Document MOpoR8LN2Q8OogDY3MJxyae9

a i MCUSII VERSION Vernal T\.* CF.ST 76/55-8 RECOMMENDATION CONCERNING THE laAs'aa^ii itkocehat ioiKplifaig in LIQUID CHLORINE EIGHTH EDITION - NOVEMBER 1983 This document can be obtained from BIT Chlorine, 250 Avenue Louise, BP 72, 1050 Brussels 00 07?9?<S CONFTDFNTTAl PHEFACE The production or chlorine has expanded rapidly ih recent years and larger and larger quantities are stored, used and transported. The Industry has a very good safety record, and any incidents which have occurred havejin general led to conscaucnces of relatively little importance. Nevertheless, ISSiiti&tyh'WMa The recommendations proposed in this code are based on the various measures taken by member companies of the BITC. They constitute a base line above which one is free to choose. They in no way are intended as a substitute f r the relevant national or international regulations, which should be respected in an integral manner. They result from the understanding and experience of the chlorine producers in their respective countries at the date of lbsue of this particular document. Established in good faith, they should not be used as a comprehensive working document, but as a guide which should, in each particular case, be adapted and utilised in consultation with a chlorine producer. Any potential user, therefore, should address their enquiries to a chlorine manufacturer before undertaking detailed implementation of the guidelines laid down in this code. This text may be modified in the future to take into account evolution of the technology and general technical progress. This edition of the document has been drawn up by a Working Group "Storage Transport and Safety", to whom all suggestions concerning possible modifications should be addressed through the offices of the BIT Chlorine. It should not be reproduced in whole or in part without the authorisation of the BITC. 0 077927 confidential f INDEX l 1 OBJECTIVES OF THE RECOMMENDATION 2 AREAS OF POTENTIAL CONCERN 2.1 Chlorine Liquiflers 2.2 Chlorine Storage 2.3 Mobile Containers 2.H Vaporisers 2.5 Other Situations - 3 3.1 3.2 3*3 3-3.1 3.3*2 3.3.3 LIMITING LEVELS OF NC13 IN CHLORINE Basic Principles Maximum Level Inside Any Equipment Maximum Levels Introduced Into Fixed and Mobile Tanks Chlorine Storage Small Mobile Containers Bulk Transport Containers 4 SAFETY MEASURES *.1 Principles. H,2 Frequency of Testing .3 Temperature Limits R.R Construction Details Tor Vaporisers and Rebollers 5. ANALYSIS OF NITROGEN TRICHLORIDE REFERENCES * APPENDIX 1 - PHYSICAL AND CHEMICAL PROPERTIES OF NIGROGEN TRICHLORIDE APPENDIX 2 - SOURCES AND DECOMPOSITION OF NITROGEN TRICHLORIDE *V DO 07?928 CONFIDENTIAL RECOMMENDATION CONCERNING THE MAXIMUM LEVELS OF NITROGEN TRICHLORIDE I LIQUID CHLORINE I OBJECTIVES OF THE RECOMMENDATION All flvatcas ror holding Hould chorine. tJj^jjjj^raui^possibl^grgggjy^jRj^h^gai^^e^jeneratcd^rro^th^Hguld or gashas?^T^he||s^s tern. Thlsprcssurewffr?Tr^generan"^fTs^^ froraTKevapour pressure of chlorine or from the gas pressure of an inert gas .additive. Howevt the chlorine system and other reactive materials (H-, hydrocarbons etc). The purpose of this recommendation Is to highlight the potential problems with accumulation of the reactive material nitrogen tri chloride (NCI ) in liquid chlorine and 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 problems which can arise from accumulation of NCI in a system should consult the chlorine supplier to confirm that there is no exposure to an unacceptable or uncertain level of risk. 2 BBiMa 2. t Most of the NCI 2.2 2-3 2.<l The operation of chlorine storage tanks can involve slow vaporisation of chlorine and concentration of impurities/ ,. _, ___ _____ rr,,___ _ ____ _Juld be a cause for concern in i^^rag%Vysteraiwhich^*is pressure controlled by recycle of gas, or in the venting down of storage containers prior to emptying. 