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In d. Eng. Chem. Chem. Res. Res. 1998, 37 , 7 34 -738
Loop Venturi Reactor sA Fea Feasible Alte Alternativ rnative e to Stirr Stirre ed Ta T ank Reactors? L aurent aurent L. van van Die Di erendon rendonck* ck* RUG, Nijenborg, 9747 AG Groningen, The Netherlands
J indrˇich Zahradnı´ k Institute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic, 165 02 Prague 6, Czech Czech R epubl ic
´ clav Linek Va Departm Departm ent of Chemi Chemi cal E ngineeri ngineeri ng, Prague I nstitute of Chemical Technology Technology,, 166 28 Prague 6, Czech Czech R epubl ic
During the past decennia, the stirred tank reactor, especiall y for gas -liquid operations, has receiv received ed much at tention. However, prope properr design of turbine-stirred turbine-stirred ga s -liquid reactors on an indust ria l sca sca le can st ill be difficult difficult to ma ke. On the lar ge sca sca le, the removal of hea t ma y become become a limiting factor. Insta llation of additional cool cooling ing coils coils into the reactor vessel vessel ma kes the design design problems even more complex. complex. The development development of loop loop Ventu Ventu ri design s offers some new solutions to t he scale-up scale-up questions, especi especially ally w hen high pressures pressures a re involved. involved. Recent Recent public publicaa tions on this t ype of of reactor demonstra te fa st mixing (inc (including luding micromixi micromixing) ng),, high ma ss-tra ss-tra nsfer ra te, a nd a n independently independently designed heat excha excha nger in the circulat circulat ion ion loop loop of of the rea ctor. ctor. The gas loop loop ensuring comp complete lete ga ga s mixing represents represents an a dditional favora ble featur e. On the w hole, hole, the loop loop Venturi Venturi rea ctors ctors can be viewed viewed a s a n effici efficient ent a lterna tive to the stirr ed ta nk reactors, offering offering ea sier scale-up. scale-up. This conclusion conclusion is supported by t he less dependency dependency of th e mas s-tr s-tr a nsfer rrat at e and mixing mixing on the reactor r eacto r scal scalee.. Introduction To design a gas -liquid reactor for commercial processes, cesses, the designer requir es a lot of informa informa tion a bout the system involved. involved. However, most of of the informa informa tion can be collected on a small scale (bench or semi-technical scale). scale). In th e selection selection proce process ss of a ga s-liquid contactor t y p e, e, t h e s t i r r ed ed t a n k r e a ct ct o r w i t h a s t a n d a r d t u r b i n e stirrer (Rushton) is the most common choice (Figure 1). This type of reactor possesses a wide application area in chemica chemica l- a nd bio-proc bio-process ess industr y a nd, a ccordingly, ccordingly, ha s been receiving receiving extensive extensive coverag coverag e in the litera tur e, including most notably classic textbooks by Uhl and Gr ay (1966 1966)), Na gat a (1975 1975)), and Oldshue (19 (1983 83)). A t h or or ou ou g h a n a l y s is is of g a s con t a c t i n g w i t h l iq iq u i d s i n stir red vessels h a s been present ed by J oshi et a l. (198 (1982) 2),, Mann (1983), and more recently by Tatterson (1991). In spite of routine routine industria l use of stirred tan k reactors, their design a nd scale-up scale-up still pose pose man y questions a nd rely to a considerable considerable extent extent on part icular icular expe experienc riencee and knowknow-how how of designers. designers. In th e industrial practice, practice, the prevailing approach is still scaling-up by empirical testing w ith t he actua l proce process ss thr ough a series of sca sca leup sta ges. Westert erp and coco-workers (1963 (1963)) performed in th e sixties comprehensive comprehensive studies both both on the physical an d chemical chemical behavior behavior of the stirred ta nk reactors. Many public publicat at ions ions of the group contributed contributed to gra nt a cita cita tion price price a nd improved improved understa nding of the ba sic d e si si g n . M a n y i n ve ve st st i g a t o r s ca ca m e u p w i t h t h e ir ir s o l uutions for for the scale-up. scale-up. An ast onishing number of papers in the literature, dealing with t he aspects aspects of stirring stirring a nd the scale-up, scale-up, confuses confuses designers. There ha s been little agreement about the approach to the scale-up among
cheme of a sta ndard geometry vessel vessel with a Rushton Figure Figure 1. S cheme type turbine stirrer.
