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Selection of Feed Filtration Scheme for Hydrocracking Units Li Fengling, Liu Hengtao (Dalian Petrochemical Branch, China National Petroleum Corporation, Dalian, Liaoning 116032) Abstract: Two design schemes for the feed filtration process in a single-stage partially recycled hydrocracking unit were analyzed and compared, and through this comparison, a more reasonable design scheme was selected. Keywords: hydrocracking; filter; fixed-bed reactor; solid particles; filtration process. Chinese Library Classification Number: TQ051.85 Document Code: B. Hydrogenation is an important process in petroleum processing; its main purpose is to remove impurities such as sulfur, nitrogen, oxygen, and metals from petroleum through hydrogenation, thereby improving the quality of the oil products and reducing environmental pollution. Another objective is to crack heavier feedstocks into lighter fuels under high hydrogen pressure. The vast majority of hydrogenation units use fixed-bed reactors, which requires that solid particles in the feed material be removed as much as possible before it enters the reactor. Otherwise, these solid particles will block the reactor, increasing the pressure drop across the bed and causing the plant to shut down. This not only increases the catalyst consumption of the device but also has an adverse effect on production, potentially causing significant financial losses for the enterprise. Therefore, solid particulate matter that could clog the bed layer should be removed by a filter as much as possible before it enters the reactor. It should be noted that oil-soluble metal compounds, gums, asphaltenes, and reaction coke present in the feedstock can also cause blockages in the catalyst bed, and these blockages cannot be removed by filters. For hydrogenation units, the amount of solid particulates contained in different feedstocks varies. For example, the mass fraction of solid particles in naphtha and kerosene is generally (0.5–10)×10-6, while in diesel and vacuum distillate wax oil it is generally (20–600)×10-6. The mass fraction of solid particles in residue oil is generally (200–1200)×10-6. Generally, solid particulate matter particles larger than 25 μm account for 80% of the mass of such particulates, while constituting only 5% of the total number of solid particles. It is generally required that the diameter of solid particles in the raw materials be less than 25 μm. In this way, the feed filter of the hydrogenation unit only needs to remove solid particles larger than 25 μm, thereby ensuring that the catalyst bed does not stop operating due to blockage by such solid particles. The performance of a raw material filter depends primarily on the flow rate, that is, the volume of fluid that passes through a unit area of the filter screen per unit of time. The design techniques of different filtration systems may affect the size and performance of specific devices, but flow rate is the decisive factor in determining the size of a filtration system. Taking the automatic backwash filtration system as an example, regardless of the type of material used, the lower the flow rate and the longer the backwash interval, the higher the filtration efficiency. By reducing the flow rate of the filter, the amount of filter feed obtained between two backwash cycles increases exponentially (as a square). A low flow rate is highly effective in improving the cleaning capacity of filters; the lower the flow rate, the better the filter can be restored to a specified level of cleanliness, regardless of its type. This is because when the flow rate decreases, the velocity of the raw material also drops, and the speed at which the solid particles in the raw material collide with the surface of the filter screen also decreases. As the velocity of the solid particles decreases, their kinetic energy as they penetrate the filter screen also decreases. When the velocity of the solid particles decreases, they tend to form a filter cake on the surface of the filter, and this filter cake can be easily removed. As the flow rate decreases further, a loose, thick filter cake forms on the surface of the filter screen; when the material to be filtered passes through this filter cake, it improves the efficiency of particle removal by the filter screen. 1 Analysis of the filtration process: A company plans to build a new hydrocracking unit with a capacity of 360×104 t/year. This unit will employ a partial recycle process in its reaction section; there will be two parallel reactors in this reaction area, and the hydrogen gas will be mixed after passing through the furnace. A thermal hydrocracking process will be used. The hydrorefining and hydrocracking catalysts are placed in one reactor, and the recycled oil from the distillation section is fed to the reactor inlet. The flow rate of the fresh feed from the upstream unit is 430 t/h, the temperature is 150°C, and the pressure (gauge pressure) is 0.69 MPa. The flow rate of the recycled oil from the fractionation section is 270 t/h, the temperature is 213°C, and the pressure (gauge pressure) is 0.69 MPa. There are 2 design options available for the process flow of raw material filtration (Figure 1): ① The recycled oil from the distillation section does not go into the raw material filter; the filter only filters fresh raw material coming from upstream units (Figure 1a). ②The recycled oil from the fractionation section and the fresh feedstock from the upstream units are combined and then fed into the feed filter (Figure 1b). The components of the raw materials and recycled oil are shown in Table 1 and Table 2 respectively. 