Patent Publication Number: US-11655418-B1

Title: Ultra purified pitch process

Description:
BACKGROUND OF THE INVENTION 
     Throughout history there has been a need for strong materials made from individual fibers with little strength. Use of such fibers to make rope and other articles was an important development in early civilization. Rope making occurred before the wheel was invented. In ancient Egypt, reed fiber was the primary source, but flax, grass, papyrus, fibers from date palms, animal hair or even leather strips were also used. Rope was made of multiple strands braided or twisted together. Although the individual fibers or strands had relatively low strength, the resulting rope would not break even if an individual strand were to fail. Multiple weak fibers could form strong rope or other material, but there was always an interest in using stronger fibers to make a rope or material which was both stronger and lighter in weight. 
     Centuries later, an unusual fiber was developed by Edison in 1879 as part of the development of the electric light bulb. Edison heated cotton threads to carbonize them. These carbonized threads were used as filaments for the first light bulbs. Although most light bulbs eventually used tungsten filaments, carbon filaments were used where their special structural properties were important. US Navy ships used carbon filament light bulbs as late as 1960, because the carbon filament withstood vibration better than other filament materials. Early carbon fibers had little strength in tension so their use was generally limited to light bulbs. 
     The potential use of carbon fibers for other materials than bulbs was not appreciated until Roger Bacon, an American physicist, produced carbon fibers in 1958 while working to discover the triple point of carbon, the conditions at which all three phases coexisted. During his experiments, Bacon made carbon fibers, up to 1 inch long, with extraordinary properties. His fibers had a high tensile strength, 20 Gigapascals (GPa) and Young&#39;s modulus of 700 GPa. Tensile strength measures pulling force to break a fiber, Young&#39;s modulus measures stiffness, or ability to resist elongation under load. Bacon&#39;s carbon fibers were far stronger than steel, on a weight basis. Bacon&#39;s incidental discovery began a modern fiber race—a race to develop ever stronger fibers which were affordable. Carbon fibers are now widely used in manufacture of airplanes, disc brakes and many consumer products where its high strength and lower weight make them the preferred material. 
     Carbon fibers can be made from many materials ranging from Edison&#39;s cotton threads to rayon. Most carbon fibers today are made from Poly Acrylonitrile (PAN). Fibers also can be made from isotropic or mesophase pitch. Modern, high performance carbon fibers can be made which exhibit graphitic crystalline structure or a turbostratic structure. In the latter structure, parallel graphene sheets are stacked irregularly or are haphazardly folded, tilted, or split. Crystalline structure can only be observed in vapor-grown carbon filaments or in carbon fibers derived from mesophase pitch. Vapor-grown fibers have excellent properties, but at present the cost of making these fibers precludes widespread use. Fibers made from mesophase pitch also exhibit crystalline structure and have excellent properties, but fibers made from mesophase pitch can be produced at lower cost than vapor-grown fibers. 
     We investigated the state of the art of producing mesophase fibers and the factors which influenced the properties of strength and specific modulus in said fibers. A search of the literature on mesophase formation showed that particulates have a profound impact on mesophase formation and fiber properties. Small particles can interfere with the complex mechanism by which mesophase is formed from isotropic pitch. Even more significant, the particles which end up in the fibers are a “weak link” in the resulting fiber, creating a spot which can break on flexing or in tension. 
     When a fluidized catalytic cracking unit is the source of the feed the particulate contamination is usually very small catalyst particles which escaped with cracked product vapors into the FCC main column. The particles end up in the bottom of the column producing an aromatic rich and significantly contaminated heavy oil product sometimes called “slurry oil” due to the particulate contamination. Refiners try to avoid some of the contamination by letting the slurry oil settle in a tank to allow some of the catalyst fines to settle to the bottom of the tank, producing a clarified slurry oil (CSO). CSO has a greatly reduced fines content but still contains some particulates, enough to contaminate any mesophase product with catalyst fines. 
     Coal tar may be used to make mesophase pitch and carbon fiber. Coal tar is usually contaminated with significant amounts of coke fines. The fines are usually coal-derived solids (coal, coke, cenospheres) and by-product-derived solids (carbon blacks, pyrolysis blacks). Coal-derived contaminants, in addition, contain the inherent mineral matter associated with the feed coal to the coke ovens. 
