Ind. Eng. Chem. Res. 1999, 38, 1877-1883
1877
An Electrically Driven Gas-Liquid-Liquid Contactor for Bioreactor and Other Applications Costas Tsouris,* Abhijeet P. Borole, Eric N. Kaufman, and David W. DePaoli Chemical Technology Division, Oak Ridge National Laboratory,† P.O. Box 2008, Oak Ridge, Tennessee 37831-6226
An electrically driven gas-liquid-liquid bioreactor is described here, in which an aqueous medium containing a biocatalyst is introduced as a discontinuous phase into an organiccontinuous liquid phase containing a substrate to be converted by the biocatalyst. A gas discontinuous phase, which may be needed to provide oxygen or a gaseous substrate to the biocatalyst, is also introduced into the bioreactor. In contrast to previous work on electrically driven contactors, it was found that the electroconvection generated by the electric field between parallel-plate electrodes may be employed to increase the volume fraction of the discontinuous gas phase in the bioreactor, providing the means for enhanced mass transfer. The electrically driven bioreactor was utilized for oil desulfurization experiments with Rhodococcus sp. IGTS8 bacteria as the biocatalyst. The organic phase used in the experiments was hexadecane containing dibenzothiophene, a model sulfur compound, that is oxidatively desulfurized to 2-hydroxybiphenyl (2-HBP) by the bacteria in the presence of air or oxygen. The gas volume fraction was increased by 60% by the application of a pulsed electric field, thus providing a means for increased transport of oxygen needed for oxidative desulfurization. The velocity of droplets and bubbles was measured by a phase Doppler velocimeter. The average rising velocity of bubbles was decreased from 13 to less than 3 cm/s and the average horizontal velocity was increased from 0 to 5 cm/s as the field strength was increased from 0 to 4 kV/cm. Desulfurization rates ranged from 1.0 to 5.0 mg of 2-HBP/g of dry cells/h. The desulfurization rate with aeration was doubled under the electric field as compared to the zero-field desulfurization under the same conditions. Introduction In many industrial systems, there is a need to facilitate the transformation of a substrate existing in an organic phase. Although some biocatalysts have been identified to perform such reactions directly in organic media (Almarsson and Klibanov, 1996; Woodward and Kaufman, 1996), in general, few biocatalysts are active in nonaqueous media. It is generally more feasible to suspend the biocatalyst in an aqueous phase and contact it with an immiscible organic phase containing the substrate. This approach involves several transport steps, which make the process relatively slow; however, the biocatalyst is more viable and active than if it were directly suspended in the organic phase. Several types of liquid-liquid contactors, developed in the past for solvent extraction processes, can be readily employed in multiphase bioprocessing to reduce mass-transfer limitations. Among these systems, an electrically driven emulsion-phase contactor (EPC) was developed by Scott and co-workers (1989, 1994) to produce a fine emulsion by means of electric fields (Figure 1). The distinguishing characteristic of this contactor is the use of high-intensity electric fields to disperse and coalesce electrically conducting (aqueous) liquids into relatively nonconducting (organic) liquids. Results of extraction experiments showed that the energy consumption, 2.4 W/L of emulsion, was much smaller than the energy consumption in conventional * Corresponding author. E-mail:
[email protected]. † Research supported by the Office of Oil and Gas Processing, U.S. Department of Energy under Contract DE-AC0596OR22464 with Lockheed Martin Energy Research Corp.
Figure 1. The emulsion-phase contactor (EPC) with the nozzle region and the operating channel.
