Introduction
1)
Exo-tetrahydrodicyclopentadiene (THDCP), a high- density liquid fuel can be easily synthesized by the hy- drogenation reaction of dicylcopentadiene isomeric mix- ture. One major disadvantage of the hydrogenation and isomerization process is the co-production of endo- THDCP, which should be separated from the product mixture. Scheme 1 shows isomerization mechanism of endo-THDCP into exo-THDCP and fundamental phys- ical properties of THDCP isomers are listed in Table 1.
Despite the industrial importance of THDCP isomers, very little is reported so far on their physical and equili- brium properties.
Primary objective of this study is therefore to develop a process for separating isomeric mixtures of exo- and en- do-THDCP by obtaining property data for each isomer
†To whom all correspondence should be addressed.
(e-mail: [email protected], [email protected])
and their binary isobaric vapor-liquid equilibrium data at different pressures. Secondary objective, a main purpose of this work, is to model and optimize the distillation col- umn to obtain each isomer with 99 wt% purity both at the top and the bottom, respectively. Our earlier pub- lished vapor pressures for both isomers and isobaric VLE, directly measured using a glass still at different pressures, were used for the simulation of distillation col- umn in this work [1]. Antoine equation and NRTL liquid activity coefficient model [2] were used for regression of the experimental VLE data.
Since exo- and endo-THDCP components are not built- in the pure component database of the general-purpose chemical process simulator, PRO/II with PROVISION [3-8], we used UNIFAC group contribution method [9]
to estimate the pure component physical properties such as normal boiling temperature, critical properties, stand- ard liquid density, standard heat of formation, and so on.
We used ‘SHORTCUT’ module built-in PRO/II with PROVISION simulator to predict the minimum number and Jeong-Sik Han
Department of Chemical Engineering, Dong-Yang University, Kyoungbuk, 750-711, Korea
*Department of Chemical Engineering, Chungnam National University, Daejeon 305-764, Korea
**Advanced Chemical Technology Division, Korea Research Institute of Chemical Technology, Daejeon, 305-600 Korea
***ADD (Agency for Defence Development), Daejeon, 305-600 Korea Received October 12, 2006; Accepted June 12, 2007
Abstract: In this study, simulation of distillation process for the mixture of exo-tetrahydrodicylcopentadiene (exo-THDCP) and endo-tetrahydrodicyclopentadiene (endo-THDCP) was carried out. Minimum reflux ratio and minimum number of stages were calculated to obtain products having 99.0 wt% of each components, respectively. Theoretical number of stages was estimated from the correlation of number of stages and reflux ratio. Optimal feed tray location that minimized the reboiler heat duty was also determined. Besides, the va- por-liquid equilibria for this mixture at 101.3 kPa were reported.
Keywords: exo-tetrahydrodicylcopentadiene, endo-tetrahydrodicylcopentadiene, separation, simulation, opti- mization
Scheme 1. Wagner-Meerwein type ionic chain mechanism.
of stages and minimum reflux ratio for separation. We al- so used ‘COLUMN’ and ‘OPTIMIZER’ module in PRO/II with PROVISION to determine the theoretical number of stages and optimal feed tray location that min- imizes the reboiler heat duty.
Experimental Works
Pure component vapor pressure of exo- and endo- THDCP and VLE data of this mixture, used in this simu- lation, were directly determined using Dr. Sieg and Roeck type recirculating still. The detailed description of the measuring procedure has been described elsewhere [1]. The experimental pressure range was ca. 10 to 101.3 kPa. Experimental works for bubble temperature and compositions were performed at 101.3 kPa. Pure compo- nent experimental vapor pressure data of exo- and endo- THDCP vs. temperature were correlated using four pa- rameters Antoine equation shown in the equation (1).
log
(1)
Table 2 shows regressed coefficients of each component. Average absolute deviations in prediction of vapor pressures for exo- and endo-THDCP are 0.9991 and 0.7719 %, respectively.
Theory
The non-random two-liquid (NRTL) activity coefficient model, equation (2), was used to correlate the binary VLE data of exo- and endo-THDCP system and for the distillation column simulations containing component exhibiting non-ideal vapor-liquid equilibrium phase be- havior [10].
In equation (2), and Gij are optimum binary inter- action parameters that minimize the deviations between the experimental data and the calculation values, re- spectively, and can be written as equations (3) and (4):
(3)
(4)
In equation (3), T refers to an absolute temperature. The NRTL model has the following three interaction parame- ters, uij, uji, andαij. The binary interaction parameters for the isobaric binary vapor-liquid equilibrium data were optimized by regressing the experimental bubble point temperatures and compositions, and were determined us- ing equation (2). The pattern search optimization algo- rithm suggested by Nelder and Mead [11] was used to minimize the objective functions in equation (5). Based on this optimization algorithm, the iteration was carried out to minimize the objective function F(Obj), equation (5). For three variables of NRTL model, a simplex is a regular tetrahedron, and the method is the pattern search that compares objective function values at the four verti- ces of a regular tetrahedron. The worst vertex, where the objective function is largest, is rejected and replaced with a new vertex. A new regular tetrahedron is formed and the search is continued. The process generates a sequence of regular tetrahedron, which might have different shapes, for which the objective function values at each vertices get smaller and smaller. The size of the regular tetrahedron is reduced and the coordinates of the mini- mum point are found.
