Solids Content of Black Liquor Measured by Online Time-Domain NMR

Ekaterina Nikolskaya,Yrjö Hiltunen,Petri Janhunen,Mikko Haapalainen

doi:10.3390/app9102169

Abstract

Black liquor, a valuable by-product of the pulp production process, is used for the recovery of chemicals and serves as an energy source for the pulp mill. Before entering the recovery unit, black liquor runs through several stages of evaporation, wherein the solids content (SC) can be used to control the evaporation effectiveness. In the current study, the time-domain nuclear magnetic resonance (TD-NMR) technique was applied to determine the SC of black liquor. The TD-NMR system was modified for flowing samples, so that the black liquor could be pumped through the system, followed by the measurement of the spin-spin relaxation rate, R2. A temperature correction was also applied to reduce deviations in the R2 caused by the sample temperature. The SC was calculated based on a linear model between the R2 and the SC values determined gravimetrically, where good agreement was shown. The online TD-NMR system was tested at a pulp mill for the SC estimation of weak black liquor over seven days without any fouling, which demonstrated the feasibility of the method in a harsh industrial environment. Therefore, the potential of the TD-NMR technology as a technique for controlling the black liquor evaporation process was demonstrated.

Highlights

Black liquor is a major by-product of chemical pulp production [1]
We investigated samples of weak, intermediate, and half-strong black liquors that were collected at different process stages in a pulp mill
A sample of each black liquor was pumped into the magnet system once, and the relaxation rates

Summary

Introduction

Black liquor is a major by-product of chemical pulp production [1]. It helps to establish economically effective pulp mills, because during the chemical recovery cycle, black liquor is used as an energy source for the pulp mills, as well as the source for inorganic process chemical recovery and reuse [1,2,3,4].In the case of the kraft process of turning wood into wood pulp, the cellulose fibers are separated from other components during digestion in aqueous solutions of sodium hydroxide and sodium sulfide at elevated temperatures and pressure [1,2]. Black liquor is a major by-product of chemical pulp production [1]. Expended cooking liquor is called black liquor, and it contains organic and inorganic compounds, as well as water [1,5,6]. The chemical composition of black liquor is dependent on the origin of the raw fiber material and determines its physical properties. These properties include density, viscosity, rheological behavior, calorific heating value, and boiling point rise [1].

Methods

Results

Discussion

Conclusion