retrofit of complex and energy intensive processes (1997)

7/29/2019 Retrofit of Complex and Energy Intensive Processes (1997)

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~ P e r g a m o nCompute~" chem. En gng, Vol. 21, Suppl., pp. $517-$522, 1997

1997 Elsevier Science LtdA l l r ig h ts r ese r v edPrin ted in Gre at Br itainPII:S0098-1354(97)00101-4 0098-1354/97 $17.00+0.00

R e t r o f i t o f c o m p l e x a n d e n e r g y i n t e n s i v e p r o c e s s e sA n i t a K o v a 6 K r a l j a n d P e t e r G l a v i 6

F a c u l t y o f C h e m i s t r y a n d C h e m i c a l E n g i n e e r i n g , U n i v e r s i t y o f M a r i b o r ,S m e t a n o v a 1 7 , M a r i b o r , S l o v e n i a

P h o n e : + 3 8 6 6 2 2 2 9 4 4 5 5 F a x : + 3 8 6 6 2 2 2 7 7 7 4A b s t r a c t - Sequent ia l s t ruc tura l and pa ramete r opt imiza t ion of re t rof i t t ed complex and energyintens ive cont inuous processes has been s tudied . A method for sequent ia l opt imiza t ion ofre t rof i t s , c o m b i n e d s e q u e n t i a l a p p r o a c h has been deve loped us ing p inch ana lys i s , an improvedop t i mi z a t i on p roc e dure a nd mi xe d i n t e ge r non l i ne a r p rog ra mmi ng (MINLP ) o r non l i ne a rprogramming (NLP) algori thms. Pinch analysis gives many al ternat ive re trofi t designs forpos tu la t ing a supers t ruc ture . The supers t ruc ture , ma te r ia l and energy f low ra tes have beenopt imized sequent ia l ly by a d i rec t sea rch method us ing ASPEN PLUS s imula tor wi th energy andmate r ia l bounds . The hea t exchanger ne twork of the supers t ruc ture obta ined, f la shes andcom pressor were opt imized s imul taneous ly wi th the M INL P or NL P a lgor ithms. W e haverea l ized an addi t iona l prof i t of 2 ,803 MUSD/yr wi th the combined sequent ia l approach in acase s tudy of an exis t ing methanol p lant .

1 . P R O C E S S R E T R O F I T SRetrof i t des ign demands s imul taneousmate r ia l and energy f low ra tes opt imiza t ionbut i t i s not ye t achievable wi th themathemat ica l me thods ava i lable for rea lp roc e s se s . W e ha ve de ve l ope d a me t hod t oopt imize re t rof i t s in complex and energyintens ive cont inuous processes , the combinedapp roa ch (Kova~ and Glavi~, 1995) but i thas been l imi ted to energy re la tedopt imiza t ion us ing r igorous mode ls of processuni t s but f ixed mate r ia l f lows . The combinedapproach was based on the pr inc ip les ofthe rmodynamics us ing a lgor i thmic opt imiza t ionme t hods .

W e ha ve e x t e nde d t he c ombi ne d a pp roa c h t osequent ia l s t ruc tura l and pa ramete ropt imiza t ion of re t rof i t s , the c o m b i n e ds e q u e n t i a l a p p r o a c h . T h e a p p r o a c h p e r f o r m s asequent ia l re t rof i t of the supers t ruc tureobtained with the pinch analysis (used in thec ombi ne d a pp roa c h ) , u s i ng t he AS P EN P LUSmate r ia l f low opt imiza t ion and MINLP orNLP algori thms.2 . C AS E S TUDYThe sugges ted combined sequent ia l approachhas been te s ted on an exist ing , complex, low-pressure Lurgi me thanol p lant (F ig . 1) , thesame plant a s was used for the combinedappro ach (Kova~. and Glavi ( : , 1995).2 . 1 . S U P E R S T R U C T U R E O F R E T R O F I T

