Process In­ten­si­fi­ca­tion Op­por­tu­ni­ties in Mul­tiphase Stirred Tank Re­ac­tors

Shankar B. Kaus­ley, Man­ishku­mar D. Ya­dav, Gau­rav G. Das­tane, Chan­drakant R. Holkar, Anirud­dha B. Pan­dit*

Chemical Industry Digest - - What’s In? -

Man­ishku­mar D. Ya­dav, Gau­rav G. Das­tane, Chan­drakant R.

Holkar, Anirud­dha B. Pan­dit, In­sti­tute of Chem­i­cal Tech­nol­ogy, Mum­bai; and Shankar B. Kaus­ley, Tata Con­sul­tancy Ser­vices Ltd. In this ar­ti­cle, an over­view of process in­ten­si­fi­ca­tion in stirred tank re­ac­tors leads to many op­por­tu­ni­ties to en­hance pro­duc­tiv­i­ties. An over­view is pre­sented along with their in­dus­trial ap­pli­ca­tions. De­sign as­pects are also dis­cuss.

Ab­stract

The mul­tiphase stirred tank re­ac­tors are widely used in in­dus­try due to their high heat and mass trans­fer co­ef­fi­cients, wide range of liq­uid phase res­i­dence times and ca­pa­bil­ity of han­dling wide range of su­per­fi­cial dis­persed phase gas velocities. The stirred tank re­ac­tors have ap­pli­ca­tion in the in­dus­try in terms of gas dis­per­sion, solid sus­pen­sion and gas-liq­uid-solid con­tact­ing. In the present ar­ti­cle, an over­view of process in­ten­si­fi­ca­tion in stirred tank re­ac­tor is pre­sented along with their in­dus­trial ap­pli­ca­tions. An over­view of stirred tank re­ac­tors and their de­sign as­pects are also dis­cussed.

Key­words: Stirred tank re­ac­tor, Gas-in­duc­ing im­peller, Multi-stage stirred tank re­ac­tor, Frac­tal im­pellers, Hy­dro­dy­namic cav­i­ta­tion.

Prof. Anirud­dha B. Pan­dit (J.C. Bose Fel­low, FNA, FNAE, FNASc, FASc, MASc) is UGC Sci­en­tist ‘C’ (Pro­fes­sor’s Grade) in chem­i­cal en­gi­neer­ing at In­sti­tute of Chem­i­cal Tech­nol­ogy, Mum­bai. He is ac­tively in­volved in work­ing with com­mit­tees in the area of har­ness­ing so­lar en­ergy & with tribal pop­u­la­tion in ex­tend­ing the chem­i­cal en­gi­neer­ing prin­ci­ples for dry­ing of farm/ for­est prod­uct & wa­ter dis­in­fec­tion for potable wa­ter. He has au­thored over 330 pub­li­ca­tions, 3 books and over 10 chap­ters and has 13 pa­tents with over 19450 ci­ta­tions with h-in­dex 76 and i10-in­dex 269 (as per Google Scholar) and is on the Ed­i­to­rial board of sev­eral In­ter­na­tional Sci­en­tific Jour­nals. *Cor­re­spond­ing au­thor: Email: ab.pan­dit@ict­mum­bai.edu.in Shankar B. Kaus­ley is Sci­en­tist at Tata Con­sul­tancy Ser­vices Ltd. He has com­pleted Ph.D. (Tech.) in chem­i­cal en­gi­neer­ing from In­sti­tute of Chem­i­cal Tech­nol­ogy, Mum­bai. He has worked in the field of devel­op­ment of low-cost so­lu­tions for pro­vid­ing safe drink­ing wa­ter to In­dian house­holds and devel­op­ment of cost-ef­fec­tive so­lu­tions for re­use of in­dus­trial waste­water.

Man­ishku­mar Ya­dav is pur­su­ing Ph.D. (Tech.) in Chem­i­cal En­gi­neer­ing at In­sti­tute of Chem­i­cal Tech­nol­ogy, Mum­bai. His area of re­search in­ter­est is nan­otech­nol­ogy, process en­gi­neer­ing and scale-up.

Gau­rav G. Das­tane is do­ing Ph.D. (Tech.) in chem­i­cal en­gi­neer­ing at In­sti­tute of Chem­i­cal Tech­nol­ogy, Mum­bai. He is work­ing on de­sign­ing of cav­i­tat­ing de­vices. His work ex­pe­ri­ence was in de­sign val­i­da­tion of solid ox­ide fuel cells us­ing CFD.

Chan­drakant R. Holkar is do­ing Ph.D. (Tech.) in chem­i­cal en­gi­neer­ing at In­sti­tute of Chem­i­cal Tech­nol­ogy, Mum­bai. He is work­ing in the field of waste­water treat­ment and waste man­age­ment.

1. In­tro­duc­tion

The mixing of mul­tiphase streams is one of the most com­mon and im­por­tant unit op­er­a­tions in chem­i­cal in­dus­tries. Uni­form mixing is im­por­tant as it af­fects the rate of chem­i­cal re­ac­tions and qual­ity (se­lec­tiv­ity) of the prod­uct. There are two types of mix­ers used in the process, one re­quires it’s own power sup­ply (us­ing ro­tat­ing mixing im­pellers) in ad­di­tion to the power sup­plied by the flow­ing stream. The oth­ers re­quire only the process stream as the en­ergy source for mixing which are called mo­tion­less or ‘static’ mix­ers. Among the dif­fer­ent ways to per­form mixing, me­chan­i­cally ag­i­tated stirred tank re­ac­tors are gen­er­ally pre­ferred con­sid­er­ing the ad­van­tages which in­clude high heat and mass trans­fer co­ef­fi­cients, good mixing ca­pa­bil­ity, a wide range of liq­uid phase res­i­dence times, ca­pa­bil­ity of han­dling wide range of su­per­fi­cial dis­persed phase gas velocities. These ben­e­fits have led to the ex­is­tence of stirred tank re­ac­tors in in­dus­try, from small, medium to large.

