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A review of scale control methods
Desalination,31(1979)309-320 OEls.evierScientific PublishingCompany,
Netherlands
Am&en&m-PrintedinTbe
A REVIEW OF SCALE CONTROL METHODS
WADE
N.M.
Preece Cardew and Rider, Brighton,
East
Sussex,
Consulting
165-167 Preston Road,
Engineers,
England.
SUMMARY
The
methods
polyphosphate,
scale
limitations, each
of scale acid
that
organic
Recent processes bed
and
control
in MSF evaporators,
used
organic
polymers
performance
and
water
reduce
are
plant
namely
reviewed_
corrosion
sodium
Temperature
are
discussed
for
method. It is shown
and
control
dosing
that
unit
polymers
developments are
briefly
evaporators
and
are
costs
now
competitive
in scale
control
discussed, the use
with
increasing
with
acid
for MSF
including
and
sulphate
brine
temperature
dosing.
other
ion
evaporation
exchange.
fluidized
of surfactants.
SYMBOLS md
=
distillate
Efh
=
brine
C
=
mean
P TI
=
brine
temperature
entering
T2
=
brine
temperature
leaving
Ah=
mean
mass
flow
recirculation specific
heat
enthalpy
flow of brine
difference
highest lowest
temperature temperature
between
vapour
on heat
transfer
stage stage
and
distillate
INTRODUCTION
Control problems scale
of scale
in
the
control Since
formation
distillation
if the
chemicals
the multistage
present
installed
review
concentrates
capacity
Plant
of seawater. used
flash of
on scale
produce
(MSF)
land control
process
based
309
accounts
used
is one
corrosion
aggressive
seawater
methods
surfaces
conditions for
over
distillation with
XSF.
of
can be
the basic linked
in the 90%
plant, Scale
of
with plant.
the this
control
in
falling
film
Three
and
other
main
scale
polyphosphate acid
the
from
developed
dosing
a given
llS°C,
polymer much
Cost
less
sulphate
risk
the
recent
from
Considerably
the
allows
plant,
with
greater
of
nankaly:
three
dosed
up
distillate
to raise
temperature
to be Recently
rates-
to about
IlO"C,
plants. It is shown
techniques.
the other
The
90°C.
corrosion
temperatures
in acid
these
to about
if it is possible
the brine
of much
than
for
is
feed
brine
methods,
despite
that
the higher
the use
make-up,
temperatures
of
ion
exchange
in conjunction should
to remove
with
be possible
acid
if this
calcium
dosing. development
OF SEAWATER
seawaters
the
solubility
limits
transfer
surfaces,
The
and
contain
heat
three
bicarbonate,
110 to
main
scale
can
be
Above combine
making
by
be
sulphate
and
the
of seawater
above
by
are
are
deposited
on
calcium
in concentrations about
50°C
dioxide
addition
at higher
with
(ref.1).
sulphate
carbon
scales
vary
in an evaporator,
performance.
in seawater
controlled
operation
magnesium some
the hydroxide Calcium
calcium
which
is heated
and
exceeded,
transfer
On heating
by acid
90°C
with
and
are
seawater
constituents
is present
can
removed
about
controlled
forming
salts
When
heat
precipitate,
carbonate
neutralisation,
salts
reducing
(as CaCOS).
which
of calcium
can
of some
in concentrations
of salts,
conditions.
bicarbonate
140 mg/l
formed.
a number
climatic
magnesium
Calcium
which
increased
the use
corrosion given
seawater
CONSTITUENTS
All
then
in MSP
chemicals.
location
be
of
development
higher
risk
competitive
the
scale
used
successful.
SCALING
is
presently
temperature
be
dosing
the
permit
are
are
can
Acid at
additives
polymer chemicals .specrfrc cost of
proves
are
brine
of plant
but
comparisons
Another
restricts
size
temperature.
to about
with
methods
discussed.
polymers
top brine
but
control
are briefly
dosing
Polyphosphate
raised
of evaporator
dosing
organic
output
types
from
calcium
carbonate Deposition
is liberated. of certain
temperatures
about
additives
or by
Carbonate
possible.
cleaning.
carbonate
ions
to produce
additives,
can hydrolise
magnesium
or by maintaining
to hydroxyl
hydroxide brine
ions, This
stole. pH below
the
which can
range
precipitates. is present
in seawater
in concentrations
up
to
about
in
311
2500 ppm.
In recirculated brines of about 70.000 mg/l total dissolved solids,
the solubility limit of calcium sulphate anhydrite can he exceeded without scale due to the slow nucleation rate of this salt.
The hemihydrate
form will
however precipitate rapidly once the solubility limit is reached, at about 120°C
The hard scale deposits are very difficult
for the above brine t.d.s.
It is therefore essential
to remove either by chemical or mechanical means.
to keep the combination of temperature and calcium sulphate concentration below the point at wbicb the hemihydrate precipitates. Scale control nmthods have been developed to control the rates of some or all these scale forming constitllents of seawater.
SODIUM POLYPEOSPEATE DOSING
Feed dosing with sodium polyphosphate was the first scale control method to be used for large multistage
flash evaporators.
About 4mgfl of a mixture
generally containing about 60X of sodium polyphosphate,
such as Albrivap A
(formerly Hagevap) is dosed into the seawater feed make-up.
Polyphosphate
retards the formation of hard alkaline scales at temperatures up to about 9O"C, precipitating a softer sludge type of scale. by keeping brine velocity above about 1.8 m/s.
Scaling rate can be reduced To achieve periods of three to
four months between acid cleans, fouling factors for heat recovery stages 2 OC-I should be about 0.00015 W/m , with a greater allowance in the brine heater. The tendency of polyphosphates
to hydrolise to ortho phosphate ions,
producing calcium orthophosphate, needs to be limited in order to maintain effectiveness of scale control.
This requires careful control of brine
temperature and the avoidance of excessive phosphate concentration. &uly plant users, particularly
in the Middle East, use polyphosphate as
their standard scale control method and have built up considerable experience with this technique in large plants.
