Ind. Eng. Chem. Res. 1998, 37, 2529-2534
2529
Hydrodynamics and Mass Transfer in a Rotating Disk Supercritical Extraction Column Antero Laitinen* and Juha Kaunisto VTT Chemical Technology, P.O. Box 1401, FIN-02044 VTT, Finland
A novel rotating disk column (RDC) designed for high-pressure extraction was tested by extracting ethanol from aqueous solutions using supercritical carbon dioxide at 10 MPa and 313 K as a solvent. Mass-transfer efficiencies and hydraulic characteristics were measured as functions of a specific power input group (N3R5H-1D-2) and solvent-to-feed ratio. Agitation generally increased the value of the overall mass-transfer coefficient (Koda) and decreased the value of the height equivalent to a theoretical stage (HETS). The Koda values ranged from 0.006 to 0.015 s-1, and the HETS values ranged from 0.37 to 0.8 m. The dispersed-phase drop size decreased with the agitator rotor speed. The measured values were between 1.3 and 1.8 mm. Agitation and increasing solvent flow rate increased the dispersed-phase holdup from 0.04 to 0.2. The measured slip velocities ranged from 0.08 to 0.14 m s-1. The highest total throughput of the RDC column was approximately 74 m3 h-1 m-2. Introduction Separations using supercritical fluids as solvents in countercurrent extractions have been of considerable interest particularly in food and pharmaceutical industry due to several reasons. Mass transfer is enhanced because supercritical fluid, such as carbon dioxide, exhibit liquidlike density while the viscosity and diffusivity remain between liquidlike and gaslike values. Because the supercritical solvent is typically a gas at NTP, solute-solvent separation after extraction is usually relatively simple and cost-effective. Additionally, carbon dioxide is nontoxic, nonflammable, and relatively inexpensive. Up to now almost all countercurrent supercritical fluid extraction studies have been carried out in spray, sieve tray, or packed columns (Peter and Brunner, 1978; Rathkamp et al., 1987; Lahiere and Fair, 1987; Seibert et al., 1988; Czech and Peter, 1990; de Haan and de Graauw, 1991; Brunner et al., 1991; Bernad et al., 1993; Meyer and Brunner, 1994; Lim et al., 1995; Simo˜es et al., 1995; Sato et al., 1997; Nagase et al., 1997). In these studies countercurrent supercritical extraction columns, particularly packed columns, have been shown to be very efficient compared to conventional liquid-liquid extraction columns. According to Rathkamp et al. (1987), height equivalent to a theoretical stage (HETS) values as low as 10% of the values of the conventional liquid-liquid system have been measured. Although the packing increases the column efficiency compared to spray columns, the capacity is greatly reduced. This means that high-efficiency packed columns have a large diameter, which is not desirable in high-pressure vessels due to pressure vessel cost. The spray column arrangement allows the construction of small diameter columns, but the efficiency will not be as high as in packed columns. One solution combining the benefits of both spray and packed columns could be a mechanically agitated extraction column. Information concerning mechanically agitated extraction columns using supercritical fluid as a solvent is very * To whom correspondence should be addressed. Fax: +358 9 456 7026. E-mail:
[email protected].
Figure 1. Experimental installation.
