Single Reaction With Recycle

[@anuragbanerjee8990]

Thank you so much, I so wish our professors could teach this as simply as this..thank you!!!🙏

[@Rahul-oy1vo]

+LearnChemE why we cant do an overall balance to find the molar flow rate of condenser pure methanol stream , If we already got to know 100mol/hr of fresh feed and 40mol/hr of purge stream , why it cant be as 100=n3 +40 ?? Please clarify this to me

[@aimanfarhan2709]

yeah totally agreed. This part is really confusing. Why can't it be 100= n3 + 40, thus the value for the product stream (methanol) is n3= 60 mol/hr. Can someone explain to me? I got CPP test tomorrow particularly on the purge and recycle stream. Thanks

[@ritobanghosh7453]

I think because a reaction is involved i.e 1 mole CO +2moles of H2 form 1 mole of CH3OH. If the mole balance doesn't work here(1+2 is not equal to 1),it will not work for overall balance.

[@UliSwag]

Awesome video learncheme

[@LearnChemE]

This screencast has been reviewed by faculty from other academic institutions.

[@stayarmy5057]

Hey isn't the extent of reaction 40mol/hr

[@wishwandd]

How come the N2 going into the reactor is not the same amount as the N2 leaving the reactor? That is Y1N*n1 should equal Y2N*n2, since N2 is an inert and doesn't take part in the reaction.

[@LearnChemE]

You are correct. Those should be equal. There is an annotation to fix this, but annotations need to be turned on and even then don't work on portable devices. The problem is that the mole fractions for CH3OH and N2 were reversed. They should be as follows: Y2N = 0.24 and Y2E=0.05 Sorry for the confusion.

[@ThetruthQ12]

does this error affect the other values?

[@IGMFSH]

Hello, I am wondering what the intuition is behind using the extent of reaction method. I like this method a lot better than the atomic balance (which I saw from previous comments can also be appropriate) because it is a bit cleaner. thanks for all the instructional videos!

[@nigelstanford4]

Why couldn't we use the "(1 - Y + 0.25)H2/mol" method when mapping out the components of the streams in the upper right?...Wouldn't this technique reduce the amount of unknowns?

[@brandonmartin3297]

was thinking the exact same thing. better for me to just skip degrees of freedom and do atomic balances at this point. these videos are super wishy-washy with the DOF analysis

[@ThetruthQ12]

can i use the reactor feed and the stream after the reactor to calculate the correct extend of the reaction( your y2n is wrong)

[@snakesghost7817]

Can you use the "Atomic Species Balance" approach for this problem?

[@LearnChemE]

Thanks for your question. Yes, you could use atomic species balances to solve this problem. However, it is easier to use molecular species balances in this case because you have a single reaction and only 4 different molecules in the system. Atomic species balances are most useful when there are multiple reactions and many different molecular species involved, because molecular species balances in that type of problem can become quite complicated.

[@rlanil5014]

Why we add +1 to overall balance for the reaction as far as I know it is only valid for dof analysis on the reactor

[@allhumans7599]

why is the compostion of N2 in the recycle stream is same as in the purge ?

[@LearnChemE]

Because we split the stream into both a purge and recycle, the compositions must be the same. Otherwise we would need a separation technique to change this (filtration maybe?)

[@michelleakure4195]

LearnChemE I still don't understand why that is the case. If the stream was split in two with the recycle being 25%Nitrogen I would've assumed the other stream would be 75% Nitrogen. But you also said what leaves the condenser is 25%Nitrogen, so I would think there would be no Nitrogen in the purge, as its all gone in the recycle stream.Please clarify this for me.

[@LearnChemE]

mimi A Thanks for asking. Here's the scenario: You have a stream coming out of the condenser. You are diverting part of that stream out of the facility, but most of it goes back into the process. You do not change the stream in any way; you only send it in two directions. The composition stays the same. The stream leaving the condenser is 25% nitrogen, therefore both streams from the splitter will also be 25% nitrogen. In this case, the 25% is not about the total amount of nitrogen, but rather the composition of the stream. To carry out the math, there are 85 mol nitrogen leaving the condenser (340mol * 0.25mol N2/mol), 10 mol nitrogen leaving the process (40mol * 0.25mol N2/mol), and 75 mol N2 going back into the process (300mol * 0.25 molN2/mol). 85 = 10+75. Please let me know if this helps.

