No abstract

No abstract

No abstract

Butanol, produced via traditional acetone-butanol-ethanol (ABE) fermentation, suffers from low yield and productivity. In this article, a non-ABE butanol production process is reviewed. Clostridium pasteurianum has a non-biphasic metabolism, alternatively producing 1,3-propanediol (PDO)-butanol-ethanol, referred to as PBE fermentation. This review discusses the advantages of PBE fermentation with an emphasis on applications using biodiesel-derived crude glycerol, currently an inexpensive and readily available feedstock. To address the process design challenges, various strategies have been employed and are examined and reviewed; genetic engineering and mutagenesis of C. pasteurianum, characterization and pretreatment of crude glycerol and various fermentation strategies such as bioreactor design and configuration, increasing cell density and in-situ product removal. Where research deficiencies exist for PBE fermentation, the process solutions as employed for ABE fermentation are reviewed and their suitability for PBE is discussed. Each of the obstacles against high butanol production has multiple solutions, which are reviewed with the end-goal of an integrated process for continuous high level butanol production and recovery using C. pasteurianum and biodiesel-derived crude glycerol.

Economic realities for the rising industrial biofuel production have changed substantially during the low oil price period starting in the mid 2010's. Increased competition requires the sector to increase productivity through the reduction of low-value by-products and full utilization of all value and energy stored in their respective feedstock. Biodiesel is produced commercially from substrates such as animal fat and vegetable oil, generating approximately 10 wt% crude glycerol as its main, currently underutilized, by-product. This crude glycerol is contaminated with catalyst, soap, free fatty acids, glycerides and methyl esters; hence only a small fraction enters the existing glycerol markets, while the purification costs for the majority of crude glycerol are simply too high. However, this presents a unique opportunity to generate additional value. One technical possibility is to use crude glycerol as a carbon source for butanol production, a compound of higher value and energy, a potential additive for gasoline and diesel fuels and bulk chemical commodity. Conversion facilities could be co-located with biodiesel plants, utilizing established infrastructure and adding significant value and productivity to the existing biodiesel industry. This review focuses on the current activities geared towards the bioconversion of crude glycerol to butanol.

Clostridium autoethanogenum is capable of converting C1 carbon sources such as CO and CO2, as well as C5 carbon sources such as xylose, into various products, rendering it suitable for syngas conversion. However, pure gas fermentations generally have low volumetric productivity caused by low cell mass concentrations, resulting from low specific growth rates and low gas–liquid mass transfer. The strong dependency on gas–liquid mass transfer causes data generated in serum bottles to often deviate from experiments in stirred tank reactors. This study therefore characterizes growth and product formation in both the serum bottle and stirred tank reactor, while investigating a sequential and simultaneous feeding of xylose and gaseous carbon substrates. The ratio of the product was shown to be independent of the initial xylose concentration in serum bottles, while in stirred tank reactor experiments the product ratio changed under xylose-limited conditions. The product ethanol caused inhibiting effects which could be quantified in a kinetic model. The comparison of feeding strategies showed clearly that a fed-batch process with simultaneous xylose and CO feeding led to higher CO conversion when compared to CO conversion in a sequential cultivation strategy. Carbon can be directed toward acetate formation via fed-batch fermentation under C-limited conditions. Moreover, the combined feed of xylose and CO is an advantageous method to significantly enhance gas conversion in comparison to sequential feeding.

In this study, the direct conversion of wet oleaginous yeast biomass to fatty acid methyl esters (FAME) using base transesterification in the presence of an ionic liquid was optimized. The ionic liquid, 1-ethyl-3-methylimidazolium ethylsulfate, was used to facilitate this process and improved the yields of FAME transesterified directly from wet biomass using potassium hydroxide (KOH) as a catalyst. Factorial screening was first used to identify critical factors affecting the transesterification yield, and subsequently, response surface methodology was employed to study the interaction of methanol, KOH, and temperature on reaction yield. The optimized conditions for dried biomass were found to be 16.9 g methanol/g yeast, 0.056 g KOH/g yeast, 2 g [C2mim­[EtSO4]/g yeast, and 65 °C, which yielded 97.1% conversion of the maximum FAME yield in only 2.5 h. The optimized system was further studied to observe the reaction profiles and FAME yield over time from both dry yeast and fresh wet yeast biomass containing varying degrees of water (from 65 to 80 wt %). The ionic liquid was found to improve total overall yield of FAME (96.9 ± 0.4%) compared to the negative control without ionic liquid (69.6 ± 5.0%) when wet yeast was used. While all the ionic liquid was recovered from the reaction, it contained only 59.3% of the catalyst, suggesting a heterogeneous catalyst may be more appropriate in future work.

No abstract