The fine chemical crystallization process includes operation units such as cooling crystallization, solvent crystallization, feed liquid filtration, solvent displacement, mother liquor transfer, and discharge. The crystallization kettle, filtration equipment, and mother liquor storage tank will continuously emit solvent VOCs waste gas, which includes various mixed organic solvents such as ethyl acetate, dichloromethane, acetone, tetrahydrofuran, methanol, isopropanol, etc. Crystallization production belongs to a typical intermittent batch operation mode. During the solvent addition, vacuum pressure breaking, and discharge and exhaust stages, the concentration of exhaust gas instantly increases. During the cooling and crystallization settling stage in the kettle, the concentration of exhaust gas decreases. Under the staggered operation conditions of multiple crystallization equipment, the fluctuation amplitude of exhaust gas volume and concentration is large, and the switching of working conditions is frequent. The exhaust gas flow is prone to carrying solvent droplets, fine crystalline particles, material aerosols, and saturated water vapor. Halogenated solvents hydrolyze with water vapor to generate acidic corrosive media, which can corrode the gas collection pipeline and internal heat exchange and separation components of the system. The crystallization section belongs to Class A explosion-proof hazardous area, where organic solvents are flammable and explosive, and some materials are toxic. Strict technical conditions are required for the particle and droplet pretreatment ability, instantaneous concentration impact resistance, multi-component solvent adaptation ability, corrosion resistance, explosion-proof safety interlock mechanism, and long-term stability of intermittent operation of the solvent volatilization exhaust gas recovery system.
There are many practical shortcomings in the application of traditional waste gas treatment processes in the solvent volatilization waste gas conditions of fine chemical crystallization processes. The incineration process can oxidize and decompose organic pollutants, but the high-value organic solvents evaporated during the crystallization process will be directly burned, resulting in high material loss costs. When the exhaust gas contains halogenated components such as dichloromethane, high-temperature incineration can easily generate toxic and corrosive by-products such as hydrogen chloride, requiring the construction of complex post-treatment systems such as rapid cooling and exhaust gas absorption. Equipment investment, fuel consumption, and daily operation and maintenance costs remain high for a long time.activated charcoalAdsorption belongs to the method of pollutant transfer treatment and cannot achieve solvent resource recovery and utilization. Faced with the alternating impact of high and low concentrations of waste gas in the crystallization process, combined with the mist droplets and crystal particle impurities carried by the intake air, the adsorption filler is prone to gel and plate formation, quickly reaching adsorption saturation. The high concentration impact working condition also poses a safety hazard of adsorption heat release, requiring frequent shutdown and replacement of adsorption consumables. The output of hazardous waste is large, and the pressure on enterprise hazardous waste disposal and operation is significantly increased. The simple condensation process relies on low temperature to achieve solvent liquefaction and precipitation, and the recovery effect is good only during high concentration exhaust periods such as pressure breaking exhaust. Under low concentration crystallization conditions, the organic vapor partial pressure is insufficient, and the solvent recovery efficiency is greatly reduced. A large number of organic components are directly discharged with the exhaust gas, making it impossible to achieve stable resource utilization throughout the entire cycle. A large number of project implementation practices have shown that a single process mode is difficult to match the actual working conditions of intermittent pulses and complex and variable components in the crystallization process. The system's processing capacity is insufficient at peak exhaust times, and exhaust emissions in low concentration ranges are prone to exceed standards. It is difficult to simultaneously meet the multiple demands of production safety, environmental standards, and solvent recovery cost reduction.
The compression condensation film is coupled to a full physical treatment route, which is free of combustion and chemical reaction in the whole process, and is suitable for the solvent volatile waste gas recovery system in the fine chemical crystallization process. The waste gas produced by crystallization kettle, suction filtration device and mother liquor storage tank is collected and transported through closed negative pressure gas collecting pipeline network. The waste gas first enters the pre-treatment unit of the system, and relies on the multi-stage filtration defoaming structure to remove the solvent droplets, fine crystalline particles, material aerosols and free moisture carried by the airflow, so as to avoid impurities from entering the back-end compressor, heat exchanger and membrane module, resulting in fouling and blockage of the heat exchange surface and irreversible pollution of the membrane element. In view of the obvious entrainment of crystalline exhaust gas particles and large fluctuations in working conditions, the pretreatment unit adopts a hierarchical interception structure to complete purification protection step by step according to the particle size of impurities, effectively protect the precision core components at the back end, and extend the service life of the whole system. After pretreatment and purification, the waste gas enters the compression unit to increase the partial pressure of organic steam, change the gas-liquid phase equilibrium condition, and create favorable conditions for liquefaction of various mixed solvents. The gas is sent to the multi-stage gradient condensation module, and gradient cooling is carried out based on the boiling point difference of different solvents. Most organic solvents are liquefied and precipitated, and the preliminary solvent recovery is completed.
After the condensation treatment is completed, the tail gas still remains part of the unliquefied organic component, enters the organic membrane separation unit, and completes the enrichment and concentration of the organic component by means of the difference in the permeation rate of the polymer membrane material to the organic solvent molecules and the air molecules. The treated exhaust gas is discharged to the atmosphere, while the high-concentration organic gas stream enriched on the membrane side is recirculated back to the compressor inlet, forming a closed-loop system that further enhances the overall recovery rate of various mixed solvents. After precipitation, water separation filtration and index detection, the recovered liquid mixed organic solvent can be sent back to the crystallization section for recycling and reuse after its quality meets the requirements of the crystallization production process, so as to recover the solvent resources originally lost with the volatilization of waste gas, reduce the amount of hazardous waste generated from the source, and reduce the enterprise's solvent procurement cost and hazardous waste disposal expenses. The whole recovery system adopts a skid-mounted integrated structure. The factory completes the assembly of the whole machine, explosion-proof calibration and pressure test. The on-site construction workload is small, and the original gas gathering pipe network of the crystallization section can be directly connected to the original gas gathering pipe network of the crystallization section, thus minimizing the shutdown period of technological transformation and reducing the interference to the crystallization separation production operation.
The entire system has been designed in strict compliance with explosion-proof standards for the fine chemical industry. All pipelines, valves, and heat-exchange components that come into contact with waste gas are fabricated from corrosion-resistant materials to withstand the acidic corrosion caused by hydrolysis of halogenated media, thereby ensuring long-term continuous operation of the system. The system is equipped with comprehensive overpressure, overtemperature, and combustible gas detection as well as automatic interlock protection mechanisms to handle the instantaneous concentration surges caused by solvent replacement and reactor pressure relief, thereby mitigating various safety risks associated with flammable and explosive media. The intelligent variable-frequency control system automatically adjusts the system’s operating load based on changes in intake air volume and concentration, thereby matching the intermittent pulsed exhaust characteristics of the crystallization process. This prevents system overload or unnecessary idling, optimizing overall energy consumption. Based on on-site measurements of exhaust gas flow rate, component composition, concentration fluctuation ranges, particulate and droplet impurity content, ductwork layout, and workshop site conditions, we provide customized design solutions. Rather than relying on standardized, off-the-shelf models, we offer a full-service package that includes on-site condition assessment, process design, system equipment manufacturing, on-site installation and commissioning, operator training, and ongoing operation and maintenance.Standardized production and manufacturing, standardized on-site construction, and routine inspection and maintenance ensure the long-term reliable operation of the entire solvent-vapor recovery system, enabling compliant treatment of exhaust gases from fine chemical crystallization processes while achieving cost reduction and efficiency gains through the resource recovery of organic solvents.
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