NON-PUFFING PETROLEUM COKE

Abstract: Very fine particle size iron oxide or calcium fluoride is dispersed in a high sulfur petroleum coker feedstock before delayed coking to produce a needle coke with low CTE and negligible puffing on heating to the temperature of graphitization.

Description: Pescription NON-PUFFING PETROLEUM COKE BACKGROUND OF THE INVENTION: Electrode grade graphite is manufactured from a commercial - grade of coke having an acicύlar, anisotropic microstructure called needle coke, see U.S. 2,775,549 to Shea, Dec. 25, 1956, Cl. 201-42, made by delayed coking of certain petroleum residues under specific conditions of heat and pressure. To produce graphite from such coke, it is necessary to heat it to a temper~ 0 ature in the range of 2000-3000 C, which has the dual function of supplying energy for the conversion of the carbon in the coke to the graphitic crystalline form and of volatilizing impurities. When carbon bodies made from such cokes are heated at temperatures in the vicinity of 1000-2000 C, various sulfur-containing co - 5 pounds decompose, attended by a rapid and irreversible expansion of the carbon body. This phenomenon is termed "puffing". During the production of graphite articles, particularly high performance graphite electrodes, puffing is extremely undesirable as it may destroy the structural integrity of the piece and render it 0 marginal or useless for its intended purpose. Puffing of a carbon article made from high sulfur cokes generally starts at about 1500°c, and may result in a volumetric expansion of as much as 25%. It is not simply an elastic expansion but should be characterized as an inelastic, irrevers- OMPI _ /., WIFO , ible expansion. ■ The generall accepted explanation of the puff ng phenomenon is that in acicular needle' cokes with a relatiyely large amount of sulfur, sulfur atoms, are bonded ' to carbon atoms, by coyalent bonds,'either in carbon ring structures or linking rings. These bonds are less stable at high temperatures than the. carbon-to- carbon.bonds. On heating, the carbon-sulfur bonds rupture, the sulfur is freed, then reacts with hydrogen to form hydrogen sulfide. The simultaneous rupture of these bonds and evolution of hydrogen sulfide and other sulfur containing materials causes the physical expansion called puffing. Puffing has been avoided in the past by using coke made from petroleum residues low in sulfur content. This approach is of only limited utility at present since the -principal petroleum crudes currently in use have high sulfur contents, and the cokes made from their residues will normally exhibit an undesirable degree of puffing. Another approach to elimination or alleviation of the puffing problem in manufacture of graphite articles has been by the use of additives. These additives have usually been added during the mixing stage when various sizes and grades of coke, particles are mixed, before being wetted with pitch, formed into the desired shape, baked at an intermediate temperature and graphitized at high temperatures. Additives have included primarily metal salts and oxides, as disclosed in British 733,073, Greenhalgh, July 6, 1955, Cl. 90 b; French 1,491,497, Gillot et al. , Aug. 11, 1967, Cl. C 01 b; French.2,035,273, Continental Oil, Dec. 18, 1970, Cl. C 10 b -57; U.S. 3,642,962,. Wallouch., .Feb. 15, 1972, Cl. 2Q1-17; U.S. 3,563,705, Grindstaff et al. , Feb. 16, 1971, C. C 0.1 b 31/04, Cl. 423 ^ -375; U.S. 3,842,165, Grindstaff et al. Oct. 15, 1974, Cl. C 01 b 31/04, Cl. 264-29.1; and U.S. 3,338,99.3, Juel et al. Aug. 29, 1967, Cl. 106-56. The patents above disclose the use of iron, sodium, chromium, nickel, cobalt, boron, aluminum, titanium, calcium, zirconium, manganese, magnesium, barium and strontium compounds as puffing inhibitors. Some compounds of this group are in general usage O Λ,. W and of these a choice is naturally made based .upon the effectiveness as a pu fing inhibitor and upon other properties of the graphite article such as electrical resistivity, tensile strength., modulus of rupture, modulus of elasticity, coefficient of thermal expansion, and cost. Of the above, French 1,491,497 discloses tha use of chromium oxide at 0.2-5% in a mixture with coke and a binder as a catalyst, enabling graphitization to occur at temperatures in the range of 1200°-2000°C. French 2,035,273 discloses a low sulfur coke produced by the addition of 0.3-5% of sodium carbonate to the coking stream mix¬ ture and subsequent hydrogenation of the coke at high temperature. British 733,073 discloses the use of oxides of chromium, iron, copper, or nickel incorporated in the grinding stage of coke, mixed with pitch, shape.d, baked at 1200 C, and graphitized at 2500 -2800 C. U.S. 3,563,705 discloses