Wood pump hydrogenation process flow

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Coking crude benzene is pumped from the crude benzene feed tank in the tank farm to the main plant area using a crude benzene feed pump. First, it passes through a crude benzene filter (with a filtration precision of 100 microns) to remove the solid particles and polymers contained in the crude benzene feedstock. There is a pressure difference display before and after the filter, allowing one to monitor its level of clogging so as to switch it in a timely manner. The raw material continues to flow into the crude benzene buffer tank equipped with a nitrogen seal. The overpressure of the buffer tank and the amount of nitrogen are controlled by instruments; when overpressure occurs, the material is discharged into the vent tank, and when the pressure is low, the opening is increased to boost the nitrogen supply. Substances such as wastewater present in the raw materials are separated in the water separation pack and sent to the water separation tank in the oil storage area. The residence time of the raw material in the buffer tank is approximately 45 minutes. The crude benzene feed is then pressurized to about 3.4 Mpa (g) using a crude benzene pump, and subsequently divided into two streams. Approximately 90% of this crude benzene feed, along with all the cyclohydrogen gas coming from the cycle gas compressor, is mixed in a pre-evaporator equipped with nozzles; there, it partially evaporates before entering the nozzles at the bottom of the multi-stage evaporator. After exchanging heat with the main reaction products in the pre-evaporator, the feed is heated to around 176°C. The material sent to the nozzles at the bottom of the multi-stage evaporator mixes with the circulating liquid from the reboiler at the bottom of the evaporator tower before entering the bottom of the multi-stage evaporator. The bottom reboiler is heated by the main reaction stream. In addition, about 10% of the crude benzene supplied by the feed pump enters the top of the multi-stage evaporator as reflux to ensure the proper operation of the multi-stage evaporator. The heavy components accumulated at the bottom of the pre-evaporator are periodically discharged to the flash tank. To achieve more complete vaporization, nozzles are installed on the multi-stage evaporator, allowing the liquid in the second stage to flow further into the nozzles, thereby improving the vaporization efficiency of the feed material. The upper part of the multi-stage evaporator is equipped with 10 layers of trays, which ensures that the maximum amount of heavy components remains at the lower part of the tower and does not enter the gas stream at the top of the tower, thereby reducing the impact on the catalyst in the hydrogenation reactor. A small amount of high-boiling-point substances at the bottom of the tower (about 2–3% of the raw material’s weight) is discharged as residue from the bottom of the multi-stage evaporator and sent to the flash tank, in order to avoid difficulties in subsequent operations, primarily to prevent clogging. After flashing, the light components are sent to the crude benzene buffer tank, while the residue containing a small amount of benzene, toluene, and xylene along with a large amount of heavy hydrocarbons is sent to the tar processing section for further treatment via a residue pump, controlled by the liquid level. The operating pressure of the multi-stage evaporator is primarily determined by the pressure drop in the hydrogenation reactor; as the operating time increases and the reactor pressure drop rises, the operating pressure of the multi-stage evaporator must also be increased in order to maintain pressure balance throughout the entire reaction system. The mixture of evaporated crude benzene and recycle gas exits the top of the multi-stage evaporator at around 192°C and enters the pre-reactor heat exchanger, where it is heated by the main reactants to the temperature required for the pre-reaction. From the bottom of the pre-reactor, it flows upward through the catalyst bed. The dienes and styrene are hydrogenated and saturated in the presence of the Ni-Mo catalyst, and the gas mixture exits from the top of the pre-reactor. Due to the exothermic nature of the reaction, the outlet temperature of the reactor is influenced by catalyst activity, system circulation conditions, feed composition, and the inlet temperature (around 217°C); thus, the outlet temperature of the reactor ranges from 205 to 240°C. During normal operation, the conversion rate is maintained by adjusting the inlet temperature; the reactor outlet temperature must not exceed 240°C, as high temperatures will cause polymer formation. Under normal conditions, the temperature difference in the reactor should be maintained at least at 10°C; when this difference reaches around 5°C and the outlet temperature reaches its maximum value, the catalyst needs to be regenerated. Substances with high boiling points are discharged from the bottom of the pre-reactor and sent to the flash tank V-103. The main reactions in the pre-reactor are as follows: Cyclopentadiene + hydrogen = Cyclopentene, C5H6 + H2 = C5H8; Cyclohexadiene + hydrogen = Cyclohexene, C6H8 + H2 = C6H10; Other dienes + hydrogen = Monoenes, CnH2n-2 + H2 = CnH2n; Styrene + hydrogen = Ethylbenzene, C8H8 + H2 = C8H10; Indene + hydrogen = Dihydroindene, C9H8 + H2 = C9H10; Methylstyrene + hydrogen = Methylethylbenzene, C9H10 + H2 = C9H12; Carbon disulfide + hydrogen = Methane + Hydrogen sulfide, CS2 + 4H2 = CH4; Ethanethiol + hydrogen = Ethane + Hydrogen sulfide, C2H6S + H2 = C2H6 + H2S. The gas mixture exiting from the top of the pre-reactor is heated by the heat exchanger in the main reactor, and then further heated by the heating furnace in the main reactor to reach the required inlet temperature. It enters the main reactor from the top, flows downward through the Co-Mo catalyst bed, where reactions such as desulfurization, denitrification, and olefin saturation of the feed materials take place. The reaction that takes place in the main reactor is an exothermic reaction, causing the outlet temperature to rise to 310–370°C. The partial pressure of hydrogen in the gas mixture at the outlet of the main reactor is 1.71 Mpag. During normal operation, the reaction activity of the catalyst is maintained by adjusting the temperature at the reactor inlet, as it is not possible to change the amount of catalyst or the partial pressure of hydrogen in the gas mixture.