Comparative analyses of transient batch operations of hollow-fiber DCMD and WGMD desalination systems at high salinity levels

This study explores hollow-fiber membrane distillation (HFMD) as an alternative for treating high-salinity RO brines using a recirculating batch process. Water gap (WG) MD offers thermal and practical benefits that need to be compared with direct contact (DC) MD during the batch operation. A novel integrated, fully coupled transient CFD model for both HFMD configurations, HF-DCMD and HF-WGMD modules, with a zero-dimensional model for the feed tank is introduced. The model is validated against experimental data with a wide range of salinities and exhibits high accuracy with deviations lower than ±5 %. The results showed that the HF-WGMD module is more energy efficient than the HF-DCMD module despite having lower freshwater productivity and requiring longer batch operating times. At 70°C feed temperature and during the brine concentration from 70,000 to 100,000 and 233,333 ppm to get a pure water recovery ratio of 30% and 70%, the cumulative specific thermal energy consumption of the HF-WGMD module is lower than that of the HF-DCMD module by 12% and 8.2%, respectively. This is affected by an average reduction of 33.3 % in conduction heat loss with the option of a water gap during the batch process, providing an efficient means to treat concentrated brine.