Self-Regulating Behavior of Hybrid Membrane Systems as Demonstrated in an Element-Scale Forward Osmosis-Reverse Osmosis Hybrid System

Abstract

Hybrid membrane systems can be difficult to design due to the requisite flow rate matching between up- and downstream unit operations. In this work, we use a forward osmosis- reverse osmosis (FO-RO) hybrid system to demonstrate how some membrane systems can exhibit self-regulating behavior due to a tendancy for systems like these to move toward thermodynamic equilibrium.  This can lessen the need for complex control system for flow balancing.   We show this behavior using a module-scale test bed that can mimic the behavior of larger scale systems. The system shows permeate flow rate near-convergence between the FO and RO modules after startup or when perturbed by a change in RO module pressure.  While not promoting FO as a technology for desalination, we believe that exploring the behavior of this hybrid system  shows how some membrane systems can leverage thermodynamics, rather than expensive control systems, to achieve steady state operation.

Keywords: process control, hybrid systems, osmotic processes, debottlenecking