Application of Dividing Wall Column Distillation (DWCD) Process for Separation of 1,2-Butanediol, 1,4-Butanediol, and 2,3-Butanediol Mixture to Save Energy

Adrian T., Schoenmakers H., and Boll M., 2004.  Model predictive control of integrated unit operations: Control of a divided wall column, Chem. Eng. Process.: Process Intensification, 43,  347-355. 

Arora S., 2014. Simulation Study of Divided Wall Distillation Column, B.A. thesis, Technology In Chemical Engineering and Processing, National Institute of Technology, Rourkela.

Barroso-Mu˜noza F. O., Hernández S., Hernández-Escoto H., Segovia-Hernández J. G., Rico-Ramírez V., and Chavez R. H., 2010. Experimental study on pressure drops in a dividing wall distillation column, Chem. Eng. Processing, 49, 177–182.

Bernad-Serra A., Jakobsson K., and Alopaeusb V., 2018. Simulation and design of a dividing wall column with an analysis of a vapor splitting device, Chem. Eng Transactions, 69.

Buck C., Hiller C., and Fieg G., 2011. Applying model predictive control to dividing wall columns, Chem. Eng. & Tech., 34, 663-672.

Errico M., Tola G., Rong B. G., Demurtas D., and Turunen I., 2009. Energy saving and capital cost evaluation in distillation column sequences with a divided wall column, Chem. Eng. Res. Des., 87, 1649–1657.

Fang J., Zhao H., Qi J., Li C., Qi J., and Guo J., 2015. Energy conserving effects of dividing wall column, Chinese J. Chem. Eng., 23, 934–940.

Filho P. B. O., Nascimento M. L. F., and Pontes K. V., 2018. Optimal design of a dividing wall column for the separation of aromatic mixtures using the response surface method, Proceedings of the 28th European Symposium on Computer Aided Process Engineering, Graz, Austria,.

Ge X., Yuan X., Ao C., and Yu K. K., 2014. Simulation based approach to optimal design of dividing wall column using random search method, Comp. Chem. Eng., 68, 38–46.

Gor N., Upkare M., Satpute S., and Mali N., 2017. Simulation and analysis of divided wall column for energy efficient and intensified distillation, International Conference on Sustainable Development for Energy and Environment, January 16th & 17th 2017, Pune, India.

Illner M., and bin Othman M. R., 2015. Modeling and simulation of a dividing wall column for fractionation of fatty acid in oleochemical industries, Perintis E-J., 5, 34-44.

Kaur J., 2012. Simulation Studies of Divided Wall Distillation Column, M.S. Thesis, Thapar University.

Khalili-Garakani A., Ivakpour J., and Kasiri N., 2016. Three-component distillation columns sequencing: Including configurations with divided-wall columns, Iranian J. Oil & Gas Sci. and Tech., 5, 66-83.

Khushalani K., Maheshwari A., and Jain N., 2014. Separation of mixture by divided wall column using Aspen Plus, Int. J. Emerging Tech. Adv. Eng., 4, 563-571.

Kim Y. H., 2017. Energy saving in a crude distillation unit with a divided wall column, Chem. Eng. Comm., http://www.tandfonline.com/loi/gcec20,.

Kiss A. A., and Rewagad R. R., 2011. Energy efficient control of a BTX dividing-wall column, Comp. Chem. Eng., 35, 2896–2904,.

Landaeta S. J. F., Kiss A. A., and Haan A. B. D., 2012. Enhancing multi-component separation of aromatics with Kaibel columns and DWC, Proceedings of the 22nd European Symposium on Computer Aided Process Engineering, London.

Le Q. K. 2014. Design and simulation of dividing wall column for ternary heterogeneous distillation." Norwegian University of Science and Technology, Master Thesis in Chemical Engineering.

Long N. V. D. , and Lee M., 2012. Dividing wall column structure design using response surface methodology, Comp. Chem. Eng., 37, 119– 124.

Nguyen M. T. D., 2015. Conceptual Design, Simulation and Experimental Validation of Divided Wall Column: Application for Nonreactive and Reactive Mixture, PhD Thèse, Institut National Polytechnique de Toulouse (INP Toulouse),.

Niggemann G., Hiller C., and Fieg G., 2010. Experimental and theoretical studies of a dividingwall column used for the recovery of high-purity products, Ind. & Eng. Chem. Res., 49, 6566-6577.

Rangaiah G. P., Ooi E. L., and Premkumar R., 2009. A simplified procedure for quick design of dividing-wall columns for industrial applications, Chem. Product Process Model., 4,  1-42.

Sangal V. K., Kumar V., and Mishra I. M., 2012. Optimization of structural and operational variables for the energy efficiency of a divided wall distillation column, Comp. Chem. Eng., 40, 33– 40.

Sangal V. K., Kumar V., and Mishra I. M., 2013. Optimization of a divided wall column for the separation of C₄-C₆ normal paraffin mixture using Box-Behnken design, Chem. Ind. Chem. Eng., 19, 107−119.

Shi P., Xu D., Ding J., Wu J., Ma Y., Gao J., and Wang Y., 2018. Separation of azeotrope (2,2,3,3-Tetrafluoro-1-propanol + Water) via heterogeneous azeotropic distillation by energy-saving dividing-wall column: Process design and control strategies, Chem. Eng. Res. Des., https://doi.org/10.1016/j.cherd.2018.05.025,.

Shojae K., keramati M., Beiki H., 2015. Simulation of divided wall column for separating dimethyl ether from water-methanol mixture, Farayandno (in Persian)10, 67-75.

Sun J., Ge H., Chen W., Chen N., and Chen X., 2015. CFD simulation and experimental study on vapour splitter in packed divided wall column, The Canadian J. Chem. Eng., 93, 2261-2265.

Szabo L., Balaton M., Nemeth S., and Szeifert F., 2008. Modeling of divided wall column, University of Pannonia, Department of Process Engineering, Hungary.

Wang E., Li Z., and Li C., 2013. Simulation on application of dividing wall column in aniline distillation, Adv. Materials Res., 602-604, 1299-1303.

Wang E., 2014. Simulation and analysis of multiple steady states in dividing wall column, Asia-Pacific J. Chem. Eng., DOI: 10.1002/apj,.

Yuqi H., Jing F., and Chunli L., 2015. Simulation optimization and experimental study of cross-wall adiabatic dividing wall column used to separate Hexane-Heptane-Octane system," China Pet. Processing and Petrochemical Tech., 17, 108-116.

Zhai C., Liu Q., Romagnoli J. A., and Sun W., 2019. Modeling/Simulation of the dividing wall column by using the rigorous model, Processes,  26, doi:10.3390/pr7010026.

Zhong Y., Wu Y., Zhu J., Chen K., Wu B., Ji L., Shen Y., 2015. The Distillation Process Design for the Ternary System 1,2-Butanediol + 1,4-Butanediol + 2,3- Butanediol." Sep. Sci. Tech. 50, 2545–2552.

