Feature and Application of Spray Drying Process

Spray drying has been employed in the food industry for concerning one hundred fifty years and is responsible for creating a number of the foremost essential ingredients and product within the food industry today—such as milk, instant low, and fine-grained flavors. Learn the key steps within the spray drying method, the highest advantages of this method, and the variables you should know for creating the perfect powder.

We encounter spray dried ingredients and food products all the time. Whenever a liquid has been converted to a shelf-stable powder, there’s a more better chance that spray drying was used. The most usually spray dried foods include:

  • Milk Powders
  • Dried Eggs
  • Instant Coffees
  • Instant Teas
  • Dried Fruit Juices
  • Honey Powders
  • Molasses Powders
  • Powdered Flavors

While there are many drying processes on the market for food, the spray drying method is distinguished by its unique equipment that enables for speedy drying with minimal heat exposure. In spray drying, a liquid is sprayed through atomizer into a chamber that contains streams of hot air. The moisture quickly evaporates, leaving behind solid powder particles that fall to bottom of the chamber.

Spray drying is ideal for heat-sensitive materials and whenever a free-flowing, uniform powder is needed. Whereas alternative drying techniques generally produces flakes that then should be ground to size, spray dryers create a free-flowing powder with a slim size distribution, making a subsequent grinding step unnecessary. Furthermore, spray drying is method of choice for commercial-scale encapsulation applications and is utilized to encapsulate flavors, carotenoids, and lipids.

The benefits of spray drying are:

  • Appropriate for heat-sensitive foods
  • Manufactures fairly uniform particle sizes
  • Makes a free-flowing powder
  • Efficient at encapsulation

Spray drying is a unique method of drying that depends on atomization to create a uniform, free-flowing powder and permits heat exposure to be kept to a minimum. The spray drying process consists of the following steps:

  • Preparation of the liquid or slurry
  • Adding the liquid feed to the spray dryer
  • Atomization of the liquid feed to create droplets
  • Drying of the droplets in a heated air stream
  • Collection of the dried particles

Two Characteristics of Spray Drying are:

  • Atomization
  • Drying Kinetics

Atomization:

Atomization is the distinguishing feature of the spray drying process and plays a critical role in determining the quality of the finished product. It involves generating a vast range of droplets from a liquid stream, thus greatly increasing its surface area and allowing faster drying rate. For example, 1 m3 of a liquid can form 2×1012 droplets with a surface area of 60,000 m2.

Atomization can be accomplished through single-fluid nozzle, two-fluid nozzle, or rotary disc atomizers which manufacture droplet sizes from 10 to 500 µm (ideally 100 to 200 µm), depending on the feed consistency and composition.

When the atomized droplets come in contact with the heated air currents entering the chamber, a series of simultaneous heat and mass transfer processes takes place. Heat is transferred to the product to evaporate moisture, and mass is transferred as a vapor into the surrounding gas.

Drying Kinetics:

The drying method can be described as having two phases: the constant-rate period and the falling-rate period. In the constant-rate period, moisture evaporates quickly from a saturated surface via diffusion through the stationary air film at a rate sufficient to maintain saturation. In the falling-rate period and as moisture removal progresses, the solute dissolved in the liquid reaches a concentration beyond its saturation concentration to form a thin shell at the droplet surface.

Kinetically, this stage marks a transformation from low- to high-temperature drying. Following this, and depending on inlet temperature, feed consistency, and atomization variables, the droplets may follow one of two principal pathways, creating either small-dense or large-hollow particles. Dried particles are recovered with separation devices such as cyclones and bag filters or are scrubbed for further collection followed by cooling and packaging.

A number of variables influence the characteristics of the finished powder, including feed properties, type of atomizer used, and airflow factors

Optimizations of these variables are often achieved through a good understanding of the spray drying method to produce particles free of imperfections and with the required properties.

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Microwave Technology for Dehydration

Dehydrating is one amongst the most common processes in industry. This executed is enforced by numerous techniques, like freeze-drying. It’s an energy-consuming method. Microwave sources are a decent option to provide the energy dehydration for this method. In reality, it’s microwave-assisted dehydration. The microwave sources will be delivered around some kilowatts. Electromagnetic energy is transformed into thermal energy because of the interaction of electromagnetic fields and materials.

