29 replies to this topic

[vista]

Dear All,

we are using MDEA to remove acid gases from sour gas streams which contains about 400 ppm of H2S and 3.2 mol % CO2. I have studied in literature that MDEA stripper top temp should be between 195F to 210F.
If low than poor regeneration would occur and if high, excess reboiler heat input.
currently at our plant this temp is 220F which remains same. I want to know how we can control this temp...I would also like to share that we have direct fired reboiler and sweet gas id being used as a fuel. steam temp remains 240F.

[gegio1960]

Dear Vista,
I think your high temperature is linked to high pressure at top of Regenerator.
You would reduce it if you could reduce one or both of the following parameters:
- pressure drop in the overhead system;
- pressure in the downstream system.
regards

[Atttyub194]

My Friend

Good day

In order to give you reasonable comments / suggesstion following may be required
- PFD of the system
-Analysis of the gas

Hope you may understand the scenario

Best regards,

[thorium90]

Hi could you tell us the bottom temperature of the reboiler too? I believe your stripper would have something like 50% MDEA 50% water. The bottom temperature would be the saturation temperature of water at the stripper pressure. The top temperature would be mainly governed by how hot it is at the bottom. If the top temperature is too high lots of water and MDEA is lost overhead which affects the concentration in the system. Can i clarify that your reboiler actually uses steam that is produced at 240F in some boiler that uses sweet gas as fuel? Or does the reboiler as you described is direct fired; column bottom liquid is in the shell side and hot burnt gas in tube side? Also, are you able to know the dP across the column? Is it packed or tray? High dP is a sign of too much reboiling.

[gegio1960]

thorium,
the temperature at top is governed by pressure in the overhead system, not by bottom temperature, neither by pressure drop in the tower.
Regards

[thorium90]

gegio,

the reboiling at the bottom causes the liquid at the bottom to boil, as it moves up, it encounters the colder rich liquid coming from the absorber unit which cools it down. The liquid at the bottom is therefore at saturation (in order to be boiling) and the saturation temperature depends on the pressure in the column which would be the a function of the pressure drop and the pressure in the overhead. For example, if you would pour more rich liquid into the stripper, the top temperature would drop.

[thorium90]

I think the TS has solved her problem? She has not replied ever since.

[vista]

thorium,
sorry 4 being late
i would like to answer your queries one by one
​reboiler skin temperature is 230F and steam temp is 240F
we are using 22% MDEA and the remaining water
the reboiler is direct fired; column bottom liquid is in the shell side and hot burnt gas in tube side
column DP is 0.5 to 0.6psig and its a valve trayed column

[thorium90]

The stripper top temperature is most directly affected by the bottom temperature since other parameters do not have much freedom of change; eg: the flow from the absorber to the stripper top I'm gonna assume is controlled by the level of the absorber column? So that cant change much if the loading didnt change. If the CO2 slippage from the absorber top is not too much, you can always reduce the stripper top temperature. How is the control system here like? Do you take the stripper top temperature as the SP and control the flow of hot burnt gas to the tube side? 22% MDEA does seem on the lower side though. DP of 0.5 to 0.6psig is quite reasonable for a valve trayed column. Like I mentioned, if CO2 slippage is not too high you can always run with a lower stripper top temperature. Too high temperature in the stripper increases the rate of solvent decomposition, more heat stable amine salts and also results in more losses of the chemical and water in the overhead. It is possible the reason why you need to run so hot to maintain CO2 slippage is that the MDEA concentration is on the lower side. Does the vendor for the MDEA chemicals give you any technical advice?

Edited by thorium90, 18 January 2013 - 02:47 PM.

[narendrasony]

Dear Vista,
As pointe out by gegio1960, Optimum stripper top temperature depends on the stripper top pressure.

To reduce it below 220 DegF, you may check your acid loading of lean amine. If it is below the value specified by amine supplier (typically 0.01-0.02 moles of acid gas/moles of amine), you may reduce the rebolier heat input. Reboiler outlet temperature will also reduce somewhat, but less compared to top temperature.

