Eutectic Freeze Crystallization with an Aqueous KNO3−HNO3

4874

Ind. Eng. Chem. Res. 2003, 42, 4874-4880

Eutectic Freeze Crystallization with an Aqueous KNO3-HNO3 Solution in a 100-L Cooled-Disk Column Crystallizer Raymond Vaessen, Marcelo Seckler, and Geert Jan Witkamp* Laboratory for Process Equipment, Delft University of Technology, Leeghwaterstraat 44, 2628 CA Delft, The Netherlands

Eutectic freeze crystallization is a novel technique for processing waste and process streams of aqueous electrolyte solutions, separating them into pure water and pure solid salt. A dedicated apparatus, a 100-L cooled-disk column crystallizer, has been developed and tested, combining crystallization and solid/solid separation of ice and salt. Experiments have been performed with a ternary system of aqueous KNO3-HNO3. Heat-transfer rates were achieved in the range of 1300-4600 W‚m-2‚K-1, which are related to the square root of the scraping rate. The ice product slurry contained 0.15 wt % salt. The ice impurity content after three washing cycles was as low as 15 ppm K+. Well-faceted KNO3 crystals are produced with high purity. Introduction The chemical industry processes a wide variety of waste and process streams composed of aqueous electrolyte solutions of highly soluble salts. The feasibility of conventional separation techniques such as evaporative crystallization, cooling crystallization, and reverse osmosis is often determined from considerations on the energy consumption, the attainable yield, and the nature and purity of the derived salts and solutions. By processing of these aqueous electrolyte solutions with eutectic freeze crystallization (EFC), energy costs can be reduced by about 70% for certain systems in relation to three-stage evaporative crystallization.1 EFC is operated close to eutectic conditions, in such a way that two separate pure phases crystallize simultaneously. In the case of aqueous binary electrolyte solutions, such phases are a salt and ice. This principle is shown in the phase diagram of a KNO3 solution, Figure 1, where the so-called salt line, ice line, and eutectic point are defined. If a solution at ambient temperature containing 0-11 wt % KNO3 is cooled, ice is formed when the temperature reaches the ice line. Further cooling results in more ice formation and a corresponding elevation in the salt concentration in the solution, until the eutectic point is reached. If more concentrated starting solutions are used (>11 wt % KNO3), saturation with respect to KNO3 is reached first, salt is subsequently formed, and the mother liquor becomes more diluted until the eutectic point is reached. At the eutectic temperature and below, progressive crystallization of ice and salt occur until no liquid phase is left. However, in a continuous process, the liquid feed and recycle, suspension withdrawal, and heat removal are adjusted to give slightly subeutectic temperatures and a stable solid/liquid suspension. Separation of salt crystals from ice can be accomplished taking into account the difference in their densities. Eutectic solutions of electrolytes typically have a density of 1100-1200 kg‚m-3, ice of 900 kg‚m-3, and salts on the order of magnitude of 2000 kg‚m-3. * To whom correspondence should be addressed. Tel.: +31-15-2786678. Fax: +31-15-2786975. E-mail: G.J.Witkamp@ wbmt.tudelft.nl.

Figure 1. Phase diagram of the H2O-KNO3 system.

These differences are so large that an integration of settling within the crystallization process is attainable. Ice forms an ice slurry layer at the top of the crystallizer, while salt settles to the bottom. Thus, an extra process step is eliminated. The necessity of further processing of these slurry streams depends on the required product specifications. Usually the salt and the ice slurries are filtered or centrifuged, washed, and dried. For production of ice with a purity higher than 99%, an ice washing in a hydraulic wash column may be applied.2 The low operating temperatures of the eutectic freeze process are favorable for the processing of thermally unstable products. Corrosion of construction materials is substantially reduced because of the lower corrosivity of the electrolyte solutions at low temperatures. Especially for systems with acidic impurities, like the aqueous KNO3-HNO3 system, material costs of the process equipment can be reduced. In the past, research has been attributed to the development of a eutectic crystallization process for NaCl production.3 It was concluded that eutectic crystallization was technically feasible. In cases where high product quality was required, it was even found to be economically feasible. However, these results have never led to an industrial application. Recent investigations

