Catalytic Vapor-Phase Oxidation of Terpenes APPARATUS C. IC. CLARK' AND J. ERSKIKE HAWICINS University of Florida, Gainesville, Fla.
Regulation and RIeasurement of Air Flow
The steps in the development of an apparatus for the laboratory investigation of the catalytic vapor-phase oxidation of volatile organic compounds are outlined. The various units found to be most satisfactory in such an apparatus are described. The method of operation is discussed.
T
HE terpenes undergo oxidation readily, and numerous compounds can be made in this way (1,10,11). A large number of investigations have been made of the action of the usual laboratory oxidizing agents on these substances, but little information is available on the action of air on the vapors of terpenes in the presence of oxidation catalysts, particularly a t elevated temperatures. For this reason, and in view of the fact that catalytic vapor-phase oxidation lends itself readily to continuous operation, a research program has been initiated which has three principal objects: to develop suitable apparatus and technique for carrying out the catalytic vapor phase oxidation of volatile organic compounds, to investigate the reaction as applied to compounds of the terpene series, and to develop, if possible, a feasible commercial process whereby new and valuable compounds may be prepared from the commercially available terpenes. This paper deals with the development of the necessary apparatus. The apparatus for carrying out the catalytic vapor-phase oxidation of volatile organic compounds operates essentially as follows (5, 7 ) : Two streams of air are provided, one designated as primary air and carrying the vapor of the compound being oxidized, and the other designated as secondary air and serving to bring the composition of the reaction mixture to the proper value. These two streams meet as they enter a reactor containing the catalyst. After passing through the catalyst zone, where oxidation is allowed t o proceed a t the desired temperature, the treated gas stream passes through condensers, scrubbers, or other suitable apparatus for recovering the reaction products. Accordingly, in assembling such an apparatus, the following units are required: a n arrangement for delivering constant measured streams of primary and secondary air to the reactor; means for feeding the vapor of the material being oxidized into the primary air stream at a constant measured rate; a reactor containing the catalyst, which must dissipate the heat of reaction and a t the same time maintain the reacting gases at the desired optimum temperature; product recovery equipment. 1
Present address, Florida, Citrus Experiment Station, Lake Alfred, Fla.
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The air supplied to the oxidation apparatus was obtained from the laboratory compressed-air line. However, the line pressure yaried over such a wide range that auxiliary regulation was necessary in order to provide a sufficiently constant rate of flow. Several methods for accomplishing this were tried before a satisfactory system was found. I n the first arrangement tried, air passed from the laboratory line through a reducing valve, thence through a capillary tube into a reservoir, and finally to the oxidation apparatus. Connected to the reservoir was a relief valve connecting to a jet opening beneath a column of mercury whose height could be adjusted by a leveling bulb. Regulation depended upon the escape of air through the jet when the pressure tended to exceed the value corresponding to the height of the column of mercury. While this arrangement greatly reduced the fluctuations of flow due to variation of line pressure, it did not maintain a rate sufficiently constant for satisfactory operation. I n the next arrangement tried, an adjustable pressure drop was introduced by inserting a stopcock in the air line leading to the oxidation apparatus. Across this stopcock was connected a manometer provided with electrical contacts, which actuated a mechanical valve so connected that air was allowed to escape from the line when the rate of flow tended to exceed the desired value. This system gave excellent regulation but was too critical and required constant attention. The arrangement finally adopted is shown in Figure 1: Air from the laboratory line passed through reducing valve Vl and entered reservoir R1via capillary CI. Connected to the reservoir was a mercury manometer, Mi, provided with electrical contacts Y. The contacts communicated with thermionic relay R which was patterned after the vacuum manostat described by Herschberg and Huntress (6) The thermionic relay operated magnetic valve V z ,which opened and allowed air to escape from the feed line when the pressure in R1became sufficiently great to close the circuit through MI. As a result, the pressure in RI oscillated over a range just sufficient to make and break the contact in M I . Insertion of C1, a 6-cm. length of 1-mm. capillary, reduced the oscillations to the point \!-here they were just discernible on mercury manometer 362. The latter was included so that the pressure in the system could be read. I n normal operation, flutter valve V2 opened and closed about sixty times a minute. I n addition to the units just described, safety valve VBwas included. This was a mercury relief valve similar to the one previously described, and it served to prevent the development of a dangerous pressure in the system, in the event the I
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regulation system failed to operate. Ru was rubber tubing and B was a leveling bulb whose height determined the pressure a t which air would escape through Vs. Towers A1 and Az contained soda lime to remove carbon dioxide from the air supply. Reservoir Rz further reduced the fluctuations in the rate of flow. Once constant pressure was attained in the system, the rates of flow of primary and secondary air were readily set and held at constant values by stopcocks SZ and SI, respectively. Capillaries C2and C3 served two purposes; they made adjustment of SIand Szless critical by supplying part of the required pressure drop, and they tended to prevent water from being blown out of the flowmeter manometers by limiting the flow when S1 and Sz were wide open. Trap T I was inserted as an added caution to prevent water from being carried from the meters into the hot reactor in case any violent surges developed in the air stream. The rates of flow in the two air streams were measured by flowmeters Fl and Fz. They were of the conventional orifice type, with water as the manometric fluid. They were calibrated by being connected to an aspirator bottle, and the manometer reading was noted concurrently with the time required for a measured volume of air to be drawn through the orifice. The accuracy of the meters was about 2 per cent. The regulation provided by this setup was such that pulsations were not noticeable on meters F1 and Fz. In runs lasting several hours, however, a slight gradual drift in rate of flow usually occurred, necessitating occasional adjustment of stopcocks SIand S2.
