Summary of example upset case consequences
Base case
Case 1
Case 2 Lower
Case 3 Higher
Case 4 Higher
Case 5 Lower
Lower cat. circulation
combustion
stripping steam flow
feed
feed
rate
air flow 37,739 508,373
steam flow
temp. 37,739 508,373
Flow rate, BPD
37,739 508,373
37,739 508,373
37,739 508,373
37,739 508,373
lb/hr
Temperature to riser, ºF Steam to riser, lb/hr Stripping steam, lb/hr Reactor Plenum temp, °F Top pressure, psig Catalyst-to-oil ratio Coke yield, wt% Delta coke, wt% Catalyst circulation rate, lb/hr
347
347
347
347
347
327
10,124 13,879
10,125 13,879
10,124 13,879
10,124 16,656
20,255 13,879
10,124 13,879
1,013
1,003
1,013
1,013
989 49.3
1,013
49.3
49.3
49.3
49.3
49.3
4,596,476
4,472,270
4,616,501
4,953,155
4,591,363
4,643,976
9.076 6.569 0.716
8.829 6.499 0.728
9.113 6.563 0.712
9.779 6.582 0.666
9.062 6.381 0.697
9.169 6.644 0.717
Regenerator Dense bed temp. ºF Outlet pressure, psig Combustion air, lb/hr Flue gas, O₂, vol% Outlet temp. °F
1,373.8 1,381.1
1,369.0 1,376.8
1,372.0 1,374.3
1,348.3 1,356.2
1,341.1 1,349.7
1,374.9 1,381.7
53.7
53.7
53.7
53.7
53.7
53.7
461,890
461,890
438,796
461,890
461,890
461,890
1.00 0.08
1.23 0.05
0.11 0.48
1.07 0.11
1.58 0.06
0.77 0.12
CO, vol% (dry) CO₂, vol% (dry) SOx, vol% (dry)
16.77
16.62
17.24
16.79
16.30
16.93
0.04
0.04
0.05
0.05
0.04
0.04
CO₂, lb/hr SOx, lb/hr
111,484
110,563
108,845
111,797
108,460
112,447
428.6
413.8
426.4
437.2
385.0
434.6
Yields/conversions Conversion, vol% Conversion, wt% Propylene, lb/hr
76.83 75.71 30,881 199,372 68,174
75.26 74.15 29,067 201,571 71,939
76.62 75.49 30,725 199,097 68,600
77.77 76.65 31,108 203,101 66,100
72.18 71.08 26,619 199,362 77,864
76.96 75.84 30,874 199,703 67,874
Cat. naphtha (gasoline), lb/hr
LCO, lb/hr
Table 1
may differ from one FCC unit to another. Applications of the model for predicting the consequences of an upset case need to be consistent with the control schemes of the system being analysed. Figure 1 is an example of an FCC control scheme where the combustion air flows to the regenerator that is on flow control, and the combustion air flow rate to the regenerator will need to be kept constant in the performance model when using the model to assess the impact of different deviating process parameters. The Hysys Version 12 FCC performance model includes physical dimensions of the reactor and regenerator, feed composition and characteristics, operating parameters including common reaction kinetic parameters, catalyst selection, and details of the reaction. Property data of hydrocarbon feeds, typically ranging from the gasoil frac - tion of the crude oil to heavier feedstocks, including atmos - pheric resid, vacuum gasoils, and/or vacuum resids, can be input into the model. The calculated results from the Hysys model include fairly comprehensive parameters similar to those normally provided by the licensor. With the proper input data, the results from the model can be useful for analysing the upset conditions or off-design operating performances. For further elaboration, examples are provided of upset events from five different deviating parameters: catalyst circulation rate, combustion air flow rate, stripping steam flow rate, steam feed rate, and feed
temperature to the riser. These five were chosen as illustra - tive examples, but other upset events can also be modelled. The simulated consequences from the upset events discussed herein are intended to show the resulting oper- ational changes at the steady-state condition and have not been checked against the actual field operating data. Moreover, for extreme or severe upset cases such as loss of flow, the model will not be applicable directly to these cases, which typically result in activating the shutdown system. However, the model can be used to generate ref - erence data likely useful for developing a dynamic model, which is typically needed to determine the process safety time available for system shutdown. Catalyst circulation rate Catalyst circulation rate is an essential parameter or vari- able determined by the heat balance between the reactor and the regenerator. An FCC reactor involves both exo- thermic and endothermic reactions, resulting in a net total endothermic reaction. The heat required for increasing the sensible heat of the feed, vaporisation, and the net endothermic reaction is sup - plied by the temperature drop of the circulating catalysts as they pass through the reactor. The resulting catalyst-to-oil ratio (C/O) affects the cracking reaction yield conversions and reactor temperatures. With increasing C/O, active sites
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