10.1021/ie020946c CCC: $25.00 © 2003 American Chemical Society Published on Web 09/06/2003

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Figure 2. Ternary phase diagram of the H2O-KNO3-HNO3 system at varying temperatures. Axis scales are in weight percent. Table 1. Composition of the Industrial Aqueous KNO3-HNO3 Waste Stream component

amount [g‚kg-1]

KNO3 HNO3 Mg2+ Na+ NH4+ Ca2+ total organic content (TOC)

150 15 0.7 0.3 0.3 0.04 1

have again shown the possible technical and economical viability of EFC for a large number of relevant industrial systems.4 A dedicated equipment design, the cooled-disk column crystallizer (CDCC), which combines EFC and solid/ solid separation, has been proposed.1,5 A laboratoryscale unit has been successfully used for simultaneous crystallization of ice and copper sulfate pentahydrate.4 This study describes a study with a new pilot-scale CDCC, evaluating the heat-transfer capacity, gravitational separation efficiency, and product properties. Experimental System: Aqueous KNO3-HNO3 Solution During the production of starch from potatoes, an aqueous acidic waste stream is generated with the composition specified in Table 1.6 Presently, this stream is neutralized with KOH and discharged to surface waters. It is desirable to recover the economically valuable KNO3 as a pure solid, thereby reducing the amount of generated waste and the consumption of chemicals. For this purpose, the conventional technique of evaporative crystallization may be applied. If EFC is additionally included, yields are increased and energy consumption is lowered. The economical feasibility of this process configuration was shown by van der Ham.4 The industrial stream can be regarded as the ternary system H2O-KNO3-HNO3 shown in Figure 2. A specific feature of a ternary system is the existence of eutectic lines. In contrast to a binary system, in which eutectic conditions are defined by a unique point, ternary systems form multiple solid phases at varying compositions of the solution and varying temperatures. At least three different solid phases may be formed. The phases of interest for the eutectic freeze process are obviously ice and solid KNO3, which are found in the top section of the phase diagram, containing 0-34 wt

Figure 3. Detail of the ternary phase diagram of the H2OKNO3-HNO3 system. Black squares indicate literature data; open triangles represent experimental data.

% HNO3 and 2-11 wt % KNO3 at temperatures ranging from -3 to -44 °C. Because the relevant eutectic composition data of ternary KNO3-HNO3 solutions at desired process temperatures of 0 and -10 °C are not available, additional eutectic compositions have been determined experimentally. On the basis of these measurements, the detailed ternary phase diagram shown in Figure 3 is constructed. Experiments have been performed with a constant HNO3 concentration of 6 wt % in the eutectic crystallizer. The eutectic solution then contains about 6 wt % of KNO3 at a temperature of -5 °C and has a density of 1070 kg‚m-3. The feed contained about 9 wt % of KNO3, which is saturated at 0 °C, and has a density of 1096 kg‚m-3. Experimental Setup Eutectic Crystallizer Design Criteria. The crystallizer should fulfill three functions. First, heat needs to be removed from the aqueous solution for cooling of the feed to eutectic temperature, for crystallization of ice and salt, and for compensation of heat losses to the surroundings. Second, the residence time should be provided for nucleation and growth of both ice and salt crystals. Third, gravitational separation between ice and salt should be achieved within the crystallizer. The residence time should allow ice and salt crystals to grow to a size suitable for solid/solid separation and their subsequent removal from the mother liquor. The overall residence time is defined by dividing the feed rate by the total crystallizer volume. However, the relevant variables in defining growth are the residence times for ice and salt crystals, defined as the solid content within the crystallizer divided by the quantity of removed product. The generally used assumption of regarding the crystallizer as mixed-solution mixedproduct solution does not hold. Gravitational separation may be accomplished if salt and ice are formed as independent phases and if agitation is sufficiently low. Therefore, a compromise has to be found for the agitation level to accommodate solids suspension, heat transfer, and solid/solid separation requirements. Gravitational separation is intrinsically associated with distribution of both ice and salt throughout the crystallizer, so that it directly affects the residence times of each of these solid phases. In this

4876 Ind. Eng. Chem. Res., Vol. 42, No. 20, 2003

coefficient is obtained:

Rtot ) Q/A∆Tlog

(2)

