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 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.
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.
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
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.
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.
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