Introduction to Mass Transfer Operations (2 of 5)

www.ChemicalEngineeringGuy.com
▪ Separation by Phase Addition/Creation
▪ Distillation
▪ Flash, Fractional, Batch Distillations
▪ Extractive, Salt, Reactive Distillation
▪ Gas Absorption
▪ Reboiled, Refluxed
▪ Stripping (Reboiled, Refluxed, etc.)
▪ Liquid-Liquid Extraction
▪ One or Two Solvents
▪ Drying, Humidification, Evaporation, Crystallization, Desublimation, Leaching*
*Technically Phase Addition/Creation →
“Separations involving Solids”

▪ We use both (Creation & Addition) since they are pretty similar and in some case
they act both at the same time:
▪ Distillation vs. Gas Absorption
▪ Extractive distillation

▪ If the feed is a single-phase solution, a second separable phase must be developed
before separation of the species can be achieved.
▪ The second phase is created by:
▪ an energy separating agent (ESA) and/or
▪ added as a mass-separating agent (MSA)
Do you need the Full Version?
Contact me if needed!
Contact@ChemicalEngineeringGuy.com
https://courses.chemicalengineeringguy.com/courses
You can also check out more content here:
My Youtube Channel →
My Fan Page →
The LinkedIn→
My website:

▪ An ESA involves:
▪ heat transfer or
▪ transfer of shaft work to/from the mixture.
▪ An example of shaft work:
▪ Creation of vapor from a liquid phase by reducing the pressure.
▪ An MSA may be partially immiscible with one or more mixture components
▪ It is frequently the constituent of highest concentration in the added phase.
▪ Alternatively, the MSA may be miscible with a liquid feed mixture
▪ BUT may selectively alter partitioning of species between liquid and vapor phases.
▪ This facilitates a separation when used in conjunction with an ESA
▪ E.g. extractive distillation

▪ Disadvantages of using an MSA:
▪ need for an additional separator to recover the MSA for recycle
▪ need for MSA makeup
▪ possible MSA product contamination
▪ more difficult design procedures.
▪ When immiscible fluid phases are contacted:
▪ intimate mixing is used to enhance mass-transfer rates so that the maximum degree-of-
partitioning of species can be approached rapidly.
▪ After phase contact:
▪ the phases are separated by employing gravity and/or an enhanced technique such as
centrifugal force.
My Fan Page →
The LinkedIn→
My website:

▪ The most common separation operations based on interphase mass
transfer between two phases.
▪ Commonly, one of which will be created by an ESA or added as an MSA.

▪ Overview, Calculations, Process, Equipment
▪ Flash Distillation
▪ Fractional Distillation
▪ Batch Distillation
▪ Azeotropic Distillation
▪ Extractive Distillation
▪ Pressure-Swing Distillation
▪ Steam Distillation
▪ Vacuum Distillation

▪ Partial condensation/vaporization and Flash vaporization.
▪ Exploiting Partial Pressures and Volatilities:
▪ When the feed mixture includes species that differ widely in
volatility, expressed as vapor–liquid equilibrium ratios (K-
values)— partial condensation or partial vaporization - may be
adequate to achieve the desired separation.
My Fan Page →
The LinkedIn→
My website:

▪ Two phases are created when a vapor feed is partially
condensed by:
▪ removing heat, and a liquid feed is partially vaporized by adding
heat.

▪ Alternatively:
▪ A partial vaporization can be initiated by flash vaporization Operation
▪ By reducing the feed pressure with a valve or turbine.

▪ In both operations:
▪ After partitioning of species has occurred by interphase mass
transfer, the resulting vapor phase is enriched with respect to the
species that are more easily vaporized
▪ While the liquid phase is enriched with respect to the less-volatile
species.
▪ The two phases are then separated by gravity (Vapor on Top as
Distillate, Liquid on Bottoms)
My Fan Page →
The LinkedIn→
My website:

▪ Typical Calculations:
▪ Inlet Feed Composition
▪ Material balance in Vapor/Liquid
▪ Energy requirements for isothermal operation
▪ The Drum:
▪ Drum Size (Diameter & Height)
▪ Dimensions (inside)
▪ Demister sizing / placing

▪ One of the most widely uses of
flashing is in the petroleum
refining:
▪ Rough separation of:
▪ Gases vs. Liquids is required prior
distillation.
▪ Another application is in the
Amine Treating Process:
▪ Removal of Vent gases
My Fan Page →
The LinkedIn→
My website:

▪ Flash Drums
▪ Horizontal / Vertical

▪ Often, the degree of separation achieved by a single contact
of two phases is inadequate
▪ It may be to the volatility differences among species not being
sufficiently large.
▪ In that case, fractional distillation is a good fit!
▪ It is the most widely utilized industrial separation method

▪ Distillation involves multiple contacts between counter-
currently flowing:
▪ liquid phase (goes downwards, due to gravity)
▪ vapor phase (goes upwards, due to gravity)
▪ Each contact, called a stage, consists of mixing the phases
▪ This promotes rapid partitioning of species by mass transfer
▪ It is then followed by phase separation.
My Fan Page →
The LinkedIn→
My website:

▪ The contacts are often made on horizontal trays arranged in a
column
▪ Vapor, flowing up the column, is increasingly enriched with
respect to the more-volatile species
▪ Liquid flowing down the column is increasingly enriched with
respect to the less-volatile species.