1. Introduction

Due to the environmental and energy crisis, the focus of scientific and industrial research is on developing process design methods for reducing energy consumption and increasing energy efficiency in chemical processes (Nguyen, 2015). Distillation is undoubtedly the most common separation process used in the chemical industry. About 95 percent of the liquid and gas separation processes in the chemical industry are subject to distillation. About 3% of the total world energy consumption is assigned to operating distillation columns worldwide. The application of new distillation column technologies resulted in substantial energy savings. The DWCD process is one of the safest technologies adopted in reducing the energy consumption in distillation towers, thus, lowering operating costs (Le, 2014).

Adrian et al. (2004) performed experiments in a laboratory mini-plant, where the feed consisted of butanol, pentanol, and hexanol, and the dividing wall column of four sections. Szabo et al. (2008) assessed the DWCD process modeling, where the influence of the main parameters of the DWCD process, such as the liquid and vapor split ratio, column height, vertical wall position, and wall heat transfer was of concern. Errico et al. (2009) evaluated the DWCD process for energy savings and investment costs, where, the feed consists of a mixture of normal paraffins from butane and heptane. The system had four products and five feed combinations. Rangaiah et al. (2009) proposed a method forrapidly designing the DWCD process for industrial applications. Their results indicated that the vapor and liquid split ratios had a significant effect on the energy required in the DWCD process. They assessed the ternary system of benzene, toluene, and p-xylene. They applied two distillation towers to simulate a rigorous DWCD process.

Barroso-Mnnoz et al. (2010) assessed the pressure drop in a DWCD process experimentally, by testing different volumes of gas and liquid velocities to measure pressure drop and identify operating areas. The adopted system was air/water as a base. Niggemann et al. (2010) assessed the separation of n-hexanol, n-octanol, and n-decanol ternary mixture. Buck et al. (2011) simulated and assessed this ternary mixture in a DWC experimentally.  Kiss and Rewagad (2011) assessed the dynamic state simulation of a DWCD process, where the mixtures of benzene, toluene, and xylene were of concern.

Kaur (2012) assessed a DWCD process simulation, where the separation of a xylene-toluene-benzene mixture was of concern. In their study, the assumption was made in the constant structural parameters and variable vapor and liquid split ratios. The design of the DWCD process by applying the Response surface methodology (RSM) is proposed by Long and Lee (2012) which was applied in the design and optimization of an acetic acid treatment process. Sangal et al. (2012) optimized the structural and operational variables of a DWCD process to increase energy efficiency. The Box-Benken Design (BBD) and RSM were applied in optimizing the parameters. Sangal et al. (2013) optimized a DWCD process to separate normal C₄-C₆ paraffin mixtures through the Box-Benken design. The parameters studied were the reflux ratio and liquid and vapor split ratios. RSM was applied in optimizing the parameters. Wang et al. (2013) simulated the DWCD process for aniline distillation.

Arora (2014) simulated a DWCD process, where the benzene-toluene-p-xylene, benzene-toluene-o-xylene, and methanol-water-glycerol mixtures were assessed. The effects of the reflux ratio, stage number, feed composition, and split ratio parameters were studied in finding optimal operating conditions. Ge et al. (2014) optimized a DWCD process by applying neural networks and genetic algorithms. They applied the n-pentane/ n-hexane/ n-heptane, benzene/ toluene/ ethylbenzene, and ethanol/n-propanol/n-butanol feeds. Khushalani et al. (2014) assessed the DWCD process simulation through the Aspen Plus for benzene-toluene-xylene mixtures, where the wall position was determined by the number of trays in the absorber columns. The height of the wall was determined by the number of trays in the rectifier and the stripper. A DWCD simulation for the separation of benzene-toluene-xylene was made by Wang (2014).

Fang et al. (2015) assessed the n-hexane, n-heptane, and n-octane mixtures separation in the DWCD process by simulation and experiment. They considered the DWC with thermal insulation (HIDWC) and DWC with wall heat transfer (HTDWC) processes. The modeling and simulation of a DWCD process for fatty acid separation in the oleochemical industry is assessed by Illner and Othman (2015), where, a 4-column model is applied to simulate DWC. The conceptual design, simulation, and experimental validation of the DWCD process are assessed by Nguyen (2015), where the FUGK model is adopted to design the DWC. Shojae et al. (2015) simulated the DWC distillation to separate dimethyl ether from a water and ethanol mixture and found that the DWC structure led to a duty reduction of about 24% for the condenser and 7% for the reboiler. They applied a three-column model to simulate the DWCD process. Sun et al. (2015) assessed the experimental study and CFD simulation of a vapor splitter in a packed DWCD process. The simulation and experimental study of the adiabatic DWCD process across the wall is assessed by Yuqi et al. (2015), where the feed consists of hexane-heptane-octane. The effects of feed stage, side product stage, liquid and vapor split ratios on energy consumption is assessed based on simulation response surface model.

Khalili-Garakani et al. (2016) assessed the three-component distillation columns sequences, in DWC. The feed mixture is examined at low, medium, and high contents of the middle component concentrations. They made comparisons among the conventional and complex thermal coupling, thermodynamic equivalence, and DWC arrangements. Gor et al. (2017) simulated the separation of the butane, pentane, and hexane mixture in a DWCD process and assessed the effect of different parameters on the purity of the components and the duties of the reboiler and the condenser. Their results indicate that the energy reduction by DWC is 34.74% for the condenser and 31.28% for the reboiler. They applied the 4-column model to simulate DWC using Aspen Plus. A DWCD process simulated for a crude oil distillation unit is assessed by Kim (2017) who found that DWC led to a reduction in remixing in the feed stage and increased the thermodynamic efficiency of the CDU. The unit performance evaluation reveals that the DWC unit gains a 37% energy saving in heat duty consumed and 17% in condenser duty compared to conventional distillation.

The simulation and design of the DWCD process along with the analysis of a vapor splitter is assessed by Bernad-Serra et al. (2018), where the mixture of benzene, toluene, and xylene prevailed.  The performance of a DWCD process for the separation of aromatic mixtures through RSM is optimized by Filho et al. (2018), where the DWC application saves energy up to 44% compared to the conventional 2-column arrangement. The design parameters for the DWC include the number of stages in the upper, lower, and prefractionator sections and the internal vapor and liquid flow to the prefractionator. Shi et al. (2018) adopted an azeotropic distillation method to separate 2,2,3,3-tetra fluorine, 1-propanol, and water mixtures in a DWCD process, where, chloroform was consumed as an azeotropic reagent. Zhai et al. (2019) simulated a DWCD process through a rigorous model and assessed the effects of the liquid and vapor ratios. The split ratio at the top of the pre-fractionator has an optimum point for energy saving, and the feed mixture consists of benzene, toluene, and xylene.

