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Fractional Composition
9.4 ADSORPTION
Adsorption is the bonding of molecules or particles to a surface. On the other hand, absorption
is the filling of pores in a solid. The bonding to the surface is usually (but not always) weak and
reversible. Compounds that contain functional groups are very often strongly adsorbed on activated
carbon.
The most common industrial adsorbents are activated clay, carbon, silica gel, and alumina,
because they present enormous surface areas per unit weight. Clay is a naturally occurring mineral. Roasting organic material to decompose it to granules of carbon produces activated carbon.
Coconut shell, wood, and bone are common sources of activated carbon. Silica gel is a matrix of
hydrated silicon dioxide. Alumina is mined or precipitated aluminum oxide and hydroxide.
Temperature effects on adsorption are profound, and measurements are usually at a constant
temperature. Graphs of the data are called isotherms. Most steps using adsorbents have little variation in temperature.
9.4.1 CHemICAl FACtors
As already stated (Chapter 8), petroleum is a complex mixture of paraffin, naphthene, and aromatic
hydrocarbons, as well as nitrogen-, oxygen-, and sulfur-containing compounds and traces of a variety of metal-containing compounds; the amounts of non-hydrocarbon compounds increase with
molecular weight. By definition, the saturate fraction consists (or should consist) of paraffins and
cycloparaffins (naphthenes). The single-ring naphthenes, or cycloparaffins, present in petroleum
are primarily alkyl-substituted cyclopentane and cyclohexane. The alkyl groups are usually quite
short, with methyl, ethyl, and isopropyl groups being the predominant substituents. As the molecular weight of the naphthenes increases, the naphthene fraction contains more condensed rings with
six-membered rings predominating. However, five-membered rings are still present in the complex
higher molecular weight molecules.
The aromatics fraction consists of those compounds containing an aromatic ring and varies from
monoaromatics (containing one benzene ring in a molecule) to diaromatics ( substituted naphthalene)
to triaromatics (substituted phenanthrene). Higher condensed ring systems ( tetraaromatics,
pentaaromatics) are also known but are somewhat less prevalent than the lower ring systems, and
each aromatic type will have increasing amounts of condensed ring naphthene attached to the aromatic ring as molecular weight is increased.
However, depending upon the adsorbent employed for the separation, a compound having an aromatic ring (i.e., six aromatic carbon atoms) carrying side chains consisting in toto of more than six
carbon atoms (i.e., more than six nonaromatic carbon atoms) will appear in the aromatic fraction.
The typical nitrogen compounds found in petroleum are generally divided into two groups,
basic and nonbasic (Chapters 8 and 9), each of which has alkyl chains and other ring systems.
The basic nitrogen compounds cause difficulty with the many acid-catalyzed processes used in
petroleum refining. For example, in catalytic cracking, the basic nitrogen adsorbs on the catalytic
acid sites and reduces the cracking activity of the catalyst. Furthermore, in reactions catalyzed by
liquid acids, the presence of basic nitrogen compounds in the feed increases acid consumption and,
thus, the cost of the process. Typical of such processes are alkylation, isomerization, and olefin
absorption.
Another type of nitrogen compound is located in the porphyrin derivatives (Chapter 8), which
consist of four pyrrole rings connected together with methylene bridges at the carbons next to the
nitrogen atoms. They are normally found in trace quantities in the high molecular weight fractions
of petroleum as metal complexes.
The oxygen compounds found in petroleum fractions are often products of exposure to air.
However, some naturally occurring oxygenated compounds do exist in petroleum; these are
Fractional Composition
9.4 ADSORPTION
Adsorption is the bonding of molecules or particles to a surface. On the other hand, absorption
is the filling of pores in a solid. The bonding to the surface is usually (but not always) weak and
reversible. Compounds that contain functional groups are very often strongly adsorbed on activated
carbon.
The most common industrial adsorbents are activated clay, carbon, silica gel, and alumina,
because they present enormous surface areas per unit weight. Clay is a naturally occurring mineral. Roasting organic material to decompose it to granules of carbon produces activated carbon.
Coconut shell, wood, and bone are common sources of activated carbon. Silica gel is a matrix of
hydrated silicon dioxide. Alumina is mined or precipitated aluminum oxide and hydroxide.
Temperature effects on adsorption are profound, and measurements are usually at a constant
temperature. Graphs of the data are called isotherms. Most steps using adsorbents have little variation in temperature.
9.4.1 CHemICAl FACtors
As already stated (Chapter 8), petroleum is a complex mixture of paraffin, naphthene, and aromatic
hydrocarbons, as well as nitrogen-, oxygen-, and sulfur-containing compounds and traces of a variety of metal-containing compounds; the amounts of non-hydrocarbon compounds increase with
molecular weight. By definition, the saturate fraction consists (or should consist) of paraffins and
cycloparaffins (naphthenes). The single-ring naphthenes, or cycloparaffins, present in petroleum
are primarily alkyl-substituted cyclopentane and cyclohexane. The alkyl groups are usually quite
short, with methyl, ethyl, and isopropyl groups being the predominant substituents. As the molecular weight of the naphthenes increases, the naphthene fraction contains more condensed rings with
six-membered rings predominating. However, five-membered rings are still present in the complex
higher molecular weight molecules.
The aromatics fraction consists of those compounds containing an aromatic ring and varies from
monoaromatics (containing one benzene ring in a molecule) to diaromatics ( substituted naphthalene)
to triaromatics (substituted phenanthrene). Higher condensed ring systems ( tetraaromatics,
pentaaromatics) are also known but are somewhat less prevalent than the lower ring systems, and
each aromatic type will have increasing amounts of condensed ring naphthene attached to the aromatic ring as molecular weight is increased.
However, depending upon the adsorbent employed for the separation, a compound having an aromatic ring (i.e., six aromatic carbon atoms) carrying side chains consisting in toto of more than six
carbon atoms (i.e., more than six nonaromatic carbon atoms) will appear in the aromatic fraction.
The typical nitrogen compounds found in petroleum are generally divided into two groups,
basic and nonbasic (Chapters 8 and 9), each of which has alkyl chains and other ring systems.
The basic nitrogen compounds cause difficulty with the many acid-catalyzed processes used in
petroleum refining. For example, in catalytic cracking, the basic nitrogen adsorbs on the catalytic
acid sites and reduces the cracking activity of the catalyst. Furthermore, in reactions catalyzed by
liquid acids, the presence of basic nitrogen compounds in the feed increases acid consumption and,
thus, the cost of the process. Typical of such processes are alkylation, isomerization, and olefin
absorption.
Another type of nitrogen compound is located in the porphyrin derivatives (Chapter 8), which
consist of four pyrrole rings connected together with methylene bridges at the carbons next to the
nitrogen atoms. They are normally found in trace quantities in the high molecular weight fractions
of petroleum as metal complexes.
The oxygen compounds found in petroleum fractions are often products of exposure to air.
However, some naturally occurring oxygenated compounds do exist in petroleum; these are