The absorption method for solvent recovery essentially involves using liquid absorbents to "wash" the solvent vapor from the exhaust gas: low concentration, high air volume exhaust gas comes into contact with gas and liquid in the tower, and solvent molecules are transferred from the gas phase to the liquid phase, becoming a low flow, high concentration rich liquid. Then, through distillation or stripping regeneration, the solvent is separated and reused. It does not pursue a single device to complete the task, but undertakes the key link of "enrichment", often forming a complete recovery chain with condensation and distillation.
When the exhaust gas and absorbent come into countercurrent contact in the tower, and the solvent partial pressure in the gas phase is higher than the equilibrium partial pressure in the liquid phase, the solvent continuously enters the liquid phase, which is the driving force for absorption. The lower the inlet concentration of the absorbent, the more complete the gas-liquid contact, and the greater the driving force, the better the absorption effect. The liquid that has absorbed solvent is called rich liquid and is sent to the regeneration unit; The lean solution with recovered concentration after regeneration is circulated back to the tower, forming a closed loop through this process.
The most typical scenario is low concentration and high air volume: direct condensation of such exhaust gases is not economical, and direct emissions do not meet standards. First, absorption is used to increase the concentration and reduce the air volume, and then the back-end condensation or distillation unit can be made smaller. Solvents with good water solubility (such as methanol, acetone, ethylene glycol ethers, etc.) have a direct absorption effect with water; Hydrophobic solvents should be selected as specialized absorbents with high solubility for the target component.
On the other hand, direct condensation of exhaust gas with high concentration is more convenient, and the positioning of absorption method is "low concentration finishing and enrichment". The two types of processes are in a relay relationship rather than a competitive relationship.
There are four basic requirements for absorbents: high solubility in the target solvent, low self volatilization, easy regeneration, and compatibility with process materials without introducing impurities. A packed tower with a large gas-liquid contact area and reduced pressure is a commonly used choice for tail gas absorption; The circulating pump should be selected according to the flow rate of the lean solution and leave a margin. The bottom liquid level and the replenishment and drainage liquid should be automatically controlled to avoid a decrease in absorption driving force caused by high concentration of the rich solution.
| Absorbent route | Applicable scenarios | Configuration points |
|---|---|---|
| Water absorption | Water soluble solvents such as methanol and acetone | The absorption solution can be directly sent for distillation separation, with the lowest cost |
| Specialized organic absorbent | Hydrophobic and insoluble solvents in water | Choose varieties with high solubility and easy regeneration to prevent impurities from being introduced by volatilization |
| Absorption+Distillation Regeneration Combination | In situations where solvent recovery is required | Rich liquid continuous regeneration, lean liquid circulation, solvent condensation recovery |
There are two common routes: one is distillation, which uses the boiling point difference of components to evaporate the solvent from the rich liquid, and condenses it at the top of the tower to obtain the recovered solvent; The second is stripping, which uses steam or inert gas to blow off the dissolved solvent, blow off the gas, and then condense and recover it. Which option to choose, how much energy consumption, and how much recovery rate should be based on actual measurement data to determine parameters, without making decisions based on estimates. Regularly test the concentration and impurities of the regenerated lean solution, and supplement or replace the absorbent.
Absorption is liquid phase enrichment, adsorption is solid phase capture. Large air volume and low concentration of tail gas can go through the two roads, specifically according to the solvent water solubility, absorbent source, adsorbent regeneration energy consumption and operating costs of comprehensive comparison.
Distillation and separation shall be arranged according to the components, and the part that can be recovered shall be recovered first; the residual liquid that cannot be reused shall be collected and disposed according to the specifications and shall not be directly discharged into the water body.
Common in packing scaling or hanging material, excessive circulation close to flooding, air volume fluctuations. First check whether the circulation volume and air volume deviate from the setting, and then check the packing status.
Depending on inlet concentrations and local emission requirements. In most cases, absorption is responsible for enrichment and preliminary purification, and condensation or adsorption is required at the back end. Finally, the measured emission data shall prevail.
This article by LuoyangWohong PetrochemicalEquipment Co., Ltd. arranges that the company provides design, manufacturing and operation and maintenance services for condensation recovery, absorption enrichment, adsorption recovery and complete sets of solvent recovery devices for the chemical and pharmaceutical industries. If you need process evaluation or scheme configuration, please contact and exchange.
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