Steam Reforming Plant – Comprehensive Exam Notes

BASICS / KEY TERMS

• Endothermic reaction: $\text{Heat is absorbed}$ to drive the chemical change.
• Exothermic reaction: $\text{Heat is released}$ during the chemical change.
• Adsorption: Temporary surface-level uptake of molecules; release achieved by changing process conditions.
• Absorption: Bulk, usually irreversible (until saturation) uptake of molecules in another phase.
• Catalyst: Substance that accelerates reaction rate without being consumed.

CHARACTERISTICS OF HYDROGEN

• Physical properties
  • Lightest chemical element, first in the periodic table.
  • $\text{No colour, odour, or flavour}$ → undetectable by human senses.
  • Highly flammable; needs only minimal ignition energy (e.g., static electricity).
  • Flame is almost invisible (light-blue).
  • Exists as liquid at cryogenic conditions; liquid H₂ causes severe cold burns.
  • Diffuses rapidly through metals (carbon steel, stainless steel) and non-metals → leakage risk.
• Chemical behaviour
  • Generally non-corrosive and non-toxic.
  • Powerful reducing agent.
  • Can create asphyxiation hazards by displacing $21\%$ of $\mathrm{O_2}$ in unventilated spaces.
• Safety envelope
  • Auto-ignition temperature for H₂/O₂ mixtures ≈ $(500\,^{\circ}\mathrm{C})$ (varies with $P,T,\text{humidity}$).
  • Flammability window in air at $1\,\text{atm}$: $4\%$–$74\%\,\text{(v/v H}_2)$.
  • Lower heat of combustion than most domestic gases → lower energy density per unit volume.

HYDROGEN GENERATION METHODS

• Steam–Methane Reforming (SMR) – dominant industrial route.
• Electrolysis – splitting water with electricity.
• Ammonia cracking – $\mathrm{NH<em>3 \rightarrow 1.5\,H</em>2 + 0.5\,N_2}$.
• By-product streams from other chemical processes.

INDUSTRIAL & EMERGING APPLICATIONS

• Classical uses: edible-oil hydrogenation, petrochemical selective hydrogenations, pharma syntheses, high-T welding/cutting, float-glass, metal oxide reduction (sponge iron), chemical & polymer syntheses (polyethylene, polypropylene, methanol, sorbitol, ammonia), electronics, cosmetics, uranium redox, quartz cutting.
• Energy transition / "New Millennium" targets:
  • Distributed power for buildings, industry, satellites, space transport.
  • Fuel-cell vehicles, buses, back-up generators.
  • Establishment of hydrogen filling stations; on-site generation units (e.g., PAE Generator 4000 delivering $10{,}000\,\text{SCFH}$ at $5000\,\text{psig}$ for quick-fill dispensers).

HIGH-LEVEL PROCESS FLOW (ONSITE SMR PLANT)

1. Feed pre-treatment / Desulfurisation
2. Primary reforming (Steam-Methane Reformer)
3. High-Temperature Shift (HTS) converter
4. Gas cooling & condensate knock-out (K.O. Drum)
5. Steam recovery (Waste-Heat Boiler; Mixed-Feed Coil)
6. Hydrogen Purification (PSA)
7. Vent/Fuel management (Reformer burners)

DESULFURISATION SYSTEM

• Purpose: Remove $\mathrm{H_2S}$ & organo-sulfur species from Natural Gas/LPG/Naphtha feeds to protect downstream catalysts.
• Sequence
  1. Hydro-desulfurisation (HDS) – catalytic, endothermic.
     $\mathrm{R{-}SH + H<em>2 \rightarrow R{-}H + H</em>2S}$ (Ni–Mo catalyst at $300\,^{\circ}\mathrm{C},\;17.1\,\mathrm{kg\,cm^{-2}}$).
  2. ZnO guard bed (adsorption/absorption)
     $\mathrm{ZnO + H<em>2S \rightarrow ZnS + H</em>2O}$
  • Outlet S-specification: <3\,\text{ppm}_{\text{w}}.
• Zinc Oxide pellets (white spheres)
  • Composition: $90!{-}95\%\,\mathrm{ZnO}$, $5!{-}10\%\,\mathrm{Al<em>2O</em>3}$.
  • Benefits: solid, safe, high capacity, low OPEX.

