Acid gas removal system of ethylene device

Abstract

The invention discloses an acid gas removal system for an ethylene device, which belongs to chemical production and comprises an alkaline washing tower and a flow control module, a cracking gas feeding pipeline is communicated with the lower part of the alkaline washing tower and is positioned at the upper part of a baffle plate, and a quenched water heater and an online analyzer a are arranged on the cracking gas feeding pipeline; the quenching water heater is connected with a regulating valve h, a butter discharging pipeline and a liquid level controller a are arranged at the butter tank, a regulating valve a is arranged on the butter discharging pipeline, the alkaline washing tower is further provided with a discharging pipeline, and a regulating valve b is arranged on the discharging pipeline; the alkaline washing tower is further provided with a circulating system a, a circulating system b and a circulating system c, the boiler water supply and feeding pipeline is communicated with the pipeline a through the pipeline b, the pipeline b is provided with a flow controller e, a regulating valve g and a summation controller, and a baffle is arranged on the lower portion of the filler b. The method is mainly applied to chemical production.

Description

Technical Field

[0001] The present invention belongs to the technical field of chemical engineering, and particularly relates to an acidic gas removal system in an ethylene plant. Background Art

[0002] With the development of the global petrochemical industry, ethylene, as an important chemical raw material, has important application values in the fields of petrochemical, plastics, rubber, synthetic fibers, etc. At present, the production scale of ethylene continues to expand, but this is accompanied by higher requirements for environmental protection and efficient utilization of resources. In the process of ethylene production, acidic gas removal is a necessary key process step, and its technical performance directly affects production efficiency, energy consumption, and product quality.

[0003] At present, there are the following common problems in the acidic gas removal technology in ethylene production: First, the alkali concentration gradient of the existing caustic scrubber is unstable, resulting in problems such as a large consumption of caustic liquor, serious corrosion of the tower body, and low operating efficiency; second, the removal efficiency is relatively low, which not only increases energy consumption but also reduces the product purity; third, the water quality of the top water washing section is difficult to control, affecting the quality of the final product. The existence of these problems seriously restricts the sustainable development of the ethylene production industry. Therefore, there is an urgent need for an advanced removal method that can not only improve the removal efficiency, reduce energy consumption, but also ensure the stability of water quality. As described in CN201921442190.7, the method of using 3-stage caustic washing and 1-stage water washing is a relatively common treatment means at present, but the method described in this patent still cannot completely solve the above three problems. Especially in the water washing section, as the operation time of the device extends, more and more butter will accumulate in the water washing section, resulting in impure feed to the downstream drying unit of the system and exacerbating the pulverization of the molecular sieve in the downstream dryer. In view of the above problems, the present invention still adopts the method of 3-stage caustic washing and 1-stage water washing, but provides an improved method and correspondingly designs a new control method to achieve the automatic control function. Summary of the Invention

[0004] In order to solve the above technical problems, the present application provides an acidic gas removal system for an ethylene plant, and the technical solution adopted is as follows:

[0005] An acidic gas removal system for an ethylene plant includes a caustic scrubber and a flow control module. The upper part of the caustic scrubber is successively provided with a demister a, a bubble cap tray, a demister b, a sieve tray, a riser a, and a downcomer a. The middle part of the caustic scrubber is provided with a packing a, a riser b, and a downcomer b. The lower part of the caustic scrubber is provided with a packing b. A butter tank is provided below the baffle. Since the above equipment belongs to the prior art, the applicant will not elaborate here.

[0006] The core improvement of the present invention is as follows:

[0007] The cracked gas feed pipeline is connected to the lower part of the caustic scrubber and is located above the baffle. An emergency cooling water heater and an on-line analyzer a are provided on the cracked gas feed pipeline. The emergency cooling water heater is used to heat the cracked gas, and the on-line analyzer a is used to detect H 2 S and CO 2 content. The emergency cooling water heater is connected to a control valve h, and the control valve is used to control the flow rate. A butter discharge pipeline and a level controller a are provided at the butter tank. A control valve a is provided on the butter discharge pipeline. The caustic scrubber is also provided with a discharge pipeline, and a control valve b is provided on the discharge pipeline. The control valve b is used to control the discharge amount.

