20 results
Frontmatter
- Alison Lewis, University of Cape Town, Marcelo Seckler, Universidade de São Paulo, Herman Kramer, Technische Universiteit Delft, The Netherlands, Gerda van Rosmalen, Technische Universiteit Delft, The Netherlands
-
- Book:
- Industrial Crystallization
- Published online:
- 05 July 2015
- Print publication:
- 02 July 2015, pp i-iv
-
- Chapter
- Export citation
11 - Precipitation and anti-solvent crystallization
- Alison Lewis, University of Cape Town, Marcelo Seckler, Universidade de São Paulo, Herman Kramer, Technische Universiteit Delft, The Netherlands, Gerda van Rosmalen, Technische Universiteit Delft, The Netherlands
-
- Book:
- Industrial Crystallization
- Published online:
- 05 July 2015
- Print publication:
- 02 July 2015, pp 234-260
-
- Chapter
- Export citation
-
Summary
Why this chapter is important
Precipitation processes are important in a number of different fields, including extractive metallurgy, where the high recovery (i.e. recovered mass/initial mass in solution) is exploited to recover valuable metals; water treatment, where the same high recovery is exploited to cause high levels of removal of contaminants; pharmaceuticals, where a high recovery of product is important; and nano-precipitation, where the small particle size and a monodisperse crystal size distribution are important.
What is precipitation?
The distinction between crystallization and precipitation is often based on the speed of the process, with precipitation usually being defined as a fast process that results in rapid solid formation of extremely small crystals (Jarvenin,8). However, a more scientific definition of precipitation is the fact that the product is formed by a chemical reaction. Thus, precipitation is often referred to as “reactive crystallization.”
In precipitation processes, two soluble reactants are mixed to form a sparingly soluble product. What makes it unique is that often, especially in high-recovery precipitation, the reagent streams are highly concentrated and thus very high supersaturations, especially local supersaturations, are created (Figure 11.1). Because of the high supersaturation, the conversion of the solutes into solid particles is (in contrast to crystallization) usually a very fast process.
What makes it unique?
Precipitation is used for sparingly soluble substances (solubility in the range 0.001−1 kg m−3) for a number of reasons, but mostly because of:
a. the requirement for a high recovery of the product;
b. the requirement for a high degree of removal of a species;
c. other techniques not being suitable. For example, cooling crystallization is not suitable if the compound has a flat or retrograde solubility curve, whilst evaporative crystallization is too expensive due to the volume of the water to be evaporated.
4 - Nucleation
- Alison Lewis, University of Cape Town, Marcelo Seckler, Universidade de São Paulo, Herman Kramer, Technische Universiteit Delft, The Netherlands, Gerda van Rosmalen, Technische Universiteit Delft, The Netherlands
-
- Book:
- Industrial Crystallization
- Published online:
- 05 July 2015
- Print publication:
- 02 July 2015, pp 71-103
-
- Chapter
- Export citation
-
Summary
Why this chapter is important
To create a population of crystals from a liquid, either a solution or a melt, supersaturation has to be induced by increasing the concentration of the solute, by decreasing of the temperature or increasing of the pressure of the liquid with respect to the equilibrium value. In a continuous crystallization process the supersaturation is sustained, while in batch crystallization the supersaturation is consumed in time. If the initial fluid is a clear liquid the formation of crystals begins with a nucleation process called “primary nucleation.” The type of nuclei and their rate of formation affect the size distribution of the crystal population, its polymorphic form and other properties of the crystals, so control of the nucleation process is crucial in obtaining the required product specifications. If new phase formation takes place by statistical fluctuations of solute entities clustering together in the solution or of molecules in the melt, it is called “homogeneous primary nucleation.” If, however, new phase formation is facilitated by the presence of tiny, mostly invisible particles such as dust or dirt particles, on which nucleation preferentially starts, it is called “heterogeneous primary nucleation.”
If, conversely, the supersaturated liquid already contains one or more crystals of the material being crystallized, these so-called parent crystals are able to breed nuclei sometimes after further outgrowth to sufficiently large sizes. This type of nucleation is called “secondary nucleation” because of the prior presence of crystals. It is the only nucleation mechanism in cooling or evaporative crystallization once crystals are present. The crystals present generally consume the supersaturation to values that are too low to induce primary nucleation, but still allow secondary nuclei to grow in the crystal population.
