(488d) Analysis of the Influence of Heat Flows in Optimal Design and Operation of Multicomponent Distillation Columns: Extractive Distillation Case Study

Distillation is a mayor separation system in chemical process industries. It uses heat supplied from a higher temperature level at the reboiler, and rejects almost equal amount of heat in the condenser at lower temperature levels. Therefore, it resembles a heat engine producing a separation work with a rather low efficiency[1]. Just in the U.S  distillation of chemicals and petroleum uses about 53 percent of the total energy used for industrial separations and is the largest energy-consuming process in industry[2]. Besides the second-law efficiency of conventional distillation is very low, only around 5-20%. This means that distillation is associated with a high entropy production (exergy loss) and hence degradation of energy. To reduce the exergy wasted on it is recommended to alter design and operation of the distillation process. This is achieved by spreading the heat requirements over the whole length of a distillation column. Such design consideration is referred to as diabatic distillation.[3]

Previous studies of this kind of systems have been considered, with some contemplations  about  heat exchange between stages, such as heat exchange in all stages[4], systems in which each stage could have whether a condenser or a reboiler[5]  and columns with sequential heat exchangers[6]. These studies compare entropy production of each of the proposed systems, against an adiabatic distillation column with same operating conditions, and only analyze binary systems (50% toluene-50% benzene in most cases).

In the present work the concept of diabatic distillation is applied into a extractive distillation system (extractive distillation of ethanol using glycerol as entrainer, an innovating distillation technique because the use of glycerol which recently has a high availability as byproduct in the production of biodiesel) in order to study how heat flows influence the design and operation of multicomponent distillation systems using Mixed Integer Nonlinear Programming (MINLP).

The mentioned distillation system is modeled applying the rigorous MESH equations (Mass balances, Equilibrium relationships, Summation of compositions and energy balance) into each separation stage within the column, taking into account a heat flow whether entering or leaving the stage. The model (assuming adiabatic operation) is compared with a simulation of the same distillation column in Aspen Plus V7.2 achieving accurate results. Then, optimization of the operating conditions was carried out for both diabatic and adiabatic extractive distillation column with an economic objective function, assuming heat flow as a binary variable which may exists or not at each stage for the diabatic case. The same energy consumption for adiabatic and diabatic systems was found, but in the diabatic system the heat is distributed all through the column reducing heat requirement for the reboiler. In this vein an exergy term is included in the model to attain a thermo - economic optimization of the mentioned case study, to identify the location, magnitude and source of the exergy destruction and energy loses in the system computing the associated cost with these terms. 

From all the analysis about operating conditions of the case study, a methodology for the optimal design and operation of distillation systems including heat flows through the column is proposed, using MINLP in order to determine number of stages and feed locations of these particular separation systems.

 REFERENCES

1.            Demirel, Y., Thermodynamic Analysis of Separation Systems. Chemical ans Biomolecular Engineering Research and Publications, 2004(Thermal Mechanics).

2.            Sciences, N.A.o., N.A.o. Engineering, and N.r. Council, Real Prospects for Energy Efficiency in The United States. 2010, Whashington, D.C.: The National Academies Press. 349.

3.            Schaller, M., Numerically Optimized Diabatic Distillation Collumns, in Natural sciences2007, Technischen Universität Chemnitz: Chemnistz. p. 86.

4.            De Koeijer, G., A. Røsjorde, and S. Kjelstrup, Distribution of heat exchange in optimum diabatic distillation columns. Energy, 2004. 29(12-15 SPEC. ISS.): p. 2425-2440.

5.            Chen, M.-C., K. Takeshita, and M. Ishida, Computational Study on Binary Distillation of Heat-Driven Distillation System. Industrial & Engineering Chemistry Research, 2005. 44(24): p. 9156-9163.

6.            Jimenez, E.S., et al., Optimization of a diabatic distillation column with sequential heat exchangers. Industrial and Engineering Chemistry Research, 2004. 43(23): p. 7566-7571.