CN102267889A - Method for recovering spirit of vinegar by combining extraction with azeotropic distillation - Google Patents

Abstract

The invention relates to a method for recovering dilute acetic acid by combining extraction and azeotropic rectification. The method first carries out liquid-liquid extraction on part of the acetic acid-water component to be separated, the extractant is injected into the bottom of the extraction tower, and is extracted by countercurrent contact with dilute acetic acid to obtain the top component (extract phase) comprising acetic acid and the extractant and meet environmental protection requirements. The waste water (raffinate phase) is discharged; then, the extract phase and another part of dilute acetic acid enter the azeotropic rectification tower to carry out azeotropic rectification. Azeotrope of boiling agent and water. The azeotropic agent and water achieve liquid-liquid separation in the phase separator, the organic phase returns to the azeotropic rectification tower as an azeotropic agent and returns to the extraction tower for use as an extraction agent, and the water phase enters the azeotropic agent recovery tower or is discharged. The extractant and entrainer used in the present invention are the mixture of alkyl acetate and corresponding alcohol. The extraction technology reduces the water content entering the azeotropic distillation, thereby reducing the amount of the entrainer and saving the heating steam flow of the azeotropic distillation column, so as to reduce the amount of the azeotrope and the energy consumption of the device.

Description

A method for recovering dilute acetic acid by combining extraction and azeotropic distillation

technical field

The invention provides a method for recovering dilute acetic acid by combining extraction and azeotropic distillation techniques. It belongs to the separation engineering field of chemical engineering, and is especially suitable for the treatment of low-concentration acetic acid wastewater. It is an acetic acid dehydration system combined with an extraction technology and azeotropic distillation technology that takes into account both efficiency and environmental protection.

Background technique

Acetic acid is widely used in industry. It can be used as a solvent for oxidation reactions and as an important raw material for organic synthesis industry. During the use of acetic acid, it is often necessary to purify and refine acetic acid, which often involves the separation of acetic acid and aqueous solution. The acetic acid-containing wastewater generated in the production process, although the acetic acid content in the wastewater is low, the total amount of acetic acid wastewater generated in industrial production is large, and the amount of acetic acid discharged every year is very high. This is not only a waste of resources, but also caused great harm to the environment. The industry has strict requirements on the discharge of waste water.

There are two main methods of general acetic acid dehydration treatment, namely ordinary rectification method and azeotropic rectification method. Since the relative volatilities of water and acetic acid are close to 1, the use of ordinary rectification requires more number of plates and a larger reflux ratio, resulting in greater energy consumption. Moreover, the content of acetic acid in the discharge water at the top of the tower is also relatively high, resulting in a large loss of acetic acid and high environmental protection treatment costs. At present, the method of heterogeneous azeotropic distillation is generally used for dehydration of acetic acid. Commonly used entraining agents include n-propyl acetate, n-butyl acetate, and isobutyl acetate. Heterogeneous azeotropic distillation has the characteristics of low energy consumption, less required number of plates, and high separation efficiency. It is economical and environmentally friendly to use an azeotropic distillation column for the separation of high-concentration acetic acid aqueous solution. But when the concentration of acetic acid in the feed is low, adopting this azeotropic distillation method requires higher energy consumption. This is because in the heterogeneous azeotropic distillation process of the acetic acid-water system, water and the entrainer leave the rectification tower in the form of gas from the top of the tower, and are separated into liquid and liquid after condensation. If the water concentration is too low, the azeotropic distillation column will need more entrainer, the gas-liquid phase load in the distillation column will increase, and the energy consumption of the system will be high, which is uneconomical. In view of the above-mentioned shortcomings in the prior art, the present invention proposes an "extraction-azeotropic distillation" technology that takes into account both cost-effectiveness and energy-efficiency and is applicable to existing manufacturing equipment.

Contents of the invention

In order to reduce the energy consumption in the recovery process of azeotropic rectification of dilute acetic acid, the present invention provides an acetic acid dehydration method combined with extraction-azeotropic rectification technology. Process improvement is achieved. This is by adding an extraction tower on the basis of the original azeotropic distillation tower to concentrate part of the dilute acetic acid to reduce the water content in the total feed to the azeotropic rectification tower, thereby reducing the operating load of the azeotropic rectification tower , to achieve the purpose of energy saving.

