FCC (fluid catalytic cracking) is a refining process that converts gas oil and other heavy feedstocks into lighter, more valuable products, such as gasoline, olefinic gases and other products. Cracking of petroleum hydrocarbons was originally done by thermal cracking, which has subsequently been almost completely replaced by catalytic cracking be- cause that type of cracking produces more gasoline with a higher octane rating. It also produces by-product gases that have a larger number of carbon-carbon double bonds (i.e. more olefins), and hence a higher economic value, than those produced by thermal cracking. The feedstock to the FCC process is usually the portion of the crude oil that has an initial boiling point of 340°C or higher at atmospheric pressure and an average molecular weight that ranges from about 200 to about 600 or higher. This portion of the crude oil is often referred to as heavy gas oil or vacuum gas oil (HVGO) . In the FCC process, the feedstock is heated to a high temperature under a moderate pressure and then brought into contact with a hot, powdered catalyst. The catalyst breaks the long-chain molecules of the high-boiling hydrocarbon liquids into much shorter mol ecules, which are collected as a vapor. Basically, the FCC unit (FCCU) consists of a reactor and a regeneration unit. The cracking of the heavy hydrocarbon molecules is carried out in the riser section of the reac tor, while the oxidation of the coke deposited on the cata lyst during cracking in the riser takes place in the regen eration unit. According to Polato et al . , Catalysis Today 133-135 (2008), pages 534-540, feeding a sulfur-containing feedstock into an FCC unit without pre-treatment will lead to about half of the total sulfur presented being released as ¾S, while a little less stays in the liquid products as sulfur compounds. About 5% of the total sulfur will be left on the catalyst as part of the coke to be converted into sulfur oxides (around 10% SO3 and 90% SO2) that are re leased to the atmosphere during the catalyst burn-off in the regeneration unit. These SO _x gases are major atmos pheric pollutants causing acid rain.

In principle, emission of SO _x gases can be addressed either by feed hydrotreating prior to the FCC process or by scrub bing flue gases. However, not all refinery plants have these capabilities, and their introduction into a refinery plant involves very high financial investments. Thus, SO _x transfer additives constitute a very convenient solution as they do not require altering of the existing infrastructure in any way. They can simply be added together with the FCC catalyst during standard FCC process operation.

Considering the reduction of SO _x emissions in the FCC unit, the least costly strategy is the addition of SO _x transfer additives to the FCC catalyst. Those additives adsorb SO _x and so transfer sulfur back into the riser, where it is re leased as ¾S which is removed the usual way (through the Claus process) . This technique is very practical since the use of additives requires almost no capital investment, ex cept for the cost of an additive loading system and the availability of a Claus plant. Three steps determine the performance of an SO _x transfer catalyst: (1) the oxidation of SO ₂ to SO ₃ under the FCC regenerator conditions (typi cally at 680-730°C); (2) the trapping of SO ₃ on the cata lyst in the form of sulfates, and (3) the reduction of sul fates to release sulfur as ¾S in the FCC riser (typically at 520-530 °C) .

While FCC processes are efficient as regards catalyst use, the regeneration step typically results in the evolution of undesirable gases such as SO _x , CO and NO _x . Various attempts have been made to limit the amounts of these gases created during the FCC regeneration step or otherwise to deal with the gases after their formation. For instance, magnesium aluminate spinel additives are often used to prevent or minimize emission of SO _x from the regenerator. Also, vari ous noble metal catalysts have been used to minimize the emission of CO from the regenerator. Unfortunately, the ad ditives used to control CO emissions typically cause a dra matic increase in NO _x evolution from the regenerator. Thus, there remains a need for more effective NO _x control addi tives suitable for use in the FCC process. US 6.143.167 de scribes NO _x reduction compositions for use in FCC pro cesses. These compositions comprise an acidic oxide sup port, an alkali metal and/or alkaline earth metal or mix tures thereof, a transition metal oxide having oxygen stor age capability and a group lb and/or lib transition metal.

Regarding the composition of the transfer catalysts, com pounds of intermediate basicity, such as Mg,Al-mixed oxides and/or spinels (MgAl204) derived from hydrotalcite-like compounds, have so far been among the most promising.

