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PROCESS DESIGN AND CONTROL Dynamic Simulation of Startup in Ethyl tert-Butyl Ether Reactive Distillation with Input Multiplicity Budi H. Bisowarno and Moses O. Tade´ * School of Chemical Engineering, Curtin University of Technology, G.P.O. Box U 1987, Perth WA 6845, Australia
Dynamic simulation is used to understand the effect of a startup policy on the ethyl tert-butyl ether (ETBE) reactive distillation. Because input multiplicity is likely to occur in every reactive distillation, its effects on startup are investigated. The ETBE reactive distillation is presented as an example to show characteristic behaviors during startup of the ETBE column. It is found that the dynamics of the startup condition are influenced by the given startup policy. An understanding of the input multiplicity is a necessary requirement before performing the startup operation to avoid excessive energy, to achieve a stable optimum process operation. 1. Introduction
Table 1. ETBE Column Simulation Input
Reactive distillation is a novel alternative to a typical reactor and distillation combination.1 It offers advantages in chemical reaction by shifting the chemical equilibrium and therefore increasing the total conversion and in separation by overcoming distillation boundaries. Economic advantages also result from direct energy integration and reduction of equipment costs. This technology is suitable for the production of ethyl tert-butyl ether (ETBE), an important oxygenate with better blending properties than methyl tert-butyl ether (MTBE).2 ETBE is also a semirenewable oxygenate because it is synthesized in a liquid-phase reaction between isobutylene and ethanol that can be produced from biomass such as agricultural waste. Reactive distillation exhibits multiplicity phenomena, which can be distinguished as input and output multiplicities and pseudomultiplicities.3 For output multiplicity, the different startup policies may bring the column to an inappropriate operating condition. Both theoretical analysis and experimental verification have been done on tert-amyl methyl ether (TAME) synthesis.4,5 The output multiplicity region was shifted according to the operating conditions so that its existence could be verified with the available laboratory column. Similar phenomena were demonstrated using dynamic simulation for three different cases.6 However, there is no reference in the open literature that discusses the importance of input multiplicity during startup. The input multiplicity has been known to impose more practical control problems than that of output multiplicity in reactive distillation.7 The startup policy for reactive distillation was developed based on that of a typical distillation column.8 The column was filled with a certain amount of feed, and then the column was heated at total reflux. The reboiler * To whom correspondence should be addressed. E-mail:
[email protected]. Tel.: +61 8 9266 7704. Fax: +61 8 9266 3554.
feed conditions temperature rate composition
30 °C 0.76 L/min 29.1% ETBE 9.1% EtOH 7.3% iBut 54.5% nBut overall excess EtOH 5.0 mol %
column specifications rectification stages reaction stages stripping stages total stages feed stage overhead pressure reflux ratio (L/D)
2 3 5 10 6 950 kPa 5
duty was kept constant at its operating steady-state value. Therefore, the column was brought into its stable batch operation at its operating pressure before introduction of the additional feed and setting of the reflux ratio/rate. Previous operation of the pilot unit showed that the premixed feed, which has the same concentration as the feed, was the best mixture to stabilize the column before introduction of the additional feed.9 The reason was that the existing column temperature profile was close to that of the steady-state condition so that the startup condition would last for the shortest possible time. In this paper, the importance of input multiplicity during startup is evaluated by using dynamic simulation. The transient responses of the reactive distillation column are used to assess and understand the effects of different variables during the startup operation. A proposed “best” startup policy is presented. 2. Process Configuration and Mathematical Model The details of the ETBE reactive distillation pilotplant column are shown in Table 1. The column is located in the School of Chemical Engineering, Curtin University of Technology, Perth, Western Australia. The column was modeled using the equation-based process simulation package, SpeedUp.10 A rigorous expression for ETBE reaction equilibrium was incorporated, and the kinetic expression that is valid over a wide range of ethanol concentrations was used.2 The side reactions
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Figure 1. Input multiplicity region at 950 kPa and L/D ) 5.
