NO174042B - FISCHER-TROPSCH CONVERSION OF SYNTHESIC GAS TO HYDROCARBONES - Google Patents

Description

The present invention generally relates to a method for converting gaseous mixtures containing carbon monoxide and hydrogen into hydrocarbons by contact with Fischer-Tropsch catalysts. More particularly, the invention relates to a method for converting carbon monoxide, hydrogen and unsaturated hydrocarbons, e.g. alkenes, to hydrocarbons by contact with Fischer-Tropsch (FT^ catalysts. In the present context, the term "Fischer-Tropsch catalyst" includes related catalysts of the type generally designated "iso-synthesis catalysts" and derivatives thereof.

The Fischer-Tropsch (F-T) synthesis originated in Germany in the 1920s, reached its peak in Germany during World War II as a supplier mainly of motor fuels and now survives only in South Africa in combination with a coal gasification process at Sasol the facilities. Immediately after World War II, most industrial nations were involved in F-T research, but interest began to wane with the advent of cheap oil and gas. In recent years, however, interest in the F-T process has blossomed again, and considerable research has been directed towards the development of new and improved catalysts. As a result of this, the number of catalytically active F-T metal components has increased beyond Iron and cobalt, which are the basis for the original catalysts.

A demand exists for a Fischer-Tropsch process for the conversion of synthesis gas to form high yields of liquid hydrocarbon products, while simultaneously minimizing the production of C 2 products and maximizing the total carbon conversion and productivity, and in particular the conversion and productivity of C3 or higher hydrocarbons.

It has now been found that this demand can be met to a certain extent by providing in the synthesis gas feed to a Fischer-Tropsch process a defined proportion of at least one unsaturated hydrocarbon, especially if the Fischer-Tropsch catalyst contains a solid, acidic component.

Processes in which mixtures of synthesis gas and unsaturated hydrocarbons are reacted over a catalyst are known in the art.

Thus describes e.g. USP-A-2,606,939 the conversion of olefinic feedstock containing carbon monoxide and hydrogen, the olefin being present in an amount greater than 60%, preferably greater than 70% by volume, by polymerization/hydropolymerization over an activated iron catalyst to higher olefinic hydrocarbons . This represents a description of an olefin conversion process as opposed to a synthesis gas conversion process.

USP-A-4,547,525, discloses that methane production in Fischer-Tropsch hydrocarbon synthesis reactions is reduced by the addition of one or more olefins to the hydrogen and carbon monoxide gas feed. A small effect regarding carbon monoxide conversion was observed. The use of a solid acidic support for the catalyst is not disclosed.

According to the present invention, there is provided a method for producing a hydrocarbon product by bringing a gaseous mixture comprising carbon monoxide, hydrogen and an unsaturated hydrocarbon into contact with a Fischer-Tropsch catalyst under Fischer-Tropsch conditions, and this method is characterized in that that as unsaturated hydrocarbon ethylene is used, which is present in an amount of 2-15 volume-#, and that as Fischer-Tropsch conditions a temperature in the range 175-450°C and a pressure in the range 20-100 bar are used .

It can be mentioned that from DE patent 896792 it is known to use an ethylene/acetylene addition to the synthesis gas in a Fischer-Tropsch process.

F-T catalysts generally comprise one or more metals from group VIII in the periodic table, possibly on a suitable support. Suitable metals include e.g. iron, cobalt, nickel, molybdenum, tungsten, rhenium, ruthenium, palladium, rhodium, osmium, iridium and platinum, preferably iron, cobalt or ruthenium. The metal can suitably be in the form of the elemental metal, an oxide or a sulphide. The catalyst may suitably include a promoter which may be an alkali metal, an alkaline earth metal or a rare earth metal. Examples of suitable promoters are the hydroxide, oxide or salt of lithium, sodium, potassium, rhubidium, cesium, magnesium, calcium, strontium, barium and thorium. The carrier can e.g. be aluminum oxide, carbon, silicon dioxide, zirconium oxide, titanium oxide, magnesium oxide, cerium oxide or mixtures thereof. Iso-synthesis catalysts generally comprise non-reducible oxides, e.g. thorium oxide, zirconium oxide, aluminum oxide, cerium oxide and gallium oxide. Conventional techniques such as e.g. impregnation, can be used for the production of such catalysts.

