maximum levels of nitrogen trichloride in liquid chlorine

7/24/2019 Maximum Levels of Nitrogen Trichloride in Liquid Chlorine

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Maximum Levels of Nitrogen Trichloride in

Liquid Chlorine

GEST 76/55

12th

Edition

February 2012

EURO CHLOR PUBLICATION

This document can be obtained from:

EURO CHLOR - Avenue E. Van Nieuwenhuyse 4, Box 2 - B-1160 BRUSSELS

Telephone: 32-(0)2-676 72 65Telefax: 32-(0)2-676 72 41


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GEST 76/5512thEdition

February 2012 page 2 of 35

Euro Chlor

Euro Chlor is the European federation which represents the producers of chlorineand its primary derivatives.

Euro Chlor is working to:

improve awareness and understanding of the contribution that chlorine chemistryhas made to the thousands of products, which have improved our health, nutrition,standard of living and quality of life;

maintain open and timely dialogue with regulators, politicians, scientists, the

media and other interested stakeholders in the debate on chlorine; ensure our industry contributes actively to any public, regulatory or scientific

debate and provides balanced and objective science-based information to helpanswer questions about chlorine and its derivatives;

promote the best safety, health and environmental practices in the manufacture,handling and use of chlor-alkali products in order to assist our members inachieving continuous improvements (Responsible Care).

***********

This document has been produced by the members of Euro Chlor and should not be reproduced inwhole or in part without the prior written consent of Euro Chlor.

It is intended to give only guidelines and recommendations. The information is provided in goodfaith and was based on the best information available at the time of publication. The information is

to be relied upon at the users own risk. Euro Chlor and its members make no guarantee andassume no liability whatsoever for the use and the interpretation of or the reliance on any of the

information provided.

This document was originally prepared in English by our technical experts. For our membersconvenience, it may have been translated into other EU languages by translators / Euro Chlormembers. Although every effort was made to ensure that the translations were accurate, Euro

Chlor shall not be liable for any losses of accuracy or information due to the translation process.

Prior to 1990, Euro Chlors technical activities took place under the name BITC (BureauInternational Technique du Chlore). References to BITC documents may be assumed to be to Euro

Chlor documents.


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RESPONSIBLE CARE IN ACTION

Chlorine is essential in the chemical industry and consequently there is a need forchlorine to be produced, stored, transported and used. The chlorine industry hasco-operated over many years to ensure the well-being of its employees, localcommunities and the wider environment. This document is one in a series whichthe European producers, acting through Euro Chlor, have drawn up to promotecontinuous improvement in the general standards of health, safety and theenvironment associated with chlorine manufacture in the spirit of ResponsibleCare.

The voluntary recommendations, techniques and standards presented in thesedocuments are based on the experiences and best practices adopted by membercompanies of Euro Chlor at their date of issue. They can be taken into account infull or partly, whenever companies decide it individually, in the operation ofexisting processes and in the design of new installations. They are in no wayintended as a substitute for the relevant national or international regulations whichshould be fully complied with.

It has been assumed in the preparation of these publications that the users willensure that the contents are relevant to the application selected and are correctlyapplied by appropriately qualified and experienced people for whose guidancethey have been prepared. The contents are based on the most authoritative

information available at the time of writing and on good engineering, medical ortechnical practice but it is essential to take account of appropriate subsequentdevelopments or legislation. As a result, the text may be modified in the future toincorporate evolution of these and other factors.

This edition of the document has been drawn up by the Storage, Transport andSafety Working Group (GEST) to whom all suggestions concerning possiblerevision should be addressed through the offices of Euro Chlor.


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Summary of the Main Modifications in this version

Section Nature

All Reorganise the structure of the document with most sectionsrewritten to add clarifications

5.2 Added some potential sources of NCl3in chlorine

5.2 Added calculation of NCl3 concentration in chlorine from ammoniain brine and ferrocyanide in salt

7 Added information on managing potential NCl3 concentrationincrease

9.3 Added information on how to dilute NCl3when emptying a tank

Appendix 1 Correction of figure 1

Appendix 2 Calculations of examples corrected

Appendix 3 Added

TABLE OF CONTENTS

1.

INTRODUCTION 6

2.

OBJECTIVES OF THE RECOMMENDATION 6

3.

SAFE PRINCIPLES FOR CHLORINE CUSTOMERS 7

4.

EXAMPLES OF NITROGEN TRICHLORIDE INCIDENTS 7

5.

NITROGEN TRICHLORIDE 8

5.1. Properties of NCl3 8

5.2.

Sources of NCl3in chlorine 9

5.3. The Effect of Chlorine Liquefaction 10

5.4. The Effect of Chlorine Evaporation 10

6.

HAZARDOUS LEVELS OF NITROGEN TRICHLORIDE 10


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7.

CONTROLLING NITROGEN TRICHLORIDE LEVELS 11

7.1. General considerations 11

7.2. Raw materials and utilities 12

7.3. Chlorine Storage 12

7.4.

Mobile Containers 13

7.5. Vaporisers 13

7.6. Other Gas/Liquid Equipment 14

7.7. Process and Operational Changes 15

7.8. Emptying Equipment and Pipelines 15

7.9. Plant Design 15

8.

OPERATIONAL LEVELS OF NCL3IN CHLORINE 16

8.1. Maximum Level of NCl3inside any Equipment 16

8.2.

Maximum Levels of NCl3in Fixed Tanks 17

8.3.

Maximum Levels of NCl3in Small Mobile Containers 17

8.4. Maximum Levels of NCl3in Bulk Transport Containers 17

9.

REDUCING LEVELS OF NCL3IN CHLORINE 18

9.1. Reduction of NCl3Formation 18

9.2. Destruction of NCl3 18

9.2.1 For Gaseous Systems 18

9.2.2 For Liquid Systems 19

9.2.3 Alternative methods 19

9.3.

Blending of Chlorine 19

10.

OTHER SAFETY MEASURES 20

10.1. Nitrogen compounds Analysis in Brine 20

10.2. NCl3Analysis in Liquid Chlorine 20

10.3. Temperature Limits 21

11.

REFERENCES 21


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1. INTRODUCTION

Nitrogen trichloride (NCl3) is a trace compound normally found in chlorine. It iscreated during the manufacturing process. When proper precautions are taken

during manufacturing and use of chlorine, it presents no hazard. However, whenconcentrated it can become explosive and is one of the most dangerous materialsencountered in the chlor-alkali industry.

At high concentrations nitrogen trichloride will spontaneously decompose with adestructive force of 30-40% of TNT. It is believed that at a concentration of 3 - 6%NCl3 in chlorine, it can decompose spontaneously, resulting in a significantpressure rise in enclosed systems. At approximately 13% concentration inchlorine, the potential for catastrophic detonation exists.

Because of the hazard presented by nitrogen trichloride, there should always be asignificant safety margin between operating levels and the concentrations known

to be hazardous ideally all operations should be carried out withoutconcentrating the NCl3above the normal manufactured levels.

