Transcript PROCESS TECHNOLOGY

Stages in Catalyst development
Preparation
Screening
Reaction
network
Kinetics
Increasing:
 time
 money
 reality
Trends:
Life tests
Scale-up
Parallel activities
Subcontracting
Catalysis and Catalysts - Catalyst Performance Testing
1
Transport Phenomena in
Packed-bed Reactor
PLUG FLOW
MIXING
DISPERSION
DIFFUSION
REACTION
TRANSPORT PHENOMENA
Catalysis and Catalysts - Catalyst Performance Testing
2
Catalytic Reaction Engineering
Stability
Mechanism
Kinetics
CATALYSIS
ENGINEERING
Reactor
Engineering
Transport
Phenomena
Catalyst
Catalysis and Catalysts - Catalyst Performance Testing
3
Classification of Laboratory Reactors:
Mode of Operation
LABORATORY CATALYTIC REACTORS
Steady state
Transient
Continuous flow
Batch
Plug flow
Integral
Semi-batch
Mixed flow
Discontinuous
Step
Pulse
Differential
Single pass
External
Riser reactor
Thermobalance
Packed bed
Catalysis and Catalysts - Catalyst Performance Testing
Recycle
Fluidization
Internal
Berty reactor
Fluid bed
TAP
Multitrack
Slurry
4
Classification of Laboratory Reactors:
Contacting Mode
PFR
CSTR
FBR
Slurry/FBR
with recycle
1
2
3
4
5
6
7
8
Riser
Riser
fluid recycle
Catalysis and Catalysts - Catalyst Performance Testing
FBR
contn. cat feed
Slurry/FBR
fluid + cat. rec.
5
Pep versus Rep
Pe 
uLb
Dax
Pe p 
Catalysis and Catalysts - Catalyst Performance Testing
ud p
Dax
6
Maximum allowable particle diameter versus (1 - x)
n = 1, single phase, Pep = 0.5
Lb
8n
 1 

 ln

d p Pep  1  x 
dt
 10
dp
10
dt = 50 mm
dt = 50 mm
Lb = 100 mm
dpd p(mm)
(mm)
1
dt = 5 mm
dt = 5 mm
Lb = 10 mm
0.1
dt = 1 mm
dt = 1 mm
Lb = 1 mm
0.01
0.01
0.1
1
1-x
(1 - x)
Catalysis and Catalysts - Catalyst Performance Testing
7
Catalyst size
Practical catalyst:
often: dp = 1 - 3 mm
large reactor needed
Option: dilution with inerts
Catalysis and Catalysts - Catalyst Performance Testing
8
Catalyst wetting in trickle-flow reactors

Determined by friction and gravity
– particle diameter
– viscosity
– linear velocity (from LHSV and Lb)
Wtr 

 l  ul
d p2  g
 5  106
Example
– LHSV = 2 m3 / (m3h)
Catalysis and Catalysts - Catalyst Performance Testing
9
Catalyst wetting in trickle-flow reactors, Example
LHSV = 2 m3/m3h
l = 10-6 m2/s
dp = 1 mm
106  ul
ul
Wtr  6

 5  106
10  10 10
ul = LHSV.Lb = 2 Lb m/h
Lb > 90 mm
Catalysis and Catalysts - Catalyst Performance Testing
10
Maximum allowable particle diameter versus kinematic
viscosity for complete wetting in trickle-flow reactors
10
LHSV = 2 m3 / (m3h)
d p (mm)
L b = 1000 mm
1
L b = 100 mm
L b = 10 mm
0.1
1.00E-07
Wtr 
 l  ul
d p2  g
 5  106
Catalysis and Catalysts - Catalyst Performance Testing
1.00E-06

l
1.00E-05
(m2/s)
LHSV 
v ,l
Vreactor
ul  A ul


Lb  A Lb
11
Dilution with Inerts
Hydrodynamics governed by
small inert particles
Kinetic performance governed by
catalyst extrudates
Catalysis and Catalysts - Catalyst Performance Testing
12
Maximum allowable particle diameter as a
function of the catalyst fraction in a diluted bed
10
Lb
= 100 mm
nx
d p (mm)
1
Lb
= 10 mm
nx
0.1
Lb
= 1 mm
nx
0.01
0.1

fraction catalyst (1-b)
xdilutedbed  xundilutedbed
xundilutedbed
Catalysis and Catalysts - Catalyst Performance Testing
1
b n xdp bn xdp


