1. Which unit operation involves the mechanically induced establishment of a flow pattern in a liquid mixture?
a) Sedimentation
b) Agitation
c) Mixing
d) Crystallization
Discussion
Explanation: Agitation is the process involving induced circular motion of homogeneous particles to prepare a liquid mixture, mechanically. Sedimentation refers to the settling down of suspension particles whereas Mixing refers to the random distribution of initially separate ingredients. Crystallization on the other hand is the formation of clumps of solid particles due to mass transfer.
2. The role of Baffles is to improve mixing and remove stratification during agitation.
a) True
b) False
Discussion
Explanation: During the process of agitation the baffles, which are one of the mechanical agitation devices used, remove stratification in the radial direction and improve mixing performance. A typical agitator vessel setup consists of 4 baffles and 1 impeller.
3. The formula Np=\(\frac{P}{n^3Da^5\rho}\) denotes which number in impeller problems?
a) Weber number
b) Froude number
c) Power number
d) Euler Number
Discussion
Explanation: Powernumber is the ratio of inertial force to viscous force. It is dimensionless and Buckingham’s Pi theorem is applied to it, thus giving the formula Np=\(\frac{P}{n^3Da^5\rho}\) , where P is the Pressure of the liquid, n is the rotation per minute of the impeller, D is the diameter and ρ is the density of the liquid.
4. Which one is not the purpose of establishment of agitation in fluids?
a) Suspension of solid particles
b) Blending of solids
c) Blending miscible liquids
d) Promoting heat transfer
Discussion
Explanation: Agitation is done to suspend solid particles that might have settled down or are not distributed throughout the fluid, for the dispersion of a gas through the liquid, for promoting heat transfer and for the blending of miscible liquids.
5.An agitator is selected depending upon the __________
a) Volume of liquid
b) Height of the tank
c) Power required
d) Fluid viscosity
Discussion
Explanation: An agitator is selected depending upon the viscosity of the fluid, since that is the only way to determine the selection from the impellers of different types. This selection is independent of fluid volume, height of the tank used, and the power required in the process.
6. Name the type of impeller used for agitation of low viscosity fluids, on high speeds?
a) Propeller
b) Concave Blade Impeller
c) Straight blade turbine
d) Paddles
Discussion
Explanation: Propellers are a type of impellers used for agitation of low viscosity fluids, on high speeds. They are effective in large vessels due to their persistence of flow currents. Paddles are used in slow or moderately sped vessels, whereas turbines are used over a wide range of viscosities.
7. Power consumption of agitators is a function of ________
a) Impeller used
b) Volumetric flow rate
c) Volumetric flow rate and Kinetic Energy
d) Kinetic Energy
Discussion
Explanation: The power consumption of agitators is a function of both, the volumetric flow rate inside the vessel and the kinetic energy. It does not depend on the type of Impeller used for the process.
8. How can swirling be prevented in agitated vessels?
a) By reducing the power provided
b) By the use of turbines
c) By the use of Baffles
d) By lowering the shaft
Discussion
Explanation: Swirling is a hinderance in agitated vessels and can be prevented by using Baffles appropriately, along with the turbines according to the viscosity of the fluid. Reducing the power in the system will only result in reducing the amount of agitation and the lowering of the shaft will result in unequal distribution of the fluid currents.
9. Ice has __________ thermal conductivity than water.
a) lower
b) higher
c) same
d) very lower
Discussion
Explanation: Ice has higher thermal conductivity than water. This is due to the molecular arrangement in solids and liquids where solid has packed structure of molecules and stronger bonding between molecules compared to liquids. Therefore the thermal conductivity of frozen foods is higher than the foods at normal temperature.
10. Which material has highest thermal conductivity?
a) Diamond
b) Copper
c) Silver
d) Bronze
Discussion
Explanation: Diamond has highest thermal conductivity around room temperature along with the graphite and graphene. The thermal conductivity of diamond is 2000 watts per meter per Kelvin, which is five times higher than copper.