MAC, as an excellent polar solvent, is widely used in polyurethane, pharmaceuticals, pesticides, high molecular membranes (such as PVDF), and electronic chemicals. However, DMAC has certain toxicity and poor biodegradability, and its waste liquid must be effectively treated and recycled. Traditional recovery methods (such as ordinary distillation) have problems such as high energy consumption, limited product purity, and easy decomposition. This article will focus on introducing several efficient, energy-saving, and environmentally friendly new processes for DMAC waste liquid recovery and compare them with traditional methods.

I. Traditional Recovery Processes and Their Limitations

Traditionally, atmospheric or vacuum distillation is mainly used.

• Process: Waste liquid → Pretreatment (filtration, decolorization) → Separation in distillation column → Obtain DMAC product.

• Limitations:

1. High energy consumption: DMAC has a high boiling point (165°C), and direct heating distillation consumes a lot of energy.

2. Easy decomposition: Long-term high-temperature heating may cause partial decomposition of DMAC, generating impurities such as dimethylamine, affecting product quality and yield.

3. Azeotrope problem: When the waste liquid contains water or certain alcohols, azeotropes may form, increasing separation difficulty.

4. Bottoms coking: For waste liquid containing high molecular impurities (such as spinning wastewater), the bottom of the column is prone to coking, affecting continuous operation.

II. Mainstream New Processes and Technologies

The core idea of the new processes is "energy conservation and consumption reduction" and "quality improvement and efficiency enhancement".

1. Molecular Sieve Adsorption-Distillation Coupling Process

This is currently one of the most mainstream and economical methods for treating DMAC wastewater, especially low-concentration wastewater.

• Process Principle:

1. Adsorption Concentration: Use molecular sieves (such as 3A, 4A type) with extremely high selectivity for water molecules in an adsorption tower to adsorb and remove the majority of water from the wastewater, highly concentrating DMAC in the liquid phase (from 5% to 10% to over 70%).

2. Desorption Regeneration: After saturation, a small amount of hot air or hot nitrogen is used to desorb and regenerate the molecular sieves. The desorbed water vapor is condensed and discharged, and the molecular sieves are recycled.

3. Distillation Purification: The concentrated DMAC solution is sent to an efficient and energy-saving distillation system (such as heat pump distillation) for further purification to over 99.5% high-purity product.

• Advantages:

• Significantly reduces energy consumption: Avoids evaporating a large amount of water from low-concentration wastewater, with energy consumption only 20% to 30% of traditional distillation.
• High product purity: Good pretreatment effect reduces the load on the distillation column.
• High degree of automation: Can be operated continuously or semi-continuously.

2. Permeation Vaporization (PV) - Distillation Coupling Process

This is an advanced membrane separation technology, particularly suitable for breaking the azeotrope and removing trace amounts of water.

• Process Principle:

1. Preliminary Concentration: First, pre-concentrate the waste liquid through ordinary distillation or other methods.

2. Permeation Vaporization Dehydration: Send the pre-concentrated liquid to a permeation vaporization membrane system. One side of the membrane is liquid, and the other side is vacuum or carrier gas. Due to the membrane's high selectivity for water, water molecules preferentially pass through the membrane surface and vaporize, being condensed and removed.

3. Distillation Purification: After obtaining DMAC with extremely low water content, it enters the distillation column for final purification.

• Advantages:

o Breaks the azeotrope: Can easily overcome the water-DMAC azeotrope limitation and directly obtain products with extremely low water content.

o Energy-saving and environmentally friendly: No phase change in the process, low energy consumption; no introduction of third components, no pollution.

o Operates at room temperature: Avoids DMAC decomposition caused by high temperatures.

3. Heat Pump Distillation (MVR) Technology

This is an efficient energy-saving distillation technology, suitable for treating medium and high-concentration DMAC waste liquid.

• Process Principle: The low-temperature steam at the top of the distillation column is compressed by a compressor to increase its temperature and pressure, and then used as the heat source for the reboiler at the bottom of the column. The steam itself is condensed into a liquid. This system recovers and utilizes the latent heat of the steam at the top of the column, significantly reducing the consumption of external steam.

• Advantages:

o Significant energy-saving effect: Energy consumption can be reduced by 60% to 80%.

o Suitable for large-scale recovery: The larger the scale, the more obvious the economy.

4. Extractive Distillation

When the DMAC waste liquid is mixed with solvents that have a boiling point close to it or can form azeotropes (such as certain alcohols), this method can be adopted.

• Process Principle:

An "extractant" (or "entrainer") is added to the system, which can change the relative volatility between the original components, making the difficult-to-separate mixture easier to separate. After separation, the extractant is recovered and recycled through another column.

• Advantages:

o Solves the problem of azeotropes/near-boiling points: It is an effective means for separating complex mixtures.

III. Process Combination and Optimization: Integrated New Process Route

In actual industrial applications, a combination of multiple technologies is often adopted to form the most optimized process package. A typical integrated new process route is as follows:

"Zeolite Adsorption/Pervaporation + Heat Pump Distillation" Combined Process

Flowchart: text

DMAC wastewater (5%-20%)

↓

[Pre-treatment unit] (filtration, oil removal)

↓

[Molecular sieve adsorption unit or pervaporation unit] → Wastewater meets discharge standards or is reused

↓

Concentrate (70%-90%)

↓

[Thermal pump distillation column]

↓

Top of column → Light component impurities (recovered or treated)

↓

Bottom of column → [DMAC product column] (optional, for further purity improvement)

↓

High-purity DMAC (≥99.5%) ← [Finished product tank]

Advantages of this combined process:

1. Front-end deep dehydration: Utilizing molecular sieves or pervaporation to remove a large amount of water at a lower temperature, significantly reducing the load and energy consumption of subsequent distillation.

2. Back-end efficient purification: Utilizing thermal pump distillation to purify the concentrate, with the lowest energy consumption.

3. Excellent product quality: The entire process avoids prolonged high-temperature heating of DMAC, resulting in minimal decomposition, good purity and color of the product.

4. Lowest overall cost: Although the initial investment is relatively high, the operating costs (mainly electricity and steam) are much lower than traditional processes, with a short payback period.

IV. Future Development Trends

1. Intelligence and automation: Utilizing big data and AI models to optimize process parameters, achieving intelligent control and predictive maintenance.

2. Application of new materials: Developing molecular sieves and membrane materials with higher selectivity, greater flux, and longer lifespan.

3. Energy integration optimization: Globally integrating the energy systems of recovery devices and production facilities to further explore energy-saving potential.

4. Full resource recovery: Not only recovering DMAC but also co-recovering other valuable components (such as salts, other solvents) from wastewater, achieving "zero discharge".

Summary

The new process for DMAC waste liquid recovery has shifted from the single "distillation" thinking to the coupled and integrated thinking of "pretreatment concentration + efficient separation and purification". The combination of molecular sieve adsorption and pervaporation as efficient pre-concentration/dehydration technologies with the core energy-saving purification technology of heat pump distillation is currently recognized as the most advanced, economical and environmentally friendly new process route for DMAC waste liquid recovery, which is very suitable for enterprises to adopt in technological transformation and new projects. When choosing a specific process, a detailed technical and economic assessment should be conducted based on the composition, concentration, treatment volume and investment budget of the waste liquid.