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Solvent crystallization of palm based dihydroxystearic acid with isopropyl alcohol: Effects of solvent quantity and concentration on particle size distribution, crystal habit and morphology, and resultant crystal purity
Industrial Crops and Products 34 (2011) 1135–1140
Contents lists available at ScienceDirect
Industrial Crops and Products journal homepage: www.elsevier.com/locate/indcrop
Solvent crystallization of palm based dihydroxystearic acid with isopropyl alcohol: Effects of solvent quantity and concentration on particle size distribution, crystal habit and morphology, and resultant crystal purity Gregory F.L. Koay a,b , Teong-Guan Chuah a,c,∗ , Sumaiya Zainal-Abidin b,d , Salmiah Ahmad b,e , Thomas S.Y. Choong a a
Department of Chemical and Environmental Engineering, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor Darul Ehsan, Malaysia Advanced Oleochemical Technology Division, Malaysian Palm Oil Board, 43650 Bandar Baru Bangi, Selangor Darul Ehsan, Malaysia c Institute of Tropical Forestry and Forest Products, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor Darul Ehsan, Malaysia d Faculty of Chemical and Natural Resources Engineering, Universiti Malaysia Pahang, 25000 Kuantan, Pahang Darul Makmur, Malaysia e Director General’s Office, Malaysian Rubber Board, 50450 Kuala Lumpur, Malaysia b
a r t i c l e
i n f o
Article history: Received 20 January 2011 Received in revised form 25 March 2011 Accepted 29 March 2011 Available online 4 May 2011 Keywords: Dihydroxystearic acid Solvent crystallization Simultaneous batch crystallizer Functional ingredient Process optimization
a b s t r a c t Crude dihydroxystearic acid was prepared from palm based oleic acid and was then solvent purified with isopropyl alcohol in a custom fabricated simultaneous batch crystallizer unit. The crystallized dihydroxystearic acid was a functional ingredient that acted as multipurpose intermediate for synthesis of various fine chemicals, cosmetics and personal care products. The effects of solvent quantity and concentration on particle size distribution, crystal habit and morphology, and resultant crystal purity were studied. The crystals were purer but smaller and the span of the distribution curve was wider at higher solvent quantity and concentration. Through scanning electron microscopy and X-ray diffraction, it was observed that the crystals agglomerated into plate-like (flaky) habit with triclinic crystal structure. Solvent crystallization with 80% IPA at 20 ◦ C and solute:solvent ratio of 1.0:1.0 was the most optimized and efficient, producing -DHSA crystals that has high resistance against fat exudation during vacuum filtration process. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Palm based 9,10-dihydroxystearic acid (DHSA) is a type of hydroxyl fatty acid that, due to its special structure, has special functionality and thus is poised as a multipurpose intermediate for synthesis of various fine chemicals, cosmetics and personal care products. However, crude DHSA is irritating to human skin (Awang et al., 2001), unsuitable for use in consumer products. Traces of chemicals used as precursors and reactants during its synthesis and thereafter adhered to its crystals rendered crude DHSA to cause dermal irritancy. The safety characteristic and/or requirement of DHSA and its potential as a functional ingredient in various consumer products have been summarized by Koay et al. (2011). Purification through solvent crystallization is required to produce nonirritant DHSA and Siwayanan
∗ Corresponding author at: Department of Chemical and Environmental Engineering, Faculty of Engineering, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor Darul Ehsan, Malaysia. Tel.: +60 389466288; fax: +60 386567120. E-mail addresses: [email protected], [email protected] (T.-G. Chuah). 0926-6690/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.indcrop.2011.03.030
et al. (2004) reported that the potency in causing dermal irritancy was nullified when DHSA crystals were purified to 80% purity. Particle size distribution (PSD) plays an important role in overall desirability of DHSA crystals. PSD asserts strong influence during filtration process where mother liquor and embedded impurities are removed (Abidin et al., 2009; Sumaiya et al., 2010; Koay et al., 2011) and is a key parameter during handling of resultant DHSA crystal powder. Uniformity in particle size is important as there is great demand for crystals with narrow PSD curve. These crystals have good storage and transportation properties, free flowing nature and pleasant appearance. Crystals with unimodal distribution are desirable within purification and separation process because they improve downstream processing efficiency and total separation economics (Mullin, 2001; Sumaiya et al., 2007, 2010; Abidin et al., 2009). For consumer products application, DHSA crystals with unimodal distribution warrant for direct inclusion into functional blends, especially into powdered, beaded and prilled products, eliminating the needs for double handling. Thus, crystallization processes that results in narrow PSD curve are highly favorable.
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An internal reference trial study established that solvent crystallization of DHSA with isopropyl alcohol (IPA) was best carried out with 70 and 80% IPA at final temperature 20 ◦ C. This study aims to investigate effects of (i) solvent quantity and (ii) solvent concentration on its (a) PSD, (b) crystal habit and morphology and (c) resultant crystal purity. This information is important for process control and optimization in the large scale production of DHSA crystals of desired yield and properties to meet the needs from the functional ingredients’ perspective and demands from the fine chemicals, cosmetics and personal care products industry.
