Delayed coking plants are the most common hardware solution for heavy oil refining. Their device includes 2 main modules - a reactor, where the raw materials are heated and coked, and machined. The design of the plants is carried out in stages and includes the calculation and selection of technological equipment, the definition of technical and economic indicators.
Appointment
The coking process is one of the methods of oil refining. Its main purpose is to obtain lump petroleum coke. In industry, 3 coking methods are used:
- Periodic, in cubes. The raw material is loaded into a horizontal apparatus, heated with the help of a furnace under it, then it is calcined for 2-3 hours. After which the furnace is cooled and the finished product is unloaded. This method is the most simple and inefficient.
- Continuous. This method is still at the stage of industrial development.
- Semi-continuous, currently the most widely used.
UZK delayed coking units belong to the latest type of technological equipment. In them, the raw materials are preheated in an oven, and then transferred to unheated reaction chambers having a layer of thermal insulation to maintain the required temperature. The number and size of reactors, the power of furnaces affect the performance of the entire installation.
The first UZK in Russia was launched in 1965 at Ufaneftekhim. The delayed coking unit of this enterprise is still operating today. After reconstruction in 2007, its productivity is about 700-750 thousand tons / year by weight of processed raw materials.
End products
At ultrasonic testing, in addition to coke, the following substances are obtained:
- coking gases (used as process fuel or processed to produce propane-butane fraction);
- petrol;
- coke distillates (fuel, cracking feedstock).
At domestic ultrasonic testing, the coke yield is 20-30% by weight. This indicator depends primarily on the quality of raw materials. The greatest demand for this product is experienced by the metallurgical industry (production of anodes and electrodes, aluminum, abrasives, carbides, carbon-graphite materials, ferroalloys). In addition to the first Ufa UZK, other delayed coking units were also built in Russia: at the Omsk Oil Refinery, Novokuybyshevsky Oil Refinery, LUKOIL-Volgogradneftepererabotka LLC, LUKOIL-Permnefteorgsintez, LUKOIL-Permnefteorgsintez, and Angarskaya Neftekhimsky Petrochemical Novoil ”(Ufa), PJSC TANECO (Nizhnekamsk).
Raw materials
The starting materials for coking are divided into 2 groups: products of primary and secondary processing. The following substances are used as raw materials in delayed coking plants:
- fuel oil;
- semi-tar;
- tar;
- coal tar pitch;
- asphalts and other products for the production of technical oils;
- liquid coal residues;
- heavy pyrolysis and shale resin;
- thermal cracking residues;
- oil bitumen and heavy oil.
The most common raw materials are currently highly aromatic oil residues.
Influence on technological parameters
The following parameters of the coking unit depend on the properties of the starting materials:
- the efficiency of the reaction chamber;
- quality of products received;
- coke yield;
- process conditions.
The most important raw material parameters are:
- Coking ability, depending on the content of asphalt-resinous substances. The value of coking ability should be in the range of 10-20%. With a smaller value, the coke yield decreases, and with a larger value, deposits accumulate inside the coils in the furnaces. Coking ability is determined by the mass of the solid residue in the crucible after heating a sample of the oil product in it.
- Density.
- Chemical composition. Of the harmful impurities that have the greatest impact on the quality of coke, sulfur is isolated (it should be no more than 1.5% by weight). Depending on the purpose of coke in the process, it is preferable to use different types of raw materials. So, to obtain the fibrous structure of the final product, materials with a paraffin base are used.
The coke yield is proportional to the density of raw materials and the content of asphaltenes in it.
Coking stages
The technological process in delayed coking plants is long and continuous, from the supply of raw materials to the unloading of the finished product. It is conditionally divided into 3 stages:
- Decomposition reactions, the formation of distillate fractions, intermediate compounds, condensation.
- A significant reduction in the content of unsaturated hydrocarbons in gases, an increase in the molecular weight of the components of the residue, and cyclization reactions.
- The increase in the content of asphaltenes in the residue up to 26%, a decrease in the number of resins and oils. Convert liquid residue to solid coke.
Classification
There are 2 main types of delayed coking plants according to their layout: one-block and two-block.
Among the two-unit installations, 4 types are distinguished, for which the following structural and technological features are characteristic:
- The inner diameter of the coking chambers is 4.6 m. Tent heating furnaces, four pairwise working chambers. The kerosene and gas oil obtained in the coking process are used for heating.
- Coke oven chambers Ø 5.5 m. The feedstock is straight-run fuel oil with the introduction of highly aromatic hydrocarbons, which contribute to an increase in the yield of a quality product.
