Primary-drying control

This system is called wet NO control. Water or steam is injected into the primary combustion zone. This method has been used ef fectively in the past. Current installations are using this system when the water or steam is readily available or if they are already part of the process. Maintenance costs are higher when compared with dry control, because this method requires high quality water. If high quality water is not used, the corrosion associated with dissolved minerals in the water may prematurely damage the turbine. 

Searles, J.A., Carpenter, J.F., Randolph, T.W. The ice nucleation temperature determines the primary drying rate of lyophilization for samples frozen on a temperature-controlled shelf. PDAJ. Pharm. Sci. Technol. 90, 860-871, 2001... 

The primary drying time of the product, which can be defined as the time elapsed from the moment when the vacuum is created in the chamber to the disappearance of the last ice crystals in the product, is obviously a dependent parameter as the rate of ice sublimation is controlled by the same factors as product temperature. This is also the case for the secondary drying time of the product, i.e., the time elapsed from the moment when ice sublimation is complete to the end of the cycle. During this period, water is desorbed from the product at a rate dictated both by technical factors and by product characteristics. 

Process control determination of the end point of primary drying... 

Primary drying Evacuate chamber, control at a target set point of 80 pHg ( 20 pHg). Ramp shelf to —10 C at an average controlled rate of 30°C/hr Control shelf at target set point of —10 C ( 5°) for 15 hr... 

The chamber pressure during the sublimation step (i.e. the primary drying process) has been found to be related to the product and shelf-surface temperatures [8] however, determining the shelf temperature required is more difficult as it depends on the nature of the heat transfer fluid used to control the shelf temperature and also on the particular design of the freeze-dryer. 

For safety reasons, the pressure is set in such a way that the temperature is 5°C below the melting point of the product. On an hourly basis, the freeze-dryer automatically uses a drain valve to allow the pressure within the system to drop. A simultaneous fall in product temperature is indicative of the presence of frozen solvent that should thus be extracted during the sublimation or primary drying step. Detecting any residual solvent in this way avoids the risk of the product melting down. Before the desorption step is started, this pressure control cycle is repeated until no further drop in product temperature is observed. 

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