Pressure control for Net Zero vapor generation system 1

[reena1957]

Hi, I have to choose a Pressure control for Net Zero vapor generation system with no light components - sort of kerosine splitter. I have the "Steady under pressure - pressure control techniques" by Sloley giving 26 different methods with plus and minus for each scheme. But I would like to know what has worked for you guys in such cases? I do not have abundant fuel gas or inert gas to go for split range presuressurizing and venting systems. Thanks

 

[sshep]

The hot vapor bypass of variable area condenser (elevated) is pretty reliable, but the dynamic performance can be slow. If you go this route I recommend installing instrumentation on the exchanger level. This will allow you to know when you are about to run out of area and otherwise diagnose pressure problems. We have at least 20 of these at this site, but unfortunately not all have the level indication that I suggest. I don't remember all the flavors Sloley mentions, but the one with a control valve at the outlet (drain) of the condenser is the most reliable.

In the reflux drum you cannot count on the "thin crust" of saturated liquid above the bulk subcooled liquid to keep your hot vapor bypass size down. The hazard of trusting to that non-equilibrium phenomenon is that you may find your hot vapor bypass is too small, or that a small level or flow upset will knock down the tower pressure. A few years ago I worked start-up on plant with a tower that used a 2" hot vapor bypass. Last visit I saw that it had been changed out to 8".

You will need a vent in any case for start-up- you should automate this with a valve (HIC if not a PIC). In spite of what Andrew says, I have seen many towers running as a net vapor generator with only the disolved N2 in the feed coming from storage (first tower in train only). You can't really count on this, but if it works then take advantage of it by using your vent for some nice pressure control of the reflux drum.

best wishes,
sshep

 

[reena1957]

Thanks sshep. Is the hot vapor bypass having a control valve apart from the one on the condenser liquid outlet OR is it a pressure balancing line? Which one: the pressure of column OR that of the vessel, is controlled?
Thanks once again

 

[sshep]

reena,

These installations have a control valve in the condenser outlet which controls the pressure via liquid level which (in my case) is unmeasured, but which I suggest you try to measure if you are building new. By measuring you will be able to know when you are almost out of condensing area, and perhaps improve the dynamic performance.

The hot vapor bypass takes off the overhead line upstream of the condenser and discharges to the reflux drum. It has a second control valve (in parallel to the condenser and its control valve)which controls the dP between the tower and the reflux drum.

All of these installations work well for maintaining the pressure, but the dynamics are unfortunately slow due to the variable area and large liquid hold-up. Of all the hot vapor bypass installations I have seen, this configuration seems to be the most fool-proof.

best wishes,
sshep

 

[OCR]

Hi sshep,

It seems that you have a good experience of this type of column.
I'm searching for a control scheme for a naphtha splitter working with no vapor at the outlet of the condenser.
I have gone through Sloley methods and also Chin paper but in my scheme I use an air condenser with a large subcooling of the overhead and it seems that its not as efficient as with a water cooler.
Presently I have a scheme with pressure control valve at the inlet of the air condenser and a pressure differential control valve on the hot bypass.
System which shall work but they are some details which are not always mentionned.
Do I need a special inlet in the drum (i.e. a deep leg in order to have the subcooled liquid entry in the bottom of the drum)
If I have a deep leg I may require an equilibrium line between the drum and the outlet of the aircooler?
Do I need a goose neck at the outlet of the air cooler in order to be sure to have a certain number of row flooded?

Thanks