ABSTRACT
Direct CO2 methylation with toluene, as one of the CO2 hydrogenation technologies, exhibits great potential for the CO2 utilization to produce the valuable para-xylene (PX), but the tandem catalysis remains a challenge for low conversion and selectivity due to the competitive side reactions. The thermodynamic analyses and the comparation with two series of catalytic results of direct CO2 methylation are conducted to investigate the product distribution and possible mechanism in adjusting the feasibility of higher conversion and selectivity. Based on the Gibbs energy minimization method, the optimal thermodynamic conditions for direct CO2 methylation are 360-420 °C, 3 MPa with middle CO2/C7H8 ratio (1:1 to 1:4) and high H2 feed (CO2/H2 = 1:3 to 1:6). As a tandem process, the toluene feed would break the thermodynamic limit and has the higher potential of >60% CO2 conversion than that of CO2 hydrogenation without toluene. The direct CO2 methylation route also has advantages over the methanol route with a good prospect for >90% PX selectivity in its isomers due to the dynamic effect of selective catalysis. These thermodynamic and mechanistic analyses would promote the optimal design of bifunctional catalysts for CO2 conversion and product selectivity from the view of reaction pathways of the complex system.
ABSTRACT
The screening of high-efficiency and low-energy consumption absorbents is critical for capturing SO2. In this study, absorbents with better performance are screened based on mechanism, model, calculation, verification, and analysis methods. The acidity coefficient (pK a) values of ethylenediamine (EDA), piperazine (PZ), 1-(2-hydroxyethyl)piperazine (HEP), 1,4-bis(2-hydroxyethyl)piperazine (DIHEP), and 1-(2-hydroxyethyl)-4-(2-hydroxypropyl)piperazine (HEHPP) are calculated by quantum chemical methods. A mathematical model of the SO2 cyclic absorption capacity per amine (αc) in the amine-based SO2 capture process is built based on the electroneutrality of the solution. Another model of desorption reaction heat (Q des) is also built based on the van't Hoff equation. Correspondingly, αc and Q des of the above five diamines are calculated and verified with the experimental data. The results show that αc of the diamine changes with the increase in the pK a value, and the increase in the pK a value directly leads to changes in Q des. The order of αc of the above five diamines is EDA > PZ > HEHPP > HEP > DIHEP, and the order of Q des is EDA > PZ > HEHPP > DIHEP > HEP. The multiobjective analysis between αc and Q des suggests that it is not advisable to simply pursue a higher αc while ignoring Q des. The compound quaternary system absorbent has a wider range of αc than the single ternary absorbent, which is the direction of absorbent development. This study is expected to strengthen absorbent screening for the amine-based SO2 capture process from flue gas.
ABSTRACT
Ethylene glycol production has been significantly augmented in recent years due to its various uses. However, conventional oil and coal-based ethylene glycol routes are forcing severe obstacles in production cost and pollutant emissions. Biomass is regarded as a potential contributor to cleaner and sustainable development of the energy sector. Herein, an efficient biomass-to-ethylene glycol (BtEG) process is firstly proposed and analyzed by the simulator, Aspen Plus software. Its key operational parameters are investigated and optimized after the validation of the established models and simulation results. Results show that the optimal oxygen/biomass ratio and temperature of biomass gasifier are 0.4 t/t and 1300 °C. The optimal reaction temperature, pressure, and H2/DMO ratio of ethylene glycol synthesis reactor are 220 °C, 4.0 MPa, and 4.5 kmol/kmol, respectively. After conducting carbon and exergy analyses, it found that the carbon and exergy efficiencies of this process are calculated as 37.64% and 38.74%.