Computer-Aided Design of Industrial Catalytic Reactor

Transcription

1 Internatonal Journal of Engneerng and echnology olume o., January, omputer-ded Desgn of Industral atalytc eactor Wordu,., Igonekwu, L.I Department of hemcal/etrochemcal Engneerng, Unversty of Scence and echnology, kpolu, ort Harcourt, gera. BS hs research attempt to develop computer program used to montor the complex seres-parallel reacton process for the upgradng of percent naphtha-lump feed n reactor desgn processes. he desgn equaton developed, calculates the length of the tuular reactor requred to otan a specfed converson. he resultng soluton yelds the converson profle, temperature profle and reactor resdence-tme as a functon of tue length. he followng functonal parameter otaned; space-tme, Space-velocty, length of the reactors, volume of the reactors, pressure effect and heat generated per unt volume of the reactors. nally, the results otaned were enchmarked wth ndustral plant data, and they les wthn the ndustral model-constrants. Hence the desgn work meets the ndustral reactor operatng condtons. eywords: omputer-ded Desgn, omplex Seres-arallel eactons, lug-flow eactor, aphtha-lump feed.. IODUIO eactor and reacton engneerng plays a vtal role n petroleum and chemcal processng ndustres. he purpose of ths work s to acquant the reader wth the nteracton etween reactor desgn/selecton and the characterstcs of the chemcal reacton of nterest. eactor selecton and desgn are the ass of economcal and safe operaton. hemcal reactons n petroleum refnng nclude a huge spectrum of unque propertes. hs ncludes how the reactants are contacted, whether a catalyst s used, how much heat s evolved or asored, and how fast the reacton takes place (aylor and rancs, 6). ommercal refneres represent huge nvestments n captal. Small errors n equpment desgn and szng translates to mllons of dollars n unnecessary expense. Hence, t s extremely mportant to do the est jo possle on desgn, szng, modelng and specfyng reacton condtons for petroleum refnng reactors (onson, 9). petroleum and petrochemcals, pharmaceutcals, specalty chemcals etc. he smultaneous developments n catalyss and reacton engneerng n 9s and 9s acted as a drvng force for the onset of ratonal desgn of catalytc reactors. hese rgorous desgn efforts, whch frmly ased on sound mathematcal prncples, n turn trggered the development of several proftale catalytc processes. In the md 9s, chemcal reacton engneerng developed nto a separate ranch of chemcal engneerng. apd expanson of petroleum and petrochemcal ased ndustres n 95s and the 96s fueled the growth of catalytc reactors. Dengh proposed the concept of deal reactors and explaned the mportance of reacton knetcs on reactor performance (Dengh, and uner, 98). Scholarly contrutons from several emnent researchers lke Dengh, Danckwarts, Levenspel, Honghen and Watson lad the foundaton of catalytc reactor modelng. Several alternatve reactor confguratons were examned for conductng ndustrally mportant catalytc reactons.. MEILS D MEHOD atalyst reactons and reactors have a wde spread applcatons n the producton of chemcals n ulk, eactons ale.: netc arameters of aphtha lumps feed ctvaton energy (E ) kj/kmol he followng functonal data were mathematcally determned usng approprate computatonal procedure and ench-marked wth ndustral plant data. re-exponental constant (o) H kg/kmol of H lerated Heat of reacton onverson of aphthenes to romatcs 75.9x ISS: 9- IJE ulcatons U. ll rghts reserved.

2 Internatonal Journal of Engneerng and echnology (IJE) olume o., January, onverson of araffns to aphthenes x 8-85 onverson of aphthenes to araffns x 5 85 ale.: olume of atalyst ale.6 eactor Operatng arameters eactors olume of catalyst (M ) eactor 6. eactor 7. eactor 65.6 arameters emperature of the reactor ressure of the reactor 5 O 98kpa H /H molar recycle rato.77 Molecular mass of feed aphtha alue.87kg/mol ale.: atalyst Dstruton n the reactor Weght Of atalyst (kg) o of drums 8, 7 5, eed flow rate eed flow rate 6797kg/hr 59.8 kmol/kg eed densty 766.6kg/m olumetrc flow rate atalyst dstruton n the reactors m /hr ::5, 6 Dynamc vscosty.9entpose ale.5: eed haracterzaton eed to reactor eed component eed composton mol% araffns.78% aphthenes 5.78% romatcs.% he prncples of energy and materal alance of process plant desgn were adopted to the functonal depletng pathway of the followng knetc sequence as follows: Dehydrogenaton, Hydrogenaton, Isomersaton, Decyclsaton and eventual H crackng nto,,, and. In a plug flow reactor desgn, reactants are fed to the reactor at the nlet and the products are removed from the reactor outlet. he reactng mxture s assumed to move as a plug and ts propertes are assumed to e unformly dstruted across the cross-secton of the reactor mantanng the fundamental mass alance equaton. elow: mass of mass of enterng leavng the volume dv the volume dv per unt tme per unt tme mass of mass of dsappearng accumulated y reacton wthn the volume dv wthn the volume per unt tme per unt tme (.) Equaton. s mathematcally transformed nto equaton. as, Smplfyng d r dv (.) onsderng, catalyst weght dw r r dv (.) (.) Equaton. s the requred desgn equaton for the computer-aded desgn of ndustral catalytc reactor studes. It s therefore sujected to catalytc reformng whch s a contnuous crackng catalytc process carred out n the presence of platnum ased catalyst at a pressure of 98a and temperature of 5 o.. ate Law he rate law follow power law on the rate of depleton or dsappearance of feed component to form the product reformate through the reactons to n the reactor, mantanng frst-order knetcs, constant catalyst actvty and operatng at steady-states process. ISS: 9- IJE ulcatons U. ll rghts reserved.

