HAZARDS IN CHLORINE MANUFACTURE & STORAGE

  

BY  

H. S. SEHGALDGM (OPERATION)

 GUJARAT ALKALIES AND

CHEMICALS LIMITEDVADODARA

INTRODUCTION 

Chorine is the by-product of Caustic-Chlorine Industry. It is generated by the electrolysis of brine in mercury, diaphragm or membrane cells. There are 500 Chloro-alkali plants in the world with a total annual capacity of over 50 million tons. DOW Chemicals is the largest producer of chlorine (6.0 Million Tons). In India hardly 2.5 Million tons chlorine is produced, 5 % of world production from 42 plants. GACL is meeting 16 % chlorine demand of India. Today, chlorine occupies a pivotal role in chemical production. Thousands of industrial and consumer products are made using chlorine. It is called “GIFT” to mankind. But in spite of these benefits, chlorine has environmental and health risks which need to be managed responsibly. Human exposure to chlorine has resulted in large number of deaths and injury. Major accidents are due to lack of knowledge and over confidence. To prevent serious injuries and inherent hazards associated with chlorine, policies and procedures has to be established. This encourages communication, good house keeping, timely maintenance, educating the persons involved, monitoring, auditing, developing, adhering to standard operating practices.

Chlorine Manufacturing process

1st stage cooling

filtration

liquification

2nd stage cooling

compressor

Drying

Storage

CL2 from

cell

90 0

Consumption

40 o H2SO4

3.0 Kgcm2

0 0 C

15 o C40 o C

Bottling

CHLORINE - MANUFACTURING PROCESS  

-      Chlorine produced by the various processes especially by electrolysis is saturated with water vapor at high temperature and may also contain traces of chlorinated hydrocarbons and is normally at slightly above atmospheric pressure. Before the chlorine can be used, it is to be cooled, dried, purified, compressed and where necessary liquefied.

 -        Liquid chlorine is stored in tanks and filled in 900 kgs. tonners and

100 kgs. cylinders by dry air padding / liquid chlorine pumps and dispatched to various consumers.

 -     In developed countries liquid chorine is transported in tankers and

railway containers of 55 MT / 90 MT capacity. -        This mode of transportation is yet to pick-up in India. -    During manufacturing operation different types of chlorine is being

experienced:- 

1.                Wet Chlorine – Moisture more than 150 ppm.2.                Dry Chlorine – Moisture less than 150 ppm.3.                Liquid Chlorine – Chlorine liquefied by way of temp.

Pressure control.4.                Aqueous Chlorine – Chlorine concentration above 750

ppm In water / solution.5.                Aqueous chlorine has more serious effect due to its

characteristics.6.               All type of chlorine has different properties from each other.

HAZARDS ASSOCIATED WITH CHLORINE  

    Impact of Chlorine (Health) :  Chlorine gas is a respiratory irritant. Its odor gives ample warning of

its presence. Its greenish – yellow color makes its visible at higher concentrations. In extreme cases, breathing difficulty may lead to death. It will react with body moisture to form acids. Liquid chlorine will cause serious burns. Exposure to chlorine gas can cause burning of eyes and nose.

 

  Aquatic Life :  Elevated chlorine level in water can create strong taste and odor if

organic matter is present. Generation of trihalomethanes damages the liver and kidney of fishes and other living organism present in water.

 

  Animals :  Same physiological response mechanism as human being takes places. 

  Vegetation : 

Plants in the path of chlorine release may be damaged. Chlorine bleaches leaves. Chlorine stops the plant from producing Chlorophyll Healthy plants recover over time.

              Fire and Explosion Hazards : -         Non combustible in air.-    Most combustible materials will burn in chlorine as they do in oxygen.-   Flammable gases and vapors will form explosive mixture with chlorine.-  Reacts explosively with acetylene, turpentine, ether, fuel gas, hydrocarbons, hydrogen and finely divided metals.-  It reacts with ammonia, forming explosive nitrogen trichloride.-    Dry chlorine reacts violently with titanium.-   Liquid chlorine + carbon di-sulphide explodes when added in iron cylinder.-   Liquid chlorine passed through polypropylene with zinc oxide reacts explosively.-   Tin with liquid chlorine at –34 Deg.C and aluminum powder at –20 Deg.C leads to explosions.-   Rubber with liquid chlorine or dry gas leads to fire and explosions.-  The vapor is heavier than air and shall travel along the ground and persists at low level. -    It is difficult to disperse.-    At local heating of steel equipments and chlorine – iron fire may develop at 251 Deg.C.-   If exposed to fire, cylinders may explode violently releasing total contents. A full cylinder may burst at heating to 95 Deg.C.-   Chlorination reactions must be carefully regulated to prevent this reaction from going out of control.