[0008] A circulation system a is provided at the lower part of the caustic scrubber. The circulation system a includes a weak base circulation pump and a washing oil injection port 1. The circulation system a is also provided with a stop valve a and a flow meter a. The circulation system a is used for the circulation of the weak base;

[0009] A circulation system b is provided in the middle of the caustic scrubber. The circulation system b includes a medium base circulation pump and a washing oil injection port 2. The circulation system b is also provided with a stop valve b and a flow meter b. The circulation system b is used for the circulation of the medium base;

[0010] A circulation system c is provided at the upper part of the caustic scrubber. The circulation system c includes a strong base circulation pump and a washing oil injection port 3. The circulation system c is also provided with a stop valve c and a flow meter c. The circulation system c is used for the circulation of the strong base;

[0011] The fresh caustic feed pipeline is connected to the strong base circulation pump. A ratio controller, a flow controller a and a control valve c are provided on the fresh caustic feed pipeline;

[0012] A cracked gas discharge pipeline is also provided at the upper part of the caustic scrubber;

[0013] The boiler feed water pipeline is connected to the upper part of the caustic scrubber. A flow controller c and a control valve d are respectively provided on the boiler feed water pipeline;

[0014] A waste water discharge pipeline is also provided at the upper part of the upper part of the caustic scrubber. A control valve e is provided on the waste water discharge pipeline;

[0015] The waste water discharge pipeline is connected to the strong base circulation pump through pipeline a. A control valve f and a flow controller d are provided on pipeline a;

[0016] The boiler feed water pipeline is connected to pipeline a through pipeline b. A flow controller e, a control valve g and a summing controller are provided on pipeline b.

[0017] Preferably: a baffle is provided at the lower part of the packing b.

[0018] Preferably: an on-line analyzer b is provided on the cracked gas discharge pipeline. The on-line analyzer b is used to detect H 2 S and CO 2 content.

[0019] The working principle of the present invention is as follows: The cracked gas from upstream is heated to 45°C by the quench water heater in the cracked gas feed pipeline and then enters the weak base section, medium base section, and strong base section of the caustic scrubber. Finally, it passes through the water wash section at the top of the tower to remove the alkali foam entrained in the cracked gas and enters the downstream device.

[0020] Online analyzers are provided in both the feed line of the caustic scrubber and the discharge line at the top of the tower to detect CO 2 and H 2 S.

[0021] Wash oil injection ports are provided at the outlets of the circulation pumps in each of the weak base section, medium base section, and strong base section to separately inject wash oil into each section to improve the separation effect of the alkali liquor and butter in each section and achieve the purpose of separately skimming the butter. At the same time, the main inlet of the wash oil can be connected to the boiler feed water to separately regulate the alkali concentration in each section to control the alkali concentration gradient.

[0022] Specifically, no separate circulation pump is provided in the water wash section of the present invention. After the boiler feed water is replenished into the water wash section, the water above the overflow port automatically flows into the strong base section and forms a certain ratio with the fresh alkali liquor replenished in the strong base section, just achieving the purpose of diluting 20% fresh alkali liquor into 10% alkali liquor. There is a stream of water continuously withdrawn from the bottom of the water wash section to continuously displace the water wash section, thereby ensuring the quality of the water wash section.

[0023] In order to ensure the stable dilution of 20% fresh alkali liquor into 10% and at the same time ensure the displacement volume of the water wash section, the specific control is as follows:

[0024] Normally, the boiler feed water is continuously replenished into the upper part of the water wash section by flow control, and the withdrawal from the bottom of the water wash section is automatically controlled by the liquid level of the water wash section.

