Drying is one of the standard unit operations in chemical and pharmaceutical industries. During drying, evaporation energy is added to dry the material and to remove the solvent in a vapor phase.
Solids can have many different forms, flakes, granules, crystals, powders, etc. The liquid can be on the surface or within the surface in cellular structures, such as wood. Pure water can exist in three states, solid, liquid, and vapor. The state in which it is at any time depends on the temperature and pressure conditions. The removal of water as vapor from the material surface depends on the external conditions of temperature, air humidity and flow, area of exposed surface, pressure, and the movement of moisture within the solid, and is a function of the physical nature of solid, the temperature, and its moisture content.
The end of drying cycle depends on the type of water present in the substrate. Bound water is the minimum water held by the material that exerts an equilibrium vapor pressure less than the pure water at the same temperature. Unbound water is the amount of water held by the material that exerts an equilibrium vapor pressure equal to that of pure water at the same temperature. Drying rate is determined by the rate at which liquid on the material surface evaporates and rate at which the liquid moves from within the material to the surface to replace the evaporated liquid.
There are a number of approaches to determine the end of the drying process. The most common one is to construct a drying curve by taking samples during different stages of drying cycle against the drying time and establish a drying curve. When the drying is complete, the product temperature will start going up indicating the completion of drying at specific desired product moisture content. Karl Fischer titration and loss on drying (LOD) moisture analyzers are also routinely used in batch processes. The water vapor sorption isotherms are measured using a gravimetric moisture sorption apparatus with vacuum-drying capability.
For measuring moisture content in grain, wood, food, textiles, pulp, paper, chemicals, mortar, soil, coffee, jute, tobacco, rice, and concrete, electrical resistance type meters are used and operate on principle of electrical resistance, which varies minutely in accordance with the moisture content of the item measured. Dielectric moisture meters are also used, which rely on surface contact with a flat plate electrode that does not penetrate the product.
For measuring moisture content in paper rolls or stacks of paper, advanced methods include the use of the Radio Frequency Capacitance method. The instrument measures the loss, or change, in RF dielectric constant as affected by the presence of moisture.
Some researchers have used micro capsulated thermochromic pigments as heat indicators in a fluid bed drying process. As the product dries, the color of the pigments changes indicating the loss of moisture. Moisture measurement thus indicated by the heat indicator pigments. Near-infrared reflectance spectroscopy for on-line measurement of moisture content in fluidized beds has been successfully applied to the automatic control of fluid bed granulation by both fuzzy logic and neural network. Triboelectric probes have been used for real-time measurement of solid moisture content in fluidized beds.
The conventional control approach is not the most efficient as output variables do not always reflect exactly the target property (moisture or solvent level), leading to long cycle times and risks of product degradation (e.g. dehydration). Alternatively, off-line analyses (loss on drying, Karl-Fisher titrations, and gas chromatography) are more accurate but significantly increase cycle time. They require manual sampling of wet products during the drying phase, which leads to potential safety and industrial hygiene issues such as operator exposure to potent drugs.
With continuous dryers, in-line dryer monitoring using near-infrared (NIR) spectroscopy is becoming increasingly popular. Inferential measurements often rely on parameters like airflow, pressure, or inlet/outlet air or product temperature. In-line moisture measurement offers several advantages, such as reduced drying time, minimized product sampling, eliminating LOD testing, minimizing or eliminating reduction of worker exposure to hazardous material, greater batch-to-batch consistency, significant time and cost savings, and improvement in product quality.
Dilip M, Parikh is president of DPharma Group Inc., Ellicott City, MD, a pharmaceutical technology consulting firm. He is an industrial pharmacist with more than 35 years of industrial experience in R&D, manufacturing, cGMP-complaint facility planning, and operational management at various pharmaceutical companies in Canada and the U.S. The editor of the “Handbook of Pharmaceutical Granulation Technology”, 3rd Edition, Parikh has authored three other book chapters and a number of scientific publications, and has spoken worldwide at scientific conferences on solid dosage technology development and manufacturing.
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