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Vacuum Distillation Column: Design And Control

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#26 Art Montemayor

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Posted 24 June 2007 - 10:10 AM


I'm on my way to church with my granddaughthers, so I have to be quick here. pardon the writing.

You have a distillation column operating under a partial vacuum. You are feeding a liquid mixture to this column for the sake of separating the components. Water and methanol will be expelled through the overheads as vapor and the heavier components will remain at the bottom - all done under a steady-state distillation operation. The universal chemical engineering rule should work here: What goes in must come out.

Now comes the BIG question: if the external (atmospheric) pressure is greater than what is in your column and the bottoms product must not be allowed to accumulate (this is a steady state operation), then how are you going to evacuate the bottoms components under a steady state if you don't use a pump (or any other device)???

I don't offer an ejector workbook because I've never put one together. I offer a Vacuum production workbook - which I believe you already have - and in it the steam jet ejector is explained. Remember that you never have responded to my query as to what you were doing with the saturated overheads vapors? There was a reason for my query and now I think you are coming to that subject. That is why I always ask for ALL of the basic data and your Scope of Work. Until I have that, I can't help in that area.


#27 merac


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Posted 24 June 2007 - 10:46 AM


The objective of the distillation column is to purify methyloleate to a minimum of 96% (weigh). In order to do that I have to remove methanol, water and triolein from the feed stream, so:
-the bottom stream is mainly triolein
-the overhead vapor is a mixture of methanol, water and methyloleate. This stream goes to a partial condenser where:
- methanol and water exits the vacuum condenser as noncondensables (and they go to a vent condenser in order to separate methanol and water and recirculate to the beginning of the process)
-methyloleate , mainly pure, exits the condenser as a saturated liquid and goes to the reflux drum and then, a part of this stream (the distillate stream), to the methyl oleate tank in order to sell it.
Was it the information you ask for?

Related to the bottom of the column, I thought that there are some cases in which a vertical termoshyphon with no pump is used. Natural circulation is provided because of density differences between the bottom liquid of the column and the liquid-vapor mixture that exits the reboiler. Does it correct? What is the reason for which I cannot use that bottom configuration for the reboiler?

Thanks for your help

#28 Art Montemayor

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Posted 24 June 2007 - 01:38 PM


As this thread grows and grows, the little migas (crumbs) of information you are furnishing are starting to form the various problems that may be present in your project. This will be Post #28 and we have just now established that you are planning to evacuate your heavier bottoms product out of your distillation column with nothing else than just thermosyphon action. If you succeed in doing this, you will make engineering history and the world will flock to your door. I don’t believe that is going to happen.

You are going to have to furnish a mechanical means (a pump) in order to evacuate the bottoms product out of your distillation column that is operating under a partial vacuum. The only other way to evacuate the bottoms product would be to elevate the sump of the distillation tower high enough in order to form a hydostatic leg of bottom product such that the static pressure at the bottom of the hydrostatic leg is larger than the outside, atmospheric pressure. I know of no other way to do it without a pumping device.

I am not familiar with the distillation of methyl oleate. I understand it has a boiling point around 218 oC. I don’t know what is the equilibrium conditions (vapor composition) at the top of your distillation column, but shouldn’t there be an equilibrium condition established between the condenser’s liquid product (which you say is essentially 100% methyl oleate) and the water and methanol vapor? Since you don’t furnish any compositions or other process data, I can’t analyze whether your system will function as you have physically described it. Presumably you have simulated the process and that is what your program states you have to do in order to effect the separation. You have not told us how you developed the distillation scheme or method of separation.

Are you in agreement with the recommendations I offered about the elevation difference between the column top and the condenser / reflux drum?

#29 merac


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Posted 25 June 2007 - 01:10 AM


I have simulated the process with Aspen Plus.

The feed composition (mass fraction) is:
-methyloleate 0,946
-methanol 0,003
-water 0,001
-triolein 0,050
-temperature 60ºC
-pressure 1,1 bar
This stream has a methanol, water and triglycerid composition higher than the one that is permited by an european code that the mehtyl oleate has to fulfill in order to sell it.

