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

Impurity Effects Heat Exchange Reactors Separation Systems Recycle Loops

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 is less or more due to impurity –Total heat of vaporization 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 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 Heat Exchange is not as effective with time

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 and higher pH leads to higher dissolution rate Solution

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

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

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

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

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 and higher pH leads to higher dissolution rate Solution

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

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

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 –Exothermic Reaction Dies –Endothermic Preventing Explosions Preventing Stalling

Equilibrium Reactor- Temperature Effects Single Equilibrium aA +bB  rR + sS –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 i s ) /RT] –γ i = activity coefficient of i –V i =Partial Molar Volume of i Van’t Hoff eq. y i (x i ) is smaller due to Impurities

Kinetic Reactors - CSTR & PFR – Temperature Effects Used to Size the Reactor Used to determine the reactor dynamics Reaction Kinetics C i 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 r K 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 r K 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, ΔH rxn Heat Balance over Reactor Cooling Adiabatic 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 A  B A Bulk Fluid External Surface of Catalyst Pellet Catalyst Surface Internal Surface of Catalyst Pellet C Ab C As 2. Defined by an Effectiveness Factor 1.External Diffusion Rate = k C (C Ab – C AS ) 3. Surface Adsorption A + S A.S 4. Surface Reaction 5. Surface Desorption B. S B + S 6. Diffusion of products from interior to pore mouth B 7. Diffusion of products from pore mouth to bulk

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)

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

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 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

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

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?

Ultra-high purity Si plant design Fluid Bed Reactor ( C) Si+7HCl  SiHCl 3 + SiCl 4 +3H 2 Si+ 2HCl  SiH 2 Cl 2 Reactor (1200C) SiHCl 3 +H 2  Si+3HCl SiH 2 Cl 2 +1/2 H 2  Si+3HCl Si at 99.97% Powder Si at % Separation Train Flash H 2 & HCl H2H2 Very Pure SiHCl 3 &SiH 2 Cl 2 H 2 -HCl Separation HCl SiCl 4 HCl Fluid Bed Reactor(600C) Si+SiCl 4 +2HCl  2SiHCl 3 HCl SiCl 4 Si H2H2 Flash

Chemical Vapor Deposition of Si

Chlorosilane Separation System ComponetBP H 2 − °C SiH C HCl −85.05°C SiHCl 3 -30°C SiH 2 Cl 2 8.3°C SiCl °C Si 2 Cl 6 145°C - polymer ImpuritiesBP BCl °C PCl °C AlCl 3 182°C Product

Ultra-high purity Si plant design Fluid Bed Reactor ( C) Si+7HCl  SiHCl 3 + SiCl 4 +3H 2 Si+ 2HCl  SiH 2 Cl 2 Reactor (1200C) SiHCl 3 +H 2  Si+3HCl SiH 2 Cl 2 +1/2 H 2  Si+3HCl Si at 99.97% Powder Si at % Separation Train Flash H 2 & HCl H2H2 Very Pure SiHCl 3 &SiH 2 Cl 2 H 2 -HCl Separation HCl SiCl 4 HCl Fluid Bed Reactor(600C) Si+SiCl 4 +2HCl  2SiHCl 3 HCl SiCl 4 Si H2H2 Flash

Separation Systems

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

Impurities in Recycle Loop

Failure of Flash to do its job, H 2 recycle is fed to Reactor Both Product 1 & 2 are liquid products so there is not place for H 2 to leave Column.