The Tank within Tank System: A Critical Pretreatment Solution for Coal-to-Liquid (CTL) Oily Wastewater

1.Core characteristics and treatment difficulties of coal to oil oily wastewater

Coal to oil (including direct and indirect liquefaction) wastewater is the most difficult type of industrial wastewater to treat, and its oil containing characteristics are particularly prominent.

(1) Complex oil composition: Wastewater not only contains traditional petroleum products, but also a large amount of coal-derived synthetic oils, catalyst fines, solvents, as well as heavy tar and asphaltene generated in high-temperature and high-pressure reactions.

(2) Severe emulsification: Intense process mixing and inherent surfactants lead to highly stable oil-in-water emulsions, rendering simple gravity separation ineffective.

(3) High concentration and high toxicity: COD (Chemical Oxygen Demand) is usually in the thousands or even tens of thousands of mg/L, and contains substances such as phenol, cyanide, and ammonia nitrogen that are toxic to microorganisms.

(4) High water temperature and large fluctuations: The sewage discharged from the device has a high temperature, which affects the material and separation efficiency of the equipment.

(5) Threat to subsequent biochemical treatment: Oil will envelop microorganisms, inhibit their activity, and block the aeration head. To protect downstream biological treatment units—where oil can inhibit microbes and foul equipment—influent oil content must be reduced to below 20-30 mg/L.

2.The role and positioning of oil-water separators in coal to oil projects

In the vast process of coal to oil wastewater treatment, the tank within tank plays a crucial pre-treatment role as a bridge between the previous and the next.

(1) Core function: Efficiently remove "easily separable" oils

The GZ-XX series tank within tank oil-water separator developed by Qingdao Bailida is a treatment system equipment consisting of an outer tank (sewage regulation homogenization tank), an inner tank, a hydro-cyclone separation device, an automatic oil skimming and discharge device, and associated piping, commonly known as the "tank within tank";  It integrates wastewater equalization, homogenization, and oil removal functions. Mainly using sedimentation and cyclone separation technology to separate free oil and large particle dispersed oil in wastewater.

l Objective: To remove the vast majority of free oil and dispersed oil (i.e. oil droplets with a particle size of 50-100μm or larger) from wastewater.

l Effect: It can effectively reduce the oil content in the influent from several thousand or even tens of thousands of mg/L to below 150 mg/L, and the removal efficiency of free oil can reach over 90%.

GZ-XX Tank-within-tank

(2)  Core value: Clearing obstacles and reducing costs for subsequent processes

Protecting backend devices: The tank in the tank first removes most of the free oil, allowing subsequent devices to focus more on demulsification. This significantly reduces the operating load of the system, directly leading to a significant decrease in operating costs.

Resource recycling: It can recycle coal based oil products with high purity and has certain economic value.