RISKS & CONTROLS OFNITROGEN TRI-CHLORIDE

Presented by

H S.SEHGAL, DY.GMGujarat Alkalies & Chemicals Ltd

Vadodara,

Gujarat (India)

PREFACE

• The Production of Chlorine has expanded rapidly in the recent years and larger quantities are stored, used and transported.

• Nitrogen tri chloride is an extremely hazardous chemical which is synthesized in small quantities in all chlorine manufacturing plants.

• Nitrogen tri chloride under certain circumstances is extremely unstable and can produce violet explosion.

• It must be handled and disposed off with extreme cautions.

AREAS OF POTENTIAL CONCERNS

1. Chlorine Liquefiers

2. Chlorine Storages

3. Mobile Containers

4. Liquid Chlorine Tonners/ Cylinders

5. Vaporizers

6. Upset Conditions in Process or Preparation for Maintenance

7. Recycling of various streams

8. Modification Jobs

9. Destruction Process of Nitrogen Tri Chloride

OBJECTIVE OF STUDY

PURPOSE - To highlight the potential problem with accumulation of reactive material Nitrogen Tri Chloride in liquid Chlorine.

- To describe the constraints which need to be observed in order to avoid the

risks associated with its presence in routine Chlorine storage, transport

and use.

Any Chlorine user who is uncertain of the potential problem which can arise from accumulation of NCl3, should consult the Chlorine suppliers to confirm acceptable level and level of risk.

OCCURRENCE & FORMATION OF NCl3

Nitrogen Tri-Chloride was first observed in 1811 from the action of Chlorine on the solution of Ammonium Chloride.

This compound is a yellow oil with a pungent Chlorine like Odour.

It is insoluble in water and soluble in most organic solvents.

A drop of oil explodes violently when touched with a feather dipped in turpentine.

Nitrogen Tri-Chloride is formed during the production of Chlorine when Ammonical Nitrogen is present in the brine fed to the electrolytic cells.

Any ammonical compound in the brine or water will be converted to NCl3 in short time. 30 ppm of ammonia in water is converted to NCl3 in 20 seconds.

NCl3 is soluble to the extent of 7.3 mg/l in liquid Chlorine.

OCCURRENCE & FORMATION OF NCl3

Once NCl3 forms in the electrolytic cells, it will pass with Chlorine gas through the coolers, scrubbers, acid seal pumps and will be condensed with the liquid Chlorine.

NCl3 Vapor pressure is a function of temperature. Under the usual brine electrolysis conditions NCl3 will be readily swept out of the cells with chlorine .

NCl3 concentrates itself in the layer of liquid Chlorine next to vapor phase. As the liquid Chlorine is used up, the concentration of NCl3 keeps increasing at vapor-liquid inter-phase.

Chemical reaction is as under---

NH3 + Cl2------NH2Cl+HCl NH2Cl+Cl2------NHCl + HCl NHCl+ Cl2------ NCl3+HCl

ASSOCIATED HAZARDS Nitrogen Tri-Chloride is one of the most hazardous material in Chlor-

Alkali industry.

Fatalities have occurred and workers have been injured by NCl3 explosions.

The explosive power of small quantity of NCl3 can be catastrophic & can cause release of Chlorine & large scale damage to personnel, plant equipment .

NCl3 is sensitive to light, impact and ultrasonic radiation A spontaneous explosive compound. Explosive potential is 30% of TNT.

The compound has +72 Kcal/mol free energy of formation and decomposition gives 54.7 Kcal/mol.

On detonation at constant volume, a temperature of 2128 0C and pressure of 5361 atm is reached.

Decomposition of as little as 0.3 gm/cm2 of exposed wall area in a confined volume can overstress steel and can create a very hazardous situation.

ASSOCIATED HAZARDS (CONTD)

Increasing the number to 1.5 gm/cm2 results in fracture.

The hazard is less when Chlorine contaminated with NCl3 is spread in a wide pool than it collects in a nozzle or section of piping.

NCl3 is much less volatile (Boiling point:710C) than Liquid Chlorine (Boiling point:-340C). It tends to accumulate at different points in the process. Any NCl3 present in the gas shall concentrate in liquid chlorine. Vapor pressure of NCl3 is 0.11 atm and for chlorine is 3.68 atm at 0 0C which shows NCl3 will remain on evaporation of liquid chlorine.

