Reactor Space Time and Space Velocity

Space Time, τ, is the time necessary to process one reactor volume of fluid based on entrance conditions. It is obtained by dividing the reactor volume by the volumetric flow rate entering the reactor:

In simple words, a space-time of 2 min means that every 2 min one reactor volume of feed at specified conditions is being treated by the reactor.

Example 1: If a reactor has a volume of 0.2m³ and an inlet volumetric flow rate of 0.01m³/s. The space time, τ, is calculated as follows.

For a PFR, space time is calculated as follows:

For a CSTR, space time is calculated as follows:

Space Velocity (SV), is the number of reactor volumes of feed at specified conditions which can be treated by the reactor. It is defined as follows: 

At first glance, it seems like SV is the reciprocal of τ, however there can be a difference between the two quantities. For τ, the entering volumetric flow rate is measured at the entrance conditions while for SV other conditions are often used.

The two space velocities commonly used in industrial applications are the liquid-hourly space velocity (LHSV) and gas-hourly space velocity (GHSV). The entering volumetric flow rate, v₀, in the LHSV is frequently measured as that of a liquid feed rate at 60°F or 75°F,