(583u) Analysis of Transport-Kinetics Interactions in Complex Commercial Catalyst Shapes for SO2 Oxidation Using Different Flux Models

Introduction. 
Development of next-generation chemical processes that have zero emissions is a
key environmental objective for sustainable development. The manufacture of H₂SO₄
by the air oxidation of SO₂ to SO₃ is an important
technology where an opportunity exists for new catalyst development and process
innovation by reducing emissions of unconverted SO₂ in process
reactor tail gases owing to the sheer number (> 1500) and scale (ca. 500 to
4500 metric tons/day) of typical plants. The global supply
of H₂SO₄ is projected to grow from 200 MM tonnes in 2006
to more than 258 MM tonnes in 2015 with a value of > $10 MMM
(British Sulphur Consultants, 2012). Hence, an opportunity exists to develop
new innovations in environmental catalysis and reaction
engineering for an important technology that has a
rich and long history with solid projected growth.

Sulfuric
acid catalyst manufacturers offer a variety of catalyst shapes for the air
oxidation of SO₂ to SO₃ using adiabatic multi-stage
reactor systems. These shapes include solid and hollow cylinders as well as
various those involving hollow multi-lobe ribs, such as 5-lobe and 6-lobed
shapes.  However, the a priori technological bases for specifying a
specific catalyst shape over another are generally lacking other than the
obvious macroscopic requirements to maximize catalyst activity and life while
minimizing pressure drop.  From a catalyst design perspective, specification of
catalyst size, catalyst shape and internal pore structure are interlinked
parameters that have received notable attention by catalyst scientists and
engineers since these properties affect the catalyst effectiveness factor, bed
pressure drop, rate of catalyst attrition, catalyst mechanical integrity,
catalyst life, amongst other key measures of performance.  It is evident that
determination of preferred catalyst design parameters requires careful
optimization since tradeoffs exist in how the observed reaction rate and other
measures of performance may be affected by altering one or both of these
parameters. 

The
primary objective of this presentation is twofold: (1) to develop a rigorous
modeling framework that accounts for non-ideal diffusion and non-isothermal
reaction in various realistic 3-D commercial catalyst shapes for the oxidation
of SO₂ to SO₃, such as hollow cylinders and multi-lobe
particles; Several different flux models, such as the Wilke model and Dusty Gas
model are deployed using COMSOL Multiphysics as the modeling tool; and (2) to employ
this framework to compare the performance of these various catalyst shapes
under typical multi-pass convertor operation.

Methods.
Some catalyst shapes that have been proposed for
SO₂ oxidation to SO₃ are illustrated in
Figure 1. The ones shown here are not exhaustive, but provide a perspective of
shapes that have been proposed in patents and the open literature.  In the case
of catalyst shapes involving multiple lobes, the shape parameters include the
number and shape of the lobes, the characteristic dimension of a given lobe,
the distance from the lobe center to true particle center, and the diameter of
the hollow region.  The lobes for these shapes are based upon either circle
functions, trigonometric functions, or a triangle, although this is not a
restriction and other shape functions will be discussed. 

Figure
1.  Illustration of Various Catalyst Shapes.

The above catalyst shapes
and others not shown for brevity were constructed using either the drawing
tools provided in COMSOL Multiphysics, or imported as a .dwg file
created using a special-purpose program that generates the exterior particle
coordinates that can be imported in AutoCAD.  Both the 2-D and 3-D shapes are
provided to show that the analysis requires a consideration of all three
spatial dimensions. 

Preliminary Results and
Discussion.  COMSOL Multiphysics is used to model
non-isothermal diffusion and reaction with compositional and temperature
dependence of all model parameters.  The reaction kinetic model is based upon
the work of Collina et al. (1971).  Both the Wilke model and Dusty Gas
model are used to describe diffusional fluxes, and comparisons are made between
the concentration and temperature profiles as well as calculated values for
particle effectiveness factor.

Typical SO₂
concentration profiles for a multi-lobe particle shape whose lobes are
described by a rounded step function are shown in Figure 2.  In this case, the
Wilke model is used to describe the diffusional fluxes. Here, the particle
surface is exposed to a constant bulk concentration and temperature. A cross
section of the temperature profiles inside the particle at the particle
mid-section is shown in Figure 3.  The particle model provides the starting
basis for coupling local particle behavior to the external particle field
equations, which is an essential component of SO₂ oxidation reactor
modeling.

Figure 2.  SO₂ Concentration Profiles for
a Novel Shape.

Figure 3.  Catalyst
Temperature Profiles at the Mid-Section.

Summary.  Identification
of optimal catalyst shapes can be approached on various degrees of complexity. 
COMSOL Multiphysics provides a platform where emphasis can be placed on
exploring models versus creating computer codes.  Detailed knowledge of
transport principles and insight into the numerical methods used is required to
obtain results.

References

British
Sulphur Consultants, Sulphuric Acid : Global Supply and Demand in the Next
Decade, Topsøe Catalysis Forum ? Denmark, August 23^rd to 24th 2007, http://www.topsoe.com/sitecore/shell/Applications/~/media/PDF%20files/Topsoe_Catalysis_Forum/2007/Peacock.ashx,
Last accessed on Nov 12, 2012.

Collina,
A., Corbetta. D. and Cappelli, A. Eur. Syrnp. "Use of Computers in the
Design of Chemical Plants," Firenze (1971).