     Another possible source of particulates in coal tar or wood tar is “semi-coke” produced as an unwanted product during coking. Regardless of the source, solids contaminated coal tar is difficult to filter and centrifuge treatment achieves only limited success. 
     Mesophase pitch is a preferred raw material for manufacture of carbon fiber, especially as lower cost processes have been developed for mesophase pitch production. https://www.compositesworld.com/news/coming-to-carbon-fiber-low-cost-mesophase-pitch-precursor. This new process, and most processes for making mesophase pitch, start with slurry oil or clarified slurry oil. Even with feed filtration, some catalyst fines remain in an amount sufficient to impact the properties of the mesophase pitch and the carbon articles made from the pitch. Carbon fibers typically have diameters in the range of 8-10 microns. Thus, when spinning carbon fiber from isotropic or mesophase pitch, solid particles that are 2-3 microns in diameter or larger can represent major flaws in the spun fibers, causing the fibers to break or significantly reducing the carbon fibers&#39; strength. Larger solid particles can even plug the spinneret holes, cutting off flow altogether. Thus, if pitch is to be used for carbon fiber production, it is extremely important that even very small solid particles be removed to very low concentration. In the past, solids in pitch and pitch precursors were removed by centrifugation or severe filtration. These processes add significantly to the cost of the process. 
     We wanted a process which greatly reduced the amount of particulates in mesophase pitch and did so at a reasonable cost. In general, pitch processes start with an aromatic liquid feed that is partially converted to mesophase pitch in a reactor. Unconverted feed is removed, after vapor and liquid separation, as an overhead vapor. The solid contaminants in the feed oil predominantly remain in the liquid mesophase pitch product. The overhead vapors, which contain 2-3 ring and heavier aromatics, are obtained as a vapor phase and have a greatly reduced, even essentially eliminated, solids content. These 2-3 ring and heavier aromatics can be recovered as a pure vapor from the pitch liquid residue fraction. The vaporized hydrocarbons are clean while the catalyst fines or semi-coke, cannot vaporize and remain in the liquid phase. 
     The vapors are often condensed and pumped back to mix with the incoming feed in pitch processes. The essentially solids free recycle material is mixed with the solids contaminated feed. The mesophase pitch liquid product contains essentially all of the solids in the feeds. When an isotropic pitch product is sought, the problems are similar in that any solids in the liquid aromatic feed end up in the isotropic pitch product. 
     We discovered a way to greatly reduce, indeed reduce to any desired level, the solids contamination in an isotropic or mesophase pitch product. We produce a vaporization purified feed at low cost, by using an existing plant process which produces such purified vapors. 
     BRIEF SUMMARY OF THE INVENTION 
     Accordingly, the present invention provides a process for producing a reduced contaminant content pitch product from a contaminated multi-ring aromatic liquid fresh feed containing solid contaminants comprising charging said contaminated fresh feed comprising multi-ring aromatics to a pitch forming reactor operating at pitch forming reaction conditions including a pitch forming temperature and pressure and converting said contaminated feed into at least one of isotropic pitch and mesophase pitch and unconverted or partially converted contaminated feed, discharging from said reactor a reactor effluent comprising a two phase mixture of liquid pitch and a vapor phase comprising unconverted and partially converted feed into a vapor liquid separation means, separating said two phase mixture in said vapor liquid separation means into a pitch rich liquid phase with an increased contaminant content relative to said contaminated feed and a vaporization purified vapor phase fraction with a reduced or eliminated contaminant content, cooling, condensing and recovering at least a portion of said vaporization purified vapor phase fraction as a vaporization purified multi-ring aromatic intermediate product, at least periodically charging said vaporization purified intermediate to a pitch forming reactor and converting therein at least a portion of said purified intermediate to an ultra-purified isotropic or mesophase pitch with a reduced solids content as compared to said contaminated feed, and recovering said ultra-purified isotropic or mesophase pitch as a product of the process. 
