Column Profile Maps Research Articles

Feed distribution in distillation: Assessing benefits and limits with column profile maps and rigorous process simulation

This contribution describes the column profile map (CPM) methodology for designing distributed feed distillation columns. For non-sharp product distributions, a case study shows that energy savings of approximately 35% can be obtained if the feed stage(s) are designed optimally. Feed distribution allows capital cost savings, expands operating leaves, and can obtain greater separation feasibility. However, this column only has benefits in ternary and higer-order systems and when product distributions are non-sharp. To validate these counter-intuitive claims, a real Benzene, p-Xylene, Toluene system is modeled using CPMs, and the resulting design parameters are transported to Aspen Plus®. Using a sum of squared errors objective function to quantify savings, a cost saving trend very similar to the one predicted by the CPM method is obtained. This article therefore describes a complete design methodology for distributed feed systems and provides convincing evidence of benefits of such a system. © 2012 American Institute of Chemical Engineers AIChE J, 59: 1668–1683, 2013

Pinch Point Calculations and Its Implications on Robust Distillation Design

Rising energy costs and growing environmental awareness motivate a critical revision of the design of distillation units. Systematic design techniques, such as the rectification body, column profile map, and temperature collocation methods, require exact knowledge of all pinch points in a particular system, because these stationary points delineate the possible composition trajectories realizable in separation columns. This paper demonstrates novel methods for rigorously determining all pinch points for the constant relative volatility, ideal and non-ideal systems. Constant relative volatility and ideal solution systems are transformed into one-dimensional polynomial and nonlinear functions, regardless of the number of the components. A deflation method is proposed to locate all zeros in ideal and non-ideal zeotropic problems. For more challenging non-ideal problems, a novel hybrid sequential niche algorithm is used to solve hard azeotropic problems successfully. Finally, the design implications of these pinch point locations are investigated to show how new separation configurations can be devised. Methodically the paper points out the use of rigorous pinch point computations in conjunction with continuous composition profiles for robust distillation design.

On Column Profile Maps: An Analysis of Sharp Splits

Column profile maps (CPMs) have recently been shown to be a useful tool in the design of distillation operations, especially complex configurations. CPMs are essentially a family of composition trajectories for a column section in a distillation column. They have been shown to be topological transforms of residue curve maps, and are dependent on two main parameters: RΔ (generalized reflux) and XΔ (a pseudocomposition vector), which effectively fixes the net flow in a column section. This paper focuses on the interaction between these parameters for the special case of sharp splits. With the CPM technique it has been shown that sharp splits behave in a unique manner, due to a phenomenon termed as “node bumping”. In particular, it has been shown that it is possible to locate specific ranges of RΔ where one may drastically influence the movement of profiles in somewhat counterintuitive directions. Although this effect may not be useful in product producing column sections, it is shown how these unique phenom...

Graphical design and analysis of thermally coupled sidestream columns using column profile maps and temperature collocation

AbstractA systematical procedure is presented to design thermally coupled sidestream units like side rectifiers and side strippers are presented in this article. The method combines the column profile map technique to assess topological characteristics of the specific configuration with temperature collocation to rigorously ensure a realizable column design, without making assumptions with regard to the phase equilibrium or product specifications. The proposed methodology offers a unique graphical insight into the challenging problem of thermally coupled column synthesis. Techniques are presented for highlighting superior designs or eliminating inferior ones, based on vapor flow rate, number of stages, and thermodynamic efficiency. Design parameters such as the feed and side‐draw trays that may require insight or experience are products of the procedure. Design solutions obtained using this methodology can be used to initialize the state of the art process flow sheeting tool, AspenPlus™, which typically leads to fast convergence to the desired product purities without further adjustments. © 2010 American Institute of Chemical Engineers AIChE J, 2011

Column profile maps as a tool for synthesizing complex column configurations

There has recently been a renewed interest in the design of distillation processes due to rising energy cost and growing environmental concerns. Column profile maps (CPMs) have been developed as a graphical tool to simplify the design procedure of distillation schemes as well as a method for analyzing existing processes. Using CPMs one is able to change topology within the composition space to suit the requirements of the separation and hence many separations that have been thought of as difficult or unviable can now be better understood and consequently new designs may be devised. The CPM technique has also been proven to be extremely useful as a design tool for complex columns, as configurations irrespective of complexity can be modeled and graphically understood. This paper aims to summarize the most important and interesting results and applications obtained using the CPM technique. It shows how CPMs may be used to synthesize complex columns like a Petlyuk or Kaibel column, as well as showing how new sharp split separations can be devised.

