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Space velocity is the ratio of the volumetric flow rate of reactants entering to the reactor volume. It can be expressed as SV=υ0/V. Different industries have their own definitions for standard temperature and pressure. Weighted hourly space velocity (WHSV) is based on mass instead of volume. Calculations are simple when the reactor volume and reactant input flow rate are known.
In chemical engineering and reactor engineering, space velocity refers to the quotient of the volumetric flow rate of reactants entering divided by the reactor volume. It is commonly regarded as the reciprocal of the space-time of the reactor. In industry, it can be further defined by the phase of the reactants under certain conditions. There are special values for this measurement for liquids and gases and for systems using solid catalysts.
By definition, space velocity can be mathematically expressed as SV ≡ υ0 / V. In this expression, υ0 represents the volumetric flow rate of the reactants entering the reactor and V represents the reactor volume. This expression is the reciprocal of the definition for the reactor spacetime, . However, space-time is measured under reactor inlet conditions, and space velocity is often measured under a number of standard conditions, so the reported velocity may differ from the reciprocal of the measured space-time.
Liquid space velocity hourly (LHSV), is a method of relating the flow rate of the reacting liquid to the reactor volume at a standard temperature. Usually, this temperature ranges from 60° Fahrenheit to 75° Fahrenheit (15.6° Celsius to 23.9° Celsius). Volumetric flow is treated as a liquid under these conditions, even though the actual material may be a gas under normal operating conditions.
Gas hourly space velocity (GHSV) is a similar method of relating reactant gas flow rate to 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 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 verify the calculation basis.
Weighted hourly space velocity (WHSV) differs from LHSV and GHSV, because volume is not used. Mass, rather than volume, provides the basis for WHSV. This measure is the quotient of the mass flow rate of the reactants divided by the mass of the catalyst in the reactor.
Calculations are simple when the reactor volume is known and the reactant input flow rate is known. For example, if 70 ft3/hour of a reactant enters a reactor with an internal volume of 250 ft3, the calculated space velocity is approximately 0.28 hour-1. This can be seen as the number of reactor changes the system is undergoing in one hour.
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