Depolarized Petroleum Naphthas CLYVEALLEN Anderson-Prichard Research & Development Laboratory, Cyril, Okla.
A discussion is given of the concept of polarity, in so far as it applies to the substances naturally present in petroleum oils. For characterization purposes petroleum constituents are divided into three classes: polar, polarizable, and nonpolar. The practically important properties of each class-odor, stability, volatility, viscosity, s o h bility, and surface activity-are reviewed. Differences in properties are clarified, between petroleum products as regularly refined and as depolarized, the term “depolarize” meaning removal of polar and polarizable compounds. The advantages of depolarized naphthas in several industrial applications are pointed out, and new uses of depolarized naphthas are indicated.
tions of N, S, and 0 with double bonds. Organic sulfur groups in general may be considered to exist as polar analogs of the oxygen groups listed (21). This view is equivalent to defining a polar compound as one containing one or more polar groups, noting the possibility of internal compensation. The term “polarizable” may be defined in an analogous manner, If the attachment or near approach of a polar grouping to a molecular structure induces polarity in the structure, the latter can be said to be polarizable. For example, a n olefinic or aromatic double bond is polarizable. Polarizable molecules may be looked upon as potentially or latently polar. I n the case of aromatics, the electrostatic charge forces are in general partly or wholly internally compensated so that the hydrocarbon molecule over-all has little or no moment ( I S , S$)-for example,
A
HIGHLY refined grade of petroleum naphthas has in recent years achieved industrial significance. This grade, termed “deodorized” or “depolarized,” is characterized by nonpolar paraffinic and cycloparaffinic composition and the absence of polar and polarizable substances. This discussion reviews the effects of polar compounds in solvents and endeavors to make plain the practical value of depolarized naphthas.
Aromatic Hydrocarbon Benzene Toluene Naphthalene MethyInaphthalene
0 0 0 0
polar forces exhibit themselves in olefinic and aromatic pounds, whether internally compensated or not, by strong attractions between atoms, and result in denser structures-that is,
POLAR, POLARIZABLE, AND APOLAR NAPHTHA CONSTITUENTS
Technical solvent constituents may be arbitrarily divided, on the basis of certain properties, into the classes polar, polarizable, and nonpolar. Polar compounds receive their name from having in their molecules one or more pairs of positive and negative.poles. The poles are created by a difference in the location of molecular centers of positive and negative charges. Polarity is associated qualitatively with molecular dipole moment, which is the product of electric charge times distance of separation ( I S ) . Dipole moment or polarity may be considered 8s localized in certain molecular bonds or groups ( 5 ) . I n a series OfComPounds which have a single polar group, the m0ments of the m m b e r s of the series are approximately constant. Characteristic magnitudes are ( I S ): Compound Saturated normal hydrocarbons Normal aliphatic acids Ethers Normal amines Mercaptans Sulfides Alcohols Phenols Esters Alkyl chlorides Ketones Nitroparaffins Kitriles
Moment 0 x 10-ia 4 0 4
Substance Aliphatic hydrocarbons Cycloparaffins Naphthalene Benzene Olefins Acetylenes
Bonding
c-c c-c c-c c-c c =c CEC
Distance between Atoms, A.
:3
1 1 1 1
41
39 34 22
Another way in which polar influences, both active and latent, become evident is in the dielectric constant. The dielectric constant of a substance is the ratio of the electrical chpacity of a condenser containing the substance to its capacity at the same electrostatic potential when the condenser is evacuated. Polar molecules tend to orient themselves in an electrical field, thus greatly increasing electrical condenser capacity and, consequently, dielectric constant (23, 25). Moreover, the electrical field induces an appreciable moment in polarizable groups. The dielectric constant is therefore one of the most useful criteria of active and iqipient polarity. Examples of the magnitude of the dielectric constant are given in Table I, with the hydrocarbons arranged in order of increasing molecular weight, so that structural contributions are immediately evident.
hl o men t 0 0 x 10-1s 0 8 1 23 1 3 1 5 1 7 1 . 78 2 0 2 7
3 1 3 4
T 4 B L E I. DIELECTRIC CONSTANTS O F HYDROCARBONS Some molecules containing polar groups may be internally compensated by equal and opposite individual group moments, and therefore have very small or zero over-a!l dipole moment. Such molecules may, however, exert unmistakable polar effects in their immediate neighborhood (28). The struct&l grouping rather than the molecular moment’ is, therefore, a better criterion of polarity. Examples of polar groups are (18): NO2 (nitro), CN (nitrile), NH2 and NHCHa (amino), NCS (iso-thiocyano), CO (carbonyl), COH (hydroxy), COR (ether), COZHand COZM (carboxyl), C 0 2 R (ester), I, Br, and C1 (halogen), and combina-
Hydrocarbon n-Pentane Benzol Cyclohexane n-Hexane Toluol n-Heptane Ethyl benzol n-Octane Kaphthalene Decalin Methyl naphthalene n-Decane
124
Molecular Weight 72 78 84 86 92 100
106 114 128 138 142 142
Dielectric Consbant 1 84 2 25 2 03 1 89 2 39 1 92 2 40 1 95 2 60
2 16 2 79 1 99
INDUSTRIAL AND ENGINEERING CHEMISTRY
January 1948
I n the case of dielectric constant, as in the case of dipole moment, if the fields of force of polar groups can be neutralized within the molecule itself, the molecular orientating force may be quite small. Compensation is exemplified in Table 11.
