Request for the process flow of purifying food-grade CO₂ from PSA decarburization exhaust gases

Operating Procedures for Carbon Dioxide Recovery: This unit utilizes adsorption technology to remove high-boiling-point impurities and moisture, and distillation to remove non-condensable gases. Its basic principle relies on the selective adsorption properties of adsorbents toward different substances, as well as the variation in their adsorption capacity with temperature, in order to achieve the separation of gas mixtures and the regeneration of the adsorbents. The principle of distillation is to utilize the difference in boiling points between carbon dioxide and other impurity gases in order to purify carbon dioxide. Feed gas: The material to be separated in this unit is the carbon dioxide gas obtained through pressure swing adsorption in an ammonia synthesis plant. Its specifications are as follows: Composition of the feed gas: See Table 1-1. Table 1-1 Composition of feed gas (on a dry basis, V%) | Parameter | Value | H2S | 589 mg/Nm3 | COS | 9 mg/Nm3 | CS2 | 8 mg/Nm3 | CO2 | 90 | Others | 10. Temperature of the feed gas: ≤40°C. Pressure of the feed gas: Atmospheric pressure. Flow rate of the feed gas: 1664 Nm3/h. Product: This unit produces a single product, namely high-purity liquid CO2. Quality requirements: Superior to the relevant specified values in the standards GB1062-89 for food-grade CO2. Purity: CO2 content ≥ 99.9% (v%) Temperature: -18 °C Output pressure: ~2.0 MPa Production capacity: 20,000 tons per year (8,000 hours of operation) Yield: 85.0% The process of this unit consists of six stages: 200# rough desulfurization stage, 300# compression stage, 400# precise desulfurization stage, 500# pretreatment stage, 600# liquefaction and purification stage, and 700# storage and tanking stage. The process flow diagram is shown in Figure 1. Raw gas → Moisture removal → Blower → Cooling. In the 200# rough desulfurization step, part of the gas from the PSA decarburization system is evacuated; after pressure stabilization and mixing in a buffer tank, it is pressurized from 3–15 KPa to 40 KPa by a blower and sent to the primary desulfurizer, which is equipped with CNJT-1 type desulfurizing agent. A large amount of H2S is removed from the feed gas (the H2S content is reduced to below 20 mg/Nm3) before it enters the 300 compression process. The primary desulfurization in the desulfurization system employs a two-tower series-parallel operation mode. It includes equipment such as the blower CO201 and the rough desulfurization tower T0201 (A, B). 2. 300# Compression Process: The function of the 300# raw gas compression process is to compress the raw gas to the appropriate operating pressure. The feed gas compressor features three-stage compression. The feed gas is pressurized to 0.98 MPa at the exit of the second stage of the compressor (at a temperature of 65–90°C), and then sent to the desulfurization unit (400) to remove impurities such as sulfides and CH3OH. The purified feed gas is returned to the inlet of the third stage of the compressor. After being compressed to 3.4 MPa in three stages, it is sent to the liquefaction and purification process to further increase the concentration of carbon dioxide, thereby obtaining high-purity carbon dioxide product. It includes equipment such as raw gas compressors (C010A, B, C) and degreasers (T0301). 3. 400# Precision Desulfurization Process: The raw gas, which has been compressed to 0.98 MPa, enters the precision desulfurization stage (secondary desulfurization). In this stage, organic sulfur compounds as well as a small amount of inorganic sulfur are removed from the gas. After being cooled by cooler E0401 and free water is separated off using a water separator, the gas proceeds to the pre-treater, where impurities such as CH3OH are removed from it, ensuring that it meets the required standards. The secondary desulfurization agent is regenerated every six months; medium-pressure steam is used for high-temperature regeneration, while pre-treated feed gas is used for cold blowing. 4. 500# Pretreatment Process: The pretreatment is carried out using a six-tower, three-group variable temperature adsorption process, with two towers forming one group. The basic principle of the variable-temperature adsorption process is to take advantage of the significant difference in the adsorption capacity of the adsorbent for impurity components across different temperatures. Under conditions that allow the adsorbent to selectively adsorb certain components, impurities are selectively adsorbed at normal temperatures, thereby achieving gas purification. At high temperatures, the impurities adsorbed by the adsorbent are desorbed, allowing the adsorbent to be regenerated. During operation, one set of preprocessors is in the adsorption stage while the other two sets are in the regeneration stage. The adsorption time is four hours, and the regeneration time is eight hours; the regeneration gas comes from the vapor at the top of the 600# purification tower. The gas first enters the lower part of the primary preprocessor, which is in the adsorption stage (T0501A or T0501B, T0501C), where the moisture contained in the gas is removed through adsorption. It then passes through the upper part of the secondary preprocessor (T0502A or T0502B, T0502C) to have any remaining amounts of H2S and •CS removed from the gas. In each cycle of the preprocessor for each group, it must go through five steps: adsorption (A), depressurization (H), cooling (C), and pressurization (R), with some isolation time in between; the sequence is shown in Table 4-1. The regeneration of the adsorbent is carried out in four steps: 1. The pressure in the dryer is reduced to atmospheric pressure (D); the pressure is relieved in the opposite direction to that of adsorption, that is, toward the inlet side, and the gas is discharged outside the device. 2. Heating to desorb impurities (H): The flash vapor from the top of the 600 purification tower is used to heat and flush the adsorbent in the opposite direction to that of adsorption. 3. Once the heating regeneration of the cooling adsorbent (C) is complete and heating is stopped, the vapor flashing at the top of the 600 purification tower continues to flush the adsorbent, cooling it essentially to room temperature. 