Application of traditional mass-transfer algorithms to a bioextraction process

1868 Ind. Eng. Chem. Res. 1989,28, 1868-1873

Application of Traditional Mass-Transfer Algorithms to a Bioextraction Process

R. Bruce Eldridge,* Sharon Booth-McGee,+ and Jim L. Turpin Chemical Engineering Department, University of Arkansas, Fayetteville, Arkansas 72701

The removal of ethanol from a fermentation broth was studied in a 2.5-cm-diameter spray tower extractor. A mixture of a commercial isoparaffinic solvent (Exxon Isopar M) (70 w t %) and tributyl phosphate (30 wt %) was used as the extractive solvent. Ethanol solubility and microbial toxicity measurements were obtained for the solvent system. The solvent exhibited an ethanol distribution coefficient of 0.21 g/g and was not toxic to the microorganism when contacted with the broth a t saturated solution concentrations. The ethanol was produced by batch fermentation using the yeast Saccharomyces cereuisiae. Results obtained from the extraction experiments were compared to the predictions of traditional liquid-liquid mass-transfer models. The agreement was good, and the results indicated that traditional approaches can be used to design a bioextractor. For comparison, data for the extraction of ethanol from pure water were also obtained. There was a significant difference between the mass-transfer results obtained for the two systems.

Liquid-liquid extraction is widely used for the recovery of dilute solutes from aqueous streams. Recently, there has been a significant amount of interest in modeling the mass-transfer process (Seibert, 1986; Rocha, 1984). This research has produced a sound understanding of the extraction process and has identified algorithms that accurately model the extractor performance.

A t the same time, there has been increased interest in biological product recovery. Liquid-liquid extraction is viewed as having a great deal of potential in this area, and a moderate amount of bioextraction development work has been conducted. Because of the difficulity encountered in finding a biocompatible solvent, with a high solute solubility, the majority of the development effort has fo- cused on solvent selection (Kollerup and Daugulis, 1985). Most of the extraction research to date has been done in contactors that have very low mass-transfer efficiencies (Minier and Goma, 1986). A very limited amount of op- timization of contactor design has been done, and no traditional mass-transfer models have been reviewed to determine if they are applicable to a bioextraction process. However, an extractive fermentation system has been developed and tested that significantly enhanced the concentration of glucose that can be fully utilized by the microorganism (Kollerup and Daugulis, 1987). This system conducted the extraction in the fermenter vessel, thus limiting the maximum removal efficiency to one theoretical stage. An extension of this work utilized a flash vaporization unit to recover the ethanol from an undisclosed solvent (Kollerup and Daugulis, 1987). Another approach involving a multimembrane bioreactor system has been developed, which allows the use of a microorganism toxic extractive solvent (Cho and Shuler, 1986). This approach will allow solvents to be selected solely based on solute solubility levels, not microorganism toxicity.

In this work, we determined the applicability of tran- ditional liquid-liquid mass-transfer models to a bioextraction process. Mass-transfer results were obtained for the bioextraction of ethanol from a fermentation broth and for the extraction of ethanol from pure water. The water system was studied in order to determine if the presence of surface active agents (salts, cell bodies, carbon source, etc.) in the fermentation broth would affect the separation.

* Present address: Phillips Petroleum Co., Bartlesville, OK

'Present address: Ethyl Corporation, Baton Rouge, LA 70801. 74004.

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Table I. Isopar M Physical Properties DroDertv value test method

solubility parameter flash point distillation

IBP 10% 50 % 90 % FBP

av mole weight API gravity specific gravity viscosity

at 25 "C at 100 "C

at 16 "C a t 66 "C

liquid specific heat

heat of vaporization (100 "C) surface tension (25 "C)

7.3 80 "C

207 "C 213 "C 223 "C 241 "C 260 O C

191 49.2 0.783

2.46 CP 0.72 CP

2.05 kJ/(kg "C) 2.26 kJ/(kg "C) 307 kJ/ kg 26.6 dyn/cm

calculated ASTM D 93 ASTM D 86

cryoscopic ASTM D 287 calculated

ASTM D 445

enthalpy data

API Project 44 du Nuoy

A degradation of mass transfer, caused by the presence of cell bodies, has been reported in the literature (Crabbe et al., 1986).