figure, it is commonly multiplied by lo4. The standard light wave lengths are ordinarily the hydrogen alpha and beta (c and F) lines; thus Specific dispersion = (nF
TABLE11. DIELECTRIC CONSTANT AND DIPOLE MOMENTOF UNSYMMETRICAL AND SYMMETRICAL COMPOUNDS Substance Nitromethane Tetranitromethane Chlorobenzene p-Dichlorobenzene Chloroform Carbon tetrachloride
Dielectric Constant 39 2.1 5.9 2.9 5.1 2.2
Moment 3.8 x lo-*'
0.0
1.6 0.0 1.0
125
- nc) d
lo4
Typical specific dispersion values are as follows (16,40) : Hydrocarbpn Class Paraffins and cycloparaffins, all boiling ranges Monoolefins in gasoline boiling range .4romatics in gasoline boiling range Conjugated diolefins in gasoline boiling range Naphthalene .4nthracene
Specific Dispersion 98-99 110-135 168-189 160-243 303 525
0.0
hpolar, or nonpolar, hydrocarbons are those having no polar or polarizable groups-that is, paraffins and cycloparaffins. Another instance of the influence of polar forces is apparent in refractive indices. The refractive index of a substance depends upon the velocity of light in that substance. Since the light velocity is diminished by the interaction of the light waves with the electrostatic-electromagnetic fields in molecules, the refractive index depends upon the polarizability. Approximate constants can be assigned to various bonds (IS,%?,as): Bond
Bond Refraativity 1.21 4.15 6.02
c-c c=c
CEC
Optical polarizability of hydrocarbons as measured by the refractive index, and electrostatic polarizability as measured by the dielectric constant, are related. Electrically symmetrical hydrocarbons obey the equation (23, 26, 38) dielectric constant = square of refractive index. Electrically asymmetrical hydrocarbons show a difference: dielectric constant > square of refractive index (Table 111).
TABLE 111. REFRACTIVEINDEX(n), DIELECTRICCONSTANT (Dk), AND DIPOLEMOMENT (u) OF HYDROCARBONS Dk
Hydrocarbon n-Hexane n-Heptane n-Octane n-Decane Cyclohexane Benzol p-Xylol Naphthalene 0-Xylol m-Xylol Toluol
Ethyl benzol Methyl naphthalene
1.3756 1.3876 1.3976 1.4120 1.4260 1.5014 1.4955 1.6140 1.5046 1.4912 1.4955 1.4951 1.6180
1.89 1.92 1.95 1.99 2.03 2.25 2.24 2.60 2.50 2.37 2.39 2.40 2.79
Dk - n z
0 0 0
0 0 0 0 0 0.32 0.13 0.15 0.16 0.17
u X 10'8
0 0 0 0
0 0 0 0 0.52 0.35 0.34 0.39 0.41
The magnitude of polar differences in the case of lubricating oil processing is shown in Table Is' ( 2 3 ) . ~
~~
TABLE IV. LUBRICATING OIL EXTRACTION RESULTS 011 S0z-raffinate SOz-ex trac t SOz-raffinate SOz-extract
hlol. Wt.
