Chinese Journal of Chemical Engineering, 16(4) 552—557 (2008)

Ice Slurry Formation in a Cocurrent Liquid-Liquid Flow*

PENG Zhengbiao (彭正标), YUAN Zhulin (袁竹林)**, LIANG Kunfeng (梁坤峰) and CAI Jie (蔡杰) School of Energy and Environment, Southeast University, Nanjing 210096, China

Abstract A new technique for ice slurry production was explored. Multiple small water-drops were formed in an-other immiscible chilled liquid by a single-nozzled atomizer and frozen in the fluidized bed by direct contact heat transfer. Experiments were conducted to investigate the dynamic behaviors of the ice crystal making system. The results demonstrate that the ice crystals could be produced continuously and stably in the vertical bed with the cir-culating coolant of initial temperature below －5°C. The size distribution of the ice crystals appears non-uniform, but is more similar and more uniform at lower oil flow rate. The mean ice crystal size rests seriously with the jet veloc-ity and the oil flow rate. It decreases with decreasing the oil flow rate, and reaches the maximum at an intermediate jet velocity at about 16.5 m·s－1. The ice crystal size is also closely related to the phenomenon of drop-coalescing, which can be alleviated considerably by reducing the flow rate or lowering the temperature of the carrier oil. How-ever, optimization of liquid-liquid atomization is a more effective approach to produce fine ice crystals of desired size. Keywords ice slurry, drop-coalescing, ice crystal size distribution, liquid-liquid atomization

1 INTRODUCTION

Environmental crises such as ozone depletion and global warming limit the use of traditional refrig-erants and lead to a revival of the natural ones due to their zero ozone depletion potential and low total equivalent warming impact [1, 2]. However, most of these ‘old’ refrigerants are known to be flammable and toxic, for example, ammonia, and hence the use of them requires enclosed systems or a secondary refrig-erant fluid for the cold distribution [3].

An additional advantage of a secondary cycle is the possibility to store cold energy [4], which becomes increasingly important, especially in extremely cold/hot areas. There, electrical energy consumption varies greatly during the day and the night, which leads to an electricity peak-load period and an off-peak period between midnight and early morning. If the cold en-ergy is provided and stored at night and then released in the day, part or all peak-load can be shifted to the off-peak period. Thus, both effective energy manage-ment and economic benefit are achieved [5].

It was just the development of the second-loop systems, which are mainly responsible for the advantages of the ice slurry technology, to which a marked attention has recently been paid. Ice slurry is a water-based liquid in which fine ice crystals of no more than 1 mm in size are presented and the liquid water can be either pure or mixed with antifreeze additives. Nowadays, ice slurry has been accepted as a promising working fluid due to its technical merits [6, 7]. First of all, for its excellent cold heat capacities, the high heat transfer coefficient of ice slurry flow allows great reduction in heat exchanger size and total operating costs. Refer-ring to its good flow ability, it can be pumped directly through distribution pipe works and heat exchangers. Moreover, it can quickly respond to the thermal load demand with its huge transient fusion energy. Last but not least, as an ideal secondary refrigerant for indirect refrigeration systems, it appears absolutely safe, envi-

ronment-friendly and highly efficient. So far, the ice slurry technology has been widely applied in several fields, just as by Wang and Kumusoto [8] exemplified in the application to multifunctional buildings.

Several techniques for ice slurry production are available. Currently, the most commonly applied and technologically developed technique is the mechanical- scraper type, by which ice slurries with ice fraction of up to 60% can be obtained. Researches on this type is mainly concentrated on the mechanisms governing ice crystallization [9-11], heat transfer coefficients [12, 13], industrial applications [8, 14], etc. The direct con-tact heat exchanger ice slurry generator was built and investigated by Chuard and Fortuin [15], in which a cool fluid was directly injected into the water tank and ice crystals formed due to the cold heat issued from the coolant evaporation. Fluidized bed with fluidized particles for ice slurry generation was suggested ini-tially by Klaren and Meer [16]. As yet, only laboratory set-ups have been developed. In recent years it was studied experimentally and theoretically by Meewisse [17] and Pronk [3]. Ice slurry generators using su-per-cooled water have been investigated for many years and already applied successfully in practical projects [18-20]. The crucial issue is to get greater su-per-cooled degree without water-freezing. Generators of spraying water into a vacuum chamber were pro-posed originally by James and Kam [21]. Recently, theoretical and experimental study was conducted to evaluate the effectiveness of the technique by a diffu-sion-controlled evaporation model [22, 23].

