Crystallization process systems synthesis

Thus, methods are now becoming available such that process systems can be designed to manufacture crystal products of desired chemical and physical properties and characteristics under optimal conditions. In this chapter, the essential features of methods for the analysis of particulate crystal formation and subsequent solid-liquid separation operations discussed in Chapters 3 and 4 will be recapitulated. The interaction between crystallization and downstream processing will be illustrated by practical examples and problems highlighted. Procedures for industrial crystallization process analysis, synthesis and optimization will then be considered and aspects of process simulation, control and sustainable manufacture reviewed. 

Chang, W.-C. and Ng, K.M., 1998. Synthesis of processing system around a crystallizer. American Institute of Chemical Engineers Journal, 44, 2240. 

The section on crystallization comprises zeolite synthesis, kinetics and mechanism of formation, stability relationships, recrystallization processes as well as the genesis of natural zeolites. Recent advances in this field have been surveyed, and some new perspectives have been outlined in the review by E. M. Flanigen. Most of the studies in this field are still empirical because of the complexity of the systems involved. Considerable progress has been made, however, towards a better understanding of the processes and mechanisms governing zeolite crystallization. It is not unreasonable to expect that conditions for synthesizing new zeolite structure types can eventually be predicted. 

This study showed that the overall crystallization processes for mor-denite, zeolite X, and zeolite A were similar. However, the physical properties of the crystallizing system determine the rate-limiting step for a particular zeolite synthesis. In the case of mordenite in which both the viscosity of the batch composition and the morphology of seed crystals were varied, it was observed that diffusion in the liquid phase was the ratedetermining step. For zeolite X the actual growth rate on the crystal-liquid interface was the rate-limiting factor as shown by identical conversion rates for the seeded and unseeded systems. For zeolite A in the system chosen, both processes influenced the conversion rate. 

Figure 4.4 shows the crystallinity of the TS-1 and silicalite-1 samples synthesized under different conditions in the YNU and method A systems (the Si/Ti ratio in the synthesis gel was 50 for both series of samples). Clearly, the crystallization rate of TS-1 was much slower in the YNU system than that in the method A system and the presence of Ti in the gel not only severely retarded the crystallization process by prolonging the induction period but also reduced the crystallinity of most of the samples. Nevertheless, higher crystallinity of TS-1 was finally achieved in the YNU system than in the method A system. 

To increase the crystallization rate and to alter the product phase, an alkaline mineralizer is sometimes added to the solvothermal reaction. Some researchers believe that, compared with the hydrothermal process, solvothermal synthesis allows the product to be free from foreign ions because the organic solution, having a low relative permittivity, is free from ionic species. When precursor gels are prepared from alkoxide, one can prepare products free of foreign ions. However, when the precursor gel is prepared by precipitation from salt solutions, or when alkali/acid mineralizer or ionic surfactant is added to the solvothermal crystallization system, the above statement is a myth. In fact, ions are easily adsorbed or occluded in the product particles because of the low dielectric constant of the organic solvent. 

In the synthesis of faujasite type zeolite, the sequence of the phase evolution amorphous faujasite "P", is well known. Dwyer and Chu( l demonstrated that when TMA ions was added to the initial synthesis mixture a new sequence amorphous faujasite —> ZSM-4, prevailed. In the present work, we utilised a nucleation gel, usually used for directing faujasite phase( ), to the new synthesis system for omega zeolite. Here, we report the effects of nucleation gel on the crystallization processes. 

Because Al and P atoms in the final stmcture come from the isolated aluminum and phosphorus sources in the parent mixture, respectively, the formation reaction of Al-O-P linkage must have happened during the crystallization process. Considering the importance of the introduction of organic amines to the synthesis system, we can... 

Ultra-fine particles of molecular sieves (0.2 0.5 pm) can also be synthesized by hydrothermal crystallization in a suitable reaction gel system. The key to this synthesis is again the study of the crystallization process and the control of crystallization conditions. 

The next two chapters deal with investigations concerning solid silicon monoxide. The application of thin films of this material is based on its unique mechanical, chemical, and dielectric properties. It is related to Si-Si systems in so far as solid SiO consists of small particles of Si and Si02. Depending on the conditions for synthesis, the material has different local structures. In the contribution of U. Schubert and T. Wieder (Chapter 18), the structure and reactivity of a special SiO modification (Patinal ) is described. This material consists of Si and Si02 regions of 0.25 - 0.5 nm in diameter, which are connected by a thin interface. Most of the SiO reactions are also observed for elemental silicon. H. Hofineister and U. Kahler (Chapter 19) show that thermal processing of solid SiO (from BALZERS) up to 1300°C leads to phase separation into Si nanocrystallites embedded in an SiOx matrix. Their internal structure is determined by solid-phase crystallization processes. 

The objective of this chapter is to review the open literature on molecular sieve zeohte synthesis, highlighting information regarding the fundamental mechanisms of zeolite crystallization in hydrothermal systems. The text, therefore, focuses on the three primary mechanistic steps in the crystallization process nucleation of new populations of zeolite crystals, growth of existing populations of crystals, and the role played by existing zeolite crystal mass in the subsequent nucleation of new crystals or the growth of zeolite crystals in the system. 

The systematic analysis of the parameters governing the crystallization of zeolite ZSM-5 also makes a chance to define the crystallization mechanism of ZSM-5 zeolites in SDA-free system. To achieve the controllable synthesis of ZSM-5 zeolites with designed particulate and chemical properties, the thorough understanding of the critical processes during the crystallization is the only way. Such goal is achieved by both efforts, the kinetic analysis of crystallization processes and the alkaline, post-treatment of final products. 

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