Regenerator heat balance

A heat balance can be performed around the reactor, around the stripper-regenerator, and as an overall heat balance around the reactor-regenerator. The stripper-regenerator heat balance can be used to calculate the catalyst circulation rate and the catalysi-to-oil ratio. 

Fig. 35. Regenerator heat balance, 950°F. reactor. [Murphree el al., Trans. Am. Inst. Chem. Engra. 41, 19 (1945). Reprinted by permission.]...

Example 21-4. Regenerator Heat Balance. Example 21-1 will be continued by determining the heat balance of the regenerator. If the 5.15 weight per cent of carbon is to be burned to produce a flue gas containing one part of COs per one part of CO, and it contains 10 per cent of hydrogen, it will have a net heating value [Eq. (21-2)] of ... 

Table 21-23. Regenerator Heat Balances for a 950 F Reactor Temperature...

The Aspen HY SYS Petroleum Refining FCC model relies on a series of submodels that can simulate an entire operating unit while satisfying the riser and regenerator heat balance. Note that the configuration is similar to the minimum submodels listed in Table 4.3 of the previous section. We summarize Aspen HYSYS Petroleum Refining submodels in Table 4.4 and highlight some key features in subsequent sections. 

Overall comparison between amine and carbonate at elevated pressures shows that the amine usually removes carbon dioxide to a lower concentration at a lower capital cost but requires more maintenance and heat. The impact of the higher heat requirement depends on the individual situation. In many appHcations, heat used for regeneration is from low temperature process gas, suitable only for boiler feed water heating or low pressure steam generation, and it may not be usefiil in the overall plant heat balance. 

Thus the ECCU always operates in complete heat balance at any desired hydrocarbon feed rate and reactor temperature this heat balance is achieved in units such as the one shown in Eigure 1 by varying the catalyst circulation rate. Catalyst flow is controlled by a sHde valve located in the catalyst transfer line from the regenerator to the reactor and in the catalyst return line from the reactor to the regenerator. In some older style units of the Exxon Model IV-type, where catalyst flow is controlled by pressure balance between the reactor and regenerator, the heat-balance control is more often achieved by changing the temperature of the hydrocarbon feed entering the riser. 

Fig. 2. Heat balance of the FCCU. Heat balance around the reactor A, heat balance around the regenerator B, and the overall heat balance around the entire...

Thus the amount of heat that must be produced by burning coke ia the regenerator is set by the heat balance requirements and not directly set by the coke-making tendencies of the catalyst used ia the catalytic cracker or by the coking tendencies of the feed. Indirectly, these tendencies may cause the cracker operator to change some of the heat-balance elements, such as the amount of heat removed by a catalyst cooler or the amount put iato the system with the feed, which would then change the amount of heat needed from coke burning. 

If FCCU operations are not changed to accommodate changes ia feed or catalyst quaUty, then the amount of heat required to satisfy the heat balance essentially does not change. Thus the amount of coke burned ia the regenerator expressed as a percent of feed does not change. The consistency of the coke yield, arising from its dependence on the FCCU heat balance, has been classified as the second law of catalytic cracking (7). 

The burning of coke in the regenerator provides the heat to satisfy the FCCU heat balance requirements as shown in equation 1. The heat released from the burning of coke comes from the reaction of carbon and hydrogen to form carbon monoxide, carbon dioxide, and water. The heat generated from burning coke thus depends on the hydrogen content of the coke and the relative amounts of carbon that bum to CO and CO2, respectively. 

The stripped catalyst is picked up by a stream of air and carried into the regenerator where the carbon is burned at temperamres about 1100-1300°F. Entrained catalyst is again removed by cyclones and the flue gas goes out the stack. The hot, regenerated catalyst leaves the regenerator and takes with it much of the heat of combustion. This is carried over to the reactor to vaporize the feed and to balance the endothermic heat of cracking. Thus, the process is heat balanced. 

A cat cracker continually adjusts itself to stay in heat balance. This means that the reactor and regenerator heat flows must be equal (Figure 5-4). Simply stated, the unit produces and bums enough coke to provide energy to ... 

If a reliable spent catalyst temperature is not available, the stripper is included in the heat balance envelope (II) as shown in Figure 5-4. The combustion of coke in the regenerator satisfies the following heat requirements ... 

Using the operating data from the case study. Example 5-5 shows heat balance calculations around the stripper-regenerator. The results are used to determine the catalyst circulation rate and the delta coke. Delta coke is the difference between coke on the spent catalyst and coke on the regenerated catalyst. 

The mass and heat balances of the process are satisfactory when coal is gasified to a 53% carbon conversion in the gasifier and the remaining 47% of carbon is sent to the regenerator. From the heat values of the produced fuel gas and the input coal, the cold-gas efficiency (91 vol% H2 with 9 vol% CH4 298 K, 0.1 MPa) was calculated to be higher than 0.77. 

Regeneration gas requirements are readily obtained once the adsorber is sized and the pressure vessel has been designed. The requirements for gas, a flow rate and duration for heating arise from a heat balance. 

In order to establish an optimized FCC operation, it is often possible to influence factors impacting the heat balance. If coke combustion produces an amount of heat that causes regenerator temperature to rise above a preferred level, refiners may choose to reduce the feed temperature or lower the heat of combustion of the coke by reducing... 

The reactor and regenerator mass and heat-balance equations for the dense phase and the bubble phase are given by equations (7.29) to (7.45). The catalyst activities in the reactor and regenerator are defined by the following two relations... 

If the residual in step 8 is too large, correct the value of T using interpolation. Then repeat steps 3 to 8. Once the residual is small enough (< 10-6), substitute ft in the heat-balance equation of the regenerator (7.37), written in the form... 

The plots in Figures 7.8 and 7.9 make both Qer and Qeg infinite and therefore the dense phase and bubble phase conditions are identical and are equal to the output conditions of the reactor and the regenerator in this example. In case of finite exchange rates between the bubble and dense phases in reactor and regenerator, the output conditions from the reactor and the regenerator can be obtained by mass and heat balances for the concentration and the temperature of both phases and these expressions use the same symbols as before, but without the subscript D (used to signify the dense phase before). 

Simple manipulations similar to those used for the reactor give the following heat-balance equations for the regenerator. 

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