Chemical and Petroleum Engineering, Vol. 48, Nos. 1–2, May, 2012 (Russian Original Nos. 1–2, Jan.–Feb., 2012)

ACCELERATING FREEZING OF DROPS OF AQUEOUS SOLUTIONS IN CRYOCHEMICAL TECHNOLOGY

N. S. Trutnev, I. A. Buzdalina, V. A. Zakrevskii, and K. A. Vikhrov

A method of producing cryogranules of water-soluble salts in a moving cryogenic liquid layer, which can be used to accelerate heat exchange process in film boiling, is examined. This method is shown to be more efficient than conventional cryogranule production methods.

Numerous studies have shown that the properties of ultra- and nano-disperse materials differ markedly from the properties of the initial crystalline form of the materials. One of the methods of producing ultra- and nano-disperse materials is cryochemical technology. This technology is based on crystallization of disperse drops of solutions in liquid nitrogen and consists of the following stages: preparation of the initial solution, dispersion of the solution in liquid nitrogen, freezing of the dispersed solution drops in liquid nitrogen with formation of cryogranules, and subsequent vacuum-sublimation drying of the obtained cryogranules. The freezing stage determines decisively the structure and dispersion characteristics of the end product. A variety of solution freezing methods is known: freezing in cryogenic liquids, freezing in refrigerated liquids, freezing on refrigerated surface, freezing in vacuum, etc. [1]. Currently, the method used widely is freezing in liquid nitrogen, which ensures high heat transfer coefficient as well as chemical purity of the end product. In the liquid nitrogen freezing process, the solution drops disperse on the static surface of the nitrogen and, having been crystallized, sink in it. As the drops fall on the refrigerant surface, film boiling process begins. The vapor interlayer formed hinders heat exchange between the liquid nitrogen and the drops, increasing thereby the drop crystallization time [1]. In order to accelerate the crystallization process, the Moscow State University of Engineering Ecology (MGUIE) developed a method where drops are dispersed in a moving liquid nitrogen layer [2]. As the drops fall on the surface of the moving cryoliquid, the latter boils up vigorously and draws the drops into forced motion, and the formed vapor interlayer of the evaporated refrigerant is entrained by the incident flow, reducing thereby the thickness of the interlayer, and this leads to an increase in the heat transfer coefficient and a decrease in the drop freezing time. The freezing method is schematically illustrated in Fig. 1. A vessel 1 is filled with liquid nitrogen 2 which is set into motion by a disk stirrer 3 immersed in it. As a result, the refrigerant moves coaxially, forming a funnel 4. The solution is delivered to the funnel surface by a jet atomizer 5. Having fallen onto the refrigerant flow, the drops move away from the atomization area and, having been crystallized, sink. After completion of the dispersion process, the cryogranules are extracted from the vessel. The experiments were conducted in a setup shown schematically in Fig. 2 [3]. The following freezing conditions were investigated: • delivery of single (individual) drops to static liquid nitrogen surface; • delivery of single drops to moving liquid nitrogen layer; Moscow State University of Engineering Ecology (MGUIE), Moscow. Translated from Khimicheskoe i Neftegazovoe Mashinostronie, No. 2, pp. 30–33, February, 2012. 0009-2355/12/0102-0109 ©2012 Springer Science+Business Media, Inc.

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Fig. 1. Schematic illustration of method of freezing in moving liquid nitrogen layer.

Fig. 2. Schematic diagram of setup for producing cryogranules: 1) Dewar vessel; 2) vessel containing solution; 3) heat-insulated vessel containing liquid nitrogen; 4) electric motor; 5) shaft with stirrer; 6) jet atomizer holder (clamp); 7) frame; 8)control panel; 9) jet atomizer.

• dispersal of drops through jet atomizer to static liquid nitrogen surface; and • dispersal of drops through jet atomizer to moving liquid nitrogen layer. In the case of static nitrogen surface, freezing was carried out in a specially designed pan filled with liquid nitrogen. Freezing of individual drops means that the next drop is delivered to the refrigerant surface only after the preceding one sinks. Single drops were created by a 15-ml Pfizer dropping bottle (drop diameter 2.8–3 mm) and dispersed by a QFJ-30 jet atomizer with drop dispersity of 0.8–1.2 mm. The experiments were performed in solutions of alkali-metal nitrates. One of the basic quality characteristics of the obtained cryogranules is their size distribution. The cryogranules were analyzed on a Haver & Boecker vibrating sieve analyzer, after which they were dried in a vacuum-sublimation unit. The aver110

Fig. 3. Exterior view of cryogranules obtained under various freezing conditions: a) delivery single drops to static liquid nitrogen surface; b) delivery single drops to moving liquid nitrogen layer; c) dispersion through jet atomizer on static liquid nitrogen surface; d) dispersion through jet atomizer in moving liquid nitrogen layer.

