Analysis of Volatile and Semivolatile Organic Compound Transport in Membrane Distillation Modules

Membrane
distillation (MD) is an emerging water treatment process
that offers near-complete removal of nonvolatile contaminants. However,
the transport of volatile compounds in the process is highly varied
and poorly understood. In this work, we investigate the transport
of a wide range of semivolatile and volatile compounds in membrane
distillation (55 compounds total) allowing us to gain broad insights
into the compound properties, system designs, and operating conditions
that impact transport rates in direct-contact MD. We first use experimentally
verified simulations to study the effects of different molecular properties
on transport and show that the Henry’s constant and diffusion
coefficient are the most important molecular properties in determining
solute flux. We then study the transport resistances across distillation
membranes and find distinct transport regimes dominated by resistances
associated with either diffusion through the membrane or boundary
layers on either side of the membrane. Simulations of large-scale
MD modules reveal the impact of membrane area, operating temperature,
and crossflow velocity on the removal of contaminants. Finally, we
comprehensively compare the removal of different classes of contaminants
between MD and RO, finding that MD is more effective in removing small
neutral compounds with low volatility. Overall, the results of this
work demonstrate important compound properties and operating conditions
that control volatile mass transport in MD.