1 Monolithic Reactors for Environmental Catalysis 朱信 Hsin Chu Professor Dept. of Environmental Eng. National Cheng Kung University

2 1. Introduction  Minimize the pressure drop associated with high flow rates  Allow the process effluent gases to pass uniformly through the channels of the honeycomb

3 2. Chemical Kinetic Control  To be controlled by chemical kinetics rather than by diffusion to or within the catalyst pore structure while the geses are cold  When the surface becomes sufficiently hot, the rate will be determined by mass transfer.  In the laboratory, when screening a large number of catalyst candidates, it is important to measure activity at low conversion levels to ensure that the catalyst is evaluated in the intrinsic or chemical rate-controlling regime.  Good laboratory practice is to maintain all conversions below 20% for kinetic measurements. (adiabatic) For highly exothermic reactions (i.e., △ H > 50kcal/mol), measurements should be made at conversions no greater than 10%.

4  A material balance across any reactor gives the following equation assuming one-dimensional, plug flow, steady-state operation: wherev=velocity (cm/s) C=molar concentration [(g ‧ mol)/cm 3 ] z=length (cm) r=rate of reaction [(g ‧ mol)/(cm 3 ‧ s)]  When the conversion or the reactant concentration is low, the reactor is considerd isothermal; hence

5  Assume the oxidation of ethane to CO 2 and H 2 O in a large excess of O 2 in a fixed bed of catalyst: We can assume that the rate is independent of O 2.  It obeys first-order kinetics (pseudo-zero-order in O 2 ), so the rate is expressed as: where k’ = the apparent rate constant  Integrating from the reactor inlet (i) to outlet (o) gives: where t = actual residence time (s)

6  t =  Volumetric hourly space velocity (VHSV) VHSV =  The rate expression then becomes: By varying the space velocity, the change in conversion can be determined. The slop of the plot yields the k” of the reaction at STP.  Next slide (Fig. 4.1) Ethane conversion versus temperature at different space velocities. Next slide velocities.

8 3. Bulk Mass Transfer  When experiments are conducted with extremely active catalyst or at high temperatures, diffusional effects are introduced, and the intrinsic kinetics of the catalytic material is not determined accurately.  The activation energy will decrease as pore diffusion and bulk mass transfer become more significant.  Stationary environmental abatement processes are designed to operate in the bulk mass transfer regime where maximum conversion of the pollutant to the nontoxic product is desired.  Diffusion processes have small temperature dependency (low activation energies). Chemical-controlled reactions have a high degree of dependence on temperature (high activation energies).

9  Important benefit of diffusion processes: the physical size and other geometric parameters of the honeycomb for a required conversion can be obtained using fundamental parameters of mass transfer. Wherek g =mass transfer coefficient (cm/s) a=geometric surface area per unit volume (cm 2 /cm 3 ) C=reactant gas phase concentration [(g ‧ mol)/cm 3 ]  Integrating, Fractional conversion = 1- exp[-(k g at)]

10  Some dimensionless numbers where D=diffusivity of pollutant in air (cm 2 /s) W=total mass flowrate to honeycomb catalyst (g/s) A=frontal area of honeycomb (cm 2 ) d ch =hydraulic diameter of honeycomb channel (cm) ρ=gas density at operating conditions (g/cm 3 ) μ=gas viscosity at operating conditions (g/s ‧ cm) ε=void fraction of honeycomb, dimensionless  Equation on last slide becomes: Fractional conversion = 1- exp where L = honeycomb length (cm)

11  Next slide (Fig. 4.2) Correlations for N sh, N sc, and N Re Next slide

13  Example 1 Calculation for Mass Transfer Conversion The removal of propane (C 3 H 8 ) in a stream air at 300 ℃ and atmospheric pressure with: Flow rate, W= 1000 lb/h (126g/s) Diameter of monolith, D=6 in. (15.24 cm) Length of monolith, L=6 in.(15.24 cm) Area of monolith, A=182.4 cm 2 Monolith geometry, 100 cpsi (15.5 cells/cm 2 ) C 3 H 8 feed fraction, X=1000 vppm (volume parts per million)  Sol: From the literature (Lachman and McNally, 1985) d ch = in. (0.21cm) ε=0.69 a = 33 in. 2 /in. 3 (13 cm 2 /cm 3 )

14  Using Hodgman’s (1960) Handbook of Chemistry and Physics The density (ρ) and viscosity (μ) of air : ρ at 300 ℃ = 6.16 × g/cm 3 μ at 300 ℃ = 297 × g/s ‧ cm Therefore,  To utilize Fig. 4.2, the following term must be determined:

15  From Bird et al., 1960, the diffusivity for a binary system: whereM=molecular weight of species, A=air; B=C 3 H 8 [g/(g ‧ mol] P=total pressure (atm) σ AB =collision diameter for binary system (Å) T=absolute operating temperature (K) =collision integral for binary system, dimensionless  Using Table B-1 from Bird et al., 1960: For air: M A = 28.97, σ A =3.617Å, For C 3 H 8 : M B =44.09, σ B =5.061Å, where σ and are Lennard-Jones parameters for the single components.

16  The binary system:  Using this value and Table B-2 from Bird (1960),  Therefore,

17  Using Figure 4.2, N Re d ch /L=9.75→N sh /N sc 0.56 =3.8 Therefore, N sh =4.4  Fractional conversion= = =0.736 = 73.6% (done)

18 4. Reactor Bed Pressure Drop  Pressure drop ( △ P) a. flow contracts within the restrictive channel diameter b. washcoat on the surface of the honeycomb channel creates friction  The basic equation for △ P derived from the energy balance: whereP=total pressure (atm) f=friction factor, dimensionless g c =gravitational constant ( cm/s 2 ) υ=velocity in channel at operating conditions (cm/s) ρ=gas density at operating conditions (g/cm 3 )  Next slide (Fig. 4.3) Friction factor correlation to N Re Next slide

20  The velocity in the channel (υ) whereε=void fraction (percent open frontal area of the honeycomb) A=cross-sectional area of honeycomb  Simplify the basic equation for △ P  Next slide (Fig. 4.4) △ P versus flow rate To select the optimum honeycomb geometry (volume, cross- sectional area, length, cpsi, etc.) for a given application Next slide