Optimizing Biogas Upgrading in a Packed Column by Enhancing Mass-transfer Operation

Abstract

Biogas is a gas produced by decomposition of organic matter by microorganisms in an environment devoid of oxygen via hydrolysis, acidobenesis, acetogenesis and methanogenesis. Biogas is mainly made up methane and carbon dioxide, whose proportions vary depending of the substrate used and the condition in the bio-digester. Due to the presence of carbon dioxide, raw biogas contributes to global warming and lowers the calorific value of the utility gas while the trace element hydrogen sulfide, is not only a corrosive gas, but a health hazard to the handlers. This increases storage and handling costs and costs, the costs incurred when seeking medical attention. To its calorific value, the gas is upgraded from one of a modest methane content of 40 to 60% to biomethane of more than 95% methane by sequestrating carbon dioxide and hydrogen sulfide in processes like physical absorption, chemisorption, pressure swing adsorption and membrane separation. The objective of this investigation is to optimizes biogas upgrading in a packed column by enhancing mass transfer of CO2 molecules across the phases. The absorption is enhanced by the chemical reaction between CO2 reacts with dilute aqueous sodium hydroxide solution in the liquid phase. In the absorption, the effects of four non-associating factors on the overall mass transfer coefficient were separately investigated using the ratio of carbon dioxide mole fraction in the gas feed to the column to that of the effluent gas. This was done by varying one factor while keeping the other factors constant and the overall mass transfer coefficient 𝐾𝑦𝑎𝑒, ccalculated. The factors considered were the gas superficial velocity 𝑢𝑦, the solvent superficial velocity 𝑢𝑥, solvent concentration 𝐶𝑥 and carbon dioxide mole fraction in the gas feed to the column 𝑦𝑎. Using the 𝐿934orthogonal array of the Taguchi method of optimization, the column’s optimum operating condition was established. This was achieved by transforming the overall mass transfer coefficient for each trial to the signal-to-noise ratio (𝜓) using the “larger-the-better” objective function. The results showed that the optimum operating condition of posted a signal-to-noise ratio of −1.5539, which is equivalent to an the overall mass transfer coefficient of 0.8444 𝑚𝑜𝑙⁄𝑚3. 𝑠 . 𝑃𝑎. It was also found that the influence of the factors on the response parameter was in the order 𝑢𝑥 > 𝐶𝑥 > 𝑦𝑎 > 𝑢𝑦 where the influence of 𝑦𝑎 and 𝑢𝑦 on the absorption process is insignificant. This trend implies that CO2 flux across the gas-liquid interface is greatly influenced by the effective area 𝑎𝑒, that is a function of turbulence in the liquid phase. The finds here can applied in biogas production units where the column is directly connected to the raw gas line from the digester, and operated with a high liquid-to-gas (𝐿⁄𝐺) ratio. The high ratio would be sufficient in eliminating the

need of chemisorption thereby making the process a purely a physical process while still retaining the benefits of eliminating the health hazard associated with biogas, increase its calorific value and reduce CO2 emission a remedy of global warming.