15 replies to this topic

[rosa]

Hi friends,

As you know generally high Pressure and Low Temp in needed for absorption and low pressure and high Temp is requierd for regeneration.in recent days we encountered with a problem.I want to know that what is the effect of pressure on regeneration section of AMINE?I mean if the pressure in regeneration section decreases even less than design value ,dose it lead to a problem in regeneration section?reducing press lead to reducing temp and finally lean amine loading.

cheers!
rosa

[Art Montemayor]

Rosita:

Without knowing any specifics about what type of Amine process you are operating, I have to answer only in generalities.

Normally, the lower the pressure (& also the temperature) in an Amine Stripper-Reboiler system, the better the regeneration (as long as you have sufficient trays, stripper and reboiler), the less the corrosion, and the less the amine degradation. These are all good effects.

However, you are going to have to confront immediate tradeoffs when you lower the regeneration system's pressure. The first effect is going to be the NPSHa for your lean amine pump. Usually, this is the biggest factor in not allowing you to lower the pressure beyond a certain range. For MEA, I never got my reboiler down to less than 9 psig without having Lean Pump problems.

Your CO₂ loading won't be a problem if your reboiler/tube bundle is big enough for the heat flux and the heating requirement.

Can you be more specific about your process and how it is operated?

[rosa]

In our sweetening units we use MDEA, methyldiethanol amine, to sweet the feed gas. As you know MDEA is a selective amine that mostly absorb H2S (and a little CO2). Normally lean amine H2S loading was around 10 ppm(wt) when our sour gas flow was 450 MMSCFD but in recent days it has increased to 50 ppm with sour gas flow rate 300 MMSCFD. I mean when the sour gas flow rate( inlet feed to absorber) decreased , lean amine H2S LOADING INCREASED. Why?
I think due to sour gas flow rate reduction,AG(Acid gas) flow rate at the top of stripper also decreased.so Pressure also decreased an finally it lead to temp reduction and increasing LEAN amine H2S loading happened that is somehow reasonable.
But also there is a big question in my mind that I will tell you.
For you information there is 2 trains in our onshore refinery. That each train has one gas sweetening unit with the same condition. But the named problem (lean amine H2S loading) happened just for one train and the second train has good H2S loading while their conditions are the same!!


cheers!
Rosa

[vicini]

My first question when someone has an amine stripper problem is what is your stipper control loop. Reboiler temp? Reboiler duty? Reflux rate? Lean Loading? or what? With that information I can start tracking down problems.

PS If you control on anything other than reboiler duty, you will have problems.

[Pronab]

Rosa
Your idea about low pressure and lean amine H2S loading is just reverse.

It seems to me that your Regenerator bottom temperature not controlling properly. At low pressure in Regn lean amine loading should be low.How you are cotrolling your Regn temperature and how much it is. We are maintaining at top it is 105 Deg.C and at bottom at 130 Deg. C.Controlled philosophy is top temperature in auto at 105 deg. C with cascade of steam flow valve.
Maintain Bottom temperature , if not in auto try to controll at manually for some time and check lean amine H2S. However at 50 ppmw of lean amine H2S is not a problem. Even we are maintaining more than 100 ppmH2S In lean amine .
Anyway how much concentration are you using . Try to maintain less than 45 wt%. If you need more specific about amine contact me .
Best of luck.

[rosa]

Dear pronab,

The steam flow rate in controlled by auto-cascate from top tempreture of stripper like as yours control.when our sour gas flow was 500000Nm3/hr,top temp=106 and lean amine H2S loading was 30 PPm(wt).but due to the plant limitation in new condition gas flow=320000nm3/hr so with top temp of 106C the lean amine H2S loading went up to 120 ppm.so we increased top temp to 109C and Lean amine H2S loading decreased from 120 to 50 ppm.but still it is more than 30 ppm.why by increasing top temp to 109C still H2s loading is 50ppm incomparing to condition when 106C and 30ppm? of course we are over the turn down ration.
I should mentioned that it is true when sour gas flowrate decreased the rate of gas(water+acid gas vapor)in stripper also decreases and mass transfer decrease and stripper efficeincy goes down.but to solve the problem we increase top temp from 106 to 109 C but still more h2s loading?

What is the effect of re4flux on h2s loading(lean amine)?more reflux lead to less H2S loading or not?

Cheers!
rosa

[Zauberberg]

Hello Rosa,

Why don't you raise the regenerator bottoms temperature and maintain total reflux regime? It looks like you are just lacking of mass transfer efficiency at lower throughput, as you mentioned.
If column shows no signs of flooding and feed composition has not been changed, simple operational testing will guide you through the problem.

Regards

[shanti1927]

Hey Rosa can you information about your gas plant, like the composition flow rate, temperature, pressure at which feed gas is entering and name of the plant and where it is located. I am trying to design a gas plant on hysys. can we design it on pro/II too ?

