ARTICLE pubs.acs.org/crystal
Continuous Mass Crystallization of Vitamin C in L(þ)-Ascorbic AcidEthanolWater System: Size-Independent Growth Kinetic Model Approach Published as part of a virtual special issue of selected papers presented at the 9th International Workshop on the Crystal Growth of Organic Materials (CGOM9). Boguszawa Wierzbowska,† Nina Hutnik,† Krzysztof Piotrowski,*,‡ and Andrzej Matynia† † ‡
Wroczaw University of Technology, Faculty of Chemistry, Wybrzeze Wyspianskiego 27, 50 370 Wroclaw, Poland Silesian University of Technology, Department of Chemical & Process Engineering, ks. M. Strzody 7, 44 101 Gliwice, Poland ABSTRACT: Experimental results concerning continuous isohydrical drowning-out mass crystallization of vitamin C in an L(þ)-ascorbic acidethanolwater system are presented. The process environment was created in a laboratory-scale draft tube, mixed suspension mixed product removal (DT MSMPR) crystallizer with internal circulation of suspension. Assuming a constant feed concentration of ethanol (20 mass %), the feed concentration of vitamin C was changed within the 3050 mass % range. The mean residence time of suspension in working volume of a crystallizer was varied from 900 to 3600 s. Combinations of the presented above input process parameters resulted in a productivity 2441692 kg/(m3 h) of crystal product, of mean size within 0.200.24 mm and of CV within the 5060% range. The supersaturation level in mother solution reached relatively high values (up to ca. 6.5 mass %), in particular at a short mean residence time of suspension (900 s). With this time elongation to 3600 s, the supersaturation level decreased, however, by ca. 40% (to 3.9 mass %). The simplest size-independent growth (SIG) kinetic model was adopted for nucleation ((3.327.8) 107 1/(s m3)) and crystal growth ((1.66.9) 108 m/s) rates estimation. Kinetic relations and feedback between nucleation and crystals growth were identified and analyzed in detail.
’ INTRODUCTION Vitamin C (L(þ) ascorbic acid, Acidium ascorbicum, (R)-3,4dihydroxy-5-(S)-1,2-dihydroxyethyl)furane-2(5H)-one, E300, LAA) is a very important component of many pharmaceuticals. It takes part in stimulation of collagen synthesis, efficient functioning of the blood system and heart, influences a decrease in cholesterol level, and generally stimulates the human immunological system. As a strong reductor, vitamin C shows antioxidative properties, removes free radicals, and takes part in fat and cholesterol metabolism, red blood production, and ferrum bioassimilation. It reduces toxic effects of Se, Cu, V, Co, and Hg in the organism, as well as influences glucose level in the blood. LAA is an important ingredient of cosmetics, responsible for skin protection against sun radiation. Its natural synthesis can be carried out in plant structures (mainly in chloroplasts) and in selected animals’ organs (liver, kidneys). It can be naturally found in vegetables and fruits. Daily dose is 75 mg/day for women and 90 mg/day for men.1,2 Production of vitamin C from natural biomass resources by leaching/extraction methods is uneconomical since it is too expensive (e.g., requires the application of HPLC technique) and ineffective with respect to the final process yield. In an industrial scale, vitamin C is produced by a complex seven-stage r 2011 American Chemical Society
ReichsteinGr€ussner synthesis method (ca. 50% efficiency of Dglucose f L(þ)-ascorbic acid conversion). This process, however, requires many organic and inorganic chemicals, as well as appropriate selection of individual temperature for each process stage.3,4 The relatively low selectivity of the biochemical synthesis process means that the implementation of an additional, purification stage for selective separation of the main product becomes thus necessary. The required chemical purity of postsynthesis vitamin C can result from multistage batch crystallization from its water solutions.58 Reduction in the necessary batch crystallization stages favoring improvement in the process yield and product crystal quality can be achieved by the introduction into the process system some third component responsible for modification of the intrinsic physicochemical properties of the multicomponent system, mainly solubility with respect to the original solvent. Considering the most advantageous final effects, methanol or ethanol is especially recommended as additive.913 Batch crystallization of vitamin C in four-component mixtures, LAAmethanol ethanolwater,1417 also provided promising results. Received: November 16, 2010 Revised: February 14, 2011 Published: March 22, 2011 1557
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Scheme 1. Scheme of Laboratory DT MSMPR Crystallizer with Auxiliary Equipment for a Continuous Isohydrical DrowningOut Mass Crystallization of Vitamin C
