VENT NON-CONDENSIBLES IN PARTIAL CONDENSER? 4

[ppeng01]

I have a question about a partial condenser. When I am condensing a liquid out of a gas mixture, I always thought there needed to be a gas outlet at the top of the partial condenser and a separate flowpath for the non-condensibles to prevent accumulation and vapor binding of the condenser. I am being told by a more experienced engineer that a liquid outlet at the bottom of the shell is all that is needed. He says the liquid draining out of the condenser to the separator drum will pull the noncondensibles with it, and there will be no accumulation problem. Besides, he argues, there is nowhere else for the noncondensibles to go. I keep seeing a big bubble of noncondensibles in the condenser in my mind's eye, thereby reducing the effective area of the condenser. Is it just me?

 

[Compositepro]

You are correct, a vent is required.

 

[StoneCold]

Let's explore the situation further. What is the application? I have a lot of condensers on batch reactors that do not have a vent for the non condensables. They operate just fine, though if you would have asked me before I had seen them run, I would have said you need a vent. It really depends on the application.

Regards
StoneCold

 

[ash9144]

I'm not sure you have fully explained what exactly what you have. If this is truly a partial condenser then where does the non condensed process vapors go? There would need to be more to your vent system than what you have presented.

Does the sytem operate under vacuum?

 

[ppeng01]

The partial condenser is going to be on an amine regenerator unit. The vapor coming off the regenerator will be water and carbon dioxide. The condensed liquid and vapor will pass from the condenser to a reflux accumulator drum. Most of the water will be condensed and will be used as reflux. The non-condensed CO2 and a little water vapor will pass out of the reflux accumulator drum through a vapor outlet.

 

[ash9144]

You just answered your original question. The vapor outlet on the accumulator drum is the outlet for other noncondensibles as well as CO2.

 

[ppeng01]

I must not be making myself clear. If I have a condenser that feeds into a reflux accumulator drum (that is vented), is it realistic to expect that I can get away with one outlet in the condenser that will convey both the condensed liquid and noncondensible vapor through the outlet piping into the reflux accumulator drum. The liquid and vapor mixture would have to pass through the same line, which would have to be at the bottom of the condenser so as not to cause a liquid build-up in the condenser and block-off the bottom tubes. Also, with the described piping configuration, what prevents the noncondensibles from separating from the condensing liquid and accumulating in the top of the condenser? What prevents them from forming a large bubble and leading to inert blanketing of tubes and resulting in less and less effective heat exchange area? Don't we need a separate vent from the top of the condenser that also leads to the reflux accumulator drum so any noncondesibles will have a natural flowpath (one that will use their buoyancy rather than fight it)?

 

[Montemayor]

ppeng01:

The answer is YES. You must allow for the CO2 to exit the cooler-condenser (as we used to call it) through a nozzle that allows for it to continue on downstream – Preferably without even going through what you call the reflux accumulator drum. I have designed and installed about 7 to 10 of these same, exact CO2 cooler-condensers in the past. I even manufactured 3 of them – together with the condensate accumulator (reflux accumulator drum). I’ve always consolidated my condensed “weak amine solution” that is condensed in the Stripper overheads cooler in such a manner in order to use it as a wash for the gases exiting the top of the Amine absorber tower. This has always helped me to reduce the normal amine losses that are experienced as leaving with the treated gases. I never use a reflux on a CO2 Stripper; a reflux is not needed in this application because it does no help in establishing an equilibrium at the top of the Stripper and it only adds another steam consumption load to the amine reboiler and increases the amount of water vapor going up the Stripper. Most – or all of the CO2 will essentially strip out in the top 2 theoretical stages in the Stripper.

I am attaching a sketch of what I mean and what I think you are doing in order to fully explain to you how the CO2 is cooled and the associated water + trace amine are condensed and separated in a CO2 removal process. I believe you are trying to do the same thing and are having trouble trying to explain it and convince others; I hope this helps to reinforce what you are trying to do.

 

[ppeng01]

Thank you for your help!

 

[CJKruger]

Montemayor, very interesting design. I always assumed you need a reflux and 2-3 trays above the feed to reduce amine losses overhead. But if you have no purge, then maybe it doesn't matter.

ppeng01, I think both design options are acceptable. There are many partial condensers where the liquid and vapour leave through the same pipe at the bottom (to the overhead receiver). On some, but not all, I have seen a small vent line from the top of the condenser to the drum.

For total condensers it is a different story, and I think a vent line is requried to remove noncondensibles from the condenser.

 

[lt5q5p]

A good source for determining proper location of noncondensible lines is "Troubleshooting Refinery Processes" 2nd edition by Norm Lieberman. You would likely see some benefit by adding a non consensible line on your reboiler to remove CO2. However if your are using 35# steam and not 135# you may not have as much of a CO2 issue. Without being able to walk the piping down and just looking at the drawing I don't think that you would have a noncondensible problem anywhere else.

 

[dcasto]

Montemayor, your use of the condensed liquids from the still overhead as a wash on the adsorber has several flaws in most natural gas applications. If you need ultra low CO2 in the exit from the absorber, the CO2 rich wash stream can recontaiminate the exit gas.If the amine is removing H2S, you definately won't meet outlet specification for H2S.

In treating really rich or gas streams with over 15% ethane and heavier, the cooled still overhead can cause cooling and condensation of heavier hydrocarbons which leads to foaming.

In smaller amine plants, we would just pump those condensed still overheads into the bottom of the still column, saves a bundle of opx and equipment if the contactor operates at 1000 psi.