Nisith K. Das, R.K. Bhandari, Prasanta Sen and Bikash Sinha
Variable Energy Cyclotron Centre

Extraction of gaseous helium from natural sources is significantly important for its use in advanced research and frontier technologies. The second lightest element, helium, owing to its unique electronic configuration, is endowed with many extraordinary properties. Helium defies chemical combination with other elements. It is widely employed in the gas discharge lasers for transfer of energy to lasing gases. Its low cross-section for nuclear reactions under neutron bombardment and high thermal conductivity makes it suitable for use in nuclear power plants. One of the most important applications of liquid helium is as refrigerant for superconducting magnets and is also used in diverse fields like magnetic resonance imaging, magneto hydro-dynamics studies etc. It may well be used widely in future for superconducting power transmission cables. The primary commercial applications of gaseous helium are : welding, purging, pressurization and generation of controlled atmospheres. Other uses are for leak detection, deep sea diving breathing mixtures, chromatography and as a lifting gas for blimps and balloons. Helium, therefore, turns to be a commodity with wide potential applications in modern technology and has assumed considerable strategic significance.

Helium is conventionally derived from petroleum gas fields, however, the geographic distribution of helium bearing natural gas deposits is singularly uneven. The recognized helium rich (> 0.3 vol %) regions include the middle eastern parts of USA. Nearly ninety percent of the world’s exploitation is concentrated over there and the average helium concentration is around 0.8 vol %. Helium is also extracted in Canada, Algeria, Poland, Russia and China. The average concentrations of helium in the fields of these countries range between 0.18 vol % to 0.9 vol %. Since in India such favourable natural gas deposits are not explored yet, it seems logical to look for it in the unconventional terrestrial sources such as thermal spring emanations and monazite sands.

Table 1

Raw Gas Collection
Bubbling gas from thermal spring vents is collected via inverted funnel and passed through an online moisture trap and stored in two gas holders (capacity of 2 Nm3 each). This gas is further passed on to the chemical traps containing Glycol-Amine solution in the ratio of 1:1 in strip off moisture and carbon dioxide present in the raw gas. Subsequently the gas stream is fed into a solid dessicant tower filled with molecular sieves 13x and compressed into gas cylinders by the recovery compressor at a pressure of 32 bar g. The chemical traps are periodically reactivated using tape heaters for reuse. Figure 1 shows the raw gas filling station at Bakreswar. These cylinders are transported to the existing helium enrichment plant at VECC / SINP, for enriching helium.

Helium Enrichment
The key components of the enrichment plant, working on the principle of cryo-condensation, are the three distinct modules; a Storage unit, a Drier unit and a Condensation unit. In the first stage the feed gas available from the spring and compressed at a pressure of 32 bar g is put into a low pressure gas receiver where from it is transferred after compression at a pressure of about 150 bar g into the high pressure gas storage unit of 50 Nm3 capacity. The dry feed gas after pretreatment in the Drier unit enters the Condensation unit

at a flow rate of 16 Nm3 / hr and passes through the main heat exchanger I which is a brazed aluminum plate fin exchanger. This acts to reduce the thermal duty (pre-cooling) of the incoming gas from 312 K to 106 K by counter-flow of the return liquid nitrogen vapour from the gas liquid separator and from liquid nitrogen bath of the second heat exchanger. At this temperature all the components like argon, methane, oxygen and small quantity of nitrogen get condensed at a pressure of 15 bar g. This gas then passes into the coiled-tube heat exchanger immersed in liquid nitrogen bath where the feed gas temperature is further lowered to 91K. Consequently, at a pressure of 14.5 bar g most of the remaining nitrogen and traces of other gases become liquefied within the heat exchanger II and enriched helium (90 vol %) with small amount of nitrogen is withdrawn as the product gas and put into the receiver vessel. The helium enrichment plant, shown in Figure 2, can process a maximum of 50 Nm3 / hr of raw natural gas containing 1.0 vol % helium concentration to yield 0.5 Nm3 / hr of highly enriched (90 vol % )helium.

The helium enrichment plant has been designed, fabricated and developed indigenously . Figure 3 shows the gas chromatogram of raw and enriched Bakreswar thermal spring gas. This enrichment plant has been successfully and routinely operating for the last three years. The plant is open to all those who would like to enrich helium from a concentration of 0.7% vol % to 90 vol %.