liquid-liquid contactors and that mass-transfer performance was high. To avoid high current and maintain high voltage on the parallel electrodes, it was required
10.1021/ie9802515 CCC: $18.00 © 1999 American Chemical Society Published on Web 04/15/1999
1878 Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999
that the continuous phase between the electrodes be of low conductivity. Hence, the EPC can only be used for aqueous-dispersed/organic-continuous systems. The nozzle region of the contactor shown in Figure 1 was used to introduce the dispersed fluid in the form of small droplets. A breakage-coalescence cycle was induced by a modulated electric field in the operatingchannel region between two parallel plates, which controlled the size of the droplets as well as their motion toward the opposite end of the contactor (Figure 1). Scott and co-workers (1994) pointed out that as the dispersion entered the operating channel, the droplets began to continuously coalesce and redisperse between the plates, because of the electrophoretic and dielectrophoretic forces. Drops were polarized under electric fields, and as a result, they broke into two or more droplets or coalesced with other polarized droplets. These smaller droplets obtained electrical charges and because of electrophoretic forces, moved toward the oppositely charged electrode. The phenomena described above contribute to intense horizontal mixing of both continuous and dispersed phases in the EPC. Axial mixing has also been observed, which is undesirable, because it decreases the driving force for interfacial mass transfer. However, axial mixing in other contactors, such as the multistage stirred contactor, is much more intense than that in the EPC (Tsouris et al., 1990; Scott et al., 1994). The main disadvantages of the EPC in extraction applications are the low flow rate and the low volume fraction (less than 2%) of the aqueous dispersed phase that are achievable in steady operation. A relatively high flow rate overwhelms the ability of the system to disperse the conductive phase and results in electrical shorting between the plates, decreasing system performance and increasing power consumption. These disadvantages of the EPC do not impact systems such as bioprocessing applications, for which it is desirable to have a relatively low flow rate and volume fraction of the dispersed phase. An example of such an application is the biodesulfurization of oil, in which the process economics demand that a vast quantity of oil be treated by a small quantity of a biocatalyst (Kaufman et al., 1998). The biological refining of oil offers an attractive alternative to conventional thermochemical treatment methods, because of the mild operating conditions and greater selectivity exhibited by the biocatalysts. Efforts in microbial screening have identified microorganisms capable of petroleum desulfurization either oxidatively or reductively. In the oxidative approach, organic sulfur is converted to sulfate and may be removed in process water. This approach is attractive because it would not require further processing of the sulfur and may be used at the well head, where process water may then be reinjected. In the reductive desulfurization approach, organic sulfur is converted into hydrogen sulfide, which may then be converted into elemental sulfur. Desulfurization rates, which have been obtained by both oxidative and reductive microorganisms, have been summarized by Kaufman et al. (1997). It was found that, regardless of the approach of biodesulfurization, the key factors affecting the economic viability of these bioprocesses are the biocatalyst activity and cost, differential in product selling price, sale or disposal of coproducts or wastes from the treatment process, and the capital and operating costs of unit operations.
Figure 2. Oxidative pathway for the desulfurization of DBT by Rhodococcus sp. IGTS8 bacteria.
It is desirable, with respect to process economics, to minimize the biocatalyst and water utilization with respect to the amount of oil. Such an operation can be efficiently carried out by the EPC, as shown in this study. The EPC developed by Scott et al. (1994) was modified to work properly with biological systems, which tend to accumulate along surfaces and thereby form conductive paths between the electrified parallel plates. The Rhodococcus sp. IGTS8 bacteria that were used in this study for the desulfurization of dibenzothiophene (DBT) fall into such a category. These bacteria follow the oxidative pathway for the desulfurization of DBT, with the reactions shown in Figure 2 (Kilbane and Bielaga, 1990). Oxygen needed by the bacteria is provided by dispersing air or oxygen bubbles into the liquid in the EPC. Thus, the modified EPC described in this work is used to contact a gas-liquid-liquid system rather than a liquid-liquid system in solvent extraction for which it was originally developed. Addition of the gas bubbles in the EPC revealed that electroconvection rather than electrophoresis is a major contributor to the axial dispersion and the serpentine droplet trajectory reported by Scott and co-workers. To provide a better understanding of the EPC operation, measurements