Modelling catalyst regeneration in an industrial FCC unit

Transcription

1 AMERIAN JORNAL OF SIENIFI AND INDSRIAL RESEARH 03, Science Huβ, ISSN: X, doi:0.55/jsir Modelling ctlyst regenertion in n industril F unit *.. Dgde, nd Y.. Puyte Deprtment of hemicl/petrochemicl Engineering, Rivers Stte niversity of Science nd echnology, Port Hrcourt, P. M. B. 5080, Port Hrcourt, Nigeri ABSRA Predictive models for process prmeters during regenertion of spent ctlyst in n industril fluid ctlytic crcking (F) unit re presented. he models dopt twophse theory where the dense region of the regenertor is divided into bubble-phse nd n emulsion-phse. he bubble-phse is modelled s plug flow rector, while the emulsion-phse is modelled s continuous stirred tnk rector (SR). Profiles for regenertor-temperture, quntity of coke burnt, nd flue gs composition, t different operting conditions re lso presented. Model-predictions re compred with plnt dt nd good greement is obtined. Simultion results indicte tht inlet-ir velocity nd ctlyst-bed height hve significnt influence on the performnce of the rege nertor. he model-estimted optimum operting conditions of the regenertor re regenertortemperture of bout 000, inlet-ir velocity of bout 3.5 m/s, nd ctlyst-bed height of 3 m. eywords: Modelling; regenertor; ctlytic crcking; coke com bustion, ctlyst INRODION Fluid ctlytic crcking (F) is one of the key processes in modern petroleum refining. While it evolved bout 50 yers go, the technology is still being improved upon (Gry & Hndwerk, 00; Sdghbeigi, 000; Jones nd Peter, 006) due to mny chllenges brought bout by environmentl regultions, product-qulity demnds, nd economics. tlytic crcking is similr to therml crcking except tht ctlysts fcilitte the conversion of the hevier hydrocrbon molecules into lighter products. se of ctlysts in the crcking rection increses the yield of improved-qulity products under much less severe operting conditions thn therml crcking. Since the crcking rections produce crbonceous mteril (referred to s coke) tht deposits on the ctlyst nd very quickly reduces the ctlysts rectivity, burningoff the deposited coke with ir blown into the regenertor regenertes the ctlyst. he regenertor opertes t temperture of bout 000 nd pressure of bout 4 kp. ombustion of coke is exothermic nd it produces lrge mount of het tht is prtilly bsorbed by the regenerted ctlyst nd lso provides the het required for vporiztion of the feedstock nd the endothermic crcking rections tht tke plce in the riser-rector. For this reson, F units re often referred to s het blnced (Ahri et l, 008; Hn & hng, 00; Ali et l, 997). he regenertor is divided into two regions, nmely n upper dilute region nd lower dense region. Most of the models proposed for ctlyst regenertion focus on the dense region which is further divided into bubble-phse nd n emulsion-phse (i.e. two-phse model). he dvntge of this two-phse model of the dense region is tht it describes the whole rnge of fluidiztion regimes tht covers the operting conditions of vrious types of F regenertors. he erliest models were single-phse, simplecontcting model with plug flow, nd dispersion of recting components with tnks in series (Arthur, 95; Rowe & Ptridge, 965; Weiz & Godwin, 966). Severl workers (Weiz & Godwin, 966; Morley & De-Ls, 987; Morley & De-Ls, 988) crried out extensive studies on the kinetics of coke burning rections for zeolite ctlyst nd estimted the kinetic prmeters for the process but did not pply the kinetic prmeters to simulte industril F 94

2 Am. J. Sci. Ind. Res., 03, 4(3): regenertor. Other workers (De-Ls et l, 98; Errzu et l, 979; rishnih et l, 007) performed stedy-stte nlysis of the regenertor using the two-phse theory (bubblephse rich in gses, nd emulsion-phse rich in ctlyst prticles). Dynmic models hve lso been developed in severl studies for F regenertor. For exmple, Ro et l. (004) developed dynmic model for F regenertor in which the dense region ws divided into two phses (bubble nd emulsion-phses), with both phses modelled s continuous stirred tnk rectors (SRs) in series. Ali et l. (997) presented dynmic model for F regenertor tht comprises severl ordinry differentil equtions, nd lso presented nlyticl solutions of the system of differentil equtions bsed on pseudo-stedy stte conditions. Hn nd hung (00) presented dynmic model for F regenertor with the ssumption tht the ctlysts