Classification and performance evaluation of external heaters

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This post was last edited by chengkang on 2010-8-15 20:23 Classification of external heat collectors and their performance characteristics 1. Classification of external heat collectors At present, there are two types of heat collectors commonly used in domestic and foreign refining units, which are classified according to the location where the heat collector is installed in the regenerator.: There are two types of internal heat collectors and external heat collectors. Internal heat collectors can be divided into two types: water tubes and vertical tubes. ； The internal heater has a simple structure, but has low operating flexibility and difficult maintenance. External heat collectors can be divided into: There are three types of heat collectors: upflow, downflow and back-mixing. These heat collectors can all meet the process requirements, but each has its own shortcomings. Although the external heater has a complex structure and a large investment, it is flexible in operation and easy to maintain. Both upflow and downflow external heat exchangers use a single-acting slide valve to adjust the amount of catalyst flowing through the heat exchanger. The equipment structure is complex and the investment is expensive. ； The back-mixed external heat exchanger overcomes the shortcomings of the upflow and downflow external heat exchangers, but because the hot and cold catalysts enter and exit through the same port, the temperature difference between the upper and lower heat exchangers is large, the heat extraction capacity is reduced, and the heat regulation is not sensitive. In order to overcome the above-mentioned shortcomings of the heater, Sinopec Luoyang Petrochemical Engineering Company has researched and designed a gas-controlled adjustable external heater, which has the following characteristics: In this way, the external heat exchangers currently in use in our country can be divided into two categories and four forms, namely valve-controlled type.: Downstream, upstream ； Air control type: Downflow, no circulation. There are two major factions of heaters at home and abroad.: Beijing Institute and Luoyang Institute. Beijing Institute: Dirty Picture-1 Luoyang Courtyard: Others are developed by Luoyang Institute. Basic features of air-controlled external circulation tube-type external heat extraction. In addition to the common features of the air-controlled external heat exchanger designed by the company, this air-controlled external circulation glance-type external heat extraction device (Figure 5) also has the following basic characteristics.: aThe internal circulation pipe of the catalyst is changed to an external circulation pipe, which makes the heat pipe bundle in the external heat collector easier to arrange in terms of structural design and the fluid distribution becomes more reasonable. ； b The steam drum is directly arranged on the upper part of the heater, and the steam drum water directly circulates naturally by gravity, which not only eliminates the need for a hot water pump, but also reduces the floor space. ； cThe external circulation pipe is easy to install and maintain ； d. The heater is easy to start and stop, and can be started and stopped at any time. The catalytic column can be stored or deadbed in the heater for a long time, and it is also very convenient when it needs to be activated. The basic type of external heat extraction technology existing in my country is a complete set of technologies. The petrochemical industry has different names for different forms of j heat collectors. Currently, there are three types of 5-line ones.: Downflow, upflow and air-controlled. It can be seen from these four terms that people use the catalytic action and control method in the external heat exchanger body to express its technical characteristics. From the patent characteristics and protection scope of foreign companies, it also focuses on the control of the fluidized state of catalysis. However, the understanding of its characteristics within the industry is not systematic and even somewhat confusing. Like the first three names, the first two are classified according to the flow direction of the catalyst in the device, while the latter one is classified according to the control method of the catalyst flow. The upper flow type and lower flow type 3 can also be pneumatically controlled. The gas-controlled type cannot determine the flow direction of the catalyst. In order to facilitate analysis, its technical characteristics are first positioned. Catalyst flow classification is determined as: Downflow, upflow, j-circulation. The control method from the fluidized state is set as: Valve-controlled and pneumatic-controlled types obviously can only be made into pneumatic-controlled ones without circulation. The gas-controlled external heat exchanger developed by Luoyang Petroleum Engineering Engineering Company has a downflow type in the heat exchange area, but the circulation method is changed to pneumatic control and the sliding valve is lost, so it can be called a gas-controlled downflow type. In this way, the external heat exchangers currently in use in our