PHE vs shell&tube in lean/rich interchanger 1

[zzznook]

Hi everyone.
Im calculating on a lean/rich interchanger for an gas sweetening system. What is done if a shell&tube with theta=1 (deltaT/deltaTL) and the desired temperature of 100C into the strippingcolumn is not achieved and the only reachable T is about 80C? Is an extra heater placed just before the strippingcolumn or is the reboiler set to manage the extra duty?

Does anybody know from an operating plant?

 

[Montemayor]

Zzznook:

I have trouble deciphering what you are saying. Your thread title infers something about a Plate Heat Exchanger (PHE) or a Shell & Tube (S&T) unit, but then you mention something about a S&T with a “theta” of 1.0. What is “theta”? For many years I operated, designed, and built Amine plants and I’ve never come across a “theta”. That doesn’t matter; it just shows my ignorance about your nomenclature or acronyms. What is important (as I gather from your other postings in the past) is that you are designing an Amine-based gas sweetening plant (I believe). You haven’t stated what Amine solution you are using (I suspect MDEA – the one in vogue today), the type of reboiler you have (I suspect steam heated as opposed to direct-fired), and the system’s configuration. I have always designed on the traditional amine configuration when using steam as my reboiler heat source and the lack of being able to display a flow diagram on this forum limits me to a verbal description. The Amine solution travels the following circuit:

1) Out of pressurized Amine Absorber bottom as rich solution;
2) Into one or two (in series) Amine Exchangers through the tube side;
3) Into the top of the Amine Stripper (at approx. 180 -200 ^oF) as feed;
4) The rich Amine solution is regenerated to lean solution as it travels down the stripper and into the reboiler where it exits at around 220 – 230 ^oF, depending on the type of Amine and the strength of the solution;
5) The hot, lean Amine solution exits the reboiler and goes directly to the Amine Exchangers’ shell side to pre-heat the rich solution;
6) The cooler lean solution then goes to the lean Amine pump where it gains the pressure to go to an Amine cooler tube side and then on to the top of the Amine Absorber to pick up H₂S and CO₂. The cycle is complete.

It would be great (& thermodynamically impossible) for the rich solution to be heated within 5 ^oF of the lean solution in the heat exchangers. However, this is never the case and the reboiler is designed to make up the sensible heat to take the solution to the set reboiler temperature.

If you are debating the use of PHE exchangers instead of S&T because you are trying to increase the approach temperature of the rich solution, that’s an idea. I and others have tried this before many times. I personally had a lot of troubles with the inevitable PHE gaskets (together with many others) and the tough insulation geometry. However, I’ve heard (& read) new gasket material and design has improved and many are content to live with the leaks (if any). I went on with S&T design and never had problems with stainless steel tubes and weaker Amine solutions and solution reclaimers and carbon filters. All this is a fall out from the constant threat of corrosion from bad or contaminated Amine solution. Everyone dealing with this system has, after a sustained time of operating experience, reached a compromise solution or a favorite manner of resolving the many problems.

I hope this response addresses the main topic or substance of your query and helps you out somewhat.

 

[zzznook]

Thank you Montemayor.
Yes that system is it. The "theta" is the rate between the difference between in and out temperarure and the logarithmic mean temperature. The graeter it is the more narrow the hot inlet and cold outlet can be. A PHE has a higher theta value than a s&t.

 

[rmw]

As a rule of thumb, PHE's always can get a much closer approach than S&T's. In theory, (if you have enough money to buy plates, they can get closer approaches than you can measure with plant instrumentation.

rmw