chapter 9 balances on reactive processes by : ms. nor helya iman bt kamaludin email:...

CHAPTER 9BALANCES ON REACTIVE

PROCESSES

By : Ms. Nor Helya Iman Bt KamaludinEmail: [email protected]

1PTT 108: Mass and Energy Balances

Introduction •The importance of energy balance on

reactor:To tell the process engineer how much heating or cooling requires in order to operate at the desired conditions.

•Consequences of unstable heat on a reactor:Failure of the reactor temperature control system which can lead to

rapid overheating possibly an explosion. 2PTT 108: Mass and Energy

Balances

Exothermic vs. Endothermic Reaction

Exothermic Reaction Endothermic Reaction

More energy is released when the product bonds form than it took to break the reactant bonds.

More energy is required/ adsorbed to break the reactant bonds than it took to form the product bonds.

Energy (heat or work) must be transferred away from the reactor to keep the temperature below a value that leads to safety and product quality problems.

Energy (heat or work) must be added to the reactor to keep the reactor temperature (and hence the reaction rate) from decreasing; unprofitable.

3

PTT 108: Mass and Energy Balances

Heat of Reaction

•Definition:

The heat of reaction (enthalphy of reaction),

(T,P), is the enthalphy change for a process in

which stoichiometric quantities of reactants at

temperature, T and pressure, P react completely

in a single reaction to form products at the same

temperature and pressure.


Heat of Reaction (cont’d)

Consider a reaction:



•One O-O bond and two H-H bonds broken. System absorbs energy, Usystem and Hsystem increase from reactants to transition state.

•4 O-H bonds are formed. System releases

energy, Usystem and Hsystem decrease from transition state to products.


Heat of Reaction (cont’d)Suppose stoichiometric quantities of the reactants (2 mol H2 + 1 mol O2) react completely, with the reactants starting at specified T and P and the products (2 mol H2O) ending at the same T and P.



The change in enthalpy from reactants to products,

is the heat of reaction.

For stoichiometric quantities of H2 and O2 reacting completely at T=25ºC and P =1 atm,


Heat of Reaction (cont’d)Negativemore energy released by product bond formation than absorbed when reactant bonds break. The reaction is therefore exothermic.

For example:



If 5 mol H2/s consumed in which the reactants and products are at 25°C, then the energy balance is


Standard Heat of Reaction

• is the heat of reaction when both the reactants and products are reacting completely at a specified reference temperature and pressure, usually at 25°C and 1 atm.

•(The “standard” part, which refers to the specified temperature and pressure, is denoted by the superscript°.)



If A is a reactant or product, vA is its stoichiometric coefficient (negative for reactant, positive for product), and nA,r is mol of A are consumed or generated at 25°C and 1 atm, then the enthalpy change is

where ξ is the extent of reaction.

For an open system, dots would go above the ∆H, nA,r, and ξ. 12PTT 108: Mass and Energy

Balances

Heat of Reaction (cont’d)•Properties of the heat of reaction (p. 443)

exothermic reaction if negative, endothermic reaction if positive; ΔĤr (T, P) nearly independent of pressure at low

and moderate pressures; the value of heats of reaction depends on how the

stoichiometric equation is written (e.g.,standard heat of the reaction 2A→2B is twice that of A→B) the value of heat of reaction depends on the states of aggregation (g, l, s) of the reactants and products.


Class Discussion

•EXAMPLE 9.1-1


Internal Energy of Reaction• If a reaction take place in a closed reactor at constant

volume, the heat released/absorbed is determined by the change in internal energy between reactants and products, not enthalpy.

• The internal energy of reaction, ΔÛr(T) is the difference Uproducts – Ureactants if stoichiometric quantities of reactants react completely at temperature T.