2.4. X-ray diffraction (XRD) analysis XRD analysis was carried out as per published procedure (Oh, 1989; Koay et al., 2009) to determine the morphology of DHSA crystals using Enraf Nonius X-ray Generator FR590 and FR522 Guinier Camera. The DHSA crystal samples were analyzed at room temperature with three-compartment cell using standard sample holder. Kodak Scientific Imaging Film was exposed to capture the XRD patterns and the spacing bands shown on the film were measured with Enraf Nonius viewer under illuminated magnification. XRD readings were interpreted according to Bragg’s Law (Eq. (1)). n = 2d × sin
2. Materials and methods 2.1. Preparation of purified DHSA crystals Purified DHSA crystals were prepared as per published procedure (Koay et al., 2009) in a custom fabricated simultaneous batch crystallizer unit. Crude DHSA was melted in oven preset at 80 ◦ C for 8 h. The molted crude DHSA was mixed with IPA (70 and 80% concentration) in static 3.5 in 600 ml glass beakers. The concoctions were left for cooling for crystallization to occur. The crystallization period was 720 min, cooling rate was −0.6 ◦ C min−1 and final temperature was 20 ◦ C. The solvent quantity (expressed as crude DHSA to IPA (w/w) ratio, thereafter referred as DHSA:IPA ratio) varied between 2.0:1.0, 1.5:1.0, 1.0:1.0, 1.0:1.5, and 1.0:2.0. No significant temperature gradient was observed in the concoctions and DHSA crystals started to form at the bottom of the crystallization vessels. Purified DHSA crystals were segregated from mother liquor through vacuum filtration. The entire concoction was filtered. The segregated crystals were collected and dried in oven preset at 50 ◦ C for 48 h. 2.2. Particle size distribution (PSD) analysis PSD analysis was carried out to measure the distribution pattern of DHSA crystals. The analysis was carried out as per published procedure (Abidin et al., 2009) using Malvern Instruments Mastersizer 2000 and Hydro 2000S particle size analyzer. The equipment was set to process particle size of 0.020–2000.000 m. Mie-Scattering analysis model was used for diffraction pattern measurement. The reflective index of DHSA was 1.520 while for water, the dispersant, was 1.330. The Mastersizer was set to record background measurement for 20 s followed by sample measurement for 20 s. DHSA crystals were then inserted to a designated vessel where they were introduced to the dispersion module until an obscuration level of 10.0–20.0 was achieved. Three measurements were done for each sample and the average value was taken as the final result. The crystals’ specific surface area, surface weighted- and volume weighted-mean diameters were determined along with d(0.1)-, d(0.5)- and d(0.9)-cumulative undersize. 2.3. Scanning electron microscopy (SEM) analysis SEM analysis was carried out as per published procedure (Sumaiya et al., 2007; Koay et al., 2009) using Quanta 400 and EDAX (GENESIS 7000) scanning electron microscope. It was carried out to observe the habit of DHSA crystals formed under different conditions. The DHSA crystal samples were mounted on a metal stub and coated under near vacuum condition (in an ambient of argon gas) with very thin layer of gold so that they could conduct electricity and be illuminated by electrons. The crystals were photomicrographed at 1000× magnification.
(1)
where n is integer; is wavelength (Å); d is inter-atomic spacing (Å); is diffraction angle (◦ ). 2.5. Fourier transform infra red (FTIR) analysis FTIR analysis was carried out as per published procedure (Koay et al., 2006) to determine the functional group(s) in DHSA using Nicolet Magna 550 series II FTIR spectrophotometer. A potassium bromide (KBr) pellet was used to determine the background signal. Since DHSA crystals were in powder form, they were mixed with KBr powder to form a pellet before being inserted into sample compartment to undergo 64 scans. The spectral range (wave-numbers) for the spectrophotometer was 4000–400 cm−1 and the final output format was in percent transmittance. 2.6. Gas chromatography (GC) analysis GC analysis was carried out to measure the purity of DHSA crystals. The analysis, using 9,10-threo-DHSA standard for comparison, was carried out as per published procedure (Awang et al., 2001; Koay et al., 2006; Sumaiya et al., 2007; Abidin et al., 2009) using Hewlett-Packard HP-6860A Plus gas chromatograph. The TMS derivatives of DHSA were prepared by weighing 0.01 g of sample into a vial and 2 ml of GC-grade N,N-dimethylformamide and N,O-bis-trimethylsilyl acetamide were added. The mixture was vortexed for 30 s and incubated at 60 ◦ C for 30 min. It was cooled for a few minutes, 1.5 ml of it transferred into a 2 ml clear wide opening crimp vial laced with sodium sulphate anhydrous, sealed, then injected into the GC equipment. The TMS derivatives of DHSA were separated on a non-polar column, HP-5 (Hewlett-Packard, 30 m × 0.25 mm × 0.25 m) with helium as carrier gas. The oven was programmed to hold at 150 ◦ C for 1 min, followed by ramping from 150 to 290 ◦ C at 10 ◦ C min−1 . The final temperature was held at 290 ◦ C for 30 min. The injector and flame-ionization detector were set at 300 ◦ C. 3. Results and discussion 3.1. Effects on particle size distribution The PSD curves for DHSA crystals formed under different DHSA:IPA ratios (1.5:1.0, 1.0:1.0 and 1.0:1.5) and IPA concentrations (70 and 80% IPA) are shown in Fig. 1. All DHSA:IPA ratio to IPA concentration combinations formed bell-shaped curves. The PSD span was determined by standard deviation, s (Eq. (2) and Eq. (3)) and tabulated in Table 1. Higher IPA quantity (lower DHSA:IPA ratio) resulted in lower median particle size, lower volume weighted diameter, D and broader PSD span. Higher IPA concentration resulted in similar trend. Water, the co-component in the solvent, acted as binder that facilitated crystal cluster formation, somewhat similar to the role of crystal nuclei. IPA in the other hand was a solvent that countered crystal cluster formation. Thus, at higher IPA quantity and concentration (tantamount
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Table 1 DHSA crystal PSD curve standard deviation. DHSA:IPA (ratio)
IPA concentration (%)
Averaged median particle size (m)
1.5:1.0
70 80 70 80 70 80
470.535 464.520 456.378 430.749 397.129 381.054
1.0:1.0 1.0:1.5
± ± ± ± ± ±
10 10 10 10 10 10
to lower water content), more relatively smaller crystal clusters were present when the crystallization process was terminated at 720 min. Nevertheless, regardless of DHSA:IPA ratios and IPA concentrations, the PSD spans were still quite similar, as indicated by only very slight differences in s values.
