Asphalt processes and products

Abstract

A process for manufacturing mesophase and / or isotropic pitch. A liquid rich in aromatics is charged into a first thermal polymerization reactor at high temperature and pressure to produce a flashed liquid rich in isotropic pitch. The residual liquid is charged into a second thermal reactor to produce mesophase pitch. These reactors can be tubular reactors or CSTRs. Carbonized or graphitized fibers, foams, blocks, sheets or balls can be produced from the mesophase pitch.

Description

[0001] Cross - reference to related applications

[0002] Two recent patents of the applicant, US9222027 and US9376626, are related and are incorporated herein by reference. Technical field

[0003] The present invention relates to the formation of isotropic pitch or mesophase pitch. Background art

[0004] Much work has been done in preparing isotropic or mesophase pitch. The above - mentioned two recent patents of the applicant respectively relate to the preparation of isotropic pitch and mesophase pitch.

[0005] Although the methods in the above patents are considered the best available techniques for preparing isotropic pitch and mesophase pitch, the applicant continues to research to improve these processes. The applicant has developed an improved two - stage process and has also provided a better single - stage process. Summary of the invention

[0006] Accordingly, the present invention provides a two - stage method for producing mesophase pitch from an aromatic liquid, comprising thermally polymerizing an aromatic liquid feed comprising at least a portion of 2 - ring and 3 - ring aromatic compounds by adding the feed to a first - stage reactor operating under thermopolymerization conditions, the thermopolymerization conditions including a temperature high enough to initiate thermal polymerization and a pressure high enough to maintain at least a portion of the 2 - ring and 3 - ring aromatic compounds in the liquid phase, thereby converting at least 20 wt% of the 2 - ring and 3 - ring aromatic compounds into isotropic pitch and light, generally gaseous hydrocarbons, and discharging the first - stage reactor effluent; flash - vaporizing the first - stage reactor effluent to remove at least most of the light, generally gaseous hydrocarbons in a flash zone, the absolute pressure of the flash zone not exceeding half of the absolute pressure in the first - stage thermal polymerization reactor, to produce a flash - vaporized effluent stream; forming mesophase pitch from the flash - vaporized effluent stream in a second - stage reactor under mesophase - forming conditions, the mesophase - forming conditions including a pressure not exceeding half of the absolute pressure in the first - stage reactor and a temperature that converts at least 1 / 3 wt of the flash - vaporized effluent liquid into mesophase pitch; and recovering mesophase pitch as a product from the second - stage reactor.

