Baosteel benzene hydrogenation

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Energy-saving Technologies for the Benzene Hydrogenation Unit at Baosteel Abstract: Based on Baosteel Chemical’s many years of experience in operating benzene hydrogenation units, this paper provides a detailed analysis of the energy-saving measures employed in such units, focusing on aspects such as the design of the residue tower, the utilization of heat from Unit 200, the recovery of steam condensate, and the use of vacuum exhaust gas to replace part of the coke oven gas. Keywords: Benzene hydrogenation, energy saving, residue tower, pressure and atmospheric distillation, steam condensate, vacuum exhaust gases. Introduction: With the widespread use of benzene hydrogenation technology, there are no longer any technical barriers to its core technologies. For example, the hydrogenation process in 100-unit systems primarily relies on BASF’s low-temperature gas-phase hydrogenation technology from Germany and AXENS’ low-temperature gas-liquid phase hydrogenation technology from the United States, while the removal of non-aromatic compounds in 300-unit systems mainly utilizes formylmorpholine gas-liquid phase extraction distillation and sulfolane gas-liquid phase extraction distillation techniques. It can be said that the devices currently in use are generally at the same level, whether in terms of product quality or total investment. Therefore, the local differences and optimizations of various systems can play a crucial role in reducing operating costs; this is true for several energy-saving measures discussed in this article. 1. Comparison of energy savings between placing the residue oil tower at the back and the separation tower for light and heavy components at the front: Currently, the main raw materials used in the benzene hydrogenation process are light benzene and crude benzene. For light benzene, due to its relatively light composition, it is sufficient to add a flash tank after the evaporator to flash the light benzene, thereby removing a small amount of heavy benzene; the specific process is shown in Figure 1. This process does not feature a residue tower or a decanting tower, so it is not discussed in detail in this paper. Figure 1 Flow diagram of the light benzene flash process. This article focuses on the differences in reconstituent removal technologies using crude benzene as the raw material. From the perspective of current technologies, there are mainly the following two approaches. 1.1 Post-residue tower process: In this process, the crude benzene feed is heated by a continuous evaporator; the benzene vaporizes, the residue tower removes the heavy benzene, while the light benzene returns to the evaporator to be completely vaporized before entering the reaction system. The process flow diagram is shown in Figure 2. Figure 2. Process flow diagram for the residue tower at the rear stage. 1.2 Process diagram for the fractionation tower at the front stage. Figure 3. Process flow diagram for the fractionation tower at the front stage. In this process, the crude benzene feed is first subjected to the removal of heavy benzene, after which the light benzene is vaporized and fed into the reaction system. 1.3 Comparison of the two processes in terms of energy savings: The heat consumption of the 100-unit system is primarily due to the energy used for heating furnace gas, the energy required for removing Reorganization components, and the energy consumed by mechanical devices such as pumps. Taking a 100,000 t/a crude benzene hydrogenation process as an example, this paper conducts comparisons mainly from the aforementioned aspects. Table 1 compares the energy consumption of two benzene hydrogenation units at a certain company, as shown in Table 1. Table 1: Comparison of energy consumption with recombination component separation before and after placement. Details: Kw. Energy consumption for gas: 5901 vs. 594; Energy consumption of the residue tower: 735 vs. ——; Energy consumption of the recombination tower: —— vs. 1918; Total energy consumption: 1325 vs. 3512. It can be seen from this comparison that the energy consumption when the recombination component separation tower is placed before the residue tower is significantly higher than when it is placed after the residue tower. This is mainly because using distillation to remove the heavy components is a distillation process with a heavy head and light tail; if a pre-distillation step is employed, approximately 90% of the crude benzene must be distilled first. The residue tower is located after the evaporator, and its feed volume is only about 30% of the raw material volume; therefore, the amount of vaporization in this tower is also much lower. 2 A comparison of energy savings between pressurized distillation and atmospheric distillation in Unit 200: Generally speaking, whether steam heating or heat transfer oil systems are used, about 70% of the energy consumption in benzene hydrogenation occurs in Units 200 and 300; therefore, how to effectively utilize the waste heat from these units is key to achieving energy savings in benzene hydrogenation. In this regard, the main difference in heat utilization at home and abroad currently lies in whether a pressure of 200 units is applied; from a technological perspective, there are mainly the following two methods. 2.1 Pressure-driven distillation technology for Unit 200: This technology makes effective use of the waste heat at the top of Unit 200’s tower, utilizing the steam from that top as energy for the reboiler in the benzene tower. In order for the steam from Unit 200’s tower to effectively heat the components at the bottom of the benzene tower, it is necessary to apply pressure to Unit 200; the specific process flow is shown in Figure 4. Figure 4. Pressurized distillation technology for Unit 200 2.2, Atmospheric distillation technology for Unit 200. Considering that pressurized technology is used in Unit 200, on the one hand its separation capacity decreases, which requires an increase in the reflux ratio and thus an increase in the steam consumption at the bottom of the column in Unit 200 ； On the other hand, the operation of the benzene tower in unit 300 is dependent on that of the pre-distillation tower in unit 200; as a result, the entire unit 300 must start operating simultaneously with unit 200, which affects production flexibility. For this reason, many current designs employ atmospheric distillation in unit 200 without making use of its energy. The specific process flow diagram is shown in Figure 5. Figure 5. 