[kankerfist]

I just encountered a process question and I am not sure how to approach it.  The question involves 2 feeds and 1 output and gives the following information:

feed 1 is 98.8% N2 and 1.2% CO2 by volume, at 10 degrees C and 105 kPa.
feed 2 is 0.0917 cubic meters of CO2 / min at 30 degrees C and 1 atm.
the output is 3.4% CO2 by volume.

No reactions occur in the process.

The question asks what the flow rate of feed 1 is.  Normally I begin these by converting all streams to either mass or moles, but I am not sure how to do that here becuase feed 1 is missing the V and n  ideal gas variables.  Can I assume 1.2% of the volume of feed 1 + the volume of feed 2 = 3.4% of the output volume?  Any hints would be appreciated

[Montemayor]

Kanker:

I don’t know where you got this problem, but from its flavor I have to assume you got it out of a text book as a school homework problem.  That makes you a student – and one that has not been paying that much attention in class.  Or, perhaps, you have a rotten prof who doesn’t take the time to teach his chemical engineering students how to dissect and operate on a simple 1st year engineering problem.  If the latter is the case, I would raise holy hell about the amount of money being paid for an education that doesn’t; teach even the basics that will be sorely needed in the later, tougher courses to come.

I refuse to do homework for students.  It’s a slur against the student and damages his/her chances of ever learning how to solve engineering problems on his/her own – a cursed malady that will surely lead to his/her flunking out of engineering or never being able to carry out their career successfully.  However, what I can do is show you how simple and direct the solution is – if you take the time and study the methodology and basic principles applied.  Hopefully, this will help you (& other students) out in future similar problems.

All experienced chemical engineers know one, universal, and often applied rule:  when you are stuck on a process problem, always stop and make a heat and material balance.  In your case, there is no reaction and there is no heat transfer.  The exercise immediately reduces itself to a simple, high school equation:

Mols of CO₂ entering  =  Mols of CO₂ exiting
(Mols CO₂ in Stream 1) + (Mols CO₂ in Stream 2)  =  (Mols CO₂ in Stream 3)
(0.012) (Mols Stream 1) + (Mols Stream 2)  =  (0.034) (Mols Stream 3)  =  (0.034) (Mols Stream 1 + Mols Stream 2)

You know (or should know) the value of Stream 2.
You now have an equation with only one unknown: Mols Stream 1 (the answer)

I hope this helps you out with similar challenges in the future.

[kankerfist]

My professor is fine, but I am blind so it takes some time to translate the concepts I've understood audibly into something applicable to these practice problems.

That makes you a student – and one that has not been paying that much attention in class.

I pay attention in class,  you need to pay attention to the fact that not every engineer is able to read and write with pen and paper.  I rely a lot on forums like this when a concept that sounds simple turns out difficult to apply.

Or, perhaps, you have a rotten prof who doesn’t take the time to teach his chemical engineering students how to dissect and operate on a simple 1st year engineering problem.

My professor is a researcher at GA Tech, they don't hire rotten professors.  Close your eyes and have your computer dictate my simple, 1st year problem to you.  Does your inability to solve it without a pencil mean your education was rotten?  This forum is a tool for information exchange, not somewhere to belittle and discourage people.

[Donaldson Tan]

kankerfist: Are you a 1st year chemical engineering undergraduate?

This problem is a standard material balance problem. The condition of Stream 1 is given, so that you can calculate the volumetric flowrate of Stream 1. However, the key in solving this problem is to perform a material balance on a molar basis. The beauty of this approach is that the volumetric composition of a gas mxture is the same as its molar composition.

Hence, assume the molar flowrate of Stream 1 is N₁

N₂ (molar flowrate of Stream 2) = P₂V₂/R.T₂ <- All these variables are known, so N₂ is solvable.

Performing material balance of CO₂

CO₂ in Stream 1 + CO₂ in Stream 2 = CO₂ in Mixed Stream

And given that Stream 2 is 100% CO₂

0.012N₁ + N₂ = 0.034(N₁ + N₂)

Rearranging this will give you the value of N₁, which allows you to find the volumetric flowrate via the perfect gas equation.