Department of Chemical Engineering, Lund University, Box 124, Lund, SE-221 00, Sweden.
Biotechnol Biofuels. 2012 Aug 6;5(1):57. doi: 10.1186/1754-6834-5-57.
A common trend in the research on 2nd generation bioethanol is the focus on intensifying the process and increasing the concentration of water insoluble solids (WIS) throughout the process. However, increasing the WIS content is not without problems. For example, the viscosity of pretreated lignocellulosic materials is known to increase drastically with increasing WIS content. Further, at elevated viscosities, problems arise related to poor mixing of the material, such as poor distribution of the enzymes and/or difficulties with temperature and pH control, which results in possible yield reduction. Achieving good mixing is unfortunately not without cost, since the power requirements needed to operate the impeller at high viscosities can be substantial. This highly important scale-up problem can easily be overlooked.
In this work, we monitor the impeller torque (and hence power input) in a stirred tank reactor throughout high solid enzymatic hydrolysis (< 20% WIS) of steam-pretreated Arundo donax and spruce. Two different process modes were evaluated, where either the impeller speed or the impeller power input was kept constant. Results from hydrolysis experiments at a fixed impeller speed of 10 rpm show that a very rapid decrease in impeller torque is experienced during hydrolysis of pretreated arundo (i.e. it loses its fiber network strength), whereas the fiber strength is retained for a longer time within the spruce material. This translates into a relatively low, rather WIS independent, energy input for arundo whereas the stirring power demand for spruce is substantially larger and quite WIS dependent. By operating the impeller at a constant power input (instead of a constant impeller speed) it is shown that power input greatly affects the glucose yield of pretreated spruce whereas the hydrolysis of arundo seems unaffected.
The results clearly highlight the large differences between the arundo and spruce materials, both in terms of needed energy input, and glucose yields. The impact of power input on glucose yield is furthermore shown to vary significantly between the materials, with spruce being very affected while arundo is not. These findings emphasize the need for substrate specific process solutions, where a short pre-hydrolysis (or viscosity reduction) might be favorable for arundo whereas fed-batch might be a better solution for spruce.
第二代生物乙醇研究的一个常见趋势是专注于强化过程并提高整个过程中水不溶性固体（WIS）的浓度。然而，增加 WIS 含量并非没有问题。例如，众所周知，预处理木质纤维素材料的粘度会随着 WIS 含量的增加而急剧增加。此外，在较高的粘度下，会出现与物料混合不良相关的问题，例如酶分布不均和/或难以控制温度和 pH 值，这可能导致产量降低。不幸的是，实现良好的混合并非没有成本，因为在高粘度下操作叶轮所需的功率要求可能很大。这个非常重要的放大问题很容易被忽视。
在这项工作中，我们在搅拌槽式反应器中监测了整个高固体酶水解过程中的叶轮扭矩（因此是功率输入）（<20% WIS），蒸汽预处理的芦竹和云杉。评估了两种不同的工艺模式，其中一种是保持叶轮速度不变，另一种是保持叶轮功率输入不变。在固定叶轮速度为 10rpm 的水解实验中得到的结果表明，预处理芦竹的水解过程中，叶轮扭矩会迅速下降（即它失去了纤维网络强度），而云杉材料中的纤维强度保留时间更长。这转化为芦竹相对较低的、与 WIS 无关的能量输入，而云杉的搅拌功率需求则大得多，并且与 WIS 高度相关。通过以恒定功率输入（而不是恒定叶轮速度）操作叶轮，结果表明功率输入会极大地影响预处理云杉的葡萄糖产率，而芦竹的水解似乎不受影响。
结果清楚地突出了芦竹和云杉材料之间的巨大差异，无论是在所需的能量输入方面，还是在葡萄糖产率方面。功率输入对葡萄糖产率的影响在材料之间也有很大差异，云杉的影响非常大，而芦竹则没有。这些发现强调了需要针对特定底物的工艺解决方案，对于芦竹，短的预水解（或降低粘度）可能是有利的，而对于云杉，分批进料可能是更好的解决方案。
