22
The Chemistry and Technology of Petroleum
To further define the terms dry and wet in quantitative measures, the term dry natural gas indicates that there is less than 0.1 gal (1 gal, US, = 264.2 m 3 ) of gasoline vapor (higher molecular
weight paraffins) per 1000 ft 3 (1 ft 3 = 0.028 m 3 ). The term wet natural gas indicates that there are
such paraffins present in the gas, in fact more than 0.1 gal/1000 ft 3 .
Associated or dissolved natural gas occurs either as free gas or as gas in solution in the petroleum. Gas that occurs as a solution in the petroleum is dissolved gas, whereas the gas that exists in
contact with the petroleum (gas cap) is associated gas.
Other components such as carbon dioxide (CO 2 ), hydrogen sulfide (H 2 S), mercaptans (thiols;
R-SH), as well as trace amounts of other constituents may also be present. Thus, there is no single
composition of components that might be termed typical natural gas. Methane and ethane constitute the bulk of the combustible components; carbon dioxide (CO 2 ) and nitrogen (N 2 ) are the major
noncombustible (inert) components.
1.5 MANUFACTURED MATERIALS
1.5.1 WAx
The term paraffin wax is restricted to the colorless, translucent, highly crystalline material obtained
from the light lubricating fractions of paraffinic crude oils (wax distillates). The commercial products melt in the approximate range of 50°C–65°C (120°F–150°F). Dewaxing of heavier fractions
leads to semisolid material, generally known as petrolatum, and solvent de-oiling of the petroleum
or of heavy, waxy residua results in dark-colored waxes of a sticky, plastic to hard nature. The waxes
are composed of fine crystals and contain, in addition to n-paraffins, appreciable amounts of
iso-paraffins and cyclic hydrocarbon compounds substituted with long-chain alkyl groups. The melting points of the commercial grades are in the 70°C–90°C (160°F–195°F) range (Brooks et al., 1938).
Highly paraffinic waxes are also produced from peat, lignite, or shale oil residua, and paraffin
wax known as ceresin, which are quite similar to the waxes from petroleum, may also be prepared
from ozokerite.
1.5.2 resId
A resid (residuum, plural residua) is the residue obtained from petroleum after nondestructive
distillation has removed all the volatile materials. The temperature of the distillation is usually
maintained below 350°C (660°F) since the rate of thermal decomposition of petroleum constituents is minimal below this temperature but the rate of thermal decomposition of petroleum
constituents is substantial above 350°C (660°F).
Resids are black, viscous materials and are obtained by distillation of a crude oil under atmospheric pressure (atmospheric residuum) or under reduced pressure (vacuum residuum). They may
be liquid at room temperature (generally atmospheric residua) or almost solid (generally vacuum
residua) depending upon the nature of the crude oil (Chapter 17).
When a residuum is obtained from a crude oil and thermal decomposition has commenced, it
is more usual to refer to this product as pitch (see also page 23) The differences between a parent
petroleum and the residua are due to the relative amounts of various constituents present, which are
removed or remain by virtue of their relative volatility.
The chemical composition of a residuum from an asphaltic crude oil is complex. Physical methods of fractionation usually indicate high proportions of asphaltenes and resins, even in amounts up
to 50% (or higher) of the residuum. In addition, the presence of ash-forming metallic constituents,
including such organometallic compounds as those of vanadium and nickel, is also a distinguishing
feature of residua and the heavier oils. Furthermore, the deeper the cut into the crude oil, the greater
is the concentration of sulfur and metals in the residuum and the greater the deterioration in physical
properties (Chapter 17).
The Chemistry and Technology of Petroleum
To further define the terms dry and wet in quantitative measures, the term dry natural gas indicates that there is less than 0.1 gal (1 gal, US, = 264.2 m 3 ) of gasoline vapor (higher molecular
weight paraffins) per 1000 ft 3 (1 ft 3 = 0.028 m 3 ). The term wet natural gas indicates that there are
such paraffins present in the gas, in fact more than 0.1 gal/1000 ft 3 .
Associated or dissolved natural gas occurs either as free gas or as gas in solution in the petroleum. Gas that occurs as a solution in the petroleum is dissolved gas, whereas the gas that exists in
contact with the petroleum (gas cap) is associated gas.
Other components such as carbon dioxide (CO 2 ), hydrogen sulfide (H 2 S), mercaptans (thiols;
R-SH), as well as trace amounts of other constituents may also be present. Thus, there is no single
composition of components that might be termed typical natural gas. Methane and ethane constitute the bulk of the combustible components; carbon dioxide (CO 2 ) and nitrogen (N 2 ) are the major
noncombustible (inert) components.
1.5 MANUFACTURED MATERIALS
1.5.1 WAx
The term paraffin wax is restricted to the colorless, translucent, highly crystalline material obtained
from the light lubricating fractions of paraffinic crude oils (wax distillates). The commercial products melt in the approximate range of 50°C–65°C (120°F–150°F). Dewaxing of heavier fractions
leads to semisolid material, generally known as petrolatum, and solvent de-oiling of the petroleum
or of heavy, waxy residua results in dark-colored waxes of a sticky, plastic to hard nature. The waxes
are composed of fine crystals and contain, in addition to n-paraffins, appreciable amounts of
iso-paraffins and cyclic hydrocarbon compounds substituted with long-chain alkyl groups. The melting points of the commercial grades are in the 70°C–90°C (160°F–195°F) range (Brooks et al., 1938).
Highly paraffinic waxes are also produced from peat, lignite, or shale oil residua, and paraffin
wax known as ceresin, which are quite similar to the waxes from petroleum, may also be prepared
from ozokerite.
1.5.2 resId
A resid (residuum, plural residua) is the residue obtained from petroleum after nondestructive
distillation has removed all the volatile materials. The temperature of the distillation is usually
maintained below 350°C (660°F) since the rate of thermal decomposition of petroleum constituents is minimal below this temperature but the rate of thermal decomposition of petroleum
constituents is substantial above 350°C (660°F).
Resids are black, viscous materials and are obtained by distillation of a crude oil under atmospheric pressure (atmospheric residuum) or under reduced pressure (vacuum residuum). They may
be liquid at room temperature (generally atmospheric residua) or almost solid (generally vacuum
residua) depending upon the nature of the crude oil (Chapter 17).
When a residuum is obtained from a crude oil and thermal decomposition has commenced, it
is more usual to refer to this product as pitch (see also page 23) The differences between a parent
petroleum and the residua are due to the relative amounts of various constituents present, which are
removed or remain by virtue of their relative volatility.
The chemical composition of a residuum from an asphaltic crude oil is complex. Physical methods of fractionation usually indicate high proportions of asphaltenes and resins, even in amounts up
to 50% (or higher) of the residuum. In addition, the presence of ash-forming metallic constituents,
including such organometallic compounds as those of vanadium and nickel, is also a distinguishing
feature of residua and the heavier oils. Furthermore, the deeper the cut into the crude oil, the greater
is the concentration of sulfur and metals in the residuum and the greater the deterioration in physical
properties (Chapter 17).