Considerations for crude unit preflash drums and preflash towers

A guide to debottlenecking, revamping, designing, and operating crude unit preflash facilities based on literature and the authors’ experience.

Henry Z Kister and Walter J Stupin (dec), Fluor
Maureen Price, Maureen Price Consulting LLC

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Article Summary

Preflash drums or towers are extensively used in crude feed trains between the desalter and the atmospheric tower heater. Preflash drums or towers have been discussed by many literature references, each focusing on one or a few important aspects and providing valuable guidelines. None of these attempted to bring together all these lessons in a manner that can guide engineers involved in the debottleneck, revamp, design, and operation of crude units.

Purpose and location in the crude train
In crude oil refining, a preflash drum or tower is a vessel that flashes a portion of the light components of the crude as well as some water upstream of the atmospheric tower charge furnace. The use of preflash drums or preflash towers between the desalter and the crude atmospheric tower is typically done to manage crude hydraulics, as part of grassroots unit design, to increase crude capacity, or to allow processing of lighter crudes as part of a revamp.

As the preflashing term suggests, the primary function of these devices is to flash the lighter (volatile) portion of the crude oil before it enters the furnace inlet control valves. These control valves distribute the feed to the various heater passes. Flashing upstream of these valves makes it impossible to distribute the feed to the heater passes adequately. Pass flow imbalances cause heater bottlenecks, rapid coking, and even tube overheating and rupture. The alternative is to use high-pressure booster pumps with expensive high-pressure piping and exchangers to prevent flashing upstream of the valves. In addition, vapourisation in the crude train dramatically increases the pressure drop, which may restrict the crude feed rate. Depending on the configuration, preflashing may also be valuable in debottlenecking the furnace and/or the atmospheric tower, especially when processing lighter crudes (>30° API).

Preflash devices can be located anywhere in the preheat train, with temperatures typically varying from 300°F to 500°F.1,2 Higher temperatures give higher preflashing rates. Preflash device pressure often ‘rides’ on the atmospheric tower pressure, but in some cases preflashing is performed at higher pressure by adding a control valve in the drum overhead vapour line. Preflash towers with condensers have their own pressure control systems.

A key consideration is where the preflash drum overhead vapour is routed. In most crude trains, it is routed to the flash zone of the atmospheric tower (see Figure 1). In this configuration, it debottlenecks neither the furnace nor the tower. Its only merit, then, is to permit lower pressures to be used upstream of the furnace control valves. Any unloading it does on the furnace is countered by the need to add heat in the furnace to make up for the cooler drum overhead vapour bypassing the furnace into the flash zone of the atmospheric tower. The bypassing of lights raises the coil outlet temperature, raising the potential for coking or encountering metallurgical limitation.

Golden3 presents a case of a unit processing 26.3° API crude, with the heater coil outlet temperature maintained at 700°F. Raising the preflash temperature from 275ºF to 400°F reduced the heater duty by 12% but increased the resid yield on the crude from 48.2% to 51.1%. The significant loss of distillates to resid was because no heat was added in the furnace to make up for the cooler drum overhead vapours entering the atmospheric tower flash zone. If one wanted to keep the resid yield unchanged for the same increase in preflash temperature, the heater coil outlet temperature would have needed an increase of 22°F (to 722°F).

An alternative configuration to debottleneck the furnace and atmospheric tower is to have the preflash drum overhead routed to a point further up in the atmospheric tower, as shown in Figure 2. In this configuration, the preflash drum unloads the furnace and the section of the atmospheric tower below the point of entry of the preflash drum vapour into the atmospheric tower, which in Figure 2 is above the diesel draw. The maximum unloading is achieved with a preflash tower (or pre-fractionator), as shown in Figure 3. This arrangement gives a large unloading both on the furnace and the entire atmospheric tower. With light crudes ( >30º API), debottlenecking of 10-20% can be achieved using a preflash tower. In some cases, some kerosene can also be drawn from a preflash tower a few trays above the feed. It has been estimated that approximately 20% of the crude distillation units in North America include an independent crude preflash tower.2

Effect on atmospheric tower stripping:
“The carrier effect”

The unloading achieved by the Figure 2 and, more so, Figure 3 alternatives is not free. The preflashed naphtha bypasses the flash zone of the atmospheric tower. Naphtha is a light, and as a light, it helps the stripping in the bottom of the atmospheric tower. Having it bypass the flash zone means less stripping of lights from the resid. Stichlmair and Fair4 present charts showing that liquid yield from a flash declines when light components are added to the mixture. Adding a light component generates a significant partial pressure in the vapour phase, reducing the partial pressures of the heavier components and promoting their stripping.

This effect was studied and discussed at length by Ji and Bagajewicz5 for the flash zone of the atmospheric crude tower. They show that when the K-value of a component is greater than 30, the light component (hexane and lighter) will have the same stripping effect on a molal basis as steam. Other naphtha components will have a smaller, yet significant, stripping effect. When some of these components are removed in a preflash drum or preflash tower and do not reach the flash zone of the atmospheric tower, there will be a greater need for stripping steam. Alternatively, especially if there is a constraint on the stripping steam, it means a greater loss of gasoil or diesel yield. In one case5 of light crude with no steam increase, the loss was shown to be as high as 2%. Typically, the steam can be increased to some extent, and the loss of gasoil to resid with light crudes is around 0.3-0.5% of the crude. With heavy crudes, the loss with no steam increase is much smaller, often around 0.3%.

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