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Author Topic: BIG PROBLEM, ABSORPTION DRYING TOWER.  (Read 9523 times)

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moleman1985

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BIG PROBLEM, ABSORPTION DRYING TOWER.
« on: March 26, 2007, 12:40:35 PM »

Sorry for the long text post but I have alot of problems with this task. :-\

I have a large problem to solve although I only wish to seek help and guidance not for someone to actually do it for me.
My problems in that I need to design an absorption tower (dryer), to remove water content from a gas stream using a stream of concentrated sulfuric acid I have to design the size, type of packing, temperature change, pressure drops ect. (packing is to be first stage randomly packed and second stage trays), there is also a recycle stream which entres between the two different transfer stages, this recycle is also cooled so will effect the temperature profile.

All I have to go off is the compositions and flow rates of the gas and liquid in and out as I have already done a complete mass and mole balance.

I have a good understanding of the process and recently sat an exam on this an got 80%, but I cannot work out where to start with such little information. I have to make assumptions that the flow rates stay constant, but in fact one stream out triples, also the temperature stays constant which is doesn't do to dilution and a cooled recycle stream, the transfer rate stays constant which it possible cannot as it has different packing. I have seen a lecturer for help but he just said to split it up into 10 sections and work at it from there, but everywhere I read it says to do an overall calculation then split it up which I think is correct, although I do understand possibly treating the two types of packing separately from the beggining.

So far I have constructed an operating line on a y vs. x graph, but this seems usless without an equilibrium line in which I could extrapolate some data, but my lecture told me they would be the same so I abandoned that idea.

Also tried to use Henry's constant as the system is gas phased controlled and find value of y*, but I do not know which constant to use, (I presume the water vapour), but I cannot even find this.

I also tried an idea that y=y*, for a total overall transfer but I cannot see how this would enable me to further split up the system to get more accurate individual sections.

Thankyou for reading and I hope you can help guide me in the correct direction.

Also I would say that if someone was willing to help me it would be great to be able to communicate over msn so there isnt the long delay of emailing, I know this would mean anyone giving up there own time and I would be willing to pay them £20+ say per hours ish, for them giving up their time. I would posted all my activity on this tread so to maybe help anyone else with a simular problem.
« Last Edit: March 26, 2007, 01:05:44 PM by moleman1985 »
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eugenedakin

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Re: BIG PROBLEM, ABSORPTION DRYING TOWER.
« Reply #1 on: March 27, 2007, 05:58:29 AM »

Hello moleman1985,

With such a large task, my first suggestion is to actually draw a diagram on a piece of paper, and if you wouldnt mind, either send the diagram to me or post it on Chemical Forums.  Drawing the diagram helps me understand more of your questions.

Fantastic!! You completed the compositions, and flow rates (thats most of the work).

Yes, you do have to make assumptions (and sometimes, these assumptions are not correct-thats the worse part).  All you can do is your-best when provided with the available information.  You mention that the flow rates are to stay constant, but one of them triples.  Could you explain this a little more for me (this is where the picture helps :) )

With the lecturers suggestion (split it up into 10 sections), he has a good idea.  Essentially, he is getting you to examine the process in gradients (its a good exercise).  Then perform the overall calculation and then split it up.  Determine your differences between the two, and then make logical adjustments.

There are MANY ways to design a dryer, and most of them are functional.  A good method of determining the best design is through practical experience, and asking many questions.  Your doing GREAT!

Feel free to ask more questions, and I look forward to your drawing.

Cheers!

Eugene



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eugenedakin

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Re: BIG PROBLEM, ABSORPTION DRYING TOWER.
« Reply #2 on: March 27, 2007, 08:27:02 PM »

Hello moleman1985,

Wow.. you definately have alot to consider... but it seems like your headed on the right path.

Do you happen to have a copy (or access to) Perry's Chemical Engineers' Handbook?

I would suggest you examine chapter 14, which outlines complete Gas absorption and Gas-liquid system design.  This includes Packed- and Plate-Tower design.

Chapter 15 outlines Liquid-Liquid Extraction Operations and Equipment, also with many different types of towers.

Here is a demister link which may help with some of your considerations (pg 7 of 16)  http://www.acsseparations.com/Files/Downloads%20Page%20PDFs/Product%20Design%20Manuals/Mist%20Eliminator%20Manual/Design%20Manual%20Mist%20Eliminator%20-%20New.pdf

Again, all of these are suggestions, and you are the person ultimately responsible for its suitability and design (sorry for the caveat).

I can only offer suggestions, and calculations will only take you so far.  There is the 'experience factor' which accounts for alot. My suggestion is to 'overbuild it', and people usually do not complain when the process is too efficient...

I am curious, why do you suggest a packed column and trays?  Also, I usually suggest the addition of a demister since there will always be a time when the tower will be pushed to its limit.

Take care, and keep the questions coming.  Hopefully, this helps.

