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I. Lift Tube Reactor The basic configuration of a lift tube reactor is shown in Figure 3-12. Based on functional segmentation, the lift pipe can be divided into the following sections: 1. Pre-lift section. The fluidization state and flow rate of the catalyst in the lift pipe are crucial for both conversion rate and product selectivity. A pre-lift section is established, using a steam-light hydrocarbon mixture as the lifting medium. This not only accelerates the catalyst and causes it to flow upward in a piston flow pattern, but it also passivates the heavy metals on the catalyst, facilitating rapid mixing with oil mist and thereby improving conversion rates and product selectivity. 2. Cracking reaction section The lift pipe above the feed nozzle serves to provide the required residence time for the cracking reaction. The function of the catalyst separation section at the top of the riser is to perform the initial separation of the product from the catalyst. The main products of catalytic cracking are the cracking intermediates, which can be further cracked into small molecules that are not the desired products, or they can be condensed to form coke. Therefore, it is essential to control the overall degree of cracking, optimize the reaction time, and carry out rapid separation of the products from the catalyst immediately after the reaction is complete. http://bbs.**.com/data/attachment/forum/201506/01/103753a6m0jgcbh6mamyna.jpg chap3_5_clip_image002.jpg (24.75 KB, Downloads: 0) Download attachment; Save to album. Uploaded on 2015-6-1 10:37. http://bbs.**.com/data/attachment/forum/201506/01/103753l82l8t4tzt2fe9yh.jpg tsgfyqjt.jpg (247.18 KB, Downloads: 0) Download attachment; Save to album. Uploaded on 2015-6-1 10:37. Figure 3–12: Schematic diagram of a riser reactor. A proper riser feed nozzle enables thorough atomization of the feed material, resulting in oil droplets with an average diameter close to that of the catalyst particles. A uniform spatial distribution of these droplets, along with an appropriate flow rate, facilitates mixing of the oil with the catalyst, which is beneficial for increasing the yield of the product and reducing the amount of coke formed. 3. Stripping Section The function of the stripping section is to use steam to remove the oil and gas adsorbed on the catalyst as well as the oil and gas present between the catalyst particles, thereby preventing them from being carried by the catalyst to the regenerator, which would increase the coking load on the regenerator and reduce the yield of the product. Measures to improve stripping efficiency: First, increase the number of stages in the stripping section and use efficient stripping trays ； Second, adjust the flow rate of the catalyst to increase the contact time between the catalyst and steam and improve the displacement effect of oil and gas ； Third, increase the number of steam inlets to improve steam distribution and stripping efficiency. In recent years, there has also been considerable research on downward tubular reactors. The advantage of a downward-flow reactor is that the oil and gas flow downward together with the catalyst, eliminating the problem of solid particles sliding down; the flow pattern can be plug flow with little backmixing ； It is possible to combine it with a tubular regenerator to save on investment, etc. This type of reactor may be more suitable for reactions that require high temperatures and short contact times. There are already some patents on research on downflow reactors, but no reports of industrialization have been seen yet. II. Regenerator The main function of the regenerator is to burn off the coke on the coked catalyst in order to restore its activity, while also providing the heat required for cracking. There are various types of regenerators in industry. Generally, it can be divided into three types: single-stage regeneration, two-stage regeneration, and rapid regeneration. The catalyst regenerator unit at Lanzhou Petrochemical adopts an overlapping two-stage regeneration process. http://bbs.**.com/data/attachment/forum/201506/01/103754cgsoiesbkhlel04g.jpg c-3x.jpg (39.42 KB, Downloads: 0) Download attachment Save to album Uploaded on 2015-6-1 10:37 Figure 3-13 Schematic of the regenerator Two-stage regeneration involves passing the catalyst to be regenerated through two fluidized beds sequentially for coking. Two-stage regeneration primarily uses two regenerators. In the first stage, approximately 80%~85% of the total carbon burned is consumed ； In the second stage, air is used along with higher temperatures to burn away the remaining amount of carbon. The two stages of regeneration can be carried out by dividing them into two sections within a single regenerator cylinder, or they can be performed in two separate regenerators. Compared with single-stage regeneration, the main advantages of two-stage regeneration are as follows: First, for a fully mixed fluidized bed reactor, from a reaction kinetics perspective, the coking rate is proportional to the carbon content of the regenerant. Since only most of the coke is burned during the regeneration in the first stage, the carbon content of the semi-regenerated material at the outlet of this stage is higher than that of the regenerated material, thereby increasing the coking rate in the first stage ； Secondly, during the second regeneration stage, fresh air (which increases the logarithmic mean oxygen concentration) and a higher temperature can be used, thereby increasing the carbon burning rate as well ； Third, the combustion rate of hydrogen in coke is higher than that of carbon; when about 80% of the carbon has burned off, almost all of the hydrogen has also been burned off. As a result, the water vapor partial pressure in the second stage can be very low, which reduces the degree of hydrothermal aging of the catalyst. Furthermore, the catalyst inventory in the second section is lower than that in a single-section regenerator, and the residence time is shorter. Both of these factors create conditions for raising the regeneration temperature. During the two regeneration phases, there is an optimization issue regarding the carbon burning ratio in the first and second phases. In addition to considering the essentially complete burnout of hydrogen in the coke in the first stage, optimization should also be carried out from the perspective of carbon burning kinetics. For industrial plants, generally 80% to 90% of the total carbon burned is consumed in the first stage. To maintain thermal balance between the two regenerators, the unit is equipped with two downward-flow external heat exchangers for heat removal. As shown in Figure 3-14: http://bbs.**.com/data/attachment/forum/201506/01/103754x1fv6xwh4gg2366x.jpg chap3_5_clip_image006.jpg (31.57 KB, Downloads: 0) Download attachment; Save to album. Uploaded on 2015-6-1 at 10:37: http://bbs.**.com/data/attachment/forum/201506/01/103755qcsuajpj8ssgg3o3.jpg xxswqrq.jpg (196.05 KB, Downloads: 0) Download attachment; Save to album. Uploaded on 2015-6-1 at 10:37. Figure 3-14: Downward-type external heat exchanger. The operation principle of the downward-type external heat exchanger is that the catalyst coming from the regenerator passes through the heat exchanger from top to bottom, while the fluidized air flows through the heat exchanger from bottom to top at an apparent flow velocity of 0.3 m/s to 0.5 m/s, thereby keeping the catalyst in a fluidized state. The heat exchanger consists of a dense-phase bed zone and a dilute-phase zone; the gas carrying a small amount of catalyst returns to the dilute-phase zone of the regenerator through the exhaust pipe at the top. The heat exchanger is equipped with a tube bundle, and softened water is passed through it to generate steam, which in turn carries away the heat. The catalyst circulation rate is adjusted by the slide valve on the outlet pipeline, while the height of the dense-phase material level in the heat exchanger is controlled by the slide valve on the hot catalyst inlet pipeline.