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Particle Size and Distribution Analysis
Deepak Parab, V R Sankar Babu Particle Size Analysis that is also known as Particle Size Measurement or Particle Sizing is an important process practiced in chemical and petrochemical organisations. The process also finds applications in mining and agriculture industries. According to American Institute of Chemical Engineers, Particle Size Analysis deals with the production, handling, modifi cation, and use of a wide variety of particulate materials. Following article draws attention to all the aforesaid processes involved in Particle Sizing.

Particle is a distinct entity of any material. These particles with a shape Spherical or Irregular may be as single entity or agglomerated as a bunch of grapes.

The measurement of particle size is an important aspect in material science. To monitor production and particle stability, efficient and rapid methods for particle sizing are needed. The electronic measurement of samples offers a convenient solution to this problem. There is various techniques viz., Sieve analysis, Sedimentation,

Electrical Sensing Zone (ESZ), Microscopy by which people measure particle size distribution of materials. However, Laser diffraction is the proven technique to measure PSDs of materials as it delivers possibility of wide size range measurement, high resolution, short measurement time, enhanced repeatability, wet & dry mode measurement and versatile application. Especially, it confers easy solution for PSD measurement of complex shape particles like pigment in paints, carbon black filler and cement particles.

Laser Diffraction Method
In Laser diffraction method, the forward diffraction of a laser beam by the particles is used to determine their size distribution. The angle of diffraction is inversely proportional to particle size, and the intensity of the diffracted beam at any angle is a measure of the number of particles with a specific cross-sectional area in the beam`s path. Two optical models are commonly used to calculate PSD, the Fraunhofer diffraction model and the Mie theory. The former is based on the approximation that the laser beam is parallel and the detector is at a distance that is very large compared with the size of the diffracting particle. The Mie theory is a solution of the Maxwell equations (i.e., a set of four fundamental equations governing the behaviour of electric and magnetic fields) describing propagation of the electromagnetic wave of light is space. The theory provides a solution for the case of a plane wave (i.e., the wave fronts of which are planes) on a homogeneous sphere of any size.

In addition, it takes into account phenomena other than diffraction (e.g., transmission through the particle) and therefore requires knowledge of the RI of the material tested. The Mie theory thus offers an exact solution to the scattering of light from a homogeneous sphere (but not from an irregularly shaped particle). The resultant PSD computed by either the Fraunhofer diffraction or the Mie theory is a volume based size distribution. Early generation laser diffraction instruments for PSD determination suffered from a size detection limit > 0.5 um. In addition, they employed mainly the Fraunhofer diffraction, which is inaccurate for particles smaller than d=10? (? is wavelength of electromagnetic radiation, laser). In newer Laser diffraction, the lower detection limit was extended to approximately 0.02 um.

In classical set up, single light source with multiple lens & detector elements are used. Limitation of long optical bench and expensive detector components is realised over the years. In Modern approach, the optical bench with multiple light sources (Lasers) is designed to overcome the limitations of classical approach. This enables invention and usage of solid state lasers, which are economical in comparison with Gas lasers. Functional advantage of instantaneous laser on and off is also benefited, which results in no warm up time for the instrument. In general, solid state laser has better durability compared to gas lasers. Main advantages of the Laser diffraction technique for PSD determination include, shorter time of analysis (10 seconds analysis time), higher repeatability, small size of sample needed for analysis and a wide range of size fractions into which the entire range of particles size can be divided.

Analysis & Its Mode
Laser diffraction method requires that the particles be in a dispersed state, either in liquid (suspension) or in air (Aerosol). The former is presently referred to as the Wet method, whereas the latter is termed the Dry method. Variation between Wet and Dry method appears primarily from the different ways in which the particles are dispersed in each case.

In liquid, it is possible to modify solution conditions by changing pH or adding chemical dispersing agents like surfactants Tween-80, Span-80, Triton X-100. Also one can disrupt aggregates using mechanical or ultrasonic energy. Thus for the very fine fraction, a better state of dispersion can be achieved in an appropriately selected dispersion medium. Generally, Water is an excellent dispersing medium for most of the particles.

In Dry mode, a stream of compressed dry air with a combination of vacuum is used to both disperse the particles and to transport them to the sensing zone. This method of dispersion works best for the coarse size fraction, where the inter-particle contacts are weak. For particles smaller than a micrometer, or highly asymmetric particles, air dispersion is generally not appropriate for sizing.

A representative sample, dispersed at an adequate concentration in a suitable liquid or gas, is passed through the beam of a monochromatic light source, usually from a laser. The light scattered by the particles at various angles is measured by a multi-element detector, and numerical values relating to the scattering pattern are then recorded for subsequent analysis.

These numerical scattering values are then transformed, using an appropriate optical model and mathematical procedure, to yield the proportion of total volume to a discrete number of size classes forming a volumetric particle size distribution (e.g., x50 describes a particle diameter corresponding to 50 per cent of the cumulative undersize distribution).

Reporting of Results
The distribution statistics are usually reported by the instrument data system. Most common parameters are calculated from the cumulative distribution. Percentile sizes, dm, represent the particle size in relation to which m percent of the distribution is smaller. Qy represents the percent smaller than y microns. Mean sizes, such as D4,3, the arithmetic volume mean diameter, can also be calculated by representing the distribution as a collection of spherical particles with diameters of the size band midpoints. Size distributions are of three types Number, Area and Volume distributions.

