Soave–Redlich–Kwong EOS Pressure Calculator

Calculator

Enter state and critical properties to compute pressure.

Temperature, T

Absolute temperature is used internally.

Molar volume, v

Use molar volume for your mixture or pure fluid.

Acentric factor, ω

Typical range: −0.3 to 1.5 (dimensionless).

Critical temperature, Tc

From property tables or datasheets.

Critical pressure, Pc

Ensure the chosen unit matches your value.

Output pressure unit

Result also shows pascals for reference.

Reset

Formula used

Single-component SRK form using molar volume.

The Soave–Redlich–Kwong equation of state is written as:

P = RT/(v − b) − aα /( v(v + b) )

Parameters

a = 0.42747 R² Tc² / Pc

b = 0.08664 R Tc / Pc

Tr = T/Tc

m = 0.480 + 1.574ω − 0.176ω²

α = [1 + m(1 − √Tr)]²

Symbols

P pressure
T temperature (kelvin)
v molar volume (m³/mol)
Tc, Pc critical properties
ω acentric factor
R gas constant (8.314462618 Pa·m³/mol·K)

For mixtures, SRK typically uses mixing rules for a and b. This page computes the pure-form expression directly from your entered properties.

How to use this calculator

Practical steps for consistent inputs.

Enter the temperature T and select its unit.
Enter the molar volume v at that state, with units.
Provide Tc, Pc, and the acentric factor ω.
Select the output pressure unit, then press Calculate Pressure.
Review intermediate values to confirm ranges, then export results.

If the output is negative or unstable, check that v is realistic and that units match. Many real-gas states require accurate v from density or a prior EOS solve.

Example data table

Sample inputs and computed output.

Fluid	T (K)	v (m³/mol)	Tc (K)	Pc (MPa)	ω	Pressure (bar)
Methane (illustrative)	300	0.003	190.56	4.5992	0.011	8.202

Values are for demonstration. Use verified property data for engineering work.

Technical article

Eight focused notes for engineering use.

1) Why SRK is used for pressure prediction

The Soave–Redlich–Kwong (SRK) equation of state is a cubic model that improves ideal‑gas pressure estimates when attraction and finite molecular size matter. It is popular because it is fast, stable, and often accurate enough for many nonpolar and mildly polar fluids.

2) Required property data and typical sources

This calculator needs temperature, molar volume, and three constants: critical temperature, critical pressure, and acentric factor. Tc and Pc come from property tables, datasheets, or simulators. The acentric factor is tabulated for pure compounds and reflects vapor‑pressure behavior.

3) The role of molar volume in practical workflows

SRK pressure depends strongly on molar volume because the repulsion term uses (v − b) while the attraction term scales with 1/[v(v + b)]. In practice, v is obtained from density, or from a separate EOS solve that returns Z and v at a given P and T.

4) Understanding the reduced temperature and alpha function

The reduced temperature Tr = T/Tc normalizes thermal energy. SRK uses a temperature‑dependent attraction factor α to represent weakening cohesion as temperature rises. As Tr increases, α typically decreases, lowering the attraction term.

5) How the acentric factor modifies behavior

The parameter m(ω) increases with ω, shifting α away from older square‑root forms. Higher ω commonly indicates greater deviation from simple spherical behavior, and SRK uses ω to better match saturation‑pressure trends across temperature.

6) Numerical checks that prevent nonphysical states

The model includes an excluded volume b. If v ≤ b, the repulsion term diverges and the state is nonphysical. The calculator flags this condition to help you confirm realistic states and correct unit conversions.

7) Interpreting the pressure output and units

Pressure is computed in pascals using an SI gas constant, then converted to your selected unit. Compare the result to expected operating ranges and trends versus temperature or volume. Large deviations usually point to inconsistent v, Tc/Pc, or ω values.

8) Practical limitations and when to upgrade models

SRK may be less reliable for strongly polar or associating fluids, and near the critical region. Mixtures also require mixing rules and binary interaction parameters for good accuracy. For rigorous phase behavior, use calibrated parameters within a process simulator or a specialized EOS.

FAQs

Quick answers for common SRK pressure questions.

1) What inputs must be in absolute units?

Temperature is converted to kelvin internally, so any provided unit is acceptable. Critical pressure is converted to pascals. Molar volume is converted to m³/mol before the SRK equation is evaluated.

2) Where do I get the acentric factor ω?

ω is tabulated for most pure compounds in property databases and datasheets. It is dimensionless and is commonly listed alongside Tc and Pc in thermodynamic property tables.

3) Why does the calculator warn when v ≤ b?

In SRK, the repulsive term uses (v − b). When v approaches b, the denominator becomes zero and the state is nonphysical. Increase v or verify your unit conversions.

4) Can this be used for liquids?

SRK can approximate liquid states, but results may be sensitive to v and near‑critical behavior. For liquids, use accurate density‑based molar volume and consider models better tuned for condensed phases.

5) What does a negative pressure indicate?

Negative pressure usually signals inconsistent inputs, such as an unrealistically large molar volume, incorrect critical constants, or a state outside the model’s meaningful range. Recheck v, Tc, Pc, and units.

6) How do I compute molar volume from density?

Convert density to molar volume using v = M/ρ, where M is molar mass and ρ is mass density. Ensure consistent units, then enter v in m³/mol, L/mol, or cm³/mol.

7) Does this calculator handle mixtures?

This page evaluates the pure‑form SRK expression from the entered properties. Mixtures require mixing rules for a and b and, often, binary interaction parameters to achieve good accuracy.