Stripping Section Research Articles

Column configurations of continuous heterogeneous extractive distillation

AbstractIncreasing regulations and constraints have renewed interest for more efficient distillation process in separating azeotropic mixtures and close boiling components. We investigate the feasibility of heterogeneous extractive distillation process in a continuous column considering several feed point strategies for the entrainer recycle stream and for the main azeotropic feed. Depending on these choices, the heterogeneous distillation column is composed of one, two, or three column sections. A differential mass balance model enables to compute continuous liquid composition profiles of the rectifying, extractive, and stripping sections. Unlike homogeneous extractive distillation, reflux policy composed by a single or both decanted liquid phases is considered as well as the external feeding influence on the composition of the top column liquid stream. Limiting operating conditions of key parameters like the entrainer/feed flowrate ratio and reflux ratio required to obtain a target top and bottom product compositions are obtained. For illustration, separation of acetonitrile–water mixture using butyl acetate as a heavy heterogeneous entrainer is selected. In this case, withdrawal of a saddle binary heteroazeotrope is the main difference of this process compared with the well‐known heterogeneous azeotropic distillation process where the top vapor product is the lower boiling point of the ternary system. © 2007 American Institute of Chemical Engineers AIChE J, 2007

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.

Analysis and optimization of enantioselective extraction in a multi-product environment with a multistage equilibrium model

Based on a single stage model and experimental results for enantioselective extraction of amino acids and amino alcohols, simulations are carried out for a fractional extractor using a multistage equilibrium model. A higher pH, lower temperature, higher concentrations and a higher excess of extractant all result in higher purities in both product streams, but a higher wash stream is required to realise good yield and good purity. The improved purity can be explained from a higher operational selectivity in the single stage, from lower variation of the extraction factors along the extractor, or from a combination of these effects. Application of ‘reflux’ results in higher product purities at a lower wash stream requirement, but a lower process capacity. It was shown that various combinations of process conditions result in the desired product specifications (99% yield and 98% enantiomeric excess for both extract and raffinate). This leads to optimization possibilities in a multi-product environment: the more stages available above the minimum number for a specific compound, the larger the process capacity for that compound. All systems for which a single stage selectivity of 1.5 was obtained can be separated in a multi-product extractor containing 50 stages, evenly distributed over the wash and strip section.

Plutonium and other actinides behaviour in NEXT process

The behaviour of Pu and other actinides in some characteristic processes of the NEXT process, i.e. the crystallization process and the U–Pu–Np co-recovery process, was investigated with referring to the experimental results obtained in our experimental facility, CPF. In the crystallization process, the existence of Pu(VI) in the dissolver solution brought the co-crystallization of Pu(VI) with U, and low DF Pu. Such co-crystallization could be prevented by adjusting the Pu valence to Pu(IV). In the crystallization with the dissolver solution containing Pu(IV), meanwhile, DF Cs showed significant low values, which might be caused by the formation and crystallization of some kinds of Cs compounds with Pu(IV). In the U–Pu–Np co-recovery process which had high [HNO 3] feed solution obtained from the crystallization process, it was confirmed experimentally that almost all the Np could be extracted with U and Pu in the extraction section. Some considerable parameters, e.g. temperature, acidity of stripping solution, were pointed out for preventing the leakage of U and Pu to the solvent and for avoiding the Pu polymer formation in the stripping section using only diluted HNO 3 as the stripping solution. Under the appropriate stripping condition taking account of these parameters, the calculation and experimental results showed that these elements could be stripped efficiently.

Design and Control of a Methyl Tertiary Butyl Ether (MTBE) Decomposition Reactive Distillation Column

The synthesis, design, and control of a reactive distillation column for decomposing methyl tertiary butyl ether into isobutylene and methanol are reported in this work. Because the reaction is highly endothermic, it is advantageous to consider cautiously internal heat integration between the reaction and separation operations during process synthesis the and design, thereby necessitating deliberate determination of the feed location and judicious distribution of reactive section along the height of the process (Huang et al. AIChE J. 2006, 52, 2518). Through extensive comparison with a simple process design in which the reactive section is located between the rectifying section and the stripping section, it is demonstrated that a substantial reduction of capital investment and energy consumption can be achieved with this strategy of process development. Process dynamics and operation of the resultant process design were further investigated, and a noticeable improvement was gained in process controllability. The major reason for this improvement can be attributed to the fact that a more synergistic relationship has been evolved between the reaction and separation operations in the pursuit of further internal heat integration during process synthesis and design.

