Monitoring Of Trace Oxygen In Co2 In Beer Brewing

Process

CO2 is used in a number of stages in the brewing process. The brewing companies have the choice of buying the CO2 or re-using the CO2generated during the fermenting process. To take advantage of the cost savings of generating its own CO2 , the breweries must have a reliable and accurate means of monitoring the purity level of the CO2

Application

During the beer making process yeast reacts with malt, hops, and sugars in a fermentation tank to produce the beer, alcohol, and fermenter off gas. The majority of this gas is CO2. The CO2 is recovered, filtered, and stored in a recovery tank for later use during the canning and bottling process. If the fermentation process is complete, the recovered CO2contains 100 ppm or less of O2.

After fermentation, the beer is pasteurised to kill bacteria present. This assures a safe product and eliminates the requirement to keep the beer refrigerated assuring a long shelf life. However, the pasteurisation process results in a flat (non- carbonated) beer. Thus, breweries must inject CO2 into the beer during the canning and bottling process to provide the carbonation. It is essential for the product quality that the injected CO2 is as pure as possible and contains a minimum of O2

The CO2 recovered during the fermentation process is passed through activated charcoal filters to remove impurities; however, some residual oxygen still remains. Oxygen, if present in the CO2 in more than trace quantities (>200 ppm), can contaminate the beer. The resultant product is often referred to as "skunky" beer, a term that relates to the beer's foul smell and taste. Therefore, to assure that the fermentation process is complete, and that the recovered CO2 contains very little oxygen, breweries must be able to measure the oxygen at levels less than 0.05% in the fermenter off gas.

Solution

Traditionally, the wet chemical methodology, ORSAT, has been used to make manual measurements of the recovered gas. This manual method is time consuming, subject to operator errors, and provides only a point in time analysis.

Since the analysis is not continuous, CO2 cannot be recovered until the manual reading indicates low oxygen readings and high CO2 . The time factor between analysis will often result in the loss of CO2 .A fuel cell oxygen analyser cannot be used for these measurements because the very high CO2 (acidic) concentrations effect the pH balance of the cell's electrolyte biasing the oxygen readings and greatly reducing the cell's life.   

The ideal solution to this measurement is the Servomex 4100 (Figure 1) equipped with a zirconia sensor. The zirconia sensor is capable of measuring 0-100 ppm oxygen to an accuracy of 0.1 ppm. With a sample flow rate of 400 ml/min, the analyser will reach its T90 in 15 seconds. The analyser can simultaneously measure from one to four separate streams providing a fast, accurate oxygen analysis with continuous analogue outputs and high oxygen alarms. The cross interference from any hydrogen, hydrocarbons, or CO present can be minimised by the use of an activated charcoal filter at the inlet of the analyser.

The sampling system used before the analyser should assure that a clean dry gas of the proper pressure and flow rate is fed to the 4100 Gas Analyser. In addition to providing filtration and protection from liquid carry over to the analyser, the sampling system removes residual alcohols from the gas stream. Servomex can provide single or multi-stream sample conditioning systems normally mounted in a free- standing rack.   

SERVOMEX 4100