Dealing with Impurities in Processes and Process Simulators
ChEN 5253 Design II
Terry A. Ring
There is not chapter in the book on this subject

• Heat Exchange
• Reactors
• Separation Systems
• Recycle Loops

• Impurities effect heat capacity
– Lower C p
• Various options
– Raise C p
• Increase H
2
• Impurities effect the enthalpy of stream
– Total heat of condensation is less or more due to impurity
– Total heat of vaporization is less or more due to impurity

• Impurities in Steam – Trouble shooting
(MicroPlant) Lecture
– Heat exchanger with Steam Trap
– Build up of Impurity with Time
• Kills Heat Exchange with Time.
– To Overcome This Problem
• Clean up steam
• Purge to remove impurity build up
• How to determine the purge flow rate?

• Impurities in Fuel
– Vanadium in Venezuelan Crude Oil
• Vanadium follows the heavy oil product that is burned to supply heat for the refinery
• Vanadium gives low temperature eutectic in weld beads
– Welds failed in process heaters
– Welds failed in process boiler
– Crude Processing (desalting & hydrotreating) to remove heavy metals before entering the refinery

• Impurities that lead to high corrosion rates
– e.g. HCl in steam
– Heat exchangers are hot so corrosion is fast
– Corrosion of Heat Exchanger surfaces
• Decreases heat transfer coefficients in U
• Heat Exchange is not as effective with time
– Cooling towers are easily corroded
• Lower heat transfer coefficients
• Heat Exchange is not as effective with time

• Pitting Corrosion
• Galvanic Corrosion
• Corrosion in General

Galvanic
Series
Least Noble metal corrodes when two metals are in contact

• Two metals are connected together
• Exposed to water with dissolved salts
• Less Noble metal is dissolved away
– Aluminum is less noble to steel
• Higher salt content and higher pH leads to higher dissolution rate
Solution

• Fe2+(aq) + 2e− → Fe(s)
• −0.44 V
• Fe with Stainless Steel
– Corrosion Potential =
+ 0.14 V
• Fe with Copper
– Corrosion Potential =
+ 0.3 V
Pourbaix diagram

Corrosion Rates-OLI Corrosion Analyzer
Pipe Flow
D= 0.1m

• Al3+(aq) + 3e− → Al(s)
• −1.68 V
• Connection with Iron
• Corrosion Potential
• = +
1.2 V

• Increase with salt concentration
• Increase with temperature
• Increase with decrease in pH

• Two metals are connected together
• Exposed to water with dissolved salts
• Less Noble metal is dissolved away
– Aluminum is less noble to steel
• Higher salt content and higher pH leads to higher dissolution rate
Solution

• Water is recycled in Stream Plant
– Steam Generator
– Process
– Return Condensed Steam
– Makeup water is DI water to eliminate impurites
• Steam Generator
– Chemical Treatment to prevent corrosion
– Corrosion Inhibitors
• Phosphates, pH control (buffers), other chemicals

Cathodic
Protection
• Zinc Protection
• Zn-Fe
– 1 mm/yr Zn loss
|z.A|*m.A
SS
Fe
Al

• Heat Exchange
• Reactors
• Separation Systems
• Recycle Loops

• Poisons for Catalysts
– Kill Catalyst with time
– S in Gasoline kills Catalytic Converter
• Impurities can cause side reactions altering
– Reactor conversion
– Generating additional undesirable products
• Impurities Impact Equilibrium Conversion
• Impurities Impact Reaction Rates
– Lower concentrations
• Impurities have Reaction Heat Effects
– Lower Cp of feed in slope of operating line

• Reaction Run Away
– Exothermic
• Reaction Dies
– Endothermic
• Preventing Explosions
• Preventing Stalling

Equilibrium Reactor-
Temperature Effects
• Single Equilibrium
• aA +bB  rR + sS
Van’t Hoff eq.
K eq
 a r
R a a s
S a
 exp
 

 o
G rxn a a RT
A B
– a i activity of component I

 ,

 d ln dT
K eq




H
RT o rxn
2
• Gas Phase, a i
– φ i=
= φ i y i
P,
= fugacity coefficient of i y i
(x i
) is smaller due to Impurities
• Liquid Phase, a i
– γ i
= γ i x i exp[V
= activity coefficient of i i
(P-P i s ) /RT]
– V i
=Partial Molar Volume of i

Kinetic Reactors - CSTR & PFR –
Temperature Effects
• Used to Size the Reactor
• Used to determine the reactor dynamics
• Reaction Kinetics
 r j
  dC j dt
 k ( T )
C 

1 i
C i
 i k ( T )
 k o exp


E
RT
A


C i is lower with Impurities

• Increasing Temperature Increases the
Rate
• Equilibrium Limits Conversion
Equilibrium line is repositioned and rate curves are repositioned due to impurities

• Used to Size the Reactor
V

F ko
X k 
0 dX
 r k
• Space Time = Vol./Q
• Outlet Conversion is used for flow sheet mass and heat balances r
K is smaller and V is larger due to impurities.

