The Catalytic Hydrogenation of Aromatic Nitro Ketone in a Microreactor: Reactor Performance and Kinetic Studies

Transcription

1 TERATOAL JOURAL OF EMAL REATOR EGEERG Volume Aticle A The atalytic ydogenation of Aomatic ito etone in a Micoeacto: Reacto Pefomance and inetic Studies Sunitha Tadepalli Adeniyi Lawal Stevens nstitute of Technology, sunithatadepalli@gmail.com Stevens nstitute of Technology, Adeniyi.Lawal@stevens.edu SS opyight c 009 The Bekeley Electonic Pess. All ights eseved.

2 The atalytic ydogenation of Aomatic ito etone in a Micoeacto: Reacto Pefomance and inetic Studies Sunitha Tadepalli and Adeniyi Lawal Abstact atalytic hydogenation of nito aomatics is an impotant class of eactions in the phamaceutical and fine chemical industies. These eactions ae extemely fast and highly exothemic in natue; hence, mass and heat tansfe limitations play an impotant ole when these eactions ae conducted in conventional batch eactos. The use of a mico-channel eacto fo such eactions povides impoved mass tansfe ates which may ensue that the eaction opeates close to intinsic kinetics. n the pesent study, the hydogenation of a model aomatic nito ketone was conducted in a packed-bed micoeacto. The effects of diffeent pocessing conditions wee studied using 5%Pd/Alumina catalyst, viz.: hydogen pessue, substate concentation, tempeatue, and esidence time on the convesion of substate, Space Time Yield (STY, and selectivity of poduct. ntenal and extenal mass and heat tansfe limitations in the micoeacto wee examined. The kinetic study was undetaken in a diffeential eacto mode, keeping the convesion of the eactant at less than 0%. The oveall eaction was teated as compising two sepaate eactions: fist, the eduction of the nito compound to hydoxylamine and then, the eduction of the hydoxylamine to amine. Two ate equations fo the two consecutive eactions assuming the Langmui-inshelwood mechanism povided the best fit to the expeimental data. These two ate equations pedicted the expeimental ates satisfactoily and the diffeences wee within 0% eo. Expeiments wee also caied out in an integal eacto, and the eacto pefomance This wok is funded by the DOE-TP unde contact DE-F36-0GO356. The authos gatefully acknowledge the suppot. We also acknowledge the technical contibutions of: D. Jale Muslehiddinoglu, Donald ientzle and D. San iang of Bistol-Myes Squibb, ou industial patne; Pof. Suphan ovenklioglu fo his helpful suggestions; and all the membes of JMS, especially D. Dongying Qian and D. Raghunath alde fo thei help thoughout the eseach wok.

3 data wee found to be in ageement with the pedictions of the theoetical models. EYWORDS: catalytic hydogenation, packed-bed micoeacto, eacto modeling, kinetics, nito aomatics

4 Tadepalli and Lawal: ydogenation of Aomatic ito etone in a Micoeacto. ntoduction The catalytic hydogenation of nito compounds is an industially impotant pocess fo the pepaation of amines that ae used as intemediates in the manufactue of polyuethanes, ubbe chemicals, agicultual poducts, dyestuffs, photogaphic chemicals and dugs, as well as vaious othe chemicals (ishimua, 00. The nito goups, especially those attached to the aomatic ings ae the most eactive functional goups and hence ae eadily hydogenated ove tansitional metal catalysts. These eactions ae vey fast, and highly exothemic in natue. Fo example, the heat of eaction fo the hydogenation of nitobenzene to aniline is 493 kj/mol. Theefoe cae must be taken to pevent the eaction fom becoming too violent. This can be done by contolling the scale of eaction, o by choosing an efficient eacto system capable of emoving the eaction heat and poviding apid mass and heat tansfe ates fo optimum pefomance. n the phamaceutical and fine chemical industies, most of the nito goup hydogenation eactions ae nomally caied out in lage scale batch eactos to obtain poducts in lage quantities fo economical easons. n the batch eactos, mass tansfe between diffeent phases is slow. As a esult, the eaction ate in the batch eacto is limited often by the ate of hydogen tansfe to the catalyst and the eaction takes much longe time than actually equied by the kinetics. n addition, the high heat elease esulting fom the lage eaction enthalpy cannot be contolled efficiently in the batch eacto and hence may lead to the fomation of seveal bypoduct(s o intemediate(s which can also eact with one anothe, and futhe lead to violent decomposition of nito-compounds o patially hydogenated intemediates. n ecent yeas, thee has been a gowing inteest in utilizing micoeactos fo vey fast and highly exothemic eactions such as fluoination (essel et al., 000, nitation (Dummann et al., 003 and othe eactions, because of thei numeous advantages ove conventional eactos. The main advantage of the micoeacto is its ability to educe heat and mass tansfe esistances due to its high suface to volume atio compaed to conventional chemical eactos. This high suface to volume atio of the micoeactos tanslates into high heat and mass tansfe ates which enable the complete utilization of the catalysts fo highly exothemic and fast eactions, poviding isothemal conditions and esulting in highe selectivity and highe poduction ates of the desied poduct. ydogenation eactions ae most suited fo implementation in micoeactos whee the advantage of high suface to volume atio can be exploited to educe the heat and mass tansfe esistances. A numbe of studies have been conducted to evaluate micoeacto fo the hydogenation of diffeent model substates which ae impotant fo the poduction of chemical intemediates and poducts (andoust et al., 989; ijhuis et al., 003; eutze Published by The Bekeley Electonic Pess, 008

