Utilization of methanol from kraft black liquor

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University of New Brunswick
The objective of this report is to determine whether or not it is feasible for Northern Pulp to recover or produce methanol from their weak black liquor stream. Currently the weak black liquor stream is passed through a set of evaporators and burned to recover the white liquor that is used in the digester. The other stream leaving the evaporators is known as foul condensate which is comprised of water, methanol, and other volatile sulfur compounds. The methanol to be recovered is sent to the chlorine dioxide generator. It is necessary that the maximum volume of water in the methanol-water mixture be 80% and free of other chemicals. With the help of a decision matrix, steam stripping was determined to be the leading technology to recover methanol from the foul condensate. It was also possible to modify the current steam stripper at the mill and further reduce the capital investment and footprint of the proposed design. Pre-treatment of the foul condensate was found to be crucial to achieve the desired purity of methanol. Hydrogen peroxide was added to the foul condensate to react with some of the sulfur compounds before it was sent to the steam stripper. Aspen Technology’s HYSYS was used to simulate the steam stripper and gave a methanol purity of 99.68 wt% excluding water with 87.7% of the methanol being recovered. Based on the findings of this study the proposed design is economically viable and should be investigated further. The total capital investment for the new equipment is $672,000 CAD, the total operating cost is $438,000 CAD and the annual cash flow is approximately $492,000 CAD/yr. From this analysis, a cumulative cash flow diagram was created and subsequently, a profitability analysis was performed. The payback period was determined to be just under 2 years (≈1.8 years). Net present value (NPV) after taxes was estimated to be $4,680,000 CAD after 15 years of operation. The return on investment (ROI) and internal rate of return (IRR) for the proposed design were calculated to be approximately 46% and 64%, respectively.