillation tower into a reaction distillation tower; controlling the volume ratio of the acid azeotrope to the methanol to be 1:0.9-1.5, returning the reaction liquid at the bottom of the reaction rectifying tower to the vacuum rectifying tower, and returning the mixture of the methanol and the methyl acetate distilled from the top of the reaction rectifying tower to the methanol rectifying tower. The invention has the advantages that: the generated methyl acetate is removed through reactive distillation, the forward proceeding of the reaction is promoted, and the yield is improved; acetic acid contained in the raw material is used as a catalyst, so that the problem of deactivation is not needed to be worried about, and excessive decomposition of DMF is not caused; the prepared methyl acetate has high purity, the product DMF has high purity, and the production cost of the sucralose is reduced.)

1. A method for treating waste DMF in sucralose production is characterized by adopting the following devices: the device comprises a vacuum rectification tower, a reaction rectification tower, a pipeline, a vacuum rectification tower, a reaction rectification tower and a vacuum rectification tower, wherein a discharge port of an azeotropic mixture of the vacuum rectification tower is connected with a lower feed inlet of the reaction rectification tower;

the method comprises the following steps:

(1) feeding waste DMF discharged from the bottom of a deacidification tower in a sucralose DMF recovery working section into a vacuum rectification tower for vacuum rectification, feeding non-condensable gas extracted from the top of the rectification tower into a methanol rectification tower for rectification again, and feeding methanol recovered by rectification into a reaction rectification tower for reaction;

(2) sending the acid azeotrope extracted from the lower part of the vacuum distillation tower into a reaction distillation tower; controlling the volume ratio of the acid azeotrope to the methanol to be 1:0.9-1.5, returning the reaction liquid at the bottom of the reaction rectifying tower to the vacuum rectifying tower, and returning the mixture of the methanol and the methyl acetate distilled from the top of the reaction rectifying tower to the methanol rectifying tower.

2. The method according to claim 1, wherein the method comprises the following steps: the pressure of the vacuum essence is 6-40 kPa, and the temperature is 85-90 ℃.

3. The method of claim, wherein the method comprises the following steps: the pressure of the reaction rectification is 7-30kPa, and the temperature is 85-110 ℃.

4. The method for treating waste DMF in the production of sucralose according to any one of claims 1 to 3, wherein: the waste DMF discharged from the bottom of the deacidification tower contains 5-15% of acetic acid, the water content, DMAC (dimethylacetamide) and tetramethylurea do not exceed 1%, and the rest is DMF.

5. The method for treating waste DMF in the production of sucralose according to any one of claims 1 to 3, wherein: the non-condensable gas extracted from the top of the vacuum rectification tower contains 30-60% of methyl acetate, 30-60% of methanol and the balance of water; the acidic azeotrope extracted from the lower part of the vacuum rectification tower contains 29-33% of acetic acid, 0-5% of DMAC and the balance of DMF.

6. The method according to claim 1, wherein the method comprises the following steps: the vacuum distillation is single-tower batch distillation, multi-tower continuous distillation or single bulkhead tower continuous distillation.

7. The method according to claim 1, wherein the method comprises the following steps: the reactive distillation is batch distillation or continuous distillation.

Technical Field

The invention belongs to the technical field of chemical production, and particularly relates to a method for treating waste DMF (dimethyl formamide) in sucralose production.

Background

In the production process of sucralose, DMF (N, N-dimethylformamide) is used as a solvent for the chlorination process, and DMF is needed to form a Wei's reagent in the chlorination process of sucralose, so DMF is also an indispensable participant in the chlorination process. Under the existing process conditions, a large amount of waste DMF is inevitably generated. Waste DMF discharged from the bottom of the deacidification tower of the sucralose DMF recovery section is acidic due to acetic acid which is an important impurity contained in the waste DMF, and is also called acidic DMF. Acetic acid and DMF may form an azeotropic mixture with a boiling point higher than that of pure DMF, so it cannot be easily separated by distillation. The waste DMF also usually contains DMAC (N, N-dimethylacetamide) and tetramethylurea in a certain proportion, and DMAC is also difficult to separate from acetic acid due to the close structural properties of DMAC and DMF.

