Mass Transfer (Set 3) Flashcards
As one of the transport phenomena, the concepts in mass transfer are crucial for design, operation, and maintenance of mass or heat & mass transfer equipment such as mixing and separation processes.
Solvent extraction is the terminology applied to the liquid-liquid extraction, which is preferred for the separation of the components of liquids, when
one of the liquid components has very high affinity towards the solvent.
Inside the distillation column, the
a. driving force for the vapour flow is the pressure drop.
b. liquids are not always at their bubble points.
c. pressure increases gradually from bottom to the top of the column.
d. none of these
driving force for the vapour flow is the pressure drop.
Diffusivity in concentrated solutions differs from that in dilute solutions, because of the changes in
both (a) and (b).
viscosity with concentration. & degree of ideality of the solution.
Occurrence of ‘case hardening’ during drying of a high moisture solid cake ____ the drying rate.
decreases
Which of the following is not a continuous drier?
Tray drier
Humidification involves mass transfer between a pure liquid phase and a fixed gas, which is
insoluble in the liquid
____ is the temperature at which a gas-vapor mixture becomes saturated, when cooled at constant total pressure out of contact with a liquid.
Dew point
Rate of leaching increases with increasing
temperature
An azeotropic mixture is a ____ mixture.
constant boiling point
Pick out the wrong statement.
For Laminar flow over a plate of length L, the local mass transfer co-efficient at a distance L from the leading edge is 1.5 x 10-2m/s. Then the average mass transfer co-efficient for the plate is 2 x 10-2m/s.
Molecular seives are regenerated by heating to ____ °C.
200-330
In continuous distillation, the internal reflux ratio (Ri) and the external reflux ratio (Re) are related as
Ri = Re/1+Re
In a counter-current extractor, as the axial mixing increases, the extraction efficiency
decreases
Heat sensitive materials like certain pharmaceuticals and food stuffs can be dried in a/an ____ dryer.
freeze
In extractive distillation, solvent is
added to alter the relative volatility of the mixture.
Radioactive nuclear waste is treated in
pulsed column extractor
Large scale drying of sugar is done in a ____ dryer.
rotary
____ temperature is the steady state temperature attained by a small amount of liquid evaporating into a large quantity of unsaturated gas-vapor mixture.
Wet bulb
The drying time between fixed moisture contents within the ‘constant rate period’ is proportional to (assuming that drying occurs from all surfaces of the solid) (where, T = thickness of the solid)
T
____ equation applies to diffusivities in liquids.
Gilliland
A packed tower compared to a plate tower for a particular mass transfer operation
incurs smaller pressure drop.
Which of the following is the most commonly used leaching solvent in vegetable oil industry?
Hexane
In an absorber, HETP does not vary with the
none of these
Large quantity of silica gel is normally dried in a ____ dryer.
through circulation
For the same system, if the same liquid used in an absorber is decreased, the tower height will
increase
Hydrogenation of oil is carried out in a/an ____ in Vanaspati manufacturing plant.
agitated vessel
Sides of equilateral-triangular co-ordinates (on which ternary liquid system is plotted) represent
a binary mixture.
In distillation column design, the McCabe-Thiele procedure is inadequate and a Ponchan-Savarit procedure is needed, when
a total condenser is used.
In an interphase mass transfer process, the lesser the solubility of a given solute in a liquid, the higher are the chances that the transfer process will be
gas phase resistance controlled.
As the reflux ratio decreases, the
number of plates increases.
Which of the following equipments is not used for gas dispersion?
Spray towers
One mole feed of a binary mixture of a given composition is flash vaporised at a fixed P and T. If Raoult’s law is obeyed, then changing the feed composition would effect
the fraction vaporised but not the product composition.
