12 replies to this topic

[TheStudent]

Hey guys great forum. Straight off the bat I'd like to say I'm not very good at anything when it comes to design, so please be kind if my questions seem very dumb.

I am currently doing a design project on production of methyl chloride. My task is the detailed design of a methanol recovery column that will recieve a mixture of large amount of water, some methanol and a very small amount of hydrogen chloride. The column will recieve this feed as a liquid from the 2nd reactor in a series of 2 (having been seperated from the product stream of methyl chloride and a small amount of dimethyl ether), and will then seperate the methanol from the other two components and feed it back to the first reactor.
However I am having a bit of trouble in terms of designing this column. I am not sure wether to treat it as a multicomponent or binary mixture (combining the water and HCl due to the amounts being so large and small respectively).

From a preliminary mass balance, the amount coming in to the recovery column is roughly as follows:-
Water: 20000kg/hr
Hydrogen Chloride: 60kg/hr
Methanol: 1600kg/hr
at 35oC

This however will more than likely change, but I can use it as a starting basis.

From here onwards I am unsure what to do. I know that the top product will need to be the methanol (and some water according to the preliminary balance), and the bottoms will be a mixture of of HCl and water (and probably some methanol) which will not be recovered, and become a waste stream. Since I am not concerned about the composition of the bottom stream, would it be fair to treat the distillation as binary, to perhaps use more simple calculation methods? Or is that not acceptable in terms of design.

What would be a good place to begin on the detailed design aspect of this column? At present I am looking at the heat loads assuming the pressure to be 1 atmosphere and the top temperature to be approx 70oC and the bottoms to be around the region of 80oC

I am getting nowhere though as I can't get my head around it all

[pavanayi]

TheStudent,

For a start, one excellent reference for design of distillation columns is a book by H. Kister titled 'Distillation Design'. It might prove helpful to you for your project and also future reference requirements.

Are you using any simulators to help with your design or are you using hand calculations?

Have you done any shortcut / priliminary calculations yet (by assuming binary or otherwise)?

How did you estimate the top and bottom temperatures?

[TheStudent]

Thanks for the reply and the reference. I assumed the top temperature as the boiling point of methanol at 1atm (approx 66) , and the bottoms was more of a guess (something in between the bpt of methanol and that of water and hcl)

I began trying to do some hand calculations however I was assuming multicomponent so I could not find any good methods that applied to multicomponent rather than binary. I havent tried any method assuming binary distillation as I thought it would be wrong.

In terms of simulation software I have access to a few, however I am unsure how to use them. I think in terms of the whole scope I will end up using both simulation software and hand calculation.

Is there a good place to start that does not rely too much on wether the system is multicomponent or binary, or must that be completely defined before going into detailed design?

Thanks

[sarah21]

hi i'm in a similar situation too, i decided to assume multicomponent distillation. This information was posted from the user Zauberburg where he described what equations etc to use for a multicomponent distillation column so check out the link below it takes you straight to that post.
hope this helps. I also recommend Coulson and Richardson Chemical Engineering Design book. Email me if you need any lecture notes.
http://www.cheresour...aration-column/

[TheStudent]

Thanks that looks very useful. I think I may already have Coulson and Richardson volume 2 and 6 actually.

So from that post:-

a. Identify light and heavy key components for each column/system;
LKC: Methanol HKC: HCl ?

b. Guess splits of the non-key components and compositions of the distillate and bottoms products;

Is there any way to make an educated guess about the splits and compositions? I have the preliminary mass balance, but for the detailed design a revised mass balance is expected, which at the moment I'm getting nowhere with. I have information of the feeds coming out of the 2nd reactor but that's it for now

c. Calculate relative volatility of light and heavy component(s);
This equation?

after that the rest seems pretty self explanatory.

So following that method I will be able to find the number of trays and the feed entry tray using hand calculations? I'm guessing I can then use simulations to work out a more accurate set of figures.


Thanks a lot for all the help!

Edited by TheStudent, 22 February 2011 - 09:55 AM.

[pavanayi]

The Post from Zauberburg should help you. You can always use a simulaion software to cross check your results.

A good place to start will be the short cut column calculations in any simulation software. Ensure that you are choosing a property method that is valid for the components in the system.

Even in simulation software, the shortcut calculation method uses the same equations. Bear in mind the basic assumptions that are behind those equations: Assumptions of
1. constant relative volatility of the components and
2. constant molar flow through the column

The results from such a short cut calculation method is usually used as the starting assumptions for the rigorous distillation calculations, as the solver will have a good initial estimate of values and hence will converge faster.

PS: if you assumed top temperature as the saturation temperature of methanol at the column pressure, why did you not use the same for the bottom? Considering the fact that more than 99% of the bottom flow will be water, you will see that the bottom temperature is very near to the saturation temperature of water at pressure in the reboiler (column pressure + the small increase in pressure due to the height of water column at the bottom of the column), which will be around 103°C

[pavanayi]

Hi,

I have not had the chance to look at your document, but in the meantime, this should be helpful:

I would assume Heavy key as water, because it is the water that you want to avoid being present too much with your methanol (right??)
Composition of distillate and bottom products are usually defined by product specifications (The reason you are doing the distillation in the first place).
An example for the distillate in your case might be to recover methanol with a water content less than 0.5%, to be recycled back, dictated by the reactants quality requirement.
Another example of the requirement might be to recover 99.5% methanol to be recycled so that you use spend less for fresh makeup methanol (both are not the same). Check your requirements, and define what you need as the distillate stream quality wise and quantity wise (That should have been the first step!!!)

