• Introduction to mass transfer:
• Mass transfer is the transport of components
under a chemical potential gradient. Transport
occurs from a region of higher concentration
gradient to lower concentration gradient.
• Mass transfer operations are based upon
differences in physico-chemical processes
such as vapour pressure and solubility.
• Various operations employed for separating
the components of mixtures are based on
transfer from one homogenous phase to
another. Eg: Leaching, drying, absorption,
distillation, humidification, liq-liq extraction,
stripping etc
•
• Absorption and stripping:
• Absorption is the transfer of material from
gas phase to liquid phase.
• Stripping is the separation of gas solute from
liquid phase.
• Adsorption and desorption: (Not true
inter-phase mass transfer, as fluid only
adheres to solid surface; doesnt dissolve in it)
transfer of mass from gas/liq to surface of
solid
• Desorption is the transfer of mass from solid
surface to gas/liquid medium.
• Extraction: Separation of constituents of a
liquid solution by contact with another liquid.
•
Solvent
Liquid added to solution:
•
Solution to be extracted: Feed
•
Solvent-rich product: Extract
•
Residual liquid separated: Raffinate
•
• Leaching is the treatment of a finely divided
solid with a liquid.
• Distillation is an operation wherein liquid
mixture of miscible and volatile substances are
separated into individual components by
partial vapourization.
• Humidification: Enrichment of vapour
content in gas stream takes place by passing
the gas over a liquid.
• Dehumidification involves transfer of
water vapour from gas phase to liquid phase.
•
• Drying: Removal of relatively small
amount of water or other liquid whereas
evaporation refers to removal of relatively
large amount of water from solutions.
•
• Crystallisation: Formation of solid from
liquid solution based on difference in solute
concentration and its solubility at certain
temperature.
•
• Membrane separation: Diffusion of solute
from liquid or gas through a semi permeable
or microporous membrane to another fluid.
•
• Molecular diffusion is movement of
individual molecules through a substance by
virtue of their thermal energy.
• It is explained by kinetic theory:
simplifying it we can say a molecule travels in
a straight line at uniform velocity until it
collides with another molecule, whereupon its
velocity and direction both change.
• MEAN FREE PATH: Avg distance
travelled by molecule between collisions
• RATE OF DIFFUSION: Net distance
travelled in one direction in a given time.
•
• Rate of molecular diffusion is v slow, to
change it:
•
• 1. It increases with decreasing pressure
since as pressure is decreased number of
molecules present per unit volume decreases
therefore number of collisions also decrease,
thus increasing net distance travelled in one
direction at a given time.
•
• 2. It increases with increasing temperature,
as with temperature molecular velocity
increases therefore increasing distance
travelled thus increasing rate of molecular
diffusion.
•
• Barriers have an immense importance for
molecular diffusions, water kept in vacuum
has rate of evaporation 3.3 kg/sm^2 but upon
placing a stagnant layer of air 0.1 mm thick,
rate is decreased by a factor of 600.
•
• Concentration terms:
• 1. Mass concentration (rho): rho1 = m1/V1
• total rho = summation of all rho
• 2. Summation of mass fraction:
• summation Wi = summation of rhoi/rho
• 3. Mass fraction:
• wi = rhoi/rho
• 4. Molar concentration:
• Ci = pi/RT
• Total = summation Ci
• Total for ideal gas mixtures: C = Pt/RT
• 5. Mole fraction
• xi = Ci/C [LIQ/SOLID]
• yi = CI/C [GASES]
• yi = pi/pt [IDEAL GAS MIXTURE]
•
•
• Formula: Avg molecular weight (M av) =
Feed weight/Total Kmol
• Total molar conentration = rho/M av
•
•
• Flux is the rate of transport of species i
through unit area normal to the transport, it is
a vector quantity.
•
•
•
Mass flux(N) = Molar flux(J) + Ci/C*N
• UNIT 3 ABSORPTION
•
• TRAY TOWERS:
• Tray towers are cylindrical towers with
trays or plates with a downspout to facilitate
the flow of liquid from one tray to another by
gravity. Gas passes upwards through openings
and passes through liquid to form froth and
discharges from the liq and moves to next tray
located above.
• Each tray acts as a stage itself, since there
is intimate contact between gas and liquid
phase in each tray.
• > To ensure longer contact time, liquid
pool on each tray should be deep enough that
gas bubbles require a longer time to rise
through the column of liquid.
•
>If Gas velocity is relatively high, it is
dispersed very thoroughly in the liquid which
is agitated into froth.
•
•
• Operational difficulties:
• 1. Entrainment of droplets of liquids in
rising gas stream
• 2. High pressure drop for gas flowing
through trays
• 3. High pressure => higher pumping cost
• 4. In case of distillation, higher pressure is
to be maintained in the reboiler which may
lead to decomposition of heat sensitive
compounds
•
• ………….Flooding: A higher pressure drop
may lead to gradual build up of liquid in each
tray and may ultimately fill the entire space
between the trays.
•
• ………….Priming: Due to high gas
velocities, foam present between the trays
liquid gets carried away with the gas, and is
recirculated between trays. This added liquid
leads to increase in pressure drop and lead to
flooding,
•
• …………Coning: If liquid rates are too
low, gas rising through the openings of tray
may push liquid away leading to poor
gas-liquid contact
•
• …………Weeping:If gas rate is too low,
liquid may rain down
•
• …………Dumping:At very low gas rates,
none of the liquid reaches downspouts
•
•
• Towers are generally made of metal but
they depend on material being dealt with
therefore they may also be glass or plastic.
