Dehydrogenation of alkanes

Abstract

A reactor system for dehydrogenation of alkanes in a given temperature range upon bringing a reactant stream including alkanes into contact with a catalytic mixture. The reactor system includes a reactor unit arranged to accommodate the catalytic mixture, where the catalytic mixture includes catalyst particles and a ferromagnetic material. The catalyst particles are arranged to catalyze the dehydrogenation of alkanes. The ferromagnetic material is ferromagnetic at least at temperatures up to an upper limit of the given temperature range. The reactor system moreover includes an induction coil arranged to be powered by a power source supplying alternating current and being positioned so as to generate an alternating magnetic field within the reactor unit upon energization by the power source, whereby the catalytic mixture is heated to a temperature within the temperature range by means of the alternating magnetic field. Also, a catalytic mixture and a method of dehydrogenating alkanes.

Claims

1. A method for dehydrogenating of alkanes in a given temperature range T in a reactor system, said reactor system comprising a reactor unit arranged to accommodate a catalytic mixture, said catalytic mixture comprising catalyst particles in intimate contact with a ferromagnetic material, wherein said catalyst particles are arranged to catalyze the dehydrogenation of alkanes, and said ferromagnetic material is ferromagnetic at least at temperatures up to an upper limit of the given temperature range T, and an induction coil arranged to be powered by a power source supplying alternating current and positioned so as to generate an alternating magnetic field within the reactor unit upon energization by the power source, whereby the catalytic mixture is heated to a temperature within the given temperature range T by means of said alternating magnetic field, said method comprising the steps of: (i) generating an alternating magnetic field within the reactor unit upon energization by a power source supplying alternating current, said alternating magnetic field passing through the reactor unit, thereby heating catalytic mixture by induction of a magnetic flux in the material; (ii) bringing a reactant stream comprising alkanes into contact with said catalyst particles; (iii) heating said reactant stream within said reactor by the generated alternating magnetic field; and (iv) letting the reactant stream react in order to provide a product stream to be outlet from the reactor, wherein the catalytic mixture is selected from one of the following: wherein the catalytic mixture comprises catalyst particles and ferromagnetic particles that are mixed and treated to provide bodies of catalytic mixture, said bodies having a predetermined ratio between catalyst and ferromagnetic particles, wherein said catalytic mixture comprises bodies of catalyst particles mixed with bodies of ferromagnetic material, wherein the smallest outside dimension of the bodies is in the order of about 1-2 mm or larger, or wherein said ferromagnetic material comprises one or more ferromagnetic macroscopic supports susceptible for induction heating, wherein said one or more ferromagnetic macroscopic supports are ferromagnetic at temperatures up to an upper limit of the given temperature range T, wherein said one or more ferromagnetic macroscopic supports is/are coated with an oxide and wherein the oxide is impregnated with catalyst particles.

2. The method according to claim 1, wherein the temperature range T is the range from between about 350° C. and about 700° C.

3. The method according to claim 1, wherein the reactant stream is preheated in a heat exchanger prior to step (ii).

4. The method according to claim 1, wherein the catalytic mixture is wherein catalyst particles and ferromagnetic particles are mixed and treated to provide bodies of catalytic mixture, said bodies having a predetermined ratio between catalyst and ferromagnetic particles.

5. The method according to claim 1, wherein the catalytic mixture is wherein said catalytic mixture comprises bodies of catalyst particles mixed with bodies of ferromagnetic material, wherein the smallest outside dimension of the bodies is in the order of about 1-2 mm or larger.

6. The method according to claim 1, wherein the catalytic mixture is wherein said ferromagnetic material comprises one or more ferromagnetic macroscopic supports susceptible for induction heating, wherein said one or more ferromagnetic macroscopic supports are ferromagnetic at temperatures up to an upper limit of the given temperature range T, wherein said one or more ferromagnetic macroscopic supports is/are coated with an oxide and wherein the oxide is impregnated with catalyst particles.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 show temperature profiles of a reactor unit heated by convective/conductive and/or radiation heating, and induction heating, respectively;

(2) FIG. 2 shows temperature profiles along the length of an inductively heated axial reactor unit according to the invention; and

(3) FIGS. 3a and 3b show schematic drawings of two embodiments of a reactor system.