3 Internatonal Journal of Engneerng and echnology (IJE) olume o., January, r H (.5) dw r (.) r H (.6) Integratn g equ aton (.) r r 5 6 (.7) (.8) W dw X r (.) Where -6 are the varous rate constants for the ndvdual reactons. hus n general: W X r (.) Ea Exp (.8a) W X r (.). ate ontrollng Step In chemcal knetcs, the rate or velocty of a reacton wth several steps s often determned y the slowest step, known as the rate determnng step (Zumdal, 5). herefore a rate determnng step s the step n a multstep chemcal reacton wth the slowest rate of reacton. hs step s rate wll set the overall rate of reacton for the mult-step reactons. hs step s manly determned expermentally and t s eng employed extensvely n the real desgn of the relevant equpment necessary for such reacton process. In catalytc reformng, the most mportant reacton s the transformaton of the cychoalkane (aphthene) nto aromatcs. It s a rapd reacton whch approaches equlrum n a very short tme. ycloalkane formaton (.e. paraffn somerzaton) s the slowest step. Once ths compound have eng formed dehydrogenaton to enzene, methyl enzenes (toluene) and dmethly enzenes (xylene) s very rapd (Smth, 959). In the course of ths desgn work, the prncple of rate, controllng step, whch s the dehydrocyclzaton reacton (paraffn somerzaton) step, wll e adopted for the real desgn snce t s the slowest step. rom the desgn equaton we have: dw r e arrangng equaton (.9) (.9) But reactor volume s manly the catalyst weght dvded y the ulk of the catalyst densty thus, W omnng equaton (.) and (.) or the paraffn somerzaton r In term of r p p O X nf rom deal gas equaton omnng converson ( x) r H equaton (.7) and ( x) (.8) H (.) (.5) (.6) (.7) (.8) ISS: 9- IJE ulcatons U. ll rghts reserved.

4 Internatonal Journal of Engneerng and echnology (IJE) olume o., January, r ( x) But y (.) omnng equaton (.9) & (.) r O y ( x) uttng equaton (.) n to (.5) O y ( x) H ( x) H (.9) (.) X y p ( x) y ( x) H (.) he shape of the reactor s cylndrcal n nature therefore equatng the volume of the reactor to that of cylnder we have D L omnng equaton (.) and (.7) (.) D L y y H X ( x) (.) L D y y H X ( x) (.5) Just as the reacton tme t s the natural performance measure for a atch reactor so are the space-tme and space velocty the proper performance measures of flow reactors: (Levenspel, 7). hese terms are defnes as follows: tme requred to process one reactor space tme ( ) volume of feed measured at specfed condtons o omnng equaton (.) and (.6) (.6) O H X ( x) (.7) Space velocty ( S) umer of condtons reactor volume of feed whch can e treated at specfed n unt tme S S O H Xf ( x) (.8) (.9) ISS: 9- IJE ulcatons U. ll rghts reserved.

