crystallization, drying of solid

ADIGRAT UNIVERSITY

COLLEGE OF ENGINEERING AND TECHNOLOGY DEPARTMENT OFCHEMICAL ENGINEERING PROJECT ON:DRYING OF SOLID, CRYSTALLIZATION AND EXTRACTION COURSE TITLE: MASS TRANSFER UNIT OPRATION COURSE CODE: ChEg3114 PREPARED BY NETWORK THREE NAME

IDNO

SECTION

1. ETAYHAILU ......................................................………..RET0457/06

ONE

2. H/MARIAM ASGEDOM .................................................RET0748/06

ONE

3. KIROS G/MEDHIN...........................................................RET0937/06

ONE

4. LETU DESALEGN ...........................................................RET0969/06

ONE

5. LICHIYA ALEM ...............................................................RET0971/06

ONE

6. WASIE KEBRIE ...............................................................RET1578/06

ONE

SUBMITTED TO: INSTRUCTOR H/MICHAEL TSEGAY SUBMISSION DATE: WEDNSDAY25, 09, 2008 E.C

ACKNOWLEDGMENT First and for most we would like to say thanks for our instructor “H/MICHAEL T.” to give this project and the formats of the project. Moreover, for all group members and for our advisor that supports by giving idea and economical aid until the project is complete.

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ABSTRACT The aim of this project is to understand the concept of drying of solids, crystallization, and extraction. Drying is the removal of moisture from a substance. Psychrometry is that branch of engineering science, which deals the study of moist air that dry air mixed with water vapor or humidity. In addition, to discuss the psychrometric term like; humidity, wet bulb temperature, dew point depression and so on. Crystallization is also a chemical solid–liquid separation technique, in which mass transfer of a solute from the liquid solution to a pure solid crystalline phase occurs. In chemical engineering, crystallization occurs in a crystallizer. Finally, Extraction is the withdrawing of active agent or a waste substance from a solid or liquid mixture with a liquid solvent. The main area of extraction is for hydro metallic processes, for pharmaceutical industry (producing active agent), for petroleum industry (production of monomers and aromatic) and for cleaning of wastewater to separate solved compounds

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LIST OF CONTENT Contents ...................................................................................................................................... Page Acknowledgment ............................................................................................................................... I Abstract ........................................................................................................................................... II

List of figures ................................................................................................................................. III 1. Drying of solids ........................................................................................................................ 1 Introduction ............................................................................................................................... 1 Objectives .................................................................................................................................. 1

1.1 Gas vapor mixture ................................................................................................................ 2 1.2 psychrometric terms ............................................................................................................. 3 1.3 psychrometric chart .............................................................................................................. 4 Conclusion ........................................................................................................................... 6 2. Crystallization............................................................................................................................. 7 Introduction ............................................................................................................................... 7 Objective .................................................................................................................................... 7

2.1 Crystallization Fundamentals ................................................................................................ 8 2.2 Solid-Liquid equibria ........................................................................................................... 8 2.2.1

Solubility and Phase Diagrams ................................................................................. 8

2.2.2

Solubility and saturation ......................................................................................... 10

2.2.3

Crystal nucleation .................................................................................................. 10

2.2.4

Crystal growth....................................................................................................... 12

2.3 Crystallization from melts .................................................................................................. 13 2.3.1

Basic techniques .................................................................................................... 13

2.3.2

Multistage-processes ............................................................................................. 13 Conclusion ............................................................................................................ 14

3. Extraction ................................................................................................................................. 15 Introduction ............................................................................................................................. 15 Objectives ................................................................................................................................. 15

3.1 Definition of Extraction ..................................................................................................... 16 3.2 Selection of the solvent ...................................................................................................... 16 Conclusion ......................................................................................................................... 17 Reference .......................................................................................................................... 18

LIST OF FIGURE

PAGE

Figure1.1 Psychrometric chart ............................................................................................................. 4 Figure1.2 Various air-conditioning processes .................................................................................... 5 Fig 2.1 Solubility curves for substances with two polymorphs I and II (2) .................................... 10 Fig 2.2 Effect of super saturation on the rates of homogeneous and heterogeneous nucleation ... 12 Fig 2.3 Concentration driving forces for crystal growth from solution .......................................... 13

