CN113915963B - Microwave vacuum freeze-drying equipment and microwave vacuum freeze-drying method thereof - Google Patents
Microwave vacuum freeze-drying equipment and microwave vacuum freeze-drying method thereof Download PDFInfo
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
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- F26B9/00—Machines or apparatus for drying solid materials or objects at rest or with only local agitation; Domestic airing cupboards
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- F26B—DRYING SOLID MATERIALS OR OBJECTS BY REMOVING LIQUID THEREFROM
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- F26B3/32—Drying solid materials or objects by processes involving the application of heat by development of heat within the materials or objects to be dried, e.g. by fermentation or other microbiological action
- F26B3/34—Drying solid materials or objects by processes involving the application of heat by development of heat within the materials or objects to be dried, e.g. by fermentation or other microbiological action by using electrical effects
- F26B3/347—Electromagnetic heating, e.g. induction heating or heating using microwave energy
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- F26B—DRYING SOLID MATERIALS OR OBJECTS BY REMOVING LIQUID THEREFROM
- F26B3/00—Drying solid materials or objects by processes involving the application of heat
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- F26B5/048—Drying solid materials or objects by processes not involving the application of heat by evaporation or sublimation of moisture under reduced pressure, e.g. in a vacuum in combination with heat developed by electro-magnetic means, e.g. microwave energy
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- F26B—DRYING SOLID MATERIALS OR OBJECTS BY REMOVING LIQUID THEREFROM
- F26B5/00—Drying solid materials or objects by processes not involving the application of heat
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- H05B6/64—Heating using microwaves
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- H05B6/64—Heating using microwaves
- H05B6/647—Aspects related to microwave heating combined with other heating techniques
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- H—ELECTRICITY
- H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
- H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
- H05B6/00—Heating by electric, magnetic or electromagnetic fields
- H05B6/64—Heating using microwaves
- H05B6/74—Mode transformers or mode stirrers
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Abstract
A microwave vacuum freeze-drying device and a microwave vacuum freeze-drying method thereof are provided, the microwave vacuum freeze-drying device comprises a microwave heating device, a vacuum device, a cold trap, a refrigerating device and a control system, the vacuum device comprises a vacuum chamber, an electric heating plate, a rotary vane vacuum pump and a water circulation vacuum pump, the rotary vane vacuum pump and the water circulation vacuum pump are connected with the vacuum chamber, the electric heating plate and the microwave heating device are both arranged in the vacuum chamber, the refrigerating device is connected with the water circulation vacuum pump, the cold trap is arranged between the vacuum chamber and a vacuum pipeline, the control system is respectively connected with the microwave heating device, the vacuum device, the cold trap and the refrigerating device, the control system collects parameters of the microwave heating device, the vacuum device, the cold trap and the refrigerating device and carries out program control and function switching on the parameters so as to realize simultaneous or sectional implementation of microwave vacuum freeze-drying and vacuum freeze-drying. The invention also provides a drying method of the microwave vacuum freeze drying equipment.
Description
Technical Field
The invention relates to microwave vacuum freeze-drying equipment and a microwave vacuum freeze-drying method, in particular to microwave vacuum freeze-drying equipment capable of realizing switching between electric heating and microwave heating and a microwave vacuum freeze-drying method thereof.
Background
The vacuum freeze drying technology is a mature processing technology, is a novel drying and dehydrating technology combining a vacuum technology and a freezing technology, and is widely applied to the food processing industry, the biological product industry, the medicine processing industry and the like. Most of heat sources for vacuum freeze drying are drying by methods such as hot air circulation, steam heating, electric heating and the like, heat is transferred from outside to inside, moisture is removed from inside to outside, heat transfer and mass transfer in the material drying process are reversed, energy loss is increased, and the sublimation process is delayed. Along with the inward transition of the drying interface, the drying rate is gradually reduced, the heating period is long, the phenomenon of external dryness and internal growth is easy to occur, the energy consumption is high, and the maintenance cost is high.
The microwave vacuum freeze drying combines the advantages of vacuum freeze drying and microwave drying, makes up for deficiencies, can improve the product quality, accelerates the drying speed, leads the product quality to be close to the freeze-dried product in the aspects of color, nutrient content, rehydration performance and the like, and has lower energy consumption. There are still many problems to be solved in the current microwave vacuum freeze-drying, which prevent the technology from being widely used.
