Novel manufacturing techniques for microstructured reactors in industrial dimensions

2.2.1 Flexibility to process needs

The main reason for the development of modular microreactors is that they facilitate flexibility to process needs. This means they can be adapted to varying throughputs and/or residence times, and to the reaction process itself, for example, by tempering each reaction module at an individual temperature level according to the desired reaction rate. Furthermore, modular microreactors can be adapted to the reaction conditions, i.e., to different pressure and temperature levels, to heat and mass transfer requirements, and to the number and nature of phases, including a single liquid phase, two immiscible liquid phases, or with an additional solid phase, e.g., if a heterogeneous catalyst is used.

2.2.2 Modularity in the design

The flexibility to process needs can be achieved due to the modularity in the design of the stacked plate reactors. On the one hand, the size and the composition of the reaction modules can be varied, e.g., by adjusting the structured width, the length and the number of plates and the size and shape of the channel cross section to the present process. On the other hand, the joining technology of the plates can be chosen, here, laser welding or vacuum brazing. In CoPIRIDE, the also applicable joining technology, diffusion welding, was not adopted since vacuum brazing offers more degrees of freedom in the design of the plate structure. Also, the application of different catalysts and supports is feasible, depending on the applied reactor design and chosen joining technology. Furthermore, the regular microstructured reaction plates can be exchanged for foam plates.

Possibilities also exist for modifications on-site, e.g., in the number of reaction modules, or the type of mixers.

3.1.1 Reactors comprising plates with regularly arranged microchannels

Two types of reactors comprising plates were developed and realized. These were comprised of microstructured plates, both at the reaction and the heat transfer side made by roll embossing. Vacuum brazed reactors are suited for applications with higher pressure but without wall coated catalysts. Currently, they are designed and successfully tested to withstand pressures up to 10 MPa which enables an operating pressure of up to 7 MPa. Laser welded reactors can only be operated at lower pressures, but comprise plates which can be coated with a catalyst, if needed. In the latter case, it is very beneficial that the microstructured plates which are coated with the catalyst can be removed and also reinserted after joining the stack by laser welding. This is impossible for the conventional design (e.g., using wet chemical etched plates). Within the CoPIRIDE project, several methods of the removal of deactivated catalytic coatings and the recycle of the microstructured substrate were investigated and compared to fresh plates, with respect to their economic efficiency and environmental impact [18]. Figure 3 exemplarily shows the basic design of a stack comprising, on both sides, plates which were microstructured by using roll embossing.

Figure 3

Basic design of a stacked plate microreactor (exploded view) comprising plates microstructured by using roll embossing.

If a higher pressure is required, as is possible for laser welded reactors, vacuum brazed reactors comprising metal foams with a catalyst coating provide a solution.

3.1.2 Reactors comprising open-cell foam plates at the reaction side

As mentioned, the reaction plates which are microstructured by roll embossing and exhibiting parallel microchannels can be exchanged for foam plates. Such metal (or ceramic) open-cell foams are commercially available, e.g., sheets of nickel foam with different pore sizes and thicknesses. These foam plates offer benefits for multi-phase reactions, e.g., if one liquid reactant is dispersed in another one and the dispersion tends to coalesce during the reaction time. In this case, the foam can act as a re-dispersion unit for a pre-dispersed reaction mixture or, at higher velocities, even as disperser. This behavior could be verified by experiments with a test system carried out at IMM (see Figure 4). After passing the micromixer and the subsequent foam reactor, the mean droplet diameter was, both for the smaller caterpillar micromixer CPMM-R600 and for the bigger CPMM-R1200 one [19, 20], almost the same as after the respective micromixers alone. However, with the foam reactor alone, significant higher throughputs were necessary to achieve the same mean droplet diameters.

Figure 4

Comparison of the mean droplet diameter as function of the total volumetric flow using water (0.01 mol SDS)/heptane at a flow rate ratio of 5:1.