various authors. Essentially, reports reports on on investigat investigat ions ions p er er f or or m e d o n a l a r g e s c a l e h a v e b e en en m i s si si n g . Th e ma in question faced faced by the designer has been been keeping keeping e it it h e r t h e p ow ow e r i n pu pu t (P g /V L ) o r t h e s t i r r er er s p ee ee d consta consta nt. For the latt er case, an effectiv effectivee stirrer speed speed was defined as a criterion for scale-up (U L ) (N - N 0)D 2/T ). The problem problem is complic complicat at ed by the effec effectt of increasing superficial gas velocity (φg /F ) U g ) w i t h t h e reactor scale. scale. In t his respect, respect, floo flooding ding of the stirrer occurs if U g g 0.05 m s -1. Smith a nd War War moeskerk moeskerken en (1985 1985)) defined defined the operation operation window for for the turbine stirrer preventing preventing gas overlo overloading. ading. It ha s been been known t h a t t h e e x t e n t o f l i q u id id m i xi xi n g d e cr cr e a s es es w i t h s ca ca l e while validat ed informa informa tion about gas pha se mixing mixing on a large scale ha ve been been still missing.
S0888S0888-5885( 5885(97) 97)003 0031313-8 8 CCC : $15.0 $15.00 0 © 1998 1998 American American C hemical Society P ubl ish ed on Web 01/01/1998
Ind. Eng . C hem. Res., Vol. 37, No. 3, 1998
Figure 2. Scheme of a loop Venturi reactor.
A general question in the whole field of gas-liquid reactors concerns prediction of t he 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 tra nsfer. The industrial processes are mostly characterized by undefined components in the liquid streams which can influence interfacial area to a la rge extent. Recently, Martin (1996)empha sized this p rob le m a g a i n i n h is t h es is w o r k a n d s h ow e d t h e complexity of the mass-transfer prediction in relation to the power input. It can t hus be concluded tha t scale-up of the st irred ga s -liquid contactors is still far from being a routine ta sk. The engineering people may look for alterna tives. In this respect, the jet loop reactors still have not been s u ff ici en t l y e va l u a t e d . Th e je t l oop r e a ct o r s o f t h e design illustrated in Figure 2 have been claimed to solve some of the a bove-ra ised questions a nd ma ke the scaleu p e a s ie r. A c h a r a ct e ri st i c f e a t u re of t h e je t l oop reactors is the requirement of a la rge pumping capa city f or l iq u i d ci r cu l a t i on , a n d t h e r ef or e , t h e p u m p i s a critical item in the reactor design. It ha s, however, to be pointed out tha t stirrers do pump equal a mounts of liquid volumes around. The relevant design para meters of the jet loop reactors w ill be elucida ted in m ore detail later.
Aspects of Scaling-up the Stirred Gas-Liquid Reactors As stated above, the scale-up of a gas -liquid reactor h a s t o b e b a s ed on d a t a ob t a i ne d i n a d ow n -s ca l ed version of the commercial unit. This postulat e calls immediat ely for a series of boundary conditions, nam ely 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 w ell-defined micromixing zone for the fastreacting streams to mix with the bulk phase components.
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(4) A mass- and heat transfer volume capacity which should not vary much with the scale. (5) A reliable sha ft sea ling a t high pressures. Beside the questions related to collecting required informa tion on t he sma ll scale, there is still a lot more to know for reliable designing stirred gas-liquid reactors on the la rge scale. Crucial questions in this respect include the following: (1) The scale-up r ules, na mely keeping power input constant, ask for an unambiguous relationship between the power number (N p ) P /FL N 3D 5) and the so-called gas flow number (N A ) φg /N D 3) (Martin, 1996), which is not the case in the related litera ture. In a ddition, it is well-known that there exists an extreme differen ce in reported va lues of th e local energy dissipation ra te, especially on large sca les (up to a fa ctor of 20). Indeed, this raises the question of how reliable this approach t o t h e r e a ct o r s ca l e -u p i s . R e ce n t ly , Wi ch t e r le a n d Svera k (1996) reviewed t he scaling rule U L ) (N - N 0)D 2/T ) constant and declared this approach as physically well based. (2) The volumetr ic ma ss-tra nsfer coefficient is n egatively influenced by the process of bubble coalescence t a k i n g p l a ce a t l a r g e d i s t a n ce s f r om t h e s t i r r er . Accordingly, t he overa ll k L a values will decrease with the reactor scale. (3) In genera l, th e 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 performa nce. Emp ir i ca l r el a t i on s f or t h e m a s s -t r a n s f er r a t e d o n ot a ppropriat ely account for th e influence of these interna ls on the interfacial area. (5) The ga s pha se is imperfectly mixed. Accordingly, there is a need for va lida ted a xial dispersion coefficients. In summary, it is thus possible to conclude that the d e si g n of a t u r b i ne -s t i r r ed g a s-l iq u id r ea c t or i s a complex problem. Therefore, the performa nce of the jet loop reactors will be examined in the next paragraph, wit h respect to the above-ra ised questions. An at tempt will be ma de at demonstra ting th eir favorable scale-up features.
