Cheap freeze dryer vegetable for sale
Drying is one of the ways to keep substances from spoilage. There are many drying methods, such as sun drying, boil drying, drying, spray drying and vacuum drying. However, these drying methods are all carried out at temperatures above 0°C or higher.
The freeze-drying method is basically carried out at a temperature below 0 °C, that is, the product is frozen. Until the later stage, in order to further reduce the residual moisture content of the product, the product is raised to a temperature above 0 °C, but generally not over 50°C. The lyophilization process includes three stages: pre-freezing, sublimation and re-lyophilization. Reasonably and effectively shortening the freeze dryer supplier cycle has obvious economic value in industrial production.
1. Freezing of products
When the solution is quick-frozen, the grains remain at the size visible under the microscope; on the contrary, when the solution is slow-frozen, the crystals formed are visible to the naked eye. Coarse crystals leave large gaps in the sublimation, which can improve the efficiency of freeze-drying, while fine crystals leave small gaps after sublimation, which hinders the sublimation of the lower layer. The structure is good, the dissolution speed is fast, and the moisture absorption of the finished product is relatively stronger.
The medicines are pre-frozen in the freeze dryer in two ways: one is to cool down the product and the drying box at the same time; the other is to cool down the shelf of the drying box to about -40°C, and then put the product in, the former is equivalent to slow freezing , the latter is between quick freezing and slow freezing, so it is often used to take into account freeze-drying efficiency and product quality.
The disadvantage of this method is that when the product is put into the box, the water vapor in the air will quickly condense on the shelf, and in the early stage of sublimation, if the plate heats up quickly, the sublimation of a large area may exceed the normal load of the condenser. This phenomenon is especially pronounced in summer. The freezing of the product is in a static state. Experience has proved that the phenomenon of supercooling is easy to occur until the temperature of the product has reached the eutectic point. However, the solute is still not crystallized. In order to overcome the supercooling phenomenon, the freezing temperature of the product should be lower than a range below the eutectic point, and it should be kept for a period of time until the product is completely frozen.
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2. Conditions and speed of sublimation
When the saturated vapor pressure of ice at a certain temperature is greater than the partial pressure of water vapor in the environment, the sublimation can begin; the suction and capture of water vapor by the condenser lower than the temperature of the product is a necessary condition to maintain the sublimation. The distance a gas molecule travels between two consecutive collisions is called the mean free path, and it is inversely proportional to the pressure. Under normal pressure, its value is very small, and the sublimated water molecules can easily collide with the gas and return to the surface of the steam source, so the sublimation speed is very slow. As the pressure decreases below 13.3Pa, the mean free path increases by 105 times, which makes the sublimation speed significantly faster, and the flying water molecules rarely change their aspect, thus forming a directional steam flow.
The vacuum pump acts in the freeze dryer to remove the permanent gas to maintain the low pressure necessary for sublimation. 1g of water vapor is 1.25L under normal pressure and expands to 10,000L at 13.3Pa. It is impossible for ordinary vacuum pumps to remove such a large volume in unit time. The condenser actually forms a vacuum pump dedicated to trapping water vapor.
The temperature of product and coagulation is usually -25°C and -50°C. The saturated vapor pressures of ice at this temperature are 63.3Pa and 1.1Pa respectively, so there is a considerable pressure difference between the sublimation surface and the condensation surface, if the partial pressure of the non-condensable gas in the system can be ignored at this time Regardless, it will promote the sublimated water vapor from the product to reach the surface of the condenser directionally at a certain flow rate to form frost.
If the sublimation process does not supply heat, then the product can only reduce the internal energy to compensate for the sublimation heat, and the sublimation stops until its temperature is balanced with the condenser temperature. In order to maintain the temperature difference between sublimation and condensation, sufficient heat must be provided to the product.
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3. Sublimation process
In the first stage of heating, the temperature of the product should be lower than its eutectic point in a range, so the shelf temperature should be controlled.
If the product is partially dried, but the temperature exceeds its eutectic point, melting of the product will occur. The melted liquid is saturated with ice but not saturated with solutes. Therefore, the dry solute will dissolve in quickly, and finally concentrate into a thin, rigid mass with a very poor appearance and a poor dissolution rate.
If the melting of the product occurs in the late stage of a large amount of sublimation, the amount of the melted liquid is small, so it is absorbed by the dry porous solid, resulting in some defects in the block after freeze-drying, and the dissolution rate is still slow when adding water to dissolve. .
In a large number of sublimation processes, although the temperature of the shelf and the product is very different, because the temperature of the plate, the temperature of the condenser and the vacuum temperature are basically unchanged, the sublimation heat absorption is relatively stable, and the temperature of the product is similar.
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The above is the introduction to the vegetable freezer. If you have needs, you can contact Zhengzhou Keda Machinery Equipment Co., Ltd. For decades, Zhengzhou Keda Machinery Equipment Co., Ltd. has been committed to the production and manufacture of laboratory instruments and pipe fittings. If you want to know about our products, or are interested in purchasing our instruments, please contact us, we will be happy to help you.
