PRINCIPLES OF ODORIZATION - ASGMT

1 2002 PROCEEDINGSPAGE 141 AMERICAN SCHOOL OF GAS MEASUREMENT TECHNOLOGYPRINCIPLES OF ODORIZATIONS ixto Ortiz phillips chemical Company LP10001 Six Pines Drive, The Woodlands, TX 77380 INTRODUCTIONThe reason gas companies odorize natural gas is simple:it is a legal requirement. In 1937, the high school in NewLondon, Texas exploded, killing over 200 men, women,and mostly children. Natural gas emanating from a leakaccumulated in the school s basement and ignited whensomeone entering the area flipped on a light switch. Noone knew about the leak because un-odorized naturalgas has no odor. This disaster led federal and stateregulators to enact new regulations that requiredsuppliers to odorize natural remains one of the most important tasksperformed by the gas company.

2 Thus, it is critical thatgas company management and employees have a clearunderstanding of the fundamentals of gas of the basic characteristics of odorantcompounds, the reasons for blending these compounds,and the potential problems that can arise is a vitalcomponent of an ODORIZATION program s success. Thelegal and human costs of natural gas accidents demandconstant vigilance from everyone involved in anodorization REQUIREMENTSThe regulations requiring ODORIZATION of natural gas areenforced in the United States by the Department ofTransportation s (DOT) Office of Pipeline Safety and areencoded within 49 CFR (a) and (f). Thelaw clearly states natural gas must be odorized so thatany leaks are readily detectable, by a person with a normal sense of smell, when the concentration ofnatural gas reaches 1/5th of the lower explosive range of concentrations bounded by the upperexplosive limit (UEL) and lower explosive limit (LEL)represent the optimal conditions at which a mixture ofnatural gas and air can ignite in the presence of an ignitionsource.

3 If the concentration of natural gas is higher thanthe UEL, the mixture is too rich to support , if the concentration of natural gas is lowerthan the LEL, the mixture is too lean to support the detection requirement of odorized gas at 1/5th(20%) of the LEL creates a safety factor that can buytime and save lives when a gas leak occurs. Leakinggas will be detected well before the mixture of gas andair is able to support studies have been conducted throughout theyears to find the best chemical compounds to use forodorization of natural gas. These studies have revealedthat mercaptans, a class of organosulfur compounds,are the best chemicals to use for ODORIZATION of naturalgas. Mercaptans have a repulsive smell that is detectableat extremely minute concentrations in the part per billionrange.

4 The vast majority of human beings can smellmercaptans at these extremely low levels, so thesecompounds are very effective at odorizing natural USED IN ODORIZATIONNot all members of the mercaptan chemical family areamenable for use in natural gas ODORIZATION . The lowmolecular weight mercaptans, such as ethyl mercaptanand methyl mercaptan, are much too reactive to use innatural gas systems. This means these compounds areeasily converted to other less odorous forms via chemicalreactions with other compounds found in natural gassystems. On the other hand, higher molecular weightmercaptans, such as nonyl mercaptan and dodecylmercaptan, are not volatile enough to be effective. Thus,the mercaptans used in ODORIZATION are middle of theroad in terms of molecular weight, volatility, andreactivity.

5 Mercaptans typically used in the gas industryinclude:1. tert-Butyl mercaptan2. n-Propyl mercaptan3. Isopropyl mercaptan4. sec-Butyl mercaptantert-Butyl Mercaptan (TBM)TBM is the most common odorant blendstock in NorthAmerica. TBM has a strong, gassy odor at lowconcentrations, and is the most oxidation resistant ofthe mercaptans. However, its relatively high freezing point(34 F) means it must be blended with other organosulfurcompounds with lower freezing points so it can be usedin climates where the temperature regularly gets belowfreezing. TBM also features the best soil penetrability ofall the Mercaptan (NPM)NPM is not a major blend component, although it is usedin blends popular in the Northwestern region of the Canada. Although NPM is the most easily oxidizedof the mercaptans, its low freezing point makes it a verygood blendstock to use with TBM.

6 A strong odor andrelatively high vapor pressure are two other goodcharacteristics of 1422002 PROCEEDINGSAMERICAN SCHOOL OF GAS MEASUREMENT TECHNOLOGYI sopropyl Mercaptan (IPM)IPM is the second most popular blendstock in NorthAmerica. IPM features a characteristic gassy odor,strong soil penetrability, and is second only to TBM inoxidation resistance. Its low freezing point ( 203 F) andhigh vapor pressure are two good features of Mercaptan (SBM)SBM is the least used blendstock in North America. SBMis primarily used in Wick odorizers, where its low vaporpressure and high boiling point help prevent thepossibility of over-odorizing with this COMPOUNDS USED IN ODORIZATIONT hiophaneThiophane, also known as tetrahydrothiophene (THT), isthe only cyclic organosulfur compound used inodorization.

7 THT is one of the most prevalent blendstocksin Europe and the Western United States, and is the onlyorganosulfur compound used in its pure form to odorizenatural gas. THT is sometimes blended with TBM, if it sblended at all. Compared to the mercaptans used inodorization, THT is the most oxidation resistant, but hasa low odor impact and poor soil other components used in odorant blends are a classof organosulfur compounds called sulfides. Sulfides areblended with mercaptans for various reasons, such aslowering the freezing point of a blend or impartingstronger anti-oxidation characteristics to the have much higher odor thresholds thanmercaptans, so their odor impact is not nearly as , sulfides are extremely resistant to oxidationand are not very likely to oxidize under pipelineconditions.

8 Two examples of sulfide compounds typicallyused in odorant blends are dimethyl sulfide and methylethyl BLEND?All organosulfur compounds used in the ODORIZATION ofnatural gas are blended to make odorants. As mentionedpreviously, thiophane is the only organosulfur compoundthat can be used in its pure form to odorize natural others must be used as part of a reason for blending is simple: no single organosulfurcompound is 100% effective as an odorant in naturalgas. While all organosulfur compounds used inodorization have good characteristics, they also haveshortcomings that preclude their use in their pure formas gas odorants. For example, TBM must be blendedwith other compounds to lower the overall freezing pointof the blend to an acceptable a nutshell, blending assures the optimal performanceof an odorant.

9 Different conditions, such as geographicalregion, the characteristics of the gas being odorized, thetype of gas system, the type of equipment used toodorize the gas, and the flow rates, to name a few, alldetermine the best type of blend to use. Blending assuresthe creation of an odorant that is best suited for thespecific usage PITFALLSO dorization of natural gas is not without its are times when natural gas loses its odor, andwhen that happens, a potentially dangerous situationarises. An understanding of the possible causes of thisphenomenon is essential for successfully troubleshootingthese problems when they occur, and also forestablishing preventive measures that will keep themfrom happening in the Affecting Odorant QualityEven though odorants feature highly repulsive smells thatare detected by a vast majority of the population, thereare instances when people are unable to smell properlyodorized gas.

10 The gas industry has conducted numerousstudies throughout the years that seek to explain thecomplexities involved in detecting and studies have revealed that factors such as aging,smoking, and allergies can affect the ability to smellodorants in natural gas. Also, there is an element ofdistraction: it has been shown that other sensorydistractions can keep the mind occupied enough so thatthe smell accompanying a gas leak is, in some cases, factor for consideration is odor fatigue. It hasbeen demonstrated that the olfactory sense quicklyacclimates itself to the smell of mercaptans, causing theinitial impact of the mercaptan s smell to quickly decreasewith time. In this case, a person entering a residence orbuilding where a gas leak is occurring has a few minutesto take action before his or her nose becomes fatiguedby the , there is the phenomenon of odor masking.