Process for hydrocarbon pyrolysis with spatially separated heating and reaction zones within the reactor chamber

Description

Field of the invention

[0001] The invention is in the field of pyrolytic decomposition of hydrocarbons, preferably methane, to hydrogen and pyrolysis carbon and relates to a device in which several reactors are connected in series and in which carbon particles are introduced from an upstream reactor into a downstream reactor. State of the art

[0002] In the near to medium term, hydrogen production will continue to rely on fossil fuels, primarily natural gas (NG). On the other hand, conventional hydrogen production processes are one of the main sources of anthropogenic CO2 emissions into the atmosphere.

[0003] In principle, hydrogen can be obtained from hydrocarbon fuels through oxidative and non-oxidative conversion processes. Oxidative conversions involve the reaction of hydrocarbons with oxidants such as water, oxygen, or combinations of water and oxygen (steam reforming, partial oxidation, and autothermal reforming processes). As a first step, these processes produce a mixture of hydrogen and carbon monoxide (synthesis gas), from which the hydrogen is separated by gas conditioning (water-gas shift reaction) and, preferably, oxidation reactions and CO2 removal steps. Total CO2 emissions from these processes can reach up to 0.4 m3 per m3 of hydrogen produced.

[0004] Non-oxidative processes involve the thermal decomposition (or dissociation, pyrolysis, cracking) of hydrocarbons into hydrogen and carbon. The thermal decomposition of natural gas has been used for several decades as a means of producing carbon black, with hydrogen being an additional valuable product in the process. In these processes, hydrocarbon vapor is decomposed into hydrogen and soot particles at a temperature of approximately 1400°C over a preheated contact. The process has been carried out, for example, as a semi-continuous (cyclic) process using two tandem reactors. US 2,926,073 describes an improved apparatus for producing soot and hydrogen from hydrocarbons through a continuous thermal decomposition process.

[0005] Electrical heat input is particularly suitable for providing the reaction enthalpy required for the reaction, and renewable electricity can be conveniently used for the process. In this case, the process is a "green" hydrogen technology. The reactor is resistance-heated via at least one pair of electrodes arranged axially in the particle bed. Although carbon particles themselves have high electrical conductivity, the electrical resistance results from the contact points between the particles and the small transfer surfaces. The electrical current flows through the carbon bed and dissipates into thermal energy due to the electrical resistance of the particle bed.

[0006] US 2,982,622 describes a process for the thermal decomposition of methane gas into hydrogen and carbon, wherein methane gas fed into a reactor is fed into a bed of coke particles. In the central region of the reactor, several electrodes are arranged, via which the coke particles are resistance-heated, so that a reaction zone with a temperature in the range of approximately 1040 to 1380°C is created in the region between the electrodes, in which the pyrolysis reaction of the fed methane to carbon and hydrogen takes place. During the process, additional carbon particles are continuously added to the reactor chamber from above, while carbon particles are removed from the bottom. Furthermore, it is possible to return the carbon particles removed from the bottom to the process at the top of the reactor.This process is intended to ensure effective heat transfer from the gas coming from the reaction zone to the carbon particles, as well as from the carbon particles coming from the reaction zone to the supplied gas.

[0007] The use of carbon particles instead of other catalyst materials, some of which allow the conversion of hydrocarbons even at temperatures lower than 1000°C (see, for example, US Pat. No. 3,284,161), has the advantage of allowing the carbon to be formulated during the reaction step. When carbon particles are present, the methane pyrolyzes preferentially on the particles, but soot can also form in the gas phase. The particle sizes can be adjusted by varying the size of the particles and the specific carbon deposition.

[0008] However, one problem with the process described in US Pat. No. 2,982,622 is that it is not easy to ensure a homogeneous heat input over an extended period. For a homogeneous heat input into the heating volume, a homogeneous electrical resistance is required across the entire cross-sectional area of the reactor. If paths with differing electrical resistances occur, the electrical current flows preferentially in the areas of lower electrical resistance, resulting in higher conversions in these areas due to higher temperatures. As a result of the operation of the pyrolysis reactor, pyrolytic carbon deposits over time, further reducing the resistance along these "preferred paths." This results in hot spots and ultimately a failure of the heating concept.