     In another embodiment, the present invention provides a process for simultaneously and continuously producing a solids contaminated pitch product and a reduced contaminant content pitch product from a contaminated multi-ring aromatic liquid fresh feed containing solid particulates comprising charging said contaminated fresh feed comprising multi-ring aromatics to a pitch forming primary reactor operating at pitch forming reaction conditions including a pitch forming temperature and pressure and converting said contaminated feed into a solids contaminated liquid phase comprising at least one of isotropic pitch and mesophase pitch and a vapor phase comprising unconverted or partially converted contaminated feed, discharging from said primary reactor a reactor effluent comprising a two phase mixture of said solids contaminated liquid phase and said vapor phase into a primary vapor liquid separation means, separating said two phase mixture in said primary vapor liquid separation means to produce a pitch rich liquid phase with an increased solids contaminant content relative to said contaminated fresh feed and a vapor fraction comprising unconverted or partially converted feed vapors with a reduced or virtually eliminated solids contaminant content, cooling and condensing said vapors with a reduced or virtually eliminated solids content in a primary vapor liquid separator to produce a vaporization purified unconverted or partially converted feed intermediate liquid product, charging said vaporization purified intermediate liquid product to a secondary reactor operating at pitch forming reaction conditions including a pitch forming temperature and pressure and converting therein said vaporization purified intermediate liquid product into a two phase mixture comprising at least one of isotropic pitch and mesophase pitch and a vapor phase, discharging from said secondary reactor said two phase mixture into a secondary vapor liquid separation means, and recovering as a product of the process from said secondary vapor liquid separation means a liquid pitch product having a reduced solids content as compared to said solids contaminated fresh feed and to said pitch recovered from said primary reactor separation means. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a process flow diagram for a closely coupled isotropic and mesophase pitch process of the present invention in blocked or cyclic operation. 
         FIG.  2    shows a continuous process flow diagram for a plant making ultra-pure isotropic pitch and, as a by-product, isotropic pitch contaminated with solids. 
         FIG.  3    shows a continuous process for making an ultra-pure mesophase pitch product and as a byproduct mesophase pitch with solids contamination. 
         FIG.  4    shows a continuous process for making ultra-pure mesophase pitch and an isotropic pitch contaminated with solids. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to  FIG.  1   , a clarified slurry oil feed in line  1 , recovered from the bottom of the main column associated with an FCC unit not shown, is charged via pump  2  and line  4  into preheater heater  10 . Preheated feed is charged via line  12  into preferred but optional filter  14  and then via line  16 , heater  18  and line  19  into reactor  20 , shown as a black box. Reactor effluent is discharged via line  22  across pressure reducing valve means  24  and line  26 , mixed with preferred but optional steam or other superheated fluid added via line  80  and the resulting mixture charged via line  28  into cyclone separator  30 . An isotropic pitch rich product is withdrawn as a liquid fraction via line  35  and may be recovered as a product by means not shown or charged to tank  36 . Isotropic pitch is withdrawn from tank  36  via line  37  and pump  38  and discharged via line  39  to pass through heater  40  and line  42  to mix with steam or other superheated fluid in line  82  to form a mixture passed via line  44  into mesophase forming reactor  50 . Preferably a significant amount of the heat input required for the feed to the mesophase forming reactor  50  is supplied by the addition of superheated fluid, reducing the amount of heat which must be added via heater  40 . Reactor effluent is discharged via line  52  and mixed with preferred but optional superheated fluid added via line  84  and the resulting mixture discharged via line  54  into cyclone separator  60 . Overhead vapor from separator  60  is withdrawn via line  64 , preferably joins overhead vapor in line  32  from cyclone  30  and charged via line  65  to cooler  66 . Cooled vapors are charged via line  67  into separator  70 , running at conditions which keep any water and light hydrocarbons present in the vapor phase, which are removed overhead via line  71 . Condensed hydrocarbon liquid in separator  70  is withdrawn via line  78  and stored in tank  79 . The overhead vapors in line  71  are cooled in cooler  72  and discharged via line  73  into vapor liquid separator  74 . A fuel gas stream is withdrawn from this separator via line  75  while a light distillate stream is recovered via line  77  and water recovered via line  76 . Mesophase rich pitch product is withdrawn via line  62  from cyclone  60 . Water, or other fluid to be superheated is charged via line  90  to heater  92  producing superheated fluid in line  93  which can be charged to the first reactor effluent via line  80  or the charged to the second reactor via line  82  or the second reactor effluent via line  84 . 
