Crystallizer Material Balance with Recycle

Organized by textbook: learncheme.com/
Performs material balances on multiple units to solve for unknowns. Focuses on crystallizer balance where a slurry is part solid and part liquid. Made by faculty at the University of Colorado Boulder Department of Chemical and Biological Engineering.
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[@stevenguzman7735]

I'm not understanding how the problem states that: "exiting the crystallizer are the crystals with 10% of the solution, and the remainder solution which forms the recycle stream." So how is it that then it says the filter cake contains 85% filter cake and the rest solution." When it had already stated that the crystals exit the crystallizer with 10% of the solution. (How is ten percent of the solution .15?) I understand you put 15% for the solution because of the 85% crystals, but I don't get how that follows what the problem stated and also how the composition of the recycler and the output stream of the crystallizer are known since the recycler only has the same composition if it splits from a stream. Which in this case it doesn't???? It only says 90% ...Thank you

[@Darwinion]

Totally agree... some of these problems are so badly described from the outset. "Recycle is 90% of the solution". What solution? The original feed 25% wt? Or the feed plus the recylcle? Totally unclear.

[@UliSwag]

The wording also had me a bit confused too

[@kennywong385]

@@Darwinion You have to understand about this is a separation process (please google function of centrifugal). In reality, we intended to get purer result. Hence, we have the recycle. We recycle the solution (again you have to understand what is solution). After several separations, we have 85% solids begin separated, but we still have another 15% of solution which is impure. In that case, we pass through another separation called drier. After this drier, we assumed to get 100% dried solid. Before question the tutor, I suggest you to think more first.

[@pepa7669]

@@kennywong385 hi, where is it said that after the separations we have 85% of solids? Because I dont read that, what i read is that "the filter cake containing 85% crystals (and therefore 15%water) and the rest of the solution (which is 10% of the solution exiting the crystalizer) are fed to a dryer".

[@137mango]

Your voice is perfect. So soothing

[@letumilechocho231]

Thank you times a million 🥺 the way the solutions and solids was explained was beyond helpful

[@ben-hurvalenciavalencia263]

By doing this, the degrees of freedom of dryer, as erroneously calculated all the teachers of the Department are: DOF = 3 (unknown) - 3 balances (water, air and solid) = 0. The mass fractions obtained for stream 4 are : solid, 0.8725 and water, 0.1275. Mass balances are trivial. Solid: 1000 = 0.8725 * m4, m4 = 1146.13 kg / h. Water: 1146.13 x 0.1275 = 0.08 * n2 * 18, then n2 = 101.48 kgmol / h. Air: 0.92 * 101.48 = n1, thus n1 = 93.36 kgmol / h. Another nice thing is that the teacher uses kg / h for mass flow and uses mol / h for the molar flow of air instead of unifying the units.

[@planomathandscience]

All of the teachers in the department? Really? All of them?

[@juanmatheodelgado1492]

Actually, the DOF for the drier is 3 Unknowns and 2 Balances because the Air balance is not independent. The leftover DOF can be easily solved by calculating the KD in m4.

[@shanto4471]

Hi, I'm not exactly sure why you ignored the recycling stream. Could you please explain it further? Thank you.

[@LearnChemE]

The recycling stream is calculated in this screencast. There are points within the calculation where we are defining the system to find degrees of freedom. Some of these defined systems encompass the recycle stream. In that case we ignore it since we are only looking at inputs and outputs to the defined system. Please let me know if this answers your question.

[@nigelstanford4]

What if I made the recycle stream come out of a splitting point between the crystallizer and the drier

[@ben-hurvalenciavalencia263]

+ Tyrone BiggumsEsa corriente está formada por sólidos que cristalizaron. NO PUEDE HACERSE. Se recicla la solución que tiene todavía sólidos disueltos para cristalizarlos.

