Advancements in osmosis- and pressure driven membrane separation processes - Optimizations of membrane module designs through computational fluid dynamics

Water stress affects an increasing part of the world population. In 2019, 2 billion people were estimated to live in water stressed countries, and around 30% of the world population doesn’t have access to save drinking water. Potable water production through water treatment is therefore essential for counteracting water stress and securing the supply of clean water.

However, water treatment is energy intensive and exhibits a significant environmental impact. The energy-efficient treatment of water is therefore among our societies’ most relevant challenges. Membrane technologies have become widely applied for the treatment of water, of which especially the desalination of seawater through reverse osmosis is substantial.

Reverse osmosis applications are predominantly very energy intensive. Through the combination with energy-neutral forward osmosis or the energy-generating pressure retarded osmosis processes, the net energy consumption of reverse osmosis filtration processes can be lowered. With regard to the high energy consumption of reverse osmosis filtration this PhD project is concerned with the efficiency optimization of reverse osmosis, forward osmosis, and pressure retarded osmosis processes.

More specifically, the comprehensive objectives of this PhD project are the quantification and optimization of the above mentioned membrane separation processes through hydrodynamic modeling of flow and solute transport mechanisms in the membrane-embedding module geometries. This thesis outlines and relates four studies which collectively depict the significance of the design of membrane modules.

The first part of this thesis focuses on the optimization of the reverse osmosis desalination process, whose process efficiency is governed by the total filtration rate, the filtrate purity, and the associated driving pressure. The most widely used module geometry for reverse osmosis is the spiral-wound module.

It is assumed that its efficiency is most significantly affected by the feed channel and its embedded spacer. Through a computational fluid dynamics analysis of the spacer geometry, the effect of the spacer size and its orientation in the feed channel is quantified. It is found that thicker spacers marginally improve the water flux and decrease the pressure drop.

In desalination applications with high salinity, the filtration efficiency is significantly increased when the spacers are not flow-aligned. Subsequently, the model of the spiral-wound module is applied in the development of a simulation tool for designing entire desalination plants, to which the same efficiency criteria apply.

Through the simulation of different desalination designs, favorable application-specific conifigurations are identified. All desalination designs are shown to be most efficiently operated at low feed flow rate and high feed pressure, disregarding membrane fouling. The second part of this thesis focuses on osmosis-driven membrane processes, of which forward osmosis and pressure retarded osmosis are addressed.

The efficiency of forward osmosis processes is governed by the permeation flux, the reverse salt flux, and the associated driving pressure. For the hollow fiber module the process efficiency is quantified against the packing density of the embedded fibers. Increasing the packing density yields a greater total membrane surface area, increases the cross flow, due to narrowing the channel, but also causes an increase of the driving pressure.

An efficient trade-off between these effects is found for a packing density of ∼ 70%. In pressure retarded osmosis, the efficiency criteria are the power density, and the driving pressure. A novel, pressure retarded osmosis-specific module design is presented. The design uses submerged sheets of helically-twisted membranes to enforce the solute mixing in the draw channel.

Due to its low packing density the design exhibits a very low pressure drop, and a low dilution level of the draw solution. The latter effect is compensated for by the recirculation of the draw solution. A computational fluid dynamics study shows that this pressure retarded osmosis -specific design is more effective compared to traditional filtration designs.