A vacuum pulse width pulse amplitude modulation drying method

Abstract

The present application relates to the field of vacuum drying, and particularly relates to a vacuum pulse width and pulse amplitude modulation drying method. The method comprises heating the material at a first vacuum degree to perform first stage drying, and then performing second stage drying at a periodically changing vacuum degree between the first vacuum degree and a second vacuum degree; performing third stage drying at a periodically changing vacuum degree between the second vacuum degree and a third vacuum degree; wherein the second pressure is lower than the first pressure; the third pressure is between the first pressure and the second pressure, and the periodically changing rate of the second stage drying is lower than the periodically changing rate of the third stage drying. The vacuum pulse width and pulse amplitude modulation drying process provided by the present application matches the drying vacuum degree variation law with the material drying rate and the water migration law, improves the drying rate, reduces the drying time, and improves the material drying quality.

Description

Technical Field

[0001] This invention relates to the field of vacuum drying, and more particularly to a vacuum pulse width and amplitude modulation drying method. Background Technology

[0002] Drying is a crucial unit operation for reducing the moisture content of materials and is an indispensable part of many food and agricultural product processing stages. Vacuum drying refers to drying materials under an environment pressure lower than standard atmospheric pressure. The saturation temperature of water vapor decreases with decreasing pressure, allowing moisture in the material to evaporate at low temperatures. This effectively preserves the heat-sensitive components of the dried material, and the anaerobic environment prevents oxidation during the drying process. However, prolonged exposure to negative pressure can cause severe deformation of the material.

[0003] To address the problems associated with vacuum drying, current methods typically employ vacuum pulse drying. This involves periodically alternating the vacuum level within the dryer, subjecting the material to alternating vacuum and atmospheric pressure. This process continuously expands and compresses the material's microscopic capillary channels, connecting previously disconnected micropores to form new ones. It also prevents severe degradation of the material due to prolonged vacuum conditions. However, the alternating pressure patterns in vacuum pulse drying do not align with the material's drying rate and moisture migration patterns, resulting in a limited improvement in drying rate. Therefore, developing novel vacuum drying processes based on drying kinetics is of significant importance. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a vacuum pulse width and amplitude modulation drying method.

[0005] Specifically, the vacuum pulse width and amplitude modulation drying method provided by this invention includes: heating the material under a first vacuum level for a first stage of drying; then performing a second stage of drying under periodic switching between the first and second vacuum levels; and performing a third stage of drying under periodic switching between the second and third vacuum levels. The second pressure is lower than the first pressure; the third pressure is between the first and second pressures; and the periodic switching rate of the second stage of drying is lower than that of the third stage of drying. This invention addresses the problems of severe deformation of materials during vacuum drying, the need to improve the drying rate of vacuum pulse drying, and long drying times. It divides the pressure (vacuum) change pattern within the dryer during the drying process into three stages based on the drying characteristics of conventional solid materials: the first stage is a high-pressure stage; the second stage involves periodic changes in pressure within the dryer between high and low pressure values, preferably maintaining a stable pressure for a period of time when high and low pressure values ​​are reached; and the third stage involves rapid periodic switching of pressure within the dryer between medium and low pressure values. By adopting the above-mentioned vacuum pulse width and amplitude modulation drying process, the variation law of drying vacuum degree is better matched with the drying rate and moisture migration law of the material, which can significantly improve the drying rate, reduce the drying time, and improve the drying quality of the material.

[0006] Preferably, the second vacuum degree is 0 to 20 kPa, and more preferably, the second vacuum degree is 8 to 15 kPa.

[0007] Further preferably, the first vacuum degree is 70-120 kPa, and more preferably, the first vacuum degree is 101 kPa.

[0008] In this invention, by controlling the vacuum level of the first stage of drying, the dried material can be heated better, especially at the preferred vacuum level of 101 kPa, which can further accelerate the temperature rise of the material.

[0009] Further preferably, the third vacuum degree is 20-70 kPa, and more preferably, the third vacuum degree is 20-30 kPa.

[0010] Further preferably, during the periodic changes of the second stage drying, when the first vacuum level and / or the first vacuum level is reached, the first vacuum level and / or the second vacuum level is maintained for a period of time; preferably, it is maintained for 2 to 20 minutes.

[0011] In this invention, by adjusting the vacuum level of the second stage of drying and maintaining the first vacuum level and / or the second vacuum level for 2 to 20 minutes, the drying rate can be further improved and severe deformation of the dried material can be prevented.