In batch crystallization from a clear liquid the process can either be started by primary nucleation or by the addition of seed crystals to the supersaturated liquid. The role and impact of the addition of seed crystals will be further addressed in Chapter 8.
12 - Melt crystallization
- Alison Lewis, University of Cape Town, Marcelo Seckler, Universidade de São Paulo, Herman Kramer, Technische Universiteit Delft, The Netherlands, Gerda van Rosmalen, Technische Universiteit Delft, The Netherlands
-
- Book:
- Industrial Crystallization
- Published online:
- 05 July 2015
- Print publication:
- 02 July 2015, pp 261-283
-
- Chapter
- Export citation
13 - Additives and impurities
- Alison Lewis, University of Cape Town, Marcelo Seckler, Universidade de São Paulo, Herman Kramer, Technische Universiteit Delft, The Netherlands, Gerda van Rosmalen, Technische Universiteit Delft, The Netherlands
-
- Book:
- Industrial Crystallization
- Published online:
- 05 July 2015
- Print publication:
- 02 July 2015, pp 284-302
-
- Chapter
- Export citation
-
Summary
Why this chapter is important
Although additives and impurities are often present in relatively small concentrations, even trace amounts can have a significant effect on the crystallization process as a whole. Thus, when working in “real” crystallization processes, understanding the influence of additives and impurities is very important.
Although the topic of additives and impurities has been the subject of extensive study by many different researchers, it is still a difficult area, partly because the field itself is very broad and diverse, but also because there are so many different applications in which additives and impurities play an essential role (Meenan et al., 2002, Sangwal, 2007).
Application and occurrence
For many centuries long chain organic additives with molecular weights of a few thousand daltons were used in particle formation processes as, for example, dispersants, flocculants and flotation agents. They were generally added, at dosages below 1 wt%, after the particles were formed.
However, in this chapter,we will discuss the role and design of additives and impurities that are present during crystallization at low dosages from parts per billion (ppb) to, at most, a few wt%. Highly charged metal ions can influence the crystallization process at only ppb concentrations, polymers at <1 wt% concentrations, whereas monomers must be present at the wt% level in order to have an influence.
Because of their low concentrations, additives do not influence the supersaturation (by coordination of ions in solution, for example), but only have the ability to act upon the crystal surface. Such additives are, for example, growth inhibitors, habit modifiers, anti-caking agents (applied as after-treatment) or templates for preferred nucleation.
Since impurities (i.e. additives that are unintentionally present) that can originate from the raw material, or can be formed as by-products during synthesis of the product, often have similar effects as additives on the crystallization process, they are also considered in the same chapter.
The solvent can also have similar effects on the growth kinetics and the shape of the crystals, as will be discussed.
Industrial Crystallization
- Fundamentals and Applications
- Alison Lewis, Marcelo Seckler, Herman Kramer, Gerda van Rosmalen
-
- Published online:
- 05 July 2015
- Print publication:
- 02 July 2015
-
Bridging the gap between theory and practice, this text provides the reader with a comprehensive overview of industrial crystallization. Newcomers will learn all of the most important topics in industrial crystallization, from key concepts and basic theory to industrial practices. Topics covered include the characterization of a crystalline product and the basic process design for crystallization, as well as batch crystallization, measurement techniques, and details on precipitation, melt crystallization and polymorphism. Each chapter begins with an introduction explaining the importance of the topic, and is supported by homework problems and worked examples. Real world case studies are also provided, as well as new industry-relevant information, making this is an ideal resource for industry practitioners, students, and researchers in the fields of industrial crystallization, separation processes, particle synthesis, and particle technology.
9 - Measuring techniques
- Alison Lewis, University of Cape Town, Marcelo Seckler, Universidade de São Paulo, Herman Kramer, Technische Universiteit Delft, The Netherlands, Gerda van Rosmalen, Technische Universiteit Delft, The Netherlands
-
- Book:
- Industrial Crystallization
- Published online:
- 05 July 2015
- Print publication:
- 02 July 2015, pp 192-209
-
- Chapter
- Export citation
-
Summary
Why this chapter is important
Industrial crystallizers may be seen as devices that create a certain level of supersaturation as the driving force for the generation of the product crystals. Measurement of the supersaturation is therefore of paramount importance to control crystallization processes, as it determines the mechanisms and the rates of elementary crystallization processes such as nucleation, crystal growth and agglomeration. These elementary processes eventually lead to the formation of crystals of a wide spectrum of sizes. The so-called crystal size distribution (CSD), or more generally particle size distribution (PSD), is also a very important product characteristic that not only determines the performance of the crystalline product, but also the efficiency of the downstream processes like filtration and drying, as well as product stability during storage, as explained in Chapter 2.
