Personal Summary of Thermal Conversion Process

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Thermal conversion process: The thermal conversion process involves the thermal cracking of residue oil under certain temperature and pressure conditions. Common thermal conversion processes mainly include visbreaking and coking. Viscosity reduction cracking is a relatively mild thermal conversion process aimed at converting residue oil into fuel oil with viscosity meeting specified standards, while also producing some gas, unstable gasoline, and a small amount of diesel. Currently, two main process flows are employed in China: one is the delayed viscosity reduction process, in which vacuum residue is directly fed into an upflow viscosity reduction reactor. The second is the upward-flow viscosity reduction process: the vacuum residue is first heated to 450 degrees in a heating furnace, and then flows from bottom to top through an upward-flow viscosity reduction reactor, with a reaction residence time of around 40 minutes. The coking process is a type of deep cracking process that converts residue oil into higher-value distillates and petroleum coke. Delayed coking is a refining process that dates back to the 1930s, and it has been widely used due to its simple process flow and low investment requirements. In recent years, delayed coking technology has also been continuously developing; by adjusting parameters such as the recycle ratio, reaction temperature, and coke drum pressure, the liquid yield can be increased by 6% to 8%, while the coke yield can be reduced by 4% to 6%. Fluidized and flexible coking is a process technology developed by the American company Exxon, whose advantages include continuous operation, low coke production, and high liquid yield. Due to the high costs of fluidization and flexible coking, this process has not yet been adopted in China. Globally, the trend toward heavier and lower-quality crude oil is accelerating, while demand for light oil products is increasing and demand for heavy fuel oil is declining. Coupled with increasingly strict environmental regulations, this is leading to a growing disparity between the supply of crude oil resources and the demand for petroleum products. Delayed coking technology has become an important method for lightening heavy oils around the world, as it requires less capital investment, has a simple process, is technologically mature, and can process various types of heavy residue oils with high levels of asphaltenes, sulfur, and metals to produce the largest amount possible of distillate products. It has seen rapid development and will play an increasingly important role in the deep processing of residue oils. In recent years, researchers around the world have focused their efforts on researching and improving delayed coking technology primarily on increasing liquid yield, reducing the production of coke and gases, optimizing operations, and improving quality. The current status and development trends of delayed coking in China 1 Technological advancements in domestic delayed coking processes over the past decade 1.1 Technology for producing petroleum needle coke 1.2 Application of combined processes (1) Catalytic cracking–delayed coking combined process. (2) Viscosity reduction – delayed coking combined process. (3) Viscosity reduction – solvent deasphalting – delayed coking combined process. 1.3 Development of new equipment: The breakthroughs in increasing the size of these devices are mainly reflected in the design of heating furnaces and coke towers. 1.4 Development of product applications 1.5 Clean production technologies 1.6 Energy-saving technologies for equipment 1.7 Automation technologies 2 Major problems in current production 2.1 Sulfur corrosion 2.2 Equipment: The main problems with coke towers include bulging and deformation of the tower body, corrosion of the tower walls in the upper and middle sections where the gas phase meets the gas-liquid phase, as well as cracks easily forming in the welds of components such as the skirt base, bottom cover oil transfer lines, and large oil and gas pipe joints used for coke removal. The main problems with hydraulic coking removal equipment are: the failure rate of the air motors used in shaft-type hydraulic coking removal is high, so the design needs to be improved and the materials used must be of better quality; the outlet pressure of high-pressure water pumps is generally low (15–18 MPa), resulting in a slow coking removal speed. 2.3 Environmental Pollution First and foremost is the issue of pollution caused by cold coking water and cutting coking water. Next is the pollution problem caused by solid powder residues. 3 Development Trends 3.1 Larger-scale delayed coking units 3.1.1 Increased scale of delayed coking units 3.1.2 Growing size of coke towers 3.2 Improved production technologies 3.2.1 Advances in delayed coking processes and equipment **These advancements have improved delayed coking technology; the focus of process development is on optimizing operating conditions, aiming to increase production capacity while achieving the highest possible liquid yield, reducing the amount of coke produced, and handling lower-quality feedstocks as much as possible. 