US 3677533 A
A method is described for preheating coke in a calcining process to a temperature between about 600 DEG to 850 DEG F. This preheating avoids shattering of the coke into undesirable small particles in the calcining kiln. The hot combustion gases from the kiln are admixed with air to burn all volatile combustible material contained in the gases and a portion of the resultant hot gases is admixed with cooler recycle gas from the preheater to obtain a heating gas for the preheater having a temperature between about 800 DEG and 850 DEG F. This heating gas is supplied to the preheater in sufficient volume to preheat the coke to above about 600 DEG F. The coke is passed through the preheater and kiln at the kiln capacity feed rate, sufficient to provide a coke residence time in the kiln no greater than about 54 to 58 minutes.
Description (OCR text may contain errors)
United States Patent [151 3,677,533 Olover et al. [4 July 18, 1972  METHOD OF USING A COKE PREHEA'I'ER Pnmary Exarmner.lohn J. Camby Attorney-Milton W. Lee, Richard C. Hartman, Lannas S.  lnventors: Robert K. Olover, San LUIS Oblspo; Henderson, Dean s df d and Robe" Strauss George L. Ford, Arroyo Grande. both of Cflhf- 57 ABSTRACT  Assign: c'momhv L05 A method is described for preheating coke in a calcining gdes process to a temperature between about 600 to 850 F. This  Filed; "h 8, 1971 preheating avoids shattering of the coke into undesirable small particles in the calcining kiln. The hot combustion gases from PP ,122 the kiln are admixed with air to burn all volatile combustible material contained in the gases and a portion of the resultant 52 hot gases is admixed with cooler recycle gas from the prea a g zg g heater to obtain a heating gas for the preheater having a tem-  Field Search i "58 R 52 perature between about 800 and 850 F. This heating gas is supplied to the preheater in sufficient volume to preheat the 6 m" coke to above about 600 F. The coke is passed through the [s l R Cm preheater and kiln at the kiln capacity feed rate, sufficient to UN TED S E PATENTS provide a coke residence time in the kiln no greater than about 54 to 58 minutes. l,303,088 5/l9l9 McCaig et al ..263/32 R 2,486,205 /1949 Prosk ..263/52 X 9 Claims, 2 Drawing Figures Patented July 18, 1972 2 Sheets-Sheet 1 ATTORNEY METHOD OF USING A COKE ramm'ma DESCRIPTION OF THE INVENTION The invention relates to improvements in coke calcination and, in particular, to improvements in preheating of the coke feed prior to the coke calcination.
Petroleum coke, such as produced by delayed coking operations, contains substantial amounts of volatile combustible material, generally from about to weight percent. This volatile material renders the coke unsuited for electrode manufacture and similar uses. It is common practice, therefore, to remove the volatile combustible material by calcining the coke. The calcination is commonly performed by passing the coke through a rotary kiln in contact with hot combustion gases to raise the temperature of the coke sufficiently to reduce its volatile combustible material content to less than about 1 percent. Calcination temperatures required to do this typically range from about 2,000 to 2,600 F.
Many petroleum cokes when introduced into a calcining kiln tend to rupture and shatter into small particles, a large percentage of which are too small for subsequent use. It has been found that this tendency can be by preheating the coke to a temperature greater than about 600 F. and preferably between about 650 and 1.000" F. in the absence of an oxidizing atmosphere prior to its introduction into the kiln. The preheating treatment has been found to be effective in limiting the shattering of the coke and has been in commercial use for several years; however, this use has been plagued with operating difficulties and, heretofore, has limited the coke rate through the kiln to less than its design capacity.
Initial attempts to preheat the coke were made in a refractory lined preheater which is similar to that designed for use with rotary lime burning kilns. The extended use of such a preheater for coke was found to be fraught with difficuities, resulting from the combustibility of the coke and its volatiles, from air leaks into the preheater which caused combustion and explosions, and from the coking tendency of volatile matter released from the coke during preheating.
