energy balance.pptx

7/25/2019 Energy Balance.pptx

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Paul Ashall, 2008

Module 9001Energy Balance

Paul Ashall, 2008


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Paul Ashall, 2008

Concerned with energy changes and

energy flow in a chemical process.

Conservation of energy first law ofthermodynamics i.e. accumulation of

energy in a system = energy input

energy output


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Paul Ashall, 2008

Forms of energy

Potential energy mgh! "inetic energy #$2 mv2! %hermal energy heat &! supplied to or removed

from a process 'or( energy e.g. wor( done )y a pump '! to

transport fluids *nternal energy +! of molecules

m mass (g!g gravitational constant, .8# ms-2

v velocity, ms-#


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Paul Ashall, 2008

Energy balance

systemmass in

Hin

mass out

Hout

W

Q


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Paul Ashall, 2008

IUP! con"ention

- heat transferred to a system is ve and

heat transferred from a system is ve

- wor( done on a system isve and wor(done )y a system is -ve


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Paul Ashall, 2008

#teady state$non%steady state

/on steady state -

accumulation$depletion of energy in

system


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Paul Ashall, 2008

Uses

eat re1uired for a process ate of heat removal from a process

eat transfer$design of heat e3changers Process design to determine energy

re1uirements of a process Pump power re1uirements mechanical

energy )alance! Pattern of energy usage in operation Process control Process design 4 development etc


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Paul Ashall, 2008

Ent&al'y balance

p.e., (.e., ' terms = 0

& = 2 # or & = 5

, where 2is the total enthalpy of output

streams and #is the total enthalpy of

input streams, & is the difference intotal enthalpy i.e. the enthalpy heat!transferred to or from the system


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Paul Ashall, 2008

continued

& ve #62!, heat removed from

system

& ve 26#!, heat supplied tosystem.


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Paul Ashall, 2008

E(am'le ) steam boiler

%wo input streams7 stream #- #20 (g$min.

water, 0 deg cent., = #29.: (;$(gne output stream7 29 (g$min. saturated

steam#: atm., 20? deg cent.!, =2:.? (;$(g


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Paul Ashall, 2008

continued

*gnore (.e. and p.e. terms relative to enthalpychanges for processes involving phasechanges, chemical reactions, large

temperature changes etc& = 5 enthalpy )alance!@asis for calculation # min.teady state

& = out in& = B29 3 2:.? B#20 3 #29.:! #:9 3 2:2!& = :.: 3 #09(;$min


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Paul Ashall, 2008

#team tables

Dnthalpy values (;$(g! at various P,

%


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Paul Ashall, 2008

Ent&al'y c&anges

Change of % at constant P

Change of P at constant %

Change of phase

olution

Ei3ing

Chemical reaction

crystallisation


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Paul Ashall, 2008

*atent &eats +'&ase c&anges,

Fapourisation G to F!

Eelting to G!

u)limation to F!


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Paul Ashall, 2008

Mec&anical energy balance

Consider mechanical energy terms onlyApplication to flow of li1uids

5P 5 v2

g 5h H = 'I 2where ' is wor( done on system )y a pump

and H is frictional energy loss in system ;$(g!

5P = P2 P#< 5 v

2

= v22

v#2

< 5h = h2h#

@ernoulli e1uation H=0, '=0!


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Paul Ashall, 2008

E(am'le % Bernoulli e-tn.

'ater flows )etween two points #,2. %he

volumetric flow rate is 20 litres$min.

Point 2 is 90 m higher than point #. %hepipe internal diameters are 0.9 cm at

point # and # cm at point 2. %he

pressure at point 2 is # atm..Calculate the pressure at point 2.


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Paul Ashall, 2008

continued

5P$I 5v2$2 g5h H = '

5P = P2 P# Pa!5v2 = v22 v#

2

5h = h2- h#m!

