Mixing Considerations In Chemical Reactor Scale-Up - COMSOL
Mixing Considerations in Chemical ReactorScale-UpAndrew Fiore, Ph.D.Andrew Spann, Ph.D.Nagi Elabbasi, Ph.D.contact@veryst.comwww.veryst.com10/11/2019
Scale-up In Chemical Reactors Scaling chemical reactors from lab scale to pilot scale to production scalerequires a detailed understanding of the physical system Coupled heat transfer, mass transfer, reaction kinetics, fluid flow Chemical reactor scale-up considerations Geometric similarity Ratio of surface area to volume Energy input, generation, and removal rates Rate-limiting transport processes Mixing efficiency10/11/20192
Mixing in Chemical Reactions Two ideal reactor models are often used to illustrate the importance ofmixing on reaction yield and selectivityContinuously Stirred Tank Reactor(CSTR) โ complete mixing (uniformconcentration everywhere)Plug Flow Reactor (PFR) โzero axial mixing (spatiallyvarying concentration)๐ถ ๐ถ0๐ถ ๐ถ ๐ง More complicated mixing models can be developed from combinations ofthese simple models10/11/20193
Mixing in Chemical Reactions The Van de Vusse reaction system demonstrates the reactor designtradeoffs inherent in the simple PFR and CSTR models๐2 ๐2 /๐1101100๐1 ๐2๐1๐2๐ถแ ๐ด ี ๐ต ๐ี3๐ด ๐ดี๐ทIII๐ต is the desired product๐ถ and ๐ท are undesired byproducts10 1IIRegion Highest Yieldof ๐ฉI10 2 110100101๐1 ๐3 ๐ด0 REitherVan de Vusse, Chem. Eng. Sci., 196410/11/20194
Non-Ideal Mixing โ Turbulence Ideal mixing models (PFR, CSTR) may notbe valid at larger scales, and are unlikely tobe useful approximations for complicatedreactor schemes Non-ideal mixing is controlled by fluidmechanics within the reactor and is oftenquantified using a residence timedistribution Dead zones Short-circuitsAdapted from Figure 13.3(Fogler, 2010) Recirculation regions Turbulence changes the flow pattern withinthe reactor Turbulence can affect mixing withoutsignificantly modifying the residence timedistribution10/11/20195
How Turbulence Affects Mixing Turbulence increases mixing through eddies and vortices โ the chaoticmotion in turbulent flows causes dissolved species to effectively diffuse farmore quickly than by molecular diffusion alone Example: Reaction in a shear flow (Breidenthal, J. Fluid Mech., 1981) Fast fluid is light grey; slow fluid is medium grey; reaction product isdark grey๐ ๐ 200๐ ๐ 1600010/11/20196
How Turbulence Affects Mixing Turbulence increases mixing through eddies and vortices โ the chaoticmotion in turbulent flows causes dissolved species to effectively diffuse farmore quickly than by molecular diffusion alone Example: Reaction in a shear flow (Koochesfahani and Dimotakis, J. FluidMech., 1986) Fast fluid is dark blue; slow fluid is red; intermediate colors indicatereactant products๐ ๐ 175010/11/2019๐ ๐ 230007
A Model Reactor Setup for Mixing Studies To model the effect of turbulent mixing onthe chemical reaction, we use a multi-inlettubular reactor, shown at right (top) Different reactants enter the reactorthrough alternating inlets, indicated bythe surfaces highlighted in blue The reactor has two planes of symmetry,so we model only one-quarter of thereactor, shown at right (bottom)10/11/20198
Yield in Bimolecular Reactions๐1 We start by considering a simple bimolecular reaction: ๐ด ๐ต ี ๐ถ The yield of species ๐ถ is shown below as a function of dimensionlessdistance along the reactorConcentration slice forspecies ๐ถ with ๐ ๐ 100Yield๐ถ๐ด0 ๐ต0Damkohler number, ๐ท๐ ๐1 ๐ด0 ๐ฟ/๐10/11/2019๐ ๐ 1.0 102๐ ๐ 5.4 103๐ ๐ 5.4 105๐ ๐ 5.4 107๐ท๐ ๐ง/๐ฟ9
