Engineering lesson
PT PH and PS flash specifications explained
A PT flash specifies pressure and temperature. A PH flash specifies pressure and enthalpy and solves for temperature. A PS flash specifies pressure and entropy under its stated energy model.
By PetroCalcHub Team · Updated October 24, 2026 · Free to read and use
Choose the physical process before the specification
An ideal adiabatic throttle is commonly modelled with an enthalpy constraint, while an ideal reversible adiabatic expansion uses an entropy constraint. These assumptions are not interchangeable.
The linked energy-flash workflow includes its documented caloric model and heat input. It does not infer equipment efficiency or heat loss from pressure alone.
Reference states matter to energy accounting
Absolute enthalpy and entropy values depend on the model's reference convention. Differences and conservation must use one consistent convention for all inlet and outlet states.
A cubic EOS property model also needs an ideal-gas heat-capacity treatment. Matching an EOS name alone does not guarantee matching caloric predictions between implementations.
Inspect the solved temperature and residual
The recorded example varies supplied heat at fixed inlet and outlet pressure conditions. The outlet temperature and phase split are solved outputs.
Use the separate PS workflow for an entropy-constrained process. This panel is explicitly PH-style and does not label its results isentropic.
Explore the example
Original recorded synthetic cases solved with the site's peng-robinson-energy-flash engine. The full inputs, outputs and diagnostic rows are in the downloadable JSON. These are discrete solved states; the lesson does not interpolate a new solve.
Select one of 5 recorded scenarios. Intermediate states are not interpolated.
Specific heat input (positive into fluid) (kJ/kg) = 0. Outlet equilibrium temperature (K): 296.256406; Temperature change (K difference): -33.7435936.
- Outlet equilibrium temperature (K)
Worked example
Peng-Robinson Heat-Duty and Isenthalpic Flash Calculator
Fixed controls: Inlet absolute pressure = 5 MPa(a); Inlet equilibrium temperature = 330 K; Outlet absolute pressure = 1 MPa(a); Methane feed mole fraction = 0.5 dimensionless; Ethane feed mole fraction = 0.15 dimensionless; Propane feed mole fraction = 0.15 dimensionless; n-Butane feed mole fraction = 0.1 dimensionless; n-Pentane feed mole fraction = 0.05 dimensionless; n-Hexane feed mole fraction = 0.05 dimensionless.
Relation and unit basis
Internal SI: hbar_out(p_out,T_out,z)=hbar_in(p_in,T_in,z)+q(J/kg)*Mz(kg/mol). Solve F(T)=hbar_out-hbar_in-q*Mz=0 over 180-450 K. UI q(kJ/kg) is multiplied by 1000. q=0 is isenthalpic, not isothermal.; PR phase enthalpy is ideal-gas component enthalpy plus cubic residual enthalpy. The bulk molar enthalpy equals (1-beta)*hL+beta*hV. Native ideal-gas functions are pinned with the component data; no constant-Cp or vapor-only latent-heat shortcut is used. Compare enthalpy differences on the same fixed reference.; At common T and p, xi*phiL_i=yi*phiV_i. zi=(1-beta)*xi+beta*yi; Ki=yi/xi and sum[zi*(Ki-1)/(1+beta*(Ki-1))]=0. CoolProp performs stability analysis and phase-split iteration; Wilson K-factors are initial guesses, not the final equilibrium solution.
Specific heat input (positive into fluid) is varied within the supported input range. 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.
1. Define the baseline
Specific heat input (positive into fluid) (kJ/kg) = 0
Fixed controls: Inlet absolute pressure = 5 MPa(a); Inlet equilibrium temperature = 330 K; Outlet absolute pressure = 1 MPa(a); Methane feed mole fraction = 0.5 dimensionless; Ethane feed mole fraction = 0.15 dimensionless; Propane feed mole fraction = 0.15 dimensionless; n-Butane feed mole fraction = 0.1 dimensionless; n-Pentane feed mole fraction = 0.05 dimensionless; n-Hexane feed mole fraction = 0.05 dimensionless.
