ChE 201 August 26, 2008 - Bangladesh University of ...teacher.buet.ac.bd/shoukat/chapter8mixingandsolutions.pdf• Ammonia-Water System in VLE ... is produced by absorbing Hydrogen

ChE 201 August 26, 2008

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ChE 201

Chapter 8Balances on Nonreactive Processes

Heat of solution and mixing

Definitions

• A solution is a homogeneous mixture

• A solute is dissolved in a solvent.l i h b b i di l d– solute is the substance being dissolved

– solvent is the liquid in which the solute is dissolved

– an aqueous solution has water as solvent

• A saturated solution is one where the concentration is at a maximum - no more solute is able to dissolve.– A saturated solution represents an equilibrium: the rate of

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A saturated solution represents an equilibrium: the rate of dissolving is equal to the rate of crystallization. The salt continues to dissolve, but crystallizes at the same rate so that there “appears” to be nothing happening.

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3 Stages of Solution Process

Separation of Solute– must overcome IMF or ion-ion attractions in solute– requires energy, ENDOTHERMIC ( + ΔH)requires energy, ENDOTHERMIC ( ΔH)

Separation of Solvent– must overcome IMF of solvent particles– requires energy, ENDOTHERMIC (+ ΔH)

Interaction of Solute & Solvent– attractive bonds form between solute particles and solvent

particles


particles– “Solvation” or “Hydration” (where water = solvent)– releases energy, EXOTHERMIC (- ΔH)

Dissolution at the Molecular Level?

• Consider the dissolution of NaOH in H2O


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• Ideal mixture/solution– Heat of mixing or solution is negligible

Id l i b h i k ll f l– Ideal gas mixture behavior works well for nearly all gas mixtures and for liquid mixture of similar compounds

• But for other mixtures or solutions– Aqueous solution of strong acids or bases or

certain gases of solids


g– Heat of solution should be included in energy

balance calculations

Δ

8.5 Mixing and Solution

• Change of Enthalpy

mixii HHxH Δ+= ∑– Heat effect accompanied by mixing of two different

species– for hydrocarbons and simple molecules. But

for some mixtures heats of mixing are substantial.

mixii∑

0≈Δ mixH


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Heats of Solution & Mixing

• Process involves mixing two fluids– Heat of Mixing

• Process involves dissolving a gas or solid in liquid– Heat of Solution

• Heat of SolutionChange in enthalpy for a process in which

ˆ ( , )SH T rΔ


1 mole of a solute (gas or liquid) is dissolved in r moles of a solvent

at a constant temp. T


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Findings from Table B.11

– Provides heat of solution and heat of mixing at 250C and 1 atm

– Enthalpies are expressed per mole of solute not– Enthalpies are expressed per mole of solute, not per mole of solution

– Heat of solution is directly depends on amount of solvent as well as amount of solute

– Heat of solution at infinite dilute solution


Example 8.5.1

Hydrochloric acid is produced by absorbing gaseous HCl in water. Calculate the heat that must be transferred to or from an absorptionmust be transferred to or from an absorption unit if HCl(g) at 1000C and H2O(l) at 250C are fed to produce 1 ton/h of 20.0 wt% HCl(aq) at 400C.


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Enthalpy Concentration (H-x or H-y) Charts

• Energy balance calculations on liquid phase systems involving mixtures can be cumbersome when heats of mixing arecumbersome when heats of mixing are significant.

• Calculations can be simplified for binary systems by using an enthalpy-concentration chart.A l t f ifi th l l /


• A plot of specific enthalpy vs. mole/mass fraction.

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Reference state:H2SO4(l) at 77 oF2 4( )H2O (l) at 32 0F


Enthalpy Concentration Charts for VLE calculation

• A plot of specific enthalpy vs. mole/mass fraction

f• Enthalpy Concentration Charts for VLE calculations

• Ammonia-Water System in VLE• DOF=2+C-P=2+2-2=2;• Fix the system pressure. So, 1 DOF is used


y p ,and specification of another DOF should determine the other system properties

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Ref State:Ref. State:Pressure 1 atm.

Isothermal lines are


Isothermal lines arecalled tie lines. They connect Vapor and liquidPhase.

