5 replies to this topic

[shan]

I know it is OK to maintain the same TEG flow circulation rate if gas flow is lower than design rate. How about Amine? Should amine circulation rate or amine strength be reduced if gas flow is lower than the design rate?

[Zauberberg]

If the plant will be operating at lower feed gas rates for continuous period of time, reducing amine circulation rate should be considered. The only thing to be kept in mind is turndown flow (or minimum wetting flow for packed towers) of lean amine.

Keeping the circulation flow constant will result in energy waste in regeneration section: up to 50% or more of total heat duty goes for heating the solvent from feed temperature to reboiler temperature. Therefore, establishing the new operating envelope (amine circulation, strength) for Gas Sweetening Unit is highly desirable.

[shan]

Thank you, Zauberberg. Your answer makes sense. Then, why should TEG circulation keep the same even if gas flow rate is lower? TEG circulation rate should be reduced also to save regeneration duty. Right?

[Art Montemayor]

Shan:

I think Zauber has indicated that turning down the TEG (or Amine) flow rate to an absorption tower is the ideal, practical thing to do when you have a reduced inlet gas flow rate.

However, there are other parameters that are involved in the successful application of an absorption system - effects such as the minimum flow that can ensure efficient "wetting" of the packing material as well as the minimum flow that can ensure that there will be good liquid distribution and no by-passing of liquid to the tower's wall.

TEG flow rates are notoriously very low as compared to other absorption systems and many times the economics involved in trying to save a few $$$ may not be justified by trying to reduce the solvent's flow rate. I have to assume that you are alluding to a positive displacelment recirculation pump in your generalized question(s). A centrifugal pump cannot be successfully controlled to reduce pumping capacity and discharge pressure at the same time. Consequently, one of the required conditions for reducing solvent flow in a pressurized absorber (such as in a natural gas sweetner or TEG dehydrator) is to employ a PD pump and control the flow rate by varying the speed with a VSD or VFD (Variable Speed or Frequency Device).

The important thing to keep in mind is that, depending on the absorber's process design, you have a "window" or "envelope" within which you can operate when it comes to flow rates. If you exceed the maximum flow rate, you flood; if you exceed the minimum, you fail to wet or also "channel" the liquid distribution and lose absorption efficiency.

[Zauberberg]

As usually, Art has given a thorough and complete answer to your query. If you compare Amine vs. TEG circulation rates in corresponding sweetening/dehydration units (in the same process plant) you'll see that TEG flow rates are tremendously lower than amine circulation rates (for feed gas with "moderate" acid gas content). For such reason, the margin for changing operating envelope in TEG units is always much smaller than it is in AGRU's. That is why bubble-cap trays are always used in TEG contactors (and not some other type of trays) since they can hold small quantities of liquid and provide sufficient residence time regardless of the total liquid flow throughout the tower. If packed towers are used, all of them require some minimum wetting i.e. the minimum flow of liquid in order to ensure good performance of absorber and avoid liquid maldistribution that results in off-spec product.

I have seen (and been operating) large, multistage centrifugal pumps used for lean amine circulation in high-pressure service, and that is something that is done successfully. Due to lower flow in TEG service, usually reciprocating pumps are used.

[shan]

I know it is OK to maintain the same TEG flow circulation rate if gas flow is lower than design rate. How about Amine? Should amine circulation rate or amine strength be reduced if gas flow is lower than the design rate?

Than you Art/Zauberberg. I am very satisfied with your explantions.