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Design of a Hybrid Distillation-Pervaporation Bio-Ethanol Purification Process Using Conceptual Design and Rigorous Simulation Tools

Patricia M. Hoch, Chemical Engineering Department, Plapiqui - UNS - Conicet, CC 717, Bahía Blanca, 8000, Argentina and José Espinosa, Universidad Nacional Del Litoral, INGAR - CONICET - UNL, Avellaneda 3657, Santa Fe, Argentina.

Optimization of both design and operation variables of a bio-ethanol purification plant intended to produce fuel grade ethanol is a challenge to make the bio-fuel a realistic alternative in the energy market.

In this work, we propose an evolutionary optimization procedure which intensively uses both conceptual and rigorous models for the design and simulation of unit operations involved in a bio-ethanol purification process. While conceptual models are used to determine initial values for design and operating variables of a given operation unit, rigorous simulation departing from initial estimates generated at conceptual design level enables to remove simplifying assumptions, interconnect equipments and calculate operating and investment costs.

Once an initial design of the purification plant is obtained, opportunities of improvement are easily recognized and then tested by performing the design and simulation steps until a cost-effective bio-ethanol purification plant is achieved.

The bio-ethanol purification plant comprises a separation sector with two distillation columns, where both the reboilers are replaced by live steam feeds, a decanter and a pervaporation unit, used to obtain fuel grade ethanol. The plant is integrated with two other sectors, a fusel plant with several liquid-liquid separators, and an evaporation plant, where a triple effect evaporator, a centrifuge and a rotary dryer are used to obtain an animal feed co-product rich in proteins. Given the plant complexity and the high non-linearity of the corresponding models, we propose an evolutionary optimisation procedure which uses intensively both conceptual and rigorous models for the design and simulation of unit operations.

Finding initial values for the column design is made easier within the conceptual design environment, which is used to obtain an initial estimation of the total number of trays of the main distillation column, placement of feed and side streams, and steam flow rate in the simulation environment. In addition, convergence of the rigorous model is enhanced by using initial estimates of internal profiles generated at the conceptual design level despite of the highly non-ideal behavior of the multicomponent azeotropic mixture.

Applying the same strategy to estimate initial values for other important values; i.e., number of liquid-liquid separators and flow rate of the wash-water stream in the fusel plant or temperature and permeate pressure in the pervaporation sector, quasi-optimal values for a cost-effective bio-ethanol purification plant are found.

The advantage of the method is that the systematic use of conceptual models allows the designer to capture the main characteristics of each operation involved in the process.

The main results obtained show a minimum in the operation costs corresponding to a minimum in the steam flow rate of the hybrid column. This minimum in steam flow rate can be only explained by the presence of the fusel component, which influences both the energy demand and feasible products of the hybrid column. Therefore, designs based on the binary system ethanol-water do not represent the system behavior in an accurate way. From this consideration emerges the importance of properly determining the amount of trace components that enter the purification process.

From the analysis of the results obtained it is also clear that the investment cost corresponding to the membrane sector is high enough to promote future research efforts in testing both selectivity and flux behavior of other commercial pervaporation membranes.

References

Hoch P.M., Espinosa J., Optimisation of a Bio-ethanol Purification Process Using Conceptual Design and Simulation Tools, 18th European Symposium on Computer Aided Process Engineering – ESCAPE 18, Lyon, France, 1-4 June 2008.