Study On Catalytic Oxidation Of Lignin By Nano-CuO In A Rotating Packed Bed

With the rapid economic and social growth,the requirement for fossil resources is growing.Lignin is the only renewable natural organic matter that can directly prepare aromatic compounds and is regarded as an ideal substitute for fossil resources.A large amount of lignin produced by the paper industry as by-products per year,but due to its complex structure,diverse products,and poor reactivity,only about 5%of lignin by-products are used.Catalytic wet aerobic oxidation(CWAO)is one of the effective technologies to realize high-value utilization of lignin.However,the stirred tank reactor is often used in the traditional oxidation process,which is limited by its mass transfer and mixing performance,resulting in low yield and long reaction time in the current oxidation process.In this work,nano-CuO was used as a catalyst in the wet aerobic oxidation of sodium lignosulfonate.Because Higee could enhance gas-liquid mass transfer,the rotation packed bed was carried as an experimental device in order to realize the intensification of this reaction.The effects of various operating conditions in a STR or RPB on the yield of single phenolic compounds were studied systematically.Additionally,the kinetics of catalytic oxidation reaction of lignin carried out on nano-CuO was studied,providing scientific support for industrialization.Main conclusions are described briefly below.(1)The oxidation process of sodium lignosulfonate catalyzed by the nano-CuO catalyst in the STR was researched.The effects of reaction time,the partial pressure of oxygen,reaction temperature,lignin concentration,rotation speed,NaOH concentration,kind and content of catalysts on the yield of aromatic aldehyde were studied systematically.The optimal operating conditions were obtained as follows:5 wt.%CuO-B catalyst,sodium lignosulfonate concentration of 50 g/L,reaction temperature of 150?,oxygen pressure of 0.4 MPa,stirred speed of 800 rpm,NaOH concentration of 2.0 mol/L and reaction time of 30 min.Under these operating conditions,the yield of vanillin can reach 1.88%(corresponding to 48.5%of G-lignin content),and the yield of syringaldehyde can reach 4.46%(corresponding to 42.2%of S-lignin content).(2)The kinetic model of catalytic oxidation of sodium lignosulfonate carried out on CuO-B catalyst in the STR has been studied experimentally from 0.2 to 0.6 MPa(oxygen pressure),30 to 60 g/L(lignin concentration)and 130 to 150? respectively.The experimental data proved that the catalytic oxidation of lignin is second-order series parallel reaction.The kinetic parameters were obtained through the fitting.The activation energy for the two steps of oxidation reaction was 30.5 kJ-mol-1 and 19.6 kJ-mol-1,respectively.The oxidation of G-lignin was regarded as a fast reaction,with the reaction rate controlled by oxygen mass transfer.The validated reaction kinetic equations are:dcG/dt=632.7e-3664/T CGPO[mol·L-1.min-1]dcV/dt=632.7e-3664/T CGPO-9.7e-2354/T CVPO mol·L-1·min-1](3)Based on the above study,the oxidation process of sodium lignosulfonate catalyzed by nano-CuO catalyst carried out in an internal circulation RPB was researched.The effects of reaction time,rotation speed and pressure of oxygen on the yield of aromatic aldehyde were studied.The optimal operating conditions were obtained as follows:sodium lignosulfonate concentration of 50 g/L,reaction temperature of 150?,oxygen pressure of 0.4 MPa,catalyst concentration of 2.5 g/L,rotation speed of 1000 rpm,NaOH concentration of 2.0 mol/L and reaction time of 40 min.Under these operating conditions,the yield of vanillin can reach 1.91%(corresponding to 49.5%of G-lignin content),and the yield of syringaldehyde can reach 3.78%(corresponding to 35.8%of S-lignin content).Comparing the above two processes,it was discovered that the time of reaching the maximum yield of vanillin was shorter in RPB process under the same reaction conditions,and the yield increased slightly,but the yield of syringaldehyde was lower.