It is preferred that a solid, acidic component is appropriately incorporated into F-T catalysts by means of a carrier or as an intimate mixture with it. Suitable acidic components include e.g. silica/alumina, layered clays, columnar layered clays, and crystalline tectometal silicate, e.g. gallium silicate and aluminosilicate zeolites. Preferred are crystalline gallium silicate and aluminosilicate zeolites having a high silicon dioxide content, molar ratio of silicon dioxide to aluminum oxide greater than 12:1, preferably in the hydrogen form. An example of a suitable zeolite is a zeolite of the MFI type, e.g. ZSM-5, as described in US patent 3,702,886. The hydrogen form for a crystalline aluminosilicate zeolite can conveniently be prepared by techniques that are well known in the zeolite field.

Examples of catalysts suitable for use in the present process include (i) an oxide of zinc, gallium or indium, and at least one other metal, together with a porous, crystalline tectometal silicate as described in EP-Å-0124999, e.g. . a crystalline aluminosilicate zeolite of the thorium/gallium/MFI type, (ii) a composition comprising ruthenium/cerium oxide as described in published EP patent application 169,743, preferably in combination with a crystalline aluminosilicate zeolite of the MFI type, and (iii) an iron/crystalline , acid aluminosilicate zeolite catalyst as described e.g. in USP-A-4,298,695.

In connection with the present process, it is preferred that the mixture of carbon monoxide and hydrogen and the olefinic hydrocarbon is derived from separate sources and is then combined in the presence of the F-T catalyst. Mixtures of gases mainly comprising carbon monoxide and hydrogen, possibly also containing carbon dioxide, nitrogen and methane, for example, are usually referred to in the technical field as synthesis gas. Synthesis gas can in theory be obtained from any carbon-containing source of which coal, natural gas and high-molecular-weight hydrocarbon residues can be mentioned as examples. The synthesis gas can thus conveniently be obtained by gasification of coal using techniques that are well known in the field, e.g. the gasifier process developed by Lurgi Kohle und Mineraloeltechnik GmbH. The synthesis gas can alternatively be obtained by steam reforming of hydrocarbon feeds, e.g. sulfur-free natural gas or paraffinic naphtha in the presence or absence of a reforming catalyst. In another alternative, synthesis gas can be produced by partial oxidation of a hydrocarbon feed, where the hydrocarbon feed is introduced into an oxidation zone maintained in a fuel-rich state, in the absence of a catalyst and the presence or absence of steam. Catalytic autothermal reforming of hydrocarbon liquids, i.e. partial oxidation in the presence of added steam, can also be used for the production of synthesis gas which is necessary for carrying out the invention.

An example of a suitable process for the production of synthesis gas is described in co-published EP patent application 164,864 which provides a process for the production of synthesis gas where (a) a gas containing saturated hydrocarbon and oxygen with a ratio of hydrogen to oxygen that is greater than stoichiometric conditions for complete combustion, is introduced into a layer of particulate material, the layer comprising the material which is catalytically active for partial oxidation and/or steam reforming reactions, (b) the upward flow rate of the hydrocarbon/oxygen-containing gas stream being sufficient large enough to cause a spattering effect of the bed material, (c) the hydrocarbon and oxygen react together, and (d) the reaction products are removed.

The unsaturated hydrocarbon may be substantially pure, or may be present in a mixture with other materials, e.g. hydrogen and paraffinic hydrocarbons, in a petroleum refinery fraction or another fraction. A suitable fraction which is rich in unsaturated hydrocarbons is e.g. the product obtained from thermal cracking of C2~C4alkanes or other petroleum fractions such as naphtha.

The olefinic hydrocarbon can be combined with the synthesis gas feed to a reactor containing an F-T catalyst or can be fed separately to such a reactor.

When carrying out the present method, it is preferred that in a first step a gaseous mixture comprising carbon monoxide, hydrogen and at least one unsaturated hydrocarbon is formed, and that in a second step the mixture formed in the first step is converted into a product comprising hydrocarbons by contact with a Fischer-Tropsch catalyst under Fischer-Tropsch conditions.

