Chem Eng: Calculate Heat Req for Temp Change from 25°C to 250°C

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Discussion Overview

The discussion revolves around calculating the heat required to raise the temperature of air from 25 °C to 250 °C before it enters a combustion furnace. Participants explore the application of the ideal gas law and enthalpy changes in this context, focusing on the specific enthalpy change and flow rates.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Technical explanation

Main Points Raised

  • One participant presents the problem statement and relevant equations, indicating the need to calculate heat in kW based on temperature change and specific enthalpy.
  • Another participant suggests determining the exit molar density using the ideal gas law, prompting further calculations.
  • A participant provides a calculated exit molar density of 34.487 mol/m³.
  • There is a repeated calculation of the molar flow rate based on the exit molar density and flow rate, resulting in approximately 44.7031 mol/min.
  • One participant expresses understanding of the calculations but questions whether it is necessary to find the enthalpy for the entering flow.

Areas of Agreement / Disagreement

Participants generally agree on the calculations related to molar density and flow rates, but there is uncertainty regarding the necessity of calculating the enthalpy for the entering flow, indicating a lack of consensus on that point.

Contextual Notes

Participants rely on assumptions of ideal behavior and specific enthalpy values, but there are unresolved aspects regarding the entering flow's enthalpy and its impact on the overall calculations.

kp3legend
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Homework Statement



Prior to entering a combustion furnace, air is heated up from ambient temperature (25 °C) to 250 °C. This change in temperature causes the specific enthalpy to change to 3745 J/mol. Assume that the pressure of the air leaving the heater is 150 kPa and the flow rate is 1.3 m3 /min. Assuming ideal behaviour, calculate the heat required in units of kW.

Homework Equations



PV=nRT
H=U+PV
dH=Q

The Attempt at a Solution


I can find P1 and V1 from the equation T1/T2=P1/P2 and V1/V2=P2/P1
P1=85.487 kpa
V1=136.863m^3
 
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Since you know the exit temperature and exit pressure, you should be able to determine the exit molar density using the ideal gas law. What is the exit molar density (moles/m^3)?
 
It will be 34.487 mol/m^3
 
kp3legend said:
It will be 34.487 mol/m^3
If the exit molar density is 34.387 moles/m^3, and the exit flow rate is 1.3 m^3/min, what is the flow rate in moles/min?
 
It should be 1.3 x 34.387 = 44.7031 mol/min. I guess
 
kp3legend said:
It should be 1.3 x 34.387 = 44.7031 mol/min. I guess

In the molar flow rate is 44.7 mol/min and the change in enthalpy per mole is 3745 Joules, how much heat is being added per min? How much heat is being added per second?

Chet
 
Omg thank you very much now I get it . but do you think I have to find H for the entering flow ?
 
kp3legend said:
Omg thank you very much now I get it . but do you think I have to find H for the entering flow ?
What do you think?
 
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