CRE Catalytic Reactor chemical reaction engineering

2 CSTR + PFR in series

Catalysis and catalyst

A Catalyst is a substance that affects the rate of chemical

reaction but emerges from the process unchanged.

Catalysis is the occurrence, study, and use of catalysts and catalytic

processes.

Different reaction paths

Activation energy

Activation energy

Objective: to increase yield and selectivity

Catalysis

Catalyst

Catalytic packed-bed reactor, schematic.

Definition:Affects the rate of reaction but remains unchanged after the process

1. Homogeneous

2. Heterogeneous

Types of catalyst

Porous-

Molecular sieves-material with very small holes of precise and uniform size-silica

Monolithic- Catalytic converter with a catalyst-coated, ceramic honeycomb monolith (monolith is a large block of stone) through which the exhaust gases pass

Supported carbon alumina silica- to maximize the area by distributing the catalyst over support

Unsupported-

Porous

Monolithic

Supported

Types of catalytic reaction

1. Alkylation and dealkylation reaction

2. Isomerization reaction

3. Hydrogenation and dehydrogenation reaction

4. Oxidation reaction

5. Hydration and dehydration reaction

6. Halogenation and dehalogenation reaction

Normal Pentane Octane Number = 62

Iso-Pentane Octane Number = 95

Ex 1: Catalytic reformer

Reforms lighter hydrocarbon tohigher octane value and hydrogen

Ex 2: Catalytic conversion of exhaust gas

CO NOCO2 1

2N2

Why catalytic reactions are different?

A----------------P

A---------Pc

Homogeneous

Heterogeneous

Steps in catalytic reaction

1. Mass transfer (diffusion) of reactant A from the bulk fluid to the external surface of the catalyst pellet

2. Diffusion of the reactant from the pore mouth through the catalyst pores to the immediate vicinity of the internal catalytic surface

3. Adsorption of reactant A onto the catalyst surface

4. Reaction on the surface of the catalyst (A---- B)

5. Desorption of product P from the catalyst surface

6. Diffusion of the product from the interior of the pellet to pore mouth at the external surface

4. Mass transfer of the products from the external pellet surface to the bulk fluid

Steps in catalytic reaction

Properties of catalyst

Active site: Reactions are not catalyzed over the entire surface but only at certain active sites or centers that result from unsaturated atoms in the surface.

An active site is a point on the surface that can form strong chemical bonds with an adsorbed atom or molecule.

Turnover frequency: Number of molecules reacting per active site per second at the condition of experiment

Catalyst

Active site S

Vacant and occupied sites

For the system shown, the total concentration of sites is

In general, Ct = Cv + CA.S + CB.S

Active site=S

Reactant=A

Cv=Concentration of vacant sitesCA.S=Concentration of sites with adsorbed ACB.S=Concentration of sites with adsorbed B

Adsorption isotherm

SASA

][atm /k

/ k-

1-

A

A-

AA

ASAVAASAvAAAD

kK

KCCPkCCPkr

Gaseous reactant,

Solid catalyst

PA =Partial pressure of the reactantCv= Concentration of vacant sitekA/k-A=Constant of proportionality for attachment and detachment

Rate of attachment vAA CPk

Rate of detachment k A- SAC

Adsorption isotherm

VAASAAAD

AD

CPKCkr

r

. 0/

0 :mequilibriu @

) 1( AAVVAAVSAVt PKCCPKCCCC AA

tV

PK

CC

1

AA

AA

t

SA

t

AA

AASA

VAASA

AA

tV

PK

PK

C

C

CPK

PKC

CPKC

PK

CC

1

1

1

Adsorption isotherm

Concentration of reactant on the catalyst surface as a function of partial pressure of the reactant

This type of isotherm equation is called Langmuir Isotherm(at a particular temperature)

Langmuir adsorption isotherm

Langmuir

Adsorption

Isotherm

AP

T

SA

C

C Increasing T

Slope=kA

Surface reaction step

One adsorbed reactant species

Langmuir-Hinshelwood kinetics

One adsorbed product species



+one vacant site



+one vacant site


Two adsorbed reactant species


Two adsorbed product species


Two adsorbed reactant species

Different types of sites S and S


Two adsorbed product species

Surface reaction step Eley-Rideal kinetics


One reactant molecule in gas phase


A BC

CS CS

rDC kD CCS PCC

KDC

rDC rADC

KDC 1

KC

rDC kD CCS KCPCC

(10-20)

(10-21)

Desorption from surface of the reaction

Adsorption

Surface Reaction

Desorption

Which step is the Rate Limiting Step (RLS)?

