Protein Concentration
and Diafiltration by
Tangential Flow Filtration
T E C H N I C A L B R I E F
Table of Contents
Purpose 1
What is TFF? 1
TFF Basics 3
Define Your Process Goals 5
Choosing the Right Equipment 5
Optimization of Key Process
Parameters 9
Characterizatiom of Performance 13
Putting the Process Together 15
System Considerations 17
Major Process Considerations 19
Glossary of Terms 23
Technical References 23
.
An Overview
PURPOSE
Membrane-based Tangential Flow
Filtration (TFF) unit operations are used
for clarifying, concentrating, and
purifying proteins. This technical brief
is a practical introduction to protein
processing using tangential flow
filtration. It is intended to help scientists
and engineers achieve their protein
processing objectives by discussing
how the choice of key components
and operating parameters will affect
process performance.
What is TFF?
Filtration is a pressure driven
separation process that uses
membranes to separate components in
a liquid solution or suspension based
on their size and charge differences.
Filtration can be broken down into
two different operational modes –
Normal Flow Filtration and Tangential
Flow Filtration. The difference in fluid
flow between these two modes is
illustrated in figure 1.
In Normal Flow Filtration (NFF),
fluid is convected directly toward the
membrane under an applied pressure.
Particulates that are too large to pass
through the pores of the membrane
accumulate at the membrane surface
or in the depth of the filtration media,
while smaller molecules pass through
to the downstream side. This type of
process is often called dead-end
filtration. However, the term “normal”
indicates that the fluid flow occurs in
the direction normal to the membrane
surface, so NFF is a more descriptive
and preferred name. NFF can be
used for sterile filtration of clean
streams, clarifying prefiltration, and
virus/protein separations and will not
be discussed in this document.
In Tangential Flow Filtration (TFF),
the fluid is pumped tangentially along
the surface of the membrane. An
applied pressure serves to force a
portion of the fluid through the
membrane to the filtrate side. As in
NFF, particulates and macromolecules
that are too large to pass through the
membrane pores are retained on the
Pressure
Filtrate
Feed Flow
Membrane Membrane
Filtrate
Feed Flow Pressure
Normal Flow Filtration Tangential Flow Filtration
Figure 1. Comparison of NFF and TFF
upstream side. However, in this case
the retained components do not build
up at the surface of the membrane.
Instead, they are swept along by the
tangential flow. This feature of TFF
makes it an ideal process for finer
sized-based separations. TFF is also
commonly called cross-flow filtration.
However, the term “tangential” is
descriptive of the direction of fluid
flow relative to the membrane, so it is
the preferred name.
How is TFF Used in Protein
Processing?
TFF can be further subdivided into
categories based on the size of
components being separated. For
protein processing, these can range
from the size of intact cells to buffer
salts. Figure 2 details typical
components that would be retained by
a membrane and that would pass
through a membrane for each of the
subdivisions. In addition, it shows the
range of membrane pore size ratings
or nominal molecular weight limits
(NMWL) that generally fall into each
category.
A membrane pore size rating,
typically given as a micron value,
indicates that particles larger than the
rating will be retained by the
membrane. A NMWL, on the other
hand, is an indication that most
dissolved macromolecules with
molecular weights higher than the
NMWL and some with molecular
weights lower than the NMWL will
be retained by the membrane. A
component’s shape, its ability to
deform, and its interaction with other
components in the solution all affect its
retention. Different membrane
manufacturers use different criteria to
assign the NMWL ratings to a family
of membranes. The technical
references at the end of this document
provide more detail on membrane
retention determination as well as
additional information on other related
topics.
Microfiltration (MF) is usually used
upstream in a recovery process to