Patent Application
20080173573
1. Technical Field
The invention relates to a process and apparatus for separation of petroleum coke on the basis of density, and particularly to producing a dense coke fraction used in the production of carbon anodes for aluminum production.
2. Description of the Prior Art
Carbon anodes are produced in a paste plant for the electrolytic reduction of aluminium. Roughly one tonne of carbon is consumed for every two tonnes of aluminium produced. A typical source of carbon for the paste plant is calcined petroleum coke. Coke density has an important impact on the anode properties. The coke raw material arrives in the plant as a material having a wide range of particle sizes and densities. In the anode plant, the coke is separated into various size fractions. A specific amount of each size fraction goes into the production of an anode in order to produce a predefined granulometry in the finished anode necessary to meet performance specifications.
WO 82/02503 by Forsberg et al., discloses an air stream separator for separating a heavier fraction such as stones, from a mixture of lighter particles with differing properties. The separator comprises a vibrating conveyor and a series of air nozzles directed in generally the conveying direction at the discharge end of the conveyor. Both lighter and heavier fractions are collected on a rocking chute having a horizontal portion and an inclined portion. The lighter fraction discharging from the conveyor is entrained in the air stream from the air nozzle onto the horizontal portion of the rocking chute, where the lighter fraction and is collected at a side outlet opening. WO 82/02503 teaches that the heavier stone fraction landing on the inclined portion of rocking chute is not able to move upward and the heavier stone fraction is discharged at the lower inclined end of the separator.
U.S. Pat. No. 3,485,361 by Adams discloses a device for separating workpieces from machining chips and turnings. The device comprises a perforated grid on which a mixture of workpieces/chips/turnings are placed, the mixture is fed onto a shaker chute, at whose discharge the mixture is exposed to a high velocity air stream directed upwards through the grid at the mixture of workpieces/chips/turnings. The lighter chips and turnings are entrained in the air stream while the workpieces are appropriately discharged without being entrained.
U.S. Pat. No. 4,793,918 by Thomas discloses a gravity separator for particulate matter deposited on a perforated separator deck. The rate of separation is controlled by varying the end raise and the side tilt of the separator and includes and air supply directed to separate areas of the deck. The deck includes at least two perforated covers separated by transverse ribs, these features improve the solids separation. The U.S. Pat. No. 4,793,918 separator discloses that the heavier portion is discharged at the lower end of the tilted separator deck.
GB427,499 by Barker discloses an apparatus for separating dirt from coal where material enters an incoming fed trough and descends along a series of tilted reciprocating decks with an air-pervious bottom. The dirt/coal mixture is subjected to an upward current of air which helps to stratify the materials. The dirt/coal stratify while tumbling down the tilt of the decks. The denser and finer dirt is discharged at a first trough outlet with the lighter coal continuing down the tilted deck in a layer above the dirt being discharged. This lighter coal is subjected to second air-pervious deck similar to the first deck where a coal/dirt stream is discharged in a second trough outlet and returned to the incoming feed trough. The lightest and largest particle size clean coal fraction passes over a weir plate for collection.
U.S. Pat. No. 6,838,024 B1 relates to a production process to obtain carbon blocks intended for aluminium production. The process of U.S. Pat. No. 6,838,024 comprises separation of an initial carbon input into at least two size fractions, and taking most of these size fractions and crushing and mixing them together to obtain a product having a controlled proportion of ultra fine particles. This patent teaches that the presence of ultra fine particle grains increases the density of carbon blocks produced.
S. Wilkening teaches in “Potentialities in the Paste Plant”, Light Metals, 1997, pages 569-576, that coke may be separated on the basis of density. Wilkening states that given the same particle size recipe a higher apparent density will have a favourable effect on the strength permeability and resistivity of the carbon produced. An air knife is suggested by Wilkening for the coke density separation.
It is an aim of the present invention to produce a densified coke product for the production of carbon anodes, having increased density.