In the process, the coke is heated to the desired temperature by passing hot gases through the preheater in the absence of any oxygen. These hot gases are generated in an afierbumer where the gas efiluent from the kiln is contacted with air to burn the volatiles released in the calcination step. A portion of the resulting hot combustion gases from the afterburner are passed to the preheater and the remainder are discharged into a vent stack. These gases, atter contacting the coke in the preheater are recycled, part to the preheater inlet and the remainder to the afterbumer, with the amount and proportion of the recycled gases being varied to control the temperature of the gases introduced into the preheater. The design and construction of a preheater that can function with gases laden with cokable volatile material has, until recently, plagued the commercial use of coke preheating.
We found that the successful employment of a coke preheating step prior to calcination required that the afterburner combustion gases, which are circulated through the preheater, do not contain any significant amount of coking precursors. The desired reduction of coking precursors in the calciner exit gases was attained by introducing air into the aflerburner in an amount sufficient to increase the temperature of the combustion gases exiting the aflerbumer to between about l,$00 and 2,000 F. In addition, we found that to avoid excwsive coke deposits from forming on the equipment, the temperature of the gas to the preheater must be less than that temperature which would volatilize a significant amount, e.g., about 1 percent, of volatile combustible material from the raw coke. The maximum allowable temperature which can be employed depends upon the type of coke in the preheater and its volatile combustible material content, but generally ranges from about 800 to about 850 F. This limits the maximum temperature to which the coke can be preheated, however, significant reduction in coke shattering can still be achieved. Although the imum value, it must, on the other hand, be sufficiently high to insure that the gas exiting the preheater has a temperature which is above its water condensation temperature, generally above about 180 F. and, preferably, above about 230 F., again to avoid excessive deposition in the equipment.
The temperature of the combustion gases withdrawn from the aflerbumer is greater than the maximum allowable temperature of the gas which can be injected into the preheater and, thus, some of the preheater gas effluent which has been cooled by the coke during its passage through the preheater must be recycled and combined with the afierburner combustion gases to reduce their temperature. Generally the amount of preheater exit gas which must be recycled for the appropriate cooling is approximately three times the amount of afierburner combustion gm which is introduced into the preheater.
The necessity to maintain a low temperature of the gases introduced into the preheater and to maintain the gas effluent from the preheater above its condensation point limits the temperature differential available for effecting the preheating. Consequently, large volumes of gas must be circulated through the preheater to raise the coke to the desired temperature without reducing the coke flow rate through the preheater. 'Ihis gas is recycled by a centrifugal fan which was designed for operation at about 1,200 revolutions per minute. It has been found that this speed is excessive and results in rapid failure of the fan bearings when deposits of coke are formed on the fan by the coking precursors in the recycle gas stream Reduction in speed of the fan, while extending operat ing time for the preheater, results in a reduced feed rate through the preheater.
We have now found that the preheater can be successfully employed with a coke calcining process without limiting the production rate of the process by the use of a recycle gas fan having a sufficient capacity to circulate from 700 to L000 standard cubic feet of gas per minute (SCFM) per ton of raw coke or about 930 to 1,300 SCFM per ton of calcined coke product at relatively low impeller speeds such as from 800 to l 100 revolutions per minute. At these low speeds, imbalances caused by coke deposition on the fan are not serious enough to require frequent interruption of the process for cleaning and repair.
The invention will be described with reference to the following figures of which:
FIG. I is a side view of the preheater, afterbumer and calciner and shows the coke and circulating gas flow; and
FIG. 2 is a cross-sectional view of the preheater taken along line 2-2 of FIG. 1.
