Spray Drying

Spray drying is not a new technology. It has been used for many years—C.E. Rogers Co., for example, has been manufacturing spray dryers since the early 1900s—and is a standard technology in the food industry, especially the dairy industry. Nevertheless, incremental improvements are always being made.

In a typical spray dryer, a solution, suspension, or emulsion is pumped to an atomizer at the top of the drying chamber. The atomizer—a rotating wheel or a nozzle—sprays the liquid into a high-velocity stream of hot air or other gas, producing droplets. As the droplets pass through the hot air flow—which can be cocurrent with the liquid, countercurrent, or a combination of both—the moisture rapidly evaporates. The large particles fall to the bottom of the chamber and are collected. Fine particles entrained with the exhaust air are generally collected by passing the air through a series of external cyclones, scrubbers, or bag filters.

According to Jeff Bayliss (phone 410-922-5900), Vice-President, Spray Drying Systems, Inc., the heat and mass transfer during drying occurs in the air and vapor films surrounding the droplet. This protective envelope of vapor keeps the particle at the saturation temperature. As long as the particle does not become completely dry, evaporation still takes place and the temperature of the solids does not approach the dryer outlet temperature. As a result, heat-sensitive products can be spray dried at relatively high air temperatures without the products’ being harmed.

The atomizer is a critical component of the spray dryer. Producing droplets of specific size and surface area by atomization is a critical step in the spray drying process, Bayliss said. The degree of atomization influences the drying rate, and therefore the required particle residence time, and therefore the dryer size. All of the atomizing techniques can give good average particle size control, but there are differences in the particle size distribution.

The most commonly employed atomization techniques are pressure nozzle atomization, in which a spray is created by forcing the fluid through an orifice; centrifugal atomization, in which a spray is created by passing the fluid across or through a rotating wheel or disk; and two-fluid nozzle atomization, in which a spray is created by mixing a compressed gas with the fluid. The pressure nozzle gives the narrowest distribution of particles, Bayliss said; the centrifugal atomizer and two-fluid nozzle give broader distributions.

According to Dennis R. Heldman (732-932-9611), Professor of Food Process Engineering at Rutgers University, the atomizer is the spray dryer component that relates most specifically to particle size and particle size distribution, which in turn relates to dispersibility of the product for rehydration. The challenge has always been to find the atomization system that provides the most uniform distribution of droplets.

The drying chamber is also important, he said. The droplet needs sufficient time in the chamber to dry without contacting the walls. From an efficiency standpoint, the challenge is to come up with ways to reduce the energy consumption as much as possible, to make spray drying competitive with other systems.

Another aspect of the drying chamber, he said, is quality. Many components of food are heat sensitive, so we want to keep the residence time to a minimum, especially once the dry particle is formed and as much moisture as wanted has been removed. We also want to keep exposure to high temperature to a minimum, and this relates to the uniformity of particle size distribution. If different size droplets are present, by the time the largest droplet is dried, the smallest droplet will have been exposed longer than necessary. Thus, a narrow particle size distribution would improve the quality of any heat-sensitive component of the product, such as vitamins, color, or flavor.

The separator is the component of the system that separates the dry product from the air. Several types of separators are used, including cyclone separators, and bag filters. These are rather significant cost components of a spray drying system, Heldman said, and it’s always a challenge to find more efficient, less costly ways to separate the dry particles from the air.

Heldman pointed out that there is also a safety issue unique to spray drying, compared to other types of dryers: the potential for spontaneous combustion of products that have explosive properties or are flammable. If small particles accumulate in corners and crevices within the drying chamber and are exposed to air, spontaneous combustion could occur, causing an explosion, as in grain elevators. All it takes is a spark from the electrical system. Manufacturers of these systems have to be careful and insulate the chamber from electrical connections.

Often, the dry particles are recirculated into the atomizer to agglomerate them. As moisture from the input liquid forms a film on the small particles, they clump together to form larger and more uniform particles. These larger particles are then dried, typically in a fluidized bed, often included in the spray dryer system.

