Practical experience in scale control

Desalination, 73 (1989) 313-325

313

Elsevier Science Publishers B.V., Amsterdam

PRACTICAL

-

-

Printed in The Netherlands

EXPERIENCE

IN

SCALE

CONTROL

MOHAMEDABDULKAREEMAL-SOFI*,SALMANKI-IALAF** AND ADNANA/.,-OMRAN* *P.O.B, 752, A1-Khobar 31952. Saudi Arabia, Fax 966-3-1952 **P.O.B. 2, Manama, Bahrain, Fax 973-533035

S......................... UMMARY

Oes~llnation is b e c o m i n g the vital source of domestic water fop the Arabian Peninsula. Reliance on desalination is oarticularly l:;ronounced along the east coast, i.e~ Gulf Coast of Saud! Arabia and other Guii: Coo,c,s~ratlon Council ~GCC:, States. Todate the orlmary ~mecess d e [ ) i o y e d alc::n,::! ~ h e G u l f is M u l t i - S t a g e Flash (MSF) evapomation. M S F oe,.'~:ormance r e l i e s primarily on hea',; t r a n s f e r between val::,our a n d b r i n e solution along a temperature range of 25i 2 1 E.'egree C. Almost all MSF evaooraI.:ers in t h i s region ape e,oerated by brine r'ec:irculatlon to im,gr:::~ve e f f i c i e n c y and thus reduc:e cost. Yet re,::::iPculation man,,:,la!:es hea1: transfer with cc:,ncentrated sea water so]otleno Due to the concentrated nature of the heat transfer medium,, s c a l i n q l i~:~ the most critical factor contrc~] ]Inc~ MSF Orocluc:tivity, esmecially upoer half of the said temoeya'bure range. Scale f:::~rmatien c a n n o t be eliminated, b u t it c a n b e c:::]mbated,, It is o a n t i c u l a r l y essential to minimize scale formation on heat tP~ns'f÷~!r surfaces i.e. tubes .'.'.nne~surf.aces. P!iri!mum scale presence in tubes is achie,.,ed by either fu, r m a b z o n prr:::,ventlon or removal,, This Darer C,:)ver'-:~ f i v e cases: A thru E. C:ase A pertairYs co a medit!m c~:,Da,:zJt 7 e v a D o r a t c ! r of 2 - - 3 M:[GD d i s t i i l a t e ~roductlor",'. C : a s e :{:{'; i.T..~ £, so~.ecial trial on ball cJ.,::eE,n i n g at a n c~pt;i.m:~.:::ed low an{-lsc.alant dose rate. This tr:~al ~,,~as o n one of the, l:4rqer caDac:[t':~ MSF ev.a,aorat:ors,, Trial B i.4a~- r ~ e p ' i : : Q p m e d o n a 5"-7 MIGD evaD(:Jrat(:H", while cases C thru E are on a [,h:lrd grod!::) of evr~l:]c)r.:-iI~or o f 5 - - 6 M.[Gi], l : : , r o d ! . J c t ' i o n ,.::apacl.t£. In this last group the three cas{~:~s a r e ' , ;

C D

E

is a n a n t i s e a l . : ~ ~ , n t : op'timizat'Jon tPia.i.. i~.,:, r e l a t e , d to oen(,-'rai c, b s e r v a t : t o n s on day to day ootimized operation of this group of evaDorat,:.~Ps and is a n e t h e P antiscalant o0tlmization trial.

0011-9164/89/$03.50 © Elsevier Science Publishers B.V.

314 !..r~!]2~D_!}!] c T 3 0 N_ F_'ti~>.'_~e,p_t_i 9 n.:

Scale prevention may be by depletion of some scale forming constituents. Depletion can be achieved b y either- r e a c t i o n o f ion e',-'change. Alternatively prevention can be by chelating i.e. h o l d i n g in solutionq the scale forming species. This- is d o n e b y c : h e l a t i n g a g e n t s ~'enet-ally t-eferred to as c h e m i c a l additives. Pceventian as it nay b e d e d u c e d is an o n l i n e task. Sulphm-ic a c i d is t h e c o m m o n e s t acid injected into r'eact w i t h c a ~ ' b o n a t e thus 9enet-ating caf-bon Hydr'ochloric acid could also be injected for r'eaction to b e g e t c a r b o n d i o x i d e .

m a k e up to dioxide. a similar

There at-e f o u r geoups of organic compounds which have a pr-oven c h e l a t i n g e f f e c t on e l e c t r i c a l l y c:ha~-ged s c a l e f o r m i n g species present in s e a w a t e e . Pair's of t h e s e g e o u p s c a n b e conside~'ed as kins. Polyphosphate and polyphosphonate ace phosphoet~s based alkalines. Polycar'boxylic and polymeliac a r e vet'v w e a k a c i d s . Polyphosphate is "the m o s t infer~io~ - o n e of 'these cc,'mpounds as it u n d e r g o e s hydrolytic r'eaction. This .~-eaction b e g e t s a sludge compoLlnd of phosphate, and at t e m p e ~ - a t u r e s of o v e r 90 Deg. C., this ~-eaction is of an appreciable magnitude, Its use in chelating is always l:i.m i t e d to top b r i n e tempei--atures (TBT) of 8 9 - 9 1 Deg. C.

