Experimental study and numerical optimization for removal of methyl orange using polytetrafluoroethylene membranes in vacuum membrane distillation process

One of the most water-intensive and environment polluting anthropogenic industries is the textile industry. Amongst textile dyes, methyl orange (MO) is an extensively used anionic azo dye that is toxic to the environment. In this study, the effect of various process parameters was studied for different operating conditions (mass flow rate, vacuum pressure, inlet feed temperature, dye concentration) to remove MO from synthetic wastewater in the vacuum membrane distillation (VMD) process. We determined the optimum conditions for operating variables using response surface methodology (RSM) and developed a regression model to describe permeate flux, dye rejection and specific energy consumption. The optimum operating parameters from the RSM study were bulk feed temperature: 70 degrees C, mass flowrate: 40 kg h-1, dye concentration: 60 mg l- 1 and vacuum pressure: 720 mm Hg. For the optimized input process parameters, the experimentally observed values were permeate flux: 19.60 +/- 1.03 l m-2 h-1, dye rejection: 99.80 +/- 0.07%, and SEC: 2.04 +/- 0.11 kW h m-3. Furthermore, for these optimized conditions, a 3D CFD model was developed to gain insight into the membrane module's thermal, velocity, and concentration field. The evaporation of feed at the membrane surface on the feed side and condensation of vapours at the membrane surface on the permeate side was captured in this model. The permeate flux profile and dye rejection for various cycles of VMD operation with intermittent washing were also performed to study the fouling behaviour of the membrane. The membrane's surface fouling was analyzed using SEM and FT-IR analysis. The SEM analysis of the fouled membrane indicated that the dye deposition was not severe due to high hydrophobicity, high liquid entry pressure, and high pore wetting resistance of the PTFE membrane. The FT-IR analysis showed that the fouled PTFE membrane sufficiently retained its initial structure and property.