Engineering lesson
Explore separator pressure and phase split on one synthetic feed
Separator pressure changes phase equilibrium and how material is allocated between gas and remaining liquid. In a train, the downstream feed is the retained upstream liquid, not the original feed reused at every stage.
By PetroCalcHub Team · Updated October 24, 2026 · Free to read and use
Follow material through the stages
Each stage flashes its incoming stream at a specified pressure and temperature. Removed vapor is counted, and the liquid becomes the next feed with a changed composition.
Adding stage vapor fractions directly is wrong if each fraction uses a different incoming-stream denominator. Convert every amount back to the original feed basis first.
Vary one pressure with other controls fixed
The recorded study changes first-stage pressure while keeping stage temperatures, later pressures, feed and EOS assumptions fixed. It reports the train's retained-liquid result on the original-feed basis.
This is a thermodynamic sensitivity, not a separator mechanical design or a search for an economically optimal pressure.
Inspect balances and phase conditions
Keep every stage's phase amounts and compositions with the final result. A single final liquid yield can conceal a failed intermediate flash.
The explorer selects recorded engine cases. Opening the workflow performs a fresh calculation for supported inputs; the lesson does not interpolate uncomputed pressures.
Explore the example
Original recorded synthetic cases solved with the site's three-stage-separator-train 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.
Stage 1 absolute pressure (MPa(a)) = 5. Total separated vapor moles per feed mole (dimensionless): 0.809755941; Final liquid moles per feed mole (dimensionless): 0.190244059.
- Total separated vapor moles per feed mole (dimensionless)
- Final liquid moles per feed mole (dimensionless)
Worked example
Three-Stage Hydrocarbon Separator Train Calculator
Fixed controls: Stage 1 absolute temperature = 330 K; Stage 2 absolute pressure = 2 MPa(a); Stage 2 absolute temperature = 300 K; Stage 3 absolute pressure = 0.5 MPa(a); Stage 3 absolute temperature = 280 K; 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
Normalize original feed moles to one. Stage j removes NV,j=Nj*beta_j and sends only Nj+1=Nj*(1-beta_j) with composition xj to the next stage. Overall zi0=sum_j NV,j*yij+NL3*xi3. Separator pressure strictly decreases, while temperatures are prescribed independently.; For j=2,3, interstage heat Qj=Nj*(stage-equilibrium molar enthalpy minus previous liquid molar enthalpy), J/mol original feed. Stage 1 establishes initial enthalpy, so upstream conditioning duty is not included. Sum all removed vapor energy and final liquid energy and require overall residual<1e-4 J/mol original feed.; Classical Peng-Robinson, standard quadratic alpha: a=sum_i sum_j xi*xj*sqrt(ai*aj)*(1-kij), b=sum_i xi*bi. The matrix is symmetric with zero diagonal. Available database coefficients and common user-assumed coefficients are distinguished; there is no volume translation or hidden parameter tuning.
Stage 1 absolute pressure is varied within the supported input range. Six components C1-C6, normal C4-C6, classical PR and all kij=0. No water, isomers, sour gas, heavy fractions, reactions, solids or volume translation. Use the expanded-mixture tools for their explicitly supported alternatives, not by inserting a new species in these fixed feed slots.
1. Define the baseline
Stage 1 absolute pressure (MPa(a)) = 5
Fixed controls: Stage 1 absolute temperature = 330 K; Stage 2 absolute pressure = 2 MPa(a); Stage 2 absolute temperature = 300 K; Stage 3 absolute pressure = 0.5 MPa(a); Stage 3 absolute temperature = 280 K; 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
Normalize original feed moles to one. Stage j removes NV,j=Nj*beta_j and sends only Nj+1=Nj*(1-beta_j) with composition xj to the next stage. Overall zi0=sum_j NV,j*yij+NL3*xi3. Separator pressure strictly decreases, while temperatures are prescribed independently.; For j=2,3, interstage heat Qj=Nj*(stage-equilibrium molar enthalpy minus previous liquid molar enthalpy), J/mol original feed. Stage 1 establishes initial enthalpy, so upstream conditioning duty is not included. Sum all removed vapor energy and final liquid energy and require overall residual<1e-4 J/mol original feed.; Classical Peng-Robinson, standard quadratic alpha: a=sum_i sum_j xi*xj*sqrt(ai*aj)*(1-kij), b=sum_i xi*bi. The matrix is symmetric with zero diagonal. Available database coefficients and common user-assumed coefficients are distinguished; there is no volume translation or hidden parameter tuning.
Stage 1 absolute pressure is varied within the supported input range. Six components C1-C6, normal C4-C6, classical PR and all kij=0. No water, isomers, sour gas, heavy fractions, reactions, solids or volume translation. Use the expanded-mixture tools for their explicitly supported alternatives, not by inserting a new species in these fixed feed slots.
3. Substitute the values
p1 = 5 MPa(a); with the fixed controls above, the workflow evaluates Total separated vapor moles per feed mole = 0.809755941 dimensionless; Final liquid moles per feed mole = 0.190244059 dimensionless
Retain intermediate precision. The displayed values are rounded only after evaluation.
