termination devices can increase residence time or the cat- alyst-to oil-ratio (cat/oil) to increase reaction severity. The implementation of a secondary or dedicated riser to crack recycled light naphtha or light hydrocarbons in gen- eral can play a fundamental role in maximising light olefins yield, especially in the range of ethylene and propylene under severe reaction conditions. Additionally, upgrading to advanced feed injection systems ensures uniform dis- persion, optimum atomisation, reduced hydrocarbon partial pressure, and optimal contact between feed and catalyst. 7 Simple adjustments can be accomplished without the addition of expensive upgrades or catalyst trials. Adjusting operating conditions such as cracking temperature, pres- sure, and feedstock composition can help in maximising desired olefins production. Increasing the severity of the cracking process can lead to higher yields of lighter prod- ucts but requires careful balancing to avoid excessive coke formation and catalyst deactivation. Maximising riser outlet temperature (ROT) is one of the first independent process variables to consider due to the relative ease and flex - ibility to change. Most FCCs will operate in the region of 520/530ºC for maximum gasoline output. Increasing the ROT will increase LPG olefin production. For maximum LPG olefins, high severity FCCs can operate >540ºC. More extreme process conditions can be applied when the FCC unit is upgraded to high severity or maximum propylene FCC where they can operate above 560ºC with reduced hydrocarbon partial pressure (>7 wt% dispersion steam) and cat/oils exceeding 15 wt/wt in some cases. 11 Investment in these new FCC technologies is an excellent prospect for olefins optimisation. Advancements involving the proprietary HS-FCC process, developed by Technip and Axens and its partners include a controlled short contact time down flow reactor (DFR) system with contact times <1 second and even more elevated cat/oil ratios. These improvements to the well-known FCC process can reach a considerably higher level of light olefins production (mainly propylene) while lowering fuel gas production to bridge the gap between refining and petrochemicals industries.⁹ By focusing on these upgrade strategies, refineries will not only boost their FCC unit’s efficiency but also enhance their capability to produce a broader range of valuable pet- rochemical products, aligning with market demands and economic trends. Investing in existing equipment is well within the framework of reaching net zero. The cost of upgrades to the FCC or new catalyst trials to enhance pro- cess conditions and optimise the unit’s severity is offset by better yields. On average, these yields can provide greater than $15 million of additional revenue annually while sup- porting decarbonisation in the refinery. Slurry oil yields and properties As might be expected, slurry oil quality is a function of such variables as the properties of the FCC feed, severity of the operation, type of catalyst, and operating conditions in the FCC unit. The marketability of slurry is penalised on its density, clarity, and contaminate contents. Slurry oil yields ranging from 1-2 vol% for easy-to-crack feeds to as much as 24 vol% on residue fluid catalytic cracking (RFCC)
Properties of typical slurry oils
Property
Range (minimum to average to maximum)⁸
API gravity
-8 to 1 to 32
Sulphur, wt% Nitrogen, ppmw
0.5 to 1.3 to 5.8
50 to 1,600 to 10,100
Aromatics
31 to 53 to 96
Asphaltenes, vol%
Nil to ~8
Solids, ppmw Nickel, ppmw
1,000 to 6,000
0 to 110 5 to 200
Vanadium, ppmw
Table 1
feeds or maximum diesel applications have been observed. Upgrading slurry oil is problematic due to its low American Petroleum Institute (API) gravity and high content of asphaltenes in resid operations. Properties of typical slurry oils can be found in Table 1 . Resid (>1,050°F), in general, and asphaltenes, in particu - lar, are large hydrocarbons with a high carbon-to-hydrogen ratio. Resids have a molecular size >25 Å, and asphaltenes have a molecular size of >100Å These compounds are especially rich in metals and contain nickel and vanadium, which promote coke generation when deposited on FCC catalysts. When cracking residual feedstocks, the zeolite pores (<14Å [7.4Å through the super cages]) are not large enough to crack these large asphaltene structures to enter the pores of the catalyst and therefore pass along to the slurry. A well-designed resid FCC catalyst will contain an active matrix with mesopores (100Å to 500Å) to increase the cracking of these large HC compounds. The level of asphaltenes conversion in an RFCC unit is then a function of the accessibility and the selectivity of the active matrix in the catalyst. 3 , 5 Catalyst particles in the slurry, besides containing nickel and vanadium, can also bring in sodium and other feed metals that are deposited onto the catalyst. Slurry oil may also contain other solid FCC particles such as SOx reduction, CO promoter, fuel sulphur reduction, metals traps, and bottoms cracking additives. The elements in these additives (such as Mg, Pt, Pd, Ce, and Ca) can change the quality of the slurry/sludge. As refiners introduce more and more resid into the FCC, slurry oil yields will increase, and the quality of the slurry oil will decrease. In addition, a larger proportion of asphaltenes and heteroatoms will enter the FCC. This is relevant because the level of asphaltenes in the slurry oil determines which technology is best for removing par- ticulate solids. Mechanical filtration becomes problematic in these scenarios due to the occlusion from asphaltenes and waxes presented in heavier resid feedstocks used to produce high levels of propylene in petrochemical devel- opment. This leaves most complex refineries looking for a more effective source of clarification and a reliable fines removal system. 1 , 5 Slurry oil particulate removal technologies Holding tanks have been used to allow solids to settle out of the slurry oil. The resultant decanted oil solids content will be a function of the sedimentation tank design, the physical
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PTQ Q3 2025
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