Optical Technique for in-Situ Measurement of Concentration Polarization in Direct Contact Membrane Distillation

Membrane Distillation (MD) is a thermal separation process that is driven by the vapor pressure difference across a hydrophobic membrane. MD employs hydrophobic membranes with a high liquid entry pressure (LEP), which prevents the passage of water below the LEP. This process can achieve complete rejection of ions and non-volatile organics as long as the membrane pores are not wetted. Like in all membrane processes, concentration polarization can significantly impact the permeate flux in MD. As MD is a thermal separation process, polarization effects include both temperature and concentration polarization. The effect of temperature polarization on MD performance has been studied extensively in the past. However, concentration polarization has been neglected by most researchers when evaluating MD performance. Previous work on treatment of high salinity produced water has shown that the concentration of salts at the membrane surface can be up to 30% higher than that in the bulk near saturation conditions.

The objective of this study was to develop a technique to facilitate in-situ probing of solute concentration in the concentration polarization layer and investigate its impact on permeate flux in direct contact membrane distillation (DCMD). To achieve this objective, a visible spectrophotometer was developed using a collimated laser beam as the light source and a CCD line camera with 14-μm resolution was used as the detector. A fully customizable membrane cell with a transparent feed channel was constructed to measure the concentration of solute near the membrane surface and within the boundary layer of the feed stream. The DCMD module was mounted on an adjustable support to precisely position it between the light source and the detector such that the incident light is parallel to the membrane surface and the transmitted light is captured by the detector. Light absorption near the membrane surface that is proportional to the concentration of dissolved ions was used to characterize the concentration polarization based on previously developed calibration curves. This experimental system was used to evaluate the extent of concentration polarization as a function of feed temperature, hydrodynamic conditions and solute concentration the impact of these parameters on the concentration polarization coefficient will be discussed at the conference.