(438d) Novel Jet-Loop Reactor for Kinetic Measurements of Gas-Solid Heterogeneous Catalyzed Reactions Involving Commercial-Scale Particles

Introduction.  Development
of new catalytic materials for the selective conversion of next-generation
feedstocks to clean fuels and chemicals, or improvement of catalyst
compositions for existing processes involving current generation feedstocks,
requires reliable kinetic models for design of new reactor configurations or
for analysis of transport-kinetic interactions in existing reactor systems
(Berty, 1999; Davis and Davis, 2012; Doraiswamy and Uner, 2013; Fogler, 2005;
2010; Froment and Bischoff, 1979; 1990; Roberts, 2008; Salmi et al.,
2010; Schmidt, 2004).  Procurement of accurate kinetic data for kinetic model
discrimination by combining statistical sequential experimental designs with
kinetic parameter estimation techniques (Schwaab et al., 2006; 2008) requires
a reactor system that can generate the required kinetic equation response
variables, such as reaction rates, specie concentrations, and temperatures for
a given sequence of user-defined input variables.    

Experimental reactors
for measurement of reaction kinetics can be broadly classified according to the
catalyst forms (powder, granule, or pellet) and the numbers of phases present
(gas, gas-liquid, gas-liquid-liquid).  A good introductory overview of various reactor
designs for both gas-solid catalyzed and gas-liquid-solid catalyzed reactions is
provided in standard texts on reaction engineering (e.g., Froment and
Bischoff, 1979, Sections 2.3 and 6.5; Levenspiel, 2002, Chapter 21, Section
IV), dedicated monographs (Berty, 1999; Chapter 2, Section 2), and review
chapters (Pratt, 1987, Chapter 4).  It is generally agreed that for the purpose
of generating kinetic data, laboratory-scale reactors that operate isothermally
and allow direct evaluation of reactant conversions, product yields, and reaction
rates from experimental measurements of flow rates and specie concentrations, such
as that provided by those where the fluid flow patterns approach perfect
back-mixing, are preferred for various practical reasons over systems where the
flow patterns approach perfect plug flow (Berty, 1984; 1999).  Among various
designs with nearly perfect fluid back-mixing, recycle reactors are preferred
as important tools for catalyst testing and kinetic studies.  They can often be
operated at the operating conditions of the commercial-scale reactor, and kinetic
rate equations developed from the data they produce can often be used directly
for predicting the performance of commercial-scale units. Recycle reactors are also
useful for the study of most commercially important reactions, although some
exceptions occur, especially for cases where the catalyst activity changes
rapidly when compared to typical fluid residence times.

Some examples of various recycle reactor designs that
have appeared in the open literature are shown in Figure 1.  Types 1, 2, 4, and
5 are internal recycled reactors, while Type 3 is an external recycle
reactor.  They are distinguished from each other by the location of the
circulating reaction fluid relative to the fixed-bed of catalyst.  The various
internal recycle reactor designs shown here depend upon a mechanical agitator
to create either an axial or radial flow internal circulation of gas. 
Conversely, the external recycle reactor design uses a compressor to create the
required flow rate of circulated gas.  One exception here is the so-called propulsive
jet internal recycle reactor shown as Type 5, which relies upon a high-speed
turbulent jet through an internal draft tube to create the required gas
circulation.  This latter design does not contain any moving parts, which
reduces the complexity and cost when compared to the other designs.  This
absence of any moving parts is particularly attractive when the reaction
kinetics must be evaluated at high temperatures or when using corrosive
reaction mixtures since issues associated with bearings and components that
would be subject to failure are eliminated.

Objectives.  The primary objective
of this work is to describe the development, application and modeling of a new
experimental reactor for studying the kinetics of gas-solid catalyzed reactions
based upon the propulsive jet concept shown above in Figure 1 as Type 5.  The
propulsive jet concept is more attractive since this approach does not involve
any moving parts and is simpler to fabricate.  The lack of any moving parts reduces
the maintenance requirements, minimizes fabrication costs, and allows for
easier set-up and operation.  The test reaction used is the oxidation of SO₂
to SO₃ over V₂O₅-based catalysts since
increasing emphasis is being placed on development of new catalysts having
higher activity at lower temperatures with resulting lower SO₂
emissions for more environmentally-friendly processes.  This reaction also
represents a challenging test case since previous experience with a
conventional Berty-type of reactor resulted in significant issues associated
with corrosion of the catalyst basket, excessive bearing wear, and operational interruptions
associated with maintenance of the Magne-Drive.     

Results and Discussion.  A photograph of the
new reactor design is shown in Figure 2.  The one shown here is constructed of
Type 316 stainless steel, although other versions have been constructed from
Hastelloy C and other advanced metal alloys.  The catalyst chamber is sealed on
both ends by SwagelokÔ VCR fittings
with metal gaskets for ease of installation. It can accommodate several
commercial size catalysts whose largest dimension is ca. 16 mm. The reaction
gases are mixed upstream and introduced at a steady flow rate from a gas
manifold and removed through the outlet whose back-pressure is maintained
constant by a back-pressure regulator.  Analysis of SO₂, O₂
and N₂ in the feed and product gases is performed with an on-line
GC.  A pitot tube located on the return leg is used to measure the internal gas
recirculation rate. A large number of experiments were conducted in which the total
gas flow rate, gas temperature, reactor total pressure, catalyst particle size,
nozzle size, and nozzle location were systematically varied to assess the
effect of these variables on the gas recycle rate.  The key conclusions from
these studies are: (1) nozzle velocity must be maintained above a certain
threshold value to generate adequate gas recycle, which was also confirmed by the
fluid mechanical calculations; (2) a nozzle I.D. of 0.015-in generates recycle
rates between 20 to 45 when gas flow rates between 50 to 950 sccm are used and
the nozzle exit is located at an optimal location.

An example of typical kinetic data is
provided in Figure 3. These data are based upon temperatures between 400 to 475^oC 
using a gas feed consisting of ca. 1% SO₂, 7.5% O₂ and
82.5% (bal) N₂ at flow rates between 100 sccm to 900 sccm. The
resulting rate data were fitted to a rate model that accounts for reversible
reaction behavior. The agreement between the reaction rates versus %SO₂
conversion data is shown below in Figure 3a.  A parity plot that shows a
comparison between the experimental and model-predicted %SO₂
conversions at a selected temperature (425^oC) is shown in Figure 3b.
Good agreement is obtained which illustrates the validity of a simple but
effective reaction rate model.  These and other detailed results will be
presented that demonstrate the utility of the proposed jet loop reactor as a
relatively simple but effective reactor type for studying the kinetics of
gas-solid catalyzed reactions.

References

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