Residence time
Encyclopedia
Residence time is the average amount of time that a particle
spends in a particular system. This measurement varies directly with the amount of substance that is present in the system.
The residence time is a representation of how long it takes for the concentration to significantly change in the sediment.
Residence time is a widely used term that is mostly seen in science, technological and medical disciplines. Every discipline that uses residence time in some way adapts the definition in order to make it more specific to the application to which it is referring. The base definition for residence time also has a universal mathematical equation that can be added to and adapted for different disciplines. This is as follows:
The generic variable form of this equation is as follows:
where
is used as the variable for residence time, V is the capacity of the system, and q is the flow
for the system.
Residence time begins from the moment that a particle of a particular substance enters the system and ends the moment that the same particle of that substance leaves the system. The system in question is arbitrary and can be defined as needed according to the application. If the size of the system is changed, the residence time of the system will be changed as well. The larger the system, then larger the residence time, assuming the inflow
and outflow
rates are held constant. The smaller the system, the shorter the residence time will be, again assuming steady-state conditions.
Inflow and outflow will also have an effect on the residence time of a system. If the inflow and outflow are increased, the residence time of the system will be shorter. However, if the inflow and the outflow of a system are decreased, the residence time will be longer. This is assuming that the concentration of the substance in the system and the size of the system remain constant, and assuming steady-state Steady-state refers to a system in which the given parameter is held constant over time. conditions.
of the substance throughout the volume of the system. It is also assumed that chemical degradation (chemical decomposition
) does not occur in the system in question and that particles do not attach to surfaces that would hinder their flow. If chemical degradation were to occur in a system, the substance that originally entered the system may react with other existing compounds in the system, causing the residence time to be significantly shorter since the substance would be chemically broken down and effectively be removed from the system before it was able to naturally flow out of the system.
In environmental engineering, residence time applies to water treatment and wastewater treatment. It refers to the amount of time that water spends in a batch reactor
, plug flow reactor, completely mixed flow reactor (CMFR), and/or flocculation
tanks.
Batch reactors, plug flow reactors, and CMFR’s are used in wastewater treatment plants as a means of treating wastewater. Flocculation tanks are part of drinking water treatment facilities where the chemically treated water needs enough time to form flocs Flocs are colloidal particles that have combined with a coagulant in order to form large enough particles that will eventually settle out in the next phase of water treatment. before reaching the sedimentation basin. These processes are dependent on an adapted version of residence time. In this situation, the important parameter is how long a concentration of fluid needs to remain in the system for in order to be adequately treated.
Here the residence time is being used to determine the changing concentration of a contaminant in a system. This residence time is based on the inflow, outflow, volume, initial concentration of contaminant, the added chemical for treatment, and the rate at which the reactions take place. This is particularly useful for a flash mixer in a water treatment facility to determine if too little or too much of a chemical is initially being introduced into the system.
) refers to the quantity of time required to conduct outgassing
of accumulated gasses in vacuum environment. The amount of residence time required to achieve outgassing is directly dependent on the temperature of the environment. The higher the temperature, the less residence time in the vacuum environment is required to outgas the same quantity of material. Many vacuum chambers are wrapped with heaters to increase the temperature and thus "bake out" the outgassing molecules.
The equation for 
is referenced incorrectly in MANY books. Pisacane and others have a negative sign before 
. This is incorrect as it would cause additional heating to increase the time required for outgassing.
The reference residence time,
, is typically assumed to be 1.7 x 10^-13 (seconds), with experimental values typically between 10^-12 and 10^-14 (seconds). The activation energy, 
, is material dependent and ranges for 400 to 100,000 (Jules/Kmole).
s, lakes, streams, and oceans. More specifically it is the time during which water remains within an aquifer, lake, river, or other water body before continuing around the hydrological cycle. The time involved may vary from days for shallow gravel aquifer
s to millions of years for deep aquifers with very low values for hydraulic conductivity
. Residence times of water in rivers are a few days, while in large lakes residence time ranges up to several decades. Residence times of continental ice sheets is hundreds of thousands of years, of small glaciers a few decades.
