This drying method consists in drying a product by putting it in contact with heated walls. Most of the time the drying happens in rotating drums for a better homogeneity. This technology is mostly used in heavy industry drying processes.
Infrared drying evaporate water or solvent at high temperature. It can also be coupled with hot air and is used when the drying effect must be concentrated on the surface of the treated product to also obtain a roasting effect.
Freeze-drying process is based on the water sublimation effect in a low temperature and low pressure environment. It is often recommended for the drying of temperature sensitive food products, as it preserve most of the organolpetic properties.
This technology consists of mixing air and solid particles so that the whole behaves like a fluid. It is mainly used to dry seeds or grainy materials. This drying method is most of the time really quick.
Last but not least, the drying by microwave and radio frequency is based on the dielectric drying technology. By heating the entire volume of the product simultaneously, it is particularly recommended in applications where homogeneous and gentle drying is required.
As most of the food ingredients in the food industry, powders can be contaminated by microorganism like molds, or bacteria, which could result in safety and quality concerns for industrials and consumers. Discover how to overcome this issue with our microwave and radio frequency solutions.
True of all process facilities where drying plays an important role, such as those in the chemical, pharmaceutical, plastics, and food industries, the purchase of improper industrial drying equipment can significantly affect a manufacturer's economic bottom line. With the guidance offered in this book, engineers will be able to confidently choose industrial drying equipment that increases profits, runs efficiently, and optimally suits their needs.
Adjustment and control of moisture levels in solid materials through drying is a critical process in the manufacture of many types of chemical products. As a unit operation, drying solid materials is one of the most common and important in the chemical process industries (CPI), since it is used in practically every plant and facility that manufactures or handles solid materials, in the form of powders and granules.
The effectiveness of drying processes can have a large impact on product quality and process efficiency in the CPI. For example, in the pharmaceutical industry, where drying normally occurs as a batch process, drying is a key manufacturing step. The drying process can impact subsequent manufacturing steps, including tableting or encapsulation and can influence critical quality attributes of the final dosage form.
Apart from the obvious requirement of drying solids for a subsequent operation, drying may also be carried out to improve handling characteristics, as in bulk powder filling and other operations involving powder flow; and to stabilize moisture-sensitive materials, such as pharmaceuticals.
This article provides basic information on the sometimes complicated heat- and mass-transfer processes that are important in drying, and discusses several technologies used to accomplish the task.
Drying may be defined as the vaporization and removal of water or other liquids from a solution, suspension, or other solid-liquid mixture to form a dry solid. It is a complicated process that involves simultaneous heat and mass transfer, accompanied by physicochemical transformations. Drying occurs as a result of the vaporization of liquid by supplying heat to wet feedstock, granules, filter cakes and so on. Based on the mechanism of heat transfer that is employed, drying is categorized into direct (convection), indirect or contact (conduction), radiant (radiation) and dielectric or microwave (radio frequency) drying.
Heat transfer and mass transfer are critical aspects in drying processes. Heat is transferred to the product to evaporate liquid, and mass is transferred as a vapor into the surrounding gas. The drying rate is determined by the set of factors that affect heat and mass transfer. Solids drying is generally understood to follow two distinct drying zones, known as the constant-rate period and the falling-rate period. The two zones are demarcated by a break point called the critical moisture content.
In a typical graph of moisture content versus drying rate and moisture content versus time (Figure 1), section AB represents the constant-rate period. In that zone, moisture is considered to be evaporating from a saturated surface at a rate governed by diffusion from the surface through the stationary air film that is in contact with it. This period depends on the air temperature, humidity and speed of moisture to the surface, which in turn determine the temperature of the saturated surface. During the constant rate period, liquid must be transported to the surface at a rate sufficient to maintain saturation.
At the end of the constant rate period, (point B, Figure 1), a break in the drying curve occurs. This point is called the critical moisture content, and a linear fall in the drying rate occurs with further drying. This section, segment BC, is called the first falling-rate period. As drying proceeds, moisture reaches the surface at a decreasing rate and the mechanism that controls its transfer will influence the rate of drying. Since the surface is no longer saturated, it will tend to rise above the wet bulb temperature. This section, represented by segment CD in Figure 1 is called the second falling-rate period, and is controlled by vapor diffusion. Movement of liquid may occur by diffusion under the concentration gradient created by the depletion of water at the surface. The gradient can be caused by evaporation, or as a result of capillary forces, or through a cycle of vaporization and condensation, or by osmotic effects.
The capacity of the air (gas) stream to absorb and carry away moisture determines the drying rate and establishes the duration of the drying cycle. The two elements essential to this process are inlet air temperature and air flowrate. The higher the temperature of the drying air, the greater its vapor holding capacity. Since the temperature of the wet granules in a hot gas depends on the rate of evaporation, the key to analyzing the drying process is psychrometry, defined as the study of the relationships between the material and energy balances of water vapor and air mixture.
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 to increase, indicating the completion of drying at a 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.
Adiabatic dryers are the type where the solids are dried by direct contact with gases, usually forced air. With these dryers, moisture is on the surface of the solid. Non-adiabatic dryers involve situations where a dryer does not use heated air or other gases to provide the energy required for the drying process
Vacuum dryers. Vacuum dryers offer low-temperature drying of thermolabile materials or the recovery of solvents from a bed. Heat is usually supplied by passing steam or hot water through hollow shelves. Drying temperatures can be carefully controlled and, for the major part of the drying cycle, the solid material remains at the boiling point of the wetting substance. Drying times are typically long (usually 12 to 48 h).
Fluidized-bed dryers. A gas-fluidized bed may have the appearance of a boiling liquid. It has bubbles, which rise and appear to burst. The bubbles result in vigorous mixing. A preheated stream of air enters from the bottom of the product container holding the product to be dried and fluidizes it. The resultant mixture of solids and gas behave like a liquid, and thus the solids are said to be fluidized. The solid particles are continually caught up in eddies and fall back in a random boiling motion so that each fluidized particle is surrounded by the gas stream for efficient drying, granulation or coating purposes. In the process of fluidization, intense mixing occurs between the solids and air, resulting in uniform conditions of temperature, composition and particle size distribution throughout the bed.
Freeze dryers. Freeze-drying is an extreme form of vacuum drying in which the water or other solvent is frozen and drying takes place by subliming the solid phase. Freeze-drying is extensively used in two situations: (1) when high rates of decomposition occur during normal drying; and (2) with substances that can be dried at higher temperatures, and that are thereby changed in some way.
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