Membrane reactors

Background

The separation of substances is nowadays performed downstream of the chemical synthesis and is usually more time and energy consuming than the synthesis itself. Thermal processes such as distillation, extraction or adsorption are dominating in this field of application Membrane separation processes are significantly less energy consuming and therefore cheaper than thermal separation processes. Due to the chemical and thermal stability of ceramic materials, it should be possible to use the membranes directly within the reactor and thus couple chemical reaction and material separation. On one hand the membrane can be used for the dosing of starting materials or on the other hand for the separation of (by-)products to shift the chemical equilibrium and thus to increase yield. It may itself be catalytically active or coated with a catalyst. This makes it possible to separate reactive intermediates and to achieve higher yields in equilibrium-limited reactions.

By coupling a catalyzed chemical reaction with a membrane that can be used for reactant-dosing or product-stripping chemical synthesis can be made much more efficient. This principle has been known for some time but has not yet been established in the industry. By using membrane reactors for chemical synthesis, new reactor and plant concepts can be developed. The combination of the catalyst with the membrane can be realized in various ways:

1. Loose contact between membrane and a fixed catalyst bed or monolith
2. Membrane acts as a support for the catalyst (coating)
3. Catalytically active membrane
   

Example 1): Membrane Reactors for Chemical Synthesis: Power-to-Gas PtG / Power-to-Liquid PtL

In chemical synthesis with equilibrium limitation, the membrane has the task of selectively removing a product, thereby increasing the conversion and a purified product is obtained. Examples of such reactions are the methanation (Power-to-Gas: PtG) and the methanol synthesis (Power-to-Liquid: PtL) using carbon dioxide (CO₂) and hydrogen (H₂) as starting materials, as both as a removable reaction product Water is created.

PtG: The Sabatier process for the production of methane (CH₄) from the waste CO₂ and hydrogen (H₂) is currently generating great interest in the production of "artificial natural gas" for energy storage. Based on this process, hydrogen can be converted into storable methane via the "power-to-gas" (PTG) route via the intermediate hydrogen. The efficiency of the process is of crucial importance for economic efficiency. One approach to increasing efficiency is to remove the equilibrium limitation of the reaction in order to achieve higher levels of substance conversion and methane yields through appropriate measures. According to the principle of Le-Chatelier (principle of least compulsion), the position of the chemical equilibrium can be shifted within wide limits by the ratio of the reactants involved. In this case, the water is ideally removed as a reaction product to obtain highly concentrated methane. For this separation task, membranes with specific selectivity for water (steam) with retention of the other components H₂, CO₂ and CH₄ are particularly suitable. The reaction is usefully carried out at high pressure (e.g., H₂ from high pressure electrolysis). When using a membrane reactor, methane remains on the high-pressure side and, if necessary, does not have to be compressed for feeding into the natural gas network.

PtL: Methanol is the simplest representative of alcohols and an important basic chemical in the chemical industry. The production usually takes place starting from synthesis gas (CO and H₂) at 50-100 bar. It is one of our targets to use CO₂ for the methanol synthesis in order to reduce overall CO₂ emissions. Compared to the use of synthesis gas, this results in specific problems, in particular the formation of water during the reaction. The complex composite catalyst (usually Cu with ZnO on a support and possibly other components) is degraded by the resulting water of reaction, this inhibition significantly reduces the productivity of the reaction. Since it is also an equilibrium reaction, comparable approaches apply as for the production of methane from CO₂ and hydrogen. As a result, the in-situ separation of water provides significant advantages in terms of catalyst activity and overall productivity of the reaction. In the scientific literature, there are mainly conceptual studies and theoretical considerations on the use of membranes in methanol synthesis, which predict consistently higher methanol yields. The requirements for the membrane are different in the synthesis of methanol compared to the Sabatier process. Thus, the pressure in the methanol synthesis is significantly higher with at least 50 bar, so that a special focus must be placed on the pressure stability.

Further information on catalysts and its coupling with membranes can be obtained here.

Pyrolysis of hydrocarbons produces carbons with different properties that can be selectively permeable to water. By depositing very thin carbon layers (<1 μm) from different precursors, the IKTS was able to prepare for the first time molecular sieve membranes of excellent selectivity and with very high fluxes. Due to the use of porous ceramic carrier tubes, the membranes can be operated at elevated pressure, which is essential for the Sabatier process and the synthesis of methanol out of H₂ and CO₂.

Further information (water-)selective membranes can be obtained here.

Example 2): Membrane Reactors for Chemical Synthesis: GtL

By means of a controlled reactant dosage via a membrane, the selectivity of a chemical reaction can be increased, and undesired components are suppressed. A potent and industrially relevant example of such controlled and homogeneous dosage is the separation of oxygen (O₂) from air by membranes and the partial oxidation of hydrocarbons (e.g., natural gas) to syngas carried out with this separated oxygen. This reaction is of particular importance for the use of accompanying petroleum gas, which is often flared off and for the use of shale gas.

The production of synthesis gas by partial oxidation (POx) from methane is an important process step in the production of synthetic fuels and raw materials for the chemical industry. Methane is partially oxidized to H₂ and CO. The provision of the required oxygen causes the bulk costs of this process. The integration of ceramic O₂-permeable membranes in syngas reactors has been the subject of intensive laboratory research worldwide for around 20 years. Corresponding laboratory membrane reactors contain as a central component the membrane in an externally heated reactor shell. The membrane prevents direct contact between methane and air, in the methane-filled gas space, a catalyst is placed, where the reforming reaction takes place.

In the context of our investigations O₂-permeable membranes are developed and characterized. The ceramic membrane becomes conductive at high temperatures for oxide ions and for electronic charge carriers (electrons or holes). If there is a difference in oxygen concentration on both sides of the membrane, the partial pressure ratio acts as a driving force for the transport of oxygen from the oxygen-rich to the oxygen-poor side of the membrane. The transport of the electronic charge carriers takes place in the opposite direction. When suitable membrane materials are selected, the reaction conditions can be controlled so that the temperature either is in the range of catalytic partial oxidation (CPOx) at temperatures of 800 ° C to 900 ° C or in the range of thermal partial oxidation (TPOx) at temperatures of 1200 ° C and above.

Further information about oxygen-selective membranes can be obtained here.

The use of hydrogen-selective membranes in syngas reactors in the GTL process can expand the possibilities of using the generated synthesis gas. By varying the CO / H₂ ratio, the composition of the product mixture of the Fischer-Tropsch synthesis is influenced. In the case of H₂ membranes, two membrane types are generally suitable for synthesis gas production: metallic membranes (usually palladium based) and ceramic membranes. Palladium and its alloys (mainly palladium / silver) have been found to be particularly suitable for separating hydrogen from gas mixtures at temperatures above 300 ° C, since they are largely stable and have high hydrogen permeability. In addition to palladium, mixed conducting ceramic membranes based on mixed oxides (eg substituted barium cerate and lanthanum tungstate) are also suitable for hydrogen separation.

Further information about hydrogen-selective membranes can be obtained here for metal based systems and here for ceramic based systems.