Each of the above types for ice slurry production has its particular merits and disadvantages. However, those which are applicable in practice and guarantee continuous operation without problems are still too expensive [7]. Thus, it is meaningful to develop an ice slurry generator working in an efficient, reliable and economical way. The main objective of this study is to design and evaluate a new technique by using a circu-lating fluidized bed in liquid-liquid system for ice slurry

Received 2007-09-24, accepted 2008-01-17.

* Supported by the Specialized Research Fund for the Doctoral Program of Higher Education of China (20060286034). ** To whom correspondence should be addressed. E-mail: zlyuan@seu.edu.cn

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production. Experiments under various conditions were carried out and the final results validated favora-bly the feasibility and effectiveness of the technique.

2 EXPERIMENTAL

2.1 Experimental set-up

A laboratory-scale ice slurry generator was con-structed to study the new technique. Fig. 1 shows a schematic diagram of the experimental set-up.

Figure 1 Schematic diagram of the experimental facility 1— fluidized bed; 2—atomizer; 3— four-outlet valve; 4—high-pressure air generator; 5—water tank; 6—ice slurry stor-age tank; 7—stirrer; 8—electric fan; 9—condenser; 10—throttle valve; 11—evaporator/oil tank; 12—compressor; 13—filter; 14— ice-crystals/oil separator; 15— floodlight; 16—digital camera & digital video

temperature sensor; flow meter; pressure sensor; control valve; pump

The oil is cooled to the desired value below 0°C in the refrigeration system, and then is pumped into the vertical fluidized bed (a glass cylinder of nominal inside diameter 40 mm and height 2 m). When the oil flows in steady status, the normal water is pumped from the tank into the bottom of the bed. Through a specially designed atomizer which is immersed in the oil, a jet forms and then breaks up into multiple small drops. These drops flow upstream together with the ambient chilled oil, and heat is transferred between phases by direct contact. Consequently, the drops freeze quickly into ice crystals. A filter with pores of no more than 100 μm is used to filter out the ice crys-tals from the oil after exiting the bed. Mix the crystals with water, either pure or mixed with antifreeze addi-tives, in a predetermined ratio to gain ice slurries of desired ice packing factor (IPF). An oil/water separa-tor is employed for the remnant water exclusion from the oil, which, with higher temperature due to heat transfer to the drops, is re-cooled in the refrigeration cycle and then circulated into the bed again.

Innovations and merits of the new technique be-come clear and are described as follows. First, the small drops formed by the atomizer greatly increase the heat transfer contact area due to their small di-ameter. Secondly, the direct contact to exchange en-ergy between phases resolves the inherent disadvan-tage of ice-scaling in traditional ice generators and enhances the heat transfer coefficient strikingly. Sim-

ple tests show that a drop of initial temperature at 2°C and diameter 1 mm will be frozen in less than 2 s dur-ing falling in the cold oil pre-cooled to － 8°C. Moreover, ice slurry of arbitrary IPF can be achieved by the new generator. After separation with the carrier oil, the pure ice crystals can be utilized directly to mix with water or aqueous solutions in any desired ratio. Finally, the simplicity of the apparatus is also techni-cally attractive. No complex or expensive equipment are required, so the initial investment is affordable, as well as the scale-up and improvement of the currently operating apparatus in the future.

The structure and location of the spray nozzle is an important design consideration. In the study, a single spray nozzle of inside diameter (ID) 0.12 mm was utilized. It is fixed at the tip of a copper jet-pipe of nominal ID 5 mm and height 350 mm, which is located vertically at the bottom of the fluidized bed, with spray directed upwards. The carrier oil enters the fluidized bed horizontally, about 300 mm below the nozzle head. When the two phases flow cocurrently, the radial mo-tion of the continuum is rather weak, and the collision between the dispersed particles is less frequent. Drop-coalescing is thus alleviated favorably. All these guarantee adequate energy exchange between phases and fine ice crystals froze thoroughly at the outlet.

However, ice agglomeration would be encoun-tered frequently in experiments for all the oil tem-perature was below 0°C. Two auxiliary devices have been taken to avoid ice clogging in the jet-pipe. With a four-outlet valve inserted into the jet-piping, high-pressure air was utilized to sweep through the pipe continuously once jetting was ceased. The other measure was to wrap plastic insulation around the jet-pipe to keep it from being frozen.