The solvent system used in our work, Isopar M and tributyl phosphate (TBP), has been previously used for ethanol recovery (Tedder et al., 1984). These experiments were conducted in a Karr reciprocating plate extractor. The Karr column has been used, with good results, for bioextraction (Karr, 1979).

After a review of solvents referenced in the open literature, the Isopar M-TBP solvent system was selected based on its ethanol solubility, low microorganism toxicity, moderate viscosity, and favorable melting and boiling points.

Materials and Equipment Materials. Isopar M from Exxon Chemical was used

as the primary solvent. Detailed properties of Isopar M are given in Table I. It is an isoparaffinic hydrocarbon mixture having a boiling range from 207 to 260 "C. Tri- butyl phosphate (TBP) was used as the cosolvent. The extractive solvent was a 70% (by weight) Isopar M-30% (by weight) TBP mixture.

A Saccharomyces cerevisiae yeast strain, ATCC 24860, was used to produce the ethanol. Product (ethanol) in- hibition has a significant effect on the yeast productivity.

0 1989 American Chemical Society

Ind. Eng. Chem. Res., Vol. 28, No. 12, 1989 1869

Receiving Tank / iermentor Extract

Extract o J

r Solvent Tanks

n

I I

Figure 1. Experimental apparatus.

A t ethanol concentrations of 12% (v/v), production and growth are totally inhibited. Experimentally, S. cereuisiae has several advantages. It is relatively resistant to con- tamination, is tolerant of a pH as low as 4.0, and is easily cultured. Its use in previous extraction experiments makes comparisons between our work and prior research possible.

Analytical Equipment. The ethanol concentration in the water and the fermentation broth was analyzed by using a Tracor 586 gas chromatograph with a 60/80-mesh Chromasorb 102 column. A 100/120-mesh Porapak Q column was used to determine the solvent-phase ethanol concentration. For both columns, the oven temperature was maintained at 120 "C with the thermal conductivity detector operated at 175 "C. Standard solutions were used for calibration. The aqueous-phase glucose concentration was measured by using a YSI Model 27 industrial analyzer. The broth cell density was obtained by using a Bausch & Lomb Spec 20 spectrophotometer. The absorbance wavelength was set at 520 nm.

Extraction and Fermentation Equipment. The fermentation was conducted in a 20-L Virtis fermenter. The temperature and agitation speeds were automatically controlled.

The extraction was conducted in a 2.5-cm-diameter glass spray tower. The mass-transfer zone height was approximately 190 cm. The extraction system consisted of the fermenter, a 40-L solvent feed tank, and 40-L receiving tanks for the broth and solvent (see Figure 1). All feed and product lines were 0.6 cm stainless steel. The fluids were transported by peristaltic pumps. Drop sizes were determined photographically. In order to eliminate dis- tortion from the drop size photographs, a small water-filled cell was placed around a section of the column. The drop size pictures were taken with a standard 35-mm camera using high-speed color film.

Experimental Procedure Equilibrium Measurements. The ethanol distribution

coefficient, for both test systems, was determined with the following experimental procedure. The solvent, ethanol, and the aqueous phase (water or fermentation broth) were placed in a 250-mL separatory funnel in a solvent to aqueous phase ratio of approximately 5/1 (volume). The mixture was shaken for 20 min and then allowed to separate into two phases. Both phases were then analyzed for ethanol content by using the analytical equipment previously mentioned. The procedure was done three times per concentration to ensure the result repeatability.

Solvent Toxicity Tests. The solvent system we used had previously been tested for microorganism toxicity and found to be nontoxic (Tedder et al., 1984). In order to confirm this finding, we conducted a series of experiments

to verify that the solvent was nontoxic. Known quantities of solvent and fermentation broth were contacted in a flask over a 24-h time period. The flasks were continually agitated by a stir bar. The effect the solvent had on both glucose utilization and cell growth was determined. Several different solvent mixtures were tested: pure Isopar M, saturated 70-30 Isopar M-TBP mixture, 50% excess solution of 70-30 Isopar M-TBP mixture, and pure TBP.