F'olyphosphonate does not have such a limiting T B T b u t it h a s indicated sludging potentials partic_~lar"ly at tempera'tur.es in e x c e s s of I I C . Deg. C. Such sludging has been ~-~elated to excessive d o s e r a t e s as it is b e l i e v e d t h a t at e x c e s s i v e dose r a t e t h e compot!nd f c ) t ~ o n e c h e l a t e s m o r e s c a l e "forming s p e c i e s than designed for. Secondly i't c,an chelate species othe~ ~ t,h a n t h o s e it is intended fcu .~ e.g. c o ~ r o s i o r i p~-oduc:ts arid electrically charged o~" even pola~'ized suspended solids :incoming w i t h seawater,. Polymer'ic acids also s h o w similarchelating effects specially when dosed excessively. Never, t h e l e s s u n d e r d o s i n g of any' one of these chemical additives e,'-'poses t h e system to potentia], risk of scale for'mat ion. Additive antiscalants other than po]yphosphate based have b e e n u s e d in MSF evapot~ators at T B T s h i g h e r t h a n 91 Deg. C. The highest repot-ted T B T in c o n j u n c t i o n wit:h additive has b e e n 118 Deg. C at t h e K h a l d i a h evapo~.-ato~'s in Jeddah. Both polycarbo>'ylic and polyphosphonate basled a d d i t i v e s were used successfully at a T B T of 113 Deg. C. As mentioned ea~"lie~thet-e a r e a l w a y s r i s k s r e l a t e d to d o s i n g rate. Under--dosing l e a d s to s c a l e formation while over'--dosing is b e l i e v e d to enh~nce sludge fot'ma'tion. Sludging potential inc:r'eases r a p i d l y at ]'BTs of over ~ ii0 Deg. C a n d b o t h sludging and scaling a~-e q u i t e probable. It is thus essential to establish the optimum dose rate, with close oper-ational and a n a l y t i c a l , m o n i t o e ~ i n g at t e m p e r a t u r e s a b o v e 110 Deg. C. Suc:h added monitoring t-equirement is f e l t to be w o r ' t h w h i l e as t h e hit]her a c h i e v a b l e pr'oduction per evaporatoras w e l l as p e r t o n of s t e a m f l o w to t h e bovine h e a t e r is b e n e f i c i a l even at higher' oper'a't

:m-,tiscalan't c:ost at hi,:]her. TB!s

i 7g

~:~er uni'i.: 7 s ,::..~tpab ] e ':,'f

,:,L, tpc.,.to f~ ] f i ] i l n g

F:'urthermc]l"e summer, peak

315 r'eQuir~ements f o p wateras well as power'. This-. advantageous situation is q u i t e p r o n o u n c e d ~ h e n M S F u n i t s a r e r e c e i v e r ' s o'f s t e a m fr~om b a c k p~'essu~'e s t e a m turbo.-gene~-etor~ s e t s .

B. ~.m._o.x_.a_], Scale removal is p o s s i b l e by the use of c h e m i c a l dissolving agents. These agents are mostly used off line. Nevertheless there are refe~ences in t h e I i t e r a t u P e to c)r~ 1 i n e c l e a n i n g a s it is c o m m o n l y ~'efer~'ed to scale ~-emova]. med~anism. Mechar~ical removal is al. t e r n a t i v e to c h e m i c a l cleaning. This method is u s u a l l y employed on iineo Very raPely it is applied as an off line task as well. On line cleaning is d o n e b y s o f t ball circulation to s w e e p tube inside surfaces. Ball cleaning is q u i t e effective not only on scale but also on sludge fo~.~med in t h e c h e l a r i n g p r'ocess. It is thus very vital at high top br'ine tempeI'~atures ( T B T ) with additives. On "the o t h e r , hand ball cleaning is n o t s o v i t a l at e v e n higher" ' t e m p e r a t u r e r e a c t i v e scale prevention method due to Ic~wer" s l u d g i n g compa~ed to additive opei.-at ion. ]"het~e is a cl(ise r e l a t i o n between sludging and e x p e ~ ' i e n c e h a s s h o w n t h e p o s s i b i l : i t y of s l u d g e s(:ale. T h i s is f e l t t o t a k e p l a c e d u e to f l o w thus the development of concent~-ation cells.

