Process for producing acetic acid in hardwood kraft pulp mills

To determine the economic feasibility of producing acetic acid from commercial hardwood chips in kraft pulp mills, laboratory experiments were conducted to obtain sufficient data to perform a preliminary economic analysis for a proposed acetic acid recovery process. The acetyl groups in northeast hardwood were hydrolyzed from the xylan polymers in the wood to obtain sodium acetate. The extraction experiments were performed by using 4%–6% sodium hydroxide at low temperature (50°C–80°C). Sodium acetate from the extract was concentrated and then converted into acetic acid and sodium hydroxide by salt splitting using bipolar membrane electrodialysis (BPMED). Flow diagrams were prepared and cost estimates made for the capital and operating costs for the proposed acetic acid recovery process. The discounted cash flow rate of return on investment was estimated for pulp mills in the range of 1000–2000 tons/day. A preliminary economic analysis showed that the discounted cash flow rate of return on investment is primarily a function of (1) the plant size, (2) the selling price of acetic acid, and (3) the content of acetyl groups present in wood species. The income for the process and thus the rate of return on investment increases with increasing acetyl content in the wood and selling price of the acetic acid. When food grade acetic acid is produced, for example, the rate of return varies between 9% and 16% depending upon the size of the pulp mill, assuming the wood contains 3.5% acetyl groups on a dry basis and the selling price of acetic acid is US$900/ton.