Avaliação de processos de separação por membranas para concentração e fracionamento de lignina

• Autor(a) principal: Mendes, Sophia Franciscani
• Data de Publicação: 2020
• Tipo de documento: Dissertação
• Idioma: por
• Título da fonte: Repositório Institucional da UFU
• Texto Completo: https://repositorio.ufu.br/handle/123456789/29045; http://doi.org/10.14393/ufu.di.2020.47
• topic: Lignina; Concentração; Membranas; Frações; Retenção; Lignin; Membranes; Fractions; Retention; Concentration; CNPQ::ENGENHARIAS::ENGENHARIA QUIMICA::TECNOLOGIA QUIMICA

Lignin is a by-product of pulp and paper industries that has been generally used for power generation. However, biorefineries based on lignocellulosic materials have sought to develop high value-added renewable products from biomass processing, bringing advantages such as low cost and easy availability of the raw material. One way of recovering the lignin is by means of membrane separation processes. The main goal of this work was to determine the best operational conditions of the membrane filtration process for the concentration and fractionation of lignin present in liquid black liquor fractions. Micro and ultrafiltrations, in dead-end and cross-flow operations, with commercial polymeric and ceramic membranes, in sequential and direct processes, were employed to evaluate the fractionation capacity of different pretreated black liquor fractions, at different pH values, besides of powdered lignin solutions. The obtained permeates were characterized by total solids, soluble solids, lignin concentration, antioxidant potential, colorimetry, 13C NMR, GPC, FTIR-ATR and/or TGA. Microfiltration of the pH 9.0 liquid fraction was performed in dead-end in a 0.22 µm polymeric membrane, with a stabilized flux of 100 L h-1 m-2. The 0.22 µm membrane permeate was sequentially ultrafiltrated in a 5 kDa membrane, also in dead-end, with relatively low stabilized flux, 0.8 L h-1 m-2. Also, in dead-end operation, the liquid fraction was microfiltrated in a 0.05 µm membrane, after centrifugation, with a stabilized flux of 1.3 L h-1 m-2, followed by 10 kDa ultrafiltration, with a stabilized flux of 5.0 L h-1 m-2. Despite the higher flux in the 10 kDa membrane ultrafiltration, solids retention was 17%. In order to increase the permeate flux, a cross-flow ultrafiltration in a 5 kDa membrane was then performed. Although the initial flux increased, there was no significant increase in final cross-flow flux when compared to dead end (0.9 L h-1 m-2 versus 0.8 L h-1 m-2). By comparison, a powdered lignin solution at a concentration of 47 g L-1 was also ultrafiltrate in cross-flow in a 5kDa polymeric membrane. The final flux of this process was 16 L h-1 m-2 with 97% solids retention, and flux difference is probably associated with feed lower solids content. Proceeding in cross-flow operation, a commercial 20 kDa alumina ceramic membrane was tested for liquid fractions at three different pH values. The pH 9.0 and 9.5 fractions had the highest final fluxes, 5.4 and 5.5 L h-1 m-2, respectively, and presented the lowest total solids concentration, the pH 10.0 fraction presented the lowest flux, 2.2 L h-1 m-2 and the highest concentration of total solids. Ultrafiltration of the pH 9.0 fraction had the highest lignin retention among the tested fractions (83%), as well as clarified permeated in relation to the feed. GPC analysis confirmed lignin fractionation with membrane treatment, obtaining lignin with more homogeneous molecular mass in the permeate. The 20 kDa membrane was also tested for ultrafiltration of a 10% lignin powder solution with final flux of 6.7 L h-1 m-2 and 75% lignin retention. Thus, as in the polymeric membrane, the permeate flux for lignin solution is higher than with the liquid fractions, evidencing the influence of other compounds present in these fractions. Hollow fiber membranes were also tested for filtration of 10% lignin powder solution. The stabilized flux for dead-end in alumina hollow fibers was 2.3 L h-1 m-2 and for cross-flow in a composite alumina with dolomite hollow fiber was 10.3 L h-1 m-2. Despite the difference in flux both fibers retained only 13% of solids and presented poor lignin retention. Thus, the use of commercial 20 kDa ceramic membranes proves to be more operationally viable due to the obtained solids flow and retention values. The pH 9.00 liquid fraction is the most promising for lignin concentration due to its relatively high flow and high efficiency for lignin separation.