Quality of a product may be defined as its fitness for a pur- pose. Once the required quality is determined by the applica- tion, it is controlled by appropriate processing, testing, and analysis. This section will outline quality criteria for signifi- cant finished product gaseous fuels and light hydrocarbons.
Appropriate ASTM International and Energy Institute (for- merly the Institute of Petroleum) methods for testing and analysis will be listed in parentheses (ASTM/IP). The full titles of the methods can be found at the end of the chapter under the heading “Applicable ASTM/IP Standards.”
Natural Gas
The primary uses for natural gas are as a fuel for the produc- tion of heat (home and commercial/industrial heating) and power generation. Significant quantities of natural gas are also used to produce hydrogen and in the production of ammonia.
Following processing (especially removal of moisture, H2S, CO2, and gases that are not condensable during ordinary processing) and conditioning into a merchantable or finished product, natural gas is composed primarily of CH4 and can contain a little ethane (C2H6), propane (C3H8), butanes (C4H10), and trace amounts of heavier hydrocarbons, depending on the source and extent of extraction of ethane and heavier hydrocar- bons. Natural gas pipelines set the quality requirements for nat- ural gas, which usually include restrictions on total sulfur, hydrogen sulfide, oxygen, carbon dioxide, nitrogen, solids, water, and minimum energy content in Btu (or MJ).
The principal quality criterion of natural gas is its heat- ing value(ASTM D1826 and D3588). It is common practice during periods of very high natural gas demand (during periods of low temperatures) to boost the energy content (Btu content) of natural gas by the addition of small quanti- ties of propane and air (also called “peak shaving”). Most gas companies sell natural gas based on the energy (Btu) con- tent, not the volume of natural gas or methane, because of constant variability in composition.
Natural gas must be readily transportable through high- pressure pipelines. Therefore, the water content, as defined by the water dew point (ASTM D1142, D4888, and D5454), must be considered to prevent the formation of ice or hydrates in the pipeline and to protect pipelines against cor- rosion, which can be severe if moisture is present as well as H2S or CO2—even at just parts per million concentrations.
Likewise, the amount of entrained hydrocarbons heavier than ethane, as defined by the hydrocarbon dew point, should be considered to prevent accumulation of condensa- ble liquids that could block the pipeline or interfere with process instrumentation and measurement.
Natural gas and its products of combustion must not be unduly corrosive to the materials with which they come in con- tact. Thus, the detection, measurement, and reduction ofhydro- gen sulfide (ASTM D4084 or D4810) and total sulfur (ASTM D1072 or D6228 or D5504) are important. When natural gas is used for fuel purposes (heating or engine fuel), it is normal practice to add one or more odorants (mercaptans or sulfides) to the natural gas so that leaks can be readily detected.
If the gas is to be liquefied by cryogenic processing and stored in liquid form (LN gas [LNG]), carbon dioxide will separate out of the cold liquid as a solid and interfere with the refrigeration system.Carbon dioxide can be deter- mined by ASTM D4984 or ASTM D1945. If the gas is to be used as a feedstock for the production of hydrogen or
petrochemicals, acomplete analysis(ASTM D1945) must be made, because some gases contain materials that are delete- rious to these processes.
Ethane
This normally gaseous paraffinic compound (C2H6) has a boiling point of approximately 88C (127F). Ethane can be handled as a liquid at very high pressures (typically above 1,800 psig) and at temperatures below 32C (90F).
Virtually all ethane produced (that is, separated from raw natural gas) is used as a feedstock for the production of high purity ethylene—a basic petrochemical whose applications are covered more fully in the section on olefins and diolefins.
Quality criteria for ethane as a petrochemical feedstock are specific to the particular process used for petrochemical manufacture. In general, however, such feedstocks must be very low in water content (ASTM D1142), oxygen (ASTM D1945), carbon dioxide (ASTM D1945), sulfur compounds (ASTM D1072), andother elementsthat will interfere with the process or the catalysts used in petrochemical manufacture.
Liquefied Petroleum Gases
A mixture of hydrocarbons, principally propane and butanes, obtained from either natural gas liquids or refinery sources, LP gas can be stored and handled as liquids at ambient temperatures and moderate pressures.
