versão impressa ISSN 0327-0793
Lat. Am. appl. res. vol.41 no.1 Bahía Blanca jan. 2011
D-limonene and geranial fractionation from lemon essential oil by molecular distillation
P. C. Rossi, A. A. Willnecker, J. Berti, A. V. Borgarello, G. N. Mezza and M. C. Pramparo
Fac. de Ing. Univ. Nac. de Río Cuarto, Ruta 36 Km 601, 5800 Río Cuarto, Córdoba. Argentina.
Abstract - D-limonene and geranial are, respectively, the most abundant terpenic and oxygenated compounds found in lemon essential oil. The main objective of this research work is to study the technical feasibility of molecular distillation, in order to separate and concentrate those thermal labile compounds of lemon essential oil and to determine the best evaporation temperature and feed flow rate values which will lead to high separation efficiency. The highest temperature analyzed allowed to obtain a residue poor in d-limonene and enriched in geranial, with low geranial yield (between 35-50%). Regarding d-limonene, the highest temperature (30 °C) used, led to higher yields of d-limonene. Lower feed flow rate (0.6 ml min-1) led to low concentrations of d-limonene (320 g kg-1) and geranial (70 g kg-1) in the residue, with low yield for geranial (23.5%).
A high yield of geranial in the residue (76.4%) can be obtained by using a feed flow rate of 1.3 ml min-1, which leads to the highest geranial concentration (113 g kg-1 ).
Keywords - Essential Oils; Lemon; Molecular Distillation; D-limonene; Geranial.
In Argentina, there is a significantly important "citrus" fruits production, from which different products are obtained - mainly juices. Essential oils are the most important by-products of juice production, and they are extracted from the "citrus" peelings. Nowadays, the production of natural essential oils is being encouraged; therefore there seems to be a brilliant future concerning this economical activity (Bruzone, 2003; Lawrence and Reynolds, 1999).
Lemon essential oil is a product of particular interest for Argentina, since it is placed in a relevant spot in the international markets. Besides that, it constitutes a versatile product since it is used as a flavored and scented agent in cosmetic, pharmaceutical and foodstuff industries.
Oils obtained from different "citrus" fruits have in common a high amount of terpenes, which are volatile-low molecular weight compounds. The most frequent terpene found in lemon oil is d-limonene, which constitutes around 700 g kg-1 of the oil weight. The remaining part of it consists of low to medium molecular weight aldehydes, unsaturated aldehydes, ketones, esters and alcohols. Given that, d-limonene can be considered as a primary solvent for scented compounds present in lemon essential oil (Buccellato, 2000).
Among the chemical substances found in the essential oil, some of them have nutraceutical properties (Miyake et al., 1998). Some isoprenoids such as d-limonene and others such as farnesol, tocotrienol and geranial have been evaluated on their chemo-protective activity. When administered to rats, dogs and humans at levels between 0.1 and 5% tumor genesis was suppressed by a direct action over carcinogens and pre-carcinogens that require activation. D-limonene was found to be the most effective chemo-protector (Wildman, 2001).
Given these important properties of the compounds found in lemon essential oil, it is necessary to study beneficial techniques of separation and concentration that allow obtaining more valuable products. Several methods exist for processing valuable compounds present in essential oils. Concentration can be carried out by vacuum fractional distillation, extraction using oxygenated solvents with diluted alcohol or other solvents, or dragging steam distillation. Despite the fact that acceptable separations are obtained using these methods, they present some disadvantages such as the formation of degradation products due to the high operation temperatures, or the presence of solvent traces in the final product (Sinclair, 1984; Haypek et al., 2000; Pino and Sanchez, 2000; Stuart et al., 2001).
In order to overcome those difficulties in the separation process, alternative separation techniques have been searched for. One of them is the molecular distillation -also called short path distillation. This operation is expected to provide satisfactory results, since it requires a short residence time and a very low operation temperature, due to the high vacuum levels in which the separation takes place. These characteristics prevent the compounds of interest from suffering thermal damage, and lead molecular distillation to become a very useful technique in the purification of thermal labile substances (Pramparo et al., 2004; Pramparo et al., 2006; Zeboudi et al., 2005).