in the dense bed re in therml equilibrium with the gses in the bubble-phse, nd the gses in both the bubble nd emulsion phses re in tubulr flow, but most of the prmeters used in this model cnnot be esily obtined without extensive experimentl studies; thus, the model cnnot be dpted for the simultion of industril F unit. In this study, the dense region of the regenertor is lso modelled s two phses (bubble nd emulsion), but the temperture in the emulsionphse is ssumed to be higher thn tht in the bubble-phse due to the high density of ctlyst in the emulsion-phse. In the works of Hn nd hung (00) nd Ali et l. (997), constnt superficil velocity ws ssumed in both the bubble nd emulsion phses. However, it is ssumed in this study tht the gses in the emulsion-phse re t minimum fluidiztion velocity while the gses in the bubble-phse re bove the minimum fluidiztion velocity, noting tht the sum of the velocities in the emulsion nd bubble phses equls the constnt superficil velocity. he ssumption of minimum fluidiztion velocity in the emulsion-phse is justified by the fct tht the ctlyst spends longer time in the emulsion-phse (with high ctlyst density) thn in the bubble-phse (with low ctlyst density) which lso fvours ctlytic combustion in the emulsion-phse. he models presented re used to simulte the regenertor of functionl F unit in Nigerin refinery. oke combustion kinetics in regenertor-rector sully, coke is mixture of different components (crbon, hydrogen, nitrogen, sulphur, etc.), but minly crbon (Ali et l. 997). hus, during ctlyst regenertion in F unit, coke is burnt to produce crbon monoxide nd crbon dioxide (Ro et l. 004; Arbel et l. (995). Also, the homogeneous O combustion rection tking plce in the bubble-phse is ssumed to be negligible compred with the ctlytic O combustion in the emulsion-phse (Ali et l. 997, Elnshie & Elshishini, 993). Hence, the following irreversible coke combustion rections occur in the emulsionphse of regenertor (Weiz & Godwin, 966). O O O Het O Het O O O O Het where is the rection rte constnt for coke burning, nd O is the rection rte constnt for the ctlytic O combustion. Eqution (3) is the fter-burning rection which tkes plce in the dense region if sufficient oxygen is supplied to support it. he rection which goes to completion in the dense region of regenertion to fully regenerte the ctlyst is clled the controlled-fter-burning rection. However, O burning is usully initited by using promoter, which is ctlyst tht speeds up the rection of crbon monoxide to crbon dioxide. he promoter, usully metl like pltinum, is ttched to the F ctlyst during mnufcturing. Pltinum-bsed combustion promoters hve been utilized in F units to ctlyze the oxidtion of O to O for over 30 yers (Gry & Hndwerk, 00; Yng, 003; Guthier et l. 000). he better the dispersion of pltinum, the more effective is the combustion of coke. he rte expressions for the component gses in the emulsion-phse re obtined s follows 95

3 Am. J. Sci. Ind. Res., 03, 4(3): oke (): ( r ) SO Oxygen (O ): ( ) O / ro SO OO yo O (5) O rbon dioxide (O ): where O is the molr concentrtion of F ( ) / ro SO OO oxygen in the feed ir, nd the prime O (6) indictes mole frction. rbon monoxide (O): he regenertor-rector model: Figure shows hypotheticl representtion of F regenertor which is divided into two regions dilute nd / ( ro) SO OSO dense regions, where (7) S nd R re the where ( r volumetric flow rtes of spent nd regenerted ) is the rte of coke combustion ctlyst respectively; E nd B re the emulsion rection; ( ri ) re the rtes of combustion of nd bubble phses respectively; mf nd individul components of the flue gs; S is the re the minimum fluidiztion velocity nd inlet-ir molr concentrtion of coke on spent ctlyst; velocity respectively; ie nd ib re the molr i re the molr concentrtions of the flue concentrtions of flue gses in the emulsion nd bubble phses respectively. he dilute region is gses (i.e. exit gses), with i O, O, O ; the section between the top of the regenertor nd,, re the rte constnts in Eqs. nd the boundry between the two regions, while the dense region extends from the boundry (4) (7) nd defined s follows (Hn & hng, between the two regions to the exit of the 00; Ali et l. 997). regenertor nd is divided into bubble-phse nd n emulsion-phse (unii & Levenspel, for O blnce 99). he mount of solids entrined (8) in the F y O dilute region is usully