AbstractAn experimental study has been carried out on a continuously operated pilot fractional distillation column equipped with an external heat pump. The distillation column was a 15 cm internal diameter glass unit containing eleven single bubble cap plates. A methanol‐water mixture was fed to the column. After operating the heat pumps with R114 as the working fluid, further experiments have been conducted with R11 as the working fluid. Plots of pressure against enthalpy, condensation pressure and latent heat of vaporization against condensation temperature and theoretical Rankine coefficient of performance against gross temperature lift and condensation temperature are presented for both R114 and R11. R11 has correspondingly higher theoretical Rankine coefficients of performance than R114. The experiments show that the actual coefficients are also higher for R11 than for R114. A maximum actual coefficient of performance of 5.3 was obtained using R11 as the working fluid with a gross temperature lift of 38.4°C. The experimental data for R11 were found to be reproducible during operation over a number of weeks. This showed that the relative thermal instability of R11 compared to R114 had not apparently affected the performance of the syst