Mass Transfer Flashcards
Which of the following is the most suitable for extraction in a system having very low density difference ?
Centrifugal extractor
Centrifugal extractor
When the temperature and humidity of air is low, we usually use __________ draft cooling tower.
natural
natural
__________ columns are used for liquid dispersion in a continuous gas phase.
Packed
Packed
Which is the controlling factor for a drum drier?
Heat transfer
Heat transfer
Air initially at 101. 3 kPa and 40°C and with a relative humidity of 50%, is cooled at constant pressure to 30°C. The cooled air has a
higher relative humidity
higher relative humidity
In batch distillation with constant reflux, overhead product composition __________ with time.
decreases
decreases
To increase the absorption factor, (where, G = gas flow rate, S = solvent flow rate)
increase ‘S’ and decrease ‘G’.
increase ‘S’ and decrease ‘G’.
In case of an absorber, the operating
line always lies above the equilibrium curve.
line always lies above the equilibrium curve.
Absorption accompanied with chemical reaction is exemplified by the absorption of
SO2 in alkaline solution.
SO2 in alkaline solution.
Which of the following liquid-vapor contacting devices provides maximum contact surface area for a particular duty ?
Wetted wall column
Wetted wall column
The equipment frequently used for adiabatic humidification-cooling operation with recirculating liquid is
spray chamber
spray chamber
The operation involved when the soluble material is largely on the surface of an insoluble solid and is merely washed off by the solvent is called
elutriation or elution
elutriation or elution
In the constant rate period of the rate of drying curve for batch drying,
surface evaporation of unbound moisture occurs.
surface evaporation of unbound moisture occurs.
It takes 6 hours to dry a wet solid from 50% moisture content to the critical moisture content of 15%. How much longer it will take to dry the solid to 10% moisture content, under the same drying conditions? (The equilibrium moisture content of the solid is 5%).
71 min
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.
Compound A is extracted from a solution of A + B into a pure solvent S. A Co-current unit is used for the liquid-liquid extraction. The inlet rate of the solution containing A is 200 moles of B/hr.m2 and the solvent flow, rate is 400 moles of S/m2. hr. The equilibrium data is represented by Y = 3X2 , where Y is in moles of a A/moles of B and X is in moles A/moles of S. The maximum percentage extraction achieved in the unit is
50%
50%
Wetted wall tower experiment determines the
mass transfer co-efficient.
mass transfer co-efficient.
With decrease in the throughput (compared with the design capacity) for a bubble cap distillation column, its efficiency
increases
increases
The equilibrium liquid composition compared to the vapor composition in case of azeotropic mixture is
same
same
Out of the following gas-liquid contacting devices, for a given set of operating conditions, gas pressure drop is the least in __________ tower.
wetted wall
wetted wall
Free flowing granular materials can be best dried in a __________ drier.
rotary
rotary
In case of an ideal solution, the total vapor pressure varies __________ with the composition expressed as mole fraction,
linearly
linearly
If the specific heats of a gas and a vapor are 0.2KJ/Kg.°K and 1.5 KJ/Kg.°K respectively, and the humidity is 0.01; the humid heat in KJ/°Kg. is
0.215
0.215
At the same gas flow rate, the pressure drop in a packed tower being irrigated with liquid __________ that in dry packed tower.
is greater than
is greater than
The partial pressure distribution of an ideal gas diffusing through another stagnant ideal gas at steady state follows a/an __________ law.
exponential
exponential
(NRe . NSc) is termed in mass transfer operation as the __________ number.
Peclet
Peclet
__________ column is the most suitable for achieving the best performance for mass transfer operations involving liquid with dispersed solids.
Plate
Plate
When the feed to a distillation column is a saturated liquid, slope of the feed line is
infinity
infinity
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.
on heating, a vapor phase will appear ; while on cooling, a solid phase will appear.