country can be divided into two categories and four forms, namely valve-controlled type.: Downstream, upstream ； Air control type: Downflow, no circulation. · The valve-controlled downflow type is mainly designed and implemented by the Beijing Design Institute of Sinopec Group, and the other three types were successfully developed by Luoyang Petrochemical Engineering Company. At present, almost all external heat exchangers in our country are used to generate steam. The cold example is the water evaporation process, and its heat transfer film coefficient is in the order of 1dw/(m2·Y) ； The thermal example is gas-solid fluidization and wall heat transfer, and the film coefficient is of the order of 1dW/(m2·Y) ； The cold side resistance can be ignored, and the heat transfer is controlled on the hot side, that is, the shell side. For single-unit regeneration and parallel two-unit regeneration, the catalyst can only be lifted and returned to the regenerator after cooling. However, the problem is more complicated for two-vessel regeneration in a high-low arrangement, the most typical one being a front burner tank. At this time, the catalyst in the heat exchanger usually comes from the second regenerator. The carbon content is low and is close to the regenerated catalyst. Entering the first mixing section will reduce the average carbon content and is not conducive to regeneration. But on the other hand, the temperature in the second regenerator is relatively high. After cooling, it may still be higher than the sustaining catalyst. Mixing with the sustaining catalyst can increase the ignition temperature. In particular, the storage capacity of the coke tank is very small, and the overall "inertia" of the system is small. The catalyst supplied will have a significant impact on the average carbon content and mixing temperature in it. ultimately affects the regeneration effect. Therefore, for regenerators arranged high and low, it is necessary to determine where the cold catalyst returns based on the overall regeneration effect and two-stage load distribution, as well as optimization of carbon content and mixing temperature. From the operation of the heat collector itself, it is most advantageous for the cooled catalyst to return to the bottom. It saves the conveying air and is more conducive to the control of the circulation volume in the heat exchanger and the adjustment of heat extraction. The operating cost of the external heat exchanger air system is considerable. Although the compressed air has been used for regeneration as much as possible during the design, the effect is poor, so it is economically meaningful to reduce the air consumption as much as possible. It can be seen from the above data: The air-controlled equipment has the highest utilization rate, and the valve-controlled downflow type has the lowest air consumption (when the cold catalyst is not lifted) ； The air-controlled type has the lowest investment. Under the condition that the regeneration process is met, the economic and technical performance of the external heat exchanger must also be considered. It saves investment, low operating costs, low energy consumption, and is safer and more reliable. Minimizing the air volume is an important means to reduce energy consumption and operating costs. Dense-phase fluidized bed operation should be used in the external heat exchanger as much as possible. Laboratory and engineering practice results have proven that the optimal gas velocity for convective catalytic cracking catalyst particles is 0.4. O. 5I/s, at this time not only the heat transfer efficiency is high, but also the adjustment is sensitive. An economic analysis must also be done on the wind used to maintain catalyst circulation. Catalyst circulation increases the heat gain, but it also consumes energy, especially for upward transport. Figure-1 External circulation pipe + valve-controlled + partial backmixing (countercurrent contact) + fluidizing air + lifting air + downflow countercurrent contact has a good heat transfer effect. The catalyst circulation volume is adjusted by changing the conveying air volume and the sliding valve to achieve the adjustment of heat extraction. Figure-2 External circulation pipe + valve-controlled type + complete back-mixing (fast bed) + lifting air + up-flow type Figure-3 No circulation + no valve + complete back-mixing + fluidizing air + same inlet and outlet Figure-4 Internal circulation pipe + air-controlled type (no slide valve) + partial back-mixing (counter-flow contact) + internal lifting air (conveying air) + down-flow type By changing the transport air volume, the catalyst circulation volume is adjusted to achieve the regulation of heat gain. Figure-5 External circulation pipe + air-controlled type (lifting air) + partial back-mixing (bubbling bed counter-current contact) + fluidizing air + downflow type. The catalyst circulation volume is adjusted by changing the conveying air volume to achieve the adjustment of heat extraction. shortcoming: If the delivery pipe is long, it will cause the bed to die and it will be difficult to start again. The above are several early types of external heat extraction in foreign countries, and a lot of improvements have been made to them in China.