Lesson 10 - Carbon Dioxide Recovery Technology

The recovery process typically involves the system by which the CO₂ emitted from a fermenter (or fermenters) is collected, passes through purification and liquefaction, and ultimately enters a storage tank. In terms of timings for the recovery process, it is common to consider this from the moment filling the fermenter with wort is finished, i.e. from when the tank is full.

As we will see, different formats of purification equipment can deal with different quantities of contaminants. Ironically perhaps, the highest concentration contaminant is one which is not even produced during fermentation yet is one which we can easily limit in raw gas being fed to a purification plant. We can do this by merely giving it time.

Vent or collect strategy for CO₂ raw gas

CO₂ raw gas is what leaves the top plate of the fermenter through the fermenter vent line (also called in some circles, the bung line). As the recovery process starts from the moment the fermenter is full of wort (and presumably pitched with yeast), the first raw gas to be emitted from the fermenter will be air displaced from the headspace of the fermenter by the first CO₂ to come out of solution. As time progresses, the raw gas will become enriched with CO₂.

Air is a significant contaminant not only because it contains oxygen, but it is also a significant contaminant in the recovery process itself, where the oxygen and nitrogen components of air can adversely affect the CO₂ liquefaction process. We know from our brewing science studies that air has the potential to induce oxidative flavour damage in beer. We will discuss this in more detail at a later stage.

The simplest way to ensure that the raw gas entering the recovery plant does not contain excessive quantities of air is to vent it to the atmosphere directly from the fermenter using a vent valve, which could be a manual or automated valve. Different strategies are available to guide us on when the raw gas should be switched from vent to being collected. These include:

• Time-based – the decision to switch from venting the raw gas to collecting it is made with reference to an elapsed duration of time. This is typically from the moment that filling the fermenter has been completed, at which point the fermenter would be put on vent, until the time that the fermenter was put on collection. The intent behind using time is to allow for sufficient fermentation activity and hence, for carbon dioxide to be produced, to have purged the air out of the headspace so that the oxygen level is low enough to not negatively impact the beer. A four to six-hour venting duration would be typical.

• Oxygen measurement – explicit measurement of the oxygen content of the raw gas can be used to trigger the vent-to-collect decision. Two locations can be used:

  • At the bung line of each fermenter

  • Within the collection main itself

The bung line of a fermenter is a traditional name for the line, or pipe, which carries the raw CO₂ gas out of the fermenter and ultimately into the collection main.

The term comes from the traditional name for a cork or stopper which was used to close or seal off a vessel, e.g. a barrel or cask, by fitting it in the bunghole of the cask. If fermentation was taking place within the vessel, then the line or pipe coming from the headspace of the vessel could be bunged up by fitting a bung in the end of the line. This would allow back-pressure to build-up in the fermenter and increase the dissolved CO₂ in the beer.

What do you think the advantages and disadvantages would be of the three different approaches of shifting the fermenter from vent-to-collect?

1. Timer

2. Oxygen measurement in each fermenter bung line

3. Oxygen measurement in the collection main

Give yourself 5-10 mins to think about this and then click ‘Next’ to continue with this lesson.