[0025] In order to ensure the displacement of the water wash section while taking into account the utilization efficiency of the boiler feed water replenished into the water wash section, a branch is separated from the withdrawal line at the bottom of the water wash section and is controlled by a summing controller to be replenished into the suction port of the strong base pump. The fresh alkali delivered by the pump from the 20% fresh alkali tank also enters the suction port of the strong base pump under the control of a flow controller. The replenishment amount of the fresh alkali is comprehensively determined by the acid gas content in the feed of the caustic scrubber, the acid gas content in the discharge at the top of the caustic scrubber, and the alkali concentration gradient in each section of the caustic scrubber. The replenishment amount of the fresh alkali and the replenishment amount of the boiler feed water are automatically matched under the action of a ratio controller.

[0026] For example: According to the comprehensive situation of the caustic scrubber, it is necessary to supplement 20% of the fresh caustic soda with a flow rate of F2. To ensure that the supplemented caustic soda solution is diluted to 10%, under the action of the ratio controller, it will automatically require a flow rate of F2 to be diverted from the extraction line at the bottom of the water scrubbing section, thereby automatically controlling the opening degree of the regulating valve f. If the opening degree of the regulating valve f is too large, it indicates that the load of the caustic scrubber is high at this time, and the liquid level of the water scrubbing section may be pulled down. At this time, the low-level override control of the liquid level of the water scrubbing section will be automatically triggered to limit the further opening of the regulating valve f. To ensure the stability of the system, the regulating valve e will be automatically triggered at this time to supplement the part of the boiler feed water that cannot be supplemented by the regulating valve f with the regulating valve e.

[0027] The water scrubbing section of the caustic scrubber is designed as a bubble-cap tray, the strong caustic section is designed as a sieve tray, and the medium caustic section and the weak caustic section are both designed as structured packings. The gas is connected through risers between each section of the caustic scrubber, and the liquid falls into the next section through the downcomers of each section.

[0028] The bottom of the weak caustic section is separated by a partition. The right side is the mixing chamber of caustic soda and butter. Since butter is lighter than caustic soda, when the butter reaches a certain liquid level, it will automatically overflow over the partition and enter the butter chamber on the right side. Further static separation is achieved in the butter chamber. The caustic soda at the bottom is intermittently discharged manually, and the butter rises automatically with the liquid level and enters the butter trough in the butter chamber, and is continuously extracted under the action of the liquid level controller.

[0029] Compared with the prior art, the present invention has the following beneficial effects: 1. Cancel the water washing cycle; 2. Improve the quality of the water scrubbing section, and at the same time introduce advanced control to realize the automatic adjustment of the water and caustic soda supplement amounts of the caustic scrubber; 3. Realize the separate adjustment of the alkali concentration gradient of each caustic scrubbing section; 4. Realize the separate skimming of butter in each caustic scrubbing section; 5. Reduce the usage amount of caustic soda solution, and reduce problems such as serious tower body corrosion and low operation efficiency.

[0030] Implementation Case 1

[0031] A large ethylene production plant processes 5000 cubic meters of cracked gas per day. Before introducing the acid gas removal system of the present invention, the caustic scrubber had a large consumption of caustic soda solution. It consumed 5000 liters of fresh caustic soda solution with a concentration of 20% per day. And due to the unstable alkali concentration gradient, the tower body was severely corroded and needed to be repaired every month. The removal efficiency was only 80%, and the product purity was affected. The water quality of the water scrubbing section was difficult to control, and the molecular sieves of the downstream dryer were severely pulverized and needed to be replaced every two months.

[0032] After introducing the new system, the cracked gas is heated to 45°C by the quench water heater and enters the caustic scrubber. The on-line analyzer a monitors the contents of H2S and CO2 in the cracked gas feed in real time, and the regulating valve h accurately controls the flow rate of the quench water heater according to the data. The weak caustic section, the medium caustic section, and the strong caustic section are circulated through the circulation system respectively, and washing oil is injected into the washing oil injection ports respectively, effectively improving the separation effect of the caustic soda solution and butter.

