How the Purity of Sparged Carbon Dioxide Affects the Oxygen Concentration of Beer

 

My last blog post discussed the importance of carbon dioxide purity when using injected CO2 to increase the CO2 concentration of your beer. The thing we learned is that the purity of CO2 must be very high (99.99% or better) when using injection, or you will at the same time significantly increase your dissolved oxygen levels. However, injection isn’t the only method for adding CO2 to beer. Sparging, in which CO2 is bubbled through beer (usually in a tank with slight over-pressure) is another common practice for boosting CO2. So in this post we will explore how sparging a finished beer tank with carbon dioxide impacts the final oxygen concentration of your product.

First, let’s do a quick review of what happens when you inject CO2. When you inject gas (usually into a pipe,) you are forcing a given weight of gas into the liquid under pressure. All of the carbon dioxide, plus any trace oxygen and nitrogen, gets pushed into the beer and dissolves completely, allowing you to calculate the weight of gas used and extrapolate from there your various gas concentrations.

On the other hand, when you sparge gas into a liquid the dissolved concentration of gas will be bound by Henry’s Law. Henry’s Law tells us that the amount of gas that will dissolve in a liquid will be proportional – at a constant temperature – to the partial pressure of gas in equilibrium with the liquid.  This means that the gasses dissolved in your beer will never be more concentrated that the partial pressure of the gas you are using to sparge.

The consequence is that, with any given CO2 concentration outcome desired, you will have significantly lower oxygen concentrations for sparged beer then for injected beer. For example, in injected beer the oxygen pickup from injecting one volume of 99.95% CO2 (at 0oC) into the beer when the oxygen concentration is 0.01% is 143 ppb. But a theoretical sparging of that same CO2 into the beer at atmospheric pressure would follow Henry’s Law, and your oxygen pickup would be about 7 ppb.

In real brewing situations, however, most brewers use tank overpressure to help get sparged CO2 into solution, so you would probably be picking up about 2 times the above amount, or 14 ppb. The table below shows the expected oxygen pickup given varied percentages of O2 traces (in your CO2) when measured at sea level and at 0oC:

Sparged CO2 at 1 V/V

0.001% O2

0.005% O2

0.01% O2

<1

3

7

My final thought is that CO2 purity isn’t nearly as important if you are sparging rather than injecting, since the amount of gas that will dissolve into your liquid is much lower. This also applies to the purity of the gas you use to flush air from tanks before filling.

How the Purity of Injected Carbon Dioxide Affects the Oxygen Concentration of Beer

When an industrial supplier sets a minimum purity for the CO2 they supply to your brewery, you need to be aware of the ramifications of that purity and whether there is any chance it will increase the dissolved oxygen concentration of your beer. CO2 specified at 99.5% or better may sound very pure, but when we do the math we find this is actually a problem. This post is specifically about carbon dioxide that is “injected” into beer. Another post will address CO2 that is “sparged” into beer.

If your CO2 has a 99.5% purity, then the impurity is 0.5%. For purchased CO2, the assumption is that the impurity is always air, so only 1/5 of the impurity – 0.1% — should be oxygen. If you were to add to your beer one Volume of CO2 with an impurity of 0.1% O2, it would increase your oxygen concentration by a whopping 1,420 ppb.

In 1985, Nick Huige and his Miller Brewing Company co-workers published a paper on this subject in the MBAA Tech Quarterly. Their findings on the affect of injecting impure CO2 are astounding and can been seen in the table below.

 

Amount of added CO2

Concentration of O2 impurity in CO2

0.001%

0.005%

0.02%

0.5 V/V

7 ppb

35 ppb

142 ppb

1.0 V/V

14 ppb

71 ppb

284 ppb

2.0 V/V

28 ppb

142 ppb

567 ppb

Dissolved oxygen added to the beer

 

So if you want to inject CO2 into your beer, you need to be mindful of the actual purity. I know a brewer whose CO2 specification from his supplier was 99.5%. Most of the time the supply was much purer >99.998%, which is excellent. But when they had an unexpected increase in their dO2 levels, the cause was eventually traced back to the CO2. What was the purity of the “problem” C02? A number that still sounds good on a cursory level – 99.97%! – but was not acceptable in the context of dissolved oxygen in the product.

Most brewers specify a minimum CO2 purity of 99.990%. This equates to an oxygen impurity of about 0.002%. If one V/V of CO2 with this oxygen content were injected into beer, the resulting increase to the beer dO2 would be about 28 ppb.

My final thought is that if you are injecting CO2 into your beer, be sure your purity specification is not too low. You don’t ever want to be in a position where you’ve been getting great CO2 but then have one “bad” batch – still within the manufacturer’s specification – adding too much oxygen to your product.

 

Gas Phase Measurement Units

In the past couple of weeks I’ve received several questions about measurement units and how they differ from one another. Have you ever tried to keep bar, mbar, atm, Kpa, %Vbar, %, torr, ppm and ppb straight? If you’re listening to someone in speed mode (I plead guilty) it can be a challenge to follow.

So let’s start by looking at the way different units present at 1 bar, the unit of pressure sometimes also referred to as “atmosphere.”

1 bar =

  • 1000 mbar
  • 750.1 Torr
  • 750.1 mm Hg
  • 29.53 inches Hg
  • 0.987 Atm
  • 14.50 psia
  • 100 kPa

As a brewer you probably won’t see much of units like Torr or mm of mercury (mm Hg), but there’s a unit called %Vbar or ppmVbar that may be helpful. I use them a lot and they can easily be interchanged with percent, but there is a specific distinction in that it is tied to atmospheric pressure and thus stands for “Percent Volume Barometric and “PPM Volume Barometric”. “

So why use Vbar instead of just percent? If you’re at a high elevation and want to specify that that the percent of the gas you are measuring is being measured at atmospheric pressure, then Vbar is your unit. For example, Denver Colorado is roughly 5280 feet. At that elevation there are about 15 percent fewer atmospheric gas molecules —  855 mbar – versus the 1013 mbar you would find at sea level in San Francisco. The Vbar units confirm that the instrument is at atmospheric pressure while the sample is being measured.

This table compares different gas percentages using some of the most common units you may encounter:

Unit

mbar

Bar

Atm

Percent (absolute)

%Vbar

PPM

100% gas

(at sea level)

1013

1.013

1.000

100.0

100.0

1,000,000

100% gas  (atmospheric at 5280 feet)

855

0.855

0.844

84.4%

100

1,000,000

1.000 % gas

10

0.010

0.010

1

1

10,000

0.100 % gas

1

0.001

0.001

0.1

0.1

1,000

0.010 % gas

0.1

0.0001

0.0001

0.01

0.01

100

0.001% gas

0.01

0.00001

0.00001

0.001

0.001

10

0.0001 % gas

0.001

0.000001

0.000001

0.0001

0.0001

1

My final thought is to understand the units available to you. If you are purging down a tank with CO2 and want a specific percentage of CO2 purity, use the units that will equate back to what could dissolve in your beer if the purge didn’t exhaust all of the contaminating gas in the tank.

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