Presentaci

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MEMBRANE TECHNOLOGY OTHER TECHNIQUES
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OTHER TECHNIQUES
MEMBRANE DISTILLATION
Introduction

• Most membrane transport processes are isothermal
  processes with either concentration pressure or
  electrical potential difference as the driving
  force.In this case we have a thermal process.

(1)

• When a membrane separates two phases held at
  different temperatures, heat will flow from the
  high-temperature side to the low-temperature
  side. This transport of heat can be described by
  Fouriers equation, where the heat flow is
  related to the corresponding driving force, the
  temperature difference.

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OTHER TECHNIQUES
MEMBRANE DISTILLATION
Definition

• Is a process in which two liquid or solutions at
  different temperatures are separated by a porous
  membrane.

• The liquid or solution must no wet the membrane
  otherwise the pores will be filled for capillary
  force. This implies that non-wettable porous
  hydrophobic membranes must be used in the case of
  aqueous solutions.

• Membrane distillation is a type of low
  temperature, reduced pressure distillation due at
  the use porous hydrophobic polymers.

• The material in this case is not wetted by the
  liquid feed and thus liquid penetration and
  transport across the membrane is prevented,
  provided the feed site pressure does not exceed
  the minimum entry pressure for the pore size
  distribution of the particular material.

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OTHER TECHNIQUES
MEMBRANE DISTILLATION
Schematic representation

• Such transport occur in a sequence of three
  steps
• Evaporation on the high-temperature side.
• Transport of vapour molecules through the pores
  of the hydrophobic porous membrane.
• Condensation on the low-temperature side.

• Membrane distillation is one of the membrane
  processes in which the membrane is no directly
  involves in separation the only function of the
  membrane is to act as a barrier between the twos
  phases. Selectivity is completely determined by
  the vapour liquid equilibrium involves. This
  means that the component with the highest partial
  pressure will show the highest permeation rate.

(M. Mulder, 1997.Basic Principles of Membrane
Technology)
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OTHER TECHNIQUES
MEMBRANE DISTILLATION
Schematic representation
2
6
1
5
3
4
Figure 3. Fractionation by membrane distillation,
1, porous hydrophobic membrane polymer 2, feed
3, vapour space 4, cooling water 5, chilled
wall 6, condensed droplets.
(K.Scott R. Hughes, 1996.Industrial Membrane
Separation Technology)
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OTHER TECHNIQUES
MEMBRANE DISTILLATION
Process parameters
Figure 4. Contact angles of liquid droplets on a
solid (nonporous) material.

• The main requirement is that the membrane must no
  be wetted. If wetting occurs, the liquid will
  penetrated spontaneously into the pores of the
  membrane.
• The wettability is the determined by the
  interaction between the liquid and the polymeric
  material, with no wetting occurring at low
  affinity. Information about wettability can be
  obtained by contact angle measurements, i.e. a
  drop of liquid is placed upon a nonporous flat
  surface and the contact angle is measured.
• For low affinity the contact angle ? will have
  values gt 90º.
• If the material is porous, the liquid will
  penetrate into the pores wetting occur (? lt 90º).

• This can be described by the Laplace equation

(2)
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OTHER TECHNIQUES
MEMBRANE DISTILLATION
Membrane materials

• The membrane hydrophobic materials typical are

• Polypropylene
• PTFE (polytetrafluroethylene)
• PVDF (polyvinylidenedifluoride)

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OTHER TECHNIQUES
MEMBRANE DISTILLATION
Some applications
The applications are determined by the
wettability of the membrane, which implies that
mainly aqueous solutions containing inorganic
solutes can be treated. The surface tension of
these solution differs little from that of water.

• The production of pure water
• Laboratories
• Semiconductor industry
• Desalination of the seawater
• Concentration of aqueous solutions
• Removal of compound organic volatiles (VOCs)
• Contaminated surface water (benzene)
• Fermentation products and volatile bioproducts
  (ethanol, butanol, acetone or aroma compounds).

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OTHER TECHNIQUES
MEMBRANE DISTILLATION
Advantage drawback
Advantage

• The major advantage of membrane distillation is
  that, with compact modules equipped with hollow
  fibres, a high surface area per unit liquid
  volume for mass transport is accessible and thus
  high overall permeation rates are attainable.

Drawback

• The main limitation in a MD is the surface
  tension of the solution that must be similar at
  tension surface water.

