Membrane distillation is a thermally-driven separation process that uses a hydrophobic microporous membrane. Only water vapor molecules transfer through the membrane, driven by a vapor pressure difference induced by a temperature difference across the membrane. There are two main membrane configurations - hollow fiber membranes and flat sheet membranes - which are commonly made from polytetrafluoroethylene, polypropylene, or polyvinylidenefluoride. Membrane distillation has applications in desalination, wastewater treatment, and food processing.

1. Subject: Advance Separation Techniques (2160507)
CHAPTER 12
MEMBRANE OR OSMOTIC DISTILLATION
INTRODUCTION
 Membrane Distillation (MD) is a thermally-driven separation process, in which only vapour
molecules transfer through a microporous hydrophobic membrane.
 The driving force in the MD process is the vapour pressure difference induced by the
temperature difference across the hydrophobic membrane. This process has various
applications, such as desalination, wastewater treatment and in the food industry.
PRINCIPLE OF MEMBRANE DISTILLATION
 Most processes that use a membrane to separate materials rely on static pressure difference as
the driving force between the two bounding surfaces (e.g. reverse osmosis - RO), or a
difference in concentration (dialysis), or an electric field (ED).
 The selectivity of a membrane can be due to the relation of the pore size to the size of the
substance being retained, or its diffusion coefficient, or its electrical polarity.
 Membranes used for membrane distillation (MD) inhibit passage of liquid water while
allowing permeability for free water molecules and thus, for water vapour.
 As water has strong dipole characteristics, whilst the membrane fabric is non-polar, the
membrane material is not wetted by the liquid.
 The driving force which delivers the vapour through the membrane, in order to collect it on
the permeate side as product water, is the partial water vapour pressure difference between
the two bounding surfaces.
 This partial pressure difference is the result of a temperature difference between the two
bounding surfaces. As can be seen in the image, the membrane is charged with a hot feed
flow on one side and a cooled permeate flow on the other side.

2.  The temperature difference through the membrane, usually between 5 and 20 K, conveys a
partial pressure difference which ensures that the vapour developing at the membrane surface
follows the pressure drop, permeating through the pores and condensing on the cooler side.
 Mass transfer in the DCMD process includes three steps: firstly the hot feed vaporizes from
the liquid/gas interface, secondly the vapour is driven by the vapour pressure difference and
crosses from the hot interface to the cold interface through the pores, and thirdly the vapour
condenses into the cold side stream. Therefore, there are two major factors controlling the
mass transfer: one is the vapour pressure difference, and the other is the permeability of the
membrane.
 If the fluid dynamics conditions on both sides of the membrane could be considered good,
mass transfer through the membrane may be the limiting step for mass transfer in MD. The
influence of the physical properties on membrane permeability includes:
(1) The effective area for mass transfer is less than the total membrane area because the
membrane is not 100% porous;
(2) For most practical membranes, the membrane pores do not go straight through the
membrane and the path for vapour transport is greater than the thickness of the membrane;
and
(3) The inside walls of the pores increase the resistance to diffusion by decreasing the
momentum of the vapour molecules.
 The mass transport mechanism in the membrane pores is governed by three basic
mechanisms known as Knudsen-diffusion (K), Poiseuille-flow (P) and Molecular-diffusion
(M) or a combination between these known as the transition mechanism.
a) Knudsen diffusion takes place when the pore size is too small, so the collision between
the molecules and the inside walls of the membrane suitably expresses the mass transport
and the collision between molecules can be ignored.
b) Molecular diffusion occurs when the molecules move corresponding to each other under
the influence of concentration gradients.
c) In Poiseuille flow (viscous flow), the gas molecules act as a continuous fluid driven by a
pressure gradient.

