Consisting of three basic steps, spray drying begins with atomization of a liquid feed into a spray of fine droplets. The spray is then contacted with, and suspended by, a heated gas stream, allowing the liquid to evaporate and leave the dried solids in essentially the same size and shape as the atomized droplet. Finally, the dried powder is separated from the gas stream and collected. The spent drying gas is treated to meet environmental requirements and then exhausted to the atmosphere or, in some cases, recirculated to the system.

Key options in atomization

Several types of atomization may be employed, including centrifugal, nozzle, pneumatic and sonic atomization. Centrifugal atomization uses a rotating disc or wheel to break the liquid stream into droplets (Figure 4). These devices normally operate in the range of 5,000 to 25,000 rpm with wheel diameters of 5 to 50 cm. The size of the droplets produced is nearly inversely proportional to the peripheral speed of the wheel.

The distribution of particle sizes about the mean is fairly constant for a given method of atomization, hut the mean itself can be varied from as little as 15 μm to as large as 250 μm, depending on the amount of energy transmitted to the liquid. The mass flow of the liquid, its viscosity, solids content and surface tension influence particle size directly, but none to the degree of peripheral wheel velocity. Consequently, an increase in feed rate may slightly increase the particle size, but use of a variable-speed drive on the centrifugal atomizer facilitates correction to the specified size.

One advantage of centrifugal atomization is that atomizing machines are available in many sizes. A small air-driven laboratory unit handles from 1 to 10 L/h of liquid feed, while the largest commercial units driven by 850-kW motors can handle in excess of 200,000 kg/h.

The second most common form of atomization is hydraulic pressure-nozzle atomization. Here the liquid is pressurized by a pump and forced through an orifice to break the liquid into fine droplets. Orifice sizes are usually in the range of 0.5 to 3.0 mm. As a result, a single nozzle is limited to somewhere in the order of 750 kg/h of feed, depending on pressure, viscosity, solids content and orifice size.

Greater pressure drop across the orifice produces smaller droplets. Therefore, to reduce the particle size for a given feed rate, the nozzle must be removed and a smaller orifice substituted. This in turn requires a higher pump pressure to achieve the same mass flow through the nozzle. Very large systems may have as many as 40 nozzles, making control of particle size dificult.

Precise control, however, is not always required, and large, multiple-noz-zle dryers are often used when the only requirement is that the mean particle size be quite large. Although nozzles are considerably less complicated than centrifugal atomizers, a high-pressure pump is required. During the drying of abrasive materials, the nozzles can pose special problems. The potential for plugging the relatively small orifices is another drawback for nozzle-based atomization systems.

A third method used primarily in smaller drying systems is two-fluid pneumatic atomization. Here atomization is accomplished by the interaction of the liquid with a second fluid, usually compressed air. Neither the liquid nor the air require very high pressure, with 200 kPa to 350 kPa being typical. Particle size is controlled by varying the ratio of the compressed air flow to that of the liquid.

The main advantage of this form of atomization is that the liquid has a relatively low velocity as it exits the nozzle, and therefore, the droplets require a shorter flight path for drying. This makes two-fluid nozzles ideal for use in pilot- or laboratory-scale equipment.

Lastly, a more recent development is sonic atomization. Here ultrasonic energy is used by passing the liquid over a surface vibrated at ultrasonic frequencies. These systems are suitable for producing very fine droplets at low flowrates. More development is needed if these atomizers are to find wider acceptance in industrial drying both in capacities handled and the range of different products to be atomized. A detailed account of atomizer designs, operating parameters and selection criteria is available elsewhere [4].

Dryer configuration

The atomized droplets that are formed a short distance from the atomizing device have a velocity and direction initially established by the atomizer (Figure 5). It is necessary for the heated gas to mix with the cloud of droplets, begin evaporation, and influence the movement of the droplets inside the dryer, so that they can dry sufficiently and do not stick on contact with the dryer walls. This is accomplished by placing the atomizer in, or adjacent to, a properly designed air-disperser.

