FOULING PHENOMENA IN MULTI STAGE FLASH (MSF) DISTILLERS 1

Transcription

1 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 - CO CO H 2 O 2-3 CO H 2 OH + (A) + Ca 2+ CaCO 3 (B) + 2 H 2 O - 2 HCO 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

2 2 OH - + Mg 2+ Mg (OH) 2 (D) - 3 HCO H 2 OH + 6 H 2 O + 3 CO 2 (E) - 4HCO 3 + Ca 2+ +Mg 2+ CaCO 3 + Mg(OH) 2 + 3CO 2 + H 2 O (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 CaCO 3 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 CaCO 3 & Mg (OH) 2 & Sponge rubber ball on-line mechanical cleaning system. 2104

3 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]. 2105

4 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 (H 2 OH + ) 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 : HCO H 2 O 3 CO H 2 OH + (A) CO CO Ca 2+ CaCO 3 (B) + 2 H 2 O 2 HCO OH - (C) 2 OH - + Mg 2+ Mg (OH) 2 (D) - 3 HCO H 2 OH + 6 H 2 O + 3 CO 2 (E) - 4HCO 3 + Ca 2+ +Mg 2+ CaCO 3 + Mg(OH) 2 + 3CO 2 + H 2 O (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 2106

5 CO 3 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 CaSO 4 ) in MSF distillers are controlled in almost all commercial plants by maintaining top brine temperature (TBT) below 122 o C 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 = CO CO 2 + H 2 O (G) This will lead to the precipitation of CaCO 3 as its solubility limits are exceeded. As temperature is increased further carbonate ions hydrolyze by the following reaction: CO H 2 O = 2OH - + CO 2 (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 = CO 2 + O H - HCO OH - = CO H 2 O (I) (J) 2107

6 They had, therefore, suggested subsequent precipitation of Ca CO 3 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 - + CO 2 (K) Mg O H - = Mg (OH) 2 (L) HCO OH - = CO H 2 O (M) Ca ++ + CO 2-3 = Ca CO 3 (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 2108

7 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 CaSO 4 contents (even after concentration) to below its saturation [17]. Very recently ( ) 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 2109

8 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) 2110

9 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. 2111

10 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 2112

11 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. 2114

12 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 ( ) 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 CaCO 3 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, 2114

13 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. 2115

14 1.8 Causes and Mechanisms The proposed reaction steps (A through E) can very well support the common understanding of CaCO 3 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 2116

15 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 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 recirculating brine is under pressure and the CO 2 generation step is self-limiting in the absence of gaseous CO 2 release off the re-circulating brine solution. Should the 2117

16 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 CO 2 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 CO 2 generation step to be occurring. That is to say, the sequence is predominantly ending with step (D) before any appreciable CO 2 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. 2118

17 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 Top Brine Temperature (TBT) Reference is to be made to the range given in Figure 1 of 90 to 115 ºC 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 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. 2119

18 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 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 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. 2120

19 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. 2121

20 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 % Poor B % Good C % 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 Brine recycle Brine Recycle Brine blow to HRJS To HRCS Ex. BH down ph Unit Conductivity µs/cm M.Alkalinity mg/l * Sulfate mg/l TDS mg/l Total Hardness mg/l Calcium mg/l Magnesium mg/l *mg/l ppm, in the case of alkalinity it is measured as CaCO

21 16 14 REDUCTION IN ANTISCALANT DOSE RATES (Before & After Optimization Program) 12 Dose Rates Recommended in 1981 Dose Rates as Optimized in 1986 Dose Rates as Optimized in 1996 Dose Rates as Proposed Optimum ANTISCALANT DOSE RATE, ppm TOP BRINE TEMPERATURE (TBT), O C Figure 1. Originally Proposed Versus Optimized Antiscalant Dose Rate TO EJECTOR SYSTEM EJECTOR CONDENSER TIC REJECT SEA WATER TO DRAIN / NF FIC MAKEUP FROM RO STEAM SUPPLY SEA WATER SUPPLY ANTI - SCALANT LIC BLOWER ANTIFOAM CONDENSATE TO FLASH TANK HEAT RECOVERY STAGES BLOWDOWN HEAT REJECT STAGES MAKEUP PUMP BRINE RECIRCULATION PUMP FLASH TANK LIC DEAERATOR CONDENSATE LIC DECARBONATOR LIC SODIUM BISULFITE ACID DISTILLATE DISTILLATE PUMPS Figure 2(a). Schematic Diagram of MSF Pilot Plant 2123

22 EJECTOR CONDENSER SYSTEM H/P STEAM X X X X L/P STEAM SEA WATER REJECT DISCHARGE SEA WATER BRINE HEATER PRODUC T WATER CONDENSATE PUMP HEAT RECOVERY STAGES CONDENSATE ANTISCALANT TANK SODIUM SULFITE ACID TANK ANTIFOAM TANK DEAERATOR BRINE RECYCLE PUMP MAKEUP TEMPERING PUMP BRINE BLOWDOWN PUMP Figure 2(b). Schematic Diagram of Al-Jubail MSF Plant Phase II Temp. FBT FBC Conc. RBC FBT - Flashing Brine Temperature RBT - Recyc le Brine Temperature SWT - Sea Water Temperature FBC - Flashing Brine Concentration RBC - Recycle Brine Concentration SWC - Sea Water Concentration RBT SWC SWT BH HRC HRJ Figure 3. Temperature & concentration gradients in MSF Distillers 2124

23 Figure 4, TYPICAL TREND OF DEPOSITION IN MSF DISTILLERS DEPOSITION TF TF - Tube Fouling FC-DPF - Flash Chamber & Demister Pad Fouling FC-DPF BH HRC HRJ Figure 4. Typical trend of deposition in MSF distillers 2125

