INTRODUCTION

Unit Operation: Steps intended primarily to perform physical transformation (opposed to chemical transformation) of the input .

Examples of unit operations:• Heat exchanger (change of temperature)• Fluid flow (transportation)• Distillation ( Separation of mixture into multiple streams)• Evaporation (Remove water from liquid)• Gas absorption (Remove one component of a gas mixture)• Classification (Divide mixture of particles into diff. classes based on size)

Absorption and Stripping

• Absorption (or scrubbing) is the removal of a component (the solute) from a gas stream by contact with liquid (the solvent or absorbent).

• Desorption (or stripping) is the removal of a component from a liquid stream by contact with gas.

• Thus, absorption and stripping are opposite unit operations, and are often used together as a cycle.

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Absorption Systems

• Absorption can occur in two ways:

– Physical – No chemical reaction

– Absorption With Chemical reaction

• In physical absorption, chemical reaction has no effect

• In absorption with chemical reaction, rate of absorption depends on nature of reaction.

• The choice of a liquid absorbent depends on the concentrations in the feed gas mixture and on the percent removal desired.

• If the impurity concentration in the feed gas is high, ~10-50%, we can often dissolve most of the impurity in a nonvolatile, nonreactive liquid. Such a nonreactive liquid is called a physical solvent.

• If the impurity concentration is lower, around 1-10%, we will tend to use a liquid capable of fast, reversible chemical reaction with the impurity. Such a reversibly reactive liquid is referred to as a "chemical solvent.“

• If the impurity concentration in the feed is lower still, we may be forced to use a liquid that reacts irreversibly, an expensive alternative that may produce solid waste.

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Absorption Systems – Physical

• Examples:CO2 and water - CO2 is separated from airNH3 and water – NH3 is separated from air

• Physical absorption relies on the solubility of a particular gas in a liquid.

• This solubility is often quite low;Relatively large amount of liquid solvent is needed.

• Because of the low solubility and large solvent amounts required in physical absorption, chemical absorption is also used.

Absorption Systems – Chemical• Chemical absorption relies on reaction of a particular gas with a reagent in a

liquid.

• Examples:HCN and aqueous NaOHHCl / HF and aqueos NaOHNH3 and H2SO4

• This absorption can often be quite high; consequently, a smaller amount of liquid solvent/reagent is needed to obtain the required separation.

• However, the reagent may be relatively expensive, and it is often desirable to regenerate when possible.

Scrubbing Examples

• In NFC, some plants use ammonia gas, chlorine gas etc. These gases are vented to atmosphere through stacks.

• APPCB limits for release of these gases into atmosphere to prevent pollution.

• Hence, to reduce the concentration of these gases to the limits, these gases are absorbed into a liquid. This is called scrubbing.

Solubility of gas

• Absorption is based on the solubility of gas in a liquid

• With the increase in temperature, solubility of a gas in liquid decreases

• Absorption is done at lower temperature

• Desorption/Stripping is done at higher temperature

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Absorber/Stripper Cycle

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Selection of solvent

• 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

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Selection of solvent

• Volatility

Low volatility or low vapor pressure of the solvent enhances the adsorption operation as solvent loss with carrier gas is very small

• 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

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Selection of solvent

• Corrosiveness

Non-corrosive or less corrosive solvent reduces equipment construction cost as well as maintenance cost

• Cost

The solvent should be cheap so that losses will be insignificant and should be easily available.

• Toxicity and Hazard

Solvent should be non-toxic, nonflammable, non-hazardous and should be chemically stable.

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Steam is generally used in desorption as stripped solute can be recovered very easily by condensing steam leaving desorption tower

Absorbers

• To achieve gas absorption, main requirement is that gas should be brought in intimate contact with the liquid.

• Feed (i.e. gas mixture) is introduced at the bottom and the solvent (i.e. liquid) is given at the top.

• The absorbed gas with the solvent leave at the bottom and unabsorbed gas leaves at top.