2. Select the relation
Internal SI: hbar_out(p_out,T_out,z)=hbar_in(p_in,T_in,z)+q(J/kg)*Mz(kg/mol). Solve F(T)=hbar_out-hbar_in-q*Mz=0 over 180-450 K. UI q(kJ/kg) is multiplied by 1000. q=0 is isenthalpic, not isothermal.; PR phase enthalpy is ideal-gas component enthalpy plus cubic residual enthalpy. The bulk molar enthalpy equals (1-beta)*hL+beta*hV. Native ideal-gas functions are pinned with the component data; no constant-Cp or vapor-only latent-heat shortcut is used. Compare enthalpy differences on the same fixed reference.; At common T and p, xi*phiL_i=yi*phiV_i. zi=(1-beta)*xi+beta*yi; Ki=yi/xi and sum[zi*(Ki-1)/(1+beta*(Ki-1))]=0. CoolProp performs stability analysis and phase-split iteration; Wilson K-factors are initial guesses, not the final equilibrium solution.
Specific heat input (positive into fluid) is varied within the supported input range. 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.
3. Substitute the values
q = 0 kJ/kg; with the fixed controls above, the workflow evaluates Outlet equilibrium temperature = 296.256406 K; Temperature change = -33.7435936 K difference
Retain intermediate precision. The displayed values are rounded only after evaluation.
4. Read the result
Outlet equilibrium temperature (K) = 296.256406; Temperature change (K difference) = -33.7435936
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.
Result and interpretation
Outlet equilibrium temperature (K) = 296.256406; Temperature change (K difference) = -33.7435936
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.
Heat input changes while the inlet state and outlet pressure stay fixed. The solver determines outlet temperature.
Exact scenario data
Original recorded synthetic cases solved with the site's peng-robinson-energy-flash engine. The full inputs, outputs and diagnostic rows are in the downloadable JSON. These are discrete solved states; the lesson does not interpolate a new solve.
| Specific heat input (positive into fluid) (kJ/kg) | Outlet equilibrium temperature (K) | Temperature change (K difference) |
|---|---|---|
| -10 | 294.0786 | -35.92139 |
| -5 | 295.1688 | -34.8312 |
| 0 | 296.2564 | -33.74359 |
| 5 | 297.3414 | -32.65858 |
| 10 | 298.4238 | -31.57619 |
Scroll the table to see all columns. The highlighted row is the worked baseline. Downloads retain full numerical precision.
Test your understanding
Work through the question before revealing its answer. Use the stated assumptions and include units in every numerical step.
1. Why does specifying outlet pressure alone not determine outlet temperature?
Show the worked answer
An additional thermal or process constraint is required, such as temperature, enthalpy, entropy or a heat/work model.
2. Reproduce the baseline Outlet equilibrium temperature (K) without rounding intermediate values.
Show the worked answer
q = 0 kJ/kg; with the fixed controls above, the workflow evaluates Outlet equilibrium temperature = 296.256406 K; Temperature change = -33.7435936 K difference. Outlet equilibrium temperature (K) = 296.256406. 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.
3. Compare the first and last recorded scenarios. Which input changes?
Show the worked answer
Specific heat input (positive into fluid) (kJ/kg) changes from -10 to 10. Other controls remain as stated in the premise. Outlet equilibrium temperature (K) changes from 294.078614 to 298.423809.
4. What would be needed to apply this example to a different physical system?
Show the worked answer
Replace the synthetic controls with compatible documented inputs and check the relation's assumptions. Specific heat input (positive into fluid) is varied within the supported input range. 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.
Before using the result
- State the process constraint.
- Use one caloric reference convention.
- Check energy or entropy residuals separately from component balance.
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.
Continue learning
Start with the broader workflow guide
Sources and method
The explanation and synthetic examples are original PetroCalcHub teaching material. The references below support the definitions and model context. They do not supply field validation for the synthetic scenarios.