The logarithmic temperature difference between coolant and crystallizer is defined by

∆Tlog )

(

)

(3)

Because there are three cooling elements in the CDCC, consequently with six cooling surfaces, six logarithmic temperature differences are defined. More details about these differences will be discussed later in the Results and Discussion section. The total heat-transfer rate can be appointed to three heat-transfer resistances, namely, the resistance in the coolant liquid, the metal cooling plate, and the resistance in the process liquid. They are related by

Figure 4. Schematic drawing of the 100-L CDCC.

way, a complex interplay exists between crystallization and solid/solid separation in EFC. Overall Geometry and Volume. The crystallizer designed to fulfill the functions just described is a vertical cylindrical vessel, divided into compartments by horizontal cooling disks. To these features, the crystallizer owes its name: cooled-disk column crystallizer (or CDCC). This design allows a linear scale-up by simply increasing the number of compartments because the volume/cooling area ratio is constant. Upward transport of ice crystals and settling of salt crystals is enabled by orifices in the cooling plates. Scrapers on the cooled surfaces provide agitation and scale removal. A schematic representation of the CDCC is shown in Figure 4. Experiments have been performed in a crystallizer containing four compartments and three cooling plates. The column diameter is 500 mm, and each compartment with a height of 140 mm approximately contains 25 L of useful volume, resulting in a total volume of 100 L. The compartments are constructed of poly(methyl methacrylate), forming a transparent column for optimal visual observations. Cooling Plates. Disk-shaped cooling elements are used, having flat horizontal cooling surfaces on the top and bottom sides. The plates are constructed of stainless steel. An even distribution of coolant flow through the plates is obtained with a labyrinth-like path for the coolant. With three double-sided cooling elements in the crystallizer, the total cooling area in the crystallizer is 0.69 m2. The specific cooling area per unit crystallizer volume in this setup is 7.24 m2‚m-3. Heat Transfer. The heat flux Q was derived using coolant flow measurements φcool, the temperature difference between in- and outflowing coolant ∆T, and the heat capacity Cp of the coolant; a heat balance is calculated in eq 1:

Q ) φcoolCp,cool∆Tin,out

Tcool,in - Tcool,out Tcool,out - Tcryst ln Tcool,in - Tcryst

(1)

Because flow and temperature data are acquired every minute, heat transfer can be monitored during the experiment. Considering the accuracy of the measurement devices, the result of the calculation above has a deviation of 9%. By division of this heat flux by the surface area of the cooling plates A and the logarithmic temperature difference ∆Tlog, the total heat-transfer

1 1 1 1 ) + + Rtot Rcool Rplate Rproc liq

(4)

The heat-transfer coefficient of the metal plate Rplate is calculated by dividing the thermal conductivity by the plate thickness and delivering a value of 3650 W‚m-2‚K-1 at 0 °C. The resistance of the coolant was independently determined and fitted to a Nu-Re-Pr relation. The Dittus and Boelter equation7 proposed for calculating heat transfer within tubes in the turbulent regime was taken as a starting point. Because the coolant flows in a duct with a rectangular cross section, the hydraulic diameter was used in the Reynolds number.8 The range of the investigated Reynolds number of the coolant is 1000-2000, which corresponds to the transient flow regime. The power of the Prandtl number is set to 0.3.7 The preexponential term was fitted to our experimental data. The following adjusted Dittus and Boelter relation was thus derived:

Nu ) 0.086Re0.8Pr0.3

(5)

Scrapers. It is commonly known that ice has a strong tendency to adhere on cooled surfaces. Once an ice-scale layer is deposited, the heat-transfer rate decreases substantially because the heat-transfer coefficient of ice is typically 10-20 times below that of stainless steel. Prevention of ice-scale formation therefore plays a major role in the design and operation of EFC processes based on indirect cooling. Ice-scale formation in electrolyte systems can be efficiently removed by scraping with hard materials.9 Elastic materials such as rubbers or silicones, which would otherwise be favored because of the associated low equipment wear, cannot exert enough mechanical force to remove ice crystals from the cooling plate. Therefore, high-density polyethylene has been chosen as the construction material for the scraper blades. Three scrapers rotate on each cooling surface. The blades are mounted on a central axis in the crystallizer. Blade springs press the scraper elements toward the metal plate in order to ensure full contact and an even force distribution. The scraper elements have slanted bottom sides, so that the contact area of metal/plastic can be regarded as a line contact. The required scraping rate is ruled by the requirement of ice-scale prevention on the cooling plate. The