FIGURE 1.
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Raw Material Feed In large industrial installations, such as are used in the production of maleic and phthalic anhydrides (6, 7), the usual practice is to vaporize the raw material by means of the heat recovered from the reaction products and then spray it directly into the primary air stream. The rate of flow is measured by the conventional types of flowmeters. I n a laboratory investigation, on the other hand, in which organic material is fed a t the rate of only a few grams per hour, spraying the liquid into the air stream and measuring its rate of flow by a flowmeter are not practicable. In this case the simplest method, and that giving the most accurate control and measurement, consists in bubbling the primary air through the organic liquid, the rate of evaporation being controlled by suitable adjustment of the temperature, and hence the vapor pressure, of the organic liquid. This was accomplished in the present investigation by carburetor C (Figure 1). It consisted of a spiral of 6-mm. glass tubing in which the incoming air was brought to the desired temperature, and two bubbler tubes 2.5 cm. in diameter and 10 om. long. In the bubbler tubes the air was saturated a t the prevailing temperature with the compound being treated. The three parts of the carburetor were connected as a unit by sealed glass joints, so that the entire assemblage could be weighed before and after a run and thus provide an accurate measure of the amount of material fed. The temperature of the carburetor was maintained constant to 0.1' C. by immersing it in water thermostat S. The latter was regulated by a mercury thermoregulator, heater, stirrer,
OXIDATIOX APP.4RATVS
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INDUSTRIAL AND ENGINEERING CHEMISTRY
and cooling coil, not shown in the diagram. The carburetor was connected to the flowmeter by rubber tubing, and to reactor U by a standard-taper glass joint. Thus i t was readily removed from the assembly for filling and weighing.
The Reactor The reactor to be used in vapor-phase oxidation must be designed with great care. This is evident when one considers that the reaction involved is highly exothermic, and yet must be conducted within a narrow range of temperature if maximum yields are to be obtained. This means that the reactor must be capable of dissipating large quantities of heat a t a high and constant temperature. This is acconipliahed industrially by the Downs type reactor (4) in which the catalyst is packed into a multitude of small tubes surrounded by a liquid such as mercury or diphenyl, the boiling point of which determines the reactor temperature. The heat of reaction which is absorbed by the boiling liquid generally is utilized in a heat exchanger for generating process steam, preheating reacting gases, etc. (5, 7 ) . The first reactor used in this investigation consisted of a straight glass tube containing the catalyst and mound with resistance wire for electrical heating. The attempt was made to control the reaction temperature by regulating the current through the heater. Satisfactory control of temperature by this means proved impossible even with constant attention, and i t finally was abandoned in favor of the arrangement shown in the diagram. The reactor shown consisted of a Pyrex U-tube, U , with legs 13 mm. inside diameter and 13 cm. long. The entrance leg was packed for a distance of 12.5 cm. with aluminum balls, P , approximately 5 mm. in diameter, to preheat the incoming gas. The exit leg was packed for an equal distance with catalyst, Ca. Standard-taper glass joints were provided for connecting the reactor to the carburetor and to the product recovery system. Connection to the secondary-air flowmeter was made with rubber tubing. The tube between the carburetor and the reactor was wound with a few turns of heating wire to prevent condensation of the vapor carried by the air. Absorption of heat from the reactor and maintenance of the proper temperature were accomplished by immersing it, in a bath consisting of a eutectic mixture of sodium, potassium, and calcium nitrate (8, 9). The bath fluid was contained in a steel can, H , wound with an electrical heating element, Z. The can was insulated by inserting i t in a second can and packing the annular space between them with asbestos powder. An insulating lid, I, of asbestos board supported a thermocouple, T,for measuring the bath temperature. The fluid was circulated by a stirrer, not shown in the diagram. The bath temperature was regulated by manual adjustment of the current passed through heating element 2. The salt mixture used as bath fluid had a melting point of about 175" C. and hence was solid a t room temperature Accordingly, i t was necessary to remove all glass parts from the bath before allowing i t to cool.