Dk
306 270 319 283
2.177 3.103 2.214 3 224
Dk
-
n* 0.00 0.64 0.00 0 76
u X 1010 0.00 1.04 0.00 1.13
Aniline Pt. 91 4 89 7
POLAR AND POLARIZABLE CONTENT
The- quantities of polar and polarizable substances ordinarily present in petroleum products w e appreciable.' For practical purposes all of the sulfur, nitrogen, and oxygen compounds can be considered as polar. The olefins and aromatics comprise the polarizable hydrocarbon group. Considering first the impurities, several generalities can be remarked. Sulfur varies widely, usually increasing with the boiling range. Ordinarily a medium-boiling mineral spirits cut runs 0.04 * 0.02% in sulfur ( 2 ) . This means a content of sulfur-bearing compounds of about 0.2%, inasmuch as sulfur itself would comprise only about of the weight of the sulfur-containing molecule. Nitrogen is, usually present in petroleum products, ordinarily being less than 0.1% (16,17);in a medium-boiling cut this represents less than 1% nitrogen-containing compounds. Oxygen is in most cases divided between cresylic and naphthenic acids. Cresylic acid content may vary considerably, usually, however, in a medium-boiling cut running about 0.6% ( 2 ) . Naphthenic acids may be expected to the extent of about 0.2% in fractions boiling above 200" C. and to a considerably less extent in lower-boiling fractions. Halogen and other inorganic impurities-sodium, calcium, lead, and copper in the form of salts-may become attached in the oil during refining but should occur only in small amounts. I n general, the total nonhydrocarbon impurities in a representative petroleum distillate can be taken in terms of polar compounds as about 1 to 2%. Similar generalizations may be made respecting polarizable content. Little or no olefinic material exists in crude oils, b u t small amounts may be formed by thermal breakdown during distillation. The olefin content of a commercially straightrrun medium-boiling fraction should not exceed 1% and is usually found to be 0.5 t o 1% ( 2 ) . Aromatics have been found in all petroleum oils examined (2, 16, 17)but usually in small quantity. Ordinarily, medium-boiling fractions will run from 0.5 t o 6% aromatics, with 3% as a fair average. The total polar and polarizable substance content in a typical mineral spirits cut may be taken to be 2 to 6%. I n dcpolarization this 2 to 6% of material is removed, and usually a n additional 6 to 18% of hydrocarbons is removed in the process for complete depolarization. INFLUENCE O F POLARITY ON NAPHTHA PROPERTIES
Probably the most direct way of differentiating between polarizable hydrocarbons (olefins and aromatics) and nonpolar (paraffins and cycloparaffins) lies in the differences in optical specific dispersion. The specific dispersion is obtained by measuring the optical dispersion, which is the difference in refractive index (n) between two standard wave lengths of light, and dividing this value by the density (d) measured at the same temperature. T o avoid inconvenient ciphers in the expressed
The foregoing treatment of polarity can be considered as a collective introduction t o the following matter, in which it is desired t o examine individual polar effects. Polar effects are actually closely interrelated, so that it is not possible t o take up one without tending to involve one or more others. An arbitrary classification can be made, however, which will serve the present purposes. The discussion will proceed according to the following listing of polar phenomena: odor, chemical reactivity (oxidation,
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INDUSTRIAL AND ENGINEERING CHEMISTRY
etc.), association (volatility, vapor pressure, vaporization and drying rates, viscosity), solubility (solvation and solvent retention), and naphtha surface activity (surface tension, absorption, adhesion) ODOR. Ordinary petroleum naphthas have odors instinctively associated with mineral spirits, kerosene, or some sort of “refinery smell,” whereas depolarized petroleum naphthas do not, I n other words, depolarization is ementially synonymous with deodorization, a t least in so far as the human perception of petroleum odors is concerned. Without entering into odor psychology or physiology, it seem obvious that odor of petroleum naphthas is intrinsically a polar effect. The relation of odor t o depolarization has been discussed (36) in the interests of the pest cohtrol industry. An article giving the point of view of the paint industry has also recently appeared (36). The desideratum of the modern paint formulator has been forcefully put (366): “It is to be emphasized that a n odorless thinner means more than merely a doctor sweet naphtha. Only the petroleum chemist is pleased with the odor of a sweet naphtha-to all others the odor is obnoxious. What is desired is a truly odorless thinner or, failing in this, a thinner with a pleasant odor similar, for example, t o the fruity odor of straight-chain paraffins.” Naphtha odor quality is also important in the dry-cleaning trade and in numerous other instances where persons come into close association with naphthas. A blanket statement can be made covering all cases: Depolarization accomplishes the removal of all disagreeable petroleum odors. CHEMICAL REACTIVITY. I n general, greater chemical reactivity is directly related t o greater polarity. Polarizable hydrocarbons should be classed with other polar compounds with respect to reactivity. Aromatic and olefinic structures conventionally represented by and C=C coiild be better represented by
when reactivity is under consideration: A variety of chemicals may add to hydrocarbon double bondsfor example, hydrogen and certain metallic salts (IS,18). Of particular importance in the case of naphthas, oxidizing agents react especially readily with open-chain ethylenic linkages. The elow development of acid or peroxides in ordinary naphthas upon long standing can be attributed primarily t o ethylenic oxidation, although nonhydrocarbon impurities are often important contributing factors. Naphtha depolarization gives a n oxygenstable product. ASSOCIATION.A universal property of atoms and molecules is the exertion of relatively weak forces of attraction (termed “cohesive”) upon other atoms or molecules. Molecules which show marked electrical dissymmetry-polarity-in their structure attract one another by virtue of other and stronger forces in addition to the nonpolar cohesive forces (18); these are called association forces. Association forces result from the mutual interaction of permanent electric dipoles, or they may result from the attraction of a dipole for a n induced dipole of a polarizable molecule (13). Association forces cause deviations from behavior c o m m h to normal or nonpolar substances; thus they are often responsible for boiling points and viscosities being higher, and other related effects, as well as for increases in density and refractive index (18). At the boiling point under atmospheric pressure, the thermal agitation of the particles of a liquid becomes so great that the
Vol 40, No. 1
particles leaving the surface of the liquid exert a pressure of 1 atmosphere. The energy absorbed from the liquid in the transition t o the gaseous state is the latent heat of vaporization. The latent heat of vaporization divided by the absolute boiling point is approximately a constant for nonpolar compounds and higher for polar compounds, because of the additional energy required to overcome electrostatic attractions and break up polar liquid molecular agglomerates. This energy relation can be seen in the following table (25). Latent Heat of Vaporization Boiling Point. K.