5 ntenational Jounal of hemical Reacto Engineeing Vol. 6 [008], Aticle A et al., 00; Liu et al., 005. ydogenation of nito compounds epesents one impotant categoy of these eactions which has been studied extensively in the micoeacto by employing catalysts in diffeent foms (Machado et al., 999; Andesson et al., 986; Boekhuis et al., 00; eutze et al., 005; Födisch et al., 00; essel et al., 003; essel et al., 004. The advantages of conducting these eactions in the micoeacto wee clealy demonstated fom ou pevious wok on the hydogenation of o-nitoanisole in a packed-bed micoeacto (Tadepalli et al., 007a; 007b, whee we have demonstated the temendous incease in the mass tansfe coefficient of the micoeacto in compaison to semi-batch eacto, and the ability to conduct kinetic study of extemely fast eactions in the micoeacto. oweve, thee wee no advantages in conducting kinetic studies of hydogenation of o-nitoanisole in the micoeacto compaed to semi-batch eacto because the eaction is elatively slow and the eaction ates in the two eactos wee expectedly the same in the kinetic-contolled egime. ence, the advantages of conducting kinetic studies in the micoeacto can only be ealized if the eaction is extemely fast. n the pesent wok, the hydogenation of a model aomatic nito ketone of phamaceutical inteest was investigated in a packed-bed micoeacto. The amine fomed fom this eaction is used as an intemediate in the manufactue of a dug. n contast to hydogenation of o-nitoanisole, this eaction is extemely fast, and contolled by mass tansfe limitations in industial eactos, which makes it moe appopiate to conduct the kinetic studies fo this eaction in the micoeacto. The pefomance of the packed-bed micoeacto fo this eaction was evaluated by studying the effects of diffeent eaction vaiables, e.g., eaction tempeatue, pessue, substate concentation in the feed on the convesion, yield and selectivity of the poduct. Futhe, the kinetics of this eaction was studied in the micoeacto so that optimum eaction conditions can be detemined fo the design of the multichannel eacto.. Expeimental n this section, a list of the chemicals used fo the expeiments, and the desciption of the analytical method ae povided. The micoeacto expeimental set-up as well as expeimental pocedue was descibed in ou pevious wok (Tadepalli et al., 007a. A schematic of the micoeacto expeimental set-up is shown in fig.. The micoeacto was made of SS36L with an oute diamete of.5875 mm and inne diamete of mm. The total length of the eacto vaied fom 6 to 8 cm of which the catalyst was packed within to 6 cm of the eacto (depending upon the catalyst loading equied fo the expeiment, and the emaining length of the eacto was filled with inet glass beads of the same size ange as the catalyst paticles. The catalyst 5% (w/w Pd suppoted on Alumina

6 Tadepalli and Lawal: ydogenation of Aomatic ito etone in a Micoeacto 3 (supplied by Johnson Matthey was sieved to obtain a paticle size distibution of micons. The catalyst has a BET suface aea of appoximately 50 m /g, a total poe volume of appoximately 0.50 cc/g, and an aveage poe diamete of appoximately 5 Angstoms. The aomatic nito ketone compound is a popietay molecule of Bistol-Myes Squibb. This compound is a cystalline yellow solid. The solid substate was dissolved in a solvent -methyl pyolidone (MP which was obtained fom Phamco Poducts nc. The hydogen and nitogen wee puchased fom Paxai. itogen was used fo diluting the hydogen steam and also to flush the eacto system at the end of each expeiment. oncentation of liquid poduct steam was detemined by igh Pefomance Liquid homatogaphy (PL, and composition of the gas phase was measued by a Shimadzu G-4B gas chomatogaph with a Mole Sieve 5A column at 35 and agon as caie gas. All the liquid poduct samples wee diluted with De-ionized wate pio to being injected into the PL. A Diode Aay detecto was used fo scanning so that the samples could be scanned at multiple wavelengths at the same time. A method was established in the PL using wate, methanol, acetonitile and ammonium acetate (buffe as the mobile phase in conjunction with an RP 8 column fom Phoenix. alibation was done fo both the eactant and the poduct using standad solutions. The concentations of eactant and the poduct in the unknown poduct samples wee detemined using the calibation. 3. Results & Discussion n this section we will fist povide a bief desciption of the eaction in geneal followed by a discussion of the study on fluid flow chaacteization, and the effects of diffeent opeating conditions on eacto pefomance using 5%Pd/Alumina catalyst, the objective being the detemination of the best opeating egime fo maximum yield of the desied amine in the micoeacto (Sections The kinetics study of the eaction is next pesented in Section 3.4. Published by The Bekeley Electonic Pess, 008

7 4 ntenational Jounal of hemical Reacto Engineeing Vol. 6 [008], Aticle A Figue. Schematic of the Micoeacto Expeimental Set-up

8 Tadepalli and Lawal: ydogenation of Aomatic ito etone in a Micoeacto Reaction and catalyst activity The catalytic hydogenation of aomatic nito ketone compound involves a theestep eduction seies eaction as shown in fig.. Fist, the eduction of the nito goup to nitoso goup, then to hydoxylamine goup and finally to amine goup. This path is usually favoed at low concentations of nito compound fo which the concentations of the intemediates ae small. At high concentations of nito compounds, the concentations of the intemediates i.e. nitoso and hydoxylamine compounds incease to the extent that they eact with each othe leading to the fomation of azo and hydazo compounds. At the concentation levels that we utilized fo the expeiments, only one intemediate was obseved in the poduct steam along with the poduct and eactant at all opeating conditions. This intemediate was found to be the hydoxylamine deivative by the Mass Spectum Analysis. Figue. Reaction pathways fo the hydogenation of nito compound Thee is no difficulty in educing the nito goup in pefeence to the cabonyl (ketone goup unless these functional goups ae spatially poximate, in Published by The Bekeley Electonic Pess, 008