Complex Column Design by Application of Column Profile Map Techniques: Sharp-Split Petlyuk Column Design

Currently employed short-cut design techniques have a tendency to be configuration-specific. Few can be applied to complex distillation configurations. In this work, we will demonstrate, in detail, the use of column profile maps (CPMs) for the comprehensive analysis and design of complex distillation systems by applying the CPM technique to the design of the fully thermally coupled (Petlyuk) distillation column at sharp-split conditions. It is shown that, for set product composition specifications and a set reflux ratio, only a small region of key parameters (vapor and liquid split ratios) result in feasible separations. These results—and, hence, the CPM design procedure—are validated by the work of Halvorsen in his 2001 Ph.D. thesis. It is also shown that the minimum reflux solution can be found using this methodology. The results are valid for all zeotropic separation syntheses.

Application of Membrane Residue Curve Maps to Batch and Continuous Processes

Membrane residue curve maps (M-RCMs) [Peters et al. Ind. Eng. Chem. Res. 2006. 45, 9080] plot the change, over time, of the retentate composition in a batch still. In this paper, the relevance of M-RCMs to both batch and continuous processes is investigated. A constant relative permeability model is used for demonstration purposes, and the theory is then extended to a real system of methanol/butene/methyl tertiary butyl ether (MTBE). It has been shown that the differential material balances over a continuously operated membrane unit are mathematically equivalent to those which describe the M-RCM. The time variable in the batch setup is analogous to the spatial variable in continuous units. The retentate composition in a nonreflux continuous unit, for example, will follow the residue curve that passes through the initial feed composition. The M-RCM, in conjunction with the necessary flux equations, allows a designer to determine permeation area (or time) required in a continuous (or batch) process. Membrane columns operating at total and infinite reflux are discussed. A novel approach in synthesizing and designing hybrid distillation−membrane processes emerges: using the M-RCM in conjunction with column profile maps (CPMs) allows one to graphically interpret hybrids in an efficient manner. The method generates the attainable region (AR) for a chosen configuration and informs the designer of parameters like membrane area and number of distillation stages required.

Reactive column profile map topology: Continuous distillation column with non-reversible kinetics

In this paper we present a topologically based approach to the analysis and synthesis of reactive distillation columns. We extend the definition of Tapp et al. [Tapp, M., Holland, S., Glasser, D., & Hildebrandt, D. (2004). Column profile maps part A: Derivation and interpretation. Industrial and Engineering Chemistry Research, 43, 364–374] of a column section in non-reactive distillation column to a reactive column section (RCS) in a reactive distillation column. A RCS is defined as a section of a reactive distillation column in which there is no addition or removal of material or energy. We introduce the concept of a reactive column profile map (RCPM) in which the profiles in the RCPM correspond to the liquid composition profiles in the RCS. By looking at the singular points in the RCPM, it is demonstrated that for a single chemical reaction with no net change in the total number of moles, the bifurcation of the singular points depends on both the difference point as introduced by Hauan et al. [Hauan, S., Ciric, A. R., Westerberg, A. W., & Lien, K. M. (2000). Difference points in extractive and reactive cascades I-Basic properties and analysis. Chemical Engineering Science, 55, 3145–3159] as well as the direction of the stoichiometric vector. These two vectors combine to define what we call the reactive difference point composition. We show that there only certain feasible topologies of the RCPM and these depend only on the position of the reactive difference point composition. We look at a simple example where the vapour liquid equilibrium (VLE) is ideal and show that we can classify regions of reactive difference point compositions that result in similar topology of the RCPM. Thus, by understanding the feasible topologies of the RCPM, one is able to identify profiles in the RCPM that are desirable and hence one is able to synthesize a reactive distillation column by combining RCS that correspond to the desired profile in the RCPM. We believe that this tool will help understand how and when reaction could introduce unexpected behaviors and this can be used as a complementary tool to existing methods used for synthesis of reactive distillation columns.

