Improved antiscaling treatment for desalination plants

Desalination,66 (1987) 285-298 Elsevier Science Publishers B.V.,Amsterdam-PrintedinThe

IMPROVED ANTISCALING

285 Netherlands

TREATMENT FOR DESALINATION

P. ZINI' , R. ZANNONI', R. LUCENTI 'Rohm and Haas European 06560 Valbonne, France

2

Laboratories,

Reggiane Officine Meccaniche, Emilia, Italy

PLANTS

Sophia

Antipolis,

V. Agosti 21, 42100

Reggio

SUMMARY The continuous evolution in polymers technology has recently provided tools for designing and industrially producing several new classes of water soluble polymers. Progresses in the usage and understanding of water soluble polymers have been important, for instance, in oil fields applications, in detergency, in mining and, of course, in industrial water treatment. Among those latter we will present a case of a new development in the area of polymers usage for multiflash evaporators plants, where high temperatures (+ 11OOC) and low risk operating conditions can co-exist in order to maximise the plant efficiency and productivity. This paper covers two aspects of this development, mainly the initial lab work and the plans for confirming those results in a newly built MFE plant. INTRODUCTION The experiments

used raw

sea

water in

a

laboratory

apparatus of the type shown by Elliot et al. to give results (1,2) which correlated well with plant experience Elliot's apparatus consisted of a glass cell, 4 in. ID and about 12 in. long, into which sea water, with or without additives, was pumped at a slow rate of about 10 l/day. cell contained a steam heated approximately maintain

"U" tube of aluminium

12 in. long x 0.35

in. OD, which was used

the sea water at the test temperatures.

usually in the region

The

brass,

93 to llO°C. An

These

air sparge into

to were the

cell continually removed CO2 released by the decomposition

286

of bicarbonate simulation

sea water, thus giving a

in the heated

of

the

water

conditions

good

in

an

evaporator, and a throttling valve balanced the flow of

the

chemistry

usually

sea water out of the cell with the feed rate. Tests

scale

continued for a period of two weeks. During this time deposited on

the heated

vessel walls and the off

was scraped

amount of

sea

of

as on

glass

the

area

scale from each

the performance

the total

quantity

water

of

the

scale on

the

feed,

usually

sea water. "Good" additives

expressed as mg scale/kg conditions

the particular

and

and weighed

unit

per

U tube

base plate. The

gauged by

additive was U-tube

as well

the

of

experiments

under

seemed

to

produce less than 1 mg/kg of deposit on the heated U-tube. The

undertaken

measurements

the apparatus were

work

present

in two ways

represent advances on Elliot's work the amounts of scale

the

in

:

firstly,

and the rest

on the U-tube heater

of the

and secondly,

measured separately,

use of hot air as the heating fluid instead of steam enabled heat transfer measurements

to be

EXPERIMENTAL

Elliot

constant rate

experimental (1)

.

et al. into

each

of

technique Sea

closely

water was

the cells

metering pump, each head drawing reservoir.

an

of additive performance.

DETAILS

The general that of

proceeded

be derived, thus providing

and scale fouling factors to extra measurement

made as the run

by

followed

pumped a

at

head

three

sea water from a

a

separate

Each cell consisted of a 6 in. nominal bore glass

pipe line, 16 in. long, sealed at each end by thick plates of 90/10 copper/nickel the end

alloy.

plates allowed

feed devices.

A

overflow weir

and liquid

0.5

in.

liquid depth of about 13

bore Cu/Ni

various measuring pipe

level controller,

served

as

an a

completely

excess sea water flowed

of the cell to another level control vessel. A sparge fed argon continuously

or

maintaining

in. in each cell which

covered the U-tube heater. The

into

Holes drilled or tapped

insertion of

into the base of the vessel.

out pipe,

281

II +-l

288 This bubbled up through the sea water, removed CO2 liberated as

a result of the decomposition

also served to deoxygenate sea

water

conditions

chemistry

desalination

plant.

of bicarbonate by heat, and

the sea water, thus similar

The injected

argon

maintaining

to

those

gas plus

in

CO2

a and

water vapour left the vessel through a single tube condenser and passed into the level control vessel. These gases,

plus

excess sea water, flowed as a two-phase mixture out of vessel

to

drain

maintained

a

via

pressure

control

valve

this which

the absolute pressure required in the test cell.

