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Conference Papers | 2005 Victorian Conference Papers
ON-SITE
GENERATION OF SODIUM HYPOCHLORITE - BASIC OPERATING
PRINCIPLES AND DESIGN CONSIDERATIONS
John
Hooper, Managing
Director,
Fluidquip Australia Pty. Ltd.
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ABSTRACT
This paper discusses the technical advances that have
been made in the equipment used for generating 0.8%
Sodium Hypochlorite on-site ( at the point of use )
and the benefits & limitations associated with this
technology.
1.0
INTRODUCTION
On-Site
Generation of Sodium Hypochlorite ( on-site at the Water
Treatment Plant ) has been in commercial use for over
30 years. Power Stations are significant users of on-site
generation systems, however the technology has not been
as widely utilised in Water Treatment Plants due to
the operating costs compared to that of Bulk Chlorine
products.
However, recent advances in Electrolyser Cell Technology
(namely, Applied Thermodynamic Management of the Cell)
have reduced capital, operating and maintenance costs
and made this technology much more viable. In addition,
Occupational Health and Safety issues have caused Water
Utilities and Consultants to specify the technology
in place of Chlorine Gas and Commercial Grade Sodium
Hypochlorite. There are now a large number of WTP's
and commercial swimming pools worldwide utilizing on-site
Sodium Hypochlorite generation systems.
2.0
DISINFECTION OPTIONS
Disinfection Technology options include:
1. Chlorine Gas
2. Commercial Grade 12.5% Sodium Hypochlorite
3. Calcium Hypochlorite Tablets ( 65% available Chlorine
)
4. Ultraviolet
5. On-Site Generation of Sodium Hypochlorite ( <1% available
Chlorine )
3.0 BASIC OPERATING THEORY
Generating
Sodium Hypochlorite onsite is a simple and straightforward
process that uses three common consumables: salt, water
and electricity. On-site generation systems operate
by feeding softened water into a brine dissolver. The
salt dissolves to form a brine solution, which is further
diluted to the desired salt solution.
The salt solution is then passed through the electrolytic
cell(s), which apply a low voltage DC current to the
brine to produce Sodium Hypochlorite.
The Sodium Hypochlorite is then safely stored in a day
tank and when it reaches the low-level set point, the
system automatically restarts to replenish its supply.
Morton Solar Salt is used in the process. The system
is fully automated and has a manual operation feature
if needed.
4.0
KEY COMPONENTS OF AN ELECTROLYTIC CHLORINATION SYSTEM
a)
System Control Panel
Mimic Panel of all components
Electrolyser Cell status
System Start / Stop & Reset
Brine Pump Status
Brine Flow Rate
Brine Tank usage to date
Water Softener status
Product Tank level
Metering Pump status
Chlorine Residual
Process Alarms
b)
Power Supply / Rectifier
Converts 415 VAC to low voltage DC to power Electrolyser
Cell(s)
c) Electrolyser Cell
UV stabilised PVC body containing Titanium Anode and
Nichol Alloy Cathode
d) Water Softener
Ion Exchange type Water Softener that removes Scaling
Calcium and Magnesium Salts with non-scaling Sodium
Salts preventing fouling of the cells. Provides water
supply to Brine Tank
e) Brine Tank ( Salt Saturator
)
Contains Raw Salt and 'soft' feed water. This Saturated
Brine Solution is the feed to the Electrolyser Cells
f)
Brine Pump
Pumps the Saturated Brine Solution to the Electrolyser
Cells
g)
Sodium Hypochlorite Storage Tank
Bulk storage of 0.8% Sodium Hypochlorite. Includes Hydrogen
Vent and Ultrasonic Level Switch
5.0
TYPICAL SYSTEM CONFIGURATION
6.0 THE PROCESS
NaCl + H20 + 2e -------------------- NaOCl + H2
Salt + Water + Energy -------------------- Hypo + Hydrogen
7.0
GENERATION COSTS ( RAW MATERIALS ONLY )
| SALT
= |
0.62
KG @ $0.34 per kg in 25 kg bags |
=
$0.21 |
| ENERGY
= |
0.9
KWH @ $0.12 / KWH |
=
$0.11 |
| WATER
= |
26
LITRES @ $0.40 per kl |
=
$0.03 |
| |
1.0
KG CHLORINE |
=
$0.35 |
8.0 SALT AND WATER QUALITY
SALT - Chemical Analysis for Salt required for Electrolytic
Cells
(Note: The above chemical analysis
should be taken as a guide only)
SALT - Sieving Analysis
PACKAGING:
Laminated woven polypropylene bags 15 kg, 25 kg and
50 kg net when packed. Also available in 1 tonne polypropylene
bulk bags.
STORAGE:
Store in a dry area out of direct sunlight.
WATER
Standard Potable Grade preferably with low Total Hardness.
Water Softener option available with Generation System.
