Reflux Ratio Control on Amine Regenerator

[EmmanuelTop]

Has anyone seen this type of control on amine regenerator towers?

The basic concept is that the ARC (Advanced Regulatory Control) will be used to calculate the quantity of acid gas absorbed by circulating solution (kmol/hr). Reflux flow will also be calculated and expressed in the same way, so that the ratio Reflux/Acid Gas can be used as the primary controller loop cascaded to fuel gas flow controllers on regenerator heaters.

It seems that the operators find conventional control scheme (regenerator overhead temperature cascaded to fuel gas flow controllers on amine heaters) rather difficult and unstable - probably due to significant lag time between the action (change in T) and the response (firing the heater).

Can someone with field experience give his thoughts about this subject?

Thanks,

 

[sshep]

Hey Emmanuel,

Howzit my friend. Is there any chance of a picture? What happens to the top temperature control (does it stay, or is it replaced equivalent inferential feedback balance control)? Is reflux just set at fixed value now?

Some sort of calculated controller is probably going to make the stability better, but the solution proposed seems like it addresses a disturbance of variable acid gas load rather than your own analysis of the cause of instability (deadtime and higher order response). If the reflux is currently set at a fixed value, it may be worse after the change because the heater and reflux will have more interaction. If the acid gas in the tower feed is changing and can be calculated (as your description implies), then maybe a feedforward control on the heater duty is a better way to address. Another idea is that the top temperature is too late, and maybe you should use a temperature a few trays down the tower.

best wishes,
sshep

 

[EmmanuelTop]

Hello Sean

Things are going better and better here. The dry season has just started in Equatorial Guinea, it's quite pleasant and almost enjoyable.

To go back on the subject: actually the feed gas rate and acid gas content/composition is essentially constant. That makes the reflux ratio as - apparently - the most suitable variable to be used for control since there are no high fluctuations that could affect the system stability.

I was also thinking to use e.g. bottom tray temperature as primary control loop cascaded to fuel gas flow controllers, but it seems that controlling temperature always has some inherent "in-built deviation" due to the nature of temperature-controlled processes.

Feed forward control would be the best option in the case you described: fluctuations of feed gas rate and acid gas content.

My only concern with RR control is the sensitivity of the variable itself - small changes of RR cause relatively high changes in heater duty. Another issue could be the thermal inertia of fired heaters: once when/if they start "bouncing" it's very difficult to bring them into stable regime.

And of course, this control system would be equipped with low/high fuel gas pressure overrides.

 

[maddocks]

I haven't seen reflux flow control used - my guess is that it's going to be slightly unstable and even slower than the current arrangement. By the time you reach a 10% change in reflux flow, you've jacked a whole bunch of heat into the tower and it will swing way past your set point. The tower will never settle out and will stay in a perpetual swing.

What you could do is move the TIC down further in the tower closer to the steam inlet and then use the overhead temperature as a remote setpoint to assist in the cascade system for fuel gas control. This should be pretty good. If you were fanatical (or a phd candidate in controls), you could sense rich amine feed temp and adjust via some feed forward but it could go unstable on you quickly.

Hope this helps.

 

[dcasto]

there is no such thing as a cost effective or quality effective reflux control on amine.

The absolute control on an amine regenerator is the lean loading of the acid gses in the amine solution. A perfect control system would adjust the heat into the reboiler in order to have the loading of the amine at the highest level to meet specifications in the absorber. In theory, the amount of energy should be proportional (nearly linear), but in application you have energy loses in the rrich lean exchangers, atmospher, ect. So constant based on flowreate plus a factor times the amount of acid gas would get you close.

I've seen 4 or 5 different control schemes, in real life, a control on energy into the still and run the reflux condensor at a low temperature to minimizer water/amine losses works best. If the acid gases can be calculated, the adjust the amine circulation rate, and then do a control where Heat input is constant + factor * amine rate in gpm.

this assumes you have a constant concentration of amine solution.

 

[EmmanuelTop]

This approach actually works in one of Total's refineries (The Netherlands, I think). There are some particular issues with regards to signal filtering requirements due to extremely noisy signals.

 

[EmmanuelTop]

dcasto: shouldn't reflux ratio reflect the quality of regenerated solvent? Constant reflux ratio = constant lean amine loading.

Reboiler heat is used for three purposes: 1) Heating the solvent from tower feed T to tower bottoms temperature, 2) Breaking the bonds between amine and acid gas (desorption), and 3) reflux evaporation. Therefore, constant reflux ratio should mean constant quality of regenerated solvent, and is independent of regenerator operating pressure.

 

[dcasto]

emmanueltop
only if loading is constant, circulation constant, reflux temp is constanant, amine qulity is constanant, trays are in constant mechanical loading.

BUT, none of these are constanant. Above you stated "There are some particular issues with regards to signal filtering requirements due to extremely noisy signals." All that noise is because amine systems are extremely complex. The heat of reaction changes the temperature of the solution, side reactions when salts are formed, they all affect the ability to get low enough lean loading to meet the specifications in the absorber.