Membrane distillation (MD) is a thermally driven separational program
in which separation is enabled due to phase change. A hydrophobic
membrane displays a barrier for the liquid phase, allowing the vapour
phase (e.g. water vapour) to pass through the membrane's pores. The
driving force of the process is given by a partial vapour pressure
difference commonly triggered by a temperature difference.
1 Principle of membrane distillation
Membrane distillation techniques
1.2 Direct-contact MD
1.3 Air-gap MD
1.4 Sweeping-gas MD
1.5 Vacuum MD
1.6 Permeate-gap MD
1.7 Vacuum multi-effect membrane distillation
3 Solar-powered membrane distillation
Principle of membrane distillation
Capillary depression of water on a hydophobic membrane
Temperature and pressure profile through the membrane considering
State of the art processes that separate mass flows by a membrane,
mostly use a static pressure difference as the driving force between
the two bounding surfaces (e.g. RO), a difference in concentration
(dialysis) or an electric field (ED).Selectivity of a membrane is
produced by either its pore size in relation to the size of the
substance to be retained, its diffusion coefficient or electrical
polarity. However, the selectivity of membranes used for membrane
distillation (MD) is based on avoiding passage of liquid water while
allowing permeability for free water molecules and thus, for water
vapour. These membranes are made of hydrophobic synthetic material
(e.g. PTFE, PVDF or PP) and offer pores with a standard diameter
between 0.1 and 0.5 µm. As water has strong dipole
characteristics, whilst the membrane fabric is non-polar, the membrane
material is not wetted by the liquid. Even though the pores are
considerably larger than the molecules, the liquid phase does not
enter the pores because of the high water surface tension. A convex
meniscus develops into the pore. This effect is named capillary
action. Amongst other factors, the depth of impression can depend on
the external pressure load on the liquid. A dimension for the
infiltration of the pores by the liquid is the contact angle Θ=180
– Θ'. As long as Θ > 90° and accordingly Θ' > 0° no
wetting of the pores will take place. If the external pressure rises
above the so-called wetting pressure, then Θ = 90°resulting in a
bypass of the pore. 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. 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.
Membrane distillation techniques
Schematic AGMD arrangement
Many different membrane distillation techniques exist. The basic four
techniques mainly differ by the arrangement of their distillate
channel or the manner in which this channel is operated. The following
technologies are most common:
Direct Contact MD (DCMD)
Air Gap MD (AGMD)
Vacuum MD (VMD)
Sweeping Gas MD (SWGMD)
Vacuum multi-effect membrane distillation (V-MEMD)
Permeate Gap MD (PGMD)
In DCMD, both sides of the membrane are charged with liquid- hot feed
water on the evaporator side and cooled permeate on the permeate side.
The condensation of the vapour passing through the membrane happens
directly inside the liquid phase at the membrane boundary surface.
Since the membrane is the only barrier blocking the mass transport,
relatively high surface related permeate flows can be achieved with
DCMD. A disadvantage is the high sensible heat loss, as the
insulating properties of the single membrane layer are low. However, a
high heat loss between evaporator and condenser is also the result of
the single layer. This lost heat is not available to the distillation
process whereby its efficiency is lowered.
In air-gap MD, the evaporator channel resembles that in DCMD, whereas
the permeate gap lies between the membrane and a cooled walling and is
filled with air. The vapour passing through the membrane must
additionally overcome this air gap before condensing on the cooler
surface. The advantage of this method is the high thermal insulation
towards the condenser channel, thus minimizing heat conduction losses.
However, the disadvantage is that the air gap represents an additional
barrier for mass transport, reducing the surface- related permeate
output compared to DCMD. A further advantage towards DCMD is the
fact, that volatile substances with a low surface tension such as
alcohol or other solvents can be separated from diluted solutions, due
to the fact that there is no contact between the liquid permeate and
the membrane with AGMD. AGMD is especially advantageous compared to
alternatives at higher salinity. Variations on it can include
hydrophobic condensing surfaces for improved flux and energy
Sweeping-gas MD, also known as air stripping, uses a channel
configuration with an empty gap on the permeate side. This
configuration is the same as in AGMD.
Condensation of the vapour takes
place outside the MD module in an external condenser. As with AGMD,
volatile substances with a low surface tension can be distilled with
this process. The advantage of SWGMD over AGMD is the significant
reduction of the barrier to the mass transport through forced flow.
Hereby higher surface-related productwater mass flows can be achieved
than with AGMD. A disadvantage of SWGMD caused by the gas component
and therefore the higher total mass flow, is the necessity of a higher
condenser capacity. When using smaller gas mass flows there is a risk
of the gas heating itself at the hot membrane surface, thus reducing
the vapour pressure difference and therefore the driving force. One
solution of this problem for SWGMD and for AGMD is the use of a cooled
walling for the permeate channel, and maintaining temperature by
flushing it with gas.