sc:aling. Past converting to stagnation and

The two on line scale cont~-ol m e t h o d s g o h a n d in hated. As discussed earlier ball cleaning is a must with high temperat.Pe additives, lilt is t h e r e f o r e mandatory t~] c-eve~a n t i s ( : a l a n t s , i.e. a d d i t i v e s when evaluating mechanical on st1.-eam b a l l c l e a n i n g . Ball cleaning is q u i t e a n a t t r a c t i v e iY~ethc:id a s long a s it remains effective. Effective cleaning can effect savings e v e n at d o u b l e ball consumption a s it c a n l e a d to s i z a b l e savinqs in a n t i s c a l a n t costs. Yet ball cleaning effectiveness is d i f f i c u l t to define. ]his paper will attempt to p r o p o s e cc)ntPolling parameter-s r,e q u i r e d to evaluate the effectiveness of scale prevention and ball c Iean inc.

The known on line parameter is bal]. l o s s e s . There are two w a y s t h a t b a l l s (:an b e l o s t . Balls e i t h e l .~ d i s a p p e a r as they are not r'eclaimed at t h e e n d of a c i r c : u l a t i o n c y c l e or' t h e y c o u l d P e d u c e in size. Size Peduction ~-enders balls ineffective in t u b e s w e e p i n g . Disappear'ing balls can be accounted fo~.~ b y inspection and investigation, Lost balls u s u a l l y e n d up e i t h e r at t h e l a s t flashing c h a m b e r - ot~ e v e n p a s s i n g o u t w i t h t h e bt~ine b l o w d o w n stream. L o s t b a l l s w h i c h e n d up in t h e l a s t f l a s h i n g chamber' would indicate r - e c l a i m i n g sc~-een malfunction. [In t h e O t h e ~ ~ hand lost h,~!],s which ~-eac::h the .-'~ t f ~ . - ~ l l c h a n r ~ e l ~,~:i.t h the i" i FI e

b ]. c:,IA!

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316 Iossess

are

There are descending I. 2. 3. 4.

partic-ularly

related

four ways of size seriousness are:

to

screen

reduction.

malfunction. These

in o r d e r

of

Splitting i.e. b r e a k a g e . Shrinking. Uneven wearing. Uniform wearing.

The most severe ball reduction problem is s p l i t t i n q , in s u c h c a s e s b a l l h a l v e s o r s m a l l e r p a r t s a r e f e u n d in t h e c a t c h m e n t baskets~ A l s o f o u n d in t h e c a t c h m e n t Oaskets are reduced in size balls. T h e n e x t t y p e ~_~f r e d u c t i o n i.e. s h r u n k b a l l s is mainly attributable to t h e b a l l m a t e r i a l and its temperature tolerance. L a s t is bali weac which occurs primarily d u e to abrasion at t u b e i n l e t s a n d w i t h i n t u b e s . This phenomenon is related to b a l l m a t e r i a l as well as system characteristics. The main parameter effecting wear is the tube inlet end configuration of rectangular versus flared. Wear is a l s o related t o t h e n u m b e r of w a t e r boxes and tube sheets balls h a v e to g o t h r o u g h . Furthermore it c o u l d b e related to t h e length and surface smoothness of t u b e s . Diameter difference b e t w e e n b a l l a n d t u b e .':ould a l s o contribute to ball wear. Ball wear of u n i f o r m s h a p e a n d r a t e is q u i t e an acceptable operating cost. But uneven or accelerated wear requires some remedial action. Ball material could be a major problem but there are cases where tube mouth and/or inside surfaces cause accele~ated or uneven wear. Operating experience often demonstrates cylinderiza-tion of balls. In su~:=h i n s t a n c e s ball roundedness is lost. They become w o r n in t h e m i d d l e like a cylinder~ with two hemispherical c a p s w h e r e nc~ w e a r i n g had occurred. Operating experience indicates that normal ball consumption is b e t w e e n 3 0 - 6 0 balls per day (200-400 per week). The n u m b e r o f b a l l s a n d t h e l e n g t h of c i r c u l a t i o n cycle are quite influential oq consumption. Rates and numbers are related to operating philosophy,, Ball materials~ density,, a n d d i a m e t e r difference between ball and tube alse affect consumption. Other causes are those related to the system. From experience the number of b a l l s c a n b e s e t at b e t w e e n 3 0 - 6 0 % c;f n u m b e r o f t u b e s . A lower percentage is i n s u f f i c i e n t while a higher percentage c o u l d l e a d to b l o c k a g e by the presen(ze of m o r e t h a n ,:ne b a l l in a t u b e at any one time. The higher perr_'entage is a l s o n o t d e s i r a b l e a s wear ~ at t h e tubes mouths would exzeed the desired level compared to wear through tubes. Conversely the lower percentage could lead to starvation and localized missed tube patches. Missed patches of t h e t u b e b u n d l e c o u l d a l s o be due to t h e brine flow regime and its velocity in the water box. It could additionally b e d u e to low tube velocity and thence blockage. In o t h e r w o r d s f l o w r e g i m e is a ma:~or contributor to inefficient ball cleaning. Rat~:~ a n d t y p e o f a n t i s c a l a n t d o s e d to t h e s e a w a t e r m a k e u p would influence sludging or scaling depending on top brine temperature. Brine content of suspended and dissolved matter could also cause problems, These fact~rs interact w i t h f l o w r e g i m e to t h e p o i n t t h a t it ~ o u ! d b e iml:::,,:)ssible to i d e n t i f y whic:h is t h e c a u s e o r e-~:fect. Ir, ~?ene~a! m l s s e 0 o r s l u d g y p a t c h e s on ~:L,b e sheets at t h e t i m e of it,t e r r a ! inspecti!::~n ,~)re symptc~ms c!~ L,n e v e n

