crystallization, drying of solid

8/16/2019 crystallization, drying of solid

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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


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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 orhumidity. 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


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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

III


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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 air-

water 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 dryair) 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)

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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

independent intensive properties.

they are used extensively in air-c

Figure1.1 Psychrometric chart.

Basic features of psychrometric

The dry bulb temperature

The specific humidity ω i

The curved line at the l

states are located on

humidity100%. Other co

t a specified pressure is completely specified b

Psychrometric charts present the moist air pro

onditioning applications.

chart are:

s are shown on the horizontal axis.

s shown on the vertical axis.

ft end of the chart is the saturation line. All

this curve. Thus, it also represents the c

stant relative humidity curves have the same g

two

erties;

the saturated air

rve of relative

neral shape.

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Lines of constant wet-bu

Lines of specific volume

Lines of constant enthal

some charts) lines of c

air, the dry-bulb, wet-bu

Thus the dew-point te

horizontal line to the sat

Air-conditioning processe

Maintaining a living space or

requires some processes calle

heating (raising the temperatu

(adding moisture), and dehumidi

Figure1.2 Various air-conditioni

Most air-conditioning processes

and thus the mass balance relatio

Mass balance for dry air: ∑ ṁ ai

Mass balance for water: ∑ ṁ

lb temperature have a downhill appearance to t

also have downhill appearance to the right wit

py lie very near to the constant wet-bulb tem

nstant wet-bulbs are used as constant-enthalpi

b, and dew-point temperatures are identical.

perature of atmospheric air can be determin

rated curve.

an industrial facility at the desired temperat

air-conditioning processes. These processes

e), simple cooling (lowering the temperatu

fying (removing moisture).

g processes

can be modeled as steady-flow processes,

n mın ̇ = ṁ out can be expressed for dry air and

= ∑ ṁ aout (kg/s)

in = ∑ ṁ wout or ∑ ma ̇ ωin = ∑ ṁ a ω

e right.

steeper slopes.

erature, thus (in

es. For saturated

d by drawing a

re and humidity

include simple

e), humidifying

ater as

ut

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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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Fig 2.1 Solubility curves for sub

Whilst transformation cannot oc

in the temperature range shown,

system.

2.2.2. Solubility and satur

Super saturation

A solution that is in thermo

temperature is a saturated solut

given by the equilibrium satura

may express as mass of anhydrat

2.2.3. Crystal nucleation

Nucleation, the creation of cry

since nuclei may be generate by

scheme distinguishes between

nucleation - in the presence of cr

tances with two polymorphs I and II (2)

cur between the metastable (I) and (I) in the m

it is possible above the transition temperature i

ation

ynamic equilibrium with the solid phase of its

ion, and a solution containing more dissolved

ion value is said to be supersaturated. Soluti

e/mass of solvent or as mass of hydrate/mass o

talline bodies within a supersaturated fluid, is

many different mechanisms. The most nucleati

rimary nucleation - in the absence of crystal

ystals.

onotropic system

an enantiotropy

solute at a given

solute than that

n concentrations

free solvent.

a complex event,

on classifications

s and secondary

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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

2.2.4. Crystal growth

In simple terms, the crystalli

diffusional step ions or molec

transported from the bulk flui

surface, and a deposition step in

n the rates of homogeneous and heterogeneous

ation process may be consider to take place

ules at the crystal surface are deposited in

through the solution boundary layer adjace

which adsorbed solute and integrated into the c

nucleation

in two stages a

which solute is

t to the crystal

ystal lattice.

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Fig 2.3 Concentration driving fo

2.3. Crystallization from

2.3.1. Basic techniques

A melt is a liquid or a liqui

crystallization is the process of

crystallized solid is deposited fr

separate, or partially separate,

differ from that of the liquid mix

Two basic techniques of melt cr

(a) Gradual deposition of a crys

melt, and

(b) Fast generation of discrete cr

2.3.2. Multistage-processe

A single-stage crystallization pr

further separation, melting, wasa) A repeating sequence of cryst

b) A single crystallization step

relatively pure liquid stream.

ces for crystal growth from solution

elts

mixture at a temperature near its freezing

separating the components of a liquid mixture

m the liquid phase. Where the crystallization

he components, the composition of the cryst

ture from which it is deposited

stallization are:

talline layer on a chilled surface in a static or

stals in the body of an agitated vessel

ocess may not always achieve the required pr

ing, or refining may be required. Two approachllization, melting, and re-crystallization;

followed by countercurrent contacting of th

point and melt

by cooling until

rocess is used to

llized solid will

laminar flowing

oduct purity and

es are used:

crystals with a

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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 gas-

liquid 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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crystallization, drying of solid

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