Does anyone know any information on negative pressure benzene distillation? Thank you!

This technology is a new energy-saving and emission-reduction technique worth promoting for widespread use. A brief introduction to the process flow is as follows: In the rich oil process, the rich oil coming from the final cooling and benzene removal unit first exchanges heat with the gas-oil heat exchanger at the top of the benzene removal tower, and then with the hot poor oil discharged from the bottom of the tower. After that, it enters the tubular furnace where it is heated, before proceeding to the negative-pressure benzene removal tower for further benzene removal. 　　Light benzene process: The light benzene vapor escaping from the top of the vacuum benzene removal tower first exchanges heat with the oil-rich stream before entering the condensation cooler; the condensed liquid then goes into the light benzene reflux tank. The light benzene fraction obtained after water separation is sent to the top of the tower as reflux, while the remainder is taken out as a product. 　　Side-line extraction: The side line of the negative-pressure benzene removal tower is used to extract high-purity benzene and naphthalene oil. 　　Lean oil process: Part of the hot lean oil discharged from the bottom of the negative-pressure debenzing tower is pumped to the tubular furnace in the debenzing tower for heating, and then returned to the bottom of the debenzing tower as a heat source ； Another portion of the hot lean oil is pumped through the lean-rich oil heat exchanger and the first-stage lean oil cooler before being sent to the lean oil tank; it is then pumped to the second-stage lean oil cooler for cooling and subsequently sent to the final cooling and benzene washing section for reuse. 　　To maintain the quality of the circulating wash oil, a portion of the hot lean oil is introduced into the regeneration tower for regeneration. Most of this washing oil is evaporated and enters directly at the bottom of the debenzing tower. The high-temperature residue oil remaining at the bottom of the regeneration tower is pumped: one part is sent to the tubular furnace in the regeneration tower for heating before being returned to the regeneration tower as a heat source, while the other part is sent to the tar tank in the tar processing plant. II. Characteristics of the process This process set has the following notable advantages over the atmospheric pressure benzene removal process: (1) The atmospheric pressure process involves the distillation of crude benzene at normal pressure, whereas this process is equipped with vacuum pumps at the top of the tower to ensure the negative pressure required in certain critical systems. A negative pressure environment helps improve the relative volatility of light benzene in the washing oil and reduce the temperature of the mixed vapor escaping from the tower, thereby facilitating the separation and condensation of light benzene. 　　(2) At present, the vast majority of atmospheric-pressure processes use superheated steam distillation for benzene removal, which results in a large amount of wastewater generated from the separation of crude benzene; however, this process does not utilize steam, but rather relies on the high-temperature hot lean oil from the bottom of the benzene removal tower as the heat source for distillation. This ensures efficient use of energy, prevents energy waste, and at the same time reduces the amount of wastewater generated from crude benzene separation. 　　(3) The atmospheric pressure process is equipped with only one tubular furnace heating system; however, due to the characteristics of tubular furnace heating and the requirements of the process, this process uses two tubular furnace heating systems to ensure stable and uniform heat supply. The total gas consumption of the tubular furnace is lower compared to a constant-pressure distillation benzene removal system of the same scale. 　　(4) The atmospheric pressure process is equipped with multiple oil-water separators; since the water separation from crude benzene occurs at a much lower level in this process, only one light benzene reflux tank is required in this setup to meet the needs for static separation of water from crude benzene. III. Economic benefits: Approximately 180 kg more crude benzene can be produced per hour, which is equivalent to an increase of 1.314 kg of crude benzene per ton of coke produced. IV. Social Benefits 　　(1) Energy savings of over 8,160 tce/year, equivalent to 6.8 kgce per ton of coke. (At a scale of 1.2 million tons of coke). V. Environmental Benefits 　　(1) The wastewater generated during the separation of crude benzene using this process reduces by about 1.5 tons per ton of benzene, compared to conventional pressure-based processes. 　　(2) The system equipment does not produce any exhaust gas leaks; the system’s exhaust gas is sent through pipes to a negative-pressure gas network, thus keeping the environment clean. Those interested can contact us by phone at: 18638751326

