Preflash tower condenser and drum Many preflash towers have their own condenser and reflux drum system, and their naphtha products are sent to the naphtha processing facilities downstream of the atmos - pheric tower (Figure 3). If the condenser and drum of the atmospheric tower have enough capacity, Capex can be saved by sending the preflash tower overhead to the atmos - pheric tower condenser and drawing reflux for the preflash tower from the atmospheric tower reflux drum. The scheme proposed by Lee, 6 Figure 4, also eliminates the preflash tower condenser and drum and saves Capex. Adding a preflash tower and condenser (as in Figure 3) debottlenecks the atmospheric tower condenser. As many atmospheric crude towers are bottlenecked by their con - denser capacities, this condenser debottleneck is invalua - ble not only for achieving a higher throughput from the unit but also for allowing better recovery of distillates (gasoil and diesel) from the resid. Additional atmospheric tower condenser capacity allows reducing pressure in the tower, which vapourises some of the resid into gasoil and diesel products. Also, atmospheric tower condenser limitations often constrain the amount of stripping steam that can be used, so debottlenecking the condenser allows more strip - ping steam to be used and again recovers more distillates out of the resid. In addition, this system provides better flexibility to keep the crude unit optimised for running vari - able API feeds depending on loading and turndown. On the debit side, this scheme aggravates the dew and salt point problems in the atmospheric tower, as discussed earlier. One important consideration is that many preflash tow - ers remove the bulk of the light gases from the crude. This again helps the atmospheric tower condenser, as these gases reduce its heat transfer coefficient. The absence of these gases may convert the atmospheric tower condenser from a partial condenser to a total condenser. This neces - sitates changes to the tower pressure control scheme, and if these are not correctly performed, the atmospheric pres - sure control may become unstable. Energy savings For refineries where energy savings in the crude unit are a desirable target, it is important to explore how adding crude preflashing devices will affect energy consumption. The crude heater is one of the highest energy consumers in a refinery, and minimising its duty is a good target. A study by Feintuch et al13 demonstrates the importance of taking the preheat train constraints into account when comparing energy usage between preflashing and no pre - flashing options. In their revamp case, adding a preflash drum with its overhead going to the atmospheric tower flash zone (similar to Figure 1) was a significant energy saver because the lower downstream pressures permitted the reuse of existing exchangers. In that case, replacing the existing exchangers with new ones was too costly to be economical. Another comparison was conducted by Lee.6 In addition to the three cases of no preflash, preflash drum, and preflash tower, he added the semi-preflash tower option in Figure 4. The crude considered was a 60.7º API Eagle Ford crude.
The preflash drum overhead vapours were directed into the atmospheric tower flash zone. Lee’s study was based on the same product yields and the same overhead condenser duty of the atmospheric tower for all cases. Product overlaps were kept identical for all cases. To counter the carrier effect, Lee raised the coil outlet temperatures in the preflash cases. The preflash temper - atures were set to comply with the atmospheric tower condenser limitation, varying from 282ºF for the preflash drum to 395ºF for the preflash tower and 353ºF for the semi-flash tower. In addition, Lee’s study adjusted the crude tower pumparound balance for each case to give the same product yield pattern as in the no preflash case. The changes in the pumparound duties were reflected as changes in furnace preheat temperatures. Per our experi - ence, Lee’s comparison has a realistic basis that is reflective of many crude systems. Lee’s study found that the preflash drum (with its over - head entering the flash zone in the atmospheric tower) had little effect on the heater duty energy consumption and did little unloading in the atmospheric tower. The preflash tower reduced the heater duty by 18% compared to the no-preflash case and achieved a very large hydraulic unloading in the atmospheric tower, especially above the kero pumparound. The semi-preflash option gave a 13% reduction in heater duty and intermediate hydraulic unloading. In this semi-pre - flash option, the hydraulic unloading above the kero pumpa - round was much less than in the preflash tower option. The energy savings from the addition of a preflash tower depends on the location of the heat transfer surface area, the cut points, the unit constraints, and the preflash scheme. A consensus among several experts7 is that adding a pre - flash tower may or may not save energy. One thing both the previous studies teach is that it is important to carry out the comparisons on a basis that correctly reflects the specific crude train under consideration and its constraints. A dif - ferent basis and different ground rules can lead to different conclusions. Each case should be considered on its own merit. Market conditions may also dictate which configura - tion is better suited for a specific project at a given time. Economics: Preflash drum, tower, or no preflash? The no preflash scheme saves the cost of the drum and piping but raises the cost of the preheat exchangers that would need to be designed for the higher pressure required to keep the feed liquid from flashing upstream of the heater feed control valves. In the revamp of an existing preheat train, being able to reuse existing exchangers and save the cost of new ones may shift the economics in favour of pre - flashing, as demonstrated in one case.13 The preflash column and semi-preflash options are not much more expensive than the flash drum, provided they make use of the atmospheric tower overhead condenser and reflux drum.⁹ The most expensive is the preflash tower option with its own condenser, reflux drum, and pumps. In a revamp, however, adding the preflash tower with the reflux drum and pumps can be more cost-effective and trouble-free than modifying the atmospheric tower and its overhead system. Further, the bulk of this construction can
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PTQ Q1 2024
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