Mass Transfer (Set 4) 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.
Perforated plate towers are unique for solvent extraction because, they provide
repeated coalescence & redispersion of the drop phase.
In case of absorption & stripping, the interface of the liquid & gas phases are present in equilibrium, when the diffusional resistance of ____ is zero.
interface
Pick out the wrong statement pertaining to the analogy between equations of heat and mass transfer operations.
Reynolds number in mass transfer is analogous to Grashoff number in heat transfer.
The slope of the operating line for a single component co-current absorber when plotted in terms of mole ratio units is
(-) ve
The individual mass transfer co-efficients (moles/m2 . s) for absorption of a solute from a gas mixture into a liquid solvent are, KL = 4.5 and KG = 1.5. The slope of the equilibrium line is 3. Which of the following resistance (s) is (are) controlling?
gas side
Pore size of the molecular seive 5A is ____ mm.
0.5
Swenson-Walker crystalliser is a ____ unit
continuous
According to the Fenske equation, what will be the minimum number of plates required in a distillation column to separate an equimolar binary mixture of components A and B into an overhead fraction containing 99 mol% A and a bottom fraction containing 98 mol% B ? Assume that relative volatility (αAB = 2) does not change appreciably in the coloumn.
12
Absorption (liquid-gas system) with evolution of heat as compared to isothermal absorption results in
all (a), (b) and (c).
With increase in temperature, the mutual solubility of two liquids
increases
Pick out the wrong statement.
The capacity of a distillation coloumn is maximum at total reflux.
The relative volatility of a binary mixture at the azeotropic composition is
1
Relative volatility varies with the concentration of component for an ideal solution. The relative volatity of a binary mixture may be defined as the ratio of vapor pressure of component ‘A’ to that of component ‘B’, when
Vapor phase obeys Dalton’s law and liquid phase obeys Raoult’s law.
Bound moisture is that liquid which exerts an equilibrium vapor pressure ____ that of the pure liquid at the given temperature.
less than
Lewis number (Le) is
Sc / Pr
Which of the following is not a continuous leaching equipment?
Pachuka tank
____ extractor is used for the concentration of radioactive nuclear waste.
Pulsed column
Moisture content of a substance when at equilibrium with a given partial pressure of the vapour is called the ____ moisture.
equilibrium
All moisture in a non-hygroscopic material is the ____ moisture.
unbound
Pick out the correct relationship. (where, Ri = internal reflux ratio Ro = external reflux ratio.)
(1 + Ro) = Ro/Ri
Separation of ____ employs gaseous diffusion process.
isotopes of uranium
For water-ethanol system, the minimum reflux ratio
is computed from the slope of the upper operating line that is tangent to the equilibrium curve.
The interfacial area per unit volume of dispersion, in a gas-liquid contactor, for fractional hold up of gas = 0.1 and gas bubble diameter = 0.5 mm is given by (in m2/m3)
500
Though, theoretically the adsorption & desorption isotherms is expected to coincide but sometimes at least a part of the isotherms does not coincide and this phenomenon is known as adsorption hysterisis. In such cases, the desorption equilibrium pressure is ____ that obtained for adsorption.
lower than
Experimental determination of ____ is done by wetted wall column method.
mass transfer co-efficient
Pick out the wrong statement:
In case of a stripper, the equilibrium curve is always below the operating line.
In liquid extraction, if the selectivity is unity, then
no separation will occur.
Paper industry employs ____ driers.
cylinder
Which of the following has the same dimension as mass diffusivity?
Both (b) and (c)
Pick out the wrong statement pertaining to the cooling towers.
Effectiveness of a mechanical draft cooling tower is reduced with increase in air wet-bulb temperature.
Experiments were conducted to determine the flux of a species A in a stagnant medium across a gas-liquid interface. The overall mass transfer co-efficient based on the liquid side for dilute systems for the above was estimated to be 4 x 10-3 kg mole/m2.s. The equilibrium data for the system is given as y = 2x. The flux across the interface (in kg mole/m2 .s) for bulk concentrations of A in gas phase and liquid phase as y = 0.4 and x = 0.01 respectively is
8.5 x 10-4
Leaching of coarse solid lumps is also termed as
percolation
The main reason for dividing a tall packed tower into series of small towers is to
avoid channeling.
Co-current absorbers are usually used when the gas to be dissolved in the liquid is
a pure substance
At what percentage (by volume) of alcohol composition, it forms an azeotrope with water?
95
Measurement of the interfacial area of mass transfer is achieved easily & accurately in case of a ____ coloumn.
wetted wall
With the lowering of equiliDrium pressure, at a given temperature, the amount of ad-sorbate on the adsorbent
decreases.
Mass transfer co-efficient is defined as
Co-efficient = Flux/concentration difference.
When an unsaturated air-water mixture is heated at constant pressure, then
the relative humidity decreases.
During sensible cooling process
both (b) and (c)
At a fixed pressure, the humidity depends upon the partial pressure of vapor in the mixture. Humidity of a vapour free gas is ____ percent.
0
In case of physical adsorption, the difference between heat of adsorption and heat of normal condensation is
equal to the heat of wetting.
Pick out the wrong statement:
The equilibrium moisture content of the solid can be reduced by increasing the absolute humidity.
The value of L/mG ratio for economical absorption operation ranges from
1.25 to 2.0
Large scale usage of flash distillation is practised in
petroleum refinning
High pressure at the bottom of a distillation tower handling heat sensitive materials results in
thermal decomposition of bottoms.
