Introduction
Thermal cracking processes (bitumen viscosity reduction) are significant methods for converting heavy feedstocks. The increase in the amount of energy consumption in the world, the reduction of renewable energy sources and environmental issues have caused more attention to the upgrading of heavy oils. These processes are carried out at low pressure and high operating temperature and do not need expensive catalysts. Today, the most common processes for converting heavy feed are viscosity reduction.
These processes are important for refineries from an economic point of view and their use will continue in the coming centuries. Thermal fracture is one of the most important methods of upgrading the rest of the world's heavy residues. 58% of heavy oil upgrading capacity includes thermal cracking processes. There are other methods, such as hydrogen conversion, catalytic cracking, and asphaltene separation, for the upgrading of heavy oil residues.
In thermal fracture, the connection between hydrocarbon components with long branches is broken by heat and heat and they turn into smaller branches. The higher the temperature, the more and faster the breakdown of hydrocarbon branches occurs.
The upgrading of bitumen to light and valuable products is a vast process that will lead to a decrease in the molecular weight and boiling point of the components inside the feed. The chemistry of turning bitumen into light and valuable materials is complicated, because bitumen consists of large amounts of organic molecules, including hydrocarbons with high carbon numbers in each branch, single atoms such as sulfur and nitrogen, and metals. These components are scattered in the food in the form of light substances such as methane to substances with a molecular weight of 15000 grams per gram mol.
The thermal conversion of bitumen to lighter components occurs due to the reactions between free radicals. Although the reaction of a radical branch alone is simple, it will be very complicated in the bitumen composition that contains a large amount of these free radical reactions.
The two main methods for improving and thermal breakdown of bitumen are viscosity reduction and coking
Reducing the viscosity of bitumen is a process of breaking and mild thermal decomposition, which was initially used to reduce the viscosity of feed and use it as fuel oil. This process includes 25% of the upgrade processes.
Reducing the viscosity of bitumen was invented in 1920 and has been widely used to promote and improve the residual vacuum and heavy oil in order to produce gas products, naphtha, as well as feed with lower viscosity. This non-catalytic thermal process is widely used to reduce the viscosity and pour point of feed and use it as heating oil. In the process of viscosity reduction, the amount of liquid product production is higher compared to gas and production coke.
Measures should be taken to obtain more liquid products from this process under milder operating conditions.
On the other hand, coking processes lead to the production of gas, liquid and solid products (coke), which depends on the temperature, pressure and type of process feed. The amount of coke production in such processes is much higher than the process of reducing the viscosity of bitumen. In hydrogen processes, by adding hydrogen to the system, lighter and better quality products can be obtained compared to the other two processes. But the supply of hydrogen source and its production cost is high. Considering that something around 40% of crude oil will be refined in the form of sludge, therefore, a lot of investments have been made to upgrade this type of feed, and depending on the economy, the type of feed, and the need for manufactured products, various processes can be chosen.
Two industrial methods of viscosity reduction
- Viscosity reduction with sugar
- Viscosity reduction with coil
The difference between these two methods is that in the Sukri process, a lower temperature and a longer residence time are used, while with the coil method, the temperature will be higher and the residence time will be shorter. Therefore, if the amount of viscosity reduction is equal, the Sukri method consumes less energy compared to other methods. In the method of reducing viscosity with a soaker, conversion is also done in a heater, but most of it is done in a reaction container or a soaker that keeps two fluid phases at a high temperature for a certain period of time. The quality and efficiency of the production products of the sukri chamber or coil are the same at the same operational intensity and are independent of the device structure.
By providing the necessary residence time to carry out the desired reaction in the Sucre chamber design, the heater operates at a lower outlet temperature. As a result, fuel consumption is saved. But this process also has disadvantages, which can be mentioned in the de-coking operation of the heater and the soaker chamber. Of course, the necessary equipment to remove coke from the soaker chamber is not as simple as the thermal coil type equipment, and in the soaker chamber type, more equipment is needed to remove the coke and remove it.
The method of removing coke from a container is to separate it with the help of high pressure water and as a result create a large amount of water containing coke that needs to be filtered for reuse.
The coil process is very different from the Sucre process. Because in the coil process, the transformation is done due to the high temperature failure in the coil placed in the furnace. The most important advantage of the coil is having heating with two heated areas, which provides better control over the heated materials. Also, descaling of heating pipes is easily done by steam-air descaling method.
In the process of reducing bitumen viscosity, the goal is to produce valuable liquid products and reduce feed viscosity, but minimizing the amount of unwanted coke production is another important goal of this process. One of the ways to reduce coke production is to add hydrogen to the system either directly or indirectly - using hydrogen-containing gases such as methane. In this method, with the addition of hydrogen to the system, the ratio of carbon to hydrogen is reduced, so the amount of production of lighter products increases.
This method is used in processes such as hydrogen conversion of heavy residues. In some processes, this method has been used for viscosity reduction with the addition of hydrogen, hydrogen thermal conversion, or water conversion processes that lead to hydrogen production during the reduction process.
Until today, many studies have been done on the viscosity reduction process, kinetics, modeling, and also the characteristics of the obtained products, which can reduce the viscosity of all reactions in the thermal fracture process depending on three factors: time, temperature, and pressure.
In some of these processes, the reduction of bitumen viscosity has been investigated from the point of view of gas, liquid and coke efficiency, as well as the amount of changes in the viscosity of the liquid product.
But most of these investigations have been done at a temperature higher than 400 degrees Celsius. There are few information and studies on the breaking process and decomposition of heavy oil and bitumen in the lower temperature range such as 340 to 400 and with different reaction times.
In this work, the effect of pressure increase on the efficiency of coke and produced gas has been investigated. In the following, the rate of change of product yield at constant temperature and pressure and 7 time periods and at the end the relationship between temperature and time and the yield of products at a temperature between 340 and 400 degrees Celsius, which is lower than the industrial temperature of this process, which is often 430 degrees, is examined. Is.
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