It is a further aim of the present invention to continuously produce the densified coke in a controllable manner.
In accordance with one aspect of the present invention there is provided a process for producing a dense coke fraction for carbon anode production, the process comprising steps of: providing a first particulate coke stream having a first average density and a first average particle size distribution; feeding the first particulate coke stream to a density separator at an incoming flow rate; and using the density separator to stratify and separate the first particulate coke stream into at least two separate particulate fractions of different average density, the at least two fractions comprising a light coke fraction and the dense coke fraction, the dense coke fraction having an average density which is greater than the first average density and an average particle size
distribution substantially equivalent to the first average particle size distribution.
In accordance with another aspect of the present invention there is provided a process for producing a dense coke fraction for carbon anode production, the process comprising steps of: providing an incoming particulate coke stream, combining the incoming particulate coke stream with a recycle stream to produce a combined feed coke stream, feeding the combined feed coke stream to a particle size separator, separating the combined feed coke stream into one oversize coke fraction, at least one mid-size coke fraction, and an undersize size fraction, wherein the mid-size coke fraction has a first average density and a first average particle size distribution; feeding the mid-size coke stream to a density separator at an incoming flow rate; and using the density separator to stratify and separate the mid-size coke stream in into at least two separate particulate fractions of different average density, the at least two fractions comprising a light coke fraction and the dense coke fraction, the dense coke fraction has an average density which is greater than the first average density and an average particle size distribution substantially equivalent to the first average particle size distribution.
In accordance with a further aspect of the present invention there is provided an apparatus for producing a dense coke fraction for carbon anode production, the apparatus comprising: an inclined oscillating table comprising: an upper end discharging the dense coke fraction, a lower end, and a particulate coke retaining deck extending between the upper end and the lower end, the deck being gas-pervious and adapted for receiving a first coke stream to be densified to the dense coke fraction; and a gas mover, producing a gas flow rate through the gas-pervious deck.
In accordance with yet a further aspect of the invention there is provided an apparatus for producing a dense coke fraction for carbon anode production, the apparatus comprising: a particle size separator having at least two decks producing an oversize coke fraction, at least one mid-size coke fraction and an undersize fraction, a coke transferring apparatus transferring the at least one mid-size coke fraction to a density separator, the density separator having an inclined oscillating table comprising: an upper end discharging the dense coke fraction, a lower end, and a particulate coke retaining deck extending between the upper end and the lower end, the deck being gas-pervious and adapted for receiving a first coke stream to be densified to the dense coke fraction; and a gas mover, producing a gas flow rate through the gas-pervious deck.
Further features and advantages of the present invention will become apparent from the following detailed description, taken in combination with the appended drawings, in which:
a is a plot of the baked anode density versus pitch % for an anode produced from a sample (Coke A) and from a sample of densified Coke A produced according to one embodiment of the present invention;
b is a plot as per
In carbon anode fabrication, the starting material, petroleum coke, is understood to be a particulate carbon containing material obtained from crude oil which has been treated to remove any volatile components. It is known that petroleum coke contains particles of various densities. Lower density particles arise from a variety of reasons but primarily because low density coke includes a larger percentage of pores. Therefore as the percentage of low density coke present in the anodes increases, the density, and the mechanical and electrical properties of the anodes decrease.
The present invention provides a process and an apparatus to increase the density of particulate petroleum coke fractions used in anode production. The process of the present invention includes the advantage that during operation both the process and the apparatus tend to be self-regulating. The skilled practitioner would understand the expression “self- regulating” to mean that the process and apparatus continue to produce a consistently densified particulate coke fraction despite fluctuations in the incoming petroleum coke raw material being fed to the process and apparatus.
A stream of calcined petroleum coke 3 is fed to a size classification system and may have a wide range particle size. An anode plant of
An incoming petroleum coke stream 3 is combined with a recycle stream 9 to produce a combined stream of petroleum coke 5. Combined stream 5 is transported by an appropriate powder conveyer or system towards a particulate size separator. The particulate size separator is typically a multi-deck vibrating screen 20 having various screen mesh sizes, which separate or cut the combined petroleum coke stream 5 into various size fractions.
