Closed reflux system

The conventional hydrothermal method (either open or closed reflux systems) for alkali activation of fly ash has been extensively utilized by previous researchers [1, 5]. A typical experimental set up employed for the closed reflux system is similar to an autoclave [18], where both pressure and temperature can be varied as per the desired experimental conditions. 

It should be noted here that the one- and two- step methods mostly employ a closed reflux system [15], which facilitate hydrothermal activation of the fly ash at elevated pressure but are expensive. In addition, the two-step method employs chemicals like NaOH and NaAlOa [15], which also add to the overall cost of synthesis of the fly ash zeolites. On the contrary, the TSA being conducted in an open reflux system (refer Fig. 5.1), the cost of synthesis of zeolites is reasonably low. Also, lower energy consumption ( 72 kWh) supports the superiority of the TSA, as compared to the two-step method. In terms of purity of the zeolites, the process adopted in the TSA (refer Fig. 5.2) results in enhanced cation-exchange capacity, CEC, of the residues of the fly ah (843 meq./lOO g) as compared to the conventional methods (388 meq./lOO g for 1-step activation and 250 meq./lOO g for 2-step activation) [15]. Apart from this, the multiple recycling of the filtrates before their final disposal, in the TSA, is helpful in reducing the pH and concentration of the heavy metal ions (viz.. Si and Al) present in them, which is not the case with the conventional methods. 

The MARS-S is constituted of a multimode cavity very close to domestic oven with safety precautions (15 mL vessels up to 0.5 L round-bottomed flasks, magnetic stirring, temperature control). The magnitude of microwave power available is 300 W. The optical temperature sensor is immersed in the reaction vessel for quick response up to 250 °C. A ceiling mounted is available in order to make connection with a conventional reflux system located outside the cavity or to ensure addition of reactants. These ports are provided with a ground choke to prevent microwave leakage. It is also possible to use a turntable for small vessels with volumes close to 0.1 mL to 15 mL vessels (120 positions for 15 mL vessels). Pressure vessels are available (33 bar monitored, 20 controlled). 

Generally, two set-ups for conducting MAE are applied in laboratories closed vessels systems allowing for temperature and pressure adjustment and control, and open vessel systems for procedures carried out under atmospheric pressure. In the open vessel systems, maximal temperature is determined by the temperature at which the extractant boils. In those systems, absorption of microwave radiation occurs in the whole sample therefore, heating is effective and homogeneous. The main disadvantage of open vessel systems is the possibility of volatile compound loss. This can be reduced by application of a reflux system fitted into the top of the extraction vessel. 

A modified domestic microwave oven with a refluxing system (Kenwood Microwave, 2.45 GHz, 900 W) is used to carry out the reactions. The experimental setup is shown in Figure 4.2. In all the experiments the microwave is operated at a specific cycling mode on 21 seconds, off 9 seconds, with the total power always at 900 W. The cycling mode was chosen in order to avoid the bumping of the solvent. The reactions were carried out in a closed hood with air-exhaust ventilation. 

The closely related N- arylazoaziridine system (278) decomposes in refluxing benzene to give aryl azides and alkenes, again stereospecifically (70T3245). However, biaryls, arenes and other products typical of homolytic processes are also formed in a competing reaction, although this pathway can be suppressed by the use of a polar solvent and electron withdrawing aryl substituents. 

Direct Scale-Up of Laboratory Distillation Ljficiency Measurements It has been found by Fair, Null, and Bolles [Ind. Eng. Chem. Process Des. Dev., 22, 53 (1983)] that efficiency measurements in 25- and 50-mm (1- and 2-in-) diameter laboratory Oldersbaw columns closely approach tbe point efficiencies [Eq. (14-129)] measured in large sieve-plate columns. A representative comparison of scales of operation is shown in Fig. 14-37. Note that in order to achieve agreement between efficiencies it is necessaiy to ensure that (1) tbe systems being distilled are tbe same, (2) comparison is made at tbe same relative approach to tbe flood point, (3) operation is at total reflux, and (4) a standard Oldersbaw device (a small perforated-plate column with downcomers) is used in tbe laboratoiy experimentation. Fair et al. made careful comparisons for several systems, utibzing as large-scale information tbe published efficiency studies of Fractionation Research, Inc. 

Intramolecular cycloadditions of substrates with a cleavable tether have also been realized. Thus esters (37a-37d) provided the structurally interesting tricyclic lactones (38-43). It is interesting to note that the cyclododecenyl system (w = 7) proceeded at room temperature whereas all others required refluxing dioxane. In each case, the stereoselectivity with respect to the tether was excellent. As expected, the cyclohexenyl (n=l) and cycloheptenyl (n = 2) gave the syn adducts (38) and (39) almost exclusively. On the other hand, the cyclooctenyl (n = 3) and cyclododecenyl (n = 7) systems favored the anti adducts (41) and (42) instead. The formation of the endocyclic isomer (39, n=l) in the cyclohexenyl case can be explained by the isomerization of the initial adduct (44), which can not cyclize due to ring-strain, to the other 7t-allyl-Pd intermediate (45) which then ring-closes to (39) (Scheme 2.13) [20]. While the yields may not be spectacular, it is still remarkable that these reactions proceeded as well as they did since the substrates do contain another allylic ester moiety which is known to undergo ionization in the presence of the same palladium catalyst. 

Fig. 3.1.2 The apparatus used in 1956 for the methanol extraction of Cypridina luciferin. The dried Cypridina (500 g) is extracted at a temperature lower than 40°C with refluxing methanol under reduced pressure for two days. The atmosphere inside the apparatus is completely replaced with hydrogen gas that was purified by its passing through a quartz tube containing red-heated copper fragments. The temperature of the mantle heater is adjusted, the system evacuated, and then all stopcocks are closed. The extraction with refluxing methanol continues for many hours without any further adjustment. From the author s 1957 notebook.

The RC1 is designed to simulate closely the operation of large scale batch and semi-batch reactor systems. The RC1 equipment consists of the reaction vessel, overhead condenser for reflux/distillation operations, receiver, metering pumps, and a heat transfer fluid heating/cooling circulating unit. Me-... 

One procedure for the synthesis of these title ring systems appeared recently <2003S1079>. Yadav and Kapoor described that the transformation of some oxadiazole and thiadiazole derivatives bearing specially substituted methylsulfinyl side chain 131, when reacted with thionyl chloride, give ring-closed compounds 134. The reaction was carried out in pyridine under reflux conditions in 74-79% yield. As shown in Scheme 25, the authors assume that the first step is the formation of the sulfonium salt 132 which undergoes cyclization with hydrogen chloride and sulfur dioxide elimination to 133 and, finally, demethylation of this intermediate leads to the final product 134. 

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