che 201 hand out 4 chapter 6
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Chapter 6
Multiphase Systems
Dr. M. A. A. Shoukat Choudhury
Email: shoukat@buet.ac.bdWebsite: http://teacher.buet.ac.bd/shoukat/
Multiphase Systems Why Study?
- Phase change operations such as freezing,, ,
Example:
1. brewing a cup of coffee or tea,
2. absorption of SO2,
3. distillation to recover methanol or ethanol
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4. L-L extraction
5. Adsorption
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6.1 Single Component Phase Equilibrium
Phase diagram A plot of one system variableagainst another that shows the conditions at whichthe substance exists as solid li uid and as.
Phase Diagram of water
- Boiling point
- normal boiling point
- melting/freezing point
- sublimation point
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- triple point
- critical temperature and pressure above whichtwo phases (liquid-vapor) never coexist.
Phase Diagram of water
Gas
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Difference of vapor and gas
Vapor:- Gaseous substance below its criticaltemperature which can be condensed bycompressing or increasing the pressure
Gas :
- Gaseous substance above its criticaltemperature which cannot be condensed by
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compressing or increasing the pressure
Vapor Pressure Definition: Vapor pressure (also known as equilibrium vapor
pressure)is the pressure of a vapor in equilibrium with its non-vapor phases.
form, and all gases have a tendency to condense back into theiroriginal form (either liquid or solid). At any given temperature,for a particular substance, there is a pressure at which the gasof that substance is in dynamic equilibrium with its liquid orsolid forms. This is the vapor pressure of that substance at thattemperature.
The equilibrium vapor pressure is an indication of a liquid's
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.atoms to escape from a liquid or a solid.
Higher the vapor pressure, the more volatile the compound is.
Source : wikipedia
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Estimation of Vapor Pressure
Clapeyron Equation / Clausius-ClapeyronEquation
Antoine Equation
Coxs Chart
Clapeyron Equation / Clausius-Clapeyron Equation
Example 6.1-1 Vapor Pressure Estimation using Clausius
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-Clapeyron Equation:
If the vapor pressure of benzene is 40 mm Hg at 7.6o
Cand 60 mm Hg at 15.4 oC, find the vapor pressure at 42.2
oc using Clausius-Clapeyron Equation
Antoine Equation log10 p
* = A - B/ (T + C)
p = 10^ A - B T + C
T is in oC
p* is in mm Hg
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Coxs Chart for Vapor Pressure Estimation
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Water Vapor
Pressure Table
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Water VaporPressure Table
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Gibbs Phase Rule How many intensive variables must be
specified to define a system?
= =, ,P=number of phases
intensive variables do not depend on thesize of the system. Example: molar volume,density
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the system. Example: mass, volume
Calculate DOF for various regions of waterphase diagram.
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Gas-Liquid Systems One Condensable Components
Examples evaporation, drying, humidifications,condensations, dehumidifications
Water
Bone ry airBDA + water
Raoults Law: pi= yi P = pi*
Saturated: pi= yi P = pi*
Superheated: pi= yi P < pi*(T)
P, T
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Dew point: if the superheated vapor is cooled
at constant pressure, the temperature at whichthe first dew forms
Degrees of superheat: Ti - Tdp
Example 6.3.2A stream of air at 100 oC and 5260 mm Hg contains
10% water by volume.
superheat of the air
b) Calculate the percentage of vapor that condensesand the final composition of the gas phase if the airis cooled to 80 oC at constant pressure.
c) Calculate the percentage of vapor that condenses
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,is compressed isothermally to 8500 mm Hg.
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= = *
Gas-Liquid Systems Multi-Component systems
A + B + C
A, B, C P, T
- works good for xAis close to 1 (i.e., the solution isalmost pure A)
- mixtures of structurally similar liquids (straight-chainalcohols, aromatic hydrocarbons,...)
