For many years, spray drying has been one of the most energy-consuming drying processes, yet it remains one that is essential to the production of dairy and food product powders. The challenge is to reduce processing costs while maintaining product quality. The solution is two-fold: Develop new technology to provide greater dryer efficiency and introduce new ways to recover and reuse energy from the primary process.
Basically, spray drying is accomplished by atomizing feed liquid into a drying chamber, where the small droplets are subjected to a stream of hot air and converted to powder particles. As the powder is discharged from the drying chamber, it is passed through a powder/air separator and collected for packaging. Most spray dryers are equipped for primary powder collection at an efficiency of about 99.5%, and most can be supplied with secondary collection equipment if necessary.
Figure 2 shows a conical-bottom chamber arrangement with a side air outlet, high pressure nozzle atomization and pneumatic product transport beneath the chamber. This single-stage dryer is well suited to making relatively large particles of dairy products or proteins for cattle feed. If the product cannot withstand pneumatic transport, it can be taken out, unharmed, directly from the chamber bottom.
Atomization TechniquesProper liquid atomization -- as dictated by the properties of the feed and the desired final powder characteristics -- is essential for satisfactory drying and to produce a prime-quality powder. To meet varying parameters, there are three atomizing systems: centrifugal disk, high pressure nozzle or steam injection.
With centrifugal or spinning disk atomization, liquid feed is accelerated to a velocity in excess of 300 ft/sec to produce fine droplets that mix with the drying air. Particle size can be controlled by wheel speed, feed rate, liquid properties and atomizer design. There are no vibrations, little noise and small risk of clogging. Furthermore, the system operates with low power consumption and provides feed-rate capacities in excess of 200 tons/hr.
With the pressure nozzle system, liquid feed is atomized when forced, under great pressure, through a narrow orifice. This approach offers more options when selecting the spray angle, direction of spray and atomizer position within the chamber. It also allows cocurrent, mixed-current or countercurrent drying with the production of powders having particularly narrow particle size distribution and/or coarse characteristics. Because particle size is dependent upon feed rate with this design, dryers using pressure nozzles have somewhat limited capabilities to change product characteristics and operating rates.
These two atomization methods are well established and often are incorporated into the same dryer. They help produce a most acceptable product and offer maximum flexibility for drying a variety of products. The third method -- steam injection technique -- can produce a better than average product with a significant increase in bulk density and fewer fines.
As feed droplets are subjected to heated air during conventional drying operations, water evaporates and diffuses to the particle surface, forming a hard shell. Beneath this shell, a small amount of residual moisture spreads to void areas, vaporizes, mixes with air, expands and penetrates weak points. This creates porous particles. When steam is added -- at the right point and in the correct amount -- to a properly designed atomizer, air encapsulation in the liquid droplet is eliminated. Instead, the drying particles collapse to produce a dense, void-free powder. Controlling the amount of steam injected permits precise adjustment in powder bulk density. Furthermore, reducing the air-exposed surfaces often reduces product oxidation and prolongs powder shelf life.
Cutting Drying CostsThe best way to reduce energy usage in spray drying is to reduce the specific energy consumption of the process. Experience has shown that for many products, the dryer inlet temperature can be raised, the outlet temperature lowered, and a larger temperature differential thus created. While this procedure substantially cuts energy needs and does not harm the most heat-sensitive products, care must be taken and a proper balance struck. The nature of the product usually defines the upper limit -- for example, the denaturation of milk protein or discoloring of other products. A higher inlet temperature requires close control of the airflow in the spray drying chamber and, in particular, around the atomizer. Furthermore, it must be noted that a lower outlet temperature will increase the humidity of the powder.
In the past, the typical inlet and outlet temperatures of a milk dryer were 356 and 203oF (180 and 95oC), respectively. Newer designs have increased the inlet temperature to 428oF (220oC) or higher and reduced the outlet temperature to around 185oF (85oC). This change was made possible by the development of multistage drying systems.
To obtain optimum drying economies combined with improved powder characteristics, some processors use a fluid-bed dryer/cooler (figure 3). In this type of dryer, powder is conveyed to the inlet chute of the vibrating fluid-bed by a metering device equipped with a variable speed drive. The fluid bed is divided into three sections, each with an air inlet system. In the first section, powder can be humidified by a wetting agent, then agglomerated to the required particle size. In the second section, the agglomerates are dried to the desired moisture content; they are cooled in the third section. During the process, air-entrained fines are recovered in a cyclone collector for return to the wetting zone. Residence time and temperature can be varied for optimum drying conditions. The fluid bed can be mounted externally with two- and three-stage dryers or the fluid-bed drying/cooling can be integrated within the drying chamber.
Consider a two-stage design as an example. Powder at approximately 7% moisture is discharged from the primary drying chamber to the fluid bed for final drying and cooling. This prolongs drying time from approximately 22 sec in a typical single-stage dryer to more than 3 min, making it possible to use lower drying temperatures and reduce the heating energy usage by 15 to 20%. At the same time, the slower, more gentle drying process produces more solid particles of improved density and solubility. If desired, the fluid bed can be designed for rewet instantizing or powder agglomeration and for the addition of a surface active agent such as lecithin.
Heat Recovery EquipmentAlthough it is possible to reduce direct power use by widening the gap between spray dryer inlet and outlet temperatures, optimum thermal conditions generally require the use of heat recuperators as well.
Although there are many recuperators available for recovering heat from drying air, only a few are suited for the spray drying process due to its dust-laden air, which tends to contaminate the heat exchange surfaces. To be effective, recuperators for spray dryer use should have the following properties: modular design, high thermal efficiency, low pressure drop, automatic cleaning, large temperature range, stainless steel construction and large capacity. Two such recuperators are the air-to-air tubular and air-to-liquid plate designs.
Air-to-Air Heat Recuperator. For optimum flexibility, this heat exchanger has a modular design, with each module consisting of 304 tubes welded to an end plate. Available in standard lengths, the modules can be used for counterflow, cross-flow or two-stage counterflow.
Air-to-Liquid Recuperator. When an air-to-air tubular recuperator is impractical due to space limitations or the length of the distance between the outlet and inlet air to be preheated, a liquid heat recovery system is available. In this plate recuperator, waste air is cooled in counterflow by circulating a water/glycol mixture. At high temperature, circulating thermal oils are used. Heat recovered by this means generally is recycled to preheat the drying air, but it also can be used for other purposes.
Other Heat Recovery Methods. Many spray drying plants are equipped with bag filters to minimize emissions and recover valuable powder. This filtration system can be coupled with a finned tube recuperator for heat recovery. Particularly effective when the air is not dust-loaded, this system is compact, flexible and, usually, inexpensive.
Plants equipped with scrubbers also provide an inexpensive way to recover heat. Because the scrubber is a heat exchanger, some heat from the waste air can be captured and recycled by passing the scrubber water through a finned tube preheater. An added benefit is lower water consumption due to a reduction in scrubber liquid evaporation. However, if the scrubber liquid is cooled intensively, such an efficient recovery of latent heat will occur that a net condensation in the scrubber will follow. This will, of course, create another pollution problem if the solids concentration in the fluid is so low that it cannot be returned to the process. To offset this effect, a recuperator can be installed immediately before the scrubber.