Propane
A normally gaseous paraffinic hydrocarbon (C3H8) having a boiling point of approximately 42C (44F), propane is handled as a liquid at ambient temperatures and moderate pressures. It may include some ethane (maximum is typically about 6 % by volume), and up to 2.5 liquid volume (LV) per- cent of butanes and heavier hydrocarbons. Vapor pressure at 38C (100F) is never to exceed 1,434 kPa (208 psig). Com- mercial propane can include varying amounts of propylene, andcommercial propane–butane mixes can include varying amounts of propane, propylene, butanes, and butylenes.
Butane
A light paraffinic hydrocarbon (C4H10), butane is usually handled as a liquid at ambient temperatures and moderate pressures. Butane can be fractionated further into normal butane (n-butane), which has a boiling point of approxi- mately 0C (32F), and isobutane, which has a boiling point of approximately12C (11F).
LP Gases
The original use for LP gas was as a fuel gas in areas where pipelined natural gas was not available. Later, most LP gases were fractionated into their separate components, princi- pally propane, n-butane, and isobutane, as preferred feed- stock for the petrochemical industry.
There are many uses for LP gases, among the most important being the following:
1. Residential, commercial, and construction fuel for heating 2. Engine fuels
3. Farm uses such as crop drying, tobacco curing, and powering irrigation pumps, and railroad uses to prevent icing of track switching gear
4. Chemical feedstock for production into ethylene and propylene
5. Manufacture of gasoline components such as alkylate
6. Gasoline blending stocks (butanes)—isobutane is a partic- ularly good, high octane blending component, with a motor octane number of 97
Special applications such as alkylateand petrochemical feedstockare not covered by ASTM D1835 because they are usually purchased and manufactured to meet specific quality requirements that differ from one process application to another.
The ASTM specifications for LP gases are designed to properly define acceptable products for historical residential and industrial uses. In many cases, it has been found that the products meeting these specifications will be usable in applications other than the ones for which they were designed. However, some new applications such as fuel cells and microturbines can have much more stringent require- ments (such as sulfur in the low ppm concentration range and essentially no heavy ends or residues).
The following descriptions will serve as a general guide to the applications of the four LP gas fuel types:
Special Duty Propane (HD-5)
This fuel type is tailored to meet the needs of internal combustion engines operating under moderate-to-severe con- ditions (modern automotive applications). Special duty pro- pane will be less variable in composition and combustion characteristics than the other fuels described in ASTM D1835. Note that the high concentration of propane and lim- itations of allowable other hydrocarbons means that HD-5 propane will have consistent octane quality; thus, it is not necessary to measure octane quality on each batch of HD-5 propane. For information, the “5” in “HD-5” refers to a maxi- mum of 5 % propylene. Similarly, an “HD-10” propane, allowed in some states, may contain up to 10 % propylene.
Since most distributors of propane handle only a single grade of propane, where HD-5 propane is available, it is also used for less demanding applications that could use com- mercial propane, such as heating.
Commercial Propane
This fuel type, sometimes called commercial LP gas, can con- tain small quantities of ethane (limited by vapor pressure restrictions) and varying quantities of propylene from refinery gases. It is preferred for residential and commercial uses (mostly heating) in geographic areas and in seasons where low ambient temperatures are encountered. Commercial propane, having a lower octane quality than HD-5 propane, is only suita- ble for low severity internal combustion engine applications (such as some agricultural irrigation pumps).
Commercial Propane/Butane Mixtures
This fuel type covers a wide range of mixtures and permits the tailoring of fuels to specific needs. This fuel finds appli- cations in warmer areas and seasons where low ambient temperatures are not frequently encountered. This fuel is not common in the North American market, but may be used in other, more tropical, parts of the world.
COMMERCIAL BUTANE
This fuel type has limited use as a heating fuel and may only be used in warm climates and in industrial applications where fuel vaporization problems are not present. High purity butane, not described in ASTM D1835, has been used as a propellant in pressurized personal care products.
LP gases are essentially propane or butane or mixtures of the two. As a result, the important characteristics of these products can be defined and controlled by relatively few measurements. Thevaporization andcombustion character- isticsof specification LP gases are defined for normal appli- cations by vapor pressure, volatility and (relative) density.
The significance of the tests is summarized here.
Sampling LP Gases
Before discussing the important properties and test methods of products in this section, a discussion of sampling is vital.