9.0 CHARACTERISTICS OF 0.8%
SODIUM HYPOCHLORITE
Very
Stable due to Low Concentration
Reduced risk of decomposition to form Chlorates due
to lower concentration
Minor
Total Dissolved Solids ( TDS ) Impact ( 3 PPM for every
1 PPM Dosed - equal to 12.5% Sodium Hypochlorite )
pH
of 9.0 versus 13.0 for 12.5% NaOCl
More
effective Oxidant than commercial Hypochlorite due to
Oxidation Reduction Potential ( ORP ).
Not
Classed as Dangerous Goods, ( < 1% ) therefore storage
bunding not required.
10.0
MSDS for 0.8% Sodium Hypochlorite ( extract only )
PRODUCT
IDENTIFICATION:
PRODUCT
NAME: SODIUM HYPOCHLORITE, 0.8% SOLUTION
FORMULA: NAOCL
FORMULA WT: 74.44
CAS No: 7681-52-9
NIOSH/RTECS: NH3486300
COMMON SYNONYMS: HYPOCHLOROUS ACID, SODIUM SALT; CLOROX
PRODUCT CODE: 9416
EFFECTIVE: 22/5/86 REVISION #0
PRECAUTIONARY
LABELLING
BAKER SAF-T-DATA (TM)SYSTEM
HEALTH -1 SLIGHT
FLAMMABILITY -0 NONE
REACTIVITY -1 SLIGHT
CONTACT -1 SLIGHT
HAZARD RATINGS ARE: 0 to 4 (0 = NO HAZARD; 4 = EXTREME
HAZARD)
LABORATORY
PROTECTIVE EQUIPMENT:
SAFETY
GLASSES; LAB COAT
PRECAUTIONARY LABEL STATEMENTS:
WARNING CAUSES BURNS, HARMFUL IF SWALLOWED
AVOID CONTACT WITH EYES, SKIN CLOTHING
AVOID BREATHING VAPOUR, KEEP IN TIGHTLY CLOSED CONTAINER
USE WITH ADEQUATE VENTILATION
WASH THOROUGHLY AFTER HANDLING.
SAF-T-DATA (TM)STORAGE COLOUR CODE: ORANGE (GENERAL
STORE)
11.0
BYPRODUCTS OF SODIUM HYPOCHLORITE GENERATION PROCESS
-
Hydrogen Gas - Vented to Atmosphere passively or by
Air Dilution
- 1.6
g of Hydrogen produced per every kg of Chlorine Equivalent
-
Concentration below detectable flame limit
- Brine
waste from water softener regeneration
- Chlorates
- With Cell Temperature Management, Chlorates will
be 3% - 4 % of Dose
- Bromates
- Salt contains Bromide. Apx. 50% of Bromide Ion content
will be converted to Bromate
12.0
FEATURES OF 0.8% SODIUM HYPOCHLORITE v CHLORINE GAS
-
Greatly reduced risk to plant personnel
- No
threat to public safety
- Low
risk management issues
- No
requirement to purchase, handle or transport hazardous
chemicals
13.0
FEATURES OF 0.8% SODIUM HYPOCHLORITE v 12.5% COMMERCIAL
SODIUM HYPOCHLORITE
- Safety
- non hazardous material
- Lower
Disinfection Byproducts
- Consistent
Solution Strength
- No
Gassing-off
- pH
9 v 13
- No
requirement to purchase, handle or transport hazardous
chemicals.
14.0
FEATURES OF 0.8% SODIUM HYPOCHLORITE v CHLORINE TABLETS
-
Safety - non hazardous material
- Better
dose rate control
-
Lower cost
- pH
9 v 13
- No
requirement to purchase, handle or transport hazardous
chemicals
- No
Calcium deposits
15.0 INDUSTRY DRIVERS FOR THE
USE OF ON-SITE SODIUM HYPOCHLORITE GENERATION
-
Occupational Health & Safety ( OH&S )
- Risk
Management associated with proximity to the public
- Transportation
of Chemicals
- Handling
of Chemicals
16.0
MAINTENANCE & MATERIAL COSTS - CASE STUDY - DAYTONA
BEACH WTP - FLORIDA, USA.
After
Four (4) years of operation, the System has proven reliable
while producing 638,220 kg of 0.8% Sodium Hypochlorite
at an average cost of $0.39 per kg. Labour and maintenance
for the on-site system has been fairly straightforward,
including mostly manufacturer-recommended actions.
Operation & Maintenance of the system consisted of the
following:
-
De-Scaling (Acid Washing) of the Electrolytic Cell(s)
from Calcium deposits ( once per year )
- Cleaning
the salt tanks ( once every four years )
- Cleaning/Changing
Water & Brine filters ( one per month )
- Hypochlorite
Storage & Brine Tank inspection ( once per year )
- Electrode
replacement every 5 - 10 years depending on use and
Salt & Water Quality
17.0
CONCLUSION
Generating Sodium Hypochlorite on-site is a viable,
cost effective and safe alternative to 12.5% Commercial
Grade Sodium Hypochlorite and Chorine Tablets. It is
more expensive per kg than Chlorine Gas, however if
safety issues are taken into account, it can still be
a viable alternative.
18.0
ACKNOWLEDGEMENTS
Severn Trent Services - ClorTec Division DOWNLOAD
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