Continuous-flow configurations

Bio)chemical sensors can be used in both the batch and the continuous mode. While this is also true of probe-type sensors, flow-through sensors can only be used in a continuous regime coupled on-line to a continuous-flow configuration. 

In conventional continuous-flow configurations [2,3], (bio)chemical reactions, separations based on mass transfer between two phases, and continuous detection occur at different places and hence sequentially. 

Figure 5.13 — Irreversible-reusable flow-through sensor for the kinetic multidetermination of phosphate and silicate based on integrated sorption of a reaction product, reaction (/ situ reduction) and photometric detection. (A) Microsensor block (1) and components (2). (B) Continuous-flow configuration coupled on-line to the sensor. P peristaltic pumps SV switching valve W waste. For details, see text. (Reproduced from [39] with permission of the American Chemical Society).

The single-pass continuous flow configuration (Figure 21.11) for medium- and large-scale applications. This mode is suitable for a continuous production and is cheaper than the two previous for buffer tanks, tubing, process control instrumentation, and energy consumption. However, a later adjustment of unit to altered process conditions is not easy. 

PSA in combination with a flow system hcis been investigated and a flow injection manifold as well as a continuous flow configuration have been described. The modulated flow technique was introduced for sensitivity enhancement of flow PSA. The enhancement depends on the design of the flow cell. The stripping time t is given by ... 

This regime is characterized by the presence of one continuous fluid phase and one discrete fluid phase in tubular systems. The existence of the discrete phase generates a large interfacial area per unit tube volume for all flow configurations included in this regime. For that reason, Regime IV is of pragmatic interest when interphase heat and mass transfer are of key importance. 

Wan et al. [61] also reported the highly effective conversion of methane to aromatic hydrocarbons over Cu, Ni, Fe, and Al catalysts. The effects of the type of catalyst, its configuration, and the microwave irradiation conditions on reaction path and product selectivity were examined under both batch and continuous-flow conditions. 

Depending on the configuration of the device and the method for sample and reagents introduction, it is possible to classify the systems into static (batch or discrete sampling instrument) or flowing stream, both using continuous-flow or stopped-flow systems. 

The process for introducing the HPLC-separated analyte peaks into the NMR probe can be configured using two different modes of operation on-flow (continuous flow) or stopped-flow. 

The philosophies for automation have been described in the foregoing sections. However, to solve an analytical problem there may well be more than one approach that offers potential. The Hterature abounds with methods that have been automated by flow-injection and by continuous-flow methodologies. Also, very often a procedure which involves several stages prior to the actual measurement can be configured by combining two of the approaches. An example of this is the automated Quinizarium system described by Tucker et al. [46]. This was a continuous extraction followed by a hatch extraction which is finally completed by a batch measurement on a discrete sample for quantification and measurement. Whereas sample preparation is almost always required, there is no doubt in my mind that the best approach to this area of activity is to avoid it totally. The application of near infra-red spectroscopy is an example of this strategy. 

Figure 5.16 — Flow-through photometric sensor for the determination of traces of copper based on the immobilization of a chromogenic ligand (PAN) in a special flow-cell coupled on-line with a flow injection (A) or continuous-flow (B) configuration. IV injection valve SV switching valve W waste TGA thioglycollic acid. For details, see text. (Adapted from [42] with permission of Elsevier Science Publishers).

Stagnation flows represent a very important class of flow configurations wherein the steady-state Navier-Stokes equations, together with thermal-energy and species-continuity equations, reduce to systems of ordinary-differential-equation boundary-value problems. Some of these flows have great practical value in applications, such as chemical-vapor-deposition reactors for electronic thin-film growth. They are also widely used in combustion research to study the effects of fluid-mechanical strain on flame behavior. 

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