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Chem Eng 2012 Lecture 5

Terms in this set (29)

What is the general expression for the first law of thermo for an open system at steady state?
delta(nH)= Q(dot) + Ws(dot)
When we deal with reaction processes, what is typically better to use, n or m?
Using a molar flow rate is typically easier to use bc reactions relate species by molar ratios
What can we assume about the shaft work for most reactors?
shaft work is zero, even stirring produces minimal shaft work
What assumptions can we make about heat for reactors?
None:

some reactors add heat, some remove heat, and some operate adiabatically
Adiabatic
no heat exchange
Why is it important to indicate the phase of each of the reactants and products in a chemical reaction?
The heat of the reaction depends on the heats of formation of each of the reactants and products, and the heats of formations depend on the phases of compounds and species
What is the difference between air and oxygen in these reactor problems?
Oxygen implies an inlet stream of pure oxygen

Air implies an inlet stream that is a mixture of oxygen and nitrogen
When we have an air inlet stream, what are some key important details?
79% N2
21% O2

MW= 29

N2 is inert, meaning it doesn't take part in the reaction
extent of reaction
The fraction of reactants that has been converted to products
What is the General formula for extent?
n(out)= n(in) + vExtent

v= stoichiometric coefficient
What is the sign of the stoichiometric coefficient?
negative for reactants

positive for products
Inorder to calculate the enthalpies of the species involved in our reaction, what must we do to give us a comparision value?
We need a reference or starting point
---->use reactants at a certain reference temperature and pressure
---> typically 1 atm and 25 celsius
If we are not given the heat of reaction, what can we use to find the heat of reaction?
1.) Heat of formation
2.) heat of combustion
standard heat of formation
the change in enthalpy that accompanies the formation of one mole of a compound from its elements with all substances in their standard states at 25 degrees celsius
What is the heat of formation of elements like O2 or N2?
0
If there is no phase change, what formula can we use to calculate the enthalpy of a species?
integral of the heat capacity from the reference temp to the measured temp
What do we usually use as our reference temperature?
298K or 25C
What is a big problem with reacting systems that can lead to waste?
In many reactors, a significant fraction of the reactants fed to the reactor do not react, which if we do nothing with is a huge waste of money
What is a solution to having unreacted reactants in the product stream?
If we separate the unreacted reactants from the product, we can recycle these reactants back into the reactor feed
What is the difference between the reactor feed and the fresh feed?
fresh feed is the original amount of fresh reactants we feed to the reactor, the rector feed is a combination of this fresh feed and the recycled, unreacted reactants
What does having a reactor with a recycle stream do to the logistics of the flow system, like the piping?
A recycle stream increases the flow rate of the reactor feed and subsequently the outlet streams

Greater flow rates means you need a larger reactor and piping with a larger diameter
What is a purge stream and why do we need it?
The feed to a reactor typically contains some amount of an inert species, and this species will also end up in the reactor outlet

These inerts will typically end up in the recycle steam with the rest of the unreacted reactants after the separation step

If we do not remove these inerts as quickly as they enter the system in the fresh feed (The flow rate of inerts in the fresh feed > flow rate of inerts in product stream) the concentration of inerts in the recycle stream and reactor feed will increases over time

Use the purge stream to control the concentration of the inerts species so it does not build up over time
Single Pass Conversion
(reactant in reactor feed - reactant output from reactor) / reactant in reactor feed
overall conversion
(reactant in fresh feed - reactant in product and purge)/(reactant in fresh feed)
Which is typically larger, the overall conversion or the single pass conversion?
the overall conversion
Why is the overall conversion larger than the single pass conversion?
The fresh feed is less than the reactor feed
What happens to the composition across the split point between the purge stream and the recycle stream?
the composition does not change
Fractional conversion
moles reacted / moles fed

(moles in - moles out)/ moles in
Fractional Excess
(moles fed - moles stoich)/(mole stoich)

----> this is for reactant in excess
----> n-stoich is the amount of moles required
----> fractional excess should be a number less than one
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