STEAM REFORMER

• Principal reactions (endothermic, Ni/La₂O₃ catalyst within $5/8''$ tubes):
  1. $\mathrm{CH<em>4 + H</em>2O \rightleftharpoons CO + 3\,H_2}$
  2. $\mathrm{CO + H<em>2O \rightleftharpoons CO</em>2 + H_2}$ (water–gas shift inside reformer equilibrium)
• Operating envelope
  • Feed: sulfur-free NG + saturated steam at $300{-}350\,^{\circ}\mathrm{C}$, $15.3\,\mathrm{kg\,cm^{-2}}$.
  • Furnace tube outlet: $820{-}830\,^{\circ}\mathrm{C}$, $15\,\mathrm{kg\,cm^{-2}}$.
• Performance notes
  • Higher temperature → higher conversion, lower methane slip but thermal-stress risk.
  • Catalyst health monitored by $\Delta T_{\text{in-out}}$, $\Delta P$, and methane slip.
  • Catalyst matrix: $20!{-}30\%\,\mathrm{NiO}$, $1!{-}2\%\,\mathrm{La<em>2O</em>3}$ on $70!{-}80\%$ α-alumina carrier.

HIGH-TEMPERATURE SHIFT (HTS) CONVERTER

• Reaction (exothermic): $\mathrm{CO + H<em>2O \rightarrow CO</em>2 + H_2}$.
• Catalyst: Iron–Chromium oxide on graphite.
  • Typical composition: $80!{-}85\%\,\mathrm{Fe<em>2O</em>3}$, $8!{-}10\%\,\mathrm{Cr<em>2O</em>3}$, $1!{-}2\%\,\mathrm{CuO}$, $1.5!{-}3\%$ graphite.
• Operating window
  • Inlet: $357\,^{\circ}\mathrm{C}$, $16\,\mathrm{kg\,cm^{-2}}$ ; Outlet: $413\,^{\circ}\mathrm{C}$, $15.8\,\mathrm{kg\,cm^{-2}}$.
  • Normal temperature rise: $+40^{\circ}\mathrm{C}$. Deviations signal deactivation.
• Attributes: high poison resistance, robust, low steam demand, best-in-class CO removal at $330{-}480\,^{\circ}\mathrm{C}$.

CONDENSATE SEPARATION (K.O. DRUM)

• Cools crude H₂ stream from $37^{\circ}\mathrm{C}$ (inlet) to $37^{\circ}\mathrm{F}$ (outlet) at $15\,\mathrm{kg\,cm^{-2}}$.
• Demister & level control essential; high-level alarms prevent liquid carry-over to PSA.

WASTE-HEAT BOILER / STEAM NETWORK

• Stack gas (reformer) enters at $870^{\circ}\mathrm{C}$, exits at $260^{\circ}\mathrm{C}$.
• Generates $17.24\,\text{bar}_\mathrm{g}$, $207^{\circ}\mathrm{C}$ saturated steam.
• Continuous blow-down at same pressure; always maintain water level & pressure to avoid tube damage and plant-wide pressure perturbations.
• Mixed-Feed Coil further heats NG+steam mix to $371^{\circ}\mathrm{C}$, $16.5\,\text{bar}_\mathrm{g}$ prior to entering reformer tubes.