200-unit atmospheric distillation technology 2.3 Energy consumption comparison of 200-unit distillation technology. The energy consumption of the 200-unit and 300-unit systems is related, not only to the energy use of the 200-unit itself, but also to the energy consumption of the various heat exchangers in the 300-unit system, as well as whether the heat exchange network of the 300-unit system is efficient. This article compares the two most common processes with a processing capacity of 100,000 tons per year, and the results are shown in Table 2. Of course, even if the heat exchange networks of other devices are not exactly the same as those mentioned above, this method can still be used for comparison. Table 2 Comparison of Energy Savings Between Pressurized Distillation and Atmospheric Distillation in the 200-unit System. Pressurized distillation technology for the 200-unit system; Atmospheric distillation technology for the 200-unit system. Energy consumption in the pre-distillation tower (Kw): 3822 vs. 22956; Energy consumption in the extraction tower (Kw): 1984 vs. 1099; Energy consumption in the separation tower (Kw): 2346 vs. 5237; Energy consumption in the benzene tower (Kw): 00; Total energy consumption (Kw): 8152 vs. 10192. As can be seen from this comparison, the use of pressurized distillation in the 200-unit system allows for a reduction in the overall energy consumption of both the 200-unit and 300-unit systems by approximately 15%. This is mainly because, after pressurized distillation, the heat required for the benzene column comes primarily from the vapor at the top of unit 200; the heat used in the stripping column is mainly intended to supply heat to the extraction column and the stripping column itself. In contrast, after atmospheric distillation in unit 200, a large portion of the heat from the stripping column is used to heat the reboiler of the benzene column, leaving much less heat available for heating the stripping column itself ； On the other hand, what determines the heat content of 200 and 300 units is also the temperature of the lean liquid after heat exchange; it can be said that the lower the temperature, the more fully the heat is utilized, which in turn leads to greater energy savings. The lean liquid from the pressure distillation unit 200 has a temperature of around 135°C after heat exchange; in the atmospheric-pressure process mentioned earlier, the temperature of the lean liquid after heat exchange is 140°C. From this perspective, it is logical that the energy consumption for pressure distillation in Baosteel’s benzene hydrogenation unit 200 is low. Of course, if normal-pressure distillation is used for unit 200, its energy consumption will definitely be lower than that of pressure distillation, which is also consistent with Table 2 above; nevertheless, overall it still requires more energy. For the recovery of steam condensate on a scale of 100,000 t/a, if steam heating is used, the total consumption is approximately 15 t/h; in addition, to remove 100 units of ammonium compounds, about 2 t/h of heated pure water needs to be injected. Due to the substantial profits from previous benzene hydrogenation, the consumption in this area did not receive sufficient attention. With the widespread use of benzene hydrogenation technology, there is an increasing emphasis on reducing energy costs. Since the temperature of steam condensate is around 80°C, it can be used directly; therefore, recycling steam condensate to replace pure water is a very effective way to save energy and reduce consumption. The remaining 13 t/h of condensate water can be integrated into the pure water system pipeline network. Taking both aspects into account, a 100,000 t/a crude benzene processing plant can save 118,800 t/a of pure water and approximately 7,900 t/a of 16 kg steam per year. 4. The vacuum off-gases are sent to the hydrogenation heater, where they partially replace coke oven gas. In the benzene hydrogenation process, vacuum distillation is generally used in the deasphalting column of unit 100 and the stripping column of unit 300; this inevitably generates vacuum off-gases. These gases are usually sent for combustion in flares. However, if the heat contained in these off-gases can be utilized effectively, and any minor amount of condensate present in them is removed before sending them to the heater for combustion, it is possible to reduce the consumption of coke oven gas. Based on the hydrogenation of 100,000 t/a of benzene, the volume of vacuum exhaust gas is 10 m3/h; considering the differences in its composition compared to coal gas, the heat content of one cubic meter of this exhaust gas is equivalent to that of 2.6 m3 of coal gas. Overall, it is possible to save 206 km3 of gas per year. 5 Conclusion (1) Depending on the method used for setting the stripping components, the process with a residue oil tower placed at the rear results in an extremely significant energy-saving effect in Unit 100. (2) By subjecting Unit 200 to pressure distillation and employing an appropriate heat exchange network, the overall energy consumption of Units 200 and 300 will be significantly reduced. (3) By taking into account both the recovery of steam condensate and the heating using pure water supplied by Unit 100, a crude benzene processing plant with a capacity of 100,000 t/year can save 118,800 t/year of pure water, as well as approximately 7,900 t/year of steam equivalent to 16 kg per ton. (4) As a type of waste gas, vacuum exhaust can be turned into a valuable resource if utilized properly.