Continuous freeze-drying

In this chapter the current state of the art as concerns monitoring of the process has been critically reviewed, pointing out that in order to be applicable in predictive control systems, measuring devices should be able to supply the full state of the system, that is, not only the product temperature but also the residual ice or moisture content, and some physical characteristics or process parameters, such as the cake resistance to flow, and the shelf to product heat transfer coefficient. 

C Stop coefficient (variation rate of the solvent in the product) hieak leakage rate 

Ki tuning parameter for an integral controller Kp tuning parameter for a proportional controller JQ overall heat transfer coefficient 

Y correction coefficient accounting for batch heterogeneity in DPE algorithm E error 

Annstrong, J. G., 1980. Use of the capadtance manometer gauge in vacuum freeze-drying. J. Parent. Drug Assoc. J4 473 3. 

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Continuous freeze-drying equipment has been developed(60), and chopped meat and vegetables may be dried in a rotating steam-jacketed tube enclosed in the vacuum chamber. A model of the freeze-drying process has been presented by Dyer and Sunderland191 , and further details are given in Section 15.8 on freeze crystallisation. 

At first sight, then, complex mechanical setups as we propose for semicon-tinuous or continuous freeze-drying seem susceptible to serious hazards in the sterility control of the operation. However, this is not as dramatic as it looks since the essential part of the process—if not the entire process—is carried out automatically and can be done entirely within a sterile environment by remote control. Indeed, today highly sophisticated automation can be achieved if we resort to the advanced technologies developed for the nuclear industry and profit by the experience of their operators. However, this sophistication will definitely bear on the cost and this can be a serious drawback for conventional low-priced products. 

Continuous freeze drying From the description of batch freeze drying, it can be seen that the utihty requirements vary considerably. During sublimation drying the requirements are 2 to 2.5 times the average requirement. To overcome this peak load and to meet the market request for high unit capacities, continuous freeze dryer designs have been developed. The special features are twofold ... 

Pyle HAA, Eilenberg H (1971) Continuous freeze drying. Vacuum 21 103-114 ... 

Whilst carrier technology appears as rather specific to the pharmaceutical and cosmetic industries, the next developments that I would like to present are, actually, the results of technology transfer from the food industry, namely, soft ice, continuous freeze-drying, and radiation treatment. 

Under these circumstances, it can be easily appreciated that a continuous freeze-drying plant is a highly sophisticated piece of equipment which most often is designed and put together by the food industry itself, which keeps the whole development strictly confidential and assembles different components purchased from multiple unconnected manufacturers. 

The road now looks much more open. It is a rough but worthwhile exercise, since it can result in a much easier processing of sterile injectables with increased security and a substantial decrease in cost. Moreover, it appears to be a smart process easily coupled with continuous freeze-drying. 

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