Compound Hexane Benzene Xylene Cresol Valeric acid Water
19.9
20.7
21 .o
22.9
23.1 26.0
The effect of polarity on evaporation rate can be deduced from consideration of association forces. Rates of evaporation of liquids of different kinds are not proportional to boiling points because of the intervention of polarity. As commonly observed. esters, ketones, alcohols, and hydrocarbons having approximately the same boiling points will have different rates of evaporation owing to the difference in chemical structure and the type of molecular aggregates that are present through polar association (f299).
Similarly, polar compounds present as impurities in regular grade naphthas-sulfur, nitrogen, and oxygen derivatives-and olefins will in general evaporate at a slower rate than the saturated hydrocarbons. This effect is often noticeable by the odor near the dry point, when the impurities have become concentrated. The effect of polarity on the drying rate of regular grade naphthas which have stood for some time in storage is frequently particularly marked. I n storage, nonhydrocarbon impurities or unsaturates may react with each other or with oxygen to yield combinations very much slower drying than to be expected from the boiling range of the naphtha. Depolarization gives a clean, odor-free dry. I n considering the effect of polarity on viscosity, it may be noted that the viscosity of a liquid is a measure of the resistance set up by intermoleeular attractive forces t o the passage of one molecule past another. Viscosity is therefore predominantly affected by polarity. Viscosity is so strongly dependent upon temperature, however, that measurements designed to separate polar influences are difficult to interpret (13). I n a special case of polarity, though, that of hydrogen bonding, association forces and the accompanying viscosity effects are so pronounced that there is no difficulty in their recognition. Hydrogen bonding can occur in the liquid state between several classes of compounds, particularly alcohols and amines. For example, a n alcohol, which may be represented as ROH, in the liquid phase actually exists as more or less giant molecules briilt up by polar forces (62, 68, SI) :
H-0. R
. , H-0. . . H-0. . . H-0 R
R
R
Liquids (pure liquids, solvents, or solutes) capable of hydrogen bonding have much higher viscosities than would be expected from the molecular size and structure. For instance, water is much more viscous than methane; ethyl alcohol and ethyl amine are much more viscous than propane; aniline and phenol are more viscous than toluene, etc. (f24). Where multiple hydrogen bonding can occur, similar considerations apply even more decidedly; for example, glycol is about twenty times as viscous as water or alcohol. SOLUBILITY.Solubility is primarily a polar phenomenon, influenced strongly by chemical and structural considerations embodied in the principle “like dissolves like.” I t is well known that, with few exceptions, a substance of high polarity will show
January 1948
INDUSTRIAL AND ENGINEERING CBEMISTRY
decreasing solubilities in a series of solvents of progressively decreasing polarity. Conversely, in general, a solute of low polarity will be less and less soluble aa solvents of higher polarity are chosen. It is equally well known that a substance is more soluble in solvents to which it is closely related in structure. (31). The influence of polarity on solubility can best be understood in terms of hydrogen bonds (9, 82, 32) and polarizability. A fuller conception of hydrogen bonding than that introduced in connection with viscosity suggests that hydrogen can coordinate two atoms of oxygen or nitrogen, or also one of these atoms and a carbon atom sufficiently activated by negative (for example, chlorine, nitro, or nitrile) atoms or groups; the coordinating bond in the latter case is relatively weak (9): Strong
0 . ..H-0 N . . .H-0 0...H-N
I
Weak
,{
127
Whh a strongly polar substance like water as one of a pair of liquids in contact, the interfacial tension of aliphatic compounds (excluding sulfur compounds) decreases in the order: saturated hydrocarbons, unsaturated hydrocarbons, alkyl halides, esters, ethers, ketones, aldehydes, amines, alcohols, acids (13), in agreement with the general order of increasing polarity. Interfacial tensions of several suhstanrep against water as one p h a w are as follows:
'
Other Phase p-Hexane n-Octane Toluene Benzene Chloroform Nitrobenzene Diethyl ether Aniline
Tendion
