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Peng-Robinson Heat-Duty and Isenthalpic Flash Calculator

Solve PR outlet temperature and equilibrium compositions after a pressure drop with prescribed heat input; zero heat input gives an isenthalpic C1-C6 flash.

Inputs

Results

Calculate to view linked results. Default values are examples, not recommended operating inputs.

Learn, compare and practice

Worked explanations, controlled studies and printable exercises connected to this calculation.

PT PH and PS flash specifications explained

Calculation sequence

  1. Enter the inlet equilibrium pressure, temperature and C1-C6 mole fractions, then a lower or equal outlet absolute pressure and specific heat input. The entire feed is retained; this is a single equilibrium flash, not a train that removes vapor between stages. Positive q heats the fluid, negative q cools it. Zero q gives the PR isenthalpic throttling case; it does not hold temperature fixed. The synthetic default is not an operating recommendation.
  2. Calculate the fully checked inlet PT equilibrium and its molar enthalpy using the pinned native ideal-gas component functions plus cubic departure enthalpy. Mixture molecular weight converts specific heat from kJ/kg into J/mol. A two-phase enthalpy is a mole-fraction-weighted phase lever rule; native bulk and separately queried phase enthalpies must agree. Enthalpy reference offsets cancel only because the overall composition and reference convention are unchanged.
  3. At the specified outlet pressure, sample 25 temperatures from 180 to 450 K and require strictly increasing equilibrium enthalpy on that checked grid. Bracket the energy residual and use the existing fmin bisection solver, then enforce an absolute energy residual below 0.0001 J/mol. The monotonic grid is a numerical screening check, not a proof that every possible mixture has a unique physical PH root. An invalid trial flash or unbracketed target stops the calculation.
  4. Recalculate and fully audit the final outlet PT state, including material, fugacity, pressure and volume closure, before presenting its temperature and composition table. Single-phase results have no invented vapor quality. Temperature change is a temperature difference, not an absolute temperature. Saved work retains inlet conditions, outlet pressure, q and every feed fraction. The PDF includes the energy residual and phase allocations; CSV exports the component table.

Worked check

The synthetic PR case starts at 5 MPa and 330 K with fractions 0.50/0.15/0.15/0.10/0.05/0.05 and flashes to 1 MPa at q=0 kJ/kg. The solved outlet temperature is about 296.256406 K and vapor mole fraction is 0.840941, rather than holding temperature at 330 K. Add heat to compare evaporation, or remove heat to compare condensation within the accepted envelope. These uncalibrated demonstration values are not a measured separator benchmark.

Synthetic examples for reproducibility, not measured field results.

Assumptions and limitations

  • C1-C6 only with normal C4-C6, zero binary interaction parameters, no volume translation and the pinned ideal-gas caloric functions. No C7+ heat-capacity assumption, wet/sour gas, arbitrary mixture, chemical reaction, multiple liquids or solid phase. Agreement with an independent implementation does not replace measured-PVT validation.
  • Both pressures must be 0.1-10 MPa(a), outlet<=inlet; inlet and every trial/outlet temperature must remain 180-450 K. q is restricted to -500 to 500 kJ/kg and must admit an energy root in that temperature envelope. The existing endpoint near-critical/vanishing-phase and closure gates still apply. No shaft/compression work, kinetic or potential-energy changes, frictional pressure traverse or critical-flow prediction.
  • Steady instantaneous vapor-liquid equilibrium with prescribed pressure and specific heat, not a coupled well heat-transfer model, PH property API with an arbitrary external enthalpy reference, separator sizing, residence time or transient simulation. The outer bounded energy root is this application's wrapper around native PT flashes, not a claim of native general mixture PH support. Failure to bracket a root is reported, never extrapolated or clipped.

Implemented equations

Steady energy balance and temperature root

hˉout(po,To,z)=hˉin+qMzF(To)=hˉout−hˉin−qMz=0Mz=∑iziMi\begin{aligned}\bar h_{\rm out}(p_o,T_o,z)&=\bar h_{\rm in}+qM_z\\F(T_o)&=\bar h_{\rm out}-\bar h_{\rm in}-qM_z=0\\M_z&=\sum_i z_iM_i\end{aligned}

Caloric phase lever rule

hˉ=(1−β)hˉL+βhˉVhˉα=∑iciαhˉi0(T)+hˉαR\begin{aligned}\bar h&=(1-\beta)\bar h_L+\beta\bar h_V\\\bar h_\alpha&=\sum_i c_i^\alpha\bar h_i^0(T)+\bar h_\alpha^R\end{aligned}

Fugacity equilibrium and component conservation

xiφiL=yiφiVzi=(1−β)xi+βyiKi=yi/xi0=∑izi(Ki−1)1+β(Ki−1)\begin{aligned}x_i\varphi_i^L&=y_i\varphi_i^V\\z_i&=(1-\beta)x_i+\beta y_i\\K_i&=y_i/x_i\\0&=\sum_i\frac{z_i(K_i-1)}{1+\beta(K_i-1)}\end{aligned}

Sources and maintenance

Published by PetroCalcHub Team. Implementation and source checks: . This does not claim independent expert review.

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