[Digestion] CO2 bubbler recycle

[Zietsman, Rex]

Hi Listers,

Let me paint a picture: imagine I can remove CO2 from biogas by, for
argument's sake, absorbing it into water under pressure, say 5 bar. If I
then depressurize the CO2 rich water into a closed tank to a lower
pressure, say 50kPa, a good deal of the CO2 will come out of solution
and go back into the gas phase. If I then duct the CO2 to bubblers in
the bottom of the digester, I would be feeding a fairly pure source of
CO2 back into the system. 

For my education, can someone speculate on what the effect this would
have on gas production? My elementary chemistry tells me that, due to Le
Chatelier's (sp?) principle, the higher CO2 presence would tend to force
additional CH4 to be made in order to keep the CH4/CO2 ratio similar. If
this is true, it would increase the overall CH4 production rate. 

Let me put it another way: let us say that the mass balance battery
limit box includes from the digester feed up to the CO2 scrubber outlet
with CO2 recycled internally to those battery limits. Would the internal
recycle cause a higher mass of methane to be produced?

If my hypothesis is true, it would give financial incentive to install
scrubbing equipment. It would also provide a means of gently mixing the
digester in a low shear manner and, finally, I believe that the mixing
would discourage floating sludge formation.

If this hypothesis has merit, what type of compressor would be suitable?
I know that we have discussed this to some extent in compressing biogas
about a month ago, but this point was not elaborated upon to any degree.

I look forward to a healthy debate.
Rex