TBP01x 5.5 Conceptual process design for 1,3 PDO production
Welcome to this unit, which is the first of two that apply the theory of the previous units to
the common case of 1,3 PDO production. This unit provides the qualitative conceptual
thinking, and the next will elaborate quantitative elements.
Let s go back to the common scheme of an integrated bioprocess, starting at the fermenter.
We assume for the moment that the fermenter feedstock is a conventional carbohydrate
solution. In a later stage, this can be modified to a 2nd generation feedstock with
hydrolysate sugars as well as its five carbon sugars, contaminants and salt load. For now, our
PDO production is high and the broth will contain mostly water, cells and PDO. It is
contaminated with organic material: unconverted substrates (for now mostly sugars), and
metabolic byproducts. These need to be removed to the specification (specs). It also has
some inorganic load, due to pH control, buffering or feedstock. The inorganics also need to
be removed.
So let s start at the conditions that resulted from modules 3 and 4. The fermenter or
fermenters contain a cumulative 2250 ton broth and produces 165 kmol of 1,3PDO per hour.
This requires 71 kton/hr of concentrated feedstock in the fermenter with 3 moles (or 50
wt%) of glucose per kg feedstock. And also a substantial flow of oxygen and ammonia. This
results in a large off gas flow that now includes CO2, some water and the remaining oxygen.
And of course the liquid product flow with the components indicated at the previous slide.
Note that the cell concentration is reasonably high with 3,275 moles /kg of broth. This is 85 g
of dry bacteria, which corresponds to 0,26 kg of wet cells/kg.
Note that this value is much higher than in many literature references, which stop at
approximately half that concentration. The high cell concentration will lead to a large
amount of adhering medium in the cell removal step. This amount is approximately as large
as the cell flow. The associated product must then be recovered by washing steps, which will
dilute the product. There are also proteins, associated with high cell density and cell lysis,
that need to be removed.
The organics are composed of unconverted substrates and metabolic byproducts.
Unconverted substrates are estimated to be 1 % of the product flow or 10 g/kg. Data on
formation of metabolic byproducts is difficult to find, but taking this literature graph as an
example you see formation of a mix of organic acids (acetic, lactic, succinic, pyruvic, and
formic ). Glycerol is another byproduct that you often find in PDO processes. Literature
estimates a total of 17 g/L, which we translate to 17 g/kg.
Similarly, other references such as patents indicate a total of 6 7 g/L of inorganic
contaminants. All of this needs to be removed to the specifications.
So, summarizing, we have the following gross broth composition. Water is calculated as the
balance. The high cell density will also lead to a high viscosity, and make it difficult to
process.
The integrated process of 1,3PDO then looks as follows. Cell disruption is not necessary. Cell
removal (with a wash to recover the product) of course is. This will be a first step to perform.
Protein may be removed to some extent by that step as well, but in general requires a
separate step. Also this will be an early unit operation, as proteins tend to foul many unit
operations. Then the PDO solution has to be cleared from its main contaminants, which are
water and the inorganic components. And afterwards purified from the  similar
contaminants the organics fraction. Formulation is not discussed yet be assume that PDO
will be produced as a clear liquid. This gives a relatively straight forward cascade of 5 process
steps, where steps 3, 4 and 5 may change order, or where several may be combined. Let us
now explore the options for each step.
Cell removal, step A. Microbial cells sediment (too) slow for a simple gravity settler.
Reasonable alternatives are centrifugation, filtration and micro filtration. The latter is
actually a membranes based separations. Our first selection is microfiltration, but of course
we have to evaluate this choice in a later stage. The stream with the separated cells will
contain quite a bit of medium with dissolved PDO. This needs to be recovered by
(countercurrent) washing of the cells, for instance in a countercurrent set up of N
membrane units. Evaluation will be by its economic and ecological numbers. We can do that
by using the simple design recipe introduced in earlier units.
Step B is protein removal. Extraction and evaporation are no serious options, can you guess
the reason? Protein precipitation and crystallisation may be, when concentrations are
sufficiently high. Sorption and chromatography form a group of related technologies where
a more or less selective solid phase is used to bind the protein. These and membrane
processes are generally rather versatile, and both may work. Patents describe specific
membrane processes (ultrafiltration) that may work for the case of PDO. We have selected
membrane processes for protein production. This has to be evaluated against e.g. sorption
and chromatography in terms of costs, emissions and practical aspects (robustness, fouling
etc)
 Inorganics are often (charged) salts remaining from pH control in the fermenter and
feedstock contamination. The pretreatment process and subsequent steps typically yield
even higher concentrations. In general they lead to scaling and corrosion in equipment. DSP
technologies such as ion exhange that bind these charged species, are generally favoured.
But membranes, and crystallization /precipitation can work as well.
Water is one of the most abundant molecules. Reducing water significantly reduces the size
of DSP equipment. Although many techniques can be used, a common choice is evaporation
if water is the most volatile species.
The last step in this example is the removal of organic molecules that are unconverted
feedstock molecules such as sugars and metabolic byproducts such as organic acids. Since
these small molecules are comparable to PDO, removing them requires more effort. We
have to employ sometimes rather small differences in polarity, solubility or boiling point. In
the case of PDO, separation based on boiling point is a common choice and a cascade of
distillation columns is frequently described. In that case, the boiling point of PDO is
intermediate to that of the remaining water, and the organic molecules.
Bringing all these initial choices A E together gives a conceptual process such as described in
a patent by one of the leading PDO manufacturers. Summarizing: it consists of the following
subsequent steps membranes for [A] cell removal (microfiltration) and step[B] protein
removal. This is followed by [C] ion exchange  a charge based sorption technology to
remove inorganics, [D] volume reduction by water removal and [E] finally polishing of PDO
by a cascade of distillation steps. The latter two steps are fairly energy intensive, and
therefore energy integration is an important target.
But of course also other choices can be made, as is described in another (US) patent. Here
the removal of the  organics and  inorganics is integrated into a specific selective sorption
technology by the name of ion exclusion. Ion exclusion is more effective at lower
concentrations. So, combining the removal of organic and inorganic contaminants in a single
step (potential CAPEX reduction) maybe offset by higher water removal (and energy) costs.
The results cannot be argued by qualitative considerations only. Therefore we need
systematic, quantitative methods to evaluate economic and ecological impacts. A good
systematic design method is staged and iterative, and will limit unnecessarily detailed
calculations and even more costly experimental work. Therefore it will zoom in on process
alternatives from rough to fine: from (block) structure and streams to detailed descriptions
of equipment and their operations and control.
This is one of these systematic iterative methodologies the Delft Design Matrix. It is too
detailed for this MOOC so come to Delft if you want to become a well trained process
engineer. In the next unit, we will compare both conceptual alternatives on quantitative
terms.
So, see you in the next unit!