Space velocity (chemistry)

In chemical engineering and reactor engineering, space velocity, S V , refers to the quotient of the entering volumetric flow rate of the reactants divided by the reactor volume (or the catalyst bed volume) which indicates how many reactor volumes of feed can be treated in a unit time (for instance, a reactor with a space velocity of 7 h⁻¹ is able to process feed equivalent to seven times the reactor volume each hour). It is commonly regarded as the reciprocal of the reactor space time. In industry, space velocity can be further defined by the phase of the reactants at given conditions. Special values for this measurement exist for liquids and gases, and for systems that use solid catalysts.

By definition, space velocity can be expressed mathematically as

Calculations are straightforward when the reactor volume is known and the incoming reactant flow rate is known. For example, if 70 feet³/hour of a reactant enter a reactor with an internal volume of 250 feet³, the calculated space velocity is approximately 0.28 hour⁻¹. This can be viewed as the number of reactor changes the system is undergoing in one hour.

Liquid hourly space velocity (LHSV = Reactant Liquid Flow Rate/Reactor Volume)

This technique involves linking the flow rate of a reactant in liquid form to the reactor's volume, with all measurements adjusted to a uniform temperature. For the purposes of this method, the flow rate is analyzed as though it pertains to a liquid, irrespective of the substance's potential to exist as a gas under standard operating conditions.

Gas hourly space velocity (GHSV = Reactant Gas Flow Rate/Reactor Volume)

It is a similar method for relating the reactant gas flow rate to the reactor volume. GHSV is usually measured at standard temperature and pressure. Different industries may have their own definitions for standard temperature and pressure and these conditions may be closer to ambient conditions than to the International Union of Pure and Applied Chemistry values of 32 °F (0 °C) and 1 bar (100 kPa). It is always important for an engineer to check the basis of calculation.

Weight hourly space velocity (WHSV = Mass Flow/Catalyst Mass)

It differs from LHSV and GHSV, because volume is not utilized. Mass, rather than volume, provides the basis for WHSV (weight basis). This measurement typically is used to describe catalytic reactors, and denotes the quotient of the mass flow rate of the reactants divided by the mass of the catalyst in the reactor.