Systems
Almost Multiphase all chemical processes involve material being transferred from one phase to another Phase-change oper ations freezing, melting, evaporation,condensation Seper ation and pur ification separate components of mixtures Often A and B are fed into a system
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Multiphase Separation
Example brewing a cup of coffee hot liquid water and solid coffee beans are contacted Soluble portions of the coffee beans transfer from solid phase to liquid solution Residual solids are removed by f iltr ation
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Other separation terms
Absor ption or Scr ubbing removal of undesirable portions of a product gas perhaps through contact with a liquid D istillation separating two liquids based on a difference in vapor pressure L iquid extr action separates liquids based on differences in solubility
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Separation
Satur ated when a species transfers from one phase to another and the new phase is holding as much of the species as it can Transfer rates of species to their new phase generally slow as saturation approaches Phase equillibr ium concentrations
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Phase Diagrams
At most a single either a a single
temperatures and pressures, pure substance exists as solid, liquid, or a gas- that is phase
At a few specific T and P, the substance can exist as two or even three phases simultaneously Phase diagr am plot one system variable versus another (usually T
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SLE
LVE
SVE
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Phase Diagram Terms
Vapor pr essur er P for a point on the VLE curve Boiling point T for a point on the VLE curve Nor mal Boiling Point the T on the VLE curve at P = 1 atm M elting point or f r eezing point T
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Estimating vapor pressures
Volatility degree to which a species tends to transfer from a liquid (or solid) state to a vapor state At a given T and P, a more volatile species is more likely to be in vapor phase than a less volatile species which is more likely to be in a condensed phase
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Vapor pr essur e measure of volatility
Antoine Equation
Vapor pressure (p*) is not tabulated for all species and temperatures, so it is often estimated from an emperical relationship A ntoine E quation A simple empirical equation relating vapor pressure and temperature data Values for A, B, and C can be found in
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Vapor Pressure Example
In your lab, you measure the vapor pressure of ethylene oxide and determine it to be 1169 mmHg. Estimate the temperature in your lab (in F) with help from your friend Antoine.
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Gibbs Phase Rule
When describing a system you cant specify all of the information independently For example: T,P mass in solid phase, mass in liquid phase, mass or mole in fraction in each phase Specifying some of these will cause to others fix by nature
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For example, setting a T and P for a
System Variables
I ntensive do not depend on the size of the system Extensive do depend on the size of the system
What type of variable is each of the following? Mass
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Volume Temperature
Degrees of Freedom
D egr ees of F r eedom the number of intensive variables that can be specified for a system at equilibrium
G ibbs Phase Rule : DF = 2 + c
DF degrees of freedom c the number of chemical species
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Example
Determine the degrees of freedom of each of the following equilibrium systems and give a possible set of variables that may be specified:
1.
NaCl crystals in an aqueous NaCl solution Humid air in equilibrium with condensed water (consider air one species)
2.
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Gas-Liquid Separations
Evaporation Drying Humidification
Transfer liquid into the gas phase
Condensation Dehumidification
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Raoults Law
Consider a system of liquid water and air. Some water will vaporize and be present in the air at equilibrium. A DOF analysis on this system shows: DF=2+2-2=2. That means if we specify two of the intensive variables (T, P, and yH2O) the third is uniquely specified. This unique relationship can be
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Raoults Law
Raoults Law for a single condensable species:
Pi =yi P=p* i (T)
If a gas at T and P contains a saturated vapor whose mole fraction is yi and if this vapor is the only species that
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Example
Air and liquid water are contained at equilibrium in a closed chamber at 75C and 760 mmHg. What is the molar composition of the gas phase?
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Points about gas-liquid systems
A gas in equilibrium with a liquid must be saturated with the volatile components of that liquid The partial pressure of a vapor in equilibrium in a gas mixture containing a single condensable component cant exceed vapor pressure condensation will occur Super heated vapor a vapor present
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Points about gas-liquid systems
Boiling point temperature at which p*= P R elative humidity : hr = (pi/p*i)*100% M olal humidity: hm = pi/(P-pi) = moles vapor/moles dry gas Absolute humidity: ha = piMi/[(Ppi)Mdry]
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Example
Humid air at 75C, 1.1 bar, and 30% relative humidity is fed into a process unit at a rate of 1000 m3/h. Determine the molar flow rates of water, dry air, and oxygen entering the unit. What is the dew point?
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Example
A gas stream consisting of 100 lbmole/h of an SO2-air mixture containing 45 mole% SO2 is contacted with liquid water in a continuous absorber at 30C. The liquid leaving the absorber is analyzed and found to contain 2.00 g of SO2 per 100 g of H20. Assuming that the gas and liquid streams leaving the absorber are in equilibrium at 30C and 1 atm,
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