ADVANCED SOLID WASTE MANAGEMENT UNIT IV

I

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Separating Plastic From

Shredded Waste

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Problem: Your facility has purchased an air classification system with a fan that can generate an air

flow rate in the system that goes up to 20 ft/second. Your Hammermill shredder is able to provide a waste stream containing plastic that passes a 3 cm grate, if the density of the plastic is 0.875 g/cm.

The facility does not have the means to conduct a drop test, but as the site consultant, they want you

to make your best estimate if the unit will separate out the plastic from the shredded waste. Show all

work.

Solution: We will use the terminal velocity empirical equation (see pp. 203-204)

Vs = 1.9 + 0.092s + 5.8A

where Vs is the terminal settling velocity of material in units of ft/sec

s is the particle density in units of lb/ft3

A is the particle area (length x width) in units of in2

(1) Convert the plastic density from g/cm to lb/ft3

s in lb/ft3 = 0.875 𝑔

𝑐𝑚3 × (2.54

𝑐𝑚

𝑖𝑛 )

3 × (

12 𝑖𝑛.

𝑓𝑡 )

3 ×

1 𝑙𝑏

454 𝑔 = 𝟓𝟒. 𝟔

𝒍𝒃

𝒇𝒕𝟑

(2) A in in2 = 3 𝑐𝑚 × 3 𝑐𝑚 × ( 1 𝑖𝑛.

2.54 𝑐𝑚 )

2 = 𝟏. 𝟒 𝒊𝒏.𝟐

(3) We can now use the terminal velocity equation and plug in our calculations:

Vs = 1.9 + 0.092s + 5.8A

= 1.9 + (0.092 x 54.6) + (5.8 x 1.4)

= 15.04 ft/sec

Therefore, since the Vs = 15.04 ft/sec, this is less than the 20 ft/sec airflow rate in the air

classification system. This means the unit will separate the plastic from the shredded refuse.