At equilibrium the concentration of water in vapour phase (C)in kg/m3 of air space and the amount of water (m) adsorbed per kg of dry silica gel are related by, C = 0.0667m. To maintain dry conditions in a room of air space 100m3 containing 2.2 kg of water vapour initially, 10 kg of dry silica gel is kept in the room. The fraction of initial water remaining in the air space after a long time (during which the temperature is maintained constant) is
0.4
0.4
Percentage saturation is __________ the relative saturation.
always smaller than
always smaller than
Under conditions of flooding in packed tower, the gas pressure drop
is maximum
is maximum
For achieving rapid drying rate in a spray dryer, the diameter of the particles in the feed should be in the range of __________ microns (1 mm = 1000 microns).
10-60
10-60
Weight of 1 m3 of humid air as compared to 1 m3 of dry air, under the same conditions, is
less
less
In a counter-current liquid extractor
only one of the liquids may be pumped at any desired rate.
only one of the liquids may be pumped at any desired rate.
Prandtl number for water at 20°C is about
7
7
Steam distillation is used to
avoid thermal decomposition of a component.
avoid thermal decomposition of a component.
A slurry is to be dried to produce flaky solid. Which dryer would you recommend ?
Spray dryer
Spray dryer
Large scale drying of wheat is done in a __________ dryer.
spouted bed
spouted bed
Azeotropic distillation is employed to separate
constant boiling mixture.
constant boiling mixture.
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 )
T^2
T^2
At the boiling point of the liquid at the prevailing pressure, the saturated absolute humidity becomes
∞
∞
Dorr thickener is an equipment used for
sedimentation
sedimentation
A 25 cm x 25 cm x 1 cm flat sheet weighing 1.2 kg initially was dried from both sides under constant drying rate conditions. It took 1500 seconds for the weight of the sheet to reduce to 1.05 kg. Another 1m x 1m x 1cm flat sheet of the same material is to be dried from one side only.Under the same constant drying rate conditions, the time required for drying (in seconds) from its initial weight of 19.2 kg to 17.6 kg is
1000
1000
In an absorption coloumn, the flooding velocity for random packing is __________ that for stacked/regular packing.
smaller than
smaller than
In a binary distillation column, if the feed contains 40 mole % vapour, the q line will have a slope of
-1.5
-1.5
Radioactive nuclear waste is treated in
pulsed column extractor
pulsed column extractor
Heat sensitive materials like certain pharmaceuticals and food stuffs can be dried in a/an __________ dryer.
freeze
freeze
Molecular seives are regenerated by heating to __________ °C.
200-330
200-330
Large scale drying of sugar is done in a __________ dryer.
rotary
rotary
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
T
Large quantity of silica gel is normally dried in a __________ dryer.
through circulation
through circulation
Hydrogenation of oil is carried out in a/an __________ in Vanaspati manufacturing plant.
agitated vessel
agitated vessel
Which of the following equipments is not used for gas dispersion ?
Spray towers
Spray towers
Which of these columns incurs the lowest pressure drop ?
Packed column (with stacked packing)
Packed column (with stacked packing)
Which of the following same diameter columns gives lowest pressure drop per unit height ?
Packed column (stacked)
Packed column (stacked)
Operating velocity in a packed tower is usually __________ the flooding velocity.
half
half
The percentage humidity is less than the relative humidity only at __________ percent humidity.
both zero and hundred
both zero and hundred
Minimum possible diameter of a packed coloumn is decided mainly by the
flooding
flooding
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
more than
Milk is dried usually in a __________ dryer.
spray
spray
At a given equilibrium pressure, with increase in temperature, the concentration of adsorbed gas on solid adsorbant
decreases
decreases
Which of the following equipments is not used for liquid dispersion ?
Agitated vessels
Agitated vessels
Diameter of raschig rings used in packed tower in industry is normally around __________ inches.
2
2
In physical terms, Schmidt number means
momentum diffusivity/mass diffusivity.
momentum diffusivity/mass diffusivity.
In case of liquids, the binary diffusivity is proportional to (where, T = temperature)
T
T
Mass transfer co-efficient is directly proportional to DAB0.5, according to __________ theory.
penetration
penetration
Diameter to height ratio for a rashig ring is
1
1
In liquid-liquid extraction, the number of phases at plait point is
1
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 entire amount of C is transferred to solvent B.
