Distillation: apparatus – Apparatus – Systems
Reexamination Certificate
2000-04-20
2002-04-23
Griffin, Walter D. (Department: 1764)
Distillation: apparatus
Apparatus
Systems
C202S168000, C202S169000, C202S153000, C203S043000, C196S014520, C208S311000, C208S313000, C208S314000, C208S322000, C208S329000, C585S804000
Reexamination Certificate
active
06375802
ABSTRACT:
FIELD OF INVENTION
The present invention relates to chemical separation processes, and, more specifically, to an improved process for separation of aromatic compounds from mixtures of aromatic and non-aromatic compounds and methods for retrofitting existing equipment for same.
BACKGROUND OF THE INVENTION
Aromatic petrochemicals, such as benzene, toluene and xylenes (collectively, “BTX”), serve as important building blocks for a variety of plastics, foams and fibers. Traditionally, these fundamental compounds have been produced via catalytic reformation of naphtha or through steam cracking of naphtha or gas oils, producing streams such as reformate and pyrolysis gasoline. BTX derived from such traditional methods typically include substantial amounts of non-aromatic compounds having similar boiling points, effectively precluding simple distillation as a means of separation of the aromatics from the non-aromatics.
Accordingly, a variety of extraction techniques have been developed in an effort to separate aromatic compounds from non-aromatic ones. Such prior art extraction techniques typically involve the use of solvents which exhibit a higher affinity for the aromatic compounds, selectively extracting the aromatic compounds from the mixture of aromatic and non-aromatic compounds. An example of such prior art extraction techniques is the sulfolane process developed by Shell Oil Company. The sulfolane process employs the use of tetrahydrothiophene 1,1 dioxide (or sulfolane) as a solvent and water as a co-solvent. The process uses a combination of liquid-liquid extraction and extractive stripping in a single, integrated design.
Despite its wide-spread use, the sulfolane process suffers from several disadvantages imposed by its design. For example, such process is restricted in its available production capacity. This is due to the fact that in order for liquid-liquid extraction to occur, a phase separation must take place between the solvent/extract and the raffinate material. The maximum aromatic content of the feedstock is restricted to approximately 80%-90%.
Additionally, in traditional sulfolane process designs, the range of feedstock choices is limited. This is due to the fact that existing sulfolane extraction units were constructed when feedstock was presumed to include total aromatic concentrations of from about 30%-60%. With improvements in new catalysts and the development of continuous catalytic regeneration (“CCR”), the aromatic content of reformate streams is significantly higher, exceeding the point where liquid-liquid phase separation, and hence simple extraction, can occur. One attempt to resolve this dilemma has been to artificially recycle non-aromatic or raffinate material in order to lower the total aromatic concentration and thus promote phase separation. Alternatively, a co-solvent composition can be increased in an effort to increase the solvent system selectivity. Both of these attempts to accommodate recent developments in catalysts and catalytic systems with prior art designs significantly decrease operating efficiency and unit capacity of the process.
Another drawback associated with the prior art sulfolane process is the concentration effect of undesired components present in the reflux stream. Extraction solvents have group selectivity favoring extraction of aromatics>naphthenes/olefms>paraffins and a light/heavy selectivity which favors lower carbon number components. Accordingly, the sulfolane process design was premised upon the theory that the extractive stripping operation would easily remove lighter non-aromatic compounds, which would flow as reflux to the main extractor and displace heavier aromatics.
In practice, the design produces at least two undesired effects: (1) difficulty in recovering the heavier aromatics into the extracted stream; and (2) buildup of light impurities in the extractive stripper and reflux system. The former undesired effect associated with such prior art designs is the incapacity of such designs to completely remove and recover the heaviest species of aromatic compounds within the mixed feedstock. For example, an operation using the prior art design and processing a BTX range feedstock may result in nearly complete benzene recovery while losing upwards of 15% or more of the xylenes within the feedstock into the raffinate due to the lower affinity of the solvent for xylenes compared with benzene. Such results require the employment of additional recovery schemes in an effort to more completely recover the xylenes present in the feedstock.
The latter undesired effect results in significant increases in the concentration of lower carbon number components (e.g., C5 and C6 naphthenes and olefins) within the reflux stream, which can lead to product contamination of the lowest carbon numbered aromatic compounds. Attempts to cope with this problem include increased efforts by the operator to strip such undesired components into the reflux stream and/or employing a drag stream from above the aromatic product fractionator to recycle to the extraction section. Both attempts result in increased energy consumption by and reduced capacity of the system.
Thus, there remains a need for a recovery process and method for retrofitting existing recovery process equipment to improve upon prior art aromatics recovery processes, and to avoid the disadvantages described above.
SUMMARY OF THE INVENTION
In accordance with the present invention there is provided an improved process for separation of aromatic compounds from mixtures of aromatic and non-aromatic compounds and a method for retrofitting existing equipment for employing said improved process. In one aspect, the improved process for separation of the present invention includes an extractive distillation operation as a primary separation step for the recovery of aromatic compounds. This embodiment of the invention is preferably used with feedstocks containing BTX fractions, but it is noted that it can also be used with feed fractions containing between 5 and 12 carbons.
It was discovered that the prior art sulfolane process and accompanying system suffered primarily in its design and implementation with respect to three main areas: (1) the main extractor; (2) the extractive stripper; and (3) the extract recovery operation. Although other incremental improvements were made to other aspects of the prior art process, the main improvements described herein are realized in these three primary areas.
In a first embodiment of the improved separation process of the present invention, a hybrid extraction/extractive distillation system is employed. A portion of the mixed hydrocarbon feedstock is routed to a new, separate extractive distillation column (“EDC”) which operates in parallel with the main extractor, extractive stripper and water-wash operations of the process. The use of an EDC allows recovery and purification of aromatic compounds to occur in a single operation. The optional use of a co-solvent further improves the recovery capability of this embodiment of the improved aromatics recovery process of the present invention.
In a second embodiment of the improved aromatic recovery process of the present invention, the hydrocarbon feedstock originates from a heartcut fractionation column (“HFC”), such as a reformate splitter column. Additional advantages of the process are realized by segregating the feedstock fractions to the extraction and extractive distillation operations. Use of a co-solvent may be practiced with this embodiment of the improved aromatics separation process of the present invention to further improve recovery of aromatic compounds from the feedstock.
In a variation of the third embodiment described above, a side cut of the feedstock including a heavier fraction is taken from the prefractionator column and processed in the EDC. The overhead portion is fed to the traditional liquid-liquid extraction portion of the system. The chief advantage associated with this variation of the third embodiment is the more complete recovery of heavier aromatic compounds, avoiding
Gentry Joseph C.
Lee Fu-Ming
Griffin Walter D.
HFM International, Inc.
Jenkins & Gilchrist A Professional Corporation
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