The ana lys i s of the combined approach gavei n fo rma t i on on none c onomi c a l c oo l i ng o f t hesynthes i s gas ; consumpt ion of the hea t f lowra te in coole r C2 could be lower and s teamproduc t ion in B1 could be h igher (F ig . 1) .The exis t ing hea t f low ra te in the coole r C2is not in tegra ted enough wi th the process ,a bou t 1 MW of he a t f l ow ra t e i s be i ng l o s tin cool ing wa te r ( the resul t comes f rom thepinch analysis). We have postulated twosupers t ruc tures . The f i rs t one which consumedthe surplus hea t f low in C2 had threea l te rna t ives . The second supers t ruc ture whichused the cooling of the synthesis gasconta ined three a l te rna t ives .2 . 2 . M I N L P / N L P O P T I M I Z A T I O NThe op t i mum s t ruc t u re o f t he s e l e c t e d he a texchanger ne twork, f la shes and compressorwe re ob t a i ne d wi t h AS P EN P LUS a nd t une dwi t h MINLP us i ng GAMS p rog ra m. W e ha dused a s tage-wise mode l of MINLP for theexis t ing hea t exchangers El , B1 and E2 (F ig .1) and for new hea t exchangers N, A, A1and Z w i th add i t iona l eq ua t ions for re t rof i t ofH E N ( Y e e a n d G r o s s m a nn , 1 9 9 0 a n d Y e e e ta l . , 1990) . The solu t ion of MINLP mode l hadno t a c h i ve d c onve rge nc e . The re fo re t he b i na ryvar iables were es t ima ted , the mode l wassui table for the NLP. The s tage-wise mode lhad va r iable hea t capac i ty f low ra tes for thecold s t reams and wi th them the s t ruc tures canbe chosen, too .

$517


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$ 5 1 8 P S E ' 9 7 - E S C A P E - 7 J o in t C o n f e r e n c e

r r m FI *~t

. . . . . : : . ' . ' .... ,otH ~ ml SMtt l . . . . . . .

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. J r .to II

~ , w , J -~ t t e t g , ,

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~ cPl to sxl f r e t 214 *t*~ |3I * SK~

[ ~ , 1 ,

Fig . 1 : P rocess f low diagram for a low-pressure Lurgi methanol p lant .The m ode l f o r t he r e t r o f i t wa s e n l a r ge d w i th

the exis t ing hea t exchangers E2, E3, AC1 andC1, the in te rmedia te adiaba t ic f la shes F2, F3 ,F 4 a nd F 5 a nd the c om p r e s so r C P 1. Thes t a ge - wi se m ode l u se d f o r r e t r o f i t o f HENwith va r iable hea t capac i ty f low ra tes and alo t o f va r i a b l e s wa s ve r y c om pl i c a t e d . Thec om ple x s t a ge - wi se m ode l c ou ld no t beenla rged wi th the exis t ing hea t exchangersn= E2 , E3 , AC 1 a nd C 1 , ye t. The a dd i t iona le qua t ions o f he a t ba l a nc e s f o r t he n th he a te xc ha nge r we r e :( Tini,n"Tuti.n)"CFi.n= ~,~n n= E2 ,E3 ,AC 1,C 1 1A I n T n - ( T i n . T o u t . ~ / T o u t " /i n ~ / - - ~ . I ,n " J , n / ' k - - i , n ' - - ] , n i li n ( C T m . T O U t. "~/ t 'Tout . Tin ~x ~, t , n " j , n / - ~ . - - t , n ' - - j , n j / 2A a = ~ n / ( A I n T n ' U n ) n =E2 , E3 , AC 1 , C 1 3

NAaddn>-An' .~4eXn'MOl/En,n n= E2 ,E 3,A C 1,C I 4n = lN~ _ , M O V E n , n < 1 n = E 2 , E 3 , A C 1 , C 1 5n = i

01 o l '

A ho t s t r e a m ( i ) wa s t he c oo l ing o f t hesynthes is gas in hea t exchangers E2, E3, AC1a nd C 1 . C o ld s t r e a m s ( j ) we r e t he he a t ing o fthe de m ine r a l i z e d wa te r i n E2 a nd E3 , t hea i r in a i r c oo le r AC 1 a nd the c oo l ing wa te rin t he c oo le r C I . The fi r st e qua t ion de no te sthe hea t f low ra te of the n th hea t exchanger .The s e c ond e qua t ion c a l c u l a t e s t he l og - m e a nte m pe r a tu r e d i f f e r e nc e . The th i r d e qua t ionp r e se n t s he a t a r e a o f t he n th he a t e xc ha nge r.The f ou r th e qua t ion s e l e c t s t he a dd i t i ona la r e a o f t he n th he a t e xc ha nge r. On ly oneexis t ing hea t exchanger can be se lec ted for

one loc a t ion ( f i f t h e qua t ion ) . The he a tc a pa c ity fl ow r a t e o f t he ho t s t r e a m in he a texchan ger E2 (CFi.E2) i s the s am e as th a t o fthe s t ream of synthes is gas in s tage-wisem ode l . The he a t c a pa c i ty f l ow r a t e o f ho ts t r e a m e s in he a t e xc ha nge r s E3 , AC 1 a nd C 1are a func t ion of vap ou r mass f low (FUt,Vu)a f t e r t he u th s e pa r a t ion ( u=F 2 , F 3 , F 4 a ndF 5) a nd o f t he m o la r he a t c a pa c i ty o f t hesynthes is gas ( inc luding gas and wa te r ) :CF i ,n=F ut 'Vu'Cpi , n n = E 3 , A C 1 , C 1 6

