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Ind. Eng. Chem. Res. 1997, 36, 4318-4324
Extraction of Squalene from Shark Liver Oil in a Packed Column Using Supercritical Carbon Dioxide Owen J. Catchpole,* Jan-Christian von Kamp, and John B. Grey Industrial Research Limited, P.O. Box 31-310, Lower Hutt, New Zealand
Continuous extraction of squalene from shark liver oil using supercritical carbon dioxide was carried out in both laboratory and pilot scale plant. The shark liver oil contained around 50% by weight squalene, which was recovered as the main extract stream. The other major components in the oil were triglycerides, which were recovered as raffinate, and pristane, which was recovered as a second extract stream. Separation performance was determined as a function of temperature; pressure; oil to carbon dioxide flow rate ratio, packed height and type of packing; and reflux ratio. The pressure, temperature, and feed oil concentration of squalene determined the maximum loading of oil in carbon dioxide. The oil to carbon dioxide ratio determined the squalene concentration in both the product stream and raffinate stream. The ratio of oil flow rate to the flow rate of squalene required to just saturate carbon dioxide was found to be a useful correlating parameter for the oil loadings and product compositions. Of the three packings investigated, wire wool gave the best separation efficiency and Raschig rings the worst efficiency. Mass transfer correlations from the literature were used to estimate the number of transfer units NTU from experimental data and literature correlations. NTU’s from the experimental data were comparable to predictions at a pilot scale but were underpredicted at the laboratory scale. The use of reflux at the pilot scale enabled the concentration of squalene in the product stream to be increased from 92% by mass to a maximum of 99% by mass at fractionation conditions of 250 bar and 333 K. Introduction Squalene (C30H50) is a commodity chemical that is present in high concentration in the liver oils of certain deep sea sharks (Tsujimoto, 1932; Hilditch and Williams, 1964). The sharks are in sufficient abundance off the coast of New Zealand for sustainable, commercial recovery of squalene, providing a suitable method for extraction from the oil can be developed (King and Clark, 1987). Squalene is also present in lower concentrations in some common edible oils such as olive oil (Sonntag, 1979). Some work has been carried out on the phase equilibrium (Simo˜es and Brunner, 1996) and separation of squalene from olive oils (Bondioli et al., 1992), and Olive oil deodorizer residues (Bondioli et al., 1993; Stodlt et al., 1996) using supercritical extraction. These works have shown that squalene can be easily separated from the heavier components of the oil such as triglycerides and mixed glycerides. The most promising process is the separation from deodorizer distillates where the initial concentration is around 10% by weight. However, squalene is not easily separated from oleic acid, which is also present in high concentration. Shark liver oil on the other hand has a very low concentration of fatty acids, and a high concentration of squalene (40-75% by mass), with the remainder of the oil being low volatile triglycerides/glyceryl ethers. This makes the oil suitable for fractionating using supercritical carbon dioxide. Phase equilibrium for squalene/carbon dioxide and squalene/triglyceride/ carbon dioxide mixtures has been reported in a companion paper (Catchpole and von Kamp, 1997). This work examines the separation of squalene from shark liver oil in packed columns at a laboratory and pilot scale. Extraction of squalene from shark liver oil is also of interest from a mass transfer point of view. Fraction* Address correspondence to this author. Phone: 64 4 5690 000. Fax: 64 4 5690 132. E-mail:
[email protected]. S0888-5885(97)00223-6 CCC: $14.00
ation of liquid streams in a packed column using a nearcritical fluid solvent bears some similarity to the atmospheric pressure unit operations of liquid/liquid extraction and gas phase stripping of solute from a liquid solvent. The “gas” phase, carbon dioxide, usually has a low capacity for the solute and is thus similar to stripping. However, the gas phase is dense and somewhat liquid like in mass transfer properties, thus resembling liquid/liquid extraction. The liquid/nearcritical fluid systems previously studied can be categorized as having low miscibility with carbon dioxide, such as water containing an alcohol (Lahiere and Fair, 1987; Lim et al., 1995), or having partial to high miscibility with carbon dioxide, such as terpenes (Sato et al., 1995; Simo˜es et al., 1995); glycerides (Brunner and Peter, 1982); and hydrocarbons (de Haan and de Graauw, 1991). The shark liver oil/carbon dioxide system is a partial miscibility system, as carbon dioxide dissolves to around 30% by weight in the liquid (Catchpole and von Kamp, 1997). The mass transfer performance of immiscible systems has been modeled by using standard correlations (Lahiere and Fair, 1987; Siebert and Moosberg, 1988; Lim et al., 1995). The situation for partially miscible systems is complicated by the fact that the viscosity and surface tension of the liquid phase of fatty substances are substantially altered by the presence of carbon dioxide at high pressure (Blaha-Schnabel et al., 1996; Jaeger et al., 1996; Brunner, 1994; Peter et al., 1987), and that the equilibrium relationship is usually nonlinear. The effects of packed height and type of packing were thus examined in this work. Experimental Section Fractionation experiments were performed in a laboratory scale (5 mL/min oil processing capability) and pilot scale (30 mL/min oil) packed column fractionation plant. The laboratory scale plant with a column internal diameter of 25.4 mm has been described elsewhere (Catchpole and von Kamp, 1996). A schematic of the © 1997 American Chemical Society
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Figure 1. Schematic of pilot scale apparatus.
