Micro reactor heat

Fuel cells Micro-reactors/heat exchangers... 

Among several types of reactors investigated, the microstructured reactor was successfully applied to the synthesis of a pharmaceutical intermediate via a fast exothermic Boc protecting reaction step. The reaction temperature was isothermally controlled at 15°C. By using the microstructured reactor the heat of reaction was completely removed so that virtually no byproducts were produced during the reaction. Conversions as high as 96% were achieved. The micro-reactor operation can be compared with other reactors, however, which need to be operated at 0°C or -20°C to avoid side reactions. 

Table 1.2 Key data for production of polymers in the compact reactor/mixer/heat exchanger, termed micro reactor, by cationic polymerization yielding propylene, piperylene, butylenes, etc. [50].

Many other, less obvious physical consequences of miniaturization are a result of the scaling behavior of the governing physical laws, which are usually assumed to be the common macroscopic descriptions of flow, heat and mass transfer [3,107]. There are, however, a few cases where the usual continuum descriptions cease to be valid, which are discussed in Chapter 2. When the size of reaction channels or other generic micro-reactor components decreases, the surface-to-volume ratio increases and the mean distance of the specific fluid volume to the reactor walls or to the domain of a second fluid is reduced. As a consequence, the exchange of heat and matter either with the channel walls or with a second fluid is enhanced. 

In order to exemplify the potential of micro-channel reactors for thermal control, consider the oxidation of citraconic anhydride, which, for a specific catalyst material, has a pseudo-homogeneous reaction rate of 1.62 s at a temperature of 300 °C, corresponding to a reaction time-scale of 0.61 s. In a micro channel of 300 pm diameter filled with a mixture composed of N2/02/anhydride (79.9 20 0.1), the characteristic time-scale for heat exchange is 1.4 lO" s. In spite of an adiabatic temperature rise of 60 K related to such a reaction, the temperature increases by less than 0.5 K in the micro channel. Examples such as this show that micro reactors allow one to define temperature conditions very precisely due to fast removal and, in the case of endothermic reactions, addition of heat. On the one hand, this results in an increase in process safety, as discussed above. On the other hand, it allows a better definition of reaction conditions than with macroscopic equipment, thus allowing for a higher selectivity in chemical processes. 

Table 1.6 Characteristic quantities to be considered for micro-reactor dimensioning and layout. Steps 1, 2, and 3 correspond to the dimensioning of the channel diameter, channel length and channel walls, respectively. Symbols appearing in these expressions not previously defined are the effective axial diffusion coefficient D, the density thermal conductivity specific heat Cp and total cross-sectional area S, of the wall material, the total process gas mass flow m, and the reactant concentration Cg [114].

One of the most often cited advantages of micro reactors is their prevention of hot spots by strongly enhanced heat dissipation. Hence higher dosing of reactants, i.e. higher reactant concentration and/or higher catalyst loading, may be possible... 

Worz et al. give a numerical example to illustrate the much better heat transfer in micro reactors [110-112]. Their treatment referred to the increase in surface area per unit volume, i.e. the specific surface area, which was accompanied by miniaturization. The specific surface area drops by a factor of 30 on changing from a 11 laboratory reactor to a 30 m stirred vessel (Table 1.7). In contrast, this quantity increases by a factor of 3000 if a 30 pm micro channel is used instead. The change in specific surface area is 100 times higher compared with the first example, which refers to a typical change of scale from laboratory to production. 

Worz et al. stress the possibility of carrying out very fast reactions with large reaction heat in micro reactors [110]. They often use the terms isothermal operation or isothermicity to describe adequately the carrying out of a reaction with a heat that is taken out of the processing volume immediately upon release. In practice, they often refer to a temperature increase of 1-2 °C as a limit for fulfilling the criterion of isothermal operation. 

Micro reactors permit high-throughput screening of process chemistries imder controlled conditions, unlike most conventional macroscopic systems [2], In addition, extraction of kinetic parameters from sensor data is possible, as heat and mass transfer can be fully characterized due to the laminar-flow condihons applied. More uniform thermal condihons can also be utilized. Further, reactor designs can be developed in this way that have specific research and development funchons. 

Calculations predict that improved heat transfer for reacting systems in micro-channel heat-exchanger reactors could lead to considerable size reduction of the equipment, by enhancing the degree of product formation per micro channel (see Figrue 1.24) [140, 141]. This was exemplarily shown for a fast, high-temperature... 

Interview with Worz/BASF in a special on heat exchangers giving expert opinion on compact heat exchangers, feasibility and problems of large-scale implementation of micro reactors measuring tool for process optimization exotic status. scale-up unit-construction kit industrial implementation in 5 years [216],... 

Nature as model for micro-reactor development general advantages of micro flow onset of industrial interest micro heat exchanger vision of mefhanol-fuel reforming costs stiU too prohibitive [231],... 

Off-the-shelf catalogue sales of micro reactors have just started [15]. With an increasing number of commercial products, quality control will become more important. Brandner et al. describe quality control for micro heat exchangers/reactors at the Forschungszentrum Karlsruhe [23]. All manufacturing steps are accompanied by quality control and documentation. Leak rates (down to 10 mbar 1 s for He) and overpressure resistance (up to 1000 bar at ambient temperature) are measured. Under standardized conditions, the mean hydraulic diameter is determined. Dynamic tests supplement this quality control. 

Accordingly, serious commercially oriented attempts are currently being made to develop special gas-phase micro and mini reactors for reformer technology [91, 247-259], This is a complex task since the reaction step itself, hydrogen formation, covers several individual processes. Additionally, heat exchangers are required to optimize the energy balance and the use of liquid reactants demands micro evaporators [254, 260, 261], Moreover, further systems are required to reduce the CO content to a level that is no longer poisonous for a fuel cell. Overall, three to six micro-reactor components are typically needed to construct a complete, ready-to-use micro-reformer system. 

The diffusion paths for heat and mass transfer are very small, making micro reactors ideal candidates for heat or mass transfer-limited reactions. 

The share of solid wall material is typically much higher than in macroscopic equipment. Hence solid heat transfer plays an important role and has to be ac-coimted for when designing micro reactors. 

The design of multichannel micro reactors for gas-phase reactions is typically based on a stack of micro structured platelets. For strongly endothermic or exothermic reactions, it lends itself to alternate between layers of reaction channels and heat-... 

In chemical micro process technology with porous catalyst layers attached to the channel walls, convection through the porous medium can often be neglected. When the reactor geometry allows the flow to bypass the porous medium it will follow the path of smaller hydrodynamic resistance and will not penetrate the pore space. Thus, in micro reactors with channels coated with a catalyst medium, the flow velocity inside the medium is usually zero and heat and mass transfer occur by diffusion alone. 

A complete reactor module was built, consisting of the actual micro reactor and an encasement that serves for temperature setting [28], The latter consists of two parts, a furnace for setting the high temperature in the reactor inlet collection zone and in the reaction zone and a cooler for the outlet collection zone. The micro reactor has a housing with standard tube connections. An electric furnace serves for heating, Temperatures can be measured in the furnace, at the furnace/micro reactor border and in the outlet collection zone. For thermal insulation, a 2 mm ceramic... 

The micro reactor can be operated at temperatures up to 480 °C [44]. Platelet exchange can be performed in a short time, needing only 15-30 min of cooling from operational to ambient temperature. Heat production rates of about 30 W can be achieved without the need for external cooling [43]. 

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