CHAPTER 3 - Polytechnique Montréal · CHAPTER 3 BASIC CONCEPTS OF MASS AND ENERGY ... Mass balance...
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CHAPTER 3 BASIC CONCEPTS OF MASS AND ENERGY BALANCES
Transcript of CHAPTER 3 - Polytechnique Montréal · CHAPTER 3 BASIC CONCEPTS OF MASS AND ENERGY ... Mass balance...
CHAPTER 3BASIC CONCEPTS
OF MASS AND ENERGY BALANCES
Mass Balance
Variations of mass contained whitin the control volume at time “t”
Total mass flow entering the system at time “t”
Total massflow leavingthe system at time “t”
= -
Where :
Δmcv : mass change
Σmi : inputs
Σme: outputs
Δmcv = Σmi - Σme
Example : Mass BalanceMass balance for a system with chemical reactionA fuel oil is analyzed and is found to contain 87%
weight carbon, 11% hydrogen and 1.4% sulfur, with the remainder being non-combustible (inert) material.
• The oil is burned with 20% excess air (based on complete combustion of the carbon to CO2, the hydrogen to H2O and the sulphur to SO2). The oil is burned completely, but 5% of the carbon forms CO instead of CO2.
• Calculate the molar composition of the exhaust gas leaving the burner.
Example : Mass Balance
NOTE : INPUT RATE = Qs * Cs + Qw * CwOUTPUT RATE = Qm * Cm = (Qs + Qw) * CwDECAY RATE = KCVINPUT RATE = OUTPUT RATE + DECAY RATE
Energy Balance for Closed SystemsConservation of energy principle:
Time intervalvariation of the total energy in a system
Net energy transfered across the system boundary by heattransfer into the system
Net energy transfered across the system boundary by workdone by the system
= -
INPUT OUTPUT
Where :
Δ KE = Kinetic Energy changeΔ PE = Gravitational Potential Energy changeΔ U = Internal Energy changeQ = HeatW = Work
ΔKE + ΔPE + ΔU = Q – W
All parameter mentioned above, are in Joules, Btu or Calories.
Total Energy Variation
Characteristics of Energy Balance Calculations
– W is the work transfered from the surroundings to the system. + Q is the heat energy transfered into the system from the surroundings.
Therefore :+ W is the work done by the system released intothe surroundings- Q is the heat energy transfered into the surroundings from the system
Example : Energy BalanceA mixture of 1 kmol of gaseous methane and 2
kmol of oxygen initially at 25°C and 1 atm burns completely in a closed, rigid container. Heat transfer occurs until the products are cooled to 900K. Determine the amount of heat transfer in kJ.
1 kmol CH4 (g)2 kmol O2T1= 25°CP1= 1 atm
State 1
Products of combustionT2P2
State 2
Assumptions :1. The contents of the closed, rigid container are taken
as the system
2. Kinetic, potential energy effects and work = 0.
3. Combustion is complete
4. The reactants and products each form ideal gas mixtures.
5. The initial and final states are equilibrium states
Example : Energy Balance
Example : Energy Balance
Example : Energy Balance
