Delayed Coking

Coking (referred to as coking) is a deep thermal cracking process, and it is also one of the means to treat residual oil. It is also the only technological process that can produce petroleum coke, which cannot be replaced by any other process. In particular, the special demand for high-quality petroleum coke in some industries has caused the coking process to occupy an important position in the oil refining industry.

1. Feedstock:

Delayed coking is a decarburization process similar to catalytic cracking to change the hydrocarbon ratio of petroleum. The raw material of delayed coking can be heavy oil, residual oil or even asphalt, and the quality of raw materials is relatively low. The main conversion processes for resid are delayed coking and hydrocracking.

2. Products:

The main products are wax oil, diesel oil, coke, crude gasoline and part of the gas, the respective proportions are: wax oil accounts for 23-33%, diesel 22-29%, coke 15-25%, crude gasoline 8-16%, Gas 7-10%, external oil 1-3%.

3. Basic Concepts

Coking uses hydrogen-depleted heavy residual oil (such as vacuum residual oil, cracked residual oil and asphalt, etc.) as raw materials, and conducts deep thermal cracking reaction at high temperature (400-500 °C). Through the cracking reaction, part of the residual oil is converted into gas hydrocarbons and light oil products; another part of the residual oil is converted into coke due to the condensation reaction. On the one hand, because the raw material is heavy and contains a considerable amount of aromatic hydrocarbons, on the other hand, the reaction conditions of coking are more severe, so the condensation reaction accounts for a large proportion, and a large amount of coke is generated.

4. Production process

The production process of the delayed coking unit is divided into two parts: coking and decoking. Coking is a continuous operation, and coking is a gap operation. Since industrial plants are generally equipped with two or four coke drums, the entire production process is still operated continuously.

a. The crude oil is preheated, and the coking feedstock (vacuum residue) first enters the raw material buffer tank.And then it is pumped into the convection section of the heating furnace to heat up to about 340~350 °C.

b. The preheated crude oil enters the bottom of the fractionation tower, and exchanges heat with the oil and gas produced by the coke tower in the fractionation tower (the temperature at the bottom of the tower does not exceed 400 ℃).

c. Feedstock and circulating oil are extracted from the bottom of the fractionation tower together, which are pumped into the radiant section of the heating furnace with a hot oil pump, heated to the temperature required for the coking reaction (about 500 ℃), and then entered into the coke tower from the lower part through the four-way valve to carry out the coking reaction. .

d. Feedstock reacts in the coke tower to form coke and accumulates in the coke tower. The oil and gas come out from the top of the coke tower and enter the fractionation tower. After heat exchange with the raw material oil, gas, gasoline, diesel oil and wax oil are obtained by fractionation. The bottom circulating oil and the raw material are then subjected to coking reaction.

5. Production equipment

a. Coke drum

The coke drum is an empty cylinder made of thick boiler steel plate, which is the place where the coking reaction takes place.

b. Hydraulic decoking equipment

The coke drum is used in rotation, that is, when the coke in one drum is coalesced to a certain height, the raw material is switched to another coke drum through a four-way valve. The coke drum where the coke is coalesced is first cooled with steam and then decoked hydraulically.

c. Flameless burner

The coking heating furnace is the core equipment of the device, and its function is to heat the rapidly flowing residual oil in the furnace to a high temperature of about 500 °C. Therefore, it is required to have a high heat transfer rate in the furnace to ensure that enough heat is provided to the oil in a short time, and at the same time, it is required to provide a uniform thermal field to prevent the furnace tube from coking due to local overheating. For this reason, delayed coking usually employs a flameless furnace.