Computational fluid dynamic modeling of a ground flare

A major ethylene plant underconstruction in Al-Jubail, Saudi Arabia found itself under pressure from neighboring facilities and regulatory agencies to reduce flare emissions, flame visibility, and noise. Plant personnel contacted a company that had pioneered and commercialized a grade mounted, multipoint flare burner system that eliminates the need for an elevated flare which historically contributes to increased noise levels, inhibited efficiency, limited service life, increased flame visibility, and increased smoke formation. Computational Fluid Dynamic (CFD) simulation of the flare system was used to aid in the design of the flare system by predicting fence temperature, grade temperature, air flow to the burners, and flame and radiation characteristics of the flare system. Installed during the plant's 2004 startup, the flare burners were designed, fabricated, and tested on a fast track schedule. Production tests and operating data show that the flare burners achieve the predicted CFD results and the required increased smokeless capacity, reduction in noise levels, decreased visibility of the flames and enhanced operational efficiency, while simultaneously resisting the failures typically associated with elevated flares.

C o m p u ta tio n a l F lu id D y n a m ic M o d e lin g o f a G r o u n d F la re B ria n D u c k , P .E . C a l l i d u s T e c h n o l o g i e s , LLC T u lsa, O K U SA E m a il: b d u c k @ c a l l i d u s . c o m ABSTRACT IN T R O D U C T IO N A m ajo r e th y len e p lan t under c o n s t r u c t i o n in A l-J u b a il, S a u d i A r a b i a fo u n d itself under pressu re fr o m n eig h b o rin g fa c ilitie s a n d re g u la to ry a g e n c i e s to r e d u c e f l a r e e m i s s i o n s , flam e visibility, and n o ise. P la n t p e rs o n n e l c o n ta c te d a c o m p a n y that h a d p io n e e re d a n d co m m ercialize d a g r a d e m o u n t e d , m u ltip o in t f l a r e b u r n e r s y s t e m t h a t e l i m i n a t e s t h e n e e d fo r a n e le v ated flare w hich h is to ric a lly c o n t r i b u t e s to i n c r e a s e d n o i s e le v e ls , in h i b it e d e f fic ie n c y , lim ite d s e r v i c e life, i n c r e a s e d f l a m e visibility, a n d i n c r e a s e d s m o k e fo rm a tio n . C o m p u t a t i o n a l F lu id D y n a m ic (C F D ) sim u la tio n of th e flare s y s t e m w a s u s e d to a i d in t h e d e s i g n of th e flare s y s t e m b y p re d ic tin g f e n c e t e m p e r a t u r e , g r a d e t e m p e r a t u r e , a ir flo w to t h e b u r n e r s , a n d f l a m e a n d r a d i a t i o n c h a r a c t e r i s t i c s of t h e f l a r e s y s t e m . I n s t a l l e d d u r i n g t h e p l a n t 's 2 0 0 4 s t a r t u p , th e flare bu rn ers w ere d esig n ed , fa b ricate d , a n d te s te d on a fa st trac k sc h e d u le . P ro d u ctio n te sts and o p e r a t i n g d a t a s h o w t h a t t h e fla r e b u rn e rs a c h ie v e th e p re d ic te d C FD re su lts a n d th e re q u ire d in cre ased s m o k e l e s s c a p a c i t y , r e d u c t i o n in n o i s e l e v e ls , d e c r e a s e d visibility of t h e f l a m e s