Crystallizers scraped surface continuous crystallization

The scraped sxmface crystallizer makes crystallization continuous. Generally, the only reason to work with batch crystallization is very low design capacity. If design capacity is above 500,0001b annually, the scraped surface continuous crystallizer will save time, energy, and manpower. 

Scraped surface continuous crystallizers offer the best approadi for low-temperature crystallizations such as the separation of meta- and paraxylenes or oleic and linoleic acids. 

Many of the processing plants are quite large and require many scraped surface continuous crystallizers, often with a number of units in a series. Larger plants usually require several parallel trains of crystallizers. 

The basic goal of designing scraped surface continuous crystallizers for dewaxing is to ensure longer time on stream between turnarounds. 

Scraped Surface Continuous Crystallizer Specifically Designed for the Growth of Easily Separable Crystals... 

In effect, the scraped surface continuous crystallizer is a heat exchanger, but quite an unusual one, because it generally performs as a cooling crystallizer. Heat transfer occurs across the wall in the inner pipe, with cold fluid outside, and process flmd inside. As cooling occurs, crjretals tend to form and foul the inner pipe wall. The scraper blades rotate on the inner pipe wall and remove the deposits that would inhibit heat transfer. The majority of the crystallization takes place in the bulk of the fluid, as opposed to the wall, thus allowing growth of easily separable crystals. 

The drive system on a scraped surface continuous crystallizer must allow free turning of the driven shaft. The shaft must also be sealed against leakage of the process fluids. The drive system used is illustrated in Fig. 054. 

This type is similar in principle to the tank type, but the cooling surfaces are continually scraped or agitated to prevent the fouling by deposited crystals and to promote heat transfer. They are suitable for processing high-viscosity liquors. Scraped-surface... 

The scraped surface, or close-clearance exchanger, illustrated in Fig. 10 is required for a few very difficult situations. An example is purification by fractional crystallization, in which a refrigerant boiling in the annulus cools a solution of various substances and certain species selectively crystallize out on the surface of the inner pipe. The crystalline deposit must be continuously scraped off of the surface in order that the heat transfer rate be maintained. The crystals are eventually removed from the remaining liquid by filtration. 

As the mix is frozen and aerated its viscosity increases both because the temperature is decreased and also because ice crystals and air bubbles are formed. The viscosity of an aerated mix increases according to Einstein s equation (equation 2.11) for low air volume fractions. Figure 7.22 shows what happens as an aerated ice cream mix is frozen. The storage modulus, G starts to rise at about — 0.5 C when the first ice crystals form, and continues to increase as the temperature decreases. This data was obtained by using a scraped surface heat exchanger that is simultaneously a rheometer G was calculated from the torque on the dasher. 

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