9 6 ntenational Jounal of hemical Reacto Engineeing Vol. 6 [008], Aticle A which case ing closue may fom a numbe of by-poducts along with the poduct of inteest (Fische and Weitz, 980. ence the cabonyl goup was not being educed and only the nito goup was being educed and the oveall hydogenation of the aomatic nito ketone was consideed as a pocess consisting of two consecutive eactions, one leading to the fomation of the hydoxylamine deivative, and the othe leading to the fomation of the amine deivative. The pofitability of any heteogeneous chemical pocess depends often on the life time of the catalyst since shot life time equies fequent egeneation o even patial o complete change of the catalyst. Duing the hydogenation of nito compounds, thee have always been issues of apid deactivation of palladium catalysts (Batholomew, 00; essel et al., 003. Deactivation of catalysts fo this class of eaction could be eithe due to leaching o poisoning of active catalytic sites. t has been epoted by Albes et al. (00 that leaching of palladium occus unde hydogen staving conditions. Unde such conditions, nito compounds oxidize metal atoms on the catalyst which fom complexes with amino compounds and ae dissolved in the liquid. Leaching of palladium catalysts in MP has also been epoted by seveal authos (Zhao et al., 000; 00. Leaching is especially significant fo cabon suppoted palladium catalyst used in sluy eactos whee the anchoing between palladium and the suppot is weak. Also, palladium catalysts ae paticulaly sensitive to poisons, especially to compounds with lone pai of electons such as S o -compounds which fom stong dono bonds to the suface. The poison may be the eactant, the intemediates o the poduct in which case it is known as self poisoned o self inhibited. Duing the hydogenation of nito compounds, it is most likely that the intemediate compounds i.e. nitoso, azo and azoxy compounds adsob stongly onto the catalyst suface blocking the catalytic sites and educing the catalyst activity. The catalyst 5% Pd/Alumina used in the pesent study did not show any signs of deactivation eithe due to leaching o poisoning of active catalytic sites as the concentation of the poduct steam did not change with time at least within the 8 hous that we used to conduct each expeimental un. oweve, polonged exposue of the catalyst to MP o the eaction mixtue could possibly lead to deactivation of the catalysts eithe due to leaching o due to accumulation of the intemediate compounds on the catalyst suface. Theefoe, life time of the catalyst and the mechanism of catalyst deactivation will be investigated as pat of futue wok.

10 Tadepalli and Lawal: ydogenation of Aomatic ito etone in a Micoeacto Fluid flow chaacteization The fluid dynamics of eactos involving multi-phase flow has significant effect on the eacto pefomance. As the two eactants ae intoduced at a T-junction, they ente the eacto as altenating gas-liquid slugs. This type of flow was also obseved in ou pevious wok on the hydogenation of nitoanisole (Tadepalli et al., 007a unde simila opeating conditions. The wok of Wada et al (006 on flow distibution in gas-liquid micoeactos also confims that slug flow exists in the micoeacto unde these opeating conditions. Duing gas-liquid eactions in mico channels, slug flow has been shown to enhance mass tansfe significantly (eutze et al., 00 Theefoe, fo all the expeiments epoted hee, the eacto was opeated in slug flow egime Effect of opeating conditions on eacto pefomance The effect of opeating conditions on the eacto pefomance was studied by compaing the convesion of the eactant, selectivity and Space-Time Yield of the poduct (amine unde diffeent eaction conditions. Yield is defined as the atio of the numbe of moles of the desied poduct fomed to the moles of the eactant fed. Selectivity is defined as the atio of the moles of the desied poduct fomed to the moles of the eactant consumed. Space-Time Yield (STY is the ate of fomation of the desied poduct pe unit mass of the active metal (Pd in the catalyst. The STY indicates an aveage eaction ate o pefomance of the eacto Effect of pessue To study the effect of pessue on convesion, selectivity and STY, expeiments wee caied out in the pessue ange of psig at a substate concentation of % (w/w in MP, tempeatue of 30 and constant esidence time. The esidence time was kept constant by vaying the catalyst loading. Figue 3 shows a plot of the convesion vs. pessue at a constant esidence time of 0.8 seconds (inlet conditions. The data indicate that the convesion and selectivity decease with incease in pessue, but the STY fist inceases and then emains constant with incease in pessue. Published by The Bekeley Electonic Pess, 008

11 8 ntenational Jounal of hemical Reacto Engineeing Vol. 6 [008], Aticle A (% Pessue (psig STY (g. pod./g. cat.h Selectivity onvesion STY Figue 3. Effect of pessue (at constant esidence time on eacto pefomance (All uns wee made at a tempeatue of 30, % (w/w substate in MP feed liquid, gas and liquid flowate of 5sccm and 0.05ml/min espectively The decease in the convesion and selectivity with incease in pessue at constant esidence time is due to the decease in the catalyst loading with incease in pessue to maintain constant esidence time. The incease in STY with incease in pessue is attibuted to the incease in the concentation of dissolved hydogen, theeby esulting in a highe eaction ate. oweve, with futhe incease in pessue, the incease in concentation of dissolved hydogen has negligible effect on the STY indicating that the dependency of eaction ate on hydogen concentation is almost zeo ode at high pessues Effect of tempeatue To study the effect of tempeatue on convesion, selectivity and STY, expeiments wee caied out in the tempeatue ange of 30-60, substate concentation of %(w/w in MP, pessue of 00psig and a catalyst (5%Pd/Alumina loading of 3mg. The liquid and hydogen gas flow ates wee 0.05ml/min and 5sccm espectively. Figue 4 shows that with incease in tempeatue, the convesion, selectivity and STY incease, and at 60 both the selectivity and convesion wee 00%.

12 Tadepalli and Lawal: ydogenation of Aomatic ito etone in a Micoeacto 9 (% Tempeatue (deg. STY (g. pod./g. cat.h Selectivity onvesion STY Figue 4. Effect of tempeatue on eacto pefomance (All uns wee made at pessue of 00 psig, % substate in MP feed liquid, gas and liquid flowate of 5sccm and 0.05ml/min espectively, catalyst loading3mg The data show that thee is vey slight incease in the eaction ate with incease in tempeatue. This may be eithe due to the convesion being close to 00% fo the ange of tempeatues consideed in which case the incease in eaction ate with tempeatue may not be significant, o it may also be that the eaction is influenced by extenal mass tansfe unde these opeating conditions. This obsevation has been claified in the following studies whee the effect of mass tansfe was studied and it was found that extenal mass tansfe is significant in this ange of tempeatues consideed Effect of esidence time Expeiments fo vaying the esidence time wee done both by changing the liquid and gas flow ates, and also by changing the amount of catalyst in the eacto. Expeiments fo vaying the liquid flow ate wee conducted to study its effect on the convesion of substate, selectivity and STY of poduct. All the expeiments wee caied out at a tempeatue of 30, substate concentation of % (w/w in MP, pessue of 00psig and a catalyst (5%Pd/Alumina loading of 3mg. The flow ate of the liquid was vaied in the ange of ml/min, and the flow ate was kept constant at 5sccm. n these expeiments, the actual gas flow ate is appoximately 0 times the liquid flow ate. Theefoe, a change in liquid flow Published by The Bekeley Electonic Pess, 008

