4 - CONCLUSIONS
Pretreatment test were conducted following a central composite design to extract optimal points and mathematical models for glucose yield and inhibitors concentration.
A set of points (X, Y and Z) were chosen, each one with different characteristics (maximum yield, minimum inhibitors, minimum cellulose degradation).
All the points tested responded with values in the interval of those predicted by the mathematical model, with the exception of the glucose yield of Z, which exhibited a value 12% above the expected.
The Z point also proved itself to be shortly after the border of the cellulose-degrading area; if we take a look at the Z cellulose percentage, we found it to be 34,47%, while that of the material as it is, was found to be 38,25%. This means that, at 240 °C, even a treatment of 30 minutes starts to throw cellulose into the liquid phase, losing glucose and, thus, ethanol. The cellulose-degrading area may then actually be a little bigger than expected, considering that the condition number 4 (223 °C / 104 min) already shows 3% less cellulose than the original material. The optimal yield point of the model (W) is then being “pushed” left, almost coinciding with X (see Fig. 3.20).
Y is the best “all-round” point; even if X and Z as well are above the limits, the literature show that even if fermentation succeed, it starts to be slowed down17 at inhibitors concentration under the limits reported above. So Y, having the best post-treatment cellulose fraction, manage to provide an absolute glucose content a little lower than X, but working in a totally safe area for fermentation.
4.1 - Observations
During the pretreatment phase, the reactor temperature controller proved to be very slow in reaching the set point, generating a “heating-up time” comparable to the actual residence time.
Fig. 4.1 - Thermal history of pretreatment condition 2 (223 °C / 26 min).
What happens, in pretreatment with high temperatures and short times, is that the heat exchanged by the biomass before reaching the set point may be actually higher than that in the pretreatment, making the
“effective” residence time longer than those inputted in the model. The relevance of this heating phase decrease with the increasing of the residence time or at lower temperatures.
On the other side, when the temperature is on the very low side, the control presents a badly oscillating trend, making hard to establish a real treatment temperature.
Fig. 4.2 - Thermal history of pretreatment condition 3 (137 °C / 104 min).
0 50 100 150 200 250
0 10 20 30 40 50 60 70 80 90
Temperature [°C]
Time [min]
0 20 40 60 80 100 120 140 160
0 20 40 60 80 100 120 140 160
Temperature [°C]
Tempo [min]
Further, pretreatment with temperatures above 200 °C, heavily changed the physical structure of the material, giving it sand-like aspect. What happens is that, due to the high humidity content left in the solid after filtering, the particles tends to form small “rocks” of biomass.
This may affect the hydrolysis, since the surface area available to enzymes, is reduced by this aggregation; it may be favorable to process the pretreated biomass with milling to assure it maintains the powder-like aspect.
Fig 4.3 - Straw pretreated above 200 °C.
The furfural model, apart from being the worst in terms of model fitting, has also a different trend from the others (see Fig. 3.16 - 3.19); in fact, it presents a peak in central temperature ranges while the other inhibitors tend to increase with temperature. This is due to furfural degradation in presence of glucose at temperatures higher than 200 °C, thus freeing this range of temperature from the presence of the inhibitor20.
4.2 - Further Developments
In this work, time and temperature were tested as they are the primary parameters to affect pretreatment, but future works could test various gas types and pressure used to pressurize the reactor.
Moreover, due to the uncertain effect of some inhibitors, in particular lactic acid, and also the interactions between them, it will be good to use, as a response variable for the model, not the inhibitors concentrations but straight the ethanol yield after fermentation.
All the liquid characterization phase would then be by-passed, with the fermentation directly used to adjust the pretreatment conditions.
Of more interest could be the development of technologies to be able to ferment C5 sugars; since the degradation of hemicellulose generates a liquid (of pretreatment) rich of xylose, instead of discarding this fraction could generate more ethanol, thus dramatically increasing the ethanol yield.
