What is sublimation drying?
Drying is the process of drying products by means of heat and wind. The heat will cause the water in the product to evaporate, and the wind will carry the moisture out of the drying chamber.
Sublimation drying is the process of drying frozen products under vacuum conditions. Under this condition, when heated, water will sublimate directly from solid (ice) to gaseous form. This helps the product after dehumidifying to retain its shape, color, taste, and nutrients.
Within the limits of the article, we will not present too academically but only explain in an easy to understand and visualized way for all customers.
Sublimation of water
To better explain the above definition, SUNSAY dryer would like to present how the sublimation process of water takes place.
The diagram above shows 3 states of water: solid phase, liquid phase, gas phase. The intersection point between the three state lines is called the “Tripple Point”.
If the pressure is higher than the pressure of the Triple Point. When heated, water will change from a solid to a liquid, and then vaporize. This is the process that we often see under atmospheric pressure conditions.
If the pressure is lower than the pressure of the Triple Point. When heated, water will change from a solid, sublimating to a gas.
Triple Point of water is O (0.0098oC; 4.58mmHg)
Thus, the sublimation of water occurs when the ambient pressure is withdrawn below the Triple Point pressure. When the temperature is low, water is solid. As the temperature rises, the water will gradually sublimate into a gaseous state.
Structure of a sublimation dryer
A freeze dryer will have many components, but we will analyze the following 3 parts.
Vacuum pump. This is a device that creates a vacuum environment for the drying process.
Freezing department. This is a device that helps to freeze the product at the beginning of the drying process. At the same time, it also absorbs moisture released from the dried product.
Heating element: When the product is frozen, the vacuum pump creates enough vacuum conditions, the resistor will heat the product so that the sublimation process occurs.
How is the sublimation drying process?
The freezing unit will freeze the product. Usually takes 6-8 hours. In some lines, to save time, the product will be frozen first in the quick freezer. Then put it in the sublimation drying.
The vacuum pump drains all the air inside the drying chamber, creating a vacuum environment. The resistor will heat the ice to sublimate into steam.
The sublimation steam released out of the product will be reabsorbed in a part called the water trap. The temperature at the water trap is usually about 200C lower than the ambient temperature of the drying chamber. When steam enters the water trap, it will immediately condense into ice.
At this stage, only moisture remains in the product deep in the core. At this point, the resistor will heat up very high, maybe up to 70 oC. The process continues until no more moisture escapes.
Is sublimation drying a drying at a negative temperature?
During sublimation, the temperature in the drying chamber will always be negative. Even in the water trap, the negative temperature is very deep.
However, on the product tray there will be a heating element design. For sublimation to occur, the product can be heated up to 60 degrees Celsius.
With a very complex drying technology such as sublimation drying, it is difficult to conclude simply whether drying is at negative or non-negative temperatures.
Advantages of the sublimation drying method
Sublimation drying technology has created products with good quality. nutritional content (protein, lipid, glucose), vitamins, enzymes and biological active ingredients, color, smell, taste, etc., are almost kept intact and not destroyed.
Especially, the product after drying has a soft porosity, when soaked in water, it completely swells and swells again and is almost the same as the original material.
Products after drying are put into bags and then vacuum pressed, stored at ambient temperature, have a long shelf life, low storage costs, and little change in quality.
The product is heated up to 60 degrees Celsius, so nutrients and vitamins are destroyed?
Actually not at all. Nutrients and vitamins are not destroyed at very high temperatures, possibly above 90 degrees Celsius.
The reason why products are degraded, discolored, and smelled when drying is usually due to strong oxidation when heated above 40 degrees Celsius. However, in the freeze-drying chamber, oxygen and air have been drained out. , so no more oxidation occurs.
Why is it necessary to distinguish between freeze drying, vacuum drying and sublimation drying?
SUNSAY freeze drying, vacuum drying and sublimation drying , all three drying techniques are drying at cool temperatures. And more or less they have similarities, so many customers easily confuse them with each other. And the quality of dried products of these 3 techniques will also be different.
Freeze drying technology
SUNSAY freeze drying is also known as heat pump drying. SUNSAY freeze drying is a method of drying using very dry air at a temperature cooler than normal drying temperature. The drying temperature range is from 35 to 60 degrees Celsius, the humidity of the drying air is about 10-30%. Drying is carried out at atmospheric pressure.
Heat pump dryers are constituted by a heat pump placed in a drying cabinet or a drying tunnel depending on the size. Heat pumps have a hot end and a cold end. The hot end will provide heat for the drying agent, and the cold end will be used to dehumidify the drying air. Therefore, cold dryer is also called SUNSAY cold dryer.
Hot and dry air is circulated through the trays of the product to be dried. At the end of this cycle the air temperature will drop and the humidity will increase. Then the air stream carrying the moisture of agricultural products will be sucked through the duct of the condenser indoor unit. Here the air will be cooled down below the condensation point. The water vapor in the air is condensed, causing the air’s vapor content to decrease to zero. This cold dry air is directed through the hot chamber for heating. The air temperature gradually increases within the allowable range (35 ÷ 60) 0C.
Then, they are led into the drying chamber containing the product. Under the difference between the specific pressure of water vapor on the surface of the product and the specific pressure of water vapor in the air (drying agent), the water vapor in the product itself evaporates and dries the product. This process is continuous and cyclical.
Due to the low temperature of the drying room, the highest is about (35 ÷ 60) 0C, so the product quality is less affected. Products have good quality, bring high economic value.
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