在甲烷火焰中合成二氧化鈦奈米顆粒之模擬研究

本研究以C語言建立了顆粒動力學模型，將其搭配FLUENT12.0.16軟體之熱流場結果預測氣相燃燒合成法之產物顆粒粒徑。顆粒動力學模型中考慮了熱流場、化學反應、布朗運動、燒結作用、顆粒間的凡德瓦力等影響，在假設產物為單分散的情形下，以聚集塊之數量密度(N)、單位質量氣體中的顆粒總體積(V)和總表面積(A)描述產物粒徑並且分析粒徑的成長過程：氣態前驅物進入火焰後會在極短的時間內反應生成氣態產物，當產物的濃度夠高時便開始凝聚成核形成奈米級的產物顆粒。隨後由於產物顆粒間的不斷的碰撞以及燒結等作用的影響，產物粒徑會不斷的增加直到被收集起來為止。此外也將模型套用至TTIP在甲烷預混火焰中合成TiO2的實驗上和實驗進行，並將火焰分成加熱區、反應區I、反應區II和純凝合區做討論，探討在不同氧氮比、總流量、和前驅物濃度的情形下對顆粒粒徑的影響。若以2.5cm處為收集高度，結果顯示隨著氧氮比從20/80增加到50/50主要會影響在反應區II中的粒徑成長，其收集粒徑會從40.9nm上升至54.2nm；當總流量從1.88x10-5kg/s增加至5.65x10-5kg/s時，各反應區域皆會被拉伸，但在2.5cm處由於三組參數所在區域不同使得粒徑沒有遞增遞減的變化；當前驅物TTIP的濃度由0.2%增加至0.6%，主要影響了反應區I中前段的TiO2生成，其粒徑會從40.5nm上升至51.0nm。

關鍵字

氣相燃燒合成法；顆粒動力學；碰撞效率

並列摘要

A computational model describing the particle kinetics in flame combustion synthesis process was presenting using the programming language C and computational fluid dynamics software FLUENT12.0.16. The thermal-flow field, chemical reaction, Brownian motion, sintering, Van der Waal interaction and diffusion had been considered in the model and assuming the spread of particles and aggregates are monodisperse. The particle size and the condition in the synthesis process were described by number concentration of particles (N), total particle volume per mass (V) and total particle surface area per mass (A). As gas-phase precursors enter the flame, they will react on high temperature and the gas-phase production is produced. Because the chemical reaction rate is very fast in the flame, the oversaturated production will form nano-size production particles. Particle size will grow continuously by coagulation and sintering until particles are collected. Using TTIP as the precursor to produce TiO2 nanoparticles in methane premixed flame to compare with the experimental result was investigated in this research and the effects of operation parameters such as oxygen/nitrogen ratio (O2/N2), total mass flow rate(Q) and TTIP concentration(XTTIP) have been discussed. Considering the collection height at 2.5cm, as O2/N2 is increased from 20/80 to 50/50, it will enhance the collision and sintering in reaction regime II and lead particle size (dp) to increase from 40.9nm to 54.2nm. As Q is increased from 1.88x10-5kg/s to 5.65x10-5kg/s, it will lengthen regimes so the collected height in three different cases are in the different regions. Therefore particle sizes are irregular as Q increases. And as XTTIP is increased from 0.2% to 0.6%, it will increase TiO2 particles produced in reaction regime I and leads to increase dp from 40.5nm to 51.0nm.

並列關鍵字

gas-phase combustion synthesis ； particle kinetics ； collision efficiency

參考文獻

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在甲烷火焰中合成二氧化鈦奈米顆粒之模擬研究

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