A comparative evaluation of design factors on bubble column operation in photosynthetic biogas upgrading

Archishman Bose,Richard O’Shea,Jerry D Murphy,Richen Lin

doi:10.18331/brj2021.8.2.2

Archishman Bose, Richard O’Shea + Show 2 more

Open Access

https://doi.org/10.18331/brj2021.8.2.2

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Journal: Biofuel Research Journal	Publication Date: Jun 1, 2021
Citations: 12	License type: cc-by

Affiliation: University College Cork

Abstract

Studies attempting to optimise photosynthetic biogas upgrading by simultaneous investigation of the bubble column-photobioreactor setup have experienced considerable variability in results and conclusions. To identify the sources of such variation, this work quantitatively compared seven design factors (superficial gas velocity; liquid to gas flow rate (L/G) ratio; empty bed residence time; liquid inlet pH; liquid inlet alkalinity; temperature; and algal concentration) using the L16 Taguchi orthogonal array as a screening design of experiment. Assessments were performed using the signal to noise (S/N) ratio on the performance of CO2 removal (CO2 removal efficiency, CO2 absorption rate, and overall CO2 mass transfer coefficient) and O2 stripping (O2 concentration in biomethane and O2 flow rate in biomethane). Results showed that pH and L/G ratio were the most critical design factors. Temperature and gas residence times had minimal impact on the biomethane composition. The interactive effect between pH and L/G ratio was the most impactful, followed by the interactive effects between superficial gas velocity and L/G ratio and pH on CO2 removal efficiency. The impact of L/G ratio, algal concentration, and pH (in that order of impact) caused up to a 90% variation in oxygen content in biomethane. However, algal concentration had a diminishing role as the L/G ratio increased. Using only the statistically significant main effects and interactions, the biomethane composition (CO2% and O2%) was predicted with over 95% confidence through regression equations for superficial gas velocity up to 0.2 cm/s.

Highlights

This was to ensure that the O2 in the biomethane is derived primarily from stripping of the Dissolved oxygen (DO) of the circulating algal liquid with minimal influence of the O2 released by the active photosynthesising microalgae
The results indicated the important influence of the algal concentration on increasing O2,%,BM at a lower liquid to gas flow rate (L/G) ratio; at a higher L/G ratio, the O2,%,BM was found to be independent of the algal concentration
The L16 Taguchi orthogonal array (OA) as a screening design of experiment was selected as the statistical tool

Summary

Introduction

The two-step photosynthetic biogas upgrading process, employing CO2 removal and subsequent utilisation of CO2 by microalgae, is considered a novel biogas upgrading technology (Meier et al, 2015; Bose et al, 2019). As shown, CO2, as well as H2S are absorbed in a bubble column by a carbonate-rich sodium carbonatebicarbonate buffer solution (pH 9 and above) via the principle governing Equation 1 (Bose et al, 2019), decreasing the pH. The bicarbonate uptake by the algae in a photobioreactor regenerates carbonate in the solution with a corresponding rise in pH (Equation 2). The re-circulation of the regenerated carbonate-rich solution into the bubble column continues the carbonate-bicarbonate cycle. In addition to the biomethane, the cultivated microalgae is a high-value product critical to improving both the economic benefits and sustainability of the biomethane produced (Bose et al, 2020)

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