- - i n o u t i nC p i , n - [ ( C p g a s , n + C p gas,n ) . W gas , n+ C p w , n ' W w , . +CpOUt,V .,.out,v - - ( ' n o u t , I u , o u t, I 1 19 u ,w , n " w w n r U p W , l l ' r r W , / i J ~ ' r r n

n = E 3 , A C 1 , C 1 7

C p i n g a s, n = a i n g a s , n + b i n g a s , n . T u t i , n . lCpOUt - n o u t .4 . b o u t T o u tg a s , n - - - g a s , n - - ~ g a s, n ' ~ i ,nC pi n - - n i n k i n " r o u tw , n - a w , n T M w,n " 1 - i ,n-1C D O Ut ,V - - a O U t v + b O U t v T O u t1 - w , n - ' w , n ' w , n " i ,nC p o u t , I - n o u t I . 4 - b o u t I T o u tW , I I - - " ' W , l l ~ ' W , l l T M i , I I

891011

12Wn =W gas ,n+ W w ,n 13wOUt , v. . . . I ,, , 14, n - - v v w , n - A , vv I1w u t ' l w , n =XI 'Wn 15X I - - n lI - h i T o u t .- - " w , n - - v w , n " - I ,n 16

T h e m o l a r h e a t c a p a c i t i e s o f c o m p o n e n t s a r ef unc t ions o f t he t e m pe r a tu r e ( e qua t ions f r om8 to 12 ) a nd the y de pe nde d on va pour o r


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PSE '97-ESCAPE -7 Joint Conferencel i q u i d p h a s e ( e q u a t i o n s 1 4 t o 1 6 ) . T h e t o t a la m o u n t f l o w f r a c t i o n s u m s t h e f l o w f r a c t i o n so f g a s a n d w a t e r i n s y n t h es i s g a s ( E q . 1 3 ).T h e o u t l e t t e m p e r a t u r e ( 7~ u tl ) o f c o o l i n gs y n t h e s i s g a s s t r e a m i n s t a g e - w i s e m o d e l , i st h e s a m e a s t h e i n le t te m p e r a t u r e o f h o ts t rea m in he a t exch ang e r E2 (T in i.E2) , and thei n le t t e m p e r a t u r e ( 7~ n 2) o f t h e s t r e a m a f t e rt h e c o m p r e s s i o n i s n o t f i x e d i n t h e e n l a r g e dm o d e l o f r e t r o fi t b e c a u s e b o t h t e m p e r a t u r e sc a n b e o p t i m i z e d . T h e h e a t c a p a c i t y f l o w r a t eo f th e s t r e a m a f t e r t h e c o m p r e s s i o n i sv a r i a b l e s u b j e c t t o v a p o u r m a s s f l o w r a t e a f t e rt h e F 5 s e p a r a t i o n a n d t o m o l a r h e a t c a p a c i t y .T h e e x i s t i n g h e a t e x c h a n g e r s n = E 2 , E 3 a r ei n c l u d e d in t s th e e q u a t i o n s o f h e a t b a l a n c e sf o r e x i s t i n g a n d a d d i t i o n a l a m o u n t f l o w r a t e so f t h e d e m i n er a l iz e d w a t e r f o r 3 7 b a r s t e am(Fd~ and AFdw) :(TaUtj,n -Tinj,n).Cpj,n. Fo w +A Fd w )=# n

n = E 2 , E 3 1 7T h e a d d i t i o n a l h e a t f l o w r a t e o f 3 7 b a r s t e a mc a n b e p r o d u c e d i n t h e n e w h e a t e x c h a n g e rN a n d i n t h e e x i s t i n g , e n l a r g e d b o i l e r B I :A ~ 3 7 = A ~ B I + ~ / 18T h e a d d i t i o n a l a m o u n t f l o w r a te o f t h ed e m i n e r a l i z e d w a t e r f o r t h e 3 7 b a r s t e a m i s :A F d w = A ~ 3 7 / ( A n v a p + A H 2 4 4 - A H 4 5 ) 19T h e o u t l e t t e m p e r a t u r e o f E 2 ( 7~ u tj ,E 2 ) is212 C. The ex i s t i ng , i n l e t s t r eam of t hed e m i n e r a l i z e d w a t e r i n E 3 i s c o o l e d i n A C 3g i v i n g o f f 1 6 3 8 k W ; w i t h o u t c o o l i n g i t t h et e m p e r a t u r e s i n E 2 a n d E 3 a r e h i g h e r . T h ei n l et a n d o u t l e t t e m p e r a t u r e s o f t h ed e m i n e r a l i z e d w a t e r i n E 3 a r e 6 9 C a n d 1 0 9 C . T h e i nl et t e m p e r a t u r e o f t h e