[0033] The fresh caustic feed pipeline conveys fresh caustic solution to the strong caustic circulation pump stably through a ratio controller, a flow controller and a regulating valve. The boiler feed water pipeline supplies water to the upper part of the caustic scrubber through a flow controller and a regulating valve. The excess water in the water washing section automatically flows into the strong caustic section and is mixed with the fresh caustic solution proportionally to dilute the 20% fresh caustic solution into 10% caustic solution. Part of the water drawn from the bottom of the water washing section is discharged by the regulating valve, and the other part is replenished into the suction port of the strong caustic pump through the pipeline under the control of the regulating valve, the flow controller and the summing controller.

[0034] The operation results show that the consumption of caustic solution is greatly reduced, and only 3000 liters of 20% concentrated fresh caustic solution is consumed every day. The removal efficiency is increased to 95%, and the product purity is significantly improved. The water quality in the water washing section is stable, the service life of the molecular sieve in the downstream dryer is extended to more than one year, and the maintenance cost is greatly reduced.

[0035] Embodiment 2

[0036] A medium-sized ethylene production enterprise originally had a high-energy-consuming and low-efficiency acidic gas removal system. It processed 3000 cubic meters of cracked gas every day. Due to the problem of caustic concentration, the caustic scrubber operated unstably, and the removal efficiency was only 75%. The water quality in the water washing section was poor, resulting in high impurity content in the product and affecting sales.

[0037] After adopting the system of the present invention, accurate control is achieved in the links such as the heating of the cracked gas feed, the circulation of caustic solution in each section, and the water supply and drainage in the water washing section. The on-line analyzer monitors the contents of H2S and CO2 in the cracked gas discharge in real time, providing data support for system adjustment.

[0038] In the weak caustic section, the weak caustic circulation pump of the circulation system makes the weak caustic circulate continuously. When washing oil is injected from the washing oil injection port 1, the separation effect of the butter is improved. The same applies to the medium caustic section and the strong caustic section. The fresh caustic solution enters the strong caustic circulation pump stably through a flow controller and a regulating valve.

[0039] The boiler feed water enters the water washing section according to the set flow rate, and the excess water flows into the strong caustic section. Part of the water drawn from the bottom of the water washing section is discharged through the regulating valve, and the other part is replenished into the suction port of the strong caustic pump under the control of the summing controller, etc.

[0040] After running for a period of time, the caustic scrubber operates stably, the consumption of caustic solution is reduced by 35%, and the removal efficiency is increased to 92%. The water quality in the water washing section is effectively controlled, the impurity content of the product is reduced, the product quality is recognized by the market, and the economic benefits of the enterprise are significantly improved. Brief Description of the Drawings

[0041] The present invention will be further described below in conjunction with the drawings and embodiments;

[0042] Figure 1 It is the working principle diagram of the present invention. Detailed Embodiments

[0043] This section will describe in detail the specific embodiments of the present invention. The preferred embodiments of the present invention are shown in the accompanying drawings. The function of the accompanying drawings is to supplement the description in the text part of the specification, enabling people to intuitively and vividly understand each technical feature and the overall technical solution of the present invention. However, it should not be construed as a limitation on the protection scope of the present invention.

[0044] As Figure 1 shown:

[0045] An acidic gas removal system for an ethylene plant includes an alkali scrubbing tower 25 and a flow control module 7. The upper part of the alkali scrubbing tower 25 is successively provided with a demister a20-1, a bubble cap tray 21, a demister b20-2, a sieve tray 22, a riser pipe a23-1, and a downcomer a24-1. The middle part of the alkali scrubbing tower 25 is provided with a packing a26-1, a riser pipe b23-2, and a downcomer b24-2. The lower part of the alkali scrubbing tower 25 is provided with a packing b26-2. The lower part of the baffle 27 is provided with a grease tank 28.

