7 replies to this topic

[mjd]

Hi all,Can anybody help me in find out how the stripping gas application increases the TEG concentration during regeneration?How this works?mjd

[JoeWong]

mjd,

Stripping gas helps by lowering the partial pressure of water in the vapor space, making it easier for water to travel from the liquid phase to vapor phase. Thus, help to increase TEG concentration during regeneration.

JoeWong

[Art Montemayor]

MJD:

You have been the recipient of what is probably one of the most concise, articulate, and accurate definitions of how TEG concentration is increased above approx. 99.5% wt. via the use of dry stripping gas in the reboiler section. Joe Wong certainly deserves Kudos for his explicit and insightful explanation. The answers to "tough" and complex problems always seem to go back to simple basics - such as partial pressures.

All I can possibly add to this great response is that efficient intimate contact within a packed bed is of importance from the practical point. The key is partial pressure; the important aspect is knowling how to apply the effect efficiently, safely, and simply. Simply injecting the dry gas into the reboiler liquid TEG is one way; but it is very inefficient. A separate, packed tower immediately downstream of the exiting Lean TEG out of the Reboiler is the most effective way I have used to bring about the desired reconcentration of the TEG.

Joe:

Excellent knowledge of a Unit Operation is one thing; the ability to explain it succinctly and expertly is quite another. Good Show!

[JoeWong]

Mr. Montemayor,
Thanks for your compliment. Appreciate.

Your statement have given this forum a life and light to young people...I would like to take this opportunity to encourage everyone do the same.

Thanks again.


JoeWong

[mjd]

Thank u to Joe & Mr. montemayor

[andiez]

Dear all,

Im having this problem here with my TEG regeneration which i hope to find some solutions.

First of all, I am having huge teg loss from the regenerator which only give me a TEG concentration of less than 98%. i am using a reboiler temperature of 204 °C with reboiler pressure and vent back pressure of around 105kpag. At this pressure, lots of TEG is lost to the vent hence lower purity. I would like to know what is usually the normal operating pressure of the reboiler and vent outlet? Is 105kPag too high?

Secondly, i am trying to simulate a packed column contactor (Sulzer Mellapak 250 Y) using HYSYS. Is there a way to simulate this in using HYSYS steady state simulation? Based on the height of the packing, is there a way to calculate the number of trays and efficiency to be used in the simulation?

Thanks a lot,
Andiez
Trainee Process Engineer

[Art Montemayor]

Andiez:

Please do not resort to “skyjacking” a thread. You have raised a good query and we are all interested in helping you resolve your problem. However, out of respect and courtesy to mjd you should start your own thread in this Forum. This will allow us all to focus on your query and problem with more direct and specific help. If you start a new thread based on your specific problem, I will transfer my current remarks there to keep all information relative to your query in one, specific place.

Your problem description is very confusing. You don’t supply all the required basic data to make a full analysis of what you are doing. 105 kpag (15.3 psig) is the highest pressure I’ve heard about in a TEG regen system. Why are you sustaining such a large back pressure on your TEG reboiler + stripper? Your TEG temperature is at the max. level you can justify without “cooking” the glycol – although, you might have cooked already and this might explain your high TEG losses. Let’s look at the points you raise, one-by-one:

“First of all, I am having huge teg loss from the regenerator which only give me a TEG concentration of less than 98%.”
The glycol losses have nothing to do with your inability to regenerate the TEG. This is confusing the way it is presented. Do you have any idea why your losses are high? Please define what you call “high” and where, specifically, the losses are located. We need to be specific in order to identify the magnitude of the problem and pick out our areas of attack. What is specific is the degree of rich TEG concentration. But since you fail to tell us basic data such as the flow rates, temperatures, pressures, and design product gas water content, we can’t really tell anything from the 98% glycol value. You can still dry natural gas (if indeed it is natural gas you are drying – you haven’t stated the gas being dried) even with 98% TEG. However, the product gas may not meet pipeline specifications – whatever those are in your area.

“I am using a reboiler temperature of 204 °C (399 ^oF) with reboiler pressure and vent back pressure of around 105 kpag (15.229 psig). At this pressure, lots of TEG is lost to the vent hence lower purity.”
As I’ve mentioned, this is a very high pressure to sustain on the TEG regen system. Usually, we try to keep the reboiler pressure as low as we can in order to ensure full TEG regeneration. A design pressure of approximately 0.5 psig (3.5 kPag) is usual. Is the 105 kPag a design value or is it a result of recent problems or modifications? Again, lack of details and basic information keeps us from making positive comments. Your assertion of TEG losses being caused by the pressure are simply not factual and I strongly doubt that it is the truth of what is occurring. A higher system pressure, of itself, doesn’t cause high liquid losses. And high TEG losses have little or nothing to do with TEG purity results. The two values are not related in the case of a TEG regen system. If you have a strong reason for making those assertions, you should have evidence or facts to back you up.

“I would like to know what is usually the normal operating pressure of the reboiler and vent outlet? Is 105 kPag too high?”
As I mentioned, this back pressure is much too high compared to normal design standards. Only you know why this high pressure is being applied or tolerated – but you haven’t told us why. Unless you have a strong need or reason for maintaining such a high pressure on the reboiler, I certainly would not do it. The goal for good and complete TEG regeneration is to try to keep the reboiler temperature and pressure as low as possible while producing the design concentration of TEG. Some people even resort to partial vacuum to keep the reboiler temperature lower than 400 ^oF.

“Secondly, I am trying to simulate a packed column contactor (Sulzer Mellapak 250 Y) using HYSYS. Is there a way to simulate this in using HYSYS steady state simulation? Based on the height of the packing, is there a way to calculate the number of trays and efficiency to be used in the simulation?”
First you describe a problem that should not be tolerated and is probably causing you grief, TEG loss, revenue losses, and out of specification product gas; then you describe as part of your solution, a simulation exercise. Please explain why you are doing basic process design when the existing, designed, and installed equipment is not functioning according to your needs and specifications (I think). Are you trying to say that you have come to the conclusion that the original design is inadequate or flawed? Are you now embarked on designing another regeneration system? This is very confusing. Normally, I would not reach the re-design stage until I had exhausted all efforts to troubleshoot the existing unit. Here, I have to assume that the original unit was properly designed, installed, and is being operated in the prescribed manner. Please furnish us with all the details and the rest of the story. I can tell you from first-hand, practical design and operating experience that you normally wouldn’t have to worry about the quantity of theoretical trays and their efficiency. Hell, a TEG regen unit is usually designed by experience anyway. The quantity of packing or trays in a TEG stripper vary between between 5 to 8 ft in height – something that is so trivial and insignificant that one doesn’t bother to run an expensive and totally useless (in my opinion) HySys simulation. As a Trainee Process Engineer I believe your time is better spent in troubleshooting the regen system. Do the normal, expected troubleshooting steps before arriving at a situation such as you are defining now. Your production unit is putting out bad product and you are losing expensive TEG. You must decide whether to remedy the situation or to replace the existing equipment. Have you arrived at the latter decision without troubleshooting?

As an example of what I think you should be doing, please refer to my comments in a previous thread in this Forum, titled “Reference Web Site - troubleshooting of all kind of system failure”. Here, you will find some specific information where to get or read about troubleshooting your TEG unit. I strongly recommend you get a copy of “Troubleshooting Natural Gas Processing – Wellhead to Transmission”; (PennWell) by Norm Lieberman. In the chapter on TEG dehydration systems you will get a complete description of what you are up against and how to troubleshoot and operate such a unit.

I hope this helps you out.

[andiez]

sorry for the inconvenient. i will repost this question again under process simulation section. thanks