10000 12000 14000 16000 18000 24000 22000 20000 28000 26000
Water - Liq2 [ppm(mass)]
0 6000 4000 2000 8000
Temperature (˚C)
Figure 1 The y-axis shows water solubility in the hydrocarbon stream (i.e., Liq2, plotted against temperature on the x-axis
Decrease the proportion of atmospheric residue (ATMRES) or imported ATMRES and imported gasoil in the FCC feed Monitor the sodium content in the residue and FCC feed and in the exhausted catalyst • Enhance efficiency of the CDU desalting stage • Implement a dedicated desalting stage for FCC operations • Increase the top temperature of the main fractionator • Inject corrosion inhibitor • Inject wash water and/or salt dispersant. These mitigation strategies could yield partial or complete effectiveness based on the understanding and resolution of the extent of corrosion-related phenomena. This is where an ionic modelling tool is valuable in pinpointing areas most susceptible to corrosion and in devising appropriate miti- gation plans. Ionic modelling Electrolyte (or ionic) models provide insights into corrosion and fouling risks in the FCC unit. Thermodynamic analysis can be used to evaluate the scaling (fouling) and corrosion potential of FCC unit streams with varying water qualities and operating conditions (tem- perature, pressure, pH, and composition). These insights allow the engineer to adjust operating conditions, which reduces inefficiencies and process bottlenecks, and avoids potential leaks and unplanned shutdowns. Similarly, corrosion risks change with changing vapour/ liquid/liquid composition and temperatures and pressures in the main fractionator. The corrosivity of individual streams can be analysed at various operating conditions to aid in the selection of materials for tower internals, heat exchang- ers, and overhead systems. This can further optimise the tower design and establish operating envelopes that avoid highly corrosive conditions, extending the life and reliability of the units. Electrolyte chemistry modelling is mission-critical in understanding the behaviour of complex water and hydro- carbon streams in downstream oil and gas process units. This is particularly true in an FCC unit that is highly suscep- tible to upsets due to minor changes in feedstock qualities
or operating conditions. An electrolyte model can provide deep insight into these complex systems, informing the engineer on operating efficiencies, as well as corrosion and scaling risks. Case study: Boosting gasoline production in an FCC unit A refinery wanted to enhance the quality of its gasoline pool, capitalise on the economic value of using opportunity crudes in its topping unit, and increase the proportion of atmospheric residue in its FCC feed. Thus, it embarked on a significant alteration to its FCC unit operations. These operational adjustments aimed to reduce the pro- duction of light-cycle and heavy-cycle naphtha, resulting in a diminished portion of heavy hydrocarbons being directed toward the gasoline fraction. Because of these modifica - tions, the tower top temperature was also affected, leading to a 15-20°C drop in tower top temperature. This adjust- ment carried the potential for two distinct side effects: Condensed acidic water accumulation on the upper tower trays Salt deposition in the tower top and TPA section. In preparation for this management of change, an ionic model based on OLI Systems tools was constructed to establish the unit integrity window and assess the poten- tial impact of these operational changes on the overall unit corrosion risk. The proposed operating conditions for the main fraction- ator were: • Continuous operation with overhead temperatures <120°C @ 1.9 BarG in the presence of condensed water at the column’s top • Operating periods with high chloride levels linked to a higher fraction of atmospheric residue in FCC bottoms and poor desalting performance at the topping section. This was documented by an increase in sodium content in the exhausted catalyst. These two factors caused several undesirable effects: • Column internals : Acidic water condensed in areas where, due to temperatures exceeding 95°C, ammo- nia (NH 3 ) is not available to neutralise it. This increased
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PTQ Q4 2023
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