Annual production of vitamin C is 80 000 Mg and approximate costs are 600 bln USD. It should be emphasized that the production rate increases by 34%/year. Main recipients of vitamin C are pharmaceutical (50%), food (25%), cosmetic (15%) and agricultural industries (10%). The systematic growth of demand stimulating its production rate favors the gradual redesign and reorganization of all stages of current production technology from a batch regime into a continuous one, potentially providing better controllability resulting in higher and stable product quality. The laboratory experiments to date covered a continuous mode of crystallization-based purification both in binary, pure water solutions of LAA,1820 and in enhanced ternary systems in the presence of methanol.21,22 A continuous regime in vitamin C crystallization, although technologically attractive, is not widely used in practice. This fact results the most often from characteristic, relatively low productability of biotechnological plants, also including these for vitamin C synthesis, compared to the classical mass crystallization processes, for example, applied in the production of mineral fertilizers. The experimental data demonstrating kinetic relations in continuous mass crystallization of vitamin C in an LAAethanol water system are presented and discussed. The tests were carried out using a typical laboratory draft tube, mixed suspension mixed product removal (DT MSMPR) crystallizer of working volume 0.6 dm3 with an internal circulation of crystal magma controlled by a propeller mixer and circulation profile. The coupled influence of LAA concentration in a feed ([LAA]rm = 3050 mass %) and a mean residence time of suspension in a crystallizer working volume (τ = 9003600 s) on the resulting size distribution of product crystals was tested and analyzed theoretically. Invariable concentration of ethanol (antisolvent additive) in a feed ([EtOH]rm = 20 mass %), irrespective of the current LAA mass fraction, was provided for each experiment. Experimental crystal size distributions were determined with analytical methods, on which basis nucleation and crystal growth rates were established for each analyzed case. The resulting size characteristics of product crystal populations with the corresponding kinetic data provide the possibility of more thorough insight into this original technological approach — a continuous
isohydrical drowning-out mass crystallization process of vitamin C in LAAethanolwater systems.
’ EXPERIMENTAL DETAILS Experimental Setup. Schematic representation of the laboratory system used for the experiments is presented in Scheme 1. Installation was based on professional elements made by IKA Labortechnik, Germany. Operation and control of the plant were realized by a PC computer using IKA Labworldsoft software. The main element of experimental plant was a laboratory-scale continuous DT MSMPR crystallizer equipped with propeller agitator responsible for internal circulation of suspension (adapted standard configuration of IKA laboratory reactor system LR-A1000). It was a hermetic, glass-made cylindrical tank (Vt = 1 dm3, D = 120 mm, H = 123 mm), inside which a circulation profile was arranged (draft tube, d = 61 mm, h = 53 mm). In the tank/draft tube axis, a three-paddle propeller mixer (dm = 55 mm) of standard geometrical proportions was located. The stirrer speed was kept constant (N = 10 ( 0.2 1/s) in all experiments satisfying hydrodynamic requirements of maintaining a stable and intensive enough circulation of suspension inside the crystallizer working volume (Vw = 0.6 ( 0.02 dm3). The bottom part of crystallizer vessel was equipped with a heating/cooling coil fed with thermostated water of adjustable temperature acting as a heat exchanger. It provided isothermal process conditions, Tcr = 293 K, with (0.2 K tolerance. Both the temperature adjustment system and the control/ regulation points of temperature (TIC), feed flow rate (FQI), and suspension level in a crystallizer vessel (LIC) are marked in Scheme 1. Inlet liquid solutions tested, considered as biochemically active multicomponent systems, were analytically prepared just before their introduction into the crystallizer working space. The following analytically pure reagents were used: commercial LAA of the main ingredient content >99.7 mass % (for analysis and for biochemistry applications, MERCK, Germany), ethanol (96 mass %, p.a., POCH Gliwice, Poland), and distilled water (Nanopure Diamond Barnstead). Feed solution reservoir temperature Trm was ca. 10 K higher with respect to feed saturation temperature, Teq (strictly dependent on the current proportions within the feed components). To prevent against undesirable spontaneous nucleation in a fresh feed before its first contact with the process environment, delivery installation was also preventively heated. To prevent eventual undesirable decomposition of vitamin C during the 1558
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Table 1. Technological Input Dataa and Corresponding Statistical Parameters of Crystal Size Distribution (CSD) of L(þ)-Ascorbic Acid (Vitamin C) Produced from LAAEtOHH2O System in a Continuous DT MSMPR Crystallizer27 no. [LAA]rm [mass %] τ [s] [LAA]eq,ml,Tcr [mass %] Δcmax [mass %] [EtOH]ml,Tcr [mass %] Δcml, Tcr [mass %] MT [kg/m3] Lm [mm] L50 [mm] CV [%] 1
30
900
17.5
12.5
21.2
6.3
61
0.203
0.196
60.1
2 3
35 40
900 900
17.2 16.9
17.8 23.1
23.0 25.2
6.3 6.6
145 234
0.200 0.218
0.181 0.197
57.9 53.6
4
45
900
16.5
28.5
28.0
6.7
330
0.239
0.230
51.2
5
50
900
16.0
34.0
31.0
6.5
423
0.210
0.180
54.0
6
40
1800
16.8
23.2
25.5
5.6
242
0.229
0.215
53.2
7
40
3600
16.7
23.3
25.8
4.4
250
0.235
0.218
53.9
8
50
