14 replies to this topic

[bag]

i'm a student and designing a vacuum distillation column, or more specifically a vacuum steam distillation column. the steam will carry some by products out and by this product is purified. and this is a one-pass through distillation.

now the steam flow rate is about 200kg/hr. how big should the vacuum line be?

the operating pressure in the column will be 10mbar.

[ankur2061]

juyi,

The line size has to be determined by you. We are only going to give you guidance.

Vacuum line sizes are sized in the same manner as for any compressible fluid (gas / vapor). All the fluid flow equations for compressible fluid are applicable for vacuum line size. Just check out your fluid mechanics book for the relevant pressure drop equations.

Regards,
Ankur.

[bag]

Does the low vacuum have an impact on line sizing?

I'm a bit confused by the word vacuum. If under such a low vacuum, and i still want to let 200 kg/hr steam out of the column, the volumetric flow rate will expand right?

[breizh]

Hi ,

To avoid mistake , use absolute pressure and then apply the right equations for gas .
Breizh

[bag]

Let's say the steam I'm using for the distillation is at 2 bara; so when it enters vacuum line (2 mbara) the volume expands 1,000 times. am I right?

I will use the equation PV = nRT.

[Zauberberg]

Vacuum applications are a bit different than other cases (when line sizing is concerned), as there are other factors which need to be addressed - primarily the capacity and performance of vacuum equipment (ejectors, liquid ring pumps) with respect to the required pressure (vacuum) in the column. In many cases, you simply cannot afford to sacrifice almost any pressure drop in the column overhead system.

General rule of thumb for vacuum applications says: pressure drop in the complete overhead circuit should not exceed 5% of the system absolute pressure, or 0.0015 bar/10m - whichever is less. Depending on performance characteristic of your vacuum equipment you might be able to achieve even better than that. But please be cautious, as the figures given above are - as their name says - just a rule of thumb.

As for the expansion of steam - you are basically correct, except that the temperature of steam will change as well (not only the pressure), due to expansion and mass transfer inside the column. The best way to obtain steam conditions when undergoing various cases of expansion is to use Steam tables. One such workbook is attached below.

Attached Files

•  XSteam_Excel_v2.6.xls   701.5KB   281 downloads

[Art Montemayor]

baq:

Here is some more guidance:

You are correct about being a bit confused by the word "vacuum"; you make a grave mistake in stating that when the 2.0 bara steam enters the vacuum line (2 mbara) the volume expands 1,000 times.

The steam in the distillation column IS NOT AT 2.0 bara. The steam inside the column is at 2 mbara. Don't forget: you are operating the distillation column under a partial vacuum of 2 mbara. Therefore, the steam inside the column is at that pressure (the entrance to the vacuum system you are required to operate is at a bit less pressure since there has to be a DRIVING FORCE to evacuate the overhead products being produced.

You are being confused about how a partial vacuum system operates. Download the vacuum design and operating workbook that I have uploaded in our Forums and you will get a better understanding of what you are up against. Use our SEARCH machine to find it.

Zauberberg is absolutely correct. DO NOT USE THE GAS EQUATION OF STATE. You have excellent and accurate steam data in the Steam Database he has furnished you. Use that for obtaining steam properties.

[bag]

Hi, Zauberberg

I think I understand what you mean, but I'm not sure if I have understood you correctly.

Since the steam is still considered saturated, its temperature and pressure change along the saturation line. I made a wrong assumption that only the pressure drops, without a temperature drop at the same time - especially when there is no external heating source.

Am I correct?

Thanks for your workbook! I'm reading it now!

[bag]

'Art Montemayor':

Thank you so much! I got a reasonable pipeline size now! .ha-ha......

[Zauberberg]

As the steam is still considered saturated so temperature and pressure change along the saturation line. I made a wrong assumption that only pressure drop without temperature dropping at the same time, especially when there is no external heating source.

You need to look at the conditions on top of the vacuum column (absolute pressure and temperature) and that will provide you with the condition of the overhead stream as well. Steam tables can then be used for calculating properties of steam required for line sizing.

[bag]

Zauberberg:


Got it! Thanks!

[Padmakar Katre]

Dear,
When you talk about the steam stripping application and the some impurities stripped out of the liquid feed, then you need to get the overhead composition also. It will water rich but the stripped impurities will also have their presence in the column overhead line, so it's essential to consider the overhead composition in total and then accordingly size the overhead line. Any single phase line sizing the two parameters are governing i.e. allowable velocity and allowable pressure drop. To get information about this you can refer the velocity and pressure drop limits based on the operating pressure and accordingly size the line. I have seen 40-60 m/sec of the velocity in crude vacuum distillation unit fractionator column overhead lines. You can find a trade-off between the pressure drop in the line and the resultant cost of vacuum pulling device, say a steam ejector.

[anpasc]

Please,

take a look to the attached paper from GEA; it will give you all the required informations about vacuum line sizing.

Regards
superandy

Attached Files

•  abl8e_09.pdf   716.82KB   278 downloads

[bag]

dear Padmakar

thank you. seems like the line size is still somehow experience based. one advisor have suggested a line size the it works well. i tried to design according to the minimum vacuum requirement but it comes out with a huge line size. when i realized vacuum would definitely change after steam is introduced. so such a big line is not necessary for relatively higher vacuum. if possible, i would rather do a small scale trial first (or pilot plant) and then size based on the real experience.

[bag]

superandy:

Thank you so much. I'm reading it!