increase to cause more cracking and higher conversion of gasoil, and the yield of fuel gases and coke increases. Common parameters such as coke yield and delta coke (wt% difference between the spent catalyst and the regenerated catalyst) can be related to C/O. For a given FCC reactor and regenerator system with the same feed rate and characteris- tics, the catalyst (regenerator) circulation rate increases with changes in process conditions such as higher reactor riser temperature, lower feed preheat, or others demanding addi- tional heat input to the reactor. While the Hysys model includes fairly complete input and output parameters commonly used in FCC performance modelling, only some are displayed in this discussion. As an example of upset cases in the normal catalyst circulation rates in Case 1, Table 1 shows the changes or consequences rela- tive to the base case when the catalyst circulation rate in Case 1 reduces by 2.7% (arbitrary, about 120,000 lb/hr reduction). This reduction decreases the C/O, the reactor temperature (1,013-1,003ºF), and, expectedly, the total conversion. Due to the decreased catalyst circulation, the resulting delta coke in Case 1 increases slightly to satisfy the system heat balance even at the reduced reaction temperature of 1,003ºF, and the resulting coke yield (equal to delta coke x C/O) decreases slightly. For FCC units with control schemes similar to Figure 1, the air flow rate from the main air blower will remain essentially unchanged when a lower set point for reactor temperature reduces the catalyst circulation rate. Compared to the base case, Case 1 in Table 1 shows a reduced yield conversion, mainly with lower propylene and slightly higher yields for cat naphtha and LCO. This yield distribution varies depending on the selectivity and activity of the catalysts selected. The Hysys model contains several options for catalyst types to select. Modern catalysts can accumulate some quantities of coke and still maintain significant activity. Recent revamp options include recycling fractions of the spent catalysts (or car- bonised catalyst) from the stripper back to the reactor riser. The Hysys model also includes this recycling option, which offers the flexibility of increasing the C/O in the reactor riser to increase conversion and selectivity without significantly impacting the system heat balance. Combustion air flow Case 2 in Table 1 shows the calculated results for operation when the combustion air flow rate from the main air blower to the regenerator is reduced by 5%, relative to that for the base case. As shown, flue gas oxygen content drops from 1 vol% in the base case to 0.11 vol% in Case 2, and CO con- tent in flue gas increases from 0.08 vol% in the base case to 0.48 vol% in Case 2, which is six times higher due to less excess oxygen. The regenerator will operate in partial burn if combustion air further reduces to below 0% excess oxygen in the flue gas. With reduced inlet air flow in Case 2, CO₂ vol% increases in Case 2 compared to that in the base case. Reduction of O₂ vol% in flue gas to 0.11 vol% in Case 2 decreases flue gas temperature rise in the regenerator dilute phase due to afterburning of CO, as depicted by a lower regenerator flue gas temperature of 1,374ºF in Case 2 vs 1,381ºF in the base case. Additionally, combustion air to the
regenerator in each case of Table 1 does not include any oxygen enrichment, but this option is available in the Hysys FCC model. Reactor feed streams (hydrocarbon and steam) and out - let temperature do not change from the base case to Case 2, and heat transferred from the regenerator to the reactor through the circulating catalyst remains about the same for both the base case and Case 2. The combusted amount of coke on spent catalysts mainly converted to CO₂ (in full burn operation) to supply required heat remains essentially the same for the base case and Case 2, as shown in the essen- tially unchanged coke yield data (coke combusted in regen- erator relative to the feed rate). Stripping steam Case 3 in Table 1 shows the calculated results from the Hysys model for operating with a stripping steam rate 20% higher than the base case. This high stripping steam rate could occur because of operator action or the control valve sticking open. The increased stripping steam rate reduces the amount of heavy hydrocarbons or coke on the spent catalyst, leaving the stripper and recycling back to the regenerator. As the reactor feed rate and outlet temperature in Case 3 remain the same as those in the base case, the reduced combus- tibles on the spent catalyst to the regenerator increase the required catalyst circulation rate by about 7.7% to generate the unchanging heat demand of the reactor and decrease delta coke by about 7%. As shown in Table 1, the Hysys model results in increasing the conversion from 76.83 vol% in the base case to 77.77 vol% in Case 3, mainly due to the increase in catalyst circula- tion rate or C/O. Relative to the base case, mass flow yields for propylene and naphtha are respectively 0.7% and 1.9% higher, with a reduced yield of -3.0% for LCO. Steam feed rate The FCC reactor typically includes several steam feed streams, including those for catalyst transfer line aeration, feed atomisation, riser lift and emergency purge, stripping, and others such as instrument purging. When the control valve for one of these steam feed streams malfunctions and becomes wide open, the total steam rate feeding the reactor will increase. Case 4 in Table 1 shows the results when the steam feed rate to the riser increases by 100%. The steam feed rate could potentially increase by higher than 100% of the normal rate if the emergency steam control valve becomes wide open. Steam feeding the reactor is superheated, but the temper- ature is much lower than the riser operating temperature. The sensible heat required to raise the steam temperature in the riser comes from the regenerated catalysts entering the riser. When the steam flow rate to the riser increases, the riser and reactor temperatures could drop before the regenerated catalyst flow rate increases to a level adequate for reaching and maintaining the reactor normal set tem- perature. Case 4, with the same catalyst circulation rate as that in the base case, shows the reactor (plenum) temper- ature drops from 1,013ºF in the base case to 989ºF when the steam feed to the riser increases by 100%. The Hysys
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PTQ Q1 2024
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