Referring now to FIG. 1, the coke treatment facilities are shown as comprising a coke preheater l0, afterbumer l2, and coke calciner 14. Raw petroleum coke, which is received from a delayed coking unit of a petroleum refinery, is classified into a suitable size range for calcining and preheating. Particles which are so small that they will result in an excessive pressure drop in the gas flow through the preheater are separated. Typically, particles having diameters of one-eighth inch or less are separated and, preferably, particles having diameters of onefourth inch or less are separated. The particles having a diameter greater than one-fourth inch are thus separated from the finer particles and are discharged onto conveyor 16. The coke is transported by conveyor 16 to the top of the preheater where it is discharged into distributor 18. This distributor comprises a conduit which narrows at its base and splits into two smaller conduits 20 and 22. A gate 24 is pivotably supported to move between positions which divert the coke flow into one or the other of the conduits 20 and 22. Each of the smaller conduits is connected to the top of the preheater through serially connected first and second feed traps 26 and 28 and slide gate 30. The feed traps provide a double lock chamber through which the coke may be transferred without allowing air to enter the preheater. These chambers are ted by trap doors 23 and 27 that are supported inside temperature of the incoming gas must not exceed the maxthechambers andare pivotable between open and closed posi- IOI045 0150 tions. Movement of the doors is effected by air pressure as described in greater detail hereinafter with reference to FIG. 2. The slide gates 30 are horizontal, impermeable plates which, when closed, slide over the entrances to the preheater and seal the preheater from the feed traps. Each slide gate is biased with a closing force such as a pneumatic ram actuated by air pressure. The slide gates are open when filling the preheater and, in the event of a power failure, or a failure in the air pressure to gates 23 and 27, these slide gates 30 will close from the air pressure available in an air reservoir, thereby retaining the preheater isolated from the atmosphere.
The preheater to, which is shown in greater detail in FIG. 2, comprises vemel 32 which contains a gravitating bed of coke solids. Baffle means, internally positioned in preheater l distribute the coke to a heat transferring section at the base of the preheater where the coke is contacted by hot combustion gases introduced therein through conduit 36. The hot gases are distributed throughout the coke bed in the heat transferring section by a gas distributing means. The gases pass through the coke bed and are collected within the preheater by a baffled collecting means and are finally removed from preheater through conduit 34.
The coke solids are discharged from the preheater by hydraulically actuated rams which force the coke into discharge hopper means 38 and conduit 40. Conduit 40 discharges the coke directly into the rotary kiln 14. The ram mechanisms for forcing the coke into discharge hopper 38 are shown in end view together with coke collection means for retaining any coke which spills behind the rarm during their forward motion and which is pulled from the bed when the rams are retracted. This backspill coke is collected in hoppers 37 which are connected by inclined conduit 39 to standpipe 4!. This standpipe discharges the coke into a screw conveyor 43 which has a water seal to prevent air leakage into the preheater. The screw conveyor thereafter transfers the coke particles to conduit 40 and kiln 14 through line 45.
The coke is calcined in kiln 14 by contacting the coke therein with hot combustion gases. The kiln is rotated at a velocity to achieve a residence time of the coke in the kiln of about 50 to about 58 minutes, preferably about 54 minutes. The kiln combustion gases are generated by burner 42 which is supplied with air from fan 44 and fuel such as methane through line 46. Air is directly injected into kiln 14 through auxiliary fan 48 which maintains a sufficient amount of air within the kiln to burn a portion of the volatile combustible material released by the coke during calcination. The amount of air injected into the kiln by fan 48 is also controlled so as to maintain the gas temperature exiting the kiln between about 1,000 and l,200F.
The coke is discharged from the revolving kiln through a stationary hood 50 maintained at the rear of the kiln. A seal is maintained between hood 50 and calciner l4 and cooled by circulating cooling air through jacket 54 which encircles the seal. The colling air is supplied to cooling jacket 54 by fan 56. The hood is a refractory lined housing which encorrmasses the end of kiln l4 and achieves the calcined coke exiting the kiln drum. The bottom of the hood is connected to a rotary coke cooler through conduit 52 to allow coke to discharge directly from kiln [4 through hood 50 and conduit 52 into the coke cooler, not shown.
