Improved high temperature MSF evaporator operation for potable water production

Desalination, 54 (1985) 361-376 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

IMPROVED

HIGH

TEMPERATURE

MSF

EVAPORATOR

361

FOR

OPERATION

POTABLE WATER

PRODUCTION BY D. P. LOGAN' and K. KURODA' 1 Calgon Corporation, P. 0. Box 1346,

Pittsburgh,

'Mitsui Engineering and Shipbuilding Minato, Tokyo, 105 (JAPAN)

Co., Ltd.,

PA 15230 (USA) l-Chrome

18-16 Shimbashi

ABSTRACT The Royal Comnission of Al Jubail and Yanbu, Kingdom of Saudi Arabia, consnissioned Mitsui Engineering and Shipbuilding to design and build dual high temperature MSF evaporators to provide fresh water 'to the industrial city of Al Jubail. An antiscalant without polymalic acid, which is intolerant of low ferric ion concentrations, has been used successfully since startup over three years ago. Unit operating data is discussed which establishes the trouble free high temperature operation possible with antiscalant dosing. Preventive maintenance measures combined with proper system component design and choices of materials of construction have resulted in minimal corrosion and excellent performance while providing fresh water on an "as needed" basis.

INTRODUCTION Two

high

temperature,

constructed

by

Commission

of Al

produce Scale

deposit

cleaning,

The

scale

and

Support

services

monitoring

and

performance discussion inspected

test

OOll-9164/85/$03.30

additive,

included

on

Kingdom

inhibitor

Foaming

the

Saudi

temperature

deaeration

is

designed for

of

dosing

by continuous

Calgon

and

on-site technical

Units

of Unit One's during

dosing.

phases,

periodic

data

by

by vacuum

is prevented

and phosphonates,

acceptance

Jubail,

(MES)

with

Arabia,

of 110°C. mechanical

and oxygen

chlorine

controlled

and Royal

scaven-

addition by

with

continuous

(Ref. l-4).

control

polycarboxylates

evaporators

each at a top brine

is minimized

shock

flash

Shipbuilding

in Al

is achieved

Biofouling

application

stage and

Yanbu

of distillate

supplemental,

antifoam

and

control

and corrosion

ger injection.

multiple

Engineering

Jubail

19,200 m3/D

and

daily,

Mitsui

One

a

proprietary

blend

of

performance

for

the

commercial

startup

and

Two.

visits. Also

including

shakedown

operation

assistance,

assistance

condition,

the critical

EL-2438,

was used for the evaporators'

period.

Elsevier Science Publishers B.V.

routine This

presented

deposit

of

units.

performance

paper

contains

is

detailed

analyses,

a

when

it was

PERFORMANCE

DATA

The twin, multiple are designed

to produce

stage flash evaporators

owned by the Royal Cormmission

800 m3/hr of distillate

each with a maximum

Each 21 stage unit contains

ppm.

18 heat recovery

tion stages, with a brine recirculation makeup

flow rate. The design

the recirculating

brine

cycles

divided

dose rate for EL-2438

Key

data

design

reflect

appear

of concentration,

antiscalant

in Appendix

operating

modes,

rejec-

defined

Document,

depending

as the TDS

makeup,

design

Volume

operating

II,

flow.

parameters

pages 164-167.

on the seawater

makeup

of

is 1.5X.

was 6 ppm based on makeup

Unit

One.

from the MES Contract

several

and 3 heat

rate of 3.2 to 3.8 times the seawater

by the TDS of the seawater

The nominal

were obtained

stages

TDS of 25

They

temperature

(Ref. 5).