$519d e m i n e r a l i z e d w a t e r i n E 2 i s v a r i e d ; 1 6 3 8 M Wo f h e a t f l o w r a t e o r l e s s w a s c h o s e n . T h em o l a r h e a t c a p a c i t y o f t h e d e m i n e r a l i z e dwa te r i n E3 i s f i xed and i t i s i nc luded in E2a s a f u n c t i o n o f t h e i n l e t t e m p e r a t u r e :Cpj.E2 = aj,E2+ bi,E2"(Tinj,E2) 20A d d i t i o n a l e q u a t i o n s f o r t h e m a s s b a l a n c e o fth e u th s e p a r a t i o n ( u = F 2 , F 3 , F 4 a n d F S ;u = l . . U ) f o r a ll th e c o m p o n e n t s in th es y n t h e s is g a s ( s = C O 2 , C O , H 2 , C I -I 4 a n d H 2 0 ;s = l . . S ) a r e :F inu= FUt'Vu+Fut 'l u = I . .U 21F i n u w i n u . s = F U t , V u w o u t , V u , + F O U t , I u w o u t , l u , ss u = l . . U s = l . . S 2 2ZwUt'Vu,s= 1 u = 1 . .U 23s S

~wut ' lu,s= 1 U= 1 . .U 24s

g u . s = d u , s + C u . s" Z U t i , n + bu,s" Tuti.n)2u = I . . U s = I . .S 2 5wUt 'vu,s=Ku,s, wUt '|u ,s; u = I . .U s= I . .S 26T h e i n le t a m o u n t f l o w r a t e f o r t h e u ths e p a r a t io n s u m s t h e o u t l e t a m o u n t f lo w r a te so f v a p o u r a n d l iq u i d p h a s e s ( E q . 2 1 ) .E q u a t i o n 2 2 i n c l u d e s t h e a m o u n t f l o wf r a c ti o n . T h e e q u i l ib r i u m c o n s t a n t o f t h e s hc o m p o n e n t i n t h e u th s e p a r a t i o n i s a f u n c t i o no f t e m p e r a t u r e .

T h e t e m p e r a t u r e a f t e r t h e c o m p r e s s o r C P 1(TUtcP1) can b e ca l cu la t ed b y the e qua t ion :Z U t c p I = a c p I + b c P l . Z u t i , c 1 27T h e o b j e c t i v e f u n c t i o n ( E q . 2 8 ) u s e s t h ep r i c e s o f 3 7 a n d 3 , 4 b a r s t e a m ( (7 3 7 , C 3 ,4 )c o o l i n g w a t e r ( C c w ) a n d f u e l ( C r u e l )r e p r o d u c e d i n T a b l e 1 .

T a b l e 1 : C o s t d a t a f o r e x a m p l e p r o c e s s .Installed costs of heat exchanger*AJSD:(8600,0 + 670-A0-S3).l,8 ; A : are a in m2Energy costs:cost of 37 bar steam ( C 3 7 ) , ' " /(USD/(kW.yr))cost of demineralized water included)cost of 3 ,4 bar steam (C3,4), " /(USD/(kW.yr))cost of cool ing water ( C c w ) , & /(USD/(kW.yr))

170,070,013,6cost of fuel (C:~d) , & /(USD/(kW.yr))cost of methanol ( C M E o t l ) , + /(USD/yr)cost o f moving h eat exchanger ++ A JSDcost of repiping ++ /USDtime fraction of operation /(h/yr)interest rate /%lifetime /),r

127,5487,5300,0400,08000,08,07, 0& A h m a d , 1 9 8 5 * T j o e a n d L i n n h o f f . 1 9 8 6 * * H u i a n d A h m a d . 1 9 9 4 + H o f f m a n , 1 9 9 5 ** Kravanja, 1990++ Ciric and Floud as, 1989

