Gas licuado de petróleo

Kirk-Othmer Encyclopedia of Chemical Technology. Copyright c John Wiley & Sons, Inc. All rights reserved.

LIQUEFIED PETROLEUM GAS
Liquefied petroleum gas (LPG) is a subcategory of a versatile class of petroleum products known as natural gas liquids (NGLs) that are produced along with and extracted from natural gas (see Gas, natural). LPG is also produced from the refining of crude oil (seePetroleum). Although LPG is commercially defined as propane [74-98-6], butane [106-97-8], and butane–propane mixtures, commercial availability is primarily limited to propane (see Hydrocarbons). There are two grades of specification propane, propane HD-5 and special-duty propane. The primary difference in the two grades is that the propylene [115-07-1] content of propane HD-5 is restricted to a maximumof 5 vol %. Propylene (qv) is found only in refinery-produced propane. The principal uses of LPGs are as fuels and feedstocks (qv) for the production of motor gasoline and a wide variety of chemicals (see Fuels, synthetic; Gasoline and other motor fuels). Other natural gas liquids include natural gasoline [8006-61-9], which is composed of the pentanes and heavier components of the natural gasstream, and ethane [74-84-0]. Most recently ethane has become the principal product of natural gas processing plants.

1. Properties
In general, LPG specifications involve limits for physical properties. Consequently, the composition of the commercial-grade products varies between wide limits. Physical properties of the principal components of LPG are summarized in Table 1 (1).

2. Manufacture andProcessing
LPG recovered from natural gas is essentially free of unsaturated hydrocarbons, such as propylene and butylenes (qv). Varying quantities of these olefins may be found in refinery production, and the concentrations are a function of the refinery’s process design and operation. Much of the propylene and butylene are removed in the refinery to provide raw materials for plastic and rubberproduction and to produce high octane gasoline components. LPG is recovered from natural gas principally by one of four extraction methods: turboexpander, absorption (qv), compression, and adsorption (qv). Selection of the process is dependent on the gas composition and the degree of recovery of ethane and LPG, particularly from large volumes of lean natural gas.
2.1. Turboexpander Process

Ethanehas become increasingly desirable as a petrochemical feedstock resulting in the construction of many plants that recover the ethane from natural gas at −73 to −93◦ C . Combinations of external refrigeration and liquid flash-expansion refrigeration with gas turboexpansion cycles are employed to attain the low temperatures
1

2

LIQUEFIED PETROLEUM GAS

Table 1. Physical Properties of LPGComponentsa
Component ethane propane isobutane n-butane 1-butene cis-2-butene trans-2-butene n-pentane
a Ref. b To

CAS Registry Number [74-84-0] [74-98-6] [75-28-5] [106-97-8] [106-98-9] [590-18-1] [624-64-1] [109-66-0]

Molecular formula C2 H6 C3 H8 C4 H10 C4 H10 C4 H8 C4 H8 C4 H8 C5 H12

Boiling point, 101.3 kPa,b ◦ C
−88.6 −42.1 −11.8 −0.5 −6.3

Vapor pressure, 37.8◦ C, kPab 1310 498356 435 314 343 107

Liquid density, g/Lc 354.9 506.0 561.5 583.0 599.6 625.4 608.2 629.2

3.7 0.9 36.0

1. convert kPa to psi, multiply by 0.145. c At saturation pressure.

Fig. 1. One-expand cycle with external refrigeration for high ethane recovery in the hydrocarbon liquid product.

desired for high ethane recovery. Figure 1 is a flow diagram of a one-expander cycle having externalrefrigeration (see Refrigeration). Dry inlet gas that has been dehydrated by molecular sieves (qv) or alumina beds to less than 0.1 ppm water is split into two streams by a three-way control valve. Approximately 60% of the inlet gas is cooled by heat exchange with the low pressure residue gas from the demethanizer and by external refrigeration. The remainder of the inlet gas is cooled by heat...