Development of Reliable Scaling Propensity Prediction Model for Direct Contact Membrane Distillation

Abstract

Membrane Distillation (MD) is a developing technology that allows the recovery of pure water from hyper-saline feed solutions, such as reverse osmosis (RO) brine. Deposition of particles becomes more prominent in the feed channel as the salt continues to build up due to water permeation. Due to this, technical challenges arises inside the feed channel leading to decreased process efficiency. Scaling is one of the main limitations in MD of seawater, which is attributed to the crystallization of low soluble salts such as CaSO4. A predictive model for scaling tendency of gypsum was built in this thesis. The predictive model incorporates the science behind fouling, saturation and solubility limits of the ionic species, and the transport phenomena inside the MD modules. Two flow configurations, counter and co-current, were studied in this thesis with both empty and spacer-filled channel modules. The channel length was 5 m with inlet feed temperature of 60 ℃ and permeate temperature of 20 ℃. The feed was a 66.7 kg/m3 concentrated brine, consisting of 5.5 kg/m3 of sulfate anion and 1 kg/m3 of calcium cation. The validation of the model was done using experimental data from literature where an average error of less than 5% was achieved. Once temperature and concentration profiles are obtained along the full-scale channel length, the activity coefficients of individual ions were assessed using Pitzer thermodynamical model, then a scaling potential index was used to assess the saturation level of the solution with respect to gypsum. Results revealed that co-current flow mode is more resilient to gypsum scaling, but flux is also lower. The addition of spacer enhanced mass transfer in the channel, resulting in lower concentration polarization. SPI for spacer-filled channel indicated that super-saturation of the solution happens at a further length of the channel at the membrane boundaries. A sensitivity analysis for inlet feed temperature, concentration, and flow rate was done. Increasing flow rate has a positive effect on the scaling propensity of gypsum. While increasing temperature or concentration has a negative effect.

Date of Award	8 May 2024
Original language	American English
Supervisor	Emad Alhseinat (Supervisor)