MASS TRANSFER
Assoc. Prof. Dr. Emel AKYOL
Stage and Continuous Gas-
LiquidSeparation Processes
TYPES OF SEPARATION PROCESSES AND METHODS
1. Absorption. When the two contacting phases are a gas and a liquid, the unit operation is called absorption. A
solute A or several solutes are absorbed from the gas phase into a liquid phase in absorption. This process
involves molecular and turbulent diffusion or mass transfer of solute A through a stagnant nondiffusing gas B
into a stagnant liquid C An example is absorption of ammonia A from air B by the liquid water C Usually, the
exit ammonia-water solution is distilled to recover relatively pure ammonia.
Absorption
Gas absorption: It is a mass transfer operation in which one or more gas solutes
is removed by dissolution in a liquid. The inert gas in the gas mixture is called
“carrier gas”. In the absorption process of ammonia from air-ammonia mixture by
water, air is carrier gas, ammonia is „solute” and water is absorbent. An intimate
contact between solute gas and absorbent liquid is achieved in a suitable absorption
equipment, namely, tray tower, packed column, spray tower, venture scrubber, etc.
Desorption or stripping operation is the reverse of absorption.
Absorption operation is of two types;
•physical
•and chemical.
𝐴𝑏𝑠𝑜𝑟𝑏𝑒𝑛𝑡
𝑆𝑜𝑙𝑢𝑡𝑒+𝐶𝑎𝑟𝑟𝑖𝑒𝑟 𝑔𝑎𝑠 𝑆𝑜𝑙𝑢𝑡𝑒 𝑎𝑏𝑠𝑜𝑟𝑏𝑒𝑑 𝑖𝑛 𝑎𝑏𝑠𝑜𝑟𝑏𝑒𝑛𝑡+𝐶𝑎𝑟𝑟𝑖𝑒𝑟 𝑔𝑎𝑠
For the determination of driving force in any mass transfer operation, the solubility of a species
in a solvent, i.e., equilibrium distribution between phases is important. With the increase in
temperature, solubility of a gas in liquid decreases.
Hence, absorption is done at lower temperature. On the contrary, desorption is done at higher
temperature.
Selection of solvent for absorption and stripping
Few criteria for the selection of an absorbent are as follows:
(A) Gas Solubility: High solubility of a gas in the solvent is preferred, utilizing low quantity of
solvent. Absorbent should not dissolve carrier gas. Similar chemical nature of solute and
absorbent (solvent) gives a good solubility. If chemical reaction takes place between solute
and solvent, rate of absorption is extremely high. But the reaction should be reversible to
recover solvent during desorption.
(B) Volatility: Low volatility or low vapor pressure of the solvent enhances the adsorption
operation as solvent loss with carrier gas is very small. Sometimes, a second less volatile
solvent is used to recover the first solvent.
(C) Viscosity: For better absorption, a solvent of low viscosity is required. In mechanically
agitated absorber, greater amount of power is required for high viscous solvent and
flooding is also caused at lower liquid and gas flow rates.
(D) Corrosiveness: Non-corrosive or less corrosive solvent reduces equipment construction
cost as well as maintenance cost.
(E) Cost: The solvent should be cheap so that losses will be insignificant and should be
easily available.
(F) Toxicity and Hazard: The solvent should be non-toxic, non-flammable, non-hazardous
and should be chemically stable.
Selection of solvent for absorption and stripping
Few criteria for the selection of an absorbent are as follows:
(A) Gas Solubility: High solubility of a gas in the solvent is preferred, utilizing low quantity of
solvent. Absorbent should not dissolve carrier gas. Similar chemical nature of solute and
absorbent (solvent) gives a good solubility. If chemical reaction takes place between solute
and solvent, rate of absorption is extremely high. But the reaction should be reversible to
recover solvent during desorption.
(B) Volatility: Low volatility or low vapor pressure of the solvent enhances the adsorption
operation as solvent loss with carrier gas is very small. Sometimes, a second less volatile
solvent is used to recover the first solvent.
(C) Viscosity: For better absorption, a solvent of low viscosity is required. In mechanically
agitated absorber, greater amount of power is required for high viscous solvent and
flooding is also caused at lower liquid and gas flow rates.
(D) Corrosiveness: Non-corrosive or less corrosive solvent reduces equipment construction
cost as well as maintenance cost.
(E) Cost: The solvent should be cheap so that losses will be insignificant and should be
easily available.
(F) Toxicity and Hazard: The solvent should be non-toxic, non-flammable, non-hazardous
and should be chemically stable.
Two common gas absorption equipments are packed tower and plate tower. The gas
and the liquid phases come in contact in several discrete stages. Thus, a stage wise
contact is there in a plate column. But in packed tower, the up-flowing gas remains in
contact with down-flowing liquid throughout the packing, at every point of the tower.
Therefore, packed tower is known as “continuous differential contact equipment
TYPES OF SEPARATION PROCESSES AND METHODS
2. Distillation. In the distillation process, a volatile vapor phase and a liquid phase that vaporizes are involved.
An example is distillation of an ethanol-water solution, where the vapor contains a concentration of ethanol
greater than in the liquid. Another example is distillation of an ammonia-water solution to produce a vapor richer
in ammonia. In the distillation of crude petroleum, various fractions, such as gasoline, kerosene, and heating oils,
are distilled off.
