NO177492B - Process for catalytic production of hydrocarbons having at least 2 carbon atoms - Google Patents

Description

The invention relates to a method for the catalytic production of hydrocarbons containing at least 2 carbon atoms per molecule, where the hydrocarbons are at least partially present as a liquid under the prevailing reaction conditions, from a gas mixture comprising hydrogen and carbon monoxide.

Methods for converting synthesis gas (that is, a gas mixture comprising hydrogen and carbon monoxide) into hydrocarbons are known. As such conversions are highly exothermic processes, aids must usually be used to remove heat from the reaction zone. For example, a suitable reactor is a multi-tube reactor in which a coolant flows through the spaces between the tubes. The tubes are filled with suitable catalyst particles. Synthesis gas flows in a downward direction through the pipes and the reaction products are removed from the bottom of the reactor.

Currently, there is a growing demand for equipment with greater capacity, not only because all chemical processes are carried out on an increasingly large scale, but also because certain processes are used to an increasing extent. In particular, the synthesis of hydrocarbons containing at least two carbon atoms per molecule by converting carbon monoxide and hydrogen, produced for example by gasification of coal, is of increasing interest.

Scaling up a reactor of the above-mentioned multi-tube type to increase its capacity can, however, have a serious adverse influence on efficiency, especially if the reactor is used to carry out highly exothermic reactions, such as the conversion of carbon monoxide and hydrogen to hydrocarbons. In this highly exothermic reaction, the released heat must be removed continuously in order to avoid undesired high temperatures which could cause a sharp increase in the reaction rate (possibly followed by deactivation of the catalyst) and/or occurrence of unwanted side reactions. The goal of continuous heat removal in exothermic reactions means, with regard to tube-type reactors, that the tubes that make up the reaction zones must have a relatively small cross-sectional area with a heat transfer medium that circulates around the outer surfaces of the tubes. If the cross-sectional areas of the reaction zones are large, then the middle parts of the zones are too far away from the heat transfer medium on the outside of said zones, and will therefore tend to experience unwanted temperature increases or temperature drops. Increasing the capacity of a tube-type reactor should therefore be done by increasing the number of tubes rather than increasing the diameter of the tubes. However, the use of a large number of tubes enclosed in a reactor vessel with a necessarily large diameter poses a number of problems, namely that, firstly, it becomes difficult to achieve uniform distribution of heat transfer medium over the entire diameter of the reactor vessel, and secondly, it becomes more difficult to get an even distribution of fluids over the different pipes. An even distribution of heat transfer medium is required to obtain a reaction product with a predetermined constituent and to prevent stresses in the tube bundles due to temperature differences.

An object of the present invention is to overcome the above-mentioned problems encountered when increasing the capacity of tube-type reactors suitable for carrying out strongly exothermic reactions such as the catalytic production of hydrocarbons from hydrogen and carbon monoxide.

It has now been found that the catalytic production of hydrocarbons containing at least two carbon atoms per molecule can very suitably be carried out in a reactor comprising a catalyst layer where one or more spirally wound cooling tubes have been installed, the reactor being kept at conversion conditions.

Use of a reactor of this type does not lead to the problem encountered with increasing the capacity of a multi-tube reactor, in particular the uniform distribution of heat transfer medium over the entire diameter of the reaction vessel and along the tubes. A higher heat transfer for the process side is also achieved, and it is not necessary to use tube sheets of very large diameter. Another advantage is that the thickness of the wall in the reactor is determined by the process pressure and not by the pressure in the coolant, as the process pressure is usually lower than the pressure in the coolant. The helical cooling coils do not cause major expansion problems, which makes the reactor less sensitive to temperature differences between the tubes and the reactor wall. Furthermore, it is easier to charge and discharge the catalyst in this type of reactor than it is in a multi-tube reactor.

The method according to the present invention therefore relates to the catalytic production of hydrocarbons which contain at least two carbon atoms per molecule, and where the hydrocarbons are at least partially present as a liquid under the prevailing reaction conditions, from a gas mixture comprising hydrogen and carbon monoxide, comprising passing the gas mixture through a reaction zone containing a stationary catalyst bed comprising catalyst particles. The method is characterized by the fact that heat is removed from the reaction zone with a cooling medium that flows in one or more spiral cooling coils.

