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keywords] Catalysis, Fractionation Column, Tray, Packing Summary] After the fractionation tower tray of the catalytic unit is transformed into packing, the pressure drop of the fractionation tower can be reduced, thereby increasing the inlet pressure of the gas compressor or reducing the outlet pressure of the main fan, reducing the energy consumption of the device. At the same time, the processing capacity and separation accuracy of the fractionation tower are improved while the height and diameter of the fractionation tower remain unchanged. 1. Introduction The internal components of the fractionation tower of the catalytic device usually use a combination of baffles and trays. The function of the baffles is to use the contact between the oil slurry and the reaction oil vapor to wash the catalyst particles entrained in the oil vapor. The superheat of the reaction oil vapor is removed to recover heat. At the same time, the oil slurry components in the oil vapor are condensed. Therefore, the baffle at the bottom of the fractionation tower is the contact between the oil vapor and the oil slurry. Mass transfer and heat transfer sites include annular baffles and herringbone baffles. The trays usually use inclined hole trays, which have the characteristics of pressure drop and difficulty in clogging. At the same time, the fractionation tower needs to separate the gasoline, diesel and recycled oil components that have a large difference in the 50% point of Engler's distillation (large difference in relative volatility). The separation effect of the inclined hole tray can meet the product separation requirements. Towers can be divided into plate towers and packed towers according to their structures. Plate towers have simple structure, low cost, strong adaptability, and easy enlargement. The research on fluid mechanics and mass transfer models is relatively mature. Before the 1970s, the development and research of trays was in a leading position. However, in the past 20 years, new packings with excellent performance have come out one after another, especially structured packings and new towers. The continuous development and application of internal parts and the continuous deepening of basic theoretical research have led to new breakthroughs in the amplification technology of packed towers, changing the situation where plate towers are the mainstay. Structured packing is very popular as a device with high mass transfer efficiency under low pressure drop. Compared with plate towers, the new packed tower has large production capacity, high separation efficiency, small pressure drop, large operating elasticity and liquid retention. It has advantages such as small quantity. But there are also some shortcomings. For example: high cost, not suitable for complex distillation towers that need to install intermediate reboilers or multi-side discharges. It is used in high-pressure distillation. Due to large axial backmixing, the efficiency is low. Specially designed packed towers are required. (1) In the field of petroleum refining, structured packing has been successfully used in crude oil atmospheric distillation, vacuum distillation towers, and catalytic absorption - Stabilization system absorption tower, desorption tower, stabilization tower, etc. The catalytic unit fractionation tower is a complex distillation tower with side extraction and mid-stage reflux heat extraction. There are few examples of domestic application of packing transformation. The industrial application of structured packing in foreign FCC fractionation towers began in 1983. After the liquid distributor design was further improved, the transformation plan became more complete and mature. (2) The actual application situation of transforming the catalytic unit fractionating tower into a packed tower at home and abroad shows that the transformation of the catalytic fractionating tower into a packed tower has played a positive role in improving the processing capacity of the unit, saving energy and reducing consumption, and improving the separation accuracy. However, there are also some problems that need to be paid attention to during the transformation process. 