• Suppose a reaction occurs by assuming ideal gas behavior and neglecting specific volume where υi is the stoichiometric coefficient of species i, thus the internal energy of reaction is related to the heat of reaction (closed system) is given by


Internal Energy of Reaction (cont’d)

•For example, for the reaction

•The internal energy of reaction is∆Ûr (T) =

=


Class Discussion

•EXAMPLE 9.1-2


Hess’s Law General statement of Hess’s Law:

If the stoichiometric equation for reaction 1 can be obtained by algebraic operations (multiplication by constants, addition, and substraction) on stoichiometric equations for reaction 2, 3,…, then the heat of reaction ΔĤr1 can be obtained by performing the same operations on the heat of reactions ΔĤr2, ΔĤr3,…

In the other words: Hess’s law states that if you can obtain a stoichiometric equation as a linear combination of the stoichiometric equations for other reactions, you can determine its heat of reaction by performing the same operations on the heats of the other reactions.


• For example, suppose we experimentally determine the following two standard heats of reaction:

• We want to determine the heat of the reaction A → B + 2D but can’t carry out that reaction experimentally. We observe, however, that we can obtain that stoichiometric reaction as [1] + 2x[2]:


Class Discussion

•EXAMPLE 9.2-1


Formation Reactions and Heats of Formation

Formation reaction: A reaction in which a compound is formed from its elemental constituents as they occur in nature [e.g., O2(g), and not O].

Standard heat of formation :The enthalpy change associated with the formation of 1 mole of the compound at a reference pressure and temperature (25˚C, 1 atm). Standard heats of formation of many species are given in Table B.1. For example, liquid benzene:

The standard heat of formation of an elemental species [C(s), H2(g), O2(g),...] is zero.

21

Formation Reactions and Heats of Formation (cont’d)

We can use Hess’s law to show that for any reaction a hypothetical process path can be drawn from reactants to elements to products:


Formation Reactions and Heats of Formation (cont’d)

It may shown using Hess’s Law that: if vi is the stoichiometric coefficient of the ith species participating in a reaction (+ for products, - for reactants) and is the standard heat of formation of this species

and since enthalpy is a state function,


Class Discussion

•EXAMPLE 9.3-1


Heat of Combustion

Standard Heat of Combustion : Heat of the combustion of any substance with

oxygen to yield specified products [e.g., CO2 (g), H2O (l), SO2 (g) and N2 (g)], with both reactants and products at 25°C and 1 atm.

The standard heats of combustion can be found in Table B-1. For example, for acetone:


Heat of Combustion (cont’d)

If a reaction only involves combustible reactants and products, then we can calculate the standard heat of the reaction from tabulated standard heats of combustion. The formula is:

where vi is the stoichiometric coefficient of the ith reactant or product species and is the standard heat of combustion of that species. The formula looks like the one involving heats of formation, except that summations are reversed (reactants - products).


Heat of Combustion (cont’d)

This formula is derived from Hess’s law in the same way that the heat of formation formula was derived:


Class Discussion

•EXAMPLE 9.4-1


Energy Balances on Reactive Processes

To perform energy balance calculation on a reactive system, you must include the following aspects:

1. Draw and label flowchart2. Use material balances and phase equilibrium

relationship to determine the amount of stream component and flow rates

3. Choose reference states for specific enthalpy/internal energy

4. Prepare and fill the inlet-outlet enthalpy table5. Calculate 6. Calculate


Energy Balances on Reactive Processes

Two methods are commonly used to choose reference states for enthalpy calculations:

1. Heat of Reaction Method - Generally preferable when there is a single reaction for which is known.

2. Heat of Formation Method - Generally preferable for multiple reactions and single reactions for which is not readily available.


Energy Balances for Heat of Reaction Method

Energy balance equation for heat of reaction

method:


The process path that leads to this expression for (recalling that the reference states are the reactants

and products at 25˚C and 1 atm) is

Writing and substituting for each of the three enthalpy changes on the right leads to the given expression for


Energy Balances for Heat of Reaction Method (cont’d)

Energy Balances for Heat of Formation Method

Energy balance equation for heat of formation method:


Energy Balances for Heat of Formation Method (cont’d)

The process path for (recalling that the reference states are the elemental species at 25°C and 1 atm) is

Writing and substituting for each of the two enthalpy changes on the right leads to the given expression for 34PTT 108: Mass and Energy

Balances

Class Discussion

•EXAMPLE 9.5-1 •EXAMPLE 9.5-2•EXAMPLE 9.5-3•EXAMPLE 9.5-4


THANK YOU….


chapter 9 balances on reactive processes by : ms. nor helya iman bt kamaludin email:...

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