System and method for recovering excess hydrogen chloride and purifying methyl chloride in methyl chloride synthesis

Abstract

The application discloses a system and a method for recovering excessive hydrogen chloride and purifying chloromethane in chloromethane synthesis, and the system comprises a hydrogen chloride recovery system and a chloromethane purification system. The hydrogen chloride recovery system adopts a high-efficiency fractionating tower, which can reduce the concentration of by-product hydrochloric acid from 20% to below 2%, and greatly reduces the yield of the by-product hydrochloric acid. The chloromethane purification system adopts a combined drying tower, which is provided with three sections of sulfuric acid with different concentrations and temperatures, and performs step-by-step purification of 'coarse removal-main decomposition-fine removal' to efficiently remove water and dimethyl ether. Compared with the prior art, the application solves the problems of large by-product hydrochloric acid yield, high treatment cost, high chloromethane purification energy consumption and low efficiency in the prior art, has remarkable energy-saving and emission-reducing effects and economic benefits, and is suitable for industrialized production of chloromethane.

Description

Technical Field

[0001] This invention relates to the field of chloromethane synthesis technology, and in particular to a system and method for recovering excess hydrogen chloride and purifying chloromethane during chloromethane synthesis. Background Technology

[0002] Currently, the commonly used industrial technology for chloromethane synthesis is the liquid-phase catalytic method. In this reaction, hydrogen chloride is usually in excess, so the chloromethane mixture exiting the reactor contains unreacted hydrogen chloride gas. Existing processes typically treat this by condensation in a cooler and absorption in an acid scrubbing tower, forming dilute hydrochloric acid with a concentration of 16%–20% as a byproduct, accounting for approximately 50%. This byproduct hydrochloric acid is produced in large quantities (approximately 200,000 tons per year), requiring a corresponding methanol removal system and deep desorption system to recover a portion of the methanol and hydrogen chloride. However, the processing consumes a large amount of steam and cooling water, and some of it still needs to be sold externally. This not only increases environmental pressure but also wastes chlorine resources and increases production costs.

[0003] Furthermore, existing chloromethane drying technologies mostly employ a three-tower series process. This method suffers from problems such as large equipment footprint, significant cold loss, high power consumption of circulating pumps, and limited absorption efficiency of the trays, resulting not only in high energy consumption but also limiting further improvements in the quality of chloromethane products.

[0004] Therefore, there is an urgent need for a new system and method that can efficiently recover excess hydrogen chloride, significantly reduce the production of by-product hydrochloric acid, and achieve efficient and low-energy purification of chloromethane. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a system and method for recovering excess hydrogen chloride and purifying chloromethane during chloromethane synthesis, achieving efficient recovery of hydrogen chloride and high-quality purification of chloromethane, thereby reducing energy consumption and environmental impact.

[0006] To achieve this technical objective, the present invention adopts the following solution: In a first aspect, the present invention provides a system for recovering excess hydrogen chloride and purifying chloromethane during chloromethane synthesis, comprising a hydrogen chloride recovery system and a chloromethane purification system. The hydrogen chloride recovery system includes a fractionation tower, with a condenser connected to the top and a reboiler and a chloromethane outlet connected to the bottom. The chloromethane outlet is connected to the chloromethane purification system via a transfer pump. The chloromethane purification system includes a combined drying tower, which consists of a lower section, a middle section, and an upper section. A first circulating pump is connected to the middle section of the combined tower, and a first cooler is connected to the first circulating pump and the middle cooler. A second circulating pump is connected to the upper section of the combined tower, and a second cooler is connected to the second cooler and the upper section. A demister is connected to the top of the upper section of the combined tower.

[0007] Furthermore, the fractionation tower adopts a composite tray structure combining a high-efficiency directional floating valve and structured packing, and a crude chloromethane feed inlet is set in the middle of the fractionation tower.

[0008] Secondly, the present invention provides a method for recovering excess hydrogen chloride and purifying chloromethane during chloromethane synthesis, comprising the following steps: Excess hydrogen chloride recovery: Crude chloromethane enters through the middle feed inlet of the fractionation tower, and the separated hydrogen chloride is discharged from the top of the fractionation tower. After being condensed by the condenser, it enters the next process. Chloromethane is collected from the bottom of the fractionation tower and then pumped into the combined drying tower for purification. Chloromethane purification: The chloromethane separated by the fractionation tower enters the lower section, middle section and upper section of the combined tower in sequence. Different concentrations of sulfuric acid are used to purify the chloromethane in different tower sections in a stepwise manner, realizing the functional division of "coarse removal-main decomposition-fine removal". After purification by the combined drying tower, it enters the demister.