the use of mixtures of iron or calcium compounds with small amounts of titanium or zirconium compounds as puffing inhibitors incorporated into the coke-binder mixture. U.S. 3,338,993 discloses the use of calcium, magnesium, strontium, and barium fluorides as puffing inhibitors with raw or calcined coke and binder, mixed, shaped, baked and graphitized. U.S. 3,642,962 discloses the use of 1-3% calcium cyanamid or calcium carbide as desulfurizing agents and puffing inhibitors, mixed with raw coke prior to calcining. At present, the most common methods of the above are those using iron oxides mixed dry in the coke-pitch binder blend as puffing inhibitors. These are effective puffing inhibitors but must be used with, caution, as their use tends, to increase the coefficient of thermal expansion or CT , of the finished product, to an undesirable level. The coefficient of thermal expansion (CTE)_, which is convention¬ ally expressed in in./in./ C or cm./cm./ C, is alsb of yital importance in the production of graphite for certain applications. Electrodes for electric furnace melting of steel must haye a low CTΞ to avoid excessive differential expansion at operating tempera¬ tures and the resultant spalling, which in turn causes excessive consumption of the electrode-and cost in operation.- Other applications, requiring- imensional stability at hijjh. temperatures are well-known although of somewhat less economic importance. In general, the addition of any foreign material to a graphit- izing carbonaceous mix will have, in addition to its desired effect, such as puffing inhibition, the ' effect of increasing the CTE of the graphite body. A needle coke is distinguished by its physical structure when microscopically examined, showing long needle-like acicular particles. Such cokes ' , to be suitable for manufacture of graphite electrodes to be used in ultra-high powered electric steel fur¬ naces, should have a graphite CTE characteristic of less than 5 x 10 / C measured over the range of 0 -50 C. Needle cokes for lower powered electric steel furnaces may have a graphite CTE characteristic of as much as 7 x 10 / C over the 0 -50 C range. •The cokes or blends of cokes must be thoroughly mixed with the puffing inhibitor to avoid the difficulties present in making uniform homogeneous blends and in thoroughly coating the particles, which are often as much as 7mm-. in diameter. Both of these difficulties can lead to non-uniform dispersion of the inhibitor and to puffing, even though there is sufficient inhibitor present in the total mix to prevent puffing. This non-uniformity is particularly troublesome when operating under the newer type of graphitization processes, which raise the temperature of the carbon bodies (i.e. electrodes) at a much higher rate than the older processes. The " combination of high sulfur with high, rate of temperature rise exacerbates the problem and requires undesirably slow heating rates to overcome puffing. It should be emphasized that oyercoming the puffing problem becomes increasingly more di ficult in the larger graphite electrode sizes (above 20 in. ( (51 cm.11 diarn). because in such sizes, larger particles of coke are used. Since the pu£f * ing inhibitor only coats the surface of the particles, the coke surface area to in¬ hibitor weight ratio decreases, for a given weight addition ratio, giving a higher concentration of inhibitor on the coke -SUR _O Λ, WI particle surfaces for the larger particle blends. Tfrus a large amount of the inhibitor is at relatively greater distance from the ' centers of the coke particles in the larger coke ' particle mixes as opposed to the. smaller particle mixes used in smaller electrodes.- Migration of the ' inhibitor into the. centers of the large particles becomes progressively more dif icult and less effectiye as the coke particles increase in size. The puffing problem is further increased with the rate of graphitization of the. carbon bodies. Optimum distribution of the inhibitor throughout the structure of the carbon body to be graphitized is essential as the degree of puffing for any coke particle blend is highly rate sensitive, being directly related to the rate of temperature increase during the graphitization cycle. Thus, the figures in certain of the examples given will show a much higher dynamic puffing at a 14 C/min. temperature rise than for a 5 C/min. rise. The amount of puffing for any giyen coke-inhibitor blend . could be expressed ' as- a proportionality in the general form: DP = K(S,P,ΔT) where S = sulfur content of coke 1 P = mean particle size ΔT = rate of temperature where DP _= dynamic puffing increase 1 = amount of inhibitor K =-• proportionality factor. Thus it may be seen that increases in