Lecture 2 Flashcards
what are the benefits of batch operation?
small production rates - a few tons/day
usually more flexible and economical
usually require less capital expenditure
what is a disadvantage to batch operation
quality control is usually a problem
what are the benefits to continuous operation?
adopted in all large-scale chemical processing
diminished labour costs: eliminations of operations such as emptying and filling of reaction vessels
automatic control of process, reducing labour costs but requiring greater capital outlay
greater constancy in reaction conditions leads to more uniform product quality
what do we neglect in isothermal reactors
we neglect:
heat release (or uptake) by the reaction
heat transfer between parts of the reactor and/or heat exchange with the surroundings
what is the mass balance in isothermal reactors
for any type of reactor, we apply the principle of mass conservation to a reactant in a certain volume ΔVr and certain time interval Δt
input to the reactor - output from the reactor - consumption due to chemical reaction = accumulation
what is r
r is the rate of disappearance of reactant moles per unit volume per unit time [kmol/ m^3 s]
why do we assume reaction starts instantly at t=0 for batch reactors
we assume t=0 at the start of the reaction because we assume the reactant is fully loaded and ready to react and we are neglecting loading time and time it will take to heat the reactor up. realistically not true because it takes time to fill the reactor would happen unless at very low temperature. then it would take a while to raise temperature to reaction temperature
what volume do we use to apply the mass balance equations in batch reactors
we assume homogenous system with infinite stirring. therefore, we are considering the whole volume of the reactor
what is the input and output for the batch reactor
the input and output are zero. at time = 0 the reactor has already neen loaded and we’re not adding anything and at the end we’re not extracting anything
what is the general mass balance
input-output+production-consumption=accumulation
what is the mass balance for a batch reactor
production- consumption= accumulation
what does the mass balance in a batch reactor apply to
the whole volume of the reactor
teh dimension of each term is mass
why do we assume steady state for CSTR
we assume steady state because that is how the reactor operates most of the time except when it stars, ends, or is there’s disturbances
what volume do we use to apply the mass balance equation to the CSTR
whole reactor volume assuming homogenous system and infinite stirring rate
what is the accumulation in CSTR
zero because steady state
what is the general mass balance for steady state CSTR
inputc
what does the mass balance for steady state CSTR apply to
the whole volume of the reactor
the dimension of each term is mass/time
why do we assume steady state for PFR
generally operates at steady state except when starting, ending and if theres any disturbances
what volume would we use to apply the mass balance equation for steady state PFR
an infinitesimally small slice perpendicular to the flow
what is the accumulation in steady state PFR
zero because steady state
what is the mass balance equation for steady state PFR
input- output +production - consumption= 0
what does the mass balance apply to in steady state PFR
a differential cross section of the reactor and then integrated
the dimensions of each term is mass/time
define rate of reaction
the rate of reaction is defined as the rate of net moles of a species produced or disappearing/ (unit of time)(unit of volume)
what is the rate of reaction of a reactant A
rA= -1/VR * dNA/dt
minus sign because reacttant disappears
1/V represents per unit volume
negative sign ensures r is always positive
what is the rate of reaction for product B
rB= 1/VR * dNB/dt
if the reaction is carried out at constant volume what is the reaction rate for reactant A
rA= -dCA/dt
why can we assume constant volume for liquid phase reactions
liquid phase reaction can be assumed to have constant volume because liquids are incompressible
how do we deal with rate of reaction for gas phase reactions
in gas phase reactions we assume the gas presents ideal behaviour and then we can use:
NART/V =P = CART
write the rate of reaction as a function of temperature
rA=-1/VR * dNA/dt = k CA ^n
where n is the reaction order
what is n
n is the reaction order. it does not necessarily correspond to the stoichiometric coefficient and there not necessarily has to be a reaction order
how are reaction rate equations determined?
they are determined experimentally
what methokds are used find reaction orders and kinetic coefficients
there are four methods: differential, integral, half times, initial rates
slide 11 lecture 2
what is the accumulation in a batch reactor in terms of a species A which can be a reactant or product in tems of number of moles of A within the system at time t and molecular weight of A and reaction rate of component A
(MANA)t+Δt- (MANA)t
where t=0 when reactor is loaded
and t+Δtis any point in time during the reaction
what is the production- consumption in a batch reactor in terms of rate of reaction
(MArAVR)Δt
where production is positive and consumption is negative
what is the derivation on slides 13 and 14 for
answer when figure out
