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Ind. Eng. Chem. Res. 1998, 37, 734-738
Loop Venturi ReactorsA Feasible Alternative to Stirred Tank Reactors? Laurent L. van Dierendonck* RUG, Nijenborg, 9747 AG Groningen, The Netherlands
Jindrˇ ich Zahradnı´k Institute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic, 165 02 Prague 6, Czech Republic
Va´ clav Linek
Ind. Eng. Chem. Res. 1998.37:734-738. Downloaded from pubs.acs.org by EASTERN KENTUCKY UNIV on 01/29/19. For personal use only.
Department of Chemical Engineering, Prague Institute of Chemical Technology, 166 28 Prague 6, Czech Republic
During the past decennia, the stirred tank reactor, especially for gas-liquid operations, has received much attention. However, proper design of turbine-stirred gas-liquid reactors on an industrial scale can still be difficult to make. On the large scale, the removal of heat may become a limiting factor. Installation of additional cooling coils into the reactor vessel makes the design problems even more complex. The development of loop Venturi designs offers some new solutions to the scale-up questions, especially when high pressures are involved. Recent publications on this type of reactor demonstrate fast mixing (including micromixing), high mass-transfer rate, and an independently designed heat exchanger in the circulation loop of the reactor. The gas loop ensuring complete gas mixing represents an additional favorable feature. On the whole, the loop Venturi reactors can be viewed as an efficient alternative to the stirred tank reactors, offering easier scale-up. This conclusion is supported by the less dependency of the mass-transfer rate and mixing on the reactor scale. Introduction To design a gas-liquid reactor for commercial processes, the designer requires a lot of information about the system involved. However, most of the information can be collected on a small scale (bench or semi-technical scale). In the selection process of a gas-liquid contactor type, the stirred tank reactor with a standard turbine stirrer (Rushton) is the most common choice (Figure 1). This type of reactor possesses a wide application area in chemical- and bio-process industry and, accordingly, has been receiving extensive coverage in the literature, including most notably classic textbooks by Uhl and Gray (1966), Nagata (1975), and Oldshue (1983). A thorough analysis of gas contacting with liquids in stirred vessels has been presented by Joshi et al. (1982), Mann (1983), and more recently by Tatterson (1991). In spite of routine industrial use of stirred tank reactors, their design and scale-up still pose many questions and rely to a considerable extent on particular experience and know-how of designers. In the industrial practice, the prevailing approach is still scaling-up by empirical testing with the actual process through a series of scaleup stages. Westerterp and co-workers (1963) performed in the sixties comprehensive studies both on the physical and chemical behavior of the stirred tank reactors. Many publications of the group contributed to grant a citation price and improved understanding of the basic design. Many investigators came up with their solutions for the scale-up. An astonishing number of papers in the literature, dealing with the aspects of stirring and the scale-up, confuses designers. There has been little agreement about the approach to the scale-up among
Figure 1. Scheme of a standard geometry vessel with a Rushton type turbine stirrer.
various authors. Essentially, reports on investigations performed on a large scale have been missing. The main question faced by the designer has been keeping either the power input (Pg/VL) or the stirrer speed constant. For the latter case, an effective stirrer speed was defined as a criterion for scale-up (UL ) (N - N0)D2/T). The problem is complicated by the effect of increasing superficial gas velocity (φg/F ) Ug) with the reactor scale. In this respect, flooding of the stirrer occurs if Ug g 0.05 m s-1. Smith and Warmoeskerken (1985) defined the operation window for the turbine stirrer preventing gas overloading. It has been known that the extent of liquid mixing decreases with scale while validated information about gas phase mixing on a large scale have been still missing.
S0888-5885(97)00313-8 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/01/1998
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Figure 2. Scheme of a loop Venturi reactor.