317 distribution I. 2. 3. 4. 5. 6. 7.

of

balls

and

are

related

to:

Development of v o r t e x e s or eddies in w a t e r b o x e s . Slow flow through t h e w a t e r b(~x. Vaporization in t h e u p p e r p a r t of t h e w a t e r b o x . Low tube velocity. Very low circulation (number) of balls. Very high circulation ( n u m b e r ) of b a l l s . Heavily sludged up system.

T h e r e is a major draw back in e v a l u a t i o n effectiveness at present. This draw back presumptuous misconceptions of r e l i a n c e on:

2~

Circulation Consumption

quantity level.

and

of ball is due

cleaning to three

duration.

T o d a t e t h e r e is n o s o u n d w a y of v e r i f y i n ~ j o p t i m u m levels of either circulation quantity~ r a t e o r e v e n cc,n s u m p t i o n . It is therefore necessary to develop an optimization scheme f(~,r t h e s e on l i n e p a r a m e t e r s . In t h e p r o c e s s ~ c~0nsumption level n e e d s to b e correlated to total distance travelled by balls. Furthermore correlation to number of possible impingement s i t e s is n e e d e d . Particular attentioq must be paid to the tube mouth.

One other variable t(i be correlated for consumption comparison is t h e ball discharge and collection system configuration. Such correlation w,~uld lead to better performance evaluati(~n as well as optimized consumption. Optimized consumption would not and should not necessarily mean lower consumption. It could be much higher~ when evaluated in c o n j u n c t i o n with antiscalant consumpti~]n, as the two are cooperators (collaborants) to combat scale and sludge b u i l d u p i n s i d e M S F e v a r ~ o r a t o r t u b e s a n d ~:.~n t u b e s h e e t s . Off line detection, evaluation and even measurements are ~]ossible,, Water boxes and tube sheets Drovlde a lot c)f useful information as to scale control effectiveness. Localized scaling or slugging of tube sheet indicate a problem. The nature of such localized observation is t h e k e y to identification of t h e p r o b l e m . F,:~P i n s t a n c e t o p tube scaling cou]d be related to brine vaporization,. Top or even bottom tubes sludging could be related to e i t h e r ball density or insufficient n u m b e r of b a l l s in c i r c u l a t i o n ~ Mid bundle or ~ide sludging are attributable to f l o w r e g i m e e . g . t h e d e v e l o p m e n t s of vortexes ,iP e d d i e s ~ Localized s c a l i n g of v e r y l i m i t e d areas could be related to tube blockage due to heavy sludging, excessive ball circulation or foreign debris e.g. shells, weeds~ w~od pieces or flaked off iron oxides i.e. corrosion products.

CAS. E______A .: This cas(0 wi]l be referred to w:ith other Cases B thi,-u E,, discussed e ~ i - l i , e r we*"e c l b s e r v e d of t h e 2 - 3 MIGD e v a p e r . a t o r .

when carr./ing out comoarisens Y e t mosi:: c.f l:r;e a b n o r m a l i t i e s d u r . sn~:~ d.7:~,/ t o d a y o o e r : ~ l . : i , o n

318

This case, as mentioned at the very beginning~ represents a very healthy operation leading to successful scale control. Table 1 lists design and operating parameters of Case B. Ball consumption rate v e r s u s n u m b e r of c y c l e s f o r C a s e B a r e found plotted i~ F i g u r e I. This evaporator was operated with o n e h o u r of b a l l c i r c u l a t i o n per shift i.e. three per day. Each one hour of ball circulation is identified by eighteen(18) cycles, thus each 378 cycle represents one week of operation. A total of 587 balls were consumed in 7 w e e k s as can be seen from Figure I. Figure 1 also shows ball diameter reduction during the 7 w e e k s o f T r i a l B. In s e v e n weeks average ball diameter reduced f r o m 2 5 d o w n t o 2 2 . 8 mm. The latter diameter is c o n s i d e r e d to represent a discarding limit for balls i.e. a size below which balls render no effective sweeping function in tubes of i n s i d e d i a m e t e r s of "2~'a mm. Antiscalant dose rate during Trial B is as low as 0.73 ppm. Trial B was designed to s t u d y b a l l c o n s u m p t i o n and size reduction at a n o p t i m i z e d antiscalant dose rate at a TBT of 92 De~.C° Figure 2 shows important operating parameters measured/obtained during this trial. Recycle total dissolved solids (TDS) w a s a r o u n d 6 3 , 1 0 0 p p m t h r o u g h o u t t h e t,-ial.