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CHAPTER ONE 1. DRYING OF SOLID Introduction Drying is perhaps the oldest, most common and most diverse of chemical engineering unit operations. Drying of solid is a mass transfer process consisting of the removal of water or another solvent by evaporation from a solid, semi-solid or liquid. This process is often used as a final production step before selling or packaging products. To be considered "dried", the final product must be solid. The separation operation of drying converts a solid, semi-solid or liquid feedstock into a solid product by evaporation of the liquid into a vapor phase via application of heat. In the special case of freeze drying, which takes place below the triple point of the liquid being removed, drying occurs by sublimation of the solid phase directly into the vapor phase. The psychrometry is that branch of engineering science, which deals the study of moist air that dry air mixed with water vapor or humidity. Generally drying is the removal of moisture from a substance.

Objectives:   To remove water present in liquid foods by evaporation  To yield solid products  To extend the shelf-life of foods by reducing their water activity  Differentiate between dry air and atmospheric air.  Differentiate between the specific and relative humidity of atmospheric air.  Define the dew-point temperature of atmospheric air

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1.1Gas vapor mixture When we are dealing with a gas–vapor mixture, however, the vapor may condense out of the mixture during a process, forming a two-phase mixture. This may complicate the analysis considerably. Atmospheric air normally contains some water vapor (moisture). The dry-air contains no water. Although the amount of water vapor in the air is small, it plays a major role in human comfort and thus in air-conditioning applications. The psychrometry is that branch of engineering science, which deals with the study of moist air that is dry air mixed with water vapor or humidity. It is also includes the study of behavior of dry air and water vapor mixture under various sets of conditions. Though the earth's atmosphere is a mixture of gases including nitrogen (N2), oxygen (O2), argon (Ar) and carbon dioxide (CO2), yet for the purpose of psychrometry, it is consider a mixture of dry air and water vapor only. Psychrometry is concerned with determination of the properties of gas-vapor mixtures. The airwater vapor system is by far the system most commonly encountered. The purpose of psychrometry is a mixture of dry air and water vapor consider only. The temperature of air in air-conditioning applications ranges from about -10 to 50°C. In that range both dry air and atmospheric air (including water-vapor) can be treated as ideal gas, with negligible error. Thus the ideal-gas relation PV = RT can be applied. The partial pressures of atmospheric air are: Also, one can write:

Note that since air is considered as an ideal-gas, the enthalpy of air (both water-vapor and dry air) is only a function of temperature, at the temperature range of interest, i.e., -10 to 50°C. Taking 0°C as the reference temperature; with the constant-pressure specific for dry-air (in the range of interest) cp= 1.005 kJ/kg.°C; one obtains: hdry air = cp T = (1.005 kJ/kg. °C) T (kJ/kg) 2

1.2 Psychrometric Terms Psychrometrics is the study of the physical and thermodynamic properties of air-water mixtures. Some terms used in psychrometrics are:  Dry Air—Air not containing any water vapor.  Moist Air—A mixture of dry air and water vapor.  Air Mixture—A mixture of dry air and water vapor.  Dry Bulb Temperature—the temperature measured by an ordinary thermometer.  Wet Bulb Temperature—the lowest temperature to which an air mixture can be cooled solely by the addition of water.  Dew Point Temperature—the temperature at which moisture starts to condense from air cooled at constant pressure and humidity ratio.  Humidity Ratio—Weight of water vapor in pounds per pound of dry air or grains of water vapor per pound of dry air or kilograms of water vapor per kilogram of dry air expressed as a decimal. This quantity may also be called Absolute Humidity.  Relative Humidity (RH)—the ratio of actual water vapor pressure to the vapor pressure of saturated air at the same dry bulb temperature. RH is expressed as a percentage.

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1.3 Psychrometric Chart  The state of the atmospheric air at a specified pressure is completely specified by two

independent intensive properties. Psychrometric charts present the moist air properties; they are used extensively in air-conditioning conditioning applications.

Figure1.1 Psychrometric chart.