 0.05
1  b 2 Lb
2 L0
13
Effect of Catalyst/Diluent Distribution
in Decomposition of N2O
1.0
X(N2O)
0.8
0.6
0.4
0.2
0.0
600
650
700
750
800
850
900
T (K)
Catalysis and Catalysts - Catalyst Performance Testing
14
Laboratory Reactors
PFR:
–
–
–
–
CSTR:
– larger amounts of catalyst and flows needed
– deactivation not determined directly
– direct rate data from conversions
FBR:
– non-ideal behaviour
– continuous handling of solids possible
TGA:
– limited to weight changes
– careful date interpretation needed
– often mass-transfer limitations
Batch:
– catalyst deactivation hard to detect
– yields conversion and selectivity data quickly over large range
deactivation noted directly
small amounts of catalyst needed
simple
yields conversion data, not rates
Catalysis and Catalysts - Catalyst Performance Testing
15
Mass and heat transport effects
catalyst particles

Mass and heat transport phenomena
– Extraparticle transport
– Intraparticle transport


Catalyst effectiveness
Generalizations
– Catalyst shape, kinetics, volume change

Observable quantities
– Criteria - transport disguises - experimental
Catalysis and Catalysts - Catalyst Performance Testing
16
Liquid film
Gas film
Ts
T
cs
c
Bulk gas
(bubble)
Tb
Catalysis and Catalysts - Catalyst Performance Testing
T
T
cb
c
c
Bulk gas
Exothermic
Gas film
Bulk liquid
Endothermic
Exothermic
17
Gradients at Particle Scale
Gas/solid Reactor
Gas film
Ts
T
Cs
c
Tb
T
Cb
c
Bulk gas
Exothermic
Catalysis and Catalysts - Catalyst Performance Testing
Endothermic
18
Gradients at Particle Scale
Gas/liquid/solid Slurry Reactor
Liquid film
Bulk gas
(bubble)
Gas film
T
c
Bulk liquid
Exothermic
Catalysis and Catalysts - Catalyst Performance Testing
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Isothermal - External Mass Transport
mass transfer
reaction
Ap  kf cb  cs   Vp  rv
rv ,obs  rv (cs )  a' kf cb  cs 
No transport limitations if:
film layer
cb
cs cb
When?
How to determine cs?
cs
Catalysis and Catalysts - Catalyst Performance Testing
20
Isothermal - External Mass Transport
kf cb  cs   Ap
observed rate

Catalyst effectiveness: e 
rate at bulk fluid conditions
rv (cb ,Tb )  Vp
Observable quantity:
rv ,obs
a' kf (cb  cs ) (cb  cs )
Ca 


a' kf cb
a' kf cb
cb
10
rV = kVCn
-1
e  1 Ca  1 0.05
n
0.05
Ca 
n

0.5
1
e
1
n=
0.1
0.01
0.001
2
0.01
0.1
1
Ca
Catalysis and Catalysts - Catalyst Performance Testing
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Nonisothermal - External Transport
Mass:
Heat:
rv ,obs  a' kf cb  cs 
( Hr )  rv ,obs  a' hTb  Ts 
T and c coupled via 
:max. T-rise over film
Catalyst effectiveness?
Catalysis and Catalysts - Catalyst Performance Testing
Ts
 1   eCa
Tb
e 
( Hr )kf cb ( T )max

hTb
Tb
 cs 
   1  Ca
 cb 
22
Nonisothermal - External Transport
General kinetics:
Ca small
 Ea
rv (cs ,Ts ) kv (Ts ) f (cs ) kv (Ts )
e 



 exp
rv (cb ,Tb ) kv (Tb ) f (c b ) kv (Tb )
 RTb
 Tb

  1  1  0.05
 Ts

Series expansion:
 Ea   ( H r )  k f  cb   rv ,obs 
  
  
  0.05
 b   e  Ca  
h  Tb
 R  Tb  
  kf  a'cb 
Catalysis and Catalysts - Catalyst Performance Testing
23
Isothermal - Internal Mass Transport
Slab
Mass balance, steady state diffusion & reaction
1st order irreversible:
Deff
d 2c
 kv c  0
dx2
Boundary conditions: x  L
x0
L
0

1.0
x+dx x
0.1
dc
0
dx
cosh( x / L)
c  cs
cosh( )
0.8
c*
c  cs
0.6
1.0
0.4
kv
L
Deff
2.0
0.2
10.0
0.0
1.0
0.8
0.6
0.4
0.2
L
Vp
Ap

