fouling phenomena in multi stage flash (msf) distillers

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FOULING PHENOMENA IN MULTI STAGE FLASH (MSF) DISTILLERS 1 Mohammad Abdul-Kareem Al-Sofi Research & Development Center, Saline Water Conversion Corporation P.O.Box # 8328, Al-Jubail 31951, Kingdom of Saudi Arabia ABSTRACT Fouling in Multi Stage Flash (MSF) distillers has been occupying researchers for many years. A lot of work has been done and more is yet to come in order to fully understand the role of various components and their interaction including the effectiveness of scale control techniques. In this paper an attempt is made based primarily upon visual and reported observations of fouling in various parts along the flow path of brine solutions in MSF distillers. This analysis is aimed at proposing certain sequence of scale forming reaction steps and to suggest certain experiments that could verify the validity of the proposed reaction mechanism. The proposed reaction steps are shown in alphabetic order to denote the sequence that could prevail inside heat transfer tubes of recovery and heat input (brine heater) sections and water boxes and thereafter in flash chambers including demister pads of MSF distillers. To predict the fate of various species and pH values one must identify reaction steps. Starting with reaction (A), which is initiated as a result of solution heating. The next three steps will proceed, thus reaction (D) is considered as the last step inside Distiller heat exchanger tubes. Then reaction (E) will occur dominantly after the release of pressure in flash chambers where carbon dioxide Evolution can take place. These five steps (A to E) plus the overall balance (F) are shown below: 3 HCO 3 - + 3 H 2 O 3 CO 3 2- + 3 H 2 OH + (A) CO 3 2- + Ca 2+ CaCO 3 (B) 2 CO 3 2- + 2 H 2 O 2 HCO 3 - + 2 OH - (C) 1 Presented at the European Conference on Desalination and the Environment, Las Palmas, Gran Canaria, Spain, 9–12 November 1999: Proc., Vol 3, pp. 61-76. 2103

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Page 1: Fouling Phenomena in Multi Stage Flash (Msf) Distillers

FOULING PHENOMENA IN MULTI STAGE FLASH (MSF) DISTILLERS1

Mohammad Abdul-Kareem Al-Sofi

Research & Development Center, Saline Water Conversion Corporation

P.O.Box # 8328, Al-Jubail 31951, Kingdom of Saudi Arabia

ABSTRACT Fouling in Multi Stage Flash (MSF) distillers has been occupying researchers for many

years. A lot of work has been done and more is yet to come in order to fully understand

the role of various components and their interaction including the effectiveness of scale

control techniques.

In this paper an attempt is made based primarily upon visual and reported observations

of fouling in various parts along the flow path of brine solutions in MSF distillers. This

analysis is aimed at proposing certain sequence of scale forming reaction steps and to

suggest certain experiments that could verify the validity of the proposed reaction

mechanism. The proposed reaction steps are shown in alphabetic order to denote the

sequence that could prevail inside heat transfer tubes of recovery and heat input (brine

heater) sections and water boxes and thereafter in flash chambers including demister

pads of MSF distillers. To predict the fate of various species and pH values one must

identify reaction steps. Starting with reaction (A), which is initiated as a result of

solution heating. The next three steps will proceed, thus reaction (D) is considered as

the last step inside Distiller heat exchanger tubes. Then reaction (E) will occur

dominantly after the release of pressure in flash chambers where carbon dioxide

Evolution can take place. These five steps (A to E) plus the overall balance (F) are

shown below:

3 HCO3- + 3 H2O ⇔ 3 CO3

2- + 3 H2OH+ (A)

CO32- + Ca2+ → CaCO3 ↓ (B)

2 CO32- + 2 H2O ⇔ 2 HCO3

- + 2 OH- (C)

1 Presented at the European Conference on Desalination and the Environment, Las Palmas, Gran Canaria, Spain, 9–12 November 1999: Proc., Vol 3, pp. 61-76.

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2 OH- + Mg2+ → Mg (OH)2 ↓ (D)

3 HCO3- + 3 H2OH+ → 6 H2O + 3 CO2 ↑ (E)

4HCO3- + Ca2++Mg2+ → CaCO3↓ + Mg(OH)2 ↓ + 3CO2 ↑ + H2O (F)

The common practice is that: pH values are measured of solutions drawn out of heat

exchanger tubes of recovery section or brine heater. This leads to the evolution of

carbon dioxide. In view of the above, variation in pH values, which could support the

above mechanism, were never addressed. Certain experiments should, therefore, be

devised where pH values of recirculating brine inside heat exchanger tubes could be

measured on-line while still under pressure. It would then be probable (especially if

particular species detection electrodes are installed) to measure variation in pH values

due to hydronium ion generation in reaction (A) and its consumption in reaction (E).

Meanwhile, the presence of hydroxyl ions will be short lived primarily due to Mg(OH)2

precipitation through reaction (D) which will consume hydroxyl ions that are generated

in reaction step (C).

The above proposed steps (A to D) support CaCO3 precipitation ahead of Mg(OH)2.

