L1-1 Slides courtesy of Prof M L Kraft, Chemical & Biomolecular Engr Dept, University of Illinois, Urbana-Champaign. CHBE 424: Chemical Reaction Engineering

L1-1

Slides courtesy of Prof M L Kraft, Chemical & Biomolecular Engr Dept, University of Illinois, Urbana-Champaign.

CHBE 424: Chemical Reaction Engineering

Introduction & Lecture 1

L1-2


Understanding how chemical reactors work lies at the heart of almost every chemical processing operation.

Design of the reactor is no routine matter, and many alternatives can be proposed for a process. Reactor design uses information, knowledge and experience from a variety of areas - thermodynamics, chemical kinetics, fluid mechanics, heat and mass transfer, and economics.

CRE is the synthesis of all these factors with the aim of properly designing and understanding the chemical reactor.

What is Chemical Reaction Engineering (CRE) ?

Chemical process

Raw material

Separation Process

Products By-products

Separation Process

L1-3


How do we design a chemical reactor?

Type & size

Maximize the space-time yield of the desired product (productivity lb/hr/ft3)

StoichiometryKinetics

Basic molar balancesFluid dynamics

Reactor volume

Use a lab-scale reactor to determine the kinetics!

L1-4


Reactor Design

ReactionStoichiometry

Kinetics: elementary vs non-elementarySingle vs multiple reactions

ReactorIsothermal vs non-isothermal

Ideal vs nonidealSteady-state vs nonsteady-state

L1-5


What type of reactor(s) to use?

in

out

Continuously StirredTank Reactor (CSTR)

Well-mixed batch reactorPlug flow reactor (PFR)

L1-6


What size reactor(s) to use?

Answers to this questions are based on the desired conversion, selectivity and kinetics

Reactor type &

size

Conversion&

selectivity

Kinetics

Material &energy

balances

L1-7


Chemical Reaction• A detectable number of molecules have lost their identity

and assumed a new form by a change in the kind or number of atoms in the compound and/or by a change in the atoms’ configuration• Decomposition• Combination• Isomerization

• Rate of reaction – How fast a number of moles of one chemical species are

being consumed to form another chemical species

L1-8


Rate Law for rj• rA: the rate of formation of species A per unit volume [e.g., mol/m3•s] • -rA: the rate of a consumption of species A per unit volume

rj depends on concentration and temperature:

A B products CkCr BAA

1st order in A, 1st order in B, 2nd order overall

kCr nAA nth order in A

A2

A1A Ck1

Ckr

Michaelis-Menton: common in enzymatic reactions

aE

RTA A

A

-r A e C Arrhenius dependence on temperature

A: pre-exponential factor E : activation energy

R : ideal gas constant T:temperature

L1-9


Basic Molar Balance (BMB)

moldtd

s

mol

smol

s

moldt

dN G F F j

jj0j

Rate of flow of j

into system

-

Rate of flow of j out of

system

+

Rate of generation of j by chemical

rxn

-Rate of

decomposition of j

= Rate ofaccumulation

combine Nj: moles j in system at time t

System volume

Fj0 FjGj

in - out + generation = accumulation

L1-10


Basic Molar Balance (BMB)

moldtd

s

mol

smol

s

moldt

dN G F F j

jj0j

Rate of flow of j

into system

-

Rate of flow of j out of

system

+

Rate of generation of j by chemical

rxn

-Rate of

decomposition of j

= Rate ofaccumulation

If the system is uniform throughout its entire volume, then:

VrG jj

Moles j generated

per unit time(mol/s)

=

Moles generated per unit time and

volume (mol/s•m3)

Volume(m3)

L1-11


Non-Uniform Generation

DV

If rj varies with position (because the temperature or concentration varies) then rj1 at location 1 is surrounded by a small subvolume D V within which the rate is uniform

Rate is rj1 within this volume

DV

Rate is rj2 within this volume

jG limm→∞DV→0

D

m

1i

V

jj dVrVr

1

11 y

x

z

1

0

1

0

1

0jj dz dy dx z,y,xrG then

Plug in rj and integrate over x, y, and z

system

L1-12


Basic Molar Balance Equations

jj0 j j

dNF F G

dt

jj0 j j

dNF F r V uniform rate n V

dti

V jj0 j j

dNF F r dV nonuniform rate in V

dt

In Out- +Generation =Accumulation

Next time: Apply BME to ideal batch, CSTR, & PFR reactors

System volume

Fj0 FjGj

L1-13


Review of Frequently Encountered Math Concepts

L1-14


yln x y ln x

xln x ln y ln

y

aln bt 1 ln x ln y

b

a bln bt 1x

lny

a b xln bt 1 ln

ye e

a b xbt 1

y a by bt 1 x

ln ae a

ln x ln y ln xy

Solve for X:

Basic Math Review n

n

1x

x p

q pqx x

aln bt 1 ln x n y

bl

Example: Problems that Contain Natural Logs

L1-15


b bn

na a

1dx x dx

x

b

n 1

ax n 1 For n≠1:

n 1 n 1b an 1 n 1

5 t

21 0

dx cdt

dx

5t0

1

1 ct

x d

1 1 c

t 05 1 d

c0.2 1 t

d c0.8 t

d d

0.8 tc

nn

1x

x

b b

ana

1dx ln x

xFor n=1:

p

q pqx x

ln b ln ab

lna

Review of Basic Integration

Solve for t:

L1-16


d

0d dd

1 1t 01 1

k ln ln ln c lk kk k

n c

d

kc

1 k tdcdt

d

kdt

1 k tdcc

0

t c

0 cd

1 dck dt1 ctk Do NOT move t or c outside of the integral

0

t

d

c

0 c

1 dck dt

c1 tk

From Appendix A:

xx

0 0

dx 1ln 1 x

1 x

xx

0 0

dx 1ln 1

x1x

cc0

0d

d

t

kk1

k ln 1 t ln c

0

d 0d

kln k t 1 ln c ln c

k d

d 0

k cln k t 1 ln

k c

cklnln k t 1d ck 0de e

k

ln k t 1dkd

0

ce

c

k

ln k t 1dkd0c e c

Solve for c:

ε is a constant

Documents

L1-1 Slides courtesy of Prof M L Kraft, Chemical & Biomolecular Engr Dept, University of Illinois, Urbana-Champaign. CHBE 424: Chemical Reaction Engineering