Energy Balances Flashcards
Why is the energy balance necessary?
The rate constant is a function of T. So, as conversion changes, T and the rate constant will also vary.
Write the energy balance for a system.
Qg = Qr + Q Qg = heat generated Qr = heat required to raise temp of T0 Q = heat removed
Write the energy balance for an adiabatic system.
Qg = Qr
How do you calculate Qg? Is this from material or energy balance?
Qg = -H(T) nA0 xA materials balance (generation)
How do you calculate Qr? Is this from material or energy balance?
Qr = -nA0 Cp (T - T0) energy balance (consumption)
What is CP (hat)?
The mean heat capacity of a species initially - used as specific heat capacity in calculations.
Write the energy balance when Cp is independent of temperature.
- (H(T0) + Cp (T - T0)) xA = CpA (T - T0)
Derive the equation from energy balances for CSTR volume.
V = [nA0 xA]/[A0 exp(-Ea/RT) CA0 (1 - xA)]
Derive the equation for reaction rate for a first order, reversible reaction.
rA = k CA0 (1 - xA/xA,eq)
How should reactors be placed in series?
Higher T with higher rate and throughput decrease to lower T
Show how the energy balance can be plotted.
T = T0 + (-H(T0)/Cp0) xA
T (x)
xA (y)
What is shown where the materials and energy balance curves intersect?
Simultaneous solutions where the reactor can operate at steady state (operating points).
What happens if the energy balance curve is above the materials curve?
Energy > materials
Required > generated
system will cool
What happens if the energy balance curve is lower than the materials curve?
Energy < materials
Required < generated
system will heat up could cause thermal runaway.
