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Flammable powders

The first special case is the problem created by powders that, once dried, become flammable or explosive. Essentially, all organic materials and many inorganics, such as pure powdered metals, can burn. The combination of air, heat and fine particles existing in most dryers presents the perfect conditions for combustion.

This problem is usually addressed in one of four ways. Provision can be made to relieve the rapid pressure rise created during a deflagration. This is done by installing blowout panels or doors. The recommended size for a relief area can be calculated from the U.S. National Fire Protection Assn. (NFPA; Quincy, Mass.) Code 68, once the properties of the dust are characterized [8].

A second method of protection is explosion suppression. Here, canisters filled with a suppressant are installed on the equipment and triggered to release the suppressant on detection of excessive heat or pressure. In the application of either relief panels or suppression systems, all equipment must be constructed for a design pressure somewhat higher than normal.

In a few cases, drying systems have been designed to withstand the full force of an explosion. This is generally considered impractical from both a fabrication and operation standpoint, but has been done successfully.

The final option for handling flammable powders is to inertize the system. With this method there are two further options. The first is a self-inertized system in which a direct-fired heater is operated with recycled drying gas, and only enough ambient air to support combustion.

Of course, the recycle gas must not only be cleansed of particles, but most of the water vapor must be condensed in order to maintain sufficiently low humidity in the drying system. An amount of gas equal to the combustion products plus slight excess air must be continuously purged from the system.

Flammable solvents

The final form of special system for flammable materials is suitable not only for flammable powders, but also for the solvent being evaporated when it is flammable. Companies dealing with pharmaceuticals, polymers, powdered metals and advanced ceramics commonly need to dry products from a solution or suspension in organic solvent. In these cases, the system must be completely inert, and there can be no ignition source present in the area. Recirculating nitrogen is usually used for the drying gas [9].

A typical closed-loop system is composed of an indirect heater, such as a finned tube or shell-and-tube heat exchanger, a drying chamber (be it spray, flash or fluid bed), product-separation equipment, and a condenser to recover the evaporated solvent (Figure 8). Fans, ducts, instrument connections, and other items in the loop must be gas-tight.

Figure 8

A closed-cycle system features drying in an inert gas (usually nitrogen) atmosphere to avoid contact of volatile organics with oxygen.

The system should be under slight positive pressure to prevent any in-leakage of oxygen. Typically, O2 analyzers are used to assure safe operation of these units. All motors, instruments and other electrical hardware must meet the appropriate area classification per NFPA 68.

The economics of drying

The difference between inlet and outlet drying-gas temperature is the driving force for evaporation in all gas suspension dryers. For a given quantity of evaporation therefore, the greater the temperature difference, the lower the gas flow required and the smaller the system. As a result, it is impossible to relate dryer size to evaporative capacity without doing so for a variety of temperature conditions. However, one certainty is that all thermal dryers are sized on the amount of evaporation with less accounting of the total solids.

Capital cost

Of all the systems discussed, the hybrid systems require the greatest amount of equipment, and hence, incur the highest capital cost. The least expensive are the conventional flash dryers, both in equipment and in installation. However, flash dryers are somewhat limited in the feed and product characteristics they can handle. Spray dryers and fluid beds fall in the middle.

The type of atomization, the preferred heat source, the drying chamber configuration and the pollution-control equipment are the main factors of overall system cost. To demonstrate the sensitivity to seemingly small changes in process specification, one can use a spray dryer as an example (Table).

Design Basis for a Spray Dryer
Feed rate 6,000 kg/h
Solids content 50%
Inlet temperature 300°C
Outlet temperature 100°C
Product rate 3,000 kg/h
Moisture in product Less than 0.1%
Plant elevation Less than 600
Table: Minor changes in the initial design data can have significant effects on capital and operating costs

For an average product, this would require a spray dryer with a drying chamber seven meters in diameter and a direct gas-fired heater with 3,000,000 kcal/h capacity. Capital cost for atomizer, air disperser, drying chamber, baghouse, ductwork, fans and instrument and control package amounts to about $750,000.

However, if the feed slurry is pre-concentrated to 65% solids, then the evaporative rate and the dryer size is only 54% of the original system. Cost is about 70%. This, of course, assumes the feed can still be atomized at 65% solids.

On the other hand, if the inlet temperature could be elevated to 450 XC, the system needs to be only 65% of the original size, costing about 80% of the original $750,000. If both increased solids and increased temperature difference were possible, the system would be only 36% the size of the original and cost about 55% as much.

Operating costs

Fuel efficiency of a gas-suspension dryer is best thought of as the ratio of heat input to heat used. This is calculated from the temperature difference across the dryer divided by the difference between inlet and ambient temperatures. The higher the inlet temperature and lower the outlet, the more efficient the process and the lower will be the heat consumed per kilogram of evaporation. As a rule, the energy requirement runs from about 670 kcal/kg of evaporation to as high as 1,400 kcal/kg.

Electrical power consumption is a relatively minor component of the overall operating cost, except possibly with large fluid beds handling dense products, and therefore, having high pressure drops. Typical power usage for medium-sized systems is 0.04 kW/kg of evaporation.

Aside from depreciation of initial installed cost, labor is the only other major operating cost. Generally, it requires one person to operate any of the systems described, regardless of size. The dryer operator can often be responsible for additional adjacent systems.

Large systems using multiple spray nozzles, such as nozzle-type spray dryers, may require additional attention, depending on the tendency of the feed to clog nozzles or the degree to which particle size analysis is performed to insure on-spec product. Today's control systems based on personal computers and programmable logic controllers provide the operator with all the needed information. Data logging and trend analysis also have greatly reduced the need for process supervisors or technicians to monitor drying systems as closely as in the past.

As with any new process, one should always obtain the assistance of people experienced in the field, whether they be from in-house technical service groups, outside consultants or equipment vendors. It is also important that laboratory and pilot facilities be available to generate the required empirical data. Based on these data, one can always modify a system to meet changing process requirements. However, if the wrong system is chosen in the first place, it can never be expected to operate satisfactorily.

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About this article
Fresh Options in Drying by Fred V. Shaw. Reprinted from Chemical Engineering

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