SU1474406A1 - Method of sublimation drying of nonorganic salt aqueous solution - Google Patents

Abstract

This invention relates to methods for freeze-drying liquid aqueous solutions of inorganic salts. The aim of the invention is to increase the dispersion of the obtained product, as well as reduce the duration of the process. The method consists in that in the method of freeze-drying of liquid materials, including freezing the liquid material by introducing it directly into the vacuum chamber with a pressure below the triple point and subsequent sublimation removal of the liquid, the liquid material being introduced into the vacuum pressure, the equilibrium temperature is lower than the temperature of the material being processed and higher than the temperature on the surface of the desublimator, which is at the point of entry of the material being processed deposited on its surface. The flow rate of the applied liquid is 0.13-0.25 of the consumption of the material being processed. Ethanol, acetone or cyclohexene are used as the applied liquid. The proposed method is possible drying of such salts as NA ₂ SO ₃ , KNO ₃ , BA (NO ₃ ) ₂ , NAHCO ₃ , KH ₂ PO ₄ , H ₃ BO ₃ .

Description

(21) 4182878 / 31-26

(22) 01/14/87

(46) 04.23.89. Bit. Number 15

(71) Moscow Institute of Chemical Machines

(72) A.Z. Volynets, A. V. Karabanov, S. M. Brazhnikov, A. V. Rozhdestvensky, S. N. Rodionov and L. A. Serova

(53) 66.047.25 (088.8)

(56) US Patent No. 3319344, cl. 34.5, 1967.

(54) METHOD OF SUBLIMATION DRYING OF AQUEOUS SOLUTIONS OF INORGANIC SALTS

(57) The invention relates to methods for freeze-drying liquid aqueous solutions of inorganic salts. The aim of the invention is to increase the dispersion of the product obtained,

as well as reducing the duration of the process. The method is that in the freeze drying process

liquid materials, including freezing the liquid material by introducing it directly into the vacuum chamber with a pressure below the triple point and subsequent sublimation removal of the liquid, the liquid material being processed is introduced into the vacuum together with a liquid 6 having an equilibrium temperature at a given pressure below the temperature of the material being processed: above the temperature the surface of the deo-duplicator, which is applied to its surface at the point of entry of the material being processed into a vacuum. The flow rate of the applied fluid is 0.13 0.25 of the flow rate of the material being processed. Ethanol, acetone or cyclohexene are used as the applied liquid. According to the proposed method, it is possible to dry such salts as HaAZO ,, KNO ,, Ba (NO,) i} NaHCO ,, KH2P04,. 1 hp ff, 1 tab.

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The invention relates to methods for freeze-drying liquid materials, in particular concentrated solutions of salts of inorganic substances, and can be used in the chemical industry.

The purpose of the invention is to increase the dispersion of the resulting product, as well as reducing the duration of the process when using concentrated solutions.

Example t. The method is to freeze the stock solution.

in vacuum at a pressure below the water troyn point, its sublimation removal and subsequent desublimation of water vapor. At the same time, the freezing of the initial solution is carried out in the presence of a liquid that at this pressure has an equilibrium temperature below the solution temperature and above the desublimation temperature. This liquid, which uses ethanol or acetone, or cyclohexene, together with the solution is introduced into the freezing stage

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coaxially located capillaries at a flow rate of 0.13–0.25 ma.ch. on 1 ma.ch. solution.

When released into vacuum, this liquid covers the surface of the original solution. Since its equilibrium temperature at a given pressure in the vacuum chamber is lower than the equilibrium temperature of the initial solution, its crystallization occurs at a higher temperature gradient than in the absence of a deposited liquid. In addition, if there is a liquid film on the surface of the initial solution, the initial evaporation (at the stage of cooling to the freezing temperature) occurs only from this film. Therefore, the removal of water from the initial solution drops occurs only in the solid phase (sublimation). The liquid 9 forming the film does not contain impurities; therefore, when it is fully paired (from the liquid or solid phase), a dense surface crust is not formed that prevents the vapor from leaving the internal vapors of the frozen drops of the initial solution. As a result, an increase in the temperature gradient and a decrease in the thermal resistance to heat transfer make it possible to achieve high values of freezing rates, sufficient to obtain an ultrafine powder and to ensure a high uniform mixing of the components. The elimination of a dense surface crust of a material that has a high resistance to the release of vapors from the inner layers of the drop reduces the duration of the drying process. Moreover, this effect plays an especially important role in processing concentrated solutions, since the permeability of the surface layer is the lower the higher the concentration of the initial solution.