Negative pressure benzene removal technology: The current atmospheric pressure benzene distillation process has two main problems. One is the high consumption of steam required for production; 1–1.5 tons of steam are needed to produce 1 ton of crude benzene, and the treatment of the water resulting from steam condensation is difficult； Secondly, the benzene removal efficiency is low, resulting in a high benzene content in the lean gas, which in turn affects the benzene absorption efficiency and leads to a high benzene content in the gas exiting the benzene washing tower. To reduce wastewater generation and benzene loss, our factory has developed a negative-pressure benzene removal distillation process in collaboration with several research institutions and universities, achieving significant environmental and economic benefits in actual production. 1 Process Overview 1.1 Process Principle The negative pressure benzene removal process is based on the relationship between liquid pressure, boiling point, and relative volatility as per distillation principles. A vacuum pump is used to create a reduced pressure in the benzene removal tower, thereby lowering the pressure at the surface of the oil-rich mixture. This reduces the boiling points of the components in the oil-rich mixture, allowing benzene compounds to be vaporized from it at temperatures lower than those used in normal pressure distillation processes. At the same time, due to the reduction in operating temperature and pressure, the relative volatility of crude benzene in the oil-rich phase increases, facilitating its desorption from the washing oil and thereby improving the benzene removal efficiency. This process can reduce heat consumption under the same production load, offering good energy-saving effects. Meanwhile, operating at lower temperatures can reduce product decomposition or polymerization losses. Figure 1 Process flow diagram of negative pressure benzene removal 1.2 Process flow The process flow of negative pressure benzene removal technology is shown in Figure 1. As shown in Figure 1, the rich oil sent from the final cooling and benzene removal unit passes through the oil-gas heat exchanger and the rich-poor oil heat exchanger (without being heated by a tubular furnace) successively, to be heated to 170°C before entering the benzene removal tower. A portion of the thermally depleted oil from the bottom of the debenzing tower is pumped out using the tower’s circulation pump, heated to 230–245°C in a tubular furnace, and then returned to the bottom of the debenzing tower as a heat source for distillation. The crude benzene vapor escaping from the top of the tower is cooled in an oil-vapor heat exchanger and a crude benzene condenser, after which it enters the crude benzene reflux tank. Part of this vapor is pumped back to the top of the tower using a crude benzene reflux pump as reflux, while the rest goes into the crude benzene intermediate tank, from where it is then pumped to the oil storage facility using a crude benzene product pump. Additionally, a hot lean oil pump is used to pump a portion of the lean oil from the bottom of the debenzene tower to the lean-rich oil heat exchanger for heat exchange; after that, it is cooled in a first-stage lean oil cooler before being sent to the lean oil tank. It is then pumped out by a cold lean oil pump and cooled to 27–29°C in a second-stage lean oil cooler before being sent to the final cooling and benzene washing unit. The non-condensable gases discharged from the top of the crude benzene reflux tank are cooled in a non-condensable gas cooler before being sent to a vacuum pump, which then pumps them into the gas pipeline ahead of the fan. Here, the vacuum pumping capacity of the vacuum pump is used to adjust the operating pressure at the top of the debenzene tower, so that it operates under appropriate negative pressure conditions. The hot lean oil pumped by the dephenolization tower circulation pump is heated in a tubular furnace, and 1% to 5% of this hot lean oil is taken out and fed into the regeneration tower for regeneration. The hot lean oil at the bottom of the regeneration tower is pumped out by the circulation