3. MEMBRANES FOR MD APPLICATIONS
 There are two common types of membrane configurations shown in Figure 3:
 Hollow fiber membrane mainly prepared from PP, PVDF, and PVDF-PTFE composite
material
 Flat sheet membrane mainly prepared from PP, PTFE, and PVDF.
 The most common materials used for MD membranes are polytetrafluoroethylene (PTFE),
polypropylene (PP) and polyvinylidenefluoride (PVDF). The porosity of the membranes used
is in the range of 0.60 to 0.95, the pore size is in the range of 0.2 to 1.0 μm, and the thickness
is in the range of 0.04 to 0.25 mm.
(a) Hollow fiber
 The hollow fiber module, which has been used in MD, has thousands of hollow fibers
bundled and sealed inside a shell tube. The feed solution flows through the hollow fiber and
the permeate is collected on the outside of the membrane fiber (inside-outside), or the feed
solution flows from outside the hollow fibers and the permeate is collected inside the hollow
fiber (outside-inside). The main advantages of the hollow fiber module are very high packing
density and low energy consumption. On the other hand, it has high tendency to fouling and
is difficult to clean and maintain. It is worth mentioning that, if feed solution penetrates the
membrane pores in shell and tube modules, the whole module should be changed.
 Compared with flat sheet membranes, hollow fiber membranes have relatively large specific
surface areas, but the main impediment of the hollow fiber module is its typically low flux
(generally 1–4 L m 2 h 1 at 40–60 °C). The low flux is related to its poor flow dynamics
and the resultant high degree of temperature polarization. However, high-flux hollow fiber
membranes with different features suitable for membrane distillation have been developed
recently, such as dual-layer hydrophilic-hydrophobic fibers with a very thin effective
hydrophobic PVDF layer (50 μm), and hollow fiber membranes with a sponge-like structure
and thin walls, which have flux of about 50–70 kg m 2 h 1 at about 80–90 °C. This flux is
as high as that from flat sheet membrane.

4. (b) Plate and frame
 The membrane and the spacers are layered together between two plates (e.g. flat sheet). The
flat sheet membrane configuration is widely used on laboratory scale, because it is easy to
clean and replace. However, the packing density, which is the ratio of membrane area to the
packing volume, is low and a membrane support is required.The flat sheet membrane is used
widely in MD applications, such as desalination and water treatment.
 The reported flux from flat sheet membranes is typically 20–30 L m 2 h 1 at inlet
temperatures of hot 60 °C and cold 20 °C. In general, the polymeric membrane shown in
Figure 3b is composed of a thin active layer and a porous support layer. This structure is able
to provide sufficient mechanical strength for the membrane to enable the active layer to be
manufactured as thin as possible, which reduces the mass transfer resistance.
(c) Tubular membrane
 In this sort of modules, the membrane is tube-shaped and inserted between two cylindrical
chambers (hot and cold fluid chambers). In the commercial field, the tubular module is more
attractive, because it has low tendency to fouling, easy to clean and has a high effective area.
However, the packing density of this module is low and it has a high operating cost. Tubular
membranes are also utilized in MD. Tubular ceramic membranes were employed in three
MD configurations DCMD, AGMD and VMD to treat NaCl aqueous solution, where salt
rejection was more than 99%.
(d) Spiral wound membrane
 In this type, flat sheet membrane and spacers are enveloped and rolled around a perforated
central collection tube. The feed moves across the membrane surface in an axial direction,
while the permeate flows radially to the centre and exits through the collection tube. The
spiral wound membrane has good packing density, average tendency to fouling and
acceptable energy consumption. It is worth stating that there are two possibilities for flow in
a microfiltration system; cross flow and dead-end flow. For cross flow, which is used in MD,
the feed solution is pumped tangentially to the membrane. The permeate passes through the
membrane, while the feed is re-circulated. However, all the feed passes through the
membrane in the dead-end type.
TYPICAL APPLICATIONS OF MEMBRANE DISTILLATION ARE:
 Seawater desalination
 Brackish water desalination
 Desalination brine treatment
 Process water treatment
 Water purification
 Removal/Concentration of ammonium
 Resource concentration