Gas Properties and Compression Behaviour

1. Direct answer and purpose

Gas composition changes compressor selection because molecular mass, density, compressibility, heat capacity, enthalpy, speed of sound and phase behaviour determine the actual inlet volume, compression work, discharge temperature, aerodynamic or volumetric performance, condensation risk and operating envelope. A flow value without composition, pressure, temperature and reference basis is therefore incomplete.

It does not select a compressor or replace a validated property package, supplier performance calculation, process simulation, materials review or project risk assessment.

2. Controlled decision and package boundary

The process owner supplies representative compositions and operating states. The thermodynamics owner approves the equation of state, mixing rules, interaction parameters, reference state and validity range. The compressor supplier applies those properties to the proposed machine and guarantees only agreed cases.

The calculation boundary begins with a defined gas sample or composition set and ends with approved property values at suction, intermediate, discharge, recycle, cooling and shutdown states. Mechanical type selection, materials, seals, lubrication, relief, controls and detailed hydrogen or carbon-dioxide service remain with their controlled pages and qualified specialists.

3. Composition is an operating variable

State every component on a mole basis unless another basis is explicitly named. Include minimum, normal, maximum, start-up and upset compositions, analytical uncertainty, detection limits, sampling point, date and wet or dry basis. Reconcile each analysis to 100%; document how balance gas and components below detection are handled.

Two mixtures can share the same average molecular mass and still have different compressibility, heat capacity and phase behaviour. A single “design molecular weight” cannot represent composition-dependent thermodynamics. Keep credible component correlations together rather than combining independent maxima into a physically impossible mixture.

4. Molecular mass and mass-flow conversion

Mixture molecular mass is the mole-fraction-weighted sum of component molecular masses. At a fixed molar flow, a heavier mixture carries more mass. At a fixed mass flow, a lighter gas carries more moles and usually more inlet volume. These relationships affect positive-displacement, centrifugal corrected flow, piping velocity, separator loading and leak calculations.

Record whether a meter or process model reports mass, molar, standard volume or actual volume. Conversion requires the same composition and state used in the property calculation. Do not infer actual inlet volume from molecular mass alone; pressure, temperature and compressibility also enter.

5. Reference-flow and pressure definitions

Use absolute pressure in thermodynamic equations. Label gauge pressure separately and record the atmospheric pressure used for conversion. Define standard or normal volume by reference temperature, absolute pressure, composition basis, moisture basis and compressibility convention. “Nm3/h,” “Sm3/h,” “SCFM” and “ACFM” are not interchangeable without these definitions.

For a stated reference state, molar flow can be obtained from volume using n = pV/(ZRT). Actual suction volume then follows from Va = nZaRTa/pa. If both reference and actual states use the same composition, Va/Vr = (Za/Zr)(Ta/Tr)(pr/pa). Preserve significant figures consistent with input uncertainty.

6. Compressibility factor and real-gas density

The compressibility factor Z expresses departure from the ideal relation pV = nRT. Real-gas density is rho = pM/(ZRT), using compatible units. Z can vary with composition, pressure and temperature, especially near saturation, critical regions and at elevated density. Using Z = 1 without a bounded screening purpose can distort inlet volume, power and flow conversion.

Use the approved property method at every relevant state, not one Z copied across a compression path. Record both the calculated value and the method that produced it. If a vendor uses a different method, reconcile the property difference before comparing machine performance.

7. Heat capacity, k and polytropic behaviour

Isobaric heat capacity Cp, isochoric heat capacity Cv and their ratio k = Cp/Cv vary with temperature, pressure and composition. They influence idealised temperature rise, work and acoustic behaviour. A constant k may support a transparent first screen, but it is not a substitute for real-gas enthalpy calculations over a wide pressure or temperature interval.

Compressor calculations may use isentropic or polytropic exponents and efficiencies defined by a particular standard or method. Do not substitute Cp/Cv automatically for a supplier’s polytropic exponent. Record the definition, property package and state range so the result can be reproduced.

8. Enthalpy and discharge temperature

For a real gas, compressor power is tied to mass flow and enthalpy rise, with the applicable efficiency and loss definitions. Discharge temperature depends on composition, suction state, pressure ratio, efficiency, heat transfer and staging. It is not determined by pressure ratio alone.

Calculate each stage with validated properties and agreed intercooler outlet conditions. Compare predicted temperatures with lubricant, seal, material, valve, coating, process and deposition limits supplied by their owners.

9. Density, sonic velocity and machine response

Density affects inlet volume, aerodynamic head, Reynolds number, separation and pressure-drop calculations. Speed of sound and thermodynamic derivatives influence centrifugal Mach number, choke, anti-surge correction and valve dynamics. Composition shifts can therefore move an operating point even when measured mass flow appears unchanged.

For positive-displacement machines, density and suction volume influence displacement and loading. For dynamic machines, molecular mass, Z, temperature and k enter corrected performance relationships. Always ask the supplier which properties and corrections its map or performance program uses.

10. Phase envelope and saturation margin

A phase envelope identifies vapor-liquid boundaries for a stated composition and property model. Dew point is the state where the first liquid forms from vapor; bubble point is where the first vapor forms from liquid. Mixtures can have separate water and hydrocarbon condensation concerns, and composition changes can move both.

Plot suction, compression, intercooling, recycle, throttling, shutdown and settle-out paths against applicable phase boundaries. State the required single-phase margin and its owner. Pressure reduction can cool a gas through Joule-Thomson behaviour; compression followed by cooling can also cross a dew line.

11. Critical, supercritical and dense-phase regions

Near a critical region, density and heat capacity may change sharply with small pressure or temperature changes. Labels such as “gas,” “dense phase” or “supercritical” do not by themselves supply design properties. Use a method validated for the composition and state range and perform sensitivity checks where gradients are steep.

