【作者】 刘山当；

【导师】 朱家骅；

【作者基本信息】 四川大学 ， 化学工程， 2004， 硕士

【摘要】 磷石膏是湿法磷酸生产过程的副产品,大量占用土地,严重污染环境,成为实现磷肥工业可持续发展的瓶颈。目前,世界各国在磷石膏的综合利用方面积极开展了大量工作,其中,磷石膏转化制备硫酸铵或进一步生产硫酸钾是实现磷石膏综合利用的有效途径之一。以石膏为原料生产硫酸铵是一种已经工业化的工艺过程,从技术的角度,磷石膏转化制备硫酸铵并无困难,目前主要存在的问题是经济性。随着70年代高浓度氮肥的发展,而石膏制硫酸铵工艺存在设备投资大和能耗高等经济问题,限制了该工艺的发展。因此,改造现有的工艺技术,提高资源与环境技术自身的经济性是最为重要的途径。本文提出利用我国特有的氨水脱碳净化合成氨原料气的过程,同步实现磷石膏转化为硫酸铵,使之成为合成氨的加工过程之一,因而可以大幅度提高磷石膏转化为硫酸铵的经济性。文献中没有发现利用氨水脱碳净化合成氨原料气的过程实现磷石膏转化为硫酸铵的相关报道,还有大量基础性的研究工作需要进行。对本文提出的石膏氨水悬浮液脱碳净化合成氨原料气并制备硫酸铵的新的工艺流程,模拟该体系气-液-固三相条件进行了初步工艺探索试验。初步考查原料配比和浓度对石膏转化率的影响,反应温度对反应速率的影响,以期对适宜的工艺条件有初步的掌握。在实验探索的基础上,通过进一步分析气-液-固三相体系的反应机理和动力学,采用渗透模型,建立了石膏氨水悬浮液<WP=5>吸收CO2的动力学模型,为进一步研究和探索奠定了基础。实验结果表明,氨的利用率不低于碳酸氢铵流程,磷石膏转化率可以达到96%以上,说明该工艺具有可行性。同时发现,反应体系气-液-固三相接触方式、接触时间对过程有明显影响,说明石膏氨水悬浮液与混合气体的气液传质问题对过程的影响很大,很可能是控制性因素,必须通过系统的深入研究掌握其基本规律。合成氨工业中原料气的CO2浓度一般在18～35%范围内,由此可见,工业过程必须充分重视石膏氨水悬浮液与原料气的气液传质设计。体系温度也是敏感参数,说明化学动力学也是应该进一步研究的重点。采用较低温度有利于降低氨损失,并为碳酸钙结晶成长提供有利条件。在实验条件下,反应温度40oC是一个适宜的选择。通过析气-液-固三相体系的反应机理可知,该体系中有十余种组份,石膏氨水悬浮液吸收二氧化碳过程由一系列反应耦合而成,总反应速率由CO2的吸收反应、NH2COO-的水解反应和石膏溶解速率共同决定,给出了相关反应的速率方程。以渗透模型为基础,建立了该体系伴有连续可逆化学反应的传质模型。 利用数值计算的方法,反应速率方程和平衡方程用Newton-Raphson方法进行线性化,用有限差分方法线性化偏微分方程组,通过计算主体平衡模型确定CO2吸收模型中偏微分方程组的初始条件和边界条件,得到了体系中各组份的浓度分布,CO2吸收过程的化学增强因子和吸收速率,表明CO2的吸收反应主要在气液界面附近进行,水解反应和石膏溶解反应在整个液相区域内进行,吸收过程的化学增强因子和吸收速率都随着碳化度的增加而降低,CO2的吸收速率随着气相主体CO2摩尔百分含量的增加而增加。以上结果为工程和工艺研究提供理论指导。更多还原

【Abstract】 Phosphogypsum, which is a by-product from wet phosphoric acid process, has restricted the sustainable development of phosphorus fertilizer industry due to its occupation of land and pollution in the environment. At present, many countries are paying more and more attention to dealing with phosphogypsum. The process which converts phosphogypsum to ammonium sulfate fertilizer with precipitated calcium carbonate as a by-product during the conversion is one of the effective approaches to the disposal of phosphogypsum. The process has already been industrialized and there is no difficulty in technology; but its cost is more expensive than high concentrated nitrogen fertilizer developed in 1970s, which seriously restrained its development. Therefore, improving the process and reducing cost are the most important way. In this paper, a new ameliorated process has been presented that the disposal of phosphogypsum is integrated into the carbonation process of synthesized ammonia, in which the utilization of phosphogypsum and the purification of raw material gas of synthesized ammonia is achieved at the same time. Preliminary cost estimates suggest that the process is economically feasible. Although some basic data can be obtained from literatures, there remains many fundamental research works to do. A process experiment has been designed to simulate the conditions in <WP=7>vapor-liquid-solid system. In order to understand the optimum process conditions, the influence of raw material proportion, material concentration, reaction time and temperature on the reaction of Phosphogypsum with aqueous ammonia and carbon dioxide to prepare ammonium sulfate and calcium carbonate under the conditions of vapor-liquid-solid system has been studied. According to the experimental investigation, the mechanism and kinetics of the carbonation process of the vapor-liquid-solid system hasve been analyzed adequately. On the basis of penetration theory, a simplified kinetics model has been proposed to calculate the dynamic absorption rate of carbon dioxide into suspension of phosphogypsum in aqueous ammonia.The experimental results indicated that the conversion of Phosphogypsum was more than surpassed 96%, which was not less than that of ammonium bicarbonate process. Such proved the new process was feasible. Meanwhile, the results revealed the conctact style and contact time played important roles in the process, suggesting that gas-liquid mass transfer be essential, even the determining step. So it is necessary to pay more attention to the mass transfer. The temperature of the vapor-liquid-solid system is also an important parameter. Lower temperature can reduce the volatilization of ammonia and promote the crystallization and growth of calcium carbonate. Under experimental conditions, 40℃ is compatible.Chemical reactions involved in the vapor-liquid-solid system were considered as three consecutive and reversible steps: the absorption reaction of carbon dioxide, the hydrolysis of carbamate and the dissolution of phosphogypsum, and other reactions included were instantaneous. The reaction rate equations and equilibrium constants were also provided. A model based on penetration theory has been developed to quantify the mass transfer within the liquid phase. The developed concentration profiles of the diffusing species and the depth of penetration have been determined for different penetration time and different carbonation degree. Moreover, <WP=8>the chemical enhancement factor and carbon dioxide absorption rate were calculated. Model simulations revealed that both the enhancement factor and the absorption rate decreased as carbonation degree increasesd from low level to high level.更多还原