9 replies to this topic

[EngrHamza]

Hi,

 

I hope you all are fine and doing great. 

 

Currently I am designing my final year design project. I am designing catalytic reforming plant and process used is CCR by UOP. I almost design all major equipment. I copes with some problem while designing the distillation column and also called de-butainzer. My questions are below;

 

1) I got K values for C1 to C6 hydrocarbon at specified temperature and pressure which are 60C at top and 200C at bottom. But there are other components in feed which are benzene, toluene, methylcyclohexane and methylcyclopentane and for these component I cant find K value and antoine coefficient temperature ranges at upto 100 C for these component. Can any one guide me how to get K values for these hydrocarbon?

 

2) How to calculated Length of column. According to me L = no of trays x tray spacing and I have read this in some literature. But As there is top space above top most tray and as well as bottom space. In one document it say top space must be between 800mm to 1000mm for man-way but what about bottom space? As I know there must be something related to residence time if I am using following equation given in literature named as "Chemical Engineering Process Design & Ecnomics by Hary Silla";

 

Ls=bottom space = 0.06 x No of actual trays + 2.0.

Actual trays are 30 

Space between trays = 0.45 m 

Bottom Dia = 1.4m

Top dia = 0.851 m 

 

By using this I got 3.8 m space at bottom is it feasible ? 

 

 

Kindly guide me please

 

 

[Pilesar]

The reboiler return should be in the vapor space of the sump at a distance of at least one tray space below the bottom tray. The return nozzle should be at least one foot above the highest liquid level. The liquid in the sump should have time for the vapor to disengage -- at least one minute of residence time. These criteria will give minimum height of sump. There may be other criteria that result in taller sumps. If the bottoms pump fails, is there time for an operator to notice and go to the field to switch on the spare pump? The trays are not designed to handle much force from below so it is important that the sump liquid does not get above its designated high level to keep from damaging the lower trays. There is not a standard sump height for all columns. The 3.8 m space you propose at the bottom is feasible. Add a manway at the feed location, at the top, and in the sump. You will have a column skirt to hold the column on its foundation. It may add 3 to 10 meters to the overall height of the column but 'column height' is often referenced only to the straight sides of the pressure vessel. The column heads are excluded from this common measurement and the height is referenced as 'tangent to tangent' (T/T) where the straight side meets the curved head or sometimes you see it as 'seam to seam' (S/S) where the head is welded to the straight cylinder. 

[Pilesar]

I notice your column has different top and bottom diameters. Consider using only the larger diameter for the entire column. Where diameters change, there will be a cone transition which adds some cost and requires additional column height. Your column diameter should be large enough for a man to work in order to install the trays. The tray supports need to be welded to the inside of the column. The few trays you will have in the top section and the small difference in diameter means there will be very little if any capital cost savings and much annoyance and additional cost later when maintenance is needed.

[breizh]

Hi,

To add to Pilesar's comments , a document to support your work.

Breizh

[Pilesar]

Designing a real debutanizer can be trickier than it may seem. There are so many C4 and C5 components to consider. Some C4 components actually act heavier than some C5 components during distillation which can be very annoying to the designer. Your temperature profile seems wrong to me with such a large temperature spread for splitting C4s from C5s. Perhaps your pressure is higher than I am used to seeing for debutanizers. Are you using such high pressure because you are forced to use air cooling for condensing? How do you know your top and bottom temperatures when you haven't modeled the distillation?

[EngrHamza]

I have performed bubble point and dew point calculations for top and bottom temperature 

[EngrHamza]

Some times reflux ratio is increased to main top temperature upto 90 to 100 C

[EngrHamza]

Which designing column I consider following components in my feed 

Methane

Ethane

Propane

Butane

I-pentane

N-pentane

Hexane

Heptane

Toluene

Xylene

Benzene

Methycyclopentane

Methycylohexane

 

In real there are many components usually isomers of main or major components

[EngrHamza]

And also components in feed depend upon the quality of naphta or composition of naphtha we are processing. Like in this we only process HSR naptha and hydrotreated naptha and in this there are no olefins and normal amount of napthenes and major amount of parrafins are present

[Pilesar]

Only you know your feed composition and the product specs. Your comment 'Some times reflux ratio is increased to main top temperature up to 90 to 100 C' makes no sense to me. I think you may need to review distillation basics before committing to this design. Ask your professor or get advice from other students familiar with your project. As a final project, it would be good to have understanding. Making a guess and hoping it is correct does not prepare you for an engineering career. You don't seem to be using a steady state process simulator so that will make trial and error much more time consuming. Consider a hypothetical column with just C4s and C5s. How would you set the distillation conditions? What would be the column pressure and what would be the top and bottom temperatures? Then add in propane and hexane into your feed as well. How could you determine the new column conditions? Would you need to change the operating pressure? Would the top temperature change? Would the bottom temperature change? Why? I do not know how much you understand, but now is a good opportunity to learn if you are concerned you have gaps in your knowledge of distillation. One hint for you: often the distillation pressure is dependent on the available cooling and heating utilities.