EP0334742A1 - Liquid phase selective hydrogenation of a normally gaseous feedstock containing ethylene, acetylene and naptha - Google Patents

Abstract

Process for selective hydrogenation in a liquid phase of an effluent originating from an ethane steam cracker, in which the said effluent is brought into contact with a catalyst consisting of at least supported palladium, characterised in that the operation is carried out in the presence of a liquid phase containing at least a proportion of the hydrogenated, condensed and recycled petrol cut of the said effluent. <??>The said effluent (1), the said liquid phase (8) and optionally hydrogen (13) pass through the hydrogenation reactor (4). The product obtained is fractionated (5) into a head gas cut (7) containing ethylene and a bottom liquid petrol cut which is partially recycled (8). <??>The process can be employed for the production of ethylene and for the production of petrol. <IMAGE>

Description

Thermal conversion processes such as steam cracking, for example, produce olefinic compounds of interest for petrochemicals, but their recovery requires a selective hydrogenation of the acetylene and diolefinic impurities co-produced by these processes.

These hydrogenations are generally carried out on partial cuts such as, for example, the C₂ cut containing ethylene and acetylene, the C₃ cut containing propylene, propyne and propadiene, the C₄ cut containing butenes and butadiene and gasoline cutter containing aromatics, other olefins and other diolefins.

Such separate treatments are only possible if the relative amounts of these cuts are close, which is the case when the charge of the steam cracker is naphtha or diesel. When the charge consists of ethane, the effluent from the steam cracker essentially comprises C₂ hydrocarbons (hydrocarbons with 2 carbon atoms), the heaviest cuts (C₄ and petrol) being clearly minority. It is common practice to roughly separate the condensable liquids and to send all of the gaseous effluent to a hydrogenation catalyst to recover the ethylene produced. The weight composition of this ethane steam cracker effluent is given in Table 1. Table 1. Overall weight composition of an ethane steam cracker effluent. Hydrogen 1.44% Carbon monoxide 0.06% Methane 24.79% C₂ 38.65% C₃ 26.70% C₄ 3.41% Essence (C₅ - C₉) 4.95%

At 15 ° C, under 20 bars (2000 KPa) of pressure, such an effluent is entirely gaseous: the hydrogenation must therefore be carried out in the gas phase. However, the heaviest parts of this cut (C₄ and petrol) contain highly polymerizable compounds such as butadiene, isoprene and styrene as indicated in Tables 2 and 3 in which the typical compositions of C₄ cuts are given. and essence. Table 2. Detailed composition by weight of the C coupe cut contained in an ethane steam cracker effluent. C dans content Total content Butadiene 43% 1.48% Butenes 43% 1.47% Butane 14% 0.46%

The catalysts used in this hydrogenation are quickly fouled by these polymerization products and the cycle times are therefore disadvantageously very short.

The object of the present invention is the development of a new process for selective catalytic hydrogenation in the liquid phase of a normally gaseous hydrocarbon charge (that is to say in vapor form at normal temperature and pressure ) containing in particular acetylene, ethylene, petrol (hydrocarbons C₅ to C₉), the liquid phase (or liquid diluent), in the presence of which (which) is operated, comprising at least part of the condensable fraction of said charge, that is to say at least part of the gasoline cut (C₅ - C₉) hydrogenated, condensed and recycled of said charge. Table 3. Detailed weight composition of the petrol cut contained in an ethane steam cracker effluent. Gasoline content Total content Pentanes + Pentenes 5.5% 0.27% Isoprene 0.4% 0.02% Hexane + Hexenes 5.5% 0.27% Benzene 51.9% 2.57% Heptane + Heptenes 1.6% 0.08% Toluene 13.5% 0.67% Octanes + octenes 0.8% 0.04% Ethylbenzene 2.6% 0.13% Xylenes 2.2% 0.11% Nonanes + Nonènes 7.9% 0.39% Styrene 8.1% 0.40%

In general, the charge (in vapor form) to be hydrogenated can contain:
* 0 to 6%, preferably 1 to 2.5% by weight of hydrogen;
* 0 to 40%, preferably 15 to 30% by weight of methane;
* 25 to 80%, preferably 30 to 45% by weight of C₂ hydrocarbons and, in particular, 0.1 to 5%, preferably 0.2 to 2% by weight of acetylene and 15 to 75%, preferably 20 to 35% by weight of ethylene (and for example 0 to 25% by weight of ethane);
* 0 to 40%, preferably 15 to 35% by weight of C₃ hydrocarbons;
* 0 to 10%, preferably 1 to 6% by weight of C₄ hydrocarbons, and
* 1 to 20%, preferably 1 to 7% by weight of gasoline, that is to say of hydrocarbons of 5 to 9 carbon atoms (C₅⁺) and, in particular, 0.4 to 11% , preferably 0.8 to 6% by weight of aromatic hydrocarbons (less than 9 carbon atoms).