Firstly, microwave heating belongs to medium heating, a microwave electric field promotes polar molecules to reciprocate at high frequency to generate internal friction heat, and the internal heat accumulation is too high, so that ice crystals in materials are easily melted, the sublimation effect is influenced, the materials are contracted and deformed, and the appearance quality of products is influenced; secondly, vacuum freeze drying is finished in a low-pressure environment, the breakdown field intensity of gas is reduced along with the improvement of the vacuum degree, gas molecules are easily ionized into ions, and therefore a glow discharge phenomenon occurs, the microwave energy is lost, equipment is possibly damaged, the service life is influenced, and a sample is burnt; thirdly, most of the existing devices place a vacuum chamber inside a microwave resonant cavity, and aiming at a specific microwave source, in order to ensure the working efficiency and the uniformity of a microwave field of the microwave resonant cavity, the volume of the resonant cavity is generally designed to be smaller, and the volume of the corresponding vacuum chamber is smaller, so that the actual production and the industrialized popularization are not facilitated; in addition, how to shorten the freeze-drying cycle to reasonably regulate and control electric heating and microwave heating, strengthen the intelligent control function of the device, reduce the energy consumption of the device, reduce the equipment cost and the like are important problems to be solved in the field.
Disclosure of Invention
The technical problem to be solved by the present invention is to provide a microwave vacuum freeze-drying apparatus capable of switching between electric heating and microwave heating, aiming at the above problems in the prior art.
In order to achieve the purpose, the invention provides microwave vacuum freeze-drying equipment which comprises a microwave heating device, a vacuum device, a cold trap, a refrigerating device and a control system, wherein the vacuum device comprises a vacuum bin, an electric heating plate, a rotary vane vacuum pump and a water circulation type vacuum pump, the rotary vane vacuum pump and the water circulation type vacuum pump are connected with the vacuum bin through vacuum pipelines, the electric heating plate and the microwave heating device are arranged in the vacuum bin, the refrigerating device is connected with the water circulation type vacuum pump, the cold trap is arranged between the vacuum bin and the vacuum pipelines, the control system is respectively connected with the microwave heating device, the vacuum device, the cold trap and the refrigerating device, and the control system is used for acquiring parameters of the microwave heating device, the vacuum device, the cold trap and the refrigerating device and carrying out program control and function switching on the microwave heating device, the vacuum device, the cold trap and the refrigerating device so as to realize simultaneous or segmented implementation of microwave vacuum freeze-drying and vacuum freeze-drying.
The microwave vacuum freeze-drying equipment comprises a microwave resonant cavity, a microwave emission source, a material tray bracket and a microwave stirrer, wherein the microwave resonant cavity is arranged in the vacuum bin and shares the bottom with the vacuum bin, the electric heating plate is positioned above the microwave resonant cavity, the material tray bracket is arranged in the microwave resonant cavity and is positioned under the electric heating plate, the microwave emission source is arranged at the bottom of the microwave resonant cavity, the microwave stirrer is arranged in a microwave energy feedback port of the microwave emission source, and a metal shielding overflow plate with sieve pores is arranged at the bottom of the microwave resonant cavity.
In the microwave vacuum freeze-drying equipment, a sealing gasket for preventing microwave and vacuum leakage is arranged at the joint of the microwave resonant cavity and the vacuum bin.
In the microwave vacuum freeze-drying device, the microwave resonant cavity is formed by enclosing a microwave shielding metal net, and a sieve mesh for shielding microwaves and communicating the microwave resonant cavity with the vacuum chamber is arranged on the microwave shielding metal net.
In the microwave vacuum freeze-drying apparatus, a microwave feed-in cover for increasing the microwave feed-in area and avoiding glow discharge is disposed above the microwave feed-in port, and the microwave feed-in cover is a polystyrene material.
In the microwave vacuum freeze-drying equipment, in the vacuum freeze-drying stage, the control system presets the variation range of the material temperature, and adjusts the electric heating plate to be opened or closed according to the material temperature data acquired by the temperature sensor in real time, so as to ensure the sublimation drying process to be carried out stably; and in the microwave vacuum freeze-drying stage, the control system calculates the material temperature rise amplitude in unit time and adjusts the on or off of a microwave emission source according to the change rule of the material temperature along with time through material temperature data acquired by the temperature sensor in real time.
In order to better achieve the above object, the present invention further provides a microwave vacuum freeze-drying method, wherein the microwave vacuum freeze-drying apparatus is adopted, and the method comprises the following steps:
s100, a vacuum freeze drying step, namely presetting a change range of material temperature according to a temperature change rule in a material sublimation process, opening an electric heating plate, a rotary vane vacuum pump, a refrigerating device and a cold trap at the early stage of drying when water is sublimated vigorously, reducing the pressure in a vacuum chamber to 100-133 Pa, and quickly sublimating most of water by adopting electric heating under a low vacuum condition; and
and S200, microwave vacuum drying, namely starting a microwave heating and water circulation type vacuum pump, closing a refrigerating device, adjusting the pressure in a vacuum bin to 2000-4000Pa, realizing heat transfer and mass transfer in the same direction by heat transfer in the material, and promoting residual moisture in the material in the vacuum bin to quickly escape to realize quick drying of the material.
