DE202021001131U1 - Electrically directly heated catalysts for endothermic tube bundle reactors - Google Patents

Abstract

Tube bundle reactor (1) for endothermic reactions, characterized in that catalytically active internals (3a, b, c) for carrying out reactions are provided in the tubes (3).

Description

In chemical engineering, tube bundle reactors are very often used for catalyzed process engineering. Catalysts are arranged as a fixed bed in up to 40,000 parallel pipes, which act as heat exchangers between the inner space filled with catalyst and the outer space. The inner diameter of such pipes can be between 10 and 50 cm and the height up to 4 m (exceptions 15th m).

Tube bundle reactors are used for strongly exothermic or endothermic reactions in which an intensive heat exchange has to be ensured.

In the exothermic process, the released heat of reaction can be dissipated in the coolant (heat transfer oils, molten salt, water, etc.) that flows around the pipes.

With the endothermic process, the required heat can be transferred to the tube bundle tubes, for example through a very large number of ceiling burners (up to 3000). In this case, the heat of the furnace flame is to be transported by radiation to the pipe wall and then further inwards by heat conduction to the ceramic catalyst bed and to the fluid flowing through. Tube bundle reactors with side burners are also used, but this requires the 5 - multiple need for side burners with corresponding peripherals. In addition, the stress on the refractory lining is higher for side-fired reactors than for ceiling-fired versions.

The performance of endothermic tube bundle reactors is heavily dependent on the generation and distribution of heat within the reactor, but, what is even more important, also on the heat conduction within the ceramic packed bed catalysts used up to now.

Due to the heat transport mechanisms due to the insufficient heat conduction of the ceramic catalysts and the simultaneously occurring endothermic reactions, a different axial and, at the same time, a radial temperature profile is established within the reaction tubes.

The endothermic process management is state of the art, but it requires a very high peripheral expenditure for the fuel supply, the heat supply and, last but not least, for the safety-relevant monitoring in the event of failure of these components.

For decades, endothermic reactions have been taking place in tube bundle reactors using ceramic catalysts. This concept is well known, relatively simple and can be scaled relatively easily by changing the number of pipes. A disadvantage of almost all fixed bed configurations, however, is the heat management, since a very large number of small pipes are required in order to even out the temperature profile within the bed and to avoid possible harmful hot spots. The heat input from the outside heated pipe wall to the inside takes place through heat conduction. The temperature difference between the outer pipe wall and the pipe center can be up to 150 ° K lower radially. Due to the very different expansion coefficients between the metallic pipes (high expansion coefficient) and the ceramic catalyst bed (low expansion coefficient), catalyst breakage occurs again and again when such reactors are heated to the operating point and when such reactors are cooled down due to malfunctions. The result is pressure losses that are far outside the operating window. Furthermore, the need to select very small pipe diameters due to the low heat conduction of the bed, additional restrictions in the catalyst particle size of the ceramic bed and / or further problems in connection with the pressure loss in these pipes. The smaller the diameter of the tubes, the smaller the catalyst sizes must be. This also significantly increases the risk of creeping currents on the pipe wall. In addition, the different temperatures inside the reactor tubes cause further complications, such as undesirable side reactions, which one tries to avoid or to minimize by various countermeasures, such as regeneration. For the necessary regeneration of the catalyst bed, the burner output must be reduced to very low values at regular intervals and the furnace chamber temperature can drop so far that the burners no longer ignite spontaneously, if necessary, but rely on external ignition sources. This requires that each of the hundredfold burners must be equipped with a flame monitor with appropriate instrumentation and maintenance. The problem of contamination of the sight glasses is also the focus of the plant operation.

In the patent literature ( EP 0247384 A2 ; DE 10357064 A1 ; EP 1178 278 A2 ; EP 1681 091 A2 ; WO 2016 106293 A1 ) numerous measures for processes and devices for optimizing the heat balance of burner-operated - mostly natural gas - endothermic tube bundle reactors are specified. Some may alleviate the problem, but one disadvantage of using natural gas as a heating gas is that it emits _{CO 2} and NO _{x when it is fired} becomes. Existing emission limits require tax-relevant consideration and / or extensive air pollution control measures.