Fermentation Procedure. Similar fermentation pro- cedures were used to generate the fermentation broth used in the toxicity tests, the equilibrium measurements, and the extraction experiments. The media consisting of 100 g/L of glucose and 3 g/L of nutrients (yeast extract), in distilled water. HC1 (1 N) was added to the solution to reduce the pH to 4.0. The mixture was sterilized and cooled to 30 "C. The solution was then inoculated with a yeast seed culture to obtain an intial cell concentration of 0.2 g/L for the toxicity test or 1.0 g/L for the equilibrium and extraction tests.

For the extraction experiments, the broth was agitated in the Virtis fermenter a t a speed of 100 rpm; the temperature was controlled at 30 "C. Prior to starting the fermentation, the fermenter and media were sterilized in an autoclave. The fermentation was continued until the ethanol concentration was approximately 35 g/L, at which time the broth was passed through the extraction column and collected in the receiving tank. After each experiment, the broth was transferred back to the fermenter. A con- centrated solution of glucose and nutrients was added to the fermenter to restore the glucose concentration to 100 g/L. The broth was fermented until the ethanol concentration reached 35 g/L, and the extraction was repeated.

Extraction Procedure. The extractions were performed at steady state (once through) for 1.5 h. The solvent/feed ratio ranged from 5 to 20 (v/v). The solvent was dispersed through the column at rates between 200 and 450 rnL/min. The continuous (aqueous) phase rate ranged from 20 to 50 mL/min. The flow rates were measured with rotameters or timed collections. Samples of the inlet and exit streams were taken at 1 h and at the end of each run. The dispersed phase holdup was determined by stopping the feed to the column and measuring the change in height of the coalesced layer interface. Drop sizes were determined photographically.

Liquid-Liquid Mass-Transfer Models The mass-transfer models we tested for applicability to

bioextraction have recently been reviewed and found to accurately predict the separation for traditional liquid- liquid extraction (Seibert, 1986). A summary of the models is given below.

For a continuous mass-transfer process, the mass- transfer zone height (2) is given by

2 = (HTU)(NTU)

The number of transfer units (NTU) is based on the concentration profile in the contactor. In order to obtain this value, an assumption must be made about the degree of axial mixing in the contactor. Commercial spray towers exhibit a large amount of continuous-phase backmixing. However, for small diameter experimental units, the level of backmixing is small and the concentration driving force can be approximated with a plug flow model. The contactor we used in this study has a length/diameter ratio of 75/1, which significantly reduces any tendency for the continuous phase to backmix. An empirical method for predicting the level of backmixing for a spray tower (Vermeulen et al., 1966) was used to confirm the validity

1870 Ind. Eng. Chem. Res., Vol. 28, No. 12, 1989

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of the plug flow assumption. The NTU is given by

Ca,top - Ca,bottom

C1m NTU =

where

In (ca - Ca*)bottom

The height of a transfer unit (HTU) is given by

HTU = Vc/(Kca) (3)

The overall mass-transfer coefficient is related to the individual mass-transfer coefficients through the two-resistance equation:

The individual mass-transfer coefficients are obtained from mass-transfer models. For the extraction experiments, we used the following models.

For the continuous-phase coefficient, we used the following equation (Ruby and Elgin, 1948):

For the dispersed-phase coefficient, two models were reviewed. The first model tested assumes a convective mass-transfer mechanism. (Note the absence of the solute diffusivity in the equation.) The model relates the level

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Time (hr) Figure 3. Toxicity test results (effect on cell growth). (+) Control batch, (+) Isopar M, (0) TBP, ( 0 ) saturated solvent, (*) 50% excess.

of solute transfer to the degree of internal drop circulation (Handlos and Baron, 1957):

0.00375V, kd = (6)

(I+:)

We also tested a model that is based on a combined diffusive-convective mechanism (Laddha and Degaleesan, 1978):

0.023 V, h d = (7)

the slip velocity was calculated from (Seibert, 1986)

4Apgdw v,, = ~

3PccD

v, = v,, exp(-1.92f#Jd) (8)

The interfacial area per unit volume is defined by

The physical properties used in these models were obtained through direct measurements. The diffusion coefficients for ethanol were obtained from a standard calculation procedure (Wilke and Chang, 19551, using pure water properties for the continuous phase. As was mentioned previously, the drop sizes were determined photographically.