In addition to providing energy, the microwave-assisted dehydration is time-saving. This technique is quick because of penetrating electromagnetic fields within the material. It leads to volumetrically heating rather than heating from the surface of the material in standard ways. Usually, the frequency of electromagnetic fields is 2450 MHz that is allotted by regulatory commissions in dielectric heating ways. Within the following, the mechanism of this technique is represented. All relations governing the transfer of mass and heat are mentioned. The way to transfer and dissipate energy is represented. Dielectric properties of various materials are listed. The effective parameters in crucial insulator properties are mentioned.

Population growth of human societies leads to increasing the demand for needs like food, clothing, housing, etc. Meeting them needs new industrial ways alternatively traditional ones. Manufacturing of foodstuff is contains in this principle. Today, totally different processes are being done on the mineral, vegetable, and animal product. A number of them are pasteurization, sterilization, conservation, etc. every of them is employed for a selected purpose. Throughout these processes, physical, chemical, and biological changes occur. They have an effect on the standard of foodstuff (color, flavor, volume).

Drying is the most typical method to increase the life of food product to form them easier to keep up. Meanwhile, microwave technology has achieved a major position among alternative ways in food business. Not only is that this technique utilized in food business however additionally in pharmaceutical business and medical sciences, for removing water from aqueous solutions and conserving the blood, bone, and skin.

In conventional technique for drying foodstuff, it’s heated, sometimes by flowing hot air, to evaporate its moisture. Also, the heating may be done by alternative ways from direct solar radiation to using microwave energy. In dehydration technique, removing the moisture content of material is completed by sublimation of water molecules with internal heating once freezing the material and making a vacuum. Compared with typical ways, it causes little irreversible changes in food and so keeps the standard of product at a superb level. Rehydration, color (browning), and volume (volume reduction and consequently shrinkage) are key parameters in crucial the standard of foodstuff and are thought-about in. low temperature during this technique helps to prevent most biological reactions, and therefore it’s appropriate for dehydrating heat-sensitive material like biological product. However, this technique is pricey. it’s appropriate for valuable foodstuffs like coffee.

Microwave energy is utilized to defrosting meat. It reduces the desired time from hours to a couple of minutes. Also, it’s utilized in sterilizing some heat-sensitive foods and cacao bean roasting.

Considered the conventional and microwave-assisted freeze-drying method. It showed that the drying time is less than 20% for microwave-assisted freeze-drying method because of volumetrically heating in this method.

Drying (or dehydrating) is removing moisture content from a material. This phenomenon, that needed phase transition in water content of material, needs plenty of energy.

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Different Types of Annealing Techniques

Annealing is a heat treatment method that alters the microstructure of a material to alter its mechanical or electrical properties. Typically, in steels, annealing is employed to reduce hardness, increase plasticity and help eliminate internal stresses. Annealing may be a generic term and should refer to subcritical, intermediate or full annealing in a very type of atmospheres.

The process of heating a metal or alloy to an acceptable temperature for an explicit amount of time and so slowly cooling (generally with the chamber cooling) is termed annealing.

The essence of annealing is that the transformation of the pearlite when heating the steel to austenitizing. Once annealing, the tissue is near to that after equilibrium.

Purpose of Annealing:

  • Reduce the hardness of steel, improve malleability, and facilitate machining and cold deformation process.
  • The chemical composition and organization of uniform steel, refining grain, to enhance the performance of steel or to arrange for extinction.
  • Eliminate internal stress and method hardening to stop deformation and cracking.
  • Annealing and normalizing are primarily used for making ready heat treatment.

For components with low stress and low performance, annealing and normalizing can even be used as final heat treatment.

According to the heating temperature, the commonly used annealing method is divided into:

Phase change recrystallization annealing above the critical temperature:

  • Complete annealing
  • Diffusion annealing
  • Incomplete annealing
  • Spherification annealing

Annealing below the critical temperature:

  • Recrystallization annealing
  • Stress annealing

The selection of the annealing method generally has the following principles:

  • The various steels of the hypoeutectoid structure usually choose complete annealing.
  • In order to shorten the annealing time, isothermal annealing will be used.
  • The spheroidizing annealing is mostly utilized in hypereutectic steel.
  • When the request isn’t high, you’ll opt for not to complete annealing.
  • Tool steel, bearing steel is usually used spheroidized annealing.
  • Cold extrusion and cold upsetting components of low carbon steel or medium carbon steel are typically used spherified annealing.
  • In order to eliminate the method hardening, recrystallization annealing is used.
  • So as to eliminate the interior stress caused by numerous process, stress annealing is used.
  • In order to enhance the inhomogeneity of the structure and chemical composition of high-quality steel, diffusion annealing is usually used.

