Computer Aided Design Module for a Binary Distillation Column

Computer Aided Design Module for a Binary Distillation Column K. R. Onifade Department of Chemical Engineering, Federal University of Technology Minna, Nigeria Abstract A Computer Aided Design (CAD) module was developed for determining some design parameters for a binary distillation column using the Lewis-Sorrel method and Visual Basic. The module made use of a data bank containing physical and thermodynamic properties of 18 substances. A sample design problem was solved using the module. Results from the module and manual calculations were very close. Keywords: Binary distillation column, CAD module, CADdc, Lewis-Sorrel method, microsoft access, visual basic, differential distillation, equilibrium distillation, rectification. Introduction The separation of liquid mixture into their several components is one of the major operations in chemical/petroleum industries and distillation is perhaps the most widely used method of achieving it. Binary distillation is a unit operation for separating two or more components in a mixture by distributing them between a vapour and liquid phase based on the difference in the volatility of the components. Three main methods of binary distillation are used in practice. These are differential distillation, flash or equilibrium distillation and rectification. All rely on the basic fact that the vapour is always richer in the more volatile component than the liquid from which it is formed. In rectification, which is the most important method, part of the vapour is condensed and returned as liquid to the still. In the other two methods, the entire vapour is either removed or it is condensed as a product. The essential merit of rectification is that it enables a vapour to be obtained that is substantially richer in the more volatile component than the liquid left in the still. This is achieved by an arrangement known as a fractionating column, which enables the successive vaporization and condensation to be accomplished in one unit. The theory and equations and correlations for determining important parameters in binary distillation is well discussed in many chemical engineering text books (Coulson and Richardson 1983; Hengstebeck 1963; Kern 1990; and Treybal 1981). Two methods are employed in calculating the design parameters in binary distillation. They are the Lewis-Sorrel and McCabe-Thiele methods. The former method can also be used for calculations in multicomponent distillation to determine the number of plates and is the basis of modern computerised methods. The algorithm for implementing the method is well documented (Himmeblau 1974; and McCabe et al. 1993). It shows that the stage-wise calculations use equilibrium data and mass balance calculations, which are represented by the operating line equations. Modern engineering practice is becoming largely dependent on computer and information technology. Computer Aided Design (CAD) is therefore used in the design, maintenance and operations of the plants (Oguntoyinbo 1993; Rooney and Steadman 1980; and Westerberg et al. 1979). Plants are generally made up of unit operation equipment, which are similar in functions and differ only in their duty or throughput. A design approach that is yielding positive results is the independent creation of modules, which can be incorporated into large 1

systems of flowsheeting (Onifade 2000; Onifade 2001a). The objective of this work is to create a similar module for a binary distillation column. Design Method Methodology Lewis-Sorrel s method was used in developing the source code. The column was assumed to operate under the following conditions. 1. Constant relative volatility. 2. Constant molar overflow. 3. Normal or reasonable temperature and pressure. 4. A single point feed. Development of Module A data bank of the physical and thermodynamic properties and Antoine constants of some substances (Carl et al. 1981; and Perry 1984) was created. The constants were used to compute the vapour pressure and the relative volatility across the column. Coding Language The design program was developed using Visual basic because of its advanced features that are well suited to modular programming. Two files are used in implementing the procedure. These are CADbdc.bas and CADbdc.mdb. The CADbdc.bas is the program for the source code. The flowchart for this source code is shown in Fig. 1. It is a menu oriented and user friendly program. There is an executable form of CADbdc.bas, Cadbdc.exe, which can be run from DOS environment. The program draws data from Cadbdc.mdb that contains Antoine s constants, temperature range, and molecular weight for 18 substances. These data are used for computing vapour pressure through Antoine equations (Coulson et al. 1991; and Sinno tt 1993). The CADbdc is a random access Microsoft Access file. Program Run The steps for running the program are as follows: 1. Load the CADbdc into the Visual basic window, click Start. A title page will be displayed on the screen showing an Architectural Design logo of the program. This will be followed by the screen in Fig.2. 2. The user will supply the input specifications in the window with heading beginning with S/No. 3. The user will input the two components by entering their names in windows with headings Fluid One Properties and Fluid Two Properties, respectively. Once the names are entered, the program will pick their properties from CADbdc. For the test problem used in this work, the module has been programmed to input the names of the two components automatically. 4. The user will click on the window with α AB (relative volatility). Two options will be displayed. One allows the user to supply the value of the relative volatility directly. The other option asks the program to calculate the relative volatility. 5. When Step 4 is done, the window with q value will be activated. When clicked, the user will be provided with five conditions for determining q. He will be asked to choose one. The q value will be displayed automatically in the window. 6. As soon as Step 5 is completed, the program will automatically calculate the necessary design parameters. The results of the module can be seen by clicking on Results at the top of the left window of Fig. 2. 7. If an optimization is required, the user can change any of the design specifications to obtain the corresponding design parameters. The module is ended by choosing exit from the window on the left. The Test Problem Results and Discussion The module was tested using the following design problem (Coulson et al. 1991). A continuous fractionating column is to be designed to separate 30,000 kg/h of a mixture 2

