Bubble Column Technology

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<p>c 2005 Wiley-VCH Verlag GmbH &amp; Co. KGaA, Weinheim10.1002/14356007.b04 275Bubble Columns 1Bubble ColumnsFor other industrial reactors and their applications, see Stirred-Tanc and Loop Reactors, TubularReactors, Fixed-Bed Reactors, Fluidized-Bed Reactors, Three-Phase Trickle-Bed Reactors,Reaction Columns, Thin-Film Reactors, Metallurgical Furnaces, and Biochemical Engineering.Peter Zehner, BASF Aktiengesellschaft, Ludwigshafen, Federal Republic of GermanyMatthias Kraume, BASF Aktiengesellschaft, Ludwigshafen, Federal Republic of Germany1. Introduction . . . . . . . . . . . . . . . . 22. Bubble Columns and Modications 42.1. Design and Applications . . . . . . . . 42.2. Gas Distribution . . . . . . . . . . . . . 52.3. Flow Regimes . . . . . . . . . . . . . . . 62.4. Fluid Dynamics . . . . . . . . . . . . . . 72.5. Bubble Size . . . . . . . . . . . . . . . . 82.6. Bubble Rise Velocity . . . . . . . . . . 92.7. Dispersion of the Liquid Phase . . . . 92.8. Dispersion of the Gas Phase . . . . . . 102.9. Gas Holdup . . . . . . . . . . . . . . . . 102.10. Specic Interfacial Area . . . . . . . . 122.11. Volumetric Mass-TransferCoefcient . . . . . . . . . . . . . . . . . 132.12. Heat Transfer . . . . . . . . . . . . . . . 142.13. Slurry Bubble Columns . . . . . . . . 142.14. Airlift Loop Reactors . . . . . . . . . . 163. Downow Bubble Columns . . . . . . 183.1. Design and Applications . . . . . . . . 193.2. Operating Conditionsand Gas Holdup . . . . . . . . . . . . . 203.3. Mass Transfer . . . . . . . . . . . . . . . 214. Jet Loop Reactors . . . . . . . . . . . . 224.1. Design and Applications . . . . . . . . 234.2. Typical Dimensions . . . . . . . . . . . 264.3. Energy Balance . . . . . . . . . . . . . . 274.4. Mixing Behavior and Fluid Dynamics 274.5. Gas Holdup . . . . . . . . . . . . . . . . 284.6. Mass Transfer . . . . . . . . . . . . . . . 304.7. Three-Phase Loop Reactor . . . . . . 315. References . . . . . . . . . . . . . . . . . 31Symbols (see also Principles of Chemi-cal Reaction Engineering and Model Reac-tors and Their Design Equations)Variablesa specic interfacial area, m1A interfacial area, m2d diameter, mdh diameter of holes, mdi inner diameter of draft tube, mdn nozzle diameter, mD diffusion or dispersion coefcient, m2/sDG, L diffusion coefcient of dissolved gas inliquid, m2/seM energy dissipation rate per unit mass,W/kgen jet power per unit volume, W/m3eV energy dissipation rate per unit volume,W/m3f fraction of cross-sectional areafi fraction of cross-sectional area of drafttubeF cross-sectional area, m2Fi cross-sectional area of draft tube, m2FR cross-sectional area of reactor, m2h height, mhR height of gas liquid mixture, mht height of reactor, mJD dispersion owkL liquid-phase mass-transfer coefcient,m/sP power, Wr radial distance from column axis, mt time, su supercial velocity, m/sv velocity, m/svrG relative velocity of bubble swarm in liq-uid, m/svrS relative velocity of particle swarm in liq-uid, m/sV volume, m3V volumetric ow rate, m3/sz axial coordinate, m2 Bubble ColumnsGreek symbols heat-transfer coefcient, WK1m2 volume fractionG gas holdup drag coefcient of circulation ow dynamic viscosity, kg m1s1 kinematic viscosity, m2/s density, kg/m3 density difference between liquid andgas, kg/m3S density difference between liquid andsolids, kg/m3 surface tension, N/m mass concentration, kg/m3Subscriptsa annular spaceb bubblebS Sauter diameterc, circ circulationD downowG gas phaseh holei inside draft tubeL liquidmax maximum valuemin minimum valueM per unit massn nozzlep particler relativeR upow, reaction mixtureslip slipS solidst reactorV per unit volume1. IntroductionBubble columns are devices in which gas, inthe form of bubbles, comes in contact with liq-uid. The purpose may be simply to mix the liq-uid phase. Far more often, however, substancesare transferred from one phase to the other, forexample, when gaseous reactants are dissolvedin a liquid or when liquid reaction productsare stripped. Both processes can take place si-multaneously. Achemical or biological reactionnearly always proceeds in the liquid phase. De-pending on the application, special measures tointensify mass transfer between the two phasesmay be useful, or the residence-time distributionof one or both phases may be modied.The liquid may also contain inert, cat-alytically active, or reactive particles in sus-pension. Oxidation, hydrogenation, chlorina-tion, phosgenation, alkylation, and other pro-cesses have long been performed in bubble-column reactors in the chemical industry. In1978, more than 107t/a of chemical productswere made in bubble columns [1]. Since then,marked growth has occurred. Industrial reac-tors for high-tonnage products have capacitiesof 100 300 m3. Larger bubble columns, withcapacities up to 3000 m3, are employed as fer-menters for protein production from methanol.The largest units (20 000 m3) are those for waste-water treatment.Scientic interest in bubble columns has in-creased considerably in the past 10 15 years.Up to the mid-1970s, only 10 to 20 publicationsappeared annually; by the mid- to late 1980s, thenumber had increased to 80 per year. This ledto the development of many empirical correla-tions and theoretical models enabling the math-ematical simulation of bubble- column reactors.Some academic research groups and commer-cial software developers have offered simulationprograms.The mixing of a liquid and a gas having onlypartial mutual solubility is one of the unit op-erations in chemical technology. As Figure 1shows, this operation takes one of three prin-cipal forms. The simplest design is the bubblecolumn (Fig. 1 A) in which gas is fed into thecolumn at the bottom and rises in the liquid, es-caping from it at the upper surface; the gas isconsumed to a greater or lesser extent (depend-ing on the intensity of mass transfer and chem-ical reaction). When the off-gas contains highconcentrations of valuable reactants, part of itis recycled to the reactor. This recycle design,however, lowers the concentration prole in thebubble column and must be optimized from aneconomic standpoint. In a simple bubble columnthe liquid is led in either cocurrently or counter-currently to the upward gas streamand has a longresidence time. The ow direction of the liquidphase has little effect on the gas-phase residencetime, which is comparatively short. Thus, in theBubble Columns 3Figure 1. Principal methods of gas liquid mixingA) Bubble column; B) Downow bubble column; C) Jet loop reactorsimple column, the ow of gas is always frombottom to top, and the stream can be made up ofboth fresh and recycle gas.Longer gas-phase residence times can beachieved with the downow bubble columnshown in Figure 1 B. The liquid is pumped downthrough the column at a velocity of more than20 cm/s, so that gas let in at the top is entrained inthe ow and can even be held in a suspension-like state until it has reacted completely. Usu-ally, however, unconsumed gas is removed withthe liquid and separated. Special designs per-mit phase separation inside the apparatus. Thedownow bubble column is used mainly whenlarge liquid streams are to be contacted withsmall gas streams and a short liquid residencetime is required. The necessary velocity cannotalways be obtained with the liquid inlet to thereactor. Thus, like the gas in an ordinary bub-ble column, the liquid in the downow bubblecolumn can be recycled. Typical applicationsfor downow bubble columns are the ozonationof drinking water and the treatment of water inswimming pools. A special use of such devicesin the evacuation and compression of gases hasalso been reported [2].In both types of column energy must be sup-plied continuously to the two-phase system tokeep the liquid and gas mixed. Only in this waycan separation of the phases be counteracted orreversed. In the rst case, the simple bubble col-umn, this energy is supplied by the gas. In thedownow bubble column the energy is suppliedby the downowing liquid.A different mechanism comes into play inthe jet loop reactor (Fig. 1 C). Here no net owof gas or liquid occurs along the column; in-stead, an internal circulating ow is produced.One way to achieve this is with a propeller, butother approaches exist. In the most commonlyused type of loop reactor, the jet loop reactor,the owis drivenbya high-velocityliquidjet. Asin the downow bubble column, gas is let in atthe top and dispersed by the jet energy. Bubblescan be distributed throughout the reactor volumeonly if the downward liquid ow velocity in theinternal tube is greater than the slip velocity ofthe bubbles. Accordingly, a minimum power in-put is required.4 Bubble ColumnsThese three basic methods of dispersing gasin liquid are generally not used in their pureforms. The variety of problems in chemical andbiotechnical processes has led to many differ-ent contacting devices that combine these basictechniques.Figure 2. Types of bubble-column reactorsA) Simple bubble column; B) Cascade bubble column withsieve trays; C) Packed bubble column; D) Multishaft bubblecolumn; E) Bubble column with static mixers2. Bubble Columns andModications2.1. Design and ApplicationsBubble columns are very adaptable gas liquidcontacting devices; possible designs are shownin Figure 2. The simplest form of bubble col-umn (Fig. 2 A) consists of a vertical