2 Comparison of the two design schemes: The difference between Scheme 1 and Scheme 2 lies in whether the circulating oil enters the raw material filter. If the circulating oil contains solid particles, it must be sent to a filter for filtration, using Option 2. Since the circulating oil contains no solid particles, it is not necessary to pass it through a filter; therefore, Solution 1 is adopted. There are generally two sources of solid particles in circulating oil: one is FeS generated by the corrosion of the distillation system, and the other is the solid particles already present in the entire system at the beginning of operation. A large amount of solid particulate matter remains in the system after the unit is put into operation for the first time or following maintenance. These particulates are discharged from the fractionation system along with the circulating oil. If circulating oil enters the feed filter system, those particles with a diameter greater than 25 μm will be filtered out. If the circulating oil enters the reaction system directly, these particulates will deposit on the catalyst bed, increasing the pressure drop across the catalyst bed. However, if the circulating oil is sent to the vacuum wax oil tank area at the beginning of operation, and after it removes all solid particles from the system, the circulating oil is then directed directly to the reaction system; in this way, the solid particles present in the system will not deposit on the catalyst bed. However, this will delay the start-up process, and it is also difficult to determine when the recycled oil can be directly fed into the reaction system. At a domestic refinery, the hydrocracking unit once stopped operating due to excessive pressure drop in the reactor bed, caused by solid particles such as FeS present in the unfiltered recycled oil. The distillation section of this hydrocracking unit employs a butane removal process; that is, the first column in the distillation section is a butane removal column, which is designed to remove all H2S and components with molecular weights below C4 from the top of the column. The unit was originally designed as a hydrocracking unit primarily for producing naphtha. In 1999, the catalysts in the unit were replaced with those intended for producing intermediate distillates, resulting in very little formation of components below C4; this led to unsatisfactory H2S removal in the butane stripping tower. This caused some of the H2S to enter the subsequent heaters and distillation towers, leading to corrosion of those equipment and the formation of large amounts of FeS. The solid particulates such as FeS generated enter the reactor directly along with the circulating oil, and accumulate on the catalyst bed, resulting in an excessive pressure drop across the bed and forcing the plant to shut down prematurely. The distillation process of the hydrocracking unit discussed here differs significantly from that of this refinery; the distillation section employs a main stripping tower approach, meaning that the first tower in the distillation system is the main stripping tower, whose function is to remove all H2S, part of the naphtha, and all components below the naphtha level from the top of the tower. To prevent corrosion at the top of the main stripping tower as well as in the entire distillation system, the design temperature at the top of the main stripping tower is set to be 14°C above the dew point of water; 99.999% of H2S must be removed. Since the feed temperature to the main stripping tower is controlled by the temperature of the material coming from the reaction section, it is very difficult to meet the aforementioned requirements by adjusting operations if there are changes in the feed to the main stripping tower. However, if the requirements regarding the yields and cut points of naphtha and kerosene are relaxed, it is possible to increase the top temperature of the tower, thereby raising the dry point of the naphtha at the tower top and ensuring that all H2S is removed there. In this way, H2S will not enter the distillation tower, and the distillation tower will not produce particulates such as FeS. An economic evaluation of the above two design options was conducted, and the results showed that Option 2 requires an additional investment of 2.2 million yuan compared to Option 1. 3 Conclusion Through analysis and comparison, it is concluded that Option 2 is a sound choice, but it requires an additional investment of 2.2 million yuan. If Plan 1 is adopted, the investment will be 2.2 million yuan less than that of Plan 2. However, the following points should be taken into account during future operation and maintenance: ① At the beginning of operation, the circulating oil should be directed to the VGO tank area; once the circulating oil has removed all solid particles from the system, it can then be sent directly to the reaction system, thereby preventing any solid particles remaining in the system from accumulating on the catalyst bed. ②During the normal operation of the plant, in order to prevent corrosion in the distillation system, the temperature at the top of the main stripping tower must be 14°C higher than the dew point of water; simultaneously, 99.999% of the H2S must be stripped and separated. The above two points are also relatively easy to implement; therefore, option 1 was chosen for the feedstock filtration process in the hydrocracking unit discussed in this paper. (