a natural by-product of the removal of nitrogen from the gas stream to increase its heating value. In other cases, process streams are designed specifically to remove helium.
The processing of helium from natural gas can generally be considered as occurring in two distinct processes, although both processes can and do occur at the same physical location. The first step is the extraction of crude helium (50 to 70 percent by volume) from the natural gas stream. The second step is further refining to purify the helium to commercial grades.
Helium is often separated from natural gases in the course of removing nitrogen to improve heating value. In the United States the lowest practical helium concentration that can economically justify extraction is typically around 0.3 percent by volume. Sometimes, however, the helium is not extracted from high-concentration natural gases and is simply vented to the atmosphere when the natural gas is burned as fuel.
Determining the feasibility of extracting helium from a particular source of natural gas is extremely complicated and is influenced by a combination of technological, logistical, and economic factors. For example, too small a reserve base may disfavor the installation of expensive helium extraction and/or purification facilities. Economic and technical considerations surrounding other products in the natural gas stream and contractual obligations can also affect the economics of helium extraction. All of these factors must be taken into account before a helium extraction site can be planned and established.
Extraction of crude helium from natural gas typically requires three processing steps. The first step is the removal of impurities. Amine and glycol absorption, dry desiccant adsorption, and/or other extraction processes typically remove water, carbon dioxide, and hydrogen sulfide from the gas. The second step is the extraction of the high-molecular-weight hydrocarbons. The third step is cryogenic processing, which removes most of the remaining methane gas. The product is a crude helium typically containing 50 to 70 percent helium, with the remainder being primarily nitrogen along with smaller amounts of argon, neon, and hydrogen.
Final purification of helium, prior to liquefaction, is typically done using either (a) activated charcoal absorbers at liquid-nitrogen temperatures and high pressure or (b) pressure-swing adsorption (PSA) processes. Low-temperature adsorption can yield helium purities of 99.9999 percent, while PSA processes recover helium at better than 99.99 percent purity (Hwang et al., 1995). PSA can be less costly for gaseous helium but may be more costly where liquefied helium production is desired. The PSA process is widely used to produce specification-pure helium in conjunction with cryogenic enrichment (Hwang and Weltmer, 1995).