In this study, the DWCD and conventional processes of separation 1,2-BD, 1,4-BD, and 2,3-BD in the ternary mixture are simulated for the first time, and the related parameters are optimized to reduce the heat duties of the system by considering proper product purities. The 2,3-BD has extensive industrial applications, in pharmaceuticals, fumigants, perfumes, and printing inks (Zhong, et al. 2015). The separation of 1,2-Bd, 1,4-BD, and 2,3-BD is essential to purify the three diols. Distillation, because the diols have similar chemical properties, is an effective method for separation. The studies on applying the DWC technology in reducing energy consumption in distillation towers are for a long time, the DWC does not lead to a reduction in energy consumption in all cases. Some parameters, such as the type, properties, composition, and percentage of feed components are influential, and each case should be assessed and simulated separately.

2- Methods and Materials

The Aspen Plus software is applied to model and simulate conventional distillation columns and the DWCD process. The shortcut and rigorous simulations are run with the DSTWU and RadFrac models, respectively. The thermodynamic model NRTL is applied for modeling the processes. The feed specifications of this study are tabulated in Table 1.

Table 1 The feed specifications

The conventional distillation model for separating the three-component mixture with two distillation

columns is shown in Fig. (1). The feed enters the first column, and at the boiling point of the components, 2,3-BD leaves the top of the first column as a distillate; the bottom product of the first column contains 1,2-BD and 1,4-BD, which enter the second column, and 1,2-BD leaves the top and 1,4-BD leaves the bottom of the second column.

Figure 1 The conventional distillation process for separating three-component mixtures consisting of two columns

The assessed parameters for the conventional distillation include: number of stages of the first and the second columns, the reflux ratios of the first and the second columns, and feed stages numbers of the first and the second columns.

DWCD integrates the two conventional columns into one with a dividing wall, usually installed in the central section, which is practical in multicomponent mixtures separation with only one column shell, one reboiler, and one condenser. DWCD is a complex process, but in commercial software Aspen Plus, there is no standard model for its simulation. The available findings indicate that - four models are applied for simulating a DWC: 1) pump around sequence, 2) two-column sequence including prefractionator, 3) two-column sequence including postfractionator, and 4) four-column sequences.

Though the last model is hard in initializing all interconnecting streams next to its slow convergence, it allows more flexibility in this simulation compared with the other models. It is the best model for simulations and the most appropriate in assessing the vapor and liquid splits. In this research this model is applied.

The diagram of the DWCD process simulation In this study is shown in Fig. (2), where, as observed the system has four distillation columns and two vapor and liquid splitter. The four-column sequence consists of two Absorbers, one Rectifier, and one Stripper. Feed enters Absorber-1 and the top product of Absorber-1column comes into the Rectifier as a vapor phase. The bottom product enters the Stripper; the top product of the Rectifier is the final product of the system which includes the lightest component.

The bottom product of the Rectifier which is in the liquid phase enters the liquid splitter, to be divided into two parts, one that enters Absorber-1 and the other that enters Absorber-2. The bottom product of Stripper is the final product of the process and contains the heaviest component. The top product of the Stripper is in the form of vapor and enters the vapor splitter to be divided into two parts, one that enters Absorber-1 and the other that enters Absorber-2. The Absorber-2 column has two output products, the top of which is in the vapor phase and enters the Rectifier column. The lower product of the Absorber-2 column is liquid and enters the Stripper column. Its side product is the final product of the process and contains the intermediate component. The Absorber-2 column has two liquid and vapor inlets, which are liquid and vapor splitters outputs, respectively.

Figure 2 Diagram of the DWCD process simulation for ternary feed separation

The assessed parameters of DWCD are the number of stages of Absorber1 and Absorber2, Rectifier, Stripper, reflux ratio, vapor split ratio, liquid split ratio, feed stage number of Absorber1, and side product stage number of Absorber2.

3- Results and Discussion

3.1. Two Conventional Distillation Columns

3.1.1. Shortcut Simulation for the Conventional Distillation Columns

Table 2  shows the input data of the two columns of DSTWU.

Table 2 the input data of the two columns of DSTWU

The Aspen Plus enable user to provide reflux ratio as a function of the minimum reflux ratio as a negative value. When user is provided with this negative value of -1.2, Aspen Plus interprets it as 1.2 x(Minimum reflux ratio) for column design. 

 The characteristics of the top and bottom products of distillation columns in the shortcut results are tabulated in Table 3. Table 4 shows the characteristics of the first and second columns in the shortcut design results.

Table 3 The characteristics of top and bottom products of distillation columns in the shortcut results

3.1.2. Rigorous Simulation of the Conventional Distillation Columns

Input data for rigorous simulation is based on shortcut results with required modifications. The top and bottom flow characteristics of the distillation columns in the rigorous simulation, are shown in Table 5, and the heat duties of the reboilers and the condensers in the two-column rigorous simulation results are tabulated in Table 6. The characteristics of the first and second columns in the rigorous simulation are tabulated in Table 7.

Table 2 The characteristics of the first and the second columns in the shortcut design

 Table 3 The product characteristics of the distillation columns in the rigorous simulation

 Table 4 The heat duties of the reboilers and the condensers in the two-column rigorous simulation

 Table 5 The characteristics of the first and the second columns in the rigorous simulation

3.1.3. Optimization of rigorous simulation of the conventional distillation columns

3.1.3.1. Number of the first column stages

The effect of the stages number of the first column on the output parameters is shown in Fig. (3); the effect of the number of the first column on the molar fraction of the components in the two columns' products is shown in Fig. (3-a), where, as observed an increase in the number of first column stages increases the concentration of all three components. The increase rate for 2,3-BD as the lightest component is more pronounced in comparison with the other two components, because an increase in the number of column stages, enhances the separation process. The effect of the number of the first column stages on the heat duties of the reboiler and the condenser of the first column is shown in Fig. (3-b), where, as observed, an increase in the number of the first column stages, increases the duties of both the reboiler and the condenser of the first column. This effect on the reboiler duty is more than that of the condenser duty, The reason for this can be explained that by increasing the number of the first column stages, the height of the column increases and requires more duties due to fixing the other parameters such as flowrates and refluxes.

As observed in Fig. (3-b), an increase in the number of stages in the first column from 26 to 34 slightly increases, the heat duties of the reboiler and the condenser by 0.011% and 0.06%, respectively. The reason for this slight increase in heat duties is due to the slight change in the product concentration at the top of the first column (2,3-BD), which increases from 0.95 to 0.98% by an increase in the number of trays from 26 to 34. Another reason for this is the big difference in the boiling point between the light component (2,3-BD, 180.7 C) and the heavy component (1,4-BD, 228 C). The reason for the fluctuating changes in the condenser duty in Fig. (3-b) is related to the convergence in the numerical simulation method, while the graph shows an increase in the condenser duty. It is also to note that the product flow rates at the top of the columns remain constant in mass units.

The effect of the number of stages of the first column on the heat duties of the reboiler and the condenser of the second column is shown in Fig. (3-c), where, as observed an increase in the number of the first column stages decreases the duties of the reboiler and the condenser of the second column. As the number of stages in the first column increases, the separation occurs more in the first column. Because the other parameters are constant, the role of the second column decreases, therefore, the reboiler and the condenser duties of the second column are reduced. The effect of the number of stages of the first column on the sum of heat duties of the reboilers and the condensers of the first and the second columns is shown in Fig. (3-d), where as observed an increase in the number of the first column stages, decrease the sum of reboilers and condensers' duties of both two columns. Based on the results of this section, 34 stages are selected for the first column.