The abundance of magnesium ions and the extreme low solubility of magnesium

hydroxide will rapidly then lead to its formation. However, scale precipitation inside

tubes are not only from initial scale formation under pressure inside the tubes but also

due to nucleates recirculation from flash chambers back into heat gain exchanger tubes

because of brine recycling. Recent analysis of variation in coloration of water boxes

came as a strong support to this hypothesis. Reaction steps suggested by other workers

will be shown for the sake of comparison. It is worth to note that the end results of

various reaction mechanisms are almost the same in the cited ones also the same to the

proposed overall reaction (F) as shown in this abstract.

Key Words

Multistage Flash (MSF) distillers, Heat input section: brine heater (BH), Heat recovery and rejection sections, Single and multi pass heat exchanger (HE) bundles, HE inlet and outlet tube sheets, Heat transfer tubes, Vapor space, flash chambers, demister pads, Scale and sludge forming species and their reaction mechanisms, Scale control chemicals, alkaline scale of CaCO3 & Mg (OH)2 & Sponge rubber ball on-line mechanical cleaning system.

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1. INTRODUCTION Evaluation of scale control methodology and optimization thereof plus the review of

plant and heat exchanger tube inspection reports [1&2] have led to comprehension of

the proposed mechanism. Some recent analysis and review of published works

particularly those of Professor Ahmad Medhat Shams El-Din, (and his coworker Rizk)

[3 & 4] had stimulative effects to inquisitively look into the commonly and currently

stipulated reaction mechanisms, more specifically the sequence of events, i.e., reaction

hierarchy or steps. Over the years there were instances when scale was predominantly

reported in certain sections of MSF distillers where no one would expect any scale.

There were also cases, yet seldom, that such predominance were of an over riding

nature in those sections where no heavy scaling could be expected, even more, or in the

absence from places where scale is believed and typically observed to be the heaviest

[2]. Such deviations, unevenness and/or abnormalities require some logical

explanation.

This paper will primarily address reaction mechanisms especially the step-wise reaction

sequence and most specifically how and at which stage the carbon dioxide release

would be coming into effect [5]. The work of professor Shams El-Din and his coworker

particularly their postulation of scale nucleus circulation back into heat gain exchanger

tubes [4] is to be given the rightly deserved recognition. This paper will also discuss

antiscalant effectiveness and causes of its deterioration [6]. In addition the paper will

go into operational aspects as well, with very brief reference to design causes of scale

and sludge formation. Some emphasis is also placed upon the on-line mechanical

cleaning of heat exchanger tubes by sponge rubber balls. Ball cleaning requirements

and terminologies are addressed in some detail.

It is worth noting that there is a wealth of experience on MSF distillation in general and

scale control in particular in the Gulf Cooperation Council (GCC) states [1 - 5 & 7 – 11]

and the world at large [6, 12 & 13]. This paper is primarily based on local knowledge

and experiences. Also worth of noting that (primarily local) experiences have led to

antiscalant dose rate reductions as shown in Figure 1 [1, 2, 6 & 8].

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2. DISCUSSION In this part of the paper seven headings (subsections) will appear. First section is under

the heading of literature review. Under the second heading an effort is made into giving

as clear a picture as possible of certain unique field reports. This will be followed by

description of graphical presentation of these cases. The fourth, subsection will offer

what is felt to represent the situations and conditions typically occurring inside various

parts of an MSF distiller. The fifth subsection is devoted to ball cleaning. Under the

sixth heading deviations are discussed. Variation off the normal operating conditions

that could lead to abnormalities are further elaborated on in the closing subsection of

this discussion under the heading of causes and mechanisms. It is worth stressing that

one of the primary objectives of this paper is to offer a logical sequence of events and

an overall view.

It is also to be noted that acidic species are referred to in this paper as hydronium

(H2OH+) and not as (H+). Because the older notation of (H+) had created some

confusion in the true sense of the word. Due to the non-existence of a free moving

proton in acidic media. With the above in mind, reaction steps in chronological order

(A to E ) plus their overall balance (F) are presented as follows :

3 HCO3

- + 3 H2O ⇔ 3 CO32- + 3 H2OH+ (A)

CO32- + Ca2+ → CaCO3 ↓ (B)

2 CO32- + 2 H2O ⇔ 2 HCO-

3 + 2 OH- (C)

2 OH- + Mg2+ → Mg (OH)2 ↓ (D)

3 HCO3- + 3 H2OH+ → 6 H2O + 3 CO2 ↑ (E)

4HCO3- + Ca2++Mg2+ → CaCO3↓ + Mg(OH)2 ↓ + 3CO2 ↑ + H2O (F)

1.1 Literature Review

Alkaline scale formation in seawater distillation begets from the decomposition

hydrolysis of seawater bicarbonate ion as process temperature is increased. The mostly

observed scales that occur in multistage flash (MSF) distillers are found to be either Ca

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CO3 or Mg (OH) 2. The two are commonly referred to as alkaline scale. The reference

notation is a result of their laboratory formation dependence on solution alkalinity.

Currently there are couple of methods of controlling alkaline scale formation in MSF

especially where they would become an obstacle to continuous (problem free, i.e.,

smooth) operation. The areas where scale and sludge formation take place are: a - heat

transfer tubes and most specifically those of the brine heater, b - demister pads, c - water

boxes and tube sheets, d – flash chamber brine gates and orifices and e - (most remotely

to occur) their ingress into the vapor space [1-4]. Common control methods are through

bicarbonate depletion where it reacts with inorganic strong acids, e.g., sulfuric or

hydrochloric, which is dosed into the seawater make-up to MSF distillers. Alternatively,

alkaline scale formation is controlled by organic polymeric additives [14 & 15].

