This invention overcomes the tradeoff between throughput and thermal efficiency inherent in single-stage VMD systems by cascading multiple VMD modules each operating at progressively lower pressures. At each stage, heated feed water is vaporized and driven through a gas-permeable membrane by a vacuum drawn on the vapor side of the membrane. The hot product water is passed through a condenser and a permeate cooler where energy is transferred to cold incoming feed water. Liquid feed water remaining in the first vaporization vessel is then transferred to the next stage where further vaporization is induced by the lower vapor-side pressure and the process is repeated. Systems can be built with arbitrary numbers of stages, with more numerous stages associated with better thermal efficiency and increased equipment cost.
This multi-stage VMD system achieves high throughput since each module produces desalinated water in parallel. Additionally, thermal efficiency is improved by vaporizing as much water as possible at early, low-vacuum, stages before proceeding to later, high-vacuum stages. This allows more heat to be recovered as incoming feed water absorbs energy from progressively hotter product water as its own temperature rises.