of 40% benzene and 60% toluene with the overhead product containing 97% benzene and bottom product containing 98% toluene. These percentages are by weight. A reflux ratio of 3.5 mole to 1 mole of the overhead product is to be used. The molar latent heats of benzene and toluene are 7,360 and 7,960 cal/g-mole, respectively. Benzene and toluene form an ideal system with a relative volatility of about 2.5. The feed has a boiling point of 95 o C at a pressure of 1 atm. (a) Calculate the moles of overhead product and bottom product per hour. (b) Determine the number of ideal plates and the position of the feed plate: (i) if the feed is liquid and at its boiling point. (ii) if the feed is liquid and at 20 o C (specific heat capacity of 0.44 cal/g o C. (iii) if the feed is a mixture of two-thirds vapour and one third liquid. (c) If steam at 20 lb f/h 2 (1.36 atm.) gauge is used for heating, how much steam is required per hour for each of the above three cases, neglecting heat losses and assuming the reflux is a saturated liquid? (d) If cooling water enters the condenser at 25 o C and leaves at 40 o C, how much cooling water are required in cubic meters per hour? (e) What is the minimum reflux ration for cases b(i), b(ii), and b(iii) (f) What is the minimum number of ideal plates for cases b(i), b(ii), and b(iii) Results The module results for the test problem are shown in Figs. 3 to 5. Fig. 3 represents the results for questions (a), b(i), c(i), d(i), e (i) and f(i). Similarly, Fig. 4 represents the results for questions (a), b(ii), c(ii), d(ii), e(ii) and f(ii) while Fig. 5 represents the results for questions (a), b(iii), c(iii), d(iii), e(iii) and f(iii). The comparison of the results obtained from the module and manual calculations is shown in Tables 1 to 4. Discussion of Results The results for the top (distillate) and bottom products are the same for each set of results in Figs. 3, 4, and 5 because the mass balance which gives rise to these values is not affected by q, which is dependent on the condition of feed. The q values for the feed conditions in b (i), b(ii) and b(iii) are: 1.0000, 1.3681 and 0.3333, respectively. The equations for determining the number of plates in the rectifying section is not affected by q, hence the number of plates in the rectifying section is the same in Figs. 3, 4 and 5. However, in the stripping section, the liquid (Lm) and vapour flow rates (Vm) are obtained from: Ym = Vn + F(q 1) (1) Lm = Ln + q F (2) where Ln and Vn are the liquid and vapour flow rates in the rectifying section and F is the feed flow rate. These equations are used in the stagewise calculations to determine the number of plates in the stripping section. The mass of steam used in the re-boiler and the minimum reflux ratio (Rm) are obtained respectively from: Ms = λ (Vn + F(q 1) (3) λ s Rm = X DN Yq (4) Yq Xq Where λ is the heat of vaporization of component B, λ s is the molar latent heat of steam, X DN is the mole fraction of component A in the distillate and Xq, Yq are the coordinates of the point of intersection of the enriching operating line and the q line. Equations (1) to (4) are functions of q, hence different values will give rise to different results for number of plates in the stripping section, the mass of steam used in the re-boiler and the minimum reflux ratio, as shown in Fig. 3 to 5. The minimum number of plates and the mass of water used in the condenser are the same in Figs. 3, 4, and 5 because q does not feature in the equations used for determining them. 3