• Smaller towers are fitted with handholes,
and larger with manways
• Trays also made of metals/alloys and
fastened suitably to prevent movement due to
gas surge
•
• Tray spacing is based on the following:
•
1. Expediency in construction
•
2. Maintenance cost
•
3. Flooding
•
4. Entrainment
•
• Varies from 15 cm, tower dia should be
large enough to handle satisfactory operating
conditions - it can always be decreased by
increasing tray spacing
•
• Therefore cost of tower depends on
height and also suitable tray spacing.
•
• Liquid is drawn into lower tray by means
of downspouts, they may be circular pipes or
vertical plates
• Since liq is agitated into froth, it must be
allowed enough time in the downspout so that
gas gets detached properly and liquid flows in
the next lower tray
•
• Short circuiting of gas is prevented by
dipping legs of downcomer in the liquid in the
next lower tray
•
• Depth of liquid required is maintained by
overflow weir (may be an extension of the
downspout plate) It may be straight, V-shaped,
or circular
•
• Weir length = 60-80% of tower diameter
•
• Tray efficiency:
• Fractional approach to an equilibrium
stage which it attained by a real tray.
•
• Conditions at various locations on the tray
differ and aren’t the same hence the efficiency
varies at various locations.
• “POINT” efficiency = (yn yn+1)/(yn-yn+1)
•
•
•
•
y is mol fraction
n is tray under consideration, n+1 is tray below nth tray
• Murphree tray efficiency: Based on bulk
average concentrations of all local pencils of
gas
•
• Murphree tray efficiency: The ratio of the
increase in mole fraction of vapour of a
volatile component passing through a plate in
a column to same increase when vapour is in
equilibrium
•
•
Overall tray efficiency = Number of ideal
trays required/Number of real trays required
•
• Wetted-wall towers: A thin film of liquid
flows down inner wall of the empty vertical
tube with gas flowing co/counter currently
• Generally flow of gas is countercurrent to
liquid flow
• Normally used for measurement of mass
transfer coefficient
•
•
• Packed towers:
• These are towers filled with packings &
used for continuous contact of liquid and gas
co-currently or counter-currently. Presence of
packing gives enormous liquid-gas contact
area. Liquid is distributed over the packings
and trickles down through the packed bed
•
• Characteristics of packed bed:
• 1. Should provide large interfacial surface
between liquid and gas
• 2. Should possess desirable fluid flow
characteristics like low pressure drop for gases
and good enough to give high value of mass
transfer coefficient
• 3. Chemically inert to fluids being
processed
• 4. Should have good structural strength
• 5. Cost effective
•
•
•
•
• HUMIDIFICATION:
•
•
• Classical example of interphase
mass and energy transfer (Gas &
Liquid brought together)
• Humidification - liquid is
transferred to gas phase
• Dehumidification - Gas phase to
liquid phase
•
• Matter transferred b/w phases in
both cases is matter from liquid phase
it can either vapourise
(humidification) or condense
(dehumidification)
•
•
•
•
•
•
•
•
•
A - Substance transferred (vapour)
B - Main gas phase
y - moles
p - partial pressure
CA - Specific heat of vapour
CB - Specific heat of gas
•
•
• Molal absolute humidity:
(Y)
•
• Moles of vapour carried by a unit
mole of vapour free gas
•
• Y = yA/yB = pA/pB
•
• Grosvenor humidity/Mass absolute
humidity(Y’): When yA and yB are
expressed in mass
•
•
• Saturated Absolute
humidity (Ys):
•
• When vapour gas mixture is
saturated, partial pressure becomes
equal to vapour pressure of that
substance.
•
YS = P /PB
A
•
•
•
DRY BULB
TEMPERATURE: (DBT)
•
•
• Temperature indicated by thermometer by
ordinary immersion in vapour-gas mixture
•
Relative humidity/Relative
saturation: (% RH)
•
•
•
Expressed as a percentage
If pA is partial pressure under a given
condition and PA is vapour pressure at DBT
of mixture, then
•
• % RH = pA/PA * 100
•
•
Percentage
saturation/humidity: (Hp)
•
• Percentage of humidity under given
conditions to humidity under saturated
condition
• Hp = (Y/Ys)*100
•
•
• Dew point:
• Temperature (tDP) at which vapour-gas
mixture becomes saturated when cooled at
constant total pressure out of contact with a
liquid.
• As soon as temperature is reduced below
dew point, vapour will condense as a liquid
dew
•
• HUMID HEAT: (CS)
• Heat required to raise unit temperature of
unit mass of gas and its accompanying vapour
at constant pressure
•
Cs = CAY’ +CB
•
•
•
•
• Enthalpy:
• Enthalpy of a vapour-gas mixture is the
sum of enthalpies of gas and vapour content
• For a gas at DBT of tG with a humidity of
Y’ enthalpy relative to reference state t0 is
•
• H’ = CB(tG-t0) +Y’[CA(tG-tDP)+lambda DP + C
A,L (tDP - t0)
•
•
• lambda DP - latent heat of vapourisation at
dew point
• C A,L - specific heat of component
A(vapour) in liquid phase
•
•
• Humid volume: (VH)
• It is the volume of unit mass of dry gas in
vapour gas mixture and it is accompanying
vapour at prevailing T&P
•
• check book for expression
•
•
•