Microreactors

Mikroreaktori su reaktorski sustavi izvedeni u mikroskopskom mjerilu koji su, u cijelosti ili barem djelomično, proizvedeni primjenom metodologije mikrotehnologije i mikroinženjerstva. Čini ih mreža mikrokanala tipičnih dimenzija 10 µm - 500 µm urezanih u čvrstu pločicu (staklo, silikon, polimeri.). Male dimenzije mikroreaktora osiguravaju višestruke prednosti naspram klasičnih makroreaktorskih sustava. Kao posljedica malog promjera kanala strujanje kapljevine u sustavu je većinom laminarno. Mikroreaktore karakterizira izrazito velik omjer međufazne površine i volumena reaktora, zbog čega je prijenos tvari i topline učinkovitiji, a količina i broj otpadnih procesnih struja znatno smanjen. Primjena mikroreaktora omogućava preciznu regulaciju procesa te uporabu malih količina reaktanata i katalizatora, a pogodni su i za obavljanje reakcija koje su izrazito egzotermne/endotermne ili čak reakcija koje su eksplozivne i zahtijevaju upotrebu opasnih kemikalija. Uporabom mikroreaktora prijenos u veće mjerilo (eng. scale-up) znatno je pojednostavljen jer se jednostavno provodi povezivanjem procesnih jedinica (eng. numbering-up), čime su uklonjeni visoki troškovi projektiranja i skraćeno je vrijeme potrebno za prenošenje iz laboratorijskog mjerila na industrijsku primjenu. Do danas su razvijeni mikroreaktori u kojima je moguće provođenje nekoliko istovremenih reakcija, separacija i analiza komponenata sustava u jednom mikrokanalu. U ovom radu dan je pregled osnovnih karakteristika mikroreaktorskih sustava (struktura, izvedba i svojstva). Prikazani su reakcijski procesi (jednofazni i višefazni sustavi) koji se mogu provoditi i njihove karakteristike. Zbog složenosti procesa u mikroreaktorskim sustavima za njihov se opis primjenjuju složeni matematički modeli, pa je u ovom radu dan pregled metodologije za postavljanje matematičkih modela procesa koji se odvijaju u mikroreaktorima.Nowadays, microreactors are finding increasing application in many fields, from the chemical industry and biotechnology to the pharmaceutical industry and medicine. They offer many fundamental and practical advantages over classical macroreactors (large surface to volume ratio, excellent mass and heat transfer, shorter retention time (Table 1), smaller amount of reagents, catalysts and waste products, laminar flow, effective mixing). Microreactors consist of a network of microsized channels etched into solid substrate (Fig. 1). Typical dimensions of microchannels are in the range from 10 µm to 500 µm. They are connected to a series of reservoirs for chemical reagents and products to form a complete device called “chip”. Microreactors can be produced from glass, silicon, quartz, metals and polymers. Optimal material depends on chemical compatibility with solvents and reagents, costs and detection methods used in process control. The most commonly used material is glass since it is chemically inert and transparent. One of the aims of today’s research in the field of microtechnolgy is developing of so-called micro-total-analysis-systems (µ-TAS; Fig. 3). Such a device would perform sampling, sample preparation, detection and data processing in integrated manner. The most µ-TAS research has been made in biomedical field (analysis of DNA and proteomics). Using microreactors, the complex process of scale up is replaced with numbering up (replication of microreactor units), eliminating time and costs necessary for transfer from laboratory to industrial production. Numbering up can be performed in two ways: external numbering up (connection of many devices in parallel) and internal numbering up (parallel connection of functional elements, incomplete devices (Fig. 2)). One of the biggest advantages of numbering up is that continuous operation is uninterrupted if one of the units fails, because it can be easily replaced with no effect on other parallel units. Research has confirmed that microreactor methodology is applicable for performing gas and liquid phase reactions. They can be used for different single/multiple phase reactions (Fig. 7–8) and even for explosive and flammable reactions or those that use highly toxic components (Table 2). Depending on the microchannel’s geometry, material and physical properties of solvents, the contact between two phases can create different flow patterns (Fig. 10). A chemical process in microchannels can be described with the same equations as the process in macroreactors. A standard approach for modeling transport phenomena (mass and heat) in the field of reactor engineering is based on convection-diffusion equations. Gas phase and liquid phase flows are usually described by Navier-Stokes equations (solution in Fig. 5–6). Due to small thermal diffusion path, microreactors allow fast heat transfer and efficient control of temperature distribution (Fig. 13–14). In cases of technical applications, multi-phase systems (gas-liquid or liquid-liquid) are mostly used. For their modeling, the detailed knowledge is required on the multiphase flow pattern, volumetric gas content, pressure drop, liquid film thickness and internal mixing. For better understanding of those processes, dimensionless numbers are used very often (Table 3). The most common flow regime in multiphase systems is the slug regime; in this regime, slugs of one phase flow through the microchannel alternately with slugs of the other phase (Fig. 15–16). Since both phases move alternately, each slug serves as an individual processing subvolume. The mass transfer takes place with two mechanisms, convection (due to the internal circulations) within the slug and diffusion (because of concentration gradients) between slugs (Fig. 19). For solving the equations describing those complex systems, CFD is used. Using CFD, the flow domain is divided into a mesh of volumes and partial differential equations are discretized over the computational mesh, yielding a set of algebraic equations which are then solved by an iterative calculation procedure (Fig. 15–16, 18). Many of the models in the literature have been developed for specific processes and reactors, and allow the prediction of flow, mass and heat transfer for that specific case with a high degree of accuracy (Fig. 20–21)