Due to the low temperature and high pH
of the recirculating brine, the risk of plant corrosion is lower than with acid dosing.
The chemical is safe to handle and can be dosed by vacuum
injection or metering into the brine or the feed make-up.
Some care is needed
to ensure adequate mixing, particularly if nbztering pumps are used. Acceptance tests on a 11365 m3/day polyphosphate
dosed plant in Bahrain
over a 2400 hour operating period gave an increase in fouling factor of 0.00011 w/In" Yz-'
for the heat recovery section compared with the design 2 OC-1 allowance of 0.00015 W/m At this rate of fouling three acid cleans per . year would be sufficient to keep the plant operating at design distillate output and within rated steam consumption.
312
UADE The Taprogge
condensers particularly
effective
in the boundary with
additive
against (ref
sponge
system
to scale
in water
layer
and
the
cost
of
particular
site.
INFLUERCE
OF BRIBE
The
distillate
this
removed
TEMPERATURE
output
temperature
fine by
in chemical
range,
md = ‘b
salt
The
a MSF
is settle
In conjunction to be evaluated
needs test
design
can
plant
facility,
stage
for
a
OUTPUT
distiller
depends
in accordance
with
01
T2
-
which
sand
means.
portable
at the plant
in power
'Ibis method
or
chemical
ON DISTILLATE
of
both
consumption
system_
evaluation
established
in distillers.
containing
the Taprogge
assist
is well
control
is not
dosing,reduction
2) would
operating
ball
as an aid
and
on brine
the
following
flow
and
equation.
1
(1)
'P
Ah
For
constant
temperature about
5OC
maximum and
range_ higher
value
the variation
flow,
The than
on
in output
upper
temperature, supplied. if steam unaltered.
curve
for The
flow
on
with
constant lower
the of
top brine
brine
to the brine
flow.
shows heater
the
T2
The
concentration,
used. TI
flash
is generally
composition,
control
to
temperature.
temperature
east
the possible
curve
seawater
seawater
scale
is proportional
temperature,
maximum
of middle
shows
output
brine
design
the method
for T 2 , typical
The
distillate
minimum
the
depends
of T,
particularly
of 40°C
brine
Figure ,
1 shows
assuming
a value
conditions. increase
assuming change
is kept
in output additional
in output
constant,
and
with steam
with
top brine can
brine
fouling
be
temperature,
margin
is
313
180
COMSTANT
so
FLOW
BRINE
1SQ w
c
ua
5
0
130
g 4 i
0
-CONSTANT
/-
l20
E 0
STEAM
FLaW
no
s
100
m
MAX.
Fig. I
ACID
WSING
Acid
scale
Variation
dosing
was
above
is injected forming
into
Carbon
dioxide
the
0.1
seawater
mg/l.
dependent
output
in
followed some
1960's
plant feed
with
brine
co2 For
this
to the
reaction
however
a typical
to be
temperature
of raising
increased
to neutralise
following
+
by a vacuum will
as a means
output
make-up
mo;z=qo
formed
oo pH.
the
according
+
tower,
in
so enabling
constituents,
in a degassing to below
of distillate
introduced
90°C,
H+
being
TEYPERhTURE
BRINE
<@.I CONTROL)
temperature Acid
-
120
110
100
90
top brine
(ref.31
the
alkaline
reaction.
co2
is normally deaerator remain
seawater
released
to reduce in solution,
alkalinity
of
to atmosphere the the
oxygen
content
concentration
120 mg/l
(as CaC03)
WADE
314 the injection rate of 98Z sulphuric acid for full neutralisation would be 120 mg/l of seawater feed make-up.
Brine pH could be below 7. due to residual
CC*, which would be corrosive to carbon steel shells.
Two alternative dosing
methods are used:-
a)
reduce the acid injection rate to leave about 15 mg/l residual bicarbonate alkalinity in the feed, or,
b)
inject sufficient acid for full neutralisation
and
raise brine pE by the addition of caustic soda or other al'kali.
Tbe first method results in some scaling and still requires careful pB control to avoid serious corrosion.
The second method is less critical but There is also some risk of
involves an appreciable extra chemical cost.
Magnesium hydroxide scale formation at higher pY. The combination of maximum temperature and brine concentration must be kept below the solubility limit of Ca S04. hard adherent scales.
AH20 to avoid precipitation of
It is sometimes overlooked that conditions in the
boundary layer in the brine heater, particularly close to a steam inlet, will be a few degrees above the bulk brine temperature.
Plants have been designed
for brine temperatures up to 121°C, but it has often been found necessary to decrease this temperature to around !lS°C to operate free of calcium sulphate scale. It is possible to reduce the fouling factor allowance in acid dosed plant. compared with plant designed for polyphosphate
dosing, since the rate of scaling
A typical fouling msrgin for the heat recovery stages in an 2 CC-1 acid dosed distiller is 0.00007 W/m . is much lower.
The main drawbacks with acid dosing are the risks of corrosion in carbon steel shells and the potential hazard to personnel handling acid.
In their
survey of a number of plants for OSW in 1972, Newton, Birkett and Ketteringham (ref.41 showed that severe attack on carbon steel shells was more prevalent in acid dosed plants than in phosphate dosed plants. corrosion measurements 5 years operation.
Morin (ref.51 reported
in the acid dosed Bluehills plant in the Bahamas after Wb;lst there was little attack in the first vessel,
which was lined with stainless steei, the floors in the second vessel had corroded at an average rate of 2.4 mm/year, with even higher rates of attach in areas of turbulence or impingement. To
contrclcorrosion,
without depositiag scale, it is necessary to keep
brine pg within a narrow bandwidth, about 7.7 - 8.0,
To achieve this accuracy
WADE
315
it is necessary
to maintain pH meters to a high standard with regular cleaning
and calibration, backed up by regular brine ssmpling aud analysis for dissolved These high standards are hard to attain,
metals, pH and residual alkalinity.
particularly where skilled operators are at a premium. Whilst the author is not aware of any serious accident due to the use of acid in desalination plant, there must be a considerable risk in the handling and use of dangerous
chemicals.