Recovery of Products Recovery of the products of vapor-phase oxidation involves the removal of small amounts of organic vapors from a large volume of air. Naturally, the choice of apparatus for this purpose depends upon the properties of the particular products to be recovered. Ordinarily, simply cooling and condensing or, for more volatile materials, cooling and scrubbing Kill suffice. In the present instance passing the treated gases through 3 Liebig condenser caused the organic material to condense
Vol. 33, No, 9
into a heavy fog, which was practically impossible to collect. It was found that passing the exit gas directly from the reactor into a water scrubber packed in ice served quite well. I n this way water-soluble products were obtained as an aqueous solution, while water-insoluble materials formed a layer which could be removed by a separatory funnel. The scrubber shown in Figure 1 consisted of two scrubber tubes, TY1 and Wz,in series. They were suspended in battery jar E, packed with ice. Tubes WI and WZwere 4.5 cm. in diameter and 15 cm. tall. Tubes with the same capacity, but taller and of smaller diameter, gave trouble as a result of frothing which occasionally occurred. As a precaution against loss of volatile material, the scrubbed gas was passed through trap T2immersed in a freezing mixture of ice and hydrochloric acid contained in Dewar flask D. After passing through the recovery apparatus, the spent gas was allowed to escape through tube X . When analyses of the spent gas were made, samples were drawn directly into the gas buret through sampling tube G. Even with the arrangement just described, a dense fog wa8 produced under certain operating conditions. Two methods were effective for collecting the material contained in the fog; one was to pass the exit gas through a tower packed with activated charcoal, and the other was to pass it through an electrical precipitator similar to that employed by Bibb and Lucas ( 3 ) . However, in no case was the amount of material in the fog sufficient to permit a chemical examination.
Operation of Apparatus The reactor bath required several hours to heat from room temperature to the usual operating point (around 400' C.) . Hence it was necessary to start heating the bath well in advance of the time for starting a run, I n the meantime, the carburetor was filled with the liquid to be oxidized until the bubbler tubes were about half full. It was then weighed to within 50 mg. The scrubbers for the product recovery were filled with distilled water, 70 ml. being placed in the first scrubber and 90 ml. in the second. The difference in the amounts used in the two scrubbers was necessitated by the fact that the water and other condensable material formed in the reaction gradually increased the volume of the liquid in the first scrubber. The total volume was such as to permit dilution to the proper mark when the scrubber contents mere transferred to a 250-ml. volumetric flask for analysis. As soon as the salt bath had melted (1'75-180" C.), the reactor, stirrer, and thermocouple were set in place, and the remainder of the apparatus was connected to the reactor. All rubber-to-glass connections were sealed with glycerol, while the standard-taper glass connections were sealed with an oil-insoluble grease compoqed of glycerol, dextrin, and mannitol (g). As soon as the carburetor, thermostat, and the reactor bath had reached the desired temperatures and the proper air pressure had been developed in the air reservoir, the apparatus was ready to be set in operation. It was necessary to start the primary air stream first; otherwise, the back pressure due to the scrubbers backed up the liquid in the carburetor. Oncc the two air streams were set at the desired rates by adjustment of stopcocks SIand Sz, they required only occasional correction. The temperature of the reactor bath could be maintained to *3' C. by manual adjustment of the rheostat in the heater circuit. Owing to the relatively large heat capacity of the bath and to the rather poor heat transfer from the heater winding through the necessary insulation into the bath container, response of bath temperature lagged considerably behind changes in the rheostat setting. Hence some practice was necessary before proper temperature control could be effected.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
At the conclusion of the run the primary air was cut off first, and the secondary air allowed to flow for a few minutes to sweep out the system. The apparatus then was taken apart and the product collected for anaIysis. The carburetor was dried and reweighed in order to determine the amount of material fed to the reaction.
