Distillation - mechanical recompression
Distillation - mechanical recompression
(OP)
I was wandering about the technical and economical feasability of using top vapours stream of a rectification column of isopropanol as heating agent of its own reboiler, after a mechanical compressor.
I need some help in order to find out who supplies such equipment and who has already implemented such modification in an existing column. Costs? Power?
By the way the vapour stream leaving the column is about 11,000 kg/h at 80oC.
Thanks
I need some help in order to find out who supplies such equipment and who has already implemented such modification in an existing column. Costs? Power?
By the way the vapour stream leaving the column is about 11,000 kg/h at 80oC.
Thanks
fvincent





RE: Distillation - mechanical recompression
HTH
Dick Russell
RE: Distillation - mechanical recompression
I've just seen a site that deals with this type of integration:
http
that may interest you.
RE: Distillation - mechanical recompression
I think the preheating of the feed is more efficient with the bottom stream, isn't it. In my case, I am considering increasing the effectiveness of this preheater, since bottom stream leaves the column at nearly 100oC and the feed after the preheater is only at 62oC (isopropanol at 60%w)
As to the compressor, I have read a lot too, but actual costs and power are still missing. I have asked proposals for the compressor but no answer came.
That is why I was wondering if someone could give a hint about the equipement to pressurize 10.5 t/h of an azeotropic mixture of isopropanol and water which leaves the top of the column at 80oC and at 0.94 bar absolute. I think I should pump the vapours at least up to 3.2 bar abs in order to obtain a 15oC temperature difference between condensing IPA steam and bottom temperature.
Thanks for the comments and suggestions you gave.
fvincent
RE: Distillation - mechanical recompression
RE: Distillation - mechanical recompression
regards
fvincent
RE: Distillation - mechanical recompression
RE: Distillation - mechanical recompression
COMPRESSOR BLOCK PC101 Overhead vapor compressor, PC-101
GAS HORSEPOWER = 443.4
POLYTROPIC EFFICIENCY = 72.0 %
DRIVER POWER = 330.6 KILOWATT AT 100% DRIVER EFFICIENCY
INLET VOLUME = 6922.8 CU.M/HR, LIQUID FRACTION =0.0000
INLET CONDITIONS OUTLET CONDITIONS
STAGE TEMP(C) PRES(BARA ) Z TEMP(C) PRES(BARA ) Z AVG.K
1 80.0 0.9400 0.9854 142.2 3.200 0.9677 1.105
POLYTROPIC HEAD PER STAGE (METERS): 8323.
HEAT EXCHANGER BLOCK TT101 Condenser/Reboiler, TT-101
DUTY= 2.1664 MM KCAL/HR
HOT SIDE INLET TEMP= 142.2 C, L/F=0.0000, FLOW= 10500.00 KG/HR
VAPOR DENSITY= 4.471 KG/M3
OUTLET TEMP= 110.9 C, L/F=1.0000
LIQUID DENSITY= 716.87 KG/M3
The efficiency will affect just the discharge temperature, not the condenser approach. Assuming the bottoms of the column is just water at 1 atm, or 100 C, and you wanted a 15 C approach, then you would need a discharge pressure of 3.63 bar(a), and the results look like this:
COMPRESSOR BLOCK PC101 Overhead vapor compressor, PC-101
GAS HORSEPOWER = 492.2
POLYTROPIC EFFICIENCY = 72.0 %
DRIVER POWER = 367.0 KILOWATT AT 100% DRIVER EFFICIENCY
INLET VOLUME = 6922.8 CU.M/HR, LIQUID FRACTION =0.0000
INLET CONDITIONS OUTLET CONDITIONS
STAGE TEMP(C) PRES(BARA ) Z TEMP(C) PRES(BARA ) Z AVG.K
1 80.0 0.9400 0.9854 148.6 3.630 0.9648 1.105
POLYTROPIC HEAD PER STAGE (METERS): 9239.
HEAT EXCHANGER BLOCK TT101 Condenser/Reboiler, TT-101
DUTY= 2.1582 MM KCAL/HR
HOT SIDE INLET TEMP= 148.6 C, L/F=0.0000, FLOW= 10500.00 KG/HR
VAPOR DENSITY= 5.009 KG/M3
OUTLET TEMP= 115.0 C, L/F=1.0000
LIQUID DENSITY= 710.99 KG/M3
HTH
RAR
RE: Distillation - mechanical recompression
Thanks a lot
you deserve my seldom given star!
fvincent
RE: Distillation - mechanical recompression
The pressure you gave is slightly in vacuum at the top. Why not make it atmospheric to avoid air leakage? Air leakage will hurt the tail of the condensation, requiring higher pressure and a vent condenser (which you likely have now anyway).
To expand on your earlier response to a question on the use of electrical work to do reboiling indirectly, the output shows the "air conditioning" effect; expending 0.316 MMkcal/hr of electric power input "lifts" the latent energy of the overhead vapor to a temperature/pressure where it can be extracted usefully, flowing "downhill" (in temperature) to boil the column bottoms. The higher the "lift", the more energy it takes and the lower the economic benefit.
Of course, the thermal condition of the feed will affect the relation between condenser and reboiler duties. If the feed is subcooled, you'll need more reboil duty than will be available by condensing the (compressed) overhead vapor. If the feed is partially vaporized, you will need a trim condenser to handle the excess vapor energy. A feed/bottoms exchanger could help improve on the first situation.
RE: Distillation - mechanical recompression
The feed preheating will be improved. Currently the exit stream leaving the bottom tray preheats the feed to only 66oC. Since bottom temperature is approximately 100oC (only water a little below 1.0 bar abs), it is possible to recover additional heat, at least preheating the feed up to the inlet tray temperature (near 90oC).
The column operates today with direct steam. So, if the heat pump is installed, the indirect boiler will be required. The total bottom stream will obviously reduce (only a small direct steam condensate will exist). Feed preheating has to be re-evaluated, of course.
I had used a simpler approach for calculating power. I considered (in fact - omitted) that the azeotropic point of isopropanol+water would not change with the pressure elevation. Furthermore, according to the saturation curve of pure isopropanol, the pressure corresponding to 115oC would be 3.16 bar abs. You showed that the pressure had to be higher.
I have also considered fully adiabatic compression instead of polytropic and finally - since it was a short calculation - I assumed a Z=1.
I thank you very much again.
Now I have to look for costs. I don't have the slightest idea of the price of such compressor (which has to be oil free). Some suppliers I contacted are really late...
I wished they could give the numbers as promptly as you gave..
regards
fvincent
RE: Distillation - mechanical recompression
I presume you are just roughing out whether or not the concept is at all in the ballpark of profitability. If it does seem so, be sure to engage the services of a good engineering outfit to do a more detailed evaluation and cost estimate. The detailed engineering costs alone could be considerable.
RE: Distillation - mechanical recompression
fvincent