13 0 ntenational Jounal of hemical Reacto Engineeing Vol. 6 [008], Aticle A ate does not change the esidence time significantly. Figue 5 shows the effect of liquid flow ate on the convesion, selectivity and STY (% STY (g. pod./g. cat.h Selectivity onvesion STY Liquid flowate (ml/min Figue 5. Effect of liquid flowate on eacto pefomance (All uns wee made at a tempeatue of 30, pessue of 00psig,% substate in MP feed liquid, gas flowate of 5sccm, catalyst loading3mg The data show that the convesion and selectivity decease with incease in liquid flow ate while the STY o the oveall eaction ate deceases vey slightly with incease in liquid flow ate. n the expeiments that wee conducted to elucidate the effect of the liquid flow ate at constant gas flowate, it was obseved that the liquid slug length inceases with incease in the liquid flow ate. As the liquid slug length inceases, the ate of convective mass tansfe fom gas to liquid deceases (eutze et al., 00. Small liquid slugs enhance the mass tansfe fom gas to the liquid. Theefoe, convesion, selectivity and STY incease with decease in liquid flow ate (o decease in liquid slug length. Anothe method of changing the esidence time is to change the gas flow ate. Expeiments fo vaying the gas flow ate wee conducted to study its effect on the convesion of substate, selectivity and STY of poduct. All the expeiments wee caied out at a tempeatue of 30, substate concentation of % (w/w in MP, pessue of 00psig, liquid flow ate of 0.05ml/min, and a catalyst (5%Pd/Alumina loading of 3mg. The effect of gas flow ate on convesion, selectivity and STY is shown in fig. 6. The data show that the convesion, selectivity and STY ae nealy unchanged with incease in gas flow ate. Although we keep the liquid flowate constant, as we incease the gas flowate the oveall flow velocity inceases thus deceasing the esidence time.

14 Tadepalli and Lawal: ydogenation of Aomatic ito etone in a Micoeacto (% STY (g. pod./g. cat.h Selectivity onvesion STY Gas flowate (sccm Figue 6. Effect of gas flowate on eacto pefomance (All uns wee made at a tempeatue of 30, pessue of 00psig,% substate in MP feed liquid, liquid flowate of 0.05ml/min, catalyst loading3mg Theoetically, if the eaction is contolled by kinetics, the convesion, selectivity and STY should decease with decease in esidence time; on the othe hand, if the eaction is pimaily contolled by mass tansfe, the convesion, selectivity and STY should incease with incease in gas flowate due to bette mass tansfe. oweve, if we assume a pseudo-homogeneous model fo the flow, then the gas flowate will not affect the convesion, selectivity and STY; on the othe hand, if we assume a two phase flow model, then the oveall (liquid + gas flowate affects the convesion, selectivity and STY. The pesent esults tend to agee with the pseudo-homogeneous model because the convesion, selectivity and STY ae unchanged with gas flowate. oweve, it is not clea if the eaction is contolled both by mass tansfe o intinsic kinetics unde these opeating conditions. The eduction of eaction esidence time and the incease of mass tansfe ate with incease in gas flowate have an opposing effect so that the eaction pefomance does not change much. This obsevation has been claified in the studies that follow by isolating the effect of esidence time and mass tansfe. The isolation of the effect of esidence time on the convesion, selectivity and STY was achieved by vaying the catalyst loading with all the othe opeating conditions emaining constant. All the expeiments wee caied out at a tempeatue of 30, substate concentation of % (w/w in MP, pessue of 00psig, liquid flow ate of 0.05ml/min, and flow ate of 5sccm. Figue 7 shows the esults; as expected the convesion and selectivity incease with incease in catalyst loading. The value of STY deceases with incease in catalyst Published by The Bekeley Electonic Pess, 008

15 ntenational Jounal of hemical Reacto Engineeing Vol. 6 [008], Aticle A loading because the aveage eactant concentation deceases at highe convesion. The effect of mass tansfe on the STY was studied by changing the oveall flow velocity at the same esidence time within the ange of flowates consideed. The esidence time was kept constant by inceasing the amount of catalyst in popotion to the incease of flow velocity. Figue 8 shows the effect of oveall velocity on STY. The fact that the eaction ate inceases with incease in flow velocity indicates that mass tansfe plays an impotant ole. Theefoe we can conclude that the eaction in micoeacto is contolled by both mass tansfe and intinsic kinetics unde the opeating conditions used fo these expeiments. (% atalyst loading (mg STY (g. pod./g. cat.h Selectivity onvesion STY Figue 7. Effect of catalyst loading on eacto pefomance (All uns wee made at a tempeatue of 30, pessue of 00psig, % substate in MP feed liquid, gas and liquid flowate of 5sccm and 0.05ml/min espectively

16 Tadepalli and Lawal: ydogenation of Aomatic ito etone in a Micoeacto 3 0 STY (g pod/g cat-h Oveall flow velocity (cm/s Figue 8. Effect of oveall flow velocity at same esidence time (All uns wee made at a tempeatue of 30, pessue of 00psig, % substate in MP feed liquid Effect of substate concentation To study the effect of substate concentation on eacto pefomance, expeiments wee caied out in the concentation ange of -0% (w/w substate in MP, tempeatue of 50, 00psig eacto pessue, and a catalyst loading of 3mg of 5% Pd/Alumina. The liquid and hydogen gas flow ates wee 0.05ml/min and 5sccm espectively. Figue 9 shows that the incease in substate concentation esults in decease in convesion. The selectivity is lowe at highe substate concentation indicating that the ate of intemediate fomation may have a stonge dependency on the substate concentation. The STY inceases up to substate concentation of 4% (w/w in MP and then stats to decease theeafte. Simila tend was obseved in ou ecent wok on the study of hydogenation of o-nitoanisole at diffeent substates concentations (Tadepalli et al., 007. We obseved that the ate of amine fomation at high concentations of o-nitoanisole is low because of substate inhibition effect pesent at high concentations as a esult of weak adsoption of hydogen on catalyst suface in compaison to nito compound. Published by The Bekeley Electonic Pess, 008