were not modeled, because the system dynamics is not significantly affected.11 The UNIFAC vapor-liquid equilibrium model was used to predict component activities due to the strong nonideality of the reaction mixtures, and the phase equilibrium calculations used recently published vapor-liquid equilibrium data.12 Further details of the mathematical model can be found in our previous publications.1,11 Previous studies have shown that this column shows input multiplicities and pseudomultiplicities, but there is no output multiplicity.13 3. Input Multiplicity and a Startup Policy 3.1. Input Multiplicity Region. The ETBE reactive distillation column under study exhibits input multiplicity as shown in Figure 1. The optimum reboiler duty with respect to ETBE purity was 8.45 kW at a constant operating pressure of 950 kPa and a constant reflux ratio of 5. The input multiplicity region below the optimum condition is controlled by the separation process while the reaction process controls that above the optimum condition.3 Increasing the reboiler duty increases separation so that the ETBE purity increases in the separation-controlled region. However, increasing the reboiler duty could reduce the ETBE purity because it decreases the chemical equilibrium constant as shown in the reaction-controlled region. Using the reboiler duty as a manipulated variable could result in conflicting effects on separation and reaction processes. It is likely that other manipulated variables could produce similar phenomena. The input multiplicity results from the conflicting effect of operating through the same manipulated vari-
able (e.g., reboiler duty or others). Therefore, its region depends on the operating conditions. For example, Figure 2 shows the effects of the operating pressure on input multiplicity. At a constant operating pressure, the reboiler duty was varied while either the reflux ratio or the volumetric reflux rate was kept constant. The results indicate that the input multiplicity region is shifted to a lower reboiler duty for achieving the optimum ETBE purity at the higher operating pressure. This is reasonable because the reactive section temperature is also higher at a higher operating pressure so that the reaction equilibrium is then lower at a constant reboiler duty. As a result, the isobutylene conversion as well as the ETBE purity decreases. Therefore, a lower reboiler duty is needed to maintain the optimum ETBE purity [Figure 2(left)]. This phenomenon is more important if the volumetric reflux rate is varied instead of the reflux ratio as shown in Figure 2(right). This result is important because the LV configuration, which uses the reboiler duty to vary the vapor rate (V) and volumetric reflux rate (L) as manipulated variables, provides a better performance than the ratio configurations, including the (L/D)V configuration.11 Figure 2 also shows that the size of the input multiplicity region is quite similar despite the different operating pressures. Therefore, it could be speculated that the input multiplicity always occurs in this reactive distillation column for any operating condition. Several reactive distillations that are obtained from any number of either separation or reaction stages exhibit input multiplicity as well. Figures 3-5 show the effect of a different number of rectification, reactive, and stripping stages on input multiplicity, respectively. These figures show the input multiplicity regions resulting from one and seven more rectification stages, two and five more reactive stages, and one and four more stripping stages than those in the column described in Table 1. Despite the different reboiler duties that result in the input multiplicity region, the size of the input multiplicity is quite similar. Therefore, input multiplicity may occur in every reactive distillation column. 3.2. Effects of Operating Pressure. Similar to a typical distillation column, the ETBE reactive distillation column was filled with a mixed feed having the same concentration as the feed. The column was then stabilized through batch operation at total reflux, until it reached a stable batch operation. At this point in the startup, the chosen reboiler duty is critical in stabilizing the batch operation because the optimum reboiler duty depends on the targeted operating pressure. Figure 6
Figure 2. Input multiplicity region at L/D ) 5 (left) and L ) 2.5 L/min (right).
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Figure 3. Input multiplicity for different rectification stages.
Figure 4. Input multiplicity for different reactive stages.
Figure 5. Input multiplicity for different stripping stages.
