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

Impurity Effects Heat Exchange Reactors Separation Systems Recycle Loops

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

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?

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

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

Corrosion Pitting Corrosion Galvanic Corrosion Corrosion in General

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

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

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

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

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

Corrosion Products Rust 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 Rust Pourbaix diagram

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

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

©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 22.12 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.

©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 22.13 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.

Impurity Effects Heat Exchange Reactors Separation Systems Recycle Loops

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

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.

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

Membrane Separations

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

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

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

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!

Ultra-high purity Si plant design Si at 99.97% Powder H2 & HCl Separation Train Fluid Bed Reactor (400-900C) 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 99.999999999%

Chemical Vapor Deposition of Si

Chlorosilane Separation System Componet BP H2 −252.879°C SiH4 -111.8C 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

Ultra-high purity Si plant design Si at 99.97% Powder H2 & HCl Separation Train Fluid Bed Reactor (400-900C) 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 99.999999999%

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

Impurity Effects Heat Exchange Reactors Separation Systems Recycle Loops

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

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.

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

Impurity Effects Heat Exchange Reactors Separation Systems Recycle Loops

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

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

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

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

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

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.

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.

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

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

Inerts Addition Effect Similar to Impurity Effects

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

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)

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?

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

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