Economic incentives to meet summer demand for fuel drive 100% maximum FCC operations. Unfortunately, peak summer gasoline demand also coincides with operational problems affecting FCC operations, from the reactor to the recovery section. One of these constraints seen at many refineries involves higher seasonal ambient air temperatures that lower air density. Considering that the FCC regenerator’s main air blower (MAB) is a volume driven machine, the ability to regenerate FCC catalyst by coke burn off quickly becomes problematic without a “dense air” solution.
In addition, tightened refined product quality requirements compel engineers to meet new specifications with decades-old equipment. In reality, enhancing the ability of existing equipment to meet competitive FCC product specifications is doable with readily available “foreign” equipment, such as mechanical chillers to overcome cooling limits. This is what one refinery with a 30,000 bpd FCC did to resolve MAB limits during peak summer time gasoline demand, by taking advantage of the thermal physical properties of air (cooling increases density and wt% of O2) and applying it to a constant volume machine.
High summer ambient temperature reduced air mass through the MAB, reducing percentage of oxygen available for coke burn-off in the regenerator. The less dense air forces a reduction in cracking severity due to the regenerator’s reduced ability to burn off coke on accumulated catalyst. Unless MAB inlet temperature can be lowered, the molar flow rate of oxygen into the regenerator will decline as summer time ambient temperatures increase.
For the 30,000 bpd FCC unit, a temporary cooling solution was engineered based on mechanical refrigeration to chill MAB inlet air from 105°F down to 60°F. The 45°F drop in temperature resulted in denser air with higher percentage O2 (by 7%) to preserve coke burn-off. According to Michael Karlin, Engineering Manager at Aggreko Process Services (APS), “We were able to study the system and provide a design package based on readily available equipment during the previous winter before implementation the following summer.” Operation based on the APS solution began in April and ran through September. According to Karlin, “The 7% increase in O2 equated to $42,000/day in profit.”
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