The first, uppermost deck 22 of the screen 20 removes the largest particle size fraction or “oversize” fraction. One or more “mid-size” fractions (in this case 2 are illustrated) 11, 13 are withdrawn from one or more intermediate decks 24,26. The “undersize” fraction that has passed through all the screens, is collected at the bottom 28 of the screen 20. The system of conveyors, the vibrating screen 20 (or screens) and the crusher 60 are all equipment types that are well known to the skilled practitioner.
The “oversize” stream 7 is transported by an appropriate conveyor or system of conveyors to a crusher 60. The crusher 60 reduces the size of the petroleum coke “oversize” fraction 7 to that more appropriate for anode production. As has been seen the crushed material stream 9 is recycled and combined with incoming stream 3 to produce combined stream 5. The multi-deck screen 20 further separates the incoming size fraction into two “mid-size” fraction streams 11 and 13. The “undersize” fraction stream 15 is collected at the bottom 28 of the screen 20 and is transported appropriately for further milling (not shown) to achieve even finer particle sizes needed for anode production. It will be understood that the apparatus may include various hoppers for temporary storage product and feed controllers.
By contrast the process of the present invention one embodiment of which is represented in
In
The uppermost screen deck 122 separates out the “oversize” coke fraction 107 which is transported to a crusher 160. The crushed particulate stream 109 leaving the crusher is combined with incoming petroleum coke stream 103.
The “mid-size” fraction streams 111, 113 collected from decks 124, 126 respectively, are each transported to the density separators 182 and 190. Only two fractions are illustrated by the same treatment can be applied to any number of “mid-size” fractions.
The incoming coke streams 111, 113 are each transported or dropped by gravity into a density separator. The density separator in a preferred embodiment is an oscillating table density separator 182, 190. The density separators 182, 190 separate the incoming petroleum coke fraction into at least two particulate fractions in the following ways.
The density separators 182 and 190 have a vibrating and inclined table. The inclined table includes an gas-pervious deck which is designed to retain the incoming petroleum coke fed into the density separator. The inclined table of separators 182, 190 is set at an oscillating frequency which can be regulated and whose action when combined with the upward flow of gas through the gas-pervious deck, stratifies the incoming coke such that particles of greatest density remain adjacent to the inclined table surface and those of lower density distribute further from the table surface. Thus coke fractions of different average density can then be separated from the stratified material.
Separators of this type may be referred to as “destoners” in which the inclination of the table is from a first upper side to an opposite lower side. The opposite lower side includes a dam or wall which is substantially parallel to the first upper side. The dam or wall at the lower side controls the depth of the bed of material to be separated. Other density separators include “gravity separator” in which the inclination of the table is from one upper lower corner (or end) to an opposite lower corner (end). Destoner equipment includes, for example, equipment manufactured by Oliver in Rocky Ford, Colo., USA (model 1205) and by Forsberg in Thief River Falls, Minn., USA (model P10). Gravity separators include, for example, Oliver Model 4800 or Forsberg Model 300V.
In a preferred embodiment of the present invention a “destoner” is used. Such equipment is known to be useful for separating particles of substantially different densities, for example, separating seeds (low density) from stone (high density) contaminants, but the “destoner” has not heretofore been applied to separation of a coke stream having a narrow particle size and density, as in the invention. It has been found that the petroleum coke having various fractions, of similar density and similar average particle size can be effectively separated in such equipment, generally considered for only very rough density separation of distinctly different materials.
