Transport Phenomena Exam 2 Key Equations

Mass flux
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Terms in this set (33)
Mass flux
mass flow rate/ area perpendicular to flow

units: g/scm^2
molar flux
molar flow rate/area perpendicular to flow

units: mol/scm^2
Flux symbol
JA
Mass transfer rate symbol
NA
Fick's Law of Diffusion
NAX = -Dab(A)(delta CA/delta x)
Diffusion coefficient or mass diffusivity of A in B symbol
Dab
For the mass diffusivity of chemical species A, which of the following is TRUE?
a. It is a property of species A
b. It is affected by other species in the system
c. It may vary with changes in temperature
d. all of the above
e. none of the above
all of the above
For 1-D, steady-state, diffusion mass transfer with no generation or consumption, and constant properties, in which of the following geometries is the mass flux constant?
a. slab geometry
b. cylindrical geometry
c. spherical geometry
d. all of above
e. none of the above
slab geometry
In Fick's Law, why is there a negative sign before the product of mass diffusivity and concentration gradient?
a. the negative sign makes the concentration gradient always positive
b. the sign of the mass flux is always the opposite of the sign of the concentration gradient
c. this makes the mass flux always go in the positive direction
d. all of the above
e. none of the above
the sign of the mass flux is always the opposite of the sign of the concentration gradient
In 3-D mass transfer, which of the following is a vector quantity/
a. the concentration gradient
b. the mass flux
c. the mass transfer rate
d. all of the above
e. none of the above
all of the above
Consider a hydrogel slab where there is 1-D, diffusion mass transfer at steady-state, constant properties, and generation of species A. If one of the boundaries is impermeable to species A, which of the following is TRUE? a. the concentration of species A at a given position is changing with time b. the concentration gradient of species A does not change with position c. the mass flux of species A is not changing at one boundary d. all of the above e. none of the abovethe mass flux of species A is not changing at one boundaryThe laplacian term in the vector form of the mass transfer equation represents a. mass transfer by diffusion b. mass transfer by convection c. accumulation of chemical species in the system d. all of the above e. none of the abovemass transfer by convectionIn our discussion of mass transfer with chemical reaction, the RA a. happens only in one part of the system b. is always negative c. is on a per-volume basis d. all of the above e. none of the aboveis no a per volume basisWhich CANNOT be a boundary condition for mass transfer problems? a. CA = 0 b. dCA/dx = constant c. CA - solubility of gas in the solid d. all of the above can be boundary conditionsall of the above can be boundary conditionsFor oxygen gas diffusing into the blood through the alveolar membrane in the lungs, which of the following is used to determine a boundary condition? a. Antoine's equation b. Raoult's law c. henry's law d. all of the above e. none of the abovehenry's lawA glass that is half-full of water is placed on a table in a room with no air circulation. The water evaporates, and the water vapor diffuses out of the glass. What can you say about the air in contact with the water's surface? a. the water vapor concentration of the air in contact with the water surface is equal to the concentration of liquid water b. the water vapor concentration of the air in contact with the water surface is in equilibrium with the liquid water at the surface c. the water vapor concentration of the air contact with the water surface can be determined using Henry's law d. all of the above e. none of the abovethe water vapor concentration of the air in contact with the water surface is in equilibrium with the liquid water at the surfaceConsider a mass transfer system where species A is simultaneously diffusing and undergoing the following reaction A->B. If the reaction is elementary, what is the expression for RA? a. RA = - constant b. RA = - kACA c. RA = - kBCB d. RA = - kBCACB e. none of the aboveRA = - kACAConsider a mass transfer system where species A is simultaneously diffusing and undergoing the following reaction 2A->B. If the reaction is elementary, what is the expression for RA? a. RA = - constant b. RA = - kACA^2 c. RA = - kBCB^2 d. RA = - kBCACB e. none of the aboveRA = - kACA^2Consider a mass transfer system where species A is simultaneously diffusing and undergoing the following reaction B->A. If the reaction is elementary, what is the expression for RA? a. RA = - constant b. RA = - kACA c. RA = - kBCB d. RA = - kBCACB e. none of the aboveRA = - kBCBFor series mass transfer through multiple resistances, which of the following is TRUE? a. the system is at steady-state b. there can be mass transfer by diffusion c. there can be mass transfer by convection d. all of the above e. none of the aboveall of the abovemass concentrationPA mass of A/unit volume units: m^2/smolar concentrationCA moles of A/unit volume units: mol of A/m^3Concentration of a chemical speciesPA = CAMAMolecular weight or molar massMADiffusionmovement of individual particles, molecules, atoms and ionsConvective mass transferthe movement of mass through a medium as a result of the net motion of a material in the mediumForced Convectiondue to the external force (e.g. fan)Free convectiondriven by a density difference in the fluid (created by concentration or temperature differences)mass transfer coefficienthm - a function of system geometry, fluid properties, flow characteristics, the magnitude of concentration difference (delta CA) - not a property of the material - experimentally calculated - always includes the effect of diffusion in the fluidFick's Law (differential form)JAX = -Dab(dCA/dx) - one dimension form - can happen in all three directionsMass flow rate equationNA1-2 = hm(A)(C1-C2) units: m/sMass transfer assumptions- Geometry (cartesian, cylindrical, spherical) - diffusion of mass transfer - unsteady or steady state - # of dimensions - generation/consumption or no generation/consumption of the chemical speciesGeneration of species ARA RA = mass or moles of A produced or consumed/ (time)(unit volume) Consumption = - generation