EXAMPLES OF MASS TRANSFER

Example of mass transfer

Mass transfer operations

The operation carried out for separating the component of the mixture, involving the transfer of (mass) material from one homogeneous phase to another, due to the difference in vapor pressure, solubility or diffusivity and in which driving force for transfer is a concentration the difference is called mass transfer operations

Example:- Distillation, Gas absorption, Liquid Extraction, Drying

Example of mass transfer operations

• Depending upon phase contact between Gas, liquid and solid phase following are the classification of the mass transfer operations

Gas-gas phase contact

• The fragrance of incense sticks after lightning diffuse from one room to another room. 

Gas-liquid contact

• A cup of water left in a room eventually evaporate as a result water molecules diffusing into the air. This is liquid to gas diffusion.
• Gas absorption:- If a mixture of ammonia and air is contacted with liquid water, a large portion of ammonia will dissolve in the liquid, and in this way air ammonia mixture can be separated.
• Desorption or stripping:- If air is brought into contact with an ammonia-water solution, some of the ammonia leaves the liquid and enters the gas phase.
• If the liquid phase is a pure liquid, containing one component while the gas contains two or more, the operation is humidification or dehumidification depending upon the direction of transfer
• Humidification of the air: - When dry air contacted with liquid water result in evaporation of some water into the air
• Dehumidification: - When very moist air contacted with pure liquid water may result in condensation of part of moist air in the air. 

Gas-liquid contact

• A piece of solid CO2 ( dry ice ) will also get smaller and smaller in time as the CO2 molecules diffuse into the air. This is a solid to gas transfer example.
• Fractional sublimation: - If a solid solution is partially vaporized without the appearance of the liquid phase, the newly formed vapor phase and residual solid each contains all the original components, but in different propositions
• Drying or Desorption:- If solid which is moistened with a volatile liquid is exposed relatively dry gas, the liquid leaves the solid and diffuses into the gas
• Adsorption: - If diffusion takes place from solid to gas phase is called adsorption.
• For example:- If a mixture of water vapor and the air is brought into contact with activated silica gel, the water vapors diffuse to solid, which retains it strongly, and the air is thus dried.

Liquid-liquid contact

• Liquid extraction operations:- This separation involving contact of two insoluble liquid phases.
• for example - Acetone water solution is shaken in separating funnel with carbon-tetra chloride and the liquid allowed to settle, a large portion of acetone will be found in the carbon chloride-rich phase and will thus have been separated from the water.
• Fractional extraction:- a solution of acetic acid and acetone can be separated by adding it to the insoluble mixture of water and carbon tetrachloride. After shaking and setting, both acetone and acetic acid will be found in both liquid phases, but in different proportions.

Liquid-solid contact

• If a spoon of sugar is added in a cup of coffee will eventually sweeten the coffee because sugar molecule diffuses into the coffee.
• Leaching:- In this process diffusion is from solid to liquid phase takes place.
• For example- Leaching of gold from its ore by cyanide solutions. Another example is the leaching of cottonseed oil from the seeds by hexane.
• Adsorption:- In this process diffusion is from liquid to solid phase takes place
• For example:- The colored material which contaminates impure cane sugar solutions can be removed by contacting liquid solutions with activated carbon.

Solid-solid contact

• The diffusion rate is very slow
• If metallic gold and metallic are placed in intimate contact, over a period of time molecules of lead will diffuse into the gold and vise versa. but the rate of diffusion is very slow.

MASS TRANSFER AND MASS TRANSFER OPERATIONS

Mass transfer

• Mass transfer means the transfer of substance through another on a molecule scale.
• The net movement of mass from one location usually by a stream, phase, fraction, or component, to another.
• This mass transfer occurs due to the concentration difference or gradient.

Hence The process of transfer of mass as a result of the concentration difference of a component in a mixture or two-phase which are in contact is called mass transfer

• Example:- evaporation of water from a pool of water into a stream of air flowing over the water surface

Mass transfer operations

The operation carried out for separating the component of the mixture, involving the transfer of (mass) material from one homogeneous phase to another, due to the difference in vapor pressure, solubility or diffusivity and in which driving force for transfer is a concentration difference is called mass transfer operations

• In the mass transfer operations, neither equilibrium phase consists of only one component. 
• Hence when two-phase is initially contacted, they will not be of equilibrium compositions. 
• The system then attempts to reach equilibrium by a relatively slow diffusive movement of the constituents, which transfer in part between the phases in the process. 
• Separations are therefore never complete, although they can be brought as near completion as desired. 
• Mass transfer occurs in many processes such as absorption, evaporation, adsorption, drying, precipitation, membrane filtration, and distillation. 

