PTQ Q1 2024 Issue

Light naphtha

Top-PA

Naphtha

Water

Preflash tower

Mid-PA

Steam

Kerosene

Bottom-PA

Steam

Diesel

Desalter

Crude atmospheric column

Steam

Gasoil

Water

Crude preheat

Crude oil

Steam

Atmospheric residue

Crude heater

Figure 3 Preflash tower scheme

Effect on atmospheric tower stripping: “The carrier effect”

crudes, the loss with no steam increase is much smaller, often around 0.3%. Effect on water dew point When using a preflash tower with an independent con- denser/reflux drum system (Figure 3), some of the naph- tha is completely bypassed around the atmospheric tower, which raises the dew point and salt point at the tower overhead. This is tempered (usually to a small extent) by the removal of the entrained water in the crude, as well as some of the water dissolved in the crude, in the pre- flash tower. As this water ends up in the preflash tower overhead, it bypasses the atmospheric tower. With most atmospheric towers attempting to keep a margin (typically 25°F) above the water dew point, the naphtha bypassing means either cutting the stripping steam (usually practised) at the expense of larger loss of diesel or gasoil yield to the resid or raising the atmospheric tower overhead tempera- ture, which loses kerosene to the naphtha. There are other means of countering the naphtha bypass - ing. One scheme is to send preflash naphtha or naphtha recycle to near the top of the atmospheric tower, but this comes at the price of loading up the upper section of the tower and reducing its thermal efficiency. Another scheme is Soun Ho Lee’s idea6 of refluxing the preflash tower with naphtha from the atmospheric tower and returning the pre- flash tower vapour to the atmospheric tower (see Figure 4 ). Finally, the metallurgy near the top of the atmospheric tower can be upgraded at a significant cost of course.

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 Fair 4 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 compo- nents 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 case 5 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

PTQ Q1 2024