Chemistry of hydrocarbon compounds – Purification – separation – or recovery – By cooling of liquid to obtain solid – e.g. – crystallization,...
Patent
1997-07-21
2000-08-29
Knode, Marian C.
Chemistry of hydrocarbon compounds
Purification, separation, or recovery
By cooling of liquid to obtain solid, e.g., crystallization,...
585813, 585814, 585815, 585479, C07C 714, C07C 522
Patent
active
061111619
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to a new economic process for recovering and purifying paraxylene from a charge of hydrocarbons which contain paraxylene at a concentration that is greater than that at thermodynamic equilibrium. It relates, in particular, to the processes that enrich their effluent with paraxylene to more than 30%, particularly the processes that provide at least one paraxylene-enrichment step by crystallization at very low temperature, for example, this crystallization step being followed by a step of purifying the paraxylene in at least one stage (U.S. Pat. No. 3,177,265 and U.S. Pat. No. 2,866,833).
The invention relates in particular to a process for preparing and purifying paraxylene from a charge of aromatic hydrocarbons having 8 carbon atoms comprising a combination of steps for selective adsorption, for purification by high-temperature crystallization and for isomerization as described in the patents of the applicant FR 2,681,066 and U.S. Pat. No. 5,284,992 that are incorporated by reference.
It applies in particular to the preparation of paraxylene with very high purity for the production of terephthalic acid for the synthesis of synthetic fabrics.
Crystallization has been used commercially for a very long time to isolate and purify paraxylene, typically from a mixture of xylenes and ethylbenzene close to equilibrium (C.sub.8 -aromatic fraction).
The C.sub.8 -aromatic fractions ordinarily come from a reforming unit or from an ethylene production unit and especially from the reformates by distillation alone or extraction in combination with a distillation as a function of the composition of the charge, of the sensitivity to impurities of the downstream technologies and of the savings of the recovery processes.
The typical composition by weight of a C.sub.8 -aromatic fraction is approximately 22% paraxylene, 16% ethylbenzene, 18% orthoxylene and 44% metaxylene. Very low temperatures are generally required to effectively recover, by crystallization, the paraxylene from a C.sub.8 fraction. Furthermore, there is a eutectic limit that prevents the complete recovery of all the paraxylene from a C.sub.8 fraction. For example, in a low-temperature crystallization unit, for a C.sub.8 -aromatic fraction containing 22% weight of paraxylene, only about 50 to 65% of the paraxylene is recovered, the remaining paraxylene will be found in the paraxylene-depleted mother liquor, which can be introduced into an isomerization unit. The latter will isomerize the metaxylene, orthoxylene and in certain processes, the ethylbenzene, and a mixture of xylenes close to equilibrium containing about 22% paraxylene will again be obtained. This recycled flow, in combination with the fresh charge, is then introduced into the crystallizer so as to recover more paraxylene. In this way, the C.sub.8 -aromatics can be recycled to extinction and recovery of a maximum amount of paraxylene, with by-products resulting from the isomerization. The production of these by-products is a significant insufficiency of the system since each time that xylenes are introduced into the isomerization unit, a part of them is converted into non-xylenes such as toluene. Actually, the chemistry of the isomerization is very complex and the main reactions which lead to losses of xylenes are the disproportionation (dismutation) of the xylenes into toluene and trimethylbenzenes, dealkylation of the xylenes, and in certain cases even the formation of non-aromatics. To do this, the total yields of a set of isomerization and crystallization units are typically 60 to 80% and a large isomerization recycling loop is necessary to maximize the recovery of the paraxylene.
In the 1970's, another process was marketed to prevent the eutectic limitation from a low-temperature crystallization. This process uses an adsorption to separate the paraxylene from a mixture of xylenes. The adsorption makes it possible to recover more paraxylene from a C.sub.8 -aromatic fraction. Thus, from a charge containing 22% weight of paraxylene, it is possibl
REFERENCES:
patent: 5284992 (1994-02-01), Hotier et al.
patent: 5329060 (1994-07-01), Swift
patent: 5329061 (1994-07-01), Swift
patent: 5401476 (1995-03-01), Hotier et al.
patent: 5659098 (1997-08-01), Beck et al.
patent: 5866740 (1999-02-01), Mikitenko et al.
MacPherson Stuart R
Mikitenko Paul
Dang Thuan D.
Institute Francais du Petrole
Knode Marian C.
LandOfFree
Process for production of paraxylene comprising a high-temperatu does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Process for production of paraxylene comprising a high-temperatu, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Process for production of paraxylene comprising a high-temperatu will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-1251773