Analysis and optimization of heat integrated distillation sequences

This work proposes a heuristic-evolutionary method to optimize heat-integrated distillation sequences. The key to this method is the use of a quantitative measure which evaluates the Overall Difficulty of Separation for a separation problem (ODOS). ODOS is defined as ODOS=(Q/dt)/F where Q is the condenser duty in the column, dt is the temperature difference between the condenser and reboiler and F is a factor, proposed by Nadgir and Liu (1983), which reflects the balance between the top and bottom products. F is defined as F = Min (V/L, L/V) where V and L are respectively the amounts of the top and bottom products.

The ODOS, developed in this work, not only enabled selection of a good upper bound for the cost of the 'optimal' sequence, but allowed prediction of a band of 'good' sequences for the problems considered in this work.

This quantitative measure was also successful in eliminating the non-competitive sequences from further consideration. The selection of a band of sequences enables the designer to choose the sequence for actual implementation from objectives other than economics, like ease of control, plant layout, etc. The method involves no intermediate evaluation of the candidate sequences, unlike traditional evolutionary methods.

This work has also confirmed the decomposition principle of Sophos et al (1981, 1982), which stated that the separation sequencing problem should be tackled first and then heat integration should be attempted for the 'good' sequences identified.

This method was tested by calculating the costs for each sequence with and without integration for seven of the nine examples considered in this work. For the other two examples, as well as for the first seven examples, values reported in the literature were used to assess this method. This method has also been compared with the standard heuristic methods in the literature. This method compares very favorably with the literature methods in terms of locating a good initial sequence, identifying competitive sequences, eliminating non-competitive sequences and providing a quantitative measure for the competitiveness of a sequence.