Molecular Sieves
Ethanol Dehydration Unit
Process Description
Doc. No. R.PD.09.01.A0
July 2004
1.0
Process Decription
1.1
General
Unit 900 - Dehydration (see DWGs R.PI.09.01.F0 and R.PI.09.02.F0 )
Operating principle
Technology proposed for ethanol dehydration is based on a physical/chemical process
called adsorption.
In this process water molecules are trapped on the surface of adsorbing solid, called
molecular sieve, removing water from the mixture to be dehydrated.
During the adsorption process molecular sieves tend to become saturated, that means, in
order to restore their dehydration capabilities, after the adsorption phase a regenerative
one must be carried out.
Proper process conditions ensure the above-mentioned operations taking place.
Plant operation
Raw alcohol is fed to the top of evaporation / regeneration column in order to be vaporised,
after being preheated by the dehydrated product.
Overhead vapours are sent, after superheating, through a bed containing the molecular
sieves.
Outcoming dehydrated alcohol is then condensed, providing heat recovery, sub -cooled and
pumped to the storage.
During the adsorption process, water is fixed by the molecular sieve, when the bed
approaches the saturation conditions, it must be regenerated, the adsorption process
continues then on a second bed.
The regeneration is carried out working under vacuum conditions and using part of the
dehydrated alcohol as regeneration media.
Two reactors, in which alternatively the adsorptive and regenerative phases are carried
out, are then required in order to guarantee continuous plant operation.
The alcohol used for the regeneration, containing the water removed from the bed, is
recycled back to the distillation column from where water is finally taken out from the
bottom and discharged.
Process Description - Rev. 0 - FS/fs - Pag. 2
1.2
Detailed Description
Column Operations . Raw ethanol from storage is pumped by means of pump P-900 to the
dehydration unit.
After preheating in E-.914 it enters at the top of column C-910.
The ethanol flow is automatically controlled by FCV-910.
The ethanol is then removed as overhead vapour form the colum and reach the molecurar
sieve after being superheated in E-.905.
The pressure control valve PCV-911, installed on vapour line allow the dampening of the
pressure variation due to the switching of the moleculare sieve reactors from vacuum to
overpressure conditions.
The temperature of ethanol superheating is controlled adjusting the steam flow by means of
control valve TCV-926.
The column heating is carried out by direct steam injection, the steam rate is controlled by
automatic valve TCV-920 that will adjust the steam requirements on the basis of the signal
coming from the column sensible tray.
Stillage from column bottom free from ethanol are pumped out by means of P-910, cooled
in E-918 and sent to the battery limits. The discharge is carried out under leve control, by
means of LCV-901.
Oil extraction. It may occur, depending on the quality of raw etanol fed to the system, that
some fusel oil tend to build-up into the distillation column. In order to avoid an excessive
accumulation that can affect negatively the equilibria inside the column, once the plant has
been operated for some time, it would be advisable, start the fusel oil extraction system.
By means of flowmeter FI-901, a purge stream rate is fixed, and send to a vaporization
system realized by a separator connected to a plate heat exchanger vaporizer. The level is
kept costant inside the separator adjusting the steam rate valve LCV-904.
Reactor Operation.
The reactors operate alternatively under adsorption and under regeneration. The presence
of two reactors allow a continuous processing switching from one phase to the other.
Process Description - Rev. 0 - FS/fs - Pag. 3
A routine programmed in the control system will allow the realization of the sequences in
accordance to the steps described in the documents : I.AF.09.00.D0 .
The column overhead vapours after superheating enter the reactor under overpressure ( in
adsorption), water molecules remain trapped into the beds, while the outcoming dehydrated
ethanol is sent to the condensation. The pressure is kept into the system by means of the
pressure control valve PCV-914.
Before the PCV, a certain amount of the dehydrated ethanol is sent to the other bed for the
regeneration. This amount is measured by flow transmitter FE-907, and manually adjusted ( only
once time ) may means of manual valve V053. Moreover this flow is not continuous but impulsive,
impulse are created opening and closing automatically valve BV-905.
The bed under regeneration is working under vacuum conditions, vacuum and low water partial
pressure in the regeneration stream will result in a desorption of the water from the bed to the
vapour stream.
Such stream after flowing through the reactor is sent to the vacuum condenser E-906, where after
condensation, is recycled back to the distillation column for reconcentration. Recycle is carried out
by means of pump P-902 that extracts diluted ethanol under level control from the accumulator D902 located below vacuum condenser.
After certain time the bed under adsorption is saturated and the other one under regeneration is
ready to start a new adsorption cycle. The switching procedure is then started in accordance to
the procedure given in document no. I.AF.09.00.D0 and a new cycle begin.
Vacuum is realized by means of a liquid ring vacuum pump which operates in a partial recycle
circuit. An inter cooler will reduce substancially the amount of purge/makeup.
Dehydrated ethanol after condenstation is collected into D-901, and from here after subcooling in
E-916, is sent by means of P-903 to the final storage. Extraction is carried out under level control
by means of LCV-903.
Process Description - Rev. 0 - FS/fs - Pag. 4