of bubble velocities in the EPC were obtained using a phase Doppler velocimeter. Measurements of the volume fraction of aqueous droplets in the absence of bubbles were also obtained using an ultrasonic method, which works only for liquid-liquid systems. The ability of the EPC to be used as a gas-liquid-liquid bioreactor was demonstrated by desulfurization experiments of DBT using Rhodococcus sp. IGTS8 bacteria. Materials and Methods EPC. The original geometry and operation of the electrically driven contactor (EPC) have been described by Scott et al. (1994) and Kaufman et al. (1997). The EPC (see Figure 1) is a square Teflon vessel of 10 × 10 cm cross section and 61 cm length. The front and backsides are open and covered with clear Lexan plates during operation to allow visualization. A thin sheet of Teflon is placed between the main body and the two Lexan plates to prevent wetting and current leakage. The original design of the EPC had two 30 × 6 cm stainless steel plates, placed parallel to each other on the two Teflon sides, which were used as high-voltage electrodes. These parallel electrodes, which formed the “operating channel” of the EPC, were connected to pulsed dc (direct current) high-voltage (up to 35 kV) peaks that were generated by a pulsed dc power supply and automobile ignition parts. A power supply (HewlettPackard 6653A, Avondale, PA) and two sweep/function generators (BK Precision 3030, Chicago, IL) were used to produce the signal, which was then passed through ignition coils (Mallory Promaster 29901, Carson City,
Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999 1879
NV) to step up the voltage. High-voltage diodes (Collmer Semiconductor CS4107X30, Dallas, TX) were placed between the ignition coils and the EPC electrodes to produce positive and negative pulses. The positive signal was applied to one of the two plate electrodes, while the negative signal was applied to the other. The pulses were synchronized by this configuration, thereby doubling the field strength (voltage difference divided by distance). The two sweep/function generators were used to generate a modulated signal of a low-frequency component (on the order of 1 Hz) and a high-frequency component (on the order of 1 kHz) superimposed on each other. The low-frequency signal dictated an on/off mode for the applied voltage. The high-frequency signal was applied only when the low-frequency signal was in the “on” mode. The high-frequency signal was mainly responsible for the drop coalescence-redispersion cycle, while the low-frequency signal was responsible for the axial motion of the droplets, which was driven by gravity. The aqueous phase is introduced in the organic phase through an electrically grounded capillary. Surrounding the capillary is a metal ring immersed in the organic phase. The ring is connected to an automobile ignition coil through a diode, in a manner similar to the plate electrification system. The region of the capillary tip and the electrified ring, which is called the “nozzle region,” is used to form small droplets of the aqueous phase into the organic phase. EPC Modifications. When the original design of the EPC was used with the Rhodococcus sp. IGTS8 bacteria in the aqueous phase, it was observed that the bacteria formed aggregates on the walls of the EPC. Some of the aggregates extended from one electrode to the other, forming conducting paths that allowed high current flow. Thus, high voltage could not be maintained between the parallel plates, resulting in poor hydrodynamic performance. This problem can be avoided by suspending the electrified plate by rods (Figure 3), reducing the possibility for the formation of conducting paths. In the modified experimental arrangement, the suspended central plate was electrified, while the side plates were grounded. The distance between the central electrified plate and the side grounded plates was half that between the two original electrodes. The potential difference, however, is also half the original; therefore, the field strength, defined as the potential difference over the distance, is the same. In this modified configuration, there are two identical operating channels between the three parallel plates. Also, two identical nozzle regions are present to equally distribute the input dispersed fluid in the two operating channels. The modified EPC provides oxygen to the bacteria by means of gas introduced at the bottom of the liquid column through an electrically grounded stainless steel bubble diffuser, positioned such that bubbles are formed in the organic fluid. Hydrodynamics Experiments. A phase Doppler velocimeter (PDV from TSI, Inc., St. Paul, MN) was employed to measure the velocities of bubbles and drops between electrified parallel plates. In these experiments, a geometry similar to that of the original EPC was used, which involved two parallel plate electrodes separated by a distance of 5 cm. Air was injected from a stainless steel bubble diffuser placed at the center of the electrodes and approximately 3 cm below the bottom end of the electrodes. Simultaneous negative and positive
Figure 3. Modified EPC with an electrified center plate and grounded side plates.