[0009] Furthermore, the system pressure of the fractionation column is controlled at 0.6 MPa, the condenser temperature is controlled at -15℃, and the reboiler temperature at the bottom of the column is controlled at 40℃.

[0010] Furthermore, after recovering hydrogen chloride in a fractionation tower, the concentration of hydrochloric acid byproduct in chloromethane is reduced to below 2%.

[0011] Furthermore, the lower section of the combined tower uses sulfuric acid with a concentration of 75-85%, an operating pressure of 0.49-0.54 MPa, and a temperature of 25-35℃.

[0012] Furthermore, the middle section of the combined tower uses sulfuric acid with a concentration of 85-90%, an operating pressure of 0.49-0.54 MPa, and a temperature of 40-50℃.

[0013] Furthermore, the upper section of the combined tower uses concentrated sulfuric acid with a concentration of 90-98%, with an operating pressure of 0.49-0.54 MPa and a temperature of 30-40℃.

[0014] Furthermore, the middle section and the upper section of the combined tower are equipped with independent circulating cooling systems to maintain the temperature of the middle section of the combined tower at 40~50℃ and the temperature of the upper section of the combined tower at 30~40℃.

[0015] Furthermore, the purified chloromethane achieved a purity of over 99.9%.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows: By using a high-efficiency fractionation tower, the concentration of by-product hydrochloric acid was reduced from 20% to below 2%, which greatly reduced the amount of acid sold and processed externally and the related subsidy costs, thus reducing environmental pressure and resource waste.

[0017] The combined drying tower replaces the traditional multi-tower series, reducing the footprint by about 60%. Through the design of "gradient concentration, segmented function, and independent temperature control", the combined drying tower achieves the optimal efficiency of dehydration and decomposition of dimethyl ether, and solves the problems of overload and heat crosstalk in the traditional multi-tower series.

[0018] This invention reduces steam and cooling water consumption, and ultimately produces high-quality chloromethane with a purity of over 99.9%, possessing excellent value for industrial application. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the excess hydrogen chloride recovery and chloromethane purification system in the chloromethane synthesis of this invention.

[0020] The following are labeled in the diagram: 1. Reboiler; 2. Fractionating column; 3. Condenser; 4. Transfer pump; 5. Lower section of combined column; 6. Middle section of combined column; 7. Upper section of combined column; 8. First circulation pump; 9. First cooler; 10. Second circulation pump; 11. Second cooler; 12. Demister. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are some embodiments of the present invention, but not all embodiments.

[0022] Please see Figure 1 This invention provides a system for recovering excess hydrogen chloride and purifying chloromethane during chloromethane synthesis, mainly comprising a hydrogen chloride recovery system and a chloromethane purification system. The hydrogen chloride recovery system includes a reboiler 1, a fractionation tower 2, a condenser 3, and a transfer pump 4. The chloromethane purification system includes a combined drying tower, a first circulating pump 8, a first cooler 9, a second circulating pump 10, a second cooler 11, and a demister 12. The combined drying tower consists of a lower section 5, a middle section 6, and an upper section 7.

[0023] The fractionation tower 2 adopts a composite tray structure combining a high-efficiency directional floating valve and structured packing. A crude chloromethane inlet is set in the middle of the fractionation tower 2. A condenser 3 is connected to the top of the fractionation tower 2. A reboiler 1 and a chloromethane outlet are connected to the bottom of the fractionation tower 2. A transfer pump 4 is connected to the chloromethane outlet of the fractionation tower 2. The transfer pump 4 is connected to the lower section 5 of the combined tower.

[0024] The combined drying tower consists of a lower section 5, a middle section 6, and an upper section 7 connected in sequence. Each section uses sulfuric acid of different concentrations to perform step-by-step purification of chloromethane gas from fractionation tower 2, realizing the functional division of "coarse removal-main decomposition-fine removal", and efficiently removing impurities such as moisture and dimethyl ether in sequence.

[0025] The lower section 5 of the combined tower is equipped with a discharge port; the middle section 6 of the combined tower is connected to a first circulating pump 8, which is connected to a first cooler 9, forming a circulation loop of "middle section 6 of the combined tower - first circulating pump 8 - first cooler 9 - middle section 6 of the combined tower"; the upper section 7 of the combined tower is connected to a second circulating pump 10, which is connected to a second cooler 11, forming a circulation loop of "upper section 7 of the combined tower - second circulating pump 10 - second cooler 11 - upper section 7 of the combined tower"; a demister 12 is connected to the top of the upper section 7 of the combined tower. The circulation pumps and coolers prevent the sulfuric acid from overheating due to the heat of reaction.

