ENHANCEMENT OF MASS TRANSFER IN SOLID-LIQUID EXTRACTION BY PULSED ELECTRIC FIELD

Transcription

1 Journal of the University of Chemial Niola Tehnology Shoilevand Metallurgy, 40, 4, 2005, ENHANCEMENT OF MASS TRANSFER IN SOLID-LIQUID EXTRACTION BY PULSED ELECTRIC FIELD M. Savova 1, H. Bart 2, I. Seikova 1 1 University of Chemial Tehnology and Metallurgy 8 Kl. Ohridski, 1756 Sofia, Bulgaria 2 University of Kaiserslautern, Institute of Chemial Engineering, Department of Mehanial and Proess Engineering, Kaiserslautern, Germany marianasavova@yahoo.de Reeived 11 November 2005 Aepted 28 November 2005 ABSTRACT Laboratory-sale equipment was developed making the Pulsed Eletri Field (PEF)-treatment possible during solidliquid extration. Preliminary experiments were performed for testing the effet of pulsed eletri energy input on mass transfer kinetis. As example the kinetis of extration of water-soluble polysaharides fration from linseeds (Linum usitatissimum L.) has been studied. Samples were submitted to PEF treatment at varying field strengths (0.5 to 5 kv m -1 ) applying a pulse numbers up to 900 with a pulse duration of 900 µs. Additional effet of moisture ontent of dried or wetted samples were also studied. The results indiated that reognizable inrease in mass transfer rate started to our at 0.5 kv m - 1 moderate field strength at short times of PEF treatment in the range of 2 min. Keywords: pulsed eletri fields, eletropermeabilization, linseed, muilage reovery, extration kinetis. INTRODUCTION In the solid-liquid extration proessing a struturally dependent diffusion is haraterized by the degree of solid matrix disintegration, whih influenes the amount of the ompounds released into the solvent. Apart the traditional pre-proessing methods of ahieving a high level of ell wall s disruption suh as mehanial, thermal or hemial treatment, a variety of new high eletri field appliations were suessfully demonstrated. To date, high-voltage fields have been used mostly for degradation of organi ompounds in water, inativation of miroorganisms in liquid produts, and for intensifiation of mass transfer in reative liquid-liquid extration [1, 2]. In reent years, there has been an inreasing interest in the appliation of pulsed eletri field as a proessing alternative for vegetable raw. Several ombinations of PEF with other treatments suh as pressing, osmoti dehydration and drying have shown advantages over individual proessing [3]. In more reent researh papers the impat of moderate eletri pulses on the mass transfer during solid-liquid extration of sugar and pigments from red beets, sugar beets, apples, arrots [4, 5], and of vegetable oils from maize kernels, olives and soybeans [6] has been investigated. 329

2 Journal of the University of Chemial Tehnology and Metallurgy, 40, 4, 2005 The effet an be attributed to the eletrial damage of the ell walls (eletropermeabilization) resulting in temporal or permanent pores formation and internal porosity inreases. The unlear mehanisms of redution in the mehanial integrity of the ell walls leads to the absene of riteria for hoosing optimal parameters of PEF treatment for the wide spreading of vegetables. The objetive of the experimental work presented here is to develop an experimental set-up making the study of PEF-indued effet possible during solidliquid extration. Effiieny of this proess was dedued by monitoring rates of mass transfer during muilage reovery from linseed - water system. THEORY Some vegetables have an effetive eletrially ondutivity σ over the range S m -1 suggesting their suitability to be proessed by PEF. Inside the heterogeneous plant tissues, the struture of the ell is maintained by the ell wall, and its seletive permeability is provided by a thin membrane (5-8 nm) overing the ell just interior to the ell wall. The largest omponents building-up the membrane are eletrially insulating lipids (mainly phospholipids onneted by polar and non-polar interations) and it may be onsidered as a dieletri interfae between the ondutive intraellular and extraellular tissues. As a result of a very low eletrial ondutivity of the membrane σ m in the range of S m -1, the voltage applied inside the tissues drops exlusively aross the ellular membrane and may provoke an eletri breakdown at ellular level. Aording to the theory proposed by Zimmerman [7] the transmembrane potential U m indued on a ellular membrane in an external eletri field is determined by the following equation: U m = αfd E (1) with: E - eletri field strength, kv m -1 ; d diameter of the ell, m; α geometry fator depending on the shape of the ell, equal to 0,75 for a spherial ell, and to 1 for a ell of ubi form; f fator, relating eletrial and geometrial properties of the ell and of the ell membrane. Several semi-empirial models of phenomenologial nature have been proposed for estimating the transmembrane potential and ell eletropermeabilization probability. Taking into aount the insulating properties of the ell membrane and simplifying the general phenomenologial model, Bazhal et all. [3] proposed an approximate estimation for the oeffiient f aording to the following expressions: σ m f = (1 + ) / K where 2σ K dσ m σ K = 1+ (2 + ) / 4 (2) d σ σ m e Considering usual onditions for vegetable tissues (ell size d of m, membrane thikness d m about 10-8 m, and σ m <<σ ) the expressions an be approximated as: K 1 and f 1 and the potential U m is proportional to the fields strength E and size of the ell d. This approximation onfirms the fat that despite the large variety of morphologial and eletrial properties of the vegetables, similar values for harging of the insulated membrane up to a ritial threshold U r on the order of 1V were reported. The orresponding eletri field E r is U r Er = Vm (3) d The most generally aepted mehanism of membrane eletropermeabilization, based on a ritial transmembrane potential onept onsists of following steps (Fig. 1): Polarization of the membrane as a result of harge separation aross the nonondutive membrane (the normal potential without external field U o is onsidered approximately 10 mv); 330