▪ Feed to the column enters on a tray somewhere between the
top and bottom trays.
▪ The portion of the column above the feed entry is the
enriching or rectification section, and that portion below is
the stripping section
▪ Vapor feed starts up the column; feed liquid starts down.
My Fan Page →
The LinkedIn→
My website:

▪ Liquid is required for making contacts with vapor above
the feed tray
▪ Vapor is required for making contacts with liquid below
the feed tray.
▪ Commonly:
▪ At the top of the column, vapor is condensed to provide
down-flowing liquid called reflux.
▪ Similarly, liquid at the bottom of the column passes
through a reboiler, where it is heated to provide up-
flowing vapor called boilup.

▪ Typical objectives:
▪ Use a Murphree vapor-stage efficiency to determine the
number of actual stages (plates) from the number of
equilibrium stages.
▪ Extend the McCabe–Thiele method to:
▪ multiple feeds
▪ Sidestreams
▪ open steam (in place of a reboiler).
▪ Estimate overall stage efficiency for binary distillation
from correlations & laboratory column data.

▪ 1. Feed flow rate, composition, temperature,
pressure, and phase condition 2. Desired degree of
component separation
▪ 3. Operating pressure (which must be below the
critical pressure of the mixture)
▪ 4. Pressure drop, particularly for vacuum operation
▪ 5. Minimum reflux ratio and actual reflux ratio
▪ 6. Minimum number of equilibrium stages and actual
number of equilibrium stages (stage efficiency)
▪ 7. Type of condenser (total, partial, or mixed)
▪ 8. Degrees of liquid reflux subcooling
My Fan Page →
The LinkedIn→
My website:

▪ 9. Type of reboiler (partial or total)
▪ 10. Type of trays or packing
▪ 11. Column height
▪ 12. Feed-entry stage
▪ 13. Column diameter
▪ 14. Column internals, and materials of construction
▪ 15. Heat lability and chemical reactivity of feed
components

▪ Fractional Distillations in the industry:

▪ Most common types of Distillations are:
▪ Petrochemicals, separation of liq-vap
▪ Petroleum Refining
My Fan Page →
The LinkedIn→
My website:

www.ChemicalEngineeringGuy.com https://demonstrations.wolfram.com/McCabeThieleMethodForMethanolWaterSeparation/

▪ As the name implies, is a Distillation.
▪ Batch refers to the concept of non-continuous “batch”
operation.
▪ Composition, flow rates, heat rates, temperature and
many other variables WILL change with respect to time.
▪ A familiar example of a batch distillation is the
laboratory distillation shown next.
My Fan Page →
The LinkedIn→
My website:

▪ The liquid mixture is charged to a still-pot and heated to
boiling.
▪ The vapor formed is continuously removed and condensed to
produce a distillate.
▪ The compositions of the initial charge and distillate change
with time; there is no steady state.
▪ The still temperature increases and the amount of lower-
boiling components in the still pot decreases as distillation
proceeds.

▪ Batch operations can be used to advantage when:
▪ The capacity of a facility is too small to permit continuous
operation at a practical rate.
▪ Seasonal or customer demands require distillation in one unit of
different feedstocks to produce different products.
▪ Several new products are to be produced with one distillation unit
for evaluation by potential buyers.
▪ Upstream process operations are batchwise and the compositions of
feedstocks for distillation vary with time or from batch to batch.
▪ The feed contains solids or materials that form solids, tars, or resin
that can plug or foul a continuous distillation column.
My Fan Page →
The LinkedIn→
My website:

▪ Time is king!
▪ Batch Operations:
▪ No Reflux (Simple)
▪ With Reflux
▪ Continuous Reflux, changing Composition of Distillate
▪ Continuous Distillate composition, changing Reflux
▪ Operation Line vs. Equilibrium Lines
▪ Number of Stages
▪ Sizing, Pressure Drops
▪ Pot Size / Equipment
▪ Reservoir, Reflux control

▪ Application of Rayleigh equation for a simple batch
distillation for differential distillation (1 stage)
▪ Calculations as a function of time:
▪ batch-still temperature, residue composition,
instantaneous comps, average distillate composition
▪ Apply the by modified McCabe–Thiele methods,
residue and distillate compositions for binary batch
rectification under conditions of equilibrium stages
▪ Application of:
▪ shortcut and rigorous equilibrium-stage methods
My Fan Page →
The LinkedIn→
My website:

▪ Type of Operations:
▪ Batch Stripper
▪ Middle Vessel Column
▪ Batch Rectifier

▪ Pharma Industry
▪ Breweries
▪ Small Plants

www.ChemicalEngineeringGuy.com https://demonstrations.wolfram.com/MultistageBatchDistillation/

▪ Steam distillation refers to a process in which
live steam is in direct contact with the distilling
system in either batch or continuous operation.
▪ The basis of steam distillation rest on the fact
that water forms immiscible mixtures with most
organic substances
▪ These mixtures will boil at a temperature
below that of either water or the other
materials.

