An azeotrope in the desert

Abstract

One of the most exciting applications of thermodynamics in chemical industry is the separation of azeotropes. The azeotrope in the system tetrahydrofuran — water seems to be an easy one, as it can be separated in two columns without the help of a selective agent by pressure-swing distillation. The topic of this paper is a case where this principle was applied. As well, a number of other components had to be separated so that two further distillation columns had to be provided. To everyone’s surprise, the water content could often not be kept below the target value of 300 ppm so that the product was off-spec. The various possible reasons were regarded. While almost everyone concluded that the azeotropic separation did not work, some thermodynamic considerations suggested that the pressure-swing distillation seemed to do its job. To find the reason of the failure, a hygrometer had been calibrated in a way that it could instantaneously indicate the water content in the tetrahydrofuran. It could be proven that the azeotrope of the tetrahydrofuran — water was properly separated. The reason for the moisture in the product could be located downstream the azeotropic separation and easily been corrected.