Hydrogen fluoride recovery process

Details Hydrogen fluoride recovery process Hydrogen fluoride recovery process

1999-10-07

2003-04-29

Keys, Rosalynd (Department: 1621)

Chemistry of inorganic compounds

Halogen or compound thereof

Hydrogen halide

C423S483000, C423S488000, C423S24000R, C568S682000, C568S683000, C570S134000, C570S164000, C570S165000, C570S166000, C570S168000, C570S169000, C570S170000, C570S177000, C570S178000, C570S179000, C570S180000

Reexamination Certificate

active

06555086

ABSTRACT:

This invention relates to a hydrogen fluoride recovery process and particularly to a process for separating hydrogen fluoride from gaseous 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 gaseous mixture of an organic compound and hydrogen fluoride which comprises contacting the gaseous mixture with a solution of an alkali metal fluoride in hydrogen fluoride, separating a gas phase depleted in hydrogen fluoride and containing the organic compound from a liquid phase enriched in hydrogen fluoride and recovering hydrogen fluoride from the liquid phase enriched in hydrogen fluoride.
Whilst any alkali metal fluoride may be used, we prefer potasssium 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 gaseous mixture, the gas phase depleted in hydrogen fluoride may subsequently be contacted again with a solution of an alkali metal fluoride in hydrogen fluoride from which a further gas phase further depleted in hydrogen fluoride and a further liquid phase enriched in hydrogen fluoride may be recovered. This procedure may be repeated as often as desired. The first and, if present, subsequent separation step(s) may suitably be carried out in one or more mixer/settler units or in a liquid/liquid extraction, for example a packed column as desired. If desired, a series of bubble columns may be employed for successive separation steps.
The gas phase depleted in hydrogen fluoride may be separated and treated in an appropriate manner such as by subjecting to distillation, for example in a distillation column, to recover the organic compound and/or to obtains recycle stream for feeding to an upstream point in the process, for example 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 typically 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, a point upstream of the reactor so as to supplement a feed stock 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 as desired. Any hydrogen fluoride which remains in the recovered organic compound can be recovered for example by distillation or removed by aqueous scrubbing.
The liquid phase enriched in hydrogen fluoride will usually be subjected to a separation process, preferably distillation, to recover essentially anhydrous hydrogen fluoride therefrom. The separation may be carried in any conventional separation apparatus 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 liquid phase can be collected for use in another reaction or recycled to an upstream process step, for example the reactor in which the organic compound is produced or a suitable feed stock line in the process. This liquid phase enriched in hydrogen fluoride extracted from the stream being treated will usually also contain some organic compound(s) extracted from the stream being treated. Distillation of the phase suitably 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, for example 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 separating the liquid phase enriched in hydrogen fluoride to recover hydrogen fluoride can be recycled to the process and re-used to extract hydrogen fluoride from the mixture being treated.
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 the solubility of the alkali metal fluoride in the hydrogen fluoride. The efficacy of the recovery of hydrogen fluoride from the process is influenced by the partial pressure of the hydrogen fluoride in the gaseous stream depleted in hydrogen fluoride which is itself influenced by the concentration of the alkali metal fluoride in the solution. The overall pressure in the separation step may influence the efficacy of re

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