Volumetric Expansion : Liquid chlorine increases considerably in volume when evaporated 1 Vol = 457.6 Vol. Liquid chlorine has high coefficient of thermal expansion, resulting in rapid increase in volume. If temperature goes 67 Deg.C the volumetric expansion of liquid chlorine goes beyond the capacity of the container leading to serious damage.  

Corrosion Hazard : Chlorine and moisture forms hydrochloric acid which attacks steel. It also forms hydrates of iron chloride which cause plugging.  

Hazards Due To NCl3 Impurity : Presence of NCl3 in liquid chlorine can result in explosion. NCl3 is soluble in liquid chlorine. As soon as liquid chlorine exhaust in the system NCl3 explode. 

Terrorist Activity Hazard : Chlorine is freely available in the market. It can be used by the terrorist. It was used in World War – I as a pulmonary choking agent. Recently in February, 2007 about 30 MT Chlorine was used in Iraq war.

HAZARDS IN CHLORINE MANUFACTURING : 

CELL HOUSE : -   Chlorine is generated in the electrolyser slightly above atmospheric pressure. In the event of failure of down stream or any abnormality it is

possible that full chlorine production rate can over pressurize the electrolyser / collection headers and large quantity of chlorine can enter the atmosphere. For this necessary control valves / safety valves are provided which are connected with the caustic scrubbing system.

-     There is the possibility of hydrogen generation along with chlorine in the electrolyser. It can form explosive mixture. Continuous analysis of

chlorine / hydrogen is a must to avoid this hazard. In membrane cells hydrogen in chlorine has reduced considerably. But some explosions have taken place due to pin holes developed in membranes.

-      All the products are at higher temperature 85 – 90 Deg.C in Cell House and can cause chemical / thermal burns. Protective equipments such

as goggles, hand gloves, boots and respiratory system must be used by the working personnel to avoid any hazard.

-   In the Cell House even the minor variation in process parameter leads to big hazard in the electrolyser / down stream. In order to avoid this

production system it is to be controlled by procedures, instrumentation competent dedicated and experienced persons only.

-   Vent gas of cell will be hot and wet and generally in large volume and low pressure. In order to provide low pressure drop to electrolyser, an

independent and individual release line is provided through safety valves to avoid the emission in the atmosphere.

-   In cell house chlorine / chlorinated depleted brine piping operate at 85 – 90 Deg. C. Normal FRP piping suddenly give way. It is made sure that pipe lines are of ALPOLITE-797 instead of Atlac- 382 (Bis Phenol Resin).

COOLING :  -  Cooling of chlorine gas can be achieved from 90 – 15 Deg.C by “Direct

Contact Cooling” or by “Indirect Cooling”. It forms a solid hydrate at 9.6 Deg.C which sets the minimum temperature for wet chlorine cooling. Formation of solid chlorine hydrate promotes corrosion of steel and plug the heat exchanger of gas piping creating back pressure in cell house. Careful control with adequate failure warning of the gas temperature is required.

 - In the direct cooling there is the possibility of ammonical impurities in water which leads to formation of explosive chemicals NCl3. Ammonical impurity must be checked in water.

 -Titanium is used as a tube material for heat exchanger in “Indirect Cooling”.

Titanium reacts violently in the presence of dry chlorine. Back flow prevention is provided to take care of intermixing. Temperature of chlorine should not be taken below 13 Deg.C. Low temperature combustion has its implication for cold weather.

   CHLORINE FILTERTATION :  - Chlorine gas evolved from cells contains water droplets, brine

mist which can cause choking in drying towers because of formation of Na2SO4 and results back pressure. In order to remove these impurities brink demister with glass wool are provided. This removes the solid and liquid impurities mechanically and is self-cleaning.

 -  Chlorinated water generated in cooling and filtration is utilized

in the system.

DRYING : -            Wet chlorine after cooling and filtration is dried with concentrated Sulphuric acid (98 %). Chlorine gas containing even small amount of water is highly corrosive. Moisture level must be checked after drying. Upper limit of moisture content is 50 ppm. -    Circulated sulphuric acid is cooled with chilled water to neutralize heat of dilution. -    Sulphuric acid is a dangerous material that attacks body tissue very rapidly. All personnel needs to be adequately trained for every phase of drying operation with particular attention given to avoidance of contract with sulphuric acid. Protective clothing and safety equipments should be mandatory. -     Use of H2SO4 required specific attention:

a)  Velocity below 1 M/Sec. To prevent excessive erosive wear.b)  Flange guards to provide on sulphuric acid lines.c)   Hydrogen is generated in sulphuric acid system. Hot work permit to

be given after thoroughly drying the system. -      Instrumentation to monitor acid flow concentration, temperature, as well as gas temperature is required. Pressure drop across the drying tower is to be checked regularly. -    There is the possibility of hydrogen generation and accumulation at the top of the drying tower with sulphuric acid system and results explosion due to electrostatic charge generated by circulating sulphuric acid. To avoid this chlorine outlet should be immediately at the top or immediately at the side. No pocket should be available for hydrogen accumulation.