Me­chan­i­cally ag­i­tated stirred tank re­ac­tors have the po­ten­tial to pro­vide an eco­nomic op­tion for mixing of liq­uid-liq­uid, gas-liq­uid, solid-liq­uid, and gas-liq­uid-solid streams, pro­vided these are de­signed ac­cord­ing to the tar­get ap­pli­ca­tion. The re­ac­tor should meet dif­fer­ent per­for­mance re­lated ob­jec­tives such as ho­mog­e­niza­tion, solid sus­pen­sion, solid sus­pen­sion in low vis­cos­ity liq­uids, gas dis­per­sion, self-in­duc­tion of gas and con­tact­ing gas/liq­uid/solid.

There is no ‘Univer­sal’ re­ac­tor which can sat­isfy all the re­quire­ments. Ad­di­tion­ally, mixing and blend­ing op­er­a­tions, are in­her­ently known to be very in­ef­fi­cient (η ~ 0.9 to 4%). Hence, there are enor­mous op­por­tu­ni­ties to en­hance the per­for­mance of ex­ist­ing stirred tank re­ac­tors.

2. De­sign as­pects of mul­tiphase stirred tank re­ac­tor

In or­der to de­sign a mul­tiphase stirred tank re­ac­tor, dif­fer­ent pa­ram­e­ters of com­po­nents af­fect­ing its per­for­mance needs to be con­sid­ered. These pa­ram­e­ters/com­po­nents in­clude i) re­ac­tor ge­om­e­try (di­am­e­ter and height), ii) im­peller type iii) im­peller di­am­e­ter iv) im­peller po­si­tion v) im­peller ro­ta­tional speed vi) num­ber of baf­fles and baf­fle width vii) num­ber of noz­zles and their size for liq­uid/gas in­jec­tion and other re­ac­tor in­ter­nals such as coil and sam­pling point. An­other im­por­tant pa­ram­e­ter which needs to be con­sid­ered is the en­ergy re­quired for the com­plete mixing of dif­fer­ent streams. 2.1. Stirred tank re­ac­tor ge­om­e­try

A typ­i­cal stirred tank re­ac­tor is shown in Fig. 1. Gen­er­ally, it’s a ver­ti­cal cylin­dri­cal tank or ves­sel with dish end at the bot­tom. Some of the old de­signs have con­i­cal shape bot­tom, but these de­signs have in­ad­e­quate mixing in the con­i­cal part. When solid is present, in­ad­e­quate mixing in the con­i­cal part can even cause chok­ing of the un­der­flow out­let.

The im­peller di­am­e­ter to the tank di­am­e­ter (D/T) is one of the im­por­tant pa­ram­e­ters and is typ­i­cally in the range of 0.2 to 0.5. The re­ac­tor with lower D/T has lower cost and re­quires low power con­sump­tion. Low D/T im­peller gives high shear and lower pump­ing rates. High D/T im­peller sys­tem is costlier and has high op­er­at­ing cost. It pro­duces low shear and high

pump­ing. The ra­tio of fluid height to the di­am­e­ter of stirred tank (H/T) is an­other im­por­tant pa­ram­e­ter and is typ­i­cally in the range of 0.5 -1.0. For stirred tank re­ac­tors with H/T ra­tio greater than 1, mul­ti­ple im­pellers are re­quired. The ra­tio of the dis­tance of the im­peller from the bot­tom to the tank di­am­e­ter (C/T) is an­other pa­ram­e­ter which is gen­er­ally in the range of 0.1 to 0.3. Low C/T is re­quired for mixing op­er­a­tion in­volv­ing solid sus­pen­sion. Baf­fles are flat plates at­tached to the in­te­rior of the tank and pro­trude in­ward to in­ter­rupt and pre­vent swirling of a fluid. The use of im­peller cre­ates sec­tions of the so­lu­tion that don’t move in the tank. In a cylin­dri­cal tank with­out baf­fles, im­peller cre­ates a vor­tex ef­fect that causes the en­tire mix­ture to move only tan­gen­tially with no real ax­ial mixing. The baf­fles pre­vent the vor­tex for­ma­tion and cause the con­tent in the tank to move from top to bot­tom. Gen­er­ally, 2-4 baf­fles are used in the tank and placed at equidis­tance in­side the tank. The baf­fle width to tank di­am­e­ter ra­tion (B/T) is in the range of 1/10 to 1/12.

2.2. Im­peller de­sign

Im­peller is one of the most im­por­tant com­po­nents of stirred tank re­ac­tor. Im­pellers are di­vided into three main cat­e­gories: ra­dial flow, ax­ial flow and mixed flow, de­pend­ing upon the flow pat­tern de­vel­oped by them in the liq­uid. The ax­ial flow im­pellers in­clude pro­pel­lers and the ax­ial flow hy­dro­foils of the Light­nin A 315. The ra­dial flow im­pellers in­clude Rush­ton tur­bine, pad­dle im­peller and curved blade tur­bine. The mixed flow im­pellers in­clude pitched blade tur­bines. The pitched blade tur­bines are avail­able as up­flow and down­flow vari­ants, both these im­pellers have greater ax­ial com­po­nent ve­loc­ity com­pared to ra­dial com­po­nent of ve­loc­ity (par­tic­u­larly when the ag­i­ta­tor di­am­e­ter to tank di­am­e­ter ra­tio is less than 0.4).