Method of the present invention mainly has following process:

1. Extraction tower extraction process

In the liquid-liquid extraction tower, dilute acetic acid is added from the top of the extraction tower, and optional extractants such as n-butyl acetate, isobutyl acetate, isopropyl acetate, n-propyl acetate and their mixtures are added from the bottom of the extraction tower. At the same time, the extraction agent selected is the same substance as the entrainer used in the azeotropic rectification tower. After extraction and separation, the top of the extraction tower obtains an extract phase whose main components are acetic acid, water and extractant, and the bottom of the tower obtains a raffinate phase with a large water content and a small amount of organic compounds. The extract phase enters the azeotropic rectification tower for azeotropic rectification, the mass fraction of acetic acid in the raffinate phase is lower than 0.1%, and the extractant content is controlled below 0.8%. Therefore, in order to better reduce the environmental pollution caused by direct discharge, the raffinate phase at the bottom of the extraction tower can be added to the solvent recovery tower to recover the extraction agent. However, due to the low mass fraction of acetic acid, the organic matter content is low And in the case of no pollution to the environment, it can be discharged directly without passing through the solvent recovery tower to recover the solvent.

2. The distillation process of the azeotropic distillation column

The extraction phase obtained in process 1 (its main components are extractant, water, and acetic acid) is injected into the upper part of the azeotropic distillation column, and is carried out under the action of the entrainer (the extractant used in process 1 is the same component) Azeotropic rectification, and at the top of the tower, an overhead fraction whose main components are water and an entrainer and a bottom solution whose main component is acetic acid are obtained. After the top mixture is condensed, it enters the phase separator for stratification to obtain the water phase whose main component is water and the organic phase whose main component is an entrainer. A part of the organic phase is returned to the bottom of the extraction tower to be recycled as the extractant, and its flow rate is determined by the content of acetic acid in the components to be separated and the extraction performance of the tower body of the extraction tower. The other part is used as the reflux of the azeotropic distillation column. The water phase is added to the solvent recovery tower to recover the entrainer (extractant).

3. Recovery process of entrainer (extractant)

The water phase in the phase separator at the top of the azeotropic distillation column contains a small amount of entrainer that needs to be recovered. In addition, the raffinate phase of the extraction tower also contains a small amount of extractant that needs to be recovered. The entrainer (extractant) recovery tower is also an azeotropic rectification tower, and the entrainer is extracted from the top of the tower or in the tower according to the process and the impurities in the feed. The tower kettle is waste water that meets environmental protection requirements.

In order to achieve the highest efficiency of the combined application of extraction-azeotropic distillation technology, the extractant can be a single component or a mixture of two or more components.

The mass content of acetic acid in dilute acetic acid using this treatment method is (10-30)%. In process 1, the extraction operation is preferably carried out at a temperature not higher than 50° C. to initially reduce the water content in the acetic acid wastewater. The extraction tower in this process can be a rotating disk tower or a packed tower, generally in the form of gravity phase separation, and can be a mechanically stirred tower, a pulse tower or a countercurrent extraction tower without a mechanically stirred tower.

The extractant is fed from the bottom of the tower, and the acetic acid-water component to be separated is fed from the top or the upper part of the tower. Usually, there is a section of area above the feed of the acetic acid-water solution for the top stream to be fully stratified.

The azeotropic distillation column in process 2 is to carry out azeotropic distillation on the extract phase that has been preliminarily dewatered and the dilute acetic acid that has not been concentrated under the action of an entrainer to obtain a high concentration that meets industrial requirements and can be recycled. Acetic acid solution. The dilute acetic acid that has been preliminarily dewatered and the untreated dilute acetic acid are respectively added to different trays in the upper part of the azeotropic distillation column.

The azeotropic rectification column of the present invention can adopt a tray column or a packed column. The feeding position of the extract phase stream obtained in process 1 is generally fed at the upper part of the azeotropic distillation column. In addition, the dilute acetic acid aqueous solution that has not been preliminarily removed is generally added in the middle and upper parts of the azeotropic distillation column.

Based on experimental data and general principles and the needs of practical applications in the present invention, the entrainer can be a single component or a mixture of two or more components that form an azeotrope with the separated components. Based on the requirements of economy and the principle of easy separation, the present invention selects a mixture of alkyl acetate and corresponding alcohol as the entrainer, such as selecting n-butyl acetate and n-butanol as the extraction agent.

The high-concentration acetic acid solution obtained at the bottom of the tower in process 2 can be recycled and returned to the production system to be recycled as a solvent or reactant. The mass fraction of acetic acid in the acetic acid solution at the bottom of the azeotropic distillation tower is 92-99%, the mass fraction of the entrainer is less than 0.1%, and the water content is generally less than 10%.

