3 replies to this topic

[82ghost82]

Good morning all,
I have to design a plant (for university) to product DME, from dehydration of methanol (2 methanol->water+DME).
My plant has to product 5000 tonne/day of DME.
Dehydration of methanol occurs in 2 reactive distallation column (2 because there's a flooding problem, and column diameter cannot be more than 8 m);each column has to product 2500 tonne/day of DME (from partial condenser, 2500 tonne/day vapor DME).

From energy, material balances and simulations I calculated that I need for each column -99026480 Watt at the condenser and 104542668 Watt at the reboiler.
Here are some datas for one column:

Number of trays: 30
Number of reactive trays: 13 (from stage 8 to 20)
Reboiler: kettle (stage number 30)
Condenser: partial (stage number 1)
Feed: on stage 8
Reflux: 9 (molar)

FEED:
3484,6 tonne/day (0.2% water, 99.8% methanol-->weight fraction)
Temperature: 120 °C
Pressure: 9 bar (equal to column pressure)

FROM THE SECOND STAGE TO PARTIAL CONDENSER:
Vapor flow: 24999,6 tonne/day
Vapor compositions (molar fraction):
-methanol: 5,44E-05
-DME: 0,9999456
-water: 7,76E-10

Condenser temperature: 40,27 °C
Condenser pressure: 9 bar
Second tray temperature: 40,29 °C

Reboiler:
from the 29th stage to reboiler:
liquid flow: 5424,5 tonne/day
Liquid compositions (molar fraction):
-methanol: 6,09E-05
-DME: 1,27E-14
-water: 0,99993908

Reboiler temperature: 175,44 °C
Reboiler pressure: 9 bar
29th tray temperature: 175,43 °C

Vapor flow from reboiler to column: 4439,92 tonne/day
Vapor compositions (molar fraction):
-methanol: 7,15E-05
-DME: 1,54E-14
-water: 0,99992846

Liquid flow out of the reboiler: 984,61 tonne/day
Liquid compositions (molar fraction):
-methanol: 1,30E-05
-DME: 4,05E-16
-water: 0,99998695

Utility: cooling water to condenser:
30 °C (IN); 42 °C (OUT)

Site temperature: -38°C [min. winter temp.]; 38°C [max summer temp.]

Problems are in condenser and reboiler: too much heat (about 100 MW/-100 MW)..I don't know how to solve the problem, how to design reboiler and condenser (too big!)..any ideas??

Thank you all.
Daniele.

[abhi_agrawa]

Daniele,
From your data it seems that you are designing quite close to the minimum number of required trays. Therefore your reflux requirement is quite large. I'd suggest that you increase the number of fractionation trays, that should decrease the duty required.

hope this helps,
abhishek

[82ghost82]

Thank you for your reply abhi_agrawa.

I think I can put a compressor that can compress the gas flow till 25-29 bar at 85-90 °C, a total condenser (in this case I have a major deltaT ml with water utility), an hydraulic turbin that expand the flow to 9 bar and a flash wich divide gas and liquid flow; with this configuration I can have a 65-70% liquid flow to reflux to the column..any other ideas?

[abhi_agrawa]

Daniele,
The scheme you are suggesting seems to me lke a vapor-recompression scheme. I'd say that instead of condensing the vapor from column overhead against cooling water, see if you can utilize it as a heating media for you reboiler.
I'll say that you investigate increasing the number of theoretical trays required for the separation. What I usually do is simulate at several number of theoretical trays, draw a curve of reflux ratio against the number of trays, and from the curve try to get the optimum number of trays for the column.

abhishek