The im­pellers can be fur­ther cat­e­go­rized based on whether they cre­ate shear field or bulk move­ment. The ax­ial flow pro­pel­ler, the hy­dro­foils and the mixed flow im­pellers (when D/T < 0.4) de­velop bulk ax­ial pat­terns. The down­flow type mixed flow im­pellers de­velop a mean flow di­rected to­wards the base of the ves­sel and is there­fore use­ful for solid sus­pen­sion in solid-liq­uid sys­tems. These im­pellers are less ef­fi­cient in three­p­hase (gas-liq­uid-solid) sys­tems both for gas dis­per­sion and solid sus­pen­sion. High shear im­pellers such as the ra­dial flow Rush­ton tur­bine gen­er­ate smaller bub­bles with lower rise ve­loc­ity. How­ever, these im­pellers con­sume higher power. As a com­pro­mise for these two con­flict­ing re­quire­ments of low power con­sump­tion and gas dis­per­sion, the pitched blade tur­bine in the up­flow mode has been sug­gested. The lat- ter has been shown to af­ford sta­ble and ef­fi­cient op­er­a­tion par­tic­u­larly in three-phase re­ac­tors[ 2]. For exam

1, ple, the up­flow pitched blade tur­bine (PTU) yields 36% higher gas holdup than its down­flow vari­ant.

2.3. En­ergy re­quire­ment

The en­ergy re­quired for the mixing, is an­other im­por­tant fac­tor which needs to be con­sid­ered dur­ing the de­sign of mul­tiphase stirred tank re­ac­tor, as the op­er­at­ing cost of the re­ac­tor de­pends on the en­ergy con­sump­tion. The en­ergy re­quired is cal­cu­lated from the power re­quired to ro­tate the im­peller, which is cal­cu­lated ac­cord­ing to equa­tion (1).

P=N ρ N3 D5 (1)

P

Where P is the power re­quired (W), N is the power

P num­ber, ρ is the den­sity of fluid (kg/m3), N is the speed of ro­ta­tion (rev/sec) and D is the di­am­e­ter of the im­peller (m). The power num­ber (N ) is the func­tion of im

P peller type and ge­om­e­try. It is a weak func­tion of re­ac­tor ge­om­e­try and strong func­tion Reynold num­ber, Re for Re<104. The power num­ber (N ) for the given

P im­peller ge­om­e­try can be found from the cor­re­la­tion curves be­tween Power num­ber and Reynolds num­ber.

The en­ergy re­quired for dif­fer­ent op­er­a­tions/steps per­formed by dif­fer­ent im­peller is sum­ma­rized in Ta­ble 1.

The power re­quired to run the im­peller de­pends upon the type of im­peller and its geo­met­ric con­fig­u­ra­tion of the tank-im­peller com­bi­na­tion.

P=φ(μ , ρL, D, T, g, N, im­peller geo­met­ric param

L eters such as blade width, blade an­gle, thick­ness and other geo­met­ric de­tails re­lat­ing to im­peller and ves­sel di­men­sions). Gen­er­ally, di­men­sion­less num­bers are used to show the re­la­tion­ship.

The dif­fer­ent di­men­sion­less num­bers used in above ex­pres­sion and that in gen­eral used in mul­tiphase re­ac­tor de­sign are given in Ta­ble 2. These di­men­sion­less num­bers help to un­der­stand the power con­sump­tion more clearly. For ex­am­ple, Froude num­ber (NFr= N2D/g), is unim­por­tant, for power num­ber (N = P/ ρ

P

N³ D⁵) es­ti­ma­tion, when re­ac­tor is de­signed in such a way that there is no vor­tex for­ma­tion (i.e. by pro­vid­ing four baf­fles of width equal to 10% of stirred tank re­ac­tor di­am­e­ter).

The power num­ber (N ) for a given im­peller geom

P etry can be found from the cor­re­la­tion curves be­tween Power num­ber and Reynolds num­ber. The dif­fer­ent pa­ram­e­ters af­fect­ing the power num­ber in­clude num­ber of blades, shape, size and align­ment (flat/pitched) and flow (up­flow and down­flow). For e.g. the power num­ber for pitched blade tur­bine (six blade) down­flow im­peller is 1.52. For im­peller con­fig­u­ra­tion with N > 10,000, the im­peller power num­ber (N ) re­mains

Re P con­stant[ 3].

Fur­ther, the torque re­quire­ment of the gear drive re­quired for stir­ring can be cal­cu­lated from the power con­sump­tion us­ing fol­low­ing equa­tion (3). The torque es­ti­ma­tion is use­ful for se­lec­tion of gear box, siz­ing of shaft and is es­pe­cially a good cri­te­ria for flow ve­loc­ity sen­si­tive op­er­a­tions.

