6 replies to this topic

[Genii]

Hi all,

What will happen if the reflux drum is installed little high (say 1.5ft) than the condenser?

As the gas comes in the air cooled condenser it will condense in the cooler and accumulate in the tubes of the condenser. Now for this mixture to flow to the reflux drum there should some pressure to overcome that liquid static pressure head. This accumulation will rise the tower top pressure.

Also that accumulated liquid will lessen the heat transfer area for the incomming gas.

With this situation can the tower work?

Regards

[joerd]

This can work, but as you rightfully observe, the pressure in the tower will be higher than in the reflux drum, to be able to push the condensed liquid up into the reflux drum. You may want to read an excellent paper by Andrew Sloley on pressure control which describes this method (among others). It can be found at http://www.distillat...cal/p081_01.htm
Be aware that the system will be sensitive to operating changes, in particular turndown, when the flow rates are low, and you (theoretically at least) could get slugging in the condenser liquid line. It is worthwhile to do hydraulic calculations for all situations and see how the pressure profile works out. Also, discuss it with the person doing the design of the air cooler.

[Genii]

Dear joerd,

For your Information.

We have a unique pressure and level controlling philosophy, sketch can be found as attachment.
Liquid level is maintained by control valve located on the vapor outlet line of the reflux drum and pressure is controlled by control valve located on the product line(Liquid Outlet).

If pressure increases PC(located on the column top vapor line) will open the PCV and liquid level will fall down, the LC (located on the liquid level) will open the LCV and release the pressure by releasing the vapors.

If the level rises the LC will close the LCV which results in increase in pressure which will open the PCV, so that level can be maintained.

Regards

Attached Files

•  Unique_Scheme.GIF   3.57KB   106 downloads

[sgkim]

Hi Genii,

It seems very very unique. But I think the control system you described and showed in your sketch will not work at all. Neither pressure nor level can be controlled easily: at high level the LC opens LCV then the pressure drops and at high pressure the PC opens PCV but the level drops. No one knows what controls what !

Do you have any particular reason why you want to control pressure with liquid flow and level with vapor flow?

Should'nt it be corrected as the attached, which is not unique but quite typical ?

Stefano

Attached Files

•  PCV_LCV.doc   26KB   94 downloads

[Genii]

Hi Stefano,

I will find the particular reason for adopting this scheme but this scheme is working and operational.

However a little correction is required: On High level LC will close the LCV which will result in increase in pressure. With this increase in pressure PC will open the PCV, hence lowering the level.


Regards

[sgkim]

Genii:

(1) Remember that the volume of vapor is several hundred or thousand times (say, 1500 times) that of liquid according to the pressure and temperature. Therefore, if the column pressure increse by ΔP, the amount of vapor equal to ΔP multiplied by the system volume shall be removed thru vapor(not liquid) discharge line or be condensed additionally in the condenser. Such a large volume cannot be removed by liquid only in a short time. Or, the pressure will accumualte until the safety valve(PSV) actuates at the set point for relieving the vapor from the system to waste.

But your pressure control system, preseltly, is to discharge liquid when pressure increases. I am sure no one will expect it ever work.

(2) The same problem exists in your system configuration for level control. If liquid level increase by ΔV, the LCV will have to discharge vapor by about 1500 times of ΔV, which will give too much vapor loss along with a great pressure drop in system pressure. To much sacrifice even by a small incremental liquid volume change with dramatic pressure and temperature fluctuations !! Who will be sure your distillation system ever work !!

Stefano

[Nirav]

hello,

I read the scheme (sketch) explained by Genii.

But I agree with what Stefano has replied.

In fact, we had similar kind of scheme in one of our EPC projects which we supplied. A solvent recovery tower pressure was controlled by the liquid control valve. And it just did NOT work. There were continuous pressure swings and it was not stable at all. Therefore, it got modified afterwards.

Following was observed during initial run of the plant.

As soon as pressure increases above set point in "vapor" overhead line of tower, the "liquid" control valve will open. It requires large amount of liquid be removed in order to decrease the pressure. But pressure goes on rising till the time large amount of liquid is removed. So, pressure profile goes UP with time.
Now, after about 90 to 100 seconds, as liquid is being removed, a point comes when a pressure suddenly starts decreasing "quickly". [something like vacuum effect when you remove liquid from close system].
Remember, this happens because we control pressure in "vapor" region by removing "liquid". As soon as pressure goes below set point, valve starts closing to maintain pressure, but again, it takes time and pressure starts going below set point. Even after lot of tuning of control valve, this did not got stable.

The above cycle of pressure UP & DOWN kept on repeating every 3 minutes.

This is particulary true if line is not "free draining" from condenser outlet to drum and elevation of liquid control valve is higher than the drum. (This is what Genii is asking in his first quote by providing "pocket" in the overhead line from condenser to drum).

In nut shell, pressure control of system should be avoided by a liquid control valve EXCEPT for "liquid tight" systems.

Any more comments ??