Documentation:CHBE Exam Wiki/Module 5 - Non-reactive Energy Balances

From UBC Wiki
CHBE 241
Exam resources wiki
Example alt text
Chemical and Biological Engineering
Welcome to the CHBE Exam Resources Wiki!

This wiki is intended to host past exams

with fully worked-out hints and solutions
Past Exams
Final Exam 2016W
Midterm Exam 1 2016W
Midterm Exam 2 2016W
Problem Sets
Module 1 - Process Basics
Module 2 - Reactors
Module 3 - Separations 1
Module 4 - Separations 2
Module 5 - Non-reactive Energy Balances
Module 6 - Reactive Energy Balances


Question 1

At 150K and 41.64 atm, oxygen has a tabulated specific volume of 4.684cm3 /g and a specific internal energy of 1706 J/mol.
Determine the specific enthalpy of O2 in this state.


Solution


Question 2

Prior to entering a furnace, air is heated from 25°C to 150°C and the change in specific enthalpy for this process is 3640 J/mol. The flow rate of air at the outlet of the heater is 1.5 m /min and the air pressure at this point is 150 kPa absolute.

Calculate the heat needed for the process in kW. Assume ideal gas behavior and that kinetic and potential energy changes from the heater inlet to the outlet are negligible.

Solution



Question 3

Superheated steam at 40 bar absolute and 500°C flowing a rate of 200 kg/min is sent to an adiabatic turbine which expands to 5 bar. The turbine outputs 1250kW. The expanded steam is then sent to a heat exchanger where isobaric heating occurs, resulting in the stream being reheated to its initial temperature. Assume no changes in kinetic energy.

Question 3a

Write the energy balance for the turbine and determine the outlet stream temperature.

Solution


Question 3b

Write an energy balance on the heat exchanger and use it to determine the required input (kW) to the steam.

Solution


Question 3c

Justify that an overall energy balance on the two-unit process is satisfied.

Solution


Question 3d

If the inlet and outlet pipes of the turbine are 0.5m, show that it is reasonable to neglect the change in kinetic energy for this unit.

Solution



Question 4

A 15 m3 tank contains steam at 280°C and 15 bar. The tank and its contents are cooled until the pressure drops to 1.3 bar. Some of the steam condenses in the process.

Question 4a

What is the final temperature of the tank contents?

Solution


Question 4b

How much steam condensed (kg)?

Solution


Question 4c

How much heat was transferred from the tank?

Solution


Question 5

The maximum pressure that a 250L water tank can withstand is 20bar (absolute). At time x, the tank contains 180kg of liquid water and the inlet and outlet valves are closed, and the absolute pressure in the space of above the liquid is 3 bar (assume that the space only contains water vapor). Out of carelessness, the tank heater was turned on and the tank was left unmonitored. Let t1 be the time when the heater is turned on and t2 be the moment before the tank explodes. Use the steam tables for the following calculations.

Question 5a

Determine the water temperature, the liquid and head-space volumes (L), and the mass of water vapor in the head space (kg) at time t2 .

Solution



Question 5b

Determine the water temperature, the liquid and head-space volumes (L), and the mass of water vapor (kg) that evaporates between t1 and t2.

Solution


Question 5c

Calculate the amount of heat (kJ) transferred to the tank contents between t1 and t2. Give two reasons why the actual heat input to the tank must have been greater than the calculated value.

Solution


Question 5d

List three different factors responsible for the increase in pressure resulting from the transfer of heat to the tank.

Solution


Question 6

Two beakers of aqueous sulfuric acid with different concentrations were mixed to form a 70 wt% product solution. The first solution at 20 wt% H2SO4 was taken from the bench top at 25°C while the second solution with 80wt% sulfuric acid was taken from the cooling bath at 15°C. Assume that there was no temperature loss between the mixing process and the initial temperature of the solutions from the respective locations.


Question 6a

If the mass of the 20wt% solution is 5 lb, what is the mass of the 80 wt% solution ?


Solution


Question 6b

Use the enthalpy-concentration chart for H2SO4/Water to estimate the product solution temperature if the mixing is adiabatic.

Solution



Question 6c

If the product solution drops to 25°C, how much heat is lost from the solution to the surrounding under a constant atmospheric pressure.

Solution


Question 6d

In terms of safety, is it safer to add a concentrated acid to a dilute acid or vice versa? Use the chart to explain your response.

Solution