ON LINE CHEMICAL CLEANING OF CRITICAL HEAT

EXCHANGERS FOR COOLING WATER DEPOSIT AND THEIR

CONTROL TO SUSTAIN HIGH PRODUCTION LEVEL OF
METHANOL. - AN EXPERIENCE AND CASE STUDY AT GNFC

Dr. R. K. Sharma, Mr. L. It. Patel & Mr. M. M. Bhatt.

GNFC, Bharuch, Gujrat

ABSTRACT
Deposit control is the most critical aspect of cooling water treatment because it is
directly related to very purpose for a cooling system heat removal. Any deposit present in
heat transfer surfaces impairs heat removal and there for reduces the efficiency of entire
system.
Cooling water deposits are of two types - scales and fouling. Scales are hard dense,
crystalline deposits formed by the precipitation of dissolved materials when their solubility
have been exceeded due to change in conditions. Foulants form softer non crystalline
deposits because suspended materials settle out or adhere to metal surfaces.

INTRODUCTION
Deposit control is the most critical aspect of cooling water treatment because it is
directly related to very purpose for a cooling system heat removal. Any deposit present in
heat transfer surfaces impairs heat removal and therefor reduces the efficiency of entire
system. Unless deposition is properly controlled, it can negate the effectiveness of an
expensive and well designed cooling water treatment and its system.
This direct effect on cooling efficiency has always required deposits control be an
integrated part of cooling water treatment programs. The potential for deposition is
increased by the trend to conserve water and the need to use more environmentally
acceptable material for controlling corrosion and deposition. To conserve water higher
cycles of concentration are being used, this increased the levels of scale forming or fouling
material in the water. The need to use Non-toxic corrosion inhibitors is also increasing the
potential for deposition. Since many of these less toxic corrosion inhibitors are more effective
at high pH conditions, scaling potential is increased, while the effectiveness of many micro
biocides is decreased. Meeting the need of these conflicting objectives, require a good,
understanding of the effects and cause of deposition and their control by adopting new
methodology like on line chemical cleaning and its monitoring, to sustain high
production level of the plant with out its shut down. This paper deals with this
subject.

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EFFECT OF DEPOSITION
Deposits affect cooling system operation both directly/indirectly. Heat transfer can
be reduced directly due to the insulating effect of a deposit as shown in Fig-1. Deposits
have lower thermal conductivities than the metal wall of tube and there after act much
like insulation to impair rapid heat transfer. Deposition can also reduce cooling because it
restricts water flow, across heat exchangers and increase AP. Reduced heat transfer
increases production loses due to reduced efficiency.
Deposits besides reducing cooling efficiency, can initiate leaks in the system by
promoting localized corrosion. The phenomenon is described as under deposit corrosion.

COOLING WATER DEPOSITES AND THEIR CONTROL

The sources of potential deposits are as diverse as the effect of these deposits.
Potential deposits can enter the cooling system through the make up water, from the air,
through process contamination, as well as being formed within the system through
corrosion. Make up water may contain both dissolved as well as suspended materials,
which can forms deposits. The scrubbing action of a cooling tower can introduce considerable
quantities of dissolved gases, dust, dirt and other particulates matter carried by the air.
Process contaminants are as diverse as the number of products manufactured and may
enter through leaks and plant upsets etc.

COOLING TOWER DEPOSITS ARE TWO. TYPES: SCALES AND FOULING.

A. Scales : Scales are hard, dense, crystalline deposits formed by the precipitation of
dissolved materials when their solubilities are exceeded due to condition changes.
Calcium Carbonate is the most common scale in cooling water, whose solubility is
very low about 13 ppm. Other common scales of cooling water are Calcium Sulfate, Calcium
Phosphate, and Calcium Fluorides etc. Scale occurrence in cooling water system is observed
as shown below in table 1.

Calcium Phosphate
Calcium Carbonate
Silica
Calcium Sulfate
Calcium Fluoride
Table 1

1. Calcium Phosphate scale : Calcium Phosphate has become leading form of scale,
due to phosphate based corrosion inhibitor. When orthophosphate is present in the
water, calcium phosphate scale is formed, inhibit heat transfer and lead to under
deposit corrosion. The formation of Ca3(PO4)2 scale is governed by the pH, Calcium
and Phosphate concentration and temperature of recirculating water.
The application of PBTC + HEDP polymer have reduced calcium phosphate deposits.
They have performed well with Ca" levels up to 450 ppm as CaCO3 with total phosphate
10-18.ppm as P043 The copolymers of poly melates and poly acrylates have exhibited
good calcium phosphate scale control.

