particularly in systems with increasing paraffinic char - acter, leading to aggregation and deposition rather than dissolution.4 , 6 In heavy-oil fouling experiments, the maximum initial foul- ing rate occurred at a resins-to-asphaltenes ratio of approx- imately 2.5 and dropped to essentially zero once that ratio exceeded approximately 5.8.2 This quantifies how readily a blend shift can move a system across the stability boundary with no change in gross contaminant concentration. Paraffinic components, while above their classical wax appearance temperature, do not behave as low-viscosity liquids in multicomponent systems. Instead, they partici- pate in structured, highly viscous organic phases that act as binding media for inorganic particles. Fouling studies have shown that these phases form gel-like networks capable of trapping fines and evolving into semi-solid deposits.² , ³ Commercial paraffin waxes melt in the range of 46-68°C (115-154°F), and crude-oil wax appearance temperatures measured by differential scanning calorimetry (DSC) fall in the 5-24°C range for heavy crudes and 30-36°C for lighter crudes, well below the 150-185°C conditioning window. 4 The relevant wax behaviour at operating temperature is therefore not dissolution but participation in viscous, gel-forming multicomponent phases. Oxidative and thermal processes further contribute through the formation of gums and polymeric intermediates, increasing polarity and promoting adhesion to both solids and surfaces. The result is a continuum of semi-deforma- ble organic-inorganic agglomerates with a structure that evolves with temperature. Within the 300-365°F range, these structures remain sufficiently mobile to be transported but sufficiently cohesive to retain integrity. The practical sig - nificance of even trace precursor levels is substantial. In a 300,000 bpd refinery, deposition of just 1 ppmw of crude as carbonaceous foulant in the preheat train corresponds to approximately 15 tons per year of solid deposit,2 with direct heat-transfer and pressure-drop consequences downstream of the conditioning system. In this regime, capture behaviour is governed by interaction with these evolving structures rather than by nominal particle size. These pathways do not occur in isolation. Heavy hydro- processing feeds are typically blended systems containing varying proportions of paraffinic, aromatic, and resid-de - rived components, where wax-rich phases, asphaltenes stabilised by aromatic content, and oxidation-derived gums may coexist. The dominant agglomeration mechanism is therefore not fixed, but depends on the balance between solvency, composition, and temperature, directly influenc - ing the structure and behaviour of the resulting organic- inorganic agglomerates. Systems with stronger paraffinic contribution tend to produce more deformable, smear- prone structures, while those with higher aromatic or polar content exhibit increased adhesion and structural integrity. Limitations of conventional filtration When subjected to conventional surface filtration, the deformable nature of these agglomerates leads to rapid performance degradation (see Figure 2 ). Hydrocarbon-rich phases deform under pressure, spread across the upstream
Figure 1 Organic-inorganic agglomeration structure
polymerisation products from cracked fractions, paraffinic associations, and asphaltenic material, forming heteroge- neous organic-inorganic agglomerates with behaviour gov- erned by their composite structure.4 , 6 Suspended inorganic species warrant particular attention individually. Ni+V, concentrated in vacuum residues from opportunity crudes at levels of 100-500 ppm or higher, deposit on hydrotreating and hydrocracking catalysts, pro- mote dehydrogenation and coke formation, and shorten cycles. Resid hydroprocessing designs commonly assume Ni+V limits near 250 ppm for acceptable cycle economics.4 Iron and calcium from corrosion and incomplete desalting contribute directly to exchanger and filter fouling and pack into grading and guard layers, providing surface area for further organic deposition. The composite nature of deposits in these systems is significant from a filtration design standpoint. Autopsies of hydroprocessing internals and crude preheat fouling studies converge on a consistent picture: deposits are gel- like organic matrices with trapped liquid and solid phases. These deposits consist of asphaltenic and coke-like frame- works with embedded inorganics, oxidised and polymeric material, and waxy phases, rather than pure metal scale or pure coke.2 , 4 This composite structure is what renders size- based filtration inadequate as a primary capture mecha - nism. The relevant contaminant cannot be meaningfully characterised by a single particle diameter. These structures are viscoelastic rather than discrete (see Figure 1 ). Their morphology evolves with temperature and solvency, and they remain deformable under hydraulic stress. This behaviour is consistent with fouling observa- tions in heavy hydrocarbon systems.2 - 4 Temperature-dependent agglomeration behaviour Agglomerate stability in heavy hydroprocessing feeds is strongly temperature-dependent and reflects the balance between solubility, association, and chemical transforma- tion of hydrocarbon species in the presence of suspended solids. Within the relevant feed-conditioning window (~300-365°F/150-185°C), asphaltene stability becomes particularly sensitive to solvency conditions. Published work shows that asphaltene solubility can decrease with increasing temperature under refinery-relevant conditions,
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PTQ Q3 2026
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