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Modelling of a low-pressure reverse osmosis system with concentrate recirculation to obtain high recovery levels

Abstract:

A big problem for the desalination plants situated far from the sea is the elimination of the concentrated brines. The volume reduction of the final concentrate could allow an economic transport and a high recovery level. In order to solve this problem, a reverse osmosis system with recirculation (RRO) is proposed. In this system, a part of the final concentrate is recirculated in order to obtain high concentration ratios at low applied pressure. The recirculated stream flows in the permeate side in counter-current reducing the osmotic pressure difference, which is the driving force for solute transport. In this way, a low pressure is needed to establish the water flux. Formally, the RRO system is similar to a continuous distillation or a liquid-liquid extraction, and the flow recirculation ratio, defined as the relation between the flows of product and recirculated stream, is the main operative parameter. In this work a model of the system is used to evaluate the efficiency and to determine the situations where it can be preferable compared to other configurations or unit operations.