CAS_E___.__C: Antiscalant manufacturers have investigated the possibility of finding not only better solvents but also some compounds that would give a dispersive action. It is felt that dispersants will improve polymer antiscalant effectiveness. S u c h an i m p r o v e m e n t w o u l d h a v e at l e a s t t h r e e a d v a n t a g e s . It would lower dose rate and hence treatment cost., consequently reducing sludginq potential as well as demister fouling. This trial was carried out on a 5 MIGD nominal capacity (at T B T o f 9 0 D e g . C) M S F u n i t b u t at a h i g h e r T B T of 110 Deg. C w i t h an a n t i s c a l a n t additive, composed of poly(=arboxylate, phosphonate and citric acid~ operates by a threshclld and dispersive mechanism. This antiscalant was used throughout the test period. It c o u l d h a v e been due to its threshold and dispersive mechanism that the dose rate was reduced d o w n to 2 . 0 p p m in r a t i o to s e a w a t e r make-up. A low calculated fouling factor of 19 x 1 0 - 6 c o m p a r e d t~i a d e s i g n f o u l e d c o n d i t i o n f a c t o r of 2 0 0 10-6 M2 D o g ..C / W was obtained. Thus the extrapolated duration of run between successive acid cleanings would be 2,.94 years., The test result showed a very s l i g h t d r o p of performance ratio from the design value of 8.85 t~) 8 . 7 4 K9/2326 KJ. T h e drop co'uld be in p a r t r e l a t e d to a h i g h e r bottom brine temperature (BBT) as well as top temperature° In e s s e n c e t h e f l a s h r a n g e is l o w e r t h a n t h e d e s i g n value of 6 8 . 2 D e g . C at 67 D e g . C. T h i s 1 . 2 D e g . C dr'op in f l a s h r a n g e would lower performance proportionally by 1.8%. thus calculated performance ratio would be around 8.73 K g / 2 3 2 KJ. Figure 3 summarizes va,~iable operating parameters measured/obtained during this trial. The seawater temperature was one degree °,5 D e q o C :~t 3 4 Deg. C, a n d m a k e - u p was

below design value of hi!::ihe~' t h a n de~ii(-Jn a s

319 would be expected for 1.6% none production than design. The bottom flash temperature is (~ne d e g r e e h i g h e r t h a n t h e d e s i g n v a l u e o f 4 1 . 8 D e g . C. The above temperatures clearly indicate that the higher operating cold end difference is b e l o w d e s i g n b y 2 D e g . C. T h i s is t o b e r e l a t e d to t h e c l e a n l i n e s s of heat transfer surfaces of t u b e s a n d their perf,~rmance. Serious attentio-~ to the ball cleaning system is r e q u i r e d fop this unit as daily losses were very high. Visual inspection revealed few facts related to hal] cleaning. Ins~:~ection revealed a p a t c h y b u i l d u p o f s l u d g e in c e r t a i n a r e a s ef t h e heat transfer bundles at the tube sheets. It a l s o r e v e a l e d high,'er s c a l i n g in t h e t o p t u b e rows in the brine heater. Furthermore inspection revealed the presence of s~me 3000 b a l l s in t h e l a s t f l a s h c h a m b e r . Dividing this number by the l e n g t h of t h e trial (one hundred days) shows that on average over 30 (present + discharged with bl(_~w d o w n ) b a l l s p e r d a y k;ere l o s t o u t of t h e b a l l c l e a n i n g cycle into the flashing brine chambers~ Such high losses indicate very ooor synchronous function of the ball collection screens. This can also be due to system obsolescence. Excessive l o s s of b a l l s w o u l d lead to poor cleaning, a s is evident from the patchy sludge present and the scaling of the top rows. Nevertheless t h e r e is a n o t h e r m a j o r c o n t r i b u t o r to local sludging and scaling, a s it (::an be,, d u e t(~ p o o r flow r e g i m e e . g . s~.~irls in t h e w a t e r b o x e s a n d c a v i t a t i o n . Table No.2 gives the chemical analysis ~:~f s l u d g e f o u n d at t h e b r i n e h e a t e r c,u t l e t . The reported result of 9.7i% phosphate as P04 could be attributed to the ~:~resence o f e:::cess antiscalant. T h i s is quite typical, particularly after an optimization program, as it is c u s t o m a r y during such a trial to s t a r t w i t h a m u c h h i g h e r d o s e rate than actually needed ( s e e f i g u r e 3). This situation7 was the case here. Seawater dissolved cations and anions of magnesium hydroxide Mg(OH) and calcium carbonate CaC03 constitL{ted alm(:st 62%. This c o u l d b e a s i g n of t h e a n t i s c a l a n t ' s chelating effectiveness. The presence of sr~me 3 4 % l o s s e s o n i g n i t i o n is attributable to the organic constituents ~:~f t h e antiscalant~ as it centains polycarboxylate and citric acid,. Marine bio and living ac:tivity could also contribute to 34% loss on ignition. Particular attention h a s to b e d r a w n to t h e j e l l y fish and coloured, especially red, tides experienced in t h e G u l f a r o u n d t h e t i m e o f T r i a l C.