Number distribution: D[1,0] = S d/n
Area distribution: D[3,2] = S d3/ S d2
Volume distribution: D[4,3] = S d4 / S d3

Certified or standard reference materials consisting of a known distribution having a range of spherical particles over one decade of size are preferred for instrument validation. They are certified to mass percentage by an absolute technique, if available, and used in conjunction with an agreed, detailed operational procedure. The response of a laser diffraction instrument is considered adequate if the mean value of x50 obtained from at least three independent measurements does not exceed the certified range of values of the certified or standard reference material by more than 3 per cent. The mean values for x10 and x90 must not exceed the certified range of values by more than 5 per cent. For repeatability, the coefficient of variation must be less than 3 per cent for x50 and less than 5 per cent for x10 and x90. Below 10 m, these maximum values are doubled. In most of the analytical methods, volume distribution is stated. D10, D50, D90 and Mean volume are the parameters will be interested. 10 per cent, 50 per cent and 90 per cent of material sizes will be controlling parameters for material quality. Specifications also will be set for these parameters. Lower and upper 10 per cent size will be always neglected considering the process variation.

Industries Applications
Knowledge of particle sizes and the size distribution of a powder system is a prerequisite for most production and processing operations. Particle size and size distribution have a significant effect on the mechanical strength, density, and electrical and thermal properties of the finished object. Significant production losses can be incurred due to high rejection rates if size and size distribution of powders being used in a process are not adequately controlled. In most instances powder suppliers provide size and size distribution information, but that information needs to be checked for quality control purposes.

Organic pigments are used to colour organic coatings and indeed they are major constituents of printing and packaging inks. They are commonly supplied in powder form, the grain size distribution of which can be determined directly by laser diffraction using dry powder feed. This is advantageous in monitoring dry milling procedures used to reduce lump dry press cake to fine powder, and for comparison and matching of different types of milling equipment. Grain size distribution can affect dispersibility of a pigment in application media, especially premix dispersion in ink media. Catalysts used in the refining industries are characterized for its size, shape and specific surface area to control its performance.

A distribution of particle sizes (PSD) is a fundamental characteristic of cement powder. Accurate PSD`s are required in computational efforts to model the hydration process and it is an important practical issue for the cement industry. The most important property of Portland cement that is likely to vary is the strength of concrete produced from it. Cement-induced concrete strength variability can be due to both physical and chemical causes. The main physical cause of strength variability ?? changes in cement particle size. The fineness to which the cement is ground will evidently affect the rate at which concrete strengths increase after mixing. Size analysis in Fillers industries viz, Carbon black, Silica, Clay etc is one of the important material characterisations. These fillers are used in polymer industries as major ingredients in the polymer composite for reinforcement purpose. The extent of polymer reinforcement depends on filler particle size. Higher size leads to poor reinforcement. Lower size leads to more energy for mixing and poor dispersion in the polymer matrix. Hence, optimum size needs to be controlled during the manufacturing. Zeolites have important industrial applications including use as catalysts, molecular sieves and ion exchange materials. Microporous aluminosilicate zeolites have widespread industrial application. Traditional uses include: as a catalyst for cracking of high molecular weight hydrocarbons to shorter chain hydrocarbons in the petrochemical industry. Its efficiency depends on particles size and its distribution. In petrochemical industries, Catalysis could play an important role in conversion of natural gas. Drivers for development of advanced catalysts include (i) production of high value products from inexpensive raw materials, (ii) energy-efficient chemical conversion processes, (iii) increasingly stringent environmental regulations, and (iv) low cost catalysts such as with reduction or replacement of precious metals. Most of these can be achieved by using nano catalysts.

Nano catalysts could be classified into four distinct types, namely: nano particulate, nano porous, Nano crystalline, and supra molecular catalysts. Particle size effects in nano catalysis are of growing interest. One goal of nano catalysis research is to understand how decreasing the size of catalytic particles alters the intrinsic catalytic performance beyond simply expanding surface area. In the case of semiconducting metal oxides and sulfides, when the size of the nano crystals is smaller than the Bohr excitation radius of the material, they exhibit properties, which are size dependent and distinct from the bulk. Versatility and effectiveness of nano catalysts for controlling the activity, selectivity, and lifetime of catalysts depends on its size and its distribution. These nano catalysts particle sizes are in the range of 50 nm to 150 nm. Metal particles stabilization through their dispersion on an oxide support. This approach has the benefit of providing ligandfree metal particles. Additionally, these supports provide large surface areas in the range of 100 to 300 m2/g, and metal loading of about 1 wt% with particles of about 2 nm in diameter, as currently used, which corresponds to having metal particles every 30nm. The support is usually an oxide such as alumina, silica, zirconia, ceria, mesoporous silica, organic-inorganic hybrid materials, polymers, etc.

Petroleum coke, as one of byproducts of the oil refining industry, has a significantly increasing production. Petroleum coke, consisting of hydrocarbons, has the following main characteristics: high carbon content, high calorific value, and low ash content. Affected by the rise of coal prices and the reduction of coal production, petroleum coke has already become a popular fuel for power generation in power plants, and begins to be used as a potential fuel for gasification. Purified terephthalic acid (PTA) is one of the most important chemical materials. It is widely used in the manufacturing of audio films, polyester fibers, molded resins and polyethylene terephthalate (PET) bottles. Particle size of PTA influences the final properties of various materials. Its size ranges from 20 m to 150 m and these values are being used as quality control parameter of PTA materials. Size characterization of materials in various industries is inevitable. There is a need to have faster, accurate, precise technique with easy interpretation of results. Laser diffraction based particle size analysis meets the industries requirement and it is well accepted across the industries.