There are well established ways of operating that minimise the possibility ofconcentrating the nitrogen trichloride to hazardous levels. These ways ofoperating are strongly recommended by Euro Chlor and should always befollowed. Exceptions should be limited to very special circumstances that havebeen thoroughly assessed from both a technical and a risk perspective.

Nitrogen trichloride is only hazardous within liquid chlorine. It is not hazardouswhen present in in chlorine gas. Nevertheless chlorine gas that condensesdeliberately or inadvertently to become liquid chlorine can be hazardous (nitrogen

trichloride being les volatile than chlorine, it will tend to accumulate in the liquidphase).

2. OBJECTIVES OF THE RECOMMENDATION

The purpose of this recommendation is to highlight potential dangers withaccumulation of nitrogen trichloride (NCl3), which is a common impurity in liquidchlorine, and to describe the actions, strategies and constraints that should beobserved in order to avoid risks associated with its presence.

Any chlorine user who is uncertain of potential problems which can arise from

accumulation of NCl3 in a system, should consult their chlorine supplier forassistance in evaluating their specific risk.

A brief collation of information available on NCl3can also be found in the WorldChlorine Council Global Safety Team newsletter 12 of December 2009.


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3. SAFE PRINCIPLES FOR CHLORINE CUSTOMERS

Although nitrogen trichloride is very hazardous at high concentrations, it is benignat concentrations normally supplied by manufacturers, which is up to 20 mg/kg.Nitrogen trichloride only becomes hazardous if it is concentrated, which can be

deliberate or inadvertent.

This section is provided to give chlorine customers some simple rules of thumbthat will minimise their risk of accumulating nitrogen trichloride. It must bestressed that it is fully acceptable to operate in alternative ways under theguidance and expertise of a knowledgeable chlorine supplier.

Do not discharge chlorine gas from a vessel containing greater than1 tonne of liquid chlorine.

Do not continue adding liquid chlorine to a vessel from which chlorine gasis taken.

Use preferably plug flow or flash type vaporisers. (see section 7.5)

Any operation outside these key principles requires both a proper risk assessmentand discussions with your chlorine supplier.

Chlorine suppliers should have a deeper understanding of nitrogen trichloride andshould base their operation on the rest of this document rather than these simpleprinciples.

4. EXAMPLES OF NITROGEN TRICHLORIDE INCIDENTS

There have been a number of incidents in which a high concentration of nitrogentrichloride is believed to have been the cause and these are documented tovarying degrees in other publications; however the following short selection givesexamples of incidents from the past. The early incidents were generally due tolittle knowledge of nitrogen trichloride and its perils within chlorine. More recentincidents have generally happened where the actual level of nitrogen trichloride inparts of the system has been higher than believed, despite the science beingunderstood. The purpose of the following information is to demonstrate that thedangers of nitrogen trichloride are real and not merely a theoretical concept.

Early scientific incident:

Pierre Louis Dulong began experimenting with nitrogen trichloride in 1812 and asa result of explosions lost not only two fingers but also an eye. The hazards oftesting explosive concentrations of NCl3 have restricted such experimentalinvestigation to a minimum.

Early industrial incidents:

In 1928 two cylinders ruptured at the Ashokan Reservoir Treatment Plant in NewYork. Without any prior understanding of the hazards of nitrogen trichloride,chlorine had been produced with around 450 mg of NCl3per litre of chlorine and


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then discharged as a gas. This had increased the concentration in the remainingliquid to dangerous levels.

In 1949 at Croton Lake Gate House a chlorine drum was discharged completelyas gas. Forty minutes after complete discharge there was an explosion resulting inthe drum fracturing for almost its full circumference and a section 450mm x

275mm was blown out. The cause is generally believed to have been a highconcentration of nitrogen trichloride.

Recent industrial incidents:

In 1994 a chlorine purifier in Taft, USA exploded and debris from the explosionwas found over 60m away.

In 1998 in Yarraville, Australia there was an explosion in a flexible copperconnecting tube. During subsequent investigative work there was anotherexplosion in laboratory test equipment. Both were caused by nitrogen trichloride.

Overview

In many of the reported incidents the concentration of nitrogen trichloride wassignificantly greater than the current typical sales specification of 20 mg/kg.

Although these higher levels were below the usually accepted dangerousconcentrations of NCl3 by a suitable safety margin, the concentrations hadunknowingly increased resulting in the incidents.

5. NITROGEN TRICHLORIDE

5.1. Prop ert ies of NCl3

The physical properties of nitrogen trichloride are detailed in Appendix 1. Thefollowing is a summary of the key features of the chemical.

Nitrogen trichloride is a yellow, oil-like liquid.

Nitrogen trichloride is practically insoluble in water but highly soluble in chlorineand other materials such as benzene, chloromethanes and carbon disulphide.

Nitrogen trichloride has a boiling point of 71C (cf. chlorine at minus 34C) and afreezing point of minus 27C.

As nitrogen trichloride is less volatile than chlorine, the relative boiling point andvolatility mean that when chlorine containing NCl3is vaporised or evaporated, the

NCl3in the remaining liquid phase will increase.In pure form or at high concentrations in chlorine, NCl 3 is extremely unstable. Itcan decompose either spontaneously or with minimal (negligible) initiation energy.The decomposition can be either rapid or explosive detonation.

If nitrogen trichloride detonates, the destructive force is estimated as 30-40% thatof the explosive trinitrotoluene (TNT).

If nitrogen trichloride is maintained at the typical concentrations in produced liquidchlorine (no more than 20 mg/kg), there is no explosive risk.


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5.2. Sou rces of NCl3in chlor ine

NCl3can be formed during chlorine manufacture by reaction between chlorine andnitrogen compounds. Nitrogen compounds in the brine used for aqueouselectrolysis are the main source of NCl3.

Considering

0.82 = the molar weight ratio between N and NH3

B = kg /h brine flow per kg Cl2/h produced (usually about 13 in a recycledbrine circuit)

C = the conversion factor of nitrogen into NCl3in the cell (usually between40 to 60%)

8.6 = the molar weight ratio between NCl3and N

1 mg/kg of NH3 in the feed brine will give 0.82 * B * C * 8.6 mg NCll3 /kg Cl2produced or about 37 to 56 mg NCl3/kg chlorine produced with the figures here

above; should the nitrogen be completely transformed into NCl3 this would give92 mg NCl3/kg chlorine produced.

Rock salt and solution mined salt using surface waters will contain varying levelsof ammonium and nitrate salts.

When rock salt is mined using explosives, the explosive used will normallycontain nitrogen compounds.

Water used for solution mining can contain traces of ammonium basedfertilisers.

The use of vacuum salt in brine recycle circuits tend to give very low levels of

NCl3, however ferrocyanide anti-caking agents can introduce nitrogen to the salt.1 mg K4Fe(CN)6/kg salt completely transformed will give about 3 mg NCl3/kgchlorine.

Unless the equipment used to store or transport the salt raw material is dedicatedto handle salt only, the salt can be contaminated by residues of other nitrogencontaining chemicals that have been inside the same equipment, such asfertilisers.

Flocculent agents for decantation of impurities in the brine purification can be apotential risk of contamination.

