Chemical Engineering November 2013 - 46

1
Assumes process is being cooled from 86°F to 76°F
by heating glycol from 36°F to 47°F
Engineering Practice
0.1
Economy of scale ends
when the heat load
exchangers, the heat-transfer coefficients,
the effective heat-transfer
area, and the temperature driving
forces are all known.
Q = (U)(A)(LMTD)
Where:
U = Overall heat-transfer coefficient,
Btu/h/°F/ft2
A = Area, ft2
Each component is known. The above
analysis shows that, for heat transfer,
the design factors are already well
understood and predictable, and thus
present a low risk to the project.
Scale down. Scale down refers to the
need to carry out certain tests at small
scale to replicate the conditions that will
exist for the large-scale units. For example,
small-scale units do not need to
use very cold coolant because the ratio
of heat-transfer surface area to liquid
volume is relatively high at small scale.
Therefore, small-scale testing should
use greatly reduced heat-transfer area
- such as might be provided by a coldfinger-type
heat exchanger. Even better,
the pilot plant should include an
external heat exchanger to simulate
both the cold surfaces and other effects
of the side-stream heat-exchange
design, which is being pursued for full
scale. The smallest heat exchanger of
the type that will be used for the production
plant will determine the size of
the pilot plant.
For instance, in the current hypothetical
100,000-gal process, the smallest
plate-and-frame heat exchanger
providing similar temperatures and
liquid velocity would run at about 10
gal/min, 0.02% of full scale, suggesting
that a pilot plant with a 200-gal capacity
could support the design. Keep
in mind that mass and momentum
transfer considerations may require
other demonstration-plant sizes.
Other considerations
The desire to employ huge fermenters
should not be interpreted as the desire
to have just one production fermenter
serving the process. Having one production
fermenter may offer the minimum
cost at first, but it may make it
difficult to operate the plant. For instance,
with only one fermenter, process
problems are much more difficult
0.01
exceeds the capacity of
the biggest commercial
heat exchanger
0.001
1
10
100
1,000
Process flowrate, gal/min
to troubleshoot and it is impossible
to run a controlled experiment to
test revised media formulations or
process conditions.
If the application uses multiple
fermenters to produce the needed capacity,
then when a process problem
occurs, many potential causes can be
ruled out immediately, based on which
fermenter(s) was (were) affected. In a
single-fermenter production plant, all
variables must be considered for every
process problem.
The more fermenters you use, the
more insight you can gain into the
process. The 1-million-gal fermenter
in the current example is just one unit
in a multi-million-gal production facility.
The number of units ultimately
required would be dependent on the
fermenter cycle time, and the plant
production rate, but as a rule of thumb,
should not be less than about six.
In the case of a large fermenter, if
batch sterilization is required, the
sterilization of the media should be
performed prior to charging the fermenter.
The reason is that the time to
reach sterilization temperature of the
media and the subsequent cool down
can take a significant amount of time,
and designing for the cool down will
require a substantial amount of chilling
capacity or a large reservoir of
chilled water. Huge-scale fermenters
should be designed for process cooling
only, not for batch sterilization, pasteurization
or bio-inactivation.
The discussion provided above deliberately
ignores the issue of maintaining
temperature uniformity within
the tank, because that topic is more a
function of momentum transfer rather
than heat transfer. Momentum transfer
in huge-scale fermentation units
is another important issue, which is
behond the scope of this article. It is
interesting to note that the use of external
heat exchange removes tank
dimensions and shape as variables
affecting heat transfer, so that those
46 ChemiCal engineering www.Che.Com november 2013
10,000 100,000
FIGURE 2. As
shown, the economy
of scale for a plateand-frame
heat
exchanger ends
when the heat load
exceeds the capacity
of the biggest
commercial heat
exchanger
variables can be optimized independently
of heat transfer.
Cooling is a big expense. A process
that is micro-aerobic or anaerobic has
a clear economic advantage because
those processes generate much less
heat. A process that is warm enough
to be cooled with 85°F cooling tower
water would also be advantageous.
Closing thoughts
The use of external heat exchangers
has distinct operational advantages,
because fermenter heat-transfer capacity
becomes independent of tank
size, shape and material of construction.
From a heat-transfer point of
view, fermenter size can grow without
bound but the economy of scale ends
at about 24 million Btu/h. Pilot-scale
testing with a fermenter above 200gal
working volume would prove heattransfer
coefficients and validate that
the microbe can tolerate the conditions
brought by external heat exchange. n
Edited by Suzanne Shelley
Acknowledgements:
Thanks to Andrew Baird and Craig
Sandstrom, both of Fluor, for peer
review.
Authors
Jim Gregory is is a process
engineer at Fluor (jim.
gregory@fluor.com). He holds
B.A in biophysics and a B.Sc.
in chemical engineering from
the University of Connecticut,
and an M.Sc in biochemical
engineering from Rutgers
University. He has experience
in the design and operation of
industrial microbiological processes
ranging from human
cell line monoclonal antibodies to diesel fuel.
C.R. Green (Bob) is director
of design development at
Fluor (bob.green@fluor.com).
He holds a B.S. in mechanical
engineering from N.C. State
and an M.E. in mechanical
engineering from the University
of South Carolina. He
is a registered professional
engineer in six states. Green
has experience in the design
and startup of microbiological
processes including human cell line monoclonal
antibodies, amino acids, bacteria and biofuels
and biochemicals.
Cost, $ per Btu/h
http://www.Che.Com
Chemical Engineering November 2013

Table of Contents for the Digital Edition of Chemical Engineering November 2013