1800
15.9
34.1
31.4
5.1
432
0.228
0.210
51.0
9
50
3600
15.8
34.2
31.8
3.9
442
0.240
0.218
52.6
Temperature in DT MSMPR crystallizer: Tcr = 293 ( 0.2 K, [EtOH]rm = 20 mass %, Δcmax = [LAA]rm [LAA]eq,ml,Tcr, Δcml,Tcr = [LAA]ml,Tcr [LAA]eq,ml,Tcr. a
purification operation, the surfaces of the process plant running a risk of direct contact with a solution/suspension of LAA crystals were composed of chemically inert glass or plastic elements. Metallic mixer was covered with a protective coating to confine both the secondary nucleation effects and chemical decomposition of vitamin C. Experimental Schedule. Laboratory tests were divided into two main groups. • The first set of experiments was carried out with the purpose of examining the influence of L(þ)-ascorbic acid concentration in a feed. The following feed compositions were investigated: [LAA]rm = 30 (corresponding to Teq = 315.5 K), 35 (Teq = 325.0 K), 40 (Teq = 333.5 K), 45 (Teq = 343.0 K), and 50 mass % (Teq = 353.5 K). For this set, a constant value of mean residence time of crystal suspension in a crystallizer working volume, τ = 900 s, was assumed. • The second set of tests focused on examining the influence of systematically increasing mean residence time 1800 and 3600 s at a selected, constant concentration of L(þ)-ascorbic acid in a feed solution (40 or 50 mass %). For uniformity of process conditions in both measurement series, the ethanol concentration in a feed stream was assumed constant: [EtOH]rm = 20 mass %. It must be emphasized that the process parameter ranges assumed for the tests overlapped practical technological limitations of isohydrical drowning-out mass crystallization of LAA ([LAA]rm, [EtOH]rm, Trm, Tcr, τ)9 providing the experimental data of direct practical utility. Physicochemical data concerning solubility (ceq = f(T, [EtOH]rm) and density of saturated solutions (Fsat = f(T, [EtOH]rm)) in the LAAEtOHH2O process system are presented elsewhere.9,23
’ MATERIALS AND METHODS Laboratory experiments (Scheme 1) applying continuous mode of mass crystallization require more complex test procedures compared to a batch regime variant. The LAAwaterethanol environment inside apparatus must be initially filled with a suspension of LAA crystals. Then the continuous inflow/outflow from the tank (regulated by the pump) must be arranged to initialize the process of self-adjustment of this heterogeneous system to the assumed technological conditions. Fresh solution of assumed chemical composition was systematically pumped into the apparatus working volume with volumetric flow rate providing the required residence time in a crystallizer space, τ. To avoid eventual accumulation effects (forbidden in a flow reactor design) product crystal suspension was removed from the tank with the identical volumetric flow rate, resulting in a stable and closely controlled free suspension level in apparatus. Stabilization of the assumed parameter values for each continuous process (here: Tcr = 293 K, Vw = 0.6 dm3, N = 10 1/s) resulting from the action of complex internal interrelations and kinetic feedbacks requires some initial startup period. In continuous mass
crystallization processes, it is usually accepted t = 5τ. After analytical confirmation of stable process conditions (steady-state mode achieved) isohydrical drowning-out mass crystallization of LAA ran through the additional time of t = 7τ. After this time, the crystallizer content was a subject of analytical tests. To avoid any problems with selection of the representative product sample crystal suspension from the whole crystallizer was collected and separated in a laboratory centrifugal device into solid and liquid phases. Vitamin C crystals were washed with cold water and ethanol, dried in the darkness, and weighed. Residual concentration of LAA in the mother solution was determined with a precise weight method (based on the mass difference in a sample: directly after the process termination and after total evaporation of the solvent mixture). Final concentration of ethanol in the sample resulted from the mass balance constraints. Both concentrations are presented in Table 1. It should be emphasized that ethanol concentration in mother solution (process environment) is higher, in some conditions even considerably, compared to its initial concentration in a feed. This fact is of fundamental meaning for continuous drowning-out mass crystallization in this specific physicochemical system. This analytically identified increment results mainly from continuous solid phase formation, resulting in establishment of some mass flux of LAA modifying component proportions in the mother liquor compared to the original feed composition. For higher MT values (423442 kg/m3), [EtOH]ml exceeds even 31 mass % (Table 1). Mass basis mi(L) (or volume basis, Vi(L)) product crystal size distributions were determined with the standard procedures of particle analyzer COULTER LS-230. Corresponding population density values, ni, were then calculated according to the formula presented below, eq 1: ni ¼
mi Vi ¼ kv FL3i ΔLi Vw kv L3i ΔLi Vw
ð1Þ
Calculated with eq 1 ni values created a basic database on which the fundamental kinetic parameters of the continuous vitamin C mass crystallization process were determined.
