RU2821115C1 - Sublimation dryer - Google Patents

Abstract

FIELD: sublimation drying.

SUBSTANCE: invention relates to devices for sublimation drying. Sublimation of thermolabile material is carried out using a device comprising a drying chamber and a condensation chamber, between which there is a multistage (two-stage) turbocompressor equipped with an intermediate steam cooler between the stages. Device is also equipped with a control unit, pressure and temperature sensors, valves and pumps. At the same time in the drying chamber conditions are provided below the water triple point, and the condensation chamber is provided with conditions above the water triple point.

EFFECT: increasing energy efficiency and accelerating the drying process.

5 cl, 2 dwg, 1 tbl

Description

The invention relates to drying equipment, namely to installations for sublimation drying.

Devices for freeze drying (or freeze dryer) ensure the removal of solvent from frozen solutions, gels, suspensions and biological objects. The freeze-drying process is based on the sublimation of a solidified solvent (ice) without the formation of macroquantities of the liquid phase. Freeze drying allows you to store products for a significant period of time without losing their beneficial properties.

Currently, many methods and devices are known for freeze-drying various products and materials. Devices for freeze-drying can be capacitive, belt, drum-type, conveyor-cyclic, etc., and the initial product for freeze-drying can be in various states of aggregation (liquid, ice, suspension, paste, granules, etc.) . The characteristics of the vast majority of methods and devices for freeze drying are the following:

1. The pressure in the drying chamber is below the pressure of the triple point of water.

2. The moisture in the material being dried is contained in the solid phase.

3. The dried material receives heat from various types of heaters (radiation, microwave and others), due to which it is heated to temperatures above the equilibrium temperature between the gaseous and solid phases of water, and the moisture in the material undergoes a process of sublimation.

4. In the drying chamber or in a volume directly connected to the drying chamber, there are surfaces cooled to temperatures below the equilibrium temperature between the gaseous and solid phases of water, and on these surfaces the steam condenses to form the solid phase of water.

5. The gaseous environment of the device is formed by water vapor with impurities of other gases, the proportion of which is small. This condition is ensured with the help of vacuum pumps that perform initial pumping of the volume before sublimation begins and continue to remove non-condensable gases from the volume during the sublimation process.

The disadvantage of methods and devices that meet these characteristics is that the layer of ice formed on the cooled surface requires periodic removal, which complicates the device and the technological process. In addition, a layer of ice on the surface of the heat exchanger creates thermal resistance to the heat transfer of condensation, which tightens the requirements for the heat removal system from the cooled surface.

A freeze-drying device is known (EnWave, Canada) US 2020/02003751A “Dehydration below the triple point of water”, which presents a method of freeze-drying using microwave heating of organic material, and the device also contains an intermediate vacuum pump (booster pump - eng) with the ability to increase the pressure to 10 times the pressure in the sublimation chamber, due to which the pressure in the condensation chamber can exceed the triple point pressure. This device is impractical due to the high energy consumption of intermediate vacuum pumps used in the industry. These pumps are of the positive displacement type and expend significant power on mechanical friction. At the same time, the steam in sublimation devices has low pressure (tens of Pa), so the share of work spent by the pump on compressing the steam will be much less than the share of work spent on overcoming friction. Vacuum pumps are used in sublimation installations to remove only a small part of water vapor mixed with non-condensable gases.

In the proposed device for freeze drying, dynamic type compressors are used - axial or centrifugal turbocompressors with a sufficiently large volumetric capacity at the inlet. As an example, we point out that a freeze drying installation with a capacity of 1 kg of raw materials per hour at 50% moisture content of the raw materials, with a pressure in the drying chamber of 100 Pa, should be provided with a turbocompressor with a capacity of about 1000 m ³ / hour. The required compression ratio is about 10. When compressed in one stage, according to the equation of the adiabatic process

Where – adiabatic index, and taking into account the imperfection of the compression process, the temperature of the steam in the turbocharger reaches at least 500-600 K, which significantly complicates the design of the turbocharger. Therefore, compression must be carried out in at least 2 stages with intermediate cooling. The operation can be carried out, for example, by two KTurboTCL-100 compressors (Korea), connected in series, through one steam intercooler.

The purpose of the invention is a freeze-drying device, which provides a practical and energy-efficient drying method, including compression of water vapor and its condensation to form a liquid phase. The device allows you to speed up the freeze drying process due to the continuous removal of condensate in the form of liquid water.

The purpose of the invention is achieved using a device comprising a drying chamber and a condensation chamber, between which a multi-stage turbocharger is installed, equipped with a steam intercooler between the stages. At the same time, conditions are provided in the drying chamber below the triple point of water, and in the condensation chamber (condenser) conditions are provided above the triple point of water.

The essence of the invention is illustrated by drawings.

In fig. 1 shows a diagram of a device for freeze drying.