3.1.3.2. Number of stages of the second column

The effect of the number of the stages of the second column on the output parameters is shown in Fig. (4). The effect of the number of stages of the second column on the mole fraction of the components in the products is shown in Fig. (4a), where, as observed, because the 2,3-BD is separated in the first column, the concentration of 2,3-BD remains constant with an increase in the number of second column stages. The increase in the number of second column stages leads to a better separation of 1,2-BD and 1,4-BD.

The effect of the number of the second column stages on the heat duty of the reboiler and the condenser of the first column is shown in Fig. (4-b), where as observed, the effect is neutral. The effect of the number of the second column stages on the heat duty of the reboiler and the condenser of the second column is shown in Fig. (4-c), where as observed an increase in the number of the second column stages decreases both the reboiler and the condenser heat duties. In Fig. (4-c), an increase in the number of stages in the second column from 12 to 21, caused a slight increase in the heat duties of the reboiler and the condenser; 1.22% and 1.36%, respectively. The reason for this slight increase in heat duties is due to the slight change in the product concentration at the top of the second column (1,2-BD), which increased from 0.94 to 0.98(4.25%). It is also to note the product flow rates at the top of the columns remain constant in mass units.  The effect of the number of the second column stages on the sum of heat duties of the reboilers and the condensers of the first and the second columns is shown in Fig. (4-d). The amounts of other parameters are constant. As observed an increase in the number of the second column stages, decreases the sum of the reboilers and the condensers' heat duties of the two columns. According to the findings in Fig. (4-d), 20 stages are selected for the second column.

3.1.3.3 The reflux ratio of the first column

The effect of the first-column reflux ratio on the output parameters is shown in Fig. (5) and The effect of the first-column reflux ratio on the mole fraction of the components in the products is shown in Fig. (5-a). As can be observed by increasing the reflux ratio of the first column, the concentration of 1,4-BD as the heaviest component is almost constant, but the concentration of the two components of 2,3-BD and 1,2-BD is increased. The reason for this can be explained that by increasing the reflux ratio, the separation is enhanced, and the concentration of the components in the products increases. The effect of the first column reflux ratio on the heat duties of the reboiler and the condenser of the first column is shown in Fig. (5-b), where, as observed, an increase in the first column reflux ratio, increases both the reboiler and the condenser heat duties. The increase in the duties is linear. This trend is perfectly logical By increasing the reflux ratio of the first column, the vapor and liquid flowrates increase at all stages of the column, so it increases the duties of the reboiler and the condenser. The effect of the first column reflux ratio on the heat duties of the reboiler and the condenser of the second column is shown in Fig. (5-c), As can be observed, with the increase of the reflux ratio of the first column, the duties of the reboiler and the condenser of the second column decreased. This decrease is almost linear. The reason for this can be explained that by increasing the reflux ratio of the first column, the separation in the first column is increased, and therefore the role of the second column decreases. The reboiler and the condenser duties of the second column are reduced. The effect of the first column reflux ratio on the sum of the reboilers and the condensers' heat duties of the first and the second columns is shown in Fig. (5-d). The values of the other parameters are constant. An increase in the first column reflux ratio increases the sum of the reboilers and the condensers' heat duties of both columns. This increase is linear. According to the results of this section, the first column reflux ratio is selected 3.57.

3.1.3.4. The second column reflux ratio

The effect of the second-column reflux ratio on the output parameters is shown in Fig. (6) where The effect of the second-column reflux ratio on the mole fraction of the components in both the two columns' products is shown in Fig. (6-a). As increase in the second column reflux ratio, increase the concentration of two other components while the concentration of the 2,3-BD is constant. The effect of the second column reflux ratio on the heat duties of the reboiler and the condenser of the first column is shown in Fig. (6-b). Figure 6 (c) shows the effect of the second column reflux ratio on the reboiler and the condenser heat duties of the second column. As can be observed, by increasing the reflux ratio of the second column the reboiler and the condenser heat duties of the second column both increased. This increase is linear and is quite reasonable. The effect of the second column reflux ratio on the sum of heat duties of the reboilers and the condensers of both columns is shown in Fig. (6-d), As can be observed by increasing the reflux ratio of the second column, the sum of reboilers and condensers heat duties of the two columns are increased. This increase is linear. The comparison between the results of Error! Reference source not found.-d) and Error! Reference source not found.-d) (as to the effect of the first column reflux ratio on the sum of the reboilers and the condensers duties of the two-columns) indicates that the effect of increasing the second column reflux ratio on total duties of the reboilers and the condensers of both the columns is greater than the effect of increasing the first column reflux ratio on this parameter. This is because the flow rate in the first column is higher, thus, a more pronounced effect in the first column. According to the results of this section, the second column reflux ratio isselected1.52.

3.1.3.5. Feed stage number of the first column

The effect of the feed stage number of the first column on the output parameters is shown in Fig. (7), The effect of the feed stage number of the first column on the mole fraction of the components in the three products is shown in Fig. (7-a). As can be observed by increasing the feed stage number of the first column, the concentration of 2,3-BD and 1,2-BD as the lightest and the medium components at first increased and then decreased, but the concentration of 1,4-BD is insignificantly increased.

. The effect of the feed stage number of the first column on the heat duties of the reboiler and the condenser of the first column is shown in Fig. (7-b). The effect of the feed stage number of the first column on the heat duties of the reboiler and the condenser of the second column is shown in Fig. (7-c), where as observed by an increase in the feed stage number of the first column, the reboiler and the condenser heat duties of the second column first decrease and then increase. The effect of the feed stage number of the first column on the sum of the heat duties of the first and the second columns is shown in Fig. (7-d). The other parameters are in constant values. By an increase in the feed stage number of the first column, the sum of the reboilers and the condensers' heat duties of the two columns first decreased with a steep slope and then increased with a gentle slope. According to the results of this section, the feed stage number of the first

column is selected18.

3.1.3.6.   Feed stage number of the second column

The effect of the second column feed stage number on the output parameters is shown in Fig. (8), the effect of the feed stage number of the second column on the mole fractions of the components in the three products is shown in Fig. (8-a) and as observed an increase in the feed stage number, the concentration of 2,3-BD remains constant, which is reasonable because 2,3-BD is separated in the first column. By this increase, the concentrations of 1,2-BD and 1,4-BD as key components of the top and bottom products, first increase and then decrease.

This effect on the heat duties of the reboiler and the condenser of the first column is shown in Fig. (8-b), where, as observed, it is null, which is reasonable. This effect on the heat duties of the reboiler and the condenser of the second column is shown in Fig. (8-c), where, as observed by an increase in the second column feed stage number, the reboiler and the condenser heat duties of the second column first decrease and then increase. This effect on the sum of heat duties of the reboilers and the condensers of the first and the second columns is shown in Fig. (8-d), where, as observed by an increase in the second column feed stage number, the sum of the reboilers and the condensers' heat duties of both columns first decrease, and then increase, and its changes is quite similar way as to the changes in the reboiler and the condenser heat duties of the second column. The other parameters are constant. According to the results of this section, the feed stage number of the second column is selected 11.

