Mass Transfer Part 4 Flashcards by Chris Wilfred Estrada

Which of the following is a suitable absorbent for removal of H2S from natural gas?
Vetrocoke solution
Monoethanol amine (MEA)
Dilute
Hot water

Monoethanol amine (MEA)

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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
1
8
9
10

Y-out = 0.05 - (0.05 * 0.98) = 0.001
N = 6 * log(0.05/0.001)-2
N = 9

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Schimdt number and Lewis number for pure air at atmospheric conditions are respectively
< 1 and = 1
1 and 0
>1 and = 1
0 and = 1

< 1 and = 1

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Below is given an equilibrium moisture curve of a substance. State which regions represent free moisture and unbound moisture respectively.
image of curve
II, III
II, IV
IV, III
IV, II

II, III

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Molecular diffusion is caused by the
transfer of molecules from low concentration to high concentration region.
thermal energy of the molecules.
activation energy of the molecules.
potential energy of the molecules.

thermal energy of the molecules.

molecular difusion thermal energy

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At __________ reflux, theoretical number of plates in a distillation coloumn is calculated using Fenske-Underwood equation.
operating
total
minimum
maximum permissible

total

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Separation of a mixture of two gases by absorption in the liquid solvent depends upon the difference in their
viscosity
density
solubility
relative volatility

solubility

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Components, having widely different boiling point in a binary mixture, can be separated using __________ distillation.
molecular
extractive
steam
simple

simple

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Which of the following equipments is not used in liquid-liquid extraction?
Pachuka tank
Agitated vessels
Centrifugal extractors
Packed towers

Pachuka tank

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Pick out the wrong statement.
Packed towers are preferred for vacuum operation, because the pressure drop through the packing is less and they (packings) also lessen the possibility of tower wall collapse.
Packed towers are preferred over plate towers for handling corrosive and foamy liquids.
Diameter of randomly packed tower is normally more than 1.2 metres.
Due to uneven supply and improper distribution of liquid, problem of channeling, loading & flooding is generally encountered in packed towers.

Diameter of randomly packed tower is normally more than 1.2 metres.

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Equilibrium-moisture curves of different solids
are different
are same
depend on the humidity of the gas
none of these

are different

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Azeotropism for ethyl alcohol-water system disappears by
increasing temperature
decreasing pressure
increasing pressure
no means

decreasing pressure

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Degree of freedom for leaching and solvent extraction process is
1
3
0
2

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Diffusion co-efficient generally depends upon the temperature, pressure & the nature of the components of the system. Its dimension is not the same as that of the
mass transfer co-efficient.
thermal diffusivity.
kinematic viscosity.
volumetric diffusivity.

mass transfer co-efficient.

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Equilibrium moisture of a solid is equal to the total moisture minus free moisture. Which of the following substances will have maxi-mum equilibrium moisture?
Anthracite coal
Rag paper
Bamboo
Inorganic solids

Bamboo

BMEM bamboo maximum equilibrium moisture

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In the absorption of ammonia in water, the main resistance to absorption is by the __________ phase.
liquid
gas
both (a) & (b)
neither (a) nor (b)

gas

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Drying of refractory is normally carried out in a __________ dryer.
open pan
tray
rotary vacuum
truck/tunnel.

truck/tunnel.

RTT refractory truck/tunnel

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The expression, ln(F/W) = https://www.indiabix.com/_files/images/chemical-engineering/mass-transfer/314-454-1.png is
differential distillation equation
for NtoG in absorption for dilute mixture
slope of q-line in McCabe-Thiele method
for relative volatility

differential distillation equation

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Heat sensitive materials with very high latent heat of vaporisation may be economically separated using
liquid extraction
distillation
evaporation
absorption

liquid extraction

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When the solvent dissolves very little of solute, then
solvent of low latent heat of vaporisation should be used.
solvent of low freezing point should be used.
large quantity of solvent is required to extract the solute.
very small quantity of solvent is required.

large quantity of solvent is required to extract the solute.

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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
9.21
4.605
100
0.001

4.605

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HETP is numerically equal to HTU, only when the operating line
lies below the equilibrium line.
lies above the equilibrium line.
and equilibrium lines are parallel.
is far from the equilibrium line.

and equilibrium lines are parallel.

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In a distillation operation, what is the effect of the temperature of the reflux stream (given below) on the condenser and reboiler loads ?
Reflux conditions:
(i) Reflux stream is completely liquid and is at its bubble point.
(ii) Reflux stream is below its bubble point.
Condenser and reboiler loads are the same in both the cases.
Reboiler load is the same in both the cases but condenser load is higher in case (ii).
Condenser load is the same in both the cases but reboiler load is higher in case (ii).
Both condensor and reboiler loads are higher in case (ii) as compared to case (i).

