2.4 Phase diagrams and their applications

Phase diagrams are essential tools in separation processes, mapping out how substances behave under different conditions. They show phase transitions, equilibrium states, and composition relationships, helping engineers design efficient separation systems.

These diagrams come in various types, each offering unique insights. From pressure-temperature plots to composition diagrams, they guide the analysis of complex systems and inform critical decisions in distillation, extraction, and crystallization processes.

Phase Diagrams and Their Applications

Types of phase diagrams

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  Txy Diagram – Foundations of Chemical and Biological Engineering I View original

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  Phase Diagram – Foundations of Chemical and Biological Engineering I View original

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  Txy Diagram – Foundations of Chemical and Biological Engineering I View original

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  Txy Diagram – Foundations of Chemical and Biological Engineering I View original

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• Pressure-Temperature (P-T) diagrams map solid, liquid, and gas phase regions pinpoint triple point and critical point illustrate phase boundaries and coexistence lines
• Temperature-composition (T-x-y) diagrams depict bubble point and dew point curves show composition of liquid and vapor phases demonstrate tie lines and lever rule applications
• Composition-composition (x-y) diagrams reveal equilibrium relationships between liquid and vapor phases include diagonal line (y = x) for reference highlight deviation from ideality (azeotropes, partial miscibility)

Phase equilibrium determination

• Gibbs' Phase Rule $F = C - P + 2$ calculates degrees of freedom (F) based on components (C) and phases (P) guides analysis of system variability
• Binary systems analysis using T-x-y diagrams determines coexisting phase compositions calculates relative phase amounts aids in distillation column design
• Ternary systems evaluation with tie lines locates overall composition point determines equilibrium phase compositions useful for liquid-liquid extraction
• Multicomponent systems interpretation uses isothermal and isobaric sections visualizes complex phase behavior applies to petroleum refining and chemical separations

Applications in separation processes

• Distillation column design leverages T-x-y diagrams determines minimum reflux ratio calculates theoretical stages optimizes feed tray location improves energy efficiency
• Liquid-liquid extraction utilizes ternary diagrams identifies immiscibility region determines distribution coefficients calculates extraction efficiency enhances solvent selection
• Crystallization process design employs solid-liquid equilibrium diagrams analyzes eutectic points and compositions determines maximum theoretical yield optimizes cooling profiles
• Three-phase distillation considers VLLE identifies heterogeneous azeotropes optimizes operating conditions for phase splitting improves separations of close-boiling mixtures

Concepts in phase behavior

• Azeotropes in binary systems appear as minimum or maximum boiling points form due to molecular interactions broken by pressure swing or entrainer addition (extractive distillation)
• Miscibility gaps in liquid-liquid systems exhibit upper and lower critical solution temperatures (UCST, LCST) vary with temperature affect extraction and purification processes
• Critical points in phase behavior mark transition to supercritical fluid exhibit unique properties utilized in supercritical extraction (caffeine removal)
• Retrograde condensation in gas-condensate systems occurs in specific pressure-temperature regions impacts natural gas processing requires careful well management