Flash Drying Hazardous Materials

In many continuous flash drying applications, the products being dried and solvents being evaporated are often non-toxic and do not present an explosion hazard. Therefore, system design is relatively straightforward and the environmental issues are of minor concern as water vapor and sometimes combustion gases are the only byproducts emitted to atmosphere.
    However, when dealing with hazardous materials, and when the wet product requires evaporation of volatile solvents such as cyclohexane, heptane, or methanol, the system design requires careful consideration from both a safety and environmental perspective.
    A typical continuous flash drying system comprises the following components: a process blower to provide the motive power; an air heater (usually a direct-fired gas burner or resistive electric heater); an inlet rotary airlock; a wet product feeder; a flash dryer; a product filter receiver; a discharge airlock; an exhaust fan; instrumentation; and control panel.
    Two control loops are required, one for temperature and one for pressure. The temperature-control loop works to maintain a specific outlet temperature on the dryer. The pressure (vacuum)-control loop is set at a level that will promote dry material flow to the collector without generating blow back or excess vacuum.
    Considering the safety aspects first, we must look at a method to reduce the possibility of fire or explosion within the dryer system. In a standard system, hot air is used as the heat-transfer medium. When drying products containing volatile solvent components it is often advantageous to use an inert gas as the heating and drying medium. Using an inert gas such as nitrogen presents two issues. Firstly, from a cost perspective it would be beneficial if the system could be a closed-loop design so that nitrogen is not wasted by using an open configuration and immediately emitting the saturated gas to atmosphere, but is instead recycled. Secondly, the options for heating the incoming gas stream are limited because the most common heat source, a direct-fired gas burner, cannot be used, and either a direct-type electric heater or an indirect gas, steam, or oil heat source must be used.
    Before commencing feed of solvent-laden wet material to the dryer system it must also first go through an initial purge cycle to ensure that the oxygen level of the atmosphere within the closed-loop dryer system is depleted to a predetermined safe level. Once a safe level has been reached, the inlet temperature of the dryer can be elevated to standard operating conditions and the wet material feeder can start.
    Once in operation, the dryer continuously discharges dried solid powder and a volatile solvent-laden outlet gas stream. First, the dried solid powder material is separated from the gas stream in a reverse-pulse filter receiver and then the solvent-laden gas stream is passed through a HEPA filter to ensure all particulates are removed. Next, the cleaned gas stream is passed through a cooling heat exchanger to condense the valuable solvent vapor into a liquid that can then be recovered and reused in the upstream chemical process. In addition to the cost benefit of reusing the solvent, the environmental issues are substantially reduced because the amount of volatile solvent emitted to the atmosphere, via any losses through the air locks used at the feed inlet and product outlet, is only a tiny fraction of the total mass of solvent present in the wet feed. The majority of the system, from the dryer exit, through the interconnecting ductwork, filter-receiver and HEPA filter, is operated under a slight negative pressure to ensure that there are no emissions of hazardous vapors to the atmosphere in the process area.
    In conclusion, although the capital cost of an inerted closed-loop condensing flash-drying system may be higher than a standard open system, it is likely to be more cost effective over the long term than a standard system requiring secondary scrubbers to clean the gas discharge to meet emission regulations. Additional cost benefits are gained from recycling expensive solvents and inert gases.
    Simon Howard is a director of business development with Fluid Energy Processing & Equipment Co.. He has been in the powder processing industry for 20 years and specializes in particle-size reduction and flash-drying system applications.