COMPRESSION :  -      There are three ways of compression :

A.   AtmosphereB.   Medium pressureC.   High pressure.

  Medium pressure compression system is to be used in view of safety. -        Turbo compressors are used for compressing 225 – 250 MT / day. These are used without sulphuric acid. This system needs care from moisture, carryover of rust particles and sulphuric acid mist from drying tower. Candles filter to be installed in suction line of compressor. -    Centrifugal compressor is used for compression of chlorine with sulphuric acid as sealing medium at medium pressure 3 – 3.5 kg/cm2 upto 80 – 90 tonnes of chlorine per day sulphuric acid concentration to be maintain 95 – 98 % / 40 Deg.C Maximum to avoid corrosion. -    Check is made to analyze the concentration of sulphuric acid on daily basis. -  The most likely hazard in compression stage is acid cooler failure resulting acid water mix up raising the temperature of acid and chlorine enormously. -   Continuous pH monitoring of the chilled water is done at individual cooler to ensure non-mixing of acid in the cooler. -    Cooling water failure is another phenomenon to be controlled. This results high chlorine temperature, low compression capacity and other chain reactions. A pressure switch is required in cooling water circuit.

-       

•-     To prevent the back flow of chlorine (in case of compressor • tripping) in to the low pressure system automatic control • valves and NRV valves are used.•- Compressor are fitted with mechanical seal to prevent any • leakage of chlorine / air entry in to the system.•-  Acid temp is to maintained below 40 Deg.C to avoid corrosion.•-  Under pressure safety seal is to be provided in the system to • safeguard under heavy vacuum.•- Proper alignment of the compressor and motor is required. • Vibration damages seals and results gas / acid leakage.•-  Provision to run the compressor with closed discharged valves • in case of start up / 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.

HAZARD IN SULPHURIC ACID MIST ELIMINATION :  -    Sulphuric acid has the tendency to form mist. Liquid particles in the mist are 10

um or less in diameter. Sulphuric acid mist particles goes down to sub-micron size. Particles larger 3 um size are collected effectively. Efficiency is very low for 1 um particles which are carried in the system and cause sludge formation. Dry chlorine mist elimination with glass wool packing works up to 99 % efficiency. Carryover of sulphuric acid mist to liquefier to storage tank results sludge formation / choking in pipe line / mal-functioning of chlorine valves.

 LIQUEFACTION : -      Chlorine is liquefied by moderate pressure and refrigeration system.-  Dry chorine vapor will always contain some inert. It is impossible to achieve

total liquefaction. After recovering of liquid chlorine, the residual gas (sniff gas) is disposed off to hydrochloric manufacturing or scrubbing system depending upon the situation.

-  There is the possibility of enrichment of hydrogen during liquefaction which can cause explosion.

-  For safety hydrogen is continuously monitored in sniff gas (less than 4%) by on line analyzer and manually.

-  Make sure that refrigerant used for liquefaction does not react with chlorine if there is a leak.

 

CHLORINE STORAGE : -    Liquid chlorine storage presents the potential of major accident hazard. Rate of

liquid chlorine leak will be 15 – 20 times greater than that of gas. - Liquid chlorine is generally stored in horizontal pressure vessels. These are

installed as per the Static and Mobile Vessel Rules, 2004 and this installation is approved through Department of Explosives, Nagpur.

ASSOCIATED HAZARDS IN STORAGE :

Maintenance Hazard :-

-  Failure of fastners at –34 Deg.C when liquid chlorine tank / pipe line achieves this temperature. CS bolts suitable up to –29 Deg.C, A 320 L7 bolts and A 104 grade 4 nuts are suitable.

 - Due to welding failure in level gauge (magnetic type) chlorine leakage is taking

place. Proper testing / replacement to radar type level gauge is suitable. -  Titanium gasket are fitted in place of haste alloy “C” due to improper check /

testing. Testing to be confirmed. -  Cleaning with hydrocarbons leads to exothermic reaction with chlorine. -  Lubrication / greasing result hazardous reaction. Trichloro ethylene cleaning

media / fluorinated grease to be used. -  Delay in maintenance line results moisture entry / corrosion / passing through

of valves. -  Welding / hot work in chlorine lines results in iron fire. Needs proper flushing. - External corrosion under insulation due to temperature change results leakage. - Repair of liquid chlorine / chlorine pipe line result frequent leakage. New pipe lines to be changed / no repair maint. For chlorine line.