Importance of Annealing:

Annealing is utilized to reverse the consequences of work hardening, which might occur throughout processes like bending, cold forming or drawing. If the material becomes too hard it can make working impossible or end in cracking.

By heating the material higher than the recrystallization temperature, it’s created a lot of ductile and thus able to be worked all over again. Annealing conjointly removes stresses that may occur once welds solidify. Hot rolled steel is additionally shaped and formed by heating it higher than the recrystallization temperature. Whereas steel and alloy steel hardening is common, alternative metals also can benefit from the method, like aluminium, brass, and copper.

Metal fabricators use annealing to help produce complicated components, keeping the material workable by returning them on the point of their pre-worked state. The method is vital in maintaining ductility and reducing hardness after cold working. Additionally, some metals are toughened to extend their electrical conduction.

Annealing with Alloys:

Annealing will be administered with alloys, with a partial or full toughen being the sole ways used for non-heat treatable alloys. The exception to this is with the 5000 series alloys, which might tends to low temperature stabillisation treatments.

Alloys are annealed at temperatures of between 300-410°C, depending on the alloy, with heating times starting from zero.5 to three hours, depending on the scale of the work piece and therefore the variety of alloy. Alloys ought to be cooled at a most rate of 20°C per hour till the temperature is reduced to 290°C, after that the cooling rate isn’t necessary.

Advantages:

The main benefits of annealing are in however the method improves the workability of a cloth, increasing toughness, reducing hardness and increasing the plasticity and machinability of a metal.

The heating and cooling method additionally reduces the bearableness of metals whereas enhancing their magnetic properties and electrical conductivity.

Disadvantages:

The main disadvantage with annealing is that it is a time intense procedure, counting on that materials are being annealed. Materials with high temperature necessities will take an extended time to cool down sufficiently, particularly if they’re being left to cool down naturally inside an annealing furnace.

Annealing is utilized across a range of industries wherever metals need to be worked into advanced structures or worked on many times.

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Moisture Analysis in the Pharmaceutical Industry

Moisture is a crucial parameter in the manufacture of bulk solid pharmaceuticals. The producing method of pricey medicine is usually sophisticated, and through a method that has crucial stages that occur over many days, quick and correct determination of moisture content is important.

Generally, moisture analysis needs to be performed in product and process development, as well as during manufacturing to specify and control the maximum allowable moisture content at each step. Knowing the moisture content at each step is part of the very careful process control required during manufacture.

Several drying ways are utilized for moisture analyses, including mathematical determination established on infrared detection and chemical titration. The Karl Fischer technique involves adding a reagent to the sample that reacts with the water present to manufacture a non-conductive chemical. However, this solely provides a reliable measure of moisture content if most of the moisture is due to water. A sample containing very little water, however high levels of alternative volatiles, can give a low moisture reading in a Karl Fischer titration, once in reality it still contains a major quantity of moisture.

There is a large list of properties of pharmaceutical product that are influenced by moisture content, and directly have an effect on how tablets are manufactured. This contains chemical stability, crystal structure, compaction, powder flow, lubricity, dissolution rate, and polymer film permeability.

The presence of moisture affects the consistency and stability of tablets. an excessive amount of moisture can cause an agglomeration of powder particles and a poor crumbly tablet; insufficient moisture can cause the tablet to fall apart. Fine-grained excipients might fail to flow if they’re too wet, and a few active pharmaceutical ingredients (APIs) would possibly crystallize or change shape if there’s an excessive amount of moisture. Solid dosage forms are created utilizing a vast range of processes as well as freeze drying, fluid bed drying, compaction, granulation, and extrusion. All of those operations rely on the quantity and the state of water present. Moisture can even influence the chemical/physical properties of individual active ingredients and excipients.

That is why it’s essential to analyze moisture content throughout manufacture and understand how moisture content affects every individual step throughout method development in order to establish specifications and parameter limits. 

The thermo gravimetric technique of moisture analysis is usually accepted as the most reliable. though numerous means of heating the sample have been utilized to try and improve accuracy, infrared radiation remains one among the most popular drying techniques.