m 1 2 s= (Xi − X) ki n
(2)
i=1
m X¯ =
X i=1 i n
(3)
where s is standard deviation; n is total volume percent (100); Xi is average mesh size; i is integer; ki is volume percent. There were only slight differences in the PSD of crystallization at 20 ◦ C with preset cooling rate −0.6 ◦ C min−1 for the different DHSA:IPA ratio to IPA concentration combinations. Had it been rapid cooling, the PSD would vary distinctively. Rapid cooling produces massive quantity of small nuclei. These nuclei increase agglomeration rate. The PSD curve would skew both left and rightward along the X-axis (Davey and Garside, 2002). Rapid cooling pushes ‘would be’ crystals in DHSA-IPA concoction from meta-stable to unstable state prematurely. The PSD would vary distinctively and have very different s values. Thus, crystallization process at cooling rate −0.6 ◦ C min−1 was operating within threshold of slow cooling mode.
D (m)
Xi (m)
s (m)
506.456 502.978 496.130 474.872 445.986 423.173
507.612 504.133 497.273 475.960 447.011 424.140
1773.847 1774.277 1775.144 1778.004 1782.288 1785.998
3.2. Effects on crystal habit and morphology The SEM images for DHSA crystals with 80% IPA are shown in Fig. 2. Concoction with lesser IPA resulted in larger crystals; concoction with more IPA resulted in smaller crystals. However, regardless of DHSA:IPA ratios, the size differences were small (<100 m). The crystal ‘lumps’ from 1.0:1.5 ratio were more compact than those from 1.5:1.0 ratio. The more compact the crystal ‘lumps’ were, the smaller the DHSA crystals, in line with earlier PSD analysis. Regardless of DHSA:IPA ratios, DHSA crystals showed platelike (flaky) habit that grew in a layer-by-layer fashion. Similar findings were reported earlier – plate-like DHSA crystals for IPA solvent crystallization; spherical-like DHSA crystals for ethanol solvent crystallization (Koay et al., 2009). The flower-like structure was similar to that of desert rose structure occurred among gypsum and barite crystals in arid areas. The desert rose were semi-spherical radiating rose-like rosette aggregates of platy barite crystals (Sunagawa, 2005). At initial glance, DHSA crystals appeared to pile up in unorganized manner. At closer evaluation, the crystal arrays were actually organized, periodically repeating themselves three dimensionally, as expected in all crystals. These platelike flakes had same crystallographic axis as those of basal crystals. The FTIR and XRD analysis results (Fig. 3 and Table 2) are in tandem with SEM analysis. FTIR spectrum for both crude and purified DHSA showed absorption bands at 1780–1620 cm−1 (carboxyl functional group), 3400–3200 cm−1 (hydroxyl functional group) and 3000–2800 cm−1 (C–H bonding). There were no differences in the spacing values between XRD spectra or diffraction patterns (intensity) in crude and all purified DHSA crystals. Crude and purified DHSA shared the same crystal structure. No polymorphic modification and crystal structure alteration took place in DHSA during crystallization. Both crude and purified DHSA crys-
Table 2 DHSA crystal XRD readings. Crude DHSA Spacing Long spacing 9.82 6.48 Short spacing 5.43 5.05 4.77 4.50 4.24 4.07 3.88 3.58 2.85 Fig. 1. PSD curves for different DHSA:IPA ratios at (a) 70% IPA and (b) 80% IPA.
Typical for purified DHSA Intensity Medium Weak Weak Medium Weak Strong Weak Strong Weak Strong Weak
Spacing Long spacing 9.82 6.48 Short spacing 5.43 5.05 4.77 4.50 4.24 4.07 3.88 3.58 2.85
Polymorphic form: strong  (and possibility of weak  presence).
Intensity Medium Weak Weak Medium Weak Strong Weak Strong Weak Strong Weak
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tals were of beta () polymorphic form (triclinic crystal structure). This supported earlier argument that the crystallization process was operating within threshold of slow cooling mode. For rapid cooling, some alpha (␣) polymorphic crystals, with lower melting point and heat stability, might form. Future research should look into the effects of different cooling modes, including temperature at extremity, on dynamic evolution of DHSA crystal habit and morphology. The formation of -DHSA crystals was necessary to avoid and/or to minimize fat exudation (the squeezing through phenomenon during filtration process, a phenomenon still considered quite poorly understood). It is only known that fat exudation might be caused by, but not limited to, the presence of unstable crystals (e.g. ␣-crystals) and/or by the presence of far too many small crystals.
3.3. Effects on crystal purity
Fig. 2. SEM images of DHSA crystals for 80% IPA with different DHSA:IPA ratios (a) 1.5:1.0, (b) 1.0:1.0 and (c) 1.0:1.5.
