04h Dew Point Corrosion.ppt
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The document discusses dew point corrosion caused by the condensation of water vapor and corrosive compounds like HCl, NH4Cl, and NH4HS. Major issues occur when these compounds dissolve in water droplets at dew points, causing fouling or acidic/basic corrosion if the water wash is insufficient. Problem areas include crude unit and FCC overheads where chlorides and ammonia from feedstocks can react. Maintaining sufficient water washing is key to preventing fouling and mitigating under deposit corrosion.
04h Dew Point Corrosion.ppt
- 1. uop Corrosion and Fouling Dew Point Corrosion Jun 2002 1 Dew Point Corrosion
- 2. uop Corrosion and Fouling Dew Point Corrosion Jun 2002 2 Dew Point • Normally defined as the temperature and pressure at which a vapor condenses – e.g. water vapor => liquid water • Sometimes refers to sublimation point – e.g. ammonium chloride vapor => solid • We will also consider boil dry condition
- 3. uop Corrosion and Fouling Dew Point Corrosion Jun 2002 3 Major Dew Point Issues • HCl dissolution into water at water dew points in unit overheads • Ammonium chloride sublimation from vapor and relation to water dew point • Ammonium bisulfide sublimation from vapor and relation to water dew point • Insufficient water wash volume
- 4. uop Corrosion and Fouling Dew Point Corrosion Jun 2002 4 HCl Formation • HCl(g) + H2O ==> H3O+ + Cl- – First drop of water will be a saturated solution • pH will be in the negative range • Additional water condensation dilutes solution and makes problem less severe
- 5. uop Corrosion and Fouling Dew Point Corrosion Jun 2002 5 Principal Problem Areas • Crude column overhead – Chlorides from crude – Water from crude and distillation column • FCC main column overhead – Chlorides from distillation column – Water from stripping steam in reactor, main column
- 6. uop Corrosion and Fouling Dew Point Corrosion Jun 2002 6 Ammonium Chloride Formation • NH3(g) + HCl(g) <=> NH4Cl(g) • NH3 may be added as a neutralizer – Overheads of crude units and main columns of other units • NH3 may be formed from nitrogen and hydrogen in high temperature reactors – N2 + 3H2 <=> 2NH3 – Reformers, Hydrotreaters
- 7. uop Corrosion and Fouling Dew Point Corrosion Jun 2002 7 Principal Problems • As temperature drops NH4Cl sublimates – Causes fouling • Water wash will correct fouling – Acidic, corrosive solution • Insufficient water wash – Damp solid – severe under deposit corrosion
- 8. uop Corrosion and Fouling Dew Point Corrosion Jun 2002 8 Ammonium Bisulfide Formation • High temperature, catalyst, nitrogen, sulfur, hydrogen – N2 + 3H2 <==> 2NH3 – S + H2 <==> H2S • As gases cool – NH3 + H2S <==> NH4HS
- 9. uop Corrosion and Fouling Dew Point Corrosion Jun 2002 9 Principal Problems • As temperature drops NH4HS sublimates – Causes fouling • Water wash will correct fouling – Basic, corrosive solution/slurry • Insufficient water wash – Damp solid – severe under deposit corrosion
- 10. uop Corrosion and Fouling Dew Point Corrosion Jun 2002 10 Ammonium Bisulfide Issues • Corrosivity of water solution function of – Kp in vapor – Liquid concentration – Temperature – Velocity
- 11. uop Corrosion and Fouling Dew Point Corrosion Jun 2002 11 Ammonium Bisulfide Issues • Kp (mol% H2S x mol% NH3 in dry gas) – No corrosion to carbon steel if Kp < 0.02 • Kp can be higher for alloy material
- 12. uop Corrosion and Fouling Dew Point Corrosion Jun 2002 12 Refinery Examples • Reactor effluent air coolers (REACs) for hydrotreaters and hydrocrackers • Sour water stripper overhead • FCC fractionation section
- 13. uop Corrosion and Fouling Dew Point Corrosion Jun 2002 13 Solutions • Water wash – Prevent fouling (NH4Cl, NH4HS) – Reduce corrosivity (NH4Cl, NH4HS, HCl)