Torque (τ) = Power/2π N=N ρ N2 D⁵/ 2 π (3)

P

Since the re­ac­tor is sup­posed to per­form mul­ti­ple op­er­a­tions si­mul­ta­ne­ously and the to­tal en­ergy sup­plied is dis­trib­uted for these var­i­ous op­er­a­tions per- formed by the im­peller de­pend­ing upon its ge­om­e­try (type of im­peller) and its lo­ca­tion, plenty of op­por­tu­ni­ties ex­ist for op­ti­miza­tion thru process in­ten­si­fi­ca­tion. 2.4. Scale-up cri­te­ria

Re­ac­tor de­sign has been one of the most im­por­tant tasks for any chem­i­cal en­gi­neer in process in­dus­try. Due to the ad­vance­ment of com­pu­ta­tional power avail­able as on to­day, sci­ence be­hind the re­ac­tor de­sign has been un­der­stood and math­e­mat­i­cally mod­elled in great de­tail. Still the plant scale de­sign is pre­ceded by pi­lot scale and lab scale data is used for the de­sign val­i­da­tion mainly due to fear of fail­ure at com­mer­cial (large) scale pro­duc­tion. Scale up has been cov­ered in great de­tail in many books and has re­ceived ex­cel­lent re­views[ 6]. The over­all con­clu­sion based on

4– the sci­ence de­vel­oped be­hind the scale-up is based on sim­i­lar­ity. Gen­er­ally, the geo­met­ric sim­i­lar­ity and kine­matic sim­i­lar­ity are be­ing con­sid­ered as the ac­cept­able scale-up cri­te­ria.

Geo­met­ric sim­i­lar­ity: In case of stirred tank, var­i­ous geo­met­ric di­men­sions can be con­sid­ered dur­ing the de­sign of bench scale or pi­lot scale plant. Re­ac­tor tank height to di­am­e­ter ra­tio, im­peller di­am­e­ter to re­ac­tor di­am­e­ter ra­tio are widely used.

Kine­matic sim­i­lar­ity: As men­tioned in the case of geo­met­ric sim­i­lar­ity, kine­matic fac­tor such as im­peller tip ve­loc­ity is kept con­stant for bench scale or pi­lot scale plant. It has been re­ported that kine­matic sim­i­lar­ity is a nec­es­sary con­di­tion but not suf­fi­cient.Since, the level of tur­bu­lence main­tained at small scale is dif-

fi­cult to achieve at a large scale, hence, in­stead of tip ve­loc­ity, power per unit vol­ume is main­tained con­stant.

Dif­fer­ent as­pects in­volved in scale up of ho­mo­ge­neous and het­ero­ge­neous re­ac­tion sys­tems

Gas-Liq­uid Sys­tem: Gas-Liq­uid (G/L) sys­tems ex­ist in a va­ri­ety of pro­cesses used in chem­i­cal in­dus­try. Hy­dro­gena­tion, chlo­ri­na­tion, ozona­tion, ox­i­da­tion, etc are few ex­am­ples of G/L sys­tems. In ad­di­tion, many waste wa­ter treat­ment plants uti­lize large sized gas liq­uid con­tac­tors. Though the de­sign of MAC/stirred tank re­main same as dis­cussed in per­vi­ous sec­tion, the gas - liq­uid mass trans­fer co­ef­fi­cient in the sys­tem is of para­mount im­por­tance. Gas-liq­uid mass trans­fer co­ef­fi­cient (KLa) is gen­er­ally rated as a func­tion of power per unit vol­ume and vol­u­met­ric flow rate of gas. Where k a is the vol­u­met­ric gas-liq­uid mass trans

L fer co­ef­fi­cient (s-1), P is the power re­quire­ment (W), V is the liq­uid vol­ume (m3), VG is the su­per­fi­cial gas ve­loc­ity, con­stants A, α and β are the re­gres­sion co­ef­fi­cients which can be ob­tained from the ex­per­i­men­tal data.Var­i­ous re­searchers have pro­posed afore­men­tioned cor­re­la­tion with dif­fer­ent con­stant and ex­po­nent val­ues. More de­tails can be found in work re­ported by Yawalkar et al.[ 7].

Gas-Liq­uid-Solid sys­tem: Three phase sparged re­ac­tors are widely used in process in­dus­try where solid par­ti­cles are usu­ally used as cat­a­lyst or re­act­ing species. Three phase sys­tems with re­quire­ment of ex­cel­lent con­trol over the heat trans­fer are gen­er­ally car­ried out in stirred tank re­ac­tors. In ad­di­tion, stirred tank re­ac­tors of­fer flex­i­bil­ity over the liq­uid phase res­i­dence time which is of ut­most im­por­tance in process in­dus­try.

De­sign of three phase sparged re­ac­tors is based on var­i­ous pa­ram­e­ters; some im­por­tant points are pre­sented as be­low:

1. Gas phase holdup in the re­ac­tor is mainly gov­erned by bub­ble di­am­e­ter and ter­mi­nal velocities of the bub­bles.

2. Par­ti­cle di­am­e­ter has strong in­flu­ence over the frac­tional gas hold-up.

3. For com­plete off bot­tom sus­pen­sion of solids, the min­i­mum su­per­fi­cial gas ve­loc­ity must be cal­cu­lated con­sid­er­ing the par­ti­cle den­sity, con­cen­tra­tion, di­am­e­ter dis­tri­bu­tion and re­ac­tor di­am­e­ter.

4. Set­tling ve­loc­ity of solid par­ti­cles is much dif­fer­ent from ter­mi­nal set­tling ve­loc­ity which must be con­sid­ered in de­sign cal­cu­la­tions. Cor­re­la­tion for solid-liq­uid mass trans­fer co­ef­fi­cient have been pro­posed by var­i­ous au­thors

Sh = 2 + A(Re) (Sc) (5)

α β

Where Sh is the Sher­wood num­ber, Re is the Reynolds num­ber and Sc is the Sch­midt num­ber and con­stants A, α and β are the re­gres­sion co­ef­fi­cients which can be ob­tained from the ex­per­i­men­tal data.

