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Cross-section/band area (ft2) 100 200 300 400 500 600 Curves 1 and 3 Shell volume (ft3) 1000 2000 3000 4000 5000 6000 Curve 2

Bed cross-section (ft2) 20 40 60 80 100 120 Curve 4

Figure 13. Approximate capital costs of (1) spray dryers, (2) rotary dryers (direct-oil-fired, mild steel, excluding product collection system), (3) continuous-band dryers steam-heated with roller extruder), (4) continuous-fluid-bed dryers (direct-oil-fired with primary cyclone).

Figure 13. Approximate capital costs of (1) spray dryers, (2) rotary dryers (direct-oil-fired, mild steel, excluding product collection system), (3) continuous-band dryers steam-heated with roller extruder), (4) continuous-fluid-bed dryers (direct-oil-fired with primary cyclone).

Dryer cross-section (ft2) 10 15 20 25 30 35 40 45 50 55 60 Curve 6 Drum surface area (ft2) 20 30 40 50 60 70 80 90 100 110 120 Curve 7 Tray area (ft2) 200 300 400 500 600 700 800 900 100011001200 Curve 8

Figure 14. Approximate capital costs of (5) pneumatic dryers (direct-fired, stainless steel, including primary cyclone), (6) double-cone dryers (stainless steel, excluding vacuum equipment), (7) drum dryers (cast-iron drums, dip feed), (8) tray dryers (steam/electric-heated).

condition. As a result, drying forms an important part of most food and chemical processes and accounts for a significant proportion of total fuel consumption.

The rapid escalation of fuel costs over the past 10 yr, together with the prevailing uncertainty of future availability, cost, and possible supply limitations, highlights the continued need to actively engage in the practice of energy conservation. Some of the factors affecting dryer efficiency and certain techniques designed to reduce the cost of the drying operation are discussed in the next few sections.

In making an appraisal of factors affecting dryer efficiency, it is useful to draw up a checklist of those items that have a significant bearing on both operation and economy. It also is appropriate prior to examining various op tions to emphasize that in the final analysis, the primary concern is the "cost per unit weight" of the dried product. This single fact must largely govern any approach to dryer selection and operation. Additionally, there is an increasing necessity to consider the unit operation of drying in concert with other upstream processes such as mechanical dewatering and preforming techniques in a "total energy" evaluation.

In considering which factors have a bearing on dryer efficiency and what can be done to maximize that efficiency, the following aims should be kept in mind:

1. Maximum temperature drop across the dryer system indicating high energy utilization. This implies maximum inlet and minimum outlet temperature.

2. Employ maximum permissible air recirculation, ie, reduce to an absolute minimum the quantity of dryer exhaust, having due regard for humidity levels and possible condensation problems.

3. Examine the possibility of countercurrent drying, ie, two-stage operation with exhaust gases from a final dryer being passed to a predryer, or alternatively, preheating of incoming air through the use of a heat exchanger located in the exhaust gases.

4. Utilize "direct" heating wherever possible in order to obtain maximum heat release from the fuel and eliminate heat-exchanger loss.

5. Reduce radiation and convection losses by means of efficient thermal insulation.

While the above clearly are basic requirements, there are a number of other areas where heat losses occur in practice, including sensible heat of solids. Furthermore, other opportunities exist for improving the overall efficiency of the process. These are related to dryer types, methods of operation, or possibly the use of a combination of drying approaches to obtain optimum conditions.

Types of Dryers

Considering the requirements for high inlet temperature, the "flash" or pneumatic dryer offers great potential for economic drying. This stems from the simultaneous flash cooling effect that results from the rapid absorption of the latent heat of vaporization and allows the use of high inlet temperatures without thermal degradation of the product. This type of dryer also exhibits extremely high evaporative rate characteristics, but the short gas-solids contact time can in certain cases make it impossible to achieve a very low terminal moisture. However, a pneumatic dryer working in conjunction with a rotary or a continuous fluidized-bed dryer provides sufficient residence time for diffusion of moisture to take place. Such an arrangement combines the most desirable features of two dryer types and provides a compact plant and conceivably, the optimum solution.

As a further example, it is common practice in the process industries to carry out pretreatment of filter-cake materials using extruders or preformers prior to drying. The primary object is to increase the surface area of the product in order to produce enhanced rates of evaporation and smaller and more efficient drying plants. It is interesting to examine the improvements in energy utilization in a conveyor-band dryer resulting from a reduction in overall size of the dryer simply because of a change in the physical form of the feed material.

Consider a typical case. As a result of preforming a filter cake, the evaporative rate per unit area increases by a factor of 2, eg, 1.9 to 3.8 lb/ft2 h. This permits the effective dryer size to be reduced to half that required for the non-preformed material and, for a plant handling 1 ton/h of a particular product at a solids content of 60%, the radiation and convection losses from the smaller dryer enclosure show an overall reduction of some 140,000 Btu/h. Although it is necessary to introduce another processing item into the line to carry out the pretreatment, it can be shown that the reduction in the number of dryer sections and savings in horsepower more than offset the power required for the extruder. This differential is approximately 15 kW and the overall saving in total energy is approximately 10% ie, 466 kW compared with 518 kW. Furthermore, the fact that the dryer has appreciably smaller overall dimensions provides an added bonus in better utilization of factory floor space.