17 4 ntenational Jounal of hemical Reacto Engineeing Vol. 6 [008], Aticle A 0 40 (% STY (g. pod./g. cat.h Selectivity onvesion STY Reactant conc. (w/w Figue 9. Effect of substate concentation on eacto pefomance (All the uns wee made at 30, 00 psig eacto pessue, liquid flowate of 0.05 ml/min, gas flowate of 5sccm, catalyst loading 3 mg 3.4. inetic studies inetic studies of fast hydogenation eactions pesent a unique challenge because these eactions ae usually limited by mass tansfe steps. The suface eaction is so apid that it is difficult to ovecome the mass tansfe esistances to obtain the tue intinsic kinetics of the eaction. The oveall hydogenation ate fo such eactions may be influenced by tanspot effects such as gas-liquid, liquid-solid mass tansfe and intenal mass tansfe. ence, pio to detemining the tue intinsic kinetics of the pesent eaction, appopiate conditions have to be selected whee the tanspot effects can be consideed to be negligible. The appoach used fo conducting kinetic studies fo this eaction was simila to that used fo the hydogenation of o-nitoanisole in a micoeacto (Tadepalli et al., 007. The fist step in this appoach is to evaluate the possible mass and heat tansfe limitations within the opeating conditions used fo the expeiments, and then study the intinsic kinetics of the eaction Analysis of intenal mass tansfe effect The effect of intenal mass tansfe was studied expeimentally by vaying the catalyst paticle sizes. Expeiments wee conducted using two diffeent catalyst paticle size anges, micon (aveage size 60 micon and micon (aveage size micon at diffeent hydogen pessue values to detemine 0

18 Tadepalli and Lawal: ydogenation of Aomatic ito etone in a Micoeacto 5 the effect of catalyst paticle size on the eaction ate (STY. The effect of paticle size on STY at diffeent pessues is shown in fig. 0. The data in this figue indicate that the values of STY at diffeent pessue levels consideed ae nealy the same fo the two paticle sizes utilized. ence, it can be concluded that the influence of intenal diffusion on the eaction ate can be neglected fo the ange of pessues and paticle sizes studied. To futhe validate this conclusion, calculation of the catalyst effectiveness facto (η eff was pefomed. The effectiveness facto calculated unde the eaction conditions used fo the above expeiments anged fom as shown in Table. Since the effectiveness facto fo these opeating conditions is close to (η eff, the calculations confim the expeimental data which indicate that intenal mass tansfe does not affect the eaction ate significantly. 300 STY (g pod/g cat-h micons micons Patial Pessue of (psig Figue 0. Effect of paticle size on STY at diffeent pessues (All uns wee made at a tempeatue of 50, % substate in MP feed liquid, liquid flow ate of 0.4 ml/min, Oveall gas flow ate 40 sccm, catalyst loading3mg Published by The Bekeley Electonic Pess, 008

19 6 ntenational Jounal of hemical Reacto Engineeing Vol. 6 [008], Aticle A Table. alculations fo Thiele Modulus and effectiveness facto fo the diffeent expeimental conditions Pessue (psig s (mol/l obs ( moles/gcat-sec φ η E E E E E E Analysis of extenal mass tansfe effect The pesence of extenal mass tansfe limitation was detemined expeimentally by vaying the flow velocity at constant esidence time. At constant esidence time, and in the absence of mass tansfe limitations, the concentation pofile and the degee of convesion will be the same fo diffeent flow velocities. n section 3.3.3, it was obseved that the effect of extenal mass tansfe was significant in the ange of opeating conditions consideed. Expeiments wee conducted beyond this ange of opeating conditions to see at what oveall flow velocity the eaction is contolled by tue intinsic kinetics. Figue shows the esults obtained fom these expeiments which wee conducted at a constant esidence time by vaying the gas and liquid flow ates popotionately with the catalysts amounts so as to obtain constant esidence time.

20 Tadepalli and Lawal: ydogenation of Aomatic ito etone in a Micoeacto 7 Figue. Effect of oveall flow velocity on eaction ate fo the same esidence time (All uns wee made at a tempeatue of 50, eacto pessue of 00 psig and % (w/w substate in MP The data show that the eaction ate inceases with incease in flow velocity up to a cetain velocity and then emains constant. Fo diffeent patial pessues, the velocity at which the eaction ate levels off is diffeent because of the change in concentation of hydogen at diffeent patial pessues of hydogen leading to diffeent intinsic eaction ates. Beyond 6 cm/s, the eaction ate is independent of the oveall velocity indicating the absence of extenal mass tansfe limitations fo the eaction within the ange of patial pessues consideed Analysis of heat tansfe effects Apat fom mass tansfe limitations, heat tansfe limitations should also be consideed to ensue that the kinetic data obtained in the expeimental eacto eflect only kinetic effects. Fo this pupose, the citeia established by Meas (97a, 97b wee used to estimate whethe heat tanspot effects wee significantly affecting the expeimental esults. Axial heat tanspot effect can be neglected if the atio of axial length of the eacto is much geate (>0 than the catalyst paticle size. n the pesent study, since the micoeacto length at typical eaction conditions is much geate than the catalyst paticle size (>30 times, axial heat conduction can be neglected. oweve, adial heat tansfe effect Published by The Bekeley Electonic Pess, 008