The the

critical

Royal

kilograms

the

enthalpy

ratios

in

the

corrected the ER. startup

report

for the boiler Results

appear

The method

under

Two.

design

nominal

Average

values

phase,

for

the

:

fluid

Three.

and period top

brine

for

initial

test,

the

boiler boiler

boiler

derating

2, the

unit was

production

Calculated of

The mass creates

the

996

for

and

a

is 12.6

efficiency

kg/m3

for

the

of distillate

was

not

a maximum

+2% error

Period one represents

temperatures,

ER

ER value

two, the acceptance

in

the initial

phase for Unit

distillate

Flow, m3Jhr Distillate

production

outlined

and evaporator temperature

under

12.5 12.7

1 is attributed

by MES.

performance control

test and the evaporator operated

ES

Kg/l0 kcal

813.3 786

flow for Period

schedule

and

of steam energy

below:

The lower distillate operations

the

calculating

densities

106.7 106.6

shakedown

and

as

distillate

conditions.

TEMP, 'C BHO

Period

MES

defined

The guarantee

flow, which

makeup

in Appendix

and performance

between

is

The steam energy used is corrected

in Appendix

used

upon

which

is the total

and 950 kg/m3 for the condensate.

rates and E R 's appear

the

produced

condensate.

appear

agreed

ratio,

(net) per 1000 kilocalories

produced

for operation

this

distillate

One.

of

kcal

parameter,

efficiency

sent to the boiler.

calculation

kg/1000

the

The net distillate

the makeup

sample

results

is

of distillate

(net) used. minus

operating

Commission,

test,

valve

minimum

relatively

Included

to the startup

the 72 hour

performance output

constant

and

in this period were

test.

conditions

acceptance

test, During

the

95%

Period

of top brine

363 temperature, 106 to 107"C, and seawater makeup, 1,800 to 2,000 m3/hr.

The

average distillate flow rate and efficiency ratio exceeded design values for this period. The expression for the brine heater heat transfer coefficient appears below: @BH

"BH =

Where 'BH = Gc X (ils - ic) A

= 3,495 Ii?

Gc

= mass of condensate in kg/hr

ils = steam enthalpy i

= C

And

condensate enthalpy ATBH

LMTD =

ln=%+HATBH

= TBHO - TBHI in "C

TTD = TSHELL - TBHO in Oc Similar expressions were used to calculate the heat recovery and heat rejection section coefficients. The fouling factor was found by subtracting the reciprocal of the clean heat transfer coefficient from the reciprocal for the observed coefficient: fouling factor =

1

-

1=1

XX-5 Microcomputer data storage and processing of evaporator operating parameters provides for efficient utilization of manpower resources. A fouling rate is calculated by obtaining the slope of the curve of the fouling factor versus time. This value has utility jn predicting the length of run between cleanings.

However, the shell side condensl-ngtemperature

must be used in the calculations, Design data indicate that a constant 3°C temperature difference should exist between the units' condensate temperature and the steam shell side condensing temperature.

Because of the major

influence of the LMTD on the HTC calculation, slight errors in reading the

364 temperatures Also,

can result

accurate

calibration

which

few

ly

are used

are

degrees.

because

surfaces,

indirectly

only

slightly

Also

the

ER

by

Operating

results

spot measurements

INSPECTION

density

occurs

Chemical

of

the

on

values, readings

unit

critical

a constant

are adverseheat

dosing

has

been

production in fouling.

alkaline

excellent,

calculations.

transfer

steam input,

distillate

prevents

both of a

indirectly,

the increase

that EL-2438

the

of the unit.

and enthalpy temperature

to reflect

inspection

routine

contrast,

will decline with a constant

to maintain

and routine mass balance

of the physical

In

condition

and the steam consumption

to date indicate

evaporators.

damaged.

cleanliness

case, the ER will decline

in the

the highlights

the

HTC values.

requiring

or the use of long stem mercury

the operating

deposition

production

to obtain,

by erroneous

production

increase

in the calculated

easily

to obtain

reflects

If

the distillate

rate. In either

scale

based

on

We can focus now on

of Unit One (Ref. 6).

REPORT

Representatives

of

Mitsui

representatives

operators'

20, 1982.

tion.

and

affected

fouling.

or the steam usage will

buildup

equipment

expensive

both the distillate

affected

July

sensing

are

changes

are difficult

ratio uses flows to determine

Temperatures of which

values

of temperature

thermometers, efficiency

in significant

temperature

Deposit

The

and the chemical

following

sample

Engineering

discussion

and

vendors'

presents

analyses

established

in excellent

condition,

Shipbuilding, inspected

highlights

the alkaline,

the

units'

Unit No. 1 on of the inspec-

non-adherent

nature

of the minor deposits.

