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MEL140Properties of pure substancesPure substanceA pure substance has the same chemical composition throughout.Are the following confined in a fixed volume pure substances:Ice (single component, single phase) A mixture of water and water vapor (single component, multiphase)Air in gas phase (multi-component, single phase)Oil in contact with water (multi-component single phase)A gaseous mixture containing N2,O2,H2O, CO2 obtained from burning kerosene (multicomponent, single phase) Liquid air in contact with gaseous air (multicomponent, multiphase) Objective: evaluating the properties for single component pure substances existing in one or more phases (multiphase). NYNYYYAlert: in chemistry a pure substance is defined such that it consists of one component (chemical species) and therefore must be non-mixture. We follow a different definition (see above) in engineering thermodynamics.2-195.806 degree C boiling point of nitrogen -182.95 degree C boiling point of oxygen

PhasesSolidLiquidGasA region within matter with distinct molecular arrangement that is homogeneousthroughout that region which is separated from other regions (if any) by distinct boundary surfaces. Physical properties (like density and refractive index) of each phase is different.The three principal phases:

http://www.chem.purdue.edu/gchelp/atoms/states.html3-195.806

Phase equilibriumA system can be composed of subsystems with different molecular arrangements separated by phase boundaries (phases). Phase equilibrium prevails when no transfer of mass happens between phases.The state postulateA property is characteristic of the system such as specific volume (v), temperature (T), pressure (P), (specific) internal energy (u).A state is the condition of a system as determined by its properties.A simple compressible system is a system whose only mode of performing quasi-equilibrium work is through a change of its volume against a pressure.The state postulate: The state of a simple compressible system consisting of a pure substance is completely specified by two independent intensive properties.The state postulate can be represented by an equation of state such as f(p,v,T)=0 (or say g(p,v,u)=0). It is often convenient to represent this functional relationship by A surface in p,v,T (or u,p,v) space or more commonly its projections on (p,v), (T,v) and (p,T) planes.Tables of propertiesThe P-v diagram

Remove just enough heat to keep temperature constantas the volume is reduced.It will be observed that exceptduring 2-3, pressure also needs to be increased for executing this process in a quasi-equilibrium manner.During 1-2 you 4-5 (notduring 2-3-4)Shows isotherms on P-v diagram123

The critical state: recapitulation At the critical state (Tc, Pc), saturated liquid and saturated vapor states are identical (SLL intersects SVL). Increasing/decreasing pressure at a given temperature leads to condensation/evaporation only if a state lies below the critical isotherm.

Phase change processes

Critical properties of common fluids Water/steam:CP: 374o C (647.1 K), 22 Mpa (~more than 200 atm)at which specific volume 0.003106 m3/kg (~three times less dense than @ STP)BP at atmospheric pressure (101.325 kPa): 100o C (1 atm) Refrigerant 134a or R134a or 1,1,1,2-Tetrafluoroethane in your freeze: CP: 101o C, 4 Mpa (~40 atmosphere) BP at atmospheric pressure : -26o C, 101.325 kPa (1 atm) Nitrogen:CP: -147o C, 3.4 MPaBP at atmospheric pressure : -196o C

Carbon-dioxide:CP: 31.05oC ,7.39 Mpa (CO2 is not a gas in Delhi for six months, i.e. Apr-Sept)BP at atmospheric pressure: -78.5oCTable A.1 (Tc,Pc,vc)How far a state is away from critical point?

Curious facts:

Critical isotherm and the gas-vapor nomenclatureSupercritical fluidsT>Tcr and P>Pcr

Principle of corresponding states (van der Waal, 1880) Reduced temperature: Tr=T/TcReduced pressure: Pr=P/PcReduced volume: vr=v/vc

my equation of state has a universal form which can be identified by its predicted behavior at critical point Other equation of states might also be given similar universal forms by the same procedure.Regardless of the substance, there is a universal equation of state connecting the reduced co-ordinates. So, thermodynamic states of different substances correspond.

Can be stated as:Principle of corresponding states (van der Waal, 1880, continued) Correspondence means the same reduced co-ordinates should mean the sameness of a third reduced property such as reduced volume.Compressibility factor is an important reduced property given by:

Z signifies departure from ideal gas behavior. More discussion on significance in notes.Principle of corresponding states: All fluids when compared at the same Tr and Pr have the same Z and deviate from the ideal gas behavior to about the same degree.This principle is the basis of classifying systematizing organizing and compacting experimental measurements on P, V and T.