Experimental Measurement of the Saddle Node Region in a Distillation Column Profile Map by Using a Batch Apparatus

A simple theoretical method for the evaluation of the separation of mixtures using distillation columns operating at finite reflux, called column profile maps (CPMs), has been developed (Tapp et al., 2004). These CPMs are simply transforms of the residue curve maps (RCMs) and are used for sequencing and synthesis of distillation columns. Thus for example, the methanol, diethyl ether and benzene system has a low boiling azeotrope between methanol and benzene which appears as a saddle point in the RCM. As a result the RCM has two stable nodes and hence two distillation regions divided by a simple distillation boundary. It can be theoretically shown that the transformation of the CPM moves the saddle point that was on the boundary of the mass balance triangle in the RCM into the mass balance triangle of the CPM. Similarly the two stable nodes, corresponding to pure component nodes, in the RCM move out of the mass balance triangle of the CPM. The CPM of this system was experimentally evaluated to verify that a saddle point node does indeed occur inside the mass balance triangle. The experimental technique uses a semi-batch apparatus and measures the boiling liquid concentration in the still as a function of time (Modise et al., 2005). The importance of this is that concentration profiles achieved in the semi-batch still are essentially the same as those of a continuous distillation column section. The experimental measurements showed that there is indeed a saddle point in the CPM.

Novel separation system design using “moving triangles”

Shortcut design techniques have been employed in the initial design of traditional distillation systems. Current techniques are not useful in the design of novel or complex configurations however. We will show that by using column profile mapping “moving triangles” to model the behaviour of column sections (CS), any distillation configuration, no matter how complex, can be modelled and its behaviour more thoroughly understood. As an example, a thermally coupled column will be modelled using column profile maps. It is suggested that by gaining an understanding of the behaviour of the configuration quickly and easily, using column profile maps, time and money can be saved by avoiding poor initial decisions and designs.

Column Profile Maps. 2. Singular Points and Phase Diagram Behaviour in Ideal and Nonideal Systems

The first paper of this series [Tapp, M.; Holland, S. T.; Hildebrandt, D.; Glasser, D. Ind. Eng. Chem. Res. 2004, 43 (2), 364] analyzed the behavior of ternary homogeneous distillation systems by using column profile maps, which are generated by using the difference point equation. Ideal thermodynamics was assumed, and it has been shown how using column profile maps one can devise more creative designs. This paper explores the mathematics and the topology of the maps for systems with ideal and nonideal thermodynamics by looking at the occurrence of singular points in ternary homogeneous systems depending on the values of the design parameters of the difference point and the reflux ratio. Eigenvalue and eigenvector maps have been introduced, and their usefulness for manipulating phase diagrams and therefore the column profiles has been shown. The focus of the third part lies in the application of the tools presented in this paper for distillation process synthesis and design.

Column Profile Maps. 1. Derivation and Interpretation

The use of ordinary differential equations as a shortcut technique for the description of distillation columns has been well established over the last 3 decades. Residue curve maps (RCMs) have been employed as a graphical representation tool in the analysis and interpretation of the behavior of distillation systems. However, RCMs enable one to gain insight into infinite-reflux column behavior only. This paper will be the first part of a series that looks at column profile maps (CPMs) obtained by using the difference point equation. CPMs represent the finite-reflux case, and the map depends on the values of the difference point and the net flow rate. This paper focuses on analyzing the behavior of distillation systems for ideal thermo and shows how using CPMs one can devise more creative designs. The focus of the second part lies in analyzing systems with nonideal behavior and develops tools for the synthesis of distillation.