PSA HYDROGEN PURIFICATION SYSTEM

• Feed ("crude" H₂):
  \begin{aligned}
  &\mathrm{H2}\;79.0\%\ &\mathrm{CO2}\;18.0\%\
  &\mathrm{CO}\;1.0\%\
  &\mathrm{CH4}\;0.5\%\ &\mathrm{N2}\;1.3\%\
  &\mathrm{H2O}\;0.2\%\;\text{(vapor)} \end{aligned} at $30{-}40^{\circ}\mathrm{C}$, $14.1\,\text{bar}</em>\mathrm{g}$.
• Product purity: $99.99\% \;\mathrm{H_2}$.
• Typical vessel configuration: $3{-}5$ identical beds cycling through Adsorption → Depressurisation → Purge (with product H₂, counter-current) → Re-pressurisation.
• Layered bed design (top→bottom): Ceramic balls → Activated alumina (water removal) → Activated carbon (CO, CO₂, CH₄) → Molecular sieves (N₂, residual CO).
• All impurities desorb during purge and exit as vent gas.
  • Vent composition (to burner): $\mathrm{CH<em>4}\;7.0\%$, $\mathrm{CO</em>2}\;45.7\%$, $\mathrm{CO}\;6.6\%$, $\mathrm{H<em>2}\;39.8\%$, $\mathrm{H</em>2O}\;0.9\%$ at $0.20{-}0.82\,\text{bar}_\mathrm{g}$.
  • Vent collected in low-pressure tank, then combusted in reformer furnace → offsets natural-gas demand.

CYCLIC ADSORBER OPERATION (MOLECULAR SIEVE STAGE ILLUSTRATION)

• Adsorption step: high-pressure feed flows downward; impurities retained in pores.
• Purge/Regeneration: Back-flush with purified H₂ at lower pressure; impurities sweep out opposite direction.
• Re-pressurise with product H₂ to align pressure for next adsorption cycle.
• Continuous sequencing via automated valves and timers ensures steady $99.99\%$ product.

CATALYSTS & ADSORBENTS – SUMMARY TABLE

• ZnO guard bed → permanent $\mathrm{H_2S}$ removal; cheap, safe.
• Ni/La₂O₃ reformer catalyst → high-activity methane cracking; sensitive to S-poisoning.
• Fe–Cr HTS catalyst → robust CO shift; highly poison-resistant.
• PSA adsorbents → multilayer selectivity for sequential impurity trapping.

PRACTICAL, ETHICAL & SAFETY IMPLICATIONS

• Hydrogen provides a low-carbon pathway for heat, transport, and power but poses unique leak/ignition hazards → rigorous detection & ventilation protocols are ethical imperatives to protect personnel and communities.
• Integrated heat recovery (waste-heat boiler, vent-gas firing) maximises energy efficiency, aligning with sustainable-operation principles.
• Catalyst stewardship (regeneration, proper disposal of spent ZnS, metal oxides) mitigates environmental impact.

KEY NUMERIC QUICK REFERENCE (EXAM CRIB)

• Desulfuriser outlet spec: <3\,\text{ppm S}_{\text{w}}.
• Reformer tube outlet T: $820{-}830^{\circ}\mathrm{C}$.
• HTS ΔT: $+40^{\circ}\mathrm{C}$ (normal rise).
• PSA product: $99.99\%\,\mathrm{H_2}$ at $14\,\mathrm{kg\,cm^{-2}}$, $32{-}40^{\circ}\mathrm{C}$.
• Flammability window: $4\%{-}74\%\,\mathrm{v/v}$.
• Auto-ignition: $500^{\circ}\mathrm{C}$.

STUDY TIPS & CONNECTIONS

• Relate steam reforming equilibrium to Le Châtelier’s principle: higher $T$ drives reforming endothermically; higher $P$ favours methane (mole reduction) → hence moderate pressure chosen.
• Compare HTS chemistry to low-temperature shift (not in slides) to understand full CO removal chain.
• PSA concept mirrors chromatography: selective adsorption by molecular size/polarity.
• Link safety data (diffusion, flammability) to design of leak-proof vessels & flame detectors in earlier process-safety lectures.