Reactor Design

S,S&L Chapter 6

Objectives

• De Novo Reactor Designs

• Plant Improvement– Debottlenecking– Increase Plant Capacity– Increase Plant Efficiency– Decrease Costs– Pollution Minimization

Reactor Types

• Ideal– PFR– CSTR

• Real– Unique design geometries and therefore RTD– Multiphase– Various regimes of momentum, mass and heat

transfer

Reactors in Process Simulators

• Stoichiometric Model– Specify reactant conversion and extents of

reaction for one or more reactions

• A model for multiple phases in chemical equilibrium

• Kinetic model for a CSTR• Kinetic model for a PFR• Custom-made models (UDF)

Used in early stages of design

Stoichiometric Reactor

• C chemical Species• υi,j stoichiometric coefficient for

ith species in jth reaction• Aj chemical formula of jth

species• R chemical reactions

C

jjji RiA

1, ,...,2,1,0

Stoichiometric Reactor Example• Reactions

– 1 Methane Synthesis– 2 Coking

• Conversion, Xk, of key component, k– Xk=(nk-in – nk-out)/ nk-in

• Extent of Reaction– ξi= (ni,j-in – ni,j-out)/ νi,j

)(1

1

11

1

12

)(

2

2

2

,2

3

2

,1

22

32

sC

OH

COH

v

OHCH

COH

v

sCOHHCO

OHCHHCO

j

j

Cjij ,...,2,1, n nR

1iiin-jout-j

CjXk

jk ,...,2,1,n- n n in-kin-jout-j

Reactions with low conversions?

• Due to slow kinetics

• Due to non-favorable Equilibrium– Solution

• Set up reactor

• Followed by Separator

• Recycle reactant to extinction

Equilibrium Reactor-1• Single Equilibrium• aA +bB rR + sS

– ai activity of component I

• Gas Phase, ai = φiyiP, – φi== fugacity coefficient of i

• Liquid Phase, ai= γi xi exp[Vi (P-Pis) /RT]

– γi = activity coefficient of i – Vi =Partial Molar Volume of i

2

ln,exp

RT

H

dT

Kd

RT

G

aa

aaK

orxneq

orxn

aB

aA

sS

rR

eq

Van’t Hoff eq.

Equilibrium Reactor-2• Total Gibbs Free Energy is minimized at T&P

– Specify components that are entering system and T&P of System– Specify possible reaction products

• Gives outlet composition at equilibrium

i

C

iiTot GnG

1

Kinetic Reactors - CSTR & PFR

• Used to Size the Reactor

• Used to determine the reactor dynamics

• Reaction Kinetics

/)exp()(

)(1

RT

EkTk

CTkdt

dCr

Ao

C

ii

jj

i

PFR – no backmixing

• Used to Size the Reactor

• Space Time = Vol./Q

• Outlet Conversion is used for flow sheet mass and heat balances

kX

kko r

dXFV

0

CSTR – complete backmixing

• Used to Size the Reactor

• Outlet Conversion is used for flow sheet mass and heat balances

k

kko

r

XFV

Catalytic Reactors

• Various Mechanisms depending on rate limiting step

• Surface Reaction Limiting

• Surface Adsorption Limiting

• Surface Desorption Limiting

• Combinations

– Langmuir-Hinschelwood Mechanism (SR Limiting)

• H2 + C7H8 (T) CH4 + C6H6(B)

TB

HTT pp

ppkr

04.139.112

Enzyme Catalysis

• Enzyme Kinetics

• S= substrate (reactant)

• E= Enzyme (catalyst)

OHS

SEOHs CkkCk

CCCkkr

2

2

321

31

Problems

• Managing Heat effects

• Optimization– Make the most product from the least reactant

Managing Heat Effects

• Reaction Run Away– Exothermic

• Reaction Dies– Endothermic

• Preventing Explosions

• Preventing Stalling

Optimization of Desired Product

• Reaction Networks– Maximize yield,

• moles of product formed per mole of reactant consumed

– Maximize Selectivity• Number of moles of desired product formed per mole of

undesirable product formed

– Maximum Attainable Region – see discussion in Chap’t. 6.

• Reactors and bypass

• Reactor sequences