Influence of column hardware on the performance of reactive distillation columns

We compare the performance of an MTBE synthesis column using two different hardware configurations: (1) a sieve tray column in which the catalyst particles, encased inside wire gauze envelopes, placed along the liquid flow path; (2) a column filled with catalytically active packing of Raschig ring shape. The columns simulations are performed using a rigorous nonequilibrium model. Using the bottoms flow rate of MTBE as continuation parameter it is shown that the two different hardware configurations exhibit significantly different bifurcation diagrams. The sensitivity of this bifurcation diagram has been studied with varying (a) methanol feed, (b) iso-butene feed, (c) inert feed and (d) reflux ratio. We show that the cross-flow contacting on the sieve tray configuration is beneficial to conversion.     

The effect of heat transfer between the phases on the performance of countercurrent distillation columns

In distillation processes net evaporation or condensation in the bulk phases or at the interface is called thermal distillation. The causes of thermal distillation, especially mismatch between the rates of heat and mass transfer in and between the phases, are discussed. By taking thermal distillation into account in heat and material balances it is shown that the performance of countercurrent equipment should depend upon composition of the mixture and should have a maximum value, as has been found experimentally in this and in previous work. The experimental results indicate that the efficiency of the equipment is very low when the mixture is dilute, and this is not accounted for by the theory.     

The best performance in spiral wired thermal diffusion columns

The Improvement in performance of a concentric tube thermal diffusion column by means of a wire spiral, having a diameter equal to the annular spacing and inserted as a spacer in the annular region, was investigated. Equations of the optimum wire angle of inclination from the vertical for the maximum separation, maximum output and minimum column height have been derived. Considerable improvements in performance are obtainable if the spiral wired columns are employed, instead of using the open column (without wire), so that the convection strength can be reduced and controlled, resulting in suppressing the undesirable remixing effect while still preserving the desirable cascading effect.     

The best performance of inclined thermal diffusion columns of the frazier scheme

The equations for the best angles of inclination for maximum separation, maximum production rate and minimum column length in inclined flat-plate thermal-diffusion columns of the Frazier scheme, have been derived. Considerable improvement in performance is obtained when a Frazier scheme is operated at the best corresponding angle of inclination, especially for the schemes of high column number.     

The dominant time constant for distillation columns

It is shown that the dominant part of the dynamic composition response of two-product distillation columns can be approximated by a linear first-order response. This applies also for large perturbations to the column. The numerical value of this dominant time constant (τ_c) can be obtained from steady-state simulations. A simple analytical expression for small peturbations is derived which provides insight into the variation of τ_c with operating conditions. The time constant τ_c does not apply when there are changes in the internal flows only.     

全指标精馏塔全塔负荷性能图分析新方法

针对传统塔板负荷性能图在现代精馏塔分析中存在的问题,提出一种精馏塔全指标全塔负荷性能图新分析方法。与单指标的全塔负荷性能图相比,全指标全塔负荷性能图用于精馏塔瓶颈分析,不同塔内件合理匹配分析和最终施工图设计塔内件评价方面更加直观、容易、便于编程。     

Thermodynamic optimisation of distillation columns

In this paper a methodology for thermodynamic analysis and distillation column ‘targeting’ is presented, with emphasis on the use of side condensers and side reboilers. Research in the past has been towards the establishment of a heat distribution curve, showing the way in which heat can be added or extracted across the different column sections. One major disadvantage of these profiles is that they refer to reversible columns, and cannot be used effectively to target for modifications in a real column.The main feature of the proposed methodology is the introduction of a minimum driving force, defined in terms of exergy loss distribution of the existing column, to set realisable targets for side reboiling/condensing in real columns, resulting in considerable energy savings. In addition to providing realisable targets, the new approach also provides the design engineer with information about the best location to place a side exchanger, and the required additional column modifications. The methodology can be applied using conventional column models in commercial process simulation programs, but can be significantly simplified by using reboiled and refluxed absorber models in a bespoke program. Simulation results for modified designs set by the new approach, for binary and multicomponent separations, verify the feasibility of the targets. This contrasts with previous approaches, which result in temperature shifts and heat load penalties after placing side reboilers/condensers, thus requiring additional simulation time and experienced judgement.     

风载荷对精馏塔分离效率的影响

以乙苯－苯乙烯精馏塔为例，探讨了风载荷对较高塔器分离效率的影响。作者统计了当地两个不同月份的风级气象资料，用曲线标绘出了乙苯－苯乙烯塔顶部苯乙烯损失量以及操作回流比随当地风级变化的实际情况，形象地说明了风载荷对高塔分离效率的影响，并在新的设计方案中，对安装方式采取了措施，增强了塔的刚度，从而提高塔板效率１３％左右，而且有明显的节能效果。     

Minimum-reflux regime of simple distillation columns

A general algorithm for calculating minimum reflux ratios in simple distillation columns used for the separation of ideal and nonideal (including azeotropic) homogeneous multicomponent mixtures is proposed. The algorithm deals with various splits: direct, indirect, intermediate, and with one distributed component. The algorithm is mathematically rigorous, developed on the basis of the geometrical theory of distillation, and makes it possible to consider the special features of nonideal mixtures (tangent pinch, necessity of using nonadiabatic columns).     

