Particle engineering technologies are usually used to enhance the performance of the particles that have production or performance challenges. By increasing the particle functionality scientists are able to overcome production issues and deliver a uniform product of a higher quality.
Particle engineering has a wide range of applications in various fields, Food, Feed, Cosmetics, Paints, and Pharma to name a few. Particle engineering has become the golden goose of industry because it offers both mathematical and empirical solutions to enhance product properties and the production process. For example in pharma, particle engineering is used to make great strides to obtain op- timal particle characteristics for pulmonary and oral solid-dos- age delivery. Homogeneity and size reduc- tion are key which is realized through various possible production techniques such as micronization, which reduces particles down to the micrometer or, in some cases, nanometer (1/1000 of a micrometer), is the most widely used size-reduction technique. Other technologies include spray drying, wet polishing, and supercritical fluid processing which for example is used to deliver drugs directly to the lungs, these recently fine-tuned techniques have made the pro- duction process as well as body absorption more effective. Micronization brings possible solutions as well as additional challenges. With smaller parti- cles the surface area increases thus creating the possibility for the smaller particles to interact with the moisture used in the production process, which leads to increased viscosity, which can create flow resistance, shear stress etc. The importance of particle size distribution is fundamental in creating product consistency. Consistent size and distribution improves dose uniformity, ho- mogeneity of blends and stabil- ity against segregation, stabilize mixtures, improve rheological (viscosity) characteristics of granules and semisolid formula- tions, reduce hygroscopicity of moisture absorbing materials, and ‘smooth out’ the granular material of solid particles.
Current particle engineering technologies include: fluid bed agglomeration, coating, encapsulation, blending, milling, and classifying. Through these technologies enhanced functionality of the particle production process leads to:
- Improved solubility and/ or dissolution
- Flowability improvement (free flowing)
- Homogeneity & stability against segregation
- Improved dose uniformity • Increasing bulk density
- Stabilization of mixtures • Reducing hygroscopicity
Particle engineering is the Golden Goose of industry, because particle engineering offers industry the possibility to tailor make particles. Particle engineering requires a deeper understanding of the particle formation process. Complex structured granules are difficult to design primarily because of the complexity in formula- tion process, that need to be tweaked in a correct manner to achieve the desired result. We briefly touched upon the various techniques. In this part of the article we will give an overview of the various tech- niques relating to spray-drying available in the particle engi- neering process.
Fluid Bed Agglomeration
The granulation of a mixture improves solids properties such as flowability, compressibility and plays a fundamental role in limiting segregation of the mixture. Fluid bed granulation is a wet granulation process that adds a specific liquid to base particles to form granules. The material is suspended in the fluid bed granulator by applying high-ve- locity cyclonic air flow current to the particles in order to setup a fluidized bed of material. The added liquid is sprayed upon the base particles in a timed manner which results in the solids sticking to each other, forming granules with a larger material size. At the end of the spraying pro- cess, the wet granule is dried within the fluid bed using hot air. During the spraying gran- ules will grow in size and sub- sequently reduce in size due to granules impacting each other, this leads to attrition, which can lead to a myriad of issues in the production and handling process such as a heterogenous particle size distribution, mate- rial waste due to the evapora- tion of smaller particles, dusti- ness et cetera.
Spray-drying is frequently used as an encapsulation method. The material to be encapsulat- ed, together with an amphipa- thic protein having both hydro- philic and hydrophobic aspects generally a starch derivative is homogenized as a suspen- sion. The slurry is then fed into a spray-drier, usually a tower heated well past 100 degrees Celsius. The slurry atomizes as it enters the tower, the small size of the droplets result in a relatively large surface area which dries the material in a relatively quick manner, as the water evaporates, the starch de- rivative forms a hardened shell around the material.
In certain cases spray-drying is used to blend materials, for example in the case of in- fant formula (baby milk). This process is known as the wet blending spray-drying process. During this process ingredients are blended together (minerals, lactose, vegetable oil, skimmed milk and whey protein), the mixture is then homogenized and subsequently pasteurized and spray dried to produce a powdered product. This process has the benefit of ensuring a uniform nutrient distribution throughout the product batch. Particle engineering requires a deeper understanding of the particle formation process. Particle engineering is about creating the optimum particle size and size distribution as well as other aspects of the particle’s morphology as well as specific surface characteristics and homogeneity. Micronization is the most widely used size-reduction technique. In this article we will address the various types of micronization and more importantly the Laws of comminution/ size reduction.
What is micronization?
Micronization is the technique of reducing the typical diameter of solid particles. There are various means of micronization such as mechanical means, i.e. milling, another method is through supercritical fluids and manipulate the principles of solubility. In this article however we will focus primarily on the mechanical means. The term micronization usually refers to the reduction of a solid to the micrometer range, but can also describe further reductions to the nanometer range. Common usages include the production of API’s (Active Pharmaceutical Ingredients, food and feed ingredients, as well as different types of chemicals. Mechanical micronization techniques are based on attrition to reduce particle size. These techniques are primarily based on techniques such as milling, crushing and grinding.
A mill is generally comprised of a cylindrical drum that usually contains spheres, and is able to grind, crush, or cut the solids material. As the drum rotates the spheres inside collide with the solid material, crushing them into the desired diameter range.
During the grinding process the solids are shaped by trapping the solids between grinding units which rub against each other, thus reducing the particle size.
Laws of comminution (size reduction)
It is almost impossible to find out the accurate amount of energy required in order to affect size reduction of a given material. This is because there is wide variation in size and shape of particles both in the process feed as well as with the base product, and second point is some energy is wasted as heat which cannot be determined exactly, therefore it’s impossible to accurately calculate the energy requirement for size reduction on a precise level. Through various formulas we are able to calculate the needed level of energy in size reduction, this however can only be calculated by approximation. The various laws do not take the mechanical losses of the grinder into account. There are three laws of comminution to be taken into account. The laws date back to the 19th and mid-20th century, yet are still widely used.
Kick’s and Rittinger’s law
Let’s start with the Kicks law, it can be applied to the crushing of solids and states that the amount of energy required to crush a given quantity of material to a specified fraction of its original size is the same, regardless of the original size. The second law we will address is Rittinger’s Law, it states that the energy required for size reduction is directly proportional, not to the change in length/ width dimensions, but to the change in surface area. It has been found, experimentally, that for the grinding of abrasive particles in which the increase in surface area is relatively small Kick’s Law is a reasonable estimate. However in the case of size reduction of fine powders, with large surface areas Rittinger’s Law fits the experimental data better and is therefore more accurate in regards to larger (newly created) surface areas. Both Kick’s as well as Rittinger’s law are determined experimentally, by running mill tests with the material to be crushed, they thus have limited application.
A more representative method for predicting power consumption in crushing as well as grinding was proposed by Bond in 1952. Bond’s law says that effort (energy) required to form particles from very large particle size is proportional to the square root of surface to volume ratio of the product. So here we have both surface as well as volume ratio and effort required for crushing the feed of large size to the required product size.
To calculate the energy requirements/ consumption needed to grind coarse sized solids materials such as gravel, Kicks law can apply and does suffice. If we deal with the cement or raw meal for example we can use Bond’s law because it is applicable at intermediate size and if we are dealing with pigments or very fine particle we can use Rittingers law, so these are the ranges of applicability of three laws of comminution/ size reduction.
Delft Solids Solutions is a privately-held contract research organization working on research and characterization of solid materials. Topics include, but are not limited to, primary physical properties as porosity, pore volume, pore size, surface area, particle size, density of powders and granules. We also investigate related characteristics such ...