TYPES OF SEPARATION PROCESSES AND METHODS
3. Liquid-liquid extraction. When the two phases are liquids, where a solute or solutes are removed from one
liquid phase to another liquid phase, the process is called liquid-liquid extraction. One example is extraction of
acetic acid from a water solution by isopropyl ether. In the pharmaceutical industry, antibiotics in an aqueous
fermentation soLution are sometimes removed by extraction with an organic solvent.
4. Leaching. If a fluid is being used to extract a solute from a solid, the process is called leaching. Sometimes this
process is also called extraction. Examples are leaching copper from solid ores by sulfuric acid and leaching
vegetable oils from solid soybeans by organic solvents such as hexane. Vegetable oils are also leached from other
biological products, such as peanuts, rape seeds, and sunflower seeds. Soluble sucrose is leached by water
extraction from sugar cane and beets.
TYPES OF SEPARATION PROCESSES AND METHODS
5. Membrane processing. Separation of molecules by the use of membranes is a relatively new unit operation
and is becoming more important. The relatively thin, solid membrane controls the rate of movement of
molecules between two phases. It is used to remove salt from water, purify gases, in food processing, and so on.
6. Crystallizalion. Solute components soluble in a solution can be removed from the solution by adjusting the
conditions, such as temperature or concentration, so that the solubility of one or more solute components is
exceeded and they crystallize out as a solid phase. Examples of this separation process are crysta!lization of
sugar from solution and crystallization of metal salts in the processing of metal ore solutions.
7. Adsorption. In an adsorption process one or more components of a liquid or gas stream are adsorbed on the
surface or in the pores of a solid adsorbent and a separation is obtained. Examples include removal of organic
compounds from polluted water, separation of paraffins from aromatics, and removal of solvents from air.
SINGLE AND MULTIPLE EQUILIBRIUM CONTACT STAGES
Countercurrent Flow
SINGLE AND MULTIPLE EQUILIBRIUM CONTACT STAGES
Countercurrent Flow
SINGLE AND MULTIPLE EQUILIBRIUM CONTACT STAGES
Countercurrent Flow
EXAMPLE
A gas mixture at 1.0 atm pressure abs containing air and CO is contacted in a single-stage mixer continuously
2
with pure water at 293 K. The two exit gas and liquid streams reach equilibrium. The inlet gas flow rate is 100 kg
mol/h, with a mole fraction of CO of Y = 0.20. The liquid flow rate entering is 300 kg mol water/h. Calculate
2 A2
the amounts and compositions of the two outlet phases. Assume that water does not vaporize to the gas phase
Solution:
Countercurrent Multiple-Contact Stages
Countercurrent Multiple-Contact Stages
EXAMPLE
It is desired to absorb 90% of the acetone in a gas containing LO mol % acetone
in air in a countercurrent stage tower. The total inlet gas flow to the tower is
30.0 kg mol/h, and the total inlet pure water flow to be used to absorb the
acetone is 90 kg mol H 20/h. The process is to operate isothermally at 300 K and
a total pressure of 101.3 kPa. The equilibrium relation for the acetone (A) in the
gas-liquid is YA =2.53xA Determine the number of theoretical stages required for
this separation.
Solution:
Analytical Equations for Countercurrent Stage Contact
EXAMPLE
Repeat previous Example but use the Kremser analytical equations for
countercurrent stage processes.
EXAMPLE
A tray tower is to be designed to absorb SO2 from an air stream by using pure water at 293 K (68°F). The entering gas contains
20 mol % SO2 and that leaving 2 mol % at a total pressure of 101.3 kPa. The inert air flow rate is 150 kg air/h·m2 and the
entering water flow rate is 6000 kg water/ h·m2.. Assuming an overall tray efficiency of 25%, how many theoretical
trays and actual trays are needed? Assume that the tower operates at 293 K (20°C).
EXAMPLE
A tray tower is to be designed to absorb SO2 from an air stream by using pure water at 293 K (68°F). The entering gas contains
20 mol % SO2 and that leaving 2 mol % at a total pressure of 101.3 kPa. The inert air flow rate is 150 kg air/h·m2 and the
entering water flow rate is 6000 kg water/ h·m2.. Assuming an overall tray efficiency of 25%, how many theoretical
trays and actual trays are needed? Assume that the tower operates at 293 K (20°C).
Equilibrium data:
x 0 0.0001403 0.000250 0.000422 0.000842 0.001965 0.00279 0.0042 0.00555 0.00698
y 0 0.00158 0.00421 0.00763 0.01855 0.0496 0.0513 0.0775 0.121 0.212
EXAMPLE
A non-volatile oil containing 4% (molar percentages) propane is directly treated in a gas separation
tower with superheated steam. The percentage of propane at the exit of the tower is 0.2%. Total
pressure is 2 atm and temperature is 1500 C. There is 3,808 kmol steam for the (oil + C3H8) stream
of 100 kmol. Equilibrium data under these conditions are given by the equation y = 25x. Where y;
Mole fraction of C3H8 in steam, x; It is the mole fraction of C3H8 in oil. Find the theoretical number
of trays needed to make the separation.
