H elium is present in much of the world’s natural gas fields at a wide range of concentrations. In the past, only gas fields containing 0.3% helium or higher could justify the high cost

of recovery.1 Decades of technology development coupled with improved economics has lowered this threshold.2 Even though one-third of the world’s helium comes from LNG operations, helium recovery is practised only at three locations out of the large number of the natural gas liquefaction plants around the globe: Qatar, Algeria and Australia. With the recent shifts in helium supply and demand, an increasing number of LNG plants may be amenable to helium recovery,3 coinciding with an “unprecedented LNG capacity expansion” around the world.4

Walter Nelson and Jin Cao, Air Products

and Chemicals, Inc., USA, present an unconventional

approach to helium recovery from LNG.

Reprinted from December 2019

Conventional helium extraction from LNGMost helium extraction techniques used today require a high level of refrigeration, and are thus energy-intensive processes. Natural gas liquefaction is an attractive point of helium extraction because only an incremental amount of energy is needed, if there is enough helium in the natural gas, normally at least 0.1 – 0.5% (1000 – 5000 ppm). In the early 1990s, Air Products became the first company to design and build a helium refining plant to process crude helium that had been extracted during the production of LNG in Algeria.

Most LNG processes require an end flash after liquefying the natural gas in the main cryogenic heat exchanger (MCHE) to reduce the nitrogen content of LNG to less than 1 – 1.5% to avoid auto-stratification in the LNG tank and the potentially dangerous roll-over.5 Several configurations of such an end-flashed LNG process with varying degrees of complexity and control are presented by

Chen et al.6 The simplest example of such a process employs an isenthalpic expansion, and a flash separator, which can be replaced with a nitrogen rejection column to provide more operating control. The vapour from the flash drum contains a large fraction of the dissolved helium in the LNG at a much higher concentration, along with the desired amount of nitrogen removed plus some methane. The vapour is normally used to satisfy the fuel requirements of the LNG plant. An added step of partial condensation of the vapour can lead to very high helium concentration (approximately 50%) suitable for purification. Such a helium upgrading step integrated with the end flash is described in the 2008 patent of Roberts and Repasky.7 The patent gives the example of upgrading an LNG stream with 0.04% (400 ppm) helium, similar to that of the feed to the Ras Laffan plant in Qatar,8 to a crude helium of approximately 46%. The upgrading factor of helium concentration can vary widely according to multiple operating parameters, as well as constraints such as plant fuel requirements, nitrogen specifications and LNG production rate.6 Often, a crude stream with a helium concentration in the range of 7 – 15% is produced from the LNG process, which is fed to a helium purifier system to produce 99.999% helium ready to be liquefied. Because the purifier system operates at a lower temperature than the LNG end flash to ensure good separation of helium from N2 and CH4, deeper removal of freezable impurities will be required, or a separate temperature swing adsorption (TSA) unit may need to be added upstream of the helium purifier.

Helium extraction from BOGRecently, attention has been paid to the possibility of extracting helium from LNG tank boil-off gas (BOG).

Natural gas with a high enough concentration of helium (>1000 ppm) and a large enough flow would have a conventional helium extraction process built-in before it gets into the LNG tank. Therefore, most LNG tank BOG, despite containing the majority of the dissolved helium in LNG before flashing into the tank, contains too little helium to be interesting in the past. However, with improved economics, the concentration threshold worthy of examination for helium extraction opportunities continues to fall. In particular, liquefaction plants without an end flash or a cryogenic distillation column after the LNG MCHE would have all the helium trapped in the LNG until it is flashed into the storage tank. Assuming a very low concentration of 200 ppm helium in the natural gas, the BOG will have a helium concentration many times higher (potentially >1%), containing greater than 99% of the total helium in the feed.