21 8 ntenational Jounal of hemical Reacto Engineeing Vol. 6 [008], Aticle A cannot be assumed to be negligible because adial tempeatue gadients can lead to eaction ates seveal thousand folds geate at the axis of the eacto than at the wall due to the absence of heat emoval pathway fom the axis of the eacto to the envionment. A citeion fo quantitative analysis of adial heat tanspot limitations is given by the Damkőhle numbe fo heat tansfe (Meas 97a (D a which is defined by the following equation: D a obs R Δ ( ( ε λtw( + b o RT < 04. Ea w ( f the left hand side of the above equation is smalle than the ight hand side, the tansvese tempeatue diffeence in the eacto would be less than 5%. n this case, the left hand side was found to be one ode of magnitude smalle than the ight hand side. ence, it can be concluded that the heat tansfe effects can be neglected in the mico eacto. With all the above consideations, appopiate eaction conditions have been obtained in the micoeacto whee the eaction is contolled by suface kinetics and not significantly influenced by mass and heat tansfe esistances Rate analysis using diffeential method Expeiments wee conducted in the micoeacto unde diffeent opeating conditions to detemine the kinetic ate expession fo this eaction. Fo these expeiments, the oveall flow velocity was maintained above 6 cm/s to ensue the absence of extenal mass tansfe limitations and, the convesion of the eactants was limited to 0% so that a diffeential eacto can be assumed to find the dependence of the eaction ates on the eactant concentations. The high oveall flow velocities used fo these expeiments esulted in a pessue dop between 0-5psig acoss the eacto. ence, the concentation of hydogen was calculated based on the aveage patial pessue of hydogen acoss the eacto. The effect of hydogen pessue on the ate of disappeaance of nito compound was investigated in a ange of 0-00psig, at tempeatues of 30, 40 and 50 o and substate concentation of % (w/w in MP. The esults ae pesented in fig.. The ate of disappeaance of nito compound fist inceases with incease in hydogen pessue and then emains constant at highe hydogen pessues.

22 Tadepalli and Lawal: ydogenation of Aomatic ito etone in a Micoeacto 9 Rate (mol/g cat-h Patial Pessue of (psig 30 deg. 40 deg. 50 deg. Figue. Effect of patial pessue on the eaction ate of disappeaance of nito compound The effect of concentation of nito compound on the ate of disappeaance of nito compound was investigated in the concentation ange of to 0% (w/w at tempeatues of 30, 40 and 50 o, and a hydogen pessue of 00psig. The esults ae pesented in fig. 3. The data show that the eaction ate fist inceases and then deceases with nito compound concentation. The decease in the eaction ate at high nito compound concentation may be because of substate inhibition effect pesent at such high concentations as mentioned in section This suggests that Langmui inshelwood expession should be consideed fo the ate equation. Published by The Bekeley Electonic Pess, 008

23 0 ntenational Jounal of hemical Reacto Engineeing Vol. 6 [008], Aticle A Rate (mol/g cat-h onc. Of the nito compound (wt% 30 deg. 40 deg. 50 deg. Figue 3. Effect of substate concentation on the ate of disappeaance of nito compound The effect of hydogen pessue on the ate of fomation of amine was investigated in a ange of 0-00psig at tempeatues of 30, 40 and 50 o, and substate concentation of % (w/w in MP. The esults ae pesented in fig. 4. The data show that the ate of fomation of amine at 30 and 40 o inceases slightly with incease in hydogen pessue wheeas at 50 o, the ate fist inceases almost linealy with incease in hydogen pessue and then levels off at highe hydogen pessues. This is because at highe tempeatues, a high concentation of eactants is equied to satuate the catalytic sites than at lowe tempeatues. n othe wods, the adsoption equilibium constants decease with incease in tempeatue. The effect of concentation of nito compound on the ate of fomation of amine was investigated in a ange of to 0% (w/w (substate in MP, at tempeatues of 30, 40 and 50 o, and a hydogen pessue of 00psig. The esults ae pesented in fig. 5.

24 Tadepalli and Lawal: ydogenation of Aomatic ito etone in a Micoeacto Rate (mol/g cat-h Patial Pessue of (psig 30 deg. 40 deg. 50 deg. Figue 4. Effect of patial pessue on the eaction ate of fomation of amine 0.8 Rate (mol/g cat-h onc. Of the nito compound (wt% 30 deg. 40 deg. 50 deg. Figue 5. Effect of substate concentation on the ate of fomation of amine Published by The Bekeley Electonic Pess, 008

25 ntenational Jounal of hemical Reacto Engineeing Vol. 6 [008], Aticle A The ate fist inceases with incease in nito compound concentation and then deceases slightly with futhe incease in concentation of nito compound. A compaison of the data of figues 3 and 5 shows that the ate of disappeaance of nito compound is highe than the ate of fomation of amine, at simila conditions. This suggests that the hydogenation of the nito compound to intemediate is faste than the hydogenation of intemediate to poduct. Fo the pupose of kinetic study, the oveall eaction was split into two-step seies eaction as shown in fig. 6. R O O + R + O O + R + O R Figue 6. Reaction pathway fo the hydogenation of nito compound inetic modeling Langmui-inshelwood (L- model was used to develop ate equations fo both the eactions. The L- model fo catalytic eactions typically involves a sequence of thee steps:. Adsoption of the eactants. Suface eaction 3. Desoption of the poducts. Fo each of the individual eactions shown in fig. 6, these steps occu on palladium sites epesented by (s. The mechanistic schemes fo both eactions ae shown in figues 7 and 8 espectively. Although the mechanisms shown in figues 7 and 8 coespond to competitive eaction (whee the two eactants compete with each othe fo the same site and non-dissociative adsoption of hydogen, othe mechanisms such as non-competitive eaction/non-dissociative adsoption of hydogen, competitive eaction/ dissociative adsoption of hydogen, and competitive eaction/ dissociative adsoption of hydogen ae also consideed. Based on these mechanisms, diffeent ate equations ae deived

26 Tadepalli and Lawal: ydogenation of Aomatic ito etone in a Micoeacto 3 consideing each step as the ate limiting step fo both eactions. The ate equations ae deived by neglecting the concentations and suface coveage of intemediates and poducts on the catalytic sites at low convesions of the eactants. These ate equations ae listed in Table. Reaction - ompetitive Adsoption Figue 7. Mechanistic scheme fo Reaction Published by The Bekeley Electonic Pess, 008

27 4 ntenational Jounal of hemical Reacto Engineeing Vol. 6 [008], Aticle A Reaction - ompetitive Adsoption Adsoption of Reactants O + (s (s O R R Suface Reaction + (s (s R (s O + (s R (s + O + (s Desoption of the poduct R (s R + (s Figue 8. Mechanistic scheme fo Reaction

