Question

In the manufacture of nitric acid, ammonia and preheated air are mixed to form a gas containing 10 mole% NH₃ at 600^oC. (The ammonia is not preheated, since if it were, it will decompose to N₂ and H₂). The mixture is then catalytically oxidized to form NO₂, which is absorbed in water to form HNO₃. If ammonia enters the gas blending (mixing) unit at 25^oC at a rate of 520 kg/h, and heat is lost from the mixer to its surroundings at a rate of 7 kW, determine the temperature to which the air must be preheated. (Use the first two terms of the C_p equation for air).

Answer 1

NH3 enetring at 25 C and Air eneter at T =?

reaction hppene at 600 C

Hence NH3 is need to be heated to 600 C

NH3 flow rate, M_NH3 = 520 kg/hr

Specific heat of NH3, CP_NH3 = 5.56 kJ/kg-k

Heat required for NH3, Q1 = mCPT = 520*5.56*(600-25) = 1662440 kJ/hr = 461.79 kJ/sec = 461.79 kW

Heat lost to surronding, Q2 = 7Kw Hence total heat from Air , Q = Q1+Q2 = 461.79+7 = 468.79 kW

mixture to rector is 10% NH3 and 90% Air mol basis

Hence mole of NH3 enetring = 520 / 17 = 30.588 kmol/hr

Hence Air flow, N_Air = 30.588*0.9 /0.1 = 275.29 kmol/hr

Hence air mass flow, M_Air = 275.29 *28.84 = 7939.48 kg/hr = 2.21 kg/sec

Specific heat of Air =

Heat provided by Air==> Q = mCPT ==> 468.79 = 2.21*1.14*(T-600) ==> T = 786 ⁰C

Hence air need to be pr-heated to 786 ⁰C