How to determine the condensation temperature of VOCs solvent recovery? Design ideas of graded condensation and refrigeration series

In the VOCs organic solvent liquefaction recovery system, the condensation temperature is the core parameter to determine the recovery rate and operating energy consumption. If the temperature is set too low, the refrigeration energy consumption and frosting risk will go up. If it is set too high, the solvent residue in the tail gas will not come down. In engineering, a single temperature is usually not used, but graded condensation is used to combine different temperature levels to find a balance between recovery rate and energy consumption. The following describes the selection basis of the condensing temperature and the idea of the series arrangement.

The condensation temperature is not photographed.

• The type and ratio of solvents determine the basic temperature level: the slope of the saturated vapor pressure of different solvents varies with temperature;
• The inlet concentration determines the difficulty of recovery: the higher the concentration, the more condensate is produced at the same temperature level;
• The target recovery rate determines the degree of deep cooling: the higher the requirement, the lower the terminal temperature level;
• The moisture content determines the pre-cooling requirements: if the water is not removed first, the cryogenic section will be frosted and blocked.

How to divide the series of graded condensation

1. Level I (pre-cooling section):Reduce the exhaust gas to below normal temperature, remove water and high boiling point components, and recover part of the solvent at the same time;
2. Secondary (main condensing section):Determine the temperature level according to the saturated vapor pressure characteristics of the main solvent, and condense and liquefy most of the solvent;
3. Class III (cryogenic section):According to the requirements of the target recovery rate, further cooling is carried out to recover the low concentration residual solvent;
4. Rear section (membrane separation):The solvent components that are still gaseous after condensation enter the membrane unit for enrichment, and are recycled back to the front end.

Relationship between series and energy consumption

The number of stages increases and the recovery rate increases, but the refrigeration work required for each temperature drop is increasing. This is also why the engineering seldom relies solely on cryogenic pursuit of extreme recovery, but after cryogenic membrane separation: a relatively economical membrane unit is used to enrich the residual solvent back to the front end, so that the cryogenic section does not have to be too deep, and the overall energy consumption is more reasonable.

Ideas of Temperature Arrangement for Different Targets

Input conditions to be checked at design time

1. Exhaust volume and concentration range, including the maximum value under fluctuating conditions;
2. Solvent type, ratio and respective boiling point characteristics;
3. Moisture content of exhaust gas and whether it contains dust and acid and alkali components;
4. Target recovery and end discharge requirements;
5. Site cold source conditions and available utility capacity.

FAQ

Q: Can I cover all solvents with one uniform temperature?
Answer: Not recommended. The saturated vapor pressure of different solvents varies greatly, and the unified temperature level is either insufficient to recover or waste energy consumption, which should be calculated according to the system.

Q: Do I have to go to the deep cold section?
A: It depends on the target recovery rate and emission requirements. If the solvent reuse is the main purpose and the emission requirements are conventional, the two-stage condensation and membrane separation can often be met.

Q: How to solve the problem of frost?
Answer: The core is the pre-cooling dehumidification and heat exchanger switching design. The water is removed from the front end, and the cryogenic section is operated alternately in two ways, which can defrost without stopping the machine.

Q: How much more expensive is graded condensation than single-grade?
Answer: It depends on the series and the form of cold source. It cannot be generalized. The increase in investment in the classification scheme is usually recovered in terms of operating energy consumption and recovery income, which needs to be accounted for on a project basis.

LuoyangWohong PetrochemicalEquipment Co., Ltd. is engaged in the design and manufacture of condensation and membrane separation solvent liquefaction recovery system, which can be calculated according to the actual working conditions. Welcome to contact the exchange.