"MIXED" Gas. a.k.a. Beer Gas

    I have been asking the same question of beverage gas providers, for more than 10 years. I am still struggling to get a concrete answer.  The question is"How do you ensure that the Nitrogen and CO2 in a Mixed Gas Cylinder, dispense in the proper mix, when these Gasses behave so differently?"  The answers I have received, over the years, have varied widely, and inevitably, when I challenge them, or request demonstrated proof, I am ghosted....every time.  

    Before I delve too deeply into this Fucking Quagmire of an issue: If you are not a Draught/Draft Beer Pro, or are relatively new to Beer, you should read INTRO TO BEER CARBONATION   before proceeding with this article. You might be able to follow most of it, without reading the article, but knowing why people wish to use beer gas, is pretty crucial. The short answer is, sometimes you need to augment pressure, but if you use pure CO2, it causes both quality and pouring problems.

    From this point on, I am assuming, that you understand why beer gas is used, or that you are taking my simple explanation, on faith.  Since we're all up to speed now, the reason I ask that particular question, "how do they mix?", Is because CO2 and nitrogen (N2), behave very differently in your compressed gas cylinder, even when sharing the same one, together.   

    All pure substances, such as CO2 or N2, are subject to a phenomenon known as the liquid-vapor Critical Point.  This is the endpoint of the Pressure - Temperature phase curve. The critical temperature of a substance is that, above which the substance cannot be liquefied, no matter how much pressure is applied. Critical pressure is that which is required to liquefy a substance at its critical temperature. As a substance falls below it's critical temperature, the pressure required to liquefy it also falls. 

While this phenomenon affects both of the gases found in a beer gas tank, it affects these two gases very differently.  The critical pressure of CO2 is 1,070 PSI and has a critical temperature of 31° c/88° f.  The critical pressure of nitrogen is lower, at only 491 PSI, but it's critical temperature is minus -148° c/-232° f. With a critical temperature that is so low, nitrogen that is compressed into a cylinder will never exist as a liquid, unless kept extremely cold, and at great expense, I might add. This is not practical for the amount of gas needed to dispense beverages.

First, none of the beverage gas providers will even tell me what a 30% blend means. It could mean either that 30% of the volume of the cylinder is CO2, or that the expanded gas from a cylinder would come to a 30% ratio of CO2. Whichever the case, we have some issues. A 30% expanded gas volume is not enough CO2 to maintain carbonation over time. Your kegs will slowly go flat. 30% cylinder volume is nearly 60% volume of CO2. This is actually a common blend of CO2 found in many beer systems, but there is no possible way, by which, these gases will magically mix, as they are being drawn out of the cylinder.  And certainly not in the ratio prescribed. Furthermore, both ratios are incorrect for nitrogenous ales such as Guinness and Boddington's. Both are too heavy, in CO2 content, and will ultimately overcarbonate nitrogenous beers, with a long enough exposure. The real issue however, goes back to the disparaging critical points of these two substances. 

    When a beer gas cylinder is at full pressure, which can range from 1800 to 3,000 PSI, depending on tank size and type, the CO2 condenses to liquid. At these pressures CO2 cannot exist in gaseous form, unless the temperature is maintained above 31° c/88° f. Even then, as it began in liquid state, it will not evaporate as a gas, but instead exist as a super-fluid . Whether super-fluid or liquid, CO2 is many times heavier than N2, and this type of cylinder draws gas from the top of the tank.  The gases will remain separate in the tank, until N2 pressure falls to the critical pressure of CO2. The tank will then remain at this pressure. As additional gas is drawn from the tank, CO2 will evaporate until it reaches its critical pressure equilibrium. The gas in the vapor space will mix, in whatever ratio is present in the vapor space, only. This process will continue until all liquid or super-fluid CO2 has boiled off, at which point, the cylinder is near empty.

    There are Gas Cylinders in the marketplace, which have built in mixing apparatus. Some employ a  draw tube which is specifically perforated. The aim of this method is to restrict nitrogen flow enough that there is a pressure drop within the draw tube. Were such a pressure drop large enough to allow the CO2 in the bottom of the tube to boil, the gases would mix, to some degree. I imagine it is possible, that some engineer might figure out how to construct such a tube, with perforations that vary in size and concentration, which would be able to produce a fairly consistent mix, in this manner. It is possible that it has even happened. To date, I have yet to find a beverage gas provider willing to prove the result.

I have also seen mixing valves, which truly are proven capable of producing a fairly accurate and consistent mixture of these gases. I have yet to see one that is larger than a 4.8 litre liquid volume equivalent.  This can work, for one or two kegs, but is not commercially viable for a large system, or one with multiple nitrogenous ales.

Yes, theoretically, if one were to heat CO2 above it s critical temperature, and then manage to maintain that temperature while forcing it into, and holding it within a cylinder, it would remain gaseous. One could also, theoretically force N2 in with this heated CO2. However, if at any point, were the temperature of the tank to fall below the critical temperature of CO2, the CO2 would condense to liquid, and never return to a full gas state until below critical pressure.  Additionally, the challenge of keeping the tank at high temperature would be exacerbated by a constant self-refrigeration taking place within the tank valve, as gas is drawn off. Both of these gases will act as refrigerants, to some degree. CO2 is actually a very efficient refrigerant, in this regard, as it is very well suited to collecting and retaining heat energy, as it expands. Many large, modern refrigeration systems, such as are found in grocery stores, use CO2 as their refrigerant.  

    The laws of physics are laws.  They are proven facts.  These aren't theories.  CO2 performs as such.  Nitrogen performs differently.  Only, in gaseous form, will they act similarly, and play well together.  

    Is Beer Gas Bullshit?  Depends on the question.  If your question is, will beer gas improve pour and reduce waste, on a system that is currently using pure CO2, at pressures exceeding 15 PSI?  If this is a yes, then Beer Gas is a good thing, if not the best thing.  

    If you are asking if Beer-Gas Cylinders provide a reasonably consistent and correct blend of CO2 and N2, from start to finish, I have explained that they cannot.  So if this is the claim of a gas salesperson, such a claim is Bullshit, unless magic exists.  As to what the salesperson knows, believes, or has been taught, I cannot say, but make sure that , if they make a specific claim, they are willing to prove it.  

    If you find yourself in an awkward situation, where Beer Gas Helps, and you don't have the ability to correctly  upgrade your beer system, in the short term, Beer Gas can help immensely.  However, within a few months, I would recommend a conversion to an On-Demand Gas Blender, fed by cylinders or a Nitrogen Generator, or Beer Pumps.  I dig into that one more deeply, in INTRO TO BEER CARBONATION  For now, just be glad that you are well prepared to do what is best for your Beer system and for your Beer.  

Cheers

Dan Broaddus