14 replies to this topic

[engineer1985]

hi all,

As we know steam is used in stripper for stripping acid gases in gas sweetning unit. i am confused with fuel gas pressure. Normally we adjust steam temperature with fuel gas pressure in reboiler.steam temperature is 240F with fuel gas pressure of 29 psig. but now a days steam is being acheived at fuel gas pressure of 16 psig.this decreasing trend of fuel gas pressure appeared in last 10 months. all the other parameters are normal.

[pavanayi]

Engineer1985,
As you know, 'we' know nothing about your system. Please upload a rough P&ID with flow, temperatures and pressures marked, and explain how it was operated, and what has changed recently.
Also, tell us what specifically is it that you are confused about.

[engineer1985]

thanks pavanayi,

we are using methyldiethanol amine (MDEA) as a solvent for absorption of acid gases from raw gas streams..MDEA after absorbing acid gases is regenerated in a regenerator (stripper) utilizing steam from reboiler...reboiler feed comes from stripper bottom...reboiler is a fired tube type utilizing sweet gas as a fuel...steam temperature (normally 240F) is adjusted by fuel gas pressure (normally 29 psig)...my question is that from last few months steam temp (240F) is being acheived at a fuel gas pressure of 16 psig...i want to know how this is possible that we are acheivng steam temp at such a low fuel gas pressure when all the other parameters are same as they were before last few months... i tried to upload a sketch but it does'nt worked..

[mav9rick]

I take that the "steam" you are reffering to is simply the vapour generated at the bottom of the stripper. You need to look at the overall heat balance around the stripper. Has the temperature of incoming rish amine changed? What other parameters are you talking about? is there a flow meter installed to measure the fuel gas flow? where is teh fuel gas pressure measured? Also, please furninsh more details about your fuel gas fired reboiler.

[engineer1985]

mav9rick,

yes u r right...rich amine temperature has also changed from 204F to 194F...stripper top temp is also reduced from 218F to 200F(max)....what,s the reason behind this?why the stripper top temp has reduced..i have attached a rough sketch of my system,,,and my primary question about fuel gas...we normally maintain our steam temp at 240F...if steam temp increses then we reduce the reboiler fuel gas pressure and vice versa....we dont have any flow meter installed at fuel gas supply line...now a days we are having fuel gas pressure of about 16 psig ...a few months ago it was 29 psig...i m confused with this behaviour of reboiler...as steam temp (240F) is acheived at both the pressures...whats the reason behind this...

Attached Files

•  pfd.jpg   831.54KB   28 downloads

[mav9rick]

engineer1985,
So afterall the parameters that you told use were constant in your earlier post, arent really constant.
There are numerous variables in your problem. First of all, the stripper top temperature has reduced because your rich amine feed temperature to the stripper has reduced. This would translate into cooler temperature at the top of teh stripper column. You need to investigate the following:

1) Has your incoming rich amine from the Flash Tank also dropped in temperature? it can occur if you are sweetning colder gas feed. If the rich amine temperature has been held contstant at 240F AND the rich amine temperature ex the Flash Drum is constant, it indicates fouling issues in the heat exchanger. I assume the amine circulation rate has not changed over the past month or so. Correct?

2) How do you know that the rich amine is at 241F. The TIT on the reboiler shows 228F and the steam/gas mix going to the stripper shows 240F. These temperatures should not be that different. Please verify that the temperatures quoted are measured values and not your estimate.

3) Has the pressure in the column dropped or has it remained more or less constant?

4) What is the source of fuel gas for the fired reboiler? The actual heat input depends on the Wobbe index and if the fuel gas composition has varied ( as is quite often the case with refinery fuel gas ), required fuel gas and hence the pressure required for the same heat input will also change.

Edited by mav9rick, 16 October 2011 - 01:38 AM.

[engineer1985]

mav9rick.,

thanks for your detailed analysis..i would like to answer your queries step by step..

