Design and Construction of Turboexpander based Nitrogen Liquefier

Cryogenic refrigerators are becoming increasingly popular particularly in the areas of superconducting magnet applications, particle accelerators and medical imaging systems, etc. It has also got wide applications in preservation of live biological materials as well as in scientific equipment. In spite of nearly half a century of R & D experience, our country is still dependent on imports for most of its needs in cryogenic refrigerators and liquefiers. These components are enormously expensive to buy and to maintain. The customers are often forced to buy equipment due to non-availability of proprietary spares. It is imperative that our country develops an indigenous nitrogen liquefier of capacity in the range 10 to 50 litre/hour. With the support from the Department of Atomic Energy, our institute has initiated a programme on development and study of a turboexpander based nitrogen liquefier of intermediate capacity (20 l/h). The focus of this project is to build a turbine based liquid nitrogen generator of capacity 20 l/h using indigenous technology. This technology and expertise will be extended for the liquefaction of helium in future. The development of the turboexpander based nitrogen liquefier begins with the process design of the cycle. The simulation of the cycles has been done using the software Aspen HYSYS. All the state points are fixed and each equipment specifications are determined. While designing the process, equipment availability, constraints and cost is to be kept in mind. Process design also includes the setting the parameters up to the optimum condition so that maximum amount of liquid will be obtained. After process design the thermodynamics parameters of all the components are available. As per process the nitrogen gas is compressed in the compressor upto 8 bar. The compressed gas passes through the first heat exchanger. Some amount of the gas is diverted through the turboexpander and remaining gas flow through the second heat exchanger. A JT valve is used to expand the liquid which is collected in the phase separator at a pressure just above ambient (1.2 bar). The vapour comes out of phase separator mixes with the cold gas from the turboexpander and the resultant stream meets at the suction side of the compressor, after passing through the second and first heat exchanger as the reversed stream.

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