Ground water residence time applications are useful for determining the amount of time it will take for a pollutant to reach and contaminate a ground water drinking water source and at what concentration it will arrive. This can also work to the opposite effect to determine how long until a ground water source becomes uncontaminated via inflow, outflow, and volume. The residence time of lakes and streams is important as well to determine the concentration of pollutants in a lake and how this may affect the local population and marine life.
Hydrology, the study of water, discusses the water budget in terms of residence time. The amount of time that water spends in each different stage of life (glacier, atmosphere, ocean, lake, stream, river), is used to show the relation of all of the water on the earth and how it relates in its different forms.
spends in the body. This is dependent on an individual’s body size, the rate at which the Drug will move through and react within the person’s body, and the amount of the Drug administered.
The Mean Residence Time (MRT) in Drug deviates from the previous equations as it is based on a statistical derivation. This still runs off a steady-state volume assumption but then uses the area under a distribution curve to find the average drug dose clearance time. The distribution is determined by numerical data derived from either urinary or plasma data collected. Each drug will have a different residence time based on its chemical composition and technique of administration. Some of these drug molecules will remain in the system for a very short time while others may remain for a lifetime. Since individual molecules are hard to trace, groups of molecules are tracked and the distribution of these is plotted to find a mean residence time. The equation for this distribution comes from the following equation:
MRT =
Particle
A particle is, generally, a small localized object to which can be ascribed physical properties. It may also refer to:In chemistry:* Colloidal particle, part of a one-phase system of two or more components where the particles aren't individually visible.In physics:* Subatomic particle, which may be...
spends in a particular system. This measurement varies directly with the amount of substance that is present in the system.
The residence time is a representation of how long it takes for the concentration to significantly change in the sediment.
Residence time is a widely used term that is mostly seen in science, technological and medical disciplines. Every discipline that uses residence time in some way adapts the definition in order to make it more specific to the application to which it is referring. The base definition for residence time also has a universal mathematical equation that can be added to and adapted for different disciplines. This is as follows:
The generic variable form of this equation is as follows:
where
is used as the variable for residence time, V is the capacity of the system, and q is the flowFlow
-Relating to the movement of material:* Fluid dynamics, or fluid flow, the motion of a gas or liquid* Environmental flow, the amount of water necessary in a watercourse to maintain a healthy ecosystem* Flow chemistry, a chemical reaction run in a continuous stream...
for the system.
Residence time begins from the moment that a particle of a particular substance enters the system and ends the moment that the same particle of that substance leaves the system. The system in question is arbitrary and can be defined as needed according to the application. If the size of the system is changed, the residence time of the system will be changed as well. The larger the system, then larger the residence time, assuming the inflow
Inflow
Inflow may refer to:*Inflow - In hydrology, the source of the water in a body of water*Infiltration/Inflow, inappropriate connection of surface runoff drains into sanitary sewers*Inflow , air that flows inwards to a thunderstorm...
and outflow
Outflow
Outflow may refer to:*Capital outflow - an economic term describing capital flowing out of a particular economy.*Bipolar outflow in astronomy represents two continuous flows of gas from the poles of a star....
rates are held constant. The smaller the system, the shorter the residence time will be, again assuming steady-state conditions.
Inflow and outflow will also have an effect on the residence time of a system. If the inflow and outflow are increased, the residence time of the system will be shorter. However, if the inflow and the outflow of a system are decreased, the residence time will be longer. This is assuming that the concentration of the substance in the system and the size of the system remain constant, and assuming steady-state Steady-state refers to a system in which the given parameter is held constant over time. conditions.
