I would put the CO2 inlet in the side of the outer long tube, near the bottom, maybe 3/4 of the way down. Then the bubbles have to flow up against the incoming water, instead of just flowing directly into the big CO2 bubble at the top. I would also consider reversing the sanitary tee, so the side leg directs the water flow down. That would reduce the head loss through the reactor. I think the basic idea is a good one.