Literature Cited (1) Aschan, O., “Naphtenverbindungen, Terpene und Campherarten”, Berlin, Walter de Gruyter and Co., 1929. (2) Bennett, H., “Chemical Formulary”, Vol. 3, New York, D. Van Nostrand Co., 1936. (3) Bibb, C. H., and Lucas, H. J., IND. ENCI.CREW.,21, 635-8 (1929). (4) Downs, C. R., U. S. Patents 1,374,720 (April 12, 1921), 1,604,739 (Oct. 26, 1926).
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(5) Groggins, P. H., “Unit Processes in Organic Synthesis”, 2nd ed., New York, McClraw-Hill Book Co., 1938. (6) Herschberg, E. B., and Huntress, E. H., IND. ENG. CHEM., ANAL.ED., 5, 344-6 (1933). (7) Marek, L. F., and Hahn, D. A., “Catalytic Oxidation of Organic Compounds in the Vapor Phase”, New York, Chemical Catalog Co., 1932. (8) Menzies, A. W. C., and Dutt, N. N.,J . Am. Chem. SOC.,33, 1366-76 (1911). (9) Morton, A. A., “Laboratory Technique in Organic Chemistry”, New York, McGraw-Hill Book Co., 1935. (10) Simonsen, J. L., “Terpenes”, Cambridge University Press, 1932. (11) Wallach, O . , “Terpene und Campher”, 2nd ed., Leipsig, Veit and Co., 1914. from a thesis submitted to the Graduate Council of the University of Florida by C. K. Clark in partial fulfillment of the requirements for the degree of dootor of philosophy, AuguRt, 1940.
AssTRlcTaD
OXIDATION OF PINENE TO MALEIC ANHYDRIDE C. IC. CLARK AND J. ERSKINE HAWKINS HE preceding paper (page 1174) presented the details of an apparatus for carrying out the catalytic vapor-
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phase oxidation of volatile organic compounds. I n a g plying this reaction to compounds of the terpene series, pinene was selected as the first compound to be studied because, as the major constituent of American turpentine (6),it is the most readily available terpene. The liquid-phase oxidation of pinene by air, both in the presence and in the absence of catalysts, has been the subject of a number of investigations (1, S, 4 , 6 , 9,14, 16, 16). However, little has appeared on vapor-phase oxidation in the presence of catalysts a t elevated temperatures. The principal published work is that of Schrauth ( I S ) : who patented a process for the oxidation of turpentine to maleic anhydride by passing its vapor, mixed with air, over a catalyst a t elevated temperature. The specification of conditions is rather vague in the patent, and no mention is made of possible yields. With sufficientlyhigh yields, such a process should be economically feasible. With this in mind and in view of the vagueness
A study of the catalytic vapor-phase oxidation of pinene to maleic anhydride over a vanadiumpentoxidecatalyst shows the effect on the yield of reaction temperature, space velocity, and oxygen-pinene ratio. Optimum conditions are as follows: catalyst support, 3 mm. pumice; total and free volume of catalyst zone, 16.6 and 10.0 ml., respectively; inside diameter of catalyst tube, 1.3 cm.: length of catalyst zone, 12.5 cm.; reaction temperature, 425’ C.; space velocity, 6500 liters per hour; mole ratio of oxygen to pinene, 100; maximum yield of maleic anhydride, 29 per cent.
of the specifications in Schrauth’s patent, it was considered of value to determine the optimum conditions for maleic anhydride formation and the maximum yield to be expected.