Molecular distillation is based on the partial vaporization of the compounds of a mix, which usually moves as a falling film in contact with a heated surface, and the subsequent condensation that takes place in a very close and cold surface. The main feature of this operation is its very low operation pressure, around 10-4-10-6 atmospheres. In these conditions, relative volatility of the compounds increases, and the operation temperature needed in order to obtain a certain separation degree decreases considerably. As a result of this, it is possible to separate thermal labile compounds.
Since the molecules travel a short distance before being condensed, molecular collisions are insignificant. This also leads to a high evaporation velocity, and a low residence time of the molecules in the equipment. Under these conditions, the separation occurs at a technologically acceptable velocity (Weissberger, 1951; Perry et al., 1984).
The main objective of this work is to study the technical feasibility of molecular distillation, in order to separate and concentrate terpenic and oxygenated compounds of lemon essential oil, and to determine the most adequate operation conditions which lead to an efficient separation.
The molecular distillation equipment used for these experiments was a UIC KDL4 falling film evaporator, with 4 dm2 evaporation surface and 2 dm2 condensation surface as well (see Fig. 1). The device has a spinning roller, whose velocity could be manipul ated. The vacuum system is conformed by a diffuser pump and a mechanical pump, able to reach a vacuum of up to 10-6 atmosphere, with a maximum feed flow rate of 0.5kgh-1. The equipment also counts with a steam trap, a cooling bath and a feed flow rate controller. The operational variables analyzed were evaporation temperature and feed flow rate, as they have been proved to constitute the greatest influence in the separation efficiency.
Fig. 1: Schematic diagram of wiped-film molecular still:(a) motor drive; (b) feed flask; (c) wiper; (d) residue receiver; (e) distillate receiver; (f) one-stage condenser inlet; (g) one-stage condenser outlet; (h) cold trap; (i) rotary vane vacuum pump.
Firstly, a characterization of three different samples of lemon essential oil -which were supplied by national industries-, took place. The analytical determinations were carried out under the recommended Standards by the American Oil Chemist's Society. The quantification of the different compounds of the essential oil was made by gas chromatography, using a Hewlett Packard HP 5890 gas chromatograph, with a HP INNOWAX column, equipped with a flame ionization detector. There are many different chromatography operation conditions found in bibliography about temperature conditions (injector temperature, detector temperature, and programs for relating temperature/time in the oven) (Pino and Sanchez, 2000; Stuart et al., 2001; Atti dos Santos and Atti-Serafino, 2000; Vekateshwarlu and Selvaraj, 2000). The chromatography operation conditions used in this work were: initial temperature 60ºC, ramp rate 2ºC/min, final temperature 250ºC, held for 60 minutes. The split relation was 1:40. The injection volume was 1 µl. Carrier gas flow rate was 45 ml/min. Injector temperature was 250ºC. FID temperature was 350ºC.
Three representative samples of lemon essential oil were distillated, with the objective of finding, on one hand, concentrated d-limonene, and on the other hand free-terpene essential oil rich in oxygenated compounds, with the best possible performance (high yields and high purity of the valuable compounds). Because of the three different raw materials have similar composition, the results shown in this paper are the average values of these samples.
Two fractions were obtained in each of the experiments: the distillate (monoterpenic hydrocarbons, mainly d-limonene) and the residue (terpene-free oil, rich in oxygenated compounds). The temperature of the condenser and the spinning velocity of the roller were set in the values usually used for this kind of applications. The operation conditions used in all the experiments are displayed in Table 1.
Table 1. Experimental operation conditions
Two groups of experiments were carried out with the three samples of essential oil, and the main objective was to find the adequate conditions in the separation. In the first group of experiments, the variation of the parameters of interest was observed by the modification of the most important variables in this use of molecular distillation. Those variables are: evaporation temperature and feed flow rate. According to the results of the first group of experiments, the second group took place. Then the temperature was constant, and the feed flow rate was modified in a certain range, in order to analyze its influence on the separation degree obtained.
B. Description of the first group of experiments
This stage of the experiment consisted of four tests: two different evaporation temperatures (22 ºC and 30 ºC) were fixed for two different feed flow rates (1.3 and 2.1 ml min-1). These temperatures were adopted because if the temperature is lower than the lowest of the range, there is no driving force in the condenser and therefore there is no separation; and if the temperature is higher than the highest of the range, a thermal degradation takes place in the sample and destroys it.
