ETHANOL FROM SUGAR BEET - Sugar Beets - page 18

In a corn dry mill where three columns operate in series seemingly without double effect
and thermo-compression, r dist,th is 190 kg w · m − 3

beer [34].
The Finnish project in Switzerland would operate with three simple columns (no men-
tion of double effect or thermo-compression) [28].

The system requires r dist,th = 125

kg w · m − 3 and r dist,e = 3 kJ e · m − 3

beer . values seem too low.
In a Maguin Interis wheat dry mill operating with a single column at low pressure, the
specific consumption is r dist,th = 180 kg w · m − 3

beer

and r dist,e = 2 . 2 kJ e · m − 3

beer , which is beer

consistent with indications from the first study.
The distillation step in UNGDA factories is performed with two coupled stripping
columns using a double-effect (the condenser of one column is the reboiler of the other)
and thermo-compression [2]. Process steam is injected directly into the first effect. The

system requires r dist,th = 80 kg w · m − 3 and r dist,e = 11 kJ e · m − 3

beer . We assume that the beer

operation is achieved with this latter and efficient process.
Ethanol content in the final product is about 91.5% or 93% in volume.

4.6.6. Ethanol dehydration

In modern factories dehydration process is achieved with molecular sieves, which absorb
selectively water, instead of the binary distillation previously used. Sieves are regenerated
by heating or swing pressure.
Although of the same order, values of r dehy found in the literature present a large dis-
persion: r dehy = 0 . 34 kg st · l − OH and 68 kJ e · l − OH in a wheat dry mill of Maguin Co. [2],
r dehy = 0 . 55 kg st · l − OH and 68 kJ e · l − OH in a UNGDA factory [2], while r dehy = 0 . 49
kg st · l − OH and 12 kJ e · l − OH for ethanol produced from beet molasses according to indica-
tions of also Maguin Co. [29].
Only the latter provides information about the process: overheated ethanol vapor passes
through sieves and latter ones are regenerated by condensing outgoing steam to produce a
vacuum. For our analysis we retain this process.

4.6.7. Stillage concentration

Because of the high chemical oxygen demand COD of the beet stillage - nearly 1 kg of
COD per kg of DM - due to its soluble matter, methane can be produced through stillage
anaerobic digestion [2, 29]. Assuming an efficiency of COD digestion of 85% and a yield
of 12 MJ HHV · kg − 1

COD of methane, and taking into account stillage DM fraction of around
37 kg DM · t − A (See Subsection 3.3.), we would obtain a gain of around 370 MJ HHV · t − A .
Because of the high residence time for anaerobic fermentation (20 days), investment on
large tanks is necessary (for an ethanol factory of 0.30 M.m 3 · y − 1 , a total volume of 165 Ml
would be required). Electricity for agitators, pumps... would amount to about 10% of the
gas heat value. For these reasons we do not take into account this possibility.
Concentrating stillage DM from C DM = 10% to 52% requires to remove more than 8
kg w · kg − 1

DM of water, or w DM conc ' 300 kg w · t − A .
The operation is supposedly carried out with the same process used to produce syrup.