You should also check stripper reflux water sample for amine content. It should be on lower side, perhaps 0.1-0.3 weight%. A higher value would indicate the need to reduce the top temperature, but lean amine loading should always be within the prescribed limits.

Is there any specific reason why amine concentration is maintained only 22% in your system? Your re-boiler outlet temperature is on lower side due to that reason also and far less than thermal degradation temperature (typically 127 DegC or 260 DegF for MDEA). You may increase amine concentration upto 45-50% and reduce amine circulation flow rate. This way, you can reduce reboiler firing a lot.


Regards
Narendra

[thorium90]

the temperature at top is governed by pressure in the overhead system

As pointe out by gegio1960, Optimum stripper top temperature depends on the stripper top pressure.

However, the effect of changing the column pressure is less pronounced than changing the amount of reboiling and hence the temperature at the bottom. What actually happens is that by changing the pressure say by increasing pressure, the saturation temperature at the bottom also increases and thus with all other parameters constant, the top temperature will also increase. Therefore, what is observed is that the top temperature appears to be governed by the pressure in the overhead. I mean if you think of it another way, changing pressure cannot possibly only change the temperature, (ideal gas law), but it can change saturation temperatures (phase diagram). Therefore what actually changed was the saturation temperature at the bottom and as the fluid gets cooled by the same amount while traveling to the top (temperature difference from top and bottom), the top temperature will become higher if the top pressure is increased. Also, the column pressure does not have much leeway to change as it typically provides the suction head for the lye pump to pump lean fluid to the absorber and is also limited by mechanical constraints since these strippers are typically low pressure.
The idea is also talked about in the link below on pg 11
http://www.bre.com/p...ulesofThumb.pdf

Do correct me if my line of thought or understanding is incorrect.

I also agree that the low amine concentration could be the main cause of the problems.

Edited by thorium90, 28 January 2013 - 09:40 AM.

[vista]

Narendra,

Thanks for your response.

I will check the lean amine loading and amine content in reflux and will come back to you very soon.
But right now I would like to ask that as mentioned in your response that the degradation temp of MDEA is 260F. I hav’nt seen the degradation temp of MDEA neither in any literature nor in MSDS provided by MDEA vendors. Please confirm this.

Also, as pointed out by you to increase the concentration upto 45-50% and reduce the amine circulation rate but unfortunately we don’t have any provisions in our system to reduce the flow rate it always remain the constant (i.e 85 USGPM). so how can we reduce reboiler firing?

Additionally, please tell me wheather the MDEA concentration effects the boiling point of MDEA in system?

Thanks

[thorium90]

woops, i think i made so many edits to my post that you missed my post.


the thermal degradation is also there on the same page

The idea is also talked about in the link below on pg 11
http://www.bre.com/p...ulesofThumb.pdf

different mdea concentrations affect the boiling point. Its a mixture, so if it is 100% water its water bp and if 100% mdea its mdea bp. You can refer to the link pg 19
http://www.arkema-in...ure/pdf/328.pdf

Also, i suggest you consult with your vendor what is the recommended concentration needed. There might be a reason for the lower amine concentration (although i highly doubt it should be that low), perhaps activator usage. MDEA has a higher viscosity than water, sudden increases might make it unstable with uneven concentration in the system. Gradual doses, gradual increases, let it mix evenly.

reduce the amine circulation rate but unfortunately we don’t have any provisions in our system to reduce the flow rate it always remain the constant (i.e 85 USGPM). so how can we reduce reboiler firing?

I dont get it, why cant the flow be changed? And why cant the reboiler firing be changed? Arent there any control valves you can play with?

Edited by thorium90, 28 January 2013 - 09:46 AM.

[vista]

thorium90,

can you please tell me how can i reduce overhead pressure?

[thorium90]

thorium90,

can you please tell me how can i reduce overhead pressure?

whats controlling the overhead pressure? Maybe some simple sketch would help?

