1 Introduction & Background

The first commercial production of LPG in the US was around 1920 by Frank Philips, the founder of Conoco Philips. He bought the patent method of separating the propane-butane mixture (gaseous fractions) from petrol from American chemist Walter O. Snelling. Shortly after that, Austrian entrepreneur Ernesto Igel founded Empresa Brasileira de Gáz a Domilicio Ltda in Brazil, the first Brazilian gas distribution network for cooking equipment through bottled LPG. By 1938 the Brazilian company changed its name to Ultragaz S.A., acquiring three distribution trucks and 166 customers, and today Ultragaz are one of the biggest LPG operators globally. In Europe, Italian company Liquigas – today a subsidiary of a leading Italian group in the distribution of LPG and LNG, SHV Energy Group – started bottled supplies of LPG near Venice in 1938.

By World War II, around 62% of all American households had LPG installations. By 1947, the first liquefied gas tanker was built and entered service exporting large LPG volumes outside the US. In 1950, LPG use as transport fuel gained popularity, and in 1965 Chevrolet introduced four new LPG-powered engines for commercial vehicles. Today, we find LPG everywhere: industrial, commercial, transport, and residential. LPG is considered the easiest fuel option for remote supplies through road tankers with the added advantage of lesser environmental pollution – a significant property for energy market penetration under clean fuel transition worldwide.

The modern LPG market distinguishes LPG supplies based on the percentage of Propane content in LPG (mixture of Propane and Butane) enclosure – the lesser the Butane content in LPG higher the price and quality of LPG. Propane is a clean-burning gaseous fuel declared clean fuel under US Clean Air Act 1990 (amended to reduce NOx / SOx emissions).

1.1 Important Technical and Safety Details – LPG Operations

Commercially produced LPG is available as a mixture of primarily propane, butane, isobutane with traces of butylene, propylene and mixtures of other gases, depending on the process and feed (crude oil or associated gas) used for its extraction. LPG components are produced/extracted during crude oil refining (through fractional distillation) and by processing natural gas. Usually, the throttling of natural gas is carried out using Jetty-Valve or Turbo-Expander, and LPG components drop out of natural gas at cooler temperatures created during the throttling process. Turbo-Expander technology is suitable when high Propane-extraction-efficiency is targeted during production. LPG components remain in liquid form, being under pressure, throughout their supply chain with a top vapor space of enclosure – vapor pressure and concentration rise with the increase in temperature and reduction in pressure of enclosure. To avoid boiling of LPG during transportation, refrigerated tankers and refrigerated storage tanks are also in use, which significantly enhances the transport capacity and efficiency of LPG delivery along the supply chain.

LPG is a colorless and odorless gas (both Propane and Butane). There is no difference in domestic LPG composition and the commercial/industrial LPG composition – being typically the same propane-butane mixture. For safety reasons, enabling a pungent smell to detect leakage, Ethyl Mercaptan is added as an Odorant. Propane has a slightly better volumetric energy density @ 91,500 BTU/US gallon than LNG @ 89,500 BTU/US gallon and is a lot easier in handling than LNG.

Some important properties of Propane are listed below for reference:

  • LPG (propane) boils at -42°C or -44°F, becoming gas vapor under atmospheric pressure.
  • LPG has an adiabatic flame temperature of about 1970°C when burned in air.
  • LPG (propane) gas boiling point temperature: -42 °C
  • LPG (propane) gas melting/freezing temperature: -188 °C
  • LPG-Propane auto-ignition temperature: 470 °C
  • Gaseous expansion of LPG: 1 L (liquid) = 0.27 m3 (gas)
  • LPG limits of flammability: LPG volumetric concentration in air is between 2.15% to 9.6%

Note: Natural Gas Liquids (NGLs) are distinguished from LPG due to the presence of a few more gases not normally included in LPG. NGL composition and LPG include traces of ethane, ethene, butylene, propylene, and strong concentrations of propene, isobutene, butadiene, pentane, pentene and pentanes plus.

1.2 Understanding Wobbe Index

Liquid LPG density is about half that of water. However, LPG is heavier than air than natural gas is lighter than air.