Drying Techniques

If, as stated, the prime concern is the cost per pound of dried product, then major savings can be achieved by reducing the amount of water in the feedstock to a minimum prior to applying thermal methods of drying. Again, since it generally is accepted that the mechanical removal of water is less costly than thermal drying, it follows that considerable economies can be made when there is a substantial amount of water that can be readily removed by filtration or centrifuging. This approach, however, may involve changing the drying technique. For example, whereas a liquid suspension or mobile slurry would require a spray dryer for satisfactory handling of the feed, the drying of a filter cake calls for a different type of dryer and certainly presents a totally different materials handling situation.

In approaching a problem by two alternative methods, the overall savings in energy usage may be considerable as illustrated in Figure 15, which is a plot of feed moisture content versus dryer heat load. As may be seen, the difference in thermal energy used in (A) drying from a moisture content of 35% down to 0.1%, or alternatively (B) drying the same material from 14% down to 0.1% is 24.3 X 10®

Figure 15. Thermal energy required for drying versus feed moisture content (duty as in Table 1).

Btu. Route A involves the use of a spray dryer in which the absolute weight of water in the feedstock is considerably greater than in route B, which involves the use of a ther-moventuri dryer. Both types, incidentally, come under the classification of dispersion dryers and generally operate over similar temperature ranges. As a result, their efficiencies are substantially the same.

It will be apparent from Table 3 that the spray dryer is at a considerable disadvantage because of the requirements for a pumpable feed. This highlights the need to consider the upstream processes and, where a particular "route" to produce a dry product requires a slurried feed, to consider whether a better alternative would be to dewater mechanically and use a different type of dryer.

Comparing these two alternative methods for drying a mineral concentrate, the spray dryer uses a single-step atomization of a pumpable slurry having an initial moisture content of 35% and employs thermal drying techniques alone down to a final figure of 0.1%. The alternative method commences with the same feedstock at 35% moisture content but employs a rotary vacuum filter to mechanically dewater to 14%. From that point, a pneumatic dryer handles the cake as the second of two stages and thermally dries the product to the same final moisture figure.

While the difference in thermal requirements for the alternative routes already has been noted, it also is necessary to take into account the energy used for mechanical dewatering in accordance with assumptions outlined in the footnote to Table 3. From reference to this chart, it will be evident that the two-stage system requires approximately one-third of the energy needed by the single-stage operation but that the savings are not limited to operating costs. There also are significant differences in the basic air volumes required for the two dryers, which means that ancil-laries such as product collection-gas cleaning equipment will be smaller and less costly in the case of the pneumatic dryer. The same applies to combustion equipment, fans, and similar items. Actually, the chart shows that capital cost savings are in the nature of 50% in favor of the filter and pneumatic drying system.

This illustration amply demonstrates the need for a more detailed consideration of drying techniques than perhaps has been the case in the past. It also points to the desirability of examining a problem on a "total energy" basis rather than taking the drying operation in isolation. Such a full evaluation approach often will prove it advantageous to change technology, ie, to use a different type of dryer than the one that possibly has evolved on the basis of custom and practice.

Operating Economies

Looking at dryer operations in terms of improving efficiency, it is interesting to see the savings that accrue if, for example, a pneumatic dryer is used on a closed-circuit basis, ie, with the recycle of hot gases instead of total rejection. Briefly, the "self-inertizing" pneumatic dryer consists of a closed loop as shown in Figure 16 with the duct system sealed to eliminate the ingress of ambient air. This means that the hot gases are recycled with only a relatively small quantity rejected at the exhaust and a correspondingly small amount of fresh air admitted at the burner. In practice, therefore, oxygen levels of the order of 5% by volume are maintained. This method of operation raises a number of interesting possibilities. It permits the use of elevated temperatures and provides a capability to dry products that under normal conditions would oxidize. The result is an increase in thermal efficiency. Furthermore, the amount of exhaust gas is only a fraction of the quantity exhausted by conventional pneumatic dryers. This is clearly important wherever there may be a gaseous effluent problem inherent in the drying operation.

A comparison of a self-inertizing versus a conventional pneumatic dryer is detailed in Table 4 with the many advantages clearly apparent. Drying with closed-circuit

Table 3. Comparison of Operating Conditions and Energy Utilization of Pneumatic-and-Spray Dryers Processing Concentrates


Pneumatic dryer

Spray dryer

Feed rate

Filtrate rate

Evaporative rate

Production rate

Initial moisture content

Final moisture content

Air inlet temperature, °F

Air outlet temperature, °F

Total thermal input

Basic air volume (BAV) at NTP

Fuel consumption

Total dryer horsepower

Total filter horsepower

Total system horsepower

Total thermal input expressed as kilowatts

Total energy input to system

88,200 lb/h 21,500 lb/h 9200 lb/h 57,500 lb/h 14% 0.1% 752 230

21.43 X 106 Btu/h 28,100 NCFM 1080 lb/h 187 kW 295 kW 482 kW 6280 kW 6762 kW

88,200 lb/h

66.2 X 106 Btu/h 86,700 NCFM 3290 lb/h 530 kW

530 kW 19,400 kW 19,930 kW

Note: An assumed volumetric flow of 3 ft3/min ft2 of fitter area with approximately 1 hplO ft3 min has been used as being typical of the flow rates and energy requirements for the filtration or mechanical dewatering equipment.

Figure 16. Self-inertizing pneumatic dryer with backmix facility.

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