Pre-tower discharged air recovery method and application

Abstract

The invention relates to the technical field of crude alcohol rectification processing, in particular to a pre-tower vent gas recovery method and application, and has the beneficial effects that gas secondarily cooled out of a pre-tower enters a purge gas washing tower, steam containing a small amount of methanol is absorbed by soft water and then is vented to a torch, and a large amount of methanol condensable components exist in vent gas, so that the gas can be recycled; a cooler is additionally arranged to further cool vent gas, a liquid phase is sent to a crude alcohol tank and then sent to a rectification system, non-condensable gas is sent to a coal grinding station hot blast stove to replace part of fuel gas, meanwhile, potential safety and environmental protection hazards existing in the vent gas are eliminated, economic benefits are remarkable, primary water serves as a cooler cold source, and the primary water is sent to a desalted water station for use after being heated; the water mixing efficiency of desalted water can be improved, and the water consumption is reduced; meanwhile, non-condensable gas is conveyed to a fuel gas pipe network, the fuel gas consumption is further reduced, the refined alcohol yield is increased, and the comprehensive energy consumption is reduced.

Description

A method for recovering pre-exhaust air and its application Technical Field

[0001] This invention relates to the field of crude alcohol distillation processing technology, specifically to a method and application for recovering pre-tower vented air. Background Technology

[0002] In the existing technology, the crude alcohol gas needs to be heated during the distillation process to form two substances: liquid and gas. The gas phase after cleaning has a complex structure and many components. The common treatment method is to directly transport it to the emergency flare location for ignition.

[0003] However, in actual processing, the pre-tower releases a large amount of vented air. This vented air not only cannot be utilized, but also causes instability in the gas pressure during processing. This wastes energy and affects the safety and stability of the processing. Furthermore, if a malfunction occurs, the non-condensable gases generated in the vented air cannot be discharged normally, which can easily lead to safety accidents such as explosions. Summary of the Invention

[0004] The purpose of this invention is to provide a pre-tower venting air recovery method to solve the problems mentioned in the background art.

[0005] To achieve the above objectives, the present invention provides the following technical solution: a method for recovering vented air from a pre-tower, characterized in that the recovery method includes the following steps: Step 1: The qualified crude methanol produced is weakly alkaline after neutralization reaction. The neutralized crude methanol is first passed through a preheater, and the condensate from the hot end outlet of the reboiler is heated to 72°C before entering the pre-tower; Step 2: The temperature at the bottom of the pre-tower is maintained at 79°C by the reboiler, and the temperature at the top of the pre-tower is controlled at 76°C by the reflux liquid. The liquid phase at the bottom of the pre-tower is pumped to a pressurized tower for distillation operation by the pre-tower reflux pump. Step 3: The vent air from the secondary cooling unit enters the purge gas scrubbing tower, where soft water absorbs the methanol vapor it contains, and then it is connected to the emergency flare. Step 4: The vent air contains a large amount of methanol-like condensable components. After pressurizing, the vent air is fed into the cooler for further cooling. After being cooled by the cooler and heat exchanged with primary water, the vent air enters the gas-liquid separator. The cooled liquid phase enters the crude alcohol tank and is pumped to the distillation system. Some of the non-condensable gas enters the gas compressor and is pressurized to 0.4 MPa, and then transported to the fuel gas pipeline network.

[0006] Preferably, in step one, the crude alcohol is metered into the distillation system pipeline by a 5% sodium hydroxide solution to neutralize the organic acid, mainly formic acid, generated in the synthesis reaction.

[0007] Preferably, in step two, reboilers A and B are installed on both sides of the lower part of the pre-tower. The two sets of reboilers are heated with 0.5 MPa saturated steam to maintain the temperature at the bottom of the pre-tower at 79°C. The gas coming out of the top of the pre-tower is cooled to 68°C and 48°C by circulating water through a series of primary and secondary condensers. The condensed methanol solution is collected in the pre-tower reflux tank. The temperature after mixing is 65°C. After being pressurized by the reflux pump, it enters the tower from the top of the pre-tower as reflux liquid. The temperature at the top of the pre-tower is controlled at 76°C by the reflux liquid.

