Weekly Topic: Renovation of the delayed coking heater? Issue 17, June 5–11, 2011

Reply to 1# lijianhuai: Let me start by talking about the modification of single-sided furnaces. The two furnaces that have been modified so far are Unit 1 at Guangxi Petrochemical and Anqing Petrochemical; both of these are single-sided radiation furnaces, and the technology used for their modification was developed by Professor Xiao from Shida University. It involves changing the single-sided furnace to a design similar to a double-sided furnace, by increasing the furnace’s strength and the number of burners. The coking furnace in a certain plant is a single-sided radiation furnace that operates using a convective-direct radiation process. It features dual-point water injection. The coking reaction involves two main processes: cracking and condensation. In the section from the furnace’s natural convection chamber up to the area ahead of the 15th furnace tube, the reaction temperature is relatively low, and cracking is the predominant reaction. In the latter part of the furnace, condensation is the main process, resulting in the formation of coke. Therefore, increasing the residence time of the material in the furnace tubes during the first phase can enhance cracking and increase the yield of light products, while reducing the residence time in those tubes during the second phase can minimize coking and extend the production cycle. By diverting the amount of water or steam injected at the original first point, the residence time of the first half of the material in the furnace tubes can be increased. By adding a second point for injecting water or steam at the 15th furnace tube, the speed of the second half of the material through the furnace tubes is increased, thereby reducing the residence time of the material flow. We can reduce the steam injection volume at the first point and increase it at the second point. The radiation outlet is located in the middle: specifically, the ninth furnace tube from the bottom serves as the radiation outlet. In a heating furnace, the tubes between the 1st and 15th positions are subjected to the strongest radiation, with the 8th and 9th tubes being the most affected by it. Using the 9th tube as the outlet helps save energy and reduces the load on the heating furnace. ) Currently, the plant’s load is 110%; the temperature of the furnace tube walls is no more than 600°C, the temperature in the furnace chamber is no more than 800°C, the temperature at the furnace outlet is 493±1°C, the temperature at the bottom of the coke tower is 488–489°C, and the oxygen content in the flue gas is 2–3%.

Reply to 2# lijianhuai: What is the water injection volume? During the expansion and renovation of the coking furnace, the original lightweight castable was replaced with a ceramic fiber lining, resulting in a reduction in the surface temperature of the furnace shell from 110 degrees to 80 degrees ； The diameter of the furnace tube was changed from 133 to 141, resulting in an increase of over 15% ；

