pyResToolbox MCP Server

Solve Rachford-Rice equation for vapor-liquid equilibrium.

PHASE BEHAVIOR TOOL - Calculates vapor fraction (beta) and phase compositions for two-phase flash at specified pressure and temperature. Essential for compositional analysis, separator design, and phase behavior calculations.

Parameters:

• zis (list, required): Overall mole fractions of components (0-1). Must sum to 1.0. Length must match Kis. Example: [0.5, 0.3, 0.2] for 3 components.

• Kis (list, required): Equilibrium ratios (K-values) for components. Ki = yi/xi where yi = vapor mole fraction, xi = liquid mole fraction. Length must match zis. Example: [2.5, 1.8, 0.6]. K > 1 = light component.

Rachford-Rice Equation: Σ[zi(Ki - 1) / (1 + β(Ki - 1))] = 0

Where:

• zi = overall mole fraction of component i

• Ki = equilibrium ratio (yi/xi) for component i

• β = vapor mole fraction (0 to 1)

Phase Behavior:

• β = 0: All liquid (subcooled)

• 0 < β < 1: Two-phase (vapor + liquid)

• β = 1: All vapor (superheated)

K-Value Behavior:

• K > 1: Component prefers vapor phase (light components)

• K = 1: Component equally distributed (critical component)

• K < 1: Component prefers liquid phase (heavy components)

• K-values depend on pressure, temperature, and composition

Solution Method: Iterative Newton-Raphson method with bounds checking (0 ≤ β ≤ 1). Converges rapidly for well-posed problems. Typically converges in 3-10 iterations.

Applications:

• Gas-Oil Separator Design: Determine separator conditions for phase split

• Phase Envelope: Calculate bubble/dew points and phase boundaries

• Compositional Simulation: Flash calculations in compositional models

• EOS Flash: Solve equation of state flash calculations

• Surface Facility Design: Design separation trains and processing units

• Material Balance: Phase split in material balance calculations

Returns: Dictionary with:

• vapor_fraction (float): Vapor mole fraction β (0-1)

• liquid_composition (list): Liquid phase mole fractions xi

• vapor_composition (list): Vapor phase mole fractions yi

• method (str): "Rachford-Rice (Newton-Raphson)"

• note (str): Interpretation guidance

• inputs (dict): Echo of input parameters

Common Mistakes:

• Mole fractions don't sum to 1.0 (must normalize)

• K-values don't match components (length mismatch)

• K-values at wrong P-T conditions (must match flash conditions)

• Using weight fractions instead of mole fractions

• Not accounting for non-hydrocarbon components

• K-values from wrong correlation/EOS

Example Usage:

Result: β ≈ 0.3-0.5 (two-phase), with light components enriched in vapor, heavy components enriched in liquid.

Note: Rachford-Rice equation assumes ideal mixing. For real systems, K-values must account for non-ideality (activity coefficients, fugacity). K-values are typically obtained from EOS (Peng-Robinson, Soave-Redlich-Kwong) or correlations (Wilson, Standing). Always ensure K-values match flash conditions.