One should also pay attention to chemicals, waters and steam condensates that

are recycled in the brine loop.

Leaks of cooling water into the brine can be a cause of contamination if the watercontains nitrogen compounds.

NCl3can also be formed from other nitrogen based impurities during manufacturein the electrolytic cell.

The concentration of NCl3 could change due to an adjustment in processconditions.


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It must be noted that not all nitrogen compounds are converted to nitrogentrichloride. Nitrogen gas introduced into the chlorine gas stream is not convertedto nitrogen trichloride under the normal operating conditions existing in currentchlor-alkali production facilities. Fully oxidized forms of nitrogen, such as nitratesalts (potassium nitrate or sodium nitrate), are not likely to form nitrogen trichloride

upon interaction with chlorine.As a consequence of one or more of the above mechanisms, nitrogen trichlorideis commonly found as a trace compound in liquid chlorine from most suppliers.

5.3. The Effect of Chlor ine Liqu efact ion

Nitrogen trichloride has a much higher condensing temperature (ie boiling point)than chlorine (71C vs minus 34C at atmospheric pressure). Therefore duringliquefaction of chlorine most of the NCl3present in the chlorine gas phase will becondensed into the liquid phase. Because all of the chlorine gas is not liquefied,the concentration of nitrogen trichloride in the liquid chlorine produced will be

greater than that in the original gas.If only a small proportion of the gas entering the liquefier is liquefied, theconcentration of NCl3in the liquid produced will be significantly higher than in thefeed gas. See section 9.1 about controlling this.

5.4. The Effect of Chlor ine Evaporat ion

The high difference in partial vapour pressure between chlorine and NCl3 alsotends to concentrate the nitrogen trichloride in the liquid phase during evaporationof chlorine.

During evaporation of chlorine, some NCl3 does evaporate although the amountdepends on the concentration of NCl3 and other possible compounds, thetemperature and the pressure.

6. HAZARDOUS LEVELS OF NITROGEN TRICHLORIDE

On the basis of experimental results, it is assessed that, on detonation, a mass of1.5 g of pure NCl3per cm of metallic surface wetted is capable of completelyfracturing the metal of a typical chlorine vessel and 0.3 g/cm2 is capable ofcausing cracks. This means that a small deep pool (eg in a sump) is morehazardous than the same quantity spread thinly over a large surface area.

Experimental results show that a concentration of NCl3in chlorine greater than 3%w/wat ambient temperature is capable of an accelerated decomposition, which isstrongly exothermic.

The levels of nitrogen trichloride that are hazardous are normally accepted asthose above 1% w/w(10,000 mg NCl3/kg chlorine).

Note: the above figures are NOT exact figures below which operation isguaranteed to be safe. The data for the dangerous concentration of NCl 3


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varies in the authoritative texts, typically between 1% and 13% w/w.Therefore the concentrations given above are those that are known to carrysignificant hazard. This needs to be considered in conjunction with thedifficulty of knowing and measuring the precise concentration of nitrogentrichloride at all points within a system. Also hazard assessments need to

consider the possibility of inadvertent concentration above the plannedfigures. Consequently operational procedures should always ensure thatthe concentration within any system is always maintained at a level that hasa large safety margin below the figures given above.

It must be stressed that the above hazardous levels are about 3 orders ofmagnitude higher than those found in chlorine produced by modernmanufacturers, which is typically no more than 20 mg NCl 3/kg chlorine. Thereforeunder normal operating conditions, every effort should be made to ensure thatNCl3levels do not increase above this limit unless fully risk assessed.

7. CONTROLLING NITROGEN TRICHLORIDE LEVELS

7.1. General consid erat ions

The low concentration of nitrogen trichloride found in chlorine produced by EuroChlor members (no more than 20 mg/kg) is totally benign. The principle hazardoccurs where the nitrogen trichloride is concentrated significantly above theselevels. Wherever possible this should be avoided.

Concentration of nitrogen trichloride will happen when the chlorine is vaporised ata higher rate than the nitrogen trichloride. The high boiling point of nitrogentrichloride (+71C) compared to that for chlorine (minus 34C) makes it possible

that NCl3could be concentrated in any circumstance involving the evaporation ofchlorine.

If the evaporation is part of a process in which chlorine is continually added to apool and evaporated, then unless appropriate designs and operational proceduresare used it can result in dangerously high levels of nitrogen trichlorideaccumulating in the liquid chlorine.

In a system vaporising chlorine, the NCl3may stabilise at a level that will dependon the particular design and operation. It is quite possible for this equilibrium levelto be unsafe and therefore the system must be designed and controlled to ensurethat it is safe. There are standard solutions for many types of equipment to

provide this safety.

Operational upsets and maintenance activities can also result in higher thanenvisaged levels of nitrogen trichloride caused by inadvertent or unexpectedevaporation of chlorine. An incident involving a large loss of containment ofchlorine could result in significant levels of NCl3remaining after evaporation of thespillage.


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7.2. Raw materials and uti l i t ies

An important factor in the potential level of NCl3 in chlorine is the raw materialsused.

Raw materials include not only the salt used, but also water, settling and filter

aids, drying acid, etc., any of which could affect the levels of NCl 3in chlorine.

Some changes of raw materials are obvious, such as changes of salt supplier.However others are less apparent, such as changes in the particular mine, movingto a new salt seam within a mining area or changing the source of dissolvingwater supply.

To ensure any changes are identified and that they cannot adversely affect thesafety of the chlorine, a regular check should be maintained on the total nitrogencompounds in the brine. Also the level of the nitrogen trichloride in the chlorineproduced should be measured on a regular basis.

Whenever any source of raw materials is changed, a series of specific checks

should be made to ensure that a new NCl3risk has not been introduced.

7.3. Chlo rine Storage

Chlorine bulk storage tanks should never be used to provide chlorine gas to auser process directly because this can quickly result in unacceptably high levels ofnitrogen trichloride in the remaining liquid. Where chlorine gas is required, thisshould either be taken from small transport containers (no more than one tonne)that are completely emptied before refilling or by drawing liquid chlorine from astorage tank and passing it through a vaporiser.

Normal operation of chlorine storage tanks sometimes involves slow orintermittent vaporisation of chlorine (in particular to control the pressure of thetank) and this can result in a steady increase of the concentration of NCl3 in theliquid phase. If the storage tank is continually topped up without removingsufficient liquid chlorine from it, the concentration of nitrogen trichloride willincrease. These activities need to be effectively controlled to ensure hazardousNCl3levels are not achieved. Where the NCl3levels are known or suspected to behigher than 20 mg/kg chlorine, care needs to be taken because:

The chlorine taken from the tank will have an elevated NCl3 concentrationlevel which may be higher than anticipated and could create a hazard indownstream equipment.

If the tank is subsequently emptied by a process involving evaporation, theNCl3concentration could be significantly higher than expected resulting ineither a safety margin smaller than estimated or dangerous conditions.

Where vapour is not routinely taken from the stock tank, care still needs to betaken during the emptying and shutdown procedures to avoid any accidentalconcentration of the nitrogen trichloride to hazardous levels.


