Hydrogen fluoride recovery process

Details Hydrogen fluoride recovery process Hydrogen fluoride recovery process

1998-04-08

2001-08-07

Nguyen, Ngoc-Yen (Department: 1754)

Chemistry of inorganic compounds

Halogen or compound thereof

Hydrogen halide

C423S483000, C423S488000, C210S753000

Reexamination Certificate

active

06270742

ABSTRACT:

This invention relates to a hydrogen fluoride recovery process and particularly to a process for separating hydrogen fluoride from organic compounds and recovering the separated hydrogen fluoride. The invention is particularly useful for recovering hydrogen fluoride from mixtures containing minor proportions, for instance less than 25% by weight, of hydrogen fluoride and for recovering hydrogen fluoride from mixtures in which the organic compound(s) and hydrogen fluoride form an azeotropic or near-azeotropic composition. A particular embodiment of the process resides in separating hydrogen fluoride from halogen-containing organic compounds, notably fluorine-containing organic compounds, and recovering the hydrogen fluoride.
Fluorine-containing organic compounds such as hydrofluorocarbons (HFCs), hydrochloro-fluorocarbons (HCFCs) and chlorofluorocarbons (CFCs) are often produced by reacting a halocarbon starting material containing one or more atoms other than fluorine, especially chlorine atoms, with hydrogen fluoride in the liquid phase or the gaseous phase in the presence of a fluorination catalyst. The product from such reactions comprises the desired fluorine-containing organic compound, organic by-products, hydrogen chloride and unreacted hydrogen fluoride and other starting materials and it is desirable to separate these materials and recover as much as possible of the hydrogen fluoride for re-use. A proportion of the hydrogen fluoride usually can be separated and recovered by distillation but the resulting distillate usually contains residual hydrogen fluoride especially in cases where the organic compound(s) and hydrogen fluoride form an azeotrope. This residual hydrogen fluoride is usually removed from the organic compound(s) by scrubbing the product stream with water or preferably aqueous alkali and the aqueous scrubbing liquor is then disposed of after appropriate waste water treatment. Whilst aqueous scrubbing is an effective way of removing hydrogen fluoride from the organic compound(s), it tends to be expensive in terms of hydrogen fluoride loss from the process and it is desirable to separate as much as possible and preferably essentially all of the hydrogen fluoride from the product stream before aqueous scrubbing.
According to the present invention there is provided, in a first aspect, a process for separating and recovering hydrogen fluoride from a mixture of an organic compound and hydrogen fluoride which comprises contacting the mixture in the liquid phase with a solution of an alkali metal fluoride in hydrogen fluoride, effecting phase separation of an upper organic compound phase and a lower hydrogen fluoride phase and recovering hydrogen fluoride from the lower hydrogen fluoride phase.
Whilst any alkali metal fluoride may be used, ewe prefer potassium fluoride or caesium fluoride, especially caesium fluoride. If desired, mixtures of two or more alkali metal fluorides may be employed.
The solution of alkali metal fluoride in hydrogen fluoride may be essentially anhydrous if desired. As an alternative, a solution of an alkali metal fluoride in aqueous hydrogen fluoride may be employed.
The mixture of organic compound(s) and hydrogen fluoride being treated may be essentially anhydrous since anhydrous hydrogen fluoride is essentially non-corrosive. Where the solution of alkali metal fluoride in hydrogen fluoride is essentially anhydrous, the mixture of organic compound(s) and hydrogen fluoride is preferably essentially anhydrous. Whilst water may be present in the process, the advantage of non-corrosivity associated with anhydrous hydrogen fluoride is reduced by the presence of water. An aqueous solution however provides the advantage that, for a given concentration of alkali metal in the solution, a higher level of recovery of hydrogen fluoride from the mixture of organic compound and hydrogen fluoride may be secured. Accordingly, an anhydrous or an aqueous solution will be selected according to the particular advantage which is desired in a given process.
In order to improve the yield of hydrogen fluoride recovered from the said mixture, the upper organic compound phase may subsequently be contacted again with a solution of an alkali metal fluoride in hydrogen fluoride from which a further upper organic compound phase and a further lower hydrogen fluoride phase may be recovered. This procedure may be repeated as often as desired. The first and, if present, subsesequent separation step(s) may suitably be carried out in one or more mixer/settler units or a liquid/liquid extraction column as desired.
The upper, organic phase can be separated and treated in an appropriate manner such as distillation, for example in a distillation column to recover the organic compound and/or to obtain a recycle stream for feeding to the reactor in which the desired organic compound is produced. This phase, whilst depleted in hydrogen fluoride, will usually contain some residual hydrogen fluoride and during distillation to recover the desired organic compound will provide one or more streams containing hydrogen fluoride which may be recycled. The recycle stream(s) may be fed to the reactor in which the organic compound is produced or to a process stream containing the organic compound for example the mixture of an organic compound and hydrogen fluoride as described in the first aspect of the invention. Any hydrogen fluoride which remains in the recovered organic compound can be recovered for example by distillation or removed by aqueous scrubbing.
The lower, hydrogen fluoride phase will usually be distilled to recover essentially anhydrous hydrogen fluoride therefrom. The distillation may be carried in any conventional distillation apparatus for example a distillation column, but is preferably carried out in a flash vessel, for example a single-stage flash vessel which is suitably equipped with a reboiler and condenser.
The hydrogen fluoride recovered from the lower phase can be collected for use in another reaction or recycled to the reactor in which the organic compound is produced. This lower phase, enriched in hydrogen fluoride extracted from the product stream being treated will usually also contain some organic compound(s) extracted from the product stream being treated. Distillation of the phase removes any such organic compound(s) together with hydrogen fluoride; this mixture may then be subjected to further separation to remove at least a part of the hydrogen fluoride from the organic compound. The organic compound(s) may then be recycled as described above to the reactor in which the desired organic compound is produced or to a process stream containing the organic compound.
The solution of alkali metal fluoride in hydrogen fluoride obtained as residue on distilling the lower phase to recover hydrogen fluoride can be recycled to the process and re-used to extract hydrogen fluoride from the mixture being treated.
The density of a solution of alkali metal fluoride in hydrogen fluoride increases with increasing concentration of alkali metal fluoride so that the extractant solution is denser than the organic compound(s) and separates as the lower phase during phase separation. The density of the extractant solution is preferably appreciably greater than that of the organic compound(s) so that as its density falls by dilution with hydrogen fluoride extracted from the mixture being treated it does not approach closely the density of the organic compound(s) and there remains a sufficient difference in density to enable efficient phase separation to be effected. The amount of alkali metal fluoride in the extractant solution may vary within wide limits, depending upon the particular organic compound(s) in the mixture being treated and hence the required density of the extractant solution and the solubility of the alkali metal fluoride in the hydrogen fluoride. As a guide, the concentration of alkali metal fluoride, for example caesium fluoride, will typically be from about 20% to about 80% by weight and preferably not more than about 70%. Where potassium fluoride is employed

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