Assumptions
When using the residence time equation, a variety of assumptions are made to reduce the complexity of the system being modeled. These assumptions include, but are not limited to: steady state inflow and outflow, constant volume, constant temperature, and uniform distributionUniform distribution
-Probability theory:* Discrete uniform distribution* Continuous uniform distribution-Other:* "Uniform distribution modulo 1", see Equidistributed sequence*Uniform distribution , a type of species distribution* Distribution of military uniforms...
of the substance throughout the volume of the system. It is also assumed that chemical degradation (chemical decomposition
Chemical decomposition
Chemical decomposition, analysis or breakdown is the separation of a chemical compound into elements or simpler compounds. It is sometimes defined as the exact opposite of a chemical synthesis. Chemical decomposition is often an undesired chemical reaction...
) does not occur in the system in question and that particles do not attach to surfaces that would hinder their flow. If chemical degradation were to occur in a system, the substance that originally entered the system may react with other existing compounds in the system, causing the residence time to be significantly shorter since the substance would be chemically broken down and effectively be removed from the system before it was able to naturally flow out of the system.
Applications
Depending on the complexity of the system being modeled and the application for which it is being used, the residence time equation can be altered significantly or even used as a factor.Engineering
Residence time is widely used across all engineering disciplines, including chemical engineering, biological systems engineering, biomedical engineering, environmental engineering and geological engineering. The residence time formula is adapted for each of these disciplines depending on the system, the complexity, and the substance involved.In environmental engineering, residence time applies to water treatment and wastewater treatment. It refers to the amount of time that water spends in a batch reactor
Batch reactor
The Batch reactor is the generic term for a type of vessel widely used in the process industries. Its name is something of a misnomer since vessels of this type are used for a variety of process operations such as solids dissolution, product mixing, chemical reactions, batch distillation,...
, plug flow reactor, completely mixed flow reactor (CMFR), and/or flocculation
Flocculation
Flocculation, in the field of chemistry, is a process wherein colloids come out of suspension in the form of floc or flakes by the addition of a clarifying agent. The action differs from precipitation in that, prior to flocculation, colloids are merely suspended in a liquid and not actually...
tanks.
Batch reactors, plug flow reactors, and CMFR’s are used in wastewater treatment plants as a means of treating wastewater. Flocculation tanks are part of drinking water treatment facilities where the chemically treated water needs enough time to form flocs Flocs are colloidal particles that have combined with a coagulant in order to form large enough particles that will eventually settle out in the next phase of water treatment. before reaching the sedimentation basin. These processes are dependent on an adapted version of residence time. In this situation, the important parameter is how long a concentration of fluid needs to remain in the system for in order to be adequately treated.
-
-
- C=Concentration
- C0=Initial Concentration
- k= reaction rate constant
= batch reactor residence time
-
Here the residence time is being used to determine the changing concentration of a contaminant in a system. This residence time is based on the inflow, outflow, volume, initial concentration of contaminant, the added chemical for treatment, and the rate at which the reactions take place. This is particularly useful for a flash mixer in a water treatment facility to determine if too little or too much of a chemical is initially being introduced into the system.
Aerospace
In Aerospace Engineering, Residence Time () refers to the quantity of time required to conduct outgassingOutgassing
Outgassing is the release of a gas that was dissolved, trapped, frozen or absorbed in some material. As an example, research has shown how the concentration of carbon dioxide in the Earth's atmosphere has sometimes been linked to ocean outgassing...
of accumulated gasses in vacuum environment. The amount of residence time required to achieve outgassing is directly dependent on the temperature of the environment. The higher the temperature, the less residence time in the vacuum environment is required to outgas the same quantity of material. Many vacuum chambers are wrapped with heaters to increase the temperature and thus "bake out" the outgassing molecules.
