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Suspension melt crystallization
2. Suspension Melt Crystallization Melt crystallization is a technique used for purification and separation of mixtures of chemicals and metals. The division between the more commonly used technique of solution crystallization and the melt crystallization cannot be precisely defined. However, the techniques used for these two cases can differ remarkably, which justifies the categorization. Commonly the fraction of the crystallizing component in melt crystallization is present in higher amount than all the other mixture components together. For example for ultra purification of benzene and naphthalene the feed concentrations can be as high as 99.5 wt.-% for benzene and 90.45 wt.-% for naphthalene [Mid69]. Actually, this point already brings with it the further aspects used to characterize melt crystallization: High viscosities and importance of heat transfer. When a mixture is brought to a temperature, at which most of the liquid is near its freezing point, the viscosity of such a mixture is high compared to solutions, where only small portion of the mixture is near its solidification temperature. Also, when the fraction of the crystallizing component is high the mass transfer of molecules to the growth sites is easy, and heat transfer will be the rate-determining factor. However, all the aspects mentioned above can be found in processes normally classified to the solution and high viscosity.
The advantages of melt crystallization are in the relatively low energy demand of the freezing process and in the high selectivity of crystallization [Ulr02]. The heats of fusion for majority of compounds with industrial importance are lower than the heats of vaporization by the term 0.2 - 0.5 [Win90]. The low energy demand can be demonstrated by the extreme example of water: The heat of vaporization is 2260 kJ/kg, whereas the heat of fusion is only 334 kJ/kg [Kir78]. However, in the most used industrial application of melt crystallization - batchwise crystallization on a cooled surface - the energy requirements of heating and cooling of the whole crystallizer equipment, as well as the heat transfer medium, has to be taken into account. Wintermantel and Wellinghoff [Win90] have stated that in such processes a multiple of the heat of crystallization is needed, which reduces the energy benefits of melt crystallization compared to distillation. It was also stated that future studies should be focused on continuous melt crystallization instead of batch processes. The high selectivity of the crystal lattice incorporation for eutectic systems allows separation of close boiling components, for example isomers. One important application in this field is the separation of p-xylene from its isomers with its production capacity exceeding ten million tons per year [Fis97]. The restriction of this selectivity to eutectic systems is a minor problem, knowing that 70% of the existing systems are eutectic [Mat91].
However, the high selectivity of the crystallization process depends strongly on how the crystallization is carried out. Melt crystallization can be carried out by a layer process, where the crystalline material to be separated forms on a cooled surface, or by crystallization from suspension. In this work the focus is given mainly to the suspension melt crystallization. By crystallization from a suspension high purities can be obtained in a single separation step due to a large surface area for mass transfer in comparison to layer crystallization processes. In suspension crystallization the surface area available for the same volume of crystals than in
Suspension melt crystallization
layer crystallization is approximately two to three orders of magnitude higher. A further benefit of crystallization from suspension is the applicability for continuous operation. Due to the huge surface area compared to the layer processes, the separation is not affected by the kinetics of the crystallization step only. In suspension crystallization processes the following solid-liquid separation plays an important role on the final product purity. This is especially so for the melt crystallization, where the melt concentrated with impurities and high viscosity of the liquid phase make the separation even more difficult. For this reason special processes for solid-liquid separation have been developed for melt crystallization. The solid-liquid separation in suspension melt crystallization processes is very often carried out in wash or crystallization columns. A thorough study of the different melt crystallization processes have been published by Özogus [Özo92], Verdoes and Nienoord [Ver03b], Arkenbout [Ark95], Ulrich [Ulr02] and Ulrich and Glade [Ulr03].
Suspension melt crystallization is discussed in the next chapters as follows: In Chapter 2.1 the principles of suspension melt crystallization are discussed. The different processes of suspension melt crystallization have been discussed in Chapter 2.2. The solid-liquid separation in melt crystallization is presented in Chapter 2.3. A summary of the suspension melt crystallization processes is given in Chapter 2.4. 2.1 Effect of Crystallization Kinetics on Suspension Melt Crystallization The kinetics in a crystallization process includes two factors: Nucleation and crystal growth. The prerequisite for both nucleation and growth are non-equilibrium conditions, at which a new solid phase will be formed as the system moves towards equilibrium. In industrial crystallization this means a liquid supersaturated by the crystallizing component. Because the supersaturation is in melt crystallization in almost every case created by reduction of temperature, the undercooling is usually used as a measure for the supersaturation. This also allows an easy tracking of the crystallization process in the phase diagram, usually presented by a temperature-concentration diagram. In every crystallization process different nucleation and growth mechanisms take place simultaneously. However, the dependence of nucleation
and growth processes on temperature and undercooling can be very different. This can be brought up with an example presented in Fig. 2.1.1 [Rit85]. From the figure it can be seen that for nucleation from a crystal free solution a certain limiting undercooling is necessary. This limiting undercooling is called the width of the metastable zone for primary nucleation, and it is Figure 2.1.1 Effect of undercooling on crystal
growth and nucleation [Rit85].
Suspension melt crystallization
strongly dependent on heat exchange, fluid dynamics and the solution composition (concentration, presence of additives). By increasing undercooling the aggregation of the molecules becomes easier. This phenomenon is counter played by the increasing solution viscosity by decreasing temperature, which hinders the movement of molecules and makes their incorporation into an ordered crystal lattice more difficult. Thus the rate of nucleation has a maximum value at a certain temperature. Similarly goes crystal growth rate through a maximum. However, the regime for crystal growth lies in a totally different temperature region. It can also be seen, that for crystal growth there exists no limiting undercooling, which has to be overcome for the growth process to start.
The above mentioned behaviour of nucleation and crystal growth is the underlying principle for many suspension melt crystallization processes, and this will be pointed out by discussion of the processes, where the practical effect of these kinetic effects can be seen. In following a description of these two kinetic phenomena is given from the viewpoint of melt crystallization. 2.1.1 Nucleation Nucleation can occur by a variety of processes. It is usually divided to homogeneous primary nucleation, heterogeneous primary nucleation and secondary nucleation. Secondary nucleation has its origins also in several different phenomena, which are usually taking place simultaneously. Nucleation by cavitation has also been presented as its own class of nucleation process [Jac65], being precisely speaking just one special type of homogeneous nucleation.
As mentioned before, nucleation is affected by various different parameters. In real industrial crystallization processes most of these parameters are not constant throughout the crystallization system. Therefore, theoretical equations to calculate the nucleation rate have hardly any use in industrial suspension melt crystallization processes. Indeed, the geometrical construction of the crystallizer has a significant influence on the nucleation rate.
In continuous crystallization processes the secondary nucleation is the dominating mechanism for the birth of new crystals [Ark95]. Usually this happens through collisions of crystals with each other and with agitator blades and crystallizer innings like walls and baffles. Especially, crystal suspensions transported through pumps are set under large mechanical impact, which greatly increases the rate of secondary nucleation. Therefore, secondary nucleation is often called contact nucleation. The magnitude of secondary nucleation due to mechanical effects depends on the crystal size (kinetic energy of collisions), the hardness of crystals, the agitation rate or the pump speed and the suspension density. The process of secondary nucleation due to collisions has been investigated e.g. by Ulrich [Ulr81] and Gerigk [Ger91], and recently by Gahn and Mersmann [Gah99], who have developed a model for the mechanical impact between crystal and impeller blade based on the crystal hardness. Secondar