a n d e n h a n c e d o p e r a t i o n a l e f fic ie n c y , w h ile s im u lta n e o u sly re sistin g th e fa ilu res ty p ic a lly a sso c ia te d w ith e le v a te d flares. In 2 0 0 4 , c o n stru c tio n of a m ajo r e t h y l e n e p l a n t in t h e M id d le E a s t f a c e d c h a l l e n g e s to fin d w o r k a b l e m e t h o d s to r e d u c e f l a r e e m i s s i o n s , f l a m e visibility, a n d n o is e . P la n t p e r s o n n e l s o lv e d th eir d i l e m m a b y t u r n i n g to a c o m p a n y w ith ex p ertise in e n v i r o n m e n t a l e m i s s i o n co n tro l and co m b u stio n th at had p io n e e re d a n d co m m ercialized a g round m o u n t e d , m u ltip o in t f l a r e s y s t e m . By e l i m i n a t i n g t h e n e e d fo r a n e l e v a t e d fla r e , t h e p r o p o s e d s y s t e m n o t o n ly r e s o l v e d all i s s u e s , b u t p r o v i d e d t h e a d d e d b e n e f i t of e n h a n c e d s e r v i c e life. B y u s i n g C o m p u t a t i o n F luid D y n a m i c (C F D ) sim u la tio n th e flare sy stem su p p lier p re d ic te d fe n c e te m p e r a tu re , g r a d e t e m p e r a t u r e , a ir flo w to t h e b u r n e r s , a n d th e fla m e a n d ra d ia tio n c h a r a c t e r i s t i c s of t h e f l a r e s y s t e m . T h e fl a r e b u r n e r s w e r e d e s i g n e d , f a b r i c a t e d , a n d t e s t e d o n a f a s t t r a c k s c h e d u l e to m e e t i n s t a l l a t i o n r e q u i r e m e n t s of t h e 2 0 0 4 p lan t startu p . P ro d u ctio n te s ts a n d o p e ra tin g d a ta c o n f ir m t h e fla r e s y s t e m a c h i e v e d t h e p r e d i c t e d C F D r e s u l t s , w ith all r e d u c t i o n c r it e r i a m e t , a n d t h e a d d e d a d v a n t a g e of e n h a n c e d o p e ra tio n a l efficiency th a t h a s r e s i s t e d t h e f a i l u r e s a s s o c i a t e d w ith e l e v a t e d fla r e s y s t e m s . M o d elin g A p p ro a c h a n d A ssu m p tio n s The d esig n en g in e e rs used s p e c i f i c a t i o n s f u r n i s h e d b y t h e p l a n t to p e r f o r m C F D m o d e l i n g of t h e m u ltip o in t g r o u n d f la r e , utilizing a f l a r e s y s t e m c o m p r i s e d of n e a r l y 1 0 0 0 b u r n e r s , a n d co n firm e d tem p eratu re fo r th e su rro u n d in g fe n c e , g ra d e te m p e ra tu re , a ir flo w to t h e fla r e b u r n e r s , f l a m e l e n g t h a n d t h e r a d i a t i o n c h a r a c t e r i s t i c s of t h e f l a r e s y s t e m . A fte r a t h r e e - d i m e n s i o n a l e q u ip m en t re p re se n ta tio n w as built usin g th e CFD so ftw are, a m esh c o m p r i s e d of h e x a h e d r a l a n d t e t r a h e d r a l e l e m e n t s w a s u s e d to a c h i e v e t h e finitev o l u m e s o l u t i o n of t h e flo w field. T h i s so lv er c o n c u rre n tly so lv ed flow, tu rb u len ce, o x id atio n c h e m istry and r a d i a t i o n m o d e l s o v e r t h e e n t i r e a r e a of ca lcu latio n . Figure 1 - P r e s s u r e b o u n d a r ie s r e p r e s e n te d by t h e blue, s e m i- tr a n s p a r e n t s u rfa c e s . W a s t e g a s flow w a s s e t a t a r a t e of 1 4 , 1 0 5 m e g a w a t t s . T h e a ir in t h e m o d e l w a s c o m p o s e d of 0 . 