CSE._..__._._D: A In t h i s c a s e on~:e a g a i n n o p a r t i c u l a r data on results ape presented as they represent the normal operation ,::if t h a t g r o u p of e v a p o r a t o r s undergone t r i a l s C ~ E. Y e t it is w o r t h stating that the normal operation of t h e g r o u p h a s s h o ~ n s o ~ e similarities t o r e s u l t s o f t r i a l s C & E Qn oI~e h a n d a n d c a s e s A or B on the other. These simila~rities will be addressed after presenting T r i a l E.

CE_______E: AS T h i s is a case of a specific t r i a l run. In this trial a r i o t h e r c::omoc~und ~,,as u s e d to effe,::t antis(:allng. Here a 0o!ymerTc a c i d ~Jas oDtimi:;~ed dov~n c o !es:s tha,7 2 p;c,m at a T B T e f :I08 D e g . C. T h e r e s u l t s of th1~:; tr1.:-~i :Jere (iuise slm:i l~r

320 to t h o s e o f T r i a l C c a r r i e d o u t on a n o t h e r e v a p o r a t o r of the s a m e group~., T h r e e of t h e s e r e s u l t s are worth highlighting° The first being the similarity in the rather high ball consumption. The other two were the differences in l o w e r sludged up system in Trial E when compared to Trial C. Likewise lower demister fouling was evident at inspection a f t e r T r i a l E. Figure 4 summarizes variable operating i]arameters measured/obtained during this trial.

C......................................... OMPARISONS Post operation internal evaporator and brine heater inspection indiceeted a lot o f s i m i l a r i t i e s b e t w e e n C a s e s A, C & E. O n t h e o t h e r h a n d c a s e s C, D & E w e r e a l s o q u i t e s i m i l a r in repcJrted o p e r a t i n g parameters including (but not limited to):-I. 2. 3. 4.

Minimal deterioration in f o u l i n g f a c t o r s . Antiscalant dose rates. Higher than designed cold end temperatures~ Rather" h i g h b a l l c o n s u m p t i o n r'ates.

Similarities were~ I. 2.. 3. 4.,

5.,

between

Cases

A,

C & E

(more

particularly

A & C)

Ball accumulation in l a s t s t a g e a s s e e n d u r i n g p o s t operation inspe(-- t i on. Patchy sludging of t u b e s h e e t s . Localized scaling of tubes in top temperatL~re s t a g e s a s w e l l a s in t h e b r i n e heater.. Sludge presence in w a t e r b o x e s a n d f l a s h c h a m b e r s . Appreciable demister fouling.

Ca~-~es A & B are presented as an abnormal versus normal (healthy) operation. T h i s is d o n e t o v e r i f y that operating abnormalities (~ould h a v e g r e a t s i m i l a r i t i e s on e v a p c : r a t o r s of different plant site, seawater condition and even capac:ities~ T h i s is e v i d e n t from comparing three distinctly different g r o u p s of e v a p o r a t o r s at t h e s e widely apart localities along the Gulf Coast.

OT__________.__.I B JEC VE S : T h e ob.iec:tive o f scale elimination of scale from n o t tc~ b e taken to task condition.

control i!s not the (:::omp Ie t e the tube surfaces. T h e i s s u e is by striving towards bare tube

Experience has proved that the presence of a thin film cif scale which does not hamper the heat transfer, efficiency, as detected b y t h e g a i n o u t p u t r a t i o , is o f a d v a n t a g e for proper plant operation.

Scale control by the c~-mr-~ot b e e,~.aluated

use of in t h e

antiscalants and cleaning balls a b s e n c e cJf -.:~c!e:~. ~nder!:.:.tandir~g

,::~f

'~.inble

:/s~e~:i'~menk

the

~:,'rocess

an!::!

a

t::~,::~c(.~du~re.