The following excel is not made by me, but i think Art won't mind if you use his file as a template to describe your system.

Attached Files

•  CO2 Plant Operating Parameters Rev7A.xlsx   809.33KB   105 downloads

[vista]

Thorium90,

Thanks for your guidelines…this stuff is really helpful.

Also there’s no control valve installed at amine charge pump discharge so flow cant be changed…however reboiler firing can be control by controlling the fuel input to the reboiler…

And one thing more, to be very honest…I cant understand how to control overhead pressure from Art’s sketch…

[vista]

different mdea concentrations affect the boiling point. Its a mixture, so if it is 100% water its water bp and if 100% mdea its mdea bp. You can refer to the link pg 19
http://www.arkema-in...ure/pdf/328.pdf

thorium90,

according to this graph what would be the BP of MDEA with concentration of 22%....
as i have already mentioned that reboiler skin temp is 230F and steam temp is 240F...

[thorium90]

frankly, i also thought the graph was not too useful, as the slope is quite gentle between 20 and 50%. You could zoom in and use a ruler...

Although I think the greater concern is to check with your vendor on the correct concentration you require.

Also there’s no control valve installed at amine charge pump discharge so flow cant be changed…however reboiler firing can be control by controlling the fuel input to the reboiler…

So you cant reduce the flow even if the gas load is reduced? Wow, thats a perfect waste of electricity, pumping so much maybe for little benefit.

I still think you should correct the concentration first then tweak the heating duty. After all, with a new concentration you will have new heat capacity and new heating requirement

Edited by thorium90, 28 January 2013 - 11:45 AM.

[Art Montemayor]

 

Vista/gegio1960/Attyub194/thorium90/narendrasony/and any other aspiring and interested young ChE grad wanting to learn how Unit Operations and Amine Plants are operated in real life:

 

I’ve been sick in bed for over 1-1/2 months and have only been following the Forums as I was able under a lot of pain and drugs.  I’ve followed this thread by reading and feeling more pain for what I identify as a lack of understanding as well as a lack of communicating in an engineering manner.  If I seem to be too blunt and critical, it is not meant that way.  My sole intent is to be truthful and call things what they truly are.  If something is incorrect and wrong, then I call it that way; if someone is lacking in cooperating or communicating in the manner called for, than I identify it as such.

 

Vista: where are the detailed PFD and process basic data that was requested?  Your lack of work and time effort in generating and supplying this vital data is what has drawn out this thread to 18 posts – AND YOU STILL DON’T UNDERSTAND THE SIMPLICITY AND COMMON SENSE OF HOW TO LOWER OR RAISE THE INTERNAL PRESSURE IN AN AMINE STRIPPER.  This has to be embarrassing to all those concerned with this thread.

You guys have been laboring with what has to be one of the most basic and simple applications of Unit Operations – an amine reboiler / stripper operation.  I have lectured and taught seminars on this subject in the past and always started by reminding everyone that it is completely analogous to what our mothers have done so many times in their kitchens to keep us fed: heating a liquid mixture until the bubble point is reached.  By bubble point, you will recall from basic heat transfer that this means that the liquid mixture starts to “boil”.  The mixture will vaporize its liquid content at the temperature that corresponds to the pressure exerted on it.  Under these conditions the liquid and the vapor are “saturated”.  Our mothers normally cook in vessels open to atmospheric pressure; however, sometimes they also use a “pressure cooker”.  This devise simply keeps a back pressure on the boiling liquid and, because of this, the boiling (saturated) pressure and temperature are increased.

 

That, simply put is what happens in a reboiler / stripper operation.  It is no more complex than that.  There is always a back pressure control valve that maintains the pressure inside the stripper.  This control valve is located downstream of the LP cooler condenser that cools the sour gas and condenses the water vapor produced with the same gas.  This condensate is what is normally called “reflux” and sent back to the top of the stripper.