H= frictional energy loss mechanical energy

loss to system! ;$(g!' = wor( done on system )y pump ;$(g!I = #000 (g$m


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Paul Ashall, 2008

continued

Folumetric flow is 20$#000.0! m$s

= 0.000 m$s

v#= 0.000$/+0.00,

, 2 13.94 m$s"2 0.000555$ /+0.00,

, 2 6.6 m$s

+1015 % P1,$1000 7 8+6.6,) +13.94,$ 7 9.:1.0 2 0

P12 63: Pa +6.3 bar,


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Paul Ashall, 2008

#ensible &eat$ent&al'y

calculations Jensi)leK heat heat$enthalpy that must )e

transferred to raise or lower the temperature of asu)stance or mi3ture of su)stances.

eat capacities$specific heats solids, li1uids,gases,vapours!

eat capacity$specific heat at constant P, Cp%! =d$d% or 5 = integral Cp%!d% )etween limits %2and

%# +se of mean heat capacities$specific heats over a

temperature range +se of simple empirical e1uations to descri)e the

variation of Cp with %


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Paul Ashall, 2008

continued

e.g. Cp = a )% c%2 d%

,where a, ), c, d are coefficients

5 = integralCpd% )etween limits %2, %#5 = Ba% )%2 c% d%?

2 ?

Calculate values for % = %2, %#and su)tract

/ote7 % may )e in deg cent or " - chec( units for CpL


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Paul Ashall, 2008

E(am'le

Calculate the enthalpy re1uired to heat a

stream of nitrogen gas flowing at #00

mole$min., through a gas heater from20 to #00 deg. cent.

use mean Cp value 2.#; mol-#"-#orCp

= 2 0.22 3 #0-2% 0.9:2 3 #0-9%22.8: 3 #0-%, where % is in deg cent!


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Paul Ashall, 2008

Heat ca'acity$s'ecific &eat data

Helder 4 ousseau pp:2$: and %a)le @#0 PerryKs Chemical Dngineers and)oo( %he properties of gases and li1uids, . eid et al, ? th

edition, EcMraw ill, #8: Dstimating thermochemical properties of li1uids part

:- heat capacity, P. Mold 4 M.>gle, Chem. Dng.,#, p#0

Coulson 4 ichardson Chem. Dng., Fol. , rd

edition,ch. 8, pp2#-2? JPhysPropsK


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Paul Ashall, 2008

E(am'le ) c&ange of '&ase

A feed stream to a distillation unit contains an e1uimolarmi3ture of )enNene and toluene at #0 deg cent.%hevapour stream from the top of the column contains

8.? mol O )enNene at 90 deg cent. and the li1uidstream from the )ottom of the column contains ?0molO )enNene at 90 deg cent.

B/eed Cp )enNene, li1uid!, Cp toluene, li1uid!, Cp)enNene, vapour!, Cp toluene, vapour!, latent heatof vapourisation )enNene, latent heat of vapourisationtoluene.


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Paul Ashall, 2008

Energy balances on systems

in"ol"ing c&emical reaction tandard heat of formation 5of! heat of

reaction when product is formed from its

elements in their standard states at 28 ", #atm. (;$mol!

aA )@ cC d

-a -) c d stoichiometric coefficients,Qi!

5ofA, 5of@, 5

ofC, 5

ofheats of formation!

5o

= c 5ofC

d 5of

- a 5ofA

- )5of@


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Paul Ashall, 2008

Heat +ent&al'y, of reaction

5ove exothermic reaction!

5ove endothermic reaction!


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Paul Ashall, 2008

Ent&al'y balance e-uation %

reactor&p = products reactants &r

&p heat transferred to or from process

&r reaction heat R 5o!, where R is

e3tent of reaction and is e1ual to Bmoles

component,i, out moles component i,

in$ Qi


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Paul Ashall, 2008

systemQr

Hreactants H'roducts

Q'

7"e

%"e;ote< ent&al'y "alues must be calculated =it& reference to a

tem'erature of deg cent


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Paul Ashall, 2008

Energy balance tec&ni-ues

Complete mass )alance$molar )alance

Calculate all enthalpy changes )etween

process conditions and standard$referenceconditions for all components at start input!

and finish output!.

Consider any additional enthalpy changes

olve enthalpy )alance e1uation


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Paul Ashall, 2008

Energy balance tec&ni-ues

Adia)atic temperature7 &p = 0


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Paul Ashall, 2008

E(am'les

eactor

Crystalliser

rier

istillation


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Paul Ashall, 2008

>eferences

The Properties of Gases and Liquids,

. eid

Elementary Principles of ChemicalProcesses, .E.Helder and

.'.ousseau

energy balance.pptx

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