Application of a thermally coupled distillation column with separated main columns to gas concentration process

A modified fully thermally coupled distillation column (FTCDC) for operability improvement is utilized in a gas concentration process. The column consists of a prefractionator and two separated main columns having high distillation efficiency and flexible control structure. The operability of the proposed column is evaluated by examining the open-loop dynamic responses of step input variations with the HYSYS simulation. The simulation result indicates that the modified system can give better control than the original FTCDC. The energy saving and reduction of construction cost are discussed, and the ease of vapor flow manipulation and the elimination of a compressor in the vapor transfer are also evaluated as possible improvements.     

Approximate design of fully thermally coupled distillation columns

An approximate design procedure for fully thermally coupled distillation columns (FTCDCs) is proposed and applied to example ternary systems. The procedure gives a fast solution of structural and operation design for a preliminary study of the FTCDC. The structural information resolves the design difficulty, caused from the interlinking streams of the column, which is encountered when a conventional design procedure is implemented. The design outcome explains that how the thermodynamic efficiency of the FTCDC is higher than that of a conventional two-column system and how the system of a separate prefractionator is different from a dividing wall structure. From the design result of three example systems with three different feed compositions, the useful performance of the proposed scheme is proved. In addition, the structural design of the FTCDC gives better understanding of the system and leads to high efficiency design of the column.     

Design of a fully thermally coupled distillation column based on dynamic simulations

A dynamic simulation of a fully thermally coupled distillation column is conducted for the design of a possible operation scheme, and its performance is examined with an example process of butanol isomer ternary system. The outcome of the dynamic simulation indicates that the column can be operated by using a 3 × 3 control structure. The structure consists of three controlled variables of the compositions of overhead, bottom and side products and three manipulated variables of the flow rates of reflux and steam and liquid split ratio between a main column and a prefractionator. This paper was presented at The 5th International Symposium on Separation Technology-Korea and Japan held at Seoul between August 19 and 21, 1999.     

Design of a fully thermally coupled distillation column for hexane process using a semi-rigorous model

An industrial scale hexane process is designed for the implementation of a fully thermally coupled distillation column (FTCDC). A semi-rigorous material balance and Peng-Robinson equilibrium relation are utilized in the structural design. The operational design is conducted with a commercial design program, the HYSYS. The design outcome of the structural design indicates it to be comparable with the practical system of a conventional two-column arrangement in field operation, which shows the effectiveness of the design procedure implemented here. The procedure is good for the system of many components found from actual field applications. In addition, an investigation of the energy requirement of the FTCDC and a conventional system shows that an energy saving of 34.1% is available with the FTCDC.     

Approximate estimation of operational variables in fully thermally coupled distillation columns using pseudo-pinch points

An approximate procedure for the estimation of operational variables in fully thermally coupled distillation columns (FTCDCs) using two pseudo-pinch points of the feed and side draw trays is proposed, and its performance is examined with two example processes. The estimates from the proposed procedure show some 20% error compared with the results of rigorous simulation using commercial design software, the HYSYS. In addition, the relation between vapor flow rate and composition at one stage above the feed tray-required in the estimation of operational variables of the FTCDC-is analyzed to give information for the selection of the feed tray composition. A preliminary evaluation of operational variables helps to screen unrealizable design obtained often from iterative trial procedures employing the mathematical programming.     

Dynamic analysis of thermally coupled distillation sequences with undirectional flows for the separation of ternary mixtures

The Petlyuk distillation system has been considered with special interest because of the high energy savings it can provide with respect to the operation of sequences based on conventional columns. The original design of the Petlyuk structure, however, shows two interconnections that seem to affect its operational and controllability properties. To overcome this problem, two alternate structures have been suggested that use unidirectional flows of the vapor or liquid interconnecting streams. In this work, a comparative analysis of the control properties of the Petlyuk column and the alternate arrangements with unidirectional interconnecting flows is presented. Through a singular value decomposition analysis, it is shown that the alternate schemes provide better theoretical controllability properties than the Petlyuk system. Closed loop tests using proportional-integral controllers were also carried out, and the results showed that, in most of the cases considered, the alternate arrangements improved the dynamic responses of the Petlyuk column. Such arrangements, therefore, show promising perspectives for its practical consideration.     

