Liquid Ethylene Density Calculator | NIST Data

What is the liquid ethylene density calculator?

The liquid ethylene density calculator estimates the density of liquid ethylene (C₂H₄) from its temperature and pressure. It is built on reference data from the NIST Chemistry WebBook, a free property database run by the U.S. National Institute of Standards and Technology. The tool works for temperatures from 105 K to 282 K and pressures from 1 bar to 100 bar. If ethylene would be a gas at the chosen conditions, the tool says so instead of giving a liquid density.

Ethylene is a gas at room temperature. Producers cool it below its normal boiling point of about 169 K (about -104°C) at atmospheric pressure, or keep it under pressure, so that it stays liquid for storage and transport. Liquid ethylene packs far more mass into the same volume than the gas. Its density at a given temperature and pressure is needed for sizing storage tanks, checking pipeline flow and balancing mass in a plant.

How the calculator works

The calculator works in three steps.

Step 1: Saturation values. Every temperature below the critical point has a saturation pressure. This is the pressure at which liquid and vapor exist together, also called the vapor pressure. The liquid at that pressure has the saturated-liquid density. The calculator stores both values from NIST tables at temperatures from 105 K to 282 K. Between table points it interpolates (estimates a value between two known points). Saturated-liquid density is interpolated in a straight line with temperature. Saturation pressure is interpolated as the logarithm of pressure against 1/T, which follows its curved shape more closely.

Step 2: Phase check. If the pressure is below the saturation pressure, ethylene boils, so it is a gas. The calculator then reports "gas" and the saturation pressure, with no liquid density.

Step 3: Pressure correction. Above the saturation pressure, the liquid is squeezed slightly and becomes denser. The calculator uses a Tait-type equation, a standard form for how liquid density rises with pressure:

$\rho = \frac{\rho_{sat}}{1 - C \ln\left(1 + \dfrac{P - P_{sat}}{D}\right)}$

Where:

$C$ and $D$ were fitted to NIST data at each table temperature, from the saturation pressure up to 100 bar. They are interpolated between table points like the other values. The user enters only temperature and pressure.

The pressure is measured from the saturation pressure, not from atmospheric pressure. This matters near the critical point. At 270 K, for example, the liquid only exists above about 38 bar, and it compresses much more than it does at 200 K.

The result is rounded to two decimal places.

Worked example

Find the density of liquid ethylene at 200 K and 100 bar.

Step 1. Read the saturation values at 200 K from the table:

• $P_{sat}$ = 4.55482 bar
• $\rho_{sat}$ = 521.222 kg/m³
• $D$ = 280.22 bar
• $C$ = 0.089762

Step 2. Check the phase. 100 bar is above 4.55 bar, so ethylene is liquid.

Step 3. Apply the pressure correction:

$\frac{100 - 4.55482}{280.22} = 0.34061$

$\ln(1.34061) = 0.29312$

$1 - 0.089762 \times 0.29312 = 0.97369$

$\rho = \frac{521.222}{0.97369} = 535.31 \text{ kg/m}^3$

The calculator returns 535.31 kg/m³. NIST lists 535.31 kg/m³ at 200 K and 10 MPa (10 MPa is 100 bar).

More examples

At 200 K, going from 10 bar to 100 bar raises the density by about 13 kg/m³, or 2.5%. At 270 K, the liquid is much softer. Its density rises from 356.39 kg/m³ at saturation (38.1 bar) to 412.70 kg/m³ at 100 bar, about 16%.

At 250 K and 10 bar, the pressure is below the saturation pressure of 2.3295 MPa (about 23.30 bar). Ethylene is a gas there, so the calculator gives no liquid density.

Valid input ranges

The calculator accepts temperatures from 105 K to 282 K and pressures from 1 bar to 100 bar.