321

Optimized scale control does not necessarily mean operation at low c h e m i c a l and ball consumption costs. Yet successful operation at l o w r a t e s of a n t i s c a l a n t dosing and ball charge as w e l l as consumption were achieved. Results of t r i a l s reported here were by far less than most reported rates elsewhere. The ultimate objective of eliminate its presen(::e on the minimum film thickness beat transfer.

s c a l e ec]ntrol s h o u l d not be to h e a t t r a n s f e r a r e a b u t to a c h i e v e which could not hamper efficient

REFERENCES: ............................................. I,,

Khumayyis, D a u d S., e t . a l . ; "Economical Evaluation of AIKhobar Phase-If 50 MIGD at T h r e e D i f f e r e n t M o d e s of Operation"~ 2 n d W o r l d Con!.':iress or-, D e s a l i n a t i o n & Water" Re-Use; Bermuda, Nov. 1985.

2.

AI-Sofi, M e h a m e d A., et. al.~ " T h e r m a l P e r f o r m a n c e x 5.2 MIGD MSF Plants, AI-Jubail Phase-II", Ibid.

3.

B u t t , F.~ Treatment

4.

Ai-Sofi, Additive F. B u t t ,

5.

N a d a , N.; " E v a l u a t i o n of V a r i o u s A d d i t i v e s at A l - J o b a i l Phase- I During Reliability Trials"; Desalination 50 (1984).

6.

Khumayyis, D a u d S. a n d O h t a n i , M.~ "Construction and Cemmissioning of 6 x 23v000 T/D MSF Plants, Ai-Jobail Phase-I; IDA C o n f e r e n c - e , F l o r e n c e , M a y IC~83.

7.

A l-Mudaiheem. Ahmed "Evaluation of C h e m i c a l MSF Plants"; Ibid.

8.

AI-Sofi, in D u a l Algeres,

9.

Ai-Sofi, Mohamed A., e t . a l . ~ "Additive Scale Control Optimization and Operation Modes": 3rd World Congress on Desalination and Water Re-Use, Cannes, France, September 1987. Also Desalination, 66 (1987) 1 1 - 3 2 .

et.al.; "Field for Control of

of

10

T r i a l s of H y b r i d Acid-Additive S c a l e in M S F P l a n t " ; Ibid.

Mohamed A. ~ "Field Trials of H y b r i d Ac:id Treatment for Control of S c a l e in M S F P l a n t b y et.al. Review"~ IDA M a g a z i n e , December' 1987.

M. and Additives

Szostak, Ronald M.~ for Scale Control in

M o h a m e d A., " M S F C h e m i c a l and Fuel Purpose Plants": 3rd Arab Energy Algeria, M a y 1985.

10.

Sitra

Plant

"Trial

C"

repc, rt

II.

Sitra

Plant

"Normal

12.

Sitra

Plant

"Trial

13.

Mokhtar, A t i f A., " 9 0 D a y s D e m o n s t r a t i o n at :!!?..33 at A ! - K h a l d i a h MSF Oesa~in~t]c.n

O~:,erating E"

Saudi Arabia",, i2th Imorovement Aes©c~at:ion blay :~ 9 V).:]. ,,

re0ort

Consumption Conference,

(Unpublished). Parameters"

(Unpublished).

(Unpublished).

Annu~! CcnfeYence (WSIA:,. O~tand-::::~

of Belgard EV P l a n P, J e d d a h ~ .::..f

W-.~,t~::~r S u ~ : ~ o I V V:lorida~ LISA,.,

322

2A_~!!=E__=_i !7,A_SE ::_~ DESIGN AND OPERATING PARAMETERS FOR CLEANING BALL TRIAL AT__________________._~______________t'ZIPTIMIZED ANTISCALANT DOSE RATE

NUMBER

OF

NO.

TUBES

OF

STAGES

OF

IN

RECOVERY

EACH

SECTION/REJECT

RECOVERY

STAGE/BRINE

TUBE

INSIDE

DIAMETER

OF

TUBE

LENGTH

RECOVERY

STAGES/B.H

TUBE

MATERIAL

BRINE

RECOVERY

HEATER

BOTTOM

LAST

TRIAL

DOSE

DURATION

NUMBER

OF

RATE

TO

......................

(m)

RUNS

PER

DAY/CYCLES

NUMBER

OF

CYCLES

TOTAL

NUMBER

OF

INITIALLY

TOTAL

NUMBER

OF

BALL

TOTAL

NUMBER

OF

REDUCED

AVERAGE

DAILY

BALL

AVERAGE

DAILY

REDUCED

AVERAGE

DAILY

BALL

AVERAGE

BALL

AVERAGE

REDUCED

AVERAGE

BALL

INITIAL

PER

(Cu/Ni)

(90/10)/(70/30)

IN/OUT ..........