In the microwave vacuum freeze-drying method, in step S100, the temperature of the electric heating plate is set to 70-80 ℃ in the initial stage of sublimation, the temperature of the electric heating plate is set to 60 ± 5 ℃ in the middle stage of sublimation, the temperature of the electric heating plate is set to be not more than 50 ℃ in the later stage of sublimation, material temperature data in the vacuum bin is collected by a temperature sensor in real time in the drying process, and a control system adjusts the electric heating plate to be turned on or off according to the material temperature data so as to ensure that the material temperature rise process is stable and efficient.
In the microwave vacuum freeze-drying method, in step S200, the material temperature rise range in unit time is set according to the material temperature data in the vacuum chamber collected by the temperature sensor in real time, when the material temperature rise range is reduced to a set value, the control system starts the microwave emission source to perform microwave vacuum drying on the material, so that residual moisture is rapidly separated out until the material moisture is reduced to below 10%, and the drying process is finished.
The microwave vacuum freeze-drying method comprises the steps that a control system controls the vacuum degree of a vacuum chamber according to the sequence of vacuum freeze-drying and microwave vacuum drying and the required pressure range, the pressure change range is adjusted through pressure data in the vacuum chamber acquired by a pressure sensor in real time, a material is put in the vacuum chamber, then the rotary-vane vacuum pump is started, when the vacuum degree of the vacuum chamber is reduced to be below a set value, the electric heating plate is started to heat, a vacuum degree switching point is set based on the material temperature rise amplitude and real-time moisture change, when the vacuum pressure of the vacuum chamber is increased to be the vacuum degree switching point, the rotary-vane vacuum pump is switched to a water circulation type vacuum pump, and microwave vacuum drying is started.
The invention has the technical effects that:
1. compared with the microwave vacuum freeze-drying equipment in the prior art, the microwave vacuum freeze-drying equipment has the advantages that the microwave resonant cavity is arranged in the vacuum bin, so that the acting volume of microwave drying under the vacuum freezing condition is increased, the material drying treatment capacity is increased, and the subsequent industrialization of the microwave vacuum freeze-drying is facilitated;
2. the pressure of a vacuum system is controlled by a rotary vane vacuum pump and a water circulation type vacuum pump with large principle difference, an electric heating plate and the rotary vane vacuum pump are started in the period of vigorous vacuum sublimation according to the precision and performance difference of the vacuum pump, a microwave heating and water circulation type vacuum pump is started in the microwave drying stage, a lower vacuum degree (100 Pa-133 Pa) can be maintained in the period of vigorous sublimation to promote the mass sublimation of water, the later stage sublimation is blocked, the microwave heating is utilized to enable the heat and mass transfer to be homodromous, so that the drying process is accelerated to improve the energy efficiency, the vacuum degree can be maintained at 2000-4000Pa in the period, and the microwave glow discharge can be avoided in the vacuum interval;
3. according to the invention, the electric heating plate is arranged above the microwave resonant cavity, and because the periphery and the top of the microwave resonant cavity are all metal screens with specified round holes, heat can enter the microwave resonant cavity through the radiation of the round holes, so that the sublimation latent heat is provided for materials; meanwhile, the microwave resonant cavity is communicated with the external vacuum environment through a screen mesh so as to ensure the condition of microwave vacuum drying;
4. the microwave radiation source adopts a solid microwave source, has small volume compared with a magnetron in the prior art, does not need high-voltage power supply, is more beneficial to microwave feed-in, and can realize continuous adjustment of microwave power and fine heat supply; the semicircular polypropylene microwave feed-in cover is matched, so that the microwave feed-in area is increased, and the drying energy efficiency is improved; the energy feed port is provided with a microwave stirrer in front, so that microwave oscillation modes in the microwave resonant cavity are mutually superposed, and the energy distribution of a microwave field is uniform.
The invention is described in detail below with reference to the drawings and specific examples, but the invention is not limited thereto.
Drawings
FIG. 1 is a schematic structural diagram of a microwave vacuum freeze-drying apparatus according to an embodiment of the present invention;
fig. 2 is a schematic structural diagram of a microwave heating device according to an embodiment of the present invention.
Wherein the reference numerals
1. Vacuum device
11. Water circulation type vacuum pump
12. Rotary vane vacuum pump
13. Vacuum cabin
14. Electric heating plate
15. Water tank
2. Microwave heating device
21. Microwave resonant cavity
22. Microwave stirrer
23. Tray bracket
24. Microwave feed-in cover
25. Metal shielding overflowing plate
26. Microwave energy feedback port
3. Control system
4. Refrigerating device
5. Cold trap
Detailed Description
The invention will be described in detail with reference to the following drawings, which are provided for illustration purposes and the like:
on the basis of a common vacuum freeze drying process, aiming at the limitations that the microwave resonant cavity of the existing microwave vacuum freeze drying equipment has small available volume and is easy to glow discharge and the like, the invention uses electric heating as a main mode for providing sublimation latent heat in the initial drying stage to promote mass sublimation of water in materials; sublimation is hindered in the drying later stage, utilizes microwave heating to provide sublimation latent heat, improves drying efficiency, shortens drying time, reduces drying cost. The specific structure and principles of the present invention are described in detail below.