scarce. However, Bunzenberger and Marr (1988) have used a Karr-type reciprocating-plate column to extract ethanol and furfural from aqueous solutions using supercritical carbon dioxide as a solvent. Further, Schultz et al. (1974) have used liquid carbon dioxide at 60 bar and 23 °C in a Scheibel column to extract volatile flavors from aqueous solutions. This paper reports mass-transfer efficiencies and hydraulic characteristics of a novel high-pressure benchscale mechanically agitated rotating disk column (RDC) operated under supercritical conditions. Experimental Section Equipment. A schematic view of the bench-scale experimental apparatus used in the experiments is in Figure 1. The extraction column (Chematur Ecoplanning Co.) has 35-mm inside diameter and 2-m height
S0888-5885(97)00658-1 CCC: $15.00 © 1998 American Chemical Society Published on Web 05/05/1998
2530 Ind. Eng. Chem. Res., Vol. 37, No. 6, 1998 Table 1. RDC Column Characteristics height of the column, mm total agitated height, mm column diameter, mm compartment height, mm rotor disk diameter, mm stator opening diameter, mm smallest cross-sectional area, mm2
2000 1400 35 20 21 24 452
and it can be used up to 40 MPa. A special pressurecompensating sealing system was developed to lead the electrically driven agitator shaft through the column head. The maximum rotating speed in this construction is around 400 rpm. The windows in the middle and at both ends of the column allowed us the observation of liquid level and fluid hydraulics. The liquid level at the top of the column was automatically controlled by an optical sensor, which pneumatically opened the raffinate stream valve. The column is equipped with a heating jacket, in which water is circulated by a thermostat (Lauda C6 CS). The column internal design consists of removable parts, allowing the use of different agitator geometries or even different types of columns. In this work the RDC design was used. The RDC geometry is summarized in Table 1. The RDC column consists of a vertical shell in which horizontal stator rings are installed. The stator rings are flat plates with a central opening. In the middle of the compartments formed by the stator rings rotor disks are installed. The calculation of the smallest cross-sectional area is based on the stator opening diameter. A commercial high-pressure extraction unit (Nova Werke AG) was used to feed carbon dioxide to the column. Carbon dioxide flow rates were measured by two high-pressure turbine flowmeters (EG&G Flow Technology) in solvent and extract streams. No devices, such as dispersers, were used to create the drops. An automatically controlled micrometering valve placed on the extract stream was used to control the pressure inside the column. The liquid feed stream was pumped to the column top by a high-pressure piston pump (Lewa AG). The liquid feed and carbon dioxide solvent streams were preheated to the extraction temperature by constant temperature water baths. Procedures. All experiments were carried out at 10 MPa and 313 K. Aqueous ethanol solution initially containing 10 wt % ethanol was used as a continuous phase and carbon dioxide as a dispersed phase in all experiments. Prior to each run, the column was heated to the operating temperature. The system was then pressurized to approximately 50 bar with carbon dioxide, after which the aqueous ethanol solution was pumped to the top of the column until the phase boundary was seen in the window at the top of the column. The carbon dioxide compressor (Nova AG) was then turned on and the system pressurized to the extraction pressure, and the extract stream valve was opened, allowing carbon dioxide to flow through the continuous liquid phase. The feed pump was then turned on again and the optical sensor controlling the liquid level was activated. A tachometer was used to measure the speed of the rotor. Each extraction experiment lasted approximately 4 h. Fresh carbon dioxide from the container was used in all experiments to avoid using a preloaded solvent. During the run the feed and raffinate tanks were weighed, and the carbon dioxide flow rates were measured every 20 min. After a 2-h run, samples were