[0008] Preferably, in step four, the venting air pressure is controlled by adjusting the variable frequency fan, and the pressure is increased to 100 kPa before being sent to the cooler.

[0009] Preferably, in step four, the pipelines for conveying primary water and demineralized water are connected to a cooler as a cold source for venting and cooling. After the venting air exchanges heat with the primary water, the water temperature rises from 17 degrees to 19 degrees. The pipelines are equipped with manual valves, regulating valves, pressure gauges, and flow meters, which are adjusted in real time according to the outlet temperature of the cooler.

[0010] Preferably, the non-condensable gas in step four is divided into two pipelines: one pipeline is sent to the fuel gas network, and the other pipeline is sent to the emergency flare. Both pipelines are equipped with shut-off valves.

[0011] Preferably, an oxygen analyzer and an online chromatographic analyzer are installed on the pipeline connecting the non-condensable gas to the fuel gas pipeline network.

[0012] Preferably, the pipeline connecting the vent gas scrubber to the emergency flare is equipped with a shut-off valve that is automatically controlled according to the pressure. The pipeline connecting the vent gas scrubber to the emergency flare is connected to the variable frequency fan through a diversion pipeline, and a shut-off valve is installed on the diversion pipeline.

[0013] Preferably, the air venting capacity of the pre-tower is 1200-1500 Nm³. 3 / h, the main components include methanol 3-8%, dimethyl ether 25-30%, non-condensable gases 50-60% and water vapor 1-3%, as well as trace impurities, including methyl formate, methane, acetaldehyde and hydrogen.

[0014] An application of a pre-tower vent air recovery method, wherein the above-mentioned pre-tower vent air recovery method is applied to the distillation processing of crude alcohol.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows: The present invention introduces the gas from the pre-cooling tower into the purge gas scrubbing tower, absorbs the small amount of methanol vapor with soft water, and then vents it to the flare. The vent gas contains a large amount of methanol-like condensable components. By adding a cooler, the vent gas is further cooled. The liquid phase is sent to the crude alcohol tank and then to the distillation system, while the non-condensable gas is sent to the hot air furnace of the coal mill to replace part of the fuel gas. At the same time, the safety and environmental hazards of the vent gas are eliminated, resulting in significant economic benefits. Using primary water as the cooler's cold source, the primary water is heated and then sent to the demineralized water station, which can improve the demineralized water blending efficiency and reduce water consumption. Meanwhile, the non-condensable gas is sent to the fuel gas pipeline network, further reducing fuel gas consumption, increasing refined alcohol production, and reducing overall energy consumption. Attached Figure Description

[0016] Figure 1 is a system flowchart of the recycling method of the present invention; Detailed Implementation