Referring to fig. 1, fig. 1 is a schematic structural diagram of a microwave vacuum freeze-drying apparatus according to an embodiment of the present invention. The microwave vacuum freeze drying equipment comprises a microwave heating device 2, a vacuum device 1, a cold trap 5, a refrigerating device 4 and a control system 3, wherein the control system 3 is preferably a PLC (programmable logic controller) control system 3, the vacuum device 1 comprises a vacuum bin 13, an electric heating plate 14, a rotary-vane vacuum pump 12 and a water circulation type vacuum pump 11, the rotary-vane vacuum pump 12 and the water circulation type vacuum pump 11 are connected with the vacuum bin 13 through vacuum pipelines, the water circulation type vacuum pump 11 is connected with a water tank 15 of the water circulation type vacuum pump, the electric heating plate 14 is arranged in the vacuum bin 13, the microwave heating device 2 is arranged in the vacuum bin 13, the vacuum bin 13 can be a cylindrical, square or rectangular metal shell, and preferably has a cylindrical structure. Refrigerating plant 4 with water circulating vacuum pump 11 is connected, cold trap 5 sets up vacuum chamber 13 with between the vacuum line, cold trap 5 be used for steam desublimation, link to each other with vacuum chamber 13, vacuum pumping system is at cold trap 5 top opening, and the vapor that sublimates out in the material is caught to cold trap 5, is taken out noncondensable gas by rotary vane vacuum pump 12, and refrigerating plant 4 uses with vacuum device 1 cooperation, maintains the required vacuum state of vacuum chamber 13 inside completion sublimation. The refrigerating device 4 comprises a refrigerating unit and a cooling grid, and the setting and control of the starting time of the compressor, the temperature of the cooler, the pre-freezing temperature and the pre-freezing time can be adjusted by the PLC control system 3. The control system 3 is respectively connected with the microwave heating device 2, the vacuum device 1, the cold trap 5 and the refrigerating device 4, and the control system 3 is used for acquiring parameters of the microwave heating device 2, the vacuum device 1, the cold trap 5 and the refrigerating device 4 and carrying out program control and function switching on the microwave heating device 2, the vacuum device 1, the cold trap 5 and the refrigerating device 4 so as to realize simultaneous or segmented implementation of microwave vacuum freeze drying and vacuum freeze drying. The PLC control system 3 comprises a microwave power, temperature, weight and vacuum degree measurement and control instrument and an automatic control program, and is composed of a temperature sensor, a pressure sensor, a gravity sensor, a PLC host computer, a control program and the like. The method specifically comprises the steps of setting a microwave power automatic regulation change mode, setting a pressure change mode and a change amplitude of a vacuum bin 13, setting a relative change matching mode of vacuum pressure and microwave radioactive source power, switching between a rotary-vane vacuum pump 12 and a water circulation type vacuum pump 11, and the like in the freeze-drying process, calculating real-time moisture content based on parameters, and controlling an electric heating and microwave heating switching mechanism according to the real-time moisture content, thereby realizing fine heat supply.
Referring to fig. 2, fig. 2 is a schematic structural diagram of a microwave heating device 2 according to an embodiment of the present invention. The microwave heating device 2 includes a microwave resonant cavity 21, a microwave emitting source (not shown), a tray bracket 23 and a microwave stirrer 22, the microwave resonant cavity 21 is disposed in the vacuum chamber 13 and shares a bottom, preferably a square bottom, with the vacuum chamber 13, the electric heating plate 14 is disposed above the microwave resonant cavity 21, heat radiation enters the microwave chamber and is absorbed by materials, the tray bracket 23 is disposed in the microwave resonant cavity 21, a material tray is supported and placed on the material bracket and is disposed under the electric heating plate 14, and is made of a material having a certain intensity, typically a resin material such as ceramic or polypropylene, and the like, the microwave emitting source is disposed at the bottom of the microwave resonant cavity 21, a microwave generator (magnetron or solid source) with continuously adjustable emitting power is selected, microwave energy is fed into the microwave resonant cavity 21 through a waveguide, so as to achieve high-precision control of heat supply and enhance microwave introduction efficiency, in this embodiment, the solid source is preferably used as the microwave emitting source, and does not require high-voltage power supply, and has a larger frequency span, better feeding effect, and more stable power output. The microwave stirrer 22 is arranged in a microwave energy feed port 26 of the microwave emission source, and a metal shielding overflowing plate 25 with sieve pores with specified sizes is arranged at the bottom of the microwave resonant cavity 21. Wherein, a sealing gasket for avoiding microwave and vacuum leakage is arranged at the joint of the microwave resonant cavity 21 and the vacuum bin 13. The periphery