Numerically optimized performance of diabatic distillation columns

Recently, the concept of equal thermodynamic distance (ETD) has been proposed to minimize entropy production in a distillation process using a diabatic column. ETD gives the optimal temperature profile to first-order in N⁻¹, where N is the number of trays. ETD, however, does not generally give the true minimum for distillation columns with few trays. We therefore apply a fully numerical, multidimensional optimization routine to determine minimum entropy production. Since this method does not depend on an underlying theory, we expect a true minimum to be revealed. We then compare the performance of ETD and numerical optimization by varying the number of trays and the purity requirements. Our results show a surprisingly good agreement between the ETD results and those obtained numerically.     

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.     

Vapor recompressed dividing-wall distillation columns: Structure and performance

Due to the topological structure of double columns and multiple separating sections in dividing-wall distillation columns (DWDCs), the development of vapor recompressed dividing-wall distillation columns (DWDC-VRHPs) represents a challenging issue with great complexities and tediousness. For the separations of light-component dominated and wide boiling-point ternary mixtures, because the purification of the light-component from the intermediate-and heavy-components incurs the primary energy dissipation, the application of vapor recompressed heat pumps (VRHP) should be aimed to reduce the irreversibility and this leads to the generation of the optimum topological structures of the DWDC-VRHPs, i.e., a DWDC plus a two-stage VRHP. The first-stage VRHP is to preheat feed, not only taking the advantages of the small temperature elevation available but also favoring the mass transfer between the vapor and liquid phases through feed splitting. The second-stage VRHP is to reduce further separation irreversibility. The philosophy can be applied to any DWDCs no matter where the dividing wall locates. Two case studies on the separations of ternary mixtures of benzene, toluene, and o-xylene and n-pentane, n-hexane, and n-heptane demonstrate the economic optimality of the proposed DWDC-VRHPs and reveal the inherent interplay between internal and external process integration.     

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.     

精馏塔的压降测量研究(英文)

Pressure drops are of major importance for distillation/absorption columns. This paper mainly discusses how to correctly measure, interpret and use pressure drop data. The possible causes of incorrect pressure drop measurements are studied including the effects of pressure tap dimensions, locations, and vapor condensation etc. The effect of the static head of vapor on the pressure drop data and column pressures is evaluated. Variations of sec-tional pressure drops along the column are investigated based on the experimental data obtained from commercial size distillation columns at Fractionation Research, Inc. (FRI). For a packed column, it is found that the spacing between the liquid distributor and the top of the bed affects the overall pressure drop measurements, which is con-firmed by a fundamental fluid dynamics analysis.     

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.     

Elevating the flexibility and operability of dividing-wall distillation columns via feed thermal condition adjustment

Because of the complicated interplay between the prefractionator and main distillation column involved,the black-hole problem might occur and prohibit the assignment of four specifications to dividing-wall distillation columns (DWDCs) (e.g.,the three main product compositions plus an impurity ratio in the intermediate product),which lowers terribly process flexibility and operability.In this paper,a feed thermal condition adjustment strategy,achieved by the installation of a pre-heater in feed pipeline,is employed to eliminate the black-hole problem and serve to enhance process flexibility and operability.Through the strong influence to the overall mass and energy balance of the DWDC,the feed thermal condition adjustment can alter the interlinking flows between the thermally coupled prefractionator and main distillation column and work effectively to coordinate their relationship.A DWDC separating a benzene,toluene,and o-xylene mixture is chosen to ascertain the feasibility of the philosophy proposed.The static and dynamic studies demonstrate that the feed thermal condition adjustment is an effective way to refine process design and can completely eliminate the black-hole problem and elevate consequently process flexibility and operability.     

The best performance of spiral wired thermal diffusion columns of the frazier scheme

Equations have been derived for the optimum wire angles of inclination for maximum separation, maximum production rate and minimum column height in the Frazier scheme of concentric‐tube thermal diffusion columns with a tight fitting wire spiral, having a diameter equal to the annular spacing, and wrapped on the entire inner tube. Considerable improvement in performance is obtained when a Frazier scheme is operated at the optimum corresponding wire angle of inclination, especially for the scheme of high column number.     

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.     

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。     

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。     

Nonlinear observers for distillation columns

A full-order nonlinear observer for distillation columns is presented. Temperatures are measured at different points of the column and compared to the observer's output temperatures. The error is weighed by a spatially distributed function. The resulting term is used to correct the mathematical model of the observer. The estimated temperature and concentration profiles are then given by the state of the observer. The mathematical model used includes material balances on each tray as well as thermodynamic state-functions. It consists of dynamic and algebraic equations, exhibits severe nonlinearities and is of high order. Physical insight is used to design the observer. The movement of the regions of high mass transfer rate determines the essential dynamic quality of a column. Knowing the location of these mass transfer regions allows observer design and positioning of measurement points. The obtained observer was successfully tested in several simulation studies. It exhibits dynamic behaviour and robustness properties.     

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。     

Closed-loop optimal operation of batch distillation columns

Often, the main source of disturbance for a batch distillation system is an upset in the feed to the process. If the operation of a batch column is carried out on the basis of the nominal value of the feed composition, a high degree of uncertainty in the initial conditions to the batch may lead to run the column suboptimally, with a possibly large economic penalty. In this paper, a three-step strategy is proposed for the closed-loop implementation of optimal operating policies for batch rectifiers. First, the optimal reflux rate is calculated off-line for several feed compositions. Then, a correlation is developed off-line between the optimal reflux rate and the composition profile in the column at the end of the startup phase. Finally, the detection of the composition profile is performed on-line during the startup phase, so that the optimal reflux rate can be calculated and implemented in a closed-loop fashion. This allows operating the column optimally even though the actual feed composition is not known. Since the calculations to be performed on-line are straightforward, the computational demand is kept to a minimum. Results for binary and ternary systems indicate that, by using the proposed procedure, the column performance can be improved by as much as 30% with respect to a conventional open-loop optimal strategy.     

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.