Gas saturated with water at 100°F and atmospheric pressure is about 6.5 percent water vapor, roughly 65,000 ppm. The beverage grade limit for moisture is 20 ppm. Closing that gap means removing more than 99.97 percent of the water, and every bit of it has to be gone before the CO₂ reaches the cold section of a liquefier.
Biogas leaves a digester wet, and the CO₂ split off it in an RNG upgrader usually stays wet. On a membrane unit, water vapor permeates along with the CO₂ and ends up in the off-gas. The figures below use a stream saturated at 100°F as a worked case. They are our own calculations from standard water vapor pressure data, and a real stream will sit at or below that line.
Compression Does Most of the Drying
The first large cut comes almost free. A liquefier needs the gas at pressure anyway, and compressing wet gas raises the partial pressure of the water until it condenses in the aftercoolers.
Compress the worked case to 300 psig and cool it back to 100°F, and an ideal gas estimate says the gas can now hold only about 3,000 ppm. Roughly 95 percent of the water drops out as liquid in the separators.
That still leaves about 150 times the beverage limit. The last 5 percent is the hard part, because everything downstream runs colder than the temperature at which that water will stay a vapor.
Water Can Turn Solid Above Freezing
Below 32°F, water leaving the gas forms ice. Under pressure it can go solid at warmer temperatures too. Water and CO₂ together form CO₂ hydrate, an ice-like solid in which water molecules cage the CO₂.
Measured equilibrium data place the lower quadruple point of the CO₂ and water system at about 1.04 MPa and 29°F, and the upper one near 4.5 MPa and 50°F. Between those two points, the temperature at which hydrate is stable climbs with pressure, above the freezing point of water. Ice or hydrate, the solid collects on heat exchanger surfaces, valves and filters and narrows the flow path until the plant has to stop and thaw.
A 2024 study by SINTEF and Equinor, published in Industrial & Engineering Chemistry Research, measured how fast that happens. Liquefying CO₂ from an amine capture plant at about 7 bar with 200 ppm of water, the team saw solids begin forming at about 6.7 bar, and a test filter blocked completely in roughly an hour. Because the pressure was above the triple point, the authors attribute the solids most likely to hydrate.
At 100 ppm, the same low pressure condition showed no rise in pressure drop. At 16 bar, 200 and 300 ppm produced no sign of solids at all. The tolerance moves with pressure and temperature, and the distance between a clean run and a blocked filter can be a few hundred ppm.
The Stripper Cannot Remove Water
In a CO₂ liquefier, the stripping column handles the light gases. Nitrogen, oxygen and methane are more volatile than CO₂, so they boil out of the liquid and leave through the vent, as our post on liquefaction vent losses walked through.
Water sits at the other end of the scale. At cold section temperatures its vapor pressure is a tiny fraction of CO₂'s, so a column treats it as a heavy component. Heavy components go to the bottom, which is where the product is drawn. Water that gets past the dryer does not leave through the vent. It ends up in the liquid CO₂, frozen onto a cold surface on the way there, or both.
That makes moisture different from most other lines on a certificate of analysis. Sulfur compounds, hydrocarbons and inert gases each have a process step that can deal with them. For water, the only step that counts is the dryer, upstream of the cold section.
What 20 ppm Means at the Dryer
Dryers on CO₂ service usually sit after compression and the aftercoolers, where most of the water has already condensed out and the gas is at pressure. A common arrangement uses two desiccant beds, one adsorbing while the other is heated to drive off what it collected.
Moisture is often reported as a dew point, so the translation matters. By our calculation, 20 ppm of water corresponds to a frost point of about minus 67°F at atmospheric pressure. The same 20 ppm at 300 psig has a frost point of about minus 20°F, because the water's partial pressure rises with total pressure. A dew point reading means nothing without the pressure it was taken at.
For a beverage plant, the specification is the tighter limit. In the SINTEF tests, liquefaction at 16 bar ran without solids at 300 ppm, which is 15 times the beverage limit. A dryer sized only to keep the liquefier running can still produce liquid that fails the moisture line.
A Bed Run Too Long Passes Water
Desiccant has a fixed capacity. A bed that runs past its loading starts letting water through, slowly at first and then quickly once the adsorption front reaches the outlet.
Two things push a bed there early. One is a warmer or wetter feed than the design assumed. The other is regeneration that leaves the bed partly loaded when it goes back in service. A moisture analyzer on the dryer outlet catches either problem before the cold section does.
Ambient conditions feed straight into this. At the same pressure, a stream saturated at 100°F holds about 36 percent more water than one saturated at 90°F. An aftercooler running warm on a July afternoon sends that difference to the dryer.
What This Means for a Project
For an RNG operator looking at CO₂ recovery, the water in the off-gas is a design input alongside its CO₂ and inert content. Temperature and pressure at the tie-in point, and whether the gas leaves saturated, set the knock-out duty, the dryer size and the regeneration energy. The summer peak of that temperature belongs in the design basis, not the annual average.
CleanCycleCarbon's patent pending cryogenic process was designed for biogas-derived CO₂, where the feed always arrives wet, and entered continuous commercial beverage grade production in December 2024. Beverage grade at 99.9 percent is the spec we certify to, and the 20 ppm moisture limit is part of it.
For a buyer, the moisture line on a certificate of analysis reports on one piece of equipment that sits upstream of everything credited with purity. A source can pass purity, sulfur and benzene and still produce wet product if its dryer runs at the margin. Purity gets made in the cold section. The moisture number is settled before the gas ever gets there.