Ejector Loop Reactors Working Characteristics. In ejector loop reactors (ELR), all power input has been supplied for pumping liquid th rough a Ventur i-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 performa nce applies generally to both alterna tive ar ra ngements. A survey of different ELR modifications for both semi-ba tch a nd continuous flow operat ions has been given by Zahr a dnik a nd Ry lek (1991). A principle of gas dispersion in ejectors is demonstrated in Figure 3, showing a schemat ic cha rt of a typical Venturi-type ejector. A deta iled trea tment of underlying theoretica l concepts can be found in the works of Witte (1969) and Cun ning ha m (1974a,b). The circula tin g liquid is forced through an ejector nozzle where the liquid is accelerated into a jet which due to its momentum entrains reaction g a s i n t o t h e m i xi n g t u b e. I n t h e m i x in g t u b e , g a s a n d liquid a re intensively mixed in t he mixing shock zone where the gas is finely dispersed as very small bubbles. As the resulting gas -liquid stream leaves the ejector
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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 w orks of Na gel a nd co-w orkers (1970, 1973), who first proposed the application of ejectors for gas dispersion in gas -liquid tower contactors, various aspects of EL R performa nce have been extensively studied by numerous aut hors both from academia and from th e i n d us t r y . Accor d i n gl y , t h e l it e r a t u r e , r e vi ew e d f or example by Kastanek et al. (1993) or most recently by Ha velka et al. (1997), offers a considerable sum of information and know-how, insufficiently acknowledged b y t h e p o t en t i a l u s e r s . Th e m o st w i d e ly k n o w n E L R has been commercialized by Buss AG in Switzerland and is known as the Buss loop reactor. D u e t o t h e ir op er a t i n g p r in ci pl e a n d con s t r u ct i on arrangement, the ELR exhibits numerous favorable features regarding their process application as well as design a nd scale-up. The mode of dispersion form a tion, described a bove, provides high intensity interfacial contact and, accordingly, a high rate of mass transfer in the rea ctor. Cra mers et a l. (1992a) reported va lues of the int erfacia l a rea from 40 000 to 70 000 m2 m -3 in the ejector and from 500 to 2500 m 2 m -3 in the whole reactor (T ) 0.3 m, H ) 1.5 m), while maximum values of k L a mea sured in t he ejector rea ched 6 s-1 for the air w a t e r s y s t e m (C r a m e r s e t a l . , 19 93). F o r t h e s a m e system and reactor size, Havelka (1997) obtained with an optimized ejector configuration the values for k L a 7.5 an d 0.2 s -1 for the ejector a nd t otal r eactor syst em. The application of ELR is, however, particularly advantageous for noncoalescing systems in which fine primary gas dispersion has been preserved in the whole reactor v es s el . F or s u ch a s y s t em (0 .3 m o l L-1 of aqueous solution of Na 2S O 4) Ha velka (1997) obta ined k L a 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 overa ll k L a values up to 1.2 s -1 for noncoalescing systems as compared with values 0.050.25 a nd 0.15-0.5 s -1 corresponding for such systems to bubble columns and stirred tank reactors, respectively. The ELR a re thus particularly suitable for fast