3.1.4.    The results of rigorous simulation optimization in conventional two-column case

The optimal conditions of rigorous simulation in the conventional two-column case are tabulated in Table 8.

The mole fractions of the components in the products of the first and the second columns in the conventional two-column RadFrac model in the optimal conditions are tabulated in Table 9.

Table 8- The optimal conditions of rigorous simulation in a conventional two-column case

Table 9- The mole fractions of the components in the products of the first and the second columns in the two-column RadFrac model in the optimal conditions

The reboiler and the condenser heat duties of the first and the second columns in the conventional two-column Rigorous simulation in the optimal conditions are tabulated in Table 10.

Table 10- The reboiler and the condenser heat duties of the first and the second columns in the two-column Rigorous simulation in the optimal conditions

The profiles at the optimal conditions are shown in Fig. (9) and the temperature changes at different stages of the first column in the optimal conditions are shown in Fig. (9-a), where, as observed, the temperature increases with an increase in stage number. In stage 18, a breakpoint is observed, which corresponds to the location of feed entry into the first column. The first column concerns the separation of 2,3-BD from the mixture of 1,2-BD and 1,4-BD. The top temperature of the column is 180.8 ⁰C, which is very close to the 2,3-BD boiling point (180.7 ⁰C). The column bottom temperature is 211.04 ⁰C, which is the temperature of the 1,2-BD and 1,4-BD mixture. The concentration changes of the components in the liquid phase at different stages of the first column at the optimal conditions are shown in Fig. (9-b), where, as observed by an increase in the number of the stages, the concentration of 2,3-BD decreases as the lightest component, the 1,4-BD concentration increases as the heaviest component, and the concentration of 1,2-BD as the intermediate component first increases and then decreases. In step 18, a breakpoint is observed, which corresponds to the location of the feed entry into the first column. Since the composition of feed differs from that of components in step 18, it results in a change in the concentrations trends. The 2,3-BD concentration at the top of the column is 0.975 and leaves as the top product of the column. The concentrations of 1,2-BD and 1,4-BD in the bottom product are 0.531 and 0.46, respectively. The concentration changes of the components in the vapor phase at different stages of the first column at the optimal conditions are shown in Fig. (9-c), where, as observed, the changes in the vapor phase concentrations are similar to those in the liquid phase. The comparison between Figs. (9-b and c) concerning the components’ concentration changes in the liquid and vapor phases at different stages of the first column reveals that in the second case, no breakpoint is observed at the feed entry stage (stage 18), the feed enters as the liquid phase on the stage. This phenomenon shows that the concentration of 1,4-BD (the heaviest component) in the liquid phase has increased from 0% at the top of the column to 46% at the bottom, while, the same change in the vapor phase is from 0% rate at the top of the column to 25% at the bottom. Therefore, the rate of change for 1,4-BD is lower in the vapor phase . The concentration of 1,2-BD in the liquid phase has increased from 0 at the top of the column to 53% at the bottom revealing a maximum value of 75% at stage 30, while, the same change in the vapor phase is from 0 at the top of the column to 73% at the bottom with a maximum value of 85% at stage 32. Therefore, the rate of change for 1,4-BD is lower in the liquid phase.

 The temperature changes at different stages of the second column at the optimal conditions are shown in Fig. (9-d), As can be observed, the temperature is increased with increasing stage number. This column corresponds to the separation of 1,2-BD and 1,4-BD. The top temperature of the column is 196.29 ⁰C, which is very close to the 1,2-BD (196.42 ⁰C) boiling point. The bottom temperature of the column is 229.95 ⁰C, which is very close to the 1,4-BD (228 ⁰C) boiling point. The concentration changes of the components in the liquid phase at different stages of the second column at the optimal conditions are shown in Fig. (9-e), where, as observed, the concentration of 2,3-BD is almost zero at all stages, and the concentration of 1,4-BD as the heaviest component increases with an increase in stage number, and at stage 20, which leaves the column this concentration is 0.977. The composition of 1,2-BD as the intermediate component decreases with an increase in stage number and leaves at the top of the column at a concentration of 0.975. The concentration changes of the components in the vapor phase at different stages of the second column at the optimal conditions are shown in Fig. (9-f), where, as observed, the trend of changes is quite similar to the liquid phase.

3.2. Dividing Wall Column Distillation (DWCD) Simulation

The initial characteristics of the four columns and the liquid and vapor splitters in the DWCD process are tabulated in Table 11; the characteristics of the products of the DWCD process are tabulated in Table 12 and the reboiler and condenser heat duties in the DWCD process are tabulated in Table 13.

Table 11- The initial characteristics of the four columns and the liquid and vapor splitters in the DWCD process

Table 12- The characteristics of the products of the DWCD process

Table 13- The reboiler and the condenser heat duties in the DWCD process

3.2.1. DWCD process optimization

3.2.1.1. The Absorber1 stage number

The effect of the Absorber1 stage number on the output parameters is shown in Fig. (10), and the effect of the same on the mole fractions of the components in the DWC process products is shown in Fig. (10-a), where, as observed, the 1,4-BD concentration in the product is constant as the Absorber1 stage number increase, and the 2,3-BD and 1,2-BD concentrations increase. The reason for this can be explained that by increasing the number of column stages, separation is increased. The same effect on the heat duties of the reboiler and the condenser of the system is shown in Fig. (10-b), where, as observed, both the reboiler and the condenser heat duties increase by an increase in the stage number.

According to Error! Reference source not found., an increase in the stage number in the Absorber1 from 7 to 16, leads to a slight increase in the heat duties of the reboiler (0.095%) and the condenser (0.049%);. This slight change is related to an increase in the product concentration from 0.915 to 0.983 (7.43%) for 2,3-BD, and from 0.932 to 0.975 (4.61%) for 1,2-BD, with the same increase in the stage number. Note that the reflux ratio remains constant in all cases.

 This effect on the sum of heat duties of the DWCD reboiler and the condenser is shown in Fig. (10-c), As can be observed, as the number of stages of Absorber-1 column increases, the sum of the reboiler and the condenser duties of the DWCD process are increased. The other parameters remain constant values. Based on the results, 16 stages are selected for Absorber-1.

3.2.1.2. The Absorber2 stage number

The effect of this number on the output parameters is shown in Fig. (11) and this effect on the mole fraction of the components in the DWC system products is shown in Fig. (11-a), where as observed an increase in this number, the concentration of 2,3-BD and 1,2-BD increase and the concentration of 1,4-BD in the product remains constant. The reason for this can be explained that by increasing the number of column stages, separation is increased.

This on the heat duties of the reboiler and the condenser of the system is shown in Fig. (11-b), and as observed, both the reboiler and the condenser heat duties increase by an increase in the stage number. This increase in stage numberleads the column height which requires more heat duty due to the stability of the other parameters.

