Drying Technology

There are three main types of gas-suspension dryers:

• Spray dryers, to convert a liquid solution or suspension to a dry, free-flowing powder
• Fluid bed dryers, used to dry wet filter cake, or for pastes and sludges with dry product recirculation
• Flash dryers, for a relatively dry, crumbly, non-sticky feed

The type of dryer chosen for any given application depends on both the feed properties and product requirements. Important feed properties are the moisture content, solids, viscosity, and density, as well as any volatile, flammable, or toxic components. Dried product specifications may include average particle size and particle size distribution, density, moisture content, and residual volatiles or solvents. Powder characteristics can be controlled and powder properties maintained constant through continuous operation.

Spray Drying

Spray drying is a three-step drying process involving both particle formation and drying. (1) The process begins with the atomization of a liquid feed into a spray of fine droplets. (2) Then a heated gas stream suspends the droplets, evaporating the liquid and leaving the solids in essentially their original size and shape. (3) Finally, the dried powder is separated from the gas stream and collected. Spent drying gas is either treated and exhausted to the atmosphere or recirculated to the system. These three steps are accomplished by three components: the atomizer, the disperser, and the drying chamber.

The selection and operation of the atomizer is of extreme importance in achieving an optimum operation and production of top-quality powders. There are four main types of atomization:

• Centrifugal atomization, the most common, uses a rotating wheel or disc to break the liquid stream into droplets. The rotational speed determines the mean particle size, while the particle size distribution about the mean remains fairly constant in a system. Centrifugal atomizers are available in a large variety of sizes, from laboratory scale to very large commercial units.
• Hydraulic pressure-nozzle atomization forces pressurized fluid through an orifice. Multiple nozzles are used to increase capacity. The particle size depends on the pressure drop across the orifice, so that the orifice size determines the capacity of the system. This type of atomization is simpler than centrifugal, but cannot be controlled as well. It is not suitable for abrasive materials, or materials that tend to plug the orifices.
• Two-fluid pneumatic atomization uses nozzles, as well, but introduces a second fluid, usually compressed air, into the liquid stream to atomize it. This type of atomization has the advantage of relatively low pressures and velocities and a shorter required drying path. It is most often used in small-scale equipment, laboratory or pilot size.
• Sonic atomization, not yet widely used, passes a liquid over a surface vibrated at ultrasonic frequencies. It can produce very fine droplets at low flow rates. Current limitations are capacity and the range of different product that can be atomized.

After atomization, a disperser brings the heated gas into contact with the droplets. The disperser must accomplish three things: mix the gas with the droplets, begin the drying process, and determine the flow paths through the drying chamber. The drying gas may be heated directly by combustion of natural gas, propane, or fuel oil, or indirectly using shell-and-tube or finned heat exchangers. Electric heaters may be used in small dryers. Industrial radial fans move the heated gas through the system.

The drying chamber must be sized to allow adequate contact time for evaporation of all of the liquid to produce a dry powder product. Factors that impact the drying time include the temperature difference between the droplets and the drying gas, and their flow rates. The exact shape of the chamber depends on the drying characteristics and product specifications, but most are cylindrical with a cone-shaped lower section to facilitate collection of the product.

Finally, proper configuration of the atomizer, disperser, and drying chamber is essential for complete drying and to avoid the deposit of wet material on the interior surfaces of the dryer. Designs may use co-current, counter-current, or mixed flow patterns.

The powder is separated from the drying gas at the bottom of the chamber. Most often, the gas exits through an outlet duct in the center of the cone. Heavier or coarser particles will be separated at this point, dropping into the cone to be collected through an air lock. Then either cyclones or fabric filters (or both) remove the remaining powder from the exit gas. In systems producing a very fine powder, most of the collection takes place at this point.

