New Single-Oxidation Adipic Acid Route Will Slash Costs

A new adipic acid route that eliminates oxidation of nitric acid, one of the two oxidation steps required by conventional processes, promises to slash capital costs by more than 30% and operating costs by more than 20%. Adipic acid the primary feedstock for nylon 66.

Virtually all the world's existing adipic acid plants use the two-step process developed by DuPont in the 1940's. They make adipic acid by oxidizing cyclohexane in air in the presence of a cobalt catalyst to yield cyclohexanone and cyclohexanol (also called KA oil). The units then purify the KA oil and further oxidize it with nitric acid. This yields adipic acid and esters, which are hydrolyzed to adipic acid and dibasic acids.

The new One-Step Process goes directly from cyclohexane to adipic acid, esters, and other byproducts. Because the process does not use nitric acid, it does not produce nitrous oxides (ozone depleters) or nitrate byproducts (water polluters). The elimination of a second oxidation stage and reduced pollution requirements dramatically lowers capital and operating costs.

Marketing (back to top)
Fluor Daniel (Greenville, SC) will market the new route, which was developed by researchers funded by RPC Inc. (Atlanta, GA), which owns the technology.

RPC, which became involved with the project in 1993, when it formed a joint-venture with Twenty-First Century Research Corp., which developed the Technology. Under the agreement, RPC agreed to fund the research while Twenty-First Century Research evolved the technology, created the patent estate, and devised a licensing strategy.

By mid-1999, says Fluor Daniel, the research program was about 85% complete. Twenty-First Century Research had validated the technology at a continuous test unit in Poulsbo, WA. It had also completed extensive batch work on various other unit operation steps.

Fluor Daniel joined the research effort in 1998 and conducted extensive due diligence studies and directed some of the research effort to validate unit operations critical to the success of the technology.

"Not only will manufacturers benefit from dramatic capital cost reductions, but they will also realize cost savings through the elimination of environmental issues related to the disposal of nitric acid by-products," says Fluor Daniel process technology VP Mag Fouad.

Because adipic acid is not a core business, RPC chose to divest the business in April 1999. Although no longer staffed, the laboratory and test facility remain intact and actively maintained. All research data, including engineers' notebooks, analytical results, etc., is well organized, protected and available for review. Fluor Daniel plans extended semicommercial testing that will lead to commercialization.

One-Step Process Overview
The One Step Process yields adipic acid from a single-stage reaction of cyclohexane with oxygen in acetic acid in the presence of a cobalt catalyst. Reaction byproducts include glutaric acid, succinic acid, and small amounts of cyclohexanol and cyclohexanone (key intermediates produced by conventional adipic acid routes).

The reaction also yields over-oxidized product and adipic esters. Hydrolyzing the latter with an acidic catalyst cleaves the ester and forms adipic and other carboxylic acids, as well as some overoxidized product and small amounts of cyclohexanone and cyclohexanol. The combination of the single oxidation and hydrolysis boosts yields to levels comparable with conventional technology.

Core Process. The core process technology involves:

• Reaction. Cyclohexane reacts with oxygen in a cooled, continuously stirred tank reactor in the presence of a cobalt catalyst dissolved in acetic acid. The unit uses oxygen rather than air (used in conventional units), which significantly reduces the size of the air abatement systems. An offgas recovery system collects, compresses, and recycles oxygen, nitrogen, and VOCs. A solvent recovery system removes water and recycles acetic acid and cyclohexane.
  
• Recovery. The reactor product contains adipic acid, byproduct dibasic acids, esters, acetic acid, and unreacted cyclohexane. The process recovers crude adipic acid through crystallization, then purifies it through recrystallization, drying, and packaging. Unreacted raw materials are recycled.
  
• Catalyst Recovery. The concentrated mother liquor from adipic acid recovery (containing dibasic acids, esters, and catalyst) goes to catalyst recovery. An extraction process recovers the catalyst and recycles it back to the reactor.
  
• Ester Hydrolysis. After catalyst extraction, the mother liquor moves to hydrolysis. Esters are contacted with water in the presence of a catalyst, converting them to adipic acid and a small amount of cyclohexanone and cyclohexanol. Adipic acid is again separated in recovery crystallization and recycled to adipic acid recovery. Such residual dibasic acids as glutaric and succinic acid, are further treated and sold as byproduct.

Pollution Issues (back to top)
Because it produces less air and water pollution, the One-Step Process significantly reduces capital and operating costs when compared with the conventional process.

Gaseous Emissions. Conventional systems must heat large vent gas streams at high temperatures in order to achieve the complex reduction of N₂O and NOx. Because the One-Step Process does not use nitric acid, of N₂O and NOx treatment is a moot issue. All that's needed is thermal oxidation of VOCs. Even then, the use of oxygen rather than air significantly reduces the load placed on the abatement system.

Liquid Effluent. Conventional two-step processes produce wastewater with high biological oxygen demand (BOD). It typically requires anoxic denitrification to remove nitrates, followed by activated sludge treatment to reduce BOD. According to Fluor Daniel, the One-Step Process requires only activated sludge treatment.

Economics
According to Fluor Daniel, the One-Step Process promises to improve the economics of both stand-alone and integrated (with nylon 66) facilities. To determine product profitability, Fluor Daniel compared a theoretical 135,000 mt/yr grassroots adipic acid plant operating at 80% yield with a comparable conventional facility.

The analysis suggests that the new process reduces capital costs 33% and operating costs by nearly 23 %. Over a 20-year lifespan, process savings will contribute more than $ 120 million towards cash flow (based on net present value, discounted at 15%).

According to Fluor Daniel, the process achieves lower total installed costs (TIC), production costs, and capital investment. This is how it breaks down process benefits:

Total installed costs (TIC):

• Elimination of the KA-oil-to-adipic-acid oxidation step.
  
• Elimination of nitric acid handling, recovery, purification, and recycling.
  
• Simplification of air abatement through elimination of N₂O and NOx emissions.
  
• Simplification of the wastewater treatment by elimination of nitrates.

Production costs:

• Reduced raw material cost by elimination of nitric acid.
  
• Reduced utility and offsite disposal costs through elimination of N₂O, NOx and wastewater nitrates.

Fluor Daniel estimates both One-Step and conventional adipic acid processes consume similar amounts of utilities. While oxygen costs marginally more than air, it finds the cost more than offset by lower capital costs for air abatement and reactor offgas and recirculation systems. For this process, the company concludes, capital drives return on investment.

Fluor Daniel's economic analysis also considered an integrated nylon 66 complex with capacity for 215,000 mt/yr nylon 66, 115,000 mt/yr hexamethylenediamine (HMDA), and 135,000 mt/yr adipic acid. It finds the One-Step Process reduces capital costs by approximately 11%.

Current Status (back to top)
RPC's test facility, begun in mid-1996, includes feed tanks and pumps for raw material, catalyst and product, jacketed product receivers, and a continuously stirred tank reactor equipped with heating jacket, internal heating/cooling coil, and gas sparge ring. The PC-controlled facility features extensive instrumentation for capturing experimental results and an advanced shutdown system to handle process hazards. The facility itself is easily dismantled for shipping and reassembly at another site.

According to Fluor Daniel, the unit produced enough crude adipic acid to test downstream processes capable of meeting stringent quality requirements. The downstream operations, however, operated in batch rather than the continuous mode required by commercial facilities.

For more information: Gert Smit, Project Manager, Chemical and Life Sciences, Fluor Daniel Corp., 100 Fluor Daniel Dr., C105R, Greenville, SC 29607. Phone: 864-281-8709. Fax: 864-676-7211.

By Alan S. Brown