Chen et al.6, 8 described a novel approach of nitrogen removal from LNG with a nitrogen rejection column taking in both the throttled, partial vaporised LNG and the cooled, partially condensed BOG. Such a process not only provides a more energy-efficient way to recycle the BOG, but also presents a way to reject nitrogen and other lights without losing methane. Helium in the original natural gas feed

Figure 1. Air Products NGP helium recovery system implemented on boil-off gas (BOG).

Figure 2. Details of Air Products NGP helium recovery and purification system shown in Figure 1.

Table 1. A capital and power comparison of the Air Products NGP helium recovery and purification process versus the traditional cryogenic alternative

Air Products NGP process

Cryogenic alternative

Helium in feed 0.5%

Source of feed Cold end flash, downstream of compression

Equipment cost 1.0 1.85

Power 1.0 7.0

Total installed cost 1.0 2.18

Reprinted from December 2019

would be concentrated in the rejected nitrogen and can then be recovered using the process described by Roberts and Repasky.7

Xiong et al. described two cryogenic cycles to recover helium from LNG tank BOG estimated at around 1% concentration.9 Both cycles require cooling down compressed BOG to cryogenic temperatures where most of the helium and nitrogen are stripped out of the methane using a distillation column. The column overhead vapour containing mostly nitrogen and helium is then cooled down further and partially condensed to separate crude helium from a nitrogen waste stream. To achieve the recovery of both helium (>95%) and CH4 (100%) the authors desired, an external refrigerator with either N2 or helium as the coolant was employed, adding to the complexity of the cycle.

Novel non-cryogenic hybrid helium extraction and purificationThe conventional approach of helium extraction from LNG requires a high degree of process integration due to the significant refrigeration required. Air Products has a long history of designing and building cryogenic helium recovery systems integrated with the main LNG process. Although feasible, retrofitting such cryogenic systems after the plant is built and running can be costly and may also impact the operation of the plant. Therefore, the cryogenic helium extraction part of the process should

ideally be included during the original design of the LNG process for it to be economical. Moreover, the process economics of cryogenic cycles generally do not extend well down to small scales, nor do such cycles handle feed flow variations or on-off operating modes easily, whereas LNG BOG flow and helium composition can vary widely as a result of ship loading and/or the operation of the upstream LNG trains.

A novel method is described here to recover helium from the LNG with the benefit of easy retrofit, minimal process integration, design and operational simplicity, and low cost. This process – Air Products Noble Gas Purification (NGP) – can be built during the construction of the LNG project, or can be added anytime afterward with minimum impact to the main process, allowing a great deal of flexibility in project execution. It can be added to accept the vapour from the end flash, fuel or nitrogen vent from the nitrogen removal column, or the BOG directly after the BOG compressor (Figure 1). No refrigeration is required, and no pressure loss is incurred on the fuel or BOG stream.

This patent-pending NGP helium recovery and purification process is depicted in detail in Figure 2. The feed gas, end flash gas taken after the fuel gas compressor or compressed BOG, is directed through a first membrane system, which preferentially permeates helium. The membrane design, membrane area and the permeate pressure, can all be used to optimise helium recovery against the downstream equipment cost and compressor

power. In general, approximately 90% of the feed stays in the non-permeate with minimal pressure drop – only a few psi. Although CH4 and N2 will co-permeate through the membrane, the selectivity of helium versus these slow gases results in a concentration of the helium in the permeate. There is a trade-off between helium recovery and the upgrading factor of the permeate, but a 90%+ helium recovery and a four to five-fold increase in the helium concentration in the permeate can be easily achieved. Referencing Figures 1 and 2, the upgrading of the helium is illustrated in Figure 3 showing the helium concentration in each of the streams labelled in the process diagrams, and in Figure 4 showing the factor of helium upgrade by each unit operation.