[0026] A method for recovering excess hydrogen chloride and purifying chloromethane during chloromethane synthesis includes the following steps: Excess hydrogen chloride recovery: Crude chloromethane enters through the middle feed inlet of fractionation tower 2. The separated hydrogen chloride is discharged from the top of fractionation tower 2 and condensed by condenser 3 before entering the next process (recycled to the chloromethane synthesis process). Chloromethane is collected from the bottom of fractionation tower 2 and then sent to the combined drying tower for purification via transfer pump 4.

[0027] The system pressure of fractionation column 2 is controlled at 0.6 MPa, the temperature of condenser 3 is controlled at -15℃, and the temperature of reboiler 1 at the bottom of the column is controlled at 40℃.

[0028] The system pressure of fractionation column 2 can be maintained stable by introducing an inert gas (such as N2); the bottom liquid level is controlled by adjusting the amount of chloromethane product withdrawn from the bottom of the column; the temperature is controlled by monitoring the temperature of the sensitive plate and adjusting the amount of steam in reboiler 1.

[0029] After recovering hydrogen chloride in fractionation tower 2, the concentration of hydrochloric acid by-product in chloromethane is reduced to below 2%.

[0030] Chloromethane purification: The chloromethane separated by fractionation tower 2 enters the lower section 5, middle section 6 and upper section 7 of the combined tower in sequence. Different concentrations of sulfuric acid are used to purify the chloromethane in different tower sections in a stepwise manner, realizing the functional division of "coarse removal-main decomposition-fine removal".

[0031] The lower section 5 of the combined tower is a pre-absorption and buffer section: It uses relatively dilute sulfuric acid with a concentration of 75-85% to pre-absorb most of the moisture and acidic impurities in the crude chloromethane gas, preventing excessively rapid dilution and temperature rise of the concentrated sulfuric acid in the upper section, thus protecting the effectiveness of the concentrated sulfuric acid in the upper section. The operating pressure of the lower section 5 of the combined tower is 0.49-0.54 MPa, and the temperature is 25-35℃.

[0032] The lower section 5 of the combined tower has a sulfuric acid concentration of 75-85%. At this concentration, the dew point of the sulfuric acid at 25-35℃ is much lower than that of the chloromethane mixture. It can absorb water in only one direction without water return, and can pre-remove more than 80% of the water and acidic impurities.

[0033] Section 6 of the combined tower is the main reaction zone: it uses sulfuric acid with a concentration of 85-90%. This concentration of sulfuric acid can efficiently decompose the impurity methyl ether in chloromethane into methanol, while simultaneously absorbing a large amount of water. The operating pressure of section 6 of the combined tower is 0.49-0.54 MPa, and the temperature is 40-50℃.

[0034] The upper section 7 of the combined tower is the final drying and protection section: concentrated sulfuric acid with a concentration of 90-98% is used for final deep dehydration and further decomposition of residual dimethyl ether. The operating pressure of the upper section 7 of the combined tower is 0.49-0.54 MPa, and the temperature is 30-40℃.

[0035] During the reaction, the acid temperature rises due to the heat of reaction and the heat of dilution. If left untreated, this will lead to a sharp drop in concentrated acid efficiency and increased acid mist. This invention incorporates a circulating pump and a cooler in the middle section 6 and upper section 7 of the combined tower, respectively, to cool the sulfuric acid and prevent it from becoming too hot.

[0036] Independent circulating cooling is provided in the middle section 6 and the upper section 7 of the combined tower. The cooling water circulation is controlled to achieve the purpose of cooling and realize the stepped temperature control of the combined drying tower (25~35℃ / 40~50℃ / 30~40℃). There is no heat crosstalk between sections, which avoids the sudden drop in efficiency and the aggravation of acid mist caused by the rise in acid temperature.

[0037] Furthermore, a demister 12 is installed after the combined drying tower to ensure that the purified chloromethane gas is free of sulfuric acid droplets. The purity of the purified chloromethane can reach over 99.9%.

[0038] The method of this invention can stably reduce the concentration of by-product hydrochloric acid to below 2%, increase the purity of chloromethane product to over 99.9%, greatly reduce the equipment footprint, and significantly reduce the consumption of steam, cooling water, and electricity.