Both condensor and reboiler loads are higher in case (ii) as compared to case (i).

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Steady state equimolal counter diffusion occurs in case of
leaching
absorption
binary phase distillation
liquid-liquid extraction

binary phase distillation

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JD factor for mass transfer is a function of Reynolds number. Mass transfer by molecular diffusion from a single drop to surrounding still air is given by Nsh = 2 Nst = 2 Nsc = 2 none of these

Nsh = 2 ## Footnote JD Nsh = 2

The relative saturation of an unsaturated mixture of gas and vapor is independent of the nature of the vapor. temperature of the mixture. composition of the mixture. none of these.

none of these.

The adiabatic saturation curve for a vapour-gas mixture is straight line. slightly concave upward. slightly concave downward. none of these.

slightly concave upward.

Pick out the wrong statement. Cellulose acetate membranes are used in dialysis process. Elutriation process refers to when the soluble material is largely concentrated on the surface of an insoluble solid and is simply washed off by the solvent. Cascade' means a group of stages interconnected so that various streams flow from one to another. Zirconia probe is used for the humidity measurement.

Zirconia probe is used for the humidity measurement. ## Footnote Zirconia wrong

The value of NA/(NA + NB), for steady state equimolal counter diffusion of two gases 'A' and 'B' is 1 ∞ 0.5 2

∞

Drift in a cooling tower is the water entrained by the circulating air. dependent on the water lost by evaporation. desirable. all (a), (b) & (c).

the water entrained by the circulating air.

With increase in the concentration of the adsorbate, the integral heat of adsorption increases. decreases. remains unchanged. may increase or decrease ; depends on the type of adsorbate.

decreases.

Small dia distillation column can be a packed column sieve tray bubble cap any of these

packed column

If mass diffusivity in a mixture is equal to the thermal diffusivity, then the Lewis number is 0 1 < 1 > 1

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 0.048 0.079 0.122 0.152

0.152

__________ column is used in gas absorption process. Wetted wall Seive/perforated tray Bubble cap Packed

Packed

Nitrobenzene (boiling point = 210.6 C) is steam distilled at 1 atm pressure.Nitrobenzene will distill off __________ C. at < 100 at > 210.6 between 100 and 210 none of these

at < 100

Pick out the wrong statement. With decrease in feed temperature, the number of plates required for binary distillation decreases for a given L/V ratio. The extraction per unit of total height for a short packed column is more effective than for a tall column, in case of liquid-liquid extraction. Natural draft cooling towers are made of ferro-concrete and its cross-section is wider at the bottom. Vanderwall adsorption is also called chemisorption.

Vanderwall adsorption is also called chemisorption. ## Footnote wrong vanderwall chemisorption

At what value of reflux ratio, number of theoretical plates in a distillation column is minimum? 0 1 infinite < 1

infinite

Batch tray dryers suffers from the disadvantage of high maintenance cost. non-uniform & low production rate. high labour cost & low heat economy. all 'a', 'b' & 'c'.

high labour cost & low heat economy.

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 > 4 :1 < 8 : 1 > 8:1 < 6:1

> 8:1

__________ column is preferred to be used, when a high liquid hold up is required in a reactor for gas-liquid reaction. Packed Bubble Spray Tray

Bubble

The 'shanks system' of leaching (i.e., counter-current multiple contact leaching) is used for recovery of tannis from the tree barks and woods. leaching sodium nitrate from Chilean nitrate bearing rock. both (a) and (b). neither (a) nor (b).

both (a) and (b).

Penetration model (theory) for mass transfer was enunciated by Danckwerts Toor and Marchello Higbie Kissinevskii

Higbie ## Footnote HP Higbie Penetration

Surface renewal model of mass transfer was enunciated by Toor and Marchello Fick Danckwerts Stefan

Danckwerts

When the __________ the relative humidity of air decreases, despite an increase in the absolute humidity. temperature rises pressure rises temperature falls pressure falls

temperature rises

For turbulent mass transfer in pipes, the Sherwood number depends on the Reynolds number as Re^0.33 Re^0.53 Re^0.83 Re

Re^0.83

The relative humidity of air decreases in-spite of an increase in the absolute humidity, when the temperature rises. pressure rises. temperature falls. pressure falls.

temperature rises.