8EJEIa5S^ The use of these containers becomes a cause for concern when chlorine is withdrawn from the gas phase, rather than by direct discharge as liquid to storage, vaporisation or other process use. CKehe^ftf lorlt iHerjory Ubef lcahflpt~'bcllccralh' that '^he^chlorlnfe^ leSo^^rwiri Suitable 'precaQtlonji must^e tak^n"1EtTensur! __ as^iJto^]NCl/pconcefl|3r^S3ESElS5S&^**" iav/Ehig"cdntain r/system does not qollect the residues in infined volume. _______________ DO Q72929 fiSCteSaerJ CONFIDENTIAL. Chi rine vaporisers, particularly constant volume vaporisers such as kettles, are equipment where high boiling Impurities -2 arc likely to collect. Careful checks must therefore be maintained on the quality of the chlorine fed to such equipment, and of the concentration of impurities in the residual liquid. Where the equipment or process is such that dangerous concentrations could occur, material must be drawn off periodically to a purge vaporiser. In the case of a constant volume vaporiser it may be preferable to operate the purge system continuously to prevent an NCI build-up and to facilitate subsequent venting down. ^ Some chlorine gas treatment processes make use of a direct contact pre-cooler and reboiler system where higher concentrations of NCI can collect. Where the system operates at low temperatures, care dust be taken to avoid a dangerous concentration being reached. Systems operating at higher temperatures need to be controlled, but in general it can be demonstrated that NCI collects in the liquid and is then decomposed at temperatures of 5o-70*C. Another safeguard is to use a system where.liquid is run off, diluted with carbon tetrachloride and then maintained at temperatures of ca 50-70*C to decompose the NCI 3* Other Situations Periods of upset conditions, or preparation for maintenance, can lead to an increase in levels of NCI . For example emptying of a container by venting down priorJto maintenance, the passage of liquid into a heated catchpot on a chlorine relief system etc, could be a potential hazard and should be taken into account in plant design and operating and maintenance procedures. LIMITING LEVELS OF WC1 IN CHLORINE Basic Principles On the basis of the experimental results Indicated in Appendix 1, it is calculated that, on detonation, a mass of 1.5 gms of pure NCI /cm2 of metallic surface wetted is capable of fracturing the metal of a typical chlorine vessel. Appropriate safety measures therefore, should have as their objective that this critical mass loading should never be reached, during either normal or abnormal operation, of the chlorine containing system. The experimental resulta show that a concentration of NCI in solution greater than 3J w/w at ambient temperature is capable^of an accelerated decomposition which Is strongly exothermic. This concentration should also nev r be reached at any time. Maximum Level Inside Any Equipment The maximum admissible concentration of NCI, at each point in a liquid chlorine inatallatlon (production, storage, transport or use) sh uld be limited in a way which always r spects the basic principles enumerated in 3.1. Consideration sh uld be given to: DO 077930 - the Inherent risk of exceeding any safety margin Confidfntiai. - the variation of measured NCI c ntents with time, and any Imprecision introduced by th 3meth d of sampling and analysis - the perl d f time betw en successive measurements -3- A supplementary safety margin should be Incorporated into the prescribed levels of NCI, for different situations. It is therefore recommended that In order3to avoid reaching a level of 3i, the NCI, concentration should be limited to IK in all parts of the installation where NCI, could concentrate during normal operation. Where frequent checks are not carried out, this limit should be reduced to 0.t or 1000 ppm. This limit of 1000 ppm should be maintained for all reboilers and vaporisers, and In the residues obtained when emptying stock tanks. 3.3 Maximum Levels of NCl^ Introduced Into Fixed and Mobile Tanks The allowed NCI, levels have been established taking into account the potential concentration of NC1_, particularly where there is intended or accidental vaporisation of the chlorine in the gas phase. All calculations carried out for a specific Installation, should take into account: - the quantity of NCI which might be present after complete vaporisation of the3chiorlne - the dimensions and the overall geometry of the container The calculated maximum levels of NCI, in liquid chlorine introduced into vessels of various dimensions, where the chlorine could be removed in its entirety as gas, ignoring the effect of any diluents, and where the quantity of NCI,/cm