pump of the regeneration tower, heated to 240–255°C in a tubular furnace, and then sent back to the bottom of the regeneration tower to serve as a heat source for distillation there. The gas from the top of the regeneration tower enters the benzene removal tower, while sludge is regularly removed from the bottom of the regeneration tower and pumped to the oil storage facility. The washing oil consumed by the system is periodically replenished into the system from the washing oil tank via the rich oil pump inlet (in the benzene washing section). The separated water leaving equipment such as the recirculation tank is discharged into the separated water vent tank, and then pumped to the final cooling intermediate tank. The non-condensable gases from each storage tank are collectively led to the coal suction pipeline in front of the blower in the condensing and ventilation section. 1.3 Process Characteristics: The negative pressure benzene removal process uses recycled heat-poor oil instead of steam to provide heat; no water vapor is introduced into the process, **which reduces the amount of water generated. It offers advantages such as energy savings, environmental protection, and reduced emissions, with an overall energy-saving effect of over 25%. A negative pressure environment helps to increase the relative volatility of benzene in the washing oil, effectively reducing the distillation temperature. This results in high benzene removal efficiency, stable benzene content in the oil stream, and an increase in the benzene recovery rate from the benzene washing tower of over 8%. Based on the comparison data of benzene removal efficiency for various systems shown in Table 1, the negative pressure benzene removal process exhibits a more significant effect; the average benzene content in the gas after passing through the benzene removal tower is 1.10 g/m3, with a benzene absorption rate of 96.1%. The benzene absorption rate in the atmospheric pressure distillation process is at most 87.8%, resulting in significant losses of benzene. Since much less water needs to be removed from crude benzene in this process compared to the atmospheric pressure process, the oil-water separation equipment used here includes only one crude benzene reflux tank. The crude benzene obtained after separation is used as reflux, while the remaining portion flows into the intermediate crude benzene tank before being sent to the oil storage facility. The vacuum pump, one of the key equipment in the system, uses crude benzene as the liquid ring medium, resulting in no wastewater discharge and excellent environmental benefits. The exhaust gases from various tanks are all directed into the coal intake pipeline before the blower, which contributes to environmental protection. Both the benzene removal tower and the regeneration tower are made of stainless steel, ensuring a long service life. The trays in the benzene removal tower are of the latest high-efficiency type, offering high distillation efficiency and low resistance; they also possess a self-cleaning function that effectively solves the problem of residue accumulation on the trays caused by the heavy components in the oil. The entire process employs dual feeding, namely feed at the middle and bottom of the tower, with the bottom of the tower being heated cyclically using a tubular furnace. The tubular furnace features a dual-chamber structure, with the radiant sections inside being two separate units that share one convective section. The circulating oil in the benzene removal tower enters the convection section and Radiator No. 1 in sequence for heating, while the circulating oil in the regeneration tower is heated only by Radiator No. 2. This allows for a reasonable distribution of heat, uniform heating, and reduced floor space requirements. ☆TEL: 13325285381 Table 1 Benzene content in gas before and after the benzene washing tower from July to November 2012 (g/m3) Item July August September October November Average System 1 (at atmospheric pressure) Before tower 29.81 29.93 28.82 29.23 31.43 29.84 After tower 3.71 4.13 3.69 3.88 4.78 4.04 System 2 (at atmospheric pressure) Before tower 29.74 29.45 29.05 28.50 32.14 29.78 After tower 3.56 3.67 3.53 3.57 3.65 3.60 System 4 (at atmospheric pressure) Before tower 29.95 29.95 29.25 29.77 31.40 30.06 After tower 3.24 3.38 2.51 3.27 3.88 3.26 System 5 (under negative pressure) Before tower 29.63 28.61 25.91 27.14 28.02 27.86 After tower 2.32 1.19 0.52 0.71 0.80 1.10 2 Production commissioning 2.1 Parameter control The ultimate goal of benzene removal distillation is to vaporize benzene