On the other hand, the formation of Non-Alkaline scale (mainly CaSO4) in MSF

distillers are controlled in almost all commercial plants by maintaining top brine

temperature (TBT) below 122 oC in order to limit its formation. Although causes and

products of alkaline scale and/or sludge are quite known, the mechanism of their

formation (according to A. Mubarak [16]) “has been a subject of a long persisting

ambiguities. Langlier and more recently Shams El Den (same as Din) et al spoke of

carbonate formation directly from bicarbonates which, in their view, will hydrolyze to

produce hydroxyl groups causing the precipitation”. The suggested mechanisms by

Shams El Din & Rizk are based on a “primary bimolecular reaction occurring at

moderate temperatures”. This reaction was cited to be:

2 HCO- 3 = CO2-

3 + CO2 + H2O (G) This will lead to the precipitation of CaCO3 as its solubility limits are exceeded. As

temperature is increased further carbonate ions hydrolyze by the following reaction:

CO2-

3 + H2O = 2OH- + CO2 (H)

They also suggested a unimolecular decomposition of bicarbonate along with its

neutralization by the following reactions (in an alphabetic respective order – though

they suggest parallel reactions):

HCO-

3 = CO2 + O H- (I)

HCO-3 + OH- = CO2-

3 + H2O (J)

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They had, therefore, suggested subsequent precipitation of Ca CO3 and (in a time lag

order behind) Mg(OH)2. In their view, the appearance of either one scale at lower

temperature stage(s) of MSF distiller by one mechanism or the other could be attributed

to circulation with the brine recycle [4].

Mubarak [16] also cited Dooly and Glatter also Harais and coworkers (in the same order of publications) who suggested the following order of reactions: HCO-

3 = OH- + CO2 (K) Mg++ + 2 O H- = Mg (OH)2 (L) HCO-

3 + OH- = CO2-3 + H2O (M)

Ca++ + CO2-

3 = Ca CO3 (N) According to Mubarak [16]: “It is clear that the ambiguity revolves around the

underlying kinetics and since the consequences to adopting either concept would, no

doubt, have a great impact on the operational as well as maintenance philosophy of

desalination plants, the need for a kinetic study that will hopefully contribute to the

understanding of such a complex system is therefore readily apparent. Likewise, since

the inhibiting action of antiscalant materials (namely, Belgard of Ciba Geigy ”-

misspelled Ciba which should have ended with an {a} rather than with an {e},

moreover Ciba Geigy long before 1997 had sold its antiscalant chemical business to

Food Manufacturing Company (FMC) which sold this year to Great Lakes, both of

USA – “and POC of Degussa)” – there is quite a long list of suppliers which Mubarak

have ignored unfairly. Mubarak continues to say that they describe their products

“under the term {Threshold Effect} has never been quantified, so to speak”. He

suggests, “a study addressing the intrinsic role of these materials in desalination

processes seemed very much in order”. Almost all earlier works in understanding of

scale and/or sludge formation, hence their control were conducted under laboratory

atmospheric or even sub atmospheric (including nitrogen purging) pressures [14 & 16].

It is worth at the end of this literature review to cite that a number of works were

published on non-conventional approaches in MSF scale prevention techniques. In these

somewhat new techniques the scale forming ions are removed ahead of exposing

seawater to heating in heat recovery section and then the brine heater of MSF distillers.

These works have taken two directions the earlier, which was proposed and tested

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during the late seventies and the eighties, was an extension to the conventional

depletion approach where by inorganic acids are used to deplete ions causing alkaline

scale. The process is then followed by ion exchange for sulfate reduction (by

approximately 50%: for example from 2900 Mediterranean water ionic content down to

1400 parts per million), hence for the Mediterranean water, the reduction of CaSO4

contents (even after concentration) to below its saturation [17]. Very recently (1998-99)

Saline Water Conversion Corporation represented by Research and Development Center

in Al-Jubail, Saudi Arabia published the bulk of its results on membrane seawater

softening by the use of commercially available brackish water softening Nanofiltration.

Nanofiltration permeate (NFP) was used as a substitute make-up water to an MSF pilot

plant distiller. NFP replaces acidified or antiscalant treated seawater [18-21]. It is

expected that this new approach will not become dominant before the end of the next

decade (2010 onwards). In view of which a better understanding of situations (as they

are today) is still much needed (see quotation from Mubarak [16] shown earlier under

this heading).

1.2 Field Observations

Certain abnormalities in sludge and scale presence in various parts of MSF distillers

were reported in the past. Heavy or uneven depositions were reported to occur inside

heat exchanger tubes from lower end to mid-section up to high temperature parts in heat

recovery stages. There were also cases when inlet sides of brine heater tubes were

fouled to a higher degree than outlet tube ends [1 & 2]. Moreover, there are references

to alkaline scale inverse temperature dependence solubility. (It is worth stressing that

the so-called inverse behavior resulting from the rate of generation of anions (as shown

by reactions A & C) rather than the temperature dependence of alkaline scale solubility

[12 & 13]).

