Biogas Condensers for Moisture and Vapor Removal

Saturated Gas, Low Pressure

Because the gas starts saturated, water begins condensing the moment it is cooled. Cooling saturated biogas from 100 F to 40 F removes roughly nine-tenths of its water. Methane and CO2 make up the bulk of the flow, so gas-side heat transfer coefficients are low and the unit behaves more like a gas cooler with a condensing film than like a steam condenser.

Pressure drop allowances of a few inches of water column are typical, which is why gas velocity, tube count and inlet nozzle size matter more here than in almost any other gas duty.

H2S, Siloxanes and Wet Corrosion

The condensate absorbs H2S, CO2 and ammonia and runs acidic, and where it evaporates and re-wets it concentrates. Carbon steel condensers can fail within a couple of years, starting at the gas inlet and the condensate collection zone. 316L on every gas-wetted surface is the baseline. Chilling also drops out part of the siloxane load, which reduces deposits on engine valves and extends carbon bed life.

Air Cooler First, Chiller Second

A Horizontal Fin Fan cooler is an efficient first stage on warm digester gas, shedding heat to ambient without a chiller. It cannot reach a 40 F dew point, and in winter it needs protection from freezing condensate. A Fixed Tubesheet shell and tube unit with glycol on the shell then sets the final dew point, and a gas-to-gas reheat exchanger after it lifts the gas back above saturation so downstream lines stay dry.

Getting the Water Out

A chilled biogas stream produces a steady flow of water that has to leave without letting gas out. A correctly sized liquid seal or drain trap, heat traced where it can freeze, prevents both gas release and condenser flooding.

Put a demister downstream of the chilling stage. Fine droplets that slip past the condenser carry dissolved H2S into the carbon bed or straight into the engine intake.

Engines, Upgrading and Flares

An engine wants gas well below saturation at its inlet so no liquid forms in the fuel train. Upgrading systems want drier gas still, since water competes with CO2 in membranes and fouls PSA media. Tell us what sits downstream, the gas flow in scfm or Nm3/hr and the methane content, because the dew point target follows from the user.

Flares need little drying, but a flare that backs up an engine still benefits from a drained header, since liquid slugs snuff pilot flames and corrode burner tips.

Digester and Weather Swings

Digester gas temperture and production swing with feedstock and season, and an air-cooled stage swings with ambient. A unit sized on a winter test can run short in July. Trend gas outlet temperature and chiller load through the year, and check the condensate drain strainer often, since sludge and siloxane fines collect there first.