1. pentane, hexane, heptane, 2. methanol, ethanol, propanol
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. , ,
Henrys Law: pA= yAP = xAHA(T)
- HA is Henrys constant for A at temp. T
- works good for xAis close to 0 (i.e., the dilutesolution of A)
Bubble Point Temperature (for solution) Bubble point: When a liquid solution is slowly heated at
constant pressure, the temperature at which the firstbubble forms is the bubble point of the solution at thegiven pressure.
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Dew point: When a vapor is slowly cooled at constantpressure, the temperature at which the first liquid dropletforms is the dew point temperature at the given pressure.
Dew Point Temperature
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Graphical Representations of VLE
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Solubility
The solubility of a solid in a liquid is themaximum amount of that substance that cane sso ve n a spec e amoun o e
liquid in the equilibrium. The solubilitystrongly depends on temperature.
Saturated
Supersaturated
Solubility curve
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Elementary Principl es of Chemical Processs, 3/E by Richard M. Felder and
Ronald W. Rousseau
Copyright 2005 by John Wiley & Sons, Inc. All rights r eserved.
Figure 6.5-1 (p. 266)Solubilities of inorganic solutes.
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Solubility Curves
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oC
Problem 6.78 A solution containing 100 lbm KNO3/ 100 lbm
H2O at 80oC is fed to a cooling crystallizer
opera e a . urry rom e crys a zeris fed to a filter where the crystals areseparated from the solution. Determine theproduction rate of crystals (lbm crystals/ lbmof feed) and the solid-liquid mass ratio (lbm
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the crystallizer. The solubility of KNO3 is 40 gKNO3/ 100 g H2O at 25
oC.
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6.78 solution
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Weather Report
What kind of humidity?
What is its meaning?
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Courtesy: The Daily Star, April 27, 2009
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Gas-Vapor Systems
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Definitions of Various Humidity
% relative saturation/humidity
- Saturation refers to any gas - vapor system
- Humidity refers to air water system
sr or hr =
Molal saturation
sm or hm =
pipi
*(T)X 100%
piP - pi
Absolute Saturation or humidity
sa or ha =
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Pi Mi(P- pi)Mdry
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Definitions of Various Humidity
Percentage saturation/humiditysp or hp =
sms *
X 100%
piP - pi pi
*
P pi*
= x 100%
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Problem 6.27On a hot summer day the temperature is 35 oC,and barometric pressure is 103 kPa, and there a ve um y s . n a r con onerdraws in outside air, cools it to 20 oC, anddelivers it at a rate of 12500 L/h. Calculate therate of moisture condensation (kg/h) and thevolumetric flow rate of the air drawn from the
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6.27 Solution
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Problem 6.38Stack gas is emerging from a furnace at 300oC and 105 kPa. It contains CO2 at a partialressure of 80 mm H and no CO O
methane or ethane. The fuel gas fed to thefurnace containing methane and ethane isburned with air. Calculate the mole fraction ofmethane in the fuel and the dew pointtemperature of the stack gas.
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Example 6.4.3 Do Yourself
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Problem 6.44Sulfur trioxide (SO3) dissolves in and reacts with water to form anaqueous solution of sulfuric acid (H2SO4). The vapor is in equilibrium
with the solution contains both SO3 and H2O. If enough SO3 is added, allof the water reacts and the solution becomes pure H2SO4. If still moreSO3 is added, it dissolves to form a solution of SO3 in H2SO4, called
.pure SO3. A 20% oleum by definition contains 20 kg of dissolved SO3and 80 kg of H2SO4 per hundred kg of solution. Alternatively oleumcomposition can be expressed as % SO3 by mass, with constituents ofthe oleum considered to be SO3 and H2O.
a) Prove that a 15.0% oleum contains 84.4% SO3 b) Suppose a gas stream at 40 oC and 1.2 atm containing 90 mol% SO3
and 10% N2 contacts a liquid stream of 98% H2SO4 (aq), producing
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.the partial pressure of SO3 in equilibrium with this oleum is 1.15 mm Hg.Calculate i) the mole fraction of SO3 in the outlet gas if this gas is in
equilibrium with the liquid product at 40 oC and 1 atm, and ii) the ratioof (m3 gas feed)/(kg liquid feed).
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Who is he?
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