Kinetic Model Size-Independent Crystal Growth (SIG).
Experimental data were interpreted with a kinetic model of continuous mixed suspension mixed product removal (MSMPR) crystallizer,24 based on the following, fundamental theoretical assumptions: • mass crystallization process in steady state runs in ideally mixed crystal suspension, • aggregation, agglomeration, attrition, breakage, and similar partial phenomena within the examined solid phase are not observed, • zero-sized nuclei can appear only, • nonclassified, fully representative to the bulk magma population of geometrically similar crystals is continuously removed from the 1559
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crystallizer, size distribution of which results only from intrinsic kinetic feedback between nucleation and crystal phase growth supplemented by diversified residence times of individual particles in the apparatus space (stochastic size composition of the removed product), • linear crystal growth rate is size-independent, SIG kinetic model (G(L) = G = const for the average supersaturation level in a process system). Under these assumptions, the crystal population balance equation can be rearranged into the following final form, eq 2:25 L nðLÞ ¼ n0 exp ð2Þ Gτ Fitting the experimental population density distribution n(L) data with eq 2 makes estimation of the fundamental kinetic parameter values of mass crystallization process in ideal MSMPR crystallizer configuration B and G possible. Graphical representation of rearranged eq 2 ln n(L) is in this case a straight line, whose intersection with the y-axis for L = 0 represents ln n0 while its slope is 1/(Gτ). Thus, for the known and precisely determined mean residence time of crystal suspension in a working volume of the apparatus, τ, the crystal linear growth rate G can be determined directly from this slope. Knowing G (slope) and nuclei population density n0 (intersection point) nucleation rate B can be calculated directly with eq 3: ð3Þ
B ¼ n0 G
As a result of complex interrelations between mass transfer and hydrodynamic conditions (development of specific surface area of crystal phase, magma density MT, productability MT/τ) and kinetics of the process (nucleation/growth rates), some Δc value self-establishes in the system simultaneously. For the engineering calculations, some convenient in form, however pure empirical correlations can be used,24,26 eq 45: B ¼ kN ðΔcÞn G ¼ kG ðΔcÞg
n>g
ð4Þ
1 600 μm, Lmax ∼ 750 μm) increases while the number of the smallest particles decreases. Crystal fractions of sizes below 40 μm in the product were 4.13 and 3.34%, respectively. Population Density Distributions. Exemplary size distributions of LAA crystals presented in Figure 1 were then converted (eq 1, kv = 1) into their corresponding population density distributions, n(L), shown in Figure 2. From their courses in the ln n L coordinate system it results that for L > 50 μm this dependency can be satisfactorily approximated with a straight line. Taking advantage of it, thus indirectly assuming validity of the SIG MSMPR kinetic model, one can estimate the values of the linear growth rate of crystals, G, and of their nucleation rate, B (all results are presented in Table 2). From Figure 2 it also results that for L < 50 μm the experimentally determined population density distributions are concave to the top. This characteristic course can be interpreted as a qualitative indicator of a more complex kinetics of crystal phase growth than corresponding to the most simplified SIG MSMPR kinetic model. In general, this type of population density distribution manifests growth rate dispersion (Gi = const,
Figure 2. Comparison of population density distributions of L(þ)-ascorbic acid crystals produced in a continuous DT MSMPR crystallizer (SIG kinetic model approach): the points experimental data, solid lines values calculated with eq 2 corresponding to crystal sizes L > 50 μm.
