Since almost heat make-up is required, the arrangement is compact and hence initial investment is minimized. They are normally used for producing large size crystals of ammonium sulphate, potassium chloride, diammonium chloride etc. When destruction of fines is not needed or wanted, baffles are not provided and the internal circulation rate is set to have the minimum nucleating influence on the suspension. When large evaporation rates are required, external heating must be provided. To achieve an effective process control it is important to control the retention time and the crystal mass, to obtain the optimum conditions in terms of crystal specific surface and the fastest possible growth.
|Published (Last):||22 August 2006|
|PDF File Size:||18.27 Mb|
|ePub File Size:||15.40 Mb|
|Price:||Free* [*Free Regsitration Required]|
Separations Chemical Crystallizers Crystallizers are used in industry to achieve liquid-solid separation. They are an important piece of chemical processing equipment because they are capable of generating high purity products with a relatively low energy input. Copyright Swenson Process Equipment, Inc. They create a super-saturated solution by evaporating the solvent of a saturated solution. The solute of this supersaturated solution then cools, forming crystals. These types of crystallizers are classified as mixed-suspension, mixed-product-removal MSMPR crystallizers.
The key assumption of an MSMPR crystallizer is that the slurry is perfectly mixed and uniform throughout the system. Vaporization occurs at the top surface of the slurry, while nucleation occurs near the bottom of the crystallizer body.
The crystals are removed and vaporized solvent is condensed and returned to the crystallizer body. Options include baffling, a conical entrance, and an elutriation step. A conical entrance promotes more thorough mixing, which creates a more uniform slurry mixture.
Baffling is used to remove fines from the mixture, so that they can be recirculated for further growth. Elutriation, like baffling, removes smaller particles from the slurry, except elutriation separates particles based on weight rather than size.
Elutriation is the process of flowing air over particles. Lighter particles are picked up by the air stream, while heavier ones remain in place. Usage Examples Forced-circulation crystallizers are commonly used to produce salts and chemicals in industry. The forced-circulation crystallizer shown on the left is used to produce sodium sulfate, while that on the right is used in the agricultural industry. The two-stage crystallizer shown below is used by an agricultural chemical company to evaporate phosphoric acid from a mixture and recover solids.
Small, unwanted crystals, called fines, are removed from larger particles using gravitational settling and recycled through the crystallizer again, resulting in maximum crystal recovery and larger crystals.
In the baffled region gravitational settling separates larger crystals from fines. The larger crystals settle between the baffle and draft tube, as shown below, and are removed in the product slurry, while the fines are recirculated after being redissolved in a heat exchanger. The evaporated solvent is then condensed, cooled, and returned to the region of active crystallization. DTB crystallizers are also used for the purification of organic compounds as well as the production of salts.
Slurry is drawn from the crystallizer body and then cooled before being pumped back into the crystallizer body. This vacuum makes it possible to generate a supersaturated solution when very low operating temperatures are needed. Vacuum crystallizers may be continuous or batch. The batch vacuum crystallizer is particularly useful when processing materials that tend to grow on the walls of continuous crystallization equipment.
The crystallizer body can be seen on the left, with the condenser and booster slightly above and to the right of it. They are used primarly for the crystallization of melts. Copyright Armstrong Chemtec Group, West Chester, PA Equipment Design A scraped surface crystallizer consists of a jacketed pipe in which a cooling medium between the pipe wall and the jacket remove heat from the slurry, causing crystallization.
Inside the pipe, large scrapers wipe the solid deposits from the wall to prevent build up. Scraped surface crystallizers are often grouped together in assemblies, as shown below.
Draft Tube Baffle (DTB) Crystallizer
Separations Chemical Crystallizers Crystallizers are used in industry to achieve liquid-solid separation. They are an important piece of chemical processing equipment because they are capable of generating high purity products with a relatively low energy input. Copyright Swenson Process Equipment, Inc. They create a super-saturated solution by evaporating the solvent of a saturated solution.
So one may identify two main families of crystallization processes: Cooling crystallization Evaporative crystallization This division is not really clear-cut, since hybrid systems exist, where cooling is performed through evaporation , thus obtaining at the same time a concentration of the solution. A crystallization process often referred to in chemical engineering is the fractional crystallization. This is not a different process, rather a special application of one or both of the above. Application[ edit ] Most chemical compounds , dissolved in most solvents, show the so-called direct solubility that is, the solubility threshold increases with temperature. Solubility of the system Na2SO4 — H2O So, whenever the conditions are favorable, crystal formation results from simply cooling the solution. In the diagram, where equilibrium temperature is on the x-axis and equilibrium concentration as mass percent of solute in saturated solution in y-axis , it is clear that sulfate solubility quickly decreases below