of residual surface wetted remains below 0.3 gm/cm3 (see3Appendlx I) is as shown in the following table for containers of various sizes. In order to provide a large factor of safety a second set of figures is provided to indicate the recommended level of NCl^. For all large storage containers, chlorine should only be removed as liquid, but a limiting NCl^ concentration is recommended to provide a safety margin to cover all possible circumstances of removal of chlorine as vapour. Vessel Capacity Maximum Concentration of NCI- (ppm w/w) in Liquid Chlorine3 Calculated Recommended* 50 kg 1000 kg 20 t 50 t >300 t 200-300 50-60 15 6 5-6 20 20 5 2 2 *20 ppm w/w NCI, is the generally accepted maximum level for liquid chlorine3for all BITC liquid chlorine manufacturers. 3.3*1 The recommended values for each type of container are as indicated below. DO 072931 Chlorine Storage CONFIDENTIAL Particular attention is required4 to cover the following circumstances. In the case f low pressure storage (reference CEST 73/17), there is a progressive loss of contents by vaporisation. Creater care theref re must be taken to ensure that th NCI- remains at a low and predictable ___________ level. Thia becomes most Important when the e ntaln r la nearly empty, particularly prior to being taken out of commission. Prior to commend nr, the emptving operation, it is wise to 3ct an upper limit' of b ppm NCl^ in the liquid chlorine .present in the vessel, and to Introduce a quality of liquid necessary to achieve this figure. If any container and associated pipework has low points (sump, branches etc) where liquid chlorine could collect, and which Is not readily removed during the emptying operation, the system should be flushed with liquid containing <2 ppm NCI, before the final contents are run down. Appropriate precautions should be taken with normal pressure storage tanks if they are emptied by vaporisation of the final contents. In all normal situations for pressure storage applications with liquid discharge the recommended maximum level for liquid chlorine introduced is 20 ppm NCl^. 3.3*2 Small Mobile Containers Chlorine cylinders or drums of up to one ton are frequently used for extraction of chlorine as gas rather than liquid. Their size however, is such that dangerous quantities of NCI, rich material will not accummulate at the later stages of emptying if a limit of 20 ppm NCl^ is set for chlorine put into small containers. 3.3*3 Bulk Transport Containers Almost all road tankers of liquid chlorine deliver directly to customer storage and it is only in well defined circumstances that gas is supplied to a point of use. Most rail tanks are also used in prescribed deliveries to customer storage where the mode of operation is guaranteed to be the direct discharge of liquid. The NCI limits therefore, are set by the use to which the chlorine will be put or by the behaviour of the residual chlorine left in th vehicle until refilled. The recommended limit for NCl^ in these circumstances is 10 ppm, possibly 20 ppm for smaller containers or for chlorine known to contain significant amounts of bromine or other diluents. On the other hand iso-containers and certain tank deliveries may be used for chlorine gas discharge to a process. In these circum stances, or in any other cases of uncertainty, the NCI- limit should be set at 5 ppm to avoid any problems with the residual material. Such containers should preferably not be fitted with a sump or any other configuration of equipment where a small heel of liquid can collect, unless the maximum level of NCI, limit Is reduced to below 2 ppm. *1 SAFETY MEASURES *1.1 Principles DO 07293? CONFTDFNTTAl The most important control over the potential levels of NCI- is at source in the chlorine producing process. Wh nevqr the chi rine pr ducing process r sourc of raw materials are changed, a check should be made to ensure that a new NCI- risk has n t been introduced (change f anode type, water, settling aids, salt source, drying acid). For electrolysis units, a regular check should be maintained on the total nitrogen comp unds in the brine. Any modification to the materials used in the process should always be accompanied by a check against any potential problems connected ---------------------------------- -5 - 4.2 Frequency of Testing for NCl^ in Liquid Chlorine The appropriate frequency of testing should be determined by experi ence and by the extent of variation in the results obtained. As an indication, however, the following frequencies are set as a guide. *1.2.1 Whenever the NCI level is capable of varying between 0.1 and 1?, an analytical check should be carried out each week, in particular on the purge line from vaporisers or condensors. 