ENVIRONMENT
Energy Ene rgy absorpt absorptio ion n probes control oily-water discharges New technology monitors monitors oil-emulsi oil-e mulsion on layer in water separation processes and reduces pollution sourcesource- points G. Agar a n d P. Clewis, Agar Agar Corp., Houst Houst on, a n d C. Spencer, Litwin Engineers & Constru ctors, ctors, Inc., Houston
E
nergy absorption instrumentation is rapidly emerging as the preferred method of interface control for separation processes. This high-frequency elec electr tr omagnetic meas urem ent technique accuaccurately senses volume percentages (not level) in phase separat ions ions su ch a s wat er an d oil. oil. Instead of searching searching for for or assu ming a clean clean int erface, erface, the instr ument s monitor percenta percenta ges of of water water at points points in t he system, and can measur e either either water in oil oil or or oil in water mixtures. This sen sitivity gives the opera tor “visio “vision” n” inside t he system an d consequ consequ ent ly, ly, more reliable cont cont rol. N o w u n i t o p e r a t i o n s c a n e f fe fe c t i v e l y m o n i t o r a n d r e d u c e t h e i r o i ly ly -w -w a t e r r e l e a s e s . R e d u c i n g t h e w o r k load load on existing existing wastewater t reatm ent systems lessens lessens oil-g oil-grea rea se levels levels in effluent wat er. Tighter Tighter hydrocarbon-releas bon-releas e monitoring can brin g discha discha rges int o comcompliance and dim inish overall emission levels. levels.
A need is met. This new technology technology confront confront s one of ma ny problems ass ociat ociat ed with pollution sour sour ce-po ce-point int cont cont rol: det ection. ection. Because Because operat ors cann ot see th rough vessel walls, walls, th ey must r ely on oth oth er met hods tha t sh ow fluid levleve l s . T h e e m u l s i o n ’s ’s n a t u r e f u r t h e r c om om p l i ca ca t e s l e v e l detectio detection n a nd adds to the dilemma. For m ost emulsions, emulsions, the interface interface is not a clean-c lean-cut line. Rather it is a hydroc a r b o n /w /w a t e r t r a n s i t i o n z on on e w h e r e c om om p o n e n t c on on cent cent ra tion var ies especially especially with vert ical ical position. position. Consequent ly, ly, tr aditional level control control techniques have n ot acknowledged th is phenomen on. Thu s th ey often often gave f a l s e in in f or or m a t i o n t h a t u l t i m a t e l y r e l e a s e d h y d r oc o ca r bons bons int o wastewater. N e w s o l u t i o n . S t a t e - of of -t -t h e -a -a r t s o u r c e -r -r e d u c t i on on s o lu lu t i on on s t h a t a r e b ot ot h e ff ff ec ec t i ve ve a n d e c on on o m i ca ca l a r e
often atta inable through improve improved d instru ment ation and process control. New technology for process control in s e p a r a t i o n p r o ce ce s s e s m u s t h e l p i n a c h i e vi vi n g c om om p l i ance with with in creas ingly rigid EPA regulat ions ions such as Benzene NESHAPS. Other inter face face co cont rol methods procedur procedur es for for s epar at ion ion pr ocess ocess cont cont rol such as sight glasses a nd capa cita cita nce probes have been ineffectiv ineffectivee in detecting the hydrocar hydrocar bon/water bon/water in ter face. face. Result— undercarr y of hydrocar hydrocar bons bons (and benzene) in wastewat er streams. A new solution, energy absorption technology, measu res hydrocar hydrocar bon bon concentra concentra tion in water, rath er tha n t he int erface. erface. This highly reliable reliable method greatly reduces hydrocarbon undercarry.