4 The stamping (R) of the dryer is used to pressurize another set of pre-treaters to the adsorption pressure for the treated carbon dioxide gas. As an example, consider the operation of the pretreater T0501(0502)A, which goes through five steps in one cycle, to illustrate the temperature-controlled adsorption process in this device. The timing sequence of the pretreatment system and the status of the valves are shown in Table 2-2. The coding rules for the programmable valves are as follows: V0iX – Number of the tower: from T0501(0502)A to T0501(0502)B. Valve functions: 1. Raw gas inlet valve; 2. Purified gas outlet valve (pressurization valve); 3. Regeneration heating gas inlet valve; 4. Regeneration waste gas outlet valve; 5. Regeneration cold blow gas inlet valve; 6. Regeneration cold blow waste gas outlet valve; 7. Main product gas outlet valve (main pressurization valve); 8. Main regeneration gas vent valve; 9. Main product gas vent valve. Switching valves: (1) Adsorption mode – Valves V01A and V02A are opened, while all other valves related to the pretreater T0501(0502)A remain closed. The desulfurized raw gas enters the pretreater T0501(0502)A through valve V01A. Impurities such as methanol in the raw gas are adsorbed under the adsorption pressure, and the purified raw gas is sent to the compression stage for three-stage compression. When the adsorption front of the impurities reaches the outlet end, valves V01A and V02A are closed, the feed gas supply to preprocessor T0501(0502)A is stopped, and the pressure inside the tower remains at the level corresponding to the adsorption state. (2) Reverse pressure relief (abbreviated as D): Immediately after the adsorption step of preprocessor T0501 (0502)A is completed, V04A is activated, and the gas with a high pressure inside the tower is discharged through V04A to the vent pipe. At this time, the non-condensable gases from the liquefaction and purification process flow directly into the vent main via valve V08 for discharge. (3) After the reverse pressure release in the heating regeneration (abbreviated as H) preprocessor T0501(0502)A is completed, valve V03A is opened; subsequently, valve V04A is also opened. The cold gas from another tower, after being heated by a heater, is used to heat and regenerate processor T0501(0502)A. The waste gas resulting from this regeneration process is discharged through valve V04A into the main vent line for disposal. (4) After the isolation (IS) heating regeneration is completed, close all valves connected to the preprocessor T0501(0502)A for approximately 30 minutes. (5) For cold blowing (C), the valves V05A and V06A are opened; the non-condensable gases from the liquefaction and purification process are used to directly cool the preprocessor T0501(0502)A. The waste gas then passes through valve V06A and is discharged to the regenerative heater for heating, so as to enable hot blowing in another tower. (6) Pressurization (abbreviated as R): After the cold blowing is completed, close valves V05A and V06A, and open valves V02A and V07. The raw material gas purified by the preprocessor T0501(0502)C is used to pressurize preprocessor T0501(0502)A to the adsorption pressure. At this time, the non-condensable gases from the liquefaction and purification process flow directly into the vent main via valve V08 for discharge. At this point, all the steps within one cycle for preprocessor T0501(0502)A have been completed, and it moves on to the next cycle. Preprocessors T0501(0502)B and T0501(0502)C carry out the same steps as tower A, only at different times to ensure that the separation process continues uninterrupted. Table 4-1: Timing Table for the Pretreatment System
Absorber – Process Steps and Times: 240, 15; 210, 15; 15, 210, 15.
Tower A: ADHISISCR; Tower B: ISCRADHIS; Tower C: DHISISCRA.
5,600# Liquidation and Purification Process: The gas that has been purified by the desulfurization and pretreatment systems is compressed three times by a compressor to a pressure of 3.4 MPa, after which it enters the liquidation and purification process. It undergoes heat exchange in heat exchanger E0601 before being liquefied in ammonia evaporation condenser E0602. For liquid CO2 products containing non-condensable gases, part of the CO2 and other non-condensable gases are removed through overhead flashing. The high-purity CO2 product obtained at the bottom of the tower is sent to Tower 700# via control valve LV602. The vapor from the tower top is reduced in pressure twice, through control valves PV601 and HV601, to around 0.1 MPa; after being preheated by heat exchanger E0601, it is sent outside the boundary zone. The principle of this process is based on distillation, which is used to separate carbon dioxide with a high boiling point from components with a lower boiling point, thereby obtaining high-purity liquid CO2 product. The packing in the upper packing layer of the purifier is made of type 4.5 stainless steel metal-rolled perforated corrugated packing. At the bottom of the tower, a coiled tube heat exchanger is used; the heat source for these coils is crude CO2 gas that has not been liquefied, with a temperature of around 40–140°C. At the top of the tower, a shell-and-tube heat exchanger is employed for heat exchange – the cold source here is the non-condensable gas resulting from flashing at the tower top, which is cooled to temperatures of around -40°C through throttling, and this gas exchanges heat with the rising non-condensable gas at temperatures of -5 to -10°C, thereby allowing the recovery of carbon dioxide gas from it. The refrigerant used for CO2 liquefaction is liquid ammonia, which enters the evaporation-condensation unit (E0602). Through the evaporation of liquid ammonia, CO2 is condensed into a liquid state, while the vaporized ammonia is directed to the gas phase of the ammonia-liquid separator. 6. 700# Storage and filling process: The high-purity liquid CO2 product is fed into a liquid storage tank, and then the liquid CO2 is pressurized using a loading pump before being filled into cryogenic tank trucks.

Reply to 2# sdsfhg_liu: Thank you very much. Why is that flowchart gone?