Solvent recovery is a key technology in modern industrial production, especially in the fields of chemical engineering, pharmaceuticals, coating, and electronics, for achieving cost reduction, emission reduction, and compliance with environmental regulations.

I. What is Solvent Recovery?

Solvent recovery refers to the process of treating used and contaminated solvents (waste solvents) through physical or chemical methods to remove impurities and restore their purity to a level suitable for reuse.

Core Objectives:

1. Economic Benefits: Fresh solvents are expensive, and recycling and reusing them can significantly reduce raw material procurement costs.

2. Environmental Protection: Avoiding the direct discharge of toxic and harmful waste solvents into the atmosphere or treating them as hazardous waste reduces environmental pollution and carbon footprint.

3. Regulatory Compliance: Adhering to increasingly strict environmental, safety, and health regulations.

4. Sustainable Development: It is an important part of green manufacturing and the circular economy.

II. Mainstream Solvent Recovery Processes

The following are several of the most commonly used and promising solvent recovery technologies in industry:

1. Distillation (Rectification/Distillation)

This is the most classic and widely applied solvent recovery method, particularly suitable for recovering mixed solvents with different boiling points.

• Principle: It utilizes the different boiling points of the components in a mixed liquid to achieve separation through heating, vaporization, and condensation processes.

• Core Equipment: Distillation column, reboiler, condenser, storage tank.

• Process: Waste solvents are heated and vaporized. The vapor rises in the distillation column and comes into contact with the descending liquid, undergoing multiple partial vaporizations and condensations. Components with lower boiling points (light fractions) are enriched at the top of the column and condensed for recovery, while components with higher boiling points (heavy fractions) are enriched at the bottom.

• Advantages: Mature technology, high separation purity, large processing capacity, and continuous operation.

• Disadvantages: High energy consumption, and difficulty in separating azeotropes or systems with very close boiling points.

• Applications: Widely used in the recovery of mixed solvents in the paint, coating, pharmaceutical, and printing industries.

2. Membrane Separation (Membrane Separation)

An emerging and highly efficient separation technology, particularly suitable for solvent dehydration or removal of trace impurities in specific systems.

• Principle: It uses selective permeation membranes, driven by pressure, to allow certain components (such as water or small molecules) to pass through while retaining the solvent, thereby achieving separation.

• Core Equipment: Membrane modules, high-pressure pumps.

• Process: Waste solvents are forced through the membrane surface under pressure. Water molecules or other small impurities pass through the membrane, while the purified solvent is concentrated and recovered.

• Advantages: Low energy consumption, no phase change, suitable for heat-sensitive substances at ambient temperature, compact equipment, and easy to scale up.

• Disadvantages: High membrane cost, high requirements for pretreatment of feed liquid, and potential membrane swelling or fouling issues.

• Applications: Dehydration of solvents such as ethanol and isopropanol; recovery of organic solvents from oily waste.

3. Adsorption (Adsorption)

Utilizing the selective adsorption of porous solid adsorbents to remove trace impurities from solvents.

• Principle: Impurity molecules (such as water, acidic/alkaline substances) are adsorbed on the vast surface of adsorbents (such as molecular sieves, activated alumina, activated carbon) through intermolecular forces.

• Core Equipment: Adsorption towers (usually two or more, alternating between adsorption and regeneration).

• Process: Waste solvents pass through the adsorbent bed, where impurities are adsorbed, and pure solvents are discharged. When the adsorbent is saturated, it is regenerated by heating (temperature swing adsorption, TSA) or depressurization (pressure swing adsorption, PSA).

• Advantages: Can be used for deep purification, removing trace amounts of water or impurities, and achieving extremely high product purity. • Disadvantages: Limited adsorption capacity, frequent regeneration required, usually used as a post-treatment unit for distillation or membrane separation.

• Applications: Deep drying of solvents (e.g., drying solvents to a water content <10ppm); decolorization and deodorization.

4. Condensation

Mainly used for recovering solvents from gas streams, it is a common unit in waste gas treatment.

• Principle: By lowering the temperature of solvent-containing vapor, it condenses into liquid and separates from non-condensable gases.

• Core equipment: Condensers (commonly shell-and-tube or plate types).

• Process: High-temperature waste gas enters the condenser and undergoes heat exchange with the coolant (usually cold water or brine), and the solvent vapor condenses into liquid for recovery.

• Advantages: Simple process, low equipment cost, stable operation.

• Disadvantages: Recovery efficiency is limited by the solvent’s dew point. For low-concentration or high-boiling-point solvents, the recovery is incomplete and often requires combination with other methods (such as adsorption or incineration).

• Applications: Recovery of solvents from the exhaust of drying ovens and reaction vessels.

III. Basis for Selecting Solvent Recovery Processes

The choice of which or which combination of processes depends on the following key factors:

1. Properties of the solvent: boiling point, whether it forms azeotropes, thermal stability, corrosiveness, etc.

2. Properties of impurities: type, content, boiling point difference from the solvent, solubility, etc.

3. Scale of treatment and recovery target: Is it intermittent small-batch processing or continuous large-scale production? What is the purity requirement for the recovered solvent?

4. Cost considerations: including equipment investment cost, operating energy consumption, maintenance costs, etc.

IV. Typical Integrated Process Systems

In practical applications, multiple technologies are often combined to achieve the best economic and technical effects.

Example: Recovery of NMP (N-methylpyrrolidone) containing water and a small amount of high-boiling-point impurities

1. Thin-film evaporator: First, remove most of the high-boiling-point polymer impurities.

2. Distillation column: Separate the azeotropic mixture of NMP and water, obtaining crude NMP and azeotrope.

3. Membrane separation/molecular sieve adsorption: Deep dehydration of crude NMP to obtain anhydrous high-purity NMP.

4. Condensation system: Treat the breathing exhaust from the distillation system and storage tanks to recover the volatile NMP.

V. Development Trends and Challenges

• Energy conservation and consumption reduction: Development of high-efficiency energy-saving technologies such as heat pump distillation and side-stream distillation.

• Intelligence: Utilizing big data and AI to optimize recovery process parameters and achieve predictive maintenance.

• Application of new materials: Development of separation membranes and adsorbents with higher flux, higher selectivity, and better chemical resistance.

• Handling of complex systems: Development of new separation processes for more complex and challenging waste solvents.

In summary,

solvent recovery is a systematic engineering task that requires the selection and design of the most suitable process route based on the specific solvent system and economic goals. With increasing environmental pressure and technological progress, efficient and low-energy-consuming solvent recovery technologies will become an indispensable core competitiveness for industrial enterprises.