5 Internatonal Journal of Engneerng and echnology (IJE) olume o., January, Holdng me (t) mean resdence tme of flowng materal n the reactor. Q ( Hr ) X (.) or constant densty systems (all lquds and constant densty gases) t (.) By nducton prncple q Q H r X (.) Heat Generated er Unt olume of the eactor s the nvolvement of heat n the desgn of the reactor, and from operatonal of pont vew, heat generated along the plug flow reactor s a functon of, Heat of reacton of the speces, feed flow rate, converson term Mathematcally stated as, q H r X (.) Equaton (.) s the requred heat generated per unt volume of the reactor. omnng equaton. and. q Hr X H X f ( x) (.). ressure Effects ressure s a determnant factor n reactor desgn. It s very mportant n characterzng the flow of reactng flud n a reactor, partcularly plug flow reactors. ressure drop s a functon of:. rctonal factor. lud densty. low velocty v. netc rate data or varale v. Length of the reactor eactor dameter and hence, mathematcally Where f gd L (.9) = ressure drop along the reactor length = lud velocty D = Dameter of the reactor L = Length of reactor f = rctonal factor Interestngly f depends on the flow regme and otan n reference lterature. where, f or e flow Where:..6 (e) QD through H ppes D H = hydraulc dameter of the ppe; Q = olumetrc flow-rate (m /s) = pe cross sectonal area (m ) = nematc vscosty of the flud omnng equaton.9 and.8 (.) (.) f gd D H X ( x) (.) ISS: 9- IJE ulcatons U. ll rghts reserved. 5

6 ractonal converson ractonal converson Internatonal Journal of Engneerng and echnology (IJE) olume o., January,. ESULS D DISUSSIO he followng fgures.,.,.,.,.5, and.6 of results are presented and dscussed accordngly olume of reactor (m ) gure.: lot of fractonal converson on volume of reactors rom the tale of volume of reactors,, aganst the fractonal converson, t shows that ncrease n the converson of the naphtha feed rngs aout ncrease n the reactor volumes necessary for ts reacton. he ncreasng volume n the dfferent reactors depends on the catalyst loadng n each of them, snce the catalyst dstruton among the three dfferent reactors ncorporated to one are dfferent. But n general oservaton, there s a general ncrease as the converson ncreases, thus at 9% converson whch s the desgn converson lmt, we have the mnmum sze of the reactor that can gve us realstc output. Length of reactor (m) g.: lot of fractonal converson aganst length of reactors ISS: 9- IJE ulcatons U. ll rghts reserved. 6

7 ractonal converson ractonal converson Internatonal Journal of Engneerng and echnology (IJE) olume o., January, rom the tale of length of reactors,, aganst the fractonal converson, ncrease n the converson of the reactant rngs aout ncrease n the length of the reactor requred for ts converson. rom oservatons the change n the length of the reactors n the three dfferent reactors are dfferent ut very close, proaly ecause of dfferent operatng condtons such as catalyst loadng and volume of the reactor. t aout 9% converson of the reactant we have 6.m, 5.96m and.56m, respectvely for the reactors,,, these desgn values represent the mnmum sze of reactor whch at the gve operatng condton produces a realstc and expected product. Space me g.: plot of fractonal converson aganst space tme Space tme s the tme necessary to process one reactor volume of feed gven a partcular set of entrance condtons. hs parameter s otaned y dvdng the reactor volume y the volumetrc flow rate enterng the reactor. rom the tale of space tme aganst the fractonal converson of the feed component, ncrease n the converson of the naphtha feed rngs aout mnmal ncrease n the space tme of the three dfferent reactors ncorporated together as one unt, ut the change n space tme as a result of change n converson are dfferent ecause of the catalyst dstruton wthn the three reactors, of course the reactor wth the hghest catalyst loadng have the smallest space tme. Space velocty g.: plot of fractonal converson aganst space tme ISS: 9- IJE ulcatons U. ll rghts reserved. 7

8 ractonal converson ractonal converson Internatonal Journal of Engneerng and echnology (IJE) olume o., January, Space velocty s the numer of reactor volume of feed at specfed condtons whch can e treated n unt tme. hs parameter s a recprocal of the space tme. rom the tale of space velocty aganst the fractonal converson, ncrease n the converson of the naphtha feed rngs aout decrease n the space velocty n the three dfferent reactors ncorporated as one unt. hs result complements the relatonshp etween the space velocty and space tme. ressure effect g.5: plot of fractonal converson aganst pressure effect on the system rom the tale of result of pressure effect as a functon of fractonal converson, t s seen clearly that generally the pressure generated wthn the three reactors ncreases as the fractonal converson s ncreasng, at aout 9% converson, the pressure effect wthn the reactors attans the desgn value. rom the graph, t shows that the pressure effect depends on fractonal converson.e the amount of feed converted to product, t then means that at hgher pressures, more of the reactant s eng converted to product. Wthn the range of % to % conversons, the pressure effect was relatvely nfntesmal, ut a sharp response n pressure effect s eng oserved from 5% converson and aove. Heat generated per unt volume of reactor g.6: lot of fractonal converson aganst heat generated per unt volume of reactors ISS: 9- IJE ulcatons U. ll rghts reserved. 8