shows the responses of ETBE purity resulting from different operating pressures when the reboiler duty was kept constant at 8.45 kW during startup. Also, the reflux ratio was held constant at 5 while the condenser duty was allowed to vary. Figure 6 shows that the existing ETBE purity is higher at a lower operating pressure. This results from lower column temperatures, including those in the reactive section that increase the chemical equilibrium. The ETBE purity responses that result from introduction of the additional feed present interesting results. Although the stable batch operation results in higher ETBE purity, the operating pressures below the opti-
mum result in a lower stable ETBE purity. Operating pressures above the optimum also produce a slightly lower stable ETBE purity despite the differences in the original ETBE purity. These results indicate that a lack of understanding of the input multiplicity could result in an inefficient startup policy. According to Figure 2, the optimum ETBE purity could result from a different combination of the reboiler duty and the operating pressure. Therefore, similar responses for the ETBE purity, as shown in Figure 6, could be derived for different constant reboiler duties. This analysis indicates the possibility for implementing a startup policy for the experimental work based on the particular reactive distillation column. For example, a lower reboiler duty could produce an optimum condition with lower condenser duty as long as the column could be run at a higher operating pressure. At constant reboiler duty, a higher operating pressure results in a lower condenser duty. Therefore, the operating pressure that is too low uses an unnecessary condenser duty. On the other hand, a high operating pressure may result in a reboiler duty that is too large. This analysis suggests that an understanding of the input multiplicity could reduce the utilities required during startup. A lower reboiler duty (Figure 2) could result in an optimum condition for ETBE purity at a higher operating pressure, which implies a lower condenser duty. Although the ETBE purity responses are quite similar during startup, the high operating pressure could reduce the isobutylene conversion because of a lower chemical equilibrium. Therefore, moderate operating pressures, in the range of 700-1000 kPa, are suggested, because these will maintain a balance between the utilities required and high chemical equilibrium, necessary for stable operation of this pilot-scale reactive distillation column. 3.3. Effects of Reboiler Duty. Because the reboiler duty is usually the main manipulated variable, it is important to understand the effects of changes on the responses of the reactive distillation column, especially during the startup. The effect of the reboiler duty on the transient condition during startup was studied at a constant operating pressure (950 kPa) for both separation and reaction-controlled regions. The responses of the ETBE purity during startup after the introduction of the feed are shown in Figure 7. Figure 7 shows that the ETBE purity increases significantly and after that decreases before increasing to a steady-state value. These phenomena could be explained by noting the changes in the internal liquid and vapor rates after introduction of the additional feed. The fast initial increases in the ETBE purity result from the introduction of additional reactants into the reactive section. This change lasts for a short period of time because the feed leaves the column almost entirely in the bottom product because of the lack of an additional heat supply. This results in less ethanol reacting with isobutylene so that the conversion and therefore the ETBE purity decrease. Without changes in the reboiler duty, the operating pressure could also be kept constant by changing the condenser duty. The condenser duty decreases gradually with the increasing feed rate, and the column temperature profile necessarily increases. As a result, the ETBE purity increases continuously until it reaches a steady-state value. Figure 7 also shows that a larger reboiler duty results
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Figure 6. Responses of ETBE purity during startup at Qr ) 8.45 kW.
Figure 7. Responses of ETBE purity during startup at 950 kPa for separation-controlled (left) and reaction-controlled (right) regions.
Figure 8. Effect of multiplicity on responses of isobutylene conversion (left) and ETBE purity (right) during startup.
in a higher stable ETBE purity in the separationcontrolled region. On the other hand, the converse applies in the reaction-controlled region. Besides, the reactive distillation column is stabilized at relatively similar times for different reboiler duties. Although the stable batch operation produces higher ETBE purity, the reboiler duty that is below the optimum value results in lower ETBE purity in the stable continuous operation. The reboiler duty that is above the optimum also produces slightly lower stable ETBE purity. This result indicates the importance of recognizing input