[0009] Another critical aspect of a process as described in US 2,962,622 is the increased tendency for blocking due to the formation of pyrolysis carbon bridges.

[0010] To solve these problems, US Pat. No. 3,254,957 proposes a process in which a reactor filled to a certain height with coke particles is used. Several electrodes are mounted in the reactor below the upper particle filling level, via which the particle material is resistance-heated. An inert gas (e.g., hydrogen) is introduced into the coke particles from below to fluidize the coke particles in the reactor. Methane gas is introduced into the coke particle bed in the area above the electrodes and reacted there, allowing hydrogen generated during the reaction process to be removed from the top of the reactor.

[0011] Although a process as described in US Pat. No. 3,254,957 avoids the above-mentioned disadvantages of the process from US Pat. No. 2,982,622, a significant disadvantage of this process concept is that heat exchange between the coke particles and the hydrocarbon material to be decomposed is not possible. Thus, the teaching of US Pat. No. 3,254,957 requires a significantly higher energy input than the process described in US Pat. No. 2,982,622, which impairs the economic viability of the process.

[0012] Against this background, there is a need for a process for the pyrolytic decomposition of hydrocarbons, and in particular methane gas, into hydrogen and carbon that, on the one hand, realizes the favorable energy transfer from hydrocarbons to carbon particles as described in US Pat. No. 2,982,622, while, on the other hand, avoiding the problems of bridging and blocking in the area between the electrodes. The present invention addresses this need. Description of the invention

[0013] The problem described above is solved by a device for the pyrolytic conversion of hydrocarbons to hydrogen and carbon, which device comprises at least two reactors connected in series with a reactor chamber, wherein each of these reactor chambers has at least two electrodes spaced apart from one another in the flow direction with respect to the flow direction of the hydrocarbons, via which electrodes the reactor can be resistance heated, and wherein each upstream reactor has in its lower region a removal device for particulate material which is connected to the upper region of the respective downstream reactor and enables the supply of particulate material to the upper region of the respective downstream reactor.

[0014] With regard to the above device, it is preferred if the removal device is designed as a rotary valve, screw conveyor, or solids valve. Furthermore, it is expedient if the at least two reactors connected in series in the device according to the invention have respective inlets for hydrocarbons and respective outlets for hydrogen gas. The outlets for hydrogen gas can then be combined into a common line at a suitable location.

[0015] The number of reactors can be selected according to the required carbon deposition or particle access. In each reaction step, hydrocarbon gas, especially natural gas, is conveniently fed into the lower section of the reactor, which is converted into carbon and hydrogen in the reaction zone. The hydrogen or product gas is withdrawn at the top of each reactor, the individual product gas streams are preferably combined, and the remaining product gas heat is conveniently utilized in a steam generator.

[0016] Due to the thermal integration of the hot carbon particles into the hydrocarbon feed, the carbon at the reactor bottom exhibits a moderate temperature level. In the context of the present invention, it is also possible to use the hot product gas stream to preheat the hydrocarbon feed. However, the disadvantage of this variant is the high carbon particle temperature, which places high material demands on the device for transferring the carbon particles to the next reactor.

[0017] The heat integration of the electrically heated carbon bed in the reaction zone requires countercurrent operation via a moving carbon bed. The high-temperature zone, and thus the reaction zone, can be positioned below the lower electrode by adjusting the velocity or heat capacity flow of the carbon bed. The pyrolysis carbon-coated particles can be discharged from the reactor floor after passing through the reactor.