If Rm is the minimum reflux ratio, the optimum reflux ratio may be around __________ Rm.
1.2 to 1.5
1.2 to 1.5
Mass transfer co-efficient varies as DAB0.5, according to the __________ theory.
surface renewal
surface renewal
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.
humidified with wet bulb temperature decreasing.
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
gas side
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
12
The relative volatility of a binary mixture at the azeotropic composition is
1
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.
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
less than
Lewis number (Le) is
Sc / Pr
Sc / Pr
Moisture content of a substance when at equilibrium with a given partial pressure of the vapour is called the __________ moisture.
equilibrium
equilibrium
All moisture in a non-hygroscopic material is the __________ moisture.
unbound
unbound
Separation of __________ employs gaseous diffusion process.
isotopes of uranium
isotopes of uranium
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
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
lower than
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
When an unsaturated air-water mixture is heated at constant pressure, then
the relative humidity decreases.
the relative humidity decreases.
Mass transfer co-efficient is defined as
Co-efficient = Flux/concentration difference.
Co-efficient = Flux/concentration difference.
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
0
The value of L/mG ratio for economical absorption operation ranges from
1.25 to 2.0
1.25 to 2.0
Large scale usage of flash distillation is practised in
petroleum refinning
petroleum refinning
Dried finished product sampled from various parts of a __________ dryer exhibits non-uniformity in the moisture content.
tray
tray
Rotary dryers cannot handle __________ materials.
sticky
sticky
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
Temperature of the solid
When adsorption hysteresis is observed, the desorption equilibrium pressure is __________ that obtained by adsorption.
always lower than
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
bound
Detergent solution is dried to a powder in a
spray dryer
spray dryer
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
liquid film
The diffusivity (D) in a binary gas mixture is related to the temperature (T) as
D ∝ T1.5
D ∝ T1.5
Tray spacing in a distillation column of dia 10-12 ft. used in petroleum refinery may be around __________ inches.
30
30
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.
the top and bottom compositions correspond to the maximum enrichment available.
Dryer widely used in a textile industry is __________ dryer.
conveyor
conveyor
Which of the following adsorbant is used in the refinning of sugar ?
Bone charcoal
Bone charcoal
The dew point of an unsaturated gas-vapor mixture does not depend upon the __________ of the mixture.
temperature
temperature
Removal of __________ exemplifies an adsorption unit operation.
water from petrol
water from petrol
To remove all the moisture from a wet solid it requires exposure to __________ air.
perfectly dry
perfectly dry
The operating cost of a distillation column at minimum reflux ratio is
minimum
minimum
The reflux to a distillation column is 100 moles/hr, when the overhead product rate is 50 moles/hr. The reflux ratio is
2
2
The wet and ‘dry bulb temperature for a vapour-gas mixture are 25°C and 30°C respectively. If the mixture is heated to 45°C at constant pressure, the wet bulb temperature will be __________ °C.
> 25
> 25
Henry’s law states that the
partial pressure of a component over a solution is proportional to its mole fraction in the liquid.
partial pressure of a component over a solution is proportional to its mole fraction in the liquid.
According to Poiseuille’s law, the permeability for gas flow through a capillary is proportional to (where, μ = gas viscosity)
1/μ
1/μ
For a binary mixture distillation process, the degree of freedom is 2. However, if the pressure is fixed in this process, the number of independent variables in this process will be
1
1
H2S is being absorbed in a gas absorber unit. The height of the transfer unit based on the overall mass transfer coefficient on the gas side is 0.4 m. The equilibrium data is given by, y = 1.5 x. The bulk concentration of H2S has to be reduced from 0.05 to 0.001 mole fraction in the gas side. The height of the tower (in metres) corresponding to an operating line given by, y = 5x + 0.001 is
2.0
2.0
The most efficient cooling tower out of the following is
induced draft
induced draft
Sodium bicarbonate is produced during soda ash manufacture using a __________ tower.
baffle
baffle
The temperature of water cooled in cooling tower is always __________ temperature of entering air.
more than the wet bulb
more than the wet bulb
Mass transfer co-efficient is directly proportional to DAB according to the __________ theory.
film
film
Tower diameter may be decreased by
use of increased tray spacing.
use of increased tray spacing.
