Drawback continuous-flow

High labor and handling costs as well as the start-up and shutdown times required to fill and empty the reactor are important drawbacks in a batch operation. Continuous flow systems are nearly always more cost-effective than batch reactors, especially when large volumes are to be treated, i.e., the main application of this reactor configuration is wastewater treatment. The removal of phenolic compounds from waters has been performed using SBP and HRP in continuous stirred tank reactor (CSTR) [49, 75, 76, 81, 83, 84],... 

To alleviate these drawbacks, alternative methodologies relying on the continuous provision of fresh extractant volumes to the solid sample under mvestigation have been developed, characterized, and contrasted with the classical end-over-end extraction procedures. The fundamental principles of these novel, dynamic (nonequilibrium) strategies, based primarily on the use of continuous-flow analysis (Ruzicka and Hansen, 1988), flow injection analysis (Ruzicka and Hansen, 1988 Trojanowicz, 2000 Miro and Frenzel, 2004b), or sequential injection analysis (Ruzicka and Marshall, 1990 Lenehan et al., 2002), are described in detail below, and their advantageous features and limitations for fractionation explorations are discussed critically. 

Chloromethane is an important industrial chemical. Olah et al. [56] have reported the selective catalytic monochlorination of methane to chloromethane over superacidic sulfated zirconia solid catalysts, for example 804 /Zr02, Pt/ S04 7Zr02, and Fe/Mn/S04 7Zr02- The reactions were conducted in a continuous-flow reactor under atmospheric pressure. At 200 °C with 30 % chlorine the selectivity to chloromethane was > 90 %.The selectivity could be enhanced by adding platinum. The only by-product was CH2CI2. The latter is formed by the subsequent chlorination of chloromethane. No chloroform or carbon tetrachloride was formed. The authors postulated that chlorination occurs by an electrophilic insertion of an electron-deficient, metal coordinated, chlorine molecule into the C-H bond of methane. One drawback of the process was that above 225 °C, part of the metal was removed as the metal chloride [56]. Formation and subsequent loss of volatile metal chlorides is a major pitfall that should be avoided during vapor-phase chlorination over solid catalysts. 

In continuous-flow pyrolysis systems, the sample is heated rapidly in a steady flow of carrier gas. The volatile pyrolysis products are diluted by the carrier gas and quickly removed from the reaction zone into the separation column. The main drawback of this method is the comparatively poor reproducibility of the heating pattern. 

This type of titration overcomes one of the most serious drawbacks of ti-trimetric analysis, namely the long operational time involved. As the results of all the continuous-flow titrimetric determinations developed to date are obtained by monitoring of the analytical signal as a function of time, this is a variable of paramount importance and therefore calls for strict control. The flowing stream Into which the sample is inserted or with which It is merged can be either stopped after each titration or kept circulating between consecutive samples, thus giving rise to different modes, all of which result In a considerable decrease in the time usually needed for conventional titrations. 

The milder conditions for A -amino temporary deprotection and final cleavage-deprotection from the solid support in Fmoc methodology have promoted more diversified work in the synthesis of protected phosphotyro-sine derivatives. Thus, methyl [94,102-104], benzyl [104-109], tert-butyl [104,105,109-111], dimethylamine [112], propylamine [113], isopropylamine [113], (methydiphenylsilyl)ethyl [114], and ally [104] 0,0-protected phosphotyrosine derivatives have been described. Using a test pp60 -de-rived peptide sequence in the continuous-flow method, JCitas et al. [104] reported no significant differences in the reactivities of the methyl-, benzyl-, tert-hutyl- and allyl-protected synthons or in the incorporation of subsequent residues. However, several drawbacks associated with some of the derivatives have been reported. 

In the dynamic headspace method, the sample is put in a thermal desorption unit in order to desorb the RS a continuous flow of a carrier gas pushes the RS into a trapping system which is refrigerated and where they are accumulated prior to analysis. Then the RS are rapidly desorbed by rapid heating and carried onto the column via the carrier gas. There are different ways to apply this technique. The arrangement when purge gas passes through the sample is often called the purge and trap technique (some other equipment uses the acronym DCI (desorption, concentration, injection)). This method is particularly useful for very low concentrations of RS as the total amount of a substance is extracted and can be applied directly to powders without need to dissolve them. The main drawback is that the dynamic headspace methods are not readily automated. ... 

Flow-through luminescent procedures have several advantages continuous flow operation, easier automation, improved precision, and selectivity by using online separation chemistries. The drawback that water has in SPP batch procedures, especially with paper, disappears with continuous procedures, making it possible to obtain signals from aqueous flows (Figure 3). 

At higher water contents of the support the water layer becomes too thick and the substrate has to diffuse into the water layer, or the catalyst has to diffuse to the interface. The result is a decrease in catalyst-product contact time leading to lower activities. This sensitivity towards water is a drawback of this otherwise attractive concept. Horvath performed experiments using substrates with different solubilities in water and showed that, under optimal conditions, this solubility did not influence the activity [18]. Furthermore, he performed a hydroformylation reaction in a continuous system and even under reaction conditions no leaching of rhodium complex was detected. The water obviously leaches if the SAPC is used in a continuous flow system, which in a practical application should be compensated for by using water-saturated organic solvents. 

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