• In DE 10 2015 013 071 A1 wird vorgeschlagen die katalytische Schüttung innerhalb der Reaktorrohre zunächst im 1. Abschnitt elektrisch induktiv über Einzelspulen und anschließend im 2. Abschnitt über Dekenbrenner zu beheizen. Der Katalysator soll hierbei über „geeignete Eigenschaften“ verfügen.
• Diese zweifache Anordnung erscheint aufgrund der großen Anzahl von Rohren und der hieraus resultierenden Peripherie hinsichtlich Versorgung sowie Meß- und Regeltechnik etwas komplex.
• Außerdem sollte die Temperatur im Grenzbereich zwischen elektrischer und Brennerbeheizung innerhalb der zahlreichen Rohre, zur Vermeidung von Nebenreaktionen, möglichst im ähnlichen Bereich regelbar sein. Zusätzlich ist zu erwähnen, dass es sich hierbei um eine indirekte Beheizung der Katalysatoren handelt.

To generate the required heat flow within endothermic tube bundle reactors, a whole series of measures are presented in the patent literature, some of which are intended to reduce the consumption of combustion media through the use of other heat sources:

• In DE 10 2015 013 071 A1 it is proposed to heat the catalytic bed inside the reactor tubes first in the 1st section electrically inductively via individual coils and then in the 2nd section via Deken burners. The catalyst should have “suitable properties”.
• This double arrangement appears somewhat complex due to the large number of tubes and the resulting peripherals in terms of supply as well as measurement and control technology.
• In addition, the temperature in the border area between electrical and burner heating within the numerous tubes should be controllable in a similar range as far as possible in order to avoid side reactions. It should also be mentioned that this is an indirect heating of the catalysts.

In DE 10 2015 004 121 A1 electrical secondary heating is also proposed, in which case all reactor tubes should not be connected to a voltage source. This arrangement also appears to be associated with a lot of effort due to the large number of pipes and the additional measures resulting therefrom with regard to supply and measurement and control technology. It should also be mentioned that this proposal also involves indirect heating of the catalytic converters.

In DE 10 2018 213 463 A1 it is proposed to operate a tube bundle reactor with ceiling burners. In addition, the reactor tubes can be equipped with a conductive (resistance) or inductive (magnetic field) heating device. The ceiling burners, on the other hand, are suggested for all pipes and should be in continuous operation.

Also, some pipes should not be equipped with a catalyst bed. No explicit details are given about measurement and control options for the different heat flows. Also the criteria according to which the selection of the pipes to be additionally equipped with electrical heating or the catalyst-free pipes is to be made are not defined.

It is also stated that: "Suitable ceramic materials should also be used as heating elements".

Further in the patent text: “Suitable ceramic materials preferably include ceramic materials known to those skilled in the art or material mixtures with ceramic components which are stable at the operating temperatures of the reformer or have only little material wear. Exemplary ceramics can be Al ₂ O ₃ , MgO, ZrO ₂ , aluminum titanates, silicides, carbides, nitrides, etc. ”.

However, the person skilled in the art is also aware that, for example, Al ₂ O ₃ , aluminum titanate (Al ₂ TiO ₅ ), MgO; SiO ₂ , ZrO _{2 and} others still have very high specific volume resistances even at high temperatures, so that they are widely used especially as electrical high-temperature insulators.

In Wismann Sebastian Thor (2019), "Electrocally heated steam methane reforming", Technical University of Denmark, it is explained that uniform heating of tube bundle reactors is a very complex process. The combustion must be a few hundred degrees higher than the desired reaction temperature, the required power must be approx. 150 kW / m2. As a result, of the total volume of the burner-fired reactor, only less than 2% can be used as total volume for the catalyst. Furthermore, it is stated that conventional burner-heated tube bundle reactors, due to the stringent requirements with regard to the heating gradient of the refractory lining, require approx. One week to achieve a constant operating state.