This hydrogenated cut can also contain a small amount of carbon monoxide, for example between 0.01 and 1% by weight, preferably between 0.02 and 0.2% by weight.

The charge to be hydrogenated can, for example, consist of the effluent from an ethane steam cracker.

The presence of hydrogen in the feed to be hydrogenated can advantageously avoid working with an external source of hydrogen.

The method according to the invention allows more satisfactory operation of the installation, the cycle times being increased in significant proportions and, surprisingly, the quality of the liquid which is recycled to the hydrogenation reactor is improved. The hydrogenated cuts produced in the process meet the most stringent specifications: indeed, the C₂ cut (after hydrogenation and separation) can easily contain less than 5 ppm by weight of acetylene, and the gasoline cut (after hydrogenation and separation) has a "Maleic Anhydride Value" (MAV), which is a measure of the content of conjugated diolefins, determined according to UOP standard No. 356, preferably less than 3.

The hydrogenation catalyst consists of at least supported palladium. Palladium is generally deposited in an amount of 0.01 to 1% by weight on a suitable support such as, for example, alumina or silica or a mixture of these two constituents.

Palladium can be associated with at least one additional metal chosen, for example, from the group formed by silver and gold, with contents which may generally be between 0.01 and 1% of the weight of catalyst. Preferably, the Au / Pd or Ag / Pd or (Au + Ag) / Pd weight ratio is less than 1.

The hydrogenation can be carried out in at least one reactor in which the catalyst is preferably placed in a fixed bed. Figure 1 shows a non-limiting example of application of the invention.

The cup (normally gaseous) to be hydrogenated (1) (for example the effluent from an ethane steam cracker), the liquid diluent (8) and possibly hydrogen (13) (in the case where the cup to be hydrogenated does not contain or contains only a very small amount) are introduced into the hydrogenation reactor (4). After cooling in the exchanger (10), the effluent from said reactor (4) is sent, via line (11), to a distillation column (5) making it possible to separate a gas fraction at the top (7) (which contains excess hydrogen and hydrocarbons with less than five carbon atoms, for example methane, the incondensable gas containing excess hydrogen, hydrogenated C₂, C₃ and C₄ cuts (i.e. cut C₄ ⁻ Hydrogenated)) a gasoline cut (C₅ - C₉) at the bottom (possibly accompanied by a small amount of the least volatile C₄), gasoline cut which will at least partly constitute the liquid diluent. This liquid diluent is partly recycled, that is to say returned to the reactor (4) by the pipe (8) through a pump (12). The other part of this solvent is preferably removed (therefore purged) before it passes through the pump (12), so that the total amount of gasoline contained in the system (reactor (4) + lines + distillation column (5 )) is substantially constant, this sample constituting the hydrogenated gasoline fraction of the process.

This hydrogenated and sampled petrol cut can be used directly as fuel, therefore without transformation, because it contains an extremely small amount of diolefins and therefore of undesirable gums, diolefins which are in fact mainly hydrogenated during the process according to the invention .

It is more easily possible, by distillation methods known to those skilled in the art, to separate the C₂ cut, then the ethylene (which is a mixture of ethylene contained in the starting charge and of ethylene produced by hydrogenation of acetylene) contained in the gas fraction discharged at the head of column (5): the process according to the invention therefore also makes it possible to produce ethylene.

The entire catalyst is permanently sprayed with the liquid phase (or liquid diluent) constituting the flow (8) and entering the reactor (4) towards its top. The fresh charge to be hydrogenated can be injected towards the top of the reactor (4), via the pipe (2) and / or halfway up the catalyst by a pipe (3). This arrangement makes it possible to vary, during operation, the quantity of catalyst which works, which makes it possible to adjust the reactivity of the total mass of catalyst.

The possible installation on the liquid inlet of the reactor (4) of a steam exchanger (9) can make it possible to adjust the inlet temperatures of the said reactor.