[TheStudent]

I believe I can define the quality myself, and more than likely I'll be choosing the methanol recovery to be as high as possible. However how would I guess the split of the non key (in this case hydrogen chloride?)

I think it's actually HCl that I'm trying to avoid as in the preliminary mass balance it was calculated as follows (this balance was not performed by me so anything not making sense I will have to clear up with that person):

Stream 1: From reactor 2 to Recycle colum,
Stream 2: Recycle Stream tops back to reactor 1
Stream 3: Waste stream bottoms

Stream 1 compositon was 0.3% HCl, 7.3% MeOH and 92.4% water on a mass basis.

An assumption was made that the recycle efficiency (Molar Basis) was 95%.

So for stream 2, the molar flow rate of MeOH would be the molar flow rate in stream 1 multiplied by 0.95. The molar flow of MeOH would then be converted to the mass flow by multiplying by Molar mass. I think another assumption was made that this made up 92% of the of the total mass flow out of stream 2 (I'm not sure why, as I didn't do this, but the number on the excel file was entered manually rather than using a formula), thus leaving the remaining 8% as water, with no HCl present. Once again I'm guessing based on the assumption that all of the HCl remains in the bottoms

Stream 3 contains the remaining MeOH, Water and all of the HCl.

This was a preliminary mass balance however and at a very early stage where a lot of assumptions had to be made. For the revised mass and energy balance I suspect a lot will change and I assume I can't be making such broad assumptions.

[pavanayi]

The split of non key components can be calculated using Hengstebeck-Geddes (1958) method, a description of which is available in several references, including, I believe, in Coulson & Richardson's series Vol 6.

I am also attaching another file which I found from the internet from the website of University of Palermo, which I think might be useful to you. All credits to the original author.

Attached Files

•  Multicomponent distillation.pdf   1.4MB   40 downloads

[TheStudent]

I've had another thought, is it possible to model the water and hydrogen chloride as very dilute Hydrochloric Acid? i.e. giving that mixture a boiling point of 110oC as an azeotropic mixture?
EDIT: It may be that the HCl is hydrochloric acid, and the remainder is just water, I am unsure of this though. I will continue to work for now on a multicomponent basis in that case,

LKC: Methanol Boiling Point 65oC
HNKC: Water boiling point 100oC
HKC: Hydrochloric Acid boiling point 110oC

and change it if any thing different comes up


My assumptions so far are as follows:
Column Top: 65oC
Column Bottom: 100oC
Assume 99% of LKC is in distillate
Assume 100% of HKC is in bottoms
Assume (for now) 100% of HNKC is in bottoms

How would I go about calculating the relative volatilities to use in the Fenske equation?
I know it's something about either the K-Values or the dew point/bubble point but I can't seem to find the right data to help me do these calculations?

Edited by TheStudent, 22 February 2011 - 10:04 AM.

[pavanayi]

come on mate!!
Don't tell me you need help with relative volatility!!!

Also, assumption of 100% recovery of any key component in distillation is primarily wrong. Would you assume 100% recovery of one component in a binary distillation? I shouldn't be telling you this
Try and read about light and heavy key in distillation and also about splits of non key components.

[TheStudent]

come on mate!!
Don't tell me you need help with relative volatility!!!

Also, assumption of 100% recovery of any key component in distillation is primarily wrong. Would you assume 100% recovery of one component in a binary distillation? I shouldn't be telling you this
Try and read about light and heavy key in distillation and also about splits of non key components.

like I said before in my first post im very stupid & not very good at anything when it comes to this topic (or to be honest anything in the world of chemical engineering) but I've been managing to scrape through. This is my final year so I just need to knuckle down and get this out of the way. I realise the volatility is simply the comparison of the K values of the heavy and light keys at the top and bottom temperatures, which when above 1 means the separation is feasible.

I've changed my mind and I'm calling water the heavy key. I'm using an initial guess outlined by many examples that all of the heavy non keys appear in the bottoms. I have assumed 99% of methanol in distillate, with 95% of water in bottoms.

I've done some bubble and dew point calculations to work out the top and bottom temperature and the results don't seem right (bubble and dew are very close no matter what pressure I use) I've attached the spreadsheet I've used to calculate them. Could this because the feed is over 90% water?

I used the antoine equation and got the coefficients from "Yaws' Handbook of Antoine Coefficients for Vapor Pressure" with pressures in mmHg and Temp in Centigrade (from the book)

Attached Files

•  Dist_Bubble_Dew.xlsx   11.17KB   26 downloads

[pavanayi]

TheStudent,
I have looked at the excel sheet, and I can see only one stream (no idea which one, because you have not labelled it). If you were estimating temperatures of top and bottom and feed streams, I expected to see three streams.

Anyway, with the stream present in the sheet, I can see a 7°C difference between bubble and dew point (for a stream that is 95% water and 5% methanol), so I don't find anything strange with it. I cant understand your concern in stating they are too close? In fact, if it was 100% water, both bubble and dew points would have been one and the same (But you already knew that didn't you??)

Also, frankly, I am not getting why you are bothered with the temperatures at top and bottom in the first place, when you can define your design based on the assumption that the relative volatility is based on the feed stream conditions. Of course, if you want to be 'more accurate', then you can find relative volatilities at top and bottom conditions and then take an average.