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.
Mechanism of drying
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.
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. This type of instrument operates on the principle of electrical resistance, which varies minutely in accordance with the moisture content of the item measured. Dielectric moisture meters are also used. They 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 (RF) capacitance method. This type of instrument measures the loss, or change, in RF dielectric constant, which is affected by the presence or absence of moisture.
Types of dryers
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
Dryer classification can also be based on the mechanisms of heat transfer as follows:
• Direct (convection)
• Indirect or contact (conduction)
• Radiant (radiation)
• Dielectric or microwave (radio frequency) drying
Direct, or adiabatic, units use the sensible heat of the fluid that contacts the solid to provide the heat of vaporization of the liquid.
With adiabatic dryers, solid materials can be exposed to the heated gases via various methods, including the following:
• Gases can be blown across the surface (cross circulation)
• Gases can be blown through a bed of solids (through-circulation); used when solids are stationary, such as wood, corn and others
• Solids can be dropped slowly through a slow-moving gas stream, as in a rotary dryer
• Gases can be blown through a bed of solids that fluidize the particles. In this case, the solids are moving, as in a fluidized-bed dryer
• Solids can enter a high-velocity hot gas stream and can be conveyed pneumatically to a collector (flash dryer)
Non-adiabatic dryers (contact dryers) involve an indirect method of removal of a liquid phase from the solid material through the application of heat, such that the heat-transfer medium is separated from the product to be dried by a metal wall. Heat transfer to the product is predominantly by conduction through the metal wall and the impeller. Therefore, these units are also called conductive dryers.
Although more than 85% of the industrial dryers are of the convective type, contact dryers offer higher thermal efficiency and have economic and environmental advantages over convective dryers. Table 1 compares direct and indirect dryers, while Table 2 shows the classification of dryers based on various criteria.
|Table 1. Comparison of Direct and Indirect dryers |
|Property||Direct/adiabatic dryer (convective type)||Indirect/non-adiabatic contact dryer (conductive type)|
|Carrier gas||Uses sensible heat of gas that contacts the solid to provide the heat of vaporization of the liquid||Little or no carrier gas is required to remove the vapors released from the solids|
|Heat transfer||Heat transfer medium is in direct contact with the surface of the material to be dried||Heat needed to vaporize the solvent is transferred through a wall|
|Risk of cross contamination||Persists||Avoided, as the heat transfer medium does not contact the product|
|Solvent recovery||Difficult as there is a large volume of gas to be cooled to recover the solvent||Easier because of limited amount of non-condensable gas encountered|
|Operation under vacuum||Not possible||Allows operation under vacuum, ideal for heat sensitive materials|
|Dusting||High||Minimized because of small volume of vapors involved|
|Explosion hazard||Higher rate||Easier to control as vapors can be easily condensed|
|Handling of toxic materials||Not suitable||Suitable because of low gas flow|
|Energy efficiency||Significant energy lost through exhaust gas||Higher energy efficiency as the energy lost through the exhaust gas is greatly reduced|
|Evaporation and production rates||Higher than contact dryers||Drying rates are limited by heat transfer area, lower production rates|
|Cost||High||Higher initial cost; difficult to design, fabricate and maintain|
|Table 2. Classification of Dryers |
|Mode of operation||Batch Continuous*|
|Heat input type||Convection*, conduction, radiation, electromagnetic fields, combination of heat transfer modes Intermittent or continuous* Adiabatic or non-adiabatic|
|State of material in dryer||Stationary Moving agitated, dispersed|
|Operating pressure||Vacuum* Atmospheric|
|Drying medium (convection)||Air* Superheated steam Fluegases|
|Drying temperature||Below boling temperature* Above boiling temperature Below freezing point|
|Relative motion between drying medium and solids||Co-current Countercurrent Mixed flow|
|Number of stages||Single* Multistage|
|Residence time||Short (60 min)|
|* Most common in practice|
The following are descriptions of various types of batch dryers.
Tray dryers. This dryer type operates by passing hot air over the surface of a wet solid that is spread over trays arranged in racks. Tray dryers are the simplest and least-expensive dryer type. This type is most widely used in the food and pharmaceutical industries. The chief advantage of tray dryers, apart from their low initial cost, is their versatility. With the exception of dusty solids, materials of almost any other physical form may be dried. Drying times are typically long (usually 12 to 48 h).
