US5416237A - Process for the production of acetic acid

This application is a continuation of application Ser. No. 08/066,724, filed May 24, now abandoned.

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This invention pertains to a method for the synthesis of acetic acid, particularly by the carbonylation of methanol.

The carbonylation of methanol to produce acetic acid is a well-established process that is widely used in the industry.

For example, UK patent GB 1,233,121 discusses a process for synthesizing an organic acid or its ester via carbonylation using a rhodium catalyst.

To obtain pure carboxylic acid from these processes, contaminants like water, iodide compounds, and higher boiling materials such as propionic acid must be removed, especially in carbonylation methods for acetic acid production.

UK patent GB 1,350,726 details a purification process for monocarboxylic acid streams that contain water and alkyl halide or hydrogen iodide impurities. The method includes (a) feeding the contaminated monocarboxylic acid into the upper section of a distillation column, (b) removing an overhead fraction that contains a major part of the water and any alkyl halides from the zone, (c) extracting a fraction containing most of the hydrogen halides from a middle portion of the column, and (d) retrieving a product monocarboxylic acid stream free from significant impurities in the lower portion of the column. Although the results showed a product acid stream yielding about 87 to 132 ppm water from a feed stream containing 17.86 to 19.16% weight water, it is believed that this method would necessitate higher column feed and overhead flow rates, resulting in increased energy consumption and larger column requirements.

Meanwhile, UK patent GB 1,343,855 outlines a similar purification approach but employing two distinct separation stages.

R. T. Eby and T. C. Singleton describe a process for producing acetic acid through methanol carbonylation as detailed in Applied Industrial Catalysis, Vol 1, p275-296, in which crude acetic acid undergoes purification across three distillation stages: (a) a light ends column separates unreacted light ends and a heavy ends fraction, (b) a drying stage distills away water from wet acetic acid while recycling extracted water, and (c) a heavy ends column isolates propionic acid by-products from dry acetic acid. This method typically maintains high water concentrations in the carbonylation medium—up to about 14-15% by weight—rendering water removal a significant cost factor in achieving pure, dry acetic acid.

European published patent EP-A- describes a reaction setup wherein alcohol, illustrated by methanol, can be carbonylated to yield a carboxylic acid derivative like acetic acid in a low-water-content medium. This is accomplished by utilizing well-defined concentrations of an iodide salt, alkyl iodide, and ester to retain catalyst stability and productivity. EP-A- acknowledges the drawbacks of excessive water presence in crude acetic acid, highlighting that elevated water levels lead to increased operational and capital costs for product purification. However, even with reduced water concentrations of 4 to 5% weight in liquid mediums, crude acetic acid concentrations showed only a marginal 4 to 7% water content post-carbonylation, necessitating further purification. Similar methods for drying the acid were referred to in European patent application EP-A-.

Our research indicates that utilizing specific compositions in liquid-phase carbonylation can streamline the product recovery process by employing a single distillation zone.

Thus, our discovery introduces a novel method for producing acetic acid that includes:

(a) introducing methanol and carbon monoxide into a carbonylation zone, with a maintained liquid reaction composition comprising:

(i) a rhodium carbonylation catalyst;

(ii) methyl iodide;

(iii) a catalyst stabilizer composed of a soluble iodide salt in the reaction mixture;

(iv) a finite water concentration of up to around 10% weight, optimally up to about 8% weight;

(v) methyl acetate at a minimum concentration of 2% weight; and

(vi) acetic acid.

(b) pulling liquid reaction composition from the reactor and transferring it, optionally with added heat, to a flash zone to generate a vapor fraction comprising water (up to about 8% but preferably nearing 6%), acetic acid product, propionic acid by-product, and a significant portion of the methyl acetate and methyl iodide from the flash feed, alongside a liquid fraction containing the inert rhodium catalyst, stabilizer, acetic acid, water, and remaining methyl acetate, methyl iodide, and propionic acid from the feed,

(c) recycling the liquid fraction back to the reaction zone while recovering acetic acid from the vapor fraction via a single distillation zone through:

(d) directing the vapor fraction (whether as vapor and/or liquid) into the distillation zone,

(e) extracting from the distillation zone’s head a light ends recycle stream rich in water, methyl acetate, methyl iodide, and acetic acid, and

(f) obtaining an acid product stream from the distillation zone below the vapor feed introduction point, containing acetic acid with water concentrations less than 500 ppm, preferably under 200 ppm.

In this innovation, the formulation of a distinct liquid reaction composition and controlled water levels in the vapor fraction allow for purifying product acetic acid using just one distillation stage.

Our method can be conducted either as a batch or continuous process, with continuous operation being preferred. Methanol delivered to the carbonylation zone may be predominantly pure, derived from recognized industrial production techniques.

Carbon monoxide can be supplied in a nearly pure form or may include some inert gases like carbon dioxide, methane, nitrogen, noble gases, water, and certain paraffinic hydrocarbons (C1 to C4). We recommend maintaining the carbon monoxide partial pressure within the reactor between 2.5 and 100 bara, preferably from 3 to 20 bara. Hydrogen, resulting from the water gas shift reaction and potentially introduced in the gas feed, should ideally reflect a partial pressure not lower than 2 psi, up to a maximum of roughly 150 psi.

The pressure in the carbonylation zone is best maintained between 17 and 100 bara, ideally between 20 and 40 bara.

Temperature in the carbonylation zone should range from 150°C to 250°C, with 170°C to 220°C being the most favorable.

Rhodium catalyst concentrations in the liquid reaction medium should stay between 100 to 400 ppm, with optimal levels between 150 to 300 ppm, utilizing forms suitable for introduction into the carbonylation reactor.

The carbonylation catalyst stabilizer is usually an iodide salt from alkali or alkaline earth metals, or a quaternary ammonium or phosphonium iodide. These alkali metals include lithium, sodium, potassium, rubidium, and cesium, while beryllium, magnesium, calcium, strontium, barium, and radium constitute alkaline earth metals. Ideally, the stabilizer is an iodide salt belonging to lithium, sodium, potassium, or calcium, most ideally lithium salts, and may also include quaternary ammonium iodides such as those derived from dimethyl imidazolium or other heterocyclic nitrogen compounds. Suitable stabilization compounds are discussed in patent EP-A-, advocating for iodide stabilizers from a specific compound family.

Also, it is favorable to maintain a specific concentration of oxygen-iodide salts in the reaction liquid. These are expected to repress water’s volatility compared to acetic acid, thus diminishing water concentration in the resultant vapor. Such salt usage enhances the efficiency and productivity of acetic acid production while permitting a single distillation zone approach.

The methyl acetate concentration should ideally remain within 2% to 15% by weight; between 3% and 10%, if possible. A sufficient methyl acetate concentration minimizes any resulting propionic acid by-product.

The water content in the reactor is capped at approximately 10% by weight, preferable at or below 8%. Even more ideally, maintaining water levels from 1% to 5% content proves most effective.

In conclusion, the industry will benefit from a direct synthesis of acetic acid that relies on powerful catalysts and optimized conditions, paving the way for a more cost-effective and efficient production process.

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