Chemical Engineering April 2012 - 65

RELATIONSHIP AMONG PROPERTIES
T
hose working in the chemical process industries (CPI) are
likely familiar with physical properties such as melting point,
boiling point, density, specific heat, density and others. However,
using physical properties to create great chemical processes
depends on their mutual relationships, and to the nuances of the
manufacturing process. Reaction chemistries produce new molecules,
and when different molecules are blended, formulated
products are produced. Some physical properties are common
If the process is continuous, maintaining
the reaction temperature at
the desired temperature can control
the reaction rate. Because the solvent
is replaced with a melt, the process productivity
improves considerably. Since
product " C " is a liquid at room temperature,
using a melt also raises process
productivity. Reduced solvent use also
lowers the solvent recovery load and
improves process sustainability.
To be viable, the alternate method
has some built-in caveats, including
that the raw material " A " has to
be available in melt form, and the
production volume has to be large
enough to warrant the melt material
handling investment.
There are numerous cases where
molten raw materials can be used
with very tight stoichiometric control.
Each situation has to be evaluated.
The resulting product can be purified
using different unit operations and
crystallized to produce products with
the desired quality. Such processes are
environmentally more sustainable,
productive and highly profitable compared
to solvent-based processes.
Use of phase separation
Case A: In some reactions, a byproduct,
such as water, must be removed.
If the solvent is insoluble in water, and
if their densities are sufficiently different,
then the solubility and density
differences can be used to separate the
two and the solvent can be recycled
back to the process.
In processes where water forms an
azeotrope with the solvent, water is
evaporated out with the solvent and
the mix is condensed. Use of chilled
water in the overhead condenser can
not only improve the condensation of
the azeotropic mix, but since the solvent
temperature will be lower compared
to that of condensation using
cooling-tower water, the reaction rate
can be improved. This will lower the
batch cycle time and improve profitability.
Case
B: Phase separation can be effectively
used to enhance process yield. In
to reaction chemistries and formulated products, while others are
not. How these properties are used in a process depends upon
our understanding of their value and relationship with other
properties. Just as each process has its own individual requirements,
each physical property is unique. The uniqueness of the
physical properties of one chemical relative to other chemicals in
the process offers engineers the opportunity to manipulate them
to create an optimal process.
❏
the following reaction, intermediates
" C " and " D " are produced. Intermediate
" C " is soluble in water and " D " is soluble
in an organic solvent. In the subsequent
reaction, these two intermediates
are reacted to produce product " E. "
However, intermediate " D " hydrolyzes
to produce an unwanted product if left
in contact with water, and process yield
is significantly lowered.
A + B -> C + D -> E
To preserve the yield, it is best to separate
the two phases and mix them just
before the production of " E " begins.
U.S. patent no. 7,078,524 discusses
a chemical reaction where two isomers
are produced. One is a desired pharmaceutical
product and the other cannot
be converted to a saleable product.
However, the overall process yield is
improved if the undesired isomer is
isolated and recycled after separation.
The process suggested in the patent
can be significantly simplified through
creative solvent selection. Were the described
process to be commercialized
as indicated in the patent, it would
be cumbersome and would extend the
batch cycle time significantly.
However, through proper solvent selection,
the " undesired " isomer can be
recycled in situ, the process could be
not only simplified, but process yield
would be improved. This is an example
of how human creativity, along with
the ability to finesse physical characteristics,
could generate a better process
- a possible " Eureka " moment.
For any reactive chemical process,
solvent selection is extremely important,
especially where phase separation
is involved. Greater difference in
densities facilitates and accelerates
phase separation. Recognize that solution
density increases as more solid is
dissolved in a solvent. If the densities
of the solutions to be separated approach
each other, separation can become
a challenge, and will take longer.
Thus it is important to choose solvents
such that there is a noticeable density
difference under the process conditions.
This difference can hasten the
separation, lowering the processing
time. From a manufacturing standpoint
reduced process time through a
faster separation is important.
Capitalizing on exotherm
Due to equipment capability limitations
during process development in
the laboratory, the exotherm is controlled
either by lowering the reaction
temperature with ice or chilled water,
or by slowing the raw-material addition
rate. When such processes are
scaled-up, the same practice continues.
These methods work, but they extend
the reaction time and reduce the
process productivity.
It is well known that raising the
reaction temperature speeds the reaction
rate and can improve yield. A
reaction exotherm can be very effectively
controlled by the efficient use of
heat exchangers. Commercial technologies
exist for doing this. What variables
can be manipulated (such as,
feed method, flow, flowrate, temperature
control, and so on) and how they
are controlled, will improve profits
through the development of a simpler
and more sustainable process.
Controlling exotherms effectively,
and as soon as they happen, using heat
exchangers can not only improve the
process yield, but makes the process
safer and prevents formation of color
bodies that can occur due to localized
heat generation. Inline heat exchangers
in a pump-around system represent
one of the methods. Other methods are
situation dependent.
Creating excellent processes
Engineers are routinely educated
about the physical and chemical properties
of common reactants, solvents
and products. However, they generally
are not taught how to creatively
exploit them to develop processes that
are simple, sustainable and economical.
Finding methodology to finesse
these properties into simplified processes
comes from a firm understanding
of the properties, and from previous
experience. Chemical properties
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Chemical Engineering April 2012

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