Ind. Eng. Chem. Res., Vol. 28, No. 12, 1989 1871

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Time (hr) Figure 4. Toxicity test results (effect on glucose utilization). (i3) Control bath, (+) Isopar M, (0) TBP, ( 0 ) saturated solvent, (*) 50% excess.

Experimental Results and Discussion The results from the ethanol solubility measurements

are presented in Figure 2. Within the accuracy of the experiment, no variation of the distribution coefficient was observed for the three separate aqueous phases. This result indicates that equilibrium measurements made for a water-solute-solvent system can be used to predict the distribution coefficient of a broth-solute-solvent system. The value of the ethanol distribution coefficient was 0.21 g/g (2.39 mol/mol).

The drop diameters for the water-ethanol-solvent system and the broth-ethanol-solvent system were 0.46 and 0.28 cm, respectively.

The toxicity test produced results similar to those previously reported (see Figures 3 and 4). The extractive solvent, the 70-30 blend, exhibited a very slight microorganism toxicity. Isopar M did not show measurable toxicity, while TBP was highly toxic.

A comparison of the experimental overall mass-transfer coefficients for both the water and fermentation broth is shown in Figure 5. The water system coefficients were significantly higher. This can be attributed to variations in the physical properties of the two systems and to the impact that surface active agents in the fermentation broth have on the behavior of the dispersed phase drop. The level of mass transfer is greatly affected by the internal circulation of the drop. Surface active agents have been shown to decrease the level of circulation and therefore reduce mass transfer. It is possible that material in the broth (salts, cell bodies, carbon source, etc.) acts as a surface active agent and causes part of the mass-transfer-coefficient reduction. Also, the broths higher viscosity decreases the solute diffusivity, thus lowering the rate of mass transfer. Due to increased slip velocities a t high

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t 0.4 0.6 0.8 1 . 0 1 . 2 1 .4 1 . 6

Solvent / Feed (molar)

Figure 5. Water system and broth system comparison. (i3) Water results, (+) broth results.

continuous-phase feed rates, the mass-transfer coefficient increased as the solvent/feed ratio decreased.

Plots that compare the overall mass-transfer coefficients predicted by the various models to the experimental results are presented. The models differ in the equation that was used to predict the dispersed-phase coefficient. Each experimental point presented is an average of two experiments.

The water results (Figure 6) show that neither model does a good job of fitting the data over the entire range of test conditions. However, there is a pattern in the results. The convective model of Handlos and Baron (1957) fits the data at low S / F (high slip velocities). The diffusive-convective model of Laddha and Degaleesan (1978) fits the data at high S / F (low slip velocities). The plot suggests that a gradual transition occurs from a diffusive-controlled mechanism to a convective-controlled mechanism. Neither equation models this transition zone accurately.

For the broth results, the model of Laddha and Dega- leesan fits the data reasonably well. However, the results indicate that the model tends to overpredict a t low levels of mass transfer (Figure 7). The model of Handlos and Baron did not fi t the broth data. These results may indicate that the presence of surface active agents in the broth suppress the convective mass-transfer mechanism even at low S / F (high slip velocities).

Conclusions and Future Work Based on our results, the Ruby-Elgin and the Laddha-

Degaleesan equations, combined through a two-resistance model, will model the extraction of ethanol from a fermentation broth with a reasonable level of accuracy. Additional research on other fermentation systems is

1872 Ind. Eng. Chem. Res., Vol. 28, No. 12, 1989

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needed to determine if this is a truly general result. The equilibrium measurements indicate that the Isopar M- TBP solvent has a significant affinity for ethanol. Within the accuracy of our measurements, the equilibrium solubility of ethanol in the solvent was not affected by the composition of the aqueous phase. We also confirmed earlier results that indicated that the solvent was nontoxic.