▪ This is a very desirable feature, especially if the
organic compound has a high boiling point at
which it may be unstable or decompose.
▪ As long as liquid water is present, the high
boiling component will vapourise at a
temperature well below its normal boiling
point.
▪ Steam is widely used because of its energy
level, cheapness, and availability.
My Fan Page →
The LinkedIn→
My website:

▪ Steam distillation is commonly used in the following
situations (1/2)
▪ To separate relatively small amounts of volatile impurity
from a large amount of material
▪ To separate appreciable quantities of higher-boiling
materials
▪ To recover high-boiling materials from small amounts of
impurity which have a higher boiling point

▪ Steam distillation is commonly used in the following
situations (2/2)
▪ Where the material to be distilled is thermally unstable
or reacts with other components associated with it at
the boiling temperature
▪ Where the material cannot be distilled by indirect
heating even under low pressure because of the high
boiling temperature
▪ Where direct-fired heaters cannot be used because of
fire hazards
My Fan Page →
The LinkedIn→
My website:

▪ Typically, you will focus on:
▪ Distillation Column
▪ Extraction
▪ Retention of organic-aqueous layer

▪ Typical crude distillation unit with steam
strippers for its side-draw products:
▪ Kerosene & gas oil.
▪ Steam distillation is most commonly employed
in petroleum refining operation, usually in
combination with vacuum distillation.
▪ Essential Oils Extraction
▪ Typically performed via batch (non-copntinuous)

▪ Steam distillation of stearic acid.
▪ Stearic acid is steam distilled at
200°C in a direct-fired still.
▪ Steam is introduced into the molten
acid in small bubbles, and the acid
in the vapor leaving the still has a
partial pressure equal to 70% of the
vapor pressure of pure stearic acid
at 200°C.
▪ Plot the kg acid distilled per kg steam added as a function of total pressure from 101.3
kPa to 3.3 kPa at 200°C.
▪ The vapor pressure of stearic acid at 200°C is 0.40 kPa.
My Fan Page →
The LinkedIn→
My website:

▪ This method of distillation is very much similar
to conventional distillation
▪ Main exception is that it operates at very low
(near vacuum) pressure.

▪ This method of distillation allows the mixture to boil
at a lower temperature
▪ This avoids the thermal degradation problem
▪ To handle the larger volume of vapours that result
from distillation at reduced pressure, a vacuum
distillation column usually have fairly large diameter
and tray spacings.
My Fan Page →
The LinkedIn→
My website:

▪ Pretty similar as any Distillation Tower.
▪ Pressure Controllers is an issue

▪ Typical VDU

▪ An example flowsheet for a crude distillation
column (using steam distillation) & the vacuum
distillation column is shown in the below.
▪ The most common type of unit:
▪ VDU (pink)
▪ Vacuum Distillation Unit
▪ After Atmospheric (cheaper) distillation is
performed
▪ VDU is further used

▪ Separates a mixture that forms a pressure-sensitive
azeotrope
▪ It uses two columns in sequence at different pressures.
▪ In this case, separation can be achieved without using an
additional entrainer (MSA)
▪ For example:
▪ Ethanol-Water mixture
▪ Azeotropic composition 89.4 mole% at 101.325 kPa
▪ Azeotropes does not form at P < 9.2 kPa.
My Fan Page →
The LinkedIn→
My website:

▪ Consider the case of a minimum-boiling azeotrope:
▪ with T-x-y curves at P2
▪ As the pressure is reduced to P1:
▪ the azeotropic concentration moves toward a smaller percentage
of A (xP2 to xP1).

▪ The set-up for a pressure-swing distillation will be then:
▪ Column 1, at P1
▪ Column 2, at P2

▪ Typical calculations are similar as in Distillation
▪ Plus: Pressure changer

▪ Planning and Sequencing are an important factor here.
▪ Correct displacement of the azeotrope is required
My Fan Page →
The LinkedIn→
My website:

▪ Separation of a mixture of 5% water with 95%
tetrahydrofuran.
▪ This azeotrope can be economically separated
using a pressure swing distillation system.
▪ The “swing” in this case between 1 atm and 8
atm.