taken from the feed, raffinate, and extract streams once every 20 min. The extract from the separator was continuously collected. The ethanol concentration in all samples was analyzed by GC. The concentration of ethanol in the extract stream was calculated by the overall material balance, and this value was used when Koda and HETS values were calculated. The concentration deviations in the consecutive feed and raffinate samples were typically within 2%, whereas the deviation of the consecutive extract stream samples was typically within 5%. Material balances were checked after each run. The relative deviation of the overall material balance was typically within 2%, whereas the relative deviations of ethanol balances around the column were within 7%. The method described by Rathkamp et al. (1987) was used to measure the dispersed-phase holdup. The data were obtained by shutting the feed pump and carbon dioxide compressor off at steady-state conditions and measuring the time required for the carbon dioxide drops that entered the bottom of the column to rise to the top of the column. This information combined with the flow rate of the dispersed phase was used to calculate the volume of the dispersed phase in the column, and the hold up for the run was calculated. The rising drops at various flow rates and rotor speeds were tape-recorded by a camera directed to the window in the middle of the column. The average diameter of the drop was calculated for each experiment from a frozen video frame. It was not possible to measure flooding points with this construction, because the optical sensor opened the raffinate stream valve at the bottom of the column when the liquid level started to rise. However, when the solvent and feed flow rates reached certain values, the raffinate stream valve was open so much that the major part of the solvent flow came out from the column with the raffinate stream. At this point the column was not any more operable. This point was used to describe the capacity of the column. Calculation Methods. Because the RDC column is a continuously operating column, the concept based on the height of a transfer unit (HTU) and the number of transfer units (NTU) was used to estimate the column efficiency and to calculate the mass-transfer coefficient. Recently, Lim et al. (1995) have measured the distribution coefficient of ethanol between supercritical CO2 and water. The value of 0.12 measured for a dilute ternary carbon dioxide-ethanol-water system at 10.1 MPa and 313 K was used in our calculations. According to the two-film theory, the resistance in the solvent phase controls the mass transfer, if the distribution coefficient is relatively small. Therefore, the calculation was based on the dispersed carbon dioxide phase. For dilute solutions and an immiscible solvent, the number of transfer units (NTUod) and the height of a transfer unit (HTUod) for the dispersed phase can be written as
NTUod )
∫yy
1
2
HTUod )
dy y* - y
Vd Koda
(1) (2)
where y is the weight fraction of solute in the dispersed phase, y* is the equilibrium value, Vd is the superficial velocity of the dispersed phase, Kod is the overall mass-
Ind. Eng. Chem. Res., Vol. 37, No. 6, 1998 2531 Table 2. RDC Column Extraction Results
no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14
S/F 13/1
13/1.5
13/2
rotor speed, rpm
ethanol concentration in feed stream, wt %
ethanol concentration in raffinate stream, wt %
ethanol concentration in extract stream, wt %
ethanol concentration in extract, wt %
Koda, 1/s × 103
HETS, m
10
2.59 2.17 0.97 0.78 0.84 0.95 2.30 2.69 2.75 2.52 3.76 3.39 3.70 3.32
0.65 0.65 0.60 0.67 0.72 0.65 0.85 0.99 0.94 0.84 0.96 0.99 0.95 1.02
78 82 84 86 80 81 84 91 89 89 89 89 89 84
6.0 7.2 9.7 12.7 11.1 10.3 13.0 12.3 11.0 11.8 11.6 13.0 13.1 14.8
0.80 0.67 0.46 0.37 0.41 0.44 0.44 0.49 0.53 0.49 0.58 0.50 0.52 0.45
0 50 158 223 301 388 0 104 207 312 0 103 204 303
10
10
transfer coefficient based on the dispersed phase, and a is the interfacial area. The total agitated height Z of the column is given by
Z ) HTUodNTUod
(3)
According to Treybal (1963) the height equivalent of a theoretical stage (HETS) and height of a transfer unit (HTUod) are related by
HETS )
ln E HTUod E-1
(4)
where E is the extraction coefficient. The effect of axial mixing on column efficiency was omitted, because the diameter of the column is only 35 mm. Slip velocity through the smallest cross-sectional area of the column Vs describes the relative velocity of the phases and is defined as
V hs )
V hd V hc + h 1-h
(5)
Figure 2. Effect of agitation on the overall mass-transfer coefficient. (O) S/F ) 13/2; (9) S/F ) 13/1.5; (b) S/F ) 13/1.
