Transfer of Gases into Solution
As seen previously, Dalton’s Law describes how, under equilibrium conditions,
a gas will exert a partial pressure above the liquid proportional to its molar
fraction.
If the gas is in contact with beer at a pressure above its equilibrium pressure,
gas will pass from the gas phase (headspace) into the liquid phase at a rate
described by the equation:
dc/dt = kL * A * (CE – C) / V
where:
kL is the mass transfer coefficient
A is the interfacial gas: liquid area
CE is the equilibrium concentration
C is the concentration of the gas in solution at time t
V is the volume of the liquid (beer)
and dC/dt is the rate of change of concentration with time
The rate of gas solution into liquids has a number of implications with respect
to beer, notably carbonation, but also dispense of keg beer and avoiding air
pickup.
So what are the factors that will lead to rapid gas pickup into beer?
Or, what can make dC/dt large?
• Interfacial Area (A) - if the size of the bubbles injected is very small, then
the interfacial area will be large and the rate of transfer high. We will see
later how this can be achieved in practice by the choice of carbonation
equipment.
If the beer is in a tank or a keg, then it is the surface area of the beer that
will have an effect on gas pickup.
• Mass Transfer Coefficient (kL) - this can be increased by having highly
turbulent flow at the gas injection point.
• Volume of Beer (V) - if the volume of beer is small, the rate of increase of
dissolved gas concentration will be faster. This is seen in dispense of keg
beer. As the volume of beer in the keg decreases, there is a rapid increase
in the ratio A/V and carbonation can quickly reach the equilibrium level
determined by the applied dispense pressure.
• Concentration Gradient (CE/C) - the further the carbonation level is from
equilibrium, the faster will be the transfer rate. We will see later how this
can also be affected by temperature and pressure.