2. Industrial application of transforming the catalytic unit fractionating tower into a packed tower 2.1 Transformation applications where the main purpose is to reduce the energy consumption of the device. FCC unit fractionating towers can use structured packing * * Improve the pressure distribution of the catalytic device. The typical pressure drop of the main fractionating tower is about 34.3kPa, while that of the packed tower is only 6.9kPa. The pressure drop of 27.3kPa can solve the stuck neck problem of the air compressor or the main fan. The use of a packed fractionating tower reduces the pressure drop from the reactor outlet to the air compressor. The benefit is: increasing the suction pressure of the air compressor. Solve the bottleneck problem of the air compressor capacity. And/or reduce the power required by the air compressor and reduce the discharge pressure of the main fan. Solve the bottleneck problem of the main fan capacity. In addition, the heat recovery solution of the fractionation tower can be improved. The heat recovery in the fractionation tower can be improved. The load of the tower top system can be reduced. The pressure drop of the system can be reduced. Bring additional benefits. Including simplifying the tower top system. (2.3) 2.1.1 Reduce the energy consumption of the pneumatic compressor. The pneumatic compressor of the FCC device is used to compress the rich gas generated by the reaction. At the same time, the inhaled gas is used to control the reaction pressure. The pressure between the reaction pressure and the inlet of the pneumatic compressor is mainly the pressure drop of the fractionation tower tray and the pressure drop of the oil and gas condensation cooling system at the top of the fractionation tower. The power of the pneumatic compressor is basically linearly related to the gas volume or pressure ratio (outlet pressure/inlet pressure) of the pneumatic compressor inlet. In Table 1 (Example 1), the reaction pressure remains unchanged. The packing of the fractionation tower is changed. The inlet pressure of the air compressor after the tower increases. The pressure ratio of the air compressor decreases. The suction flow rate (volume) of the air compressor inlet can be increased. The inlet gas density increases with the increase of the inlet pressure, thereby increasing the mass flow rate of the air compressor. After the fractionation tower was transformed into a packed tower, the power of the air compressor was not increased much (11.4%), but the processing capacity of the device was increased significantly (22.5%). This is especially practical for devices that limit the processing capacity due to insufficient load of the air compressor. Table 1 Comparison of data before and after the transformation of a certain FCC (2) The upper limit of the change range of air intake volume before and after the project transformation/103m3.h-151.768.9 increased by 33.3%. The suction pressure/kPa82.0114.4 increased by 39.5%. The discharge pressure/kPa1537.31537.3/ air compressor power/MW5.355.96 increased by 11.4%. Fractionating tower pressure drop/kPa39.39.6 decreased by 75.6% and unit capacity/m3.d-11272015104 increased by 22.5% 2.1.2 Reduce the energy consumption of the main fan. The reactor pressure of the FCC device matches the regenerator pressure. When the pressure of one side rises or falls, the other side must also change synchronously. Otherwise, the reverse pressure balance cannot be maintained. In Table 2 (Example 2), the pressure at the air compressor inlet remains unchanged after the fractionation tower is replaced with a packed tower. Because the pressure drop of the fractionation tower decreases, the reaction pressure can be reduced. , the regenerator pressure is simultaneously reduced, that is, the outlet pressure of the main fan supplying air is reduced. After the outlet pressure of the main fan is reduced, the air supply volume can be increased to achieve the purpose of improving the regenerator's burning capacity and increasing the processing capacity. For devices whose device capacity is limited by the load of the main fan (the main air flow is the bottleneck), transforming the fractionation tower into a packed tower is also an effective way to increase the processing capacity of the device. Table 2 Comparison of data before and after the transformation of a certain FCC (2) Comparison effect before and after the project transformation Gas compressor suction pressure/kPa68.968.9/ Fractionation tower top pressure/kPa96.596.5/ Fractionation tower bottom pressure/kPa131.0103.4 (tray pressure drop reduced by 27.5kPa) Reaction pressure/kPa151.7124.1/ Regeneration pressure/kPa137.9110.3/ Main fan outlet pressure/kPa220.6193.1 (main air volume can be increased by 14%) 2.1.3 Optimize the energy utilization of the fractionation system. Reduce the tower top condensation cooling load. The fractionation tower top condensation cooling system needs to condense and cool the gasoline, liquid hydrocarbons, dry gas, and water vapor in the gas phase coming out of the top of the tower. Reducing the pressure drop of the top oil and gas condensation cooling system can also increase the pressure of the gas compressor inlet. After the fractionation tower is changed to a packed tower, heavy gasoline circulation reflux and extraction are increased. Exit the side line. Make part of the gas in the