Secondary drying process control

In addition, the secondary drying process must be controlled in such a way that overdrying of the product is not possible. 

Test run the curves 2 in Fig. 1.71 are taken from the test run, as shown in Fig. 1.63, but with pressure control 0.36 mbar (total pressure measured with Capacitron). The ice temperature has been -22 °C (constant) for 3 h and DR reached 0.05 %/h after 10 h. Secondary drying could have been started much more early, thus shortening the drying process. 

The freezing of a product is a very important step. The structure in the frozen product decides whether the product can be freeze-dried at all and under which conditions it can be done. For this reason, the consequences of the freezing rate, layer thickness of the product and excipients are discussed in some detail. The second main point is the measurement and control of the two drying phases the main and secondary drying and the third concentrates on the residual moisture content, its measurement and the consequences during storage of the dry product. There will be critical opinions that some of the processes are unilaterally represented. aim was to show the, hmrts and the advantages ot certain procedures to enable the reader to decide whether the ideas of the quoted authors, or my own can be applied to his tasks. 

At the PC level, the freeze-drying process and all secondary processes are visualized, controlled and documented. Such secondary processes include condenser defrosting, CIP/SIP, plant tests, failure statistics, calibration values, counter of operating hours, plant schematic, table of valves, actual temperature and pressure values, recipes, start parameters and machine parameters. 

Hot plates are also used to heat one type of material so that a second may be heated. These secondary heaters may include water baths, oil baths, sand traps, or aluminum plates. Water and oil baths are typically used to heat a drying flask on vacuum evaporators. Oil baths are more messy and require special clean up, but the oil (typically a silicone oil) will not evaporate during drying processes that can take up to several hours. Stronski came up with a very simple and easy solution to limit water evaporation. Stronski8 recommended that the user float Styrofoam popcorn or chips up to three inches deep on water. This suggestion not only limited evaporation, but extended the maximum controlled temperature range. 

Typically, the drying process can be divided into primary and secondary phases. During the primary phase, the drug solution is filled into vials and then placed within a temperature-controlled drying chamber. There, the solution is frozen according to physiochemical principles as the shelf temperature is lowered to below freezing. The shelf temperature is... 

It has already been mentioned that there exists no absolute dividing line between primary and secondary drying. However, for practical purposes of process control and economy, it is important to establish the point of completion of sublimation, i.e. the removal of all ice from the product. The interested reader is directed to the literature, where available techniques have been described and compared in detail.The most direct indicator of ice removal is the chart recorder output of the product temperature. As sublimation nears completion, the product temperature rises, dually to reach the shelf temperature. A practical problem lies in the difficulty of measuring the temperature of the dried cake in a reliable manner, and in the atypical drying behaviour of any vial that carries a temperature probe. 

The cyclone collector is extremely common and can be found with many design variations incorporated to increase efficiency or to meet specific needs. Almost all spray drying processes employ cyclones for primary or secondary product collection. Other industrial processes employ them to control the discharge of unwanted aerosols into the environment. 

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