A NANOFILTRATION (NF) MEMBRANE
                          PRETREATMENT OF SWRO FEED
                              AND MSF MAKE-UP
                          Ata M. Hassan, M. AK. Al-Sofi, Ahmed Al-Amoudi,
                             A. T. M. Jamaluddin, N. Kither Mohammad,
                                Ghulam Mustafa and Ibrahim Al-Tisan

                                      Research and Development Center
                                     Saline Water Conversion Cooperation
                                      P. O. Box # 8328, Al-Jubail 31951
                                           Kingdom of Saudi Arabia.


Removal or significant reduction of hardness in seawater, lowering of TDS and removal of
turbidity from the feed to seawater desalination plants should lead to an improvement in the
conventional seawater desalination processes by lowering of their energy requirement and
chemical consumption, by increasing water recovery with the ultimate benefit of lowering the
cost of fresh water production. A process that allows for the achievement of the above
objectives should represent a milestone in seawater technology. This has been shown at
SWCC RDC to be feasible by a combination of NF with the conventional seawater
desalination processes. The seawater treatment first with the NF membrane process is
significantly beneficial, it produced a new seawater product considerably different from Gulf
seawater and for the purpose of desalination is far superior to it in qualities and without the
problems normally associated with high concentration in seawater of scale forming ions,
high TDS and high turbidity. The results obtained from a combined fully integrated NF-
SWRO pilot plant demonstrated that hardness is removed by the NF unit to an ion rejection
values of : Ca++ 80.7%, Mg++ 87.7%, total hardness 86.5%, SO4= 93.3%, HCO3- as CaCO3
63.3%.       In addition the Cl-, Na+ and K+ ions rejection is 26.7%. This latter effect in
combination with hardness removal resulted in a decline in Gulf Seawater TDS from 44,046
ppm to 27,619 ppm for a net lowering of TDS by 37.3%. The seawater pH is also reduced
from 8.2 to 7.85. Moreover, this NF secondary treatment to media filtration of raw Gulf
Seawater feed produced high quality feed from which nearly all the turbidity and
microorganism are removed.

  Abstract of Paper presented at IDA World Congress on Desalination and Water Reuse, Oct. 6-9, 1997 Madrid, Spain.
Paper is in Press for publication in the International Desalination & Water Reuse Quarterly. Detail of this investigation shall
appear in next year Vol. 9 of SWCC Research Activities and Studies.

Operation of SWRO pilot plant unit with this new NF product for over 2100 hours produced
excellent results. Product water recovery from the SWRO unit was raised from less than 25%
when operated alone at an applied pressure (Pappl) of 60 bars to over 46% when operated at
the same Pappl in combination with a NF-SWRO system and rose to an amazing recovery of
58% at Pappl of 70 bars. Similarly, the quantity of product increased significantly by over
70%, while the differential pressure across the SWRO membrane (∆P) remained at a steady
constant low value. Better results are expected from a commercial NF-SWRO system.

Likewise, because of the same effects, it is possible to operate MSF plants without use of
antiscalant at high top brine temperature of 120 oC and above with an increased gain in
distillate output. In addition low content of hardness ions it is also possible to operate MSF
plants on SWRO reject from NF-SWRO system as part of a hybrid NF-SWROreject/MSF plant.
Preliminary trails of NF product as make-up to an MSF pilot plant unit operating at 120 oC
and comprising 2 and 4 stages of heat rejection and recovery, respectively, gave very
encouraging results of high throughput at reduced scale control treatment. Use of the
SWRO, reject from the NF-SWRO unit as feed to the MSF pilot plant is in progress.

A simulated desalination model, which compares the operation of Jeddah SWRO plant,
capacity 56880 m3/day, with and without NF modification shows that it is possible in the
former case to increase significantly the quantity of water product and water recovery at
lower energy consumption per m3 than those values when the plant is operated alone without
NF pretreatment. Technoeconomic analysis shows also the same trend. A saving of up to
30% in cost of water production may be realized by SWRO operation in a NF-SWRO system
than without the NF pretreatment.      This is in addition to raising the plant production
capacity by up to 70% or more while continuing to operate with the same SWRO equipment.

