INDUSTRIAL METHODS FOR PRODUCTION OF NAPHTHALENE 1) INTRODUCTION 2) PROPERTIES OF NAPHTHALENE 3) PROCESS DESCRIPTION a) COAL TAR METHOD b) HYDEAL PROCESS 4) FLOW DIAGRAMS 5) ADVANTAGES
Introduction Naphthalene is an organic compound with formula C10H8. It is the simplest polycyclic aromatic hydrocarbon, and is a white crystalline solid with a characteristic odor. As an aromatic hydrocarbon, naphthalene's structure consists of a fused pair of benzene rings. Most naphthalene is derived from coal tar . From the 1960s until the 1990s, significant amounts of naphthalene were also produced from heavy petroleum fractions during petroleum refining, but today petroleum-derived naphthalene represents only a minor component of naphthalene production. Approximately 1.3M tons are produced annually.
Properties of Naphthalene
In the early 1820s, two separate reports described a white solid with a pungent odor derived from the distillation of coal tar . In 1821, John Kidd cited these two disclosures and then described many of this substance's properties and the means of its production. He proposed the name naphthalene, as it had been derived from a kind of naphtha (a broad term encompassing any volatile, flammable liquid hydrocarbon mixture, including coal tar).
Recovery from coal tar
Process Description In industrial practice, distillation of coal tar yields an oil containing about 50% naphthalene, along with a variety of other aromatic compounds. This oil, after being washed with aqueous sodium hydroxide to remove acidic components (chiefly various phenols), and with sulfuric acid to remove basic components (in quencher) and later undergoes fractional distillation to isolate naphthalene. The tar, acid-free chemical oil, is charged to the system where most of the low boiling components, e.g., benzene, xylene, and toluene, are removed in the li ght-solvent column (i.e. column 1). The chemical oil next is fed to the solvent column, which is operated under vacuum, where a product containing the prenaphthalene components is taken overhead (i.e. from column 2 to column 3). (In column 3) This product, which is called coal-tar naphtha or crude heavy solvent, typically has a boiling range of 130-200 C and is used as a general solvent and as a feed stock for hydrocarbon-resin manufacture because of its high contents of resins, e.g., indene and coumarone. The naphthalene – rich bottoms from the solvent column then fed to the naphthalene column where a 78 C freeze – point product (78 C naphthalene) is produced. The naphthalene column is operated at near atmospheric pressure to avoid difficulties in distillation of this product . The crude 78 C naphthalene that is produced by distillation is not of suitable quality for many applications and must be further upgraded. The main impurity in crude 78 C coal-tar naphthalene is sulfur, which is present in the form of thio-naphthalene (1-3%). Methyl and dimethyl naphthalene, crude coal-tar naphthalene are also presents (1-2 wt. %) with lesser amount of indene, methylindenes, tar acids, entire bases. The liquid product is distilled to separate naphthalene, gasoline, and fuel oil (in distillation column). Some of the aromatic concentrate may be recycled. The naphthalene produced is usually better 99% pure and low in sulfur content.
FLOW CHART COAL-TAR METHOD
Figure 9: Manufacture of naphthalene using Coal Tar method
Advantages of Coal Tar Method
Environment friendly method, where unused petroleum factions are used to produce naphthalene instead of dumping them and causing degradation.
Hydrodealkylation process
Hydrodealkylation is a chemical reaction that often involves reacting an aromatic hydrocarbon, such as toluene, in the presence of hydrogen gas to form a simpler aromatic hydrocarbon devoid of functional groups. An example is the conversion of 1, 2, 4-trimethylbenzene to xylene. This chemical process usually occurs at high temperature, at high pressure, or in the presence of a catalyst. These are predominantly transition metals, such as chromium or molybdenum.
The reaction is a hydrocracking one where alkyl side-chain breaks and simultaneously hydrogenated.
Fluidized bed reactor: Is a type of reactor device that that can be used to carry out a variety of multiphase chemical reactions. In this type of reactor, a fluid (air/water) is passed through a granular solid material (usually a catalyst) at high enough velocities to suspend the solid and cause it to behave as it was a fluid. The solid substrate (the catalytic material upon which chemical species react) material in the fluidized bed reactor is typically supported by a porous plate, known as a distributor. The fluid is then forced through the distributor up through the solid material. At lower fluid velocities, the solids remain in place as the fluid passes through the voids in the material. This is known as a packed bed reactor. As the fluid velocity is increased, the reactor will reach a stage where the force of the fluid on the solids is enough to balance the weight of the solid material. Once this minimum velocity is surpassed, the contents of the reactor bed begin to expand and swirl around much like an agitated tank or boiling pot of water. The reactor is now a fluidized bed.
Figure 10: Fluidized bed reactor
PROCESS DESCRIPTION: The feed is LCO (light cycle oil). The hydrodealkylation process with two fluid-beds, one for hydrodealkylation reaction and the other for catalyst (preferably Molybdenum) regeneration. In the fluid-bed reactor, highly exothermic hydrodealkylation reaction can be conducted at a uniform temperature in the bed, and the catalyst is transferred to the regenerator to burn the coke deposited on the catalyst in the reactor. The fluid-bed reactor is operated at pressures between 1.0 and 1.6 MPa to maintain hydrogen partial pressure necessary for promoting the hydrodealkylation and preventing excess coke formation. Depending on the main product required, the process is operated either in naphthalene mode or in methylnaphthalene mode. Operational differences between the two modes are the reaction temperature, separation and recycling of the product oil. Catalyst used in the process should have characteristics as follows: high dealkylation activity, sufficient hydro-desulfurization and denitrogenation activity resistance against concentrations of sulfur and basic nitrogen in the feed, resistance against high amount of coke deposit on the catalyst, and thermal stability in the repeated reaction and regeneration. The catalyst is recycled by burning the deposited coke on the catalyst in the regenerator section and is recycled. The catalyst can be recycled up to 100-150 times.
FLOW CHART HYDEAL PROCESS
Figure 11: Manufacture of naphthalene using Hydeal process
Advantage of Hydeal Process
The hydrodealkylation reaction can be continued over long periods without detrimental coke deposit.