The equation for is referenced incorrectly in MANY books. Pisacane and others have a negative sign before . This is incorrect as it would cause additional heating to increase the time required for outgassing.-
-
= Residence Time (seconds)
= Residence Time at 273.15 degrees Kelvin (seconds)
= The activation energy for desorption of contaminant (Jules/Kmole)
= The universal gas constant (J/(Kmole * Degrees Kelvin))
= Temperature (Degrees Kelvin)
-
The reference residence time,
, is typically assumed to be 1.7 x 10^-13 (seconds), with experimental values typically between 10^-12 and 10^-14 (seconds). The activation energy, , is material dependent and ranges for 400 to 100,000 (Jules/Kmole).Environmental
In environmental terms, the residence time definition is adapted to fit with ground water, the atmosphere, glacierGlacier
A glacier is a large persistent body of ice that forms where the accumulation of snow exceeds its ablation over many years, often centuries. At least 0.1 km² in area and 50 m thick, but often much larger, a glacier slowly deforms and flows due to stresses induced by its weight...
s, lakes, streams, and oceans. More specifically it is the time during which water remains within an aquifer, lake, river, or other water body before continuing around the hydrological cycle. The time involved may vary from days for shallow gravel aquifer
Aquifer
An aquifer is a wet underground layer of water-bearing permeable rock or unconsolidated materials from which groundwater can be usefully extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology...
s to millions of years for deep aquifers with very low values for hydraulic conductivity
Hydraulic conductivity
Hydraulic conductivity, symbolically represented as K, is a property of vascular plants, soil or rock, that describes the ease with which water can move through pore spaces or fractures. It depends on the intrinsic permeability of the material and on the degree of saturation...
. Residence times of water in rivers are a few days, while in large lakes residence time ranges up to several decades. Residence times of continental ice sheets is hundreds of thousands of years, of small glaciers a few decades.
Ground water residence time applications are useful for determining the amount of time it will take for a pollutant to reach and contaminate a ground water drinking water source and at what concentration it will arrive. This can also work to the opposite effect to determine how long until a ground water source becomes uncontaminated via inflow, outflow, and volume. The residence time of lakes and streams is important as well to determine the concentration of pollutants in a lake and how this may affect the local population and marine life.
Hydrology, the study of water, discusses the water budget in terms of residence time. The amount of time that water spends in each different stage of life (glacier, atmosphere, ocean, lake, stream, river), is used to show the relation of all of the water on the earth and how it relates in its different forms.
Pharmaceutical
For the medical field, residence time often refers to the amount of time that a DrugDrug
A drug, broadly speaking, is any substance that, when absorbed into the body of a living organism, alters normal bodily function. There is no single, precise definition, as there are different meanings in drug control law, government regulations, medicine, and colloquial usage.In pharmacology, a...
spends in the body. This is dependent on an individual’s body size, the rate at which the Drug will move through and react within the person’s body, and the amount of the Drug administered.
The Mean Residence Time (MRT) in Drug deviates from the previous equations as it is based on a statistical derivation. This still runs off a steady-state volume assumption but then uses the area under a distribution curve to find the average drug dose clearance time. The distribution is determined by numerical data derived from either urinary or plasma data collected. Each drug will have a different residence time based on its chemical composition and technique of administration. Some of these drug molecules will remain in the system for a very short time while others may remain for a lifetime. Since individual molecules are hard to trace, groups of molecules are tracked and the distribution of these is plotted to find a mean residence time. The equation for this distribution comes from the following equation:
MRT =
-
-
=total number of groups
=the average time in the body
=number of molecules in the ith group
=total number of molecules introduced into the system
-
See also
- Residence time distributionResidence Time DistributionThe residence time distribution of a chemical reactor is a probability distribution function that describes the amount of time a fluid element could spend inside the reactor. Chemical engineers use the RTD to characterize the mixing and flow within reactors and to compare the behavior of real...
- Lake retention timeLake retention timeLake retention time is a calculated quantity expressing the mean time that water spends in a particular lake. At its simplest this figure is the result of dividing the lake volume by the flow in or out of the lake...
- Water cycle: residence times