Freeze Concentration of Aqueous Solutions - ACS Symposium Series

Chapter 27

Freeze Concentration of Aqueous Solutions Tasoula C. Kyprianidou-Leodidou and Gregory D. Botsaris

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Chemical Engineering Department, Tufts University, Medford, MA 02155

The development of the freeze concentration process for fruit juices has been hampered by the fact that solute concentrate is entrained by the ice crystals. This incomplete separation of the entrained concentrate from the ice results in a considerable increase of the cost of the process. In this investigation sucrose solutions were concentrated by the formation of an ice layer on the externally cooled walls of the crystallizer. The formation of the layer was initiated by secondary nuclei induced by rotating ice seeds, at subcoolings smaller than the critical subcooling needed for spontaneous nucleation. A minimum in the amount of sucrose entrapped in the ice layer was observed at a subcooling smaller than the critical subcooling for spontaneous nucleation. The effect of soluble pectins on the minimum was also studied.

Freeze concentration involves the concentration of an aqueous solution by partial freezing and subsequent separation of theresultingice crystals. It is considered to be one of the most advantageous concentration processes because of the many positive characteristicsrelatedwith its application. Concentration processes such as evaporation or distillation usually result in removal of volatiles responsible for aroma; in addition the heat addition in these processes causes a breakdown in the chemical structure that affects flavor characteristics and nutritive properties. In contrast freeze concentration is capable of concentrating various comestible liquids without appreciable change in flavor, aroma, color or nutritive value (JL2*2) The concentrate contains almost all the original amounts of solutes present in the liquid food. The application of thefreezeconcentration process has been successful in various cases such as the preconcentration of wine (4). The application of the process for the concentration of fruit juices, which are mainly consumed for theirflavorand taste, seems to be ideal because of the improved volatileretention.Nevertheless, the process is not as widely accepted because of its increased economic requirements. These are related to the difficulty of effectively separating the resulting solid phase from the liquid phase (mother liquor). The formation of a large number of very fine 0097-^156/90A)438-0364$06.00A) © 1990 American Chemical Society

Myerson and Toyokura; Crystallization as a Separations Process ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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crystals makes their complete separation from the concentrate almost impossible and results in losses of valuable material in the discarded ice. This increases considerably the operating cost of the process. A control of the ice production stage is required in order to minimize the total loss of entrapped material per unit weight of ice. The freeze concentration process has been mainly studied in suspension crystallizers (5.6) as a crystallization process where secondary nucleation is the prevailing mechanism of ice formation. Since primary and heterogeneous nucleation are of little importance in this process, investigators have focused theirresearchon understanding the mechanisms of the secondary nucleation process and the factors that affect it. It is believed that the processes which contribute to the formation of secondary nuclei are mainly collisions of existing crystals with solid surfaces such as the crystallizer walls, other crystals and the stirrer blades (7.8). In systems which involve relatively high flow velocities of the solution over the crystal surfaces, secondary nuclei are produced because of fluid shear (2JQ)- Dendrites or other entities are detached from the surfaces of the existing crystals. Some of these entities are larger than the critical size and grow to form new particles. Supersaturation, stirring rate and additives are some of the factors that influence the kinetics of secondary nucleation (5 6.) In some applications of thefreezeconcentration process, ice formation is taking place on the cooling surfaces of scraped surface heat exchangers (1). Scraping paddles mounted on the rotating shaft subsequently scrape the solid ice layer and push it into the liquid where crystal growth is taking place. A combination of an external heat exchanger and a stirred tank is adopted in some industrial applications such as the Grenco process (1). In this process the goal of minimizing the loss of concentrate in the ice is promoted by feedingfinecrystalsfromthe scraped surface heat exchanger to arecrystallizerthat contains a suspension of larger crystals. The difference in the melting temperatures of the various sizes of crystals results in growth of the larger crystals and dissolution of the smaller ones (11). Thus Ostwald ripening is taking place (12). Our study involved the combination of an ice layer formation on the cooled walls of a crystallizer and of secondary nucleation. The latter was initiated by rotating ice seeds. The crystallizing liquids were aqueous solutions of sucrose and of pectins and sucrose. Fruit juices are composed of various organic species the most important of which are sugars in the form of sucrose, glucose andfructose.It is clear that the study offruitjuices is to a large extent a study of water and sugar solutions. The nucleation and growth processes are greatly affected by the presence of additives that affect the surface properties of the crystals. Pectic compounds are present in mostfruitjuices and therefore the effect of pectins on ice crystallization is of particular importance and worth to study. Previous studies on ice nucleation in the presence of macromolecular compounds such as polymers, dextran and pectins (5.13.14) showed that there is a considerable effect of these solution additives on the ice nucleation rate. It was observed that probably due to an increase in the viscosity of the solution, the nucleation rate decreases with an increase in the macromolecule concentration. In our study, the amount of liquid entrapped was obtained by measuring the sucrose in the ice layer. The conditions that minimize the liquid entrapment were then identified. EXPERIMENTAL STUDY