Implications in Reactor Design and Scale-upThe residence time required to achieve 80% conversion of the productdecreases with increasing Reynolds number, and is much lower forturbulent flows than laminar ones, as highlighted in the table below๐ผ๐ณ๐1.0 102๐น๐ Yield 102๐ ๐ 1.0 ๐ ๐ 5.4 103๐ ๐ 5.4 105๐ ๐ 5.4 107๐ท๐ ๐ง/๐ฟ10/11/2019๐ซ๐ ๐๐ ๐จ๐๐ณ๐ผ230.25.4 103127.35.4 10590.65.4 10759.0In the table above, the Damkohlernumber, ๐ทa, provides the desiredreactor size10
Yield in Bimolecular Reactions with ProductDecomposition Suppose that the original bimolecular reaction is accompanied by decomposition of theproduct to an undesired byproduct๐1๐ด ๐ตี๐ถ๐2๐ถี๐ท ๐ ๐ 1.0 102๐ ๐ 5.4 103๐ ๐ 5.4 105๐ ๐ 5.4 107๐ท๐ ๐ง/๐ฟ10/11/2019SelectivityYieldThe yield (left) and selectivity (right) of species ๐ถ is shown below as a function ofdimensionless distance along the reactor๐ ๐ 1.0 102๐ ๐ 5.4 103๐ ๐ 5.4 105๐ ๐ 5.4 107๐ท๐ ๐ง/๐ฟ11
Implications in Reactor Design and Scale-up YieldThe reactor size for optimal yield at each Reynolds number is summarizedin the table below๐ ๐ 1.0 102๐ ๐ 5.4 103๐ ๐ 5.4 105๐ ๐ 5.4 107๐ท๐ ๐ง/๐ฟ10/11/2019๐ผ๐ณ๐น๐ ๐๐ซ๐OptimalYield1.0 102150.157%5.4 10323.569%5.4 10556.170%5.4 10759.373%๐ซ๐ ๐๐ ๐จ๐๐ณ๐ผ12
Summary Chemical reactor scale-up is acomplex problem involving detailedunderstanding of fundamentalphysics Simulations are a useful tool tounderstand how physics changewith scale Mixing and turbulence affect theyield of chemical reactions, even ingeometrically similar reactors Multiphysics simulations usingCOMSOL can be used to optimizereactor designs at scale๐ ๐ 1.0 102๐ ๐ 5.4 103๐ ๐ 5.4 105๐ ๐ 5.4 107๐ท๐ ๐ง/๐ฟ10/11/201913
Summary At Veryst, we combine insight into fundamental physics of chemical reactorswith computational models to help our clients solve reactor scale-upproblems, including Stirred tank reactors Packed bed reactors Flow reactors (laminar flow, plug flow) Microreactors10/11/201914
References Broadwell, J.E. and Breidenthal, R.E., 1981. Structure in turbulent mixinglayers and wakes using a chemical reaction. Journal of FluidMechanics, 109, pp.1-24. Koochesfahani, M.M. and Dimotakis, P.E., 1986. Mixing and chemicalreactions in a turbulent liquid mixing layer. Journal of Fluid Mechanics, 170,pp.83-112. Fogler, H.S., 2010. Essentials of Chemical Reaction Engineering. PearsonEducation. Van de Vusse, J.G., 1964. Plug-flow type reactor versus tankreactor. Chemical Engineering Science, 19(12), pp.994-996.10/11/201915
Scale-up In Chemical Reactors Scaling chemical reactors from lab scale to pilot scale to production scale requires a detailed understanding of the physical system Coupled heat transfer, mass transfer, reaction kinetics, fluid flow Chemical reactor scale-up considerations Geometric similarity Ratio of surface area to volume
mechanical mixing (rotating, vibrating) hydraulic mixing pneumatic mixing pipeline mixing (turbulent flow, static mixer) Method of mixing fluids A โmechanical mixing using turbines B โmechanical mixing using blade impellers C โhydraulic mixing D โpneumatic mixing with stationary inputs
2. TUBULAR REACTOR DESCRIPTIONS 3 3. GENERAL EVALUATION OF TUBULAR REACTORS 10 3.1 Design Basis and Requirements 10 3.2 General Evaluation 11 4. EVALUATION OF SPECIFIC REACTOR DESIGNS 19 4.1 Modec Constant Diameter Tubular Reactor 19 4.2 Dickinson Tubular Reactor Designs 22 4.3 Welch and Slegwarth Annular Reactor 23 4.4 Li and Gloyna Reactor 24
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