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7.4. Mob ile Con tainers

Concentration of NCl3occurs when chlorine is discharged in the gaseous phasefrom a system containing liquid chlorine. This operation is not recommended,except for small containers which are fully vented before refilling: decades of

experience has shown that cylinders and drums containing up to 1 tonne ofchlorine with NCl3levels up to 20 mg/kg can be safely discharged this way.

Larger containers (in particular rail tanks, road tankers and ISO tank containers)should not be discharged in the gas phase. If the large container has to be fullyemptied, this should be done in liquid phase as far as technically possible (with amaximum of one tonne remaining in the liquid phase); the remaining chlorine canbe removed by complete evaporation.

Where exceptional circumstances require gas discharge from containers greaterthan 1 tonne, such as in an emergency, suitable precautions must be taken, toassess the specific equipment design and procedures and determine themaximum concentration of NCl3 that could be reached throughout the dischargeprocedure taking into account:

the initial concentration of NCl3in the chlorine.

the increase in concentration during the discharge.

the shape of the equipment and consequent maximum mass per unit areaof the NCl3.

Good communication between supplier and customer is essential to assesspossible problems due to NCl3accumulation.

7.5. Vaporisers

Vaporisers can be classed in different basic groups, according to their design (seeGEST 76/47 Design and operat ion of chlor ine vapor isers), but for our actualpurpose, vaporisers are split here into three categories.

Plug flow vaporisers (typified by a coil in bath, a double envelope or avertical tube vaporiser) where the chlorine passes through the vaporiserlinearly as a slug that is progressively boiled during its passage. There isno recirculation or remixing of the chlorine with fresh chlorine entering thevaporiser.

Constant volume vaporisers, typically with a heating device inside them,

like double envelope or kettle vaporisers. Chlorine liquid is added to thevessel where it is boiled with the vaporised gas taken away from thevessel.

Flash type vaporizers (like bayonet bundle) where chlorine is injected in anoverheated system that immediately flashes (evaporates) all incomingliquid. In this kind of vaporizers, there is no liquid chlorine phase toaccumulate NCl3.


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Constant volume vaporisers maintain a pool of liquid chlorine. In such vaporisers,liquid chlorine containing nitrogen trichloride is added to the pool and chlorine gaswith relatively little nitrogen trichloride is removed. As a result NCl3will accumulatein the liquid chlorine pool until equilibrium is reached. The equilibrium compositioncan be calculated from the relative volatility (vapour pressures) of NCl3 and Cl2

(see Appendix 1). In these calculations, the thermal destruction of the NCl3shallbe neglected. In the Appendix 1, the distribution of NCl3between gas and liquidphase is expressed as a concentration factor. The maximum level of NCl3accumulation will therefore be directly related to the vaporiser temperature andthe NCl3concentration in the Cl2 feed.

The vaporisation is generally carried out at temperatures at which some thermaldestruction of NCl3 will occur. The amount destroyed depends on parameterssuch as liquid chlorine hold up, temperature, residence time and the equilibriumgas-liquid composition.

The destruction of NCl3 (first order reaction) is expressed using a reaction rateconstant k, which is a function of the temperature. It is difficult to assess the exactlevel of destruction because of the differing temperatures involved (chlorine bulk,chlorine local, vessel exterior and heat exchange surface). It is therefore difficult topredict the actual NCl3 levels that will result in the vaporiser. Ignoring NCl3destruction in the vaporiser gives anextra operating safety margin. Nevertheless,careful checks must still be maintained on the NCl3levels in both the chlorine fedto such equipment and the residual liquid. Where the equipment or process issuch that dangerous concentrations could occur, liquid must be drawn offcontinually or periodically from the lowest point of the vaporiser - normally into apurge vaporiser or a separate tank as described below.

For constant volume vaporisers, a greater safety margin can usually be achieved

by installing a purge system that continually removes liquid chlorine from the poolto prevent NCl3 build-up. The continuous purge should have appropriatemonitoring to make sure it is functioning correctly. The purge flow is normallyvaporised completely in a relatively small dedicated plug flow vaporiser (typically asmall coil high temperature vaporiser in adequate material) to destroy the NCl3.

An alternative approach is for the purge flow to be returned to a storage tank thathas output to other destinations. This will not be suitable if the only output fromthe stock tank is back to the vaporiser. Sufficient dilution shall be confirmed byanalysis.

Particular care should be taken when removing constant volume vaporisers fromservice. It is essential to assess the effect this will have on nitrogen trichloride

concentrations, especially where the residual chlorine has to be vented down orvaporised.

7.6. Other Gas/Liqu id Equipm ent

Some chlorine gas treatment processes make use of a liquid chlorine directcontact pre-cooler and reboiler system which can lead to collection of highconcentrations of NCl3. Where the system operates at low temperatures, care


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must be taken to avoid a dangerous NCl3concentration being reached. It must bekept in mind that such a pre-cooler could contain more than one equilibrium stageleading to much higher NCl3concentrations in the pre-cooler bottom. In this case,a monitored purge is necessary to control the NCl3level.

7.7. Process and Operat ional Changes

Changes to the chlorine manufacturing process or operation can affect theamount of nitrogen trichloride produced. There are many possible changes thatcan affect NCl3 levels. They include new plant, upgrading plant or equipment,production rate changes and shutting down parts of a plant. Activities andoccurrences such as taking items of equipment out of service, recommissioning,flow of liquid into a relief stream, flow of liquid into a heated trap vessel andsampling can result in changes to NCl3 levels. These can not only affectnitrogen trichloride levels in the overall process, but may also result indisproportionately high levels of nitrogen trichloride in small parts of the system orinside items of equipment or pipework.

Such factors should be accommodated in the plant design, but if the level ofnitrogen trichloride in the plant is unexpectedly high, the design philosophy maybecome invalid. To control this, the level of the nitrogen trichloride in the plantchlorine should be measured on a regular basis.

Whenever the chlorine producing process is changed, specific checks should bemade to ensure that the effect on the concentration of NCl 3is assessed.

7.8. Emp tying Equipment and Pipel ines

When it is necessary to totally empty chlorine from a piece of equipment or

pipeline, the safest method is to remove all, or as much as possible, as liquid. Thisis typically achieved by using the equipments normal discharge route or bytransferring the liquid chlorine into a storage tank. If the chlorine in the equipmentis to be vaporised and already has increased levels of nitrogen trichloride, itshould be diluted by blending (adding and purging of liquid). The target should bethat before any final vaporisation begins, the liquid chlorine pool has no more than1000 kg and has a nitrogen trichloride concentration of no more than 20 mg/kg.

7.9. Plant Design

Nitrogen trichloride is most hazardous when there is a possibility of complete (or

almost complete) evaporation of significant quantities of chlorine. It is especially aproblem when the final residual pool is confined to a small volume. Thereforeplant design should always consider how to minimise this risk associated withnitrogen trichloride.

In particular the following should be considered:

How equipment will be emptied. Where possible it should be feasible tocompletely empty the equipment as liquid into the process or a receivingvessel.


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There should be no low points or sumps inside the equipment where smallquantities of liquid could accumulate.