1)Rich mine temp from flash drum is 139F not 240F..raw gas temp entering the Absorber is held constant (110F)...rich amine after L/R exchanger is 194F now...earlier it was 205F...there may be a fouling issue in L/r exchanger...we have replaced the plates of exchanger...after that rich amine temp reduced to 194F insted of 205F...amine circulation rate is held constant also..


2)rich amine temp going to stripper is not 241F...its a lean amine temp after reboiler...the steam temp (240F), TIT showing 228F is the reboiler skin temp...and all these temps are measured values not assumed...

3)column bottom pressure is also reduced from 9.10 psig to 8.72psig...


4)we are using processed gas as a fuel...and i dont think there's any change in its composition...

[mav9rick]

Thanks engineer1985. I am still trying to gather enough information in order to diagnose the problem.

Have you seen any variation in the temperature of rich amine coming from the Flash Drum? You quote 139F now. What was it before the heat exchanger plate replacement? Also, what is the temperature of lean amine going to the absorber? And lastly, have you seen any variation in the

The reduction in pressure at the stripper bottom, for the same reboiler temperature, indicates a reduction in vapour traffic to the Stripper. This could be because of reduced rich amine loading. Due to the reduced acid gas content of rich amine, less energy is required to regenerate amine while maintaining the sensible heat increase in the reboiler.

[pavanayi]

Engineer1985,
Has there been any changes to the raw gas flow rate into the absorber?

[engineer1985]

pavanayi,

as indicated in the PFD the temp of rich amine from flash drum is 139F...this temp is almost constant...however rich amine temp at L/R exchanger outlet is 194F....earlier it was 204F before replacing the plates of L/R exchanger...

lean amine temp going to absorber is 134F...

and also the stripper bottom pressure is reduced for the same reboiler temp...please explain this also...raw gas flow rate also reduced to about 1 MMSCFD....

[mav9rick]

Ok it is quite clear that the rich amine loading (which is a measure of H2S or CO2 content of rich amine) has reduced. This could happen either because the plant feed gas quality or quantity has changed.
Amine regeneration process os nothing but evolution of H2S and CO2 from the rich amine by application of heat. Amount of heat input is directly proprtional to fuel gas presdure in your case. So if the rich amine acid gas loading has reduced because the gas flow into the plant has reduced AND you have kept the lean amine circulation constant, you are no longer going to need the same amount of energy to drive the acid gases from amine. This means you will have to drop the fuel gas pressure.
To prove this, you can try returning the plant to original throughput and provided the gas compisition is similar, you will find that you are going to have to increase the fuel gas pressure.

Let us know how you go
Cheers
Karan

[engineer1985]

mav9rick,

as explained by you...

"gas flow into the plant has reduced AND you have kept the lean amine circulation constant, you are no longer going to need the same amount of energy to drive the acid gases from amine"

does it means that the amount of acid gases in raw gas streams have been reduced, so that rich amine loading has also reduced?

[mav9rick]

Morvnlight, 
In the case at hand, it appears that the plant throughput has been reduced. I am assuming that the composition of the incoming gas has NOT been changed. This means that the acid content of the raw gas jas not changed but the flow of gas going through the absorber has. 
Typical rich amine loading is in the range of 0.45-0.55 mol/mol of amine(MDEA assumed for the case at hand for example).
 The acid gas absorption by amine is exothermic in nature. The heat of reaction is ~ 450 btu/lb of H2S. The regeneration process is endothermic on the other hand.
 Therefore, if the net loading of rich amine has reduced due to lower quantity of gas flOwing though the plant, we will need less enery to regenarate a fixed quantity of amine. 

[engineer1985]

mav9rick,


thanks a billion for delivering your valuable knowledge...

[operator.alotibi]

Hi all

On this subject,

rich amino formula RN H3 HS ?? , and If we set reboiler before regenerator Does this help reduce the H2s

like Our friend did

# reboile

Attached Files

•  regenotor .....jpg   85.32KB   9 downloads

Edited by operator.alotibi, 27 December 2011 - 02:55 PM.