very smll compred to the totl mount of ctlyst retined in the regenertor vessel. Most of the coke on the for O blnce ctlyst pellets is combusted in (9) the dense region; tht is, full combustion of coke to O is ssumed in the dense region. Accordingly, the for O blnce effect of the dilute region on (0) the overll O O F F (4) performnce of the regenertor is ignored (Ali et l. 997). he ir distributors, spent ctlyst, nd where O / O is the intrinsic rtio of cyclones recycle pipes in the emulsion-phse, crbon dioxide to crbon monoxide. he mole produce enough turbulence tht justifies this frctions of the vrious flue gses re expressed phse to be modelled s continuous stirred with respect to the molr concentrtion of oxygen tnk rector (SR). Hence, it is ssumed tht in the feed ir, in the form the emulsion-phse is bed t minimum fluidiztion velocity nd coke combustion O yo rections occur in this phse, while () the bubble- O phse moves s plug flow nd exchnges mss nd het with the emulsion-phse without coke combustion rection due to its deficiency in ctlyst prticles (hrisnih et l. 007). In the derivtion of the mthemticl models for coke 96

4 Am. J. Sci. Ind. Res., 03, 4(3): combustion nd ctlyst regenertion, the following ssumptions re mde: i. nstedy-stte conditions for the energy nd coke combustion blnces in the emulsion-phse due to high density of ctlyst (Ali et l. 997), nd stedy-stte condition for the sme opertions in the bubble-phse due to low density of ctlyst. ii. he homogeneous combustion rection tking plce in the bubble-phse is negligible compred with the ctlytic O combustion in the emulsion-phse (Ali et l. 997, Morley & De-ls, 988). Mss blnce for coke combustion in the emulsion-phse: Appliction of the lw of conservtion of mss to coke combustion in the emulsion-phse bsed on the bove ssumptions, gives dys R yr y S S d L A ( ) GI G E r L y L S GI dlgi d where the dimensionless rte of chnge of ctlyst-bed height is obtined s ib ie y ib, y ie G G dlgi S R (5) d AG ct ( E) with i O, O, O ; ib nd ie re the mss densities of gses in the bubble nd he dimensionless vribles in equtions (4) emulsion phses respectively, nd G is the nd (5), nd the volume of the regenertor, re defined s totl mss density of gses in the regenertor. S y S, y R R, VG AG LG ), ct ( E ct LG t LGI, L L where ys is the mss frction of spent ctlyst, yr is the mss frction of regenerted ctlyst, S is the mss density of spent ctlyst, R is the mss density of regenerted ctlyst, ct is the mss density of ctlyst, VG is the volume of the regenertor, is the dimensionless time, L GI is the dimensionless ctlyst-bed height, LG is ctlyst-bed height, A G is the crosssectionl re of the regenertor, E is the void frction in the emulsion-phse, nd L is the stedy-stte ctlyst-bed height. Mss blnce for gses in the bubble-phse: Appliction of the lw of conservtion of mss to gses in the bubble-phse, with the ssumptions of no ccumultion of gses nd without coke combustion rections, gives the mteril blnce for the flue gses s dy dl ib GI be L ( B)( yib yie) where be is the mss-trnsfer coefficient between the bubble nd emulsion phses, B is the void frction in the bubble-phse, y ie nd y ib re mss frctions of gses in the emulsion nd bubble phses respectively nd (4) re defines s Mss blnce for gses in the emulsionphse: Appliction of the lw of conservtion of mss to gses in the emulsion-phse, with coke combustion rections nd ccumultion of gses, gives the mteril blnce (6) for the flue gses s dyie d L ieo GI ( yieo yie ) ( ) E r y y where ieo is the incipient velocity into the emulsion-phse, y ieo is the initil mss frction of the respective gses entering the emulsion- i L be ib ie L 97

5 Am. J. Sci. Ind. Res., 03, 4(3): phse, nd ( ri ) re the rection rtes of the flue gses in the emulsion-phse. o o, ref R R ref Energy blnce in the bubble-phse: In the bubble-phse, stedy-stte opertion is ssumed becuse of the high velocity of gses, nd it is lso ssumed tht no combustion rection tkes plce due to low ctlyst density. Appliction of the lw of conservtion of energy to gses in the bubble-phse bsed on the bove ssumptions, gives the energy blnce in this phse s with d b Hbe ( b e ) ( B) L dl GI b b ref, where, b nd e re the dimensionless tempertures in the bubble nd emulsion phses respectively, b is the bubble-phse temperture, e is the temperture in the emulsion-phse, ref is reference temperture