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GRD), secondary nucleation effects, and a size-dependent difference in relative fluidcrystal velocity defining specific mass transfer conditions. All these effects are, however, undistinguishable from each other disposing the population density distribution data only. Nevertheless, for design calculations they all can be treated as an integrity, producing an identical external effect in the ln n L coordinate system. Thus, the final effect of these complex contributions may be a subject of general description using any size-dependent growth (SDG) kinetic model, mathematically convenient for the (substitute-) kinetic data evaluation.22 Application of the SDG kinetic approach for these data elaboration will be presented in the next authors’ work. Accompanying Processes. Figure 3 shows microscope images (scanning electron microscope JEOL-5800 LV) of exemplary vitamin C crystals produced in continuous isohydrical drowning-out mass crystallization process. From their analysis it can be concluded that continuous mass crystallization of L(þ)-ascorbic acid in presented process conditions is accompanied by co-running agglomeration and secondary nucleation processes. Agglomerates and aggregates composed mainly of small and medium-size crystal fractions are present in both populations. Some of the larger crystals show morphology defects and rounded edges. Some broken, both smaller and larger fragments, of parent crystals can be observed, as well, suggesting relatively intensive attrition and breakage processes within the population. Comparing these observations with the corresponding population density distributions (see Figure 2) one can conclude that attrition and breakage within the crystal phase partly compensate their agglomeration and aggregation effects. Together with possible GRD or/and SDG phenomena, it produces characteristic, concave course of ln n(L) function, especially within the smallest crystals range. Process Kinetics. Resulting from regression analysis, final equations of population density distributions (SIG model applied, eq 2) fitted to experimental ni(L) data restricted to L > 50 μm and determined on this basis the values of process kinetic parameters are presented in Table 2. An increase in LAA concentration in a fresh feed stream (roughly correlated with MT rise), for a constant value of mean residence time τ = 900 s (Table 12, nos. 15), provides an increase in crystal linear growth rate, G, from 6.22 108 (for [LAA]rm = 30 mass %) to 6.89 108 m/s (for [LAA]rm = 45 mass %), followed by a slight drop to 6.35 108 m/s (for [LAA]rm = 50 mass %). Such
Figure 3. Scanning electron microscope images (magnification 300) of L(þ)-ascorbic acid crystals produced in a continuous DT MSMPR crystallizer: (left) test no. 3, (right) test no. 5; see corresponding data in Tables 1 and 2 and in Figure 1.27 1562
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modification in feed composition results also in a nearly 7-times increase in nucleation rate, B. Elongation in the mean residence time of suspension produces, by general lowering of the supersaturation level with its consequences, a significant decrease in both kinetic parameter values. In particular, an increase in τ = 900 f 3600 s, regardless of the LAA concentration in a feed (40 or 50 mass %), results in about (3.83.9)-time decrease in linear growth rate, G, and about (4 4.5)-time decrease in nucleation rate, B. As a result of advantageous adjustment of the G/B ratio by higher reduction in the nucleation rate, the mean crystal size Lm increases about 7.8% ([LAA]rm = 40 mass %) and about 14% ([LAA]rm = 50 mass %). Application of multivariable regression methods for general fitting of all available experimental data vectors: MT, G, B, N (Tables 1 and 2) to eq 8 template makes calculation of its parameter (especially exponent) values possible. For Tcr = const = 293 ( 0.2 K and N = const = 10 ( 0.21/s these are i = 1.04, j = 0.91. Equation 8 can be now presented in a final form as eq 9: B ¼ 3:03 1013 G1:04 MT0:91 R 2 ¼ 0:952
ð9Þ
It may be noticed that in mass crystallization processes: 0.5 < i < 3 (usually i > 1) and 0.4 < j < 2 (usually j = 1).24,26 The exponent values in eqs 89 can also provide some further, theoretical insight concerning the dominating collision type observed during vitamin C crystals attrition and breakage, especially which stage of secondary (contact) nucleation is a limiting one.26 For j < 1 and i > 1 conditions (see eq 9) it can be expected that the nucleation process of L(þ)-ascorbic acid in the presented experimental conditions (process regime, crystallizer type and mode, feed composition) is kinetically limited by the rate of potential nuclei formation on the surface of parent vitamin C crystals. However, their incorporation into crystal lattice dominates over their removal back into bulk suspension. Some aspects of compatibility between experimental data with SIG MSMPR model predictions (more precisely, with predictions of its derivative in a form of design equation, eqs 8 and 9) require a more detailed discussion. An increase in MT value for Tcr = const, N = const, τ = const, and j = 1 should theoretically result in constant Lm, G, and Δcml values. However, from the experimental data it is evident that these are not. The mean size of crystals, Lm, as well as their linear growth rate, G, increase (tests nos. 14, Tables 1 and 2). Moreover, exponent j = 0.91 < 1 (see eq 9). The main reason explaining this is an increase in a working supersaturation Δcml value (by 6%), despite the simultaneously observed increase in crystal specific surface area arising from higher suspension density (MT), facilitating also more intensive attrition within the bulk magma additionally contributing the surface development tendency. This larger specific surface area of crystals in suspension through effective reduction of mass transfer resistances should theoretically correspond to a lower working supersaturation level. In contrast, an evident increase in its value was observed, which can be connected with high values of maximal supersaturation at a feed inlet (Δcmax = 12.528.5 mass %). This effect is additionally, directly intensified by a relatively short mean residence time of suspension, τ (900 s), constant (not adjusted to current MT value) stirrer speed, N, as well as an increase in antisolvent (ethanol) concentration in a mother solution after solid phase formation and growth. An increase in suspension density provides evident economical benefits since crystallizer productivity is thus considerably higher. In practice,
