8 replies to this topic

[venki1985]

Dear all
I have designed a crude column with wash zone efficiency of 40%
Typical V/L ratio in wash zone is 10-12% for a 2% overflash.
Kindly please can anyone guide me with what could be the max allowable V/L ratio in wash zone.
IS it also possible to calculate the mass transfer efficiency of the wash zone with available data of V/L Traffic and their respective physical properties.

Thanks in advance
Bye
Venki

[Himanshu Sharma]

I am a bit confused,you have used term wash zone which is particular to vacuum column zone just above RCO(column feed) inelt,please confirm my understanding.

 

Regards

Himanshu

[venki1985]

Thanks for ur reply
Here I have mentioned the term wash zone for AGO fractionation zone , just above flash zone of crude column.

[S.AHMAD]

V/L ratio should be limited by tthe allowable flooding on the tray... high V causes jet flooding. Check the tray performance/flooding curve..

[PingPong]

Typical V/L ratio in wash zone is 10-12%

V/L ratio? I assume you mean L/V ratio?

 

Read my comments regarding dirty washoil and overflash in this topic: http://www.cheresour...cdu-simulation/

[venki1985]

dear all

thanks for your reply, i wish to put my question in an another way.

As we observe the V/L traffic in a crude column from top to bottom the ratio of V/L increses from zone to zone  , for example in Hy.naptha fractionation  zone its in the range of  1 and for kero frac its 2-4 and for wash zone its 10-12.

by thumb rule we take wash zone efficiency as 50-60%(which coulkd be due to less vapor liquid interface in this zone which is again due to less liquid load).

 

It would be of great help if  you could help with with how this efficiency number is arrived or  is there any correlation from which it could be calculated.

 

Thanks in advance

regards,

Venki

Edited by venki1985, 25 November 2013 - 10:37 PM.

[PingPong]

Largest resistance for mass transfer is in the liquid, and determined by liquid diffusivity.

Liquid diffusivity is highest at low liquid viscosity, and lowest at high liquid viscosity.

To put it simple: liquid viscosity is the most important factor determining the tray efficiency in a crude column. The higher the liquid viscosity, the lower the diffusivity, and the lower the tray efficiency.

 

Other factors that have some effect on tray efficiency are related to tray design: type of tray, outlet weir height, length of liquid flow path, ....

 

Also %Flood has some inpact on tray efficiency: at very high and low %Flood efficiency is less. Obviously entrainment and weeping have negative impact on tray efficiency.

 

Any numbers on tray efficiency you find in literature, or in your company design manuals, are based on experience.

[venki1985]

Thanks ping
Your explanation is very clear and precise.
As you rightly said different valve types has varying mass transfer efficiency and at normal flooding factor almost all valve types gives more or less same efficiency and this made me to think of what could be the most most governing factor to ascertain the zone efficiency and your has provided with insights of fundamental understanding of the application.
Thanks
Venki

[Himanshu Sharma]

Hello All,

 

i am a bit late joining on this topic but here is my two cents ,wherein i have  tried to summarize some of the Physical factors known to me, which have an impact on tray efficiency.

 

Liquid Viscosity: General trend ,as liquid viscosity increases, efficiency decreases.

 

Relative Volatility: General trend ,When relative volatility increases, efficiency decreases.

 

Vapor Diffusivity: General Trend,Increased diffusivities will increase efficiency. Keep in mind, as pressure increases,
vapor diffusivities decrease.

 

Liquid Diffusivity: General Trend is that higher liquid diffusivities will result in higher efficiencies. Liquid diffusivities generally increase withpressure which countervail the change in vapor diffusivity. However, in most distillation systems the influence of the vapor diffusivity will dominate the influence of the liquid diffusivity.

 

Liquid Surface Tension: Debatable but decrease in surface tension appears to be resulting in increase in efficiency.This effect is not visible for high pressure distillation column.

 

I have posted this information earlier also in a related thread.