GRE were contracted by the Physics Department at the University of Manchester to design a machine for scrubbing/cleaning/purifying argon gas resulting from a test process.
Project Goals
The requirement was to take contaminated gas, pass it through a scrubber, while monitoring and guaranteeing the efficacy of the cleaning process as the gas passed through the system, and then liquefy it and store it in a Dewar, ready for re-use.
Cleaning the contaminated argon is very desirable, because the only other alternative is to use a 100% liquid to waste system (using imported Dewars of liquid argon and simply letting it evaporate and then letting it vent to air. This method is expensive both in terms of capital cost and environmental impact, due to the high energy use in the production of cryogenic gasses.
Additionally, the university specified that the system must be energy efficient, with minimal environmental impact.
GRE engineers had multiple technical meetings with Manchester University professors and researchers, along with specialists from Harvard University in the USA, which collectively determined that it should be possible by utilising a chain of conjoined processes, such as evaporation, heating, pumping, cleaning, condensing and then storing (each element coming together in a holistic system).
Concept Design and Thermodynamic Analysis
GRE then then analysed the thermodynamic properties of the fluid (argon) and decided upon the actual mechanical process that could achieve the client’s requirements while incorporating the scrubber. This involved first boiling the contaminated liquid/gas mix completely (without any entrained liquid whatsoever), to produce a cold gas stream, which would be moved with a specialised pump (again, the selection and use of the chosen pump proved a risk, as it would be used outside of its specification). Secondly, it was heated to ambient temperature, and then heated again to around 400°C, a temperature suitable for it to be cleaned. From here it was passed through the scrubber. Following which, it would then be cooled back down to ambient temperature and returned to the condensing, where it was reliquefied and returned to the Dewar, for the cycle repeated.
The system can recycle perpetually, until such time as the argon purity is at an acceptable level, after which the entire argon inventory is returned to the cryogenic storage vessel for storage as a liquid and subsequent use.
Additional Project Requirements
An additional part of the challenge was to make the process energy efficient whilst achieving the deliverables of the process requirements. The initial idea was to use a series of heat exchangers, with resistive electric heaters as the source of heat, to first boil the argon, and then heat the gaseous argon to ambient temperature. The gas would then be scrubbed and returned to the storage vessel as warm (ambient temperature) liquid. Finally, it would have to have then be cooled and condensed to liquid ready for storage, using a cryo-refrigerator.
This could have worked well, but would have been extremely inefficient, because both electric heaters and, more so, cryo-refrigeration is very energy hungry, which was against the design brief from the University.
It also meant that we would have been simultaneously heating and cooling within the process cycle, thus wasting thermal energy.
Solution
Through detailed analysis work and calculation, GRE designed a recovery heat exchanger that used the heat of the returning ambient gas to boil and heat the outgoing liquid / gas argon.
This reduced the theoretical energy consumption by 80%, and meant that a much smaller, and less energy-consuming cryo-refrigerator could be used, because it’s function would simply be to “finish off” the re-condensing element of the process, rather than cool to the saturated vapour temperature and condense the entire inventory.