4 replies to this topic

I am working on onshore treatment project located in Canada and I am working with a consultant having 25 years of experience.

Process description of the initial treatment is very simple. We have hydrocarbon vapors (containing light, heavy hydrocarbon, CO2, H2S, steam) condensed (from 265 Deg F to 140 Deg F, pressure @60 psia) in air cooler (Approx. 45 MMBtu/hr). Resulting condensate and gas is separated in 3 phase separator. Oil and water is transported for further processing i.e. stabilization. Gas is compressed in 3 stage compressor and transported for further processing.

Project is located in extremely cold climate (-50 F in winter time) and there is lot of turn down of incoming vapor.

Consultant is concerned of possible freezing during the winter time in the air cooler especially when there is major turn down and he is in favor of indirect cooling. But Indirect cooling increases the cost of project and this is an optimization project. Client is also concerned of cost of project.

Question:
1. Which is preferable method of cooling? Direct air cooling or indirect cooling with antifreeze (glycol/water mixture) and then anrifreeze/air cooling. and Why?

2. What are the methods of control during the winter time to avoid the freezing?

3. Is anyone has experience of operating air cooler in cold climate?

I have suggested possible solution such as variable speed fan drive, louver control, use of space heater.

Thanks,

Suren

[Andrei]

My feeling is that your consultant may have 25 years experience overall but not so much, if any, in Canada.
There is a procedure called "winterization" of the air coolers to prevent freezing. A qualified consultant would know what's all about. Definitely a canadian consultant would know.
The decision of using air coolers versus glycol cooling is something that the client should figure out during the feasibility studies phase of the project. Even if you use glycol the circulated solution cooling is done also by air-cooling, so winterization is required on that one. It's a matter of cost / reliability analysis that can indicate what is the best solution: glycol versus air-cooling. Each individual project has it's own particulars and there is no general rule to tell what cooling solution is better.
There is one aspect that net differentiate the two solutions: glycol cooling allow heat integration, air-cooling not. So, if you think about recovering some heat in some part of your plant and use it somewhere else than a glycol solution may be suitable. That can give you significant operating costs advantages, upsetting sometimes higher capital costs.

Regards

[djack77494]

Suren,
Air cooling is quite common, even in severe cold weather climates. With concerns about potential overcooling, several approaches are common. These include the ones you mention. In Alaska, where conditions are likely similar, many air coolers are configured for air recirculation. The air that actually flows around the tubes is a mixture of fresh and recirculating air blended so that good (but not excessive) cooling is obtained. In your case, you have a good margin between your cooler outlet temperature (140F) and freezing, so I wouldn't think you'll have a problem. If you have steam available, add a steam coil to the air cooler to permit heating of the air prior to cooling. (Sounds kind of crazy, doesn't it? Still, it's a common practice.)
Good luck,
Doug

[dynetix]

There are MANY examples of aerial coolers in applications in which water is condensed operating in winter conditions here in Canada where the temperatures reach the extremes you mention. Standard practice is to fix the aerial cooler not only with louvre control but also with recirculating louvre controls to allow recirculation of the cooler discharge air to the air inlet side of the cooler. In addition, the heating coil option allows for maintenance of temperatures above freezing during shutdown periods. Coils and header boxes should, of course, be designed to facilitate discharge of water from the cooler and adequate drains should be provided.

Thank you very much for all your replies. I am more confident to justify direct cooling approach to my consultant.

Also I have found today an API RP 632 "Winterization of Air-cooled Heat Exchangers". I glanced through this API and I think it has information regarding winterization as well as economic comparision between direct and indirect cooling.

Again Thanks a lot.

Suren