[0007] In another embodiment, the present invention provides carbon fibers, graphite fibers, or carbon foams prepared from mesophase pitch by the following steps: thermally polymerizing an aromatic liquid feed comprising at least a portion of 2-ring and 3-ring aromatic compounds by adding the feed to a first stage reactor operating under thermopolymerization conditions, the thermopolymerization conditions including a temperature high enough to initiate thermopolymerization and high enough to maintain at least a portion of the 2-ring and 3-ring aromatic compounds in the liquid phase to convert at least 20 wt% of the 2-ring and 3-ring aromatic compounds to isotropic pitch and light, generally gaseous hydrocarbons, and discharging the effluent of the first stage reactor; flashing the first stage reactor effluent to remove at least most of the light, generally gaseous hydrocarbons in a flash zone, the absolute pressure of the flash zone not exceeding 1 / 2 of the absolute pressure in the first stage thermopolymerization reactor, to produce a flash effluent stream; forming mesophase pitch from the flash effluent stream in a second stage reactor under mesophase forming conditions, the mesophase forming conditions including a pressure not exceeding 1 / 2 of the absolute pressure in the first stage reactor and a temperature to convert at least 1 / 3 wt of the flash effluent liquid to mesophase pitch; and recovering the mesophase pitch from the second stage reactor and converting the mesophase pitch to at least one of carbonized or graphitized fibers, foams, blocks, sheets, or spheres by conventional processes for the mesophase pitch, and recovering at least one of the carbonized or graphitized fibers, foams, blocks, sheets, or spheres as a product of the process. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The figure is a simplified diagram of an embodiment of a two-stage process for producing mesophase pitch from aromatic liquids. Fresh feedstock consisting of filtered aromatic liquid 1 is fed into feed tank 2 and then enters pipeline 103 through pump 3, where it is mixed with the recycled heavy aromatic compound stream in pipeline 4 and discharged into mixing feed drum 5. The mixed feed is discharged through pipeline 106, and pump 7 increases the pressure for discharging into flame heater 8. Then, the heated mixed feed is charged into the first-stage tubular reactor 9 through pipeline 108, where the feed is partially converted into a mixture of isotropic pitch and mesophase pitch. The reactor effluent is discharged from the first-stage reactor through pipeline 10 and passes through pressure reducing valve 11 to reduce the pressure. Then, the flash partially converted mixture is mixed with the superheated steam added through pipeline 12, which is produced by passing the boiler feed water in pipeline 113 through steam boiler / superheater 13. The molar ratio of steam to hydrocarbon is generally greater than 1:1, for example, 3:1. The mixture obtained in pipeline 14 is fed into the second-stage tubular reactor 15, where additional mesophase is formed. The second-stage reactor effluent is discharged into vapor / liquid separator 17 through pipeline 16. The liquid stream 18 leaving this separator contains 85 - 90 mass% of mesophase pitch (the balance is mainly isotropic pitch), and represents a yield of 88 mass% of the fresh feedstock. By feeding stream 18 into asphalt cooling and solidification system 19, stream 18 is recovered for storage and distribution. Using the ethylene glycol / water cooling stream 22 from ethylene glycol cooling system 23, the steam 20 from vapor / liquid separator 17 is cooled to 155 °C in shell-and-tube heat exchanger 21. Any other conventional cooling method can be used, for example, feed / product exchange, air cooling through a finned fan cooler, or cooling water from a cooling tower. The cooled steam stream 24 enters vapor / liquid separator 25, where the heavy aromatic compound stream 26 is drawn from the bottom and briefly stored in heavy aromatic compound tank 27. Then, this heavy aromatic material is pumped through pipeline 127 and pump 28, and recycled to feed drum 5 through pipeline 29 or cooled by air cooler 30, sent to heavy aromatic compound tank 31 for storage through pipeline 131, and pumped out through pipeline 133, pump 32, and pipeline 132 for sale as a valuable by-product, for example, a solvent rich in aromatic compounds. The overhead vapor removed from vapor / liquid separator 25 through pipeline 33 is cooled to 49 °C in air cooler 34 and discharged into three-phase separator 35 through pipeline 134. The gas phase is removed from the top of the tower through pipeline 36. The water of the steam condensate is removed from the bottom of the separator through pipeline 37. The light hydrocarbon stream is removed through pipeline 38 and discharged into light hydrocarbon storage tank 39, where it is pumped out through pipeline 140, pump 40, and pipeline 141. Detailed Description of the Invention

[0009] Details regarding feeds, products, and reaction conditions in tubular reactors are described in the applicant's related earlier patents, which are incorporated herein by reference and not repeated here.

[0010] First stage

[0011] Preferred are tubular reactors and relatively high pressures. The difference from the applicant's earlier work is to promote the conversion of aromatic compounds into pitch molecules. In the applicant's earlier patents, the applicant tried to limit the mesophase content in isotropic pitch products to 1 wt% or less. This is an effective and excellent method when isotropic pitch is the desired product, but it is not always optimal when some or all of the isotropic pitch products are to be converted into mesophase pitch. When mesophase is the ultimate goal, it may be beneficial to operate the first stage of the process to produce isotropic pitch "contaminated" with more than 1 wt% mesophase, preferably more than 2 - 5 wt% mesophase, and ideally more than 10 - 25 wt% mesophase, or even more.

[0012] This contaminated product from the first stage is "neither fish nor fowl" and has little or no value as an isotropic pitch product, but it is an ideal feedstock for the second reaction stage, where additional thermal polymerization will produce a marketable mesophase product. Flash distillation in the first stage removes light gases formed as by-products of thermal polymerization and thermal dealkylation, and also preferably removes bicyclic and tricyclic aromatic compounds. In the second stage reactor, a lower pressure promotes the formation of mesophase. The bicyclic and tricyclic aromatic compounds are removed because they are thought to interfere with the formation of mesophase.

[0013] When the first stage is carried out in a continuous stirred tank reactor (CSTR), the chemistry and general methods are similar, but there are different limitations and concerns. The thermal reactions for forming isotropic pitch and to a limited extent mesophase pitch are the same, but the reactants are slightly different, and the amount of mesophase that can be tolerated is usually significantly reduced.

[0014] The reactants in the CSTR may be different because a CSTR typically consists of a stirred vessel with a stirred liquid at the bottom and a vapor space above. Materials in the gas phase are more difficult to react with materials in the liquid phase because the contact is greatly reduced compared to the strong contact between vapor and liquid in a tubular reactor.