[@michelleakure4195]

LearnChemE Okay, I understand that now..Thanks

[@charlieni7872]

LearnChemE I still don't understand. The gases in the recycle stream are including CO, N2, and H2. Doesn't that mean the 25% Nitrogen is equal to (mole N2 )/ (mole CO+mole N2+mole H2) ? How do you know the purge stream has the same mole fraction ? so is the exit stream from condenser? Or is this just something the case for inert gas ? when you purge the inert gas, you let it keep the same percentage in all streams?

[@nicolasmino9169]

Hi! Could you explain me why do I need to calculate purge streams via recycle/splitting point instead of the overall process balance? I'm wondering this because the amount entering the fresh feed is not equal to the amount exiting including the purge stream (in many textbook problems)... Thanks!

[@LearnChemE]

Thanks for the question. You need to calculate the purge stream via those points because you will miss them altogether in the overall process. I'm guessing the confusion here is that the streams do not seem to add up because they are in mol/hr, not mass/hr. The reaction taking place in this problem goes from 3 moles to 1 mole, so it seems off. Does that answer your question?

[@nicolasmino9169]

@@LearnChemE Actually, it helped me a lot... So if the balance is in terms of mass/hr, then the overall process balance would consider the purge stream instead of just the splitting point?

[@jamessiazon6457]

The N2 before it enters the reactor should just be equal as it leaves the reactor since it does not participate to the system reaction, which gives Y2N = 0.23611 or simply 0.24 mol N2. Also, Y2E should be 0.0555 mol right?

[@LearnChemE]

You are correct on both accounts. The numbers were switched for Y2N and Y2E. And, yes, Y2E should be 0.055 or 0.06. Thank you for your attention!

[@kutlwanomalekutu4298]

can you please help me figure out how you got the extend and the compositions of the purge stream, do we simultaneously solve the 3 equations because I did that and it's not giving me the same answer

[@khamisaltobuli8482]

The single pass conversion of CO : Entering the reactor 30 moles from the fresh feed and 75 moles from recycle stream(0.25R=0.25x300=75) total 105 moles CO Entering the reactor. Leaving the reactor 10 moles in the Purge stream and 75 moles in the recycle totaling 85 moles CO Leaving the reactor. Therefore single pass conversion is (105-85)/105=0.19 (19% NOT 17%. I think the Methanol content should not be included in the gases leaving the condenser. ONLY unconverted (Unreacted species).

[@joycyl02]

I also got 19%. I think the difference is that they rounded off the mole fractions entering the reactor?

[@aaminahrabaab635]

When I solved the sum I got 19% too! But I don't know how he found out the gross feed composition..

[@karinas.1320]

Yep I also got 19%. They rounded too much, should have kept more decimal places.

[@owenfazza6349]

Hey mate on 7:22 how did you get 20 moles/hour on you’re excess reaction?

[@braincellium]

at 7:02 he has 3 equations and 3 unknown values. he just solves the system.. you probably don't need this knowledge now since it's been 3 years since you commented, sorry :,)

[@m.hamzarahid2293]

Your values for N2 and CH3OH mol fractions in reactor outlet are flipped. Please fix it.

[@LearnChemE]

Thank you. We do need to fix this.

[@danielejohnson4505]

Hi, could you please help me to understand why you say that the purge should add to one when you are doing the degrees of freedom, making the D0F analysis for the Overall Balance equals to zero?

[@LearnChemE]

+Daniele Johnson In this case, we mean that the mole fractions of the different species should add to 1. y5C + y5H + 0.25 = 1. That is another relationship to help us get to the solution and therefore brings our degrees of freedom to zero.

[@alifsyafiq]

ok so my question is, it is not necessary that the feed coming in plus the recycle is equal to whats coming out? i mean if overall balance, 400 is not = 20 + 340? this is really confusing

[@LearnChemE]

+Alif Syafiq Thank you for your question. This can be confusing. In this case, the reaction is the difference. In the reaction, 3 moles of reactants form 1 mole of product. If you look at the reactor, there are 400 moles entering, but only 360 leaving. I hope that helps.

