Study On Optimal Design Of Reaction Process Based On Gibbs Free Energy Minimization Method

Chemical reaction process is a major part of chemical engineering process. Optimal problem of chemical reaction process is an important content of chemical process system engineering all the way. Optimal design of chemical reaction process can provide theoretical basis for developing reaction process. It is an instant requirement for sustainable chemical industry.The major objective of this paper is optimal design of energy coupling for methane partial oxidation to synthesis gas and the closed loop process of polysilicon with SiHCl₃ method. Focusing on this topic and starting from the characteristics of reaction themodynamics, a two-tier optimal model for energy coupling is built for containing endothermic reactions and exothermic reactions simultaneously, that is, on the inner level, equilibrium composition is obtained by solving minimization of Gibbs free energy, and on the outer level energy coupling could be achieved by changing feed quantity. Optimal design of reaction technology is carried under different temperature, pressure and feed ratio. The majar contents of this work are as follows:1. Study on energy coupling technology during the reaction of partial oxidation of methane to synthesis gasEnergy coupling technology (that is, heat effect of system is zero at equilibrium) during the reaction of partial oxidation of methane to synthesis gas, contained steam-carbon dioxide reforming, is investigated. The results indicate that effect of temperature and feed O₂ on reaction heat is obvious. Energy coupling could always be achieved by optimizing O₂ content in the feed through fixing the others, it is also found that feed O₂ could react completely at equilibrium and subsequent separating technology is not influnced. The H₂/CO ratio at equilibrium is mainly determined by feed H₂O/CO₂ ratio, but it has little relation to oxygen content. Furthermore, the relationship among variables and carbon elimination conditions are also obtained when achieving energy coupling. The thermal neutrality experimental result in quartz tube reactor and the result in fluidized bed reactor are similar to this paper, respectively.2. Study on the closed loop process of polysilicon with SiHCl₃ methodThe closed loop process of polysilicon with SiHCl₃ method includes three reaction subsystems. They are SiHCl₃ reduction subsystem, SiHCl₃ synthetic subsystem and SiCl₄ conversion subsystem, respectively. The major problems of this closed loop process are high raw material consumption and high energy consumption. Taking the efforts decreased raw material consumption and saved energy as the goal, new production technologies are proposed. The major innovations are as follows:(1) SiHCl₃ reduction subsystemThe traditional SiHCl₃ reduction subsystem was carried through heating by electricity under high temperature, which brought a lot of inconvenience for process design and high enengy consumption. At the same time a lot of byproducts SiCl₄ were produced, which increased raw material consumption. Based on these, Energy coupling achieved by adopting Cl₂ partial oxidation in SiHCl₃ reduction subsystem is proposed, that is, polysilicon production process could be achieved without supplying exterior energy and electricity consumption could be saved. At the same time it is also found that feed Cl₂ amount could react completely at equilibrium and subsequent process is not influenced.A new SiHCl₃ reduction subsystem without producing SiCl₄ is proposed, that is, former and latter of SiCl₄ during the reaction is equal by adding certain SiCl₄ in the feed, which can restrain byproduct SiCl₄ produced. The results indicate that raw materials consumption can be reduced and the selectivity of polysilicon can reach above 97%.A new SiHCl₃ reduction subsystem that both no SiCl₄ produced and without supplying exterior energy is proposed, that is, it can make that former and latter of SiCl₄ during the reaction process is equal by adding certain SiCl₄ in the feed and reaction heat is zero at equilibrium by adopting Cl₂ partial oxidation simultaneously. The results indicate that feed Cl₂ amount could react completely at equilibrium,high temperature and high H₂/ SiHCl₃ are favorable, Cl₂ consumption of per polysilicon is lower under above conditions, the selectivity of polysilicon can reach 93%.(2) SiHCl₃ synthetic subsystemHCl and Si are as raw materials in traditional SiHCl₃ synthetic subsystem. A lot of byproducts SiCl₄ are produced during production process, which increases raw materials consumption. Based on this, a new SiHCl₃ synthetic subsystem without SiCl₄ produced is proposed, that is, former and latter of SiCl₄ during the reaction process is equal by adding certain SiCl₄ in the feed. The results indicate that the selectivity of SiHCl₃ can reach above 98.6%.(3) SiCl₄ conversion subsystemIn SiCl₄ conversion system it is fit for increasing the conversion of SiCl₄ under high pressure and high H₂/SiCl₄ ratio. High selectivity of SiHCl₃ is abtained under lower temperature, pressure and H₂/SiCl₄ ratio.(4) Study on the closed loop process of polysilicon with SiHCl₃ methodThe traditional closed loop process of polysilicon with SiHCl₃ method had a lot of byproucts SiCl₄ produced and high energy consumption. Base on these, a new closed loop technology is procduced. It is composed of two subsystems. They are SiHCl₃ synthetic system without SiCl₄ produced and SiHCl₃ reduction system that both no SiCl₄ produced and without supplying exterior energy, respectively. In this new process no SiCl₄ is produced and energy coupling is achieved by adopting Cl₂ partial oxidation in SiHCl₃ reduction system. So raw material consumption is reduced, and the investment of equipment in SiCl₄ conversion system is avoided. At the same time energy consumption is saved.(5) In order to achieve high-efficiency and rational utilization of SiCl₄ a new SiCl₄ reduction technology of producing liquid polysilicon at superior high-temerature is proposed. The results indicate that above 2500℃the conversion rate of SiCl₄ is close to 100% and the selectivity of liquid polysilicon can reach 99%. Based on these, a new technology to produce liquid polysilicon with SiCl₄ method is proposed at superior high temperature, that is, firstly, SiCl₄ is synthesized with Si and Cl₂ as raw materials. Secondly, energy coupling is achieved by adopting Cl₂ partial oxidation in SiCl₄ reduction system at superior high-temerature. The results indicate that the selectivity of SiCl₄ in SiCl₄ synthetic system is close to 100%. The selectivity of liquid polysilicon can reach 99%. So it can reduce separation system investment and make the process simple.