As observed in Error! Reference source not found. (11-b), an increase in the stage number in the Absorber2 from 7 to 16, increases the heat duties of the reboiler and the condenser slightly at 0.086% and 0.039%, respectively. This slight increase is due to slight changes in the concentration of the products: 1) as to 2,3-BD, from 0.935 to 0.983 (5.13%), and 2) as to 1,2-BD from 0.938 to 0.975 (3.94%) when the tray number increases from 7 to 16. Note that the reflux ratio remains constant in all cases.

 This effect on the sum of heat duties of the DWC system reboiler and the condenser is shown in Fig. (11-c), where, as observed, as the number of stages increases, the sum of the reboiler and the condenser heat duties increase. The other parameters are constant. Based on the results of this section, stages selected for Absorber2 is 16.

The separation process of benzene, toluene, and o-xylene in a DWC system is assessed by Szabo et al. (2008). They evaluated the effect of wall height, by involving the number of stage of feed and side product sections. In their study, the wall had a central vertical position, and the feed and the side product stages were the same; as the wall height increased, the reboiler's heat duty decreased first and then increased slightly. At the wall height of nine stages, energy consumption was at its lowest.

 3.2.1.3. The Rectifier stage number

The effect of the Rectifier column stage number on the output parameters is shown in Fig. (12). The same effect on the mole fraction of the components in DWC system products is shown in Fig. (12-a), where, as observed an increase in the stage number of increases concentration of 2,3-BD and 1,2-BD, thus, an increase in the separation process. The concentration of 1,4-BD in the product remains constant. This effect on the heat duties of the reboiler and the condenser of the system is shown in Fig. (12-b), where, an increase in stage number from 10 to 24, slightly increases the heat duty of the reboiler first; by 0.0080% and then decreases. For the condenser, the heat duty is decreased slightly; 0.03% with an increase in the stage number from 10 to 34. The reason for the slight changes in heat duties is due to the slight changes in the product concentration as to 2,3-BD increased from 0.959 to 0.983 (2.5%), and as to 1,2-BD increased from 0.961 to 0.975 (1.46%) with the same increase in the stage number. Note that the reflux ratio remains constant in all cases.  This effect on the sum of heat duties of the reboiler and the condenser of the DWC system is shown in Fig. (12-c), and As observed, an increase in the stage number decreases the sum of the reboiler and the condenser heat duties. The other parameters remain constant. The changes are almost linear and based on the results of this section, the selected stages are 21.

3.2.1.4. The effect of the Stripper stage number

The effect of the Stripper column stage number on the output parameters is shown in Figure 13 (a) shows the effect of the number of stages of the Stripper column on the mole fraction of the components in the DWC system products. As can be observed by increasing the number of stages from 10 to 12, the concentration of 1,2-BD and 1,4-BD components slightly increases and then remains constant, still the concentration of 2,3-BD is the same at all stages.

This effect on the reboiler and the condenser heat duties of the DWC system is shown in Fig. (13-b), where as observed, an increase in column stage number, makes the effect on the heat duty slight. Figure 13 (c) shows the effect of the number of stages of the Stripper column on the sum of heat duties of the reboiler and the condenser of the DWC system. where as observed an increase in the stage number, the total heat duties do not change. The other parameters are in constant values. Based on the results of this section, the stages selected for the Stripper are 13.

Szabo et al. (2008) assessed the effect of the relative position of the wall on the reboiler heat duty in the DWC separation process where, the wall height was nine stages and the total stage number was assumed constant, with equal numberof stages in the feed and the side product sections. They concluded that the energy consumption is lower when the stage number at the top and bottom of the wall are the same. One reason for this phenomenon is the fact that the components in the feed had a similar mass fraction.

3.2.1.5. The effect of reflux ratio

The effect of the column reflux ratio on the output parameters is shown in Fig. (14) and this effect on the mole fractions of the components in the DWC process products is shown in Fig. (14-a), where, as observed, an increase in the reflux ratio, increase the concentrations of all the components in the products. This increase is especially noticeable in the light and the middle components. The reflux ratio in the DWC is essential. As the reflux ratio increases, more liquid rich in volatile compounds returns to the column, and the operating line gradient for the enrichment section of the column moves toward a maximum value of one. thus, increasing the liquid flow rate in the column and enhanced separation.

Gor et al. (2017) assessed the effect of the reflux ratio on the components’ mole fraction in DWC system consisting of the butane-pentane-hexane separation. According to their results, a change in the reflux ratio, keeps the butane mole fraction as the lightest component almost constant. The pentane and hexane mole fractions, as intermediate and the heaviest components increase by an increase in reflux ratio.

This effect on the reboiler and the condenser heat duties of the system is shown in Fig. (14-b), where, as observed an increase in the reflux ratio, increases both the reboiler and the condenser heat duties, which is reasonable. These changes are linear. With an increase in reflux ratio (due to fixing top product and side stream flow rates), the flow rate increases in all stages of the column, thus, an increase in the heat duties of the reboiler and the condenser.

The effect on the sum of heat duties of the reboiler and the condenser of the DWC system is shown in Fig. (14-c), where, as observed, an increase in the reflux ratio of the column, increase the sum of heat duties of the reboiler and the condenser linearly. The other parameters are in constant values.

Kaur (2012) assessed the reboiler heat duty variations in terms of a DWC system’s reflux ratio on involved xylene-toluene-benzene and concluded that an increase in column reflux ratio increases the heat duty of the reboiler linearly. Gor et al. (2017) assessed the reboiler heat duty variations in terms of reflux ratio in a DWC system and found the reboiler heat duty of the column increases linearly upon an increase in reflux ratio.

Overall, it can be concluded that the appropriate reflux ratio leads to better separation in different parts of the column and reduces the heat duties of the reboiler and the condenser. According to the results of this section, the reflux ratio of the DWCD process is5.6.

3.2.1.6. The effect of vapor split ratio

The effect of the vapor split ratio on the output parameters is shown in Fig. (15) and the same effect on the mole fractions of the components of the products in the DWC system is shown in Fig. (15-a). This is the vapor input ratio into Absorber2 to the total inlet vapor to vapor splitter at the bottom of the two parallel columns of Absorber1 and Absorber2 where, as observed, the mole fractions of the products depend highly on the values ​​of the vapor split ratio and so the interconnected flows of the process. With an increase in the vapor split ratio, the molar concentrations of all three components first increase and then decrease. These changes are particularly noticeable for the light, 2,3-BD and the intermediate 1,2-BD component. These changes in the vapor split ratio have a maximum point of 0.3.

Arora (2014) assessed the effect of the vapor split ratio on the purity of products, and evaluated the separation process of ternary mixtures including benzene-toluene-paraxylene, benzene-toluene-orthoxylene and methanol-water-glycerol in the DWC system. They found that the highest influence of the vapor split ratio is on the concentration of the intermediate component in the side product, and the lowest effect is on the concentration of the heaviest component at the bottom product of the column.

The separation process in a DWC system is assessed by Gor et al. (2017), where the effect of the vapor split ratio on the mole fractions of the components is of concern and concluded that there exists a maximum point for all the components.

