Microwave Heat Treatment of Black Liquor for the Degradation of Hemicellulose

Abstract

The largest use of fossil petroleum is the production of fuels. Hence, finding sustainable alter-natives is an important step to reduce the emissions of greenhouse gases. One such alternative is to produce fuel from biomass, such as waste products from agriculture and the forest indus-try. In Sweden, the forest industry is an important part of the economy. The pulp mills pro-duce an excessive amount of a by-product called black liquor, which has a high content of the biopolymer lignin. The company SunCarbon is developing a process to separate the lignin from the black liquor, purify it and recycle the remaining chemicals to the pulp mill. The purified lignin is to be mixed with a carrier oil to a lignin-rich oil that can be used in refineries to pro-duce fuel such as gasoline and diesel.
One of the steps in the process is the heat treatment, which has the purpose to degrade espe-cially hemicellulose, but also lignin, into smaller molecule fractions. A problem with the heat treatment is the corrosion of steel in the reactor vessel, causing the release of metals that pol-lute the product oil and that are difficult to remove. To solve this problem, the idea to use mi-crowave heating has been introduced. Traditional heating methods require high temperature of the reactor vessel to heat up the medium by conduction and convection. Microwaves heat the material directly and thus the prior heating of the reactor vessel is not required. This means that the temperature of the reactor vessel should be lower and enable for the use of Teflon vessels that do not leach metals to the liquor. The direct heating of the material should also enable faster heating rate. In this Master Thesis, microwave heat treatment was investigated and compared to traditional heat treatment in a steel autoclave. Evaluation was made by ana-lysing the degradation of hemicellulose and lignin with chromatographical methods and lignin yield in the subsequent separation by acidulation in the SunCarbon process.
The microwave treatment resulted in degradation of lignin of the same degree as the heating in the autoclave, at temperatures up to 200°C. The microwave had a faster heating rate than the autoclave. When varying the heating time and the time at the target working temperature, the results indicated equal degradation of lignin independent of treatment time. Analysis of sugar content after treating with acid hydrolysis indicates that the degradation of hemicellu-lose was greater in the autoclave for treatment at 200°C. This could be explained by the short-er heating rate or the instability of the pressure in the microwave vessels. The sugar concentra-tions were very similar in material treated with the two heat-treatment methods at lower tem-peratures, where no leakage had occurred, and the treatment time was equal. Neither of the two heat-treatment methods degraded hemicellulose as much as expected, based on previous studies by SunCarbon. This might be due to insufficient temperatures or treatment times, as temperatures above 200°C could not be investigated with the current laboratory microwave equipment. Conclusively, only a short heating time is sufficient to degrade lignin, but to de-grade hemicellulose longer treatment times or higher temperatures are required. The degrada-tion of both lignin and hemicellulose seem to be independent of heat treatment method, indi-cating that the use of microwaves can be as efficient as other heating methods. This was also confirmed by the nearly equal yields of lignin in the subsequent separation process.
When treating black liquor retentate with acid hydrolysis in a pH range of pH 1-3, a substance is formed that precipitate and form crystals when dissolving the mixture with 50% acetoni-trile/water. Analysis indicates that the crystals are composed of oligomers of five monomers, each with the molecular weight 142 Da. Further investigation is required to identify the com-position of the crystals, for example using NMR analysis.

Popular Abstract

Popular Science Summary: Production of biofuel – processing of paper mill by-products to fuel components
A new process is being developed to produce fuel from the paper mill by-product black liquor. In this Thesis, a new heat treatment method was investigated to degrade large molecules in the black liquor to smaller ones, using microwaves.
The production of fuel is the largest use of fossil petroleum. Therefore, finding sustainable alternatives is an important step to reduce the emissions of greenhouse gases that are causing climate change. One such alternative is to use biomass, for example waste products from the agriculture and the forest industry. In Sweden, the forest industry is an important part of the national economy. The pulp and paper mills produce excessive amounts of a by-product called black liquor, which today is used to produce steam for energy production in the mills. However, the black liquor has a high content of the macromolecule lignin, which has the potential to be used for more refined and valuable products. Lignin is one of the main components of wood, providing the wood with its firm structure. The lignin molecules consist of various rings of carbon atoms linked together in complex irregular networks. Breaking down these networks into smaller fragments, the resulting components are similar to molecules found in petrol.
An idea is to use black liquor to produce biofuel similar to gasoline or diesel. SunCarbon is developing a process that can be integrated with pulp mills with the intention to extract the lignin from the black liquor and recycle the other chemicals to the mill. The lignin is separated from the black liquor by a series of processes starting with membrane filtration, after which follows a heat treatment that breaks down lignin and other large molecules into smaller components. It is especially important to degrade hemicellulose, which is another molecule found in wood. Otherwise, the hemicellulose may cause problems downstream in the process. After the heat treatment, carbon dioxide is added, which acidifies the liquor and makes the lignin precipitate. The precipitated lignin can then be separated from the remaining liquid. The lignin is further purified in several steps before it is mixed with an oil that dissolves the lignin. What kind of oil will be used is still a topic of investigation; it could be a fossil or a biobased oil. The formed suspension is then further processed in a petroleum refinery to fuel.
During the process development, some problems have been encountered in the heat treatment. The high temperature causes corrosion of the steel reactor in which the heating of the liquor takes place. The corrosion causes leaching of metals, such as molybdenum, that are difficult to remove in the purification step and that pollute the product oil. To tackle this problem, an idea is to do the heating using microwaves. Microwaves heat up the liquid directly, compared to traditional heating, which requires the reactor vessel to heat up first and then transfer the heat to the liquid. There are two important benefits with the direct heating of microwaves. Firstly, it can enable lower working temperature of the reactor vessel, allowing for the use of Teflon coating in the reactor to avoid leaching of metals to the liquor. Secondly, the heating can be faster since there is no need to heat up a heating medium.
In this Master Thesis, microwave heating was tested and compared to standard heating in an autoclave. The results indicate that microwaves degrade lignin and hemicellulose to the same degree as autoclave heating does. The hemicellulose was not degraded as much as expected, neither with microwaves nor traditional heating. This could be due to a shorter heating time. The microwave oven could not be operated at temperatures above 180°C for a long time, since the high pressure caused leakage. Conclusively, microwave heating appears to work equally well as the autoclave heating, although a microwave equipment that can work at temperatures above 200°C is required for sufficient degradation of hemicellulose.

• author: Rydgård, Maja
• supervisor: Mats Galbe; Linnéa Kollberg; Caroline Ottosson
• organization: Chemical Engineering (M.Sc.Eng.)
• course: KETM05 20201
• year: 2020
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering; Chemistry
• keywords: lignin, biofuel, black liquor, heat treatment, microwave heating, chemical engineering, kemiteknik
• language: English
• id: 9020135
• date added to LUP: 2020-06-24 13:25:11
• date last changed: 2020-06-24 13:25:11