9 Internatonal Journal of Engneerng and echnology (IJE) olume o., January, rom the tale of results of heat generated y the reactors per unt volume of the reactor as a result of change n fractonal converson of the feed components, t wll e clearly seen that reactor and reactor generated a negatve heat effect and reactor has a postve heat effect, these effects s a functon of the chemcal reactons takng place wthn the system. In the reactors and, much heat s consumed resultng n a negatve heat effect whle n the reactor ; heat s eng generated resultng to a postve heat effect wthn the reactor. rom the graph of heat effect as a result of fractonal converson, the graph show clearly that the level of heat generated or consumed depends so much on the level of feed conversons.. OLUSIO Over the past two decades much progress has een made on the theory and practce of knetc lumpng, largely n response to ncreasngly strngent requrements for fuels/lues qualty, crude slate flexlty and reactor control and optmzaton. Several new approaches have een developed for constructng roust process desgns used n petroleum refnng. In certan areas, hgh resoluton desgns have proved ther worth y makng molecular management realty. he desgn of a typcal catalytc has eng carred out nvolvng most of ts key consttuent unts. netc modelng of the reactons occurrng n the ed of the reactors connected n seres formed the most sgnfcant part of the overall smulaton effort. reactor for the reacton schemes nvolvng aout pseudo-component connected together y a network of reactons n the c 5- c range has eng desgned consderng the necessary operatng condtons and parameters. he desgn method was parameterzed y ench markng the knetc model aganst plant data. he research utlzes the fundamental prncples of materal component alance approach to formulate the requred steady state desgn equatons. he desgn parameters were analyzed and solved numercally wth the help of mat-la software. he desgn result also showed that catalyst has a greater nfluence on the rate and speed of the reacton mechansm and equpment desgn n general. OMELUES he molar flow rate of the feed at the nlet (moles/tme) he molar flow rate of the feed at the ext (moles/tme) olumetrc flow rate at the nlet (volume per tme) olumetrc flow rate at the ext (volume per tme) -r ate of reacton of the feed araffn hydrocaron aphthene hydrocaron romatcs hydrocaron H Hydrogen G Hydrocaron gases ate constant of forward reacton n the aromatzaton reacton ate constant of ackward reacton n the formaton of aphthenes ate constant of forward reacton for the dehydrocyclzaton reacton ate constant of ackward reacton n the formaton of paraffns 5 ate constant for aphthene crackng 6 ate constant for paraffn crackng -r ate of romatzaton reacton -r ate of dehydrocyclzaton reacton -r ate of aphthene crackng -r ate of paraffn crackng Molar concentraton of aphthene Molar concentraton of romatc O re-exponental constant E a ctvaton energy Gas constant emperature () X I onverson term for spece W Weght of catalyst atalyst densty ISS: 9- IJE ulcatons U. ll rghts reserved. 9

10 Internatonal Journal of Engneerng and echnology (IJE) olume o., January, D Dameter of the reactor L Length of the reactor Space tme S Space velocty t Holdng tme q Heat generated per unt volume of reactor H Heat of reacton ressure drop enold numer Densty of flud lud flow velocty rctonal factor Q olumetrc flow-rate pe cross sectonal area nematcs vscosty EEEES [] gular.e., ncheyta J.J. 99, ew rocess Model roves ccurate n ests on atalytc eformer, Ol Gas, vol 5, p 8-8. [] nsar,. M., ade, M.O. (998). onstraned nonlnear multvarale control of a catalytc reformng process. ontrol Eng ractce. 6,695. [] I ales, Selected values of propertes of hydrocarons and adatcd compounds, esearch roject; 975, hennodynamc eseards ast., exas& Unv. [] Bednarova L., Lyman.E., ytter E., Ho]men., J. atal, vol, p 5. [5] Bolz, Grazan, Harand M, Ozawa, Sorensen M. adal-low eactors ontanng Multple atalyst Beds, Especally for aphtha eformng and Upgradng Int. ppl. WO 998. [6] lorence E, hrstelle., atrce M. 5, atalytc ropertes n n-heptane eformng of t- Sn and t- Ir-Sn/ L O atalysts repared y Surface edox eacton, J. pp. atalyss : vol 95, p [7] lu,., Sn, L, hu, J. (). he revew of research on catalytc refonnng unt smulaton. ontrol and Insrwnents to hem. Intl. 9,,9. [8] ogler S.. 997, Element of hemcal eacton Engneerng, nd Edton, rentce-hall of Inda rvate Lmted. [9] roment, GE, Bscholl. (99) hemcal eactor nalyss and Desgn. Edema, ew ork: W. [] roment; GE(9*. he netc of omplex atalytc eactons, hem, Eng. Sc,,5,7. [] Gates B.., atzer J.., Schult G , hemstry of atalytc rocesses, McGraw-Hll ook o. ew ork, p 8. ISS: 9- IJE ulcatons U. ll rghts reserved.