Spray-Drying Innovative Use of an Old Process

 Spray-Drying
Innovative Use of an Old Process
May 1997 -- Design Elements

By: Ronald C. Deis, Ph.D.
Contributing Editor

  Food product developers come into daily contact with spray-dried products, sometimes without realizing it.   Spray-dried dairy products-- milk, whey, cheese, buttermilk, sodium caseinate, coffee whitener, butter, ice-cream mixes-- comprise a large industry. Many are sold commercially or used in commercial products. Spray-dried whole egg, egg yolk and albumen also are prevalent ingredients. Spray-drying highly volatile flavors minimizes loss. Savory products often utilize spray-dried meat purees.   Many protein sources (soy powders, isolated soy protein, whey proteins and various vegetable sources) come in spray-dried form. Fruit and vegetable pulps, pastes and juices are spray-dried as whole powders or as blends, with common sources being tomato, banana and citrus. Maltodextrins also are spray-dried in various forms-- powdered, granulated or agglomerated for use as a bulking aid.   When one considers the range of products used daily, it becomes easy to recognize the value of spray-drying as a powerful tool for delivering cost-effective, high-quality ingredients.   Consumers also see spray-dried products daily. One might wake up to instant coffee, coffee whitener and sweeteners, and continue the day with instant soups, dried-egg products, baby foods and powdered cheeses. Even condiments used in food preparation might be spray-dried (such as garlic and pimento). Spray-drying can be used to create nonfood products. Vitamins or medications might be made of compressed, spray-dried powders. Bathroom tiles might be composed of spray-dried clay, and wall paint contains spray-dried pigments. Spray-dried dyes color clothing and fabrics.   The spray-drying process is older than might commonly be imagined. Earliest descriptions date to the 1860s, and the first patent of note is dated 1872. Refining the process is ongoing.   Spray-drying involves transforming a fluid, pumpable medium into a dry-powdered or particle form. This is achieved by atomizing the fluid into a drying chamber, where the liquid droplets are passed through a hot-air stream. The objective is to produce a spray of high surface-to-mass ratio droplets (ideally of equal size), then to uniformly and quickly evaporate the water. Evaporation keeps product temperature to a minimum, so little high-temperature deterioration occurs.   The feed can be a solution, a suspension or a paste in the simplest form. The dried product can be powdered, granulated or agglomerated. The dry product form can be varied depending on the feed, dryer design and process conditions. Though quite energy-intensive in many cases, spray-drying is often the drying method of choice because of its continuous design and flexibility. It delivers a powder of specific particle size and moisture content regardless of the dryer capacity or product heat sensitivity. In a continuous operation, the spray-dryer delivers a highly controlled powder quality with relatively easy control. 'Nozzling' up  In its simplest form, spray-drying consists of four separate process stages: atomization of the feed; spray-air contact; drying; separation of the dried product from the drying air.  Atomization is generally accomplished by one of three basic devices: a single-fluid (or pressure) nozzle, a two-fluid nozzle, or a rotary atomizer (also known as a spinning disc or a wheel). The single-fluid nozzle allows more versatility in terms of positioning with the spray chamber, so the spray angle and spray direction can be varied.   Since particle size is partially dependent on the feed rate, nozzles have more limitations in terms of product characteristics and operating rates. Once the nozzle is in place, rate can only be varied by pressure; changing the orifice requires removing the nozzle. In high-volume operations, several nozzles are located within the chamber and positioned so constant evaporation conditions are maintained around each nozzle. For more viscous or abrasive feeds, two-fluid nozzles are utilized, with air being the second medium to move the feed and effectively atomize it. Air can be mixed internally within the nozzle or externally to the nozzle. In situations in which small particle sizes might not be possible with a single-fluid nozzle, the two-fluid nozzle can provide the necessary additional atomization ( however, this produces a much wider particle size range.   Fluid feeds also can be dispersed and atomized by centrifugal force on a rotary or spinning disk. Liquid feed is accelerated to greater than 300 ft./sec. to produce very fine droplets. Particle size is primarily controlled by the wheel speed.   Simply explained, the liquid feed is distributed to the center of the wheel or disc, travels over the surface as a thin film, and is flung from the edge as small droplets. Vanes or a rough-surface wheel can minimize slippage of fluid as it's flung to the outside of the wheel.   What method is best? Experts consider nozzles and wheels capable of producing virtually identical particle types. However, when high production rates are expected, the general choice is a wheel, unless a high percentage of fats are present, as in dairy products. Then nozzles would be preferred to reduce the free fat content of the surface. Frequently, both types are used in the same drying chamber for versatility. The choice depends on experience as much as desired product characteristics.   