Correct sampling of LP gases, especially propane, presents dif- ferent challenges from sampling other fuels. Most fuels, like gasoline, diesel fuel, and aviation fuels, are liquids and are used by the application as liquids. Thus, the importance of most conventional fuel sampling is to obtain a representative sample of the liquid fuel. In the case of LP gases, the prod- ucts are stored as liquids under moderate pressure, with a vapor phase above the liquid. However, in most applications, LP gases are actually used as gases. Improper sampling, sam- ple storage, even removal of an aliquot for analysis, and the way the test specimen is introduced to an analyzer can alter the composition of the test specimen and give a false result for certain properties, especially vapor pressure. Thus, correct sampling according to ASTM D1265 or D3700 is critical. Fur- ther, some properties such as corrosion toward copper alloys, caused by aggressive sulfur species in the LP gas, can be changed by the sampling procedure or sample container, again giving a false test result. Sample containers with an inert internal coating must be used for LP gases for certain tests when determining reactive materials.
Test Methods
All the test methods cited in this chapter are ASTM test meth- ods. Note, however, that the Gas Processors Association (GPA;
http://gpaglobal.org/) has many test methods for properties described here, and some laboratories in North America will use GPA methods. In Europe, EN or ISO methods may be used.
VAPOR PRESSURE
For most purposes, LP gases will be stored, handled, and trans- ported as a liquid (under moderate pressure) but used as a gas.
To store, handle, and transport it safely, the vapor pressure must be known (ASTM D1267 and recently ASTM D6897).
Vapor pressure gives an indication of the low temperature con- ditions under which initial vaporization will take place. It can also be considered a semiquantitative measure of the amount of the most volatile material present in the product. The pre- dominant volatile component in propane is ethane, and the predominant volatile component in butane is propane. Thus, the vapor pressure limit in HD-5 propane is an effective limit on the maximum ethane (and methane) content in the pro- pane. Vapor pressure is also used as a means for predicting the maximum pressures that can be experienced in storage and ensures trouble-free performance in commercial equipment.
Vapor pressure can also be calculated from a composi- tional analysis using ASTM D2598. The accuracy of the vapor pressure calculation is entirely dependent on the accu- racy and completeness of the compositional analysis.
VOLATILITY
Volatility in LP gases (ASTM D1837) is expressed in terms of the 95 % evaporated temperature and is a measure of the
amount of the least volatile fuel components present in the product. This specification controls the heavy ends in the fuel and is, in effect, a restriction on higher boiling fractions that might not vaporize for use at system temperatures.
DENSITY AND RELATIVE DENSITY
Density and relative density are important in measurement and custody transfer calculations. It is not a specification property but is usually determined for commercial or trans- port reasons. ASTM D1657 is the test method to actually measure the density or relative density of LP gases. Relative density of liquefied LP gases is the density of the LP gas rela- tive to the density of water (density of the LP gas/density of water) at a stated temperature, usually 15C (60F). More commonly, density or relative density is calculated from a compositional analysis using ASTM D2598. In addition to vapor pressure, volatility, and density, there are other limits imposed on the LP gas that provide assurance that the prod- uct will perform dependably under actual conditions of use.
RESIDUE
Residue (ASTM D2158) is a measure of the concentration of contaminants boiling above 38C (100F) that can be present in the LP gas, usually as a result of contamination during distribution, downstream of the original production. Con- taminants that have frequently been seen and identified are compressor oils, lubricants from valves, plasticizers from hoses, corrosion inhibitors, and other petroleum products from pumps, pipelines, and storage vessels that are used in multiple service applications. Contaminants in the gasoline boiling range are usually not a problem, but contaminants of diesel fuel (from transfer of LP gas in a multiproduct pipeline) can create low volatility liquids in vaporization units, depending on the temperature of vaporization.
Contaminants, whatever their source, can be particularly troublesome in liquid withdrawal systems followed by vapor- ization, such as those used in internal combustion engine fuel systems where the materials accumulate in the vapor- izer or at low points in a fuel gas transfer line and will ulti- mately plug the fuel system. The oil stain observation portion of D2158 can provide some insight into the nature of the residue and show the presence of oily contaminants that might not be detected visually in the first step of the test (evaporation of 100 mL of propane in a centrifuge tube).
Note that the temperature of the residue separation step has a huge impact on the quantity of residue collected. Thus, a residue test performed at 100C can show a very much smaller amount of residue than a test performed at 40C on the same LP gas fuel.