Optimal operation of Kaibel distillation columns

The objective of this paper is to study the Kaibel distillation column from an operability point of view. Two different objectives, namely minimizing energy requirement at fixed product purities and maximizing product purities with a fixed boilup are considered. We have visualized the objective functions for the two cases as a function of operational degrees of freedom and conclude that operation with fixed product purities is the more difficult case from control point of view.     

精馏塔进料位置探索

精馏塔进料位置的选择极为重要 ,不正确的进料位置会使全塔效率下降。不能用下移进料口位置来增大塔顶产品浓度。     

Design and control of energy-efficient distillation columns

Distillation is the best option for the separation of hydrocarbon mixtures, unless the boiling points of the constituents are close together. Despite being widely utilized in field applications, the high energy demand of distillation calls for efficient columns in order to save energy. The efficient divided wall column (DWC), diabatic distillation column, and internally heat-integrated distillation column (HIDiC) are introduced here, and the design and control of the columns are briefly reviewed. The practical applications of the columns in the processes of natural gas production from raw gas drawn from underground and benzene separation from naphtha reformate are presented to show the energy-saving performance of the energy-efficient distillation columns. The side-rectifier DWC reduced the heating duty of the conventional system by 5.9%, and provided a compact construction, replacing the three-column conventional system with a single column suitable for offshore application. Moreover, the controllability of DWC was improved by utilizing the side-rectifier. The benzene removal process utilizing the extended DWC lowered the heating duty of the whole conventional process by 56.8%.     

A note on the identification and control of batch distillation columns

A method for the identification and control of a batch distillation process is presented in this note. The proposed model consists of a first-order integrating process in composition with a high-frequency gain. The feedback controller is designed in the framework of robust nonlinear control with modeling error compensation techniques for the control of distillate composition via manipulations of the reflux ratio. The proposed identification and control procedures are illustrated via numerical simulations.     

Nonlinear model-based feedforward control of distillation columns

An approach to feedforward distillation control based on inverse computation of nonlinear stage models is presented. The feedforward model calculates dynamic trajectories of manipulated variables from measured disturbances and product purity set points independently of the control configuration. Because the model includes the dominant dynamics and nonlinearities of the column, dynamic decoupling of the control loops is achieved. A superimposed linear controller only has to compensate model uncertainties and disturbances that cannot be measured. The proposed approach improves the control performance. Simulation studies show the applicability of the method to multicomponent distillation as well as to distillation trains. Experiments on a pilot plant scale binary distillation column verify the simulation results.     

Understanding the dynamic behaviour of middle-vessel continuous distillation columns

The fundamental dynamic behaviour of a middle-vessel continuous distillation column is studied in this paper. The interaction between the middle vessel (MV) design and control parameters is identified by means of a linear analysis. Transfer functions relating the dynamics of the product composition responses to changes in the disturbance inputs and manipulated inputs are derived. The role of the MV holdup and MV level controller gain in altering the product composition responses is identified. It is shown that the MV level controller gain can significantly affect the control performance when feed flow disturbances must be compensated for. However, it has a much lower impact in the control performance when feed composition disturbances need to be rejected; in this case, exploiting the “buffering” effect of the MV is more important for control. The analysis is carried out in detail for a LV control configuration, and is subsequently extended to an “on-demand” DB configuration. Finally, simple guidelines for the choice of the MV level controller gain and MV holdup are provided.     

Solution of problems involving systems of distillation columns

A convergence method (called the Θ-method) and calculational procedure have been developed which may be applied to any system of distillation units. The procedure makes use of the programs of the individual units without modification. This amounts to the use of the programs of the individual units as building blocks for the construction of the model for the system. Any number of independent feeds to any one of the distillation units may be specified, and any number of sidestreams may be either withdrawn from the system or fed to any other unit of the system.     

Intelligent control system for extractive distillation columns

We developed and implemented an intelligent control system to be used in an extractive distillation column that produces anhydrous ethanol using ethylene glycol as solvent. The concept of artificial neural networks (ANN) was used to predict new setpoints after disturbances, and proved to be a fast and feasible solution. The developed control system receives data from temperature, flowrate and composition measurements of the azeotrope feed, and the ANN estimates the new set-points of the controllers to maintain 99.5 mol% of ethanol at the top and less than 0.1mol% at the bottom; feed composition was also estimated using an ANN. All ANN were trained to provide output data corresponding to an optimized operating condition. The results showed that the intelligent control system can predict a new operating condition for any disturbance in the column feed and presented superior performance when compared with the control system without ANN.     

Efficiency-based nonequilibrium modeling of industrial-scale multicomponent distillation columns

An efficiency-based nonequilibrium model is proposed to simulate two industrial distillation columns. In this model, the behavior of trays is studied by considering the mass transfer at interphase. The interphase mass transfer is simply evaluated by the simultaneous solution of the vapor material balance equation and the definition of the Murphree efficiency. A MATLAB code is developed to implement the simulation procedure. For verification, the product compositions and plate temperatures are compared with the reported experimental data. The discrepancy of the experimental and simulation results is about 0.5% and 20% for the temperature and the product compositions for both columns, respectively.