Chemical Engineering interview questions and answers - part 5


Chemical Engineering interview questions and answers - part 5

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24. What are the reasons of removal of particles from effluent gas?

The main reasons for the removal of particles from effluent gas are:
> In order to protect and maintain the health of the workers and operators in the plant. It is also done to protect the surrounding population. The primary danger is due to the inhaling of the dust particles which can cause health issues.
> It is also done to minimize the chances of explosions. Many carbonaceous materials remain suspended in air and when mixed with finely powdered metals can form a highly combustible mixture.
> Particles are also removed to reduce the loss of valuable materials.
> It can also be used to recycle the gas for ex in case of blast furnaces the gas is used to fire the stoves.

25. In a crusher where is the energy provided to it is used up for?

The energy fed to a crusher is utilized for the following purposes:
> To produce the elastic deformation of the particles prior to the fracture of the particles.
> In order to produce inelastic deformation this causes the particles to reduce in size.
> The energy is also used to cause the equipments elastic distortion.
> In the friction of the particles between themselves and between the machine.
> The energy also gets used up in the form of heat, noise and vibration in the plant.
> The energy also get used up in the friction losses in the plant.
Most of the energy is lost and only some of it is properly utilized. Nowadays it is of prime importance to reduce the energy losses.

26. On what basis are materials to be crushed evaluated apart from size?

The choice of the crusher machine to be employed to crush the materials depends on the following properties apart from the size of it:
> Hardness: The hardness affects the power consumption and the wearing of the machine. With hard materials it is necessary to use a low speed machine along with pressure lubrication.
> Structure: Normal granular materials can be crushed by using compression and impact methods. On the other hand for fiber like materials compression and impact methods don't work instead a tearing action is required.
> Moisture content: The moisture content in the materials can greatly affect the choice of machines. In case of excess moisture the materials flow gets hindered and it is essential to prevent them from caking together and form balls.
> Crushing Strength: The amount of power that is required to crush a material can be derived directly from the crushing strength of the material.
> Friability: This property indicates the tendency of a material to fracture.
> Stickiness: As the name suggests this property is used to evaluate how sticky a material is as a very high sticky material can clog the machinery.

27. What are the advantages of using a ball mill over other conventional methods of crushing?

The advantages of using a ball mill over other conventional methods are as follows:
> The mill can be used in multiple modes. It can be used in both wet and dry modes. The wet mode helps in the removal of the product.
> The installation cost and the power consumption of the mill is low.
> Since the mill can be used in an inert atmosphere hence it could be used for the crushing of explosive materials.
> It can be used for any type of hardness materials and also the grinding medium is cheap.
> It can also be used for continuous and batch operations.
> The ball mill supports two types of grinding:
> Open circuit grinding
> Closed circuit grinding

28. Mention some of the specialized grinding and crushing methods.

Some of the specially developed grinding and crushing methods are as follows:
> Electrohydraulic crushing: In this method an underwater discharge is created from a high capacity capacitor.
> Ultrasonic Grinding: This type of grinding the material id placed between a drive roll and plate both of which are ultrasonically activated. This process is used for grinding micro sized materials.
> Cryogenic grinding: For certain materials effective grinding is not possible by conventional means such as rubber, textiles etc. In order to cope with this problem the material is frozen to ultra low temperatures using liquid nitrogen. Once the material is frozen it is grounded as per requirements.
>Explosive Shattering: This type of shattering is still on experimental basis, it involves energy to be transmitted to the particles in the form of shockwaves from an explosion chamber.

29. What are the assumptions made for Kynch Theory?

The following assumptions are made for the Kynch Theory:
> Across the horizontal layer the particle concentration is uniform
> The effects of the wall can be ignored.
> No differential settings of particles are considered due to shape, size or composition changes or differences.
> The velocity at which particles fall depends only on the local concentration of the particles and nothing else.
> The concentration is considered to be uniform throughout or it can be assumed to increase towards the bottom.
> The velocity of sedimentation approaches zero as the concentration reaches the limiting value. This limiting value corresponds to the sediment layer at the bottom which gets deposited.

30. What are the merits of using a falling film evaporator?

The advantages of using falling film evaporators are as follows:
> These types of evaporators have very high heat transfer coefficients ranging from 2000-5000 W/m(square) for water and 500 to 1000 for organic liquids
> The residence times are short in case of heated surfaces, 5-10 seconds without recirculation
> They have very low pressure drops, 0.2-0.5 kN/m(square) ,
> These evaporators are well suited for vacuum operations as well.
> The evaporation ratios are very high. 70 per cent without and 95 per cent with recirculation,
> They have a very wide operating range, they can provide as much as 400% of the minimum throughput,
> In addition to the above advantages they have a low cost of operation and are less susceptible to fouling

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