Chloroalkali technology is one of the largest electrochemical industries in the world. Main products being Caustic, Chlorine and Hydrogen is generated by the electrolysis of Sodium Chloride, which is energy intensive. Chloroalkali industries is the 2 nd largest consumer of electricity next to aluminum. Chloroalkali plants are notable for wide variety of hazards inherent in their operation.
To hazards – chlorine is a notoriously toxic gas.
Explosive hazards – Hydrogen forms explosive mixtures with Air or Chlorine.
Corrosive hazards – These results from stray electric currents as well as wet chlorine hypo chlorite and strong acids.
Electrical hazards – High direct current are used in the electrolytic process
Totally safe chloro-alkali plant does not exist. It is reasonable, however, to aim for design which will be no more hazardous than the general run of human activity. Experience show that considerable progress has been made towards this goal. Yet the accidents involving chlorine still feature in the world.Major accidents are taking place during transportation & at customer’s end.
Chlorine is extremely irritating to the mucous membranes, the eyes and the respiratory tract. In extreme cases lung tissues may be attacked resulting in pulmonary edema. Splashes of liquid chlorine on the eyes, skin and clothing may cause immediate irritation and chemical burns as well as severs damages to body tissues. In the presence of moisture it is highly corrosive and attacks common metals.
Human exposure to chlorine in the past has resulted in large no of deaths & injury. As per study mechanical failure contributes 32% share in accident. Major accidents are due to lack of knowledge & over confidence.
About 55 million tons Chlorine is generated in the world. 2.5 million tons Cl2 is produced in India through 42 caustic-chlorine plants based on different technologies.
M/s. Gujarat Alkalies & Chemicals Ltd. meeting the 16% demand of India.
Chlorine manufactured by caustic-chlorine industry is liquefied by cooling filtration, compression, refrigeration.
Liquid chlorine is stored in storage tanks and filled in Tonners (900 kg), cylinders (100 kg) and dispatched to various customers.
In developing countries liquid Cl2 is transported in tankers and railway containers of 55-90 MT capacity.
In India this mode of transportation yet to pick up.
During operation following type of Chlorine is being experienced
1) Wet Chlorine – moisture more than 150ppm
2) Dry Chlorine – moisture less than 150 ppm
3) Liquid Chlorine – chlorine liquefied by way Temp-Pressure control
4) Aqueous Chlorine – moisture more than 750 ppm
Aqueous chlorine has more serious effect due to higher inhaling capacity due to its characteristics.
All type of chlorine has different properties from each other.
Chlorine Manufacture process and the bullet
1 st stage cooling filtration liquification 2 nd stage cooling compressor Drying Storage CL2 from cell 90 0 Consumption 40 o H2SO4 3.0 Kgcm2 0 0 C 15 o C 40 o C
Chlorine is very reactive gas, hardly found in free state.
0.15% is available in combined state only.
Threshold value are as under
Odor threshold – 0.31 ppm
PEL (OSHA) – 1 ppm
TLV-STEL (ACGIH) – 3 ppm
IDLH (NIOSH,MSHA) – 10 ppm
Chlorine was used during world war 1 as a choking (pulmonary) agent. Recently in 2007 about 30 tons chlorine was used in Iraq war.
Extent of poisoning depends on amount of chlorine, how exposure take place and length of time.
burning sensation in hose, throat, eyes
Nausea & vomiting
Burning pain, redness, blisters on skin, frost bite in case of liquid chlorine.
Difficulty in breathing
Fluid in the lungs with 2-4 hrs.
HAZARDS ASSOCIATED WITH CHLORINE
Long terms effects are not found in people who survive sudden exposure.
High potential for terrorist use, as readily available.
Natural element extremely oxidizing.
Combine with water forms hydrochlorous and hydrochloric acid and oxygen free radicals. All cause major tissue damages.
Possibility of public exposure during transportation and water treatment use.
Corrosion due to leakage.
Contamination with other liquid chlorine in storage can cause explosion.
Formation of NCl3 in liquid chlorine in storage can cause explosion.
Chlorine toner valve leakage.
Exposure of chlorine toner to high temperature or fire.
Immediate reaction of dry chlorine with titanium.
Over chlorination is absorption and water chlorination.
Explosive with hydrogen in presence of moisture.
Iron fire at 251 o C.
Exothermic reaction with organic/oil/grease.
Cl2 gas liquefies in pipeline under external temp condition.
Liq Cl2 expands with rise in temp Also pressure rises with temp.