however, crystal attrition/breakage intensity increases what devaluates the final product properties, especially its characteristic size values. From the data presented in Table 2 (nos. 14) it can be, however, concluded, that linear growth rate of crystals increased about 11% only while nucleation rate increased nearly 4-times. Interesting behavior in the continuous process mode was identified analyzing the test data nos. 4 and 5 in Tables 12. Beyond the feed LAA concentration of 45 mass % (in this case it was 50 mass %) significant devaluation in Lm (0.239 f 0.210 mm) and L50 (0.230 f 0.180 mm) was observed (Table 1, nos. 45). It can be explained on the kinetic basis using corresponding data from Table 2 (nos. 45). An increase in a feed LAA concentration from 45 to 50 mass % produces a slight decrease in the G value (6.89 108 f 6.35 108 m/s, stopping thus its growing tendency) and simultaneously provides a significant increase in nucleation rate, B (15.1 107 f 27.8 107 1/(m3 s)). The resulting, appropriate shifts of Lm and L50 toward lower values are also confirmed by a corresponding increase in adequate n0 values (2.19 1015 f 4.37 1015 1/(m m3)). It may be assumed that beyond [LAA]rm = 45 mass % some negative tendencies in kinetic B T G feedbacks impose practical technological constraint, which is worth paying attention to in design works. From theoretical analysis it results that for i > 1 higher values of mean residence time, τ for Tcr = const, MT = const, and N = const should correspond to a decrease of linear growth rate, G. Mean crystal size, Lm, should theoretically grow (the effect is more pronounced when i is higher than 1), while supersaturation Δcml is expected to devaluate (eq 5). Laboratory test data (for experimentally determined i = 1.04, eq 9) confirm these theoretical presumptions (see No. 3 and nos. 59, Tables 1 and 2). It is confirmed on experimental and theoretical fields that in real crystallizer attrition and breakage effects of various intensities depending on its constructional details and operating mode are usually observed. Secondary nucleation intensity, however, decreases with the mean residence time elongation (eqs 89). Its source, general disintegration intensity, as a pure mechanical effect depending on the mixing power dissipated within the suspension volume, remains constant. Additional, an experimentally confirmed factor lowering the working supersaturation level, is the increase in specific crystal surface area resulting from attrition effects. This phenomenon influences both crystal growth rate and product mean size disadvantageously. Supersaturation decreased also by 3040% (see Table 1) as a result of MT parameter growth. The MT(τ) relation was thus not constant it can be regarded as another, however relatively small deviation from the theoretical restrictions of the SIG MSMPR kinetic model. However, from the own experimental data it results that the maximal mean residence time tested, τ = 3600 s did not result in any disadvantageous consequences with respect to the crystal size characteristics (see Table 1). Mean size of vitamin C crystals and their homogeneity (CV) identified in the precisely defined laboratory test conditions are still acceptable. Comparison with Literature Data. In the open literature, according to the best authors’ knowledge, there is no information on the continuous mode for vitamin C purification by means of a mass crystallization process. Thus, the experimental data concerning kinetics of the crystal phase growth G = 1.63 108 to 6.89 108 m/s (with ethanol present in the feed) can be strictly comparable only with the authors’ own laboratory data covering pure binary LAAwater system (G = 1.80 108 to 7.10 108 m/s)19 or ternary LAAmethanolwater system (G = 1.63 108 7.24 108 m/s).21 For theoretical 1563
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Crystal Growth & Design purposes, these data may be roughly compared with some other results, however what must be strongly emphasized corresponding to a batch regime, with all consequences of its interpretation. Wierzbowska et al.16 exploring kinetics of the vitamin C crystallization in various LAAMeOHEtOHwater systems identified G = 1.41 108 to 3.94 108 m/s. Freitas and Giulietti7 for comparable physicochemical systems reported G = 1.26 108 to 8.38 108 m/s. Omar in his study12 confirmed similar kinetic behavior of these crystals' growth. For a pure water solution of LAA, he observed G = 2.4 108 to 5.4 108 m/s, in a solution composed of (excluding LAA) water (80 mass %) and ethanol (20 mass %) G = 2.4 108 to 1.31 107 m/s, in a water (80 mass %) and methanol (20 mass %) mixture G = 1.20 108 to 3.37 107 m/s and in a water (80 mass %) and propanol (20 mass %) mixture it was G = 8.17 109 to 4.68 107 m/s. All G values reported in the literature, regardless of batch or continuous mode of the process, as well as number and individual concentrations of drowning-out agents introduced into the LAAwater system, are generally of the same magnitude. Minor differences result probably from except chemical composition of the process environment different experimental conditions and the test equipment precision.
