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[KR]

Hello friends,

Firstly, I am very new in designing packed column and really need help.
I would design packed column using structured packing (Mellapack e.g 500Y, 350Y, 250Y ) as wish to have low pressure drop.

I am using HETP for finding height of a column. My knowledge says HETP depends on physical properties of the fluid handled and packing charactoristics. I have seen in some of the literature,HETP has been given for different packings.

I am wondering how would I get HETP for Mellapack packing. OR do I need to compute using some equation?

Thanks in advance for your help.

[Art Montemayor]

Komal:

Help us with the basics first. Are you a degreed Chemical Engineer? If not, have you studied or taken any formal Unit Operations courses?

In order to furnish you with some help or guidelines, we have to know if we can start communicating with you at the same level of expertise or learnings. Otherwise, we are going to either confuse you more or we are going to lose you.

Give us all the detailed background on this application and how you find yourself dealing with something you don't know where it originates.

[KR]

Hi Art,

I agree with you. I am a degreed Chemical Engineer and working at present. It's really amazing to get your quick response. Thanks.

[Art Montemayor]

Komal:

If you are going to be involved in the basic process design of packed towers, then it is essential that you do voluminous research and study of the previous work that has been done in the past on this subject. And I consider it almost mandatory that you obtain a copy of Ralph F. Strigle's classic book: "Random Packings and Packed Towers - Design and Applications". My copy was published in 1987 by Gulf Publishing Co. I don't know if it is still being published or if you can buy it used on the Internet. You should also study every thing that John S. Eckert and Robert Treybal wrote on the subject. Ernest Ludwig is another excellent source for information in his tome, "Applied Process Design for Chemical and Petrochemical Plants".

HETP stands for height equivalent to a transfer plate (or "stage"). The height of a transfer unit is defined as that depth of packing required to produce a change in the composition equal to the mass transfer driving force causing that change.

Despite the great amount of data available, no method yet exists for HETP prediciton with a high degree of accuracy. The prudent designer resorts to commercial experience whenever and wherever he/she can. Your supplier of "Mellapack" should be able to help you with empirical HETP's.

Norton Chemical Process Products developed a proprietary correlation for prediction of HETP values based upon a regresssion of their own data, plus that others. This correlation for atmospheric pressure distillation is found in Streigle's book on p. 152.

Good luck in finding all the right literature.

[David Sugiman]

Hi Komal,

I happened to have a few projects with Sulzer and have familiarized myself these few years with Sulzer trays and structured packing.

In the link to Sulzer structured packing below, you can find expected HETP for various types of Sulzer packing:
http://www.sulzerche...uments.11.2744/

A word of caution though. When we did revamping of our distillation section by using Sulzer's Mellapak Packing, we don't take HETP figure seriously as there are many factor that might influence this figure. Real plant data (if available) is definitely the information that we can trust the most!
After installation, I did manage to find a few distillation columns with HETP value quite significantly differ from what Sulzer published in the brochure.

To add up on Art's book recommendation, my personal favourites for these subjects are Distillation Operation and Distillation Design written by Henry Z. Kister.

[KR]

Hi Art,

Thanks for all the books you recommended.Will take into account.

David,

Thanks very much for your link and as you said, I won't rely completely on the data given as I wanted that info for the rough calculation.

[smalawi]

Hi,

HETP should be independent from the system as far as I recall, but the real value might differ significantly from what vendors claim. Sulzer has a tray & column sizing software free to use and will give you some informaiton. You can download it from thier site.

As for the refernce books, try the Kister books, free on google books ! go go google !!!

Cheers,

SM

[David Sugiman]

Hi,

HETP should be independent from the system as far as I recall, but the real value might differ significantly from what vendors claim.
Cheers,

SM

Hi SM,

Based on my experience (or my lack of it since I'm still a young engineer with three year experience), I have to disagree with your opinion on some cases:
1. Two liquid phase
Due to phenomena called as underwetting, liquid with high surface tension tend to reduce HETP value of structured packing especially at high surface tension liquid rich area. I have personally seen this in my plant at water - organic phase region. Kister distillation design reports that the drop of HETP value can be as much as 50%

2. Physical properties issue
This is a little bit more problematic. For plant process engineer like me, we put more importance on actual plant performance and at times, we change binary parameters in order to fit with actual plant data.
As the result, calculated number of theoretical stages required for column can be different from the theoretical stages required if calculated by tray/packing vendor due to difference in physical properties.
The question of which one is more accurate is difficult to answer for me, but at least it doesn't pose a big problem for me as I can rely on actual plant data when I need to design one particular column (as opposed to rely on HETP value).

Best regards,
David