Future work will attempt to develop a commercially viable ethanol extraction process. A process has been designed and will be outlined in a publication once ap- propriate patents are in place. Additional fermentation systems will be studied and modeled to determine if the conclusions obtained in the present work can be extended to other fermentation systems. Also, the use of various extractor internals will be studied to determine the most efficient bioextractor design.

Nomeinclat ure a = average interfacial area per unit volume, cm2/cm3 a, = total interfacial area for mass transfer, cm2 C, = concentration of solute in the heavy phase C,* = equilibrium concentration of the solute in the heavy

C1, = log-mean concentration gradient d, = dispersed drop diameter d,, = Souder mean drop diameter k , = continuous-phase film coefficient k d = dispersed-phase film coefficient Kd = overall mass-transfer coefficient m = solute distribution coefficient, mol/mol V, = continuous-phase velocity, cm/s V, = slip velocity Vex. = volume of extractor, cm3 2 = height of mass-transfer zone, cm

Greek Symbol 4 = fractional dispersed-phase holdup Subscripts c = continuous phase d = dispersed phase

trihutyl phosphate, 126-73-8; water, 7732-18-5.

Literature Cited Cho, T.; Shuler, M. L. Multimembrane Bioreactor for Extractive

Fermentation. Biotechnol Prog. 1986, 2(1), 53-60. Crabhe, P. G.; Tse, C. W.; Munro, P. A. Effect of Microorganisms

on Rate of Liquid Extraction of Ethanol from Fermentation Broths. Biol. Bioeng. 1986,28, 939.

Handlos, A. E.; Baron, T. Mass and Heat Transfer from Drips in Liquid-Liquid Extraction. AZChE J. 1957, 3(1), 127-136.

Karr, A. E. Extraction of Whole Fermentation Broth with a Karr Reciprocating Plate Extraction Column. Presented a t the 29th Canadian Chemical Engineering Conference, Oct 1979.

Kollerup, F.; Daugulis, A. J. Screening and Identification of Ex- tractive Fermentation Solvents Using a Data-base. Can. J. Chem. Eng. 1985, 63, 919.

Kollerup, F.; Daugulis, A. J. Process Development of a Prototype Extractive fermentation System. Ann. N . Y. Acad. Sci. 1987,506, 478.

Laddha, G. S.; Degaleesan, T. E. Transport Phemonina in Liquid Extraction; McGraw-Hill: New York, 1978.

Minier, M.; Goma, G. Ethanol Production by Extractive Fermenta- tion. BiotechnoL Bioeng. 1986, 24, 1565.

Rocha, J. A. Mass Transfer Efficiency of Sieve Tray Liquid-liquid Extraction Columns. PbD. Dissertation, The University of Texas at Austin, Dec 1984.

Ruby, C. L.; Elgin, J. C. Mass Transfer Between Drops and a Con- tinuous Liquid Phase in a Countercurrent Fluidized System: Liquid-liquid Extraction in a Spray Tower. Chem. Eng. Prog. Symp. Ser. 1948,51, (16).

Seibert, A. F. Hydrodynamics and Mass Transfer in Spray and Packed Liquid-liquid Extraction Columns. Ph.D. Dissertation,

phase

Registry No. Ethanol, 64-17-5; Exxon isopar M, 12619-80-6;

Ind. Eng. Chem. Res. 1989,28, 1873-1878 1873

Wilke, C. R.; Chang, P. Correlation of Diffusion Coefficients in Di-

Receiued for review December 9, 1988 Revised manuscript received June 26, 1989

Accepted August 15, 1989

The University of Texas a t Austin, Aug 1986. Tedder, D. W.; Ekkles, A. J.; Ferster, P. J.; Tawik, W. Y. Continuous

Fermentations and Product Recovery by Liquid-liquid Extrac- tion. Proceedings of Biotech 84, p 177, 1984.

Vermeulen, T.; Moon, J. S.; Hennico, A.; Miyauchi, T. Axial Dispresion in Extraction Columns. Chem. Eng. Prog. 1966,62,95.

lute Solutions. AZChE J. 1955, 1 , 264.