• The greater practical limitation in membrane
  distillation applications is the required
  pressure upstream of the membrane.

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OTHER TECHNIQUES
OSMOTIC DISTILLATION
Introduction

• Similar to membrane distillation.

• Both phases at the same temperature.

• The partial pressure gradient due to the osmotic
  pressure is the driving force.

• The osmotic pressure is risen by adding
  appropriate compounds to the receiving phase.

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OTHER TECHNIQUES
OSMOTIC DISTILLATION
Transport mechanism

• Osmotic distillation is a separation process in
  which a liquid mixture containing a volatile
  component is contacted with microporous,
  non-liquid-wettable membrane whose opposite
  surface is exposed to a second liquid phase
  capable of absorbing that component.

Feed Channel
Dilute Feed In
Concentrated Feed Out
Semipermeable Membrane
Concentrated Brine In
Diluted Brine Out
Brine Channel
Figure 5. In osmotic distillation, a
semipermeable membrane acts as a vapor gap that
allows migration of volatiles in a single
direction.
Chemical Engineering Progress, 1998
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OTHER TECHNIQUES
OSMOTIC DISTILLATION
Some applications

• This technique to play an important role in the
  processing of foods, pharmaceutical and
  biological products.

Examples

• Fruit juice enrichment.
• Alcohol removal from wine and beer.

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OTHER TECHNIQUES
OSMOTIC DISTILLATION
Transport mechanism (solute concentration)
The driving potential for such transport is the
difference in vapor pressure of each component
over each of the contacting liquid phases. If the
sole or primary volatile component in solution is
the solvent, then evaporation of solvent from the
solution of higher vapor pressure into that of
lower vapor pressure will result in concentration
of the former and dilution of the latter. If the
solvent vapor pressure over the liquid being
concentrated drops to a value equal to that over
receiving phase, no further transport will occur.
Increasing Water Vapor pressure
Diluted Brine Out
Concentrated Brine In
Membrane
Dilute Feed In
Concentrated Feed Out
Decreasing Water Vapor pressure
Figure 6. Mechanism of osmotic distillation
through a microporous hydrophobic membrane
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OTHER TECHNIQUES
OSMOTIC DISTILLATION
Transport Mechanism (Alcohol removal )
In this case, the solute of interest is
evaporated from the feed at the membrane surface,
transported by vapor diffusion through the
membrane pores, and condensed into a strip liquid
on the opposite face of the membrane. Most
commonly, the stripping liquid is pure water or
an aqueous solution containing a lesser
concentration of the solute being transferred.
Increasing alcohol partial pressure
Dilute Alcohol Solution Out
Water In
Water
Membrane
Alcohol
Wine In
Dealcoholized Wine Out
Decreasing alcohol partial pressure
Figure 8. Mechanism of selective alcohol removal
by evaporative pertraction
Chemical Engineering Progress, 1998
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OTHER TECHNIQUES
OSMOTIC DISTILLATION
Process design

• The objective of an OD process for concentration
  of an aqueous feed is to remove water from the
  feed via transfer of a large fraction of the
  water into a saline strip solution, yielding a
  product of the desired solute concentration and
  diluted strip. The essential design parameters
  are

• The required plan capacity in daily volume of
  feed to be concentrated.
• The solute concentrations in the feed and final
  concentrated.
• The water vapor pressure/concentration
  relationship for the feed stream.
• The water vapor pressure/salt concentration
  relationship for the strip solution.
• The intrinsic water vapor permeability of the OD
  membrane, expressed as

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OTHER TECHNIQUES
OSMOTIC DISTILLATION
Process design
The process employs partial batch recycle on the
feed side (to minimize large feed viscosity
changes through the membrane) and continuous
countercurrent recycle plus evaporative
reconcentration of the brine strip.
Figure 9. Typical OD system for juice
concentration
Chemical Engineering Progress, 1998
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OTHER TECHNIQUES
OSMOTIC DISTILLATION
Process design
In the figure 10, we can see the following, the
wine from a chilled storage tank is continuously
recycled through the shell side of the OD
contactor array, while strip solution (water)
from a second storage tank is continuously
recycled.
Figure 10. Dealcoholization of ferments by
evaporative pertraction
Chemical Engineering Progress, 1998
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OTHER TECHNIQUES
OSMOTIC DISTILLATION
Advantage drawback
Advantage

• Retains flavours and fragrances better than
  thermal techniques.

