Generally the dew point required are as follows:

 

Use in internal combustion engine for electric power generation:

Dew point <12°C @ 1 atm

Use as low pressure pipeline gas for residential/industrial use:

Dew point <5°C @ 1 atm

Use as CNG for motor vehicle:

Dew point <-10°C @ 1 atm

Use as LNG :

Dew point <-165°C @ 1 atm

 

 

Subcooling and Reheating Process

The process consist in cooling the biogas to a temperature lower, by a few degrees, than the desired dew point temperature, to condense the humidity along with desolved compounds such as hydrogen sulfide and siloxanes as well as low boiler hydrocarbons.

 

The gas is then re-heated to the desired temperature through a compression stage compression.

 

 

Vacuum Pressure Swing Adsorption (VPSA) Process

 

This process is primarily used for the dehumidification of biogas destined to the production of compressed natural gas (CNG) whether for transportation CNG or pipeline CNG.

 

The process is very similar to the VSA process with the exception that the feed gas is under pressure and that the desorption sequence is designed to minimize the loss of methane as well as the energy usage (loss of pressure).

 

Generally the process consists of four (4) towers in parallel which can be expanded up to eight towers. At all times one tower is in the adsorption mode while others are each in one of  depressurization, vacuum, re-pressurization modes.

 

 

Pressure Swing Adsorption (PSA) Process

 

This process is used at high pressure for the drying of gas from the discharge of carbon dioxide removal system, to very low dew point prior to the liquefaction of natural gas.

 

The duty of the PSA in this application is to remove all humidity from a gas saturated with humidity as it exits the amine scrubbing system (CO2 removal system).

 

The dehumidification load is high and the required output is very stringent. PSA at high pressure is the only recommended process for this duty.

 

The saturated gas enters the first tower where all humidity will be remove in a single pass. The other towers are to recover the most pressure and methane in a sequence similar to the VPSA sequence, only at a much higher pressure.

 

Dew point of -70˚C are obtained with this process.