5 replies to this topic

[flangecoupling]

Dear All,

 

I am studying deisobutaniser column in alkylation unit of a refinery. As I looked at the column bottom configuration / temperature profile in the documents the situation looked unusual, Following are my observations / queries.

 

1. Column bottom is baffled.

2. The liquid from the bottom tray is at around 160 deg C.

3. The reboiler outlet is at 186.3 deg C.

4. Reboiler inlet is at 173 deg C.

5. Bottom product is at 186.3 deg C.

 

The column bottom configuration is given in the attachment.

 

My queries are as follows

 

1. Query 01:

That even though the bottom product and reboiler outlet is at same temperature the composition of the reboiler outlet liquid and the bottom product is not same as per the H&MB document. How is this possible?.

 

1. Query 02:

After a long run the temperature of the bottom should come to saturation and bottom product and reboiler inlet should be at same temperature, but the H&MB shows different temperatures as mentioned above.

 

Attached Files

•  Deisobutaniser.docx   37.37KB   20 downloads

[shan]

1.  The reboiler outlet stream is two phase flow with vapor and liquid.  The bottom product is one liquid phase flow only.  The bottom production composition should be identical with reboiler outlet stream liquid phase composition.

 

2.  The baffle divides the tower bottom into two sections.  The left section is for supplying reboiler inlet and the right section is for receiving reboiler outlet.  The temperature difference and vapor generation is because of reboiler heat.

[rabindra]

1. The reboiler outlet stream is a mixture of liquid and vapor. On entry to the column bottom section after reboiling, it flashes and the liquid and vapor gets separated. While liquid comes down to be drawn off as a product, vapour goes up. In the heat and mass balance, the product should be a little heavier than the reboiler outlet stream.

 

2. May be because part of the reboiler outlet liquid mixes with last tray bottom liquid and the resultant temp is in between the two. Not sure about it though.

[flangecoupling]

Dear Shan, 

 

1. Yes you are right, and I compared the liquid phase composition of the reboiler outlet with the bottom product. But the composition is not same yet the temperature is exactly same.

 

2. Yes I agree. But if you see the attached document there is a deflector baffle which will deflect all the liquid coming from reboiler to reboiler inlet side. My question is that than how the bottom product has same temperature as reboiler outlet (while the deflector baffle is there).

[flangecoupling]

Dear Rabindra,

 

1. Lets assume that flashing occurs sufficiently so that composition of reboiler outlet and bottom product becomes different, but than why the temperature are exactly same?.

 

2. Yes this point is correct reboiler outlet mixes with last tray liq so that temperature of reboiler inlet is in between the two temperatures. But the temperature of bottom product is same as reboiler outlet despite of the deflector baffle ( the figure is in the attached document ). 

[shan]

1.  If there is pressure difference between reboiler and bottom of tower, there may be some light compositions flashing into vapor phase or heavy compositions condensing into liquid phase, which may result the composition difference between the reboiler outlet liquid and the bottom product.

 

2.  Your deflector baffle should not across the baffle to extend to the left section. All the liquid from the reboiler outlet should contain in the right section as the bottom product.  The reboiler outlet vapor condenses into the bottom tray liquid in the left section and heats up the bottom tray liquid in the left section from 160C to 173C as the reboiler inlet.