Batch dryers, control

O. Fadum and G. Shinskey, Saving energy through better control of continuous and batch dryers. Control Engineering, 27(3) 69-72 (March 1980). 

Typical automatic batch-dryer control systems use the exhaust-air temperature as the controlled variable to determine when to end the drying process. Shinskey [37] and... 

Typical automatic batch-dryer control systems use the exhaust-air temperature as the controlled variable to determine when to end the drying process. Shinskey [37] and Fadum and Shinskey [38] have described an alternative control scheme employing inferential control. The system has proven to be an effective substitute for online moisture analysis in terminating drying. This method is based on the following equation ... 

Batch dryers have high labour requirements, but close control can be maintained over the drying conditions and the product inventory, and they are suitable for drying valuable products. 

Process designs vary among manufacturers, however, there is generally an air flow from the top or sides blowing down or across the bed of material in a zoned area. This zone may have its own batch dryer with a fan, a heater, instrumentation and duct work, or it may be manifolded such that the air flow is regulated to maintain a certain temperature in that section using the evaporative cooling effect to control the outlet temperature from that zone. 

Robinson described a control system based on a temperature-drop model to determine and control the product moisture content at any appropriate point inside batch dryers [2]. It is based on the model... 

Batch drying is a time-dependent and repeatable process usually used for the drying of small volumes, sensitive products, or valuable materials (e.g., pharmaceutical products). Manual control of batch dryers requires higher labor costs per unit product throughput than continuous processes, necessitating the need of automatic control. The major benefits of automatic control of batch-drying processes are increased safety, increased production through a reduction in cycle time, and increased consistency of product quality. In... 

The maximum temperature at which the drying material may be held is controlled by the thermal sensitivity of the product and this varies inversely with the time of retention. Where lengthy drying times are employed, as for example in a batch shelf dryer, it is necessary to operate under vacuum in order to maintain evaporative temperatures at acceptable levels. In most continuous dryers, the retention time is very low, however, and operation at atmospheric pressure is usually satisfactory. As noted previously, dryer selection is considered in some detail in Volume 6. 

A conventional wastewater treatment system with an average flow rate of 160,000 gpd produces effluent suitable for NPDES discharge. Metal hydroxide sludges are dewatered in a 15 cu. ft filter press producing more than one half ton of filter cake per day. The filter cake is further dewatered in a 7 cu. ft, batch-type sludge dryer. Based upon recommendations by their consultant, the firm also uses the sludge dryer to dehydrate nickel strip solutions. Two reverse osmosis systems are used for partial nickel recovery. Trivalent chromium is recovered by drag-out control and evaporation. 

Mean drying time for the oven tray process is 19.2 hr. All 15 batches were dried within the specified time of 16 to 20 hr. No seasonal influence was apparent. The average moisture content of these batches is 1.2% the standard deviation is 0.3%. The 15 batches dried using the fluid bed dryer had a residual moisture of 0.8% (SD = 0.1%). Drying time is mechanically controlled and not recorded. The statistics favor the fluid bed process it is more efficient and uniform. There is nothing in these data to disqualify the oven tray dryer from further use, however. 

In conclusion, drug A production was shown to be within established specifications, and there is no reason to believe this will not be the case for future production as long as all practices are continued in their present form. Furthermore, there is no significant difference between batches produced by the tray dryer process and the fluid bed process. A validation report should memorialize these findings. The report should also recommend eliminating the option to use a no. 5 screen for the wet milling step and a no. 12 screen to pulverize the dried granulation. There is no experience or only limited experience with this equipment that supports its continued availability. In the same vein, the final blend time should be standardized at 10 min and automatically controlled by means of a timer. 

Motor-driven process compressors Turbine-driven process compressors Complex multi-unit packages TDC/PLC and batch controllers Rotary dryers / filters... 

ODORS PROM MEAL DRYER AND OIL DEODORIZATION—Odor is a qualitative parameter from the standpoint of both measurement and control. Motivation for abatement usually arises out of complaints of citizens residing nearby a facility. Sources of odor can be meal driers and rehned oil deodorization. Acidulation has been an odor producer in some isolated instances however, is most often because of basic poor design and operation usually coincident with batch processing in open-topped kettles. 

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