4 replies to this topic

[raghav]

hi,

what may be the reason for variation in diameter of distillation column between stripping and enriching section..

pls share your views.

regards
raghav

[Zauberberg]

The only reason for a change in tower diameter is substantial change in liquid/vapor loads inside the tower, and/or tower internals type. Transition points are usually located in the feed zone or, sometimes, few trays above the reboiler return when reboiling ratio is extremely high (crude stabilizers, for example). In any case, detailed hydraulic calculation of fractionation tower will point the places where significant change in vapor/liquid traffic occurs, asking for different tower diameter or different type of column internals.

[riven]

Another take on the above excellent post

'Column Diameter
Column diameter is found based on the constraints imposed by flooding. The number of ideal stages isn't needed to find the diameter -- only the vapor and liquid loads. You do need the number of actual stages to get the column height.

Before beginning a diameter calculation, you want to know the vapor and liquid rates throughout the column. You then do a diameter calculation for each point where the loading might be an extreme: the top and bottom trays; above and below feeds, sidedraws, or heat addition or removal; and any other places where you suspect peak loads.

Once you've calculated these diameters, you select one to use for the column, then check it to make sure it will work. Some columns will have two sections with different diameters -- consider this possibility if you end up with regions where the estimated diameter varies by 20% or more, but realize it will be more expensive than a column that is the same all the way up.

One issue that ought to be considered is the validity of your design numbers. If you are following the "traditional" approach, you've probably designed your column for reflux rates in the range of 1.1 to 1.2 times the minimum. This may not give you a column that can handle "upsets" well, so you may want to design for a capacity slightly greater than that -- increasing the flows by about 20% might be wise. '
Taken form http://www.cbu.edu/~...till7.html#diam

[raghav]

Dear both,

Thanks a lot for your valuable inputs.

raghav

[katmar]

Unless you have pump arounds in the column, it basically comes down to the condition of the feed. If you have a saturated vapor feed, the introduction of the feed will do little to the liquid flowrate down the column, but it will result in an increased vapor load in the upper part of the column. In this case the diameter of the enriching (upper) section is likely to be larger than the stripping (lower) section.

On the other hand, if your feed is a subcooled liquid it will cause additional condensation at the feed point and decrease the vapor load in the upper part of the column and increase the liquid load in the lower part. In this case the stripping section is likely to have a larger diameter than the enriching section.