3.2.2 Foster-Wheeler Company’s SYDEC process 3.2.3 MaxCoking technology developed by American Coking Technology Company 3.2.4 Application of defoamers in coke towers 3.2.5 Application of anti-coking agents in heating furnaces 3.3 Advancement of control systems: The use of advanced and optimized control technologies can **increase processing capacity and liquid yield, improve product quality, save energy, and enhance the economic efficiency of the facility. Many facilities abroad have adopted such technologies, and this represents an important development direction for production technologies in delayed coking units. 3.4 Cleaning of the plant environment – Achieving a favorable environmental condition in delayed coking units, minimizing various types of emissions, and realizing clean production are important challenges in the development of such units. Problem to be solved: As crude oil becomes increasingly heavy, the processing and utilization of vacuum residue are receiving growing attention from refineries. The delayed coking process has always been the main method for upgrading heavy oil due to its high degree of thermal conversion and strong adaptability to feedstocks. The coker is the core equipment of a delayed coking unit, determining the operating cycle and energy consumption level of the entire unit. Among the various factors that affect the long-term operation of heating furnaces, coking of the furnace tubes is one of the main causes. The coke deposited on the furnace tubes arises from the condensation reaction of the raw materials, and its coking rate is equal to the difference between the rate of coke formation and the rate of its detachment. Among them, the formation rate of tube fouling is related to the temperature of the inner wall of the tube and the properties of the feed materials; whereas the shedding rate of tube fouling is related to the thickness of the boundary layer and the concentrations of the coking precursors on both sides of the boundary layer. The condensation reaction of heavy oil is a radical reaction; at the beginning of the reaction, the radicals generated are trapped by the asphaltenes in the heavy oil, which prevents the combination of these radicals to form coke. As the conversion rate increases, the radical concentration rises, the cage effect of the gum is disrupted, and the likelihood of radical polymerization leading to coking increases. Controlling the residence time of the oil in the tubes and its thermal conversion rate to ensure that it leaves the furnace outlet before the heavy oil shielding effect is disrupted, while increasing the rate at which coking precursors are removed, is key to reducing the coking rate in the furnace tubes. To predict the distribution of medium products at different cross-sections within coking furnace tubes, the research team on tubular heating furnaces at China University of Petroleum developed a 12-element lumped heat conversion reaction model. Through static experiments, the relationship between oil samples from various sources and the model constants was established, and process simulation techniques were used to solve many practical problems in production. However, it takes a long time to experimentally determine the constant values of the distribution model for the products of heavy oil thermal cracking. After the conversion rate of the oil at the furnace outlet is determined using process simulation software, the properties of the feedstock in the actual plant change, making it difficult to optimize the operating coking furnace in a timely manner. Moreover, the parameters of the thermal conversion reaction distribution model are fundamental for calculating the residence time of the oil inside the pipes as well as its thermal conversion rate. Due to the complex composition of residue, and the difficulty in conducting residue thermal conversion experiments on-site, it was proposed to take advantage of the low conversion rate of heavy oil within coking furnace tubes. By using mathematical transformations, data were collected in the laboratory, and simulation software was employed to derive universal kinetic constants that can be applied to different feedstocks, thereby providing foundational data for the optimization and design of coking furnaces. Reaction apparatus 1: Schematic diagram of the reaction apparatus. Using the aforementioned reaction apparatus and a 12-particle lumped kinetic model for analyzing the reaction products of heavy oil, the correlation coefficient R and F-test results show that it is feasible to disregard the secondary reactions of narrow fractions and treat all thermal conversion reactions as first-order reactions at low conversion rates. The model can be used to predict the pyrolysis and condensation behavior of heavy oil under non-reactive conditions. However, this is only reliable when the conversion rate is low, and the results do not take into account the secondary reactions of narrow fractions. What if the conversion rate is high and there is a secondary reaction? How should it be handled? What kind of model would be more appropriate? If the 12-element lumped power model is still to be used, how should it be improved? Working steps for Reaction Unit 2, the reaction