De­tails re­gard­ing de­sign of three phase sparged re­ac­tor de­sign can be found in re­view by Pan­dit and Joshi[ 8].

3. Ap­pli­ca­tions of mul­tiphase stirred tank re­ac­tor

The mul­tiphase stirred tank re­ac­tor has ap­pli­ca­tions in many chem­i­cal and al­lied in­dus­tries, min­ing in­dus­try, waste­water treat­ment plants etc.

For ex­am­ple, hy­dro­gena­tion of or­gan­ics such as ani­line to cy­clo­hexy­lamine, ben­zene to cy­clo­hex­ane etc. are few ex­am­ples of in­dus­tri­ally im­por­tant re­ac­tions car­ried at a scale of more than 10,000 met­ric tonnes per an­num. In hy­dro­gena­tion re­ac­tions, about 20 – 50% ex­cess hy­dro­gen is sup­plied due to its low sol­u­bil­ity. Hence, in these pro­cesses, the com­plete uti­liza­tion of

hy­dro­gen is one of the ma­jor chal­lenges in de­sign of stirred tank re­ac­tor. In con­ven­tional re­ac­tors it is re­cy­cled us­ing ex­ter­nal com­pres­sors. The stirred tank re­ac­tors with gas in­duc­ing im­pellers are found to re­cy­cle the hy­dro­gen within the re­ac­tor and thus elim­i­nate the re­cy­cle step (dis­cussed more in sec­tion 4.1.2). This min­i­mizes the cap­i­tal and op­er­at­ing cost of the process.

Stirred tank re­ac­tors are also widely used in bio­chem­i­cal pro­cesses such as biomass pro­duc­tion (e.g. sin­gle cell pro­tein, Baker´s yeast, an­i­mal cells, mi­croal­gae), for me­tab­o­lite for­ma­tion (e.g. or­ganic acids, ethanol, an­tibi­otic, aro­matic com­pounds, pig­ments), to trans­form sub­strates (e.g. steroids) or even for pro­duc­tion of an ac­tive cell mol­e­cule (e.g. en­zymes). Some of the im­por­tant ap­pli­ca­tions of stirred tank re­ac­tors in bio­chem­i­cal pro­cesses are pre­sented in Ta­ble 3.

The dif­fer­ent fac­tors to be con­sid­ered dur­ing con­struc­tion of stirred tank re­ac­tor for bi­o­log­i­cal ap­pli­ca­tions in­clude, steril­ity, aer­a­tion, mixing, tem­per­a­ture and ph con­trol. The main re­ac­tor de­sign chal­lenge here is to re­duce the power cost re­quired for long fer­men­ta­tion pe­ri­ods and to per­form uni­form mixing in the re­ac­tor with­out any phys­i­cal dam­age to the cells. Gen­er­ally, multi-stage stirred tank re­ac­tors are found to be more suit­able un­der these ap­pli­ca­tions (dis­cussed in de­tail in sec­tion 4.1.1).

4. Process in­ten­si­fi­ca­tion op­por­tu­ni­ties in mul­tiphase stirred tank re­ac­tor

Mul­tiphase stirred tank re­ac­tors need to be de­signed to per­form the spe­cific op­er­a­tions in the most en­ergy ef­fi­cient ways. The dif­fer­ent steps in­volved are i) iden­ti­fi­ca­tion of the rate con­trol­ling step, ii) Es­ti­ma­tion of the frac­tion im­peller power dis­si­pa­tion used for the above iii) Se­lec­tion/de­sign of the im­peller and op­er­at­ing con­di­tions to max­i­mize the per­for­mance.

4.1 Stirred tank re­ac­tor with mod­i­fi­ca­tions in im­peller ge­ome­tries and lo­ca­tion

4.1.1 Multi-stage stirred tank re­ac­tors

The sin­gle stage stirred tank re­ac­tor has lim­i­ta­tions in terms of liq­uid and gas phase back­mix­ing. Fur­ther, with an in­crease in tank di­am­e­ter the power con­sump­tion is in­ef­fec­tive in the wall re­gion and gas dis­per­sion be­comes lower. These lim­i­ta­tions are over­come by us­ing mul­ti­ple im­pellers and height to di­am­e­ter ra­tio greater than one. The mul­ti­stage con­tac­tors need thin­ner wall as com­pared to sin­gle stage con­tac­tor for the same con­tac­tor vol­ume and thus can be used ad­van­ta­geously for high pres­sure op­er­a­tions. The ra­tio of the height of each com­part­ment (Hs) to the col­umn diam- eter varies in the range of 0.5 to 1.5. The com­part­ments are usu­ally sep­a­rated by ra­dial baf­fles to re­duce ex­tent of gas and liq­uid phase back­mix­ing. Multi-stage stirred tank re­ac­tors are mainly used in dif­fer­ent in­dus­trial ap­pli­ca­tions such as fer­men­ta­tion, crys­tal­liza­tion and poly­mer­iza­tion.Mul­ti­ple im­peller sys­tems are pre­ferred over sin­gle im­peller sys­tem, in biore­ac­tors where shear sen­si­tiv­ity to mi­cro-or­gan­isms is an im­por­tant cri­te­rion for de­sign. Mul­ti­ple im­pellers of­fer lower shear as com­pared to sin­gle im­peller sys­tems due to an abil­ity to oper­ate at lower op­er­at­ing im­peller speeds and al­low the free­dom of con­trol­ling the dis­persed phase hold-up and the res­i­dence time over a wide range[ 18, 19].