MODE OF CHLORINE PRODUCTION
Cell Room Operations
Cooling and drying
Compression and liquefaction
Chlorine Storage and container filling
Destruction of waste Cl2
Dealing with emergencies like power failure, container leakage and failure of pipelines etc.
Material of Construction & Piping design/maintenance
Chlorine is generated in the electrolyzer slightly above atmospheric pressure. In the event of down stream failure it is necessary to relieve the full Cl2 production rate without over pressurizing the cell. This is most suitably achieved with the help of control valves connected to this scrubbers. Also individual electrolyzer is connected with the depressurizing line with safety valves to take care any abnormality of the system.
In electrolytic cell, the production of chlorine and hydrogen must be regulated by procedures and instrumentation, to prevent the emission of chlorine to the atmosphere and to prevent hydrogen and chlorine forming a combustible mixture.
Air must be excluded from the hydrogen stream so that it can not combine with hydrogen to form an explosive hydrogen-oxygen mixture.
Protective equipments such as goggles, hand gloves, boots and respiratory protection must be provided to the persons working in the area to avoid chemical and thermal burns.
Continuous analysis of chlorine is vital in cell room operation to avoid formation of explosive mixture.
SAFETY FACTORS IN OPERATIONS
Rectifiers and transformers are normally operated by personnel outside the cell area. Operations should be as little time as possible near the cubicles of rectifiers that are in service because an electrical fault in this equipment could expose them to an electrical flash.
Extreme care should be taken, never to work on them while they are electrically energized.
Breakers & relays must be kept reliable by periodic inspection, cleaning & testing.
Maintenance must be done by trained & qualified personnel only.
COOLING AND DRYING
The chlorine gas produced in the cell room is hot and saturated. The temp. of the gas as high as 90 o c.
Aspects of safety are as under:
As titanium is used as a tube material for heat exchanger, a careful review must be made to ensure, even in upset conditions that dry chlorine does not come into contact with the tubes. Dry chlorine reacts violently with titanium. Back flow preventor is provided to take care this problem
Careful control with adequate failure warning, of the gas temperature from the second coolers is required to prevent the formation of solid chlorine hydrate (9.6 o C). The hydrate formation may completely plug the heat exchanger or gas piping creating a big hazard in cell room.
Facilities for periodic or continuous washing of them demister must be provided to prevent excess salt crystallization due to carryover from the cell room. This can lead to high pressure drops and stoppage of gas flow.
To take care high temp of Cl2 before cooling the FRP line should be of Alpolite (797) resin instead of Atlac 382 (Bis-phenol resin)
The method used for chlorine drying is by contact with H2SO4. Dry Cl2 is defined the moisture less than 150 ppm by volume of water.
Aspects for safe operation
Conc. Of H2SO4 is a major factor when selecting materials of construction. Plastics or lined equipment and pipes must be considered.
Outlet conc. to be maintained 78 – 80% by continuous dosing of 98% H2SO4 to ensure proper drying.
The use of H2So4 requires specific attention. Velocities must be kept low 1 m/sec to prevent excessive erosive wear. Due to corrosive nature of dilute H2SO4 seals or rotating parts become rapidly worse.
Flange guards to be provided on H2SO4 pipe lines to avoid splashing of H2SO4.
Hot work permit to be issued on dilute H2SO4 after thoroughly washing/drying that system. H2 generates in dilute H2SO4 system during washing which can lead to explosion with hot work.
During above mentioned cooling & demisting operation, chlorinated water is generated which is effectively utilized in vacuum De-chlorination system.
Continuous monitoring of moisture in Cl2 by analysis/ by online instrumentation is a must.
Instrumentation to monitor acid flow conc., temperature, as well as gas temp is required. The capabilities of monitoring the pressure difference across the drying towers and cooling equipments is important for detection of restriction to flow with in the system.
Significant hazards involved in handling and storing H2SO4. All personnel need to be adequately trained for every phase of drying operation with particular attention given to avoidance of contact with the H2SO4. Protective clothing & safety equipments should be mandatory.
Chlorine outlet from the drying tower should be at the top or immediately in line with the top portion to avoid accumulation of hydrogen at the top & to avoid explosion due to electrostatic charge
Compression and Liquefaction
There are three ways of compression
Medium pressure compression system is to be used in view of safety.
Turbo compressors are used for compressing 225–250 tonne/day. These are used without H2SO4. This system needs extra care from moisture, carryover of rust particles & H2SO4 mist from drying tower. Candle filter to be installed in suction line of compressor.
