Chemistry of hydrocarbon compounds – Purification – separation – or recovery – By cooling of liquid to obtain solid – e.g. – crystallization,...
Reexamination Certificate
2000-12-01
2002-04-23
Griffin, Walter D. (Department: 1764)
Chemistry of hydrocarbon compounds
Purification, separation, or recovery
By cooling of liquid to obtain solid, e.g., crystallization,...
C585S805000, C585S812000, C585S813000
Reexamination Certificate
active
06376736
ABSTRACT:
The present invention relates generally to an improved process for the production of high purity meta-xylene as part of an integrated xylene separation and recovery system utilizing an improved arrangement of fractionation, adsorption and crystallization steps.
BACKGROUND OF THE INVENTION
Commonly, para-xylene is commercially produced from C-8+ aromatic streams, usually the C-8+ fraction of reformate from catalytic reformers. The C-8+ aromatics are typically fed to a distillation column, commonly referred to as a xylene splitter, where the ethylbenzene, para-xylene, meta-xylene, and some or virtually all of the ortho-xylene are taken overhead. Ortho-xylene is normally left in the xylene splitter bottoms stream in a significant amount only if a purified ortho-xylene product is to be produced. If it is, then the xylene splitter bottoms stream is typically fed to an ortho-xylene product column where the ortho-xylene is distilled from the C-9+ aromatics.
The xylene splitter overhead stream is then fed to a para-xylene purification (PXP) unit for the recovery and purification of para-xylene product. This PXP unit separates out the para-xylene, typically either by crystallization or by adsorption onto a molecular sieve, utilizing conventional technologies. The para-xylene-depleted stream withdrawn from the PXP unit is then fed normally to an isomerization unit where the meta-xylene and ortho-xylene are partially converted catalytically to para-xylene. Depending on the type of catalyst, the ethylbenzene in this stream is partially converted in the isomerization unit primarily to either benzene or xylenes. After distilling off toluene and lighter compounds, the C-8+ aromatics from the isomerization unit are fed back to the xylene splitter.
If it is desired that meta-xylene be separated out as a purified product in this type of plant, U.S. Pat. No. 3,773,846 (Berger '846), which is incorporated herein by reference, and other patents (described below) have proposed that meta-xylene purification be done between the PXP unit and the isomerization unit. As with para-xylene, meta-xylene can be purified by either crystallization or adsorption. If crystallization is used, however, the purification of the meta-xylene can be hindered by the para/meta and ortho/meta eutectics. Thus, U.S. Pat. Nos. 3,798,282 (Bemis et al.) and U.S. Pat. No. 3,825,614 (Bemis et al.), which are incorporated herein by reference, teach methods for purifying meta-xylene downstream of a para-xylene crystallization unit by crystallization at temperatures below the para/meta eutectic. These patents teach crystallization techniques whereby the para-xylene crystals will be smaller than the meta-xylene crystals, and this allows for some degree of rough separation. After this first separation, the concentrated meta-xylene can be melted and recrystallized in a second stage to produce high-purity meta-xylene. Using adsorption to purify the para-xylene, as taught Berger '846 is probably a more practical commercial approach when meta-xylene is to also subsequently be purified by crystallization because adsorption can reduce the para-xylene concentration in the meta-xylene crystallization unit feed to well below the para/meta eutectic point and, thus, avoid this eutectic problem.
Berger '846 does not, however, efficiently address the issue of avoiding the meta/ortho eutectic which can form during meta-xylene purification by crystallization. In Berger '846, the ortho-xylene removal is accomplished by fractionation, in particular in a “third fractionation zone”, located between the adsorption step and the meta-xylene purification step. Also in Berger '846, a “second fractionation zone” is recommended to be located just upstream of the third fractionation zone. This second fractionation zone is used to concentrate the meta-xylene prior to crystallization by distilling away much of the ethylbenzene. This approach is expensive in terms of both capital and operating costs.