The inverted U-shaped heater was made from 0.25 in.

OD

tubing, also of 90/10 Cu/Ni alloy to minimise corrosion. The total length of pipe

in this heater was

27.1 in. The

base

plate was fixed by a number of stainless steel screws to the thicker end plate at the bottom of the cell and sealed by

a

rubber gasket. At the end of an experiment the entire heater assembly could be inspection.

unscrewed and removed

from the cell

for

In contrast to the steam heating used by Elliot,

in this work

air was used

as the heating

medium. Air

was

separately supplied to each heater via a commercial electric heater

("Sikkens"

paint

raised its temperature

stripper,

to c.

outlet from the U-tube was to the sea water with an air

rotameters were used

of 105 l/min. to monitor the

Low voltage

wound round the outside of These were

to

speed up

temperature and, during

kW

rating)

determined by the heat

and was typically llO°C

flow rate

U-tube heater.

1.3

200°C. The temperature at

Three calibrated air flow through tapes

the glass section of each the

attainment of

a run,

the

were operated

the

transfer

for a clean

electrical heating

which

tube air each were celi.

operating

at very

low

power to maintain this temperature. The overall

heat transfer

were calculated from the air outlet of

coefficients

in

each

temperatures at the inlet

cell and

each U-tube heater, and the sea water temperature.

289

For

these

O.OlOC.

measurements

platinum

used, calibrated

against a

three

thermometers were

resistance

The

directly as temperatures

thermometer

on each cell to hot air

were

outputs

box. Two thermocouples

on a

were

chart recorder

of used

sea water and

provide measures of the

inlet temperatures

to read

on the digital scale

to O.Ol°C

an automatic potentiometer

resistance standard

as

the quick

checks on the operation of the cells. Each

cell

was

filled

with

sea

water

containing

inhibitor at the appropriate dilution and gradually up to

temperature by

the operating

tapes. This took

about four

means of

hours, during

brought

the

heating

which time

argon flow to each cell and the pressure relief valve the level

control

vessel were

pressure reading of about

adjusted

0.5 bar on

to give

the after

a

gauge

each cell. Once

this

temperature was attained the feed pumps were switched on and the hot

directed through

air streams

heaters. As soon as the few minutes after air flow rates readings almost

the

U-tube

feed commenced) air temperatures were taken for

immediately.

each of

air temperatures had stabilised

These

first

each cell and readings

compute overall heat transfer coefficients, U

repeated used

were 0’

between

U-tube heater and sea water before any significant

coefficients

for each

to the

amount of

scale was deposited. These U o readings were regarded as "clean" heat transfer

(a and

the

U-tube

of the

exchangers. Hourly readings of temperature and air flow rates

were

taken, from which overall heat transfer coefficients

(Ut)

were computed. For

the thermal

fouling

follow the increase of the

fouling

resistance R W

R

W

=1

11

-t

each value

of u

could be calculated

t' from

k 11

-0

It was thus possible to

resistance with time as the run proceeded,

290

Apart from reducing the amount of scale deposited from sea water on heated surfaces an anti-scaling additve may have the further useful property of

modifying the

deposit

structure so that it is only loosely held on the surface and amenable to removal by shear forces from the flowing fluid. Since many miles

of heat

exchange tubing are

water distillation plants the could

be

significant in

effect of

relation

to

used in

deposit the

sea

strength

overall

heat

transfer coefficient and

hence tubing capital costs. When scale deposits qualitative observations were therefore made of their relative hardness and adherence scraping off

the

to the heater surface, as well as of their colour. After

each

series, the

scale

characteristics were

summarised under three headings : (i) Qualitative examination of colour, hardness and Scanning

Electron

structure.