Patent publication No. CN106831471A discloses that DMF is purified by reactive distillation using a rectifying column, but the use of an NKC-9 catalyst has a problem of catalytic deactivation. In patent publication No. CN109503409A, sulfuric acid is used as a catalyst, but sulfuric acid accelerates the decomposition of DMF, and the produced dimethylamine reacts with sulfuric acid to produce dimethylamine sulfate which has no catalytic ability, resulting in high catalyst consumption, reduced DMF recovery, and generation of useless waste salt.

Disclosure of Invention

The invention aims to solve the defects and provide a method for treating waste DMF in sucralose production; the invention firstly obtains relatively pure (99.7-99.9%) DMF by rectification, and then DMF and acetic acid are generated into methyl acetate by a reactive rectification mode.

In order to achieve the above object, the invention adopts the following technical scheme:

a method for treating waste DMF in sucralose production is characterized by adopting the following devices: the device comprises a vacuum rectification tower, a reaction rectification tower, a pipeline, a vacuum rectification tower, a reaction rectification tower and a vacuum rectification tower, wherein a discharge port of an azeotropic mixture of the vacuum rectification tower is connected with a lower feed inlet of the reaction rectification tower;

the method comprises the following steps:

(1) feeding waste DMF (5-15% of acetic acid, water, DMAC (dimethylacetamide) and tetramethylurea and the balance of DMF) discharged from the bottom of a deacidification tower at a sucralose DMF recovery working section into a vacuum rectification tower for vacuum rectification, controlling the pressure in the tower to be 6-40 kPa and the temperature to be 85-90 ℃, feeding noncondensable gas (30-60% of methyl acetate, 30-60% of methanol and the balance of water) extracted from the top of the rectification tower into a methanol rectification tower for rectification again, and feeding the rectified and recycled methanol into a reaction rectification tower for reaction;

(2) sending acidic azeotrope (acetic acid 29-33%, DMAC 0-5% and DMF in balance) extracted from the lower part of the vacuum distillation tower into a reaction distillation tower; controlling the volume ratio of the acid azeotrope to the methanol to be 1:0.9-1.5, controlling the pressure of the reaction rectifying tower to be 7-30kPa, controlling the temperature to be 85-110 ℃, returning the reaction liquid at the bottom of the reaction rectifying tower to the vacuum rectifying tower, and returning the mixture of the distilled methanol and the methyl acetate at the top of the reaction rectifying tower to the methanol rectifying tower.

Further, the vacuum distillation is single-tower batch distillation, multi-tower continuous distillation or single more complex bulkhead tower continuous distillation.

Further, the reactive distillation is batch distillation or continuous distillation.

If the reactive distillation is intermittent operation, the acidic azeotrope is added at one time, methanol is added in batches along with the reaction, an azeotropic mixture of methanol and methyl acetate is distilled out, the weight content of the methanol is about 18%, and the proportion of the methanol in the distillate rises after a period of time to form a non-azeotropic mixture. If the reactive distillation is a continuous operation, the acid azeotrope is added from the middle of the column, methanol is added from the bottom of the column, and the non-azeotropic mixture is distilled from the top of the column.

The invention utilizes methanol and acetic acid to generate methyl acetate, and the methyl acetate has a boiling point lower than that of the methanol, is easy to distill away and remove from a reaction system, and promotes the forward reaction. The boiling point of the methanol is lower than that of water, and the methanol does not azeotrope with the water, so that the methanol is easy to remove and recycle, and the sewage cannot be mixed with a large amount of organic matters to be difficult to treat. The reaction does not need additional acid catalysis, the acidity of acetic acid in the reaction system is enough to catalyze the reaction, and the problems of strong acid concentration in high-boiling impurities and strong acid catalytic DMF decomposition are avoided. After passing through the reactive distillation tower, the content of acetic acid is reduced to about half of the original content, and the acetic acid is returned to be decompressed and distilled again to obtain a DMF product. The by-product is an azeotropic mixture of methyl acetate and methanol, although the azeotropic mixture cannot be separated by using a rectification mode, the azeotropic ratio is different under different pressures, so that two rectification towers with different pressures can be used for realizing separation to obtain pure methyl acetate.