COPPER CORROSION
Copper strip corrosion (ASTM D1838) limits provide assur- ance that difficulties will not be experienced in the deteriora- tion of copper and copper alloy (brass) fittings and connections commonly used in LP gas systems. Reactive sul- fur compounds such as elemental sulfur, hydrogen sulfide, and some other sulfur species or decomposition products of those species are corrosive to copper. The copper strip cor- rosion test is an extremely sensitive test that will detect virtu- ally all species of corrosive sulfur, including minute traces of hydrogen sulfide. As little as 0.5 to 2 ppm of H2S can cause a copper strip test failure. Mixtures of H2S and elemental sulfur below 1 ppm can be very corrosive to copper. It is
important that the product being tested does not contain any additives (such as “anticorrosion” additives) that can diminish the reaction with the copper strip, or a “false pass”
can be obtained. Note that batches of LP gases can pass the copper strip corrosion test at point of manufacture, but through mixing with other batches of LP gases, reactions of sulfur species during transit (COS to H2S, for example), or contamination during transportation and distribution can become corrosive to copper in the field.
ASTM D2420 is a simple pass/fail test forhydrogen sul- fideand is based on the discoloration of alead acetate solu- tion on a filter paper exposed to propane vapors. The test method is required by ASTM D1835 as an additional safe- guard that hydrogen sulfide is not present.
SULFUR CONTENT
Total sulfur content is determined by ASTM D2784, D6667, and other test methods. Sulfur content is limited to ensure that the sulfur oxides formed during combustion are low enough that there will not be acidic condensates in exhaust gases. It should be noted that ASTM D2784 does not include the usual precision statement. This was in recognition of the difficulty of shipping stable samples of LP gas containing various concentrations of sulfur to a number of cooperating laboratories. Studies have confirmed that the sulfur species can change during transit. Recent developments in shipping LP gas samples for laboratory proficiency testing are expected to allow the development of precision data for many LP gas test methods that lack such data.
Note that for decades, LP gases had much lower sulfur content than gasoline or diesel fuel. However, most gasolines now contain less than 30 ppm sulfur, and many diesel fuels now contain less than 15 ppm sulfur, making LP gases higher in sulfur content than other major fuels. Thus, HD-5 propane, with up to 123 ppm sulfur by mass, could not be used with current gasoline or diesel engine emission control technologies, which cannot tolerate such higher sulfur fuels.
If LP gas were to be used in engines with current emission control technologies, the sulfur content would have to be reduced significantly, and a new warning mechanism would be needed to replace the current sulfur-containing odorants.
WATER CONTENT
Moisture content (ASTM D2713) is a measure of the approxi- mate water content of the product. Liquid fuels like diesel fuel, jet fuel, gasoline, and even butane must contain no free water, but may be saturated with dissolved water. Only commercial and special duty propane types of LP gas must actually besub- saturated with waterat normal ambient temperature. In fact, the allowable water content in propane is equivalent to the water saturation content of propane at about25C. Water content is controlled to provide assurance that pressure reduc- ing regulators and similar equipment will operate consistently without freeze-ups caused by autorefrigeration at points of pressure reduction in LP gas systems, leading to separation of dissolved water from the product and freezing.
Water content must also be controlled to low concentra- tions to avoid hydrate formation in pipelines. Gas hydrates are formed as a granular solid or slushy substance when liq- uid water is intimately mixed with some of the light hydro- carbons, principally methane, ethane, and propane under certain conditions of temperature and pressure. Hydrates can, and do, form in pipelines well above the freezing point
of water, if sufficient moisture is present in the stream and the pressure is high enough.
COMPOSITIONAL ANALYSIS
A component analysis can be performed using ASTM D2163.
A gas chromatograph is used to obtain a component distri- bution of the LP gas. Precise compositional data are often needed for chemical feedstock applications and specification analysis. The component distribution data may be used to calculate certain physical properties, such as relative density, vapor pressure, and octane number (ASTM D2598).
Odorization
LP gases are odorless, and leaks cannot be detected by smell.
Thus, for safety, LP gases to be used for fuel purposes are usu- ally odorized prior to distribution to end users so that leaks in residential and commercial distribution systems can readily be detected “at one fifth the lower explosive limit (LEL) in air” by most people. The only two odorants currently used in North America for LP gases are ethyl mercaptan and tetrahydrothio- phene (thiophane). To ensure that the LP gas is properly odor- ized with ethyl mercaptan, ASTM D5305, a “stain tube” test method, may be performed, either in the field or in a laboratory.
Note that LP gas is not odorized when intended for feed- stock use or further processing. When users require unodorized products, suppliers generally require written instructions from the end user to document that they do not want odorized prod- uct and that the product will not be used for fuel purposes.
