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Equipment for crystallization from solutions and melts The aim of the present work is to present the general knowledge about the equipment which now exists for crystallization from solutions and melts. The research will be based on the extensive study of the scientific literature and other related sources. It is necessary to consider the most important and widely spread kinds of equipment, looking at them from the viewpoint of easy data acquiring. Introduction To start with, it is necessary to describe the process of crystallization itself, for the better understanding of the methods the described equipment works.
Mullin (2001) describes crystallization as the process of formation of solid crystals from homogeneous solutions. The crystals, which are formed as a result of crystallization, can bear various shapes, starting from cubic or mono clinic, finishing with trigonal, for example. Crystallization is used for the production, purification and recovery of solids. Crystalline products have an attractive appearance, are free flowing, and easily handled and packaged. (Coulson & Richardson, 2003) However, it should be born in mind that in order for the solution to be ready for crystallization, it must me in the state of supersaturation.
Supersaturation is the state in which the solution has in its contents more solids dissolved, than it is normally at the same temperature. Equilibrium is as important as saturation is, for the successful crystallization of solutions. In reality, crystallization usually takes place in large scales, but to consider it on the small example, it is easy to take the water-salt solution and create supersaturated mixture (as soon as no more salt is dissolved, it will mean that the solution is supersaturated). Adding additional salt into the solution, the basis for the crystallization is created, as its first stage is usually called nucleation.
In industrial scales, this primary nucleation usually needs much driving force to start, and in fact the origin and laws of this driving force are not yet understood until the present moment. This stage of crystallization is usually followed by the secondary nucleation, when the growth of crystals is initiated through contact with any device or with other crystals. Secondary nucleation is also unclear and is not fully represented in any theory. Mathematic relationships do exist to correlate experimental data.
However, correlating experimental data to model crystallization is time consuming and often considered extreme for batch operations, but can easily be justified for continuous processes where larger capital expenditures are necessary. For batch operations, only preliminary data measurements are truly necessary. (Price, 1997) The main types of equipment used for crystallization It is necessary first of all, to distinguish the main types of the equipment, which is usually used for the crystallization from solutions and melts. These are: tank crystallizer's scraped surface crystallizer's forced circulating liquid evaporator-crystallized circulated magma vacuum crystallized. (Gilbert, 1991) For the aims of the present work, I will here pay attention to the main two types of crystallizer's, which are of interest to me these are tank crystallizer's and scraped surface crystallizer's. Tank crystallized Tank crystallized is supposed to be the oldest type of equipment used for crystallization, thus it may be considered to be the simplest in operation, though it often appears that using this equipment it is not always possible to acquire the necessary clear correlated data, and to use it for practical aims. Considering tank crystallizer's, it is interesting to note, that the above mentioned example of the salt-water solution is appropriate here to imagine the main principles, according to which tank crystallizer's work. It is often, that in the construction of tank crystallizer's heat transfer coils are used.
This type of equipment is mostly used for the pharmaceutical industry, where high quality of crystals is required, because this method is highly labor-intensive, and is not suitable for universal use. Tank crystallizer's produce the product of high quality, and its suitability for the pharmaceutical and related industries is based on the fact, that crystals have better preservation properties. (Person, 2004) Speaking about tank crystallizer's, it is also necessary to remember that they also differ between each other mainly through their types and positions, and in relation to tank crystallizer's there are certain methods, which ultimately will make crystals grow better and more efficiently. Among the most widely-spread methods can be suggested decreasing the attrition and mechanical stresses, as well as increasing retention time. (Geankoplis, 1993) It is known that higher temperatures during the process of crystallization produce better results, but it should be remembered that the choice of the temperature appropriate will be based on the specific material used, and thus too high temperatures should be supposed as possible causes of the crystallization failure. The size of the crystals, which may be supposed to be of high quality through the use of the tank crystallizer's, is no more than 0. 2 mm, which is usually produced by using high-speed impeller pumps. (Ulrich & Jones, 2004) If the size of the crystals is to be bigger, and the crystals themselves are to be coarser, it is possible to use the draft-tube-baffled crystallizer's; the pumps which are used in this equipment are internal, their diameter is higher, and they possess less tip speed. This type of crystallizer's is more energy-saving, than that of high-speed impeller pumps. Scrape surface crystallized Scrape-surface crystallizer's are of different designs, constructions, but are similar in their functions and principles of operation.