The invention has the advantages that:

1. the device is simple and easy to operate; according to the invention, the generated methyl acetate product is removed by reactive distillation, so that the forward progress of the reaction is promoted, and the yield is improved; according to the invention, acetic acid contained in the raw material is used as a catalyst, so that the problem of inactivation is not needed to be worried about, and the acidity of the acetic acid is moderate, so that excessive decomposition of DMF is avoided;

2. the methyl acetate prepared by the method has high purity (the normal-pressure azeotropic ratio is about 83 percent, and the total content after methanol is added is 99.8-99.9 percent), the pure methyl acetate and the azeotropic mixture have good economic values, the economy of the waste DMF treatment process can be improved (the purity of the DMF product is more than or equal to 99.7 percent, and the recovery rate is more than or equal to 95 percent), and the production cost of the sucralose is finally reduced.

Drawings

FIG. 1 is a schematic diagram of batch distillation treatment of waste DMF in sucralose production;

FIG. 2 is a schematic diagram of a continuous distillation process for waste DMF in sucralose production.

Detailed Description

The invention is further illustrated with reference to fig. 1:

a method for treating waste DMF in sucralose production comprises the following specific implementation steps:

example 1

In the embodiment, a vacuum rectification tower, a methanol rectification tower and a reaction rectification tower are all subjected to single-tower batch rectification, the height of a filler is 50 cm, the diameter of the filler is 4 cm, and a glass spring is filled in the filler;

(1) feeding 2500 mL of waste DMF (10% of acetic acid, water, DMAC (N-dimethylacetamide) and tetramethylurea and the balance of DMF) discharged from the bottom of a deacidification tower of a sucralose DMF recovery working section into a reduced pressure rectifying tower for reduced pressure rectification, distilling low-boiling impurities (methyl acetate and methanol) at normal pressure, keeping the temperature of the kettle unchanged, reducing the pressure in the tower to 30kPa, distilling water, and keeping the volume of the low-boiling impurities and the water to be 21 mL; reducing the pressure in the tower to 7-9 kPa, controlling the temperature at the top of the reduced pressure distillation tower to 75-80 ℃ to obtain 1700 mL (the purity is 99.8%) of DMF, reducing the pressure to 6-8 kPa, and distilling an acid azeotrope, wherein the components of DMF, DMAC and acetic acid =2.4:0.08:1 (determined by gas chromatography, abbreviated as GC) are used for distilling the acid azeotrope, the rectification is continued, the DMAC content in the acid azeotrope is slowly increased firstly and is rapidly increased finally, when the temperature at the bottom of the reduced pressure tower to 5 kPa is increased to 110 ℃, the proportion of DMF, DMAC and acetic acid =1.01:1.08:1 is reached, and 645 mL of the acid azeotrope is obtained totally; stopping rectification, wherein the residual kettle bottom liquid is 87 mL of high-boiling impurities;

(2) 2000 mL of acid azeotrope (GC containing 30 percent of acetic acid, 2 percent of DMAC and the balance of DMF) is taken and sent into a reaction rectifying tower; adding 750 mL of methanol, carrying out reactive distillation under normal pressure, controlling the tower bottom temperature of a reactive distillation tower to be 94-95 ℃, and the tower top temperature to be 48-50 ℃, distilling an azeotropic mixture (containing 18% of methanol) of the methanol and the methyl acetate, adding 250 mL of methanol every 250 mL of distilled liquid, gradually increasing the temperature along with the progress of the reaction, and increasing the methanol content of the distillate; the bottom temperature of the kettle is 98-105 ℃, the overhead temperature is 65-71 ℃, the distillate methanol content is 70% (by weight), the cumulative addition amount of the methanol is 1500mL, 1850 mL of bottom fraction of the reactive distillation tower remains in the kettle, and DMF, DMAC, acetic acid =4.3:0.2:1 (GC);

(3) adding 2500 mL of a non-azeotropic mixture of methanol and methyl acetate into a methanol rectifying tower for normal pressure rectification, controlling the temperature of the bottom of the methanol rectifying tower to be 70-74 ℃ and the temperature of the top of the methanol rectifying tower to be 48-55 ℃, distilling 1100 mL of an azeotropic mixture (containing 18% of methanol) of the methanol and the methyl acetate, and leaving 1400 mL of the methanol in the kettle.

