Inherent Safety in Process Design

Basic Concepts

  • Inherent Safety vs. Extrinsic Safety
    • Inherent safety is the philosophy of designing processes that inherently eliminate hazards away such that external safety systems are not needed.

Examples of Inherent Safety Failures

  • Example #1: The Bhopal Disaster (1984)

    • Location: Bhopal, India.
    • Accident: Thousands killed due to the release of methyl isocyanate (MIC).
    • Reactions involved:
    • Desired reactions:
      • extCH3extNH2+extCOCl2<br/>ightarrowextCH3extN=extC=extO+2extHClext{CH}_3 ext{NH}_2 + ext{COCl}_2 <br /> ightarrow ext{CH}_3 ext{N}= ext{C}= ext{O} + 2 ext{HCl} (Methylamine and Phosgene produces Methyl Isocyanate)
      • extCH3extN=extC=extO+extb−Naphthol<br/>ightarrowext1−Naphthyl−N−methylcarbamate(Carbaryl)ext{CH}_3 ext{N}= ext{C}= ext{O} + ext{b-Naphthol} <br /> ightarrow ext{1-Naphthyl-N-methylcarbamate (Carbaryl)}
    • Undesired reactions could occur if conditions change, all reactions being exothermic.
    • Design Failures:
    • Poor choice of reaction scheme.
    • Large storage of methyl isocyanate.
    • Plant located in a residential area.
    • Multiple operational and extrinsic safety errors during operations contributed to the disaster.

  • Example #2: Texas City Explosion (2005)

    • Scenario: A raffinate splitter tower in an isomerization unit.
    • Issues during startup led to rising liquid levels and excessive temperatures.
    • Reaction: Increasing heat and bottoms flowrate along with the operating conditions.
    • Consequences: Attempting to generate vapor to lower liquid levels created a series of operational failures, leading to an explosion.

Conceptual Design Approaches to Improve Process Safety

  • Timing for Safety Consideration:
    • Safety must be part of the conceptual design stage right from the beginning and be revisited in final designs.
Heuristics for Designing Inherently Safe Plants

  1. Material Selection:

    • Choose safer solvents, separating agents, intermediates, and materials of construction.
    • Avoid the use of hazardous substances that can leak (corrosives, unstable materials).

  2. Pressure Control:

    • Risks of High/Low Pressure:
      • High pressures can lead to rupture; low pressures can cause vacuum conditions, risking tank collapse.
    • Underpressure scenarios causing issues include:
      • Vents blocked causing vacuum from discharge pumps.
      • Reaction events due to contamination or external temperature conditions.
      • Vessel corrosion due to inadequate material or environment.
      • Physical collision with the vessel thus causing rupture.

  3. Control of Ignition Sources:

    • Common ignition sources in chemical plants include:
      • Furnaces and operations involving flames.
      • Layouts allowing people or cars to enter flammable zones.
      • Electrical equipment sparking; even non-conductive materials moving can generate static charge.

  4. Temperature Deviations:

    • Conceiving designs that consider risks of runaway reactions, operations after shutdown, or contaminant-driven reactions.
    • Prepare for potential fires from other units or failures in control systems.
    • Material selection must withstand worst-case temperature scenarios to maintain safety.

Conclusions

  • Safety in chemical processing must begin from conceptual design.
  • The selection of process chemistry is crucial for ensuring plants are inherently safe.
  • Efficiency must be balanced with the flexibility to handle operational deviations.
  • Careful control of temperature and pressure is vital, underscoring safety as the primary consideration in chemical processes.