[0046] The cracked gas feed pipeline is connected to the lower part of the alkali scrubbing tower 25 and is located above the baffle 27. An emergency cooling water heater 30 and an on-line analyzer a19-1 are provided on the cracked gas feed pipeline. The emergency cooling water heater 30 is connected to a regulating valve h31. A grease discharge pipeline and a liquid level controller a29 are provided at the grease tank 28. A regulating valve a34 is provided on the grease discharge pipeline. The alkali scrubbing tower 25 is also provided with a discharge pipeline, and a regulating valve b33 is provided on the discharge pipeline.

[0047] The lower part of the alkali scrubbing tower 25 is provided with a circulation system a. The circulation system a includes a weak base circulation pump 12 and a washing oil injection port 14-1. The circulation system a is also provided with a stop valve a38 and a flow meter a39.

[0048] The middle part of the alkali scrubbing tower is provided with a circulation system b. The circulation system b includes a medium base circulation pump 13 and a washing oil injection port 14-2. The circulation system b is also provided with a stop valve b35 and a flow meter b36.

[0049] The upper part of the alkali scrubbing tower is provided with a circulation system c. The circulation system c includes a strong base circulation pump 11 and a washing oil injection port 14-3. The circulation system c is also provided with a stop valve c16 and a flow meter c15.

[0050] The fresh alkali feed pipeline is connected to the strong base circulation pump 11. A ratio controller 8, a flow controller a9, and a regulating valve c10 are provided on the fresh alkali feed pipeline.

[0051] The upper part of the alkali scrubbing tower 25 is also provided with a cracked gas discharge pipeline.

[0052] The boiler feed water pipeline is connected to the upper part of the alkali scrubbing tower 25. A flow controller c17 and a regulating valve d18 are respectively provided on the boiler feed water pipeline.

[0053] The upper part of the caustic scrubber 25 is also provided with a waste water discharge pipeline, and a regulating valve e4 is arranged on the waste water discharge pipeline;

[0054] The waste water discharge pipeline is communicated with the strong base circulation pump 11 through pipeline a, and a regulating valve f5 and a flow controller d6 are arranged on pipeline a;

[0055] The boiler feed water inlet pipeline is communicated with pipeline a through pipeline b, and a flow controller e2, a regulating valve g3 and a summing controller 1 are arranged on pipeline b;

[0056] An on-line analyzer b19 is arranged on the cracked gas discharge pipeline.

[0057] The working principle of the present invention is as follows: The cracked gas from the upstream is heated to 45°C by the quench water heater 30 in the cracked gas feed pipeline and then enters the weak base section, medium base section, and strong base section of the caustic scrubber, and finally passes through the water washing section at the top of the caustic scrubber 25 to remove the alkali foam entrained in the cracked gas and enter the downstream device;

[0058] On-line analyzers are provided on both the feed line and the discharge line at the top of the caustic scrubber to detect CO 2 and H 2 S in the cracked gas.

[0059] Washing oil injection ports are provided at the outlets of the circulation pumps in the weak base section, medium base section, and strong base section respectively, to inject washing oil into each section separately to improve the separation effect of the alkali liquor and butter in each section and achieve the purpose of skimming butter separately. At the same time, the main inlet of the washing oil can be connected to the boiler feed water to adjust the alkali concentration in each section separately, so as to control the alkali concentration gradient.

[0060] In the water washing section of the present invention, a separate circulation pump is no longer provided. After the boiler feed water is replenished into the water washing section, the water higher than the overflow port automatically flows into the strong base section and forms a certain proportion with the fresh alkali liquor replenished into the strong base section, just achieving the purpose of diluting 20% fresh alkali liquor into 10% alkali liquor. There is a stream of water continuously extracted from the bottom of the water washing section to achieve the purpose of continuous replacement of the water washing section, thus ensuring the quality of the water washing section.

[0061] In order to ensure that 20% fresh alkali liquor is stably diluted to 10%, and at the same time ensure the replacement amount of the water washing section, the specific control is as follows:

[0062] Normally, boiler feed water is continuously replenished into the upper part of the water washing section by flow control, and the extraction at the bottom of the water washing section is automatically controlled by the liquid level of the water washing section.