On exposure of the sample to infrared radiation, the surface of the sample is heated 1st. The energy is then conducted from the surface through the complete volume of the sample. this point for the heat to conduct throughout the whole sample has been the limiting factor of standard infrared loss-on-drying moisture analyzers. If the sample has high dielectric properties, the drying time can increase. This result is compounded by the partial reflection of the infrared energy, preventing efficient heat transfer.

Since effective moisture determination depends on the speed at which measurements are obtained, this absorption delay makes it impossible to see the moisture content of high-moisture samples utilizing standard infrared loss-on-drying analyzers during a production atmosphere.

In addition, it’s impossible to make sure that the heat has effectively permeated through the whole sample since the temperature measuring is created in the cavity instead of rather than sample itself. This carries the chance that moisture remains in the middle of the sample, giving an underestimation of the moisture content. Conversely, the surface of the sample continues to be heated for the whole time required for the heat to be absorbed throughout the sample that might lead to scorching of those areas.

Furthermore, the analyzers can’t be used directly on the manufacturing line, because of the need for a fume hood to get rid of the water vapor and different volatiles. This necessitates a delay whereas samples are transported to the analyser, therefore any production parameters that need real-time feedback won’t be optimally controlled, probably impacting product quality and variability.

Heating utilizing halogen components has been adopted in preference to standard infrared loss-on-drying moisture analyzers since the best heating temperature may be achieved more quickly. Halogen drying, however, still carries the danger of uneven heat exposure which may undermine the results obtained.

For manufacturers, it’s essential to think about the impact of moisture in bulk materials additionally because the finished product. Moisture content fluctuates from batch-to-batch, and to attain consistency in formulation there should be a reliable technique to see moisture content accurately. To be effective, moisture determination strategies should be quick, repeatable, and precise.

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Drying of Food Materials by Microwave Energy

Microwave energy has very successful application in the field of food processing particularly for food drying to preserve the quality of the precious food materials. Various food materials dried using microwave energy was extensively reviewed. Microwave drying appears to be a viable drying method for the rapid drying of food materials. It was noticed that at the higher microwave output power considerably lower drying time took place. The application of pulsed microwave energy was found more efficient than the continuous application. The microwave-vacuum drying could reduce drying time of vegetable leaves by around 80-90%, compared with the hot air drying. Microwave drying maintained a good green color close to that of the original fresh green leaves with surface sterilization in most of the vegetables. The microwave heating of vegetable seed reduces the moisture content and anti-nutritional factor with maintaining the natural color of the valuable seed.

Drying is the oldest and traditional methods of food preservation and is the most widely used technique of preservation, which converts the food into light weight, easily transportable and storable product. Although the origin of drying goes back to antiquity, there is a constant interest and technological improvements in the process of drying keeping this mode of preservation still as new. The specific objective of drying is to remove moisture as quickly as possible at a temperature that does not seriously affect the quality of the food. Drying can be accomplished by a number of traditional and advanced techniques.

Microwave heating is based on the transformation of alternating electromagnetic field energy into thermal energy by affecting the polar molecules of a material. Many molecules in food (such as water and fat) are electric dipoles, meaning that they have a positive charge at one end and a negative charge at the other, and therefore, they rotate as they try to align themselves with the alternating electric field induced by the microwave rays. The rapid movement of the bipolar molecules creates friction and results in heat dissipation in the material exposed to the microwave radiation. Microwave heating is most efficient on water (liquid) and much less on fats and sugars which have less molecular dipole moment.

In drying of food materials, the aim is to eliminate moisture from food materials without affecting their physical and chemical structure. It is also important to preserve the food products and increase their storage stability which can be accomplished by drying. Microwave drying is a newer addition to the family of dehydration methods.

In Microwave drying tomato slice was sampled, from the starting of the drying the change in the sample weight was recorded at the time intervals of 2 minutes. The drying tests were terminated when the moisture content indicated 10%. The final moisture content of each sample was measured in order to calculate the moisture content at each weighing interval. Among several subjective quality attributes of dried tomato slices the colour is an important one which indicates the level of effects of different drying methods or conditions.