[@nigelstanford4]

Wish I could read that

[@LearnChemE]

+Tyrone Biggums If the recycle stream splits from another stream, then the compositions would be the same in those streams. In this case, the solution compositions are the same anyway, so it wouldn't make a difference here. However, it could make a big difference if the recycle stream comes from a splitting point versus comes from another process unit. Does that answer your question?

[@LearnChemE]

+LearnChemE You would, however, have to be careful that the solids weren't included in the recycle stream or that would make the process a bit different.

[@bashaledelicia6974]

thank you very much this is really helpful

[@mindfreak22full]

how can i decide whether the recycle stream is coming out of the product stream or the crystallizer it self ?

[@planomathandscience]

You don't decide. That's how it works. Feeds -> reactor -> then streams to the dryer and recycle to the feed.

[@kerrimac31]

my head hurts

[@engineermaithamoil3712]

What is your country

[@daniyard2012]

Hello, thanks for good video. I need explanation for flow between Crystallizer/Centrifuge and Drier. There is one stream but you gave 4 specifications that are : 0.85 kg solids/kg - 0.15 kg soln/kg; 0.15 kg KD/kg - 0.85 kg H2O/kg. So 0.85 kg KD/kg means it is a total weight of KD that is present in both solids and solution? And 0.85 kg H2O/kg is only present in 0.15 kg soln/kg? Also, do we always have solids formed after compounds passing through crystallizer/centrifuge? Thanks a lot.

[@LearnChemE]

Thank you for your question. That stream has two parts: there is a solid part and solution. The solid part, 85%, is all KD. The other 15% is a solution that also has some KD in it. The solution itself has 15% KD, with the remainder being water. Please listen around 6:50 as the narrator starts the balance around the drier. Does that make it more clear to you?

[@rafliiman7238]

when calculating n2 why you devided it by 18 x 10^-3 ... why not just devided by 18?

[@kelsietaylor23]

Why did you not consider m2 and m5 to find m3?

[@farahatiqa9164]

My head and my heart are hurt

[@tatendaVIDZ90]

if m2 was required how would you calculate it?

[@LearnChemE]

+Collins Saguru m2 = m1 + m5.

[@minjinseo3681]

Good day :) Just want to ask if it did matter if you use Dry Air Balance @ Dryer and OMB @ Dryer to find n2? I tried this one but didn't know why it gave me a different answer. Thank you very much sir :)

[@johnsethole3741]

does the recycle stream always have the same composition as the other prod stream

[@LearnChemE]

If the recycle steam splits off of another stream, at a split point, they would have the same compositions. However, if the recycle stream comes from a process unit where a separation takes place, they might not necessarily be the same composition.

[@tanhuiwun2216]

+LearnChemE what if the stream enters a separator and is divided into recycle stream and purge stream? Does these two divided stream have same composition as the stream before the separator?

[@stevensmith3257]

At around 7:00, you start to do the KD balance for the Drier. Since 90% of the solution is going to the recycle, wouldn't only 10% of the solution be going to the drier, and therefore the KD balance be (0.85)m4 + (0.1)(0.15)(0.15)m4 = 1000 ?

[@stevensmith3257]

Or I guess in this case, the stream isn't splitting. The solution is exiting as two separate streams from the crystallizer. Does that mean that you wouldn't care about it when calculating stream m4, and only take it into account when calculating the recycle?

[@LearnChemE]

This is tricky. Thanks for the question. It looks like you've gotten to the answer from your next comment. So, yes, in this case we are doing a balance around the drier (dryer?). Therefore, we are not interested in the recycle percentage from the crystallizer. Where it gets confusing is that this input stream to the drier is a mixture of solid and liquid. And that is why it seems to be two streams. First all of the solid is considered (0.85m4), then the solution, which is 15% of the stream and only 15% KD (0.15*0.15*m4). I hope that helps.

[@wanda7658]

when calculating for n2 (water leaving the drier) why are we dividing with 18x10-3

[@LearnChemE]

+Wanda small boi Thank you for your question. In that equation, the right side is in units of moles and the left side is in units of mass. To get the units to match, the left side must be divided by the molecular weight of water. Then both sides are molar units.