An advantage of carrying out the method in this way is that the total carbon converted per passage can be greatly increased compared to producing carbon monoxide/hydrogen separately and converting this over an F-T catalyst. Furthermore, certain catalysts show a synergistic effect, i.e. the CO conversion is increased in the presence of the unsaturated hydrocarbon, and/or the proportion of CO2 and methane formed is reduced.

The first step in which a gaseous mixture comprising carbon monoxide, hydrogen and at least one unsaturated hydrocarbon is formed can be carried out in a number of different ways. It can e.g. is carried out by partial oxidation/cracking of one or more gaseous hydrocarbons or easily volatile hydrocarbons. A suitable method involves partial oxidation of gaseous or vaporized hydrocarbons with oxygen by preheating the reactants together or separately, feeding the mixture into a reaction chamber, reacting the reactants in a flame and rapidly cooling the reaction gases, such as described in GB patent 835,676.

A particularly preferred method is described in published EP patent application 163,385 which provides a process for the production of synthesis gas and higher hydrocarbons wherein (a) a gas containing a saturated hydrocarbon and oxygen with a ratio of hydrogen to oxygen greater than the stoichiometric ratio for complete combustion, is introduced into a layer of an inert particulate material, (b) the upward flow velocity of the hydrocarbon/oxygen-containing gas stream is sufficiently large to fluidize or produce a splashing effect of the layer material whereby at least part of the particulate the material is thrown up over the bed surface and then falls back into the bed, (c) the hydrocarbon and oxygen-containing gas is ignited and reacted together, and (d) the reaction products are removed.

Yet another process is described in published EP patent application 178,853 which provides a process for the production of synthesis gas and/or higher hydrocarbon wherein (a) a gas containing saturated hydrocarbon and oxygen with a ratio of hydrocarbon to oxygen greater than the stoichiometric ratio for complete combustion, is introduced with hydrogen into a bed of particulate material, (b) the upward flow velocity of the gases is sufficiently great to cause a splashing effect of the bed material above the sunken level of the bed, (c) the hydrocarbon, oxygen and hydrogen are reacted together, and (d) ) the reaction products are removed.

It is preferred that the gaseous mixture that is mixed in the first step of the method can be transferred without significant intermediate treatment directly to the second step. Under certain conditions, however, it may be desirable to subject the gaseous mixture formed in the first step to, for example, an intermediate step in which water is removed from the gaseous mixture and/or an intermediate step in which alkynes present in the gaseous mixture, is selectively hydrogenated to alkenes.

As mentioned, a temperature in the range 175-450°C and a pressure in the range 20-100 bar are used as Fischer-Tropsch reaction conditions. The optimal reaction conditions will vary from catalyst to catalyst, but will be within the above-mentioned ranges.

The method is preferably carried out on a continuous basis.

The nature of the hydrocarbon product will depend, among other factors, on the specially chosen F-T catalyst, the specific reaction conditions used and on the particular composition of the gaseous mixture fed to the F-T catalyst.

The invention will now be further illustrated only by way of example and with reference to the accompanying drawing.

(A) Preparation of mixture of CO. H2 and unsaturated hydrocarbon

The drawing shows a schematic diagram of a reactor and auxiliary equipment.

The reactor 1 has the form of an isolated elongated quartz column 2 with a conical base part 3, the cone angle from the vertical plane being 20°. The base part of the reactor contains a collapsed layer 4 of particulate material with a particle size of the order of 1-1.5 mm in diameter. The particulate materials were crushed refractory rock, silicon carbide, quartz and zirconium oxide. The base of the column is adapted to receive a nozzle 5 for introducing reactants. The nozzle outlet can be adjusted vertically in the layer of particulate material. The upper part 6 of the reactor is open so that an outlet is formed for the removal of the product gases. A line 10 enables samples of the products to be removed from the product gas stream.