For this study (15) a batch crystallization system was employed. The experimental set-up is shown infigure1. The basic parts of the apparatus were the following;


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CRYSTALLIZATION AS A SEPARATIONS PROCESS

1. The Crystallizer Cell Where All The Experiments were Performed. This was a 1 liter pyrex vessel covered by a 1 cm thick plexiglass cover. Holes in the plexiglass cover permitted the introduction of a Beckman thermometer and a specially constructed stirrer capable of accommodating two ice seed crystals on its blades. These seed crystals were attached to the tips of the stirrer blades by a slow freezing process of water in contact with the tips of the stirrer. 2. The Constant Temperature Bath. This was a glass container of a 30 cm diameter and 20 cm high, covered by a 1 cm thick plexiglass cover. The bath was filled with a 50% solution of ethylene glycol in water and it was cooled by continuously circulating ethylene glycol from a refrigerator unit (VWR model 1140) through a 1/2" I.D. copper tubing. The temperature of the bath was controlled by a temperature controller that regulated the power of a 150 Watt, heating element submerged in the cooled water-glycol mixture. The external surfaces of the bath were covered by a 2" thickfiberglass insulation. In this way the temperature of the bath was stable within a 0.005 K interval. A Beckman thermometer was used to measure the relative temperature differences between the crystallizing cell and the constant temperature bath. The whole set of experiments was conducted using the following solutions: 1. Pure water. 2. 5% sucrose solution. 3. 10% sucrose solution. 4. 5% sucrose solution with 1 g/1 pectins. After the bath attained its equilibrium temperature, the crystallizer was charged with about 400 ml of liquid and was inserted into the bath. After about 30 minutes the system attained a constant temperature and a subcooling (difference of equilibrium temperature and constant temperature before initiation of crystallization) was established. Introduction of die seed crystals (after being allowed to warm for a period of a few seconds) on the specially prepared stirrer initiated crystallization (secondary nucleation) and resulted in a change in the temperature of the crystallizer (figure 2). The temperature of the crystallizer attained an equilibrium value of a few minutes after nucleation occurred. The concentration of the sucrose solutions was measured using a refractometer (.1% accuracy). RESULTS AND DISCUSSION OF RESULTS During the experimental procedure the following observations concerning the behavior of the system were made: When pure water was used we observed that primary nucleation occurred at subcoolings higher than 1°K. For smaller subcooling values no primary nuclei were formed, and the system remained in a metastable state. Introduction of the seed crystals caused the formation of a large number of secondary nuclei and a change in the temperature of the crystallizer was observed that followed a sigmoidal curve (figure 2) and is characteristic of a system moving from a metastable to a stable equilibrium state. Because of this behavior of the system, a temperature difference (driving force for heat transfer) is established between the bulk of the outside constant temperature bath (and as a result of the walls of the crystallizer) and the bulk of the crystallizer (this will be expressed as ATfromnow on). The relatively


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KYPRIANIDOU-LEODIDOU & BOTSARIS

Frew Concentration

Figure 1. Experimental set-up.