3 Niola Shoilev Temporal destabilization and redution in the thikens of the membrane as a result of indued intraellular stress; Reversible breakdown of the membrane nearby the ritial potential U r resulting in small ondutive hannels formation, whih leads respetively to an immediate disharge at the membrane, re-sealing of pores, followed by a repeated harging proess; Pores formation of an irreversible nature obtained by inreasing the intensity of the eletri field and pulse duration and number. Fig. 1. Mehanism of dieletrial breakdown of the ell membrane. The earlier experimental studies in the proessing of vegetables [3-6] were onduted in stati treatment hambers applying field strengths on the range kv m -1 with pulse duration of µs and up to 1000 pulse numbers, suggesting that the impat of both reversible and irreversible eletropermeabilization had found appliations. by the seeds. The muilage is reovered from the extrats EXPERIMENTAL Sample Materials Water-soluble non-starh polysaharides fration (prinipally muilage) reovery from dry, ripe seeds of Linum usitatissimum L. was studied as example. Apart the fatty aids (30-40%) linseeds are rih soures of vegetable muilage (about 5-6 % of the seed weight). The linseeds are flat, oval, with an average weight of 5.5 mg per seed, length being about 4-5 mm, and are approximately 2.5 mm wide and 1.5 mm thik. The muilage is present on the surfae of the seed, and an be easily extrated by water from the whole seeds. Fig. 2. Steps of muilage reovery from Linseed. Extration proedure The overall proess of muilage reovery from linseeds is presented in Fig. 2. The kineti experiments were arried out in a bath-type stirred thermostated vessel. At speifi intervals, the extration was stopped and the solvent-dissolved muilage bulk phase was then removed. The muilage dissolved forms a visous hydrogel surrounding the seed when it is wetted, the solution is extremely visous and its removal required several separations. The seeds were separated by filtration using a hand sieve, brief washing with additional water, and entrifugation (at rpm) of the olleted seedssolution filtrate to reover the residual solution retained Fig. 3. Experimental set-up. 331

