Conventional Distillation Sequences Research Articles

Effect of Recycle Streams on the Closed-Loop Dynamics of Thermally Coupled Distillation Sequences

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 in this work by using rigorous dynamic simulations. The incorporation of thermal links into conventional distillation sequences can lower the energy consumption up to 40% without introducing additional control problems to the resulting thermally coupled distillation sequences. In some cases, the thermally coupled distillation sequences outperformed the dynamic behavior of the conventional distillation sequences for set point tracking. This result is important to establish that thermally coupled distillation sequences not only can have significant energy savings but also good dynamic properties.

Analysis of Control Properties of Thermally Coupled Distillation Sequences for Four-Component Mixtures

The control properties of thermally coupled distillation sequences for the separation of quaternary mixtures of hydrocarbons were compared to those of conventional distillation sequences. Two thermally coupled systems were investigated: a thermally coupled distillation sequence with side columns and a Petlyuk-type column. The preliminary steady-state design of the thermally coupled distillation sequences was obtained by starting from a conventional distillation sequence and then optimizing for minimum energy consumption. The control properties of the sequences considered were obtained by using the singular value decomposition technique and closed-loop dynamic simulations under feedback control. It was found that, for some frequencies, the thermally coupled distillation sequences present theoretical control properties similar to those of conventional distillation sequences, i.e., similar minimum singular values and condition numbers. Also, both types of distillation sequences show interactions when relative gain arrays were obtained. Then, when both types of distillation sequences were studied under the action of proportional integral controllers, the thermally coupled sequences also presented optimum dynamic responses similar to those obtained in the conventional distillation scheme. As a result, it can be concluded that thermally coupled distillation sequences can present theoretical control properties similar to those obtained in conventional distillation sequences. This result is significant because it lets one establish that the energy savings predicted for thermally coupled distillation sequences are achieved without introducing additional control problems.

A comparison of the feedback control behavior between thermally coupled and conventional distillation schemes

A comparative analysis of the feedback control responses to set point changes of three thermally coupled distillation schemes and two conventional distillation sequences for the separation of ternary mixtures is presented. Designs for the thermally coupled schemes were obtained and optimized for energy consumption to link their energy characteristics to their dynamic behavior. For the comparison of the dynamic behavior, responses to set point changes under closed loop operation with proportional-integral (PI) controllers were obtained. For each separation scheme, the parameters of the PI controllers of the three composition control loops were optimized using the integral of the absolute error criterion. The effects of feed composition and of the ease of separability index were considered. The dynamic tests showed that in many cases the thermally coupled distillation schemes outperformed the dynamic responses of the conventional distillation sequences. The results indicate that there exist cases in which the energy savings provided by the thermally coupled systems do not conflict with their dynamic properties.

Thermodynamic Analysis of Thermally Coupled Distillation Sequences

Thermodynamic efficiency calculations have been performed for the separation of ternary and quaternary mixtures of hydrocarbons in both conventional and thermally coupled distillation sequences. When ternary mixtures were considered, energy savings achieved in the thermally coupled distillation sequences were between 10 and 30% in comparison to the two conventional distillation sequences. Regarding thermodynamic efficiency, thermally coupled distillation sequences presented the highest values in almost all of the cases considered. When the analysis was extended to the separation of quaternary mixtures, the thermally coupled distillation sequences show acceptable energy savings and the thermodynamic efficiency of a thermally coupled distillation sequence linked to a side stripper and a side rectifier was better than that obtained in the conventional distillation sequence; however, for the thermally coupled distillation with prefractionator (Petlyuk-type column), the second law efficiency was low when compared to that of the conventional distillation sequence for the separation of a quaternary mixture with a low content of intermediate components.

Energy-Efficient Designs of Thermally Coupled Distillation Sequences for Four-Component Mixtures

A design procedure for thermally coupled distillation sequences for the separation of four-component mixtures is presented. The schemes analyzed include a sequence with a side rectifier and a side stripper and a sequence with a prefractionator (Petlyuk-type column). Initial designs for the thermally coupled distillation sequences are obtained from the tray distribution of a conventional distillation sequence and then optimized for energy consumption using two interconnecting flows as search variables. Several tray arrangements for each thermally coupled design were analyzed, and the results show that the sequence with side columns can reach a lower energy consumption.

Analysis of Control Properties of Conventional and Nonconventional Distillation Sequences

In this paper a controllability analysis of seven distillation sequences for the separation of ternary mixtures using singular value decomposition and closed-loop responses under feedback control is presented. The results show that nonconventional distillation sequences such as thermally coupled distillation sequences can have better control properties than nonintegrated schemes, such as the conventional direct and indirect sequences, distillation systems with side streams, and sequences with three conventional distillation columns.

Reducing energy requirements in unit operations

This paper discusses the role of the chemical engineer in the management of energy through process design with particular reference to multicomponent distillation combined with heat exchanger networks. Since there is no analytical solution it is necessary to consider actual cases. Four systems of four components and one of five are considered in detail. Both are near-ideal in their vapour—liquid equilibrium relationships and for each system a wide range of feed compositions is considered for each of two degrees of recovery (90% and 99%). For no heat interchange between streams, it is found that the conventional sequential distillation sequence is the optimum in energy terms for all but four cases. When heat interchange is considered, however, nearly half the four-component cases and one-third of the five-component cases require configurations other than this simple sequential one for minimal energy consumption. The rules suggested by Rathore et al. are shown to be valid in including all feasible matches but do not exclude all non-feasible ones.