In this thesis, the isolation and recovery of lignin from black liquor from a kraft mill was investigated, an in-depth polymer characterization was carried out, and finally a method was identified for the possible valorization of lignin from a South African mill for a kraft mill. The SEC data revealed a narrow molecular weight distribution with a low polydispersity index in the lignin sample—all desirable properties for further lignin valorization.
INTRODUCTION
- General Overview of Thesis
- Context of the Research
- Research Questions
- Aims and Objectives
- Layout of Thesis
This study has focused specifically on the black liquor obtained during the kraft pulping process. A stepwise centrifugal recovery and washing procedure was investigated as an improved method for the recovery of lignin precipitated from power mill black liquor.
LITERATURE REVIEW
Chemical Composition of Wood
Lignin
- Background
- Isolation of lignin
The lignin makes up the majority of the mass lignin content found in most biomass materials. Unlike MWL, MWEL and native lignins, Klason lignin is unsuitable for characterization and studies of depolymerization of lignin, as it is not representative of the protolignin found in wood.
The Pulping process
- Introduction
- Kraft pulping
- Black Liquor
At the end of the preparation in the power mill, the pulp is separated and further processed into a desired product. The properties of black liquor depend on the raw materials used for pulping (coniferous wood, hardwood, fiber), the conditions of the pulping process (temperature, duration, pressure, etc.), the type of digester used (batch, continuous) and the treatment of the liquid after processing the pulp.
Lignin Recovery
- Lignin recovery from black liquor
- Precipitation of lignin from black liquor
The solubility of lignin is controlled by the pH, temperature and ionic strength of the black liquor. Lowering the pH of black liquor below a certain critical value (pH ~ 9) results in the precipitation of lignin.
Biorefinery
- Valorisation of lignin
It has been proven that hemicelluloses and cellulose components are volatile fractions (Gergova et al., 1994). Reactivity studies of the gasification reaction have shown that the reaction proceeds as a combination of catalyzed and uncatalyzed reactions (Rodríguez-Mirasol et al., 1993b).
CENTRIFUGAL WASHING AND RECOVERY AS AN IMPROVED METHOD
- Introduction
- Materials and methods
- Black Liquor Characterization
- Lignin Precipitation from Black Liquor
- Analytical Procedures
- Results and discussions
- Black liquor analysis
- Conclusions
Size exclusion chromatography was used to determine the molecular weight distribution of the lignin samples. After repeatability and reproducibility studies on the sample, the moisture content of the black liquor was approximately 85%. The amount of lignin in the first supernatant was further analyzed to obtain a better material balance of the precipitation procedure.
It was found that approximately 6% of the lignin dissolved in the first mother liquor during precipitation. Centrifuging the sample enables a better separation of the colloidal suspension, which is formed as a result of the precipitation of lignin from the black liquor. In their study on the precise measurement of lignin dn/dC, Contreras et al.
SEC curve showing three different methods to determine the molecular weight distribution of the lignin sample.
CHARACTERISTICS OF LIGNIN PRECIPITATED WITH ORGANIC ACIDS AS
- Introduction
- Materials and Methods
- Lignin Isolation
- Characterization
- RESULTS AND DISCUSSION
- Elemental Analysis
- Thermogravimetric Analysis
- FTIR
- CONCLUSIONS
The objective of the contribution was to investigate the thermal behavior of lignin samples precipitated from kraft liquor with three organic acids (formic, acetic and citric acid). An efficient procedure for the precipitation, isolation and recovery of lignin is described in Chapter 3 and a published paper was used (Namane et al., 2015). The sample to KBr ratio, as well as the disc weight, was kept constant for the FTIR samples.
A representative elemental composition of the lignin samples precipitated from different acids is shown in Table 4.1. Thus, the sulfur (sulfites and sulfates) retained during the cooking process could react with the -OH groups in the syringyl and form -SO2H groups in the lignin samples. FTIR was performed to analyze for differences in the functional groups of the different lignin samples obtained (Figure 4.3).
The assignment of the main IR peaks observed in the lignin samples is shown in Table 4.2.
PREPARATION OF ACTIVATED CARBON BY PARTIAL GASIFICATION OF
Introduction
It is readily available, easily isolated and modified, and has the potential to be a feedstock for chemical production (Fitigăue et al., 2013; Kouisni et al., 2012; Norberg, 2012; Gårdfeldt and Svane, 2011). Due to its low cost and limited water solubility, raw lignin has also been studied as an adsorbent for heavy metals in water purification (Šćiban et al., 2011; Guo et al., have shown that lignin can be incorporated into inorganic substrates, such as silica, to produce biocomposites that can act as polymer fillers or absorbers of harmful chemicals, as well as have good electrokinetic ability. The conversion of biomass to activated carbon has been extensively researched and has there have been a number of studies that have shown different procedures and applications that have used biomass waste for the production of activated carbon for functions such as adsorption (García-Mateos et al., 2015; Cotoruelo et al., 2012), catalysis ( Bedia et al., 2009) and electrochemical processes (Berenguer . ., 2015).
Activated carbon from lignin can be prepared by chemical or physical activation (Montané et al., 2005; Rodriguez-Mirasol et al., 1993; Rodríguez-Mirasol et al., 1993a; Rodríguez-Mirasol et al., 1993b). Different activation methods result in different porosities as well as specific surface areas of lignin-based activated carbon. Physical activation, on the other hand, is a two-step process achieved by first carbonizing the raw lignin to obtain carbon, followed by partial gasification of the carbon with carbon dioxide, steam, air, or flue gases (Fu et al., 2013; Carrott et al., 2008).
