corrosion in tower internals, domes, heat exchangers, and associated piping. The impact of the phenomenon extends to downstream equipment. Salt deposition on heat transfer surfaces reduces thermal efficiency, affecting the performance of critical equipment such as the depropaniser tower reboiler and lim- iting propylene glycol (PGR) recovery. Likewise, the pres - ence of salts in the process streams can cause premature mechanical seal failures and contribute to deposition in the wet gas compressor, affecting its performance. At the global level, the build-up of deposits can increase pressure in the flash zone, requiring higher bottom tempera - tures to maintain the yield of products such as light cycle oil (LCO), worsening fouling conditions in the slurry circuit, and affecting the unit’s heat balance. Given the multiplicity of impacts generated by salt for - mation and deposition, its management should not be approached solely from a corrective perspective. Strategies such as water washing or the use of dispersants can partially mitigate the problem, but they do not eliminate its root cause (see Figure 1 ). In this context, effective control of the phe - nomenon requires a preventive approach, based on continu- ous monitoring of process variables and management of the system’s chloride balance, including all its potential sources. Case study: Mitigating salt formation With the aim of analysing the operational impact associated with salt formation in the FCC unit main fractionation sys - tem, a case is presented based on the operation of a Latin American refinery, where this phenomenon began to emerge progressively as of 2019. The FCC unit, designed for processing heavy feeds with high operational flexibility, plays a central role in naphtha production, with a fractionation scheme that requires strict control of thermal and separation conditions. In this context, the main fractionator constitutes a critical point for the unit’s operational stability. Starting in 2019, a sustained increase was observed in the differential pressure in the upper section of the fractionator, particularly in the area associated with cracked naphtha handling. This behaviour was accompanied by operational evidence consistent with salt deposition, including the pres- ence of salts in wash streams and the increasing need for interventions to restore operating conditions. As the phenomenon progressed, the operational impacts intensified. Restrictions in vapour-liquid traffic were recorded in the overhead section, affecting separation efficiency and causing deviations in the distillation cuts, especially between light and heavy naphtha. Under more severe conditions, the accumulation of deposits led to episodes of instability in the reflux system and an increased tendency for heavy hydro - carbons to be carried over into lighter streams. The problem was not limited to the fractionator. The prop- agation of the phenomenon to downstream equipment gen- erated additional impacts on the unit’s operation. A decrease in heat transfer efficiency was observed in exchangers asso - ciated with the heavy naphtha circuit and in the reboiler of the depropaniser tower, affecting product recovery and the thermal balance of the system.
• Operating conditions: Carryover or the accumulation of species in certain areas of the system. • Catalyst: When technologies based on chloride-containing binder are used. The combined effect of these sources determines the chloride balance in the unit, which is a critical factor in the propensity for NH₄Cl formation. In practice, monitoring the chloride content and its correlation with operational varia- bles, such as the differential pressure in the overhead sys- tem, is a key tool for identifying the phenomenon. Salt precipitation mechanism and operational impacts The formation of NH₄Cl salts in FCC units occurs mainly through the vapour-phase reaction between NH₃ and HCl: NH₃ (g) + HCl (g) ⇌ NH₄Cl (s) The equilibrium of this reaction depends on the partial pressure of the reactants and the temperature of the system. Under typical operating conditions, NH₄Cl can form in the vapour phase and subsequently deposit when the system reaches temperatures below its thermodynamic stability limit. This phenomenon occurs primarily in areas where there is a progressive cooling of the vapours, such as the top of the main fractionator and the overhead lines. Under these con- ditions, the combination of the thermal profile and the avail - ability of precursors determines the propensity for deposit formation. From a thermodynamic standpoint, the precipitation of NH₄Cl is governed by the product of the partial pressures of NH₃ and HCl. In systems where NH₃ is in relative excess, salt formation is predominantly controlled by the availability of chlorides. In this scenario, increases in chloride content result in a direct shift of the equilibrium towards the formation of deposits. Once formed, NH₄Cl salts deposit on metal sur - faces, even in the absence of a visible liquid phase, and due to their high hygroscopicity, they can create highly corrosive environments. From an operational standpoint, their accumulation causes a progressive increase in the pressure drop (ΔP) in the top section of the fractionator, reducing the active area of the trays and deteriorating the vapour-liquid contact. As a result, phenomena such as liquid entrainment (flooding), reflux instability, and loss of separation efficiency occur. Furthermore, the presence of salts promotes the formation of sour solutions in the presence of moisture, causing localised Figure 1 Washing effluent from the pumparound pump and HCN product
12
PTQ Q4 2026
www.digitalrefining.com
Powered by FlippingBook