Simulation and Analysis of a Reactive Distillation Column for Removal

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Simulation and Analysis of a Reactive Distillation Column for Removal of Water from Ethanol−Water Mixtures Weizhong An,† Zixin Lin,*,† Jun Chen,† and Jianmin Zhu‡ †

College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao, 266100, China Liaoning Oxiranchem Group, Liaoyang 111003, China

‡

ABSTRACT: A novel approach to removing water from near-azeotropic ethanol−water mixtures is proposed based on the hydration of ethylene oxide (EO) to produce ethylene glycol (EG) in a reactive distillation (RD) column. Steady-state simulations using the Aspen Plus software package were carried out to investigate the feasibility of the suggested approach, and a sensitivity analysis was carried out to obtain the optimal design parameters. The results showed that, using the optimal operating conditions, a reactive distillation column is capable of circumventing the azeotropic limitation to obtain anhydrous ethanol. Compared with traditional approaches, the proposed approach is promising because of its great potential for reducing energy consumption and capital costs.

1. INTRODUCTION Driven by several world energy crises that have taken place since the 1973 oil crisis, demand for ethanol continues to be high, because ethanol is an excellent alternative clean-burning fuel to conventional hydrocarbon-based fuels such as gasoline. Because of the existence of a minimum-boiling azeotrope of ethanol (95.6 wt %) and water (4.4 wt %), special processes have to be employed to remove the remaining water from the mixture. Azeotropic distillation and extractive distillation are two widely used processes, both of which involve the introduction of a third component into the mixture.1 For azeotropic distillation, the added component forms new binary and/or ternary azeotropes in the system to purify ethanol, as long as the distillation proceeds in a feasible region of the residue-curve map.2,3 For extractive distillation, the added component changes the relative volatility of ethanol with respect to water, to the extent that separation becomes practicable.4−7 Technologies of pervaporation and adsorption are also commercially used. The former represents a new generation of membrane separation process involving a phase change to form a gaseous permeate and liquid retentate using a semipermeable membrane,8 whereas the latter employs molecular sieves or biomaterials (e.g., corn grits, cassava starch) to selectively adsorb water.9−11 Another process for dehydrating ethanol is supercritical fluid extraction, in which carbon dioxide is employed as the solvent. Liquid CO2 is used to extract the ethanol, and then CO 2 is flashed off by depressurization to recover anhydrous ethanol.12 However, regardless of whether they are distillation or nondistillation technologies, all of the above processes involve either considerable energy consumption or huge capital investments because of their complex process configurations. In addition, for some of these processes, large-scale production of anhydrous ethanol can hardly be achieved because of their batchwise operation. Taking into account the characteristics of the separation process in this study, we considered that the removal of water from ethanol−water mixtures is well-suited to be carried out in a reactive distillation column. © 2014 American Chemical Society

Reactive distillation (RD), also known as catalytic distillation (CD) when a catalyst (heterogeneous or homogeneous) is employed, can be considered as a new unit operation that couples reaction with distillation.13 Because of its potential for improved process intensification and successful commercial applications, reactive/catalytic distillation has attracted increasing interest from both academia and industry.14,15 As suggested by Terrill and co-workers, one application of RD is the separation of close-boiling mixtures, where a reactive entrainer is introduced into the column to react selectively with the original components.16,17 This theory implies the potential for similar application to azeotropic mixtures, which was, indeed, confirmed to be feasible.18,19 Dirk-Faitakis and Chuang successfully proposed a CD process whereby water in a nearazeotropic ethanol−water mixture was removed by reaction with isobutylene (IB) to form tert-butyl alcohol (TBA) and, meanwhile, a portion of ethanol reacted with IB to form ethyl tert-butyl ether (ETBE).20 As around 90% of the water could be removed by reaction, the final product was a mixture containing mainly ethanol, TBA, and ETBE that was suitable for use as an oxygenated fuel additive. However, it is noted that this approach cannot directly obtain high-purity ethanol, because the reactions to form both TBA and ETBE are reversible and equilibrium-limited. In the field of RD/CD, a number of articles have already been published on the synthesis of ethylene glycol (EG) by the hydration of ethylene oxide (EO), as EG is an important raw material used in the manufacture of poly(ethylene glycol)s with different molecular weights, as well as other polyester resins, antifreezes, and solvents. Parker’s 1958 patent pointed out that the chemical reactions are highly exothermic and the reaction conditions are mild enough to allow EG production by RD.21 Ciric and Gu described two reasons for producing EG by RD:22 Received: Revised: Accepted: Published: 6056