Design of pipework should also consider the implications of low points thatcould act as sumps for a pipe system or other equipment.

Ensuring that there are operating instructions that state the correct way tostart up and shut down plants, as well as to empty equipment.

Note: Horizontal chlorine drums (up to 1000 kg) and upright chlorine cylinders(typically up to 100 kg) containing up to 20 mg/kg nitrogen trichloride are routinelyand safely discharged in the gas phase.

8. OPERATIONAL LEVELS OF NCl3IN CHLORINE

During normal production and use of chlorine the levels of nitrogen trichloride willbe several orders of magnitude below the levels at which it is hazardous.

Historically the maximum level for chlorine sold and transported has beenmaintained at a level not greater than 20 mg/kg (20 ppm w/w). At thisconcentration nitrogen trichloride does not present a hazard.

As discussed in previous chapters, there are some particular processes operatingwith a higher level of nitrogen trichloride (like chlorine vaporisers for example, seealso paragraph 8.1). In these circumstances special assessment mustbe madeto ensure that dangerous levels can never be approached.

The shutdown of equipment (for maintenance, etc.) carries a particular risk if thereis a need to evaporate significant quantities of chlorine during emptying of theequipment. Special assessment must be made to ensure that dangerous levels of

NCl3are never approached.

8.1. Maximum Level of NCl3inside any Equipment

The maximum admissible concentration of NCl3at each point in a liquid chlorineinstallation (production, storage, transport or use) should wherever possible belimited to 20 mg/kg. Where this is not achievable, detailed risk assessments mustbe carried out to ensure there are significant safety margins which prevent theconcentrations from approaching the known dangerous levels - which are a massof 0.3 g of pure NCl3 per cm of wetted metallic surface and an overallconcentration of 1% w/w. The assessment should consider both normal and

abnormal operations.The safety margin should be as great as possible and under no circumstancesshould operation be carried out in a way that could lead to concentrations ofgreater than 1% w/w. When operating at levels approaching this, theconcentration of NCl3should be monitored frequently. Where such monitoring isnot possible this limit should be reduced to 0.1% w/w or 1000 mg/kg.


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Furthermore when assessing the risk of operating at higher levels of nitrogentrichloride, consideration should be given to:

The risk of unknowingly exceeding any safety margin a significant factorin many historical incidents.

The accuracy of NCl3 concentration measurements for the method ofsampling and analysis used and the effect of the possible differencebetween actual and measured levels on safety margins.

The measurement frequency and the possible variation with time ofnitrogen trichloride levels in the chlorine.

8.2. Maximum Levels of NCl3in Fixed Tank s

Nitrogen trichloride is normally controlled during chlorine production before theliquefaction stage. Therefore fixed tanks will normally contain chlorine that hasnitrogen trichloride at levels below 20 mg/kg. This would normally only be

exceeded by a process upset or by evaporative procedures (see point 7.5).Chlorine should not deliberately be stored with nitrogen trichloride at a levelgreater than the 20 mg/kg. During any emptying procedures where the residualchlorine will be removed by evaporation, the concentration of NCl3should not begreater than 20 mg/kg when there is 1 tonne liquid chlorine remaining in the tank.

8.3. Maximum Levels of NCl3in Smal l Mobi le Containers

Chlorine is commonly taken in gaseous form from cylinders and drums containingup to one tonne capacity. Experience shows that the standard NCl3 limit of20 mg/kg in these small containers does not result in accumulation of dangerous

quantities of NCl3 in the later stage of emptying. Therefore small mobilecontainers should not be filled with chlorine containing more than 20 mg/kg NCl3.

8.4. Maximum Levels of NCl3in B ulk Transpor t Containers

It is strongly recommended that all rail tanks, road tankers and ISO tankcontainers of liquid chlorine are emptied by discharge in the liquid phase directlyinto the customersstorage, as recommended by Euro Chlor. The recommendedlimit for NCl3in these circumstances is 20 mg/kg.

In some emergencies, transport tanks may need to discharge chlorine gas. Inthese circumstances, or in any case where there is a need to discharge chlorine inthe gas phase, a nitrogen trichloride limit should be set based upon a specificanalysis of the situation. Careful assessment of the risk is required if NCl3 levelscould exceed 20 mg/kg chlorine and, if the operation is performed out side thechlorine producers premises, he will be involved in this risk assessment.


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9. REDUCING LEVELS OF NCl3IN CHLORINE

The nitrogen trichloride in manufactured chlorine is normally controlled to beno more than 20 mg/kg using one or more of the following methods. Themethods selected will depend upon the practicality and cost effectiveness for

the particular manufacturing operation.

9.1. Reduc tion of NCl3Format ion

Nitrogen compounds present in salt, water etc. used to make the feed brineshould be reduced as much as practicable.

Chlorination of the brine at a pH higher than 8.5 or treatment with sodiumhypochlorite is capable of destroying a large proportion of ammoniumcompounds, producing hydrochloric acid and nitrogen, which can theneasily be removed by air scrubbing the brine.

In situations where a small proportion of the manufactured chlorine gasstream is required as liquid, there are two possible ways to achieve this:

a) Pass all of the chlorine gas through a liquefier and liquefy a smallproportion of it.

b) Pass a small proportion of the gas stream through the liquefierand liquefy most of it. The remainder of the gas does not passthrough a liquefier.

The liquid produced by method (b) will contain less NCl3 than (a). This isbecause the NCl3is liquefied preferentially in method (a).

9.2. Destru ctio n of NCl3

9.2.1 For Gaseous Systems

Thermal decomposition of nitrogen trichloride can occur above 40-50C, but theextent of decomposition will depend on the residence time. Centrifugal and dryvolumetric chlorine compressors contribute to the reduction of NCl3 content inchlorine gas.

Ultra violet lights in the range 36004800 will decompose NCl3in gas streams.This can be done in units similar to those sometimes used to reduce the hydrogencontent of chlorine gas.

Activated carbon and metals, particularly copper base alloys at temperatures of80-100C, can also promote the decomposition of NCl3. Great care must be takento avoid the risk of fire due to overheating as both NCl3destruction and residualwater absorption on these catalysts are exothermic and iron will burn in chlorine atelevated temperatures.

Remark: fresh activated carbon must be chlorinated slowly, starting with highlydiluted chlorine (with nitrogen for example), to avoid excessive temperatureincrease.


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Chlorine gas can be scrubbed with liquid chlorine or carbon tetrachloride to extractthe NCl3; the liquid is then maintained at 50-70C for progressive destruction ofNCl3as described in the following section.

9.2.2 For Liquid Systems

If operating at temperatures of 50-70C (such as in vaporisers, distillation units),some of the NCl3will be destroyed. However the evaporation of chlorine may stillresult in higher levels of NCl3than are acceptable and in these circumstances anadditional system to reduce the NCl3is required.

Where there is accumulation of nitrogen trichloride in equipment such as avaporiser, removing periodically or continuously a small proportion of the chlorineas liquid can control the maximum NCl3level. The NCl3in this side stream is thendestroyed separately and this is often referred to as a purge system.

