Tank crystallisers

This is the simplest type of industrial crystallising equipment. Crystallisation is induced by cooling the mother liquor in tanks which may be agitated and equipped with cooling coils or jackets. Tank crystallisers are operated batchwise, and are generally used for small-scale production. 

Installation of an agitator in an open-tank crystalliser gives smaller and more uniform crystals and reduces batch times. Because less liquor is retained by the crystals after filtration... 

The sodium sulfite precipitates first and is removed by centrifugation, washed with water, and dried. The mother Hquor containing ammonium chloride is sent to crystallising tanks and the salt thus formed is washed and dried, giving a product said to analyse weU over 99%. 

Tanks are used in innumerable ways in the chemical process iadustry, not only to store every conceivable Hquid, vapor, or soHd, but also ia a number of processiag appHcations. For example, as weU as reactors, tanks have served as the vessels for various unit operations such as settling, mixing, crystallisation (qv), phase separation, and heat exchange. Hereia the main focus is on the use of tanks as Hquid storage vessels. The principles outlined, however, can generally be appHed to tanks ia other appHcations as weU as to other pressure-containing equipment. 

In the Sulser-MWB process the naphthalene fractions produced by the crystallisation process are stored in tanks and fed alternately into the crystalliser. The crystalliser contains around 1100 cooling tubes of 25-mm diameter, through which the naphthalene fraction passes downward in turbulent flow and pardy crystallises out on the tube walls. The residual melt is recycled and pumped into a storage tank at the end of the crystallisation process. The crystals that have been deposited on the tube walls are then pardy melted for further purification. Following the removal of the drained Hquid, the purified naphthalene is melted. Four to six crystallisation stages are required to obtain refined naphthalene with a crystallisation point of 80°C, depending on the quaHty of the feedstock. The yield is typically between 88 and 94%, depending on the concentration of the feedstock fraction. 

Phenol is supplied commercially either in the solid (crystalline) state or as a solution in water (water content 8-20%). Where supplied as a solid it is usually handled by heating the phenol, and the molten material is pumped into the resin kettles or into a preblending tank. If the solution is used care must be taken to avoid the phenol crystallising out. 

The flow diagram of the enzyme reactor for continuous production of the L-amino add is given in Figure A85. The acetyl amino add is continuously charged into the enzyme column through a filter and a heat exchanger. The effluent is concentrated and the L-amino add is crystallised. The acyl-D-amino add contained in the mother liquor is racemised by heating in a racemisation tank, and reused. 

Semi-continuous crystallisation processes which often combine the best features of both batch and continuous operation are described by Nyvlt(35), Randolph(37), Robinson and Roberts(70) and Abbeg and Balakrishnam(7I ). It may be possible to use a series of tanks which can then be operated as individual units or in cascade. Mullin(3) suggests that for production rates in excess of 0.02 kg/s (70 kg/h) or liquor feeds in excess of 0.005 m3/s, continuous operation is preferable although sugar may be produced batch-wise at around 0.25 kg/s (900 kg/h) per crystalliser. 

Iminobisacetonitrile crystallised in a holding tank of a production plant. Cleaning was by filling with water and heating to dissolve the nitrile. It was heated to a supposed 75°C. An hour later the tank exploded causing severe structural damage. The... 

Fig. 100. Production diagram of a-naphthyl ether of pyrocatechinphosphorous acid 1 - reactor 2, 5, 10, 13 - cooler 3, 8, 12 - batch boxes 4 - vacuum distillation tank 6,7 - receptacles 9 - etherificator 11 - crystalliser 14 - nutsch filter 15 - vacuum draft...

Fig. 102. Production diagram of phosphonitrilechloride trimer 1,2- batch boxes 3 - agitator 4- reactor 5, 11, 17, 20- coolers 6- separator 7 - tower 8, 13 - nutsch filters 9, 12, 21 - receptacles 10, 19 - distillation tanks 14, 15 -collectors 16- extractor 18- pressure filter 22- crystalliser 23 - ultracentrifuge...

The obtained saturated solution is filtered in pressure filter 18, which heated with vapour to avoid crystallisation. The oily liquid remaining on filter cloth is collected and added to the product in collector 15 the hot filtered crystal solution enters tank 19 for partial distillation of light petroleum and preparation of oversaturated solution. The hot solution is sent from the tank into crystalliser 22. 

Apparatus 22 is cooled with salt solution (-15 °C) to crystallise phosphonitrilechloride trimer the mixture is held for some time to let ciystals grow and precipitate. Then the ciystals are separated in centrifuge 23, and the mother solution is sent back to tank 19. The obtained crystals of phosphonitrilechloride trimer can be given additional purification, i.e. reciys-tallisation from fresh light petroleum or vacuum distillation at 127 °C and a residual pressure of 17 GPa. 

Drainage water from coal mines is frequently charged with ferrous sulphate consequent upon the oxidation of pyrites in the coal, and upon evaporation yields the impure salt. The salt is readily obtained in a pure state by dissolving electrolytic iron in dilute sulphuric acid and allowing to crystallise, preferably in an inert atmosphere. Commercially ferrous sulphate is obtained by exposing heaps of pyrites to the oxidising action of the air. Ferrous sulphate and free sulphuric acid drain off into tanks, the acid being neutralised with scrap iron —... 

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 crystallisers can be operated batchwise, with recirculation of the mother liquor, or continuously. A disadvantage of this type is that they tend to produce very small crystals. A typical unit is the Swenson-Walker crystalliser shown in Volume 2. 

The crystals of nitrate deposited in the crystallising tanks are covered with a little water, drained, and allowed to dry in the sun for five or more days. They have then the following composition —... 

Sodium Nitrite, NaNO.,.—The liquid from the fifth absorption tower is evaporated, run into shallow tanks, allowed to crystallise, centrifuged, and the crystals dried in a current of hot air. The product contains 99 per cent. NaNO., being sold as a fine white powder (see p. 14 for properties). 

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