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First, rapid separation of EO from glycols and improved overall selectivity can be expected because of the large volatility difference between EO and EG. Second, the heat of reaction can be directly absorbed into the heat required for separation, achieving a natural heat integration that might reduce operating costs. In their article, they conducted the synthesis and design of an RD column in terms of a mixed-integer nonlinear programming (MINLP) formulation. Okasinski and Doherty addressed the synthesis and design of an RD column for EG production and presented useful insights into the process development.23 However, we should point out that nearly all of the publications regarding EG production by RD emphasized the use of distillation to enhance (or intensify) reaction (i.e., to improve selectivity and utilize heat of reaction). To our knowledge, there have not yet been any reports applying the hydration of EO in an RD column to break azeotropic limitations to separate azeotropic mixtures containing water. This work proposes a novel application of RD technology for the removal of water from near-azeotropic ethanol−water mixtures based on the hydration of EO. By contrast to the process employing IB as the reactive entrainer suggested by Dirk-Faitakis and Chuang,20 the chemical reactions presented in this work are irreversible and not equilibrium-limited. Thus, complete conversions of EO and water together with anhydrous ethanol are expected theoretically. The main objectives of this work were to investigate the feasibility of the proposed approach and to identify suitable parameters for preliminary process design by means of computer simulations.

C2H4O (EO) + C4 H10O3 (DEG) → C6H14O4 (TEG) (3)

2.2. Ethoxylation Reaction. Ethoxylation is the addition of EO to compounds with active hydrogen atoms (alcohols, glycols, acids, esters, amines, amides, alkylphenols, polyols), which is usually catalyzed by acids and bases and mainly produces nonionic surfactants with numerous practical applications.27 As shown in eqs 4 and 5, ethanol reacts with EO in a manner similar to the hydration of EO. The products are a series of short-chain polymers containing ethylene glycol monoethyl ether (EGME), diethylene glycol monoethyl ether (DEGME), and so on C2H4O (EO) + C2H6O (EtOH) → C4 H10O2 (EGME) (4)

C2H4O (EO) + C4 H10O2 (EGME) → C6H14O3 (DEGME)

However, according to our knowledge, no study has focused on the kinetics of the ethoxylation of ethanol. Hence, we carried out experiments in a stainless steel 2 L batch reactor at Liaoning Oxiranchem, Inc. (Liaoyang, China), and obtained the relevant kinetic data. Detailed information regarding the apparatus and experimental procedure were described previously.28 From the kinetics experiments, we found that, in the absence of catalysts, the ethoxylation rate of ethanol is much less than the hydration rate of EO. However, because a high concentration of ethanol is maintained in the RD column, the formation of EGME and DEGME cannot be neglected. Therefore, the kinetics of the ethoxylation of ethanol is also considered in this study, as listed in Table 1. Based on the mechanism of ethoxylation, the two reactions have approximately the same rate constant and activation energy.29−31 It is pointed out that all of the reactions (reactions 1−5) are irreversible and highly exothermic (approximately −100 kJ/mol of EO reacted).

2. CHEMICAL REACTION KINETICS 2.1. Hydration Reaction. The reaction kinetics of hydration of EO has been studied for many years.24−26 One of the most recent kinetic models of EO hydration was developed by Altiokka and Akyalcin,26 who studied the kinetics experimentally using a pressurized batch reactor. In the cited work, the authors provided detailed kinetic data with and without a catalyst. Because the noncatalytic hydration of EO for the production of EG is a well-known process that is widely used in industrial practice (in most cases within a tubular reactor),26 it was used in this work for the removal of water. The corresponding uncatalyzed reaction kinetic equations are listed in Table 1, as provided by Altiokka and Akyalcin.26

