| a | effective interfacial area per unit volume of packing | m2/m3 |
| A | cross-sectional area | m2 |
| A m | membrane surface area | m2 |
| c p | specific heat capacity at constant pressure | kJ/(kg K) |
| c v | specific heat capacity at constant volume | kJ/(kg K) |
| d g | abundance of gas component g | kg |
| F | Faraday constant | — |
| h | specific enthalpy on mass basis | kJ/kg |
| | specific enthalpy on molar basis | kJ/kmol |
| | specific enthalpy of formation on molar basis | kJ/kmol |
| H | enthalpy | kJ |
| H 0 | enthalpy of formation | kJ |
| He | Henry's law constant | Pa/kmol fraction gas in liquid |
| He ′ | Henry's law constant | (Pa m3)/kmol gas in liquid |
| HHV | higher heating value | kJ/kg |
| HR | heat rate | kJ/kWh |
| J i | flux through a membrane | m/s |
| LHV | lower heating value | kJ/kg |
| m | mass | kg or t |
| | mass flow rate | kg/s |
| MW | molecular weight | kg/kmol |
| n | polytropic coefficient | — |
| n i | number of moles for specie i | mol |
| Nu | Nusselt number | |
| p | absolute pressure | Pa, bar |
| p f | partial pressure of the permeated gas at the feed side of a membrane | Pa, bar |
| p i | partial pressure of component i | Pa, bar |
| p p | partial pressure of the permeated gas at the permeate side of a membrane | Pa, bar |
| P | membrane permeability | m3(STP)/(m h bar) |
| Pr | Prandtl number | |
| q i | gas adsorbed per unit mass of adsorbent | mol gas/kg adsorbent |
| q p,i | volumetric flow rate through membrane | m3(STP)/h |
| Q | heat | kJ |
| | heat transfer rate | kW |
| r | radiative forcing | W/(kg m2) |
| R | specific gas constant | kJ/(kg K) |
| R u | universal gas constant | kJ/(kmol K) |
| Re | Reynolds number | |
| s | specific entropy on mass basis | kJ/(kg K) |
| S | entropy | kJ/K |
| S ads | adsorption selectivity | — |
| t | time | s or h |
| t m | membrane thickness | m |
| T | temperature | °C or K |
| u | absolute velocity | m/s |
| U | internal energy | kJ |
| v | specific volume on mass basis | m3/kg |
| V | volume | m3 |
| | volumetric flow rate | m3/s |
| w | specific work on mass basis | kJ/kg |
| | specific work on molar basis | kJ/kmol |
| W | work | kJ |
| | power | kW |
| x | vapour quality | kgvapour/kgvapour+liquid |
| x i | mole fraction in liquid phase for component i | — |
| y i | mole fraction in gas phase for component i | — |
| Z | compressibility factor | — |
