Thursday, 15. November 2007

Use of conceptual design for the optimization of bioethanol processes

The benefits to large chemical processes gained by conducting conceptual design studies in advance are well known. Because technical decisions made at an early stage of process development have the greatest influence on profitability, these studies have become de rigueur for chemical processes. Bioethanol plants have comparable energy and investment requirements, and significant savings can be achieved with a moderate amount of effort. Key to the result are mature and integrated tools, a systematic and efficient workflow and integrated expertise in a variety of disciplines.

Influence profitability with conceptual design
Opportunities for significantly improving the earnings situation of a bioethanol plant can be identified in particular by considering local parameters as part of a conceptual design study. These are decisive for the selection of the proper raw material, the by-products with the highest value-added and the optimal energy concept.

Fig. 1: Early general optimization in conceptual design

The success of a conceptual design study depends primarily on the following factors:

• Analysis of the current process, including patent search and definition of the cost structure, in preparation for further work
• Structured procedure
• Staffing of the team with suitable expertise
• Use of suitable tools for rapid generation and evaluation of ideas
• Use of suitable experiments to clarify open issues (only as needed)
• Definition of the objective
• Involvement of the customer/investor

BTS's proven procedure and the required tools are shown in Fig. 2.

Fig. 2: Procedure and tools for a conceptual design study

The concept generation phase (Phase 1) comprises the analysis of the local parameters and a creative phase characterized primarily by the identification of market prices for various by-products of biomass production and a survey of available utilities and their prices on site. In addition, the large number of process alternatives must be investigated in superficial technical detail and with an uncertainty factor that is still high.

In Phase 2, evaluation and prioritization, the emphasis is on separating the promising (and feasible) concepts from unprofitable ones with the assistance of experts and suitable tools for the rapid assessment of process and cost data. This phase must be performed after the creative Phase 1 so that the generation of ideas is not stopped prematurely. More detailed development follows in Phase 3. Greater effort can then be devoted to the assessment of process alternatives, since the cursory assessment in Phase 2 has greatly reduced the number of concepts under consideration. The evaluation and prioritization phase as well as the elaboration and refinement phase for the remaining concepts requires the input of experienced experts and suitable integrated tools for

• Simulation of the complete process as the basis for apparatus design and for the calculation of energy requirements and other utilities
• Short cut design of apparatus
• Estimation of investment costs
• Calculation of manufacturing costs and profitability
• Laboratory trials to evaluate the fundamental feasibility of new ideas
• Pilot-scale trials to generate reliable data in the later phase for scale up.

The use of such tools in combination with specific information from experienced experts enables a rapid comparison of various process concepts, a reliable comparison of economic potential and thus targeted steering and acceleration of the conceptual design study.

Bioethanol from rye
In the study presented, this methodology was applied to the production of bioethanol from rye.

In this specific study performed by BTS, the emphasis was on a possible enhancement of value by dividing the by-products and process improvements as a result of reduced fouling. No variation of the energy concept was considered, however.

The standard process for the production of bioethanol from grain (Fig. 3) is characterized by the fact that all of the grain is milled after cleaning, and all components of the grain, including the fiber-rich husk, is dragged through the entire process. The residual biomass fraction is dried and marketed as dried distillers grains with solubles (DDGS).

Rye is a raw material whose components are particularly sensitive to thermal loads, as confirmed by BTS experts in internal laboratory trials.To improve the handling of these problematic components (e.g. fibers, proteins, pentosans), the standard process requires the use of expensive enzymes to reduce the viscosity as well as additional process-related effort (vacuum operation, redundant apparatus) to enable production.

BTS therefore performed a conceptual design study for the production of bioethanol from rye.

Fig. 3: Standard process and optimized bioethanol process from the BTS study

Fig. 3 shows the process that the study identified as advantageous. What sets this process apart is that the low-starch husk fraction that interferes with the milling process is separated out and decantation was placed in front of fermentation, resulting in simplifications to the downstream process steps. In addition, the thermal coupling of distillation and dewatering as well as the evaporation and drying steps was improved, resulting in significantly decreased energy consumption. Further economic improvement was achieved in this case by the separate production of a fiber-rich animal feed fraction (DDG) and a protein-rich animal feed fraction (DDS). The feasibility of the deviations from the standard process was then investigated.

Cost-optimized process
The result is a process offering significant improvements in terms of both investment costs and operating costs. It should be noted, however, that the process was optimized only for the parameters of this specific project and entirely different processes were identified as optimal in other conceptual design studies for bioethanol processes.

Fig. 4: Comparison of the profitability of the alternatives investigated