• Near 104 K, ethylene freezes. Its triple point is 104.0 K, the temperature where solid, liquid and gas coexist. At 104 K, NIST lists the liquid only at its saturation pressure, so the tool starts at 105 K.
• At 282.35 K, ethylene reaches its critical point. Above it, liquid and gas can no longer be told apart. The tool stops at 282 K.
• Below 1 bar or above 100 bar, the calculator gives a validation error.

Within this range, the calculator stays within about 0.3% of NIST data up to 280 K. Between 280 K and 282 K the liquid becomes very compressible, and the error can reach about 1%. At low temperatures, the saturation pressure is far below 1 bar, so every allowed pressure gives a liquid. Above about 169 K, low pressures give gas.

Why liquid ethylene density matters

Density calculations support several everyday tasks in chemical plants:

• Storage tank sizing. Liquid ethylene expands as it warms. Tanks need enough headspace for that expansion.
• Pipeline and pump design. Pressure drop and pump suction calculations depend on the density of the liquid in the line.
• Mass balance. Flow meters often measure volume, not mass. Converting volume to mass requires a density value.
• Custody transfer. When ethylene changes hands, measured volumes are converted to mass using density. The mass sets the amount billed.

Limitations

The calculator interpolates stored NIST values. It is not a full equation of state. It does not handle mixtures of ethylene with other gases. It does not give gas density. Safety-critical work that needs certified data should use NIST's own software, such as REFPROP.

Near the critical point, small changes in temperature or pressure cause large changes in density. Results between 280 K and 282 K are the least accurate.

Frequently asked questions

What is liquid ethylene density measured in? The calculator reports density in kilograms per cubic meter (kg/m³), the SI unit of density.

What temperature and pressure units does the calculator use? Temperature in kelvin (K) and pressure in bar. To convert from Celsius, add 273.15. To convert from psi to bar, multiply by 0.06895. One megapascal (MPa) equals 10 bar.

Why does the calculator sometimes say "gas"? At each temperature, ethylene stays liquid only above its saturation pressure. Below that pressure it boils. At 250 K, for example, the saturation pressure is about 23.3 bar, so 10 bar gives gas.

Why does pressure matter more at higher temperatures? Close to the critical point, the liquid is loosely packed and easy to compress. At 200 K, raising the pressure from 10 to 100 bar adds about 2.5% to the density. At 270 K, the change from saturation to 100 bar is about 16%.

Can this calculator be used for ethylene gas? No. It only gives liquid density. Gas-phase ethylene needs a different model, such as an equation of state.

How accurate is this calculator? It stays within about 0.3% of NIST values up to 280 K. Between 280 K and 282 K, close to the 282.35 K critical point, the error can reach about 1%.

References

1. National Institute of Standards and Technology. Ethylene: Phase change data, NIST Chemistry WebBook, SRD 69. NIST WebBook phase change data for ethylene — normal boiling point about 169 K, triple point 104.0 K.
2. National Institute of Standards and Technology. Saturation properties for ethylene, 150 K to 250 K, NIST Chemistry WebBook, SRD 69. NIST saturation table for ethylene — saturation pressure of 2.3295 MPa at 250 K.
3. National Institute of Standards and Technology. Isothermal properties for ethylene at 200 K, NIST Chemistry WebBook, SRD 69. NIST 200 K isotherm for ethylene — liquid density of 522.13 kg/m³ at 1 MPa and 535.31 kg/m³ at 10 MPa; critical temperature 282.35 K.
4. National Institute of Standards and Technology. Isothermal properties for ethylene at 270 K, NIST Chemistry WebBook, SRD 69. NIST 270 K isotherm for ethylene — saturated liquid 356.39 kg/m³ at 3.8125 MPa; 412.70 kg/m³ at 10 MPa.
5. National Institute of Standards and Technology. Isothermal properties for ethylene at 280 K, NIST Chemistry WebBook, SRD 69. NIST 280 K isotherm for ethylene — 387.76 kg/m³ at 10 MPa.