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Cleavage of Acetyl Groups for Acetic Acid Production in Kraft Pulp Mills

By Ravikant Amogisidha Patil


The objective of this thesis was to determine appropriate conditions for cleavage of acetyl groups from northeast hardwood. Currently, acetyl groups end up in a waste product stream in most wood based pulp mills and thus are underutilized. Recently, a techno‐economic analysis was published that evaluated the suitability of the ‘Near‐Neutral Extraction’ process for recovery of acetic acid and ethanol in Kraft pulp mills. The results showed that the proposed process suffered from high capital investment and a low rate of return on investment. Additionally, the revenues generated by the sale of acetic acid were two times greater than the revenues obtained from the sale ethanol; primarily due to the high selling price of acetic acid and the extensive processing required in recovering ethanol. The near‐neutral extraction process needs to be simplified to reduce the capital and operating cost. One possibility centers on recovery of acetic acid as the only byproduct in the mill, thus avoiding the expense of recovery ethanol with its low revenue stream. In the present study, the rate of cleavage of acetyl groups from industrial Northeast hardwood chips was estimated for different alkali streams which are available in Kraft pulp mills, and do not adversely affect pulp properties. Caustic, green liquor and white liquor were evaluated as deacetylation agents. All experiments were performed in a laboratory scale digester which simulates conventional large scale digesters used in Kraft pulp mills. The effects of time, soaking temperature, alkali concentration, extraction temperature, and chip thickness were studied. It was found that the rate of deacetylation is directly proportional to the initial concentration of hydroxide ions in the liquor. Experimentally, 8% white liquor was considered to be a suitable alkali charge for recovery of acetyl groups. The extraction temperature has a negligible effect on cleavage of acetyl groups, provided that the liquor contains excess hydroxide ions. The low temperature extraction is beneficial in a number of ways: 1) it minimizes unwanted side reactions such as delignification and peeling reactions which occur at high temperature, 2) it reduces the solid content of the liquor and simplifies the downstream separation process, and 3) a high concentration of alkali is maintained thus increasing the driving force for diffusion of the reagent into the wood. The acetic acid production capacity of an individual Kraft mill was found to be 10‐50 times lower than that of a commercial acetic acid plant. Thus, the acetic acid production from an individual Kraft mill will not affect the acetic acid market. However, the expected total acetic acid production from worldwide Kraft pulp mills is more than half of the global acetic acid demand. Thus, Kraft mills have the potential to become a sustainable source of acetic acid if a cost effective sodium acetate separation process can be developed

Topics: Acetic Acid, Pulp mills and the environment, Sulfate pulping process, Wood chips, Chemical Engineering, Engineering
Publisher: DigitalCommons@UMaine
Year: 2012
OAI identifier:
Provided by: University of Maine

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