Equipment for scrubbing

• Two types of columns are used for scrubbing (gas absorption):

– Packed columns– Plate/tray columns

Other absorption equipments are, spray column, agitated contactor, venture scrubber, etc

Plate columns have less efficiency.

Hence,most commonly used are Packed

Columns.

Tray columns

Packed Columns

A packed tower is a pipe/column filled with some packing.• Suited to applications where high gas removal efficiency is required• Exhaust gas is relatively free from particulate matter

Packings

• Packing materials are utilized to provide large interfacial area of contact between two phases.

• These are made from either of ceramics, metals or plastics.

• A number of packing materials with various size, shape and performance are available

• Design depends on corrosiveness of gas & scrubbing liquid, size of absorber, static pressure drop and cost

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Types of packings

• Random or dumped – Raschig rings, lessig rings, berl saddles, intalox saddle & pall rings

• Structured - low gas pressure drop and improved efficiency. Corrugated metal sheet packing and Wire mesh packing

• Grid - used for high gas or vapor capacities at low pressure drop. Mellagrid series; Flexigrid series; Snap grid series

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Random Packings

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Structured & Grid Packing

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Mellagrid F-grid Nuttergrid

Packing characteristics

• Cost – Low cost

• Surface area: A large interfacial area of contact

• Void volume:high void volume –low Press. drop

• Fouling resistance: should not trap suspended solids present in liquid.

• Mechanical strength: Good mechanical

• Uniform flow of streams: Stack of packing materials should have uniform void spaces through which both the streams (gas &liquid) can flow uniformly. Non-uniform flow of streams leads to stagnant liquid pool which in turn gives low mass transfer.

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Liquid Distributors

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Gas Distributors

Factors affecting performance

• Gas velocity - The rate of exhaust gas from the process determines the scrubber size to be used. The scrubber should be designed so that the gas velocity through it will promote good mixing between the gas and liquid phases. However, the velocity should not be too fast to cause flooding.

• Liquid-injection rate - Generally, removal efficiency is increased by an increase in the liquid-injection rate to the vessel. The amount of liquid that can be injected is limited by the dimensions of the scrubber. Increasing liquid-injection rates will also increase the operating costs. The optimum amount of liquid injected is based on the exhaust gas flow rate.

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• Packing size - Smaller packing sizes offer a larger surface area, thus enhancing absorption. However, smaller packing fits more tightly, which decreases the open area between packing, thus increasing the pressure drop across the packing bed.

• Packing height - As packing height increases, total surface area and residence time increases, enhancing absorption. However, more packing necessitates a larger absorption system, which increases capital cost.

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Factors affecting performance

Problems in packed bed flow

• Channeling: Gas or liquid flow is much greater at some points than at others

• Flooding: Liquid stops flowing altogether and collects in the top of the column due to very high gas flow

Liquid flow should be high enough to avoid channeling and achieve loading.

Gas flow should be low enough to avoid flooding

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Equilibrium curve

• When two phases are brought into contact they eventually reach equilibrium.

• For ex., water in contact with air evaporates until the air is saturated with water vapour, and the air is absorbed by the water until it becomes saturated with the individual gases.

• In a mixture of gases, the degree to which gas is absorbed depends on solubility of gas in liquid.

• The equilibrium concentrations are shown on equilibrium curve

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McCabe-Thiele Plot – Absorber

Notes on Absorbers

• Note that the operating line for an absorber is above the equilibrium curve. For a given solute concentration in the liquid, the solute concentration in the gas is always greater than the equilibrium value, which provides the driving force for the separation.

• Also note that the OL is linear. This results because of the form of the operating line where L/G is a constant. L and G are based upon the nonvolatile solvent and insoluble carrier gas, respectively, which do not change.

• If we had used mole fractions and total gas and liquid rates, the OL would be curved.

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McCabe-Thiele Plot – Stripper

Notes on Strippers

• The OL will be the same as that used for absorbers.

• The difference, compared to an absorber, is that the equilibrium curve will be above the operating line.

• Otherwise, the design approach for strippers is the same as that for absorbers.

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