DETAILED DESCRIPTION OF THE FIGURES

(4) FIG. 1 is a graph showing temperature profiles of a reactor unit 10 heated by convective/conductive and/or radiation heating, and induction heating, respectively, during an endothermic reaction within the reactor unit 10. The temperature profiles in FIG. 1 are indicated together with a schematic cross-section through a reactor unit 10 having walls 12 holding a catalyst bed 14 with a catalytic mixture for endothermic reactions. In the case of induction heating, the catalyst mixture in the catalyst bed 14 is susceptible to inductive heating. Means for heating the reactor unit 10 and/or the catalyst bed 14 are not shown. In the case of convective, conduction and/or radiation heating, the means for heating could e.g. be fired burners; means for induction heating would typically be an electromagnet, e.g. an induction coil. A temperature scale is indicated at the right side of FIG. 1. The reactor unit 10 is an axial flow reactor unit and the temperature profiles shown in FIG. 1 indicate the temperatures at the center of the catalyst bed within the reactor unit. The horizontal dotted line indicates a temperature of 550° C. at the centre of the catalyst bed both in the case of convective, conduction and radiation heating (curve 16) and induction heating (curve 17). The dotted curve 16 indicates the temperatures outside the reactor unit, at the reactor unit walls as wells as within the catalyst bed 14 when heated by convective/conductive and/or radiation heating, whilst the solid curve 17 indicate the temperatures outside the reactor unit, at the reactor unit walls as well as within the catalyst bed 14 when heated by convective/conductive and/or radiation heating, and induction heating, respectively.

(5) It is clear from FIG. 1, that in the case of convective/conductive and/or radiation heating, the temperature is higher outside the wall 12 than within the wall 12, and that the temperature within the catalyst bed 14 is lower than that at the wall 12. At the center of the catalyst bed, the temperature is at its lowest. This is because the temperature at the heat source must be higher than the reaction zone and due to the temperature loss through the walls and due to the endothermic nature of the reaction within the reactor unit 10. In contrast, the temperature profile as indicated by the curve 17 shows that for induction heating the temperature is higher at the wall 12 compared to outside the reactor unit, whilst the temperature inside the catalyst bed increases from the wall 12 to the center of the catalyst bed 14.

(6) In general, performing endothermic reactions is limited by how efficient heat can be transferred to the reactive zone of the catalyst bed 14. Conventional heat transfer by convection/conduction/radiation can be slow and will often meet large resistance in many configurations. Moreover, heat losses within the walls of the reactor play a role. In contrast, when heat is deposited inside the catalyst bed 14 by the induction concept, the catalyst bed will be the hottest part of the reactor 10 in contrast to conventional heating where the exterior heat source has to be significantly hotter than the internal part to have a driving mechanism for the heat transfer.

(7) To make the catalyst bed susceptible for induction, different approaches may be applied. One approach is to heat the catalyst by induction by making the catalytically active particles of the catalyst ferromagnetic at reaction temperatures.

(8) In addition to the possibility of delivering heat directly to the catalyst mixture, induction heating offers a fast heating mechanism, which potentially could make upstart of a dehydrogenation reactor relative fast.

(9) FIG. 2 shows temperature profiles along the length of an inductively heated axial reactor unit according to the invention. FIG. 2 shows two different temperature profiles: an isothermal profile I. and an increasing temperature profile II, along the axial direction of the reactor unit. The reactant stream reaches the catalytic mixture at the reactor length L=0 and leaves the catalytic mixture at the reactor length L=1. In the isothermal profile I, the temperature is held constant throughout the reactor length. This is achievable by designing the induction coil and/or the catalytic mixture accordingly. In the temperature profile II, the temperature increases along the path of the reactant stream through the reactor unit. This is advantageous, in that a relatively low inlet temperature (at L=0), reduces the risk of cracking of the reactant stream, and in that a high temperature towards the end of the reactor unit (L=1) provides an improved thermodynamic equilibrium for the dehydrogenation reaction. In the temperature profile II, it is noted that the maximum reactant stream temperature is the outlet temperature. Even though FIG. 2 is shown for an axial flow reactor unit, similar profiles are relevant for radial flow reactor units along the path of the reactant stream through the catalytic mixture.

(10) FIGS. 3a and 3b show schematic drawings of five embodiments 100a and 100b, of a reactor system. In FIGS. 3a and 3b, similar features are denoted using similar reference numbers.

(11) FIG. 3a shows an embodiment of the reactor system 100a for carrying out dehydrogenation of alkanes upon bringing a reactant stream comprising alkanes into contact with a catalytic mixture 120. The reactor system 100a comprises a reactor unit 110 arranged to accommodate a catalytic mixture 120 comprising catalyst particles and a ferromagnetic material, where the catalyst particles are arranged to catalyze the dehydrogenation of alkanes to alkenes and/or dienes and the ferromagnetic material is ferromagnetic at least at temperatures up to about 500° C. or 700° C.