The hot gases exit kiln 14 through opening 60 at the coke inlet end of kiln l4 and discharge directly into afterburner 12. The afierbumer I2 is a stationary, refractory lined vesel with one end thereof connected to the coke inlet end of kiln 14. As the gases enter this vessel from the kiln they pass over dam means 58 which is an annular baffle formed of refractory bricks and peripherally mounted within aflerburner l2 and extending radially inwardly a sufficient distance to shield the steel tail ring of kiln 14. This refractory dam shields kiln l4, and, in particular, tail ring 62 of kiln 14 from radiation of the gases in afierburner 12 and thereby avoids warping of this steel ring and resultant failure of its refractory lining.
The hot gases in after-burner 12 are crmtacted with air that is forced therein through several tangential jets circularly located around the afierbumer. A circular plenum formed by bussle ring68 en the afierbumer and bears tangential jets 69 through which air is forced by fan 66 and connecting conduit 64. The air, entering tangentially into the afterburner, creates vortices within the chamber and improves intermixing of the gaseous constituents. A sufficient amount of air is supplied to the afterburner to insure nearly complete combustion of all combustible material in the gases. The resultant combustion generally raises the temperature of the gases to about 1,500 F.
a portion of the gases in afterburner 12 is discharged through damper means 70 into a refractory lined, pyrolytic scrubber 72. The damper means 70 comprises a refractory lined plate which when fully inserted into the afierburner isolates the afterburner from the pyrolytic scrubber. The amount of gases vented to the scrubber can be adjusted by simply raising or lowering damper means 70. Air is injected into the scrubber 72 from line 74 and fan 76. Line 74 passes through preheater 10 so as to cool the internal preheater baffles and raise the temperature of the air passing therethrough to about 150 F. An excess of air is injected into the scrubber so that all of the unburned volatile material is completely combusted along with any coke particles which my be entrained in the exiting afterburner gas stream. The purified gases are discharged into the base of exhaust stack 78 through a refractory lined breeching 80.
The remainder of the afterbumer gases is pulled through large diameter duct 36 to the preheater. A butterfly type damper 82 is disposed within duct 32 to provide means for isolating the preheater 10 from the afterburner 12 when shutdown of the facilities is necessary. After the hot gases contact the raw coke in the preheater, they are removed through duct 34 and drawn into the intake of a large circulating fan 84. A slide gate 86 is maintained wifl'iin this duct to isolate the preheater from the afierburner during shutdown. The circulating fan discharges the gases to two locations; a portion is passed directly into the afierburner through line 88 which enters at the side of the alterbumer so that particles in the afterburner cannot fall into the line when the fan is shut down, and the remainder of the gases is discharged through line 90 into duct 36 with the relative amounts and portions of the preheater gases being controlled by damper 92.
The damper 92 is adjusted so that the amount of gas recycled to the preheater is sufficient to maintain the temperature of the gas entering the preheater between about 800 and 8 50 F., below the temperature at which any objectionable amount of volatile combustible material is released by the raw coke. This temperature will vary somewhat, depending on the coke source and type. The maximum temperature which can be used can be determined for any particular coke by a thermogravimetric analysis and selecting the temperature to be no greater than that which will volatilize about 1 percent of the total volatile combustible material from the coke.
The preheating of the incoming raw coke with circulation gas having the proper temperature requires the use of large volumes of recycle preheater gas, and fan 84 must therefore be adequately sized for handling the large volumes of circulation gas. It has been found that for an incoming preheater gas temperature within the desired range, the gas rate should be from 3,000 to 4,000 pounds per hour per ton of raw coke.
Some volatile combustible material (VCM) can be released from the coke in the preheater as well as minor amounts of small particulate coke. These volatiles and fine particles are entrained within the circulating gas and form coke deposits on the various surfaces contacted by the gas. The coke deposition is the greatest on the bafiles and obstructions which are directly impinged by the gas stream, such as on the blades of fan 84 and on damper 86. Coke deposits are also often found within lines 34 and 90 and, particularly, at each bend or curvature in these lines. Of particular importance is the necessity of maintaining fan 82 relatively free of coke buildup since minor imbalances on the fan blades cause severe fan vibrations and necessitate preheater shutdown for fan cleaning. It has been found that successful operation of the preheater over extended run periods requires that the fan 84 be massively oversized with enlarged shaft and sleeve bearings to withstand min ror imbalances from coke deposits. It has also been found that the fan must be operated at relatively low rpm, e.g., SOD-l ,100 rpm, since at low rpm the fan can withstand minor vibrations over prolonged periods. The fan housing should also be provided with quick opening doors that can be opened to permit cleaning of the housing and fan blades.