[0015] The amount of tolerable mesophase in the first-stage product is typically lower than that tolerable in a tubular reactor. The tubular reactor keeps all reactants in motion, and its walls are largely wiped clean by the flowing fluid. This situation is similar to what a refinery experiences when trying to use cooling water with a high solids content from the Mississippi River. The dirty cooling water can be used, but only in a tubular heat exchanger and only when maintaining a high flow rate. In a typical CSTR, the walls of the vessel and the blades of the impeller or other mixing devices can tolerate some mesophase, but usually not as much as can be tolerated in a tubular reactor.

[0016] The pressure in the first stage should be high to retain more two-ring and three-ring aromatic compounds in the liquid phase. These aromatic materials can be converted to isotropic pitch, but usually only when they are in the liquid phase.

[0017] Second stage

[0018] When using a tubular reactor, the second stage can be operated in the manner described in the applicant's earlier patents on mesophase production.

[0019] Compared to the pressure in the first stage, the absolute value of the pressure in the second stage should be reduced by at least 50%. When maximum mesophase production and concentration are the goal, it is preferred that the pressure be low enough and the temperature be high enough to evaporate at least most of the molar amounts of the two-ring and three-ring aromatic compounds. These two-ring and three-ring aromatics tend to interfere with mesophase formation, so it is best to remove them.

[0020] When using a CSTR reactor, more care is needed in design and operation to avoid plugging the hot reactor. The mesophase content is typically higher than 50 wt%, and this material is hot enough to easily form coke and viscous enough to easily plug the vessel or impeller. By using a higher impeller speed and / or a rocking reactor, the operation of the CSTR can be extended to some extent.

[0021] One advantage of a CSTR over a tubular reactor is that they are cheaper to build and require only a small footprint.

[0022] Preferably, little or no externally heated flash effluent is discharged into the second-stage reactor. Preferably, the second-stage reactor has no external heating. This is because these mesophase-rich and coke-prone materials tend to coke on hot surfaces.

[0023] Heat balance

[0024] The heat requirement for the second stage of the process is preferably and mainly provided by the feed. The feed and product of the first stage reactor can tolerate significant conventional heating, e.g., in a fired heater or immersed in a molten metal or molten salt bath. The thermal reaction occurring in the first reactor is mainly controlled by time and temperature. For every 10 °C increase in temperature, the reaction rate approximately doubles, so operating the first reactor hotter allows for a reduction in the reactor size.

[0025] When the light fractions and 2- and 3-ring aromatic compounds are removed by flash evaporation, the first stage reactor effluent will cool significantly. The residual liquid from this flash is still very hot, and in many cases hot enough to initiate thermal polymerization in the second stage reactor. Additional heat can be added to the second stage reactor in the form of a superheated fluid, preferably superheated steam. The second stage reactor operates at a much lower pressure than the first stage, so construction costs can be significantly reduced due to the lower pressure operation. The residence time in the reactor can be increased by using a tubular reactor with a larger inner diameter and / or longer length or by using a larger CSTR.

[0026] Additional residence time can be achieved by using multiple reactors in series or by recycling some of the second reactor effluent back to the first reactor. Usually, once-through operation is preferred, both to reduce capital and operating costs and because the liquid effluent from the second reactor is rich in mesophase and prone to coking.

[0027] When using a CSTR reactor, heat can also be increased by mixing or mechanical energy. The energy can be added through a conventional mixing impeller. There is no free energy, but rather an electrically or steam-driven pump transfers energy into the second reactor by intense mixing.

[0028] Other forms of energy can be added, such as ultrasonic or microwave energy, e.g., the use of microwave heating of coal as disclosed in US3503865 (COALLIQUEFACTION PROCESS, Stone).

[0029] Illustrative Embodiments

[0030] The following data and discussion are based on limited laboratory experiments, with a lot of extrapolation and estimation for different parts of the process. They are fairly reliable estimates but not actual experiments.

[0031] Example 1 - Two Tubular Reactors

[0032] This is the reactor design shown in the attached drawing. Although the discussion related to this figure mainly focuses on the mesophase product, this design allows for the recovery of relatively pure isotropic pitch, either as an intermediate product or as a final product. When the conversion in the first tubular reactor is limited by the amount of mesophase tolerated in the product, relatively pure isotropic pitch can be recovered as an intermediate product. In this case, the first-stage reactor is operated to convert most of the feed into isotropic pitch, but the conversion is limited so that the intermediate pitch product has less than 2 wt% mesophase, or 1 wt% mesophase, or 0.5 wt% mesophase or less. This does not maximize the overall production of mesophase, but it will allow for the recovery of some isotropic pitch product from the intermediate separator and some mesophase pitch product downstream of the second thermal reactor.

