It's been about 3 months since we started our PCP2 class with our lecturer, PM Dr. Ramli Bin Mat. Alhamdulillah, i've succesfully passed my PCP1 last semester. for this sem, we discovered how to do energy balances calculation on non-reactive and also in reactive system. As for energy balances, writing reference state is the basis that ‘must have’ in order to solve energy balance problem.
The objective of nonreactive process balances is to calculate enthalphy,H or internal energy,U from heat capacity equation using steam table, hypothethical path, psychrometric chart and other tables with the change in T, P, phase and chemical reaction mixing.
H = U + PV
∆U = U - U ref
∆H = H - H ref
Hypothetical path is use to calculate enthalpy in different T,P, phase or mixing process of energy balances. In this cases, boiling point, cv and cp are very important to develop the path. Based on the first law of thermodynamics for closed and open system,
Closed system : Q – W = ∆KE + ∆PE + ∆U
Open system : Q – W = ∆KE + ∆PE + ∆H
Normally, Q = ∆H in the absence of work shaft, potential and kinetic energy.
Q = n out . H out – n in . H in
While H = cp (T2-T1)
Energy balance table will be developed with the reference stated, to get the heat absorbed/released, Q.
We also learn how to read psychometric chart based on dry T/wet T/Absolute humidity/relative humidity/humid volume given. From the graph, we can get enthalphy value and saturation T.
For energy balance on reactive process, we have learn standard heat of reaction, heat of formation and heat of combustion.
Standard heat of reaction is the heat of reaction (enthalphy change when reactants consumed completely at temperature T and pressure P to form product at the same T and P ) at specified reference temperature and pressure, usually at 1 atm, 25°c. when ∆Hr° > 0, the reaction is exothermic (heat is released during process) while ∆Hr° < 0, the reaction is endorthermic (heat absorbed during process). Standard heat of reaction can also be calculated using hess law, which can be obtain as a linear combination of equations for other reactions. We also can get Hr from table B.1. if the product produced at different state/temperature/pressure than it’s reactant, hyphothethical path is needed to solve the problem.
While standard heat of formation is enthalpy change (heat of reaction) associated with the formation of 1 mole of the compound at a reference temperature and pressure (usually 25oC and 1 atm). To calculate heat of reaction from heat of formation using hess law is
∆Hr° = Hf°i = ( Hf°)product - Hf°)reactant
where v is stoichiometric coefficient of reactant/product species I and ∆Hf° is standard heat of formation for that species. standard heat of formation of substances also listed in table B.1.
The last one is the standard of heat of combustion, ∆Hc°. standard heat of combustion, ∆H°c, is the heat released when one mole of a substance reacts with oxygen to yield specified products in a specified phase. standard heats of reactions that involve only combustible substances and combustion products can be calculated from tabulated standard heats of combustion, in another application of Hess’s law which is
∆Hr° = Hc°i = ( Hc°)reactant - Hc°)product
Heat of combustion of substances also listed in table B.1.
To solve energy balance in reactive process, there are 2 common approach to choose reference state/condition.
- heat of reaction method
Reactants and products are considered as molecular species at To, which is the reference temperature for the heat of reaction (usually 25 oC)
Example of reference state : n-C4H12 (g), i-C4H12 (g) , HCl (g) at 25oC and 1 atm
This method is generally preferable when where is a single reaction for which ∆Hor is known
For multiple reaction,
- Heat of formation method
reference is taken as the elements that make up the reactants and products at reference temperature To.
example of reference state : N2 (g), O2 (g), H2 (g) at 25oC and 1 atm
This method is generally preferable for a single reaction and multiple reactions where ∆Hor is not readily available.
Comparison of the hypothetical process paths that correspond to the heat of reaction and heat of formation methods :