The purity of DHSA crystals formed under different DHSA:IPA ratios (1.5:1.0, 1.0:1.0 and 1.0:1.5) and IPA concentrations (70 and 80% IPA) is shown in Fig. 4. The minimum targeted purity was 80% (Siwayanan et al., 2004; Sumaiya et al., 2007). Regardless of IPA concentration, DHSA crystal purities from 1.5:1.0 ratio were well below 80%. By extrapolation, even 90% IPA would not result in DHSA crystals with 80% purity as inadequate solvent was present. DHSA was a fatty acid, solid at room temperature. Molted crude DHSA was highly viscous. When added with IPA, the concoction viscosity reduced. For 1.5:1.0 ratio (and all other combinations with higher DHSA content), inadequate IPA was present to dilute the concoction for higher diffusivity (which would lead to better selectivity and phase separation during crystallization) (Krishnamurthy and Kellens, 1996) and thus the failure to sufficiently remove the impurities in crude DHSA. For 70 and 80% IPA, DHSA crystal purity for 1.0:1.0 ratio was 80.54 and 86.26%; the purity for 1.0:1.5 ratio was 82.51 and 87.79%. DHSA crystals from 1.0:1.5 ratio and 70% IPA required 1050 g IPA for every 1000 g of crude; DHSA crystals from 1.0:1.0 ratio and 80% IPA required 800 g IPA for every 1000 g of crude. However, the purity achieved by the latter was higher (Table 3). For 70% IPA, increasing IPA from 700 g to 1050 g, crystal purity improved 1.97%; for 80% IPA, increasing IPA from 800 g to 1200 g, crystal purity improved merely 1.53%. Solvent crystallization of DHSA with DHSA:IPA 1.0:1.0 ratio and 80% IPA was preferred for its higher reliability (compared to DHSA:IPA 1.0:1.0, 70% IPA) and economical competitiveness (compared to DHSA:IPA 1.0:1.5, 80% IPA). It commands higher level of confidence in consistently producing DHSA crystals with purity higher than 80% at acceptable solvent usage level. Increment in total solvent (by quantity and concentration) should result in higher DHSA crystal purity. However, the filtration step posed as a limiting phase as it was standardized for all combinations. During filtration, for concoction with 1.0:1.0 ratio, most mother liquor was removed before the crystals settled down and formed a wet cake; for concoction with 1.0:1.5 ratio (and all other combinations with higher IPA content), a wet cake was formed before most mother liquor was removed. This caused some mother liquor (with impurities) to re-include and re-embed into DHSA crystals. The mother liquor adsorbed within the crystals and/or remained between individual crystals (Sun, 1971; Saito et al., 2000). The dropout of the 1.0:1.5 ratio was mainly attributed to filtration and this was not foreseen. After all, crystallization was more of an art than a science, developed through years of experience (Hartel, 1991) and thus, future research should consider and explore this factor.
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Fig. 3. FTIR spectrum for (a) crude DHSA and (b) purified DHSA crystals. Table 3 Solvent quantity and concentration versus resultant DHSA crystal attainable purity (reference basis: 1000 g crude DHSA). IPA concentration
DHSA:IPA 1.0:1.0
70% 80% Increment
(g) (%)
DHSA:IPA 1.0:1.5
Increment
Solvent
IPA
Purity
Solvent
IPA
Purity
IPA
(g)
(g)
(%)
(g)
(g)
(%)
(g)
(%)
(%)
1000 1000
700 800
80.54 86.26
1500 1500
1050 1200
82.51 87.79
350 400
50.00 50.00
1.97 1.53
– –
100 14.29
– 5.72
– –
– 5.28
– –
– –
– –
150 14.29
Purity
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full scale industrial mass production of DHSA crystals to meet the needs from the functional ingredients’ perspective and demands from the fine chemicals, cosmetics and personal care products industry. References
Fig. 4. Purity of DHSA crystals formed under different DHSA:IPA ratios and IPA concentrations.
4. Conclusion The study on effects of solvent quantity and concentration was successfully carried out by solvent crystallizing palm based crude DHSA with IPA under five different DHSA:IPA ratios and two different IPA concentrations. Higher solvent quantity resulted in smaller purified DHSA crystal median particle size and the broadening of the bell-shaped PSD curve. SEM images revealed that DHSA crystals formed in higher IPA quantity were more compact and smaller than those formed in lower IPA quantity. Regardless of crystallizing conditions, DHSA crystals formed were of ‘desert rose’ conformation (plate-like flakes that grew in layer-by-layer fashion which eventually formed semi-spherical rosette aggregates). Total solvent was directly correlated to attainable purity of DHSA crystals. The current study has shown that crystallization of DHSA with IPA as solvent was most optimized and efficient when carried out with 80% IPA at 20 ◦ C and DHSA:IPA 1.0:1.0 ratio, commanding the highest level of confidence in consistently producing DHSA crystals with purity higher than 80% at acceptable solvent usage level. The -DHSA crystals formed were of the most desired as they are known to be highly resistant against fat exudation. This study has also shown that the filtration step was a limiting phase. To boost DHSA crystal attainable purity, more efficient filtration is needed. Further study for optimized filtration step is a precursor to
Abidin, S.Z., Ling, G.K.F., Abdullah, L.C., Ahmad, S., Yunus, R., Choong, T.S.Y., 2009. Effects of temperature and cooling modes on yield, purity and particle size distribution of dihydroxystearic acid crystals. Eur. J. Sci. Res. 33, 471–479. Awang, R., Ahmad, S., Kang, Y.B., Ismail, R., 2001. Characterization of dihydroxystearic acid from palm oleic acid. J. Am. Oil Chem. Soc. 78, 1249–1252. Davey, R., Garside, J., 2002. From Molecules to Crystallizers – An Introduction to Crystallization. Oxford University Press, New York. Hartel, R.W., 1991. Crystallization process. Manuf. Confectioner 71, 61–66. Koay, G.F.L., Abdullah, L.C., Yunus, R., Choong, S.Y.T., Siwayanan, P., Salmiah, A., 2006. Crystallization of dihydroxystearic acid (DHSA) produced from commercial grade palm oil based crude oleic acid employing isopropyl alcohol as solvent. Int. J. Eng. Technol. 3, 115–124. Koay, G.F.L., Guan, C.T., Zainal-Abidin, S., Ahmad, S., Choong, T.S.Y., 2009. Habit and morphology study on the palm-based 9,10-dihydroxystearic acid (DHSA) crystals. Mater. Chem. Phys. 114, 14–17. Koay, G.F.L., Chuah, T.