24 % Dist a) 100% Balls to Tubes % Dist. 51 b) 50% Balls to Tubes % Dist. c) 25% Balls to Tubes Figure 5. Ball Distribution Probabilities with different Percent of Balls to Tubes (in any one cycle) 2126

25 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. 2127

26 Figure 7A. Uneven Slugging Figure 7B. Heavy Slugging Figure 7C. Blockage of Heat Recovery Section Inlet Tube Sheet Figure 8. Blockage of Heat Recovery Section Inlet Tube Sheet 2128

27 REFERENCES 1. Al-Sofi, M.AK., Al-Hussain M. A. and Al-Zahrani S. G., (1987), Additive Scale Control Optimization and Operation Modes, Desalination, 66, Al-Sofi, M. AK., Khalaf, S. and Al-Omran, A. A., (1989), Practical Experience in Scale Control, Desalination, 73, Shams El-Din, A. M. and Rizk, A. M., (1988), on the thermal stability of the HCO - 3 and CO 2-3 ions in aqueous solutions, Desalination, 69, Shams El-Din, A. M. and Rizk, A. M., (1994), Brine and scale chemistry in MSF distillers, Desalination, 99, Nada N., (1986), Operating Experience of MSF units, Topics in Desalination, SWCC. 6. Finan, M. A., Smith, S., Evans, C. K., Muir, J. W. H., (1989), Belgard - 15 years Experience in Scale Control, Desalination, 73, Al-Sofi, M.AK., Hamed, O.A. and Mustafa, G.M., (1997), Evaluation of a modified polyphosphonate as an additive in Multistage Flash distillers, 2 nd O&M Acquired Experiences in SWCC, Al-Jubail, Saudi Arabia. 8. Al-Gobaisi, D. M. K., (1994), A quarter century of seawater desalination by large multistage flash plants in Abu Dhabi, Desalination, 99, Al-Sofi, M.AK., Al-Hussain, M. A. Al-Omran, A. A. and Farran, K. M., (1993), A full decade of operating experience in Al-Khobar Phase II MSF plants, 6th IDA World Congress, Yokohama, Japan. 10. Al-Sofi, M. AK., El-Sayed, E. F., Imam, M., Mustafa G. M., Hamada, T., Haseba, S. and Goto T., (1995), Heat Transfer Measurement as a Criterion for Performance Evaluation of Scale Inhibition in MSF plants, 7th IDA World Congress, Abu Dhabi, UAE, 3, Al-Sofi, M.AK., Hamada T., Tanaka Y. and Al-Sulami S. A., (1994), Laboratory testing of antiscalant threshold effectiveness, 2nd WSTA Gulf Water Conference, Manama, Bahrain, 2, Hodgkiss, T. (1980), A short course on desalination technology, chapter 19, King Abdul Aziz University, Jeddah, Saudi Arabia, p Harris, A., (1983), Desalination technology, developments and practice, chapter 2, Applied science publishers, London and New York, 31-9 (Edited by: Andrew Porteous). 14. Shams El-Din, A.M. and Rizk, A.M., (1989), The problem of Alkaline scale formation from a study on Arabian gulf water, Desalination, 71, Shams El-Din, A.M. and Rizk, A.M., (1988), A 700-Day Experiment with Belgerd EV 2000 as anti-scale agent in MSF distillers, Desalination, 69, Mubarak, A., (1997), A kinetic model for scale formation in MSF desalination plants effect of antiscalants, Third WSTA Gulf Water Conference, Muscat, Sultanate of Oman, 2,

28 17. Maio, A. De., Zannoni, R., Ronzoni, A., Boari, G., Siberti, L. and Santoni, (1983), Desalination of seawater make-up to MSF, Desalination, 45, Hassan, A.M., Al-Sofi, M.AK., Al-Amodi, A.S., Jamaluddin, A.T.M., Dalvi, A.G.I., Kither, N.M., Mustafa, G., Al-Tissan, I.A.R., (1998), A two part article, in Desalination and Water Reuse Quarterly, in May-June & September October issues, Vol. 8/1&2, & Hassan, A.M., Al-Sofi, M.AK., Al-Amoudi, A.S., Jamaluddin, A.T.M., Farooque, A.M., Rowaili, A., Dalvi, A.G.I., Kither, N.M., Mustafa, G.M., and Al-Tissan, I.A.R., (1998), A new approach to membrane and thermal seawater desalination processes using nanofiltration membranes (part 1), Desalination, 118, Hassan, A.M., Al-Sofi, M.AK., Al-Amoudi, A.S., Jamaluddin, A.T.M., Farooque, A.M., Rowaili, A., Dalvi, A.G.I., Kither, N.M., Mustafa, G.M., and Al-Tissan, I.A.R., (1999), A new approach to membrane and thermal desalination processes using nanofiltration membranes (part 2), 4 th WSTA Gulf Water Conference Proceedings, Vol. 2, Manama, Bahrain. 21. Al-Sofi, M.AK., Hassan, A.M., Mustafa, G.M., Dalvi, A.G.I., Kither, N.M. (1998), Nanofiltration as a means of achieving higher TBT of 120 C in MSF, Desalination, 118, Dalvi, AG. I., Kither, M.N.M., AL-Sulami, S.A., Sahul, K. and Al-Rasheed, R.A., (1999), Effect of some chemical constituents in recycle brine on the performance of scale control additives, SWCC RDC Report No. TR 3803 / APP Dalvi, AG. I., Kither, M.N.M., Al-Sulami, S.A., Sahul, K. and Al-Rasheed, (1999), Effect of various forms of Iron in recycle brine on the performance of scale control additives in MSF Desalination plant, WSTA 4 th Gulf Water Conference, Bahrain, Vol. 2,