The present invention relates to a process for selective removal of diolefins from a gas stream, preferably from a fuel gas stream, which contains both diolefins and monoole- fins, prior to hydrodesulfurisation in a hydrotreatment re ^¬ actor. The selective removal of diolefins is carried out in a pre-treatment reactor located upstream the hydrotreatment reactor, in which organic sulfur compounds are converted to H ₂ S.

Diolefins can react to form polymers when a fuel gas stream containing them is passing through a hydrotreatment pre- heater and reactor. This diolefin reaction may cause severe fouling in the gas hydrotreatment preheater and reactor, and therefore it is necessary to remove the diolefins as completely as possible before they enter the hydrotreatment preheater and reactor.

In the refining industry, sulfur removal or recovery is a very important issue. Sulfur is among the most dominant contaminants in petroleum fractions, and legislation not only limits the permissible sulfur content of finished products, but also limits refinery emissions to the atmos ^¬ phere. Furthermore, there is a tendency towards the imposi ^¬ tion of even more stringent sulfur requirements for fuel gas streams. Therefore, sulfur removal and recovery is a vital process for refineries and gas plant operations. In most locations, the sulfur is hydrotreated and thus con ^¬ verted to hydrogen sulfide, which can be scrubbed from the various liquid or gas streams. The hydrogen sulfide collected from the hydrotreaters and/or gas plants can subsequently be treated, e.g. by the Claus process. As regards prior art in the field, US 8.921.630 describes a process for the removal of sulfur from a fuel gas stream that, in addition to organic sulfur compounds, contains di- olefins and oxygen. The fuel gas stream is treated in a pre-treatment reactor in order to significantly reduce the amounts of any diolefins and oxygen contained therein prior to the hydrodesulfurisation in a hydrotreatment reactor wherein organic sulfur compounds are converted to ¾S. More specifically, in the pre-treatment reactor the fuel gas stream is brought into contact with hydrogen in the pres- ence of a catalyst comprising a group VIb metal, such as

Mo, and a group VIII metal selected from Co and Ni on a po ^¬ rous refractory oxide support under mild hydrotreating conditions. These conditions include a temperature of 150 to 350°F (66 to 177°C), a pressure in the range of 50 to 400 psig and a gaseous hourly space velocity in the range from 0.05 to 4000 hr- ¹ .

A number of prior art documents specifically mention sup ^¬ ported catalysts comprising a group VIb metal and a group VIII metal as being well-suited for hydrotreating purposes. Thus, US 5.507.940 discloses a catalyst in which a liquid form of a silicon compound is incorporated into an alumina- supported group VIb and group VIII metal based catalyst and calcined in an oxidizing atmosphere. US 7.557.062 and US 7.749.937 also disclose hydrotreating catalysts based on a group VIb and group VIII metal based catalyst and a refrac- tory oxide material, which comprises more than 50 wt% tita- nia .

WO 2014/087364 discloses a hydrotreating catalyst espe- cially suited for preparing diesel-range hydrocarbons from a feed containing vegetable oils, comprising a group VIb and a group VIII metal impregnated on a non-refractory oxide as support. US 6.686.309 describes a catalyst for selective hydrogena- tion of unsaturated diolefinic compounds, which can also eliminate mercaptans . The catalyst comprises a particulate support selected from alumina, silica, silica-alumina, mag ^¬ nesia or mixtures thereof, palladium and at least one metal selected from molybdenum and tungsten.

Other supported catalysts of this type are disclosed in WO 2014/033653, US 7.968.069, US 6.306.289, US 7.557.062 and US 2013/0153467.

Catalysts based on a group VIb metal only, i.e. not con ^¬ taining a group VIII metal, are not known specifically for use in connection with gas hydrotreatment processes. They are, however, well known for use within other process fields. Thus, WO 02/32570 describes Mo-catalysts supported on bimodal alumina, which are useful for hydrodemetallation of heavy hydrocarbons. As sulfur components are converted into ¾S during a hydrotreatment process, metals will be deposited onto the catalyst as metal sulfides, which will poison or occlude catalytic metal sites that are predomi ^¬ nantly located in the catalyst pores, leading to rapid deactivation of the catalyst. Applicant's EP 2 334 757 Bl describes a process for the production of a hydrocarbon fuel from a renewable organic material by hydrodeoxygenation (HDO) . The HDO catalyst is an unpromoted supported Mo-catalyst with a Mo content of

0.1 to 20 wt%, which does not comprise Co and Ni . The support is selected from alumina, silica, titania and combina ^¬ tions thereof, and it has a bimodal porous structure with pores having a diameter as measured by mercury intrusion porosimetry larger than 50 nm, that constitute at least 2 vol% of the total pore volume. By using a carrier with a bimodal pore distribution, the catalyst is more resistant to pore plugging and minimizes increases in pressure drop and deactivation rate.

