Isolation of Carboxylic Acids and NaOH from Kraft Black Liquor with a Membrane-Based Process Sequence

Abstract

In kraft pulping, large quantities of biomass degradation products dissolved in the black liquor are incinerated for power generation and chemical recovery. The black liquor is, however, a promising feedstock for carboxylic acids and lignin. Efficient fractionation of black liquor can be used to isolate these compounds and recycle the pulping chemicals. The present work discusses the fractionation of industrial black liquor by a sequence of nanofiltration and bipolar membrane electrodialysis units. Nanofiltration led to retention of the majority of lignin in the retentate and to a significant concentration increase in low-molecular-weight carboxylic acids, such as formic, acetic, glycolic and lactic acids, in the permeate. Subsequent treatment with bipolar membrane electrodialysis showed the potential for simultaneous recovery of acids in the acid compartment and the pulping chemical NaOH in the base compartment. The residual lignin was completely retained by the used membranes. Diffusion of acids to the base compartment and the low current density, however, limited the yield of acids and the current efficiency. In experiments with a black liquor model solution under optimized conditions, NaOH and acid recoveries of 68-72% were achieved.

Keywords: biorefinery; bipolar membrane electrodialysis; black liquor; carboxylic acids; lignin; nanofiltration.

Figure 1

Schematic of membrane configuration and separation mode in the EDBM membrane stack.

Figure 2

Power over treatment time for the batch EDBM treatment of BL permeate, U_cell = 10 V (constant).

Figure 3

Development of carboxylic acid concentrations and pH value over the course of the EDBM experiment with BL permeate. (a) Salt compartment; (b) acid compartment; (c) base compartment. For (a–c): ■ glycolic acid, ● lactic acid, ♦ formic acid, ▲ acetic acid, X pH value. (d) DM (■) and ash (■) content measured in the initial and final samples from the three compartments.

Figure 4

Power over treatment time for the batch EDBM treatment of BL model solution, U_cell = 10 V (constant). The red circles mark the points of NaOH addition.

Figure 5

Development of carboxylic acid concentrations and pH value over the course of the long-term EDBM experiment with BL model solution. (a) Salt compartment; (b) acid compartment; (c) base compartment. For (a–c): ■ glycolic acid, ● lactic acid, ♦ formic acid, ▲ acetic acid, X pH value. (d) DM (■) and ash (■) content measured in the initial and final samples from the three compartments.

Figure 6

Current efficiency development during the EDBM experiment with BL permeate (●) and BL model solution (●).

Figure 7

Yields of total acids in the acid compartment for the EDBM experiment with BL permeate (●) and BL model solution (●).