2student_CHAPTER 5 (1)

7/23/2019 2student_CHAPTER 5 (1)

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CHAPTER 5-ENERGY BALANCE

FOR REACTIVE SYSTEM

CHE243-MATERIAL AND ENERGY BALANCE AND

SIMULATION


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HEAT OF

REACTIONHESSS LAW

HEAT OF

FORMATION

HEAT OF

COMBUSTION


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Heat of reaction (or enthalpy of reaction) Hr

(T,P) is the enthalpychange for a process in which stoichiometric quantities of reactants attemperature T and pressure P react completely in a single reaction to formproducts at the same temperature and pressure.Refer Felder pp. 442

Stoichiometric quantities of reactions molar amounts of the

reactants numerically equal to their stoichiometric coefficients.

Consider the reaction between solid calcium carbide and liquid waterto form solid calcium hydroxide and gaseous acetylene:

HEAT OF REACTION

gHCsOHCalO 2HsCaC 22222 The heat of the calcium carbide reaction at 25 0C and 1 atm is:

mol

kJ125.4HHH rreactantproduct


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HEAT OF REACTION

Several important terms and observations related to heats of reactions:

If Hr(T,P) isnegative

, the reaction is exothermic

If Hr(T,P) ispositive, the reaction is endothermic

At low and moderate pressures,Hr(T,P)is nearly independent of pressure.

The value of heat of reaction depends on how the stoichiometric equation iswritten.Refer Felder pp. 443

The value of a heat of reaction depends on the state of aggregation (gas,

liquid, or solid) of the reactants and products.

The standard heat of reaction, H0ris the heat of reaction when boththe reactants and products are at specified reference temperatureand pressure, usually 250C and 1 atm.


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Hesss Law If the stoichiometric equation for reaction 1 can be obtained byalgebraic equations (multiplication by constants, addition, and subtraction)on stoichiometric equations for reaction 2, 3,then the heat of reactionH0r1can be obtained by performing the same operations on the heats of reactions

H0r2, H0r3.Refer Felder pp. 446

HESSS LAW


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Example HesssLaw(Felder pp 446)

.

The standard heats of the following combustion reactions have beendetermined experimentally:

1. C2H6 + 3.5O2 2CO2 + 3H2O H0

r1 = -1559.8 kJ/mol

2. C + O2 CO2 H0

r2 = -393.5 kJ/mol

3. H2 + 0.5O2 H2O H0r3 = -285.8 kJ/mol

Use Hesss law and the given heats of reaction todetermine the standard heat of reaction:

4. 2C + 3 H2 C2H6 H0

r4 = ???

mol

kJ84.6H

0

r4

^


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A formation reaction a compound is formed from its elementalconstituents as they normally occur in nature. Refer Felder pp. 447

Standard heat of formation, H0f the enthalpy change associatedwith the formation of 1 mole of the compound at a referencetemperature and pressure (usually 25 0C and 1 atm)

The H0ffor elemental species (eg. O2, N2, C, etc..) is zero (0).

The standard heat of reaction, H0r

is:

HEAT OF FORMATION


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Example Determination heat of reaction from heats of formation(Felder pp 447)

.Determine the standard heat of reaction for the combustion of liquidn-pentane, assuming H2O (l) is a combustion product

C5H12(l) + 8O2(g) 5CO2(g) + 6H2O(l)

mol

kJ3509.54H

0

r

^


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Standard heat of combustion, H0cthe heat of combustion of thatsubstance with oxygen to yield a specified products, with bothreactants and products at 25 0C and 1 atm.Refer Felder pp. 448

The standard heat of reaction,

H0

r that involve only combustiblesubstances and combustion products that can be calculated fromtabulated standard heats of combustion.

HEAT OF COMBUSTION


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Example Determination heat of reaction from heats of combustion(Felder pp 449)

.Determine the standard heat of reaction for the dehydrogenation ofethane:

C2H6 C2H4 + H2

mol

kJ136.93H

0

r

^


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PROCEDURE CALCULATION

SIMPLE ENERGY BALANCE

CALCULATION


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HEAT OF

REACTION

METHOD

HEAT OF

FORMATIONMETHOD


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For single reaction where H0ris known.Refer Felder pp. 450

Complete the material balance calculations on the reactor to the greatestextent possible.

