An optimization of reverse osmosis (RO) networks for seawater desalination with spiral-wound modules (SWM) was presented in this work. The membrane transport model, which was based on the mass and momentum transport equations, took into consideration the longitudinal variation of the velocity, the pressure, and the salt concentration in the membrane modules. The pressure exchanger (PX) was included in the RO superstructure, and salinity increase caused by volumetric mixing in the PX was considered. The results obtained from the presented model were compared with the actual plant operational data from literature and found to be in good agreement with relative errors of 0.81%∼2.15% and 0.01%∼0.09%, in terms of water recovery and salt rejection, respectively. The optimum design problem was formulated as a mixed integer nonlinear programming (MINLP) problem. The variation of feed salinity was studied using the RO networks model. For the feed concentration higher than 32 kg/m 3 , one-stage RO system is favored. When the feed concentration is below 28 kg/m 3 , two-stage RO system is the better choice. The unit product cost increases with the decreases of permeate concentration requirement. For the looser permeate concentration requirement (0.30 kg/m 3 ), one-pass configuration can meet the required quality of desalted water. When the lower permeate quality requirement of concentration is from 0.050−0.20 kg/m 3 , a two-pass system is more suitable. The influence of system recovery rate on the plant performance was discussed. Finally, sensitivity analysis showed that the total annualized cost is highly sensitive to the feed flow rate, the operating pressure, and electricity cost, while the energy consumption is highly sensitive to the operating pressure, the feed salinity, and the feed temperature.
Increasingly
strict constraints on the boron concentration for
safe drinking and irrigation water present a tremendous challenge
for the design of seawater reverse osmosis (RO) desalination systems.
This work presents an optimization study of a seawater reverse osmosis
RO network with permeate split (PS) design under boron concentration
restrictions. Front part permeates with better quality and higher
flux are sent directly to the product, and back part permeates are
reprocessed in pass 2 with high pH value. The irreversible thermodynamic
model is employed to describe the membrane transport behavior of boron.
Constraints for the system flow and operation conditions are added
to guarantee safe operating of the RO system. Both single-product
and two-product RO systems are optimized for different types of feed
seawater. Results show that the PS design is mainly dominated by the
boron constraints, while the system recovery is mainly controlled
by the feed salt concentration. Due to the upper bound of pH for pass
2, PS design could be introduced for pass 2 to improve the boron rejection.
For a two-product output system, the permeates from both RO pass 1
and pass 2 could be split and sent to different products. In general,
PS design could offer lower water cost, lower energy consumption,
and smaller system size compared with normal design. Not only the
operation conditions but also the flow structure of the RO system
should be adjusted according to both membrane fouling and degradation
with time.
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