(12) Reactant is introduced into the reactor unit 110 via an inlet 111, and reaction products formed on the surface of the catalytic mixture 120 are outlet via an outlet 112.

(13) The reactor system 100a further comprises an induction coil 150a arranged to be powered by a power source 140 supplying alternating current. The induction coil 150a is connected to the power source 140 by conductors 152. The induction coil 150a is positioned so as to generate an alternating magnetic field within the reactor unit 110 upon energization by the power source 140. Hereby the catalytic mixture 120 is heated to a temperature within a given temperature range T relevant for dehydrogenation of alkanes, such as between 350° C. and about 500° or 700° C., by means of the alternating magnetic field.

(14) The induction coil 150a of FIG. 3a is placed substantially adjacent to the inner surface of the reactor unit 110 and in physical contact with the catalytic mixture 120. In this case, in addition to the induction heating provided by the magnetic field, the catalyst particles 120 adjacent the induction coil 150a are additionally heated directly by ohmic/resistive heating due to the passage of electric current through the windings of the induction coil 150a. The induction coil 150a may be placed either inside or outside the catalyst basket (not shown) supporting the catalytic mixture 120 within the reactor unit 110. The induction coil is preferably made of kanthal.

(15) The catalytic mixture 120 may be divided into sections (not shown in the figures), where the ratio between the catalytic material and the ferromagnetic material varies from one section to another. At the inlet of the reactor unit 110, the reaction rate is high and the heat demand is large; this may be compensated for by having a relatively large proportion of ferromagnetic material compared to the catalytic material. The ferromagnetic material may also be designed to limit the temperature by choosing a ferromagnetic material with a Curie temperature close to the desired reaction temperature.

(16) Placing the induction coil 150a within the reactor unit 110 ensures that the heat produced due to ohmic resistance heating of the induction coil 150a remains useful for the dehydrogenation reaction. However, having an oscillating magnetic field within the reactor may cause problems, if the materials of the reactor unit 110 are magnetic with a high coercivity, in that undesirably high temperatures may be the result. This problem can be circumvented by cladding the inside of the reactor unit 110 with materials capable of reflecting the oscillating magnetic field. Such materials could e.g. be good electrical conductors, such as copper. Alternatively, the material of the reactor unit 110 could be chosen as a material with a very low coercivity. Alternatively, the induction coil 150 could be wound as a torus.

(17) To make the catalyst bed susceptible for induction, different approaches may be applied. One approach is to support the catalyst particles on the ferromagnetic material. For example, the ferromagnetic material comprises one or more ferromagnetic macroscopic supports susceptible for induction heating, and the one or more ferromagnetic macroscopic supports are ferromagnetic at temperatures up to an upper limit of the given temperature range T. The one or more ferromagnetic macroscopic supports is/are coated with an oxide and the oxide is impregnated with catalyst particles. Another approach is to mix catalyst particles and ferromagnetic particles and treat the mixture to provide bodies of catalytic mixture. Additionally or alternatively, the catalytic mixture comprises bodies of catalyst particles mixed with bodies of ferromagnetic material, wherein the smallest outside dimension of the bodies are in the order of about 1-2 mm or larger.

(18) The catalyst particles may comprise gallium, a noble metal catalyst, a metallic sulfide or Cr.sub.2O.sub.3. The catalyst particles may be impregnated on to a carrier. The catalyst particles may be promoted with an appropriate promoter, for example gallium could be promoted with platinum. The metal of the metallic sulfide may e.g. be Fe, Co, Ni, Mn, Cu, Mo, W and combinations thereof. The catalyst particles may be mixed with a ferromagnetic material with a high coercivity and a high Curie temperature, such as AlNiCo or Permendur.

(19) The catalytic mixture preferably has a predetermined ratio between the catalyst particles and the ferromagnetic material. This predetermined ratio may be a graded ratio varying along a flow direction of the reactor.

(20) In another approach, ferromagnetic macroscopic supports are coated with an oxide impregnated with the catalytically active material. This approach offers a large versatility compared to the ferromagnetic nanoparticles in the catalyst, as the choice of catalytic active phase is not required to be ferromagnetic.

(21) FIG. 3b shows another embodiment 100b of the reactor system for carrying out dehydrogenation of alkanes upon bringing a reactant stream comprising alkanes into contact with a catalytic mixture 120. The reactor unit 110 and its inlet and outlet 111, 112, the catalytic mixture 120, the power source 140 and its connecting conductors 152 are similar to those of the embodiment shown in FIG. 3a.

(22) In the embodiment of FIG. 3b, an induction coil 150b is wound or positioned around the outside of the reactor unit 110.

