Design limit for CY structured packing in vacuum distillation

[plantprowler]

I was trying to size a column for vacuum distillation (10 mmHg) that uses Sulzer CY type packings. I'm using Sulocol, the vendor's sizing program and one aspect confuses me:

For my column size the flood percent is quite low (40%) yet the pressure drop per unit meter (3.04 mbar/m) seems to be getting flagged in red.

Are these independent constraints? Will I be forced to operate at 40% of capacity? Or is the "red" flagging of mbar/m only an indicative warning?

If it matters the Pressure is 10 mm Hg and the column traffic is 1200 kg/hr gas & liq. Dia I am trying is 1200 mm. F factor is 1.32.

 

[don1980]

don1980 editing note: I edited this post to strike-through statements I made which are incorrect. The minimum pressure is at the top of the column. Sorry for the misleading and incorrect info in my original message.

Recognize that your operating pressure (10 mm Hg abs) is extremely low - that's a very deep vacuum. At this pressure, the vapor density is also extremely low which means you're handling a very high volume of vapor. The amount of allowable pressure drop (from bottom to top) is physically limited to 10 mm Hg. If the dP was higher than 10 mm Hg, then you obviously can't be operating at 10 mm Hg abs at the top of the column.

According to the Sulzer algorithm, your vapor load isn't impeding the downward flow of liquid (it's not close to flooding), but this vapor load is resulting in a physically impossible set of operating conditions. That is, you can't operate at 10 mm Hg at the top of the column while also experiencing 3.04 mm Hg pressure drop per meter. You have 6.6 m of packing height, so the calculated pressure drop is 6.6m * 3.04 mm hg per m. Thus, that amount of pressure drop isn't possible if the column is operating at 10 mm Hg abs.

 

[georgeverghese]

The unit pressure drop of 3.04mbar/m seems okay, which converts to about 0.38inH20/ft. When you read through the narratives on page 14-41 and 42 in Perry 7th ed when compared with the generic limit of 1.5inH20/ft for 100% flood. Try to locate this operating point on the Eckert flood factor map and see if this tells you why this dp/m comes up red.There may be some advice in the simulator help manual also.

So, if the column top pressure is 10mmHg abs, then column bottom pressure would be 25.14mmHg - are you comfortable with this ? Where is the feed and what pressure would it be at this location - is there a pressure discrepancy here?

By the way, this Sulzer CY structured packing is listed with a packing factor Fp of 69/m in table 14-7a in this 7th edn of Perry. What is the meaning of this factor F in your output ?

 

[plantprowler]

@don

Thanks very much. So essentially under these conditions my column diameter is not flooding constrained but pressure drop constrained?

A follow up question:

For arguments sake assume this was a 100 mm Hg abs. pressure distillation. Still Sulcol warns me of the pressure drop the moment it exceeds 3 mbar/m. The column is still far from flooding (62% of flood point) (see screenshot below)

Which begs the question: In addition to the flooding limit is there an independent limit on the absolute pressure drop per m of packing? Why? (In fact, there seems to be a tooltip that's saying for this packing that Max dp is 3 mbar/m)

Also, note that the Sulcol interface never really asks me for the absolute pressure I want to operate at. Just the density that I compute for it. So it has no way of flagging (say) an unrealistic 18 mm Hg drop over a column operating at 10 mm Hg. What it seems to be doing is just flagging the dp-per-m of packing.

 

[plantprowler]

@george

Thanks for the tips.

I checked out the manual. No help there regarding this particular limit.

Regards your other question: I believe the F here is what's called the "F factor". My second screenshot shows this more clearly.

 

[georgeverghese]

Strange why this program doesnt re compute the gas density value at the bottom of the tower when it has computed the pressure drop. For instance, in the new output for column operation at 100mmHg at column top, why is the gas density the same for both column top and bottom ? This discrepancy would havea bigger effect for the previous run where you had 10mmHg at column top.

Okay, it looks like this F factor is the square root of rho-V2, but I still dont see what the significance of this is in this column hydraulics. If this Sulzer CY packing also has an upper limit of 3mbar/m in addition to the typical flood factor limit, then it would be better to keep within both these limits. Would be good if you could get some clarification from Sulzer why this dp/m limit is so low for CY packing (and why there is no internal adjustment to column gas prpessurre at col bottom ) in the meanwhile.