Space Time and Space Velocity

Space time is simply vessel volume divided by volumetric feed rate, the time it would take to process one reactor volume of feed. Two cubic metres fed at 5 L/s gives $\ au = 400\ \ ext{s}$. Space velocity is its reciprocal, and it is the form catalyst vendors quote: 9 per hour in this case. The same quantity turns up under a dozen names depending on the trade, including hydraulic retention time in wastewater, contact time in disinfection, and LHSV or GHSV in refining.

The wrinkle that catches people is that space time and mean residence time are only the same thing when density is constant through the reactor. Space time is defined on the feed rate measured at the inlet, so for a gas-phase reaction that changes mole count, or one running non-isothermally, the actual average time a molecule spends inside differs from $\ au$, sometimes substantially. In liquid-phase work the two coincide and nobody worries about the distinction, which is exactly why it trips people up the first time they meet a gas-phase reactor.

Space velocity also carries a units trap worth flagging. GHSV is normally quoted with the gas volume measured at standard conditions while the catalyst volume is the actual packed bed, so it is not a pure reciprocal time despite having units of reciprocal hours. WHSV replaces volume with mass of feed per mass of catalyst per hour and is a different quantity again. When a vendor quotes a space velocity, find out which denominator they mean before you size anything.