Another advantage of foam plates is that they can be coated with high loads of a catalyst. Both for laser welded reactors and vacuum brazed reactors, these catalyst coated foam plates can be inserted after joining and removed when the catalyst has to be renewed. This enables the use of vacuum brazed reactors also for heterogeneously catalyst reactions, since catalysts are normally not able to withstand the high temperatures applied for joining (above 1000°C).

A special variant of the foam reactor, however, with comparatively thick plates made by wet chemical etching, was manufactured especially for a reaction under supercritical conditions. It is applicable at a pressure of 20 MPa and at temperatures up to 400°C (Figure 5).

Figure 5

Vacuum brazed foam reactor for high pressure and temperature applications.

3.2.1 Development steps

At the beginning, it was challenging to find out the shape and the dimensions of microchannels which had to meet several demands, which in parts, contradict each other. From the point of view of the reaction, the channels had to be small for optimum heat transfer properties, but large enough to minimize pressure drop and risk of blockage; from the manufacturing point of view, the challenge was to enable roll embossing and joining by vacuum brazing, without filling the channels with the brazing alloy. The channel width should be large enough according to the achievable tolerances in the case of screen printing with brazing paste. As a first compromise, manual embossing tests with several channel shapes and dimensions were made and exemplary simulation calculations for the heat transfer properties for a real, highly exothermic reaction were performed, as can be seen in Figure 8.

Figure 8

Temperature profile for oil (hot side) and water (cold side) as result of computational fluid dynamics simulation of the reactor’s channel geometry.

Since it was necessary to fix the brazing alloy in a defined position to avoid an uncontrolled partial filling of the channels, two different channel shapes for laser welded and vacuum brazed reactors were developed, the latter with a groove at the top, which is able to fix the brazing alloy (see Figure 9).

Figure 9

Cross section view of two microstructured layers comprising vacuum brazed plates microstructured by roll embossing.

With the manually embossed plates, first test reactors with plates of 17 mm structured width were manufactured to prove the principle of design.

After the respective channel shapes and dimensions were found, different mating rolls for microstructuring of precision stainless steel strips of 60 mm width were manufactured by Wetzel, which were applicable both for laser welded and vacuum brazed reactors. Using the microstructured coils, Laserzentrum Schorcht (laser cutting, laser welding) and IMM (screen printing, vacuum brazing at a subcontractor, refinishing and connectors) manufactured laser welded and vacuum brazed reactors comprising roll embossed plates with 60 mm width and 200 mm length.

With the experiences of the manufacture of these first devices, a second generation of slightly redesigned embossing rolls for brazed reactors was manufactured, and used for an advanced version of these microreactors. Furthermore, the dimensions were increased and a mating roll for the microstructuring of strips with 150 mm width was made (Figure 10).

Figure 10

Downstream side of mating roll 150 mm with microstructured strip.

With plates of this width, reactors for the pilot scale or lower production scale with respect to fine and specialty chemistry can be manufactured. The manufacturing process is executed by means of rollers in a special embossing calender, which was redeveloped as part of the CoPIRIDE project. This in-house developed embossing calender was incorporated in the test line in order to also test the required complete manufacturing process from coil to coil.

In addition, the Wetzel Company designed a large scale embossing calender system which is suited for strips with further increased width for microreactors on the production scale. As an example, for ductile metals with higher corrosion resistance, titanium sheets were microstructured by manual embossing and a first test reactor with the same channel and outer dimensions as the stainless steel test reactor was manufactured by using vacuum brazing.

3.3.1 Manufacture of the components

In a first step, the structured plates for the reaction and the heat transfer side have to be separated from the microstructured strip coil in the desired length and shape. Next, the additional frames and intermediate plates have to be prepared. In our case, both were done by Laserzentrum Schorcht, by using laser cutting. Finally, the stack has to be completed by the top plate, temperature sensing plates and the bottom plate, the latter also functioning as a stacking device. Prior to stacking, the separated plates microstructured by roll embossing have to be flattened by using a sheet metal straightening machine; this was done at IMM.