tken to be 960 (NHPR, 987), H be is the het-trnsfer coefficient between the bubble nd emulsion phses, nd b is the bubble velocity. Energy blnce in the emulsion-phse: Appliction of the lw of conservtion of energy to coke on spent nd regenerted ctlysts s well s the gses in the emulsion-phse, with coke combustion rections, ccumultion of gses, nd trnsfer of het between the bubble nd emulsion phses, gives the energy blnce in the emulsion-phse s d ( e ieo d A L n ( i with G ieo GI Hie)( ri ) O p o ie iepiee ) ieo ( S p ) S SR Rp R Re ( )( ) A L ( ) E ie pie R pr G GI ie pie R pr 6.77 mf Dbr ce 3 ( H )( r ) R L db () ( ie pie Rp ref ) R HbeL( b e ) e dl GI ( ie pie Rp ) LGI d R where ieo nd ie re the volumetric flow rtes of flue gses t the inlet nd outlet of the emulsion-phse; o is the dimensionless inlet ir temperture of the regenertor, ieo nd ie re the mss densities of the flue gses t the inlet nd outlet of the emulsion-phse respectively, is the mss density of gs (oxygen) in the emulsion-phse, pieo nd pie (0) re the specific het cpcities of the flue gses b t the inlet nd outlet of the emulsion-phse respectively, p nd S p re the specific het R cpcities of spent ctlyst nd regenerted e e ctlyst respectively which re tken () to be equl, ref ΔH ie re the het of rection of the flue gses in the emulsion-phse, ΔH is the het of rection O for coke combustion, is the specific re for het trnsfer between the bubble nd emulsion phses, R is the dimensionless temperture of the riser, nd R is the riser-temperture. Note tht the inlet temperture of the regenertor is prtly the temperture of the spent ctlysts entering the regenertor from the riser, nd prtly the inlet mbient ir temperture. Hydrodynmic specifictions: he interchnge mss-trnsfer coefficients between the bubble nd emulsion phses re relted in the form (unii & Levenspel, 99; Dvidson & Hrrison, 963). with nd be ce 4.5 mf bc db bc / / / D g 5/ 4 db 98

6 Am. J. Sci. Ind. Res., 03, 4(3): where ce is the mss-trnsfer coefficient between the cloud nd emulsion phses, bc is mss-trnsfer coefficient between the bubble nd cloud phses, d b is the effective bubble dimeter, D is the ir diffusivity through the ctlyst, g is the ccelertion due to grvity, mf is the voidge t minimum fluidiztion, nd br is the rise-velocity of single bubble in the bed nd is given by (Froment & Bischoff, 990). / y.7( gd ) O yo b yo (7) br 0 b Accordingly, the interchnge hettrnsfer coefficients between the bubble nd emulsion phses my be expressed s H mf (8) be Hce Hbc with (unii & Levenspel, 99) / / mf Go pg 5.85( GoGo pg) g Hbc d 5/ 4 Estimtion of kinetic prmeters, (9) feedstock nd b db ctlyst properties: he kinetic prmeters nd where Go is the density of the gs mixture, pg is the specific het cpcity of the gs mixture, Go is the therml conductivity of the gs mixture, H ce is the het-trnsfer coefficient between the cloud nd emulsion phses, nd H bc is the het-trnsfer coefficient between the bubble nd cloud phses. By compring the expressions for bc nd H bc bove, corresponding expression for H ce my be obtined through the expression for ce s H ce 6.77 Go he flow velocities in the nlysis re relted in the form (Froment & Bischoff, 990). b mf br where mf is the minimum fluidiztion velocity in the emulsion-phse of the regenertor, nd is the superficil inlet-ir velocity into the regenertor. he exit concentrtions of the flue gses nd exit tempertures from the emulsion nd bubble phses re given s (Dgde, 009) where y y O O e y y e O b O y b y e O b O e e nd the subscripts e nd b indicte emulsionphse nd bubble-phse respectively. O re not known, nd they re functions of temperture generlly given by the Arrhenius reltionship where O), i i io exp( E / R ) is the rection rte constnt ( i, is the pre-exponentil constnt, E is io the ctivtion energy, R is the universl gs constnt, nd is the bsolute temperture. ble shows the kinetic prmeters for coke nd crbon dioxide used in this study. he dimensions of some components of typicl F / unit, nd some physicl properties of the recting Go pgmfbr species, re presented in bles 4. (30) he het of 3 d b rection of the flue gses nd the vrious combustion rections were estimted using expressions presented by Hn nd hung (00). Results nd Discussion ble 5 shows the comprison between (3) plnt dt nd predictions of the models (Eqs. (7) nd (9) combined through Eqs. (3) (34) for mss frction 99

