Trim Coolers-- How to determine the requirement?

I believe the reason is for easier column pressure control for a refluxed tower stable operation, considering the prevailing weather conditions.

Most straight-run towers, operate with total condensing of overheads.
Reformer stabilizers, which by being fed from flash drums, may contain 1% or 2% equilibriun quantities of hydrogen and methane, cannot be total condensers. The same with de-ethanizers.

The bubble point pressure of the distillate product in the reflux drum depends on the reflux drum temperature. Its pressure will set the tower pressure profile. The dew point temperature of the distillate product at the tower top pressure will be the tower-top temperature.

A finfan condenser (sometimes used in areas with small ambient-temperature changes) might possibly suffer large transient upsets, such as when exposed to a sudden rainstorm, even large day-night temperature variations in some places may unfavourably affect fractionator pressure control.

Water-cooled condensers, on the other hand, have a narrower range of temperature variation and are preferred for this reason.

Is this answer satisfying ?

As usual, 25362 gave you a well-thought out response. Another, much simpler reason, might possibly be:

Many refineries have had many revamps and many expansions at various times by various process designers after their original construction. One process designer may have concluded that trim coolers were either economically benefical or needed for some other reason. Other process designers may have thought they were not economically beneficial or had some other reason for not using them. After all, there is no standarized "cookie cutter" way of designing refinery units.

Milton Beychok
(Visit me at www.air-dispersion.com)
.

Milton Beychok! Now there is a name I remember from my days at Fluor, Irvine.
Milt and some of his collogues taught me (and many others) some of the facts about process. This informal teaching included, for me where Trim Coolers are used (required). I was taught that Trim Coolers are used in plants where there are unfavorable atmospheric conditions (high temperature and low humidity) and the shortage of usable cooling water. This condition typically exists in most countries in the Middle East.
The Air cooler is used to bring the temperature of the commodity down as far as possible in the hot summer months. The commodity is then run through the water cooled shell and tube Trim Cooler to lower the product temperature to the proper point.
The cooling water for the Trim Cooler was supplied from a closed loop system and was sometimes run through a chiller section to achieve the optimum temperature.
A retired piping designer

With all due respects the question -as I understood chillboy- wasn't why are there trim coolers, but why these aren't used on straight-run naphtha stabilizers.

"Why we have condensers (trim coolers) with cooling water after air fin coolers in Crude overhead, Pre-Fractinator overhead, Reformer reactor effluent cooling system, HDT systems reactor effluent cooling system and why do not we have the trim cooler but only condensers in Naphtha stabilizer overhead?"

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25362, the above is a direct cut and paste quote from Chillboy's question. It reads to me as if he is very plainly asking "Why are trim coolers used in some places (crude tower, pre-fractionator, Reformer ...) but not used here(naphtha stabilizer)?"

Milt Beychok

I may be wrong in my interpretation. English is not my mother tongue.

However..., Milton, you quoted only the first part of chillboy's posting which, as I see it, is more a rhetoric statement of facts than a question.

The following paragraph, which you omitted, clarifies and summarizes -to my grasping- the leitmotif:

I am not able to figure out why these trim coolers are not designed for LPG condensation...I would like to know the details of the Why question, the design basis.

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Gents
Our NHT stabiliser doesnt have a trim cooler (and I am working in the ME). Looking on the amount of LPG in the Overhead these are fairly low, so for recovering LPG a trim cooler wouldn't add that much to the LPG production. We can in wintertime produce some smaller amounts of LPG out of the NHT stabiliser but the problem is that C5 content is difficult to control. I guess if properly designed this can be done on a large NHT and a trim cooler could pay off. For smaller NHT's this might not be optimal. If the feed contains a lot of LPG for some reason it might be beneficial to recover this out of the NHT stabiliser if it isnt economical to do this upstream the NHT.
Another type I have encountered is to recompress the offgas and send it to the LPG plant, seen this on a combined NHT/KHT plant. This solution recovered the LPG in the gasphase and "substituted" the trim cooler. Also this was in a cold climate condition and on a 30 year old plant where the feed to the NHT/KHT contained a lot of LPG.
Roger

25262, I can see that you and I have a different perception of Chillboy's question.

Be that as it may, my main point is that I don't think there is any scientific or technical reason why any fractionation tower overhead system should or should not include a trim cooler. It is simply a matter of how the original designer of the system perceived the cost-benefit economics of using a trim cooler. It has nothing to do with the tower being or not being a naphtha stabilizer or any other type of fractionating tower. Nor does it have anything to do with the tower overhead being LPG or any other hydrocarbon stream. I repeat that it is simply a matter of how the designer perceived the economics of the system ... or perhaps even failed to consider the economics.

Milton Beychok
(Visit me at www.air-dispersion.com)
.

I pressume a good design would have to consider:

[→] the initial and final temperatures of the process fluids, air and water,
[→] water quality and availability,
[→] proper control to achieve stability and safety of the operation,
[→] doing an economical examination including first costs as well as operating and maintenance expenses.

One more note. Although for close control of condensate outlet temperature water coolers follow air coolers, the reverse has been chosen in ethylene primary fractionators, for heat transfer optimization.

Hello All!
Let me first express my thanks to all who took time to reply and discuss. As I was travelling, I could not reply.

My question is we have AFC and trim coolers in our crude fractionators, and other similar column overhead systems. But we have two Naphtha Stabilizers, which are not having AFCs and trim coolers but a set of cooling water condensers. I thought there could be a reason for not having them in LPG condensing systems.

But because of 25362, rogerH and pennpiper and mbeychok discussions, we come to know many things that we did not know. Thanks for all and let me come back with more details, if you want any specific info, reg this question, pls reply.
thanks for everything,
cheers
atm