Lesson 11 - Carbon Dioxide Recovery Technology SAQ

**Your company’s flagship lager, Brand-X, has a fermentation profile template as shown in the table below. Currently, the fermenter is vented for the first 48 hours post-filling. After that, CO₂ is collected for the next 48 hours. Then it is vented for the remainder of the fermentation. Your recovery plant operator wishes to increase the recovered CO₂ into the storage tank by adopting one of three approaches, namely:

a)    Turning the tank onto collection 24 hours earlier only.

b)    Venting for the first 48 hours, as-is, and then leaving the tank on collection until fermentation has completely ceased.

c)     Or a combination of both a) and b).

For each approach, estimate the additional idealised quantity of CO₂ which would be available for collection. Advise the plant operator of any changes in the raw gas composition which they would need to be aware of.

Response:

The first thing to do is work out the idealised CO₂ available for collection with the as-is situation, i.e. tank on collection from 48 to 96 hours, as this will be the case against which the three alternatives will be measured against. We can do this by calculating the weight of sugar consumed by the fermentation using drop in Plato.

At an OG of 12.5°P, wort has a density of 1,050.5 kg/m³, so 1 m³ of wort weighs 1,050.5 kg/.

12.5°P wort is 12.5% w/w sugar so 1 m³ contains 12.5/100 x 1,050.5= 131.3125 kg of sugar.

At a PG (real attenuation) of 7.5°P, wort density is 1,029.8 kg/m³ so 1 m³ of wort weighs 1,029.8 kg.

7.5°P wort is 7.5% w/w sugar so 1 m³ contains 7.5/100 x 1,029.8 = 77.235 kg of sugar.

So the mass of sugar fermented is 131.3125 – 77.235 = 54.0775 kg.

Since for every 1 kg of sugar fermented we get 0.49 kg of CO₂, the CO₂ evolved = 54.0775 x 0.49 = 26.497 kg per m³ of wort or 2.65 kg/hL.

For the rest of the options we will shorten how the calculation is displayed but the method is the same as above. 

Option a) 24 to 96 hours on collection:

Option b) 48 to 144 hours on collection:

Option c) 24 to 144 hours on collection:

Each of the three approaches makes available:

• (3.74 – 2.65) = 1.09

• (4.19 – 2.65) = 1.54

• (5.28 – 2.65) = 2.63

kg/hL additional quantities of idealised CO₂ available for collection.

For any changes in raw gas composition associated with the three approaches, we would want to be mindful of the potential for increasing oxygen (O₂) and nitrogen (N₂) in our raw gas for approaches a) and c), which involve reducing the vent period.

It is not possible to be certain that O2 and N2 values would increase. The tank would still have had a minimum of 24 hours to vent, which may be sufficient to adequately vent the headspace (assuming a typical 20-25% fermenter headspace).

Nor would it be possible to state whether the recovery plant can adequately handle any increase in O₂ or N₂ concentrations. We could raise this potential issue and suggest the operator investigate it further through plant measurement, for instance.

Approach b) is unlikely to change the raw gas composition, at least from the perspective of O₂ or N₂, because it involves leaving the tank on collection later, rather than turning it on earlier. The CO₂ recovered from the tank would have long ago vented the O₂ or N₂.