Following is a short explanation of the typical process units required in most modern CO2 plants.

Inlet Cooling and Scrubbing
This is the pre-cooling and initial clean-up stage. The CO2 is usually received from the source at a temperature between 100° F and 120° F, saturated with water. The gas is usually cooled to approximately 100° F with tower water and may be further cooled to approximately 50° F with refrigerant. This process is for the purpose of removing the bulk of the contained water and many of the water soluble components.

Compression
CO2 compression is usually performed in two stages from approximately atmospheric pressure to a pressure in the area of 300 PSIA with intercooling between stages. Screw compressors were first used for food grade CO2 compression in 1981. They have since become the standard in the industry. The key to their application is lubricant separation systems, which lower the lubricant levels in the CO2 product by orders of magnitude. The lubricant content of CO2 compressed in a properly designed screw compressor system is far less than in a "mini-lubed" reciprocating compressor and is typically comparable to many types of "non-lube" reciprocating systems. Laboratory analysis typically shows quantities of lubricant in the CO2 in single digit parts per billion - normally in the 5 PPB range or less in many cases. Other advantages of screw compressors are:

• 30,000 hours operating time between inspections
• ability to "float" on line with changing flows
• minimal foundation requirements providing the ability to completely skid plants, thus making the purchaser investment in bricks and mortar minimal and also making plants easy to move if a source is lost
• plants more adaptable to automation and unattended operation
• screws are easy to monitor via vibration readings with more predictable Mean Time Between Failures

Oxidation
This stage is typically not required unless heavier hydrocarbons are present in the CO2 stream. The system heats the CO2 gas to approximately 1000° F to 1250° F, injects oxygen into the CO2, takes the CO2 through a platinum catalyst at high temperature, during which the hydrocarbons are converted to CO2 and H2O. Heat recovery is an integral part of the process. In many cases, additional heat is not required after start-up.

Aftercooling
Typically uses shell and tube exchangers in two stages. First, the CO2 is cooled in a water cooled exchanger to about 100ºF; secondly, in a refrigerant cooled exchanger to about 50° F to remove as much water as possible prior to dehydration.

Adsorption
In this process several different contaminants may be removed using adsorptive materials such as plain carbon, coated carbon, zinc oxide or others. The contaminants removed would include hydrogen sulfide, mercaptans, and a range of aromatic hydrocarbons such as acetaldehyde, amyl alcohol, and benzene. This unit may be placed at different points in the system depending on the component being removed.

Drying
CO2 is typically dried to minus 80° F dew point (7.8 PPMV of water). Drying systems are normally controlled by moisture content of the leaving CO2 or may be on a timed cycle. The standard desiccants are alumina and molecular sieve, with silica gel being used occasionally.

Condensing
CO2 condensing is normally accomplished in the region of 265 to 300 PSIA at minus 20 to minus 30° F, depending of the plant preference and the system requirements. The condenser is typically a shell and tube exchanger with CO2 on the tube side.

Distillation
Liquid CO2 from the condenser is processed in this unit to remove air, methane, hydrogen and other low boiling point elements to provide the required CO2 purity. Vent condensing (cooling the column vent to a lower temperature, thus partially condensing the CO2 contained in the vent stream) may be used for higher percentage of CO2 recovery, particularly if the inlet CO2 stream has a high non-condensable content.

Subcooling
This stage cools the liquid CO2 stream below the associated saturation pressure prior to entering storage. This cooling lowers both tank and transport vent losses.

Storage and Loading
The liquid CO2 is usually stored in a carbon steel "bullet" type tank with a minimum of two tanks per plant. The total minimum capacity should be no less than three days plant production (100 tone per day plant would use two 150 tone storage tanks minimum). Loading of the trucks typically requires 30 minutes to an hour based on a nominal 20 ton tank truck. The loading pumps used may be positive displacement or centrifugal.

Refrigeration
The refrigerant of choice in the majority of plants is ammonia, which is very efficient, readily available, low cost, and self-alarming. The refrigeration normally takes place at two levels: approximately minus 20 to minus 30 for the low stage and at 35 to 50° F for the high stage. Finally, the refrigerant condensing could be either air-cooled, water-cooled shell and tube or evaporative type.