Regeneration of adsorbent beds 2

[amar122amar]

Adsorbent beds are designed for a certain capacity based upon design flow rates of fluid and impurity loading. In actual conditions fluid flow rates may vary depending upon plant throughput variation, short outage etc. Can we use fluid flow rate through bed to find time to regenerate the bed? Or regeneration frequency shall be fixed based upon design conditions.

 

[don1980]

Effectively, you're asking whether significant channeling occurs at lower flow rates. That's possible, especially if the bed height is small and the diameter is large. If you're analyzing the contaminant concentration in the outlet stream (inlet stream too if the contaminant concentration isn't constant) then you can use that data to answer this question and to maximize bed life. Ideally you want uniform distribution of the inlet stream throughout the cross-section of the bed, and that's most likely maximized (at least it's intended to be maximized) at the design rate.

 

[georgeverghese]

Theoretically, it would appear that regen time could be reduced if the adsorption step process stream flow is lower than design. Best to ask the regen unit design licensor. Do not attempt this on your own. My attempts to vary regen step parameters (on my own in my younger days) failed dramatically - it ended up in a plant shutdown several days later !

 

[amar122amar]

Thanks don1980 and georgeverghese
Actually we were trying to optimize regeneration frequency to save cost. Some times our beds operate at reduced flow rate as compared to design. So is it better to regenerate the bed based upon total flow passing through it or we shall fix a time (no. of days)based frequency for regeneration?

 

[chemengbr]

Amar,

That’s a hard questions for us to answer (IMHO). Could you regen at a different frequency then design? Yes, I’d say so. If proper sampling and monitoring you can probably create some type of totalizer calc or watch performance and have that trigger your regens. Will that save you cost? My guess is probably, but the risk/reward also needs to be considered. If this causes out of spec product, than how much could that cost you etc? In the end got find the right balance or a margin of safety that you and your facility are comfortable with.

 

[georgeverghese]

Agree it is very tempting to reduce regen frequency based on totalised flow during adsorption, which may save you some OPEX for utilities consumption. But if you take an overall look at annual OPEX in your plant, what fraction of total OPEX be with these savings? Is it then worth the trouble and risk?

 

[georgeverghese]

@amar,
Thinking about this a little more, it looks like there is only a low risk of any operating failure if you were to reduce the regen cycle frequency to suit the process fluid flowrate, assuming
a)you have a reliable FIQ on the process fluid feed side
b)feed fluid contaminant composition is always lower than design
c)there is no co adsorption of other components during adsorption
d)feed fluid P/T is relatively constant (if either of these parameters affects adsorption capacity).

 

[amar122amar]

Thanks for valuable inputs.

 

[DominicLee]

Very interesting question. Thanks. I have the same question in the exam.