The effect of the vapor split ratio on the heat duty of the reboiler and the condenser of the DWC system is shown in Fig. (15-b), where as observed, the reboiler and the condenser heat duties are depend highly on the vapor split ratio. An increase in this ratio, makes the heat duty of both the reboiler and the condenser increase first and then decrease with a maximum point of 0.3.

This effect on the sum of heat duties of the reboiler and the condenser of the DWC system is shown in Fig. (15-c) where, as observed an increase in the vapor split ratio, makes the sum of the reboiler and the condenser heat duties increase first and then decrease with a maximum point of 0.3. The other parameters are in constant values.

The effect of the vapor split ratio on the heat duty of the reboiler and the condenser in a DWC system is assessed by Yuqi et al. (2015) experimentally and running simulations of the hexane-heptane-octane separation process; concluded that there exists an optimum value of the vapor split ratio at which the heat duty of the reboiler and the condenser is at its minimal. Kaur (2012) assessed the effect of the vapor split ratio on the reboiler heat duty in a DWC system and concluded that there exists an optimum value of the vapor split ratio at which the reboiler heat duty is at its minimal.

 3.2.1.7. The effect of liquid split ratio

The effect of the liquid split ratio on the output parameters is shown in Fig. (16), and the same effect on the mole fractions of the components in DWC products is shown in Fig. (16-a). The liquid and vapor split ratios are important parameters for obtaining the proper concentrations in DWCD process products. In this study, the liquid split ratio is the liquid input to Absorber2 to the total inlet liquid flow to the liquid splitter at the top of the two parallel columns of Absorber1 and Absorber2 As observed, the mole fractions of the components at first increased and then decreased as the liquid split ratio increased. Changes in this ratio have a maximum point of 0.6. Because the feed enters the Absorber1, it is reasonable that a lower percentage of liquid enters the Absorber1 column to maintain the liquid balance in both parts.

The separation process at DWC is assessed by Arora (2014) where the effect of the liquid split ratio on the mole fractions of the products is of concern. They concluded that the change in the liquid split ratio has no effect on the purity of the column bottom product, but is effective on the purity of the other two products.

This effect on the heat duty of the reboiler and the condenser of the system is shown in Fig. (16-b), where as observed an increase in this ratio makes the heat duty of both the reboiler and the condenser increase first and then decrease with a maximum point of 0.6. The same effect on the sum of heat duties of the reboiler and the condenser of the DWC system is shown in Fig. (16-c), where, as observed, an increase in the liquid split ratio, makes the sum of heat duties of the reboiler and the condenser increase first and then decrease. The internal flows in Absorber1 are increased by the feed flow, and the same in Absorber2 decreases by the side product flow which makes the optimum vapor split ratio less than 0.5, and the optimum liquid split ratio greater than 0.5. According to the results, the liquid split ratio of the DWCD process is selected0.6.

The significant influence of the liquid and vapor split ratios on the molar concentration of the products, and the heat duties of the reboiler and the condenser are illustrated in Figs. (15 and 16).

Gor et al. (2017) assessed the effect of the liquid split ratio on the mole fractions of the components of the products in the separation process of a DWC system and concluded that there is no effect on butane mole fraction as the lightest component, but pentane and hexane mole fractions change as the intermediate and the heaviest components have a maximum point and changed with the liquid split ratio. They assessed the effect of the liquid split ratio on the reboiler heat duty and found that an increase in the liquid split ratio increases the reboiler heat duty and then decreases.

Zhai et al. (2019) simulated the DWC system. They investigated the effects of the liquid and vapor split ratios and showed that the heat duties were dependent on the liquid and vapor split ratios. In their work, the feed consisted of benzene, toluene, and xylene. There was an optimal point for energy-saving and could be used as an important parameter.

3.2.1.8. The effect of feed stage number of Absorber1

The effect of feed stage number in the Absorber1 column on the output parameters is shown in Fig. (17) and the same on the mole fraction of the components in the DWC system products is shown in Fig. (17-a), where, as observed for the 1,4-BD component, an increase in the feed stage number from 2 to 7 increase the product concentration. Changing the feed stage number decreases the product concentrations 2,3-BD and a maximum is observed in 1,2-BD.

The separation process of ternary mixtures in DWC is assessed by Arora (2014), where the effect of feed stage number on the purity of products is of concern and concluded the feed stage number has no significant effect on product purities at the top of the column.

This effect on the reboiler and the condenser heat duties is shown in Fig. (17-b), where as observed, an increase in the feed stage number reduces the heat duties of the reboiler and the condenser. These changes are initially slow and gradually followed by a steeper slope.

Yuqi et al. (2015) assessed the separation process in DWC by evaluating the effect of feed stage number on reboiler heat duty, and concluded, thus, a decrease in feed stage number, would increase the reboiler heat duty.

The effect of feed stage number in Absorber1 on the sum of heat duties of the reboiler and the condenser of DWC is shown in Fig. (17-c), where, as observed, an increase in the feed stage number decreases the sum of heat duties of the reboiler and the condenser. These changes begin slowly and then become fast. According to the results of this section, the feed stage number of Absorber1 in the DWCD process is selected 8.

3.2.1.9. The effect of Absorber2 side product stage number

The effect of Absorber2 side product stage number on the output parameters is shown in Fig. (18), where this effect on mole fractions of the components of products is shown in Fig. (18-a). By an increase in the stage number: 1) the mole fraction of the 2,3-BD component first increases and then remains constant, 2) the mole fraction of the 1,4-BD component decreases, and 3) for the mole fraction of 1,2-BD a maximum is observed. This effect on the heat duties of the reboiler and the condenser of DWC is shown in Fig. (18-b), where as observed an increase in the side product stage number, first increases the reboiler and the condenser heat duties, and then remains constant. This effect on the sum of heat duties of the reboiler and the condenser of DWC is shown in Fig. (18-c), The other parameters are in constant values. As can be observed by increasing the number of side product stage, the sum of heat duties of the reboiler and the condenser is first increased and then remains constant. According to the results of this section, the side product stage number of Absorber2 in the DWCD process is selected 10.

3.2.1.10. The optimal conditions of DWC

The profiles of DWC at optimum conditions are shown in Fig. (19) and the temperature changes of 4 parts of the DWC at optimum conditions are shown in Fig. (19-a), where, the temperature increases with an increase in stage number.

The temperature difference of the different stages of the two parallel sections of the Absorber1 and Absorber2 columns is slight and the diagram is S-shaped where the upper part represents the Stripper column, of the separation of 1,2-BD and 1,4-BD, and the lower part represents the Rectifier column, of the separation of 2,3-BD and 1,2-BD at a lower temperature. The top temperature of the column is 180.73 ⁰C, which is close to the 2,3-BD boiling point (180.7 ⁰C). The bottom temperature of the column is 229 ⁰C, which is similar to the 1,4-BD boiling point (228 ⁰C). The temperature in step 31 (stage 10 of Absorber2) corresponds to the 1,2-BD side product stage at 198.08 ⁰C, which corresponds to the 1,2-BD boiling point (196.42 ⁰C).