[0017] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0018] Please refer to Figure 1. This invention provides a technical solution: a method for recovering vented air from a pre-distillation tower. The recovery method includes the following steps: Step 1: Crude alcohol is metered and injected with a 5% sodium hydroxide solution into the pipeline leading to the distillation system. This neutralizes the organic acid, mainly formic acid, generated in the synthesis reaction. The qualified crude alcohol produced is weakly alkaline after neutralization. The neutralized crude methanol is first heated to 72°C by the vapor condensate from the hot end outlet of the reboiler and then enters the pre-distillation tower. Step 2: The temperature at the bottom of the pre-distillation tower is maintained at 79°C by the reboiler, and the temperature at the top of the pre-distillation tower is controlled at 76°C by the reflux liquid. The liquid phase at the bottom of the pre-distillation tower is pumped to the pressurized tower for distillation. Reboilers A and B are installed on both sides of the lower part of the pre-distillation tower. Two sets of reboilers are heated with 0.5 MPa saturated steam to maintain the temperature at the bottom of the pre-tower at 79°C. The gas exiting the top of the pre-tower is condensed in stages by circulating water through a series of primary and secondary coolers, reducing the temperature to 68°C and 48°C respectively. The condensed methanol solution is collected in the pre-tower reflux tank. The temperature of the mixture is 65°C. After being pressurized by the reflux pump, it enters the tower from the top as reflux liquid. The temperature at the top of the pre-tower is controlled at 76°C by the reflux liquid. Step 3: The vent air from the secondary cooler enters the purge gas scrubbing tower, where the methanol vapor it contains is absorbed by soft water. Then, the emergency flare is connected. The vent air volume of the pre-tower is 1200-1500 Nm³. 3 / h, the main components include methanol 3-8%, dimethyl ether 25-30%, non-condensable gases 50-60%, and water vapor 1-3%, as well as trace impurities, including methyl formate, methane, acetaldehyde, and hydrogen. A pressure-controlled shut-off valve 01 is installed on the pipeline connecting the purge gas scrubber to the emergency flare. The pipeline connecting the purge gas scrubber to the emergency flare is connected to a variable frequency fan via a branch line, on which a shut-off valve 03 is installed. Step four: The vent gas contains a large amount of methanol-like condensable components. After pressurization, the vent gas is fed into a cooler for further cooling. After cooling in the cooler and heat exchange with primary water, the vent gas enters the gas-liquid separator. The cooled liquid phase enters the crude alcohol tank and is then pumped to the distillation system. Part of the non-condensable gas enters the gas compressor and is pressurized to 0.4 MPa, then transported to the fuel gas pipeline network. The vent air pressure is controlled by adjusting the variable frequency fan. After the pressure is increased to 100 kPa, it is sent to the cooler. The primary water and demineralized water pipelines are connected to the cooler as a cold source for cooling the vent air. After the vent air exchanges heat with the primary water, the water temperature rises from 17 degrees to 19 degrees. The pipelines are equipped with manual valves, regulating valves, pressure gauges, and flow meters, which are adjusted in real time according to the cooler outlet temperature. The non-condensable gas is divided into two pipelines: one pipeline is sent to the fuel gas pipeline network, and the other pipeline is sent to the emergency flare. Both pipelines are equipped with shut-off valves 04 / 02. The pipeline connecting the non-condensable gas to the fuel gas pipeline network is equipped with an oxygen analyzer and an online chromatograph.

[0019] An application of a pre-tower vent air recovery method, wherein the pre-tower vent air recovery method is applied to the distillation processing of crude alcohol.

[0020] Working principle: The qualified crude methanol produced by the synthesis unit is metered and injected with a 5% sodium hydroxide solution into the main feed pipe of the refined methanol system to neutralize the organic acids, mainly formic acid, generated in the synthesis reaction, making the crude methanol weakly alkaline (pH=8-9). The neutralized crude methanol first passes through a preheater, and the steam condensate from the hot end outlet of the pre-tower reboiler is heated to about 72°C before entering the pre-tower. Pre-tower reboilers A / B are installed on both sides of the lower part of the pre-tower, and the reboilers are heated with 0.5 MPa saturated steam to indirectly heat the liquid crude methanol at the bottom of the pre-tower, maintaining the bottom temperature of the pre-tower at about 79°C. The top temperature of the pre-tower is controlled at about 76°C by the reflux liquid. The gas coming out of the top of the pre-tower is condensed by circulating water in the first and second pre-tower condensers in series, and the temperature is reduced to 68°C and 48°C respectively. The condensed methanol solution is collected in the pre-tower reflux tank (mixing temperature 65°C), and after being pressurized by the reflux pump, it enters the tower from the top of the pre-tower as reflux liquid.

[0021] The pre-tower venting volume is approximately 1200-1500 Nm³ / h, temperature 42 degrees Celsius, and pressure 42 kPa (main components are methanol (3-8%), dimethyl ether 25-30%, non-condensable gases (N₃+CO+CO₂) 50-60%, and water vapor 1-3%). The venting pressure is controlled by a variable frequency fan. After the pressure is increased to 100 kPa, the venting is sent to the cooler. This pipeline is equipped with pressure, temperature, and flow meters. The venting air exchanges heat with primary water (17-20 degrees Celsius) throughout its journey through the cooler before entering the gas-liquid separator. The cooler is equipped with a secondary regulating valve, which mainly controls the cooler outlet temperature. The cooled liquid phase enters the crude alcohol tank, and part of the non-condensable gas is pressurized to 0.4 MPa by the gas compressor and sent to the fuel gas pipeline, indirectly reducing the membrane hydrogen extraction load, increasing the synthesis cycle volume, and improving the refined alcohol yield.

