5 replies to this topic

[GasPro]

To all

I would like to know if anyone has experience this problem. I am treating bulk H2S and CO2 removal using MDEA solvent. I am operating below design feed gas temperature which fluctuate with the ambient temperature. The unit operates more stable suring the night as the temperature is low. I am seeing that there is H2S in the product gas however no CO2 is noticed.

With these readings there is high flas gas that is being produced and Amine being carried over to the HC side of the flash drum when the circulation rate approaches 80% of design. Has anyone seen this before? Is there a link between CO2 absorption and flash gas make?

Desperate to know...

Thanks in advance
GasPro

[Zauberberg]

There is too little data for making comments. How much is H2S/CO2 content in the feed gas? What is the acid gas content in the Absorber overhead stream (sweet gas)? What is the amine circulation rate and how much is the rich amine loading, mol/mol? Is this a generic MDEA plant, or you use some kind of formulated solvent?

Furthermore, what do you mean by excessive flash gas? How do you measure it? What is the rich solvent temperature entering flash drum in (both) cases? What is the difference (in exact figures) in treated gas composition, amine circulation rate, amine inlet temperature to Absorber? In which way the amine carryover is correlated to Absorber conditions and lean amine temperature? Do these effect change with amine circulation rate only, or based on CO2 pickup, or any other parameter?

[GasPro]

Thanks for your response to this issue. The feed gas is approx 3 mol % H2S and 4.29 mol% CO2. The sweet gas has 7ppmv H2S and 0ppmv CO2. What I mean by excessive flash gas is design spec is 960 Nm3/hr and I am getting in the area of 1400 Nm3/hr. This flow is measure by a FI on the flash stack.

The amine concentration is 45% with a acid gas loading of 0.019 mol AG/mol MDEA. The lean gas temperature is approx 61 degC (which fluctuate with ambient as well), with feed gas at 44-55 degC range from night to day. At circulation rates at 75% of design ( approx 690 m3/hr) there is carry over of amine to the HC skimming compartment.

There is no basis for associating this carry over with the absorber. I just wanted to get an understanding if with the temperature fluctuating and the complete absorption of CO2 and break through of H2S, if somehow all these things are related. The unit is designed to have a tight H2S spec and a more relax CO2 absorption (H2S to ppmv levels, CO2 to mol%)

Any idea as to why there is a complete consumption of CO2 and not a complete comsuption of H2S considering MDEA has an affinity for H2S over CO2?

[Zauberberg]

I must say I am surprised to hear about H2S slip while CO2 is being removed completely in MDEA-based systems. The reaction of MDEA with H2S is almost instantaneous while the removal of CO2 largely depends on reaction kinetics which is transferred to amine circulation rate, amine temperature, tray residence time etc. How do you check treated gas for H2S/CO2 content? Only by process analyzers, or you actually submit gas sample(s) to the laboratory?

As for the "carryover", do you actually circulate more amine (i.e. having higher amine-to-feed-gas ratio) in those cases, when compared to "normal" operation? And at what times do you experience this phenomenon - when feed gas is hotter, or colder? What is the solvent flash drum temperature on both cases?

It would be really convenient if you can prepare an Excel sheet showing both cases (low and high flash gas rates), and corresponding process parameters - feed gas composition, temperature, pressure, flow, treated gas composition(s), lean amine loading, temperature and flow, flash drum temperature and pressure, flash gas flows. Can you do that?

[GasPro]

Zauber


I will get the requested information together and post it. The CO2 is an online analyzer and this reading correlates with the lab analysis. One interesting thing that I found out is there was a breakthrough of H2S due to a high flow of sour gas to the absorber. This caused a slippage of H2S beyond soec and at that time the CO2 analyzer had a reading. Not much though however there was a reading. Thing that I will do however is to do a mass balance on the unit to see where the CO2 is going. I believe that the lab should be able to accomplist this. It should be in the acid gas but is there a way that the CO2 could desolve in the amine and remain in solution even after the regenerator? Just spit-balling as this does not make sense...

There is an elaborate calc tool here for calculating the circulation rate which is based on acid gas loading, mol% CO2, mol%H2S feed gas etc. The method that I am using to see if it is that my circulation rate is too high is to increase feed rate until a break through occurs ie max h2s in product is 20 ppmv, I increase feed until my analyzer reads eg. 8 ppmv. I then increase circulation rates until that h2s level in product reduces to say 3 ppmv. I am confident at that point that I have consumed all or most of the free active sites in the amine solution. Doing this I wonder if I should increase my circulation rates to reduce residence time. What concerns me is stripping out of HC with an increase in circulation rates.


How are things in the middle east? I hope you are maintaining your body's water balance as I had to when I was there.


Regards
GP

[Zauberberg]

GasPro,

Things are going fine, thank you for asking. This is my second venture in the Middle East, so I guess I have become quite resistant to heat and humidity. West Africa was much more interesting though, and I always look to go back there if any new opportunity arises. As for the water balance, I prefer maintaining it with cold beer and from time to time boost it with some spirits as well. It keeps my mind awake in this place.

Yes, there is always some CO2 in the lean amine and you actually measure it (lean amine loading). Instead of thinking of amine being a "CO2 thief" I tend to suspect on process analyzers, particularly when they measure in the ppm range. Remember, we are speaking here about molecules/hour in treated gas when concentrations are in such a low range. Another thing that makes me think in that particular direction, is H2S slippage, which really shouldn't occur, especially if CO2 pickup is 100% as you say.

More circulating amine - more hydrocarbons co-absorbed. Depending on design/operational margins, this could be an issue in your plant but I have to admit that I have never encountered such phenomenon before, not even in the literature. So I presume something else must be going on. When collecting plant data, please include % opening of the pressure control valve on amine flash drum, and add another column with process design data (for all those parameters we mentioned before).