An ideal gas stream (Stream A), C_P = 5R/2, 50 mole/h, is heated by a steady-sta

Question

An ideal gas stream (Stream A), C_P = 5R/2, 50 mole/h, is heated by a steady-state heat exchanger from 20 degree C to 100 degree C by another stream (Stream B) of another ideal gas, C_P = 7R/2, 45 mole/h, which enters at 180 degree C. Heat losses from the exchanger are negligible. For concurrent flow in the heat exchanger, calculate the molar entropy changes (S^out - S^in) for each stream, and for the heat exchanger. For countercurrent flow in the heat exchanger, calculate the molar entropy changes (S^out - S^in) for each stream, and S_gen for the heat exchanger. Comment on

Explanation / Answer

we consider that the flow rate of two streams are same in both cases (cocurrent and countercurrent)

a) molar entropy changes for ideal gas stream is given by delS = nCpln(T2/T1)

here T1 and T2 are the initial and final temperatures

for the first stream (cold gas)

T1=273 +20 = 293K

T2= 273 +100 = 373K

n=1

del S = 5R/2 ln (373/293) = .60351R

for the second gas stream (hot gas)

T1= 273 +180 = 453K

due to same flow rate T2= T1-(100-20) =453-80K = 373K

n=1

delS = 7R/2ln (373/453) =-.680098R

Therefore Sgen = .60351R +(-.680098R ) = -.076588R

b) the values will be same as the flow in both cases are parallel but directions are different

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