7 Am. J. Sci. Ind. Res., 03, 4(3): of exit gses, nd Eqs. (0) nd () combined through Eq. (35) for exit temperture). It my be seen from ble 5 tht the predicted dt gree resonbly well with the plnt dt. For proper ctlyst regenertion with low crbon content on the regenerted ctlyst, nd complete burning of O to O, there must be excess oxygen concentrtion of to 4 mol % (Bi et l. 998) in the regenertor which is consistent with the plnt vlue nd modelprediction in ble 5. Although crbon monoxide ws not detected in the flue gs of the plnt dt (ble 5), the model-predicted 5.7 mol.% concentrtion of crbon monoxide in the flue gs is recommended for combustion in the O boiler to generte superheted stem nd energy for the plnt. Figure shows plots of mole frction of the flue gses ginst dimensionless time, indicting tht the concentrtion of oxygen increses rpidly with time to mximum vlue of corresponding to dimensionless time of 0.05, nd then decreses to 0.03 t corresponding dimensionless time of unity. We note in Fig. tht initil concentrtions of oxygen, crbon monoxide, nd crbon dioxide were chosen (Hn & hng, 00) in order to simulte the models. hus, the initil rpid increse in the concentrtions of theses gses my indicte tht the chosen initil concentrtions of the gses re less thn their ctul concentrtions t the beginning of coke combustion. However, s the coke combustion process progresses, oxygen from the inlet ir rects with coke on the spent ctlyst to produce crbon monoxide nd crbon dioxide ccording to the combustion rections () nd (), so the concentrtion of oxygen in the exit gses decreses with time while the concentrtions of crbon monoxide nd crbon dioxide increse with time. But the concentrtion of crbon monoxide in Fig. increses to mximum vlue nd then decreses from the mximum vlue with time. Although the decrese in concentrtion of crbon monoxide with time fter the mximum vlue my be due to its conversion to crbon dioxide, this conversion process my not be ttributed to the presence of significnt quntity of oxygen in the regenertor since oxygen lso decreses with time during the period of crbon monoxide depletion. Hence, the decrese in concentrtion of crbon monoxide with time fter the mximum vlue my be due to the contribution of combustion promoter which sustins nd speeds up the rection of crbon monoxide to crbon dioxide. Figure 3 shows the vritions of regenertortemperture nd quntity of coke burnt with dimensionless time. he coke on spent ctlyst is burnt continuously leding to its decrese from n initil vlue of 0.05wt.% to bout 8wt.% (t dimensionless time of unity). he regenertortemperture increses grdully from n initil vlue of bout 797 to 000 (t dimensionless time of unity). he increse in regenertor-temperture with time is due to ccumultion of het produced by the exothermic coke combustion rections tking plce in the regenertor. Figure 4 shows the influence of inlet-ir velocity on the mole frction of the flue gses (i.e. oxygen, crbon dioxide, nd crbon monoxide) in the regenertor. It my be seen from Fig. 4 tht the mole frctions of both oxygen nd crbon monoxide decrese s the inlet-ir velocity increses, while the mole frction of crbon dioxide initilly decreses slightly from vlue of 0.49 t 0.5 m/s to 0.38 t 3.5 m/s, nd then increses to vlue of 0.68 t 8.5 m/s. he decrese in the mole frctions of oxygen nd crbon monoxide s the inlet-ir velocity increses indictes tht these gses re consumed in the regenertor nd their rtes of consumption increse s the inlet-ir velocity increses. Since the only sources for consumption of oxygen nd crbon monoxide in the regenertor re the coke combustion rections () (3), it mens the rtes of these chemicl rections increse s the inlet-ir velocity increses. his is similr to the effect of velocity of drying medium on drying process where drying rte (especilly in the constnt-rte period) increses with increse in velocity of the drying medium. As the inlet-ir velocity increses, the rte of oxygen supply to the regenertor lso increses nd the combustion rections () (3) re enhnced, so tht the mole frction of crbon dioxide is expected to increse s the inlet-ir velocity increses nd not for the mole frction of crbon dioxide to decrese s the inlet-ir velocity increses to 3.5 m/s s obtined in Fig. 4. hus, the initil decrese in concentrtion of crbon dioxide suggests tht both crbon dioxide nd crbon monoxide re not produced in the regenertor by the combustion rections for inlet-ir velocity less thn 3.5 m/s. In other words, combustion of coke nd crbon monoxide do not tke plce in the regenertor t inlet-ir velocity less thn 3.5 m/s, such tht the minimum inlet-ir velocity required to initite combustion rections in the regenertor my be tken to be 3.5 m/s. For proper ctlyst regenertion nd complete burning of crbon monoxide to crbon dioxide, the inlet-ir velocity must lwys be in excess of the theoreticl or exct mount required (NPHR, 987). Air velocity must 300