gas phase of the gasoline component that originally came out of the top of the tower condense in the upper part of the fractionating tower and then extract it. Reduce the gas flow entering the condensation and cooling system at the top of the tower. Reduce the pressure drop of this system. Example 3 illustrates this point. The increase in the inlet pressure of the gas compressor is contributed by the reduction of the pressure drop of the fractionation tower and the reduction of the pressure drop of the oil and gas condensation and cooling system at the top of the fractionation tower. Example 3: After a certain FCC was modified with structured packing, a (heavy) crude gasoline reflux line was added to the top of the fractionation tower. (The heavy crude gasoline extraction process was added), which reduced the heat load of the overhead condenser by 22.4MJ/h. The inlet pressure of the gas compressor was increased by 14.5kPa. The inlet volume flow rate was reduced by 20%. The power dropped by 10%. (3) The extraction temperature of the heavy gasoline extraction cycle set up in the fractionation tower is higher than the oil and gas temperature at the top of the tower. This makes the temperature level of the heat recovered from the cooling heavy gasoline higher than the temperature level of the heat recovered from the oil and gas at the top of the cooling tower. According to reports: the FCC main fractionation tower can be recovered after optimization Collect more energy. After optimization, the gas and liquid phase load in the tower will increase, which requires the use of high-efficiency packing or large-capacity trays. After the original plate fractionation tower is transformed into a packed tower, the fractionation tower has 5 side lines from bottom to top to extract reflux. They are oil slurry. The second middle section ( Heavy cycle oil) cycle return. A middle section (light cycle oil) cycle return. Heavy gasoline cycle return. Top cycle return. The extraction position of heavy gasoline is above the heavy gasoline cycle return return tower mouth. This will help to increase the extraction temperature of heavy gasoline and improve energy efficiency. The temperature level used. Before the transformation, the fractionation tower was a plate tower. The heavy gasoline extraction position is on the same tray as the heavy gasoline circulation reflux extraction outlet (4). Document 5 also introduces the method of optimizing energy utilization by extracting heavy gasoline after converting the fractionation tower from a plate tower to a packed tower. It should be pointed out that the extraction of heavy gasoline from the fractionation tower can not only be implemented on the packed tower. For the design of the heavy gasoline extraction process for the plate tower, it is necessary to increase the number of trays and replace the high-efficiency trays to improve the separation accuracy of the upper tower. The use of packing modification is an optimal solution to improve the separation effect of the upper tower. The renovation design of the fractionation tower using structured packing instead of trays should consider the optimal utilization of all internal components including the liquid distributor. Experience has proven that using packing instead of trays can ensure the quality of gasoline and diesel separation. It can also reduce the pressure drop of the entire tower. Solve the bottleneck problem of the main fan or air compressor (reduce the energy consumption of the main fan or air compressor) and optimize the energy utilization of the fractionation tower. 2.2 Renovation applications with the main purpose of increasing the processing load of the unit 2.2.1 The processing capacity is increased from 50,000 tons/year to 150,000 tons/year (6) The 50,000 tons/year atmospheric distillation-resid catalytic cracking combined unit of Jilin Province Nong'an Refinery was renovated and expanded to a 150,000 tons/year unit. Designed by Daqing Petrochemical Design Institute. Transforming the plate fractionation tower into structured packing not only doubled the processing capacity, but also improved the separation efficiency. The original fractionation tower has a total of 28 trays. The diameter of the upper section is 1.2 meters. The diameter of each section below the top circulation section is 1.6 meters. Consider the trays below the middle section reflux and the refining section. It is easy to occur during long-term operation at high temperatures. Scaling and blockage. Still using solid tongue trays. The whole tower has three sections of packing. The top circulation section is 3.0 meters of 125X packing, the light diesel section is 3.8 meters of 250Y packing, and the middle reflux section is 0.8 meters of 250 Y and 0.8 meters of 125Y packing. There are 8 trays below the mid-section reflux. A highly elastic trough-type multi-stage liquid distributor is designed. The modified device started operation in September 1991. The pressure drop of the fractionating tower is 9kPa, which is 9kPa less than the pressure drop of the plate tower. Practice has proved that the structured packing fractionation tower has the advantages of large processing capacity, large operating flexibility, high separation accuracy, large pressure drop, high separation efficiency, and low energy consumption. 