**4. Sources of hydrogen in helium**

**3.3. Pumping two-phase liquid: vapor helium through a capillary impedance tube**

<sup>=</sup> *<sup>π</sup><sup>H</sup>*<sup>2</sup> (*T*) \_\_\_\_\_ *πHe*(*T*)

expression [Eq. (2)] in the interval 3–4.2 K (solid line). Similarly, the molar fraction of H2

the work of Jewel and McClintock [12] (dashed line), is also shown. Both are very similar.

vapor phase is obtained from the vapor pressure of solid hydrogen. Since solid hydrogen can be considered as a volatile solute (i.e., the solute vapor pressure is not negligible) for T > 3 K, there is a well-defined minimum solubility in the liquid phase for each temperature. This minimum solubility is also obtained from the vapor pressure. Furthermore, to know whether

not relevant because this already justifies the experimentally observed blockage times. In fact,

On the other hand, at the very low concentration levels under discussion, solid hydrogen may

(*T*) |*eq*

dissolve in the liquid [12]. In that case, the molar fraction of solid H2

if it is higher, it will just reduce the blockage time of the impedance.

(*T*), may be estimated from classical solubility theory.

equilibrium saturation line, *π<sup>H</sup>*<sup>2</sup>

74 Superfluids and Superconductors

*yH*<sup>2</sup>

tion, *yH*<sup>2</sup> (*T*) |*eq*

phase, *xH*<sup>2</sup>

saturation molar fraction of molecular H2

the vapor pressure saturation line of hydrogen, *π<sup>H</sup>*<sup>2</sup>

**Figure 3** shows the saturation molar fraction of H2

Thus, a well-defined lower limit for H2

the actual value of the solubility of H2

**Figure 3.** Low-temperature H2

(*T*) |*eq*

function of T in the range 3–4.2 K, at πHe(T).

[12]), and, from the H2

(dashed line, *xH2*

from its theoretical limiting solubility in the liquid phase, *xH*<sup>2</sup>

When two-phase liquid–vapor He is pumped from a bath at 4.2 K and 100 kPa (105 Pa), through a capillary impedance tube, the He stream cools down through its P–T vapor–liquid

. This can be calculated from cryocondensation theory (see Section 5.1), in this case,

(*T*):

(*T*). Thus, if there is enough hydrogen to form solid clusters, the

in the vapor phase, *yH*<sup>2</sup>

concentration as a function of temperature in helium

in the liquid phase is higher than the minimum value is

(*T*) |*eq*

saturation molar fraction in helium, obtained from the limiting solubility of H2

equilibrium-saturation vapor pressure (solid line, *yH2*

in the vapor phase will be the starting concentra-

, *T* ≤ 4.2*K* (2)

(*T*) |*eq*

, obtained from expression (1) in

dissolved in the liquid

, calculated using

obtained

in He

<sup>|</sup>*eq* [19]), as a

(*T*)

Helium is a nonrenewable and scarce resource on Earth. It is formed by natural radioactive decay from some thorium and uranium minerals. Today, commercial helium is predominantly extracted from natural gas sources. Alternative sources of helium production have been investigated over the years such as the ability of extracting helium from non-hydrocarbon sources. In 2016, scientists from the United Kingdom reported the discovery of a large helium reserve, 54 BCN (1.53 × 1012 sL, i.e., 2.7 × 105 Tm) in Tanzania, trapped in ancient rocks and not intermixed with natural gas [20]. Additionally, the possibilities of helium extraction from the atmosphere [21] or from CO2 stream [22] are being studied. Despite these future opportunities, helium is still a nonrenewable resource that must be used responsibly by mankind. This implies recycling helium when it is possible.

The present commercial helium production is extracted from a few natural gas fields around the world (located in Canada, the USA, Algeria, Poland, Qatar, China, Russia, Australia and Indonesia). These sources have a considerable amount of helium-rich gas (around 1%) to make extraction economically feasible.

Let us consider the Linde Group helium extraction facility in Darwin, Australia [23]. In this facility, the raw feed gas flow is 20,730 Nm3 /h with up to 3 mol% helium. The purification process of the feed gas consists of partial condensation of nitrogen in two stages, cryogenic adsorption and finally, catalytic oxidation of hydrogen followed by a dryer system.

