Civil Engineering homework help
CE 4354/5358
Solid Waste Engineering Project – Spring 2022
Due Monday May 2, 11:59 p.m. (midnight) Late submissions: 10% off per day
The Problem
Green City, population 550,000, is going to build a Material Recovery Facility (MRF) for its recyclables. Your first task is to design a MRF for the city.
The city is also considering combustion and anaerobic digestion as alternatives to landfilling. The by-products from any of these processes would still need to be landfilled. Your second task is thus to compare the following two options and recommend one to the city:
A. MRF + anaerobic digester + landfill (you will only be able to have the yard waste and food waste go to the digester)
B. MRF + combustor + landfill.
The waste composition for Green City is:
Component |
Percent by Weight |
Food |
19 |
Paper |
27 |
Textiles |
8 |
Yard |
12 |
Wood |
5 |
Steel |
3 |
Glass |
5 |
Aluminum |
6 |
Plastic |
15 |
I recommend setting up a spreadsheet to do repetitive calculations. Pay attention to significant digits – points will be subtracted for using more than 3 significant digits.
Part 1. MRF Design (43%)
Citizens separate out paper, steel, glass, aluminum, and plastics. Assume a 45% recycling participation rate (45% of the recyclables in the waste stream are separated out and sent to the MRF; the other 55% remain part of the waste stream with the refuse and go to the combustor, digester, or landfill).
The MRF will include the following unit operations, targeted to separate out the material in parentheses:
· Hand sorting (plastics)
· Trommel screen (glass)
· Eddy current separator (aluminum)
· Magnet (steel)
· Air classifier (paper)
· Shredder after the air classifier, to shred the separated paper.
· Roll crusher after the magnet, to flatten cans.
f-values (rejection fractions) for the separation processes are given in the following table:
Component |
Air Classifier |
Trommel Screen |
Magnet |
Hand Sorting |
Eddy Current Separator |
Paper |
0.1 |
0.9 |
1.0 |
0.9 |
1.0 |
Glass |
0.8 |
0.1 |
1.0 |
1.0 |
1.0 |
Ferrous/steel |
1.0 |
1.0 |
0.0 |
1.0 |
0.9 |
Aluminum |
0.9 |
0.9 |
0.9 |
1.0 |
0.1 |
Plastic |
0.7 |
0.9 |
1.0 |
0.2 |
0.9 |
The shredder and roll crusher process 100% of the waste fed to them (f = 0 for all materials).
1. (3 points) Estimate the mass per day of paper, steel, glass, aluminum, and plastics that enter the MRF.
2. (17 points) Specify the order of the 5 separation processes (air classifier, trammel screen, magnet, hand sorting, and eddy current separator), if the objective is to obtain the highest effectiveness for separation of paper. As a measure of effectiveness, multiply recovery of paper by purity of paper (the 2 equations in the book do not apply because we have more than one separation process and more than two materials). Recovery of paper should be calculated as follows:
Recovery of paper = (paper extract separated by air classifier)/(total paper entering
MRF
) * 100%
There are 5 separation processes, which means there are 5! = 120 possible orders. To simplify the analysis, estimate the effectiveness of paper removal for 5 possible orders:
A. air classifier first (which should give maximum recovery of paper),
B. air classifier second and hand sorting first (less recovery of paper than Option A but greater purity),
C. air classifier second and trommel screen first (less recovery of paper than Option A but greater purity),
D. air classifier second and eddy current separator first (less recovery of paper than Option A but greater purity),
E. air classifier last (which should give the maximum purity of paper).
The order in which you place the other separation processes does not matter for this analysis.
Hint: For some of the options, you don’t need to calculate all of the coefficients in the matrix.
3. (3 points) Draw a schematic of the MRF process train (example: Fig. 5-36), including all 7 unit processes in the order chosen in #2. Label the kind of material that is separated by each process.
4. (4 points) Determine the capacity of each of the 7 unit operations (5 separators plus the shredder and roll crusher). The capacity is the mass of material that each unit will process per day.
Hint #1: The 5 separators are in series. So the first separator will process all of the waste that goes to the MRF. The second separator will process all of the waste minus the waste extract separated out by the first process, and so on.
Hint #2: From #2, you already have the matrix of masses of each component exiting each separator (reject). Simply sum the masses of the various components to find the total mass entering the next unit.
Hint #3: All of the material separated out by the magnet (extract) goes to the roll crusher. All of the material separated out by the air classifier goes to the shredder.
5. (5 points) Using your matrix with the chosen order of separation processes from #2, estimate the quantity (mass/day) and purity of the 5 materials recovered (paper, steel, glass, aluminum, and plastics). (Purity of paper was already determined in 2.) The quantity recovered for sale is what ends up in the target extract only (paper separated by the air classifier only, for example).