This
risk can be reduced by correct design of
storage and transfer systems and by the use of protective cloth&.
Operators
are hovever reluctant to wear face masks and gloves, particularly in hot climates.
Bulk
transport
and
handling
is safer than small drums or carboys.
of acid in special purpose containers
Bulk purchase is also likely to be
cheaper for the large quantities of acid required, for example around 800 tonnes/year for a 12000 m3/day plant at '1.65 load factor. Until recently acid dosing was the only method of operating distillers at high temperatures, giving savings in water cost compared with polyphosphate. which many operators considered to be worthwhile, particularly
in areas of
high fuel cost.
POLYMBR ADDITIVBS
A number of feed treatment chemicals have been introduced in recent years as the result of research and development work to extend the range of additive dosing to higher brine temperatures. Chemicals based on polyacrylates were found to produce a hard scale if certain temperature limitations were exceeded.
Research work by Ciba Geigy
and the DRABA (ref.6) resulted in the development of Belgard EV, a low molecular weight polymeric carboxylic acid. introduced chemicals of a similar type.
Other manufacturers have since
These polymers may act either as
threshold inhibitors or by means of distorting the lattice structure of scale crystals,or both,so as to reduce adhesion to other crystals or to metal surfaces_
Plant trials and regular operation with Belgard EV (ref.71 have
shown that it is possible to.control scale formation at brine temperatures up to llO°C and possibly higher.
Dosing rate must be increased with temperature,
from about 2 mg/l at 90°C to about 6/8 mg/l at llO°C.
The actual dose rate
required depends on seawater composition and plant detail design and should be established by trials on clean plants at each location. Although some polymer additives are weak acids, the Lou dosing rate has little effect on brine pH. typical values being in the range 8.5 to 9.0, depending nn seawater alkalinity_
Corrosion rates with polymer dosing have
been found to be as lov as with polyphosphate, except for some increase in the
WADE
316
first 316L 213
few
stages
stainless
The
temperature Polymer
treatment
and
pump
The
by
use
some
reduction
fall
in output,
venting
to polymer
of acid
operation
with
consequent
acid
dosing.
rate
and
a small
corrosion
plant The
is usually
recent Acid
Spanhaak
dosed
and
in flash
for plants
method
should
to operate
COST
that
polymer
be
80%
scale
is now
for new
on polymer
up
of of
The
gases
acid
dosed
from plant
rates, cost. and
stoicheometric
this
method.
described
is satisfactory
in acid
a viable
to about
installations.
should
of Belgard
the
control
dosed
and
outpubif
of capital
charges
to acid
that
still
lower
and
plant.
alternative llO°C
It would
or polyphosphate,with
supply
to compensate.
in corrosion
than
using
involve
in maintenance
Trials
that
at temperatures
considered
at 90°C
the
to a corresponding
of a combination
is added.
dosing
first
from
25%
to extract
saving
to about
of
may
in converting
a reduction
use
order
flow
modification
and
by
steam
plant
lead
brine
is substantially.lower
operating
the
dosed
could
purpose
is the
showed
(ref.81
chambers
It is considered dosing
10S°C.
need
is reduced
of polymer
others
of
this
plant
design,
to around
to achieve
development
plant
increases
in reliability
injection
dosage
may
the
CO2
It is often
a polyphosphate
to raise
main
type
or as an alternative
distillers.
in acid
and
possible
stages.
improvement
A further
by
temperature
if it is not
temperature
from
polymers
for
to extract
plant,
Providing
temperature
temperature
consideration
with
plant.
dosed
output
adequate. brine
in brine
system
the high
are
raising
of high
output
temperatures.
Lining
release.
designed
for new
or acid
distillate
capacities
achieved
either
CO2
be worth
to be
as in polyphosphate
phosphate
at high
and
may
needs
can be used
existing
dosing
system
stages,
to increase
polymer
temperatures
or cupro-nickel
venting
dosing
for
possible
to high
steel
stages.
high
be
due
this
be possible required.
COMPARISON
Costs operating
of seawater costs,
maintenance. specific shell
capital
due
a greater
control
for
have
main
cost,
temperature
steam
consumption
the
same based
on extraction increases
vith
power,
brine
chemicals,
temperature
power pumping
operators
and
is a reduction
transfer
Auxiliary
recirculation
and
surface
and
consumption power
in
distiller also
required
with
range.
estimated
considered.
up
in the heat
rating.
in brine
made
auxiliary
o.f raising
output
decrease
been
are
d-tieto savings
method
are
fuel,
effect
a given
to the flash
Costs
pressure
comprising
The
needed
falls,
desalination
for
In each at
from brine
12000 case
the brine a 6OMU
m3/day the
MSF
The
heater.
turbo-generator.
temperature,
distillers,
performance
the
fuel
for
ratio costs Since
input
each
is 8:l.
of providing
scale
giving steam
the extraction
to the boiler
will
WADE
317
also increase.This
is reflected
in
fuel
top brine temperature than for 90°C. E60 per tonne.
costs aboct 10% higher for ll5OC Fuel costs are based on a fuel Price of
Operating costs are calculated on a basis of 65% load factor.
Annual capital changes at early 1979 price levels are based on amortization at 10% interest rate over a plant life of 20 years, and include the distillers, foundations and the proportion of turbine and boiler costs appropriate to the provision of extraction steam. The estimated costs are given in Table 1 and illustrated in Figure 2, as unit costs in pence per cubic metre of water produced.
TAEILE I
Comparative cost estimate for 12000 m3/MSF distillers with 8:l performance ratio, for various types of feed treatment, at 0.65 load factor.