Other advantages of this combined dual NF-SWRO process and with the reduction in TDS
and hardness of Gulf Seawater by the NF pretreatment, the second stage desalination of
product from first SWRO stage is no longer required. Moreover, the NF-Seawater
desalination processes require fewer chemicals in their operation than that required by the
conventional seawater desalination processes making them friendlier to the environment,
particularly the marine environment.

                      Osman A. Hamed, Mohammad AK. Al-Sofi,
                     Ghulam M. Mustafa, and Abdul Ghani Dalvi
         Research & Development Center, Saline Water Conversion Corporation,
              P. O. Box 8328, Al-Jubail 31951, Kingdom of Saudi Arabia

The Saline Water Conversion Corporation (SWCC) is an internationally recognized
authority on scale control in multi-stage flash (MSF) distillers. This international
recognition came about as a result of many years of work on this topic. There are
publications of SWCC on MSF scaling phenomena over the past couple of decades.
During the last decade, there was a monumental growth in SWCC’s installed MSF plants
capacities. The majority of SWCC MSF distillers were designed to operate with additive
scale control method rather than the previously predominant method of carbonate
depletion through chemical reaction with acid, e.g., sulfuric or hydrochloric.

The Research and Development Center (RDC) in Al-Jubail has been involved during the
last three years in the evaluation of available commercial scale inhibitors. A total of eight
tests have been carried out at RDC MSF pilot plant, Jeddah Phases III & IV and Al-Jubail
Phases II to evaluate the effectiveness of three antiscalants. These tests by antiscalant
types and places are (i) polycarboxylates in Al-Jubail II, RDC pilot plant and Jeddah IV,
(ii) polymaleic acid in Jeddah IV and RDC pilot plant and (iii) polyphosphonate in Jeddah

This paper presents the results of both qualitative and quantitative performance of these
three different antiscalants. The threshold effect of these antiscalants was tested on
laboratory scale. On pilot and commercial plants the variation of the fouling factor with
time is examined and quantified. The evaluation tests conducted on the commercial plants
revealed that all the examined antiscalants were successful in inhibiting alkaline scale
formation and improving plant performance at top brine temperature ranging between
105 and 110oC and with respective dosing rates of 1.5 and 3.0 ppm.

Presented in Second Acquired Experience Symposium, September 1997, Al-Jubail


Control of scale formation on heat transfer surfaces is one of the basic problems in the
distillation processes. Formation of scale on heat transfer surfaces will impede the rate of
heat conducted. The main scale forming constituents of seawater are calcium bicarbonate,
magnesium salts and calcium sulfate. On heating, bicarbonate will yield carbonate, which
can precipitate with calcium if the saturation limit is exceeded. As a result magnesium
hydroxide will also form. Calcium carbonate and magnesium hydroxide are termed
“alkaline scales” while calcium sulfate is known as “non-alkaline scales”. Great deal of
efforts have been put to understand the phenomena of scaling and to develop substances
which could prevent the formation of scale.

Alkaline scale formation in condenser tubes is a major problem encountered in the
operation of MSF distillers. Two methods are employed to control the formation of
alkaline scales. The first involves the prevention of scale deposition by reaction hence
depletion of carbonate through pH adjustment by acid addition. The second method
controls scale precipitation through the addition of much less than stochiometric quantities
of special chemicals. These are inorganic or organic polymer compounds which, when
adsorbed on a scale crystalline, interfere either with the nucleation or the crystal growth
process [1]. Mixtures of sodium tripolyphosphate dispersing agent have been used to
inhibit alkaline scale deposition in seawater evaporators since the 1950s [2]. Addition of
polyphosphate retards the formation of alkaline scale by preferentially combining the
calcium and magnesium into non-adherent complexes. This treatment is named as
threshold because of the small quantities of chemical used.

