ION TRANSPORT BEHAVIOUR THROUGH ELECTROMEMBRANES LEADING TO INDUSTRIAL PROCESS OPTIMIZATION

Abstract

Increasing population, rapid industrialization couple with greed for energy have resulted severe environmental challenges and acute shortage of fresh water. Awareness among scientific community helped in implementation of stringent pollution control board norms to encourage industries opt for reduction, reutilization, and recycling of energy/raw materials, where paradigm shift in process technology is limited. This report takes up two unique industrial challenges: (i) Pulp and Paper Mills, and (ii) Selective recovery of carboxylic acid from fermentation broth and address them by applying electromembrane techniques. In case of pulp and paper mills the chemical recovery section uses lime (CaO) based causticization to convert green liquor (Na2CO3 is the major constituent followed by Na2S and NaOH) to white liquor (NaOH and Na2S) along with precipitation of CaCO3 (lime mud) as solid waste. White liquor is recycled for digestion of wood, while recycling of lime mud needs multistage washing, filtration, drying, and high tempe to regenerate lime (CaO). Use of fossil fuels in kiln releases greenhouse gases making the lime based causticization water/energy intensive beside leaving carbon footprint in the environment. This lime based causticization may be avoided by one-step electroelectrolysis (EED) of green liquor using cation-exchange membrane (CEM) to obtain pure NaOH, the major constituent of white liquor. In case of fermentation broth containing mixture of carboxylic acids pose separation challenges due to their weak nature and pH dependent dissociation. In conventional route series of unit operations involving precipitation, filtration, washings, precipitate dissolution, etc. are adopted resulting solid/liquid wastes besides energy consumption and time. This challenge is also addressed applying electromembrane technique using a mono-selective anion exchange membrane (AEM). As electromembrane based techniques were applied for addressing separately both these problems of two different industries, in Chapter 1 a brief introduction on electromembranes, their classification, characterization techniques adopted are discussed. With a brief outline of the origin of green liquor in pulp and paper mills and its conventional lime based causticization torecycle cooking liquor (white liquor) along with formation of lime mud is described. Recycling of lime mud is water/energy intensive process leaving huge carbon foot print. Thus, based on the chemical composition of green liquor a lime free electromembrane technique is proposed that can convert green liquor to pure NaOH.