▪ Recall from previous lecture that:
▪ an azeotrope is a special class of liquid mixture that
boils at a constant temperature at a certain
composition.
▪ It behaves as if it were one component with one
constant boiling point.
▪ Such mixture cannot be separated using conventional
distillation methods.
My Fan Page →
The LinkedIn→
My website:

▪ The problem:
▪ A temperature is reached at which the compositions of the
vapour phase and liquid phase become the same.
▪ The compositions of the liquid and vapour remain the
same until all of the liquid is eventually vapourised.

▪ Separation of Azeotropic Mixtures can be broadly classified into the following
methods:
▪ changing system pressure (Pressure-Swing Distillation)
▪ by addition of an entrainer
▪ Azeotropic Distillation
▪ NOT → Extractive Distillation
▪ by combination with other processes (Hybrid systems)
My Fan Page →
The LinkedIn→
My website:

▪ Main focus:
▪ Separation of Binary Azeotropes by Addition of Entrainer
▪ A new component (called the entrainer) is added to the
original feed mixture
▪ This form (or nearly form) an azeotrope with one (or more)
of the feed components.
▪ The azeotrope is then removed as either the distillate or
the bottoms.

▪ Azeotropic distillation also refers to those processes in
which a new component is added to an original feed
mixture
▪ This will break an azeotrope that otherwise would be
formed by the feed components.
▪ Thus, the purpose of deliberately adding the entrainer is:
▪ Either:
▪ to separate one component of a closely boiling pair
▪ Or:
▪ to separate one component of an azeotrope.
My Fan Page →
The LinkedIn→
My website:

▪ To illustrate the basic concepts consider the following set-
up:
▪ Separation of a mixture A-B that forms minimum-boiling
azeotrope (MinBA)
▪ The entrainer “E” is either:
▪ A medium boiler
▪ i.e. its boiling point in intermediate between components A and B
▪ or is a low boiler
▪ that can form an intermediate boiling maximum azeotrope with A.
*This is the main difference between Extractive distillation
My Fan Page →
The LinkedIn→
My website:

▪ The FEED (A and B) is mixed with the entrainer E
(recycled/recovered from C2) before entering column C1
▪ Component B (which is essentially free of the azeotrope
A-E) is removed from the bottom of column C1
▪ The overhead vapour from C1 is fed to column C2.

▪ Component A is removed as overhead product and
entrainer E as the bottoms product.
▪ The entrainer is recycled back to column C1.
▪ This assume 100% removal of Entrainer “E” in C2.

▪ Alternatively, component A can be separated first as
overhead from column C1.
▪ Column C2 then splits the bottoms from C1 into:
▪ the entrainer E (as overhead product)
▪ component B (as bottoms product).
▪ For a maximum-boiling azeotrope:
▪ the entrainer should either be a medium boiler
▪ or a high boiler that forms an intermediate-boiling
azeotrope with component B.
My Fan Page →
The LinkedIn→
My website:

▪ The problem of such processes lies in the existence of an
appropriate entrainer.
▪ The probability of azeotrope formation increases with
decreasing boiling point differences of A and B
▪ Most azeotropic mixtures to be separated by distillation are
close boiling ones.
▪ In many cases it may be difficult or even impossible to find a
substance that boils between AND does not form new
azeotropes.

▪ But even if such an entrainer exists:
▪ the process might be uneconomical because
of the small boiling point difference in the
system.
▪ The entire process has to be performed in
the boiling point range of the A-B mixture.
My Fan Page →
The LinkedIn→
My website:

▪ The system acetone-heptane-benzene is one of the few
examples in which the process works economically.
▪ Example:
▪ Acetone-Heptane + benzene as the entrainer.
▪ The respective boiling points are:
▪ A - acetone (56.2°C)
▪ B - heptane (98.4°C)
▪ E - benzene (80.1°C)
▪ A-B Minimum-boiling azeotrope (55.6°C).

www.ChemicalEngineeringGuy.com https://demonstrations.wolfram.com/SeparatingBinaryAzeotropesUsingPressureSwingDistillation/

▪ When the volatility difference between two species to be
separated is so small as to necessitate more than about 100
trays
▪ Consider Extractive Distillation
▪ Here, a miscible MSA, acting as a solvent, increases the
volatility difference among species in the feed
▪ This reducing the number of trays
My Fan Page →
The LinkedIn→
My website:

▪ The boiling point of the solvent is generally:
▪ much higher than the boiling points of the feed mixture that
formation of new azeotropes is impossible.
▪ The high boiling point will also ensure that the solvent:
▪ will not vapourise in the distillation process.
My Fan Page →
The LinkedIn→
My website:

▪ Generally:
▪ the MSA is the least volatile species and is introduced
near the top of the column.
▪ Reflux to the top tray minimizes MSA content in the
top product.
▪ A subsequent operation, usually distillation
▪ is used to recover the MSA for recycling.