-G., Zainal-Abidin, S., Ahmad, S., Choong, T.S.Y., 2011. Development, characterization and commercial application of palm based dihydroxystearic acid and its derivatives: an overview. J. Oleo Sci., in press. Krishnamurthy, R., Kellens, M., 1996. Fractionation and winterization. In: Hui, Y.H. (Ed.), Bailey’s Industrial Oils & Fats Products, Edible Oil & Fats Products: Processing Technology, vol. 4, fifth ed. John Wiley and Sons, New York, pp. 301–338. Mullin, J.W., 2001. Crystallization, fourth ed. Elsevier Butterworth-Heinemann, Oxford. Oh, F.C.H., 1989. X-ray diffraction study on palm oil and palm stearin polymorphism. In: Reports for Experiments Carried Out as a Visiting Scientist at Department of Food Science. University of Guelph, Ontario, Canada, pp. 1–27. Saito, N., Yokota, M., Fujiwara, T., Kubota, N., 2000. Liquid inclusions in crystals produced in suspension crystallization. Chem. Eng. J. 79, 53–59. Siwayanan, P., Shaari, N.Z.K., Maurad, Z., Aziz, H.A., Hassan, H.A., Ahmad, S., Awang, R., 2004. Scale-up production and purification of palm oil-based dihydroxystearic acid (DHSA). In: Proc. 18th SOMChE 294–300. Sumaiya, Z.A., Koay, G.F.L., Chuah, T.G., Parthiban, S., Robiah, Y., Salmiah, A., Choong, T.S.Y., 2007. Effect of cooling modes on solvent crystallization of palm oil based dihydroxystearic acid using ethanol. J. Appl. Sci. 7, 2222–2225. Sumaiya, Z.A., Chuah, A.L., Koay, G.F.L., Salmiah, A., Choong, T.S.Y., 2010. Effect of solvent concentration and cooling modes on morphology, particle size distribution, and yield of dihydroxystearic acid (DHSA) crystals. Part. Sci. Technol. 28, 236–246. Sun, Y.-C., 1971. Water: key to new crystallization process for purifying organics. Chem. Eng. 78, 87–90. Sunagawa, I., 2005. Crystals – Growth, Morphology and Perfection. Cambridge University Press, Cambridge.
Contents lists available at ScienceDirect
Industrial Crops and Products journal homepage: www.elsevier.com/locate/indcrop
Solvent crystallization of palm based dihydroxystearic acid with isopropyl alcohol: Effects of solvent quantity and concentration on particle size distribution, crystal habit and morphology, and resultant crystal purity Gregory F.L. Koay a,b , Teong-Guan Chuah a,c,∗ , Sumaiya Zainal-Abidin b,d , Salmiah Ahmad b,e , Thomas S.Y. Choong a a
Department of Chemical and Environmental Engineering, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor Darul Ehsan, Malaysia Advanced Oleochemical Technology Division, Malaysian Palm Oil Board, 43650 Bandar Baru Bangi, Selangor Darul Ehsan, Malaysia c Institute of Tropical Forestry and Forest Products, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor Darul Ehsan, Malaysia d Faculty of Chemical and Natural Resources Engineering, Universiti Malaysia Pahang, 25000 Kuantan, Pahang Darul Makmur, Malaysia e Director General’s Office, Malaysian Rubber Board, 50450 Kuala Lumpur, Malaysia b
a r t i c l e
i n f o
Article history: Received 20 January 2011 Received in revised form 25 March 2011 Accepted 29 March 2011 Available online 4 May 2011 Keywords: Dihydroxystearic acid Solvent crystallization Simultaneous batch crystallizer Functional ingredient Process optimization
a b s t r a c t Crude dihydroxystearic acid was prepared from palm based oleic acid and was then solvent purified with isopropyl alcohol in a custom fabricated simultaneous batch crystallizer unit. The crystallized dihydroxystearic acid was a functional ingredient that acted as multipurpose intermediate for synthesis of various fine chemicals, cosmetics and personal care products. The effects of solvent quantity and concentration on particle size distribution, crystal habit and morphology, and resultant crystal purity were studied. The crystals were purer but smaller and the span of the distribution curve was wider at higher solvent quantity and concentration. Through scanning electron microscopy and X-ray diffraction, it was observed that the crystals agglomerated into plate-like (flaky) habit with triclinic crystal structure. Solvent crystallization with 80% IPA at 20 ◦ C and solute:solvent ratio of 1.0:1.0 was the most optimized and efficient, producing -DHSA crystals that has high resistance against fat exudation during vacuum filtration process. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Palm based 9,10-dihydroxystearic acid (DHSA) is a type of hydroxyl fatty acid that, due to its special structure, has special functionality and thus is poised as a multipurpose intermediate for synthesis of various fine chemicals, cosmetics and personal care products. However, crude DHSA is irritating to human skin (Awang et al., 2001), unsuitable for use in consumer products. Traces of chemicals used as precursors and reactants during its synthesis and thereafter adhered to its crystals rendered crude DHSA to cause dermal irritancy. The safety characteristic and/or requirement of DHSA and its potential as a functional ingredient in various consumer products have been summarized by Koay et al. (2011). Purification through solvent crystallization is required to produce nonirritant DHSA and Siwayanan
∗ Corresponding author at: Department of Chemical and Environmental Engineering, Faculty of Engineering, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor Darul Ehsan, Malaysia. Tel.: +60 389466288; fax: +60 386567120. E-mail addresses: [email protected], [email protected] (T.-G. Chuah). 0926-6690/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.indcrop.2011.03.030
et al. (2004) reported that the potency in causing dermal irritancy was nullified when DHSA crystals were purified to 80% purity. Particle size distribution (PSD) plays an important role in overall desirability of DHSA crystals. PSD asserts strong influence during filtration process where mother liquor and embedded impurities are removed (Abidin et al., 2009; Sumaiya et al., 2010; Koay et al., 2011) and is a key parameter during handling of resultant DHSA crystal powder. Uniformity in particle size is important as there is great demand for crystals with narrow PSD curve. These crystals have good storage and transportation properties, free flowing nature and pleasant appearance. Crystals with unimodal distribution are desirable within purification and separation process because they improve downstream processing efficiency and total separation economics (Mullin, 2001; Sumaiya et al., 2007, 2010; Abidin et al., 2009). For consumer products application, DHSA crystals with unimodal distribution warrant for direct inclusion into functional blends, especially into powdered, beaded and prilled products, eliminating the needs for double handling. Thus, crystallization processes that results in narrow PSD curve are highly favorable.