Microscopy

the

deposits

(The latter

and

X-ray

for

employed

diffraction

measurements). (ii) The mass of scale deposited on each of the

U-tube

heat exchangers. (iii) The

heat

transfer/fouling characteristics of

these heat exchangers under scaling conditions. The results also show that approvimately twice as scale was deposited on the cooler outlet legs of

the

much

U-tube

as on the inlet ones. Comparisons of the amounts of scale deposited

per

unit surface

parts of

the

cells

area of

the various component one exception, the

show that, with

amounts per unit area on

the unheated surfaces were

larger

than those on the heated surfaces. The exception is the test scale with untreated sea water where the highest amount of per unit area was found on the U-tube, as would be expected if the scaling

mechanism involved direct crystallisation of the scale on the surface, since the temperature and thus the driving force for scaling should be highest there.

291

It would be expected that scale would be formed on hot surface. temperature

Since

the

sea water

at

is fed directly into the cells at the

temperature,

any

approximately

a very high supersaturation

room

operating

is quickly reached,

which may well be sufficient to cause nucleation

in the bulk

sea water as well as on the hot surfaces. This suggestion is supported by the observation of scale crystals in the

level

control vessels through which excess sea water flows out

of

the test cells ; since these are unheated they should be

at

a lower temperature

be

than the cells and thus there should

or zero

a smaller

driving force

of

for nucleation

scale

inside them, and so any scale present is likely to have been carried there in the flow from the cell. The higher amounts of scale found on unheated

surfaces

than on heated ones could be the result of either or both of : (1)

two processes

thermophoresis,

which

means that

the

small crystal nuclei are less likely to deposit on a surface the higher its temperature, stress at the convective

flow

dislodge poorly

along them,

the

hot

air

which might

from the side.

shear

increased

be sufficient

Calculations

of

the

90% of the

total water

With

inlet

on the waterside

and

outlet

respectively,

air

the

should initially be

about 10°C and l°C above the bulk sea water temperature, that there would be a much greater convective flow of on the inlet side. If

them before

cementation.

they become

more

firmly

so

water

the scale nuclei were initially

weakly attached to the tube surface, this flow could some of

to heat

hot air to the sea

of about 210°C and 117V

surface temperatures

from the

suggest that about

transfer in the cells

temperatures

(2) a higher surface

adherent scale.

heat transfer resistance lies on

and

hotter surfaces, arising

only remove

attached

by

The additives might play an important role here

by producing a porous deposit which was more easily by convection.

removed

292

TEST RESULTS decided to test two

As starting point we available additives, one,

polymer C, is

several operating MFE plants ; good

antiscaling

agent

commercially

already in use

in

the second, polymer A, is

for

general

water

a

treatment

applications but not actually used in MFE plants. A "blank", i . e . a cell

without polymeric additive was

also

run to establish a negative control against which to measure performance

differences.

Among the several parameters

checked, table 1 shows the

results on the scale collected on the U-tube heat exchanger, together with the relevant test conditions. It can be seen that : 1.

The "blank" performed very of scale deposited.

poorly, with a high

This scale was

weight

also very hard

to

remove. The

2.

"general-purpose"

antiscaling

polymer,

although

three the weight of

deposited

scale, did not compared favourably with the

commercial

reducing by a factor of

polymer already in use in the MFE plants. The scale on the heat exchanger protected by polymer

A

was also hard and difficult to remove.

TABLE 1 First test series

Polymer

A,

Polymer C, blank Cell 1

Sea water To Sea water rate Air flow rate Air inlet To

(OC) (l/day) (l/day) (*Cl

Additive level

(ppm)

Scale on U-tube per kg of sea water feed

(mg/kg)

Cell 2

105 29 105 220

105 29 105 220

Polymer A

None

6

1.7

Cell 3 105 29 105 220 Polymer C 6

4.5

0.8

Since between polymer A there were differences moved

composition, we designed

to

and the successful polymer

evaluate

back the

series

to a

relative

One key difference

which

of

bench

importance

tests

of

those

emerged from the bench

tests

parameter on the final performance

of the polymer.

was that the various classes of water soluble polymers significantly

C,

in both molecular weight and chemical

different

salt

tolerances respect

to

have water

containing high Ca++ levels associated with high Na+ and Cllevels. Figure 1 describes

in

chemical composition may tolerance of

play on the

polymers. Low

generally more

tolerant

weight homologues.

how molecular weight

summary

salt ti

molecular weight

to

salts

than

However a relatively

and

temperature polymers

higher

are

molecular

small percentage

of

"ad hoc" comonomer into the main backbone chain, is far more effective than salt

molecular weigh

tolerances

can

variations. Extremely

therefore

be

obtained

also

good for

relatively high molecular weight polymers.