2 1 % o x y g e n a n d 0 .7 9 % n itro g en , and set at a t e m p e r a t u r e of 2 7 ° C . T h e g a s l o w e r h e a tin g v alu e w as set at 1 4 ,0 9 2 k c a l / N m A3, th e m o le fraction c o m p o s itio n a t 1 .0 0 C 2 H 4 , a n d m a tc h e d to t h e a ir t e m p e r a t u r e of 2 7 ° C . T h e larg e s c a le d iffe re n c e b e tw e e n th e fu e l g a s o r i f i c e s in t h e m u ltip o in t tip, a n d t h a t of t h e d o m a i n lim its r e q u i r e d to c a p t u r e t h e p e r f o r m a n c e of t h e fla r e , is s u c h t h a t it is c o m p u t a t i o n a l l y i n f e a s i b l e to c a p t u r e t h e d e t a i l a t a s u f f i c ie n t level n e a r th e b u r n e r tips a n d c a rry th e m e s h th ro u g h o u t th e d o m ain . T h e re fo re , th e a s s u m p t i o n m a d e , in o r d e r to m o d e l t h e f l a r e a s a c o m b i n e d s y s t e m , is t h a t t h e a r e a im m e d iately a r o u n d th e b u rn e rs c a n b e r e p r e s e n t e d b y "s o u r c e t e r m s " a p p l i e d to a g rid s t r u c t u r e t h a t is s o m e w h a t c o a r s e r t h a n t h a t r e q u i r e d to d im en sio n ally c a p tu re t h e fu e l g a s o rific e s . T h e m o d e l fo r t h e M id d le E a s t e t h y l e n e p l a n t v e r if i e d flo w to t h e f l a r e w a s a d e q u a t e , p e a k t e m p e r a t u r e s fo r t h e flare s y s t e m w ere a c c e p ta b le , and r a d i a t i o n w a s b o t h w ith in a c c e p t a b l e lev e ls and d issip a te d w ith in th e b o u n d a r i e s of t h e f l a r e f e n c e . T o v e rify t h a t visibility lim its w e r e m e t , c o n t o u r s of s t a t i c t e m p e r a t u r e , d i f f e r e n t c o n t o u r s of t h e m o l e f r a c t i o n of C O 2 , v e l o c it y v ec to rs, and iso -su rfa ce r a te s th at r a n g e d fr o m 1 . 5 8 k W / m 2 to 1 5 . 7 7 k W / m 2 w e r e u s e d in t h e m o d e l i n g . P e a k f e n c e t e m p e r a t u r e s w e r e m e a s u r e d to b e l e s s t h a n 2 5 0 °C o n t h e i n te rio r of t h e i n n e r f e n c e s tru c tu re . T h e p e a k m o le fraction of e t h y l e n e m e a s u r e d a t t h e t o p of t h e f e n c e w a s 3 . 5 7 x 1 0 -5. T h e m o d e l p r o v e d a lm o s t c o m p le te d e stru c tio n b efo re th e f l a r e m e t t h e t o p of t h e f e n c e , a s w ell a s a d e q u a t e o x y g e n s u p p ly to th e b u rn e rs . P e a k r a d i a t i o n l e v e l s t h a t w e r e d ir e c tly b y t h e f e n c e w e r e p r e d i c t e d to b e 3 . 1 4 x 1 0 -1 k W / m 2, d i s s i p a t i n g to 1 . 8 6 x 1 0 -1 k W / m 2 in t h e fa r-fie ld r e g i o n s . I 1.15e+03 1.096+03 1.046+03 9.79e+02 9.236+02 8.676+02 8.11e+02 7.556+02 6.996+02 6.436+02 5.876+02 ■ 5.316+02 ■ 4.756+02 4.196+02 3.636+02 3.076+02 I 2.516+02 I 1.956+02 I 1.396+02 H 8.296+01 ■ 2.696+01 Figure 2 - C ontours of Static Tem perature (°C). E t h y l e n e F ie ld F la r e S p e c i f i c a t i o n s W ith t h e e m p l o y m e n t of g r o u n d lev e l l i n e a r b u r n e r c o n f i g u r a t i o n s , v i s i b le f la r e e m