4.2.2 Whenever the NCI content is normally >100 ppm (0.01?) and less than 1000 ppm (0.1?) the analytical checks should be carried out at least once/month. This frequency is also appropriate for chlorine from a liquefaction system where the NCl^ lies between 20 and 100 ppm. 4.2.3 For chlorine systems where the NCI level is typically 5-20 ppm the analytical frequency should be3onee/3 months on liquefaction output and on randomly chosen storage/mobile containers. In other liquid systems where the level is < 5 ppm the frequency of testing is satisfactory at once/year. During periods of commissioning, maintenance, process modification etc, whenever there is a risk of higher NCI, levels, the above frequencies should be Increased. Regular checks, at an appropriate frequency, should be maintained on the output from any NCI, destruction system, eg U/V irradiation of gaseous chlorine. 4.3 Temperature Limits Purge vaporisers are used industrially to destroy NCI, and temperature limits arc set to achieve a controlled decomposition rate, while at the same time ensuring that local overheating of metal surfaces is avoided. Because of the influence that increasing temperature has on the relative explosive decomposition rates of NCI , care should be taken to control the temperature of equipment^handling liquid chlorine containing high levels of NCI- (0.1-1?). Overheating of the chlorine should be avoided and therefore direct electrical heating systems should not be used. A purge vaporiser can be designed to operate with low pressure steam at 120*C to facilitate slow decomposition of NCI . Otherwise, one should employ a heat transfer fluid, for which the temperature can be limited to 80-90*C, thereby ensuring that the liquid chlorine containing NCI does not exceed 80aC. 3 DO 072933 4.4 Construction Details for Vaporisers and Reboilers CONFTDFNTTai, All equipment where there is a risk of NCI. accumulating, as a result of the progressive vaporisation of liquid chlorine, should be fitted with a purge point at the low point of the system, to enable the residues to be purged in any instance where there is an excessive Increase in NCI, levels. It is als possible to treat these residues by dilution in carb n tetrachloride befof'e thermal decompo sition. Such dilutl n permits the system to remain always at a NCI. concentration of less than 1?, ven aft r vaporisatl n of the chlorine. -6 ANALYSIS OF NITROGEN TRICHLORIDE The analysis of NCI in liquid chlorine requires specialist analytical equipment as described in the BITC publication*. The analysis of low concentrations is the most difficult and care must be taken to avoid contamination of the samples with other nitrogen compounds. REFERENCES Analytical method - BITC Analytical Procedure. "The Determination of itrogen Trichloride in Liquid Chlorine" 20 May 1981. Chlorine Institute Members Information Report 21 "Nitrogen Trichloride - A Collection of Papers" Edition 2 1975. Chlorine Institute Literature Review C E Vogler Nov 8 1962. Mellor Volume VIII Inorganic and Theoretical Chemistry, p 603* 604. 00 070934 CONFIDENTIAL * . `APPENDICES 1 PHTSICAL AND CHEMICAL PROPERTIES OP NITROGEN TRICHLORIDE Nitrogen trichloride is formed during the electrolytic production of chlorine, due to side reactions between the chlorine and various nitrogen compounds In the brine solution. It is a liquid with a boiling point of 71*C, which is miscible in all proportions with liquid chlorine and other chlorine compounds such as carbon tetra chloride. Nitrogen trichloride can also decompose exothermically, and in the concentrated form the liquid behaves as a sensitive explosive, capable of rapid deflagration and detonation. The explosive nature of the material when mixed with liquid chlorine (and similar materials) Is very dependent on both its concentration and on the total amount present. Pure liquid NCI has