Successful operation of the preheater also requires that all gas circulation lines and dampers be sized for minimum flow resistance. The preheater is designed for minimum pressure drop through the coke bed at the aforementioned flow rates, e.g., about 12 to l8 inches of water. The pressure drop through line 34 to the fan is about l to 3 inches of water.
The preheater is illustrated in more detail in FIG. 2 which is a cross-sectional view taken along line 22 of HG. l and displays the intemal components of the preheater. An A-shaped bafi'le 100 longitudinally traverses the lower center of the preheater and extends from one side of the preheater to the other. The baffle 100 is supported within the preheater by refractory lined support columns 101 which vertically extend from the preheater base 118 and connect with each leg of the baflle. Cooling tube 102 pass through the center of each support column 101 and connect with each leg of the baffle. These cooling tubes provide a means for cooling the baflle and supports during operation of the preheater. The baffle is protected by refractory bricks 104 which line the lower half of the baffle exposed to the hot circulating gases. A triangular passageway 103 passes through the preheater and upper portion of baffle 100 and communicates wifli the atmosphere. This passageway is fluid-tightly sealed from the internals of the preheater so as to prevent air from entering the preheater. A cooling line 106 is longitudinally disposed within triangular passageway 103 and communicates with the upper portion of ballle 100. This line provides a cooling means for the upper parts of the baffle by circulating air therethrough from an externally located fan shown as 76 in FIG. 1. The air passes through the battle and is removed through line 74 shown in FIG. 1 which discharges the heated air into pyrolytic scrubber 72.
A gas distributor 108 is disposed immediately beneath A- shaped baffle 100 and longitudinally extends the length of the baffle and preheater 10. The distributor comprises a permeable conduit with one end sealed against one side of the preheater wall and the other end extending through the opposite side of the preheater and connecting to gas conduit 36. The hot combustion gases from the afterbumer enter the preheater through the gas distributor 108 and pass through the coke around the base of the A-shaped baflle 100.
The gas, after contacting the coke around the base of baffle 100, is collected by a baffled collecting means located above the gas distributor I08 and removed from the preheater by conduit 34. The collecting means comprises baffle plates 110 which are longitudinally disposed along each side of the preheater wall above A-shaped conduit 100. Each baffle plate is inclined downwardly towards baffle 100 to form a longitudinally disposed inverted V-shaped enclosure between the baffle plate and wall of the preheater. A short skirt 112 on each baffle plate towards the legs of the A-shaped baffle 100 and constricts the area between the bafile 110 and the legs of baffle 100. A permeable duct 114 is located under each baffle plate "0 within the V-shaped enclosure and extends longitudinally the length of the baffle. One end of each permeable duct 114 is connected to efiluent conduit 34 through a cornmon header, not shown, and the other end terminates at the wall of the preheater. The baffle plates 1 as well as the skirts 112 are refractory lined for protection from the hot circulating gases.
The preheater is equipped with hydraulic rams which push the particulate coke under the leg of bafi'le 100 and into a discharge hopper 38. The hydraulic rams 116 extend through wall 32 on each side of the preheater near the refractory lined preheater base 118. Each ram is comprised of a hydraulic cylinder 120 and associated piston, housing seal 124, push rod 126 and ram piston 130. The seal 124 is a rubber sleeve that is secured at its opposite ends to the hydraulic cylinder 122 and to the outside wall of hopper 37 so that air is precluded from leaking through the sliding joint around rod 126. The rams 1 16 are disposed at a slight angle from horizontal so that when ram piston is extended it moves downwardly along slide plate 132 towards the center of the preheater and discharge hopper 38. The actuation cylinders 122 as well as seal 124 are located outside the preheater walls with push rods 126 extending through spill hole 134 in preheater wall 32 to the ram piston 130 within the preheater. The lower portion of the preheater is lined with refractory bricks 136. The ram pistons 130 are stainless steel.
