Coke (referred to as coking) is a deep thermal cracking process and one of the means of treating residual oil. It is the only 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 refining industry.
- Raw materials:
Delayed coking and catalytic cracking similar to the decarburization process to change the hydrocarbon’s hydrocarbon ratio, the delayed coking feedstock can be heavy oil, residual oil or even asphalt, the quality requirements of raw materials are relatively low. The main conversion processes for residual oils are delayed coking and hydrocracking.
The main products are wax oil, diesel oil, coke, crude gasoline and some gases. The respective proportions are: wax oil accounts for 23-33%, diesel oil 22-29%, coke 15-25%, crude gasoline 8-16%. Gas is 7-10%, and external eucalyptus oil is 1-3%.
- Basic concepts
It is hydrogen lean heavy coker resid (e.g., vacuum residuum, cracked residue and asphalt, etc.) as a raw material, the depth of thermal cracking reaction at a high temperature (400 ~ 500 ℃). A portion of the residue is converted to gaseous hydrocarbons and light oils by a cleavage reaction; another portion of the residue is converted to coke due to the condensation reaction. On the one hand, due to the heavy raw materials, a considerable amount of aromatic hydrocarbons are contained, and on the other hand, the reaction conditions for coking are more severe, so the condensation reaction accounts for a large proportion and generates more coke.
- Production process
The production process of the delayed coking unit is divided into two parts: coking and decoking, coking is continuous operation, and decoking is a gap operation. Since industrial plants typically have two or four coke drums, the entire production process is still continuous.
The crude oil is preheated, and the coking raw material (vacuum residue) first enters the raw material buffer tank, and then is pumped into the heating furnace to raise the convection section to about 340~350 °C.
The preheated crude oil enters the bottom of the fractionation column and exchanges heat with the oil produced by the coke drum in the fractionation column (the bottom temperature does not exceed 400 ° C).
The feedstock oil and the circulating oil are taken out from the bottom of the fractionation tower, and the hot oil pump is used to drive into the radiant section of the heating furnace, heated to the temperature required for the coking reaction (about 500 °C), and then enters the coke drum from the lower part through a four-way valve to carry out the coking reaction. .
The raw materials are reacted in the coke drum to form coke, which is accumulated in the coke drum. The oil and gas flows from the top of the coke drum to the fractionation tower, and after heat exchange with the feedstock oil, the gas, gasoline, diesel oil and wax oil are obtained by fractional distillation. The bottom circulation oil and the raw materials are further subjected to a coking reaction.
- Production equipment
The coke drum is an empty cylinder made of thick boiler steel plate and is a place for coking reaction.
2.Hydraulic decoking equipment
The coke drum is used in rotation, that is, when the coke in one tower is coalesced to a certain height, the raw material is switched to another coke tower through a four-way valve. The coke drum that coalesces coke is first cooled with steam and then hydraulically defocused.
The coking furnace is the core equipment of the device, and its function is to heat the rapidly flowing residue 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 sufficient heat is supplied to the oil in a short time, and at the same time, it is required to provide a uniform heat field to prevent coking of the furnace tube due to local overheating. For this reason, delayed coking usually employs a flameless furnace.
The basic principle of heavy oil lightening is to change the relative molecular mass and hydrogen to carbon ratio of the oil, and the relative molecular mass and hydrogen to carbon ratio are often changed simultaneously. There are two ways to change the hydrogen to carbon ratio of oil. One is decarbonization and the other is hydrogenation.
- Raw materials:
Heavy oil, etc.
Light oil (gasoline, kerosene, diesel or catalytic cracking, cracking of olefins)
- Basic concepts
Hydrocracking belongs to the hydrogenation route of petroleum processing. It is to add hydrogen from the outside in the presence of catalyst to increase the hydrogen to carbon ratio of the oil.
Hydrocracking is essentially an organic combination of hydrogenation and catalytic cracking processes. On the one hand, heavy oils can be converted into light oils such as gasoline, kerosene and diesel through cracking reactions, and on the other hand, they can be prevented like catalytic cracking. A large amount of coke is generated, and sulfur, chlorine, and oxygen compound impurities in the raw material can also be removed by hydrogenation to saturate the olefin.
- Production process
According to the state of the catalyst in the reactor, it can be divided into several types such as fixed bed, bubbling bed and suspended bed.
(1) Fixed bed hydrocracking
A fixed bed means that a particulate catalyst is placed in the reactor to form a static catalyst bed. The feedstock oil and hydrogen gas are heated and pressurized to reach the reaction conditions, and then enter the reaction system, and then hydrorefined to remove sulfur, nitrogen, oxygen impurities and diolefins, and then hydrocracking reaction. After the reaction product is cooled, separated, depressurized and fractionated, the intended product is sent to the device to separate a gas containing a high hydrogen content (80%, 90%) for use as circulating hydrogen.
Unconverted oil (called tail oil) can be partially recycled, fully circulated or not circulated once.
(2) Fluidized bed hydrocracking
The fluidized bed (also known as the expanded bed) process drives the catalyst movement with a certain granularity by means of the fluid flow rate to form a gas, liquid and solid three-phase bed, thereby fully contacting the hydrogen, the feedstock oil and the catalyst to complete the hydrogenation reaction process. .
The bubbling bed process can process raw materials with high metal content and carbon residue values (such as vacuum residue). And can make heavy oil deep conversion; but the reaction temperature is higher, generally in the range of 400~450 °C.
This kind of process is more complicated and has not been industrialized in China.
(3) Suspension bed (slurry bed) hydrogenation process
The suspended bed process is a hydrogenation process that has received renewed attention in order to accommodate very inferior materials. The principle is similar to that of a fluidized bed. The basic procedure is to premix the raw material with a fine powder catalyst and then flow from the bottom to the top with hydrogen. The catalyst is suspended in the liquid phase for hydrocracking reaction. The reaction products flowed together from the top of the reactor.
The device can process a variety of heavy crude oil and ordinary crude oil residue, but the equipment investment is large. The process is currently in the research and development stage in China.
- Production equipment
The main equipment of the hydrogenation process production equipment is operated under the conditions of high temperature, high pressure and hydrogen and hydrogen sulfide. Therefore, the design, manufacture and material selection are very high, and the control of production operation is also very strict.
High-pressure hydrogenation reactors are key equipment in the plant, operating conditions are harsh, manufacturing is difficult, and expensive.
According to whether the medium directly contacts the metal wall, it is divided into two structures: a cold wall reactor and a hot wall reactor. The reactor consists of a cylinder and an internal structure.
1.Hydrogenation reactor cylinder
The reactor cylinder is divided into two types: a cold wall cylinder and a hot wall cylinder.
2.Hydrogenation reactor internals
The hydrogenation reaction is operated under the conditions of high temperature and high pressure and corrosive medium (H2, H2S). In addition to the prevention of hydrogen corrosion and corrosion of other media in the material, the hydrogenation reactor should also ensure: the reactants (oil and gas and Hydrogen) is evenly distributed in the reactor to ensure good contact between the reactants and the catalyst; the reaction heat is removed in time to avoid excessive reaction temperature and overheating of the catalyst. In order to ensure the optimal reaction conditions and prolong the life of the catalyst; under the premise of uniform distribution of the reactants, the pressure drop inside the reactor is not excessive, so as to reduce the load of the circulating compressor and save energy.
For this reason, it is necessary to provide necessary internal components inside the reactor to achieve uniform distribution of gas and liquid. Typical reactor internals include: inlet diffusers, gas liquid distribution trays, descaling baskets, catalyst support trays, quench hydrogen tanks and redistribution trays, outlet manifolds, and the like.