Yellow oil of NCl3 explodes violently : When heated in greater than 93 0C Exposed to light Brought in contact with Ozone, Phosphorous, Arsenic, Alkali and

organic matter.

Spill of NCl3 is a hazardous residue. Spilled NCl3 can detonate on simple walking through it.

ASSOCIATED HAZARDS (CONTD)

The use of containers are cause of concern. When liquid chlorine is exhausted in containers and gas is withdrawn to storage, vaporization or other process use then decomposition of NCl3 starts. Once decomposition begins, the exothermic heat of reaction causes to propagates very rapidly and move on to detonation.

I NCI DENT LOCATI ON DATEStorage Tank (3) Explosions China April, 2004

Detonation in copper tubing Australia July, 1998

Explosion of an empty railcar which had been Belgium February, 1995

unloaded via the gas phase

Chlorine suction chiller explosion; Major Louisiana (USA) March, 1994

equipment damage

Compressor suciton chiller drain piping

explosion killing two people and severing Colombia November, 1993

one person's leg above the knee

Purification column pot exploded killing one West Virginia (USA) March, 1983

Cooling tower (believed to be compressor Michigan (USA) November, 1978

suction chlorine cooler) exploded killing one

Two explosions in chlorine vaporizers have Brazil Not known

been rerported, no details available

One-ton container exploded Not known Febryuary, 1949

One-ton container exploded Not known November, 1948

Tank car cover blew off killing two Norway January, 1940

Three interconnected storage tanks (45 tons

total capacity) at a pulp mill exploded Romania December, 1939

releasing 21 tons of chlorine

Two one-ton containers exploded Not known July, 1928

An evaporator explosion was reported India 1965

Several evaportators explosions investigated South America 1981

(NCl3 concn. 50 to 300 mg/ltr.)

SUSPECTED NI TROGEN TRI CHLORI DE I NCI DENTS

SOURCES OF NITROGENOUS IMPURITY

Nitrogen compounds may be present in salt and other chemicals added during processing.

Nitrogenous explosives used to break the beds in the mines.

Anti-caking agent present in ppm concentration.

Salt delivery vehicle is a potential source of contamination.

Dissolving water is the source of nitrogenous matter in solution mining.

Calcium Chloride used in some brine systems – a by-product of Solvay Process of Soda Ash is a source of Ammonical impurity.

SOURCES OF NITROGENOUS IMPURITY

KCl salt recovered by the use of Amines as separator can add Nitrogenous impurity.

H2SO4 used in Chlorine drying is also source of Ammonia compounds sometimes.

Plant utilities are likely sources like use of impure water to prepare treatment chemicals, flushing water on rotating equipments seals, water recycle from waste minimization systems, direct contact cooling water.

Steam and its condensate are also potential sources. The culprits are usually the amines used as corrosion inhibitors in steam system.

SOURCES OF NITROGENOUS IMPURITY

Higher risk of Nitrogen Tri-Chloride formation or accumulation during periods of start up, shut downs, process modifications, maintenance, process upsets or process fluctuations

Not all compounds are converted to Nitrogen Tri-Chloride. Water contaminated with nitrogenous material like Caprolactum does not contribute rise in NCl3 (used in mine brine).

Nitrogen gas introduced into Chlorine is not converted to NCl3.

Fully oxidized form of Nitrate salts (Potassium Nitrate or Sodium Nitrate) do not form NCl3 with interaction with Chlorine.

Awareness to the suppliers

Suppliers to be made aware about the importance of preventing contamination of the materials and the hazards involved in chlor-alkali industry.

1 ppm of Ammonia in the feed brine can produce >50 ppm of NCl3 in dry Chlorine and 1 ppm of Ammonia in salt can produce about 12 ppm of NCl3

in the Chlorine. Salt to be mined with the help of an explosive which is free from

Nitrogenous matter.

Salt to be stored without anti-caking agent.

Safer type of water for the manufacture of salt.

Rigid analytical programme for the analysis of the salt.

Dedicated vehicles for the transportation of salt to avoid any nitrogen compound contamination.

To Control the formation of NCl3 in liquid chlorine, it is essential to limit the entry of Nitrogen compound in Salt and Brine solution in process continuously.

Analysis on line :- An on line analysis has been developed by a French organization

website :- www.seres-france.com. It works in on principle of sequential analysis

It can measure Ammoniac salt(NH4) Concentration in Brine & Ncl3 Concentration in Cl2 gas at the exit of Cl2 Compressor.