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).
For more on vacuum drying, see Batch Drying with Vacuum Contact Dryers https://www.chemengonline.com/batch-drying-with-vacuum-contact-dryers-2/
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.
Microwave vacuum dryers. High-frequency radio waves with frequencies from 300 to 30,000 MHz are utilized in microwave drying (2,450 MHz is used in batch microwave processes). Combined microwave-convective drying has been used for a range of applications at both laboratory and industrial scales. The bulk heating effect of microwave radiation causes the solvent to vaporize in the pores of the material. Mass transfer is predominantly due to a pressure gradient established within the sample. The temperature of the solvent component is elevated above the air temperature by the microwave heat input, but at a low level, such that convective and evaporative cooling effects keep the equilibrium temperature below saturation. Such a drying regime is of particular interest for drying temperature-sensitive materials. Microwave-convective processing typically facilitates a 50% reduction in drying time, compared to vacuum drying.
Continuous dryers are mainly used in chemical and food industries, due to the large volume of product that needs to be processed. Most common are continuous fluid-bed dryers and spray dryers. There are other dryers, depending on the product, that can be used in certain industries — for example, rotary dryers, drum dryers, kiln dryers, flash dryers, tunnel dryers and so on. Spray dryers are the most widely used in chemical, dairy, agrochemical, ceramic and pharmaceutical industries.
Spray dryer. The spray-drying process can be divided into four sections: atomization of the fluid, mixing of the droplets, drying, and, removal and collection of the dry particles (Figure 2). Atomization may be achieved by means of single-fluid or two-fluid nozzles, or by spinning-disk atomizers. The flow of the drying gas may be concurrent or countercurrent with respect to the movement of droplets. Good mixing of droplets and gas occurs, and the heat- and mass-transfer rates are high. In conjunction with the large interfacial area conferred by atomization, these factors give rise to very high evaporation rates. The residence time of a droplet in the dryer is only a few seconds (5–30 s). Since the material is at wet-bulb temperature for much of this time, high gas temperatures of 1,508 to 2,008C may be used, even with thermolabile materials. For these reasons, it is possible to dry complex vegetable extracts, such as coffee or digitalis, milk products, and other labile materials without significant loss of potency or flavor. The capital and running costs of spray dryers are high, but if the scale is sufficiently large, they may provide the cheapest method.
For more on spray drying see Optimizing Analysis for Spray-Drying https://www.chemengonline.com/optimizing-analysis-for-spray-drying/
With increasing concern about environmental degradation, it is desirable to decrease energy consumption in all sectors. Drying has been reported to account for anywhere from 12 to 20% of the energy consumption in the industrial sector. Drying processes are one of the most energy-intensive unit operations in the CPI.
One measure of efficiency is the ratio of the minimum quantity of heat that will remove the required water to the energy actually provided for the process. Sensible heat can also be added to the minimum, as this added heat in the material often cannot be economically recovered. Other newer technologies have been developed, such as sonic drying, superheated steam, heat-pump-assisted drying and others.
Drying is an essential unit operation used in various process industries. The mechanism of drying is well understood as a two-stage process and depends on the drying medium and the moisture content of the product being dried.
Batch dryers are common in chemical and pharmaceutical industries, while continuous dryers are routinely used where large production is required. Since the cost of drying is a significant portion of the cost of manufacturing a product, improving efficiency or finding alternative drying routes is essential.
Edited by Scott Jenkins
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Dilip M. Parikh is president of the pharmaceutical technology development and consulting group DPharma Group Inc. (Ellicott City, MD 21042; Email: firstname.lastname@example.org). As an industrial pharmacist, Parikh has more than 35 years of experience in product development, manufacturing, plant operations and process engineering at various major pharmaceutical companies in Canada and the U.S. Prior to staring DPharma Group, he held the position of vice president of operations and technology at Synthon Pharmaceuticals in North Carolina and vice president and general manager at Atlantic Pharmaceuticals Services in Maryland. He is the editor of “Handbook of Pharmaceutical Granulation” 3rd ed. He has authored several book chapters and articles on various pharmaceutical technologies, including quality by design, process assessment and contract manufacturing. He has been an invited speaker at scientific conferences worldwide on solid-dosage technologies development and manufacturing.