electrical pulses, produced by automobile ignition coils as described above, were provided to the two electrodes, respectively, to increase the electric-field strength. The plates were immersed in an organic solvent, such as kerosene or hexadecane. To provide charge carriers and increase the electric-field effect, a small quantity of versatic acid was added until the conductivity of the solution was increased to 0.4 pmho/cm. The conductivity was measured by a conductivity meter (model 627, Scientifica, Princeton, NJ). Both horizontal and vertical components of bubble velocities were measured by the PDV, 8 mm above the bubble diffuser. In these experiments, air was injected at a flow rate of 15 mL/min. Other hydrodynamics measurements, obtained from the original, two-plate EPC, include visualization of color tracers and bubbles and volume-fraction measurements of bubbles with and without the influence of the electric signal. Volume Fraction of Drops. Aqueous-phase holdup was measured in the EPC by an ultrasonic technique (Tsouris and Tavlarides, 1993). In the present work, this technique utilizes a pulse generator (model 5055PR, from Panametrics, MA) that sends a series of electrical pulses to an ultrasonic transducer (model V301, from Panametrics, MA) mounted on the outside wall of the EPC. The electrical pulses are translated into sound waves that travel through the liquid in the EPC and are received by an identical receiver transducer mounted on the outside wall opposite the transmitter transducer. The sound waves are translated into an electrical signal by the receiver transducer and sent to a digital oscilloscope (Pro 50 from Nicolet, WI) which is also connected to the pulse generator for triggering purposes.
1880 Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999
The periods of time taken by sound waves to travel through pure aqueous phosphate buffer and pure hexadecane were measured initially. The temperature of each phase was controlled at 30 °C by circulating the organic and aqueous phases through a water bath. The time taken by sound waves to travel through the dispersion of buffer solution in hexadecane was used to determine the aqueous holdup. Measurements were made in the absence of the biocatalyst at two positions in the EPC: 3 in. (top of plates) and 10 in. (bottom of plates) below the nozzle region. Biocatalyst, Solvent Systems, and Analytical Methods. Rhodococcus sp. wild strain IGTS8 (ATCC 53968), provided by Energy Biosystems Corp. (The Woodlands, TX), served as the biocatalyst for the oxidation of DBT in hexadecane. The enzymatic steps in the pathway of DBT oxidation by these bacteria have been discussed by Gray et al. (1996). Cells were supplied as a frozen paste and had a cell dry weight of 0.28 g/g of original frozen material. In each experiment, the aqueous phase consisted of 0.156 M, pH 7.5, potassium phosphate buffer. Dibenzothiophene (DBT) was dissolved in hexadecane, which was the organic (continuous) phase, to form a solution approximately 0.6 wt % in DBT. The samples collected from the EPC were centrifuged at 14 000 rpm for 5 min to separate the cells from the aqueous phase. Concentrations of both DBT and the desulfurization product 2-hydroxybiphenyl (2-HBP) in hexadecane were measured by gas chromatography using a Hewlett-Packard 5890 gas chromatograph (GC) with a flame ionization detector (Kaufman et al., 1997). In the experiments reported in this work DBT and 2-HBP concentrations in the aqueous phase were below the detection level of our instruments; thus, only organic-phase concentrations of these substances are reported. Biodesulfurization Experiments in the EPC. The EPC (total capacity, 2.4 L) was demonstrated as a bioreactor for the oxidative biodesulfurization of DBT. The temperature was controlled at 30 °C by pumping liquid from the top of the EPC through a stainless steel coil submerged in a heated bath and then recycling it to the bottom of the EPC. The biocatalyst as a frozen cell paste (26.7 g) was added in potassium phosphate buffer to form a 100 mL volume suspension, which was introduced in the EPC through two 1.6 mm o.d., 1 mm i.d. metal capillaries (V-140 Upchurch Scientific, Oak Harbor, WA) at a flow rate of 5 mL/min and removed from the bottom of the EPC to be recycled. Air was introduced at the bottom of the EPC at a flow rate of 36 mL/min. Samples from the top of the EPC were drawn every hour, centrifuged, and run in triplicate through the GC for DBT and 2-HBP analyses. Results and Discussion Hydrodynamics. Various electrohydrodynamic phenomena occurring in electrically driven solvent extractors have been discussed by Thornton (1968), Bailes (1979), Kowalski and Ziolkowski (1981), Scott and Wham (1989), and Scott et al. (1994). These phenomena refer only to the drop phase and include drop polarization, breakup, electrophoresis, and coalescence. Axial mixing occurring in the continuous phase under an electric field was attributed to the presence and motion of the drops. Another mechanism of mixing in a single-phase fluid is by electroconvection (Cropper and Seelig, 1962; Mc-
Figure 4. Bubble flow in the EPC. Top left: bubbles in the middle of the EPC at 0 V. top right: bubbles in the middle of the EPC when the field is on. Bottom left: bubbles near the bubble diffuser at 0 V. Bottom right: bubbles near the bubble diffuser when the field is on.
