Paul R Orlowski
In many respects, what defines under-utilization applies to when FCC feedrates are down or when operations are normal; the difference is a matter of degree. In general, under-utilization can be defined whenever:
– Pumps & compressors are unloaded (feed, liquid & vapor products, combustion air)
– Catalyst loading equipment has unused capacity
– Emissions control equipment is unloaded
– Regenerated/spent slide or plug valve DPs are exceptionally high, or
– The unit pressure balance has wider rangeability.
Where to look for profit improvement? Three places should be evaluated:
1) The sources and types of feed components that are cracked
2) The technology, formulation and blend of catalyst and additives cracking the feed
3) The operation of the cracking, combustion, flue gas and product recovery equipment.
Let’s discuss the opportunities that come with each of these as well as potential obstacles hindering your success in pursuing them.
Crudes or purchased feedstocks with lower qualities than what are normally cracked offer the opportunity to improve profitability. Higher sulfur, acid number, carbon, and/or metal feeds can be economically attractive while being processed at levels that minimize your risk. Conversely, some refiners introduce feedstocks with better than normal quality in order to fill out their recovery sections while cracking less feed. Internally to the refinery there are incremental feedstocks, like atmospheric or vacuum resid, that can be cracked to raise overall volume gain for the facility. Also, FCC products can be recycled to obtain more valuable yield distributions, like gasoline re-cracked to generate more olefins or heavy cycle oil re-cracked to more valuable liquid products.
Be mindful that an opportunity feedstock may be less crackable than what its reported bulk properties suggest. Excessive yields of fuel gas and coke could result, negatively impacting the heat balance and reducing the feedstock profitability. If the hydrothermal stability of the catalyst is insufficient, then the catalyst may not be able to tolerance additional metals. Additional or new process chemicals for mitigating fouling or corrosion may be needed when cracking more difficult feedstocks.
Optimization of catalyst selectivities, activity or both is worth reviewing with your supplier. Through reformulation or blending ratio changes, the yield structure or metals tolerance can be shifted in a favorable direction. Introduce cracking additive technology (e.g. ZSM-5, bottoms reducer) to shift the yield structure quicker or respond to feed quality changes better. Consider trialing new catalyst or catalyst additive technologies to pursue further product distribution improvement. Lowering catalyst additions would reduce your expenses. Raising catalyst additions may improve cracking selectivities. Trial each to determine what fits your operation better. Also consider adding equilibrium catalyst on a regular basis to reduce your expenses.
Adjusting catalyst technology or formulation could result in physical property changes of the circulating inventory. Monitor the fluidization parameters since fines retention or catalyst attrition may be less. If catalyst additions are reduced, monitor the particle size distribution (PSD) of the inventory…you may have to adjust the fresh catalyst size grade. Standpipes may be over- or under-aerated depending on the operating conditions…adjust aeration rates as necessary for proper pressure build. Closely review the properties of any equilibrium catalyst under consideration for purchase…incompatibility with the fresh catalyst technology could lead to under-performance.
Equipment Operation Opportunities
It is likely that the reduced rate operation will free up combustion air. This opens up the opportunity to operate differently and consume the unused coke burn capacity. Pertaining to the riser operation, lowering or removing feed preheat will increase catalyst circulation, conversion and volume gain. Feed dispersion steam can be conserved to reduce sour water production, or increasing the steam/feed ratio may actually improve feed/catalyst contacting. Being too aggressive with dispersion steam conservation can lead to poor feedstock atomization, feed nozzle plugging, or “wetted” spent catalyst going to the regenerator. Conduct steam/feed trials to determine what works better. Also consider taking feed nozzles out-of-service to improve overall atomization – review the design and procedures before trying this.
By introducing or increasing riser lift gas you may reduce the net dry gas production by “conditioning” the metals on the regenerated catalyst. You can also introduce or increase cracked naphtha injection to promote light olefin production. Under reduced feed rate conditions there will be more riser contact time, which should promote bottoms cracking. You may realize higher slurry ash content as a result, which may impact product blending or increase the erosion rate of system piping. Also, the riser velocity may get too low, which promotes backmixing, resulting in higher fuel gas and coke yields, gasoline overcracking to LPG and other selectivity shifts.
