I n d . E n g . Chem. Res. 1987, 26, 1087-1092 Pageau, L.; Sourirajan, S. J. Appl. Polym. Sei. 1972, 16, 3185. Reid, R. C.; Prausnitz, J. M.; Sherwood, T. K. T h e Properties of Gases and Liquids, 3rd ed.; McGraw-Hill: New York, 1979. Rossi, C.; Bianchi, E. Chim. Znd. (Milan) 1958, 40,992. Sourirajan, S. Reverse Osmosis; Academic: New York, 1970; Chapter 3. Sourirajan, S.; Matsuura, T. Reverse Osmosis/ Ultrafiltration Pro-
1087
cess Principles; National Research Council of Canada: Ottawa, Ontario, 1985; Chapter 4. Sourirajan, S. Lectures on Reuerse Osmosis; National Research Council of Canada: Ottawa, Ontario, 1983; p 316. Received for review February 3, 1986 Accepted November 13, 1986
Flow Regime Transitions in a Bubble Column with a Paraffin Wax as the Liquid Medium Dragomir B. Bukur,* Dragan PetroviE,' and James G. Daly Department of Chemical Engineering, Texas A&M University, College Station, Texas 77843
Gas hold-up measurements were made in a 0.051-m-diameter by 3.05-m-long glass bubble column with a molten paraffin wax as the liquid medium. For temperatures between 230 and 280 "C, there is a range of gas velocities where two modes of operation are possible, and they are referred to as the "foamyn and the "turbulent bubbling" flow regimes. The start-up velocity determines which flow regime will be obtained. Transitions between these two flow regimes occur and are influenced by the temperature (i.e., the liquid viscosity) and the gas distributor design. Lower temperatures and/or perforated plate distributors with larger holes favor the existence of the turbulent bubbling flow regime. Bach and Pilhofer's correlation provides a very good fit for gas holdups obtained in studies with orifice plate distributors in the absence of foam. Fischer-Tropsch (F-T) reaction represents an important route for indirect coal liquefaction. A slurry bubble column reactor appears to have significant economic advantages (Gray et al., 1980; Thompson et al., 1981) over fixed and entrained bed reactors currently used by Sasol in South Africa. In the slurry bed process, very small catalyst particles are suspended in a molten wax and a mixture of H2 and CO (synthesis gas) is passed through in the form of gas bubbles. An important factor in the operation of bubble column reactors is the rate of mass transfer between the gas and the liquid. Dependence of the specific gasliquid interfacial area on operating conditions and reactor geometry is needed for the reactor design and scale-up. Very few experimental studies have been conducted using molten wax as the liquid medium, and considerable differences in results were obtained. Calderbank et al. (1963) measured the gas holdup and the gas-liquid interfacial area in a 0.051-m-diameter column with a ball and cone distributor using Krupp wax as the liquid medium. Deckwer et al. (1980) determined effects of column diameter (0.041 and 0.10 m), superficial gas velocity (up to 0.04 m/s), temperature (143-285 "C), pressure (400-1100 kPa), and solids concentration (up to 16 wt %) on gas holdup using a hard paraffin wax as the liquid medium. They found that the gas holdup is essentially independent of temperature for T > 240 "C, column diameter, and column pressure, and it decreases slightly with the addition of solids. They observed holdups higher than the ones obtained in the Calderbank et al. study. Quicker and Deckwer (1981) studied the effect of distributor design on the gas holdup and the Sauter mean bubble diameter in a 0.095-m column at 130 and 170 "C. They found that the bubble size is independent of distributor type, while higher holdups were obtained with a *Author to whom the correspondence should be addressed. On leave from Institute of Petrochemistry, Faculty of Technology, University of Novi Sad, 21000 Novi Sad, Yugoslavia.