In this chapter, you will learn various techniques used to measure the supersaturation and the CSD/PSD. In addition, you will have the knowledge to select the most convenient measuring technique according to a given measuring objective and crystallization system. With these tools, you will be able to understand and improve the process as well as the product quality of an industrial crystallization process.
Sampling and dilution procedures
Process measurements can be classified according to the sampling procedure: (a) inline, with a probe inserted in the process; (b) online, recirculating a side stream between the process and the instrument; (c) offline, in a laboratory. In the latter two procedures, a suspension sample must be withdrawn from the crystallizer, diluted and sometimes separated into solids and pure liquid. For the solids characterization, careful attention has to be paid to these procedures; otherwise the measurements are drastically affected (Allen, 2003, Gerla, 1995, Jager, 1990).
7 - The population balance equation
- Alison Lewis, University of Cape Town, Marcelo Seckler, Universidade de São Paulo, Herman Kramer, Technische Universiteit Delft, The Netherlands, Gerda van Rosmalen, Technische Universiteit Delft, The Netherlands
-
- Book:
- Industrial Crystallization
- Published online:
- 05 July 2015
- Print publication:
- 02 July 2015, pp 151-177
-
- Chapter
- Export citation
-
Summary
Why this chapter is important
The prediction of the product quality in industrial crystallization processes is not simple and requires detailed knowledge of the crystal size distribution (CSD). To obtain such knowledge, the population balance equation (PBE) has to be solved, describing the evolution of the CSD in the crystallizer.
In this chapter PBE-based models of the crystallization process will be discussed in detail, describing the formulation of the PBE based on either the length or the volume of the crystals, explaining the different terms of this important equation and their relation to the different kinetic processes in the crystallizer. Growth- and nucleationdominated systems and processes that are predominantly agglomeration and breakage will be discussed separately, as they require a different formulation. The coupling of the population to the enthalpy and material balances, to obtain a realistic description of the process is discussed, as well as solution methods for a PBE-based crystallizer model. Multi-compartment models are also introduced that can be used to describe the effects of profiles in the process conditions on the performance of an industrial crystallizer, due to the large volume or complex geometry. A few examples of such multi-compartment models are given.
Having a crystallization model is extremely valuable for the development, optimization and design of a process that meets specific product requirements. To be more precise, having a PBE-based crystallization model will allow the user
• to predict the resulting particle size distribution even at a different scale of operation, on the basis of the known kinetic models;
• to estimate the parameters of a kinetic model based on an experimentally determined particle size distribution knowing the process conditions under which it was obtained;
• to optimize the process conditions to obtain a desired particle size distribution given a validated kinetic model.
Evolution of the crystal size distribution in a crystallizer
A crystalline product consists of a distribution of crystals (or of agglomerates of crystals that are often referred to as particles) of different sizes.
Industrial crystallization in practice: from process to product
- Alison Lewis, University of Cape Town, Marcelo Seckler, Universidade de São Paulo, Herman Kramer, Technische Universiteit Delft, The Netherlands, Gerda van Rosmalen, Technische Universiteit Delft, The Netherlands
-
- Book:
- Industrial Crystallization
- Published online:
- 05 July 2015
- Print publication:
- 02 July 2015, pp xxiii-xxviii
-
- Chapter
- Export citation
-
Summary
Scope of the book
Crystallization refers to the phase transformation of a compound from a fluid or an amorphous solid state to a crystalline solid state. However, a crystallization process is not just a separation process; it is also a production process and a purification technique, as well as a branch of particle technology. It thus encompasses key areas of chemical and process engineering (Davey and Garside, 2000).
Crystallization is an extremely old unit operation, but is still used to produce highly specified speciality chemicals, and pharmaceuticals. In fact, there are few branches of the chemical and process industries that do not, at some stage, employ crystallization or precipitation for production or separation purposes (Mullin, 2003). Crystalline products include bulk chemicals such as sodium chloride and sucrose, fertilizer chemicals such as ammonium nitrate, potassium chloride, ammonium phosphates and urea; valuable products such as pharmaceuticals, platinum group metal salts and organic fine chemicals; products from the new and rapidly expanding field of engineered nanoparticles and crystals for the electronics industry, as well as biotechnology products such as protein crystals.
