267732 A Hybrid Simulation-Optimization Approach for the Design of Internally Heat-Integrated Distillation Columns
A Hybrid Simulation-Optimization Approach for the Design of Internally Heat-Integrated Distillation Columns
Juan A. Reyes-Labarta*, Miguel A. Navarro, José A. Caballero
Department of Chemical Engineering, University of Alicante, Ap. Correos 99, Alicante 03080, Spain (e-mail: email@example.com)
This work introduces a systematic method for the optimal and rigorous design of internally heat-integrated distillation columns (HIDiC). The total number of intermediate heat exchanger and their optimal location are determined. The number of trays of each section can be optimised in an outer loop providing the method a great flexibility. The commercial process simulator Hysys® has been used to implement the process and solve the rigorous VLE using the available thermodynamic models such as NRTL, and MatLab® to implement the optimization algorithm.
Keywords: Distillation, hybrid simulation-optimization, optimal processes design, HIDiC.
Distillation is still one of the most important separation techniques, even though it is an expensive operation in terms of capital and energy costs. This fact with the major current environmental problems such as the global warming and depleting resources have led to growing interest in more efficient and sustainable chemical processes. In these sense, since the distillation columns normally present sections with different heat requirements (to vaporize a liquid in the bottom and to condensate a vapour in the top), heat-integrated distillation columns (HIDiC) can be an efficient way of saving energy [1-3].
HIDiC columns are characterised by the existence of internal heat integration between the rectifying section and the stripping section. In order to allow the heat transfer from the rectifying to the stripping section, the temperature of the rectifying has to be increased with a compressor. Normally, the stripping-section pressure is the same as the feed pressure, while the rectifying-section pressure should be elevated enough that the different between the dew-point temperature of the distillate and the bubble point of the bottoms product were significant (larger than 10 ºC).
In this kind of designs, once we have defined the operating pressures, the main variables to get the best column configuration for a specified separation are: the number of theoretical stages of each section (nRS, nSS), reboiler duty (QR), distillate flow rate (D), number and location of the intermediate heat exchangers, overall internal heat transfer (QIHT) and compressor shaft work (WS).
Due to the inclusion of very efficient simulation algorithms in commercial packages the design of distillation columns has been usually performed by successive simulations [4,5]. However, this trial-and-error procedure is very time consuming and of doubtful utility when trying to evaluate complex configurations or sequences of interrelated columns.
On the other hand, the use of new design methodologies based on superstructure models and mathematical programming (GDP or MINLP techniques) present difficult convergence due to the high non-linearity of the equations used (especially when rigorous equilibrium calculations are modelled). Additionally, these methodologies normally have a strong dependence of the initial values used to start the optimization calculations and need the utilization of good initialization procedures and tight bounds on the variables to convergence [6-8].
Thus, in the present paper, we present a hybrid simulation-optimization approach for the optimal and rigorous design of heat integrated distillation columns. This approach allows easily analyse and quantify the effect of the presence of intermediate heat exchangers over the overall heat duty, the total annualized cost or even over the environmental impact, by combining the capabilities of process simulation packages with optimization tools and life cycle assessment (LCA) [9,10].
2. System description
Figure 1 represents the general configuration of a HIDiC. The basic components are the rectifying section (RS), the stripping section (SS), the compressor (Comp), the reboiler (R), the condenser (cond), the expansion valve (EV) and the intermediate heat exchanger (IHE). An additional pre-heater (p-H) is also show in Figure 1, just before the compressor, to avoid the possibility of presence of a liquid phase in the compressor. This pre-heated is controller by the hysys adjustment tool (A) that introduce the minimum amount of heat in order to have a saturated vapour in the stream leaving the compressor.
Figure 1. General configuration of a internally heat-integrated column.
The system operation at two pressure levels is as follows. The feed steam (F) is introduced in the top of the stripping section. The distillate (D) and residue (R) streams are the products living its corresponding section. The vapour stream leaving the stripping section (V1,SS) is compressed before to be introduced in the rectifying section. As commented before, the stream V1,SS could be preheated in order to avoid the presence of any liquid phase in the compressor. The liquid stream leaving the rectifying section (Ln,RS) is expanded before to be introduced in the stripping section. The overall internal heat transfer (QIHT) is exchanged between the two rectifying and stripping section by the IHE equipment.
In order to show an example of the advantages that the proposed approach offers, Figure 2 shows the results of a sensitivity analysis where, for a defined configuration of a HIDiC (i.e. concrete number of trays and pressures in both sections,) the overall internal heat transfer (QIHT) has been varying from 0 to 2000 kW.
Figure 2. Condenser and reboiler duties, and compressor shaft work vs overall internal heat transfer (QIHT).
As we can observe in Figure 2, the existence of only one internal heat exchange between both sections can produce interesting reduction in the total energy demand and therefore significant cost saving, without being necessary heat exchange between each tray of both sections. This results are consistent with those proposed by different authors [2,11,12].
A hybrid simulation-optimization design method has been suggested and applied to analyse the advantages and benefits of using internal heat exchangers in the distillation columns. The effect of the overall internal heat transfer on the energy requirements has studied for a defined HIDiC configuration. This procedure is based on the robustness of the commercial simulators.
The proposed approach can also be applied to vapour recompression columns (VRC) and where
Acknowledgements: The authors would like to acknowledge financial support from the Spanish Ministerio de Ciencias e Innovación (PPQ, CTQ2009-14420-C02-02).
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GDP General Disjunctive Programming
MINLP Mixed-integer non-linear programming