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Figure 5. Photograph of the cooling element: orifice ring, cooling plate, and scrapers.

minimal scraping rate required for keeping a plate clear of an insulating ice layer is among others related to the supersaturation level, ionic system, and concentration.9 The scrapers also have a function in improving heat transfer. The higher the rotational speed, the higher the turbulence within the crystallizer, the higher the process liquid heat-transfer coefficient. However, this effect is in conflict with the requirement of an undisturbed ice/salt separation. A basic model describing the influence of the scraping rate on heat transfer is the penetration theory.10,11 This theory assumes that the scraper blades wipe off the thermal boundary layer mixing it completely with the bulk. After a scraper blade has passed, heat penetrates from the cooled surface into the process liquid as if it were a stagnant medium. The heat-transfer coefficient is related to the scraping rate by the following equation:

Rpen ) 2xλFCpnN/π

(6)

in which λ, F, and Cp represent the thermal conductivity, density, and heat capacity of the process bulk fluid, respectively. The number of scrapers is indicated by n and N is the rotational speed in rps. Orifice Rings. The orifices allowing free vertical flow of liquid and solids through the column have been placed in a thermally nonconducting ring surrounding the cooling element. In this way a cooling area is created that is as smooth as possible. This allows effective surface scraping and little unscraped cold surface on which ice can form a scale layer. The surrounding ring provides a frame in which the cooling elements are fixed in the crystallizer. The holes in the ring cover approximately 15% of the cylinder’s cross section. Orifices, cooling plates, and scrapers of a single cooling element are depicted in Figure 5. Ice Baffles. In the top section of the CDCC, relatively large volumes of ice are present, which are removed through the crystallizer overflow. To obtain a homogeneous and freely flowing ice slurry, baffles are inserted in the top compartment of the column. Measurement and Control System. Mass and heat balances around the crystallizer are calculated from continuously monitored flows and temperatures. A Fisher-Rosemount DeltaV Fieldbus system is used for control and data acquisition. The temperatures of the feed stream, of each crystallizer compartment, and of the in- and outgoing coolant were measured. These temperatures are registered by Pt-100 elements with an accuracy of 0.1 °C. The flows of the feed, of the salt slurry, and of the coolant in each cooling element are

Figure 6. Flow sheet of the CDCC setup. TT instruments represent temperature transmitters, and FT instruments are flow transmitters.

determined by Fisher-Rosemount magnetic flowmeters with an accuracy of 0.5%. Experimental Setup Flow Sheet. The feed to the CDCC is precooled in a stainless steel plate heat exchanger to 2-3 °C, a temperature slightly above the saturation temperature. Heat removal is provided by a Lauda RK8-KP thermostat using an ethylene glycol mixture as the coolant. The feed stream enters the crystallizer in the second compartment from the bottom. The salt slurry flow is removed from the bottom of the crystallizer with a peristaltic pump and consequently leads to a 0.1 m2 semicontinuous belt filter. The filtrate is recycled to the storage vessel. The ice overflow is indirectly controlled by feed and salt slurry flow rates. No mother liquor recycle is applied. Instead, all crystallizer outlet streams are recycled to the feed storage vessel. A 10 kW (at 0 °C) Lauda (UKS 12000) cooling unit circulates a secondary refrigerant through the CDCC cooling plates. The total coolant flow of the unit is regulated with a shortcut valve, while the flow in each plate can be controlled by separate needle valves. A 40 wt % Freezium solution, containing potassium formate, was used as the refrigerant. The flow sheet of the experimental setup is given in Figure 6. Experimental Procedure The CDCC is first filled with a feed solution at ambient temperature, and a low feed rate between 20 and 40 L‚h-1 is applied. It is intended to perform experiments with logarithmic temperature differences ranging from 1.5 to 7 °C. Considering the eutectic process temperature of approximately -5 °C, the cooling unit set point was varied between -7 and -17 °C. When the reaction temperature reaches about 0 °C, salt crystallizes. To keep the solid salt content at a steady level before ice is formed within the crystallizer, the salt product pump discharges salt slurry at its maximum flow rate of 12 L‚h-1. The eutectic temperature is subsequently reached, so ice crystallization starts. When the ice layer fills the top compartment of the crystallizer, the feed flow is increased to a rate of 60-100 L‚h-1 in order to keep the volume content of ice at a constant level. The system reaches a steady-state condition after some time, characterized by a constant process temperature, feed flow, and solids content in the crystallizer. Scraping rates were applied between 12 and 23 rpm. Considering the construction of three scraper blades per cooling plate, this is equivalent to 0.60-1.15 passages of scraper blades per second. The scraping speed is

