M. Gfllbe, M. Larsson, K. Stenberg, C. Tengborg, and G. Zacchi1

Department of Chemical Engineering I, Lund University,
S-221 00 Lund, Sweden

Ethanol is an attractive alternative for replacing gasoline as a motor fuel and it can be produced from a number of cellulosic materials. However, processes in which biomass is involved are often very complex. By a combination of computer simulations and experiments in a bench-scale unit different process strategies can be evaluated, and the results from experiments can serve as feed-back to the process simulations. The results in this study show the impact of recycling process streams and the effect of different energy-integration options.

Ethanol, which is believed to be an interesting alternative for replacing gasoline as a motor fuel can be produced from a number of cellulosic materials including agricultural products (1-4). The main reason for turning towards processes utilising cellulosic materials is the abundance of various cellulose sources, such as forest waste (4-6). Ethanol production from sugar — or starch-containing crops is an industrially well-established technique which has been used for many years, mainly for the production of alcoholic beverages. However, the process technology for the conversion of cellulosic materials into ethanol has not yet been fully optimised. A number of different technologies have been proposed, the major difference being the way in which the material is hydrolysed and the fermentable sugars extracted.

Basically, the production of ethanol from, for example, wood can be performed by hydrolysing the material, thus releasing fermentable sugars. The sugars are fermented to ethanol using various micro-organisms, and the diluted ethanol is recovered in a distillation unit. Hydrolysis can be performed either by the use of dilute or concentrated acids (7-9), or by using cellulose-degrading enzymes (3,10-12). The advantages of acid hydrolysis are the well-established technology and short reactor residence times. The main drawbacks are the corrosive action of the acid on equipment, the production of large amounts of salts, such as gypsum, from the neutralisation of the acid and the non-selectivity of the acid. Enzymatic hydrolysis has

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© 1997 American Chemical Society

the potential to give higher yields, and it is performed at temperatures only slightly above room temperature, 40-50°C. However, enzymatic hydrolysis requires pre­treatment in order to open up the compact structure of the cellulosic material (13-16).

When developing a new industrial process for ethanol production based on enzymatic hydrolysis, it is essential to have access to reliable experimental data. Most data found in the literature are based on laboratory-scale equipment, and determined under carefully controlled conditions, although some pilot facilities have been used to verify some of the laboratory data (17-19). Most of the data are also for individual steps, e. g. pretreatment, hydrolysis or fermentation, and are not always valid for a fully integrated process. This leads to many assumptions when estimating the ethanol production cost. This is reflected in the large discrepancy between estimated production costs. Several cost estimates for the production of ethanol can be found in the literature with prices ranging from 0.18 to 1.51 US$/L (20-22). Accurate design requires mid-scale process facilities with integrated process steps, e. g. bench-scale or pilot plants. A bench-scale unit is attractive, due to its flexibility and low equipment cost, for investigations of a variety of process parameters and process configurations. The more expensive pilot-plant facilities are, however, necessary, for final scale-up of the process.

Process development also requires technical and economic calculations in order to estimate equipment and energy requirements, to assess the overall production cost and to discriminate between various process configurations. Process simulations are also valuable for the identification of cost-critical steps and for the planning and configuration of experimental investigations in small-scale equipment. The data thus gathered can be used to refine the technical and economic models for more accurate calculations.

This chapter presents the combined use of a bench-scale unit and process simulation for the development and optimisation of an integrated bio-ethanol plant based on enzymatic hydrolysis of lignocellulosics.