Continuous surface cooling crystallization

Continuous surface cooling crystallization is not a modern process. In this method, the supersaturation is created using the surface of a heat exchangee As soon as this surface is covered by incrustations, the production output is directly affected. The... 

High levels of supersaturation at the liquid surface or at the tube walls in a surface-cooled crystallizer are the dominant cause of wall salting. Although some types of crystallizers can operate for several months continuously when crystallizing KCl or (NH4)2S04, most... 

The sorbitol solution produced from hydrogenation is purified in two steps [4]. The first involves passing the solution through an ion-exchange resin bed to remove gluconate and other ions. In the second step, the solution is treated with activated carbon to remove trace organic impurities. The commercial 70% sorbitol solution is obtained by evaporation of the water under vacuum. The solid is prepared by dehydration until a water-free melt is obtained which is cooled and seeded. The crystals are removed continuously from the surface (melt crystallization). The solid is sold as flakes, granules, pellet, and powder forms in a variety of particle size distributions. 

In a continuously operated vacuum crystallizer the feed solution should reach the surface of the liquor in the vessel quickly, otherwise evaporation and cooling will not take place, because, owing to the hydrostatic head of solution, the boiling point elevation becomes appreciable at the low pressures (<20mbar) used in these vessels, and the feed solution will tend to migrate down towards the bottom outlet. Care must be taken, therefore, either to introduce the feed near the surface of the liquor in the vessel or to provide some form of agitation. 

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. 

Into a 1-litre beaker, provided with a mechanical stirrer, place 36 - 8 g. (36 ml.) of aniline, 50 g. of sodium bicarbonate and 350 ml. of water cool to 12-15° by the addition of a little crushed ice. Stir the mixture, and introduce 85 g. of powdered, resublimed iodine in portions of 5-6 g, at intervals of 2-3 minutes so that all the iodine is added during 30 minutes. Continue stirring for 20-30 minutes, by which time the colour of the free iodine in the solution has practically disappeared and the reaction is complete. Filter the crude p-iodoaniline with suction on a Buchner funnel, drain as completely as possible, and dry it in the air. Save the filtrate for the recovery of the iodine (1). Place the crude product in a 750 ml. round-bottomed flask fitted with a reflux double surface condenser add 325 ml. of light petroleum, b.p. 60-80°, and heat in a water bath maintained at 75-80°. Shake the flask frequently and after about 15 minutes, slowly decant the clear hot solution into a beaker set in a freezing mixture of ice and salt, and stir constantly. The p-iodoaniline crystallises almost immediately in almost colourless needles filter and dry the crystals in the air. Return the filtrate to the flask for use in a second extraction as before (2). The yield of p-iodoaniline, m.p. 62-63°, is 60 g. 

Mix 42 5 g. of acetone cyanohydrin (Section 111,75) and 75 g. of freshly powdered ammonium carbonate in a small beaker, warm the mixture on a water bath FUME CUPBOARD) and stir with a thermometer. Gentle action commences at 50° and continues during about 3 hours at 70-80°. To complete the reaction, raise the temperature to 90° and maintain it at this point until the mixture is quiescent (ca. 30 minutes). The colourless (or pale yellow) residue solidifies on coohng. Dissolve it in 60 ml. of hot water, digest with a little decolourising carbon, and filter rapidly through a pre-heated Buchner funnel. Evaporate the filtrate on a hot plate until crystals appear on the surface of the liquid, and then cool in ice. Filter off the white crystals with suction, drain well, and then wash twice with 4 ml. portions of ether this crop of crystals of dimethylhydantoin is almost pure and melts at 176°. Concentrate the mother liquor to the crj staUisation point, cool in ice, and collect the... 

Double-Pipe Scrapea-Surface Crystallizer This type of equipment consists of a double-pipe heat exchanger with an internal agitator fitted with spring-loaded scrapers that wipe the wall of the inner pipe. The cooling hquid passes between the pipes, this annulus being dimensioned to permit reasonable shell-side velocities. The scrapers prevent the buildup of solids and maintain a good film coefficient of heat transfer. The equipment can be operated in a continuous or in a recirculating batch manner. 

The ether is allowed to evaporate on the water-bath and th e nitrile is then hydrolysed by continuing to heat it on the water-bath with the addition of 4—5 times its volume of cone, hydrochloiic acid until crystals appear on the surface. Water is added and the hot liquid decanted and filteied from any oil. On cooling, the crystals are filtered, washed vvith a little cold ivaterand dried. A fuither quantity can be extiacted from the filtrate with ether. It may be recrystallised from benzene. Yield, 10—15 grms. 

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