In fig. Figure 2 shows the P-T phase diagram near the triple point of water and an example of the behavior of steam in the described freeze-drying installation.

The freeze-drying device contains the following components, which are shown in Fig.1. The device contains a drying (sublimation) chamber 1, in which the dried material 2 is located, thermally insulated steam lines 3, a multi-stage turbocompressor with at least two stages of vapor compression (Fig. 1 shows two stages 4 and 12), a condenser 5, intercoolers steam between the stages of the turbocharger (Fig. 1 shows one cooler 6 between two stages). Inside the condenser and intercoolers there are heat exchangers 7, which receive cooling fluid from the refrigeration machine (external component, not shown). Connected to the condensation chamber 5 is a pipe for draining condensate 8, leading to a pump 9, which increases the pressure of the condensate to atmospheric, and a pipe 10 for removing a mixture of steam and non-condensable gases using a vacuum pump 11. The device also includes a device for heating the material being dried, not indicated in the figure. , temperature and pressure sensors (installed on the drying chamber, intercoolers and condenser), valves for admitting external atmosphere, and a control unit connected to the material heating device, sensors, turbocharger, pumps and shut-off valves (not marked).

In this case, the freeze-drying device is equipped with a multi-stage turbocompressor (the number of stages is at least two), and steam intercoolers are located between each pair of stages. Without limiting the generality of the invention, this document provides, as an example, a freeze drying apparatus with a two-stage turbocharger and one steam intercooler.

The drying device operates as follows. The material to be dried is placed into the drying chamber 1. Preferably, previously frozen material is placed into the drying chamber. The cooling fluid from the refrigeration unit cools the intermediate heat exchanger 7 and the final heat exchanger 13. The vacuum pump 11 is turned on to remove air from the system. When the turbocompressor is not running, air freely passes from the drying chamber 1 through the steam lines 3 and turbocharger stages 4 and 12, then enters the condensation chamber 5 and is sucked out using the vacuum pump 11.

A two-stage axial turbocharger (first stage 4 and second stage 12), designed for low inlet pressure, is started by the control unit after the system pressure reaches the upper limit of its operating range. Under conditions of low pressure in the drying chamber 1, provided by the operation of the vacuum pump 11 and a multi-stage turbocompressor (stages 4 and 12), the transition of moisture from material 2 to the state of steam begins. When the pressure in drying chamber 1 falls below the triple point of water (P _tr = 611.7 Pa), then the moisture in material 2 is in a solid state (ice) - regardless of whether material 2 was previously frozen. After the pressure in the drying chamber 1 reaches a value according to the technical conditions of drying, the material heating device is turned on. The water vapor in the drying chamber 1 has a temperature and pressure below the temperature and pressure of the triple point of water (<611.7 Pa). Steam through steam lines 3 enters sequentially into the first stage 4 of the turbocompressor, the steam intercooler 6, then into the second stage 12 of the turbocompressor. As the water vapor passes through the stages of a multi-stage turbocharger (stages 4 and 12), it increases its temperature and pressure in a process that is close to isentropic. As steam passes through the intercooler 6 at constant pressure, it reduces its temperature, but remains above the water condensation point. The steam, after passing through the second stage 12 of the turbocompressor, enters the condenser chamber 5 (the pressure here exceeds the pressure of the triple point of water), where it condenses on the heat exchanger 7 to form liquid water. In this case, the conditions for steam in the condenser are above the triple point of water, which is determined using a temperature and pressure sensor in the condenser chamber 5. The desired temperature in the heat exchanger 7 is maintained above zero by regulating the refrigerant supply rate.

During operation of the installation, water is pumped out from the condensation chamber 5 by a liquid pump 9, which increases its pressure to atmospheric pressure and then the water is removed for subsequent use (or drainage). The operating mode of the vacuum pump 11 is adjusted in such a way as to maintain the content of non-condensable gases in the condenser 5 at an acceptable level; an excess of this level can be detected, for example, by the excess of the difference between the surface temperature of the heat exchanger in condenser 5 and the temperature of saturated steam at a pressure equal to the pressure in condenser 5 by 10 K.

Upon completion of the freeze-drying process (monitoring according to the readings of material temperature sensors 2 and pressure in the drying chamber 1), the turbocompressor (stages 4 and 12) and the vacuum pump 11 are stopped, then atmospheric air is injected into the drying chamber 1, into the steam intercooler 6, into the condensation chamber 5, and into the drying chamber 1, which allows you to extract the dried (sublimated) material 2.

The advantages of this device over freeze-drying devices, in which the pressure in the sublimation chamber and in the condensation chamber are close, are the following.

1. Condensation in the condenser occurs without the formation of ice and with the release of liquid water, which does not accumulate on the condensation surface, but is removed from the chamber by a water pump during the drying process. This speeds up the overall drying process.