The mechanism of threshold agent has not yet been fully explained. It might be due to
adsorption of the threshold agent on the scale crystal nuclei and results in the formation of
distorted crystal, and subsequent inhibition of its growth. Certain threshold chemicals tend
to form more sludge than others after some time of operation. It thus requires mechanical

treatment to remove the sludge. The major problem with polyphosphate-based inhibitors
was found to be the thermal degradation of polyphosphate at temperatures above 90oC and
the subsequent loss of threshold effect of the product. This restriction in top brine
temperature to 90 oC limited the thermal efficiency of evaporator designed for threshold
treatment during the 1950’s and 1960’s.
Acid dosing was introduced in the 1960s as a means of overcoming the temperature
limitations and the poor performance of polyphosphate [2]. Acid dosing, by removing the
bicarbonate from the feed water, allowed evaporators to operate at increased top
temperatures, close to the calcium sulfate solubility limits. It was found to have drawbacks
such as careful control and monitoring of the dose level was essential to minimize the risks
of plants corrosion or scale formation and ensure a reasonable plant life.

As problems associated with acid operation started to become known, the opportunity for
high temperature scale control additives to replace acid while giving commensurable
performance was noted since 1970 [2]. Low molecular weight polymeric carboxylic acid
and phosphorous base alkaline were developed as high temperature additives. It has
become a common belief that these chemicals distort the lattice structure and minimize the
rate of growth of the alkaline scales. Polyphosphonate "phosphorous base alkaline group"
does not hydrolyze as easily as the polyphosphate group due to the greater stability of the
c-p bond in phosphonates as compared with the p-o bond in phosphates.

The dosing rate of antiscalant is one of the most important operating parameters.
Underdosing leads to scale formation while overdosing is believed to enhance sludge
formation [3,4]. It is thus essential to establish an optimum dose rate.

Although a number of methods have been developed to minimize or prevent the formation
of alkaline scales, formation of scales due to calcium sulfate salts at high temperatures and
brine concentrations can not be avoided. The only method which is currently used to
prevent the formation of calcium sulfate is to operate the plant below the solubility limits
of calcium sulfate.

SWCC’s Experiences on Scale Control

Saline Water Conversion Corporation (SWCC) has been actively involved in the inhibition
of scale formation in order to improve the performance of its distillers. A number of
optimization tests has been carried out resulting in successful operation of MSF distillers
at low antiscalant dosing rates. This can be attributed to several factors such as plant
operators awareness to reduce chemical dosing while maintaining effective plant
performance, adoption of on-line sponge ball cleaning and competition amongst various
additive suppliers to provide the most cost effective dose levels to meet the needs of

A direct comparison on the performance characteristics and economics of three additives
tested on the MSF distillers of Al-Jubail phase I has been reported [5]. The tested additives
included polyphosphate, polyphosphonate and polycarboxylates. The performance testing
revealed that the best scale control method was operation at low temperature (90.6 oC) and
low additive dosing. The three tested antiscalants proved to be successful during the 30
days test period. Individual dose rates varied between 3 and 5 ppm. The sponge rubber ball
on-line cleaning system was found to be effective with polyphosphate additive. However,
the slight soft scale accumulation with the polycaboxylate and polyphosphonate additives
did not warrant extensive use of the ball cleaning system. The design heat transfer fouling
resistance is more than required by either type of chemical additive. If a lower design
fouling factor is used, smaller heat transfer area will be required which will then have a
significant decrease in total evaporator installed equipment cost.

Another study was reported on the evaluation of various additives at Al-Jubail phase I
during reliability tests [6]. Performance ratio calculations were used to predict the time
span between acid cleaning for three additives Belgard EVN, Albrivap B and Flocon 247.
Assuming a linear relationship between performance ratio and time, the test results
showed that the predicted time span in days before acid cleaning is required was 310 for
Flocon, 590 for Belgard and 1466 for Albrivap. It has been stated that it would be of great
interest if chemical instruments will be fixed on the MSF distillers to measure in site both

alkalinity and pH under operating conditions. Such measurements will give a direct
estimate of the quantity of scale deposited.