Two case studies. A midwestern midwestern U.S. refinery refinery an d a large petroleum petroleum bulk term inal in Taiwan Taiwan demonstr ate the benefit of energy absorption. Both facilities exper i e n c e d s i gn gn i f i ca ca n t i m p r o v em em e n t i n l o s s co co n t r o l a n d reductions reductions in effluent tr eatm ent costs costs from th is techtechnique. The refinery refinery met t he latest Benzene NESHAP sta ndar ds an d reduced tota tota l benzene benzene dischar dischar ged 82% 82%.. Oth er pr ojec ojectt benefit s included reduced capital costs costs for for t he sa me project. project. This r efinery spent a pproximat pproximat ely $ 4 m i l l i on on o n a s o u r c e r e d u c t i o n p r o je je ct ct ( l e s s t h a n $400,000 $400 ,000 was spent on relat ed instr umen ts) and a voided voided investing over over $70 millio million n on a was tewater tr eatm ent unit (WWTU) (WWTU) projec projectt t o meet th e sam e regulat ory comcompliance. A petroleum petroleum bulk t ermin al in Taiwan a chieved similar r esults a nd cut oil-disc il-dischar har ge co concentra tions to less than 10 ppm. Source Source reduction—prevention reduction—prevention is bet ter than cure. Num erous stu dies detail the advan ta ges of of sour sour ce-reduce-reduct i on on o ve ve r t r e a t m e n t p r o gr gr a m s . 1– 3 A good good examp le is compliance with with th e NESH AP Benzene Waste Operations regulation (40 (40 CFR 61, 61, Subpart FF—revised Ja nua ry 7, 1993, 1993, 58 FR 3072). 3072). This regulat ion ion r equires t ha t all facilities facilities dischar dischar ging 10 metric tons per year (mtpy) or m ore total benzene benzene must treat all wastestreams containing 10 ppm or m ore benzene. In a refinery, benzene originates from hydrocarbon undercarr y in in wast ewater st ream s. Therefo Therefore, a sourcesourcereduction program that segregates total wastewater
Reprinted from HYDROCARBON PROCESSING® magazine, August 1993 issue, pgs. 55-59. Used with permission.
Outlet collector
Transformer
To chemicals injection control
Interface level
2
4
Dump valve controller
1
P+1 Solids buildup alarm
To recorder
3 Inverter pan distributor and supports
Brine Crude
Installation at tank farm
Fig. 1. Bottom injected desalter or dehydrator.
flow flow into individual str eams by using advanced co cont rol str at egies egies can can m eet the r equired 10 ppm limit. SegreSegregat ed low-c low-conc oncent ent ra tion (< 10 ppm) st rea ms would be exempt exempt from from t reatm ent. Even in instan ces where stream e x e m p t i o n i s n o t t h e g oa oa l , l e s s e n i n g t h e l o a d o n t h e WWTU just just ifies sour sour ce redu ction. ction.
Defining sources. These EPA references show separa tion processes processes as th e most signific significan an t cont cont ribut ors to g e n e r a t e d w a s t e w a t e r . T h e s e c a n i n c l u d e b ot ot h b a t c h processes processes (tank dewat ering, batch separa tors, etc.) etc.) an d c on on t i n u o u s p r o ce ce s s e s ( d es es a l t e r s / d e h y dr dr a t o r s , i n - li li n e separa tors, etc.). etc.). When m anu ally co cont rolled, th ese processes processes offe offerr t he great est opport opport un ity fo for impr ovement ovement . Separ Separ at ors with older older t echn echn olog ology y can can contr ibute significant polluta nt s to wast ewater. Typic Typical al oilyoily-wat wat er cont cont ribut ors for for a refinery are: • Desalters—40% • Storage—20% • Slop oil oil recovery recovery an d ta nks —15% • Oth er p rocesses—25% rocesses—25%.. Thus, by cut cut ting hydrocar hydrocar bon bon un dercarry from the prim ar y contribut contribut ors, one one can a chieve chieve fewer fewer losses losses a nd mu ch less pollution. pollution. Traditional approaches. Separ at ion ion cont cont rol schemes schemes ha ve tra ditionally been been designed to