In addition to the above, uneven sludge depositions were reported especially in water

boxes and on the face of tube sheets. Such uneven presence were either restricted to

certain areas of the water box and the tube sheet or very heavy in some specific stages

along the flow path of recirculating brine from cold to hot end of the recovery section or

across the brine heater. Uneven depositions of somewhat similar pattern were also

reported to take place inside water boxes and on inlet tube sheets and those of heat

recovery or rejection section inlets in particular. There were also cases when heavier

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depositions reported in lower temperature flash chambers and more specifically carry

over of scale into their demister pads [2].

1.3 Graphical Presentation In this subsection graphical presentation of reported configurations, behaviors and

observations will be specified. Figure 2 shows MSF distiller flow patterns [1 & 11].

Figure 3 shows the trend of temperature and concentration gradients in various

sections of a typical MSF distiller as the one shown in Figure 2b. Figure 4

represents the trend of commonly accepted deposition of sludge and/or scale along the

flow path of recirculating brine inside heat exchanger tubes of heat recovery and heat

input (brine heater) sections of MSF distillers as well as deposition in flash chambers

(including demister pads) of successive stages along the flow path of flashing brine

from hot to cold end of MSF distillers. Figure 5 is a group of graphical presentation

based on a correlation developed by a ball supplier for percent distribution as they are

influenced by variation in ball to tube ratios. Based on (Figure 5 a to c) (Table 1) was

prepared to summarize level of cleaning effectiveness.

In Figure 6a abnormalities in deposition in various parts of heat recovery and brine

heater tubes are shown. Then abnormalities in deposition in flash chambers including

demister pads are shown in Figure 6b.

Figures 7a, b & c show, respectively, some of the reported situations of (a) uneven

sludge build-up (b) heavy build up in water boxes and as (of detrimental effect) shown

on tube sheets and last (c) blockage of inlet tube sheet of MSF distillers, heat recovery

section. 1.4 Typical Deposition As shown in Figure 4 the commonly accepted trends of deposition should

progressively be in direct relation with temperature rise. This norm as was stated earlier

should not be misinterpreted as an inverse solubility relation but must be properly

identified as being due to the temperature dependence of bicarbonate and carbonate

hydrolysis as shown in reaction steps A through D. Deposition inside water boxes or

flash chambers including demister pads are to be related to antiscalant hydrolysis or the

rise in concentration (Total Dissolved Solids: TDS, usually in parts per million: ppm)

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through successive stages. This is found to be acceptable and thus can be viewed as one

of the contributing factors to deviations off commonly accepted norms. Increased

concentration is a result of distillate production by evaporation off flashing brine

solution in a step-wise.

1.5 Ball Cleaning as Scale Control Technique The most ingenious aspect of scale control could be the on-line mechanical scale and

sludge removal technique. This technique utilizes sponge rubber balls which are

slightly of larger diameter than the tube internal. They are pushed through heat transfer

tubes under the influence of solution pressure. For some heat exchanger tube bundles

these balls go through all the passes of a single exchanger bundle such as the ones in

most power plant condensers, for which this technique was initially invented. As

thermal desalination started to flourish specially by multistage flash (MSF) process, this

technique was introduced into the process. Its applicability and success was primarily

due to the close resemblance between multi pass power plant condenser and MSF heat

rejection or recovery sections as well as the mostly single pass heat input tube bundles

of MSF brine heaters (BHs). This technique came to MSF process at the peak of its

development while most of the operating units were employing polyphosphate as

antiscalant. In such time the success of ball cleaning was due to the heavy sludge

forming characteristic of the polyphosphate. Ball cleaning was extremely beneficial to

brine heater tubes and quite useful for heat recovery section tubes and of certain benefits

to heat rejection section tubes.

From 1980 onwards the majority of MSF installed (new) plants were equipped with

such systems for on-line mechanical tube cleaning. The system was also introduced to a

number of older plants as retrofits. Almost all newer installations contain either a

combined or separable cleaning loops for heat input (BH) and recovery sections. It is

evident from the above that ball cleaning started to take an appreciable role in scale and

more specifically sludge build up control inside heat transfer tubes of MSF distillers. It

is also evident that the system started to appear during the last couple of decades out of

3.5 decades of MSF since its inception. The reason for highlighting the age difference is

to prepare the reader for some misconceptions, which have surrounded this on-line

mechanical tube cleaning technique.

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One of these misconceptions is the thought that rubber balls are so orderly in their

movement. That is to say, when a group of successive bundles (for example) of 3000

tubes each are charged with 3000 balls, could each ball go through one of the 3000

tubes? Such a naive orderly flow of balls is still one of the misconceptions that occupies

the minds of some of the users. In such case, the user would be naively expecting that

these 3000 balls would march (in a militarily ordered fashion) into the 3000 tubes of

each successive bundle. In fact, ball system designers and manufacturers have come up

with correlations that do not look at the matter in this naive way. Such correlations

predict ball to tube probability distribution for a given number ratios of balls to tubes

similar to the ones shown in Figure 5a to c. Manufacturers would, therefore, suggest

to keep the number ratio of balls to tubes around 0.3 ± 0.1, i.e. 20 to 40% of tubes. This

aspect (of ball to tube ratio) is stressed upon because it has been established that missing

even a large number of tubes in any one cycle is (by far) of lower negative impact on

cleaning than the presence of more than one ball in any particular tube per any one cycle

of ball cleaning. The argument is further supported by stating that the missed tubes in

any one-ball cycle (again in a probabilistic view) would be cleaned in one of the

successive cycles. While multiple balls presence in one tube would cause appreciable

drop in velocity through that particular tube thus causing scaling due to low velocity,

see reference [2]. Moreover, subsequent i.e., following, balls are in fact going through a

tube, which is supposedly had just been cleaned by the front ball. The orderly march of

balls towards tubes is the most dangerous misconception. Another aspect of ball

cleaning belonging to the category of misconceptions needs to be clarified and thus

abolished. This second misconception is related to a parameter which most probably

has been blindly copied out of single pass power plant condensers, where the issue of

balls to tube ratio and distribution are of by far lower impact than they are in multi pass