Optimization of scale control additive dose levels at different top brine temperatures in Al-
Khobar II and Al-Jubail II desalination plants has been reported [7]. Both plants were
originally designed to operate at low or high temperature using polyphosphates or liquid
polymers. Suggested dose rates by design were from 3-5 ppm for low temperature
additives (LTA) and 7-12 ppm for high temperature additives (HTA). Antiscalant dose
rates were reduced in a stepwise procedure. Dose levels were reduced to as low as 2.2 and
0.8 ppm for effective and safe operation at high and low temperature respectively. The
economical saving achieved by dose rate optimization were gigantic in magnitude.

Trial tests on the performance of polymaleic acid (PMA) and polyphosphonate (PPN)
antiscalants at Al-Jubail phase II desalination plant was reported [8]. The results revealed
that polymaleic acid was performing better compared to polyphosphonate. The ball
cleaning operation had a significant effect on the polyphosphonate group over the
polymaleic group. Also visual inspection for selected units after a predetermined time span
revealed that polymaleic acid had a better performance.

Gained operational experiences on Al Khobar Phase II plant during the period 1982-1992
was reported [9]. The water production, top brine temperature, antiscalant type and dose
rate and performance ratio of three distillers were reported. The distillers were initially
dosed with polyphosphonate antiscalant for a period of one year (1984) and with a TBT of
85 oC and a dose rate of 3 ppm. Polyphosphonate was then replaced by polymaleic acid
during the year 1985 with a dose rate of 2 ppm and TBT of 90 oC. From 1986 up to 1992
polyphosphonate liquid with a reasonably low dose rate of around 1 ppm and TBT
ranging between 85 to 100 oC was used. Up to 1990, the performance ratio (PR) was
ranging between 6.6 to 7.0. From 1990 onwards the PR dropped to as low as 6.3 which
was attributed mainly to postponed acid cleaning. It has been concluded that reasonably
low antiscalant dose rate (1-2 ppm of makeup sea water) was adequate, ideal TBT for
operation with proven capabilities of polymeric acid was 105 oC and no safe operation

could be endorsed without good sponge ball cleaning. Close monitoring of the MSF
distiller would mandate acid cleaning of heat transfer tubes once or as a maximum twice
per decade.

Basic Approach

This paper reports the results of the evaluation tests of commercial scale control additives
performed by Al-Jubail Research and development center during the last three years.
Testing of scale inhibitors characteristics and effectiveness has been carried out at three
levels : laboratory, pilot plant and finally testing on commercial plants. At the laboratory
level the threshold effect of different antiscalants was first tested on bench scale
equipment. At the pilot plant level the effectiveness of polymaleic and polycarboxylic
acids was tested. The MSF pilot plant is of 20 ton per day capacity and its schematic
diagram is shown in Figure 1. It consists of a brine heater, four heat recovery and two heat
rejection stages. The plant is also equipped with external deaeration and on-line ball
cleaning facilities. At the commercial levels, tests were conducted in Jeddah III and IV and
Al-Jubail II to evaluate the performance of polycarboxylic acid, Polymaleic acid and
polyphosphonate as scale control additive. Design characteristics and test parameters of
the commercial plants are summarized in Table 1.


Laboratory tests

Antiscalants have two principal effects: threshold and crystal distortion. Threshold effect
is responsible for preventing scale initiation and inhibiting or slow down the deposition of
soft scale consisting mainly of calcium carbonate and magnesium hydroxide. Crystal
distortion effect is responsible for suppressing the adhering ability of scale. Antiscalant
addition results in the distortion of scale crystals into spheroid structures, which have less
contact surface between particles and the metal surface than a flat shaped untreated scale
crystal. Spheroid shaped particles tends to rotate around its own axis while moving with

the liquid stream along pipe axis. This combined motion results in a radial force that
causes the particle to migrate towards the pipe center leaving a particle free pipe wall.

