Vacuum solutions for drying process
The freeze-drying process is used in the food and pharmaceutical industries for a wide variety of products to increase their shelf life and sometimes also to reduce their weight. This article will explain why vacuum plays a decisive role and how the operator goes about finding a customized vacuum solution for his process.
The main applications for freeze-drying are in the pharmaceutical and food industries. In the food industry, for example, high-quality fruits and coffee are freeze-dried as bulk goods, so both the aroma and color of the products are largely preserved. Due to the high throughput rates, the largest freeze-drying systems are used for these applications. A special feature is the continuous drying process often used for drying coffee, which involves the use of vacuum load-locks.
In the pharmaceutical and biotechnology industries, the freeze-drying process is used for temperature-sensitive vaccines, antibiotics and bacteria, for example. These are dried directly in glass vials or syringes for storage and can later be dissolved in seconds when required. When drying parenteral products, there are particularly high demands on the purity and sterility of the systems.
For the freeze-drying process, it is imperative to achieve a pressure below the triple point of the solvent used (e.g. water). In addition, unwanted substances such as oxygen are removed from the drying chamber during evacuation. While the working pressure for the duration of the main drying process is typically between 0.5 hPa and 1×10-2 hPa, it can be up to 1×10-3 hPa during subsequent drying or for the conditioning of the chamber.
Evacuation with vacuum pumps
The criteria for selecting the vacuum pumps include a sufficiently low final pressure, a high pumping speed to achieve the desired pump-down time, reliability and long maintenance intervals. Dr Stefan Merita, Application & Project Manager explains: “For the final pressure, it should be noted that the value provided in the technical data of the vacuum pump corresponds to the value at which the vacuum pump no longer possesses any effective pumping speed. As far as possible, therefore, the final pressure of the vacuum pump should be a decade below the desired working pressure.” Due to its pumping effect, the ice condenser has a positive effect on the achievable pressure in the freeze dryer. This influence increases as the cooling surface temperature decreases.
Rotary vane pumps have a robust design that is particularly advantageous in freeze-drying applications
One component of the specification of a freeze dryer is usually the pump-down time to a defined pressure (usually 0.1 hPa). The nominal pumping speed of the vacuum pump only gives an indication here, since it represents the maximum pumping speed at only one pressure point.
“We assist the operator with the design and dimensioning of the vacuum system, taking into account the entire characteristic curve of the vacuum pump(s), and losses caused by pipes and leaks. For this, we use our own specially developed modern calculation programs,” adds Merita.
Technologies for pressure measurement
Accurate and repeatable pressure measurements in the drying chamber are essential for the development and control of freeze-drying processes. Due to the relevant pressure range in freeze-drying, two main technologies come into question:
Pirani vacuum gauges
Pirani vacuum gauges measure the pressure indirectly via the pressure-dependent heat conductivity of gases. A common mode of operation is to keep a filament at a constant temperature, where the required heat input is an indicator of the surrounding pressure. Pirani vacuum gauges can measure in the pressure range from atmospheric pressure to about 1×10-4 hPa, by which a meaningful accuracy can only be achieved in a much narrower range (about 10 hPa to 1×10-3 hPa). Patrick Walther, Product Manager for Gauges and Mass Spectrometer: “Thanks to their pulse technology, our TPR 270 and TPR 271 vacuum gauges offer better accuracy than conventional Pirani vacuum gauges. While the TPR 270 is ideal for all standard freeze-drying applications, the TPR 271 is particularly suitable for more challenging applications. The helical shape and unique material properties of the platinum-rhodium filament ensure a longer service life and resistance to solvent vapors.”
Capacitive vacuum gauges
The capacitive vacuum gauges offer far better accuracy than Pirani vacuum gauges. They are also independent of the type of gas. The measuring range of a capacitive vacuum gauge usually extends over four pressure decades. Accuracy is best for each of the upper decades, because at low pressure, the constant factors influencing the measurement inaccuracy are greatest. Therefore, for freeze-drying, capacitive vacuum gauges with a maximum measuring range of 1 hPa or 10 hPa offer the best performance. The capacitive vacuum gauges of the CLR range were developed for vapor-sterilizable freeze dryers. The CLR gauges are actively heated to 160 °C, which reduces the risk of condensation within the vacuum gauges. Also, the electronics are not located in the immediate vicinity of the diaphragm and measurement chamber and are therefore not affected by the hot vapor.
Gas analysis for quality assurance
The mass spectrometer is a very effective process analytical technology (PAT) tool for freeze-drying. The adjustment plates of production freeze dryers are predominantly tempered with silicone oil. The circulation of the heat transfer fluid is sealed off from the drying chamber. However, the silicone oil supply and discharge lines, in particular, are subject to high stress.
Temperature and pressure fluctuations as well as mechanical stress during the positioning of the adjusting plates cause leaks over time. Even though the amount of leaked silicone oil may be small at first, several batches may already have been contaminated before the leak is detected. It is then difficult to subsequently determine exactly how many batches have already been affected by the silicone oil leakage and will have to be discarded. This can result in high consequential costs. In order to counteract this problem effectively, it is necessary to detect the silicone oil leakage when it first occurs, so that valuable batches can be saved from contamination.
Due to its high flexibility, the mass spectrometer can also be used to monitor the water vapor concentration as well as the concentration of other gases present (such as nitrogen or oxygen). By monitoring the water vapor content, the end point of the main and subsequent drying processes can be determined much more accurately than is possible with the help of comparative pressure measurement, for example. This provides new opportunities for optimization, particularly in the development of freeze-drying processes.
“We work closely with the manufacturers of freeze-drying systems when integrating the mass spectrometer into the production system. We deliver the complete mass spectrometer system including turbopumping station, measuring technology and control valve, and provide support for integration into the controller of the freeze dryer,” adds Patrick Walther.
Localization of leaks
If the leak rate exceeds the desired threshold, it is important to locate and correct the leaks. With their high detection sensitivity, short test time and easy operation, helium leak detectors are ideal for localizing leaks. With this method, the freeze-drying system needs to be evacuated. Helium is then sprayed locally onto sealing points, weld seams and other potential leakage points, from the outside, using a spray gun. If there is a leak, the helium flows into the evacuated vacuum chamber and is sucked in and detected by the leak detector. In order to realize short response times in large systems, the leak detector is used in partial flow to the existing vacuum system.
This powerful leak detector can be used by the service technician on site
The ASM 310 is a powerful and universally applicable leak detector that is portable and can be used by service technicians on site.
Source: Pfeiffer Vacuum