Analysis Of The Process Of Lignocellulose Pretreatment By ScCO₂ Explosion And Its Effect On The Separation Of Components

Lignocellulose,as a major category of renewable biomass,has not yet been fully utilized.Because of the natural composite barrier structure,the appropriate pretreatment of lignocellulose is important.Due to its strong permeability and no pollution,supercritical carbon dioxide?scCO₂?is used for the extraction of non-polar components of lignocellulose and bioethanol pretreatment.Based on the previous study of steam explosion on biomass,scCO₂ is used as a new gas-phase explosion medium with water vapor,and the lignocellulose can be destroyed by the explosion force during instantaneous high pressure relief of scCO₂.The organizational structure can be destroyed by the effect of pretreatment.It is important to analyze the mechanism of scCO₂ explosion pretreatment and to establish the mass transfer and heat transfer process model,and further to guide the process control and optimization of scCO₂explosion pretreatment of different lignocellulosic raw materials.This paper initially analyzed the physical and chemical processes of the pretreatment of SCS explosion.According to the temperature and pressure changes and the permeation of water and CO₂ in the treatment process,the treatment process can be divided into the material rehydration stage,the heating-up and pressurization stage,the material cooking stage and the instantaneous pressure relief stage.After calculating the energy consumption of each stage,it has been found that low moisture content weakened the treatment effect and high moisture content increased unnecessary energy consumption.The initial moisture content should be at least the sum of the mass of saturated vapor at this temperature and the moisture content of the fiber saturation point?FSP?at a certain temperature to achieve a better explosion effect.Because of large energy consumption required for the scCO₂ pressurization,choosing a lower pressure is beneficial to energy conservation.Secondly,this paper analyzed the physical process of lignocellulose treated by SCS explosion during the instantaneous pressure relief stage and establish the explosion model.Due to the variety of lignocellulose,it required different gas-phase explosive forces for different raw materials.In order to explore the physical mechanism of explosion,the typical parenchyma cells and thick-walled cells in lignocellulose were selected to establish single cell spherical and cylindrical models,respectively.For the parenchyma cells,the polymer-elasticity model was used to describe the stress-strain behavior of the cell wall.The relationship between cell pressure and cell deformity in the parenchymal cells during SCS explosion was analyzed,and the deformation and the critical yield of the parenchyma cells were obtained.The pressure difference indicates that explosion can effectively destroy the structure of parenchyma cells.Based on the elastic mechanical analysis method of thick-walled pressure vessels,the stress condition of thick-walled fiber cells was investigated.It was found that the scCO₂ explosion effect on the fiber cells was insufficient.In the primary wall,the strength of the pitted membrane is similar to the strength of the parenchyma cells and pit is a weak spot on the cells.scCO₂ was released through the pits and had a significant destructive effect on the pit membrane.The scCO₂ explosion pretreatment of beet pulp and eucalyptus were carried out respectively.It was found that the cell structure of the surface of beet pulp was completely destroyed after explosion,and the damage was enhanced with the increase of pressure.The surface of the beech wood was mainly destroyed at the pit.The results of the explosion experiment were consistent with the analysis of the theoretical model,indicating that the model can be applied to the surface structure of lignocellulose.The SCS explosion process is a complex physico-chemical process.Due to the coupling effect of high-temperature water and scCO₂,the degradation of hemicellulose and lignin in the cell wall is enhanced.As a result,the strength of the cell wall was decreased and the effect of SCS explosion was enhanced.In order to study the physico-chemical coupling mechanism of SCS explosion,the sugar beet pulp rich in parenchyma cells with obvious explosion effect was selected and analyzed by scanning electron microscopy,infrared spectroscopy,thermogravimetric analysis,and X-ray diffraction.The increase of pressure,depressurization rate,moisture content and temperature can enhance the destruction of the material morphology during rapid explosion pressure relief.The increase of temperature,pressure and water content contributed to the degradation of components such as pectin and hemicellulose.The optimal treatment conditions for SCS explosion pretreatment of beet pulp are at the temperature of 80?,under the pressure of 14 MPa and with 50%moisture content under rapid pressure relief condition.The components such as hemicellulose and pectin were degraded.After hot water extraction,half of the water-soluble substances were separated from the beet pulp and these substances were mainly hemicellulose and pectin.