**3.2. The impedance blocking problem**

The impedance-blocking problem arises when liquid helium, containing traces of H2 , is transferred to a cryostat in which the liquid is pumped through a very small capillary or impedance tube, to produce temperatures below 4.2 K using evaporation cooling.

To reduce any impedance blocking, a widespread and generally accepted "low-temperature best practice" is to stop any solid impurity at strategic locations along the helium supply chain with submicron metal-sintered filters. The first opportunity to stop solid impurities in a typical laboratory workflow is while transferring helium from a storage Dewar to the application cryostat for the first time, i.e. during initial cooldown. To this end, many laboratories incorporate a metal filter at the outlet "tip" of the helium transfer line so that the solid impurities are trapped in the line and not transferred into the application cryostat. Once the helium transfer is complete, the line is warmed up, the impurities are flushed away and the tip is ultrasonically cleaned. If any impurities should make it past the first filter, and, furthermore, to filter any other solid impurities already present in the application apparatus, a second "best practice" employed by cryostat designers is to incorporate a similar type of filter at the inlet of the impedance tube at the cold end of the apparatus cryostat.

It is important to appreciate, however, that mechanical filtering of this type is limited in its effectiveness and cannot selectively discriminate and separate H2 molecules from their helium carrier flow (a two-phase liquid and vapor helium flow), neither during liquid transfer nor during pumping. This is because despite the relatively high binding energies reported for hydrogen with the surface of some solids [14, 15], which involve potentials of the order of several hundred K, the specific surface area per unit volume of the metal-sintered filters commonly used in this application is below 0.5 m2 /g. This is about three-to-four orders of magnitude smaller than the area per unit volume exhibited by state-of-the-art solid H2 storage devices. Moreover, the porous size of media grade selected (0.5 microns) is also more than three orders of magnitude larger than the H2 molecular radius. Based on these considerations, the contribution of H2 physical sorption on the walls of the mechanical filter is estimated to be very small and, furthermore, limited by the very small vapor pressure of solid hydrogen at the temperature of liquid He.

Thus, in light of these considerations, we postulate that despite the adoption of these simple "best practices," H2 molecules will inevitably first enter the application cryostat during helium refills and then, the impedance fine capillary tubes during continuous operation below 4.2 K. As a result, part of the H2 molecules, carried by the helium flow, will freeze (or precipitate) inside the capillary. This is a consequence of the reduction in temperature and total pressure of helium, which is accompanied by a sizable reduction in solid hydrogen vapor pressure and in the hydrogen limited solubility in liquid helium. So that, sooner or later, depending on the specific dimensions and helium flow rate pumped through the capillary, a blockage will appear. When this occurs, the whole set up has to be warmed up, at least up to about 14 K (hydrogen melting point) but more often to room temperature, with a dramatic loss of time and liquid helium.

**Figure 2** illustrates molecular H2 , present in the liquid helium bath, flowing through a submicron-sized metallic-sintered filter (e.g., 500 nm as average pore size) placed to stop solid impurities entering the fine capillary impedance tube. When the temperature in the capillary is reduced below 3 K by evaporation cooling, the H2 vapor pressure, as well as the limiting solubility of H2 in helium, becomes negligibly small (*xH*<sup>2</sup> <10−14). Therefore, all the H2 present in the liquid helium heterogeneously nucleates along the walls of the impedance tube. A similar mechanism in a completely different working fluid and temperature range, for the freezing of water molecule impurities in nitrogen gas, has been proposed to explain blocking in micromachined Joule-Thomson coolers operating approximately at 100 K [16, 17].

precautions to avoid contamination during liquid helium refills. On the other hand, there are a considerable number of low-temperature applications that require achieving temperatures below 4 K [5], which are very sensitive to impurities present in the liquid and, therefore, those applications need extreme pure liquid helium for proper operation [13].