[0033] Relatively pure isotropic pitch product can also be obtained as the product from the second thermal reactor. The first thermal reactor converts the required amount of aromatic feed into isotropic pitch, typically 10 - 60 wt%, preferably less than 50 wt% converted into isotropic pitch, and in the second thermal reactor to the required isotropic pitch content, typically about 70 wt%, or 80 wt%, or 90 wt%. Flash evaporation is usually required between the two thermal reactors because typically the second thermal reactor will be designed to operate at a lower pressure than the first thermal reactor, but it is beneficial to retain more of the 2-ring and 3-ring aromatic compounds in the liquid phase to facilitate their conversion in the second thermal reactor. A good flash evaporation method is to reduce the pressure sufficiently to remove most of the lighter by-products and most of the 2-ring aromatic compounds, while retaining at least most of the molar amount of the 3-ring aromatic compounds in the residual liquid phase removed from the flash evaporation. This residual liquid phase can then be charged to the second thermal reactor, preferably injecting a superheated fluid after flash evaporation.

[0034] Example 2 - Tubular Reactor to CSTR

[0035] This method can be similar to the method of Example 1. The first thermal reactor is a tubular reactor and the second is a CSTR. The lower pressure in the CSTR is beneficial for the evaporation of 2-ring and 3-ring aromatic compounds and for the formation of mesophase.

[0036] Example 3 - CSTR to Tubular Reactor

[0037] The first thermal reactor is a stirred tank reactor. Although a robust reactor design is required to handle the operation at high pressure, at least in terms of coking problems, the reactor will be relatively easy to operate. The first reactor can leave most of the feed unconverted, so the fluid is easily agitated. The coking tendency of the material increases with the increase in the pitch percentage, especially the mesophase pitch percentage. Therefore, most of the coking problems are transferred to the second thermal reactor downstream, which is a tube operating in fully developed turbulence.

[0038] The first reactor should be operated at a relatively high pressure sufficient to maintain at least most of the three-ring aromatic compounds in the liquid phase, and preferably to keep most of the two-ring aromatic compounds in the liquid phase. These aromatic compounds can be converted to pitch, but liquid-phase operation is preferred. Flash evaporation or some pressure-reducing means between the first and second reactors are usually necessary to remove at least most of any remaining unconverted two- and three-ring aromatic compounds.

[0039] This method (i.e., from CSTR to tubular reactor) allows for a relatively compact and simple first reactor, and the increased fluidity largely offsets the coking risk, and the tubular reactor can rely on turbulence to reduce coking.

[0040] Example 4 - Two CSTRs

[0041] In this method, stirred tank reactors are used for the first and second thermal reactors. The pressure in the first reactor is relatively high to keep the two- and three-ring aromatic compounds in the liquid phase. The flash evaporation step removes most of the two- and three-ring aromatic compounds, which is beneficial for thermal conversion to mesophase in the second thermal reactor. Since the mesophase is close to coke, the coking problem in this method is significant. For this reason, having multiple CSTRs such that if one CSTR cokes, it can be swapped out for decoking is also beneficial.

[0042] Effect of residence time on product performance

[0043] In some applications, it is important that the final mesophase or isotropic pitch product has a relatively narrow molecular weight range. Using a tubular reactor with precisely controlled residence time in the thermal reactor will produce a product with the narrowest molecular weight range. In some applications, having some molecular weight distribution may be beneficial. To facilitate the mesophase pitch process, having a certain amount of isotropic pitch (i.e., 5 wt%, 10 wt%, or 15 wt%) as a softening agent is usually beneficial. CSTR reactors will have a range of liquid residence times, but tubular reactors do not.

[0044] The process of the present invention provides what is believed to be the most cost-effective method for producing mesophase pitch from aromatic liquids. Most of the benefits are achieved through the close coupling of the first and second reactor stages. There is no need to cool the isotropic pitch material, and little or no preheating of it is required upstream of the second stage of the reactor.

[0045] In addition, the process is flexible and can be used to recover isotropic pitch from the flash vessel between the two stages.

[0046] In addition, the process provides a method for producing pure isotropic pitch from an aromatic-rich liquid in a single stage or even from two hot reactors. If desired, the process can also be used to produce mesophase pitch from an isotropic pitch feedstock.

[0047] Tube reactors, which have excellent mixing characteristics, fouling resistance, and the ability to add some heat by electrically heating the tube walls, have proven to operate well in our laboratory. In some applications, when using certain types of electrical heating, the relatively large footprint, relatively low capacity, and the need for careful fabrication may be of sufficient concern to warrant the use of a CSTR in the first or second reactor or both.