Rayleigh’s equation applies to __________ distillation.
differential
differential
The Reynolds number of the liquid was increased 100 fold for a laminar falling film used for gas-liquid contacting. Assuming penetrating theory is applicable, the fold increase in the mass transfer co-efficient (Kc) for the same sytem is
10
The slope of 9-line is determined by the
thermal condition of the feed
thermal condition of the feed
In steam distillation, the
product must be immiscible with water.
product must be immiscible with water.
Which of the following can be calculated theoretically using steam table and Dalton’s law of partial pressure; if the relative humidity and dry bulb temperature of moist air is known ?
Wet bulb temperature
Wet bulb temperature
A mixture of 10% C6H6 vapour in air at 25°C and 750 mm Hg has a dew point of 20°C. Its dew point at 30°C and 700 mm Hg will be around __________ °C.
18.7
18.7
Moisture in a solid exerting an equilibrium vapour pressure equal to that of the pure liquid at the same temperature is called the __________ moisture.
Unbound
Unbound
Refractory bricks are usually dried in a __________ dryer.
Tunnel
Tunnel
What is the degree of freedom of an absorption process in which only one component is transferred between phases ?
3
3
Acetone is to be removed from air in an isothermal dilute absorber using pure water as solvent. The incoming air contains 5 mole% of acetone (yin = 0.05). The design equation to be used for obtaining the number of trays (N) of the absorber is, N+2 = 6 log (yin/yout).For 98% recovery of acetone, the number of trays required is/are
9
9
Degree of freedom for leaching and solvent extraction process is
3
3
An alkaline solution is used to reduce the concentration of carbon dioxide in a stream from 10% to 0.1% by absorption with irreversible chemical reaction. The overall number of transfer units based on gas phase is
4.605
4.605
The adiabatic saturation curve for a vapour-gas mixture is
slightly concave upward.
slightly concave upward.
Small dia distillation column can be a
__________ column is used in gas absorption process.
packed column
packed column
An air-water vapour mixture has a dry bulb temperature of 60°C and a dew point temperature of 40°C. The total pressure is 101.3 kPa and the vapour pressure of water at 40°C and 60°C are 7.30 kPa and 19.91 kPa respectively.The humidity of air sample expressed as kg of water vapour/kg of dry air is
0.152
0.152
In a moderately sized packed absorption tower, channeling (which is most severe with stacked packings) can be minimised by taking the ratio of tower diameter to packing diameter
> 8:1
> 8:1
__________ column is preferred to be used, when a high liquid hold up is required in a reactor for gas-liquid reaction.
Bubble
Bubble
Which of the following provides maximum contact surface for a liquid-vapour system?
Packed tower
Packed tower
In stripping section of continuous distillation column, the
liquid is enriched with high boiler.
liquid is enriched with high boiler.
In rectifying section of a continuous distillation column, the
vapour is enriched with high boilers.
vapour is enriched with high boilers.
A pure drug is administered as a sphere and as a cube. The amount of drug is the same in the two tablets. Assuming that the shape and size do not influence the mass transfer, the ratio of rate of dissolution in water at t = 0 for the cubic to spherical tablet is
1.24
1.24
In extraction, as the temperature increases, the area of hetrogeneity (area covered by binodal curve)
decreases
decreases
In case of an unsaturated mixture of gas and vapor, the percentage saturation is __________ its relative saturation.
lower than
lower than
Use of packed towers for distillation is generally limited to the
small sizes.
small sizes.
For absorbing a sparingly soluble gas in a liquid, the
liquid side coefficient should be increased.
liquid side coefficient should be increased.
Desulphurisation of petrol is done in a
rotating disc extractor
rotating disc extractor
At 750°K and 1 atm, the approximate value of Schmidt number for air is
0.1
0.1
Calculation of mass transfer co-efficient is mostly/normally done using __________ theory.
film
film
Which of the following is used for the concentration of rubber latex ?
Agitated film evaporator
Agitated film evaporator