Although crystallization is an increasingly important industrial process, one that is governed by thermodynamics of phase separation, mass and heat transfer, fluid flow and reaction kinetics, it is not usually explicitly covered in any of the existing core chemical engineering material.
A large percentage of final or intermediate industrial products consist of a product of a crystallization process, i.e. tiny crystals or particles that have to conform to product specifications with respect to crystal size and shape, crystal size distribution, degree of agglomeration and uptake of either liquid or solid impurities. These product properties relate to the selected type of crystallization process as well as to the specific crystallization mode and type of hardware used for production. Crystallization is therefore much more than just a simple separation process. Unfortunately, the technology to design, operate and optimize crystallization processes is usually covered only very briefly as part of a broader overview on separations or particle technology, such as the chapter on Crystallization and Precipitation by Mullin in Ullmann's Encyclopaedia of Industrial Chemistry (Mullin, 2003) or other, similar, volumes (Richardson et al., 2002 and Ruthven, 1997).
2 - Characterization of a crystalline product
- Alison Lewis, University of Cape Town, Marcelo Seckler, Universidade de São Paulo, Herman Kramer, Technische Universiteit Delft, The Netherlands, Gerda van Rosmalen, Technische Universiteit Delft, The Netherlands
-
- Book:
- Industrial Crystallization
- Published online:
- 05 July 2015
- Print publication:
- 02 July 2015, pp 26-50
-
- Chapter
- Export citation
-
Summary
Why this chapter is important
In a competitive market a product has to meet increasingly stringent quality demands, so a number of particle characteristics must be carefully controlled, such as:
• crystal size distribution of the product
• shape of the crystals
• occurrence of polymorphism
• mother liquor inclusions in the crystals
• uptake of impurities in the crystal lattice
• degree of agglomeration.
These characteristics determine most functional features of particulate materials during their use as products. In the food industry, for example, both the size distribution and the shape of ice crystals in ice cream are important (Myerson, 2001). The mean size of the ice crystals should be between 35 and 40 μm in order to give the required smooth texture and melt properties, whilst the crystals themselves must be round and smooth in order to give the correct mouthfeel. Obviously the impurity concentrations in food products must be very carefully controlled.
In the pharmaceutical industry, because of the final use of the compounds, strict specifications as regards size, morphology, dissolution properties and polymorphic form are enforced.
In the bulk chemical and extractive metallurgy industries, the “particle design” is crucial (Söhnel and Garside, 1992). The crystal size distribution and corresponding particle surface area are of particular importance, since these, together with the particle morphology, have a major impact on the particle processing characteristics. For example, solid–liquid separation by centrifugation or filtration, drying rates, particle flow properties, bulk density and thus packing characteristics, as well as propensity to cake are all critically dependent on these particle properties (Söhnel and Garside, 1992).
Crystal size distribution (CSD) or particle size distribution (PSD)
One of the main characteristics of a product is its crystal size distribution or, in the case of agglomerated or non-crystalline particles, its particle size distribution.
6 - Agglomeration
- Alison Lewis, University of Cape Town, Marcelo Seckler, Universidade de São Paulo, Herman Kramer, Technische Universiteit Delft, The Netherlands, Gerda van Rosmalen, Technische Universiteit Delft, The Netherlands
-
- Book:
- Industrial Crystallization
- Published online:
- 05 July 2015
- Print publication:
- 02 July 2015, pp 130-150
-
- Chapter
- Export citation
-
Summary
Why this chapter is important
In the context of crystallization, agglomeration is the process in which two or more particles are brought in contact and stay together for a sufficiently long period such that a crystalline bridge between the particles can grow. Thus, a stable particle or agglomerate is formed. Particle agglomeration plays an important (and not always desirable) role in the formation of larger particles in precipitation and crystallization processes. Because of its significant effect on product quality, control over agglomeration is important for industrial crystallization (Hollander, 2002).
