Browsing by Author "Border, Allistair Fraser"

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    Techno-economic analysis of seaweed biorefinery scenarios for production of high-value products and biogas
    (Stellenbosch : Stellenbosch University, 2020-12) Border, Allistair Fraser; Goosen, N. J.; Gorgens, Johann F.; Stellenbosch University. Faculty of Engineering. Dept. of Chemical Engineering. Process Engineering.
    ENGLISH ABSTRACT: This study addressed the potential extraction of high-value compounds from an underutilized seaweed resource through techno-economic analyses. The South African seaweed sector is currently limited to low-tech, low return activities and its viability is subject to international market fluctuations. Brown seaweeds contain diverse, unique compounds that have high bioactivities. Research has been conducted into the extraction and isolation of these compounds, namely fucoidan, laminarin, and alginate, for use in functional foods. These compounds are high-value and have growing markets. A cascading biorefinery approach was applied to Ecklonia maxima, a species of brown seaweed native to South Africa, in order to maximize the inherent value of all compounds in the biomass through a fractionation process.Multiple biorefinery scenarios were developed for the production of products that contain fucoidan, laminarin, and alginate for sale in the functional food market. The process utilized novel green technologies to extract and purify target compounds in order to develop a feasible and sustainable business case. Collaborative experimental studies were conducted that investigated the effectiveness of these technologies to extract target compounds, and the data generated in those studies were used as a basis for techno-economic analyses in this study. Primarily, three potential extraction technologies were compared: enzyme-assisted extraction (EAE), ultrasound-enzyme-assisted extraction (UEAE), and subcritical water extraction (SWE). Experimental data generated at bench scale in four collaborative studies (two for SWE) were integrated into process mass and energy balances in this study for realistic process simulation, which was performed in Aspen PlusTM using the Aspen Simulation Workbook approach. The cascade valorization approach incorporated waste minimization by utilizing process waste for energy from biogas, for which experimental data were generated in this study and integrated into simulations. A processing capacity of 3000 ton p.a. fresh seaweed was used based on expected supply from an industry partner, and the impact of scale on profitability was assessed within harvesting allowances of the seaweed. At this scale, production rates of 34-72 ton p.a. high molecular weight product (27-45 wt.% fucoidan and laminarin), 43-58 ton p.a. low molecular weight product (10-17 wt.% fucoidan and laminarin), and 74-99 ton p.a. purified alginate were achieved for respective scenarios. The greatest capital expenses lay within downstream drying, with the falling film evaporator and spray dryer comprising 70-75% of the equipment purchase cost. Discounted cash flow economic models were developed from mass and energy balances, using the minimum fucoidan selling price (MFSP) as a profitability metric, for a 25% nominal IRR. Experimental design (ED) data surrounding the respective extraction technologies, provided by collaborative experimental studies, were integrated into Aspen Plus simulations and an MFSP for each data point was calculated. Optimal processing conditions for the respective extraction technologies were then determined by applying response surface methodology to the ED data sets and corresponding sets of MFSP’s. All experimental design optimization results of respective datasets yielded minimum MFSP’s that were attractive relative to market price data of fucoidan, for a 25% nominal IRR. Economically optimized simulation MFSP results were compared to the minimum MFSP obtained in the ED data provided for each specific scenario, to determine whether or not the predicted regression model had successfully aided in an improved design. This was the case for EAE ($184/kg MFSP, 20% reduction from ED minimum MFSP), UEAE ($111/kg MFSP, 49% reduction from ED minimum MFSP), and SWE 1 cases ($119 MFSP/kg, 30% reduction from ED minimum MFSP). The result of the SWE 2 validation run ($209/kg MFSP) was in-between the minimum and lower-quartile MFSP for the SWE 2 ED data MFSP results ($189-218/kg). Economically optimal extraction conditions of EAE, UEAE, and SWE 1 scenarios were identical to optimal extraction conditions to maximize the recovery of fucoidan. Process sensitivity analyses of Aspen Plus process parameter design assumptions identified the proportion of fucoidan recovered by the 3 kDa ultrafiltration unit as the most sensitive process parameter for all scenarios, where a 15% decrease in recovery of fucoidan in the Low-MW fractionation product resulted in MFSP’s of $222/kg for EAE, $122/kg for UEAE, $145/kg for SWE 1, and $230/kg for SWE 2. Fractional factorial experimental designs quantified significant interaction effects caused by the cascade nature of the process, with the largest observed span in MFSP for these interaction sensitivity analyses being $138/kg for the SWE 2 scenario, between the +10% fucoidan extraction and +10% 3kDa and 10kDa UF fucoidan recoveries ($143/kg) versus the -10% fucoidan extraction and -10% 3kDa and 10kDa UF fucoidan recoveries ($281/kg). Economic sensitivity analyses of the optimized scenarios were conducted using single-variable and Monte Carlo analyses to identify and quantify major risks of 30 primary economic model assumptions. Monte Carlo analyses resulted in market-competitive MFSP’s of below $203/kg for EAE, $123/kg for UEAE, $138/kg for SWE 1, and $214/kg for SWE 2 at an 80% probability and therefore were concluded to be at an acceptable level of investment risk. The greatest sources of economic risk to the MFSP of all scenarios, for economic parameters investigated, were the sales price of alginate as a co-product (27.8% average contribution to risk), feedstock cost (10.8% average contribution to risk), Rand-Dollar exchange rate (9.5% average contribution to risk), and the inflation rate (8.0% average contribution to risk). Based on profitability results and robustness to changes in process parameter performance estimations and economic parameter risk analysis, the UEAE scenario was recommended as the best performing extraction technology at optimal conditions. Biogas production estimates were made from experimental data and included in Aspen Plus simulations. A central composite design was used to explore the effects of temperature, solids loading and ISR on biogas production for both raw seaweed and hydrolyzed seaweed residue. Optimal biogas production results were estimated as 256 ml CH4/g VS and 259 ml CH4/g VS for raw seaweed and hydrolyzed residue respectively (62% CH4 and 70% CH4) based on RSM results. Pilot scale runs were conducted in order to validate the results achieved at bench scale, with similar quantities but lower qualities being achieved (88% methane yield of bench scale tests for both feedstocks). The inclusion of anaerobic digestion as an ancillary process supplied approximately 6-7% of the required process steam for the respective scenarios. The process inclusion increased MFSP between 1.3-1.9%, and therefore could be included as a means of waste reduction without major impact on MFSP, however the system did not pay for itself nor contribute significantly to supplying the energy demands of the process.