The threshold characteristics of three different antiscalants were examined. These were
polyphosphonates, polymeric maleic acid and polymeric carboxylic acid. The method of
confirming the threshold effect was performed through the addition of a certain quantity of
inhibitor in a supersaturated brine solution. The ability of the scale inhibitor to prevent the
deposition of scale was checked by observing the change of M-alkalinity with time. The
threshold effect of the three examined antiscalants was carried out at a constant 2 ppm
dosing rate and at two different temperatures of 95 and 110 oC. The change of M-alkalinity
as CaCO3 (ppm) with time in minutes is shown in Figures 2 and 3. Both figures show that
the alkalinity of the supersaturated solution with no scale inhibitor (blank), which can be
considered as a reference state, decreased rapidly with time. At 10 minutes time the
alkalinity dropped to 140 ppm at 95 oC and 80 ppm at 110 oC.

Figures 2 and 3 also reveal that all scale inhibitors have the ability to prevent the
deposition of scale when they contact supersaturated brine solution but their effects
decreases with the increase in retention time. Modified polyphosphonate and polymaleic
acid were found to exhibit the highest threshold effect compared to polycarboxylic acid
especially at conditions of 95 oC with a retention time boundary of 20 minutes and 110oC
with retention time boundary of 15 minutes. All scale inhibitors lost their threshold effect
within 30 minutes at 90 oC and 15 minutes at 110 oC under the above laboratory

Pilot Plant Tests

Two different antiscalants polycarboxylic acid (PCA) and polymaleic acids (PMA) were
tested on the RDC MSF pilot plant. Tests were conducted with top brine temperature
range of 110-112 oC, concentration ratio of 1.38 -1.4, antiscalant dose rate of 2 ppm and
cyclic ball cleaning. The increase of fouling factor in the brine heater with time was
calculated to demonstrate the thermal effectiveness of antiscalants. Figure 4 shows the
time dependence of the fouling factors. It shows that both antiscalants are quite effective in
their performance and maintaining fouling factors well below the design values. Results of
visual inspection conducted before and after these tests revealed that there was a very thin
layer of soft sludge on the tubes. The increase of the fouling factor in the brine heater (RD,
m2 K/kW) with time (t, hours) is quantified. Using a regression analysis the following
linear relationships are obtained assuming that the rate of fouling remains constant.

For Polycarboxylic acid,      RD = 0.00002 t + 0.0349 ......................(1)

For Polymaleic acid           RD = 0.00004 t + 0.0217 ......................(2)

The linear fouling factor versus time indicates that the rate of increase of scale factor is
very small which is reflected in the low values of the time coefficients. Assuming the
linear behavior of fouling factor, the allowable periods required for the fouling factor to
reach design value and before acid cleaning becomes necessary, are 375 and 483 days for
polycarboxylic and polymaleic acids respectively. Based on the experience especially
those reported in reference [9] such allowable periods are found to be very short. The rate
of change in fouling was linear during those limited duration tests. However for longer
operating periods the asymptotic fouling model could be better representative to the
behavior of fouling curve [10]. Moreover in view of cyclic ball cleaning this asymptotic
behavior is expected to follow the saw-tooth trend.

Commercial Plant Tests

Tests were conducted in Jeddah III and IV and Al-Jubail II to evaluate the performance of
polycarboxylic and polymaleic acids and polyphosphonate as antiscalant agents. In these
tests pre, interim and post visual inspections were conducted as frequent as was deemed
essential and as production schedules could allow. During each test relevant flow rates,
temperatures and pressures were monitored. Calculations were then made to determine the
heat transfer characteristics induced by each examined antiscalant. Design characteristics
and test parameters of four commercial units are summarized in Table 1.

