TBP01x 5.3 Separation and formulation
Welcome to this unit, in which I will give you some insight in the separation and formulation
steps that are available for making biobased products.
This is the process overview for producing PDO.
So far, we have taken you through feedstock choice, the pretreatment, and the
fermentation. This unit, you will learn more about how to convert fermentation broth into
final product.
In downstream processing, we recover a valuable product by separating it from a
complicated mixture flowing out of the conversion process, like fermentation broth, and
ensuring that it has the right purity and concentration and is ready for the market.
You will recognise this diagram from previous units. The main objective in designing DSP is
to understand capital and operational costs and optimize them, while still achieving the
required product specifications.
We need to do this with the simple recipe that always works, which is the equilibrium model
that you have seen before.
Let s have a look at how typical downstream processes are built up. They are generally
composed of the following steps. First, a step that deals with the biological content  the
cells. Sometimes the product is contained inside the cells, which must therefore be
disrupted to release the product.
In other cases, the product is secreted from cells and this disruption step is not necessary. In
both situations, the cells must be removed.
The stream without cells needs to be concentrated to smaller volumes prior to purifying it
further. Finally, formulation ensures that the product is ready for the market.
In biotechnological processes, a lot of attention usually goes out to the biological conversion
step, such as fermentation. In terms of volume, fermentation processes are often much
larger in volume than the streams dealt with in downstream processing. But in terms of
energy consumption and costs, the majority of efforts are often dedicated to downstream
processing.
Let us first see what inflows we have for downstream processes. These can come from a
fermenter or another biological source, and they are often quite a bio mess. It is a multi
component and dilute soup with mostly water, but also cells that are intact or broken, with
their contents floating around in the mixture. Then there are remainders of substrate and
nutrients that were not converted, salts from pH control buffers, and antifoam that was
added to prevent foaming during fermentation.
Amidst all these components, there is some product that must somehow be filtered out.
One of the bigger issues with biotechnological processes is the fact that mixtures are
instable. This means that products are degraded by oxidation or by enzymes such as
proteases and lipases that are released when cells are disrupted.
It is good to have an idea of the typical scales in and of separation processes. There are
micro organisms, and cell debris at a factor ten smaller scale. There are multiple
technologies that effectively filter out particles that are larger than 1 10 micron, these are
called mechanical separations. Then there is a good set of molecular separation techniques
 such as extraction and chromatography  that isolate small dissolved compounds of
several nanometers in diameter. In between, in the colloidal range, there is a gap for which
there are still few suitable separation techniques available.
What we also see is that biotechnological products often have a mixed set of properties.
Taking penicillin G as an example, you see that part of the product is desired, while the other
part may need to be recycled. For instance, they may have both polar and apolar groups,
and negative or positive charges depending on the pH. Some parts of the molecule may be
more stable than others. It is not always simple to predict their behaviour in separation
processes and often empirical optimization is required.
Let s go through the various steps of down stream processing one by one and give some
characteristics. For instance, cell disruption is vastly different between laboratory and full
scales.
There is a whole range of products that require cell disruption technologies. Possible unit
operations include homogenisers and bead mills.
The second step is the removal of microbial cells. They are very small (in the range of
micrometers) and thus do not sediment very well under gravity alone, unless they form flocs
such as often in waste water treatment or beer brewing. You need other technologies such
as filtration and centrifugation processes. Even these are typically too slow to deal with
particles in the colloidal gap that I mentioned earlier.
In the concentration step, what you try to do is reduce the volume and remove most of the
abundant contaminants  water, salts, and so on. We have a range of processes that are
based on adsorption to a solid phase, extraction to a liquid phase, evaporation to a vapour
phase, or precipitation or crystallization for the product to its own solid form. There is also a
range of membrane processes that can deal with concentration.
IF we move on one step further, we get to the purification. We have a mixture of very similar
molecules which are, for example, oxidation or other degradation products or misfolds and
multimers of your final desired products. Again we have a range of possible techniques that
could be used, but for practical reasons chromatography and crystallization are most
frequently used. We focus on those in this course.
Formulation is important, but it is not a part of this course. It brings the product to the final
specifications as required for the market. This can be a business to business, or a consumer
market. In many of the cases, this deals with the composition such as purity and
concentration, but sometimes additives are required. When dealing with a solid product,
certain shape and size distributions may be desired, such as pellets or tablets. You may
therefore need binding agents or so on. Again, this is not part of the course but it can
influence the choice of certain separation techniques earlier on in the process.
These are many steps, and the question is which order to take them. You can only answer
this question by looking at the integral process, not only at each of the unit operations.
We will get back to this at the end of this week, and consider the yields and selectivity,
capacity, energy consumption, robustness and so on, and we see that the optimization
depends on the interplay of all unit operations. In the next unit we will first go into a bit
more detail of several separation processes.
So see you in the next unit!