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البوقعي عبدالله أحالماالهدل علي غادهعبدالله احمد ثامر

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الفراص

CRYSTALLIZATIONCRYSTALLIZA

TION

CRYSTALLIZATION:-

Crystallization is a solidification process. It is the process in which particles will get arranged themselves into regular geometric patterns. The product obtained after crystallization is called CRYSTAL.

CRYSTALS:  -

In Crystals, the atoms. ions molecules are arranged into a fixed geometric pattern. This geometric pattern is called a Crystal Lattice. The sites at which the atoms molecule situate are called as Lattice Sites. The angle between perpendiculars of two Faces is called as an Interfacial Angle. The visible external shape (morphology) of a crystal is called as a Crystal Habit.

Crystal habit:*This term is used to denote the relative development of different types of faces.

*Example: Sodium chloride crystallizes from aqueous solutions with cubic facesonly. On the other hand, if NaCL is crystallized from an aqueoussolution containing small amount of urea, the crystals obtained willhave octahedral faces. Both types of crystals belong to the cubic system but differ in habit. The word habit refers to the type of faces developed and not to theshape of the resulting crystal.

Crystal habit is affected by the following factors:1. Presence of impurities2. Temperature3. Degree of supersaturation. In some respects crystallizeation can be regarded as the inverse ofdissolution but there are important differences. The number of particles present during dissolution will remain constant ordecrease whereas in crystallization the number of nuclei on which materialis deposited may continuously increase.

A. Formation of small particles or nuclei.B. Growth of the nuclei.And for crystallization to occur, saturation andSupersaturation must happen.3.Crystal Growth – Once the crystals are formed, nuclei formation stops and crystal growth begins

The crystallization process consistsessentially of twostages:

2. Rate of agitation: The rate of crystallization is improved by increasing the rateof agitation. The crystallization rate initially rises very rapidly by increasingagitation but a point is reached where further increase in the agitation producesno effect on the rate of crystal growth.3. Degree of supersaturation: This increases the crystallization rate.4. Presence of impurities: Any foreign solid bodies act as a nucleus and enhancecrystallization.5. Viscosity: As the viscosity increases, the rate of crystal growth decreases,because the viscosity decreases the rate of diffusion of solute to the crystalsurface.

Factors affecting rate of crystal growth :

1- Temperature and concentration of the liquid at the crystal face:These conditions are not generally the same as those in the bulk of thesolution because(1) Concentration gradient is necessary for the transfer of solutetowards the face(2) Temperature gradient for the dissipation of the heat ofcrystallization.Thus the problem involves both heat transfer and mass transfer.Thus the concentration gradient is the driving force for crystal

Types of crystallization:

1-Crystallization from vapor:

-This occurs naturally in the formation of snow form water-vapor.

-It is also employed in the process of sublimation and for the condensation of water –vapor during freeze-drying.

2-Crystallization from Amelt:

Amelt may be defined as the liquid form of asingle material or the homogeneous liquid form of two or more materials which solidifies on cooling .An examples is freezing water to form ice..Another example is the production of crystalline sulfur by melting pure sulfur in a vessel and allowing it to cool slowly nutil a crust form over the top.

3- Crystallization from a solution:In pharmaceutical industry crystallization from a solution is the most important as compared to crystallization from a vapor or a melt.

Factors Affecting the Size of crystal:

1-Very Small Crystal: are obtained by the rapid cooling with frequent stirring of solutions almost saturated at their boiling point. The production of very small of substance is to be avoided. Because they tend the cohere into masses which are difficult to wash and may less pure than when obtained in crystals of medium size.

2-Medium Sized Crystals:

Are obtained when crystallizing from aqueous solutions saturated at about 60 to 90 the hot solution is allowed to cool slowly and without mechanical disturbance in a warm room.

