Biogas upgrading – Review of commercial technologies - SGC

SGC Rapport 2013:270
Appendix II Theory on refrigeration cycles
In the context of liquefied methane, quantities are often specified in units other
than standard cubic meters, e.g. kg or gallons. In order to facilitate the comparison
of the different technologies, the conversion coefficients for the most common
units are shown in Table 1 below.
Table 1: Conversion coefficients between different units for gas quantities.
Nm³ kg US liq. gallon litre
Nm³ 1.00 0.72 0.45 1.71
kg 1.39 1.00 0.63 2.38
US liq. gallon 2.21 1.59 1.00 3.79
litre 0.58 0.42 0.26 1.00
The Joule Thomson effect and inversion temperature
When compressing and expanding real, non-ideal gases, temperature will change
if the pressure is changed adiabatically (i.e. without the exchange of thermal energy,
e.g. in an insulated valve). This effect is called the Joule Thomson effect. The
sign and magnitude of the temperature change is normally expressed by the Joule
Thomson coefficient which is defined as
∂ T
μJT =( ∂ p) H
The coefficient depends on the type of gas as well as its pressure and temperature
before expansion. At a certain temperature, the so-called inversion temperature,
µJT is zero, so the temperature does not change upon a pressure change.
Above the inversion temperature, µJT is negative, so the gas gets warmer on expansion,
while the coefficient is positive at temperatures below the inversion temperature.
At room temperature, almost all gases have a positive coefficient and will
be cooled down by expansion, the only exceptions being hydrogen, helium and
neon. The existence of the inversion temperature implies that it is not possible to
cool a gas beyond this temperature by expansion valves. An additional cooling
effect can be achieved by replacing the throttle valve by a turbine which additionally
extracts work from the gas.
Standard cooling cycles
The elemental steps in direct cooling cycles are as follows:
1. Compression: As gases are compressed, the temperature rises.
2. Cooling: The compressed hot gas is cooled against some other medium.
This may be at room temperature or against another stream at low temperature.
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