Multiple Steady-States in a Heat Integrated Distillation Column (HIDiC)

A heat integrated distillation column (HIDiC) is a new and highly energy-efficient distillation process. In the present work, multiple steady-states in HIDiC are analyzed. In HIDiC, the pressure in the rectifying section is kept higher than that in the stripping section by using a compressor to enhance heat transfer from the rectifying section to the stripping section through the wall. Therefore, an energy balance, particularly the influence of the compressor, must be taken into account for the analysis. In this research, two types of operation policies are investigated: P1) constant compressor power and P2) constant pressure ratio. First, the conditions for making multiple steady-states are derived on the basis of the first principle model of the HIDiC. Then, the analysis results are validated through simulations. The results show that: 1) the instability condition depends largely on the compressor operation policies; 2) in case P1, multiple steady-states appear when the top product is highly pure, the bottom product is relatively impure, vapor flow rate is large, and compressor power is small; 3) in case P2, multiple steady-states appear when the bottom product is highly pure; and 4) multiple steady-states appear in a wider range of operation in case P2 than in case P1.

Production of n-Propyl Acetate by Reactive Distillation : Experimental and Theoretical Study

First steps of the development of a catalytic reactive distillation process for the production of n-propyl acetate based on experiments and simulations are proposed. The kinetics for homogeneously (sulphuric acid) and heterogeneously (Amberlyst 15) catalysed reaction were investigated and the constants for a pseudo-homogeneous model are presented. Pilot plant experiments were performed using a homogeneous strong acid catalyst in a packed column. A top-column decanter is used to withdraw the aqueous phase and to reflux the organic phase. Simulation results are in good agreement with experimental data. Thermodynamics non-idealities are taken into account using VLE and LLE NRTL interaction parameters. Alcohol conversion and n-propyl acetate purity may be dramatically increased just by adding to the pilot plant a stripping section in an additional column: six different configurations are identified to achieve such a production. The startup is studied in order to determine the best strategy to achieve steady-state conditions. The strong influence of the composition of the initial charging in the decanter can be seen and an initial charging of the two-phase top product leads to the fastest startup.

Conceptual Design of Equilibrium Reactor−Reactive Distillation Flowsheets

Economic and environmental reasons have led to process intensifications in the process industries. Reactive distillation is the most prominent example. However, often it is a challenge to satisfy the requirement for sufficient catalyst volume for the reaction while providing the interfacial area needed for mass transfer. The combination of a reactive distillation column with a pre-reactor is a valuable alternative. This paper presents an approach to identify promising designs for such flowsheets and the optimum distribution of the reaction extent between the pre-reactor and the reactive distillation column. The methodology uses a boundary value method for the design of the column; chemical equilibrium is assumed. The column usually consists of a reactive “core”, two rectifying sections, and one stripping section. The methodology will be demonstrated for the production of ethyl-tert-butyl-ether (ETBE).

Planar Miniature RFID Antennas Suitable for Integration With Batteries

A class of planar, electrically-small UHF antennas suitable for direct integration with electronic components such as batteries is introduced. The new design approach combines a meander line section and a capacitive strip section. The geometries of the two sections can together be scaled in size over a wide range of planar form factors while still maintaining self-resonance and practically realizable line widths and spacings. No external matching network is required. Moreover, batteries can be mounted above or below the capacitive strip section, significantly reducing the total size of a wireless device. Three designs are demonstrated on printed circuit board at 433 MHz. Measurements show that the antennas provide good gain and excellent bandwidth, omnidirectional radiation patterns, and electrically small size. The results of this work have numerous uses in radiofrequency identification (RFID)

Seawater deaeration at very low steam flow rates in the stripping section

The model discussed in a previous work for the simulation of a deaerator to remove the oxygen from seawater in desalination plants has been improved. The conservative character of that model has been revised on the basis of the experimental industrial experience of FISIA Italimpianti S.p.A. In particular, the behaviour of the stripping section packing when very low, or null, steam flow rates are used has been investigated. An interesting interpretation of the stripping efficiencies has been used to demonstrate how the typical technical requirement of 0.02–0.03 ppm as maximum oxygen content in the outlet water stream can often be achieved also operating without steam feeding in the stripping section.