28 Table. Rate equations deived fom L- mechanism ompetitive Reaction/ on-dissociative adsoption of hydogen Reaction o. Rate limiting step Rate equation Adsoption of nito compound k + Adsoption of ydogen k + 3 Suface Reaction ( ( ( k Suface Reaction ( ( ( k Suface Reaction (deived by omitting the factional coveage of intemediates and poducts (atziantoniou et al., ( ( ( k Desoption of intemediate ( ( ( ( b a k Reaction Adsoption of intemediate k + Adsoption of ydogen k + 3 Suface Reaction ( ( ( k Desoption of amine ( ( k Tadepalli and Lawal: ydogenation of Aomatic ito etone in a Micoeacto Published by The Bekeley Electonic Pess, 008

29 6 ntenational Jounal of hemical Reacto Engineeing Vol. 6 [008], Aticle A ompetitive Reaction/ Dissociative adsoption of hydogen Reaction o. Rate limiting step Rate equation Adsoption of nito compound Adsoption of ydogen 3 Suface Reaction 4 Suface Reaction 5 Suface Reaction (deived by omitting the factional coveage of intemediates and poducts (atziantoniou et al., Desoption of intemediate k + k + k ( ( + + k ( ( ( k( ( 3 ( + + k ( ( ( a ( + b Reaction Adsoption of intemediate Adsoption of ydogen 3 Suface Reaction 4 Desoption of amine k k + k + ( + + k ( + ( +

30 Tadepalli and Lawal: ydogenation of Aomatic ito etone in a Micoeacto 7 on-ompetitive Reaction/ on-dissociative adsoption of hydogen Reaction o. Rate limiting step Rate equation Adsoption of nito compound Adsoption of ydogen 3 Suface Reaction 4 Suface Reaction ( ( + k( + k k k( ( + ( ( + ( + a 5 Desoption of intemediate k( ( ( + + ( + a b Reaction Adsoption of intemediate Adsoption of ydogen 3 Suface Reaction 4 Desoption of amine ( k( ( + k k k( + + ( ( + ( a Published by The Bekeley Electonic Pess, 008

31 8 ntenational Jounal of hemical Reacto Engineeing Vol. 6 [008], Aticle A on-ompetitive Reaction/ Dissociative adsoption of hydogen Reaction o. Rate limiting step Rate equation Adsoption of nito compound Adsoption of ydogen 3 Suface Reaction 4 Suface Reaction ( k k k + ( ( + k ( ( ( + ( + + a 5 Desoption of k intemediate ( ( ( + + ( + a b Reaction Adsoption of intemediate k Adsoption of ydogen 3 Suface Reaction 4 Desoption of amine k k ( + ( + k ( + + a The expeimental ate data obtained wee fitted to these ate equations and wee subjected to non-linea egession analysis to estimate the kinetic constants. Based on the best fit in the egession analysis, it was found that suface eactions

32 Tadepalli and Lawal: ydogenation of Aomatic ito etone in a Micoeacto 9 assuming competitive adsoption and non-dissociative adsoption of hydogen wee the ate contolling steps fo the two eactions. Fo eaction (suface eaction contolling k ( ( 3 ( + + ( Fo eaction (suface eaction contolling k ( ( ( + + (3 The concentation of hydogen in the eaction medium was obtained fom the solubility data fo in MP fom the wok of Bunne (985. The values of the kinetic constants obtained using the optimization pogam fo these models ae given in Table 3. Table 3 shows that the equilibium constants of hydogen fo the two eactions ae diffeent. This may be due to the influence of othe eactants competing fo the same palladium sites as hydogen. Table 3. inetic constants and egession coefficients fo the eactions Reaction Tempeatue (deg. k (mol/g-h (L/mol (L/mol R ± ± ± ±.89.73± ± ± ± ± Reaction Tempeatue (deg. k (mol/g-h (L/mol (L/mol R 30.6± ± ± ± ± ± ± ± ± Published by The Bekeley Electonic Pess, 008

33 30 ntenational Jounal of hemical Reacto Engineeing Vol. 6 [008], Aticle A The ates pedicted fom the ate equations agee with the expeimental ates within 0% eo. The activation enegies and heat of adsoption of the eactants fo eactions and wee detemined fom the kinetic constants using the Ahenius equations: Fo ate constant k koe Ea RT (4 Fo equilibium constants oe Δ RT (5 whee k is the ate constant fo the eaction, k 0 is the pe-exponential facto fo the eaction, Ea is the activation enegy fo the eaction, is the equilibium constant fo adsoption of the eactant in the pesence of othe eactant, 0 is the pe-exponential facto fo the adsoption of the eactant in the pesence of othe eactant, Δ is the heat of adsoption fo the eactant in the pesence of othe eactant and T is the tempeatue in elvin. The pe-exponential factos, activation enegies and heat of adsoption of eactants calculated using the Ahenius equations fo vaious kinetics constants ae given in Table 4. Table 4 shows that the heats of adsoption of hydogen fo the two eactions ae diffeent. This diffeence may also be due to the pesence of othe competitive eactants that ae competing fo the same sites. n ode to futhe establish the validity of the above ate equations and associated kinetic paametes of the oveall eaction, a compaison was made between the expeimental data obtained unde integal conditions and the theoetical pedictions fom the above ate equations. Expeiments wee caied out at diffeent hydogen pessues and highe amount of catalyst loading unde kinetically contolled conditions. The model pedictions and expeimental data fo the yield of the poduct and the convesion of the eactant unde these eaction conditions ae shown in fig. 9, and ae within 5-0% of each othe. This excellent ageement suggests that the ate paametes detemined hee epesent the eacto pefomance data obtained unde both diffeential and integal conditions.