1
a'
0.0
x*
Catalysis and Catalysts - Catalyst Performance Testing
24
Catalyst Effectiveness
Vp
dc
 Ap
dx
observedrate
x L
i 
 0

rate w ithoutinternal gradients rv ,chem (cs ,Ts )  Vp
rv (cs ,Ts ) Vp
 rv (c,T )dV
Slab:
i 
Deff
tanh 

1st order
1
Limits:
0
i  1

i 
i
1

0.1
0.1
1
10

Catalysis and Catalysts - Catalyst Performance Testing
25
Diffusion Control?
Kinetics unknown
effectiveness cannot be calculated
2
observedrate L rv ,obs
Wheeler-Weisz:   i 

' diffusionrate' Deff cs
2
 n21
(nth order)
1st order
0th order
i
3rd order
Weisz-Prater Criterion:
2nd
order
rv ,obs  L2
i 
Deff  cb
2
 n  1

  0.15
 2 
i 2
Catalysis and Catalysts - Catalyst Performance Testing
26
Nonisothermal - Internal Transport
Ts
T
T
Ts
cs
cs
c
c
Exothermal
Endothermal
similar profiles
c and T
determined by Prater number
i 
( Hr )Deff cs
 p,eff Ts
Catalysis and Catalysts - Catalyst Performance Testing
Typical values:
0-0.3 (exothermal)
27
Nonisothermal - Internal Transport
Slab
Heat and mass balance, steady state
Effective conductivity
0.1-0.5 J/m.K.s
d 2c
Deff
 rv
dx 2
d 2T
  p,eff
 rv ( H r )
dx 2
Boundary conditions:
L
xL
c  cs
T  Ts
x0
dc
dT
0
dx
dx
0
 p,eff d 2T
d 2c
Deff

dx 2
(Hr ) dx 2
x+dx x
T  Ts 
(Hr )Deff
cs  c 
 p,eff
Prater number
T i ,max
Ts
 i 
Catalysis and Catalysts - Catalyst Performance Testing
(Hr )Deff cs
 p,eff Ts
temperature and concentration
profile similar (scaling)
28
Nonisothermal - Internal Transport
Internal effectiveness factor:
i 
 E T
rv (c,T )
k (T ) f (c ) kv (T )

 v


 exp a  s  1  1  0.05
rv (cs ,Ts ) kv (Ts ) f (cs ) kv (Ts )

 RTs  T
i
10
i
s= 10
i varied
0.6
Criterion:
0.4
1
 Ea    H r   Deff  cs   rv ,obs  L2 



  

  D c 
 p ,eff  Ts
 R  Ts  
  eff s 
0.2
-0.2
0.1
0.1
1

Catalysis and Catalysts - Catalyst Performance Testing
10
  s   i i 2  0.1
29
Criterion bed T-gradient
Analogous to particle T-gradient:
2
Ea rV ,obs  ( H r )  rt

RTw
b,eff  Tw
1
1 rp 
  
   0.05
 8 Bi h,w rt 
Bi h,w 
hw  d p
b,eff
rV ,obs  rv ,obs  (1  b )  (1 b)
Compare with:
 s   i  i  2
2
 0.05
 Ea    Hr   Deff  cs   rv ,obs  L2   1 

    0.05

  




p,eff  Ts
 R  Ts  
  Deff  cs   2 
Catalysis and Catalysts - Catalyst Performance Testing
30
Mass Transport Limitations?
Internal / External
Criterion:  = 1 ± 0.05
Internal transfer:
  i   0.15
rv ,obs  L2  n  1
i  

  0.15
Deff  cb  2 
External transfer:
0.05
Ca 
n
Ca 
Bi m
 Ca
s
Bi m 
Also:

2
while Bim>~10
s=1,2,3 (geometry)
Catalysis and Catalysts - Catalyst Performance Testing
2
rv ,obs
a' kf cb
kf  d p
Deff
Weisz-Prater more severe than
Carberry criterion
31
Heat Transport Limitations?
Internal / External
Criterion:  = 1  0.05
Series expansion of  expression around 1 for slab, first order irreversible
reaction results in:


 1
 1   eCa 

e  1  Ca exp  b 
n

External transfer:
1
b e Ca  0.05
strongest influence
Internal transfer:


s i i 2  0.1
External gradient criterion more
severe than internal gradient criterion
Catalysis and Catalysts - Catalyst Performance Testing
32
Heat Transport Limitations?
Internal / External
Largest T-gradient ?
Internal:
T  Ts 
External: Ts  Tb 
Deff (Hr )
p,eff
cs  c 
k f  H r 
cb  cs 
h
For x=0 c=0 largest T-gradient
T e
T i
Bi  Ca 
 m

