example 2 10

HOME CHAPTERS CASE STUDIES AUTHORS Order the book Select thermodynamic models for process simulation A Practical Guide to a Three Steps Methodology Download CHAPTER 2: From Fundamental to Properties Abstract Read the abstract Table of contents See the table of contents List of examples 2.1: Refrigeration system 2.2: VLE observation 2.3: Flexfuel model 2.4: Phase envelope of a natural gas with retrograde condensation 2.5: Entropy rise in a ideal gas expansion 2.6: Cryogenic plant 2.7: Distillation column 2.8: Energy balance in a column feed 2.9: Risk of condensation of water in a gas stream 2.10: Effect of the feed composition on the watergas shift reaction 2.11: Effect of temperature on the reaction constant 2.12: Chemical looping Example 2.10: Effect of the pressure on the methane reforming reaction In the methane reforming reaction, water and methane react to yield hydrogen gas and carbon monoxide. The reaction is catalyzed so that it can be considered in thermodynamic equilibrium. It occurs in the vapour phase. The equilibrium constant at 800 K is 0.032 if the reference state is taken at 1 bar. What is the effect of pressure on this reaction? Analysis: Pressure and temperature are assumed to be known. The reaction occurs in the vapour phase, and at rather high temperature (this is an endothermic reaction, so its advancement is increased at high temperature). As a first approximation, we can therefore state that the reaction occurs in an ideal gas. The equation that relates pressure with the equilibrium condition is then: Solution: See complete results in file (xls): Some help on nomenclature and tips to use this file can be found here . The exact stoechiometry of the reaction must first be determined: H 2 O + CH 4 =3 H 2 + CO The sum of the stoechiometric coefficients, =3+111 = 2 is positive (more products than reactants). The mole number of each component is written as: where is the extent of reaction. The molar fractions are calculated as: It is now easy to calculate , for any pressure, if the extent of reaction is known. In the example sheet, we find this value by numerically solving Note that is taken as 1 bar, since the reference state is 1 bar if pressure is expressed in the same unit. The solution is found in figure 1:

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Thermodynamics

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6/1/2015 Example210

http://books.ifpenergiesnouvelles.fr/ebooks/thermodynamics/02/exo_02_10.html 1/2

HOME CHAPTERS CASESTUDIES AUTHORS Orderthebook

SelectthermodynamicmodelsforprocesssimulationAPracticalGuidetoaThreeStepsMethodology

Download

CHAPTER2:

FromFundamentaltoProperties

AbstractReadtheabstract

TableofcontentsSeethetableofcontents

Listofexamples

2.1:Refrigerationsystem

2.2:VLEobservation

2.3:Flexfuelmodel

2.4:Phaseenvelopeofanaturalgaswithretrogradecondensation

2.5:Entropyriseinaidealgasexpansion

2.6:Cryogenicplant

2.7:Distillationcolumn

2.8:Energybalanceinacolumnfeed

2.9:Riskofcondensationofwaterinagasstream

2.10:Effectofthefeedcompositiononthewatergasshiftreaction

2.11:Effectoftemperatureonthereactionconstant

2.12:Chemicallooping

Example2.10:Effectofthepressureonthemethanereformingreaction

In themethane reforming reaction,water andmethane react to yieldhydrogengasand carbonmonoxide.The reaction iscatalyzedsothatitcanbeconsideredinthermodynamicequilibrium.Itoccursinthevapourphase.Theequilibriumconstantat800Kis0.032ifthereferencestateistakenat1bar.Whatistheeffectofpressureonthisreaction?

Analysis:

Pressureandtemperatureareassumedtobeknown.

The reaction occurs in the vapour phase, and at rather high temperature (this is an endothermic reaction, so itsadvancementisincreasedathightemperature).Asafirstapproximation,wecanthereforestatethatthereactionoccursinanidealgas.Theequationthatrelatespressurewiththeequilibriumconditionisthen:

Solution:

Seecompleteresultsinfile(xls):

Somehelponnomenclatureandtipstousethisfilecanbefoundhere.

Theexactstoechiometryofthereactionmustfirstbedetermined:

H2O+CH4=3H2+CO

Thesumofthestoechiometriccoefficients, =3+111=2ispositive(moreproductsthanreactants).

Themolenumberofeachcomponentiswrittenas:

where istheextentofreaction.Themolarfractionsarecalculatedas:

Itisnoweasytocalculate ,foranypressure,iftheextentofreactionisknown.Intheexamplesheet,wefindthisvaluebynumericallysolving

Notethat istakenas1bar,sincethereferencestateis1barifpressureisexpressedinthesameunit.Thesolutionisfoundinfigure1:
6/1/2015 Example210

http://books.ifpenergiesnouvelles.fr/ebooks/thermodynamics/02/exo_02_10.html 2/2

Figure1:Effectofpressureontheextentofreaction

Theconversiondecreaseswithpressure,whichisinaccordancewiththeLeChatelierprinciple(thesystemreactssoastocounteract the change that is imposed). Since in the reaction that is considered here more molecules are created thanconsumed,pressureshouldresultinadecreaseoftheextentofreaction.

Contacts TermsofUse 2011IFPEnergiesnouvelles