In the preferred “destoner” embodiment, the density separators 182 and 190 include a gas mover (not illustrated) associated with them, which can take any one of many forms. In a preferred embodiment the gas mover is selected from a blower, a fan, or a reservoir of compressed gas. In a particularly preferred embodiment the gas mover is a centrifugal fan. The centrifugal fan may also include an air dryer and filter at the inlet of the fan. The blower has a pressure and flow rate sufficient to fluidize the mid-size coke stream retained on the gas-pervious deck, and produces a bed of graduated (or graded) density with a denser layer of particles at the bottom of the bed adjacent the deck. The oscillation of the inclined table is such that the dense coke fraction 125 and 133, will climb the inclined surface of the gas-pervious deck so that the denser layer discharges at an upper end of the inclined deck of separators 182 and 190 respectively. In a preferred embodiment the oscillation may be eccentric. A fluidized lighter coke layer tumbles down the inclined deck towards the lower side where a light coke fraction is discharged as streams 123 and 131 from separator 182 and 190 respectively.
Operating parameters of the density separator may be varied manually or automatically via a controller. The operating parameters of the density separator which effect the density of the dense coke fraction include but are not limited to: the feed rate of the incoming coke stream (into the density separator); the oscillation speed (speed, frequency and eccentricity); the vibrational amplitude; the height of the exit dam or wall at the lower side of the deck; the inclination of the gas-previous deck, and the gas flow from the gas mover or blower. These parameters may be adjusted to control the relative flowrates of the dense and the light coke fractions. For a particular embodiment the incoming coke stream flow 111, 113, the average density of the dense fraction 125, 133 are controlled by the relative flowrate of the dense coke fractions. In the present invention, it is preferred that the flowrate of the dense fraction (125, 133) P1, be fixed or established at a preset flow rate. In a preferred embodiment the preset flow rate should be at least 50% of the incoming coke flowrate of the incoming coke stream (111, 113), and more preferably at least 70% of the incoming coke flowrate. The flowrates are generally given in terms of “weight(mass)/time”. The separators 182 and 190 are self-regulating with regards to any density fluctuations in the density of the incoming coke streams 111 and 113. That is, the average density of streams 125 and 133 remains relatively constant even as the average density of the incoming streams 111 and 113 varies, requiring no compensating controls to maintain a target average density in streams 125 and 113 in many cases. It will however, be appreciated that if the average density of the incoming streams 111 and 113 falls, for example, the relative flowrate in streams 125 and 133 will fall while the average density remains relatively constant.
It must be emphasized that the stratification in the density separators is such that the average particle size distribution of incoming streams 111 and 113 are substantially equivalent to that of outgoing streams 125 and 133. The expression “substantially equivalent” with respect to the particle size distribution of stream 125 and 133 is defined herein for greater clarity, as a particle size distribution which is similar to that of incoming streams 111 and 113 with respect to ultimate value and ratio of the various size fraction within the distribution with respect to each other, and is such that streams 125 and 133 can be used in the same way (and in the same proportions) as streams 111 and 113 would have been for carbon anode production.
In a preferred embodiment illustrated in
In a preferred embodiment the level control of the system operates in the following manner. The density separators 182 and 190 are set, as explained above, to operate at an established coke flow set point in the dense coke streams 125 and 133 (preferably at least 50% of the total flow 111 and 113 as noted above), which thereby establishes the average coke density in the streams 125 and 133. The operating parameters of the density separators are varied to obtain the preset density value. As has been noted the average density of the dense fraction tends to remain constant once the aforementioned operating parameters above have been appropriately adjusted, even if the densities of the incoming streams 111 and 113 fluctuate.
For clarity, only one flow stream 125 will be discussed, but the same control can be applied to any of the dense streams (125, 133). The flow rate of streams 125 will be identified as P1, and will be preset using the controls on the separator 182 as noted above, and the output flow rate of stream 127 will be identified as P2. The flowrate P2 is controlled by requirements of the downstream anode production process and its flowrate from the hopper 184 is controlled for example by either a weight (mass) or volume feeder. Under some conditions, where the flow rate P1, to hopper 184 is lower than the controlled flow rate P2 out of the hopper, the level of material in hopper 184 would gradually fall and cause the process to stop once the material in the hopper was depleted (or attained a low level shut off switch). The following control method is proposed to avoid the stoppage of the process due to a low level or the reverse situation which would stop the process when the hopper was full.