A closer look at the figure reveals that the temperature profiles of both the columns of Absorber1 and of Absorber1 column (step 8 of the Absorber1 column and step 29 of the entire DWC), and the other is the location of the side product stage number from Absorber2 column (step 10 of the Absorber2 column and step 31 of the entire DWC).

The maximum temperature difference between the Absorber1 and Absorber2 columns corresponds to stage 5 of the two columns, (stage 26 of the entire DWC column). At this stage, the temperature of Absorber-1 is 191.95 ⁰C, and the same of Absorber 2 is 196.63 ⁰C, and a temperature difference of 4.68 ^°C, which makes the effect of heat transfer across the dividing wall negligible.

Dung Nguyen (2015) assessed the effect of the components’ concentration in the feed on the temperature difference on both sides of the wall. They found that when the concentration of the intermediate component is equal to or greater than the other components of the feed, the temperature difference on the two sides of the wall is low. When the intermediate component concentration in the mixture is lower than the other components the temperature difference is high. They concluded that feed composition has a significant effect on DWC operation. The temperature profile in a four-component mixture of aromatics with two DWC columns in different arrangements is assessed by Landaeta et al. (2012). The trend of temperature change with stage number in this work follows the S-shape diagram.

The mole fraction changes of 2,3-BD in the liquid phase at various stages of the four parts of the DWC column under the optimum conditions is shown in Fig. (19-b), where as observed, the 2,3-BD concentration decreases as the lightest component in the four parts with an increase in stage number. The comparison between the mole concentration of 2,3-BD in the liquid phase of Absorber1 and Absorber2 reveals that the concentration of 2,3-BD in Absorber1 is higher than that of Absorber2.  2,3-BD leaves at the top of the column at a molar concentration of 0.982%, and at the bottom of the column, the same is approximately zero. The changes in the concentration of 1,2-BD in the liquid phase at different stages of the four parts of the DWC column subject to the optimum conditions are shown in Fig. (19-c) where, as observed, by an increase in the stage number, the concentration of 1,2-BD in the liquid phase first increases and then decreases. The comparison between the mole concentration of 1,2-BD in the liquid phase in two parallel sections Absorber1 and Absorber2, reveals that the concentration of 1,2-BD is greater in the Absorber2. The concentration of 1,2-BD is 0.018 at the top and 0.017 at the bottom of the column, reaching a maximum of 0.975% in stage 31. The upper part of the column is related to the separation of 2,3-BD and 1,2-BD, and the lower part is related to the separation of 1,2-BD and 1,4-BD. In the middle part, the concentration of 1,2-BD reaches a maximum value that is obtained at the side product. This result corresponds consistent with the findings of other researchers (Landaeta et al., 2012).

The changes in the mole fraction of 1,4-BD in the liquid phase at different stages of the four parts of the DWC column subject to the optimum conditions are shown in Fig. (19-d), where, as observed an increase in the stage number, makes the 1,4-BD concentration in the liquid phase in the Rectifier column approximately zero, which increases towards the bottom of the Stripper column and reaches its maximum value, exits as the bottom product. The comparison between the mole fraction of 1,4-BD in the liquid phase in Absorber1 and Absorber2 reveals that the concentration of 1,4-BD in Absorber1 is more significant. The diagram is S-shaped here. The concentration of 1,4-BD at the top of the column is approximately zero and at the bottom of the column is 0.983, from which the 1,4-BD product exitsThe trend of composition profiles throughout the column is consistent with the results of (Landaeta, et al. 2012). The changes of the mole fractions of 2,3-BD, 1,2-BD, and 1,4-BD components in the vapor phase at different stages of the four parts of DWCD subject to the optimum conditions, are shown in Figs. (19-e to g), where, as observed, the changes in the mole fraction of the components in the vapor phase are similar to those in the liquid phase. Observations on the concentration diagrams in the vapor and liquid phases indicate that there exists a breakpoint in Absorber1, which corresponds to the feed stage of this column (stage 8 of Absorber1 and stage 29 of the entire DWCD).

3.2.1.11. Results of optimization of the DWCD process

The characteristics of the output parameters in the DWCD process simulation after optimization are tabulated in Table 14 and the heat duties of the reboiler and the condenser in the DWCD process simulation after optimization are tabulated in Table 15.

3.2.2. The comparison of simulation results of DWCD and the conventional processes

The heat duties of the condensers and the reboilers of DWCD and the conventional processes are compared in Table 16.

The parameters studied here are divided into structural and process categories.

• Structural parameters:
• Absorber1 column stages number
• Absorber2 column stages number
• Rectifier column stages number
• Stripper column stages number
• Absorber1 feed stage number
• Absorber2 side product stage number
• Process parameters:
• The Rectifier column reflux ratio
• Vapor split ratio
• Liquid split ratio

Table 14- The product characteristics in DWCD process (after optimization)

Table 15- The heat duties of the reboiler and the condenser in DWCD process simulation (after optimization)

Table 16- The comparison of the heat duties in DWCD and the conventional processes.

As to economic efficiency, the available studies reveal that the DWC process is one of the best-proven methods because it has lower operating and investment costs in comparison with conventional processes. The DWC system converts two (or more) distillation columns by placing a dividing wall into one. Applying the DWC, the investment cost is reduced by 20-30% and the operating cost is reduced by 25% (Illner and Othman, 2015), (Nguyen, 2015). The simulation of the DWC for a crude oil distillation unit is assessed by Kim (2017) and the findings indicate that applying DWC leads to reduced mixing in the feed tray and increased thermodynamic efficiency of the CDU. The evaluation of the unit performance reveals that the DWC unit has 37% energy savings in the heat duty of reboiler and 17% in the condenser heat duty compared to conventional distillation. The economic analysis indicate a 9% investment saving.

Conclusions

The focus here is on the performance of a DWC process for separating a ternary mixture, that includes 1,4-BD, 2,3-BD, and 1,2-BD. This work is divided into the simulation of two conventional distillation columns and a DWCD process parts. Because there exists no standard model in commercial softwares, a four-column model consisting of Absorber1, Absorber2, Rectifier, and Stripper, next to two vapor and liquid split ratios are applied in simulating the DWCD process. The optimization of different parameters is performed through the sensitivity analysis, with the objective to minimize the condenser and the reboiler heat duties and obtaining appropriate product purities for all three components. In the distillation process, the main operating cost is the heat duties of reboiler and condenser. To minimize operating costs, the heat duties should be reduced. The studied parameters are of two categories of structural and process parameters. Structural parameters consist of the stages number of the four columns, feed stage number of Absorber1, side product stage number of Absorber2. The process parameters include reflux ratio, vapor and liquid split ratios. The parameters are optimized for achieving 98% mole purity of the components. In the optimized conditions, a saving of 18.32 % for reboiler and 25.18% for condenser heat duties, are obtained for the DWCD process in comparison with the conventional method.

Acknowledgements

The author appreciates Iranian Research Organization for Science and Technology (IROST) for financing the research (Grant Number: 34619).