Chapter 9Balances on Reactive Processes

Dr M. A. A. Shoukat ChoudhuryDepartment of Chemical Engineeringepa t e t o C e ca g ee gBUET, Dhaka – 1000

Heat of Reaction or Enthalpy of reaction

• The heat of reaction, (T, P), is the enthalpy change for a process in whichenthalpy change for a process in which stoichiometric quantities of reactants at temperature T and Pressure P react pcompletely to form products at the same temperature and pressure.

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Total Enthalpy Change

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Hess’s Law for Calculation of Heats of reaction

• Hess’s law states that if you can obtain a stoichiometric equation as a linear combination of the stoichiometric equations for other reactions, you can determine its heat of reaction by , y yperforming the same operations on the heats of the other reactions. For example, suppose we experimentally determine the following two standard heats of reaction:

• We want to determine the heat of the reaction A = B + 2D but can’t carry out that reaction experimentally. We observe however that we can obtain thatWe observe, however, that we can obtain that stoichiometric reaction as [1] + 2x[2]:

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Example 9.4-1 Calculate the standard heat of reaction for theThe dehydrogenation of ethane

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C2H6 = C2H4 + H2

Energy Balance

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Energy Balance – Open System

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EB – HEAT of REACTION METHOD (Reference – compounds at 25 oC and 1 atm

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EB – HEAT of FORMATION METHOD (Reference – Elements at 25 oC and 1 atm(

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Energy Balance

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Two Methods for Energy Balance

• Heat of Reaction Method – Good if there is only one reactiononly one reaction

• Heat of Formation Method – Good for multiple reactionmultiple reaction

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Heat of Reaction Method

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Heat of Reaction Method (cont’d)

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An Example

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C H (g) + 5 O (g) = 3 CO (g) + 4 H O (l)C3H8 (g) + 5 O2 (g) = 3 CO2 (g) + 4 H2O (l)

Example - Heat of reaction methodExample Heat of reaction method

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000

Heat of Formation Method

Example - Heat of formation methodExample Heat of formation method

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Problem 9.13The production of most of the steel manufactured in USA begins with the reduction of hematite ore (mostly ferric oxide) with coke (carbon) in a blast furnace to obtain pig iron. The basic reaction isbasic reaction is

Fe2O3(s)+3 C(s) 2 Fe (s) +3CO(g): ΔHr(770F) = 2.111×105 Btu/lb-mol2.111 105 Btu/lb mol

Suppose that stoichiometric amounts of ferric oxide and carbon are fed at 77 0F, the reaction is complete, the iron emerges as a , p , gliquid at 28000F and CO emerges at 5700F. Perform the following calculations for a basis of 1 ton of iron produced.

Draw and label a flowchart and perform all the material balance calculations needed to determine the amounts (1b-mole) of each feed and product stream component.

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Problem 9.13 (cont’d)

b) Taking the reactant and product species in their normal states at 770F as references, prepare an inlet-outlet enthalpy table and calculate and fill in all unknown component specificand calculate and fill in all unknown component specific enthalpies (Btu/lb-mole).

Use the following physical property data for iron:Use the following physical property data for iron:Fe(s): Cp[Btu/(1b-mole.0F)] = 5.90 + 1.50×10-3 T (0F)Tm = 27940F, ΔHm (Tm) = 6496 Btu/1b-moleFe(l): Cp [Btu/(1b-mole 0F)] = 8 15Fe(l): Cp [Btu/(1b-mole. F)] = 8.15

c) Estimate the furnace heat requirement (Btu/ton Fe produced).d) List the assumptions that make the value calculated in part (b)d) List the assumptions that make the value calculated in part (b)

only an approximate estimate of the furnace heat requirement. (One of the assumptions has something to do with the reactor pressure).

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9.13 (solution) – part d)

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Problem 9.21Ethanol is produced commercially by the hydration of ethylene:

C2H4(g) + H2O(v)= C2H5OH(v)Some of the product is converted to diethyl ether in the undesired side

reaction2 C2H5OH(v) = (C2H5)2O(v)+H2O(v)The combined feed to the reactor contains 53.7 mole% C2H4, 36.7% H2O

and the balance nitrogen which enters the reactor at 310 0C. The reactor operates isothermally at 310 oC An ethylene conversion of 5%reactor operates isothermally at 310 oC. An ethylene conversion of 5% is achieved, and the yield of ethanol (moles ethanol produced/mole ethylene consumed) is 0.90.

Data for Diethyl EtherData for Diethyl EtherΔH0

f = -272.8 kJ/mol for the liquidΔH0

v = 26.05 kJ/mol (assume independent of T)C [kJ/mol oC]=0 08945+40 33*10-5 T (oC) – 2 244*10-7 T2Cp[kJ/mol. C] 0.08945+40.33 10 T ( C) 2.244 10 T(a) Calculate the reactor heating or cooling requirement in kJ/mol feed.(b) Why would the reactor be designed to yield such a low conversion of

ethylene? What processing step (or steps) would probably follow the

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ethylene? What processing step (or steps) would probably follow the reactor in a commercial implementation of this process?

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9.6 Fuels and Combustion

• Heat from combustion reaction produces steam which drives turbine to producesteam, which drives turbine to produce electricity

• Analysis of fuels, combustion reaction andAnalysis of fuels, combustion reaction and reactor is an important activity for chemical engineers

• Three types of fuels - Solid (coal, coke, wood, solid waste)( , , , )- Liquid (hydrocarbons, coal tars, bio-ethanol)- Gaseous fuels (NG, acetylene, hydrogen)Gaseous fuels (NG, acetylene, hydrogen)

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Heating Value of a Fuel

HHV = LHV + n * ΔHv (H2O)

HV = Σ xi (HV)iFor a mixture of Fuel:

Do Yourself Example 9.6-1

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o ou se a p e 9 6

Heating Values of Common Fuels

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Adiabatic Flame Temperature

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Problem 9.66 (F&R, 3rd edition)

Methane is burned with 25% excess air in a continuous adiabatic reactor The methane enters thecontinuous adiabatic reactor. The methane enters the reactor at 25 oC and 1.1 atm at a rate of 5.50 L/s , and the entering air is at 150 oC and 1.1 atm. gCombustion in the reactor is complete, and the reactor effluent emerges at 1.05 atm. Calculate (a) the

d (b) h d f h f htemperature and (b) the degrees of superheat of the reactor effluent (consider water to be the only condensable species in the effluent)condensable species in the effluent)

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9.66 Solution

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Known Q and unknown outlet temperature

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Waste Heat Boiler

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