[@alifsyafiq]

LearnChemE Thanks for answering, however does it make any difference if i do the mass balance in terms of weight rather than mole even though with reaction? You see, my reaction, all the stoichiometric ratio is 1. Will the input have to be same as output as well?

[@AvianYuen]

Around the mixing point, we have 100(0.1)+300(0.25) = 85mol/hr of N2 entering the mixing point, but we have 400(0.21) = 84mol/hr of N2 leaving. Where did that extra 1mol/hr of N2 go?

[@MzgydedAngel]

This is simply because he rounded off the value. The mole fraction of N2 in the stream leaving the mixing point is 0.2125. You'll find your extra mol/hr there.

[@ashzRo]

how did you decide that your relationship is "purge stream mol comp must add up to 1" instead of the given relationships such as the stream ratio between recycle/feed (3:1) and 100% of output stream to be CH3OH? On 2nd thought, I see you're considering the entirety of the system so recycle/feed ratio is out the window, but what about the second one I mentioned, that 100%of output stream is CH3OH? Why can't that be your relationship?

[@LearnChemE]

The 100% output as CH3OH is counted in a way. It is simply a stream value that is already defined in the problem. We already know the composition of that stream. I hope that clears this up

[@ashzRo]

hmm I feel thats the hardest part for me for DOFs but thanks for the speedy reply

[@LearnChemE]

Said another way, it's not information that relates different unknowns to each other, so it does count in reducing the number of degrees of freedom after the unknowns and reactions are counted

[@rlanil5014]

actually we could write the mole compostions as a x1 CO 0.75-x1 H2O and 0.25 N2 like this we can decrase number of unknowns. Like this there is no neccesarry to additional info.

[@chillinwitheugene9014]

How is it possible that the flowrate of Carbon monoxide IN is 30 mol/hr but for Carbon monoxide out it is (0.25)(40) which is 10 mol/hr ?

[@b0d00d]

That's because some CO gets consumed during the reaction to form methanol

[@rocio7064]

Hi, why did you consider the 25% N2 as mol fraction for the recycle stream composition? If the problem says only 25% N2 and not 25 mol%N2, thanks

[@LearnChemE]

That is a great question. It should have been specified as 25 mol% N2. Since the rest of the information is in moles, it's generally safe to assume mole percent. But it is always good to check!

[@esthero1396]

Why did you only do the overall and single pass conversion for CO?

[@aaminahrabaab635]

Maybe because the overall and single pass conversion is done for the limiting reactant.

[@chiprecious9012]

i understand how you use the extent of reaction method to find the composition in stream 5, but when i try to use the normal method of in equals out,its not working out the same answer, why is that so? thanks

[@LearnChemE]

I'm not sure what you mean by "normal method". Does this method take the reaction into account? Please clarify and we'll make it work out.

[@chiprecious9012]

if i try to use that in equals out by taking the overall balance around entire system

[@LearnChemE]

There is a reaction in the reactor. If you don't account for that, you won't have any CH3OH coming out. A simple in=out balance over the whole system would miss that. You must use one of the methods to account for a reaction.

[@GeorgeWBushDid911]

did 40 moles just disappear from n1 to n2 or

[@GeorgeWBushDid911]

I'm very confused how it went from 400 moles to 360 moles without there being any place for moles to leave

[@shoushou5763]

In a reaction balance, the # of moles aren’t conserved. That is why we need to figure out the extent of reaction or perform an atom balance

[@archydragon5213]

but there is a recycle of n how is n in = n out? from 5.37 of the vdio

[@LearnChemE]

This is a balance around the whole system which would only include the fresh feed, n3, and n5. In this case, there is no nitrogen in n3, so for a steady state system, the amount of nitrogen going into the system equals the amount of nitrogen coming out in stream 5. Thanks for your question.

[@user-ci9te3yt6t]

@@LearnChemE If that's true then how can I have a nitrogen in n6? If the whole nitrogen is gone then how can we recycle any of it back into the reactor?!

[@hamzadragonball]

why cant we use overall balance on reactor

[@LearnChemE]

+hamzadragonball We can't start with a balance around just the reactor because we have too many degrees of freedom. However, at some point we do get enough other information to do that balance. It was simply done in a different way.

[@joelbiju7829]

Is the conversion to methanol 100 percent

[@LearnChemE]

If the conversion was 100%, we wouldn't need a recycle stream.