3-Very large Crystals:

Are obtained by setting aside a large volume of a solution and allowing it to evaporate spontaneously.The liquid should have been rendered perfectly clear by filtration and the access of dust must be guarded against.The formation of large crystals may be facilitated by seeding.That is suspending a well formed crystal of the solute in the solution the crystal acting as a .nucleus

Crystallization equipment (crystallizers):Crystallization equipment is classified according to the method bywhich super saturation is brought about:1. super saturation by cooling2. super saturation by evaporation3. super saturation by adiabatic evaporation (cooling andevaporation)4. Salting out by adding a substance that reduces the solubility ofthe substance in question (High cost of production).

Cooling crystalliz

er

Wulf-Bock

crystallizer

Swenson-

walker crystalli

zer

Oslo cooler

crystallizer

Howard crystalli

zer

1-cooling crystallization:-Swenson-walker crystallizer:

In this crystallizer, super saturation of the solution is carried out by cooling. It consists of a trough surrounding which a cooling jacket is present. In this solution and cool water moves in opposite directions. A scrapper is present attached to a shaft. Its rotation will help in avoiding deposition of crystals on the walls.

It consists of an open trough A, which is wide, a water jacket welded to the:outside of the trough. It also contains a slow speedspiral agitator set as close as possible to thebottom of the trough. A number of units may be joined together to give increased capacity. The hot concentrated solution to be crystallized is fed at one end of the troughand cooling water usually flows through the jacket in counter currentto thesolution. In order to control crystal size, it is sometimes desirable to introduce an extraamount of water into certain sections in the jacket.

Functions of the spiral stirre1. It Prevents the accumulation of crystals on the cooling surface.2. It lifts the crystals that have already been formed and shower them downthrough the solution. In this manner, the crystals grow while they are freely suspended in the liquidand therefore they are:1. Fairly perfect individuals.2. Uniform in size3. Free from inclusions or aggregations. At the end of the crystallizer there may be an overflow gate where crystals andmother liquor overflow to a drain box from which the mother liquor is returnedto the process and the wet crystals are fed to a centrifuge to remove mother liquor.

Advantages:1. Large saving in floor space.2. Large saving in material in process.3. Saving in labor.4. Uniform size crystals.5. Free from inclusions and aggregations.

2-Wulf–Bock crystallizer:

2-Wulf–Bock crystallizer:

It has similar characteristics to the swenson walkerbut it depends onair cooling and gives more uniform crystals. It consists of a shallow trough set inclined and mounted on rollers sothat it can be rocked from side to side. The slow rate of cooling in this crystallizer results in low capacity but itgives uniform crystals.

General disadvantages crystallizers:1-Need:more workers so they are of high cost.2-Need:large floor space.3Nocontrol of the size and shape of crystals.

3-Howard crystallizer:

This crystallizer consists essentially of a vertical conical device throughwhich solution flows in an upward direction. The upper end of thecrystallizer is the wide part of the cone. A Concentric outer conical chamber serves as a cooling water channel. Crystals that are suspended in the upward flowing stream of solution mustgrow to such a size that they will settle at the apex of the cone (bottom ofcrystallizer) beforethey can escape. By regulating the velocity of flow at the bottom of the crystallizer, the sizeof the product is controlled.

This crystallizer consists essentially of a vertical conical device throughwhich solution flows in an upward direction.

The upper end of the crystallizer is the wide part of the cone. A Concentric outer conical chamber serves as a cooling water channel.

Crystals that are suspended in the upward flowing stream of solution mustgrow to such a size that they will settle at the apex of the cone (bottom ofcrystallizer) before they can escape.

By regulating the velocity of flow at the bottom of the crystallizer, the size of the product is controlled.

It is composed of two main compartments:1-lower horizontal.2-upper upright.(wide)*Functions:1-As crystallizer.2-As hydraulic classification device.*As the solution flow up through the crystallizing coneIts super saturation is increases is because of the cooling.