▪ Typically, Requires 2 Equipment:
▪ Extractive Distillation Column
▪ Entertainer/MSA Separation Column
▪ Aka Solvent Recovery Column
▪ Calculations are pretty similar to those in Fractional Distillation
▪ Ternary Diagrams might be required for Extraction
▪ Solvent Selection
▪ Volatility Change
My Fan Page →
The LinkedIn→
My website:

▪ Petrochemical Applications:
▪ Separation of acetone & alcohol
▪ Separation of Toluene & i-Octane

▪ In general, Distillation Columns
▪ Trayed Columns

▪ A classic example to be cited here is the separation of
an azeotropic mixture of benzene and cyclohexane
▪ Here:
▪ Aniline is one suitable solvent (MSA)

▪ Salt Distillation:
▪ A variation of extractive distillation in which a of the key
components is altered by adding to the top reflux a soluble,
nonvolatile ionic salt
▪ This material stays in the liquid phase as it passes down the
column.

▪ Quick Answer:
▪ In azeotropic distillation →
▪ the formation of an azeotrope is required to
separate the components of a mixture
▪ In Extractive distillation →
▪ no azeotrope formation takes place.

▪ Point 1:
▪ In azeotropic distillation (AC) the third component
added is referred as entrainer
▪ In extractive distillation (EC) third component added is
referred as solvent.
▪ Point 2:
▪ Azeotropic Distillation:
▪ The third component added in AC forms low boiling
azeotrope
▪ Extractive Distillation:
▪ The third component added has low volatility (high boiling
point)

▪ The main difference between the two is in the
process of separating the mixture.
▪ Azeotropic distillation:
▪ It will forms an azeotrope right after combining the
separation solvent.
▪ This azeotrope can be a complicated way of
separating the mixture because it cannot be easily
changed by just a simple distillation.
▪ Extractive distillation
▪ A specific separation solvent is used in every
mixture that should not form an azeotrope.
▪ Typically, extractive distillation is a better and
easier method than the azeotropic distillation.
My Fan Page →
The LinkedIn→
My website:

▪ Many of the entrainers used in azeotropic distillation
are carcinogens/hazardous pollutants.
▪ For this case, let us select the ethanol-water system
▪ Here, benzene is the entrainer.
▪ In Azeotropic Distillation:
▪ Ethanol is removed as the bottoms product from the
column.
▪ BUT:
▪ Benzene is too hazardous for various reasons ranging from
workplace to product to environmental safety.

▪ An alternative to recover ethanol is to use
▪ Extractive distillation.
▪ The solvent used is:
▪ Propylene Glycol
▪ Recall also that:
▪ Water-ethanol forms a minimum-boiling azeotrope
▪ 89.4 mole% (96 wt%) ethanol

▪ For Water – Ethanol Separation, let us use:
▪ Propylene glycol:
▪ It meets all the requirements of an ideal extractive solvent:
▪ It is miscible with water at all concentrations
▪ It has a higher boiling point than water (will remain liquid)
▪ 187°C at 1 atm
▪ It does not form an azeotrope with water
▪ It has a molecular affinity for water
▪ Hydroxyl -OH group forms a weak bond with water molecule
▪ Great for Extraction!
▪ It is a relatively safe workplace material
My Fan Page →
The LinkedIn→
My website:

▪ The first column:
▪ is the ordinary azeotropic distillation
▪ Top: ethanol-water azeotrope
▪ Bottoms: nearly pure water

▪ The second column:
▪ Is an extractive distillation
▪ Propylene glycol is added
▪ Separations:
▪ Top, Middle, Bottoms

▪ Top
▪ Ethanol is the Distillate

▪ Middle
▪ Rectifying section
▪ Ethanol is purified by the removal of water.
▪ Bonding of the water molecules with glycol raises
ethanol's relative volatility with respect to water
▪ This facilitates separation.
▪ The top section reduces the concentration of
propylene glycol in the ethanol distillate to negligible
level.
My Fan Page →
The LinkedIn→
My website:

▪ Bottom
▪ Here is the striping section - ethanol from water.
▪ These are sent to the third column.

▪ The Third Column:
▪ A glycol stripper, where the glycol is recovered.
▪ The propylene glycol leaves the stripper as a
bottoms product
▪ It is is recycled back to the extraction column as
the source of solvent.
▪ The overhead from the glycol stripper:
▪ Contains mainly water & some ethanol
▪ is sent back to the first column
▪ It is combined with fresh feed.

▪ This technique embodies simultaneous:
▪ Reaction
▪ Separation.
▪ Catalysts are enclosed in special packing inside a
distillation column.
▪ This method uses distillation to remove products as they
are formed
▪ This maximizes a shift to the chemical equilibrium of a
reaction toward 100% conversion.