Lever Rule

•


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Example 8.5.5 : Equilibrium Flash Vaporisation

A 30% wt% ammonia solution at 100 psia is fed at a rate of 100 lbm/h to a tank in which the pressure is 1 atm The enthalpy of the feed solution relative to theatm. The enthalpy of the feed solution relative to the reference conditions used to construct Figure 8.5-2 is 100 BTU/lbm. The vapor composition is to 89 wt% NH3. Determine the temperature of the stream leaving the tank, the mass fraction of NH3 in the liquid product, the flow rates of the liquid and vapor product


product, the flow rates of the liquid and vapor product streams, and the rate at which heat must be transferred to the vaporizer


ChE 201 August 26, 2008




ChE 201 August 26, 2008


Energy balance

Component min, lbm/h Hin, Btu/lbmmout, lbm/h

Hout, Btu/lbm

Input-Output Enthalpy TableReference State: Liquid water at 32 oF and Liquid NH3 at -40 oF

Feed 100 100 - -

Vapor - 16 728

Liquid - 84 45


Problem – 8.32A natural gas containing 95% methane and the balance

ethane is burned with 20% excess air. The stack gas, which contains no unburned hydrocarbons or carbon monoxide, leaves the furnace at 900 oC and 1.2 atmand passes through a heat exchanger. The air on its way to the furnace also passes through the heat exchanger, entering at 20 oC and leaving at 245 oC.

(a) Taking a basis 100 mol/s of the natural gas gas fed to the furnace, calculate the required molar flow rate of air, the molar flow rate and the composition of the stack gas, the required rate of heat transfer in the preheater and the temperature at which the stack gas leaves the preheater.

(b) What would be, Q. if the actual feed rate of the natural gas were 350 SCMH?10/17/2012 ChE 201/[email protected] 26

ChE 201 August 26, 2008


8.32 Solution:

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Soln 8.32 (cont’d)


ChE 201 August 26, 2008


Soln 8.32 (cont’d)


Problem 8.86

An 8-molar hydrochloric acid solution [SG=1.12, Cp=2.76 J/g. oC] is produced by absorbing Hydrogen Chloride (HCl(g)) in water. Liquid water enter the b b d l f dabsorber at 25 oC and gaseous HCl is fed at 20 oC

and 790 torr (absolute). Essentially all HCl fed to the column is absorbed. Take one litre of product solution as a basis of calculation.

• (a) Estimate the volume (litres) of HCl that must be fed to the absorber

• (b) Estimate the heat (KJ) that must be transferred from the absorber if the product solution is to emerge at 40 oC.

• (c ) Estimate the final solution temperature if the absorber operates adiabatically10/17/2012 ChE 201/[email protected] 30

ChE 201 August 26, 2008


Problem 8.86


8.86 soln (cont’d)


ChE 201 August 26, 2008


Problem 8.97

You have anlyzed an aqueous ammonia solution and find that it contains 30 wt% NH3.

• (a) Use figure 8.5-2 to determine the mass fraction(a) Use figure 8.5 2 to determine the mass fraction of NH3 in the vapor that would be in equilibrium with this solution in a closed flask at 1 atm and the corresponding system temperature.

• (b) If the liquid phase in part (a) accounts for 90% of the total mass of the system calculate the overall system composition and specific enthalpy using y p p py gbalances.


Prob. 8.97 solution


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Problem 8.98An Ammonia water mixture containing 60 wt% NH3 is broughtto equilibrium in a closed container at 140 oF. The total mass of the mixture is 250 g. Use Figure 8.5-2 to determine the masses of ammonia and water in each phase of the system


8.5a Heat of Solution and Mixing

• Calculation of Enthalpy ˆ

(n is the moles of solvent at the given conc. )

sH n HΔ = Δ

solute

• Two reference state – Based on pure solvent

and solute – Based on pure solvent

and infinite dilution solution

S

S S

H H r

H H r H

= Δ

= Δ Δ ∞

ˆ ˆ ( )ˆ ˆ ˆ( ) - ( )


)(ˆ ∞Δ sH ˆ ( )sH rΔ

(1mol HCl, 106 mol H2O) (1 mol HCl, 10 mol water) + ((106 – 10) mol water)

0

* Note enthalpies are expressed per solute not moles of solution.

ChE 201 August 26, 2008


Reference State• Enthalpy of hydrochloric acid solution

containing 10 moles of H2O per mole of HCl(g).

• Answer: • - 69.49 KJ/mol HCl• + 5.65 KJ/mol HCl

SH H r= Δˆ ˆ ( )Reference state

• Pure solute & solvent at 250C and 1 atm


S

S S

H H r

H H r H

Δ

= Δ Δ ∞

( )ˆ ˆ ˆ( ) - ( )

25 C and 1 atm• pure solvent & an

infinitely dilute solution at 250C and 1 atm

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ChE 201 August 26, 2008 - Bangladesh University of ...teacher.buet.ac.bd/shoukat/chapter8mixingandsolutions.pdf• Ammonia-Water System in VLE ... is produced by absorbing Hydrogen