An oxidation reactor is being fed with air to aid in the combustion. The air mus

Question

An oxidation reactor is being fed with air to aid in the combustion. The air must first be heated from 25°C to 300°C at 1 atm pressure and the specific enthalpy change associated with this temperature change for air is 8170 J/mol. The inlet velocity of air is 5 m/s in pipe of 0.5 m ID (inside diameter). You may consider the gas to be ideal.

a. What is the inlet flow rate of air (mol/min)?

b. Given that the outlet diameter from this heater is 0.5 m ID, then what is the outlet velocity (m/s)?

c. Calculate the flow rate of heat required to heat the gas (kW)?

Explanation / Answer

a. Flow = pipe area x velocity

    inlet flow = 3.14 x (0.5 m/2)2 x 5 m/s = 0.9813 m3/s

or 0.98 m3/s x 60 s/min = 59 m3/min

The molar volume of air at 25°C is

22.4 L/mol x 298 K/273 K = 22.45 L/mol or 0.02245 m3/mol

Inlet flow = 59 m3/min / 0.02245 m3/mol = 2630 mol/min

b.

you have same pipe diameter at inlet and outlet.

By heating, the air volume is increasing (see gas law):

   573 K/ 298K = 1.9 times

You have the same increase for the outlet velocity

5 m/s x 1.9 = 9.6 m/s

c.

8170 J/mol x 2360 mol/min / 60 sec/min = 321 kJ/s = 321 kW

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