Magnetic fields & photo sensibility of Ncl3 influence the analyzer. Therefore the analyzer must be installed:- At a sufficient distance from the Cells House. In a close Cabinet with air conditioning & with no external light. All Transparent piping (Teflon etc) has to be covered.

DETECTION

DETECTION

ANALYSIS OF NCL3 IN LIQUID CL2 :-

Instrument technique & chemical determinations based on conversion to ammonium ions are available.

Instrument technique is based on absorption of ultra violet or infrared radiation, Gas chromatography or liquid chromatography. The Instrument technique can not analyze Ncl3 that has decomposed to Ammonium chloride & also not applicable for whole concentration range.

Chemical determinations of Ncl3 are based on the determinations of ammonium ions that are formed When the equilibrium is moved to the right. Chemical principle method is preferred as compared to the other methods.

The Chemical method is suitable for 0.2-20000mg/kgs Ncl3 in Cl2.

DETECTION

In this process Liquid Cl2 sample is taken in a refrigerated special steel trap (-60 C) . Max. sample capacity is 500ml.

Liquid Cl2 is evaporated in the presence of Hydrochloric acid & the gas is scrubbed by hydrochloric acid to form Ammonium chloride.

After removal of the dissolved Chlorine ,the hydrochloric acid solution is carefully neutralized & the Ammonium ion is determined either by Spectrophotometry of the Indophenols Complex or by an Ammonia gas sensing Electrode for level up to 200mg/kg Ncl3 Or by Distillation & titration for higher levels.

For lowest level of Ncl3 5mg/kg of Cl2, the Spectrophotometry method is preferred.

Results achieved by these methods are claimed to be reliable.

DETECTION NCl3 is assayed by the old Kjedahl Nitrogen test. It is

Time consuming (6.5 hours) Not very sensitive at low concentration. Is off-line process Can be used for grab samples only. This method of analysis is a three distinct process –

digestion, distillation titration moreover, it over-reports Nitrogen.

Chromatographic method Is rapid and precise. Time required for analysis is 20 minutes. Detector uses ultra-violet light. Has been certified by ASTM (E2036-99) and has been further modified to take

low concentration of sample for better results. This process is suitable for carbon tetrachloride system and require dilution 100:1 for safety reasons.

Depends on partial evaporation of Chlorine from chilled sample of liquid followed by addition of chromatographic elluent (methanol + dilute acetate buffer at PH-4) and injection into column.

METHODS OF CONTROLL

When Ammonia compounds are in brine despite preventive measures, destruction can be done by

controlled Chlorination at pH > 8.5 to form very volatile monochloroamine

allowing Nitrogen content to be removed by venting or surging with air.

NCL3 IN CL2 GAS IS CONTROLED BY ;

Irradiating the Chlorine gas exiting in the cells with ultra-violet light in the spectrum of 3600-4400 before the Chlorine enters the scrubbers ,

UV system is used to remove the small quantities of NCl3 formed.

METHODS OF CONTROLL

Due to thermal instability of NCl3, decomposition becomes useful at 70oC.Temperature in excess of this are found in Chlorine compressors. (Reciprocating and centrifugal)

Iron and its compounds can also cause decomposition. Destruction of NCl3 by scrubbing with cold HCl and contact with reducing

agents can be done. But this practice is not followed widely. Because of the possibility of combustion of Chlorine with Iron as well as higher

activity of copper alloys, the grease free Monel shaving would be preferred. Treatment of filtered dry gas

with activated carbon—this process is not in use. It was used in some plants, few years ago.

Silica and Alumina are other candidates. These may be impregnated to increase their reactivity.

This process is not in commercial use as it requires careful management due to possible exothermic reaction

METHODS OF CONTROLL NCl3 in liquid cl2 is Removed by passing through carbon tetrachloride solvent or equivalent as under :- Cl2 gas containing Ncl3 is passed through purifying tower containing liquid cl2. Ncl3 impurity in gas is washed out by liquid cl2 This liquid cl2 + Ncl3 is passed through purge drain tank containing carbon tetrachloride maintained at 50 c-60 c. Ncl3 impurity accumulates carbon tetrachloride & cl2 gas recycled through dryer system. Contaminated carbon tetrachloride is reacted with Naoh or suitable reducing agents. Waste Solvent is incinerated & caustic is neutralized appropriately. The Solvent carbon tetrachloride has to be replaced at scheduled interval & conc. Of C4 vapors to be minimized & recorded.

