Membrane Separations for Green Chemistry - Process Intensification for Green Chemistry - page 309

Membrane separation processes can differ greatly with regard to membranes, driving
forces, areas of application and industrial or economic relevance. The term
'microfiltration' is used when particles with a diameter of 0.1-10.0 m m are separated
from a solvent or other low-molecular component. Particles are separated according to
their dimensions, and the separation mechanism is based on the molecular sieving effect. In
ultrafiltration the components to be retained by the membrane, mainly of asymmetric
structure, aremacromolecules or submicron particles. Themembranes used for nanofiltration
are mostly asymmetric and the components to be separated are molecular mixtures and ions.
The mechanisms involved in the separation process are molecular size exclusion, diffusion
and Donnan dialysis. Reverse osmosis is a membrane process that permits separation of
particles, macromolecules and low-molecular-mass compounds (salts, sugars, etc.) from
a solvent.
Feed solutions, therefore, often have a significant osmotic pressure, which must be
overcome by the applied hydrostatic pressure.
Gas separation and pervaporation are the other two important membrane processes
used in many applications. Some of these, together with their principles, are reported
in Table 12.3.
In gas separation the membranes used are both porous and dense. With porous
membranes the transport is based on the so-called 'Knudsen diffusion', while with dense
solid membranes the gas transport is based on a solution-diffusion mechanism. Both
Knudsen diffusion and solution-diffusion transport can result in a selective transport of
gases and thus in a separation of gases. However, the extent of the separation - that is, the
separation factor - is much higher in a solution-diffusion transport than in Knudsen
diffusion.
Pervaporation is a process in which the liquid mixture to be separated is in direct contact
with one side of a dense membrane and the permeated product is removed as vapour from
the other side by the application of a lower pressure, inert gas or freezing. The principle
behind this process is selective sorption, diffusion of the selected components from
the liquid solution through the membrane and then evaporation from the permeate side
of the membrane at the downstream side with a low partial pressure. In this case the driving
force is the chemical potential gradient.
Another important family of membrane operations is the membrane contactors. A
membrane contactor is a device that achieves gas/liquid or liquid/liquid mass transfer
without dispersion of one phase within another. The interface is established at the
membrane mouth and the transport is guided by simple diffusion through the membrane
pores. Membrane contactors can be used to separate both immiscible and miscible
liquids, depending on their type (Table 12.4). Examples of membrane contactors in
which the two solvents are miscible are: membrane-distillation, osmotic-distillation and
membrane crystallization. In a membrane distillation process, a porous hydrophobic
membrane is in contact with aqueous solutions of different temperatures on either side
of it. The high interfacial tension between the hydrophobic membrane surface and the
aqueous phase prevents the passage of the aqueous phase through the membrane as a
liquid phase. The temperature gradient promotes evaporation, forming a vapour-liquid
interface at the pore entrance. At the interface, the more volatile compound evaporates,
diffuses and/or convects across the membrane and is condensed and/or removed at the
othersideofthesystem.