O-xylene is an isomer of xylene and it denotes Ortho-xylene which is also referred to as O-Xylol, 1,2-xylene or O-methyltoluene. It has the chemical formula of C6H4(CH3)2 and a relative formula mass of 106.168g/mol. The chemical has a colorless liquid or watery appearance and is characterized by a sweet aromatic odor. It is further characterized by being having a lower density than water and also being insoluble in water but highly soluble in organic solvents such as acetone and carbon tetrachloride. It forms an irritating vapor if subjected to heat where it has a boiling point of 144.5 degrees Celsius and a melting point of -25.16 degrees Celsius (4). O-xylene is a major demand and importance within in the world today it has many applications. For instance, it is used to produce Phthalic Anhydride The chemical is produced as a major petrochemical complex. There are a number of processes used to produce o-xylene such as the ISOMAR process which involves the distillation of o-xylene from the other xylenes and then crystallizing the p-xylene. The resulting mixture is the passed through an isomerization reactor under a high pressure of hydrogen with the catalyst, and high temperature so that the m-xylene and the ethylbenzene components are converted to p-xylene and o-xylene. The o-xylene and the mixture is then fractionated in a series of pre-fractionators that will result to over 90% o-xylene. This essay will majorly dwell on the description of the ISOMAR process, the equilibrium mechanisms involved in the process and also production.
ISOMAR process is patented by the Universal Oil Products (UOP) Provision Division. There are other variants of this process patented by the Maruzen Oil Co., Mitsubishi Gas Chemical Co., AFRO technology Inc., Atlantic Richfield Co., in which the trans-alkylation of xylenes is carried out under more stringent conditions in which an acid is used for catalysis, and many others. As already mentioned, the ISOMAR process involves the distillation of o-xylene from the other xylenes followed by the crystallization of its counter isomer, para-xylene. The resulting liquid mixture is the passed through an isomerization reactor in which there are extreme conditions of high pressure (Hydrogen gas) and high temperature. There also is a catalyst to speed up the isomerization process. At the isomerization reactor, the ethylbenzene and the m-xylene constituents are thereby converted to p and o-xylene at these conditions.
At this stage, a feed is added which may constitute any C8 aromatic mixture such as any contents of pyrolysis gasoline or from the catalytic reformates. The catalytic reformates are preferred to the pyrolysis gasoline since they are composed of lower ethyl benzene contents. Comparatively, pyrolysis gasoline has over 40% of ethylbenzene which is not favorable for the process. Also, it is good to note that the feedstocks can be fractional heart-cuts or pure solvent extracts which have over 25% saturates. The feed also requires a continuous hydrogen supply which can be acquired from any suitable source such as the reforming of the catalyst. The added feed is passed through the through the first pre-fractionator in which the C8 - C9 are deposited at the bottom and the remaining feed passed through to the next pre-fractionator. In the latter pre-fractionator, the more C9+ compounds are decanted i.e. deposited at the bottom. After that, the boiling point difference between the o-xylene and the other isomers is used so as to separate the mixture and result in a purer mixture of o-xylene and C9. In this case, the C9 impurities are majorly tri-methylbenzene which are now send in a fractionator for further separation. In this fractionator, the C9 compounds are deposited as the residues and the o-xylene is yielded as a pure o-xylene distillate. The resulting distillate also composes of other xylene isomers and must be further processed so as to extract only the o and p-xylene which majorly involves crystallization.
The crystallization process can be attained through either: the GNBH, the ARCOs or the Krupp-Koppers procedures. For example, the Krupp-Koppers procedure is composed of two major stages of crystallization which yield up to 99% of pure p-xylene and thus highly pure o-xylene. The first stage involves the crystallization of p-xylene over a platinum metal catalyst at high hydrogen gas pressure. The method is referred to as the Engelhard-Atlantic Octafining. Notably, the system is engineered such that it is maintained at 425 degrees Celsius since the p-xylene and o-xylene isomers are at their highest concentrations under this temperature. It is important to mention that the catalyst in the second stage is set by mixing of platinized alumina and silica-alumina cracking catalysts in equal proportions. It is worthwhile mentioning that the purity of the product, o or p- xylene in the ISOMAR process is majorly dependent on the separation process involved. In this case, even when there is the use of high concentrations of saturates as the feedstocks, the process can still yield up to 99% pure o-xylene as long as the ISOMAR is coupled with efficient fractionation. This is in comparison to the processing of p-xylene which would require the coupling of the ISOMAA process with the UOP Parex process unit or with the conventional crystallization for 99% purity yield. The major impurities in the final o-xylene yield are higher aromatics and higher C9 aromatics. The two are also by-products of the process and can be recycled back as feedstocks, can be used as blending components for gasoline and can also be used as organic solvents.