The vapor composition (mass fraction) at the top of the distillation column is:
-methyloleate 0,9965
-methanol 1,77E-3
-water 6,77E-4
-temperature 162ºC
-pressure 0,1 bar

The bottom liquid composition n(mass fraction) is:
-methyloleate 0,02
-methanol 0
-water 0
-triolein 0,98
-temperature 282ºC
-pressure 0,1 bar
-density 389,98 kg/m3
-liquid fraction 1

The liquid vapor mixture that exits the termosyphon has the following properties:
-methyloleate 0,157
-methanol 0
-water 0
-triolein 0,843
-temperature 294ºC
-pressure 0,086 bar
-liquid fraction 0,5
-density 2,45kg/m3

Of course, there is an equilibrium between the condenser’s liquid product (I said it is mainly pure because it has a mass fraction of 0,998 methyloleate, 3,9E-3 methanol, 3E-3 water) and the water and methanol vapor.

The column is elevated with a skirt of 4,50m height taking into consideration:
-that the termosyphon is vertical and has a total length of 2m
-it is necessary to have a vertical space below the reboiler of 2 m minimum in order to be able to clean the tubes of the reboiler

With this elevation, and the density differences between the liquid bottom product and the vapor liquid mixture that exits the reboiler, is possible to have a natural circulation of the liquid that exits the distillation column?

#30 Art Montemayor

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Posted 25 June 2007 - 10:47 AM


Without a mechanical means of ensuring that the bottoms product in your vacuum distillation column is removed as fast as it is forming, you will not be able to keep the process going.

You must furnish a positive method of evacuating the bottoms product. The normal, conventional way of doing this is to furnish a bottoms product pump as I have shown in the attached work book. I continue to emphasize that you cannot transport a fluid from a low pressure to a high pressure without a force (work effort). Water will not flow uphill on its own. You must pump it up to the top of the hill or carry the pails of water yourself.

Note that I've identified your streams in order to form some type of Process Flow Diagram to show what is happening in your process.

Attached File  Vacuum_Distillation_Column_Design_and_Control.zip   458.19KB   364 downloads

#31 merac


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Posted 27 June 2007 - 07:19 AM


I'm calculating the pump power of the reflux pump so I need to know the suction and discharge pressure and I have some doubts related to the calculation of that pressures.

- The pump suctions the liquid of the reflux drum and it pumps to the top of the distillation column.
This liquid has the following properties:
T (ºC) 162
P (bar) 0,094
Mass fraction
Methyloleate 0,998
Methanol 3,65E-04
Water 2,88E-04
Triolein 1,04E-03
Mass flow rate m (kg/h) 9779
Density (kg/m3) 772,82
Viscosity (cP) 0,502

- Diameter and length line:
Nominal Diameter 2"
Geometric length 9 m and total length (taking into consideration the equivalent length) 31 m

- The reflux drum pressure is 0,094 bar and the column pressure is 0,1 bar

My calculations are the following and I'm not sure if they are correct or not:

A1.Presure loss :with Fanning equation 0,13 bar.

A2.hidrostatic pressure: 0,68 bar (multiplying gravity per density per geometric length = 9,81*773*9)

A3. suction pressure= reflux drum pressure - pressure loss in line + hidrostatic pressure
= 0,094 - 0,13 + 0,68 = 0,65 bar

Is this suction pressure calculation correct?

B1. Pressure loss: with Fanning equation 0,06 bar.
B2. Hidrostatic loss pressure: 1,55 bar (multiplying gravity per density per geometric length = 9,81*773*20,4)
B3. discharge pressure = column pressure+ pressure loss in line + hidrostatic pressure loss =
= 0,1 + 0,06+ 1,55 = 1,71 bar

Is this dicharge pressure calculation correct?

Thanks you in advance

#32 Art Montemayor

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Posted 27 June 2007 - 08:16 AM


The quality of your basic data is great; the quantity is lacking.

I can't comment on your pressure drops if you don't furnish any heights or lengths of the individual suction and discharge lines.

I can't comment on whether your calculations are correct if you don't furnish the subject detailed calculations.