Other techniques also can be used to further vary particle appearance. Steam injection eliminates air in the droplet, resulting in a highly dense, higher bulk-density powder. Normally, water evaporates to the surface of the particle, forming a hard shell on the surface. Residual moisture within this shell expands, creating highly porous particles. Control of steam injection removes the air pockets in the center of the particle and prevents early formation of the shell. Drying the droplet  Several drying-chamber designs also have been used. A cylindrical flat-bottom dryer is an old design, popular for use with egg products, ice cream powder, toppings, and fruit and vegetable pulps and juices. A pneumatic powder discharger removes product, and an air broom can cool chamber walls to prevent sticking. This design allows easier access for cleaning.   Conical bottom chambers are commonly used for many products, including maltodextrins, flavors and dairy items. This design was created for the spray pattern produced by a rotary-type atomizer. Air within the chamber maintains a flow pattern, preventing deposition of partially dried product on the wall or atomizer. Air movement can be co-current (commonly, the feed and the air enter from the top); counter-current; or mixed flow (typical in some of the multistage dryers). Air movement and temperature of inlet air influence the type of final product. Maintaining the surface wetness of the particle is very important to constant-rate drying; too high an air temperature forms a dried layer at the surface, decreasing evaporation.   Drying occurs in two phases, and air-temperature control is vital to their control. The first phase is the constant-rate step, in which moisture rapidly evaporates from the surface, and capillary action draws moisture from within the particle. In the second, or "falling-rate" period, diffusion of water to the surface controls the drying rate. As moisture content drops, diffusion rate also decreases. Removing the last few percent of moisture in a single-stage dryer is responsible for most of the residence time in the dryer. As a rule, the residence time of the air and the particle in a single-stage co-current dryer are about the same. Since the moisture level is still decreasing toward the end of the process, the outlet temperature must be high enough to continue the drying process. This can be avoided by adding a fluid bed after the dryer.   The final phase of spray-drying is removing the dried product from the air in an economical and pollutant-free manner. Generally, economy depends on the ability to recycle the drying air, so removing fines from the air is very important. Depending on dryer design, the dried product can be separated at the base (as in a flat-bottomed dryer), and fines collected in some type of collection equipment. Or, the entire product and air can be removed to equipment designed to separate particles from air. Heavier product is removed by gravity, but fines need more encouragement. These fine particles are removed with cyclones, bag filters, electrostatic precipitators or scrubbers. Fines are bagged or returned to an agglomeration process; air is returned to the system. Planning a powdered product  Several considerations are critical in the assessment of a material to be spray-dried, according to Dan Donhowe, plant engineer at Spray-Tek, Inc., Middlesex, NJ. Questions to ask include: What is the material? Is there any historical data on spray-drying this material? Is it temperature-sensitive? Viscosity, percent solids, and absence or presence of particulates also are important considerations, as is particle size. Is the material hazardous in any way? Is it aqueous- or solvent-based? Does the powder present an explosion hazard? A listing of product specifications is needed, especially percent moisture, bulk density and final particle size. Viscosity and presence of particulates help determine which type of atomizer is needed -- whether it's a rotary or pneumatic nozzle (single-fluid, two-fluid, three-fluid).   The next step is evaluating the product in a small lab dryer to determine starting parameters for plant scale. For example, Spray-Tek has capabilities for using rotary discs or nozzles, and has conical and flat-bottomed dryers. Donhowe says aqueous-based products, such as maltodextrins, corn syrups, milk products, fruit products, as well as multiple-phase (encapsulated) products -- such as fragrances and flavors-- are routinely handled in single-stage spray dryers.   In running a product, a number of operating variables are open to the operator. Airflow is generally static, says Donhowe -- dryers are designed for certain airflow ranges. The infeed air temperature is varied as high as possible (depending on product sensitivity), so the feed rate can be kept as high as possible. A wide differential between the inlet and outlet temperatures takes full advantage of the energy delivered to the spray dryer.   Once infeed temperature has been set, feed rate is a function of desired moisture. If the feed rate is too high and/or the outlet temperature too low, product moisture and, usually, bulk density will be higher. Increased feed (fluid product) temperature also can increase the powder's bulk density by forming smaller spherical droplets or by de-aerating the fluid. Dilution of the feed can have an identical effect.   Coarseness of the spray also will influence bulk density. Small particles pack together with fewer voids, while coarse particles fit together more loosely. Looser packing means lower bulk density. For most applications, the best powders contain a range of both small and large particles. The small particles completely fill in the space between the coarse particles, enhancing flow. Generally, a rotary atomizer will produce a higher bulk-density product than will a nozzle atomizer. From a physical standpoint, one also can see that an increase in feed solids increases bulk density, while aeration of the feed decreases bulk density. Suspensions also produce higher density.   