N-butane is to be liquefied to make liquid petroleum gas (LPG). The butane is available at 25^◦C and 1 bar, it will be compressed to 15 bar in a compressor that has an isentropic efficiency of 85%, cooled to 0^◦C in a heat exchanger, and then expanded adiabatically to 1 bar, with the vapor recycled to the compressor. Determine the work required per kg of LPG produced.