In order for the vapors of the applied liquid to be captured in the desublimator, the equilibrium temperature of this fluid at working pressure must be higher than the surface temperature of the cusublimator.

To maintain a high speed, -

freeze drops throughout the entire process. The optimal flow rate of the liquid should be selected so that the amount of heat is reached.

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The amount of heat required for evaporating this liquid was equal to the amount of heat that must be removed from a drop of the initial solution in order to completely freeze it.

The calculation of such a heat balance shows that the flow rate of the applied fluid mainly depends on the specific heat of crystallization of the treated material and the specific heat of evaporation of the applied fluid.

The faster the applied liquid evaporates, the faster the freezing of the initial solution occurs and the greater the flow rate of the applied liquid needed to effect the complete freezing of the droplets of the initial solution. The specific heat of crystallization of aqueous solutions is approximately the same, and among liquids whose equilibrium temperature is significantly lower than the equilibrium water temperature and higher than the desublimation temperature, the specific heat of evaporation is less than all of cyclohexene and higher than all of ethanol. Therefore, the flow rate of these liquids is the minimum and maximum flow rate that must be supplied so that all the droplets freeze at the same high speed.

Experiments carried out on freeze drying of concentrated aqueous solutions of sodium sulfite, potassium nitrate, barium nitrate, potassium monophosphate using cyclohexene and ethanol as the applied liquid show that the minimum consumption of cyclohexene required for. complete freezing of the droplets at the same high speed is 0.13 mas.h, at T mac.h. of the initial solution, and the minimum consumption of alcohol is 0.25 mas per hour per mac. source solution. At lower consumption of these liquids in all the experiments carried out with sufficient speed to obtain a highly dispersed product, only a partial freezing of a drop of the initial solution, i.e. the applied liquid completely evaporates before all of the solvent (water) from the droplet passes into the solid phase. Consequently, the possibilities of the method with respect to increasing the freezing rate and obtaining a fine powder in this case are not fully realized.

The delivery of high costs of cyclohexene and alcohol is unjustified, since, as experience shows, the increment in freezing time due to an increase in the total mass of the drop (resulting from the mass of the initial solution and film weight) begins to exceed the reduction in the process duration due to high intensity evaporation of the film material, which results in an increase in the freezing path length. At 15, the receipt of substances with high

this increases the duration of the drying process and increases the load on the desublimator. The experiments also show that, using any other substances as the applied liquid, whose equilibrium temperature is lower than the equilibrium temperature of the initial solution and higher than the temperature in the desublimator, for example ethyl nitrate, their minimum consumption required for obtaining a highly dispersed product is higher than that of cyclohexene. and lower alcohol consumption under the same conditions.

Consequently, when using the method for each particular material to be deposited on its surface, it is necessary to select a technologically acceptable liquid, while to obtain the effect of an increase in the freezing rate, its consumption, related to the flow rate of the initial solution, is in the range of 0, TZ-0.25. Moreover, the minimum

the flow rate of the particular fluid being applied, about sodium, becomes insufficient for

It is economically more advantageous to use the method, since along with an increase in the freezing rate, the method allows, with a slight increase in energy consumption for vapor recovery of pre-introduced liquid in the installation, to significantly increase the concentration of the initial vapor, ie, reduce energy consumption for solvent vapor recovery. the substance obtained.

Thus, the use of the freeze-drying method makes it possible to achieve an increase in the speed of the freezing, living, thus with a considerable increase in the dispersion of the obtained product, to reduce the duration of the drying process, the energy consumption for collecting the solvent vapors during

using concentrated solutions.

As a result, the application of a film of a liquid to a lower equilibrium temperature on the surface of a material that is frozen in vacuum allows one to get rid of a poorly permeable surface layer of material and increase the temperature gradient at which freezing occurs. This leads to an increase in the freezing rate of the solution. Increase speed - morzhizakil provide0

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dispersion. Dispersed analysis shows that with this method of obtaining sodium sulfite, the minimum consumption of alcohol, in which the size of the particles obtained does not exceed 1 µm, is 0.25 wt.h. on 1 ma.ch. source solution. From this we can conclude that the segregation of components does not occur, i.e. during the time until the droplet is completely frozen, evaporation occurs only from a film of alcohol.