Piping Design Hazard:- -  Liquid chlorine pipe lines become weak due to erosion. Velocity max. 2.0 M/Sec. and schedule 80 pipe up to 6” size is required. -   Leakage from the blow holes due to failure of welding takes place which results gas nuisance. More stress on testing is required.  

Shut down Hazard :-  - High inventory of liquid chlorine / stoppage of chorine neutralizing system during long plant stoppage can results chlorine emissions. Minimum inventory / continuous of neutralize system is the solution. -  During long shut down liquid chlorine between two valves expands result building / leakage. Expansion vessel in liquid and emptying out liquid chlorine line is the solution.  

Operational Hazard :- -  Impurity in dry air leads to explosion in the tank. Impurity of hydrocarbon gases to be checked. -  Carry over of sulphuric acid in pipe line results chocking in pipe lines / tanks. Dry chlorine filter working to be checked.

TRANSFER OF LIQUID CHLORINE TO TONNERS / CYLINDERS : -      Liquid chlorine is filled in 900 kgs. tonners / 100 kgs. cylinder by padding the tank with dry air (-40 Deg.C dew point, oil and rust free). -     Instrument air used for storage tanks results chlorine in control loops. -   Liquid chlorine can be bottled with liquid chlorine pumps. By this system

iron fire can take place due to any friction in the temperature. More over the tank has to be emptied out by padding on during maintenance / testing. Double system has to be kept.

 -   Moisture in air causes choking in pipe lines / pitting of pipe lines and tanks.

Oil and fine iron powder causes exothermic reaction. -  Chlorine can travel back in air receiver during loading / unloading of air

compressor.

LIQUID CHLORINE FILLING : -  Liquid chlorine filling system in tonners / cylinders and storage facility is made as per Gas Cylinder Rules, 2004 and approved from Department of Explosives, Nagpur. 

-  While handling, storing or transporting chlorine in tonners and cylinders, loss of containments is the hazardous event. Leak is to be contained quickly or sudden release of large quantity. Preventive steps needed to avoid loss of containment. 

-  At filling station over filling can result bulging / explosion of tonner. Filling is to be regulated by audio visual alarms on the weigh balances and at the time of dispatch. -  Retention of tonners for long duration can result hardening of valve packing and consequent leak. - Tonners involved in high temperature / fire results hydraulic rupture of tonners emptying out total chlorine in the atmosphere. -  Spray of water on chlorine leaks enhances the chlorine release. Water is not be used. -  Leaky chlorine tonner when thrown in pit for neutralization floats on the surface. -  Presence of any impurity / foreign material in empty chlorine tonners, possibility of exo thermic reaction in tonner is there. It may lead to explosion also. Thorough checking of empty tonners is required. -  Spraying of liquid ammonia on the leaky valves damages the chlorine valves of the tonner. Valves are fit for dry chlorine only. Direct ammonia is not be sprayed on the valve. - Entry of unauthorised persons in chlorine handling area can result terrosit activities.

SCRUBBING OF WASTE CHLORINE :

  -    For the scrubbing of waste vented chlorine, a scrubbing system with caustic circulation is used with emergency power back up.

 -   Two / Three stage scrubber are provided in series to absorb total gas even during emergency. Bleach liquor in the circulation is regularly checked for excess caustic to effect changeover at proper time by anaylsis / redox potential analyser. -   Auto control valve is to be provided to maintain uniform vacuum in the system. -   Full over head tank with 20 % Caustic solution should be available to meet any emergency. It should be linked with ON / OFF control valve in order to dump caustic from control room also. -  Temperature alarm for Sodium Hypo System is required to maintain temperature below 35 Deg.C. -  Titanium pumps, MSRL pipe line and towers with titanium coolers are to be used for the service to give reliable emergency services. - 15 minutes chlorine neutralizing material at full load for assessment of the emergency taking appropriate action.

ASSOCIATED HAZARDS WITH PLANT SHUT DOWN / MAINTENANCE :

  

-    In running plant all process parameters are controlled very closely to

avoid any hazards. -    During plant stoppage, opening of process pipe lines, tower, pumps

and tanks etc. are involved. Both process and maintenance personnel have to be extremely careful in isolation procedure, adherence of safety rules, knowledge of fluids processed in the

system etc.

 -    Any by passing of rules or over confidence can result in any hazard

probability.

CONCLUSION :

BREADTH AND DEPTH OF DEFENCE FOR CHLORINE SAFETY :  PREDICTION  PREVENTION  DETECTION  MANAGEMENT  AUDITING  IMPROVEMENT REFRESHER TRAINING TO OPERATING PERSONNEL THINKING GLOBALLY AND WORKING LOCALLY.

T H A N K Y O U