A typical SO _x additive comprises an alumina spinel rich in Mg or Ca as well as a small amount of a metal oxide having sufficient redox properties (e.g. Ce) . During the FCC pro cess, the catalyst gradually gets deactivated due to the deposition of coke, prompting a need for the catalyst to be regenerated. This is done in the regeneration unit, where the catalyst is exposed to an oxygen-rich atmosphere. In this step, any sulfur present in the coke is oxidized to SO2 and SO3. The role of the FCC additive is (i) to oxidize remaining SO2 to SO3 and (ii) to capture any S03-forming metal sulfate (e.g. MgS0 ₄₎ . Subsequently, the regenerated FCC catalyst together with the sulfate-containing additive is transferred back to the FCC riser operating under oxy gen-deficient conditions. In the riser, the sulfates pre sent on the additive are reduced to ¾S which is processed in downstream Claus units.

It has now turned out that a specific metal-loaded magne sium aluminate spinel is very well suited as an additive to conventional FCC catalysts. More specifically, the metal to be loaded onto the magnesium aluminate spinel is copper.

Alkaline earth metal, aluminum-containing spinels and their use in reducing the SO _x content of gases are disclosed in US 4.492.677, while the preparation of anionic clay com pounds, such as hydrotalcite-like compounds, to be used i.a. for decreasing SO _x emissions from FCC units, is de scribed in US 7.112.313.

US 6.074.984 deals with SOx additive systems based upon use of multiple particle species. It is shown that the life of SO _x additives having a SCg SO ₃ oxidation catalyst compo nent and a SCg absorption component can be extended by em ploying each of these components as separate and distinct physical particles, pellets etc.

As such, the use of copper in connection with a magnesium aluminate spinel as an additive to FCC catalysts is known from the prior art. For example, US 4.522.937 discloses a process for preparing spinel compositions of an alkaline earth metal and aluminum for FCC use, preferably Mg,Al-con- taining spinel compositions and at least one additional metal component selected from Bi, Sb, Cr, Cu, Mn, V, Sn or mixtures thereof. Here, Cu is used in small concentrations only .

The optimization of SO _x additives of FCC catalysts based on Mg0-Al203 mixed oxides produced from hydrotalcites is dis closed in Appl . Catal . B Environmental 4, 29-43 (1994) . In this paper, magnesium-rich Mg,Al spinels were prepared from hydrotalcites and characterized. If Ce0 ₂ was incorporated, they showed a very good SO _x adsorption as FCC SO _x additives but their regeneration capacity was limited. Different transition metal oxides were tried as co-catalysts and, among them, CuO showed excellent properties to catalyze SO2 oxidation and also good catalyst regeneration properties. However, for 5 wt% CuO both absorption and regeneration ca pacity of the catalyst are very good, but when the amount of copper is increased, the SO _x absorption capacity clearly decreases while the regeneration percentage increases. It is further stated that the initial SO _x absorption actually is higher for the sample that does not contain Cu . The behavior of SO _x traps derived from ternary Cu/Mg/Al hy- drotalcite materials has been investigated (Catalysis Today 127 , 219-227 (2007)) . Such materials show improved SO _x trap performances with respect to binary Cu/Al materials in FCC applications, but there is no mentioning of free CuO. How ever, in Journal of Catalysis 170 , 140-149 (1997) formation of CuO is mentioned after calcination, but the spinel used cannot be considered stoichiometric (Mg/Al ratio being 3:1) .

Thus, summing up, the prior art indicates that Cu should only be used in small amounts or perhaps should not be used at all. Now it has surprisingly been found that, contrary to expec tations, the SO _x adsorption can in fact be improved by raising the amount of free CuO. So the present invention relates to a stoichiometric copper-loaded magnesium alumi- nate spinel and the use of that spinel as an additive to a conventional FCC catalyst.

More specifically, the present invention relates to an ad ditive to a conventional fluid catalytic cracking (FCC) catalyst with SO _x reducing properties, characterized in that

- it contains Cu as metal component, and

- it is prepared on a stoichiometric spinel having only negligible reactivity towards SO _x removal.

