Riser outlet temperature

The regenerated catalyst supplies enough energy to heat the feed to the riser outlet temperature, to heat the combustion air to the flue gas temperature, to provide the endothermic heat of reaction, and to compensate for any heat losses to atmosphere. The source of this energy is the burning of coke produced from the reaction. 

Post-riser quench can be used if a reactor vessel has a metallurgical limit and a higher riser outlet temperature is desired. Higher octanes and more alkylation feed may be the result. Improved vaporization of the feed could lower delta coke. 

Metals passivation also allows refiners to increase reactor severity to increase FCCU conversion. One refinery increased C4 production (valuable alkylation feed) 17.2% by increasing conversion with the injection of antimony. Another refiner was able to increase the FCCU riser outlet temperature 8°F through decreased dry gas production with metals passivation. Yield of gasoline increased, and gasoline RON clear increased 0.7. 

The typical effects of adding heavy aromatic and metals contaminated residstothe normal VGO feedstock at constant riser outlet temperature are ... 

By August 1942, the unit had been converted to avgas production. At this point the unit was operated with a synthetic DA-1 catalyst (Davison Division of W. R. Grace) and a light Coastal gas oil feed (31 API). The unit was operated with the 975°F riser outlet temperature and a 10.5 cat-to-oil ratio. The conversion of the feed was 65 vol% with dry gas and coke yields of 11.0 and 4.7 wt% respectively. The avgas yield was 25,7 vol% along with 14 vol% heavy naphtha and 35 vol% gas oil. 

Due to the initial low catalyst activity, no attention was placed on a quick separation of catalyst and hydrocarbons in the early days of FCC. However, after the development of zeolite-based catalysis and riser cracking combined with improved catalyst stability, riser outlet temperatures of 970°F and greater were observed. It was observed that quick disengaging of hydrocarbons reduced dry gas and delta coke yields. 

As the regenerator temperatures drop, the regenerated catalyst flowing back to the reactor through the regen slide valve cools. This in turn drops the riser outlet temperature, unless the catalyst circulation rate is increased. The cooler riser results in less conversion and hence less coke make. This effect further reduces the coke on the spent catalyst and lowers the regenerator temperature. 

Table 11 shows the properties of the four FCC feed blends that were used in the combined hydrotreater/FCC simulations. Table 12 presents FCC conversion and yield data. In each case, we held the FCC riser outlet temperature at 1030 F (544°C). 

REACTOR OUTLET TEMPERATURE — the higher the value, the higher the conversion. Table 2 shows a 5 Wt % FF conversion gain for a 43 F increase in riser outlet temperature. 

The next step is to specify the operating variables for the riser and reactor as show in Figure 4.67. In most FCC units, control strategies generally fix the riser outlet temperature (ROT) as a setpoint, so the ROT is a natural specification for the riser, ft is also possible to specify the Cat/Oil ratio or circrdation rate, but these specifications make the model quite difficult to converge. We recommend using the ROT as an initial specification and then shifting to other possible specifications. 

Aspen HYSYS will pull all the feedstock information and process operating after we confirm the calibration data overwrite. The status bar now indicates that we must specify product measurements to begin the calibration process. If necessary, we can modify the operating variables (such as Riser Outlet temperature, etc.) of the FCC unit in addition to the measured values. However, we recommend creating a new model file if the operating scenarios are very different... 

The Variable Navigator allows us to add variables and parameters from a given unit operation for observation during the case study. In this case study, we want to study the effects of feed rate and riser outlet temperature (ROT) on the overall conversion and yield distribution of products from the FCC. Since, we are only focused on the yield, we use the square cuts from the model directly. It is possible to perform the same case study on the basis of plant cuts. In that case, we would add a simple component splitter to separate the reactor effluent on the basis of initial and end points of the cuts. However, for this example, we will use square cuts exclusively. 

To Study the effect of riser temperature at higher unit throughput, we must create a case where will vary the riser outlet temperature. First, we increase the feed flow rate to the unit Reactor Section of the FCC unit operation window. For this example, we set the feed flow rate to 115 tons/hr as shown in Figure 4.102 and solve the model. If the model does not converge, we can increase the number of creep and total iterations in the Solve Options Section. 

Figure 4.102 Increase feed flow rate for riser outlet temperature case study.

Figure 4.103 Case study setup for riser outlet temperature.

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