7 replies to this topic

[raza0kid]

Hello,

I am a chemical engineering student working on my final year project which is the "production of different grades of lube base oil through the vacuum distillation of atmospheric residue". The products coming out of my column are GAS OIL , SPINDLE OIL, SAE-20 & SAE-40 whereas the vacuum residue leaves the bottom.

I am using a tray column and after having literature survey I fixed the following parameters for my column:

• Flash zone pressure = 95 mm Hg
• Flash zone temperature = 750 F
• top pressure = 50 mm Hg
• Number of trays = 20
  
  • Spindle oil draw tray = 4th tray
  • SAE 20 draw tray = 8th tray
  • SAE 40 draw tray = 12th tray (trays are numbered from top to bottom)
• Pressure drop per tray = 3 mm Hg

I have calculated the side drawl temperatures for the column. After applying heat balance I came across with a certain amount of heat that I must extract through hot or circulating refluxes in order for the column to be thermally balanced.

Can anyone suggest me a scheme for extracting the desired amount of heat from my column through top reflux and pumparounds?

Attached are the temperature/pressure profiles and other relevant information.

I will be very thankful for your kind help.

Raza.

Attached Files

•  fyp vacuum.xlsx   174.92KB   54 downloads

[Zauberberg]

The schemes I am familiar with always have P/A draw trays as product draw-off trays at the same time. This simplifies the design (extra spacing required for these types of trays) and also ensures direct control of the product quality/rate, through adjustments of the pumparound flow and return temperature. Unless there are some really case-specific reasons, your product draw-off trays will be the P/A draw-off trays.

Depending on how much heat has to be removed at each section, what is capacity of the trays, and also depending on flow characteristics of distillates (i.e. pour point) you can see how much margin you have for setting up P/A flow and return temperature.

There are some interesting articles on vaccum distillation at: http://www.revamps.c...alArticles.aspx

[Padmakar Katre]

Hello,

I am a chemical engineering student working on my final year project which is the "production of different grades of lube base oil through the vacuum distillation of atmospheric residue". The products coming out of my column are GAS OIL , SPINDLE OIL, SAE-20 & SAE-40 whereas the vacuum residue leaves the bottom.

I am using a tray column and after having literature survey I fixed the following parameters for my column:

• Flash zone pressure = 95 mm Hg
• Flash zone temperature = 750 F
• top pressure = 50 mm Hg
• Number of trays = 20
  
  • Spindle oil draw tray = 4th tray
  • SAE 20 draw tray = 8th tray
  • SAE 40 draw tray = 12th tray (trays are numbered from top to bottom)
• Pressure drop per tray = 3 mm Hg

I have calculated the side drawl temperatures for the column. After applying heat balance I came across with a certain amount of heat that I must extract through hot or circulating refluxes in order for the column to be thermally balanced.


Can anyone suggest me a scheme for extracting the desired amount of heat from my column through top reflux and pumparounds?

Attached are the temperature/pressure profiles and other relevant information.

I will be very thankful for your kind help.

Raza.

Hi,
This is the non-energy refinery VDU i.e. a lube grade VDU. From the skecth it is clear that there is no reflux/liquid flow at the column top which is generally by a Top Pump Around. You can extract this excess heat through the pump arounds at top which is must to have the liquid traffic inside the column, now based on the % vaporization at the flash zone P and T and % yield required you can calculate the amount of heat to be recovered through pump arounds. The balance it will go the ejector condenser since you need to keep the column ovhd temperature a minimum value which will essentailly be above the water dew point temperature. Model the column in the commercially available simulator like Pro-II, with 2 pump arounds i.e. top and bottom and compare the results with your design specifications.

[raza0kid]

The schemes I am familiar with always have P/A draw trays as product draw-off trays at the same time. This simplifies the design (extra spacing required for these types of trays) and also ensures direct control of the product quality/rate, through adjustments of the pumparound flow and return temperature. Unless there are some really case-specific reasons, your product draw-off trays will be the P/A draw-off trays.

Depending on how much heat has to be removed at each section, what is capacity of the trays, and also depending on flow characteristics of distillates (i.e. pour point) you can see how much margin you have for setting up P/A flow and return temperature.