3. THERMODYNAMIC CHARACTERISTICS The Aspen Plus software package32 was employed for the calculation of the thermodynamic properties and phase behavior of the studied systems. As was done by Dirk-Faitakis and Chuang,20 the UNIQUAC (universal quasichemical) activity coefficient model for the liquid phase and the Redlich−Kwong equation of state for the vapor phase were used for phase equilibrium calculations, where UNIQUAC binary interaction parameters were taken from Aspen’s database or estimated by Aspen Plus using the UNIFAC (universal functional activity coefficient) model. The thermodynamic parameters were obtained directly from the Aspen Plus database and compared with literature data.33 The boiling points at atmospheric pressure of the eight components involved in this study are as follows (from low to high): EO, 283.6 K; EtOH, 351.4 K; H2O, 373.2 K; EGME, 408.2 K; EG, 470.5 K; DEGME, 475.1 K; DEG, 518.0 K; TEG, 561.5 K. Note that there are obvious differences in the boiling points among pure components and that the products are much less volatile than the reactants, which suggests that the separation of ethanol from products using an RD column should be applicable. Moreover, it was found that, in addition to the ethanol−water azeotrope, the reactant water and product EGME can also form a homogeneous azeotrope. Table 2 lists the boiling points and azeotropic compositions of the existing

Table 1. Reaction Kinetics reaction

rate [mol/(L·min)]

1

8.22 × 105 exp(−8220/T)CEOCH2O

2 3 4 5

5.78 9.44 7.60 7.60

× × × ×

106 106 104 104

exp(−8700/T)CEOCEG exp(−8900/T)CEOCDEG exp(−9340/T)CEOCEtOH exp(−9340/T)CEOCEGME

Future work will attempt to extend this study to catalytical EO hydration processes. As shown in eqs 1−3, EG is formed from the hydration of EO and can continue to react with EO to form DEG and TEG. In this work, EG is the desired product; hence, the RD process should be designed to obtain the highest possible selectivity to EG. C2H4O (EO) + H 2O → C2H6O2 (EG)

(1)

C2H4O (EO) + C2H6O2 (EG) → C4 H10O3 (DEG)

(2)

(5)

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Table 2. Azeotropic Data for the System mixture

Taz (K)

xaz,1

waz,1

Paz (MPa)

ethanol (1)−water (2)

351.3 404.7 372.7 441.7

0.8968 0.9146 0.9321 0.8869

0.9569 0.9648 0.7329 0.6106

0.1013 0.6 0.1013 0.6

water (1)−EGME (2)

azeotropes at 0.1013 and 0.6 MPa33 (where 0.6 MPa is the recommended operating pressure of the RD column, as will be seen later in this article). However, in fact, the formation of a water−EGME azeotrope occurs only in theory: It is not possible in the proposed RD column because the concentration of water along the column is too low (less than 0.03, as will be seen later in this article). In other words, the azeotropic limitation of the water−EGME mixture need not be considered in the preliminary design and simulation analysis of the proposed approach. Figure 1 shows the binary ethanol−water diagram at 0.6 MPa as calculated with Aspen Plus using the UNIQUAC model,

Figure 2. Schematic of the RD column.

reboiler. For simplification, the RD column has only one feed inlet, where the ethanol−water mixture is first mixed with EO and then enters the column. Based on the feed location and the volatility of the EO reactant, the RD column can be divided into two sections, namely, a reactive zone and a stripping section, as shown in Figure 2. In the reactive zone, because EO can occur on each stage because of its high volatility and because the reactions are homogeneous without catalysts, the reaction and separation actually proceed simultaneously. In the stripping section, because of the high volatility of EO, the concentration of EO in the liquid phase is so low that this section mainly performs the separation of the products from the reactants. The overhead product is anhydrous ethanol, whereas the bottom product is a mixture containing all of the reaction products. The key operating parameters of the RD column include the column pressure, molar EO/water feed ratio, distillate-to-feed ratio, reflux ratio, and holdups, which have significant effects on the column performance. When designing the RD column, both objectives of reaction and separation should be considered. The objectives of reaction include the conversion of water and EO and the selectivity of EG, all of which are preferred to be as high as possible. The objectives of separation mainly include the separation efficiency of ethanol, in other words, the recovery of ethanol in the overhead product, which is the ratio of the molar flow rate of ethanol from the top of the column to the molar flow rate of ethanol in the feed. The objective of ethanol recovery is greater than 99.0% separation efficiency (the column bottom product should not contain any ethanol). It is noticed that, to achieve these objectives, water should be converted almost completely. In addition, energy consumption is also an important target in this study: Lower energy consumption indicated by the usage of the reboiler heat duty is desired. Table 3 lists the operating parameters and the values for the base case used in the simulation. These conditions are close to the optimal values, as discussed in the following sections.