T h e o b j e c t i v e f u n c t i o n m a x i m i z e s a d d i t i o n a la n n u a l p r o f i t , w h i c h i s a d i f f e r e n c e b e t w e e nt h e a d d i t i o n a l a n n u a l i n c o m e a n d t h ea d d i t i o n a l a n n u a l i n v e s t m e n t c o s t . T h ea d d i t i o n a l a n n u a l i n c o m e s u m s u p t h eO~C$21:I]-R

a d d i t i o n a l h e a t f l o w r a t e s o f 3 7 b a r s t e a m i nh e a t e x c h a n g e r s N a n d B 1 ( ~ r + A ~ l ;6 ~ 1 = ~ t - 1 6 , 0 6 7 . 1 0 6 W ) a n d 3 ,4 b a r s t ea m i nh e a t e x c h a n g e r A 1 ( ~ A 1 ) , t h e h e a t f l o w ra t e so f c o o l i n g w a t e r i n c o o l e r C 2 ( 2, 70 4 .1 (Y W -


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s520~A) and fue l saving in hea t exchager Z (~z) .The inve s tm e n t c os t s i nc lude ne w he a texchangers , m=A, A1, Z and N, as we l l a se n l a r ge m e n t s o f t he e x i s t i ng he a t e xc ha nge r s ,n - - B 1 , E l , E2 , E3 , AC 1 , C 1 a nd C 2 ( thea dd i t i ona l a r e a o f a n e x i s t i ng he a t e xc ha nge rn i s ne w a r e a m inus t he e x i s t i ng one ; A n "AeXn). This re t ro f i t i s opt im ized in te rm s o f 7years p lan t l i f e t ime wi th 8 % in te res t r a te .The a dd i t i ona l i nve s tm e n t c os t s a r e d iv ide d bythe l i f e t ime and m ul t ip l ied by the des i red ra teo f r e tu r n ( r =0 ,2448 ) , t o ob t a in a dd i t iona la nnua l i nve s tm e n t c os t s ( Ahm a d , 1985 ) .A n n u a l P R O F I T ( x , y ) =C37.( A ~BI + r/)N)+ C 3 .4 .~A I + C cw . ( 2 ,704" l O6~A) +Cfud '~z'[4"860 0+ 670.,4 m0'83) 1,8 +7.670.(An-AeXn) .83.1 ,8] . r m = A , A 1,Z ,N

n = B 1 , E l , E 2 , E 3 , A C I , C 1 , C 2 2 8The so lu t ion o f NLP m ode l u s ing e s t im a te db ina r y va r i a b l e s f o r s t r uc tu r e a nd e ne r gypa r a m e te r op t im iz a t ion c on ta ine s t he f o l lowinghe a t e xc ha nge r s : B1 w i th 1 6215 kW o f he a t

PSE '97-ESCAP E-7 Joint Conferencef lOW ra te , E1 w i th 3044 kW , N wi th 1901kW , E2 w i th 8 41 1 kW , E3 w i th 2991 kW ,AC1 wi th 8242 kW, C1 wi th 670 kW, A wi th1 1 0 0 k W a n d C 2 w i t h 1 7 6 8 k W . T h e h e a te xc ha nge r E3 i s m ove d in t he p l a c e o f C 1 .Th e a dd i t i ona l a nnua l e n e r gy p r o f i t wa s306 200 US D~ r .2 .3 . C O M B I N E D S T R U C T U R E

The s t r uc tu r e s a nd p r o f i t s o f op t im aa p p r o ac h e d b y A S P E N P L U S a n d N L P h a v eb e e n c o m p a r e d a n d c o m b i n e d i n a n e wre t rof i t (F ig . 2) . F rom the re t rof i t ted s t ruc tureo b t a i n e d b y A S P E N P L U S t h e s p l i t te r S 1 h a sbe e n t a ke n ove r , i t ha s p r oduc e d 591 kW o f3 , 4 b a r s t e a m , t h e s a m e a s b y A S P E N P L U S .An a dd i t iona l a nnua l p r o f i t o f e ne r gyin t e g r a t ion a c h ie ve d by the c om bine d r e t r o f i to f A S P E N P L U S a n d G A M S w a s 3 13 2 0 0US D/y r . W e ha ve ob ta ine d h ighe r e ne r gysa v ing a nd a dd i t i ona l p r o f i t w i th t he c om bine ds t ruc tures .