There are some interesting articles on vaccum distillation at: http://www.revamps.c...alArticles.aspx

Many thanks for your reply. I will surely check these articles. You mentioned "depending on how much heat has to be removed at each section", does it mean that I will have to apply P/A at each section (i.e. the top P/A, then one between spindle oil & SAE-20 and so on) ?
What I have done at the moment is that I have applied an overall heat balance on the column and got a value of reflux heat that has to be removed. Now how should i divide this value among different P/A? Also guide me about the return point of P/A back to the column.

Secondly, the number of trays that i have fixed for my column is 20 (including 4 bottom stripping trays) after going through "Watkins" and "Nelson", but i searched and found that Lube Refineries in my region have 32-35 trays in their column for the same products that i have in my project. So i would like to ask that is my column design practical?

[raza0kid]

Hello,

I am a chemical engineering student working on my final year project which is the "production of different grades of lube base oil through the vacuum distillation of atmospheric residue". The products coming out of my column are GAS OIL , SPINDLE OIL, SAE-20 & SAE-40 whereas the vacuum residue leaves the bottom.

I am using a tray column and after having literature survey I fixed the following parameters for my column:

• Flash zone pressure = 95 mm Hg
• Flash zone temperature = 750 F
• top pressure = 50 mm Hg
• Number of trays = 20
  
  • Spindle oil draw tray = 4th tray
  • SAE 20 draw tray = 8th tray
  • SAE 40 draw tray = 12th tray (trays are numbered from top to bottom)
• Pressure drop per tray = 3 mm Hg

I have calculated the side drawl temperatures for the column. After applying heat balance I came across with a certain amount of heat that I must extract through hot or circulating refluxes in order for the column to be thermally balanced.




Can anyone suggest me a scheme for extracting the desired amount of heat from my column through top reflux and pumparounds?

Attached are the temperature/pressure profiles and other relevant information.

I will be very thankful for your kind help.

Raza.

Hi,
This is the non-energy refinery VDU i.e. a lube grade VDU. From the skecth it is clear that there is no reflux/liquid flow at the column top which is generally by a Top Pump Around. You can extract this excess heat through the pump arounds at top which is must to have the liquid traffic inside the column, now based on the % vaporization at the flash zone P and T and % yield required you can calculate the amount of heat to be recovered through pump arounds. The balance it will go the ejector condenser since you need to keep the column ovhd temperature a minimum value which will essentailly be above the water dew point temperature. Model the column in the commercially available simulator like Pro-II, with 2 pump arounds i.e. top and bottom and compare the results with your design specifications.

Many thanks for your reply. If I wish to remove heat through P/A and not through top reflux, will I still be requiring a pre-condenser before the ejector system? What are the benefits of using a pre-condenser? Will it improve the ejector performance?

Edited by raza0kid, 29 August 2011 - 12:48 PM.

[Zauberberg]

Yes, the overall amount of heat to be removed should be redistributed between circulating refluxes as this results in optimized distribution of column vapor/liquid loads. If all the heat is to be removed by the top reflux or P/A alone, column diameter would be significantly higher than in the case when heat removal is distributed between several pumparounds.

Apart from the product draw-off rate, P/A are used for controlling product separation sharpness and their distillation range. For total draw-off trays (e.g. chimney) and towers with structured packing, this is the only concept I have seen so far - that each product must have its dedicated P/A as well, in order to achieve control of product quality. For columns equipped with trays it may be somewhat different as there is internal reflux (overflow) of liquid cascading down the trays - however there still must be an external heat sink where the required amount of heat must be removed from the column.

P/A return point (for trayed towers) is usually 3-5 trays above the draw-off tray. For packed towers the return point is on the top of dedicated packing, of course.

Not sure about the tray number, but check if Watkins was referring to theoretical or actual number of stages. Each column/system is different, particularly with respect to feedstock quality and fractionation requirements/product purity, so there is no universal answer on how many trays there should be in a vacuum tower.

Regards,

[raza0kid]

Hmm.. thanks alot Zauberberg.

[djack77494]

I would make two comments regarding this system. Trays are rarely used in high vacuum systems because of the high pressure drops consumed across a tray. My suggestion is to use packing instead, unless there are compelling reasons otherwise. Secondly, the overhead system (vacuum jets) withdraw small quantities of non-condensable gases from the vacuum column, but do not result in any sort of reflux stream. You may have a precondenser in the overhead system, and, if you do, it condenses mainly water to reduce the loading on the vacuum producing system. The water does not return to the vacuum column.