▪ In reactive distillation:
▪ one or more feeds can be used
▪ the reactive zone can be anywhere in the column
▪ Optimizing the the product
▪ Reactive distillation is used mainly for:
▪ Etherification
▪ Esterification
▪ It has also been successfully applied to:
▪ Alkylation
▪ Nitration reactions.
My Fan Page →
The LinkedIn→
My website:

▪ An example is the production of MTBE:
▪ methyl-tert-butyl-ether (MTBE)
▪ an octane booster for gasoline.
▪ The set-up is shown the Figure below.
▪ Reactive D – MTBE
▪ The pre-reactor provides a reaction equilibrium of over 90%
MTBE
▪ The reactive distillation pushes the reaction another 5-10%
▪ This gets almost total conversion of the reactants.

▪ In the reactive zone, bales containing beads of
catalyst resins are stacked on top each other, as
shown in the Figure below.
▪ Reactive D – Catalyst
▪ The advantage of reactive distillation is:
▪ the elimination of complicated product recovery
▪ The separation and recycling of unconverted reactants,
▪ All these lead to savings in equipment and energy
costs.
My Fan Page →
The LinkedIn→
My website:

▪ Check out this Lecture on MTBE:
▪ Important Facts
▪ Raw Materials
▪ Production
▪ Process
https://www.youtube.com/watch?v=MSbEnBMmZkM

▪ Exploiting Solubility of Gases in liquids/gases
▪ Several Gas-Liquid Operations
▪ They Requires MSA!
▪ Examples:
▪ Gas Absorption
▪ Stripping aka Gas Desorbption
▪ Refluxed Absorption/ Stripping
▪ Reboiled Absorption/ Stripping
▪ Refluxed & Reboiled Absorption/ Stripping
My Fan Page →
The LinkedIn→
My website:

▪ If you are not sure about the difference
between a Gas and a Vapor, check this video!
▪ In Gas Absorption, we use… GASES
▪ In Distillation & Evaporation we use VAPORS!
https://www.youtube.com/watch?v=fqXXe9wnVFQ

▪ Absorption is used to:
▪ separate gas mixtures
▪ remove impurities
▪ Contaminants
▪ Pollutants
▪ catalyst poisons from a gas
▪ recover valuable chemicals.

▪ If the feed is vapor and the stripping section of the column
is not needed
▪ The operation is referred to as absorption
▪ Similar to the “enriching section” of distillation
▪ Absorbers generally do not require an ESA
▪ They are frequently conducted at ambient temperature and
elevated pressure.
▪ Liquid Absorbent (MSA) is required
▪ Species transferred to the liquid absorbent are called
solutes or absorbate.
My Fan Page →
The LinkedIn→
My website:

▪ Species in the feed vapor dissolve in the absorbent to
extents depending on their solubilities.
▪ We are exploiting Solubility rather than Volatility
▪ The idea is to “clean” or “remove” gases from the inlet
feed.
▪ Those gases will be dissolved in the MSA (Liquid)
▪ Most typical Operation:
▪ High Pressure
▪ Low Temperature
▪ Counter-Current Flow

▪ Types of “Absorptions”
▪ physical absorption
▪ chemical absorption
▪ stripping
▪ Calculate the minimum MSA flow rate to achieve a specified key-
component recovery.
▪ Packing Selection or Tray Selection
▪ Column Height + Diameter
▪ Material Balances of inlet/outlet Gases and Liquid Streams
▪ Derive the operating-line equation based on the equilibrium-line

▪ Determine the required number of
equilibrium stages
▪ Total Stages/Trays
▪ Total Packing Height (HTU/NTU Model)
▪ Define overall stage efficiency
▪ Efficiencies are low for absorbers
▪ Efficiencies moderate for strippers.
My Fan Page →
The LinkedIn→
My website:

▪ Typical mechanisms of flooding in trayed
column
▪ Calcualtions of the loading point and
flooding point in a packed column.
▪ Pressure drop in a trayed column.

▪ Typically, the tray efficiency are calculated from correlations
of mass-transfer coefficients using two-film theory.
▪ For a packed column:
▪ The Concept of Height equivalent to a theoretical (equilibrium)
stage or plate (HETP or HETS) is the main goal.
▪ It is require for the number of:
▪ height of a transfer unit, HTU
▪ number of transfer units, NTU.
My Fan Page →
The LinkedIn→
My website:

▪ Stack Gas “Cleaning”
▪ Typically, industries will remove pollutants
before sending the off-gases to the
atmosphere
▪ This reduces pollution to the surroundings
▪ CO2, CO, NOx, SOx and even ashes are
recovered here

▪ Stripping aka Gas desorption is the inverse of absorption is
▪ Here,
▪ Liquid mixtures are separated, at elevated temperature and
ambient pressure
▪ This is done by contacting the feed with a vapor.
▪ Here, the vapor is acting as the stripping agent (MSA)
▪ This MSA eliminates the need to re-boil the liquid at the
bottom of the column
▪ This may be important if the liquid is not thermally stable.
▪ Applications “Cleaning” Liquids
My Fan Page →
The LinkedIn→
My website:

