PTQ Q1 2024 Issue

be performed while the crude unit is in operation, reducing the downtime during the turnaround. An optimisation study by Sloley 2 concluded that for new units in general, independent preflash towers should be avoided, as building additional capacity in the crude tower and associated equipment lowers Capex and improves yields and heat recovery. In the cases where a preflash may be beneficial in a new unit, the preferred option would be to use a preflash drum. A study by Gomez-Prado et al 14 for a grassroots design of a 120,000 BPD North American crude unit supports including a preflash drum. The benefits were more crude preheat, flexibility to process alternative lighter crudes, reduced preheat pressure to allow the use of a 300# flange rating instead of 600#, and reduced operating costs of the hot crude charge pumps. Specifically, they noted that with no preflashing, water entrainment had a high influence on the preheat exchangers’ cost. For example, with a water content of 0.8% LV, the difference due to water in the required pressure rating (preflash vs no preflash) reached about 200 psi vs 100 psi at the typical design point of 0.2- 0.3% LV. As water upsets occur, the exchangers and piping need to be designed to withstand the pressure produced with the higher water content, The cost savings more than paid for the preflash drum and its associated facilities.14 For revamps, the factors that would favour independent preflash towers include:2 , 7 • Severe capacity constraints in the entire atmospheric crude tower • Strict limits on the duty available in the existing furnace • Strict limit on furnace outlet vapourisation with very light crudes (excessive coking at heater outlet) • Severe limits on the existing overhead system – a preflash tower would add a new overhead system (Figure 3) • Removing the C 5 and lighter paraffins reduces asphaltene precipitation and fouling in downstream exchangers. This is far from a comprehensive list, and each case should be considered on its own merit. Sometimes, a preflash device is economically attractive in the least expected circumstances. Waintraub et al 15 presented a case in which a preflash drum was attractive for pro - cessing a heavy high naphthenic crude (API 16º). With a preheat temperature of 590ºF and 0.6% LV water in the desalted crude, the operating pressure to prevent flashing before the furnace control valves would have been 640 psig. The exchangers would have needed to have special metallurgy to withstand the presence of chlorides in an aqueous phase and naphthenic acids. The cost of pre - flashing the water and the light ends was much lower. The light ends content was too low to justify a preflash tower, so the economic solution was a preflash drum with its overhead going to the transfer line. Often, there are unique considerations, such as when processing synthetic crudes or bitumen, which are mixed with naphtha diluent to cut viscosities and permit adequate flow, as described in detail by Grande and Gutscher.16 In dil- uent recovery units (DRUs), preflash drums are often used upstream of the furnace and DRU tower. The large volatility gap between the naphtha and the gasoil seldom justifies a

preflash tower. In this case, free water is a major issue and can largely raise the pressure requirement for the preheat exchangers and cause damage to the furnace and tower due to rapid vapourisation. To accommodate this, there is an incentive to perform preflashing using two preflash drums at different temperatures. 16 The first drum removes most of the free water, and both remove diluent from the feed. Economics for preflash towers need to be examined care - fully. It is conceivable that an entire parallel crude unit may be a more cost-effective expansion option. Each case needs to be considered on its own merits, with the broad picture kept in mind. This is illustrated by one of the case studies by Gomez-Prado et al. 14 In that case, the preflash tower already existed, and the challenge was optimising the pre- heat temperature, with a constraint on the preflash tower overhead condenser. While additional preheat reduced the crude furnace duty, it also decreased the yield of the more valuable distillate. A model of the entire refinery was needed to optimise the preflash temperature correctly. Plot space considerations may also play a role in favouring one scheme or another. Reliability and environmental considerations There is one more important reliability consideration favouring preflash towers. Environmental and safety con - cerns have recently forced refiners to route discharges from small relief valves, previously going into atmospheric drums, back into the process. Desalter relief valves, as well as relief valves from liquid crude-handling circuits, have been routed into the atmospheric tower flash zone. Pockets of water in the reliefs or sitting in the relief valve discharge piping are common. In some instances, these pockets have ended up in the hot flash zone of the atmospheric tower, rapidly vapourising and causing a pressure surge. This can damage several trays in the fractionator, causing yield losses and forcing a premature crude unit shutdown. When a preflash tower exists, these discharges are routed to near the top of the tower, where the temperatures are low enough to tolerate water without pressure surges. Takeaways Preflash drums and towers are invaluable for debottleneck - ing preheat exchangers, crude heaters, and atmospheric towers, particularly when processing light crudes. Potential penalties are some loss of gasoil or diesel to the atmos- pheric resid and aggravation of the water dew point and salt point issues near the top of the atmospheric crude tower. To be effective, these devices need to be adequately designed, with special emphasis on containing foam and, in towers, resisting fouling. Regarding their economics, each case needs to be con- sidered on its own merit, with unit constraints taken into account. This article was originally presented at the Distillation Experts Conclave Meeting, Mumbai, India, October 12-13, 2023. This article is dedicated to the legacy of Walter Stupin, an expert, mentor, and friend who con- tributed so much to advance chemical engineering, distillation, and to help and encourage young engineers. May his memory be blessed.

PTQ Q1 2024