Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acids and salts thereof
Reexamination Certificate
2001-07-13
2003-01-14
Keys, Rosalynd (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acids and salts thereof
C562S606000
Reexamination Certificate
active
06506935
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a novel two-step method for treating a liquid stream contaminated with an iodine-containing compound using a pretreatment step in combination with an adsorption step. The adsorption step is performed by contacting the liquid stream with a solid adsorbent material comprising a zeolite having a silica to alumina molar ratio from about 5 to less than 15 which has been cation exchanged with a metal selected from the group consisting of silver, mercury, copper, lead, thallium, palladium or mixture thereof.
BACKGROUND OF THE INVENTION
Methanol carbonylation, the reaction of methanol with carbon monoxide, is used to produce a significant share of the world's acetic acid and represents the basis for virtually all new acetic acid capacity. The fundamental process, whereby methanol and carbon monoxide are reacted in the presence of a rhodium catalyst and methyl iodide promoter, is disclosed in U.S. Pat. No. 3,769,329 B1 and has become well-known as the “Monsanto process”. Although numerous improvements have since been developed, the use of an iodine-containing promoter, either as an organic iodide or metal iodide salt, has proven necessary to obtain industrially-competitive reaction rates and production economies.
Unfortunately, the use of any suitable iodine-containing promoter invariably results in the incorporation of trace iodine and organic iodide impurities into the final acetic acid product. These contaminants result from numerous transformations (thermal cracking, recombination, isomerization, etc.) of the iodine-containing catalyst promoters, which occur not only in the reactor, but also in downstream equipment, such as distillation column reboilers and recycle lines. A resulting array of C
1
to C
10
organic alkyl iodide species is produced, which are removed from the acetic acid product with varying degrees of effectiveness via the standard distillation steps used in down stream purification. Additionally, iodine may be present in the acetic acid product in the form of hydrogen iodide or iodide salts. Ultimately, without supplemental treatment to remove trace iodine-containing contaminants, product acetic acid made using methanol carbonylation technology with even the most careful fractionation steps, will still contain a small amount, typically below 100 parts per billion (ppb) of total iodine (both organic and inorganic) by weight.
The interest in a process for essentially complete removal of iodine-containing contaminants from acetic acid stems from the large share (about 40 to 50%) of its use as a precursor for vinyl acetate monomer (VAM) synthesis. Current methods of VAM production rely on a catalyst which is intolerant to even minute levels of iodine-containing compounds in the acetic acid feedstock. Therefore, the VAM production costs associated with reduced catalyst life increase dramatically with increasing feed iodine concentration.
Several disclosures in the prior art present techniques for the selective removal of iodine-containing species from process streams such as nuclear reactor containment environment off gases as well as emissions from spent nuclear fuel reprocessing operations. For example, U.S. Pat. No. 3,658,467 B1 addresses the removal of radioactive iodine-containing materials from the gaseous waste streams generated either during normal nuclear fuel reprocessing operations or even in the event of a fuel element cladding failure whereby radioactive methyl iodide is formed in significant amounts. The solution proposed in the '467 patent is a zeolite X molecular sieve exchanged with silver for treating the gaseous waste stream. All cited examples referring to the adsorptive ability of this formulation are based on performance in a dry air stream contaminated with trace radioactive methyl iodide. The structures of X-type zeolites are known to have aluminosilicate frameworks with maximum silica to alumina molar ratios, expressed as the ratio of SiO
2
to Al
2
O
3
in the fundamental zeolite framework, of about 3 and pore openings typically in the range of 7 to 8 Å.
In U.S. Pat. No. 4,735,786 B1, an alternate solution for filtering radioactive iodine-containing compounds from nuclear facility exhaust gases in the event of an accident is proposed. In offering an improvement over the prior art, the '786 patent recognizes the practical deficiencies of silver-exchanged zeolite X adsorbent for this service under high humidity conditions. The improvement offered is a different type of adsorbent, characterized as a high silica to alumina molar ratio pentasil zeolite. The adsorbent specified is exemplified by the well-known ZSM-5 type material, which is clearly described in U.S. Pat. No. 3,702,886 B1 as having ten-member rings forming medium-sized pores in the range of 5.1 to 5.6 Å. The teachings and specific examples of the '786 patent are restricted to pentasil zeolites having silica to alumina molar ratios in the range of 15 to 100, preferably 20 to 50.
In U.S. Pat. No. 4,913,850 B1, another solution for methyl iodide removal from gaseous streams is presented, whereby a silver-exchanged “binderless” zeolite material, composed of 80 to 90% zeolite X and 10 to 20% zeolite A, is used. Among the possible candidates for zeolite X materials, those having the faujasite structure are of particular interest. As mentioned previously, zeolite X formulations generally have a maximum silica to alumina molar ratio of 3. In U.S. Pat. No. 5,075,084 B1, the progress of treating radioactive iodine-containing gas streams is continued, where the problem of the proposed silver-exchanged zeolite material catalyzing the highly exothermic reaction of hydrogen and oxygen and, in the extreme case, causing catalytic ignition of hydrogen, is solved. According to the '084 patent, this undesired side reaction is suppressed when a heavy metal such as lead is added to the silver-exchanged adsorbent. The underlying zeolite compositions of the preferred materials in this patent and the previously mentioned '850 patent are identical.
In U.S. Pat. No. 4,088,737 B1, gaseous radioactive methyl iodide removal is further addressed in a multi-step treatment procedure where the initial gas purification is performed with a silver-exchanged zeolite exemplified by zeolite X. After iodine-compound breakthrough, regeneration and concentration steps are undertaken, which involve i) withdrawing the spent adsorbent from contact with the gaseous waste stream, ii) subjecting the adsorbent to desorption conditions with a hydrogen-rich stream to produce a hydrogen iodide containing off gas, and iii) treating this effluent gas with a lead-exchanged zeolite to re-adsorb and concentrate the desorbed hydrogen iodide. Lead-exchanged zeolite X is specifically cited as achieving the desired result for the final adsorption step. The advantage of the multi-step treatment is that the long-term storage of the contaminated material is less expensive for the lead-exchanged zeolite, compared to a silver-exchanged material.
In spite of these continuing developments and improvements in trace iodine and organic iodide removal from gaseous effluent streams, the methods employed have been found unsuitable for the more difficult problem of iodine-containing compound adsorption from corrosive liquids, such as commercial acetic acid product streams. Adsorbent carrier materials of the prior art such as zeolite X and zeolite A, which are classified as having low silica to alumina molar framework ratios (typically below 5), have experimentally been proven to be unstable in acetic acid. This means that the dissolution (or leaching) rate of framework components into the liquid is sufficiently large to render such materials ineffective for iodine-containing compound adsorption service in corrosive liquid media. Depending on the specific silica to alumina framework molar ratio, the pentasil zeolites, exemplified in prior art gas-phase iodine-containing compound removal using ZSM-5, are significantly more stable in acetic acid than zeolite types
Kulprathipanja Santi
Leet William A.
Vora Bipin V.
Gooding Arthur S.
Keys Rosalynd
Molinaro Frank S.
Tolomei John G.
UOP LLC
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