Example 2

Returning the bottom material of the reaction rectifying tower in the example 1 to the system for carrying out vacuum rectification again, wherein the specific operation steps are as follows:

mixing 1600mL of tower bottom fraction of the reactive distillation tower with 1000mL of waste DMF discharged from the bottom of the deacidification tower, feeding the mixture into a reduced-pressure distillation tower, repeating the operation steps in the example 1 to obtain 130 mL of distilled low-boiling impurities, and combining the distilled low-boiling impurities into a non-azeotropic mixture of methanol and methyl acetate; 170 mL of distilled water, 1750 mL of DMF product (purity 99.72%), 650 mL of acid azeotrope and 70 mL of bottom liquid.

The content (GC) of the product of example 2 at each step is as follows:

composition (I)	Acetic acid methyl ester	Methanol	DMF	DMAC	Tetramethylurea	Acetic acid
							Retention time (min)	2.8	3.4	9.8	11.9	13.8	14.4
Waste DMF (%)			84.3	4.1	1.86	8.7
							Bottom fraction (%) -of reactive distillation column	0.3	5.4	71.7	3.6	1.9	14.1
Product MF (%)			99.72	0.175

Example 3

In this embodiment, the vacuum rectification tower, the methanol rectification tower and the reactive rectification tower all adopt continuous rectification, and the vacuum rectification tower is replaced by a light component removal tower, a heavy component removal tower and a refining tower which are connected in sequence, and the specific process flow is shown in fig. 2:

(1) waste DMF (acetic acid 9 percent, water content, DMAC (dimethylacetamide) and tetramethylurea not more than 1 percent) discharged from the bottom of the deacidification tower in the sucralose DMF recovery section,the rest is DMF) and the bottom fraction of the reactive distillation column are mixed at a ratio of 0.9 m³The amount of the catalyst is pumped into the middle part of the lightness-removing column (the theoretical plate number of the lightness-removing column is 25, the feeding position is the 18 th plate), the top temperature of the lightness-removing column is controlled to be 85 ℃, the pressure in the column is controlled to be 30kPa, the top temperature is controlled to be 85 ℃, the reflux ratio is 1:1, obtaining a mixture containing methyl acetate, methanol and water (wherein the water can be condensed) from the top of the light component removal tower, and pumping the methyl acetate and the methanol into a methanol rectifying tower in a gaseous state by a vacuum pump;

(2) the mixture of DMF, DMAC, tetramethylurea and acetic acid with the thickness of 0.8 m is obtained at the bottom of the light component removal tower³The speed of/h is pumped into a de-weighting tower (the theoretical plate number of the de-weighting tower is 35, and the feeding position is 15 th plate); controlling the pressure in the de-heavy tower to be 30kPa, the temperature at the top of the tower to be 110 ℃, and the distillate at the bottom of the de-heavy tower to be high-boiling impurities (containing tetramethylurea, acetic acid, DMAC, DMF and the like); feeding the overhead distillate (DMF, acetic acid and DMAC) of the heavy component removal tower into a refining tower (theoretical plate number of 45 and feeding position of 20 th plate), controlling the pressure in the tower to be 30kPa and the temperature at the top of the tower to be 95-100 ℃, and taking the overhead distillate as a product DMF (purity)>99.7%) and the bottoms are acidic azeotropes;

(3) acid azeotrope at 0.3 m³The reaction product is pumped into the middle part of a reaction rectifying tower at a speed of/h (the number of theoretical plates of the reaction rectifying tower is 30, the feeding position of the middle part is a 20 th plate), the temperature of the reaction rectifying tower is controlled to be 95-100 ℃, the temperature of the top of the reaction rectifying tower is controlled to be 55 ℃, and the reflux ratio is 1:1, the pressure in the tower is normal pressure, and the methanol is 0.3 m³The velocity of the reaction tower is added from the bottom of the reaction rectifying tower; the mixture of methanol and methyl acetate was distilled off at the top of the column at a rate of 0.3 m³The flow rate of the mixed solution is pumped into a methanol rectifying tower (the theoretical plate number is 45, and the feeding positions are all at the 25 th plate), the pressure in the tower is normal, the top temperature is 53 ℃, and the reflux ratio is 1:1, the azeotropic mixture of the methanol and the methyl formate at the top of the column is 0.1-0.2 m³The rate in/h distilled off.