White Light Reflectance Spectroscopy as a novel and noninvasive method for in situ wetting monitoring in membrane distillation

Abstract

Wetting, in membrane distillation (MD), is a major drawback appearing as a dynamic mechanism consisting of multiple stages. Normally, the liquid-air interface is placed on top of the membrane surface and pores are only occupied with vapor. If liquid starts to penetrate, surface wetting is followed by partial wetting that develops until the fully wetted state occurs, compromising the operation. Usually, only the fully wetted state is assumed, indirectly, as liquid from the feedwater stream, containing soluble salts, passes through the pores and contaminates the permeate, thus increasing its conductivity. In this work, we present White Light Reflectance Spectroscopy (WLRS) as a novel method for in-situ wetting monitoring in MD. In WLRS, light incidents vertically onto the membrane and reflectance is continuously recorded. Thus, it is possible to observe any movement of the liquid-vapor interface in real-time, via changes in the reflectance spectrum, without compromising the structural integrity of the membrane nor the module. Notably, the proposed method is applicable to all standard MD modes, without operating limitations and restrictions. With this noninvasive technique we can predict and observe all wetting stages, not only in the presence of surfactants in the feed stream, but also due to hydraulic pressure increase.