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2478 results in Chemical engineering

Page 45 of 124

  • 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

  • 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:

    1. • crystal size distribution of the product

    2. • shape of the crystals

    3. • occurrence of polymorphism

    4. • mother liquor inclusions in the crystals

    5. • uptake of impurities in the crystal lattice

    6. • 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

  • 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.

  • 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

  • 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

  • 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.

  • 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

  • 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

  • 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

  • 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.

  • 13 - Thermodynamics

  • Shikha Agarwal
  • Book:
    Engineering Chemistry
    Published online:
    11 May 2018
    Print publication:
    16 May 2015, pp 644-742

  • Summary

    Introduction

    In our day to day life we come across several instances in which one form of energy is converted into the other form. When you rub your hands they become hot; similarly, if a metal is hammered, it becomes hot. In both the above examples, mechanical energy is converted into heat energy. On the other hand, you are well aware that when water is heated steam is generated that is used to move turbines for the generation of electricity. Here heat energy produces mechanical work. There can be numerous examples of the conversion of one form of energy into another. The science that deals with the interconversion of heat to work or any other form of energy is termed as thermodynamics. The term is self-indicative of what is happening – ‘thermo’ from temperature, which means energy and ‘dynamics’ pertains to motion or work. Chemical thermodynamics is that portion of thermodynamics that deals with the study of the processes in which chemical energy is involved.

    The principles of interconversion of energy in various forms are summarized in the three basic laws of thermodynamics. The first law is a statement of the law of conservation of energy, i.e., energy cannot be created or destroyed, it can merely be converted from one form to another. The second law of thermodynamics explains the occurrence of various reactions spontaneously. It introduces the concept of ‘entropy’ or ‘disorder’ or ‘randomness’ to predict the spontaneous occurrence of chemical reactions. The third law of thermodynamics relates, among other things, to the experimental approach to absolute zero.

    Thermodynamics is a powerful tool for chemists. It studies the relationship between the effects of temperature on physical systems at the macroscopic scale. It helps in predicting whether a reaction will occur or not. It also helps to predict the direction in which a chemical reaction will occur.

    However, thermodynamics does not show how fast a reaction will proceed. Let us now deal with the fundamental concept of thermodynamics

    Terminology of Thermodynamics

    It is necessary to define certain terms and expressions commonly used in thermodynamics.

    System and surroundings

    System A thermodynamic system may be defined as that portion of the universe which is under observation or study.

    Surroundings It is that portion of the universe that is not a part of the system.

  • 9 - Solid State

  • Shikha Agarwal
  • Book:
    Engineering Chemistry
    Published online:
    11 May 2018
    Print publication:
    16 May 2015, pp 479-517

  • Summary

    Introduction

    Solids are substances that have a definite shape and volume. They are highly incompressible and rigid and have high mechanical strength. The atoms, molecules and ions constituting the solid are closely packed and held together by strong intermolecular forces. The constituent particles occupy fixed positions in the solid and cannot move about freely. They can only oscillate about their mean positions. A true solid is characterized by a sharp melting point, a characteristic heat of fusion, incompressibility and a definite three-dimensional arrangement.

    Types of Solids

    Solids are of two types—crystalline and amorphous.

    Crystalline solids

    In crystalline solids, the atoms, molecules or ions constituting the solid are arranged in a regular manner to form a characteristic three-dimensional pattern. The regular arrangement of atoms is repeated several times throughout the molecule. Crystalline solids have long-range order and the arrangement of particles is so regular that if the arrangement of one area is known, the arrangement of particles in the entire crystal can be predicted. Moreover, a crystalline solid has the same geometry irrespective of the source from which it is obtained. Some examples of crystalline solids are NaCl, KCl, etc.

    Amorphous solids

    Amorphous (from the Greek word, amorphos meaning no form) solids have no regular arrangement of their atoms or molecules. In other words, they are characterized by a random arrangement of particles. They lack long-range order and can be regarded as supercooled liquids having structural units with a short-range order.

    However, unlike liquids they have rigidity due to their enhanced viscosities and their constituent particles occupy fixed positions. Amorphous solids do not have sharp melting points—they melt over a range of temperature. Common examples of amorphous solids are glass, rubbers, plastics etc.

  • 20 - Nanochemistry

  • Shikha Agarwal
  • Book:
    Engineering Chemistry
    Published online:
    11 May 2018
    Print publication:
    16 May 2015, pp 1032-1055

  • Summary

    Introduction

    The introduction of ‘nano’ car by TATA motors has greatly popularized the term ‘nano’ among the Indian masses. The word nano signifying very small objects of the range of 1–100 nm has gained importance in the last two decades. Nano literally means dwarf (10–9) and can be used as a prefix to the fundamental properties such as mass, time and length to give important scientific properties such as:

    nanogram 10–9 g

    nanosecond 10–9 s

    nanometer 10–9 m.

    The science of small objects has grown rapidly in the past two decades as the behavior of matter at ‘nano’ level is considerably different from the bulk behavior of the matter. The physical, chemical, electrical, optical and magnetic properties of nanomaterials are significantly different from the properties of the same material of bigger size. For example, a metal with a grain size of 10 nm is seven times harder and tougher than its counterpart with the size of hundreds of nanometers.