Alcohol is dehydrated using ____ distillation.
azeotropic
Separation of two volatile liquids by distillation makes use of their
relative volatility
Which of the following increases during evaporative cooling process with recirculated water supply?
a. Wet bulb temperature
b. Relative humidity
c. Partial pressure of vapour
d. Both (b) and (c)
Both (b) and (c)
Dried finished product sampled from various parts of a ____ dryer exhibits non-uniformity in the moisture content.
tray
____ is used as an ‘entrainer’ for the separation of acetic acid-water mixture by distillation.
Butyl acetate
Rotary dryers cannot handle ____ materials.
sticky
Pick out the correct statement.
none of these.
An example of elution is
separation of uranium oxide from its ore by H2SO4 in a Pachuca tank.
The operating line for an absorber is curved, when plotted in terms of
partial pressure
Relative volatility does not change appreciably with the change in
temperature
Boiling point diagram is not affected by the ambient
both (b) and (c)
The dew point of a saturated gas phase equals the ____ temperature.
gas
A solid material shows case hardening properties while drying. Which of the following should be controlled to control the drying process?
Temperature of the solid
In case of liquid-liquid binary diffusion, diffusivity of one constituent into another is not dependent on the
none of these
For continuous drying of grannular or crystalline material, the dryer used is the ____ dryer.
rotary
When adsorption hysteresis is observed, the desorption equilibrium pressure is ____ that obtained by adsorption.
always lower than
Moisture in a substance exerting an equilibrium vapour pressure less than that of pure liquid at the same temperature is called the ____ moisture.
bound
For a binary mixture with low relative volatility, continuous rectification to get pure products will require
high reflux ratio
The efficiency of a plate column will be maximum, when the flow is ideal ____ the plate.
both (a) & (b)
Heat load in a cooling tower
both (a) & (b).
In case of absorption with exothermic reaction, for fluids having
both (a) and (b)
Detergent solution is dried to a powder in a
spray dryer
Which of the following mixtures does not form an azeotrope at atmosheric pressure?
None of these
During bottling of cold drinks (e.g. Pepsi), the main resistance to mass transfer for the absorption of carbon dioxide in water lies in the
liquid film
The overall mass transfer co-efficient for the absorption of SO2 in air with dilute NaOH solution can be increased substantially by
increasing the gas film co-efficient.
Flooding results in
low tray efficiency
For the air water system under ambient conditions, the adiabatic saturation temperature and the wet bulb temperature are nearly equal, because
water has a high latent heat of evaporation.
In an operating distillation column, the
driving force for the vapor flow is the pressure drop, as the pressure decreases gradually from the bottom to the top of the column.
In a forced draft cooling tower, water is cooled from 95 to 80°F by exposure to air with a wet bulb temperature of 70°F. In this case, the
both (a) & (b).
The diffusivity (D) in a binary gas mixture is related to the temperature (T) as
D ∝ T^1.5
For which of the following unit operations, Lewis number is of significance?
Humidification
Tray spacing in a distillation column of dia 10-12 ft. used in petroleum refinery may be around ____ inches.
30
If a1 and a2 are the relative volatilities when the pressure in the distillation column is 1 and 2 atm respectively. Pick out the correct statement.
none of these
The dimension of diffusivity is same as that of the
kinematic viscosity
The temperature to which a vapour gas mixture must be cooled (at varying humidity) to become saturated is called the ____ temperature.
none of these
Entrainer used in azeotropic distillation should
form a low boiling azeotrope with one of the constituents of the mixture.
In case of steam distillation, the steam leaving the liquid is not completely saturated with distillate vapour, because
mixing of steam with the material being vaporised is not so intimate as to result in equilibrium condition.
Pick out the wrong statement.
The operating pressure in solvent extraction system should be below vapor pressures of all components.
Leaching of coffee from coffee beans is done by
hot water
For contacting a highly soluble gas with a liquid
bubble the gas through liquid.
During drying operation, it is easier to remove the ____ moisture.
unbound
With increase in solvent rate, the number of transfer units, NtoG, for a fixed degree of absorption from a fixed amount of gas
decreases
Ion exchange process is similar to
adsorption
A distillation column with N plates is being operated under normal conditions. At some point of time, the operation is shifted to total reflux condition (i.e., no product and residue are being withdrawn and feed to the column is stopped). At the new steady state,
the top and bottom compositions correspond to the maximum enrichment available.
Dryer widely used in a textile industry is ____ dryer.
conveyor
Solvent used in extractive distillation
changes the relative volatility of the original components.
Air at a particular humidity is heated in a furnace. The new dew point
increases
In a packed tower, the value of HETP equals HTUOG, when the equilibrium and the operating lines are (where, HETP = height equivalant to a theoretical plate HTUOG = overall gas phase height of a transfer unit)
both (a) & (b)
The vapour pressure exerted by the moisture contained in a wet solid depends upon the
all (a), (b) and (c).
The absorption factor is defined as (where, L = liquid flow rate, G = gas flow rate and, m = slope of the equilibrium line)
L/mG
Smoke is an example of
solid dispersed in gas
Which of the following adsorbant is used in the refinning of sugar?
Bone charcoal
Stacked packing compared to dumped packing
both (a) and (b).
In McCabe-Thiele method of theoretical plate calculation for a distillation column, the operating lines of stripping and rectifying sections coincide with the diagonal at ____ reflux.
total
