Effect of Thermodynamic Restriction on Energy Cost Optimization of RO Membrane Water Desalination

Abstract

Advances in highly permeable reverse osmosis (RO) membranes have enabled desalting operations, in which it is practically feasible for the applied pressure to approach the osmotic pressure of the exit brine stream. However, energy cost remains a major contributor to the total cost of water produced by RO membrane desalination. Reduction of the overall cost of water production represents a major challenge and, in the present work, various elements of water production cost are evaluated from the viewpoint of optimization, with respect to various costs (energy, membrane area and permeability, brine management, and pressure drop), as well as the important thermodynamic cross-flow constraint, utilization of energy recovery devices, and operational feed and permeate flow rate constraints. More specifically, in the present study, an approach to the optimization of product water recovery at pressures that approach the osmotic pressure of the exit brine stream is presented via several simple RO process models that utilize highly permeable membranes. The results suggest that it is indeed feasible to refine RO processes to target for operation under the condition of minimum energy consumption, while considering the constraint imposed by the osmotic pressure, as specified by the thermodynamic cross-flow restriction. Although it is shown that multistage RO provides energy savings, this is at the expense of greater membrane area cost. Overall, as process costs above energy costs are added, the operational point for achieving minimum water production cost shifts to higher recoveries. Although the newer generation of RO membranes can allow high recovery operations at lower pressures, limitations due to mineral scaling and fouling impose additional constraints. The incorporation of these phenomena in the optimization approach is the subject of ongoing research.

Energy consumption for desalination — A comparison of forward osmosis with reverse osmosis, and the potential for perfect membranes

“…Recently, forward osmosis (FO) has been receiving increasing interest from academia and industry as a potentially lower energy alternative to SWRO. Given that energy consumption makes up a major portion of the SWRO cost, reaching as high as ~45% of the total permeate production cost [2], it is useful to take a step back and compare the practical energy needs for RO and FO, where FO employs various draw solution recovery methods.…”

“…Published research on SWRO has investigated reducing the SEC by optimising the membrane module [2,[5][6][7][8][9][10][11][12][13] and/or using more permeable membranes [8,[14][15][16]. However, these studies have utilised modelling tools [1,2] without process simulation tools.…”

Predicting the Specific Energy Consumption of Reverse Osmosis Desalination

“…These desalination factors, summarized in Table 2, represented the explanatory variables in our multiple linear regression model for small-scale desalination facilities, referred to here as the small-scale model. Recovery was included as "inverse recovery" in the models as 1 1−R since SEC is proportional to this value [40,47]. The use of energy recovery systems was included as a binary variable with 0 indicating no energy recovery technology and 1 indicating the use of at least one energy recovery device, since the amount of energy savings is not often reported and different energy recovery technologies save similar percentages of operational energy [42].…”

“…Many operational and water quality factors can influence SEC for a given desalination technology [38][39][40]. For example, RO facilities with larger treatment capacity often observe economies of scale in terms of SEC due to efficiency gains associated with larger pumps [16].…”

Effectiveness-mass transfer units (ε-MTU) model of a reverse osmosis membrane mass exchanger

“…Other studies have applied the solution-diffusion model for the design of RO modules such as spiral wound, hollow fiber, and crossflow long channels [25,[28][29][30][31][32][33]. The solution-diffusion model has also been used together with relevant conservation laws to optimize the operation of RO systems and minimize the specific power consumption and cost of a plant [34][35][36][37][38][39][40][41][42][43].…”