NAMS 2002 Workshop - ICOM 2008

Desalination I – 1 – Keynote
Monday July 14, 2:15 PM-3:00 PM, O’ahu
Energy Cost Optimization in RO Desalting and the Thermodynamic
Restriction
R. Zhu (Speaker), University of California, Los Angeles, Los Angeles, CA, USA
P. Christofides, University of California, Los Angeles, Los Angeles, CA, USA
Y. Cohen, University of California, Los Angeles, Los Angeles, CA, USA, yoram@ucla.edu
Modern RO and NF membranes can be operated at remarkably low pressures.
However, these pressures are still significantly above the thermodynamic
osmotic pressure. Although various studies have advanced a variety of
approaches to evaluate the energy cost of reverse osmosis membrane
desalination, such studies have not offered a simple mathematical formalism that
considered the effect of the lower bound on the feed pressure on energy cost
optimization. In the present study, a rigorous theoretical formalism was
developed that clarifies the thermodynamic restriction on RO energy cost and
provides a basis for RO process optimization. The present approach enables
direct analytical solution for the minimum specific energy cost with respect to
water recovery, feed and permeate flow rate. The additional impact of pressure
drop within the membrane module, energy recovery devices, membrane
hydraulic permeability and brine disposal cost were incorporated into the
theoretical model. Specific results will be presented for simple RO configurations
to demonstrate the impact of multi-stage RO operation on energy efficiency in
relation to membrane cost. In addition, an analytical approach was developed to
enable optimization, with respect to energy efficiency, for multi-pass RO and NF
membrane desalting. The implications of the present work to lowering RO
desalination cost by optimization of process configuration will be presented with
reference to specific recently developed large-scale process configurations.