A general question in the whole field of gas-liquid reactors concerns prediction of the effect of physical properties of the gas and the liquid phases on the behavior of gas-liquid dispersions and on the rate of interfacial mass transfer. The industrial processes are mostly characterized by undefined components in the liquid streams which can influence interfacial area to a large extent. Recently, Martin (1996) emphasized this problem again in his thesis work and showed the complexity of the mass-transfer prediction in relation to the power input. It can thus be concluded that scale-up of the stirred gas-liquid contactors is still far from being a routine task. The engineering people may look for alternatives. In this respect, the jet loop reactors still have not been sufficiently evaluated. The jet loop reactors of the design illustrated in Figure 2 have been claimed to solve some of the above-raised questions and make the scaleup easier. A characteristic feature of the jet loop reactors is the requirement of a large pumping capacity for liquid circulation, and therefore, the pump is a critical item in the reactor design. It has, however, to be pointed out that stirrers do pump equal amounts of liquid volumes around. The relevant design parameters of the jet loop reactors will be elucidated in more detail later. Aspects of Scaling-up the Stirred Gas-Liquid Reactors As stated above, the scale-up of a gas-liquid reactor has to be based on data obtained in a down-scaled version of the commercial unit. This postulate calls immediately for a series of boundary conditions, namely the following: (1) A minimum scaleable size (T ) 0.15-0.2 m). (2) A well-defined degree of mixing both in the gas and the liquid phase. (3) A well-defined micromixing zone for the fastreacting streams to mix with the bulk phase components.
(4) A mass- and heat transfer volume capacity which should not vary much with the scale. (5) A reliable shaft sealing at high pressures. Beside the questions related to collecting required information on the small scale, there is still a lot more to know for reliable designing stirred gas-liquid reactors on the large scale. Crucial questions in this respect include the following: (1) The scale-up rules, namely keeping power input constant, ask for an unambiguous relationship between the power number (Np ) P/FLN3D5) and the so-called gas flow number (NA ) φg/ND3) (Martin, 1996), which is not the case in the related literature. In addition, it is well-known that there exists an extreme difference in reported values of the local energy dissipation rate, especially on large scales (up to a factor of 20). Indeed, this raises the question of how reliable this approach to the reactor scale-up is. Recently, Wichterle and Sverak (1996) reviewed the scaling rule UL ) (N - N0)D2/T ) constant and declared this approach as physically well based. (2) The volumetric mass-transfer coefficient is negatively influenced by the process of bubble coalescence taking place at large distances from the stirrer. Accordingly, the overall kLa values will decrease with the reactor scale. (3) In general, the influence of physical properties on the mass-transfer rate still cannot be accurately predicted. (4) The insertion of coils for heating or cooling has a tremendous impact on the reactor performance. Empirical relations for the mass-transfer rate do not appropriately account for the influence of these internals on the interfacial area. (5) The gas phase is imperfectly mixed. Accordingly, there is a need for validated axial dispersion coefficients. In summary, it is thus possible to conclude that the design of a turbine-stirred gas-liquid reactor is a complex problem. Therefore, the performance of the jet loop reactors will be examined in the next paragraph, with respect to the above-raised questions. An attempt will be made at demonstrating their favorable scale-up features. Ejector Loop Reactors Working Characteristics. In ejector loop reactors (ELR), all power input has been supplied for pumping liquid through a Venturi-type ejector. According to the ejector position, the reactors can be operated both in the downflow (Figure 2) and upflow regime. Though, indeed, these two reactor modifications exhibit some different operating features; the following analysis of ELR performance applies generally to both alternative arrangements. A survey of different ELR modifications for both semi-batch and continuous flow operations has been given by Zahradnik and Rylek (1991). A principle of gas dispersion in ejectors is demonstrated in Figure 3, showing a schematic chart of a typical Venturi-type ejector. A detailed treatment of underlying theoretical concepts can be found in the works of Witte (1969) and Cunningham (1974a,b). The circulating liquid is forced through an ejector nozzle where the liquid is accelerated into a jet which due to its momentum entrains reaction gas into the mixing tube. In the mixing tube, gas and liquid are intensively mixed in the mixing shock zone where the gas is finely dispersed as very small bubbles. As the resulting gas-liquid stream leaves the ejector
736 Ind. Eng. Chem. Res., Vol. 37, No. 3, 1998
Figure 3. Principle of ejector performance.
