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Abstract: The vacuum extraction effect under reduced pressure never meets the design requirements; the maximum vacuum level achieved is 93 KPa. Technological upgrades to the reduced-vacuum system of China National Petroleum Da Gang Petrochemical Company’s 200,000 t/year lubricant-type atmospheric and vacuum distillation unit can effectively improve the vacuum level. Keywords: vacuum evacuation; vacuum level; diameter expansion; top-of-tower vacuum; vaporization section vacuum 1 Introduction: As a result of the modifications made, the vacuum level has improved to an extent that meets the requirements of production. Currently, petrochemical refineries are striving for distillation and vacuum processing conditions characterized by \"high vacuum, low temperature, light color, and narrow boiling range\", with the odor gas containing H2S being discharged from the deodorization tank of the vacuum pump. However, this process has a severe negative impact on the environment in the plant area, which is why it is rarely used. The purpose of distillation under reduced pressure is to address the problems arising from the first renovation by carrying out a second renovation of the vacuum system in China National Petroleum Corporation’s Dagang Petrochemical Plant’s 200,000 tons per year lubricant-type atmospheric and vacuum distillation unit. Following in-depth studies of the process in 2003, it was determined that a second renovation of both the vacuum system and the pressure-driven vacuum extraction system was necessary (see Figure 3). Implementing such technical upgrades can effectively improve the level of vacuum achieved. No investment was made in this device in 1984). The two-stage steam vacuuming process remains unchanged; since the first stage was put into operation, the vacuuming effect achieved through reduced pressure has never met the design requirements. The steam vacuuming + mechanical vacuuming process has been modified to rely on steam vacuuming from the first stage, resulting in a maximum top vacuum level of 93 KPa. A three-stage vacuum system consisting of vacuum pumping, mechanical vacuum extraction, and secondary steam vacuum extraction was implemented for the first time as part of the renovation of the vacuum system; in this setup, the outlet of the vacuum pump was connected to a secondary steam ejector. According to research data, during the first renovation carried out in 2002, the deodorization tank associated with the original vacuum pump functioned as a buffer tank. The original process, which combined mechanical vacuum pumping with steam vacuum pumping and used a secondary steam ejector to remove the non-condensable gases in the tank to a tertiary condenser (see Figure 1), was modified to a two-stage steam vacuum pumping process; in this new system, the condensed liquid is discharged into a water seal tank, while only a very small amount of non-condensable gases remains. This system utilizes both steam vacuum pumping and mechanical vacuum pumping (see Figure 2). Use air discharge. After the modification, the vacuum degree at the reduced roof reached a maximum of 99.5 KPa, and remained at 99 KPa. Figure 1: Flow diagram of the reduced-pressure vacuum system (before modification). Figure 3: Flow diagram of the reduced-pressure vacuum system (after the second modification). During this modification, in order to reduce the consumption of circulating water in the vacuum system after the modification while ensuring that the condensation performance remained unaffected, two adjustments were made to the vacuum system: first, the principle of expanding the diameter using an oil transfer line was applied to further expand the outlet of the diffuser tube of the primary steam ejector; second, a series connection was added to the circulating water circuits of the original secondary and tertiary condensers. As a result, the circulating water flow rate was reduced by 25 m3/h, thereby saving on water consumption. The second renovation was relatively successful: on the one hand, it increased the vacuum level during pressure reduction, and on the other hand, it resolved the odor problem within the device, thereby improving both the conditions for pressure reduction operations and the working environment. 4 Impact of the modification on the vacuum reduction process Figure 2 Flow diagram of the vacuum reduction system (first modification) After the modification, the vacuum level at the top of the tower increased to 99 KPa. During the vacuum extraction process using steam at the vaporization section, the vacuum level at the top of the tower could reach up to 94 KPa, while the overall vacuum level increased to 96 KPa. Before and after the modification, the vacuum reduction system operated using a combination of steam-based vacuum extraction and mechanical vacuum extraction; the details are shown in Table 1 for comparison. (Mixed process), the vacuum level at the top of the tower can reach 96 KPa. As shown in Table 1, a comparison of the operating conditions of the vacuum system before and after the modifications is provided: two-stage steam evacuation process for the vapor at the top of the tower, two-stage (or three-stage) mixed process. Temperature and pressure values for the vacuum stage at the top of the tower are as follows: design values – 70 °C, 1000 KPa, 96 KPa, 92 KPa; after the first modification – 65 °C, 950 KPa, 94 KPa, 93 KPa, 96 KPa, 94 KPa; after the second modification – 99 KPa, 96 KPa. It is well known that the key to effective deep distillation under reduced pressure lies not only in increasing the temperature of the vaporization stage as much as possible but also in achieving a high vacuum level in that stage. Under the same conditions of bottom vaporization rate and furnace outlet temperature, the vaporization rate in the feed section of the vacuum tower increased by about 2% compared to before the modification, thereby improving the vacuum distillation efficiency. Considering that an excessively high superheating rate affects the color of the product at the lowest point of the vacuum distillation column, in order to maintain a superheating rate of 2%–3%, the temperature of the furnace used in the distillation process was reduced from 402 °C to 398 °C through experiments (this is the lower limit required by the process). Meanwhile, the amount of vapor removed from the bottom of the column was decreased from 270 kg/h to 210 kg/h. Production practice shows that the experimental effect is significant, with the reduced-pressure drawing rate being 0.7% higher than before the modification. On the other hand, lowering the furnace temperature and reducing the amount of stripping steam further decreases the load on the vacuum system used for top removal; this in turn helps to increase the vacuum level in that system, while also **reducing the energy consumption of the plant**. 5 Conclusion Based on the data from production practice, the effects of the modification experiments are quite evident; the restructuring of the top-removal vacuum system was highly successful. Since the experimental results showed significant improvements after expanding the outlet of the diffuser tube in the primary steam ejector, these findings from this modification experiment can be applied to the reduced-pressure secondary steam ejector. Building on the existing secondary expansion at the outlet of the diffuser tube, a third-level expansion can be carried out, and it is predicted that this experiment will be successful. References] Hou Fusheng. Handbook for Refining Engineers. Petroleum Industry Press, 1995. This post was last edited by bingyu252 on 2009-2-12 at 14:54.]