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Application of a new mass transfer and separation device in the crude benzene processing process Wei Gaixia, Shang Xiufang, Shang Enxia (Handan Iron and Steel Group Co., Ltd., Tianjin Chuangju Technology Co., Ltd.) In the gas purification systems of coking plants, the recovery of crude benzene is a very important process, as it has a direct impact on the economic efficiency of the enterprise as well as its energy consumption during production. In recent years, with the rapid advancement of coking technology, companies have placed even greater emphasis on gas purification. Various new technologies and equipment have been introduced and applied in chemical product recovery, which not only improves the yield of crude benzene but also reduces energy consumption during production. In the crude benzene recovery section, the most critical equipment are the benzene washing tower and the benzene removal tower, and the mass transfer efficiency of these devices directly determines the yield of crude benzene, as well as the consumption of washing oil and steam. The author will introduce the mass transfer characteristics of mass transfer separation devices and their application in the crude benzene section. 1 Introduction to Mass Transfer Separation Devices. As the most common type of mass transfer separation device, towers come in many varieties and can generally be divided into two categories: trays and packing. Common types of trays include bubble trays, floating valves, sieve trays, and vertical sieve trays ； Packings can be divided into structured packings and random packings, with a wide variety of forms available. For the same production process, there are various options available for tower design; it is necessary to adhere to general technical and economic principles while also taking into account the specific requirements of the manufacturing process. In the specific selection process, considerations can generally be drawn from the following three aspects: First, operational feasibility must be ensured to guarantee the normal and stable operation of the tower ； Secondly, from a chemical engineering perspective, high efficiency, large throughput, low pressure, and good operational flexibility are required ； Finally, an optimization selection must also be carried out from an economic perspective. 2 Application of new mass transfer and separation devices in the crude benzene process 2.1 Benzene washing tower The function of the benzene washing tower is to absorb benzenes from the gas using a washing oil; the washing oil and the gas flow in opposite directions within the tower, and mass transfer and separation are achieved through gas-liquid contact devices. Based on the operating volume ratio of the liquid to gas load, this tower operates with a relatively low liquid-to-gas ratio, approximately 1/500. From this perspective, when selecting a mass transfer device, tray plates are the most suitable. Because the tray has a large liquid holding capacity and a long liquid retention time, it can maintain the liquid level on the tray by adjusting the height of the overflow weir, whereas the packing cannot meet this requirement. Therefore, from the perspective of enhancing mass transfer efficiency, trays should be chosen. However, fillers were chosen to meet the process requirements, as the pressure drop across the tray is high, which has a significant impact on the energy required for gas transport. Early benzene washing column fillers included wooden grids and steel mesh screens; due to their low efficiency and low flow rate, the equipment used was relatively large, with some systems employing two or three columns in series for absorption. With the increasing demands for gas purification and the emphasis on chemical product recovery, it has gradually been replaced by high-efficiency fillers. The more common types include perforated plate corrugated packing, lightweight ceramic packing, plastic wreath packing, and stepped ring packing. Among the corrugated packing materials for stainless steel orifice plates, the 250Y type is the most suitable for use in benzene washing towers due to its high efficiency and large flow capacity. But the downside is that it is more expensive. Therefore, some manufacturers choose 250Y-type fillers made of polypropylene, which not only offer high efficiency but are also inexpensive. The coking and gas production plant at Hebei Tangsteel has been in use for 4 years with excellent results; the benzene content in the gas remains below 2.5 g/m3. Its drawback is that it cannot be purged using steam. Light porcelain fillers are also widely used nowadays; their advantages include low cost, good resistance to clogging, and the ability to be cleaned with steam. However, their disadvantages are low flux and susceptibility to breaking. The plastic wreath packing and the stepped ring packing fall between the first two in terms of efficiency and flux, with the stepped ring packing having slightly better anti-clogging properties and flux than the plastic wreath packing. For high-efficiency fillers to achieve their full efficiency, the choice of liquid distributor becomes even more important, especially in towers with large diameters and a low liquid-to-gas ratio. And the operation of the benzene washing tower falls into this category of operations. Therefore, special attention should be paid to the design, selection, and