single nozzle distributor (0.9 mm in diameter) than with a perforated plate distributor (19 holes, 1.1mm in diameter). A comprehensive study of this system has been made recently by researchers at Mobil (Kuo, 1983; Smith et al., 1984; Kuo et al., 1984a,b). They reported results illustrating effects of distributor type, liquid static height, wax type, operating conditions (temperature, gas velocity, and pressure), gas type, and column diameter on the average gas holdup. Their findings can be summarized as follows. The wax type, the distributor design, and the temperature have significant effect on the gas holdup. The effect of the liquid static height is very pronounced with SMP-type distributors (holdup increases as the static height decreases), but it is less significant with perforated plate distributors. The column diameter (0.032-0.10 m) has some effect on the holdup, whereas the effect of the pressure (0.1-1.48 MPa) and the gas used (N2, H2, or H 2 / C 0 mixture) is negligible. In this paper new experimental data obtained in a bubble column with a paraffin wax as the liquid medium are presented. The existence of two flow regimes, for a given set of operating conditions, has been observed for the first time in this system. Transitions between these two flow regimes, which are typical for hysteresis type of behavior, take place as one changes the gas flow rate. These observations have been used to explain results obtained in previous studies.
Experimental Equipment and Procedure A schematic representation of the experimental apparatus is shown in Figure 1. Nitrogen, from gas cylinders, passes through the mass flow meter (FC) and enters the preheater (PH, electrically heated U-shaped tube). The preheated gas enters the glass column (BC) through a sparger at the bottom of the column. The wax in the storage tank (1)is heated to a temperature between 150 and 200 "C before it is transported to the column using a slight nitrogen overpressure. The column is preheated
0888-588518712626-1087$01.50/0 0 1987 American Chemical Society
1088 Ind. Eng. Chem. Res., Vol. 26, No. 6, 1987 SYMBOLS VIV
01.
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F L O W CONTROLLER PRESSURE INDICATOR PREHEATER TEMP. INDICATOR TEMP CONTROLLER E L E C T R I C A L L Y HEATED
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Figure 1. Flow diagram of experimental apparatus: 1, wax storage tank; 2, gas-liquid separator; 3, scrubber; BC, bubble column (0.051-m diameter, 3.05-m long).
to a temperature of 150 "C before the wax is transferred into the column. The flow of nitrogen through the column is maintained during the entire preheating period. The disengagement zone on the top of the column, tubes connecting the column and the gas-liquid separator (2), and the separator itself are maintained at temperatures above the melting point of the wax (-110 "C) to prevent solidification of any entrained liquid. The hot gas leaves the separator and passes through the scrubber (3) filled with a mineral spirit before it is vented to the atmosphere. The average gas holdup is calculated from visual observations of expanded (including any foam present) and static liquid heights. The first reading of the expanded liquid height is made 10-15 min after the desired column temperature and the gas flow rate are reached. These readings are repeated several times (minimum of three readings for a given set of operating conditions) with 1020-min time intervals between the two successive measurements to ensure that steady state is achieved. This was particularly necessary in cases where the foam accumulated at the top. The foam level tends to rise with time, and erroneous results would be obtained if the measurements were completed over a short period of time. The static liquid height for most runs was in the range 1.5-2.4 m, but as the holdup increases, the liquid has to be drained into the wax storage tank in order to maintain the expanded liquid level within the glass column. In the extreme case of gas holdups around 70%,the minimum value of static liquid height was 0.75 m.
