News 2007

Munich, April 11, 2007

A convincing system

Helium recovery: tailor-made and absolutely gastight

A company working for more than 60 years successfully in the field of high pressure system engineering for air for respiration, industrial air and technical gases has been intensively engaged in the process of helium condensation for many years. As a result, the company supplies complete recovery systems, mostly to research laboratories of universities and institutes, worldwide - tailor-made and absolutely gastight. The system offered comprises gas compressor plants as well as the gas balloon and gas processing and storage.

Circuit: from helium liquefaction via recovery to reutilisation

Liquid helium as a cooling medium is an increasing important factor within the natural sciences during research and operation of sensitive equipment. However, only limited quantities of the coveted resource helium can be obtained by natural gas extraction. Therefore, efforts are being made to operate helium in closed circuits. In liquefaction plants, helium is cooled down until it is a liquefied gas at -269 °C. With this cryogenic liquid, corresponding equipment such as supermagnets (e.g. for nuclear magnetic resonance tomograph) and highly sensitive sensors with low noise etc. are operated. The exhausted helium gas is collected, condensed and re-fed to the liquefaction process.

Properties and extraction

After hydrogen, helium is the lightest element; it is chemically inert, has a high diffusibility and with -269 °C (4.2 Kelvin) the lowest boiling point of all gases. Because of its low density, helium is very light and is therefore very well suited as an inert filling gas for Zeppelins. 1 l liquid helium gas has a weight of about 130 g and results in about 730 l of gas when it is heated to room temperature.

Unlike many other industrial gases which are directly extracted from the air by distillation, this is not possible with helium. Helium can only be extracted economically from helium-rich natural gas sources (helium content exceeding 2 %). Such sources are rare. At present, there are only four relevant sources worldwide. Wyoming/USA, Orenburg/Russia, Sonatrach/Algeria and a small source in Poland.

The helium is separated from the rest by gas distillation, stored first and then transported at ambient pressure as a volume-saving liquid in special vacuum-superisolated tank containers (about 40 000 l). In local distribution centres, the helium is decanted into smaller containers for delivery to the end user; 1 l liquid helium costs at present about 7 to 10 Euros.

Fields of application

Gaseous helium is used, as already mentioned, in aerial navigation, as a protective gas for diverse processes (chemistry, metallurgy, semiconductor production), as search gas for leakage measurements (among others for testing air conditioning plants in the automobile industry), during diving as a diving gas mixture as well as an inert coolant for special machine applications.

Helium in its liquid form is almost the exclusive choice as a cooling medium for optimising certain material characteristics at low temperatures. By the transition from the normally conductive into the superconductive state, for example strong magnets and, as a result, giant magnetic fields can be generated (e.g. nuclear magnetic resonance tomograph). Also, with a decreasing temperature, the electronic noise is reduced so that the signal-noise ratio can be considerably optimised with increasingly low temperatures (helium-cooled detectors).

Helium liquefaction plant

Helium compressor plant in gastight execution with condensate collecting tank and HP store, gas production volume 720 l/min

The University of Konstanz has already been operating for nearly 30 years its own helium liquefaction plant with subsequent helium recovery. The annual production of over 70 000 l of liquid helium supports 42 experiments and associated equipment in the field of natural science. Besides applications in physics, such equipment is used in the specialities chemistry, biology and psychology. About 100 collaborators work directly on experiments with liquid helium.

The complete system of the University of Konstanz contains (converted) 5 500 l liquid helium. Approximately 2 300 l have to be refilled each year in order to compensate for losses. This means that each litre of liquid helium is liquefied and recovered about 30 times before the gas finally escapes through unavoidable leaks.

It is technically very difficult and expensive to liquefy helium. Unlike air separation plants, where liquefaction takes place solely by gas decompression and countercurrent cooling, in helium liquefaction plants, an additional precooling has to be carried out by a mechanical expansion machine (turbine). The resulting technical complexity of the control system leads to it being economically much more efficient to operate a central liquefaction plant and to transport the cold liquid via containers to the individual users.

Gas liquefaction processes also require a huge amount of energy. The liquefaction plant installed in Konstanz has an hourly output of 20 l liquid helium; it is fed by a compressor with 10 bar output pressure and a capacity of 800 Nm³/h, and is driven by a 90 kW electric motor.

Helium liquefaction is, however, only one half of the helium plant. The other half comprises the recovery of the helium gas which evaporates or is exhausted from plant and equipment. For this purpose, a pipe network is installed in all buildings, transporting the helium gas from the experiments to the central gas balloon. When full, the high pressure gas compressors are started via a level control sensor; they take in the helium gas, compress it to 300 bar and store it in high pressure containers. The helium liquefaction plant is fed from these high pressure containers. In this way, the helium circuit is closed.

Recovery plant

Due to its high diffusibility (very small atomic diameter), helium is able to penetrate the smallest openings and pores which leads to gas losses within the helium system. The pipe network is constructed accordingly in order to minimise these losses. In Konstanz, the pipes are made of copper, connected with brazed fittings. It is true that the pipes do not have to resist to a high pressure, but the thermal fluctuations of the transported helium gas (e.g. when evacuating cryogenic gas) lead to enormous mechanical stresses. Meanwhile, recovery systems are also made of orbital-welded special steel pipes.

Even the high pressure compressors have to meet higher standards than compressors operating with air. Besides the required airtightness, the helium compressors have to be better cooled due to the higher heat capacity compared to air, and also require a higher quality standard of the individual components (e.g. valves).

Proven system engineering

The company BAUER KOMPRESSOREN has already been working for many years in the field of helium compression. All high pressure gas compressor units, vary from type Verticus (production volume 80 l/min) to large-scale plants (1 650 l/min), have a closed gastight system and show leak rates of less than 1.2 to max. 20 mbar l/s. The high pressure compressors are controlled by the balloon management system, depending on the gas balloon fill level. They are switched on and off accordingly.

Conclusion

Due to the increasing demand and the limited extraction capacity, the prices for helium will increase on the market. However, technically there are no alternatives yet to helium liquefaction, except for the hydrogen liquefier with its high risk potential. This means ecologically that the resource helium has to be used very carefully. BAUER KOMPRESSOREN contributes to this with high pressure compressors of highest quality.

Author:
Dipl. Ing. Ludwig Kühlwein
Project Engineer