How can the chlorine gas exhaust treatment system in the chlor-alkali industry be optimized and improved?

The chlorine waste gas produced as a byproduct in the chlor-alkali industry is highly corrosive and toxic. If not effectively treated, it poses serious risks to the ecological environment and human health.Currently, most chlor-alkali chemical enterprises use the caustic soda absorption method to treat exhaust gas. The exhaust gas treatment system requires relevant improvements in system design, process control, media management, and equipment upgrades. These measures aim to enhance system operational stability and environmental performance, providing a feasible technical pathway for chlor-alkali enterprises to achieve efficient treatment of chlorine exhaust gas and the resource utilization of by-products.

During the operation of alkali electrolysis cells, saturated wet chlorine gas at 85–90°C is continuously generated and must undergo cooling and compression before it can be used as an industrial raw material. The core reaction equation for the alkali solution absorption process, which is currently the industry standard, is:2NaOH + Cl₂→ NaClO + NaCl +H₂O.This process utilizes sodium hydroxide solution to absorb chlorine gas, generating sodium hypochlorite solution with commercial applications, thereby achieving the dual objectives of treating exhaust gas and producing by-products.

1. Existing Operational Issues in Chlorine Waste Gas Treatment Systems

Considering the operating conditions of chlor-alkali production, the existing chlorine alkali solution absorption treatment system faces four major core issues that directly impact environmental control effectiveness and production safety, as detailed below:

2. Optimization and Improvement Measures for the Chlorine Waste Gas Treatment System

To address the aforementioned production defects, we have developed a systematic and targeted technical improvement plan based on process principles and equipment operating characteristics to comprehensively enhance the stability, safety, and resource utilization efficiency of the exhaust gas treatment system.

2.1 Adding a High-Level Recirculation System to Improve Absorption Efficiency:

A high-level sodium hypochlorite storage tank was added to continuously feed the absorption liquid into the secondary absorption tower via gravity flow, enabling a thorough secondary reaction with residual chlorine gas. This design extends gas-liquid contact time, improves chlorine absorption efficiency, reduces alkali consumption, inhibits the decomposition of sodium hypochlorite, stabilizes the redox potential, and ensures product quality.The traditional single-stage end-of-line absorption process is upgraded to a buffer storage tank system with a significant height difference, integrating multiple functions such as emergency buffering, deep reaction, and constant-pressure feeding.Leveraging the DCS system to achieve automated intelligent control, a dual-mode operation system is established: "homogeneous treatment during normal production" and "emergency venting during accident conditions." This strengthens the system’s ability to respond to operational fluctuations and sudden accidents, transforming exhaust gas treatment from passive end-of-pipe processing to an integrated model of active process control and resource recovery.

2.2 Optimizing Process Parameters to Enhance Safety Performance:

The lower limit for circulating alkali solution (NaOH) concentration has been explicitly raised from a broad empirical value to no less than 6.0%, thereby enhancing the system’s chemical buffering capacity and operational fault tolerance. During implementation, an online alkali concentration analyzer (or high-precision pH meter/conductivity meter) is installed on the main outlet pipe of the alkali circulation pump (in a section with stable temperature and uniform mixing). Measurement signals are transmitted in real time via a 4–20 mA signal to the DCS system.The control room program continuously compares the measured value with the set target of ≥6.0% and implements a two-level safety response mechanism: an audible and visual alarm is triggered when the concentration approaches the target value; if it further drops to a lower limit, the alkali replenishment program is automatically activated, proportionally opening the 32% concentrated alkali replenishment valve. If the system is equipped with a pure water dilution facility, the dilution valve is adjusted simultaneously to prevent significant concentration fluctuations.After the alkali supplement is mixed by the circulation pump, it is re-measured by the online instrument, forming a closed-loop control system until the concentration returns to the specified range.  

2.3 Upgrade the purified water piping to prevent scale formation:

The water source for alkali preparation and replenishment in the exhaust gas treatment system is fully replaced from mineral-containing process water with purified water having a conductivity <10 μS/cm and a total calcium and magnesium ion concentration ≤0.50 mg/L. This eliminates Ca²⁺, Mg²⁺, and other ions at the source, preventing the formation of scale such as CaCO₃ and Mg(OH)₂ on surfaces like spray distributors.

1) Construction of a Dedicated Pure Water Pipeline Network: Pipelines made of UPVC, PPH, or 316L stainless steel are connected to the product water outlet of reverse osmosis or ion exchange units, physically isolating the system from the production water system. An online conductivity meter and flow meter are installed at the pipeline network inlet. Data is uploaded in real-time to the DCS; when conductivity exceeds the limit, the system automatically cuts off the water supply and triggers an alarm, ensuring the water quality of the makeup water consistently meets standards.  

2) Precise Mixing and Scale Prevention: The alkali solution preparation tank employs an electric control valve linked to a mass flow meter to automatically and precisely mix purified water with 32% liquid caustic soda.A micro-continuous makeup water branch, interlocked with level, temperature, and crystallization trends, is added to the circulation tank. The DCS dynamically fine-tunes the makeup water volume based on heat exchanger temperature difference, spray pressure drop, and circulation flow rate, maintaining salts in a subsaturated state. This achieves continuous, low-intensity scale prevention, avoids disturbances caused by traditional high-volume flushing, and stabilizes spray distribution, heat transfer efficiency, and circulation pump operating conditions, thereby extending continuous operation cycles.

The stable and efficient operation of the chlorine gas exhaust treatment unit is a core guarantee for compliant production and sustainable development in the chlor-alkali industry, and it also impacts the surrounding ecological environment and public health and safety.To address the shortcomings of traditional caustic solution absorption processes, optimization measures—such as adding an elevated buffer system, optimizing automated control parameters, upgrading the purified water supply system, and implementing source-level prevention of scaling and corrosion—can effectively resolve existing production challenges. These measures enable efficient compliance treatment of chlorine gas emissions, improve byproduct quality, and ensure long-term stable system operation, thereby achieving efficient treatment of chlorine gas emissions in the chlor-alkali chemical industry.

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