[0012] Preferably, in the periodic transformation of the third stage drying, when the second vacuum degree is reached, the process directly switches to the second vacuum degree; when the third vacuum degree is reached, the process directly switches to the second vacuum degree.

[0013] More preferably, the periodic change rate of the third stage drying is 0.5 to 2 kPa / s.

[0014] In this invention, the periodic change rate of the third stage of drying is higher than that of the second stage. Especially under the preferred change rate, the dried material can be compressed quickly, making it easier to remove the remaining moisture in the later stage of drying.

[0015] Further preferably, the dried material is selected from one or more of food, medicine and agricultural products; preferably, the material is wolfberry, bitter orange peel or polygonatum, etc.

[0016] Further optimization is that the material is heated to a temperature of 50–90°C.

[0017] According to the present invention, material drying is divided into three stages based on the aforementioned vacuum level: the first stage is a heating stage, the second stage is a constant-rate drying stage, and the third stage is a falling-rate drying stage. In the heating stage, the material rapidly heats up during the heating process. In the constant-rate drying stage, internal moisture migrates outward at a constant rate, and the heat and mass transfer rate depends on external environmental conditions. In the falling-rate drying stage, the material surface becomes wetted, internal moisture diffuses to the surface, and moisture evaporates on the surface. As the drying process proceeds, the moisture content and drying rate of the material gradually decrease. In particular, the vacuum pulse width and amplitude modulation drying process of the present invention adjusts the vacuum level inside the dryer based on the aforementioned drying kinetic characteristics, thereby accelerating the drying rate. In the first stage, the material is kept under high or normal pressure. Because the material temperature is affected by the water vapor saturation temperature, the lower the pressure, the lower the water vapor saturation temperature, which to some extent hinders the material's temperature rise. In the second stage, moisture migration mainly occurs through liquid diffusion and capillary flow diffusion under the influence of temperature and humidity gradients, as well as water vapor diffusion. In this stage, the vacuum level within the dryer is significantly and periodically changed to increase the temperature and humidity gradient. Pressure changes also continuously expand and compress the material's microscopic capillary channels, connecting previously disconnected micropores to form new capillary channels and increasing the capillary flow rate of moisture. In the third stage, the material has a lower moisture content, making drying more difficult. This causes the pressure within the dryer to fluctuate rapidly within a medium-low pressure range, lowering the moisture evaporation temperature and increasing internal moisture diffusion.

[0018] The beneficial effects of this invention are at least as follows: This invention provides a vacuum pulse width and amplitude modulation drying process. Based on the drying characteristics of materials, by changing the vacuum degree and pulse width and amplitude values ​​in the dryer during the drying process, the change law of the drying vacuum degree is matched with the drying rate and moisture migration law of the material. This enhances the temperature and humidity gradient and capillary flow attraction, increases the migration rate of moisture and water vapor in the three stages, improves the drying rate, reduces the drying time, and improves the drying quality of the material. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 A schematic diagram of the material drying process provided for an embodiment of the present invention (1. First stage, 2. Second stage, 3. Third stage). Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. The following embodiments are used to illustrate the present invention, but are not intended to limit the scope of the present invention.

[0022] Where specific techniques or conditions are not specified in the examples, they shall be performed in accordance with the techniques or conditions described in the literature in this field, or in accordance with the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased through legitimate channels.

[0023] Example 1

[0024] like Figure 1As shown, the material drying method provided in this embodiment of the invention comprises three stages: a first stage, a second stage, and a third stage. Specifically, it includes: placing the material into a dryer, stabilizing the pressure inside the dryer at atmospheric pressure or a high pressure value slightly below atmospheric pressure, starting to heat the material, and once the material temperature stabilizes, turning on the vacuum pump to create a vacuum. After the pressure inside the dryer reaches the target low pressure value, the low pressure value is maintained stable by adjusting the vacuum pump power and the opening of the vent valve. After this stabilization period, the vent valve is opened to allow ambient air into the dryer. When the pressure reaches the high pressure value, the low pressure value is again stabilized by adjusting the vacuum pump power and the vent valve opening. After this stabilization period, the vent valve is closed, causing the pressure inside the dryer to drop. These steps are repeated to achieve periodic pressure changes. In the third stage of drying, the pressure inside the dryer is periodically and rapidly fluctuated between medium and low pressure ranges by adjusting the vacuum pump power and the vent valve opening until drying is complete.