Unit of molal diffusivity is cm2/sec gm . mole gm moles/cm2 . sec gm moles/cm. sec gm moles/cm2. sec

gm moles/cm. sec

The term "approach" in a cooling tower refers to the difference in the temperature of the cold water leaving the tower and the wet bulb temperature of the surrounding air. hot water entering the tower and the wet bulb temperature of the surrounding air. hot water entering and the cooled water leaving the cooling tower. none of these

cold water leaving the tower and the wet bulb temperature of the surrounding air.

Binary distillation involves the mass transfer by __________ at the gas-liquid interface. unidirectional diffusion from liquid to gas phase unidirectional diffusion from gas to liquid phase either 'a' or 'b' a counter diffusion at an almost equal molar rate

a counter diffusion at an almost equal molar rate

In a gas-liquid absorption coloumn, for obtaining the maximum absorption efficiency liquid stream should be distributed uniformly. gas stream should be distributed uniformly. both gas as well as liquid streams should be distributed uniformly. by passing should be completely avoided.

both gas as well as liquid streams should be distributed uniformly.

Reverse osmosis is also called dialysis electrodilysis diffusion ultrafiltration.

ultrafiltration.

Which of the following is not a batch drier? Truck drier Agitated pan drier Fluidised bed drier Vacuum shelf drier

Fluidised bed drier

Steam distillation is not recommended to be used, if the azeotropic mixture is to be separated and the final product is miscible with water. liquids decompose, if distilled directly at atmospheric pressure. material can not be distilled by indirect heating even under low pressure, because of the high boiling temperature. material to be distilled is thermally unstable or has the tendency to react with other components associated with it, at the boiling temperature.

azeotropic mixture is to be separated and the final product is miscible with water.

Rayleigh equation applies to __________ distillation. differential flash equilibrium molecular

differential

Mass transfer co-efficient 'K' according to penetration theory varies with mass dif-fusivity as D^0.5 D 1/D D^1.5

D^0.5

Most important factor to be considered in the selection of packings for absorbers is the __________ of packing. size durability porosity cost

porosity

__________ is the separation technique used for desalination of sea water. Thermal diffusion Reverse osmosis Adsorption absorption

Reverse osmosis

The relative saturation of a partially saturated mixture of vapour and gas can be increased by __________ of the mixture. reducing the total pressure increasing the total pressure reducing the temperature both (b) and (c)

both (b) and (c)

Danckwerts developed the __________ theory. penetration surface renewal film none of these

surface renewal ## Footnote DSR Danckwerts Surface Renewal DLSR camera

Molecular distillation is high temperature distillation for heat-sensitive materials very low pressure distillation both (b) and (c)

both (b) and (c)

Drying of a wet solid under constant drying conditions means the exposure of the wet solid to the air of constant humidity velocity temperature all (a), (b) & (c)

all (a), (b) & (c)

Which of the following provides maximum contact surface for a liquid-vapour system? Packed tower Bubble-cap plate column Seive-plate column Wetted wall column

Packed tower

For experimental determination of mass transfer co-efficient by wetted wall tower, the mass transfer area is calculated unknown known not required

known

In stripping section of continuous distillation column, the liquid is stripped of high boiler. liquid is enriched with high boiler. vapour is stripped of low boiler. none of these.

liquid is enriched with high boiler.

Molality is defined as the number of gm moles of solute per __________ of solvent. litre gm Kg m3

__________ is concerned with the adsorption equilibria. Fick's law Gibb's equation Freundlich equation none of these

Freundlich equation ## Footnote FA Freundlich Adsorption

Fractional solvent extraction employs only one solvent employs two solvents results in low interfacial tension none of these

employs two solvents

In the layout plan for a vacuum distillation unit, operating at 60 mm Hg, supported by a barometric condenser, the appropriate place for the location of vacuum drum for collecting the distillate will be at ground level. 2 metres above the ground. 5 metres above ground. 10 metres above ground.

at ground level.

Raschig ring made of procelain cannot be used for treating concentrated hydrochloric acid nitric acid alkalis sulphuric acid

alkalis

H2S present in naphtha reformed gas is removed by absorbing with ethanolamine K2CO3 HCl vacuum gas oil

ethanolamine ## Footnote ethanolamine H2S ethanolamine

During constant rate drying period, vaporisation rate per unit drying surface area decreases with time. increases with time. does not change with time, does not affect the moisture content of the wet solid.

does not change with time,

Pick out the wrong statement. Mass transfer co-efficient does not account for gase-liquid diffusion. Diffusion co-efficient and mass transfer co-efficient are not same in any mass transfer operation. Contacting cold vapor-gas mixture with hot liquid exemplifies humidification process. Contacting cold liquid with warm vapor-gas mixture exemplifies dehumidification process.