Figure 1. Binary diagram for the EtOH−H2O system at P = 0.6 MPa.

which is divided into two regions by the azeotropic composition (ethanol molar fraction of 0.9146). In this study, the composition of the ethanol−water feed is located in region 1, which is fixed at a molar ratio of 80:20 (ethanol/water), near the azeotropic composition. After the feed has entered the column, the concentration of water decreases sharply to a low level as a result of its consumption in the hydration of EO. Once the molar composition of ethanol is greater than 0.9146, the azeotropic limitation is circumvented. Thus, both reaction and separation of the reactants and products occur in region 2 simultaneously. Meanwhile, it is noted that, in region 2, water is more volatile than ethanol (as can be seen later in Figure 14), which means that water will be distilled together with ethanol from the top of the RD column. Thus, to obtain absolute ethanol, a nearly complete conversion of water must be achieved. The thermodynamic properties and phase behavior described above provide useful insights into the process design, as discussed in the next section.

5. RESULTS AND DISCUSSION Simulation studies were conducted with the steady-state fractionation model RADFRAC from the process simulator Aspen Plus, which is based on a rigorous equilibrium stage model for solving the MESH equations (material balances, equilibrium relationships, summation equations, and heat

4. RD COLUMN CHARACTERISTICS Figure 2 shows a schematic of the proposed RD column used in the simulations. It includes a total condenser and a partial 6058

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indicating that reactions 2−5 are favored at high temperatures. (This is because the rates of reactions 2−5 are more sensitive to the temperature than that rate of reaction 1, as can be seen from the higher activation energies.) (3) The recovery of ethanol shows a trend similar to that of water conversion (the slight drop after 0.6 MPa can also be attributed to the increased rates of side reactions). (4) The selectivity of EG displays a continuing decrease, because the operating pressure affects the concentration of EO in the liquid phase in the reactive zone, which varies the reaction rates as well as the product selectivity. With a lower operating pressure, the concentration of EO in the liquid phase is maintained at a lower level, which benefits the suppression of subsequent side reactions. Conversely, a higher operating pressure means a higher EO concentration in the liquid phase, which results in increased rates of side reactions. Thus, the operating pressure should not be too high. To ensure high conversions of water and EO and a high EG selectivity, 0.6 MPa was selected as the optimum column pressure. 5.2. Effects of EO/Water Molar Feed Ratio. The feed ratio of EO to water, an important operating parameter for the studied problem, affects both the reactions and separation. To achieve a nearly complete conversion of water, the molar feed ratio of EO to water should be at least greater than 1, according to their stoichiometric coefficients in the reactions. Within this range, a low EO/water feed ratio favors a high EG selectivity because of the low concentration of EO, whereas a high ratio is beneficial to a high water conversion and a low EG selectivity. Figure 4 illustrates this analysis well. As the EO/water molar

Table 3. Parameter Values for the Base Case and the Final Design

a

parameter

base case

final design

column pressure (MPa) feed rate of ethanol−water mixture (kmol/h) molar composition of ethanol−water feed number of stagesa feed stage location (above stage) liquid holdup (m3) feed rate of EO (kmol/h) molar feed ratio of EO to water distillate rate (kmol/h) reflux ratio

0.6 100 80:20 22 18 0.5 25 1.25:1 78 5

0.6 100 80:20 22 17 1.1 30 1.5:1 80.5 7

Including the condenser (stage 1) and reboiler (stage 22).

balances). Taking into account the complexity of the integration of reaction and separation in the same column, a sensitivity analysis was conducted primarily to analyze how the key parameters impact the column performance. In each analysis of a certain parameter, other variables remained constant at the values listed in Table 3. 5.1. Effects of Pressure. Column pressure is one of the most important design parameters for RD. It not only affects the relative volatilities of the components and the phase equilibrium, but also has significant impacts on the chemical kinetics. The column pressure sets the temperature range within the column, which consequently determines the reaction temperature (because chemical reactions take place in the liquid phase, the reaction temperature is close to the boiling point of the liquid phase) and then further affects the reaction rate. A low temperature makes the reaction rate low, which would cause very large liquid holdups. A high temperature is beneficial to high reaction rates, but this could affect the selectivities of desired products, given that the rates of side reactions also increase. A sensitivity analysis was performed to illustrate the effects of pressure, as shown in Figure 3. It can be seen that, as the column pressure increases from 0.2 to 1.0 MPa, the following changes occur: (1) The conversion of EO increases because of the increased reaction rate as the temperature increases. (2) The conversion of water increases initially, because the hydration rate increases. However, when the temperature is too high, the water conversion decreases,