Results of the performance evaluation of polycarboxylic acid at Jeddah IV unit 15
commercial plants are shown in Figure 5. The fouling factors of both the brine heater and
heat recovery section are well below the design values which indicates that the distiller
was working satisfactorily. Analytical results of pH, M-alkalinity, turbidity, chlorine and
total dissolved solid indicate that the plant was working smoothly with no abnormal
behavior. Analysis for residual antiscalant in the samples taken before flashing shows that
the antiscalant was active throughout the brine recycle passage. The assumed linear
regression analysis reveals that the allowable operating periods for the brine heater and
heat recovery sections, before acid cleaning becomes essential are 403 (compared to 375
days in the pilot test) and 867 days respectively at a dose rate of 1.8 to 2.0 ppm and a TBT
of 110oC.

Results of the performance of polymaleic acid at Jeddah IV unit 19 commercial plant is
shown in Figure 6. Trends of fouling factors in heat input and heat recovery sections show
an increase with time but remain below their respective design fouling factors. The rate of
increase of fouling factors for both units is very low indicating a minimum deposition
inside the brine heater and heat recovery tubes. These findings are supported by the interim
and post inspections of brine heater and the first three stages of the recovery section which
show that the brine heater tubes were in good clean condition. These results are found to
support the validity of the asymptotic behavior of fouling buildup. Post inspection at the
end showed that demister fouling in the first stage had blocked all the pads. In the next two
stages, demister was less fouled compared to the first stage and the scale was also soft.
These results are found to support the proposed asymptotic growth behavior in Jeddah

Results of performance of polyphosphonate at Jeddah III unit 10 commercial plant is
shown in Figure 7. The fouling factor of heat recovery section remains at almost constant
value during the whole period of the test, which indicates negligible deterioration in its
thermal performance. This was confirmed by the visual inspection, which revealed that
only a very thin layer of scale had been deposited in the first stage and a dusting of scale
inside the tubes of the second stage was apparent. The GOR and PR remained virtually

unchanged throughout the trial period. The fouling factor of the brine heater was increased
from 0.05 to 0.1 m2K/kW during 2100 hours, which was well below the design value
(0.325 m2K/kW). The visual inspection of the brine heater revealed that there was no
evidence of excessive sludge and only a thin layer of scale was observed on the tubes inner
surfaces. The demister pads in the first three stages showed some of the deposition, similar
in degree to that experienced under normal operation. The assumed linear regression
analysis predicts that it will take 4.37 and 3.2 years for the fouling factors of the brine
heater and heat recovery sections to reach the design fouling factors at a dose rate of 3 ppm
and a TBT of 108oC.

Evaluation tests were performed in Al-Jubail II to demonstrate the effectiveness of
polycarboxylic acid and at a TBT of 105oC. The tests results are depicted in Figure 8
which reveals slight increase of fouling factor with time. The distiller was also inspected
before and after test trial. The post trial inspection revealed that no sludge was found in
tubes of brine heater and heat recovery section. The demister pads were found in good and
clean condition. Thus both the calculated test results and visual inspection confirmed that
the tested antiscalant is a highly effective scale control antiscalant. The linear regression
analysis revealed that it would take 415 days for the fouling factor of the brine heater to
reach the design value.


1. Literature search revealed that one of SWCC’s main achievements in controlling
   alkaline scale formation in commercial MSF distillers was successful operation at low
   antiscalant dose rates. Dose levels were reduced down to as low as 2.2 and 0.8 ppm for
   high and low temperature respectively.

2. The threshold laboratory tests revealed that modified polyphosphonate and polymaleic
   acid inhibitors are having higher threshold effects compared to polycarboxylic acid. All
   scale inhibitors lose their threshold effects within 30 minutes at 90oC and 15 minutes at
   110oC during laboratory testing.

