Ideal Heat Integrated Distillation Column Research Articles

A hybrid heat integration scheme for bioethanol separation through pressure-swing distillation route

In this contribution, a hybrid thermal integration scheme is proposed for a pressure-swing distillation (PSD) column by combining an internally heat integrated distillation column (HIDiC) with vapor recompression column (VRC). The purpose of this article is three fold: first, it designs a HIDiC for a PSD system with a reduced number of internal heat exchangers. Second, it develops a hybrid column configuration by integrating that HIDiC, which is devised based on the thermal driving force existed between the two diabatic columns, and VRC that is devised based on the thermal driving force existed between the top and bottom of the same high pressure column, yielding an ideal HIDiC–VRC configuration. Third, it provides a comprehensive comparison between the HIDiC-alone and its hybrid HIDiC–VRC structure with reference to a conventional standalone PSD column. To evaluate the performance of all these configurations, we estimate the two performance indexes, namely energy consumption and total annual cost, for an example of a bioethanol dehydration system.

Simplified design and control of an ideal heat-integrated distillation column (ideal HIDiC)

Three kinds of process simplification are derived for an ideal heat-integrated distillation column (ideal HIDiC: S–0) using three heat exchangers between its rectifying and stripping sections. These include one with the heat exchangers located in the top, middle, and bottom of the ideal HIDiC, respectively (S–1), one with the vapor flows from the rectifying section and compressor as hot flows in the three heat exchangers (S–2), and one with the vapor flow from the rectifying section as sole hot flows in the three heat exchangers (S–3). The impact of process simplification is examined in terms of the separation of a binary mixture of ethylene and ethane. The S–1 appears inferior to the S–0 in thermodynamic efficiency, but with comparable dynamic behaviors. The S–2 and S–3 need lower capital investment and operating cost than the S–1, but with a sharp degradation in process dynamics and controllability. Although the augment of heat transfer areas leads to a reduction in operating cost, almost no favorable effect is identified on process operation. To derive a feasible design of the ideal HIDiC, one must therefore exercise a careful trade-off between design and operation via the locations of these heat exchangers. © 2012 Curtin University of Technology and John Wiley & Sons, Ltd.

Assessment of the Implementation of Heat-Integrated Distillation Columns for the Separation of Ternary Mixtures

An ideal HIDiC (internal heat-integrated distillation column) makes the reboiler heat duty equal to zero, with the heat requirements for the separation transferred to a compressor. In real applications, there is a limit on the energy integration that can be achieved depending on the separation problem. In this work, a design method based on the column grand composite curve (CGCC) is presented. The CGCC is used to establish the integration capabilities between the rectifying and stripping sections of the system. The method is used to develop HIDiC separation sequences for ternary mixtures, and the effects of HIDiC implementation on energy, economics and greenhouse-gas emissions savings are addressed. The results of several case studies show that a good deal of energy savings can be achieved with this type of arrangement, but that the work required by the compressor can overcome the energy savings achieved through the process integration.

REDUCING STATIC DEVIATIONS OF PRODUCT QUALITIES IN THE TEMPERATURE CONTROL OF AN IDEAL HEAT-INTEGRATED DISTILLATION COLUMN

Two strategies are proposed in this work for the reduction of static deviations of product qualities in the dual-point temperature control of a simulated ideal heat-integrated distillation column. The key to achieve this purpose is to sense the changes in operating conditions and make appropriate adjustments simultaneously to the set-points of the top and bottom control loops. The first method is based on the inferential signals extracted from the composition of products and the second one from the temperatures of the top and bottom stages. Both strategies are intensively studied through the operation of the simulated ideal heat-integrated distillation column separating a binary equimolar mixture of benzene and toluene, and it is found that they could work effectively to decrease the static deviations in product qualities. The strategies are characterized by great simplicity in principle and a relatively small effort in process modeling, thereby allowing wide applications in the operation of various distillation columns with a dual-point temperature control scheme.

