Calculation and analysis of the characteristics of a claude cycle with a turboexpander-compressor for natural gas liquefaction

The article presents a method of thermodynamic calculation of the Claude cycle with a turboexpander exploiting a pressure drop for small-scale LNG production without the use of any external refrigerants. The pressure in the direct gas stream is less than the critical pressure for natural gas (about 4.6 MPa). The unit diagram and the thermodynamic process are described. The calculation method is based on the heat balance equations of the cycle loops which are transformed to the final version and solved using iteration. The results of the calculations are presented as graphic characteristics of the liquefaction cycle from the pressure of the incoming natural gas stream, energy conversion efficiency of the turboexpander, temperature of the direct stream downstream of the compressor at different pressures in the return streams. The impact of the parameters is analyzed and conclusions are drawn. It is demonstrated that with an increase in the incoming gas pressure and energy conversion efficiency of the turboexpander, the turboexpander stream— while remaining significant in total—decreases to 64 %, which leads to an increase in the liquefaction ratio to 15 %. The drop in the pressure in the return gas stream can be used effectively at low incoming stream pressures.