(661e) Stream Recycling and Mixing in Reverse Osmosis Desalination

Recycling and mixing of RO desalination process streams options were studied in this work in order to determine their effectiveness in affecting the specific energy consumption (SEC) of a single-stage (single-pass), two-pass and two-stage RO process operated at the limit of the thermodynamic restriction. Optimizations of the various operational configurations were carried out both analytically and numerically providing a formalized framework to determine the minimum possible SEC and the corresponding water product recovery. The results show that in a single-stage RO process, partial retentate recycling to the feed stream does not change the SEC, while partial permeate recycling to the feed stream increases the SEC if targeting the same overall water recovery. For the two-stage RO process, diverting part of the raw feed to the second stage, in order to dilute the feed to the second-stage RO, does not decrease the minimal achievable SEC of a two-stage RO process. The energy optimization of two-pass membrane desalination, with second-pass retentate recycling to the first-pass feed stream and operated at the limit imposed by the thermodynamic restriction, revealed the existence of a critical water recovery, which is a function only of the efficiency of energy recovery device (ERD) used in the process when the high pressure feed pumps are ideal (i.e., hundred percent efficiency). When desalting is accomplished at recoveries above a critical water recovery, two-pass desalination with recycling is always less efficient than single-pass desalination. When desalting is accomplished at recoveries below the critical water recovery, the analysis shows that an operational sub-domain exists in which the SEC for a two-pass process with recycling can be lower than for a single-pass counterpart, when the latter is not operated at its globally optimal state. The present approach is well-suited for assessing RO processes design alternatives with respect to minimization of the energy consumption in water desalination.