Esterification of fatty acids in a thermally coupled reactive distillation column by the two-step supercritical methanol method

Biodiesel fuel has been shown as a clean energy alternative to petroleum diesel. Conventional biodiesel production involves the use of catalyst, which implies high energy consumptions for the separation of both the catalyst and the by-products of the reaction, including those of the undesirable reaction of saponification. Recently, a process involving the use of short-chain alcohols at supercritical conditions has been proposed (Saka-Dadan process); one of the main advantages of that process is that it avoids the need for a catalyst as well as the occurrence of the saponification reaction. However, although the process requires less pieces of equipment than the conventional one, its energy requirements are still high, making biodiesel fuel more expensive than petroleum diesel. This work proposes the use of reactive distillation and thermally coupled reactive distillation configurations to produce biodiesel fuel by the supercritical methanol method. First-order kinetics is used to represent the esterification reaction, obtaining high conversions in a single shell. Both of the configurations proposed reduce energy requirements when compared to the conventional (Saka-Dadan) process. Calculations were also performed to estimate CO₂ emissions, thermodynamic efficiency and cost. The thermally coupled reactive distillation configuration shows to be the best alternative in terms of energy consumption, CO₂ emissions and thermodynamic efficiency. Further, cost estimations also show that the use of a thermally coupled scheme considerably reduces both utilities and capital costs.     

Towards further internal heat integration in design of reactive distillation columns—Part III: Application to a MTBE reactive distillation column

According to the principle introduced in the first two papers of this series, seeking further internal heat integration between reaction operation and separation operation during the synthesis, design, and operation of a reactive distillation column synthesizing methyl tertiary butyl ether (MTBE) from methanol and isobutylene is investigated. Although the MTBE reactive distillation column is characterized by complicated thermodynamic properties and multiple steady states, a substantial reduction of energy requirement and capital investment can still be achieved with the consideration of further internal heat integration between the reaction operation and the separation operation in the two existing steady states. Dynamics and operation of the resultant process designs are then examined in terms of static and dynamic analysis and sharp improvement in process dynamics and controllability is clearly identified through intensive comparison against the simple process design without the consideration of further internal heat integration between the reaction operation and the separation operation involved. It is demonstrated that the more synergistic relationship evolved during the reinforcement of internal heat integration should account for the dramatic improvement in process dynamics and controllability.     

A hydraulics-based heuristic strategy for capacity expansion retrofit of distillation systems and an industrial application on a light-ends separation plant

A new indicator to represent a maximum capacity expansion rate of distillation processes is presented in this article. A hydraulics-based heuristic strategy consisting of three levels is proposed to address the capacity expansion retrofit redesign of distillation systems. The first level is to evaluate an existing process to get the indicator which reflects quantitatively the capacity expansion rate, where a fractional utilization of area (FUA) method is also employed to screen the process bottlenecks. In the second level, the indicator is used to find the available debottlenecking measures from a measure library which is based on heuristics. In the last level, a new economy indicator called specific annual cost (SAC) is used to sort the available debottlenecking measures, and the corresponding process flowsheets and operation parameters are determined. An industrial capacity expansion retrofit for a light-ends separation plant is conducted so as to evaluate the performance of the proposed indicators and strategy in screening the available retrofit alternatives. Modifications on feeding location, operating pressure, number of theoretical stages, and feeding thermal condition, etc. are proposed to remove the bottlenecks. The maximum capacity expansion rate of the plant is about 40.0%, and the near-optimum retrofit options for different capacity expansion rates such as 20%, 25%, 30%, 35% and 40% are also presented. The results show that the proposed capacity expansion strategy is capable to resolve the issues associated with the retrofit redesign in a systematic and relatively simple way.     