Factors affecting the gasification rate are carbon porosity, particle size, inorganic material content, temperature and partial pressure of the gasifying agent (Mani et al., 2011).
Material and Methods
- Precipitation of lignin from kraft black liquor
- Carbonisation process for preparation of chars
- Reactivity Studies
- Preparation of activated carbon
- Characterization of lignin, chars and activated carbon
The samples were then allowed to cool to room temperature while maintaining the N2 flow. The carbonized samples were washed with a 2% aqueous HCl solution at 60 °C to remove as much inorganic material as possible, and then also washed with deionized water at a similar temperature. The charcoal samples were then dried in an oven overnight at 80 °C and collected to calculate carbonization yields.
Physical activation experiments were performed by gasifying the chars with CO2 in the same furnace used for the carbonization experiments. After the gasification reaction, the samples were left to cool inside the furnace under an N2 atmosphere. The activated carbon samples were collected and measured to calculate the burn-off, and then characterized.
Morphologies of charred samples were analyzed by scanning electron microscopy (SEM) in a JEOL JSMV 6940 instrument (Malaga, Spain).
Results and Discussion
- Preparation and characterization of chars
- Reactivity Studies
- Preparation and characterization of activated carbon
The contrast in the physical appearance of the characters for all samples (C-HW and C-SW) in this study was interesting. The photomicrographs of the swelling of C-SW samples during carbonization showed spherical particles, while a mixture of the lignin particles was observed in the C-HW samples (however, in smaller amounts than in C-HWAA). Therefore, the activation of the chars by partial gasification with CO2 was performed only on SW samples.
Activated carbon isotherms clearly show that partial gasification with CO2 brought about a significant increase in the volume of adsorbed N2, due to the development of the porous structure. Pore size distributions of letters (a) and activated carbon (b) are compared in Figure 5.7. The increases observed for the diameter of the pore size from the letters on the activated carbon further revealed the higher development of the porous structure with.
The data obtained from the analysis of the porous structure of activated carbon could enable the use of SW samples in several applications, for example, in liquid-phase adsorption, catalysis, and energy storage.
Conclusions
Adsorption capacity of lignin-based activated carbon for the removal of p-nitrophenol from aqueous solutions. Preparation, characterization and application of lignin-based activated carbon from black liquor lignin by steam activation. Removal of paracetamol on biomass-derived activated carbon: modeling the fixed bed breakthrough curves using batch adsorption experiments.
Removal of water pollutants with activated carbon produced from H3PO4 activation of lignin from power black liquor. Activated carbons from lignin: Kinetic modeling of the pyrolysis of kraft lignin activated with phosphoric acid. Centrifugal washing and recovery as an improved method for obtaining lignin precipitated from South African power mill black liquor.
SUMMARY AND DISCUSSIONS
Further discussion on the findings
- Discussion based on PI
- Discussion based on PII
- Discussion based on PIII
Furthermore, there was an improvement in the physical quality of the lignin sample, which was observed at the last filtration step after the centrifugal washing. This was due to a decrease in the colloidal nature of the lignin suspension during the washing steps. Washing of the lignin could also be done in the same system, where spray nozzles could be mounted in the centrifuge.
This will result in a reduction in the use of organic acid in the process. Thermogravimetric analysis of lignin samples in air showed differences in thermal behavior for each acid used for precipitation. The differences observed in the FTIR absorption peaks indicated that the different organic acid lignins were slightly different, implying differences in lignin functionalization.
In the first stage of the experiment, the temperature in nitrogen (and in air for (ii)) was increased only to 500 °C, followed by cooling to temperatures below 100 °C and finally heated again to the final stage. temperature of 900 °C in air for both experiments (i) and (ii).
CONCLUSIONS AND RECOMMENDATIONS FOR FURTHER RESEARCH 105
Lignin isolation was performed by acid precipitation using 6 M sulfuric acid, followed by centrifugation, resulting in the estimated recovery of 83% of the lignin sample. Thermogravimetric analysis (TGA) of the HW and SW lignin samples in air revealed differences in the main degradation region of the TG curve. The strength of the acids appeared to play an important role in the thermal degradation of the lignin samples; that is, the weaker organic acid samples showed greater mass losses compared to sulfuric acid.
Thermogravimetric analysis (TGA) of the HW and SW lignin samples in air revealed differences in the main thermal decomposition region of the TG curve. The strength of the acids appeared to play an important role in the thermal degradation of the lignin samples; that is, the lignin precipitated with the weaker organic acid showed greater mass losses compared to the lignin precipitated with sulfuric acid. The aim of the present work was to investigate the thermal behavior of lignin samples precipitated from kraft black liquor with sulfuric acid and three organic acids (formic acid, acetic acid and citric acid).
Finally, the lignin samples were carbonized and the properties of the products were examined by scanning electron microscopy (SEM). Representation of the TG curves (dry basis), performed in air, for hardwood (left) and softwood (right) lignin samples precipitated with four acids. Sulfuric and formic acid precipitated lignin samples showed higher degradation in the main region of the TG curve, while acetic and citric acid samples showed less.
