Utente:Ub/Sandbox: differenze tra le versioni

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====The DTB Crystallizer====

[[Image:DTB_Xls.png|frame|left|DTB Crystallizer]][[Image:DTB_2.PNG|thumb|right|Schematic of DTB]]

Whichever the form of the crystallizer, to achieve an effective process control it is important to control the retention time and the crystal mass, to obtain the optimum conditions in term of crystal specific surface and the fastest possible growth. This is achieved by a separation - to put it simply - of the crystals from the liquid mass, in order to manage the two flows in a different way. The practical way is to perform a gravity [[settling]] to be able to extract (and possibly recycle separately) the (almost) clear liquid, while managing the mass flow around the crystallizer to obtain a precise slurry density elsewhere. A typical example is the DTB (''Draft Tube and Baffle'') crystallizer, an idea of Richard Chisum Bennett at the end of the '50s. (rumors say the name derives from the initial of the inventor - Dick Bennett - but mistery still surrounds the letter T). The DTB crystallizer (see images) has an internal circulator, typically an axial flow mixer - yellow - pushing upwards in a draft tube while outside the crystallizer there iais a settling area in an annulus; in it the exhaust solution moves upwards at a very low velocity, so that large crystals settle - and return to the main circulation - while only the fines, below a given [[granulometry]] are extracted and eventually destroyed by increasing or decreasing temperature, thus creating additional supersaturation. A quasi-perfect control of all parameters is achieved. This crystallizer, and the derivate models (Krystal, CSC, etc.) could be the ultimate solution if not for a major limitation in the evaporative capacity, due to the limited diametre of the vapour head and the relatively low external circulation not allowing large amounts of energy to be supplied to the system.