and enters the reactor vessel, a secondary gas dispersion of bubbles is obtained in the bulk fluid. Following the pioneering works of Nagel and co-workers (1970, 1973), who first proposed the application of ejectors for gas dispersion in gas-liquid tower contactors, various aspects of ELR performance have been extensively studied by numerous authors both from academia and from the industry. Accordingly, the literature, reviewed for example by Kastanek et al. (1993) or most recently by Havelka et al. (1997), offers a considerable sum of information and know-how, insufficiently acknowledged by the potential users. The most widely known ELR has been commercialized by Buss AG in Switzerland and is known as the Buss loop reactor. Due to their operating principle and construction arrangement, the ELR exhibits numerous favorable features regarding their process application as well as design and scale-up. The mode of dispersion formation, described above, provides high intensity interfacial contact and, accordingly, a high rate of mass transfer in the reactor. Cramers et al. (1992a) reported values of the interfacial area from 40 000 to 70 000 m2 m-3 in the ejector and from 500 to 2500 m2 m-3 in the whole reactor (T ) 0.3 m, H ) 1.5 m), while maximum values of kLa measured in the ejector reached 6 s-1 for the airwater system (Cramers et al., 1993). For the same system and reactor size, Havelka (1997) obtained with an optimized ejector configuration the values for kLa 7.5 and 0.2 s-1 for the ejector and total reactor system. The application of ELR is, however, particularly advantageous for noncoalescing systems in which fine primary gas dispersion has been preserved in the whole reactor vessel. For such a system (0.3 mol L-1 of aqueous solution of Na2SO4) Havelka (1997) obtained kLa values of 10 and 1.5 s-1 for the ejector and reactor vessel in agreement with Nardin (1995) who reported for the Buss loop reactor overall kLa values up to 1.2 s-1 for noncoalescing systems as compared with values 0.050.25 and 0.15-0.5 s-1 corresponding for such systems to bubble columns and stirred tank reactors, respectively. The ELR are thus particularly suitable for fast
reactions in which the liquid phase mass transfer is the reaction limiting step of the process. The existence of gas and liquid circulation loops provides perfect mixing in both phases. In addition, an external heat exchanger can be suitably inserted into the liquid circulation loop, eliminating thus the disadvantages of internal coils installation. Gas recirculation ensures complete gas utilization. Accordingly, the ELR can be operated at large values of gas throughput, providing large intensity interfacial contact, without losses of the active component or requirements for installation of a circulation compressor. Complete gas utilization eliminates problems of safety control on the off-gas streams, and moreover, the gas circulation loop circumvents the problem of the removal of undesired volatile components from the gas phase. The liquid circulation mode and high degree of macroscale turbulence in the reactor vessel provide favorable conditions for catalyst suspension which may be one of the critical issues in large scale stirred tank reactors. Further advantages of ELR, as compared with aerated stirred tanks, include the absence of moving parts, eliminating the sealing problems and allowing easier operation at elevated pressure. Cramers et al. (1992b) reported an increase of the gas entrainment rate and gas holdup with increasing pressure due to the favorable effect of increasing gas density on these ELR characteristics. Design and Scale-up. The intensity of mass transfer in ELR has been decisively determined by the rate of energy dissipation in the ejector defined as ∆PφL or, related to a unit of liquid mass in the reactor, ∆PφL/ VLFL, where ∆P denotes ejector pressure drop, φL liquid flow rate, VL liquid volume in the reactor, and FL liquid density. The secondary gas dispersion occurring at the entrance of the gas-liquid stream from the ejector to the bulk fluid in the reactor vessel, in connection with the intensive gas and liquid mixing in the vessel, ensures uniform radial and axial distribution of gas bubbles over the entire reactor content (Zahradnik et al., 1997). As a result, scale-up of ELR has been considerably safer in comparison