╱ (5)In the above equation (5), Tjexp and yjexp are ex- perimental bubble point temperature and composition for
(a) (b)
Figure 1. Isobaric vapor-liquid equilibrium data for the system of exo- and endo-THDCP at 101.3 kPa with predicted values by NRTL liquid activity coefficient model.
Table 3. Isobaric Vapor-Liquid Equilibria Data for exo- THDCP (1) + endo-THDCP (2) at 101.3 kPa
model u12 u21 α12 Δy
NRTL -263.5712 249.8241 0.3 0.0031
T/K x1 y1 T/K x1 y1
463.27 463.17 463.04 462.87 462.64 462.30 462.07 461.86 461.74 461.49 461.17 461.04 460.89
0.2576 0.2858 0.3284 0.3583 0.3978 0.4484 0.4899 0.5306 0.5606 0.6061 0.6474 0.6756 0.7070
0.2824 0.3163 0.3575 0.3864 0.4298 0.4800 0.5236 0.5628 0.5924 0.6374 0.6788 0.7036 0.7316
460.67 460.37 460.20 460.03 459.82 459.60 459.55 459.48 459.43 459.41 459.40 459.39 459.38
0.7433 0.7733 0.8106 0.8417 0.8728 0.9046 0.9224 0.9387 0.9579 0.9644 0.9728 0.9806 0.9872
0.7679 0.7945 0.8289 0.8576 0.8874 0.9142 0.9328 0.9469 0.9609 0.9666 0.9734 0.9820 0.9906
j’th data, respectively. Superscript ‘cal’ means calcu- lated (or regressed) values for vapor and liquid phase compositions. Table 3 shows the NRTL binary inter- action parameters regressed from binary vapor-liquid equilibria at 101.3 kPa with standard deviation of corre- lation results using NRTL equation. Figure 1 shows x-y and p-x-y diagram for exo- and endo-THDCP mixture at 101.3 kPa.
Process Simulation Results
The simulation and optimization for the separation of exo-THDCP from endo-THDCP can be classified into four major parts: (1) determination of the column operat- ing pressure; (2) determination of the minimum number of stages and minimum reflux ratio for separation; (3)
simulation of the rigorous distillation column; and, (4) optimization of the distillation column.
Determination of the Column Operating Pressure Determination of the distillation column operating pres- sure depends on the type of condenser and the type of re- frigerant available. We assumed that cooling water was used for the cold utility and 3.0 Kg/cm2G of low-pressure saturated steam was used for hot utility. Cooling water was assumed to be supplied at 305.15 K and to be re- turned at 313.15 K. We used sub-cooled type of con- denser and assumed an overhead reflux drum temper- ature operating at 318.15 K. Since the 3.0 Kg/cm2G of low-pressure saturated steam temperature is about 416.15 K, column bottom temperature should be operated around 393.15 K considering the temperature difference between column bottom and the steam. So, we calculated the bubble point pressure of the bottom liquid product having a composition of 1.0 wt% of exo-THDCP and 99.0 wt% of endo-THDCP. Bubble pressure calculation result showed that the bubble point pressure is about 9.239 kPa.
Determination of the Minimum Number of Stages and Minimum Reflux Ratio for Separation
Next step is to determine the minimum number of stages based on the column operating pressure, 9.239 kPa. Table 4 shows feedstock stream information. Ac- cording to Table 4, feedstock flow rate is 1,000 kg/h and 50 wt% of exo-THDCP and 50 wt% of endo-THDCP.
Feed stream is fed to the column at saturated liquid state.
According to PRO/II SHORTCUT module, minimum number of stages is 41.32 and the corresponding mini- mum reflux ratio is 7.842. Case studies for the relation of theoretical number of stages and reflux ratio were per-
Figure 2. Plot of theoretical number of stages versus reflux ratio.
formed to determine the optimum reflux ratio and the corresponding theoretical number of stages. Figure 2 shows a plot of theoretical number of stages versus re- flux ratio. In general, for the higher reflux region, utility consumptions are very large even though initial equip- ment costs are relatively low, while for the low reflux re- gion, initial investment costs are very large even if oper- ating costs are low. Therefore, optimum reflux ratio that minimizes the total investment costs will be located at the very point that the curvature is most sharply changing. Figure 2 shows us that the optimum reflux ra- tio is slightly higher than 14 and the corresponding theo- retical number of stage is 60.