Figure 4. Effects of the molar feed ratio of EO to water on the water conversion, EO conversion, EtOH recovery, and EG selectivity of the column.

feed ratio increases, the EG selectivity decreases, whereas the water conversion increases. The recovery of ethanol also increases, because an increased EO concentration is beneficial to reactions 2 and 3, whose reaction rates are more sensitive to the EO concentration. With these specified parameters, a nearly complete conversion of EO can be achieved. A molar ratio of 1.5 is finally suggested to strike a balance between high water conversion and EG selectivity as well as EO conversion. 5.3. Effects of Distillate-to-Feed Ratio. According to the simulation results, we found that the ratio of distillation to feed (ethanol−water mixture) is more critical to separation than to reaction, especially the separation in the stripping section. The analysis results are shown in Figure 5, from which one can make the following conclusions: (1) The distillate-to-feed ratio

Figure 3. Effects of the column pressure on the water conversion, EO conversion, EtOH recovery, and EG selectivity of the column. 6059

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water conversion and EG selectivity as well as the boiler heat duty, a reflux ratio of 7 is suggested as the optimal condition. 5.5. Effects of Feed Location. The location of the feed stage has significant impacts on both separation and reaction, because it determines not only the number of stages in the stripping section but also the reaction volume in the reactive zone of the column. As shown in Figure 6, when the feed

Figure 5. Effects of the distillate-to-feed ratio on the water conversion, EO conversion, EtOH recovery, and EG selectivity of the column.

(D/F) has little effect on the EO conversion, because EO is a light component that is mainly concentrated in the reactive zone and rarely enters the stripping section below the feed stage. (2) For ethanol, a smaller D/F impairs the effect of separation, leading to a higher concentration of ethanol in the bottom product, so that the recovery of ethanol is decreased. Conversely, a higher D/F is beneficial to a higher recovery of ethanol. (3) A large D/F decreases the water conversion, mainly because a portion of water is distilled from the top of the column together with ethanol. (4) The trend for EG selectivity is the same as that for water conversion, because a high water conversion indicates a high generation rate of EG and, consequently, a high EG selectivity, and vice versa. Above all, D/F is an important operating parameter. According to the analysis, the recommended value of the distillate-to-feed ratio is around 0.80 for high conversions of water and EO, as well as a desired ethanol recovery. 5.4. Effects of Reflux Ratio. In RD columns, the reflux ratio plays an important role in both reaction and separation. Table 4 describes the relationships between the reflux ratio and

Figure 6. Effects of the feed location on the water conversion, EO conversion, EtOH recovery, and EG selectivity of the column.

location is too close to the top of the column, EO can easily escape from the top because of its low boiling point, which leads to low conversions of EO and water. At the same time, the separation effect of the reactive zone is impaired. Thus, more EO is brought into the stripping section, where EG can further react with EO to form DEG and TEG. As a result, the selectivity to EG decreases. On the other hand, if the feed location is too close to the bottom of the column, more water and ethanol are withdrawn from the bottom because of the weakened separation effect of the stripping section, which also results in low water conversion and ethanol recovery. The EG selectivity shows a trend similar to that of water conversion. The above analysis explains well why there exists an optimal feed location with the best water conversion and EG selectivity. Similarly, the recovery of ethanol initially increases and then decreases, also because of the decreased separation effects of the two extreme cases. However, the EO conversion always increases as the feed location moves downward, because of the increase in its residence time in the column. To guarantee a complete conversion of EO as well as high water conversion and EG selectivity, the favored feed location is at stage 17. 5.6. Effects of Liquid Holdups. The holdup is one of the most critical design parameters for RD processes, which mainly affects reactions because it refers to the reaction volume. In this study, the liquid holdup on each tray was assumed to be the same. Figure 7 illustrates the effects of different holdups on water conversion and other characteristics. As expected, the larger the liquid holdup, the larger the EO conversion, because the residence time of EO in the reactive zone increases. However, a large holdup also favors side reactions, which reduces the conversion of water as well as the selectivity of EG and the recovery of ethanol. To ensure a high conversion of EO, a volumetric holdup of 1.1 m3 on each tray is finally selected as the optimal parameter. 5.7. Simulation Analysis. Figure 8 shows the final design of the RD column. The optimal operating variables gained from the sensitivity analysis are reported in Table 3. The simulation