installation of distributors. The efficiency of the filler depends on its specific surface area, and for it to function effectively, its surface must be properly wetted by the liquid to form a film; fillers that are not wetted by the liquid will not be effective. The spray density in the benzene washing tower is relatively low, generally around 6 m3/(m2·h). If the liquid distribution is uneven, a \"dry plate\" phenomenon can easily occur, which is equivalent to a short circuit in the airflow and results in a decrease in absorption efficiency. Therefore, when selecting a distributor, it is necessary to consider not only the spraying density of the liquid distributor but also pay special attention to its levelness during installation, in order to ensure even distribution of the liquid; this requires a higher level of precision than that needed for ordinary towers. Currently, slotted and slot-disc distributors are gradually being adopted by manufacturers; these distributors offer excellent uniformity properties and have begun to replace tubular, hole-disc, and nozzle-type distributors. Reducing oil entrainment is also one of the tasks that tower equipment needs to address. In some coking plants, the entrainment of washing oil in the gas flowing out of the benzene washing tower is relatively severe. The reasons for this are mainly as follows: (1) Packing blockage leads to an increase in gas velocity, which results in the entrainment of washing oil ； (2) A nozzle-type distributor is used; due to the poor performance of the nozzles, mist droplets are formed, which are easily entrained by the gas ； (3) Entrainment is caused by an excessively small tower diameter and a high gas velocity in the empty tower. The entrainment that occurs can be addressed using mist capture devices; bulk packing materials or structured packing with a large specific surface area can be employed, with wire mesh demisters being the most effective option, though they require regular cleaning with steam. To achieve a more satisfactory benzene washing effect, in addition to selecting an appropriate tower design, it is also necessary to pay attention to the control of process operating conditions, such as the temperature of the gas, the quality of the washing oil, and the circulation rate of the washing oil. 2.2 Benzene removal tower: Benzene removal involves evaporating the crude benzene present in the rich oil using heating or stripping methods. The rich oil from the bottom of the benzene washing tower is heated to 180°C and then fed into the middle section of the benzene removal tower, where it is stripped step by step by the vapor entering from the bottom of the tower, thanks to the gas-liquid mass transfer mechanism, in order to separate crude benzene from the washing oil. The crude benzene obtained from the rich oil accounts for about 1/50 of the amount of rich oil; if direct distillation is used, it is necessary to heat the rich oil to a high temperature. According to calculations, the temperature of the oil-rich feed entering the tower needs to be raised to 250–300°C, while the temperature at the top of the tower should be maintained at 90–95°C in order to ensure the quality of crude benzene. This results in high consumption of heat and cooling energy, as well as high requirements for the equipment and operating procedures. The use of steam stripping reduces the temperature required for benzene removal and, at the same time, increases the evaporation rate of crude benzene. In the stripping section of the benzene removal tower, the liquid phase contains almost no condensed water; it exists entirely in the form of water vapor. When the total pressure is constant, a higher water vapor partial pressure results in a lower partial pressure of crude benzene in the gas phase. From a mass transfer perspective, it increases the driving force for the mass transfer of crude benzene, which facilitates the mass transfer process. The mass transfer separation for oil-rich benzene removal is a process involving large volumes of liquid at low concentrations, with the mass transfer being controlled by liquid film resistance. Therefore, improving mass transfer efficiency must start with the liquid phase. The amount of component transferred per unit time is determined by the transfer rate and the effective phase interface; a higher transfer rate and a larger phase interface result in a greater amount of transfer. The transfer rate is also related to the transfer distance; the shorter the distance the component travels to the phase interface, the higher the rate. For mass transfer devices, it is their own goal to convert the liquid phase into multi-interface, small-sized droplets in order to improve mass transfer efficiency. The separation efficiency of the benzene removal tower directly determines the yield of benzene and the steam consumption. Over the past few decades, these towers have mostly been made of cast iron, with the tower trays consisting of strip-shaped cast iron bubbles; the advantage of this is that the equipment has strong corrosion resistance and a long service life. The disadvantages include low separation efficiency and low throughput, which result in a high benzene content in the oil-rich stream, a low yield of crude benzene, high steam consumption, large equipment size, high investment costs, heavy maintenance tasks, and long maintenance cycles. The yield of crude benzene is generally only about 0.8% of the dry