A variety of other approaches to the purification of meta-xylene from mixtures of C-8 aromatic hydrocarbons are known in the art. For example, one familiar approach is a liquid/liquid extraction process as taught by U.S. Pat. Nos. 2,528,892; 2,738,372; 2,848,517; 2,848,518; 3,309,414; 3,515,768; and 3,584,068. The liquid/liquid extraction process of these patents is based on the knowledge that boron trifluoride (BF
3
) and hydrofluoric acid (HF) will form a complex with meta-xylene that is more stable than the corresponding complexes with para- or ortho-xylenes. When mixed xylenes are mixed with limited amounts of BF
3
and large amounts of HF, the xylene/BF
3
/HF complexes form a heavy, acid, liquid phase that separates from a lighter, hydrocarbon, liquid phase. The acid phase has a high percentage of meta-xylene with lesser amounts of para- and ortho-xylenes. Non-aromatic hydrocarbons do not form complexes and thus stay in the hydrocarbon phase. Depending on conditions, ethylbenzene either stays with the light phase or disproportionates to benzene and diethylbenzene, which can also complex with BF
3
/HF. Through the use of counter-current contacting, the heavy complex can be stripped of nearly all of the non-meta-xylene hydrocarbons and most of the meta-xylene can be extracted from the light hydrocarbon phase. The complex can then be broken, and the meta-xylene separated out. The ortho-xylene and ethylbenzene can be separated from the para-xylene and residual meta-xylene by distillation.
One of the principal disadvantages of this type of liquid/liquid extraction process is that boron trifluoride and hydrofluoric acid are both extremely corrosive and dangerous chemicals. The environmental and safety risks associated with this type of process are so high as to be unacceptable to many companies and countries today. Also, this type of plant is expensive because of the many pieces of equipment and because of the cost of metals needed to withstand the corrosive nature of the chemicals.
Other known exaction-type purification processes use different chemicals but suffer from similar drawbacks. Thus, U.S. Pat. No. 2,830,105 teaches an extraction with phosphorus pentafluoride and hydrofluoric acid; U.S. Pat. No. 3,707,577 uses lithium chloride and aluminum chloride as part of a somewhat more complex extraction process; and, U.S. Pat. No. 2,562,068 employs a double solvent extraction using sulfur dioxide and pentane.
It is also known to purify meta-xylene by selective reaction followed by some type of separation. In U.S. Pat. Nos. 2,889,382 and 3,644,552, meta-xylene is selectively halogenated in the presence of para-xylene. Subsequent processing by distillation, crystallization and/or adsorption can be used to separate the halogenated meta-xylene from other C-8 aromatic hydrocarbons. The halogenation process, however, is not entirely selective for meta-xylene, and the patented processes employ expensive catalysts and/or highly corrosive materials such as molecular chlorine, hydrochloric and nitric acids. U.S. Pat. No. 2,511,711 teaches selectively sulfonating meta-xylene. As with selective halogenation, this process is not entirely selective for meta-xylene and utilizes highly corrosive chemicals. Still a third variety of selective reaction is selective alkylation of meta-xylene with propylene as taught by U.S. Pat. No. 3,539,650. This process also has many disadvantages. First, the alkylation conversion is very low which results in an expensive alkylation unit. Second, there are reaction yield losses not just once but twice. Third, propylene is needed. Fourth, cumene is made, and usually making additional products is not desirable. Fifth, the processing is very complicated and expensive.
Yet another conventional approach to purification of meta-xylene is an extractive distillation process. U.S. Pat. No. 2,763,604 teaches the use of benzonitrile and similar compounds and mixtures as an extractive distillation solvent. Benzonitrile forms loose complexes with aromatics with the strongest effect being with meta-xylene. The addi
Hubbell Douglas S.
Law James S.
Rutten Philippe W. M.
Andover-IP-Law
Griffin Walter D.
Nguyen Tam M.
Silverstein David
Washington Group International, Inc.
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