• Used to Size the Reactor
V

F ko

X r k k
• Outlet Conversion is used for flow sheet mass and heat balances r
K is smaller and V is larger due to impurities.

Exothermic Reaction
Impurities effect these curves
And areas under these curves
=size of reactor

rxn
Adiabatic
Cooling
Adiabatic
Heat Balance over Reactor
Q = UA ΔT lm
Impurities effect the Operating Curve same as inert effects

Inerts Addition Effect Similar to
Impurity Effects

Review : Catalytic Reactors –
Major Steps
Bulk Fluid C
Ab
1. External Diffusion
Rate = k
C
(C
Ab
– C
AS
)
A
External Surface of Catalyst Pellet
C
As
2. Defined by an
Effectiveness Factor
Internal Surface of Catalyst Pellet
3. Surface Adsorption
A + S <-> A.S
Catalyst
Surface
B
7 . Diffusion of products from pore mouth to bulk
6 . Diffusion of products from interior to pore mouth
A  B
4. Surface Reaction
5. Surface Desorption
B. S <-> B + S

• Various Mechanisms depending on rate limiting step
• Surface Reaction Limiting
• Surface Adsorption Limiting
• Surface Desorption Limiting
• Combinations
– Langmuir-Hinschelwood Mechanism
(SR Limiting)
• H
2
+ C
7
H
8
(T)  CH
4
+ C
6
H
6
(B) r
T
  k ( T )
1

1 .
39
C p
B v p
T p
H
2

1 .
04 p
T

Catalytic Reactors – Impurity Implications on design
1. How the surface adsorption and surface desorption influence the rate law?
2. Whether the surface reaction occurs by a single-site/dual –site / reaction between adsorbed molecule and molecular gas?
3. How does the reaction heat generated get dissipated by reactor design?

• Enzyme Kinetics r s
  k
1 k
1 k
3
C
H
2
O
C
E
C
S
C
S
 k
2
 k
3
C
H
2
O
• S= substrate (reactant)
• E= Enzyme (catalyst)

• Heat Exchange
• Reactors
• Separation Systems
• Recycle Loops

• Non-condensible Impurities
– Build up in Distillation column – Big Trouble!!
• Condensible Impurities
– Cause some products to be less pure
• May not meet product specifications
• Can not sell this product – Big Trouble!!
– Rework cost
– Waste it
– Sell for lower price

Processes are tested for Impurity
Tolerance
• Add light and heavy impurities to feed
– Low concentration
• All impurities add to 0.1 % of feed
• (may need to increase Tolerance in Simulation)
– Medium concentration
• All impurities add to 1% of feed
– High concentration
• All impurities add to 10% of feed
• Find out where impurities end up in process
• Find out if process falls apart due to impurities
– What purges are required to return process to function.

18
Q-4
17
1
XCHG-102
SPLT-101
19
• Non-condensable
Impurities
– Products of Side reactions
– Impurities in reactants
• Cause Trouble in Column with Total Condenser
– No way out
• Use Partial Condenser
• Add Flash after Reactor
– Non-condensables to flare
• Cooling required for Flash from reactant heat up
12
Reactor 22
MIX-101
21
RCYL-2
2
DTWR-102
21
20
K-101
24
23
Q-5
19
31
REAC-103
26
25
XCHG-103
22
32
Q-8
XCHG-101
ToFlare2
11
16
1
16
26
40
VSSL-101
17
50
DTWR-101
14
K-101
Q-6
15
20
Q-7
PUMP-101
Q-4

• High M w
Impurities
– Foul Membranes
– Lower Flux
• Low M w
Impurities
– Molecules will pass without separation
– Ions rejected by membrane
• Concentration polarization
• Lower Flux
• Same M w
Impurities
– causes poor separation

Impurities In Adsorption Systems
• Carbon Bed
• Ion Exchange
• Dessicant Columns
– Impurities that stick tenaciously
• Can not be removed in regeneration step
• With repeated cycles foul bed

Impurities in Absorption Systems
• Scrubber Columns
• Liquid-Liquid contacting columns
– Impurities that stick tenaciously
• Can not be removed in regeneration step
• With repeated cycles are not removed and cause product purity problems

• It is important to know where the impurites will accumulate in the train
• Which products will be polluted by which impurities
– Is that acceptable for sale of product?