4- Oslo cooler crystallizers: (or krystal crystallizers)In this crystallizer, supersaturating take place by cooling under vacuum. Hot solution is allowed to flash in the vapor head where super saturation takes place. A separate chamber is present for crystal growth. In this crystallizer, large crystals remain at the bottom of the crystal growth chamber and small ones float up and again enter into the circulating pump. Thus grading of crystals according to their sizes is observed.

The mother liquor is withdrawn near the feed point of the crystallizer by acirculating Pump and is passed through the cooler H where it becomessupersaturated and then fed back to the bottom of the crystallizer through thecentral pipe B Some nuclei form spontaneously in the crystal bed and some forms as a result ofbreakage of the crystals. A vessel G can be used to remove very small nuclei that reach the upper layers ofthe vessel E. These nuclei pass again in the cooler and then to the vessel E through the tube B. The nuclei circulate with the mother liquor until they have grown sufficiently largeto be retained in the fluidized bed (liquid fluidization). The final product is removed from the bottom of crystallizer though a valve M anda uniform product is therefore obtained because the crystals are not dischargeduntil they have grown to the required size that settle opposing the flow from tubeB.

Advantages:1. Its a continuous crystallizer2. Give uniform size crystals.3. The size of crystals can be controlled by the pump flow rate.• It is used where large quantities of crystals of controlled size are required.• It is used for crystallization of KNO3• Crystallization can be initiated by adding crystals to act as nuclei.

Melt Crystallization :Melt crystallization can be subdivided into two distinct methods. These are suspension based and solid layer based processes. The different techniques are described in detail in the literature cited, for example [Arkenbout] or [Ulrich/Glade].

The second type:Evaporative crystallizer: have twp types:1-crystallizing evaporator:

In all evaporative crystallizers the liquid layer just beneath the evaporation zone is the region with the highest super saturation. As the liquid interface is subject to periodic variations in height due to the effect of motion in the equipment, this area is prone to encrustation due to variable wetting and resultant nucleation. Large crystals require long retention times, long retention times require large equipment and large equipment results in expensive processes.

2-Oslo evaporative crystallizer (crystal evaporation crystallizer):

This method is used for substances not affected by heat.

1 .Small unclei reach the upper portion of the crystallizer body and enter

again in the heater and the cycle repeated till the desired size is

obtained.2 .So the size of crystals can be controlled.3 .It is a Continuous crystallizer.

2-Oslo evaporative crystallizer (crystal evaporation crystallizer):This method is used for substances not affected by heat.1. Small unclei reach the upper portion of the crystallizer body and enteragain in the heater and the cycle repeated till the desired size isobtained.2. So the size of crystals can be controlled.3. It is a Continuous crystallizer.

*The basic principle of all oslo crystallizer is to pass a supersaturated solution upwards through abed of crystals which is maintained in fluidizing state.*This type can be operated under reduced pressures for the production of hydrated crystals.

In this apparatus, the solution is first passed through a heater and then to aflash evaporator before being returned to the crystallizer.

This method is called adiabatic cooling. The solution is heated and then introduced into a vacuum where the totalpressure is less than the vapour pressure of the solvent at the temperature atwhich it is introduced.

The solvent must flash and the flashing must produce adiabatic cooling, i.e.when the solution is introduced into a vacum, Flash evaporation occur. Thisresults in drop in temperature and cooling which helps supersaturation, andremoving of a part of the solvent leading to crystallization.

The third type:3-Vaccum crystallizer:1-Batch vaccum crystallizer:

It acts by partial removal of vapor by vacuum application whichleads to superation and crystal formation. It is used mainly forcrystallizing thermolabile substances.

Supersaturation takes place by allowing hot solution to flash under vacuum. These consists of a crystallization chamber in which flashing is carried out. Propellers are present which will prevent the flow of unsaturated material into discharge pipe.

*The principle of adiabatie evaporation and cooling is a sim one which depend upon introducing a hot saturated solution sudenly in to a chamber maintained under sufficient vaccum.*Solution will become supersaturated both due to :1-loss of part of the solvent by evaporation.2-cooling.

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