Spiral-wound tubes or tube bundles are used to achieve the spiral-shaped cooling coils in the cooling medium. Preferably, a cylindrical reaction vessel is used which is equipped with one or more spirally wound tubes or tube bundles, each tube bundle comprising two or more spirally wound tubes with substantially the same dimensions, and located in a concentric ring or a number of concentric rings around the reaction vessel's central axis . Thus, the coolant flows via one or more spiral-shaped cooling windings located concentrically around the central axis, each pattern containing one or more spirals. When two or more concentric tubes or tube bundles are used, the screw direction of the spirals in two adjacent tubes or tube bundles is preferably opposite to each other. When two or more tube bundles are used, it is preferred to use an increasing number of spirally wound tubes in bundles located at a greater distance from the central tube and to maintain essentially the same length for each tube.

The spiral flow pattern in the coolant enables the ratio heat exchanger surface/reactor volume to be varied over a large area. The pipe diameter can be varied as well as the distance between two layers of pipes in both the axial and radial directions. The diameter of the cooling pipes is suitably chosen between 4 and 55 mm and especially between 10 and 35 mm. The distance between two adjacent rings of tube or tube bundle (distance in the radial direction) is suitably chosen between 10 and 50 mm, especially between 15 and 25 mm, and the distance between two adjacent spiral windings located in a concentric ring (distance in the axial direction) is suitably chosen between 10 and 200 mm, especially between 10 and 50 mm. The spirally wound tube bundles make it possible to use hemispherical tube sheet designs, thereby avoiding the less suitable flat tube sheets.

The heat exchanger tubes are preferably placed at such a distance in the reaction zone or zones that an optimal temperature profile is achieved therein in the radial direction. Furthermore, each group (for example concentric ring) of heat exchanger tubes can be in communication with separate inlet and outlet devices for cooling fluid which can be operated independently of other groups of heat exchanger tubes in order to achieve optimal control over the temperature profile in the reaction zone or zones.

Water is usually used as a cooling medium. Preferably, the water evaporates at least partially in the pipes. This enables the reaction heat to be removed from the reaction zone by producing steam. Other cooling media such as organic compounds, for example biphenyl, thermal oils or liquid metals can also be used.

The method according to the invention is particularly suitable for converting a synthesis gas starting material (at least partially) into hydrocarbons having at least 5 carbon atoms per molecule, preferably at least 10 carbon atoms per molecule; most preferably, paraffinic hydrocarbons with at least 20 carbon atoms per molecule. It is noted that when a substantial part of the hydrocarbons contains 20 carbon atoms per molecule (which will be the case when the main part of the hydrocarbon molecules contains at least 10 carbon atoms per molecule), then a significant part of the reaction product is a liquid under the usual conditions maintained during the reaction. Especially when a (partially) liquid product is obtained, problems could have been expected due to liquid retention and occurrence of flow patterns. In the case of a reaction product containing an average of 10 carbon atoms, most of the reaction products will be gaseous under the usual reaction conditions, and only a small amount of liquid product will be formed.

The synthesis gas feedstock referred to above contains as major components hydrogen and carbon monoxide; in addition, it may contain carbon dioxide, water, nitrogen, argon and small amounts of compounds with 1-4 carbon atoms per molecule, for example methane, methanol or ethylene.

The synthesis gas starting material can be produced in any manner known in the art, for example by steam/oxygen gasification of a carbonaceous material, for example lignite, anthracite, coke, crude mineral oil and fractions thereof, as well as oil extracted from tar sands and bituminous shale. Alternatively, steam-methane reforming and/or partial catalytic oxidation of a hydrocarbon-containing material with an oxygen-containing gas can be used to produce synthesis gas excellently suitable for use in the method according to the invention.

The present method is preferably carried out at a temperature of 100 - 500°C, a total pressure of 1-200 bar abs. and a space velocity of 200-20,000 m<3> gaseous feed per m<3 >reaction zone per hour. Particularly preferred process conditions for the production of hydrocarbons include a temperature of 150-300°C, a pressure of 5-100 bar abs. and a space velocity of 500-5000 m<3> (S.T.P.) gaseous feed/m<3 >reaction zone/hour. The term "S.T.P." as referred to above means Standard Temperature (in 0°C) and Pressure (1 bar abs.) In case synthesis gas is used as gaseous feed, the H20/C0 molar ratio therein is preferably 0.4-4 and most preferably 0.8- 2.5.

Suitable catalysts for the production of (paraffinic) hydrocarbons from synthesis gas contain at least one metal (compound) from group 8 in the periodic table, preferably a non-noble metal, especially cobalt, possibly in combination with a noble metal, for example ruthenium, on a refractory oxide carrier, such as silicon dioxide, aluminum oxide or silicon dioxide/alumina, especially silicon dioxide or aluminum oxide. Furthermore, the catalysts preferably contain at least one other metal (compound) from group 4b or 6b, most preferably selected from the group consisting of zirconium, titanium and chromium. The catalysts preferably contain 3-60 parts by weight of cobalt, optionally 0.05 - 0.5 parts by weight of ruthenium and 0.1 - 100 parts by weight of another metal or metals per 100 parts by weight carrier.

