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.

 

 

 

 

 

 

 

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