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G.F.L. Koay et al. / Industrial Crops and Products 34 (2011) 1135–1140
An internal reference trial study established that solvent crystallization of DHSA with isopropyl alcohol (IPA) was best carried out with 70 and 80% IPA at final temperature 20 ◦ C. This study aims to investigate effects of (i) solvent quantity and (ii) solvent concentration on its (a) PSD, (b) crystal habit and morphology and (c) resultant crystal purity. This information is important for process control and optimization in the large scale production of DHSA crystals of desired yield and properties to meet the needs from the functional ingredients’ perspective and demands from the fine chemicals, cosmetics and personal care products industry.
2.4. X-ray diffraction (XRD) analysis XRD analysis was carried out as per published procedure (Oh, 1989; Koay et al., 2009) to determine the morphology of DHSA crystals using Enraf Nonius X-ray Generator FR590 and FR522 Guinier Camera. The DHSA crystal samples were analyzed at room temperature with three-compartment cell using standard sample holder. Kodak Scientific Imaging Film was exposed to capture the XRD patterns and the spacing bands shown on the film were measured with Enraf Nonius viewer under illuminated magnification. XRD readings were interpreted according to Bragg’s Law (Eq. (1)). n = 2d × sin
2. Materials and methods 2.1. Preparation of purified DHSA crystals Purified DHSA crystals were prepared as per published procedure (Koay et al., 2009) in a custom fabricated simultaneous batch crystallizer unit. Crude DHSA was melted in oven preset at 80 ◦ C for 8 h. The molted crude DHSA was mixed with IPA (70 and 80% concentration) in static 3.5 in 600 ml glass beakers. The concoctions were left for cooling for crystallization to occur. The crystallization period was 720 min, cooling rate was −0.6 ◦ C min−1 and final temperature was 20 ◦ C. The solvent quantity (expressed as crude DHSA to IPA (w/w) ratio, thereafter referred as DHSA:IPA ratio) varied between 2.0:1.0, 1.5:1.0, 1.0:1.0, 1.0:1.5, and 1.0:2.0. No significant temperature gradient was observed in the concoctions and DHSA crystals started to form at the bottom of the crystallization vessels. Purified DHSA crystals were segregated from mother liquor through vacuum filtration. The entire concoction was filtered. The segregated crystals were collected and dried in oven preset at 50 ◦ C for 48 h. 2.2. Particle size distribution (PSD) analysis PSD analysis was carried out to measure the distribution pattern of DHSA crystals. The analysis was carried out as per published procedure (Abidin et al., 2009) using Malvern Instruments Mastersizer 2000 and Hydro 2000S particle size analyzer. The equipment was set to process particle size of 0.020–2000.000 m. Mie-Scattering analysis model was used for diffraction pattern measurement. The reflective index of DHSA was 1.520 while for water, the dispersant, was 1.330. The Mastersizer was set to record background measurement for 20 s followed by sample measurement for 20 s. DHSA crystals were then inserted to a designated vessel where they were introduced to the dispersion module until an obscuration level of 10.0–20.0 was achieved. Three measurements were done for each sample and the average value was taken as the final result. The crystals’ specific surface area, surface weighted- and volume weighted-mean diameters were determined along with d(0.1)-, d(0.5)- and d(0.9)-cumulative undersize. 2.3. Scanning electron microscopy (SEM) analysis SEM analysis was carried out as per published procedure (Sumaiya et al., 2007; Koay et al., 2009) using Quanta 400 and EDAX (GENESIS 7000) scanning electron microscope. It was carried out to observe the habit of DHSA crystals formed under different conditions. The DHSA crystal samples were mounted on a metal stub and coated under near vacuum condition (in an ambient of argon gas) with very thin layer of gold so that they could conduct electricity and be illuminated by electrons. The crystals were photomicrographed at 1000× magnification.