tube withno internals. Gas is fed in at the bottom whileliquid is led through the apparatus cocurrentlyor countercurrently. This simple form is seldomused in practice; instead, a number of modi-cations are employed. The back-mixing of gasand liquid phases in the simple bubble columnand the nonuniform distribution of gas bubblesover the cross section can be reduced by the in-stallation of trays (Fig. 2 B), packings (Fig. 2 C),or shafts (Fig. 2 D). All these devices can oper-ate either cocurrently or countercurrently. To setup the most homogeneous possible bubble ow,static mixer elements can also be placed in theascending ow section (Fig. 2 E).Figure 3. Hydroformylation of propenea) Stripping zone; b) Reaction zoneHydroformylation. The hydroformylationof propene is carried out in simple bubblecolumns. The reaction is homogeneously cat-alyzed by rhodium complexes. Usually thepropene and the CO/H2 gas mixture are let inat the bottom of the reactor. Incompletely re-acted gas, saturated with the reaction product,exits the reactor. The hydroformylation productis separated fromthe gas streamby condensationand forwarded to downstream processing, whilethe gas is recycled to the reactor. Because theheat of reaction cannot be completely removedby evaporative cooling using the enthalpy of va-porization of the product, the bubble column isalso equipped with an external cooling loop.One great advantage of the process is that theproduct is recovered from the reaction mixturewithout additional separation operations whichwould damage the expensive catalyst system.The close coupling between the product and therecycle gas necessary to discharge it (i.e., a cer-tain quantity of gas is required for product dis-charge for thermodynamic reasons), however,presents some problems. First, the gas ow ratecauses a high gas holdup, which reduces the re-action volume and thus decreases the productiv-Bubble Columns 5Figure 4. Oxidation of montan waxes in cascade bubble columnsa) Cascade bubble-column reactors; b) Separators; c) Final purication of wax oxidate; d) Off-gas treatmentity of the reactor. Second, large bubbles occur,which limit the delivery of gaseous reactants tothe liquid phase in the reactor. For these reasons,recycle gas is admitted to the bubble column attwo levels (Fig. 3) [3]. About half of the recy-cle gas is fed via the bottom sparger to dispersereactants into the overlying reaction zone. Theremaining recycle gas is let in via the top sparger,which lies slightly below the liquid surface, tofacilitate separation of the reaction product. Fi-nally, the CO/H2 reactant streamis fed at variouslevels to supply CO that has been consumed bythe reaction in the liquid phase.Oxidation of Montan Waxes. Bubblecolumns are used in a cascade when a narrowresidence-time distribution is required, for ex-ample, to prevent or limit undesired consecutivereactions. Reducing back-mixing (i.e., a narrowresidence-time distribution) is also useful whenreaction-engineering considerations dictate thatthe gas must be fed to various points in the reac-tor or when a liquid reactant must be degradedto the greatest extent possible.Montan waxes from brown coal must bederesinied, oxidatively bleached, and esteried(optional) [4], [5]. Oxidation of the waxes con-sists of several consecutive reactions; the rstthree steps (oxidation of resins and dark- coloredsubstances, saponication of montan waxes, ox-idation of wax alcohols) are desirable, whereasthe fourth (oxidative degradation of wax acids)is not. The residence-time distribution in the re-actor must be controlled so that the desired re-actions go as far as possible without the unde-sirable reaction occurring to any marked extent.Oxidation is performed in four cascaded bub-ble columns connected in series (Fig. 4). In therst bubble column, the crude wax for bleach-ing is metered in along with half of the requiredamount of chromic acid. Air is supplied to en-hance mixingof the reactants. The spent chromicacid is separated from the wax downstream ofboth the rst and the second bubble columns.Another 25 % of the total acid required is addedto the second and third columns. The reactionpreferably takes place at 100 125C and 1 5bar, with a residence time of 1 3 h for the en-tire cascade. The enthalpy of reaction is removedby partial evaporation of the water contained inthe chromic acid. After exiting the fourth bub-ble column, the oxidizedproduct, spent acid, andoff-gas are separated in two separators.2.2. Gas DistributionUsually, the gas is dispersed to create small bub-bles and distribute themuniformly over the crosssection of the equipment to maximize the in-tensity of mass transfer. The formation of n...</p>