The choice between electric heating and silicone oil heating for freeze dryers

Freeze-drying technology (lyophilisation), as a key technology in modern biopharmaceutical, food processing and chemical industries, is centred on the sublimation of water in materials from solid state to gaseous state directly through low-temperature vacuum environment, so as to avoid the damage of high temperature to the activity of materials. Freeze dryer as a realisation of this process equipment, the choice of its heating system is directly related to the freeze drying effect, product quality and production efficiency. In the heating mode of freeze dryer, electric heating and silicone oil heating are two mainstream technologies, this article will be from the technical principle, performance characteristics, application scenarios and the advantages and disadvantages of the two detailed comparative analysis.

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◆ Electric heating technology: The electric heating system directly heats the material by installing electric heating pads at the bottom of the material tray of the lyophiliser and converting electric energy into thermal energy. Its advantage lies in the simple process and low cost, but the temperature distribution in the heating process is not uniform, which can easily lead to different drying time of the samples and difference in drying effect.

◆ Silicone oil heating technology: Silicone oil heating system is set up in the middle of the material disc with multiple circulating channels, and the heated silicone oil circulates back and forth in the channels, heating the material evenly through heat conduction. Silicone oil becomes an ideal heat exchange medium due to its excellent thermal conductivity and good thermal stability, which can ensure the uniformity and stability of temperature in the freeze-drying process.

◆ Biopharmaceutical field
In the field of biopharmaceuticals, heat-sensitive substances such as antimicrobials, vaccines and blood products are extremely sensitive to temperature. Silicone oil heating is preferred by pharmaceutical units because of its temperature uniformity and process flexibility. Although the electric heating system is less costly, it may lead to the loss of drug activity or change in nature due to uneven temperature, so its application is relatively limited.

◆ Food processing field
The food processing field has high requirements for the flavour, nutrients and taste of freeze-dried products. Silicone oil heating can better preserve the original characteristics of food, so that freeze-dried food can quickly return to the taste and texture close to the fresh state after rehydration. The electric heating system may lead to a decline in food quality due to uneven temperature, affecting market competitiveness.

◆ Chemical industry
In the chemical industry, compounds with high purity and stability requirements need to be dried and preserved by freeze-drying technology. Silicone oil heating system, due to its excellent temperature control performance, can ensure that the compound does not decompose or deteriorate during the freeze-drying process to ensure the quality and stability of the product. Although electric heating system can meet the basic drying requirements, it may not be able to meet the requirements in the case of very high temperature control requirements.

There are advantages and disadvantages of electric heating and silicone oil heating in lyophilisers, as follows:

► Advantages and disadvantages of electric heating
Advantages
1) Simple process: electric heating is installed at the bottom of the material tray of the freeze dryer with electric heating piece, the structure is relatively simple, easy to install and maintain.
2) Low cost: compared with the silicone oil heating system, the equipment cost and running cost of electric heating are lower, suitable for enterprises or laboratories with limited budget.
3) Rapid refrigeration: the shelf refrigeration of the electric heating freeze dryer is directly from the evaporating end of the compressor, the refrigeration speed is fast, and the temperature can be reduced rapidly.
Disadvantages
1) Uneven temperature: during the heating process, there is a significant difference in temperature between the edge and the centre of the material tray, which may lead to different drying time of the samples and difference in drying effect, and it is not recommended to use it for individual products that have high requirements on colour and luster.
2) Easy to cause material damage: in the process of heating and warming, due to the rapid refrigeration, resulting in refrigeration and heating conflicts, uneven temperature control, temperature error. For example, when the shelf is controlled at -20℃, when the refrigeration end is turned on, it may drop to -30℃ in an instant, and then slowly rise to -20℃ through the heating plate, the electric heating heats up faster, resulting in sudden cold and sudden heat, the general temperature-sensitive materials will cause melting or collapsing phenomena, and some medicines will even be inactivated.
3) Limitations of the process: electric heating sublimation stage of the temperature of the partition can only be warmed up can not be achieved cooling, so electric heating equipment can do the freeze-drying process in silicone oil equipment can be done, but silicone oil equipment can be done on some of the process in the electric heating equipment can not be carried out.

► Advantages and disadvantages of Silicone Oil Heating
Advantages
1) Good temperature uniformity: There are several circulating channels in the middle of the material disc, the heated silicone oil circulates back and forth to heat the samples on the material disc, and the temperature is very uniform, and the general error is not more than 1 degree.
2) High precision of temperature control: the precision of temperature control of silicone oil series is higher than that of electric heating, and the temperature uniformity of silicone oil is higher than that of electric heating from the point of view of equipment processing.
3) Short drying time: due to the uniform temperature and high precision of temperature control, the drying time of silicone oil series is shorter than that of electric heating.
4) High process flexibility: Silicone oil heating in the sublimation drying stage of the spacer can be warmed up, can be cooled down, so the freeze-drying process can also be warmed up to maintain a certain period of time and then cooled down, to support the search for and implementation of the freeze-drying process more.
5) Small damage to materials: Silicone oil as the medium of the freeze dryer, refrigeration and heating are very reliable, the temperature can be controlled within 1 ℃, will not harm the material, suitable for temperature-sensitive materials, such as pharmaceuticals, microbiological products, precious herbs and so on.
Disadvantages
1) Technical difficulty: the design and manufacture of silicone oil heating system is relatively complex, requiring a higher level of technology.
2) Higher cost: due to the technical difficulty and high cost of equipment, the price of silicone oil heating lyophiliser is higher than electric heating lyophiliser.