There is a noticeable difference in the number of plates obtained from module and manual calculations. The stage-wise calculations are usually sensitive to rounding off errors. The manual calculations are more prone to these errors, so the results using this method are often higher than those from CAD module, which relies on computer calculations. This type of difference has been observed in CAD module for multi-component distillation column (Onifade 2000), and shell and tube heat exchanger (Sinnott 1993). The correlation coefficient for each set of values obtained using manual calculations and CADdc range from 0.87 to 1.00. These values are adequate for engineering design. It is significant to note that the worst value of 0.87 arises from the comparison of plates in the stripping section due to reasons adduced above for stage-wise calculations.the temperature and vapour pressure profile of each plate are important parameters for the eventual mechanical design of the column (Onifade 2001b). Future work on the module will therefore include the temperature profile of each plate and the vapour pressure of each component on the plates. Conclusion The Lewis-Sorrel method was used in developing a CAD module for implementing the design of binary distillation column. The module makes use of a databank for properties (such as enthalpy, specific volumes, density) of 18 substances. The CAD module was tested with a design problem. The results obtained from the module for certain design parameters were found to agree with theory and close to those obtained from manual calculations. References Carl, L.Y., Ku,Y.L.; and Fang, C.S. 1981. Chemical Engineering International News, p.20c. McGraw-Hill, New York, pp.63-65, 153-156. Coulson, J.M.; and Richardson, J.F., 1983. Chemical Engineering, Volume 6, 2 nd ed., Pergamon Press, Oxford. Coulson, J.M.; Richardson, J.F.; Backhurst, J.R.; and Harker, J.H. 1991. Chemical Engineering, 2 nd ed., Pergammon Press, Oxford. Hengstebeck, R.J. 1963. Distillation: Principles and Design Procedure. Reinhold, New York. Himmeblau, D.M. 1974. Basic Principles and Calculations in Chemical Engineering. 6th ed., Prentice-Hall, Englewood Cliff, NJ. Kern, D.O. 1990. Process Heat Transfer. McGraw Hill, New York. McCabe, W.L.; Smith, J.C.; and Harriott, P. 1993. Unit Operations of Chemical Engineering, 5th ed. McGraw-Hill, New York. Oguntoyinbo, S. 1993. Computer Aided Design and Its Applications in Nitel Research and Development 2(2): 41-44. Onifade, K.R. 2000. Computer aided design module for multi-component distillation column. AU. J.T. 4: 26-38. Onifade, K.R. 2001a. Computer aided design module for an absorption column. AU J.T. 5: 13-20. Onifade, K.R. 2001b. Computer aided design module for a binary distillation column. Proc. 3 rd Annual Conference of School of Engineering and Engineering Technology, Federal University of Technology, Minna, Nigeria, Oct. 2001. Perry, J.H. 1984. Chemical Engineers Handbook, 7 th ed. McGraw-Hill, New York. Rooney, J.; and Steadman, P. 1980. Principles of Computer Aided Design. Pitman, London. Sinnott, R.K. 1993. Chemical Engineering, Vol. 6, 2 nd ed. Pergamon Press, Oxford. Treybal, R.E. 1981. Mass Transfer Operations. 3 rd ed. McGraw-Hill, Auckland, New Zealand. Westerberg, A.W.; Hutchison, H.P.; Motard, R. L.; and Winter, P. 1979. Process Flowsheeting. Cambridge University Press, Cambridge. 4

Start Specify Feed components A and B Feed, bottom and top composition in % weight Feed rate in % weight Heat capacities of the feed components Initial and boiling temperature of the feed Vapor fraction of the feed Heat of vaporization of the components Calculate Feed, top and bottom product rate in kg/h Feed, top and bottom composition in moles Average molecular weight of the feed Liquid, vapour flow rates, number of theoretical plates in rectifying section Specify NO Is Saturated temperature Relative volatility given? Call Databank Calculate Relative volatility YES Specify Relative volatility Select appropriate nature of the feed Calculate q-value (Heat fraction) Liquid and vapor flow rates and number of theoretical plates in the rectifying section Liquid and vapor flow rates and number of theoretical plates in the stripping section Total number of theoretical in the column Mass of steam required in the reboiler Volume of cooling water required in the condenser Output the Overall result Stop Fig. 1. Flowchart for the source code of CADbdc.bas 5

Fig. 2. Screen for inputting design specification Fig. 3. CADdbc results for the problem b(i), c(i), d(i), e(i), and f(i) 6

Fig. 4. CADdbc results for the problem b(ii), c(ii), d(ii). E(ii), and f(ii) Fig. 5. CADdbc results for the problem b(iii), c(iii), d(iii), e(iii), and e(iii) 7

Table 1. Solution to test problem (a) COMPONENTS Benzene Toluene Total Correl. Coeff. F (kg/hr) Manual 154 196 350 FEED CADdbc 153.8461 195.6521 349.4983 1 x F (mole) Manual.44.56 1 CADdbc.4401.5598 1 1 D (kg/h) Manual 149.4 3.98978 153.4 DISTILLATE CADdbc 149.2308 3.9130 153.1438 1 x D (mole) Manual.97.026 1 CADdbc.9744.0256 1 0.999992 BOTTOM PRODUCT B (kg/h) Manual 4.7184 191.88 196.6 CADdbc 4.6154 191.7391 196.3545 1 x B (Mole) Manual 0.024 0.9764 1 CADdbc 0.023514 0.9765 1 1 Table 2. Solution to test problem (b) No of plates Position of feed (Plate) Cases Manual CADbdc Manual CADbdc B(i)) 11 8 4 4 B(ii) 10 8 4 4 B(iii) 12 9 4 4 Corr. Coeff. 0.866025 1 Table 3. Solution to test problem (c) and (d) Mass of steam (kg/h) Volume of Cooling water (m 3 /h) Cases Manual CADbdc Manual CADbdc b(i)) 10,520 10508 367.68 367.06 b(ii) 12,500 12470 367.68 367.06 b(iii) 6,960 6955 367.68 367.06 Corr. Coeff. 1 1 Table 4. Solution to test problem (e) and (f) R m Cases Manual CADbdc Manual CADbdc B(i)) 1.43 1.399 7 7 B(ii) 1.25 1.269 7 7 B(iii) 1.84 1.805 7 7 Corr. Coeff. 1 1 N m 8