General

Condition

The unit was heat transfer evidence

surfaces.

of mechanical

with

No brine or distillate blockages

or severe

surfaces

were clean and free of corrosion

chambers

were considered

normal

no scale buildup

on critical

leaks were noted. There was no corrosion.

and stains.

Exterior Deposits

evaporator

in the flash

in amount and location.

The Brine Heater The brine heater white,

nonadherent

the waterbox with

minor

hydroxide,

inlet tubes and tubesheet

deposit

on the tube sheet.

and tubesheet amounts

silica,

of

was

and aluminum

18 are made of aluminum

composed

magnetic

brass,

iron

present, a possible

were clean with a very light, A deposit

of calcium oxide,

sample obtained

carbonate,

sodium

also. The tubes

from

as aragonite,

chloride, in Stages

source of the aluminum.

magnesium 3 through Silica can

365 enter the system through the seawater inlet.

It can deposit as water borne

silica or combine with metal ions, for example aluminum or magnesium, to precipitate as metal silicates. The slight deposit coating on the inlet tube sheet was easily removed by rubbing with the fingers.

No hard scale was

present in the inlet tubes. The brine heater outlet tubesheet and waterbox were clean, with the base metal clearly visible through a light, tan dust-like deposit.

A deposit

analysis of a composite sample evidenced calcium carbonate as aragonite. The tubes were free of adherent deposits. The Flash Chambers The first eight flash chambers had some scale on the walls that was easily chipped or wiped off by hand. The bottoms of the demisters were also coated with a light deposit which did not hinder vapor passage. They were soft and pliable with the nonadherent deposit easily removed by rubbing with the fingers. scale.

The remaining stages, 9 through 21, were clean and free of

Scale in the first flash chamber, which had the heaviest deposit

buildup in the unit, contained calcium carbonate and magnesium hydroxide. The presence of magnesium hydroxide at this location is a consequence of the increased hydroxide ion concentration produced in the recirculating brine when the sudden release in pressure occurs in the flash chamber. The subsequent release of carbon dioxide gas results in the formation of hydroxide ions, which

can

combine with magnesium ions to

precipitate magnesium

hydroxide:

co;+$0

+ C02(gas)t+ 20H-

Mg++ + 20H- + Mg(OH)2 The first stage demister pads had clearly visible base metal and a light dusting of deposit, which was composed of magnesium hydroxide and calcium carbonate, along with minor amounts of sodium chloride and silica. The deposit analysis indicated some mist entrainment occurred, but the quantity of deposit is indicative of good foam control in the flash chambers. Additional analyses of the minor deposits from the 2nd, 3rd and 8th flash chambers evidenced aragonite as the major constituent, with minor amounts of magnesium hydroxide also present.

366 The Heat Reject Section A section of the tubesheet, Zlst stage heat reject

section

water box and 70/30 cupronickel was inspected.

No scale or corrosion

apparent.

The Heat Recovery The

tubes

inlet

and

water

boxes

the

heat

analyses

outlet

tube

recovery

section

a

evidenced

light,

tan

by rubbing with the fingertips.

Deposit

Analyses

Elemental the

of deposits

Detailed

analysis

deposit's

particular

were

elemental enable

elemental

compound

taken

and

from

pattern.

Amorphous, identifiable

deposit

elemental

composition

and diffraction

can

by

In

of

Table

each

sample.

identified contained nent

One

appear

from Unit No. 1.

a major

following

phosphorus

amount

waterborne removed seawater

silt.

during

The

iron

electron

inlet and outlet Electron

principally

which

aragonite,

crystalweight

results

of

of the chemical

the

compo-

of the unit. results

was

stage

(l-4%)

the

on

the

The sodium

deposits

composition

major

compound

deposit, chloride

of brine

of silica,

which compo-

as the unit

iron oxide

and

can enter the system as

substrate

phosphorus

the

deposit

demister

The silica

from

The

incorporation

microscope

brine heater

analyses

combining

a

a recogniz-

can

oxidation

be

of the phosphonate

present scale

layer in

the

inhibitor

scale.