2 4 5 , 5 * C 4 0 0 ' C 2 4 5 , 5 * C 2 1 2 JC 109"C 35"c 410C 1 : . l ~ , ~ ; E 2 ~ . r 2 c z t I r 3 A C I t 3 . . . . t ' . . . . .8 6 5. . c 4 4 0 * C 3 5 2 , 4C 311, *C 134, 5 C 126 C 62 *c 43,? c

~ ~ 1 0 6 ' c0' c

F ig . 2: Op t im a l s t r uc tu r e f o r t he H E , f l a she s a nd c om pr e so r a pp r oa c h e d by c om bine d r e t r o f i t .Table 2 : Cost ana l ]

ExchangerB1ElE2C1C3

'sis for opt im al re t rof i t obta in ed b ,Existin[ area /m2124,875~7410,0178,0728,00,0

New area /m 2130,3

0,0

77~9582r3230,2833,0A 41,4A1 0,0 70,4NCost of moving CI,E3Cost of repiping29,0

c om bine d se que lAdditional area /m25 ,52,2172,352,2105,041~470,429,0

i t ia l approach.Investment AJSD4 9642 32086 58932 13857 40341 99 356 67435 211600800

318 692Additional annual investment cost:Cooling water saving:Additional methanol production:Additional 37 bar steam production:Additional 3,4 bar steam production:Total additional annual income:A d d i t i o n a l a n n u a l p r o f i t :

78 000 USD/yr18 610 USD/yr2 504 300 USD/yr317 220 USDbyr41 370 USD/~jr2 881 500 USD/yr2 8 0 3 5 0 0 U S D / y r


5/6

PSE '97-ESCAPE -7 Joint ConferenceThe op t im um r e t r o f i t a pp r oa c h a c h ie ve d byc om bin ing e ne r gy in t e g r a t i on a nd m a te r i a lba l a nc e c a r r i e d ou t w i th AS P EN P LUS ha sinc r e a se d the p r oduc t ion o f m e tha no l by 3 , 2% r e qu i r ing a sm a l l t e m pe r a tu r e c ha nge inthe f l a sh F 6 a nd a dd i t i ona l a r e a o f t he c oo le r

$521C 3 . The be s t e ne r gy s t r uc tu r e ha s be e nob ta ine d by the s e que n t i a l u sa ge o f AS P ENP LUS a nd GAM S ( F ig . 3 ) . The ove r a l la dd i t i ona l a nnua l p r o f i t ob t a ine d w i th t hec om bine d se que n t i a l a pp r oa c h wa s 2 803 500US D/y r ( Ta b le 2 on P a ge 4 ) .

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F ig . 3 : Op t im a l r e t r o f i t a pp r oa c h f o r m e tha no l p r oc e s s f l ow d i a g r a m .3 . C O N C L U S I O N SW e ha ve u se d the s e que n t i a l s t r uc tu r a l a ndpa r a m e te r op t im iz a t ion m e tho d to r e t r o f i t ac om ple x a nd e ne r gy in t e ns ive c on t inuousp r oc e s s w i th t he c o m b i n e d s e q u e n t i a la p p r o a c h using pinch analysis, dire ct searcho p t i m i za t io n an d M I N L P o r N L P a lg o r it h m .We have re t rof i t ted the exis t ing , nontr iv ia lm e tha no l p l a n t w i th t he s e que n t i a l a pp r oa c hus ing s e r i e s op t im iz a t ions by AS P EN P LUSa nd GAM S . P l a n t s t r uc tu r e ha s c ha nge d w i ththe re t rof i t . Addi t iona l 1 ,866 MW of 37 ba ra n d 0 , 5 9 1 M W o f 3 ,4 b a r s t e a m c o u ld b eproduced, inc reased y ie ld gave addi t iona lprod uc t ion of 5137,0 t /~ r of meth ano l and1 , 368 M W of c oo l ing wa te r c ou ld b e s a ve d .The to t a l a dd i t i ona l a nnua l p r o f i t , a c h i e ve d bythe c om b ine d se que n t i a l a pp r oa c h wa s2 803 500 USDhjr .A C K N O W L E D M G E M E N T

T h e a u t h o r s w o u l d l ik e t o a c k n o w l e d g ef i n a n c i a l s u p p o r t t h e M i n i s t r y o f S c i e n c e a n dT e c h n o l o g y o f S l o v e n i a .N O M E N C L A T U R EM I N L P = m i x e d i n t eg e r n o n l in e a rp r o g r a m m i n g