The object of the invention is therefore to provide the heat demand of endothermic tube bundle reactors centrally via conductive, directly heated catalysts without having to use hundreds, in some cases thousands, of failure-prone and maintenance-intensive individual burners.

This object is achieved in that the tubes of the tube bundle reactors are not filled with ceramic overlay catalysts, as established in the prior art, but with metallic, oxide or non-oxide ceramic materials of uniform shape and structure that are electrically conductive and thus generate Joule heat Target temperatures are heatable. These materials can also serve as a catalyst carrier or be directly catalytically active. The positive effect of this measure for the entire system can be considerably intensified by a further catalytic inner coating of the individual reaction tubes.

The metallic, oxide or non-oxide ceramic, electrically directly heated catalytic converters ensure that the heat transport can be evened out both axially and radially within the tubes. As a result, tubes with a considerably larger diameter and lower heights and, as a result, very compact tube bundle reactors can be used. Another advantage is that by eliminating the large number of burners, which cause a large amount of radiant heat, the refractory structure of these tube bundle reactors can be made considerably smaller.

It is also known that, for example, metallic catalyst supports have a low heat capacity and high thermal shock resistance. As a result of this and the high thermal conductivity of these candidates, the reaction device can be heated to its operating temperature more quickly and more evenly than when using previously known catalyst supports. A person skilled in the art recognizes that by heating the system up to its operating temperature more quickly, the capacities can be increased enormously and thus the operating costs can be reduced.

In addition, the electrically heatable catalytic converters mentioned here can be positioned as flexible compression packings within the tubes. This ensures that there is always intensive contact and thus heat transfer between the inner pipe wall and the built-in element.

Overall, the known susceptibility of endothermic catalytic tube bundle reactors used to date can be reduced to a large extent and the efficiency can be increased significantly by the invention presented here.

The specific energy consumption of direct electrical heating of the catalytic converters is also considerably lower than that of natural gas heating with ceiling burners. This is of course also due to the fact that these systems can be set up much more compactly. The use of electricity from renewable energy sources is particularly advantageous here, especially in times of low demand from other consumers. Therefore, they could also be considered for decentralized use. Under the premise that the development in the direction of the increased use of electric cars will be accelerated worldwide, bidirectional charging of the electric cars connected to the power grid as a power storage (buffer) for wind and solar power, if generation is not constant to use. There is also the possibility of rechargeable batteries from electric cars, which can no longer be used, but still have a sufficient capacity of 70-80% and whose recycling is still uneconomical due to the fact that the performance potential is still too high, can also be used as a power storage device for the energy requirements of the invention presented here.

Electrically heatable catalysts or catalyst carriers are materials that are electrically conductive. They can consist of one or more components, each of which, due to its relatively high specific electrical resistance, fulfills the required property of converting electrical energy into thermal energy. One of the main requirements, in addition to resistance to high temperatures and oxidation, is that the specific resistance remains as independent of the operating temperature as possible. Alloying can reinforce the positive properties of the individual catalysts or catalyst carriers and differentiate them from their use in tube bundle reactors.

Suitable material components from the metal sector include austenitic CrFeNi alloys and ferritic CrFeAl alloys. For example, metals such as Ru, Ir, Rh, Ni, Co, Os, Pt, Fe, Mo, Pd, Ag are used as catalytically active substances in methanation (exothermic reaction). In addition to the optimal catalytic effect and sufficient selectivity for the product methane, economic efficiency also plays an important role in the selection of the catalyst. Catalysts based on nickel have proven particularly suitable. They have adequate selectivity and good catalytic action and, above all, have hitherto been available in sufficient quantities and at low cost. Nickel-based catalysts are widespread in chemical reaction management and can therefore be assessed as the state of the art. The coating of different catalyst carriers by means of washcoat processes is also known to the person skilled in the art as a very suitable option and method of choice.

However, it is advantageous to dispense with a catalyst carrier and instead strive to provide e.g. nickel as an electrically directly heatable catalytic component in a suitable material composite.

In the field of non-oxide ceramics, silicon carbide (SiC) and molybdenum disilicide (MoSi ₂ ) should be mentioned in particular for direct electrical heating of catalyst carriers.