It has now surprisingly been found that a supported Mo-containing catalyst as described in EP 2 334 757 Bl, that does not contain Ni or Co, i.e. a group VIb metal without a group VIII metal, can effectively convert diolefins to ole- fins at a temperature between 140°C and 180°C, a pressure in the range of 20 to 45 barg and a gaseous hourly space velocity up to 22500 NL/kg/h.

More specifically, it has been found that a Mo-only cata- lyst, such as the one developed for fixed-bed hydrodeoxygenation (HDO) service, effectively converts diolefins into olefins with only negligible olefin conversion in the temperature window T = 140-180°C. This is relevant for fuel gas streams, which contain high levels of olefins, because of the highly exothermic nature of the olefin hydrogenation . If a substantial olefin con ^¬ version takes place over the pre-treatment reactor having high levels of olefins (i.e. > 4 vol%) , a considerable temperature rise may take place, reaching temperatures where diolefins start to polymerize. Furthermore, since the olefin conversion is minimized and primarily diolefin hydro ^¬ genation takes place, the exotherm over the pre-treatment reactor can be controlled meticulously. This enables de ^¬ signing the system layout with just a simple low-cost fired heater.

The main technical novelty of this approach lies in a modification of the pre-treater catalyst to selectively treat diolefins rather than monoolefins in order to provide ap- propriate temperatures in the hydrotreatment reactor in a cost-effective way.

Thus the present invention relates to a process for hy ^¬ drotreatment of a gas stream containing both olefins and diolefins as well as organic sulfur compounds, said process comprising :

- introducing the gas stream into a pre-treatment reactor, where diolefins are reacted with hydrogen in the presence of a supported Mo-catalyst not containing Co or Ni at a temperature of 140-180°C, a pressure of 3-45 barg and a gaseous hourly space velocity up to 22500 NL/kg/h, whereby the diolefins are substantially converted to olefins, - introducing the gas stream, now depleted in diolefins, into a hydrotreater reactor having a higher inlet temperature than the pre-treatment reactor, in which the fuel gas stream is contacted with hydrogen in the presence of a hy- drotreating catalyst under hydrodesulfurisation process conditions, whereby the olefins are substantially converted to paraffins and the organic sulfur compounds are converted to H ₂ S, and

- subjecting the hydrotreated gas to a chemisorption or physisorption treatment to remove the ¾S. The gas stream is preferably a fuel gas stream.

It is preferred that the gas stream has a diolefin content between 2 ppmv and 2 vol%, and it is further preferred that the gas stream contains olefins up to a level of 20 vol%.

It is especially preferred that the gas stream has an ole ^¬ fin content of 1-15 vol%.

The support is preferably selected from alumina, silica, titania and combinations thereof.

The supported Mo-catalyst preferably has a Mo content of 0.1 to 20 wt%. The gaseous hourly space velocity in the pretreatment reac ^¬ tor preferably is between 500 and 10000 NL/kg/h, most pref ^¬ erably between 1000 and 7000 NL/kg/h.

The invention is illustrated further by the example which follows.

Example A simulated fuel gas containing 325 ppmv 1 , 3-butadiene, 1.3% ethane and 1.3% propene was passed over a catalyst bed of an alumina-supported catalyst containing Mo, but not containing Ni or Co, at a gaseous hourly space velocity of 22472 Nl/kg/h.

The detailed gas composition is given in Table 1 below.

Table 1

Detailed gas composition

The operating conditions are given in Table 2 below.

Table 2

Operating conditions

Pressure 20-40 barg

Temperature 140-220°C Catalyst load 4.45 g

Flow 100 Nl/h

The measured conversions of 1.3-butadiene and olefins (eth- ene and propene) at a pressure of 20/40 barg for a tempera- ture of 140-220°C are shown in the figure. The butadiene is primarily converted to the corresponding olefin. It was found that the selectivity of the conversion of butadiene to butenes was above 85% at all conditions tested.