Eugene
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moleman1985

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Re: BIG PROBLEM, ABSORPTION DRYING TOWER.
« Reply #3 on: March 28, 2007, 02:15:12 AM »

yes I have an electronic version of perry's which is very helpful and also coulson volume 6 which is just as good if not better as it goes into the structure and structural support I am going to have to design after. The reason for the packed bed and trays is because that is what it do in this column, I have visited at working plant and we all presumed that we would have to carry on the up todate technologies they use, also perry's says its done like this too. Just trying to do a temperature profile of the unit split up into ten sections so I'll give the transfer a bash to although intertrgration is not my strongest topic.
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Donaldson Tan

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Re: BIG PROBLEM, ABSORPTION DRYING TOWER.
« Reply #4 on: March 29, 2007, 02:57:28 PM »

You can assume a linear temperature profile in each section I guess.
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moleman1985

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Re: BIG PROBLEM, ABSORPTION DRYING TOWER.
« Reply #5 on: March 29, 2007, 03:41:56 PM »

yes i have a spread sheet on excel which has the in and out streams of the liquid and gas stream (presuming constant transfer) and also the heating due to dilution and condensation (presuming that is constant throughout) split up into ten thoretically stages, although I have not put the individual temperature input and out puts as I found this would be harder than I originally thought. I will find this hard to explain so bear with me, I think the easiest way to explain would be to say I could do it easily do if the flows were co-current and it would be a simple aprrox 27oC at the start and 55oC at the end as I have calculated that the two stream together would reach 55oC, but in a counter-current flow where would this 55oC be hit if at all, cannot be at the top as the 30oC acid stream is added and not at the middle as the heat of dilution is not complete and at the bottom the gas is at 15oC, I am thinking that presuming the two stream will have the same temperature at the middle might help me.

I just keep going around in circles in my head every time I think a stream would reach this temp. the other stream is / should be cooling/heating it, also I really would like to know if the 55oC would be reached or if this would be the average temp in the column.

Any advise or thoughts would be absolutely amzing thankyou for any responses they are helping me.
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moleman1985

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Re: BIG PROBLEM, ABSORPTION DRYING TOWER.
« Reply #6 on: April 05, 2007, 07:50:06 AM »

Hello, and sorry it's been a while I had to put the calculation for this section down, been busy making a whole scaled plant layout, HAZOP study, enironmental review, and pumping and piping specifications. I have started my calculations I think i'm on the right tracks even worked out a recycle rate, but have stopped here as I am unsure about an intergration.

The equation I came to is:-

dh= -(Gm*dy)/(NA*a*(1-y))

I need to interrgate this so I can work out h, I think I've done it,

h= - (Gm*y1)/(NA*a*(1-y1)) - (Gm*y2/(NA*a*(1-y2))

can anyone confirm this, I know I should be better at intergration but it baffles me, when I know h I think am pritty much finnish my first time round calculations, as I will guess 'a' as I have selected a packing and will get a good ratio of h and diammeter, well what I think is good but its my column so ...
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allanf

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Re: BIG PROBLEM, ABSORPTION DRYING TOWER.
« Reply #7 on: April 05, 2007, 08:23:04 AM »

[...] I am unsure about an intergration.

The equation I came to is:-

dh= -(Gm*dy)/(NA*a*(1-y))

I need to interrgate this so I can work out h, I think I've done it,

h= - (Gm*y1)/(NA*a*(1-y1)) - (Gm*y2/(NA*a*(1-y2))

That strikes me as wrong, in my head I seem to think it should be:
h = (Gm/NA*a) ln [(1-y2)/(1-y1)]
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moleman1985

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Re: BIG PROBLEM, ABSORPTION DRYING TOWER.
« Reply #8 on: April 05, 2007, 10:33:09 AM »

thankyou very much for that im terrible at integration, if anyone could confirm it that would be top notch!
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moleman1985

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Re: BIG PROBLEM, ABSORPTION DRYING TOWER.
« Reply #9 on: April 07, 2007, 06:50:22 AM »

hello, I think I may have made a big leap but not too sure, I hope/think I have derived a graph showing the equilibrium line, and the operating line together.
The straight line is the operating line, using just the two points (x1,y1) and (x2,y2) and joined them together, which I presume has an approx. gradient of L/G.
The curved line which I hope is the equilibrium line has been drawn from x and y data as I split the column up into 10 sections presuming a constant mass transfer, the begining and end points of the curve are (x1,y1) and (x2,y2), and the rest is constructed from the intermediated values I have calculated.

Can any one confirm this is an equilibrium vs. operating line graph, as I fear that the curved line is just my operating line in more detail, thankyou.

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moleman1985

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Re: BIG PROBLEM, ABSORPTION DRYING TOWER.
« Reply #10 on: April 07, 2007, 06:56:14 AM »

hello i tried to add an image but do not have a clue i press the image button and it comes up with
[img]/[img]
i though that when you pressed post, it would ask you for an image location. Thankyou.
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Borek

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Re: BIG PROBLEM, ABSORPTION DRYING TOWER.
« Reply #11 on: April 07, 2007, 07:50:40 AM »

[img ] tags are to show image linked from other site. If you want to add image and upload it to the chemicalforums server use Additional Options... link below edit field.
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moleman1985

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Re: BIG PROBLEM, ABSORPTION DRYING TOWER.
« Reply #12 on: April 07, 2007, 08:09:09 AM »

hello this is my graph
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Borek

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Re: BIG PROBLEM, ABSORPTION DRYING TOWER.
« Reply #13 on: April 07, 2007, 08:14:21 AM »

doc files are hardly pictures. Export your plot as png, gif or jpg.
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moleman1985

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Re: BIG PROBLEM, ABSORPTION DRYING TOWER.
« Reply #14 on: April 07, 2007, 10:27:44 AM »

hello the graph is a jpeg now, dont know if it'll be an attachment or it will show up on the link.

Edit: modified picture so that it will fit the screen...
« Last Edit: April 07, 2007, 01:24:09 PM by Borek »
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