4. Read the result
Total separated vapor moles per feed mole (dimensionless) = 0.809755941; Final liquid moles per feed mole (dimensionless) = 0.190244059
Every flash trial must remain at 180-450 K and 0.1-10 MPa absolute, with consistent mole fractions summing to one. Near-critical, vanishing-phase or failed closure states are rejected. Single-phase states are not given an invented vapor quality. Independent engine agreement is not experimental validation. Instantaneous equilibrium and the documented steady balance only. No machinery efficiency, transient behavior, separator sizing, phase residence time, mechanical safety approval or claimed field calibration. A rejected or unbracketed root is not extrapolated into an unsupported range. Exactly three once-through prescribed PT equilibrium stages, no recycle, heat-exchanger sizing, droplet carryover or residence-time design. Each must remain two-phase. No water decanter, C7+, gas recompression, upstream stage-1 conditioning duty or assumed separator efficiency.
Result and interpretation
Total separated vapor moles per feed mole (dimensionless) = 0.809755941; Final liquid moles per feed mole (dimensionless) = 0.190244059
Every flash trial must remain at 180-450 K and 0.1-10 MPa absolute, with consistent mole fractions summing to one. Near-critical, vanishing-phase or failed closure states are rejected. Single-phase states are not given an invented vapor quality. Independent engine agreement is not experimental validation. Instantaneous equilibrium and the documented steady balance only. No machinery efficiency, transient behavior, separator sizing, phase residence time, mechanical safety approval or claimed field calibration. A rejected or unbracketed root is not extrapolated into an unsupported range. Exactly three once-through prescribed PT equilibrium stages, no recycle, heat-exchanger sizing, droplet carryover or residence-time design. Each must remain two-phase. No water decanter, C7+, gas recompression, upstream stage-1 conditioning duty or assumed separator efficiency.
Select a recorded first-stage pressure and compare the original-feed-basis retained liquid.
Exact scenario data
Original recorded synthetic cases solved with the site's three-stage-separator-train 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.
| Stage 1 absolute pressure (MPa(a)) | Total separated vapor moles per feed mole (dimensionless) | Final liquid moles per feed mole (dimensionless) |
|---|---|---|
| 3 | 0.8530002 | 0.1469998 |
| 4 | 0.8253355 | 0.1746645 |
| 5 | 0.8097559 | 0.1902441 |
| 6 | 0.8017106 | 0.1982894 |
| 7 | 0.798593 | 0.201407 |
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 can stage vapor mole fractions not simply be added?
Show the worked answer
Each is based on that stage's incoming amount. Convert stage amounts to a common original-feed basis before summing.
2. Reproduce the baseline Total separated vapor moles per feed mole (dimensionless) without rounding intermediate values.
Show the worked answer
p1 = 5 MPa(a); with the fixed controls above, the workflow evaluates Total separated vapor moles per feed mole = 0.809755941 dimensionless; Final liquid moles per feed mole = 0.190244059 dimensionless. Total separated vapor moles per feed mole (dimensionless) = 0.809755941. Every flash trial must remain at 180-450 K and 0.1-10 MPa absolute, with consistent mole fractions summing to one. Near-critical, vanishing-phase or failed closure states are rejected. Single-phase states are not given an invented vapor quality. Independent engine agreement is not experimental validation. Instantaneous equilibrium and the documented steady balance only. No machinery efficiency, transient behavior, separator sizing, phase residence time, mechanical safety approval or claimed field calibration. A rejected or unbracketed root is not extrapolated into an unsupported range. Exactly three once-through prescribed PT equilibrium stages, no recycle, heat-exchanger sizing, droplet carryover or residence-time design. Each must remain two-phase. No water decanter, C7+, gas recompression, upstream stage-1 conditioning duty or assumed separator efficiency.
3. Compare the first and last recorded scenarios. Which input changes?
Show the worked answer
Stage 1 absolute pressure (MPa(a)) changes from 3 to 7. Other controls remain as stated in the premise. Total separated vapor moles per feed mole (dimensionless) changes from 0.853000217 to 0.798592954.
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. Stage 1 absolute pressure is varied within the supported input range. Six components C1-C6, normal C4-C6, classical PR and all kij=0. No water, isomers, sour gas, heavy fractions, reactions, solids or volume translation. Use the expanded-mixture tools for their explicitly supported alternatives, not by inserting a new species in these fixed feed slots. Every flash trial must remain at 180-450 K and 0.1-10 MPa absolute, with consistent mole fractions summing to one. Near-critical, vanishing-phase or failed closure states are rejected. Single-phase states are not given an invented vapor quality. Independent engine agreement is not experimental validation. Instantaneous equilibrium and the documented steady balance only. No machinery efficiency, transient behavior, separator sizing, phase residence time, mechanical safety approval or claimed field calibration. A rejected or unbracketed root is not extrapolated into an unsupported range. Exactly three once-through prescribed PT equilibrium stages, no recycle, heat-exchanger sizing, droplet carryover or residence-time design. Each must remain two-phase. No water decanter, C7+, gas recompression, upstream stage-1 conditioning duty or assumed separator efficiency.
Before using the result
- Carry upstream liquid composition forward.
- Use one amount basis for totals.
- Keep pressure ordering and stage temperatures explicit.
Every flash trial must remain at 180-450 K and 0.1-10 MPa absolute, with consistent mole fractions summing to one. Near-critical, vanishing-phase or failed closure states are rejected. Single-phase states are not given an invented vapor quality. Independent engine agreement is not experimental validation. Instantaneous equilibrium and the documented steady balance only. No machinery efficiency, transient behavior, separator sizing, phase residence time, mechanical safety approval or claimed field calibration. A rejected or unbracketed root is not extrapolated into an unsupported range. Exactly three once-through prescribed PT equilibrium stages, no recycle, heat-exchanger sizing, droplet carryover or residence-time design. Each must remain two-phase. No water decanter, C7+, gas recompression, upstream stage-1 conditioning duty or assumed separator efficiency.
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.