Advantageous directly heatable catalyst geometries are uniform structures such as turbulators, cylindrical brush-like shapes, cylindrical foam-like packings, etc.

• 1 eine vereinfachte Schnittzeichnung durch einen Rohrbündelreaktor gemäß Stand der Technik mit keramischen Katalysatoren innerhalb der Reaktionsrohre und Deckenbrennern (nur 4 dargestellt),
• 2 eine vereinfachte Schnittzeichung durch einen erfindungsgemässen Rohrbündelreaktor mit elektrisch direktbeheizten Katalysatoren innerhalb der Reaktionsrohre,
• 3 - 5 eine vereinfachte Schnittzeichung eines erfindungsgemässen endothermischen Reaktionsrohres mit unterschiedlichen elektrisch direktbeheizten Katalysatoren.

Further advantages, details and features of the invention emerge from the following description and on the basis of the drawings, this is shown in:

• 1 a simplified sectional drawing through a tube bundle reactor according to the state of the art with ceramic catalysts within the reaction tubes and ceiling burners (only 4 shown),
• 2 a simplified sectional drawing through a tube bundle reactor according to the invention with electrically directly heated catalysts within the reaction tubes,
• 3 - 5 a simplified sectional drawing of an endothermic reaction tube according to the invention with different electrically directly heated catalysts.

The in 1 The tube bundle reactor, generally referred to as 1, consists of a large number of tubes ( 2 ). Inside these pipes there is a ceramic catalytic bed ( 3 ). The fluid to be treated enters (4) and after the reaction enters ( 5 ) out again. The heat required for the endothermic reaction is generated by burning the combustion medium ( 6th ) inside the ceiling burner ( 7th ) generated. In ( 8th ) the exhaust gases flow out.

In 2 is, also shown as 1, a tube bundle reactor, which consists of a large number of tubes ( 2 ) is constructed. Inside these pipes are highly thermally conductive, electrically directly heated catalysts ( 3a, b, c ) positioned. The fluid to be treated enters (4) and after the reaction enters ( 5 ) out again.

In 3 is a so-called turbulator / twisted tape ( 3a) shown, which is electrically directly heated and can be used as a catalyst in the required temperature range.

In 4th is a brush-shaped, electrically directly heated catalyst ( 3b) shown.

In 5 is a catalytically coated foam structure ( 3c) shown, which is also electrically directly heated. In addition, a metal mesh ( 9 ) shown. This fabric consists of the same material as the directly heated catalyst and is used for the intensive connection between the catalyst and the reactor tube.

Of course, the examples described can still be modified and supplemented in many respects without departing from the basic concept of the invention. The invention thus also relates to the method for optimizing catalytic reactions by using electrically heatable catalytic devices.

QUOTES INCLUDED IN THE DESCRIPTION

This list of the documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent literature cited

• EP 0247384 A2 [0009]
• DE 10357064 A1 [0009]
• EP 1178278 A2 [0009]
• EP 1681091 A2 [0009]
• WO 2016106293 A1 [0009]
• DE 102015013071 A1 [0010]
• DE 102015004121 A1 [0011]
• DE 102018213463 A1 [0012]

Claims (5)

Tube bundle reactor (1) for endothermic reactions, characterized in that catalytically active internals (3a, b, c) for carrying out reactions are provided in the tubes (3). Tube bundle reactor (1) according to claim 1, that the catalytically active internals are designed as turbulators (3a), brush-like (3b) or foam-like (3c) structures. Tube bundle reactor (1) according to one of the preceding claims, characterized in that the catalytically active internals (3a, b, c) are electrically conductive and directly provide the heat required for the reaction via conductive heating. Tube bundle reactor (1) according to one of the preceding claims, characterized in that the energy for direct heating of the catalytically active internals is also supplied via renewable energy sources. Tube bundle reactor (1) according to one of the preceding claims, characterized in that the supply of energy for direct heating of the catalytically active internals also takes place via renewable energy sources and these, if required, are supplied via electricity storage.

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