(1)
where n is integer; is wavelength (Å); d is inter-atomic spacing (Å); is diffraction angle (◦ ). 2.5. Fourier transform infra red (FTIR) analysis FTIR analysis was carried out as per published procedure (Koay et al., 2006) to determine the functional group(s) in DHSA using Nicolet Magna 550 series II FTIR spectrophotometer. A potassium bromide (KBr) pellet was used to determine the background signal. Since DHSA crystals were in powder form, they were mixed with KBr powder to form a pellet before being inserted into sample compartment to undergo 64 scans. The spectral range (wave-numbers) for the spectrophotometer was 4000–400 cm−1 and the final output format was in percent transmittance. 2.6. Gas chromatography (GC) analysis GC analysis was carried out to measure the purity of DHSA crystals. The analysis, using 9,10-threo-DHSA standard for comparison, was carried out as per published procedure (Awang et al., 2001; Koay et al., 2006; Sumaiya et al., 2007; Abidin et al., 2009) using Hewlett-Packard HP-6860A Plus gas chromatograph. The TMS derivatives of DHSA were prepared by weighing 0.01 g of sample into a vial and 2 ml of GC-grade N,N-dimethylformamide and N,O-bis-trimethylsilyl acetamide were added. The mixture was vortexed for 30 s and incubated at 60 ◦ C for 30 min. It was cooled for a few minutes, 1.5 ml of it transferred into a 2 ml clear wide opening crimp vial laced with sodium sulphate anhydrous, sealed, then injected into the GC equipment. The TMS derivatives of DHSA were separated on a non-polar column, HP-5 (Hewlett-Packard, 30 m × 0.25 mm × 0.25 m) with helium as carrier gas. The oven was programmed to hold at 150 ◦ C for 1 min, followed by ramping from 150 to 290 ◦ C at 10 ◦ C min−1 . The final temperature was held at 290 ◦ C for 30 min. The injector and flame-ionization detector were set at 300 ◦ C. 3. Results and discussion 3.1. Effects on particle size distribution The PSD curves for DHSA crystals formed under different DHSA:IPA ratios (1.5:1.0, 1.0:1.0 and 1.0:1.5) and IPA concentrations (70 and 80% IPA) are shown in Fig. 1. All DHSA:IPA ratio to IPA concentration combinations formed bell-shaped curves. The PSD span was determined by standard deviation, s (Eq. (2) and Eq. (3)) and tabulated in Table 1. Higher IPA quantity (lower DHSA:IPA ratio) resulted in lower median particle size, lower volume weighted diameter, D and broader PSD span. Higher IPA concentration resulted in similar trend. Water, the co-component in the solvent, acted as binder that facilitated crystal cluster formation, somewhat similar to the role of crystal nuclei. IPA in the other hand was a solvent that countered crystal cluster formation. Thus, at higher IPA quantity and concentration (tantamount
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Table 1 DHSA crystal PSD curve standard deviation. DHSA:IPA (ratio)
IPA concentration (%)
Averaged median particle size (m)
1.5:1.0
70 80 70 80 70 80
470.535 464.520 456.378 430.749 397.129 381.054
1.0:1.0 1.0:1.5
± ± ± ± ± ±
10 10 10 10 10 10
to lower water content), more relatively smaller crystal clusters were present when the crystallization process was terminated at 720 min. Nevertheless, regardless of DHSA:IPA ratios and IPA concentrations, the PSD spans were still quite similar, as indicated by only very slight differences in s values.
m 1 2 s= (Xi − X) ki n
(2)
i=1
m X¯ =
X i=1 i n
(3)
where s is standard deviation; n is total volume percent (100); Xi is average mesh size; i is integer; ki is volume percent. There were only slight differences in the PSD of crystallization at 20 ◦ C with preset cooling rate −0.6 ◦ C min−1 for the different DHSA:IPA ratio to IPA concentration combinations. Had it been rapid cooling, the PSD would vary distinctively. Rapid cooling produces massive quantity of small nuclei. These nuclei increase agglomeration rate. The PSD curve would skew both left and rightward along the X-axis (Davey and Garside, 2002). Rapid cooling pushes ‘would be’ crystals in DHSA-IPA concoction from meta-stable to unstable state prematurely. The PSD would vary distinctively and have very different s values. Thus, crystallization process at cooling rate −0.6 ◦ C min−1 was operating within threshold of slow cooling mode.
D (m)
Xi (m)
s (m)
506.456 502.978 496.130 474.872 445.986 423.173
507.612 504.133 497.273 475.960 447.011 424.140
1773.847 1774.277 1775.144 1778.004 1782.288 1785.998
3.2. Effects on crystal habit and morphology The SEM images for DHSA crystals with 80% IPA are shown in Fig. 2. Concoction with lesser IPA resulted in larger crystals; concoction with more IPA resulted in smaller crystals. However, regardless of DHSA:IPA ratios, the size differences were small (<100 m). The crystal ‘lumps’ from 1.0:1.5 ratio were more compact than those from 1.5:1.0 ratio. The more compact the crystal ‘lumps’ were, the smaller the DHSA crystals, in line with earlier PSD analysis. Regardless of DHSA:IPA ratios, DHSA crystals showed platelike (flaky) habit that grew in a layer-by-layer fashion. Similar findings were reported earlier – plate-like DHSA crystals for IPA solvent crystallization; spherical-like DHSA crystals for ethanol solvent crystallization (Koay et al., 2009). The flower-like structure was similar to that of desert rose structure occurred among gypsum and barite crystals in arid areas. The desert rose were semi-spherical radiating rose-like rosette aggregates of platy barite crystals (Sunagawa, 2005). At initial glance, DHSA crystals appeared to pile up in unorganized manner. At closer evaluation, the crystal arrays were actually organized, periodically repeating themselves three dimensionally, as expected in all crystals. These platelike flakes had same crystallographic axis as those of basal crystals. The FTIR and XRD analysis results (Fig. 3 and Table 2) are in tandem with SEM analysis. FTIR spectrum for both crude and purified DHSA showed absorption bands at 1780–1620 cm−1 (carboxyl functional group), 3400–3200 cm−1 (hydroxyl functional group) and 3000–2800 cm−1 (C–H bonding). There were no differences in the spacing values between XRD spectra or diffraction patterns (intensity) in crude and all purified DHSA crystals. Crude and purified DHSA shared the same crystal structure. No polymorphic modification and crystal structure alteration took place in DHSA during crystallization. Both crude and purified DHSA crys-
Table 2 DHSA crystal XRD readings. Crude DHSA Spacing Long spacing 9.82 6.48 Short spacing 5.43 5.05 4.77 4.50 4.24 4.07 3.88 3.58 2.85 Fig. 1. PSD curves for different DHSA:IPA ratios at (a) 70% IPA and (b) 80% IPA.
Typical for purified DHSA Intensity Medium Weak Weak Medium Weak Strong Weak Strong Weak Strong Weak
Spacing Long spacing 9.82 6.48 Short spacing 5.43 5.05 4.77 4.50 4.24 4.07 3.88 3.58 2.85
Polymorphic form: strong  (and possibility of weak  presence).