    Moreover, the nanomaterials have a relatively larger surface area than ordinary materials and this has a significant effect on their physical properties and chemical reactivities. It has been observed that certain substances that are ordinarily unreactive become reactive in their nanoscale form because of their high surface area. The quantum effects of matter at bulk level and nano level varies significantly. (The specific surface area increases three times when the dimensions decrease from μm to nm).

    Nanoscience and Nanochemistry It is the science of nanoparticles or the particles whose size varies from 1 to 10 nm in at least one dimension. Nanochemistry exploits this size range for various chemical reactions and chemical purposes, which would not be possible otherwise at the bulk level. Nanotechnology utilizes the behavior of particles at the ‘nano’ level in various production processes.

    How much is a nanometer?

    One nanometer is equivalent to one billionth (one thousand millionth) of a meter. These particles are not visible to the human eye and can be seen only under a powerful microscope. The size of various objects can help to have a visual perception of a nano-sized objects.

  • • Human hair is about 100000 nm wide.

  • • Red blood cell is about 2000–5000 nm wide.

  • • Human DNA is 2.5 nm wide.

  • • Diameter of carbon nanotube is 1.0–1.3 nm.

  • • Water molecule is about 0.3 nm across.

  • 4 - Phase Rule

  • Shikha Agarwal
  • Book:
    Engineering Chemistry
    Published online:
    11 May 2018
    Print publication:
    16 May 2015, pp 226-275

  • Summary

    Introduction

    The phase rule was given by Gibbs and explains the equilibrium existing in heterogeneous systems. It states that the equilibrium between different phases is influenced by temperature, pressure and concentration only and not by gravity, electrical or magnetic forces. The number of degrees of freedom (F), which will be explained later, is related to the number of components (C) and phases (P) by the following phase rule equation.

    F = C − P + 2

    Explanation of Terms

    Phase A phase is a homogenous, physically distinct and mechanically separable portion of a system which is separated from other parts of the system by a definite boundary.

    Examples

  • 1. A gaseous mixture, being thoroughly miscible in all proportions constitutes one phase only.

  • 2. If two liquids are immiscible (for example, oil and water), they will constitute two separate phases.

  • 3. Two miscible liquids (for example, water and alcohol) constitute one phase only.

  • 4. A solute completely dissolved in a solvent constitutes a single phase, for example, a solution of glucose in water, salt in water, etc.

  • 5. At freezing point, water consists of three phases.

  • Ice (s) ⇌ Water (l) ⇌ Water vapour (g)

  • 6. Each solid constitutes a separate phase. A mixture of rhombic and monoclinic sulphur is a two phase system.

  • A solid solution irrespective of the number of solids present is a single phase system.

  • Calcium carbonate decomposes as follows

  • CaCO3(s) ⇌ CaO(s) + CO2(g)

  • The above equilibrium has three phases (two solid and one gaseous)

  • Similarly,

  • Fe(s) + H2O (g) ⇌ FeO(s) + H2(g)

  • Here there are two solid phases, Fe and FeO and one gaseous phase consisting of H2O(g) and H2(g). Hence, there are three phases in equilibrium.

  • 7. A homogenous solution of Mohr's salt [FeSO4.(NH4)2SO4.6H2O] constitutes a single phase.

    • Components The number of components of a system at equilibrium is the smallest number of independently variable constituents by means of which the composition of each phase present can be expressed either directly or in the form of a chemical equation. While expressing the composition of a phase in terms of its components, zero and negative quantities are permissible. The components of a system do not represent the number of constituents or chemical individuals present in the system.

  • 17 - Fundamentals of Organic Chemistry

  • Shikha Agarwal
  • Book:
    Engineering Chemistry
    Published online:
    11 May 2018
    Print publication:
    16 May 2015, pp 913-995

  • Summary

    Introduction

    Organic chemistry is defined as the study of compounds of carbon or the chemistry of hydrocabons and its derivatives. The term ‘organic’ is misleading. Earlier the term organic chemistry was used to describe the study of compounds obtained from living organisms, while the term inorganic chemistry was used for the study of compounds obtained from non living sources. With the synthesis of urea, the first organic compound in the laboratory it was proved that organic compounds can also be prepared artificially. Urea was synthesised from an inorganic compound i.e., from ammonium cyanate (NH4CNO). Later on Kolbe (1845) succeeded in preparing acetic acid, the first organic compound in laboratory from its elements. Today most of the organic compounds can be synthesized. Still the division between organic and inorganic is retained till date. Presently the term organic is used to describe the compounds of carbon. Carbon atoms have a unique property of attaching themselves to one another to an extent not possible for atoms of any other element. This property is called ‘catenation’. The carbon atoms can form long chains (linear, branched and cross link) or rings of all sizes. Organic molecules have ‘covalent bonds’ formed by the sharing of electrons between the bonded atoms. Covalent bonding imparts properties to organic compounds which are quite different from those of inorganic compounds. The general concepts and principles underlined in the forthcoming sections will give a better understanding of the organic compounds, their mechanism of bond cleavage and organic reaction mechanisms.

    Cleavage of Covalent Bonds

    Organic reactions involve breaking and formation of covalent bonds. It is essential to understand the ways in which a covalent bond can break. A covalent bond generally breaks in two ways depending on the nature of organic compound, nature of attacking reagent and the reaction conditions. The different possibilities are illustrated below-

    (i) Homolytic cleavage As we know that a covalent bond is formed by sharing of electrons between the two atoms. In a homolytic cleavage, the bond breaks symmetrically in such a way that each species retains one electron of the shared pair.

Page 45 of 124