Such crystallized may consist of multiple double pipe elements, their size may vary from 6 to 12 inch in diameter, and the space between the inner and the external pipe is usually filled with cooling liquid. Inner and external pipes are different in their construction, as inner pipe usually carries the rotating scrape blade element, which is absent in external pipes and is usually used for mixing the fluid. This type of crystallized also serves as heat exchanger, but it should be admitted that the way this heat exchanger operates is rather unusual, as traditionally this type of crystallizer's have been used for cooling the crystals; but it appears, that while the process of cooling is going, on the inner pipe wall there are the crystals which are formed and accumulated. The blade mechanism is used for eliminating these accumulated crystals from the pipe walls, and this involves the heat transfer. The bigger part of crystallization takes place in the liquid, which assists in formation of the easily separable crystals. (Borodin, 2000) As it has already been noted, tank crystallized, being highly labor-intensive, can be used for limited goals, and though it is involved into the formation of the high quality crystals, it can hardly be widely-spread type of crystallization equipment.
However, scraped-surface crystallizer's also have their advantages, and they should be noted for the better understanding of their operation and for coming to clearer conclusions as for the best choice in crystallization equipment. First of all, Thomsen & Gang (1998) write, that scraped surface crystallized is smaller in size, and thus the costs for its installation are lower. It is not as labor-intensive, as the tank crystallized, and it does not need so much space for operation. This kind of crystallized allows to better control the whole process of crystallization, while the use of expensive and hazardous materials in this equipment is minimal.
Which is also important, is that in case the expansion of production scales is necessary, this equipment is easily expansion without major changes. Such types of crystallizer's are usually used when the boiling point of the solution is very high, or when the solution is caustic. When the temperature of the solution is low, and vacuum crystallizer's are inappropriate for the use, as they may cause evaporation, surface-scraped equipment appears to be the best choice. In production of sodium-chloride this equipment has proved to be the most efficient of all similar types. It is also suitable for the crystallization of sodium carbonate and sodium chlorate. (Brown, 1991) Among other advantages of this equipment the following can be noted: The equipment can be used through the extremely wide ranges of temperature; The percentage of solids in the solutions for crystallization may at times reach 65 % and not cause any problems in the equipment operation; High viscounties of the solutions appear not to be problematic at all for this type of crystallization equipment. Comparison and effectiveness of the crystallization equipment described Literary sources relating to the description of the crystallization as a process, and the crystallization equipment in particular, differ in their definitions of the advantages for this type of equipment, making accent on their various aspects.
For example, Borodin (2000) writes, that the main advantage of the surface-scraped crystallized is in the fact, that it does need any auxiliary equipment; Konig, Rechsteiner & Trusch (1997) state, that the low cost of the equipment is one of its essential advantages. In fact, it is hard to argue against these opinions, but it should be born in mind, that even accounting these advantages, this equipment is far from being universal, and in relation to the process of crystallization Ulrich & Jones (2004) appear to be the most objective: before choosing the type of crystallized and in order to properly evaluate it being an asset for the production process, it is necessary to have at least basic information about the solutions and mother liquors which will be used in the process. Any inquiry for the similar types of equipment is usually to be accompanied by the data sheet, in which the basic characteristics of the products for crystallization are to be noted. Because the pattern or arrangement of the atoms is repeated in all directions, there are definite limitations on the shapes which crystals may assume. For each chemical compound, there are unique physical properties differentiating that material from other, so the formation of a crystalline material from its solution, or mother liquor, is accompanied by unique growth and nucleation characteristics. (Ulrich & Jones, 2004) Having looked through the various literary sources related to crystallization, I have come to the conclusion that the main advantage of the surface-scraped equipment for melts and solutions, is that it is economically feasible for use when vacuum equipment may cause deficiencies of the mother material. For the production of the good quality crystals, the use of the draft-tube baffle at times appears to be limited, as operating through the low levels of saturation, the growth of crystals may be produced only through the use of the extensive and costly dissolving of fines.
This important aspect has been described by Mullin (2001). In case there is the additional level of evaporation which is needed, the installation of the additional external equipment may work against the competitive cost of the equipment at its starting point. These are the disadvantages of the tan crystallizer's in comparison to the surface-scrape equipment; and as modern laboratories and industrial entities at present prefer cost-saving technologies, the surface-scrape solution appears to be the best for them. However, if the products to be acquired through the use of crystallization equipment are initially to be high, the enterprise may afford using tank crystallizer's, as they have really proved themselves to be highly efficient. Draft Tube Baffle has an advantage, which is mentioned only in the one literary source reviewed by me (Thomsen & Gang, 1998), and this advantage is displayed through the superior control over particle size when excessive fine crystals are present; this type of crystallized is used primarily in the production of a variety of large-size crystalline materials such as ammonium sulfate, potassium chloride and di ammonium phosphate for the fertilizer industry.