Not much research has been done on improving spray drying, Heldman said, because many foods are low margin, and the process is expensive. We need products to justify emphasis on improving the process. Spray drying really needs to get a new look and extra-special research attention. One challenge is to develop good simulation models for design of spray dryers. We don’t even have good enough models to do scaleup from pilot plant to commercial sizes, he said. This is an opportunity for young researchers looking for an area to work in.

Spray dryers are manufactured by a number of companies, among them Niro Inc., Hudson, Wis.; Carlisle Friesland (formerly Stork Friesland), Cockeysville, Md.; C.E. Rogers Co., Mora, Minn.; Spray Drying Systems, Inc., Randallstown, Md.; and others. Here’s what company representatives said about their spray drying systems and the challenges ahead.

Greg Haugen (715-386-9371), Sales Manager, Niro, Inc., said that the biggest new area is use of bag filters that can be cleaned in place instead of cyclones. The company has sold several units throughout the world but is in the process of obtaining 3-A approval for use in the United States dairy industry.

The company’s newest spray dryer design is the Multistage Spray Dryer, the MSD. It was first introduced to the dairy industry in the late 1980s for instantizing dairy products but has undergone changes which improve the instantizing by recycling the powder back to the dryer. The company began selling it for instantizing dairy protein in the U.S. in 1997, and several dryers are being used in the U.S.

Although not new, heat recovery systems are becoming more popular now as energy costs skyrocket. Air-to-air recovery systems preheat the inlet process air but are very hard to clean, he said. If a bag breaks in the bag house, it can cause a lot of problems. Niro’s heat recovery system consists of a rectangular box with stainless-steel tubes inside. “Dirty” process air passes through the inside of the tubes, and clean air going into dryer passes along the outside of the tubes, exchanging heat.

Niro’s most popular dryer in the dairy industry is the compact spray dryer with static fluid bed on the bottom, integrated with the drying chamber. It takes less room and is very energy efficient.

The challenge, Haugen said, is to develop a spray dryer to dry many types of products with a variety of different drying characteristics and produce “value-added” qualities in the final powders.

Jay Gilbert (410-628-2466), Manager of Drying and Concentration at Carlisle Friesland, said that in the past a lot of people used spray dryers to burn off the water and make a powder and didn’t realize they could do other things. They were making very simplistic, single-stage-drying products. In the past few years, he said, more and more people have recognized that they can do value-added manufacturing by spray drying and agglomerating in one piece of equipment. More and more people are doing that, from the dairy industry for skim milk and whey protein concentrate, to other segments of the food industry.

The equipment recycles fines and reintroduces them into the drying process. While doing that, it’s a simple thing to add other dry ingredients to produce value-added agglomerated products. For example, while agglomerating, trace quantities of liquid ingredients such as lecithin can be added to whey protein concentrate, to make the product more readily dispersible in water.

Carlisle Friesland uses a wide-body drying chamber, typically as big in diameter as it is high. Air goes in at the top of the dryer and is discharged out the top after passing down the center of the dryer and back up the walls. This design (see next page) provides maximum flexibility in keeping the walls of the dryer cool, controlling particle size, and keeping the dry product off the walls. This gives it a chance to get better classification of powder, with large particles falling out the bottom and fewer fines to have to pass through the cyclone. There is less sticking to the drying chamber walls because the walls are relatively cold. A fluidized bed at the bottom is an integral part of the dryer. Typically, the powder exits the drying chamber at something above finished product moisture content, and the balance of the water is removed by the fluid bed dryer, which provides the most gentle drying possible. All the fines from the fluid bed dryer and spray dryer are then put through a special conveying system back into the top of the spray dryer with the spray. Fines are stuck back onto droplets, so agglomeration takes place at the top of the drying chamber.

Gilbert concurred that the challenge ahead is to efficiently utilize energy to make high-quality products.