Figures 6.1 to 6.4 show examples of different types of agglomerates. It should be apparent from the pictures that the primary crystals that form agglomerates can be glued together in rather random ways. The agglomerates in the images lack the symmetry and the “esthetic beauty” of crystals formed due to growth. In fact, it is often the lack of symmetry that can be used to, at least qualitatively, identify the presence of agglomeration as a size enlargement process. The MgSO4 · 7H2O agglomerate in Figure 6.1 has a degree of symmetry that suggests that the individual “roses” are in fact a result of twinned growth, whilst the overall agglomerate is a consequence of the individual roses becoming agglomerated together. In contrast, both the agglomerates in Figure 6.2 have sufficiently disordered and random morphologies to be classified as true agglomerates.
In this chapter the physicochemical steps that lead to the formation of these agglomerates are described. In addition, the necessary mathematical models for describing the agglomeration process are discussed.
Agglomeration or aggregation?
A number of terms are used in literature to describe phenomena in which particles come together to form one entity. These include agglomeration, aggregation, conglomeration, coalescence, coagulation and flocculation. In this text, the convention adopted by Randolph and Larson (1988) is used.
Nomenclature
- Alison Lewis, University of Cape Town, Marcelo Seckler, Universidade de São Paulo, Herman Kramer, Technische Universiteit Delft, The Netherlands, Gerda van Rosmalen, Technische Universiteit Delft, The Netherlands
-
- Book:
- Industrial Crystallization
- Published online:
- 05 July 2015
- Print publication:
- 02 July 2015, pp xiii-xxii
-
- Chapter
- Export citation
14 - Polymorphism
- Alison Lewis, University of Cape Town, Marcelo Seckler, Universidade de São Paulo, Herman Kramer, Technische Universiteit Delft, The Netherlands, Gerda van Rosmalen, Technische Universiteit Delft, The Netherlands
-
- Book:
- Industrial Crystallization
- Published online:
- 05 July 2015
- Print publication:
- 02 July 2015, pp 303-319
-
- Chapter
- Export citation
-
Summary
Why this chapter is important
Polymorphism is a widely spread phenomenon in solid substances (Bernstein, 2002, Hilfiker, 2006, Brittain, 2009). A substance exhibits polymorphism when it can exist in more than one crystalline state. These various crystalline states consequently have a different thermodynamic potential, and therefore a different solubility in a given solvent. These various states also possess different physical and chemical properties. Polymorphs have, for example, different crystal shapes and can have different colors and tastes.
Occurrence and consequences
For simple substances, as in many mineral compounds, the basic entity has a fixed atomic, molecular or ionic structure and the different lattice structures result from different packing arrangements in the crystal lattice.
An example of an inorganic mineral substance is calcium carbonate, which can crystallize in three polymorphic crystalline forms, calcite (trigonal), aragonite (orthorombic) and vaterite (hexagonal), depending on the crystallization conditions (see Figure 14.1). An amorphous phase can also be directly precipitated from highly supersaturated solutions. The most stable form at ambient conditions is calcite.
Molecular structures of organic compounds are often flexible and expose rotational degrees of freedom around a single bond. They are therefore prone to polymorphism. An isolated molecule in the gas phase has one or more equilibrium structures. These equilibrium structures are at a different local or total minimum in potential energy and are called conformers. To go from one conformer to another, an energy barrier has to be crossed. A particular arrangement of atoms in a molecular crystal lattice cannot be far from an equilibrium structure in the gas phase. Its conformation in a crystal lattice is only adjusted to minimize the sum of the intra- and inter-molecular energy. A variation of any torsion angle of a molecule in a crystal lattice is a new conformation. In this way different polymorphs can be formed by only a conformational adjustment with respect to the gas-phase conformation. If, in addition to a change in torsion angle, there is a change in potential energy well in the new conformation, the new conformation is also a conformer.
3 - Basic process design for crystallization
- Alison Lewis, University of Cape Town, Marcelo Seckler, Universidade de São Paulo, Herman Kramer, Technische Universiteit Delft, The Netherlands, Gerda van Rosmalen, Technische Universiteit Delft, The Netherlands
-
- Book:
- Industrial Crystallization
- Published online:
- 05 July 2015
- Print publication:
- 02 July 2015, pp 51-70
-
- Chapter
- Export citation
-
Summary
Why this chapter is important
This chapter discusses the basic design of industrial crystallizers. The major design tasks are the selection of the crystallization method, the crystallizer equipment and the mode of operation, the calculation of the flow sheet of the process and of the dimensions of the equipment, the area for heat transfer and evaporation, and the power requirement for the circulation devices. With the help of a costing model, the basic design also yields a first cost estimate of the process in terms of both capital and operational costs.