, is trans-

molecules from their helium car-

storage devices. Moreover, the

phys-

/g. This is about three-to-four orders of magnitude smaller than

The impedance-blocking problem arises when liquid helium, containing traces of H2

ance tube, to produce temperatures below 4.2 K using evaporation cooling.

ferred to a cryostat in which the liquid is pumped through a very small capillary or imped-

To reduce any impedance blocking, a widespread and generally accepted "low-temperature best practice" is to stop any solid impurity at strategic locations along the helium supply chain with submicron metal-sintered filters. The first opportunity to stop solid impurities in a typical laboratory workflow is while transferring helium from a storage Dewar to the application cryostat for the first time, i.e. during initial cooldown. To this end, many laboratories incorporate a metal filter at the outlet "tip" of the helium transfer line so that the solid impurities are trapped in the line and not transferred into the application cryostat. Once the helium transfer is complete, the line is warmed up, the impurities are flushed away and the tip is ultrasonically cleaned. If any impurities should make it past the first filter, and, furthermore, to filter any other solid impurities already present in the application apparatus, a second "best practice" employed by cryostat designers is to incorporate a similar type of filter at the inlet of the imped-

It is important to appreciate, however, that mechanical filtering of this type is limited in its effec-

rier flow (a two-phase liquid and vapor helium flow), neither during liquid transfer nor during pumping. This is because despite the relatively high binding energies reported for hydrogen with the surface of some solids [14, 15], which involve potentials of the order of several hundred K, the specific surface area per unit volume of the metal-sintered filters commonly used in

porous size of media grade selected (0.5 microns) is also more than three orders of magnitude

ical sorption on the walls of the mechanical filter is estimated to be very small and, furthermore, limited by the very small vapor pressure of solid hydrogen at the temperature of liquid He.

Thus, in light of these considerations, we postulate that despite the adoption of these simple

refills and then, the impedance fine capillary tubes during continuous operation below 4.2 K. As

the capillary. This is a consequence of the reduction in temperature and total pressure of helium, which is accompanied by a sizable reduction in solid hydrogen vapor pressure and in the hydrogen limited solubility in liquid helium. So that, sooner or later, depending on the specific dimensions and helium flow rate pumped through the capillary, a blockage will appear. When this occurs, the whole set up has to be warmed up, at least up to about 14 K (hydrogen melting point)

but more often to room temperature, with a dramatic loss of time and liquid helium.

molecular radius. Based on these considerations, the contribution of H2

molecules will inevitably first enter the application cryostat during helium

molecules, carried by the helium flow, will freeze (or precipitate) inside

**3.2. The impedance blocking problem**

72 Superfluids and Superconductors

ance tube at the cold end of the apparatus cryostat.

this application is below 0.5 m2

larger than the H2

"best practices," H2

a result, part of the H2

tiveness and cannot selectively discriminate and separate H2

the area per unit volume exhibited by state-of-the-art solid H2

As an example, a typical two-phase He flow of only 1 sL/min, having *xH*<sup>2</sup> =0.35 ppb (3.5∙ 10−10) of H2 molecules [i.e., corresponding to the vapor pressure of solid hydrogen in liquid helium under typical laboratory conditions (4.2 K and 100 kPa)], pumped through a cylindrical tube impedance of 66-μm effective diameter [e.g., the low temperature impedance of a Quantum Design, Physical Properties Measurement System (PPMS)] [18], may produce a solid hydrogen cylinder block of 66-μm diameter that, in about 24 h, will have 132 μm of height. The exact time for the blocking to occur will depend on the exact solid hydrogen distribution in the impedance. Instead, several years would be necessary to produce the same effect when pumping helium with a lower concentration of H2 molecules similar to the vapor pressure of solid hydrogen at 3 K, *xH*<sup>2</sup> =0.0075 ppt (7.5∙10−15). This is the reason why we consider the vapor pressure of solid hydrogen at 3 K negligibly small regarding impedance blockage.