[0018] Kinetic pyrolysis of hydrocarbons is relevant at temperatures above 800°C, so that the available reaction enthalpy results from the product of heat capacity and temperature difference (the temperature difference here refers to the difference between the temperature of the particles heated between the electrodes and the temperature of 800°C required for a relevant reaction). It is therefore necessary to ensure that in the reactor of the device according to the invention, the carbon particles in the region of the electrodes are heated to a temperature above 800°C. To enable a shift in the reaction zone below the electrodes, it is expedient if the carbon particles between the electrodes are heated to a temperature in the range of 1000°C to 2000°C, preferably 1200°C to 1800°C, and more preferably 1400°C to 1600°C.An upper limit of 1600°C has the advantage that the reactor lining can be carried out with conventional materials, which has a positive effect on the costs of the corresponding reactor.

[0019] The hydrocarbons are not subject to any relevant restrictions as long as the release of hydrogen and the formation of carbon are possible in the temperature range above 1000°C. Suitable hydrocarbons include, for example, gaseous or liquid hydrocarbons such as methane, propane, gasoline, diesel, residual oil or crude oil at standard temperature and pressure. Preferred hydrocarbons are gaseous hydrocarbons such as methane and propane, of which methane is the most preferred. The conversion of these hydrocarbons takes place according to the reaction equations: CH 4 → C + 2 H 2 , or C n H m → nC + m / 2H 2 , where n is greater than 1 and m is equal to or less than (2n + 2). Both reactions are endothermic.

[0020] The device according to the invention can be operated economically, especially when using methane as the starting material, since methane is available in large quantities as a component of natural gas and is also inexpensive. Therefore, the hydrocarbons are preferably methane or natural gas.

[0021] For the hydrocarbons or methane, it is preferred if they are fed into the reactor at a speed of 0.1 m / s to 10 m / s, preferably 0.2 m / s to 5 m / s.

[0022] The carbon particles are preferably pyrolysis carbon or coke particles, most preferably pyrolysis carbon particles. Alternatively or additionally, the carbon particles expediently have a particle size in the range of 0.5 mm to 16 mm, and preferably 1 mm to 8 mm.

[0023] Carbon particles are used that promote pyrolytic decomposition of hydrocarbons at temperatures above 1000°C and are electrically conductive. Therefore, electrically conductive particles or packings with a conductivity of 0.001 to 100 S / m are particularly suitable. Commercial products that meet this requirement include DARCO®< KB-B (from Norit Americas Inc.), Black Pearls2000 (from CABOT Corp.), or XC-72 (from CABOT Corp.), as well as calcined petroleum coke with a low sulfur content (< 1 wt%) or the pyrolysis carbon generated during methane pyrolysis.

[0024] It is advantageous if the carbon particles are passed through the reactor at a speed of 0.1 m / h to 100 m / h, preferably 0.1 m / h to 20 m / h, and particularly preferably 1 m / h to 10 m / h (the speed here refers to the migration speed of the particles in the reactor).

[0025] Furthermore, it is expedient if the reactor is operated at a pressure in the range of 1 bar to 40 bar, preferably 5 bar to 30 bar.

[0026] It was stated above that the reactor should have at least two electrodes. In a preferred embodiment, the reactor has exactly two electrodes. In an alternative embodiment, the reactor has three to ten, and preferably four to eight, electrodes.

[0027] In order for the carbon particles used to be usable as an additional valuable product, a certain particle size is often required. The size of the particles or the specific carbon deposition can be adjusted within certain limits in a reaction step by changing the temperature level or the heat capacity of the solid bed. If larger particle sizes are to be produced from relatively small particles, a single pass of the particles through the reaction zone may not be sufficient to achieve the desired particle size. In order to still be able to produce larger particle sizes from small particles, it is possible to add the particles taken from the lower area of the reactor again to the upper area of the reactor so that the particles pass through the reaction zone several times and can thus build up to a larger particle size.