from the oil-rich mixture; after benzene removal, the oil-poor mixture is sent to the benzene washing section for reuse. Pressure and temperature control are the key control points in a distillation system, directly determining the efficiency of benzene removal. A vacuum pump is installed in the benzene vapor circuit to create a negative pressure inside the benzene removal tower, which effectively reduces the operating temperature during the benzene removal process from oil-rich mixtures. The greater the negative pressure, the lower the temperature required for distillation. The adjustable range of negative pressure generated by the vacuum pump is 0 to -99 kPa. The negative pressure inside the tower can be controlled through the backflow control valve at the vacuum pump’s outlet, with the pressure at the top of the tower maintained between -70 and -80 kPa. Operating under either low or high loads is unfavorable for the vacuum pump. The main components of crude benzene in this product are benzene, toluene, and xylene; among them, benzene accounts for 55%–75%, toluene for 12%–22%, and xylene for 2%–6%. As can be seen from Table 2, at a tower top pressure of -70 to -80 kP (absolute pressure of 21 to 31 kPa), the boiling points of the three components are 40°C, 70°C, and 90°C respectively. Therefore, to achieve a high recovery rate, the temperature at the top of the debenzing tower should be controlled at around 90°C, which is the boiling point. Excessively high control of the tower top temperature helps to reduce the benzene content in the lean oil, but it increases the gas consumption of the tubular furnace. At the same time, the light components in the washing oil are vaporized, which affects the efficiency of benzene removal and also increases the consumption of washing oil. ☆TEL: 13325285381 Table 2 Boiling points of benzene, toluene, and xylene at different pressures. Absolute pressure in kPa: Benzene in °C, Toluene in °C, p-Xylene in °C, m-Xylene in °C, o-Xylene in °C. 0.13 –36.7 –26.7 –8.0 –6.9 –3.8; 0.66 –19.6 –4.4 15.5 16.8 20.2; 1.33 –11.5 6.4 27.3 28.3 32.1; 2.66 –2.6 18.4 40.1 41.1 45.1; 5.32 7.6 31.8 54.4 55.3 59.5; 7.98 15.4 40.3 63.5 64.4 68.8; 13.3 26.1 51.9 75.9 76.8 81.3; 26.6 42.2 69.5 94.6 95.5 100.2; 53.2 60.6 89.5 115.9 116.7 121.7. At a constant oil circulation rate, the factors that affect the benzene content in the lean oil include the temperature of the rich oil entering the tower, the temperature of the lean oil entering the tower, tower pressure, and the gas load (benzene content in the rich oil). Among these factors, the temperature of the rich oil entering the tower is primarily determined by factors such as the heat exchange area of the heat exchangers. The gas load remains relatively stable over a certain period of time. The higher the vacuum level inside the tower, the more effective the benzene removal process becomes; however, in order to maintain stable production, the tower pressure is kept constant once it has been set. The temperature of the hot dilute oil entering the benzene removal tower after being heated in a tubular furnace is the main factor determining the benzene content in the dilute oil. The temperature control is too low, resulting in insufficient evaporation within the tower; thus, the oil-rich stream contains high levels of benzene ； If the temperature control is too high, the light components in the washing oil are vaporized, resulting in increased fuel consumption, greater load on the vacuum pump, and difficulty in maintaining the tower pressure. Our plant requires that the distillate yield at 180°C for crude benzene be ≥91% (the higher this value is, the greater the output of crude benzene). Based on the preliminary production testing, control ranges for the key parameters of negative pressure distillation were established: the temperature at the top of the benzene removal tower was kept between 88 and 91°C, while the pressure at the top of the tower was between -74 and -76 kPa. The temperature of the oil-poor gas entering the benzene removal tower after being heated in a tubular furnace was controlled between 235 and 245°C, allowing the benzene content in this gas to remain at a low level. The benzene removal efficiency was good, with the lowest benzene content in the gas exiting the tower reaching 0.5 g/m3. The distillate yield of crude benzene before 180°C remains stable at 91%–92%, which ensures quality while also enabling a high recovery rate of crude benzene. 