In the reactor section, with the air available, reactor temperature can be raised for more conversion and volume gain. Lowering the reactor pressure will result in less hydrogen transfer (better octane, olefin yield), better stripping, and reduced rotating equipment cost. Longer reactor residence time could also lead to product mix deterioration and vessel coking. Potentially higher butadiene yield may impact the alkylation plant. Slurry fouling rate may accelerate from higher conversion operations – you may need an antifoulant program. LCO properties will shift and may impact distillate blending for cetane, sulfur, and gravity.
In the stripper section, longer stripper residence time (from lower catalyst circulation) can lead to better stripping efficiency, product recovery, and lower coke hydrogen content, especially if reactor temperature is increased. Conserving stripping steam may help with sour water management but be careful of going too low. Like the feed nozzles, getting too aggressive with stripping steam conservation can result in steam distributor nozzle plugging and lower actual stripper bed level, which may uncovering cyclone diplegs. Operating at lower feed rate can also result in higher density spent catalyst, leading to higher slide/plug valve differentials – this presents the opportunity to shift the pressure balance in a positive way by lowering the reactor pressure.
In the product recovery section, generally the lower feed rate condition opens up wet gas capacity. This means you can handle lower suction pressure or lower molecular weight gas streams – this flexibility may expand your feedstock and catalyst options. Downstream recovery sections are likely to be under-loaded, resulting in better liquid/vapor product separations, product purities and improved treating conditions.
When it comes to the regenerator, combustion air will likely be available. You could continue to conserve in the interest of compressor energy savings or apply the excess for higher coke burns. You could also reduce or eliminate the cost of oxygen enrichment if it is part of your base operation. If the refinery needs steam and you have a catalyst cooler increase the coke burn to produce extra steam. If superficial velocities are down and there is less regenerator afterburn then your feedstock and catalyst options grow. You can also save on combustion promoter which may also result in lower emissions. Too low of a combustion air rate can lead to air distributor nozzle erosion and lower regenerator bed penetration. The burn will become uneven with the radial temperature differences growing. The catalyst regeneration will be less uniform resulting in a “salt & pepper” appearance that could impact the catalyst cracking selectivities. The fluidized density will also vary throughout the bed, potentially impacting the stability of the catalyst circulation returning to the riser. Consider plugging off “extra” air distributor nozzles during the next outage if the operation is expected to last a very long time.
In the flue gas system, turboexpander vibrations from fines deposits could be reduced or eliminated. Less or no walnut shelling would be required. However, turboexpander power generation may be lost if the regenerator pressure is lower than the normal operation. Fuel to fired boilers could be reduced or burners modified to reduce emissions. Lower fines accumulation on boiler internals may mean less opacity spiking during sootblowing cycles. Lower chemical cost would be expected for NOx and SOx reduction equipment technologies.
Realize that lower vapor and flue gas rates may result in the loss of cyclone efficiency and less reactor/regenerator catalyst retention. Larger particle sizes may be preferentially lost. Consider sealing off “unnecessary” cyclone pairs during an outage if efficiencies are expected to be low for a very long time. Lowering operating pressure should help in this situation. High slide/plug valve differentials may also lead to high valve erosion rates. See if the pressure balance could be adjusted to address this. Process control valves may perform with less stability since they may be operating near the low end of their range. Retuning may be necessary.
Consider riser, reactor, stripper, recovery, regenerator and flue gas system modifications. Alter or upgrade equipment to address plant constraints and reliability issues. Add new equipment/technology to expand processing capabilities and maximize profitability under reduced rate conditions.
Lastly, take the opportunity to improve your LP vectors for better representations of feed quality and operating condition changes. Also develop various business scenarios of interest and assemble the model projections for each. Conduct plant trials that simulate the cases and provide the necessary data. Update the LP as necessary.
Posted for B. L.