Method for recovering off-gases from ester synthesis

Abstract

The application provides a recovery method of waste gas generated in ester synthesis, which comprises waste gas absorption in a three-stage countercurrent absorption tower, gas circulation by using nitrogen as a medium, classification recovery of salt in the absorption liquid by using the solubility difference of different salts, and liquid circulation use in each step. After the components in the waste gas generated in ester synthesis are absorbed, the application classifies and recovers the components into high-purity salt, generates certain economic benefits, reduces the waste gas treatment cost, realizes closed gas circulation, does not discharge the waste gas, and realizes complete treatment of the waste gas in a small scale.

Description

Technical Field

[0001] This invention relates to a method for the comprehensive utilization of industrial waste gas, and more specifically, to a method for recovering waste gas generated from ester synthesis. Background Technology

[0002] In some ester synthesis processes, a large amount of waste gas containing volatile strong acids and sulfur oxide byproducts is generated. Sulfur oxides are one of the main sources of air pollution. In order to reduce air pollution, the existing waste gas is generally absorbed by a spray tower, that is, an absorption solvent is sprayed into the waste gas, so that the absorption solvent reacts with the acid and sulfur oxides in the waste gas, thereby removing them from the waste gas.

[0003] Existing methods for absorbing and treating waste gas using spraying suffer from several drawbacks. The high flow rate of the waste gas, the intermittent nature of the production process, and abnormal situations result in inconsistent concentrations of acids and sulfur oxides at different times. This means that sulfur oxides may remain in the discharged waste gas without reacting with the absorption solvent, leading to intermittent and insufficient absorption and causing emissions to exceed standards. Furthermore, existing spraying equipment often fails to completely remove moisture from the waste gas after treatment, leaving a large amount of water droplets containing the absorption solvent discharged with the exhaust gas. This causes secondary pollution and wastes recyclable resources.

[0004] Invention content / Utility model content

[0005] The purpose of this invention is to provide a method for recovering waste gas generated during ester synthesis.

[0006] To achieve the objective of this invention, a method for recovering waste gas generated during ester synthesis is provided, comprising the following steps:

[0007] (1) The waste gas generated from ester synthesis is discharged from the reaction tank along with nitrogen and enters the first-stage condenser to condense and recover the alcohols in the waste gas;

[0008] (2) The exhaust gas discharged from the first-stage condenser enters the three-stage countercurrent absorption tower for step-by-step spray absorption. The absorbent used for spraying is 5%-7% (preferably 5%, mass percentage concentration) calcium hydroxide suspension (which can be replaced by alkaline solutions such as sodium hydroxide and potassium hydroxide). The feed liquid of each absorption tower is circulated and sprayed to control the pH of the absorbent discharged from each absorption tower within a certain range. When the pH of the absorbent circulated and sprayed in the first-stage absorption tower drops to 6.5-7.5, the absorbent is discharged from the first-stage spray tower for salt recovery. When the pH of the absorbent circulated and sprayed in the second-stage absorption tower drops to 9.5-10.5, the absorbent in the second-stage absorption tower is transferred to the first-stage absorption tower for continued circulation and spray absorption of exhaust gas. When the pH of the absorbent circulated and sprayed in the third-stage absorption tower drops to 13.0-13.5, the absorbent in the third-stage absorption tower is transferred to the second-stage absorption tower for continued circulation and spray absorption of exhaust gas.

[0009] (3) After the waste gas is absorbed by the three-stage countercurrent absorption tower, it enters the two-stage condenser to remove the moisture entrained in the nitrogen. The recovered condensate is used for the preparation of the absorbent, and the nitrogen is recovered and compressed into the nitrogen buffer tank for storage and recycling.

[0010] (4) The absorbent discharged from the primary absorption tower is filtered and washed with water to obtain wet calcium sulfite powder, which is then dried to obtain calcium sulfite.

