Simplified Design Requirements for Gas and Liquid Dehydration and Hydrate Inhibition Systems

Introduction

Gas and liquid dehydration, along with hydrate inhibition systems, are critical for ensuring hydrocarbon streams meet downstream processing and quality requirements. Without these systems, facilities such as gas plants, refineries, and fractionation units face challenges like corrosion, hydrate formation, and costly equipment damage.

This article provides clear, simplified design guidelines based on international standards. It covers adsorption, absorption, and hydrate inhibition processes, making it easier for engineers and operators to apply best practices.

Scope

These requirements apply to:

• Dehydration of natural gases, hydrocarbon liquids, refinery off-gases, and CO₂ streams.
• Processes covered:
  • Adsorption: Molecular sieve, activated alumina, silica gel.
  • Absorption: Di-ethylene glycol (DEG), tri-ethylene glycol (TEG), tetra-ethylene glycol (TREG).
  • Hydrate Inhibition: Mono-ethylene glycol (MEG) injection.
• Exclusions: Crude oil dehydration, kinetic hydrate inhibitors, and instrument air dehydration.

The overall goal is simple: remove water and prevent hydrate formation to guarantee safe, efficient operations.

Key Design Requirements

1. General Principles

• Assume water saturation: Always design as if hydrocarbon streams are fully saturated unless accurate data exists.
• Process selection:
  • Adsorption (molecular sieve): For ultra-dry gas (≤0.1 ppmv water).
  • Absorption (TEG): For cost-effective gas dehydration (≤7 lb/MMSCF).
  • Hydrate inhibition (MEG): For pipelines or facilities exposed to hydrate risk.
• Standards to follow:
  • API 12 GDU (Glycol-Type Gas Dehydration).
  • NACE MR0175 / ISO 15156 (Materials for H₂S service).

Process Design Requirements for Produced Water Treatment and Disposal at Gas Oil Separation Plants

2. Solid Desiccant Systems (Molecular Sieve, Activated Alumina, Silica Gel)

• Material selection:
  • Use 4A molecular sieves for gas/liquid dehydration.
  • For sweet gas (H₂S <16 ppmv), standard 4A is acceptable; for sour gas, choose acid-resistant sieves.
  • Never mix desiccant suppliers, types, or sizes in the same bed.
• System setup:
  • At least two beds per system (3–5 towers for gas: one regenerating, others drying).
  • Vessels must include bottom manways (for removal) and top spools (for loading).
  • Ensure lifting access for safe handling.
• Operating conditions:
  • Feed gas temperature: <80°F (27°C) and ≥5°F (3°C) above hydrate point.
  • Pressure drop: ≤0.44 psi/ft (≤9.95 kPa/m); total ≤10 psi (69 kPa).
  • Regeneration gas: ≥550°F (288°C).
• Safety & instrumentation:
  • Install flow meters, moisture analyzers, and upstream/downstream filters.
  • Use zero-leakage valves rated >600°F (316°C).
  • Store desiccants in dry, shaded, dust-free areas.

3. Liquid Desiccant Systems (TEG)

• Material & process:
  • Use ≥98.5% pure TEG.
  • Aim for ≤7 lb/MMSCF water in dry gas.
  • Minimize TEG losses (1–3.5 lb/MMSCF).
• System design:
  • Use structured packing or trays in the contactor.
  • Add stripping gas to reach ≥99.5% lean TEG.
  • Reboiler capacity: 130–140% of design duty.
  • Install dual 100% pumps for circulation reliability.
• Operating conditions:
  • Circulation: 3 gal TEG/lb water removed.
  • Reboiler: 380–400°F (with stripping gas).
  • Inlet gas: 80–100°F.
• Ancillary features:
  • Filters for gas and glycol.
  • Proper disposal of condensed water (flare, oxidizer, or evaporation pond).
  • Nitrogen/dry gas blanket in TEG storage tanks.

4. Hydrate Inhibition Systems (MEG)

• Material: Use regenerated MEG at ≥80% purity.
• Injection design:
  • Base rates on worst-case pressure/temperature with a 5°F (3°C) safety margin and 15% extra dose.
  • Avoid MEG use below -40°F (-40°C).
• System design:
  • Reboiler: 130–140% duty, ≤324°F (162°C).
  • Use dual circulation pumps.
  • Include reflux condenser, reclaimer (if salts >1,000 ppmw), and filters.
• Ancillary systems:
  • Handle vapor/condensed water like TEG.
  • Nitrogen/dry gas blanket in MEG tanks.
  • Methanol injection as a backup for hydrate prevention.

5. Documentation & Responsibilities

• Engineers must provide:
  • Loading diagrams, data sheets, procedures, and PFDs/P&IDs.
• Responsibilities:
  • Design Engineer: Define process & mechanical specs.
  • Manufacturer: Build to spec and test.
  • Technical Team: Review and validate.
  • Project Team: Select vendors & manage integration.

Conclusion

By following these simplified design requirements and adhering to international standards such as API 12 GDU and NACE MR0175/ISO 15156, engineers can ensure:

• Safe, reliable dehydration (molecular sieve, TEG).
• Effective hydrate prevention (MEG).
• Optimized downstream performance without corrosion or equipment damage.

These systems provide the backbone of efficient and safe hydrocarbon processing, ensuring facilities run smoothly under demanding conditions.