Basic features of psychrometric chart are:  The dry bulb temperatures are shown on the horizontal axis.  The specific humidity ω is shown on the vertical axis.  The curved line at the left end of the chart is the saturation line. All the saturated air states are located on this curve. Thus, it also represents the curve of relative humidity100%. 100%. Other constant relative humidity curves have the same general shape.

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 Lines of constant wet-bulb bulb temperature have a downhill appearance to the right.  Lines of specific volume also have downhill downhill appearance to the right with steeper slopes.  Lines of constant enthalpy lie very near to the constant wet-bulb bulb temperature, thus ((in some charts) lines of constant wet-bulbs wet are used as constant-enthalpies. enthalpies. For saturated air, the dry-bulb, wet-bulb, bulb, and an dew-point temperatures are identical.  Thus the dew-point point temperature of atmospheric air can be determined by drawing a horizontal line to the saturated curve.

Air-conditioning conditioning processed Maintaining a living space or an industrial facility at the desired temperature and humidity requires some processes called air-conditioning air conditioning processes. These processes include simple heating (raising the temperature), simple cooling (lowering the temperature), re), humidifying (adding moisture), and dehumidifying (removing moisture).

Figure1.2 Various air-conditioning conditioning processes Most air-conditioning conditioning processes can be modeled as steady-flow steady processes, and thus the mass balance relation mın ̇ = ṁoutcan be expressed for dry air and water as Mass balance for dry air: ∑ ṁ ain = ∑ ṁ aout Mass balance for water:

∑ ṁ win = ∑ ṁ wout

(kg/s) or ∑ ma ̇ ωin = ∑ ṁa ωout out

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Conclusion Drying is a mass transfer process consisting of the removal of water or another solvent by evaporation from a solid, semisolid, or liquids. Generally drying is defined as the application of heat under controlled condition. Psychrometry is that branch of engineering science, which deals the study of moist air that dry air mixed with water vapor or humidity. There are so many psychrometric terms; like humidity, relative humidity, absolute humidity, wet bulb dispersion, and so on. Psychrometric charts present in a graphical form physical property data for gas systems involving one or more non-condensing gas components and a single condensing vapor component.

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CHAPTER TWO 2.CRYSTALLIZATION Introduction Crystallization is a separation and purification technique employed to produce a wide variety of materials. Crystallization may be defined as a phase change in which a crystalline product is obtained from a solution. A solution is a mixture of two or more species that form a homogenous single phase. Solutions are normally thought of in terms of liquids, however, solutions may include solids suspension. Typically, the term solution has come to mean a liquid solution consisting a solvent, which is a liquid, and a solute, which is a solid, at the conditions of interest .The solution to be ready for crystallization must be supersaturated. A solution in which the solute concentration exceeds the equilibrium (saturated) solute concentration at a given temperature is known as a supersaturated solution. There are four main methods to generate super saturation that are the following: 

Temperature change (mainly cooling),



Evaporation of solvent,



Chemical reaction, and



Changing the solvent composition

Objective  To separate a solute from a solvent  For purification and separation process  For recovery of solid materials  To form a liquid solution based on difference in solute concentration and its solubility at a certain temperature.

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2.1 Crystallization Fundamentals Crystallization may be defined as a phase change in which a crystalline product is obtained from a solution. A solution is a mixture of two or more species that form a homogenous single phase. Solutions are normally thought of in terms of liquids, however, solutions may include solids suspension. In evaluating a crystallization operation, data on phase equilibria are important as this indicates the composition of product that might be anticipate and the degree of super saturation gives some idea of the driving force available. The rates of nuclei formation and crystal growth are equally important as these determine the residence time in, and the capacity of a crystallizer. These parameters also enable estimates to be made of crystal sizes, essential for the specification of liquor flows through beds of crystals and also the mode and degree of agitation required.

2.2 Solid-Liquid equilibrium This section provides an overview of solid-liquid equilibrium and the type of information useful to crystallization that can be glean from their representation in the form of phase diagrams. Different types of – idealized – phase and solubility diagrams shall be discuss together with associated phenomena.