MSF recovery and brine heater tube bundles as a flow loop. This is mainly due to the

fact that repeated exists and entries of balls into such bundles could aggravate the

situation (on a probability basis). To illustrate this misrepresentative parameter, (which

is referred to as balls per tube-day or even in some mathematically incorrect term as:

balls per tube per day) the following tabulation is shown.

This tabulation highlights the fact that balls per tube-day (b/t-d) are not only a

meaningless parameter but also a quite misleading one. The reader may convince

himself further by developing additional similar tabulations using Figure 5 for the

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presence of more than one ball per tube which would propagate its negative impact as

the number of balls per cycle is increased.

1.6 Alkalinity Based on classical perception of scale formation, it is customary to measure overall

change in alkalinity as loss in total alkalinity (LTA). Where in distillation processes

particularly in MSF chemical monitoring and LTA are used as a scaling indication.

This practice is found to be valid yet it is to be highlighted that such indication is not to

be understood as a confirmation of scale formation. Since loss of alkalinity is due to

successive reaction steps. That is to say reactions (G) & (H) will lead to a drop of

approximately 20% of converted bicarbonate ions to carbonate. The second step in

LTA reduction is due to precipitation of calcium carbonate. Then the third step is the

conversion of carbonate to hydroxide, which would have no measurable effect on

alkalinity, i.e., no measurable LTA. Fourth step is the formation of magnesium

hydroxide. This fourth step would be responsible for measurable LTA.

On the other hand, the proposed alternative mechanism shown in reaction (A) through

(E) would cause changes in alkalinity in a slightly different stepwise fashion. The only

difference is in the magnitude of change associated with carbonate and hydroxide

formation steps, in the proposed mechanism. From this it could be seen that the overall

effects will not be any different whether embracing the conventional or the proposed

(alternative) approach.

Chemical analysis results especially those taken at extremely low antiscalant dose rate,

while operating MSF pilot unit; are shown in Table 2.

Positive LTA across the tubes suggests scale formation yet there is no change in either

calcium or magnesium concentration. This clearly demonstrates that even though the

LTA across MSF pilot unit tubes are of +11 there is no loss of calcium or magnesium

shown in Table 2, i.e., no scale is formed. This may be due to either the proposed

step-wise alkalinity change of the earlier proposed concept of weathering at the time of

sampling from brine heater outlet due to pressure release.

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Sulfate results shown in Table 2 were used to calculate the alkalinity of brine blow

down. The alkalinity of brine blow down by calculation found to be 147 ppm while the

measured value is only 138. Thus LTA by difference (147-138) should equal + 9.

Sulfate results were also used to establish that there was no depletion of calcium and

magnesium in flash chambers. Thus, the step-wise alkalinity change is further

supported by calculated calcium and magnesium expected concentration in blow down

when compared with the measured value of its concentration. Yet the magnesium loss

of 20 ppm seems surprising! It could, therefore, be attributed to analytical errors as LTA

change dose not support the loss of 20 ppm of magnesium. Conversely, it could be said

that magnesium dioxide is formed even in the absence of CaCO3 formation. The zero

change in this unique concentration confirms that no scale is formed in flash chambers

It is, therefore, suggested that scale formation is not to be directly related to positive

LTA. That is to say:

“Positive LTA is not to be taken as a confirmation of scale formation but as An indication of increased scale formation potential”. 1.7 Deviations Figures 6 through 8 show trends of deviations off the normally reported situations. In

Figure 6a deposition of scale at various parts of recovery tubes as well as the inlet tube

ends of the brine heater are shown. These deviations can be due to some variation, most

probably; of approximately less than one (to few) hour(s) duration in make-up, brine

recycle or antiscalant dosage flow. Should there be starvation of make-up or antiscalant

then concentration of scale forming species or their potential will rise, thus their

chelation threshold limits are surpassed. In such events scaling will take place very

rapidly. And depending on where the threshold limits are exceeded (depending on

concentration and/or mixing effects), then scaling will occur at that location at a given

temperature regardless of how hot the brine is. Potential for further scaling down

stream of the said location would be very low even when solution temperature or the

TDS is increased. That is to say, should scaling in some specific location along the flow

path predominate then a marked reduction in the concentration of scale forming species

will characterize the brine solution thereafter along its flow path inside heat gain tubes

or through rising TDS flashing brine. Lower flash chambers and demister pads fouling

as shown in Figure 6b could be attributed to the above phenomenon. Nevertheless,

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there are other reasons, which could have enhancing influence along the flashing brine

stream. These are (i) the effect of rising concentration due to distillate production in

successive stages, i.e., thus increasing the TDS until it would reach the brine blow down

level, (ii) operation with abnormal flashing brine level(s) in such stage(s) and (iii)

excessive splashing that would be occurring in that (those) particular stage(s) regardless

of how low (or high) its (their) temperature(s) (or TDS, respectively) might be. Figure 7 shows abnormalities, which are mostly related to design but could also be

due to mal-operation. Uneven flow regime in water boxes could lead to deposition of

mainly sludge in a patch wise fashion on inlet tube sheets as those shown in Figure 7a.