There are several options for NCl3destruction:

Vaporisation of liquid chlorine containing NCl3at high temperature in a plug

flow vaporiser system. Dissolving the NCl3 in a solvent such as carbon tetrachloride. The solvent

is then maintained at a temperature of 50-70C for a period long enough toallow the decomposition of NCl3 (See TSEM 01/274 NitrogenTr ichlor ide: a Cont inuo us Chal lenge). NCl3 within carbon tetrachloridecan also be destroyed by combustion of the mixture.

An indication of the thermal decomposition rate constant of NCl3is given in Figure3 of Appendix 2.

Heating equipment containing liquid chlorine above 80C should be avoided if itcontains very high levels of NCl3 because the fast exothermic reaction could

initiate explosive decomposition.

9.2.3 Alternative methods

NCl3 can be destroyed by reaction in a number of chemical processes, e.g.absorption of chlorine containing NCl3in caustic soda.

9.3. Blendin g of Chlor ine

Where the concentration of NCl3 in liquid chlorine is too high, the NCl3concentration can be reduced by the addition of liquid chlorine containing lowerlevels of NCl3. This will have the effect of reducing the NCl3 concentration but it

will increase the mass of NCl3. The equipment will then contain more NCl3although it will be at a lower concentration. Consequently if this liquid chlorinewere then evaporated back to the original volume, the NCl3concentration wouldbe even higher. Therefore this measure is only appropriate where it then enablesthe diluted liquid to be removed / drained and subsequently handled safelyelsewhere.

If necessary the process of blending and draining can be carried out repeatedlyuntil the concentration of NCl3 in the equipment is low enough to be safely


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evaporated to dryness. The target should be that before final vaporisation todryness begins there is a pool of chlorine that is no more than 1000 kg and has anitrogen trichloride concentration of no more than 20 mg/kg.

10. OTHER SAFETY MEASURES

10.1. Ni trogen compounds An alys is in Br ine

Periodic analysis of nitrogen compounds in the brine allows the consequentvariations of NCl3 in liquid chlorine to be determined. The acceptable limits fornitrogen compounds in the brine should take into account the local circumstancessuch as electrolysis technology, the level of destruction in the gaseous chlorine,etc.

The appropriate analysis frequency and maximum acceptable concentrationshould be determined by experience taking into consideration the extent of

variation in the results obtained.In case of change in water supply, raw materials and chemicals quality, theanalysis frequencies should be temporarily increased and the schedule adapted ifnecessary.

On-line analysers have been developed, allowing continuous monitoring of thenitrogen compounds in brine.

10.2. NCl3Analys is in Liqu id Chlor ine

Analysis of NCl3concentration is usually made on the liquid chlorine, but can also

be performed on the dry compressed gas (to give predictive information) and isrecommended on the gas flow from any NCl3 destruction unit to monitor itsperformances.

The analysis of NCl3 in liquid chlorine requires special analytical equipment asdescribed in the Euro Chlor publication Analyt ical 2 Determinat ion ofNi trogen Tr ichlor ide in Liqu id Chlor ine. The analysis of low concentration ofNCl3 is the most difficult and care must be taken to avoid contamination of thesamples with other nitrogen compounds like ammonia. These analyses areusually performed by the chlorine producers, but can be appropriate for someconsumers (for example if a constant volume vaporisation unit is used).

Regular checks at an appropriate frequency should be carried out to assess theNCl3levels in the system.

The appropriate analysis frequency should be determined by experience takinginto consideration the level of NCl3, the particular equipment, operating regimeand the extent of variation in the results obtained.

The following frequencies are suggested as a guide:

1. Where the peak NCl3level is between 0.1 and 1%, analytical check shouldbe carried out at least once per week.


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2. Where the NCl3 level is normally between 20 mg/kg and 1000 mg/kg(0.1%), analytical checks should be carried out at least once per month.

3. Where the NCl3 level is normally between 5 mg/kg and 20 mg/kg, theanalytical frequency should be at least once per 3 months on liquefactionoutput or on the chlorine storage.

4. Where the NCl3 level is normally below 5 mg/kg, the frequency of testingshould be at least once per year.

During periods of commissioning, maintenance, process modification, processfluctuations, shutting down and changes of raw material quality there can be a riskof higher NCl3levels and the frequency should be increased.

On-line analysers have been developed, allowing continuous monitoring of theNCl3concentration in chlorine and detect possible variations (see TSEM 05/299 -On-Line Determinat ion of Ni trogen Tr ichlor ide In Solvay Chlor ine Product io n

Uni ts).

10.3. Temperatu re Lim its

Vaporisers should be designed to operate with low pressure steam at 120Cmaximum (temperature to avoid chlorine-iron reaction) or using a heat transferfluid between steam and chlorine in which the temperature can be limited to 80-90C, ensuring that the liquid chlorine containing NCl3does not exceed 80C toprevent rapid decomposition of NCl3.

Dedicated small coil purge vaporisers are used industrially to destroy NCl3 bycompletely vaporising the plug flow. Temperature limits are set to achieve acontrolled decomposition rate, while at the same time ensuring that localoverheating of metal surfaces is avoided (for indication of the NCl

3decomposition

rate constant, see Appendix 1). Chlorine can be brought to higher temperatureusing high nickel alloy vaporisers and adequate gaskets (up to 300C, dependingof the type of alloy), but precautions must be taken to cool the evaporated chlorinebefore sending it into any steel equipment.

Because increasing temperature also increases the explosive decomposition rateof NCl3, care should be taken to control the temperature of equipment handlingliquid chlorine containing high levels of NCl3(0.1 - 1%). In this case, overheatingof chlorine should be avoided and direct electrical heating systems should not beused.

11. REFERENCES

GEST 73/17 Low Pressure Storage of Liquid Chlor ine

GEST 75/47 Design and operat ion of chlor ine vapor isers

TSEM 01/274 - Nitrogen Trichlor ide: a Continuing Challenge (H.Piersma)


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TSEM 05/299 - On-Line Determinatio n o f Nitrog en Trichlo r ide In SolvayChlor ine Produ ct ion Uni ts (N. Breton)

Analyt ical 2 Determinat ion o f Ni trogen Tr ichlor ide in Liqu id Chlor ine

World Chlorine Council Global Safety Team newsletter 12 of December 2009


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APPENDIX 1: PHYSICAL AND CHEMICAL PROPERTIES OF NITROGENTRICHLORIDE

Some physical properties are:

Molecular weight 120.38

Melting point 246 K (at 101.3 kPa) = minus 27CBoiling point 344 K (at 101.3 kPa) = + 71C

Specific gravity (liquid) 1640 kg/m

Heat of vaporisation 31.07 kJ/mole

Vapour pressure can be calculated with the DIPPR databank (see graph 1 below)

with P = vapour pressure NCl3(Pa)

T = absolute temperature (K)

A = 131.24 B = -8010 C = -16.91 D = 1.9677 10-5

Graph 1: Calculated pressure vapour of NCl3as a function of temperature


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Experimental data (The Electrochemical Society Proceedings, May 19781) showvalues somewhat higher than the prediction for temperatures below 50C, andanother study

2determined that the activity coefficient of diluted NCl3in chlorine (to

correct the molar concentration) was about 13.4.