Control System Design for a Single Feed ETBE Reactive Distillation Column

Reactive distillation (RD) is advantageous for the Ethyl Tert-Butyl Ether (ETBE) synthesis. The steady state model of an ETBE reactive distillation column created using the simulator HYSYS is analyzed to synthesize effective control structures. Since the column exhibits input multiplicity with the dual process objectives of ETBE RD (isobutene conversion and ETBE purity), inferential variables are selected. A control structure that organizes a sensitive tray temperature in the stripping section using the reboiler duty and maintains the temperature difference of reactive trays using the reflux flow, is found to be most suitable. A decentralized PI controller and constrained Model Predictive Controller (MPC) are implemented, and performances are compared for set point tracking and disturbance rejection. MPC control algorithms are implemented in MATLAB and interfaced with HYSYS. Constrained MPC (CMPC) is found to be effective for load disturbance rejection, which frequently occurs in the single feed configuration.

Simulation of ternary distillation in a heat integrated distillation column (HIDiC) with a rate-based model

The proposed of the present work is to develop a method for rigorous prediction of separation performance of the HIDiC. A process simulator (gPROMS) for prediction of performance of the HIDiC with random packing is developed by rate-based model using Maxwell-Stefan equations. The vapor and liquid flow rates of HIDiC increased from the bottom to the top in the stripping section. In the rectifying section, however, the flow rates decreased from the bottom to the top. Evaporation of liquid in the stripping section and the condensation of vapor in the rectifying section take place. Comparison was made observed data from a bench-scale separation (benzene-toluene system) HIDiC plant, about 16 m in height and 254 mm in diameter (concentric R&A sections) and simulation results. The energy-saving ratio shows a good agreement with observed data. The bench HIDiC showed about 40% reduction of energy consumption compared with the conventional distillation column. The effect of nonequilibrium of temperature in the HIDiC is also discussed.

Distillate Cascade Composition Control Using a Two-Temperature Measurement Secondary Component

The intent of this note is to show that the incorporation of a temperature measurement in the stripping section improves the performance of distillate cascade controller. The proposed controller regulates the distillate composition by manipulating the reflux flow rate, based on a primary component, which is driven by the distillate composition measurement, and a secondary component, which is driven by two temperature measurements located at the most sensitive trays of the stripping and rectifying sections. Compared to the standard single-temperature cascade control scheme, the proposed two-temperature cascade controller has better behavior, because of the improvement of the feed-forward-like disturbance rejection capability of the secondary control component. Numerical simulations are used to illustrate the performance of the control scheme in the face of feed flow and composition disturbances.

Equipartition of entropy production as an approximation to the state of minimum entropy production in diabatic distillation

We show that the theorem of equipartition of entropy production is important for the understanding of the state of minimum entropy production in diabatic distillation. The theorem is not valid in a strictly mathematical sense. We explain why, when and in what sense this theorem is a good approximation to the optimal state in diabatic distillation. In order to make these predictions, we use a hypothesis for the state of minimum entropy production of an optimally controlled system, which was formulated on the basis of results of entropy production minimisation in chemical reactors. The hypothesis says that the state of minimum entropy production is characterised by approximately constant local entropy production and thermodynamic forces, given that there is sufficient freedom in the system. We present numerical results which are in agreement with the predictions. The results show that a column with constant tray entropy production in the stripping section and in the rectifying section is a good approximation to the optimal column, except when the total heat transfer area is low. The agreement between the two columns becomes better and better as the total heat transfer area and the number of trays increase. The fact that the predictions and the numerical results agree very well gives support to the validity of the hypothesis.

Nondestructive estimation of wood chemical composition of sections of radial wood strips by diffuse reflectance near infrared spectroscopy

The use of calibrated near infrared (NIR) spectroscopy for predicting the chemical composition of Pinus taeda L. (loblolly pine) wood samples is investigated. Seventeen P. taeda radial strips, representing seven different sites were selected and NIR spectra were obtained from the radial longitudinal face of each strip. The spectra were obtained in 12.5 mm sections from pre-determined positions that represented juvenile wood (close to pith), transition wood (zone between juvenile and mature wood), and mature wood (close to bark). For these sections, cellulose, hemicellulose, lignin (acid soluble and insoluble), arabinan, galactan, glucan, mannan, and xylan contents were determined by standard analytical chemistry methods. Calibrations were developed for each chemical constituent using the NIR spectra, wood chemistry data and partial least squares (PLS) regression. Relationships were variable with the best results being obtained for cellulose, glucan, xylan, mannan, and lignin. Prediction errors were high and may be a consequence of the diverse origins of the samples in the test set. Further research with a larger number of samples is required to determine if prediction errors can be reduced.

