Incorporating Negative Values in AHP Using Rule- Based Scoring Methodology for Ranking of Sustainable Chemical Process Design Options

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1 20 th Europen ymposum on Computer Aded Process Engneerng ECAPE20. Perucc nd G. Buzz Ferrrs (Edtors) 2010 Elsever B.V. All rghts reserved. Incorportng Negtve Vlues n AHP Usng Rule- Bsed corng Methodology for Rnkng of ustnble Chemcl Process Desgn Optons Mohmd R. Othmn,,b Jens-Uwe Repke, Günter Wozny Chr of Process Dynmcs nd Operton, Berln Insttute of Technology, Germny. guenter.wozny@tu-berln.de b Fculty of Chem. Eng. & Nt. Res., Unverst Mlys Phng, Mlys. rzz@ump.edu.my Abstrct Adoptng Anlytcl Herrchy Process (AHP) for selecton nd rnkng of sustnble chemcl process desgn tht consders the trde-off between economc fesblty, envronmentl frendlness nd socl dvntges offers good decson methodology for mult crter problem. Typcl AHP problem however re lmted to hndle only postve preferences. The ntroducton of negtve preferences nto AHP often cretes vrous contrdctng scenros tht result n spurous nd nconsstent decson. To overcome such contrdcton rule-bsed scorng methodology s proposed. The system works by ntlly defne ech ndctor ccordng to ts vlue-desrblty behvour. Next, usng specfc converson fctors tht ct ccordngly to tht behvour the ndctors vlue s converted nto credt-penlze scorng concept. Postve preference s credted wth postve vlue whle negtve preference s penlzed wth negtve vlue tht shows ts undesrblty. Usng rule-bsed pproch the score whch spn over postve nd negtve vlue re treted to elct the fnl selecton nd rnkng soluton. The functonlty of the proposed methodology wll be demonstrted to selecton nd rnkng of severl bodesel cse scenros n presence of postve nd negtve preferences. Keywords: Anlytc herrchy process (AHP), process desgn, multcrter decson mkng, 1. Motvton Fesblty of process desgn lterntves s usully nfluence by techno-economc crter. Wth the ncresng wreness of sustnblty development (D) t s mportnt to tke nto ccount the trple bottom lne of sustnblty nmely envronment frendlness nd socl dvntges long sde wth economc fesblty. These conflctng objectves pose mult crter problem n decson mkng. ome of the commonly used technques for mult crter decson mkng (MCDM) re the nlytc herrchy process (AHP), dstnce functon method nd the mult ttrbute utlty theory (MAUT). Among the three technques, AHP s the most sutble for MCDM problems (Nrynn et l., 2007). In order to evlute sustnble desgn opton, the three crter hve to be quntfed usng pproprte ndctors. Vrous ndctors hve been proposed n the pst by vrous reserchers nd orgnztons. A few exmples re proftblty nd rte of return (ROR) to mesure economc fesblty, wste reducton (WAR) lgorthm nd gs emsson to mesure envronmentl mpcts nd sfety nd hzrd nlyss for socl relted crter. Normlly, decson mkng

2 Othmn et l. usng AHP nvolve ndctors wth postve vlue wthout relzng few ndctors re constrned to be non-postve such s debts, ssets, loss nd expresson of undesrblty (Mllet & choner, 2005). ome of the ndctors mentoned bove despte hvng postve vlue my spn over negtve vlue. Clcultng proftblty for nstnce could result n negtve vlue whch ndctes losses. Other thn tht, socl ndctors could lso hve negtve preferences to express undesrblty. In ddton, the decson ndctors my lso hve dfferent or contrdctble vlue-desrblty behvour. Argubly t cn be ctegorzed to ether hghervlue-hgher-desrblty (HVHD) or lowervlue-hgher-desrblty (LVHD). Whle HVHD behvour s obvous for exmple n mesurng proftblty, LVHD refers to decson ndctors whch prefers lower vlue for exmple envronmentl ndctors.e. potentl envronment ndctor (PEI) vlue, CO 2 emsson etc. Applyng conventonl AHP n presence of the bove menton condtons Fgure 1. Flow dgrm of the method wll become chllengng nd could elct spurous nd nconsstent results unless some modfctons re mde to the AHP process. Mllet & choner (2005) mentoned two typcl pproches hve to hndle postve nd negtve vlues n the AHP. The frst s to hndle postve nd negtve vlue seprtely nd to clculte beneft to cost rto. The second pproch, whch s stndrd method, nvolves nvertng negtve vlues nto postve preferences. These two pproches however, cuse computtonl complexty nd elct nconsstent results. They then proposed new pproch clled Bpolr AHP (BAHP) tht ntroduces modfctons to AHP softwre user nterfce nd lso ts computtonl process. It provdes smple soluton to ccommodtng negtve preferences whle mntnng true zero reference pont. The pproch however s hghly softwre dependble nd t does not consder the vlue-desrblty behvour of the ndctors. In ths work new pproch s proposed usng rule-bsed scorng methodology. Ths pproch requres severl smple modfctons to the rnkng nd evluton step of AHP. The former step nvolves defnng ech ndctor ccordng to ts vlue-desrblty behvour. Usng specfc converson fctor the ndctors vlue s converted nto credt-penlze score reflectng desrblty-undesrblty. In the evluton step, tretng both postve nd negtve vlue smultneously however my crete scenros whch cn pose numercl nconsstences. Therefore, rule-bsed pproch s proposed to ccordngly tret ech scenro to elct the fnl soluton. 2. Overvew of the methodology Fgure 1 shows the overll AHP process ncorportng the proposed methodology. The modfctons ntroduced re shown n the rnkng nd evluton step (step 3 nd 4 of the fgure). As prevously menton decson ndctors my spn to postve-negtve

