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What Is Space Velocity?

By C.H. Seman
Updated May 21, 2024
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In chemical engineering and reactor engineering, space velocity refers to the quotient of the entering volumetric flow rate of the reactants divided by the reactor volume. It is commonly regarded as the reciprocal of the reactor space time. In industry, it 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 SV ≡ υ0 / V. In this expression, υ0 represents the volumetric flow rate of the reactants entering the reactor and V represents the volume of the reactor itself. This expression is the reciprocal of the definition for the reactor space time, τ. The space time is measured at the conditions of the reactor entrance, however, and the space velocity is often measured at a set of standard conditions, so the reported velocity may be different from the reciprocal of the measured space time.

Liquid hourly space velocity (LHSV), is a method for relating the reactant liquid flow rate 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). The volumetric flow rate is treated as a liquid at these conditions, even though the actual material may be a gas under normal operating conditions.

Gas hourly space velocity (GHSV) 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) differs from LHSV and GHSV, because volume is not utilized. Mass, rather than volume, provides the basis for WHSV. This measurement is the quotient of the mass flow rate of the reactants divided by the mass of the catalyst in the reactor.

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.

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Discussion Comments

By anon1005256 — On Jul 21, 2021

The definition of GHSV in this article is unclear - like so many others, it uses 'volumetric flow' when referring to a flow at standard conditions, which is (by definition) a molar flow, not a true volumetric flow. This is a better way of explaining why GHSV will not usually match reciprocal space time - one is actually a volumetric residence time (though not always even then!), the other is usually a molar space velocity (i.e. units of Nm³/m³·h or SCF/ft³·h etc.) - note that Nm³ and SCF are NOT volumes, but pseudo-volumetric proxies for a molar quantity.

By anon999728 — On Mar 04, 2018

The "space" in space velocity refers to space as a volume. The inverse of space velocity is "space time" and you can think of it as how long it takes for the feed to pass through the reactor volume. Space velocity can be based on any feed criteria. For example, you may have a feed to the reactor that contains a reactive species and and some inert species. You can based the space velocity on the reactive species only or on the sum of the reactive and inert components. However you do it, you must specify to others what the basis is. Reactor space time and reactant residence time are not necessarily the same thing and this point causes a lot of confusion in reaction engineering. Space velocity is a way to relate the feed rate to the reactor volume (or catalyst quantity).

By MrMoody — On Dec 21, 2011

@Mammmood - I don’t know much about nuclear reactors. But I do think this has a lot of practical applications. Any technology that requires the rapid injection of a reactant would benefit I think.

I am thinking of automotive applications where you have fuel injection. I am not an expert on cars but I would think that you would want to measure how quickly the fuel enters the chamber to make sure that you are getting just the right amount of fuel.

You would have problems if it the space velocity of the fuel was too more or too less.

By Mammmood — On Dec 20, 2011

Why do they call it space velocity? I thought this would be about velocity in outer space. This has to do with the velocity of reactants in a chemical chamber. I think they should call it reactant velocity or something like that.

At any rate, I do admit it sounds like it would be an important concept. I suppose it would serve as a good fail safe mechanism in some cases. For example, a nuclear reactor undergoing a meltdown might experience a rapid increase in the space velocity in the reactor chamber, and this would signal that something was wrong.

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