Hydrocarbon Processing - September 2021 - 62

Catalysts
similar to FIG. 3. Finally, experiments are
carried out at sufficiently high fluid velocity
so that the last term of Eq. 14 may
be ignored. By varying reaction temperature
T, a plot similar to FIG. 3 can be used
to obtain the value of κ from the slope
and the earlier obtained value of EA,true
. In
this way, the handy correlation (Eq. 14)
for the non-isothermal reactor can be determined.
An example is given in Eq. 15.
Case study. Consider the case of a
gas-phase, fixed-bed reactor involving
the production of olefins from an
isomerization reaction using a typically
porous catalyst. The reaction is mildly
exothermic and the proprietary catalyst
was tested extensively in three different
scales (laboratory, pilot and demonstration
units) using the above-mentioned
variety of experimental conditions. The
obtained correlation is shown in Eq. 15:
1
=
kobs
exp
0.0234
⎡
⎢⎣
−165,250
2RT
⎤
⎥⎦
+
1.192 × 107
Rep
0.5
(15)
TABLE 2 compares the experimental
and predicted % conversion for the four
scales of the fixed-bed reactor. Obviously,
the correlation was reasonably accurate
for the laboratory, pilot and demonstration
reactors. Though the actual conversion
data for the commercial unit is
unavailable, the predicted conversion is
expected to be quite satisfactory.
Takeaway. The unsuspected shift of the
rate-controlling step is a common pitfall
during comprehensive new process/
product development, beginning from
catalyst development and testing at laboratory
and pilot scales to the up-scaled
design and adiabatic operation of the
commercial unit. The most effective approach
is to " begin with the end in mind. "
More specifically, a viable commercial
unit should be conceptually designed as
soon as basic catalyst performance data
in laboratory scale has been obtained.
To make the commercial unit viable
(technically and economically feasible),
the necessary and essential targets of catalyst
performance and improvement can
be reasonably determined. These scientific
targets are valuable preliminary requirements
for the catalyst developer to satisfy
before more extensive catalyst testing is
carried out in the laboratory and subsequent
pilot scales. To enhance the successful
design and operation of the commercial
reactor with mass throughput by
operating in adiabatic (non-isothermal)
mode and turbulent flow regime, the
most scientifically sound approach is to
develop
a handy practical
correlation
similar to Eq. 14, which can handle exothermic/endothermic
reactions taking
place using catalyst of optimal pellet size
and shape. Of course, the crucial issue of
catalyst stability and deactivation must
also be addressed adequately and the requirements
incorporated into the above
catalyst performance targets. Note: Although
the present contents are based on
the assumption of irreversible first-order
reaction, the same or at least a similar
methodology is applicable to more complex
reaction types and pathways.
LITERATURE CITED
1
2
Levenspiel, O., Chemical reaction engineering, 3rd Ed.,
John Wiley & Sons, New York, New York, 1998.
Worstell, J., Adiabatic fixed-bed reactors practical
guides in chemical engineering, 1st Ed., Elsevier, 2014.
TABLE 2. Comparison of actual and predicted catalyst performance from reactor model
in four different scales of reactors
Parameters
Diameter, mm
L/dp
Mass throughput range
Catalyst particle, mm
Feed type
Operation
Catalyst used
Flow behavior
% conversion:
expected/predicted
Laboratory
15
59
g/hr
~2
Real feed
Isothermal
g-scale
Laminar
79/72
Pilot plant
21
266
g/hr
~2
Real feed
Isothermal
g-scale
Transition
77/78
62 SEPTEMBER 2021 | HydrocarbonProcessing.com
Demonstration
66
1,087
kg/hr
~2
Real feed
Isothermal
kg-scale
Turbulent
72/71
Commercial
3,000
2,048
metric t/hr
~2
Real feed
Adiabatic
ton-scale
Turbulent
.../74
3
Fogler, H. S., Element of chemical reaction engineering,
5th Ed., Prentice Hall, Upper Saddle River,
New Jersey, 2016.
NOMENCLATURE
ap
Aobs
Atrue
EA,obs
Cross-sectional area of the pore, m2
Observed or apparent pre-exponential factor, s-1
True pre-exponential factor, s-1
Observed or apparent activation energy, J/mol∙K
EA,true True activation energy, J/mol∙K
CAs
dp
De
kobs
kPD
kIntr
kFD
Catalyst particle diameter, m
Effective diffusivity, m3
/sec
Observed overall rate constant, s-1
Mass transfer rate constant within the
catalyst pore, m/sec
Intrinsic reaction rate constant, s-1
Mass transfer rate constant within at
fluid-solid film, m/sec
Κ Characteristic constant
n
η
Sc
Vc
R Gas constant
Rep
Characteristic constant/reaction order
Effectiveness factor, dimensionless
Packed-bed Reynolds number, dimensionless
Exterior surface area of the catalyst particle, m2
T Temperature, K
vp
Average pore volume, m3
Catalyst pellet volume, m3
ψ Characteristic constant.
BAMRUNG SUNGNOEN is a Process
Technology Engineer for Olefins
and Operations Technology for
SCG Chemicals Co. Ltd., Thailand.
He provides technical support for
scale-up from laboratory to pilot
and commercial scale involving
heterogeneous catalyst and adsorbent. He has worked
in process design, research and development sections.
WIWUT TANTHAPANICHAKOON
is a Technology Adviser, Olefins
and Operations Technology for
SCG Chemicals, and is an Emeritus
Professor at the Tokyo Institute
of Technology, Japan, and
Chulalongkorn University in
Thailand. He has 40 yr of teaching and research
experience in heat and mass transfer operations,
particle technology, aerosol engineering, and process
analysis and simulation. Dr. Tanthapanichakoon
was the founding Executive Director of the National
Nanotechnology Center, National Science and
Technology Development Agency, Thailand. He holds
a B.Eng degree in chemical engineering from
Kyoto University and a PhD in chemical engineering
from The University of Texas at Austin.
KHAVINET LOURVANIJ is a Chief
Process Technology for Olefins
and Operations Technology
Department for SCG Chemicals Co.
Ltd., Thailand. He is responsible
for leading new process technology
scaling up and application toward
the company's business interests. Dr. Lourvanij
has more than 20 yr of experience in the petroleum
and petrochemical industries, covering technologies
and operations, process development and execution,
and the deployment of advanced technologies.
He earned a BS degree in chemical engineering
from Chulalongkorn University, Thailand, and an
MS degree and PhD. in chemical engineering from
Oregon State University.
Concentration of A at the surface of catalyst,
mol/m3
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