sulfur content, par¬ ticle size, and temperature rise will increase puffing, while an increase in the inhibitor level will decrease puffing. * SUMMARY OF THE INVENTION: A petroleum coker feedstock which, would normally produce a puffing coke due to its high, sulfur content is rendered non- puffing by the addition of an effective amount of 'puffing inhibitor to the feedstock as a fine particle size powder. Puffing inhibitors.such as iron oxide and/or calcium fluoride may be pre-dispersed in a high concentration in a small quantity of the feedstock (fresh feed or coker furnace feed)_, or in compatible -^T-ΣEXIJ- OMPI £ material miscible with the eedstock., or dispersed in the total coker stream and added either batchwise to a batch, type coker, continuously to.the main stream in a delayed coker, or near the tpp of a delayed coker (. as in the case of anti-foam additiyes while the coker stream 5 is admitted into the ' coker at or near the bottom of the unit. The use of a fine particle size powder of 100% less than 5 micron diameter, predispersed in a portion of the feedstock, insures that th final product will be a homogeneous coke with, puffing inhibitor uniformly distributed throughout. 10 A current of inert gas or steam bubbled slowly through the hydro¬ carbons in a batch type coker during the run aids in keeping the puffing inhibitor in suspension without significantly increasing the CTE of the finished product. In a commercial delayed coker this is not essential. For a description of delayed coking, see R. J. Diwoky, 15 Continuous Coking of Residuum by the Delayed Coking Process, Refiner and Natural Gasoline Manufacturer, Vol. 17, No. 11, Noy. 1938. Iron oxide is formed when any of numerous iron bearing materials is calcined, including organometallic compounds , and salts. Minerals such, as magnetite (Fe 0 ) , limonite C2Fe O -3H 01; and pyrites (FeS ) 20 and salts such as ferric sulfate and nitrate when roasted in air are converted to ferric oxide, and may be used to form the oxide. The reactive species may be elemental iron, produced by re¬ duction of the Fe«0_ by coke during graphitization. Calcium fluoride is also highly effective as an inhibitor 25.with slightly superior performance as compared to iron oxide. Mixtures of the two inhibitors have shown a synergistic result, being more effective than either of the two when used alone. The addition of inhibitor in this manner to the f edstock to the coker produces a coke which, is lower puffing and produces a 30 graphite which has a lower CTE than from a coke conyentionally inhibited by a dry mix. The mode of operation of these puffing inhibitors is prob¬ ably a scavenging reaction, sulfur reacting with the metallic ions to form the sulfides, then decomposing slowly at higher 35 temperatures to give a controlled evolution of yaporized elemental sulfur. • In.general the use of any-of the additiyes listed aboye, when added to a coke particle-pitch- binder mix, will lower the extent of puffing, but at the same time significantly increase 5 the CTΞ, of the graphite bodies made from such cokes. I. haye found that the use of puffing inhibitor dispersed in the coker feedstock in appropriate amounts prior to coking gives an non- expected advantage in that it controls puffing of the coke while increasing the CTE only to a smaller degree* or in some instances 10 not at all). , when compared to the CTE of a graphite body in which the puffing has been eliminated by adding the same additive to the electrode mix by the conventional dry-mix practice. CTE of the graphitized coke was determined by preparing small 5/8" x 5" (.1.6 x 12.7 cm.) electrodes by the procedure disclosed in 15 U.S. patent 2,775,549, (except for calcination of the coke to 1250 C) , and measuring their elongation over the temperature range of 0 to 50°C. DESCRIPTION OF THE PREFERRED EMBODIMENT In figure 1 a decant oil, the fractionater tower bottoms from 20 a catalytically cracked gas oil fraction, also termed slurry oil, or other equivalent hydrocarbon residue, is conveyed from the fractionater 33 through line 10 and meter 14 to diversion valve 17, where a portion of the feedstock is diverted through valve 13, and meter 15 to disperser 18. Simultaneously a ' portion of inhibi- 25 tor 12 is weighed in scale 16 and conyeyed to disperser 18 where it is dispersed in the feedstock, to a specific concentration by weight. Alternately a compatible liquid and additiyes from supply 19 are metered through yalye 11 to yalye 13 and meter 15 to dis¬ perser 18. The inhibitor is dispersed and discharged through line 30 22 and meter 23 preferably to yalve 34 and through, line 36 to top inlets 38 and 38A of the coker drums. The inhibitor may also be fed through valve 34 to mixer 24 where it is mixed with, the principal portion of the feedstock through pump 27, line 26; furnace 29 and line 31 to the bottom inlets of the coker drums 28 and 28A. 35 The overheads are taken off through line 32 and sent to the OMPI ' ι- fractionater 33. In the above flowsheet, 18 is the disperser which,.may be any of several types of ' quipment well known in th -art, preferably a high shear or colloid mill. Alternately, a sand or ball mill 5 could be used. In practice, a dispersion of approximately 5-50.