Organic compounds -- part of the class 532-570 series – Organic compounds – Heterocyclic carbon compounds containing a hetero ring...
Reexamination Certificate
2002-02-19
2003-05-27
Solola, Taofiq (Department: 1626)
Organic compounds -- part of the class 532-570 series
Organic compounds
Heterocyclic carbon compounds containing a hetero ring...
C203S041000, C203S086000
Reexamination Certificate
active
06570026
ABSTRACT:
This application is a 371 of PCT/EP00/07759 filed Aug. 10, 2000.
The present invention relates to a process for the preparation of on-spec phthalic anhydride by distillative purification of crude phthalic anhydride.
Phthalic anhydride is a key chemical of the chemical industry. It is used in considerable quantities as a starting material for dialkyl phthalates, which are employed in large amounts as a plasticizer for plastics such as PVC. Crude phthalic anhydride is produced industrially from naphthalene and/or o-xylene by catalytic oxidation in the gas phase. Preferably a crude phthalic anhydride prepared in this manner from o-xylene is used. The discharges of these processes usually contain more than 99.5% by weight, based on their total weight, of phthalic anhydride. The phthalic anhydride is generally isolated in liquid form or as a solid on separators and then usually purified by distillation. For this purpose, it is fed to the distillation column in liquid form or after vaporization.
Depending on the chosen preparation process and in particular on the starting materials and the catalysts, the product in each case contains a specific range of impurities and byproducts (cf. for example H. Suter: “Wissenschaftliche Forschungsberichte: II. Anwendungstechnik und angewandte Wissenschaft”, Dr. Dietrich Steinkopff Verlag, Darmstadt, 1972, page 39 etc.; abbreviated below to “Suter”).
On the market, a phthalic anhydride quality having the following specification limits is expected:
Solidification point (° C.) min. 130.8
Mass fractions (% by weight):
phthalic anhydride min. 99.8
maleic anhydride max. 0.05
benzoic acid max. 0.10 or max. 0.01 in the case of food quality
phthalic acid max. 0.1
Melt color number (Hazen) max. 20
Heat color number (Hazen) max. 40
Because a phthalic anhydride without discoloring impurities is required for most intended uses, characterization by the color numbers—primarily the melt color number and the heat color number—is particularly important. Color changes of the phthalic anhydride under thermal load are of practical importance because phthalic anhydride is usually stored and transported in the molten state—at about 160° C.
In industry, over the long period in which phthalic anhydride has been produced on an industrial scale, the removal of these byproducts by means of distillation has become established (cf. for example: “Ullmann's Encyclopedia of Industrial Chemistry”, 5th. Edition, Vol. A20, VCH Verlagsgesellschaft mbH, Weinheim, 1992, pages 181 to 189; abbreviated below to “Ullmann”; Kirk-Othmer “Encyclopedia of Chemical Technology”, 4th. Edition, Vol. 18, John Wiley & Sons, New York, 1996, pages 997 to 1006, abbreviated below to “Kirk-Othmer”). Low-boiling and/or azeotropic impurities, some having an intense natural color, present a person skilled in the art with considerable problems, in spite of comparatively small amounts. In practice, the procedure therefore adopted in industry is to subject the crude phthalic anhydride to combined purification comprising thermal pretreatment and distillation.
The thermal pretreatment is carried out at 220-280° C. and with a residence time in the reactor of from several hours to a day. The other boundary conditions of the thermal pretreatment depend in general on the origin and hence composition of the crude phthalic anhydride. It serves various purposes familiar to a person skilled in the art; for example, the byproduct phthalic acid is evidently dehydrated to give the desired product phthalic anhydride, which, even at low contents of phthalic acid, is of considerable economic importance in view of the large industrially produced amounts of phthalic anhydride. The water formed or other water is removed in the thermal pretreatment because it may interfere with the subsequent distillation. Furthermore, certain byproducts of the synthesis reaction are converted into resins by the thermal pretreatment, which facilitates the subsequent distillative purification of the phthalic anhydride (cf. for example “Suter”, pages 41-45). In the thermal pretreatment, it is also possible to add certain chemical substances in order to change the range of byproducts selectively before the distillation step.
The distillation—especially when it is effected by continuous method frequently of particular interest from economic points of view—is usually carried out by means of two columns in order to obtain a sufficiently pure phthalic anhydride. In the first step, as a rule the low boilers (for example benzoic acid, maleic anhydride and citraconic anhydride) i.e. substances having a boiling point below the boiling point of the phthalic anhydride, are separated off; in a second step, phthalic anhydride is then distilled off from the high boilers (for example, phthalic acid, certain color-imparting components, condensates of ingredients of the crude phthalic anhydride), i.e. substances having higher boiling points than the boiling point of phthalic anhydride or of undistillable components.
In “Suter” (loc. cit., page 45) reference is already made to a one-stage continuous distillation of phthalic anhydride (Ruhröl, Europa-Chemie 21, 7 (1965)), no further details being mentioned.
In summary, it may be said that the purification of crude phthalic anhydride is a very expensive process with respect to the plant and operating costs, especially if it is a question of obtaining the product quality required for many practical applications. The thermal pretreatment step has nevertheless been retained to date in industry in spite of the associated higher costs.
It is an object of the present invention to provide a technically simple and hence economical process by means of which crude phthalic anhydride can be purified so that the specifications demanded on the market are achieved.
We have found that this object is achieved by a process for the preparation of on-spec phthalic anhydride by distillative purification of crude phthalic anhydride, in which crude phthalic anhydride is fed to a distillation column which is operated at reduced pressure, the low boilers are removed at the top or in the vicinity of the top of the distillation column and the on-spec phthalic anhydride is removed from the column via a side take-off.
By means of the novel process it is possible, without the thermal pretreatment which has become usual in industry, to obtain a phthalic anhydride of high purity which fulfils the generally known specifications of pure phthalic anhydride or even surpasses them in particular in the color numbers: as a rule, a phthalic anhydride having a melt color number of less than 10 APHA and a heat color number of less than 20 APHA is obtained.
Suitable distillation columns (also abbreviated below to “columns”) in the context of the present invention are tray columns, columns containing dumped packings and columns containing stacked packings as well as columns in which the technical features of these column types have been combined. Tray columns are preferably used. Conventional internals, such as commercial trays, packings, for example bubble trays, tunnel trays, valve trays, sieve trays, dual-flow trays and lattice trays, Pall-Ringe®, Berl® saddles, wire mesh rings, Raschig-Ringe®, Intalox® saddles Interpak® packings and Intos®, as well as stacked packings, for example Sulzer-Mellapak®, Sulzer-Optiflow®, Kühni-Rombopak® and Montz-Pak®,and fabric packings, can be used in said column types. In the region below the column feed, internals which are also suitable for solids are preferably chosen, particularly dual-flow trays. Trays and packings of the abovementioned designs are generally suitable for this purpose.
The column is generally equipped with a bottom evaporator and also with a condenser at the top of the column.
The diameter of the column depends on the throughputs strived for in each case and can be readily determined by a person skilled in the art according to the conventional rules of industry.
The height of the column and the positions of feed and side take-off can be determined using the concept of the number of theoretical pl
Bessling Bernd
Knab Jean Werner
Kummer Matthias
Lorz Peter Michael
Peschel Werner
BASF - Aktiengesellschaft
Keil & Weinkauf
Solola Taofiq
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