Continuous tubular precipitator

Recently it was demonstrated that the rate of oxidation can be increased by the introduction of surface under basic conditions [111]. This work has introduced a new catalyst concept that meets the above criteria for use under moderate SCWO conditions in a continuous tubular flow reactor. The concept involves -in situ precipitation of the catalyst (e.g., sodium carbonate) under SCWO conditions, but the catalyst is otherwise soluble under ambient conditions. -In situ precipitation is a unique way to generate a high-surface-area catalyst in the reaction zone, thereby ensuring maximum surface contact with the medium while minimizing catalyst poisoning. 

Models for continuous, stirred precipitation behavior are at a reasonably successful stage, though the expression for breakup can be improved in the light of recent studies. Batch and tubular reactor models must additionally include an explicit accounting for primary particle formation and behavior of small particles. Some of the data necessary to do so have been collected, but number density data at the small sizes is still lacking. Finally it remains to be seen how successful the models developed for isoelectric precipitation will be in describing precipitation with other classes of precipitants. 

Tubular Precipitator. This type of continuous operation may be employed to reduce polydispersity of precipitates (Raphael et al. 1997 Raphael and Rohani 1999). The tubular precipitator may operate either under the turbulent flow or laminar flow regime. The reactants are added into the inlet section equipped with static mixers and may also enter as a multi-port feed along the length of the tubular precipitator. If the reactant feeding streams are too concentrated or if too excessive formation of precipitate occurs in the inlet section of the precipitator, a third stream of solvent is also fed to dilute the flowing suspension. The latter may contain a protective colloid or surfactant that prevent agglomeration of precipitate. 

Finally, we touch upon selected practical aspects of typical precipitation operations, using continuous MSMPR and tubular precipitators, and a semi-batch precipitator as illustrative examples. 

Recent Developments. A considerable amount of cellulose acetate is manufactured by the batch process, as described previously. In order to reduce production costs, efforts have been made to develop a continuous process that includes continuous activation, acetylation, hydrolysis, and precipitation. In this process, the reaction mixture, ie, cellulose, anhydride, catalyst, and solvent, pass continuously through a number of successive reaction zones, each of which is agitated (92,93). In a similar process, the reaction mass is passed through tubular zones in which the mixture is forced through screens of successively small openings to homogenize the mixture effectively (94). Other similar methods for continuous acetylation of cellulose have been described (95,96). 

In those cases where the particles are small and/or the viscosity of the fluid is high, filtration is not very effective. In such cases, centrifugation is the most common and effective method for separating microorganisms, cells, and precipitates from the fermentation broth. Two major types of centrifuge - the tubular-bowl and the disk-stack - are used for continuous, large-scale operation. 

Layers of 4,4 -bipyridyl (0.3 mol T1 in dichloromethane) and ethyl bromoacetate (0.3 mol 1 1 in dichloromethane) and a separation layer of dichloromethane are fitted into each other by means of the concentric separation mixer [53]. The reaction temperature is 22 °C. The reaction solution is inserted as droplets or a continuous stream either directly or via the tubular reactor in the beaker. The precipitate solution yielded is passed through a frit and the remaining solid is washed with dichloromethane and dried at elevated temperature and weighed. The quatemized product is characterized by NMR spectroscopy. 

Donnet, M., Bowen, P., Jongen, N., Lemaitre, J., Hofmann, H., Schreiner, A., Jones, A. G., Schenk, R., Hofmann, C., Successful scale-up from millilitre batch optimization to a small scale continuous production using the segmented flow tubular reactor example of calcium carbonate precipitation, Chem. Eng. Trans. 2002, 1,1353-1358. 

Japanese researchers have developed a method for electromachining using a tubular cathode, either a alkaline or neutral salt solution, and employing ultrasound with the ultrasonic vibration being continuously supplied to prevent the precipitation of chiefly metal hydroxides within the cathode. Clogging of the cathode tube can thus be prevented [115],... 

Extensive experience has proved that a bundle of smooth tubes continuously flooded by massive quantities of water will not develop encrustations provided the water is properly treated and the surface temperature does not exceed 65 C. A continuous water blanket over the tubular surfaces prevents precipitation of dissolved minerals. Cocurrent flow of water and air provides better operation. 

Chlorinating a slurry of lime in a sodium hydroxide solution forms crystals of calcium hypochlorite dihydrate, which can be filtered and dried. The precipitation of basic compounds is avoided by incremental addition of the calcium hydroxide to the sodium hydroxide solution during chlorination. In a batch process, all of the sodium hydroxide and 10-80% of the calcium hydroxide are introduced into a stirred reactor. Chlorine is then added. When the chlorination is nearly complete, the remainder of the calcium hydroxide is added without interrupting the flow of chlorine. The solids are separated and dried to from CalOCOj. In a continuous process, the sodium hydroxide solution is fed into one end of a long tubular reactor, and the calcium hydroxide slurry and chlorine are separately injected at a number of sites along the length of a jacketed reactor. The calcium hypochlorite is removed from the end of the reactor and dried. When impurities in the lime interfere with the crystallization of calcium hypochlorite, potassium hydroxide is substituted for up to 25% of the sodium hydroxide. ... 

Numerous variations of the interfacial process have been published. The reactions can be carried out in batch in stirred tank reactors or continuously in series of CSTRs and tubular reactors. Intensive mixing with dispersion and redispersion is required throughout the reaction stages. After the reaction is complete, the brine phase is separated and the polymer solution washed to remove residual amine and base. Several processes for devolatilization are in use, including solventless precipitation, steam precipitation, spray drying, falling-strand devolatilization, and vacuum extrusion in devolatilizing extruders. 

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