Centrifugal compressor is used for compression of Cl2 with H2SO4 as sealing medium at medium pressure 3-3.5 kg/cm2 upto 80-90 tonnes of chlorine per day H2SO4 conc. To be maintain 95-98 o C to avid corrosion.
To ensure complete drying of CL2 after drying tower operation check is made to check the conc. Of H2SO4 in compressor by analysis on daily basis to avoid corrosion.
H2SO4 mist is to be removed with glass wool candle filters to avoid chocking in liquefier & pipeline
Continuous PH monitoring of the chilled water is done at individual cooler to ensure non-mixture of acid in the cooler. It can create enormous problem.
To prevent the back flow of chlorine (in case of compressor tripping) in to the low pressure system automatic control valves and check valves are used.
Compressor are fitted with mechanical seal to prevent any leakage of CL2.
Acid temp is to maintained below 40 o C to avoid corrosion.
Under pressure safety seal is to be provided in the system to safeguard under heavy vacuum
Proper alignment of the comp & motor is required. Vibration damages seals and results gas/acid leakage.
Provision to run the compressor with closed discharge valve in case of startup/stoppage/maintainer (local by pass valve).
Centrifugal compressor leads to unstable operation at high pressure and low volume. It can also occurs at low chlorine density. This results surging. Speed control/by pass control is required to ensure stable operation.
After compression Cl2 is liquefied by moderate pressure and refrigeration system.
Dry Cl2 vapor will always contain some innerts. It is impossible to achieve total liquefaction. After recovering of liquid chlorine, the residual gas (sniff gas) is disposed to hydrochloric manufacture or scrubbing system depending upon the situation.
Hydrogen is continuously monitored in sniff gas (less than 4%)
Make sure that refrigent used for liquefaction does not react with Cl2 if there is a leak.
CHLORINE STORAGE AND CONTAINER FILLING
Liquid chlorine is generally stored in horizontal pressure vessels. There are installed as per the static and mobile vessel rules 2004 and this installation is approved through Dept. of explosive.
Tanks are to be designed as per IS 2825
MOC is low temperature carbon steel SA 516 grade 60/70 (-35 o C to 55 o C)
Valve of forged carbon steel & spindle of monel metal or SS 316 with bellow hastalloy C 276.
Pressure Gauge with Haste alloy C diaphragm, Haste alloy C/ SS316 dip pipe.
100% stress relieved and X-rayed.
3 mm corrosion allowance
Inspection by Lloyd & approved by CCE, Nagpur.
Design pressure 19.9 Kg/ cm2 at 65 o C
Hydraulic test pressure 23.0 Kg/cm2
Retesting after two years
Average life 20-25 years
Bolts, nuts & studs of proper MOC strictly to be used.
(Stud - A320 L7), (Nut – A194 grade-4)
SAFETY CONTROLS ON BULLETS
Low and High level alarm, 20% low & 80% high & provision of load cells in individual tank.
High & low pressure alarm, 2.5 Kg/cm2 low and 11.5 Kg/cm2
Temperature alarm –15 o C low and 30 o C high.
Double Rupture disc, 13.5 Kg/cm2
Double Safety valve opens at 13.0 kg/cm2 & closes at 12.7 kg/cm2
Alarm pressure for rupture disc puncture – 1.0 kg/cm2
Double valve of each line of international standards
Remote type valve in liquid chlorine line
One tank is always kept empty
Connection with emergency release line to neutralize the system
Cl2 detector in the storage area
Adequate distance between two storage tanks
Tanks locations at sufficient distances away from hazardous processes or storage with potential fire/explosion risk to minimize risk of damage
Provision of dyke wall, impervious flooring with 1:100 slope
Containment system in case of toxic release at tank area
Transfer of liquid Cl2 to empty tank. Time for transfer – 2 hrs by dry air compression at 10.8 kg/cm2. Tank volume – 100 M3
Foam is used to contain chlorine, expansion factor probably in range 75-250.
Chilled water below 10 o C can be used to control liquid Cl2 spillage.
Very cheap but effective cover for toxic liquefied gas pool is, large sheet of poly ethylene. Vapor evaporating from pool below cover can be withdrawn by large bore hose to a destruction system with on line connected vacuum hose.
Neutralization of chlorine with caustic soda (3 stage system)
Chlorine gas sensor.
Dyke wall is deeper rather than long for less liquid evaporation.
Nature of substrate is impervious.
Preventing passage of liquid chlorine into drainage system.
Dyke wall kept dry during rain fall season.
Substrate or bottom surface of dyke is of low heat transfer material for less evaporation.
