(395ap) Denatured Bio-Ethanol Feedstock Desulfurization By Adsorption Onto a Nickel Containing Solid

INTRODUCTION

IFP Energies nouvelles
has developed a new hydrogen production system from denatured bio-ethanol
liquid feedstock. Ethanol appears as a very promising hydrogen source due to
its low toxicity, easy handling, transportation and storage and its wide
availability. Moreover, using bio-ethanol produced from biomass sources has
almost no impact on the greenhouse effect.

The objective is to produce pure hydrogen flow
between 50 and 100 Nm³/h from this renewable feed for medium scale
industrial customers such as electronic, metals processing and oil
hydrogenation industries. The conversion of bio-ethanol to hydrogen is
performed using a catalytic auto-thermal reformer (ATR) working under pressure
which combines the exothermic reaction of ethanol with oxygen (partial
oxidation) and the endothermic reaction of ethanol with water (steam
reforming). The reactions, leading to synthesis gas (syngas
: mixture of H2, CO and CO2) production, can be described as follow :

C₂H₅OH + ½O₂ => 2CO + 3H₂

C₂H₅OH + H₂O
=> 2CO + 4H₂

At the outlet of this reforming section, the
hydrogen-rich effluent gas contains impurities, in particular carbon monoxide
CO. In order to purify this syngas and to increase
the hydrogen yield, the conversion of carbon monoxide is performed using the
water gas shift reaction (WGS) :

CO + H₂O => CO₂ + H₂

A pressure swing adsorption unit is then used
to purify the reformate gas. Using adsorption and desorption cycles over adsorbent beds (zeolite
LTA 5A) allows to remove contaminants. Pure hydrogen stream (purity of 99.95 %
vol.) is then produced.

For legal reasons, the ethanol that is not used
in the food sector is denatured by the addition of a chemical substance, called
the denaturing agent, and that makes it unfit for consumption. This denaturing
agent can be a natural gas condensate, or, more often, a gasoline. In this
case, this ethanol feedstock very often contains undesirable substances such as
sulfides or mercaptans
(condensate), or thiophenic compounds (gasoline). The
presence of these sulfur-containing compounds is very
disturbing because they can rapidly poison the catalysts used either in the ATR
reactor or in the Water Gas Shift one. Therefore the feedstock has to be desulfurized before its reforming.

The aim of this study is to develop an
adsorption process in liquid phase able to remove thiophenic
compounds from the denatured bio-ethanol, by using adsorption over a nickel
based solid.

EXPERIMENTAL

Typical denatured ethanol has to contain at
least 92,1 vol. % of ethanol, a denaturant amount comprised between 2 and 5
vol. %, and no more than 1,7 vol. % of aromatics and 10 ppm
mass of total sulfur. In this study, experiments have
been carried out with synthetic model feeds, composed of pure ethanol (98 vol.
%), toluene (2 vol. %) as the aromatic component of the denaturant and possible
adsorption competitor, and thiophene (100 and 20 ppm "S") as the sulfur
component.

All the experiments have been performed on a
laboratory scale breakthrough curve unit in dynamic conditions, with a column
of around 20 cm3, placed in an oven (350 °C max.). The synthetic feed is introduced
under pressure, in order to be always in liquid phase, at a constant flow-rate
at the column inlet containing the adsorbent, and the effluent is collected at
the column outlet and then analysed by gas chromatography in order to determine
the thiophene concentration versus time. A simple
mass balance leads to the amount of thiophene
adsorbed on the solid.

The solid used in this study is mainly composed
of a mixture of nickel and nickel oxide deposited on a silica and alumina
support, and is supplied in a stabilized form (cylindrical extrudates).
The amount of nickel is around 55 mass %, and the specific surface area is 175
m2/g.

The main parameters which have been taken into
account are:

- solid pre-treatment, consisting either in an
activation step under nitrogen or a reduction step under hydrogen, at
temperatures comprised between 120 and 300 °C

- adsorption temperature, comprised between 30
and 160 °C, with both activated and reduced solids

- influence of the presence or not of the
aromatic inhibitor (toluene)

- thiophene
concentration in the feed

RESULTS

a/ Adsorption temperature influence with an activated
or reduced solid at 300 °C

A first set of experiments performed on a feed
with toluene and 100 ppm "S" shows that a
minimum adsorption temperature of 120 °C is necessary with a solid activated at
300 °C. The sulfur breakthrough point occurs after
elution of 17 column volumes at 120 °C, and is not observed after elution of 34,0
column volumes at 160 °C. At lower adsorption temperatures, sulfur
breakthrough is quite immediate.

A second set of experiments carried out with
the same feed but with a reduced solid at 300 °C shows that the sulfur breakthrough occurs after elution of around 20
column volumes with adsorption temperatures from 30 to 120 °C, while it is not
observed at 160 °C. Reduction appears therefore more efficient than activation.

b/ Activation or reduction temperature

With an activated solid at 120 or 150 °C, sulfur breakthrough is observed immediately or rapidly
whatever the adsorption temperatures, 120 or 150 °C.

When the solid is reduced at 120 °C, sulfur breakthrough is also observed immediately at 120°C.
Its efficiency increases dramatically when it is reduced at 150 °C, sulfur breakthrough being observed after elution of 23,6
column volumes with an adsorption temperature of only 80 °C, and after more
than 34,0 column volumes at 150 °C.

c/ Toluene concentration

Toluene acts as a slight sulfur
adsorption inhibitor. With a reduced solid at 300 °C and an adsorption
temperature of 80 °C, the sulfur breakthrough occurs
after the elution of more than 34,0 column volumes without toluene and 28,6
column volumes in presence of toluene (2 vol. %).

d/ Thiophene
concentration

With a sulfur
concentration of only 20 ppm "S" in the
feed, an activation or reduction temperature of 300 °C and an adsorption
temperature of 80 °C, sulfur breakthrough occurs
after elution of larger feed volumes : 3,4 (100 ppm
"S") and 32,5 (20 ppm "S") column
volumes with the activated solid, 29,6 (100 ppm
"S") and more than 34,0 (20 ppm
"S") with the reduced solid.

CONCLUSIONS

The aim of this work was to study and develop a
pre-treatment adsorption step in liquid phase in order to desulfurize
a denatured bio-ethanol feedstock prior to ATR and WGS reactions in order to
produce hydrogen.

Experiments carried out on a laboratory scale
with a model synthetic feed have shown that the use of a solid containing both
nickel and nickel oxide deposited onto an alumina-silica support was efficient
for this purpose. Nevertheless the pre-treatment of the solid is very important
in order to improve its efficiency in the sulfur
removal. A reduction step at a temperature of 150 °C is necessary, and the
adsorption temperature has to be at least of 80 °C. Higher temperatures of
reduction and adsorption lead to a better efficiency. By comparison, a simple
activation step does not lead to a great efficiency of the solid.