▪ Strippers are best operated at low pressure and high
temperature
▪ Still, solubility of gases is considered
▪ Low Pressure → Favors solubility in gases
▪ Strippers are frequently coupled with absorbers to permit
regeneration and recycle of the absorbent
▪ Typically:
▪ If water is used as the absorbent, it is common to separate the
absorbent from the solute by distillation rather than by stripping

▪ Most calculations are similar to those of Gas Absorption
▪ The reverse must be designed
▪ Solvent Selection is of vital importance
▪ Gas/Vapor MSA is also of vital importance for the
selectivity
▪ Packed Columns are typically preferred
My Fan Page →
The LinkedIn→
My website:

▪ Air stripping is:
▪ transferring of volatile components of a liquid into an air
stream.
▪ Typical materials stripped:
▪ BTEX compounds (benzene, toluene, ethylbenzene, and
xylene found in gasoline)
▪ Solvents (trichloroethylene and tetrachloroethylene)
▪ Ammonia
https://en.wikipedia.org/wiki/Air_stripping

▪ Stripping is commonly used in industrial
applications to remove harmful contaminants
from waste streams.
▪ The Feed is mixed with water to make a slurry
and then stripped with steam.
▪ This process is able to decontaminate soils
almost completely.
My Fan Page →
The LinkedIn→
My website:

▪ Steam is also frequently used as a stripping agent for water
treatment.
▪ VOC - Volatile organic compounds - are partially soluble in water
▪ Because of environmental considerations and regulations, these
contaminants must be removed from groundwater, surface water, and
wastewater.
▪ These compounds can be present because of industrial, agricultural,
and commercial activity.

▪ Check this Article!
▪ Directly from Rashig!
▪ YES the packings!
http://www.raschig.de/editor/assets/RaschigRings/Literature%20-
%20Literature/Literature%20-%20Air%20Stripping%20of%20VOC's%20from%20Water.pdf

▪ Mostly applied for Rectifiers
▪ If it is difficult to condense the vapor leaving the top of a
distillation column:
▪ A liquid MSA called an absorbent will be fed to the top tray in
place of reflux.
▪ The resulting operation is called reboiled absorption
▪ Main Product:
▪ “Clean” Vapor Stream
▪ Liquid contains mostly MSA + Impurities
▪ In reboiled absorbers:
▪ partial stripping of the lighter components is performed
▪ This occurs in the lower part of the equipment.

▪ The Addition of a Reflux (Condenser) to a Stripper will
convert it to a Refluxed Stripper
▪ Typically Common when Impurities and/or MSA
Recovery is important
▪ Main Product:
▪ Liquid
▪ By-Products:
▪ Impurities from feed, either condensed as Liquid or left
as vapors/gases
▪ MSA Recovery
▪ Examples:
▪ Steam Distillation
▪ Air Stripping
My Fan Page →
The LinkedIn→
My website:

▪ In these case:
▪ Several “Liquid” Products:
▪ Steam Addition as MSA inlet
▪ Sour Water as MSA outlet
▪ Gas Stream as impurities

▪ The column contains only a reboiler & the feed as a liquid.
▪ Two product lines:
▪ Vapor – Stack
▪ Liquid – Clean Main Product
▪ If the bottoms product from a stripper are thermally stable
▪ This requires ESA (Enery Separating Agent) in the Reboiler
▪ Common Example is the Recovery of Amine Absorbent

▪ Check this lecture:
▪ Identify:
▪ Gas Absorption
▪ Stripping
▪ Reflux
▪ Reboiling
https://www.youtube.com/watch?v=LaQ26JEFuec
My Fan Page →
The LinkedIn→
My website:

▪ Gas-Liquid and/or Vapor-Liquid Operations are very common in the Industry
▪ (a) Flash vaporization or partial condensation.
▪ (b) Absorption*
▪ (c) Rectifier
▪ (d) Stripping*
▪ (e) Reboiled stripping.
* Main Interest of this Section

▪ (f) Reboiled absorption*
▪ (g) Refluxed stripping*
▪ (h) Extractive distillation
▪ (i) Azeotropic distillation
* Main Interest of this Section

▪ If you are interested on:
▪ Vapor-Liquid & Gas-Liquid Operations
▪ Check this Course out!
▪ Basic Principles for Mass Transfer
▪ Unit Operations involved:
▪ Flashing
▪ Distillation
▪ Absorption
▪ Strippers
▪ Scrubbers
https://courses.chemicalengineeringguy.com/p/mass-transfer-principles-for-vapor-liquid-unit-operations
My Fan Page →
The LinkedIn→
My website:

▪ Extraction Overview
▪ Liquid-Liquid Extraction:
▪ 1-Solvent Extraction
▪ 2-Solvent Extraction

▪ In liquid–liquid extraction (aka solvent extraction or extraction)
▪ a liquid feed of two or more components is contacted with a second liquid
phase, called the solvent
▪ The solvent is immiscible or only partly miscible with one or more feed
components
▪ It will completely or partially miscible with one or more of the other feed
components.
▪ Thus, the solvent partially dissolves certain species of the liquid feed,
effecting at least a partial separation of the feed components.
My Fan Page →
The LinkedIn→
My website:

▪ The solvent may be a pure compound or a mixture.
▪ If the feed is:
▪ aqueous solution, an organic solvent is used
▪ if organic, the solvent is often water.

