Clean production renovation project (1): Absorbent solution for off-gases from small-scale chlorohydrogen acid production

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Clean production improvement project (I): Absorbent solution for exhaust gases from small chlorohydrogen acid production. The absorbent solution used to handle the exhaust gases from such systems is one of the sources with high levels of acidic water discharge; the average discharge amount is 5,000 T per month, with a pH value of 1–2. It puts pressure on environmental protection; during treatment, it needs to be neutralized with caustic soda to a pH value of 6–9 before being discharged. It leads to waste of raw materials and increased costs for pollution control; therefore, upgrading this project is not only beneficial but also helps reduce pollution. I. Current Process 1. Simplified process flow diagram: Figure 1 – T2201: Secondary falling film absorber; T2202: Primary falling film absorber; P2201: Water flushing pump; V2201, V2202, V2203: Finished acid tanks. The HCL gas passes sequentially through the secondary falling film absorber T2201, the primary falling film absorber T2202, and then the water flushing pump P2201 before being discharged. At the top of the primary falling-film absorption tower T2202, a certain amount of tap water is added via a flow meter to absorb HCL gas and form dilute acid. This dilute acid is discharged from the bottom of the primary falling-film absorption tower T2202 and flows directly into the top of the secondary falling-film absorption tower T2201 for further absorption, resulting in 31% concentrated acid that is then sent to a storage tank. At the top of the water flushing pump P2201, tap water is used to absorb any remaining HCL gas from T2202, producing acidic water which is discharged directly, while the remaining exhaust gas is vented. 2. Disadvantage analysis: (1) Significant water waste. The tap water enters from above the water flushing pump P2201 and is discharged directly; the monthly flow rate is 5,000 tons, which amounts to 60,000 tons per year. At a cost of 1 yuan per ton for tap water, the annual expense is approximately 60,000 yuan. ⑵. Waste of raw materials: The exhaust gas discharged from the secondary falling film absorption tower T2201 still contains a certain amount of HCL gas. This portion of HCL gas is absorbed by the tap water entering from above the P2201, forming acidic water (pH value ~1) which is then discharged directly. It is estimated that the acidic water lost each month amounts to about 50 tons of 31% commercial hydrochloric acid; over the course of a year, this totals around 600 tons, with a value of over 300,000 yuan. (See Figure 1 for details.) II. Process Modification Concepts 1. Simplified process flow diagram Figure 2: T2201—Second-stage falling film absorber; T2202—First-stage falling film absorber; P2201—Water flushing pump; V2201, V2202—Finished acid tanks; E2201—Graphite cooler; V2203—Acidic water tank. The HCL gas passes through the second-stage falling film absorber T2201 and then the first-stage falling film absorber T2202. The HCL that has not been completely absorbed forms acidic water in the water flushing pump P2201. This acidic water is cooled by the graphite cooler E2201 before flowing into the acidic water tank V2203. It is then pumped back to the water flushing pump P2201 using an acid-resistant pump P2202 to form a cycle; once it reaches a certain concentration, it is sent to the first-stage falling film absorber T2202. (See Figure 2 for details.) 2. Advantages and disadvantages of the process: ⑴ Separation of clean and dirty water streams: As can be seen from the flow diagram, the system only discharges the cooling water from a few heat exchangers, allowing for complete recycling of this water ； There will be no more discharge of acidic wastewater, and the HCl gas in the exhaust gases will be fully absorbed. ⑵It prevents waste of raw materials: it increases the yield of the finished acid, reduces pollution control costs, and lowers pollutant emissions. ⑶The workload increases relatively: As can be seen from Figure 2, due to the addition of an acid-resistant pump, it is necessary to adjust the flow rate of the acidic water in tank V2203 so that it can be sent to the primary falling film absorber T2202, and this requires manual operation. III. Investment Estimate: This project involves modifications to the existing process, so the investment required is relatively low. The main investments include: one 10M2 graphite cooler E2201, costing 12,000 yuan ； Two acid-resistant pumps (one in operation and one as backup, 50-FS-35): 10,000 yuan ； Other piping valves: approximately 30,000 yuan. A total of 50,000 yuan needs to be invested. IV. Benefit Analysis 1) Annual additional cost: ⑴ The 4KW acid-resistant pump operates for 120 hours per month, with an annual operating time of around 1500 hours; it requires 6000 KWh of electricity, resulting in costs of approximately 3600 yuan. ⑵ The maintenance cost has increased by 4,000 yuan. The total is approximately 40,000 yuan. 2) The benefits that can be achieved include: ⑴ 60,000 tons of water saved per year, worth 60,000 yuan. ⑵ Reduction in waste emissions of 60,000 tons, worth 84,000 yuan. ⑶ An additional approximately 500 tons of commercial acid are produced each year, worth over 300,000 yuan. The total additional economic benefit is 450,000 yuan. 3) The actual annual economic benefit generated shall be no less than 400,000 yuan. PVC Workshop 2005.10.26