pilot scale apparatus is shown in Figure 1. The basic apparatus consisted of a high pressure compressor for recirculating carbon dioxide, a packed column (packed height 2.5 m, internal diameter 56 mm) with three temperature controlled sections, a high pressure air driven piston pump for the supply of the liquid shark liver oil, an electrically driven piston pump for the supply of liquid reflux, and two jacketed separation vessels for the recovery of squalene and fish odors/ pristane, respectively. The experiments were performed by passing supercritical carbon dioxide upward through the packed column at the fractionation pressure, which was controlled to (0.5 bar by a pressure controller, and the fractionation temperature(s), which were controlled to (0.5 °C by the temperature controllers. When the carbon dioxide flow rate, pressure, and temperature had stabilized, the feed liquid was pumped into the top of the first (with no reflux) or second (with reflux) section of the column at a known volumetric rate. Similarly, the reflux liquid was pumped into the top of the first section at a known rate. The raffinate was collected at regular time intervals, by draining the liquid reservoir under the bottom of the packing. The liquid level interface was thus maintained below the bottom of packing in the column, so that carbon dioxide was the continuous phase and the oil the dispersed phase. It was not possible to visually observe whether the oil passed through the column as droplets, or as a thin film which wets the packing. The carbon dioxide and dissolved oil fraction passed to the first of two separation stages. In the first, which was maintained at a pressure of 90 bar at 313 K, or 100 bar at 333 K, the bulk of the squalene was recovered. In the second, which was maintained at a pressure of 50-60 bar and an average temperature of 313 K, the fish odors and pristane were recovered. After leaving the second separation vessel, the carbon dioxide was recycled back to the column via a water cooled heat exchanger, a coriolis type mass flow meter, the recycle compressor, and a further heat exchanger. At a laboratory scale, the second separator was operated at 30 bar and 313 or 333 K, and the carbon dioxide was not recycled. Laboratory scale experiments were performed to determine the loading of shark liver oil at selected
temperature and pressure combinations as a function of oil to carbon dioxide mass ratios and the effect of packed height and type of packing on separation performance. The experiments were performed at a fixed carbon dioxide flow rate, and variable shark liver oil flow rate. The packings used were 6 mm glass Raschig rings, 4-mm glass Fenske helices, and stainless steel wire wool at a laboratory scale. Pilot scale experiments were performed using 8.5-mm glass Raschig rings to determine the separation performance with varying carbon dioxide flow rates and the effects of reflux and carbon dioxide recycle on product purity and to observe any differences in separation behavior at a larger scale of operation. Raw shark liver oil was supplied by Ocean View Fisheries, New Zealand. The oil was filtered prior to extraction experiments. Two batches were supplied for use in the experiments. The first batch used for laboratory scale experiments contained 55% by weight squalene, 45% by weight triglycerides/glyceryl ethers, 0.1% by weight pristane, and < 0.1% by weight odor compounds. The second batch used for pilot scale experiments contained 50% by weight squalene, 50% by weight triglycerides/glyceryl ethers, 0.2% by weight pristane, and < 0.1% by weight odor compounds. Results and Discussion Oil Loading as a Function of Oil to Carbon Dioxide Ratio. The loading of shark liver oil in carbon dioxide as a function of oil to carbon dioxide ratio was investigated at the following temperature and pressure combinations: 125 bar and 313 K; 200 bar and 313 K, 200 bar and 333 K, and 200 bar and a temperature gradient of 313-333 K at a laboratory scale; and 200 bar and 333 K; 200 bar and a temperature gradient of 313-333 K, and 250 bar and 333 K at a pilot scale. The packing and packed heights were stainless steel wool and 1.5 m at the laboratory scale and glass Raschig rings and 2.5 m at the pilot scale. The temperature and pressure conditions were chosen to give a range of selectivities and solubilities (Catchpole and von Kamp, 1997) and the possibility of internal reflux due to a temperature gradient. The total loading is the ratio of
4320 Ind. Eng. Chem. Res., Vol. 36, No. 10, 1997
Figure 2. Loading of shark liver oil in carbon dioxide as a function of O/S ratio (T grad is temperature gradient of 313-333 K). Solid symbols are pilot scale, hollow symbols are laboratory scale.
