PAMANTASAN NG LUNGSOD NG MAYNILAIntramuros, ManilaCollege of Engineering and TechnologyChemical Engineering Department

Crystallization equipment (crystallizers)Crystallization equipment is classified according to the method by which supersaturation is brought about:1. Supersaturation by cooling2. Supersaturation by evaporation3. Supersaturation by adiabatic evaporation (cooling and evaporation)

SUPERSATURATION BY COOLING

Swenson-Walker Crystallizer

The Swenson-Walker Crystallizer is an equipment used for continuous operation of crystallization in which the supersaturation is achieved solely by cooling. It is an example of the scraped surface crystallizer and probably the most widely used crystallizer. It consists of an open rectangular trough with a semi-cylindrical bottom of width 0.6m and length of 3 to 6m. The trough is jacketed externally for circulation of cooling water during operation. A spiral agitator/stirrer is provided for the following purposes:1. To prevent the accumulation of crystals on the cooling surface for better heat transfer2. To lift the crystals that have already been formed and shower them down through the solutionIn this manner, the crystals grow while they are freely suspended in the liquid and therefore they are:1. Fairly perfect individuals 2. Uniform in size3. Free from inclusions or aggregation

The setup consists of a top open jacketed round bottom trough. A ribbon mixer is provided in crystallizer trough, which rotates at low rpm for agitation of saturated solution. The saturated solution is prepared in a tank fitted with heater and a stirrer. This hot saturated solution is fed at one end of the crystallizer and flows slowly towards the other end while being cooled and is allowed to flow back in hot feed tank making the process continuous. The crystals sediments at the bottom of the trough, and can be washed out at the end of process. Arrangement for Cooling water circulation and flow measurement in jacket is made by using storage type water tank, a pump and a rotameter. Necessary instrumentation is done for temp measurement. In order to control crystal size, it is sometimes desirable to introduce an extra amount of water into certain sections in the jacket.

Other Swenson CrystallizersSwenson has an unusually broad background in the crystallization field and can supply a wide range of crystallizers and evaporators to meet the requirements for almost any application. Thousands of evaporators, crystallizers, dryers and other process equipment have been installed around the world and bear the Swenson name. Swenson equipment is designed and fabricated to provide optimum results under customer specified operating conditions. Each project is unique and requires its own unique process design solution.The Swenson Draft Tube Baffle (DTB) crystallizer is the most successful of the crystallizers designed to make the large, uniform crystals required for fertilizer and similar applications where superior filtration, centrifugation, washing, drying, and storage characteristics are required. This design is the result of years of research and development as well as proven plant scale operation in hundreds of installations.Associated Products Potassium ChlorideAssociated Equipment Draft Tube Baffle (DTB) Crystallizer Zero Liquid Discharge Surface Cooled CrystallizerAssociated Industries Pulp & Paper

Pan Cooler

Crystallization pan is a vessel equipped with heat exchange areas (for heating or cooling) and providing agitation. The heat exchange areas may be provided by a jacket or by a tubular heat exchange element. Agitation may be provided either by a mechanical stirrer or by recycling. Crystallization pans can operate in batch or continuous mode. These pans are widely used in food industry, especially in sugar production. Recycling pans with external heat exchangers are frequently used in the production of salt and citric acid but not in the sugar industry because of the high viscosity of material. Agitation must be sufficiently vigorous to facilitate mass transfer and good mixing, but gentle enough to prevent crystal fracture, abrasion, and secondary nucleation.Even though the sugar industry increasingly changes to continuous pan boiling, batch pans are still required, e.g. for the production of seed.The first step of seeding proceeds in a cooling crystallizer which is equipped with a stirrer specifically designed for a high shear rate. The low temperature and the clearly defined supersaturation ensure uniform growth of the added slurry seed crystals, avoiding the formation of aggregates. The syrup used (preferably thick juice) is subjected in the cooling crystallizer to evaporation to the required dry substance content, and the concentrated syrup is then cooled. When a supersaturation of 1.1 has been reached, it is seeded with the required quantity of slurry. Thereafter, the suspension is further cooled (slowly) to approx. 30C. The massecuite-to-cooling-water temperature difference serves to control the cooling gradient and, consequently, to adapt the supersaturation to be maintained during the cooling crystallization process. After the crystals have grown to a mean size of approx. 0.08 to 0.11mm (subject to requirements) at a crystal content of approximately 20% at the end of the cooling phase, the 1st-seed massecuite is dropped into a receiver. In a second seed production step, batch pans produce crystallizate with a mean crystal size of approx. 0.3 to 0.5mm. Optimum plants use stirrers providing a high shear rate. New batch pans are optimally designed for the production of 2nd seed. The feed solution is concentrated until the seeding point is reached at a supersaturation of approx. 1.1 and the 1st-seed massecuite is drawn in as quickly as possible. After a short phase of homogenization and temperature equalization, growing can start immediately. After growth, the resultant 2nd-seed magma containing approx. 45% to 50% crystals is dropped into the receiver. By adjusting the quantities of slurry and seed, the mean crystal size of white sugar can be varied within a range of approx. 0.5 to 1mm. For raw sugar, the product crystal size usually is 0.4 to 0.5mm, and for low-grade sugar approx. 0.3 to 0.35mm.

Agitated Batch CoolerIn these crystallizers, supersaturation takes place by cooling with the help of cooling coils. In this crystallizer, feed is introduced into the crystallizer from the top. Supersaturation by cooling is done through the cooling coils. An agitator is present which keeps the solution in random motion throughout the process. Blender does not only accelerate the heat, it also brings the solution of some more uniform temperature that can promote the emergence and nucleating of crystal growth. The main disadvantage in this is the deposition of crystals around the coils hindering heat exchange. In short, water is circulated though the cooling coils and the solution is agitated by the propellers mounted on the central shaft. Product is collected at the bottom of the crystallizer.This agitation performs two functions:1. It increases the rate of heat transfer and keeps the temperature of the solution more uniform. 2. It keeps the fine crystals in suspension, thus it gives them an opportunity to grow uniformly instead of forming large crystals or aggregates. (Production of uniform crystals)

Advantages1. The agitation increases the rate of heat transfer and keeps the temperature of the solution uniform throughout the crystallizer.2. Agitation keeps the smaller crystals in suspension and allows them to grow uniformly thus finer crystals can be obtained.

Disadvantages1. It is a batch or discontinuous apparatus.2. Since the solubility is least at the cooling surface hence the crystals growth is more rapid on the cooling coils. Thus the crystals deposited on the cooling coils reduces the heat transfer rate.