’ CONCLUSIONS Technical concept of continuous regime in vitamin C purification technology based on isohydrical drowning-out mass crystallization from LAAwaterEtOH mixtures was investigated in a laboratory scale. It was observed that two technological input parameters influence the process kinetics considerably. These were concentration of L(þ)-ascorbic acid in a feed, as well as the mean residence time of suspension in a crystallizer working volume. Controlled adjustment of their values results in the required crystallizer productivity and product quality (crystal size distribution). Mass crystallization process kinetics, in particular represented by nucleation and crystal growth rates, was estimated with the simplest sizeindependent growth (SIG) model. Before that, validity of MSMPR conditions in a laboratory test vessel was confirmed (inspection of CV = 50%). However, detailed analysis of population density courses indicates that this first approach is unable to describe the experimental data in a whole size range using one model equation. It can be, however, still regarded as a useful tool for identification of some general features of the process, especially observed in larger crystals fraction. The experimental data and their kinetic interpretation can be a useful reference material providing information about physicochemical fundamentals, as well as technical capabilities and limitations of a continuous isohydrical drowning-out mass crystallization variant in vitamin C purification technology. ’ AUTHOR INFORMATION Corresponding Author
*Fax/phone: (þ48 32) 237 14 61; e-mail: krzysztof.piotrowski@ polsl.pl.
’ ACKNOWLEDGMENT Size distributions of LAA crystals were determined by means of particle size analyzer COULTER LS-230 in the Institute of Inorganic Chemistry, Gliwice, Poland. ’ SYMBOLS B, nucleation rate, 1/(m3 s)
ARTICLE
ceq, solubility, mass % Δc, supersaturation of LAA in a process system, mass % Δcmax, maximal supersaturation of LAA in a process system, mass % Δcml, supersaturation of LAA in a mother solution, mass % CV, coefficient of variation (of crystal size), % d, internal diameter of draft tube element, m dm, diameter of propeller mixer, m D, internal diameter of laboratory crystallizer, m [EtOH]ml, concentration of ethanol in a mother solution, mass % [EtOH]rm, concentration of ethanol in a feed, mass % g, exponent in kinetic equation of crystal growth, eq 5 G, linear growth rate of crystals, m/s Gi, individual linear growth rate of a single crystal, m/s h, height of draft tube element, m H, height of laboratory crystallizer, m i, coefficient of sensitivity of nucleation in respect to crystal growth, eqs 68 j, coefficient of sensitivity of nucleation in respect to crystal magma concentration (MT), eq 8 k, coefficient of sensitivity of nucleation in respect to stirrer speed (N), eq 8 kBG, combined kinetic constant in eq 8, 1/(m3 s) kG, kinetic constant in crystal growth equation, eq 5, m/s kN, kinetic constant in nucleation equation, eq 4, 1/(m3 s) kNG, combined kinetic constant in eq 6, 1/(miþ3 s1i) kv, volumetric shape factor of crystal L, characteristic size of crystal, m Ld, dominant crystal size, m Li, mean size of ith crystal fraction, m Lm, mean size of crystal population, m Lmax, maximal size of crystal population, m L50, median size of crystal population (corresponding to 50% cumulative weight undersize fraction), m ΔLi, size range width of ith crystal fraction, m [LAA]eq, equilibrium concentration (solubility) of LAA, mass % [LAA]ml, concentration of LAA in a mother solution, mass % [LAA]rm, concentration of LAA in a feed, mass % MT, suspension density (mass of crystals per unit volume of suspension), kgcryst/m3 mi, mass of ith crystal fraction, kg n, population density (number of crystals within the defined size range ΔL divided by this size range width per unit volume of suspension), 1/(m m3) n, exponent in kinetic equation of nucleation, eq 4 ni, population density of ith crystal fraction, 1/(m m3) n0, population density of nuclei (zerosize crystals), 1/(m m3) N, stirrer speed, 1/s qv, volumetric outflow rate of crystal suspension, m3/s R2, correlation coefficient t, process time, s T, temperature, K Tcr, process temperature, K Teq, feed saturation temperature, K Trm, feed solution temperature, K Vi, volume of ith crystal fraction, m3 Vt, total volume of laboratory crystallizer, m3 Vw, working volume of laboratory crystallizer, m3