Preparation of Materials
CATALYGT. Only one catalyst was investigated. This was vanadium pentoxide deposited on pumice and was chosen because of the success attending its use in other oxidation reactions (7, 11). It was prepared by a method similar to that of Milas and Walsh (12). The pumice was screened so as to provide 5-mm. grains, which pack evenly in the reactor. I n a typical preparation 20 grams of technical ammonium metavanadate were dissolved in a hot mixture of l liter of water and 500 ml. of ammonium hydroxide. This solution was added gradually to 45 grams of the pumice in an evaporating dish on the steam bath until all the water had evaporated and the pumice was impregnated with ammonium metavanadate. The impregnated pumice was heated over a Meker
Formaldehyde and carbon dioxide were formed in considerable amounts under conditions favorable for maleic anhydride formation, but no carbon monoxide or olefins were formed. Maleic anhydride, formaldehyde, and carbon dioxide accounted for 55-60 per cent of the hydrocarbon oxidized. Pinene, dipentene, and p-cymene gave similar results when oxidized under similar conditions. Vanadium pentoxide deposited on pumice did not diminish in catalytic activity after oxidizing 275 times its weight of hydrocarbon.
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burner in an air bath made of two concentric crucibles until decomposition of the ammonium salt was complete, and the pumice was coated with brick-red vanadium pentoxide. ilccording to Milas and Walsh ( l a ) , this freshly prepared material is too reactive and tends to promote complete oxidation. Consequently, before being used it was packed into the reactor and a pinene-air mixture v a s passed through it for several hours a t 350-400" C. This treatment changed the color of the catalyst to a steel blue. The free volume of the catalyst was determined by packing a sample into a glass tube of the same diameter as the reactor, and measuring the amount of xylene necessary to fill the voids. The free volume was thus found to be 60 per cent of the total. Since the total volume of the catalyst space in the reactor was 16.6 ml., the free volume was 10.0 ml. PINENE.Commercial a-pinene, derived from wood turpentine, was given a simple distillation to remove most of the high-boiling constituents and to decompose the oxidized portion which yields water when heated. The distilled material was fractionated, and the pinene thus obtained had the following constants: boiling point a t 760 mm., 155-156' C.; density-at 29" C., 0.8532; refractive index a t 29" C., 1.4647. DIPENTENE.Commercial dipentene, derived from wood turpentine, was distilled and fractionated. The purified material had the following constants: boiling point a t 760 mm., 176.0-176.5" C.; density a t 29" C., 0.8371; refractive index a t 29" C., 1.4690. CYMENE. Eastman spruce turpentine was distilled to remove the bulk of the high-boiling materials. The distillate was extracted with 95 per cent sulfuric acid to remove unsaturated compounds. The extracted material was washed with 10 per cent sodium hydroxide solution, distilled, and fractionated. The p-cymene so obtained had the following constants: boiling point a t 760 mm., 176.0-176.2' C.; density a t 29" C., 0.8501; refractive index a t 29" C , 1.4860.
Analysis of Product MALEICACID. The products of oxidation were collected by passing the gas from the reactor directly into two water scrubbers immersed in ice, thence through a trap immersed in a freezing mixture as described in the previous paper ( 2 ) . At the conclusion of a run the contents of the scrubbers and the trap were transferred quantitatively to a 250-ml. volumetric flask. The brown resinous deposit which formed in the reactor exit and the crystals of maleic anhydride which sublimed onto the walls mere dissolved out with alcohol and added to the flask. The solution then was made up to volume. I n runs which produced an oil insoluble in water, this was drawn off and weighed before the solution was made up to volume. Analyses were carried out on aliquots withdrawn from the solution thus obtained. Maleic anhydride formed in the oxidation was converted to maleic acid when the reaction products were scrubbed out with water. The acid was identified qualitatively in the following manner: The scrubber solution was evaporated to a small volume and treated with decolorizing carbon. Maleic acid was separated from this solution either by extraction with ether or by precipitation as the monohydrated barium salt. The crude acid was dissolved in hot alcohol, benzene was added to incipient precipitation, and the solution was allowed to cool and crystallize. The crystals were filtered off and dried a t 110' C. The product melted at 134-135' C. An additional portion of the crude acid solution was placed in bright sunlight, and a drop of bromide was added. This isomerized the maleic acid to fumaric acid, which precipitated. It was crystallized from water and dried a t 110' C., and i t melted a t 295-297' C.
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TEMPERATURE MOL RATIO OXYGENlPlNENE
I
3 4
380 26
1
400
420
440
I
I
I
c
20
I SPACE VELOCITY 1050
1
2 SPACE VELOCITY 2900 3 SPACE VELOCITY 4660 460
480
1
500
I
1
$20
1
MOL RATIO O X Y G E N / P I N E N E = W I SPACE VELOCITY
2970
2 . S P A C E VELOCITY
4600
e 3 30
26 Y (r
E22 0'
F p la 0
MPERATURE
u