Nitrogen trichloride is an oil-like liquid, with a pungent odour; it has a low solubility

in water (2 g/kg) but is miscible in all proportions with liquid chlorine and otherchlorine compounds, such as carbon tetrachloride, chloroform, carbon disulphideand benzene.

Nitrogen trichloride decomposes exothermically, and in the concentrated form, theliquid behaves as a sensitive explosive, capable of rapid deflagration ordetonation. The explosion can be triggered by temperature (> 93C), shock andlight. Additionally, the explosion of NCl3 can also result from the contact withsubstances like ozone, phosphorous, arsenic, alkali and organic matters.

The explosive nature of the material when present in liquid chlorine (and similarmaterials) is dependent on both its concentration and on the total amount present.

Pure liquid NCl3 has been calculated as capable of generating a detonationpressure of 5.5 to 7.5 x 10 atm. At very high concentrations in liquid chlorine itcould generate an energy release equivalent to 30-40% of the explosive force ofTNT. Concentrations > 35% by weight are readily capable of detonation by shock,high temperature or ultra-violet light. 13% NCl3 appears to be the limitingconcentration to achieve detonation. Even at low concentrations, NCl3will slowlydecompose; this decomposition rate increasing with temperature or in thepresence of metals such as copper or high nickel alloys. Ultra violet light can beused to decompose NCl3safely, when it exists as a minor component in the gasphase.

As the concern over the explosive behaviour of NCl3is in the context of mixtures

with liquid chlorine, the important factor is its explosive force when containedwithin a typical chlorine pressure vessel with a wall thickness of 10-12 mm.Private reports indicate that detonation of 1.5 g NCl3(as 100%) /cm within a liquidfilm would be capable of fracturing the metal, and 0.3 g/cm of surface area iscapable of overstressing the metal to the point of cracking or fissurisation. Thecalculation of the potential explosive capability of an NCl3 / Cl2 mixture within avessel is only an approximation. As the chlorine evaporates, the concentration ofNCl3will progressively increase, but if the residual pool covers a large area, or theNCl3 concentration is still < 13%, the explosive potential remains small. On theother hand, if the residual liquid is collected in a sump or bottom drain connection,the critical mass sufficient to cause total metal failure may be reached.

1Argade S.D., Balko E.N., Kramer D.A. anf J.F. Louvar Nitrogen TrichlorideControl in Chlorine Manufacture, Industrial Electrolytic Division, TheElectrochemical Society Proceedings, May 22, 19782 Zeller, R.L., Non-ideal solution behaviour of NCl3 in liquid chlorine, Annual

meeting of the Chlorine Institute Nitrogen Trichloride workshop, March 23, 2000.


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Experimental work has also shown that chemical decomposition of NCl3 in liquidchlorine and at > 3% is rapid and strongly exothermic.

The above summarises the properties of NCl3 in the presence of the much morevolatile liquid chlorine. In the presence of higher boiling impurities, which act asinert diluents and are also found in commercial chlorine (such as bromine, C1 or

C2 chlorinated carbon compounds), the hazards from NCl3 accumulating in theresidues from evaporation are reduced by dilution. However care must be taken toensure that other reactive materials are not formed that could increase theexplosive hazard.

The maximum level of NCl3 that can accumulate in the liquid phase of acontinuous chlorine vaporiser can be calculated with the vapour pressure data forNCl3 and chlorine. Assuming a perfect gas, this distribution - or concentration

factor (333

/ NClNClNCl yxm ) is calculated as follows with the abbreviations meaning:

pp = partial pressure

vp = vapour pressurePt = total pressure

y = mole fraction in gaseous phase

x = mole fraction in liquid phase

Dalton:33

* NClNCl yPtpp and 22 * ClCl yPtpp

Raoult:333

* NClNClNCl xvppp and 222 * ClClCl xvppp

The concentration factor3NCl

m can be calculated from the Dalton and Raoult

equations:

33

* NClNCl yPtpp =33

3

333

*

NClNCl

NCl

NClNClNClvp

Pt

y

xmxvp

As NCl3 concentration is very small compared to chlorine concentration, we have:

12

Cly , so Dalton equation gives: Pt 2Clpp

and 12

Clx , so the Raoult equation gives 22 ClCl vppp

The concentration factor becomes:

33

3

3

NClNCl

NCl

NClvp

Pt

y

xm

3

2

NCl

Cl

vp

pp =

3

2

NCl

Cl

vp

vp

The values of distribution or concentration factor as function of the temperatureare shown in the Figure 1 below.


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Ideal theoretical solution (vapour pressure from DIPPR data bank)

Experimental values3 (concentration NCl3 in liquid hold-up related toconcentration in feed, supposing equal to the gas phase one, i.e. no NCl3destruction in vaporiser)

Non ideal theoretical solution4

Figure 1: Distribution of NCl3 in equilibrium between liquid chlorine and

gaseous chlorine expressed by the concentration factor3NCl

m

3Chlorine Institute Pamphlet 21, Nitrogen Trichloride - A collection of reports andpapers, Edition 5, October 2002, page 187.4 Zeller R.L., Non-ideal solution behaviour of NCl3 in liquid chlorine, Annual

meeting of the Chlorine Institute Nitrogen Trichloride workshop, March 23, 2000.


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APPENDIX 2: THERMAL DESTRUCTION OF NCl3 IN A CHLORINEVAPORISER

Fkg Cl2/h

bmg NCl3/kg

F kg Cl2/h

amg NCl3/kg

V kg Cl2

c mg NCl3/kg

Figure 2

Mass balance of a vaporiser

In the vaporiser two phenomena take place,

1) Thermal destruction of NCl3

The destruction is a 1storder reaction (mg/kg/h) and can be described with the

reaction velocity : R = k . c [eq 1]

The destruction rate in a volume V is: G = R . V = k . c . V [eq 2]

expressed in mg NCl3/h

2) Equilibrium NCl3concentration between the liquid and gaseous Cl2phase

The equilibrium is defined as the concentration factorb

cm

NCl

3

[eq 3]

Calculation

The NCl3mass balance for the vaporiser is: F . aF . b = G [eq 4]

Substitution of [eq 2] in [eq 4] gives: F . aF . b = k . c . V

Rearrangement results in: a - b = k . c . (V/F) [eq 5]

Introduction of the equilibrium relation [eq 3] in [eq 5] gives:

a - b = k . b .3NCl

m . (V/F) [eq 6]

with (V/F) = the residence time in the vaporiser,

[eq 7]


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Values for k (reaction velocity constant) are given in Figure 3 here below.

reaction rate constant k

-200

-175

-150

-125

-100

-75

-50

-25

0

25

50

75

100

30 40 50 60 70 80 90 100 110 120

TemperatureoC

100

*

log

k

Figure 3: Reaction rate constant k in h-1 for the decomposition of NCl3 inliquid chlorine as a function of temperature

Examples are shown for the two following cases: a vertical shell and tubevaporiser and a kettle type vaporiser.

