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CHEMICALENGINEERING

PROCESS ANALYSISCHE 32O

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ENERGY BALANCES

• ENERGY BALANCES THATINCLUDE THE EFFECT OF

CHEMICAL REACTION• ANALYSIS OF DEGREES OF

FREEDOM• APPLICATION OF ENERGY

BALANCES• HEAT OF SOLUTION AND

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Terminology associated with energybalances

• Surrounding• System• Boundary• Open and closed systems• Property• State• Steady state• Unsteady state• Phase• Equilibrium• Adiabatic system• Isothermal system• Isobaric system• Path Variable

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U!I"IT#• U!I"IT# $PS#% &O!ET&I%' % A&TS

A(" T EI& USE• TE&!I(O)O*#• PS#% &O!ET&I% % A&T• APP)I%ATIO(S

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TE&!I(O)O*#• T E U!I" EAT• T E U!I" VO)U!E• T E "+ BU)B TE!PT,• T E -ET . BU)B TE!PT,• U!I"IT# O/ SATU&ATE" *AS• 0 U!I"IT#• SATU&ATE" VO)U!E• "E- POI(T• 0 &E)ATIVE U!I"IT#

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P&OB)E! 1,• I2 in a process ben3ene is used as a

sol4ent and it is e4aporated into dry(itrogen, At 5678 and 191,:;(ture has a 0 relati4ehumidity o2 ?9, It is required to reco4er @9per cent o2 the ben3ene present by coolingto 5@:8 and compressing to a suitablepressure, -hat should this pressure be

The 4apor pressure o2 ben3ene is15,5;(

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E(E&*# BA)A(%ES T AT I(%)U"E T EE//E%TS O/ % E!I%A) &EA%TIO(

• A(A)#SIS O/ "E*&EE O/ /&EE"O! TOI(%)U"E T E E(E&*# BA)A(%E -IT&EA%TIO(

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"E*&EE+O/+ /&EE"O! A(A)#SIS /O& A S,S/)O- S#STE!

$A' Types o2 4ariables in4ol4ed Temperature o2 a streamPressure o2 a stream

Species Cow in a stream= orcomposition o2 a stream plus total Cow

E>tent o2 reaction

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$B' Types o2 Equation used• Independent materials balances• Energy balance• Phase equilibrium relations• %hemical equilibrium relations• Variables with speciDed 4alues• %on4ersion or e>tent o2 reaction speciDed• eat Cow and wor; speciDed• &ecycle ratio speciDed

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E>ample•

The 2ollowing e>ample illustrates a degree+o2+2reedom analysis that includes an energybalance when a reaction occurs,

• $1' Analysis o2 the degree o2 2reedom 2or a

%ombustion Process!ethane is burned with 0 e>cess air in a2urnace, /ig below shows the stream compositionsand those 4ariables whose 4alues are speciDed,

The process occurs with each stream at 1atm,"etermine i2 the number o2 degrees o2 2reedom2or the process is 3ero,

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/urnace

F

P : G 1 atm T :

/:

%O5%O speciDedO5( 5

5 O

% H 1990

P1 G 1atm T 1 speciDed

P5 G 1 atm T 5 speciDed

O5( 5

/ 1

/ 5

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Solution

A table is needed to ma;e the count o2 the4ariables and equilibrium,

The Energy balance is assumed to reduce toF G J replaced as a 4ariable with p and T(umber o2 4ariables in the process Species in /11

/55/:

Total no o2 species is @

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contd

Total Stream /lows :Stream temperature :

Stream Pressure :F 1E>tent o2 reaction

$5 reactions' 5 Total 59

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(umber o2 EquationsIndependent species materials balance

?Sum o2 species in each o2 the 5streams

$∑nK=I G / K' 5Energy balance 1SpeciDcation o2 the 4alue o2 a 4ariable

Total stream Cows $/ 1 = the basis and / 5 comes 2rom 0 e>cess air' 5Species 4alues $%O' 1

Pressures $p 1 Gp 5 G p : G 1 atm' :

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%O(T", Temperatures $T 1 and T 5 ' 5O5 to ( 5 ratio speciDed in / 5 1%omplete reaction $no % H in e>itstream e>ist'

ence the e>tent o2 reaction is I plied2or both reactions $to %O and %O 5 '

5 TOTA) 59

Thus= the degrees o2 2reedom are 3ero

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APP)I%ATIO(S O/ E(E&*#

BA)A(%ES I( P&O%ESSES T AT I(%)U"E &L(• ere we shall be illustrating the solution o2

continuous and s,s processes 2or which thegeneral energy balance reduces to 5 eqns,

$1'-ith the eMect o2 chemical r>n mergedwith sensible heat

$5'-ith the eMect o2 chemical r>n lumped inthe heat o2 r>n

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A"IABATI% &EA%TIO(• "E/(" as the tempt, Obtained inside

the process when$a'The r>n is carried out under adiabatic

condition= i,e there is no heat

interchange between the 4essel inwhich the reaction is ta;ing place andthe surroundings

$b'(o other eMects occur$c')imiting reactant reacts completely

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EAT O/ SO)UTIO( A("!ILI(*

• Enthalpy . conc, "iagram• eat o2 solution at Dnite dilution• eat $enthalpy' o2 dissolution• eat $enthalpy' o2 mi>ing• eat $enthalpy' o2 solution• Ideal solution• Incremental heat o2 solution• Integral heat o2 solution• &eal solution• Solute• Sol4ent