[0022] Non-condensable gas is normally supplied in two lines: one line is sent to the fuel gas network, and the other line is sent to the emergency flare. This ensures that if either line fails, the non-condensable gas can be discharged normally without affecting the recovery and utilization of the preheater vent air.

[0023] The existing fuel gas pipeline network is intelligently controlled by PIC. The original logic of pressure balance control is as follows: 1. The normal fuel gas pipeline network is controlled by the PIC pressure controller, which directly adjusts the fuel gas delivery valve. When the pressure is high, the vent valve is adjusted to the flare to ensure the pressure of the fuel gas pipeline network is stable.

[0024] 2. The pre-tower pressure controller directly adjusts the air vent valve.

[0025] Adjusted logic split-range control: 1. When the fuel gas pipeline pressure is low and recovery is required, the PIC controls the recovery regulating valve to send fuel gas to the fuel gas system, and the recovery system sends fuel gas to the flare, which automatically adjusts the valve according to the venting pressure; 2. When the fuel gas pipeline pressure is high or the recovery system malfunctions, the PIC automatically switches to flare venting to ensure that the pre-tower venting pressure is normal.

[0026] Low-selection control: A low-selection device is introduced into the system. The input signals are the output of the pre-tower pressure controller and the high-limit inverse logic of the fuel gas pipeline pressure. When the fuel network pressure is lower than the set value, the pre-tower vented air is allowed to be recovered to the fuel network to prevent reverse flow when the fuel gas pressure is high, thus ensuring safety.

[0027] Safety interlock logic: 1. High oxygen content interlock: Install an oxygen analyzer AT1101 on the pipeline before the vent enters the fuel gas main. If the oxygen content is greater than or equal to 2%, the interlock closes shut-off valve 02 and opens shut-off valve 04; 2. Abnormal composition interlock: If the online chromatographic analysis AT1102 shows low combustible components, the interlock closes shut-off valve 02 and automatically switches to shut-off valve 04; 3. When the blower trips due to a malfunction, the interlock closes shut-off valve 03 on the blower inlet pipeline. Shut-off valve 01 is automatically controlled according to the pressure. The interlock closes shut-off valve 02 on the non-condensable gas to fuel gas pipeline and shuts off the non-condensable gas to the emergency flare pipe. 4. When the air venting compressor pressure trips or the recovery pipeline pressure is low, the shut-off valve 02 for non-condensable gas to fuel gas network is interlocked closed, and the shut-off valve 04 for non-condensable gas to the emergency flare is opened, ensuring the normal air venting pressure of the distillation pre-tower is maintained; 5. If the pre-tower air venting pressure value is too high, the shut-off valve 01 is interlocked open, the shut-off valve 03 is interlocked closed, and the blower is interlocked to stop. The shut-off valve 01 is automatically controlled according to the pressure, and the shut-off valve 02 for non-condensable gas to fuel gas network and the shut-off valve 04 for non-condensable gas to the flare are interlocked closed, achieving safe and stable operation of the pre-tower venting system.

[0028] The essence of the above-mentioned crude methanol process is safety, energy saving and environmental protection. After the vent air of the pre-tower, which has a capacity of about 1200-1500 Nm3 / h, is modified and recycled, based on an annual production of 700,000 tons of refined methanol, about 500 tons of methanol can be recovered from the vent air annually, resulting in significant economic benefits. Using primary water as the cooling source for the cooler, the temperature of the primary water can be raised by 3-5 degrees Celsius after heat exchange. After the temperature is raised, it is sent to the demineralized water production station to prepare demineralized water, thereby improving the demineralized water production efficiency and reducing the amount of primary water used. Non-condensable gases in the vent air of the pre-tower are sent to the fuel gas pipeline network, further reducing the amount of fuel gas used and increasing the yield of refined methanol. Through this modification, the energy consumption of the entire system is reduced, and the impact of vent air on the site environment is mitigated, resulting in significant environmental benefits.

