PTQ Q1 2024 Issue

Naphtha

Top-PA

Water

Mid-PA

Semi-preash tower

Steam

Kerosene

Bottom-PA

Steam

Diesel

Desalter

Crude atmospheric column

Steam

Gasoil

Water

Crude preheat

Crude oil

Steam

Atmospheric residue

Crude heater

Figure 4 Preflash tower scheme with reflux from and vapour going to the atmospheric tower⁶

Foaming Foaming is a prime consideration for preflashing devices in crude oil distillation. In the words of one expert,7 ‘it is not a matter of if foaming is occurring, but rather to what degree it is occurring’. The foaming severity tends to vary with the crude, with some crudes generating much more severe foaming problems than others.⁸ Simple ‘bottle shake’ tests can often provide information on the degree of foaminess. The foaming could be due to traces of components with surface active properties combined with the effects of fine solid particles. Some of these components may originate in the desalter chemicals or those used for crude recovery at the well. Many of these chemicals decompose in the fur - nace, which is why preflash devices foam while stripping sections in atmospheric towers usually do not. In some cases, desalter upsets cause episodes of carryover from the preflash drum. When foam is carried over with the vapour to the upper sections of the atmospheric crude column, it adversely affects the quality of the products. Typical concerns are poor kerosene and diesel quality and high carbon residue and metals in atmospheric gasoil (AGO). Flashed crude is dark and has a high endpoint. When it enters the atmospheric tower above the flash zone, all product streams below the entry point will contain flashed crude. Even small amounts of foam carryover will cause colour and endpoint problems with kerosene and diesel. Once the crude gets into the upper sections of the atmos - pheric tower due to a foamover, it gets into the pumpa - rounds and stays in the system for lengthy periods of time,

causing the product quality issue to linger. The crude also gets into the diesel and kero hydrotreaters and rapidly deac - tivates their catalysts. Combined diesel and AGO product yield losses as high as 6 vol % on crude have been reported due to flashed crude entrained with the preflashed vapour.1 , 9 The foam also brings naphthenic acids and sulphur into places not designed to handle them. Additional adverse effects of foamovers are cavitation of the preflash drum or tower bottom pump, which can inter - rupt the feed to the furnace and atmospheric tower and initi - ate a shutdown. If the drum overhead vapour passes through furnace convection coils, crude carryover can severely coke the convection coils. If the drum overhead goes to the atmos - pheric tower flash zone, the carryover will generate a cold spot upon tower entry that will suck vapour downward. Foaming can be controlled, to some degree, by antifoam injection, typically silicones, upstream of the preflash drum. This is not favoured by most refiners due to contamination of products, adverse effects of antifoams or their degrada - tion products on hydroprocessing catalysts downstream, and high antifoam costs. For fear of foamovers and their consequences, most refin - ers route the overhead of their preflash drums to the flash zone of the atmospheric tower rather than to the upper sections of the tower (see Figure 1), at the price of forfeit - ing many of the unloading benefits that preflash drums can offer. Even when the drums are well-sized and contain de-foaming devices, many refiners still prefer to route the drum overhead into the flash zone for fear of the severe implications of even a few foamover events.

PTQ Q1 2024