Crystallization operation, industrial

Liquid-liquid and liquid-solid equilibria also find industrial applications in liquid-liquid extraction and fractional crystallization operations. 

Many industrial crystallizers operate in a weU-mixed or nearly weU-mixed manner, and the equations derived above can be used to describe their performance. Furthermore, the simplicity of the equations describing an MSMPR crystallizer make experimental equipment configured to meet the assumptions lea ding to equation 44 useful in determining nucleation and growth kinetics in systems of interest. 

Tailoring of the particle size of the crystals from industrial crystallizers is of significant importance for both product quality and downstream processing performance. The scientific design and operation of industrial crystallizers depends on a combination of thermodynamics - which determines whether crystals will form, particle formation kinetics - which determines how fast particle size distributions develop, and residence time distribution, which determines the capacity of the equipment used. Each of these aspects has been presented in Chapters 2, 3, 5 and 6. This chapter will show how they can be combined for application to the design and performance prediction of both batch and continuous crystallization. 

The design and operation of industrial crystallizers is where developments in the laboratory are confirmed and their practical significance determined. In recent years, crystallization processes involving specialty chemicals and pharmaceuticals have increased. This has led increased interest in batch crystallization operation, optimization and desigrt At the same time, the advent of powerful computers and their routine avaUabilily has stimulated interest in the area of on-line control of crystallization process (both batch and continuous). Progress in batch crystallization is surrunarized in a number of recent papers and reviews 173-801. In this section I will discuss two areas which I think will have an impact in the next decade. 

The aim of crystallization is to separate the observed component into higher quality crystals. The crystal size and probability density distribution of its size become very important factors for the product or the following processes. Although multi-phase mixing is fairly common in industries, there have been few investigations on the mixing performance of operations/equipment. In crystallization operation, the assumption of MSMPR has been used to design a crystallizer without a detailed discussion. Therefore, the assumption of MSMPR must be studied quantitatively. 

The ease with which nuclei can be produced by contact nucleation is a clear indication that this mechanism is dominant in many industrial crystallization operations. Research on this nucleation mechanism is continuing with the objective of building an understanding of the phenomenon that will allow its successful inclusion in models describing commercial systems. 

Extraction-crystallization Extraction often is used in association with a crystallization operation. In the pharmaceutical and specialty chemical industries, extraction is used to recover a product compound (or remove impurities) from a crude reaction mixture, with subsequent crystallization of the product from the extract (or from the preextracted reaction mixture). In many of these applications, the product needs to be delivered as a pure crystalline solid, so crystallization is a necessary... 

Ciystallization has been the most important separation and purification process in the pharmaceutical industry throughout its history. Many parallels exist in the fine chemicals industry as well. Over the past several decades the study of crystallization operations has taken on even higher levels of importance because of several critical factors that require increased control of the crystallization process. These levels of control require better understanding of the fundamentals as well as of the operating characteristics of crystallization equipment, including the critical issue of scale-up. 

Three different mechanisms for nucleation may occur in any crystallization operation, as discussed by Mersmann (2001, pp. 45ff.) homogeneous primary, heterogeneous primary, and activated secondary. Industrial crystallizers are usually operated under conditions of the last two simultaneously. For simplicity, nucleation as used in this discussion refers to any or all three of the mechanisms that may be important in a particular crystallization. 

This book has two goals. One is to facihtate the understanding of the fundamental properties of crystallization and the impact of these properties on crystaUization process development. The second is to aid practitioners in problem-solving using actual industrial examples under real process constraints. This book begins with fundamental thermodynamic properties (Chapters 2 and 3), nucleation and crystal growth kinetics (Chapter 4), and process dynamics and scale-up considerations (Chapters 5 and 6). Subsequent chapters cover modes of crystallization operation cooling (Chapter 7), evaporation (Chapter 8), antisolvent (Chapter 9), reaction (Chapter 10), and special cases of crystallization (Chapter 11). As mentioned, real industrial examples are provided in each chapter. 

Virtually all industrial crystallization processes involve solutions. The development, design, and control of any of these processes involve knowledge of a number of the properties of the solution. This chapter will present and explain solutions and solution properties, and relate these properties to industrial crystallization operations. 

To control a crystallizer of this type in such a way that cycling can be avoided is extremely difficult and for the most part has only been conducted with limited success. The retention time required to grow large KCl or (NH4)2S04 crystals in industrial equipment is in the order of 4-6 h. The time required for overnucleation when the system exceeds the metastable zone is a matter of minutes. The kind of operator attention required to make adjustments in the... 

Despite the large volumetric differences in this example (100 1), the vessel diameter differences are really quite small (5 1). Therefore, since the vessel diameter is an important parameter in a crystallizer, laboratory-scale crystallization operations may not be vastly different from those in pilot-plant work, and pilot scale from industrial scale. 

A number of industrial crystallizers operate on the fluidized-bed principle, in which a mass of crystals is suspended in an upward flowing stream of liquor Figures 8.38 and 8.44). One of the important parameters needed in the design... 

The design and implementation of control systems for both batch and continuously operated industrial crystallisers can be achieved by mathematical and physical structured models for the process dynamic behaviour and from on-line measurements of the crystal distribution (CSD). 

Large-scale recovery of light oil was commercialized in England, Germany, and the United States toward the end of the nineteenth century (151). Industrial coal-tar production dates from the earliest operation of coal-gas faciUties. The principal bulk commodities derived from coal tar are wood-preserving oils, road tars, industrial pitches, and coke. Naphthalene is obtained from tar oils by crystallization, tar acids are derived by extraction of tar oils with caustic, and tar bases by extraction with sulfuric acid. Coal tars generally contain less than 1% benzene and toluene, and may contain up to 1% xylene. The total U.S. production of BTX from coke-oven operations is insignificant compared to petroleum product consumptions. 

Other Industrial Applications. High pressures are used industrially for many other specialized appHcations. Apart from mechanical uses in which hydrauhc pressure is used to supply power or to generate Hquid jets for mining minerals or cutting metal sheets and fabrics, most of these other operations are batch processes. Eor example, metallurgical appHcations include isostatic compaction, hot isostatic compaction (HIP), and the hydrostatic extmsion of metals. Other appHcations such as the hydrothermal synthesis of quartz (see Silica, synthetic quartz crystals), or the synthesis of industrial diamonds involve changing the phase of a substance under pressure. In the case of the synthesis of diamonds, conditions of 6 GPa (870,000 psi) and 1500°C are used (see Carbon, diamond, synthetic). 

Creep of Thick-walled Cylinders. The design of relatively thick-walled pressure vessels for operation at elevated temperatures where creep caimot be ignored is of interest to the oil, chemical, and power industries. In steam power plants, pressures of 35 MPa (5000 psi) and 650°C are used. Quart2 crystals are grown hydrothermaHy, using a batch process, in vessels operating at a temperature of 340—400°C and a pressure of 170 MPa (25,000 psi). In general, in the chemical industry creep is not a problem provided the wall temperature of vessels made of Ni—Cr—Mo steel is below 350°C. 

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