Mass Transfer Flashcards
With increase in pressure, the relative volatility for a binary system
decreases
The McCabe Δ L law states that the
linear crystal growth rate DOES NOT depend on the crystal size.
For the same feed, feed quality and separation (in a distillation column), with the increase of total pressure, the number of ideal plates will
increase
In the constant rate period of the rate of drying curve for batch drying
surface evaporation of unbound moisture occurs.
The reason for preferring packed towers over plate towers in distillation practice is that the packed tower operation gives
low pressure drop and low hold up
Weeping in a distillation column
results due to very low gas velocity.
With decrease in the throughput (compared with the design capacity) for a bubble cap distillation column, its efficiency
increases
Pick out the system with minimum boiling azeotrope at 1 atm
ethyl alcohol-water
Free flowing granular materials can be best dried in a __________ drier.
rotary
Corresponding to Nusselt number in heat transfer, the dimensionless group in mass transfer is the __________ number.
Sherwood
Liquid diffusivity is of the order of __________ cm2 /second.
10-5 to 10-6
The partial pressure distribution of an ideal gas diffusing through another stagnant ideal gas at steady state follows a/an __________ law
exponential
The binary diffusivity in gases and liquids vary respectively as
T3/2 and T
Pick out the correct statement.
Dialysis process is used for the production of fresh water from brine.
Electrodylsis is used in the manufacture of NaOH.
Electrodylsis is used for the refin-ning/purification of petrol.
both (b) and (c).
Electrodylsis is used in the manufacture of NaOH.
Moisture contained by a substance in excess of the equilibrium moisture is called the __________ moisture.
free
With increase in temperature, the solubility of gases in liquids, at fixed pressure
decreases
On moving the feed line (q-line) from saturated liquid feed (vertical position) to saturated vapor feed (horizontal feed), if the slope of both the operating lines are to be increased, then it will result in
increased reflux ratio
__________ column is the most suitable for achieving the best performance for mass transfer operations involving liquid with dispersed solids.
Plate
Use of raschig rings in place of crushed stones as packing in packed beds (other things being same)
decreases pressure drop, increases surface area.
Pick out the wrong statement:
Reboiler is not used in open steam distillation.
The separation of solutes of different molecular sizes in a liquid solution by use of a suitable membrane is called dialysis.
Freeze drying is used for the drying of fish.
Fenske-Underwood equation is used for calculating theoretical number of plates in a distillation column at normal operating reflux condition.
Fenske-Underwood equation is used for calculating theoretical number of plates in a distillation column at normal operating reflux condition.
Pick out the wrong statement.
Critical moisture content is not a property of the material itself.
A sharp increase in pressure drop gives an indication of the flooding in a distillation column.
Separation of components is not possible in liquid extraction, if selectivity is unity.
Dehydration of ethyl alcohol is most economically done by molecular distillation at very high vacuum.
Dehydration of ethyl alcohol is most economically done by molecular distillation at very high vacuum.
Absorption factor method is used to calculate the number of ideal stages, when
operating line lies above the equilibrium line.
operating line lies below the equilibrium line.
both operating and equilibrium lines are parallel.
pressure drop in the column is very high.
both operating and equilibrium lines are parallel.
Flights in a rotary dryer are provided to
lift and shower the solids thus exposing it thoroughly to the drying action of the gas.
If the solubilities of different components (in a liquid-liquid extraction system) increase with rise in temperature, then the temperature above which they dissolve completely is known as the critical solution temperature (CST or consolute temperature). If solubilities increase with decrease in temperature, then CST is the temperature below which they dissolve completely. If a binary system has no critical solution temperature, it implies that
on heating, a vapor phase will appear ; while on cooling, a solid phase will appear.
Under conditions of flooding in packed tower, the gas pressure drop
is maximum
For absorbing a sparingly soluble gas in a liquid
liquid side mass transfer co-efficient should be increased.