During the forward motion of ram 116, a portion of the particulate coke falls behind ram piston 130, between the back of the piston and preheater wall. When the piston is retracted, the ram discharges this coke through hole 134. Hoppers 37 are connected to the outside of the preheater around ram l 16 to collect the coke forced through hole 134 during the retraction period. The hoppers are sealed around holes 134 so as to prevent air from entering the preheater. The hoppers are connected to an inclined conduit 39 and standpipe 4! shown in FIG. I.
Coke is introduced into the preheater through a set of first and second feed traps 26 and 28 and slide gate 30. The feed traps when connected in series, as shown in FIG. 2, provide a double lock chamber through which the coke can be transferred to the preheater without allowing any significant amount of air to enter the preheater. Each feed trap comprises a rectangular chamber which has a large cross-sectional area near the top and which tapers inwardly to a smaller crosssection near the bottom of the trap. The feed conduit 20 projects into the top of first trap 26 for a short distance and is completely encompassed by chamber 140. The tapered bottom of the trap projects into the top of the second trap 28 and is similarly encompassed by chamber 142 of feed trap 28. 'lhe bottom of feed trap 28 discharges into the top of the preheater through slide gate 30. A trap door 23 closes against the bottom of conduit 20 and seals chamber 140 from the conduit. Similarly, trap door 27 closes against the bottom of feed trap 26 and seals chamber 142 from chamber 140. These trap doors 23 and 27 are respectively connected by crank means 148 and 150 to pneumatically actuated cylinders 152 and 154 so that when the pistons are retracted in these cylinders, the trap doors are opened, allowing communication between the preheater and conduit 20. These trap doors are sequentially operated so that when trap door 27 is closed and chamber I42 is isolated, trap door 23 can be opened and chamber 140 can be filled with coke. When trap door 23 is closed, trap door 27 is opened, thereby allowing the coke in chamber I40 to empty into the preheater. With the sequential operation of the trap doors, the coke can be delivered to the preheater with a minimum influx of air.
As previously mentioned, slide gate 30 is disposed between feed trap 28 and preheater 10 to provide means for isolating the preheater from the feed traps. Slide gate 30 comprises a horizontal impemteable plate 156 which, when closed, seals the entrance to preheater 10. The slide gate 30 is actuated with a pneumatic cylinder and piston assembly which is supplied with air under pressure from an air reservoir. Any failure in the actuating pressure which results in a simultaneous opening of both trap doors 144 and 146 also results in an automatic closing of slide gate 30, thereby maintaining preheater l0 isolated from the atmosphere.
In operation, particulate coke having a mean diameter greater than one-fourth inch and a size range from one-fourth to about 6 inches is conveyed to the preheater and discharged into feed conduit 18. Sequential operation of dividing gate 24- and feed traps 26 and 28 fills preheater 10 with coke. The
filling procedure continues automatically until the preheater is completely filled and the coke contacts level indicator 160 which interrupts the filling procedure. The coke flows downwardly within preheater through the constricted area between baffles 100 and 110 and forms a compact bed of solids beneath skirted baffle 1 10. The coke is forced under the legs of A-shaped baffle 100 towards the center of the preheater by rarm 116. Simultaneously with the flow of coke through the preheater, hot combustion gases are introduced into the preheater from gas distributor 108. These gases pass around the bottom of the A-shaped baffle and up through the coke bed to gas collector 114. During the time the coke is preheated, generally between about 35 and 50 minutes, the coke temperature is raised from ambient to approximately 600 to 700 F. and almost all of the water contained within the raw coke (between about 8 and I2 weight percent) is vaporized and removed with the effluent gas.
