**5.1. Cryocondensation by advanced technology purifier**

Purification by cryocondensation [35] is a method to separate undesired components (impurities) from a given mixture, by freezing them. The effectiveness of this method depends on the working temperature of the purifier; it must be low enough to ensure that the vapor pressure of the impurities is negligible. The cryocondensation method can provide high levels of purification at low temperatures, even at high-input gas flows and without the need of consumable items.

For this first stage, we use the advanced technology purifiers (ATPs) [34]. These purifiers are equipped with a 10 K class cryocooler (Sumitomo CH-208R) as the refrigerator element. 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 down to the Dewar bottom.

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

Helium Recovery Plants: Large Scale (LS-HRP) or Small Scale (SS-HRP), up to the ppm range

Up to this point, we have described the impedance blocking problem, and we have shown

used to achieve temperatures below 4.2 K in helium-pumped cryostats. We have seen that hydrogen is naturally present in raw helium sources. Therefore, the production of hydrogenfree "Clean" helium is necessary to reliably operate cryostats with small impedances for long periods without interruptions. In the following paragraphs, we present a helium recovery

We propose a helium purification and liquefaction system layout using small-scale helium liquefiers based on closed-cycle refrigerators (cryocoolers). The commercial Advanced technology liquefiers (ATLs) [31, 32] have a liquefaction rate of 30 L/Day with a performance of 0.16 (L/h)/kW, close to the performance of industrial size Collins liquefiers (0.5–1.2 (L/h)/kW) [33]. This technology adapts the liquefaction rate to the consumption, it is modular and scalable and it covers needs of consumption from a few liters per day up to liquefaction rates of

The purification stage of the "Clean helium" recovery plant proposed is based on a combina-

• the cryocondensation, performed with an advanced technology purifier (ATP) [34], for the elimination of all the impurities present in the recovered helium, except hydrogen and

Purification by cryocondensation [35] is a method to separate undesired components (impurities) from a given mixture, by freezing them. The effectiveness of this method depends on the working temperature of the purifier; it must be low enough to ensure that the vapor pressure of the impurities is negligible. The cryocondensation method can provide high levels of purification at low temperatures, even at high-input gas flows and without the need of

For this first stage, we use the advanced technology purifiers (ATPs) [34]. These purifiers are equipped with a 10 K class cryocooler (Sumitomo CH-208R) as the refrigerator element.

• the chemisorption of hydrogen by a non-evaporable getter alloy.

**5.1. Cryocondensation by advanced technology purifier**

<10−10) is enough to produce the blocking of fine capillary tubes

in laboratory

[29] through plastic pipes and gas bags [30]. Thus, the presence of traces of H2

H2

(*yH*2

=10−6), seems to be unavoidable.

**5. Clean helium recovery plant**

plant capable of producing "Clean" helium.

the Collins industrial technology >240 L/Day.

tion of two purification techniques:

consumable items.

how a small amount of H2 (*yH*<sup>2</sup>

76 Superfluids and Superconductors

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

$$y\_j(T) = \frac{\pi\_j(T)}{p\_r}, \quad T \le T\_j \tag{3}$$

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 particle impurities toward the output.

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

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

**Figure 4.** Partial pressures πj (T) and molar fractions *y*<sup>j</sup> (*T*) of H2 , Ne, N2 and O2 in a gas mixture at 240 kPa [19]. The arrow lines indicate examples of impurities cooldown paths [e.g., initial impurities concentration in the mixture: H2 (1 ppm), Ne (0.1 ppm), O2 (100 ppm) and (N2 1000 ppm)]. Starting from the high temperature side, the molar fraction of each impurity is constant until its vapor pressure line is reached, after which it decreases exponentially. Black dashed lines indicate the working point of the purifier filter, at 240 kPa and 15 K.

This system can purify gas flows up to 30 sL/min with 10,000 ppm of impurities. The output flow quality is about six orders of magnitude better for the main contaminants (i.e., air in the case of recovered helium).

depends on the concentration of the hydrogen and the temperature of the material. Sieverts'

log*P* = *A* + 2 log*Q* − *B*/*T* (4)

in torr∙L/g, T is the temperature of the getter in K and A and B are constants for different NEG alloys (e.g., A = 4.8, B = 6116 for Zr(70%)-V(24.6%)-Fe(5,4%), commercialized under the name St707 [38]). When the hydrogen concentration exceeds 20 torr∙L/g, a phenomenon called "hydrogen embrittlement" occurs due to the change of the lattice parameters [37]. With enough time under these conditions, the getter alloy becomes a fine powder that can cause

An NEG material working at ambient temperature is an ideal candidate for the elimination of the remaining molecular hydrogen in helium that has been purified by cryosorption in the ATP. **Figure 5** shows that the hydrogen concentration in helium after passing through the

at 400°C. Even more, the hydrogen capacity of the getter is higher, and the sorption speed is

concept is depicted in **Figure 6**. The plant is initially fed with commercial grade 5 (99.999%

purified by cryocondensation by one or more cryo-refrigerator-based purifiers (ATPs), each with a total effective volume to store solid impurities of several liters and a maximum purifi-

concentration in the NEG alloy

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

"Clean" Liquid Helium

79

) low-pressure (P < Pc) SS-HRP

molar fraction of 10−6 [25]. The gas is further

, calculated from equilibrium isotherms of the St 707 getter alloy obtained from

increases accordingly when the material captures hydrogen molecules until it reaches the embrittlement

<<10−14), that is, several orders of magnitude lower than

equilibrium pressure in torr, Q is the H2

law describes this relationship:

problems in the getter application.

getter will be better than grade 14 (*yH*<sup>2</sup>

**5.3. Clean helium recovery plant configuration**

pure) helium gas that may contain up to a H2

cation flow rate of around 30 sL/min at 20 K.

**Figure 5.** Hydrogen molar fraction, *yH*<sup>2</sup>

Sievert's law. *yH*<sup>2</sup>

area.

Our "Clean helium" (extreme pure helium free of molecular H2

still reasonably high [38].

where P is the H2

The purifier can operate without interruptions during, at least, 1 month, and can purify more than 1 million sL of recovered helium (with a typical average impurity volume concentration of 300 ppms in total). The regeneration procedure is totally automated and it takes 7 h. Thus, the operational down-time ratio is only 1.25%.

Once the main contaminants have been removed, the second purification stage needs only to eliminate the remaining hydrogen via chemisorption by the non-evaporable getter (NEG) material.