multiplicity. A similar behavior was also observed in the isobutylene conversion responses. Similar plots could also be constructed for other constant operating pressures. Based on the response shown in Figure 2, it would be expected that a lower operating pressure requires a lower reboiler duty as shown by the optimum operating condition. The lower operating pressure also implies a lower condenser duty. However, smaller internal rates inside the column that result from lower reboiler and condenser duty could increase the potential risk of flooding in the column and
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reduce the availability of reactants in the reactive section. Therefore, the isobutylene conversion as well as the ETBE purity could decrease. As a result, any attempt to minimize the utilities required should be compromised with the reaction considerations. The input multiplicity region shown in Figure 2 shows that two reboiler duties result in the same ETBE purity at certain operating conditions. For example, both reboiler duties of 8.00 and 9.11 kW produce targeted ETBE of 96 or 84.2 mol % at an operating pressure of 950 kPa and a reflux ratio of 5. The transient responses of isobutylene conversion and ETBE purity during startup for both situations stabilized at 8.00 and 9.11 kW after introduction of the additional feed (Figure 8). Although the ETBE purity responses show that the higher reboiler duty has a slightly better performance, it produces a much lower isobutylene conversion. This result indicates that the reactive distillation column operation should be stabilized based on separation consideration rather than reaction-controlled consideration, if there are difficulties in obtaining the optimum condition. 4. Conclusions Dynamic simulation is a useful tool for assessing and understanding the transient responses of a reactive distillation column during startup. The input multiplicity could increase or decrease the overall process performance during startup depending on the chosen operating condition. However, an understanding of the input multiplicity could prevent stabilization of the column at an inappropriate operating condition. The startup study results recommend stabilization the column at the optimum operating condition before introduction of the additional feed. The results also show that if there is a problem stabilizing the column at the optimum operating condition, it could be stabilized in the separation-controlled region rather than the reaction-controlled region. This startup policy would result in the targeted ETBE purity within the shortest possible time without using excessive energy. Literature Cited (1) Sneesby, M. G.; Tade´, M. O.; Datta, R.; Smith, T. N. ETBE Synthesis via Reactive Distillation: 1. Steady-State Simulation and Design Aspects. Ind. Eng. Chem. Res. 1997, 36, 1855.
(2) Jensen, K. L.; Datta, R. Ethers from Ethanol. 1. Equilibrium Thermodynamic Analysis of the Liquid-Phase Ethyl tert-Butyl Ether Reaction. Ind. Eng. Chem. Res. 1995, 34, 392. (3) Sneesby, M. G.; Tade´, M. O.; Datta, R.; Smith, T. N. Multiplicity and Pseudo-Multiplicity in MTBE and ETBE Reactive Distillation. Trans. Inst. Chem. Eng. 1998, 76, Part A, 525. (4) Mohl, K. D.; Kienle, A.; Gilles, E. D. Multiple Steady States in a Reactive Distillation Column for the Production of the Fuel Ether TAME. I. Theoretical Analysis. Chem. Eng. Technol. 1998, 21 (2), 133. (5) Rapmund, P.; Sundmacher, K.; Hoffmann, U. Multiple Steady States in a Reactive Distillation Column for the Production of the Fuel Ether TAME. II. Experimental Validation. Chem. Eng. Technol. 1998, 21 (2), 136. (6) Scenna, N. J.; Ruiz, C. A.; Benz, S. J. Dynamic Simulation of Start Up Procedures of Reactive Distillation Columns. Comput. Chem. Eng. 1998, 22, Suppl., S719. (7) Sneesby, M. G.; Tade´, M. O.; Smith, T. N. Implication of Steady-State Multiplicity for Operation and Control of Etherification Column. Proceedings of Distillation and Absorption ‘97, Maactricht, The Netherlands, 1997. (8) Kister, H. Z. Distillation Operation; McGraw-Hill Pub. Co.: New York, 1990. (9) Bisowarno, B. H.; Tade´, M. O. Simulation of Start Up Strategies in Reactive Distillation for ETBE Synthesis. CHEMECA Conference ’99, Newcastle-Australia, 1999. (10) Aspen Technology Inc. The SpeedUp User’s Manual; Cambridge University Press: Cambridge, MA, 1993. (11) Sneesby, M. G.; Tade´, M. O.; Datta, R.; Smith, T. N. ETBE Synthesis via Reactive Distillation: 2. Dynamic Simulation and Control Aspects. Ind. Eng. Chem. Res. 1997, 36, 1870. (12) Kra¨henbu¨hl, M. A.; Gmehling, J. A. Vapor Pressures of Methyl tert-Butyl Ether, Ethyl tert-Butyl Ether, Isopropyl tertButyl Ether, tert-Amyl Methyl Ether and tert-Amyl Ethyl Ether. J. Chem. Eng. Data 1994, 39, 759. (13) Sneesby, M. G.; Tade´, M. O.; Smith, T. N. Two-point control of a reactive distillation column for composition and conversion. J. Process Control 1999, 9, 19.
Received for review August 26, 1999 Revised manuscript received March 22, 2000 Accepted March 24, 2000 IE9906417