2.2.1 Solubility and Phase Diagrams A solution is a homogeneous mixture of two or more chemical species. For a liquid solution, saturation is reach when the liquid phase, in contact with the solid phase, no longer changes its composition. A saturated solution therefore has a constant composition that not changed by the addition of further amount of the dissolved material. For a two-component system, the solubility of one component in the other is dependent on temperature and pressure. For three and more component systems, the solubility of one component also depends on the relative amounts of the other components present.

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For liquid solutions the pressure dependence is negligible if the pressure difference is small (which is the case in most applications) and will not be considered here. However, the temperature dependence (and the pressure dependence providing the difference is sufficiently large, see [Moritoki] of the solubility can be considerable and it is therefore important to state the temperature for which the solubility is reported even under „normal‟ conditions. In the following, the discussion will focus on two component systems only. Phase transformations Metastable crystalline phases frequently crystallize to a more stable phase in accordance with Ostwald’s rule of stages, and the more common types of phase transformation that occur in crystallizing and precipitating systems include those between polymorphs and solvates. Transformations can occur in the solid state, particularly at temperatures near the melting point of the crystalline solid, and because of the intervention of a solvent. A stable phase has a lower solubility than a metastable phase.

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Fig 2.1 Solubility curves for substances with two polymorphs I and II (2)

Whilst transformation cannot occur between the metastable (I) and (I) in the monotropic system in the temperature range shown, it is possible above the transition temperature in an enantiotropy system.

2.2.2. Solubility and saturation Super saturation A solution that is in thermodynamic equilibrium with the solid phase of its solute at a given temperature is a saturated solution, and a solution containing more dissolved solute than that given by the equilibrium saturation value is said to be supersaturated. supe Solution concentrations may express as mass of anhydrate/mass of solvent or as mass of hydrate/mass of free solvent.

2.2.3. Crystal nucleation Nucleation, the creation of crystalline bodies within a supersaturated fluid, is a complex event, since nuclei may be generate by many different mechanisms. The most nucleation classifications scheme distinguishes between primary nucleation - in the absence nce of crystals and secondary nucleation - in the presence of crystals. 10

Primary nucleation Classical theories of primary nucleation are bases on sequences of bimolecular collisions in addition, interactions in a supersaturated fluid that result in the build-up of lattice-structured bodies which may or may not achieve thermodynamic stability. Such primary nucleation is knows as homogeneous, although the terms spontaneous and classical have also been used. Primary nucleation

may be initiating by suspended particles of foreign substances, and this mechanism has referred to as heterogeneous nucleation. In industrial crystallization, most primary nucleation is almost certainly heterogeneous, rather than homogeneous, in that it is induced by foreign solid particles invariably present in working solutions. Homogeneous nucleation Is a consideration of the energy involved in solid-phase formation and in creation of the surface of an arbitrary spherical crystal of radius r in supersaturated fluids.

Heterogeneous nucleation The presence of foreign particles or hetero nuclei enhances the nucleation rate of a given solution, and equations similar to homogeneous nucleation have been propose to express this enhancement. Secondary nucleation Secondary nucleation can be the definition, take place only if crystals of the species under consideration are already present. Since this is usually the case in industrial crystallizers, secondarynucleation has a profound influence on virtually all industrial crystallization processes.

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Fig 2.2 Effect of super saturation on the rates of of homogeneous and heterogeneous nucleation

2.2.4. Crystal growth In simple terms, the crystallization process may be consider to take place in two stages a diffusional step ions or molecules at the crystal surface are deposited in which solute is transported from the bulk fluid through the solution boundary boundary layer adjacent to the crystal surface, and a deposition step in which adsorbed solute and integrated into the crystal lattice.