Example 1: vertical shell and tube vaporiser

The essential characteristics of this vaporiser are:

shell diameter D = 310 mm

tube diameter d = 30 mmwetted tube length L = 1400 mm

number of tubes n = 46

Liquid volume = /4 . L . (D2n . d2) = 0.06 m3

With density2Cl

= 1500 kg/m3V = 90 kg.

If the capacity of the vaporiser F= 3000 kg/h(V/F) = = 0.03 h.


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If the vaporiser is operating at an equilibrium liquid temperature of 50C, the wall

temperature of the tubes can be about 90C but the residence time at this

temperature is much shorter than (V/F) = .

Considering the average vaporiser temperature (50C) and neglecting thedestruction at the wall temperature, we obtain log k = -1.8 k = 0.0158 h

-1from

Figure 3 here above.

From Figure 1 (see Appendix 1, experimental curve)3NCl

m = about 26

With a = 10 mg/kg, b is calculated from [eq 7]:

b = 9.88 mg/kg

and c iscalculated from[eq 3]:

c = 26 . b = 257 mg/kg

The destructed amount is calculated from [eq 4]:

G = 365 mg/h.The supplied amount of NCl3is (F . a) = 3000 . 10 = 30000 mg/h

This means that only (365 / 30000) . 100% = 1.2% is destructed in the vaporiser.

If the wall temperature of the tubes is also considered for a part of the NCl 3

destruction (at 90C, log k= 0.6 and k = 4 h-1) and the corresponding volume /local residence time is hypothetically assumed to be 5% of the global volume /

residence time (* = 0.05 . (V/F)), then the total destructed amount of NCl3,

relative to the supplied NCl3, becomes 14 % (with3NCl

m being about 20).

The results of this calculation will of course vary according to the hypothesisconsidered.

Example 2: kettle type vaporiser

This kind of vaporiser (see Figure 4) contains more liquid hold up V, resulting in a

residence time = (V/F) higher than for a vertical shell/tube vaporiser.

F kg Cl2 /hb mg NCl3 /kg Cl2

c mg NCl3 /kgF kg Cl2/ha mg NCl3/kg

Figure 4 - Kettle type vaporiser


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Dimensions of the kettle type vaporiser:

shell diameter D = 800 mm

shell length Ls = 1600 mm

tube diameter d = 30 mm

tube length Lt = 1400 mm

number of tubes n = 46

Liquid chlorine hold up, assuming the shell half filled with liquid (and all tubes arein the liquid phase):

liquid volume V = /4 . (0.5 . D2 . Lsn . d2 . Lt)V = 0.356 m3.

With density Cl2 = 1500 kg/m3 V = 535 kg.

The capacity of the vaporiser is F=3000 kg/h and = (V/F) = 0.178 h.

We suppose that the vaporiser is operating at an equilibrium temperature of 50C.

From Figure 3 here above log k = -1.8 k = 0.0158 h-1

From Figure 1 (see Appendix 1)3NCl

m = 26

With a = 10 mg/kg, b is calculated from [eq 7]:

b = 9.3 mg/kg

and c iscalculated from[eq 3] :

c = 26 b = 242mg/kg

The destructed amount is calculated from [eq 4]:

G = 2054 mg /h

The supplied amount of NCl3is F . a = 3000 . 10 = 30000 mg/h

This means that (2054 / 30000) . 100% = 7% is destructed in the vaporiser.

If the wall temperature of the tubes is also considered for a part of the NCl 3

destruction (90C k = 4 h-1) and the corresponding volume / residence time is

hypothetically assumedto be also 5% of the global volume / residence time (* =0.05 . (V/F)), than the total destructed amount of NCl3relative to the supplied NCl3becomes 49%.

The results of this calculation will of course vary according to the hypothesisconsidered.

Care must also be taken here as there is in this case a much higher volume athigh temperature than for shell and tubes and the quantity of NCl3 destroyed ishighly dependent of this hypothesis.


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Conclusion

Some thermal destruction of NCl3occurs in the vaporiser. To be at the safe side itis recommended to calculate the destruction based on the bulk liquid chlorinetemperature (temperature in equilibrium with the operating pressure) rather thanthe wall temperature because of the uncertainty of the real residence time at that

temperature..

Literature

Chlorine Institute Members Information Report Pamphlet 21 "NitrogenTrichloride - A Collection of Reports and Papers" Edition 5, 2002.

Zeller, R.L., Non-ideal solution behaviour of NCl3 in liquid chlorine, Annualmeeting of the Chlorine Institute Nitrogen Trichloride workshop, March 23,2000

Mellor, Inorganic and Theoretical Chemistry, Volume VIII, pp 598 - 604.

T. Dokter - Formation of NCl3 and N2O in the reaction of NaOCl and

nitrogen compounds. Journal of Hazardous Materials 12 (1985) pp. 207,224.

Physical properties are derived from the GMELIN and DIPPR databanks


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APPENDIX 3: CALCULATION OF THE MASS / WETTED AREA RATIO OF ARESIDUAL POOL OF NITROGEN TRICHLORIDE IN A HORIZONTALCYLINDRICAL TANK

This calculation makes the following approximations:

Only the cylindrical section is considered (not the dished ends). This shouldprovide a conservative assessment.

The size of the pool is small - reasonable considering large pools of NCl3are too hazardous

The vessel is absolutely horizontal - this is a potentially significant errorbecause the pool depth will be so small (see below for how the depth iscalculated). Inclinations of the vessel that would be totally negligible from aprocess perspective may be large when compared to the pool size.

Where:

M = mass of nitrogen trichloride (g)

SG = specific gravity of fluid (= 1.64 for nitrogen trichloride)

V = volume of the pool (cm3)

A = cross sectional area of the pool (cm2)

D = vessel diameter (cm)L = length of vessel cylindrical section (cm)

H = depth of pool (mm)

Q = angle the pool subtends at the centreline of the vessel (radians)

W = wetted area of metal (cm2)

R = ratio of mass to wetted area (g / cm2)


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Calculate the volume and cross sectional area of the pool

Calculate the area of the sector subtending angle Q

Calculate the area of the triangle above the pool area

Calculate the cross sectional area of the pool (sector minus triangle)

As Taylor series for small angle gives

The cross sectional of the pool can be approximated as

Calculate the angle in radians the pool subtends at the centreline of the vessel


34/35



Calculate the depth of the pool in mm

Calculate the wetted area of metal

Calculate the ratio of mass to wetted area


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Industrial consumers of chlorine, engineering and equipment supply companiesworld-wide and chlorine producers outside Europe may establish a permanent

relationship with Euro Chlor by becoming Associate Members or TechnicalCorrespondents.

Details of membership categories and fees are available from:

Euro Chlor

Avenue E Van Nieuwenhuyse 4, Box 2

B-1160 Brussels

Belgium

Tel: +32 2 676 7211Fax: +32 2 676 7241

e-mail: [email protected]

Internet: http://www.eurochlor.org
http://www.eurochlor.org/http://www.eurochlor.org/http://www.eurochlor.org/

maximum levels of nitrogen trichloride in liquid chlorine

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