Dried fruits are widely used as components in many food formulations such as pastry, confectionery products, ice cream, frozen desserts and yogurt. Among them, dried apples are a significant raw material for many food products. The drying process was progressed through two stages, in the first stage the samples were put in a microwave oven until drying took place mainly in constant rate period; approximately 55% of the water was removed in this period. After that the forced draft oven was used until the apple samples reached the final moisture content. The second stage, the apple samples were put in forced draft oven to reach the final moisture content. For one hour or two hours the value of the drying constant increased with increased microwave output power. The change in color values was dependent on the pretreatment. The 45% sugar solution showed decrease drying rate than the other treatment. The increasing on the density power (W/g) the drying rate increased by 35%.

Many new dimensions came up in drying technology to reduce the energy utilization and operational cost. Selective and volumetric heating effects, microwaves bring new characteristics such as increased rate of drying, enhanced final product quality and improved energy consumption. Combination drying with an initial conventional drying process followed by a microwave finish or microwave vacuum process has proven to reduce drying time while improving product quality and minimizing energy requirements. However, several factors should be taken into consideration when developing drying system for the fruits and vegetables.

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Fundamentals of Thermal Radiations

Consider a hot object that is suspended in an evacuated chamber whose walls are at room temperature. The hot object will eventually cool down and reach thermal equilibrium with its surroundings. This mechanism is radiation. Radiation transfer occurs in solids as well as liquids and gases. But heat transfer through an evacuated space can occur only by radiation. For example, the energy of the sun reaches the earth by radiation. It is interesting that radiation heat transfer can occur between two bodies separated by a medium colder than both bodies. The theoretical foundation of radiation was established in 1864 by physicist James Clerk Maxwell, Who postulated that accelerated charges or changing electric currents give rise to electric and magnetic fields. These rapidly moving fields are called electromagnetic waves or electromagnetic radiation.

Thermal Radiation

Thermal radiation is electromagnetic radiation generated by the thermal motion of particles in matter. All matter with a temperature greater than absolute zero emits thermal radiation. Particle motion results in charge-acceleration or dipole oscillation which produces electromagnetic radiation.

Infrared radiation emitted by animals (detectable with an infrared camera) and cosmic microwave background radiation are examples of thermal radiation.

If a radiation object meets the physical characteristics of a black body in thermodynamic equilibrium, the radiation is called blackbody radiation.[1] Planck’s law describes the spectrum of blackbody radiation, which depends solely on the object’s temperature. Wien’s displacement law determines the most likely frequency of the emitted radiation, and the Stefan–Boltzmann law gives the radiant intensity.

Thermal radiation is also one of the fundamental mechanisms of heat transfer. That is, everything around us such as walls, furniture, and our friends constantly emits (and absorbs) radiation. The type of electromagnetic radiation that is pertinent to heat transfer is the thermal radiation emitted as a result of energy transitions of molecules, atoms, and electrons of a substance. Thermal radiation is continuously emitted by all matter whose temperature is above absolute zero.

Thus, thermal radiation includes the entire visible and infrared (IR) radiation as well as a portion of the ultraviolet (UV) radiation.

There are 4 main properties that characterize thermal radiation:

  • Thermal radiation emitted by a body at any temperature consists of a wide range of frequencies. The frequency distribution is given by Planck’s law of black-body radiation for an idealized emitter.
  • The dominant frequency (or color) range of the emitted radiation shifts to higher frequencies as the temperature of the emitter increases.
  • The total amount of radiation of all frequency increases steeply as the temperature rises; it grows, where the absolute temperature of the body.
  • The rate of electromagnetic radiation emitted at a given frequency is proportional to the amount of absorption that it would experience by the source, a property known as reciprocity. Thus, a surface that absorbs more red lights thermally radiates more red lights.

Thermal radiation is one of the three principal mechanisms of heat transfer. It entails the emission of a spectrum of electromagnetic radiation due to an object’s temperature. Other mechanisms are convection and conduction.

Radiation heat transfer is characteristically different from the other two in that it does not require a medium and, in fact it reaches maximum efficiency in a vacuum. Electromagnetic radiation has some proper characteristics depending on the frequency and wavelengths of the radiation. The phenomenon of radiation is not yet fully understood.

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Importance of Drying in Agricultural Produce

In most Indian countries, agriculture is that the engine of economic growth. However growing crops is just a part of the story when it comes to making money and feeding the family. What happens after food cultivation and harvest is simply as important. Profit is all regarding value addition –Processing your crops or livestock into one thing that matches a niche market.

One way to add value to agricultural manufacture is to dry it. With the correct equipment and information, manufacture can be sold locally or perhaps internationally for a decent value. However there’s a secret to good drying. It must be done effectively, thoroughly, and losing minimal vitamins that are so necessary in our food and that of our livestock.