The equation, https://www.indiabix.com/_files/images/chemical-engineering/mass-transfer/315-483-1.png is for (see attached picture)
steady state equimolar counter diffusion
With increase in the liquid flow rate at a fixed gas velocity in a randomly packed counter current gas-liquid absorption column, the gas pressure drop
increases
The equation, https://www.indiabix.com/_files/images/chemical-engineering/mass-transfer/314-467-1.png is for (see attached ppicture)
stready state diffusion for stagnant case.
Which of these columns incurs the lowest pressure drop?
Packed column (with stacked packing)
In the desorption of highly soluble gas from the liquid, the main resistance will be in the ____ phase.
liquid
If G = insoluble gas in gas stream and L = non-volatile solvent in liquid stream, then the slope of the operating line for the absorber is
L/G
Which of the following remains constant during evaporative cooling process with recirculated water supply?
Wet bulb temperature
For stripping of a gas in a counter current stripper, the operating line
can lie above or below the equilibrium curve.
Which of the following same diameter columns gives lowest pressure drop per unit height?
Packed column (stacked)
Diffusion in concentrated solutins differs from that in dilute solutions, because of the change in the ____ with the concentration of the solution.
both ‘a ‘& ‘b’
Stage efficiency in a leaching process depends on the
both (a) and (b)
Operating velocity in a packed tower is usually ____ the flooding velocity.
half
The Knudsen diffusivity is dependent on the
molecular velocity and por radius of the catalyst.
Minimum possible diameter of a packed coloumn is decided mainly by the
flooding
Solvent used in extractive distillation
forms a low boiling azeotrope.
Capacity of a rotary dryer depends on its
both (a) and (b)
The cooling effect in a cooling tower can be speeded up by
all (a), (b) and (c).
The following plot gives the saturated humidity (He) versus temperature (T).Line joining (H1, T1) and (H2, T2) is the constant enthalpy line. Choose the correct one from among the alternatives A, B, C and D. The following plot gives the saturated humidity (He) versus temperature (T).Line joining (H1, T1) and (H2, T2) is the constant enthalpy line. Choose the correct one from among the alternatives A, B, C and D. https://www.indiabix.com/_files/images/chemical-engineering/mass-transfer/335-677-1.png
T1-Dry bulb temp; T2-Wet bulb temp; T3-Dew point temp.
When both the fluids flow concurrently in an absorber, the slope of the operating line is
(-) ve
The percentage humidity is less than the relative humidity only at ____ percent humidity.
both zero and hundred
Back-trapping in a distillation column
decreases tray efficiency.
Flooding in a vapor-liquid contacting equipment occurs in a tray, when the pressure drop through a tray is ____ the liquid head available in the downcomer.
more than
Milk is dried usually in a ____ dryer.
spray
At a given equilibrium pressure, with increase in temperature, the concentration of adsorbed gas on solid adsorbant
decreases
Which of the following adsorbent is used to decolourise yellow glycerine?
Activated carbon
Which of the following liquid temperature (T) Vs. vapor-gas mixture enthalpy (H) diagram is valid for humidification operation?
https://www.indiabix.com/_files/images/chemical-engineering/mass-transfer/317-499-2.png
In a distillation column, with increase in the reflux ratio, the heat removed in the cooler
increases.
Pick out the wrong statement.
Relative volatility of a binary mixture changes appreciably with the minor change in temperature.
Which of the following is not a unit of mass transfer co-efficient?
None of these
The falling rate period in the drying of a solid is characterised by
increasing temperatures both on the surface and within the solid.
In which of the following unit operations, the selectivity is an important parameter?
Solvent extraction
Which of the following is an undesirable characteristic for the solvent used in gas absorption?
None of these
Which of the following equipments is not used for liquid dispersion?
Agitated vessels
Diameter of raschig rings used in packed tower in industry is normally around ____ inches.
2
The diffusion co-efficient in m2 /s. of acetic acid in benzene (liquid in liquid) is
2.09 x 10-9
Pick out the wrong statement.