H EN L PWIndices :iJmnu

= h e a t e x c h a n g e r= n o n l i n e a r p r o g r a m m i n g= w a t e r=ho t p r oc e s s o r u t i l i t y s t r e a m=c o ld p r oc e s s o r u t i l i t y s t r e a m=inde x f o r ne w he a t e xc ha nge r= inde x f o r e x i s t i ng he a t e xc ha nge r= inde x f o r e x i s t i ng s e pa r a to r

P a r a m e te r s :Aezn = a r e aacPlaingas,nainw,nalw,naUtgas,naUtw,nbcp1bingas,n

of an exis ting hea texchang er , m 2= t e m p e r a t u r e c o n s t an t f o rc o m p r e s s o r C P 1

=he a t c a pa c i ty c ons t a n t o f t he ga sf o r t h e i n l e t s t r e a m H E n= h e a t c a pa c it y c o n s t a n t o f t h ew a t e r f o r t h e i n l e t s t r e a m H E n

= l iqu id ph a se c ons t a n t o f wa te ri n H E n

= h e a t c a p a c i t y c o n s t a n t o f t h e g a sf o r t h e o u t l e t s t r e a m H E n

= h e a t c a p a c i t y c o n s t a n t o f t h ew a t e r f o r t h e o u t l e t s t r e a m H E n= t e m p e r a t u r e c o n s t a n t f o rc o m p r e s s o r C P 1

=h e a t c a pa c i ty c ons t a n t o f t he ga sf o r t h e i n l e t s t r e a m H E n


6/6

$ 5 2 2bm w .n = h e a t c a p a c i t y c o n s t a n t o f t h ew a t e r f o r t h e i n l e t s t r e a m H E nblw,n = l i q u id p h a s e c o n s t a n t o f w a t e r

i n H E nbUtgas,n = h e a t c a p a c i ty c o n s t a n t o f t h e g a s

f o r t h e o u t le t s t r e a m H E nb U t n = h e a t c a p a c i ty c o n s t a n t o f t h ew a t e r f o r t h e o u t l e t s t r e a m H E nb u , s = e q u i l i b r iu m c o n s t a n t o f s hc o m p o n e n t i n U h s e p a r t i o n

c u , s = e q u i l i b r i u m c o n s t a n t o f s hc o m p o n e n t i n u th s e p a r a t i o nd u . s = e q u i l i b r i u m c o n s t a n t o f s h

c o m p o n e n t i n u th s e p a r a t i o nK u ,s = e q u i l i b r i u m c o n s t a n t o f S h

c o m p o n e n t i n u th s e p a r a t i o nU n = o v e r a l l h e a t t r a n s f e r c o e f f i c ie n t

f o r h e a t e x c h a n g e r n , W / ( m 2 K )V a r i a b l e s :

A a d d n = a d d i t i o n a l a r e a o f n th H E , m 2A n = a r e a o f h e a t e x h a n g e r n , m ECFi,n = h e a t c a p a c i ty f lo w r a t e o f h o t

s t r e a m i i n H E n , W / KC p i . n = m o l a r h e a t c a p a c i t y f o r h o ts t r e a m i o f n th H E , J / m o l . K

C p in ga s,n = m o l a r h e a t c a p a c i t y f o r t h ei n l e t g a s i n n th H E , J / m o I . KinCp w ,n = m o l a r h e a t c a p a c i t y f o r t h ei n l e t w a t e r i n n tl~ H E , J / m o I . K

CpUtgas,n = m o l a r h e a t c a p a c it y f o r t h eo u t l e t g a s i n n th H E , J / m o l . KCpUtw,n = m o l a r h e a t c a p a c i ty f o r t h eo u t l e t w a t e r i n n th H E , J / m o I - K

F dw = a m o u n t f lo w r a t e o f d e m i n e r a l iz e d

PSE '97-ESCAPE-7 Joint Conferencew a t e r , m o l / s

F~u = i n l e t a m o u n t f l o w r a t e o fs e p a r a t i o n u , m o l / sla'Ut,vu = o u t l e t a m o u n t f lo w r a t e o f u ths e p a r a t i o n i n v a p o r p h a s e , m o l /s

F~t,tu = o u t l e t a m o u n t f lo w r a t e o f u ths e p a r a t i o n i n l i q u i d p h a s e , m o l / s

M O V E n , n = b i n a r y v a r i a b l e s f o r l o c a t i o n o fe x i s t i n g h e a t e c h a n g e r n

Tini,n = i n l e t t e m p e r a t u r e o f h o t s t r e a m ii n h e a t e x c h a n g e r n , K

Tinj.n = i n l e t t e m p e r a t u r e o f c o l d s t r e a m ji n h e a t e x c h a n g e r n , K

