Enzymatic saccharification of cellulose: a study of mixing and agitation in an oscillatory baffled reactor and a stirred tank reactor

Abstract

Cellulose saccharification has been shown to be a function of agitation. Here, the effect of agitation by oscillatory mixing in an oscillatory baffled reactor (OBR) has been assessed and contrasted with a stirred tank reactor (STR). After 168 h of saccharification at 200 Wm ¡3 , 91% conversion of the cellulose (»25 g/L glucose) was observed in the OBR, as against 74% conversion (»21 g/L glucose) in the STR. At 120 Wm ¡3 in both systems, the conversion in the OBR was 69% (»19 g/L glucose) within the first 24 h, and 88% conversion (24 g/L glucose) after 168 h. The STR yielded 55% (15.3 g/L glucose) and 67% (»18.6 g/L glucose) within the same time scale respectively, differences of 14 and 21% respectively. At equivalent power density of 10 Wm ¡3 , the two reactors exhibited the same mean strain rates of 6.65 s ¡1 , but as the power densities increased the mean strain rates in the STR became significantly higher than that in the OBR. This observation could be partly responsible for the seemingly better saccharification observed in the OBR. Although agitation is essential for optimum saccharification, the nature of the agitation is perhaps a more important factor owing to shear stresses on enzymes. BackgroundMixing is a physical process that governs the rate of change of a chemical environment. To achieve suitably high ethanol titres during fermentation, adequate levels of mixing are very important. Adequate mixing can increase the overall rate by reducing external mass transfer resistance in the rate-limiting saccharification step. This is even more imperative in the saccharification of cellulosic materials. With the projected decline in petroleum reserves and the attendant global energy crises, cellulosic materials have been recognised as some of the most promising alternative resources to supply our chemical and energy needs sustainably.[1] Cellulose is the most abundant carbohydrate polymer in nature,[2] probably the most abundant organic compound on earth,[3] and as a result has evoked long-term interest as a potential source of plentiful food and energy.[4] Cellulose is a linear homopolymer of anhydroglucose units linked by b-(1-4)-glycosidic bonds, with amorphous and crystalline regions. [5,6] It is hydrolysed into glucose during saccharification by enzyme complexes referred to as "cellulases".The hydrolysis of cellulose, however, is difficult. It is influenced by a number of factors, including the structural characteristics of the cellulosic substrate, the amorphous, easily hydrolysable and recalcitrant crystalline regions, [7] sugar inhibition,[8À11] the quality of the enzyme complex which determines the relative performance of its individual components (i.e. the speed of enzyme action), [9,12À15] enzyme and substrate concentrations,[12,14] temperature,[13] pH,[16] the degree of agitation,[17] and

Kinetics of Cellulose Hydrolysis in an Oscillatory Baffled Reactor

“…There is a clear relationship between shear stress and the extent of cellulose hydrolysis. This had earlier being demonstrated by Ikwebe and Harvey (2015). Although the model slightly underestimated the reducing sugar yields in the 22.5 s -1 ASR level (Average shear rate), it agrees well with the 45 s -1 ASR.…”

“…For example, 91% conversion of the cellulose, representing ~25 g L -1 glucose was observed in the OBR at 200 Wm -3 after 168 h of saccharification, while 74% conversion (~21 g L -1 glucose) was observed in the STR, a difference of 17%. The significantly higher saccharification yields in the OBR were due to reduced shear in the mixing environment compared to the STR (Ikwebe and Harvey, 2015). At 200 Wm -3 the ASR in the OBR was half the ASR in the STR (22.5 s -1 :45 s -1 ).…”

“…The interaction of the oscillating fluid with the baffles generates short-lived vortices resulting in uniform mixing in each of the inter-baffle regions, as each behaves as a stirred tank reactor (Mackley et al, 1990;Ni et al, 2003). The OBR design is described elsewhere (Ikwebe and Harvey, 2015). The oscillatory Reynolds number and the power densities of the OBR were calculated using the following Equations 1 and 2.…”