Catalytic Plate Reactors

Supplying heat directly into an endothermic reaction, rather than via inter-stage heaters or by radiation to a packed tube is a vital key to intensifying many important chemical processes. The catalytic plate reactor (CPR) offers an attractive route for achieving this. In a CPR, metal plates coated with a suitable catalyst are arranged in such a manner that exothermic and endothermic reactions take place in alternate channels (Fig. 1). These channels typically have a height of order of millimetres and a catalyst thickness of the order of microns.

Fig. 1 - A pair of adjacent channels in the catalytic plate reactor

The research work at the Process Intensification Group of Newcastle University has focused on the development of a detailed theoretical study on CPR's with the aim of providing a base line for a general design procedure. The advantages of CPR designs over conventional reactors arise due to excellent heat transfer characteristics and minimal intra-catalyst diffusion resistance. The heat transfer mechanism within a CPR is via conduction through the plates separating alternate process channels and as such is largely independent of the process gas superficial velocity. The catalyst layers within a CPR are thin which results in minimal diffusion limitations and thus high catalyst utilisation. These advantages result in reactors which are smaller, lighter and with a small associated pressure drop than conventional alternatives. The potential saving in reactor volume can be seen in Table 1, below:

Table 1 - Size reduction through CPR utilisation

Catalytic Steam Reformer

The feasibility of the concept of coupled endo and exothermic reactions has been demonstrated using steam reforming of methane as the fast and highly endothermic reaction with the energy being provided by the catalytic oxidation of methane. Potential exploitation of this system includes on-board hydrogen production for fuel cell powered vehicles, which are of particular interest due to stringent legislation for the control of automobile exhaust gases. This is possible due to the significant size reduction (see Table 1). Another important application is the production of syngas, which is the feed-stock for many industrial processes. Methods for preparing and coating the catalysts (based on sol-gel technology) have been developed and the necessary activity to achieve a targeted heat flux of 10kW/m² demonstrated.

The replacement of the homogeneous combustion used in conventional reactors by the catalytic one brings several advantages. It proceeds at lower temperature than conventional combustion, posing fewer constraints for materials of construction and producing virtually no NOx. Since it is a flameless process, long radiation paths needed in conventional fired furnaces are replaced by channel dimensions of one or two mm, with an obvious impact on reactor size.

Fig. 2 - Design of simple CPR by using stacked diffusion bonded shims (manufactured by Chart Exchangers)

An example of the reactor design where methane combustion and reforming are integrated is shown in Fig. 2. The device used in experimental work is shown in Fig. 3. These devices are produced from patterned metal shims which are stacked together and diffusion bonded yielding a compact metal block.

Fig. 3 - Bench scale CPR for methane steam reforming

An additional important benefit of this concept is that the production scaling can be handled through replication rather than re-sizing. This leaves the reactor performance effectively the same at all scales, thus reducing the time requirement from development to commercial production.

Methane Reforming in a Catalytic Plate Reactors

The potential of the CPR for dry or mixed reforming is currently being investigated. In the process methane, steam and carbon dioxide are simultaneously passed over a nickel based catalyst. The technique has been shown to strongly influence both the the CO:H₂ ratio and rate of carbon laydown. A parametric study is underway in an effort to minimise coke deposition whilst producing a CO/H₂ ratio which is suitable as a feedstock for Methanol or Fischer Tropsch processes.

Fischer-Tropsch synthesis in a CPR

A second application of the CPR is product enhancement for catalytic reactions where the product spectrum is highly dependant upon catalyst temperature. In such an application alternate channels contain a boiling heat transfer fluid to maintain an isothermal catalyst temperature. To demonstrate the concept the Fischer-Tropsch (FT) reaction has been investigated.

The hydrocarbon product spectrum produced by a FT catalyst is highly dependant upon catalyst temperature and rate of diffusion of reactants into the catalyst matrix. The reaction is highly exothermic and if rates of heat removal from the catalyst are not sufficiently high "hot-spots" will form which will result in degradation of the product spectrum. Studies have revealed that thin catalyst coats attached to heat transfer surface areas within a CPR can greatly enhance the yield of desirable products per unit volume as compared to conventional fixed bed technology. This volume saving coupled with an overall lighter design, requiring less ancillary equipment and with a low pressure drop make the FT CPR a potential reactor for the recovery of stranded gas reserves.