[0028] A disadvantage of such a process can be that carbon particles of different sizes are present in the reactor, which, due to a possibly differing conductivity behavior of the mixture, may require readjustment of the flow rates of the carbon particles and the supplied hydrocarbons. This disadvantage can be avoided according to the invention by using reactors arranged in series, in which the carbon particles removed from the lower end of a first reactor are introduced into the upper region of a directly downstream reactor.According to the invention, at least two reactors arranged sequentially in series are therefore provided, with carbon particles being taken from an upstream reactor in the region below the reaction zone (also referred to as the bottom), in which hydrocarbons are converted to hydrogen and carbon, and introduced into a directly downstream reactor in the region above the two electrodes (also referred to as the head). A process with at least two reactors arranged sequentially in series also enables the generation of particles that are significantly larger than the carbon particle starting material.

[0029] It is possible to fractionate the carbon particles obtained from the reactors connected in series (i.e., to separate them into particles with a predetermined particle size and particles smaller than a predetermined particle size). The particles smaller than the predetermined particle size can then be returned to the first reactor. By conducting the process in this way, the amount of carbon particle material that needs to be fed into the process can be reduced, or even reduced to zero. The particles that have the predetermined particle size can be removed as product. Furthermore, it is possible to feed the particles that are smaller than a predetermined particle size, or a portion of the product particles, to a reduction step, such as a roller mill, in which the particles are broken up and crushed. This ensures that a sufficient number of small particles are present in the process.

[0030] Figure 1 A shows an exemplary reactor, which is not in itself according to the invention, with a reactor chamber 1, from a reactor wall 2 The reactor chamber contains a carbon bed 3, two electrodes 4, and an area 5, in which the pyrolysis reaction takes place. In B An exemplary temperature profile for such a reactor setup is given.

[0031] In Figure 2 An exemplary circuit according to the invention with three reactors is shown. In this reaction scheme, carbon particles 6 at the head of the first reactor 7a and undergo a first pyrolysis step. The pyrolysis carbon is then transported gravimetrically via a conveyor unit 8a, such as a rotary valve, into a second reactor. The first reactor is connected to the line9a supplied with hydrocarbon starting material, while the product gas generated in the reactor is fed via the line 10 In the second reactor 7b, which is over the line 9b supplied with hydrocarbon feedstock, the carbon particles undergo a second pyrolysis step. During the reaction, care must be taken to ensure that the product gas from the second reactor 7b the inflow of the hydrocarbon feedstock of the first reactor 9a not contaminated, as this would dilute the reactor and thus reduce the conversion in the first reactor. The rotary valve thus regulates the moving bed velocity in the reactor and thus the position of the reaction zone, and also isolates the gas phases of the reaction steps from each other. At the lower end of the second reactor 7b is in turn a conveyor unit 8b,such as a rotary valve, through which the carbon particles are transported to the third reactor 7c This in turn is transferred via the line 9c supplied with hydrocarbon feedstock. After passing through the third reactor 7c the carbon particles are transported via the line 11 removed. List of reference symbols

[0032] 1Reactor chamber 2Reactor wall 3Carbon particle bed 4Electrodes 5Main reaction zone 6Carbon particle feed line 7a, 7b, 7cReactor 8a, 8bFeed unit 9a, 9b, 9cHydrocarbon feedstock feed line 10Product gas discharge 11Carbon particle discharge

Claims

1. Apparatus for the pyrolytic conversion of hydrocarbons to hydrogen and carbon, comprising at least two reactors connected in series with a reactor chamber, each of these reactor chambers having at least two electrodes spaced apart from each other in the direction of flow of the hydrocarbons, via which the reactor can be resistance-heated, and wherein each upstream reactor has a removal device for particulate material in its lower region, which is connected to the upper region of the respective downstream reactor and enables the supply of particulate material to the upper region of the respective downstream reactor.

2. Apparatus according to claim 1, characterized in that the removal device is a rotary valve, screw conveyor, or solid matter-valve.

3. Apparatus according to one of claims 1 or 2, characterized in that the at least two reactors connected in series have respective supply lines for hydrocarbons and respective discharge lines for hydrogen gas.