and the top of the microwave resonant cavity 21 are enclosed by a microwave shielding metal net, the metal material can be selected from iron, aluminum, copper, stainless steel and the like, an aluminum cover plate is generally selected in consideration of cost and heat conductivity, and the microwave shielding metal net is provided with a sieve hole according to the specification for shielding microwaves, and simultaneously, the microwave resonant cavity 21 can be communicated with the external vacuum environment of the vacuum bin 13. A microwave feed-in cover 24 for increasing the microwave feed-in area and avoiding glow discharge is arranged above the microwave feed-in port 26. In order to ensure the uniformity of the field intensity in the microwave resonant cavity 21, a microwave stirrer 22 is arranged at the entrance of the waveguide to make the introduced microwaves irregularly reflected and fed into the microwave resonant cavity 21, and different microwave oscillation modes are mutually overlapped to improve the energy distribution uniformity of the microwave field. A microwave feed-in cover 24 is added above the microwave feed-in port 26, preferably in a hemispherical shape, so that the feed-in area can be increased, and glow discharge can be avoided; the microwave feeding cover 24 is preferably made of polystyrene, which is a microwave transparent material, has high strength, can complete microwave introduction, reduce microwave loss, prevent vacuum leakage under the condition of high vacuum degree, ensure that microwave introduction components avoid impurity pollution and corrosion of water vapor, dust and the like, and prolong the service life of components such as a microwave source and the like.
In the vacuum freeze drying stage, the control system 3 presets the variation range of the material temperature, and adjusts the opening or closing of the electric heating plate 14 according to the material temperature acquired by the temperature sensor in real time to ensure the stable operation of the sublimation drying process; in the microwave vacuum freeze-drying stage, the control system 3 calculates the material temperature rise amplitude in unit time according to the change rule of the material temperature along with time through the material temperature signals acquired by the temperature sensor in real time, and when the temperature rise amplitude is reduced to a set value, the control system 3 adjusts the on or off of the microwave emission source.
The microwave vacuum freeze-drying method adopts the microwave vacuum freeze-drying equipment, and comprises the following steps:
step S100, a vacuum freeze drying step, namely presetting the change range of the material temperature according to the temperature change rule in the sublimation process of the material, opening an electric heating plate 14, a rotary vane type vacuum pump 12, a refrigerating device 4 and a cold trap 5 at the early drying stage with vigorous water sublimation, regulating and controlling the pressure in a vacuum chamber 13 according to the working principle of the vacuum pump, reducing the pressure in the vacuum chamber 13 to be below 100-133 Pa, and rapidly sublimating most water by adopting electric heating under the low vacuum condition; and
and S200, microwave vacuum drying, namely blocking sublimation and drying of moisture in the later stage of drying, starting microwave heating, switching a water circulation type vacuum pump 11, closing a refrigerating device 4, changing the heat transfer direction in the material, promoting the residual moisture to rapidly escape, and realizing rapid drying of the material, adjusting the pressure in the vacuum bin 13 to 2000-4000Pa, heating by matching with a microwave emission source, realizing heat and mass transfer homodromous by heat transfer in the material, promoting the residual moisture in the material in the vacuum bin 13 to rapidly escape, and realizing rapid drying of the material.
In step S100, the temperature of the electric heating plate 14 is set as follows: in the initial stage of sublimation, in order to accelerate the water overflow and additionally provide latent heat of sublimation, the temperature of the electric heating plate 14 is set to be 70-80 ℃; during the strong dehydration period in the middle stage of sublimation, the temperature of the electric heating plate 14 is set to be 60 +/-5 ℃ or 50-60 ℃; the temperature of the electric heating plate 14 at the later stage of sublimation is set to be less than or equal to 50 ℃, and is preferably kept at 35-50 ℃. The temperature of the materials is collected by the temperature sensor in real time in the drying process, and the control system 3 performs feedback adjustment on the opening or closing state of the electric heating plate 14, so that the stability and high efficiency of the temperature rising process of the materials are ensured. In step S200, a material temperature rise range in unit time is set according to a material temperature signal acquired by a temperature sensor in real time, when the temperature rise range is reduced to a set value, the control system 3 starts a microwave emission source to perform microwave vacuum drying on the material, so that residual moisture is rapidly separated out until the moisture of the material is reduced to below 10%, and the drying process is finished.