Simulation and analysis of extractive distillation process in a valve tray column using the rate based model

Valve trays are becoming popular in the chemical process industries owing to their flexibility to handle a wide range of vapor throughputs. Using the rigorous rate based model, the importance of the non-equilibrium approach is demonstrated for a typical extractive distillation process in a Glitsch V-1 valve tray column. Simulation results based on an in-house developed code indicated that the rate based model predictions for a valve tray column operation showed significant differences relative to the equilibrium model. Even small errors in product purities translated into nonoptimal feed stage locations and inaccurate number of stages required. The counter-intuitive effect of high reflux ratio on separation is explained.     

Application of the dividing wall column to olefin separation in fluidization methanol to propylene (FMTP) process

Dividing wall column (DWC) is shown to be energy efficient compared to conventional column sequence for multi components separation, which is used for olefin separation in fluidization methanol to propylene process in the present work. Detailed design for pilot DWC was performed and five control structures, i.e. composition control (CC), temperature control (TC), composition-temperature control (CC-TC), temperature difference control (TDC), double temperature difference control (DTDC) were proposed to circumvent feed disturbance. Sensitivity analysis and singular value decomposition (SVD) were used as criterion to select the controlled temperature locations in TC, CC-TC, TDC and DTDC control loops. The steady simulation result demonstrates that 25.7％and 30.2％duty can be saved for condenser and reboiler by substituting conventional column sequence with DWC, respectively. As for control structure selection, TC and TDC perform better than other three control schemes with smal er maximum deviation and shorter settling time.     

Towards further internal heat integration in design of reactive distillation columns—Part IV: Application to a high-purity ethylene glycol reactive distillation column

In the first three papers of this series, it has been shown that strengthening internal heat integration within a reactive distillation column involving reactions with high thermal effect is really effective for the reduction of utility consumption and capital investment besides the improvement in process dynamics and operation. One important issue that remains unstudied so far is the influences of reaction selectivity upon the reinforcement of internal heat integration, since the reaction operation is often accompanied by side-reactions and the maintenance of a high selectivity is extremely necessary in process synthesis and design. A reactive distillation column synthesizing high-purity ethylene glycol through the hydration of ethylene oxide is chosen and studied in this work. Because of the unfavorable physicochemical properties of the reacting mixture separated (e.g., the fairly large volatility between the reactants and the existence of a consecutive side-reaction), the process represents a challenging problem for the reinforcement of internal heat integration. Intensive comparison is conducted between the process designs with and without the consideration of further internal heat integration between the reaction operation and the separation operation involved, and it has been found that seeking further internal heat integration still leads to a substantial reduction of energy requirement, in addition to a further abatement in capital investment. Moreover, a favorable effect is again observed upon the process dynamics and operability. These striking outcomes manifest evidently that seeking further internal heat integration should be considered in process synthesis and design irrespective of what a reaction selectivity has been assigned.     

Simulation of CO<Subscript>2</Subscript> removal with aqueous sodium glycinate solutions in a pilot plant

In this paper, density, viscosity, surface tension and vapor pressure of aqueous sodium glycinate solutions of different mass fractions (0.1–0.5) at different temperatures (20–100°C) were simulated using Pro/II (version 6.01), a commercial process simulator, and compared with corresponding experimentally measured data. It was found that simulated data of physicochemical properties compared well with corresponding experimental data. We have also predicted concentration of CO₂ with each ideal stage in an absorber/stripper tower.     

Adsorptive separation of rhenium and rhodium in nitric acid solution using a column packed with an extractant impregnated resin

An adsorptive separation of rhenium and rhodium was performed by using a jacketed glass column packed with an extractant impregnated resin (EIR). The EIR bed showed a successful separation of rhenium and rhodium with about 122 BV of a breakthrough volume. The breakthrough behavior in the column was modeled to assess the mass transfer resistances in the column. The model predicted the column dynamics for rhenium quite well by assuming a homogeneous diffusion in the particle phase. The effective diffusivities of rhenium were in the order of 10⁻⁷ cm²/min. The EIR loaded beds could be eluted with a high purity of more than 99% by using a nitric acid solution.