Reactive distillation design with considerations of heats of reaction

AbstractAlthough reactive distillation columns allow direct utilization of heat of reaction to separation operation, the effectiveness of internal heat integration appears generally to be unsatisfactory and their thermodynamic efficiency could quite often be improved substantially through seeking further internal heat integration between the reaction and separation operations. Prudent arrangement of reactive section, effective determination of feed location, and deliberate distribution of catalyst constitute the three methods that can complement internal heat integration within a reactive distillation column. The reactive section is suggested to superimpose properly onto the stripping section for exothermic reactions and onto the rectifying section for endothermic reactions. Feed location and distribution of catalyst should be determined so that the effect of internal heat integration can be maximized to its fullest extent. A sequential procedure is proposed to determine an appropriate process configuration for internal heat integration within a reactive distillation column. Five reactive distillation systems, involving not only equilibrium‐limited but also kinetically controlled reactions, are used to evaluate the design philosophy proposed. It has been found that a substantial improvement in system performance can be achieved even for some reaction systems with side reactions and/or unfavorable thermodynamic properties. Seeking further internal heat integration has been demonstrated to be an effective method for refining process design of a reactive distillation column involving reactions with highly thermal effect. These conclusions are of great significance and can provide process designers with additional latitude to elaborate their process designs. © 2006 American Institute of Chemical Engineers AIChE J, 2006

The Recovery of Carbon-14 from the Graphite Moderator of a Dismantled Gas-Cooled Reactor through Plasma Chemical Reactions in CO Glow Discharge

The application of the carbon isotope separation method using plasma chemical reactions in carbon monoxide plasma for the recovery of 14C from the graphite moderator of a dismantled Japanese gas-cooled reactor, Tokai Power Station, was evaluated numerically. The 14C separation factor of 4.6 in one unit stage was estimated through the numerical simulation of a kinetic model in CO plasma. The calculated results show that the 14C concentration in the cascade tail flow can be reduced to the environmental standard recommended by the International Commission on Radiological Protection (3 Bq/cm3) and to the natural abundance level (1.3×10−4 Bq/cm3) in 3 and 16 stages in the stripping section, respectively, and the amount of 14C-enriched CO flow can be reduced to 1/103 and 1/106 of the feed CO amount in 9 and 19 stages in the enriching section, respectively. In the case of 3-stage processing in the stripping section and 9-stage processing in the enriching section, it is estimated that the total energy consumption by the plasma generator would be about 15 kW and the maximum inner diameter of the plasma reactor in the cascade would have to be about 60 cm.

Aperture-Backed Microstrip-Line Stepped-Impedance Resonators and Transformers for Performance-Enhanced Bandpass Filters

A novel class of microstrip bandpass filter is configured using the impedance transformers and an improved stepped impedance resonator (SIR). This SIR is composed of a central narrow strip section with an aperture on ground and two wide strip sections at the two sides. This low-high-low SIR resonator has a promising capability in achieving an extremely large ratio of first two resonant frequencies for design of a bandpass filter with ultra-broad stopband. The two quarter-wavelength transformers with low and high impedances, referred as to impedance- and admittance-inverters, are modeled and utilized as alternative types of inductive and capacitive coupling elements with highly tightened degrees for wideband filter design. After extensive investigation is made on the two transformers and the proposed SIR, the two novel bandpass filters are constructed, designed and implemented. Two sets of predicted and measured frequency responses over a wide frequency range both quantitatively exhibit their several attractive features, such as ultra-broad stopband with deep rejection and broadened dominant passband with low insertion loss.

The Recovery of Carbon-14 from the Graphite Moderator of a Dismantled Gas-Cooled Reactor through Plasma Chemical Reactions in CO Glow Discharge

The application of the carbon isotope separation method using plasma chemical reactions in carbon monoxide plasma for the recovery of 14C from the graphite moderator of a dismantled Japanese gas-cooled reactor, Tokai Power Station, was evaluated numerically. The 14C separation factor of 4.6 in one unit stage was estimated through the numerical simulation of a kinetic model in CO plasma. The calculated results show that the 14C concentration in the cascade tail flow can be reduced to the environmental standard recommended by the International Commission on Radiological Protection (3 Bq/cm3) and to the natural abundance level (1.3×10−4 Bq/cm3) in 3 and 16 stages in the stripping section, respectively, and the amount of 14C-enriched CO flow can be reduced to 1/103 and 1/106 of the feed CO amount in 9 and 19 stages in the enriching section, respectively. In the case of 3-stage processing in the stripping section and 9-stage processing in the enriching section, it is estimated that the total energy consumption by the plasma generator would be about 15 kW and the maximum inner diameter of the plasma reactor in the cascade would have to be about 60 cm.

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.