[0011] (5) The absorbent discharged from the primary absorption tower in step (4) is filtered to obtain the filtrate. The chloride ion content is detected. If the chloride ion content in the filtrate is less than 50 g / L, the filtrate is reused for the preparation of the absorbent so that it can continue to enrich chloride ions in the tertiary countercurrent absorption tower. If the chloride ion content in the filtrate is greater than 50 g / L, the filtrate is concentrated through a reverse osmosis membrane to a chloride ion concentration of more than 150 g / L. The resulting concentrate is then evaporated to recover calcium chloride. The reverse osmosis dialysis solution contains some permeated chloride ions and is reused for the preparation of the absorbent.

[0012] In this invention, the waste gas generated from ester synthesis includes, but is not limited to, alcohols, thionyl chloride, sulfur dioxide (sulfur dioxide is a byproduct of the synthesis reaction and constitutes the main component), and volatile hydrochloric acid.

[0013] For ester synthesis processes, please refer to CN116178192A, CN112830882A, etc.

[0014] In this process, the gas circulation velocity is 30,000 m / s. 3 The gas flow rate can be adjusted to approximately 100 m / h, depending on the type of alkaline solution used, the type of waste gas absorbed, or the pH parameter.

[0015] The circulating spray velocity of the alkaline solution (calcium hydroxide suspension) in the absorption tower is 20-30 m / s. 3 / h, the spray rate of the three-stage absorption tower gradually decreases, which can be adjusted according to the production situation and adapted to data such as waste gas flow rate, alkali concentration, and pH control.

[0016] Furthermore, the 5%-7% calcium hydroxide suspension is prepared by adding quicklime to water.

[0017] The equipment and pipelines involved in this invention use corrosion-resistant materials. The volume of each absorption tower in the three-stage countercurrent absorption tower can be reduced step by step according to the content of waste gas components, thereby reducing equipment investment and energy consumption.

[0018] By employing the above technical solution, the present invention has at least the following advantages and beneficial effects:

[0019] This invention provides a method for recovering waste gas generated during ester synthesis. The waste gas is absorbed in a three-stage countercurrent absorption tower to ensure more thorough absorption under normal production conditions. Nitrogen is used as a medium for gas circulation to prevent the formation of impurities from oxygen and carbon dioxide, resulting in higher purity of the recovered salt, generating certain economic benefits, and reducing waste gas treatment costs. The absorbent liquid is classified to recover salts, and the liquid generated in each step is recycled. No gas is discharged, which can thoroughly treat the waste gas and achieve higher recovery efficiency. Attached Figure Description

[0020] Figure 1 This is a process flow diagram of the method for recovering waste gas generated during ester synthesis according to the present invention. Detailed Implementation

[0021] This invention provides a method for recovering waste gas generated during ester synthesis, comprising waste gas absorption in a three-stage countercurrent absorption tower, using nitrogen as a gas circulation medium, and utilizing the differences in solubility of different salts to classify and recover salts from the absorbent liquid. The liquid generated at each step is recycled. This invention absorbs and classifies the various components in the waste gas generated during ester synthesis into high-purity salts, generating certain economic benefits and reducing waste gas treatment costs; it also achieves closed-loop gas circulation, preventing waste gas from being discharged externally, and enabling thorough waste gas treatment on a small scale.

[0022] The present invention adopts the following technical solution:

[0023] 1. In the ester synthesis process of this invention, the main components of the waste gas generated from the solid ester synthesis tank (reaction tank) are alcohols, thionyl chloride, sulfur dioxide (a byproduct of the synthesis reaction, accounting for the main proportion), and hydrochloric acid. After the waste gas is discharged, the alcohol organic matter is first condensed and recovered.

