tanks and long holding times are required to meet higher value product specifications in decanting processes, and smaller particles can pass through mechanical processes. 2 The bottom line Achieving net zero will come at a cost, but optimising processes and utilising advanced technology will provide offsetting dividends in the bottom line of the refiner. Some of the value generated from an electrostatic separator in removing FCC catalyst fines from slurry oil can be illus- trated using the following example. An 80,000 BPD gas oil FCC unit has a slurry oil yield of 4 vol%, or 3,200 BPD. Catalyst content of the slurry oil is 4,000 ppm. All cases are compared against the base case, where the refinery uses a holding tank to reduce its solids. The slurry holding tank is assumed to require cleaning once per year at 2,000 ppmw slurry solids at a cost of $1.5 MM. Increased catalyst loads will incur higher frequency slurry holding tank cleaning and total costs. A portion of the FCC feed is used to backwash the electrostatic separator, after which it and the associated catalyst are fed back to the FCC unit, thus reducing fresh FCC catalyst costs. FCC catalyst costs are assumed to range from $3,000 to $7,000/metric ton. The average product upgrade value for this clarified slurry oil can is estimated to be between $8 and $12/bbl. The benefits of not having to purchase chemical settling aids were not considered, even though such costs are esti- mated to be in the order of 12-40 cents per barrel treated.4 Heating costs for maintaining the holding tank at tempera - ture are not considered but can be substantial. The reduc - tion of environmental waste and the reduction of catalyst loss supports the refinery goal of net zero while providing increased profits in the process. The operation and reliability are supported by the ability of the electrostatic separator to maintain operation without blocking or plugging, so no downtime is realised. Another point not considered is the increased FCC production by not using heavy cycle oil (HCO) or light cycle oil (LCO) as back flush oil required by mechanical filters. In this case, the electrostatic separator uses raw FCC feed for back flush, therefore increasing the middle distillate production and the bottom line of the refinery.1 Profitable markets and new applications of slurry oils can be found in Table 3. Conclusion Petrochemicals are the traditional extension of the refinery value chain, as there are synergies in the production of both fuels and commodity chemicals in an integrated facility. Petrochemicals typically achieve significantly higher prices than transport fuels and thus increase FCC profitability. By using heavier resid cracking units alongside high severity complex reactors with specialised catalysts, refiners can profitably maximise propylene product. These integrated sites are highly competitive compared to standalone fuels refiners, with the flexibility to switch yields between fuels and chemicals. This suggests that the shift towards pet - rochemicals needs to be methodical and well planned to capture significant value and position a site in its individual decarbonisation plan.
Typical particle size distribution in slurry oils
Particle diameter, microns
% in range
0-5
30-60 30-55
5-15
15-25
2-12
25+
1-5
Table 2
characteristics of the slurry, the temperature of the storage tank, and whether settling aids are used. It should be noted that another product is being generated along with decanted/ clarified oil, and that is sludge. In most cases, this sludge is recycled back to the riser. Slurry oil holding tank sludge has been classified as hazardous waste and therefore requires special treatment and disposal expenses. Depending on the tank size and rate of slurry oil production, costs can be millions of dollars per cleaning. Without countermeasures, increasing resid feed to the FCC will tend to increase slurry oil production and the sludge formation rate. 3 Mechanical filtration, first put into slurry oil service around 1990, operates at temperatures up to 315ºC and employs tubular porous metal elements. The solids collect on the inside of the elements while the filtrate passes through to the outside. Some filters use porous sintered woven wire mesh metal filters and operate at 232ºC-343ºC. Others employ a 2-5-micron woven wire filter element, using LCO as a backwash at 176ºC, and claim 85-95% solids removal from the feed slurry. 4 Electrostatic separation of FCC catalyst fines from slurry oil has been in commercial operation for more than 30 years. Electrostatic separation uses a charge, causing the catalyst particles to become trapped in beds of glass beads while maintaining flow without a significant pressure drop. In this way, electrostatic separators have been able to remove more than 97% of the catalyst present in slurry oils. As the concentration of vacuum tower bottoms in FCC feeds grows, modern techniques used for the selection of catalyst removal from slurry oil will increasingly favour electrostatic separation because it is inherently less likely to foul or coke due to increasing asphaltene levels in the slurry. Particle size distribution (PSD) ranges from a variety of slurry oils are shown in Table 2 . Note that for these slurry oils, more than 90% of the particles range in size from 0 to 25 microns. In comparison, this means that very large holding
Typical permissible solids contents for various slurry oil product applications
Market
Clarified slurry oil solids (ppm)
Carbon black feedstock
50-500 50-150 50-150 25-100 25-100
Refinery use, fuel or coker feed
Marine fuel, #6 Pitch feedstock
Needle/anode coke feedstock Hydroprocessing feedstock
10-50
Carbon fibre feedstock
5-10
Table 3
32
PTQ Q3 2025
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