residence time: one construct, many systems

In chemical reaction engineering, residence time is how long an element of fluid spends inside a vessel between entering and leaving, and because different elements take different paths there is a residence-time distribution, not a single value. Danckwerts's 1953 Continuous flow systems: Distribution of residence times defined that distribution and the tracer experiments used to measure it, giving reactor design its account of mixing and contacting. The referent is the spread of transit times through a flow system, with a mean equal to the vessel volume divided by the volumetric flow rate, the same quantity that governs how long water sits in an ocean basin or a drug stays in the body.[1]

Unit

The distribution (and mean) of times fluid elements spend in the vessel

Recovery means

Not applicable

Measured by

Measured by pulse- or step-tracer experiments at the outlet, from which the residence-time distribution and its mean are computed

In oceanography and the environmental sciences, residence time is the average time a parcel of water, or a dissolved substance, remains within a reservoir such as an ocean basin, a lake, or the atmosphere before leaving it. Bolin and Rodhe's 1973 A note on the concepts of age distribution and transit time in natural reservoirs distinguished residence (transit) time, age, and turnover time for such reservoirs and showed they coincide only in special cases. It is the identical reservoir-and-flux construct as the reactor's residence time (content divided by flux), realised in a natural water body and used to judge how quickly a basin flushes or a pollutant clears.[2]

Unit

The mean time water or a substance remains in a reservoir (reservoir content ÷ flux)

Recovery means

Not applicable

Measured by

Estimated as reservoir inventory divided by throughflux, or from radioactive and transient tracers that date the water

In pharmacokinetics, the mean residence time is the average length of time a drug molecule stays in the body before being eliminated, obtained from the statistical moments of the plasma concentration-versus-time curve. Yamaoka, Nakagawa, and Uno's 1978 Statistical moments in pharmacokinetics brought this moment-based, model-independent treatment into drug analysis, where the mean residence time equals the area under the first-moment curve divided by the area under the concentration curve. It is the same transit-time construct as the reactor's and the ocean basin's, applied to the body as a system through which the drug flows.[3]

Unit

The average time a drug molecule remains in the body, in hours

Recovery means

Not applicable

Measured by

Computed from the concentration-time curve as the area under the first-moment curve divided by the area under the curve (AUMC/AUC)