Carbon Dioxide Compression

Carbon dioxide is a compound of carbon and oxygen in proportions by weight of about 27.3 percent carbon to 72.7 percent oxygen.  A gas at normal atmospheric temperatures and pressures, carbon dioxide is colorless, odorless, and about 1.5 times as heavy as air.  A slightly acid gas, it is considered by some people to have a slightly pungent odor and biting taste when, for example, it is used in “carbonated” soft drinks. 

Carbon dioxide gas is relatively nonreactive and nontoxic. It will not burn, and it will not support combustion or life.  When dissolved in water, carbonic acid (H2CO3) is formed.  The pH of carbonic acid varies from 3.7 at 1 atmosphere to 3.2 at 23.4 atm.  Carbon dioxide may exist simultaneously at its triple point as a solid, liquid, and gas at a temperature of -69.9oF (-56.6oC) and a pressure of 60.4 psig (416 kPa). 

Carbon dioxide is normally present in the atmosphere at about 0.035 percent by volume.  It is also a normal end-product of human and animal metabolism.  The exhaled breath contains up to 5.6 percent carbon dioxide.  The greatest physiological effect of carbon dioxide is to stimulate the respiratory center, thereby controlling the volume and rate of respiration.  It is able to cause dilation and constriction of blood vessels and is a vital constituent of the acid-base mechanism that controls the pH of the blood. 

Carbon dioxide acts as a stimulant and a depressant on the central nervous system.  Increases in heart rate and blood pressure have been noted at a concentration of 7.6 percent, and Dyspnea (labored breathing), headache, dizziness, and sweating occur if exposure at that level is prolonged.  At concentrations of 10 percent and above, unconsciousness can result in 1 minute or less.  Impairment in performance has been noted during prolonged exposure to concentrations of 3 percent carbon dioxide even when the oxygen concentration was 21 percent. 

At temperatures and pressures below the triple point, carbon dioxide may be either a solid (dry ice) or a gas, depending upon temperature conditions.  The triple point is the equilibrium of all three phases: solid, liquid and gas.  Solid carbon dioxide at a temperature of -109.3oF (-78.5oC) and 1 atmosphere transforms directly to a gas (sublimes without passing through the liquid phase).  Lower temperatures will result if solid carbon dioxide sublimes at pressures less than atmospheric. 

At temperatures and pressures above the triple point and below 87.9oF (31.1oC), carbon dioxide liquid and gas may exist in equilibrium in a closed container.  Within this temperature range, the vapor pressure in a closed container holding carbon dioxide liquid and gas in equilibrium bears a definite relationship to the temperature.  Above the critical temperature, which is 87.9oF (31.1oC), carbon dioxide cannot exist as a liquid regardless of the pressure. 

Safety, Storage and Handling:

Carbon dioxide is contained, shipped, and stored in either liquefied or solid form.  Additional information regarding the safe storage, handling, and use of carbon dioxide can be found in CGA G-6, Carbon Dioxide [6].  Applications using gaseous carbon dioxide are supplied by gas converted from liquid or solid carbon dioxide. 

Carbon dioxide is one and one-half times heavier than air and tends to accumulate in low or confined areas.  Positive ventilation may be required with the gas being exhausted near floor level where the carbon dioxide concentrates.  Appropriate warning signs should be affixed outside those areas where high concentrations of carbon dioxide gas may accumulate.

When entering low or confined areas where a high concentration of carbon dioxide gas may be present, use a self-contained breathing apparatus (SCBA) or supplied-air respirator.  Do not use a cartridge-type respirator. 

To dispose of carbon dioxide, vent slowly from the liquid phase of the cylinder to a well-ventilated outdoor location away from personnel or work areas.  This typically requires inverting the cylinder. 

Venting vapor could self-refrigerate the cylinders to a hazardously low temperature.  Return the emptied cylinders to the supplier with some residual pressure, with the cylinder valve tightly closed and with the valve protective cap(s) in place. 

Avoid contact of the skin or eyes with solid carbon dioxide (dry ice) or objects cooled by solid carbon dioxide.   If you come in contact with carbon dioxide, use the following first aid guidelines.

• Inhalation: Persons having inhaled large amounts of carbon dioxide and exhibiting adverse effects such as rapid respiration and headaches should be removed to fresh air immediately.  Perform artificial respiration if breathing has stopped.  Keep the victim warm and at rest and obtain medical attention at once.  Fresh air and assisted breathing (if required) is appropriate for all cases of overexposure.  Recovery is usually complete with no residual effects. 
• Skin Contact: In case of frostbite from contact with solid carbon dioxide (dry ice) or cold gases, place the frostbitten part in warm water, 100oF  to 105oF (37.8oC to 40.6oC).  If warm water is not available or is impractical to use, wrap the affected part gently in blankets.  Do not rub.  Consult a physician [5].
• Eye Contact: If the eyes are involved, obtain prompt medical attention.  The only appropriate first aid measure is a soft sterile pad held in place over both eyes.

Liquefied carbon dioxide is shipped in un-insulated cylinders, insulated portable tanks, insulated cargo tanks, and insulated tanks cars.  In high-pressure supply systems, it may be stored and used from single or manifolded un-insulated cylinders.  In low-pressure systems, it is stored in insulated pressure vessels with controlled refrigeration systems and auxiliary heating when required.  Normally, solid blocks of carbon dioxide weighing 1 lb to 50 lb (0.45 kg to 22.7 kg) are wrapped in plastic or heavy paper bags.  They are stored and shipped in insulated containers and storage boxes of varying size.  Extruded dry ice pellets are normally stored and shipped in bags or bulk in insulated containers. 

Large quantities of carbon dioxide for commercial use are primarily obtained from geological reserves or from by-product gas streams from one of the following processes: 

• Ammonia Production
• Ethyl Alcohol Production
• Hydrogen Production
• Ethylene Oxide Production 
• Synthetic Natural Gas Production 
• Acid Neutralization
• Power Plant Combustion

Solid carbon dioxide or dry ice is typically produced in three forms: blocks, pellets, or bulk “snow.”  It is manufactured by decreasing the pressure of the liquid below its triple point, forming a mixture of dry ice (snow) and cold vapors.  The solid may then be compressed into block or pellets.  Dry ice blocks are generally available in 10-inch (25.4 cm) 50-pound (22.7kg) cubes while pellets are available in various sizes.  The cold gases can be vented or compressed and reliquefied. 

Uses:

Solid carbon dioxide is used quite extensively to refrigerate dairy products, meat products, frozen foods, and other perishable foods while in transit.  It is also used as a cooling agent in many industrial processes, such as grinding heat-sensitive materials, rubber tumbling, cold-treating metals, shrink fitting of machinery parts, vacuum cold traps, and so on. 

Gaseous carbon dioxide is used to carbonate soft drinks, for pH control in water treatment, in chemical processing, as a food preservative, as an inert blanket in chemical and food processing and metal welding, as a growth stimulant for plant life, for hardening molds and cores in foundries, and in pneumatic devices. 

Liquid carbon dioxide is used as an expendable refrigerant for freezing and chilling food products; for low-temperature testing of aviation, missile, and electronic components; for stimulation of oil and gas wells; for rubber tumbling; and for controlling chemical reactions. 

Liquid carbon dioxide is also used as a fire extinguishing agent in portable and built-in fire extinguishing systems.

Reference the table below for examples and industries and applications using Carbon Dioxide