A lot of the food that small-scale farmers grow will go to waste within the market because of surpluses at harvest time. Thus drying will be a very good method of food and nutrient preservation, additionally as the way of increasing value and ultimately profits. But drying fruits, vegetables, meat and fish isn’t without risk. In contrast to alternative food products, dried produce will be eaten raw. And since they’re raw, they are a source of food-borne infection. Thus hygiene at each stage – handling, drying, sorting and packaging – is critically necessary.

Some of the health and hygiene issues once getting ready and drying agricultural turn out area unit lined during this pack. Victimization poached water is very important once laundry foods before cutting and cookery them. You ought to check that that you just use separate areas for laundry raw turn out and cookery, as a result of germs will be simply transferred.

Sun drying

For centuries, sun drying has been a way of preserving and saving foods so that they can be eaten during times of scarcity. It is a cheap method of drying produce – sunlight is free and the only costs involve drying mats or platforms, and cutting equipment. But there are disadvantages too. The temperature of sunlight cannot be controlled, leading to over- or under-dried goods. The weather is not always sunny, and sometimes the drying process can take several days.

Solar drying

Another way of using the sun’s rays to dry food is solar drying. Instead of leaving produce in the heat to dry on its own, heat is trapped inside a box, and used to heat the produce enough to dry it, but not enough to cook it. Building a solar drier needs investment. Plastic sheets, wire mesh and wooden frames all need to be bought or found locally. But the technology is not expensive, and the value added to the produce could result in a good profit.

Pre-drying treatments

Before drying your produce, there are treatments that can be used to make the drying process more effective. Examples of natural ingredients used in pre-drying include soaking briefly in sugar or salt solution to reduce moisture content and prevent the growth of bacteria and fungi.

Preparing dried produce for export

There is a big international market for dried fruits and vegetables. But supermarkets have strict standards, which small-scale farmers often find too expensive to enforce, such as the European Retail Standards for Good Agricultural Practice, or EUREPGAP. Strict levels of hygiene, and accounting are important, so that those suppliers who buy dried products can see exactly how they were dried, sorted, and where they come from.

Selling dried products for sale locally

Any buyer or consumer will tell you that one of the most important things to consider when supplying to local markets, is local demand – what your customers want. This is especially true of dried produce, because while dried mango may be very popular in one area, for example, it may not be very popular in another area. If you cannot supply to local markets and you have already invested in solar drying technology, you may be disappointed. Before you grow or invest, do your research. When it comes to selling dried produce locally, you are in a better position than if you wanted to sell to export markets because you will have a good idea what people in your area like.

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Method for Moisture Analysis Techniques

Many industries, including the food, chemical, and pharmaceutical product industries, rely heavily on precise information of the moisture content of their products as a part of quality control. Such measurements required to be fast and reproducible in order to permit for immediate correction of the work flow if any flaw appears in the final product. Both direct and indirect methods are used. Only the direct methods actually measure the moisture content; the indirect techniques only calculate it from alternative indirect indicators.

Direct Methods :

Many devices for LOD measurement use the thermo gravimetric principle. This uses the total loss of weight incurred by the sample on drying to calculate the moisture content.

Drying ovens are based on convection heating of the sample by circulating hot air. Sometimes a vacuum is added to speed up the drying process.

Infrared radiation is used in many moisture analysers, such as halogen moisture analysers which are used to produce infrared radiation from a halogen lamp. The weight of the sample is measured and recorded continuously and once it becomes constant the drying is stopped.

Microwave radiation is also an extremely rapid method of drying up a sample but the temperatures achieved are very high, making it suitable only for very thermostable materials. Larger samples can be used but the level of control of heating is reduced. Like the infrared method, the sample is typically destroyed by the analysis. It is also not useful if the moisture content is below 2%.

Developing a Method :

In all these cases, a method must be developed. This refers to setting the parameters such as time, drying temperature, and the weight of the sample, according to the requirement for each sample. This can be saved as a program for the next measurement of a similar sample. This type of adjustment is necessary to achieve values which agree with those set by the reference methods, such as the oven drying method or the Karl Fischer titration method, whichever is used. In other cases, a deviation is acceptable as long as it is measured and repeatable with each measurement.