Experimental Results and Discussion All experiments were done at atmospheric pressure using nitrogen as the gas and the Fischer-Tropsch-derived paraffinic wax designated FT-300 (also known as SH-105 Vestowax; average molecular weight 730) as the liquid medium. Effects of operating conditions (temperature and gas flow rate) and distributor type (sintered metal plate (SMP) and a single hole orifice plate) on the average gas holdup were determined in these experiments. The operating conditions chosen (ug= 0.01-0.13 m/s, T = 230-280 "C), the bed geometry, and the distributor types employed in the present study are similar to the ones used in pre-
vious studies of the F-T synthesis in slurry bubble column reactors with promoted iron catalysts (e.g., Schlesinger et al., 1951; Calderbank et al., 1963; Kolbel et al., 1955; Kuo, 1983). Effect of Temperature. Results obtained for the average gas holdup (e,) measurements with a 1.85-mm single orifice plate distributor are shown in Figure 2. For a given temperature in the range 230-280 " C , there is a range of superficial gas velocities where two values of eg are possible. These two values correspond to two modes of operation which will be referred to as the "foamy" regime (open symbols) and the "turbulent bubbling" regime (solid symbols). A stable foam layer exists on the top of the liquid level in the foamy regime, giving rise to higher holdups. In the foamy regime, gas holdup increases with temperature, with the exception of data obtained a t 250 and 265 "C which can be represented by a single curve. However, in the turbulent bubbling regime the effect of temperature, in the range 160-280 "C,on the gas holdup is small. The turbulent bubbling regime is typical for pure liquids, and it also has been referred to as the liquid circulation, heterogeneous flow, and churn-turbulent and froth flow regime. In our experiments a t T > 230 "C,this flow regime could be maintained only at relatively high values of the superficial gas velocity, i.e., ug> 0.04 m/s. According to Deckwer et al. (1980) and Shah et al. (1982), under these conditions the slug flow regime is possible. Visual observations of the flow field show the presence of slugs or slug-type bubbles at velocities greater than about 0.03 m/s. These large bubbles were observed in both flow regimes, and thus the lower gas holdups in the turbulent bubbling regime cannot be attributed to the presence of slugs. At high superficial gas velocities, there is intensive turbulent mixing in the liquid phase, and the bubble size distribution is nonuniform in both flow regimes. Fine bubbles (less than 1mm in diameter) that are present in the bulk liquid rise slowly to the top when the gas flow is stopped. Small bubbles (- 1 mm in diameter) as well as the large ones (1-2 cm) have been observed near the column wall. The main distinction between the two flow regimes at high superficial gas velocities is the presence of foam in the upper part of the gas-liquid dispersion. The past history, i.e., the start-up procedure, determines which of the two flow regimes will be obtained. The foamy regime is obtained if one starts at a low velocity and gradually increases the gas flow rate. Formation of the foam is related to the existence of small bubbles that ac-
Ind. Eng. Chem. Res., Vol. 26, No. 6, 1987 1089 80.0
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cumulate at the top of the expanded liquid layer. As the superficial gas velocity increases, the foam region increases, and high gas holdups are obtained. The turbulent bubbling regime can be obtained by starting at a gas velocity which is greater than a critical velocity where transition from the turbulent bubbling to the foamy regime takes place. These transitions are shown in Figure 2 by the broken lines with arrows connecting points in the two flow regimes. After making the transition from the turbulent bubbling to the foamy regime, subsequent increases and decreases in the gas velocity yield holdups in the foamy regime (i.e., the hysteresis type of behavior). At sufficiently low temperatures, the turbulent bubbling regime is the only stable flow regime over the entire range of superficial gas velocities, as shown in Figure 2 for T = 160 "C. This behavior can be qualitatively explained in terms of the liquid viscosity effect (pL = 9.8 mPa.s at 160 O C vs. 2.4 m P a . s at 265 "C). Bubble coalescence increases with liquid viscosity (i.e., as the temperature decreases); thus, the stable foam layer is not produced at low temperatures due to the absence of a large number of small bubbles. Figure 3 shows the gas hold-up results obtained with the 4-mm-orifice plate distributor. Once again, two values of cg are possible for a given set of operating conditions with