Cluskey and Atten, 1988; Sharbaugh and Walker, 1985; Yabe et al., 1995; Tsouris et al., 1998). Indicative of this mechanism in this work was the behavior of the kerosene interface under an electric field. It was observed that when a pulsed-dc voltage of 20 kV pulses was applied, intense interface motion occurred. This fluid flow is due to the motion of charge carriers, such as electrons and ions, caused by the electric field. Although organic fluids, such as kerosene or hexadecane, have low conductivity, some ions are always present in the bulk fluid. In addition, charge carriers are also injected by the pulsed dc signal. Charge, in the form of electrons, is transmitted to the fluid at each pulse. The motion of the electrons between the two electrodes causes the molecules of the fluid to move, thereby generating electroconvective flow. Tracer experiments were also conducted to visualize electroconvection in the bulk continuous phase between the parallel electrified plates. In these experiments, oil red was used as a tracer for flow observations in kerosene under a pulsed-dc electric field. The maximum peak of the pulses was 20 kV, and the measured current was 0.2 mA. Oil-red/kerosene solution was injected into the kerosene between the parallel plates, and a homogeneous mixture was obtained approximately 3 s after the voltage was turned on, indicating a strong electroconvective flow. Gas-Phase Volume Fraction. The fluid flow caused by the electric field forces the drops and bubbles to move between the two plates. This movement is easier to observe when bubbles are used as flow tracers because drops undergo a continuous breakage-coalescence cycle that complicates the flow. Bubbles in organic fluids are not polarizable under the conditions studied in this work, therefore bubble breakup and coalescence are considered insignificant and the bubble size distribution does not change significantly. In Figure 4, the photographs on the left were for 0 V at the middle of the EPC (top, left) and at the bottom near the bubble diffuser (bottom, left). As shown, the
Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999 1881 Table 1. Aqueous Holdup in the EPC aqueous holdup (%) aqueous flow rate (mL/min)
applied voltage (kV)
duty cycle (%)
3 in. below nozzle (near top of plates)
5 5 5 5 8 12 12
15 18 18 22 18 18 18
50 50 90 90 50 50 90
1.07 0.89 0.90 0.90 0.81 0.90 1.26
a
Figure 5. (a) Horizontal velocity distribution measurements of bubbles between parallel plates under electric fields. (b) Horizontal average velocity measurements of bubbles between parallel plates under electric fields.