is s io n s w e r e e lim in ated . D e d ic a te d g a s l i n e s s e n d w a s t e g a s to t h e f l a r e fie ld s . T h e s e l i n e s a r e in t u r n f e d b y a fl a r e s t a g i n g m a n i f o l d . T h e first-stag e b u r n e r s a r e a ir a s s i s t e d to e n s u r e t h e o p e r a t i o n r e m a i n s s m o k e l e s s a t low r a t e s of flow. S u b s e q u e n t b u r n e r s t a g e s a c h ie v e s m o k e le s s b u rn in g th ro u g h th e u s e of g a s p r e s s u r e . T h e m u l ti p o i n t b u r n e r s w e r e o r g a n i z e d in t e n s o lid r o w s a n d o n e s t a g g e r e d row , w ith e a c h b u r n e r r o w s e r v e d b y t w o co n tin u o u s b u rn in g p ilo ts . Two a d d i t i o n a l p ilo ts w e r e i n s t a l l e d o n t h e fie ld 's s i n g l e a ir a s s i s t e d f la r e , to h a n d l e e t h y l e n e flo w r a t e s o c c u r r i n g b e l o w t h e s t a g i n g p r e s s u r e fo r t h e first s t a g e of t h e m u ltip o in t a r r a y . T h e a ir f l a r e d e s i g n m a in tain s co n tin u o u s o p eratio n w h e n e v e r t h e e t h y l e n e f l a r e field is o n l i n e , h a n d l i n g lo w p r e s s u r e m in im a l flo w r a t e s , a s w ell a s h i g h e r g a s p r e s s u r e r a n g e s . O n e a c h ro w , t h e r e is a n a u t o m a t e d s t a g i n g v a l v e w ith a p r e s s u r e relief d e v i c e fo r b y p a s s in t h e e v e n t t h e s t a g i n g v a l v e d o e s n 't o p e n . Figure 3 - C ontours of Wall Tem perature (°C). D e s i g n f e a t u r e s a r e built-in to a d d b u rn e r c a p a c ity th ro u g h th e s ta g in g c o n t r o l s y s t e m , a n d limit t u r n d o w n b y rem o v in g b u rn e r c a p acity at lo w er pressures. T he m odel u sed a p re­ c a l c u l a t e d e t h y l e n e flo w r a t e of m o r e t h a n 1 .0 million k g /h r, a n d a p p l i e d it o n a v o lu m e av erag e b asis over th e a n a l o g o u s c o a r s e - g r i d b u r n e r r e g io n . O n e a c h b u r n e r tip, t h e p o r t a r e a w a s u s e d to a l l o c a t e t h e t o ta l flo w t o e a c h ro w o f b u r n e r s in t h e f l a r e s y s t e m . T h e a d i a b a t i c f l a m e t e m p e r a t u r e of e t h y l e n e at 1 2 % o x y g en w a s p re -c a lc u la te d a n d u s e d a s th e p e a k allo w a b le te m p e r a tu r e in t h e m o d e l in o r d e r to c o m p e n s a t e fo r t h e in ability of t h e c o a r s e grid to p r o p e r l y r e p r e s e n t t h e m ix in g in t h e n e a r b u r n e r d o m a i n . A d d itio n a lly , a s i n g l e flare tip w as m o d e le d and ru n in d iv id u a lly in o r d e r to u s e t h e v o l u m e a v e r a g e t u r b u l e n c e k in e tic e n e r g y a n d tu rb u len ce d issip a tio n ra te sc a la rs c a l c u l a t e d f r o m t h e f i n e - s c a l e m o d e l in th e la rg e -s c a le m o d el. L o w V isib ility D e s ig n P r o v i s i o n s T h e b u r n e r s a r e e n c l o s e d b y a ra d ia tio n fence. B e y o n d s h i e l d i n g s i g h t of t h e fla re s d u rin g b u rn e r o p e ra tio n , th e fe n c e also p ro tec ts p e rso n n e l a n d e q u ip m e n t