On the other hand, heavy silt content and possibly hydrolysis of certain brands of

antiscalants could lead to heavy deposition in water boxes or on tube sheets as shown in

Figure 7b. Heavy sludge could digress into the tubes. This could be enhanced by

nucleation in water boxes or even as a result of brine recycling. The later could be the

most probable cause of fouling inside tubes of low temperature stages of heat recovery

section as well as at or close to tube inlets of brine heater. Such fouling would also be

enhanced by variation in flow i.e., starvation, hence differential concentration which

was mentioned earlier. Scale and/or sludge may also form within the tubes where it

could deposit. Furthermore, it could travel through tubes then appear on outlet tube

sheets.

It is worth to draw attentions to the similarity in nature as well as causes of blockage of

heat recovery and heat rejection inlet tube sheets as shown in Figures 7c & 8,

respectively. In spite of their similarities there are couple of differences which are also

worthwhile to highlight. The nature of re-circulation can lead to inclusion of (i) lose

debris, e.g., scale and corrosion products; also (ii) sponge ball unwise application,

which could characterize recovery section inlet tube sheet fouling. These aspects are

believed to be the primary causes that could lead to recovery section tube mouth

blockage. On the other hand, the blockage of heat rejection inlet tube sheet as shown in

Figure 8 is mainly due to poor filtration and/or active seawater feed giving rise to the

growth of barnacles as a result of improper disinfection.

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1.8 Causes and Mechanisms The proposed reaction steps (A through E) can very well support the common

understanding of CaCO3 precipitation ahead of Mg(OH)2. On the other hand, the

proposed steps especially (A through D) would not preclude precipitation of Mg (OH)2

right after the precipitation of calcium carbonate due to magnesium hardness. This is

further enhanced by the suggested starvation, i.e., reduction in seawater make-up flow

or antiscalant dosing. It can be explained by the traditional concepts of inhibition by

nucleus chelation of scale and crystal distortion. Moreover, some recent observations

and chemical analysis came as strong supports to the suggested higher temperature zone

precipitation of Mg(OH)2.

Alternatively, it is proposed that there is higher affinity of antiscalant active sites

towards higher valency ions such as di- (or higher) valent cations of calcium,

magnesium or even ferric, ferrous and cupric ions. The higher affinity is suggested in

comparison to monovalent cations such as those of sodium (and anions of chloride,

hydroxide or bicarbonate). It is also worth noting that this affinity is controlled by the

sizes and shapes of scaling ionic species hence their attachment at active sites on the

molecular chain of polymer additive [22-23]. Above approach can thus support the

scale nucleation (as a step) on the parameter of antiscalant by attraction of scaling

species to sites where oppositely charged species are already in place. This could be the

way by which antiscalant effectiveness is reduced yet there are other reasons for the

antiscalant effectiveness deterioration. It is commonly accepted that antiscalant

especially phosphorous-based brands undergo hydrolysis [1 & 10]. In addition, there is

yet a third way by which antiscalant activity deterioration could occur. This third mean

of deterioration is postulated on the suggested higher affinity, which can be applied to

highly polarized species and charged corrosion products, e.g., silt and ferric ions,

respectively. Such mechanism is still a proposed concept.

Furthermore, heavy sludge was also reported in flash chambers with detrimental effect

on demister pad performance. On the other hand, it was established that certain brands

of antiscalant lead to higher sludge formation than other brands especially in turbid

seawaters. This can be related to the third cause of antiscalant deterioration discussed in

the previous paragraph. In such cases there are situations when sludge (if left in the

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system) could solidify. It is worth to mention that sludge solidification into scale takes

place predominantly in low flow zones especially on outlet tube sheets [1 & 2].

Flow path blockage either as shown in Figure 7c or Figure 8 of inlet tube sheets of

heat recovery and rejection section, respectively. This is primarily caused by lose

derbies that are carried through by the solution, e.g., broken pieces of shells, scales,

corrosion products or even somewhat foreign matters like sea weeds or wood (and

sometimes peculiar things such as plastic bags and nylon robs), particularly on the inlet

tube sheet to heat rejection section. It is to be said that sponge balls could turn devilish

hence further escalate such blockage (see Figure 7c), especially on the inlet tube sheet

to heat recovery section. They could have an over toning effect in the case of uneven

distribution of flow or due to the presence of more than one ball in a particular tube

leading to blockage under heavily slugged situations as shown in Figure 7a. Also

blockages by balls similar to that shown in Figure 7c [2]. As was discussed earlier,

misinformed users could turn ball cleaning into a heavy liability on heat transfer

through recovery section and brine heater tubes of MSF distillers. Reference is made to

Figure 5 and Table 1.