4878 Ind. Eng. Chem. Res., Vol. 42, No. 20, 2003 Table 2. Amount of Ice Formed in the Early Stages of Ice Nucleation crystallizer volume heat capacity solution heat of crystallization ∆T ice crystal mass ice crystal volume ice mass fraction ice volume fraction

75 3.667 334 0.7 0.68 0.74 0.8 1

L kJ‚kg-1‚K-1 kJ‚kg-1 °C kg L wt % vol %

related to the applied logarithmic temperature difference in the crystallizer. Thus, high scraping rates correspond to large temperature differences. Solid-phase residence times were roughly estimated using product slurry solid content measurements and visual observations. In the experiments overall residence times between 1 and 2.5 h were realized. Samples of KNO3 are taken from the bottom outlet of the crystallizer after 6-7 h of eutectic operation at an overall residence time of 1.7 h, i.e., 3.5-4 residence times. They were immediately separated from the mother liquor in order to minimize product alterations due to dissolution, agglomeration, or attrition during sampling processing. The salt slurry is filtered on a glass filter and subsequently washed and dried in an oven at 40 °C. Water, ethanol, and saturated KNO3 solutions were used for different samples as washing liquids in amounts of 100 mL per washing step. Ice samples were taken from the CDCC overflow after the process had been eutectic for 4 h with an overall residence time of 1.7 h. The samples were directly filtered on a glass filter. Washing was performed manually in several steps by pouring slightly subcooled demineralized water over the residual crystals on the glass filter. In each wash step, 100 mL of wash water was used. Optical microscopy was used for the determination of ice crystal shapes and sizes. Considering the sensitivity of the ice slurry to exposure to ambient temperatures and the rapid solubility increase of KNO3 as a function of temperature, all samples are quickly transported to a climate chamber kept at a temperature of -10 °C, where further processing is carried out. Samples of both ice and salt are chemically analyzed on cationic impurities by ICP-AES spectrometry. Cationic impurities found in the KNO3 crystals are determined by ion chromatography. The nitric acid content of the liquid samples is found by an acid-base titration. Results and Discussion Ice Nucleation. The onset of nucleation is observed visually by a sudden appearance of ice crystals throughout the complete crystallizer. Nucleation can also be accurately identified by an abrupt crystallizer temperature increase. In an experiment where the eutectic temperature condition was -5.2 °C, the solution was at a subcooled state of -5.9 °C. The nucleation temperature rose from -5.9 to -5.2 °C within 2 min, caused by the release of crystallization heat. This is accompanied by an observed increase in the heat-transfer rate going from approximately 380 to 640 W‚m-2. The amount of crystallized water can be calculated from the temperature increase, the solution’s heat capacity, and the heat of crystallization of ice. The results are summarized in Table 2. Crystallizer Temperatures. Slight temperature differences are found for different compartments. The

Figure 7. Heat flux through the cooling plates during eutectic operation.

