Haibo Mao

Date of Award

12-2007

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Chemical Engineering

Advisor

Joseph M. Genco

Second Committee Member

Adriaan Van Heiningen

Third Committee Member

Hemant Pendse

Abstract

A technical analysis was performed for a new process that produces ethanol and acetic acid from hardwood in addition to bleached hardwood Kraft pulp. The new process, termed the "near neutral" hemicellulose extraction process, involves the extraction of wood hemicellulose using green liquor prior to conventional Kraft pulping. This thesis assumes that the new process is located at an existing kraft pulp mill. Ancillary unit operations in the process include hydrolysis of the extracted carbohydrates using sulfuric acid, filtration of the extract to remove lignin that was extracted with the carbohydrates, liquid-liquid extraction of acetic acid and by-product furfural, liming of the hydrolyzed extract, separation of gypsum which is the product of the liming reaction, fermentation of C5 and C6 sugars and upgrading of the acetic acid and ethanol products by distillation. After the extraction step in the process, the final pH of the extraction liquor varies between 5 and 6 and preserves the yield and physical properties of the final pulp product which are equivalent to market Kraft pulp. In the thesis reported here, a design model was developed using WinGEMS and ASPEN Plus software. The integrated mass and energy balances were obtained from the computer model and used to size important process equipment in the process. Information from the process model and the flow sheet were used to estimate capital and operating cost for the process. A major economic factor in the analysis is whether a new extraction vessel is required or if an existing digester can be retrofitted into a vessel suitable for the extraction. The economic analysis was performed as a function of the size of the pulp mill being retrofitted to accommodate the hemicellulose extraction process. Alternative cases that were evaluated considered whether the utilities and waste water treatment systems were sufficient to accommodate the new process. In the economic analysis, the Discounted Cash Flow Rate of Return (DCFROR) was used to judge the economic merits of the new process. The rate of return on investment was found to vary between 9.9 and 14.0 percent depending upon the size of the pulp mill (750 to 1,500 tonne per day pulp production rate) for the case where the extraction vessel is available and the utilities and waste treatment facilities are sufficiently large to handle the additional requirements for the process. If a new extraction vessel must be purchased, the process is not economically justified regardless whether the utilities and waste water treatment facilities are sufficient to handle the new process. A very important advantage of the "near-neutral" extraction process is that the recovery cycle is off-loaded because the amount of organic matter in the black liquor is reduced and less white liquor is needed for pulping. This change in operation would potentially allow a Kraft pulp mill to significantly increase its pulp production rate if the recovery cycle is the bottleneck in the plant. In the hemicellulose extraction process the methanol content of the black liquor is thought to be reduced by about 40%, while the TRS content is also diminished. These advantages result because of the lower white liquor charge and shorter pulping time during the modified Kraft cook. A major disadvantage of the process is that the production rates for the two co-products, ethanol and acetic acid, are low when compared to corn to ethanol processes. Less steam is produced in the recovery boiler because a portion of the organic matter in the wood chips is extracted. Consequently, additional steam must be supplied from a biomass boiler or by burning fossil fuels such as natural gas, coal or fuel oil. Lastly the near neutral extraction process is capital intensive unless an extraction vessel is available for use.

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