3.3.2 Screen printing of the brazing paste

The nickel-based brazing paste was coated on both sides of the flat intermediate plates between the microstructured ones, by using a precision screen printing machine at IMM (Figure 11). The brazing metal connects the single parts of the stack along the outline and between each of the microchannels. The challenge is to apply the paste evenly, at the right position and in an optimal thickness to achieve the best results with respect to pressure stability and to avoid filling of the microchannels with the brazing alloy. After several basic in-house tests and thanks to the experiences gained during the development of the novel manufacturing techniques within CoPIRIDE, IMM is now able to achieve reproducible results in the required quality. The now patent protected [15] special shape of the channel, which exhibits a dent at the top for inclusion of the brazing alloy, successfully prevents it from clogging by the brazing alloy (see Figure 14).

Figure 11

Precision screen printing machine at IMM for coating plates with brazing solder.

After drying of the coated plates in a drying furnace, all components are then ready for stacking.

3.3.3 Stacking of the components

Using pins for adjusting the parts, the stack is built from alternating reaction and heat transfer layers, respectively, (Figure 12). Each layer consists of a microstructured plate made by roll embossing, frames and a separation plate coated with the brazing paste (see Figure 3). The latter has the additional function of closing the micro dents formed into the corrugated sheets (see Figure 9). Plates with several openings for the potential insertion of temperature sensors, like thermocouples, are provided in the improved versions. This allows information to be obtained about the temperature distribution along the length, the width and the height of the reactor.

Figure 12

Stacking of a reaction module (first version) at IMM. View on the reaction side with still uncovered channels.

Stacking starts from the bottom plate in which the adjusting pins are inserted. After a plate for temperature sensing, the first layer is for heat transfer, the second for reaction and so on. After a certain number of layers, additional plates for temperature sensing can be included. After a last plate for temperature sensing, the stack is completed with the top plate.

Currently, during the development of the technology, stacking is carried out manually at IMM, but in principle also assembly robots can be applied for stacking if for industrial applications, if a large number of stacks are required.

3.3.4 Vacuum brazing of the stacks

Vacuum brazing requires high-vacuum furnaces and operates at high temperatures. Since such devices are expensive and are not available on site of the project partners, IMM co-operates with a subcontractor. However, due to the novelty for the subcontractor with respect to vacuum brazing of stacks with such a high content of brazing paste, IMM was also involved in the development of the brazing technology, i.e., finding out optimal sequences and operating conditions (mounting, evacuation/pressurization, temperature profiles, duration). The individual steps are warm-up, debinding, fusing and cooling down. For vacuum brazing, the stacks have to be burdened with weights (see Figure 13).

Figure 13

Stacks burdened with weights inside the vacuum oven.

After vacuum brazing, the reactor looks like a monolith, but consists of layers of different materials, e.g., of stainless steel and nickel. Figure 14 demonstrates the achieved equability and high quality of the joinings.

Figure 14

Cross cut view after vacuum brazing of the stacked plate reactor.

3.3.5 Post-processing and completion with connecting parts

After vacuum brazing, the sealing surface has to be treated and the brazed stack has to be coupled with other modules and the distribution and sampling connectors at the reaction side. For this purpose, special coupling components have to be manufactured. The flanges have to ensure tightness under the intended operating conditions and the distribution and sampling connectors shall enable the distribution of the reaction media to the reaction channels as uniformly as possible. In addition to the picture of a complete assembled reactor of the first design (Figure 2), Figure 15 shows the last version, which requires further reduced treatment of the brazed stack.

Figure 15

Vacuum brazed reaction module (improved version) assembled with distribution and sampling connectors.

Before use, all single reaction modules, as well as the assembled reactor, are tested with respect to tightness and pressure stability, to ensure the suitability under the intended operating conditions.