Abstract
隨著文明科技不斷地進步，生物技術亦日新月異。如何花費較低成本 ，得到高產量、高附加價值的生化產品，已成為目前必然的趨勢。有鑑於 此，為了試圖克服傳統攪拌槽的諸多缺點，各研究紛紛提出改良型式的生 化反應器，例如氣泡塔及氣舉式反應器。針對好氣性醱酵系統，Tung等人 於1997年提出一同心網管氣舉式反應器，以改善傳統氣泡塔的性能。其實 驗結果顯示，在同一操作條件下，其氣體佔有率及氧氣質傳係數皆優於傳 統氣泡塔甚多；此一新式反應器並成功地利用於酵母菌(Saccharomyces cerevisiae)的醱酵程序上，縮短了18%的醱酵時間。 為了使此一新式 反應器能實際應用於工業規模的醱酵程序上，我們以空氣－水為系統，以 氣體佔有率、液相混合、剪應力及氣液質傳作為評估性能參數，對新式反 應器作規模放大性能測試。小規模之新式反應器的縱向放大顯示起始之液 位高度對氣體佔有率、氣液質傳係數的影響不大，而新式反應器在液相混 合時間及剪應力的表現上亦皆優於傳統氣泡塔。內徑增加至19公分後，新 式反應器在氣體佔有率及氧氣質傳係數上依然超過同規模之氣泡塔。因此 ，我們再以一對同心網管為一模組，提出一內徑29公分，內含四模組網管 的複式同心網管氣舉式反應器，並對此反應器作一連串的性能測試。實驗 結果顯示，內徑29公分之新式反應器與小規模之新反應器相比，具有表現 相似性；與同規模氣泡塔相比，在氣體佔有率及氧氣質傳係數上皆超出許 多；在液相混合時間上則與氣泡塔相近。故此一新式反應器可謂放大成功 。 為了探究新式反應器性能提昇的原因，本研究亦觀察了反應器內部 的氣泡動態表現，包括氣泡數量密度及氣泡尺寸。結果顯示，新式反應器 與氣泡塔的氣泡尺寸相近，而同一操作條件下新式反應器中的氣泡數量密 度比氣泡塔中增加了許多，顯示網管可以增加更多的氣體，因此提供了更 多的氣液界面面積，改善了質傳的能力。由於此種新式反應器的性能優異 ，在未來，我們希望能將此一反應器直接應用於醱酵工業的生產程序上， 改善現有的傳統氣泡塔及氣舉式反應器，以求能源的節約與產量的提昇。 Because of the swift and continuous progress in biotechnology, it hasbecome a necessary trend to maximize the production or additive value of biochemical products with minimal cost. Consequently a few of modified bioreactors of different types have been proposed to overcome the disadvantages of stirred tanks, such as bubble columns and airlifts. For the aerobic fermentation systems, Tung et al. (1997) proposed an airlift reactor with double concentric net draft tubes to improve the performance of the conventional bubble column. The experimental data showed that under the same operational conditions, both the gas holdup and the oxygen transfer coefficient of the proposed reactor were superior to those of the bubble column. This reactor was also applied to cultivation of Saccharomyces cerevisiae, and successfully shortened the cultivation time by 18%.In order to apply this reactor to the fermentation processes in industrial scales, we chose the gas holdup, liquid mixing, shear stress and gas-liquid mass transfer coefficient as parameters to evaluate the performance on the scale up of the proposed reactor in an air-water system. Axial scale up of the small reactor showed minor effect on the gas holdup and mass transfer, but both the shear stress and mixing time were less than the same scale bubble column. After increasing the inner diameter of reactor from 13 to 19cm, the gas holdup and mass transfer coefficient were still higher than those of the bubble column (id 19cm). Therefore we set a pair of concentric net draft tubes as one module and proposed an airlift reactor with multi-net draft tubes, which was 29cm in diameter and contained 4 modules of net draft tubes. Experimental results showed that there is a similar performance in the large reactor (id 29cm) and the small one (id 13cm) with net draft tubes. The large reactor with net draft tubes was much superior to the same scale bubble column in gas holdup and mass transfer coefficient. The mixing time of the large scale new reactor was also comparable with that of the bubble column. Thus scale-up of the reactor with net draft tubes was successfully achieved.In order to figure out the improvement of the performance of the proposed reactor, we observed the bubble dynamic behavior in the large scale reactor by measuring the bubble size and the bubble number. Experimental results showed that the sizes of bubbles in the proposed reactor and the bubble column were about the same, but in the proposed reactor the bubble number was much higher than that in the bubble column under the same operational conditions. This implied that the net tubes could hold more air and provide more gas-liquid interfacial area, which improved the mass transfer ability.Because of the excellent performance of the new reactor, we hope that it will be applied to fermentation processes in the future, and can improve the conventional bubble columns or the airlift reactors for more energy saving and higher production.