With the increase of global energy demand, the natural gas will play a key role both for energy production and for transports. Typically, natural gas is extracted and liquefied in large-scale plants to be later transported by ship or, when it is possible, by pipeline. In this study, a plug & play solution for natural gas liquefaction to be directly installed at the vehicle's filling stations, in order to avoid the transport costs of liquefied natural gas, is analyzed. The system analyzed in the paper consists in a single stage expansion process and the aim of the study is to improve the small-scale liquefaction process efficiency through the use of a cryogenic expander in replacement of a more common Joule-Thomson valve. A thermodynamic study has been carried out to optimize the process parameters with the aim of minimizing the energy consumption. This optimization study, starting from a reference case, allowed to identify an optimal case, which leads to a total energy saving of about 12% compared to the reference case. Furthermore, considerations relating to the cryogenic expander, which is a key component of the system, have been done. This device guarantees a higher thermodynamic efficiency than Joule-Thomson valve and it allows to integrate the produced shaft power into the process. This study represents a preliminary thermodynamic and parametric investigation on a low pressure LNG production process. The results of this study are the basis for the realization of a prototype which is actually under construction. Thus, further investigations by Authors will determinate the techno-economic feasibility of the optimized system also considering future experimental investigations.