temperatures in the range 230-280 O C . At 230 "C the turbulent bubbling regime could be maintained over the entire range of gas velocities (up = 0.013-0.093 m/s), whereas, with the 1.85-mm orifice (Figure 2), a transition from the turbulent bubbling regime to the foamy regime occurred as the velocity was decreased from 0.044 to 0.022 m/s. Another type of transition was observed with the 4-mm orifice at 230 and 265 "C, namely, a transition from the foamy to the turbulent bubbling regime. For example, at 230 "C, upon increasing the gas velocity from 0.033 to 0.053 m/s, the foam disappeared. Farley and Ray (1964) also reported a transition from the foamy to the turbulent bubbling regime using a paraffin wax as the liquid medium in a 0.235-m-i.d. reactor. They reported that the increase of superficial gas velocity from 0.03 to 0.06 m/s eliminated foaming within 1 h after the change was made. Results obtained with the sintered metal plate (SMP) distributor with an average pore size of approximately 40 Fm are shown in Figure 4. A transition region where two values of cg are possible for a given set of operating conditions was observed again. Due to the fact that the SMP produces much smaller bubbles than the orifice plate
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distributors, the gas holdups obtained with the SMP in both the foamy and the turbulent bubbling regime are considerably higher. Also, the turbulent bubbling regime could only be maintained over a smaller range of velocities. At 230 "C, where larger bubbles are expected (viscosity effect), the turbulent bubbling regime exists for velocities between 0.05 and 0.09 m/s. The foam breakup occurred in the experiments conducted in the order of increasing gas velocity, when the velocity was changed from 0.05 to 0.09 m/s. In the same experiment, the transition from the turbulent bubbling to the foamy regime was observed when the velocity was decreased from 0.05 to 0.03 m/s, giving rise to a typical hysteresis loop. The existence of two flow regimes for a given set of operating conditions, which was found in our study with all three distributors, has not been reported in the earlier studies with a molten wax as the liquid medium. A theoretical basis for the existence of two values of the gas holdup for a given set of operating conditions was established earlier (e.g., Wallis, 1969, p 92; Riquarts, 1979), but experimental evidence demonstrating this type of behavior is rather scarce. In studies by Anderson and Quinn (1970) and Maruyama et al. (1981) with the air/tap water system, the hysteresis type of behavior was observed. The foamy regime, in both studies, was obtained in experiments conducted in the order of increasing gas flow rate. A transition to the turbulent bubbling regime (slug flow according to Anderson and Quinn or the liquid circulation flow regime according to Maruyama et al.) occurred at a sufficiently high gas flow rate. When the velocity was decreased, the gas holdups in the turbulent bubbling regime were obtained. Anderson and Quinn (1970) suggested that the observed behavior might be caused by concentration gradients of surface-active impurities present in the tap water which promote the bubble coalescence. Effect of Gas Distributor Type. From the data shown in Figures 2-4, plots illustrating the effect of distributor type, for a given temperature, have been constructed and are shown in Figure 5. The main features that are common for all cases are as follows. The gas holdups with the SMP distributor are significantly higher than the ones obtained with the orifice plate distributors in both regimes. The gas holdups at the same operating conditions (Le., temperature and superficial
1090 Ind. Eng. Chem. Res., Vol. 26, No. 6, 1987
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Figure 7. Comparison of gas holdups obtained in studies with perforated plate distributors: 1, Kuo et al. (1984a), 1-mm single orifice; 2, Kuo et al. (1984b), 2-mm single orifice; 3, Calderbank et al. (1963), ball and cone distributor; 4, Quicker and Deckwer (1981), perforated plate (19 holes, 1.1 mm in diameter).
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gas velocity) for the two orifice plate distributors are similar in both regimes, except for T = 280 "C and ug > 0.05 m/s in the foamy regime (Figure 5c), where the gas holdups with the 4-mm orifice are significantly lower than the ones obtained with the 1.85-mm orifice plate. Comparison of Results with Experimental Data from Literature. Values of gas holdups obtained in our study have been compared with existing literature data obtained under similar conditions. Results for SMP distributors and perforated-plate-type distributors are shown in Figures 6 and 7 , respectively.