Capital Costs
Polyphospbate
Polymer
Acid
8.363 0.734 1.370 1.570
6.836 0.759 1.370 1.345
6.398
12.037
10.310
9.835
E Million
Distiller and auxiliaries Power station plant (cost increase) Civil works Engineering and contingencies (15%)
Total fMillion
0.784 1.370 1.283
-Unit Cost of Water P/m3 Capital charges (at 10% over 20 years)
-
Distiller Others
Fuel at f60/tonne Auxiliary power at fO.O2/kWH Maintenance Operation Chemicals - Polyphosphate - Polymer - Acid at
34.5 15.2
26.2 14.3
26.4 14.2
18.7 9.5 3.4 6.7
19.6 7.9
20.6 7.4 3.4 6.7
at fSOO/tonne at E2000/ 11 LiZOf "
Total unit cost P/m3
3.4 6.7
3.0 3.6
88.5
___-
83.1
82.3
318
80
88-S
-CHEMICALS
WHTEHAta 83.1 U’ERATOR!
80
AUX. POWER
:HEYICAL!
‘PERATORS
,HRMICAL!
FERATOR!
70 AUX. POWER
80
AUX. POWER
FUEL
"f P K 0) 2 z
FUEL FUEL
50
b t : r
CA?lTAl
CHARBC -OTHER PLANI
CO
CAPITAL CHARMS -OTHER PLANT
z
CAPITA1 CHARK! -OTHER PLART
30
20
CAPITAL CnAROES DISflLLEF
CAPITAL CHARRRS IISTILLER
CAPITAL CHARRES DISTILLER
10
0
POLY PHOSPHATE
Figure
2. Comparison of unit control method
ACIO
HIGH TEN POLYYEI
water
costs
for each
scale
319
WADE
Opera11 water costs are highest for the polyphosphate
dosed plant.
Savings of approximately 6.1% are indicated for a polymer dosed plant and 7.0% for an acid dosed plant, compared with polyphosphate savings
in
treatment.
The
capital cost with high temperature operation are more significant
than the extra costs of dosing chemicals involved.
In estimating these costs
plant maintenance has been taken as independent of the type of-feed treatment As discussed earlier, it is probable that an acid dosed plant would
used.
require greater maintenance expenditure than additive dosed plant.
The small
saving in overall water cost with acid dosing, compared with high temperature polymer dosing,
would be lost if the acid dosed plant were to require heavy
maintenance on distiller shells. This cost comparison shows that high temperature polymer dosing would give a significant saving compared vith polyphosphate
and would be competitive
with acid dosed plant. Costs for polymer dosing have been estimated with the same fouling factor as used with polyphosphate.
If the fouling factor for a plant designed 2 OC-1 to use organic polymers were to be reduced to 0.0001 W/m , the overall
water cost
would fall by about 2 p/m3, making this the lowest cost method.
OTEERSCALE
CONTROLTRCRRIQUJ3S
This review has discussed the scale control methods presently in use with MSF plant.
Several other techniques are, however, under development or in
limited use. A recent development in MSF technology is the use of small beads acting as a fluidised bed inside vertical tubes. to aid heat transfer and control scale.
Pilot plant tests showed promising results in the prevention of
alkaline and calcium sulphate scaling at high temperatures
(ref.91 and a
prototype 500 tonne/day plant in the Netherlands entered service in 1978 using this method, with no chemical addition to the feed other than antifoam.
The
saving in chemical costs, together with claimed capital cost reduction could make this process a strong competitor, once fully developed. Removal of sulphate ions from the feed make up, in conjunction with acid dosing, is reported by de Maio at this Congress (ref.10).
A prototype
plant is operating with 150°C brine temperature and higher temperatures are claimed to be possible.
The economics will depend on resin performance and
life aud on material selection for high temperatures conditions.
The method
might also find application in existing plants prone to calcium sulphate scaling. Falling film evaporators of the VTR type need to operate scale free to achieve the high heat transfer coefficients on which these designs are based.
320
Acid
WADE dosing
additives avoid
is normally
sulphate
distribution by Sephton
effect
scale
at and
6_ef.
(HFF) low
and can
use
water
The
from
promising acid
without
in the
flow
use
a foam
either
compression
turbulence
good
11).
shows
tempernture
vapour
since
and
by evaporating (ref.12)
evaporators operate
used
to be effective,
of
surfactants
inside
the
results or polymer
chemicals
film
is too
distribution
to promote
tubes
in pilot
has
been
for to
good developed Horizontal
plants.
dosing,
as used
lov
is essential
or alternatively
in a range
of multiple
plants.
ACKNOWLEDGEMENT
The permission for
their
Author
wishes
to present assistance
to thank this
paper,
in preparing
the Partners and
Mr.
the
cost
Cardew
of Preece,
M.R.
Homsby
estimates
and Mr. and
and M.
chemical
Rider
for
Greene data.
REFERENCES
1. 2.
3. 4.
5. 6.
7.
8.
9_.
10.
II.
12.