In summary, the typical ISOMAR process involves the feeding of the C8 aromatic reactor in which there if the deficiency in one or more components of the feedstock relative to the equilibrium composition. The processing then occurs over a fixed catalyst bed with a continuous supply of hydrogen. Subsequently, the liquid portion of the effluent becomes fractionated such that the heavy and the light aromatic by-products in addition to the cracked materials originating from the saturates within the feeds are removed. The desirable products o-xylene or para-xylene are then separated from the yielded fractionating hear-cut. The produced o-xylene can be recycled back as a whole or a portion of it into the ISOMAR reactor to which fresh feedstock is introduced into the circuit for the production of more of the desired process. It is necessary to keep the pressure and temperature conditions as moderate as possible in relation to the stability of the conventional equipment and also to the carbon steel used.
The major advantage of this process is that it is possible to regenerate the catalyst at least once through the combustion of the accumulated carbon over the period of its use. It is also important to mention that the ISOMAR process ensures that the production of the xylenes occurs at the equilibrium position of the major C8 aromatic mixture. In this case, the isomerism process is conducted in the recently mentioned temperature and pressure conditions and in the presence of catalysts such that the isomerism occurs at equilibrium (3). Keeping the system at equilibrium ensures that the desired isomer (o-xylene) is in large concentration which makes the separation of the yielded product easier. Moreover, to ensure that the process is cost-effective and highly efficient, some of the produced product is recycled back into the system so that it acts as a seed in the production and consequently resulting in optimal yield. Also, the hydrogen consumption is minimal in the processing and thus, the available hydrogen is re-channeled back into the process which makes it more sustainable.
The large scale aromatic complex used to process xylenes, such as the o-xylene, is engineered to comprise of the following major sections. Firstly, the complex is made such that it can attain low pressure (3.5kg/cm2g) so that there is continuous catalyst regeneration reformer. This reformer processes a broad boiling range feedstock so that more of the desired product (such as the o-xylene) is yielded (4). Secondly, there is a section specifically contained for the extraction of toluene and benzene through the sulpholane extraction method. Thirdly, there is a section in which the trans-alkylation and disproportionation of C9 aromatics and toluene to xylenes. Fourthly, there exists an absorption section in which the para-xylene produced can be recovered. Finally, there is the isomerization section in which the o-xylene can be processed by the manipulation of the equilibrium position.
Lately, the complex has been evolving whereby there have been commercialized mono- and bi-metallic and the reforming catalysts which are suited for semi-regenerative processes. Also, there has been the development and commercialization of the zeolite-based xylene to act as the isomerization catalyst. These novel catalysts have helped improve the catalysis processes during the production of the xylene isomers.
Further, there has been lots of improvements on the heat recovery and integration sections over the last two decades. The actual heat requirement in the modern complex can be as low as 110% of the theoretical minimum which means that the heat integration has been greatly enhanced and also that it is possible to get separations of xylenes and other aromatics over a very wide range of heat differences. For instance, there have been improvements on the amount energy required for the production of aromatics. In this case, the complex back in the 70s operated on double the energy required by the modern complex. Notably, the previous complex spent more than 60% of the energy in fractionation but there have been great improvements in the use of chromatographic techniques which just requires the right feed into the system for the separations to occur (1). Whats more, here have been great improvements in the computer algorithms which enable rigorous calculations of the temperatures and other parameters necessary for efficient production. In other words, the algorithms have enabled the processors to make easier and come up with more accurate equations of state which allow easier fractionation or chromatographic separations. This also marks the ability of the recent use of sophisticated and more effective computer program software to enhance and control the processes with optimal automation and minimal errors.
There are issues surrounding the current large-scale processing of xylene which are majorly due to the fact that the continuous catalyst regeneration is still a major sector that has yet to be efficiently improved. Further, the processing is quite costly since the yield varies a lot with a minimal fluctuation of the processing conditions (2). Also if not well checked and regulated, the processing can become very energy consuming which can make processing too cost-ineffective.
Notably, the processing of xylene and its isomers such as o-xylene is associated with lots of pollution resulting from various emissions at the processing point, the storage and also resulting from equipment leaks. There are plenty of emissions of this substance from the process vents which contribute to a lot of air pollution. The emissions amount and nature of emissions are reliable on the configuration of the processing set up, the mix of the manufactured products and also the nature of the crude oil feedstocks. Additionally, the disposal of waste is a common source of emission and pollution from the xylene products. For this reason, there arises a dire need to control the emissions and of all, control and treat the waste resulting from the processing (2).
The major form of regulation of emissions is the venting of the emissions into fuel gasses and most importantly, the recycling of the gasses so as to minimize the waste content...
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