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Fig 2.3 Concentration driving forces for crystal growth from solution

2.3. Crystallization from melts 2.3.1. Basic techniques A melt is a liquid or a liquid mixture at a temperature near its freezing point and melt crystallization is the process of separating the components of a liquid mixture by cooling until crystallized solid is deposited from the liquid liquid phase. Where the crystallization process is used to separate, or partially separate, the components, the composition of the crystallized solid will differ from that of the liquid mixture from which it is deposited Two basic techniques of melt crystallization crystalli are: (a) Gradual deposition of a crystalline layer on a chilled surface in a static or laminar flowing melt, and (b) Fast generation of discrete crystals in the body of an agitated vessel

2.3.2. Multistage-processes processes A single-stage stage crystallization process may not always achieve the required product purity and further separation, melting, washing, or refining may be required. Two approaches are used: a) A repeating sequence of crystallization, melting, and re-crystallization; re b) A single crystallization step followed by countercurrent contacting of the crystals with a relatively pure liquid stream. 13

Conclusion Crystallization is a thermal separation, and purification process that yields a solid product from a melt, from a solution or from a vapor. As for all thermal separations, non-equilibrium conditions are required as a driving force for the process. Crystallization is used as some stage in nearly all process industries as a method of production, purification, or recovery of solid materials. Crystallization provide a comprehensive over view of the subject and will prove in valuable to all chemical engineer and industrial chemists in the process industries are well as crystallization workers and students in industry and academia. Two basic techniques of melt crystallization are gradual deposition of a crystalline layer on a chilled surface in a static or laminar flowing melt and Fast generation of discrete crystals in the body of an agitated vessel.

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CHAPTER THREE

3.EXTRACTION Introduction Extraction is a separation of the constituents of a liquid solution by contact with another insoluble liquid. The liquid which is added to the solution to bring about the extraction is known as the solvent. Extraction is a separation process aiming to purify the feed or to recover one or more compounds from it. Since the mass transfer is much slower than the reaction rate. The liquid-liquid extraction is a mass transfer process betweentwo phases. One liquid phase is the feed consisting of a solute and a carrier. The other phase is the solvent. The extraction is understood to be a transfer of the solute from the feed to the solvent. During and at the end of the extraction process, the feed deprived of solute becomes a raffinate and the solvent turns into extract. Depending on the phases following types of extraction, exist:  Solid-liquid extraction (leaching)  Liquid-liquid extraction  Gas-liquid extraction also called absorption.

The main area of extraction is for hydro metallic processes, for pharmaceutical industry (producing active agent), for petroleum industry (production of monomers and aromatic) and for cleaning of wastewater to separate solved compounds. Objectives:  It is aiming to purify the feed or to recover one or more compounds from it.  For separation of the constituents of a liquid solution by contact with another insoluble liquid.

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3.1 Definition of Extraction Extraction a separation of the constituents of a liquid solution by contact with another insoluble liquid.The solution which is to be extracted is called the feed. The solvent-richen product of the operation is called the extract and the residual liquid from which the solute is separated is called the raffinate. The following are some examples techniques of extraction.  The separation of aromatics from kerosene based fuel oils,  The production of fuels in the nuclear industry and  The separation of penicillin from fermentation mixtures

3.2 Selection of the solvent The solvent for extraction has to withdraw the active agent from a mixture. Any solvent can be select based on the following points;  Selectivity: only the active agent has to be extract and no further substances, which means that a high selectivity is required.  Capacity: To reduce the amount of necessary solvent the capacity of the solvent has to be high.  Difference in density: After extraction, the two phases have to be separate in a separator and for this, a high difference in density is positive.  Low price and no or low toxicity

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Conclusion Extraction is the withdrawing of active agent or a waste substance from a solid or liquid mixture with a liquid solvent. The solvent is not or only partial miscible with the solid or the liquid. Extraction types depend on phases are Solid-liquid extraction, Liquid-liquid extraction and gasliquid extraction. Extractions are classifies in to two steps, which are single step extraction and multi-step extraction

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Reference Drying of solids ,Oldrich Holecek, Martin Kohout Extraction, lecturer, Dr.Gamse NONHEBEL,G. and MOSS,A.A.H.:Drying of Solids in the Chemical Industry(Butterworth, London, 1971). Psychrometry, Ray A. Bucklin and Dorota Z. Haman2 SHERWOOD,T.K. and PIGFORD,R.L.: Absorption and Extraction, 2nd ed. (mcgraw-Hill, New York,1952). TREYBAL, R.E.: Liquid Extraction.2nd ed. (mcgraw-Hill, New York, 1963).

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