In the (keeping

test

second

as

reference

series we a

tested

repetition

of

two the

variables test

run

previously with the MFE polymer C) : 1.

Higher level of the general antiscaling polymer A (12 ppm vs 6 ppm as in test #l).

2.

A new polymer B (at 6 ppm) designed to be able to tolerate the high salt/high temperature conditions

of a

MFE unit. The results of this series of tests show :

1.

Doubling the level

2.

The new polymer B, dosed at 6 ppm, is almost equivalent

of the general-purpose

antiscaling

agent does not improve the scale control.

to the polymer C, at the bench

indicating that the experiments

scale have an

this phenomenom.

important significance

run to

The

3.

important

polymer C

(from

variation

in

quantity

of

scale

for

0.8

sea

water

1.2

mg/kg

sea

mg/kg

to

water) may be due to a test fluctuation but also to weakeness

inherent product

(commonly recommended for reasonable) may level is no

be on

i.e.

the dosage

keeping the treatment

the edge

of a

longer sufficient to

an

at 6

ppm costs

cliff were

cope with the

the scale

formation.

scale

mglk

-_

-_

‘my 4000 . copolymer

l

\

g

.i

+ooo

s 5 '\

I

‘-

mw 4000

6

temperature

9

12

pm

polymer FIGURE 1 : SALT

TOLERANCE

FIGURE 2 : DOSAGE RESPONSE

The final test series was run with two objectives

:

1)

to test in a single variable mode the effect of dosage level for the higher

new polymer

5:

and 2)

to assess

the effect

of

(115OC) temperatures.

Figure

2

shows the

performance/dosage

response

for

this

polymer. Optimum dosage occurs at 9 ppm, but all the range between ppm

and

12

ppm

gives

reasonable

allowing a certain flexibility To note

that the

protection,

6

therefore

in "dosage malfunctioning".

absolute level

of scale

since we operate at 115'Y. not at 105V

here is

higher

as in test 1 and 2.

295

TABLE 2 Second test series - Polymer A, Polymer B, Polymer C Cell 1 Sea Sea Air Air

water To water rate flow rate inlet To

(OC) (l/day) (l/day) (OC)

Additive level

(ppm)

Scale on U-tube per kg of sea water feed

(mg/kg)

105

Cell 2

Cell 3

220

105 29 105 220

Polymer A

Polymer B

Polymer C

12

6

6

1.9

1.3

1.2

72% 28%

13% 21%

105 29 105 220

Out of which :

on colder outlet on hotter inlet

The

polymer

polymer C. to

the

variations

is

of potentially lower cost than the

This adds an additional safety

MFE

envisaged

B

66% 34%

operations

:

higher

levels

target

factor can

be

(i.e. 9 ppm vs 6 ppm) for the same cost and dosage seem to

have a

smaller negative

effect on

the

scale deposited.

Third test series - Polymer B, Polymer B, Polymer C Cell 1 Sea water To Sea water rate Air flow rate

(OC) (l/day) (l/day)

Additive

115 29 105 Polymer B

level

(ppm)

Scale on U-tube per kg of sea water feed

(mg/kg

12

2.2

Cell 2 115 29 105 Polymer B 9

1.9

Cell 3 115 29 105 Polymer C 6

2.0

PLANT TESTING The plant trial will be run for a minimum period of months at the

electrical power station

of AL-MUKHA

12

(Yemen

Arabic Republic). The tests will be conducted in a single-variable of the

MFE

four

units available.

temperature of the installation units, built

The four boilers and the

The

be used

operating

is expected at llO°C.

to provide

general services of

been released to

mode in two

maximum

water to

both

the

the power plant,

in the test

by the

have

manufacturer

(Reggiane O.M.I. - Italy) and by the owner of the plant. The start of the testing initial tests and

phase will immediately follow

controls needed to

the

check respondence

of

the MFE units to the project requirements. This approach conditions 1.

