Residence time
Then, in 1953, Peter Danckwerts resurrected the axial dispersion model and formulated the modern concept of residence time.
[7] In an ideal plug flow reactor (PFR) the fluid particles leave in the same order they arrived, not mixing with those in front and behind.
A non-zero variance indicates that there is some dispersion along the path of the fluid, which may be attributed to turbulence, a non-uniform velocity profile, or diffusion.
If the RTD curve shows more than one main peak it may indicate channeling, parallel paths to the exit, or strong internal circulation.
Just as was noted for a plug-flow reactor, an early mean will indicate some stagnant fluid within the vessel, while the presence of multiple peaks could indicate channeling, parallel paths to the exit, or strong internal circulation.
Short-circuiting fluid within the reactor would appear in an RTD curve as a small pulse of concentrated tracer that reaches the outlet shortly after injection.
This method required the introduction of a very small volume of concentrated tracer at the inlet of the reactor, such that it approaches the Dirac delta function.
[10][8] Although an infinitely short injection cannot be produced, it can be made much smaller than the mean residence time of the vessel.
can be transformed into a dimensionless residence time distribution curve by the following relation: The concentration of tracer in a step experiment at the reactor inlet changes abruptly from 0 to
In a homogeneous, first-order reaction, the probability that an atom or molecule will react depends only on its residence time: for a rate constant
[11] Hydraulic residence time (HRT) is an important factor in the transport of environmental toxins or other chemicals through groundwater.
The amount of time that a pollutant spends traveling through a delineated subsurface space is related to the saturation and the hydraulic conductivity of the soil or rock.
This is because the pore sizes are much larger in gravel media than in clay, and so there is less hydrostatic tension working against the subsurface pressure gradient and gravity.
Groundwater flow is important parameter for consideration in the design of waste rock basins for mining operations.
[5] When water flows through a volume at a slower rate, less energy is available to keep solid particles entrained in the stream and there is more time for them to settle to the bottom.
Typical HRTs for sedimentation basins are around two hours,[5] although some groups recommend longer times to remove micropollutants such as pharmaceuticals and hormones.
The types of pathogens that occur in untreated water include those that are easily killed like bacteria and viruses, and those that are more robust such as protozoa and cysts.
Atoms and molecules of gas or liquid can be trapped on a solid surface in a process called adsorption.
If the chamber can be heated, the above equation shows that the gases can be "baked out"; but if not, then surfaces with a low residence time are needed to achieve ultra-high vacuums.
[16]: 195 In environmental terms, the residence time definition is adapted to fit with ground water, the atmosphere, glaciers, lakes, streams, and oceans.
Residence times of continental ice sheets is hundreds of thousands of years, of small glaciers a few decades.
This can also work to the opposite effect to determine how long until a ground water source becomes uncontaminated via inflow, outflow, and volume.
Drugs with long residence times are desirable because they remain effective for longer and therefore can be used in lower doses.
[17]: 88 This residence time is determined by the kinetics of the interaction,[18] such as how complementary the shape and charges of the target and drug are and whether outside solvent molecules are kept out of the binding site (thereby preventing them from breaking any bonds formed),[19] and is proportional to the half-life of the chemical dissociation.
[18] One way to measure the residence time is in a preincubation-dilution experiment where a target enzyme is incubated with the inhibitor, allowed to approach equilibrium, then rapidly diluted.
[20]: 196 If the drug is delivered through a mucous membrane in the mouth, the residence time is short because saliva washes it away.
Strategies to increase this residence time include bioadhesive polymers, gums, lozenges and dry powders.
[20]: 274 In size-exclusion chromatography, the residence time of a molecule is related to its volume, which is roughly proportional to its molecular weight.
[1] Biofuel cells utilize the metabolic processes of anodophiles (electronegative bacteria) to convert chemical energy from organic matter into electricity.
[23] Shorter HRTs support the development of non-exoelectrogenous bacteria which can reduce the Coulombic efficiency electrochemical performance of the fuel cell if the anodophiles must compete for resources or if they do not have ample time to effectively degrade nutrients.