     In this embodiment of our invention, the process runs in blocked or cycling operation. The fresh feed, when taken from an FCC unit or other industrial source, will contain particulates such as catalyst fines. These can be removed to some extent by heated filtration in filter  14 , but there will still be troublesome amounts of solid contaminates. The mesophase pitch, and the isotropic pitch if produced, will have significant solids contamination, indeed the solids contamination will be higher in the product than in the feed because the solids end up in the liquid phase while the vapor phases have greatly reduced, or eliminated solids content. To make an ultra-pure pitch product, we accumulate the overhead vapors produced in both the first and the second reactors and cool and collect them in storage tank  79 . After a pre-determined period, which can be days or weeks, and when a purer pitch product is required, the liquid in storage tank  79  is withdrawn via line  100  and charged to feed line  1  by means not shown. When ultra-pure pitch product is required all the feed in line  1  will be taken from storage tank  79 . If some, or an increased level of, contaminants can be tolerated in the feed or if the process works better because of alkyl substituents in the FCC feed cycle oil, then a mixture of fresh feed from an FCC unit and vapor purified heavy distillate in line  100  may be used. 
       FIG.  2    shows a process for continuously making ultra-pure isotropic pitch and some solids contaminated isotropic pitch as a byproduct. Much conventional equipment is not shown, such as pumps and coolers, but those skilled in the pitch arts will understand the simplified process flow. Fresh feed, typically an FCC slurry oil, is charged via line  101  and pumps, optional filters and the like not shown, to heater  103  and discharged via line  105  into isotropic pitch forming reactor  107  also called the primary reactor. Reactor effluent is discharged via line  108  and passed through pressure reducing valve  110 . The reduced pressure reactor effluent passes through line  112  and is mixed with superheated vapor, preferably steam, in line  186  and the resulting mixture charged via line  114  into cyclone separator  116 . The reactor effluent vapors and the added superheated fluid are recovered overhead via line  118 , mixed with vapors in line  156  and charged via line  119  to cooler  120  and line  122  into vapor liquid separator  124 . The vapor phase is removed overhead via line  126  through cooler  128  and via line  130  into separator  132 . A fuel gas stream is removed overhead via line  134  while light distillate is removed as a bottoms stream via line  138 . When the superheated fluid added via line  186  is water the condensed water is removed from separator  132  via line  136 . The liquid phase removed from separator  116  is removed via line  117  as an isotropic pitch product which contain most, usually essentially all, of the solids in the feed in line  101 . Although this isotropic pitch contains solids, it has much value for many applications. 
     Ultra-purified isotropic pitch is made by taking the heavy distillate phase liquid from separator  124  via line  140  and pump  142  and sent via line  144 ,  143  and  147 , optionally mixed with alkylated aromatic feed in line  146 , preferably mixed with superheated fluid from line  189  and the resulting mixture in line  148  charged to isotropic pitch reactor  150  also known as the secondary reactor. An isotropic rich pitch stream is discharged from this reactor via lines  152  and  153  into cyclone  154  or any other type of vapor liquid separator. An ultra-purified isotropic pitch product is withdrawn from this cyclone or separator via line  158  and recovered as a product of the process. Overhead vapors from separator  154  are removed via line  156  and mixed with overhead vapors derived from the primary isotropic pitch reactor and the combined vapors processed through the cooler and separator  124  as discussed above. If desired a vaporization purified heavy distillate byproduct may be recovered as a separate product via line  145 , though preferably much or all of this heavy distillate is charged to the secondary reactor  150 . 
     Boiler feed water, or other fluid which is inert under these conditions, is charged via line  180  to heater  182  producing superheated fluid in line  184 . This superheated fluid may be charged via line  186  to mix with primary reactor effluent, line  188  and  189  to the inlet of the secondary reactor or via line  188  and  190  to the secondary reactor effluent. 