In this embodiment, the control system 3 controls the vacuum degree of the vacuum chamber 13 according to the sequence of vacuum freeze drying and microwave vacuum drying and the required pressure range, adjusts the pressure change range through the pressure signal acquired by the pressure sensor in real time, starts the rotary-vane vacuum pump 12 after the material is put in, starts the electric heating plate 14 to heat when the vacuum degree is reduced below a set value, sets a vacuum degree switching point based on the material temperature rise range and the real-time moisture change, switches the rotary-vane vacuum pump 12 to the water circulation type vacuum pump 11 when the vacuum pressure is increased to the vacuum degree switching point, and starts the microwave vacuum drying. Because the vacuum degree is outside the critical interval of breakdown discharge, the possibility of glow discharge generated in the microwave vacuum drying stage is extremely low. The real-time temperature of the material is measured by the thermocouple temperature sensor, so that the temperature of the material in the sublimation stage is lower than 0 ℃, the temperature of the material in the drying later stage is lower than 50 ℃, and the loss of thermosensitive components and the browning of the material color are prevented.
Compared with the microwave vacuum freeze-drying equipment in the prior art, the microwave resonant cavity 21 is arranged in the vacuum bin 13, so that the acting volume of microwave drying under the vacuum freezing condition is increased, the material drying treatment capacity is increased, and the subsequent industrial test of microwave vacuum freeze-drying is facilitated. The pressure of the vacuum device 1 is controlled by a rotary-vane vacuum pump 12 and a water circulation type vacuum pump 11 with large principle difference, according to the precision and performance difference of the vacuum pump, an electric heating plate 14 and the rotary-vane vacuum pump 12 are opened in the vigorous vacuum sublimation period, the microwave heating and water circulation type vacuum pump 11 is opened in the microwave drying period, the lower vacuum degree (preferably 100Pa-133 Pa) can be maintained in the vigorous sublimation period, the mass sublimation of water is promoted, the later sublimation is blocked, the heat mass transfer is syncroponized by utilizing the microwave heating, the drying process is accelerated, the energy efficiency is improved, the vacuum degree can be maintained at 2000-4000Pa in the drying period, and the microwave glow discharge can be avoided in the vacuum interval. The electric heating plate 14 is arranged above the microwave resonant cavity 21, and because the periphery and the top of the microwave resonant cavity 21 are all metal screens with specified round holes, heat can enter the microwave resonant cavity 21 through the radiation of the round holes to provide sublimation latent heat for materials; meanwhile, the microwave resonant cavity 21 is communicated with the external vacuum environment through a screen to ensure the vacuum drying condition of the microwave. The microwave radiation source is preferably a solid microwave source, and compared with the traditional magnetron, the microwave radiation source has small volume, does not need high-voltage power supply, is more beneficial to microwave feed-in, and can realize continuous adjustment of microwave power and fine heat supply; the semicircular polypropylene microwave feed-in cover 24 is matched, so that the microwave feed-in area is increased, and the drying energy efficiency is improved; the energy feed port is provided with a microwave stirrer 22 in front, so that microwave oscillation modes in the microwave resonant cavity 21 are mutually superposed, and the energy distribution of a microwave field is uniform.
The working process and the technical principle of the microwave vacuum freeze drying method are specifically described below by taking sea cucumbers as raw materials, and the adopted technical scheme comprises the following steps:
the method comprises the steps of raw material preparation, namely selecting sea cucumber materials with similar shapes, sizes and weights, ensuring the synchronization of the temperature rise process of the materials in the drying process, and avoiding incomplete drying or burnt phenomenon caused by overlarge or undersize volume and weight;
blanching with boiling water, wherein the sea cucumber body contains autolytic enzyme, and is subjected to intestine digging, viscera removing, cleaning, and blanching in boiling water for inactivating enzyme for 15-20min to inactivate enzyme activity and prevent autolysis;
a cooling and draining step, which specifically comprises the steps of cleaning and cooling by flowing clear water, removing substances adhered to the body surface of the sea cucumber, cooling the materials and draining;
pre-freezing, specifically comprising early-stage basic parameter measurement, wherein early-stage test detection shows that the eutectic point of the sea cucumber is about-18 ℃, the initial moisture (marked by R0 and unit g/100 g) of the sea cucumber is about 70 +/-10 g/100g (different according to the variety and the blanching draining effect), and the pre-treated sea cucumber is put into a quick-freezing box at the temperature of below-20 ℃ for freezing, so that the internal moisture of the sea cucumber is frozen in a short time and is kept for 4-6 hours, and the internal moisture of the sea cucumber material is completely formed into ice crystals;
a drying step, wherein the sea cucumber drying process mainly comprises 2 stages, and the first stage is a vacuum freeze drying stage (a water sublimation stage); the second stage, microwave vacuum drying stage (moisture analysis stage), is as follows:
the first stage, according to the material change rule (-25 to-10 ℃ and slow rise completion within 4 to 5 hours) in the sea cucumber drying process, along with rapid sublimation of water, the supplement of corresponding sublimation heat is slowly reduced, namely the temperature of a heating plate is set to be a gentle descending trend, the temperature is generally reduced from 70 +/-5 ℃ to 60 +/-5 ℃ within 4 to 5 hours, a heating plate temperature change rule program is preset in the actual drying process, a temperature sensor collects a heating plate temperature signal in real time, and a PLC system is fed back in real time to control, so that the dynamic provision of sublimation heat is realized, and the stable implementation of sublimation drying is ensured;
the second stage completes the sea cucumber microwave vacuum drying process, and along with the proceeding of the sublimation process, the moisture is gradually reduced, the sublimation process is gradually weakened, and the material temperature is shown as rapidly rising. When the rising amplitude of the material temperature exceeds the set temperature (generally 6-8 ℃) within unit time (generally 30 min), the sublimation drying stage is finished, and the sea cucumber drying enters the analysis drying period. At the moment, the temperature sensor collects the temperature change signal in real time and uploads the temperature change signal to the PLC control system 3, the PLC control system 3 closes the rotary-vane vacuum pump 12, the water circulation vacuum pump is started, the pressure of the drying bin is adjusted, the pressure rises from below 133Pa to 3000-3500 Pa (the pressure is finished within 3 min), the pressure sensor feeds back the pressure signal to the PLC control system 3, the PLC control system 3 starts the microwave source, the heating plate is closed, the microwave vacuum drying stage is started, in order to ensure the drying quality, the temperature of the material is controlled to be between 50-60 ℃, the program is set according to the requirement, the temperature sensor collects the temperature signal in real time, the PLC control system 3 is fed back to adjust the starting and closing states of the microwave source, the microwave vacuum drying is finished until the real-time moisture content is less than or equal to 10%. Real-time moisture content R t Calculation by PLC based on initial moisture content (R) 0 ) And initial mass M collected by the weight sensor 0 And real-time mass M t The calculation is carried out according to the following formula:
wherein, M 0 The initial mass of the sea cucumber before drying is g; m t The real-time mass of the sea cucumber monitored by a mass sensor probe in the drying process is g; r 0 The unit is the initial water content of the sea cucumber before drying, and the unit is g/100g; r is t The unit is g/100g of the real-time water content of the sea cucumber calculated by the PLC control system 3 in the drying process.
And (3) sealing and packaging, namely sealing and packaging the materials subjected to microwave vacuum freeze drying by using a food-grade 4-layer composite PET aluminum foil bag or other opaque food-grade packaging materials, storing the materials in a light-proof and low-temperature isolated oxygen manner, keeping the spongy texture of the materials, and avoiding the reduction of sensory quality caused by the decomposition of pigments in the materials.
According to the invention, the space of the microwave resonant cavity 21 is enlarged, the industrialized popularization is facilitated, according to the vacuum freeze drying rule of most materials, in the early stage of drying with vigorous water sublimation, the electric heating plate 14 is opened, the refrigerating system and the rotary-vane vacuum pump 12 are started, the coordination effect is realized, the electric heating is taken as the main mode for providing latent heat of sublimation, the mass sublimation of the water in the materials is promoted, and the sublimation of most water is realized under the low vacuum condition; when the speed that hinders in dry later stage moisture sublimation descends rapidly, open microwave heating and water circulating vacuum pump 11, heat transfer direction in the change material promotes to remain moisture and escapes rapidly, utilizes microwave heating to provide the latent heat of sublimation, improves drying efficiency, shortens drying time, reduces drying cost, realizes the high-efficient drying of material. Because the sublimation process begins from the outer surface of the material, the heat transfer and mass transfer resistance is small at the initial drying stage, the water content is rapidly reduced, and the sublimation rate is rapidly increased. Along with the sublimation interface constantly inwards passes, the whole coefficient of heat conductivity of material reduces gradually, and mass transfer resistance crescent can appear more obvious constant speed drying stage, and drying rate maintains at higher level, and most moisture desorption in this stage, and this process adopts the electrical heating can keep higher drying efficiency. In the later stage of vacuum freeze drying, as a drying interface is pushed inwards, heat transfer and mass transfer resistance is increased, the sublimation drying speed is reduced rapidly, ice crystals in the material are completely sublimated and disappear, a small amount of water remained in a porous structure tissue of the material needs to be removed by analysis and drying and escapes in a diffusion and permeation mode, and finally a small amount of chemical crystal water remained cannot be removed to become residual water in the material.
The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof, and it should be understood that various changes and modifications can be effected therein by one skilled in the art without departing from the spirit and scope of the invention as defined in the appended claims.