The simplest embodiment of the present invention comprises a low level measurement and controller on each of the hoppers 184 and 192 which controls the regulator of the density separators 182 and 190 this arrangement maximizes the density enhancement of the dense coke fraction produced while maintaining a constant flow rate of the dense coke fraction out of the hoppers. The separators 182 and 190 may be adapted to receive a signal or signals from one or more controllers, so as to regulate the particulate separation, through the adjustment of the previously discussed operating parameters. Considering again only one separator 182 as a representative example, when the densified coke product in the hopper 184 falls below the low level measurement, the preset flow value, P1 may be varied or adjusted to a higher value which then slightly lowers the average density of the stream 125. This is done by adjusting one or more of the density separator's operating parameters, and maintains the amount of densified coke within a predetermined level. As the influence of the operating parameters on the flow value P1 is generally known, it is not necessary to actually measure the change in P1 to achieve the necessary control. In a preferred embodiment, the gas flow is adjusted, in such a way as to deliver a larger amount of material to the densified coke streams 125 from the density separators 182. The amount of densified coke in hopper 184 is slowly refilled to a higher level, albeit at a slightly lower density level, but still higher than if the separation is conducted without a density separator 182. As noted the same control method may be applied to any of the density separator herein discussed. The hoppers 184 and 192, may in some embodiments each be replaced by multiple hoppers in parallel in which case the average of levels in the multiple hoppers is compared to the low level limit.
More complex control methods can be envisaged which include two or more stages of low level detection with two or more predefined adjustment strategies for P1. In a preferred embodiment the control method would include two level measurements and adjustment strategies.
Clearly, the hoppers 184 and 192 may also have a complimentary system to that of a low level, with a one or more high level measurement and control system, which would work in an analogous yet opposite manner as the low level measurement. The high level measurement would increase the preset flow value and thus reduce the flow of densified coke product leaving the separators 182 and 190.
Therefore the separators 182, 190 produce the dense coke fraction in streams 127, 135, and light coke fraction in streams 123, 131 respectively.
Considering
The system receives an incoming petroleum coke stream 303, which is combined with a recycle stream 309 to produce a combined stream 305, which is then fed to a multi-deck screen 320. The screen 320 in a preferred embodiment includes three decks 322, 324, 326 and bottom 328 from which the following sized coke streams are taken respectively: “oversize” stream, 307; a first “mid-size” stream, 311; a second “mid-sized, 313; and an “undersized” stream 315.
The particles of the “mid-size” streams 311 and 313 are then separated in density separators 382 and 390, which split the incoming streams 311, 313 into lower density fraction 323 and 331 and high density fractions 325 and 333 respectively.
The under-size fraction 315, and the two lower density fractions 323 and 331 are combined into stream 341 and transported for further milling (not shown).
The high density fractions 325, 333 are combined with “oversize” stream 307 into stream 405, which is conveyed to the existing system 400 of petroleum coke classification. The existing system is similar to that described in
Stream 405 is placed on a multi-deck screen 420, comprising three decks 422, 424, 426 and bottom 428, where the following sized coke streams are collected respectively: “oversize” stream, 407; a first “mid-size” stream, 411; a second “mid-sized, 413; and an “undersized” stream 415. The “oversize” stream 407 is transported to a crusher 460 where its size is reduced. The crushed stream 309 is combined with incoming stream 303. “Mid-size” streams 411 and 413 are then fed to hoppers 484 and 492. These hoppers are equipped with the same level control systems as described in
This example will illustrate the benefit of the use of the process and apparatus of the present invention in increasing the density of the petroleum coke processed and that without substantially altering the average particle size distribution of the densified coke product produced as compared to the initial average particle size distribution of a first or feed coke fraction received into the density separators of the present invention. Thus the density separator will produce a dense coke fraction which has a higher density than the coke of the prior art but will have a substantially equivalent particle size distribution as the coke fraction entering the density separator.
