Mass transfer operation SUB: Application of gas absorption. Group : 2 Prepare by: Mihir Patel Harsh Raiyani Drumik Rana Mohit Rana Harsh Sadhu 1

Topic Outline Introduction Concept of absorption Basic Principles General application Application of Unit operation for Absorption: a) Packed tower b) Plate Column.

INTRODUCTION Absorption, or gas absorption, is a unit operation used in the chemical industry to separate gases by washing or scrubbing a gas mixture with a suitable liquid. The fundamental physical principles underlying the process of gas absorption are the solubility of the absorbed gas and the rate of mass transfer. One or more of the constituents of the gas mixture dissolves or is absorbed in the liquid and can thus be removed from the mixture. In some systems, this gaseous constituent forms a physical solution with the liquid or the solvent, and in other cases, it reacts with the liquid chemically.

Basic Principles The type of contacting liquid chosen depends on the: 1.Solubility of solute (contaminant gases) in the chosen contacting liquid. - pure water : NH 3, acetic acid 2.Chemical reactivity between gas and liquid. - caustic solution: acid gases, HCl & SO 2 - produce a salt

General Applications 1.Absorbing SO 2 from the flue gases by absorption in alkaline solutions. 2. Hydrogenation of edible oils in food industry - hydrogen gas is bubbled into oil and absorbed. 3.Removal of CO 2 from synthesis gases by absorbing it with hot potassium carbonate solution. (in ammonia production) 4. Absorbing dimethyl sulfide from the food processing industry

Application of unit operation for various types of absorption The absorber may be avialable in variuos form which is describe following. 1. packed column, 2. Try column, 3. spray column, 4. venturi scrubbers, 5. bubble column, 6. wet scrubbers, 7.stirred tanks.

PACKED COLUMN

The packed column is a shell either filled with randomly packed elements or having a regular solid structure designed to disperse the liquid and bring it Dumped-type packing elements come in a great variety of shapes and construction materialsa, which are intended to create a large internal surface but a small pressure drop. A Structured,or arranged packings may be made of corrugated metal or plastic sheets providing a large number of regularly arranged channels,but a variety of other geometries exists.

Usage examples Packed columns are used mostly in air pollution. The water soluble ethylene gas ishydrolyzed to ethylene gylcol. Packed columns are also used in the chemical,petrochemical,food, pharmaceutical,paper, and aerospace industries Tray absorbers are used in applications where tall columns are required, because tall, random-type packed towers are subject to channeling and maldistribution of the liquid streams.

TRAY COLUMN

Tray absorbers are used in applications where tall columns are required,because tall,random- type packed towers are subject to channeling and maldistribution of the liquid streams. Plate towers can be more easily cleaned. Plates are also pereffered in applications having large heat effects since cooling coils are more easily installed in plate towers and liquid can be withdrawn more easily from plates than from packings for external cooling.

Usage examples Tray columns are used in a refinery dehexanizer to decrease the benzene content in the naptha feed to the process. This results in lower automobile exhaust emissions.

STIRRED TANKS If the absorbtion process includes a slow liquid- phase chemical reaction, or close control of the process is needed, stirred tanks are used. The gas is introduced directly into the liquid and mixed by the stirred in a stirred tank.

Usage examples Stirred tanks can be used in lime slurry carbonation, paper stock chlorication, regular oil hydrogenation fermation brothaeration,penicilin production, citric acid production, and aeration of activated sludge.

BUBBLE COLUMN Structured catalytic bubble columns are new, very promising types of multiphase reactors. Their configuration lies basically between slurry reactors and trickle bed reactors. The solid phase, consisting of catalyst particles, is enclosed in fixed wire gauze wraps, which are mounted along the height of the column. The gas phase is dispersed into the liquid phase and it flows in the empty passages between adjacent envelopes. The liquid phase may be operated in a batch manner or it may also circulate in co-current or counter- current manner to the gas flow.

Usage Examples Bubble columns can be used to purify nitroglycerin with water, in the chemical industry for hydrogenation, oxidation, chlorination, and alkylation, and in the biotechnological field for effluent treatmet, single-cell protein productin, animal cell culture, and antibiotic fermentation.

Bubble columns can be used for radioactive elements because there are no moving parts.

Venturi Scrubbers

Adjustable-throat venturi scrubber with movable plate Venturi scrubbers can be used for removing gaseous pollutants; however, they are not used when removal ofgaseous pollutants is the only concern. The high inlet gas velocities in a venturi scrubber result in a very short contact time between the liquid and gas phases. This short contact time limits gas absorption. To maximize the absorption of gases, venturis are designed to operate at a different set of conditions from those used to collect particles. The gas velocities are lower and the liquid-to-gas ratios are higher for absorption.

SPRAY COLUMN Spray columns are differetal contactors. The liquid stream enters the coloumn through one or more spray nozzles at different heights in the column. The droplets formed provide a large surface area for exposure to the gas stream,with smaller droplets resulting in a greater Exchange area. The liquid and gas streams can flow counter-currently or in paralel. An optimum droplet velocity is essential because low velocity will lead to low contact or turbulence and high velocity may cause flooding.

A mist eliminator is used to separate any liquid that is entrained into the gaseous phase. Spray columns are used to absorb SO2 from coal- fired boiler exhaust gases.

Wet Scrubber

Wetted packed towers are the simplest and most commonly used approaches to gas scrubbing. The principle of this type of scrubber is to remove contaminants from the gas stream by passing the stream through a packed structure which provides a large wetted surface area to induce intimate contact between the gas and the scrubbing liquor. the contaminant is absorbed into or reacted with the scrubbing liquor.

Wetted packed towers can be designed for very high efficiencies with relatively low capital and running costs. The low pressure drop associated with packed bed scrubbers permits the use of smaller more economical fans. Although efficiency may be affected, a packed tower will usually function when gas or liquor flows vary from its original design parameters. Usage examples Wet scrubbers are used by the food industry,such as in cheese proessing for dust and ambient moisture removal.

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