[0024] 2. The waste gas enters a three-stage countercurrent absorption tower for absorption. The absorbent is quicklime mixed with water to form a calcium hydroxide suspension of approximately 5%. This suspension is sprayed to absorb thionyl chloride, sulfur dioxide, and hydrochloric acid, producing a calcium sulfite and calcium chloride suspension (thionyl chloride can decompose into sulfurous acid and hydrochloric acid in water). The feed liquid in each absorption tower is circulated and sprayed to control the pH of the absorbent liquid discharged from each absorption tower within a certain range. The absorbent liquid from the first-stage absorption tower can be discharged for salt recovery after the pH drops to 6.5-7.5. The absorbent liquid from the second-stage absorption tower can be introduced into the first-stage absorption tower for further absorption after the pH drops to 9.5-10.5. The absorbent liquid from the third-stage absorption tower can be introduced into the second-stage absorption tower for further absorption after the pH drops to 13.0-13.5.

[0025] 3. After being absorbed by the three-stage countercurrent absorption tower, the exhaust gas is first condensed to remove the moisture it carries, and then recovered and compressed into a nitrogen buffer tank for storage and recycling.

[0026] 4. The absorbent liquid discharged from the primary absorption tower is filtered and washed with water to obtain wet calcium sulfite powder, which can be dried to obtain calcium sulfite.

[0027] 5. After recovering calcium sulfite from the absorbent discharged from the first-stage absorption tower, the chloride ion content of the filtrate should be tested to see if it reaches 50 g / L. If it is lower, the filtrate should be reused in the preparation of calcium hydroxide so that it can continue to enrich chloride ions in the third-stage countercurrent absorption tower. If it is higher, it should be concentrated more than 3 times by reverse osmosis membrane. The concentrated liquid should then be evaporated to dryness to recover calcium chloride. The reverse osmosis dialysate contains some permeated chloride ions and should be reused in the preparation of calcium hydroxide.

[0028] Furthermore, the composition of quicklime directly affects the purity and content of recovered calcium sulfite and calcium chloride, as well as the content of other salts.

[0029] Furthermore, the calcium hydroxide suspension is not limited to a 5% content and can be adjusted according to the usage requirements of the sprayer.

[0030] Furthermore, pH control in the three-stage countercurrent absorption tower directly affects the thoroughness of waste gas absorption and the residual calcium hydroxide in the recovered salt.

[0031] Furthermore, using nitrogen in the exhaust gas can prevent oxidation and the generation of sulfate and carbonate salts, thereby improving the purity of the recovered salt.

[0032] Furthermore, after recovering calcium sulfite, the filtrate needs to be tested to see if the chloride ion concentration reaches 50 g / L. If the concentration is higher, it will result in high residual calcium chloride in the recovered calcium sulfite and cause corrosion to the equipment. If the concentration is lower, it will affect the calcium chloride recovery efficiency.

[0033] Furthermore, the filtrate, dialysate, and condensate produced in each step are reused in the preparation of calcium hydroxide suspension.

[0034] Furthermore, the equipment and pipelines in the system must be made of corrosion-resistant materials. The volume of each absorption tower in the three-stage countercurrent absorption tower can be reduced step by step according to the content of the waste gas components, thereby reducing equipment investment and energy consumption.

[0035] The process flow for the recovery of waste gas generated from the above ester synthesis is shown in [link to process flow chart]. Figure 1 .

[0036] The following examples are used to illustrate the present invention, but are not intended to limit the scope of the invention. Unless otherwise specified, the technical means used in the examples are conventional means well known to those skilled in the art, and the raw materials used are all commercially available products.

[0037] Example 1: Method for recovering waste gas generated during ester synthesis

[0038] This embodiment provides a method for recovering waste gas generated during ester synthesis, and the specific implementation method is as follows:

[0039] In the synthesis process of esters (levorotatory p-hydroxyphenylglycine methyl ester, see CN112830882A), the main components of the waste gas generated are alcohols, thionyl chloride, sulfur dioxide (a byproduct of the synthesis reaction, accounting for the majority of the composition), and volatile hydrochloric acid.