Intensity Medium Weak Weak Medium Weak Strong Weak Strong Weak Strong Weak
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tals were of beta () polymorphic form (triclinic crystal structure). This supported earlier argument that the crystallization process was operating within threshold of slow cooling mode. For rapid cooling, some alpha (␣) polymorphic crystals, with lower melting point and heat stability, might form. Future research should look into the effects of different cooling modes, including temperature at extremity, on dynamic evolution of DHSA crystal habit and morphology. The formation of -DHSA crystals was necessary to avoid and/or to minimize fat exudation (the squeezing through phenomenon during filtration process, a phenomenon still considered quite poorly understood). It is only known that fat exudation might be caused by, but not limited to, the presence of unstable crystals (e.g. ␣-crystals) and/or by the presence of far too many small crystals.
3.3. Effects on crystal purity
Fig. 2. SEM images of DHSA crystals for 80% IPA with different DHSA:IPA ratios (a) 1.5:1.0, (b) 1.0:1.0 and (c) 1.0:1.5.
The purity of DHSA crystals formed under different DHSA:IPA ratios (1.5:1.0, 1.0:1.0 and 1.0:1.5) and IPA concentrations (70 and 80% IPA) is shown in Fig. 4. The minimum targeted purity was 80% (Siwayanan et al., 2004; Sumaiya et al., 2007). Regardless of IPA concentration, DHSA crystal purities from 1.5:1.0 ratio were well below 80%. By extrapolation, even 90% IPA would not result in DHSA crystals with 80% purity as inadequate solvent was present. DHSA was a fatty acid, solid at room temperature. Molted crude DHSA was highly viscous. When added with IPA, the concoction viscosity reduced. For 1.5:1.0 ratio (and all other combinations with higher DHSA content), inadequate IPA was present to dilute the concoction for higher diffusivity (which would lead to better selectivity and phase separation during crystallization) (Krishnamurthy and Kellens, 1996) and thus the failure to sufficiently remove the impurities in crude DHSA. For 70 and 80% IPA, DHSA crystal purity for 1.0:1.0 ratio was 80.54 and 86.26%; the purity for 1.0:1.5 ratio was 82.51 and 87.79%. DHSA crystals from 1.0:1.5 ratio and 70% IPA required 1050 g IPA for every 1000 g of crude; DHSA crystals from 1.0:1.0 ratio and 80% IPA required 800 g IPA for every 1000 g of crude. However, the purity achieved by the latter was higher (Table 3). For 70% IPA, increasing IPA from 700 g to 1050 g, crystal purity improved 1.97%; for 80% IPA, increasing IPA from 800 g to 1200 g, crystal purity improved merely 1.53%. Solvent crystallization of DHSA with DHSA:IPA 1.0:1.0 ratio and 80% IPA was preferred for its higher reliability (compared to DHSA:IPA 1.0:1.0, 70% IPA) and economical competitiveness (compared to DHSA:IPA 1.0:1.5, 80% IPA). It commands higher level of confidence in consistently producing DHSA crystals with purity higher than 80% at acceptable solvent usage level. Increment in total solvent (by quantity and concentration) should result in higher DHSA crystal purity. However, the filtration step posed as a limiting phase as it was standardized for all combinations. During filtration, for concoction with 1.0:1.0 ratio, most mother liquor was removed before the crystals settled down and formed a wet cake; for concoction with 1.0:1.5 ratio (and all other combinations with higher IPA content), a wet cake was formed before most mother liquor was removed. This caused some mother liquor (with impurities) to re-include and re-embed into DHSA crystals. The mother liquor adsorbed within the crystals and/or remained between individual crystals (Sun, 1971; Saito et al., 2000). The dropout of the 1.0:1.5 ratio was mainly attributed to filtration and this was not foreseen. After all, crystallization was more of an art than a science, developed through years of experience (Hartel, 1991) and thus, future research should consider and explore this factor.
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Fig. 3. FTIR spectrum for (a) crude DHSA and (b) purified DHSA crystals. Table 3 Solvent quantity and concentration versus resultant DHSA crystal attainable purity (reference basis: 1000 g crude DHSA). IPA concentration
DHSA:IPA 1.0:1.0
70% 80% Increment
(g) (%)
DHSA:IPA 1.0:1.5
Increment
Solvent
IPA
Purity
Solvent
IPA
Purity
IPA
(g)
(g)
(%)
(g)
(g)
(%)
(g)
(%)
(%)
1000 1000
700 800
80.54 86.26
1500 1500
1050 1200
82.51 87.79
350 400
50.00 50.00
1.97 1.53
– –
100 14.29
– 5.72
– –
– 5.28
– –
– –
– –
150 14.29
Purity
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full scale industrial mass production of DHSA crystals to meet the needs from the functional ingredients’ perspective and demands from the fine chemicals, cosmetics and personal care products industry. References
Fig. 4. Purity of DHSA crystals formed under different DHSA:IPA ratios and IPA concentrations.
4. Conclusion The study on effects of solvent quantity and concentration was successfully carried out by solvent crystallizing palm based crude DHSA with IPA under five different DHSA:IPA ratios and two different IPA concentrations. Higher solvent quantity resulted in smaller purified DHSA crystal median particle size and the broadening of the bell-shaped PSD curve. SEM images revealed that DHSA crystals formed in higher IPA quantity were more compact and smaller than those formed in lower IPA quantity. Regardless of crystallizing conditions, DHSA crystals formed were of ‘desert rose’ conformation (plate-like flakes that grew in layer-by-layer fashion which eventually formed semi-spherical rosette aggregates). Total solvent was directly correlated to attainable purity of DHSA crystals. The current study has shown that crystallization of DHSA with IPA as solvent was most optimized and efficient when carried out with 80% IPA at 20 ◦ C and DHSA:IPA 1.0:1.0 ratio, commanding the highest level of confidence in consistently producing DHSA crystals with purity higher than 80% at acceptable solvent usage level. The -DHSA crystals formed were of the most desired as they are known to be highly resistant against fat exudation. This study has also shown that the filtration step was a limiting phase. To boost DHSA crystal attainable purity, more efficient filtration is needed. Further study for optimized filtration step is a precursor to
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