Three samples of coke of different initial average particle size distribution were processed in the apparatus of the density separator of the present invention and by using an air knife as suggested in the prior art.
The initial or first average particle size distribution (mesh size) and the percent “coarse” fraction were determined for each sample and the percent “low density” separated, the gain in average density between the light layer (the lower density fraction) and the dense layer (the higher density fraction) were determined.
The results of Example 1 are presented in Table 1 and show that although the lower and higher density fractions split was about the same in both methods (the % low density), the separator of the present invention gave a greater gain in density in the higher density stream. Importantly, the amount of coarse material in the exit streams from the separator of the present invention was almost unchanged from the incoming stream, which indicates that there was a density enhancement without a change in size distribution, that is input and output streams were substantially equivalent. By comparison the density separation with the air knife, showed that the coarse fraction was almost missing from the low density stream, thus indicating that the air knife performed a size classification at the same time as the density separation was occurring. This finding with respect to the air knife is consistent with an air knife separation being based on weight rather than density.
In Example 2 the benefits of the densification of petroleum coke by the process and the apparatus of the present invention will be illustrated, particularly the positive effects of the increased coke density on baked carbon anode (electrode) properties.
The production of electrodes using the process of the present invention according to Example 2 is schematically represented in
The incoming petroleum coke (C) is screened at (S) into two size fraction F1 and F2. The coarse fraction Fl having an average particle size distribution of 0.371 inches to +4 mesh is produced. The intermediate size fraction F2 having an average particle size distribution of −4 to +14 mesh is also produced. F1 and F2 with each respectively treated in the density separators of the present invention D1 and D2. Both D1 and D2 produced a denser and a lighter fraction H1/L1 and H2/L2 where the ratio of H1/L1 and H2/L2 was approximately 80%/20% by weight. This separation weight ratio may vary from 76%/24% to 84%/16%. The density separators were a Forsberg Inc. Destoner Model G2 type, adjusted to give these high density/low density splits. This required, for example, D1 to be operated with a dam height of 9 mm, an elevation of 8.7 degrees and a pitch (degree of oscillation) of 1.0 mm at the lower (light) end and 1.5 mm at the upper (heavy) end. Airflows of about 470 cfm are required. The lower density fractions L1 and L2 are ground or crushed in G1 and G2 to finer fractions and combined. This combined ground fraction is further combined with H1 and H2 with varying amounts of pitch (P) binder and manufactured (M) into carbon anodes (electrodes) suitable for aluminum production.
Two samples of (C) were independently tested in the process represented in
a and 5b represent the plots of the baked density versus percentage pitch (P %) binder by relative increase in baked density respectively (∝ρ %) for sample Coke A and sample Coke B. Furthermore, in
Example 3 illustrates the ability of the density separator of the present invention to compensate and self regulate for variations density in the feed coke stream without the need for control loop (such as a feedback control loop).
The density separator of the present invention treated continuously two petroleum coke samples having different average density (Coke 1 where the density ρ=0.80 g/cm3 and Coke 2 where ρ=0.76 g/cm3). The two petroleum cokes were fed alternatively to the density separator of the present invention without adjusting any parameter of separation during operation. The period of treatment for each coke was 10 minutes. The input coke and the densified coke product fraction were sampled every minute and their density measured. The total percentage of separation (% low density fraction) was 19.6%
The results presented in Table 2 and in
The embodiment(s) of the invention described above is(are) intended to be exemplary only. The scope of the invention is therefore intended to be limited solely by the scope of the appended claims.
This Non-Provisional Application claims benefit to U.S. Provisional Application Ser. No. 60/883,939 filed Jan. 8, 2007.
| Number | Date | Country | |
|---|---|---|---|
| 60883939 | Jan 2007 | US |
