Open reflux system

SiC fiber was produced from polycarbosilane (PCS) by Yajima et al. " in 1975, which is the earliest case of organosilicon polymer utilization for an industrial structural material. In the Yajima process, PCS was mainly synthesized from polydimethylsilane (PDS) by a thermal conversion process using an autoclave or an open reflux system. This is the commonly available PCS. Its melt spinability, solubility in various orgaiuc solvents, and stability for storing at room temperature are critically important for industrial uses. 

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. 

Every so often you have to add a compound to a setup while the reaction is going on, usually along with a reflux. Well, you don t break open the system, let toxic fumes out, and make yourself sick to add new reagents. You use an addition funnel. Now, we talked about addition funnels back with separa-... 

A library of 2-(arylamino)benzimidazoles was prepared in a microwave-accelerated cyclocondensation of PEG-supported orffto-phcnylenediamines 31 with isothiocyanates, followed by the product cleavage from polymer support [55,56]. The quantitative cyclization required either microwave heating (in an open vessel system) for 10 min or reflux in MeOH for 4 hours (Scheme 20). The use of a soluble PEG matrix substantially simplified the... 

There are a number of published methods for the safe modification of domestic microwave ovens. The improved safety of this type of system is particularly attractive, the advantage of modifying an oven in this way is that the reaction vessel is neither sealed nor directly open to the microwave oven. Thus, reactions can be carried out using a flask attached to a reflux condenser. There are two advantages of using this type of system (i) a wide variety of solvents can be used because the fire hazard is significantly reduced compared to an open vessel system, (ii) reactions can be carried out under inert conditions. 

In further exeunination of the hydrogen peroxide - ethyl ohioroformate initiation system, with emulsions as known at the time, it was found that polymerisation of methyl methacrylate or vinyl acetate could be controlled readily at ambient ten erature, there being some inhibition period. This tended to be prolonged when emulsions were stirred under open reflux, but vinyl acetate emulsions commenced to polymerise with extreme vigour at the azeoC tropic reflux temperature (66.5°) These experiments, performed before the symposium in London in which the nature of reduction activation was disclosed, were not pursued further at the time ... 

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. 

The use of 2M to 4M H2SO4 instead of water improves the performance of this part of the reflux system (31). An optical cell 4 with a photoelectric relay was used for automatic control of the flow of SO2 (39, 43) by opening a solenoid valve to increase the flow of SO2 when the brown colored reaction zone descended to the optical cell. The NO produced by the above reactions, along with that present in the liquid phase as N2O3, passed into the exchange column B through a jacketed section at —12° to — 16°C. which condensed and returned most of the unreacted NO2 to the reactor. 

After these preheat treatment processes, the two treated (cross-linked) PMS resins were pyrolyzed at 1273 K. Resin recovery after preheat treatment and ceramic yield of these cross-linked resins at 1273 K are summarized in Table 19.2. Use of the reflux system is effective to increase resin recovery and ceramic yield. Figure 19.4 shows the overall ceramic yield of the starting PMS with different thermal histories. The dotted line indicates the intrinsic ceramic yield from PMS by direct pyrolysis up to 1273 K (about 30%). The filled circle indicates the ceramic yield at 1273 K with two-step pyrolysis, in which reflux treatment is the first step. Overall ceramic yield begins to increase at 423 to 523 K, and is saturated (approximately 75%) at 623 K. Even when a preheat treatment step on PMS is a simple heat treatment in an open argon gas flow (open circle), the overall... 

Within the conventional inline process unit, drums and their related items are generally located on either side of a central pipe rack serviced by auxiliary roads for maintenance access. In certain cases (e.g., for flash drums and deaerators), drums can be located above the pipe rack. In chemical plants, drums are generally located at all levels of enclosed or open-sided structures. For eixample, Exhibit 5-4 shows the drum location in a tower reflux system. Exhibit 5-5 shows the typical location of feed surge and compressor suaion drums, and Exhibit 5-6 shows the drum locations in an enclosed chemical plant stnicture. 

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. 

The parent compounds undergo facile hydrolysis to aminoaldehydes subsequent to the covalent hydration and reversible ring-opening as described above for pyrido[4,3-d]pjrrimidines (Section IV, B). 2-(3-Pyridyl)pyTido[2,3-d]pyrimidine undergoes hydrolysis to yield 2-aminonicotinaldehyde and nicotinamide when treated with N—HCl under reflux for 3 hours. This mechanism also probably involves a covalent hydrate. 2-Methylpyrido[4,3-d]pyrimidin-4(3H)-one, although much more stable than the parent compound, is readily hydrolyzed with dilute acid, whereas the isomeric compounds from the other three systems are stable under such conditions. 

On the other hand, samples can be irradiated at constant microwave power over a certain fixed period, for example at 100 W for 10 min. As there is no control over the resulting temperature or pressure, care has to be taken not to exceed the operational limits of the system and this type of program should only be used for well-known reactions with non-critical limits, or under open-vessel (reflux) conditions. Since in this method only the applied energy and not the resulting temperature is controlled, the quality of reaction control is often superior employing a temperature-controlled program. 

Most chemists perform experiments in which the contents of our beaker, flask or apparatus are open to the air - obvious examples include titrations and refluxes, as well as the kinetic and electrochemical systems we consider in later chapters. The pressure is the air pressure (usually pe), which does not change, so any pressure-volume work is the work necessary to push back the atmosphere. For most purposes, we can say w = pAV. 

In addition to the basic control loops, all processes have instrumentation that (1) sounds alarms to alert the operator to any abnormal or unsafe condition, and (2) shuts down the process if unsafe conditions are detected or equipment fails. For example, if a compressor motor overloads and the electrical control system on the motor shuts down the motor, the rest of the process will usually have to be shut down immediately. This type of instrumentation is called an interlock. It either shuts a control valve completely or drives the control valve wide open. Other examples of conditions that can interlock a process down include failure of a feed or reflux pump, detection of high pressure or temperature in a vessel, and indication of high or low liquid level in a tank or column base. Interlocks are usually achieved by pressure, mechanical, or electrical switches. They can be included in the computer software in a computer control system, but they are usually hard-wired for reliability and redundancy. 

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