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.