contr contr ol th e height or level of a su pposed clear clear -cut -cut int erface between a h ydrocarbon phase a nd an aqueous aqueous ph ase in a separator. With With very light light h ydroc ydrocar bons bons t hat ra pidly pidly and cleanly separa te from the aqu eous eous pha se (e.g. (e.g.,, gasoline gasoline and wat er), a n d w i t h ou ou t a n y t u r b u l e n ce ce , t h i s a p p r o a ch ch i s a ca p a b l e , a c ce ce p t a b l e c on on t r o l f or or m . H o w e v e r , i n p r o ce ce s s e s w h e r e m i x in in g e n e r g y a n d p h y s i ca ca l p r o p e r t i e s p l a y a g r e a t e r r o l e, e, t h e p h a s e s t e n d t o m i x, x, i n t e r - d is is p e r s e an d/or d/or emu lsify. lsify. When th is occurs, occurs, t he concept concept of level becomes becomes mean ingless, becau becau se no distin ct point point of phase
change (i.e., (i.e., no clear-cut clear-cut in ter face) face) exists. exists. I nst ead, a tr ansition zone zone or rag layer exists exists between th e phases. S i g h t g l a s s . A bas ic level-report ing te chnology, chnology, th e sight glas s, is intend ed to give give visua visua l indicat indicat ion ion of th e interface. This method rarely, if ever, shows the presence or size of an emu lsion lsion t hat may exist in t he vessel. If the emulsion is positioned positioned between between t he u pper a nd lower lower sight glass connections, connections, it cann ot enter t he sight glass a nd, t her efore, efore, cann ot be detected. Also, Also, poor poor fluid exc exchan ge betwee between n t he sight glass and t he tan k ensur es longer longer residence time. E ven if some of of th e emu lsion lsion does does enter the sight glass, th is level level is not indicat indicat ive ive of the actu al level. level. Other technologie technologiess indicate a supposed int erface level level based on differential specific gravity. Some examples a r e f l oa oa t s , d i s p l a ce ce r s a n d d i f fe fe r e n t i a l p r e s s u r e c el el l s . However, However, all th ese met hods give false false in dications of a clear-cut interface when there is an emulsion. These indicat indicat ions ions ar e unr eliable in emulsions, and th e hydroc a r b o n s d i s p e r s e d i n t h e w a t e r w i l l n ot ot m e a s u r a b l y affec affectt t heir output . C a p a c i t a n c e p r o b e . Ano Anoth th er tr aditional technoltechnology ogy is m easu rem ent of th e liquid’s liquid’s capa cita cita nce or dielecdielectr ic with with a capacita capacita nce probe. This technique’s technique’s benefits ar e direct process process cont cont act a nd n o dependence dependence on specific gravity. However, when installed vertically the capacita capacita nce probe probe acts as an a veraging device device,, reporting t o t a l w a t e r a n d o i l a l on on g t h e a ct i v e a n t e n n a b u t p r oviding viding no info inform ation as to th e distribut ion ion of the t wo phases. For example, if it it is immersed in a 50% emulsion, sion, it could could give th e sam e readin g as if it it wer e half immer sed in water a nd ha lf in oil, oil, with a clear-cut clear-cut int erface. When installed horizontally the capacitance probe can s ometimes act a s a point alar m, but it suffers suffers from an inability to detect the presence of hydrocarbons in the aqueous phase. A capacitance probe relies on an insu lat ing media (e.g., (e.g., oil oil or or oil with with wat er dr oplets) oplets) b e t w e e n t h e c a p a c i t o r ’s ’s p l a t e s . W h e n p r o ce ce s s w a t e r (which (which is condu condu ctive) ctive) becomes becomes th e cont cont inuous pha se, even if th ere is significan significan t h ydrocar ydrocar bon bon in t he wat er, th e capacitan capacitan ce will short-circuit short-circuit a nd t he outpu t will peg at full scale an d falsely indicat indicat e 100% wat er.
