Small chemical reactors

The design and control of very small chemical reactors (reactors on a chip) have received considerable attention in recent years, particularly in academic research. The paper by Kothare19 provides a broad review of work in this area. 

Mass spectrometer ion sources are quite small chemical reactors. It can easily be shown that the reaction time of an ion in the source will normally be in the order of a microsecond, and unless the pressure in the source is well over 100 microns only a small fraction of the ions can undergo collision. It early became apparent that where secondary ions were observed they must, in most instances, have resulted at almost every collision of ion and neutral. Further, in many instances, the heats of formation of ions and neutrals were well established, and in all such instances it was possible to show that the reactions were exothermic. Indeed, it would be impossible under most circumstances to observe a... 

JSA/JHA on Cleaning a Small Chemical Reactor by Experienced and Trained Personnel... 

Reformer. A small chemical reactor carried on board some fuel cell vehicles used to extract hydrogen from alcohol or hydrogen fuel. 

Cleaning a small chemical reactor by experienced trained personnel... 

A number of factors limit the accuracy with which parameters for the design of commercial equipment can be determined. The parameters may depend on transport properties for heat and mass transfer that have been determined under nonreacting conditions. Inevitably, subtle differences exist between large and small scale. Experimental uncertainty is also a factor, so that under good conditions with modern equipment kinetic parameters can never be determined more precisely than 5 to 10 percent (Hofmann, in de Lasa, Chemical Reactor Design and Technology, Martinus Nijhoff, 1986, p. 72). 

The equivalent of radial flow for flat-plate geometries is Vy. The governing equations are similar to those for Vy. However, the various corrections for Vy are seldom necessary. The reason for this is that the distance Y is usually so small that diffusion in the y-direction tends to eliminate the composition and temperature differences that cause Vy. That is precisely why flat-plate geometries are used as chemical reactors and for laminar heat transfer. 

Values for the various parameters in these equations can be estimated from published correlations. See Suggestions for Further Reading. It turns out, however, that bubbling fluidized beds do not perform particularly well as chemical reactors. At or near incipient fluidization, the reactor approximates piston flow. The small catalyst particles give effectiveness factors near 1, and the pressure drop—equal to the weight of the catalyst—is moderate. However, the catalyst particles are essentially quiescent so that heat transfer to the vessel walls is poor. At higher flow rates, the bubbles promote mixing in the emulsion phase and enhance heat transfer, but at the cost of increased axial dispersion. 

This response time should be compared to the turbulent eddy lifetime to estimate whether the drops will follow the turbulent flow. The timescale for the large turbulent eddies can be estimated from the turbulent kinetic energy k and the rate of dissipation e, Xc = 30-50 ms, for most chemical reactors. The Stokes number is an estimation of the effect of external flow on the particle movement, St = r /tc. If the Stokes number is above 1, the particles will have some random movement that increases the probability for coalescence. If St 1, the drops move with the turbulent eddies, and the rates of collisions and coalescence are very small. Coalescence will mainly be seen in shear layers at a high volume fraction of the dispersed phase. 

The development of modern surface characterization techniques has provided means to study the relationship between the chemical activity and the physical or structural properties of a catalyst surface. Experimental work to understand this reactivity/structure relationship has been of two types fundamental studies on model catalyst systems (1,2) and postmortem analyses of catalysts which have been removed from reactors (3,4). Experimental apparatus for these studies have Involved small volume reactors mounted within (1) or appended to (5) vacuum chambers containing analysis Instrumentation. Alternately, catalyst samples have been removed from remote reactors via transferable sample mounts (6) or an Inert gas glove box (3,4). 

A detailed characterization of micro mixing and reaction performance (combined mixing and heat transfer) for various small-scale compact heat exchanger chemical reactors has been reported [27]. The superior performance, i.e. the process intensification, of these devices is evidenced and the devices themselves are benchmarked to each other. 

Quality control tests or improvement of existing processes. Raw materials from various sources can be used in the manufacture of fine chemicals and pharmaceuticals. The raw materials can contain different impurities at various concentrations. Therefore, before the raw material is purchased and used in a full-scale batch its quality should be tested in a small-scale reactor. Existing full-scale procedures are subject to continuous modifications for troubleshooting and for improving process performance. Laboratory reactors used for tests of these two kinds are usually down-scaled reactors or reactors being a part of the full scale-reactor. 

The fine chemicals business is characterized by a small volume of products manufactured. Therefore, batch production predominates and small-scale reactors are used. The need to implement fine chemistry processes into existing multiproduct plants often forces the choice of batch reactors. However, safety considerations may lead to the choice of continuous processing in spite of the small scale of operation. The inventory of hazardous materials must be kept low and this is achieved only in smaller continuous reactors. Thermal mnaways are less probable in continuous equipment as proven by statistics of accidents in the chemical industries. For short reaction times, continuous or semicontinuous operation is preferred. 

A schematic of a small chemical flowshop. The circles and squares represent units, such as reactors, dryers, or centrifuges. 

Batch reactors are often used for liquid phase reactions, particularly when the required production is small. They are seldom employed on a commercial scale for gas-phase reactions because the quantity of product that can be produced in reasonably sized reactors is small. Batch reactors are well suited for producing small quantities of material or for producing several different products from one piece of equipment. Consequently they find extensive use in the pharmaceutical and dyestuff industries and in the production of certain specialty chemicals where such flexibility is desired. When rapid fouling is encountered or contamination of fermentation cultures is to be avoided, batch operation is preferable to continuous processing because it facilitates the necessary cleaning and sanitation procedures. 

In general, each form of ideal flow can be characterized exactly mathematically, as can the consequences of its occurrence in a chemical reactor (some of these are explored in Chapter 2). This is in contrast to nonideal flow, a feature which presents one of the major difficulties in assessing the design and performance of actual reactors, particularly in scale-up from small experimental reactors. This assessment, however, may be helped by statistical approaches, such as provided by residence-time distributions. It... 

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