The recycled liquid (or diluent) phase generally contains at least 25%, preferably 50 to 85% and, even more preferably, 60 to 75% by weight of aromatic hydrocarbons (styrene is not counted in the aromatic hydrocarbons).

The operating conditions for hydrogenation, according to the invention, according to usefully chosen as follows:
- total pressure: 10 to 50 bars;
- temperature: 10 to 150 ° C;
- space speed expressed in gas flow volume flow rate to be hydrogenated, at normal temperature and pressure (TPN), by volume of catalyst and per hour (VVH gas): 500 to 20,000, preferably 1,000 to 10,000;
- volume flow of recycled liquid at normal temperature and pressure (TPN), by volume of catalyst and per hour (VVH liquid): 1 to 15, preferably between 4 and 12.

Under these VVH gas and VVH liquid conditions, the ratio, at the inlet of the reactor (4), between the weight flow rate of recycled liquid and the weight flow rate of gaseous feedstock to be hydrogenated is usually between 0.5 and 20, preferably between 1.0 and 10 and, even more preferably, between 1.5 and 5.

The following examples illustrate, without limitation, the present invention.

Example 1.

In this example which illustrates a technique of the prior art, a normally gaseous cut is treated, the composition by weight of which is given in Table 4. No liquid diluent is used. Table 4. Weight composition of the gaseous fraction to be hydrogenated. Hydrogen 1.44% Isoprene 0.02% Carbon monoxide 0.06% Hexanes + Hexenes 0.27% Methane 24.79% Benzene 2.57% Acetylene 0.37% Ethylene 28.25% Heptane + Heptenes 0.08% Ethane 10.03% Toluene 0.67% Propadiene 0.12% Octane + Octenes 0.04% Propyne 0.28% Ethylbenzene 0.13% Propylene 15.22% Xylenes 0.11% Propane 11.08% Butadiene 1.48% Nonane + Nonènes 0.39% Butenes 1.47% Styrene 0.40% Butane 0.46% Pentane + Pentenes 0.27% Detailed weight composition of the C₂ cut contained in the cut to be hydrogenated. C dans content Total content Acetylene 1.0% 0.37% Ethylene 73.1% 28.25% Ethane 25.9% 10.03%

The catalyst contains 500 ppm by weight of palladium deposited on an alumina support with a specific surface area equal to 9 m² / g and a pore volume equal to 0.5 cm³ / g. The catalyst is placed in a fixed bed in a tubular reactor.

The section to be hydrogenated is passed through this reactor under the following operating conditions:
- VVH gas: 2500 (TPN);
- Pressure: 20 bars;
- Temperature: 40 ° C.

The weight composition of the effluent leaving the reactor after 2 days and 15 days of operation is shown in Table 6 for the C₂ cut and Table 7 for the gasoline produced (C₅ - C₉). Table 6. Weight composition of cut C coupe in the effluent leaving the reactor. after 2 days after 15 days Acetylene 4.5 ppm 0.2% Ethylene 73.6% 73.5% Ethane 26.4% 26.3% Weight composition and properties of the gasoline fraction contained in the effluent leaving the reactor. Composition (by weight) After 2 days After 15 days Paraffins 22.4% 22.2% Diolefins + Styrene 0.3% 6.0% Olefins 10.3% 4.8% Aromatic 67.0% 67.0% MY V 3 60 Octane number 98 not measured * * because the conjugated diolefins and therefore gums are present in significant quantities.

It can thus be seen that, under these conditions, the catalyst is rapidly deactivated by fouling and that the hydrogenation reaction is insufficient: in fact, on the one hand, the conversion of acetylene is only 80% after 15 days of operation (acetylene content in the C₂ cut: 1.0% by weight at the inlet, 0.2% by weight at the outlet) and, on the other hand, the conversion of the diolefins (and of styrene) has clearly decreased after 15 days of operation (content of diolefins (and styrene) in the petrol cut: 0.3% by weight after 2 days of operation, 6.0% by weight after 15 days of operation).

The only possibility to increase the conversion would be to raise the operating temperature, which would inevitably harm the olefin yield and further accelerate fouling of the catalyst.

Example 2 (according to the invention).

The same charge is treated as in Example 1, the catalyst used also being the same.

This catalyst is placed in a fixed bed in a tubular reactor; the unit also includes a distillation column containing 10 trays, the operation of this column is such that the gasoline cut (C₅ - C₉) and therefore all the benzene in the feed are found in the bottom racking and at least the major part of the C₄⁻ (hydrocarbons with four carbon atoms at most) is found at the top. The bottom liquid is taken up by a pump and constitutes the liquid inlet of the reactor, the cut to be hydrogenated being mixed with this liquid at the inlet of the reactor.

