Looking at the three points above,
1. Increase agitation:
a. Agitation may improve but remember the precipitate can get broken/crushed by a centrifugal pump that will make the grain small or mixed sizes therefore make filtering difficult.
b. There should be good agitation created already by the gas in the saturator. This may be sufficient for the saturators. MPPV is the only refinery I have seen with mechanical agitators, it is good, but they also have the gas distributed with the agitator, this is probably more important. I agree MPPV is a good system but not many refineries do this.
2. Seeding of vessel.
a. I think that with the high pH of feed, pH 11, and good CO2 there will be sufficient chemical reaction for seed to form. By recirculation maybe the small precipitate will end up making mixed grain size, like false grain in a pan and therefore also make filtering difficult. ie not recirculating will have one size grain leaving the saturator, by recirculation you will have nixed grain.
b. Remember the bigger the pH is the better chemical reaction in the saturator. By circulation we are taking liquor leaving the saturator with pH 9, mixing this with the feed pH of 11. This will drop the pH and therefore less efficient reaction?
3. Increase Retention
a. This is not really true. With a fixed flow into the saturator at a fixed level, the retention will be the same with or without circulation. Maybe very slightly more with circulation because of the volume in the recirculation lines and pump, but this is minimal and insignificant, not worth considering.