testing apparatus: Weigh the reaction tube and place it in a tubular furnace, then connect the feed pipe and the pressure measurement pipe. After weighing, the 2 receiving flasks were connected to the outlet of the reaction tube and the outlet tube of the condenser, respectively. After installing and checking the other instruments and equipment, purge with nitrogen to test for leaks, and check whether the pressure gauges, flow meters, etc., are functioning properly. Press the corresponding power switch to preheat the reaction tube. Take about 650 g of residue sample (record the weight carefully), place it on the lifting platform, and heat it using electricity to maintain a temperature of around 150°C. Heat the pump body of the plunger metering pump to 100°C. The upper, middle, and lower sections of the reaction tube are heated to 500°C and held at this temperature for 10 minutes. After that, a small amount of nitrogen is introduced to conduct a pressure test first, and then all the air inside the reaction tube is removed. Start the metering pump (which has been preset in advance, so there is no need to adjust the flow rate), keep the reaction temperature constant, and record the experimental data every 15 minutes. Pay close attention to the receipt of the reaction products, and keep the second receiving flask in an ice bath. Feed for 2 hours (approximately 300 grams of sample oil), take a gas sample for gas chromatography analysis. After sampling the gas, stop the feed pump and maintain the upper, middle, and lower sections of the reaction tube at 500°C for 30 minutes. Finally, heating of the reaction tube is stopped, and a small amount of nitrogen is introduced from the feed pipe inlet to purge the pipeline and the reactor; after purging, the two receiving flasks are removed, dried of water, and weighed. A sample of oil from the 100cn1 heavy oil bottle was taken for Enneberg distillation analysis to calculate the yields of the fractions at ≤190°C (with the oil sample from the light oil bottle included in the ≤190°C fraction) and ≤350°C. The reaction tube is removed, weighed, and the weight difference relative to the weight of the tube itself is used to determine the amount of raw coke and the coke yield. The reaction unit 3 uses solvent sedimentation and liquid chromatography column separation to separate vacuum residue into 6 group components. First, asphaltene was precipitated using n-heptane as a solvent (1∶40); then, resins were obtained by precipitation using n-butanol as a solvent (1∶40). Subsequently, activated Al2O3 was used as an adsorbent, and column chromatography was employed to separate the compounds. The chromatography column was successively washed with dearomatized petroleum ether, petroleum ether containing 15% benzene, petroleum ether containing 50% benzene, benzene, benzene+ethanol (50∶50), and benzene alone, thereby yielding saturated hydrocarbons, light aromatics, heavy aromatics, and soft resins. Through multiple separation experiments, the amounts of raw materials for the 6 species required in the thermal pyrolysis reaction experiments can be determined. A schematic diagram of the experimental setup is shown in Figure 1. The reactor is made of carbon steel with an inner diameter of 14 mm; the composition of the salt bath is: 73% NaNO3, 20% KNO3, and 7% NaNO2. It is heated by electric wires, with temperature control achieved using a DWT-702 precision temperature controller, keeping the temperature within ±1°C. All pipelines are made of stainless steel pipes with an inner diameter of 3 mm. During the reaction, nitrogen carries the pyrolysis gas and pyrolyzed gasoline upward through the reactor to the condenser; the gasoline fraction is condensed into a liquid at -10°C, while nitrogen and the pyrolysis gas go into a collection vessel for volume measurement and composition analysis. The pyrolysis gas was analyzed using a 102G type gas chromatograph, with Apiezon M/active γ-Al2O3 as the stationary phase, a hydrogen flame as the detector, nitrogen as the carrier gas, and quantitative capillary injection; the hydrocarbon content was determined by the external standard method. The boiling range of the middle distillate oil was determined using simulated chromatographic distillation; the chromatograph model was GC-14A, the column used was a 1m packed column, the initial temperature was 40°C, and the temperature increase rate was 10°C/min. The experimental procedure involves adding about 1 g of precisely weighed vacuum residue to the reactor. The air in the system is repeatedly displaced with nitrogen. Once the pressure in the reaction system, the flow rate of the nitrogen gas, and the temperature of the salt bath have all been adjusted to certain values and stabilized, the reactor is quickly inserted into the salt bath, and timing begins. After the reaction has proceeded for the specified time, the reactor is swiftly removed, and it is cooled using an electric fan. The amount of gas emitted during the reaction is measured, and its composition is analyzed. The liquid present in the condenser is precisely weighed, and its boiling range is determined. The remaining material in the reactor is also precisely weighed, with its boiling range and coke content analyzed.