with the stirred tank reactors, and the transfer of data from laboratory or bench scale units to full-size reactors does not pose serious problems. In general, ELR scale-up can thus be based on a constant value of the specific rate of energy dissipation, ∆PφL/VLFL, keeping constant decisive geometrical characteristics of the ejector distributor, namely the ratio of the mixing tube and nozzle diameters(d2/d1) and the ratios of the mixing tube and diffuser lengths to the mixing tube diameter (L1/d2 and L2/d2, respectively). For practical design purposes, these geometrical parameters of Venturi-type ejectors should be kept within the following limits, proved (Henzler, 1983; Zahradnik and Rylek, 1991) to ensure efficient performance of such gas dispersing devices: d2/d1 ) 1.5-4.5; L1/d2 ) 5-8; L2/d2 ) 8-12. Regarding typical ratios of diffuser outlet to reactor cross-section area, ejectors can be viewed as typical “point” (spaceconcentrated) distributors and their distributing efficiency is thus dependent on the reactor height to diameter ratio (H/T). Obviously, higher H/T values should be always preferred, at given reactor volume, to achieve more uniform primary distribution of gas over the whole reactor cross section. For H/T < 2, a multijet arrangement has been recommended for full-size reactors and, similarly, such a reactor configuration should be considered at vessel diameters above 1.5 or 3 m, for
Ind. Eng. Chem. Res., Vol. 37, No. 3, 1998 737
slow bioprocesses (e.g., aerobic fermentations). In this latter case, the existence of gas and liquid circulation loops can be advantageously employed for foaming suppression (Kastanek et al., 1981; Zahradnik and Rylek, 1991). A favorable comparison of the Buss loop reactor with stirred reactors with respect to reaction times, yields, and catalyst concentrations, presented for selected types of reaction processes by Nardin (1995), indicates wide potential applicability of ELR, including notably the high-pressure operations. Obviously these reactors deserve wider attention of designers and process engineers as a feasible efficient alternative to the standard types of gas-liquid and gas-liquid-solid reactors. Conclusions
Figure 4. Time course of rape-seed oil hydrogenationscomparison of productionsscale data from the stirred tank and ejector loop reactors.
upflow or downflow ELR, respectively (Zahradnik and Rylek, 1991). Alternatively, configurations with a central draught tube can be advantageously employed in large diameter units. Due to the principle of ELR operation, proper selection of a circulating pump is of crucial importance, regarding namely its flexible working characteristics, sealing system, and erosion resistance for gas-liquid-solid applications. An illustrative example of a full-scale ejector design from laboratory data has been presented, for example, by Zahradnik and Rylek (1991), who reported on the design of an upflow Venturi-type ejector distributor for an industrial reactor (T ) 1.6 m, Vr ) 8 m3) for the rapeseed oil hydrogenation catalyzed by Ni on kieselguhr. The ejector design was based on the results of hydrodynamic and mass-transfer measurements performed in an ELR 0.3 m in diameter with the liquid volume 0.120 m3 (“cold model” data) and on the reaction experiments carried out at real process conditions in a laboratory scale unit 0.094 m in diameter with an effective volume of 0.003 m3. The authors reported superior performance of the industrial ELR over the equal-size stirred autoclave with propeller agitator (D ) 0.55 m, N ) 3 s-1) used originally for the process, regarding both the rate of the process (time of a singlebatch hydrogenation) or, alternatively, the catalyst load corresponding to particular process requirements (reaction time and hydrogenation degree). Typical data from the industrial units are presented in Figure 4 in the form of time dependence of the iodine value (I.V.) commonly employed as the characteristics of oils and fatty acid hydrogenation degree (Patterson, 1983). Application. Examples of mass-transfer-limited processes representing a typical application area for the ELR include various hydrogenation processes (hydrogenation of double and triple bonds, ring hydrogenation, hydrogenation of aliphatic or aromatic nitrocompounds, hydrogenation of aldehydes and ketones, etc.), amination, alkylation, carbonylation, chlorination, oxidation, and dehydrogenation. Due to their superior gas utilization, the ELR can be, however, suitably used even for