34 Tadepalli and Lawal: ydogenation of Aomatic ito etone in a Micoeacto 3 Table 4. Pe-exponential factos, activation enegies fo the eactions and heats of adsoption of the eactants Pe-exponential facto Reaction ntinsic onstants Values R k 0 (mol/g-h 9.46E (L/mol (L/mol Reaction k 0 (mol/g-h.57e (L/mol (L/mol 9.86E Activation enegy and heats of adsoption Reaction ntinsic onstants Values R Ea (kj/mol Δ (kj/mol Δ (kj/mol Reaction Ea (kj/mol Δ (kj/mol Δ (kj/mol Published by The Bekeley Electonic Pess, 008

35 3 ntenational Jounal of hemical Reacto Engineeing Vol. 6 [008], Aticle A 00 Expeimental (% onvesion Yield Pedicted (% Figue 9. Expeimental and pedicted yield and convesion fo an integal eacto The main significance of the wok pesented hee lies in the multi-channel eacto engineeing and design fo high thoughput of desied poduct. The kinetic model along with hydodynamics and tanspot effects as obtained in this study can be combined with FD simulations to design the multi-channel eacto that can poduce kilogam o even ton quantities of desied poducts. This appoach to micoeacto design and engineeing can be used to: Evaluate the benefits and disadvantages associated with new eacto designs ntepet expeimental data of the multi-channel eacto and 3 Avoid iteative, costly and time-consuming expeimental wok. 4. onclusions A packed-bed micoeacto fo the hydogenation of aomatic nito ketone at modeate tempeatue and pessue has been demonstated. The effect of diffeent opeating conditions on the eacto pefomance was studied using 5% Pd/Alumina catalyst. The expeimental esults indicate that at low oveall flow velocities (<6 cm/s the eaction is contolled by both mass tansfe and intinsic kinetics in the micoeacto. Beyond 6cm/s, extenal mass tansfe esistances wee found to be negligible. Also, intenal mass tansfe and heat tansfe

36 Tadepalli and Lawal: ydogenation of Aomatic ito etone in a Micoeacto 33 esistances wee established to be negligible in the micoeacto. ence, the eaction is kinetically contolled in the micoeacto unde these eaction conditions. Two sepaate eaction steps wee identified fom the kinetic studies: The eduction of nito compound to hydoxyl amine and the futhe eduction of hydoxyl amine to amine. Langmui-inshelwood type ate expessions wee found to be suitable fo these eaction steps with suface eactions contolling the two eaction steps. A set of ate equations was deived based on L- model and the kinetic ate constants wee obtained fom egession of the expeimental data in a diffeential micoeacto. The obtained kinetics wee able to pedict the integal eacto behavio unde kinetic-contolled egime. n spite of the excellent mass tansfe chaacteistics of the micoeacto, thee ae limited studies on the use of micoeacto fo obtaining intinsic kinetics of fast eactions. The pesent study has demonstated the use of a packed-bed micoeacto fo obtaining the intinsic kinetics of a fast hydogenation eaction that is contolled by mass tansfe in conventional eactos. otation b d p Da D e D m E a atio of diluent to catalyst volume, dimensionless concentation of hydogen in the liquid mixtue, mol/l concentation of nito compound in the liquid mixtue, mol/l concentation of intemediate in the liquid mixtue, mol/l diamete of the catalyst paticle, μm Damkőhle numbe fo heat tansfe, dimensionless effective diffusion coefficient of hydogen in the paticle, cm /s hydogen diffusion coefficient in the liquid eactant, cm /sec activation enegy, kj/mol heat tansfe coefficient fo the eactants,w/m - 0 pe-exponential facto fo eaction, mol/g-h Published by The Bekeley Electonic Pess, 008

37 34 ntenational Jounal of hemical Reacto Engineeing Vol. 6 [008], Aticle A k k k 0 k 0 intinsic kinetic ate constant fo eaction, mol/g-h intinsic kinetic ate constant fo eaction, mol/g-h pe-exponential facto fo the eaction, mol/g-h pe-exponential facto fo the eaction, mol/g-h Equilibium constant, L/mol a b R R R R o Equilibium constant, L/mol Equilibium constant, L/mol pe-exponential facto fo eactant in the pesence of othe eactant, L/mol pe-exponential facto fo hydogen in the pesence of nito compound fo eaction, L/mol equilibium constant fo hydogen in the pesence of nito compound fo eaction, L/mol pe-exponential facto fo hydogen in the pesence of nito compound fo eaction, L/mol equilibium constant fo hydogen in the pesence of nito compound fo eaction, L/mol equilibium constant fo intemediate in the pesence of hydogen, L/mol pe-exponential facto fo intemediate in the pesence of hydogen, L/mol equilibium constant fo nito compound in the pesence of hydogen, L/mol pe-exponential facto fo nito compound in the pesence of hydogen, L/mol eaction ate fo eaction, mol/g-h eaction ate fo eaction, mol/g-h univesal gas constant, J/mol. egession coefficient, dimensionless adius of the tubula eacto, m

38 Tadepalli and Lawal: ydogenation of Aomatic ito etone in a Micoeacto 35 R obs STY T Tw Geek symbols obseved eaction ate, mol/m 3 s Space-Time Yield, g pod./(g cat. h eaction tempeatue, eacto wall tempeatue, ρ p density of catalyst paticles, g/m 3 Ε void faction of catalyst bed, dimensionless Δ enthalpy of eaction, kj/mol Δ Δ Δ Δ Λ φ η eff heat of adsoption of nitoanisole in the pesence of hydogen, kj/mol heat of adsoption of hydogen fo eaction in the pesence of nito compound, kj/mol heat of adsoption of hydogen fo eaction in the pesence of intemediate, kj/mol heat of adsoption of intemediate in the pesence of hydogen, kj/mol effective themal conductivity of poous catalyst, W/m- Thiele Modulus, dimensionless catalyst effectiveness facto, dimensionless Refeences Albes, P.; Pietsch, J.; Pake, S.F. Poisoning and deactivation of palladium catalysts. J. Mol. at A: hem (00 73, 75. Andesson, B.; atziantoniou, V.; Schöön,. Mass tansfe and selectivity in liquid-phase hydogenation of nito compounds in a monolithic catalyst eacto with segmented gas-liquid flow. nd. Eng. hem. Pocess Des. Dev. (986 5, 964. Batholomew,.. Mechanisms of catalyst deactivation. Applied at. A: Geneal (00, 7. Boekhuis, R.R.; Machado, R.M.; odquist, A.F. The ejecto diven monolith loop eacto expeiments and modelling. atal. Today (00 69, 93. Published by The Bekeley Electonic Pess, 008