A high-resolution digital CCD (charge couple device) camera by NIKON (Japan) was employed to monitor in real-time the drops formation and the par-ticle flow patterns in the fluidized bed. Five K-type thermocouples (0.2 mm OD and accuracy ±0.1°C) were placed at five different heights along the bed to measure the temperature of the slurry mixture.

Besides, the thermal capacities and fluidity of the carrier fluid at low temperature are quite critical to the new ice slurry generator. Finally, the No. 25 trans-former oil was selected among several solutions. Usu-ally, it appears transparent without any suspended matter, and its relevant thermo-physical properties are listed in Table 1.

2.2 Experimental procedure

The experiments are performed with the follow-ing procedure. Initially, run the refrigeration system to cool the oil to the desired temperature below 0°C. The chilled oil was circulated by using a centrifugal pump, and the flow rate was gauged by a gear wheel meter (LC-40). Once the oil flow stabilized, the normal water was pumped (a booster pump, 12WZ-8, the maximum flow rate: 20 L·min－1) through a glass rotameter into the bed. Photographs were taken by the CCD camera set to 1/2000 s, f4.8, spot, 100 frames per second. Opera-tion and physical parameters are also listed in Table 1.

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Table 1 Physical and operated parameters in experiments

Parameter Value po/MPa 6 ρc/kg·m－3 895 μc/kg·(m·s)－1 0.179 (25°C), 2.685 (－10°C)kc/W·(m·K)－1 0.124 cc/J·(kg·K)－1 1892

Tc,con/°C ＜－45°C σs/N·m－1 0.062 ρd/kg·m－3 998.12

dn/mm 0.12 Gc,in/L·s－1 0.25, 0.35, 0.42

Tc,in/°C －5, －7, －10 vjet/m·s－1 7.1, 11.8, 16.5, 23.6

3 RESULTS AND DISCUSSION

Experiments under various conditions (Tc,in≤－5°C) were carried out to produce ice crystals continuously and stably. Compared to static ice-making systems, in which the evaporation temperature should be as low as －10°C and even lower with the increase of ice con-centration [24], this new dynamic one shows superior heat integration efficiency. Fig. 2 displays the ice crystals produced in one of experimental conditions.

Figure 2 Ice crystals produced in experiments (Tc,in＝－5°C, po＝6 MPa, dn＝0.12 mm)

3.1 Size distribution of the ice crystals

The size distribution of the ice crystals is a criti-cal factor that intensely influences the rheological properties of ice slurry [25]. In order to investigate the size distribution, ice crystals produced under each condition were measured manually (100 sampling particles for each test), and the results were gathered statistically by dividing the range into 9 sub-parts: (0, 0.99), (1, 1.49), (1.5, 1.99), (2, 2.49), (2.5, 2.99), (3, 3.49), (3.5, 3.99), (4, 4.49), (4.5, 4.99) in unit of mm. Then, the Gaussian distribution was applied to analyze the ice particle size:

2

21 ( )( ) exp

22x nG xσσ

⎡ ⎤−= −⎢ ⎥

⎣ ⎦π (1)

where the standard deviation of Gaussian function is denoted by σ.

Figure 3 illustrates the size distribution of the ice crystals produced under conditions of Tc,in＝－7°C. It is observed that the ice crystal size differs even under

the same condition, due to the following two reasons. First, the previous results obtained from both experi-ments and numerical simulations of liquid-liquid at-omization show that the drops formation is a dynamic process of much uncertainty [26, 27]. The drop size formed after jet disruption differs greatly under the complicated effects of viscous force, buoyancy, inertia force, surface tension, jet contraction, velocity profile relaxation and ambient fluid disturbance. Moreover, the drop size would be varied again by the subsequent phenomenon of drop-coalescing, which is induced by drop collisions and determined by several factors, just as to be shown in Section 3.3. However, the ice crystal size distributes more uniformly with increasing the jet velocity at the maximum oil flow rate Gc,in＝0.42 L·s－1. Then, as the oil flow rate decreases, the discrepancy in the size distribution for different jet velocities diminishes, and the ice crystal size uniformly concentrates in the range of 1-2 mm at Gc,in＝0.25 L·s－1 in Fig. 3 (c).