C. Description of the second group of experiments
Temperature was set at one determined value, obtained from the first group of experiments (22 ºC); and different experiments took place using different feed flow rates in a determined range (0.5 to 4 ml min-1).
In both cases, the process needed to be run using moderate temperatures for the essential oil, to prevent the formation of thermal reaction products with undesirable odors.
D. Results and discussion
Figure 2 shows the geranial and d-limonene location in the gas chromatogram of the raw material, and the different compositions obtained from it are presented in Table 2. D-limonene and geranial are, respectively, the most abundant terpenic and oxygenated compounds found in this sample. Because of this, the specific results of these two key compounds were analyzed. Their concentrations were determined in the feed flow rate, as well as in the distillation products.
Fig. 2: D-limonene and geranial location in the gas chromatogram of the raw material.
Table 2. Key components present in the feeding mixture
In addition to this, the yield of d-limonene in the distillate and the yield of geranial in the residue were calculated. The equations used are the following:
where: D = distillate mass, W = residue mass, i = d-limonene, j = geranial, xD = distillate composition, xW = residue composition
Another important evaluation parameter in order to analyze the molecular distillation process is the separation ratio, which refers to the amount of distillate divided by the amount of residue, D/W.
The results of the experiments at different feed flow rates are displayed in Fig. 3 to 5.
Fig. 3: Separation ratio D/W and W/F vs. feed flow rate.
Fig 4: Influence of the feed flow rate variations over d-limonene yield and d-limonene concentration in the distillate.
Fig 5: Influence of the feed flow rate variations over geranial yield and geranial concentration in the residue.
Table 3 shows the results obtained from analyzing evaporation temperature and feed flow rate. In conclusion:
- Variations of evaporation temperature do not imply great variations in the amount of d-limonene found in the distillate for both flow rates.
- Modifications of feed flow rates lead to more significant changes on the concentration of the compounds of interest.
- The highest temperature analyzed allowed to obtain a residue poor in d-limonene and enriched in geranial, with low geranial yield (between 35-50%).
- Regarding d-limonene, the highest temperature (30 ºC) used, led to higher yields of d-limonene.
- The concentration of d-limonene obtained in the distillate under these conditions is very similar to the concentration found by using a lower temperature and the same feed flow.
- High yields of geranial in the residue can be obtained by using the lowest temperature.
Table 3. Results of the experiments
Based on these conclusions, the second group of experiments was designed at the lowest temperature analyzed while the feed flow rate was modified.
As the feed flow rate increases (F), the separation ratio decreases remarkably (Fig. 3). When a big amount of sample is fed, a larger volume is occupied in the evaporator, and the contact surface decreases. Therefore, heat transfer efficiency between fluid and the walls of the evaporator is reduced and the amount of distillate obtained is smaller.
Percent yield of d-limonene present in the distillate tends to decrease in the case of higher feed flow rates (Fig. 4); meanwhile, higher yields of geranial present in the residue are obtained when the feed flow rate increases (Fig. 5).
Although the d-limonene concentration in the distillate does not considerably change with the feed flow rate (Fig. 4), the geranial concentration in the residue presents a maximum of 113 g kg-1 at 1.3 ml min-1 (Fig. 5). Under these conditions, a high yield of geranial in the residue (76.4%) can be obtained.
Lower evaporation temperatures can be used to get a residue highly enriched in geranial and with a small amount of d-limonene. This allows the residue to become a valuable product, because of its scent properties.
Since the yield obtained for geranial in the residue was better at the lowest temperatures, a second group of experiments was carried out at this temperature, in order to analyze the impact of feed flow rate variation more deeply.
In conclusion, the variation of the feed flow rate in molecular distillation allows obtaining a wide range of products which have different properties and characteristics as well. According to this, the most adequate operational conditions depend on the objective of the operation, that is to say, it is important to analyze whether it is more relevant to reduce the loss of a valuable component (by maximizing its yield) or to obtain a product enriched in one of the components of the mix.
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Received: December 29, 2009.
Accepted: April 21, 2010.
Recommended by Subject Editor Ricardo Gómez.