Heat integration study of a conceptual lignocellulosic ethanol plant
Examensarbete för masterexamen
Ethanol based on lignocellulosic raw material has the potential to be an important alternative in the transition from fossil to renewable fuels in the transport sector. Today’s ethanol is produced from sugar or starch rich crops, often sugar canes, sugar beets, corn or wheat and thereby competes with food industry in terms of feed stock and land use. Ethanol from lower value materials, such as lignocellulosic agricultural and forest residues, will most likely not cause the same conflict of interest. Pretreatment and degradation of lignocellulose into fermentable sugars is, however, energy consuming and the process is therefore currently not economically competitive. There is a lot of ongoing research on the subject of how to make each step in the process more efficient and, to some extent, on how to combine units to save energy. In this thesis, a conceptual lignocellulosic ethanol plant is studied and by using pinch analysis methodology possibilities of heat integration are evaluated. Three parts stood out as the most important to improve in order to reduce energy demand, namely flash steam from pretreatment, purification of ethanol in the distillation and drying of residues in evaporation. By utilizing the flash steam at multiple levels, using unconventional evaporation setups and adding units to the distillation, substantial amounts of steam could be saved. Solutions including heat pumping of flash steam and distillate were also evaluated. The heat demand of the process can be covered by incineration of internal fuels, mainly lignin and evaporator syrup, and the excess steam produced is expanded through a turbo generator and electricity not used on-site is sold. Except direct comparison of steam demand in each developed case, an economic comparison was performed. The alternatives utilizing excess steam for increased electricity production, or just covering the process steam demand and selling excess lignin, were studied separately. It was shown that large amounts of steam could be saved by heat integration of thermal separation units and by redesigning the process. The study could, however, not determine in what way the saved steam is best utilized. Neither production of more excess electricity nor extracting excess lignin (despite large investment savings for the boiler) proved to be beneficial for the overall economy. The main reasons for the difficulties of making profits from the steam savings were concluded to be the impact of capital investment and raw material on the plant economy.
Kemisk energiteknik , Chemical energy engineering