h c represent the relative velocities of the where V h d and V dispersed and continuous phases through the smallest cross-sectional area and h is the dispersed-phase holdup. Analysis. Aqueous ethanol samples were analyzed by a Perkin-Elmer 900 gas chromatograph (GC) containing a Porapak Q column and equipped with a flame ionization detector. Helium was used as a carrier gas. n-Butanol was used as an internal standard. Materials. Ethanol (99.5%) was obtained from Primalco, n-butanol (99.5%) from Merck, and carbon dioxide (99.7%) from AGA. Ion-exchanged water was used in the experiments. Results and Discussion Extraction Results. The aqueous feed initially contained 10 wt % ethanol, and after extraction the raffinate contained 0.8-3.8 wt % ethanol depending on the solvent-to-feed ratio and the rotor speed. The extract coming out from the separator, which was operated at approximately 35 bar and 278 K, contained 80-90 wt % ethanol. The RDC column extraction results are seen in Table 2. Column Efficiency. The values of the overall masstransfer coefficients Koda and the height equivalent of theoretical stages (HETS) are seen in Figures 2 and 3 as a function of the specific power input group (N3R5H-1D-2). The specific power input group describes
Figure 3. Effect of agitation on the height equivalent of a theoretical stage (HETS). (O) S/F ) 13/2; (9) S/F ) 13/1.5; (b) S/F ) 13/1.
the power input of one rotor disk per unit mass in the column compartment. It is normally used to correlate the performance of RDC columns (Kosters, 1983).
2532 Ind. Eng. Chem. Res., Vol. 37, No. 6, 1998
Generally, the Koda values increased when the agitation was turned on. Rathkamp et al. (1987) and Lim et al. (1995) have measured Koda values for spray and packed columns. Their values generally ranged from 0.006 to 0.04 s-1, depending on the type of column, packing, and solvent flow rate. The values measured in our work for the RDC column are in the same order of magnitude as those measured for small-scale spray and packed columns, even though values as high as 0.04 s-1 were not measured. The measured HETS values for the RDC column were in the range of 0.37-0.8 m. At the highest solvent-tofeed ratio (S/F ) 13), agitation enhanced column efficiency significantly. Introduction of agitation effectively reduced the drop size from approximately 1.8 to 1.3 mm, and the HETS value decreased from approximately 0.8 to 0.37 m. At a lower solvent-to-feed ratio, particularly at S/F ) 9, agitation did not seem to enhance the column efficiency significantly. It is possible that in this case equilibrium was approached, and increasing the rotor speed could not any more enhance the mass transfer or decrease the HETS value significantly. Increasing the rotor speed from 150 to 300 rpm did not reduce the dispersed-phase drop size significantly, and this explains why the Koda or HETS values change only very little when rotor speed is increased. Similar values for small-scale spray and packed columns have been measured (Rathkamp et al., 1987; Seibert et al., 1988; Bernad et al., 1993; Lim et al., 1995), although Rathkamp (1986) has obtained HETS values as small as 0.08 m. It was found that carbon dioxide preferentially wets ceramic and metal packings instead of forming drops (Seibert et al., 1988). Consequently, according to Johnston (1997) sieve trays are more efficient than packings. The reported HETS values for sieve tray columns are in the range of 0.20.5 m (Lahiere and Fair, 1987; Seibert et al., 1988). It is interesting to notice that small-scale spray columns are relatively efficient. However, according to Seibert et al. (1988), the effect of column diameter on the masstransfer efficiency is evident, especially for the spray column. The HETS value for the 2.54-cm diameter spray column was approximately 0.3 m, whereas the HETS value for the 9.88-cm diameter spray column was 1.1 m at similar extraction conditions. According to the authors, the difference in efficiencies is due to the decreased holdup and increased backmixing experienced in the 9.88-cm diameter column. The dispersed-phase holdup versus the specific power input group is seen in Figure 5. As expected, mechanical agitation increases the holdup with all experimental solvent-to-feed ratios. At the smallest solvent flow rate (S/F ) 2) the effect is very moderate, and the largest effect is seen at the highest solvent flow rate (S/F ) 10). Rathkamp et al. (1987) and Lim et al. (1995) measured hold-up values for spray and packed columns as a function of the superficial velocity of the dispersed phase. The measured values ranged from 0.03 to 0.12. The measured values in this work were in the same range, except that at the highest solvent flow rate (S/F ) 10) the measured hold-up values were higher than the above-mentioned values for spray and packed columns. Column Capacity. As the flooding point measurements were not possible due to the reasons described above, the highest flow rate where the column was still
Figure 4. Effect of agitation on the dispersed-phase drop size. (O) S/F ) 13/2; (9) S/F ) 13/1.5; (b) S/F ) 13/1.
