Introduction to Delayed Coking Wastewater and Its Conventional Treatment Processes

Introduction to Delayed Coking

Crude oil generally enters the first processing step of petroleum refining—atmospheric and vacuum distillation—after electric desalting. This process yields only about 10–40% light products such as gasoline, kerosene, and diesel. The remaining heavy fractions and residual oils must undergo secondary processing to produce additional light products. Catalytic cracking is one of the most commonly used methods and represents a key production unit in most petroleum refineries. However, catalytic cracking imposes strict requirements on feedstock quality. Even for units designed for heavy oil catalytic cracking, the Conradson carbon residue of the feed is typically required to be no more than 8%, and the metal content no more than 30 μg/g. In practice, most heavy oils significantly exceed these limits. During processing, coke and heavy metals deposit within the pores and on the surface of the catalyst, leading to catalyst poisoning and deactivation. 

Delayed coking, by contrast, uses hydrogen-deficient heavy residual oils (such as vacuum residue, cracking residue, and asphalt) as feedstock and has relatively low requirements on feed quality. For this reason, it has been widely adopted by refineries as a preferred solution for processing inferior residual oils. 

Delayed coking typically adopts a one-furnace–two-drum or two-furnace–four-drum process configuration. The heating furnace operates continuously, while the coke drums are fed alternately. The feed oil or recycle oil is heated in the furnace and rapidly raised to the coking temperature (approximately 773 K) within a short time, with operating conditions controlled to prevent coke formation inside the furnace. After entering the coke drum, coking reactions occur, ultimately converting the feed into light products such as raw gasoline and diesel. 

Wastewater Generated from Delayed Coking

Based on composition and characteristics, wastewater generated from delayed coking can be classified into oil-bearing wastewater (quench water from coke cutting), high-oil-content wastewater (blowdown condensate), and sulfur-containing wastewater. Sulfur-containing wastewater must undergo ammonia stripping and hydrogen sulfide recovery in a stripping unit, after which it is either reused as make-up water for electric desalting or discharged after meeting wastewater discharge standards.

Inferior heavy oils contain natural surfactants such as asphaltenes, resins, and petroleum acids. In addition, various additives introduced during oil production (e.g., alkylbenzene sulfonates, fatty alcohol polyoxyethylene ethers), demulsifiers added during electric desalting, and corrosion inhibitors added during atmospheric and vacuum distillation contribute to the formation of stable oil-in-water emulsions in sulfur-containing wastewater. These emulsions are highly stable, do not readily separate even after long periods of standing, and exhibit high oil content.

The presence of large amounts of coke fines and oil in sulfur-containing wastewater leads to oil accumulation and coking on stripping tower trays, as well as blockage of float valves, thereby impairing stripping operations. In severe cases, this can result in unit shutdown, significantly reducing the efficiency of steam stripping. Therefore, strict oil and coke fines removal is required before coking sulfur-containing wastewater enters the stripping unit.

Traditional Treatment Technologies for Delayed Coking Wastewater

At present, conventional treatment technologies for oil and coke fines removal from sulfur-containing wastewater mainly include mechanical filtration, gravity separation, chemical demulsification, and hydrocyclone separation.