     The primary reactor achieves some conversion of aromatic liquid feed to isotropic pitch. This pitch may then be separated in separator  116  to produce a solids contaminated liquid phase and a reduced solids content vapor in line  118 . Careful cooling and condensation of this vapor can recover a heavy distillate fraction with a reduced solids content which makes an ideal charge stock to the secondary reactor. The process shown in  FIG.  2    preferably operates continuously, always producing some ultra-purified isotropic pitch and, as a byproduct, some solids contaminated isotropic pitch. 
       FIG.  3    shows a continuous process for making both ultra-pure mesophase pitch product and a byproduct mesophase pitch with solids contamination. An isotropic pitch feed, contaminated with modest or significant amounts of solids such as FCC catalyst fines, is charged via line  301  through preferred but optional pumps, preheaters and filters not shown and preferably mixed with a superheated fluid from line  376  and the resulting mixture charged via line  302  to primary mesophase forming reactor  305 . Both the primary and the secondary reactor, to be discussed hereafter, form mesophase and the designation primary and secondary refers more to their placement in the process than any difference in reaction conditions or performance. Primary reactor effluent passes via line  307  into separator  309 , which is preferably a cyclone separator or other type of vapor liquid separator. A mesophase pitch rich fraction is withdrawn from the bottom of separation means  309  via line  313  for recovery as a product of the process. This fraction can have many uses, but it will have most, or essentially all of the solids in the isotropic pitch feed as the solids remain in the liquid phase. A vapor phase is removed overhead from separator  309  via line  311  and charged via line  315  into cooler  317  and into vapor liquid separator  321  via line  319 . The separator is run at a temperature and pressure which condenses a majority of the multi-ring aromatics present but keeps water vapor, normally gaseous hydrocarbons and light distillate in the vapor phase. A liquid phase, comprising heavy distillate hydrocarbons is withdrawn from separator  321  via line  334  and  335  for recovery as a heavy distillate byproduct, if desired. Preferably most, or more preferably all, of the liquid withdrawn from separator  321  is fed to the secondary mesophase forming reactor  341  via lines  334 ,  337  and  339 . The vapor phase from separator  321  is withdrawn overhead via line  322  and cooled in cooling means  323  which discharges via line  325  into vapor/liquid separator  327 . This separator is run at a temperature and pressure which condenses water and light distillate. A normally gaseous vapor phase is withdrawn overhead via line  329  for use as a fuel gas or for other treatment, while water is removed via line  331 . A light distillate hydrocarbon fraction is recovered as a liquid in line  333  which may be burned as fuel or used as a blending agent in fuel or solvents or for other refinery purposes. 
     The heavy distillate fraction recovered from separator  321  has a greatly reduced or eliminated solids contaminant content. This material is converted in secondary mesophase forming reactor  341 , at least in part, to ultra-purified mesophase pitch. This reactor discharges via line  343  into vapor liquid separation means  345  such as a cyclone separator. An ultra-purified mesophase pitch product is withdrawn via line  349  as a product of the process. Our preferred mesophase forming reactor achieves significant mesophase formation, but leaves significant amounts of heavy distillate feed which was partially converted in reactor  341 . The unconverted feed from secondary pitch forming reactor  341  is recovered as an overhead vapor phase from separator  345  and charged via line  347  to mix with overhead vapor in line  311  from the primary mesophase pitch forming reactor  305 . We prefer to commingle the vapor phases from both the primary and secondary mesophase forming reactors as they are very similar in composition and properties and may easily be processed as a combined vapor stream. 
     Superheated fluid, preferably superheated steam, may beneficially be added to several parts of the process. Boiler feed water in line  370  is superheated in heater  372  to form superheated fluid in line  374 . This superheated fluid is charged via line  376  to mix with feed to the primary mesophase pitch forming reactor  305  or via line  378  to mix with the feed to the secondary mesophase pitch forming reactor  341 . 