[0048] When used in the first-stage reactor, the chemical efficiency of a CSTR may be slightly lower than that of a tube reactor. Two-ring and three-ring aromatic compounds will concentrate in the gas phase above the liquid in the CSTR. These aromatic compounds have the potential to be converted to isotropic pitch, but to do so they must come into close contact with the larger aromatic compounds in the liquid phase. For this reason, therefore, the CSTR is at a slight disadvantage in the first reactor. In the second-stage reactor for forming mesophase pitch, the presence of two-ring and three-ring aromatic compounds is found to tend to interfere with the formation of large mesophase molecules, and thus agitating the vapor space above the liquid is somewhat beneficial in the second-stage reactor.

[0049] Discussion / Optimization

[0050] Although the discussion and claims sometimes refer to two-ring and three-ring aromatic compounds, in many applications it may be sufficient to have only one of them to help optimize the process. Thus, it may be sufficient to design and maintain the first hot reactor at a sufficient pressure to keep the three-ring aromatic compounds mainly in the liquid phase, preferably maintaining 60%, 70%, 80%, 90% or more of these compounds in the liquid phase. If the two-ring aromatic compounds are sometimes or continuously in the gas phase, the process will still be effective. Alternatively, a refinery may focus on keeping at least a majority, preferably 60%, 70%, 80%, 90% or more of the two-ring aromatics in the liquid phase, ensuring the vapor / liquid equilibrium to keep the three-ring aromatic compounds to a greater extent in the liquid phase. Considerably oversimplifying, it is generally beneficial to have a pressure high enough to keep most of the two-ring aromatic compounds in the liquid phase in the first reactor and low enough to exclude most of the three-ring aromatic compounds to the gas phase in the second reactor.

[0051] The reactor is preferably operated to convert most of the feedstock to isotropic pitch in a first thermal reactor and most of the first reactor effluent to mesophase pitch in a second thermal reactor. Profitable operation can be achieved at much lower conversion rates if there is a market for the by-products or sufficient capital and operating funds to recover the unconverted materials. The threshold for a viable commercial plant may be a conversion of liquid aromatic feedstock to isotropic pitch of about 20%. Similarly, it is not necessary to convert most of the feed to the second thermal reactor to mesophase pitch, and conversions of 20%, 30%, 40% or higher are satisfactory.

[0052] The preferred method is to push the first reactor quite hard and convert most of the aromatic liquid feed to isotropic pitch, preferably pitch that is heavily "contaminated" with mesophase, at least 1 wt%, preferably 2 wt%, 3 wt%, 4 wt%, 5 wt%, 7 wt%, 10 wt%, 15 wt%, 20 wt% or more. This method will ensure that most of the readily convertible molecules in the feed are converted to the pitch product. Generally, the aromatic liquid feeds contemplated herein will contain a large amount of 2-ring and 3-ring aromatic compounds, which have a relatively low value / price. If this low-cost feed can be converted to a higher-value mesophase pitch, the economics of the process will be better.

[0053] The conditions in the first thermal reaction zone preferably include a temperature of 455 - 540 °C, preferably 480 - 510 °C, and desirably 495 - 500 °C. The pressure should be sufficient to maintain the required amount of 2-ring or 3-ring aromatic compounds in the liquid phase and is preferably 7 - 210 bar, more preferably 35 - 170 bar, desirably 70 - 140 bar. The residence time depends mainly on the temperature and the required conversion rate, but generally ranges from 10 seconds to 10 minutes, preferably 0.5 - 5 minutes, and desirably 2 - 3 minutes.

[0054] The conditions in the second thermal reaction zone generally involve significantly lower pressure and shorter residence time. The pressure should be less than 1 / 2 of the absolute total pressure in the first thermal reactor and is generally 7 bar or less, preferably less than 3.5 bar, desirably 2 bar or less, or even atmospheric or below atmospheric. The residence time required to effect the desired conversion of the feed to mesophase is generally less than one minute, preferably 0.1 - 10 seconds, desirably 0.2 - 2 seconds.

[0055] The pressures and temperatures in the first and second zones are suitable for relatively simple commercial process designs, with all or substantially all of the thermal and pressure energy added at the inlet of the first reactor. There will be sufficient pressure to pass the reactants through the first stage reactor and into the flash or second thermal reactor. The temperature in the first reactor can be chosen high enough such that after flashing, the liquid phase is at or near the temperature required in the second stage reactor. In this way, all of the heat required can be added to the feed of the first reactor. All of the pressure required for the reactants by the process can be added upstream of the first reactor.

