Chamber primary-drying control

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. 

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. 

The effects of pressure variations, as they occur when pressure is controlled independently via calibrated leaks, are more complicated [6-12]. In many cases, an increase in chamber pressure favors ice sublimation and is reflected in a shorter primary drying time because of the improved thermal exchanges and of the higher product temperature [6,10 12]. 

Fig. 5.9 Comparison of (a) product temperature as estimated by pressure rise test technique and (b) chamber pressure profiles during primary drying for , solid line) uncontrolled and (O, dashed line) controlled...

Attempts to obtain regular spherical shaped particles by spray drying were unsuccessful. Apparently, collisions of not completely solidified particles in the jet stream supplying primary droplets into the drying chamber result in their coalescence and/or distortion of shape. The perspective of obtaining microspheres with well-controlled shape, diameter, and diameter... 

Gieseler et al. utilized tunable diode laser absorption spectroscopy to detect water vapor concentrations, gas velocities and mass flow during freeze-drying of pure water at different pressure and shelf temperature settings and of a 5%w/w mannitol solution. The analyzer was interfaced to the spool that connected the dryer chamber to the condenser. The reported method was advantageous in that primary and secondary drying end-point control based upon mass flow rate was independent of freeze-dryer size and configuration. ... 

A chromel-alumel thermocouple is used to monitor the temperature of the ceramic boat and also acts as a sensor for the furnace temperature controller. A constant flow of clean, dry N2 gas is maintained through the quartz tube. The metal vapor is carried out of the furnace with N2 and condensed to form a polydisperse aerosol. The primary aerosol is diluted by mixing with a filtered dry air stream. The diluted aerosol is then routed to the chamber as required and the excess aerosol is vented out through a glass fiber filter. 

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