Fixed Bed Reactor Quak Foo Lee Chemical and Biological Engineering The University of British Columbia

Fixed Bed Reactor Solids take part in reaction  unsteady state or semi-batch mode Over some time, solids either replaced or regenerated 1 2 CA,in CA,out Regeneration Breakthrough curve CA,out/CA,in t

Isothermal Reaction: Plug Flow Reactor Plug flow of fluid – no radial gradients, and no axial dispersion Constant density with position Superficial velocity remains constant

Plug Flow Model z + dz CA,f + dCA,f z CA,f U0 (m/s) superficial velocity

Input – Output – Reaction = Accumulation Mass Balance Input – Output – Reaction = Accumulation Divide by ∂z and take the limits as ∂z  0 ε is void fraction in bed

For first order reaction, fluid only: Void fraction For first order reaction, fluid only: For steady state: Volume of reactor Therefore,

Conversion as a function of Height Integrating with CA,f = CA,f,in at z = 0 Note 1: Same equation as for catalytic reactor with 1st order reaction Note 2: Can be used in pseudo-homogeneous reaction

Balance on Solid aA (fluid) + S (solid)  Products Input – Output – Reaction = Accumulation Over increment of dz: input = 0, output =0 Volume fraction of solid = m3 of solid m3 of reactor volume mol m3 of solid · s

Balance on Solid

Solve These Equations = 0 (In quasi steady state, we ignore the accumulation of A in gas) Substitute rAv

Non-Isothermal Packed Bed Reactor For mass continuity  did balance on fluid and on solid For energy balance, we do balance on each phase

Non-Isothermal Packed Bed Reactor Assumptions: Adiabatic reaction – no heat lost through shell to surroundings (no radial temperature gradients) q = 0 Biλ is small – uniform T within particle (an exothermic reaction Tp > Tg) Plug flow of gas and use Tref =0 for enthalpy calculations Assume an average density can be used (ρg = constant)

Modeling q =0 Tf + dTf z + dz Tf z Tf,0 U0

Governing Equations Fluid Solid Solve all these equations together. Quasi Steady State

For Heat Transfer Z=0 all t Tf = Tf,0; Ts = Ts,0 T = 0 all z Ts = Ts,0; Tf = Tf,0 Generally, require a numerical solution for the solution of the set of equations.