RU2422196C1 - Device for cryogenic granulation of solutions and suspensions - Google Patents

Abstract

FIELD: process engineering. ^ SUBSTANCE: proposed method comprises forming coolant revolving layer in vessel and feeding initial material into liquid coolant by spraying for granulated product to be removed from coolant. Vessel is filled with coolant to preset layer height. Subsequent mixing of coolant is performed in turbulent conditions corresponding to Reynolds number of 1*106-2*107 to produce bell-like layer with curved concave paraboloid-shaped surface at layer bottom that changes into hyperboloid at its top. Initial material is sprayed in finely-dispersed drops tangentially to coolant layer surface at 20-70 degrees to horizon. ^ EFFECT: intensified freezing, higher quality of single-sized granules. ^ 1 dwg, 2 ex

Description

The invention relates to the technology of granulation of various chemically single-phase or multiphase liquid substances, for example, solutions and suspensions, mainly for their subsequent freeze-drying and obtaining materials in the form of ultrafine powders.

There is a known granulation method, which consists in freezing a solution of a mixture of nitric acid salts by spraying the solution into a water-immiscible liquid, such as partially frozen hexane, through a die with a hole diameter of 50-150 μm and exposing the solution stream to axial vibrations (SU No. 635071, 30.11. 1978).

The granular product obtained by this method is characterized by an inhomogeneous particle size distribution and instability of chemical uniformity. During freezing, a partial agglomeration of particles of the material takes place, since when spraying the solution through the die, sufficiently fast cooling of the granules cannot be ensured.

A granulation method is also known, which includes supplying an initial solution to the surface of a liquid refrigerant and forming a rotating refrigerant layer, followed by removing a granular product from it (SU No. 1155835, 05.15.1985).

The material is fed into a tank filled with refrigerant - liquid nitrogen. The height of the liquid nitrogen column was chosen such that, having reached the bottom of the tank, the solution droplets turn into granules. From the bottom of the container, the granules under the action of hydrostatic pressure and inertia created by rotating the hollow inverse cone and the screw connected to it move upward to the unloading device.

In the known method, drops of material are fed into the bath on a fixed refrigerant layer, which cannot provide the maximum freezing rate, which determines the size of the formed structural elements of the solid phase and the uniform distribution of components in the granule. Part of the supplied droplets can come into contact with partially frozen droplets floating on the surface of the refrigerant and crystallize on their surface, which leads to the coalescence of granules and the formation of agglomerates.

The fixed refrigerant layer does not provide sufficiently quick freezing of product droplets and obtaining a product with specified parameters. If you supply material to a refrigerant that is stationary, then when droplets of solution get into the refrigerant, the latter will boil violently. The resulting vapor maintains the droplets in suspension, and they float on the surface. In this case, the droplets make random movements with simultaneous rotation around their axis. The resulting product is a heterogeneous easily crumbling granules, as well as agglomerates of fused granules.

The closest in technical essence to the invention is a method of freezing drops of a liquid product, which consists in forming a rotating layer of refrigerant in the tank, which is created by spraying the latter on the inner surface of the walls of the rotating Dewar vessel (SU No. 976234, 11/23/1982). The supply of the original liquid material is carried out by hydraulic spraying in the working volume of the Dewar vessel. Under the action of inertia and centrifugal forces created by the vortex flow of refrigerant vapor, the material is fed to the surface of the cryogenic liquid layer, frozen, precipitated and removed from the process. To increase the contact time of the frozen material with the refrigerant, an additional capacity is provided in the device.

However, the film layer of the refrigerant formed on the walls is quite thin and the heat exchange process that takes place in it does not provide quick and uniform freezing of the product, which necessitates additional contact of the granules with the refrigerant in the settler and, ultimately, leads to fluctuations in the particle size distribution and possible agglomeration granules.

The objective of the invention is the intensification of the freezing process, improving the quality of the product with obtaining one-dimensional spherical granules.

The problem is solved in that in the method of cryogenic granulation of solutions or suspensions, including the formation of a rotating refrigerant layer in a tank, feeding the initial solution into a liquid refrigerant by hydraulic spraying, followed by removal of the granular product from the refrigerant, according to the invention, the tank is initially filled with refrigerant to a predetermined layer height, followed by stirring the coolant in a turbulent flow, the Reynolds number corresponding to 1 * 10 ⁶ -2 * 10 ^7, to form a funnel-shaped layer a curved concave surface in the form of a paraboloid bottom layer, rolling in its upper portion a hyperboloid, while spraying a starting material is produced in the form of fine droplets tangentially to the surface of the refrigerant and at an angle of 20-70 degrees to the horizontal layer.

The invention consists in the following.

At moderate freezing rates, structure formation at the phase boundary occurs under conditions close to equilibrium (equilibrium crystallization). The structure of the solid phase is formed under the influence of thermodynamic instability of the interface, which develops under conditions of rapid cooling of the liquid. The hardening solution undergoes a transition from a metastable to a stable thermodynamic equilibrium state.

The characteristic sizes of the formed structural elements of ice crystallites and the target product will depend on the degree of supersaturation and supercooling of the liquid, the magnitude of the surface energy, and the rate of advancement of the solid phase front (freezing rate).

With an increase in the intensity of heat removal, the structure formation during rapid freezing ceases to obey equilibrium crystallization. There is known evidence of a sharp jump in the dependence of the characteristic particle size of the solid phase on the freezing rate. The structure formed under the influence of this effect, for example, from water-salt solutions, is characterized by the presence of particles of the order of 0.1 μm or less in combination with a high degree of uniformity of their size distribution at a cooling rate exceeding 30 K / s.

At a cooling rate of about 50 K / s, the particle sizes, for example, for an aqueous solution of sodium nitrate, are in the order of magnitude from tenths to hundredths of a micrometer, and their distribution is close to monodisperse.