Instrument uncertainty can become important when a small temperature or pressure error crosses a phase boundary. Define measurement tolerance, property uncertainty and operating margin separately; do not hide all three inside an undocumented design factor.

12. Water, condensables and trace components

Water content can change molecular mass, phase behaviour, corrosion risk, hydrate or icing potential and whether a reported flow is wet or dry. Hydrocarbons, solvent vapors and other condensables can form liquid during cooling or recycle. Trace acid gases, oxygen or reactive species may control materials and safety even when their effect on bulk thermodynamics is small.

The property composition must represent the stream at the compressor boundary, including upstream removal or addition. If condensate is intentionally excluded from a gas analysis, separately specify its possible carryover and the separator or protection basis.

13. Property methods and equations of state

Name the property package, version, equation of state, mixing rules, binary interaction parameters and component database. Record whether the calculation is equilibrium, metastable or forced single phase. Model choice can matter more than numerical precision when the mixture is outside the data used to fit it.

NIST REFPROP provides critically evaluated models for pure fluids and mixtures and can calculate molecular mass, density, Cp, Cv, compressibility factor, enthalpy, phase boundaries and other properties. It also warns that estimated interaction parameters require skepticism and confirmation against measurements. See https://www.nist.gov/srd/refprop and https://pages.nist.gov/REFPROP-docs/.

14. Validation, range and uncertainty

For every property set, retain the composition, pressure and temperature range, phase, units, method version, source, uncertainty or fitness statement, calculation date and checker. Confirm that each operating point lies within the model’s validated range and component coverage. Extrapolation requires explicit specialist approval.

Compare selected states against an independent trusted source, laboratory data or supplier calculation when consequences justify it. A converged software result is not proof of physical validity. Investigate discontinuities, negative or implausible values, unexpected phase flags and large differences between methods.

15. Required asset - Gas Property Comparison with Stated Conditions

Complete one row per composition and operating state. Values shown as illustrative are not approved design data.

16. Property responsibility and evidence matrix

Use this matrix to stop unowned property assumptions from passing into procurement or commissioning.

17. Worked case - question and assumptions

Question: how do three gases differ under the same explicitly stated inlet and outlet conditions? This bounded screen compares pure nitrogen, carbon dioxide and hydrogen at 100 kmol/h, 2.00 bar(a) and 300 K inlet, with 8.00 bar(a) outlet. Pressure ratio is 4.00. The calculation assumes ideal-gas inlet volume, Z = 1 and constant illustrative k values of 1.40, 1.30 and 1.41 respectively.

Molecular masses are 28.0134, 44.0095 and 2.01588 kg/kmol. The nitrogen value is consistent with the NIST Chemistry WebBook entry at https://webbook.nist.gov/cgi/cbook.cgi?ID=C7727379&Mask=1. The other values are used here solely as traceable screening inputs and must be verified in the approved project database.

Method: mass flow equals molar flow times molecular mass. Ideal inlet volume equals nRT/p using R = 0.08314 bar.m3/(kmol.K). Ideal isentropic discharge temperature equals T2 = T1(P2/P1)^((k-1)/k). No efficiency, heat loss, real-gas enthalpy or phase calculation is included.

18. Worked case - calculated comparison

19. Worked case - result and interpretation

Equal molar flow at the same ideal inlet state gives the same inlet volume, yet mass flow differs by more than an order of magnitude because molecular mass differs. The constant-k temperature screen also differs. This demonstrates why one flow number or one air-based correction cannot describe three gases.

The result is not a compressor duty point. Carbon dioxide may require a real-gas phase and property review; hydrogen requires its own containment, materials and leakage scope; all three require real efficiencies and supplier performance methods. Replace Z = 1 and constant k with validated state-dependent properties before selection or guarantee.

20. Reference-flow conversion example

Suppose the same hypothetical 100 kmol/h is expressed at 1.01325 bar(a), 288.15 K and Z = 1. The reference volume is 2364.4 m3/h. At the worked inlet state it is 1247.1 m3/h. The ratio follows temperature and inverse pressure, not a change in molar flow.

If a project property package instead gives Zr and Za, multiply the ratio by Za/Zr. Record both Z values and their method. This example must not be used to infer an actual meter correction without the meter’s base-condition, moisture and compressibility conventions.

21. Measurement and data methodology

Locate composition, pressure, temperature and flow measurements at defined boundaries and align timestamps. A laboratory analysis may not represent a fast transient; an online analyser may need lag, calibration and validation treatment. Record sample conditioning because condensation or adsorption can bias composition.

Use calibrated absolute-pressure and temperature instruments with ranges appropriate to the state. Trend suction, discharge, intercooler and recycle points during stable and transient cases. Store raw values, units and quality flags before conversion. Recalculate properties when composition or state crosses an approved trigger.

22. Diagnostics and discrepancy matrix

23. Commissioning and acceptance

Before testing, freeze the approved composition cases, property method, reference conditions, instruments, data frequency, stable-period definition and acceptance calculations. Obtain simultaneous composition, absolute pressure, temperature, flow, speed, valve position, recycle, cooling and power data at agreed cases.

Reconcile measured flow to the contractual basis without overwriting raw data. Use the same property package and version as the accepted performance method. Record phase status and inspect separators or drains. Supplier acceptance must name permitted composition and state tolerances and the treatment of data outside them.

24. Change control and operating triggers

Define triggers for composition, molecular mass, water, condensables, suction pressure, temperature, Z, dew-point margin and property-package revision. A triggered change requires impact review of compressor performance, driver load, discharge temperature, phase risk, controls, seals, materials and metering.

Keep prior property files and calculations with revision metadata. Do not silently update an EOS database or interaction parameter in a live performance model. Rebaseline only after technical review and approval.