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

2. Impurity Effects Heat Exchange Reactors Separation Systems
Recycle Loops

3. Impurities in Heat Exchange
Impurities effect heat capacity
Lower Cp
Various options
Raise Cp
Increase H2
Impurities effect the enthalpy of stream
Total heat of condensation (CpΔT-ΔHvap) is less or more due to impurity
Total heat of vaporization (CpΔT+ΔHvap) is less or more due to impurity

4. Impurities in Heat Exchange
Impurities in Steam – Trouble shooting (MicroPlant) Lecture
Heat exchanger with Steam Trap
Build up of Non-Condensible 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?

5. Impurities in Heat Exchange
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

6. Impurities in Heat Exchange
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

7. Corrosion
Pitting Corrosion
Galvanic Corrosion
Corrosion in General

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

9. Galvanic Corrosion 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 leads to higher dissolution rate
Solution

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

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

12. Aluminum Corrosion Rates
Increase with salt concentration
Increase with temperature
Increase with decrease in pH

13. Corrosion Products Rust Fe2+(aq) + 2e− → Fe(s) −0.44 V
Fe with Stainless Steel
Corrosion Potential = V
Fe with Copper
Corrosion Potential = V
Rust
Pourbaix diagram

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

15. Cathodic Protection Zinc Protection Galvanized Steel Zn-FeSS Fe Al
Zinc Protection
Galvanized Steel
Zn-Fe
1 mm/yr Zn loss
|z.A|*m.A

16. ©2003 Brooks/Cole, a division of Thomson Learning, Inc
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
Figure Zinc-plated steel and tin-plated steel are protected differently. Zinc protects steel even when the coating is scratched, since zinc is anodic to steel. Tin does not protect steel when the coating is disrupted, since steel is anodic with respect to tin.

17. ©2003 Brooks/Cole, a division of Thomson Learning, Inc
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
Figure Cathodic protection of a buried steel pipeline: (a) A sacrificial magnesium (or zinc) anode assures that the galvanic cell makes the pipeline the cathode. (b) An impressed voltage between a scrap iron auxiliary anode and the pipeline assures that the pipeline is the cathode.

18. Impurity Effects Heat Exchange Reactors Separation Systems
Recycle Loops

19. Impurities in Separation Trains
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

20. 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 good function.

21. Reactor directly into Distillation
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
Reactor

22. Membrane Separations

23. Membrane Separations High Mw Impurities Low Mw Impurities
Foul Membranes
Lower Flux
Low Mw Impurities
Molecules will pass without separation
Ions rejected by membrane
Concentration polarization
Same Mw Impurities
causes poor separation

24. Impurities In Adsorption Systems
Carbon Bed
Ion Exchange
Desiccant Columns
Impurities that stick tenaciously
Can not be removed in regeneration step
With repeated cycles foul bed
Lower adsorption capacity after many cycles

25. 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

26. Impurities in Separation Trains
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?
Probably not!

27. Ultra-high purity Si plant design
Si at 99.97% Powder
H2 & HCl
Separation Train
Fluid Bed Reactor ( C)
Si+7HCl  SiHCl3 + SiCl4 +3H2
Si+ 2HCl  SiH2Cl2
Flash
HCl
H2-HCl Separation
SiCl4
HCl H2
SiCl4
Very Pure
SiHCl3&SiH2Cl2
Fluid Bed Reactor(600C)
Si+SiCl4+2HCl 2SiHCl3
Flash
CVD Reactor (1200C)
SiHCl3+H2  Si+3HCl
SiH2Cl2+1/2 H2  Si+3HCl Si H2
HCl+H2
Si at %

28. Chemical Vapor Deposition of Si

29. Chlorosilane Separation System
Componet BP
H2 − °C
SiH C
HCl −85.05°C
SiHCl3 -30°C
SiH2Cl2 8.3°C
SiCl4 57.6°C
Si2Cl6 145°C - polymer
Impurities BP
BCl3 12.5°C
PCl3 75.5°C
AlCl3 182°C
Product

30. Ultra-high purity Si plant design
Si at 99.97% Powder
H2 & HCl
Separation Train
Fluid Bed Reactor ( C)
Si+7HCl  SiHCl3 + SiCl4 +3H2
Si+ 2HCl  SiH2Cl2
Flash
HCl
H2-HCl Separation
SiCl4
HCl H2
SiCl4
Very Pure
SiHCl3&SiH2Cl2
Fluid Bed Reactor(600C)
Si+SiCl4+2HCl 2SiHCl3
Flash
Reactor (1200C)
SiHCl3+H2  Si+3HCl
SiH2Cl2+1/2 H2  Si+3HCl Si H2
HCl
Si at %

31. Separation Systems
Discuss PCl3 recycle back and fourth between Separation and HPC

32. Impurity Effects Heat Exchange Reactors Separation Systems
Recycle Loops

33. Purging Impurities
Find the point in the process where the impurities have the highest concentration
Put Purge here
Put a purge in almost all recycle loops

34. Failure of Flash to do its job, H2 recycle is fed to Reactor
If No Purge,
Both Product 1 & 2 are liquid products
so there is not place for H2 to leave Column.

35. Impurities in Recycle Loop
Set Purge flow rate so that the impurity concentration is sufficiently low to not effect reactor or flash separator performance.

36. Impurity Effects Heat Exchange Reactors Separation Systems
Recycle Loops

37. Impurities in Reactors
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

38. Managing Heat Effects Reaction Run Away Reaction Dies
Exothermic
Reaction Dies
Endothermic
Preventing Explosions
Preventing Stalling

39. Equilibrium Reactor- Temperature Effects
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
Van’t Hoff eq.
yi (xi) is smaller due to Impurities

40. Kinetic Reactors - CSTR & PFR – Temperature Effects
Used to Size the Reactor
Used to determine the reactor dynamics
Reaction Kinetics
Ci is lower with Impurities

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

42. PFR – no backmixing Used to Size the Reactor Space Time = Vol./Q
Outlet Conversion is used for flow sheet mass and heat balances
rK is smaller and V is larger due to impurities.

43. CSTR – complete backmixing
Used to Size the Reactor
Outlet Conversion is used for flow sheet mass and heat balances
rK is smaller and V is larger due to impurities.

44. Temperature Profiles in a Reactor
Exothermic Reaction
Impurities effect these curves
And areas under these curves
=size of reactor

45. Feed Temperature, ΔHrxn
Adiabatic
Adiabatic
Cooling
Heat Balance over Reactor
Q = UA ΔTlm
Impurities effect the Operating Curve same as inert effects

46. Inerts Addition Effect Similar to Impurity Effects

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

48. Catalytic Reactors Langmuir-Hinschelwood Mechanism (SR Limiting)
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)

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

50. Enzyme Catalysis Enzyme Kinetics S= substrate (reactant)
E= Enzyme (catalyst)

51. Galvanic Corrosion 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 leads to higher dissolution rate
Solution