A novel simplified configuration for an ideal heat-integrated distillation column (ideal HIDiC)

Although the ideal heat-integrated distillation column (ideal HIDiC) is much more thermodynamically efficient than its conventional analogues, its applications in the chemical and petrochemical process industries have been restrained due to the great difficulties and complexities in the design and implementation of internal heat integration between the rectifying section and the stripping section. For the avoidance of these difficulties and complexities, a novel simplified configuration for the ideal HIDiC, termed the SIHIDiC, is proposed and studied in this paper. Only three internal heat exchangers are used to approximate the internal heat integration, and their locations and sizes are key decision variables for process synthesis and design and should be considered to enhance thermodynamic efficiency in process development. A simple stepwise procedure is thus derived for process synthesis and design, and the SIHIDiC is then evaluated through intensive comparison with conventional distillation columns and the ideal HIDiC in terms of the separations of ethylene/ethane and benzene/toluene binary mixtures. The results obtained indicate that the SIHIDiC could be an excellent candidate to approximate the ideal HIDiC with reduced capital investment and somewhat similar (if not smaller) operating cost. The SIHIDiC offers essentially a much simpler way than the current available methods to design and implement the concept of the ideal HIDiC into separation processes.

Dynamic simulation for internally heat-integrated distillation columns (HIDiC) for propylene–propane system

This paper reports a dynamic simulation study of the internally heat-integrated distillation column (HIDiC) using equilibrium-based models. First, three different HIDiC structures, i.e. an ideal HIDiC, a HIDiC with a pre-heater, and a HIDiC with a reboiler, are analyzed by control degrees of freedom (DOF). The reboiler is considered to be a necessary part of the HIDiC from DOF analysis, thermodynamic analysis, and engineering judgment. Then, a heuristic HIDiC control configuration including a bottoms reboiler control is proposed. A modular structured simulator for dynamic distillation columns using MESH equations is developed. The simulator also considers: (1) variable column pressure on each tray of the rectifying section, (2) dynamic vapor holdup, and (3) dynamic energy balance. In addition, the SRK equation of state is employed for estimating thermodynamic properties. A typical medium-pressure HIDiC for separation of propylene and propane explored by Olujic et al. [Olujic, Z., Sun, L., de Rijke, A., & Jansens, P. J. (2006). Conceptual design of an internally heat-integrated propylene–propane splitter. Energy, 31, 3083] is adopted as numerical examples for dynamic simulation studies.

A totally heat-integrated distillation column (THIDiC) – the effect of feed pre-heating by distillate

An ideal heat-integrated distillation column (ideal HIDiC) is characterized by external zero-reflux and zero-reboil ratio operation. Since the distillate is a high-pressure vapor phase flow, it can be used to pre-heat the feed to be separated, thereby giving rise to a totally heat-integrated distillation column (THIDiC). Although the THIDiC is more thermodynamically efficient than the ideal HIDiC, it is found that the heat integration between the distillate and feed turns it into an open-loop integrating process and poses additional difficulties to process operation. Therefore, a careful decision must be made on the selection between the ideal HIDiC and the THIDiC during process development. In this paper, separation of a binary equimolar mixture of benzene and toluene is selected as an illustrative example. Both process design and operability analysis are conducted, with special emphasis focused on the characteristics of feed pre-heating with distillate. The results obtained show deep insight into the design and operation of the THIDiC.

Design and control of an ideal heat-integrated distillation column (ideal HIDiC) system separating a close-boiling ternary mixture

Despite the fact that a stand-alone ideal heat-integrated distillation column (ideal HIDiC) can be thermodynamically efficient and operationally stable, the application of an ideal HIDiC system to separate a close-boiling multi-component mixture is still a challenging problem because of the possibility of strong interactions within/between the ideal HIDiCs involved. In this work, employment of two ideal HIDiCs to separate a close-boiling ternary mixture is studied in terms of static and dynamic performance. It is found that the ideal HIDiC system can be a competitive alternative with a substantial energy saving and comparable dynamic performance in comparison with its conventional counterpart. The direct sequence appears to be superior to the indirect sequence due to the relatively small vapor flow rates to the compressors. Controlling the bottom composition of the first ideal HIDiC with the pressure elevation from the stripping section to the rectifying section helps to suppress the disturbances from the feed to the second ideal HIDiC. Special caution should, however, be taken when the latent heat of the distillates is to be recovered within/between the ideal HIDiCs involved, because a positive feedback mechanism may be formed and give rise to additional difficulties in process operation.