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high stirring rates of our experiments caused a uniform temperature distribution in the bulk of the crystallizer. An ice layer is formed at the cooled walls of the crystallizer as a result of the established temperature difference. AT is the driving force for the process that is heat transfer controlled. This is different than the temperature difference T^-TQ (where T is the temperature at the ice layer-solution interface and Tg is the equilibrium temperature that is the melting point of ice) which is the driving force for the surface integration process (this is not the controlling step). The ice layer formed is either a large transparent ice monocrystal, for the case of pure water crystallization, or a non-transparent, polycrystalline ice layer where some sucrose is entrapped, for the case of the crystallization of sucrose solutions. Pictures of these two types of layers are shown infigure3. If we examine the variation of the sucrose entrapment in the ice layer (for ice layer thickness equal to about 1 cm) the following observations are made: 1. The amount of sugar entrapped in die ice layer varies with the overall temperature difference between the bulk of the crystallizer and the surrounding cooling medium (figures 4,5). 2. A minimum is observed at a subcooling value smaller than the subcooling required for spontaneous nucleation. For instance in figure 4 spontaneous nucleation in the absence of the seeds occurred at AT>1.7°K. The minimum at AT = 1.2°K is observed in the region in which only secondary nucleation takes place. This result points out that in processes in which the ice is formed by spontaneous nucleation, as in most of the industrial processes involving scraped icefromcooled surfaces, the entrapment will be always higher than in processes that involve secondary nucleation. A possible explanation for the variation of the sucrose entrapment in the ice layer as a function of the temperature difference between crystallizer and cooling bath and the existence of the minimum is the following. For high subcoolings the nucleation rate is high. The ice layer consists of a large number of crystals which grow fast and as aresultthere is a higher entrapment of the liquid which contains not only the initially dissolved sucrose but also that rejectedfromthe growing ice crystals. For low temperature differences the heatremovalrate (driving force for the formation of the ice layer) is not adequate and the ice layer does not start forming from the beginning but after sometimeand after a large amount of secondary crystals has been formed. These secondary crystals, thrown against the wall,findthemselves in a region of lower temperature and start growing to form a "layer" of secondary crystals. At low AT the growth is rather slow and the layer grows mostly by capturing secondary nuclei, a condition which favors entrapment. As AT increases the secondary nuclei grow faster and a layer is formed consisting of larger crystals. At larger AT the number of nuclei is higher. This combined with faster growth leads to a more porous layer and increased entrapment Thus at intermediate AT we have a minimum. The effect of soluble pectins was also examined. Addition of pectins in aqueous solutions affects nucleation and growth rates. A comparative study of the system in the presence and without the presence of pectins (figures 6,7) will give some insight to whether it would be desirable to depectinizefruitjuices of high pectin content or to use pectins as additives for juices that contain little or no pectins. Infigure7, where the results for a 5% sucrose solution with and without pectins are compared, a relative displacement of the curve to the higher temperature difference region is observed when pectin addition is taking place. Minimum losses of sucrose occur at much higher heatremovalrates (higher temperature differences between the bulk of the crystallizer and the walls) and the concentration of sucrose entrapped at the point of minimum loss, has a slightly higher value. This may not be significant since the concentration difference is within the experimental accuracy. These results can be s


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Figure 3. Photographs of ice layers. Top, pure water, bottom, 5% sucrose solution.


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4.00

spontaneous nucleation

1.00 1.00

1.38

1.75 AT

2.13

2.50

CKD

Figure 4. Concentration of sucrose in ice layer versus AT for a 5% sucrose solution.

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6.40

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g

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i

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Figure 5. Concentration of sucrose in ice layer versus AT for a 10% sucrose solution.


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4.00

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Figure 6. Concentration of sucrose in ice layer versus AT for a 5% sucrose solution where pectins have been added.

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1

1.38

No pectins added

•

Pectins added

1

1.75 AT

A

2.13

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Figure 7. Concentration of sucrose in ice layer versus AT for a 5% sucrose solution with and without pectins.


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explained as follows. Pectins are high molecular weight compounds. Their addition to a crystallizing system increases its viscosity and causes lower nucleation rates. In order to have nucleation rates comparable to those of the sucrose solutions that have no pectic additives, higher subcooling values are required. As a result, minimum entrapment is observed at higher subcooling values. These results show that it may be more efficient to depectinizefruitjuices before their concentration byfreezingbecause this would give minimum losses at lower heat removal rates and thus at conditions of more economical operation. The implication of these results for the design of a scraped surface crystallizer are currently being examined. Literature Cited 1. Sulc D. Confructa studien 1984,3,258. 2. Thijssen H.A. C.J. Food Technol. 1970, 5, 211. 3. Deshpande S.S.; Bolin H.R.; Salunke D.K. Food Technology 1982, May, 68. 4. Muller J. G. Food Technology 1967, 21, 49. 5. Omran A. M.; King C.J. AIChE J. 1974, 20, 795. 6. Stocking J.H.; King C.J. AIChE J. 1976, 22, 131. 7. Evans T.W.; Margolis G. and Sarofim A. F. AIChE J., 1974, 20, 950. 8. Strickland-Constable R.F. Chem. Eng. Progr. Symp. Ser., 1972,68,1. 9. Garabedian H.; Strickland-Constable R.F. J. Crystal Growth, 1974, 22, 188. 10. Estrin J.; Wang W.L. and Youngquist G.R. AIChE J., 1975, 21, 392. 11. Huige N.J.J.; Thijssen H.A.C. J. Crystal Growth, 1972, 13/14, 483. 12. Nyvlt j.; Sohnel O.; Matuchova M.; Broul M. The Kinetics of Industrial Crystallization; Chemical Engineering monographs 19, Elsevier, 1985, 310. 13. Shirai Y.; Nakanishi K.; Matsuno R. and Kamikubo T. AIChE J., 1985, 21, 676. 14. Shirai Y.; Nakanishi K.; Matsuno R. and Kamikubo T. J. FoodScience,1985, 50, 401. 15. Kyprianidou-Leodidou T. C. M.S. Thesis, Tufts University, 1987. RECEIVED June 5, 1990


Freeze Concentration of Aqueous Solutions - ACS Symposium Series

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