2.2.2 The processing capacity is increased from 2.0Mt/year to 2.8Mt/year (7) Dalian West Pacific Petrochemical Co., Ltd. (WEPEC) 2.0Mt/a heavy oil catalytic cracking unit. The main fractionation tower tray of the unit adopts 30 layers of sieve holes and 6 layers of herringbone baffles. According to the needs of the unit expansion to 2.8Mt/a, the fractionation tower is modified with Glitch high-efficiency separation technology. While the original tower height and tower diameter remain unchanged, the 1-17 layers (from top to bottom) and the desuperheated section are replaced with 5 sections of packing. According to the ultimate load test conducted on the fractionation tower before the transformation, the gas phase load in the top circulation reflux section, the gasoline-light diesel separation section, and the first and middle circulation sections is relatively high. The mass transfer and heat transfer areas are obviously insufficient. The gas phase load of the first and lower trays is relatively normal. The mass transfer and heat transfer of the first and lower trays are relatively high. The area can meet the operation requirements before the transformation. The requirements for the transformation are: the tower height and tower diameter remain unchanged, the positions of each extraction outlet remain unchanged, and a heavy gasoline extraction system is set up. It can operate normally whether the heavy gasoline is extracted, and when the refining oil is not extracted, the entire tower can operate normally, and the pressure drop of the entire tower is not greater than 20kPa. The contents of the transformation are as follows: (1) The upper part of the packing of the light diesel section and the first intermediate circulation section is a trough-disk distributor, the upper part of the packing of the top circulation section and the bottom desuperheating section of the tower adopts a narrow slot distributor, and the upper part of the heavy gasoline is a narrow-slot oil collection tank distributor. The functions of the heavy gasoline distributor and the top circulation oil collection tank are integrated into one. The heavy gasoline is extracted from one side of the trough plate distributor at the lower part of the heavy gasoline section. The extraction bucket is an arcuate trough. (2) The top circulation section was originally a 4-layer sieve tray. It was changed to a 240mm Glitch grille (the purpose is to prevent salt formation and corrosion problems that occur when the temperature at the top of the tower decreases) and 1560mm GEMPAK packing and heavy gasoline separation section. 1980mmGEMPAK packing. The light diesel section was originally a 9-layer sieve tray, but was changed to 2860mmGEMPAK packing. The first intermediate circulation section was originally a 4-layer sieve tray, but was changed to 1800mmGEMPAK packing. (3) The light diesel extraction layer tray was replaced with a Glitch tray. The fixed parts were reused. The 6-layer sieve tray of the heavy diesel section, the 4-layer sieve tray of the second intermediate cycle section and the 3-layer sieve tray of the refining section were respectively transformed into Glitch trays. (4) In order to prevent coking, reduce the pressure drop and provide sufficient heat exchange area, replace the original 4 layers of herringbone baffles in the upper part of the superheat removal section with 1320mm Glitch grilles. The remaining 2 layers of herringbone baffles are retained. After the transformation, the load of the fractionating tower can meet the requirement of the unit's 2.8Mt/year processing capacity. The pressure drop of the fractionation tower is reduced from the original 2.5Mt/a processing capacity of 35kPa to 2.9Mt/a with a processing capacity of 12.1kPa. After the transformation, the fractionation tower can operate at 60% load. Gasoline and diesel are degassed. After the transformation, there is an evacuation problem of the top circulation return pump. The inlet must be replenished with crude gasoline. The analysis shows that the top return extraction tank is a combination of a conventional oil collection tank and a distributor. The extraction plate of the oil collection tank is assembled from 52 distribution plates. There are no seals between the distribution plates. The gaps will