9 replies to this topic

[ceciliatan]

Hi all,

I am currently doing process design for a sieve plate column. I found some deviation on my design from literature (base diameter bigger than top diameter). Which data should I be considering for top & base part of the column (Data which includes the liquid & vapor flow rate, surface tension, density of liquid & vapor) ?

***The column consists of 32 tray, feed at the 21st tray from top. So, can I just be taking the data from tray 1 for top diameter calculations while data from the 32nd tray for base calculation?

[Zauberberg]

It is possible to have different diameters for the top and the bottom section - it all depends on conditions of the feed, reboiler and condenser duty, and physical properties of vapor and liquid, all being translated into column/tray loadings. You will find many towers out there in the real world, designed with different diameters of the top/bottom sections.

[ceciliatan]

Zauberberg

***The column consists of 32 tray, feed at the 21st tray from top. So, can I just be taking the data from tray 1 for top diameter calculations while data from the 32nd tray for base calculation?

[katmar]

As Zauberberg has said, there are many columns out there with different diameters for the top and bottom sections. This is the best way to deal with widely differing flows in the top and bottom, for example with a vapour feed and relatively small reflux. But another very common method of accommodating smaller differences is to adjust the individual tray designs. This can be done by varying the hole pitch, or by having un-punched areas - and other ways too.

All 32 trays have to work, and at the finest level of detail every tray has its own unique conditions. In the "olden days" of nomographs and slide rules it could take a day or more to do a thorough tray design, so it was simply not feasible to design each tray individually. And in any case there were no simulators to give the conditions on each tray. This meant designers had to go for safe designs that would accommodate a wide range of conditions. This is why bubble cap trays were so popular.

Today it is easy and quick to design a tray using computer software. But it is still not feasible to build a column with 32 different tray designs. The chances of getting it wrongly assembled, either at original construction or (more likely) during maintenance are just too high. So you have to group the trays into sections based on your own judgement and experience. All the trays in a single section will have the same tray design. This means doing a detailed check on (at least) the top and bottom tray in each section. You should adjust the bolting or some other aspect of the mechanical design that makes it impossible to install the trays in the wrong section.

Having said all that, it is still very common to find a column with a single tray design from top to bottom.

[ceciliatan]

Katmar

So does it means that I have to choose the tray which was the most critical for top & bottom respectively? So whats the criteria to be considered in choosing those parameters? Is it the liquid & vapor flow rate at particular tray?

[katmar]

For any particular tray design there is a range of gas and liquid flowrates that will give satisfactory operation. You have to ensure that for each tray the operation fits in this range. There is a very good discussion of this in Coulson and Richardson's series on Chemical Engineering, Vol 6. I have attached a section of page 566 from the 4th edition, but you should try to get the book and read this section in detail.

Attached Files

•  C and R Vol 6 4th Ed Pg 566.gif   43.78KB   33 downloads

[ceciliatan]

Katmar

So, do you mean that the parameter considered would be the tray which has the highest flow rate?

In other words, rectifying section consisting of tray 1-20. Hence, is it HYSYS data for any of the tray which has the highest flow rate be considered for top section diameter determinations? The book from Coulson & Richardson did not guide us on which tray to be considered initially. Hope to get some feedback on how to determine which tray to be considered initially for column diameter determination.

[katmar]

The concept "highest flow rate" is not well defined. What do you mean by that? Mass flow rate? Molar flow rate? Volumetric flow rate? Liquid? Vapour? Just to get you thinking... There is no one parameter that you can say dominates tray design.

Check the top, middle and bottom tray in each section - probably good enough for an initial estimate. If you find big variations check more trays.

[ceciliatan]

what i meant is the mass flow rate of both liquid and vapor. Alright, I will try it on the top,bottom & middle. Thanks.

[alicia88]

Hi all,

If I would assume the top and bottom diameter to be equivalent, the weeping check, plate pressure drop, downcomer liquid back up and entrainment check is done once. How if the top and bottom has different diameter? Should all those mentioned be done separately for top and bottom?

what i meant is the mass flow rate of both liquid and vapor. Alright, I will try it on the top,bottom & middle. Thanks.