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Comment PDF Water Treatment

Purifying Coke-Cooling Wastewater

By Zhi-shan Bai, Shang-Tung Tu and Hua-lin Wang,  East China University of Science and Technology |

The delayed coking process is a fundamental part of the petroleum refining and petrochemical industries, and is among the main technical means for transforming inexpensive heavy oil into more valuable, light-oil products. Delayed coking units are common in petroleum refineries, and the process has been developed significantly as a heavy oil processing method in many countries, including China and the U.S.

Figure 1. Typical coke-cooling wastewater
treatment system. 1–coke drum; 2–gravity settling
pit; 3–cooling tower; 4–hot coke-cooling water pump;
5–water storage tank; 6–high-pressure pump;
7–coke settling pit

 

The importance of delayed coking and its advanced development has not, however, eliminated certain disadvantages. The process uses high-sulfur heavy oil as its raw material, and requires a large amount of cooling water. This wastewater is referred to as “coke-cooling wastewater.” The large volume of wastewater produced by a delayed coking process at a typical petroleum refinery not only contains solid coke breeze (residue from screening heat-treated coke) and liquid heavy oil, but also contains organic and inorganic sulfides — all potential sources of environmental pollution.

The problem of cleaning coke-cooling wastewater has vexed engineers for decades, but effective solutions have remained elusive. Without a low-cost technique to treat coke-cooling wastewater, environmentally inocuous production of petroleum is impossible. The process discussed here represents an improved method for purifying coke-cooling wastewater and offers a path toward cleaner petrochemical production.

Delayed coking

Essentially, delayed coking is a high-temperature process involving extensive use of direct heat to generate higher-value products from crude oil. The coking process combines severe thermal cracking and condensation reactions, and requires a large amount of high-grade energy. The process employs a heater designed to raise temperatures of the residual feedstock above the coking point without significant coke formation. The term “delayed coking” is derived from the fact that an insulated coke drum is provided for the heater effluent, so that sufficient time is allowed for coking to occur before subsequent processing.

Figure 2. Fluid flow in a hydrocyclone
occurs in a swirling fashion

 

In delayed coking, heater effluent flows into the coke drum in service. When the drum is filled to within a safe margin of capacity, the heater effluent feed is switched to an empty coke drum. The full drum is then isolated, steamed to remove hydrocarbon vapors, and filled with cooling water. Next, the drum is opened, drained and emptied, yielding a petroleum coke product. All the coke-cooling wastewater produced by the delayed coking process normally drains from the coke solids and is collected and recycled for drilling and drum cooling. A water-flow schematic diagram is shown in Figure 1.

A de-oiling and clarifier system may be used before pumping water back to its storage tank. A cooling tower is used to lower the temperature of the coke-cooling wastewater. The oil and coke removal has traditionally been achieved by a gravity settling pit, but the gravity-separating efficiency is low at temperatures of around 85–125oC.

Water containing fine coke and oil can cause costly operational and maintenance problems. Solids can settle in the water storage tank, requiring routine flushing or cleaning. The concentration of fine coke then becomes abrasive and can damage the high-pressure pump, coke drilling tools, as well as the control and water-isolation valves.Damage to the pump and valves can cause a unit outage or shutdown, which raises costs significantly. Since the system is open to the atmosphere, the oil and sulfides in the cooling tower wastewater become environmental pollutants via evaporation.

Separating phases

To move past the difficulties associated with purifying wastewater from delayed coking processes, we have designed a closed process that offers a reliable and cost-effective method to treat wastewater from industrial cool-coking. The method comprises the following steps (Figure 2): cooling the wastewater mixture produced from delayed coking to between 5 and 50C; subjecting the cooled wastewater to a solid-liquid separation step to obtain a coke breeze phase and a liquid phase; further separating the resulting liquid phase into oil and water phases; and discharging water from the oil phase.

