The report should include:
- An Excel spreadsheet with detailed workings
- There should be logic and detail to the way you have set up your calculations, to enable anyone to check through your working
- A Word/PDF document that includes:
- a Title page including the project title, unit name and names of team members
- assumptions and basis for each calculation
- key equations used to solve the problem
- a summary of your answers (the detail should be in the Excel file)
- references
There should only be 1 Word/PDF document and 1 Excel submission for each group. It is your collective responsibility to ensure that your group member(s) have submitted both these before the deadline. Please submit the Word/PDF document to the Turnitin link on the Blackboard and submit the Excel spreadsheet to the Excel submission link on the Blackboard.
Process Principles Project
PROCESS DESCRIPTION
The diagram below shows a system that heats up 1 kg of subcooled water from room temperature to its boiling point (saturated liquid). The heat required for the water is provided by burning a fuel with air.
Natural gas is used in a significant number of households for heating and cooking. Biogas is an alternative fuel source as a fuel used in combustion, for the production of heat.
Your task is to provide answers using a detailed analysis of the process of boiling water from natural gas, and other fuels as described, from an engineering perspective. In questions where you have to source information from elsewhere, please state your reference source (a credible reference!).
PROBLEMS
1. What is the average molecular weight of (a) natural gas, and (b) biogas, according to the composition allocated to your group?
2. Choose a person in your group who has a stainless steel/aluminium pot/pan (with a lid) that can hold 1 L of water. Measure/estimate the temperature of the water at your current room temperature (in °C).
3. What is the average moisture content of air in your city (Perth/Miri), in September? (Please state a credible reference source.)
4. What is the total mass of moist air required for stoichiometric and complete combustion of 1 kg of (a) natural gas, and (b) biogas?
5. If your fuel is burnt with 25% excess air, what is the extent of each separate reaction for 1 kg of natural gas? Use the extents to calculate the mole fraction composition of your exhaust gas on a dry basis. State all your assumptions.
6. If your fuel is burnt with 25% excess air, how many moles of each element enters your combustion chamber, for 1 kg of biogas? Use element balances to calculate the mole fraction composition of your exhaust gas on a dry basis. State all your assumptions.
7. If there is a leak in the air supply valve to the combustor and instead of 25% excess air, insufficient air is available for combustion. Calculate the exhaust gas mole fraction composition if only 75% of the air required is available. What is the new exhaust gas composition change if 50% of the exhaust produced is recycled back to the feed? What benefit is there to recycle some of the products?
8. Sketch a p-T and a p-V diagram for H2O, showing the saturated liquid/vapour points/lines. On each diagram, mark your initial state and your final state for H2O as it is heated to its boiling point (saturated liquid). Assume that the lid on your pot/pan is OFF.
9. If you heat the H2O with the lid on rather than off, will it require less/more energy to reach its boiling point? Use your sketch in Q7 to mark a potential new final state for H2O if you heat it with its lid ON, in order to justify your answer.
10. Using Appendices D and G in your textbook, calculate the total enthalpy of reaction to burn 1 kg of each of your fuels, if the exhaust gas temperatures are maintained at 150 °C.
11. Using your answer to Q9, what mass of each fuel is required to heat 1 kg of water in your pan with the lid OFF? (Ignore the energy required to heat the pan itself.) Assume that 25% excess air has been used for combustion.
12. What is the adiabatic flame temperature for complete combustion of 1 kg of each of your fuels?
13. If instead of burning fuel to produce energy, you used an electric heater to heat up your water, how many kWh do you need to heat up your water?
14. Estimate the dewpoints of your wet exhaust gas, if you only combust 90% of each of your gaseous fuels with 5% excess air.
Project Tips
1. Average MW - think about whether you use wt% or mol% for calculating the average molecular weight. Both methods do NOT give the correct answer.
2. Moisture content in air - RH (relative humidity) is NOT the same as % moisture content! There are many different ways to measure moisture content in air: wt% H2O, mol% H2O, absolute humidity, relative humidity etc. They all have different meanings, so please ensure you are interpreting the correct information.
3. Use moist air (based on your answer to Q3) for all the calculations for air content and therefore also product gas compositions.
4. Please do your working on EXCEL! It is efficient, and easy to change mistakes. Do NOT attempt Q4 onwards using hand calculations, as it will waste your time to repeat them, especially if you have minor changes. You also need to submit your Excel file, so please ensure it is clear in the way you have structured it.
5. Q7 is open-ended and it does not have zero degrees of freedom. You therefore have to make some assumptions in order to solve it. Questions you might want to ask yourselves are:
(a) What are the likely products of combustion, with insufficient air?
(b) Will some or all of my combustibles remain in the exhaust gas?
(c) What ratios of extents/product fractions do I need to fix in order to have zero degrees of freedom?
6. For Q8 and Q9, they are theoretical questions, but you should be able to fix your state for Q8 with data from steam tables. For Q9, you can argue your logic based on Q8. The idea is that you think about the concepts of saturated liquid/vapour, sensible heat vs latent heat, and the types of energy that are involved in the heating of water.