4 replies to this topic

[vinod]

Sir


Situation:
A side draw stream is taken form a distillation column and was being earlier used as a fuel for a fired heater, but now the client has decided to remove the fired heater, so he need s to send this stream to another vessel .The flow is by gravity. And it is proposed to have an on/off valve which will open and dump the side stream for 10 minutes in an hour.

My question?
When the on/off valve opens and dumps the fluid to the vessel, the pressure drop across the on/off valve is a minimum since on/off valves are generally like full open pipes, either close or open. Isn’t it?


Regards

Vinod

[Zauberberg]

Vinod,

It's hard to imagine that you can draw an intermediate stream from distillation tower and control it only by means of on/off valve. I believe it's practically impossible, and probably will cause frequent upsets of the tower itself. I also believe that main problem is not in the pressure drop through the valve, but rather in the philosophy of taking the intermittent draw-off stream from the column which is in continuous operation mode. What kind of side-stream is the one which requires "10 minutes of dumping within one hour"?

You should provide a sketch of the system, and give some background as well. What is the logic of operating on/off valve?

[djack77494]

What you're describing is very unusual. Normally it would be best to measure the level in a drawoff portion of the tray and modulate a level control valve to maintain a continuous flow of side product from the column. The only time I've seen things done any differently is for a very small scale operation (e.g. pilot plant) when your situation might be applied.

[Qalander (Chem)]

Sir


Situation:
A side draw stream is taken form a distillation column and was being earlier used as a fuel for a fired heater, but now the client has decided to remove the fired heater, so he need s to send this stream to another vessel .The flow is by gravity. And it is proposed to have an on/off valve which will open and dump the side stream for 10 minutes in an hour.

My question?
When the on/off valve opens and dumps the fluid to the vessel, the pressure drop across the on/off valve is a minimum since on/off valves are generally like full open pipes, either close or open. Isn’t it?


Regards

Vinod

Dear Vinod,
Although you have got some really good/relevant responses to your query.

I feel like adding slightly varied concept.

Actually at times level trolls and LCV controls are employed (though not very common) for situations with very low side stream quantities or where level increase may disturb process.This in a way works similar to On and Off system
Best Regards
Qalander

[JoeWong]

Yes. Agree with Zauberberg with this statement. Without viewing your scheme , it is hard to imagine. The scheme could be

- A level control loop with level control valve, provide additional on-off valve downstream of this control valve where operate it manually OR

- a level control loop control the on-off valve where OPEN on HIGH and CLOSE on LOW level...

- no level control but on-off control manually (DANGEROUS !)

These types of operation may subject to production campaign.

Critical factors you have to consider are
- is the system balance with side draw?
- is the system balance without side draw?
- is the system stabil during the transition from/to side draw operation ?

First and second points may be easy to understand from steay state simulation. But third point may require dynamic simulation for checking.