Preliminary Design of Sieve Tray Extraction Columns. 1. Determination of the Column Diameter. Flooding Velocities in Sieve Tray Extractors

J. Antonio Rocha,* J. Carlos CBrdenas, Cbar Sosa, and Jorge Rosales Instituto Tecnol6gico de Celaya, Departamento de Zngenieria Quhica , Celaya, Gto., Mexico

Experimental data from this project and other sources have been used to obtain three correlations for predicting the flooding velocities in sieve tray extractors. One correlation was obtained using the analogy between distillation and extraction. This correlation showed an average relative deviation of 22%. The second and third equations were obtained by dimensionless analysis and a least-squares fit of the physical properties and geometric and hydrodynamic parameters that most affect the performance of sieve tray extractors. These equations presented an average relative deviation of 11 ’3’0 and 13 % . The three correlations were used to estimate the flooding velocities and the diameters of the sieve tray extraction columns working a t 60% flooding.

In recent years, liquid-liquid extraction has gained increased attention as a commercial separation method in the process industry, as has been shown by Humphrey et al. (1984). Although liquid-liquid extraction has been practiced for many years, its study, investigation, and development have been rather poor. Pilot plant experi- mentation is still needed to design most industrial equipment.

In the classification of nonmechanically agitated contactors, the sieve tray extractor has an important role due to the relatively high throughputs, the moderate efficiency, and the simplicity of construction and operation, which is similar to the well-known sieve tray distillation column.

The operation of a sieve tray liquid-liquid extraction column, where the light liquid is the dispersed phase, is shown in Figure 1. The heavy liquid flows downward through such a extractor horizontally across each tray and through the downcomers from tray to tray. The light liquid issues from the perforations in each tray in the form of jets or drops (drop formation), rises through the heavy liquid in the form of drops (drop rise), enters into a flocculation zone, and subsequently coalesces into a layer of light liquid which accumulates immediately under each tray.

In a previous paper, Rocha et al. (1986) have shown that the optimum mass-transfer efficiency in sieve tray extractors is obtained at a high velocity of the dispersed phase, but if this velocity is increased, the extractor may flood.

Flooding in sieve tray extractors occurs when the flow rate of the dispersed phase is prevented from flowing through the column and is dragged out by the flow rate of the continuous phase. Flooding can also arise if the flocculation zone expands to fill the stage.

Correlations to predict the flooding velocities in sieve tray extractors seem important because they could permit us to fix the proper flow to a good mass-transfer efficiency and also to estimate the column diameter.

Previous Work The studies and investigations on flooding velocities in

sieve tray extractors are scarce. The experimental data

0888-5885/89/2628-1873$01.50/0

reported in the literature are limited. A small comment about the most important work done in this field of research is as follows:

Mewes and Pilhofer (1979) presented the load limits and the operational range of sieve tray extractors as a function of extractor geometry and physical properties of the system. Although no experimental data are reported, their “load diagrams” plot the volumetric flow rates of both phases in a dimensionless form by using eq 1-A and 1-B.

r 1113

L - J

Hirschmann and Blass (1984) performed investigations with the test system 1-butanol/succinic acid/water, on account of its low interfacial tension, namely, 1.75 dyn/cm. The investigations started by studying the drop formation as the main process. The results thus obtained were ap- plied to a specified design of the sieve tray column with downcomer.

Holden (1984) performed an experimental project to study the influence of geometric variables and physical properties of the system on the flooding velocities. He used two different systems (toluene/water, and methyl isobutyl ketone/water) in a 0.1-m-diameter glass column.

Dawodu et al. (1984) and Oloidi and Mumford (1985) carried out some studies on the hydrodynamic parameters of sieve tray extractors in a 0.45-m-diameter glass column. The correlations they proposed to predict the drop diameter distribution and operational holdup were mostly applicable to the system they used.

Rocha et al. (1985), using basically the experimental data of Holden (1984), proposed an approximate correlation to predict the flooding velocity of the continuous phase. The equation is as follows:

0 1989 American Chemical Society

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Application of traditional mass-transfer algorithms to a bioextraction process