2.2 Operation optimization: After operating for a period of time, as impurities are introduced and some light components are lost, the quality of the circulating wash oil gradually deteriorates. The regeneration of this wash oil and the addition of new wash oil have a direct impact on the efficiency of benzene washing. A portion of the wash oil is taken from the outlet of the debenzene tower’s circulation pump and sent to the regeneration tower for distillation and regeneration. The light components in this wash oil enter the debenzene tower from its top, while the heavy components at the bottom of the tower are discharged as residue to the oil storage facility. The system was originally designed for continuous slag discharge from the regeneration tower; that is, a portion of the fluid coming from the outlet of the circulation pump in the regeneration tower is directed to the slag tank, and the flow rate is controlled via a control valve. The amount of washing oil used is 100 kg/t or more per ton of crude benzene. Given that the oil consumption for continuous slag discharge is high, and the oil temperature in the regeneration tower reaches 250°C, when the oil enters the residue tank, insufficient insulation from low-pressure steam leads to poor oil quality; upon condensation, this can cause blockages in the pipes and tanks. Therefore, drawing on other atmospheric distillation systems, intermittent slag discharge three times a week was tried out. The residue is discharged directly from the bottom of the regeneration tower into the oil tank, without passing through a residue tank, thereby avoiding blockages caused by a drop in oil temperature. After trying intermittent slag discharge, the fuel consumption of the system was significantly reduced; it was 61 kg/t in August and 42 kg/t in September, which is not much different from the fuel consumption for washing in the atmospheric distillation process. As can be seen from the wash oil samples in Table 3, intermittent slag removal three times a week is sufficient to meet production requirements, and the quality of the recycled wash oil remains stable. Table 3: Oil washing quality and slag discharge for the five-system (negative pressure) process from August to September
Date | Oil washing density, g/cm³ | Initial boiling point, °C | Percentage of distillate before 300°C from recycled oil | Percentage of residue distilled before 300°C
20120730 | 1.062 | 252 | 93 | 43
20120803 | 1.061 | 252 | 92 | –
20120810 | 1.062 | 254 | 91 | –
20120817 | 1.069 | 254 | 90 | 31
20120824 | 1.064 | 254 | 93 | 39
20120831 | 1.065 | 256 | 92 | –
20120907 | 1.060 | 254 | 95 | –
20120914 | 1.064 | 258 | 94 | 31
20120921 | 1.069 | 256 | 93 | 30
20120928 | 1.072 | 262 | 92 | –

3. Benefit analysis
3.1 Economic benefits
(1) Assuming that each shift in the five-system process produces 16 tons of crude benzene, and that the atmospheric-pressure distillation unit consumes 1.3 tons of steam per ton of crude benzene produced, negative-pressure benzene removal can save 2.6 tons of steam per hour. However, compared to atmospheric-pressure distillation, it requires three additional pieces of equipment, which increases power consumption (55 kW for the vacuum pump, 110 kW for the circulation pump in the benzene removal tower, and 37 kW for the circulation pump in the regeneration tower). (2) The average gas consumption of the five-system tubular furnace is 800 m3/h, while under the same load the average gas consumption of the two-system (atmospheric distillation) tubular furnace is 1300 m3/h; consequently, the gas savings achieved by the negative-pressure system are quite significant. Annual savings on steam costs: 2.6×110×24×365 = 2.5 million yuan (at 110 yuan per ton of steam). Annual savings on wastewater treatment costs: 2.6×10×24×365 = 228,000 yuan (at 10 yuan per ton of wastewater). Increased annual electricity consumption: 202×24×365×1 = 1.77 million yuan (at 1 yuan per kWh). Annual savings on gas costs: 500×24×365×0.3 = 1.314 million yuan (at 0.3 yuan per m3 of gas). Total annual savings: 2.5 + 228,000 – 1.77 + 1.314 = 2.272 million yuan. 3.2 Environmental benefits: (1) Compared to atmospheric pressure processes, the negative pressure system can reduce the amount of crude benzene separation wastewater generated by 22,776 tons per year. (2) The system’s exhaust gases are not released outside; instead, they are pumped via a vacuum pump through pipes to the negative-pressure gas network in front of the blower, keeping the environment clean. 4 Conclusion The negative pressure benzene removal process has been successfully applied in the recovery and production of coking by-products; it operates stably and has yielded good economic and environmental benefits. ☆TEL: 13325285381