The nozzle 5 can be connected to an air supply 7 or another oxygen-containing gas under suitable pressure, and to a methane supply 8. A suitable supply can comprise cylinders of hydrocarbon, e.g. methane, and air or oxygen connected to the nozzle through a mixer and measuring devices for gas pressure and flow rate such as manometers and rotameters (not shown).

The reactor can also have an additional nozzle or nozzles for supplying additional methane or other hydrocarbon to the layer (not shown in the drawing). The reactor 1 is lined with a suitable insulating material 9.

A number of techniques can be used to start up the reactor. In the present example, the ignition source was a gas burner (not shown) arranged at the outlet part 6 of the column.

During start-up of the reactor, a premixed gas flow of hydrocarbon and air was led under pressure to the nozzle 5 in the base part 3 of the column. The speed of the gas flow was sufficient to cause a fountain 11 of bed particles in the free bore above the bed.

The gas stream used was very rich in fuel and consequently the gas mixture was ignited by the gas burner, and a flame stabilized at the exit of the reactor. The air flow rate was increased, bringing the mixture closer to the stoichiometric ratio, until the flame began to move slowly back down through the reactor. A flame was stabilized at the surface of the collapsed bed and the fuel flow rate reduced somewhat to achieve a nearly stoichiometric mixture. When the bed temperatures were at equilibrium, the fuel flow rate was increased and a low flow rate of oxygen was supplied to the bed. The air flow rate was then reduced and both the fuel and the oxygen increased to maintain the stable flame and the spray effect of the layer. This procedure was repeated until the feed composition was completely fuel and oxygen. The total feed mixture was always maintained on the rich side of the stoichiometric near or beyond the rich flammability limit. At atmospheric pressure, the rich limit of flammability corresponds to a methane/oxygen molar ratio of 1.5.

The constituents and composition of the reactant gases were determined using conventional techniques. This method was repeated for a variety of fuel-rich hydrocarbon/oxygen reactant compositions and different particulate bed materials. The products obtained from the reaction may include carbon monoxide and hydrogen (synthesis gas), acetylene and ethylene.

Table 1 shows results for the reaction for methane and oxygen in a reactor using different layer materials. The layer materials usually had a particle size of 1-1.5 mm in diameter and were crushed refractory stone, quartz and silicon carbide. The increased carbon molar selectivities to C2 and higher carbons are shown for all bed materials for fuel-rich conditions as the feed composition is made increasingly fuel-rich up to and above the rich flammability limit.

Table 2 shows results for the reaction of methane and air in a reactor. The layer material used was zirconium oxide spheres of the order of 1-1.2 mm in diameter. The obtained carbon molar selectivities and feed conversions are similar to those obtained with oxygen.

Calculations have shown that the residence time for the reactant gases in the hot zone is preferably less than 1 millisecond to avoid an undesirably high degree of cracking of the products into soot or other compounds. It may be possible to use longer residence times at low or atmospheric pressures.

Alternatively, a lower speed of the gas flow can be used to thereby effect fluidization of the layer material and/or surface bubbling of the layer material. The fluidization and/or bubbling is sufficient to cause the particulate material to be thrown up into the freeboard, the material returning to the bed. The bubbling can also be associated with total fluidization of the bed.

Table 3 shows results obtained in a fluidized bed reactor with a methane/oxygen feed. The layer comprised particles of zirconium oxide with a size of 0.25-0.85 mm. The reactant gases were introduced to the reactor through a distributor plate at a sufficient velocity to achieve turbulent fluidization with the bed material thrown randomly into the gas phase or freeboard above the bed.

Experiments with fuel-rich mixtures near or above the flammability limit using conventional fluidized bed operation at a pressure of 1 bar and with an inert bed material generally resulted in flame loss or instability. At the onset of surface bubbling (or localized spatter formation) a more stable combustion course followed and partial oxidation with formation of CO/H2 and C£ materials resulted. This effect continued into the complete spray sequence. It is believed that the stable combustion of mixtures near or above the flammability limit results from the countercurrent heat transfer between the descending hot particulate material and the rising gases thereby enabling preheating of incoming feed gases.

