12 replies to this topic

[DCHE]

Hi guys,

I am required to size a vertical flash drum either having a demister or not.

Operating Condition:
Temperature : 194 F
Pressure : 18.13 PSIA
Vapor Density : 0.205 lb/ft3
Liquid Density : 49.221 lb/ft3
Vapor Mass Flowrate : 112986.640 lb/hr
Liquid Mass Flowrate : 308871.040 lb/hr
*More information can be provided upon request

I have calculated my design vapor velocity factor (K) for separation to be 0.366ft/sec.
However, base on literature review a flash drum with mesh pad has a K of 0.350ft/sec.

These are the problems that I am in doubt,
- What is the significance of K?

- If I am required to add on a demister, do I size my flash drum diameter base on the K of vessel or demister?

- The momentum criteria for inlet nozzle, I have calculated it to be 8630.886 base on SI unit. This far exceed any literature review that I am came upon which is 1000/1500. What can I do about it?

- Literature review provides a 1/2 full residence time for liquid to be 3-5min for separators. If the value I have chosen is 3min, do I obtain the liquid level of my vessel using 3mins or 6mins. I am confuse due to the word 1/2, so to get full I need to times 2?

- My vessel diameter calculated base on K of vessel is 6ft, total height is 26ft. Is this dimension reasonable? My inexperience of chemical industrial plants could not give me a gauge.


Your help is deeply appreciated,
Qing Wei

[breizh]

Hi ,
Let you use the search button , a lot of info are available on the forum .
I've added an other resource:
http://www.chemsof.c...l_Separator.xls

hope this helps

Breizh

[DCHE]

Dear Sir,

Thank you for the excel file. I've searched through the forum and books such as the "Rules of Thumb for Chemical Engineering" & "Chemical Process Equipment Selection and Design". I have completed my manual calculations of flash drum and it fits the solution provided by simulation design II

However, there are some areas which I am still confuse and would like to seek clarification.

Vapor velocity factor (K)
1. This term appears in vessel design calculation, where Souders-Brown equation is use to obtain vapor-liquid separation factor. K could then be determine through a "vapor velocity design factor vapor liquid separators at 8% flooding" chart.

2. This term also appears in demister, where a recommended K could be use i.e. 0.35 for sizing purpose.

Am I right to say that (1) is use to size flash drum without demister while (2) is use for flash drum with demister.

The reason I could not tell which to use is because the K i gotten from Souders-Brown equation is 0.36 which is close to 0.35. Thus, my calculation appears to be the same as the simulation solution.

And finally, how does this vapor velocity factor effects on separation? I have yet to see any books/website/forum/PDF files that talk about its significance.

Can any kind soul please enlighten me?
Thanks!

[breizh]

Some more info to support your query .

Breizh

[DCHE]

Thank You Mr Breizh! The attachment you provided explains it very well, now I understand the calculation that I am performing. Thanks again for all help

[Florentina]

Dear Breizh,

can you provide us (if it's possible) the passwork from the Visual Basic...

Thank you in advance.
Florentina

[chouchane]

hi
i am a student at Aston University Birmingham, in my 3rd year of a chemical engineering masters MEng. as part of my group dissertation we designed a biodiesel production plant which produces biodiesel from used cooking oil and methanol.

for my individual dissertation i chose to design the methanol recovery column. for the past few weeks i was trying to design a distillation column but now i realised i have to design a flash separator or a flash drum. i have 4 weeks left and iam still not sure on how to start designing this flash column.

what i know:

the feed compositions : methanol, fame, glycerol and triglyceride.

can someone please help me with the design calculations including the operating conditions of the flash drum or any decent reference which has good information. thank you very much!!!

[Arpit_Jain]

Refer "Systematic Methods of Chemical Process Design by Biegler, Grossmann, Westerberg for flash calculations

Edited by arpit.jain, 16 February 2012 - 02:36 AM.

[chouchane]

Refer "Systematic Methods of Chemical Process Design by Biegler, Grossmann, Westerberg for flash calculations

mate...your a legend. that book is very useful...do you know wether the conditions are calculated or assumed.

[Arpit_Jain]

The conditions are calculated by doing iterations using the goal seek solver in MS Excel.

Assume a temperature and pressure. Keep on iterating by changing these values till you get the desired separation.

Hope you understood !

Edited by arpit.jain, 17 February 2012 - 02:28 AM.

[chouchane]

The conditions are calculated by doing iterations using the goal seek solver in MS Excel.

Assume a temperature and pressure. Keep on iterating by changing these values till you get the desired separation.

Hope you understood !

im not too familiar with goal seek solver. iam not really sure how to work out the operating conditions

[suhailah2505]

do u know how to design the vapor liquid separator.first thing first what should i do in order to design in term of chemical design??for mechanical design the sizing i need to calculated right if i'm not mistaken.but i want to know for chemical design what should i calculated or put in the chemical design?

[S.AHMAD]

Dear Suhailah
1. First of all, we normally call this "Process Design".
2. The purpose of process design is to determine the "right size" of process equipment such as pump. compressor, heat exchanger, flash drum etc. that meet the process requirement.
3. The first step in process design is to establish the heat and material balance. For flash drum, we need to know the flowrate of vapor and liquid to be separated.
3. Then determine the size namely the diameter and the height (for vertical vessel) or the length (for horizontal vessel).
4. Refer to attachments by Breizh for details.
5. If you have any more specific problem on the sizing methodology, post your question in a new thread