Howard Rogers (320-679-2172), President, C.E. Rogers Co., said that his company has built a dryer called the VSD that is being used by Agri-Mark, Middlebury, Vt., exclusively to dry whey permeate for use as animal feed. The system, however, is designed to produce product for human consumption as well.

It’s a two-stage drying process, in which the critical design criterion is operating temperature. In the first stage, the main chamber dries product from 60% solids to 10% moisture, then the second stage takes it from 10% moisture to about a 3.5% moisture powder, which is then packaged. Between the two stages, the product is conveyed through an intermediary drying system. The secondary dryer is a fluidized bed. The system can dry more than 5,000 lb/hr.

One challenge ahead, he said, is to make the systems bigger, as the market dictates. With increasing size comes all types of challenges, not so much drying the powder but conveying the large vol-Spray Drying Papers at the IFT Annual Meeting ume to where you want it. Dryers can now dry more than 15,000 lb/hr, and just moving and cooling that volume is a challenge.

Another challenge is complying with federal regulations regarding sanitary aspects, he said, an ever-changing, ongoing challenge. And another is incorporating a fire suppression system for these dryers that is functional and in compliance with sanitation requirements. It needs to address every area that could be exposed to fire. As spray drying systems become larger and more complex, there is more area for exposure, and these systems need to put out a fire with the least damage to the equipment. In addition, the systems need overpressure vents on the main chambers and bag houses to minimize the potential for explosions. All spray drying equipment is now designed with overpressure vents and fire suppression systems, he said.

The company makes various types of spray dryers, including vertical, horizontal, and rotary. The newest version is a developmental dryer called the RDD, a low-process-volume dryer designed to dry specialty products such as proteins and high-fat food products.

PRODUCTS & LITERATURE
Phase Transition Analyzer™ measures the controlling glass transition (Tg) and melt transition (Tm) of a complex mixture of biopolymers such as food to help characterize an extrusion operation. The controlling Tg or Tm is the temperature at which a sufficient amount of sample is softened to allow for particle compaction (Tg) or melted to allow for flow (Tm). When a sample’s Tg and Tm are combined with a mass and energy audit of an extrusion system, the user can accurately map the process. The instrument is a closed-end capillary rheometer, which uses a combination of pressure, temperature, time, and moisture to measure Tg and Tm. For more information, contact Wenger, 714 Main St., Sabetha, KS 66534-0130 (phone 913-284-2133, fax 913-284-3771, www.wenger.com) —or circle 308.

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Standard for Materials, design, and construction requirements for equipment used in meat and poultry processing has been adopted by the NSF International as an American National Standard. ANSI/NSF/3-A Standard 14159-1-2000, entitled “Hygiene Requirements for the Design of Meat and Poultry Processing Equipment,” is the only standard supported by the American Meat Institute, Packaging Machinery Manufacturers Institute, Food Processing Machinery & Supply Association, and Meat Industry Suppliers Association. It has also been selected by the U.S. Dept. of Agriculture as the standard of choice for equipment evaluation. For more information, contact NSF, International, P.O. Box 130140, Ann Arbor, MI 48113-0140 (phone 800-673-6275 or 734-769-8010, fax 734-769-0109, www.nsf.org) —or circle 310.

Porous Metal Media can be used for a variety of applications in the food and beverage processing industry. A high-precision sintering process, porous metal media can be made with strictly controlled, uniformly sized and distributed pores as small as 0.2 um. The media can be used for gas/liquid and gas/emulsion contacting, filtration, catalyst recovery, precoat filtration, and other applications, including juice clarification, yeast removal from beer production,clarification of sugar, aeration, carbonation, hydrogenation, oxygen stripping, bulking, fermentation, steam injection, pH control, and others. For a 6-p brochure describing the media, contact Mott Corp., 84 Spring Ln., Farmington, CT 06032-3159 (phone 800-289-6688 or 860-677-7311, fax 860-674-1489, www.mottcorp.com) —or circle 311.