Fluid Bed Drying

Fluid bed drying is a process in which a gas is forced upward through a bed of moist particles to achieve a fluidized state. The particles are suspended in the gas stream and dry as they flow along with the gas. Fluid beds can be either cylindrical or rectangular. There are two basic types of fluid bed designs:

• Plug flow fluid beds are used for feeds that are directly fluidizable. Baffles in the bed limit mixing in the horizontal direction to maintain plug flow. This type of bed is ideal for removal of bound volatiles or for heating and cooling. The volatile content and temperature vary uniformly as the solids pass through the bed. Baffle design depends on the shape and size of the bed, with spiral or radial baffles used in circular beds and straight baffles in rectangular.
• Back-mixed fluid beds are used for feeds that cannot be fluidized in their original state, but become fluidizable after a short time in the dryer. The feed is distributed over the bed surface, designed to allow total solids mixing. Product temperature and moisture are uniform across the fluidized layer. Heating surfaces may be immersed in the fluidized layer to improve thermal efficiency and performance.

A combination system uses a back-mixed fluid bed to reduce the moisture level of the wet feed, followed by a plug-flow section to achieve final specifications. This type of arrangement is quite common.

The advantages of fluidized-bed drying are: relatively long residence times allow high heat-transfer coefficients between the particles and the gas; the ability to closely control product temperature makes fluidized beds ideal for processing temperature-sensitive solids; and they have the highest thermal efficiency of any gas-suspension drying system.

Disadvantages are: they can process only a limited range of materials; product particles are relatively large; and there may be difficulty processing needle- or platelet-shaped particles.

Flash Drying

Flash drying forces drying gas through a heater and upward through a duct or flash tube. The high-velocity gas stream instantly suspends the feed, which enters just after the heater, and carries it to the collection equipment, usually cyclones or bag collectors.

Flash dryers are the simplest gas-suspension dryers, and require the least space. Residence time within the dryer is very short, usually less than 3 seconds. Particles must be quite small, and the best feed is reasonably dry, crumbly, and not sticky. There are several ways to obtain the required feed qualities:

• A cage mill may be used to break up the feed into the required small particles.
• If the feed is too wet or pasty, dry solids may be backmixed to create the proper consistency.
• An agitated design, using a high-speed disintegrating rotor, will keep all particles moving. This design is shorter and larger in diameter than a flash tube, creating a very compact system.

Hybrid Dryers

There are a number of hydrid systems used in applications where a single system cannot handle the requirements of both the feed and product. The most common are:

• Fluidized spray dryers (FSD™) combine spray with fluid bed drying to produce agglomerated products. The top of the system is a spray dryer, atomizing the liquid and contacting it with heated gas. Additional heated gas is introduced at the bottom to create a fluidized bed portion of the drying chamber. This type of dryer will produce a dustless, free-flowing agglomerated product. It is ideal for products that must dissolve easily, e.g. food colors, dyestuffs, pigments, and some agricultural chemicals.
• A flash dryer may be used to remove surface moisture, followed by a fluid bed for removal of bound moisture.

Niro Dryers

The MOBILE MINOR™ is a laboratory-scale spray dryer known for its flexibility and different levels of control systems. It is used to dry small quantities of solutions, suspensions, and emulsions into representative powder samples. Test results provide important information for selecting the design and technical specification of a given drying project.

The PRODUCTION MINOR™ is a larger spray dryer that can be used for pilot testing or small-scale production. It has a choice of atomizers, heating systems, and powder discharge.

The Fluidized Spray Dryer (FSD™) was invented and patented by Niro in the early 1980s. It combines fluidization and spray-drying technologies to dry a wide variety of products, including many that cannot be dried using conventional equipment. Advantages include easy control of the size and structure of the particles, making it ideal for agglomerated products, and low powder temperatures for thermally sensitive materials. It is also very energy efficient.

Niro Inc.

Niro is an international company specializing in the development, design, and engineering of liquid and powder processing equipment for the manufacture of products in liquid, powder, granulate, or agglomerate form. Niro products include spray dryers and coolers, fluid bed systems, evaporators, packaging equipment, homogenizers, membrane filter systems, coaters, agglomerators, and granulators.

Niro was founded in 1933 by the distinguished Danish engineer Johan E. Nyrop. Spray-drying was its original specialty, and in the 1950s the company added fluid bed technology. The company has supplied tens of thousands of drying systems. It is a member of the GEA Group of companies, headquartered in Bochum, Germany.

For further information:

Phone : 410-997-8700
Fax : 410-997-5021
Email: gea.pe.na@geagroup.com