Q: Given a drying operation, which formula correctly expresses the moisture content on a wet basis?

$$\%m(wb) = \frac{\text{mass of water}}{\text{mass of wet material}} \times 100$$

Q: Which of the following is the correct conversion from dry basis to wet basis moisture content?

$$\%m(wb) = \frac{\%m(db)}{100 + \%m(db)} \times 100$$

Question 1

Which of the following best describes a 'system' in the context of chemical engineering calculations?

Accepted Answer

An arbitrary portion or the whole process chosen for analysis.

Question 2

A process is classified as 'steady state' if:

Accepted Answer

The system is always at equilibrium and no accumulation occurs.

Question 3

Which of the following is NOT a valid reason for $$\dot{m}_{in} 
eq \dot{m}_{out} in a $$process unit?

Accepted Answer

The process is at steady state.

Question 4

Given a drying operation, which formula correctly expresses the moisture content on a wet basis?

Accepted Answer

$$\%m(wb) = \frac{	ext{mass of water}}{	ext{mass of wet material}} 	imes 100$$

Question 5

A 1000 kg batch of wet sand contains 30% moisture (wet basis). How much water (in kg) does it contain?

Accepted Answer

300 kg

Question 6

Which of the following is a correct simplification for the material balance equation when the system is at steady state?

Accepted Answer

Set accumulation = 0

Question 7

In a continuous process, 20 kg/min of a 10% acetic acid solution is mixed with 80 kg/min of a 30% acetic acid solution. What is the mass fraction of acetic acid in the product stream?

Accepted Answer

$$0.26$$

Question 8

Which of the following best describes the difference between evaporation and boiling?

Accepted Answer

Evaporation occurs at the surface at all temperatures, while boiling occurs throughout the liquid at the boiling point.

Question 9

A 1500 kg solution contains 20 kg-mol NaCl (MW = 58.45) per tonne of water. After adding 475 kg NaCl, what is the new mass fraction of NaCl? (Express your answer to 4 decimal places.)

Accepted Answer

0.2467

Question 10

Which of the following is the correct conversion from dry basis to wet basis moisture content?

Accepted Answer

$$\%m(wb) = \frac{\%m(db)}{100 + \%m(db)} 	imes 100$$

Question 11

A process stream contains 10% water by volume at 100°C and 5,260 mmHg. If cooled to 80°C at constant pressure, what happens to the water vapor?

Accepted Answer

Some water vapor condenses, reducing its percentage in the gas phase.

Question 12

Which of the following is NOT a step in the material balance calculation strategy?

Accepted Answer

Guess the unknowns without defining them.

Question 13

In a drying operation, 2100 kg/hr of wet feed is dried from 110% dry basis to 10% wet basis. Which equation would you use to find the mass of water to be removed?

Accepted Answer

$$m(wb) = \frac{m(db)}{1 + m(db)}$$

Question 14

Which of the following scenarios is most likely to require an unsteady-state material balance?

Accepted Answer

A batch process during start-up.

Question 15

Which of the following is a correct statement about condensation?

Accepted Answer

At the dew point, the partial pressure of the vapor equals the vapor pressure.

Question 16

A tunnel drier is used to dry 1000 kg/hr of wet sand from 30% moisture (wet basis) to 9% moisture (dry basis). What is the first step in solving for the amount of water evaporated per hour?

Accepted Answer

Convert the final moisture content to wet basis.

Question 17

Which of the following is a valid basis for a material balance calculation?

Accepted Answer

A specified quantity of material or time of operation.

Question 18

In the general balance equation, which term accounts for the buildup of material within the system?

Accepted Answer

Accumulation

Question 19

Which of the following is a plausible reason for a discrepancy between measured input and output flows in a process?

Accepted Answer

There is a leak in the system.

Material Balances and Phase Change Operations in Chemical Engineering

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