distorted. deposit

by

of

identify

A portable

hydroxide.

is

determination

or poorly

aragonite,

samples.

inspec-

distorted,

to the evaporation

oxide

The

performed.

studies

the probable

first

amounts

acquisition.

or through

into the precipitating Scanning

Trace

in several

sample

makeup,

as

is attributed

scale

easily

the

were

if it creates

the knowledge

the

during

analyses

diffraction

of magnesium

shutdown.

were found

X-ray

except

was

technique.

conditions

carbonate,

sample

of the deposits

cooled

this

Table Two contains

Calcium

in every

the

of

carbonate.

which

diffraction

generated

with

sition of the brine and the operating

removed

deposit

locations

chemically

be

analyses

free

from the first stage

calcium

in the deposit,

are

and

or quantitative

X-ray

lized percent

of

nine

semiquantitative composition.

not

removed

diffraction

able diffraction compounds

was

section.

No hard scale was present.

X-ray

or constituent

clean

principally coating

removed

tion.

were

of the minor deposits

sheets

contained

Samples

of biofouling

in the entire

Section

of

Deposit

buildup.

No evidence

damage was visible

tubes in the

studies

indicated

diffraction indicated

and the brine

the

deposit

samples

that the scale formed

X-ray that

of

analyses

support

the

brine

heater

heater

inlet

deposit

from

the

in the unit was the

outlet

analytical sample

contained

was

calcium

367 TABLE

COMPOUNDS

Sample Location

Magnesium Hydroxide

First Stage Demisters

MA

Calcium Carbonate

1

2

LMA

T

Brine Heater Inlet

MA

Brine Heater Outlet

MA

Stage 3 Flash Chamber

LMI

MA

Stage 1 Flash Chamber

MI

MA

First Stage Water Box Inlet

MA

Stage 8 Flash Chamber

MA

Stage 2 Flash Chamber

LMI

First Stage Water Box Outlet

IDENTIFIED

1

BY X-RAY DIFFRACTION

Sodium Chloride

Iron Oxide -1

Aluminum Silicate

Silica

-2

T

T

MI

T

T

T

T

LMI

T

T

MA

T

T

T

T

MA

LMI

T

T

T

LEGEND

MA LMA MI LMI T

= = = = =

Major Low Minor Minor Low Minor Trace

% 7!6?O-30 8-15 4-8 1-4

CaCO CaCOi

1 = aragonite 2 = calcite

Iron Oxide 1 = magnetite Iron Oxide 2 = hydrated ferric oxide

368 TABLE 2

PROBABLE

Sample Location

Magnesium Hydroxide

First Stage Demisters

65

Brine Heater Inlet

8-15

Brine Heater Outlet

cl.5

DEPOSIT

COMPOSITION

Calcium Carbonate 25

>30

90

Silica as SiO,

Iron Oxide

1

20-30

Sodium Chloride 4

4-8

-

1

-

1

Stage 3 Flash Chamber

4

90

Stage 1 Flash Chamber

15

75

2

4

-

cl.5

80

1

3

1

3

75

1

2

4

Stage 8 Flash Chamber

5

>30

Stage 2 Flash Chamber

6

75

1

2

3

First Stage Heat Recovery

Inlet

First Stage Heat Recovery

Outlet

Note: 1.

Trace amount (2-3%) of sulfate were present in the deposits from the 2nd and 3rd flash chambers, the 1st stage demisters, and the recovery tube section samples.

2.

Trace amounts (l-2%) of phosphorus were identified in the deposits from the demisters, flash chambers, brine heater outlet and recovery section.

369

carbonate, magnesium hydroxide, sodium chloride, silica, aluminum and iron oxide.

The relatively amorphous nature of both deposits indicated crystal

distortion by inhibitors. SUMMARY Performance data collected on the Royal Commission MSF Units established the effectiveness of EL-2438 antiscalant to prevent alkaline scale deposition on critical heat transfer surfaces of the evaporators.