Table 4. Effects of Reflux Ratio reflux ratio

water conversion (%)

EG selectivity (%)

EtOH recovery (%)

boiler duty (MW)

4 5 6 7 8 9 10

87.98 89.73 91.01 91.97 92.71 93.29 93.73

64.89 66.54 67.76 68.72 69.52 70.22 70.86

95.37 95.66 95.87 96.01 96.11 96.16 96.19

3.01 3.77 4.53 5.28 6.03 6.79 7.54

water conversion, ethanol recovery, and reboiler heat duty. In all of these scenarios, EO is converted completely. It can be seen that (1) water conversion increases as the reflux ratio is increased, because unreacted reactants are forced to recycle back to the column for further reaction; (2) the recovery of ethanol increases with increasing reflux ratio because of strengthened separation effects; and (3) the selectivity to EG exhibits a trend similar to that of water, because the EG concentration is closely related to the water concentration. In addition, the reflux ratio has a greater impact on the heat duty, as indicated in Table 4. To arrive at a compromise between 6060

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Figure 9 displays the profiles of the component generation rates through the length of the column, illustrating that the

Figure 7. Effects of liquid holdups on the water conversion, EO conversion, EtOH recovery, and EG selectivity of the column.

Figure 9. Profiles of component generation rates along the RD column.

reactions mainly proceed between stages 11 and 17. It can be seen that the consumption rates of water and EO reach a maximum at the feed stage (stage 17) because of the maximum concentration of reactants. On every other stage, reactions also occur although with relatively low rates. This suggests that water is removed mainly by reaction with EO, because it is difficult to directly separate water from ethanol at nearazeotropic compositions by normal distillation. Figure 10 shows the composition profiles of water and EO along the column. The mole fraction of water exhibits a

Figure 8. Schematic of the RD column with simulation results under optimal operating conditions.

results, shown in Figure 8, confirm the feasibility of the proposed approach. First, anhydrous ethanol (99.6 wt %) is obtained from the overhead product. The bottom byproduct is a mixture of EG, DEG, and TEG, plus a small quantity of EGME and DEGME, which can be either separated further to obtain pure components or used directly as a solvent for nitrocellulose, resins, dyes, or other applications. In this work, further separation of the bottom mixture was not considered. Second, high conversions of water and EO are realized simultaneously, as a result of the combination of reaction and separation. (Under the effects of separation, unreacted water in the stripping section is recycled to the reactive zone for further reaction.) Third, the heat released by reactions is utilized in the process, indicated by the greater heat duty of the condenser compared with that of the reboiler. Finally, because of the low concentration of water in the column, the selectivity of EG is 53.5%, not as high as that in the conventional RD process to produce EG from EO and water. However, our primary objective was the removal of water, and the main point of this study was to emphasize the enhancement of separation through reaction.

Figure 10. Composition profiles of H2O and EO in the liquid phase along the RD column.

maximum at the feed stage and then decreases sharply as a result of consumption in the reaction. From stage 5, the mole fraction of water increases slightly stage by stage until the top of the column, as a result of the shifted volatilities of ethanol and water after the circumvention of the azeotropic limitation, as well as reduced reaction rates. In this process, the concentrations of EO and water throughout the column are very low, indicating complete conversions of both components. The composition profiles of other components are shown in 6061

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Figure 11, illustrating that heavy components are enriched at and near the bottom of the column.

Figure 13. Profiles of vapor and liquid molar flow rates along the RD column.

utilization of reaction heat in the RD column helps to reduce the heat duty of the reboiler. The relative volatility of ethanol with respect to water along the column is shown in Figure 14. An interesting phenomenon

Figure 11. Composition profiles in the liquid phase along the RD column.

The temperature profile is mainly determined by the composition of the components, as demonstrated in Figure 12. The temperature changes little above stage 17, because of

Figure 14. Profile of the relative volatility of EtOH with respect to that of H2O along the RD column. Figure 12. Temperature profile along the RD column.