K.S. SPIEGLER, Principles of Desalination, Academic Press (1966) 497. K. EIMER and H.W. SCHILDMANN, Optimisation of M5F plant chemical treatment and on load sponge ball cleaning at site by means of transportable pilot plant, Sixth International Symposium on Fresh Water from the Sea, 2 (1978) 157. R.A. TIDBALL and R.E. WOODBURY, Methods of Scale Control in flash systems, First International Symposium on Water Desalination, 1965, Washington D.C. E-H. NEWTON. J.D. BIRKETT and J.M. KETTERINGHAM, Survey of Materials Behaviour in Large Desalination Plants around the World, report to OSW (1972). O.J. MORIN. Bluehills Desalination Plant - Engineering Investigation, IDEA Congress on Desalination and Water Reuse, 1, (1977) 13. M.N. ELLIOTT, T.D. HODGSON and A. HARRIS, Additives for alkaline scale control at temperatures above 93OC. Fourth International Symposium, 2 (1973) 97. T-D. HODGSON and S. SMITH, The Appraisal of additives for scale inhibition during seawater distillation, Fifth International Symposium on Fresh Water from the Sea, 1 (1976) 305. G. SPANHAAK, M.A. FINAN and A. HARRIS, The application of a high temperature scale control additive to a european MSF plant, IDEA Congress on Desalination and Water Reuse, 2 (1977) 455. A.W. VEENMAN, J.D. BROEICENS, C.M. VAN DISSEL and G. SPANHAAK, The fluidised bed heat exchanger in an MSF/FBE, its self-cleaning and de-scaling capabilities, Sixth International Symposium on Fresh Water from the Sea, 2 (1978) 61. A. DE MAIO, E_ PALMISANO, R. ZANNONI, New Method for sea water chemical treatment on MSF plant, IDEA Congress on Desalination and Water Reuse, (1979). N-M. WADE, A Practical comparison between multistage flash and falling Sixth International Symposium on Fresh Water film evaporator processes, from the Sea, 1 (1978) 327. H.H. SEPHTON, New developments in vertical tube evaporation of seawater, Fifth International Symposium on Fresh Water from the Sea, 2 (1976) 279.
Netherlands
Am&en&m-PrintedinTbe
A REVIEW OF SCALE CONTROL METHODS
WADE
N.M.
Preece Cardew and Rider, Brighton,
East
Sussex,
Consulting
165-167 Preston Road,
Engineers,
England.
SUMMARY
The
methods
polyphosphate,
scale
limitations, each
of scale acid
that
organic
Recent processes bed
and
control
in MSF evaporators,
used
organic
polymers
performance
and
water
reduce
are
plant
namely
reviewed_
corrosion
sodium
Temperature
are
discussed
for
method. It is shown
and
control
dosing
that
unit
polymers
developments are
briefly
evaporators
and
are
costs
now
competitive
in scale
control
discussed, the use
with
increasing
with
acid
for MSF
including
and
sulphate
brine
temperature
dosing.
other
ion
evaporation
exchange.
fluidized
of surfactants.
SYMBOLS md
=
distillate
Efh
=
brine
C
=
mean
P TI
=
brine
temperature
entering
T2
=
brine
temperature
leaving
Ah=
mean
mass
flow
recirculation specific
heat
enthalpy
flow of brine
difference
highest lowest
temperature temperature
between
vapour
on heat
transfer
stage stage
and
distillate
INTRODUCTION
Control problems scale
of scale
in
the
control Since
formation
distillation
if the
chemicals
the multistage
present
installed
review
concentrates
capacity
Plant
of seawater. used
flash of
on scale
produce
(MSF)
land control
process
based
309
accounts
used
is one
corrosion
aggressive
seawater
methods
surfaces
conditions for
over
distillation with
XSF.
of
can be
the basic linked
in the 90%
plant, Scale
of
with plant.
the this
control
in
falling
film
Three
and
other
main
scale
polyphosphate acid
the
from
developed
dosing
a given
llS°C,
polymer much
Cost
less
sulphate
risk
the
recent
from
Considerably
the
allows
plant,
with
greater
of
nankaly:
three
dosed
up
distillate
to raise
temperature
to be Recently
rates-
to about
IlO"C,
plants. It is shown
techniques.
the other
The
90°C.
corrosion
temperatures
in acid
these
to about
if it is possible
the brine
of much
than
for
is
feed
brine
methods,
despite
that
the higher
the use
make-up,
temperatures
of
ion
exchange
in conjunction should
to remove
with
be possible
acid
if this
calcium
dosing. development
OF SEAWATER
seawaters
the
solubility
limits
transfer
surfaces,
The
and
contain
heat
three
bicarbonate,
110 to
main
scale
can
be
Above combine
making
by
be
sulphate
and
the
of seawater
above
by
are
are
deposited
on
calcium
in concentrations about
50°C
dioxide
addition
at higher
with
(ref.1).
sulphate
carbon
scales
vary
in an evaporator,
performance.
in seawater
controlled
operation
magnesium some
the hydroxide Calcium
calcium
which
is heated
and
exceeded,
transfer
On heating
by acid
90°C
with
and
are
seawater
constituents
is present
can
removed
about
controlled
forming
salts
When
heat
precipitate,
carbonate
neutralisation,
salts
reducing
(as CaCOS).
which
of calcium
can
of some
in concentrations
of salts,
conditions.
bicarbonate
140 mg/l
formed.
a number
climatic
magnesium
Calcium
which
increased
the use
corrosion given
seawater
CONSTITUENTS
All
then
in MSP
chemicals.
location
be
of
development
higher
risk
competitive
the
scale
used
successful.
SCALING
is
presently
temperature
be
dosing
the
permit
are
are
can
Acid at
additives
polymer chemicals .specrfrc cost of
proves
are
brine
of plant
but
comparisons
Another
restricts
size
temperature.
to about
with
methods
discussed.
polymers
top brine
but
control
are briefly
dosing
Polyphosphate
raised
of evaporator
dosing
organic
output
types
from
calcium
carbonate Deposition
is liberated. of certain
temperatures
about
additives
or by
Carbonate
possible.
cleaning.
carbonate
ions
to produce
additives,
can hydrolise
magnesium
or by maintaining
to hydroxyl
hydroxide brine
ions, This
stole. pH below
the
which can
range
precipitates. is present
in seawater
in concentrations
up
to
about
in
311
2500 ppm.
In recirculated brines of about 70.000 mg/l total dissolved solids,
the solubility limit of calcium sulphate anhydrite can he exceeded without scale due to the slow nucleation rate of this salt.
The hemihydrate
form will
however precipitate rapidly once the solubility limit is reached, at about 120°C
The hard scale deposits are very difficult
for the above brine t.d.s.
It is therefore essential
to remove either by chemical or mechanical means.
to keep the combination of temperature and calcium sulphate concentration below the point at wbicb the hemihydrate precipitates. Scale control nmthods have been developed to control the rates of some or all these scale forming constitllents of seawater.