Percentage Fill of LP Gas Cylinders
While not a specification property, regulations pertaining to safe and proper handling of cylinders of LP gases, such as propane barbecue cylinders, propane storage tanks, and even sampling cylinders, typically restrict the fill level of LP gas cylinders and tanks to 80 % of the total capacity. This restric- tion is a safety requirement because light hydrocarbons like propane and butane have a very high coefficient of expan- sion as a function of temperature. If a cylinder were filled above 80 % at a cool temperature, and the cylinder was then exposed to a high ambient temperature, the liquid LP gas could expand to totally fill the cylinder, leaving no vapor space. Any further expansion would result in product release through overpressure protection devices (pressure relief valves). Should these devices fail or the heat input be so great that the relief devices cannot relieve pressure within safe lim- its, the container itself could rupture, releasing all remaining product at once—a very serious fire hazard. Thus, considera- tion should always be given to maximum temperatures that could be experienced during transportation and storage of LP gases and initial fill percentages adjusted accordingly. For example, if a rail car is likely to sit in a rail yard in the desert, it could experience extremely high summertime temperatures and fill levels may need to be restricted to 70 % maximum.
Natural Gasoline
Natural gasoline is a mixture of hydrocarbons (extracted from natural gas) that consists mostly of pentanes (C5H12) and heavier hydrocarbons (up to about C10).
In the beginning of the natural gasoline industry, the only use for natural gasoline was as low octane motor fuel or as a blending agent in the production of automotive gaso- line. Even today, natural gasoline is still used as a blending component in automotive gasoline and is also used as a
denaturant in ethanol used for fuel purposes. Some of the individual components of natural gasoline, namely isobu- tanes, butanes, pentanes,andisopentanes, may be separated as feed stocks for reforming, alkylation, and production of synthetic rubber and other petrochemicals.
For use as motor fuel or as a component of motor fuel, the primary criteria for the quality of natural gasoline are its volatility and knock performance (combustion characteris- tics). The basic measures of volatility are vapor pressure (ASTM D4953, D5190, and D5191) and distillation. ASTM D216/IP 191 was originally used for distillation of natural gasoline, but D216 has been withdrawn and replaced by ASTM D86 (IP 123). Knock performanceis measured by rat- ing in knock test engines by both the motor octane number (ASTM D2700/IP 236) and research octane number (ASTM D2699/IP 237) methods.
Other considerations for natural gasoline used in motor fuels are copper corrosion (ASTM D130/IP 154), density or API gravity(ASTM D1298/IP 160), andsulfur content(ASTM D1266/IP 191 or D6667). Although density/API gravity has only a minor relationship with quality, it is necessary to determine density or gravity for measurement (purchase–
sale agreements) and shipping.
When natural gasoline is used as a feedstock for further processing or petrochemicals, the list of quality criteria is tied to the processes to be used in the end application. For nearly all petrochemical uses, composition by hydrocarbon types is needed, and, frequently, acomplete analysis of spe- cific components is made (ASTM D2427). If a catalytic pro- cess is involved, total sulfur(ASTM D1266/IP 191 or D6667) andnitrogen are very important, even down to sub-part per million concentrations, because sulfur and nitrogen will destroy some catalyst activity.
Olefins and Diolefins
Many light olefins and diolefins that are produced by refin- ery processes are isolated for petrochemical use. Some satu- rated light hydrocarbons are specifically processed to generate olefins. The individual products are as follows:
ETHYLENE (C2H4)
This is a normally gaseous olefinic compound having a boil- ing point of approximately 104C (155F). It can be handled as a liquid at very high pressures and low tempera- tures. Ethylene is normally made by cracking an ethane or naphtha feedstock in a high-temperature furnace (thermal cracking) and subsequent separation from other compo- nents by distillation.
The major uses of ethylene are in the production ofeth- ylene oxide, ethylene dichloride, and polyethylenes. Other uses include the coloring of fruit, EP rubbers, ethyl alcohol, and medicine (anesthetic).
Since ethylene is a high-purity product (normally sup- plied at 99.5 mol percent purity or higher), the quality crite- ria of interest aretrace components (or contaminants). The components of greatest concern arehydrogen, carbon mon- oxide, carbon dioxide, oxygen,andacetylene. Waterandsul- fur content are also critical to ethylene-based processes. All of the above impurities are generally catalyst poisons to polymerization processes, even in the low ppm concentra- tion ranges on a mol basis. Therefore, the analytical prob- lems are magnified greatly. To date, ASTM International does not have a standard method for moisture at these low