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.