reactions in which the liquid phase mass transfer is the reaction limiting step of the process. The existence of ga s and liquid circulation loops provides perfect mixing in both pha ses. In a ddition, an external heat exchanger can be suitably inserted into the liquid circulat ion loop, eliminating thus the disa dvant ages of internal coils insta llation. Ga s recirculat ion ensures complete gas utiliza tion. Accordingly, the ELR ca n b e o pe ra t e d a t l a r g e v a l u es of g a s t h r ou g hp ut , providing large intensity interfacial contact, with out losses of t he active component or r equirements for insta llat ion of a circula tion compressor. Complete ga s 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 t he ga s pha se. The liquid circulation mode and high degree of macroscale turbulence in t he rea ctor vessel provide favora ble conditions for catalyst suspension which may be one of the critical i s su e s i n l a r g e s c a l e s t i r r ed t a n k r e a ct o r s. F u r t h e r a d v a n t a g es o f E L R , a s c om p a r e d w i t h a e r a t e d s t i r r e d ta nks, include the absence of moving par ts, eliminat ing the sealing problems and allowing easier operation at eleva ted pressure. Cra mers et al. (1992b) reported a n increase of the gas entrainment rate and gas holdup with increasing pressure due t o the fa vora ble effect of increasing ga s density on th ese ELR chara cteristics. 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 ∆P φL or , r e l a t e d t o a u n i t o f l i q u i d m a s s i n t h e r e a c t o r , ∆P φL / V L FL , where ∆ P denotes ejector pressure drop, φ L liquid flow r at e, V L liquid volume in the reactor, and FL liquid density. The secondar y gas dispersion occurring at t he e n t r a n ce o f t h e g a s -liquid stream from the ejector to the bulk fluid in the reactor vessel, in connection with t h e i n t en s iv e g a s a n d l iq u id m i xi n g i n t h e v es se l, ensures uniform radial and axial distribution of gas bubbles over the entire reactor content (Zahra dnik et a l . , 1 99 7). As a r e su l t , s c a l e-u p o f E L R h a s b ee n considerably safer in compar ison w ith th e stirred ta nk reactors, and the transfer of data from laboratory or bench scale units to full-size reactors does not pose serious problems. In general, E LR scale-up can thus b e b a s e d o n a con s t a n t v a l u e o f t h e s p e ci fi c r a t e of energy dissipation, ∆P φL /V L FL , keeping consta nt decisive geometrical characteristics of the ejector distributor, n a m e l y t h e r a t i o of t h e m i xi n g t u b e a n d n oz z le d i a m eters(d 2/d 1) a n d t h e r a t i o s o f t h e m i x i n g t u b e a n d diffuser lengths to the mixing tube diameter (L 1/d 2 a n d L 2/d 2, respectively). For pra ctica l design purposes, these geometrica l pa ra meters of Venturi-type ejectors sh ould be kept within the following limits, proved (Henzler, 1983; Zahradnik and Rylek, 1991) to ensure efficient performance of such gas dispersing devices: d 2/d 1 ) 1.5 -4.5; L 1/d 2 ) 5-8; L 2/d 2 ) 8-12. Regarding typical ratios of diffuser outlet to reactor cross-section area, e je ct o r s ca n b e v i ew e d a s t y p ica l “ poi n t ” (s pa c econ ce n t r a t e d ) d i s t r i bu t or s a n d t h e ir d i s t r ib u t i ng e ff ici en cy i s t h u s d e pe n d en t on t h e r e a ct o r h e i g h t t o d i a m e t e r r a t i o (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 w hole 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
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slow bioprocesses (e.g., aerobic ferment a tions). In t his latter case, the existence of gas and liquid circulation l oop s ca n b e a d v a n t a g e ou s ly e m pl oy e d f or f oa m i n g s u pp r es s ion (K a s t a n e k e t a l . , 1 981 ; Z a h r a d n i k a n d Rylek, 1991). A fa vorable compa rison of the B uss loop reactor with stirred reactors with respect to reaction times, yields, and cat alyst concentr at ions, presented for selected types of reaction processes by Nardin (1995), indicates wide potential applicability of ELR, including nota bly the high-pressure operat ions. Obviously these reactors deserve wider a ttention of designers a nd process engineers a s a feasible efficient a lternat ive to the s t a n da r d t y pes of g a s-l iq u id a n d g a s-liquid -solid reactors.
