Crystallizers open tank

No particular industrial hazards are encountered in handling liquid alum. However, a face shield and gloves should be worn around leaking equipment. The eyes or skin should be flushed and washed upon contact with liquid alum. Liquid alum becomes very sick upon evaporation and therefore spillage should be avoided. Storage tanks may be open if indoors but must be closed and vented if outdoors. Outdoor tanks should also be heated, if necessary, to keep the temperature above 450F to prevent crystallization. Storage tanks should be constructed of type 316 stainless steel, FRP, steel lined with rubber, polyvinyl chloride, or lead. Liquid alum can be stored indefinitely without deterioration. 

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... 

A detailed historical development of crystallization equipment is not the purpose of this presentation, but a few general remarks are appropriate. Crystallization has been practiced for centuries and its beginnings, like those of other classical unit operations, are lost in antiquity (Cl, S10). The earliest crystallizers were shallow open tanks, operated batch-wise and cooled by surface evaporation. Simple, inexpensive equipment... 

The oldest and simplest representative is the tank crystallizer. It consists of an open tank, either rectangular or circular in section, exposed to the atmosphere, which provides the necessary cooling. Frequently, ropes, rods, or lead strips are suspended in the bath to provide a base... 

The installation of an agitator in an open-tank crystallizer generally results in smaller, more uniform crystals and reduced batch time. The final product tends to have a higher purity because less mother liquor is retained by the crystals after filtration and more efficient washing is possible. Vertieal baffles may be fitted inside the vessel to induce better mixing, but they should terminate below the liquor level to avoid excessive encrustation. For the same reason, water jackets are usually preferred to coils for cooling purposes and, where possible, the internal surfaces of the crystallizer should be smooth and crevice-free (section 9.5). 

Tank crystallizers. In tank crystallization, which is an old method still used in some specialized cases, hot saturated solutions are allowed to cool in open tanks. After a period of time the mother liquor is drained and the crystals removed. Nucleation and the size of crystals are difficult to control. Crystals contain considerable amounts of occluded mother liquor. Labor costs are very high. In some cases the tank is cooled by coils or a jacket and an agitator used to improve the heat-transfer rate. However, crystals often build up on these surfaces. This type has limited application and sometimes is used to produce some fine chemicals and pharmaceutical products. 

India, Goa and Bombay two plants built ca 1950 batch-type open-hearth roasters ceUs are operated discontinuously, the KMnO is crystallized separately in agitated tanks 1,200 114... 

A semi-open impeller has exposed blades, but with a support plate or shroud on one side. Some people prefer the name semi-enelosed. These types of impeller are generally used for liquids with a small percentage of solid particles like sediment from the bottom of a tank or river, or crystals mixed with the liquid (Figure 6-17). 

Several refineries in Europe employ the Pro-Abd refiner to upgrade whizzed naphthalene to phthalic-grade quality or to convert the latter into the purer chemical grade. The device consists of a rectangular tank fitted with a nest of coils through which either steam or water circulates. The tank is filled with the feedstock, which is crystallized by circulating cold water in the coils. When the contents of the tank have solidified, a tap at the base is opened and hot water is circulated until the temperature is just below the desired crystallizing point of the product. This condition is maintained until no more oil drains from the base. The bottom tap is then closed and the contents of the tank are melted by steam circulation and drained. 

Grainer salt is made by surface evaporation of brine in flat pans open to the atmosphere. Heat usually is furnished by steam pipes located a few inches below tlie tank bottom. Crystals form at the surface of the brine and are held tliere temporarily by surface tension. Thus, they grow laterally for awhile and form thin flakes. But, as they grow, they tend to sink and this process imparts a peculiar, hollow pyramid-like structure to them. Such crystals are called hopper crystals. Ultimately, the crystals sink to the bottom where they are scraped to one end of the pan. The crystals are fragile and during handling they break up, finally assuming a flake-like shape. Thus, tlie term flake salt. 

The simplest type of cooling crystallizer is the unstirred tank a hot feedstock solution is charged to the open vessel where it is allowed to cool, often for several days, predominantly by natural convection. Metallic rods may be suspended in the solution so that large crystals can grow on them and reduce the amount of product that sinks to the bottom of the crystallizer. The product is removed by hand. 

CN and CS are white, tan, or yellow crystals or dust. They are solids that vaporize easily. They are dispersed as fine particles or in solution. These agents are frequently dispersed using a flammable propellant such as isopropyl alcohol. They may ignite if there is an open flame. They may be dispersed by small spray cans, large spray tanks, or by explosive devices (tear gas grenades or shells) (Figure 2.14). 

---