In fact, I don't even know where your reflux pump is located. This is the first time you have mentioned a reflux pump. I thought we were still struggling with the need for a bottoms pump. You have not yet furnished a sketch or drawing showing your PFD, your P&ID, or your equipment elevations. Without these drawings or sketches (and the related detailed calculations) it is next to impossible to comment on whether your pressure calculations are valid or credible.

I wish I could help you on this, but without the basic data and drawings, there is not much one can add to your information. I have tried to show you how to draw an elevations sketch in order to show you how a pump calculation is carried out. The elevations drawing is critical to determining the hydrostatic and NPSH calculations as well as the pressure drops expected in any pumping operation.

#33 merac


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Posted 27 June 2007 - 08:53 AM

I have also put a bottom pump, but I haven't designed yet, as I've begun to calculate the reflux pump first

I have attached a preliminar layout of the distillation column (alhough it is not finished it is useful to estimate pipe lengths). It is shown where the reflux pump is located

Also I have attached an excel sheet with the diameters and lengths of the lines

Related to the pipe lengths, I have calculated them adding geometric length and equivalent length:
1. Geometric length:
With the estimated lengths of the layout drawing, I have considered a security factor of 1,2 (multiplying the drawing length by 1,2) in order to calculate the geometric length of the line. For example: line 3001013 is 7,50*1,2=9 m

2. Equivalent length
2a. Leq suction(m) = 122*(Dinternal/12) where D internal is the internal diameter in inches
2b. Leq discharge (m)=Lgeometric(m)*(1+(0,347*D^0,5)+0,216) where D is the nominal diameter in inches

I think that's the information needed, doesn't it?

Attached Files

#34 Accio


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Posted 12 August 2007 - 12:19 PM


I am sorry to inform you that maybe there is a Macro virus in the Excel file

that is attached to your posts:

When I try to download it, my Antivirus program NOD32 shows the alert for "a probably unknown MACRO virus" and does not allow to open this file.

I have sent the above file to ESET (NOD32's producer) for confirmation/ identification of this unknown MACRO virus and I am waiting for their answer.


#35 Art Montemayor

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Posted 16 August 2007 - 09:14 AM


Thank you very much for the notice on the existing Excel 4 macro in my workbook. I have also been aware that it is there, but I can't explain how it got into my Excel template in Excel. I've been told that it is harmless, but I would certainly appreciate any news from you and anyone else on whether it is indeed a harmful virus or not.

Thank you.

#36 Suryakant Randeri

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Posted 20 January 2009 - 05:24 AM

Dear Sir,

I tried to down load file name :Vacuum Distillation Column design and control twice but every time my computer indicates error in opening the file.

Third time when I down load the zip file , my computer asked me whether I should continue for down loading it contains virus.

May I request you to please send me original excel file with out virus to my email address:
1) scranderi@yahoo.co.in
2) randeri@hpfl-india.com

This would help me to under stand the design for the vacuum distillation column set up and use it for the design.

Appreciate your early reply
with kind reagards,

Suryakant Randeri

#37 Qalander (Chem)

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Posted 20 January 2009 - 11:59 AM

Dear Respected Art.

May be there is really some virus infecting your Vacume distillation file. Does'nt permit normal download; Accordingly either admin to help or somebody capable of helping you with better excel knowledge and skills.

Best Regards

#38 hgandhi02


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Posted 18 June 2012 - 03:17 AM

dear montemayor,

Can u send me zip file " producing and maintaining vacuum.zip" on my mail id ?

thanks a lot for you help.

#39 sulabh


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Posted 03 July 2012 - 06:18 AM

Dear Administrator
I have a very basic and simple doubt which is written every where bu the reason is not clear to me.
WHY is low pressure drop preferred in vacuum distillation?

#40 KIshor


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Posted 04 July 2012 - 09:15 AM

We need vacuum distillation in order to prevent degradation or polymerization of high boiling components by carrying out distillation under vacuum. If we talk about petroleum products , they generally polymerize or under go thermal cracking at around 380 deg C. This tells us that we need to carry out distillation at temperature below 380 deg C which corresponds to 10 to 40 torr of vacuum. If the pressure drop is higher in vacuum column then bottom products may be exposed to higher temperatures leading to degradation. Hence lower pressure drop is preferred in vacuum distillation.

Edited by KIshor, 04 July 2012 - 09:19 AM.

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