u^- 9
2. Calcium Carbonate: Calcium carbonate is the most common scale. Fig. 2 shows the
relation ship of temperature to the solubility of calcium carbonate. The calcium
carbonate deposition is influenced by pH, Calcium and alkalinity level in cooling water.
Several new phosphonates are providing exceptional control to calcium carbonate scale,
because most of these polymers act as crystal modifier. Outstanding among these is
one known as Phosphono- butane-tricarboxylic acid (PBTC). The use of 50:50 PBTC:
HEDP, with 3-5 ppm level is found effective under severe scaling conditions. The blend
has enabled our system to tolerate the calcium hardness up to 450 - 500 ppm, with
total hardness up to 900 - 1000 ppm level, in circulating cooling water in different
plants at GNFC.
3. Calcium sulfate: - Specially sulfate ions replaces alkalinity when sulfuric acid is fed
to cooling water system to control system pH, but sulfate scale poses limited solubility,
exhibit serious problem to cooling water system operation, since its solubility
decreases as temperature increases *(Fig. 2). Sulfate scale control has been achieved
by charging the polymer like poly aoylate /poly acrylamide at 2-3 ppm level in cooling
water containing 500 ppm calcium as calcium carbonate.
4. Silicate scales : - High operating cycle of concentration based on silica and the rising
use of reclaimed water are accountable for high silica levels in circulating cooling
water. The formation of silicates can be prevented by limiting silica levels in circulating
water max. up to 150 ppm as SiO , although precise limit is dependent to make up
water SiO with respect to the oper7ation of cooling tower on high cycle of concentration
based on Ailica.
To prevent magnesium silicate deposition, magnesium and silica concentrations are
controlled, so that the following solubility product is not exceeded.
(Mg, ppm as CaCOs) x (SiO , ppm as SiO ) is < 40,000
2 2
New treatment (ter polymers) has been developed to increase silica solubilities
attainable in cooling water system.

B. FOULING : - Foullants form softer non-crystalline deposits because of suspended

materials settle out or adhere to metal surfaces. Silica besides being scale is also one of the
common foulants. Fouling with materials high in silica is common because suspended
solids, mud, slit and dirt are common in most surface water. The extent of fouling from
sediments in circulating water is a result of the time afforded the particle to settle. Since
cooling water basins, water boxes and shell side flow area involve appreciable holding
times as a result of their size and the low velocities through them, the settling of sediments
remains extensive, in above areas.
Polymers of high molecular weight (300,000 to 15 million) at doses level of 0.5 to 1.0
mg/lit. are commonly used. They attract fine foulant particles on to polymeric chain, forming
large puffy particles that are easily removed from heat exchangers surfaces. Non-toxic low
foaming wetting agents such as poly oxyalkylane have provided improved control on
gelatinous and oil base foulants.

Functional mechanism of organophosphonate / polymer in

reduction of scale & fouling in cooling water
Organophosphonates are one of the best deposit control agents . The Threshold and
crystal distortion property of these compounds interferes with nucleation of hardness of
crystals causing much higher levels of hardness to stay in solution. When scales are formed
they are distorted that they are non-adherent and form very soft sludge.

90 01

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f NWI ^^" " "1 I I '"r I II II rl T- 1 7 -1
 Phosphonates also provides excellent control of hydrated ferric oxides deposits, which
are formed as a result of corrosion. They adsorb on the particle surfaces and reduce the
attraction forces between individual iron particles. The sequestering ability of phosphonate
enables it to control heavy metals ( Fe,Cu , Zn) deposits . Phosphonates also help to disperse
suspended particles.
Low molecular weight polycarboxylic polymers also exhibit threshold and crystal
distortion property. When used in conjunction with organophosphonate, poly phosphate
and zinc, they aid in corrosion inhibition / deflocculation, dispersion and crystal distortion,
leading to soft scale.

Leading scale control agent and methods are shown in Table 2.

CONTROL ACTION
Agent Solubilization Dispersion Crystal modification

Phosphonate
HEDP/AMP Primary - -
PBTC Primary Moderate -
Polvacrvlate Copolymers
Acrylate / maleate Slight Moderate Primary
Acylate/acrylamide Primary Moderate Slight
Ter Polymer
Maleate/Acrylate/sulfonate Slight Slight Primary
Acrylate/Acrylamide/sulfonate Slight Moderate Primary

