Selectivity
and Mass Transfer Limitations in Pressure-Retarded
Osmosis at High Concentrations and Increased Operating Pressures
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Abstract
Pressure-retarded
osmosis (PRO) is a promising source of renewable
energy when hypersaline brines and other high concentration solutions
are used. However, membrane performance under conditions suitable
for these solutions is poorly understood. In this work, we use a new
method to characterize membranes under a variety of pressures and
concentrations, including hydraulic pressures up to 48.3 bar and concentrations
of up to 3 M NaCl. We find membrane selectivity decreases as the draw
solution concentration is increased, with the salt permeability coefficient
increasing by a factor of 2 when the draw concentration is changed
from 0.6 to 3 M NaCl, even when the applied hydraulic pressure is
maintained constant. Additionally, we find that significant pumping
energy is required to overcome frictional pressure losses in the spacer-filled
feed channel and achieve suitable mass transfer on the feed side of
the membrane, especially at high operating pressures. For a meter-long
module operating at 41 bar, we estimate feedwater will have to be
pumped in at a pressure of at least 3 bar. Both the reduced selectivity
and increased pumping energy requirements we observe in PRO will significantly
diminish the obtainable net energy, highlighting important new challenges
for development of systems utilizing hypersaline draw solutions