Fixed-Bed Reactor for studying the Kinetics of Methane Oxidation on Supported Palladium Objectives:The general goal is to understand: the influence of the presence of catalysts on the mechanism and rate of chemical reactionthe catalytic materials, their activities and the chemical and physical nature of their surfaces.The specific goals are:To understand the kinetics of the catalyzed oxidation of methane in a gradientless (perfectly mixed) reactor by: Determining the rate constantFinding the activation energyEvaluating the order of the reaction through experimentsThus obtaining the rate expressionb)To apply the chemical engineering concepts to compare and interpret the observed performance of Fixed-Bed Reactor with and with out backmixing (PFTR & R-R).

Theoretical backgroundsThe global rate of a solid catalyzed reaction is normally expressed as: Moles of reactant consumed per unit time per unit mass of catalyst (or per unit volume of the reactor containing the catalyst)The oxidation of Methane is envisioned as a single step reactionTr is the reference temperature, taken to be 250C

How to measure the global reaction rate [r(C,T)]?We need to start from species mass balance equation for particular reactor.accumulation = flow in - flow out +generation by reaction(2)PFRPFR-R

Equations For a Gradientless Reactor (CSTR or PFRR)CSTRPFRRWhere m is the weight of the catalystGradienless reactor(Perfectly mixed) in which all physical rate limitations are neglected, and uniform temperature and constant pressureWhere X, the fractional conversion of the reactant, is given by(3)(4)(5)(6)PFR-R

Substitution from eq (1) for r(C,T) and from eq (5) for Cr casts eq (4) in the following form, which relates the reactant (Methane) conversion to rate constant and operating conditions(7)

An Ideal Plug Flow ReactorThis model can be described by the following differential equation for the methane material balancewhere x is the axial position, made dimensionless by the reactor length. The initial condition for Eq 8. is the concentration at x = 0.

For the constant temperature case, Eq. (8) is readily integrated to give the concentration as a function of position. Application of Eq. (9) and substitution of the fractional conversion x (now the conversion at x = 1) from Eq. (6) leads to the following equation for the reactor conversion. (8) (9) (6) (10)xCoC

If the reaction order (n) is 1/2then the fractional conversion of Methane will be in the following form11

The Experimental Setup

Experiment ProcedureFirst session Run the reactor with zero recycle (PFR)Second sessionRun the reactor with recycle flow above 7 liters/minThird sessionRun the reactor as in the second session with different Methane concentration.P.S. Keep same feed composition in the first and second sessions

Feed gases are: Oxygen Flows at rate of(30ml/min),Mixture of 2% (by volume) Methane in Helium Flows at different rates (30,25, 20, or 16 ml/min)Helium for balanceTotal flow to the reactor is 100ml/min

Activation energy without catalystActivation energy with catalystProductReactantProgress of the reaction

How to analyze our dataApply the data on Eq. (6) to calculate experimental conversion (x) for PFR and R-PFR. Plot x vs T (reaction temperatures) for both reactors for same initial concentration of CH4. 2.To evaluate the reaction order (use the experimental data of R-PFR):Construct curves by plotting x versus Cor (initial concentration of CH4) for varying temperatures, see if the slopes of these lines show negative, zero or positive values.Use Eq. (5) [r(C,T)=qr/mCorx] to calculate r(C,T) (reaction rate) as a function of reaction temperature for all initial concentrations of methane.

Plot rate data versus C (i.e., CrTr/T) for varying temperatures on logarithmic coordinates. According to Eq. (1) the straight lines should have slope equal to the reaction order n and the intercept equal to k(T).

3. Use k(T) values and corresponding temperatures to create an Arrhenius plot. The ko and E/R (activation energy) values can be find from the intercept and the slope of the straight line.To test the kinetic model, Eq. (7) and, n=1/2, ko and E/R values can be used to calculate the conversion x as a function of the reactor temperature. Plot x(Cor/1-x)1/2 vs T of observed and predicted . See if all data points fall on same line, if so then the model is correct.5.To test the consequences of varying the initial concentrations of methane on the reaction rate, plot r versus T for various Cor.

7.Plot experimental and calculated conversion values versus T for both reactors (P.S. for same initial feed concentration).