Cryogenics 101

Cryogenic technology makes many gas applications possible.

Every time I get on a plane with my “cryogenics” logo shirt on, someone invariably asks me, “Do you have Ted Williams frozen?”

The general public often confuses “cryonics,” the freezing of bodies and body tissue, with “cryogenics,” the science that involves very low temperatures, usually regarded as below -150° F (-101° C). Rather than frozen .400 hitters, this discussion will deal with cryogenics as it relates to the production and distribution of industrial, medical and welding gases. Cryogenic technology is the keystone of our industry because it allows for the efficient and cost-effective production and distribution of the gases that our customers need.

Cryogenic Air Separation
The most common gases we deal with on a daily basis are the atmospheric gases: oxygen, nitrogen and argon. These gases are liquefied and separated from the atmosphere most frequently in large-scale cryogenic air separation plants. In simple terms, an air separation plant draws in air and compresses and cools the air using several different refrigerants and a multi-stage compressor. When certain temperature and pressure requirements are met, the air is fed through a special valve that rapidly drops the pressure of the air, causing it to cool to the point of liquefaction. This phenomenon is known as the “Joule-Thompson Effect.” You have probably seen an example of this effect when you vent a high pressure cylinder and observed the valve frosting up. Again, this cooling is caused by the rapid expansion of the compressed gas in the cylinder as it is released to the atmosphere.

transport
Cryogenic transport filling Photo courtesy of Cee Kay Supply

After liquefaction, the liquid air is introduced to a vertical column for fractional distillation. This distillation process is the same as the experiment you performed in high school or the “still on the hill,” only the temperatures are much lower. Fractional distillation uses the differences in boiling points of individual components of a mixture to separate the components. Air is a mixture of gases, with over 99.9 percent made up of nitrogen, oxygen and argon. Nitrogen boils at the lowest temperature, about -320° F, and moves up the column and is recovered, re-liquefied and sent to storage. Since argon and oxygen have boiling points only a few degrees apart (-303° F and -297° F), argon is separated from the oxygen in a special separate column. In very large air separation plants (approximately 1000 tpd), some of the minor components, such as neon, krypton and xenon, can be recovered as well. Carbon dioxide, helium and hydrogen also exist in small quantities in the air, but are not economical to recover in this process. These gases are produced more economically as byproducts of other industrial processes.

Distributors often ask me why the bulk gas products they purchase are so expensive when the raw material (air) is free. While the process described above sounds simple, a modern air separation plant is actually very complex. The real raw material in the process is electrical power. Today an efficient scale plant costs in excess of $30 million and has more than 20,000 connected horsepower. Power is generally responsible for more than 50 percent of the cost of producing a volume of product.

Cryogenic Storage and Transport
In addition to the ability to separate air into its components as a cryogenic liquid, another equally important reason exists to liquefy these gases. Large volumes of gases are much easier to store and transport as liquid than as a gas. A given volume of liquid oxygen expands by over 800 times when vaporized to a gas at atmospheric temperature and pressure. A modern liquid oxygen transport trailer can haul over a half-million cubic feet of liquid oxygen at 15 psi, while a jumbo gas tube trailer pressurized to 3,000 psi would only carry 160,000 cubic feet. It is much safer and cost effective to load, unload, store and transport the low-pressure liquid than the high-pressure gas.

The biggest problem in the transport and storage of cryogenic liquefied gases is preventing heat from the ambient surroundings from getting to the liquid and vaporizing it back to a gaseous state. Heat naturally flows from hotter objects to colder objects. The rate at which this occurs is known as the heat transfer rate. All the thermodynamic work put into the gas at the air separation plant will be wasted if heat is allowed to vaporize the cryogenic liquid.

To prevent this, cryogenic storage vessels and transports are constructed out of specialized materials and use vacuum jacketing technology to minimize heat introduction. Materials such as high nickel stainless steel and special aluminum alloys do not embrittle at cryogenic temperatures and are used in the construction of the inner vessels of storage tanks and transports. A second outer vessel contains insulation and provides an annular space that is evacuated to a high vacuum. The insulation and the high vacuum serve to reduce the rate of heat transfer to the cryogenic liquid by interrupting the mechanisms of heat transfer.

dewar
Cryogenic dewar filling Photo courtesy of Cee Kay Supply

The primary heat transfer mechanisms at low temperature are conduction and convection. Conduction is the transfer of heat by molecule-to-molecule contact, similar to what you might experience when you place an iron skillet on a hot stove. Heat is conducted from the hot burner into the skillet, then to your hand on the handle. Convection is the heat transferred from the flow of molecules past other molecules. The warm air from a hair dryer warming your skin is an example of convection. By maintaining a high vacuum on the annular space of a cryogenic tank or transport, we remove almost all of the molecules available for heat transfer. By slowing the rate of heat transfer to a minimum, we can transport and store our cryogenic gas until we are ready to use it.

Cryogenic Pumping for Cylinder Filling
Transporting and storing gases as cryogenic liquids is critical to the economics of cylinder filling as well. It is far less expensive to pressurize a cryogenic liquid than to compress a gas to the pressure required in industrial, medical and welding gas cylinders. A typical liquid oxygen DPD style cryogenic pump operating at 2.2 gallons per minute can fill 60 standard 251 cubic foot cylinders per hour with a 10 horsepower motor. To compress the same volume of oxygen as a gas would require a large compressor with a motor in excess of 200 horsepower. In addition to the much higher operating expense, the compressor would have a capital cost over ten times the cost of a same capacity cryogenic pump. Many of the gases used by our customers today would be much more expensive if we had to compress them as a gas.

Cryogenic pumps can be very reliable and low maintenance if they are installed and maintained properly. These pumps are designed to pump low-temperature liquid cryogens and must be properly cooled down and have a consistent and unrestricted supply of liquid to them. Trying to pump before the pump has cooled down, or supplying the pump with two-phase flow (both gas and liquid), will result in pump cavitation and decrease the life of the cold end wear components.

A lot of common pump problems can be eliminated with a proper installation. The pump should be located very close to the cryogenic tank supplying the liquid feed source. The tank should be elevated on tank legs a minimum of two feet to provide a continuous downward slope for the feed and return lines and to ensure the pump has sufficient suction head when the tank is low on product. When installing the feed and return lines connecting the pump, care should be taken to avoid 90-degree elbows and any excess fittings that can cause turbulence and result in cavitation. Flexible vibration eliminating sections should be installed to isolate vibration from the pump from reaching the tank. Lastly, a modern electrical cryogenic pump control panel with auto cool down and cavitation protection can eliminate accidental operator errors and keep those cylinders pumping.

Cryogenic technology has made possible many of the customer applications of gases we serve today. While cryogenics may not get the same public attention as cryonics, it is far more important to those of us still alive who deal with cryogenic applications every day.

Gases and Welding Distributors Association

Kent Buzard Meet the Author
Kent Buzard is vice president of business development for CTR Inc., located in Rock Hill, South Carolina, and on the Web at www.ctrinc.com.