Figure 3. Coke-cooling wastewater treatment system with
hydrocyclone. 1–coke drum; 2–hot coke-cooling wastewater
tank; 3–coke settling pit; 4–hot coke-cooling water pump;
5–solid-liquid separating hydrocyclone; 6–deoiling
hydrocyclone; 7–oil tank; 8–air cooler; 9–water storage
tank; 10–high-pressure pump

 

Figure 4. A de-oiling hydrocyclone is shown with an air
cooler in a plant facility

 

Figure 5. A solid-liquid separating hydrocyclone helps
prevent sulfur pollutants

 

Figure 6. A de-oiling hydrocyclone helps improve coke-cooling
efficiency
 

The solid-liquid hydrocyclone separator (Figures 3–5) is deployed between a coke-cooling hot wastewater tank and an oil-water separator. The coke-cooling wastewater is pumped into the hydrocyclone, wherein a majority of solid fine coke is separated from the coke-cooling wastewater at separation efficiencies of up to 70–80%. Under stable operating conditions, the separated fine coke is recycled back to the gravity settling pit for recovery. After the separated water phase is cooled by the air cooler to below 55oC, according to engineering requirements, or preferably lower than 50oC, it enters the water storage tank, and then is pumped into the coke drum.

After the separated oil phase (heavy oil) enters into an oil tank for further purification, it either returns to the coke tower for reprocessing, or it is stored in the oil tank to be pumped into the coke tower for eventual oil refining when needed.

Hydrocyclone treatment

A key component of the process is a single-stage, high-efficiency de-oiling hydrocyclone, a centrifugal separation device that, unlike other centrifugal machines, has no moving parts. The driving force for the separation comes from transforming the static energy of the fluid (fluid pressure) into dynamic energy (fluid velocity). Because of considerable research and development effort in this area, hydrocyclones are now widely used in various industries to separate two components of different densities. The devices were originally applied to particle-liquid separations, and have been used more recently for liquid-liquid and air-liquid separation as an alternative to gravity-based conventional separators. Hydrocyclones have several advantages that have led to wide industry acceptance. These include the equipment’s ease of operation, capability of generating high throughput, and requirements for less maintenance and floor space.

The hydrocyclone consists of cylindrical and conical components. The liquid with suspended particles is injected tangentially through an inlet opening in the upper part of the cylindrical section. As a result of the tangential entry, a strong swirling motion develops within the hydrocyclone device.

Figure 3 shows the fluid flow in liquid-liquid hydrocyclones. As the fluid is injected tangentially at the top of the hydrocyclone, centrifugal forces accelerate particles toward the walls. The fluid’s spiral movement forces large or dense particles against the wall of the cyclone and they migrate downward toward the underflow. Fine, or low-density, particles are swept into a second inner spiral, which moves upward to the overflow. Figures 4–6 show the installation of the hydrocyclone and air cooler in the treatment process for coke-cooling wastewater.

Advantages

The method and equipment of the new system have a host of advantages over previous treatment systems for coke-cooling wastewater. Among them is the prevention of sulfur pollution and foul odor while purifying wastewater. The new system also saves energy and cost resources by improving the efficiency of cool coking. The system also has the potential to reduce unit outage and shutdown time.

Utilizing hydrocyclones in coke-cooling wastewater operations helps to solve several long-standing problems associated with purifying the coke-cooling hot wastewater produced in cool coking processes. The method enables cleaner production of petrochemicals through a closed system with pollution-free discharge.

Edited by Scott Jenkins

 

Authors

Zhi-shan Bai is a professor in the School of Mechanical and Power Engineering at the East China University of Science and Technology (130 Meilong Road, Shanghai, China 200237; Phone: 86-021-64252748; Email: fbaizs@yahoo.com.cn). Bai’s research is focused on filtration and related separation processes. He has written numerous articles on hydrocyclone separation.

Shan-tung Tu is a professor in the School of Mechanical and Power Engineering at the East China University of Science and Technology. (Email: sttu@ecust.edu.cn). Tu has contributed considerably to the progress of design and life assessment methods of high-temperature structures, particularly creep and fracture behavior of welded structures, mechanical reliability and development of novel heat transfer equipment.

Hua-lin Wang is a professor in the School of Mechanical and Power Engineering at the East China University of Science and Technology. (Phone: 86-021-64252748; Email: wanghl@ecust.edu.cn). Wang has contributed to the progress of liquid-liquid and liquid-solid separation methods for the chemical engineering industry. The hydrocyclone technology has been applied in various chemical engineering units.


 

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