( D e g . C)

85/92

S T A G E . . . . . . . . . . . . . . . ( D e g . C~,

IN

IN

PER SIZE

CONSUMPTION DIAMETER

(p~m>

0.72

(WEEKS/DAYS/HOURS) RUN ..................

586

SIZE

914

BALLS ...................

.............................. SIZE

12

BALLS .....................

18.6

.........................

30.6

CYCLE ..........................

0.22

BALLS

0.35

PER

PER

CYCLE .................

CYCLE .....................

...............................

DIAMETER

BALLS

HARDNESS

........................................... SURFACE

2646

..............................

DISCARD

BALLS

3/18

1500

BALLS

INITIAL

7/49/1126

BALLS ................

CONSUMPTION

LOSSES

60

TRIAL .......................

LOSSES

LOSSES

PER

32

( D e g . C)

UP ...................

CHARGED

IN

12.186/10.58

...............

IN . . . . . . . . . . . . . . . . . . . .

TOTAL

BALL

MAKE

4364/4415

....... (mm>

RANGE .....................................

ANTISCALANT

13/3

...

........

BRINE

TEMPERATURE

HEATER

STAGES/B.H

STAGES/B.H

TEMPERATURE:

FLASH

FLASH

RECOVERY

SECTION

LIMIT .......................

FINISH

.........................

0.57

~mm)

25

(mm)

22.8 MEDIUM

NORMAL

TOTAL

DISTILLATE

PRODUCTION

(CM/HR:,

TOTAL

D!~i~TILLATE

PRODUCTION

........................... , M I G D )

ROUGH

TYPE :iI]<, 5.iiii

323

]15;.!..~.L. _-:_.J;. ~.L.~.~.~.Q!L...E!L.. ~.E!..!~_E_.H~.~.:[~E... gg.EL~,]::

======================================================= .............~'~........................................ % ANAL.YSIS --~iI~ ANALYSIS

Loss

on

Calcium

ignition @ 9 0 0 Deg. C. @ 5 ( ) 0 Deg. C. as

Magnesium

as

Phosphate~

6. (34%

M9 CaCo~

hyd~-o" ide, as

% %

8.8%

Ca

Calc:iulm C a r b o n a t e , Magnesium

13.6 20.0

F'04

Mg

(OH)~

22.0

%

38.9

%

9. 7 1 %

324 FIGURE 1. CASE B, BALL CLEANING TRIAL BALL REDUCTIONIN SIZE AND QUANTITYVERSUS NUMBER OF CYCLES

1500

25 .~0 \ \

N U

Iz,00

M BE R

i

\ \

24,50

\ \

1300

O F

2,',.00

'%

B

t20o

I

1100

A L L 5

A V E R A G E

\

B A L L 23..-'0 D

I

A m rn

N ,S Y S

T

23.C0

1ooo

E M 22.50

900

22.C0

800 0

378 NUMBER OF CYCLES

~1160 , 1150

FIGURE 2. TRIAL B. ANTISCALANT DOSE RATES OPTIMISATION

t~ 114o ~II30 tO ~1120

ii!:l

®

C.

~

~"

/

'~

9sl

ou 94 P- 93 eo 52 P 91 90 150

~DIES~N FOULED CONDITION

'°o,1 IOQ

~

7-0 7"101 0.9 I'

378

' I ' 756

'1

'1

1134

NUMBER

1512 OF

CYCLES

I

t898

'

I'

725e

I

Z54~

325 FIGURE 3. TRIAL

C. ANTISCALANT

DOSE

RATES

OPTIMISATION

~ 14001 C~1350

~

1300

w

g

"1

ou 11o I--

10g

m i,- tO8 107 '~O 200 100 , J.

DESIGN FOULED CONDITION

~, 9"0~- . . . . . . . . . . . . . . .

"°-I /

~.........

,' ......... ,' ......... ~'o' ........ :o ......... 'o ......... :o' ........ ,'o......... :o' ........ ,~

NUMBER OF DAVS FIGURE 4. TRIAL E. ANTISCALANT DOSE RATES OPTIMISATION

1400

~

1~$16N IO ~ PERFORI~ANCE .... R A T,.~

1300

3.01 <~ 2-5 a: 0 2.0

o ml~

~- 11o m p-

'0 -

lO9 200--~

DE~-J6N FOULEDCONDITION

so,,:

4O-

~

2o

~"

0 8-8/J ~

8~JO ~.

8-00

7.50 I , , , , , , ~ , , I , ' ' ' ' ' ' ' ' i

o

5

10

15

20

' ' ' ' ' ' ' ~ 1

25 NUMBER

'~

. . . . . . .

30 35 OF DAYS

I

,~0

. . . . . . .

t.5

~'i

5o

~ ' F ~ ' ;

55

'

''i

60