  • Specific Gravity of Pipeline Quality Natural Gas = 0.6
  • Lower Heating value of Pipeline Quality Natural Gas = 950 Btu / SCF
  • Specific Gravity of Propane vapor = 1.55
  • Lower Heating value of Propane vapor = 2370 Btu / SCF
  • Specific Gravity of Butane vapor = 2.08
  • Lower Heating value of Butane vapor = 2975 Btu / SCF

Wobbe Index (WI) is defined hereunder:

Using values of LHV and SG provided above, following values of WI are calculated:

  • Wobbe Index of Natural Gas = 1226
  • Wobbe Index of Propane Gas = 1904
  • Wobbe Index of Butane Gas = 2063
  • Wobbe Index of LPG (20% Butane mix with 80% Propane) = 1935

Controlling Wobbe Index is important while designing combustion nozzles for gaseous fuels. A nozzle designed for natural gas burning (WI = 1226) will not provide efficient combustion when it receives LPG (WI = 1935). So, it is important to reduce the Wobbe Index of LPG to be used as a replacement fuel for natural gas fuel systems. Wobbe Index of LPG can be reduced to the level similar to natural gas by creating an LPG-Air Mix, also known as Synthetic Natural Gas (SNG).

2 Skid-Mounted Synthetic Natural Gas Supply Systems

SNG can be directly used as a backup fuel when natural gas supplies get interrupted for any reason since Wobbe Index indicates the heat content per unit volume relative to air density at Standard Conditions of temperature and pressure. Once appropriate LPG-Air Mix is achieved that has a Wobbe Index close to that of natural gas, same natural gas sized fuel tubing, feed systems and fuel nozzles can serve efficient combustion of SNG. SNG with slightly lower WI will require high volume to provide a given heat output, and SNG with higher WI requires low volume to provide the same heat output. A homogeneous mixture that is achieved through proper mixing of LPG with compressed air can be used in any gas-fired equipment as a direct replacement for natural gas combustion applications – only such systems are restricted that are intolerant to any nitrogen (like hydrogen atmosphere generators) and very high purity methane (as natural gas) is burned in those systems.

Skid-mounted SNG Supply Systems – also known as Propane-Air or LPG-Air – can supply WI optimized SNG at remote locations and in the urban metropolis and industrial zones. Experts believe that such skid-mounted SNG Supply Systems can be economically deployed for gas loads over 40 – 50 MMBTU per day.

A typical Skid-mounted SNG Supply System consists of LPG storage facilities, truck unloading stations, transfer pumps, propane vaporizers, air compressors, propane-air mixers requiring control valves, gas flow rate meters, flow computers, calorific value measurement device, and system controls.

2.1 Use As Alternate Economic Fuel or Back-up Fuel

Skid-mounted Propane-air, also called LPG-Air or SNG, supply systems require LPG vapor supply from the storage tank and compressed air to produce SNG at the site continuously. The propane air mixture, supplied by Skid-mounted SNG Supply System, can be easily and compatibly connected to the natural gas piping – downstream of the grid supply metering station and pressure regulating assembly. Such systems need to be placed in an open atmosphere to ensure that any leaking LPG vapors do not accumulate in a low-lying pocket (being heavier than air LPG usually takes time to disperse and thus creates explosion hazard is leakage occurs). Such connectivity of Skid-mounted SNG Supply System provides live back-up – these systems, if appropriately designed, are capable of taking over upon falling grid supply pressure. These systems are most economical and beat diesel, fuel oil or propane supplies due to no requirement related to duplication of fuel supply systems – SNG does not require additional gas trains, piping, regulators, or special fuel delivery systems at the site serve as a backup energy source.

It is worth mentioning that Skid-mounted SNG Supply Systems are not just good as backup energy supply systems during interruption of primary energy sources. Still, they can also be used in winters for significant economic benefits when grid natural gas supplies become too expensive.

Process industries, like glass or chemical industries, essentially require operating continuously. There could be a significant economic loss to the industrial concern in case of natural gas interruption from the grid. Skid-mounted SNG Supply Systems play a significant role in covering fuel supply risks for any going concern. This enables the business to increase the reliability of fuel supplies and sustain its competitive position in the market.

Another advantage of SNG over other liquid fuel supplies is an environmentally friendly energy source – SNG is a clean-burning fuel like natural gas. Thus Skid-mounted SNG Supply Systems enable companies to benefit from interruptible SNG flow rates while relying on environmentally clean fuel sources.