Instrumentation requirements. requirements. W h e n e va va l u a t i n g con t r o l i n s t r u m e n t a t i on on t o m i n i m i ze ze e ff ff l u e n t u n d e r ca r r y, y, a n d d e t e ct ct / co co n t r o l e m u l s i o n s a n d d i s p e r s i on on s , certa in guidelines guidelines m ust be conside considered: red: • Direct conta conta ct with t he process process • Measu rem ent of 0% to 100% hydr ocar ocar bon/water concentration (not level) in both oil-continuous (water in hydrocarbon) and water-continuous (hydrocarbon in water) phases. • Loc Local or point point measu rement , instead of averaging ov e r a l a r g e a r e a . T h i s m e t h o d a v oi oi d s e r r o r s d u e t o hydrocar hydrocar bon/water bon/water distribution or rag layer. layer. • Minimal effec effectt on meas ur ement from from fluid properties (specific gravity, pressure, temperature, viscosity an d coating coating buildups ). Innovative technique. A new technology, known as energy absorption, has been developed specifically to meet t he previously previously described described requirem ents . The The output of energy absorption absorption inst rum ents is expressed expressed in u nits
H Y D R O C A R B O N P R O C E S S I N G / G / AUGUST 1993
of volume volume percent ages (concentr (concentr at ion, ion, n ot level) of th e water in t he nea r vici vicinity nity of the probe’s a nten nae. The i n s t r u m e n t s a r e p o s it it i on on e d t o p e n e t r a t e t h e v e s s el el a t point point s where th e measu remen t is desired. Consequent Consequent ly, ly, th e instr ument s not only serve serve to monitor monitor th e positio position n of an inter face, face, but also to tra ck cha cha nges in th e size, size, rate of growth or shrinkage and water content of rag layers, emulsions emulsions a nd dispersions. The energy absorpt absorpt ion ion pr obe obe tra nsm its a ban d of of very very high frequency electromagnetic electromagnetic energy impulses into t h e f l u i d a r ou ou n d i t s a n t e n n a a n d m e a s u r e s t h e e n e r g y a b s o r b e d . E n e r g y a b s o r p t i on on a l l o w s fo fo r r e m a r k a b l y a c cu cu r a t e m e a s u r e m e n t u n d e r v a r y i n g p r o ce ce s s c on on d i tions of the hydrocar hydrocar bon-inbon-in-water water phase. It s a bility bility to measur e a sm all amount of hydroc hydrocarbon arbon in water makes th e most s ignific ignificant ant contr ibution ibution when contr contr olling olling the s e p a r a t i on on p r o ce ce s s e s w i t h o u t a l l o w in in g h y d r o c a r b o n undercarry.
Crude tank farm
Desa ltlter
Tank dewatering system no. 1
Tank dewatering system no. 2
Refining
Finished product tank farm Tank dewatering system no. 3
Wastewater
Recovered oil Slop oil tanks
Product
Recovered slop oil
API separator
Effluent
Skimmer control system no. 5
Tank dewatering system no. 4
Fig. 2. Oily-water process flow diagram.
Example—desalter control. Ener gy absorpt absorpt ion ion t echechnology nology has been u sed with gr eat success success in r efineries an d petrochemical petrochemical operat operat ions ions t hr ougho oughout ut t he world. world. 4 C on on t r o l a p p l i ca ca t i on on s h a v e v a r i e d f r om om t h e r e l a t i v el el y simple stora ge tan k dewat ering pr ocesses ocesses to complex complex desalter control systems. The typical typical cont cont rol str at egy for for a low low veloci velocity ty desa lter is sh own own in Fig. 1. In th e desalter cont cont rol system, probes provide provide concontinu ous 4 to 20 mA out out put signals t hat are pr oport port ional ional to the wat er concentra concentra tion at t heir locat locat ions. ions. Pr obe obe 1 cont cont rols the br ine outlet valve, using its ability to measur e small amounts of oil in in wat er to mainta in a very h i g h ( a n d u n s t a b l e ) p er er c e n t a g e o f w a t e r s e v er er a l f ee ee t above the bott om of of the vessel. This a llows llows su spen ded oil in th e water pha se to separate, thus inhibiting inhibiting oil oil underc a r r y ( a s a p r i m a r y c on on t r o l f u n c t i on on ) . W h i l e p r o be be 1 esta blishes th is lower lower limit for for t he emu lsion lsion layer, probe 2 monitors t he wat er cont cont ent below below th e lower lower electr electr ical grid grid t o detect detect a nd a larm on emulsion growth growth (which (which must, by control, occur in the upward direction). This monitoring function allows allows t he operat or to avoid avoid downstr eam u pset by advance warn warn ing of of