Process simulations cannot replace experiments, but constitute a useful tool in the planning and evaluation of experiments. Furthermore, they highlight factors that are sometimes neglected in experimental studies, for example, the amounts of chemicals needed in the process (catalyst in pretreatment, acid/base for pH adjustment, nutrients and not least enzymes and yeast), which constitute a significant contribution to the production cost. The over­all demand of steam, process water and cooling water are other important factors.

Optimization of ethanol production from lignocellulosic feedstock requires a model that includes all the major process steps, since changing the condi­tions in one process step is likely to affect other parts of the process. Although no full-scale plant based on enzymatic hydrolysis has yet been built, most of the process steps (e. g. distillation, evaporation, drying and incineration) are considered to be technically mature, i. e. their operational performance is well known. Of course, the application of these unit operations in a lignocellulose — to-ethanol plant still requires to be verified on pilot scale before a full-scale plant can be constructed. However, the ethanol process includes other process steps, which are associated with greater uncertainties regarding design and performance on full scale. This is definitely true for the pretreatment step, ir­respective of the pretreatment method chosen, or how it is configured (Galbe, in this volume). It also applies to enzymatic hydrolysis or SSF at high solids concentrations, as well as solid-liquid separation of the stillage.

The modeling of a lignocellulose-to-ethanol process poses a number of unique challenges. In contrast to well-defined systems, such as pure ethanol — water systems, it involves not only vapor and liquid phases but also a solid phase, including atypical compounds like cellulose, lignin and yeast. There­fore, when simulating such a process it is necessary to use a flowsheeting program that is able to handle solid components. In most techno-economic evaluations of the lignocellulose-to-ethanol process that have been performed during the past 10 years, Aspen Plus from Aspen Technologies has been used [16—20]. In Aspen Plus, a separate solid stream is used that does not interact with the liquid phase and never ends up in the vapor phase. There is thus no need to estimate vapor phase data such as heat of vaporization or vapor pressure for components treated as solids (lignin, glucan, yeast, etc.).

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