6. (1 point) Estimate the amount of waste from the MRF that must go to the landfill, as all materials are not recovered. The waste that must go to the landfill is the waste that passes through the last separation process, without being separated out (reject).
7. (1 point) Estimate the critical speed of the trommel screen (2.75 m diameter).
8. (2 points) Calculate the characteristic size of the shredder, assuming that it must produce a product such that 95% of the material passes the 1” sieve (n=1.15).
9. (3 points) Calculate the velocity in the air classifier needed to suspend the paper pieces, assuming a 4” aerodynamic diameter. CD = 2.5, void fraction = 55%. (2 points)
10. (4 points) Calculate the size of rollers required for the roll crusher. Use 7.4 cm for the can diameter. Desired crushed particle size = 0.9 cm. Coefficient of kinetic friction for steel on steel is 0.57. Calculate the roller width W needed to meet the required capacity. Assume M = 42 rpm and = 250 lb/yd3.
Part 2. SWOLF Life Cycle Inventory Modeling (24%)
11. (10 points) Use SWOLF to compare the 2 options A and B. For option A, you will only be able to have the yard waste and food waste go to the digester. You will need to run the model twice.
SWOLF will provide output to compare the 2 options in terms of total cost, total greenhouse gas emissions, and percent diverted. Create a table of your own to summarize the comparison.
12. Use a decision-making matrix with weightings and ratings to compare the 2 options A and B based on the quantitative values in the table created in #11, as well as your semi-quantitative assessment of the following:
· air emissions (besides greenhouse gases),
· water pollution potential,
· water use,
· land impacts, and
· any other appropriate factors.
Explain your weightings of all criteria, and your ratings of the semi-quantitative criteria based on information about the options discussed in class. Based on your decision-making matrix, which option would you recommend? (13 points)
13. In a real-world situation, what other factors would need to be considered in making a decision among alternatives? Consider the 3 pillars of sustainability. (1 point)
Part 3. Combustor or Digester (16%)
Depending on whether you chose anaerobic digester or combustor in #12, address one of the following sets of questions below. For example, if you chose combustor, address the combustor questions only, NOT the digester questions.
Combustor
14. (4 points) Assuming the percents given in the composition table on p. 1 are on a wet basis, estimate the heat value of the waste.
15. (11 points) The wastes are being burned to produce electricity. Estimate the maximum amount of power production, given the following assumptions:
a. Air flow rate is 9500 kg/h, and the air contributes negligible heat.
b. Radiation losses are 5%.
c. 6% of the fuel remains uncombusted in the ash, which exits the combustion chamber at 700°C.
d. The specific heat of ash is 0.837 kJ/kg/°C, and the specific heat of air is 1.0 kJ/kg/°C.
e. The lowest temperature at which the exhaust gases can exit the stack is 25°C.
f. The efficiency conversion of steam to electricity is 38%.
16. Estimate the mass of material that must go to the landfill (mass of ash) (1 point).
Digester
17. (2 points) What is the C/N ratio of the waste mixture? What could be done to adjust the ratio to 30? Assume C/N values as follows: food waste = 15:1, Assume all information for yard waste is the same as for grass.
18. (4 points) Estimate the actual methane production from the waste (assume biogas is 60% methane).
19. (3 points) What amount of water would need to be added to bring the overall moisture content to 85%?
20. (5.5 points) Find the volume of the digester to handle the waste mixture and added water.
21. (1.5 points) The heating value of biogas is anticipated to be 600 Btu/ft3. Assume that the thermal efficiency of the turbine used to generate electricity will be 40%. Estimate electricity production in kW.
Part 4. Landfill (12%)
22. Run the CLEEN model to estimate methane production from the waste. Use rainfall of 30 in./year and temperature of 70°F. Assume the landfill opened in 2000 and use year 20187 gas production for your values. (8 points)
23. Assume that the thermal efficiency of the turbine used to generate electricity will be 40%. Estimate electricity production. (2 points)
24. Estimate the mass of recyclables that will not degrade. (2 points)
Part 5. Executive Summary (5%)
Prepare an executive summary (around 1-2 pages single spaced using a word processor) which includes:
· A brief summary of the problem,
· Main features of your designs for the MRF, combustor, and digester (a table would be a good way to summarize the quantitative values) and
· A summary of your comparison of the 23 options and your recommendation to the city and rationale.
What to submit:
Electronic:
· Spreadsheet of calculations (Clearly define/label each columns with units)
· CLEEN model
· SWOLF model file for each run
Electronic or hard copy:
· A solution to each part of the project
· Hand-written sample for each type of calculation in the spreadsheet (If a number of cells use the same formula, only one written example calculation is needed)
· Executive summary (2-3 pages max)
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