Preferably, the content of Cu is from 5 wt% or 10 wt% to 20 wt% or 25 wt%. The stoichiometric spinel is preferably hy- drotalcite with an Mg:Al ratio of 0.5 ± 0.1.

The invention also relates to a fluid catalytic cracking (FCC) process using the copper-loaded magnesium aluminate spinel additive, where SCg and SO ₃ are adsorbed onto the additives as sulfates, and where the sulfates are reduced to release sulfur as ¾S. The reduction of sulfates to re lease sulfur as ¾S is preferably done at a temperature of 520-530 °C .

The process according to the invention for fluidized cata lytic cracking (FCC) of a feed stream comprises

- feeding a heavy feedstock to an FCC reactor through a riser while adding steam,

- contacting a regenerated FCC catalyst with said feedstock stream in the upstream portion of the riser, - passing the mixture of catalyst and feedstock through the

FCC reactor to crack hydrocarbons in the feedstock, convert sulfur compounds to ¾S and deposit coke on the catalyst,

- separating the cracked hydrocarbons and the ¾S from the catalyst to obtain a cracked product stream comprising the cracked hydrocarbons and ¾S,

- passing the catalyst containing coke deposits to a regen erator and contacting it with an oxygen containing gas at elevated temperature to regenerate the catalyst by combus tion of the coke and to produce a flue gas containing the by-products of said coke combustion, and

- separating regenerated catalyst particles from said flue gas and passing the regenerated catalyst particles to the feedstock stream in the upstream portion of the riser.

Thus, the additives of the present invention differ from other commercial additives in that they are prepared on a stoichiometric spinel which is, in itself, inactive towards SO _x and NO _x removal. The presence of copper in the formula tion amounts to around 3-20 wt%. It is not only involved in the oxidation of SCy to SCy, but also in the SCy capturing step as evidenced by an increased deSCy activity with in creasing copper load.

The deSO _x activity is directly related to the amount of copper in the range between 3 and 20 wt% with pure magne sium aluminate spinel displaying only a very limited activ ity.

The deSO _x activity as a function of the copper load, which has been observed, is shown in the table below.

Adsorption capacity (ml SCy per g of additive)

The preparation and characterization of the FCC additives according to the invention was carried out as described in the experimental section below.

Experimental

The FCC additives according to the invention were prepared by impregnating 100 g of commercial hydrotalcite (Mg:Al ra tio: 0.5) with a solution of Cu (NO ₃ ) 2 3H ₂ 0 and subsequently air calcining the resulting material at 750°C for 2 hours with a heating rate of 5°/min.

The particle size of all catalytic materials was between 63 and 106 ym. The materials were characterized by powder X- ray diffraction (XRD) showing the reflections characteris tic for the spinel phase (Fig. 1 and Fig. 2) at 2Q = 37,

45, 60 and 65.5. The CuO phase only appears for additives with a Cu content of 12% or higher as identified by charac teristic reflections at 2Q = 35.5, 38.8 and 48.7 (Fig. 2). The testing of the additives was carried out using a proce dure as follows:

A fluidized bed reactor was loaded with 20 g of a mixture consisting of the FCC base catalyst and each of the five deSOx additives in a concentration of 1 wt%. The loaded re actor was heated up to 732°C with a flow of helium. As soon as the desired temperature was achieved, a flow (840 cm ³ /min) containing 1435 ppm SCg, 2 vol% Cg and He was in- serted into the reactor. This gas mixture passes upwards through the heated reaction tube and reacts with the solid. Then it exits through the SCg analyzer which continuously measures the corresponding concentrations in the effluent gas (breakthrough curves) . An adsorption cycle is finished in 100 minutes.

The tests show a positive effect of Cu when going from 0 to 20% Cu; see Fig. 3 which shows adsorbed SCy/ml as a func tion of time for five different Cu contents, ranging from zero to 20 wt% Cu . The tests were done on stoichiometric Mg/Al spinel material (Mg:Al ratio 0.5) . The effect of Cu was specifically tested at levels of 3%, 6%, 12% and 20% and it was actually found that the activity of the additive rises with rising Cu content (most markedly between 3%, 6% and 12%) .