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Carbon Dioxide Recovery & Liquefying
May 2017

Carbon dioxide gas is used in many food industries, particularly in those producing beverages. It is primarily used for carbonation of packaged drink products as well as a blanket gas in storage to exclude oxygen, which can cause deterioration of a product’s shelf-life. Breweries are able to produce their own CO2 as part of the fermentation process. The CO2 can be collected, liquefied and stored prior to vaporisation for use in product. In many cases breweries can be self-sufficient in CO2 supply. For those industries without a regular source, CO2 can also be collected from boiler flues or generated in diesel plants. Critical for all beverage producers is that the CO2 is of high purity with little oxygen contamination.

Gordon Brothers have supplied, installed and commissioned CO2 recovery plants for several breweries in Australia.

In breweries, the CO2 systems include the following stages:

      • Collection
      • Scrubbing
      • Compression
      • Deodorising & Drying
      • Liquefaction
      • Stripping & Purging
      • Storage
      • Vaporisation

 

Collection

CO2 is collected from the fermentation vessels. Collection typically starts after 24 hours, so that the CO2 produced has time to displace the air from the head space in the tank which is vented to atmosphere for this initial period. The gas quality is typically monitored to ensure minimal air content before collection starts. Collect pressure is just above atmospheric pressure.

CO2 is collected from the fermentation vessels. Collection typically starts after 24 hours, so that the CO2 produced has time to displace the air from the head space in the tank which is vented to atmosphere for this initial period. The gas quality is typically monitored to ensure minimal air content before collection starts. Collect pressure is just above atmospheric pressure.

Scrubbing

The CO2 is “washed” in a scrubbing tower to remove any soluble contaminants such as alcohol vapour. The gas passes up a packed column with water counter flowing from the top. The water can be recirculated a few times before going to waste.

Sometimes a secondary brine refrigerant is used to cool the gas to 2°C in a heat exchanger to condense and remove as much water vapour as possible prior to the compression stage.

Compression

The CO2 gas must be compressed to storage pressure before it can be liquefied. Typically the storage pressure is about 1850 kPag. The compression is typically done in two stages, either with a two stage compound compressor or with two compressors in series. The compressors need to be oil free or use food grade oil with carbon bed filtration to prevent any oil carryover. The gas is cooled at the intermediate pressure stage and at the final pressure stage with cooling water to about 32°C with condensate traps after each cooler to remove moisture.

Deodorising & Drying

The compressed gas goes through a deodoriser which is a vessel filled with activated carbon. The carbon absorbs any remaining odour from the gas.

Before the gas can be liquefied, any remaining water vapour must be removed. The gas passes through a vessel with activated alumina which absorbs the moisture. The leaving gas should have a dew point of -60°C or below to ensure that no moisture will freeze out in the liquefier.

Typically both of the above processes consist of two vessels in parallel so that whilst one is active, the other can be regenerated with a heat source to purge out the impurities and moisture.

Liquefaction

In this stage the cleaned high pressure CO2 gas is condensed into liquid. A refrigeration system operating with a suction temperature of -30°C is used in order to liquefy the CO2 to -22°C. If an ammonia refrigeration system is used then this will be a discrete system separate to other systems on site. Most breweries do not have an existing refrigeration system that operates at this low temperature and if there is a leak across the liquefier heat exchanger then calcium carbonate which will form as a result cannot contaminate the main refrigeration system.

Stripping & Purging

A stripping column can be used to further reduce the oxygen content from the freshly liquefied CO2. The oxygen does not condense in the liquefier and can remain entrained in the liquid. A small amount of CO2 is boiled to vapour in a reboiler vessel at the bottom of a stripping column and counter flows through the newly condensed liquid, which drags out the non-condensed gasses. The heat source can be electric or use warm CO2 gas from the compressor discharge, effectively precooling it prior to liquefying. The “dirty” CO2 gas is either vented directly to atmosphere or through a purger.

A purger can be used to recover most of the CO2 gas (that would otherwise be lost) out of the gas to be vented. This is done by vapouring some liquid CO2 at -46°C to liquefy the CO2 out of the foul gas.

Storage

After liquefaction and stripping, the liquid CO2 is transferred by pump or gravity to insulated storage vessels. As CO2 collection and brewery usage do not always coincide enough storage volume is required to cover this non-alignment as well as a safety buffer.

Vaporisation

Before the CO2 can be reused in the brewery, the stored high pressure liquid must be vaporised to gas and reduced to a usable pressure. This is normally done in a vaporiser which utilises a heat source to boil off the liquid in a heat exchanger. The heat source can be electric, steam, warm cooling water, or even discharge gas from the CO2 compressors. The last two options transfer unwanted heat to the CO2 liquid that would otherwise need additional external cooling to remove.

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