BLBS102-c40 BLBS102-Simpson March 21, 2012 14:23 Trim: 276mm X 219mm Printer Name: Yet to Come
774 Part 7: Food ProcessingTable 40.2.Size of Materials Retained, Driving Force, and Type of MembraneProcess Size of Materials Retained Driving Force Type of MembraneMicrofiltration 0.2–10μm (microparticles) Pressure difference (<2 bar) Porous
Ultrafiltration 1–100 nm MWCO 10^3 –10^6 Da
(macromolecules)Pressure difference (1–10 bar) MicroporousNanofiltration 0.5–5 nm (molecules) Pressure difference (10–100 bar) Microporous
Reverse osmosis <2nmMWCO10^3 Da (molecules) Pressure difference (10–100 bar) Nonporous
Dialysis 1–3μm (molecules) Concentration difference Nonporous or microporous
Electrodialysis <1 nm (molecules) Electrical potential difference Nonporous or microporousseparation medium. The technology involves separating com-
ponents from fluid streams by means of forcing the stream to
flow under pressure over the surface of the membrane. Mem-
brane properties (pore size, membrane material, and membrane
configuration) and operating conditions (pressure, temperature,
and feed velocity) play important roles for successful separation
(Table 40.2). The introduction of cross-flow membrane system
made large-scale continuous separation possible. The technol-
ogy enables processors to concentrate, fractionate, and purify
products simultaneously. The operation is usually conducted at
ambient temperature, although they also have the capability to
be run at higher or lower temperatures.MicrofiltrationThe microfiltration membrane separation process is the most
common pressure-driven membrane separation process and is
defined as the separation of a retained particle size between 0.2
and 10μm by the membrane. Two principal types of membrane
filters are used: depth filters and screen filters, running at very
low pressures owing to the open structure of the membranes.
On the basis of the multi-hole membrane, microfiltration can
effectively remove suspended particles, bacteria, colloids, and
solid proteins. The common membrane modules include spiral-
wound membranes, plate and frame membranes, tubular mem-
branes, and hollow fiber membranes.UltrafiltrationUltrafiltration is a pressure-driven filtration separation occurring
on a molecular scale and can separate high-molecular weight
solutes from a solvent, typically retaining macromolecules with
a molecular weight cutoff of more than 1000 Da. The pore size of
an ultrafiltration membrane normally is from 0.001 to 0.02μm
in order to provide purification, separation, and concentration.
The membranes used in this process have a much smaller pore
size for solvent passage than do microfiltration membranes, with
a relatively small pore area per membrane surface area.
By using an ultrafiltration membrane with a different pore
size, we can separate the contents in the solution with different
molecular weight and shape, such as the purification and concen-
tration of enzymes, proteins, cells, pathogenic organisms, and
polysaccharides, and the clarification and decolorizations forantibiotic fermentation (Li et al. 2004, Feins and Sirkar 2005,
Krstic et al. 2007, Susanto et al. 2008, Cuellar et al. 2009). Ul- ́
trafiltration mainly has the following advantages: steady high
permeated flux, resists free chlorine, wide range for pH and
temperature, easy operation, low energy and operation cost, less
pollution discharge, and compact equipment.
Ultrafiltration is used to separate solvents from solutions of
very large molecules, as well as the liquid from a suspension
of colloidal solids. Ultrafiltration membranes are commercially
fabricated in sheet, capillary, and tubular forms. The liquid to
be filtered is forced into the assemblage and dilute permeate
passes perpendicularly through the membrane while concentrate
passes out the end of the media. This may prove useful for the
recovery and recycle of suspended solids and macromolecules.
A key factor determining the performance of an ultrafiltration
membrane is concentration polarization due to macromolecules
being retained on the membrane surface.NanofiltrationA nanofiltration membrane is between a reverse osmosis mem-
brane and an ultrafiltration membrane in pore size, which can
remove NaCl under 90% concentration. A reverse osmosis mem-
brane has a high removal rate for nearly all solutes. But a nanofil-
tration membrane only has a high removal rate for the special
contents. Nanofiltration membranes mainly remove the particle
whose diameter is near 1 nm, with a molecular weight cutoff
of 100∼1000 Da. In the drinking water area, nanofiltration is
used to deal with quite large inorganic ions such as Ca^2 +,Mg^2 +,
peculiar smells, pigments, pesticides, synthesized surfactants,
dissoluble organics, and vaporized rudimental materials (Bates
2000, Braeken et al. 2004, Baisali et al. 2007). The character
of nanofiltration is that it holds the charge itself, so under low
pressure, it can also have a high desalination rate. The greatest
field for the nanofiltration is to soften and desalt brine water.
Nanofiltration has some advantages, including good chemical
stability, long life, and high rejection selectivity.Reverse OsmosisReverse osmosis is a liquid/liquid separation process that uses
a dense semipermeable membrane, highly permeable to water.
A pressurized feed solution is passed over one surface of the