Channelling in a packed tower results from the
high pressure drop
maldistribution of liquid
non-uniformity of packing
both (b) and (c)
both (b) and (c)
The change in enthalpy per unit weight of adsorbed gas when adsorbed on gas free or “outgassed” adsorbent to from a definite concentration of adsorbate is called its
integral heat of adsorption.
The wetted wall tower is used to determine
individual mass transfer co-efficient (M.T.C.) in gaseous system.
The caking of crystals can be prevented by
maintaining high critical humidity.
maintaining low critical humidity.
coating the product with inert material.
both (a) and (c)
both (a) and (c)
The diffusivity (D) in a binary gas mixture is related to the pressure (P) as
D ∝ P0.5
D∝ 1/P0.5
D ∝ 1/P
D∝ 1/P1.5
D ∝ 1/P
Which of the following psychrometric processes is followed by water vapour laden unsaturated air, when it is passed through solid or liquid adsorbent ?
a. Cooling and dehumidification.
b. Heating and dehumidification at almost constant wet bulb temperature.
c. Dehumidification with dry bulb temperature remaining constant.
d. None of these.
b. Heating and dehumidification at almost constant wet bulb temperature.
The value of NA/(NA + NB) for steady state molecular diffusion of gas ‘A’ through non-diffusing gas ‘B’ is
1
∞
0.5
2
1
Large scale drying of wheat is done in a __________ dryer.
spouted bed
The drying time between fixed moisture content within diffusion controlled ‘falling rate period’ is proportional to (assuming that drying occurs from all surfaces of the solid) (where, T = thickness of the solid )
T2
At the boiling point of the liquid at the prevailing pressure, the saturated absolute humidity becomes
∞
Pressure drop through plate tower as compared to that through packed tower, for the same duty will be
more
Dorr thickener is an equipment used for
sedimentation
Steam distillation is used to separate
high boiling substances from nonvolatile impurities.
Water entrained by circulating air in cooling towers is termed as
drift
Humidity chart is useful for the solution of problems concerning condensation, vaporisation and air conditioning. At a given dry bulb temperature, value of humidity obtained from the humidity chart directly, is in terms of __________ humidity.
molal
The Kundsen diffusivity is proportional to (where, T = absolute temperature)
sqrtT
In an absorption coloumn, the flooding velocity for random packing is __________ that for stacked/regular packing.
smaller than
The relation among various mass transfer co-efficients (M.T.C) for ideal gases is given by (where, Kc & Km are M.T.C. for equimolar counter diffusion with concentration & mole fraction respectively as the driving force. and, Kp = M.T.C. for diffusion of a gas through a stagnant inert gas with pressure as driving force.)
Kc = Kp . RT = Km . RT/p
The fixed cost of a distillation column at minimum reflux ratio is
infinite
In a binary distillation column, if the feed contains 40 mole % vapour, the q line will have a slope of
-1.5
Pick out the wrong statement.
A. Mechanical agitation is required, if the system has low interfacial tension.
B. Seive tray towers are generally used for systems having low interfacial tension.
C. When Henry’s law constant is very small, then the mass transfer rate is controlled by the gas film resistance.
D. Taylor-Prandtl analogy for heat and mass transfer considers the transfer through both laminar and turbulent layers.
A. Mechanical agitation is required, if the system has low interfacial tension.
Acetic acid will be most economically separated from a dilute solution of acetic acid in water by
A. solvent extraction
B. continuous distillation
C. evaporation
D. absorption
A. solvent extraction
Design calculation for multiple component distillation is done by
tray to tray calculations.
To get high tray efficiency
Interfacial surface between liquid and gas phase should be large.
The mass transfer co-efficient for a solid sphere of radius ‘a’ dissolving in a large volume of quiescent liquid, in which D is the diffusivity of solute, is
dependent on the Reynolds number
Pick out the wrong statement. Generally for physical adsorption, a gas of
A. higher molecular weight is adsorbed in preference to a gas of low molecular weight.
B. high critical temperature is adsorbed in preference to a gas of low critical temperature.
C. low volatility is adsorbed in preference to a gas of high volatility.
D. none of these.
D. none of these.
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
driving force for the vapour flow is the pressure drop.
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