% by wt. of puffing inhibitor in the feedstock is used as a concentrate. The puffing inhibitor dispersion and eedstock are metered in the correct proportions to give a concentration of approximately 100.05-0.5 wt. % puffing inhibitor in the feedstock. At the operating temperatures the yiscosity of the feedstock is extremely low and some means is necessary to minimize settling and a concentration of the puffing inhibitor in the lower portion of the coker during batchwise coker operation. By the introduction of a 15 small flow of inert gas bubbled up from the bottom of the coker drum the puffing inhibitor is maintained in a uniform suspension without significantly raising the CTΞ of the finished product or lowering the acicular crystal content of the coke. It is preferable to add the inhibitor at or near the top of the coke drum, through 20 either the ports normally used to inject anti-foam or a special fitting. The following are examples of specific methods of practicing the invention: EXAMPLE 1 5 The micronized puffing inhibitors, calcium fluoride and iron oxide (having approximately the same particle size distribution) were individually mixed with, samples of a fresh, f ed decant oil coker feedstock, at Q.l wt. % level in a high, speed blender for about 5 minutes. The mixtures were coked -under identical con- 0 ditions in 4 liter resin flasks. In an insulated glass resin flask, an inert.gas at the rate of 0.16 SCFH/kg (4.5 l./hr./kg.) mixture was bubbled up from the bottom of the coking pot to keep the inhibitor uni ormly dispersed in feedstock. The following .time-temperature cycle was- used: O Λ Temperature Elapsed Time Rate. ΔC /hr Room to 350°C 3 hours u - 350-450°C .5-5 hours 2Q . 450°C 16 hours • — ■450-530°C 4.0-4.5~hours 20 53.0°C 1 hour — 530°C-RT Cool-down, power off Dynamic puffing of the cokes was then determined in compari¬ son with uninhibited samples, and with, samples inhibited in the normal manner with dry-mixed iron oxide. The coke samples had 50% < 200 mesh (79 mesh/cm.), particles and 10Q% ^ 65 mesh (26 mesh/cm.) particles. Puffing was measured by taking representative samples by the method of ASTM D346-35, crushing, mixing 100 g coke and 25 g 2 pitch, and molding plugs at 12,500 psi ( . 879 kg./cm. )_. The plugs were measured- by micrometer and placed in a dilatometer. The temperature was raised to 1200 C over a period of 50+10 min. , then the test was run at a temperature increase of 5 or 12-16 C/min. over the 1200 -2900 C range, with measurements taken eyery five minutes. The reported DP is the maximum degree of elongation (or shrinkage) measured, _o. All of the DP's below were at 14 C/min. rise except as noted. Coke Properties Inhibitor Added to Coke Yld. Ash. S CTE _X ιo ~7 /°c** DP, % Δ L*** Feedstock % % % pph pph Fe 2°3* Fe 2°3* 0 Q h 0. 23. .3 0.Q1- 1.17 3.0. +6.8 — 0 23. .3 0..Q1 1.17 3.8 +1.7 0.1% CaF 24. .9 0.65 1.26 4.2 +1.2 0.1% e 2 0 3 23. .0 Q..5Q 1.22 4.1 +1.6. *(Iron oxide dry-mixed into the coke). **(Coefficient of Thermal Expansion over the range of 0°-50°c. x 10~ 7 /°c, 5/8" x 5" (1.6 x 12.7 cm.) , sample, 10 min. time) ***(Dynamic Puffing over the range of 1200°-2900°C) . Of the two puffing inhibitors, the calcium fluoride was found to be the more effective. The addition of either, inhibitor to the feedstock, significantly decreased puffing of the resulting coke. The CTE of the resulting cokes was within the range (τwder 5 x 10' / Cl considered necessary for a needle coke. ' For less demanding applications, a CTE of the graphite body of as much- as 7 x 10 / C may be acceptable, but for -ultra high power electrodes for electric steel furnaces the upper limit is generally 5 x 10 /°C. EXAMPLE.2 Another fresh, feed sample was tested as in Example 1 -using the inhibitors at a higher level of addition, with the following results: Inhibitor Coke Properties Added to CTE Feedstock ώ- C DP , % Δ L 0 2.9 8.3 CaF 2 (0. 2%) - 4.7 0.7 Fe 2 O 3 (0 .2%) 5.1 0.7 Some feedstocks may well need and be beneficially treated with inhibitor additions of as much as 0.5%, resulting in a 2% ash level in the final coke. The examples above are not shown as limitations but merely samples from the wide variety of petroleum residues currently available.