leak. In addition, there are 378 Φ4 small holes at the bottom of the extraction tank. The total leakage is greater than the internal return flow of the top circulation. There is an evacuation problem of the top circulation oil pump. Because it cannot be established The normal liquid level makes the distributor unable to play its due role and seriously affects the separation efficiency of the heavy gasoline distillation section. As a result, the heavy gasoline distillation range is less than the design requirements. The gasoline and light diesel fractions have a large overlap. Although the device can operate safely, it has a great impact on the processing load of the device, the separation accuracy of the heavy gasoline distillation section, the cooling load of the oil and gas at the top of the tower, and the smoothness of the operation. The narrow groove distributor needs to be modified after shutdown. 2.2.3 The Beihai Petrochemical Plant uses a tray/packing hybrid form to transform the fractionation tower. The processing capacity of the catalytic cracking unit of the Beihai Petrochemical Plant has increased from 0.15Mt/year to 0.34Mt/year. The fractionation tower has always used a plate tower. There is a problem of excessive vapor and liquid load in the upper part of the tower. .The separation of gasoline and diesel is unclear. The operation is difficult. The fractionation tower is modified in the form of tray/packing mix: the original 9-layer herringbone baffles at the bottom are retained, the opening rate of the fixed tongue trays on the bottom 1-3 layers is increased from 17.8% to 20%, and the 8-11 layers in the middle adopt JF angular double-acting bar valves. Layers 12-28 are changed to JP type metal orifice plate corrugated packing. The 1750mm space between the top circulation inlet and outlet is JP-1A. The 1920mm space between the top circulation top circulation extraction and the return of rich absorption oil to the tower is JP-4A. The extraction of rich absorption oil and diesel oil is 800m The m space is JP-4A. The 1500mm space between the inlet and outlet of the middle cycle is JP-1B. A conical sieve plate vapor-liquid distributor is used. After the transformation, the FCC processing capacity is increased to 0.36Mt/year. The product separation accuracy of the fractionation tower is improved. The operation difficulty is reduced. (8) 2.3 The problem of transforming the baffle plate of the desuperheating section at the bottom of the fractionation tower into structured packing. According to reports, the desuperheating section at the bottom of the fractionation tower uses structured packing. Coking problems rarely occur. Valero Refinery uses Koch Fle * -grid type II 410 stainless steel structured packing, Sun Company (Refining and Marketing Inc) uses Glish Grid (Glish Grid) structured packing, Lummus Grest Company also replaced several fractionation tower mid-plates with structured packing. No clogging problems were found, and the Cenex refinery also installed Koch Fle * -Grid structured packing. Its operating temperature is 370°C. These devices that transform the baffle system that contacts the oil slurry at the bottom of the fractionation tower with the reaction oil and gas into a grid system do not have the problem of coking of the bottom of the tower and the oil slurry system. To install structured packing, it is necessary to ensure that there are certain The flow rate of fluid flows through the packing. (9) There are many factors that can cause coking in the slurry system. However, the above example can prove that transforming the bottom baffle into regular packing will not cause coking and packing clogging problems. It shows that it is feasible to use grid packing in the desuperheating section at the bottom of the fractionation tower. 2.4. Problems arising from converting fractionation tower into packed tower Lyondell Petrochemical Company Houston The processing capacity of the original plate fractionation tower of the refinery FCCU is 12561m3/d. It is hoped to increase it to 14628m3/d. The regeneration system is limited by the pressure balance and cannot increase the catalyst circulation volume and conversion rate. In addition, the new grid distributor replaced by the regenerator has a high pressure drop. Affects the processing capacity of the main fan. We also hope to reduce the pressure drop of the fractionating tower. Transform it into a packed tower. The initial transformation uses a combined liquid collector/redistributor. The purpose is to increase the height of the packing as much as possible. After the transformation, the gasoline dry point appears to be extremely high. And the gasoline dry point does not increase with the reflux volume of the fractionating tower. The main reasons for the failure of the tower to operate