Supercritical Carbon Dioxide Pretreatment
Carbon dioxide behaves as a supercritical fluid at critical temperature (31 °C) and critical pressure (7.39 MPa.). Supercritical CO2 (SC-CO2) is becoming an important commercial and industrial solvent due to its ability in solvent extraction and its low toxicity and environmental impact. The relatively low operating temperature of the process and the stability of CO2 also allow most compounds to be extracted with little transformations. Supercritical carbon dioxide has the ability to penetrate the crystalline structure of lignocellulosic biomass overcoming the mass transfer limitations encountered in other pretreatments. Additionally, supercritical fluids show tunable properties such as partition coefficients and solubility. In the supercritical carbon dioxide pretreatment process, SC-CO2 is delivered to biomass placed in a high pressure container at a pressure of 1000-4000 psi [111]. Then the temperature of the pretreatment vessel is increased to about 200°C and held for a short period of time, which will allow CO2 to penetrate the biomass at high pressure; it is believed that once dissolved in water, CO2 will form carbonic acid, which catalyzes the hydrolysis of hemicellu — lose. Then the pressure is released resulting in the explosive disruption of the lignocellulosic biomass structure, which will increase the accessible surface area for the cellulases in the hydrolysis step [111].
Supercritical CO2 has been tested on woody biomass as well as non-wood soft biomass forms such as hardwood [112], wheat straw [113], rice straw [114], sugarcane bagasse [115], switchgrass [116, 112], and big blue grass [112].
Kim et al. [117] studied the use of SC-CO2 pretreatment of hardwood aspen and southern yellow pine with varying moisture contents followed by enzymatic hydrolysis. They reported significant enhancements in sugar yields in supercritical CO2 pretreated samples when compared to thermal pretreatments. In another study, Alinia et al. studied the effect of pretreatment of dry and wet wheat straw by SC-CO2 alone and by a combination of CO2 and steam under different operating conditions such as temperature and residence time in the pressure vessels [113]. During these studies they reported that a combination of supercritical CO2 and steam gave the best overall sugar yields [113]. Luterbacher and coworkers have recently investigated the high pressure (200 bar) CO2-H2O pretreatment for a wide variety of biomass forms such as corn stover, switchgrass, big bluestem, and mixed perennial grasses [112]. These pretreatments were investigated over a wide range of temperatures (150-250°C) and residence times of 20 seconds to 60 minutes. During these studies they found that under these operating conditions, a biphasic mixture of H2O-rich liquid phase and CO2- rich supercritical phase coexists and this greatly aids in the pretreatment process. Furthermore, they reported that such biphasic pretreatment produced glucose yields of 73% for wood, 81% for switchgrass and 85% for corn stover [112].
There are a number of attractive features in SC-CO2 pretreatment, which include the low cost of carbon dioxide as a pretreatment solvent, the formation of very little or no inhibitory compounds, high
solid loading, and the use of low temperatures. Nevertheless, the high cost of equipment that can withstand high pressure conditions of SC-CO2 pretreatment is a strong limitation to the application of this process on a large scale. Moreover, the effects on biomass carbohydrate components have yet to be elucidated. Even though SC-CO2 is being investigated by a number of researchers for pretreatments of hardwoods and soft biomass forms with a fairly high degree of success, the whole process has not proven to be economically viable with the high pressures involved being a deterrent. Improvements are needed to implement the process on a large scale.