The Unexpected Performance of Highly Permeable SWRO Membranes at High Temperatures

Abstract

In recent years, the development of new seawater reverse osmosis elements has focused on maximizing membrane area and membrane permeability. Such developments led to a new generation of SWRO elements with significantly increased permeate flows and lower pressures. When tested at the same pressures and temperatures, today’s membranes produce nearly twice as much permeate compared to SWRO membranes from ten years ago and compared to today’s tighter, higher rejecting SWRO membranes. Utilizing the highly permeable membranes leads to lower energy consumption and lower operating cost for the SWRO plants. Membrane permeability is further increased with increasing feed water temperature. As temperature increases, the membrane structure is affected and seawater viscosity decreases. This leads to a further reduction in energy consumption and operating cost. While permeability of the membrane increases with increasing temperature, the seawater osmotic pressure is also increasing. At temperatures above 25 C, osmotic pressure is more sensitive to temperature changes than membrane permeability. This leads to the diminishing influence of permeability and the increasing influence of osmotic pressure on the system feed pressure. In some situations, the net effect leads to very little change in feed pressure as temperature increases above 25C. This paper will review the theory behind the competing influences of membrane permeability and osmotic pressure on overall feed pressure. The paper will demonstrate how the evolution of the SWRO permeability has resulted in the increasing influence of osmotic pressure on the membrane performance. The theoretical results will be supported with actual membrane tests performed in the laboratory and on a pilot unit. The paper will identify the temperature ranges and the membrane permeabilities that have the least and greatest influence on the operating cost of an SWRO system.