SASA

A

k

SAvAAdAdA

CCPkrr

SBSA

SBSB

C

k

SBSASSA

CCkrr

BBBSBDDA CPkCkrr

Steps in a single site catalytic reactor

Electrical analog to heterogeneous reactions

Which one is the rate limiting step?

Which step has thelargest resistance ?

Reactor design

Catalytic reformer: Mechansim and rate eqns ?

n-pentane i-pentane 0.75 wt% Pt

Al2O3

n-pentene i-pentene Al2O3

N-P I-P

n-pentane n-pentene -H2

Pt Al2O3

i-pentene +H2

Pt

i-pentaneIsomerization

Octane number 62 Octane number 90

1.Ads2.Rxn3.Des

1.Ads2.Rxn3.Des1.Ads

2.Rxn3.Des

Isomerization of n-pentene (N) to i-pentene (I) over alumina +platinum

N IAl2O3

Mechanism for 3 steps (Mechanism: Single Site)

1. Adsorption on Surface: SNSN

2. Surface Reaction: SISN

3. Desorption: SISI

Treat each reaction step as an

elementary reaction when writing rate laws.

STEP 1: Catalytic reformer

1.

Adsorption

Surface Reaction

Desorption

Which step is the Rate Limiting Step (RLS)?

SNSN

N

K

SNvAAdAdA

CCPkrr

SISN

S

K

SISNSSA

CCkrr

SISI IIISIDDA CPKCkrr

STEP 2: Rate equation for the 3 processes

n-pentane i-pentane Al2O3

2. ----(1)

----(2)

----(3)

Assume a rate-limiting step.

Choose the surface reaction first, since more than 75% of all heterogenous reactions that are

not diffusion-limited are surface-reaction-limited. The rate law for the surface reaction step is:

S

SI

SNSS

'

INK

CCkrrr

SSISSN

Step3: Catalytic reformer

----(2)

4. Find the expression for the concentrations of the adsorbed species , CN.S and CI.S. Use the other steps that are not limiting to solve for CN.S and CI.S. For

this reaction:

CKPC NNSN:0k

r

A

AD From

CPKK

CPC II

D

ISI

:0k

r

D

D From

STEP4: Catalytic reformer

SNSN

N

K

SNvAAdAdA

CCPkrr

SISI IIISIDDA CPKCkrr

----(4)

----(5)

CKPC NNSN


CPKK

CPC II

D

ISI

4. Write a Site Balance. SISNt CCCC

IINNt

vPKPK

CC

1

CKPCKPCC IINNt

Calculate Cv in terms of Ct

S

SI

SNSS

'

INK

CCkrrr


6. Write the adsorbed reactant and product in terms of Ct

IINNtNN

SNPKPK

CKPC

1 IINNtII

SIPKPK

CKPC

1

6. Derive the rate law.

IINNPIN

SN

IINN

PIN

k

NtsSN

PKPK

KPPkrr

PKPK

KPPKCkrr

1

1

Step 7: Catalytic reformer

(Rate equation-rate of surface reactionin term of PN, PI, K)

Measurable quantity

Analogies

A B C

(A) (B)

rA kPA

1 KAPA KBPBrA

kPA

1 KAPA KCPC

(A) (B)

AS AS AS

AS

AS BS C AS B CS

BS BS CS

CS

Catalytic reaction mechanism

Catalytic reaction mechanism

2A B C

(C) (D)

rA kPA

2

1 KAPA KCPC 2

rA kPA

2

1 KAPA KCPC 2

(C) (D)

AS AS AS

AS

AS A g B CS AS AS BS CS

CS CS CS

CS

Estimation of model parameter

NN

PINSN

IINN

PINSN

IINN

PINSN

PK

KPPkrr

PKPK

KPPkrr

PKPK

KPPkrr

1

1

1Model 1

Model 2

Model 3

What are the parameter values and how to select a model?

Minimize =0 (Ydata f(xA, xB.))2

r PN PIData

Parameter estimation: example

Minimize =0 (Ydata f(xA, xB.))2

One Option : to use fmincon

2

Option: to use fmincon3

Output

CRE Catalytic Reactor chemical reaction engineering

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