Table 2

ONLINE CHEMICAL CLEANING AND ITS GAIN : -

On line chemical cleaning of E-405 (Reactor gas condenser) and E 509 (Refining
column condenser) was done first time in GNFC on 15th Sept. 2000 and subsequently done
on 30th Nov. 2000 and 191 Feb. 2001. Sulfamic acid was used with anticorrosive material
and pH of the recirculating water was brought down to 2.8 - 4.0 for about 2 hrs.
The acid cleaning was followed by flushing the heat exchangers with low molecular
weight polymer along with biodispersent to remove any available loose scale / fouling,
either in tube side or shell side.
On completion of chemical cleaning the cooling water equipments & piping were
passivated with double dose of treatment chemicals charged.
During chemical cleaning the Turbidity of E 405 out let shot up to 77 NTU and E
509 out let Shot up to 61 NTU. The E405 & E509 out let pH and Turbidity is shown in Fig.
3 & Fig. 4. The turbidity observed high due to removal of scale of Mg, Zn, Fe phosphate etc.
The total hardness of circulating water has gone up from 360 ppm to 500 ppm, total solids
from 916 ppm to 1620 ppm, Fe from 0.44 ppm to 1.28 ppm. The water analysis before,
during chemical cleaning and after passivation is shown in Table-3. The relationship of

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Zn, Mg & P04 is shown in Fig 5 & 6 for E405 & E509 heat exchangers, which were
responsible for poor heat transfer and limiting the plant, load, resulting in low productivity
of methanol. On their successful cleaning plant production has gone up. The condenser
and refining column data before and after chemicals cleaning are placed in Table-4. These
data shows the effectiveness of on line chemical cleaning of deposits of scale on shell sides
of E405 and tube sides of E509.

ADVANTAGES OF ON LINE CHEMICAL CLEANING

1. Plant shut down has been averted.
2. Production of methanol has gone up by 5 to 6 MT / day, as shown in Fig. 7.
3. The reactor gas condenser (E 405) cooling water out let temperature has gone down
from 54 to 49°C i.e. 5°C, which enhance gas processing capacity of exchanger.
4. E 509 refining column condenser pressure dropped from 0.94 bars to 0.41 bars on
30th November cleaning and was hold there. The %valve opening of cooling water
reduced from 100 to 26 %, hence the refining column efficiency has gone up to process
more crude methanol.
5. The return header cooling water temperature increased by 1°C, which indicates
high heat recovery with circulating cooling water rate of 3500M3/Hr.
6. Commercial gain, due to in-house technical development & its execution.
The details of chemical cleaning, with removal of different type of scales and foulants,
with variation in cooling water pH and application of low molecular weight polymer with
biodispersent to ensure the control of scale and biofouling with planned dose of corrosion
& scale inhibitor to overcome the corrosion & scale formation and to sustain high production
of methanol, without plant shut down, is over all theme of this paper.

ACKNOWLEDGEMENT
The authors are thankful to GNFC management for allowing to carryout on line
cleaning of critical heat exchangers like E405 / E509 and MeOH II plant operation group
members and M/s. Chembond chemicals personnel for their valuable support in making
on line chemical cleaning successful.

REFERENCES
1. Sharma R.K., Patel L.R. & Bhatt M.M.:"Monitoring & Evaluation of cooling water of NP
Complex plant, based on improved inhouse treatment"- Indian Fertilizer Scene Annual 2000-2001.
2. "The role of organophosphates in cooling water treatment."D.J.Banerjee; vol. 4, No.3,1981,
corrosion & maintance.

3. "Principles of Industrial water treatment." Drew chemical corporation.

4. "Scale & Deposits control in cooling water systems."Jeffery R. Townsend, 1978,

Drew Chemical Corporation.
Analysis report of MeOH II Cooling Water during on line chemical cleaning of
the E405 / E509 exchangers by sulfamic acid ( HSO3Nh2 ) on 01/02/2001.

No Test Unit Result

Before E-405 01H0112/01) E-509 Basin

0112101 15: 45 16:00 16: 35 16:45 16:45 11:30

1 P' 7.0 2. 9 2.7 3. 6 2.8 3.1 7.1

2 Conductivity p. mhos/cm 1118 1519 1642 1467 1660 1380 1164

3 Turbidity NTU 22 61 61 66 77 61 43

4 Total alkalinity as CaCOs PPM 46 - - - 56

5 Total Hardness as CaCOs PPM 360 500 490 500 500 490 428

6 Calcium Hardness as PPM 234 260 250 250 260 250 228
7 Magnesium Hardness as PPM 126 240 240 250 240 240 200
CaCOs
8 Iron as Fe PPM 0. 44 0.91 1. 17 1.01 1.28 1. 01 0.63