4.1.2 Stirred tank re­ac­tor with gas-in­duc­ing im­peller

The stirred tank re­ac­tor, when used for gas-liq­uid op­er­a­tion in a semi-batch op­er­a­tion (where gas is con­tin­u­ous phase and liq­uid is sta­tion­ary phase) has lim­i­ta­tions in terms of lim­ited sol­u­bil­ity of gas in liq­uid phase and hence the per pass con­ver­sion of gas is very low. Hence, it is nec­es­sary to re­cy­cle the un­re­acted gas back to the re­ac­tor, specif­i­cally for the case where gas may be highly toxic, ex­pen­sive or may pose safety prob­lem. Con­ven­tion­ally, the gas is re­cir­cu­lated us­ing link­ing mul­ti­ple tanks in series or by pro­vid­ing an

ex­ter­nal loop with com­pres­sors. How­ever, both these meth­ods re­quire ex­ter­nal ac­ces­sories and in­crease the fixed as well as op­er­at­ing ex­pen­di­ture.

Stirred tank re­ac­tor with gas-in­duc­ing im­peller over­comes this lim­i­ta­tion by re-cir­cu­lat­ing the gas in the re­ac­tor from the headspace of the re­ac­tor to the bulk liq­uid. A typ­i­cal stirred tank re­ac­tor with gas-in­duc­ing im­peller is shown in Fig. 3. The gas-in­duc­ing im­peller con­sists of hol­low shaft at­tached with hol­low tube im­peller. This re­ac­tor is a closed re­ac­tor with hol­low im­peller with hole at the top of the shaft and the tip of the im­peller. When ro­ta­tional speed of im­peller in­creases, and reaches to its crit­i­cal speed, the ki­netic head at the tip of the im­peller blade over­comes the static head of the liq­uid above it. At this ro­ta­tion speed the gas from the headspace gets in­jected into the hol­low shaft and then trans­ferred to the liq­uid through hol­low im­peller. The gas-in­duc­ing im­peller is op­er­ated above the gas-in­duc­tion speed, so that the gas is dis­trib­uted and dis­persed in the liq­uid. Gen­er­ally, a down­ward flow pitched blade im­peller is pro­vided above the gas-in­duc­ing im­peller to uni­formly dis­perse the gas in­duced in the bulk liq­uid. The stirred tank re­ac­tor with gas-in­duc­ing im­peller is found to be use­ful in dif­fer­ent pro­cesses such as alky­la­tion, ethoxy­la­tion, froth-flota­tion, hy­dro­gena­tion, chlo­ri­na­tion, am­monoly­sis and ox­i­da­tion[ 23].

21–

4.1.3 Some re­cent ad­vances: Stirred tank re­ac­tor with Frac­tal Im­pellers

The pri­mary aim of any im­peller is to re­duce nonuni­for­mi­ties with min­i­mum power con­sump­tion. Frac­tal im­pellers (FI) is one of the im­peller fit­tings in the afore­men­tioned cri­te­ria. Here, the im­peller blades are placed in such a man­ner that in­stead of sweep­ing across the fluid, the blades just cut the fluid and hence, re­duc­ing the fric­tion through­out the stirred tank. FI de­sign is based on self-sim­i­lar struc­ture which as­sist in cre­at­ing chaotic ad­vec­tion. FI help in achiev­ing uni­form dis­tri­bu­tion of en­ergy through­put the re­ac­tor by oc­cu­py­ing less than 0.4% of the to­tal vol­ume of the re­ac­tor.

In com­par­i­son to other im­pellers (ra­dial or ax­ial), the flow pat­terns de­vel­oped by FI are ma­jorly tan­gen­tial flow in ad­di­tion to small cir­cu­lat­ing ed­dies. Power num­ber (N ) of FI is found to be 0.38 which is

P much lower in com­par­i­son to other con­ven­tional ra­dial or ax­ial flow im­pellers i.e rush­ton tur­bine (N =

p 6) and pitched bladed tur­bine (N = 1.84) re­spec­tive

p ly. Kulka­rni et al have stud­ied FI ex­ten­sively and found that for both solid sus­pen­sion and gas dis­per­sion FI out­per­formed the con­ven­tional im­pellers in terms of per­for­mance at lower power con­sump­tion[ 24].

Stirred tank re­ac­tor with hy­dro­dy­namic cav­i­ta­tion sys­tems

Hy­dro­dy­namic cav­i­ta­tion is emerg­ing as an ef­fec­tive process in­ten­si­fi­ca­tion tool in re­cent years. In this process, liq­uid is passed through a con­stric­tion such that the lo­cal pres­sure of the liq­uid drops and is lower than or equal to the vapour pres­sure. In such con­di­tion the liq­uid par­tially evap­o­rates, form­ing cav­i­ties which grow as long as the pres­sure re­mains low, and even­tu­ally col­lapse down­stream when the pres­sure is re­cov­ered. When the cav­i­ties col­lapse, a large amount of en­ergy is re­leased in the form of high tem­per­a­ture and pres­sure. If the cav­ity col­lapses asym­met­ri­cally, it also re­sults in for­ma­tion of a high ve­loc­ity liq­uid mi­cro jet, thus gen­er­at­ing a large amount of shear[ 25].