[0029] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for recovering pre-tower vented air, characterized in that: The recovery method includes the following steps: Step 1: The qualified crude methanol produced is weakly alkaline after neutralization. The neutralized crude methanol is first heated to 72°C by the steam condensate from the hot end outlet of the reboiler and then enters the pre-tower. Step 2: The temperature at the bottom of the pre-tower is maintained at 79°C by the reboiler, and the temperature at the top of the pre-tower is controlled at 76°C by the reflux liquid. The liquid phase at the bottom of the pre-tower is pumped to the pressurized tower for distillation. Step 3: The vent air from the secondary condenser enters the purge gas scrubbing tower, where the methanol vapor it contains is absorbed by soft water, and then connected to the emergency flare. Step 4: The vent air contains a large amount of methanol-like condensable components. The vent air is pressurized and fed into the cooler for further cooling. After cooling by the cooler and heat exchange with primary water, the vent air enters the gas-liquid separator. The cooled liquid phase enters the crude methanol tank and is pumped to the distillation system. Some non-condensable gases are pressurized to 0.4 MPa by the gas compressor and then transported to the fuel gas pipeline.

2. The method for recovering pre-tower vented air according to claim 1, characterized in that: In step one, the crude alcohol is metered into the distillation system via a 5% sodium hydroxide solution, which neutralizes the formic acid and generates an organic acid, primarily formic acid, in the synthesis reaction.

3. The method for recovering pre-tower vented air according to claim 1, characterized in that: In step two, reboilers A and B are installed on both sides of the lower part of the pre-tower. The two sets of reboilers are heated with 0.5 MPa saturated steam to maintain the temperature at the bottom of the pre-tower at 79°C. The gas coming out of the top of the pre-tower is cooled to 68°C and 48°C by circulating water through a series of primary and secondary condensers. The condensed methanol solution is collected in the pre-tower reflux tank. The temperature after mixing is 65°C. After being pressurized by the reflux pump, it enters the tower from the top of the pre-tower as reflux liquid. The temperature at the top of the pre-tower is controlled at 76°C by the reflux liquid.

4. The method for recovering pre-tower vented air according to claim 1, characterized in that: In step four, the venting air pressure is controlled by adjusting the variable frequency fan. After the pressure is raised to 100 kPa, the venting air is sent to the cooler.

5. The method for recovering pre-tower vented air according to claim 1, characterized in that: In step four, the pipelines for conveying primary water and demineralized water are connected to a cooler as a cold source for venting air to lower the temperature. After the venting air exchanges heat with the primary water, the water temperature rises from 17 degrees to 19 degrees. The pipelines are equipped with manual valves, regulating valves, pressure gauges, and flow meters, which are adjusted in real time according to the outlet temperature of the cooler.

6. The method for recovering pre-tower vented air according to claim 1, characterized in that: In step four, the non-condensable gas is divided into two pipelines: one pipeline is sent to the fuel gas network, and the other pipeline is sent to the emergency flare. Both pipelines are equipped with shut-off valves 04 / 02.

7. The method for recovering pre-tower vented air according to claim 6, characterized in that: An oxygen analyzer and an online chromatographic analyzer are installed on the pipeline connecting the non-condensable gas to the fuel gas pipeline network.

8. The method for recovering pre-tower vented air according to claim 1, characterized in that: The pipeline connecting the vent gas scrubber to the emergency flare is equipped with a shut-off valve 01 that is automatically controlled according to the pressure. The pipeline connecting the vent gas scrubber to the emergency flare is connected to the variable frequency fan through a diversion pipeline, and a shut-off valve 03 is installed on the diversion pipeline.

9. The method for recovering pre-tower vented air according to claim 1, characterized in that: The air venting capacity of the pre-tower is 1200-1500 Nm. 3 / h, the main components include methanol 3-8%, dimethyl ether 25-30%, non-condensable gases 50-60% and water vapor 1-3%, as well as trace impurities, including methyl formate, methane, acetaldehyde and hydrogen.

10. An application of a pre-tower venting air recovery method, characterized in that: The pre-tower venting recovery method according to any one of claims 1-9 is applied to the distillation of crude alcohol.

Images