Conclusions
Figure 4. Time course of ra pe-seed oil hydr ogenation scomparison of production sscale data from the stirred tank and ejector loop reactors.
upflow or downflow ELR, respectively (Zahradnik and R y l ek , 1 99 1). Al t e r n a t i v el y , con f i gu r a t i o n s w i t h a centra l dra ught t ube can be adva nta geously employed i n l a r g e d i a m e t e r u n i t s . D u e t o t h e p r i n ci pl e of E L R operation, proper selection of a circulating pump is of crucial importance, regarding namely its flexible working characteristics, sealing system, and erosion re sistance for gas -liquid -solid applications. An illust ra tive exa mple of a full-scale ejector design from la bora tory da ta ha s been presented, for exam ple, by Zahradnik and Rylek (1991), who reported on the design of a n upflow Ventur i-type ejector dist ributor for an industrial reactor (T ) 1.6 m, V r ) 8 m 3) for the ra peseed oil hydrogenation cata lyzed by Ni on kieselguhr. The ejector design wa s ba sed on t he results of hydrodynamic and mass-transfer measurements performed in an ELR 0.3 m in diameter with the liquid volume 0.120 m 3 (“ col d m od el ” d a t a ) a n d on t h e r ea c t ion experiments carried out at real process conditions in a l a b or a t o ry s ca l e u n it 0. 094 m i n d ia m e t er w i t h a n effective volume of 0.003 m 3. Th e a u t h or s r e p or t e d superior performance of the industrial ELR over the equal-size stirred a utoclave w ith propeller agita tor (D ) 0.55 m, N ) 3 s -1) used originally for the process, rega rding both t he ra te of the process (time of a singlebatch hydrogenation) or, alternatively, the catalyst load corresponding to particular process requirements (reaction time and hy drogena tion degree). Typica l dat a from the industrial units are presented in Figure 4 in the f or m of t i m e d e pe n d en ce of t h e i od i n e v a l u e (I . V. ) commonly employed as the characteristics of oils and fatty acid hydrogenation degree (Patterson, 1983). Application. Exa mples of ma ss-tra nsfer-limited processes representing a typical a pplicat ion a rea for t he ELR include various hydrogenation processes (hydrogenat ion of double an d t riple bonds, ring hydrogenation, hydrogenat ion of alipha tic or a romat ic nitrocompounds, hydrogena tion of a ldehydes a nd ketones, etc.), a mination, alkylation, carbonylation, chlorination, oxidation, an d dehydrogenation. Due to their superior gas utilization, the EL R can be, however, suitably used even for
(1) Despite many decades of intensive research, scaleup of a st irred gas -liquid rea ctor still poses considera ble 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 consta nt. (3) The ejector-type loop r eactors a llow a simpler scale-up a pproach a nd show better ma ss-tra nsfer perf or m a n c e t h a n t h e s t i r r e d r e a c t or s . Th e s y s t e m i s , according to the literature, economically attractive at pressures a bove 10 bar. (4) The ejector loop rea ctors offer flexible d esign w ith many additional favorable features discussed in this paper. (5) Upon conclusions 1-4 , a s s e s s m e n t o f t h e E L R potential should become a standard part of the process of reactor selection for reactions in gas-liquid and ga sliquid -solid systems.
Acknowledgment The Czech co-a uth ors (J .Z. a nd V.L.) gra tefully acknowledge support given to t he resea rch of ejector loop reactors by the Grant Agency of the Czech Republic th roug h G ra nt No. 104/97/1170.
Nomenclature D ) impeller diameter, m d 1 ) nozzle diameter, m d 2 ) mixing t ube diameter, m F ) vessel cross section, m 2 H ) reactor height, m k L a ) volumetric liquid-side m ass-transfer coefficient, s -1 L 1 ) mixing t ube length, m L 2 ) diffuser length, m N ) impeller rota tion speed, s -1 N 0 ) minimum st irring speed ensuring gas dispersion, s -1 N A ) aerat ion number, N A ) φ g /N D 3 N p ) power number, N p ) P /FL N 3D 5 P ) total power input, W P g ) shaft power input under gassing conditions, W ∆P ) ejector pressure drop, P a T ) vessel diameter, m U g ) superficial gas velocity, m s -1 U L ) characteristic liquid circulation velocity, m s -1 V L ) liquid volume in vessel, m 3 V r ) reactor volume, m 3
738 Ind. Eng. Chem. R es., Vol. 37, No. 3, 1998 Greek Symbols FL ) liquid density, kg m -3 φg ) gas volumetric feed rat e, m 3 s -1 φL ) volumetric liquid flow rate through ejector, m 3 s -1
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