(120g) Effect of Recycle Streams on the Closed-Loop Dynamics of Thermally Coupled Distillation Sequences

Josué Santos-Méndez and Salvador Hernández* Universidad de Guanajuato Facultad de Química Noria Alta s/n Guanajuato, Gto., 36050, México Abstract The effects of thermal links on the closed loop dynamics of thermally coupled distillation sequences for the separation of quaternary mixtures have been studied by using rigorous dynamic simulations. The introduction of thermal links in conventional distillation sequences can lower the energy consumption up to 40% without additional control problems in the corresponding thermally coupled distillation sequences. In some cases, the thermally coupled distillation sequences outperformed the dynamic behaviour of the conventional distillation sequences for set point tracking. This result is very important to establish that thermally coupled distillation options not only can have significant energy savings but also good dynamic properties.

Keywords: Thermally coupled distillation, energy savings, dynamic behaviour

1. Introduction It is well known that conventional distillation sequences consume large amounts of energy in the reboilers; as a result, researchers are interested in developing distillation schemes that can reduce both energy requirements and capital costs. One choice of great interest is the thermally coupled distillation sequences (TCDS), which can lower the energy consumption around 30% in contrast to conventional distillation trains for the separation of ternary mixtures [1,2,3,4] and also have proper dynamic properties [5,6,7]. It has been explained in the works of Triantafyllou and Smith [1] and Hernández et al. [8] that the TCDS options can reduce the energy consumption because they do not have re-mixing, in contrast to conventional distillation sequences. The re-mixing can be explained if one considers the separation of a ternary mixture (A,B,C) by the conventional direct distillation sequence. In the first column of the direct distillation sequence, component A is obtained as overheads and components B and C are obtained as bottoms product which is separated in the next column. If the composition profile of the intermediate component B in the liquid phase of the first column is analysed, the composition of component B increases below the feed stage until a maximum and then the composition decreases. This is known as re-mixing and is associated with higher energy consumption because in order to re-purify the mixture, additional energy will be required in the next column. Energy savings are obtained with the introduction of optimal values of the flowrates of the recycle streams. Also, the introduction of a vapour or a liquid recycle stream eliminates a reboiler or a condenser respectively. When quaternary mixtures (A,B,C,D) are considered, there are five conventional distillation sequences (Figure 1) as reported by King [9]. At least five TCDS options can be obtained by the introduction of thermal links as shown in Figure 2. The main objective of this work is to compare the closed loop dynamic of the optimal conventional and thermally coupled distillation sequences. The dynamic behaviour is compared in terms of the integral of the absolute error (IAE).

2. Steady State Design of Conventional and Complex Distillation Schemes The design and optimisation methods for the conventional distillation sequences of Figure 1 are well known and can be implemented by using process simulators like Aspen Plus. The retrofit method used in this work in order to obtain TCDS options (Figure 2) with minimum energy consumption is very simple and only requires the introduction of recycle streams between the distillation columns. For example, for the direct distillation sequence, the reboilers in the first and second columns are eliminated by two vapour recycle streams. All the heating needed is supplied in the reboiler of the third column as shown in Figure 2a. When the indirect distillation train is considered, the condensers in the first two columns of the sequence are replaced by two liquid recycle streams, and the total reflux required is introduced in the last distillation column. The other three distillation sequences use liquid and vapour recycle streams (Figures 2c-2e). It is important to state that in order to guarantee optimal energy consumptions, the interconnecting streams must be varied until the minimum energy requirements are obtained. The energy-efficient designs of the conventional and thermally coupled schemes were obtained in Aspen Plus 11.1TM.

3. Dynamic Study and Case of Study The optimum conventional and thermally coupled distillation sequences were subjected to a study of their dynamic responses under the action of feedback PI controllers. Changes in the set points were considered and the optimal responses were chosen in terms of their IAE values. This study was completed in Aspen Dynamics 11.1TM. The separation of a mixture of n-pentane, n-hexane, n-heptane, n-octane with flows of 13.62, 9.08, 9.08 and 13.62 kmol/h respectively was studied. Compositions of 0.98, 0.97, 0.97 and 0.98 in mole fraction for products A, B, C and D respectively were assumed. The pressures in the columns were set in order to use cooling water in the condensers. The pairings in the control loops were selected according to practical aspects, the overheads compositions were tied to the reflux rates (Figure 3 and 4) and the bottoms compositions were controlled by manipulating the heat demands in the reboilers.

4. Results In the first part of the study, energy-efficient designs were obtained for the conventional and thermally coupled distillation sequences. The design and optimisation of the conventional distillation sequences followed standard methods in the process simulator Aspen Plus 11.1TM, but for the case of the thermally coupled distillation schemes, the optimisation task included a complete search in the recycle streams. In the optimised design of the conventional distillation sequence, thermal links were introduced as described above and the minimum energy consumption was detected in the thermally coupled distillation sequence. For the case of study considered in this work, Figure 5 shows the search for the optimal value of heat duty (QR); we can observe that the energy consumption depends strongly on the values of the vapour recycle streams (VF1 and VF2 in Figure 5). In this case, savings in energy of up to 40% were obtained in the thermally coupled distillation sequences in contrast to conventional distillation sequences (Table 1). Also, all the thermally coupled distillation sequences demanded less energy consumption in comparison to the corresponding conventional distillation sequence. Regarding the dynamic responses under the action of feedback controllers for changes in the set points, Table 2 shows the IAE values for a positive set point change in each product composition. The TCDS I outperformed the dynamic responses of the CDS I for set point changes in components A and B as shown in Table 2, but for components C and D the CDS I presented better values of the IAE. When a negative set point change of the same magnitude was implemented in each composition, a similar result was obtained. The CDS I presented the best dynamic behaviour according to its lower values of the IAE (Table 2) in comparison to the TCDS I for components B and D; on the other hand, when the set points of compositions of the products A and C were perturbed, the TCDS I presented better dynamic responses than those obtained in the CDS I. Figures 6 and 7 present the dynamic responses for the CDS I and TCDS I respectively when a positive change of 0.005 was implemented in the product composition A. It can be observed that the dynamic response of the TCDS I is better than that obtained in the CDS I. The dynamic response reached the new steady state faster. When a positive set point change in component D was considered, Figures 8 and 9 show the dynamic behaviour and for this case the CDS I presented better dynamic response than the TCDS I, but it is important to say that both types of distillation sequences can achieve the change in the set point in a short period of time. It can be stated that both types of distillation sequences present similar dynamic behaviour but the integrated distillation sequences have significant energy savings in contrast to the original uncoupled distillation sequences. This of course is an incentive for the retrofitting of conventional distillation sequences through the use of thermal links. Also, in the design of new plants, thermally linked distillation sequences can have important savings in both energy and capital costs, and the savings are obtained without creating operational and control problems.

5. Conclusions The effect of recycle streams on energy consumption and dynamic responses was studied for the separation of a quaternary mixture of hydrocarbons. The five conventional distillation sequences were subjected to retrofit by introducing mass recycle streams between the columns. Energy savings of up to 40% were achieved when the recycle streams were optimised. The dynamic responses of the thermally coupled distillation sequences were as good as those obtained in the conventional distillation sequences, and in some cases the dynamic responses of the thermally coupled distillation sequences were better than those of the conventional distillation schemes. These results establish that the energy savings can be achieved without introducing additional control problems as it might be expected originally.