3 replies to this topic

[jaffrey]

Hi

Can anyone tell me why as the D/F ratio becomes bigger the distillate composition becomes less and less. I thought as D/F is bigger more is evaporated and hence the distillate composition is larger. Can anyone help me with my understanding.

thanks

[siretb]

It is critical that you learn to formulate precise questions, if you want relevant answers.
What composition? the light key fraction?

Is by composition you mean the mass or mole fraction of the lightest components in the distillate, the explanation is simple.
The greater the D:F ratio, the more you tend to suck the heavy components towards the distllate. If the reflux does not chancge the light keys will be more diluted. THink of what happens if you feed a column with infinite separation capacity, should that exist, 2 components 20% light, 80% heavies.
with D:F=0.2 you get pure lightkey in the distillate. with D:F=0.4 you get 50%, by mass balance.

I hope this helps

[jaffrey]

It is critical that you learn to formulate precise questions, if you want relevant answers.
What composition? the light key fraction?

Is by composition you mean the mass or mole fraction of the lightest components in the distillate, the explanation is simple.
The greater the D:F ratio, the more you tend to suck the heavy components towards the distllate. If the reflux does not chancge the light keys will be more diluted. THink of what happens if you feed a column with infinite separation capacity, should that exist, 2 components 20% light, 80% heavies.
with D:F=0.2 you get pure lightkey in the distillate. with D:F=0.4 you get 50%, by mass balance.

I hope this helps

Sorry thanks a lot for your reply but still confused. From my workbook notes it says that as the the D:F ratio increases the top product's (or distillate's) composition of the MVC falls which strikes me as odd because if you are evaporating more of the feed you are applying more heat and so the D:F ratio should be higher and richer in MVC rather than falling in composition. please help

Edited by jaffrey, 02 November 2009 - 07:17 PM.

[Gautam P]

It is critical that you learn to formulate precise questions, if you want relevant answers.
What composition? the light key fraction?

Is by composition you mean the mass or mole fraction of the lightest components in the distillate, the explanation is simple.
The greater the D:F ratio, the more you tend to suck the heavy components towards the distllate. If the reflux does not chancge the light keys will be more diluted. THink of what happens if you feed a column with infinite separation capacity, should that exist, 2 components 20% light, 80% heavies.
with D:F=0.2 you get pure lightkey in the distillate. with D:F=0.4 you get 50%, by mass balance.

I hope this helps

Sorry thanks a lot for your reply but still confused. From my workbook notes it says that as the the D:F ratio increases the top product's (or distillate's) composition of the MVC falls which strikes me as odd because if you are evaporating more of the feed you are applying more heat and so the D:F ratio should be higher and richer in MVC rather than falling in composition. please help



If you vaporize more, then along with MVC, the LVC ( Low Volatile Component ) will also go into the distillate. In this case, Distillate Quantity will increase but purity will reduce. The bottom product quantity will reduce, but quality ( i.e. % of LVC) will increase. The Relation between the Distillate and Bottom Liquid Composition is given by following equation (for a binary system)

y = (xf/n) – x (1-n)/n --- (1)

Where :

y is the mass fraction of MVC in distillate

xf is the mass fraction of MVC in the feed

x is the mass fraction of MVC in bottom product

n is the fraction of feed vaporized (same as D:F for flash distillation without reflux)

Above equation has two unknown ( x and y). To determine the distillate composition for a given feed composition and D:F ratio, another relationship between y and x is required. Such a relation is provided by the Vapor – Liquid Equilibrium (VLE ) data. If the system is assumed to be ideal, then the VLE relation can be derived from relative volatility between the MVC and LVC.

y = ax / {1 + (a - 1) x } ----(2)

where a is the relative volatility. You can solve equation (1) and (2) simultaneously for various D:F ratio (i.e. n ) for a fixed feed composition and see that y decreases as n increases.

This is the fundamental of flash distillation.