I'm a chemical engineer student and I have just started to learn about mass balance without reaction.

This is my problem -

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I want to find Out the mass composition and mass flowrate of stream Y (the stream into the divider).

How am I suppose to do a mass balance around process 1 (involving the divider, before process 2) ?

From the problem , I don't know any of the compositions of the recycle 1 , I only know the flow rate .

This only tells me that the flowrate into process 1 is $50000+ 1500 $ . With this alone, how do I find mass composition and mass flowrate of stream Y


2 Answers 2


In generel for N species you have N mass balances. So in your case 2. Also mass is conserced ( other than moles when it comes to chemical reaction). So you definitely know that the mass flow rate of Y is 1500. Also have in mind that the divider doesn't change the composition of your species and you also know the composition of product 1 ( have in mind that all species together sum up to 1). I think from this point on it's not so hard to find out what Y is.


Let $F$ be the mass rate of feed 1, $x$ be the mass fraction of $N_2$ in feed 1, $y$ the mass fraction of $N_2$ in stream $Y$ and $R$ the recycle rate.

From conservation of mass over process 1:

$$ F + R = F_Y + R $$

the flow rate $F_Y$ of stream $Y$ is equal to the feed rate $F$, i.e. $F_Y = F = 1500$. From an $N_2$ component balance we deduce:

$$ Fx + Ry = (F + R)y = Fy + Ry$$


$$ Fx = Fy $$

And so we conclude $x = y$ and so the composition in $Y$ is the same as in the feed $F$. We could have concluded this directly since it is given that no reaction occurs in process 1.


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