Figure 5 flow patterns in spray drying
Figure 5 flow patterns in spray drying
A cocurrent configuration with nozzle atomizer is suited for commodity chemicals; a countercurrent design with a nozzle atomizer is best suited for products requiring heat treatment; a mixed-flow unit with a nozzle atomizer is ideal for coarse powders of heat-stable products.

Atomizer, disperser and drying chamber must all be properly configured to allow complete drying of all the droplets without deposits of wet material on the interior surfaces of the dryer. In addition, the total volume of the drying chamber and the flow patterns of the droplets and the air through the dryer must provide for sufficient contact time to allow evaporation of essentially all of the liquid. As a result, centrifugal atomizers are usually installed at the center of the roof of a relatively large-diameter spray dryer.

The heated air is introduced through a roof-mounted air disperser around the atomizer, creating a cocurrent flow of air and product. By coming in contact with the droplets as soon as they are formed, the heated air causes rapid surface evaporation, and keeps the solids relatively cool.

By the time evaporation slows down and becomes limited by diffusion of liquid from the center of the droplet to the surface, the particles have passed to a cooler region of the dryer. Therefore, heat-sensitive products can often be spray-dried using elevated temperatures in the inlet gas, even though those temperatures would damage the product in an oven or other processes that are not cocurrent, or as fast as spray drying. The larger the particle size desired in the final powder, the larger must be the diameter of the drying chamber, regardless of the unit's total throughput.

When coarse powders are needed in small production rates, a pressure-nozzle spray, in fountain configuration, is often found to be a lot more practical. Here the spray travels upward until overcome by gravity and the downward flow of air. It then reverses direction and falls, finally landing in the bottom cone of the drying chamber.

The big drawback in fountain-nozzle dryers is that the process is not cocurrent. Rather, it is mixed flow, and drying actually begins in a cooler part of the dryer and continues into the hottest zone. Since each droplet is already partly dried, the evaporative cooling effect is lessened and the chance of thermal degradation becomes greater. Sometimes lower inlet temperatures solve this problem, but also reduce total evaporation capacity.

The third most commonly used configuration has pressure nozzles at the top of a dryer, spraying cocurrently with the heated air. This takes advantage of evaporative cooling, but often requires the dryer to have a cylinder height of about 20 m. These "nozzle towers" are often used for foodstuffs, dyes, pesticides and other heat-sensitive products that must also be in a coarse, free-flowing powder form.

Collecting the dried powder

Once the product is dried to a free-flow-ing powder, it must be separated from the drying gas, which is now cooled and contains the evaporated liquid. Coarser powders are most easily collected directly from the bottom of the drying chamber cone. In this arrangement, the spent drying gas exits through an outlet duct in the center of the cone. The reversing of the gas flow allows the greatest fraction of the powder to settle in the cone, and slide to the bottom outlet often equipped with an air-lock.

Because the spent drying gas has some entrained powder, cyclones or fabric filters are often used to clean the gas. In some cases, the combination of cyclones followed by a wet scrubber proves more effective. If the powder is very fine, little is collected in the drying chamber. In this case, the cyclones or even the bag collector can become the primary collection point. Chamber collection is eliminated by using a U bend at the outlet for both gas and powder from the chamber to the other collectors.

Process gas flow

The flow of drying gas through the system is much the same as for any gas-suspension drying system. Heating by direct combustion of natural gas turns out to be the most efficient. Fuel oil or propane backup is often provided when gas curtailment is possible. If indirect heating is required, shell-and-tube or finned-tube exchangers are used with steam or a heat-transfer fluid as a heat source. Electric heaters are used on small-scale dryers. In some instances, however, waste heat from another process is recovered either by direct injection into the drying gas stream or by heat exchanger.