8 Am. J. Sci. Ind. Res., 03, 4(3): be djusted to ensure proper coke combustion nd ctlyst regenertion without fter-burning or behind-in-burning rections. Figure 5 shows the effect of inlet-ir velocity on the regenertor-temperture nd quntity of coke burnt, indicting tht the quntity of coke burnt increses s the inlet-ir velocity increses. his is due to increse in the rte of coke combustion rections resulting from corresponding increse in the rte of oxygen supply to the regenertor s the inlet-ir velocity increses, which is consistent with the trend of oxygen profile in Fig. 4. he regenertor-temperture increses to mximum vlue of bout 000 s the inlet-ir velocity increses to corresponding vlue of bout 4.0 m/s, nd then remins pproximtely constnt t this mximum temperture for higher vlues of the inlet-ir velocity. At the optimum regenertor-temperture, the opertion of the regenertor is sid to hve reched totl combustion regime nd ny crbon monoxide present in the regenertor is converted to crbon dioxide. At inletir velocity bove the optimum vlue, the spent ctlyst spends less time (low residence time of spent ctlyst) in the regenertor cused by chnnelling nd by-pssing effect inherent in typicl fluidized-bed rectors (heremisinoff & heremisinoff, 984). Some portion of the spent ctlyst lso escpes with the flue gses without proper contct with the combustion ir. It is very essentil tht the coke be burned off the spent ctlyst t the sme rte s it is produced in the riser rector. his cn be chieved by mintining smll mount of excess oxygen in the regenertor bove tht which is required to burn the coke. When ll the coke is not burnt, the unit is sid to be behind-inburning with the result tht the ctlyst turns grey (Bi et l. 998) nd loses its ctivity thereby decresing the yield of desired products in the riser. o void this, the velocity of ir entering the regenertor should be incresed grdully. Figure 6 shows the effect of ctlyst-bed height on the mole frctions of the flue gses. It is obvious from Fig. 6 tht the mole frction of crbon dioxide decreses s the ctlyst-bed height increses, reching minimum vlue of bout 0. t ctlystbed height of bout 4 m; therefter, the mole frction of crbon dioxide increses slightly s the ctlyst-bed height increses. nlike crbon dioxide, the mole frction of crbon monoxide increses s the ctlyst-bed height increses, reching mximum vlue of 0.6 t ctlyst-bed height of bout 4m; therefter, the mole frction of crbon monoxide decreses s the ctlyst-bed height increses. he mole frction of oxygen, on the other hnd, increses continuously s the ctlyst-bed height increses. At low ctlyst-bed height, the quntity of coke to be burnt is smll nd there is sufficient mount of oxygen supply from the inlet ir (t constnt velocity) so tht ll three combustion rections () (3) tke plce in the regenertor. As the ctlyst-bed height increses, the quntity of coke to be burnt lso increses, but O combustion rection (3) decreses due to insufficient mount of oxygen to support it, so tht incomplete combustion of coke to produce O (i.e. combustion rection ()) previls in the regenertor. Hence, s the ctlystbed height increses, the concentrtion of oxygen in the regenertor increses prtly due to incomplete combustion of coke nd prtly from its constnt supply from the inlet-ir, the concentrtion of crbon dioxide decreses, while the concentrtion of crbon monoxide increses. At ctlyst-bed height of bout 4m, the concentrtion of oxygen in the regenertor is high enough to support nd sustin O combustion rection (3) to form crbon dioxide, so tht the concentrtion of crbon monoxide decreses while the concentrtion of crbon dioxide increses slightly for ctlyst-bed height greter thn 4m s obtined in Fig. 6. Figure 7 shows the effects of ctlyst-bed height on the quntity of coke burnt nd the regenertortemperture. It my be seen