       FIG.  4    shows a continuous process for making ultra-purified mesophase pitch and an isotropic pitch contaminated with solids. A multi-ring aromatic hydrocarbon feed such as an FCC slurry oil contaminated with catalyst fines is charged via line  401  to heater  403  and discharged via line  405  into isotropic pitch reactor  407  where significant conversion of multi-ring aromatics to isotropic pitch occurs. Reactor effluent passes via line  409  through pressure reducing means  411  and lines  412  and  414  into vapor liquid separation means  416  such as a cyclone or other type of vapor liquid separator. Unconverted, or partially converted feed is withdrawn as a vapor via line  418 , along with any steam or other superheated fluid added and lighter products produced in the isotropic pitch forming reactor  407 . A liquid phase comprising isotropic pitch is withdrawn from separator  416  via line  420 . This isotropic pitch contains a majority, preferably at least 90% and ideally essentially all of the solids in the feed in line  401 . Although contaminated with solids this isotropic pitch fraction has considerable value for many industrial uses, such as binder pitch for the aluminum industry, driveway sealers, and diluted with solvent to tar and feather politicians. 
     The vapor phase recovered from separator  416  is charged via line  418  to cooler  430  with cooled vapors discharged via line  432  into vapor and liquid separation means  434 . Temperature and pressure therein are set to condense at least a majority of heavy distillate material, typically 2 and 3 ring aromatic hydrocarbons. The vapor phase removed from separator  434  is charged via line  436  to cooler  438  and discharged via line  440  into vapor and liquid separator  442 . Temperature and pressure are set to condense at least a majority of light distillate hydrocarbons, recovered as a liquid via line  448 . Water condensate is removed via line  446  while a vapor phase comprising normally gaseous hydrocarbons is withdrawn overhead via line  444 . 
     The heavy distillate liquid recovered from separator  434  is rich in multi-ring aromatic hydrocarbons and has a greatly reduced or eliminated solids content. A portion of this material may be recovered, if desired, as a reduced solids aromatic liquid hydrocarbon product via line  450  and  452 , but preferably most or all of the liquid removed from the separator is charged to the isotropic pitch forming reactor  470  via lines  450  and  454 . If desired, a small portion of fresh feed, even feed contaminated with solids, may be added via line  460  when some solids can be tolerated in the pitch products or when required to ensure that sufficient alkyl groups are present on the aromatic rings charged to the isotropic pitch forming reactor. Other low-solids or solids free alkyl aromatics may be added, as removal of an alkyl group from an aromatic ring creates a reactive molecule which is believed to foster pitch formation. The heavy distillate, alone or mixed with additional alkyl aromatics in line  460 , is charged to heater  462  via line  461  and discharged into isotropic pitch forming reactor  470  via line  464 . 
     Reactor  470  effluent is discharged via line  472  into separator  474 , preferably a cyclone separator. A vapor phase is withdrawn via line  476  and mixed with vapor from separator  416 . A liquid phase is withdrawn via line  478 , mixed with optional superheated fluid in line  498  and charged to mesophase forming reactor  480 . Reactor effluent is discharged via line  482  into vapor liquid separator  484 , preferably a cyclone separator. An overhead vapor stream comprising unconverted multi ring aromatic hydrocarbons and lighter materials formed in the reactor is removed via line  488  to mix with overhead vapor from the first separator  416 . An ultra-purified mesophase pitch product is recovered from separator  484  via line  486  as a product of the process. Superheated fluid, preferably superheated steam may beneficially be added to multiple parts of the process. Boiler feed water in line  490  is heated in heater  492  to form superheated fluid in line  494  which can be charged via line  496  to mix with first reactor effluent, via line  497  and  499  to the inlet of the second reactor, via line  497  and  495  to the effluent from the second reactor  470 , or via line  497  and  498  to mix with the charge to the mesophase forming reactor  480 . 
     Pitch Process 
     The process of the invention may be used for production of purer isotropic pitch or purer mesophase pitch or both. The process works especially well when the reactor effluent vapor phase from a mesophase forming reactor is recovered and charged to an isotropic pitch reactor. This is because the vapor phase recovered from mesophase formation has a high molecular weight, indeed much of this vapor is partially polymerized multi-ring aromatics, and is well suited as a feed to the isotropic pitch reactor. 
     The feeds and reaction conditions in a pitch forming reactor may be conventional. Our preferred isotropic pitch forming process is disclosed in U.S. Pat. No. 9,222,027. Our preferred mesophase pitch forming process is disclosed in U.S. Pat. No. 9,376,626. These patents are incorporated by reference. Many other pitch processes have been developed and may be used as well. 
