Apex:
Who designed and built your TEG unit? Why don’t you correct some errors in your control room display? TI 1209 = 69 oC? FIC 1208 = 0.3 m3/h? PI 1219 = 8.4 kg/cm2g? TI 1220 = 58 oC? FI 1202 = 10 Nm3/h? Are you certain that you personally checked the accuracy of this data? I don’t believe that it is accurate. Allow me to explain ....
You now explain that you are reducing the water dew point (or water content) of the feed gas going into the TEG contactor by pre-chilling it to -5 oC. You fail to explain how or where you heat up the feed gas to 36.6 oC prior to its entrance into the TEG contactor, but I have to assume that you do. This feature allows you to reduce the amount of water content in the feed gas before subjecting it to TEG absorption. It also allows you to cool down the lean TEG entering the contactor to 15 oC and obtain better and more efficient dehydration. Why this is not done I don’t understand. That is why I ask who designed this system. Or could it be that the pre-chilling is an after-thought and a process modification AFTER the unit was installed?
The temperature of the inlet gas into the contactor will dictate the total amount of water fed to the unit – if the gas is saturated with water (which is the normal case). However, in your case, this is not happening. You are feeding un-saturated gas. That is why I specifically noted the term “saturated” in my flow diagram. Look again at my diagram. Yet you say that my flow diagram & data is accurate. That is not so.
You are operating your reboiler at 191.5 oC (376 oF) and using stripping gas from your flash drum. At this reboiler TEG temperature, the very best lean TEG purity you can obtain (without stripping gas) is 98.5%. And that is at atmospheric pressure. You haven’t given us the reboiler pressure and I have to assume it is at least 5 psig – or perhaps more. Any reboiler pressure increase over atmospheric reduces this TEG purity value substantially. I would estimate your reboiler TEG purity to be 98% or less. Using steam to heat TEG reboilers is not normal – and this application shows why this is so. Normal operations call for heating the reboiler at 400 oF and atmospheric pressure to obtain about 98.4 to 98.7 wt% TEG.
Therefore, you need to apply stripping gas to the 98% TEG in order to have a decent 99.5 or better purity at the TEG contactor. But your process shows that you are using flash gas to strip – and not dry product gas. In order to reduce the TEG water content it is important to use DRY stripping gas. Your flash gas is a result of flashing wet TEG – which means the resultant flash gas is not dry. This makes your stripping step very ineffective. That is why I ask what I ask.
You normally would use 3 standard ft3 per gal of TEG circulated to bring the TEG purity up to 99.5. If you are circulating 5.9 m3/h (26 gpm) of TEG, then I calculate you should be using approximately 125 Nm3/h of dry stripping gas. You report a stripping gas rate of only 10 Nm3/h. This indicates you are not regenerating your TEG sufficiently to meet pipeline specifications – which you haven’t identified.
I can’t address why your simulation doesn’t agree with your results. We haven’t been given a copy of your simulation inputs and outputs and we don’t know how you ran it. I also can’t explain why you don’t have hydrate problems. We don’t know the temperature and pressures you are operating at. You also don’t identify the basis of your dewpoint. Is it atmospheric dewpoint, or is it the process pressure dewpoint?
All in all, this analysis can’t be specific because we lack all the required basic data on this application. Your pre-chilling may be doing more benefit in removing water than the TEG unit. See the attached dewpoint curves to see where Bobby Strain estimates the expected dew point. Although I am unable to answer your direct concerns, I hope that some of the details I have outlined help you understand what is happening in your described process.
Equilibrium Water Dewpoint in TEG Contactor.pdf 1.03MB
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