ChemSep for Windows Features
ChemSep is a program for performing multicomponent separation process calculations. ChemSep is designed for use in courses on thermodynamics and/or separation processes and features an easy to use interface for Windows, equilibrium and rate-based column models, integrated graphics (with GNUplot), and export capabilities (e.g. to Excel (see below), Word, and html). ChemSep integrates flash, the classic equilibrium stage column model and a nonequilibrium or rate-based column model in one easy to use program. ChemSep-Lite (a version with some limitations) is available free.
Residue Curve Maps and ChemSep
Residue curve maps (RCMs) now are considered an established part of an undergraduate chemical engineering education with coverage in widely used textbooks on thermodynamics (Elliott and Lira, 1999), separation processes (Seader and Henley, 1998, 2005; Doherty and Malone, 2001), and design (Biegler, Grossman, and Westerberg, 1997; Seider, Seader, and Lewin, 1999).
The simple distillation of a liquid mixture is modeled by:
These are the equations that describe residue curves in nonreacting systems and they have been very extensively studied (for reviews of the research literature see Pöllmann and Blass, 1994; Fien and Liu, 1994; Widago and Seider, 1996; Kiva, Hilmen and Skogestad, 2003). The (numerical) solution of these equations provides a family of curves known as a residue curve map, and they have become an important tool in separation process synthesis and design. ChemSep now includes a tool for creating residue curve maps for ternary systems such as the one shown below for a simple ternary system with constant relative volatilities.
In what follows we demonstrate how to create these maps with ChemSep. As a first step to the creation of a residue curve map with ChemSep it is necessary to select components (in the series of illustrations to follow they will be acetone, chloroform, and methanol) and thermodynamic model (UNIFAC activity coefficients and Antoine vapor pressures in this example); it is not necessary first to solve a simulation problem (although that it is certainly possible). Then click on Analysis and select Residue Curves as shown in the screen shot below.
This brings up the Residue Curve Map window:
The composition triangle is shown in the top right of this panel. To the left is the list of components (the first three if more than that number was included in the list if components selected earlier). The component selection (and order) can be changed here if desired. There is a cell to enter the pressure (showing a * in the illustration above), and two cells lower down to enter mole fractions. Next to the composition cells is a pull down menu for selecting the type of composition line that is to be drawn; more on this below. Above the composition cells are various control buttons and check boxes that are used to indicate what is to be calculated and/or displayed on the diagram (more on some of these below as well).
The next step is to set the pressure in the empty cell provided. We may then do nothing more than click on the Calc+Draw button for ChemSep to initiate the calculations needed to draw a map with the number of lines equal to that set in the cell above the Calc+Draw button (the default is 9). Since this does not always produce a visually appealing diagram (the lines may not be well spaced) we do not do that here. Instead, we set the number of lines to zero, click off the distillation boundaries checkbox and click on the Calc+Draw button. This leads to the calculation of the boiling points of the pure components, and any binary azeotropes and the diagram shown below.
The portion of the window below the triangle provides the details of each curve shown in the triangle. The list includes the details of the computations used to determine the boiling points of the pure component and binary azeotropes.Next, we simply move the mouse over the triangle and click on a point (this will enter the coordinates directly in the composition cells to the left of the triangle). Then click on the Add button. This will lead to the calculation of the residue curve that passes through those coordinates.
Click on Done and the curve will be displayed in the composition triangle. To add lines we simply repeat the process of locating a starting point as described above and clicking on the Add button each time. The illustration below shows the map after 11 lines have been drawn. Single lines can be highlighted (as can be seen below)
Further Reading about Residue Curves:
We illustrate the new Parametric Study feature of ChemSep with the help of a butane-pentane spliiter from Seader and Henley (1998) and shown below.
Specifications and calculated product stream flows for butane- pentane splitter. Flows are in lbmol/h.
The Peng-Robinson equation of state was used to estimate K-values and enthalpy
With 11 stages and 5 components the equilibrium stage model has 143 equations
to be solved for 143 variables (the unknown flow rates, temperatures, mole fractions).
ChemSep solved this problem in just 4 iterations. Computed product flows are shown
in the figure above.
A McCabe-Thiele diagram for this multicomponent system is shown below. For systems
with more than two components these diagrams can only be computed from the results
of a computer simulation. The axes are defined by the relative mole fractions:
Multicomponent McCabe-Thiele diagram for butane - pentane splitter.
Click on the Analysis menu and select Parametric Study to bring up the window. Then add specification variables from the pull down menu to the list of parameters that will be varied. Here we have selected the reflux ratio and specified that it be varied from 1.5 to 6 over 21 steps. In the center section we select results that we wish to monitor from another pull down list. In this illustration we have selected the mole fractions of isopentane in the overhead and n-butane in the bottoms, and the reboiler duty.
After we click the Run button ChemSep will execute 21 simulations at different reflux ratios and the values of the select output parameters will be tabulated in the bottom part of the parametric study panel as can be seen in the screen image shown above. ChemSep can export these results to Excel, or to the plotting package provided with the program. Below left we show the results of our calculations after formatting the graph (with ChemSep) so that the mole fractions use the left vertical axis and the reboiler duty the right vertical axis.