3 Incorportng Negtve Vlues n AHP Usng Rule-Bsed corng ystem for Prortzton of ustnble Chemcl Process Desgn Optons vlues nd n ddton my lso hve contrdctble vlue-desrblty behvour. The soluton de s to convert the ntl ndctors vlue to scorng system wth credtpenlze concept reflectng the ccommodton of both HVHD nd LVHD behvour. In ths concept ny desrble vlue s credted wth postve score whle undesrblty s penlzed wth negtve score. uch concept wll ensure tht negtve preferences re tken nto consderton n the overll score nsted of postvely mkng t to lower preferble vlue. The converson of the ndctor vlue v, nto ts correspondng score, depends on ts behvour whch cn be ctegorzed s follows, Ctegory 1: Credt score wth HVHD behvour for v 0 nd T b >0 v = ( ), > b, T >T b T (1) Ctegory 2: Penlze score wth HVHD behvour for v 0 nd T b <0 v = ( b ), < b, T <T b Tb (2) Ctegory 3: Credt score wth LVHD behvour for v 0 nd T b >0 = v T ( ), < b, T >T b Ctegory 4: Penlze score wth LVHD behvour for v = T ( ), - <- b, T >T b Ctegory 5: Credt score wth LVHD behvour for v 0 nd T b >0 v 0 nd T b <0 (3) (4) v = T b ( ) b, < b, T <T b where v s the orgnl vlue of -th ndctor for j-th crter. The ndctors vlue mrgn, T b s set from the hghest to the lowest vlue tht corresponds to score mrgn of b. In the rnkng stge (step 3.1) requres the ssessor to defne ech ndctor to ether one or two of the Ctegory 1 to 5 reflectng the ndctor s vlue-desrblty behvour. Ths s mportnt so tht the ndctors re evluted ccordngly tht reflects ts true behvour. In the evluton step (step 4) nvolves frstly determnng the rnge of T b nd ts correspondng score mrgn of b. Note tht settng the score mrgn b however depends on ts vlue-desrblty behvour. For HVHD hgh vlue s ssgn wth hgh score whle LVHD ssgn low score to hgh vlue. It s lso mportnt to note tht negtve score s gven to negtve preferences s penlzton for ts undesrblty. After the ndctors vlue hs been defned (step 4.2) step 4.3 nvolve convertng ths vlue nto ts correspondng score usng equton (1) to (5). The next step nvolves clculton of normlzed score usng the weghtng vectors determned n the weghts set-up step nd the score determned prevously. The clculton s however not strght forwrd snce the score spn over negtve nd postve vlue nd ths (5)

4 Othmn et l. cretes contrdctng scenros tht could ffect the overll result. In order to correctly response to ech scenro rule-bsed pproch s proposed. It consst sets of rules to hndle specfc scenro s follows, RULE 1: IF > 0 THEN N = 1 j ( n ) RULE 2: IF < 0 THEN N = 1 j ( n ) RULE 3: IF < 0 AND > 0 THEN N = pos 1 1 neg j ( n ) where N s the normlzed score for -th lterntve n the j-th crter, n j s the normlzed vlue for crter j. nd pos nd neg re the postve nd negtve score, respectvely. Applyng these rules helps to properly hndles vrous stuton of mx postve nd negtve scores for correct nd consstent soluton rnkngs. Adoptng the rule-bsed scorng methodology offers the opportunty for utomted decson support usng computer progrms.e. spredsheet, Vsul Bsc etc. 3. Applcton of the methodology In 2003 Zhng et l. conducted good techno-economc ssessment on four smulted bodesel processes nmely lkl-ctlyzed system usng vrgn ol (Cse 1), lklctlyzed system usng wste cookng ol (Cse 2), cd-ctlyzed process usng wste cookng ol (Cse 3) nd cd-ctlyzed system usng hexne extrcton (Cse 4). Ther smulton results concluded tht ll of these processes re proved to be technclly nd economclly fesble but ech hd ts lmttons. Ther conclusons however re lmted to only techno-economc crter nd furthermore, no selecton nd rnkng of lterntves methodology were ppled. To test the functonlty of the proposed methodology the four bodesel processes wll be used s cse study n ths work. As the work focuses only towrds techno-economc ssessment ddtonl work on envronment nd socl ssessment hve been performed utlzng the dt nd revews ncluded n ther work. Note however, for the envronmentl ssessment the effect of energy usge ws not consdered. The lsts of ndctors nd ts ctegorcl behvor re shown n Tble 1. Ths tble lso ncludes the rnge of ndctors vlue nd ts correspondng score used n ths work. A spredsheet progrm hs been developed embeddng ll the equtons menton erler to ssst decson mkng. The user only needs to provde the dt or prmeters s n Tble 1 nto the spredsheet. The results re shown n Fgure 2. The proposed rule-bsed scorng methodology ws ble to recognze the postve-negtve vlues nd perform overll rnkng of lterntves (6) (7) (8)

