Cycle Test 3

8/14/2019 Cycle Test 3

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KAMARAJ COLLEGE OF ENGINEERING AND TECHNOLOGYDEPARTMENT OF POLYMER TECHNOLOGY

THIRD INTERNAL TEST 2010

SUB CODE: PT 44 SUB NAME : PRINCIPLES OF CHEMICAL ENGINERING

SUB INCHARGE: K.AGATHIAN

TOTAL MARKS: 50 MARKS TIME: 1 HOURS

PART A 5 X 2 = 10 Marks

1. Define relative humidity?

2. Name various packing materials used for absorption?

3. What are the general requirements of adsorbents?

4. Define humid heat & humid volume?5. Define mass transfer coefficient and & Relative volatility?

PART B 4X 10 = 40 Marks

6. Explain the working of Rotary drier and Tunnel drier?

7. Discuss in detail the principle and equipment used for absorption?

8. Explain the working of cooling towers?

9. Discuss in detail about distillation process?

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7. Absorption: 10 Marks

The separation of solute gases from gaseous mixtures of noncondensables by transfer into aliquid solvent .

This recovery is achieved by contacting the gas stream with a liquid that offers specific or selective solubility for the solute gas or gases to be recovered.

The operation of absorption is applied in industry to purify process streams or recover valuable components of the stream.

It is used extensively to remove toxic or noxious components (pollutants) from effluent gasstreams

The absorption process requires the following steps:

Diffusion of the solute gas molecules through the host gas to the liquid boundary layer basedon a concentration gradient

Salvation of the solute gas in the host liquid based on gas-liquid solubility Diffusion of the solute gas based on concentration gradient, thus depleting the liquid

boundary layer and permitting further salvation.

The removal of the solute gas from the boundary layer is often accomplished by addingneutralizing agents to the host liquid to change the molecular form of the solute gas. This process is called absorption.

8. PACKED BED 10 Marks

A packed bed is a hollow tube , pipe, or other vessel that is filled with a packing material. The packing can be randomly filled with small objects like Raschig rings or else it can be a

specifically designed structured packing . The purpose of a packed bed is typically to improve contact between two phases in a

chemical or similar process. Packed beds can be used in a chemical reactor ,a distillation process, or a scrubber , but packed beds have also been used to store heat inchemical plants.

In this case, hot gases are allowed to escape through a vessel that is packed with a refractorymaterial until the packing is hot.

Air or other cool gas is then fed back to the plant through the hot bed, thereby pre-heating theair or gas feed.

9. Cooling towers : 10 Marks

The most obvious form of humidification equipment is the spray chamber. Here, the contacting liquid is sprayed as a mist into the gas stream. Gas velocity is kept low so that the contact time is high so that there will be only small

amount of liquid physically entrained in the gas stream. These units are usually restricted to the small-scale operations and are frequently used in

humidity control of a room or plant where either humidification or dehumidification of theinlet air is required.

TYPES OF COOLING TOWERS

Cooling towers are generally used for humidification operations.

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Cooling towers fall into two main sub-divisions: natural draft and mechanical draft. Natural draft designs use very large concrete chimneys to introduce air through the media. Due to the tremendous size of these towers (500 ft high and 400 ft in diameter at the base)

they are generally used for water flow rates above 200,000 gal/min. Usually these types of towers are only used by utility power stations.

Mechanical draft cooling towers are much more widely used. These towers utilize large fansto force air through circulated water.

The water falls downward over fill surfaces that help increase the contact time between the water andthe air. This helps maximize heat transfer between the two

10. Distillation: 10 Marks

The boiling point of a liquid is the temperature at which the vapor pressure of the liquidequals the pressure in the liquid, enabling bubbles to form without being crushed.

A special case is the normal boiling point , where the vapor pressure of the liquid equals the

ambient atmospheric pressure . It is a common misconception that in a liquid mixture at a given pressure, each component boils at the boiling point corresponding to the given pressure and the vapors of eachcomponent will collect separately and purely.

This, however, does not occur even in an idealized system. Idealized models of distillation areessentially governed by Raoult's law and Dalton's law, and assume that vapor-liquid equilibria are attained.

Raoult's law assumes that a component contributes to the total vapor pressure of the mixturein proportion to its percentage of the mixture and its vapor pressure when pure, or succinctly:

partial pressure equals mole fraction multiplied by vapor pressure when pure. If onecomponent changes another component's vapor pressure, or if the volatility of a component is

dependent on its percentage in the mixture, the law will fail. Dalton's law states that the total vapor pressure is the sum of the vapor pressures of each

individual component in the mixture. When a multi-component liquid is heated, the vapor pressure of each component will rise,

thus causing the total vapor pressure to rise. When the total vapor pressure reaches the pressure surrounding the liquid, boiling occurs and

liquid turns to gas throughout the bulk of the liquid. Note that a mixture with a givencomposition has one boiling point at a given pressure, when the components are mutuallysoluble.

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BATCH DISTILLATION

Heating an ideal mixture of two volatile substances A and B (with A having the higher volatility, or lower boiling point) in a batch distillation setup (such as in an apparatus depictedin the opening figure) until the mixture is boiling results in a vapor above the liquid whichcontains a mixture of A and B.

The ratio between A and B in the vapor will be different from the ratio in the liquid: the ratioin the liquid will be determined by how the original mixture was prepared, while the ratio inthe vapor will be enriched in the more volatile compound, A (due to Raoult's Law,).

The vapor goes through the condenser and is removed from the system. This in turn means that the ratio of compounds in the remaining liquid is now different from

the initial ratio (i.e. more enriched in B than the starting liquid). The result is that the ratio in the liquid mixture is changing, becoming richer in component B.

This causes the boiling point of the mixture to rise, which in turn results in a rise in thetemperature in the vapor, which results in a changing ratio of A : B in the gas phase (asdistillation continues, there is an increasing proportion of B in the gas phase). This results in aslowly changing ratio A : B in the distillate.

If the difference in vapor pressure between the two components A and B is large (generallyexpressed as the difference in boiling points), the mixture in the beginning of the distillation

is highly enriched in component A, and when component A has distilled off, the boilingliquid is enriched in component B.

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http://en.wikipedia.org/wiki/File:BatchDistill.svg

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