Process for recovering rhodium catalyst

Details Process for recovering rhodium catalyst Process for recovering rhodium catalyst

1999-03-24

2001-01-30

Bos, Steven (Department: 1754)

Chemistry of inorganic compounds

Treating mixture to obtain metal containing compound

Platinum group metal

Reexamination Certificate

active

06180071

ABSTRACT:

The present invention relates to a process for recovering rhodium catalyst from tar (or “polymer”) formed during the preparation of acetic anhydride and/or acetic acid by the rhodium catalysed carbonylation of methyl acetate and/or dimethyl ether in the presence of methyl iodide as co-catalyst.
The use of catalyst systems comprising rhodium and an iodine compound in the preparation of acetic anhydride by the carbonylation of methyl acetate is well known from for example U.S. Pat. Nos. 3,927,078; 4,046,807; 4,374,070; and 4,559,183 and European Patents Publication Nos. 8396 and 87870. EP-A-87870, for example, discloses a process for the production of acetic anhydride with or without the net co-production of acetic acid from methanol and carbon monoxide in a series of esterification, carbonylation and separations steps comprising:
(1) reacting methanol with recycle acetic acid in an esterification step to form an esterification product containing predominantly methyl acetate, water and optionally unreacted methanol,
(2) removing part of the water from the esterification product,
(3) reacting the esterification product still containing water with carbon monoxide in a carbonylation step in the presence as catalyst of free or combined metallic carbonylation catalyst and as promoter of free or combined halogen to form a carbonylation product containing acetic acid and acetic anhydride,
(4) separating the carbonylation product by fractional distillation into a low boiling fraction containing carbonylation feed and volatile carbonylation promoter components, acetic acid and acetic anhydride fractions, and a higher boiling fraction containing carbonylation catalyst components,
(5) recycling the low boiling fraction containing carbonylation feed and carbonylation promoter components and the higher boiling fraction containing carbonylation catalyst components to the carbonylation step and,
(6) recycling at least part of the acetic acid fraction to the esterification step.
In practice, in step (4) of the aforesaid process, flash separation/distillation of a stream from the reactor removes methyl iodide, methyl acetate, acetic acid, acetic anhydride and other volatile components from a return stream containing the catalyst and promoter components, acetic acid, acetic anhydride and other relatively high boiling components. The overheads/volatile stream from the flash separation/distillation is fed to a fractional distillation where methyl iodide, methyl acetate and other light boiling components are separated from acetic acid, acetic anhydride and other heavy boiling components. The low boiling fraction from the distillation is then recycled to the reactor as described and the high boiling catalyst-containing fraction from the flash is recycled to the reactor.
A problem commonly encountered in the aforesaid processes for the production of acetic anhydride is the formation of tar (or “polymer”). The tar accumulates in those high-boiling fractions also containing carbonylation catalyst components. It is known from, for example, U.S. Pat. Nos. 4,388,217; 4,945,075; 4,944,927 and 5,364,822 to be removable therefrom by solvent extraction.
U.S. Pat. No. 4,388,217 relates to a process for the recovery of catalyst values from a catalyst-tar solution derived from a production system in which acetic anhydride is prepared by carbonylating methyl acetate in the presence of rhodium, lithium and methyl iodide wherein the catalyst-tar solution is submitted to an extraction using methyl iodide and aqueous hydrogen iodide and recovering catalyst values in the aqueous phase.
U.S. Pat. No. 4,945,075 relates to a process for the recovery of rhodium catalyst values from a catalyst-tar solution derived from a production system in which acetic anhydride is prepared by contacting a mixture of methyl acetate and/or dimethyl ether and methyl iodide with carbon monoxide in the presence of a rhodium catalyst comprising the steps of:
(1) submitting the catalyst-tar solution to an extraction with a combination of methyl iodide and aqueous hydrogen iodide and recovering (a) the aqueous phase containing most of the rhodium catalyst values, and (b) the methyl iodide phase containing most of the tar which contains minor amounts of the rhodium catalyst values;
(2) treating the methyl iodide phase of step (1) with an oxidant selected from peracetic acid, hydrogen peroxide and ozone; and
(3) submitting the treated methyl iodide phase of step (2) to an extraction with aqueous hydrogen iodide to recover in the aqueous phase rhodium catalyst values present in the treated methyl iodide phase.
The presence of aqueous hydrogen iodide in the aforesaid process is said to be necessary to stabilise the water-soluble rhodium compound or compounds, thereby preventing the loss of insoluble rhodium which can plate out on the extraction equipment and/or the walls of pipes, vessels, etc. A disadvantage of using hydrogen iodide is that at high concentrations it can result in corrosion of metal plant. Moreover, it is not generally employed in carbonylation processes for the production of acetic anhydride.
In the commercial operation of the process described in EP-A-87870, for example, polymer is continuously removed from the reactor by taking a small bleed from the catalyst recycle stream (CRS).
U.S. Pat. No. 4,944,927 describes a process in which tar, for example in CRS, undergoes an initial wash/contact with methyl iodide and optionally water after which the resulting rhodium/tar/methyl iodide rich phase is fed into an extraction column (e.g. a Kuhni column) in which it is counter-currently extracted using acetic acid/water extractant. Recovered from the extraction column is the majority of the rhodium and any co-promoters. The methyl iodide employed for this purpose may be at least in part methyl iodide recovered from the extraction tar/methyl iodide process stream.
U.S. Pat. No. 5,364,822 describes a carbonylation catalyst recovery system as described in U.S. Pat. No. 4,944,927 in which the carboxylic acid/water extracting solution also comprises (iii) iodide salt co-promoter derived from the carbonylation process and (iv) alkyl halide. According to U.S. Pat. No. 5,364,822 the iodide salt co-promoter may be derived from the carbonylation process in a pre-extraction step. Thus by reference to
FIG. 1
therein U.S. Pat. No. 5,364,822 describes a process in which a catalyst recycle stream (CRS) being a side stream of liquid phase from a carbonylation product recovery flash stage from a rhodium-catalysed carbonylation process for the co-production of acetic acid and acetic anhydride and comprising rhodium catalyst, methyl iodide carbonylation promoter, iodide salt of a quaternary amine carbonylation co-promoter (such as N,N′-dimethyl imidazolium iodide and its alkylated derivatives), tar and carbonylation reaction products is withdrawn from the carbonylation process, diluted with methyl iodide (MeI) and fed into static mixers (
1
). Water is also fed into the mixers. In the mixers the water, CRS and MeI are mixed and pass through cooler (
10
) and chiller (
11
) into setting vessel (
2
) where they are separated to produce a pre-extraction aqueous phase (
12
) and a pre-extraction methyl iodide phase (
13
). Prior to the coolers, in the pre-extraction mixers acetic anhydride is hydrolysed to acetic acid. The pre-extraction aqueous phase is saturated with methyl iodide. The pre-extraction methyl iodide phase comprising tar and Group VIII noble metal catalyst is fed alone line (
3
) to the top of a multistage extraction Kuhni column (
4
). The pre-extraction aqueous phase comprising acetic acid, water, methyl iodide and iodide salt of quaternary amine co-promoter has its methyl iodide and acetic acid concentrations adjusted to ensure that it is saturated with methyl iodide by addition of methyl iodide and acetic acid in line (
5
) and is then fed as extracting solution to the bottom of the Kuhni column (
4
). In the Kuhni column the methyl iodide-containing composition from the pre-extraction and aqueous extracting solution are contacted t

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