Mass Transfer (Set 2) 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.
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
Deairation (removal of O2) of water is done by
ion-exchange
Make up water is supplied during cooling tower operation to compensate for water losses due to
all (a), (b) & (c)
evaporation, blowdown & entrainment
Pick out the wrong statement.
Azeotropes obey Raoult’s law at all temperature.
A minimum boiling azeotrope is exemplified by
acetone-carbon disulphide
Which of the following solutions will follow Raoult’s law most closely?
A solution of benzene, toluene and oxylene.
Use of raschig rings in place of crushed stones as packing in packed beds (other things being same)
decreases pressure drop, increases surface area.
Critical humidity of a solid salt means the humidity
both (a) and (b).
The assumption made in Elis method in distillation is that enthalpy concentration lines of vapor and liquid are
both parallel and straight
As complete saturation of an adsorbent is approached, the differential heat of adsrop-tion approaches
that of normal condensation.
Absorption with chemical reaction’ is involved in the removal of
carbon dioxide from gaseous stream using alkaline solution.
The-relative volatility for separation of a non-ideal binary mixture by distillation should be
> 1
Pick out the wrong statement:
Fenske-Underwood equation is used for calculating theoretical number of plates in a distillation column at normal operating reflux condition.
In paper industry, paper is dried in a ____ dryer.
heated cylinder
Pick out the wrong statement.
Dehydration of ethyl alcohol is most economically done by molecular distillation at very high vacuum.
Only small amount of evaporation of water produces large cooling effects because of its
large latent heat
The most common packing used in industrial operations is ____ rings.
raschig
When the feed to a distillation column is a saturated liquid, slope of the feed line is
infinity
Leaching of sugar from sugar beets is done by
hot water
Absorption factor method is used to calculate the number of ideal stages, when
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 f = moles of vapour present per mole of feed, then the slope of feed line is (Mcabe-Thiele method)
-1/f
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.
The rate of mass transfer is not dependent upon the
none of these.
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
Dry bulb temperature of unsaturated air is more than its ____ temperature.
both ‘a’ & ‘b’
For the case of flow of air past a wet bulb thermometer (air water vapour system), the approximate value of h2/ky.Cz is around
1
The reciprocal of stripping factor is termed as
absorption factor
Ponchan-Savarit method analyses the fractional equipment based on
both enthalpy and material balances.
The binary diffusivity in gases at atmospheric conditions is about
10^-1 cm2/sec
Percentage saturation is ____ the relative saturation.
always smaller than
In case of a desorber (stripper), the
operating line always lies below the equilibrium curve.
What is the reflux ratio at total reflux?
Infinity
Under conditions of flooding in packed tower, the gas pressure drop
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
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
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 minimum number of theoretical plates is required for achieving a given separation in distillation column with
total reflux
A good solvent used for absorption should not have very high
all (a), (b) & (c)
In a counter-current liquid extractor
only one of the liquids may be pumped at any desired rate.
Which of the following is an undesirable property for an absorbing solvent ?
None of these
The wetted wall tower is used to determine
individual mass transfer co-efficient (M.T.C.) in gaseous system.
Weight of 1 m3 of humid air as compared to 1 m3 of dry air, under the same conditions, is
less
Positive deviation from Raoult’s law means a mixture whose total pressure is
greater than that computed for ideality.
With increase in gas rate, the number of transfer units, NtoG, for a fixed degree of absorption by a fixed amount of solvent
increases
Schmidt number is given by
μ/ρDAB
The caking of crystals can be prevented by
both (a) and (c)
Prandtl number for water at 20°C is about
7
Boiling point diagram is
affected by pressure.
When acetone is added in a two layer mixture of methyl isobutyl ketone and water at 30°C, the acetone distributes between the two layers and the composition of the layer follows two solubility curves. For this system,
all (a), (b) and (c).
Steam distillation is used to
avoid thermal decomposition of a component.
Freundlich equation applies to the adsorption of solute from
dilute solutions, over a small concentration range.
For ethanol-water system, the lowering of distillate quality from 95% to 92% will cause ____ plate requirement.
marginal decrease in the number of
The apex of an equilateral-triangular coordinate (in ternary liquid system) represents a/an
pure component
The diffusivity (D) in a binary gas mixture is related to the pressure (P) as
D ∝ 1/P
A slurry is to be dried to produce flaky solid. Which dryer would you recommend?
Spray dryer
Which of the following psychrometric processes is followed by water vapour laden unsaturated air, when it is passed through solid or liquid adsorbent?
Heating and dehumidification at almost constant wet bulb temperature.
In gas-liquid contact operation, the number of ideal stages, https://www.indiabix.com/_files/images/chemical-engineering/mass-transfer/331-646-1.png . This is true when the stripping factor ‘S’ is (see picture)
S > 1
The cooling effect in a cooling tower can not be increased by
none of these.
The value of NA/(NA + NB) for steady state molecular diffusion of gas ‘A’ through non-diffusing gas ‘B’ is
1
The humid volume may be increased by increasing the
both (a) and (b).
Pick out the wrong statement
In actual practice, absorption is an en-dothermic process, while stripping is an exothermic process.
When the mixture to be distilled has a very high boiling point and the product material is heat sensitive, the separation technique to be used is ____ distillation.
steam
Large scale drying of wheat is done in a ____ dryer.
spouted bed
Azeotropic distillation is employed to separate
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
At the boiling point of the liquid at the prevailing pressure, the saturated absolute humidity becomes
∞
Use of natural draft cooling tower is practised, when the air has low
both ‘a’ & ‘b’
Pressure drop through plate tower as compared to that through packed tower, for the same duty will be
more
Dimension of mass diffusivity is the same as that of
kinematic viscosity
Dorr thickener is an equipment used for
sedimentation
Steam distillation is used to separate
high boiling substances from nonvolatile impurities.
In distillation where q is defined as the moles of liquid flow in the stripping section per mole of feed introduced, for saturated liquid feed
q = 1
Which of the following is an undesirable property in a tower packing?
Large weight of liquid retained.
Water entrained by circulating air in cooling towers is termed as
drift
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
With increase in temperature, the rate of leaching increases, because the
diffusivity of solute increases.
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
molal
The Kundsen diffusivity is proportional to (where, T = absolute temperature)
sqrt(T)
In case of constant underflow extraction operation, the ____ at all solute concentration.
both(a) & (b)
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
With increase in temperature, the leaching rate increases because of the
both (a) and (b).
Leaching of uranium ore and gold ore is done in
Pachuka tanks
Blowdown in a cooling tower
means discarding a small fraction of circulating water to prevent and limit the concentration of salt and impurities.
During constant rate period, the rate of drying decreases with the
both (a) and (b)
Antibiotics are best handled in a
Podbielniak extractor.
The fixed cost of a distillation column at minimum reflux ratio is
infinite
____ extractor uses centrifugal force for separating the two phases.
Podbielniak
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.
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
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
none of these.
In most of the mechanically agitated liquid-liquid extractors, baffles or horizontal compartmental plates are provided, which helps in
all ‘a’, ‘b’ & ‘c’.
In a vapor-liquid contacting equipment, the overall gas phase mass transfer co-efficient (M.T.C), KG is related to individual co-efficients (KG and KL) as
1/KG = 1/KG + m/KL