Critical compressibility of real gases

1212Source: http://books.google.com/books?id=SdIQtl7boA8C&pg=PA141&dq=%22critical+compressibility%22+hydrocarbons&hl=en&ei=jUg5TcfvBsPXrQfVjqjyCA&sa=X&oi=book_result&ct=result&resnum=2&ved=0CCwQ6AEwAQ#v=onepage&q=Table%203.1&f=false

Phase change processes

Some terminologyCompressed liquid or sub-cooled liquid: Liquid which is not about to vaporize (State 1)Saturated liquid: liquid which is about to vaporize (State 2)Saturated vapor: vapor which is about to condense (State 4)Saturated liquid-vapor mixture: a mixture of saturated liquid and saturated vapor (State 3)Superheated vapor: vapor that is not about to condense (State 5)

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Phase change processes

SaturatedliquidSaturated vaporSaturated liquid vapor mixtureSuperheated vaporCompressed/subcooled liquidLatent heatThe energy absorbed by a system during a phase change process at a given pressure/temperature is called latent heat.Latent heat of fusion (melting)Latent heat of vaporization (boiling)Latent heat goes to change the molecular potential energy; in-fact temperature, a measure of molecular kinetic energy remains constant during a phase change process.

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Saturation temperature, saturation pressure and saturation curvePhase change processes (e.g. saturated liquid boiling to saturated vapor) under a given pressure ( saturation pressure or Psat) take place at a given temperature ( saturation temperature or Tsat).Therefore Psat=f (Tsat). A plot of this function is the saturation curve

Saturation curve for water17-195.806

Property diagram for phase change processes: the T-v diagram.

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Construct at different pressures18-195.806

The critical point

The state (point) at whichthe saturated liquid andthe saturated vapor states are identical.For water19-195.806

The T-v diagram: saturated liquid line and the saturated vapor line

Saturated liquid and saturated vapor lines meet at the critical point.Shows isobarson T-v diagram20-195.806

The P-v diagram

Remove weightsto change pressureduring 1-2, 4-5 (notduring 2-3-4)Shows isotherms on P-v diagramExtending the P-v diagram to include solid phaseP-v diagram of a substance whichcontracts on freezing (mostexcept water)

aa solid at temperature lowerthan melting pointbb solid begins melting

cc solid completely melteddd liquid begins to vaporizeee liquid completely vaporized

Extending the P-v diagram to include solid phaseP-v diagram of a substance whichexpands on freezing(e.g. water)Saturated liquid linesSaturatedsolid lineaa ice at -10oC, 1 atmbb ice begins melting (0oC), 1 atmcc ice completely melted(0oC), 1 atmdd water begins to vaporize(100oC), 1 atmee water completely vaporized (100oC), 1 atmSOLIDLIQUIDThe triple lineThe states where all three phases co-exist in equilibrium lie on a straight line on the P-V or T-v diagram known as a triple line.All the triple states appear as a point on the p-T diagram and the corresponding (T,v) is called a triple point.Triple point of water: (0oC, 0.61 kPa) The P-T diagram (phase diagram)

The P-v-T surface

For substances which contract on freezingFor substances which expand on freezing (such as water).Enthalpy: a combination propertyEnthalpy (h):h=u+pvEnthalpy is useful for studying processes (such as vaporization, heat transfer) taking place at constant pressure and processes that involve flow work

ObjectiveEvaluate properties of states corresponding to:saturated liquid and saturated vapor saturated liquid-vapor mixturessuperheated vaporcompressed/sub-cooled liquid

Saturated liquid and saturated vaporSubscript f represents saturated liquid stateSubscript g represents saturated vapor stateSubscript i represents saturated solid state.Propertyfg=Propertyg-Propertyfe.g. vfg =vg-vf represents volume change on vaporization hfg = =hg-hf represents the latent heat or enthalpy of vaporization.From Table A-4Specified

Saturated liquid and saturated vaporSaturated states lie on the curve f(Psat, Tsat)=0 and can therefore be specified by specifying either Psat, or TsatTable A-4 for water: (Psat,vf,vg,vfg,uf,ug,ufg,hf,hg,hfg,sf,sg,sfg) listed against Tsat Table A-5 for water: (Tsat,vf,vg,vfg,uf,ug,ufg,hf,hg,hfg,sf,sg,sfg) listed against PsatSame data in Tables A-4 and A-5From Table A-4Saturated liquid and saturated vaporSubscript f represents saturated liquid stateSubscript g represents saturated vapor statePropertyfg=Propertyg-Propertyfe.g. vfg =vg-vf represents volume change on vaporization hfg = =hg-hf represents the latent heat or enthalpy of vaporization (for vaporization under constant pressure)

From Table A-4Saturated liquid-vapor mixturesRefer to same Tables A-4 and A-5.The proportion of saturated vapor in the mixture is indicated by a new property quality or dryness fraction:

The average value of a specific extensive property y (such as v,u,h) etc. for the mixture can be calculated from

Saturated liquid-vapor mixturesRefer to same Tables A-4 and A-5.The proportion of saturated vapor in the mixture is indicated by a new property quality or dryness fraction:

x=0 for saturated liquid0ug @ P/Th>hg @ P/TVisit Table A-6 for water

Superheated vaporProperties of pure substances (continued)MEL140Compressed liquidAt a compressed liquid stateP>Psat @ given TT