2. CONCLUSION

Scale and sludge formation have negative impacts on heat transfer, pressure drops, as

well as flow starvation and above all deterioration of distillate product water yield and

quality. Such formation can by no means be stopped or totally eliminated. Even though,

safe prolonged operation can be achieved by bringing deposition of sludge and scale

under control [9]. For such control to be effective deposition mechanisms, causes and

passivation techniques are to be clearly understood. This understanding would be the

primary initial step in any full sludge and scale deposition control program.

The proposed mechanism suggests that the formation of low temperature (instead of the

so called) alkaline scale inside heat gain exchanger tubes of heat recovery section and

the brine heater could proceed without the need for carbon dioxide generation step to

take place. By this, one can explain scaling inside heat gain tubes even where the re-

circulating brine is under pressure and the CO2 generation step is self-limiting in the

absence of gaseous CO2 release off the re-circulating brine solution. Should the

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proposed mechanism is proven to be correct then the so-called alkaline scales are to be

renamed as suggested above.

Furthermore, sludge formation is primarily due to (i) salt growth around antiscalant

molecular chain, (ii) hydrolysis of antiscalant molecular chains and (iii) silt and

corrosion product polarized species accumulation on the molecular chain of antiscalant

material. All these types of build-ups on antiscalant molecular chains would render the

additive less effective. It is, therefore, important to monitor Residual Antiscalant

Potential (RAP) [1]. Taking into consideration that the suggested mechanism and the

deliberated upon causes will lower production cost through reduced use of

consumables, e.g., antiscalants, cleaning balls and acids; plus better prolonged operation

thus shorter down time hence minimized production losses and longer plant productive

life. Moreover, improper approach to ball cleaning may also give rise to sludge and/or

scale deposition inside heat transfer tubes, which would also add to the above negative

impacts.

One of the primary observations, which this analysis would put forward, is that the

carbon dioxide evolution step (E) could have very little effect on scale formation inside

heat exchanger tubes. This proposition is put forward in order to give way to explain

scale formation even if CO2 gas is not released where its reaction is suppressed due to

its concentration. Reaction steps suggested in this paper differ from other proposed

mechanisms in the sense that scale formation inside heat exchanger tubes can proceed in

an Acidic media (see reaction step A) without the need for CO2 generation step to be

occurring. That is to say, the sequence is predominantly ending with step (D) before

any appreciable CO2 is released off the flashing brine, i.e., in flash chambers; where

carbon dioxide generation hence Evolution is controlled by reaction step (E). In order

to verify the validity of the proposed mechanism some well thought-out experiments are

to be conducted. The most detrimental checks on this validity verification are felt to be

by the results of pH measurements and detection hence the fate of various species.

The commonly introduced concept referring to: “Positive LTA as scaling yard stick” is

to be replaced with “positive LTA is not a confirmation sign of scale formation but it

is only an indication of increased scale formation potential”.

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3. RECOMMENDATIONS

3.1. Process Control

Based on this review certain process parameters are to be placed under scrutiny, hence

closely monitored and controlled within clearly specified ranges. These parameters are

outlined hereafter.

3.1.1 Top Brine Temperature (TBT)

Reference is to be made to the range given in Figure 1 of 90 to 115 ºC.

3.1.2 Antiscalant Dose Rate

Again reference is made to Figure 1. In this respect, it is recommended that the

proposed optimum dose rates are to be targets that could be reached through careful

investigation at each site based on seawater and plant design specifics. It is also

recommended to maintain a sufficiently healthy safety margin of antiscalant (above

minimum requirements), i.e., to apply an optimum rather than the minimum required

dose rate. In order to coupe up with variations in conditions of the plant and the

environment, some of the due most crucial changes, which deserve close attention, are:

1. Antiscalant preparation and dosing system malfunction

2. Make-up flow rate

3. Brine recycle flow rate hence brine tube velocity

4. Malfunction in ball cleaning system

5. Sea roughness hence seawater turbidity

6. Steam de-superheating, flow and temperature hence the TBT

3.1.3 Residual antiscalant potential (RAP)

RAP has not become a common controlling parameter because it would require

elaborate and tedious chemical analysis, yet it is felt to be quite detrimental. Its

adaptation as a monitoring control parameter is strongly recommended.

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3.1.4 Ball Cleaning Philosophy

It is strongly recommended to upgrade the understanding of ball cleaning aspects.

Misconceptions, e.g., highball to tube ratios, and ball per tube per day are to be

abolished.

3.2 Future Work

As a result of this analysis a number of tests are recommended. These tests are to be

aimed at verifying the proposed reaction steps (and their sequence). The proposed tests

can be divided into three categories as shown below.

3.2.1 Laboratory testing

1. Bench top tests to verify the validity of the proposed mechanism by

detection of various species and more importantly pH value

measurements.

2. Pressurized plug flow device is to be designed with proper selective ion

electrode detection and pH value measurements to study reaction

constants, temperatures, pressures and time dependence; in order to

verify the proposed mechanism at various ionic species and antiscalant

concentrations.

3.2.2 Pilot plant testing

1. Some tests (to verify points raised in 2a 2 above) are to be devised on a

pilot plant MSF distiller. In Figure 2a locations of additional

instrumentation are identified by crosses.

