Choices affecting loss Membrane NMWL
Operating parameters
Buffer selection
Membrane material
System materials
System design
Recovery method
Operating parameters
System components
Protein susceptibility
Buffer selection
Table 4. Typical product yield losses during a TFF process
Solubility Losses
Solubility losses are a third mechanism
of product loss during protein processing. The final bulk concentration
of a product may not be beyond its
solubility limit. However, polarization
at the membrane surface and feed
stream volume reduction along the
channel result in areas of higher
concentration throughout the TFF unit.
In addition, inadequate mixing will
increase concentration differences.
Product concentration in these areas
could potentially exceed a solubility
limitation. Since higher fluxes lead to
higher localized concentrations,
reducing flux is one way to minimize
solubility losses if they are significant
in a particular process. Alternatively,
using a process control scheme where
the concentration of product at the
membrane surface is controlled can
maximize flux without exceeding
solubility limitations.
Holdup Volume Losses
Finally, unrecoverable holdup volume
in a unit operation leads to product
loss. After processing, a certain
amount of liquid remains in both the
filter modules and the system piping.
In cases where the final product
concentration is high and/or when the
final product volume is small, these
losses could be significant. Careful
design of the piping, optimization of
total membrane area, and
development of an efficient product
recovery step will help to minimize the
product loss incurred due to
unrecoverable holdup.
Product Quality
During the course of a TFF process,
the quality of a protein could be
compromised due to aggregation or
denaturation caused by
. Micro-cavitation
. Air/liquid interfaces
. High protein concentrations
. Temperature effects
The potential for this damage
depends, in part, on the susceptibility
of the particular protein being
processed. However, even for
delicate products, damage can be
minimized or eliminated by designing
a robust process and system.
Micro-Cavitation
To prevent cavitation, which occurs
when a pump pulls a vacuum at its
inlet and fluid subsequently degasses,
the feed pump should always be run
with a minimum inlet pressure equal to
the manufacturer’s recommendation.
However, micro-cavitation will still
occur to some extent when the protein
feedstock makes multiple passes
through the feed pump and/or
through a partially closed pressure
control valve. The selection of
appropriate pumps and valves can
help prevent the protein denaturation
that microcavitation can cause.
Air/Liquid Interfaces
Other air/liquid interfaces can occur
in several places throughout a TFF
system. In the recycle tank, the
retentate stream should always be
returned below the liquid surface to
prevent foaming, and vortex formation
should be avoided by using an off
center drain or baffles in the tank.
Finally, filling the system with buffer
before introducing protein solution will
minimize any air entrainment during
startup.
High Protein Concentration
As described in the previous section,
there is the potential for highly
concentrated areas to exist within the
TFF unit. Since protein aggregation is
a result of protein-protein interactions
that are concentration-dependent,
higher concentrations could result in
more aggregated protein. The same
considerations for process control
should be made as mentioned above.
Temperature Effects
Lowering the process temperature at
which a TFF process is run is a method
often used to attempt to minimize
product quality degradation.
However, it can exacerbate any
solubility problems, since proteins are
typically less soluble at lower
temperatures. In addition, filtrate flux is
reduced at lower temperatures
because of the corresponding viscosity
increase and mass transfer coefficient
decrease. Flux decreases
approximately 2 – 3% per degree
Celsius of temperature reduction.
Therefore, for equal membrane area,
process time will be longer at a lower
temperature. Since protein
degradation can be a kinetic
phenomenon, a longer process time
may eliminate the benefit of the lower
temperature. Alternatively, if the
membrane area was increased to
compensate for the flux decrease, a