3. Evaluation tests of different antiscalants performed in Jeddah and Al-Jubail
  desalination plants showed that polyphosphonate, polymaleic and polycarboxylate
  polymers are all very effective and successful in controlling scale formation at a TBT
  range of 105 and 110oC and with corresponding dose rates of 1.5 and 3.0 ppm. The
  fouling factor of brine heater and heat recovery section induced by the three antiscalants
  were always well below the design values. The linear regression analysis reveals that
  the allowable operating periods for the brine heater and heat recovery sections before
  acid cleaning is required, are generally more than a year. These findings were
  confirmed by visual inspection which revealed in all conditions that brine heater and
  heat recovery tubes were normally maintained in clean conditions.


1. Shams El Din, A. M. and Mohammed R. A. (1994) Brine and scale chemistry in MSF
   distillers, Desalination, 99, 73-111.

2. Finan, M. A., Smith S., Evans C. K. and Muir J. W. H. (1989) Belgard EV-15 year’s
   experience in scale control. In the Proceedings of the fourth world congress on
   desalination and water reuse, 341-357.

3. Al-Sofi, M. AK., Khalaf S. and Al-Omran A. (1989) Practical experience in scale
   control, Desalination, 73, 313-325.

4. Nada, N. (1986) Comparison of cost of three MSF desalination plants using different
   chemical treatment, Topics in Desalination, SWCC, Saudi Arabia.

5. Al-Mudaiheem, A. M. A. and Szostack R.M. (1986) Evaluation of chemical additives
   for scale control in MSF evaporators, Topics in Desalination, SWCC, Saudi Arabia.

6. Nada, N. (1986) Evaluation of various additives at Al-Jubail Phase I during reliability
   trials, Topics in Desalination, SWCC, Saudi Arabia

7. Al-Sofi, M. AK., Al-Hussain M. A. and Al-Zahrani S. (1987) Additive scale control
   optimization and operation modes, Desalination, 66, 11-32.

8. Al-Zahrani, S., Al-Ajlan A. M. and Al-Jardan A. M. (1993) Using different types of
   antiscalants at the Al-Jubail power and desalination plant in Saudi Arabia, In the
   Proceedings of 6th IDA World Conference , Yokohama, Japan, 421-431.

9. Al-Sofi, M. AK., Al-Hussain M. A., Al-Omran A. A. and Farran K. M. (1994) A full
   decade of operating experience on Al-Khobar II multistage flash (MSF) evaporators
   (1982-1992), Desalination, 96, 313-323.

10. Malik, A. and Aleem F. A. (1994) Scale formation and fouling problems and their
    predicted reflection on the performance of desalination plants in Saudi Arabia,
    Desalination, 96, 409-419.

Table 1. Design Characteristics and Test Parameters of the Commercial Plants.

S. No.   Design & Test         Jeddah     Jeddah       Jeddah    Jubail
         Parameters            IV Unit    IV Unit        III     II Unit
                                 19         15         Unit 10      8
    1.   Top brine              110            110      108       105
    2.   Concentration ratio     1.4      1.3 - 1.32     1.4       1.4
    3.   Antiscalant dose        1.8            2        3         1.5
         rate, ppm
    4.   Ball cleaning ,         2              2        14        21
    5.   No. of                  9              9        3         10
    6.   Ball to tube ratio      0.4           0.4      0.75       0.3
    7.   Production, MGID        5             5         5        5.75
    8.   Number of stages        24            24        16        19
    9.   Gain output ratio       7              7        7         9.0
    10. Flash range, C           72            72        70        69
    11. Design fouling
         BH (m2K/kW)            0.395       0.395       0.325      0.2
         HRS (m2K/kW)           0.176       0.176       0.178     0.15

                                                                                                               T IC
                                                                                                                                                                         R EJEC T SEA W ATER

                                                       R E C Y C L E B R IN E
     S T E A M T O B R IN E H E A T E R

                                                                                                                                     TO EJEC TO R C O N D EN C ER
                                                                                                                                                                                       F IC