is observed that the azeotropic limitation is actually stepped past twice: once at stage 14 and again at stage 19. This can be explained by the effects of the combination of reaction and distillation. Initially, the volatility of ethanol is higher than that of water at the feed stage, because the concentration of ethanol is less than the azeotropic composition. However, the concentration of water decreases dramatically after entering the column as a result of the hydration reaction, which changes the molar ratio of ethanol to water to 0.98:0.02, greater than the azeotropic composition (0.91:0.09) at the specified operating pressure. Thus, the volatilities of ethanol and water are shifted, and the azeotropic limitation is circumvented at stages 14 and 19. Between stages 1 and 14, the fact that relative volatility of ethanol to water is quite close to 1, as indicated in Figure 14, implies that the direct separation of water from ethanol by distillation is difficult. This again suggests that the removal of water from the system is accomplished mainly by

the insignificant change in component composition. However, below stage 17, the temperature increases dramatically stage by stage, because of the enrichment of heavy components under the effects of separation. Figure 13 shows the profiles of the vapor and liquid molar flow rates along the column. It should be noticed that, between stages 17 and 21 (below the feed location), the molar flow rate of vapor is quite close to that of liquid or even higher, which is different from the situation in a conventional distillation column. This can be attributed to the heat effect of the reaction. Because all reactions in the system are highly exothermic, the heat released by reaction vaporizes part of the liquid directly, which leads to the change in flow rate profiles of vapor and liquid along the column. As is known, one substantial advantage of an RD column is that the heat of reaction can be utilized in the column, which is well reflected in this study. Such direct 6062

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reaction. Between stages 19 and 22, where the volatility of ethanol is also lower than that of water, a minimum relative volatility of ethanol with respect to water exists at stage 22. This is mainly because the separation effect leads to changes in the component composition and, consequently, influences the activity coefficients of ethanol and water. Moreover, as the products of hydration and ethoxylation reactions are heavy components with higher boiling points than water, these heavy components can further affect the relative volatility of water and ethanol. This will be further studied in future work on process development. 5.8. Energy Considerations. To illustrate the advantages of the proposed process, the energy consumption is compared with other reported processes for purifying ethanol.34 The reboiler heat duty in the proposed process is 5.32 MJ per kilogram of anhydrous ethanol, implying a reduction of 45− 75% in energy consumption compared with azeotropic distillation (10.05−15.49 MJ/kg) and extractive distillation (9.21−18.84 MJ/kg). In addition, the bottom product, containing EG, DEG, TEG, EGME, and DEGME, also generates great economic benefits for its wide usage in the fine-chemistry industry, which also shows its outstanding advantage over other separation process. For the present, detailed economic analysis and comparisons are not considered. The energy consumption of the RD process proposed by DirkFaitakis and Chuang20 is reported as 0.22 kWh/kg of product (0.80 MJ/kg), which is much less than that of our proposed approach. However, in their study, the molar ratio of ethanol to water was 88:12, which is higher than what we used in this study, and the purity of ethanol in the final product was only approximately 50% by weight. Extra energy will be required to obtain absolute ethanol. Thus, the two processes cannot be compared directly in a simple way without further detailed analysis.

ACKNOWLEDGMENTS

This research was financially supported by the National Natural Science Foundation of China (21306179) and the State Key Laboratory of Chemical Engineering of China (No. SKL-ChE12B03).

■

NOMENCLATURE Ci = molarity of component i (mol/L) P = pressure (MPa) T = reaction temperature (K) xi = mole fraction of component i wi = mass fraction of component i

Abbreviations

■

CD = catalytic distillation DEG = diethylene glycol DEGME = diethylene glycol monoethyl ether EG = ethylene glycol EGME = ethylene glycol monoethyl ether EO = ethylene oxide ETBE = ethyl tert-butyl ether IB = isobutylene MINLP = mixed-integer nonlinear programming RD = reactive distillation TBA = tert-butyl alcohol TEG = triethylene glycol

REFERENCES

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6. CONCLUSIONS This work proposes a novel RD process to remove water from ethanol−water mixtures at near-azeotropic compositions by reaction with EO, which was confirmed to be feasible according to the simulations. As water is consumed in the hydration reaction, the azeotropic limitation is stepped past, and dehydrated ethanol is distilled from the top of the column. Optimal operating conditions, including operating pressure, reflux ratio, and liquid holdups, were obtained based on a sensitivity analysis, subjected to specified conversions of water and EO as well as other objectives. A reduction in energy consumption can be achieved by utilizing the heat of reactions directly in the distillation column. Compared with other traditional separation processes removing water from ethanol, the proposed approach shows its promising applications and great potential in savings in capital investments and energy consumption. The main objective of this study was to obtain confidence in the feasibility of the proposed process, which was accomplished successfully. Future work on experimental studies is being carried out.

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