Choose reference states for specific enthalpies calculationthe best choice is250C and 1 atm.

HEAT OF REACTION METHOD

100 mol C3H8 (g)/sT=25

0C

600 mol O2 (g)/s

2256 mol N2 (g)/s

T=3000C

Q (kJ/s)

100 mol O2 (g)/s

2256 mol N2 (g)/s

300 mol CO2 (g)/s

400 mol H2O(g)/s

T=10000C


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For a single reaction in a continuous process, calculate the extent of reaction,

Prepare inletoutlet enthalpy table.

Substances min( mol/s)

Hin(kJ/mol)

mout( mol/s)

Hout(kJ/mol)

C3H8 100 0 0 0

O2 600 H2 100 H4

N2 2256 H3 2256 H5

CO2 - - 300 H6

H2O - - 400 H7

References: C3H8 (g), O2 (g), N2 (g), CO2 (g), H2 O (l), at 250C, 1 atm


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Calculate the unknown stream component enthalpy

Calculate H for the reactor. Use one of the following formula:

From energy balance, calculate Q


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Preferably for multiple reactions and single reactions where H0ris notavailable.Refer Felder pp. 451

100 mol C3H8 (g)/sT=25

0C

600 mol O2 (g)/s

2256 mol N2 (g)/s

T=3000C

Q (kJ/s)

100 mol O2 (g)/s

2256 mol N2 (g)/s

300 mol CO2 (g)/s

400 mol H2O(g)/s

T=10000C

HEAT OF FORMATION METHOD

Complete the material balance calculations on the reactor to the greatestextent possible.

Choose reference states for specific enthalpies calculation the bestchoice is 25 0C and 1 atm.


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Prepare inletoutlet enthalpy table.

Substances nin

( mol/s)

Hin

(kJ/mol)

nout

( mol/s)

Hout

(kJ/mol)C3H8 100 H1 0 0

O2 600 H2 100 H4

N2 2256 H3 2256 H5

CO2 - - 300 H6

H2O - - 400 H7

References: C3H8 (g), O2 (g), N2 (g), CO2 (g), H2 O (l), at 250C, 1 atm


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Calculate the unknown stream component enthalpy

Calculate H for the reactor. Use one of the following formula:

From energy balance, calculate Q


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Example Energy Balance About an Ammonia Oxidizer(Felder pp 453)

.

The standard heat of reaction for the oxidation of ammonia is givenbelow:

4NH3 (g) + 5O2 (g) 4NO (g) + 6H2O(v) : H0

r=-904.7 kJ/mol

100 mol NH3 /s and 200 mol O2 /s at 250C are fed into a reactor in which

the ammonia is completely consumed. The product gas emerges at300 0C. Calculate the rate at which heat must be transferred to or fromthe reactor, assuming operation at approximately 1 atm.

kW19,701.45Q


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Example Energy Balance About a Methane Oxidizer(Felder pp 454)

.

Methane is oxidized with air to produce formaldehyde in a continuousreactor. A competing reaction is the combustion of methane to form CO2

1. CH4 (g) + O2 (g) HCHO (g) + H2O(v)

2. CH4 (g) + 2O2 (g) CO2 (g) + 2H2O(v)

A flow chart of the process for an assumed basis of 100 mol methanefed to the reactor is shown in Figure 1. Calculate the rate at which heatmust be transferred to or from the reactor, assuming operation atapproximately 1 atm.

kJ15,382Q


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Example Simultaneous Material and Energy Balance(Felder pp 458)

.

The ethanol (C2H5OH) dehydrogenation reaction is to be carried out

with the feed entering at 3000C. The feed contains 90.0 mol% ethanoland the balance acetaldehyde and enters the reactor at a rate of 150mol/s. To keep the temperature from dropping too much and therebydecreasing the reaction rate to an unacceptably low level, heat istransferred to the reactor. When the heat addition rate is 2440 kW, the

outlet temperature is 2530C. Calculate the fractional conversion ofethanol achieved in the reactor. The reaction of dehydrogenation ofethanol to form acetaldehyde is stated as:

C2H5 OH (v) CH3 CHO (v) + H2(g)

The fractional conversion of ethanol:

0.313n

nn

feed

unreactedfeed

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2student_CHAPTER 5 (1)