4 Journal of the University of Chemial Tehnology and Metallurgy, 40, 4, 2005 olleted by preipitation (three part of alohol to on part of extrat) in 95,6 % ethanol water azeotrope. The alohol pereptible muilage forms a stable blo. The flos were filtered and the weight of muilage was measured gravimetrially after drying to a onstant mass. Pulsed Eletri Field treatment The simplified iruit of the experimental set-up developed for this purpose is represented in Fig. 3. Basially it onsists of a generator that forms the pulses and of a treatment hamber with a disharging zone. The high field strengths are ahieved through storing a large amount of energy in a harging iruit from a DC power supply, whih is then disharged in the form of pulses through the solid samples in the treatment hamber. The pulses are monopolar retangular of duration of 900 µs. The time between the pulses of 75 ms is signifiantly longer than the duration of the pulses. The hamber onsists of 2 parallel stainless steel eletrodes of area of 9 m 2 plaed in a polystyrene spaer. The upper one was positively harged while the other one was grounded for safety reasons. The external applied voltages may be extended up to 1800 V and field strengths an be varied gradually by the distane between the eletrodes. The experimental proedure has been divided in two groups. Samples of one group were to be used diretly in solvent extration in order to determine the kinetis of extration at the following onditions: solidliquid ratio 1:10, native ph of the suspension ph» 7, and rotation speed of 500 rpm that eliminates the external mass transfer resistane. Kinetis experiments were performed at temperature of 25, 35, and 45 o C using samples with varying moisture ontent: dried seeds (3 %), seeds with initial moisture (7 %) and wetted seeds (12 %). The seond group investigates the effet of samples PEF pre-treatment prior to the onventional extration. The PEF treatment time was about 2 min orresponding to 900 pulse numbers. Eletrial field strengths ranging from 0.5 to 5 kv m -1 were used whih fell in two main ategories. In one, the samples are treated with a mean power of up to 0.5 kvm -1. Higher power outputs ranging from 1 to 5 kv m -1 were used in another group of experiments to study the effet of higher eletri strengths. Fig. 4. Conventional versus PEF-assisted solid-liquid extration. Fig. 5. Effet of PEF treatment at different moisture ontents. RESULTS AND DISCUSSION Results of omparative extration tests with or without PEF pretreatment are reported in Fig. 4. The yields were reported in terms of mass of dried preipitate to the mass of the solid phase. An extration time of at least 3000 s at room temperature was required for near equilibrium onditions under onventional extration, giving about 50 % separation effiieny for one stage extration. The inrease in temperature yielded fairly similar equilibrium onentrations and sensible inrease in the rate of extration, the effet was more pronouned at 45 o C. This effet is due mainly to the strong temperature dependene of the high muilage visosity whih dereased at inreased temperatures, or more omplex effet of solubility of polysaharides, again dependent on the temperature used. Three different voltages giving the same field strength E of 1 kv m -1 were used to ompare the effe- 332

5 Niola Shoilev fiulty of its removal; possible PEF-indued fast kinetis of eletrial ondutivity inrease, whih in turn dereased the resistane of the hamber and redued the pulse width. Additional effets are the heterogeneous struture of the solid and the presene of ondutive debris in the samples, whih may provoke nonuniform treatment as well as operational problems. CONCLUSIONS Fig. 6. Comparison of effetiveness of PEF treatment at different field strengths. tiveness of the PEF treatment. It is apparent that the PEF pre-proessing aelerated extration in a similar manner as that exhibited by an inreasing of temperature up to 45 o C, or at room temperature within the first 15 minutes produed yields similar to those following 1h extrations without PEF treatment. The yields of extration without and with PEF were ompared together at different moisture ontents in Fig. 5. The results presented onern the first 15 minutes of extration after PEF treatment (applied in this ase at voltage of 1.25 kv). Experiments arried out indiated that partially removing moisture from the material seemed to enhane the PEF-indued effet. This dependene has not been leared yet and it may be orrelated with hanges in eletrial ondutivity and the influene of exess quantities of air and extrapartile liquid in the pores of untreated samples [3]. Several trials were arried out to loate a suitable level of PEF treatment. Fig. 6 presents some examples of muilage yields versus time at different times of extration after the PEF appliation, and different values of field strength, E. It was observed from the results that pratially reliable experimental data were reorded at 0.5 kv m -1 field strength beyond the generally aepted eletrial breakdown level. However, an inrease from 1 to 5 kv m -1 did no result in further inrease in the muilage yield onfirming earlier studies [4, 6]. The main problem onsisted in the poor reprodution and inonsisteny of ertain experimental data that may be attributed to some unfavorable fators: great affinity of jelly-like muilage for the seeds and the dif- PFE treatment was shown to be effetive for mass transfer enhanement during aqueous extration of polysaharide ompounds from linseeds, inreasing the rate of extration by % under ertain proessing onditions. Further work will be needed to determine preonditions of an effetive PEF treatment: mehani and thermal pretreatment of the solid phase (dehulling, milling and frationating of the linseeds, optimal moisture ontent) in a larger range of proess parameters (field strength and pulse number) resulting in both reversible and irreversible strutural hanges. The major problem arising is the determination of onditions of steady PEF-treatment regime without any disruption of eletrial treatment aused by overflow of aeptable maximum urrent value. For this purpose aurate measurements of sample eletrial ondutivity and possible strutural hanges before and after PEF treatment would prevent inonsistent results due to appliability of the PEF systems. Anowledgement The authors aknowledge the finanial support from the University of Chemial Tehnology and Metallurgy, Sofia, under Contrat No and the Tehnial University Kaiserslautern. REFERENCES 1. D. Knorr, A. Angersbah, M. N. Eshtiaghi, V. Heinz, D. Lee, Trends in Food Siene & Tehnology, , 2001, A. Wildberger, S. Shmidt, H.-J. Bart, ECCE 3, Nuernberg, Germany, M. I. Bazhal, N. I. Lebovka, E. Vorobiev, eprint arhiv, physis/ ,

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