A New Pulse for Spray Drying
Pulse combustion spray dryers were designed to overcome this limitation. They produce powders that are comparable to conventional systems but are far less affected by corrosion or abrasive wear. This is because pulse dryers use “gas dynamic” atomization rather than mechanical shear, and they require very low pressure (less than 1 pound per square inch [psi]) in the liquid feed system. With a pulse combustion spray dryer, manufacturers that work with abrasive materials can reduce their maintenance costs while generating powders with increased surface area and functionality, as well as a tighter particle size distribution.
How Pulse Drying WorksAir is first pumped into the pulse combustor’s outer shell at low pressure, where it flows through the patented unidirectional air valve. The air then enters a tuned combustion chamber (known as a “Helmholtz Resonator”) where fuel is added, and the air valve closes. The fuel-air mixture is ignited by a pilot and explodes, creating hot air, pressurized to about 3 psi above combustion fan pressure. The hot gases rush down the tailpipe toward the atomizer, and the air valve reopens, allowing the next air charge to enter. The fuel valve admits fuel, and the mixture explodes in the hot chamber. This cycle is controllable from about 80 to 110 Hz.
Just above the atomizer, quench air is blended in to achieve the desired product contact temperature. The proprietary atomizer in the pulse combustion system releases the liquid into a carefully balanced gas flow, which dynamically controls atomization, drying and particle trajectory. The atomized liquid then enters a conventional tall-form drying chamber. Downstream, the suspended powder is retrieved using standard collection equipment, such as cyclones and baghouses. Figure 1 illustrates the operation of a pulse combustion drying system.
Benefits of Pulse DryingThe main differences between pulse and conventional spray dryers are in atomization and heat transfer (see Table 1). In a pulse combustion spray dryer, the feed material is atomized and dried in less than one second. This rapid drying is accomplished by the system first co-locating the point of atomization and the application of heat for evaporation, then exposing the atomized droplets to high heat for a very short time, and finally providing an environment of extreme turbulence that promotes very high rates of heat transfer and drying. In a conventional spray dryer, by contrast, the atomization and application of heat are separate functions in space and time, and the heat transfer rate is lower and less turbulent. Thus, drying times are often longer and gas and electrical input are higher.
Another major difference affecting powder quality has to do with the shearing of the feed material. Conventional spray dryers atomize the feed material either by forcing it through a spray nozzle at high pressure or by introducing it onto a high-speed rotary disk. In either case, high mechanical shear is applied to the feed, and shear-sensitive materials are damaged. By contrast, a pulse combustion dryer imparts zero mechanical shear because it uses the differential velocity of the high-speed combustor exhaust gases and the low-speed, low-pressure feed to atomize the material. An example of this type of atomization can be observed by pouring a drink out of a car window while driving on the highway. To simulate pulse combustion gas dynamic atomization, the car would have to be driven at 500 mph in a 1000 degrees F desert.
Other advantages of pulse combustion spray drying over conventional spray drying include the ability to atomize and dry feed materials with much higher solids and viscosities (up to 100,000 cps), thus greatly reducing drying costs; higher overall thermal efficiency (due to the high differential between their inlet and outlet temperatures, known as the DT); lower airflows; and a smaller drying chamber footprint.
Applying the TechnologyPulse combustion spray drying systems are not new. However, recent advances in the technology are making it viable as a superior drying method to many existing drying systems. For all abrasive or corrosive feed materials, maintenance costs are far lower than with a conventional spray dryer because the feed system of the pulse dryer contains no wear parts. According to Michael Vincler, manager of special projects for OMG Americas, “Our production unit has been running for nine years. During this time we have experienced very little maintenance problems with the pulse drying system. Not having to use high-pressure feed pumps or high-speed wheels has saved us thousands of dollars in parts and down time.”
The pulse combustion technology is also providing advantages in applications where superior particle surface characteristics, a uniform mean particle size and a tight particle size distribution are key to the performance of the final product. Germany’s Degussa-H?ls AG, one of the world’s largest specialty chemical companies, installed a pilot-sized pulse dryer in 1998 to investigate the advantages of this technology on its products and quickly discovered the benefits. “The greatest benefit is the production of finer particles compared with regular spray dryers, and it also offers new possibilities in designing physical product properties,” said Stefan Schulze, who is in charge of the process technology group’s pilot plant near Frankfurt.
Another advantage of the technology is its low-pressure, open pipe feed system, which provides the ability to process higher-solids feeds without dilution, yielding higher powder production rates and much lower processing costs per finished pound.
Production-Scale SuccessIn addition to the proven performance of the pilot-scale systems (40 lbs per hour evaporation), the new pulse combustion drying technology has recently been successfully scaled up to a production size. The production-scale dryer is 32 ft tall and composed entirely of stainless steel. It operates at a heat release of 600,000-800,000 BTU/hour and evaporates water at up to 400 lbs per hour. It features a state-of-the-art control system and three remote monitoring cameras that allow manufacturers to view all stages of powder production. The system also has a built-in modem for remote monitoring and troubleshooting.
With today’s advanced pulse combustion spray drying technology, manufacturers can make better powders at a lower cost.