Tuti,n = o u t l e t te m p e r a t u r e o f h o ts t r e a m i i n h e a t e x c h a n g e r n , K

TUtj,n = o u t l e t t e m p e r a t u r e o f c o lds t r e a m j i n h e a t e x c h a n g e r n , K

w n = t o t a l a m o u n t f l o w f r a c t i o nW g a s . n = a m o u n t f lo w f r a c ti o n o f g a s i nn th h e a t e x c h a n g e rW w . n = a m o u n t f lo w f r a c ti o n o f w a t e r i nn th h e a t e x c h a n g e rw U t.V u,s = o u t l e t v a p o r a m o u n t f l o w f r a c t i o n

o f s h c o m p o n e n t i n u th s e p a r a t i o nw U t,tu, = o u t l e t l i q u id a m o u n t f l o w f r a c t i o n

o f s h c o m p o n e n t i n u th s e p a r a t i o n~ = h e a t f l o w r a t e o f n th H E , Wx t = a m o u n t f r a c t i o n o f l iq u i d p h a s eA In T = l o g - m e a n t e m p e r a t u r e d i f f e r e n c e

o f n th h e a t e x c h a n g e r , KA H = m o l a r e n t h a lp y , J / m o lA~37 = h e a t f lo w r a te o f t h e a d d i t o n a l 3 7

b a r s t e a m , W

RE F ERENCEA h m a d S ., H e a t e x c h a n g e r n e t w o rk s : C o s t t r a d e o f f s i n e n e r g y a n d c a p it a l. P h . D . t he s is ,U n i v e r s i t y o f M a n c h e s t e r , I n s t i t u te o f S c i e n c e a n d T e c h n o l o g y , M a n c h e s t e r , p p . 1 1 3 a n d 3 0 6(1 9 8 5 ) .C i r i c A . R . a n d C . A . F l o u d a s , A r e t r o f i t a p p r o a c h f o r h e a t e x c h a n g e r n e t w o r k s . Comput. chem.Engng 1 3 / 6 p p . 7 0 3 -7 1 5 (1 9 8 9 ) .

H o f f m a n J . , M e t h a n o l p r i c in g g o e s i n " f r e e f a l l" , Ch emic al Markethzg Reporter 2 4 7 / 1 2 , p p . 1 7(1 9 9 5 ) .H u i C . W . a n d S . A h m a d , T o t a l s i te h e a t i n t e g r a t i o n u s i n g t h e u t il it y s y s t e m , Comput. chem.Engng 1 8 / 8 , p p . 7 3 7 (1 9 9 4 ) .

K o v a ~ . A . a n d P . G l av i~ ., R e t r o f i t o f c o m p l e x a n d e n e r g y i n t e n s i v e p r o c e s s e s - I , Comput.chem. Engng 1 9 / 1 2 , p p . 1 2 5 5 -1 2 7 0 (1 9 9 5 ) .K r a v a n j a Z . , H e a t i n t e g r a t i o n o f c h e m i c a l r e a c t o r s i n v i e w o f f l e xi b le e n e r g y i n t e g r a t i o no f p r o c e s s f lo w s h e e t . P h . D . t he s is , F a c u lt y o f C h e m i s tr y a n d C h e m i c a l E n g i n e e ri n g , M a r i b o r ,p p . 2 8 ( 1 9 9 0 ) .

T j o e T . N . a n d B . L i n n h o f f , U s i n g p i n c h t e h n o l o g y f o r p r o c e s s r e t r o f it . C h e m . E n g n g 2 8 , p p .4 7 - 6 0 ( 1 9 8 6 ) .

Y e e T . F . a n d I. E . G r o s s m a n n , S i m u l t a n e o u s o p t im i z a t i o n m o d e l s f o r h e a t i n t e gr a t io n II . H e a te x c h a n g e r n e t w o r k s y n t h e s i s . Comput. chem. Engng 1 4 /1 0 , p p . 1 1 6 5 -1 1 8 4 (1 9 9 0 ) .Y e e T . F . , I . E . G r o s s m a n n a n d Z . K r a v a n j a , S i m u l t a n e o u s o p t i m i z a ti o n m o d e l s f o r h e a t

i n t e g r a t i o n - I . A r e a a n d e n e r g y t a r g e t i n g a n d m o d e l i n g o f m u l t i - s t r e a m e x c h a n g e r s . Comput.chem. Engng 1 4 / 1 0 , p p . 1 1 5 1 - 1 1 6 4 (1 9 9 0 ) .

retrofit of complex and energy intensive processes (1997)

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