Figure 5. Effect of agitation on dispersed-phase holdup. (b) S/F ) 3.2/1.6; (9) S/F ) 12/1.6; (O) S/F ) 16/1.6.
operable was measured. Without agitation the steadystate condition was disturbed, when the total throughput calculated as feed plus solvent flow divided by the smallest cross-sectional area of the column increased to approximately 81 m3 h-1 m-2. Agitation did slightly lower the value. The total throughput was approximately 74 m3 h-1 m-2, when the rotor speed was 300 rpm. Rathkamp et al. (1987) have measured flooding velocities for 2.54-cm diameter packed and spray columns for the carbon dioxide/2-propanol/water system. For a constant Vc ) 1.3 m/h, the packed column flooded at Vd ) 10.7 m/h, while the spray column reached flooding at 73 m/h. The RDC column was operable at approximately Vc ) 19 m/h and Vd ) 56 m/h. These values are significantly higher than the values measured for packed column, and approximately in the same order as those measured for the spray column for the carbon dioxide/2-propanol/water system. Typical operating slip velocities are seen in Figure 6. The values of slip velocities decrease with increasing rotor speed due to the decreasing dispersed-phase drop size and increasing holdup.
Ind. Eng. Chem. Res., Vol. 37, No. 6, 1998 2533 HTUod ) height of a transfer unit based on the dispersed phase, m Kod ) overall mass-transfer coefficient based on the dispersed phase, m/s m ) distribution coefficient ) concentration in solvent phase/concentration in feed phase N ) rotor speed, 1/s NTUod ) number of transfer units based on the dispersed phase R ) rotor disk diameter, m S ) flow rate of the solvent, kg/h y ) weight fraction of solute in the dispersed phase V ) superficial velocity, m/s Vs ) slip velocity, m/s Z ) total agitated height of the column, m Subscripts
Figure 6. Effect of agitation on slip velocity. (b) S/F ) 3.2/1.6; (9) S/F ) 12/1.6; (O) S/F ) 16/1.6.
Conclusion A novel rotating disk column (RDC) designed for highpressure conditions was tested by extracting ethanol from aqueous solutions using supercritical carbon dioxide at 10 MPa and 313 K as a solvent. Mass-transfer efficiencies and hydraulic characteristics were measured as functions of a specific power input group and solventto-feed ratio. Agitation generally increased the value of the overall mass-transfer coefficient (Koda) and decreased the value of the height equivalent to a theoretical stage (HETS). The Koda values ranged from 0.006 to 0.015 s-1, and the HETS values ranged from 0.37 to 0.8 m. These values are in the same range as those typically measured in small-scale spray, sieve tray, and packed columns operated under supercritical conditions. Mechanical agitation decreased the dispersed-phase drop size and increased the dispersed-phase holdup. The total throughput of the RDC column was approximately 74 m3 h-1 m-2, which is significantly larger than the capacity at flooding measured for packed columns. A mechanically agitated column seems to successfully combine the benefits of spray and packed columns (i.e., high mass-transfer efficiency and capacity). The high capacity is particularly important, because it allows the construction of small-diameter high-pressure columns, which is desirable due to the cost reasons. Mechanically agitated columns can also be applied, when the feed is viscous or contains solids. Acknowledgment M. Alkio and M. Ja¨ntti provided valuable advice and counsel during this work. Nomenclature a ) interfacial area, m2/m3 D ) column diameter, m E ) extraction coefficient ) mS/F F ) flow rate of feed, kg/h H ) compartment height, m h ) holdup HETS ) height equivalent to a theoretical stage, m