[0025] Example 2

[0026] This embodiment provides a vacuum pulse width modulation (PWM) drying method for wolfberry materials. The specific steps include: placing wolfberries in a dryer, introducing 50°C hot water into the heating plate inside the dryer, maintaining a pressure of 101 kPa inside the dryer for 10 minutes at the start of drying, then turning on the vacuum device to evacuate the vacuum. When the pressure inside the dryer reaches 10 kPa, adjusting the opening of the vent valve to maintain the vacuum at 10 kPa for 9 minutes, then fully opening the vent valve until the pressure inside the dryer is the same as the ambient atmospheric pressure, i.e., 101 kPa, maintaining this pressure for 2 minutes, then closing the vent valve to reduce the pressure inside the dryer to 10 kPa and maintaining this pressure for another 9 minutes. Repeating the above process until drying for 240 minutes, the final stage begins (the time for each stage needs to be determined according to the wolfberry drying process). When the vacuum reaches 10 kPa, immediately opening the vent valve to increase the pressure, and immediately closing the vent valve to decrease the pressure when the pressure reaches 30 kPa. This process is repeated to adjust the vacuum until the moisture content of the wolfberries is ≤13%. The results showed that the drying time for wolfberries was 330 minutes, and the rehydration rate was 3.1 ± 0.1%.

[0027] Comparative Example 1

[0028] The difference between this comparative example and Example 2 is that the vacuum level is changed periodically at the beginning of drying, which is not matched with the drying characteristics of the material.

[0029] The comparative example used wolfberry as the drying material, with a drying temperature of 50℃. The drying cycle was as follows: maintaining a vacuum of 10 kPa for 9 minutes, then drying at normal pressure for 2 minutes, followed by maintaining a vacuum of 10 kPa for 9 minutes. This cycle was repeated until the moisture content of the wolfberry was ≤13%. The results showed that the wolfberry drying time was 360 minutes, and the rehydration rate was 2.8 ± 0.2%. Compared to the comparative example, the wolfberry material in Example 2, using the vacuum pulse width modulation drying method of this invention, had an 8% shorter drying time and an 11% higher rehydration rate.

[0030] Although the present invention has been described in detail above with general descriptions and specific embodiments, modifications or improvements can be made to it, which will be obvious to those skilled in the art. Therefore, all such modifications or improvements made without departing from the spirit of the present invention fall within the scope of protection claimed by the present invention.

Claims

1. A vacuum pulse width pulse amplitude modulation drying method, characterized by, The application relates to a method for drying materials, comprising: heating the materials at a first vacuum degree to carry out first-stage drying, then carrying out second-stage drying under periodic transformation between the first vacuum degree and a second vacuum degree; and carrying out third-stage drying under periodic transformation between the second vacuum degree and a third vacuum degree; wherein the first vacuum degree is 70-120 kPa, the second vacuum degree is 0-20 kPa, the third vacuum degree is 20-30 kPa, the periodic transformation rate of the third-stage drying is 0.5-2 kPa / s, the periodic transformation rate of the second-stage drying is lower than that of the third-stage drying, in the periodic transformation of the second-stage drying, when the first vacuum degree is reached, the first vacuum degree is maintained for a period of time; when the second vacuum degree is reached, the second vacuum degree is maintained for a period of time; in the periodic transformation of the third-stage drying, when the second vacuum degree is reached, the third vacuum degree is directly transformed; and when the third vacuum degree is reached, the second vacuum degree is directly transformed.

2. The vacuum pulse-width pulse-amplitude modulation drying method according to claim 1, characterized in that, The second vacuum degree is 8-15 kPa.

3. The vacuum pulse-width pulse-amplitude modulation drying method according to claim 2, characterized in that, The first vacuum degree is 101 kPa.

4. The vacuum pulse-width pulse-amplitude modulation drying method according to claim 1, characterized in that, The maintaining time is 2-20 min.

5. The vacuum pulse-width pulse-amplitude modulation drying method according to claim 1, characterized in that, The dried materials are selected from one or more of food and agricultural products.

6. The vacuum pulse-width pulse-amplitude modulation drying method according to claim 1, characterized in that, The materials are Chinese wolfberry, immature bitter orange or yellow ginseng.

7. The vacuum pulse-width pulse-amplitude modulation drying method according to any one of claims 1 to 6, characterized in that, The heating temperature of the materials is 50-90 DEG C.

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