[0063] In order to ensure the replacement of the water washing section while taking into account the utilization efficiency of the boiler feed water replenished into the water washing section, a branch line is separated from the extraction line at the bottom of the water washing section and fed into the suction port of the strong base pump controlled by a summing controller. Fresh alkali transported by a pump from the 20% fresh alkali tank also enters the suction port of the strong base pump under the control of a flow controller. The replenishment amount of fresh alkali is comprehensively determined by the acidic gas content in the feed of the alkali washing tower, the acidic gas content in the discharge at the top of the alkali washing tower, and the alkali concentration gradient in each section of the alkali washing tower. The replenishment amount of fresh alkali and the replenishment amount of boiler feed water are automatically matched under the action of a ratio controller.

Claims

1. An acid gas removal system for an ethylene device, comprising an alkali scrubber (25) and a flow control module (7), wherein the upper portion of the alkali scrubber (25) is provided with a demister a (20-1), a bubble tray (21), a demister b (20-2), a sieve tray (22), a gas riser a (23-1), and a liquid downcomer a (24-1) in sequence, the middle portion of the alkali scrubber (25) is provided with a filler a (26-1), a gas riser b (23-2), and a liquid downcomer b (24-2), the lower portion of the alkali scrubber (25) is provided with a filler b (26-2), and the lower portion of the baffle (27) is provided with a butter tank (28), characterized in that: The cracking gas feed pipeline is connected to the lower part of the alkali washing tower (25) and is located on the upper part of the baffle (27), wherein a quenching water heater (30) and an online analyzer a (19-1) are provided on the cracking gas feed pipeline, the quenching water heater (30) is connected to a regulating valve h (31), a butter discharge pipeline and a liquid level controller a (29) are provided at the butter tank (28), a regulating valve a (34) is provided on the butter discharge pipeline, and the alkali washing tower (25) is also provided with a discharge pipeline, and a regulating valve b (33) is provided on the discharge pipeline; A circulation system a is provided at the lower part of the alkali washing tower (25), and the circulation system a includes a weak alkali circulation pump (12) and a washing oil injection port 1 (14-1). The circulation system a is also provided with a stop valve a (38) and a flow meter a (39); A circulation system b is provided in the middle of the alkali washing tower, the circulation system b comprises an alkali circulation pump (13) and a second washing oil injection port (14-2), and the circulation system b is also provided with a stop valve b (35) and a flow meter b (36); A circulation system c is provided at the upper part of the alkali washing tower, the circulation system c comprises a strong alkali circulation pump (11) and a third washing oil injection port (14-3), and the circulation system c is also provided with a stop valve c (16) and a flow meter c (15); The fresh alkali feed pipeline is connected to the strong alkali circulation pump (11), and a ratio controller (8), a flow controller a (9) and a regulating valve c (10) are arranged on the fresh alkali feed pipeline; A cracking gas discharge pipeline is also provided at the upper part of the alkali washing tower (25); The boiler feed water feed pipeline is connected to the upper part of the alkali washing tower (25), and a flow controller c (17) and a regulating valve d (18) are respectively provided on the boiler feed water feed pipeline; A wastewater discharge pipeline is also provided at the upper part of the alkali washing tower (25), and a regulating valve e (4) is provided on the wastewater discharge pipeline; The wastewater discharge pipeline is connected to the strong alkali circulation pump (11) through pipeline a, and a regulating valve f (5) and a flow controller d (6) are provided on pipeline a; The boiler feed water feed pipeline is connected to the pipeline a through the pipeline b, and the pipeline b is provided with a flow controller e (2), a regulating valve g (3) and a summing controller (1).

2. An acid gas removal system for an ethylene plant according to claim 1, characterized in that: A baffle (27) is provided at the lower part of the filler b (26-2).

3. The acid gas removal system for an ethylene plant according to claim 1, characterized in that: An online analyzer b (19) is provided on the cracking gas discharge pipeline.

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