10 replies to this topic

[Nikolay_]

Hello,

 

I have to design separator of crude preheat train. It’s intended to separate part of vaporized flow and reduce inlet pressure of fired heater. Vaporized flow is added to bottom stream of separator and follows to main column.

Does anybody have an experience in operating of separator before fired heater?

 

Kind regards,

 

Nikolai

[Chemitofreak]

I am unable to comprehend. Please attach a diagram for ease.

 

Inconvenience regretted. 

[pavanayi]

Nikolai,

Did you have a pre-flash drum in mind? It is normally found between desalter and furnace. The preheated crude is flashed and the vapour goes directly into the atmospheric column. The liquid part from the separator goes through a few more exchangers, and then into the furnace?

 

As mentioned above, a quick sketch will enable everyone to understand easily.

[Nikolay_]

The sketch is attached.  In addition, client requires to mix vapor flow with liquid flow before fired heater, because he wants to save hydrodynamics inside column.

[Nikolay_]

 sketch jpg.jpg   34.22KB   3 downloads

[rdcrags]

1. A horizontal drum is usually used in this service. The usual criteria are used for sizing.

2. The pump should not be needed with the pump at the desalter outlet.

3. Recombining the vapor and liquid is unusual but will work with proper line sizing.

[Nikolay_]

I apologize. It is required to mix vapor flow with heated liquid flow before column (after fired heater).

[Chemitofreak]

Based on my experience about the Crude Distillation Unit, the crude it pre-heated, desalted, then again heated (in heat-ex or fired heater) before it is routed to the distillation column for separation.

 

Since, the crude is desalted, the pressure of the system is above the vapour pressure of the liquid by at least 2 bar i.e. there is no chance of two phase till the outlet of the desalter.

 

I would rather have liquid crude to the maximum extent possible till it is feed to the column, simply because it is easy to carry out single phase liquid hydraulics. Also, the piping support required for the single phase is less stringent than the two phase flow.

 

Now coming to your point, I would rather pump the crude through the heater in liquid phase instead of separating the gas-liquid, then heating the liquid and again mixing it with the gas.

 

Is your system already existing one, or you are preparing FEED for the system ?

 

Maybe a little more information would be helpful.

[pavanayi]

Nikolai,

A pre-flash drum like you showed is common in crude pre-heat trains. What exact help are you looking for? A few pointers:

1. Sizing of the drum is like any other separator.

2. The removal of lighters in the pre-flash drum results in reduced flashing in the liquid line control valve upstream of the furnace

3. Normally there are heat exchangers between the pump discharge and the furnace u/s control valve. Absence of lighters will mean a risk of 2-phase flow will be avoided when exchangers foul up and pressure drops increase.

4. Mixing the light vapours from the top of the pre-flash drum into the furnace effluent is not very frequent. I have only seen one such installation. A variant is that the vapour is directly introduced into the flash zone (and not into the pipeline)

5. The pressure of the pre-flash drum has to be selected appropriately. It should be enough to take off the lighters, but not too low (see 6)

6. Since the mass flow through the furnace is reduced, outlet temperature has to be increased to compensate for the loss of energy input into the atmospheric column so that the overall energy input into the column remains the same. There may be a limit on what this can be. If not properly accounted for, this can affect the yield from the column.

[Nikolay_]

Thank you for answers.

Yes, we are preparing FEED now. At present, we have done calculations of exchangers before the separator (pre-flash drum). Now we need to size pre-flash drum then to do checking calculations of fired heater. Which internals must be installed into drum, preferred orientation (vertical or horizontal)? May someone have sketch of same existing drum or literature about it?

[Nikolay_]

 

Pavanayi, Chemitofreak, rdcrags, thank you so much!

It was very important to receive feedback from you.

I like Cheresourses and hope the quantity it's fans will continue grow.

I have posted this thread to hear about practice of operating of similar type of separator in plant.

If someone can give detail description an existing separator (preflash drum) please write me.

Thanks again.