To avoid percolation, a steep slope is provided leading to a sump with small pumps or airlifts.
Provision of prevention free access of wind and airflows to pool area.
The storage tanks may be fitted inside a close room with emergency exhaust blower. Exhaust blower leads to neutralization system.
Provision of high capacity blowers in storage and bottling area connected with neutralizing system in open storage.
MATERIAL OF CONSTRUCTION
Body: Carbon steel SA 516 Grade 60/70 low temperature steel.
Valve: Aluminum Silicon Bronze (BIS Approved)
Spindle: Monal Metal with Teflon gland packing
Inspection Plugs: Forged brass material
Hydraulic testing every two years
5% weight loss
Any physical damage
subjected to any heat treatment
New tonners must pass drop test, burst test, 100% radiography, stress test, Approval by LIyod & CCE
Toner Filing Operation Sequences
Depressurization of empty toner
Physical checking for
1. Accessories and bushes.
2. Testing date
3. Any dent on surface
Internal inspection of toner with 12V light
Preparation & Approval of check chart “for fit for filling tonners”
Filling of toner after inspection.
Over filling is to be controlled by alarm, Interlock with control valve in filling line & rechecking with crane weigh balance.
Hydraulic testing if due for testing
Scrubbing of Waste Cl2
For the scrubbing of waste vented chlorine, a scrubbing tower system is used. Since in Cell gas is hot and wet and generally large volume and at low pressure, a low pressure drop system is required.
Two/Three scrubber are provided in series so that is capable of absorbing all gases even during emergency. Bleach liquor in the circulation is regularly checked for excess caustic to effect changeover at proper time.
Auto control valve is to be provided to maintain uniform vacuum in the system
Full Overhead tank with 20% caustic should be available to meet any emergency it should be linked with ON/OFF control valves in order to dump caustic from control room also.
Temp alarm for sodium hypo system is required to maintain temp below 35 o C
Titanium pumps, MSRL pipeline & towers with titanium coolers are to be used for the service to give reliable emergency services
The most common emergency is “Power failure”. To avoid leakage of CL2 gas in the atmosphere, a number of critical equipments are connected to the generator set with automatic changeover.
Equipments generally connected with DG set are
1) DC blower
2) Caustic circulation pumps
3) Emergency lights
4) Inst. Air, cooling water, chilled water & Nitrogen system
Apart from this, regular inspection and changeover of pipelines, hydraulic testing of storage tanks and containers are done to minimize chance of any Cl2 leakage due to corrosion.
Instrumentation plays a very important role in chlorine manufacture.
some of this important instruments are as under:
Rectifier Tripping Interlocks
High Current of electrolyzer
Tripping of 2 or more electrolyzer
One of Cl2 compressor running
High level of anolyte tank
Low level of overhead pure brine tank
High temp of catholyte vent pot
High Cl2 pressure
Low Cl2 pressure
Low H2/Cl2 differential pressure
High H2/Cl2 differential pressure
Liquid Cl2 pumps not running
Full opening of Hypo valves
In addition to this a multipoint chloro alarm sensors are provided at various places within the complex to detect Cl2 leakage easily at 3.00 ppm conc.
These instruments are checked & calibrated regularly.
Breathing apparatus and other emergency kits are available at different locations.
MATERIAL OF CONSTRUCTION
1) Stone ware
3) hard rubber
4) Unplasticised PVC
5) Glass fiber (FRP)
2) Tantalum inert up to 150 deg.c
3) Haste alloy ‘C’
4) Monel metal ‘B’
DRY CHLORINE GAS OR LIQUID
1) Seamless carbon steel
2) Corrosion allowance 1 mm
3) Service temp –35 to 650c
4) Pressure- 19.9 kg/cm2
5) Tounge and groove flanges
7) Nut Bolts-Alloy Steel.
1) Drop forged steel
2) Monel spindle/haste alloy ‘c’
3) Cast steel seat
4) Teflon Gland packing.
Carbon steel is suitable upto 120 o C.
SS with 10% nickel is suitable upto 150 o C.
SS with more than 10% nickel is suitable upto 250 o C.
Monel is suitable upto 350 o C.
Nickel, Inconel & Hastalloy C is suitable upto 400-500 o C.
Nickel is used as lining material as it has poor mechanical properties.
Piping arrangements simple with min. flange joints/ screwed joints
Above ground layout
Well supported and adequately sloped
Expansion vessel in liqcl2(20% vol)
Min 20 mm pipe diameter to be utilized
Lines cleanly labeled and painted as per color code (yellow)
Linseed oil and graphite,linseed oil and white lead ,litharge and glycerin can be used for permanent joint.