• The solutes are concentrated at low temperature
  and pressure, with minimal thermal or mechanical
  damage to or loss of those solutes (thermal
  labile)

Drawback

• Is a process very expensive for water removal
  from solution than more-conventional processes
  such as distillation, UF, and RO.

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OTHER TECHNIQUES
MEMBRANE EXTRACTION
Introduction

• These membrane processes are generally referred
  to as membrane contactor.
• (other names pertraction, gas adsorption,
  membrane based solvent extraction, liquid-liquid
  extraction, hollow-fiber contained liquid
  membrane, etc.)

• The separation performance in these processes is
  determined by the distribution coefficient of a
  component in two phases and the membrane acts
  only as an interface, similar to membrane
  distillation.

• In general, it is not the enhanced mass transfer
  but rather the large area per volume as can found
  in hollow fiber and capillary modules, that makes
  this process more attractive than convectional
  dispersed-phase contactor

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OTHER TECHNIQUES
MEMBRANE EXTRACTION
Membrane contactor
Figure 11 .Schematic drawing of various membrane
contactor.
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OTHER TECHNIQUES
MEMBRANE EXTRACTION
Transport mechanism

• If a component i is transferred from feed phase
  to the permeate phase three steps can be
  considered in general
• Transport from feed phase to the membrane.
• Diffusion through the membrane
• Transfer from the membrane to the permeate phase

The flux of component i is expressed in terms
of an overall mass transfer coefficient
with
If the mass transfer resistance is completely in
the membrane phase then the first equation is
reduces to
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OTHER TECHNIQUES
MEMBRANE EXTRACTION
Gas-liquid membrane contactor
Porous membrane
Feed
Permeate
Gas phase
Liquid phase
Liquid phase
Gas phase
Liquid phase boundary layer
Gas phase boundary layer
Membrane
a)
b)
Figure 12. Gas-liquid contactors with a
non-wetted membrane (left side) and a wetted
membrane (right side ) and the corresponding
concentration profile.
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OTHER TECHNIQUES
MEMBRANE EXTRACTION
Liquid-liquid membrane contactor

• This process is characterised by two liquid
  stream separated by a porous or nonporous
  membrane.

• In case of a porous membrane the feed phase may
  either wet or not wet the membrane.

• Firstly we will consider the case where the feed
  is an organic solvent from which a solute has to
  be removed while the permeate phase is an aqueous
  phase.

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OTHER TECHNIQUES
MEMBRANE EXTRACTION
Liquid-liquid membrane contactor
Liquid phase
Gas phase
Liquid phase
Gas phase
Liquid phase boundary layer
Liquid phase boundary layer
Liquid phase boundary layer
Liquid phase boundary layer
Membrane
Membrane
a)
b)
Figure 13. Liquid-liquid membrane contactors with
a wettable liquid feed (left side) and a
non-wettable liquid feed phase (right side ) and
the corresponding component concentration profile.
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OTHER TECHNIQUES
MEMBRANE EXTRACTION
Nonporous membrane contactors
Gas-liquid and liquid-liquid membrane contactors
due to shear stresses, osmotic flow, and pressure
gradients. This may be overcome by using either
nonporous membrane contactors (G-L, L-G or L-L
contactor) or by applying a coating onto the
porous membrane.
l2
l1
Liquid phase
Gas phase
Liquid phase
Gas phase
Liquid phase boundary layer
Liquid phase boundary layer
Liquid phase boundary layer
Liquid phase boundary layer
Membrane
Membrane
a)
b)
Figure 14. Gas-liquid membrane contactors with a
composite membrane (porous membrane left) and
dense membrane (right)
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OTHER TECHNIQUES
MEMBRANE EXTRACTION
Some application

• G-L CONTACTORS
• CO2 and H2S from natural gas
• CO2 from biogas
• O2 transfer (blood oxygenation, aerobic
  fermentation)
• CO2 transfer (beverages)
• NH3 from air (intensive farmery)

• L-G CONTACTORS
• Volatile bioproducts (alcohol, aroma compounds)
• O2 removal from water

• L-L CONTACTORS
• Heavy metals
• Fermentation products (citric acid, acetic acid,
  lactic acid penicillin)

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OTHER TECHNIQUES
MEMBRANE EXTRACTION
Advantage drawback
Advantage

• The used of the hollow Fiber membranes given a
  bigger area to flux.

Drawback

• Instability of the system, the pressure applied
  must no be greater than the wetting pressure or
  the liquid penetration may occur.