Parametric studies of product compositions as a function of reflux ratio (left) and distillate flow rate (right).
It is clear that increasing the reflux ratio has the desired effect of improving product purity. This improvement in purity is, however, accompanied by an increase in both the operating cost, indicated by the increase in reboiler duty, and capital cost, because a larger column would be needed to accommodate the increased internal flow. Note, however, that the curves that represent the mole fractions of the keys in the overhead and bottoms appear to flatten showing that product purity will not increase indefinitely as the reflux ratio increases. Further improvement in product purity can only be made by changing a different specification.
Our next step is to examine what happens to product purity when we change the specified distillate flow rate, maintaining all other specifications at the values specified in the base case. We carry out a separate parametric study (window not shown) that leads to the graph shown above right. We see that the best overall product purity is obtained when the distillate rate is 45 lbmol/h. This should not come as a surprise since the flow rate of the light key (n-butane) and all components with a higher volatility is 45 lbmol/h. However, even with the distillate flow rate set to 45 lbmol/h there remains room for improvement in the separation.
The other key design specifications here are the total number of stages and the location of the feed stage. In most cases, increasing the number of stages will improve the separation. On increasing the number of stages to 26, with the feed to stage 12, increasing the overhead vapor flow to 195 lbmol/h and decreasing the distillate rate to 45 lbmol/h we obtain the McCabe-Thiele diagram shown below in which the feed now appears to be in the optimum location; the product purities also have significantly improved.
Multicomponent McCabe-Thiele diagram for butane - pentane splitter after optimization to improve product purities.
CAPE OPEN ChemSep
Despite considerable interest for many years, ChemSep has not been widely used in industry. The primary reason for the lack of commercial users is that ChemSep did not function well with major commercial flowsheet simulation programs. The main requirement for an industrial user is for ChemSep to use the thermodynamic models available in the flowsheet simulator. The protocols developed under the CAPE OPEN movement have made it possible to link make ChemSep function as a user model in any CAPE OPEN compliant process simulation system. This includes the ability to use the thermodynamic property packages of the favored simulation system, but ANY CAPE OPEN compliant thermodynamics package could be used with ChemSep. The illustration at the end of this article shows ChemSep used to replace a distillation column model in an HDA flowsheet in Aspen Plus.
ChemSep used to replace a distillation column in an HDA flowsheet in Aspen-Plus. Background is the Aspen-Plus screen. Top right is the CAPE OPEN ChemSep interface through which Aspen-Plus and ChemSep communicate. Bottom right is the ChemSep program for the column that appears as a tagged square labeled ChemSepUO in the lower center of the Aspen Plus flowsheet.
ChemSep is part of the COCOsimulator. Here a flowsheet is shown for the separation of the methanol-acetone azeotrope by means of pressure swing operation. The first column operates at 1 bar and separates pure methanol, whereas the second column operates at 6 bar and separates pure acetone over the bottoms. The azeotrope is recycled. The size of the recycle stream is dependent on the pressure sensitivity of the azeotropic composition.
ChemSep is integrated with the column rating tool of Sulzer Chemtech, SulCol. ChemSep's column rating option will make a preliminary estimate of regular sieve tray column diameters for each section in the column. Sections are trays inbetween feeds and draws. ChemSep assigns sections by default but these can be added and removed at will. To make estimates of the column diameter a system factor, fraction of flood, and default tray spacing needs to be given.
Note that ChemSep computes the column diameter of each section taking into account the different vapor and liquid flows on each stage! Especially in columns with different types of compounds, or reboiled absorbers, these flows can change a lot. Typically the largest diameters occur at the top or bottom stage of the section but that does not need to be the case.
Upon clicking the SulCol button, ChemSep will save the column information in a SulCol XML file and call up the Sulzer Chemtech column rating tool. ChemSep writes the flow rates and properties for the top and bottom stage of each section
SulCol can rate the column sections for trays and packings of different types offered by the Sulzer Chemtech company. These include regular sieve trays, but also high capacity trays with fixed MVG valves or moving BDH valves. Normal, sloped, and truncated downcomer designs are supported. For packings the full pallette of Sulzer packings can be used to rate the column.
Recently Added FeaturesWe typically release new sub-versions twice a year and a major release every two to three years. Detailed descriptions of recently added features you can find in:
ChemSep-Lite Now Available for Nothing!
A version of ChemSep with limited functionality is available for free download from www.chemsep.org/downloads/index.html. ChemSep Lite is limited to 40 components, a databank with 400+ components, and up to 300 stages in a column simulation but no nonequilibrium model. A CAPE OPEN compliant version of ChemSep-Lite has been available from January 2006 onwards. We believe that the availability of a free unit operation enables people to test the CAPE OPEN implementations and strengthen the standard.
Availability, Further Information
ChemSep is available for educational use from the CACHE corporation (www.cache.org). For licenses for non-educational use please contact the authors. Additional information about the program can be found at this site. The authors can be contacted at firstname.lastname@example.org and email@example.com. For more information on CAPE OPEN visit www.colan.org.
Copyright © Wed Feb 1 00:30:36 2012 ChemSep