[0040] The generated waste gas is discharged from the synthesis tank along with nitrogen and first enters the primary condenser to condense and recover the alcohols in the waste gas.

[0041] The waste gas then enters a three-stage countercurrent absorption tower for sequential absorption. The absorbent is quicklime mixed with water to form a calcium hydroxide suspension of approximately 5%. This suspension is sprayed to absorb thionyl chloride, sulfur dioxide, and hydrochloric acid, producing a CaSO3 and CaCl2 suspension (thionyl chloride easily decomposes in water into sulfurous acid and hydrochloric acid). The feed liquid is circulated and sprayed in each absorption tower. The absorbent discharged from the first-stage absorption tower can be discharged for salt recovery after the pH drops to 6.5-7.5. The absorbent discharged from the second-stage absorption tower can be introduced into the first-stage absorption tower for continued absorption after the pH drops to 9.5-10.5. The absorbent discharged from the third-stage absorption tower can be introduced into the second-stage absorption tower for continued absorption after the pH drops to 13.0-13.5.

[0042] After being absorbed by a three-stage countercurrent absorption tower, the exhaust gas enters a two-stage condenser to remove moisture entrained in the nitrogen. The condensate can be used to prepare the absorbent, and the nitrogen is then recovered, compressed, and stored in a nitrogen buffer tank for recycling.

[0043] The absorbent liquid discharged from the primary absorption tower is filtered and washed with water to obtain wet calcium sulfite powder, which can be dried to obtain calcium sulfite.

[0044] After recovering calcium sulfite from the absorbent discharged from the first-stage absorption tower, the chloride ion content of the filtrate is tested to see if it reaches 50 g / L. If it is lower, the filtrate is reused in the preparation of calcium hydroxide, allowing it to continue to enrich chloride ions in the third-stage countercurrent absorption tower. If it is higher, it is concentrated more than 3 times by reverse osmosis membrane, and the concentrated liquid is then evaporated to recover calcium chloride. The reverse osmosis dialysate contains some permeated chloride ions and should be reused in the preparation of calcium hydroxide.

[0045] In the above process, the gas velocity of the exhaust gas passing through each piece of equipment is 30,000 m / s. 3 The circulating spray velocity of the alkaline solution (calcium hydroxide suspension) in the absorption tower is approximately 20-30 m / h. 3 / h.

[0046] The above process flow is shown in [link to process flow chart]. Figure 1 .

[0047] Route 1 indicates that the alkaline solution is circulated and sprayed in the absorption tower;

[0048] Route 2 indicates the direction of exhaust gas flow using nitrogen as the carrier medium;

[0049] Route 3 indicates the path of the prepared alkali absorption solution in the absorption tower;

[0050] Route 4 indicates the destination of the absorbent solution after it meets the process requirements;

[0051] Route 5 indicates the destination of the residual liquid generated in each treatment step, which is used as water for the preparation of Ca(OH)2;

[0052] Route 6 represents the CaSO3 recovery route;

[0053] Route 7 represents the CaCl2 recovery route.

[0054] The recovery of calcium sulfite and calcium chloride using this process is directly affected by the calcium oxide content used in the preparation of the absorbent solution. The resulting calcium sulfite and calcium chloride contents are comparable to or slightly higher than the calcium oxide contents.

[0055] This process has the following main advantages:

[0056] (1) There is a habitual way of thinking in the industry regarding the treatment of waste gas and wastewater. All waste gas and wastewater in the factory area must be treated centrally. While centralized treatment can reduce the total cost of factory waste treatment due to scale, it also has problems such as complex composition, increased procedures, and low purity of recycled materials. Therefore, it does not have an advantage over other centralized treatment methods for waste gas and waste liquid with less complex composition. This invention adopts a method of treating a single waste gas separately.

