Research On The Mechanism Of The Plant Fiber Components Separation Strengthened By The Density Fluctuation Of Supercritical CO₂ Combined Medium

Plant fiber is a kind of carbon sink type natural polymer material with cellulose,lignin and hemicellulose as the main components.Due to the complex micro-nano structure,it has certain anti-biological erosion characteristics and anti-chemical corrosion characteristics,its large-scale resource utilization and further high-value utilization are greatly restricted.In this paper,supercritical CO₂（sc CO₂）,ethanol（Et OH）and water（H₂O）are configured as combined-media（SCOM）.According to the characteristic that SCOM density can fluctuate with the change of the temperature,a method（PFCS-S）was proposed to promote the separation and extraction of hemicellulose and lignin in plant fibers by using the characteristic of SCOM density fluctuation.The method can not only effectively strengthen the separation efficiency and decomposition effect of plant fiber components,but also effectively regulate the microstructure of the solid-phase product formed during the separation process.The work of this paper laid the theoretical and technical equipment foundation for the large-scale application of supercritical CO₂ combined-media in the field of plant fiber resource treatment.In this paper,the correlation between SCOM density and temperature-pressure is explored through calculations and experiments,the interaction characteristics between the separation products of plant fiber components and SCOM are explored in combination with molecular dynamics simulation analysis.The TPC fitting equation is obtained by fitting the"temperature-pressure"correlation curve（TPC）of SCOM during the heating process,and the coordinate system（TPD）is constructed with temperature,pressure and SCOM gas phase separation density as the coordinate axes.Combining the TPC fitting equation,the equation of state and the fugacity coefficient equation,the SCOM gas phase density and phase equilibrium point distribution map are drawn in the TPD coordinate system.The calculation results show that there are two basic phases in SCOM,which are liquid phase and gas phase.There is a sub-layered state based on density gradient in the gas phase.The sub-layer includes the SCOM layer composed of sc CO₂,Et OH and H₂O and a single sc CO₂ layer.Taking hemp stalks and eucalyptus as the research objects,through control experiments,it is found that when the SCOM was used to disposes of the two plant fibers under the conditions with T=475.24K and p=16.70MPa,which belonged to the state parameter of phase balance point calculated,The chemical structure,microscopic morphology and thermal stability of the solid-phase product present typical gradient distribution along the longitudinal direction of the reactor which indicates that the disposal capacity of SCOM gas relative to plant fibers is different based on the density gradient,and verifies the inference that the gas phase of SCOM is stratified based on the density gradient.Based on the extraction rate of xylan and the yield of Klason lignin,combined with molecular dynamics simulations,it is found that in the SCOM system,sc CO₂ can assist the separation of plant fiber components to move to the SCOM liquid phase from plant fiber through the SCOM gas phase.A method for enhancing the separation and removal of hemicellulose and lignin in plant fibers（PFCS-S）by using the SCOM density fluctuations mechanism is proposed.The separation product mass transfer mode of“medium circulation–product one way”with the“on-off”feature was realized through designing the linear fluctuation path of temperature parameters.By integrating the TPC fitting equation,equation of state and Helmholtz’s calculation function,and combing the Carnot cycle,the SCOM dissociation intensity coefficient（SCOM DIS）is proposed,and the SCOM ISI liner superposition（SCOM ISLS）model was constructed.In the SCOM ISLS model,SCOM has the ability to do external work through temperature fluctuations,for the unidirectional migration of solute in the SCOM through the MCSU mass transfer method occurring synchronously,this feature can enhance the efficiency and effect of the transferring.Based on the PFCS-S method,a“hemicellulose-lignin”stepwise separation process was designed.Taking the hemp straw fiber treated by continuous flash explosion and the eucalyptus fiber treated by continuous flash explosion as the research object,the organic solvent step-by-step separation process which was constructed by the acid ethanol separation hemicellulose method and high temperature ethanol separation lignin method as the control process.The effect and mechanism of PFCS-S strengthening the stepwise separation of“hemicellulose-lignin”was explored through control experiments,In the hemicellulose separation process,with the improvement of SCOM ISI,the extraction rate of xylan by PFCS-S has shown a linear increase trend.Compared with the separation of hemicellulose by acidic ethanol,when PFCS-S uses a combined medium composed of sc CO₂ and H₂O to separate hemicellulose,the extraction rate of xylan increased from 1.78g per 100g hemp straw raw and2.54g per 100g eucalyptus raw by acid ethanol method to 9.49g per 100g hemp straw raw and10.8g per 100g eucalyptus raw by PFCS-S,respectively.Similarly,the yield of Klason lignin also shows a linear increase with the increase of SCOM ISI.Compared with the separation of lignin with high-temperature ethanol,when PFCS-S uses a combined medium formed of sc CO₂,Et OH and H₂O to separate lignin,The yield of Klason lignin was increased from21.40%and 24.00%by the high-temperature ethanol method to 52.03%and 78.03%by PFCS-S,respectively.In the lignin separation process,compared with the medium environment provided by the high-temperature ethanol solution,the medium environment provided by SCOM can effectively prevent the condensation reaction between the lignin fragments formed during the lignin separation process,and strengthen the ability to solve the weaker S’type lignin.Compared with the lignin from two kinds of plant fibers based on high temperature ethanol,the ratio ofβ-O-4 bonds in the lignin from two kinds of plant fiber based on the SCOM increased from 27.11%and 51.01%to 33.04%and 80.43%,the ratio of S’-type lignin to S-type lignin in both lignin from two kinds of plant fiber based on the SCOM increased from 1.88%and 0.43%to 3.69%and 5.79%,the mass average molecular weight of the lignin from two kinds of plant fiber based on the SCOM increased from 1705 g/mol and2622 g/mol to 2572 g/mol and 3946 g/mol,the thermal stability of both lignins from two kinds of plant fiber based on the SCOM has been greatly improved.The lignin obtained by SCOM can be used to prepare the high-quality polyether polyols.The effects of PFCS-S on the microstructure characteristics of the solid phase products formed during the stepwise separation of two plant fibers were explored through a variety of characterization and testing methods.Comprehensive the characterization results,with the linear increase of SCOM ISI,the lignin content and surface lignin concentration of the two plant fibers linearly decreased,the absolute number and relative content of surface hydroxyl groups linearly increased,the specific surface area linearly increases,the crystallinity and grain size increased,the residual carbon rate and the maximum thermal decomposition rate are effectively increased.The above results show that although the initial component content of the two plant fibers is different and the cell wall structure is quite different,PFCS-S can effectively control the microstructure of the solid phase product formed during the separation of the two plant fibers.The solid phase product finally obtained has great application potential in the field of carbon materials.