the total mass of top product from the column to the carbon dioxide mass that has passed through the column over a given time period. The ratios of the total loadings in grams of extract/kilograms of carbon dioxide to the initial mass fraction of squalene in the feed oil are shown in Figure 2 for the temperature and pressure conditions given above. The loadings are at steady state, which was deemed to have occurred when the oil mass balance over the column closed to within 5% and the composition as measured by GC analysis was constant. The loadings represented by each point on the diagram are an average of four to six measurements. The maximum deviation between individual loading measurements was on the order of 5%. The loadings have been presented as a function of the ratio given by eq 1:
Rs ) O/S
(1)
where O is the oil mass flow rate and S is the squalene mass flow rate required to just saturate carbon dioxide at the given temperature and pressure. An equation to predict S is given elsewhere (Catchpole and von Kamp, 1997). The loadings fit an inverted exponential decay curve and tend toward an asymptotic value as the oil to carbon dioxide ratio increases. The total loadings fit in between the measured solubility of squalene and predicted solubility of triglyceride oil. The loadings followed the pattern of squalene solubility, with the highest values obtained at 250 bar, 333 K, and lowest at 125 bar, 313 K. The loadings achieved with the temperature gradient were higher by up to 15% by mass than those at 200 bar and 313 K. The loadings M fit an equation of the general form:
M ) M0(1 - exp[-p(O/S)])
(2)
The symbols are given in the nomenclature. The loadings in terms of the oil to carbon dioxide mass flow ratio follow a similar pattern. The O/S ratio has been used as a correlating parameter instead of the oil to carbon dioxide mass flow ratio, as the extract and raffinate composition can also be correlated as described below. Extract Composition as a Function of Oil to Carbon Dioxide Ratio. The composition of the raffinate, first separator, and second separator products was determined by GC for the components squalene and pristane, with the assumption that the nonvolatile
Figure 3. Extract and raffinate squalene content as a function of O/S ratio (TG is temperature gradient of 313-333 K).
triglyceride/glyceryl ether fraction makes up the remaining components. The results were correlated in terms of the oil to squalene solubility mass ratio Rs. The usefulness of this ratio is demonstrated in Figure 3 where the squalene content of the top product and raffinate streams is shown as a function of this correlating parameter. The squalene content increases with increasing Rs until the equilibrium concentration is approached, at a ratio of approximately 1. Above 1, the squalene concentration hardly changes, within the experimental error of the measurements, and tends very closely toward the equilibrium concentration. The concentration of squalene is effectively zero in the raffinate until the same ratio is reached and then begins to increase and then flatten out as the amount of oil passing through the column becomes so large that the exit concentration approaches the feed concentration. The optimum oil to solubility ratio to obtain maximum product purity and minimum loss of squalene in the raffinate without using reflux of top product is thus in the range 1-1.2. The pristane content of the top product extract was minimized by selecting suitable pressure and temperature combinations to operate the first separator. Pristane (C19H40) is known to be a skin irritant and is thus undesirable in squalene that is destined for use in cosmetic applications. The separator conditions were chosen to minimize the loss of squalene due to residual solubility in the exit gas phase, while still retaining the majority of the pristane. The enhancement of the concentration of pristane in the second separator over the feed concentration as a function of Rs is shown in Figure 4. The separator temperature and pressure combinations of 90 bar, 313 K, and 95 bar, 333 K, were used at the laboratory scale for column temperatures of 313 and 333 K, respectively. At the pilot scale, the second separator conditions were 90 bar and 313 K and 95 bar and 318 K for column temperatures of 313 and 333 K, respectively. Although there is some scatter in the data, the concentration increases almost linearly with Rs for both laboratory and pilot scale experiments in the second separator product. The enhancement in concentration in the squalene rich product from separator 1 stays constant at around 1, which ensures that a level of 0.5% by mass is not exceeded. The enhancement in separator 2 is to be expected, as pristane is the most soluble component of the oil and is virtually completely extracted from the feed oil even at high oil to carbon dioxide mass ratios. The concentration of pristane in
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Figure 4. Enhancement in pristane concentration in second separator product as a function of O/S ratio.
the raffinate stream was very low (