The metals can be introduced into the catalyst using any method known for this in the art, for example (gas) impregnation (for example in the form of chlorides or carbonyls), ion exchange, cracking or precipitation. Kneading and impregnation are preferred methods, the latter being particularly suitable for introducing cobalt. The resulting catalyst preparation is preferably calcined at temperatures of 350-370°C after each impregnation or kneading step.

Other suitable catalysts for the production of hydrocarbons, especially hydrocarbons boiling in the gasoline range and which are rich in aromatic hydrocarbons, are bifunctional catalysts comprising a component with activity for converting synthesis gas to acyclic hydrocarbons and acyclic oxygen-containing hydrocarbons such as methanol and dimethyl ether, in combination with a component which has activity to convert at least part of the previously mentioned products into aromatic hydrocarbons. Suitable components for converting synthesis gas to hydrocarbons and oxygenated hydrocarbons contain at least one metal from group 8 of the periodic table, especially iron. Suitable components for the production of aromatic hydrocarbons are crystalline silicates, for example crystalline aluminum silicates (zeolites), crystalline iron silicates and crystalline gallium silicates.

The catalysts are preferably used in a present method in the form of spherical, cylindrical or lobed particles with a diameter of 0.1-15 mm, and especially 0.5-

5 mm. The catalyst carrier particles can be produced using any method known in the art, for example pressing or extruding powdered catalyst material, if desired together with a binder material. Catalyst carrier spheres, particularly silica-containing spheres, are conveniently prepared by the "oil drop" method whereby said spheres are formed as droplets of a silica gel which solidifies when dropped into an oil bath. Alumina-based carriers are preferably made by extrusion. The catalyst present in the reaction zone or zones can be kept in contact with liquid product in case relatively heavy paraffins (with more than 20 carbon atoms per molecule) are synthesized with the present method in order to avoid the formation of carbonaceous deposits on the catalysts. Distribution devices for liquid (for example in the form of plates or layers of material that have a relatively low permeability for liquid and/or gas) can be arranged over the reaction zone or zones in order to induce substantially equal distribution over the catalyst layer and the desired optimum contact with liquid product.

Furthermore, the invention relates to liquid products produced by a method as described above.

Illustrative example

A mixture of hydrogen and carbon monoxide (H2/C0 ratio 2) is fed to a 50 ml reactor equipped with one helical cooling tube. The reactor comprises a stationary layer of a Co/Zr/SiO2 catalyst (25 parts by weight Co and 18 parts by weight Zr per 100 parts by weight SiO2, prepared by impregnating silicon dioxide with a solution of zirconium tetra-n-propoxide in n-propanol/benzene , followed by impregnation of the zirconium-coated carrier with an aqueous cobalt nitrate solution). The reaction is carried out at a temperature of 220°C, a pressure of 20 bar and a GHSV of 2000 NI gas/l cat/hour. A conversion of approx. 75% of CO is achieved, resulting in 300-350 g hydrocarbons/1/- cat/hour.

Claims (5)

1. Process for the catalytic production of hydrocarbons containing at least 2 carbon atoms per molecule, where the hydrocarbons are at least partially present as a liquid, under the prevailing reaction conditions, from a gas mixture comprising hydrogen and carbon monoxide, the gas mixture being passed through a reaction zone containing a stationary catalyst layer comprising catalyst particles, characterized in that heat is removed from the reaction zone with a refrigerant flowing in one or more spiral cooling coils. 2. Method as stated in claim 1, characterized in that the coolant flows through 2 or more concentric spiral cooling windings. 3. Method as stated in claim 2, characterized in that the screw direction of adjacent spiral cooling windings is opposite to each other. 4. Method as set forth in any of the preceding claims, characterized in that it is carried out at a temperature of 100-500°C, a total pressure of 1-200 bar abs. and a space velocity of 200-20,000m<3> (S.T.P) gaseous feed/m<3 >reaction zone/hour. 5. Method as set forth in any of the preceding claims, characterized in that a catalyst is used which comprises 3-60 parts by weight of cobalt and 0.1-100 parts by weight of at least one other metal selected from the group consisting of zirconium, titanium and chromium, per 100 parts by weight silicon dioxide, alumina or silicon dioxide/alumina carrier.