such such growth, and allows allows t ime for for corr ective ective measu res pr event event ing under carry or car ryover. ryover. Probe 3, installed on t he crude oil feed feed line near t he ta nk far m, monitors th e line line for for excesexcess i v e w a t e r i n t h e f e e d t o t h e u n i t ( a l s o p r o v id id i n g a n advance warning). Probe 4 monitors the water phase of th e desalter below below the dist ributor, alarming on th e presen ce of suspen ded oil oil tha t does not rea dily separat e and thr eatens conta minat ion ion of the brine (“reverse” (“reverse” emulsions). Case studies. A U.S. U.S. midwestern r efinery is an example of of an economic economic sour sour ce redu ction ction pr ogra ogra m u sing t he ad van ced techn ology ology.. The ref iner y’s y’s t ota l level of benzene discha discha rge was n early 17 mtp y. Initially additional s t r i p p i n g ca c a p a c i t y f or or W W TU T U w a s c on on s i d e r e d a n d planned. However, the new stripping system would redu ce th e benzene discha discha rge by 41% 41% an d cost cost several million dollar dollar s. A project project tea m consist ing of compa compa ny engineering engineering an d refinery personnel and a major engineering firm evaluat ed the a vailable level level cont cont rol techtechnologies and selected energy absorption to bring the refin ery in to complia complia nce more economically economically.. Fig. 2 sh ows
Table 1. Refinery benzene source Sour c e Crude and product storage tanks (50 total) Crude unit desalters (2) Slop oil tankage and others
Contribution,% 45 45 10
a t ypical ypical block block dia gram for for r efining. A review of of the p rima ry sources contr contr ibut ing to the refiner y’s benzene dischar ge befo before re m odific odificat at ions ions is su mm ar ized in in Table 1. A t t i m e s , t h e e xi xi s t i n g r e f i n e r y co co n t r o l s y s t e m s allowed allowed un dercar ry of free hydrocar hydrocar bons bons from desa lter opera opera tions because of their in ability to accur accur at ely detect detect the interface between the hydrocarbons and aqueous phases. This project used two methods to improve the u n d e r c a r r y q u a l i t y : a h y d r oc oca r b on on d e t e c t i on on i n s t r u ment at ion ion system an d the a ddition of of recyc recycled led water t o th e desalters. Testing Testing showed tha t ener gy absorpt absorpt ion ion i n s t r u m e n t a t i on on w a s a b l e t o d et et e ct ct t h e f i r s t t r a ce s of of suspen ded hydrocar hydrocar bons bons a bove bove the wa ter dra w-of w-offf in the desalter s tha t virtu ally eliminated free-hydroc free-hydrocar ar bon bon dischar dischar ge in th e un dercarry. dercarry.
Results. To contr contr ol cost cost s, all crude a nd pr oduct storage tan ks were m odifie odified d to use t he energy absorption absorption p r o b e s a s a p or o r t a b l e s y s t e m . T h e p r o b e s w ou ou l d b e installed on the vessels for tank-bottom-draw operations. This m ethod significant significant ly lowered lowered tota l capital costs on the project. End-of-project results yield the refinery these benefits: • Total benzene discha rge level dropped to appr oxioximat ely 3 mtpy • Impr oved oved operations yielded yielded a 82% dec decreas e in total benzene emissions • Project investment cost less than 5% of the original est imat e for for th e additional str ipping ipping capacity capacity.. Consequently, the refinery avoided a capital wastew a t e r t r e a t m e n t p r o je je ct ct e s t i m a t e d ov ov e r $ 7 0 m i ll ll i on on . Tota l capita capita l investment for for energy absorption absorption inst ru ment ation was less th an $400,000. $400,000. Bulk storage. Another rigorous test for tank-dewater ing contr contr ol was condu condu cted at a la rge petr oleum bulk s t o r a g e op op e r a t i o n i n T a i w a n . Av Av e r a g e h y d r o ca ca r b on on
H Y D R O C A R B O N P R O C E S S I N G / G / AUGUST 1993
undercarry in effluent water from mixed-crude tankage was several percent. Sidewalls of of th e tan ks were hot t apped t o allow allow ent ry of two energy energy absorpt ion ion conconcentr centr at ion ion co contr ol instr ument s. The inst inst rum ents were inserted at a 45-degree angle downward to allow for adjustment of the points points of measurement (antenna loc locations). The setpoints t ha t closed closed water -draina ge valves valves The authors Gideon Agar is president of Agar Corp. Mr. Agar holds a BS degree in computer science from Brown University, Providence, Rhode Island.