Claims (7)
1. A microwave vacuum freeze drying device is characterized by comprising a microwave heating device, a vacuum device, a cold trap, a refrigerating device and a control system, wherein the vacuum device comprises a vacuum bin, an electric heating plate, a rotary vane vacuum pump and a water circulation type vacuum pump which are connected with the vacuum bin through a vacuum pipeline, the electric heating plate and the microwave heating device are both arranged in the vacuum bin, the refrigerating device is connected with the water circulation type vacuum pump, the cold trap is arranged between the vacuum bin and the vacuum pipeline, the control system is respectively connected with the microwave heating device, the vacuum device, the cold trap and the refrigerating device, and the control system is used for acquiring parameters of the microwave heating device, the vacuum device, the cold trap and the refrigerating device and carrying out program control and function switching on the microwave heating device, the vacuum device, the cold trap and the refrigerating device so as to realize simultaneous or sectional implementation of microwave vacuum freeze drying and vacuum freeze drying;
the microwave heating device comprises a microwave resonant cavity, a microwave emission source, a material tray bracket and a microwave stirrer, wherein the microwave resonant cavity is arranged in the vacuum bin and shares the bottom with the vacuum bin, the electric heating plate is positioned above the microwave resonant cavity, the material tray bracket is arranged in the microwave resonant cavity and is positioned right below the electric heating plate, the microwave emission source is a solid microwave source and is arranged at the bottom of the microwave resonant cavity, the microwave stirrer is arranged in a microwave energy feed port of the microwave emission source, and a metal shielding overflowing plate with sieve pores is arranged at the bottom of the microwave resonant cavity; the microwave resonant cavity is formed by enclosing a microwave shielding metal net, and a sieve for shielding microwaves and communicating the microwave resonant cavity with the vacuum bin is arranged on the microwave shielding metal net; and a microwave feed-in cover for increasing the microwave feed-in area and avoiding glow discharge is arranged above the microwave feed-in port, and the microwave feed-in cover is a polystyrene material piece.
2. The microwave vacuum freeze-drying apparatus of claim 1, wherein a sealing gasket for preventing microwave and vacuum leakage is disposed at the connection between the microwave resonant cavity and the vacuum chamber.
3. The microwave vacuum freeze-drying device according to claim 1, wherein in the vacuum freeze-drying stage, the control system presets the variation range of the material temperature, and adjusts the electric heating plate to be opened or closed according to the material temperature data acquired by the temperature sensor in real time, so as to ensure the sublimation drying process to be carried out stably; and in the microwave vacuum freeze drying stage, the control system calculates the material temperature rise amplitude in unit time and adjusts the on or off of the microwave emission source according to the change rule of the material temperature along with time and the material temperature data acquired by the temperature sensor in real time.
4. A microwave vacuum freeze-drying method, characterized in that, the microwave vacuum freeze-drying equipment of any one of the above claims 1-3 is adopted, comprising the following steps:
s100, a vacuum freeze-drying step, namely presetting the change range of the material temperature according to the temperature change rule in the sublimation process of the material, opening an electric heating plate, a rotary vane vacuum pump, a refrigerating device and a cold trap at the early stage of drying when the water is sublimated vigorously, reducing the pressure in a vacuum bin to 100-133 Pa, and quickly sublimating most of the water by adopting electric heating under the low-vacuum condition; and
and S200, microwave vacuum drying, namely starting a microwave heating and water circulation type vacuum pump, closing a refrigerating device, adjusting the pressure in a vacuum bin to 2000-4000Pa, realizing heat transfer and mass transfer in the same direction by heat transfer in the material, and promoting residual moisture in the material in the vacuum bin to quickly escape to realize quick drying of the material.
5. The microwave vacuum freeze-drying method according to claim 4, wherein in step S100, the temperature of the electric heating plate is set to 70-80 ℃ in the initial stage of sublimation, 60 ± 5 ℃ in the middle stage of sublimation, 50 ℃ or less in the later stage of sublimation, and the temperature data of the material in the vacuum chamber is collected by a temperature sensor in real time in the drying process, and a control system adjusts the electric heating plate to be opened or closed according to the material temperature data to ensure the stability and high efficiency of the material heating process.
6. The microwave vacuum freeze-drying method according to claim 4, wherein in step S200, a material temperature rise amplitude in unit time is set according to material temperature data in the vacuum chamber acquired by a temperature sensor in real time, when the material temperature rise amplitude is reduced to a set value, a control system starts a microwave emission source to perform microwave vacuum drying on the material, so that residual moisture is rapidly separated out until the material moisture is reduced to below 10%, and the drying process is finished.
7. The microwave vacuum freeze-drying method according to claim 4, 5 or 6, characterized in that a control system controls the vacuum degree of the vacuum chamber according to the sequence of vacuum freeze-drying and microwave vacuum drying and the required pressure range, adjusts the pressure change range through pressure data in the vacuum chamber acquired by a pressure sensor in real time, starts the rotary-vane vacuum pump after the material is put in, starts the electric heating plate to heat when the vacuum degree of the vacuum chamber is reduced below a set value, sets a vacuum degree switching point based on the material temperature rise range and real-time moisture change, switches the rotary-vane vacuum pump to a water circulation type vacuum pump when the vacuum pressure of the vacuum chamber is increased to the vacuum degree switching point, and starts the microwave vacuum drying.
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