[0056] The concept of the flash zone is to allow the evaporation of 2-ring and 3-ring aromatic compounds from the isotropic pitch "product", but the flash vessel itself is not necessary. In a tubular reactor, it is possible to rely solely on hydrodynamics to flash the isotropic pitch liquid, preferably using an enlarged inner diameter tube in the second reactor section. The isotropic pitch-rich liquid discharged into the tube forming the second thermal reactor will flash in the tube such that the 2-ring and 3-ring aromatic compounds in the liquid phase will be depleted. There can be a flash tank with a relatively high pressure, low enough to allow the removal of most of the 2-ring and 3-ring aromatic compounds from the isotropic pitch discharged into this high-pressure flash. The liquid phase from the high-pressure flash can then be charged into the second thermal reactor. There can be a flash tank at a relatively low pressure to facilitate the removal of an increased amount of 2-ring and 3-ring aromatic compounds.

[0057] Direct contact heating / stripping

[0058] Superheated fluid can be added, preferably superheated steam, to supply any heating or stripping requirements of the process. When little heating is required and moderate stripping or removal of 2- and / or 3-ring aromatic compounds is needed, a small amount of superheated steam can be added, such as 1 to 10 wt% of the liquid. There is no upper limit to the amount of superheated steam added, and when a certain amount of heating of the liquid is required, amounts up to 50 wt%, 100 wt%, 200 wt% or more can be considered. While steam is preferred, other superheated fluids can be used when needed, such as hydrogen, normally gaseous hydrocarbons, etc.

[0059] The process of the present invention has various applications. The process can be used to prepare mesophase pitch from an aromatic oil feed using 2 plug flow reactors in series. Preferably, the pressure drop between the two reactors is sufficient to evaporate at least most of the 2-ring aromatic compounds in the feed or generated by thermal polymerization in the first stage reactor.

[0060] In other embodiments, the process can be used to produce isotropic pitch and mesophase pitch using two thermoreactors in series. The first thermoreactor should be operated at a pressure sufficient to maintain at least most of the 3-ring aromatic compounds in the feed in the liquid phase in the first reactor and at a temperature sufficient to thermally polymerize at least most of the 3-ring and heavier aromatic compounds into isotropic pitch to produce a first reactor effluent containing isotropic pitch and no more than a predetermined amount of mesophase pitch (usually less than 1 wt%). The first reactor effluent is flashed into a flash tank that is operated at a low enough pressure to evaporate at least most of the 2-ring aromatic compounds present in the first reactor effluent. A portion of the flash tank liquid is withdrawn as the isotropic pitch product of the process and the remainder is charged to a second thermoreactor that is operated at a low enough pressure to evaporate at least most of the 2-ring aromatic compounds present and for a sufficient time and at a temperature sufficient to convert at least most of the liquid feed by weight to the second reactor into mesophase pitch. In a preferred embodiment, a superheated fluid such as steam or hydrocarbon is charged to the second reactor for heating. The first thermoreactor can be a continuously stirred tank reactor or a tubular reactor. The second thermoreactor can be a continuously stirred tank reactor or a tubular reactor.

[0061] Preferably, the pressure in the first stage reactor is 14 to 210 bar, more preferably 35 to 140 bar, ideally about 68 to 70 bar. Preferably, the pressure in the second stage reactor is one tenth or less of the first stage reactor, more preferably 7 bar to subatmospheric pressure, ideally atmospheric pressure to 5 bar.

[0062] To maximize the production of mesophase pitch, the first stage reactor is preferably operated under thermal polymerization conditions that are sufficient to contaminate the isotropic pitch intermediate in the first stage reactor with a sufficient amount of mesophase pitch such that the intermediate is not suitable for use as isotropic pitch. Preferably, the isotropic pitch intermediate product of the first stage reactor effluent contains more than 1.0 wt%, 1.5 wt%, 2 wt%, 5 wt%, 10 wt%, 15 wt%, 20 wt%, 25 wt% or 30 wt% mesophase pitch.

[0063] In some cases, the "flash zone" is simply a pressure reducing valve that does not remove any vapor. In these cases, the two-phase flow is fed to the second stage reactor. In other cases, a vapor / liquid separator can be used to remove the vapor and then only the liquid is fed to the second stage reactor. In plants operating a flash separator, the overhead vapor from the first or flash vapor / liquid separator and the vapor recovered from the second stage reactor effluent are combined and used as fuel or for other product recovery.

