Expanded Definition

WHAT IS CRYSTALLIZATION?

Crystallization is a complex but highly valued process in the petrochemical and manufacturing industries. Crystallization is described as the process of producing crystals through the precipitation of a solute from a solvent, a melt, or by being directly deposited from a gas. This can be accomplished through a variety of methods including, but not limited to, cooling, evaporation or chemical processes.
With the fast growth of the chemical industry in the last 100 years, the demand for the study of crystallization dynamics has increased greatly. Variations of this process have been used for thousands of years, and today many types of equipment are used depending on the product needed and the cost involved. From plastics, petrochemicals, pharmaceuticals, and food products, the process of crystallization is considered an important industrial process due to the large number of materials marketed as crystalline particles. Some of the many products which rely on crystallization in the manufacturing process include:

• Ascorbic Acid
• Bisphenol-A-Adduct
• Boric Acid
• Caffeine
• Calcium Chloride
• Potassium Phosphate
• Salicylic Acid
• Silver Nitrate
• Vitamin C
A sample of boric acid crystals.
The crystallization process is a type of unit operation that begins by dissolving a chemical compound into a given solvent creating a mixture called mother liquor. The solute substance is precipitated from the mother liquor under controlled conditions. In order to obtain effective control over the process, importance is placed on controlling retention time and crystal mass. The goals are to achieve the most favorable conditions for the development of crystal specific surface and the fastest possible growth.
The crystallization process is made up of two major events. These are referred to as the nucleation phase and the crystal growth phase. Nucleation is the step where the dissolved molecules which have been dispersed in the solvent begin to form into clusters that become stable under the current operating conditions. These stable clusters constitute the nuclei. Whenever these clusters remain unstable, they begin to redissolve. Therefore, the clusters need to reach a critical size in order to become stable nuclei. Such critical size is dictated by the operating conditions of the particular process used (temperature, pressure, supersaturation, etc.). It is at the stage of nucleation that the atoms become arranged in a defined and periodic manner that will define the crystal structure. Crystals are formed following a well-defined structure that is established by forces that work on the molecular level. As a consequence, during its formation process, the crystal is in an environment where the solute concentration reaches a certain critical value before changing phases.
The main factors influencing solubility are concentration and temperature. The formation of solid crystals, which cannot take place below the solubility threshold at the given temperature and pressure conditions, takes place at a concentration higher than the theoretical solubility level. This fundamental factor in the crystallization process, referred to as supersaturation, is the difference between the actual value of the solute concentration at the crystallization limit and the theoretical solubility threshold. Supersaturation is the main factor behind the initial nucleation step and the crystal growth that follows, neither of which can occur in saturated or undersaturated conditions.