(1) Despite many decades of intensive research, scaleup of a stirred gas-liquid reactor still poses considerable problems. (2) Conclusion 1 is supported by the many disagreements between researchers investigating stirred gasliquid reactors. The approach to their scale-up is based on either power input correlations or proceeds via the more physically justified route by keeping the effective stirrer speed constant. (3) The ejector-type loop reactors allow a simpler scale-up approach and show better mass-transfer performance than the stirred reactors. The system is, according to the literature, economically attractive at pressures above 10 bar. (4) The ejector loop reactors offer flexible design with many additional favorable features discussed in this paper. (5) Upon conclusions 1-4, assessment of the ELR potential should become a standard part of the process of reactor selection for reactions in gas-liquid and gasliquid-solid systems. Acknowledgment The Czech co-authors (J.Z. and V.L.) gratefully acknowledge support given to the research of ejector loop reactors by the Grant Agency of the Czech Republic through Grant No. 104/97/1170. Nomenclature D ) impeller diameter, m d1 ) nozzle diameter, m d2 ) mixing tube diameter, m F ) vessel cross section, m2 H ) reactor height, m kLa ) volumetric liquid-side mass-transfer coefficient, s-1 L1 ) mixing tube length, m L2 ) diffuser length, m N ) impeller rotation speed, s-1 N0 ) minimum stirring speed ensuring gas dispersion, s-1 NA ) aeration number, NA ) φg/ND3 Np ) power number, Np ) P/FLN3D5 P ) total power input, W Pg ) shaft power input under gassing conditions, W ∆P ) ejector pressure drop, Pa T ) vessel diameter, m Ug ) superficial gas velocity, m s-1 UL ) characteristic liquid circulation velocity, m s-1 VL ) liquid volume in vessel, m3 Vr ) reactor volume, m3
738 Ind. Eng. Chem. Res., Vol. 37, No. 3, 1998 Greek Symbols FL ) liquid density, kg φg ) gas volumetric feed rate, m3 s-1 φL ) volumetric liquid flow rate through ejector, m3 s-1 m-3
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Nagel, O.; Kurten, H.; Sinn, R. Strahldusenreaktoren Teil I: Die Anwendung des Ejektorprinzips zur Verbesserung der Gasabsorption in Blasensaulen. (Jet Loop Reactors Part I: Application of the Ejector Principle for Gas Absorption Enhancement in Bubble Columns.) Chem.-Ing.-Tech. 1970, 42, 474-479. Nagel, O.; Kurten, H.; Hegner, B. Die Stoffauataschflache in Gas/ Flussigkeits-Kontaktapparaten. Auswahlkriterien und Unterlagen zur Vergrosserung. (Interfacial Area in Gas/Liquid Contactors. Selection Criteria and Scale-up.) Chem.-Ing.-Tech. 1973, 45, 913-920. Nardin, D. Trends and opportunities with modern Buss loop reactor technology. Chemspec Europe 95 BACS Symposium, 1995. Oldshue, J. Y. Fluid Mixing Technology; McGraw-Hill: New York, 1983. Patterson, H. B. W. Hydrogenation of Fats and Oils; Applied Science Publ.: London, 1983. Smith, J. M.; Warmoeskerken, M. M. C. G. The dispersion of gases in liquids with turbines. Proceedings of the 5th European Conference on Mixing, Wurzburg, 1985, BHRA: Bedford, U.K., 1985; Vol. 107, pp 115-126. Tatterson, G. B. Fluid Mixing and Gas Dispersion in Agitated Tanks; McGraw-Hill: New York, 1991. Uhl, V. W.; Gray, J. B. Mixing: Theory and Practice; Academic Press: New York, 1966, 1967; Vols I and II. Westerterp, K. R.; van Dierendonck, L. L.; De Kraa, J. A. Interfacial areas in agitated gas-liquid contactors. Chem. Eng. Sci. 1963, 18, 157-176. Wichterle, K.; Sverak, T. Surface aeration threshold in agitated vessels. Collect. Czech. Chem. Commun. 1996, 61, 681-690. Witte, J. H. Mixing shocks in two-phase flow. J. Fluid Mech. 1969, 36, 639-655. Zahradnik, J.; Rylek, M. Design and scale-up of Venturi-tube gas distributors for bubble column reactors. Collect. Czech. Chem. Commun. 1991, 56, 619-634. Zahradnik, J.; Fialova, M.; Linek, V.; Sinkule, J.; Reznickova, J.; Kastanek, F. Dispersion efficiency of ejector-type gas distributors in different operating modes. Chem. Eng. Sci. 1997, in press.
Received for review May 2, 1997 Revised manuscript received August 15, 1997 Accepted August 24, 1997X IE970313B
X Abstract published in Advance ACS Abstracts, Xxxxxxxxx YY, ZZZZ.