The delayed coking unit is one of the important facilities for processing high-sulfur heavy oil in refineries. The coking process is a complex series of thermal decomposition and condensation reactions; it is an irreversible process that requires the consumption of energy. Our company’s coking unit uses reduced-pressure residue oil, atmospheric naphtha wash oil, and catalytic slurry as raw materials, and carries out deep thermal cracking and condensation at high temperatures of 492–500°C to produce gas, rich gas, gasoline, diesel, wax oil, and coke. The thermal cracking reaction is a highly endothermic reaction, and all the heat required for the reaction is provided by the coking heater. The oil and gas generated by the high-temperature reactions in the coke tower enter the distillation tower in a superheated state. The various products obtained are released from the distillation system in liquid and gaseous form at lower temperatures; as a result, this facility involves significant consumption of high-quality energy, alongside large amounts of low-quality energy. Energy conservation and reduction thus become an important factor that directly affects a company’s economic efficiency. 　　We adopted a process energy optimization approach to carry out technical upgrades to the energy consumption of the coking unit, focusing on three aspects: reducing energy use in the production process, improving energy recovery efficiency, and enhancing energy conversion efficiency. 1 Current Situation Analysis By analyzing and comparing energy consumption over the years (see Table 1), some factors that have a significant impact on the energy consumption of the facility were identified. 1.1 Energy consumption of coking units over the years As can be seen from Table 1, the energy consumption of this unit was still at a relatively high level before the renovation in 2001, indicating that there was significant potential for improvement. 1.2 Plant load The production load of a plant is directly related to its energy consumption level. When the load is at a high level, the energy consumption of the device is relatively low; when the load is low, the energy consumption increases accordingly. 1.3 Low-temperature Heat Recovery First, we conducted a survey on the distribution of low-temperature heat sources in coking. The low-temperature heat sources for coking mainly include the oil and gas from the top of the distillation tower, the oil circulating from the top of the distillation column, the diesel before entering the cooler, the wax oil before entering the cooler, as well as the oil and gas from the top of the coke drum during large-scale steam injection and at the initial stage of small water supply during coke cooling. The specific temperature, flow rate, and heat capacity of these low-temperature heat sources are shown in Table 2 below. 　 　　Among the aforementioned low-temperature heat sources, the heat from the oil and gas at the top of the coke tower during coke cooling is difficult to utilize, as its temperature changes as the coke tower cools. The available bituminous heat sources include wax oil, diesel, top recycle oil, and overhead gas in the distillation tower. Based on the temperature of the medium after heat exchange being reduced to 70°C, the amount of heat that can be recovered by these low-temperature heat sources through heat exchange is shown in Table 3. 　 　　Based on the above calculations, the amount of heat that can be recovered from low-temperature heat sources is 77.48 GJ/h. 2.4 Fuel Consumption The heating furnace is a major energy consumer in coking plants; all the energy required for the coking reactions is provided by these furnaces, with fuel consumption accounting for over 80% of the total energy usage. Our investigation into the energy consumption of the heating furnaces revealed that due to the long-term processing of high-sulfur oil, the air preheaters in Furnaces No. 1 and No. 2 were severely damaged as a result of sulfur corrosion; the temperature at the hot air outlet was only 30–40°C, rendering them completely incapable of preheating the air or recovering heat. Additionally, the burners in the heating furnace have low combustion efficiency, resulting in fuel waste. Due to its early commissioning, the inner wall of the furnace is made of high-temperature refractory bricks, resulting in poor heat insulation and higher temperatures on the outer wall of the furnace. 2 Improvement Measures In light of the above analysis, we have taken corresponding improvement measures. 2.1 A new furnace and two towers were installed, increasing the plant’s processing capacity to 1.5 million tons per year. The original design capacity of our company’s coking plant was 800,000 tons per year; it was upgraded to 1.1 million tons per year in 1997, and further improvements were made in 1999, resulting in a significant increase in the capacity of various parts of the plant, though the degree of coordination between these components was not very high. In 2001, we carried out modifications to the coke tower, heater, and fractionation tower; by adding another furnace, two towers, and related equipment, we further increased the unit’s capacity for processing residue oil. 2.2 Implementation of the CFB and coking heat combined project To recover a large amount of low-temperature heat energy, we searched for \"cold sources\" throughout the plant; ultimately, secondary deionized water was chosen as the intermediate medium. First, the low-temperature waste heat from areas such as the overhead gases, diesel, and wax oil in the coking distillation tower is used to heat the deionized water. Then, this heated water is sent back to the power plant to heat the boilers, during which it cools down again before returning to the coking process to absorb more heat, and this cycle repeats itself. The specific process is shown in Figure 1: 2.3 Energy-saving upgrades to the heating furnaces and auxiliary equipment. To address the corrosion and failure issues of the air preheaters in Furnaces No. 1 and No. 2, two new air preheaters of the SOO-052R model were installed during the upgrades carried out in 2001. In addition, all high-efficiency and energy-saving YQ-200 burners have been installed in Furnace No. 1 to improve combustion efficiency. Eight temperature-sensing thermocouples were added to the surface of the furnace tubes in the heating furnace, to achieve a more reasonable distribution of temperature measurement points. 　　In the design of the newly built No. 3 heating furnace, advanced technologies are employed wherever possible; for example, multi-point water injection is used for the radiant furnace tubes, high-alumina fiber plastic is used for the lining of the radiant chamber, and lightweight insulating cast materials are used for the convection chamber. Increasing the height of the fire wall and expanding the radiant surface area all contribute to ensuring that the heating furnace has a high thermal efficiency. 2.4 Reducing other energy consumptions Taking into account the characteristics of the installation, we implemented a number of technical measures that have proven effective in saving energy and reducing consumption: ① Using drainage sludge as a substitute for high-pressure steam in the coke tower. When cooling coke in the coke tower, 1 hour after the end of the low-pressure steam injection, the drainage sludge is introduced into the coke tower instead of using high-pressure steam, saving 8 tons of steam per hour. 　　②Rainwater from the rainwater pump room is used as a supplementary source of water for cooling coke water, thereby saving fresh water. 　　③Recovering the 0.5 MPa condensate water from the desulfurization system to the deaerator not only recovers heat but also reduces the amount of water that needs to be softened. 　　All the above measures were implemented between April and June 2001. 3 Equipment calibration calculations In March 2002, we calibrated the CFB system of the facility; the raw materials used for this calibration were Yiqing and Xirui residue oil. The calculations regarding the heat extracted from the CFB demineralized water are shown in Table 4. As can be seen from these calculations, after implementing the combined renovation project involving CFB and coking processes, the temperature of the 207 t/day of demineralized water supplied to the CFB boiler increased from 37°C to around 74°C after passing through the coking unit. The amount of heat extracted was 32.4 GJ/h, and this alone enables a reduction in the facility’s energy consumption by approximately 4 kg of oil equivalent per ton. 4　Conclusion 　After the implementation of technical upgrades, the energy consumption of the coking unit decreased from 28.9 kg of standard oil per ton in 2000 to 22.5 kg of standard oil per ton in 2001. In 2002, the processing volume increased to 1.4022 million tons, and the energy consumption dropped to 17.2 kg of standard oil per ton, reaching a level that is among the best in the country. 4.1 Increasing the processing capacity of the device and reducing energy consumption through the scale effect of the device is a good approach. 4.2 The process characteristics of the coking unit result in a large amount of low-temperature heat remaining within the unit; by implementing a heat integration project that combines coking with CFB boilers, this heat can be fully recovered, thereby increasing the energy recovery rate of the unit. 4.3 Based on the production characteristics of the equipment, targeted measures can be taken; through small-scale technical upgrades and improvements, it is also possible to effectively reduce the equipment’s consumption of gas, water, and steam.