Vacuum MD contains an air gap channel configuration. Once it has
passed through the membrane, the vapour is sucked out of the permeate
channel and condenses outside the module as with SWGMD. VCMD and DWGMD
can be applied for the separation of volatile substances from a watery
solution or for the generation of pure water from concentrated salt
water. One advantage of this method is that undissolved inert gasses
blocking the membrane pores are sucked out by the vacuum, leaving a
larger effective membrane surface active. Furthermore, a reduction
of the boiling point results in a comparable amount of product at
lower overall temperatures and lower temperature differences through
the membrane. A lower required temperature difference leaves a lower
total- and specific thermal energy demand. However, the generation of
a vacuum, which must be adjusted to the salt water temperature,
requires complex technical equipment and is therefore a disadvantage
to this method. The electrical energy demand is a lot higher as with
DCMD and AGMD. An additional problem is the increase of the pH value
due to the removal of CO2 from the feed water. For vacuum membrane
distillation to be efficient, it is often run in multistage
In the following, the principle channel configuration and operating
method of a standard DCMD module as well as a DCMD module with
separate permeate gap shall be explained. The design in the adjacent
image depicts a flat channel configuration, but can also be understood
as a schema for flat-, hollow fibre - or spiral wound modules.
The complete channel configuration consists of a condenser channel
with inlet and outlet and an evaporator channel with inlet and outlet.
These two channels are separated by the hydrophobic, micro porous
membrane. For cooling, the condenser channel is flooded with fresh
water and the evaporator e.g. with salty feed water. The coolant
enters the condenser channel at a temperature of 20 °C. After
passing through the membrane, the vapour condenses in the cooling
water, releasing its latent heat and leading to a temperature increase
in the coolant. Sensible heat conduction also heats the cooling water
through the surface of the membrane. Due to the mass transport through
the membrane the mass flow in the evaporator decreases whilst the
condenser channel increases by the same amount. The mass flow of
pre-heated coolant leaves the condenser channel at a temperature of
about 72 °C and enters a heat exchanger, thus pre-heating the
feed water. This feed water is then delivered to a further heat source
and finally enters the evaporator channel of the MD module at a
temperature of 80 °C. The evaporation process extracts latent
heat from the feed flow, which cools down the feed increasingly in
flow direction. Additional heat reduction occurs due to sensible heat
passing through the membrane. The cooled feed water leaves the
evaporator channel at approximately 28 °C. Total temperature
differences between condenser inlet and evaporator outlet and
condenser inlet and evaporator outlet are about equal. In a PGMD
module, the permeate channel is separated from the condenser channel
by a condensation surface. This enables the direct use of a salt water
feed as coolant, since it does not come into contact with the
permeate. Considering this, the cooling-or feed water entering the
condenser channel at a temperature T1 can now also be used to cool the
Condensation of vapour takes place inside the liquid
permeate. Pre-heated feed water that was used to cool the condenser
can be conducted directly to a heat source for final heating, after
leaving the condenser at a temperature T2. After it has reached
temperature T3 it is guided into the evaporator. Permeate is extracted
at temperature T5 and the cooled brine is discharged at temperature
An advantage of PGMD towards DCMD is the direct use of feed water as
cooling liquid inside the module and therefore the necessity of only
one heat exchanger to heat the feed before entering the evaporator.
Hereby heat conduction losses are reduced and expensive components can
be cut. A further advantage is the separation of permeate from
coolant. Therefore, the permeate does not have to be extracted later
in the process and the coolant's mass flow in the condenser channel
remains constant. The low flow velocity of the permeate in the
permeate gap is a disadvantage of this configuration, as it leads to a
poor heat conduction from the membrane surface to the condenser
walling. High temperatures on the permeate side's membrane bounding
surface are the result of this effect (temperature polarisation),
which lowers the vapour pressure difference and therefore the driving
force of the process. However, it is beneficial, that the heat
conduction losses through the membrane are also lowered by this
effect. This poor gap heat conduction challenge is largely removed
with a variant of PGMD called CGMD, or conductive gap membrane
distillation. Compared to AGMD, in PGMD or CGMD, a higher surface
related permeate output is achieved, as the mass flow is not
additionally inhibited by the diffusion resistance of an air layer.
Vacuum multi-effect membrane distillation
The hydrophobic membranes (or PP foils) are welded at both sides of
the memsys frame. This frame are designed to combine and distribute
vapor, feed, non condensable gas and distillate flows.
Different numbers of memsys frame are friction welded as memsys module
(e.g. steam raiser, membrane stage and condenser). GOR and capacity of
memsys module can be easily modified deponding on the application or
Diagram of memsys V-MEMD process
The typical vacuum multi-effect membrane distillation (e.g. the memsys
brand V-MEMD) module consists of a steam raiser,
evaporation–condensation stages, and a condenser. Each stage
recovers the heat of condensation, providing a multiple-effect design.