Upon contact of the solution droplets with the surface of the refrigerant, a vapor layer is formed around the droplet, which prevents the granules from rapidly cooling. The rotating turbulent refrigerant layer “breaks” this vapor layer, and due to a significant increase in the heat transfer coefficient and increased mass of the refrigerant layer on the walls and in the bottom of the tank, in comparison with the known method, conditions are created for high values of the droplet cooling rate.

The supply of the sprayed starting material tangentially and at an angle to the surface of the rotating refrigerant layer contributes to the intensification of the process, since the droplets of the solution under the influence of the energy of rotation of the refrigerant are most trapped by the latter and carried away into the layer. Depending on the properties of the source material, its angle of supply to the surface of the refrigerant is selected experimentally in each case. The angle of supply of the solution and suspension within 20-70 degrees is optimal. At a smaller angle of material supply, droplets are worse captured by the refrigerant, and at a larger angle, the average size of the obtained cryogranules increases significantly (up to five times).

The turbulent regime corresponding to a Reynolds number of 1 * 10 ⁶ -2 * 10 ⁷ forms a funnel-shaped layer with a curved concave surface in the form of a paraboloid in the bottom of the layer, passing in its upper part into a hyperboloid. When droplets get into a rotating funnel-shaped layer with the indicated curved surface due to the formation of a layer of sufficient thickness (more than 20 average cryogranule diameters), the sphere-shaped shape of the drop does not change, which makes it possible to obtain uniform granules. Drops freeze quickly and evenly, turning into almost one-dimensional solid spherical granules. During cryogranulation, the fine-grained structure is intensively formed in the solid phase, characterized by a large number of small crystallites uniformly distributed in size in spherical granules.

With an increase in the freezing rate, the size of the formed structural elements of the solid phase decreases, and the uniformity of the distribution of components increases.

Hydraulic spraying of the material in the form of fine droplets also contributes to the production of fine cryogranules of uniform particle size distribution and the elimination of particle aggregation.

The implementation of the method is illustrated by examples and a schematic drawing.

Example 1. Technological capacity 1 with a volume of 5.0 l was filled with liquid refrigerant - nitrogen 2 to a predetermined level. The refrigerant layer by means of a paddle mixer 3 was brought into rotational turbulent motion corresponding to a Reynolds number of 1 * 10 ⁶ . The speed was sufficient for the formation of a funnel 4 in the refrigerant layer with a curved concave surface — the paraboloid a, which in the upper part of the layer becomes a hyperboloid of revolution b.

After stabilization of the surface shape of the refrigerant funnel 4, the supply of the initial solution was started.

An aqueous solution of lanthanum nitrate with a concentration of 51.9% (single-phase liquid) was preliminarily prepared.

The finished solution with a temperature of 20 ° C was pumped into the spray nozzle 5, which sprayed the material in the form of fine droplets. The flow rate of the solution was set at 25 l / h. The nozzle nozzle diameter was 0.4 mm, and the solution pressure at the nozzle exit was 0.2 MPa. The solution was supplied tangentially to the rotating layer at an angle α = 70 degrees to the horizontal.

When lanthanum nitrate solution gets into the liquid refrigerant, it quickly freezes and sinks to the bottom of the tank. Finished frozen granules were removed from the process. The size of the obtained granules ranged from 0.5 to 1.5 mm, while they had a spherical shape and a uniform composition.

Example 2

The parameters of the process were similar to example 1. Pre-prepared aqueous suspension of zirconium hydroxide with a 12% concentration of solid component. The refrigerant layer by means of a paddle mixer 3 was brought into rotational turbulent motion, corresponding to the Reynolds number 2 * 10 ⁷ . The speed was sufficient for the formation of a funnel 4 in the refrigerant layer with a curved concave surface — a paraboloid a, which in the upper part of the layer transforms into a hyperboloid of revolution b. Drops of the suspension when they get into liquid nitrogen were frozen and discharged from the tank. The solution was supplied tangentially to the rotating layer and at an angle α = 20 degrees to the horizon.

The obtained granules had a spherical shape, and their size ranged from 0.8 to 1.6 mm. The granules had a uniform composition.

It was experimentally established that the above Reynolds number is optimal for obtaining uniform spherical granules with a minimum dispersion of granule sizes. For other values of the Reynolds number, it is impossible to stably maintain the necessary shape of the rotating layer and, as a result, to obtain the required particle size distribution of cryogranules.

When frozen drops fall onto a rotating refrigerant layer with the specified curved surface due to the formation of a refrigerant layer of the required thickness extended over the height, the spherical shape of the droplet does not change, which makes it possible to obtain uniform granules.

The high heat transfer coefficient during freezing in a rotating refrigerant allows you to intensify the process of formation of granules. The granules in the finished product are chemically homogeneous, have a spherical shape and are characterized by a stable particle size distribution.

The stage of granulation by freezing is the most critical in the technological cycle. The regime of this particular stage determines the structure and properties of the final product obtained after freeze-drying of granules.

Claims (1)

The method of cryogenic granulation of solutions and suspensions, including the formation of a rotating layer of refrigerant in a tank, feeding the starting material into a liquid refrigerant by hydraulic spraying, followed by removal of a granular product from the refrigerant, characterized in that the tank is initially filled with refrigerant to a predetermined layer height, followed by mixing of the refrigerant in a turbulent mode corresponding to a Reynolds number of 1 × 10 ⁶ -2 × 10 ^7, to form a funnel-shaped layer with a curvilinear concave surface in the form of a paraboloid bottom layer, rolling in its upper portion a hyperboloid, the feed atomization produced tangentially to the surface layer of the refrigerant in the form of fine droplets and an angle of 20-70 ° to the horizontal.

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