As the operating pressure is increased, the upper limit of flammability is extended. Thus, the need for particle heat recycling for stable combustion is greater at lower operating pressures than at higher operating pressures. When using the present invention, however, the flammability limit can be further extended at increased pressures by particle heat recycling in a similar way as at atmospheric pressure.

(B) Conversion of a mixture of CO, H2 and unsaturated carbon Comparative experiment 1

A tubular reactor was charged with 10 ml of a Th0 2 /Ga 2 O 3 /H-MFI zeolite catalyst (bonded with silica) which had previously been used for 20 hours with a synthesis gas feed at approx. 400°C. Feed gas (synthesis gas comprising 65% by volume hydrogen and 35% carbon monoxide designated feed gas X) was fed to the reactor at a gas space velocity (GHSV) of 2000 hour-<1>. The reactor was heated externally to maintain an average bed temperature of 400° C at a pressure of 50 bar manorneter pressure. The reaction products were analyzed by gas chromatography. The conditions and results are summarized in table 4.

This is not an example according to the invention, because an unsaturated carbon was not present.

Example 1

Following Comparative Experiment 1, a feed stream Y containing hydrogen 57 vol-#, carbon monoxide 33.5 # and ethylene 9.5% (ie simulating the composition of the product formed in A) was fed to the reactor under substantially identical conditions. The conditions and results are summarized in table 4.

The presence of ethylene in the feed clearly has a synergistic effect: -ethylene is completely converted while carbon monoxide conversion is increased, and total carbon conversion is increased. Methane and carbon dioxide selectivities are reduced. The net result is a fivefold increase in productivity of CE3 and higher hydrocarbons.

Comparison test 2

A reactor was charged with 10 ml of a Ru/K/CeCl 2 catalyst. Feed gas X (cf. comparative test 1) was passed over the catalyst at 40 bar, 2500 GHSV, while the maximum bed temperature was maintained at 331°C. The results are given in table 4.

This is not an example according to the invention because the absence of an unsaturated hydrocarbon.

Example 2

Ethylene-containing synthesis gas (feed gas Y in example 1) was used instead of the synthesis gas feed (X) over the catalyst in comparative experiment 2. The results are shown in table 4. Under identical conditions, there is a significant increase in C3+ productivity due to the addition of ethylene, and a reduction in methane and carbon dioxide selectivities. However, ethylene conversion is relatively low over this catalyst.

Comparison test 3

The reactor was fed a physical mixture of Ru/K/CeO (6 ml) and silica-bound zeolite (H-MFI) (4 ml). Feed gas X (cf. comparative experiment 1) was fed to the reactor at 40 bar, 2500 GHSV, 370°C. The results are shown in table 4.

This is not an example according to the invention because the absence of unsaturated hydrocarbon.

Example 3

Feed gas Y (cf. example 1) was used instead of feed gas X in comparative experiment 3, and the reaction products over the catalyst used in comparative experiment 3 were analysed. The results in table 4 show that there is a greater improvement in C3+ productivity in this example where an acidic zeolite has been incorporated into the catalyst layer and a greater increase in total carbon conversion. Ethylene conversion is considerably higher while the selectivity to C3+ hydrocarbons is simultaneously improved.

Comparison test 4

The procedure in comparison experiment 1 was repeated with the exception that a magnetite/H-MFI catalyst was used instead of ThOg/GaOos/H-MFI zeolite catalyst. The reaction conditions and the results obtained are given in table 4.

This is not an example according to the invention because the absence of an unsaturated hydrocarbon in the feed.

Example 4

Comparison trial 4 was repeated with the exception that instead of supply gas X, supply gas Y was used.

The results are given in table 4.

From the above results, it is clear that olefin conversion is maximized in the presence of an acidic (zeolite) component.

Claims (1)

Process for producing a hydrocarbon product by bringing a gaseous mixture comprising carbon monoxide, hydrogen and an unsaturated hydrocarbon into contact with a Fischer-Tropsch catalyst under Fischer-Tropsch conditions, characterized in that the unsaturated hydrocarbon used is ethylene, which is present in an amount of 2-15% by volume, and that a temperature in the range 175-450°C and a pressure in the range 20-100 bar are used as Fischer-Tropsch conditions.