When the unit starts, the loop is filled with toluene and a small continuous purge on the liquid drawn off at the bottom of the column is carried out during operation, in order to have a constant liquid level in the column.

The operating conditions are as follows: - VVH gas: 2500 (TPN);
- Pressure: 20 bars;
- Temperature: 40 ° C;
- VVH liquid: 10 (TPN).

With these VVH gas and liquid conditions, the weight flow rate of recycled liquid is equal to approximately 2.8 times the weight flow rate of gaseous feed to be hydrogenated. Samples to analyze the purged liquid were taken and give the results presented in FIG. 2 (content (% weight) of toluene (continuous curve) and benzene (discontinuous curve) in the withdrawal liquid as a function of time (hour) ). It can be seen that after 200 hours the liquid phase has a constant composition which corresponds to the condensable part of the cup to be hydrogenated. The weight composition of the gaseous and liquid effluents respectively at the head and at the bottom of the column after 10 days and after 2 months of operation is indicated respectively in Tables 8 and 9. Table 8. Weight composition of section C₂. after 10 days after 2 months Acetylene 3.2 ppm 4 ppm Ethylene 73.9% 73.7% Ethane 26.1% 26.3% Weight composition and properties of the gasoline cut. Composition (by weight) After 10 days After 2 months Paraffins 22.33% 22.21% Diolefins + Styrene 0.27% 0.29% Olefins 10% 10.1% Aromatic 67.4% 67.4% MY V 2.5 2.7 Octane number 98 98

It is found that, unlike Example 1, the hydrogenation performance is stable. Indeed, after 2 months, the results are close to the initial results (see Table 10). Table 10. Conversions and weight returns. after 10 days after 2 months Conversions: Acetylene 99.97% 99.96% Propyne + Propadiene 94.2% 93.2% Butadiene 93.8% 93.1% Isoprene + Styrene 96.6% 95.4% Yields: Ethylene 101% 100.8% Propylene 101.9% 101.7% Butenes 193% 190%

Claims (10)

1. A process for the selective hydrogenation in the liquid phase of a charge of normally gaseous hydrocarbons containing at least ethylene, acetylene and a gasoline cut, in which said charge in vapor form is brought into contact with a catalyst consisting of at least supported palladium, the method being characterized in that one operates in the presence of a liquid phase, said liquid phase containing at least part of the hydrogenated petroleum cut, condensed and recycled from said charge. 2. The method as claimed in claim 1, in which said hydrocarbon feed contains 0 to 6% by weight of hydrogen, 0 to 40% by weight of methane, 25 to 80% by weight of C₂ hydrocarbons, in particular 0, 1 to 5% by weight of acetylene and 15 to 75% by weight of ethylene, 0 to 40% by weight of C₃ hydrocarbons, 0 to 10% by weight of C₄ hydrocarbons, and 1 to 20% by weight gasoline weight. 3. The method of claim 1 wherein said hydrocarbon feed contains 1 to 2.5% by weight of hydrogen, 15 to 30% by weight of methane, 30 to 45% by weight of C₂ hydrocarbons, in particular 0.2 to 2% by weight of acetylene and 20 to 35% by weight of ethylene, 15 to 35% by weight of C₃ hydrocarbons, 1 to 6% by weight of C₄ hydrocarbons and 1 to 7% by weight of gasoline. 4. Method according to one of claims 1 to 3 wherein said hydrocarbon feed is an effluent from an ethane steam cracker. 5. Method according to one of claims 1 to 4 characterized in that said liquid phase contains at least 25% by weight of aromatic hydrocarbons. 6. Method according to claim 5 characterized in that said liquid phase contains 50 to 85% by weight of aromatic hydrocarbons. 7. Method according to one of claims 1 to 6 wherein said catalyst consists of palladium and at least one additional metal selected from the group formed by gold and silver, the mixture of palladium and at least an additional metal being deposited on at least one support chosen from the group formed by alumina and silica. 8. Method according to one of claims 1 to 7 characterized in that the ratio between the weight flow rate of said liquid phase and the weight flow rate of said charge to be hydrogenated is between 0.5 and 20. 9. Method according to claim 8 characterized in that said ratio is between 1 and 10. 10. Use of the method according to one of claims 1 to 9 for the production of ethylene and for the production of gasoline.

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