normally are poor liquid distribution, poor gas distribution and the presence of liquid flooding. The combined top circulation liquid collector/redistributor used in the initial transformation has not been successfully applied to large-diameter fractionation towers. The liquid in the entire tower The distribution is seriously uneven, and only a small amount of liquid is remixed on the collector tray. The component concentration gradient caused by uneven distribution cannot be improved in the lower parts of the tower. The combined collector/redistributor has a wide groove with no openings on it, so that most of the packing below does not have primary distribution of liquid. Glitsch Company successfully carried out the second modification of the sub-fractionation tower. The liquid collector and distributor are separate. The distributor is equipped with a separation box. Distributors with multiple separation boxes should avoid being used in fractionation towers with high liquid flow rates and large diameters. The horizontal position of the groove on the distributor and the separation box should be guaranteed to be no more than 0.3cm. Otherwise, uneven distribution will occur. Carry gas and have water. The flat velocity is the main reason for poor liquid distribution. Therefore, some stable zones should be set up in the separation box to reduce the influence of the above factors. The separation box also needs to have a certain height. For large diameter and high liquid flow rate fractionation towers, great attention should be paid to the design of the distributor. The performance of the packing is directly controlled by it. The reason for the failure of all such fractionation tower modifications is due to improper design of the liquid collector and redistributor system. (10) The packed tower of Dalian West Pacific Petrochemical Co., Ltd. (WEPEC) 2.0Mt/a heavy oil catalytic cracking unit after modification also has the problem of improper design of the top reflux oil collection tank, which leads to the evacuation of the top reflux pump. The above two examples also illustrate a problem: the collection and distribution of liquid during the transformation using packing are important factors that affect the success of the transformation. It is mentioned in the literature that some problems occurred after the fractionation towers of some catalytic cracking units were switched to regular packing. The main reason is poor design of the liquid distributor. For example, some fractionation towers only make simple modifications to the liquid distributor. The height of the equal plate of the structured packing is reduced from 1320mm to 610mm. It can be seen that the high fractionation efficiency of the structured packing is obvious. The design of the liquid distributor is also crucial. (10) In a plate tower, the gas or liquid is almost completely mixed every time it passes through a layer of actual trays. Therefore, the channel flow of the liquid on the tray or the uneven distribution of the gas on the tray only affects the efficiency of one layer of the tray. But it is different in the packed tower. Each packing has its own liquid division ability. If the original distribution uniformity is worse than its own uniformity, it is difficult to compensate by increasing the height of the packing because the speed of restoring its own uniformity is slow. It may not be possible. It can compensate for the concentration difference that has been formed. Therefore, the liquid distributor is an important part of the packed tower. In addition, for the plate tower, the uneven distribution of the feed gas has little impact on the fractionation efficiency. Because the pressure drop of the tray is large, the gas is basically evenly distributed after passing through a layer of trays. After the tray is changed to packing, the pressure drop of the first bed above the feed section is often only 1/3 of the pressure drop caused by the kinetic energy of the feed. Unevenly distributed gas passes through the packed bed. * * Reduce packed bed separation efficiency.(12) 3 Conclusion: The fractionation tower of the catalytic unit is transformed into a packed tower. Part of the trays can be transformed, or the entire tower can use structured packing (including the tower bottom baffle system). This can reduce the pressure drop of the fractionation tower, reduce the energy consumption of the air compressor or main fan, and improve There are various benefits such as device processing capacity, optimizing the energy utilization of the fractionation tower, etc. But at the same time, attention should also be paid to the tower feed distributor, liquid collector and distributor. Excellent design is the key to the success of structured packing transformation (especially for large diameter columns).