[@calvinyau8933]

can somone answer this is the next few days please. its for my 3rd year design project. m4 = 1146 of feed but 0.85 of it is solids = 0.85 x 1146 = 974kg solids gooing into dryer. so how is 1000kg solids coming out

[@slayshady6752]

you are forgetting the 15% solution. 1146*(0.15 the solution at m4)(0.15 KD in the solution at m4)= 25.785 kg KD. 974+25.785=999.785 or roughly 1000 if you round it.

[@azmarose6399]

The overall DOF for this question is 1= unsolvable

[@farahatiqa9164]

Unbelievable

[@LearnChemE]

This is a valid question, and it turns out that you can't solve the overall system. So the narrator moves on to use the system around the dryer, where there are 3 unknowns and 3 balances. He jumps around a bit, but that's why it works.

[@ben-hurvalenciavalencia263]

Consider the large errors in the solution of this exercise: General: Teachers do not teach how are the degrees of freedom of process, which is done with all the currents (if you did not know), the sole criterion to determine whether a problem has a solution and therefore do not teach the solution strategy. The particular errors of this solution are appalling: 1) The "teacher" Global Balances performed despite remaining with 1 degree of freedom. Unheard. 2) The "professor" joining two units: the mixer and the crystallizer to carry balances. That is to attack the mass balance. 3) The teacher wanders from unit to unit doing balances to see how finds the unknowns and solve the problem. Without any criteria. Instead of making these juggling, who decry the teaching should calculate the composition of the current No. 4, based on solid and water from the amount of solid and the amount of solution in a kilogram of mixing and composition solution, all known. Mostrar menos RESPONDER

[@artemisc749]

hello, for the overall balance at crystalliser ,aint it suppossed to be m2=m3+m4+m5 ..?? hope you reply to this..thank you in advanced

[@LearnChemE]

If I'm looking at the correct part of the video, you are correct if a "box" is drawn in which m2, m3, m4, and m5 enter or leave the crystallizer. However, the way it's drawn, the recycle stream is in the "box", which makes the balance m1 = m3 + m4 as stated in the video. Does that answer your question?

[@artemisc749]

i think i got it..so basically both equation are correct..i love your teaching videos..keep it up ..

[@nhaz652]

7:25 : May I know why is the main flow rate is 0.15? I couldn't picture it. Kindly explain. Thanks!!

[@LearnChemE]

Great question. Stream 4 has both solids and a solution. 15% of stream 4 is solution; the rest is solids. Of that solution, 15% is KD. Does that help? This is a bit confusing due to the percentages being the same.

[@nhaz652]

@LearnChemE Thanks for your kind explanation! Ahh I see it now. So technically whatever comes out of the Crystallizer, will contain some liquid which has some KD, and that shall be counted too? Am I understanding this correctly?

[@minjinseo3681]

Good day :) Uhm do you have lecture videos about material and energy balances for reactive processes with emphases on combustion of fuels and reactions in common chemical process industries? much thanks :)

[@azmarose6399]

Just find another question😂

[@nadirahsukri3803]

This question is like women, so complicated 😊

[@mphilisindlovu839]

Sorry I think you made a mistake when calculations m4: 0.84m4 + 0.15×0.15m4 = 1000 This gives m4 = 869.57 kg/hr

[@LearnChemE]

So let's work through this. 0.85m4 + (0.15)(0.15)m4 = 1000 --> 0.85m4 + 0.0225m4 = 1000 --> 0.8725m4 = 1000 --> m4 = 1000/0.8725 --> m4 = 1146 kg/hr. Does that make sense?

[@fivepercent8209]

Easy stuff. ..

[@furkanertugral3621]

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[@alidemir6608]

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[@olebogengramorola773]

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[@furkanertugral3621]

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