from ra d ia n t h e a t, e s p e c ia lly a t th e s ta g in g m anifold w h e r e o p e r a to r a c c e s s is r e q u i r e d fo r m a i n t e n a n c e . c o m b u stio n z o n e , a n d th e b u rn e r d esig n d e t e r m i n e s its ability to p r o v i d e m ix in g o v e r t h e w i d e s t p o s s i b l e r a n g e of flow. T h e b u r n e r is a s p i d e r t y p e c o n f i g u r a t i o n w ith a c e n t e r h u b a c t i n g a s a s ta b ility p o i n t f o r t h e a s s e m b l y , to e n s u r e c o m b u s t i o n t h r o u g h a w i d e r a n g e of c o m p o sitio n s a n d tu rndow ns. Figure 4 - C o n to u rs of Mole Fraction of CO 2. A c c e s s a n d ra d ia tio n th ro u g h v e n te d s e c t i o n s is e l i m i n a t e d b y f e n c i n g p a n e l s s u p p o r te d b y th e m ain f e n c e k n e e braces. T h e f o u r th s i d e of t h e f e n c e f a c e s s t a g i n g h e a d e r s , a n d is s o l i d to p ro v id e plan t personnel c o m p le te access to th e area d u rin g fla r e o p eratio n s. S e r v i c e L ife C o n s i d e r a t i o n s In a d d i t i o n to t h e e x p e r t i s e of t h e d e s i g n t e a m , t h e m o d e l i n g p r o v i d e s t h e d a t a to fin e -tu n e th e p la n t flare s y s t e m th ro u g h f e a t u r e s a n d c o m p o n e n t s th a t p re d ic t th e p o te n tial for tro u b le - fre e o p e ra tio n o v e r a lo n g s e r v i c e life. P e r t h e t e a m 's sp e c ific a tio n s, th e bu rn ers are i n v e s t m e n t c a s t to e n s u r e d i m e n s i o n a l a c c u r a c y a n d p e r f o r m a n c e re p eata b ility . A n a d d e d b e n e f i t of utilizing c a s t i n g s is t h a t t h e y r e m o v e t h e n e c e s s i t y to drill orifices. C a stin g th e h o les le a v e s b e h in d no s h a r p e d g e s , elim inatin g th e ris k o f s t r e s s r i s e r s a n d , t h u s , t h e c r a c k s c o m m o n l y f o u n d in c o n v e n t i o n a l f l a r e b u rners. T o p r o m o t e m ix in g , it is critical fo r m u l ti p o i n t flares to o p erate w ith s u f f i c i e n t k i n e t i c e n e r g y l e v e l s in t h e i r F igure 5 - Is o -s u rfa c e of 2000 p pm CO (dry) b e lie v e d to b e a re a s o n a b l e re p re s e n ta tio n of t h e visible flam e s u rfa c e . T he s c a le g iv es t h e radial c o o r d in a te from t h e flare c e n terlin e in m eters. A d d itio n a lly , c o n v e n tio n a l in a h ig h d e p a rtu re fr o m p ressu re bu rn er d e s i g n , a w e b u n d e r e a c h a r m of t h e b u r n e r p r o v i d e s r e i n f o r c e m e n t to t h e a r m lim its t h e r m a l e x p a n s i o n . T h i s a l s o lim its t h e r e s u l t a n t s t r e s s e s w h i c h m i g h t o t h e r w i s e fo r m c r a c k s w h e r e e a c h a r m m e e t s t h e c e n t r a l h u b . T h e h o llo w w e b s u p p l i e s o p t i m u m flo w to t h e o u t e r m o s t h o le s on th e sp id e r, w h ich e n s u r e s th a t t h e g a s will r e a c h t h e a r c o f t h e b u r n e r w ith t h e g r e a t e s t a c c e s s to air. c y c l in g a n d u l t im a te l y l e a d to p r e m a t u r e b u r n e r fa ilu r e . Figure 6 - I s o - s u r f a c e of 1.58 kW/m2. s c a le is th e radial p o sitio n from c e n te rlin e of t h e flare in m eters. The th e C o n t r o l li n g t u r n d o w n , t h r o u g h t h e u s e of th e b u rn e r s ta g in g s y s te m , p ro p o rtio n s t h e n u m b e r of b u r n e r s in s e r v i c e to t h e flo w r a t e of w a s t e g a s to b e f l a r e d . T h i s o v e r c o m e s t h e lik e lih o o d of i m p r o p e r s t a g i n g , w h i c h c a n r e s u l t in e x c e s s i v e b u rn e r te m p e ra tu re s, re d u c e d b u rn er life, a n d s m o k i n g b u r n e r s . P erform an ce S u m m ary C F D m o d elin g a n d s u b s e q u e n t testin g p r o v e d t h a t b y u s i n g a v e r ti c a l , r a t h e r t h a n a n g u l a r b u r n e r o rific e p a t t e r n , f l a m e l e n g t h is r e d u c e d a n d n o t o n ly i m p r o v e s m ix in g , b u t a l s o d r a m a t i c a l l y d e c r e a s e s re circu la tio n o n th e h u b . T h i s , in tu r n , l o w e r s h u b t e m p e r a t u r e a n d l e s s e n s t h e p o te n tia l for b u rn e r c ra c k in g a n d p lu g g in g . P u lle d b u rn er c o n n e c tio n s on th e m a n i f o l d s m i n i m i z e p r e s s u r e d r o p to i m p r o v e g a s flo w into t h e b u r n e r r i s e r s . Full p e n e t r a t i o n w e l d s e n s u r e s o l i d m e c h a n i c a l c o n n e c t i o n s . In c o m p a r i s o n , c o n v e n t i o n a l b u r n e r c o n n e c t i o n s , u n lik e th e sp id e r d e sig n b u rn e rs e m p lo y e d at t h e M id d le E a s t p lan t, c a n only p ro v id e a c c e s s fo r w e l d i n g f r o m a s i n g l e s i d e , le a v in g crev ice s th a t invite th e p r o p a g a tio n of c r a c k s d u rin g th e rm a l F igure 7 - C o n to u rs of m o le fraction of O2 C o n c lu sio n T h e flare s y s t e m t h e e n g in e e r in g t e a m d e s i g n e d a n d i n s t a l l e d fo r t h e M i d d l e E ast p la n t has su cc essfu lly d e m o n stra te d its ability, under all o p eratin g co n d itio n s, to m eet th e o w n e r 's r e q u i r e m e n t fo r s a f e o p e r a t i o n a n d r e d u c e n o is e , flare e m i s s i o n s a n d f l a r e visibility. A l t h o u g h t h e s y s t e m h a s a l r e a d y p r o v e n h ig h ly r e l i a b l e in a c t u a l p r o d u c t i o n o p e r a t i o n s , it is a l s o c a p a b l e of fully a u t o m a t e d o p e r a t i o n a n d is b e i n g c o s t e f f e c ti v e ly m a i n t a i n e d w ith m i n im a l d o w n t i m e . The e f fic ie n t d e s i g n , s t r a i g h t f o r w a r d c o n s t r u c t i o n a n d p r o v i s i o n s fo r f u t u r e e x p a n s i o n , p r o m i s e s to c o m b i n e w ith lo w o p e r a t i n g c o s t s a n d lo w b u r n e r f l a m e visibility to e n a b l e e n v i r o n m e n t a l l y f r ie n d ly m u l ti p o i n t f l a r e o p e r a t i o n s t h a t a r e c a p a b l e of m e e t i n g t h e M i d d l e E a s t p l a n t 's n e e d s f o r m a n y d e c a d e s . Brian Duck has been a flare system designer for more than 25 years. He is presently the Vice-President of Flares and Vapors Control for Callidus Technologies, L.L.C.