2. The temperature of the cooling coolant in the condensation chamber is higher, which reduces the energy consumption of the refrigeration machine for cooling the coolant or, depending on the degree of vapor compression by the compressor, allows the use of recycled water as a cooling coolant.

An example of the invention.

Figure 2 shows a P-T diagram of water, on which are plotted (for comparison) the lines of P-T processes occurring with steam in a device without steam compression (for comparison) and in an open device with a two-stage turbocharger. The processes are calculated for the following conditions: an initial pressure of 50 Pa and a water vapor temperature of –10°C, the cooling coolant ensures cooling of the steam in the intercooler to +5°C and condensation in the condenser at the same temperature, the turbocharger has two stages, the adiabatic efficiency of each stage compressor is 80%.

In a device without a compressor (dashed line), the steam undergoes a process close to isobaric cooling, eventually reaching the solid-gas phase interface.

In the open device, the steam undergoes processes shown by the dashed line: first, close to isentropic compression (first stage 4 of the turbocharger), then a process close to isobaric cooling (carried out in the intercooler 6), then a second process close to isentropic compression (works second stage 12 of the turbocharger), and a second process close to isobaric cooling (condenser), leading to the line of separation of the liquid and gaseous phases. In the drying device proposed in the application US 2020/02003751A “Dehydration below the triple point of water” (see prior art), under these conditions the steam after compression by the pump will have a temperature of 319 ° C, while in the open device the maximum temperature of the steam in the system is 145°C, which gives an advantage in practical implementation.

In fig. Figure 2 shows a P-T diagram of water in the operating range of temperature and pressure (pressure is plotted on a logarithmic scale).

The dashed line displays the processes of changing the state of water vapor in the described device. Marker “•” is the triple point of water (ice-water-steam phase boundary). The triple point for water occurs at temperature T _tr = 0°C and pressure P _tr = 611.7 Pa. The “o” marker shows the initial state of the steam in the sublimation chamber. The solid lines in the diagram are the lines of phase separation (solid, liquid, vapor - indicated on the diagram). The dotted line is the cooling and condensation process of steam in a traditional vacuum pump freeze drying device. The dashed line shows the process in the freeze drying device using vapor compression in two compressor stages: first vapor compression stage, first cooling stage, second compression stage, second cooling stage, water condensation.

The cycle parameters of an exemplary freeze-drying device under the above conditions are shown in Table 1.

The pressure after the first compression stage is chosen such that the steam temperatures at the outlet of the first and second stages of the turbocharger are the same. To compare the energy characteristics of the disclosed device with a device without a compressor, line 12 of the table shows the operating costs of a refrigeration machine for condensing 1 kg of steam in a device without a compressor. As can be seen, the use of vapor compression using two stages of a turbocharger requires work costs to drive the compressor, but significantly reduces the work costs of the refrigeration machine, resulting in the total work costs being reduced by 1912/1336 ≈1.4 times.

Table 1

No. Parameter Meaning 1 Temperature in the drying chamber –10°С 2 Pressure in the drying chamber 50 Pa 3 Temperature at the outlet of the first stage of the turbocharger (at the inlet to the intercooler) 145°С 4 Intercooler pressure 229 Pa 5 Intercooler outlet temperature +5°С 6 Temperature at the outlet of the second stage of the turbocharger (at the inlet to the condenser) 145°C 7 Condenser pressure 863.5 Pa 8 Condensation temperature +5°С 9 Work consumed by the compressor 545 kJ/kg 10 Work consumed by the refrigeration machine (refrigeration coefficient at a coolant temperature of -5°C is 3.8) 791 kJ/kg eleven Work consumed to remove steam from the drying chamber (jointly by the compressor and refrigeration machine) - according to the invention 1336 kJ/kg 12 Work consumed by a refrigeration machine in a sublimation unit without vapor compression (refrigeration coefficient at a coolant temperature of –40°C is 1.48) 1912 kJ/kg

The given parameters and mode of operation of the device are selected to consider a specific example and do not express restrictions on the principle of operation of the device or restrictions on the features of the invention.

Claims (5)

1. A device for freeze drying, including a control unit, a drying chamber, steam lines, a condenser with a heat exchanger, a refrigeration unit for refrigerant, pressure and temperature sensors, a vacuum pump, a liquid pump, steam coolers with heat exchangers, a multi-stage turbocharger, characterized in that the The turbocharger connects the drying chamber and the condenser and is equipped with steam intercoolers between successive stages of the turbocharger. 2. A device for freeze-drying according to claim 1, characterized in that the multi-stage turbocompressor is a two-stage turbocompressor. 3. A device for freeze drying according to claim 1, characterized in that a pressure is provided in the drying chamber below the pressure of the triple point of water. 4. A device for freeze drying according to claim 1, characterized in that the condenser is provided with a pressure higher than the pressure of the triple point of water. 5. A device for freeze drying according to claim 1, characterized in that the steam temperature at the outlet of the steam intercooler is higher than the condensation temperature of water vapor.