Our data, in the foamy regime, with the 40-pm SMP (curve 2 in Figure 6) are in fairly good agreement with Mobil's data obtained by using a 20-pm SMP distributor (curve 3) for ug > 0.02 m/s. Mobil's data (Kuo et al., 1984a) were obtained in a 0.051-m-i.d. by 9.1-m-tall bubble column using the Fischer-Tropsch-derived paraffinic wax designated FT-200 (average molecular weight 600) at 260 "C. A discontinuity in curve 3 is due to the fact that measurements were taken at different static liquid heights (4.83-6.40 m for low values of ugand 3.05 m for ug> 0.02 m/s). Curve 1 (60-pm SMP) and curve 4 (100-pm SMP) were constructed from experimental data reported by Mobil's workers (Kuo, 1983). These experiments were conducted in a short hot flow column (0.053-m i.d., 1.9-m tall) a t 200 "C using FT-200 as the liquid medium. Deckwer et al. (1980) data for a 75-pm SMP were obtained in two bubble columns having diameters of 0.041 and 0.10 m, using hard paraffin wax as the liquid medium at temperatures between 250 and 285 "C. The gas holdups reported by Deckwer et al. (curve 5) are significantly lower than the values obtained in our study (curve 2) or in Mobil's study (curves 1,3 and 4), even
Ind. Eng. Chem. Res., Vol. 26, No. 6, 1987 1091 though the operating conditions, the reactor geometry, and the liquid medium, were similar in all cases. In Mobil's experiments with the 100-pm SMP distributor at 200 "C (curve 4), lower holdups were obtained than in our study (curve 2). This results from the use of the larger pore size (100 vs. 40 pm) and the lower operating temperature (200 vs. 265 OC). Mobil's gas holdups obtained with the 60-pm SMP at 200 "C (curve 1) appear to be too high in comparison to their own data as well as our data. This might be caused by the use of a small liquid static height (0.6 m) in experiments with low superficial gas velocities. Gas holdups, obtained with perforated-plate-type distributors, are shown in Figure 7. In the foamy regime for ugI0.04 m/s, there is no effect of the orifice size, in the range 1-4 mm, on the gas holdup, but at higher velocities the holdups obtained in Mobil's study (Kuo et al., 1984a) with the 1-mm orifice plate (curve 1)are higher than the ones obtained in our study with the 1.85- and 4-mm orifice plate distributors. This is probably caused by the fact that the process of foam breakup starts earlier with larger orifices. In the turbulent bubbling regime, the gas holdup is essentially independent of the orifice size (1.85-4 mm), and agreement between Mobil's data (Kuo et al., 1984b) and our data is excellent. In Mobil's study the turbulent bubbling regime was the only stable flow regime over the entire range of velocities investigated (i.e., ug = 0.01-0.12 m/s). This is probably due to the use of a very tall column which provides long residence time and thus facilitates the bubble coalescence. On the other hand, in their experiments with the 1-mm orifice plate, all data obtained are in the foamy regime. This is consistent with observations reported in the literature that in systems with foaming capacity the foamy regime is obtained with SMP distributors or perforated plate (PP) distributors having small hole diameters, while the turbulent bubbling regime occurs with PP distributors having larger hole diameters (e.g., Zahradnik and Kastanek, 1979; Pilhofer, 1980). The holdups obtained in the Calderbank et al. study (curve 3), with a ball and cone (B & C) distributor, and in the Quicker and Deckwer (1981) study (curve 4), with a perforated plate distributor, lie between the foamy data (curve 1) and the data obtained in the absence of foam (curve 2). Quicker and Deckwer's data with a one-holenozzle sparger (0.9-mm SN) agree very well with the data in the foamy regime. The holdups in Quicker and Deckwer's study, with both distributors, were obtained at 170 OC, while all other data were obtained at temperatures of 260 and 265 O C . In view of this fact, these holdups are higher than expected. On the other hand, Quicker and Deckwer conducted experiments in a 0.095-m-diameter by 1.35-m-longcolumn, while in the other studies experiments were done in 0.051-m-diameter columns and with static liquid heights between 1.5and 7 m. Thus, the differences in the bubble column geometry and the use of different distributors might have caused discrepancies in the results obtained. Additional experimental studies are required to clarify this point. Gas Hold-Up Correlations. Various correlations for gas holdup have been proposed in the literature, and a summary of the ones based on comprehensive experimental data is presented by Shah et al. (1982). These correlations are not applicable to foaming systems. Deckwer et al. (1980) proposed an empirical correlation that fits their experimental data obtained with the 75-pm SMP distributor. This correlation predicts much lower holdups than those obtained in our study or Mobil's study with SMP distributors, and it also predicts lower holdups
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Figure 8. Comparison between experimental gas holdups and predictions from the Bach and Pilhofer correlation (solid line, Bach and Pilhofer correlation with physical properties at 265 "C; dashed lines, *20% relative deviation from the correlation).