Density Meter, the LQ300 Sanitary Microwave Total Solids Meter, uses microwaves to accurately determine the density or percent total solids of food in water or other solutions. The technology exploits the way the total amount of food in the process affects the propagation of microwaves as they pass through the fluid. By observing the microwave phase after it passes through the fluid, an accurate measurement of the total food solids present can be determined. The meter measures the amount of total food solids, including both suspended and/or dissolved concentrations. Typical applications include determination of amount of starch in water, density of food slurries, amount of dairy solids present, amount of malt extract for brewing, percent sugar concentration, percent water in evaporated milk, and percent water in dough and extruded solids. The 3A-approved meter can be mounted in the process line horizontally or vertically. Among its advantages are no pressure drop in the line, no vibration of the product, and no fouling of optical sensors. For more information, contact Toshiba International Corp., 13131 W. Little York Rd., Houston, TX 77041 (phone 800-231-1412 or 713-466-0277, fax 713-896-5225, www.toshiba.com) —or circle 312.

Cook/Chill Unit, the TurboJet™ Model TJ-100–CC, is a multitask machine that functions as a tumble chiller for cooling kettle-produced products, as a cook/chill tank for low-temperature water-bath cooking of meats, and as a sous vide unit for the production of home-meal replacements. It has a capacity of 100 gal for pumpable (kettle-produced) products and 700 lb of meat in the cook tank mode. Most pumpable products packaged in pouches or casings can be cooled from 170ºF to 40ºF in less than an hour. For a 4-p brochure describing the unit and its advantages, contact Cleveland Range Inc., 1333 E. 179th St., Cleveland, OH 44110 (phone 216-481-4900, fax 216-481-3782, www.clevelandrange.com) —or circle 313.

Molds for the manufacture of all types of molded chocolate products are available in various styles, as well as in various colors to help manufacturers keep track of them during production. Injection molds for bars and pieces can be made in standard shapes or custom designs and come in sizes 275 mm x 135 mm up to 2,000 mm in length. Thermoformed mold styles include single and double molds, book molds, and molds for one-shot depositing. For more information on the B.V. Vormenfabriek molds, contact American Chocolate Mould Co., Inc., 3194 Lawson Blvd., Oceanside, NY 11572 (phone 516-766-1414, fax 516-766-1485, www.americanchocolatemould.com) —or circle 314.

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Spray Drying Papers at the IFT Annual Meeting
Several papers related to spray drying will be presented during the IFT Annual Meeting in New Orleans, June 23-27, 2001.

• In paper 15D-15, “Spray-Drying Microencapsulation of the Natural Colorant Bixina,” on Sunday morning, June 24, P.B.L. Constant of the Dept. of Food Technology at the Federal University of Viçosa, Brazil, will report on use of spray drying to microencapsulate a natural colorant to enhance its stability and increase its use as a food ingredient. The colorant tested was bixin, a liposoluble fraction of the pigment urucum (Bixa orellana, L), and the encapsulating agents were maltodextrin, gum arabic, and beta-cyclodextrin.

• In paper 94-6, “Mathematical Representation of Milk Spray Drying Dynamic Using a Mechanistic Model,” on Wednesday morning, June 27, M.A. Garcia from the Chemical Engineering Dept. of the Technological Institute of Veracruz, Mexico, will report on the development and validation of a mathematical model for the dynamic behavior of spray drying of milk. Milk spray drying is one of the separation processes most applied in the food industry, but it has a high energy demand. The energy efficiency can be increased with the development of advanced control algorithms. The model consisted of four differential equations for the time dependence of four variables: air moisture and temperature, and product moisture and temperature.

• In paper 94-7, “The Influences of the Composition of the Carrier Material on the Retention of Volatiles During Spray Drying,” on Wednesday morning, J.G. Brennan of the University of Reading, U.K., will report on the effect on retention of volatiles of adding milk proteins and sugar to a maltodextrin carrier during spray drying in a laboratory spray dryer. The materials added included skim milk powder, whey protein concentrate, sodium caseinate, and D-lactose in various proportions, giving a total solids content of 40%.

by NEIL H. MERMELSTEIN
Senior Editor