Both the average

production and efficiency ratio values met design criteria during periods of relatively steady state operation of the units.

A physical inspection of

Unit One after approximately two month's operating time revealed only minor accumulation of deposits in the flash chambers, water boxes, and on the fluid transfer surfaces. Deposit analyses confirmed that the scale deposited in the unit was principally calcium carbonate, as aragonite, with lesser amounts of magnesium hydroxide in

the

flash chamber samples. No

calcium sulfate

hemihydrate or anhydrite was identified in the deposits. Accurate dosing of the antiscalant was maintained throughout extended operating periods.

Mechanical cleaning remains effective in keeping tube

surfaces free of adherent deposits. The efficiency ratio was found to be a reliable indicator of system performance. Proper selection of system component materials of construction has minimized corrosion problems with the evaporators. Detailed wet and dry evaporator standby procedures provide for safety and economy. Monitoring unit operations is redundant to insure positive control of the evaporator systems. For more than three years, the Royal Commission MSF units have been providing essential potable and industrial fresh water to the town of Al Jubail, even though routine and frequent up and down operation of both units is the normal situation.

REFERENCES : : 5 6 7

N. Nada, Desalination, 41 (1982) 57-69. D. S. Khumayyis, M. Ohtani, Desalination, 45 (1983) 155-165. K. Izumi and A. Yamada, Industrial Crystallization,84 (1984) 489-492. D. P. Logan and S. P. Rey, Scale Control in MSF Evaporators, Proceedings of Corrosion 85, March 25-29 (1985) Pa er 360. MES Contract Document, Volume II (1983P D. P. Logan, Technical Report No. IP-82zD"i4(!98a) W. R. Hollingshad, Technical Report No. IP-82-D-4, Calgon Corporation (1982).

370 APPENDIX

ONE (Ref. 7)

DESIGN TEMPERATURE,

FLOW AND HTC DATA

Seawater 35°C TEMPERATURES,

"C

Clean

BHI BHO

103

Steam in (oressure. at mG) Condensate' . Brine to Recovery Distillate from Stage 21 Distillate after Cooler

110 111(.47) 108 41.8 40.0 37.9

Fouled 101.3

110

116.1(.74) 113.1 44.9 43.6 40.0

Temperature 20°C Clean 97.1

Fouled 98.0

104.9

107.7

105.9(.23)

113.2(.50)

102.9

26.8 25.0 22.9

110.2 29.3

28.1 24.7

FLOWS, m3/hr Recirculation Brine Blowdown Makeup Distillate, Total Distillate to Boiler

ER, Overall, kg/1000 kcal ER, less dist. to boiler Velocities, m/set Brine Heater Recovery Section Reject Section

Concentration Ratio Brine Heater Blowdown

Distillate

Density,

ppm TDS

7061 1147 1949 814.2 6.2

7419 1145 1947 815.7 6.8

6285 1143 1945 810.2 6.2

6289 1141 1943 811.7 6.8

16.4 16.3

12.6 12.5

16.6 16.5

13.4 13.3

1.90 1.91 1.85

2.00 2.01 1.85

1.70 1.70 1.84

1.70 1.70 1.84

1.49 1.67

1.50 1.67

1.47 1.68

1.47 1.68

20

20

20

20

4790 4620 3630

2180 2900 2290

4510 3880 3220

2110 2670 2120

Design HTC's, w/M2'K Brine Heater Recovery Section Reject Section

371 APPENDIX

TWO (Ref. 6)

METHOD

FOR CALCULATION

EFFICIENCiFRATIO

(E.R.)

GIVEN: E.R.

=

GP2

X 1000 in kg/1000

kcal

9, Where, - GM in kg/hr

GP2

=

GPl

Q,

=

GC X (i,s - ic) in kcal

And, flow, kg/hr at Tpl

=

GM

=

Total boiler makeup

Gc

=

Total condensate

i,s

=

Enthalpy

of steam, kcal/kg @ Tls,

i

=

Enthalpy

of condensate,

kcal/kg

appears

on the following

page.