Other Physical Methods :

Phosphorous Pentoxide Method – In this method, phosphorous pentoxide is placed with the sample in a closed container which is then heated. Phosphorous pentoxide is a powerful desiccating agent if placed in proximity to materials with which it is chemically non-reactive. It absorbs water from the sample. The final increase in weight of the chemical is measured to give the moisture content of the desiccated sample. Phosphorous pentoxide is a dangerous chemical.

Distillation – Another method of moisture determination is inexpensive but uses toxic solvents while yielding only relatively accurate results. It is based upon separating and measuring the moisture directly after separating it from the sample by heating.

Chemical Methods for Moisture Analysis :

Karl Fischer Titration – The most accurate and specific method for determining the water content of a substance is Karl Fischer (KF) titration. It is based upon the reaction of iodine with sample water, in presence of alcohol solvent, sulfur dioxide, and a base. It uses up the total sample water, including the water of crystallization and surface absorbed water, in the redox reaction.

Calcium Carbide Method – The calcium carbide method is cost-effective and uses a combination of materials to react with water. The end product is potentially explosive, and so the method requires great care. Moreover, the total water in the sample does not take part in the reaction and this means that repeated calibration is a necessity.

Other Methods :

These include gas chromatography, density determination, and refractometry.

Indirect Methods :

These are based upon taking measurements of moisture in different grains of the sample by a moisture sensor, for example, which are then used to calculate the moisture content of the sample. An advantage is that instant measurements are taken and thus changes in the moisture content can be monitored over time.

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Different Type of Drying Methods for Powder

Typically, powder-drying operations involve the application of heat to a solution, wet powder or slurry. Bulking and packaging of the dried powder usually follows. Common dryer types include tray, fluidized bed, and spray, rotary and vacuum dryers. In any type of dryer, the dry powder can build up as a bulk or layer in various locations within the dryer, or in downstream process equipment or ultimately in hoppers, silos, big bags or smaller packages.

Safe Powder Drying

Evaluation of self-heating hazards of powders

The first step in ensuring safety from fires and explosions in drying operations is having a proper understanding of the thermal instability properties of the powder (including its potential for gas generation), dust cloud explosibility and gas flammability.

Powder self-heating hazards can occur when the temperature of the powder in bulk or layer is raised to a level at which the heat generation rate by the exothermic reaction exceeds the rate of heat lost to the surroundings. Temperature increase follows, which frequently results in smoldering and eventually fire. As previously mentioned, several factors can affect the onset temperature for self-heating. Other variables such as air flow through the bulk powder or over the powder surface also influence the transition from smoldering to glowing and flaming as well as the onset temperature for self-heating.

Isothermal Basket Test:

Measurement of exothermic activity involves heating the sample under controlled conditions to determine the point at which its temperature starts to increase independently of the external heat source.

Isothermal basket testing is performed by heating the powder samples in cubical wire baskets of varying sizes (typically three sizes) in an oven to determine the minimum temperature at which each sample size self-heats. This test allows one to observe the effect of scale (that is, powder size/quantity) on the powder’s onset temperature for self-heating more precisely. Each trial involves placing a stainless-steel mesh basket filled with the powder sample in an oven, which is heated and maintained at a preselected temperature until self-heating is detected or for a duration of 24 hours or longer depending on the actual heating or storage cycle under study, whichever occurs first. The trials are repeated at various temperatures and basket sizes until the sample’s minimum onset temperature for self-heating is determined for each basket size.

Bulk Powder Test:

A bulk powder test is used to evaluate self-heating properties of powder in quantities not exceeding 1 ton in situations when it is heated in bulk form. Examples include powder accumulations in bulk in some dryers, hoppers, silos or packaging.

A glass cylinder with a height of 80 mm and diameter of 50 mm, which is closed at the base by a sintered glass, is filled with the test powder and placed in a uniform temperature oven. The temperature of the oven as well as the powder temperature at four different heights within the glass cylinder is monitored.

Aerated Powder Test:

The aerated powder test simulates conditions during drying (heating) operations of powder in quantities not exceeding 1 ton in which a hot air stream flows through the bulking powder — for example, in fluid bed drying.

The glass test cell used for this test is identical to the bulk powder test, however, in the aerated powder test, an air stream, which is at the same temperature as the oven temperature, flows at a rate of 0.6 1/min through the sample during the entire test cycle. As in the bulk powder test, the sample temperature is measured at several locations in the cell to detect any exothermic activity and the activity’s onset temperature.