will

to meet

enable

two

key

testing

(and therefore unscaled) MFE units

working

:

new, unused in parallel

2.

a

regular,

routine

functioning

not

interrupted

by

checks and start-up problems. units are

The four

absolutely indentical.

been designed to work at 90°C with a polyphosphate

They

have

treatment

or at llO°C with a polymeric treatment. The tables

3,4 and

5

show the

key design

data

materials. The MFE units of this plant are relatively

and small

and, as such, easy to run and to check. Their

intrinsic

flexibility,

conditions, makes these

in

units ideal

design

and

operating

for experiments,

the one we plan, where realistic, industrial conditions

like and

rigourous testing parameters are both needed.

Two

of

the

four

units

at

high

as possible, kept at

this

will

temperature

(llO°C) and, as long

temperature

for a

prolonged period

be

run

in order

long term effects of the polymer treatment.

only

to check

the

297

TABLE 3 AL MUKHA P.S. - MsF units manufacturer

Reggiane design data

Maximum brine temperature OC Total production ton/h I, Sea water make-up 0 Excess sea water II Recycle brine II Blow down II Brine to heater Sea water concentration WT% II Blow down concentration Sea water temperature OC Blow down temperature OC kg Prod/Kg Steam Gained output ratio

12.5 36.2 138.8 119.8 23.7 156.0 4.27 6.52 35.0 40.33 7.26

110 17.1 46.7 129.32 109.3 29.6 156.0 4.27 6.74 35.0 41.11 7.34

Reggiane

O.MI.1

90

TABLE 4 AL MUKHA materials

P.S.

-

MSF units

manufacturer

Total number of stages heat recovery section - number of stages - outside diameter of tubes - material

18 15 mm

Cu/Ni 90/10 and albrass

Heat reject section - number of stages - outside diameter of tubes - material Demisters

19

3 mm

Cu/Ni 70/30

Type York N.421, 4.0 inches, AISI 316 L

Shell, Heat recovery and reject sections Tube plates Waterboxes Distillate (tray and duct.) Air vents and pipe

Before actually starting will be

acid-washed

conditions and, tubes surfaces

to

the test

re-set the

more importantly, differences

due,

different start-up programmes, units.

19

or

AISI Cu/Ni Cu/Ni AISI AISI

316 L go/10 go/10 316 L 316 L

trials, both

initial project to cancel

for example,

units design

any

possible

to

slightly

lengths, in the two

test

298

TABLE 5 AL MUKHA P.S. - MSF chemical performances

units, manufacturer

Distallate quality : Total dissolved solids Total iron (Fe++) Total chlorides (Cl-1

9pm 99m 99m

Silica

99m

PH

(SiO2)

Antiscale dosing : Polyphosphate (top brine temp.OC 90) Polymeric formulation (Top brine temp. oc 110)

The test mixed Rohm & crew

and

will be

supported for

Haas and Reggiane

control

effectiveness

of

procedures the

Reggiane

O.M.I.

15.0 0.1 < a.0 6.6-7.0 0.2

9pm

4

99m

5-10

a first

period by

team. Then, normal will

polymeric

be

allowed

treatments

a

(local) to

under

check routine

conditions. Periodical checks of during and

at the

the operating end of

parameters will

the test

period, to

allow,

completely

assess the role played by both polymeric treatments.

CONCLUSION Bench experiments, fairly

small,

plant

pilot testing testing

programme of this new MFE polymeric experimentations

and

the

evaluation

treatment. Both rigorous

day-to-day

allowed

although

and a full,

constitute

routine

conditions will test accurately how this polymeric

plant approach

works for this application. The ability variety

of

the

and

conditions

the anti-scaling

is

a

recognized as key

variability parameter to judge

treatment to cope with of

that

the should

the overall

real be

plant

the life

increasingly

usefulness of

the

treatment itself. REFERENCES 1 2

Elliot, M.N., Hodgson, T.D. and Harris, A., Desalination, 14 (1974) 43-55. Elliot, M.N., Jordan, T.W.J. and Hutchinson, M. Proc. Third Int. Symp. on Fresh Water from the Sea, 1 (1970) 461.