air bubbles in kerosene moved directly upward. The photographs on the right were taken at the corresponding locations when 20 kV pulses were applied to the electrodes. As shown, bubbles were present throughout the space between the two electrodes. Continuous visualization of the flow during the experiments revealed that the bubbles followed an overall S-shaped trajectory, similar to the flow of droplets that was characterized as a “serpentine trajectory” by Scott et al. (1994). Buoyancy should be the dominant force since the bubbles are not expected to be charged in an organic medium; however, the observed motion of the bubbles indicates that electroconvection of the fluid occurred, which has a significant effect on the trajectory of drops and bubbles under an electric field. The S-shape trajectory of the bubbles led to an increase in the volume fraction of bubbles in the EPC. The bubble volume fraction was obtained by measuring the volume of the displaced liquid. At a 15 mL/min airflow rate, it was found that 20 kV pulses increased the bubble volume fraction by more than 60%. Bubble-Velocity Measurements by PDV. Velocity measurements of the bubbles were obtained approximately 2 cm above the bubble diffuser. Measurements of the horizontal velocity component are presented in Figure 5a,b. The cumulative velocity distribution in Figure 5a shows that when the electrical signal was applied, the bubbles consistently moved horizontally toward one of the plate electrodes at an average velocity of 4-5 cm/s (Figure 5b). These measurements were obtained for a pulsing frequency of 4.7 kHz; however, the frequency was not found to significantly affect the velocity of the bubbles. Also, it is shown in both parts a and b of Figure 5 that, above 10 kV, the bubble
10 in. below nozzle 0.30 0.15 0.21 NAa NA NA NA
NA ) not available.
horizontal velocity does not have a strong dependence on the intensity of the applied voltage. The axial (vertical) bubble velocity was found to be significantly affected by an applied voltage, as shown in Figure 6a,b. Both the cumulative distribution and the average velocity indicate that the bubble axial velocity decreased as the voltage was increased. A velocity decrease from 13 to less than 3 cm/s as the voltage was increased from 0 to 20 kV was observed at a frequency of 1.2 kHz. This variation, however, depends on the location at which the measurements are obtained. For example, velocity variations near the surface of the plates may be much different than at the center plane between the plates. From PDV measurements, it can be concluded that the horizontal velocity of the bubbles increased when the field was on, while the average axial velocity decreased as the voltage was increased. As a result of this behavior, the volume fraction of the bubbles in the EPC increased with the applied voltage. Similar results were observed with drops, except that there is an upper limit in the volume fraction of aqueous droplets (on the order of 1%, depending on the electrical conductivity of the dispersed phase), above which the droplets form conductive paths between the electrodes, causing sparking and high currents. Volume Fraction of Drops. Volume-fraction (holdup) measurements of drops were made for various aqueous flow rates, applied voltages, and duty cycles. Holdup data are presented in Table 1. An increase in the aqueous flow rate did not result in significant changes in aqueous holdup over the range measured at 50% duty cycle. The effect of the duty cycle on the holdup was also insignificant. However, at the higher flow rates, visual observations indicated arcing between the two electrodes. A 50% duty cycle corresponded to a voltage application 50% of the time for the low-frequency cycle. In any given frequency cycle, a fine spray of the aqueous phase was formed during the time when voltage was applied, resulting in the formation of fine droplets. The average holdup in the upper region of the EPC was on the order of 1%; in the lower region of the EPC, it was 0.3%. The reason for this behavior is that droplets at the bottom region are attracted by the electrically grounded aqueous layer and therefore move fast out of the lower part of the operating channel. In addition to the model system, hexadecane/ phosphate buffer, experiments were also conducted with actual crude oil for holdup measurements. The aqueous phase used in these experiments was the same phosphate buffer solution described earlier. The holdup measurements were made at 18 kV applied voltage, 50% duty cycle, and two different aqueous flow rates, 3 and 5 mL/min. After any change in the flow rate, the system
1882 Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999
Figure 7. Biodesulfurization of dibenzothiophene in the EPC. Rates of desulfurization observed were typically between 1 and 5 mg of HBP/g of dry biocatalyst/h. The bioconversion requires oxygen, and this requirement was met by the aeration supplied by the EPC. Aeration using pure oxygen (oxygen in an aqueous res.) did not enhance bioconversion versus aeration using air (standard aeration). Aeration at no-field conditions (no spraying) resulted in a lower conversion rate than aeration under an electric field (standard aeration or oxygen in an aqueous res.).
