Refining Petroleum for Chemicals

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Carbon Black from Petroleum Oil T. A. RUBLE

Downloaded by UNIV LAVAL on April 27, 2016 | http://pubs.acs.org Publication Date: June 1, 1970 | doi: 10.1021/ba-1970-0097.ch016

Continental Carbon Co., Houston, Tex. Over 450 million gal/year of petroleum oil are consumed in the United States for producing carbon black, an amorphous form of carbon used principally in automotive tires. Broadly, the process consists of completely burning a fuel with air, injecting the atomized feedstock oil into the hot products of combustion, and recovering the carbon black by filtration. Reactor configuration and dimensions, residence time, and temperature are the critical factors. Oil feedstock is characterized by high degree of aromaticity and low ash components, particularly, sodium, potassium, lithium, iron, and copper. /^arbon black, a relatively pure form of amorphous carbon, is now ^ being produced in the United States at a rate of almost 3 billion pounds annually. Of this, at least three-fourths is made from petroleum refinery streams, utilizing some 480 million gallons of oil; the remainder is produced from natural gas (4). Four of the seven major producers are affiliated with major oil refining companies and are considered part of their petrochemical complexes. Certainly then, the classification of carbon black as a petrochemical should be justified. Carbon black finds its way into many products: inks, paints, paper, fertilizer, plastics, and explosives to name a few. By far the major use, however, is in automotive tires which consume 65% of the total production. The present day tire contains roughly 1 pound of carbon black for each 2 pounds of rubber and provides both the bounce and wear characteristics desired by the user. The properties carbon black imparts to rubber compounds are so critical that there are currently more than 20 classified grades of oil blacks. Most distinctions between grades are mainly a function of particle size and structure, although surface chemistry is sometimes a factor for specialty uses. The more important grade designations are illustrated in Table I. Each of those listed are also subdivided according to their structure levels. 264 Spillane and Leftin; Refining Petroleum for Chemicals Advances in Chemistry; American Chemical Society: Washington, DC, 1970.

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Carbon Black Table I.

Grade Designations for Carbon Black Classification


Super Abrasion Furnace (SAF) Intermediate Super Abrasion Furnace (ISAF) Intermediate-Intermediate Super Abrasion Furnace (I-ISAF) High Abrasion Furnace ( H A F ) Fast Extruding Furnace ( F E F ) General Purpose Furnace ( G P F ) Semi-Reinforcing Furnace (SRF)

240 280 470 520 650

The over-all layout for oil black plants is more or less standard as shown in Figure 1, a typical flow diagram. F u e l gas, preheated combustion air, and preheated o i l feedstock are fed continuously into a reactor where the carbon black is formed at temperatures ranging from 2350° to 2750°F. After the reaction is complete, the carbon-laden effluent gases are shock-cooled to 1000° F by direct water quenching prior to entry into the heat exchanger and bag filter. The carbon is removed by glass fabric filtration, then micropulverized to eliminate any hard gritty particles, and conveyed to the pelletizer. Before pelletization the apparent density of the fluffy black is from 3-6 lbs/cu ft. Pelletizing is accomplished by mixing the fluffy black with approximately an equal amount of water in a pug-like mill with extreme agitation. The wet spherical granules containing 30-55% water are dried in a heated rotating drum and discharged over a magnetic separator into storage tanks. The pelleted black is free flowing and relatively dust free, having a bulk density of 20-30 lbs/cu ft. Shipments are made either i n covered hopper cars, in 50-lb bags by boxcar or truck-trailer (5, 7). Needless to say, the focal point of an entire carbon black plant is the reactor. The basic conversion process is universal in that all reactors utilize the concept of burning a fuel with excess air to completion, atomizing oil feedstock into the hot products of combustion for thermal decomposition, and quenching the reaction with direct water sprays. The earlier oil black reactors were large and unwieldy, containing tons of brick refractory, and several weeks were required to build or repair them. Today, however, by using the heat exchange principle, reactors can be constructed from pieces of standard black iron pipe with castable refractory weighing less than 1 ton (3). Complete repair is accomplished in less than one 8-hour shift. The necessarily tight control of particle diameter, structure level, and surface area, however, is not quite so simple. Particle size is a function of reaction temperature and time; the higher the temperature, the

Spillane and Leftin; Refining Petroleum for Chemicals Advances in Chemistry; American Chemical Society: Washington, DC, 1970.

266

REFINING P E T R O L E U M FOR C H E M I C A L S


smaller the particle, and a decrease i n residence time w i l l produce a smaller particle. These relationships are shown i n Figures 2 and 3. Since the heat for conversion is furnished by burning fuel and a portion of the feedstock, reaction temperature depends on the hydrocarbon-to-combustion air ratio and the speed with which the atomized feedstock is mixed with the hot products of combustion. Rapid mixing is i n turn governed by the internal reactor configuration and the dynamic flow pattern as well as the temperature and pressure profile of the elongated reaction chamber. Residence time is controlled b y the position of the water quench and total feed rates. The smaller the particle size, the greater the reinforcing effect i n rubber as measured by abrasion resistance and tensile strength.

Figure 1.

Flow diagram for oil black phnts

The chain structure or clustering of particles results from collision and coherence of the carbon particles during their formation. Structure levels can be increased by altering the reactor aerodynamics, changing the feedstock atomization pattern, and/or increasing the feedstock concentration i n the reaction zone. The reverse, of course, is the case for decreasing structure within limits. Lower structure may also be obtained by imparting the proper electrostatic charge to the carbon particles during formation, causing them to repel each other ( J , 2, 6 ) . Structure contributes to the stiffness and viscosity of the compounded rubber stock which is desirable to varying degrees, depending on end use.


