Feedstock Purification Flowsheet Options
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Consider hydrocarbon purification section * consists of three parts in sequence 1) Hydrogenation or Hydrodesulfurization * catalyst converts organic sulfur compounds (and any organo-chlorides) to H2S (and HCl) using H2 added just upstream (usually product recycle) 2) Chloride removal - required only if Cl in hydrocarbon feed * any Cl in the hydrocarbon should be now HCl * solid alkaline absorbent removes HCl by chemical reaction 3) Sulfur removal * any S in the hydrocarbon should now be H2S * ZnO based absorbent removes H2S by reaction to form ZnS
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Feedstock Purification Flowsheet Options
- 1. Feedstock Purification Flowsheet Options By: Gerard B. Hawkins Managing Director, CEO
- 2. Simplified Steam Reforming NH3 Plant H2O H/C feed H/C purification Removes impurities (S, Cl, metals) Steam reforming Converts to H2, CO, CO2 + H2O + CH4 H2O Shift WGS reaction: H2O + CO <=> CO2 + H2 H2 Hydrogen purification Removal of CO, CO2 + maybe CH4
- 3. Hydrocarbon Purification Section Historically – up to three parts • Hydrogenation or hydrodesulphurisation catalytic breakdown of organic sulphur compounds to H2S (also RCl to HCl) • Chloride removal (only if Cl present) - absorb HCl • Sulfur removal - absorb H2S Additionally – a fourth optional part • Ultrapurification Various designs depending on • feed composition; plant design
- 4. Hydrocarbon Purification Section H/C feed HDS Breaks down organo-S and RCl H2 HCl absorption Removes HCl by chemical reaction H2S absorption Removes H2S by chemical reaction Ultra- purification Polishes out trace S impurities
- 5. Hydrocarbon Feed Hydrogenation Sulfur Removal Chloride Removal Hydrocarbon Feed Sulfur Removal Hydrogenation Hydrocarbon Feed Hydrogenation Chloride Removal Sulfur Removal Hydrocarbon Purification Section - Typical Flowsheet Examples
- 6. Hydrocarbon Feed HDS ZnO Ultrapurification Hydrocarbon Purification Section - Ultrapurification Used with ZnO at usual operating conditions • ZnO removes bulk of sulfur (H2S) • Follow with a layer of ultrapurification for polishing
- 7. Hydrocarbon Purification Section - Flowsheet Different variants found across syngas plants HDS usually installed • Occasionally left out when total S is low and organo-S is very low (< 2 ppm as mercaptan, RSH) HCl removal less usual • More common in refinery H2 plants using off gas feed H2S removal always present
- 8. Hydrocarbon Purification Section - Flowsheet H2S removal – single bed or lead/lag ? • Single bed found where H2S (or total S to HDS) is low and predictable • E.g. gas purified to a pipeline spec’n of << 10 ppm • Bed must be a realistic size to last T/A interval • Otherwise lead/lag: design bed life 6 – 12 months Ultrapurification • Special situations – NOT for all • AND not installed in all the “special situations”
- 9. Hydrocarbon Purification Section – Flowsheet (cont.) Ultra-purification applications • Pre-reformers • Natural gas fed steam reformers stressed high heat flux; low steam:carbon ratio • Naphtha fed steam reformers low steam:carbon ratio • Precious metal steam reforming catalysts • GHRs