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3.2.3 Commercial plant testing

1. The third phase of the work is to be carried out on some commercial

MSF distillers. They are to be equipped with essential components to

carry out tests of similar nature to those proposed in a pilot plant (in 2b1

above). Figure 2b shows monitoring locations for the proposed tests as

marked (again) by crosses.

ACKNOWLEDGMENT The author acknowledges the support as well as the fruitful discussion with Dr. Dalvi

and Dr. Osman, heads of Chemistry and Thermal Departments, respectively, at SWCC

Research and Development Center (RDC), Al-Jubail, Saudi Arabia. Also the assistance

provided by Messrs Mustafa M. Ghulam, Khalid Bamardoof, Rafique Mubarak

Imambakhsh and Syed Mohammed Iqbal.

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Table 1. Ball per (day-tube) as a misrepresentative parameter

Case No. of tubes No. of balls

per cycle No. of cycles/day

Balls per (tube-day)

Single ball per tube-day*

Effective cleaning

A 3000 3000 1 1 37% Poor B 3000 1500 2 1 50% Good C 3000 750 4 1 54% Very good

• See Figure 5 as effective cleaning is a cumulative value. With a 10% reduction factor to take care of probable overlaps in multi cycle operation.

Table 2. Pilot Plant Chemical Analytical Results Parameter Unit Seawater

to HRJS Brine recycle To HRCS

Brine Recycle Ex. BH

Brine blow down

pH Unit 8.2 8.40 8.30 8.44 Conductivity µS/cm 58000 67800 67500 72000 M.Alkalinity mg/l* 128 137 126 138 Sulfate mg/l 3200 3795 3795 4015 TDS mg/l 44000 58260 57780 62820 Total Hardness mg/l 6700 9600 9600 10200 Calcium mg/l 490 621 621 641 Magnesium mg/l 1420 1958 1958 2079 *mg/l ≅ ppm, in the case of alkalinity it is measured as CaCO3

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Figure 1. Originally Proposed Versus Optimized Antiscalant Dose Rate

DISTILLATE

DISTILLATE PUMPS

SEA WATER SUPPLY

REJECT SEA WATER TO DRAIN / NF

MAKEUPFROM RO

HEAT RECOVERY STAGES

BRINE RECIRCULATION PUMP

MAKEUP PUMP

HEAT REJECT STAGES

BLOWDOWN

LIC

LIC

TIC

DEAERATOR

FLASHTANK

TO EJECTOR SYSTEM

CONDENSATE

LIC

LIC

DECARBONATOR

BLOWER

FIC

ANTIFOAM

ANTI -SCALANT

SODIUMBISULFITE

ACID

STEAMSUPPLY

CONDENSATETO FLASH TANK

EJECTOR CONDENSER

Figure 2(a). Schematic Diagram of MSF Pilot Plant

REDUCTION IN ANTISCALANT DOSE RATES(Before & After Optimization Program)

0

2

4

6

8

10

12

14

16

80 85 90 95 100 105 110 115 120

TOP BRINE TEMPERATURE (TBT), O C

AN

TIS

CA

LAN

T D

OS

E R

ATE

, ppm

Dose Rates Recommended in 1981

Dose Rates as Optimized in 1986

Dose Rates as Optimized in 1996

Dose Rates as Proposed Optimum

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Figure 2(b). Schematic Diagram of Al-Jubail MSF Plant Phase II

SEA WATER

X X X X

SEA WATER REJECT DISCHARGE

BRINE HEATER

ANTISCALANT TANK

SODIUM SULFITE

ACID TANK ANTIFOAM TANK

CONDENSATE PUMP HEAT RECOVERY STAGES

DEAERATOR

H/P STEAM

L/P STEAM

BRINE BLOWDOWN PUMP

BRINE RECYCLE PUMP MAKEUP

TEMPERING PUMP

EJECTOR CONDENSER SYSTEM

CONDENSATE

PRODUCT WATER

FBT FBC

RBC

SWC

BH HRC HRJ

RBT

SWT

FBT - Flashing Brine Temperature RBT - Recyc le Brine Temperature FBC - Flashing Brine Concentration RBC - Recycle Brine Concentration SWC - Sea Water Concentration

SWT - Sea Water Temperature

Temp. Conc.

Figure 3. Temperature & concentration gradients in MSF Distillers

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Figure 4, TYPICAL TREND OF DEPOSITION IN MSF DISTILLERS

DE

PO

SIT

ION

TF

FC-DPF

BH HRC HRJ

TF - Tube Fouling

FC-DPF - Flash Chamber & Demister Pad Fouling

Figure 4. Typical trend of deposition in MSF distillers

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a) 100% Balls to Tubes

b) 50% Balls to Tubes

c) 25% Balls to Tubes

32.1

37

18

9

3

0.9

% Dist.

51

27.5

15

5

1

0.5

% Dist.

66.4

20

10

2

0.6

0.1

% Dist.

Figure 5. Ball Distribution Probabilities with different Percent of Balls to Tubes (in any one cycle)

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Figure 6 A. Deviations off normal deposition tendency in heat exchangers Tubes of MSF distillers

Figures 6b. Deviations off normal deposition tendency in demister pads of MSF distillers.

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Figure 7A. Uneven Slugging Figure 7B. Heavy Slugging

Figure 7C. Blockage of Heat Recovery Figure 8. Blockage of Heat Recovery Section Inlet Tube Sheet Section Inlet Tube Sheet

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