结晶器原理及设计CHAPTER 8 CRYSTALLIZATION

Chapter 6:cRYSTALLIZATION

MISS. RAHIMAH BINTI OTHMAN (Email: rahimah@unimap.edu.my)

COURSE OUTCOMES

CODESCRIBE the basic principles and applications of crystallization process. CALCULATE the yields, material and energy balance in crystallization.

OUTLINES Basic principles and applications of crystallization process.

CALCULATION of yields, material and energy balance in crystallization.

CRYSTALLIZATION

Process of producing crystals from a homogeneous phase which is obtained from a solution. Capable of producing bioproducts at very high purity and considered to be a polishing step and a purification step Two different application of crystallization:i. ii.

Crystallization for polishing and purification Crystallization for crystallography

C OMPARISON OF C RYSTALLIZATION AND P RECIPITATIONDescription Solubility Relative supersaturation Crystallization Precipitation Wide range, usually Sparingly soluble medium to high Low High Ill-defined Small Primary High Low Difficult to control

Product morphology Well-defined Product crystal size Large Nucleation mechanism Nucleation rate Growth Rate Controllability Secondary Low Wide Range Controllable

C RYSTALLIZATION FROM S UPERSATURATED S OLUTIONS OF S ODIUM A CETATE

Description: A supersaturated solution of sodium acetate is crystallized by pouring it onto a seed crystal, forming a stalagmite-like solid. Heat is radiated from the solid.Source: Shakhashiri, B.Z. Chemical Demonstrations: A Handbook for Teachers of Chemistry

Crystals

C RYSTALLIZATION P RINCIPLES

Can appear as polyhedrons, or solids formed by plane faces when allowed to form freely.

Angles made by the corresponding faces of the same material do not vary – can be classified by this characteristics.Relative sizes of the faces of a crystal in a particular system can vary considerably – resulting in a variety of crystal shapes. This variation is called a habit. Crystal habit is influence by the conditions of crystallization, particularly by the impurities present and by the particular solvent or solvents used. Impurities can stunt the growth of a crystal in certain directions.

POLYHEDRONS SHAPE

TABULAR Book-like (tablets) that are thicker than platy but not as longated as bladed. Wulfenite forms crystals that are a good example of tabular crystals. PRISMATIC One of the most common of crystal habits. Prismatic crystals are "pencil-like", elongated crystals that are thicker than needles (see acicular). Indicolite (a variety of elbaite) forms good examples of prismatic crystals.ACICULAR Long and needle-like, thinner than prismatic but thicker than fibrous. Natrolite crystals can be good examples of acicular crystals.

C RYSTALS GROW IN 2 STEPS :Nucleation - first aggregation. 2. Growth. Thermodynamically distinct Want a few nuclei to grow big Use thermodynamics to understand the required conditions1.

Nucleation

Crystallization Princip

les

The generation of ultramicroscopic particles in the process of nucleation is the sum of contributions by primary nucleation and second nucleation. Primary nucleation : occurs in the absence of crystals, secondary nucleation: attributed to the influence of existing crystals Primary nucleation can be either homogeneous (no foreign particles are present) or heterogeneous (foreign particles present during heterogeneous nucleation) Rate of primary nucleation has been modeled by the following power law expression:(1)B: number of nuclei formed per unit volume per unit time; N: number of nuclei per unit volume; kn : rate constant; c: instantaneous solute concentration; c*: solute concentration at saturation. (c-c*) term : supersaturation, the exponent of n can range up to 10 but typically is in the range of 3 to 4.

C RYSTALLIZATION P RINCIPLES

Two types of secondary nucleation : shear nucleation (occurs as a result of fluid shear on growing crystal faces), contact nucleation ( happens because of crystals colliding with each other and with the impeller and other vessel internal surfaces. Rate of secondary nucleation in crystallization is the following:(2)

k1 : rate constant; MT : suspension density, b : can range up to 5 but has a most probable value of 2; j: ranges up to 1.5 with 1 being the most probable value

Crystallization PrinciplesThe supersaturation must be above the a certain value before nucleation will begin Metastable region : the supersaturation is low that nucleation will not start Once the supersaturation has been raised enough to be in the labile region, nucleation can begin.

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At this point, crystals begin to grow, and the supersaturation decreasesIf the supersaturation becomes too high, the nucleation rate will be too great, and amorphous precipitate will result.

Figure 1: Typical phase diagram. The components in solution consist of the product (ordinate) and the precipitating reagent (abscissa). The lines with arrows out line one possible way of performing the crystallization.

Nucleation

Crystallization Principles

Supersaturation

P HASE

DIAGRAMS

Protein concentration

Precipitation zone

Nucleation zone Metastable zone Crystals grow, but Nuclei form only infinitely slowly

Solubility curve

Under-saturation (protein remains soluble; crystals dissolve)

Precipitatant concentration (salt, PEG etc.)

C OURSE OF C RYSTALLIZATION E XPERIMENTMetastable Precipitation Protein concentration

Nucleates here

Nucleation

Crystal grows Sequesters protein [protein] drops

Start w/ soluble protein(undersaturated or metastable)

Crystal stops growing @ solubility curve Expt incr. [protein], [precipitant] Xtl grows again, until hits curve Repeats as follo …… 此处隐藏：3605字，全部文档内容请下载后查看。喜欢就下载吧 ……