[michal]

.

[mbeychok]

Read my posting of "Design of vapor-liquid separator" on May 29, 2006, 05:09:34 PM which recommends using an L/D of 3 to 4 and then also checking to make sure that the separator contains sufficient liquid hold-up volume. It isn't a matter of "either L/D or liquid hold-up", but rather a balancing of both factors.

[Donaldson Tan]

This is the thread mbeychok is referring: http://www.chemicalforums.com/indexphp?topic=8913.0

Anyway, the general rule of thumb is it depends on what set of data you have.

There is no point trying to do by method 1 when your given data is suitable for method 2.

[Ahmed]

In a V/L seperator design we use lenght-to-diameter ratio of 3 to 4.What if i use the ratio of 2?
There are two conditions to design wheter to use residence time of 3sec. or to use L/D = 4.Which one is a good practice?

[mbeychok]

Ahmed:

Have you read the thread in this forum at http://www.chemicalforums.com/index.php?topic=8913.0 that describes how to size a vapor-liquid separator?

The suggested L/D ratio is just a rule-of-thumb and you may use other ratios if you wish.  Why is it a rule of thumb?  Because many years ago it was determined to be the most economical ratio for balancing the cost of small-to-moderate size pressure vessels against the design strength of such vessels. But like all rules-of-thumb, it is not necessarily the best choice in all cases.

In fact, after you have calculated the maximum velocity of the up-flowing vapor and thus determined the minimum diameter of the upper section of the flash drum ... you can use a larger diameter for the bottom half of the vessel in order to provide more liquid hold-up volume if you need it.  In other words, you can use a swaged vessel (different diameters for the upper and lower sections) as long as you are aware that a swaged vessel is more expensive. Using a swaged vessel means you have a smaller overall length-to-diameter ... with the upper section diameter being that required by the calculated maximum vapor velocity and the lower section having a larger diameter to give you more liquid hold-up volume.