In the present invention, part or all of the water phase in process 2 can also be added to the solvent recovery tower together with the raffinate phase in the extraction tower to recover the entrainer.

The present invention also includes injecting a part of the water phase obtained in process 2 into the azeotropic rectification tower as part of the water reflux, so as to ensure the operational stability of the azeotropic rectification tower when the acetic acid content in the feed varies. The flow rate of the water phase injected into the azeotropic rectification column is different according to different operations, and zero reflux of the water phase can be realized in the present invention. The position of reflux is usually the upper part of the azeotropic distillation column.

The present invention uses the extraction technique and the azeotropic rectification technique in combination, and through the preliminary separation of the water in the stream to be separated by the extraction technique, the content of the treated water in the azeotropic distillation tower is reduced, thereby reducing the energy of the azeotropic distillation Consumption, thereby fundamentally reducing the steam consumption of the azeotropic distillation column.

Description of drawings

Fig. 1 is the extraction-azeotropic rectification-solvent recovery process flowchart of acetic acid dehydration.

Detailed ways

The description of the following examples will help to understand the present invention, but does not limit the content of the present invention. The present invention adopts the mixed solution of n-butyl acetate and n-butanol as extractant and entrainer, and uses the solution whose mass fraction of acetic acid is below 30% as the solution to be treated.

Example

Please refer to Figure 1 for the specific flow chart of the embodiment. The material composition in this embodiment is based on the mass fraction.

In the mixture stream 1 to be separated, the injection rate of dilute acetic acid is 29225 kg/h, the mass fraction of acetic acid is 0.15, and the mass fraction of water is 0.85. Stream 2, as a part of stream 1, is extracted in the upper part of the extraction tower with a flow rate of 10000kg/h; stream 3 is another part of stream 1, which directly enters the azeotropic distillation tower for heterogeneous azeotropic distillation , the flow rate is 19225kg/h.

The extraction tower 1 adopts a packed tower, the theoretical plate number of the extraction tower 1 is 30, the selected operating condition is normal temperature 25°C, and the stream temperature can also be selected as 40-50°C, and the operating pressure is generally atmospheric pressure.

Stream 9 is the extraction agent required for extraction, that is, the main component is a mixture stream of n-butyl acetate and n-butanol, wherein the n-butyl acetate content is 91%, the n-butanol content is 4%, and the water content is 4%. The injection rate is 26000kg/h.

After the separated mixture stream 2 and stream 9 are extracted in the extraction tower, the raffinate phase stream 10 is obtained at the bottom of the extraction tower, and enters the solvent recovery tower 5, and its flow rate is 25374kg/h, wherein the mass fraction of acetic acid is lower than 0.05%. The concentration of n-butyl esters is less than 0.8%.

The extract phase flow 4 of the extraction tower 1 enters the 15th layer tray plate of the azeotropic rectification column 2 whose number of plates is 61 plates (serial number is counted from the top of the tower), and another untreated mixture stream 3 to be separated enters The 35th tray of the azeotropic distillation column 2.

The operating conditions of the azeotropic rectification column 2 are selected as an operating temperature of 90-115° C. and an operating pressure of 5-25 kPag. The heat required by the azeotropic distillation column 2 (ie, the heat required for evaporating the bottom liquid) is provided by a reboiler.

After the azeotropic rectification, the still liquid stream 8 obtained at the bottom of the azeotropic rectification tower 2 is an acetic acid solution with an acetic acid concentration of more than 95%, and the flow rate is 4508 kg/h.

The overhead fraction 5 obtained from the top of the azeotropic distillation column 2 is mainly composed of water and an entrainer, and is introduced into the separation tank 4 after being condensed by the condenser 3 using cooling water. In the separation tank 4, the condensate is separated into layers, and is divided into an organic phase stream 15 mainly containing n-butyl acetate and n-butanol and an aqueous phase stream 13 mainly containing water.

Stream 9 is a part of organic phase stream 15, and its mass flow rate is 27000kg/h, and wherein n-butyl acetate content is 91%, and n-butanol content is 4%, and water content is 4%, and this stream circulates back to extraction tower 1 The bottom, tenth tray, is fed as the extractant. The remaining part of the organic phase, that is, the stream 7 whose mass flow rate is 15719 kg/h, is recycled to the 25th plate of the azeotropic distillation column 2 as an entrainer for recycling.