1 ) at the top of the column 2 ) at the bottom of the column c ) continuous phase d ) dispersed phase Superscripts * ) equilibrium value - ) through smallest cross section
Literature Cited Bernad, L.; Keller, A.; Barth, D.; Perrut, M. Separation of Ethanol from Aqueous Solutions by Supercritical Carbon DioxideComparison Between Simulations and Experiments. J. Supercrit. Fluids 1993, 6, 9-14. Brunner, G.; Malchow, Th.; Stu¨rken, K.; Gottschau, Th. Separation of Tocopherols from Deodoriser Condensates by Countercurrent Extraction with Carbon Dioxide. J. Supercrit. Fluids 1991, 4, 72-80. Bunzenberger, G.; Marr, R. Counter Current High-Pressure Extraction in Aqueous Systems. In Proceedings of the 1st International Symposium on Supercritical Fluids, Nice, France; Perrut, M., Ed.; Socie´te´ Francaise de Chimie: Nice, France, 1988; pp 613-618. Czech, B.; Peter, S. Efficiency of Different Packings in Counter Current near Critical Fluid Extraction. In Proceedings of the 2nd International Symposium on High Pressure Chemical Engineering, Erlangen, Germany; DECHEMA Chemische Technik und Biotechnologie e.V.: Frankfurt am Main, Germany, 1990; pp 419-424. de Haan, A. B.; de Graauw, J. Mass Transfer in Supercritical Extraction Columns with Structural Packings for Hydrocarbon Processing. Ind. Eng. Chem. Res. 1991, 30, 2463-2470. Johnston, K. P. Supercritical Fluid Separation Processes. In Perry’s Chemical Engineers’ Handbook, 7th ed.; Perry, R. H., Green, D. W., Maloney, J. O., Eds.; McGraw-Hill: New York, 1997. Kosters, W. C. G. Rotating-Disk Contactor. In Handbook of Solvent Extraction, 1st ed.; Lo, T. C., Baird, M. H. I., Hanson, C., Eds.; John Wiley & Sons: New York, 1983. Lahiere, R. J.; Fair, J. R. Mass Transfer Efficiencies of Column Contactors in Supercritical Extraction Service. Ind. Eng. Chem. Res. 1987, 26, 2086-2092. Lim, J. S.; Lee, Y.-W.; Kim, J.-D.; Lee, Y. Y. Mass-Transfer and Hydraulic Characteristics in Spray and Packed Extraction Columns for Supercritical Carbon Dioxide-Ethanol-Water System. J. Supercrit. Fluids 1995, 8, 127-137. Meyer, J. T.; Brunner, G. Apparatus for Determinination of Hydrodynamic Behaviour in Counter Current Columns and Some Experimental Results. In Proceedings of the 3rd International Symposium on Supercritical Fluids, Strasbourg, France; Perrut, M., Brunner, G., Eds.; I.N.P.L: Nancy, France, 1994; pp 217-222. Nagase, Y.; Tada, T.; Ikawa, N.; Fukuzato, R. Development of New Process of Purification and Concentration of Ethanol Solution using Supercritical Carbon Dioxide. In Proceedings of the 4rd International Symposium on Supercritical Fluids, Sendai, Japan; Takahashi Printing Co. Ltd.: Sendai, Japan, 1997, pp 617-619.
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Seibert, A. F.; Moosberg, D. G.; Bravo, J. L.; Johnston, K. P. Spray, Sieve Tray, and Packed High-Pressure Extraction ColumnsDesign and Analysis. In Proceedings of the 1st International Symposium on Supercritical Fluids, Nice, France; Perrut, M., Ed.; Socie´te´ Francaise de Chimie: Nice, France, 1988; pp 561570. Simo˜es, P. C.; Matos, H. A.; Carmelo, P. J.; de Azevedo, E. G.; Nunes da Ponte, M. Mass Transfer in Countercurrent Packed Column: Application to Supercritical CO2 Extraction of Terpenes. Ind. Eng. Chem. Res. 1995, 34, 613-618. Treybal, R. E. Liquid Extraction, 2nd ed.; McGraw-Hill: New York, 1963.
Received for review September 15, 1997 Revised manuscript received March 11, 1998 Accepted March 12, 1998 IE970658U