Claims: Claims 1. In a process or the manufacture of non-puffing needle coke suitable for conyersion ' to. electrode grade graphite by delayed coking of ' petroleum feedstocks haying a leyel of sulfur content-.combined molecularly in components of said .feedstock sufficiently high enough, to contribute to an irreyrsible yolu- metric expansion on heating to. a temperature of 1400 C or higher, the improvement comprising the " addition of an eff ctive amount of puffing inhibitor selected from the group consisting of iron oxide and calcium fluoride or mixtures thereof to said feedstock prior to or during the coking of said feedstock in a delayed coker. 2. In a process for the manufacture of non-puffing needle coke, having a graphite CTE characteristic of not higher than 7 x 10 / C over the range of 0 to 50 C by delayed coking of petroleum feedstocks having a level of sulfur content combined molecularly in components of. said feedstock sufficiently high enough to contribute to an irreversible volumetric expansion on heating to a temperature of 1400 C or higher, the improvement comprising the addition an effective amount of puffing inhibitor selected from the group consisting of iron oxide and calcium fluoride or mixtures thereof to said feedstock prior to or during the coking of said feedstock in a delayed coker. 3. The process of Claims 1 or 2 wherein from 0-0.5 to 0.5% of puffing inhibitor is. added to the feedstock. 4. The process of Claims 1 or 2 wherein from 0.05 to 0.5% of puffing inhibitor is added to the feedstock in the form of a pre¬ dispersed liquid concentrated dispersion of said puffing inhibitor in a liquid medium compatible with, said feedstock. 5. The process of Claims 1 or 2 wherein the puffing inhibitor used has a particle size diameter such, that IQ.0.% of said particles are less than 5 microns and 70% are less than 2 microns.. 6. The process of Claims 1 or 2 wherein the iron oxide is added in the form of naturally occuring iron bearing ore. 1.- The process-of Claims 1 or 2 wherein the ' puffing inhibitor is dispersed in a portion of the coker feedstock. OMPI /,. WIPO 8. The process of Claims 1 or 2 wherein the iron oxide used is produced in situ-by. calcination of a compound yielding iron oxide on high temperature calcination of the coke. 9.- In a process-for the manufacture of non-puffing coke suitable for conversion to graphite with, a CTE. of " not higher than 5 x 10 / C over the range of 0 to 50 C by delayed coking of petroleum feedstocks having sulfur molecularly combined in com¬ ponents of said feedstock in sufficient quantity to contribute to an irreversible volumetric expansion on heating to a temperature of 1400 C or higher, comprising selecting an effective amount of puffing inhibitor selected from the group consisting of iron oxide and calcium fluoride or mixtures thereof of particle size -6 * * less than 5u (5 x 10 m) . to be added to said feedstock, preferably 0.05 ' to 0.5% of said feedstock, dispersing said puffing inhibitor in a portion of said feedstock or other compatible material to form a uniform dispersion of said puffing inhibitor; adding said dispersion to the principal portion of said feedstock near the top of a delayed ' coker drum and coking said"feedstock in said delayed coker. - 4> '

Inventors: HSU H
Applicants: GREAT LAKES CARBON CORP
Application year: 1981
Main classification: C10G9
IPC codes: C10B55