Distillate is produced in each evaporation–condensation stage and in
Steam raiser: The heat produced by the external heat source (e.g.
solar thermal or waste heat) is exchanged in the steam raiser. The
water in the steam raiser is at lower pressure (e.g. 400 mbar),
compared to the ambient. The hot steam flows to the first
evaporation–condensation stage (stage 1).
Evaporation–condensation stages: Stages are composed of alternative
hydrophobic membrane and foil (Polypropylene, PP) frames. Feed (e.g.
seawater) is introduced into stage 1 of the module. Feed flows
serially through the evaporation–condensation stages. At the end of
last stage, it is ejected as brine.
Stage 1: Steam from the evaporator condenses on a PP foil at pressure
level P1 and corresponding temperature T1. The combination of a foil
and a hydrophobic membrane creates a channel for the feed, where the
feed is heated by the heat of condensation of the vapour from the
steam raiser. Feed evaporates under the negative pressure P2. The
vacuum is always applied to the permeate side of the membranes.
Stage [2, 3, 4, x]: This process is replicated in further stages and
each stage is at a lower pressure and temperature.
Condenser: The vapour produced in the final evaporation–condensation
stage is condensed in the condenser, using the coolant flow (e.g.
Distillate production: Condensed distillate is transported via the
bottom of each stage by pressure difference between stages.
Design of memsys module: Inside each memsys frame, and between frames,
channels are created. Foil frames are the ‘distillate channels’.
Membrane frames are the ‘vapour channels’. Between foil and
membrane frames, ‘feed channels’ are created. Vapour enters the
stage and flows into parallel foil frames. The only option of for the
vapour entering the foil frames is to condense, i.e. vapour enters a
‘dead-end’ foil frame. Although it is called a ‘dead-end’
frame, it does contain a small channel to remove the non-condensable
gases and to apply the vacuum.
The condensed vapour flows into a distillate channel. The heat of
condensation is transported through the foil and is immediately
converted into evaporation energy, generating new vapour in the
seawater feed channel. The feed channel is limited by one condensing
foil and a membrane.The vapour leaves the membrane channels and is
collected in a main vapour channel. The vapour leaves the stage via
this channel and enters the next stage. memsys has developed a highly
automated production line for the modules and could be easily
extended. As the memsys process works at modest low temperatures
(<90 °C) and moderate negative pressure, all module
components are made of polypropylene (PP). This eliminates corrosion
and scaling and allows large-scale cost efficient production.
Typical applications of membrane distillation are:
Brackish water desalination
Process water treatment
Concentration of ammonium
Solar-powered membrane distillation
Plant design of a compact system
Plant design of a two loop system
Membrane distillation is very suitable for compact, solar powered
desalination units providing small and medium range output <10000
l/day. Especially the spiral wound design patented by GORE in the
year 1985 suits this application. Within the MEMDIS project, which
kicked off in 2003, the Fraunhofer Institute for Solar Energy Systems
ISE began developing MD modules as well as installing and analysing
two different solar powered operating systems, together with other
project partners. The first system type is a so-called compact system,
designed to produce a drinking water output of 100-120 l/day from
sea-or brackish water. The main aim of the system design is a simple,
self-sufficient, low maintenance and robust plant for target markets
in arid and semi-arid areas of low infrastructure. The second system
type is a so-called two-loop plant with a capacity of around 2000
l/day. Here, the collector circuit is separated from the desalination
circuit by a saltwater resistant heat exchanger. Based on these two
system types, a various number of prototypes were developed, installed
The standard configuration of today's (2011) compact system is able to
produce a distillate output of up to 150 l/day. The required thermal
energy is supplied by a 6.5 m² solar thermal collector field.
Electrical energy is supplied by a 75 W PV-module. This system type is
currently being developed further and marketed by the Solar Spring
GmbH, a Spin -Off of the Fraunhofer Institute for Solar Energy
Systems. Within the MEDIRAS project- a further EU-project, an enhanced
two-loop system was installed on the Island of Gran Canary. Built
inside a 20 ft container and equipped with a collector aray size
of 225 m², a heat storage tank makes a distillate output of up to
3000 l/day possible. Further applications with up to 5000 l/day have
also been implemented, either 100% solar powered or as hybrid projects
in combination with waste heat.
The operation of membrane distillation systems faces several major
barriers that may impair operation, or prevent it from being a viable
option. The principal challenge is membrane wetting, where saline feed
leaks through the membrane, contaminating the permeate. This is
especially caused by membrane fouling, where particulates, salts, or
organic manner deposit on the membrane surface. Techniques to mitigate
fouling include membrane superhydrophobicity, air backwashing to
reverse or prevent wetting,, choosing non-fouling operating
conditions, and maintaining air layers on the membrane
The single biggest challenge for membrane distillation to be cost
effective is the energy efficiency. Commercial systems have not
reached competitive energy consumption compared to the leading thermal
technologies such as Multiple-effect distillation, although some have
been close, and research has shown potential for significant
improvements on energy efficiency.
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