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Example 7 – Recycle Material Stream

Problem

Material is known as follows

Type: solid particles

Initial moisture content = 0.25 kg/kg wet basis.

Initial temperature = 20 ^oC.

Product temperature = 75 ^oC

Product moisture content = 0.05 kg/kg wet basis.

Specific heat of the absolute dry material = 1.26 kJ/kg. ^oC.

Mass flow rate wet basis = 1000 kg/hr

Drying air has the following conditions

Initial pressure = 101.3 kPa.

Initial temperature = 20 ^oC.

Initial Absolute Humidity = 0.009 kg/kg

Mass Flow Rate wet basis = 10000 kg/hr

Drying air needs to go through an air filter first. Pressure drop in the air filter is 0.3 kPa. Assume dust volume concentration is 0.1 g/m³, collection efficiency of the air filter is 99.8% and filtration velocity is 2.5 m/s. Drying air then goes through a fan (the efficiency of the fan is 0.7) to gain 3 kPa static pressure, then through a heater to be heated to 110 ^oC before going to the dryer. Pressure drop of air in heater and dryer is 0.8 kPa and 1.2 kPa respectively. The exhaust air of the dryer entrains 0.1% of the total material into the dryer’s gas outlet stream. The gas outlet stream needs to go through a cyclone to collect the entrained dust material. Collection efficiency of the cyclone is 95%. Pressure drop of air in the cyclone is 0.6 kPa.

The dryer requires that the feed moisture content (wt basis) can not be greater than 0.15 kg/kg. As is known, initial moisture content (wet basis) of the material is 0.25 kg/kg. One possible solution is to mix a portion of the dried material product with the fresh material to decrease the moisture content to the required level and then feed the dryer.

A tee is required to split the product material into two halves. One half goes through a Recycle and mixes with the fresh material in a mixer and then introduced into the dryer’s material inlet.

Solution:

Similar to recycling a gas stream in Example 6, a portion of the dryer’s material outlet stream is recycled in this example. To get this problem solved an auxiliary recycle unit operation is necessary. The flowsheet is constructed as in Figure 7a and the results are shown is Figure 7b.

Figure 7a

Figure 7b