2.2 Use as Peak Shaving Fuel by Utilities

Although SNG supplies to consumers through distribution grids are not recommended for various safety and operational reasons, utilities can use Skid-mounted SNG Supply Systems to serve bulk consumers at their premises. During winters, when the leading source of energy for domestic heating in the world is natural gas, the option of procuring LNG is expensive and capital extensive. Thus, Natural Gas Distribution Companies can improve their limited number of options available to help balance and minimize the effects of winter demand constraints by arranging SNG supplies for bulk consumers through Skid-mounted SNG Supply Systems. No doubt, the drawback to using SNG is the higher cost of fuel and the need for compressed air when mixing. Still, compared to regasification and transportation of LNG, the differential in initial capital investment is so extreme that propane-air peak shaving systems often prove economical.

3 Compliance and Standards

Several international standards and recommended guidelines ensure safe production, transport, storage, and use of LPG. Significantly, Codes of Practice by LIQUID GAS UK LIMITED (Formerly LPGITA – LPG Industry Technical Association, UK) provide essential information in this regard. Additionally, the following are some relevant standards:

  • GPA 2140-17 standard describes the specifications related to physical properties and characteristics of LPG used for domestic, commercial and industrial applications
  • DOT special permit as per NFPA 58 – Standard for storage and handling of LPG
  • DOT special permit as per NFPA 58, ISO 11119-3 Part 3 – Manufacturing standards of LPG composite cylinders
  • US Government CFR – Title 49-Transportation, Part 178 – 199, DOT specifications 4B, 4BA and 4BW – Recommendations for welded steel cylinders
  • BS 5430: Part 3 – Periodic Inspection, Testing and Maintenance of Transportable Gas Containers
  • DIN EN 14678-1 – LPG Equipment and Accessories – Construction and performance of LPG equipment for automotive filling stations – Part 1: Dispensers

The API standards areas provide useful experience-based facts and solutions to LPG measurement issues in storage tanks – one of the most important considerations for efficient management of the LPG supply chain.

  • API MPMS CH3.1A Ed. 3 (2013) – Manual of Petroleum Measurement Standards – Chapter 3.1A: Standard Practice for The Manual Gauging of Petroleum and Petroleum Products
  • API MPMS CH3.1B Ed. 2 (2001/R2016) – Manual of Petroleum Measurement Standards – Chapter 3 – Tank Gauging – Section 1B – Standard Practice for Level Measurement of Liquid Hydrocarbons in Stationary Tanks by Automatic Tank Gauging
  • API MPMS CH3.3 – Manual of Petroleum Measurement Standards – Chapter 3 – Tank Gauging – Section 3 – Standard Practice for Level Measurement of Liquid Hydrocarbons in Stationary Pressurized Storage Tanks by Automatic Tank Gauging
  • API MPMS CH3.6 Ed. 1 (2001/R2011) – Manual of Petroleum Measurement Standards – Chapter 3 – Tank Gauging – Section 6 – Measurement of Liquid Hydrocarbons by Hybrid Tank Measurement Systems
  • API MPMS CH7 – Manual of Petroleum Measurement Standards – Chapter 7: Temperature Determination
  • API MPMS CH7.3 Ed. 2 (2011/R2016) – Manual of Petroleum Measurement Standards – Chapter 7.3: Temperature Determination – Fixed Automatic Tank Temperature Systems
  • API MPMS Ch 2.2E / ISO 12917-1:2002 – Calibration of Horizontal Tanks. (Petroleum and Liquid Petroleum Products—Calibration of Horizontal Cylindrical Tanks—Part 1: Manual Methods supersedes API 2551)
  • API MPMS Ch 2.2F / ISO 12917-2:2002 – Calibration of Horizontal Tanks. (Petroleum and Liquid Petroleum Products—Calibration of Horizontal Cylindrical Tanks—Part 2: Internal Electro-Optical Distance-Ranging Method)
  • API 2552 – Calibration of Spherical Tanks.
  • API MPMS Ch 2.7 (Also required API MPMS Ch 2.8A & 2.8B) – Calibration of Barge Tanks.
  • API Std 2554 (R2012) – Measurement and Calibration of Tank Cars.
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