 

The temperature at the top of the stripper has very little to do with the Mdea composition, stripper trays.  It has nothing to do with the Mdea flow rate or the firing rate of the direct fired reboiler, the acid gas loading of the Mdea solution, the ideal gas law, or the Mdea content in the LP condensate (“reflux”).  It has a lot to do with what setting the back pressure control valve has.  It is this instrument that controls the pressure (and consequently, the temperature) at the top of the stripper.  There is always an excess of water in the stripper sour gas produced in order to ensure that the lowest gas loadings are obtained in the lean Mdea.  This is what also ensures that the Lean Mdea will absorb all the required H₂S and CO₂ in the absorber – and meet specs.

 

What seems to be lacking in this thread is the common, horse sense engineering tool of stepping back and identifying the elementary and basic requirements that identify the application:

• What are the specifications for the product gas exiting the amine absorber?  What amount of H₂S and CO₂ has to be removed from the processed gas by the MDEA solution?
• temperatures?  What is the inlet temperature of the Lean Mdea and the exiting Rich Mdea?
• guaranteed composition of H₂S and CO₂ in the product gas?  What is the recommended Mdea Lean flow rate, the Mdea reboiler pressure, the Mdea Lean temperature entering the top of the stripper, and the Mdea Lean leaving the reboiler?  What are the recommended Mdea gas loadings?  What is the recommended reboiler operating pressure?
• This is an MDEA amine process for removal of sour gas.  But WHAT are the required product gas specifications at the absorber outlet? 
• What is the flow rate of the process gas in the absorber?  What are its inlet and outlet temperatures and pressures?
• Who (not a “vendor”) designed, fabricated, and supplied this Mdea process?  What is the guaranteed composition of H₂S and CO₂ in the product gas?  What is the recommended Mdea Lean flow rate, the Mdea reboiler pressure, the Mdea Lean temperature entering the top of the stripper, and the Mdea Lean leaving the reboiler?  What are the recommended Mdea gas loadings?  What is the recommended reboiler operating pressure?
• Where is the produced sour gas flowing, and what is the expected pressure downstream of the stripper back pressure control valve?  Presumably, there is a Claus process downstream or a tail gas facility to remove the toxic H₂S before it gets to atmosphere.

The above are all important questions that must be addressed and Vista has the full responsibility to furnish such information if this is to be a successful and intelligent thread.
 

Vista: how can you expect to “understand how to control the overhead pressure from my sketch?  It is a very simplified flow diagram; it wasn’t meant to do that.  Firstly, the overhead pressure at the stripper is supposed to be and remain CONSTANT.  You don’t “control” (or vary) it.  You want to keep the stripper pressure as low as you can – and this keeps the reboiler temperature as low as possible.  You should be well below the 260 ^oF temperature at which MEA degradation starts.  Keep the Lean Mdea solution at the absorber as cool as you can (normally 10 ^oF above the exit gas temperature) as well as the sour gas exiting the LP cooler-condenser.  Also, you don’t have any “steam” supply in your process: you have a direct-fired reboiler.  Are you importing steam for any other purpose?  If not, you only confuse the issue by asserting you have steam involved.

 

There are many, many things I could relate and explain to you on how an amine process is designed, installed, operated, and maintained.  I wish I had the resources, time, opportunity, and availability to do so.  However, I don’t think that can happen.  Nevertheless, I offer all that I have via our Forums in order to give you guys as much of the experience that I’ve accumulated for the past 53 years.  I hope that you take this information to heart.  If you’re smart, you will and you’ll profit greatly by it because it will save you a great deal of time obtaining it with your own sweat and tears out in the field – the way I did.

 

If you guys have further, detailed queries on this topic, make them known in specific details and with supporting basic data and detailed sketches on Excel.  I will freely contribute as much effort as you – and many times much more effort in answers, calculations, and drawings than you make.  This I have clearly shown in countless other threads in the past.  But I am now getting much older and have accumulated priorities much more precious and important in my grandchildren, so I need to have you guys demonstrate that you deserve the type, quality, and accuracy of help that I furnish.  Otherwise, I can’t justify furnishing such quality, when there is no effort or response from the OP.