Temperature control of an ideal heat-integrated distillation column (HIDiC)

In this work, a novel temperature control scheme is derived for an ideal heat-integrated distillation column (ideal HIDiC), in which a temperature difference between two stages is designated as the controlled variable to circumvent the side-effect of continuous pressure variations in the rectifying section. For making an appropriate compensation to the changes in operating conditions, an inferential signal extracted from the feed stage, n / 2 + 1 , is used to adjust the set-point of the temperature difference controller. Control of two ideal HIDiCs, separating, respectively, a binary mixture of benzene and toluene and an ideal ternary mixture of hypothetical components A–C is studied. It is demonstrated that the proposed temperature control scheme can retain a stable operation around the vicinity of the nominal steady state with improved dynamic performance and tolerable steady state discrepancies in comparison with the direct composition control scheme. Moreover, the proposed temperature control scheme is also found to be quite robust to the selection of temperature measurements.

Seeking synergistic effect—A key principle in process intensification

Process intensification calls for the combination of two or more conventional unit operations within one framework, thus giving rise to a challenging coordination problem in process synthesis and design. Seeking synergistic effect among all the conventional unit operations involved is found to be an important principle for process development. Three cases of process intensification are studied to evaluate the principle, including: (i) a hypothetical reactive distillation system; (ii) a reactive distillation column for the hydration of ethylene oxide; (iii) an ideal heat-integrated distillation column. It is demonstrated that simply combining two or more conventional unit operations together does not necessarily tap out the full potentials of capital investment reduction and energy saving. Only after the synergistic effect has been deliberately considered in process synthesis and design, can process intensification be carried out in the most effective manner. Furthermore, seeking synergistic effect appears frequently to yield improved process dynamics and provide additional redundancy to process operation because the underlying conflicts could be attenuated among all the conventional unit operations involved.

The Influences of Pressure Distribution on an Ideal Heat-Integrated Distillation Column (HIDiC)

In this paper, the static and dynamic influences of pressure distribution are examined on an ideal heat-integrated distillation column. In the aspect of process design, pressure distribution is found to have an effect upon the thermodynamic efficiency and it appears beneficial to explore an optimum pressure distribution during process development. From the viewpoint of process operation, the dynamics of pressure distribution may give rise to a severe deterioration on process controllability. Therefore, it should be taken into account in control system design for an ideal HIDiC.

A Simple Method for Modeling Process Asymmetry

Process asymmetry often introduces severe difficulties to process modeling and worsens control system performance. In this work, a simple modeling framework is proposed, which can effectively trade-off the difference between positive and negative process dynamics. It is characterized by small modeling effort in the consideration of process asymmetry. Modeling and control of a nonlinear high-purity ideal heat-integrated distillation column is conducted and the results obtained demonstrate the effectiveness of the modeling method proposed.

Interpreting Design of an Ideal Heat-Integrated Distillation Column through Exergy Analysis

Exergy analysis of an ideal heat-integrated distillation column is conducted in this paper. It is found that the process is most favourable to the separation of binary close-boiling mixtures. Pressure elevation from stripping section to rectifying section appears to be a critical design variable that can affect the thermodynamic efficiency of the process. Feed composition presents almost no effect upon internal heat integration between the rectifying section and the stripping section. Although an implicit limitation has been imposed on the process throughput, it can be obviated through a careful trade-off between capital investment and operating cost during process development. Comparison against a conventional distillation column is also conducted, which shows the great potential of internal heat integration between the rectifying section and the stripping section.

Reducing CO 2 emissions of internally heat-integrated distillation columns for separation of close boiling mixtures

A model developed originally for crude oil distillation units has been applied to a standalone internally heat integrated distillation column (HIDiC) to evaluate emissions levels and to generate design options for direct carbon dioxide emissions reduction. Simulations indicate that for propylene–propane separation, an ideal (no reboiler) HIDiC enables a reduction in emissions of 83% and of 36%, compared to conventional and heat pump alternatives, respectively. Integrating a turbine to drive the compressor, in conjunction with a suitable fuel is the key to the minimization of the emissions associated with the operation of a HIDiC. Importantly, while substantial emission reductions are achieved, the process economics are improved.