[Pronab]

Rosa
As I told before you should maintain bottom temperature to get specific lean amine. Increasing top temperature will increase reflux. You should maintain water balance in Regn also to maintain amine concentration.How much bottom temperature of Regn? Even at low gas flow ( at minimum throughput)your operating parameters should not go off spec.
Higher reflux will bring down amine concentration and also H2S in amine should come down.
Regards.

[rosa]

Dear pronab,

After decreasing of the feed gas flow rate the bottom temp also decrease from 130 C to 125C.that effected the amine H2S loasding.

kind regards,
rosa

[Pronab]

Rosa
It is interesting to me that at 64% ( 500000NM3/HR VS. 3200000 KNM3/HR)feed rate you are facing problem. Column should run even at 30% of design.
As Zauberberg told if column not flooding or reboiler working properly bottom temperature should maintain.However try to maintain manually and increase bottom temperature. Can you provide your plant Design and Operating data. We never face such problems., even we run our plant at less than 64% design . Another thing, even at 50 ppmw H2S in lean amine is not a problem. More interesting you need some H2S in lean amine aswell that is another issue.

Regards.

[Megamass]

Dear Rosa,

Maybe your problem is not in the stripper. There is also possibility of leaking of the cross exchanger. Take the samples of lean amine before and after the cross exchanger and check it for LAL (if it is possible).

One of the few control schemes employs reflux ratio for optimisation of the steam consumption.There are good rules of thumb for every type of amine system which provide efficient stripping without unnecessary losses. Reflux ratio can be easily calculated and linked in control loop for steam flow.

[Megamass]

...and I forgot to ask: How do you control the pressure in your regenerator?
Normally, you should have PIC valve on the acid gas line from the condenser vessel or somewhere in the SRU. This valve should provide steady pressure for different gas loads.
Setting the overhead pressure you set also the pressure in your reboiler (top pressure+pressure drop through the condenser and column). Only pressure in the reboiler affects the temperature of the reboiler. For different acid gas flows it should produce small changes in reboiler temperature.

[rosa]

Dear megamss,

You are right.the top regenerator column controlled by PIC at top of reflux drum.(On acid gas stream).
and as you said I think that the reason could be the amount of reflux value because more reflux means more mass transfer and more stripper efficiency but the amount of steam consumption increases.I should mentioned that the sweetening units that has more reflux we have less H2S lean amine loading
now my question;
In your opinion is it possible that by reduction temp in bottom of reboiler(around 1-2 C)The lean amine h2s loading increase around 40 ppm??or no it is the effect of reflux flow rate?

Rosa
Cheers!!

[Megamass]

Dear Rosa,

Temperature of reboiler is not linked with stripping efficiency. I will try to clear it up for you: What happens in the reboiler is you vaporize the water from the amine solution. Water will vaporize on different temperatures dependent on the pressure of reboiler. And I as I mentioned before, this pressure depends on the top column pressure plus the pressure drop through cooler and column itself (trays). If you keep constant pressure on the top of your regenerator than only pressure drop through the system will affect the pressure in the reboiler. Anyway, if the temperature is changing that only means that you generate steam at lower or higher pressure and nothing else. You still generate steam and steam is STRIPPING the amine. If you try to raise the steam flow to your reboiler, you will still maintain the same temperature in the reboiler (or SLIGHTLY higher) but you will generate more steam through the column. This will give you lower LAL and more reflux rate to the top of your column. Reflux serves only to wash out the amine from the overhead vapor. If you overstripp the amine, that means you will need more reflux and this is just energy dissipation.
So the conclusion is that temperature of reboiler doesn't effect your LAL. Just amount of steam you generate. It is normal to have lower reboiler temperatures with lower acid gas rates, because you have lower pressure drop through the column and the overhead cooler. Reflux ratio for MDEA services should be around 1.25 if my memory serves me well. Try to calculate it and find where you are. My proposition to you is to put back your control system on manual, raise or lower the steam rate to reboiler dependent on calculation of the reflux ratio, and follow the LAL. If your reflux ratio is higher than 1.25 it means you have more steam than you need and opposite.

Again, cross echanger leakage is still an option, don't forget it.

Regards

[rosa]

Dear megamass,

Thanks for your helpful and perfect guidlines.I think there is not any problem with Cross Heat exchangers.because the problem of high lean amine H2S loading happenes just in case of low sour gas feed.I think it is not related to cross exchangers(I think if it relates why we don not have any problem if high sour gas feed?)

You mentioned reflux ration=1.25 .would you please the sources of the dedicated reflux ratio number(1.25)?you know.ACID GAS leave top of regenerator then after cooler its temp decrease and ented reflux drum.then vapor(acidgas+water)goes to SRU.and liquid comes back to stripper.1.25?

Rosa
Cheers!!