The coke forced to the center of the preheater by rams 116 falls into hopper 38, through conduit 40 to kiln 14. During the retraction period of the pneumatic rarm a small portion of particulate coke is trapped behind the retracting ram piston 130 and forced into discharge hoppers 37. The coke in the hopper 37 falls through conduit 41 by gravity to screw conveyor 43. The screw conveyor has a water seal of from 4 to about 12 inches of water to prevent any introduction of oxygen into the preheating apparatus. The coke is transported from the screw conveyor and discharged through conduit 40 to kiln 14. The coke after dropping into the kiln contacts hot combustion gases which exit from the kiln at about l,000l ,200" F. The coke is calcined within the rotary kiln for approximately 50 to 60 minutes at which time the coke temperature increases to about 2,000 F. to 2,400 F. At these conditions almost all of the volatile combustible materials have been driven out of the coke and are partially combusted in the kiln. The amount of volatile material combusted in the kiln is controlled by the amount of air introduced into the kiln from fan 48 which is operated to control the temperature of the combustion gases exiting the kiln at the aforestated range and, preferably, at about 1,000 F. The calcined coke falls from kiln 14 through hood 50 and conduit 52 into a forced drafi, rotary cooler where the coke particles are water cooled to about 300 F.
The combustion gases exiting from the kiln have temperatures which range from 800 to about [200 F. These gases are passed directly into afterbumer 12. In the afterbumer, the gases are mixed with air and a portion of the unburned volatile material released from the coke in the kiln is burned. The combustion in the afterburner raises the temperature of the gases to about L500 to 1,800 F. A portion of the resulting hot gases is then pulled into pyrolytic scrubber 72 where it mixes with an excess of warmed air and the mixture burned to remove any remaining volatiles and entrained coke particles in the gases. The remaining portion of the hot combustion gases is mixed with a large volume of cool recycle gas from the preheater to reduce the temperature of the gas stream to about 800 to 850 F. This gas stream is then injected into the gas distributor 108 within the preheater 10. The gas effluent is removed from the preheater through collector 114 and conduit 34. A portion of this gas effluent is recycled and the remainder is injected into the afterburner.
The invention is further illustrated by the following example which is illustrative of a specific mode of practice of this invention and which is not intended as limiting the scope of the invention as defined by the appended claims.
EXAI'VIPLE This example demonstrates the improvement of preheating coke prior to calcination in accordance with this invention. In this example several runs are conducted to illustrate the criticality of operating the preheater under the proper conditions.
A preheater is constructed as substantially shown in FIG. 2 and installed in combination with an afterburner and rotary kiln as shown in FIG. 1. The kiln is 9 feet in diameter and feet long and is placed at a slightly downward incline to permit the incoming coke to pass to the discharge end of the kiln within about 54 minutes at the design capacity of 15 tons per hour of calcined product coke. The aflerburner is 30 feet long and has a diameter of approximately 9% feet. The preheater is rectangular in shape having a width of 28 feet, depth of 18% feet and a height of approximately 30 feet. The inlet conduit 36 connecting the after burner with the preheater has a diameter of 4.67 feet, and the preheater efiluent conduit 34 has a diameter of about 3.67 feet. The circulating fan 84 has an operating capacity of about 28,000 cubic feet per minute at l ,000 rpm.
in each of the runs, about 20 tons per hour of raw coke having a particle size between one-fourth inch and 6 inches are charged into the preheater through the double lock feed traps. The raw coke contains approximately 10 weight percent moisture and 14 weight percent of volatile combustible material.
The hot combustion gases which are circulated through the preheater are initially generated in the kiln by burning in the kiln, approximately 400 pounds per hour of methane and 27,000 pounds per hour of air. The total amount of air injected into the kiln is controlled so that the temperature of the gases exiting the kiln is maintained at about 1,100 F. Approximately 27,000 pounds per hour of air are injected into the afterbumer to raise the gas temperature to about L600 F.
A portion of the hot afterbumer gases, approximately 20,000 pounds per hour, is withdrawn from the afterburner and mixed with a sufficient amount of recycle, preheater effluent gas to reduce the temperature of the gas mixture to about 800 F. This gas mixture is pulled into the preheater, which is under negative pressure, and forced through the bed of particulate coke. The gas effluent is withdrawn from the preheater by fan 84 and approximately two-thirds of the effluent stream is recycled to the preheater inlet gas stream. The remaining third of the gas effluent is discharged into the afterburner.