(a) Gc,in＝0.42 L·s－1

(b) Gc,in＝0.33 L·s－1

(c) Gc,in＝0.25 L·s－1

Figure 3 Size distribution of the ice crystals for different jet velocities and oil flow rates (Tc,in＝－7°C, po＝6 MPa, dn＝0.12 mm) vjet/m·s-1: ■ 23.6; ● 16.5; ▲ 11.8; ▼ 7.1

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3.2 Mean ice crystal size

Figure 4 shows the effects of the jet velocity on the mean ice crystal size at different oil flow rates. The similar variation tendency was observed at dif-ferent oil flow rates: the mean ice crystal size in-creases first and decreases afterwards as the jet veloc-ity increases. When the jet velocity is relatively low, insufficient residence time allows drop-coalescing being enhanced. However, at high jet velocity, turbu-lent dominance, generation of smaller drops, and weakened drop-coalescence all favor smaller ice crys-tals. The above two aspects achieve an equilibrium at vjet＝16.5 m·s－1, and accordingly the ice crystal size reaches its maximum. Moreover, as consistent with the indication in Figs. 3(a)-3(c), the mean ice crystal size decreases as the oil flow rate decreases. On the one hand, the increasing relative velocity between the two phases give rise to smaller drops [26]. The drag force exerting on the drops falls at lower oil flow rate, and likewise the sufficient drop residence time in the bed will weaken drop coalescence. Consequently, the mean size of the ice crystals produced at the condition is smaller.

Figure 4 Effects of the jet velocity on the mean ice crystal size at different oil flow rates (Tc,in＝－7°C, po＝6 MPa, dn＝0.12 mm) Gc,in/L·s-1: ■ 0.25; ● 0.33; ▲ 0.42

3.3 Phenomenon of drop-coalescing

From above analyses, it is observed that the phenomenon of drop-coalescing is a critical factor that influences the ice crystal size.

Several researchers have reported on the phe-nomena of drops coalescence and breakup. A few years ago, Kitron et al. [28] pointed out that the coalescence and breakup of drops influenced the size distribution intensely. Eggers and Lister [29] clarified the mecha-

nism of coalescence via investigating the coalescing process of two drops with the same diameter on a solid surface. Marion et al. [30] experimentally exam-ined the influence of turbulent intensities on drops coa-lescence. In recent years, David and Rutland [31] de-veloped a new algorithm to search the colliding drops and an effective model to simulate drop-coalescing. In this study, multiple liquid drops flow up simultane-ously in the fluidized bed. Collisions between drops, the drop and the particle (either pure ice or a mixture of liquid-drop and ice), the drop and the wall, would induce coalescence directly.

Figure 5 shows the tracking of drop-coalescing at the bottom of the fluidized bed. It can be seen that the process of drop-coalescing ends in no more than 0.03 s. Fig. 6 illustrates the instantaneous snapshots taken at the height of 0 m, 0.55 m, 1.25 m at different oil flow rates (0.25 L·s－1, 0.33 L·s－1, 0.42 L·s－1), where the oil-temperature and the jet velocity keeps constant at －7°C and 7.1 m·s－1, respectively. It is clear that the statistical volume of particles increases along the bed. At the bottom (hb＝0 m), most drops remain to be liquid, and collision between drops easily leads to coalescence. At hb＝0.55 m, larger particles emerge as compared with that at hb＝0 m because of coalescence. At hb＝1.25 m, particles appear much larger. It is indi-cated that some large drops not frozen thoroughly would still coalesce due to the cumulated collisions with others during moving through the domain of 0.55-1.25 m.

In order to quantitatively examine the effects of drop-coalescing, experimental snapshots were nu-merically processed to gain the mean diameter of par-ticles along the bed height. Fig. 7 shows the effects of the flow rate and temperature of the oil on drop-coalescing. It is clear that for the same oil tem-perature, the larger oil flow rate always results in the higher frequency of drop-coalescing and larger mean particle diameter. The result validates the analyses on Fig. 4. It should also be noted that for the same oil flow rate, the lower the oil temperature, the smaller the mean particle diameter. This is expected because most drops can complete phase change in a shorter time due to the larger temperature difference between phases. Likewise, the possibility for ice crystals to coalesce is lowered accordingly.

3.4 Discussion

Above results amount to that drop-coalescing can

0.01 s 0.02 s 0.03 s 0.07 s 0.08 s 0.09 s 0.16 s 0.17 s 0.18 s

Figure 5 Tracking of drop-coalescing at the bottom of the fluidized bed (Tc,in＝－7°C, vjet＝7.1 m·s－1, po＝6 MPa, Gc,in＝0.42 L·s－1, dn＝0.12 mm)

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be alleviated considerably via reducing the flow rate or lowering the temperature of the carrier oil. How-ever, reducing the oil flow rate implies that the drops will move more slowly under the smaller carrying force. For the time being, provided that the tempera-ture difference between phases is not large enough to freeze drops in a short time, the drops will collide and coalesce frequently. As a result, large particles emerge and clog the ice generation circuit. So the lower am-bient temperature in the fluidized bed is in pressing need. Nevertheless, the low oil temperature depends on a much lower evaporation temperature, which re-duces the coefficient of performance (COP) of the refrigerator directly. Therefore, the implementation of

above means to decrease the ice crystal size is limited by the continuity and cost related factors of the system operation.