[vista]

Art, First of all I pray you may get well as soon as possible because the experience you share here in this forum is awesome.

I have uploaded the PFD along with current operating parameters. please have a look.

As indicated by you, there’s a back pressure control valve in our system which controls the pressure inside the stripper. But as we lower this pressure, the steam temp and reboiler skin temp also drops so in order to produce adequate steam we have to increase fuel input to the reboiler. Is this economical?

 

Now I will answer your questions one by one:

·         Product gas specs exiting the absorber are 3-4 ppm of H2S and 1.4 mole % CO2 where as in raw gas   we have 400 ppm of H2S and 3.8 mol% CO2.

·         Lean MDEA temp entering the absorber is 132F and rich amine outlet temp is 138F.

·         Guaranteed H2S composition is 16 ppm and CO2 1.6 mol %. Recommended flow rate of amine is 85 USGPM and reboiler pressure not less than 8 psig. Lean MDEA entering the stripper is 200 F and reboiler pressure is 10-11 psig. However am not sure about the recommended gas loadings.

·         Required product gas specs are same H2S 16 ppm and CO2 1.6 mol %.

·         Processed gas flow rate is 18 MMSCFD. Gas inlet pressure is 700 psig and outlet pressure is 695 psig. Temp of gas entering the absorber is 85F and outlet is 132F.

·         The produced sour gas is flared and the downstream pressure at back pressure control valve is 8 psig. And there’s no Clauss process downstream.

Attached Files

•  plant schmetic.pdf   181.01KB   109 downloads

Edited by vista, 30 January 2013 - 06:17 AM.

[thorium90]

Your absorber is missing.

The fluid coming out from the reboiler is technically not called steam. So that is the "steam" you were talking about... haha

And it does seem odd that the skin temperature is 230F but you can get 240F out of it. Anyway, how do you know what is the skin temperature? Im gonna assume its the temperature of the hot gas in the tubes?

 

And get well soon, Art.

Edited by thorium90, 30 January 2013 - 06:41 AM.

[vista]

thorium90,

 

i have uploaded only a portion of stripping section in the thread which is of my concern.

 

and what would you call this fluid if it is not a steam? can you please elaborate this for me?

[thorium90]

well, i think the term steam is normally reserved to imply water vapor, and not just anything that boils into vapor but strictly for water that turns into vapor. In this case, the lean mixture wouldnt exactly constitute steam. Also, this did confuse me at first because there are alot of MDEA units out there that use steam to provide the heat for reboiling and are not direct fired.

[Bobby Strain]

All this chatter. I don't understand the problem/maloperation that you are trying to correct.

 

Bobby

[narendrasony]

Vistas,

Here is a quote from theoperating  manual of one of the Amine (MDEA) suppliers (Sorry I cann't provide the whole document as it is treated confidential) :

 

"In order to prevent thermal degradation of the amine solvent , heating medium temperatures above 350 °F (177 °C) should be avoided.  It is best not to exceed a maximum bulk solution temperature of 260 °F (127 °C) when operating the regenerator."

 

Similar temperatures are specified in the attachment provided by thorium90 in post-11. So, Thermal degradation temperature may be as high as 176-177 DegC, but bulk temperatures above 127 DegC are to be avoided at column bottom. 

 

Stripper overhead temperature at a given stripper pressure is the dew point of overhead vapors which is a function of overhead composition. Higher temperatrure (at a given O/H pressure) would mean higher concentration of amine vapors in the overhead and potential loss of some amine (vapors) with acid gas. 

This could be due to somewhat excess reboiler firing. So reducing reboiler throughput is an option, but with a caution. It should not increase the acid gas loading of lean amine at column bottom. Art , please correct me if my understanding is wrong.

 

Regards

Narendra