5 replies to this topic

[Zauberberg]

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,

[Art Montemayor]

Zauber:

What kind of "amine" system are you dealing with?

If you are simply stripping out CO2 (+H2S?), why worry about any reflux ratio? What is the function of your existing and/or reflux ratio? How does the Acid gas stripper reflux ratio affect the amount of acid gas absorbed in the amine absorber? This statement has me perplexed. Perhaps in my old age I have lost the ability to relate one issue to the other.

I have never given any importance to amine (particularly MEA) acid gas strippers' so-called "reflux". In my experience, this reflux has no useful function in the regeneration of the rich amine. It might be good for keeping the top of the stripper cool, but it certainly is no "reflux" in the classical definition of what a reflux is and what it is intended to do. The overheads product of an amine acid gas stripper is a non-condensable at the process conditions and, as such, serves no purpose in being able to fix the purity of the overheads coming out of stripper - which is the classical purpose of a reflux stream. I have removed all condensate return ("reflux") to acid gas strippers in the past to simplify the process and have never noted any difference in the amine regeneration system.

Await your detailed description and/or diagram of the proposed instrumentation change and/or proposal.

[Zauberberg]

Hello Art

The concept of Reflux Ratio control is another way of controlling heat input into regenerator tower, in the sense that constant reflux ratio corresponds to the same quality (lean solvent loading) of regenerated solvent. I apologize for not having sketches and P&ID's with me, I am quite busy these days and didn't have time to prepare flow diagrams and basic parameters list in Excel; on the other hand, I cannot scan the original P&IDs since it is a proprietary material. But I believe you are more than familiar with amine (activated MDEA in this case) units, and basic control philosophy of regenerator reboilers. In any case, I'm attaching some of the related materials.

Originally, fired heater (regenerator reboiler) control has been envisaged as tower overhead temperature control (primary loop) cascaded to fuel gas flow controllers on fired heaters (secondary, slave loop). However, it appears that controlling temperature always presents a difficult task when plant optimization/energy efficiency are concerned - I guess due to nature of temperature-controlled processes (persistent deviation around the controller set point). That has been confirmed in our plant, in spite of the fact that we run feed gas with essentially constant acid gas (CO2) composition. Controller tuning did not result in almost any improvement in control stability. From that point of view, having a calculated variable (reflux ratio) should bring some more stability into the system. ARC works just fine in the absorber section of the plant, where absorber overhead CO2 analyzer is cascaded to the Ratio controller (lean amine vs. feed gas).

My only concerns are relatively high sensitivity of RR to fuel gas flow to the heater, and large thermal inertia of fired heaters in general - so instead of combating against control instability, we might be making it even worse. Or, to put my question in other way, what is the most reliable heat input control system in amine regenerators you have seen in your career? Temperature control (anywhere inside the tower) simply doesn't appear to be the best solution.

Attached Files

•  H-1204AB SECONDARY FUEL GAS CONTROL LOGIC.pdf   57.94KB   208 downloads
•  Reflux Ratio Control - Concept.pdf   7.4KB   259 downloads

Hello Art,

Can you provide any answers/hints ?

Regards!

[engg]

see below

Edited by engg, 17 October 2010 - 04:10 AM.

[engg]

hi;

How does increasing the reflux ratio affect lean solution loading? increases or decreases?

and in the second pdf, the units of the formula used to calculate the moles of reflux doesent cancel out...or am i mistaken?

furthermore, when you say feed acid gas to calculate the RR, do you mean the feed coming in the absorber or the feed (rich amine) coming in the stripper?

regards

Edited by engg, 17 October 2010 - 04:10 AM.