Micro unit operation

MICRO UNIT OPERATIONS AND CONTINUOUS-FLOW CHEMICAL PROCESSING... 

Figure 1 Schematic illustrations of micro unit operations (a) mixing and reaction (b) solvent extraction (c) phase separation (d) two-phase formation (e) solid-phase extraction (f) heating (g) cell culture.

In the late 1990s (1998-1999), the CPAC Summer Institute focused on the advances in miniaturization that were influencing areas that are related to micro-analytical and micro-instrumentation developments. These included presentations and discussions on developments in micro-unit operations (microfluidics, micropumps, microfilters, etc.). A concern that continues to arise in these discussions was the question as to how one would sample and subsequently monitor these micro-operations. The topic of sampling has always been an area of concern to the traditional macro-operations as well. Sampling seemed to be a major problem in almost every PAT application discussed. An action item at these Summer Institutes was to address potential sampling system improvements that could work across industries. 

The preceding chapters in Parts I and 11 show that the value of micro-instrumentation is being demonstrated broadly and that recent research results continue to improve on the ability of these devices to play an increasingly important role in both research and manufacturing environments. As the value of micro-unit operations is realized, there will be an increased evaluation of these devices to replace macro-units or to be incorporated within macro-imit operations. The superior mass and heat transfer capability frequently leads to significant quality, energy and environmental benefits. 

The given examples indicate that much more effort will be needed to develop micro unit-operation devices and to couple microreactors with such devices to get micro production plants. 

If microscaled workup methods were available the general use of microreactors would speed up, but micro unit operations alone wiU not contribute to greener chemistry, because in purification processes such as adsorption the equihbrium concentrations govern the amounts of solvents that have to be used. This is independent of the scale of the system. The same holds for distillation. Microreactors only increase the surface area for phase contact. This can speed up the process, but equi-... 

Thermal Lensing, Detection, Rgure 4 Realization of micro-unit operations (MUOs) (a) example of solvent extraction and (b) MUOs developed for... 

Within the chemical industry, micro-organisms and enzymes are often used as catalysts. It is possible for a unit operation in an essentially chemical production process to be a biochemically catalysed step giving rise to a mixed chemical/biochemical production process. The products of these reactions include organic chemicals, solvents, polymers, pharmaceuticals, and purfumes. Mixed chemical/biochemical production processes are continuously innovated and optimised, mainly for economical reasons. 

The main aims of MicroChemTec are the development of a unit construction kit for micro reactors, definition of standardized interfaces, investigations of modules on the market for their suitability for affiliation in the unit construction kit, documentation for this purpose, and demonstration of functioning of the concept with the example of selected unit operations or processes. 

Oroskar a. R., VandenBussche, K., Abdo, S. F., Intensification in micro-structured unit operations performance comparison between mega and microscale, in Matlosz, M., Ehreeld, W, Baselt, J. 

At this point, the final specifications for the one or two chosen product(s) after selection process should be identified. This can be done using a three-step procedure. First, we define the product micro and macrostructure. Second, we rank the product s most important attributes, an effort that forces a review of how the product will be used. Third, we review any chemical triggers, that is, chemical stimuli which cause major changes in product properties. Finally, we turn to the manufacture itself, which relies on technical know-how of thermodynamics, chemical reaction kinetics, transport phenomena, and unit operations. Some of these ideas are illustrated in the following example. Except for a schematic of the manufacturing process, the many details related to the manufacturing plant are omitted in this discussion. 

Micro-organisms present in feed water may adsorb to carbon beds, deionizer resins, filter membranes, and other unit operation surfaces and initiate the formation of a biofilm [2,8],... 

A huge number of publications meanwhile have included the term micro reactors and their applications [9] and some attempts were made to maintain order in the field, e.g. by sorting from the point of design and manufacturing issues [5, 13], by a classification of unit operations [14] or by sorting of the chemical reactions performed [15]. The latter becomes more common than the others, but a generic look seems to be necessary. 

Owing to the small size of micro devices, new design methods are required. Similar to conventional unit operations, micro-scale pendants exist, named micro unit... 

In a conventional design, each unit operation is modeled using terms and values which do not depend on the location inside the device. On the one hand, convection and diffusion in a micro structured device strongly influence the functioning of the device, and on the other hand the convection and diffusion conditions are affected by the shape of the device. To obtain an optimized fluidic micro device, some constraints on the shape of the device are necessary. These are constraints, e.g., on the average residence time, the residence time distribution and the temperature distribution [13]. 

The designer of a plant needs information about the availability of devices and unit operations. A market study executed by the Institut fur Mikrotechnik Mainz and YOLE Developpment [51] helps to make a first survey of commercially available devices. It also estimates future needs and objectives of the chemical industry and delivers a comparison between offered and required components (Figure 4.6). The providers of micro structured devices can deliver most of the components required by the chemical industry. On the other hand, there is a lack of separation devices but this is not fully transparent in Figure 4.6. Extraction devices are under represented and the important rectification units were not asked for by the interviewers, possibly because they hardly exist. 

The toolkit does not artificially fragment unit operations such as mixing and heat exchange across different modules, but integrates them into individual function-oriented micro reaction modules. A controlled thermal environment and continuous monitoring of the micro reaction process with sensors and analyzers are other crucial elements of the concept. 

Micro structured devices are often specialized products designed for a single purpose. Costs for individually developed and manufactured devices are certainly higher than those for products from the shelf. The non-existent compatibility of devices from different suppliers represents another restraint, especially because many suppliers offer only a limited number of devices and thus cannot provide the complete range of unit operations. 

As a more critical example concerning the transfer of macroscopic modeling to micro-scale applications, the following example of a simulation of a homogeneous catalytic reaction is described [133], This example also represents a typical approach in process simulation if a new reactor model or a model for a new unit operation... 

These examples underline the fact that macro-scale process simulation tools such as Aspen Plus will have to be supplemented by micro-type unit operations as introduced by the FAMOS initiative which consider the location of a fluidic cell in the device and does not assume a perfect mixing, piston flow or uniform heat transfer coefficient [13]. 

The A scale instruments have a frustoconical indentor, whereas the D scale has a pointed conical indentor. The scales overlap but the stresses on the material are different. There are also micro units available for measurements on small pieces of material, including "0"-rings. Figure 9.1 illustrates the method of operation of the Durometer hardness testers. 

Nowadays, the most common small-scale application of hydrogen is the use in residential or mobile fuel cell systems. Special requirements of this application are compact design, integrated CO-removal, high energetic efficiency, quick start-up and fast transient behavior. The proposed solutions comprise unit-operation-based concepts as well as multifunctional, micro-structured reactors. 

There are excellent HPLC systems available on the market today, yet there is one area of concern with this instrumentation, and this rests with the detection units. Certainly the most widely used detector system employs a low dead-volume micro-ultraviolet detector. This latter unit operates near 200 nm and detects mainly unsaturated linkages in phospholipids (or lipid) samples. Some contribution by carbonyl functions can be expected. This approach is an advantage when the sample under study contains olefinic groups, but will not detect those with saturated side (hydrocarbon) chains. An alternative detector is the refractive index monitor which is often called a universal detector, since it is based on the concept that the refractive index of the solvent changes when a solute is present. The drawback of the latter unit lies in its sensitivity, which is approximately 15- to 20-fold less than that of the ultraviolet monitor. 

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