                                                                                                                                                 SEA W ATER SU PPLY

                                                                                                                                                                                 C H E M IC A L IN J E C T IO N

                                          HEAT RECO VERY STAG ES                                 HEAT     R E J E C T S E C T IO N

                                                                                                                                                                                L IC

                                                                                                                                                                  DEAER ATO R

                                                                                                         MAKEUP PUMP

                                                   BLO W DO W N                                                                         FLASF TAN K

                                                                                         B R IN E R E C IR C U L A T IO N P U M P                          L IC

                                                                     D IS T IL L A T E

                                                                                                   D IS T IL L A T E P U M P

Figure 1. Schematic Diagram of the Pilot MSF Plant

        M-Alkalinity as CaCO (PPM)


                                     200                                        PMA

                                           5   10   15     20         25   30   35       40

                                                    Time in Minutes

Figure 2. Laboratory Test of Various Antiscalants for their
          Threshold Effect at 95oC & 2 ppm Concentration

        M-Alkalinity as CaCO (PPM)



                                           5   10   15     20         25   30   35       40

                                                    Time in Minutes

Figure 3. Laboratory Test of Various Antiscalants for their
          Threshold Effect at 110oC & 2 ppm Concentration


     Fouling Factor in Brine Heater,
                                                                                                            PCA                       PMA
                                                        Rd (BH) = 0.215                                     Linear (PCA)              Linear (PMA)


                                          m2 K/ kW               0.1

                                                                                              y = 2E-05x + 0.0349
                                                                                                                       y = 4E-05x + 0.0217
                                                                        0          100         200        300          400      500          600     700

                                                                                                      Time in Hours

Figure 4. Pilot Plant Tests of Polycarboxylic & Polymaleic Acids

                                                                                                                    FF-BH               FF-HRS
                                       Fouling Factor, m K/kW

                                                                            Rd (BH) = 0.395
                                                                                                      y = 1E-05x + 0.2983


                                                                 0.20          Rd (HRS) = 0.176

                                                                                              y = 9E-06x - 0.0113


                                                                        0            500           1000         1500         2000           2500     3000

                                                                                                  Test Duration in Hours

 Figure 5. Test of Polycarboxylic Acid in Jeddah IV Unit # 15

 Fo                                                                                                           FF-BH                      FF-HRS
 uli                                                0.40
 ng .K/                                             0.30
 Fa                                                 0.20
     W                                                                                                     y = 1E-05x + 0.0978
 ct 2m
 or,                                                0.10
                                                    0.00                                       y = 8E-06x + 0.005
                                                               0     500       1000       1500         2000       2500          3000       3500    4000
                                                                                        Test Duration in Hours

Figure 6. Test of Polymaleic Acid in Jeddah IV Unit # 19

                                                   0.50                                                                                            0.20
  Brine Heater Fouling

                                                                                                                  FF-BH                FF-HRS
                               Factor, m .K/kW

                                                   0.40                                                                                            0.15
                                                   0.30                            y = 6E-06x + 0.0851                                             0.10

                                                   0.20                                                                                            0.05
                                                                                                                      y = 2E-05x + 0.0509
                                                   0.10                                                                                            0.00
                                                   0.00                                                                                            -0.05

                                                   -0.10                                                                                           -0.10
                                                           0                 500                1000                  1500                2000
                                                                                      Test Duration in Hours

Figure 7. Test of Polyphosphonate in Jeddah III Unit # 10

                                                   0.25                                                                                            0.50
                         Fouling Factor, m .K/kW

                                                                                                                       FF-BH             FF-HRS
                                                   0.20                                                                                            0.40

                                                   0.15            y = 7E-06x + 0.1302                                                             0.30

                                                   0.10                                                                                            0.20
                                                                                                                y = 1E-06x + 0.1534
                                                   0.05                                                                                            0.10

                                                   0.00                                                                                            0.00
                                                           0           500              1000             1500                2000           2500

                                                                                        Test Duration in Hours

Figure 8. Test of Polycarboxylic Acid in Al-Jubail II Unit # 8


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