According to film theory for equimolar counter diffusion, the mass transfer coefficient is given by sqrt(DAB)(b)P - 3, Q - 2
In case of non-ideal gases and liquids, the molal diffusivity
is independent of pressure.
Condensation of a vapour-gas mixture just begins, when (where, p = partial pressure of the vapour P = vapour pressure of the liquid)
p = P
____ number indicates the ratio of the rates of the heat and mass transfer incase of a cooling tower.
Lewis
The expression, y(1 - x)/x(1 - y) is for
relative volatility
In physical terms, Schmidt number means
momentum diffusivity/mass diffusivity.
In case of liquids, the binary diffusivity is proportional to (where, T = temperature)
T
The diffusivity, DAB (for component A diffusing in B) is equal to the diffusivity DBA (for component B diffusing in A) for a binary mixture of
ideal gases
Mass transfer co-efficient is directly proportional to DAB0.5, according to ____ theory.
penetration
Sand is dried in foundaries in a___drier.
fluidised bed
Which of the following employs an accessory known as ‘drift eliminator’?
Mechanical draft cooling tower.
Pick out the correct statement.
For a non-reacting binary mixture of ideal gases, the partial pressure distribution of both components is linear in the case of steady state equimolal counter-diffusion.
Pick out the correct statement.
all (a), (b) and (c).
Humid volume is the total volume in m3 of 1 kg of
gas plus its accompanying vapour at 1 atm. and gas temperature.
Diameter to height ratio for a rashig ring is
1
The equipment used in the extraction of highly corrosive radio active liquids is
pulse column
Advantage of continuous drying over batch drying is that the
all ‘a’, ‘b’ & ‘c’
With increase in temperature, the rate of extraction in leaching (solid-liquid extraction system)
increases
In a packed bed absorption column, the channelling will be noted by the
sharp drop in pressure drop.
Gaseous diffusivity at atmospheric pressure is of the order of ____ cm2/second.
1
In liquid-liquid extraction, the number of phases at plait point is
1
In a liquid-liquid extraction, 10 kg of a solution containing 2 kg of solute C and 8 kg of solvent A is brought into contact with 10 kg of solvent B. Solvent A and B are completely immiscible in each other whereas solute C is soluble in both the solvents. The extraction process attains equilibrium. The equilibrium relationship between the two phases is Y* = 0.9X, where Y* is the kg of C/kg of B and X is kg of C/kg of A. Choose the correct answer.
The entire amount of C is transferred to solvent B.
The surface renewal frequency in Danckwerts’ model of mass transfer is given by (KL: mass transfer co-efficient)
sqrt(KL2.DA)
If Rm is the minimum reflux ratio, the optimum reflux ratio may be around ____ Rm.
1.2 to 1.5
During dehumidification of unsaturated air wet bulb temperature & partial pressure of vapor are not constant. However, during evaporative cooling process with recirculated water supply, the ____ remains constant.
wet bulb temperature
In case of absorption, both the operating and equilibrium lines will be straight for
dilute solutions and isothermal operation.
Mass transfer co-efficient varies as DAB0.5, according to the ____ theory.
surface renewal
For an absorber, both equilibrium and operating line will be straight for
dilute solution and isothermal operation.
A distillation column separates 10000 kg/hr of a benzene-toluene mixture as shown in the figure below: https://www.indiabix.com/_files/images/chemical-engineering/mass-transfer/335-680-1.png In the figure xF, xD and xW represent the weight fraction of benzene in the feed, distillate and residue respectively. The reflux ratio is
0.6
Unsaturated air (with dry bulb and wet bulb temperatures being 45°C and 25°C respectively) is passed through a water spray chamber maintained at 23°C. The air will be cooled and
humidified with wet bulb temperature decreasing.
Humid volume, at a given temperature is a/an ____ function of humidity.
linear
The Schmidt number which is defined as, μ/ρ D, is the ratio of the
momentum diffusivity to the mass diffusivity.
If f is defined as above, then which of the following applies to a feed at dew point ?
f = 1