[0064] The recovery of heavy liquids is generally beneficial. If the conversion rate in the first-stage reactor is relatively low, for example, 20 to 50 wt%, it is generally beneficial to recover some of the first-stage reactor effluent for mixing with the reactor feed. If the conversion rate in the second reactor is low, for example, 35 to 60% for the conversion of the mesophase, the heavy distillate from the second reactor can be recovered. Preferably, the recycle of all liquids is to the first reactor, although in some cases, it may be preferred to recycle the effluent liquid of reactor 1 to reactor 1 and recycle the effluent liquid of reactor 2 for mixing with the feed of the second reactor. Recycling may be necessary to improve the economics of the process, especially when the conversion rate per pass is relatively low.

[0065] It may be necessary to add steam or some other vapor, preferably superheated steam, to the feed entering the second-stage reactor. The presence of this steam reduces the partial pressure, provides additional thermal inertia, generates high velocity, and provides better dispersion of the pitch droplets. Although any vapor can be used, low molecular weight, condensable gases that are non-reactive and have very low solubility in the mesophase pitch are preferred. Steam is most preferred. The amount of steam, as a weight percentage of the liquid feed, can vary from 10% to 1000%, preferably 150 to 400%, more preferably 250 to 350%.

[0066] In some applications, having two second-stage reactors (in series) or second-stage and third-stage reactors would be beneficial. This allows the mesophase pitch to be sent from the second-stage reactor to a small "annealing" CSTR, or even to another tubular reactor, before or after the removal of light gases, to provide additional residence time to "fine-tune" the final mesophase pitch product.

[0067] The pressure drop in the second-stage reactor can be high. In the experiments and simulations of the present invention, the outlet pressure is about 2.75 bar plus the pressure drop of the downstream equipment. If necessary, the final discharge gas can be evacuated to further reduce the outlet pressure. The recommended inlet pressure range for the second reactor can be 3.5 to 70 bar, preferably less than 35 bar, more preferably 7 to 14 bar.

[0068] The suitable operating temperature range for the first-stage and second-stage reactors can be 400 to 595 °C, preferably 425 to 525 °C, more preferably 482 to 510 °C.

[0069] In the mesophase process of the present invention, the gas phase is continuous and the liquid phase is discontinuous. At high superficial velocities in the tubular reactor where the mesophase is formed, the shear rate in the reactor is very high. Typically, the pressure drop in a 9.5 mm (3 / 8”) reactor tube is 4 - 6 bar (60 - 90 psi). At this high shear rate, only small (<20 micron) droplets are present. In multiphase flow analysis, this flow regime can be described as annular mist. This relatively small spherical size combined with very turbulent (Reynolds number > 5×10 5 ) allows for very rapid mass and heat transfer between the liquid and vapor phases. Although these spheres may undergo some collisions with other spheres resulting in coalescence, due to the high shear forces, they will quickly break down again into smaller spheres. The vapor fraction is greater than 99% by volume. It appears that in most cases, the volatilization of the lighter hydrocarbons and subsequent organization of the large polycondensed aromatic compounds into the mesophase occur in isolated small spheres.

[0070] Important benefits are also achieved when the second or mesophase forming process is combined with the first stage or isotropic pitch forming process. The heavy distillate by - product of the mesophase forming process is not well - suited for recycling to the mesophase forming process as it will mostly just evaporate and there will be little conversion to the mesophase. This heavy distillate stream can be recycled to the feed of the isotropic pitch forming reactor, where some of it can be converted to isotropic pitch, which will be converted to mesophase pitch in the second stage of the present invention's process. This additional conversion can be a key factor in an economically viable process, especially when the conversion rate per pass is low and maximum conversion is required. Recycling of the un - converted heavy aromatic liquid in the mesophase forming reactor is largely non - productive, but recycling this heavy liquid to the first reactor in a two - stage process is very effective.

[0071] The present invention preferably uses a tubular reactor rather than a continuous stirred tank reactor (CSTR) because the CSTR is not efficient in converting isotropic pitch to mesophase pitch. The CSTR can be used to adjust the final mesophase content in the tubular mesophase forming reactor. A CSTR reactor for adjusting the mesophase content and other properties can be used for quality control of the mesophase product.

[0072] The process method and results of the present invention are surprising. The present invention forms isotropic pitch in the first - stage reactor and then produces liquid mesophase pitch, preferably which is 99 LV% vapor, in the second - stage reactor. The present invention recovers the un - converted heavy aromatic material from the mesophase forming reactor for recycling, but not back to the mesophase forming reactor.

[0073] As described in the applicant's related patent US9376626, the mesophase pitch of the present invention can be used to manufacture carbon fibers, graphite fibers, carbon foams, etc. using known and conventional processes.