than those obtained in studies with single-hole orifice plate distributors in the foamy regime. Several empirical correlations (Akita and Yoshida, 1973; Bach and Pilhofer, 1978; Hikita et al., 1980; Mersmann, 1978) were used to analyze data obtained in the absence of foam. The physicochemical properties (i.e., liquid density, viscosity, and surface tension) required in these correlations were taken from Deckwer et al. (1980, 1982). It was found that Bach and Pilhofer's correlation provides the best fit for experimental data obtained with perforated-plate-type distributors. This correlation is given by
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Summary and Conclusions The effect of operating conditions (temperature and gas flow rate) and gas distributor types (40-pm SMP, and the 1.85- and 4-mm single-hole orifice plate distributors) on the average gas holdup was studied in a glass bubble column (0.051-m i.d., 3.05-m tall) using the FischerTropsch-derived paraffinic wax (FT-300) as the liquid medium. For temperatures in the range 230-280 "C, which is typical for the F-T synthesis reaction over iron catalysts, and for all three distributors, a transition region has been found where two values of gas holdup exist for a given set of operating conditions. The higher holdups are accompanied by the presence of foam, and thus this type of operation is referred to as the foamy regime. The oper-
I n d . Eng. Chem. Res. 1987, 26, 1092-1099
1092
ating mode which is characterized by the virtual absence of foam (lower holdups) is called the turbulent bubbling regime. This type of behavior has been observed for the first time in a system with molten paraffin wax as the liquid medium. The start-up procedure determines which one of the two regimes will be attained. A transition from the foamy to the turbulent bubbling regime occurs when ugexceeds a certain critical value, and transition from the turbulent bubbling to the foamy regime occurs as the superficial gas velocity drops below a certain critical value. These critical velocities are influenced by the temperature (i.e., the liquid viscosity) and the gas distributor type. Other factors which might influence these transitions, are column diameter, liquid static height, and impurities, but their effect has not been determined. Some of previous results for gas holdups that either were not well understood (e.g., the foam breakup reported by Farley and Ray (1964)) or appeared to be in a poor agreement (i.e., small vs. high gas holdup values obtained in different studies with different types of gas distributors) can be explained in terms of the existence of these two flow regimes. Bach and Pilhofer’s (1978) empirical correlation provides a very good fit for the gas holdups obtained in studies with orifice plate distributors in the absence of foam.
Acknowledgment This work was supported by DOE under Contract DEAC22-84PC 70027. We are grateful to William Deutchlander, Robert Drummond, Mike Noak, and Dr. Khan Nguyen-tien for their help with design and construction of the experimental apparatus.
Literature Cited Akita, K.; Yoshida, F. Ind. Eng. Chem. Process Des. Dev. 1973, 12, 76. Anderson, J. L.; Quinn, J. A. Chem. Eng. Sci. 1970, 25, 373.
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Received for review February 3, 1986 Revised manuscript received February 6, 1987 Accepted February 28, 1987
Simulation of Continuous-Contact Separation Processes: Unsteady-State, Multicomponent, Adiabatic Absorption David M. Hitch,’ Ronald W. Rousseau,* and James K. Ferrell Department of Chemical Engineering, North Carolina State University, Raleigh, North Carolina 27695- 7905
A rigorous algorithm has been developed for the unsteady-state simulation of multicomponent, adiabatic absorption in packed columns. The simulation uses an implicit integration scheme to solve the partial differential model equations which may include both nonideal vapor-liquid equilibrium relationships and nonlinear mass-transfer expressions. All physical parameters used in the model are obtained from empirical correlations available in the literature. Simulation predictions, both dynamic and final steady-state, compared favorably with experimental results. The numerical simulation of continuous-contact separation devices is a topic that has long been neglected. In a previous article (Hitch et al., 1986),we pointed out that until recently most research efforts in the area of the design and analysis of separation processes have concentrated on *Author t o whom correspondence should be addressed a t School of Chemical Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0100. Current address: Eastman Kodak Company, Eastman Chemicals Division, Bldg. 152A, Kingsport, T N 37662.
staged operations. Coincident with the development of our SIMCAL algorithm, which is described by Hitch et al. (1986),other workers also produced numerical simulations that can handle rigorously the process of steady-state, adiabatic absorption in a packed column. Chief among these are the “nonequilibrium stage model” algorithms of Krishnamurthy and Taylor (1985). In fact, the nonequilibrium stage approach has been shown to be highly effective for steady-state simulation of a variety of both staged and continuous-contact separation processes. In contrast to the above-referenced work, the present article
0888-5885/87/2626-1092$01.50/0 0 1987 American Chemical Society