**Figure 2.** Schematic description of low-temperature impedance blockage by molecular H2 present in liquid He.

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

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 equilibrium saturation line, *π<sup>H</sup>*<sup>2</sup> (*T*). Thus, if there is enough hydrogen to form solid clusters, the saturation molar fraction of molecular H2 in the vapor phase will be the starting concentration, *yH*<sup>2</sup> (*T*) |*eq* . This can be calculated from cryocondensation theory (see Section 5.1), in this case, the vapor pressure saturation line of hydrogen, *π<sup>H</sup>*<sup>2</sup> (*T*):

$$\left. y\_{H\_i}(T) \right|\_{eq} = \frac{\pi\_{H\_i}(T)}{\pi\_{\mu\_i}(T)'} \quad T \le 4.2K \tag{2}$$

Thus, when the pumped helium stream expands and cools down inside the capillary imped-

through the impedance decreases by four orders of magnitude (from ≈ 10−10 to ≈ 10−14), and,

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

intermixed with natural gas [20]. Additionally, the possibilities of helium extraction from the

ties, helium is still a nonrenewable resource that must be used responsibly by mankind. This

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

Let us consider the Linde Group helium extraction facility in Darwin, Australia [23]. In this

process of the feed gas consists of partial condensation of nitrogen in two stages, cryogenic

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 pro-

[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

of this gas, the liquid helium will contain solid hydrogen in equilibrium with a molar fraction

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 block-

adsorption and finally, catalytic oxidation of hydrogen followed by a dryer system.

freezes or precipitates and blocks the capillary.

molar fraction in the two-phase liquid–vapor helium flowing

"Clean" Liquid Helium

75

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

Tm) in Tanzania, trapped in ancient rocks and not

/h with up to 3 mol% helium. The purification

= 5 ∙ 10−7 in He gas)

=500 ppb (5 ∙ 10−7), after the liquefaction

stream [22] are being studied. Despite these future opportuni-

is naturally present in helium gas as obtained from natural gas sources

(i.e., a hydrogen molar fraction *yH*<sup>2</sup>

=0.35 ppb (3.5 ∙ 10−10) (i.e., corresponding to the vapor pressure of

ance, from 4.2 to 3 K, the H2

consequently, the excess H2

**4. Sources of hydrogen in helium**

reserve, 54 BCN (1.53 × 1012 sL, i.e., 2.7 × 105

implies recycling helium when it is possible.

facility, the raw feed gas flow is 20,730 Nm3

make extraction economically feasible.

cessed 99.999% helium has up to 1 ppm.

contain up to 500 ppb in volume of H2

If the purified helium gas has a molar fraction of *yH*<sup>2</sup>

ages in thin impedances in only a few hours.

Therefore, molecular H2

of H2 molecules given by *xH*<sup>2</sup>

[21, 26–28].

atmosphere [21] or from CO2

On the other hand, at the very low concentration levels under discussion, solid hydrogen may dissolve in the liquid [12]. In that case, the molar fraction of solid H2 dissolved in the liquid phase, *xH*<sup>2</sup> (*T*), may be estimated from classical solubility theory.

**Figure 3** shows the saturation molar fraction of H2 in the vapor phase, *yH*<sup>2</sup> (*T*) |*eq* , calculated using expression [Eq. (2)] in the interval 3–4.2 K (solid line). Similarly, the molar fraction of H2 obtained from its theoretical limiting solubility in the liquid phase, *xH*<sup>2</sup> (*T*) |*eq* , obtained from expression (1) in the work of Jewel and McClintock [12] (dashed line), is also shown. Both are very similar.

Thus, a well-defined lower limit for H2 concentration as a function of temperature in helium 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 the actual value of the solubility of H2 in the liquid phase is higher than the minimum value is not relevant because this already justifies the experimentally observed blockage times. In fact, if it is higher, it will just reduce the blockage time of the impedance.

**Figure 3.** Low-temperature H2 saturation molar fraction in helium, obtained from the limiting solubility of H2 in He (dashed line, *xH2* (*T*) |*eq* [12]), and, from the H2 equilibrium-saturation vapor pressure (solid line, *yH2* (*T*) <sup>|</sup>*eq* [19]), as a function of T in the range 3–4.2 K, at πHe(T).

Thus, when the pumped helium stream expands and cools down inside the capillary impedance, from 4.2 to 3 K, the H2 molar fraction in the two-phase liquid–vapor helium flowing through the impedance decreases by four orders of magnitude (from ≈ 10−10 to ≈ 10−14), and, consequently, the excess H2 freezes or precipitates and blocks the capillary.
