I. Background of Oily Wastewater from PLA Production

On the green map of the rapidly expanding polylactic acid (PLA) industry, the "oil and grease" in production wastewater is shifting from a troublesome waste into a resource that urgently needs to be identified. Conventional treatment regards all the oil in the wastewater as an "enemy", spending heavily on indiscriminate oxidative decomposition or adding large amounts of chemicals for flocculation and sedimentation, while overlooking the recoverable "free oil" fraction. This is not only a huge cost leak, but also a double waste of resources and energy.

To improve the resource utilization of free oil, this article mainly introduces a new process for treating the free oil generated in PLA production.

Main sources of free oil in the oily wastewater from PLA production:

1. Direct discharge and leakage of vacuum pump working fluid:

Periodic replacement of waste fluid: To maintain the performance of vacuum pumps (such as liquid-ring pumps and Roots pumps), their working fluid (mostly white oil and synthetic ester oils) must be replaced regularly. The discharged waste working fluid contains a large amount of un-emulsified free oil that can be separated quickly by static settling; this is the stream with the highest concentration and the easiest recovery.

Shaft seal and mechanical seal leakage: After long-term operation of the pump body, the seals wear, causing the working oil to leak inward on the negative-pressure side and enter the pre-stage condenser together with the process gas, forming an oil-water mixture after condensation. In the early stage, the oil in it still exists mostly in a free state.

Working fluid "carry-over" and atomization: Under high vacuum and turbulence, part of the working fluid is "carried over" by the gas stream or enters the process side as extremely fine mist droplets, and then coalesces into larger oil droplets in the condensation system.

2. Leakage of lubricating oil from process equipment:

The bearing lubrication systems of agitator reducers and gearboxes on polymerization reactors, as well as large rotating equipment such as extruders and pelletizers, may suffer seal failure, causing lubricating oil (grease) to slowly seep onto equipment foundations or platforms and eventually be washed into the wastewater system.

3. Leakage from heat transfer systems:

In some heating reactors that use heat transfer oil (organic heat medium), if weld cracking or seal failure occurs, the heat transfer oil may leak into the jacket cooling water or directly contaminate the material, and then enter the cleaning stage. In the early stage of leakage, the heat transfer oil exists mostly in a free phase.

II. SINOKLE's New Treatment Process

For free oil with a droplet size usually greater than 30 microns, SINOKLE uses an oil-water separation skid whose core technology is the "high-efficiency coalescing oil remover" to treat the oily wastewater. Its design principle is a new process combining collision coalescence, wetting coalescence and cyclonic micro-nano flotation; by using high-efficiency coalescing inclined-plate packing, the oil remover can coalesce small dispersed oil droplets into large ones, greatly shortening droplet residence time and achieving rapid oil-water separation. Without adding any chemicals, the process index requirements are met by purely physical means.

High-efficiency coalescing oil remover

1. High efficiency and stability, excellent effluent

The oil content of the system effluent is stably ≤30 mg/L, fully meeting the "Emission Standard of Pollutants for Synthetic Resin Industry" (GB 31572-2015) as well as stringent local sewer-entry or direct-discharge requirements; in some cases, in-plant reuse can be achieved.

2. Strong shock resistance, flexible adaptation

Designed specifically for the intermittent, multi-batch drainage characteristics of PLA production, it withstands sharp fluctuations in water quality and volume, ensuring continuous production with no worries.

3. Energy saving and cost optimization

The whole process is dominated by physical separation with no chemical consumption, saving operating costs; the modular design saves 50% of floor space; and the recovered waste oil reaches a purity of up to 95%, which can be utilized as a resource and generate additional revenue.

4. Intelligent O&M, easy management

Equipped with the "Cloud Intelligent Control" remote management platform, it enables unattended operation, intelligent early warning, fault diagnosis and data analysis, greatly reducing O&M difficulty and labor costs.

III. Technical Comparison