Kraft Mill Biorefinery to Produce Acetic Acid and Ethanol: Technical Economic Analysis
The “near neutral hemicellulose extraction process” involves extraction of hemicellulose using green liquor prior to kraft pulping. Ancillary unit operations include hydrolysis of the extracted lignin, liquid-liquid extraction of acetic acid, liming followed by separation of gypsum, fermentation of C5 and C6 sugars, and upgrading the acetic acid and ethanol products by distillation. The process described here is a variant of the “near neutral hemocellulose extraction process” that uses the minimal amount of green liquor to maximize sugar production while still maintaining the strength quality of the final kraft pulp. Production rates vary between 2.4 to 6.6 million gallons per year of acetic acid and 1.0 and 5.6 million gallons per year of ethanol, depending on the pulp production rate. The discounted cash flow rate of return for the process is a strong function of plant size, and the capital investment depends on the complexity of the process. For a 1,000 ton per day pulp mill, the production cost for ethanol was estimated to vary between $1.63 and $2.07/gallon, and for acetic acid between $1.98 and $2.75 per gallon depending upon capital equipment requirements of the new process.To make the process economically attractive, for smaller mill sizes the processing must be simplified to facilitate reductions in capital cost.