Powder Layer Test:

The powder layer test (also called air-over-layer test) simulates the conditions in which hot air passes above a layer or deposit of powder in a dryer. Examples include tray dryers and powder deposits on the internal surfaces of all dryer types.

Precautions for avoiding smoldering, fires and explosions include:

  • Keeping the powder temperature at a safe margin below the temperature for the onset for self-heating obtained by appropriate test methods.
  • Facility and equipment design should avoid ledges, corners, dead zone, etc., where powder could inadvertently build up inside process equipment.
  • Avoid accumulation of hazardous levels of powder deposits on the inside surfaces of process equipment.

The prevention of thermal runaway in drying operations can be achieved through careful application of the tests described in this article; they form an important route to achieving safe powder drying. Of course, fire and explosion protection measures and measures to avoid other sources of ignition must also be examined through good process safety practices to achieve safe plant operation.

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Types and Methods of Wood Drying

Wood drying (also seasoning lumber or wood seasoning) reduces the moisture content of wood before its use. When the drying is done in a kiln, the product is known as kiln-dried timber or lumber, whereas air drying is the more traditional method.

There are two main reasons for drying wood:

Woodworking:

When wood is used as a construction material, whether as a structural support in a building or in woodworking objects, it will absorb or expel moisture until it is in equilibrium with its surroundings. Equilibration (usually drying) causes unequal shrinkage in the wood, and can cause damage to the wood if equilibration occurs too rapidly. The equilibration must be controlled to prevent damage to the wood.

Wood burning:

When wood is burned (firewood), it is usually best to dry it first. Damage from shrinkage is not a problem here, as it may be in the case of drying for woodworking purposes. Moisture affects the burning process, with unburnt hydrocarbons going up the chimney. If a 50% wet log is burnt at high temperature, with good heat extraction from the exhaust gas leading to a 100 °C exhaust temperature, about 5% of the energy of the log is wasted through evaporating and heating the water vapour. With condensers, the efficiency can be further increased; but, for the normal stove, the key to burning wet wood is to burn it very hot, perhaps starting fire with dry wood.

Types of Wood:

Wood is divided, according to its botanical origin, into two kinds: softwoods, from coniferous trees, and hardwoods, from broad-leaved trees. Softwoods are lighter and generally simple in structure, whereas hardwoods are harder and more complex. However, in Australia, softwood generally describes rain forest trees, and hardwood describes Sclerophyll species (Eucalyptus spp).

Wood – Water Relationship:

The timber of living trees and fresh logs contains a large amount of water which often constitutes over 50% of the wood’s weight. Water has a significant influence on wood. Wood continually exchanges moisture or water with its surroundings, although the rate of exchange is strongly affected by the degree to which wood is sealed.

Wood contains water in three forms:

Free water

The bulk of water contained in the cell Lumina is only held by capillary forces. It is not bound chemically and is called free water. Free water is not in the same thermodynamic state as liquid water: energy is required to overcome the capillary forces. Furthermore, free water may contain chemicals, altering the drying characteristics of wood.

Bound or hygroscopic water

Bound water is bound to the wood via hydrogen bonds. The attraction of wood for water arises from the presence of free hydroxyl (OH) groups in the cellulose, hemicelluloses and lignin molecules in the cell wall. The hydroxyl groups are negatively charged. Because water is a polar liquid, the free hydroxyl groups in cellulose attract and hold water by hydrogen bonding.

Vapour

Water in cell Lumina in the form of water vapour is normally negligible at normal temperature and humidity.

Drying defects

Drying defects are the most common form of degrade in timber, next to natural problems such as knots. There are two types of drying defects, although some defects involve both causes:

Defects from shrinkage anisotropy, resulting in warping: cupping, bowing, twisting, crooking, spring and diamonding.

Defects from uneven drying, resulting in the rupture of the wood tissue, such as checks (surface, end and internal), end splits, honey-combing and case hardening. Collapse, often shown as corrugation, or so-called wash boarding of the wood surface, may also occur (Innes, 1996). Collapse is a defect that results from the physical flattening of fibres to above the fibre saturation point and is thus not a form of shrinkage anisotropy.

The five measures of drying quality include:

  • Moisture content gradient and presence of residual drying stress (case-hardening);
  • Surface, internal and end checks;
  • Collapse;
  • Distortions;
  • Discolouration caused by drying.

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