The world economy currently relies on oil as an energy source for fuels and petrol, and as the raw material source for almost any carbon-based commodity chemical. However, its soaring price, along with the new concerns about carbon dioxide and global warming, has triggered in recent years an unprecedented interest on renewable energy, which has been translated into an enormous effort invested in the development of new industrial biological processes. Biorefineries can be defined as a tightly integrated set of entwined biochemical and chemical processes that produce a portfolio of chemicals, fuels and energy from a renewable raw material. A general, simplified scheme of a biorefinery would consist of a pretreatment stage, a bioconversion stage and downstream processing (which can include many of the traditional unit operations).

The success of petrorefineries after the petroleum crisis in the late 1970's came from making the most of every drop of oil through process integration; in a parallel way, biorefineries will only become competitive by fractionating the very last drop of the ‘barrel' of biomass using the same process integration tools. However, biorefineries will become even more complex than petrorefineries, as they will include pretreatment and bioconversion stages on top of the traditional chemical engineering unit operations. That implies that process integration for biorefineries will face inherited, as well as inherent, challenges.

Process engineering software packages are now major players in process integration, offering visual representation of the processes, and a full database of chemical compounds and unit operations. These characteristics offer invaluable insight into the studied processes. One of the main challenges biorefinery integration will encounter is the inclusion of solids handling in the representation of processes: although 70% of the intermediate products and 60% of the final products handled by established process industries are solids (Charpentier 2005), either the associated models and flowsheets extremely simplify their behavior, or are plainly not included. This strategy clearly leaves out a whole range of improvements in the processes. In the case of biorefineries, almost all of the raw material will be solid, as well as many of the intermediate products. Thus, the right framework to deal with solids handling within process engineering packages is of utmost importance to underpin the development of competitive biorefineries.

Cereal biorefineries will play a key role in the new economic scenario: cereals offer large scale production, high energy density and preexisting infrastructures for handling and transportation. Among them, wheat, which has accompanied mankind in its evolution, will be one of the most important ones. In a cereal biorefinery, wheat, being composed of solid granular kernels, requires as first steps appropriate milling and separation into different fractions. The effectiveness of the fractionation at this stage of the process greatly affects the performance and efficiency of later stages. While milling and sifting are usually neglected in the simulation of industrial processes, they are highly complex and interacting operations, and have proved to be a real challenge for simulation.

This research is focused on the use of commercial process engineering software for solids simulation. Four different general, commercial, process engineering packages (Aspen Plus®, Hysys®, PRO/II® and Chemcad®) have been evaluated in order to asses their general adequacy for the implementation of solids operations, and the degree and flexibility of customization they support. The evaluated parameters encompass the definition of solid component to the limitations posed by the programs as a result of the introduction of solids in the flowsheet. As a result, some deficiencies and some directions for further improvement are pointed out. Finally, an example of the integration of solids handling in Hysys® is presented: a simplified sifting unit, and a roller milling unit for wheat are included in the process simulator. The model used in the roller mill is based on the concept of the breakage equation relating the particle size distribution resulting from roller milling of wheat to the input characteristics of the wheat and the design and operation of the roller mill (Campbell and Webb, 2001; Campbell et al., 2001). The work highlights the need to add population balance capabilities alongside the mass and energy balance facilities that currently form the basis of process simulators.

References

Campbell, G.M. and Webb, C., 2001, On predicting roller milling performance. I. The Breakage Equation, Powder Technol, 115: 234-242. Campbell, G.M., Bunn, P.J., Webb, C. and Hook, S.C.W., 2001, On predicting roller milling performance. II. The Breakage Function, Powder Technol, 115: 243-255. Charpentier, J.C., 2005, Four main objectives for the future of chemical and process engineering mainly concerned by the science and technologies of new materials production. Chemical Engineering Journal 107(1-3): 3.