However, according to the theoretical and ex-perimental study by Liang et al. [26] and Richards et al. [32] regarding drop formation in liquid-liquid sys-tems, the jet becomes turbulent and eventually dis-rupts intensely into a large number of smaller drops when the relative Reynolds number is high. These smaller drops need less cold energy and are much easier to freeze. Therefore, coalescence of the drops is restrained during their passage through the fluidized bed. In addition, the permitted high temperature of the carrier oil allows the raising COP of the refrigeration system. Thus, it can be concluded reasonably that ac-quisition of small water-drops in liquid-liquid atomi-zation is a more effective way to produce fine ice crystals of desired size. Two aspects should be im-proved to form small water-drops. One is to utilize a spray nozzle of smaller nominal ID, the other is to raise the driven pressure to increase the jet velocity and achieve the turbulent jetting pattern of multiple drops formation, which is preferred to the new ice slurry generator.

4 CONCLUSIONS

Systematic experiments have been conducted to investigate the new technique for ice slurry formation. The following preliminary conclusions were drawn

Gc,in＝0.42 L·s－1 Gc,in＝0.33 L·s－1 Gc,in＝0.25 L·s－1

Figure 6 Snapshots of the flow regimes of particles at different heights (Tc,in＝－7°C, vjet＝7.1 m·s－1, po＝6 MPa, dn＝0.12mm)

Figure 7 Variation of the mean particle size along the bed at different conditions (po＝6 MPa, vjet＝7.1 m·s－1, dn＝0.12mm) Tc,in/°C, Gc,in/L·s－1: ■ －5, 0.25; □ －5, 0.42; ● －7, 0.25; ○ －7, 0.42; ▲ －10, 0.25; △ －10, 0.42

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within the parameter range covered in the present study: (1) The new system allows stable ice crystal for-

mation in the vertical fluidized bed with the circulat-ing coolant of initial temperature below －5°C.

(2) The size distribution of the ice crystals ap-pears non-uniform but becomes more similar and more uniform for different jet velocities as the oil flow rate decreases. At the minimum oil flow rate 0.25 L·s－1, the ice crystal size uniformly congregates in the range of 1-2 mm.

(3) Ice crystal size rests seriously with the jet ve-locity and the oil flow rate. It decreases with decreas-ing the oil flow rate. It reaches the maximum at an intermediate jet velocity at about 16.5 m·s－1.

(4) Ice crystal size is also closely related to the phenomenon of drop-coalescing, which can be allevi-ated considerably via reducing the flow rate or lower-ing the temperature of the carrier oil. However, the implementation of the means is limited by the conti-nuity and costs of the system operation.

(5) Optimization of liquid-liquid atomization to gain small water-drops is a more effective approach to produce fine ice crystals of desired size.

Many other important issues need to be investi-gated, such as improvement of the quality of drops formation, understanding multiphase flow behaviors and enhancing energy exchange in the fluidized bed, control of the phenomenon of drop-coalescing, and uncovering the economical features of the whole sys-tem. Further work is required to the development of the new technique and its practical application.

MOMENCLATURE

cc heat capacity of the carrier fluid, J·(kg·K)－1 dac mean diameter of the ice crystals, mm dap mean diameter of particles, mm dc diameter of the ice crystal, mm dn diameter of the spray nozzle, mm dp diameter of particles (either drop or a mixture of drop and ice), mm Gc,in flow rate of the carried fluid when entering the fluidized bed, L·s－1 hb height of the fluidized bed, m kc thermal conductivity of the carrier fluid, W·(m·K)－1 po pressure of the ambient liquid outside the spray nozzle, MPa Rer relative Reynolds number between the two phases Tc,con condensation point of the carried fluid, °C Tc,in carried fluid temperature when entering the fluidized bed, °C vjet velocity of jetting, m·s－1 μc dynamic viscosity of the carried fluid, kg·(m·s)－1 ρc density of the carried fluid, kg·m－3 ρd density of the dispersed phase, kg·m－3 φ amplification ratio through the liquid medium σ mean error of Gaussian function σs surface tension coefficient, N·m－1

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