9 Chlorides as Cl PPM 188 200 200 200 200 200 154

10 Free Chlorine PPM 0.5 nil nil nil nil nil 0.5

11 Silica as SiO2 PPM 56 53 55 56 56 57 57

12 Ortho Phosphate as P04 PPM 2.7 6.1 7.0 8.3 7.5 7.1 11.8

13 Poly Phosphate as PC>4 PPM 1.9 3.8 3. 5 3.9 3.6 3.8 5.7

14 Organo Phosphate as P04 PPM 3.0 3.8 4.2 7.3 7.1 5.5 8.2

15 Zinc as Zn PPM 0. 8 2.14 2. 00 2.43 2. 29 2.20 2.32

16 Anunonia as NHa PPM nt nt 1.0 2.0 2.0 8.1

17 Sulfate as S04 PPM - 250 225 200 175 188 275

18 Total Solids PPM 916 1424 1498 1492 1620 1296 1202

19 Total dissolved solids PPM - 1360 1434 1420 1540 1232 1158

20 Total suspended solids PPM - 64 64 72 80 64 44

Table 3
 DATE E 405 E-509
UL temp. 0/L temp. Press % Valve open
27-01 01 103 52 0.41 52
28-01 01 103 52 0.41 50
103 53 0.41 62
N
29-01 01
30 01-01 103 53 0.51 100
31-01-01 102 53 0.51 94
01-02-01 103 54 0.46 100
02 02-01 102 51 0.41 29
03-02-01 102 51 0.41 30
04-02-01 102 49 0.41 19
05 02-01 101 49 0.41 25
06-02 01 102 48 0.41 24
07-02-01 102 48 0.40 27
08-02 01 102 48 0.42 24
09-02-01 104 48 0.41 25
10-02-01 102 48 0.41 26
11-02-01 103 51 0.41 43
12-02-01 102 49 0.41 31
13-02-01 101 48 0.41 26
14-02-01 104 50 0.41 25
15-02-01 102 50 0.41 26
16-02-01 102 49 0.42 26
17-02-01 102 49 0.42 24
18-02-01 103 51 0.41 39
19-02-01 104 54 0.42 47
20-02-01 104 52 0.42 50
21-02-01 103 52 0.41 41
22-02 01 102 49 0.40 27
23-02-01 100 49 0.42 22
24-02 01 102 50 0.42 59
25-02-01 102 51 0.42 59
26-02 01 102 51 0.42 57
27-02-01 101 51 0.42 53
28-02-01 100 47 0.42 26

Table 4

9^^ i^^r i^ 2001

^^^111i NNW ^ ^^^IIN^^ ^ ^R{pVIpM iP i WHIM p N

7T -7 1^^
 INSULATION EFFECT OF A DEPOSIT

Fig. 1

2 400
---- - -- - - - - -
U 7000
\ ,

O 1600

3
5 1200

^ 800

^ •O
A 400

sn Citta'a l e CACO, - -
O
32 GO 104 14 0 116 212 246 764 320 J5& 392
50 Be 122 158 114 730 208 302 3319 314

Fig. 2
Comparison of solubilities of calcium sulfate and calcium carbonate solubility in
equilibrium with normal carbon dioxide content of the system

95
 E 405 OIL pH & turbidity
8.0 90
7.0 80
6.0 70
60 z
5.0
50
a 4.0
40
3.0 30
2.0 20 H
1.0 10
0.0 0
15:35 15:45 15:50 16:00 16:10 16:30 16:50 17:00 18:00 19:30 20:30 20:40

Time -a-pH -^-Turb

Fig. 3

E-509 OIL pH &Turbidity

8.0
7.0
6.0
5.0
CL 4.0
3,0
2.0
1.0
0.0
15:35 15:45 15: 50 16:00 16 :10 16: 30 16 :50 17: 00 18 : 00 19: 30 20: 30 20:40
Time -U-pH -t-Turb

Fig. 4

E-405 OIL Mg-HZn,P04

300.0 r 25.00
250.0 20.00 -
CL 200.0 E
a 15.00 a
150.0
x
131 100.0
10.009
C
50.0 - 5.00 N

0.0 1 4 0.00
15:35 15:45 16:00 16:35 17:00 18:00 20:30
Time f Mg-H --*--Zinc -*-T-P04

Fig. 5

96
 E-509 OIL Mg-H,Zn,P04
300.0 25.00
250.0 20.00
CE 200.0
15.00 a
150.0
10.00 0
100.0
50.0 5.00 N

0.0 0.00
15:35 15:45 16:00 16:35 17:00 18:00 20:30
Time
-*--Mg-H -Zinc -A--T-PO4

Fig. 6

N 420
g 400
z 380
0
360
34 0
0 320
w
n- 300 27- 31- 02- 04- 08- 10- 12- 14
29- 06- 16- 18- 20- 22- 24- 26- 28-
Jan Jan Jan Fab Fab Feb Feb Feb Feb Feb Fab Fab Fab Feb Feb Feb Feb

DATE

Fig. 7

NSCP - 2001 97