MASS TRANSFER ADSORPTION | Adsorption Types of Agitators

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Adsorption:

Adsorption is a separation process in which certain components of a fluid phase are transferred to the surface of a solid adsorbent. This phenomenon is now widely used in designing water treatment systems and pollution control systems for liquid effluents. The phenomena of adsorption may occur in gas-solid or solid-liquid contact. The adsorption of a gas onto a solid is an exothermic process. Adsorption of a gas onto a solid can be conducted most efficiently at high pressure and low temperature for bulk separations adsorption becomes competitive with distillation when the relative volatility is less than about 1.25. A good adsorbent should have a high selectivity for the solute, sufficiently high capacity, and long life. 

The separation factor in adsorption varies with temperature and for an ideal Langmuir system, the separation factor is independent of composition. In adsorptive separation processes, physical adsorption will occur. Physical adsorption is nonspecific and significant at relatively low temperatures. In certain adsorptive separation processes, differences in diffusion rates between molecules of comparable molecular weight are exploited to achieve useful separation between molecules. This type of separation is known as kinetic separation. Kinetic separation is generally accomplished with zeolites and carbon sieves.

A toxic solute present in industrial wastewater is to be removed by its adsorption onto adsorbents in a single-stage operation. The equilibrium relation is given by the Freundlich equation: Y* = mXn, where Y* = mass solute/mass solvent and X = mass solute/ mass adsorbent.

Now keeping m constant, as n is varied from n > 1 to n < 1, the adsorbent requirement for the same degree of separation will increase. For crosscurrent two-stage treatment of liquid solutions by contact filtration, when the adsorption isotherm is linear, the least total adsorbent results if the amounts used in each stage are equal The forces involved in physical adsorption are van-der-Waals forces electrostatic interactions and chemical interactions. The hypersecretion process is a countercurrent adsorption process regeneration of adsorbent by purge gas stripping is possible when the adsorbed species are weakly held. Regeneration of adsorbents containing several adsorbates of widely different adsorption affinities adsorbed on them is preferably done by thermal swing.

Types of Agitators

Propeller agitators are commonly made of three-bladed attached to the main shaft. They are flexible in operations and mostly used in the mechanical mixing of low to medium-viscosity fluids. These types of propellers are also called marine-type propellers. The diameter of the propellers depends on the rotational speed and diameter of the batch reactor or the agitator’s vessel. Depending on the agitator vessel size and the fluid viscosity the power consumption of the propeller agitator may exceed more than 50kW.

Turbine impellers operate at low speeds and are much larger than propellers. A turbine has an excellent feature in designing the flow pattern where a change in design can divert the flow pattern of fluid by radial flow or axial flow in the reactor vessel. Based on the configuration of the impeller blades these flow patterns can be achieved. The radial design makes the fluid flow at high velocity in the radial direction whereas axial impellers use pitched blades, make the fluid flow parallel to the shaft in the downward direction, and then push the fluid toward the wall of the agitator’s vessel. For gas dispersion operation radial turbine impeller is used and an axial turbine impeller is used for chemical reactions, suspension solid, and miscible liquid mixing. 

Types of agitators models, application, and comparison:

Agitator modelsApplicationAdvantagesDisadvantages
Paddle:
Flat paddle finger paddlesGate paddles
Solid mixing slurry mixingHeavy duty mixingAdjustable to 2 or 4 blades excellent for low-speedHigh power consumption efficient liquid circulation
Counter-rotating paddlesPaste mixingEfficient in the laminar condition lendingVibrates at high speed not suitable for liquid mixing
TumblingBlendingPaste and viscous material mixingNot suitable for  fluid solutions
Disk and conePolymers and dispersion preparationViscous solution mixing with 60 revolutions per secondPaste mass cannot be handled
Free shaft suspensionSugar processingSuspension, Thickening operationHigh power requirement
Impeller typeEmulsion preparationsGood temperature control creates axial flowGood phase mixingNot for viscous materials
Turbine agitator
Straight bladePitched bladeVaned diskCurved blade
Liquid and gas reactionsExcellent for dispersion operations creates a good radial flowOnly for less viscous liquid below 15 to 20 Ns/m2
Slotted rotary
Rotating disk
Powders and
cosmetics
Unique particle size and homogeny product formationMinimum axial flow operates better only for 0.1 to 0.01mm particle size solids
Screw
Screw in cone
Food and snack processingHomogenization of highly viscous materialsNot suitable for miscibility operations
Helical
Ribbon typeHelical screw
Polymer and paints processingHandles viscoelastic liquids that are more than 20 Ns/m2Less radial flow patterns
GateBlending operationsGood speed control handles pseudoplastic liquidsNot suitable for gas-to-liquid operations
Anchor
Round anchorCombine anchor and gate
Milk and fat processingEfficient heat exchange between the reactor walls and reaction mass (fluids)High power consumption requires heavy-duty gearbox
PropellerDairy and food processing chemicals processingLess  Metzner โ€“Ott shear rate constantHomogenizationGood miscibilityThe motion of liquid starts on one spotted spots formation at high rotation speeds occurrence of solids settling at low speeds

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