After the purification, the refined helium is liquefied using a Bryton process and stored for further distribution. The raw gas has 0.1 mol% of hydrogen (1000 ppm), and the final processed 99.999% helium has up to 1 ppm.

Therefore, molecular H2 is naturally present in helium gas as obtained from natural gas sources [24], and, in general, different methods are used to eliminate it, prior to large-scale helium liquefaction, for worldwide distribution [23, 25]. However, despite the effort to eliminate it completely, very precise analytical methods indicate that even ultra-high pure commercial grade He gas, 99.9999% pure, thus, containing less than 1000 ppb in volume of total impurities, may contain up to 500 ppb in volume of H2 (i.e., a hydrogen molar fraction *yH*<sup>2</sup> = 5 ∙ 10−7 in He gas) [21, 26–28].

If the purified helium gas has a molar fraction of *yH*<sup>2</sup> =500 ppb (5 ∙ 10−7), after the liquefaction of this gas, the liquid helium will contain solid hydrogen in equilibrium with a molar fraction of H2 molecules given by *xH*<sup>2</sup> =0.35 ppb (3.5 ∙ 10−10) (i.e., corresponding to the vapor pressure of solid hydrogen in liquid helium under typical laboratory conditions [4.2 K and 100 kPa)], and, as we have seen in the previous section, this small amount of hydrogen may produce blockages in thin impedances in only a few hours.

Apart from natural gas sources, there are other possibilities to introduce small amounts of hydrogen in the helium recovery system. These include oil degradation in high-pressure compressors or pumps, outgassing of metallic pipes or diffusion of naturally present atmospheric H2 [29] through plastic pipes and gas bags [30]. Thus, the presence of traces of H2 in laboratory Helium Recovery Plants: Large Scale (LS-HRP) or Small Scale (SS-HRP), up to the ppm range (*yH*2 =10−6), seems to be unavoidable.

The gas input flows into the Dewar neck at room temperature, and it is cooled in direct contact with the cold head and the output heat exchanger while it descends through the neck

When the gas reaches the condensation temperature for the component "j" (see **Figure 4**), at some point, near the cold head first stage, the component "j" will start to solidify by impingement on the metallic cold surfaces of the cold head cylinder and heat exchanger walls. Below the cold head, the gas temperature decreases further, and the molar fraction in the vapor

When a region of temperature of ≈15 K is reached, the helium can be considered pure from all impurities except for hydrogen and neon. At this point, the gas passes through a mechanical filter with a passage in the micron range, which will avoid the possible dragging of solid

After the filter, to be energy efficient, the clean and cold helium is forced to exchange the enthalpy from 15 to 300 K with the warm and dirty helium that enters the purifier. To do that, the helium output path consists of a heat exchanger in the form of a thin-walled stainless-steel

Thanks to the heat exchange, the cold outgoing gas cools the warm incoming gas, and therefore, the required power of the cold head is minimized. So, the system can manage high flows. In addition, the coldhead excess power during purification will counteract the growing inef-

(*T*) of H2

impurity is constant until its vapor pressure line is reached, after which it decreases exponentially. Black dashed lines

lines indicate examples of impurities cooldown paths [e.g., initial impurities concentration in the mixture: H2

, Ne, N2

1000 ppm)]. Starting from the high temperature side, the molar fraction of each

, *T* ≤ *Tj* (3)

"Clean" Liquid Helium

77

http://dx.doi.org/10.5772/intechopen.74907

and O2 in a gas mixture at 240 kPa [19]. The arrow

(1 ppm),

(*T*) <sup>=</sup> *<sup>π</sup><sup>j</sup>* (*T*) \_\_\_\_ *pT*

phase of the component "j" will decrease rapidly with T, as πj (T):

tube coiled with the form of a solenoid around the cold head.

(T) and molar fractions *y*<sup>j</sup>

ficiencies caused by the solid impurities' coating around the cold surfaces.

down to the Dewar bottom.

*yj*

particle impurities toward the output.

**Figure 4.** Partial pressures πj

(100 ppm) and (N2

indicate the working point of the purifier filter, at 240 kPa and 15 K.

Ne (0.1 ppm), O2