Mass transfer co-efficient does not account for gase-liquid diffusion. ## Footnote wrong does not account for gase-liquid diffusion

In rectifying section of a continuous distillation column, the vapour is enriched with low boilers. vapour is enriched with high boilers. liquid is stripped of high boilers. none of these.

vapour is enriched with high boilers.

For a fixed number of ideal stages in a distillation column, as the reflux ratio is increased, the difference in composition between the top and bottom product streams increases decreases remains unaffected passes through a maximum

increases

According to Chilton-Colburn analogy for mass transfer, NSt.NSc2/3 is equal to f f/2 2f 1/f

f/2

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 0.54 1.04 1.24 1.94

1.24

Minimum reflux ratio in a distillation column results in optimum number of trays. minimum reboiler size. maximum condenser size. minimum number of trays.

maximum condenser size.

Flooding in a distillation column is detected by a sharp increase in Murphree plate efficiency. decrease in pressure drop. decrease in liquid hold up in the coloumn. increase in pressure drop.

increase in pressure drop. ## Footnote flooding increase pdrop

In extraction, as the temperature increases, the area of hetrogeneity (area covered by binodal curve) decreases increases remains unchanged none of these

decreases

Pick out the wrong statement. The mass diffusivity, the thermal diffusivity and the eddy momentum diffusivity are the same for NSc = NPr = 1. 1 Nm3 of dry air is lighter than 1Nm3 of humid air. The Lewis number of a mixture is unity, when the thermal diffusivity is equal to the mass diffusivity. An azeotropic mixture of two liquids has boiling point higher than either of them, when it shows negative deviation from Raoult's Law.

1 Nm3 of dry air is lighter than 1Nm3 of humid air.

In case of an unsaturated mixture of gas and vapor, the percentage saturation is __________ its relative saturation. lower than higher than equal to either (a) or (b), depends on the mixture composition

lower than

At constant pressure, with increase of temperature, the dew point will increase decrease remain unchanged increase/decrease ; depends on the temperature

increase

For a single component absorber, the operating line is straight only when plotted in term of __________ units. partial pressure mole ratio mole fraction none of these

mole ratio

In case of unsaturated air dew point < wet bulb temperature wet bulb temperature < dry bulb temperature both (a) and (b) neither (a) not (b)

both (a) and (b)

For a binary mixture with low relative volatility, use steam distillation. use molecular distillation. use high pressure distillation. an azeotrope may be formed during distillation.

an azeotrope may be formed during distillation.

Plate efficiency is a function of the mass transfer between liquid and vapour. increases due to foaming. increases due to liquid entrainment. increases due to weeping and dumping of liquid.

is a function of the mass transfer between liquid and vapour.

Absorption accompanied by heat evolution results in increased capacity of the absorber. increase in equilibrium solubility. decrease in equilibrium solubility. none of these.

decrease in equilibrium solubility.

Even though bubble cap towers are very effective for humidification operation, they are not used commonly in industries, because of the high evaporation losses of water. high pressure drop of the gas. difficulty in its fabrication. none of these.

high pressure drop of the gas.

For a distillation column operating at minimum reflux, the concentration of liquid and vapour leaving a plate will be same. reflux ratio will be maximum. number of plates required will be maximum. none of these.

number of plates required will be maximum.

A solid is being dried in the linear drying rate regime from moisture content Xo to XF. The drying rate is zero at X = 0 and the critical moisture content is the same as the initial moisture Xo. The drying time for M = (Ls/ARc) is (where, L = total mass of dry solid, A = total surface area for drying Rc = Constant maximum drying rate per unit area X = moisture content (in mass of water/mass of dry solids)) M(Xo - XF) M(Xo/XF) M ln(Xo/XF) MXo ln(Xo/XF)

MXo ln(Xo/XF)

Flash distillation is suitable for the separation of components having very close boiling points. which form maximum boiling azeotrope. having very wide boiling points. which form minimum boiling azeotrope.

having very wide boiling points.

In Mcabe-Thiele method, at infinite reflux ratio number of plates is maximum. overhead product is maximum. both the operating lines coincide with the diagram. none of these.

both the operating lines coincide with the diagram.

Leaching rate is independent of the agitation temperature particle size none of these

none of these

Use of packed towers for distillation is generally limited to the small sizes. multicomponent distillation. high pressure operation. vacuum distillation.

small sizes.

Pick out the wrong statement. The binary diffusivity in liquids is of the order of 10-5 cm2 /sec. Molecular diffusion in solid is much faster than that in liquids. Particles movement from higher concentration to lower concentration takes place in diffusion process. According to Poiseuille's law, the permeability decreases with increase in temperature for flow of a gas through a given capillary.

