8 replies to this topic

[aanita]

In a gas sweetening unit, where H2S& CO2 are removed using MDEA solvent, due to plant throughput reduced to 35% ~ 40%, we reduce solvent flow to Amine contactor to save energy in reboiler. This solvent flow reduction is by trail and error, we reduce solvent flow and observe sweet gas H2S/CO2 and go further reduction of flow. However there are other parameters to observe also specially Column tray dry-out, rich amine loading factor. I want to know how can I define the minimum solvent flow to the amine contactor.

 

Thanks in advance.

[Zauberberg]

The best thing is to contact your chemical supplier (BASF?) and ask this question. Minimum circulation rate usually corresponds to one of the two factors you mentioned (i.e. the minimum liquid rate or maximum amine loading), but the supplier can give you the right asnwer for your particular application.

[Linda1978]

Dear Anita

 

usually turn down ratio of amine contactor is 50% and higher, so it would be difficult to work at 35% of its normal capacity. Amine capacity for absorbing H2S and CO2 is important but not necessarily sufficient. 

 

Try to rate your column at specified new condition. you may need higher amine circulation than you expect (if 35% turndown ratio is allowed)

 

also refer to operating manual of your plant. you may find some valuable data there.

 

good luck

[aanita]

Linda,

In manual it is written tray will operate satisfactorily between 30% to 100% of plant load, due to shortage of gas supply most of the time we have to operate the plant 35% of design capacity. Out sweet gas spec is OK, to reduce steam flow in reboiler my concern is how lower flow rate we should go for solvent flow. How we can confirm tray is not dry-out. To save energy we should not make problem in others.

[RockDock]

The easiest and most robust way to answer this question is to look at your ProMax model. With MDEA, you do not want to go above 0.5 rich loading. Since the column was designed to function between 30-100%, your trays should be fine. You also want to make sure you do not invert the temperature profile in your stripper. Monitor the overhead temperature of the stripper. Be sure that does not drop.

 

Another note is that as the circulation rate decreases, you may run into a rich CO2 pinch in the absorber. That should be monitored using the ProMax model. If you are pinched on the rich end, you may slip a lot more CO2 than previously designed. That could be good or bad.

 

I have seen many amine units which have seriously corroded their plant by trying to guess these operating conditions. You need to be very careful about reducing the reboiler duty. There is no excuse to operating in the dark. Use the model to help you find the optimum. All of our suggestions are based on our experience with plants we have seen. Your plant is different and deserves to be evaluated as such.

[aanita]

Rock,

We maintain rich amine loading between 0.5 to 0.7 mol/mol (inlet H2S is 6 mol% and CO2: 6.5 mol% , out H2S is 20 ppm & CO2: 2 ~ 4 mol%). However due to low throughput and lower solvent flow, reflux to stripper is very low. My concern is internals of Amine absorber/stripper column due to low solvent flow.

[RockDock]

The rich loading seems a little high to me. What is the temperature at the top of the stripper? It needs to be around 100C. That should be saturated with water. The condenser should operate at about 50C. That should generate enough reflux for you.

 

Also, you need to rate your columns. The ProMax model will tell you your gas to liquid ratio, the glitsch flood and the percent flooding of the column. That information can be used to see how your column is performing. The model will also show you how much H2S and CO2 you have on each tray. You can use that information to see see if you are stripping your amine on stainless steel trays or carbon steel trays. This information will help you determine if corrosion is a concern.

[aanita]

Rock,

You are right, rich amine loading(0.7 mol/mol) is quiet high. Regenerator top temperature is 92 ⁰C and condenser is 4~ 50  ⁰C. All problems happen due to very low feed to the plant. I have seen all the trays in column are SS 316 L (Stainless steel). However, I don't have any ProMax software, I have very little knowledge of HYSYS.

 

Thank you for your time. 

[RockDock]

At 92C, I would consider increasing the reboiler duty to avoid corrosion. However, the model would confirm whether you need to do that, or not.