Fixed-Bed Reactor for studying the Kinetics of Methane Oxidation on Supported Palladium Objectives:

1.1. The general goal is to understand: The general goal is to understand: a)a) the influence of the presence of catalysts on the mechanism and rate of chemical reactionthe influence of the presence of catalysts on the mechanism and rate of chemical reactionb)b) the catalytic materials, their activities and the chemical and physical nature of their the catalytic materials, their activities and the chemical and physical nature of their

surfaces.surfaces.

2.2. The specific goals are:The specific goals are:a)a) To understand the kinetics of the catalyzed oxidation of methane in a gradientless To understand the kinetics of the catalyzed oxidation of methane in a gradientless

(perfectly mixed) reactor by: (perfectly mixed) reactor by:

Determining the rate constantDetermining the rate constant Finding the activation energyFinding the activation energy Evaluating the order of the reaction through experimentsEvaluating the order of the reaction through experiments Thus obtaining the rate expressionThus obtaining the rate expression

b)b) To apply the chemical engineering concepts to compare and interpret the observed To apply the chemical engineering concepts to compare and interpret the observed performance of Fixed-Bed Reactor performance of Fixed-Bed Reactor with and withwith and with outout backmixing (PFTR & R-R). backmixing (PFTR & R-R).

Theoretical backgrounds

The 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)

CH4 2O2 Pd

CO2 2H2O

The oxidation of Methane is envisioned as a single step reaction

Tr is the reference temperature, taken to be 250C

r(C, T) k(T )Cn k0 e ERT [

Tr

T]n Cr

n (1)

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)

PFR PFR-R

Equations For a Gradientless Reactor (CSTR or PFRR)

qr Cor qrCr Vr

r(C, T) qr

V(Cor Cr )

qr

V can be

qr

m

CSTR PFRRWhere m is the weight of the catalyst

Gradienless reactor(Perfectly mixed) in which all physical rate limitations are neglected, and uniform temperature and constant pressure

r(C, T) qr

mCorX

Where X, the fractional conversion of the reactant, is given by

(3)

(4)

(5)

(6)

X 1Cr

Cor

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

X(1 X )n

mqr

Corn 1(Tr

T)n k0e

E

RT (7)

An Ideal Plug Flow Reactor

This model can be described by the following differential equation for the methane material balancedCd

mqr

Tr

Tk(T)Cn

where is the axial position, made dimensionless by the reactor length. The initial condition for Eq 8. is the concentration at = 0.

C(o) CorTr

TFor 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 = 1) from Eq. (6) leads to the following equation for the reactor conversion.

(8)

(9)

X 1 [1nmqr

( 1Cor

Tr

T)n k(T)]

1n

(6)

(10)

Co

C

Co C

X 1Cr

Cor

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

X 1 [1m

2qr

( 1Cor

Tr

T)

12 k(T)]2

11

FlowController

GasChromatograp

h

Reactor

Pre-heater

Integrator

TemperatureController

He

HeCH4

O2

R.P.

T T T

Vent

EXPERIMENT SETUP

The Experimental Setup

Key words 1 = rotameters 2 = bubble flowmeter3 = control valves 4 = 4-way valve 5 = mass flowmeters6 = temperature indicators controls 7 = thermocouples8 = recycle pump 9 = preheating zone 10 = furnace 11 = catalyst section12 = moisture trop13 = gas chromatograph

Experiment Procedure

First session

Run the reactor with zero recycle (PFR)

Second session

Run the reactor with recycle flow above 7 liters/min

Third session

Run 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 balance

Total flow to the reactor is 100ml/min

AFresh Catalyst (high dispersion; high surface area)

Pd Sites

Al2O3

Al2O3 -Al2O3

Cintered PdPore cintering

BOld Catalyst

Low dispersion (low activity)

COld catalyst

Low surface area (low activity)

CATALYST DEACTIVATION DIAGRAM

Activation energy without catalyst

Activation energy with catalyst

Product

Reactant

Progress of the reaction

How to analyze our data1. Apply 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.

x

T

R-R

PFTR

x

Cor

1<n<1

R-R

2. To evaluate the reaction order (use the experimental data of R-PFR):A) 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.

B) 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.

C) 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).

Cr Tr/T

R-R

n & k (T)

T1 T2

T5r

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.

4. 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.

T

R-Rx(

Cor/1

-x)1/

2

If n=1/2

5. To test the consequences of varying the initial concentrations of methane on the reaction rate, plot r versus T for various Cor.

r

T

R-R

1/T

Slope = -E/RIntercept=ko

R-R

Ln k

6. Use Eq. (11) to calculate conversion x for PFTR.

X 1 [1m

2qr

( 1Cor

Tr

T)

12 k(T)]2

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

x

T

R-R

PFTR

x

T

R-R

PFTRx

Cor

1<n<1

R-R

CrTr/T

R-R

n & k (T)

T1 T2

T5

r

1/T

Slope = -E/RIntercept=ko

R-R

Ln k

T

R-R

x(C

or/1

-x)1/

2If n=1/2

r

T

R-R

x

T

R-R

PFTR