As mentioned before, the NGP process requires no refrigeration and a very low level of integration to the main LNG process. The first membrane unit effectively isolates the helium recovery flow sheet from the bulk of the BOG. After helium extraction, the remainder of the approximately 10% of the BOG having permeated across the membrane is recompressed back to the desired pressure and returned to the original BOG circuit. The helium recovery process is designed to be gas-tight and ‘invisible’ to the LNG operation. In addition, the first membrane unit also dampens the effect of large BOG flow fluctuations caused by ship loading operations on the extraction system. An approximate three-fold surge in the BOG flow leads to only a 1% increase in the feed to the adsorption unit (stream four in Figure 2), ensuring stable operation of the recovery system with or without ship loading.

Figure 3. The helium concentration at points along the Air Products NGP process.

Figure 4. Helium upgrading factor by each step in the Air Products NGP process.

Reprinted from December 2019

Although this process is perfect for helium extraction from BOG, it can be used on the fuel gas from the LNG end flash as well. The principles of operation remain the same in this case, with the same benefits of no need for refrigeration, a low level of process integration, and easy retrofit. Assuming an end flash gas compressed for fuel usage containing 0.5% helium, Table 1 shows a comparison of the process economics between the NGP process and the traditional route of partial condensation followed by a cryogenic helium purifier as described before. As is evident from the table, the economic advantage of the NGP process over the traditional cryogenic alternative is convincing. In addition, this new membrane and adsorption-based system can be installed in a shorter time given the highly skidded and packaged nature of the equipment. Faster ramp-up and ramp-down can also be expected as compared to the alternative cryogenic process.

ConclusionWith the global helium supply continuing to be tight into the foreseeable future, and the imminent exit of US BLM from the helium business, the industry must continue to creatively look for future helium supply. The helium extraction model is experiencing a paradigm shift, and recovery can be justified at previously unthinkable source concentrations. Natural gas liquefaction is a perfect place for additional helium recovery. Nevertheless, because the economics of LNG production far outweighs that of helium extraction, it is important to ensure any new helium recovery cycle is designed to be as ‘invisible’ to the main LNG process as possible. The NGP process satisfies

that requirement and is suitable for retrofit, as well as newbuilds. The NGP process is further distinguished from the traditional approach with lower capital and operating costs at excellent recovery, as well as easier deployment and operation. This new membrane and adsorption-based process is truly marked by its flexibility and simplicity.

References1. National Research Council, ‘The Impact of Selling

the Federal Helium Reserve’, Washington DC: The National Academies Press, (2000).

2. Bureau of Land Management, ‘Helium Resources of the United States—2007’, Technical Note 429, (2008).

3. International Gas Union (IGU), ‘World LNG Report’, 27th World Gas Conference, (2018).

4. CHATTERJEE, N. and GEIST, J. M., ‘Effects of Stratification on Boil-Off Rates in LNG Tanks’, Pipeline & Gas J., 199, 40, (1972).

5. CHEN, F., OKASINSKI, M. J., and SABRAM, T. M., ‘Novel Nitrogen Removal Schemes for LNG Plants with Electric Motor Drive and Varying Feed Composition’, LNG18, (Perth, Australia), (2016).

6. ROBERTS, M. J. and REPASKY, J. M., ‘Method and Apparatus for Producing Products from Natural Gas Including Helium and Liquefied Natural Gas’, US Patent 7,437,889, (2008).

7. CHEN, F., LIU, Y., KRISHNAMURTHY, G., OTT, C. M., and ROBERTS, M. J., US Patent 9,816,754, (2017).

8. RUFFORD, T. E., CHAN, K. I., HUANG, S. H., and MAY, E. F., ‘A Review of Conventional and Emerging Process Technologies for the Recovery of Helium from Natural Gas’, Ads Sci. & Technol., 32, 49, (2014).

9. XIONG, L., PENG, N., LIU, L. and GONG, L., ‘Helium Extraction and Nitrogen Removal from LNG Boil-Off Gas’, IOP Conf. Series: Mat. Sci. & Eng., 171, 012003, (2017).

Contact Air Products for more information on:Helium recovery: +1 610-706-4730 or

email: gigmrktg@airproducts.comLNG liquefaction processes & equipment:

+1 610-481-4861 or email: info@airproducts.com