In order to stabilize the operation of the azeotropic distillation column, a part of the stream 6 of the water phase is recycled back to the first tray at the top of the azeotropic distillation column 2 with a mass flow rate of 100 kg/h. The remaining part of the water phase, that is, the stream 14, enters the solvent recovery tower 5 for recovery of organic matter and waste water treatment.

The entrainer lost in the whole recovery process is supplemented by adding the top of the separation tank 4 from the stream 11.

The solvent recovery tower 5 is an azeotropic rectification tower with 27 plates. The water phase stream 14 and the raffinate phase stream 10 are fed to the 14th tray of the solvent recovery column. The main components are n-butyl acetate and n-butanol organic phase recovery stream 16 on the eighth plate for sideline extraction, and add to the top of the separation tank 4. The stream 18 obtained at the top of the tower is the impurity methyl acetate in the dilute acetic acid feed, and the stream 19 at the bottom of the tower obtains waste water that meets environmental protection discharge requirements.

Claims (11)

1. the method for purification of a dilute acetic acid is characterized in that, adopts extraction tower, and azeotropy rectification column and solvent recovery tower are realized the purification of dilute acetic acid jointly.Before azeotropy rectification column and solvent recovery tower, set up an extraction tower, under the extraction of extraction tower, reduce the water-content of azeotropy rectification column combined feed total feed.Dilute acetic acid is divided into two strands, and one joins the extraction tower top, and another strand joins the middle part of azeotropy rectification column.In phase splitter, be divided into organic phase and water after the overhead gas phase mixture condensation of azeotropy rectification column.An organic phase part is injected the extraction tower bottom as extraction agent, extracts with the dilute acetic acid counter current contact.A part is returned the top of azeotropy rectification column as the backflow of azeotropy rectification column.The water small part is returned the azeotropic distillation top of tower as backflow, and major part enters into the solvent recovery tower middle part and reclaims solvent.The extraction phase at extraction tower top enters the top of azeotropy rectification column and carries out azeotropic distillation.Solvent recovery tower be can inject according to circumstances through the waste water after the extraction and solvent or directly discharging reclaimed.

2. method according to claim 1 is characterized in that, handled dilute acetic acid mass content is (10～30) %.The mass flux ratio that enters the dilute acetic acid of the dilute acetic acid of extraction tower and azeotropy rectification column is 1: (1～4).

3. method according to claim 1 is characterized in that, the mass flux ratio of handled dilute acetic acid and extraction agent is 1: (2～4).

4. method according to claim 1, the acetic acid quality branch in the tower base stream of extraction tower is lower than 0.5%, and the content of extraction agent is lower than 0.8%.

5. method according to claim 1, extraction agent is selected from the acetic acid alkyl ester, forms as the mixture of isobutyl acetate, n-butyl acetate, Iso Butyl Acetate, n-propyl acetate and correspondent alcohol thereof.

6. method according to claim 1 is characterized in that, the acetic acid mass content in the tower base stream of azeotropy rectification column can be 80～99%, and overhead stream is a water, the mixture of entrainer and acetic acid, and wherein the acetic acid massfraction is less than 0.2%.Overhead stream obtains the higher logistics of solvent after water is removed in the phase splitter layering, this logistics part turns back to azeotropy rectification column, and a part turns back to extraction tower and uses as extraction agent.

7. want 5 described methods according to right, entrainer and extraction agent are elected the mixture of identical acetic acid alkyl ester and corresponding alcohol thereof as, as the extraction agent and the entrainer of n-butyl acetate and propyl carbinol composition, extraction agent and entrainer that isobutyl acetate and isopropylcarbinol are formed.

8. method according to claim 1, the logistics that enters azeotropy rectification column comprises the dilute acetic acid of handling without extraction tower, and the organic phase reflux of extracting rectifying column overhead extraction phase and azeotropy rectification column and water reflux.

9. method according to claim 8, the water part in the phase splitter is returned azeotropy rectification column as backflow, and another part enters solvent recovery tower and reclaims the solvent cycle use.

10. method according to claim 8, the part of organic phase phlegma are returned extraction tower and are made extraction agent and recycle, and make entrainer and recycle and another part enters azeotropy rectification column.The mass flux ratio that enters extraction Tahe azeotropy rectification column is (0.5～2.5): 1.

11. method according to claim 1, the part of the water in the tower still extracting phase of extraction tower and the azeotropy rectification column cat head phase splitter together joins solvent recovery tower and reclaims solvent, recovered solvent can turn back to the phase splitter of azeotropy rectification column from side line extraction or cat head extraction.