highest temperature is usually found in the compartment where the relatively warm feed stream is entering the CDCC. The lowest temperature is measured in the third compartment, between the feed and the top compartments. The top and bottom compartments show higher temperatures, which might be attributed to the fact that these compartments are only cooled by a single cooling plate. Besides, because of the presence of large amounts of salt and ice crystals in these compartments, the crystallization rate is high, resulting in a large release of crystallization heat. The differences between the maximum and minimum temperatures in the crystallizer are typically in the range of 0.5 °C. Heat-Transfer Rates. An important characteristic of a crystallizer is the heat-transfer rate because it determines the cooling surface area required for processing a certain feed flow. Of course, this has direct consequences on the size of the EFC equipment. The top-plate overall heat-transfer coefficient is approximately 540 W‚m-2‚K-1, while the middle element transfers 720 W‚m-2‚K-1. These values are obtained by calculating the slope of the linear heat-transfer curve over a range of logarithmic temperature differences between 2 and 10 K, which are depicted in Figure 7. The lower heat transfer in the top section of the crystallizer might be a result of the presence of large amounts of ice crystals in the top section of the crystallizer hampering efficient mixing. The Nu numbers of the coolant have a magnitude of approximately 30-60, leading to an Rcool on the order of 500-1000 W‚m-2‚K-1. Using eq 4, Rprocess is determined to be in the range of 1000-3500 W‚m-2‚K-1. The differences are explained by the higher scraping rates that were applied at larger subcoolings. The heattransfer coefficient, shown in Figure 8, increases linearly with the square root of the scraping rate, which is in agreement with the penetration theory. Because both the coolant and the process liquid are of the same order of magnitude, the total heat-transfer coefficient is likely to increase substantially by introducing a turbulent flow through the cooling plates. Scraping Rates. The scraping rate was adjusted based on visual observation of the crystallizer content. A high rate promoted total homogenization of the suspension, while a low scraping rate resulted in inefficient ice removal from the cooled surfaces and the subsequent appearance of ice lumps. An optimal scrap-

Ind. Eng. Chem. Res., Vol. 42, No. 20, 2003 4879

Figure 8. Average process-side heat-transfer coefficient under crystallizing conditions as a function of the scraping rate.

Figure 10. KNO3 crystals produced in the CDCC, washed in ethanol. Table 4. KNO3 Feed, Product, and Mother Liquor Impurities (in ppm) element feed KNO3 mother liquor

Figure 9. Ice crystals produced in the CDCC. Table 3. Ice Impurity Level of K+ Ions mother liquor

K+ concn [ppm]

mother liquor

K+ concn [ppm]

unwashed ice ice washed 1×

2620 190

ice washed 2× ice washed 3×

39 15

ing rate leads to the formation of two separate settling zones for salt and ice, neither with salt entrapment in the ice slurry zone nor with the appearance of scaling. Ice Product. The ice content in the slurry had a constant value of approximately 10 wt % at different scraping rates. This slurry density is likely to be related to the crystal size and shape of the ice and the upward force on the ice crystals. Agitation in the applied range is of secondary importance for the slurry density because the solid content remained constant at different scraping rates. Ice crystals have a flat circular disk shape, with sizes ranging from 100 to 200 µm. The morphology of the ice crystals is shown in Figure 9. The ice purity is expressed by the amount of K+ ions present after washing because this is the largest source of impurity in the mother liquor, 6 wt % in eutectic conditions. The potassium contents in unwashed samples and samples that had undergone up to three washing cycles are presented in Table 3. The fact that the impurity level rapidly decreases after several washing steps indicates that ionic impuri-

Ca

Cu

Fe

Mg

Na

Zn

40 4 47

65 2 80

20 2 22

452 1 520

173 160 200

24 6 29

ties are found in the adhering liquid and not within the crystals. It is known from the literature that ice crystallization is very selective and the level of ion uptake is low.12 Potassium Nitrate Product. The underflow of the CDCC contains approximately 7 wt % of crystals. Similar to the solids content level in the ice overflow, the salt content is independent of scraping rates. The particles are agglomerates of well-faceted primary crystals and have average sizes of 100-200 µm. An example of the KNO3 product is depicted in Figure 10, showing crystals after washing with ethanol. The formation of these agglomerates may be attributed to the high supersaturation levels that are present in the crystallizer with respect to KNO3. The concentrations of the elements present in the largest quantities of a salt sample that has undergone two washings are given in Table 4. This table also shows the composition of the mother liquor. Gravity Separation Efficiency. The efficiency of the gravity separation is determined by taking slurry samples of approximately 1 L from the top and bottom crystallizer flows. The stirring rate in these experiments was set at 23 rpm, which is the maximum rate at which eutectic experiments have been operated. Thus, worst case settling data are obtained. The samples were placed in the climate chamber, to obtain settling of the solid phases. It was evident that the bottom samples contained no ice at all. The top samples, mainly containing ice, were stirred manually in order to ensure that KNO3 crystals caught in the ice slurry could also settle. When the slurry is separated and weighed and an ice density of 917 kg‚m-3, a KNO3 density of 2109 kg‚m-3, and a eutectic solution density at -5 °C of 1078 kg‚m-3 are used, the slurry density given in Table 5 was calculated. With a feed flow rate of 100 L‚h-1 and a salt slurry removal rate of 12.5 L‚h-1, the fraction of salt product recovered through the bottom outlet compared to the total salt production is calculated to be 87.5%.

