Take a CO₂ stream carrying 1 percent nitrogen, compress it to 300 psig, and chill it to 0°F, the conditions inside an ordinary bulk storage tank. About 8 percent of the CO₂ leaves through the vent, roughly eight moles of CO₂ for every mole of nitrogen. The condenser is working exactly as designed. Physics sets the result, and it catches anyone who treats liquefaction as the easy last step after purification.
Every liquefier has a vent, because the gases that will not condense at CO₂ conditions have to leave somewhere. Nitrogen, oxygen, argon and methane slip all go out that way. None of them leaves alone.
Liquid CO₂ Has a Narrow Window
CO₂ is only a liquid inside a fixed band. Below 75 psia, about 60 psig, it cannot exist as a liquid at any temperature, which is why dry ice turns straight to gas on a loading dock. The floor of the band is the triple point, minus 69.8°F. Colder than that, CO₂ freezes onto heat exchanger surfaces. The ceiling is the critical point, 87.8°F and 1,070 psia. Above that temperature no amount of pressure will make it condense.
Industrial liquefaction works in the lower part of that band. Pure CO₂ saturates at 291 psig at 0°F and at 200 psig at minus 20°F. Our own process overview puts liquefaction at roughly minus 20°F and about 300 psi, and that is the case worked through below.
The Vent Is Saturated With CO₂
When a mixed gas is chilled in a condenser, the vapor that stays behind is not pure inert gas. It is in equilibrium with the liquid CO₂ sitting below it, so it carries as much CO₂ as that temperature allows. At minus 20°F and 300 psig, the vapor leaving a CO₂ condenser is about 74 percent CO₂.
Put another way, every mole of nitrogen that leaves takes about 2.8 moles of CO₂ with it. By volume, that is nearly three cubic feet of CO₂ for every cubic foot of nitrogen. Oxygen, argon and methane behave almost identically at these conditions, at about 2.7. What sets the loss is how much inert gas is in the feed, not which gas it is.
These figures come from equilibrium calculations with a standard mixture equation of state for CO₂ paired with each inert gas. A real vent condenser never fully reaches equilibrium, so a working plant does no better than this and usually somewhat worse.
Inert Content Sets Recovery
Once the ratio is known, recovery follows from the feed composition. Under those same condenser conditions, a feed with 1 percent inerts loses about 3 percent of its CO₂ out the vent. At 2 percent inerts the loss is about 6 percent. At 5 percent it is 15 percent, and at 10 percent it is close to a third.
That last case is not exotic. RNG upgrader off-gas can arrive anywhere from 90 to 98 percent CO₂, with methane slip plus whatever nitrogen and oxygen the upgrading step picked up. On a system sized for 30 tons of CO₂ per day in the feed, a stream at 5 percent inerts sends about 4.5 tons a day out the vent at these conditions. The feed gas tonnage and the tonnage that reaches the tank are two different numbers, and the inerts decide the difference.
Colder or Higher Pressure
There are two ways to shrink the ratio, and both cost energy.
The first is temperature. At minus 40°F and the same 300 psig, the vent drops to about 53 percent CO₂ and the ratio falls to 1.1. The 5 percent inert feed now loses about 6 percent instead of 15. At minus 60°F the ratio is about 0.56. Each step down costs more refrigeration, and the triple point at minus 69.8°F is a hard wall. Any cold surface that reaches it starts collecting solid CO₂.
The second is pressure. Holding minus 20°F and raising the condenser to about 385 psig brings the ratio down to 1.6. That comes out of the compressor, on every ton, every hour the plant runs.
Why the Stripper Does Not Help
Colder has a catch that shows up in the product. The same equilibrium that pulls CO₂ out of the vent pushes inert gas into the liquid. At minus 20°F and 300 psig, liquid CO₂ in contact with nitrogen holds about 1 percent dissolved nitrogen. In contact with methane it holds about 2.6 percent. At minus 60°F those climb to about 2 percent and 6 percent.
Beverage grade is 99.9 percent CO₂ minimum. That leaves a budget of 0.1 percent for everything else combined, so liquid straight off a condenser fails the spec by an order of magnitude. A stripping column fixes this by boiling the dissolved gases back out of the liquid. Stripping gives those molecules somewhere to go, and the only exit is the vent.
So at steady state, every molecule of inert gas that enters with the feed and does not stay in the product leaves through the vent. A column hitting 99.9 percent has to send nearly all of the inerts that way. That makes the vent condition the number that matters, since it is the coldest point the inert gas passes before it leaves. The purity target sets how many inert molecules have to go out, and the vent temperature and pressure set how much CO₂ each one takes along.
Where the Vent Gas Goes
On an RNG site the vent stream is mostly CO₂ with methane mixed through it. It cannot be released as is, so it needs a destination that can handle methane, and the thermal oxidizer is already on site. CO₂ the liquefier failed to keep ends up back at the same equipment a capture project was built to bypass.
What This Means for a Project
Liquefaction is usually described as the settled part of CO₂ capture. Purification gets the engineering attention, and the condenser is assumed to turn whatever arrives into tank inventory. The equilibrium numbers say otherwise. On a lean feed, the condenser's operating point decides whether the plant keeps 97 percent of its CO₂ or closer to two thirds.
That changes the first question to ask about a source gas. A single CO₂ percentage is not enough, because the balance can be water and sulfur that pretreatment removes, or nitrogen, oxygen and methane that it does not. The analysis needs those three broken out individually, since together they fix the recovery before any equipment is chosen. Our cryogenic purification, now patent pending, was designed around biogas streams where those inerts are always present. CleanCycleCarbon entered continuous commercial beverage grade production in December 2024 on exactly that kind of feed.
Two sources reported at the same CO₂ percentage can carry very different inert fractions once the water and sulfur are out. The one that looks slightly worse on paper can put more product in the tank, because the vent takes its share in proportion to the inerts, ton for ton, every day the plant runs.