[0039] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; if these modifications and variations fall within the scope of the claims of the present invention and their equivalents, they should all be considered to be within the protection scope of the present invention.

Claims

1. A system for recovering excess hydrogen chloride and purifying chloromethane during chloromethane synthesis, comprising a hydrogen chloride recovery system and a chloromethane purification system, characterized in that, The hydrogen chloride recovery system includes a fractionation tower (2), with a condenser (3) connected to the top of the fractionation tower (2), a reboiler (1) and a chloromethane outlet connected to the bottom of the fractionation tower (2), and the chloromethane outlet connected to the chloromethane purification system via a transfer pump (4); the chloromethane purification system includes a combined drying tower, which consists of a lower section (5), a middle section (6) and an upper section (7) of the combined tower; the middle section (6) of the combined tower is connected to a first circulation pump (8), the first circulation pump (8) is connected to a first cooler (9), and the first cooler (9) is connected to the middle section (6) of the combined tower; the upper section (7) of the combined tower is connected to a second circulation pump (10), the second circulation pump (10) is connected to a second cooler (11), and the second cooler (11) is connected to the upper section (7) of the combined tower; a demister (12) is connected to the top of the upper section (7) of the combined tower.

2. The system for recovering excess hydrogen chloride and purifying chloromethane in chloromethane synthesis according to claim 1, characterized in that, The fractionation tower (2) adopts a composite tray structure combining a high-efficiency directional floating valve and structured packing. A crude chloromethane inlet is provided in the middle of the fractionation tower (2).

3. A method for recovering excess hydrogen chloride and purifying chloromethane in chloromethane synthesis, comprising the chloromethane synthesis excess hydrogen chloride recovery and chloromethane purification system as described in claim 1 or 2, characterized in that, Includes the following steps: Excess hydrogen chloride recovery: Crude chloromethane enters through the middle feed port of the fractionation tower (2), and the separated hydrogen chloride is discharged from the top of the fractionation tower (2). After being condensed by the condenser (3), it enters the next process. Chloromethane is collected from the bottom of the fractionation tower (2) and then enters the combined drying tower for purification via the transfer pump (4). Chloromethane purification: The chloromethane separated by the fractionation tower (2) enters the lower section (5), middle section (6) and upper section (7) of the combined tower in sequence. Different concentrations of sulfuric acid are used to purify the chloromethane in different tower sections in a stepwise manner, realizing the functional division of "coarse removal-main decomposition-fine removal". After purification by the combined drying tower, it enters the demister (12).

4. The method for recovering excess hydrogen chloride and purifying chloromethane in chloromethane synthesis according to claim 3, characterized in that, The system pressure of the fractionation tower (2) is controlled at 0.6 MPa, the temperature of the condenser (3) is controlled at -15℃, and the temperature of the reboiler (1) at the bottom of the tower is controlled at 40℃.

5. The method for recovering excess hydrogen chloride and purifying chloromethane in chloromethane synthesis according to claim 3, characterized in that, After recovering hydrogen chloride via fractionation tower (2), the concentration of hydrochloric acid by-product in chloromethane is reduced to below 2%.

6. The method for recovering excess hydrogen chloride and purifying chloromethane in chloromethane synthesis according to claim 3, characterized in that, The lower section (5) of the combined tower uses sulfuric acid with a concentration of 75-85%, an operating pressure of 0.49-0.54 MPa, and a temperature of 25-35℃.

7. The method for recovering excess hydrogen chloride and purifying chloromethane in chloromethane synthesis according to claim 3, characterized in that, The middle section (6) of the combined tower uses sulfuric acid with a concentration of 85~90%, with an operating pressure of 0.49~0.54MPa and a temperature of 40~50℃.

8. The method for recovering excess hydrogen chloride and purifying chloromethane in chloromethane synthesis according to claim 3, characterized in that, The upper section (7) of the combined tower uses concentrated sulfuric acid with a concentration of 90~98%, with an operating pressure of 0.49~0.54MPa and a temperature of 30~40℃.

9. The method for recovering excess hydrogen chloride and purifying chloromethane in chloromethane synthesis according to claim 3, characterized in that, Independent circulating cooling is provided for the middle section (6) and the upper section (7) of the combined tower, respectively, to maintain the temperature of the middle section (6) of the combined tower at 40~50℃ and the temperature of the upper section (7) of the combined tower at 30~40℃.

10. The method for recovering excess hydrogen chloride and purifying chloromethane in chloromethane synthesis according to claim 3, characterized in that, The purified chloromethane has a purity of over 99.9%.

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