Hy­dro­dy­namic cav­i­ta­tion can re­sult into phys­i­cal as well as chem­i­cal trans­for­ma­tions. The high lo­cal tem­per­a­ture gen­er­ated due to col­lapse of a cav­ity re­sults into dis­so­ci­a­tion of wa­ter mol­e­cules and for­ma­tion of highly reac- tive hy­droxyl rad­i­cals. These rad­i­cals have very high ox­i­da­tion po­ten­tial and hence cav­i­ta­tion finds an ap­pli­ca­tion as an ad­vanced ox­i­da­tion process. Al­ter­na­tively, the high ve­loc­ity mi­cro­jets also gen­er­ate a lot of mi­cro-tur­bu­lence and high shear, which re­sults in phys­i­cal break­age of any sus­pended ma­te­rial. The tur­bu­lence also en­sures uni­form mixing[ 26]. These char­ac­ter­is­tics of a hy­dro­dy­namic cav­i­ta­tion re­ac­tor make it a per­fect can­di­date for use along with a stirred tank re­ac­tor. A schematic rep­re­sen­ta­tion of the stirred tank re­ac­tor as­sem­bly with ex­ter­nally con­nected hy­dro­dy­namic cav­i­ta­tion re­ac­tor is as shown in Fig 5.

The mix­ture from the stirred tank re­ac­tor is pumped through the cav­i­tat­ing de­vice and is sent back to the stirred tank, form­ing a closed loop. The cav­i­tat­ing de­vices typ­i­cally used in such ar­range­ment are ven­turi or ori­fice plates. Typ­i­cal ap­pli­ca­tions of such hy­brid sys­tems are in prepa­ra­tion of nano-emul­sions and in gas-liq­uid op­er­a­tions.

Nano-Emul­si­fi­ca­tion: A typ­i­cal stirred tank re­ac­tor when used for emul­si­fi­ca­tion process, re­sults in a wide droplet size dis­tri­bu­tion. Also the droplet size is coarse and is typ­i­cally in mi­cron range. If the stirred tank re­ac­tor is used along with hy­dro­dy­namic cav­i­ta­tion re­ac­tor, the re­sul­tant emul­sions have a much sharper size dis­tri­bu­tion and the droplet size of nano-scale can be ob­tained eas­ily[ 27]. This is pos­si­ble due to break­age of the droplets in the cav­i­ta­tion re­ac­tor to very fine size. The stirred tank is used as pri­mary re­ac­tor where

coarse droplets are formed and the cav­i­ta­tion re­ac­tor can then be used to ob­tain nano-emul­sions. The ad­di­tion of cav­i­ta­tion re­ac­tor re­duces the process time as droplets are re­duced to nano-size within 20-30 passes through the cav­i­tat­ing de­vice. Since the tur­bu­lence gen­er­ated in the cav­i­ta­tion re­ac­tor en­sures uni­form mixing, the power re­quire­ment for the stirred tank re­ac­tor is also re­duced.

Sim­i­lar ap­proach can also be used in crys­talli­sa­tion process to ob­tain nano-crys­tals, where cav­i­ta­tion can be used to re­duce the crys­tal size by in­creas­ing the break­age rate.

Gas-Liq­uid Re­ac­tors: In stirred tank re­ac­tors with gas-liq­uid re­ac­tion sys­tems, hy­dro­dy­namic cav­i­ta­tion re­ac­tor can be used as a process in­ten­si­fi­ca­tion tool to in­crease the ef­fi­ciency. Cav­i­tat­ing de­vice can break down the gas bub­bles as the gas liq­uid mix­ture passes through it. This in­creases the avail­able sur­face area for the re­ac­tion and ac­cel­er­ates the re­ac­tion. The mi­cro­tur­bu­lence gen­er­ated due to cav­i­ta­tion helps in over­com­ing mass trans­fer re­sis­tances. The high tem­per­a­ture and pres­sure con­di­tions can also help in pro­vid­ing the ac­ti­va­tion en­ergy for the re­ac­tion and re­duce the over­all en­ergy re­quire­ment of the stirred tank re­ac­tor. In case of ox­i­da­tion re­ac­tions, typ­i­cally in waste wa­ter treat­ment, cav­i­ta­tion along with ozone can be used as an ad­vanced ox­i­da­tion process for achiev­ing bet­ter ef­fi­ciency.

4.1.4 Stirred tank re­ac­tor with draft tube cum heat ex­changer

In case of gas-solid-liq­uid or solid-liq­uid re­ac­tions, the den­sity of solid phase may be lower than the liq­uid phase re­sult­ing in float­ing of the solid phase. In case of light weight solid (re­ac­tant or cat­a­lyst), the non­uni­form mixing of solids re­sults in lo­cal­ized exother­mic hot spots (pop­corn -type burst­ing fol­lowed by gas emis­sion).

In or­der to avoid such con­di­tions, the draft tube stirred tank re­ac­tors (Fig. 6) are much ef­fi­cient. Here ax­ial flow im­pellers are gen­er­ally used and solids are added near the eye of the im­peller. The im­peller sucks the fluid from the an­nu­lus into the draft tube and then back into the an­nu­lus. The low-den­sity solid par­ti­cles are trans­ported down­ward into the draft tube and then into the an­nu­lus. This re­sults in cir­cu­la­tion and uni­form mixing of lighter weight solid par­ti­cles. The ar­range­ment is also found to avoid im­peller flood­ing.