SODIUM POLYPEOSPEATE DOSING
Feed dosing with sodium polyphosphate was the first scale control method to be used for large multistage
flash evaporators.
About 4mgfl of a mixture
generally containing about 60X of sodium polyphosphate,
such as Albrivap A
(formerly Hagevap) is dosed into the seawater feed make-up.
Polyphosphate
retards the formation of hard alkaline scales at temperatures up to about 9O"C, precipitating a softer sludge type of scale. by keeping brine velocity above about 1.8 m/s.
Scaling rate can be reduced To achieve periods of three to
four months between acid cleans, fouling factors for heat recovery stages 2 OC-I should be about 0.00015 W/m , with a greater allowance in the brine heater. The tendency of polyphosphates
to hydrolise to ortho phosphate ions,
producing calcium orthophosphate, needs to be limited in order to maintain effectiveness of scale control.
This requires careful control of brine
temperature and the avoidance of excessive phosphate concentration. &uly plant users, particularly
in the Middle East, use polyphosphate as
their standard scale control method and have built up considerable experience with this technique in large plants.
Due to the low temperature and high pH
of the recirculating brine, the risk of plant corrosion is lower than with acid dosing.
The chemical is safe to handle and can be dosed by vacuum
injection or metering into the brine or the feed make-up.
Some care is needed
to ensure adequate mixing, particularly if nbztering pumps are used. Acceptance tests on a 11365 m3/day polyphosphate
dosed plant in Bahrain
over a 2400 hour operating period gave an increase in fouling factor of 0.00011 w/In" Yz-'
for the heat recovery section compared with the design 2 OC-1 allowance of 0.00015 W/m At this rate of fouling three acid cleans per . year would be sufficient to keep the plant operating at design distillate output and within rated steam consumption.
312
UADE The Taprogge
condensers particularly
effective
in the boundary with
additive
against (ref
sponge
system
to scale
in water
layer
and
the
cost
of
particular
site.
INFLUERCE
OF BRIBE
The
distillate
this
removed
TEMPERATURE
output
temperature
fine by
in chemical
range,
md = ‘b
salt
The
a MSF
is settle
In conjunction to be evaluated
needs test
design
can
plant
facility,
stage
for
a
OUTPUT
distiller
depends
in accordance
with
01
T2
-
which
sand
means.
portable
at the plant
in power
'Ibis method
or
chemical
ON DISTILLATE
of
both
consumption
system_
evaluation
established
in distillers.
containing
the Taprogge
assist
is well
control
is not
dosing,reduction
2) would
operating
ball
as an aid
and
on brine
the
following
flow
and
equation.
1
(1)
'P
Ah
For
constant
temperature about
5OC
maximum and
range_ higher
value
the variation
flow,
The than
on
in output
upper
temperature, supplied. if steam unaltered.
curve
for The
flow
on
with
constant lower
the of
top brine
brine
to the brine
flow.
shows heater
the
T2
The
concentration,
used. TI
flash
is generally
composition,
control
to
temperature.
temperature
east
the possible
curve
seawater
seawater
scale
is proportional
temperature,
maximum
of middle
shows
output
brine
design
the method
for T 2 , typical
The
distillate
minimum
the
depends
of T,
particularly
of 40°C
brine
Figure ,
1 shows
assuming
a value
conditions. increase
assuming change
is kept
in output additional
in output
constant,
and
with steam
with
top brine can
brine
fouling
be
temperature,
margin
is
313
180
COMSTANT
so
FLOW
BRINE
1SQ w
c
ua
5
0
130
g 4 i
0
-CONSTANT
/-
l20
E 0
STEAM
FLaW
no
s
100
m
MAX.
Fig. I
ACID
WSING
Acid
scale
Variation
dosing
was
above
is injected forming
into
Carbon
dioxide
the
0.1
seawater
mg/l.
dependent
output
in
followed some
1960's
plant feed
with
brine
co2 For
this
to the
reaction
however
a typical
to be
temperature
of raising
increased
to neutralise
following
+
by a vacuum will
as a means
output
make-up
mo;z=qo
formed
oo pH.
the
according
+
tower,
in
so enabling
constituents,
in a degassing to below
of distillate
introduced
90°C,
H+
being
TEYPERhTURE
BRINE
<@.I CONTROL)
temperature Acid
-
120
110
100
90
top brine
(ref.31
the
alkaline
reaction.
co2
is normally deaerator remain
seawater
released
to reduce in solution,
alkalinity
of
to atmosphere the the
oxygen
content
concentration
120 mg/l
(as CaC03)
WADE
314 the injection rate of 98Z sulphuric acid for full neutralisation would be 120 mg/l of seawater feed make-up.
Brine pH could be below 7. due to residual
CC*, which would be corrosive to carbon steel shells.
Two alternative dosing
methods are used:-
a)
reduce the acid injection rate to leave about 15 mg/l residual bicarbonate alkalinity in the feed, or,
b)
inject sufficient acid for full neutralisation
and
raise brine pE by the addition of caustic soda or other al'kali.
Tbe first method results in some scaling and still requires careful pB control to avoid serious corrosion.
The second method is less critical but There is also some risk of
involves an appreciable extra chemical cost.
Magnesium hydroxide scale formation at higher pY. The combination of maximum temperature and brine concentration must be kept below the solubility limit of Ca S04. hard adherent scales.
AH20 to avoid precipitation of
It is sometimes overlooked that conditions in the
boundary layer in the brine heater, particularly close to a steam inlet, will be a few degrees above the bulk brine temperature.
Plants have been designed
for brine temperatures up to 121°C, but it has often been found necessary to decrease this temperature to around !lS°C to operate free of calcium sulphate scale. It is possible to reduce the fouling factor allowance in acid dosed plant. compared with plant designed for polyphosphate
dosing, since the rate of scaling
A typical fouling msrgin for the heat recovery stages in an 2 CC-1 acid dosed distiller is 0.00007 W/m . is much lower.