Flow sheet of the process is attached for reference.

NCl3 in liquid Cl2 can be controlled/destroyed by, Purging Purging NCl3 + liquid Chlorine from the vaporizer is a method of reducing concentration of NCl3. In this process NCl3 enriched liquid chlorine is bottled in Tonners / Cylinders in a scheduled way. Toleration in Cl2 tonners is

higher(20ppm) than storage tanks(2ppm). Moreover the tonners becomes empty frequently at the customer end which eliminate the possibility of NCl3 accumulation in the system. Purging and destruction process. Collection of NCl3 + liquid chlorine from the bottoms of chiller/evaporator gives another opportunity for thermal decomposition. Part of the Material is collected in a separate pot and decomposed by heating with steam upto 80

0C. Spill of NCl3 is a hazardous residue. Spilled NCl3 can detonate on simple walking through it. A spray of approximately 5% solution of reducing agents and washing with water will remove all traces of NCl3.

FLOW SHEETPROCESS FLOW DIAGRAM FOR THE CHLORINE HANDLING SYSTEM

H2SO4

DIRECT CONTACT COOLER

WET CL2

CELL GAS 90 - 95 0C

WET GAS DEMISTER

DRAWING TOWER

PURIFICATION LIQUID CHLORINE TOWER

FREON LIQUIFIER

90-95 0C

CL2 to H2SO4 Towers

NAOHDRY GAS DEMISTER

WASTE

INCINERATION

PURGE DRUM

LIQUID CL2

COMPRESSOR

DISPOSAL

15 TO 17 0C

25 TO 30 0C

CCL4

METHODS OF CONTROLL

NCl3 in liquid Cl2 can be controlled/destroyed by, Purging Purging NCl3 + liquid Chlorine from the vaporizer is a method of reducing

concentration of NCl3. In this process NCl3 enriched liquid chlorine is bottled in Tonners / Cylinders in a scheduled way. Toleration in Cl2 tonners is higher(20ppm) than storage tanks(2ppm). Moreover the tonners becomes empty frequently at the customer end which eliminate the possibility of NCl3 accumulation in the system.

Purging and destruction process. Collection of NCl3 + liquid chlorine from the bottoms of chiller/evaporator gives

another opportunity for thermal decomposition. Part of the Material is collected in a separate pot and decomposed by heating with steam upto 80 0C.

Spill of NCl3 is a hazardous residue. Spilled NCl3 can detonate on simple walking through it.

A spray of approximately 5% solution of reducing agents and washing with water will remove all traces of NCl3.

DECOMPOSITION OF NCl3 THERMAL

2NCl3 N2 + 3Cl2 (CHAIN REACTION)

NCl3 NCl2 + Cl, Cl + NCl3 NCl2 + Cl2

NCl2 + NCl3 N2 + 2Cl2 + Cl

VAPOUR PHASE - MAY LEAD TO EXPLOSION

LIQUID PHASE - HIGHLY UNSTABLE

SOLID PHASE - MAY DETONATE ON THAWING

SOLUTION NCL3 - STABLE 1.5 WT % < 30 O C

BEYOND INITIATION TEMPERATURE

SELF ACCELERATING – EXPLOSION

AT ~ 100OC. CATALYZED BY HEAT,

LIGHT, TRANSITION METAL OXIDES.

DECOMPOSITION OF NCl3

REDUCTION - BY A NUMBER OF COMPOUNDS

NCl3 + 4 HCl NH4Cl + 3Cl2

3Na2SO3 + NCL3 + 3H2O NH4Cl + 3Na2SO4 + 2HCl

RADIATION

2NCl3 N2 + 3Cl2

HEAT

U.V.

NCl3 SAFE OPERATION LIMITS

VESSEL CAPACITY

MAXIMUM CONCENTRATION OF NCL3 PPM W/W

CALCULATED RECOMMENDED

50 KG 200-300 20

1000 KG 50- 60 20

20 T 15 5

50 T 8 2

• 20 ppm w/w NCl3 is generally accepted maximum level for liquid Chlorine.

