Fundamental Study Of Chemical Reactions And Mass Transfer In CaC₂ Production

Calcium carbide, produced by the reaction of coke and CaO at high temperatures, was regarded as the mother of organic synthesis industry, and its conversion to acetylene has been the dominant route for the production of polyvinyl chloride (PVC) in China in decades. In recent years, however, the calcium carbide industry was restrained by the advancement of methanol-to-olefins (MTO), the low-cost Middle East ethylene, as well as the Calcium Carbide Industry Conditions of Access (2014) which set the maximum energy consumption and pollution emission. However, the short production route in comparison to the ethylene-based processes still makes the calcium carbide-acetylene route attractive. Therefore, modified or new calcium carbide production processes of low energy consumption and emission, and high throughput have attracted much attention.Since the major drawbacks in the current electric arc calcium carbide production process are the large energy consumption in power generation(greater than 60%), the low mass transfer rate between the two large size reacting solids, coke and lime (CaO), and the limit to enlarge the productivity from the electric arc structure, the newer processes should get rid of electric arc process, rely on thermal heating of powdery reactants which are in close contact and use reactors without complex internal structuresThis requires better understanding on fundamentals in reaction and mass transfer behaviors in the calcium carbide production process. However literatures are very limited on this subject, although the fundamentals are also applicable to the electric arc process. This dissertation studies the chemical reactions and mass transfer during calcium carbide (CaC2) production. The main conclusions are:1. An equilibrium composition model based on the total Gibbs free energy minimization, with the inclusion of the CaC2-CaO eutectic, is built to predict the optimum conditions (including temperature and pressure) of CaC2 production under various conditions. The model simulation shows that the initial C/CaO ratio influences the reaction mechanism. An initial C/CaO ratio of less than 3 (the stoichiometrical ratio) results in a three-stage mechanism while an initial C/CaO ratio of greater than 3 results in a two-stage mechanism,which agrees with the experimental results. The optimum conditions for the electric arc process are the ambient pressure and temperatures higher than 2000 ?, which agrees with the literatures and the industrial process. The optimum conditions suggested for the auto-thermal process are about 0.2-0.3 MPa at 1927-2000 ? and 0.4 MPa or higher at 2100 ?.2. Thermodynamic calculation shows that the different reaction routes reported in the literature are not contradictive with each other and can be attributed to the differences in reaction pressure and temperature. A low pressure promotes the formation of Ca vapor. The dominant mechanism at temperatures higher than 1300 ? is the CaC2 formation directly from CaO and C.3. During CaC2 production, the silicon-bearing minerals (mainly SiO2 and Al2O3·2SiO2·2H20) in the raw materials are directly reduced to SiC or react initially with CaO to form calcium silicates (mainly Ca3SiO5). Ca3Si05 reacts with C to form Ca2SiO4 and consequently forms CaC2 and SiC. The CaC2 formation via Ca3Si05 is slower than that via CaO. The calcium silicates can also be reduced by CaC2 to form SiC. Low-temperature eutectics are formed from calcium silicates and CaC2, which may reduce the purity of CaC2.Calcium silicates inhibit the reaction of C and CaO to CaC2 and cause some side reactions to form Ca vapor. SiC can react with Fe to form ferrosilicons which mainly accumulate in the furnace bottom. During CaC2 production, the aluminum-bearing minerals (mainly Al203·2SiO2·2H20) in the raw materials are directly reduced to SiC and Al2O3 or react with CaO to form calcium aluminates (mainly Ca3Al2O6)- Compared with calcium silicates, calcium aluminates are more difficult to be reduced by C, and the solid products are Al4C3 and CaC2. Calcium aluminates also inhibit the reaction of C and CaO to CaC2, and are much easier to lead to the formation of Ca vapour than calcium silicates, which does not favour CaC2 production. Considering that Al2O3 from kaolinite and calcium aluminates are difficult to be reduced, the aluminum-bearing minerals may possibily accumulate in the furnace bottom or mix with CaC2 to reduce the purity of CaC2.4. CaC2 can be synthesized in the solid state at temperatures below the eutectic temperatures of CaC2-CaO. The mechanism is elaborated by construction of a high-temperature CaC2 crystal structure and a mass transfer model based on the diffusion of ionic defects, which involves diffusion of C-22 and O2- ions from the opposite directions in CaC2 as indicated by DFT calculation. On the macroscopic scale, the mechanism is observed as diffusion of C to CaO, suggesting that a higher activity of carbon-containing feeds would reduce the energy consumption of CaC2 production.5. A physical model is established on pellets made by compressing powdery carbon and CaO. It involves mass and heat transfer as well as chemical reaction. It shows that diffusion of CO in the pellet does not limit CaC2 formation; the chemical reaction is the rate-determining step for pellet sizes of 0.03-0.06 m at environmental temperatures of 1650-1800 ?; heat transfer has little effect on CaC2 formation because the temperature in a pellet reaches equilibrium in less than 9 min.