The main drawbacks with acid dosing are the risks of corrosion in carbon steel shells and the potential hazard to personnel handling acid.
In their
survey of a number of plants for OSW in 1972, Newton, Birkett and Ketteringham (ref.41 showed that severe attack on carbon steel shells was more prevalent in acid dosed plants than in phosphate dosed plants. corrosion measurements 5 years operation.
Morin (ref.51 reported
in the acid dosed Bluehills plant in the Bahamas after Wb;lst there was little attack in the first vessel,
which was lined with stainless steei, the floors in the second vessel had corroded at an average rate of 2.4 mm/year, with even higher rates of attach in areas of turbulence or impingement. To
contrclcorrosion,
without depositiag scale, it is necessary to keep
brine pg within a narrow bandwidth, about 7.7 - 8.0,
To achieve this accuracy
WADE
315
it is necessary
to maintain pH meters to a high standard with regular cleaning
and calibration, backed up by regular brine ssmpling aud analysis for dissolved These high standards are hard to attain,
metals, pH and residual alkalinity.
particularly where skilled operators are at a premium. Whilst the author is not aware of any serious accident due to the use of acid in desalination plant, there must be a considerable risk in the handling and use of dangerous
chemicals.
This
risk can be reduced by correct design of
storage and transfer systems and by the use of protective cloth&.
Operators
are hovever reluctant to wear face masks and gloves, particularly in hot climates.
Bulk
transport
and
handling
is safer than small drums or carboys.
of acid in special purpose containers
Bulk purchase is also likely to be
cheaper for the large quantities of acid required, for example around 800 tonnes/year for a 12000 m3/day plant at '1.65 load factor. Until recently acid dosing was the only method of operating distillers at high temperatures, giving savings in water cost compared with polyphosphate. which many operators considered to be worthwhile, particularly
in areas of
high fuel cost.
POLYMBR ADDITIVBS
A number of feed treatment chemicals have been introduced in recent years as the result of research and development work to extend the range of additive dosing to higher brine temperatures. Chemicals based on polyacrylates were found to produce a hard scale if certain temperature limitations were exceeded.
Research work by Ciba Geigy
and the DRABA (ref.6) resulted in the development of Belgard EV, a low molecular weight polymeric carboxylic acid. introduced chemicals of a similar type.
Other manufacturers have since
These polymers may act either as
threshold inhibitors or by means of distorting the lattice structure of scale crystals,or both,so as to reduce adhesion to other crystals or to metal surfaces_
Plant trials and regular operation with Belgard EV (ref.71 have
shown that it is possible to.control scale formation at brine temperatures up to llO°C and possibly higher.
Dosing rate must be increased with temperature,
from about 2 mg/l at 90°C to about 6/8 mg/l at llO°C.
The actual dose rate
required depends on seawater composition and plant detail design and should be established by trials on clean plants at each location. Although some polymer additives are weak acids, the Lou dosing rate has little effect on brine pH. typical values being in the range 8.5 to 9.0, depending nn seawater alkalinity_
Corrosion rates with polymer dosing have
been found to be as lov as with polyphosphate, except for some increase in the
WADE
316
first 316L 213
few
stages
stainless
The
temperature Polymer
treatment
and
pump
The
by
use
some
reduction
fall
in output,
venting
to polymer
of acid
operation
with
consequent
acid
dosing.
rate
and
a small
corrosion
plant The
is usually
recent Acid
Spanhaak
dosed
and
in flash
for plants
method
should
to operate
COST
that
polymer
be
80%
scale
is now
for new
on polymer
up
of of
The
gases
acid
dosed
from plant
rates, cost. and
stoicheometric
this
method.
described
is satisfactory
in acid
a viable
to about
installations.
should
of Belgard
the
control
dosed
and
outpubif
of capital
charges
to acid
that
still
lower
and
plant.
alternative llO°C
It would
or polyphosphate,with
supply
to compensate.
in corrosion
than
using
involve
in maintenance
Trials
that
at temperatures
considered
at 90°C
the
to a corresponding
of a combination
is added.
dosing
first
from
25%
to extract
saving
to about
of
may
in converting
a reduction
use
order
flow
modification
and
by
steam
plant
lead
brine
is substantially.lower
operating
the
dosed
could
purpose
is the
showed
(ref.81
chambers
It is considered dosing
10S°C.
need
is reduced
of polymer
others
of
this
plant
design,
to around
to achieve
development
plant
increases
in reliability
injection
dosage
may
the
CO2
It is often
a polyphosphate
to raise
main
type
or as an alternative
distillers.
in acid
and
possible
stages.
improvement
A further
by
temperature
if it is not
temperature
from
polymers
for
to extract
plant,
Providing
temperature
temperature
consideration
with
plant.
dosed
output
adequate. brine
in brine
system
the high
are
raising
of high
output
temperatures.
Lining
release.
designed
for new
or acid
distillate
capacities
achieved
either
CO2
be worth
to be
as in polyphosphate
phosphate
at high
and
may
needs
can be used
existing
dosing
system
stages,
to increase
polymer
temperatures
or cupro-nickel
venting
dosing
for
possible
to high
steel
stages.
high
be
due
this
be possible required.
COMPARISON
Costs operating
of seawater costs,
maintenance. specific shell
capital
due
a greater
control
for
have
main
cost,
temperature
steam
consumption
the
same based
on extraction increases
vith
power,
brine
chemicals,
temperature
power pumping
operators
and
is a reduction
transfer
Auxiliary
recirculation
and
surface
and
consumption power
in
distiller also
required
with
range.
estimated
considered.
up
in the heat
rating.
in brine
made
auxiliary
o.f raising
output
decrease
been
are
d-tieto savings
method
are
fuel,
effect
a given
to the flash
Costs
pressure
comprising
The
needed
falls,
desalination
for
In each at
from brine
12000 case
the brine a 6OMU
m3/day the
MSF
The
heater.
turbo-generator.
temperature,
distillers,
performance
the
fuel
for
ratio costs Since
input
each
is 8:l.
of providing
scale
giving steam
the extraction
to the boiler
will
WADE
317
also increase.This
is reflected
in
fuel
top brine temperature than for 90°C. E60 per tonne.
costs aboct 10% higher for ll5OC Fuel costs are based on a fuel Price of
Operating costs are calculated on a basis of 65% load factor.