The kiln, afterburner and preheater are operated at subatmosphen'c pressure which is achieved by the draft from the stack. During operation the following pressures exist at the indicated locations:
When the preheater facilities are operated at the above rates and conditions, it is found that the coke leaving the preheater is approximately 650 R, which is sufficient to reduce significantly the amount of shattering of the particulate coke in the kiln. It is found that operation of the preheater-kiln facilities can be conducted over prolonged periods without shutdown of the preheater for cleaning or repair.
When the preheater is operated at the above rates and conditions except that the inlet temperature of the gas mixture discharged into the preheater is maintained at 1,500 F., it is found that the coke leaving the preheater has a temperature of about 1,000 F. At these conditions, however, successful continuous operation can not be attained because the equipment rapidly becomes laden with coke deposits and the process must be interrupted after about 4-5 days to clean and repair the equipment.
Although we have illustrated the present invention in connection with specific embodiments thereof, it is not intended that the illustrations set forth herein shall be regarded as limitations on the scope of the invention, but rather, it is intended that the invention be defined by the steps set forth in the claims and their equivalents.
1 in the method of preheating and calcining particulate petroleum coke to produce calcined coke in a preheater and kiln that are maintained at subatmospheric presure, wherein hot gases from calcination of said coke in a calciner kiln are mixed with air and combusted in an afterburner to obtain combustion gas having a temperature of from 1,500 to 2,000 E, and a portion of the combustion gas is introduced into a preheater vemel containing said petroleum coke and pamed therethrough in contact with said coke to heat said coke to a temperature geater than 600 F. and wherein effluent gases are withdrawn from said preheater, repressured by a fan, and a portion thereof is recycled and mixed with said combustion gas from said afterburner to obtain a preheater hot gas stream having a temperature of 800 to 850 F., the improvement comprising: passing said coke through said preheater and said kiln at a rate sufficient to maintain a residence time of coke in said kiln from to about 58 minutes and passing said pre heater hot gas stream through said preheater at a flow rate of from 930 to about 1,300 standard cubic feet per minute per ton of product calcined coke and sufficient to maintain the temperature of said effluent gases above about 200 F.
2. The method defined in claim 1 wherein air is supplied to the inlet of said kiln to control the temperature of said gases from the kiln between about 1,000 and 1,500 F.
3. The method defined in claim I wherein from 50 to 75 percent of the effluent gases from said preheater are recycled and mixed with said portion of said combustion gases from the afierburner.
4. The method defined in claim 1 wherein said particulate calcined petroleum coke is separated to obtain a particle size greater than one-fourth inch.
5. The method defined in claim I wherein the temperature of said efi'luent gases is maintained above about 250 F.
6. The method defined in claim 1 wherein said fan is operated at a speed of from about 500 to about l,l00 revolutions per minute.
7. The method defined in claim 1 wherein said coke is introduced into said preheater while excluding air from entering said preheater by transferring said coke through a plurality of serially connected chambers which are intermittently and alternately opened and closed.
8. The method defined in claim I wherein said coke is discharged from said preheater by reciprocating rams to discharge the coke into said kiln and wherein air is precluded from entering the preheater by flexible sealing means maintained about said rams.
9. The method of claim I wherein coke particles which are pulled from said preheater by said ran's during their rearward movement is collected by means sealed to the atmosphere and recovered by passage through a water seal that prevents air entry into said preheater.
UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION July 18, 1972 Patent No. 3 6 77 5 33 Dated cofl Robert K. Oliver and George L. Ford It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:
On the Abstract page, two occurrence s,
"Olover" should be Oliver Signed and sealed this 2nd day of January 1973.
EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Commissioner of Patents Attesting Officer USCOMM-DC O0376-P69 FORM PO-IOSO (IO-69] u s sovnnulm nmmuc. ornc: 19" 0-3ee-u4