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標題: | 雙套管膜反應器應用在甲醇水蒸氣重組之數值模擬 A Simulation Study of Double-jacketed Membrane Reactors on Methanol Steam Reforming |
作者: | Chi-Hua Fu 傅啟華 |
指導教授: | 吳紀聖(Jeffrey Chi-Sheng Wu) |
關鍵字: | 雙套管膜反應器,甲醇水蒸氣重組,數值模擬,鈀膜, Double-jacketed membrane reactors,methanol steam reforming,simulation,palladium membrane, |
出版年 : | 2008 |
學位: | 博士 |
摘要: | 本研究是以數值模擬探討雙套管膜反應器應用在甲醇水蒸氣重組反應。外管部分為甲醇或氫氣氧化反應以提供熱能給重組器端之甲醇水蒸氣重組反應。最內管為鈀膜氫氣純化端,而重組反應產生之氫氣可在反應時即被分離至純化端。研究發現當氧化端與重組端之氣體流向為同向比反向有更佳的表現。同向操作比反向操作有更高的甲醇轉化率及重組溫度。當反應器啟動時,我們使用兩種啟動狀況並模擬其表現:(1)重組觸媒溫度設定為433K,進料氣體溫度設定為543K;(2)重組觸媒溫度設定為543K,進料氣體溫度設定為433K。結果顯示兩種操作條件都在無因次時間150之內即達穩態。操作條件(1)在穩態時比操作條件(2)有較高之甲醇轉化率與重組器溫度。而操作條件(2)可在啟動初期即有高甲醇轉化率,但最後達穩態時甲醇轉化率較低。另外使用高溫水蒸氣作為重組器預熱氣體可大幅降低預熱所需時間。當反應器在穩態操作條件下,有額外氫氣需求時,可使用兩種操作情況以生產額外的氫氣:提高甲醇進料流率或提高重組器的溫度。研究發現提高甲醇進料流率比提高重組器溫度有更佳的表現。
在穩態操作時,研究結果發現增加純化端的氫氣體積流率可以提高氫氣透過鈀膜的速度,而較適的純化端與重組端之流速比為10。另一方面,氫氣的移除亦會降低重組器的溫度。在一個固定之丹姆寇勒數(Damköhler number)下,提高溫度可以增加氫氣產量,但是甲醇轉化率及氫氣產率會降低。在3種操作溫度下,較適之氧氣與甲醇莫耳流率比為1.3。另外我們使用甲醇轉化率,氫氣產率與氫氣產量來比較雙套管膜反應器與自身熱平衡重組器(Autothermal reformer)的表現。研究發現在相同操作條件下,雙套管膜反應器可比自身熱平衡重組器有較高之甲醇轉化率。 第三部分是有關於輸送限制在之雙套管膜反應器探討。研究參數有:重組器與觸媒之直徑比、重組器直徑與管長比、空隙度、純化端與重組端出口之壓力比。在壓降與熱傳阻力限制下,較適之觸媒與重組器之直徑比為10;較適之重組器直徑與管長比為0.1;較適之空隙度範圍為0.3~0.5;較適之純化端與重組端出口之壓力比為10。我們使用兩種策略來降低重組器內之溫度梯度:降低進料溫度或在純化端內部增加一個氧化反應管。降低進料溫度雖然可以減低熱傳限制,然而甲醇轉化率及氫氣產率亦會降低。因為具有較高之熱傳效率,三套管膜反應器可以大幅降低熱傳梯度使得反應趨向於恆溫操作。 The methanol steam reforming on double-jacketed palladium membrane reactors was studied by numerical simulation. A catalytic oxidation of methanol and/or hydrogen was used to provide the necessary heat for steam reforming, and hydrogen permeation by palladium membrane takes place simultaneously. The molar fractions of species and reformer temperatures were analyzed under co-current and counter-current operation between oxidation and reformer sides. The results indicated that the co-current operation outperforms the counter-current operation because of higher methanol conversion and reformer temperature than those under counter-current operation. In the transient study, the gaseous compositions and reformer temperatures were simulated under two conditions: (1) starting a cold catalyst bed at 433K and a hot inlet at 543K; (2) starting a hot catalyst bed at 543K and a cold inlet at 433K. The results revealed that both operating conditions reached thermal equilibrium within 150 dimensionless time. The operating condition (1) yielded higher methanol conversion and reformer temperature than those under condition (2) after reaching steady state. The operating condition (2) had higher methanol conversion than those of condition (1) in the beginning but low conversion at final steady state. Using hot steam to heat reformer can decrease the required time during the preheating period. In addition, two strategies were compared to analyze the reformer response when a temporary extra hydrogen demand is required. The results showed that increasing methanol mass flow rate outperforms increasing reformer temperature. The methanol conversion, hydrogen recovery yield and production rate were further analyzed at steady state. The simulation results exhibited that increasing the volumetric flow rate of hydrogen in permeation side could enhance hydrogen permeation rate cross the membrane. The favorable velocity ratio from permeator to reformer was 10. However, hydrogen removal could lower the temperature in the reformer. The hydrogen production rate increases as temperature increased at a given Damköhler number, but the methanol conversion and hydrogen recovery yield decreased. In addition, a suitable molar ratio of air to methanol was 1.3 under three air inlet temperatures. The performance of a double-jacketed membrane reactor was compared with an autothermal reactor based on methanol conversion, hydrogen recovery yield and production rate. Under the same feeding conditions, the double-jacketed reactor could convert more methanol at a given reactor volume than the autothermal reactor. The transport limitations of double-jacketed membrane reactors were studied by the ratios of reformer diameter to catalyst particle diameter, ratios of reformer diameter to length, void fractions, and ratios of exit permeator to reformer pressure. From the analysis of pressure drop and radial temperature gradient profiles, we suggested a set of favorable ratio of catalyst particle diameter to reformer diameter, ratio of reformer diameter to length, void fraction, and ratio of exit permeator to reformer pressure to be 10, 0.1, 0.3~0.5 and 10, respectively. The minimization of temperature gradients was explored by either decreasing the inlet temperature or applying an oxidation tube inside the permeator. Although decreasing the inlet temperature reduced the heat transfer limitations due to decreasing the reforming rate, the methanol conversion and hydrogen yield were also decreased. A triple-jacketed membrane reactor was proposed. The heat transfer limitations were significantly reduced due to an effective heat transfer. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/27018 |
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顯示於系所單位: | 化學工程學系 |
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