1E202 Module 11 Energy Balances in reactive processes

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1E202Module 11Energy Balances in reactive
processes
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This Week

• Solution to example from last week
• Phase change operations
• Mixing and solutions
• Energy Balances on Reactive Processes
• Fuels and Combustion
• Adiabatic Flame Temperature

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Energy balance on a gas preheater
HEATER
2000 L (STP)/min 0.100 mol CH4/mol 0.900 mol
air/mol 300oC, 1 atm
2000 L (STP)/min 0.100 mol CH4/mol 0.900 mol
air/mol 20oC, 1 atm
Use tables (B.8, B.2)
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Exercise procedure steps 1 to 6
4 Table B.8 lists data for air at 25oC and 1 atm
Methane not listed, so choose the conditions of a
process stream (g, 20oC, 1atm) 5
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Exercise procedure steps 6 to 9
6 Calculate and insert all
enthalpies in the table
7 8 work, kinetic energy, potential energy? 9

Heat must be transferred to the heater at a rate
of 12.9 kW (776 kJ/min) to achieve the required
temperature
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Phase change operations

• Consider liquid water and water vapour at 100oC
• Which U is larger?
• Latent heat DH associated with phase change
• Heat of fusion heat of melting
• Heat of vaporisation
• Latent heat is very temperature dependent!
• Remember you then choose a process path
• You may ignore moderate changes in pressure
• If not tabulated, use Troutons rule and/or
  Watsons correlation
• see Felder and Rousseau
• For gas-vapour mixtures you can also use
  psychrometric charts

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Mixing and solutions

• Heat of mixing or solution
• negative if energy released, i.e. solution must
  be cooled to keep same T
• heat of solution of 1 mole gas or solid
  in r moles liquid
• heat of mixing two fluids

Use Perrys pp 2-201 to 2-204 for heats of
solution in water or table B.11 in FR (caution
the values given are negatives of the heats of
solution as defined above)
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Example energy balances for solutions
5,480 mol HCl (g)/h 100oC
ABSORBER
1000 kg/h, 20wt HCl (aq) 5,480 mol HCl/h 44,400
mol H2O/h 40oC
44,400 mol H2O (l)/h 100oC
Choose reference states HCl (g), H2O (l), 25oC,
1 atm
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Procedure for solution example
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Energy balance for solution example
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Multiple-unit process with a recycle stream

• Drawing and labeling a flow sheet
• Perform degree of freedom analysis using
  spreadsheet
• Write in order equations you need to solve
• Use the procedure to calculate the required
  enthalpies
• Perform the calculations

Exercise handed out
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This Week

• Solution to example from last week
• Phase change operations
• Mixing and solutions
• Energy Balances on Reactive Processes
• Fuels and Combustion
• Adiabatic Flame Temperature

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Enthalpy of Reaction

• Heat (or Enthalpy) of reaction is the enthalpy
  change for a process in which stoichiometric
  quantities of reactants at temperature T and
  pressure P react completely in a single reaction
  to form products at the same pressure and
  temperature.

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Heat of Formation

• A formation reaction is the reaction in which the
  compound is formed from its elemental
  constituents as they normally occur in nature.

What is does this mean? i.e. which reaction does
this refer to?
Standard heat of reaction is determined from
standard heats of formation
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Heat of Combustion

• The standard heat of combustion of a substance is
  the heat of combustion of that substance with
  oxygen to yield specific products (e.g. CO2 (g)
  and H2O (l)) at 25oC and 1 atm.

Assumptions Carbon forms CO2 (g) Hydrogen
forms H2O (l) Sulfur forms SO2 (g) Nitrogen
forms N2 (g)
What is does this mean? i.e. which reaction does
this refer to?
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Energy Balances on Reactive Processes

• 1 Draw and label the flow chart
• 2 Use material balances and phase equilibrium
  relationships to determine as many stream
  components as possible
• 3 Choose reference states for specific enthalpy
  to prepare an inlet-outlet table
• 4 Calculate DH to substitute in the energy
  balance and complete balance calculation.

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Propane combustion process
1 Draw and label the flow chart ? 2 Use material
balances and phase equilibrium relationships to
determine as many stream components as possible ?

• 3 Choose reference states for specific enthalpy
  to prepare an inlet-outlet table
• Heat of reaction is known at 25oC 1 atm for O2
  (g), C3H8 (g), CO2 (g), H2O (l)
• Extent of reaction x (C3H8 out - C3H8 in ) / n
  (C3H8) 100 mol/s

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Input-Output Enthalpy Table
32.47
8.47
81.71
48.60
8.12
30.56
O2 (25oC)?O2 (300oC)
H2O (l, 25oC)? H2O (l, 100oC)? H2O (g, 100oC)?
H2O (g, 300oC)
4 Calculate DH to substitute in the energy
balance and complete balance calculation.
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Alternative Method

• In the following cases
• Multiple reactions
• Heat of reaction not readily known
• Use the heat of formation method
• The reference states for the enthalpy
  calculations are the elements that constitute the
  reactants and products, in the state that theyre
  found naturally, e.g. C(s), H2 (g)
• Include the heat of formation of the reactant or
  product starting with the elemental species.
• In this case heat of reaction terms are not
  required!

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Processes with unknown outlet conditions

• Thus far we knew the inlet and outlet conditions
• What if we know the inlet conditions and the heat
  input and product composition, but not the outlet
  temperature?
• Must evaluate Enthalpies in terms of the unknown
  temperature and then substitute the expressions
  in the energy balance to calculate Tout.
• For an adiabatic reactor QDH0

.
.
e.g. in adiabatic dehydrogenation of ethanol,
for the three species in the product
stream yields, for the total process
Tad145oC
?
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Simultaneous material and energy balances

• Calculate the fractional conversion achieved in
  this process

• Start with very quick
• degrees of freedom analysis.
• Dont forget mass balances should
• be on atomic species for reactions!
• Input-output enthalpies are already given
• inclusive heat of formation.

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Fuels and Combustion

• Use of heat by combustion to produce steam to
  drive electricity turbines is single most
  important commercial application of chemical
  reactions.
• Types of fuels
• Solid coal, coke, wood, solid waste
• Liquid hydrocarbons (petroleum), shale oil,
  alcohols...
• Gaseous natural gas (mainly methane), light
  hydrocarbons, hydrogen.
• The heating value of a combustible material is
  the negative of the standard heat of combustion
• higher heating value (gross heating value) based
  on H2O (l) as product
• lower heating value (net heating value) based on
  H2O (v) as product

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Typical heating values
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Adiabatic Flame Temperature

• The highest achievable temperature of the
  products in a combustion reaction is reached if
  the reactor is adiabatic (no heat loss through
  walls).
• All of the energy released by combustion reaction
  then goes to the combustion products.
• This is called the adiabatic flame temperature,
  Tad

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Calculation of an Adiabatic Flame
TemperatureLiquid methanol with 100 excess air
1 mol CH2OH (l) 25oC
FURNACE
n2 (mol CO2 ) n3 (mol H2O (v)) n4 (mol O2) n5
(mol N2 ) Tad (oC)
n1 mol O2 3.76 n1 mol N2 100oC
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Calculation of an Adiabatic Flame
TemperatureLiquid methanol with 100 excess air
Calculate enthalpy of product stream
Example FR 9.6-2