Bi h  1  Ca 
Bi m e

Bi h
i
10-104
gas-solid
10-4-0.1 liquid-solid
Industrial:
internal gradient largest
Laboratory:
external gradient largest
external gradient negligible
Catalysis and Catalysts - Catalyst Performance Testing
33
Heat Transport Limitations?
External / Bed
Comparison of external and bed gradient
(neglecting wall contribution and bed dilution):
 rv ,obs  ( H r )  rt2 (1   b ) 

 w  
2

8b,eff  Tw

   rt    p,eff
r  
 b   e  Ca
 p   b,eff
> 100
>1
  s 2 (1   b ) 

  1
 
8

 
~1
Bed gradient criterion more severe
than external gradient criterion
Catalysis and Catalysts - Catalyst Performance Testing
34
Summary Dependence rv,obs
Observed reaction rate:
rv ,obs    rv ,chem (cb ,Tb )  e i  rv ,chem (cb ,Tb )
1. Kinetics:
rv ,obs  rv ,chem (cb ,Tb )  kv cbn
does not depend on L, n reaction order, Eaapp= Eatrue
2. Internal mass transfer: rv ,obs 
rv ,chem

n 1
 E 
1
1 2
n 1

kv Deff cs   cs exp  a 
L
L
 2RTs 
depends on: 1/L, (n+1)/2 reaction order, Eaapp= ½Eatrue
3. External mass transfer: rv ,obs
1 um
 a' kf cb 
c
1m b
LL
depends on: L, flow rate, 1st reaction order, Eaapp= 0
How to check whether limitations are present?
Catalysis and Catalysts - Catalyst Performance Testing
35
Observed Temperature Behaviour
Catalysed steam gasification of carbon (coke) on Ni catalyst
5
Ni
C + H2O
CO + H2
Ea(kJ/mol)
0
61
1
1
0.75
r(obs)
• p(H2O)=26 kPa
• thermobalance
• coked catalyst:
Ni/Al2O3
164
0.1
0.6
order n
0.01
0.9
1.0
1.1
1.2
1.3
1.4
1000/T
Catalysis and Catalysts - Catalyst Performance Testing
36
Apparent Rate Behaviour
Controlling process
Apparent
order
Apparent
activation energy
Dependence
L
Dependence u
Kinetics
n
Ea(true)
-
-
Internal diffusion
n 1
2
½ Ea(true)
1/L
-
External mass transfer
1
Lm-2 *
um *
Catalysis and Catalysts - Catalyst Performance Testing
0
37
Diagnostic Tests - Mass-Transport Limitations
1. Particle size variation
observed rate
egg-shell catalysts?
particle size
2. Flow rate variation at constant space time!
xA,1
xA,2
xA,3
x
W
W
1
F
0
A ,1
W
2
F
0
A,2
Catalysis and Catalysts - Catalyst Performance Testing
3
F
0
F
0
A ,1
F
0
A,2
F
0
A,3
A,3
38
What’s observed?
intraparticle limitation
0.1
Limiting case: ‘Falsified kinetics’
dp/mm
kv
0.38
activation energy: Ea(true)/2
0.01
1.4
2.4
0.001
1.90
1.95
2.00
rv ,obs 
2.05
rchem


1
n 1
kv Deff cs
L
2.10
1000/T
reaction order (n+1)/2
wide pore silica
effect dp
Catalysis and Catalysts - Catalyst Performance Testing
particle size dependent
39
Proper Catalyst Testing

Adhere to criteria
– Ideal reactor behaviour: PFR or CSTR
– Isothermal bed
– Absence of limitations: observables, diagnostic tests


Compare catalysts at low conversion;
For high conversions use feed/product mixtures
Compare selectivities at same conversion level
Catalysis and Catalysts - Catalyst Performance Testing
40
Consecutive irreversible first order reaction
ARS
1
CA
Concentration
0.8
CS
0.6
0.4
Same CR
CR
0.2
0
0
20
40
60
W /F
80
100
0
i
Catalysis and Catalysts - Catalyst Performance Testing
41
More Efficient Catalyst Testing
• PC-controlled microreactor set-up
• Parallel reactors in one oven:
Sixflow reactor set-up
• Experimental design
ANALYSIS
P
MFC
FEED
MFC
VENT
MFC
MFC
MFC
SV
MFC
MFC
MFC
MFC
BPC
Catalysis and Catalysts - Catalyst Performance Testing
42