Industrial radial fans are used to move the gas through the system, employing a combination of forced and induced draft, or induced draft only. If ambient air is the drying gas, it may be filtered by coarse filters to remove leaves, dirt and so on. If a very clean process is required, high-efficiency particulate air filters can be used. Ductwork with appropriate dampers, expansion joints, vibration isolators and noise-abatement devices is supplied with most dryers. All equipment is usually insulated and clad to minimize heat loss and condensation, and personnel hazard.

Process design and control

Evaporation rate in a spray dryer is directly proportional to the product of the temperature difference from inlet to outlet and the mass flow of gas through the system. Outlet temperature is established by the desired moisture content in the product according to that product's equilibrium isotherm. Since true equilibrium is never reached, actual values are usually determined experimentally.

Inlet temperature is also determined by experience and should be as high as possible without product degradation. Then, for a given evaporation rate, the required process gas flow can be determined from the temperature difference. All system components can be sized based on gas flow. A gas residence time must be selected from experience, based on particle size desired and the product's known drying characteristics. This permits direct calculation of a chamber volume.

At this point, the method of atomization must be selected and matched with chamber dimensions to obtain the desired volume and configuration with respect to the atomized cloud. If nothing is known about the product, one needs to conduct pilot-scale experiments.

Once designed and built, the drying system needs fairly simple controls. Although one should have a rough estimate of the actual gas flow through the dryer, it is usually best to fix the flow at the design rate. Since outlet temperature determines the moisture content in the final product, the temperature must be controlled and modulated with respect to other changes in the system. In the simplest case, the outlet temperature controls the heat input to the feed and thereby the inlet temperature, while holding the feed rate constant. In fact, dryers with a small nozzle atomizer do just that, using a single feedback control loop. One slightly more advanced approach is to use a "cascade control configuration" in which the outlet temperature controller can change the inlet controller's set point to achieve correct final moisture level in the product.

Pressure drops across filters and cyclones, and the pressure in the drying chamber are usually monitored, not controlled, to assure that the system is operating properly. Centrifugal atomizers require monitors for lube-oil flow, temperature, and vibration. On the other hand, nozzle systems require feed-pressure monitoring.

Although a spray dryer can be operated with simple controllers, it is becoming normal practice to use programmable logic controllers (PLCs), which offer greater capability in monitoring and alarming functions. In addition, these PLCs can initiate programmed startups and shutdowns. Inclusion of a personal computer offers data logging, trend analysis and other features used in statistical process control and other quality-assurance programs.

'Kinetics' of drying

The relationship between the moisture content of solids being dried and the relative humidity of the drying gas is best described by the isothermal equilibrium diagrams, or isotherms for short (Figure 2). Clearly, the higher the temperature of a solid in contact with a gas of constant humidity, the drier the solid.

Thus, one can control the moisture content of the solids product by controlling its temperature in the presence of a gas at constant humidity. Conversely, if the maximum temperature is limited by product or process requirements, the product's moisture content can be controlled by adjusting the gas humidity.

However, in order to create a driving force for evaporation of the moisture in the wet solids feed, one must contact the solids feed with a gas stream having a considerably higher temperature than that required to achieve equilibrium at the desired final moisture. The temperature difference determines the amount of evaporation a given quantity of gas can accomplish.

Another important factor is the rate at which moisture leaves a product as the material passes through a dryer. This dynamic relationship is best illustrated graphically (Figure 3). From point A to point B, evaporation is rapid and at a constant rate. This is referred to as zero-order drying. This form of drying occurs when free moisture is evaporated from the surface of solid particles.

Figure 2
Figure 2: data for the isotherms are obtained by exposing a sample of the material to an environment maintained at a known relative humidity until the sample reaches a steady weight.
Figure 3
Figure 3: the 'kinetics' of drying can be influenced by the drying time as well as the product temperature.

Beyond point B, drying becomes slower and moisture content often reaches an equilibrium with the gas, even at temperatures above the boiling point of the liquid being evaporated. This occurs when the residual moisture is trapped in micropores or capillaries, or is physically or chemically adsorbed. This stage is called first-order drying. Examination of the drying curve provides critical insights into the behavior of the various types of gas-suspension drying systems.

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