from Fig. 7 tht the quntity of coke burnt decreses s the ctlyst-bed height increses, which is due to increse in the quntity of coke to be burnt with constnt supply of oxygen from the inlet-ir. At ctlyst-bed height of bout 4m nd higher, the quntity of coke burnt is theoreticlly zero, mening tht lmost no coke on the spent ctlyst is burnt for this rnge of ctlystbed height (see Fig. 7). Hence, the lower the ctlystbed height, the better nd more effective the coke burning process, such tht the optimum ctlyst-bed height for regenertion of spent ctlyst my be estimted from Fig. 7 to be 3m, beyond which, negligible coke combustion tkes plce. Figure 7 lso indictes tht the regenertor-temperture increses s the ctlyst-bed height increses, reching mximum vlue of bout 50 t ctlyst-bed height of bout 3m; therefter, the regenertortemperture decreses s the ctlyst-bed height increses. he increses in regenertor-temperture s the ctlyst-bed height increses is, of course, due to the exothermic combustion rections () (3) tking plce in the regenertor. At the mximum temperture of bout 50, the opertion of the 30

9 Mole F rction of F lue G ses Am. J. Sci. Ind. Res., 03, 4(3): regenertor hs reched totl combustion regime nd ny crbon monoxide present in the regenertor is converted to crbon dioxide. he decrese in regenertor-temperture for ctlyst-bed height greter thn 3m indictes tht some of the coke on the spent ctlyst re not burnt, which reduces both the het produced by the exothermic combustion rections nd the temperture of the regenertor for the sid rnge of ctlyst-bed height s obtined in Fig. 7. It my be observed tht the mximum regenertor-temperture in Fig. 7 is higher thn its mximum vlue of bout 000 in Figs. 3 nd 5, where the difference my be ttributed to dditionl het resulting from the effect of ctlyst-bed height on the combustion rections () (3) when the inletir velocity is mintined t constnt vlue. ble. inetic prmeters for coke burning (Morley & De-Ls, 987). Rection oke combustion O ctlytic combustion Pre-exponentil Activtion constnt energy (kj/kmol) m kmol s (m 3 ) kmol 0.5 s kg ble. Dimensions of some components of F unit (NPHR, 987). omponent Height (m) Dimeter (m) Regenertorrector Riser-rector.9.9 Prticle seprtor vessel ble 3. F operting conditions (NPHR, 987). Mteril Flow rte Inlet emperture Gs-oil kg/s 797 Air m 3 /s 370 tlyst kg/s 975 ble 4. Some physicl properties nd het of rection of recting species (Ali et l. 997).. Prmeter Vlue Density of ir.03 kg/m Specific het cpcity of ir.06 kj/kg Bubble-emulsion mss-trnsfer 0.5 s coefficient Bubble-emulsion het-trnsfer 0.84 kj/m s coefficient Bulk density of ctlyst 975 kg/m 3 Prticle size of ctlyst Specific het cpcity of ctlyst Holdup in the regenertor m.5 kj/kg kg ble 5. omprison between plnt dt (NHPR, 987) nd model predictions. Prmeter Plnt dt Model prediction Regenertor emperture () oke (wt. %) O (mol %) O (mol % ) O (mol %) R S Fig.. Hypotheticl representtion of regenertor Mole% Oxygen Mole% rbon monoxide E Dimensionless ime Dilute Phse Dense Phse ie mf Mole% rbon dioxide Fig.. Vrition of mole frction of flue gses with dimensionless time.. O Air, B ib i 30

10 Mole F rction of F lue G ses Mole F rction of F lue G ses oke B urnt (wt.% ) R egenertor emperture ( ) oke B urnt (wt.% ) R egenertor emperture ( ) Am. J. Sci. Ind. Res., 03, 4(3): oke Burnt (Wt% ) R egenertor emperture oke Burnt (wt% ) R egenertor emperture Dimensionless ime Inlet-ir velocity (m/s) 00 0 Fig. 3. Vritions of quntity of coke burnt nd regenertor-temperture with dimensionless time.. Fig. 5. Vritions of quntity of coke burnt nd regenertor-temperture with inlet-ir velocity Mole % Oxygen Mole % rbon dioxide Mole % rbon monoxide Mole % Oxygen Mole % rbon monoxide Mole % rbon dioxide Inlet-ir velocity (m/s) tlyst-bed height (m) Fig. 4. Vrition of mole frction of flue gses with inlet-ir velocity. Fig. 6. Vrition of mole frction of flue gses with ctlyst-bed height. 303

