PTQ Q4 2023 Issue

Identifying high catalyst losses Multiple approaches to assessing FCC catalyst losses require an assessment of the unit’s mechanical system’s impact on fluidisation efficiency

Warren Letzsch Warren Letzsch Consulting PC

H igh catalyst losses cost the refiner in many ways. Economically, the cost of extra catalyst can be sub - stantial, with fresh catalyst selling for $3,000 to $4,000/short ton. Yields may also deteriorate, which might cost more than the extra catalyst. When catalyst losses go up, the fines (0-40 microns) content usually goes down. The viscosity of the fluid beds increases while the quality of fluidisation declines. Bubbles are larger, and mass trans - fer declines, affecting catalytic reactions, stripping, and regeneration. Catalyst circulation may become erratic; more hydrocarbons could enter the regenerator, and afterburn might be higher. Higher catalyst losses cause additional wear in the flue gas system. More catalyst goes to the secondary cyclones, plenum chamber, flue gas slide valve, orifice chamber, and waste heat steam generators. The tertiary recovery system also sees higher costs, and increased catalyst going to tank - age can be expensive. Data is required to answer some fundamental questions about the problem. This includes the catalyst densities and bed levels in the unit, the catalyst circulation rate, the catalyst entrainment rates, and the level of catalyst in each cyclone dipleg. Superficial velocities and the transport disengaging heights need to be determined. Cyclone efficiencies and the losses from each side of the unit are needed, and these num - bers represent the loss history of the unit. Assessing catalyst losses The catalyst inventory of the unit can be calculated from the pressure balance around the reactor, regenerator, and con - necting piping equipment. Pressure differentials around the gas distributors and grids should be a part of the balances since changes in the delta Ps could indicate a plugged or eroded piece of equipment. The reactor side must include the gas plant since the wet gas compressor suction pressure is the starting point for the reactor pressure. The catalyst lev - els in the regenerator and stripper are determined from the taps in the side of the vessels. The equation for the bed level contains the bed density and is:

density but assume one. It should be measured because the value changes with catalyst type, gas velocity and density, and particle size distribution. A foot difference can affect the dipleg seals and the catalyst level in the cyclone diplegs. The catalyst dipleg height is given by:

Catalyst height in dipleg = (cyclone delta P + bed density x length of submerged dipleg)/dipleg density

It has been recommended that losses be plotted vs the dipleg level to find the point where excessive losses occur to ensure cyclone flooding is avoided. The levels in the diplegs are not constant and fluctuate with the delta P across each cyclone. The total losses are equal to the total catalyst additions minus the catalyst withdrawals and minus the gain in unit inventory. The inventory will be constant if the catalyst den - sities stay constant and the regenerator bed level does not change. Changes in the fresh and equilibrium catalyst silo inventories are a part of the calculations. When catalyst is added to the silo, it packs with a density equal to the ABD of the catalyst. If it sits for days, the density approaches the compacted density. If some aeration of the silo is possible, a more consistent density can be used for silo calculations. Catalyst and additive analyses are needed for the fresh materials as well as the equilibrium catalyst and fines from the reactor and regenerator. This will help track the losses of each catalyst and additive added and the possible cause of increased losses. The particle size distributions give clues to the loss mechanism. If the APS of the ecat increases significantly, the losses are probably mechanical or operational. An increase in the 0-20 micron content of the fines is due to attrition. If the losses increase and the 0-40 micron content of the ecat does not go down or even increases, then attrition is occurring. Measuring accuracy The losses from the reactor are the bottoms yield times (multiplied by...) the catalyst content. Since most units have a close-connected riser termination system, the loading to the cyclones is the catalyst circulation rate. Therefore, the effi - ciency of the separation system is easy to obtain. It is worth - while to look at the efficiency vs the catalyst circulation rate to see if lower loss rates occur at lower catalyst circulation or if losses go up when operated past the design feed rate. Regenerator losses are the total losses minus the reactor

Bed height = 62.4 (meter reading)/bed density

If the meter is a manometer containing water (62.4 lb/ ft³ density), and the level taps are placed 62.4 in apart, the reading will give a direct measure of the bed height above the lowest tap. Many refiners do not measure the bed

PTQ Q4 2023