Parameter analysis and optimization of ideal heat integrated distillation columns

Parametric analysis is performed for ideal heat integrated distillation columns (HIDiC) and comparative studies are made with conventional distillation columns. Implications of process design and operation variables are clarified and heuristics are provided for the effective process design. A generalized process configuration is suggested, which is demonstrated to have both higher energy efficiency and higher flexibilities than its original configuration. Simulation studies are conducted and the obtained results confirm the conclusion.

Operation of a bench-scale ideal heat integrated distillation column (HIDiC): an experimental study

Experimental study of an ideal heat integrated distillation column (HIDiC) is introduced in this work. It is found that the ideal HIDiC can be operated very smoothly, with no special difficulties compared with its conventional counterparts. The higher energy efficiency of the ideal HIDiC is confirmed by the bench-scale experiments. Reflux-free and/or reboil-free operations of the ideal HIDiC are also demonstrated to be feasible by the experiments.

A new configuration of ideal heat integrated distillation columns (HIDiC)

A new configuration for ideal heat integrated distillation columns (HIDiC) is proposed by further heat integration between its overhead product and feed. This modification makes the ideal HIDiC more self-support and imposes fewer constraints to the environment. The added heat integration is different in nature from the one between the rectifying and the stripping sections. The latter is self-regulating, while the former is not. Open-loop integrating process is produced by the added heat integration, which makes the process more difficult to control than before. It is therefore extremely important to explore the interaction and tradeoff between process design and operation. Simulation studies are conducted to evaluate the process operation feasibility and it is found that the process can be well controlled through manipulations of pressure difference between the rectifying and the stripping sections and feed thermal condition.

On the Startup of Ideal Heat Integrated Distillation Columns (HIDiC).

Startup of an ideal heat integrated distillation column (HIDiC) is considered in this paper. A procedure for the startup operation is suggested by taking the special characteristics of the ideal HIDiC into account. An overhead trim-condenser and a bottom trim-reboiler look necessary to be added to the process configuration during this period. However, it is found that the ideal HIDiC can also start smoothly without the presence of the bottom trim-reboiler.The transition from semi-continuous phase to normal operation mode is investigated and compared by employing two control systems. One is a gain-scheduled decentralized PI controller, the other is a nonlinear process model-based one (NPMC). It appears that the ideal HIDiC could start quite smoothly with no special difficulties compared with its conventional counterparts. The NPMC holds more advantages than the gain-scheduled PI controller, since the former provides safer and more economic startup operation of the ideal HIDiC than the latter.

Simulation and Nonlinear Control of an Ideal Heat Integrated Distillation Column (HIDIC)

The ideal heat integrated distillation column (HIDiC) is a highly interactive and non-linear process, which necessties efficient nonlinear control methods for the process operation. An efficient steady state model is proposed for the fast simulation of the process. A nonlinear process model-based controller, derived in the framework of generic model control, is designed and applied to the process. Simulations show that the controller outperforms PI controllers in operation of the ideal HIDiC. Furthermore, the regulatory performances are significantly better than those of linear model-based controllers, indicating the preference of using the controller to operate the ideal HIDiC.

Evaluating control structures for a general heat integrated distillation column (general HIDiC)

The assessment of control configurations for a general HIDiC (an ideal heat integrated distillation column incorporated with a overhead condenser and bottom reboiler structure) is addressed in this work. It is found that the double ratio control configuration, (L/D, V/B) is still the best one among all the possibilities. Moreover, the pressure difference between the rectifying and the stripping sections and the feed thermal condition are expected to be consistent manipulative variables for the control of the general HIDiC. The control configuration, (pr-ps, q), appears to be a feasible one for the process operation. The performances of the general HIDiC can be substantially improved by employing efficient multivarible control algorithms.