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SMJC 2243 Chemical Engineering

ThermodynamicsSemester 2

Session 2014/2015

16th

 March 2015Room 4.37.01/02

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Chapter 3:HEAT EFFECTS

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•

Heat transfer is a common operation in the chemicalindustry

• Example; – Manufacture of ethylene glycol (an antifreeze agent)

•

Ethylene

 ethylene oxide

 ethylene glycolOxidation Hydration – Catalytic oxidation reaction  temp.:  250C

 – Reactant (ethylene + air) heated before enter the reactor

» Preheater design depends on the ‘Rate of Heat Transfer ’ 

 – Combustion reaction (ethylene + O2) raise temperature

» Heat removed from the reactor, temp. doesn’t rise much above250 C

» Higher temp.; produce CO2 (unwanted product)

 – Design the reactor requires knowledge  heat transfer rate; dependson the heat effects assoc. with chemical reaction

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INTRODUCTION

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• Ethylene  ethylene oxide  ethylene glycol

Oxidation Hydration

 –Ethylene oxide ; hydrated to glycol byabsorption in water

» Heat evolved;

• Phase change + dissolution process

• Hydration reaction between dissolved ethylene

oxide + water

 –Glycol recovered from water distillation

(process of vaporization + condensation» Separate solution into its components

 Simple chemical-manufacturing process

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1. Temperature changes

2. Heat effects of chemical reaction

3. Phase transition

4. Formation of solution

5. Separation of solution

 determined from experimental measurement atconstant temperature

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Characteristic of Sensible Heat Effect

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SENSIBLE HEAT EFFECTS

• When heat transfer to a system; – No phase transitions

 – No chemical reactions

 – No changes in composition

 Cause the changes of temperature (system)

• When the system is a homogeneous substance(constant composition), – phase rule: fixing the values of 2 intensive properties

establishes its state

 – Molar @ specific internal energy of a substance:expressed as a function of 2 other state variable

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• Total internal Energy, 

= ,  

=

 

  +

 

   

= + 

   

where  is constant-volume heat capacity

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•The final term may be set ‘= 0’ in 2circumstances;

= +

 

   

1. For any constant-volume process, regardless of substance

2. Whenever the internal energy is independent of volume,

regardless of the process this is exactly true for ideal gases & incompressible

fluids, approximately true for low-pressure gases

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= 0

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• In either case,

=  ∆ =  

 

• For a mechanically reversible constant-volume

process, = ∆ 

= ∆ =  

 

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• Similarly, the molar @ specific enthalpy may be expressed as afunction of temperature & pressure, then = ,  

=     +      

= +    

 where  is constant-pressure heat capacity

• Again, 2 circumstances allow the final term to be set ‘= 0’ 1. For any constant-pressure process, regardless of substance

2. Whenever the internal energy is independent of volume,regardless of the process

 this is exactly true for ideal gases & incompressible fluids,approximately true for low-pressure gases

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• In either case,

=  

∆ =   

• Moreover,

= ∆for a mechanically reversible

constant-pressure, closed-system processes,

= ∆ =  

 

• The common engineering application of this

equation is to steady-flow heat transfer

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Temperature Dependence (Heat Capacity)

• Evaluation of the integral in below equation requires knowledge of thetemperature dependence of the heat capacity

= ∆ =  

 

Given by empirical equation

  = + +  + − 

1. 

  = + +  

2.     = + + − where either C  @ D is zero, depending on the substance considered

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• For gases,

 – Ideal-gas heat capacity  ( actual heat capacity) is

used in the evaluation of such thermodynamicproperties as the enthalpy

• Reason: thermodynamic-property evaluation is mostconveniently accomplish in 2 steps

1. Calculation of values for hypothetical ideal gas state, ideal-gas

heat capacities are used;

2. Correction of ideal-gas-state values to the real-gas-values

» A real gas becomes ideal in the limit as P   0,

• Ideal-gas- state het capacities (designated by  and) are therefore different for different gases; althoughfunctions of temperature, they are independent ofpressure

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• The influence of temperature on  for Ar, N2, H2O and CO2 is illustrated inFig. 4.1

•

Temp. dependence is expressedanalytically by equation, such as:

  = + +  + − 

• Values of parameters are given in TableC.1 (Appendix C) 

• Two ideal-gas heat capacities arerelated

  = 

  1 

 Temp. dependence of  follows fromthe temp. dependence of

 

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• As with gases, the heat capacities of solids &

liquids are found by experiment

• Parameters for the temperature dependence

of  are given for a few solids & liquids in Table C.3 & C.4 (Appendix 3)

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• Mean heat capacity ;

  = +

20   + 1 +

30

 + + 1

+   0 

∴ ∆ =   0  

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