ChE 201 August 26, ChE 201. Chapter 8 Balances on Nonreactive Processes Heat of solution and mixing

Transcription

1 ChE 201 Chapter 8 Balances on Nonreactive Processes Heat of solution and mixing Definitions A solution is a homogeneous mixture A solute is dissolved in a solvent. solute is the substance being dissolved d 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 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. 10/17/2012 ChE 201 shoukat@buet.ac.bd 2 ifthekerahmed@buet.ac.bd 1

2 3 Stages of Solution Process Separation of Solute must overcome IMF or ion-ion attractions in solute 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 Solvation or Hydration (where water = solvent) releases energy, EXOTHERMIC (- ΔH) 10/17/2012 ChE 201 shoukat@buet.ac.bd 3 Dissolution at the Molecular Level? Consider the dissolution of NaOH in H 2 O 10/17/2012 ChE 201 shoukat@buet.ac.bd 4 ifthekerahmed@buet.ac.bd 2

3 Ideal mixture/solution Heat of mixing or solution is negligible 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 Heat of solution should be included in energy balance calculations 10/17/2012 ChE Mixing and Solution Change of Enthalpy Δ H = x i H i + ΔH mix Heat effect accompanied by mixing of two different species ΔH mix 0 for hydrocarbons and simple molecules. But for some mixtures heats of mixing are substantial. 10/17/2012 ChE 201 shoukat@buet.ac.bd 6 ifthekerahmed@buet.ac.bd 3

4 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 Solution ΔH ˆ S ( T, r ) Change in enthalpy for a process in which 1 mole of a solute (gas or liquid) is dissolved in r moles of a solvent at a constant temp. T 10/17/2012 ChE 201 shoukat@buet.ac.bd 7 10/17/2012 ChE 201 shoukat@buet.ac.bd 8 ifthekerahmed@buet.ac.bd 4

5 Findings from Table B.11 Provides heat of solution and heat of mixing at 25 0 C and 1 atm 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 10/17/2012 ChE 201 shoukat@buet.ac.bd 9 Example Hydrochloric acid is produced by absorbing gaseous HCl in water. Calculate the heat that must be transferred to or from an absorption unit if HCl(g) at C and H 2 O(l) at 25 0 C are fed to produce 1 ton/h of 20.0 wt% HCl(aq) at 40 0 C. 10/17/2012 ChE 201 shoukat@buet.ac.bd 10 ifthekerahmed@buet.ac.bd 5

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7 10/17/2012 ChE Enthalpy Concentration (H-x or H-y) Charts Energy balance calculations on liquid phase systems involving mixtures can be cumbersome when heats of mixing are significant. Calculations can be simplified for binary systems by using an enthalpy-concentration chart. A plot of specific enthalpy vs. mole/mass fraction. 10/17/2012 ChE 201 shoukat@buet.ac.bd 14 ifthekerahmed@buet.ac.bd 7

8 Reference state: H 2SO 4 4() (l) at 77 o F H 2 O (l) at 32 0 F 10/17/2012 ChE 201 shoukat@buet.ac.bd 15 Enthalpy Concentration Charts for VLE calculation A plot of specific enthalpy vs. mole/mass fraction 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 and specification of another DOF should determine the other system properties 10/17/2012 ChE 201 shoukat@buet.ac.bd 16 ifthekerahmed@buet.ac.bd 8

9 Ref. State: Pressure 1 atm. Isothermal lines are called tie lines. They connect Vapor and liquid Phase. 10/17/2012 ChE 201 shoukat@buet.ac.bd 17 Lever Rule 10/17/2012 ChE 201 shoukat@buet.ac.bd 18 ifthekerahmed@buet.ac.bd 9

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11 Example : 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 the reference conditions used to construct Figure is 100 BTU/lbm. The vapor composition is to 89 wt% NH 3. Determine the temperature of the stream leaving the tank, the mass fraction of NH 3 in the liquid product, the flow rates of the liquid and vapor product streams, and the rate at which heat must be transferred to the vaporizer 10/17/2012 ChE 201 shoukat@buet.ac.bd 21 10/17/2012 ChE 201 shoukat@buet.ac.bd 22 ifthekerahmed@buet.ac.bd 11

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13 Energy balance Input-Output Enthalpy Table Reference State: Liquid water at 32 o F and Liquid NH 3 at -40 o F Component m in, lbm/h H in, Btu/lbm m out, lbm/h H out, Btu/lbm Feed Vapor Liquid /17/2012 ChE 201 shoukat@buet.ac.bd 25 Problem 8.32 A 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 o C and 1.2 atm and passes through a heat exchanger. The air on its way to the furnace also passes through the heat exchanger, entering at 20 o C and leaving at 245 o C. (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/shoukat@buet.ac.bd 26 ifthekerahmed@buet.ac.bd 13

14 8.32 Solution: 10/17/2012 ChE 27 Soln 8.32 (cont d) 10/17/2012 ChE 28 14

15 Soln 8.32 (cont d) 10/17/2012 ChE 29 Problem 8.86 An 8-molar hydrochloric acid solution [SG=1.12, Cp=2.76 J/g. o C] is produced by absorbing Hydrogen Chloride (HCl(g)) in water. Liquid water enter the absorber b at 25 o C and gaseous HCl is fed at 20 o C 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 o C. (c ) Estimate the final solution temperature if the 10/17/2012 absorber operates ChE 201/shoukat@buet.ac.bd adiabatically 30 ifthekerahmed@buet.ac.bd 15

16 Problem /17/2012 ChE soln (cont d) 10/17/2012 ChE 32 16

17 Problem 8.97 You have anlyzed an aqueous ammonia solution and find that it contains 30 wt% NH 3. (a) Use figure to determine the mass fraction of NH 3 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 balances. 10/17/2012 ChE 201/shoukat@buet.ac.bd 33 Prob solution 10/17/2012 ChE 201/shoukat@buet.ac.bd 34 ifthekerahmed@buet.ac.bd 17

18 Problem 8.98 An Ammonia water mixture containing 60 wt% NH 3 is brought to equilibrium in a closed container at 140 o F. The total mass of the mixture is 250 g. Use Figure to determine the masses of ammonia and water in each phase of the system 10/17/2012 ChE 201/shoukat@buet.ac.bd a Heat of Solution and Mixing Calculation of Enthalpy Δ H = nδhˆ s (n is the moles of solute solvent at the given conc. ) Two reference state Based on pure solvent and solute Based on pure solvent and infinite dilution solution Hˆ =ΔHˆ S ( r) Hˆ = ΔHˆ S( r) - ΔHˆ S( ) (1mol HCl, 10 6 mol H2O) (1 mol HCl, 10 mol water) + (( ) mol water) ΔHˆ ( ) ΔHˆ () r s s 0 * Note enthalpies are expressed per solute not moles of solution. 10/17/2012 ChE 201 shoukat@buet.ac.bd 36 ifthekerahmed@buet.ac.bd 18

19 Reference State Enthalpy of hydrochloric acid solution containing 10 moles of H 2 O per mole of HCl(g). Answer: KJ/mol HCl KJ/mol HCl Reference state Pure solute & solvent at 25 0 Cand1atm atm pure solvent & an infinitely dilute solution at 25 0 C and 1 atm Hˆ =Δ Hˆ S ( r ) Hˆ = ΔHˆ S( r) - ΔHˆ S( ) 10/17/2012 ChE 201 shoukat@buet.ac.bd 37 ifthekerahmed@buet.ac.bd 19