Seamless schedule 80 pipeline upto 6 inches above that schedule 40 is acceptable
Regular NDT testing of chlorine pipeline
Velocity to be maintained 2 m/sec
100% stress relieving & radiography for liquid Cl2 service pipeline
All pipelines are to be cleaned.
No oil/grease/hydrocarbon/alcohol is to be used for cleaning
Cleaning can be done with trichlorethylene /carbontetrachloride
Chlorine line can be washed, steam heated and dried before use
Chlorine lines to be hydro tested before drying (Dry Air –400c Dew point)
After drying lines to be tested with
1)Dry Air-/nitrogen - check for leak with soap solution
3)Dry Air + chlorine - check leakage with Ammonia solution
4)chlorine - DO -
Never attempt to repair leak by welding until all chlorine has been purged.
The line to be retested after repair
Contact of Ammonia with Brass is to be avoided
All old & unused pipelines must be identified and removed.
Neutralization system by caustic soda to form sodium hypochlorite products with emergency power supply (Three stage neutralization)
Use of safety kit to stop release.
On line detection sensors for chlorine gas.
All PPE’s provided.
Training of employees at shop floor and at consumers premises.
Training of drivers and conductors.
Rescue team availability on effected site on call.
Provision of high capacity blowers in tanks and liquid Cl2 filling area (Connected with neutralizing system)
Minimum inventory in liquid Cl2 tank
Public announcement facility and local phones
Central Control Room for Co-ordination
Remote control valve with local control
Double valve in each line
Emergency release line
Seamless piping with tongue and groves flanges.
Expansion chamber in liquid CL2 line.
100% instrumentation control in CL2 system manufacture
Competent persons to handle liquid Cl2 system approved by CCE
Liquid Cl2 tank insulated with 75 MM thick thermocole insulation and cladded with Aluminum sheet
Flame proof lights
Adequate wind socks are provided to know wind direction.
Tank replacement every 20-25 years irrespective of tank health/condition or after the report by third party or in case of physical damages.
Assembly points identified and marked properly for emergency.
Periodical mock drill exercise carried out.
Availability of On site/Off site emergency plan
Other Safety Measures in complex
License for liquid Cl2 storage in tanks.
License for liquid Cl2 storage in Tonners/Cylinders.
Approved layout plan indicating surrounding area and Cl2 neutralizing facilities.
Liquid Cl2 filling license.
Recognition for hydraulic testing facilities of Cl2 Tonners/cylinders
List of competent persons handling Cl2 operation
Environment protection Act.
The Factories Act
ON site/ OFF site emergencies Plan.
Mock drill (every Six month)
Hydraulic testing of Tonners/storage tank every two year.
Compliance of GAS CYLINDER RULES 2004 and STATIC AND MOBILE VESSEL RULES.
Lloyd inspection report of every Tonner and CCE approval before filling.
SAFETY MANUAL with MSDS and safety permit system.
Central Motor Vehicle Transport Act.
Storage and impact of Hazardous chemicals Rules.
The Public liabilities Insurance Act.
REFERENCE FOR SAFE OPERATION OF CL2
INDIAN EXPLOSIVE ACT 1884
Gas Cylinder Rules 2004
IS 8867 - Hydraulic Stretch Test – test pressure
IS 3710 - Values of saturated VP of different gases. (FILLING RATIO)
IS 3224 - Specifications of Industrial gas cylinders.
IS 5903 - Details of safety devices in cylinder valves fitted with safety devices.
IS 4379 - Identification colours for industrial gases.
IS 356 - (IS Standard Colour No. 356) Colour of gas Non-flammable but toxic.
IS 5844 - Hydrostatic stretch test – Procedure (welded cylinder)
IS 3196 - Specification of low pressure liquefiable gas service.
ASME - Specification of Cl2 tonners
(SEC. VIII WITH LATEST AMENDMENT CHIMED ENDS SPECIFICATION NO. 179.300 OF CFR 49)
State & Mobile Vessel Rules 2004
IS 2825 - Design code of pressure vessels.
IS 4263-1967 - Code of safety for chlorine
IS 10553 - Withdrawal of Cl2 from tonn container.
PART:I, 1983 - (Requirements of chlorination equipments, general Guidelines for chlorination plant and safety of chlorine.
IS 2379-1963 - Colour code for identification for pipelines.
IS 10553 Part(2) – Vacuum feed type chlorination.
IS 10553 Part(4) – Gravity feed gaseous chlorination.