Fig. 4. Cryogranule size distribution under various freezing conditions: green curve – delivery of single drops to moving liquid nitrogen layer; black curve – dispersion through jet atomizer on static liquid nitrogen surface; red curve – delivery of single drops to moving liquid nitrogen layer; blue curve – dispersion through jet atomizer in moving liquid nitrogen layer.

age size and dispersity of the particles of the dried powders were determined on a DAS 2702 diffusion aerosol spectrometer and the specific surface area was determined on a Quantachrome NOVA 2200e apparatus. Cryogranules having varied size compositions were obtained in the experiments. The exterior view of the cryogranules is shown in Fig. 3. In the case of freezing of single drops on calm (static) liquid nitrogen surface, the freezing time is 18–20 sec. The obtained cryogranules have an average size of 3 mm and a regular shape close to spherical (Fig. 3a). Freezing of single cryogranules by delivering them to a moving liquid nitrogen layer reduces the freezing time to 3–5 sec. The cryogranules so obtained have an average size of 2.5 mm and a regular shape (Fig. 3b). The reason for the shorter freezing time compared to the preceding case is that under conditions of cryoliquid motion the vapor layer moves away from the drop surface. Because of this the heat exchange process accelerates. In the case of freezing of solution by dispersing through jet atomizer on static liquid nitrogen surface (Fig. 3c), single drops agglomerate in the atomizer mouth area due to overlapping of drop drift areas, forming an incrustation of cryogranules on the refrigerant surface. The agglomerates are 3–10 mm in size and regular in shape. 111

Fig. 5. Histograms of particle distribution by size in end product under various freezing conditions: a) delivery of single drops to static liquid nitrogen surface (average particle size 138.6 nm); b) delivery of single drops to moving liquid nitrogen layer (average particle size 56.5 nm); c) dispersion through jet atomizer on static liquid nitrogen surface (average particle size 102.4 nm); d) dispersion through jet atomizer in moving liquid nitrogen layer (average particle size 42.8 nm).

In the case of freezing of drops by dispersion through the jet atomizer in moving liquid nitrogen layer (Fig. 3d), the average cryogranule size is 1–3 mm, thanks to which the drops getting into the moving refrigerant layer begin to move along with the layer and leave the atomization area. The results of cryogranule size analysis are plotted in Fig. 4. Freezing of single drops in moving liquid nitrogen layer made it possible to reduce the average particle size by 0.7 mm relative to cryogranules obtained by freezing in static liquid nitrogen layer. In this case, the granule size distribution is narrower. The content of the primary fraction in drop freezing in moving layer reaches 54%, which is 8% higher than for cryogranules obtained by freezing in static layer. In the case of freezing of dispersed solution in moving layer, the average cryogranule size was found to be 2.6 mm smaller compared to the drops frozen on static nitrogen surface under the same conditions and there was a 31% increase in the primary fraction content. Freezing of solution drops in static nitrogen layer in single drop mode does not affect the cryogranule size, but this mode is unproductive and is therefore hardly used. Experiment demonstrated that cryogranules comparable in size with initial drops can be produced by freezing solutions under condition of dispersion in moving liquid nitrogen layer. In this case, the dispersion mode is more productive than the drop mode. The advantages of the method of freezing by dispersion in moving liquid nitrogen layer are also supported by the results of investigation of powders obtained by cryochemical technology. Histograms of particle distribution by size and average particle size of the powders obtained under various freezing conditions are shown in Fig. 5. 112

TABLE 1 Specific surface, m2/g Solution freezing conditions Single drop mode

Dispersion mode

In static nitrogen layer

1.450

0.330

In moving nitrogen layer

2.850

3.700

Initial KNO3 powder* *

0.056

Initial KNO3 powder was not produced by cryochemical technology and is listed for comparison.

Freezing of solution in moving and static cryoliquid layers using QFJ-30 jet atomizer reduces average particle size by 24% compared to freezing of single drops. The distribution pattern does not change. The granule size characteristic becomes narrower and tends to be monodisperse if the method of freezing in moving layer is used. Thanks to this method, the content of the primary fraction increases on the average by 5%. The investigations of the influence of various freezing conditions on the specific surface of the obtained products were conducted in eutectic KNO3 solution concentration. The investigation results are furnished in Table 1. Much better specific powder surface values are obtained by dispersion of solution in moving cryoliquid layer than by all other methods. The obtained investigation results showed the advantage of the method of aqueous solution freezing by dispersion of the solution in moving cryoliquid layer. Compared to the popular method of freezing in static layer, this method helps reduce drop freezing time and increase specific surface area of powders produced by cryochemical technology.

REFERENCES 1. 2. 3.

M. B. Generalov, Cryochemical Nanotechnology [in Russian], IKTs Akademkniga, Moscow (2006). Russian Federation Patent No. 2422196, Method of Cryogenic Granulation of Solutions and Suspensions, dated 05.17.2010. Russian Federation Patent No. 2421272, Device for Cryogenic Granulation of Solutions and Suspensions, dated 05.17.2010.

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