’ GREEK LETTERS F, crystal density, kg/m3 1564
dx.doi.org/10.1021/cg101521k |Cryst. Growth Des. 2011, 11, 1557–1565
Crystal Growth & Design
ARTICLE
Fsat, density of saturated solution, kg/m3 τ, mean residence time of suspension in a crystallizer working volume, defined as Vw/qv, s
’ REFERENCES (1) Asard, H.; May, J. M.; Smirnoff, N. Vitamin C. Functions and Biochemistry in Animals and Plants; Garland Science, Routledge, 2004. (2) Linster, C. L.; Van Schaftigen, E. FEBS J. 2007, 274, 1–22. (3) Boudrant, J. Enzyme Microb. Technol. 1990, 12, 322–329. (4) Snajdman, L. O. Proizvodstvo Vitaminov; Piscevaja promyslennost, Moskva, 1973 (in Russian). (5) Matynia, A.; Wierzbowska, B. Przem. Chem. 1986, 65, 613–615 (in Polish). (6) Matynia, A.; Wierzbowska, B.; Bechtold, Z.; Kozak, E. In Proceedings of the 14th Symposium on Industrial Crystallization; Institution of Chemical Engineers: UK, 1999; CD-ROM No. 0090. (7) Freitas, A. M. B.; Giulietti, M. In Proceedings of the 15th Symposium on Industrial Crystallization; Chianese, A., Ed.; Chemical Engineering Transactions: AIDIC: Milano, Italy, 2002; Vol. 1, CDROM No. 232. (8) Wierzbowska, B.; Koralewska, J.; Piotrowski, K.; Matynia, A.; Wawrzyniecki, K. Chem. Proc. Eng. 2007, 28, 475–484. (9) Matynia, A.; Wierzbowska, B.; Szewczyk, E. Pol. J. Chem. Technol. 2000, 2 (1), 14–18. (10) Omar, W.; Mohnicke, M.; Ulrich, J. In Workshop on Advance in Sensoring in Industrial Crystallization WASIC 2003; Bulutcu, N.; G€urb€uz, H.; Ulrich, J., Eds.; Istanbul Technical University: Istanbul, Turkey, 2003; pp 113120. (11) Omar, W.; Ulrich, J. In 10th BIWIC 2003; Coquerel, G., Ed.; Universite de Rouen: France, 2003; pp 311318. (12) Omar, W. Chem. Eng. Technol. 2006, 29 (1), 119–123. (13) Omar, W.; Ulrich, J. Cryst. Res. Technol. 2006, 41 (5), 431–436. (14) Wierzbowska, B.; Matynia, A.; Piotrowski, K.; Koralewska, J. Chem. Eng. Proc. 2007, 46, 351–359. (15) Piotrowski, K.; Wierzbowska, B.; Koralewska, J.; Matynia, A. Pol. J. Chem. Technol. 2006, 8 (4), 23–26. (16) Wierzbowska, B.; Piotrowski, K.; Koralewska, J.; Matynia, A. Chem. Biochem. Eng. Q. 2008, 22 (3), 327–337. (17) Wierzbowska, B.; Piotrowski, K.; Koralewska, J.; Matynia, A. Pol. J. Chem. Technol. 2008, 10 (1), 60–65. (18) Smirnov, N. J.; Streltsov, V. V.; Rumiancev, M. B.; Zarucki, V. V. Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol. 1975, 18, 818–824 (in Russian). (19) Wierzbowska, B.; Piotrowski, K.; Koralewska, J.; Matynia, A.; Hutnik, N.; Wawrzyniecki, K. Cryst. Res. Technol. 2008, 43 (4), 381–389. (20) Wierzbowska, B.; Piotrowski, K.; Koralewska, J.; Hutnik, N.; Matynia, A. In: Proc. 17th International Symposium on Industrial Crystallization; Jansen, J. P.; Ulrich, J., Eds.; Maastricht, Netherlands, 2008; pp 787789. (21) Wierzbowska, B.; Koralewska, J.; Piotrowski, K.; Hutnik, N.; Matynia, A. Chem. Proc. Eng. 2008, 29, 345–360. (22) Wierzbowska, B.; Piotrowski, K.; Hutnik, N.; Matynia, A. Chem. Proc. Eng. 2008, 29, 1083–1094. (23) Matynia, A.; Wierzbowska, B. Przem. Chem. 1989, 68 (2), 81–83 (in Polish). (24) Mullin, J. W. Crystallization: ButterworthHeinemann, Oxford, 1992. (25) Randolph, A. D.; Larson, M. A. Theory of Particulate Processes: Analysis and Techniques of Continuous Crystallization; Academic Press: New York, 1988. (26) Mersmann, A., Ed. Crystallization Technology Handbook; Marcel Dekker: New York, 1995. (27) Wierzbowska, B.; Hutnik, N.; Koralewska, J.; Matynia, A.; Piotrowski, K. In_z. Ap. Chem. 2009, 48 (4), 132–133 (in Polish). (28) Wierzbowska, B.; Piotrowski, K.; Hutnik, N.; Matynia, A. In_z. Ap. Chem. 2009, 48 (3), 161–165 (in Polish). 1565
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