Chapter 12: Distillation and Vapor Pressure Data in Liquefied Petroleum Gas (LPG)

Abstract

LIQUEFIED PETROLEUM GAS (LPG) IS A GENERIC term for ethane (C2) to butane (C4) hydrocarbon mixtures that can exist as liquids under modest pressures at ambient temperatures. Methane (C1, natural gas) must be refrigerated to less than −162 ° C (−259 ° F) to be condensed by compression to liquefied natural gas (LNG). Pentane and heavier hydrocarbons (C5 +, condensate) are liquids at ambient temperature and pressure, and are used in the manufacture of gasoline, naphtha fuels, and solvents. Ethane, propane, and butane are gases at standard temperature and pressure, but can be liquefied by compression and condensation of the vapor at or below ambient temperature. Propane (C3), butane (C4), and butane/propane mixtures (C3/C4 or B-P mix) have ideal properties for a fuel, widely used throughout the world in an amazing variety of applications. They are stable, high energy content, relatively low sulfur, clean burning fuels that can be transported economically as a liquid, and be used either as a liquid or a gas. Propane can be used from about −40 to 45 ° C, and butane from 0 ° C to about 110°C (about 0 to 250 psig vapor pressure) or higher depending on the pressure ratings of the equipment being used. The contents of the LPG tank are always under pressure at temperatures above the normal boiling point of about −42 ° C for propane and 0 ° C for butane, so there is no need for a fuel pump or electrical components for most applications. This makes LPG ideally suited to a wide variety of portable, mobile, or remote applications, using mechanically reliable and simple equipment. Propane applications tend to be robust and reliable as a result. This chapter deals mostly with ASTM D1835 “Standard Specification for Liquefied Petroleum (LP) Gases” [1] Liquefied Petroleum Gas (LPG), “Commercial Propane,” and “Special Duty Propane” grades (commonly known as “spec” products). The same sampling and test methods can apply to other Natural Gas Liquids (NGLs) that can have a wider range of compositions. The same distribution equipment, rail/truck tanks, and storage vessels can be used for propane or B-P mixtures so it is not outwardly apparent what grade of LPG is being used in a particular region or application. The terms “propane,” “LPG,” and “HD5” are commonly used interchangeably in North America (although this is not technically correct). ASTM D1835 and Gas Processors Association GPA 2140 specifications exist for “Commercial B-P Mixtures,” but this is rarely used for consumer applications in North America. There is no current Canadian General Standards Board (CGSB) specification for B-P mixtures, as the winter temperatures are too cold, and butane demand is high for winter gasoline production. Polar climates must use propane year round for low-temperature operability. Tropical climates (no winter temperatures, no winter gasoline) tend to use B-P mixtures year round to utilize the butane. Temperate climates with large seasonal temperature changes could use propane in the winter and B-P mixtures in the summer. However, the logistics of seasonal distribution and air/fuel calibration changes, coupled with a more than adequate propane supply generally favors the use of propane throughout the year. The same properties that make LPG so useful contribute to some of the unique challenges in using it safely. Water/ice/hydrate properties are unique. Pressurized systems are more prone to leaks, even when the equipment is idle. The heavier than air vapor density allows accumulation in low points and cavities in the absence of ventilating air flow. When mixed with air in the right ratio in the narrow range, it has the potential for high destructive power in the event of an ignition source that results in an explosion or fire. Consequently, persons handling LPG and install equipment are typically required to be trained and licensed. A variety of safety devices and procedures are used, and LPG is odorized to help detect potentially dangerous leaks.