Transcript Document
Materials of Construction in the CPI A Continuing Story of Success Characteristics of the CPI • Major contributor to world economy • Producer of a variety of products • Produces key components for many industries • 200 + years and growing Chemical Processes Conditions • Extreme temperature range: –250 to 1400°C (LH2, POX ) • Extreme pressure range: vacuum to 4000 + psi (TDA distillation, Urea synthesis) • Large volumes of materials: 1 million gallons or more Characteristics of Chemicals Produced • • • • • Toxic (HCN, H2S) Corrosive (HF, HCl, H2SO4, NaOH) Flammable (Propane, Benzene) Explosive (DNT, NH4NO3) Carcinogenic CPI Operations • • • • • Raw materials handling Unit operations Storage Shipment Utilities Containment is Vital • • • • • Prevent leaks Secondary containment of leak Remediation of area affected Replacement of defective parts Disposal of contaminated product Materials of Construction • We rely upon M/C for containment • Very few loss of containment incidents from M/C failures • Reliability has increased dramatically in the last 40 years Question: How has industry managed to achieve this in view of the difficult conditions and cost pressures? Answer: (Later) Example: H2SO4 in Nitration Process • Nitration of toluene: C7H8 + 2HNO3 C7H6(NO2)2 + 2H2O • Sulfuric Acid ties up H2O H2O + H2SO4 H2SO4•H2O Nitration of Toluene • Dinitrotoluene, DNT, is important urethane intermediate • Sulfuric acid regeneration is key to process economics • Thermal regeneration used • Problem: How to contain hot acid? Chemico Process • Uses hot flue gas in countercurrent exchange • Acid brick + membrane • High silicon iron (Duriron®) or high silicon nickel (Hastelloy D®) fire tubes Problems with Chemico Process • Tubes often fracture – brittle material + thermal shock • Brick lining is difficult to maintain • Process is inefficient • Acid mist in exit gas pollutes Chemetics Process • • • • 5 Stage vacuum evaporation process Glass lined vessels Tantalum heater tubes (steam) Venturi steam jet + barometric condenser vacuum system Problems with Chemetics Process • Corrosion of expansion bellows (321SS+Teflon® lining) • Corrosion of venturi expander (Alloy 20Cb3 general thinning) Solution to Chemetics Problems • Up[grade expansion bellows to Alloy 20Cb3 • Upgrade venturi expander to Hastelloy B-2® Storage Tanks for Acid • Vertical cylindrical tanks • Carbon steel construction • Required to hold 72%, 93%, and 98% sulfuric acid Corrosion of Steel in Acid • 93% and 98% acid: H2SO4 + Fe FeSO4(s) + H2(g) • 72% acid: H2SO4•H2O + Fe FeSO4•H2O(s) + H2(g) • Reactions are very fast Conclusions from Rate Studies • Corrosion rates are reasonable for all three acids • Corrosion allowance required • Corrosion rate determined by dissolution of FeSO4 • Issue of velocity accelerated corrosion • Issue of corrosion product wedging Design Issues for Acid Tanks • • • • • • Inlet and exit nozzles Impingement onto walls of tank Hydrogen grooving Eliminate crevices Sunny side corrosion Prevent acid dilution! Sulfuric Acid Pipelines • • • • • Carbon steel – heavy wall Limit velocity – 2 ft/sec max Limit temperature Use long radius elbows Minimize flow disturbances from welds Problems with C Steel Pipelines • Maximum velocity limits production • Weld roughness causes damage • Accelerated corrosion at inlet end Corrosion Rate Prediction • Dissolution of iron sulfate • Mass transfer analogy (Chilton, Colburn) • Prediction confirmed by measurements CR = F(T,V,,,d)[w – Fe] Upgrades • • • • For 72%, use Alloy 20Cb3 For 93 and 98%, use 316L SS Reliability improved Iron content of acid reduced Answer to Question • Designs that incorporate features to prevent problems • Using industry standards (NACE, API) • Maintaining awareness of industry experience (MTI) • Inspection and upgrade to eliminate problems Conclusions • Higher cost materials are often justified by improved safety and reliability • Newer materials solve traditional problems • Designs can minimize materials problems THANK YOU! References 1. Shreve,N., “The Chemical Process Industries”,McGraw-Hill, 1956 2. Dillon,C.P.”M.S.for H.C.”, Vol.1 Sulfuric Acid, MTI, 1997 3. NACE RP02-94 “Design, Fabrication and Inspection etc.” 1994