Claims

1. A two-stage process for producing mesophase pitch from an aromatic liquid, comprising: a. thermally polymerizing an aromatic liquid feed comprising at least a portion of 2-ring and 3-ring aromatic compounds by adding the feed to a first-stage reactor operating under thermopolymerization conditions, the thermopolymerization conditions including a temperature high enough to initiate thermal polymerization and a pressure high enough to maintain at least a portion of the 2-ring and 3-ring aromatic compounds in the liquid phase, thereby converting at least 20 wt% of the 2-ring and 3-ring aromatic compounds into isotropic pitch and light gaseous hydrocarbons, and discharging a first-stage reactor effluent, wherein the first-stage reactor is selected from a tubular reactor and a continuous stirred tank reactor; b. flash vaporizing the first-stage reactor effluent to remove at least most of the light gaseous hydrocarbons in a flash zone, the absolute pressure of the flash zone not exceeding 1 / 2 of the absolute pressure in the first-stage reactor to produce a flash effluent stream; c. forming mesophase pitch from the flash effluent stream in a second-stage reactor under mesophase formation conditions, the mesophase formation conditions including a pressure not exceeding 1 / 2 of the absolute pressure in the first-stage reactor and a temperature to convert at least 1 / 3 wt of the flash effluent stream into mesophase pitch, wherein the second-stage reactor is selected from a tubular reactor and a continuous stirred tank reactor; and d. recovering mesophase pitch from the second-stage reactor as a product.

2. The method according to claim 1, wherein At least a portion of the flash effluent stream is recovered as an isotropic pitch product.

3. The method according to claim 1, wherein The pressure in the first-stage reactor is 15 to 350 bar.

4. The method according to claim 1, wherein The pressure in the second-stage reactor is less than 1 / 10 of the pressure in the first-stage reactor.

5. The method according to claim 1, wherein, The inlet pressure in the second-stage reactor is 3.5 to 70 bar.

6. The method according to claim 1, wherein The pressure in the second-stage reactor is low enough to maintain at least most of the 2-ring and 3-ring aromatic compounds in the gas phase.

7. The method according to claim 1, wherein, The pressure in the first-stage reactor is high enough to maintain at least most of the 2-ring and 3-ring aromatic compounds in the liquid phase.

8. The method according to claim 1, wherein The conditions in the first-stage reactor are sufficient to produce isotropic pitch having 1 to 20 wt% mesophase pitch.

9. The method according to claim 1, wherein Adding a superheated fluid or steam to the second-stage reactor.

10. The method according to claim 1, wherein, Adding sufficient superheated fluid or steam to the second-stage reactor, and wherein the second-stage reactor is a tubular reactor, to maintain a continuously dispersed droplet stream of steam in the second-stage reactor.

11. The method according to claim 10, wherein, The second-stage reactor discharges into a cyclone separator.

12. A method for preparing carbonized or graphitized fibers, foams, blocks, sheets or spheres from mesophase pitch, the method comprising the following steps: a. Thermally polymerize an aromatic liquid feed comprising at least a portion of a 2-ring and 3-ring aromatic compound by adding the feed to a first-stage reactor operating under thermopolymerization conditions, the thermopolymerization conditions including a temperature high enough to initiate thermal polymerization and a pressure high enough to maintain at least a portion of the 2-ring and 3-ring aromatic compounds in the liquid phase, to convert at least 20 wt% of the 2-ring and 3-ring aromatic compounds to isotropic pitch and light gaseous hydrocarbons, and discharging the first-stage reactor effluent, wherein, The first-stage reactor is selected from a tubular reactor and a continuous stirred tank reactor; b. Flash the effluent from the first-stage reactor to remove at least most of the light gaseous hydrocarbons in the flash zone, where the absolute pressure of the flash zone does not exceed 1 / 2 of the absolute pressure in the first-stage reactor to produce a flash effluent stream; c. Form mesophase pitch from the flash effluent stream in a second-stage reactor under mesophase formation conditions, where the mesophase formation conditions include a pressure not exceeding 1 / 2 of the absolute pressure in the first-stage reactor and a temperature that converts at least 1 / 3 by weight of the flash effluent stream into mesophase pitch, and wherein the second-stage reactor is selected from one of a tubular reactor and a continuous stirred tank reactor; and d. Recover the mesophase pitch from the second-stage reactor and convert the mesophase pitch into at least one of carbonized or graphitized fibers, foams, blocks, sheets, or spheres by a conventional process of the mesophase pitch, and recover at least one of the carbonized or graphitized fibers, foams, blocks, sheets, or spheres as a product of the method.

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