coal, with steam consumption as high as 2 t per ton of crude benzene. The application of new high-efficiency tray plates has brought about a significant breakthrough in benzene removal technology. In 2003, the coking plant of Jilin Tonggang took the lead in adopting the high-efficiency spray tray (CJST) developed by Tianjin Chuangju Company. By using the existing cast-iron benzene removal tower and replacing its bubble trays with CJST trays, the plant not only doubled its processing capacity but also increased the yield of crude benzene by about 15%, while reducing steam consumption by 30%. Xuzhou Huanyu Coking Plant upgraded its existing Φ1200mm cast iron tower, and as a result the processing capacity of the benzene removal tower was doubled. The benzene content in the lean oil remained at 0.1%–0.2%, while the consumption of direct steam was reduced to ≤1.0 t/t of crude benzene. At present, it has been applied in over a hundred coking plants, including Xingang, Tangsteel, Shougang, and Shanxi Coking. Taking the newly built gas purification unit at Tangsteel’s coking plant, which has an annual coke production capacity of 1.35 million tons, as an example, the original design called for an iron casting-based benzene removal tower with a diameter of Φ2.8m; after switching to CJST trays, the diameter of the tower was reduced to Φ2.0m. Once the crude benzene treatment unit came online, its performance metrics were **superior to those of traditional equipment. The excellent performance of CJST trays is primarily attributed to their jet-type mass transfer structure, which lifts, mixes, impacts, sprays, and disperses the liquid on the tray into droplets. This results in a large number of small droplets in the tray area, thereby greatly increasing the interfacial area between the gas and liquid phases and creating favorable conditions for gas-liquid mass transfer. Molecular transfer is achieved through gas-liquid contact, and the amount and rate of transfer depend on the area of the interface between the two phases and the distance to that interface. The characteristics of the CJST tray are: (1) a large surface area for gas-liquid phase contact, which creates favorable conditions for mass transfer ； (2) The liquid phase changes from a continuous phase to small droplets, which reduces the distance that molecules must travel within the liquid phase and thus accelerates the rate of transfer. Since the speed at which molecules move in the liquid phase is much lower than that in the gas phase, increasing the movement speed of molecules in the liquid phase becomes the decisive factor for improving the overall mass transfer rate ； (3) The turbulence resulting from high-speed collisions between the gas and liquid phases also enhances the renewal of the phase interface, which is more conducive to mass transfer. The gas-liquid contact type in traditional cast-iron bubble trays is bubbling; when gas enters the bubbles, it bubbles up through the gaps between the teeth into the liquid layer, where the gas and liquid phases come into contact. The liquid phase remains the continuous phase, enclosing the bubbles and enabling molecular transfer. Both the phase interface where the two phases come into contact and the distance over which molecules move within the liquid phase are inferior to those in jet-type trays. In addition to the tower itself, attention should also be paid to controlling the temperature of the rich oil entering the benzene removal tower, which should generally be maintained around 180°C. Excessively high temperatures not only increase the thermal load on the tubular furnace but also affect the quality of the washing oil. This is because the washing oil contains a large amount of unsaturated compounds in the crude benzene it absorbs, and the high local temperatures in the tubular furnace can cause these compounds to polymerize, resulting in an increase in the molecular weight of the washing oil and its viscosity, which directly affects the quality of the washing oil. When the temperature is too low, the volatility of crude benzene decreases, increasing the difficulty of separation and resulting in a higher benzene content in the lean fraction. Relying on steam from the bottom of the tower to raise the temperature not only leads to poor separation results but also increases steam consumption. Therefore, special attention must be paid to controlling the temperature of the rich fraction as it enters the tower during operation. 3 Conclusion In coking plants, the gas purification process is receiving increasing attention from enterprises, and it is crucial for industrial production to select chemical mass transfer and separation equipment in a proper and rational manner. With the development of distillation technology and the continuous emergence of new, high-efficiency tray components, a new wave of technological innovation efforts needs to be vigorously promoted in the coking industry. Based on the practical application of the high-efficiency spray trays developed by Tianjin Chuangju Company in coking plants such as Tonggang, Tanggang, and Xuzhou Huanyu, it can be seen that the use of these trays in the recovery process has been successful. They have enabled technological innovation in these enterprises, improved yield and reduced consumption, thereby generating significant economic benefits; hence, they are worth promoting for wider use. This post was last edited by ryn on 2009-4-19 18:42]