Study On The Engineering Application Of Lignin Lean Black Liquor Recovery

The situation of domestic pulp industry and alkali recovery system of different raw materials is analyzed and the technique of lignin extraction from black liquor,lignin utilization and industry applications are summarized in this paper.Related process and equipment of lignin extraction is studied based on one of the real case of domestic relocation project.System design and process selection of lignin extraction,lignin lean black liquor evaporation and recovery boiler after lignin removal are also discussed aiming to meet the adoption of up-to-date technology and requirement of the environmental protection.Lignin separation from black liquor of pulping process is an excellent choice of chemical pulp mill due to the reduction of heat load of recovery boiler which is the bottleneck of yield boosting of pulp mill.Lignin separation eliminates the bottleneck of capacity expansion,achieving the goal of low cost and high output of enterprise.Another advantage of lignin separation is that the lignin product of high quality can be produced by process selection and optimization.Extra benefit will be obtained through high value-added processing of alkali lignin separated from black liquor.Separation of lignin from black liquor is a best way to integrate the biorefinery technology to traditional pulp and paper industry as lignin is one of the commodities in short supply,which is meaningful for social benefit and popularization.Membrane-Acid precipitation process is appropriate for existing and planned pulp mill,from which the high quality lignin product can be obtained.The first project in domestic large-scale modern unbleached softwood kraft pulp mill combining lignin extraction with alkali recovery is selected to be the reference,of which the productivity of unbleached softwood kraft pulp reaches 155000t/a and extracted alkali lignin 10000t/a,equaling to 10.3% lignin recovery rate from black liquor while 5305t/a sulfuric acid to be consumed.Total heat input of recovery boiler is reduced from 10248.7GJ/d without lignin extraction to 9381GJ/d which equals to reduction of 8.47%.Evaporated water and steam consumption in the evaporation plant slightly increase due to the recovery of wash water.The returning of spent liquor from lignin precipitation plant supplement the sulfur losses of the whole pulp mill.On the basis of the balance calculation of pulp mill according to the design of the alkaline recovery system without lignin extraction which keeps a invariant heat value,the productivity of pulp will upgrade from155000t/a to 166374t/a,leading to 7.34% increasement when 10000t/a alkali lignin extracted from black liquor compared to the existing pulp mill of original design without lignin extraction,in addition,no additional technical reform funds are required for alkali recovery system.