Among other advantages of these crystallizer's are capability of producing large singular crystals and longer operating cycles. In fact, there exists a wide range of crystallization equipment at present, and the choice is mainly based on the crystallization material, the requirements towards the costs and installation, etc. I have become interested in the two specific kinds of equipment, described above, for the reasons of their popularity and the necessity to point out the advantages and disadvantages of both. I may also assume, that despite the extensive information about crystallization equipment present in literature, still more research is needed, as it has been stated before that certain processes in the crystallization are not yet clear. In any equipment, chosen for crystallization, it is also important to consider the means through which the state of supersaturation is achieved. These are the four main means of creating this state of materials cooling, solvent evaporation, drowning and chemical reaction.
Obviously, that each of them will have benefits and disadvantages, but it is essential to remember that as supersaturation is the state, preceding any crystallization, accounting its properties is crucial for the outcomes of crystallization. As Brown (1991) writes, that for cooling and evaporative crystallization, supersaturation can be generated near a heat transfer surface and usually at moderate rates. Drowning or reactive crystallization allows for localized, rapid crystallization where the mixing mechanism can exert significant influence on the product characteristics. Supersaturation has been already subjected to extensive learning and does not have any dark or unknown sides.
In discussing crystallization equipment the state of supersaturation is not to be underestimated. It does not depend on the equipment itself, but rather on the material to be crystallized, which of the saturation method to be chosen; thus it must correlate with the equipment most appropriate among the available types. In relation to any crystallization equipment, it is also necessary to know that the mechanical design defines the nucleation rate, (Konig, Rechneister & Trusch, 1997) that is, through the contact of crystals with each other, with pumps and propellers (if present). Simultaneously, the nucleation rates for the similar equipment will also depend on the material used.
This is why before creating large-scale production it is necessary to perform pilot tests, as without them it will be impossible to successfully perform crystallization. Conclusion The aim of the work was to discuss the available crystallization equipment, which is of interest specifically for me, through the extensive study of the literary sources and comparing the information available on the topic. We have here come to the conclusion, that despite the large amount of the crystallization equipment existing, the choice of this equipment should be based on the material which is to be subjected to crystallization. The comparison of the equipment for this process was necessary to distinguish the main advantages and principal disadvantages, which will then be used in my practical work. It has been concluded, that the two types of crystallization equipment described (tank crystallizer's and surface-scraped crystallizer's) are suitable for crystallization from solutions and melts, but the exact choice is to be made based on pilot experiments and trials. It has been discovered, that of all literary sources reviewed, different authors tend for concentrating on different advantages of certain crystallization equipment, but the conclusion has been made that modern laboratories and industrial entities base their choice not exactly on the material to be crystallized but on the cost-effectiveness of the project.
It is necessary to understand, that both tank crystallizer's and surface-scrape crystallizer's can be used for a wide range of materials, solutions and melts, but there are still processes in crystallization, which need more extensive research. Works cited Borodin, V. A. Development of New-Generation Equipment for Crystal Growth from Melt.
PHYS-USP, 43 (2000): 929 - 931 Brown, T. Chemistry: The Central Science, 5 th edition. New Jersey, Prentice Hall, 1991. Christensen, S. G.
Modeling of Vapor-Liquid-Solid Equilibria in Acidic Aqueous Solutions. Ind. Eng. Chem. Res. , 42, (2003): 4260 - 4268 Coulson, J.
M. and Richardson, J. F. Coulson and Richardson's Chemical Engineering. Elsevier Science, Oxford, 2003 Geankoplis, Christie J. Transport Processes and Unit Operations, 3 rd edition.
New Jersey, Prentice Hall, 1993. Gilbert, S. W. Melt Crystallization: Process Analysis and Optimization. AIChE J. , 37 (1991): 1205 - 18 Konig, O. , Rechsteiner, P. , Trusch, B. Equipment for Controlling Nucleation and Tailoring the Size of Solution-Grown Single Crystals.
J. Appl. Cryst. , 30 (1997): 507 - 509 Mullin, J. W. Crystallization, 4 th edition. Butterworth-Heineman, Oxford, 2001 Person, J.
C. Literature Survey for Fractional Crystallization Study. Environmental Management, 2004 Price, Chris J. Take Some Solid Steps to Improve Crystallization. Chemical Engineering Progress, September 1997, p. 34 Rajagopal, S. and Douglas, J.
M. Design and Economic Trade-Offs of Extractive Crystallization Processes. AIChE J. 37 (1991): 437 - 47 Thomsen, K. and Gang, R. Simulation and Optimization of Fractional Crystallization Processes. Chem.
Eng. Sci. , 53 (1998): 1551 - 1564 Ulrich, J. and Jones, M. J. Industrial Crystallization: Developments in Research and Technology. Chem.
Eng. Res. Des. , 82, 12 (2004): 1567 - 1570
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Research essay sample on Equipment For From Solutions And Melts