Molecular diffusion in solid is much faster than that in liquids.

Which of the following decreases during evaporative cooling process with recirculated water supply? Wet bulb temperature Relative humidity Partial pressure of vapour None of these

None of these

Laboratory determination of the dif-fusivity of vapor is done by __________ method. wetted wall column Gilliland's Winkel mann's Wike's

Winkel mann's

The minimum liquid rate to be used in an absorber corresponds to an operating line of slope = 1 ofslope = 0.1 tangential to the equilibrium curve none of these

tangential to the equilibrium curve

Unsaturated air (with dry and wet bulb temperatures being 35 and 25 C respectively) is passed through water spray chamber maintained at 35 C. The air will be cooled humidified both (a) & (b) dehumidified

humidified

For absorbing a sparingly soluble gas in a liquid, the gas side co-efficient should be increased. liquid side coefficient should be increased. gas side co-efficient should be decreased. liquid side co-efficient should be decreased.

liquid side coefficient should be increased.

A natural draft cooling tower is filled with wooden grids, which covers about __________ percent of the tower height. 10-15 30-40 70-80 90-100

10-15

The boiling points of pure components of a binary system __________ with increase in total pressure of the system. decreases increases remains unchanged may increase or decrease, depends on the system.

increases

The change in enthalpy, when a unit quantity of gas is absorbed by relatively large quantity of adsorbent (on which a definite concentration of the adsorbed gas already exists) is termed as the differential heat of adsorption. heat of wetting. integral heat of adsorption. none of these.

differential heat of adsorption.

Penetration theory relates the average mass transfer co-efficient (K) with diffusivity (D) as K is proportional to D K is proportional to sqrt(D) K is proportional to D^1.5 K is proportional to D^2

K is proportional to sqrt(D) ## Footnote penetration sqrt(D)

Desulphurisation of petrol is done in a Bollman extractor rotating disc extractor packed extraction column plate type extraction column

rotating disc extractor ## Footnote PDRD Petrol Desulphurisation Rotating Disc

Chemisorption compared to physical absorption has lower adsorption rate. lower capacity of the solvent for the solute gas. increased utilisation of stagnant zones of the liquid phase. none of these.

increased utilisation of stagnant zones of the liquid phase.

Rate of adsorption increases as the temperature increases. temperature decreases. pressure decreases. size of adsorbent increases.

temperature decreases.

Pick out the system with maximum boiling azeotrope at 1 atm. acetone chloroform ethyl alcohol-water benzene-ethyl alcohol none of these

acetone chloroform ## Footnote maximum boiling acetone chloroform

At 750 K and 1 atm, the approximate value of Schmidt number for air is 0.01 0.1 1 10

0.1

Calculation of mass transfer co-efficient is mostly/normally done using __________ theory. surface renewal film penetration none of these.

film

The velocity, concentration and temperature boundary, for the boundary layer development on a flat plate during connective mass transfer will be same, if NSc = 1 NSc = NPr = NLe NPr = NLe NSc = NLe

NSc = NPr = NLe

For the gas absorption, the height of a transfer unit, based on the gas phase is given by (G: superficial molar gas velocity, L: superficial molar liquid velocity, FG:mass transfer co-efficient, mols/m2 , a: in-terfacial area per unit volume of tower) G/FG.a FG/G.a G.a/FG L/FG.G

G/FG.a

Which of the following is used for the concentration of rubber latex? Agitated film evaporator Long tube vertical evaporator Short tube evaporator Calendria

Agitated film evaporator

As the reflux ratio, in a continuous counter-current extraction is increased, the number of stages increase decrease remain unchanged can either increase or decrease, depends on the system

decrease

In triangular co-ordinates, the ternery composition point falls __________ of the triangle. in the corners inside on the sides none of these.

inside

In a single stage extraction process, 10 kg of pure solvent S (containing no solute A) is mixed with 30 kg of feed F containing A at a mass fraction xf = 0.2. The mixture splits into an extract phase E and a raf-finate phase R containing A at xB = 0.5 and xR = 0.05 respectively. The total mass of the extract phase is (in Kg) 6.89 8.89 10 8.25

8.89

Penetration theory states that the mass transfer co-coefficient is equal to (where, De is diffusivity and 't' is time) (De.t)^1/2 (De/t)^1/2 (4De/πt)^1/2 (4De/t)

(4De/πt)^1/2

Reboiler is considered as one theoretical plate, because of the assumption that vapour and liquid leaving the reboiler are in equilibrium. vapour is recycled to the column. reboiler itself contains one plate. none of these.

of the assumption that vapour and liquid leaving the reboiler are in equilibrium.