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Reaction


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Carbon Black Time

.5 .2 Reaction Figure 2.

Influences Diameter

.1 .05 Time, Sec.

Effect of reaction time on particle size

Reaction Temperature

2200

Figure 3.

2400 Reaction

Influences Diameter

2600 Temperature °F

2800

Effect of reaction temperature on particle size

Surface area is naturally determined b y the particle diameter unless an abnormally high value is required for adsorptive purposes. Then a porous particle can be produced by allowing a longer residence time after the reaction zone, causing surface oxidation and pitting. Usually surface oxidation is detrimental to rubber reinforcement.



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Exact control of product quality is complicated by the fact that simple tests on blacks alone w i l l not adequately measure their performance i n rubber. It is necessary, therefore, to monitor the day-to-day quality by actually incorporating the black into rubber and comparing the compound with a control. Periodically, tread blacks must be compounded and road tested i n multisection retreaded tires. Because of the wide variations i n operating conditions, tread and carcass blacks cannot be made i n the same reactor; however, all tread grades can be produced in the same reactor as is the case with all carcass grades. Unfortunately, exact reactor construction details and operating details are proprietary information and cannot be revealed; however, exact internal configuration, dimensions, oil spray pattern, and burner design are all extremely critical for optimum quality and yields. One reactor w i l l produce from 20,000-40,000 lbs/day, and a unit consists of from 4 to 5 reactors, discharging into common downstream facilities. Although carbon black i n some form can be made from any hydrocarbon refinery stream, many years of experience and experimentation have narrowed the feedstock specifications greatly from the standpoint of both quality and yield of product. Typical inspection tests are listed in Table II. Table II.

Feedstock Properties

A P I gravity Viscosity, 122°F, S S F BS&W, % Ash, % Asphaltenes, % Carbon, % Hydrogen, % Sulfur, % Sodium, ppm Potassium, ppm Lithium, ppm Iron, ppm Copper, ppm B.M.C.I.

-2.0 70.0 0.08 0.05 3.0 90.5 7.5 1.5 2.50 0.24 0.30 7.30 0.10 132

Basically the o i l must be highly aromatic and relatively free of i m purities, such as sulfur, BS&W, asphaltenes, sodium, potassium, and other ash components. Aromaticity is indicated by U O P characterization factor and carbonto-hydrogen ratio; however, the most commonly used indicator is the Bureau of Mines Correlation Index ( B . M . C . I . ) . Carbon black yields i n general improve as the aromatic content increases.



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Carbon Black

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Approximately 60% of the sulfur content of the feedstock is converted to gaseous sulfur compounds which are discharged with the effluent stack gases and could cause pollution and corrosion problems. Asphaltenes, or pentane-insoluble materials, increase the grit content by forming undesirable hard coke particles. The presence of sodium and potassium, even i n minor amounts, w i l l reduce structure level, thus making it difficult to produce the high structure blacks ( I , 2 ) . The presence of lithium and sodium above relatively low limits w i l l lower the ignition temperature of the black and w i l l tend to promote oxidation. Most feedstocks are residuals produced by thermally cracking cat cracker cycle oils and stripping the light ends. Satisfactory feedstocks have also been made by solvent extraction of aromatics from various other refinery streams. W i t h the advent of more active cat cracking catalysts, the heaviest product or decant o i l from the cat cracker is suitable without further processing, provided the catalyst is sufficiently removed. Since uniform quality of black depends largely upon the feedstock uniformity, the installation of adequate storage with auxiliary blending equipment is required to eliminate variations. Although the efficiency of the carbon black process has improved steadily, there is still much that can be done. Depending upon the grade of black and type of feedstock, from 50-65% of the theoretical carbon content is now being recovered from the o i l as quality carbon black. A typical stack gas analysis is shown i n Table III. Table III.

Stack Gas Composition (Dry Basis)

Gas

Per Cent by Volume

C0 CO H C H CH N

4.50 12.10 12.80 0.61 0.34 69.65

2

2

2

4

2

2

The composition i n Table III w i l l vary according to the grade being produced; the smaller the particle, the leaner the gas. Water vapor content is usually about 45%. These gases can and are being burned under waste heat boilers to generate steam. One European plant utilizes the steam for power generation i n sufficient quantity to furnish all the power necessary for plant operation and sells the excess to the local utility company. Some consideration has been given to the utilization of the effluent gaseous components as other chemical raw materials; however, thus far the separation and purification steps pose a most difficult problem, particularly with the large amounts of nitrogen and water present.


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The future of oil carbon blacks, at least at the moment, looks bright since it is tied so closely with the rubber industry. M a n y other materials have been tried as substitutes, but as yet their performance has fallen far short of carbon black.


Literature Cited (1) Cabot Corp., U. S. Patent 3,010,794. (2) Cabot Corp., U. S. Patent 3,010,795. (3) "Capacity at Dutch Carbon Black Plant Doubled," Oil Gas Intern. 1964, (4) "Carbon Black in 1968," Mineral Industry Surveys, U. S. Bureau of Mines, Washington, D. C. (5) "Carbon Black (Oil Black)," Hydrocarbon Processing 1967, 46, 11, 158. (6) Continental Carbon Co., U. S. Patent 3,223,605. (7) "Oil Black," Ind. Eng. Chem. 1952, p. 685. RECEIVED January 12, 1970.


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