The method described in this chapter has been developed and tested for evaporative and cooling crystallization processes, but can be adapted relatively easily for melt crystallization processes from suspensions. Examples are given for evaporative and cooling processes, for a DTB evaporative crystallizer including mother liquid recycling and for an Oslo evaporative crystallizer.
The proposed procedure does not include a detailed design, in which not only the average particle size and purity have to be assured, but also the size distribution, the particle shape and degree of agglomeration, among other features that are specific to each application. In those circumstances, a detailed design procedure has to be followed that is based on a model for the industrial crystallizer that takes into account a number of compartments and the crystallization kinetics for the system of interest, followed by model validation in a pilot unit.
The basic design procedure
A hierarchical detailed design procedure has been proposed (Bermingham et al., 2000, Kramer et al., 1999) based on the pioneering work of Douglas (1985) with focus on a precise prediction of the final product quality through detailed mathematical modeling of the crystallization phenomena. Although this approach is suitable for consistent and reproducible design, it requires detailed knowledge of the kinetics and the hydrodynamics of the process, as well as substantial computing and experimental facilities. In Chapter 7 the detailed modeling needed to describe the product quality is discussed.
Index
- Alison Lewis, University of Cape Town, Marcelo Seckler, Universidade de São Paulo, Herman Kramer, Technische Universiteit Delft, The Netherlands, Gerda van Rosmalen, Technische Universiteit Delft, The Netherlands
-
- Book:
- Industrial Crystallization
- Published online:
- 05 July 2015
- Print publication:
- 02 July 2015, pp 320-323
-
- Chapter
- Export citation
Contents
- Alison Lewis, University of Cape Town, Marcelo Seckler, Universidade de São Paulo, Herman Kramer, Technische Universiteit Delft, The Netherlands, Gerda van Rosmalen, Technische Universiteit Delft, The Netherlands
-
- Book:
- Industrial Crystallization
- Published online:
- 05 July 2015
- Print publication:
- 02 July 2015, pp v-xii
-
- Chapter
- Export citation
5 - Crystal growth
- Alison Lewis, University of Cape Town, Marcelo Seckler, Universidade de São Paulo, Herman Kramer, Technische Universiteit Delft, The Netherlands, Gerda van Rosmalen, Technische Universiteit Delft, The Netherlands
-
- Book:
- Industrial Crystallization
- Published online:
- 05 July 2015
- Print publication:
- 02 July 2015, pp 104-129
-
- Chapter
- Export citation
10 - Industrial crystallizers
- Alison Lewis, University of Cape Town, Marcelo Seckler, Universidade de São Paulo, Herman Kramer, Technische Universiteit Delft, The Netherlands, Gerda van Rosmalen, Technische Universiteit Delft, The Netherlands
-
- Book:
- Industrial Crystallization
- Published online:
- 05 July 2015
- Print publication:
- 02 July 2015, pp 210-233
-
- Chapter
- Export citation
-
Summary
Why this chapter is important
Once the type of crystallization process has been chosen, it is necessary to select the most suitable crystallizer. This chapter focuses on the various types of commercially available crystallizers for evaporative and cooling crystallization. The list of crystallizers makes no pretence at being complete, and only serves as a means to highlight important features of various types of crystallizers to achieve the desired product specifications. These features are related to actuators for control (fines removal, classified product removal or a combination of both) that are available in the various types of crystallizers to manipulate the CSD. There are also other features of the crystallizers that inherently affect the CSD through their impact on the hydrodynamics of the crystallizer design. In this chapter, the various designs and their effect on the final CSD of the product will be discussed.
Criteria for the choice of a crystallizer
To understand how the differences in the design between the various crystallizers affect the product quality, detailed models are needed, as discussed in Chapter 7 on population balance modeling (Bermingham et al.,8a, Kramer et al., 1999). As explained in Chapter 7, the models divide the crystallizers into a number of compartments to be able to describe the effects of the hydrodynamics on the crystallization kinetics.
So, apart from giving guidance on the selection of the most appropriate crystallizer, the most appropriate compartment structures for the modeling of these crystallizers will be discussed.
In this chapter we will only discuss crystallizers for cooling and evaporative crystallization processes. Precipitation and anti-solvent crystallizers in which mixing of feed streams together or with the bulk liquor strongly affects the product properties requires a different approach in the selection of the nozzles as well as the vessel and stirrer design, and will be treated in Chapter 11. Melt crystallization, which is merely applied as an ultra-purification process, also requires a different selection procedure and will be treated in Chapter 12.