Paul Clewis is Clewis is regional applications manager at Agar Corp. and has 12 years of experience in petrochemical and production markets. He has extensive experience in refining unit oper- ations, specializing in the chemical and equip- ment technologies of oil/water treatment and peripheral operations. Mr. Clewis holds a BS degree in chemical engineering from Rice Uni- versity, versity, Houston. Calvin Spencer is Spencer is technology director at Litwin Engineers Engineers & Constructors, Constructors, Inc. He has 21 years of multi-media environmental regulatory and technology experience in refining, petrochemi- cal and polymer industries. Mr. Spencer holds a BS degree in chemical engineering from the University of Texas, Austin.
ha d a ra nge of 80% to 90% 90% water (in (in h igh-wat igh-wat er-coner-continu ous or oil-i oil-in-water n-water regime). The ran ge contr contr ol would would shut down down th e system at th e first signs of hydrocarbon hydrocarbon mixtur e near ing th e vessel’s effluent dischar dischar ge point. point. A series of of tests were conducted conducted by th e En vironment vironment al I n s p e c t i on on D i v i s i on on , a n i n d e p e n d e n t a u d i t i n g g r ou ou p tha t found oil oil and grease concentra concentra tion in th e effluent effluent wat er decreased from percenta ge level levelss to a r esidual of 7 to 8 ppm.
The challenge. Refining Refining a nd petrochemical petrochemical industr ies mu st ba lance environment al responsibility, responsibility, tougher ecoeconomic co compet ition an d increasin gly rigid rigid r egulations governing governing air an d water dischar ge limits. In some areas, new and/or larger units and other large-scale capital projects projects ma y be required t o achieve achieve th e legislat legislat ed polpollutan t r emoval emoval levels. levels. Howeve However, r, in man y systems, th e best place to start is th e sour sour ce(s) ce(s) generat ing those polpolluta nt s. Advanced Advanced technologies technologies such as energy absorption tion allow allow contr contr ol approaches tha t can eliminat eliminat e ma ny str eams a s pollutan pollutan t sources. Only Only after reviewing reviewing the potentials for for source reduction reduction is ther e a cert cert aint y that cost cost -effec -effective tive complian ce can can be achieved. LITERATURE CITED 1 2
3 4
Internal and Environm ental Audits of the Indu strial and Transportation Operations Operations Can Identify Areas that Need Improved Control Management, 1987. U.S. EPA, Develo Development pment Document for Interim Fin al Effluent L imitations Gu idelines and N ew Source Performance Performance Stan dard s for the Significant Organ ics Products Products S egment of the Organic Chemical Man ufacturin g Point Source Category, Category, EPA 440/175/045, 1975. U.S. EPA, Development Document for Effluent Limitations Guid elines for the December 1979. Petroleum Petroleum Refining Point S ource Category Category,, EPA 440/1-79/014b, December Putm an, “What’ “What’ss th e Best Way to Control an In terface When an Emulsion Tends to Form Between the Ph ases?” Control Magazine, Ju ly 1992, pp. 47–49 .
Article copyright © 2000 by Gulf Publishing Company. All rights reserved. Printed in USA.