Simulation of the Rigorous Distillation Column For the rigorous simulation for the separation of exo- and endo-THDCP using conventional distillation col- umn, we used the distillation configuration illustrated in Figure 3. In Figure 3, tray number was counted from top to bottom. So, overhead condenser corresponds to tray number 1, column top tray is tray number 2, column bot- tom tray is 59, and the bottom reboiler is counted to tray number of 60. Feed tray location is assumed to be fed in- to the middle stage number of 31, which will be opti- mized to minimize the reboiler heat duty. Overhead re- flux drum operating pressure was set to 9.237 kPa and condenser or column pressure drop was neglected. Over- head reflux drum operating temperature was set to 318.15 K in order to be sub-cooled using cooling water.
Figure 3. A schematic diagram of conventional distillation col- umn for the separation of exo- and endo-THDCP.
Table 4 reveals the heat and material balance around the distillation column. According to the material balance furnished in Table 5, exo- and endo-THDCP purities are all 99.0 wt% at column top and bottom product, res- pectively. Table 6 shows input design data, product spec- ifications and output summaries for the simulated dis- tillation column. According to Table 6, the heat duties of condenser and reboiler are -2.6155 × 106 and 2.5754 × 106Kcal/h, respectively. Column reflux ratio to obtain a desired purity both at column top and bottom product is 12.027, which is slightly smaller than shortcut case of study results.
Optimization of the Distillation Column
In most distillation columns, the major operating cost will be steam consumption in reboiler. Of course, if the cryogenic refrigerant used in the overhead condenser, this cold utility cost would also be quite large. For our exo- and endo-THDCP separation problem, the column top pressure was set so that cooling water could be used in the overhead condenser. Therefore, reboiler heat duty is objective function that should be minimized.
The simulation has been run using different feed stages.
The purities of both top and bottom product were kept constant. The feed stage that minimizes the reboiler heat duty could be the optimum stage. Figure 4 gives the re- sults of these calculations. Feeding on stage 30 gives the minimum reboiler heat duty, 2.5677 × 106Kcal/h.
Conclusion
This study was modeled and demonstrated to optimize the process for the separation of exo- and endo-THDCP through conventional distillation column using experi-
Table 5. Heat and Material Balance Around Distillation Column
1 2 3
Stream name Feed Stream Top Product Bottom Product
Component Kg/h Weight% Kg/h Weight% Kg/h Weight%
exo-THDCP endo-THDCP
500.00 500.00
50.00 50.00
494.00 5.00
99.00 1.00
5.00 495.00
1.00 99.00
Total Rate (Kg/h) 1,000 500.00 500.00
Temperature (K) Pressure (kPa) Enthalpy (106Kcal/h)
Molecular Weight Heat Capacity (Kcal/kgoC)
381.91 10.000 0.1434 136.16 1.281
318.15 9.237 0.0302 136.16 1.317
382.82 9.237 0.0725 136.16 1.284
Table 6. Input Data, Product Specificationsand Simulation Results for Columns
Input, specifications & output Values Tray number
Condenser type Condenser temperature (K)
Cooling medium Column top temperature (K) Condenser duty (106Kcal/h)
Reflux ratio Reboiler type Heating medium Column bottom temperature (K)
Reboiler temperature (K) Reboiler duty (106Kcal/h)
Feed stage
60 Sub-cooled
318.15
Cooling water (305.15 K/313.15 K) 376.65
-2.6155 12.027 Kettle
3.0 Kg/cm2G saturated steam 382.85
382.85 2.5754 31
Figure 4. Plot of reboiler heat duty versus feed tray location.
mental binary isobaric vapor-liquid equilibrium data.
Phase-equilibrium calculation data was decided by re- gression that used NRTL liquid activity coefficient for better prediction of experimental isobaric vapor-liquid equilibrium data. The absolute average deviation be- tween the simulated and experimental data was only 0.0031 %.
By using pure component vapor pressure data and bina- ry vapor-liquid equilibrium data, we could perform short- cut modeling to obtain minimum number of stages and reflux ratio, rigorous simulation for the reflux ratio, con- denser and reboiler heat duty calculation. The optimal feed stage location that minimized the reboiler heat duty could be determined using this approach.
Nomenclature
A, B, C, D : coefficients in vapor pressure correlation R : gas constant
T : absolute temperature (K) x : liquid phase mole fraction y : vapor phase mole fraction
u, G : binary interaction parameter in NRTL model Greek Letters
α : interaction parameter in equation (4) γ : activity coefficient in equation (2) τ : interaction parameter in equation (2) Superscripts
exp : experimental value in equations (5) cal : calculated value in equations (5) Subscripts
i, j, k : component i, j, and k
References
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