Dealing with Impurities in Processes and Process Simulators

Transcription

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 C p Various options Raise C p Increase H 2 Impurities effect the enthalpy of stream Total heat of condensation (C p ΔT-ΔH vap ) is less or more due to impurity Total heat of vaporization (C p ΔT+ΔH vap ) 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 Heat Exchange is not as effective with time

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 Al 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 Fe2+(aq) + 2e Fe(s) 0.44 V Fe with Stainless Steel Corrosion Potential = V Fe with Copper Corrosion Potential = V 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 Steam Generator Chemical Treatment to prevent corrosion Corrosion Inhibitors Phosphates, ph control (buffers), other chemicals

15 Cathodic Protection Zinc Protection Galvanized Steel Zn-Fe 1 mm/yr Zn loss z.a *m.a SS Fe Al

16 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. 16

17 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. 17

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 Q-4 18 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 MIX-101 REAC RCYL DTWR XCHG-102 SPLT XCHG K XCHG-101 Q-8 23 Q-5 ToFlare2 13 VSSL PUMP-101 Q DTWR VSSL K-101 Q-6 Q-7

22 Membrane Separations

23 Membrane Separations 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

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

28 Chemical Vapor Deposition of Si

29 Chlorosilane Separation System Componet BP Impurities BP H C BCl C SiH C PCl C HCl C AlCl C SiHCl 3-30 C SiH 2 Cl C SiCl C Product Si 2 Cl C - polymer

30 Ultra-high purity Si plant design Si at 99.97% Powder H 2 & HCl Fluid Bed Reactor ( C) Si+7HCl SiHCl 3 + SiCl 4 +3H 2 Si+ 2HCl SiH 2 Cl 2 Flash Separation Train HCl H 2 -HCl Separation SiCl 4 HCl H 2 Si SiCl 4 Fluid Bed Reactor(600C) Si+SiCl 4 +2HCl 2SiHCl 3 Flash H 2 Very Pure SiHCl 3 &SiH 2 Cl 2 Reactor (1200C) SiHCl 3 +H 2 Si+3HCl SiH 2 Cl 2 +1/2 H 2 Si+3HCl HCl Si at %

31 Separation Systems MIX-3101 HPC-Feed LPC-Feed HE Q HE TW SPLT Q HE VS PU-3100 TW VS Q HE Q TW HE HE Q RCYL-2 HE SPLT Q HE SPLT-3200 HE VS HE Q HE HE Q SPLT TW Q TW HE HE Q HE VS HE HE SPLT-100 Q (to Reduction) HE HE Q HE SPLT TW HE (TCS Grade II) (to HPC) Q HE Q SPLT TW HE HE Q SPLT-101 HE (to Reduction) 60 TW HE (TCS Grade II) HE (STC to HPC) Q HE HE HE Q To Reduction MIX-100 HE (polymer waste)

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 there Put a purge in almost all recycle loops

34 Failure of Flash to do its job, H 2 recycle is fed to Reactor Purge Recycle SPLT-101 Feed MIX REAC VSSL Q-1 5 XCHG-100 SPLT Product If No Purge, Both Product 1 & 2 are liquid products so there is not place for H 2 to leave Column. DTWR K-100 Product 2 Q-2

35 Impurities in Recycle Loop Purge Recycle SPLT-101 Feed MIX REAC VSSL Q-1 5 XCHG-100 SPLT Product 1 Set Purge flow rate so that the impurity concentration is sufficiently low to not effect reactor or flash separator performance. 2 DTWR K-100 Q-2 Product 2

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 Catalyst Degradation Coking, Sintering 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 Exothermic Reaction Dies Endothermic Preventing Explosions Preventing Stalling

39 Single Equilibrium aa +bb rr + ss K eq = a a r R a A a a Equilibrium Reactor- Temperature Effects s S a B = exp G RT o rxn a i activity of component I Gas Phase, a i = φ i y i P, φ i= = fugacity coefficient of i Liquid Phase, a i = γ i x i exp[v i (P-P is ) /RT] γ i = activity coefficient of i V i =Partial Molar Volume of i, d Van t Hoff eq. ln dt K eq = H RT o rxn 2 y i (x i ) 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 r j k( T ) = = k dc o dt j = k( T ) E exp RT A i= 1 C C αi i C i 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 V = F ko X k dx r 0 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.

43 CSTR complete backmixing 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.

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

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

46 Inerts Addition Effect Similar to Impurity Effects

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

48 Catalytic Reactors 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 = p ( T ) Cv pt ph 2 B p T

49 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?

50 Enzyme Catalysis Enzyme Kinetics r s = k S= substrate (reactant) E= Enzyme (catalyst) 1 k C 1 k S 3 + C k H 2 2 O + C k E 3 C C S H 2 O

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