5 Incorportng Negtve Vlues n AHP Usng Rule-Bsed corng ystem for Prortzton of ustnble Chemcl Process Desgn Optons ccordng to the normlzed score prortes. Fgure 1 shows the segregton of the results ccordng to the three sustnblty crter. In economc evluton Cse 3 nd 4 re the most promsng. Although ll optons hve negtve net nnul proft but bsed on fter-tx rte of return nd brek-even prce of bodesel, the cd-ctlyzed processes (Cse 3 nd 4) were economclly compettve lterntves compred to the lkl process systems. These results re consstent wth the work from Zhng. Envronment ssessment shows sgnfcnt dfference of envronmentl performnce between dfferent desgn systems whereby the lkl-ctlyzed processes s envronmentlly frendler thn the cd-ctlyzed systems. Ths result s contrbuted mostly by the lrge mount of methnol used by the cd-ctlyzed system for trnsesterfcton recton. Hgh mount of clcum sulphte deposted lso contrbute to the hgh mpct to the envronment. Among cses n ech system however the dfference s smll. Tble 1. ummry of the ndctors vlues nd the score converson specfcton Indctors Ct. Cse Cse Cse Cse core Converson Vlue, T b core, b T T b b Econ. Net nnul proft, ($x10-6 ) After-tx rte of return, % Brek-even prce, $ Env. frendlness Totl rte of PEI output Totl PEI output/product Totl rte PEI gen Totl PEI gen./product ocetl fety durng operton Operblty of the plnt fe strt-up nd shutdown Ft for purpose Desgn should meet locton specfc demnds Control of product qulty nd quntty Mntennce () (b) Fgure 2. AHP results () normlzed ndvdul score (b) overll normlzed rnkng result

6 Othmn et l. ocl crter show lmost smlr score for Cse 1, 3 nd 4. The fct tht Cse 2 ws the most complex process wth the gretest number of equpment becuse of the ddton of pretretment unt for free ftty cd removl (Zhng et l., 2003) mke t the lest preferble. The overll rnkng of the four bodesel processes s shown n Fgure 1b. Introducton of penlzton concept mkes lkl-ctlyzed processes re less preferble thn the cd-ctlyzed systems mnly becuse of ts huge economc dsdvntges. The most sustnbly fesble desgn opton s Cse 3. Although envronmentlly unttrctve but tkng nto consderton the trde-off between the two other crter t perform better the others. However, t s mportnt to note tht the decson results usng AHP s very senstve. Any modfctons mde ether to the process models or weghts n the AHP weghts set-up step could sgnfcntly ffect the decson outcome. Overll, from the results obtned t shows the proposed methodology s ble to successfully consder both desrblty-undesrblty n desgn ssessment, whch cnnot be done n the conventonl AHP. 4. Conclusons & future work A rule-bsed scorng methodology hs been proposed to hndle postve nd negtve preferences n process desgn selecton nd rnkng. The functonlty of the pproch hs been successfully demonstrted through the use of four bodesel desgn optons. It ble to correctly evlute the credt-penlze score nd provde consstent result despte presence of vrous contrdctng scenros. The useful feture of ths methodology s tht the modfcton took plce t the numercl clculton step of AHP specfclly t the rnkng nd evluton step thus enble decson mkers to focus more on the rel ssues n process desgn development. Potentlly, such pproch offers the opportunty for utomted decson support e.g. usng spredsheet. 5. Acknowledgements The uthors would lke to thnks Unverst Mlys Phng, Berln Insttute of Technology nd The Mlysn Government for sponsorng ths reserch. References [1] D. Nrynn, Y. Zhng, M.. Mnnn, 2007, Vol 85 (B5) [2] I. Mllet, B. choner, 2005, Computers & Opertons Reserch, Vol. 32, pp [3] Y. Zhng, M.A. Dub, D.D. McLen, M. Ktes, 2003, Boresource Technology, 89, pp