Figure 6. (a) Axial velocity distribution measurements of bubbles between parallel plates under electric fields. (b) Axial average velocity measurements of bubbles between parallel plates under electric fields.
was allowed to come to equilibrium for 15 min before the measurement was taken. The crude oil was maintained at 30 °C by circulating it through a constanttemperature bath. The experiment was conducted over a period of 8 h. Aqueous holdup for the buffer solution in crude oil was determined to be 4.84% ( 0.52% at 3 mL/min and 7.65% ( 1.76% at 5 mL/min. The standard deviation was determined on the basis of triplicate samples. The increased holdup in the oil system is believed to be mainly due to lower interfacial tension (12 dyn/cm at 22 °C) for the oil-buffer system as compared to that of the hexadecane-buffer system (27 dyn/cm). The lower interfacial tension results in smaller droplets because of higher breakup and lower coalescence rates. These droplets fall through the oil continuous phase at a lower rate, resulting in a higher aqueousphase holdup. Biodesulfurization. Time-course experiments for the biodesulfurization of DBT performed in the EPC are shown in Figure 7 where the production of 2-hydroxybiphenyl (in mg) is plotted with time. The production of 2-HBP was concomitant with the removal of DBT. Rates of desulfurization calculated from these data were typically between 1 and 5 mg of 2-HBP/g of biocatalyst/ h. As seen in Figure 7, while the biocatalyst does require oxygen for the desulfurization reaction, adequate aeration is provided by the use of ambient air (noted as standard aeration in Figure 7). Pure oxygen does not significantly enhance the desulfurization reaction. A comparison of the no-field (no spraying) to the field (standard aeration or oxygen in aqueous reservoir) experiments in Figure 7 shows that the production of 2-HBP is approximately doubled under spraying condi-
tions, as compared to that under the nonspraying conditions at any time. This result is due to a higher mass-transfer rate of oxygen from air bubbles to the continuous organic phase and from the organic phase to the aqueous droplets, as well as higher mass transfer of sulfur species from the organic phase to the droplets. The EPC has also been demonstrated to have no detrimental effect on cell viability or activity because of the electric fields or shear forces employed. Conclusions In summary, an EPC that had been developed earlier by Scott and co-workers as a liquid-liquid extractor was modified for use in this work as a bioreactor for oil desulfurization using Rhodococcus sp. IGTS8 bacteria as the biocatalyst. An aqueous medium containing the biocatalyst was introduced as the discontinuous phase in an organic-continuous liquid phase, between two parallel steel plates. Air or oxygen was also introduced into the bioreactor to be utilized by the bacteria. An electrical signal of a high pulsing frequency, in the kilohertz range, superimposed on a low frequency, in the hertz range, was used to form a liquid-liquid emulsion with an additional dispersed gas phase, with the objective of enhancing the mass-transfer rate of sulfur compounds from the organic phase to the aqueous phase. Hydrodynamic phenomena affecting the aqueous droplets in the bioreactor included (i) polarization, (ii) breakup and formation of charged drops, (iii) motion and collision due to electrophoresis, (iv) collision and coalescence due to the attractive force between polarized droplets approaching each other, (v) collision and dispersion due to Coulombic electroconvection, and (vi) axial motion due to gravity forces. Tracer studies, velocity measurements of bubbles, and ultrasonic measurements of pure liquids and liquid dispersions showed that significant electroconvection was induced between the parallel plates. This electroconvection may be employed to increase the volume fraction of gas bubbles as well as aqueous droplets in the bioreactor. The effectiveness of this three-phase bioreactor approach was demonstrated in desulfurization experi-
Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999 1883
ments using hexadecane containing DBT, a model sulfur compound, that was oxidatively desulfurized to 2-HBP by the bacteria in the presence of air or oxygen. Desulfurization rates ranged from 1.0 to 5.0 mg of 2-HBP/g of dry cells/h. No detrimental effect on cell viability or activity was observed. Acknowledgment Support provided by the Office of Oil and Gas Processing, U. S. Department of Energy, under contract DE-AC05-96OR22464 with Lockheed Martin Energy Research Corp., is gratefully acknowledged. Partial support was provided by the Environmental Management Science Program, Office of Environmental Management, and the Division of Chemical Sciences, Office of Basic Energy Sciences, U. S. Department of Energy. The authors also express thanks to Dr. Marsha Savage and Ms. Martha Stewart for editing the manuscript. The biocatalyst, Rhodococcus sp. IGTS8, used in this study was provided by Energy Biosystems Corp., The Woodlands, TX. Literature Cited Almarsson, O.; Klibanov, A. M. Remarkable Activation of Enzymes in Nonaqueous Media by Denaturing Organic Solvents. Biotechnol. Bioeng. 1996, 49, 87. Bailes, P. J. Electrostatic Extraction for Metals and Non-Metals. Proc. ISEC 77 Conf. (Toronto) 1979, 2, 233. Cropper, W. P.; Seelig, H. S. Mixing with an Electrostatic Field. Ind. Eng. Chem. Fundam. 1962, 1, 48. Gray, K. A.; Pogrebinsky, O. S.; Mrachko, G. T.; Xi, L.; Monticello, D. J.; Squires, C. Molecular Mechanisms of Biocatalytic Desulfurization of Fossil Fuels. Nature Biotechnol. 1996, 14, 1705. Kaufman, E. N.; Harkins, J. B.; Rodriguez, M.; Tsouris, C.; Selvaraj, P. T.; Murphy, S. E. Development of an Electro-Spray Bioreactor for Crude Oil Processing. Fuel Process. Technol. 1997, 52, 127. Kaufman, E. N.; Borole, A. P.; Shong, R.; Sides, J. L.; Juengst, C. Sulfur Specificity in the Bench Scale Biological Desulfurization
of Crude Oil by Rhodococcus IGTS8. J. Chem. Biochem. Technol. 1999, submitted for publication. Kilbane, J. J.; Bielaga, B. A. Toward Sulfur-Free Fuels. CHEMTECH 1990, Dec, 747. Kowalski, W.; Ziolkowski, Z. Increase in Rate of Mass Transfer in Extraction Columns by Means of an Electric Field. Int. Chem. Eng. 1981, 21, 323. McCluskey, F. M. J.; Atten, P. Modifications to the Wake of a Wire Across Poiseuille Flow Due to a Unipolar Space Charge. J. Fluid Mech. 1988, 197, 81. Scott, T. C.; Wham, R. M. An Electrically Driven Multistage Countercurrent Solvent Extraction Device: The Emulsion Phase Contactor. Ind. Eng. Chem. Res. 1989, 28, 94. Scott, T. C.; DePaoli, D. W.; Sisson, W. G. Further Development of the Electrically Driven Emulsion Phase Contactor. Ind. Eng. Chem. Res. 1994, 33, 1237. Sharbaugh, A. H., II; Walker, G. W. The Design and Evaluation of an Ion-Drag Dielectric Pump to Enhance Cooling in a Small Oil-Filled Transformer. IEEE Trans. Ind. Appl. 1985, IA-21, 950. Thornton, J. D. The Application of Electrical Energy to Chemical and Physical Rate. Rev. Pure Appl. Chem. 1968, 18, 197. Tsouris, C.; Tavlarides, L. L. Volume Fraction Measurements of Water in Oil by an Ultrasonic Technique. Ind. Eng. Chem. Res. 1993, 32, 998. Tsouris, C.; Ferreira, R.; Tavlarides, L. L. Characterization of Hydrodynamic Parameters in a Multistage Column Contactor. Can. J. Chem. Eng. 1990, 68, 913. Tsouris, C.; Shin, W.-T.; Yiacoumi, S. Pumping, Spraying, and Mixing of Fluids by Electric Fields. Can. J. Chem. Eng. 1998, 76, 589. Woodward, C. A.; Kaufman, E. N. Enzymatic Catalysis in Organic Solvents: Polyethylene Glycol Modified Hydrogenase Retains Its Sulfhydrogenase Activity in Toluene. Biotechnol. Bioeng. 1996, 52, 423. Yabe, A.; Mori, Y.; Hijikata, K. Active Heat Transfer Enhancement by Utilizing Electric Fields. Annual Review of Heat Transfer; Begell House: New York, 1995; Vol. 7, Chapter 4.
Received for review April 24, 1998 Revised manuscript received January 5, 1999 Accepted March 11, 1999 IE9802515