Si at 99.97% Powder
H
2
& HCl
Si
Fluid Bed Reactor (400-900C)
Si+7HCl  SiHCl
3
+ SiCl
4
Si+ 2HCl  SiH
2
Cl
2
+3H
2
HCl
HCl
SiCl
4
Fluid Bed Reactor(600C)
Si+SiCl
4
+2HCl  2SiHCl
3
Flash Separation Train
Flash
H
2
-HCl Separation
SiCl
4
H
2
Very Pure
SiHCl
3
&SiH
2
Cl
2
H
2
Reactor (1200C)
SiHCl
3
+H
2
 Si+3HCl
SiH
2
Cl
2
+1/2 H
2
 Si+3HCl
HCl Si at 99.999999999%

Chemical Vapor Deposition of Si

• Componet BP
• H
2
• SiH
4
−252.879°C
-111.8C
• HCl −85.05°C
• SiHCl
3
• SiH
2
Cl
2
• SiCl
4
• Si
2
Cl
6
-30
8.3
145
°C
°C
57.6
°C
°C -
Product polymer
• Impurities BP
• BCl
• PCl
3
3
• AlCl
3
12.5
°C
75.5
182
°C
°C

Si at 99.97% Powder
H
2
& HCl
Si
Fluid Bed Reactor (400-900C)
Si+7HCl  SiHCl
3
+ SiCl
4
Si+ 2HCl  SiH
2
Cl
2
+3H
2
HCl
HCl
SiCl
4
Fluid Bed Reactor(600C)
Si+SiCl
4
+2HCl  2SiHCl
3
Flash Separation Train
Flash
H
2
-HCl Separation
SiCl
4
H
2
Very Pure
SiHCl
3
&SiH
2
Cl
2
H
2
Reactor (1200C)
SiHCl
3
+H
2
 Si+3HCl
SiH
2
Cl
2
+1/2 H
2
 Si+3HCl
HCl Si at 99.999999999%

HPC-Feed
LPC-Feed
HE-401-106-1
401-101
MIX-3101
HE-401-106-2
401-180
Q-401-106
401-103
401-181
401-202
100
401-280 401-281
Q-401-206
401-203
SPLT-3200
HE-401-206-1
HE-401-206-2
401-204
401-102
100
401-104
75
50
TW-401-103
1 401-107
SPLT-3100
401-105-1
PU-3100
401-201-1
TW-401-203
401-201-2 40
VS-401-201
Q-3100
1
401-205
HE-401-204-1
401-207
Q-401-204
401-271
HE-401-204-2
23
401-106
Q-401-104
HE-401-104-1
401-171
RCYL-2
401-206
401-270
401-405
401-480
Q-401-406
401-403
401-481
HE-401-406-2
401-108
401-401-1
SPLT-100
VS-401-401
100 401-404
401-401-2 40
TW-401-403
1
401-407
401-470
Q-401-404
HE-401-404-1
401-406
401-501
401-471
401-380 401-381
HE-401-306-1
401-302
Q-401-306
401-303
HE-401-306-2
SPLT-3300
50 401-304
TW-401-303
20
401-305
(to HPC)
1
401-306
401-307
HE-401-304-2
401-370 401-371
Q--401-304
HE-401-304-1
401-308
(TCS Grade II)
60
TW-401-503
401-580 401-581
401-502
HE-401-506-2
Q-401-506
401-503
SPLT-101
100
HE-401-506-1
401-504
401-505 (to Reduction)
1
401-506
401-507
To Reduction
MIX-100
VS-401-601
401-601
HE-401-606-2
401-680
HE-401-606-1
401-602
Q-401-606
401-603
401-681
SPLT-3700
401-801
401-880 401-881
HE-401-806-1 Q-401-806
401-802 401-803
HE-401-806-2
SPLT-3900
80 401-804
Q-501-504
HE-401-504-1
401-570 401-571
401-508 (TCS Grade II) HE-401-504-2
70
TW-401-603
40
401-604
401-605 TW-401-803
60
401-805 (to Reduction)
1 401-807
HE-401-804-1
Q-401-804
401-806 401-870 401-871
HE-401-706-2
HE-401-706-1
401-780
Q-401-706
401-702
401-703
50 401-704
401-731
SPLT-3800
1
401-606
401-607
HE-401-604-1
Q-401-604
401-608
VS-401-701
401-808
401-701
HE-401-804-2
10
TW-401-703
401-705
(STC to HPC)
2
1 401-707 401-770
401-706
HE-401-704-1
HE-401-704-2
Q-401-704
401-771
401-670 401-671
401-708
(polymer waste)
HE-401-604-2

• Heat Exchange
• Reactors
• Separation Systems
• Recycle Loops

• Find the point in the process where the impurities have the highest concentration
– Put Purge here
• Put a purge in almost all recycle loops

Impurities in Recycle Loop
Feed
Recycle
MIX-100
1
REAC-100
2
Purge
SPLT-101
9
5
Q-1
4
1
XCHG-100
SPLT-100
6
Product 1
2
DTWR-100
8
7
K-100
Q-2
Product 2

Failure of Flash to do its job,
H
2 recycle is fed to Reactor
Feed
Recycle
MIX-100
1
REAC-100
2
Purge
SPLT-101
9
5
Q-1
4
1
XCHG-100
SPLT-100
6
Product 1
Both Product 1 & 2 are liquid products so there is not place for H
2 to leave Column.
2
DTWR-100
8
7
K-100
Q-2
Product 2