EXPERIENCES WITH NCl3 MANAGEMENT Different types of compressor being used for

compressing chlorine in chlor - alkali industries. Reciprocating Chlorine Compressor. Centrifugal chlorine Compressor. Liquid seal ring type Compressor with sulphuric acid as sealing

material. The management of Nitrogen tri chloride is well managed in

reciprocating and centrifugal compressor due to the achievement of decomposition temperature 90 0C .NCl3 is not controlled in liquid seal ring type compressors as the operating temperature range is 40 to 50 0C

Practices adopted to control NCl3 Concentration in liquid chlorine evaporator:- Purging system – part of the quantity at the bottom of the

evaporator is filled in tonners/cylinders at regular intervals Purging & decomposition of part quantity of NCl3+Liquid

Chlorine at regular intervals is collected in a separate pot and heated with steam at 80 0C/25 Kg/cm2.

The decomposed material is released to chlorine neutralization system.

EXPERIENCES WITH NCl3 MANAGEMENT Problems with liquid seal type compressors:

Some sulphuric acid mists are carried over to liquid chlorine evaporators & cause iron sulphate formation.

This problem becomes more serious with the break down of mist eliminator filter candle.

Under these conditions liquid chlorine + NCl3 drain line (used for transferring material) to tonners / cylinders gets chocked.

To continue the NCl3 monitoring process, liquid chlorine + NCl3 at the bottom of the evaporators is transferred in the main 100m3 storage tank through the evaporator liquid chlorine inlet line during plant stoppage.

No suitable online analyzer available for analysis of NCl3 or to check grab sample of NCl3 control.

Details of agency to carry out analysis of NCl3 is also not available.

Withdrawal and collection of NCl3 sample for analysis is also difficult.

By using other type of compressors, discharge temperature can go upto 900C (NCl3 decomposition point) against 40 0C with liquid seal ring type compressor. NCl3 decompositions is not appreciable with this type of compressor

EXPERIENCES WITH NCl3 MANAGEMENT In the second process

Part of the quantity of liquid chlorine +NCl3 mixture is decomposed at 800C in a steam heated jacketted catch pot.

The passage of liquid chlorine+NCl3 catch pot can be reason of explosion due to local hotspot.

The operation is done in gradual and cautious way. After decomposition chlorine is released to chlorine neutralizing

systemUnreliability factor of analyzer and non availability of analysis facility

makes the case serious.

In order to avoid a problem of NCl3 in the system all specifications of raw material and chemicals are analyzed in the laboratory on schedule basis. Possibility of online analyzers for brine is being explored to check nitrogen compounds in the brine system.

No new material is added without thorough analysis for nitrogenous matter.

NEED FOR DEVELOPMENT Need is felt for

Availability and installation of online analyzer for brine to check nitrogen compound

Development of foolproof system of collecting samples of NCl3

On line analysis at various NCl3 accumulating pockets in chlorine handling system.

To set up authorized agency for rechecking of on line analysis

Safe carriage of sample to external areas.

Development of a speedy and reliable test, coupled with more frequent monitoring,

A mechanism to call for repeat analysis,

More frequent checks after one analysis has exceeded the usual range.

CONCLUSION AND RECOMMENDATIONS

To establish practices & policies which will ensure the safety of the process.

IT IS RECOMMENDED THAT ALL PRODUCERS AND CONSUMERS SHOULD REVIEW THEIR SYSTEM AND OPERATION PERIODICALLY.

The process review should inspect/analyze the various NCl3 pockets in this process vessels.

In this process review, knowledge of the operating condition to: eliminate hazardous conditions, for example, keep the NCl3 concentration below 1.5% and

temperature below 30oC in NCl3 accumulating pockets.

CONCLUSION AND RECOMMENDATIONS

The most important control over the potential level of NCl3 is At source in Chlorine producing process.

Use of salt without anti-caking agent and mined salt without nitrogenous explosives.

Use of safer type of water. Whenever any raw material is changed a check should be made

to ensure that new NCl3 risk has not been introduced.

Frequency level of NCl3 analysis Should be once in 3 months when NCl3 is in 5-20 ppm.

Once in one year for NCl3 concentration below 5 ppm is suggested as guideline.

The Chlorine industry has gone several years with only one serious incident associated with Nitrogen

Tri-Chloride. We always need to work aggressively to prevent

NCl3 from accumulating and reaching dangerous levels.

THANK YOU