11 oke B urnt (wt.% ) R egenertor emperture ( ) Am. J. Sci. Ind. Res., 03, 4(3): oke Burnt (wt% ) tlyst-bed height (m) R egenertor emperture Fig. 7. Vritions of quntity of coke burnt nd regenertortemperture with ctlyst-bed height. ONLSION: Models which incorporte coke combustion kinetics nd hydrodynmic prmeters hve been developed for simultion of industril F regenertor. Profiles for regenertor-temperture, quntity of coke burnt, nd flue gs composition, t different operting conditions hve lso been presented. F ctlyst regenertion is very complex process nd mny fctors (solid mixing, inlet-ir velocity, temperture of spent ctlyst, crbon content on spent ctlyst, etc) influence the performnce of the regenertor. Simultion results indicte tht ctlyst-bed height nd inlet-ir velocity hve significnt effects on ctlyst regenertion process. emperture plys very importnt role in the regenertion of spent ctlyst. It is preferble to mintin n optimum temperture of the regenertor tht gives high rection rte of coke combustion nd low crbon content on regenerted ctlyst. But cution should be tken not to operte the regenertor t such low temperture tht would quench the rection or t such high temperture tht the F ctlyst becomes permnently dectivted due to stem generted from hydrogen combustion (Grry & Hndwerk, 00, Bi et l. 998). REFERENES Ahri J. S., A. Frshi nd. Forst, A Mthemticl Modeling of the Riser Rector in Industril F nit. Petroleum nd ol, 50(), 5 4 (008). Ali, H., Rohni, S., orriou, J. P. (997) Modeling nd ontrol of Riser ype Fluid tlytic rcking (F) nit. rnsctions of the Institution of hemicl Engineers, 75, Pp Arbel, A. Z., Hung, Z., Rinrd, I. H., Shinnr, R., Spre, A. V. (995) Dynmics nd ontrol of Fluid tlytic rckers-modelling of the urrent Genertion F s, Industril Engineering hemicl Resources, 34, pp Arthur, J.R. (95) Rections between rbon nd Oxygen. rnsction of Frdy Society, 47, pp Bi, D., Zhu, J. X., Yin, Y., Yu, Z (998) Simultion of F tlyst Regenertion in Riser Regenertor. hemicl Engineering Journl, 7, pp heremisinoff, N.P., heremisinoff, P.N. (984) Hydrodynmics of gs-solid fluidiztion. Gulf Publishing ompny Book Division, Houston, 984. Dgde,.. (009) Development of models for the simultion of fluid ctlytic crcking rectors. PhD hesis, Deprtment of hemicl/petrochemicl Engineering, Rivers Stte niversity of Science nd echnology, Port Hrcourt. Dvidson, J. F., Hrrison, D. (963). Fluidized Prticles, mbridge niversity Press, Newyork, 984. De-Ls, H.I., Errzu, A.F., Brreiro E., Solioz, S.(98) Anlysis of Fluidized Bedntlytic rcking Regenertor Models in n Industril Scle nit ndin Journl of hemicl Engineering, 59, pp Elnshie, S.S.E.H., Elshishini, S.S. (993) Digitl Simultion of Industril Fluid tlytic rcking nits IV: Dynmic Behviour. hemicl Engineering Science, 48, pp Errzu, A.F., De Ls, H.I.,Srti, F. (979) A Fluidized Bed tlytic rcking Regenertor Model Grid Effects, ndin Journl of hemicl Engineering, 57, pp Froment, G. F., Bischoff,. B. (990). hemicl Rector Anlysis nd Design, nd Ed.,John Willey & Sons Inc. SA, pp Gry, J.H. nd Hndwerk, G.E. (00). Petroleum Refining: echnology nd Economics, 4 th Edition. R Press. New York Guthier,., Byle, J.,Leroy, P. (000) F: Fluidiztion Phenomen nd echnologies, Oil nd Gs Science nd echnology, 55 (), pp

12 Am. J. Sci. Ind. Res., 03, 4(3): Hn, I. S., hng,.b. (00) Dynmic Modeling nd Simultion of Fluidized tlytic rcking Process. Prt : Process Modelling, hemicl Engineering Science, 56, pp Jones, D.S.J. nd Peter P. Pujdo, P.P. (006). Hndbook of Petroleum Processing, First Edition. Springer. London. rishnih D., Gopikrishn V., Awng B. nd Roslm S., Stedy Stte Simultion of Fluid tlytic rcking nit, Journl of Applied Science, 007, 7(5), pp unii, D., Levenspiel, O. (99) Fluidiztion Engineering Butterword Heinemnn, London, Morley,., De Ls, H. I. (987) On the Determintion of inetic Prmeters for the Regenertion of rcking tlyst. ndin Journl of hemicl Engineering, 65, pp Morley,., De Ls, H. I. (988) Regenertion of rcking tlyst Influence of the Homogenous O Post ombustion Rection, ndin Journl of hemicl Engineering, 66, pp NPHR, (987) New Port Hrcourt Refinery ompny: Specific ourse for Senior Stff, Are 3, Process Description, omerint. Ro, R.M., Rengswrmy, R., Suresh, A.., Blrmn,. S. (004) Industril Experience with Object Oriented Modeling F se Study, rns IchemE, Prt A, hemicl Engineerng Reserch nd Design, 8 (A4): pp Rowe, P.N., Ptridge, B.A. (965) An X-ry Sudy of Bubbles in Fluidized Beds. rnsction of Industril hemicl Engineering, 43: 57 Sdeghbeigi, R. (000). Fluid tlytic rcking Hndbook, nd Edition. Gulf Publishing, New York. Weiz, P.B., Godwin, R.B.(966) ombustion of rbonceous Deposits within Porous tlyst Prticles II, Intrinsic Burning Rte. Journl of tlysis A, 6: 7-38 Yng,W.. (003) Hndbook of fluidiztion nd Fluid Prticle Systems,R Press, London 305