Helium Purification
A helium purification plant, working on the principle of cryo-adsorption, has been installed and commissioned at VECC / SINP campus with a view to purify the enriched or impure helium to Grade-A (99.995 vol %) helium. The uniqueness of the plant rests on its capability of tackling impurity as high as forty volume percent. The purification skid is equipped with two phase separators, adsorber columns, vacuum jacketed cold box and the process logic controller with a local operator panel. The feed gas

impurities such as moisture and gross level of air and oil droplets (if any) are condensed in the two phase separators. In the purification phase, the impure helium gas at an elevated pressure of 125-150 bar g passes through a counter-stream heat exchanger where it is pre-cooled by the purified helium gas originating from the adsorbent bed. The gas stream is then led through four numbers of column where all gases except helium are adsorbed on active charcoal maintained at liquid nitrogen temperature (-196oC) resulting in the purified Grade-A helium. Figure 4 shows a typical result from quadrupole mass spectrometer showing the quality of product output. Since adsorption is a cumulative and reversible phenomena, after each run of 7 hours with a through-put of 17 Nm3 / hr at 1800 psi g, the adsorbents become saturated and call for regeneration and this is achieved by heating the adsorbents at 110oC for three hours followed by purging with pure helium. Figure 5 shows the helium purification plant which runs routinely giving Grade-A helium. This plant is also open to one and all who would like to purify helium from a concentration of 60 vol % to Grade-A (99.995 vol %) level.

Figure 3: Gas chromatogram of raw and enriched Bakreswar thermal spring gas	Figure 4 : Typical result from quadrupole mass spectrometer

Pressure Swing Adsorption
The transportation of cylinders filled with raw thermal spring gases containing helium about 1.4 vol % from field sites at Bakreswar and Tantloi to the enrichment plant at VECC / SINP campus (about 250 km away) constitutes a massive component of the entire process. It is always desirable to lessen the impurities right at the field sites thereby reducing the cost of transportation and improve the performance of the enrichment plant. For the said purpose a twin bed pressure swing adsorption (PSA) system using organic crystal materials was designed, fabricated and successfully demonstrated in the helium laboratory. This PSA system is accountable for reducing impurities like nitrogen leading to pre-enriching helium concentration to the tune of 15 vol % from about 1.4 vol %. This amounts to transporting about seven cylinders from Bakreswar in place of one hundred cylinders at present to the enrichment plant at the VECC / SINP at Kolkata. Figure 6 shows, of a twin bed PSA unit routinely working. The basic PSA process uses two fixed beds which operate 180o out of phase with each other and involves adsorption, desorption and re-pressurization steps. Molecular sieves 13x and activated alumina are employed for removal of the water vapour and traces of carbon-dioxide present in the feed gas available from thermal springs. This is followed by two stainless steel columns packed with specialized adsorbent, zeolite molecular sieve (ZMS). The nitrogen gets retained in the ZMS column as the feed gas keeps moving upwards. The cycle time, length to velocity ratio and high to low pressure ratio are optimized for efficient operation of the PSA. The two columns work alternately, while separation of helium occurs in one column at a predetermined pressure and time period, the second column is put under reactivation. Diversion of the inlet gas stream from one bed to the other is automatically actuated by solenoid operated valves and are precisely sequenced by a programmable logic controller. The PSA system is currently located at the thermal spring sites at Bakreswar. One of the advantages of the PSA system is that unlike the enrichment and purification plants where liquid nitrogen is utilized as external refrigerant, no liquid nitrogen is required to pre-enrich helium through the PSA system.

Exploration of Helium from Geothermal Springs
A state-of-the-art mobile helium exploration laboratory, funded by the Department of Science and Technology (DST) and DAE, has been custom built and recently put into operation. This mobile laboratory has a micro-gas chromatograph, radon monitor, water analyzer unit, helium leak detector, a generator set etc. for quantitative determination of helium concentration present at the various thermal spring sites. Preliminary explorations have been completed in Orissa (Taptapani, Attri and Trarbalo), Jharkhand (Tantloi), Assam (Garampani, Borpung, Gelepung, Borlungfar), and Meghalaya (Jakrem). The next phase of helium exploration would include the Himalayan Belt : Jamunotri and Monikaran
(Kulu Valley).

Extraction of Helium from Natural Gas
Looking at the national needs of Grade-A Helium in advanced technological programme, the extraction of helium from terrestrial sources such as thermal spring emanations and monazite sands does not look to be commercially viable proposals since the total source available for helium extraction from those sources is very limited. To ensure continued and reliable supply of Grade-A helium, one has to approach the extraction of helium from natural gas sources where the flow rates are extremely robust although the concentration of helium is very low, typically 0.05 to 0.10 vol %.
The major contaminants in the natural gases are primarily nitrogen and methane in addition to small amount of carbon dioxide and some higher hydrocarbons. Two procedures namely, adsorption and cryogenic, or their combination are, generally, employed for the purpose. The pretreatment of feed gas involves drying and removal of CO2 irrespective of the separation technique pursued. Adsorption process calls for the selection and availability of sieves and getting the isotherms particularly from a multi-component gas mixture, which is indeed, highly intricate issue. Even though, the entrapment of nitrogen is possible, the sieves for the methane adsorption, carbon molecular sieves, is simply tricky to obtain commercially. However, cryogenic process, in principle, can eliminate methane without much difficulty. Figure 7 gives a schematic view of helium extraction from natural gases.

In India such efforts have not yet started and it is high time that serious efforts should be made towards extraction of helium from available natural gas reserves to sustain helium related activities and become self-reliant. The experience gained by developing helium enrichment and purification systems for hot spring gases may be utilized for this purpose.