Hydrogen-Free Liquid-Helium Recovery Plants: The Solution for Low-Temperature Flow Impedance Blocking

The blocking of fine-capillary tubes used as flow impedances in ${}^4\mathrm{He}$ evaporation cryostats to achieve temperatures below 4.2 K is generally attributed to nitrogen or air impurities entering these tubes from the main bath. The failure of even the most rigorous low-temperature laboratory best practices aimed at eliminating the problem by maintaining the cleanliness of the helium bath and preventing impurities from entering the capillary tubes suggests that a different cause is responsible for the inexplicable reduction of impedance flow. Many low-temperature research laboratories around the world have suffered this nuisance at a considerable financial cost due to the fact that the affected systems have to be warmed to room temperature in order to recover their normal low-temperature operation performance. Here, we propose an underlying physical mechanism responsible for the blockages based upon the freezing of molecular ${\mathrm{H}}_2$ traces present in the liquid-helium bath. Solid ${\mathrm{H}}_2$ accumulates at the impedance low-pressure side, and, after some time, it produces a total impedance blockage. The presence of ${\mathrm{H}}_2$ traces is unavoidable due its occurrence in the natural gas wells where helium is harvested, forcing gas suppliers to specify a lower bound for impurity levels at about 100 ppb even in high-grade helium. In this paper, we present a simple apparatus to detect hydrogen traces present in liquid helium and easily check the quality of the liquid. Finally, we propose a solution to eliminate the hydrogen impurities in small- and large-scale helium recovery plants. The solution has been implemented in several laboratories that previously experienced a chronic occurrence of blocking, eliminating similar occurrences for more than one year.

Figure 1

Schematic description of low-temperature impedance blockage by molecular ${\mathrm{H}}_2$ present in liquid He.

Figure 2

Left: Schematic setup of the hydrogen detection device. Right: Photograph of the impedance at the end of a transfer line.

Figure 3

Top: Temperature and gas flow measured with the test impedance during the blocking process. The blockage occurs after about 10 h. Bottom: Detail of the impedance unblocking. The impedance is heated to about 15 K by moving it up in the Dewar neck where the flow is partially reestablished and, then, is cooled again.

Figure 4

Top view of a liquid-helium Dewar through a methacrylate cover. The hydrogen clusters project a shadow on the Dewar bottom.

Figure 5

Molar fractions $y_i(T)$ and partial pressures $p_i(T)$ of ${\mathrm{O}}_2$ (blue) and ${\mathrm{H}}_2$ (red) in recovered helium as a function of $T$, for initial molar fractions of ${10}^{-4}$ and ${10}^{-7}$, respectively, at STP (${10}^5\mathrm{Pa}$ and 300 K). Black solid lines are the equilibrium vapor pressure $P\text{−}T$ line of He (I) and the vapor-pressure lines of some typical impurities ${\pi }_i(T)$ as a function of $T$ [33], in the range 3–300 K.

Figure 6

Low-temperature ${\mathrm{H}}_2$-saturation molar fraction in helium obtained from the limiting solubility of ${\mathrm{H}}_2$ in He [dashed line, ${x_{{\mathrm{H}}_2}(T)|}_{\mathrm{eq}}$ [22] ] and from the ${\mathrm{H}}_2$ equilibrium-saturation vapor pressure [solid line, ${y_{{\mathrm{H}}_2}(T)|}_{\mathrm{eq}}$ [33] ], as a function of $T$ in the range 3–4.2 K at ${\pi }_{\mathrm{He}}(T)$.

Figure 7

Schematic configuration of a small-scale low-pressure ($<250\mathrm{kPa}$) green helium recovery plant (free of hydrogen). Gasbag, compressor, and recovery helium bottles are not completely free of ${\mathrm{H}}_2$ (orange). The commercial He bottles are the main source of contamination (red). The bypass is closed when the ATP operation temperature is $T>3\mathrm{K}$.

Figure 8

ATP output pressure and temperature during regeneration. Purification temperature remains around 3 K in normal operation and reaches around 130 K during a regeneration process. Then, after releasing all the collected impurities into the atmosphere, the purifier returns to a normal operation (240 kPa, 3 K).

Figure 9

ATP purification temperature remains around 4 K in normal operation and reaches around 25 K during a soft-regeneration process. At this moment, a great quantity of hydrogen accumulated is released and is trapped by the getter material. Then the purifier returns to a normal operation.

Figure 10

On-line monitoring of ${\mathrm{H}}_2$ content in the He (g) at the outlet of ATP (a) and getter material (b).

Figure 11

Red circles: ${\mathrm{H}}_2$ concentration in arbitrary units after about six days purifying recovered helium in a SS HRP recovery plant without a getter element. Green circles: ${\mathrm{H}}_2$ concentration after five days purifying recovered green helium gas. The amount of purified gas is of the order of ${10}^5\mathrm{sL}$ in both cases.

Figure 12

Leiden Institute of Physics cryogenic installation. Before 2016, there was one single liquefaction machine, a PSI KOCH liquefier, connected to a large storage (4000 l) of liquid helium. Helium gas from the recovery system is purified from oils, water, and hydrogen traces, and it enters the liquefaction unit as helium 99.8%, where the residual contamination is air. Air (nitrogen and oxygen) is removed by cold traps within the liquefier. From 2016, the department has substituted the old machine with five Quantum Design ATL160 small-scale helium liquefiers and three ATP30 purifiers. Helium purified from oils, water, and hydrogen traces, by the same sequence of filters, enters the Quantum Design ATPs, which remove air by cryocondensation before entering the ATLs.