If too high, an increase in inlet air can boost bulk density by retaining moisture within a case-hardened shell. If this is avoided, small upward changes in temperature usually decrease bulk density. Outlet temperature air is important to final moisture -- if it is allowed to drop too low, moisture increases, as does bulk density. If the lowest bulk density is desired, it's important for the product to contact the hottest air as it leaves the atomizer (known as co-current drying).   Only two types of continuous dryers are in use for drying liquids or liquid suspensions: film drum dryers (usually approximately 600 lbs./hour) and spray dryers (in excess of 23,000 lbs./hour). The rate differences make it easy to understand the economic benefits of spray-drying, including decreasing maintenance needs with increasing capacity. Most temperature-sensitive liquids must automatically be dried in a spray-dryer. Creating a crowd  Spray-dried powders can, if extremely fine, cause some in-process problems. Dusting is a common production complaint. Another is the occurrence of "fish-eyes" when the product is wetted -- the powder starts to dissolve, but forms a thick, gel-like mass around dry powder, resisting further hydration.   This is a sight familiar to most product formulators who've worked with any very fine powder, especially hygroscopic powders. One solution is to consider an agglomerated product. Agglomeration reduces powder surface area, and provides an open, heavier structure which allows for more even hydration. The particle sinks below the water surface and breaks apart, allowing smaller particles within the agglomerate to completely hydrate. In short, this means better and faster dispersion. Agglomeration can be accomplished in several ways. The most basic method, often used for products such as agglomerated maltodextrin, is the "straight-through" process. As the feed is being dried, the outlet air is drawn off into cyclones, where fines are collected. These are fed back into the top of the dryer (above the atomizer), where they are fed into the spray cloud. These dry fines stick to the moist new particles, producing an agglomerate of particles, which is dried, cooled and packaged.   A more easily controlled continuous method of agglomeration is what is commonly termed a fluid bed or "re-wet" system, often used for such products as instantized baby foods. The wettability and dispersability of the particle can be improved by forming a more complex agglomerate than can be achieved in the "straight-through" method. Typically, products are re-wet in a tightly controlled manner by fine spray, steam humidification, heating (if the material is thermoplastic), or spraying in some type of binder material. It is important to avoid over- or under-wetting, for obvious reasons. Many agglomerators are designed to agitate the agglomerates to build their strength and to control their size. Typically, the binder is sprayed onto a dried powder in an agglomeration zone. The powder is put into a turbulent state to allow particles to stick together. From this zone, the powder enters a fluidized-bed drying zone, where moisture is removed. The final zone is a fluidized cooling zone, where the powder is cooled to packaging temperature. This type of process has been used successfully for a number of food products, including dairy products, baby formula, flavors, fruit extracts, maltodextrins, corn syrup solids, sweeteners, starches and cocoa mixes.   In many cases, the "binder" is simply water, bu -- as in flavors -- maltodextrin or gum arabic can be used to agglomerate and encapsulate volatile flavors. Agglomeration improves wettability, dispersability and flowability of fine powders, and decreases dusting. While individual powder particles are less than100 microns, agglomerates are typically 250 to 400 microns. Bulk density decreases from approximately 43 lbs./cubic foot to approximately 28 lbs./cubic foot. Protection from degradation  The subject of spray-drying and fluid-bed drying leads to another popular food application of these technologies: microencapsulation. This is a process commonly used to protect a core material from degradation, to control the release of a core material, or to separate reactive components within a formulation.   Within the food industry, the spray dryer is the most common means of encapsulation, because it's commonly available, economical, fast, and produces good-quality material. Flavors encapsulated with gum arabic have been produced by spray-drying since the 1930s. The encapsulation process is simple, and similar to the one-stage drying process. The material to be encapsulated, which is referred to as the "active" material-- usually an oil or flavor in an oil base -- is dispersed in a hydrocolloid carrier (gelatin, modified starch, dextrin or maltodextrin, or vegetable gum such as gum arabic). An emulsifier is added, then the mixture is homogenized to form an oil-in-water emulsion, which is fed to the atomizer. Within the dryer, the aqueous phase dries, encapsulating the oil within a hydrocolloid or protein film. To improve dispersability and flowability, these particles can be agglomerated, as previously discussed. The active material encapsulated does not have to be oil-based. An aqueous active material can be homogenized within a polymer, forming a matrix. On drying, the active material is entrapped as particles within the polymer film.   As an aside, there is a modification of this that more closely resembles fluidized bed processing. Common terms for this are "air-suspension coating," "Wurster process," "spray coating" or "fluidized-bed coating." Core material particles are suspended in an upward-moving air stream, which may be heated or cooled. The coating material is atomized from above to uniformly coat the core. Particles flow up and down several times through this coating cycle as they are borne upward, then drop toward the outside of the chamber. This method was developed in the 1950s by Dale Wurster, a professor of pharmacy at the University of Wisconsin, and is one of several methods used to encapsulate aspartame.   