Biomedical Engineering Reference
In-Depth Information
Pressure-driven membrane processes are characterized by their low specific energy
consumption. Since they constitute a mechanical separation without phase or temperature
change, they consume least energy (Mirza, 2008). In addition to energy, these methods have
the advantages as follows: (Cheryan and Rajagopalan, 1998):
(1) This technology is more widely applicable across a wide range of industries;
moreover, requirement of high temperature can be avoided.
(2) The membrane is a positive barrier to rejected components. Thus, the quality of the
treated water (the permeate) is more uniform, regardless of influent variations. These
variations may decrease flux, but it does not generally affect the quality of its output.
(3) No extraneous chemicals are needed, making subsequent recovery easier.
(4) Membranes can be used in-process to allow recycling of selected waste streams
within a plant.
(5) Membrane equipment has a smaller foot print.
(6) The plant can be highly automated and does not require highly skilled operators.
2. Fundamentals of Pressure-Driven Membrane Process
The pressure-driven membrane process universally relies on the preferential retention and
passage of at least one solute and one solvent. Both of these processes often involve selective
separation of multiple solutes. The primary objectives of multiple-solute separations are to
achieve both a desired yield of the product and an acceptable level of purification, from one
or several impurities (Reis and Saksena, 1997). However, one cannot achieve complete
retention of retained solute with current membrane technology. There is a compromise
between yield and purification. Separating solutes with similar retention properties, such as
selective protein filtration, is challenging task (Nakassuka and Michaels, 1992). The function
of pressure-driven membrane process is affected by a series of factors. Major factors include
pressure, feed velocity, solute concentration and temperature. Interaction between feed
solution and membrane, characteristic of solute and cleaning operation should be also
considered in many cases.
During pressure-driven membrane process, the flux (
J
) of permeate is normally described
as a function of the driving force and the total resistance in equation (1), called Darcy's law.
Δ
P
(1)
J
=
μ
to l
where,
ΔP
is the differential pressure(or transmembrane pressure);
µ
is the dynamic viscosity
of the permeate;
R
tol
is the total resistance.
In separation of small molecules or salts from water or solvent by reverse osmosis,
nanofiltration and ultrafiltration, the effect of osmotic pressure on permeate flux is significant.
In order to obtain a flux from bulk side (higher solute concentration ) to permeate side (lower
solute concentration), the applied pressure have to be greater than the osmotic pressure. Thus
an osmotic pressure term should be added to Darcy's law as the following equation:
