BP Chemicals reveals effective alternative acetic acid production

23 November 1998 00:00  [Source: ICB]

In our bi-monthly Process monitor, Art Brownstein* looks first at a new catalyst system using iridium for acetic acid production

In contrast to the long-used rhodium catalyst system for acetic acid production, BP Chemicals now discloses the effective use of iridium as an alternative system (European Patents 0 849 248, -249, -250 and -251).

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The new catalyst is most effective when used in combination with lithium and ruthenium as shown in the table.

The reaction rate more than doubles when all three catalyst components are used compared to a system from which lithium is eliminated. Using iridium and lithium alone actually impedes the reaction rate.

The reaction system has the approximate composition of methyl iodide/methyl acetate/ methanol/water (8.4/30/59.6/2 w/w) into which carbon monoxide is pressured (28atm) at 190°C. Iridium is charged as H₂IrCl₆. As expected, reaction rate is a function of both reaction pressure and water content, as shown in the figure. Maximum selectivity to byproduct propionic acid (0.6%) coincides with the conditions for maximum reaction rate.

Iridium has historically been considerably less expensive than rhodium and is, therefore, more economically attractive. BP Chemicals reportedly anticipates an operating cost saving of 10-30% for new plants using the new catalyst compared to rhodium-based systems. The catalyst, named Cativa, will be used in its 310 000 tonne/year plant at Hull, UK.

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The primary use for acetic acid is vinyl acetate whose global demand has been growing at 3.5%/year. BP Chemicals is the primary licensor of acetic acid process technology and is the second largest producer.

An interesting technique for producing vinyl acetate has been disclosed by Eastman Chemical (World Patent Applications 98/25879 and 98/25880). Contrary to current technology based on the vapour phase addition of acetic acid to ethylene, Eastman's system is based on acetic acid as the sole feedstock.

The three-step liquid phase process comprises cracking acetic acid to ketene followed by its hydrogenation to acetaldehyde. The latter is combined with additional ketene to produce vinyl acetate in yields as high as 95%. The process sequence can be summarised as follows:

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Although it is possible to carry out acetaldehyde and vinyl acetate production in a single step, superior results are obtained if the reactions are carried out in tandem with the crude acetaldehyde from one reactor being passed directly into a second reactor for condensation with additional ketene. In this last step, a stream of ketene/acetaldehyde/N₂ (0.7/1/92m/m) is passed through a solution of 8.8% p-toluenesulphonic acid as a catalyst in acetic anhydride.

The acetic anhydride is reportedly an inert solvent for the system which also contains 0.2% t-butylhydroquinone as a polymerisation inhibitor. Vinyl acetate is obtained in 95% yield over a period of 300 minutes. No information is given for selectivity and conversion, nor are any data provided for the efficiency of acetaldehyde production other than a Pd on carbon catalyst is employed.

Production of ketene in 85-90% yield by pyrolysis of acetic acid at 700-800°C has been reported. With the use of the appropriate catalyst to avoid hydrogenation, production of acetaldehyde in high selectivity is also likely. With the current low price for ethylene at $370/tonne (contract), the current vapour phase process which proceeds in a single-step via acetoxylation of ethylene appears to be more cost effective.

Low cost nitrous oxide derived as byproduct from adipic acid production is the key to Solutia's new route to phenol.

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The economic incentive notwithstanding, proliferation of this system is hampered by the lack of generally available low cost nitrous oxide. Now Solutia discloses that nitrous oxide can be produced in very high selectivity by catalytic oxidation of ammonia (World Patent Application 98/25698).

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Passage of a stream of NH₃/N₂/O₂ (9:82:9 v/v) over a manganese-bismuth oxide on alumina catalyst at 300°C (1.6 second contact time) results in 88.6% selectivity to nitrous oxide at 99.4% conversion. Other nitrogen oxides only total 0.3%. The oxygen can be ostensibly employed as air. No information is given regarding other reaction products. Shorter residence times of 0.7 seconds offer much the same results.

In a related development, Solutia discloses that a ruthenium containing HZSM-5 catalyst (Si/Al=30) is effective in the production of phenol from benzene and nitrous oxide (World Patent Application 98/05616). Phenol is obtained in 98% selectivity at a benzene conversion of 0.33m mol/g catalyst/hour.

To achieve this, a stream of He/N2O/benzene (240/65/16 v/v) is passed over extrudates of the catalyst. The latter requires periodic regeneration after 15 hours of use by heating it at 500°C under a stream of He and O₂ (4:1 v/v). The presence of ruthenium essentially eliminates the production of carbon monoxide during regeneration.

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Solutia plans to construct a 150000 tonne/ year plant at Pensacola, Florida, which is scheduled to be onstream by 2000.

Disclosure

A partial disclosure of Mitsubishi Chemical's efforts to produce acrylonitrile by ammoxidation of propane appears in World Patent Application 98/22421. Using a promoted vanadium molybdate catalyst (MoV_0.3Nb_0.12Te_0.23O_n), acrylonitrile is obtained in 61.6% selectivity at 24.5% propane conversion.

The process comprises the passage of propane/ammonia/air (1/0.44/5.40 v/v) over a fixed bed catalyst at 434°C, and at a weight hourly space velocity of 2.53 hr_-1. Propylene and acrylic acid are produced as byproducts in selectivities of 15.9% and 2.0%, respectively. The overall reaction is as shown below.

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It is likely that the byproduced propylene can be recycled for additional acrylonitrile production. If so, this would increase the overall yield of the system. The patent application does not mention hydrogen cyanide and acetonitrile production, both of which are commonly byproduced in current propylene-based processes.

Acrylic acid production appears to be contingent upon the propane/ammonia molar ratio. A decrease in this ratio from 2.3 to 1.8 eliminates acrylic acid formation entirely at the expense of a slight decline in acrylonitrile selectivity from 61.6% to 59.8%. Propylene production rises proportionately from 15.9% to 17.9%.

The Mitsubishi Chemical data apparently focus on catalyst life studies in which the catalyst is first run for 3800 hours in a fluidised bed and subsequently for 7175 hours in a fixed-bed mode. Over this last period, selectivity to acrylonitrile declines 15%. The 61.6% selectivity cited by Mitsubishi Chemical may, therefore, be a low number since the catalyst had already had 3800 hours of use in a fluidised bed.

BP Chemicals and Asahi Chemical Industry have been independently developing their own propane-based processes for acrylonitrile. Asahi plans to undertake commercial trials of its new system at Kawasaki. BP Chemicals is building a $10m demonstration plant at Green Lake, Texas. Mitsubishi Chemical's effort is being pursued in conjunction with BOC.

The driving force for these efforts is the low cost of propane ($130/tonne) relative to propylene ($276/tonne chemical grade). A propane-based process for acrylonitrile has been variously claimed to have a 30-50% variable cost advantage by these companies.

Acrylonitrile technology has undergone an interesting evolution since the early 1950s when acetylene-based systems displaced those using ethylene oxide. In the 1960s, acetylene-based technology was, in turn, displaced by that using propylene. A sea change to propane may now be in the offing.

The use of polymers, such as polyaniline, in semiconducting applications is growing rapidly. Now Dow Chemical discloses that polyaniline can be produced in high yield and with high conductivity when aniline is treated with hydrogen peroxide in the presence of hydrogen chloride and a catalytic amount of ferrous sulphate (World Patent Application 98/25993).

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In an example, an aqueous solution of aniline and HCl (1:5 molar ratio) containing FeSO₄ (0.33% based on aniline) is treated with 35% aqueous H₂O₂ for 3.5 hours at 25°-29°C. The hydrogen peroxide is used in 27% molar excess over the aniline. Polyaniline hydrochloride is obtained in 78% yield and with a conductivity of 0.63 Siemens/cm.

It is insoluble in methanol and water. By varying the process conditions, yields and conductivities as high as 85.3% and 3.64 Siemens/cm, respectively, are attainable. Doping polyaniline and other semiconducting polymers, such as polyacetylene, leads to large increases in conductivity, frequently as large as several orders of magnitude.

Omecron Chemie of Ammersbek, Germany, reports that it is collaborating with DuPont and Philips, the electronics firm, to develop new uses for polyaniline. Philips is reported to have used Omecron's polyaniline to produce the first all-plastic computer chip.

Single-step process

A single-step process for the synthesis of ethyl acetate from ethanol has been developed by the Chinese National Petroleum Corporation (World Patent Application 98/21173).

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The process comprises the simultaneous partial oxidation of ethanol to acetic acid and its subsequent esterification with excess ethanol. To achieve this, a trickle bed reactor is employed in which a mixture of oxidation and esterification catalysts are packed. These are 10% Pd on styrenedivinylbenzene and Amberlyst 15.

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Liquid ethanol and oxygen under 35.9atm. pressure are metered (2.4 hr-1 WHSV) into the top of the reactor after first passing through a static mixer. The reactants flow downward over the catalysts which are packed between layers of glass beads. At 95°C, 93.5% aqueous ethanol is converted (56.1%) to ethyl acetate, acetic acid and acetaldehyde in selectivities of 68.5%, 30.6% and 0.9% respectively.

The selectivities are ostensibly on a weight per cent basis. While not noted, the acetic acid can apparently be esterified with unreacted ethanol and so increase the yield of ethyl acetate to nearly 100%.

In addition to direct esterification, a commonly-used route to ethyl acetate is the Tischenko reaction in which acetaldehyde is converted directly to this product by aluminum alkoxide catalysis.

Ethyl acetate is primarily used as a solvent for paints, coatings and inks. Global production is reportedly about 500000 tonne/year. BP Chemicals recently announced a new plant by an undisclosed technology.

ACETIC ACID BY METHANOL CARBONYLATION

Catalyst system	Rate, moles/hr
Ir	12.1
Ir/Li(1:1)	6.3
Ir/Ru(1:2)	15.1
Ir/Ru/Li(1:2:1)	30.8

Source: BP Chemicals



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