C

An example

Total distillate

GPl

calculation

flow, kg/hr at TM

flow, kg/hr at Tc

@

pls TC

312 EXAMPLE Sample Calculation

of Efficiency

Ratio

(ERl

1

All flows are taken from integrators

2

All volume flows are converted to mass flows by multiplying by the appropriate density (p) of water at the applicable temperature.

3

The portion of distillate used for boiler makeup, Gm, is subtracted total distillate flow, Gpl, to give net product, Gp2.

4

Enthalpy

GIVEN:

values

(Typical conditions

Total distillate Makeup

in kcal/Kg

to boiler, m3/hr

Total condensate

are obtained

for process

flow, m3/hr

to m3/hr.

- 12.26 @ 28.8"C;

from stream tables.

= 0.996 g/cm3

= 0.996 g/cm3

flow, m3/hr = 122.6 P 116°C; = 0.946 g/cm3

of superheated

low pressure

Enthalpy

of condensate:

i

to flowing

C

=

steam:

ils = 648 kcal/Kg 9 0.45 Kg/cm2

116.3 kcal/Kg

brine:

ils - ic = 531.7 kcal/Kg

THEN: GPl

= 813.9 m3/hr x 996 Kg/m3 = 810,644

Gm = 12.26 m3/hr

- 12,211 = 798,433

Gc = 122.6 m3/hr x 946 Kg/m3 = 115,979

ER =

GP2 Q,

=

Kg/hr

x 996 Kg/m3 = 12,211 Kg/hr

GPP = GPl - Gm = 810,644

Q, = Gc X (ils -

from

flow diagram)

= 813.9 @ 28.8"C;

Enthalpy

Net enthalpy

and converted

Kg/hr

Kg/hr

ic) = 115,979 Kg/hr x 531.7 kcal/Kg = 61,666,034 kcal/hr (or 61,666.0 1000 kcal/hr)

798,433 61,666.0

Kg/hr

1000 kcal/hr

= 12.95 Kg/1000

kcal

373

APPENDIX THREE

Unit 1 Period 1

Data Point

Temp, "C 8. Htr. Outlet

Dissillate m /hr

E.R. kg/1000 kcal

1

110

823

12.6

2

109

820

12.8

3

108

824

13.1

4

108

823

12.9

5

107

857

12.9

6

101

610

12.3

7

103

710

12.6

8

108

750

13.0

9

107

800

12.4

10

107.5

800

12.0

11

106

800

13.0

12

107

810

12.1

13

107

800

12.2

14

106

800

12.2

15

105

750

12.2

16

106

800

12.3

374 Unit 1 Period 2

Data Point

8 9 10 11 :: 14 17 :: 20 21 22 23

Temp, "C B. Htr. Outlet 107.5' 107 107 107 107 107 106 107 107 107 107 107 107 106.5 107 106 106 106 106 106 107 106.5 106.5 106 107 107 107 107 107

Dissillate m /hr 850 820 815 820 820 820 815 820 825 820 815 810 810 812 815 810 800 800 790 790 800 802 805 815 825 830 818 805 810

E.R. kg/1000 kcal 13.0 13.1 12.6 12.4 12.5 12.5 12.2 12.2 12.4 12.3 12.3 12.4 12.2 12.4 12.4 13.1 12.9 12.9 12.9 12.9 12.9 12.8 13.0 13.0 13.2 13.0 13.1 13.3 13.4

375

!oooT

600-r

iii

) I

90

I

I

2

I

I

I

3

4

I 5

I

I

6

7

8

I 9

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I

)

,

IO

II

I2

I3

I4

I5

16

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I

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1

3

4

5

6

7

B

9

IO

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2

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1

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Ii

Time Fig.1.

Operating

data

for

Unit

1,

Period

1.

I

I

I2

I I

I

I3

I4

I5

1

I

16

376

I

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6

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10

12

I I 14

I I 16

I 16

I I 20

I I 22

Time Fig.2.

Operating

data

for

Unit

1, Period

2.

I I 24

26