1 - Thermodynamics, crystallization methods and supersaturation
- Alison Lewis, University of Cape Town, Marcelo Seckler, Universidade de São Paulo, Herman Kramer, Technische Universiteit Delft, The Netherlands, Gerda van Rosmalen, Technische Universiteit Delft, The Netherlands
-
- Book:
- Industrial Crystallization
- Published online:
- 05 July 2015
- Print publication:
- 02 July 2015, pp 1-25
-
- Chapter
- Export citation
-
Summary
Why this chapter is important
As crystallization is concerned with the phase change in solid–liquid systems, analysis of crystallization processes starts with consideration of phase diagrams. In this chapter we will show how phase diagrams help to select a crystallization method, and to determine the yield and the temperature of a crystallization process. The industrially relevant features of the main crystallization methods are also presented.
We next consider the state of the liquid phase during crystallization processes. The solution is said to be supersaturated with respect to the crystallizing compound, meaning the solute concentration is higher than the solid–liquid equilibrium value. The degree of supersaturation is important because it is the driving force for the elementary rate processes of crystallization, such as nucleation and crystal growth. Therefore, expressions to determine the degree of supersaturation are presented, both rigorous expressions based on thermodynamics and less rigorous expressions commonly found in practice.
In order to calculate the degree of supersaturation, thermodynamic models that provide the activity coefficients of the solute are required. The main models available are compared, so that the most suitable model may be chosen, depending on the accuracy, the ease of obtaining model experimental parameters and the types of building units (simple organic molecules, biomolecules, electrolytes, etc.).
Phase diagrams
Phase diagrams display all the possible thermodynamic states of a system: the proportion and the composition of each coexisting phase. The thermodynamic states are described by a set of independently fixed variables, such as the pressure, the temperature and the mass fractions of all components but one (since the sum of the mass fractions of all components must be unity). For a binary system at constant pressure, the phase diagram may be represented by a two-dimensional T–x plot, where T is the system temperature and x is the mass fraction of one of the components, as exemplified for the silver nitrate–water system at atmospheric pressure in Figure 1.1.
8 - Batch crystallization
- Alison Lewis, University of Cape Town, Marcelo Seckler, Universidade de São Paulo, Herman Kramer, Technische Universiteit Delft, The Netherlands, Gerda van Rosmalen, Technische Universiteit Delft, The Netherlands
-
- Book:
- Industrial Crystallization
- Published online:
- 05 July 2015
- Print publication:
- 02 July 2015, pp 178-191
-
- Chapter
- Export citation
-
Summary
Why this chapter is important
There are a number of situations where batch operation is chosen instead of continuous operation (see Chapter 3). Batch processing is more economical for small production capacities of approximately 1 m3 of product per day or less, for processing of expensive materials (because product offspec losses are low) such as pharmaceuticals, as well as for processing batches of different materials in the same industrial unit. Batch crystallization is also chosen for processing of compounds that form encrustations on the crystallizer walls, because the encrustations can be washed off after each batch cycle. The major advantage of batch crystallization is the ability to produce uniformly sized particles.
Seeding is an important tool to control the product size, so the seeding technique will be treated in detail. Batch crystallization can be quantitatively described by means of population balances coupled with mass and energy balances as well as with kinetic expressions for the elementary processes. These mathematical models can be used to help understand batch processes, as well as to develop operational policies (temperature, evaporation and reactant addition trajectories throughout a batch process) aiming at improved product quality, low cost, and low raw material and energy usage.
Phenomenological description of batch crystallization processes
A batch cycle starts with a solution that is slightly undersaturated with respect to the solute to be crystallized. Crystallization is achieved by any of the methods described in Chapter 1, i.e., cooling, solvent evaporation, anti-solvent addition or chemical reaction (precipitation). Usually seeds of the crystallizing material are added early in the batch process in order to improve reproducibility and product quality. When the desired amount of solid has been formed, the slurry is transferred to a solid–liquid separation unit. The crystallizer is then washed, and fresh solution is added and brought to the desired temperature to start a new batch cycle.
The main elementary processes taking place during batch crystallization are described next. Cooling crystallization will be treated here, but the analysis can equally well be applied to evaporative crystallization.