[0057] (2) In the industry, waste gas treatment only needs to meet national emission requirements. However, production abnormalities inevitably occur during waste gas treatment, resulting in substandard exhaust gas. Existing spray equipment discharges a large amount of water droplets containing absorbent solvents along with the exhaust gas, causing secondary pollution to the environment and wasting recyclable resources. This is a drawback of the current waste gas treatment methods in the industry. In the initial process design of this invention, the aim was to improve the purity of the recovered salt and increase the selling price to reduce the treatment cost. After research, the process design was changed to use nitrogen as a medium to avoid the generation of impurities. Since the use of nitrogen increases costs, nitrogen is recycled and reused, and the entire system is closed to allow nitrogen to circulate within the system, thus solving the above problems.

[0058] The application scope of this invention is limited to the treatment of waste gas with relatively simple composition, and where the physicochemical properties of the components in the waste gas are relatively easy to separate.

[0059] Although the present invention has been described in detail above with general descriptions and specific embodiments, modifications or improvements can be made to it, which will be obvious to those skilled in the art. Therefore, all such modifications or improvements made without departing from the spirit of the present invention fall within the scope of protection claimed by the present invention.

Claims

1. A method for recovering waste gas generated during ester synthesis, characterized in that, Includes the following steps: (1) The waste gas generated from ester synthesis is discharged from the reaction tank along with nitrogen and enters the first-stage condenser to condense and recover the alcohols in the waste gas; (2) The exhaust gas discharged from the first-stage condenser enters the three-stage countercurrent absorption tower for step-by-step spray absorption. The absorbent used for spraying is 5%-7% calcium hydroxide suspension. The liquid in each absorption tower is circulated and sprayed to control the pH of the absorbent discharged from each absorption tower within a certain range. When the pH of the absorbent circulated and sprayed in the first-stage absorption tower drops to 6.5-7.5, the absorbent is discharged from the first-stage spray tower for salt recovery. When the pH of the absorbent circulated and sprayed in the second-stage absorption tower drops to 9.5-10.5, the absorbent in the second-stage absorption tower is transferred to the first-stage absorption tower for continued circulation and spray absorption of exhaust gas. When the pH of the absorbent circulated and sprayed in the third-stage absorption tower drops to 13.0-13.5, the absorbent in the third-stage absorption tower is transferred to the second-stage absorption tower for continued circulation and spray absorption of exhaust gas. (3) After the waste gas is absorbed by the three-stage countercurrent absorption tower, it enters the two-stage condenser to remove the moisture entrained in the nitrogen. The recovered condensate is used for the preparation of the absorbent, and the nitrogen is recovered and compressed into the nitrogen buffer tank for storage and recycling. (4) The absorbent discharged from the primary absorption tower is filtered and washed with water to obtain wet calcium sulfite powder, which is then dried to obtain calcium sulfite. (5) The absorbent discharged from the primary absorption tower in step (4) is filtered to obtain the filtrate. The chloride ion content is tested. If the chloride ion content in the filtrate is less than 50 g / L, the filtrate is reused for the preparation of the absorbent. If the chloride ion content in the filtrate is greater than 50 g / L, the filtrate is concentrated through a reverse osmosis membrane to a chloride ion concentration of more than 150 g / L. The resulting concentrate is then evaporated to recover calcium chloride. The reverse osmosis dialysis solution contains some permeated chloride ions and is reused for the preparation of the absorbent. Among them, the waste gases generated from ester synthesis include alcohols, thionyl chloride, sulfur dioxide, and volatile hydrochloric acid.

2. The method according to claim 1, characterized in that, The 5%-7% calcium hydroxide suspension is prepared by adding quicklime to water.

3. The method according to claim 1 or 2, characterized in that, The equipment and pipelines involved in the method are made of corrosion-resistant materials. The volume of each absorption tower in the three-stage countercurrent absorption tower can be reduced step by step according to the composition of the waste gas, thereby reducing equipment investment and energy consumption.

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