Stirred tank re­ac­tor with draft tube cum heat ex­changer are found to elim­i­nate the re­frig­er­a­tion cost, re­duces the over­all op­er­at­ing cost at least by 50% and found to give sta­ble and safe op­er­a­tion of the re­ac­tor. These are found to be use­ful in crys­tal­liza­tion (draft tube crys­tal­lizer), fer­men­ta­tion (draft tube fer­menter/ pro­pel­ler loop re­ac­tor), es­ter­i­fi­ca­tion and trans­es­ter­i­fi­ca­tion (ion ex­change resin cat­alyzed re­ac­tions) [28].

Con­clu­sions 5.

Mul­tiphase stirred tank re­ac­tors are widely used in in­dus­try due to their high heat and mass trans­fer co­ef­fi­cients, wide range of liq­uid phase res­i­dence times and ca­pa­bil­ity of han­dling wide range of su­per­fi­cial dis­persed phase gas velocities.

The re­quire­ment of mul­ti­ple ob­jec­tives from the ex­ist­ing stirred tank re­ac­tors in the in­dus­try in terms of gas dis­per­sion, solid sus­pen­sion and gas-liq­uid-solid con­tact­ing etc, from the ex­ist­ing stirred tank re­ac­tor gives rise to de­sign op­ti­miza­tion and process in­ten­si­fi­ca­tion op­por­tu­ni­ties.

The dif­fer­ent stages in­volved in the de­sign of stirred tank re­ac­tor for spe­cific op­er­a­tion in­cludes, de­ter­mi­na­tion of rate lim­it­ing step, es­ti­ma­tion of power dis­si­pa­tion re­quired for this step and then se­lec­tion/de­sign of im­peller and op­er­at­ing con­di­tions to max­i­mize the re­ac­tor per­for­mance.

The process in­ten­si­fi­ca­tion can be car­ried out in the ex­ist­ing stirred tank re­ac­tor by de­sign­ing novel tank and im­peller ge­ome­tries or by pro­vid­ing the ex­ter­nal ac­ces­sories to en­hance the per­for­mance in terms of in­crease in heat and mass trans­fer rates.

Process in­ten­si­fi­ca­tion based on de­sign of re­ac­torimpeller ge­om­e­try in­clude multi-stage stirred tank re­ac­tor (multi-im­peller re­ac­tor), stirred tank with gas-in­duc­ing im­peller and re­cent ad­vances like stirred tank with frac­tal im­pellers. Stirred tank re­ac­tors with mul­ti­im­peller sys­tem are found to be ef­fec­tive in fer­men­ta­tion, crys­tal­liza­tion and poly­mer­iza­tion re­ac­tions. The stirred tank re­ac­tors with gas-in­duc­ing im­peller are found to be ef­fec­tive in hy­dro­gena­tion, alky­la­tion and ox­i­da­tion re­ac­tions. Use of frac­tal im­pellers have the po­ten­tial to re­duce the power con­sump­tion per unit vol­ume of the liq­uid.

Process in­ten­si­fi­ca­tion us­ing ex­ter­nal ac­ces­sories in­cludes, com­bi­na­tion of stirred tank re­ac­tor with cav­i­tat­ing de­vice (flow con­stric­tion de­vice in the form of ven­turi, ori­fice or valve) and stirred tank with draft tube cum heat ex­changer. Stirred tank re­ac­tor with hy­dro­dy­namic cav­i­ta­tion are found to be ef­fec­tive for prepa­ra­tion of nano-emul­sion, crys­tal­liza­tion and waste­water treat­ment us­ing ad­vanced ox­i­da­tion pro­cesses. The stirred tank re­ac­tor with draft tube is found to be ef­fec­tive in highly exother­mic re­ac­tions.

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Fig. 1 Schematic of a typ­i­cal stirred tank re­ac­tor ge­om­e­try

Ta­ble 2 Dif­fer­ent di­men­sion­less num­bers and their phys­i­cal sig­nif­i­cance

Three phase sparged re­ac­tors are widely used in process in­dus­try where solid par­ti­cles are usu­ally used as cat­a­lyst or re­act­ing species. Three phase sys­tems with re­quire­ment of ex­cel­lent con­trol over the heat trans­fer are gen­er­ally car­ried out in stirred tank re­ac­tors. In ad­di­tion, stirred tank re­ac­tors of­fer flex­i­bil­ity over the liq­uid phase res­i­dence time which is of ut­most im­por­tance in process in­dus­try. Ta­ble 3. Ap­pli­ca­tions of stirred tank re­ac­tor in bio­chem­i­cal pro­cesses

Fig. 2. Mul­ti­stage stirred tank re­ac­tor[ 19, 20] com­part­ment of stirred tank re­ac­tor

Fig. 3 Schematic of stirred tank re­ac­tor with gas-in­duc­ing im­peller Hy­dro­dy­namic cav­i­ta­tion is emerg­ing as an ef­fec­tive process in­ten­si­fi­ca­tion tool in re­cent years. Hy­dro­dy­namic cav­i­ta­tion can re­sult into phys­i­cal as well as chem­i­cal trans­for­ma­tions.

Fig. 4 Im­age of (a) frac­tal im­peller and (b) stirred tank with frac­tal im­peller[ 24]

Fig. 6 Stirred tank re­ac­tor with draft tube cum heat ex­changer

Fig. 5 Stirred tank re­ac­tor with hy­dro­dy­namic cav­i­ta­tion sys­tem

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