4880 Ind. Eng. Chem. Res., Vol. 42, No. 20, 2003 Table 5. Top and Bottom Slurry Solids Contents product

wt % ice

wt % KNO3

vol % ice

vol % KNO3

top bottom

8.58

0.15 7.00

9.94

0.08 3.70

(2) van Oord-Knol, L. Hydraulic wash columns: solid-liquid separation in melt crystallization. Ph.D. Dissertation, Delft University of Technology, Delft, The Netherlands, 2000. (3) Swenne, D. A. The eutectic crystallization of NaCl‚2H2O and ice. Ph.D. Dissertation, Eindhoven University of Technology, Eindhoven, The Netherlands, 1983.

This calculation shows that a reasonable separation efficiency is achieved even at a higher scraping rate.

(4) van der Ham, F. Eutectic Freeze Crystallization. Ph.D. Dissertation, Delft University of Technology, Delft, The Netherlands, 1999.

Conclusions

(5) van der Ham, F.; Witkamp, G. J.; de Graauw, J.; van Rosmalen, G. M. Eutectic freeze crystallization simultaneous formation and separation of two solid phases. J. Cryst. Growth 1999, 198/199, 744.

The performance of a 100-L CDCC has been evaluated in eutectic operation. Process conditions have been determined for optimal operation. Both ice with impurity levels of 15 ppm after three washings and pure salt have been crystallized without inclusions. Process-side heat-transfer coefficients increased from 1000 to 3500 W‚m-2‚K-1 with the square root of the scraping rate. An acceptable solid/solid separation between ice and salt is achieved, leading to an entrainment of 0.15 wt % KNO3 in the ice slurry product flow. Acknowledgment The authors thank DSM Research, Pannevis Solid Liquid Separation, Kemira Agro Ventures, Nedmag, the Ministry of Economic Affairs, the Ministry of Housing, Spatial Planning and Environment and the Ministry of Education and Science of The Netherlands through the EET program for their (financial) support. Literature Cited (1) van der Ham, F.; Witkamp, G. J.; de Graauw, J.; van Rosmalen, G. M. Eutectic freeze crystallization: application to process streams and waste water purification. Chem. Eng. Process. 1998, 37 (2), 207.

(6) Eichner, P.; Vorage, M. Process for producing high-purity potassium salts. U.S. Patent 6,274,105B1, 2001. (7) Dittus, P. W.; Boelter, L. M. Heat transfer in automobile radiators of the tubular type. Reprinted in Int. Commun. Heat Mass Transfer 1985, 12, 3. (8) Kakac¸ , S.; Shah, R. K.; Aung, W. Handbook of single-phase convective heat transfer; Wiley: New York, 1987. (9) Vaessen, R. J. C.; Himawan, C.; Witkamp, G. J. Scale formation of ice from electrolyte solutions on a scraped heat exchanger plate. J. Cryst. Growth 2002, 237-239, 2172. (10) de Goede, R. Crystallization of paraxylene with scraped surface heat exchangers. Ph.D. Dissertation, Delft University of Technology, Delft, The Netherlands, 1988. (11) Beek, W. J.; Mutzall, K. M. K. Transport Phenomena; Wiley: New York, 1977. (12) Gross, G. W.; Svec, R. K. Effect of ammonium on anion uptake and dielectric relaxation in laboratory-grown ice columns. J. Phys. Chem. B 1997, 101, 6282.

Received for review November 25, 2002 Revised manuscript received July 17, 2003 Accepted August 4, 2003 IE020946C