Can anyone answer question 1? Thanks. A new gaseous fuel has been identified by

Question

Can anyone answer question 1? Thanks.

A new gaseous fuel has been identified by researchers studying the decaying dog droppings that I am too lazy to remove from my back yard. There was a Craig's List add that offered to remove them for free and I could not resist. The researchers have burned the effluent in a stream of preheated air, using the exhaust combustion gasses to heat the preheater. This preheater uses heat from the combustion exhaust gasses to heat the incoming air. The following data has been recorded for the preheater inlet and outlet streams: The temperature of the air leaving the preheater is not known because the company is cheap and the sensor is broken. Calculate the molar flows of two gaseous streams in the preheater and the temperature of the preheated air. To do this you must make an assumption about the composition of the exhaust gas. What is a reasonable chemical species to use to model the exhaust gas if combustion is complete and air was fed in significant excess? When the outlet flow thermocouple is repaired, it measures 161 C. What could explain any differences to your calculated temperature? Your roommate did a bad job cleaning the hydroflask after sneaking drinks into the movie theater last week. Now there is humid air trapped in the 2.0L flask, while sitting in your car, it has

Explanation / Answer

A.

1)

Flow rate of exhaust gas out, Vout = 1.98 x 105 L/s

Pout = 912 torr = 912 / 760 atm

= 1.2 atm

Tout = 850 °C = 850 + 273.15 K

= 1123.15 K

Molar flow rate of exhaust gas = Pout * Vout / (R * Tout)

= 1.2 * 1.98 x 105 / (0.0821 * 1123.15) mol/s

= 2576.71 mol/s

2)

Flow rate of air in, Vin = 5.53 x 104 L/s

Pin = 684 torr = 684 / 760 atm

= 0.9 atm

Tin = 30 °C = 30 + 273.15 K

= 303.15 K

Molar flow rate = Pout * Vout / (R * Tout)

= 0.9 * 5.53 x 104 / (0.0821 * 303.15) mol/s

= 1999.71 mol/s

3)

Exhaust gas will contain CO2, O2, H2O and N2

Assuming air was fed in significant excess, we can take the specific heat of exhaust gas nearly equal to that of air.

Specific heat of air @~400 K, CPa = 1.013 kJ/kg-K

Specific heat of air @~1100 K = Specific heat of exhaust gas @~1100 K, Cpe = 1.15 kJ/kg-K

MW of exhaust gas ~ MW of air = 29 g/mol

Heat balance:

Heat gained by air = Heat lost by exhaust gases

Ma * Cpa * dTa = Me * Cpe * dTe

1999.71 * MW * 1.013 * (T – 30) = 2576.71 * MW * 1.15 * (945 – 850)

T – 30 = 139

T = 169 °C

B.

Differences could be because of following reasons:

1. Assumption of exhaust gas specific heat taken to be equal to that of air.

2. Assumption of exhaust gas molar mass taken to be equal to that of air.

3. Heat losses are not accounted in the heat balance equation.

4. Specific heat is a function of temperature. We have considered it constant while performing heat balance.

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