In mass transfer operations, the Mass transfer may occur

• In one direction (gas absorption)
• In both direction (distillation)
• With simultaneous heat transfer( drying and crystallization)
• With simultaneous chemical reaction (Absorption of CO2 in an aqueous solution of KOH)
• With the exchange of one or more components
• Isothermally and non-isothermally

MODES OF HEAT TRNSFER

Modes of heat transfer

Heat transfer may be defined as the transmission of the energy from one region to another region as a result of the temperature gradient.

Heat transfer takes place in three modes.

1. Conduction

Conduction is the transfer of heat from one part of substance to another part of same substance or from one substance to another which is in physical contact with it. 

• While transferring heat there is no appreciable displacement of molecules forming that substance is occur.
• In solid, the flow of heat is result of vibrational energy.
• In liquid, the flow of heat is result of kinetic energy.

2. Convection

Convection is the transfer of heat within a fluid which is arises due to mixing of one portion of the fluid with another. 

• In a fluid, the flow of heat results in the microscopic motion of a fluid.
• There are two types of Convection.

1. Natural Convection:- Heat transfer is caused by differences in density arising from a temperature gradient
2. Forced Convection:- Heat transfer is caused by use of external means of agency

3. Radiation

Radiation is the transfer of heat through space or matter by means other than conduction or convection.

• Transfer of heat is occurred without any material medium or through a perfect vacuum.
• Transfer of heat is in the form of electromagnetic waves.
• When this wave falls on another body, a part of the energy is reflected, transmitted through the body and remaining absorbed by the body.

Figure of Conduction Convection and Radiation 

CLASSIFICATION OF FLUID

Classification of fluid

Ideal fluid

Ideal fluid is incompressible and possesses no viscosity.

• Such a fluid is only an imaginary fluid.
• All existing fluids have some velocity.

Real fluid : -

A fluid that possesses viscosity is known as a real fluid.

• In actual practice, all fluids are real fluids

Newtonian fluid

A real fluid in which the shear stress is directly proportional to the rate of shear strain (or velocity gradient) is called Newtonian fluid.

Non – Newtonian fluids

A real fluid in which the shear stress is not proportional to the rate of shear strain (or velocity gradient) is called Non -Newtonian fluid.

Ideal plastic fluid

A fluid in which the shear stress is more than the yield value and the shear stress is proportional to the rate of shear strain (or velocity gradient) is called ideal plastic fluid.

VISCOSITY / DYNAMIC VISCOCITY

Viscosity/Dynamics viscosity

It is defined as the property of fluid which offers resistance to the movement of one layer of fluid over another adjacent layer of the fluid.

• Two liquid layers at distance y and (y+dy) from the surface have velocity values u and (u+du) 
• The top layer causes a shear stress on the adjacent lower layer while the lower layer causes a shear stress on the adjacent top layer.
• The shear stress is proportional to the rate of change of velocity with respect to y.
• Mathematically

Or

Where,

  = is constant for proportionality and is known as the coefficient of dynamic viscosity
 = is called velocity gradient

• M. K. S. unit : kgf-sec/m3
• C. G. S. unit : dyne-sec/m2 , 1 dyne-sec/m2 is called one poise
• S. I. unit : newton-sec/m2 = Ns/m2

KINEMATIC VISCOSITY

Kinematic viscosity

Kinematic viscosity is the ratio of the two physical properties of the fluid.

It is also defined as the ratio between the dynamic viscosity and density of a fluid.

Mathematically

• M. K. S. and S. I. unit: - m²/s
• G. S. unit: - cm²/s
• One cm²/s is known as one stroke
• one stroke = One cm²/s = (1/100)²/s= 10^-4  m²/s
• The practical unit of kinematic viscosity is centistoke
• 1 centistoke = (1/100) stoke

SPECIFIC GRAVITY

Specific gravity

It is defined as the ratio of weight density of a fluid to the weight of density(or density) of a standard fluid.

• The standard liquid is water and the standard gas is air. It is a dimensionless quantity.
• Mathematically
• For liquids

• For gases

SPECIFIC WEIGHT AND SPECIFIC VOLUME

Specific weight and specific volume

Specific weight

• It is weight of fluid per unit volume.

• M. K. S. unit: kg/m3
• S. I. units: N/ m3
• To convert in SI units multiply (MKS) by 9.81. the specific weight of water is 9810 N/ m3
• Specific weight of a fluid varies due to

1. Change of gravity 
2. Effect of pressure and temperature 

Specific volume

• It is defined as volume per unit mass
• M. K. S. unit: m3/ slug
• S. I. units: m3/ kg

MASS DENSITY

Mass density

• It is mass per unit volume

• Dimensionally

• M. K. S. unit: metric slugs per meter cube (slug/m3)
• S. I. units: kilograms per meter cube (kg/ m3)
• The density of liquid may be considered as constant but the density of gases varies with the variation of pressure and temperature.