been calculated to be capable of generating a detonation pressure of 5-5 to 7-5 x 10* atm. At very high concentrations in liquid chlorine it could generate an energy release equivalent to 30-40$ of the explosive force of TNT. Concentrations >35$ by weight are readily capable of detonation by shock, high temperatures or ultra-violet light, 13$ NCI appears to be the limiting concentration to achieve detonation. Even at low concentrations, NCI will slowly decompose, the decomposition increasing with the*temperature, or in the presence of metals such as copper or alloys like monel. Ultra violet light can be used to decompose NCI safely, when it exists as a minor component in the gas phase. * As the concern over the explosive behaviour of NCI is in the context of mixtures with liquid chlorine, the important factor is its explosive force when contained within a typical chlorine pressure vessel - with a wall thickness of say 10-12 mm. The available evidence from the published literature indicates that detonation of 1.5 g NCI (as 100$)/cra within a liquid film would be capable of fracturing the metal, and 0.3 g/cmz of surface area is capable of overstressing the metal to the point of cracking or flssurlsatlon. The calculation of the potential explosive capability of an NCI /ci_ mixture within a vessel is only an approximation. As tne chlorine evaporates the concentration of NC1_ will progressively Increase, but if the residual pool covers a large area, or the NCI concentration is still <13$, the explosive potential remains small. On the other hand if the residual liquid collects in a sump or bottom drain connection, the critical mass may be reached sufficient to cause total metal failure. Experimental work has also shown that chemical decomposition of NCI in liquid chlorine at >3$ is rapid and strongly exothermic.-* do 07P935 The above summarises the properties of NCI In the presence CONFTDENTTAl of the much more volatile liquid chlorine. In the presence of high r boiling impurities which act as inert diluents and which are also found In commercial chlorine, such as br mine and C r C chlorinat d carbon compounds, the hazards from NCI. accumulating in the residues from evaporation are reduced by dilution. On the ther hand. If it Is possible In certain processes to f re th r reactive materials in liquid chlorine, In these circumstances the expl slv energy f the mixture would be lncr ased. -2- 2. SOURCES AND DECOMPOSITION OF NITROGEN TRICHLORIDE 2.1 Sources of its Formation NCI could be formed from the introduction of nitrogen compounds into various parts of the brine or chlorine handling systems. Nitrogen compounds in the brine used for aqueous electrolysis are the main source of NCI and only 1 ppm of NH, in brine is needed to give >50 ppm NCI rn liquid Clj. Rock salt, or in particular solution mined salt using surface waters, will contain varying levels of ammonium and nitrate salts, whereas the use of vacuum salt in the brine recycle circuit will give very low levels of NCI,, except where ferroeyanides are added to avoid caking. '3Chlorination or hypochlorite treatment of the brine is however, capable of destroying a large proportion of the ammonium salt impurity. NCI can also be formed from other nitrogen based impurities in the electrolytic cell and for example the proportion of NCI impurity could change due to a change In process conditions such as conversion from graphite to metal anodes. In plants which use direct contact water cooling of the chlorine gas before drying and compression, it is also possible to form NClj. 2.2 Destruction of Nitrogen Trichloride Certain chlorine handling and liquefaction systems are capable of removing NCI from the chlorine stream. Various processes which can be used Co reduce the hydrogen content of cell gas - eg ultra violet light or active carbon can also preferentially decompose NCI,. High metal temperatures, particularly of copper based alloys at temperatures of 80-100*C, will also decompose the material. Liquid chlorine and mixtures of chlorine with carbon tetrachloride maintained at 50-70*C leads to a progressive destruction of NCI,. Alternatively, NCI, can be eliminated by reaction in a number of chemical processes, eg absorption of chlorine containing NCl^ in caustic soda. DO 077936 CONFTDFNTTAL