     Our preferred method of making isotropic pitch is to use a tubular reactor, operating at 800-1000° F., 800-2000 psi inlet pressure, one to 20 minutes residence time, 1-20 ft/sec average velocity and 30-80 vol % vapor (avg) 
     We prefer to make mesophase pitch in a tubular reactor operating at 750-900° F., 30-100 psi inlet pressure, 200-1000 ft/sec velocity (avg) and 99.9+vol % vapor. 
     Feed Filtration 
     Feed filtration is practiced now to some extent both on the 2 and 3 ring and heavier feed. Using our process, we can avoid, or at least use much less filtration by using recycled vaporization purified feed mixed with filtered fresh feed. If ultra high purity is required in the pitch product then little or no fresh feed should be added. 
     Alkyl Groups 
     When it is desired to use isotropic pitch as a precursor for the production of mesophase pitch, producing isotropic pitch with fewer alkyl groups on the multi-ring aromatics may be advantageous. This is because alkyl groups can cause steric hindrance when the multi-ring aromatic molecules self-assemble into spherical crystal clusters to form mesophase pitch. Isotropic pitch with few alkyl groups will self-assemble to form mesophase pitch faster and reach higher mesophase contents under less severe reactor conditions. Most isotropic pitch forming reactors cause a significant amount of dealkylation of the multi-ring aromatic molecules in the feedstock. As a result, a vaporization purified multi-ring aromatic intermediate product recovered from a first isotropic pitch forming reactor has a significantly lower concentration of alkyl groups than the original feedstock. It should be noted that the same effect would be true if the feed contained no solids and purification is not necessary. When this intermediate product is fed to a second isotropic pitch forming reactor, it will thereby produce isotropic pitch with significantly fewer alkyl groups in the isotropic pitch product. The unreacted multi-ring aromatics recovered from the second isotropic pitch will contain an even lower concentration of alkyl groups and, if recycled, to the second reactor feed, will reduce the alkyl group concentration of the isotropic pitch product even further. When producing a low alkyl group isotropic pitch product, the reaction rate may be improved by adding a solids free low-boiling alkylated aromatic compound such as toluene or methyl naphthalene to the feed to the second isotropic pitch forming reactor since one of the pitch forming reactions is dealkylation of an aromatic ring to form a reactive site which subsequently reacts with another multi-aromatic ring molecule to form a larger multi-aromatic ring molecule. Unreacted low-boiling alkylated aromatic compounds are preferred because they can be easily recovered from the isotropic pitch using their low boiling points. If extreme purity isotropic pitch product is not required, a small amount of contaminated fresh feed could be added to the feed to the second reactor instead of solids free low-boiling alkylated aromatic compounds. 
     Vapor Liquid Separator Conditions 
     An important factor in running the process is flash conditions. The amount and composition of the heavy distillate recovered in the vapor liquid separators downstream of the pitch forming reactors will depend upon the temperature, pressure and amount of superheated stripping gas used. The total amount of heavy distillate, as well as the average molecular weight of the heavy distillate, is generally increased by increasing the flashing temperature, reducing the pressure and increasing the amount of stripping gas used. In general, reactor effluent, or a heated fresh feed, is flashed at relatively low pressures, theoretically possible but difficult 0.1 to 10 atm, preferably 1 to 5 atm and most preferably 20 to 50 psia. Temperature in the flash separators is usually high, from 500 to 1100° F., preferably from 600 to 1000° F. and ideally 750 to 850° F. The stripping gas to hydrocarbon weight ratio is usually from 0.1 to 9.0, preferably from 0.3 to 3.0 and ideally 0.5 to 1.5. 
     Steam Addition 
     Steam, or other superheated fluid may beneficially be added to multiple points of the process. To clarify, any fluid which is generally inert at the conditions used may be superheated and used herein, but steam is preferred. Steam may also clean the tubular, or other, reactors out to some extent by reacting with, or preventing formation of, coke deposits with the reactor. Steam also performs other important functions such as ensuring turbulent flow in the mesophase pitch forming reactor and steam stripping of any heavy hydrocarbon liquid. Steam stripping has historically been used to extract the essence of herbs and flowers, but it is also effective at vaporizing and removing 2 and 3 ring aromatic hydrocarbons from an isotropic or mesophase pitch liquid fraction.