Preflash Drum Installation

[fdomin]

Hi,
I´m working in a crude unit refluxed preflash column installation project to reduce heater firing and extend unit run lengths.
The column was installed in another crude unit but is now non-operative due to foaming problems and black distillates products.

Our client requires the installation of preflash drum upstream the column to minimize foaming risks.

I would appretiate experiences that leaded to both succesfull or failures in these units.

For example, we proposed interconnecting column and flash drum bottoms to use a single pump for produced liquids.
Is saturated liquid from column bottoms, mixed with hotter flashed crude in this pipe, going to cause problems due to vaporization?

Thanks

 

[25362]

I have some questions which may help you in finding answers to your queries by yourself.

[♣] Have you identified the causes of foam formation ? Such as excessive caustic addition to the desalter, water emulsions, solid particles, wrong temperatures, injected slops containing surfactants, etc.

[♣] At what point in the preheat train will the preflash drum be located ? Will it be "floating" on the fractionator's pressure ?

 

[0707]

In our case, after preheating in some exchangers, crude temperature will reach values of 175ºC approximately. At this temperature, crude is decompressed of more or less 17,5 bar to approximately 0,7 bar, in the preflash Drum, where occurs a partial vaporization. The gaseous phase flows directly to distillation column and the liquid phase is sent through pumps and through the exchangers (with atmospheric residue) to the heaters to raise the temperature for the convenient value of about 375º C. At this temperature the crude feed enters the distillation column.

To avoid foaming we use antifoam additives because the flash drum by itself doesn’t avoid foam it only lowers the pressure, promotes a partial vaporization and reduces heaters coke formation.

luis

 

[fdomin]

25362,

Our client found that foam formation is coincident with high solids quantities in crude.

The drum will be located at the end of the preheat train, with only one heat exchanger between the fired heater and the drum.

Drum pressure will be floating on the preflash column that will be installed along with the drum. This column will have its own overhead condensation & reflux system to obtain in spec ligth naphta.

What are recommended vapor velocities and liquid residence time in the drum?

 

[25362]

For vapor velocities in vertical drums you may find interesting discussions in thread124-10444, and thread391-95730 (for steam). For bottom holdup residence times, see thread124-159905.

 

[mbeychok]

fdomin:

You may find it useful to read faq798-1153 on how to design vapor-liquid separation drums (or knock-out drums or flash drums).

Milton Beychok
(Visit me at www.air-dispersion.com)
.

 

[fdomin]

Thanks for your answers.
Even I designed several vapor-liquid separators I want to know if there are particular tips for crude flash drums, specially when foaming can occur.
Our customer is very worried and remains skeptical, even when we are designing an split flow drum with typical knock out drums vapour velocities (K=0.157), and installing chevron-type demister (vane-pack) with 2" blades spacing

For example I´ve seen articles that recommends using 50% of the above mentioned velocity, thus resulting in a very large drum.

 

[25362]

My thoughts: I see that elimination of foam causes aren't considered. Why, then, not proceed, as 0707 indicated, with the use of antifoam chemicals, usually in a ~1 ppm dose.

Frequently the operating conditions of preflash drums are above the boiling point of water. If, for example, the conditions are 3.5 bara and 150^oC, water would boil. Any shot of water will cause the oil to foam, darkening the condensed light naphtha vapors.

Thinking laterally, since the rate of foam dissipation is roughly proportional to the liquid surface area, a tangencial entrance making the liquid to swirl around the curved wall with directional fins, until it settles on the bottom may provide additional disengaging surface to speed up the rate of foam dissipation. Similar solutions are being applied with horizontal KO vessels.