Another CO2 Reactor Design

I'm borrowing some ideas KB Koh described to me about his refinements
to the powerhead CO2 reactor. Most powerheads contain a spigot where
an airline can be attached to a venturi (jet-pump) on the outlet of
the pump. Typically the other end of a short tube is open to the air
and this provides a continuous stream of very fine air bubbles to
provide oxygen to a tank. You can attach the PVC hose from your yeast
bottles in this coupling and the result is a periodic jet of very fine
CO2 bubbles. Some powerheads have an attachment which allows you to
direct this stream of bubbles at a downward angle. Another design
has a tubular output to which you can connect a ~1/2" tube of PVC
which you can direct down into the tank. KB has a submersible powerhead
and he directs the output of the powerhead into an inverted plastic
container which tends to collect CO2 bubbles. The continuous current
through the container greatly enhances the dissolving of the CO2. 

My idea:

An ideal reactor would have a flow diffuser or expander section which
directed the flow downwards and such that small CO2 bubbles will tend
to find a meta-stable equilibrium point where the downward flow
resistance is roughly equal to the upward buoyant force. Let me try
to make a picture of this hypothetical reactor:
                                               <-- CO2
                                             ||    _______
                                 Venturi---->||   [-------]==]
                       /=================----++---/       | |]=+
    1/2" PVC tube---->/ . <-o  <-o  <-o  <-o              | |
                     / . ./==============---------\       | |
                     | o  |          Powerhead---->|      | |
  Max flow velocity--->|. |                        [______] |
                     /.V .\                             [ ] 
                    /.O. O \<---- Inverted Cone Reactor [ ]
     Entrained     / ./.\ . \                           [_]
     CO2 bubbles--->O .. o . \
                 /.././. \.\ .\     .
                /  . o  .  o . \    .
 Diffuser vanes-->/ ./ . .\. \. \   .
              /./ ./. /.\.\ \..\.\  .
              .  .   . ||     \\  . .
  Flow direction------>||   .  \\<----Min flow velocity
             .   . .  \||/.  . \||/
               .  .  . \/  .  . \/

As the chamber fills with bubbles, there would be a limiting
condition where bubbles start to escape out the sides. Before
this state is reached, the pressure in the CO2 inlet tube will
change from negative (relative to ambient) to positive. That
means that if you opened the CO2 line (say by unscrewing the
cap on the CO2 yeast bottle) water would flow out the tube
and start siphoning so a check-valve would be a good plan.
Otherwise, shutoff the powerhead before disconnecting the
yeast bottle. If it works really well, the CO2 should pretty
much dissolve and not too much accumulate. If the diffuser
chamber is not large enough, some bubbles will get forced
right out. Very tiny bubbles tend to be very easy to push
around with a low flow rate. OTOH they dissolve better due to
a high surface to volume ration.

Big bubbles have a higher ratio between buoyant force and
viscous force (lower surface to volume) hence their meta-stable
equilibrium position is higher in the diffuser in the higher
velocity region, where they get dissolved faster by more

This design would work equally well with a pressurized CO2 
bottle + regulator + needle valve.

How would you construct such an apparatus? One method would be to
glue together flat pieces of plexi-glass. It would be more like
a flattened pyramid than a cone but that wouldn't matter. Another
method is to use a large funnel and don't bother with diffuser
vanes. It'd still work but less efficient since the bubbles
would tend to congregate at the sides and join together while
the majority of flow would remain in a circular column in the
center of the funnel. You could create diffuser vanes by
gluing strips of thin plastic into wooden guides designed
to fit inside the funnel cone. Cut slots at an angle into
the pieces of wood to maintain the proper angle for the
vanes. Get the idea?

Steve     (day-dreaming about hypothetical equipment again)