Experience in the renovation of crude benzene distillation systems

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The crude benzene distillation unit of the third gas purification system at Magang Coal Coking Company uses a tubular furnace to heat the rich oil, and employs a single-column benzene removal distillation process; light benzene is produced at the top of the column, while heavy benzene and naphtha are obtained from the side streams. This unit was put into operation in March 2007, and numerous problems arose during the trial production phase. 1 Problems Existing 1.1 Excessively high temperature after lean oil cooling Both the first and second stage lean oil coolers are plate-and-frame heat exchangers, with the lean oil and cooling water exchanging heat in a counterflow manner. There are 4 lean oil coolers, each with a heat exchange area of 138.06 m2; these cool the lean oil from 105°C to 40°C. There are 3 two-stage lean oil coolers, each with a heat exchange area of 80.34 m2, which cool the lean oil from 40°C to 27°C. In actual production, the lean oil is cooled in a first stage, with its temperature dropping from 88°C to 50°C; after a second stage of cooling, the temperature reaches 37°C, neither of which meets the requirements specified in the process design. Not only is the benzene removal efficiency poor, but also the naphthalene in the lean oil transfers to the gas, resulting in an excessive level of naphthalene in the gas leaving the plant. 1.2 Poor discharge of non-condensable gases leads to large fluctuations in tower pressure. The non-condensable gases generated during the distillation of crude benzene are discharged through the vent pipes on the light benzene outlet pipeline; due to poor discharge, the top pressure of the benzene removal tower reaches 16–20 kPa, while the bottom pressure is 36–40 kPa ； At the lowest level, the pressure at the top of the tower is 0.5–9 kPa, while it is 20–29 kPa at the bottom of the tower. This leads to disruptions in the operation of the benzene removal tower; the final boiling point of light benzene fluctuates between 131 and 163°C, making it difficult to ensure consistent quality. When non-condensable gases are released, a large amount of light benzene is carried away, which not only pollutes the environment but also poses a risk of safety accidents. 1.3 Severe liquid leakage from the trays: The distillation section of the benzene removal tower consists of 39 trays, with each tray being made up of 10 plates (the 50th tray is composed of 12 plates). Liquid on the trays leaks to the lower tray through the semi-circular bolts of the detachable connectors on both sides of adjacent trays, resulting in an insufficient thickness of the liquid layer on those trays; this prevents the liquid from covering the gaps between the baffles and the lower end of the overflow pipes. Steam beneath the trays can directly enter the upper space through these gaps or overflow pipes, preventing adequate contact between the vapor and liquid phases inside the tower. As a result, mass and heat transfer are impaired, leading to poor benzene removal efficiency. 1.4 Low oil washing circulation rate: The heat exchangers for rich and lean oil in the first and second stages are both of the spiral plate type, with 5 units each. Due to the deposition on the inner walls of the heat exchanger of dust, rust, slag in the pipes, as well as coal powder, coke powder, graphite, naphthalene, etc. brought in from the benzene washing tower, the thickness of this deposit is about 2–3 mm. This results in an increased resistance in the heat exchanger; the flow rate of the cleaning oil is only 140–150 m3/h, which is far below the designed value of 180 m3/h. 1.5 It is difficult to continuously extract heavy benzene and naphtha oil. The production rate of heavy benzene is around 40 kg/h, and it is not easy to control the amount extracted using valves. Valves are provided on the naphthalene oil side line, but there are no naphthalene oil extraction pipelines or storage tanks. If heavy benzene and naphtha oil cannot be extracted properly, it will affect the quality of the circulating wash oil and the stability of production operations. 1.6 Excessively high temperature of superheated steam after the tubular furnace: The temperature of the superheated steam after being heated by the tubular furnace is too high; this is in conflict with the preheating temperature required for the rich oil, which is generally maintained at around 180°C. Meanwhile, the temperature of the superheated steam reaches 510°C, which is far higher than the designed value of 400°C. This causes the regenerator temperature to be high, severely affecting the benzene removal production process. 2 Renovation Measures 2.1 Renovation of the lean oil cooler 1) The primary lean oil cooler was originally a single-pass design; by adding 4 partitions on both the lean oil side and the cooling water side, it was transformed into a five-pass design. After the modification, the temperature of the lean oil after being cooled in a cooler dropped to 55°C, an increase of 17°C compared to before the modification. 2) The two-stage lean oil coolers were modified from 3 plate-and-frame heat exchangers to 3 spiral-plate heat exchangers, with each having a heat exchange area of 180 m2. After the modification, the temperature of the lean oil after cooling in the second cooler can reach 26–28°C, which is about 10°C lower than before the modification. 2.2 Addition of a gas-liquid separator: A gas-liquid separator (DN600, H1200) was added to the light benzene outlet pipeline of the light benzene condenser. After the modification, non-condensable gases and light benzene are effectively separated in the gas-liquid separator; light benzene flows from the bottom of this separator to the light benzene oil-water separator, while the non-condensable gases are discharged continuously and evenly through the exhaust pipe at the top, allowing the pressure at the top of the debenzing tower to be maintained at 5–8 kPa and the pressure at the bottom to be maintained at 25–28 kPa, thus ensuring stable operating conditions for the debenzing tower. 2.3 Welding of the trays in the distillation section of the debenzene tower: 39 trays in the distillation section of the debenzene tower were welded to prevent leakage from these trays. 2.4 Cleaning of heat exchangers: Chemical cleaning is performed on the rich-lean oil heat exchangers in stages 1 and 2. Not only was the oil washing circulation rate increased to 180–185 m3/h, but the heat exchange efficiency was also improved; the temperature of the rich oil after passing through the two-stage heat exchangers reached 140°C, an increase of 20°C compared to before cleaning. 2.5 Modifying the extraction methods for heavy benzene and naphthalene oil: An extraction pipeline is installed after the valve at the outlet for naphthalene oil, connecting it to the crude benzene residue tank. Orifice plates are placed on the pipelines for extracting heavy benzene and naphthalene oil in order to control the amount of fluid extracted. By selecting appropriate orifice sizes, it is possible to carry out continuous extraction of heavy benzene and naphthalene oil over extended periods of time. This reduces the benzene content in the oil with low benzene content, as well as the naphthalene content in the recycled washing oil, thereby improving the quality of the recycled washing oil and reducing the naphthalene content in the gas produced after the benzene washing tower. 2.6 Modification of steam pipes: The steam pipes are modified so that some of the steam bypasses the convection section of the tubular furnace and enters the radiation section directly. While ensuring the safety of the steam pipes in the tubular furnace and maintaining a rich-oil preheating temperature of 180°C, the superheated steam temperature can be controlled between 400 and 420°C. 3 Reform effects: As a result of the aforementioned reforms, the temperature at which low-oil conditions occur was reduced, and the circulation volume of the washing oil was increased. This led to an improvement in the efficiency of benzene removal, with the benzene content in the gas exiting the benzene washing tower being reduced to around 2 g/m3 ； At the same time, the operating conditions of the benzene removal tower were stabilized, ensuring stability in the quality and output of light benzene; the final boiling point of light benzene could be maintained at 142–148°C.