Annual capital changes at early 1979 price levels are based on amortization at 10% interest rate over a plant life of 20 years, and include the distillers, foundations and the proportion of turbine and boiler costs appropriate to the provision of extraction steam. The estimated costs are given in Table 1 and illustrated in Figure 2, as unit costs in pence per cubic metre of water produced.
TAEILE I
Comparative cost estimate for 12000 m3/MSF distillers with 8:l performance ratio, for various types of feed treatment, at 0.65 load factor.
Capital Costs
Polyphospbate
Polymer
Acid
8.363 0.734 1.370 1.570
6.836 0.759 1.370 1.345
6.398
12.037
10.310
9.835
E Million
Distiller and auxiliaries Power station plant (cost increase) Civil works Engineering and contingencies (15%)
Total fMillion
0.784 1.370 1.283
-Unit Cost of Water P/m3 Capital charges (at 10% over 20 years)
-
Distiller Others
Fuel at f60/tonne Auxiliary power at fO.O2/kWH Maintenance Operation Chemicals - Polyphosphate - Polymer - Acid at
34.5 15.2
26.2 14.3
26.4 14.2
18.7 9.5 3.4 6.7
19.6 7.9
20.6 7.4 3.4 6.7
at fSOO/tonne at E2000/ 11 LiZOf "
Total unit cost P/m3
3.4 6.7
3.0 3.6
88.5
___-
83.1
82.3
318
80
88-S
-CHEMICALS
WHTEHAta 83.1 U’ERATOR!
80
AUX. POWER
:HEYICAL!
‘PERATORS
,HRMICAL!
FERATOR!
70 AUX. POWER
80
AUX. POWER
FUEL
"f P K 0) 2 z
FUEL FUEL
50
b t : r
CA?lTAl
CHARBC -OTHER PLANI
CO
CAPITAL CHARMS -OTHER PLANT
z
CAPITA1 CHARK! -OTHER PLART
30
20
CAPITAL CnAROES DISflLLEF
CAPITAL CHARRRS IISTILLER
CAPITAL CHARRES DISTILLER
10
0
POLY PHOSPHATE
Figure
2. Comparison of unit control method
ACIO
HIGH TEN POLYYEI
water
costs
for each
scale
319
WADE
Opera11 water costs are highest for the polyphosphate
dosed plant.
Savings of approximately 6.1% are indicated for a polymer dosed plant and 7.0% for an acid dosed plant, compared with polyphosphate savings
in
treatment.
The
capital cost with high temperature operation are more significant
than the extra costs of dosing chemicals involved.
In estimating these costs
plant maintenance has been taken as independent of the type of-feed treatment As discussed earlier, it is probable that an acid dosed plant would
used.
require greater maintenance expenditure than additive dosed plant.
The small
saving in overall water cost with acid dosing, compared with high temperature polymer dosing,
would be lost if the acid dosed plant were to require heavy
maintenance on distiller shells. This cost comparison shows that high temperature polymer dosing would give a significant saving compared vith polyphosphate
and would be competitive
with acid dosed plant. Costs for polymer dosing have been estimated with the same fouling factor as used with polyphosphate.
If the fouling factor for a plant designed 2 OC-1 to use organic polymers were to be reduced to 0.0001 W/m , the overall
water cost
would fall by about 2 p/m3, making this the lowest cost method.
OTEERSCALE
CONTROLTRCRRIQUJ3S
This review has discussed the scale control methods presently in use with MSF plant.
Several other techniques are, however, under development or in
limited use. A recent development in MSF technology is the use of small beads acting as a fluidised bed inside vertical tubes. to aid heat transfer and control scale.
Pilot plant tests showed promising results in the prevention of
alkaline and calcium sulphate scaling at high temperatures
(ref.91 and a
prototype 500 tonne/day plant in the Netherlands entered service in 1978 using this method, with no chemical addition to the feed other than antifoam.
The
saving in chemical costs, together with claimed capital cost reduction could make this process a strong competitor, once fully developed. Removal of sulphate ions from the feed make up, in conjunction with acid dosing, is reported by de Maio at this Congress (ref.10).
A prototype
plant is operating with 150°C brine temperature and higher temperatures are claimed to be possible.
The economics will depend on resin performance and
life aud on material selection for high temperatures conditions.
The method
might also find application in existing plants prone to calcium sulphate scaling. Falling film evaporators of the VTR type need to operate scale free to achieve the high heat transfer coefficients on which these designs are based.
320
Acid
WADE dosing
additives avoid
is normally
sulphate
distribution by Sephton
effect
scale
at and
6_ef.
(HFF) low
and can
use
water
The
from
promising acid
without
in the
flow
use
a foam
either
compression
turbulence
good
11).
shows
tempernture
vapour
since
and
by evaporating (ref.12)
evaporators operate
used
to be effective,
of
surfactants
inside
the
results or polymer
chemicals
film
is too
distribution
to promote
tubes
in pilot
has
been
for to
good developed Horizontal
plants.
dosing,
as used
lov
is essential
or alternatively
in a range
of multiple
plants.
ACKNOWLEDGEMENT
The permission for
their
Author
wishes
to present assistance
to thank this
paper,
in preparing
the Partners and
Mr.
the
cost
Cardew
of Preece,
M.R.
Homsby
estimates
and Mr. and
and M.
chemical
Rider
for
Greene data.
REFERENCES
1. 2.
3. 4.
5. 6.
7.
8.
9_.
10.
II.
12.
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