delayed coking

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Delayed coking is a relatively thorough process for heavy oil processing; it is a thermal treatment process that involves deep thermal cracking and condensation reactions at high temperatures of 500°C, using vacuum residue and cracked residue as raw materials. Produces gasoline, diesel, intermediate distillates (as feedstock for catalytic cracking), gases, and coke. Process Introduction: The process flow for delayed coking varies depending on the type. In terms of production scale, there are processes such as the one-furnace two-tower layout and the two-furnace four-tower layout. Its characteristic is that the residue oil is passed at a very high flow rate, under high-temperature conditions, in a violent turbulent flow through the heating furnace tubes; it crosses the critical cracking temperature range of the residue oil in a very short time, reaches the temperature required for coking reactions, and then quickly leaves the heating furnace tubes to enter the reaction space of the coke tower, thereby delaying reactions such as cracking and condensation until they take place inside the coke tower. The crude oil is heated to around 350°C in the convection section of the heater through heat exchange at 190°C. It then enters the heat exchange section at the lower part of the distillation tower, where it exchanges heat directly with the hot oil and gas coming from the top of the coke tower; this process vaporizes the light components in the crude oil, while simultaneously heating the crude oil itself. The temperature in the evaporation section is 360–380°C. If the temperature is low, the amount of circulating oil that condenses is large, resulting in a high circulation ratio; conversely, the circulation ratio is low. The temperature at the bottom of the distillation tower is maintained at 385–390°C, and a bottom circulation oil pump ensures continuous circulation of the oil to minimize the risk of coking at the tower bottom. The heavy fractions in the crude oil, above the wax oil range, along with the recycled oil that has condensed from the gases coming from the coke tower, flow to the bottom of the tower. They are then pumped out by the radiant feed pump of the heater and sent into the radiant section of the heater, where their temperature is raised rapidly to around 500°C (with a pressure of 3.0–4.0 MPa at the inlet of the radiant section). To reduce coking in the heating furnace tubes, about 2% of the amount of crude oil is injected as softened water at the inlet, causing the oil to pass through its critical reaction zone in a state of intense turbulence. The flow velocity of the gas-liquid mixture is increased to 30–45 m/s, thereby delaying the coking reaction. The oil at the outlet of the heating furnace enters the coke tower via a four-way valve; it undergoes reactions thanks to the sensible heat released as it cools down. The oil and gas produced as a result have a temperature of around 430°C, and they are drawn out from the top of the coke tower. There, they exchange heat with the feed oil, after which they are separated into gasoline, diesel, wax oil, and coking gases. The coke layer formed through condensation gradually cokes on the inner wall of the coke tower from bottom to top. There is a foam layer above the coke layer, which is caused by the foaming of intermediate products during the reaction at high temperatures; this foam layer should be below the safe height of the coke tower. The coke tower operates on a batch basis, with a switching cycle of generally 48 hours; of this, 24 hours are spent on coking, while the remaining 24 hours are used for operations such as coke removal. During production, feeding is stopped when the level of coke and the foam layer in the coke tower reaches 2/3 to 3/4 of the total height of the tower, after which operation is switched to another coke tower. After feeding to the coke tower is stopped, steam must be blown in immediately to displace any remaining oil and gas, thereby reducing the volatiles in the coke. After being heated, it is cooled with water, and finally decoking is carried out. After being separated in the liquid separation tank at the compressor inlet, the coking rich gas enters the compressor where it is compressed to a pressure of 1.2 MPA. It is then cooled by a cooler and sent to another liquid separation tank. The gas enters the absorption tower where it is absorbed by diesel; after absorption, the dry gas is sent to a gas desulfurization unit. The diesel that has been absorbed at the bottom of the absorption tower returns to the distillation tower under its own pressure as reflux. Introduction to the control schemes: 1. Control of the liquid level at the top of the fractionation tower – LIC1105, using cascade control. 2. Control of the outlet temperature at the top of the coke tower – TIC1110, also using cascade control. The outlet temperatures at the tops of coke towers A and B are generally controlled through cascade regulation in conjunction with the flow rate of quench oil entering the coke tower, FIC1109. 3. The feed temperature of the crude oil to the distillation tower, TIC1115, is controlled using proportional control. For the adjustment of this temperature, proportional control is employed, with settings for 0-50% and 50-100% respectively; a certain dead zone is established at the midpoint for control purposes. 4. The water injection flow rate for D-1102/B is controlled via a single-loop control system using FIC1105. 5. The outlet temperature of the heating furnace is controlled through cascade control; this control system utilizes both the outlet temperature and the furnace interior temperature, with the outlet temperature being regulated by the main control loop ； The regulating mechanism can be either fuel gas or fuel oil. 6. Thermal efficiency of the heating furnace. 7. Anti-surge control of the air compressor can be achieved either through fixed-flow control or adaptive flow control systems. 8. Interlock circuit for air compressor and hydraulic coking removal – Process flow diagram