C. N. ZELLNERl and FRED LISTER2 Research and Development Department, Tide Water Associated Oil Co., New York, N. Y.
Difunctional Acids by Petroleum Hydrocarbon Oxidation Primary oxidation products of difunctional acids from petroleum hydrocarbons, obtained in the laboratory and pilot plant, give promise for use in alkyd resins, polyesters, and plasticizers.
T H E purpose of this article is to describe a method of oxidation of primarily paraffinic hydrocarbons, such as can be found in various petroleum fractions of high molecular weight. The method involves oxidation in the liquid phase with air or oxygen and in the presence of a catalyst in a manner that an essentially “difunctional acid” product is obtained a t high conversion levels. A difunctional acid is a compound which reacts as having two acid functions per molecule. Its purpose is to present a preliminary summary on the probable composition of the oxidation product. I n view of the extent of the literature on the general subject of hydrocarbon oxidation, no attempt is made here to refer exhaustively to this literature. Probably the most useful summaries on the subject are the review articles by Bawn (2), Waters (8), and Zuidema ( 74). These publications, as well as numerous patents (7, 3, 4, 7, g), indicate that oxidation should be stopped at relatively low conversion levels with saponification values of 6 or 7 milliequivalents per gram-that is, permitting Present address, Celanese Corp. of America, Summit, N. J. 2 Present address, Wood Machinery & Chemical Corp., Princeton, N. J.
1938
appreciable amounts of charge stock to remain unreacted. The reason for this appears to be that a t higher conversion levels, resinification and discoloration of the products resulted. These methods require some means of separation of the acidic products from unreacted hydrocarbon, commonly by saponification and reacidification. The present work was first directed toward the attainment of high levels of conversion, without resinification of the product, so that the saponification step would not be necessary. Such complete oxidative conversion of hydrocarbons to acidic material of light color and essentially devoid of unsaponifiables has not been reported in the literature. Process
Early in the program, paraffin wax (melting point 120’ F.) was oxidized to various levels of oxidation, as measured by saponification value. A correlation was made between the saponification value and the per cent breakup into water soluble acids, water insoluble acids, and unsaponifiables. Figure 1 presents graphically the results of this correlation. Extremely high saponification values must be reached before the percentage of unsa-
INDUSTRIAL AND ENGINEERING CHEMISTRY
ponifiables in the product drops to a negligible amount. Oxidation must be continued beyond a saponification value of 9 meq. per gram (504 mg. KOH per gram) in order to obtain a product with less than 5% of unsaponifiables. (Saponification value is used throughout this article to mean total saponifiables, including free acids.) The water soluble acid content--after saponification and liberation-gradually increases a t the expense of the water insoluble acids as the oxidation proceeds. At high conversions, the water solubles were found to consist in part of crystalline dibasic acids, and in part of noncrystalline dibasic acids in a ratio of about 1 : 1. The water insolubles, although largely monofunctional during the early stages of the oxidation, become dibasic, as well during later stages-i.e., they behave chemically as compounds with two acid functions per molecule. The oxidation reaction generally is continued to a saponification value of 10 meq, per gram (560 mg. KOH per gram) or higher to produce a mixture of difunctional acids. A convenient apparatus for the laboratory study of reaction variables is shown in Figure 2. Air or oxygen is metered into the reaction vessel and dispersed through a sintered-glass tube into the
Figure 1.
rapidly stirred hydrocarbon charge containing the catalyst. (The motor is capable of speeds up to 9000 r.p.m.) Volatile products are removed through a series of condensers and traps and the substantially acid-free gas is vented into the atmosphere or into a gas sampling apparatus. Oxidation reaction variables studied in this type of apparatus on a given charge stock include : Oxidizing gas distribution Oxidizing gas rate Reaction temperature Type of catalyst used While all these variables affect the rate of reaction, the most important from the standpoint of product quality is the oxygen distribution, which determines the amount of oxygen available to the reaction. Insufficient oxygen availability results in dark, resinous products, which cease oxidizing a t relatively low conversion levels with correspondingly low final saponification values. Further treatment with oxygen causes no further increase in acid production, but merely increases the molecular weight through polymerization.
Oxidation level vs. breakup
Table I shows a series of runs a t 160' C. using paraffin wax as starting material and 1% manganese naphthenate as the catalyst. The oxygen feed rate and stirrer speed were varied, causing different rates of oxygen utilization, as determined by measuring the inlet and outlet gas velocities, and by analysis of the outlet gas. The oxygen utilization rate, therefore, signifies the total oxygen consumed by a unit weight of charge per unit time, and is related to the oxygen availability and method of distribution. The ultimate saponification value reached before cessation of oxidation increases as the oxygen utilization rate is increased. Thus, a high oxygen utilization rate must be maintained in order to reach the desired high conversion (70). The minimum oxygen utilization rates which allow oxidation to proceed to about 7.0 meq. per gram saponification value, before complete cessation, were then determined for various reaction temperatures. This saponification value was chosen because it represents the highest value which had been reported
in the literature, and also because it is the point a t which difunctional material begins to be produced in significant amounts. Table I1 illustrates that minimum oxygen utilization rates to reach a given conversion, rise sharply with increasing temperature. Charge for these runs was paraffin wax containing 1% manganese naphthenate as catalyst. With higher oxygen feed rates and stirring speeds, high saponification values can be reached in short reaction periods to produce light-colored oxidation products. Table I11 shows laboratory oxidation runs a t 150' C. in which the oxygen feed was 360 liters per hour per kilogram and stirring speed was about 8000 r.p.m. Catalyst was added in three portions-namely, a t 0, 3, and 4.5 hours, the total being 1% by weight. Thus, under conditions of high oxygen distribution, higher final saponification values a t accelerated reaction rates and products of better color can be obtained.
Charge Stocks Data reported so far are based primarily on paraffin wax oxidations. HowVOL. 48, NO. 10
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OCTOBER 1956
1939
Figure 2.
Laboratory oxidation equipment
Table I.
Oxidative Conversion Level as a Function of Oxygen Utilization Rates Temperature = 160' C. Stirrer, Approximate Average Final Acid Final 0 2 Feed Rate in Speed, 02 Utilization Rate, Value, Saponification Liters/Hr./Kg. R.P.M. Liters 0 2 / H r . / K g . Charge M e q . / G . a Value, ilfeq./G.a of Hgdrocarbon Charge 7.1 2.8 23 450 108 7.6 3.2 35 70 900 10.2 4.7 45 120 900 Milliequivalents per gram (multiply by 56 to obtain mg. KOH/gram).
Table 11.
Temp.,
C. 120 140 180
Minimum Average Oxygen Utilization Rates to Reach Saponification Value of 7.0 Meq. Oxidizing Gas and Stirrer Appros. Av. Oxygen Final Utilization Rate, Saponification Speed, Gas Feed Rate, R.P.X. Litem Oz/Hr./Kg. Charge Value, Meq./G. L i t e r s / H r . / K g . Charge 7.3 1.5 Air 13 (2.6 02) 900 7.6 7.0 Oxygen 12 900 6.6 135.0 Oxygen 240 900
Table 111.
Oxidation at High Oxygen Utilization Rates Temperature 150' C.
-4pprox.
Av. Oxygen Charge to
Oxidation Paraffin wax, m.p. 120' F.
Catalyst Manganese naphthenate (1% by wt.) Manganese naphthenate cocatalysta Manganese naphthenate (1% by wt-) Manganese naphthenate cocatalyst'
Utilization Rate, L/Hr./r
