for processes in which the product is
in the retentate. To illustrate how to use
figure 6, consider a process where the
goals are to perform a 20-fold volume
concentration factor (VCF), a 7 diavolume
buffer exchange, and lose less than
7% of the product to the filtrate. For
this example, the natural log (ln) of the
VCF is 3 and N is 7, so the value of
the term (ln VCF+N) is10. If a
membrane is chosen that has a
retention of 0.99 for the product, the
product loss to the filtrate will be
9.5%, as indicated by point “A” on
the graph. Therefore, the yield loss
goal is not met. In order to reduce the
product loss while still using the same
Membrane Material Protein Adsorption
[-] [g m-2]
membrane, the amount of diafiltration
and/or volume concentration has to
be reduced. For example, if the
number of diavolumes is reduced to
4.3, the value of the term (ln VCF + N)
is now 7.3 and the amount of product
lost to the filtrate is 7.0%, as indicated
by point “B”. However, the extent of
buffer exchange is drastically reduced.
To reduce the product loss without
changing the process, a membrane
with higher retention of the product
must be chosen. If the retention is
increased to 0.999 while the value of
(ln VCF + N) remains at 10, product
loss to the filtrate drops to only 1.0%,
as indicated by point “C”. In many
cases, product retention is different
during the UF and DF sections of a
process. It is important to check this for
each process. When retention
changes, product loss to the filtrate is
determined separately for each section
by following the appropriate retention
curve and summing the two results.
Low Protein Case High Protein Case
[% Yield Loss] [% Yield Loss]
for a particular process. As a rule,
Polyethersulfone 0.5 5.0 0.10
choose a membrane that has a
NMWL one-third to one-fifth of the Biomax (polyethersulfone) 0.2 2.0 0.04
molecular weight of a product that is
Regenerated cellulose 0.1 1.0 0.02
to be retained. Also, a minimum size
difference of approximately five-fold
between components that are being Table 1. Typical protein adsorption onto UF membranes
separated is optimal.
Highly fouling feedstocks tend to
Product Loss (%) = 100 * (1 - e^(R-1)*[In VCF + N])
have higher retention of like-sized
50
proteins than cleaner feedstocks. In
45
R = 0.8
R = 0.9
B
A
C
R = 0.99
R = 0.999
addition, a process operating at very
high TMPs has lower retentions due to
an increased protein concentration at
the membrane surface. Since each
protein feedstock and process is
unique, two or more membranes may
need to be tested before choosing an
optimal one.
Choose a membrane that has
sufficiently high retention to meet
Product Loss to the Filtrate (%)
40
35
30
25
20
15
10
5
your yield goal. Product loss to the
0
filtrate due to incomplete retention is 0 246 810 12 14
cumulative for the concentration and LN (VCF) + N
diafiltration sections of a process.
Figure 6. The effect of product retention on product yield during a batch ultrafiltration/
constant-volume diafiltration process where the product is in the retentate and the
retention is constant throughout the process.
7
Modules
Membranes are fabricated into
modules in several formats. The most
common formats used for tangential
flow filtration are:
. Flat plate
. Spiral wound
. Hollow fiber
The basic flowpaths for each of
these modules is shown in figure 7.
Screens are often inserted into the
feed and/or filtrate channels in spiral
wound and flat plate modules to
increase turbulence in the channels
and reduce concentration
polarization. This is not an option with
hollow fiber modules. The turbulencepromoted
channels have higher mass
transfer coefficients at lower crossflow
rates, meaning that higher fluxes are
achieved with lower pumping
requirements. Turbulence-promoted
feed channels are, therefore, more
efficient than open channels. Using a
suspended screen in a flat plate
module gives some of the benefits of
both open and turbulence-promoted
channels. Figure 8 illustrates the
different types of channel
configurations.
Flat Plate
(Often referred to as Cassettes)
In a flat plate membrane module,
layers of membrane either with or
without alternating layers of separator