Another type of encapsulation closely related to spray-drying is spray-cooling, which, in turn, is closely related to spray-chilling. Two principles separate them: temperature within the chamber, and choice of coating material. Spray-drying has been described as the use of warm-to-hot air to remove moisture from an aqueous coating material. For spray-cooling, an active material (such as aspartame) may be encapsulated within a high-temperature melt-point vegetable fat, such as a stearine or wax (melting points of 45° to 67°C). Spray-chilling involves use of lower temperatures in the drying chamber to harden lower-melting-point fats (32° to 42°C). The fat alone can be chilled to produce powdered shortenings. Active materials, such as acidulants, vitamins, solid flavors, sodium bicarbonate or yeast can be protected by encapsulation within the fat. Spray-chilled products are commonly used in bakery products and dry mixes. Far from dried-up  Spray-drying has always been an energy-intensive process. For standard single-stage drying, the best way to control energy usage is raising the inlet temperature as high as possible, keeping outlet temperature as low as possible, taking full advantage of the energy introduced. The downside of this is potential degradation or discoloration of the product, and potential final moisture problems.   Within the past 15 years, multistage dryers have been introduced to cut energy costs while better controlling quality of temperature-sensitive products, according to Stewart Gibson, product manager, dryers, at APV Anhydro, Tonawanda, NY. A decade ago, Gibson states, the typical inlet/outlet temperatures of a milk dryer were 356°/203°F. Today, outlet temperature can be lowered to 185°F, while inlet temperature is raised to 428°F. This takes full advantage of the energy delivered to the dryer, yet doesn't damage the milk product because the temperature of the product never reaches the air temperature. The cooling effect of evaporation protects it at the top end.   Humidity is controlled by dropping the product from the single-stage dryer into a fluid bed dryer/cooler. Most of the drying occurs within the spray-dryer, but the final moisture can be higher than in the single-stage dryer -- final drying occurs over more time and at a lower temperature in the fluid bed. Energy costs can easily be decreased 15% to 20%, and product quality is more constant. An added benefit, due to the fluid bed, is the ability to re-wet and agglomerate if this type of product is needed. This technique had been applied to several dairy products, Gibson says, including skim-milk powder (low, medium and high heat); instantized skim-milk powder; whey powder; permeate; whey/fat blends; cheese powder; ice-cream powder; and whole-milk powder. Air-to-air or air-to-liquid heat recuperators also could be used to recover energy from dryer and fluid-bed exit air to further reduce energy costs.   The idea for two-stage drying then evolved into a three-stage dryer for production of agglomerated particles. APV Anhydro modified this into what is referred to as a spray bed dryer. This type of dryer has an integrated fluid bed at the bottom of the cone. Air enters and exits at the top of the chamber. The integrated fluid bed is agitated vigorously so that particles can enter the zone at high moisture (10% to 15%), and can be dried to 5%. Naturally, since the particles enter wet, they agglomerate with the dryer particles. The final (third stage) of drying and cooling is relegated to an external fluid bed. If a product has spray-drying limitations (extremely hygroscopic, sticky or high fat), this type of drying might be ideal. Drying and design  Now that many of the options available to the food product or ingredient processor have been explored, it becomes easier to relate to certain products and how they are commonly handled. For most milk products, nozzle or rotary atomizers can be used, and single-stage to multistage dryers are used. Skim milk is concentrated to approximately 50% solids in evaporators prior to introduction to the atomizer, and dried to about 3.5% moisture. Naturally, skim-milk powder can be instantized by re-wetting and agglomerating. Whole milk introduces some additional problems due to its fat content. Electric hammers prevent deposition of product on the chamber walls. The product fluid is about 50% solids, and is dried to approximately 3.5% moisture.   When whole-milk powder is instantized, the final agglomerate is finely coated with lecithin, which improves the wettability of the agglomerates. (The high level of fat normally prevents adequate wetting of the product.) Fruit juices contain a high percentage of sugar, and cannot be spray-dried in pure form. Usually a diluent such as a maltodextrin or a lower-D.E. (dextrose equivalent) corn syrup solid can be added. Even with the filler or diluent, certain minimum chamber sizes are required due to the stickiness of the concentrate, and solids-content of the feed is kept to 30% to 35%.   Design and operating characteristics of spray dryers are as varied and wide-ranging as the number of food products and ingredients on the market today. Due to unique characteristics (hygroscopicity, high fat content, particulates, multiphase characteristics), these products demand unique settings and handling characteristics to deliver them into the most readily usable dry form(s). Energy considerations have continued expanding the number of equipment options available for drying, agglomerating, encapsulating and spray-cooling new foods and ingredients, so the potential for new innovative products remains unlimited. Back to top