In addition, the duration of the drying process with 1 liter of 50% sodium sulfite solution is reduced by 0.5 h due to the fact that it is applied to the surface of the sodium sulfite solution to prevent the formation of a poorly permeable surface layer of sodium sulfite, in which the main resistance to the vapor output at the sublemation stage is concluded.

With a cost ratio of less than 0.25, the amount of alcohol evaporating from the surface of the sulfite solution

freeze the whole drop. Therefore, when it freezes, part of the water evaporates from the liquid state of the sodium sulfite solution, which leads to segregation of the droplet components and the whole droplet freezes not at the same high rate. This significantly increases the size of the particles. Analysis of variance shows that

At this flow rate, the particle size distribution becomes much wider, there are particles with a size of up to 7 microns.

When feeding alcohol with a flow rate

more than 0.25% of the sodium sulfite consumption, the shortening of the freezing time due to intensive evaporation begins to overlap with an increment of the freezing time due to an increase in the total mass of the droplet formed from the mass of the sodium sulfite solution and the mass of alcohol applied to the surface. This leads to an increase in the length of the droplet freezing path and to an increase in the load on the desublimimdtor.

Example 2. To illustrate the method, the same sublimation unit is used as in example T. The working pressure in the unit is maintained at 0.1 mm Hg, the desublimator temperature is -80 ° C. v 50% aqueous solution of potassium wasted is used as the processed material, cyclohexene is used as applied liquid. The material being introduced into the vacuum chamber together with cyclohexene is carried out through a special device described in Example 1.

The coating of potassium nitrate with a film, cyclohexene, evaporates, acquires a temperature equal to the equilibrium under this vacuum (-75, 9 ° С). In the absence of cyclohexene, the temperature of the solution is -32 ° C. Thus, the freezing of potassium nitrate begins at a lower temperature on the surface of the drop. This determines a higher freezing rate and, as a result, the production of a fine product. Analysis of variance of the product shows that upon receipt of potassium nitrate by the proposed method, the minimum consumption of cyclohexene, at which a narrow particle size distribution is observed and the particle size does not exceed 1 µm, is 0.13 wt.h. on T h., the original solution. Consequently, the segregation of components does not occur and all the drops. freezes at the same high speed.

When the cost ratio is less than 0.13, the amount of cyclohexene evaporating from the surface of the potassium nitrate solution becomes insufficient to freeze the entire drop and not the entire drop freezes at high speed, which significantly increases the particle size of the resulting product. Dispersion-ion analysis shows that when the relative consumption of cyclohexene is less than 0.13, the distribution of particles becomes wider,

due to the presence of large particles whose size reaches 3 microns. It is economically unprofitable to serve cyclohexane at a rate of more than 0.13 from the consumption of potassium nitrate solution.

Example 3. Use sublimation installation and device

vacuum, described in example 1. The operating parameters were maintained the same as in example 1. As the material being processed, use 50% aqueous solution

5 barium nitrate, acetone as a liquid.

The coating of the barium nitrate solution with a film,. Acetone, evaporates, acquires an equilibrium temperature at

This vacuum is -77.7 ° C. Without acetone, the temperature of an aqueous solution of barium nitrate is -32 ° C. Thus, the freezing of the solution begins at a lower temperature on the surface of the drop. This determines a higher freezing rate and, as a result, a higher quality of the produced product. The minimum consumption of acetone, at

0 of which the maximum freezing rate is observed (the minimum height of the drop span, ensuring complete freezing), is 0.16 of the consumption of the material being processed. The maximum particle size of the resulting product does not exceed 1 micron.

The results of the analysis of variance of the powders of the salts obtained by the proposed method in comparison with the known one are shown in the table (the concentration of the solutions is 50 wt.%).

Claims (2)

1. The method of freeze-drying aushki aqueous solutions of inorganic salts, including freezing the original solution in vacuum at a pressure 0 below the triple point of water, its sublimation removal and the subsequent desublimation of water vapor, characterized in that, for the purpose of increasing: the dispersion of the product, as well as 5 reducing the duration of the process when using concentrated solutions, the freezing of the initial solution is carried out in the presence of a liquid that has at this pressure 0 This equilibrium temperature is lower than the solution temperature and higher than the desublimation temperature, the liquid and the solution being introduced at the freezing stage through coaxially arranged capillaries at the flow rate of the liquid 147440610 whith 0.13-0.25 mac. on P. solution. 2. pop.1, I differ from u and with the fact that ethanol or acetone or cyclohexene is used as said liquid.