- Alloy steel reactors Ø 5.5 m, height 27.6 m, tubular furnaces with volume-spreading torch, through-passage cranes of increased cross-section, radioactive level gauges, which make it possible to record the location of the phase separation “coke-foam”. The latest innovation helps make better use of the useful volume of the reactor. The supply of turbulators with detergents to reduce the coking of the coils, cooled gas oil in the helmet pipes.
- Reaction chambers Ø 7 m, height 29.3 m. Axial input of raw material into reactors, hydraulic system for unloading coke with remote control, electric cranes, warehouses with outdoor storage.
Applied Equipment
The equipment with which the units of this type are equipped are divided into the following groups, depending on the purpose:
- Technological, directly involved in the coking process (furnaces, column equipment, heat exchangers, reactor chambers, cubes, refrigerators, pumps, pipelines, instrumentation, valves and other shutoff and switching valves).
- Purification - collection and purification of water to return to the working cycle (cooling and extraction of coke).
- Equipment for unloading coke from chambers (cubes). In modern mechanized installations it can be of a mechanical and hydraulic type (slings, winches, combs, cutters, rods, towers, rubber sleeves).
- Devices for transportation and processing of the finished product (receiving chutes and ramps, hoisting cranes, conveyors, feeders, crushers, warehouses).
- Machines and equipment for mechanization of work.
When designing delayed coking plants, it is especially necessary to design the structures of reaction chambers and furnaces, since the duration of the working cycle depends on the reliability of their operation.
Process parameters
The main parameters of coking technology are:
- The recirculation coefficient, defined as the ratio of the total load of the reaction coil of the furnace to the load of the entire plant for raw materials. With an increase in its value, the yield of high-quality coke, gas, and gasoline increases, but the amount of heavy gas oil decreases.
- The pressure in the reaction chamber. Its decrease leads to an increase in the yield of gas oil, a decrease in the yield of coke and gas, and an increase in foaming.
- Process temperature. The larger it is, the higher the quality of coke in terms of the amount of volatile substances, its strength and density. The maximum value is limited by the danger of coking of the furnace and pipelines, reducing the durability of the coils. For each type of raw material, its own optimal temperature is selected.
The construction of delayed coking plants is associated with high capital costs. Therefore, most often, reconstruction of an already functioning equipment complex is carried out. This is achieved by reducing the coking cycle, introducing new reaction chambers or reducing the recirculation coefficient.
Operating principle
Delayed coking plants consist of one or several pairwise groups of chambers, in which one chamber operates at the stage of coke production and the other at unloading or in an intermediate state. The process of decomposition of the starting material begins in a tubular furnace, where it is heated to 470-510 ° C. After this, the raw material enters the unheated chambers, where it is deeply coked due to the heat that came with it.
Gaseous and liquid hydrocarbons are diverted to fractional separation in a distillation column. Coke enters the department for machining, where it is unloaded, sorted and transported. A well is drilled in the finished product layer, and a hydraulic cutter is placed in it. Its nozzles operate under pressure up to 20 MPa. Pieces of divided coke fall on the drainage site where the water is drained. Then the product is crushed into smaller parts and divided into fractions. Next, the coke is moved to the warehouse.
Schematic diagram of ultrasonic testing is shown in the figure below.
Coke Chambers
The cameras are a reactor, which is the basis of the entire installation. The cycle of the camera is usually 48 hours, but in recent years, ultrasonic testing, working in 18- and 36-hour mode.
One cycle of the reactor consists of the following operations:
- loading of raw materials, coking process (1 day);
- shutdown (1/2 h);
- hydrothermal treatment (2.5 hours);
- water cooling of the product, removal of water (4 hours);
- product unloading (5 hours);
- manhole sealing, pressure testing with hot steam (2 hours);
- heating with oil product vapors, switching to a duty cycle (3 hours).
Design
The design of the delayed coking unit is developed in the following order:
- determination of required productivity, t / year;
- analysis of the raw material base;
- drawing up a theoretical material balance of the coking process for various types of raw materials;
- definition of the main material flows;
- study of the installation concept;
- substantiation of the size and number of reactors;
- determination of the duration of coke filling of one chamber and its hydraulic calculation, scheduling of the reactor;
- calculations of the temperature load on the camera;
- calculation of convection and radiation chambers;
- development of the layout of the production line;
- technological calculations of other basic equipment (distillation column, furnaces, refrigerators and others);
- development of a control and management system, selection of automation tools;
- description of emergency protection schemes;
- development of environmental aspects and safety measures;
- determination of economic indicators (capital costs, number of staff, payroll, production costs of raw materials and auxiliary materials, annual economic effect, calculation of production costs).
The calculation of the capacity of the delayed coking unit on an annual basis is carried out according to the formula:
N = P x t
where P is the capacity of the installation, t / day .;
t is the number of work days in a year.
The output in physical terms according to the basic and design options is determined on the basis of the material balance of the installation.