▪ Liquid–liquid extraction with one or two solvents:
▪ can be used when distillation is impractical
▪ especially when the mixture to be separated is temperature sensitive.
▪ A solvent selectively dissolves only one or a fraction of the
components in the feed.

▪ In a two-solvent extraction:
▪ each has its specific selectivity for the feed components.
▪ Several counter-currently arranged stages may be necessary.
▪ As with extractive distillation:
▪ additional operations are required to recover solvent from the streams
leaving the extraction operation.

▪ The raffinate:
▪ is the liquid stream which remains after solutes from the original liquid
are removed through contact with an immiscible liquid.
▪ Raffinating (metallurgy):
▪ refers to a process in which impurities are removed from liquid material.
▪ Extract:
▪ Material containing most of the solute and the solvent

▪ Supercritical-fluid extraction
▪ If the extraction temperature and pressure are only slightly above the critical
point of the solvent
▪ In this region:
▪ solute solubility in the supercritical fluid can change drastically with small
changes in temperature and pressure.
My Fan Page →
The LinkedIn→
My website:

▪ Following extraction:
▪ the pressure of the solvent rich product is reduced to release the solvent,
which is recycled.
▪ For the processing of foodstuffs:
▪ the supercritical fluid is an inert substance, with CO2 preferred because it
does not contaminate the product.

▪ Solid–liquid extraction (also called leaching) involves recovery of
substances from a solid by contact with a liquid solvent
▪ such as the recovery of oil from seeds by an organic solvent
▪ Liquid–liquid extraction has grown in importance since then because
of the demand for:
▪ temperature-sensitive products
▪ higher-purity requirements
▪ better equipment
▪ availability of solvents with higher selectivity

▪ The simplest liquid–liquid extraction:
▪ Involves only a ternary system consisting of two miscible feed
components:
▪ the carrier, C
▪ the solute, A + plus solvent, S, a pure compound.
▪ Components C and S are at most only partially soluble, but solute A is
completely or partially soluble in S.
My Fan Page →
The LinkedIn→
My website:

▪ During extraction:
▪ mass transfer of A from the feed to the solvent occurs
▪ with less transfer of C to the solvent, or S to the feed.
▪ Nearly complete transfer of A to the solvent is seldom
achieved in just one stage.
▪ In practice:
▪ a number of stages are used in one-section or two-section
countercurrent cascades.

▪ Important to know when Extraction is recommended:
▪ Ensure that Liquid Extraction is preferred to distillation
▪ Selection of a solvent using group-interaction rules
▪ Define the distribution coefficient of the ternary system
▪ Identification of its relationship to activity coefficients
and selectivity of a solute between carrier and solvent.
▪ Distinguish & Apply the required ternary mixtures:
▪ Type I → One immiscible pair
▪ Type II → Two immiscible pairs

▪ Application of the Hunter and Nash method
▪ Use of the ternary diagram (triangular diagram)
▪ Calculations on:
▪ minimum solvent requirement
▪ equilibrium stages
▪ Cascade operation

▪ Design a cascade of mixer-settler units based on
mass-transfer considerations.
▪ Size a multi-compartment extraction column
▪ Effects of axial dispersion.
▪ Determine effects on the system:
▪ pH, temperature, salt
My Fan Page →
The LinkedIn→
My website:

▪ Typical Cases:
▪ Solvent Recovery
▪ Aromatics Removal

▪ Acetic Acid Production
▪ Requires Extraction of acetic acid from
feed (aqueous)
▪ Solvents
▪ MTBE, Et.Ac. i-Propyl Ac.
▪ Raffinate
▪ Goes to Stripping Column
▪ Separates the water and acetic acid leftover
▪ Extract
▪ Goes to Solvent Recovery
▪ Separates acetic acid and solvent
My Fan Page →
The LinkedIn→
My website:

www.ChemicalEngineeringGuy.com https://demonstrations.wolfram.com/ConstructSingleStageLiquidLiquidExtraction/

Introduction to Mass Transfer Operations (2 of 5)

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Similar to Introduction to Mass Transfer Operations (2 of 5)

Similar to Introduction to Mass Transfer Operations (2 of 5) (20)

More from Chemical Engineering Guy

More from Chemical Engineering Guy (20)

Recently uploaded

Recently uploaded (20)

Introduction to Mass Transfer Operations (2 of 5)