(23) In both embodiments shown in FIGS. 3a-3b, the catalytic mixture can be any catalytic mixture according to the invention. Thus, the catalytic mixture may be in the form of catalyst particles supported on the ferromagnetic material, e.g. where in the form of ferromagnetic macroscopic support(s) coated with an oxide, where the oxide is impregnated with catalyst particles, miniliths, a monolith, or bodies produced from a mixture of catalyst particles powder and ferromagnetic material powder. Thus, the catalytic mixture is not limited to catalytic mixture having relative size as compared to the reactor system as shown in the figures. Moreover, when the catalytic mixture comprises a plurality of macroscopic supports, the catalytic mixture would typically be packed so as to leave less space between the macroscopic supports than shown in the FIGS. 3a and 3b. Furthermore, in the two embodiments shown in FIGS. 3a and 3b, the reactor unit 110 is made of non-ferromagnetic material. In the two embodiments shown in FIGS. 3a and 3b, the power source 140 is an electronic oscillator arranged to pass a high-frequency alternating current (AC) through the coil surrounding at least part of the catalyst particles within the reactor system.

EXAMPLE

(24) Catalyst bodies for propane dehydrogenation reaction are made by impregnating an alumina carrier with gallium, typically about 1 wt % gallium. The alumina carrier may be shaped as a cylinder or as an extrudate with an equivalent diameter around 3 mm. The catalyst bodies are physically mixed with ferromagnetic material having a high coercivity and a Curie temperature above about 600° C. The ferromagnetic material may e.g. be a cast iron or Alnico pretreated by oxidation in steam and hydrogen at temperatures above 700° C. Preferably, the ferromagnetic material could be ferromagnetic bodies in the form of small galvanized iron spheres, using either tin or zinc as galvanizing agent. The iron oxide may e.g. be magnetite and should have a suitable coercivity, e.g. a relatively high magnetic coercivity, .sub.BH.sub.C, e.g. .sub.BH.sub.C>20 kA/m.

(25) The catalyst bodies and the ferromagnetic material, e.g. galvanized iron spheres, are physically mixed. The mixing can be graded so the concentration of ferromagnetic material in the mixture differs throughout the path of the reactant stream through the reactor unit, viz. along the length of the reactor unit in the case of an axial flow reactor unit. For example, in the inlet region of the reactor unit, where the heat consumption is the highest due to heating of the incoming gas stream as well as conversion of the incoming gas stream, the catalytic mixture may be arranged to have a relatively higher amount of heat generating material, viz. ferromagnetic material, compared to sections of the reactor unit further downstream. Alternatively, in the inlet region of the reactor unit, the concentration of ferromagnetic material in the catalytic mixture may be lower than downstream sections of the reactor unit, since it may be advantageous to have a temperature gradient within the reactor unit where the temperature increases along the path of the reactant stream through the reactor unit. This is due to the fact that a low temperature at the inlet region of the reactor unit reduces the risk of cracking of the reactant stream. Moreover, a higher temperature towards the outlet region of the reactor units provides a better thermodynamic equilibrium. Thus, the grading of the ferromagnetic material within the catalytic mixture may be used to optimize the exit temperature, giving a high thermodynamic potential for conversion. Ideally the choice of material with high coercivity may be used to tune the exit temperature, since no heat will be generated above the Curie temperature. This will furthermore remove the risk of overheating the catalyst bodies, with resulting reduced parasitic reaction such as coking and cracking.

(26) For propane dehydrogenation, the catalytic bodies within the reactor are preheated to 580° C. and kept at this temperature by means of inductive heating. The propane gas for dehydrogenation, which could be diluted with a carrier gas, typically nitrogen, hydrogen or steam, is preheated by a feed effluent heat exchanger to about 500° C. The pressure is kept around 1 bar by pumping on the exit stream. The reaction mixture is further heated as the dehydrogenation takes place. If equilibrated at 550° C. a pure C.sub.3H.sub.8 gas will experience a 36% conversion into propene and hydrogen. The resulting reaction product outlet from the reactor unit is cooled by heat exchange.

(27) Even though the present invention has been described in connection with dehydrogenation of alkanes, primarily the dehydrogenation of alkanes to alkenes and/or to dienes, it should be noted that the invention is also suitable for dehydrogenation of other hydrocarbons.

(28) Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. The scope of the present invention is set out by the accompanying claim set. In the context of the claims, the terms “comprising” or “comprises” do not exclude other possible elements or steps. Also, the mentioning of references such as “a” or “an” etc. should not be construed as excluding a plurality. Furthermore, individual features mentioned in different claims may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is not possible and advantageous.