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Improved Energy Efficiency of a Hybrid Pervaporation/Distillation Process for Acetic Acid Production: Identification of Target Membrane Performances by Simulation Clément Servel,*,† Denis Roizard,† Eric Favre,† and Dominique Horbez‡ †

Laboratoire Réactions et Génie des Procédés, CNRS, Université de Lorraine, ENSIC, 1 rue Granville, 54000 Nancy, France Solvay, Research & Innovation Center Paris, 52 rue de la Haie Coq, 93308 Aubervilliers, France

‡

S Supporting Information *

ABSTRACT: The continuously rising cost of world energy is pushing the chemical process industry to find alternative processes to conventional separation processes. Previous studies on hybrid processes coupling pervaporation or vapor permeation with distillation have claimed interesting decreases in energy consumption compared to conventional processes. However, most of the time, it remains difficult to really evaluate the actual potential and the energy gain achievable for a membrane process for a particular separation. This paper reports an innovative simulation methodology that can be applied to determine the membrane performances that have to be achieved to replace conventional processes with a hybrid process while respecting constraints that are fixed by industrial specifications. This method is applied to the case of acetic acid dehydration. A hybrid process consisting of a pervaporation module equipped with a hydrophilic membrane coupled with a distillation column is proposed and studied. The minimum membrane performances for industrial use of pervaporation are determined and compared to membrane performances that have already been reported in the literature. As a result, this work shows that commercially available membranes have a selectivity that is too low to induce significant economic savings for this application, although some materials that are not yet commercialized show very interesting separation results. (OPEX). Many studies have been performed to find suitable alternatives to solve the problem. Among them, azeotropic distillation deserves special attention:5 it involves the addition of a third component. This option provides some reduction in the operating costs but increases the amount of separation equipment required. Moreover, it generates additional operating and environmental problems due to the presence of the added component. In this context, pervaporation appears to be an attractive alternative method to dehydrate acetic acid. This study will focus on distillation and hybridization between pervaporation (and vapor permeation) and distillation. The conventional (distillation column) and hybrid processes (distillation and pervaporation module) compared are presented in Figure 1.

1. INTRODUCTION Distillation is one of the most used but also one of the most energy-intensive separation processes. The increase in energy demand and the reduction in fossil fuels are strong incentives to find alternatives to traditional separation processes. Pervaporation and vapor permeation are used to separate components in the ranges of the composition that show thermodynamic limitations, such as azeotropes and pinches of the vapor/liquid equilibrium curve. The use of pervaporation and vapor permeation in hybrid processes with distillation has shown attractive energy-saving possibilities in many cases.1 Distillation and pervaporation hybridization processes may be an option to reduce energy consumption in chemical processes. Acetic acid is a carboxylic acid used in industry as an intermediate in many processes (vinyl acetate, terephthalic acid, acetic anhydride, cellulose esters, etc.). It is mainly produced by methanol carbonylation. In this process, the separation between water and acetic acid is energy intensive. Furthermore, the purification of bioacetic acid, which is produced by hydrothermal treatment of green waste plants2 or by an aerobic fermentation by Clostridium thermoaceticum,3 has the same problem of the high energy requirement of acid dehydration. Despite the fact that, at atmospheric pressure, a water/acetic acid binary mixture does not form an azeotrope,4 the relative volatility of water and acetic acid is close to unity and thus requires a large number of trays in the distillation column. In addition, at atmospheric pressure, the boiling temperature of the acid is 117 °C, which implies large energy consumption. Thus, the use of distillation to separate this mixture generates large capital expenditures (CAPEX) and operating costs © 2014 American Chemical Society

Figure 1. Schema of (a) conventional process and (b) hybrid process. Received: Revised: Accepted: Published: 7768

February 1, 2014 April 6, 2014 April 7, 2014 April 7, 2014 dx.doi.org/10.1021/ie500467k | Ind. Eng. Chem. Res. 2014, 53, 7768−7779

Industrial & Engineering Chemistry Research

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The use of pervaporation for the dehydration of acetic acid has been already studied, especially for the development of new membranes. A large number of candidate materials have been proposed. Here, we want to note the difficulty of a material comparison. The reported data are, in most cases, flux and separation factors, and as underlined by Baker,6 membrane comparison cannot be done on fluxes, which depend on operating conditions and differ in any two publications. Thus, materials will be compared based on the value of the water permeance and membrane selectivity. From the fluxes, inlet temperature, inlet composition, and separation factor data, driving forces for each component are calculated, which allows the determination of water and acetic acid permeances. Membranes can be either hydrophilic or hydrophobic depending on their compound/membrane affinity. Both cases will be presented and investigated in this study. 1.1. Hydrophilic Membranes. Two types of membranes have been tested: polymeric and inorganic ones. Poly(vinyl alcohol) is the most widely studied polymer;7 some authors added copolymers such as poly(acrylic acid)8 or inorganic compounds such as tetraethylorthosilicate.9 Other hydrophilic material based membranes, such as polysulfone,10 poly(vinyl chloride),11 sodium alginate,12 polyimide,13 polybutadiene,14 polybenzimidazole,15 and polyphenylsulfone,16 have also been tested for the dehydration of acetic acid by pervaporation. Some studies tested ion exchange membrane materials such as Nafion17 or inorganic membranes.18 However, the majority of these studies have not led to membrane commercialization or large-scale development, most often due to their lack of mechanical strength and stability. Nevertheless, Gorri et al.19 tested commercially available membranes. 1.2. Hydrophobic Membranes. Some authors have reported acetic acid selective membranes. The most studied material is polydimethylsiloxane, despite the fact that this membrane is not polar at all. Some modifications of this material have been tested, such as the addition of silicalite20 and cross-linking with 3-aminopropyltrimethoxy21 or poly(ether imide).22 Some authors tested liquid-supported membranes with trioctylamine23 on a polypropylene porous support or trilaurylamine.24 There are still some problems with the stability of these types of membranes that prevents their use on a large scale. Most of the published data do not discuss the reproducibility of the experimental results and report data in a narrow range of composition and temperature. A comparison of membrane performances is done to estimate the potential separation of these materials in Figure 2, where each point represents a membrane that has been published. To compare membrane materials, we decided to limit the ranges of temperature (30− 65 °C) and water composition (hydrophilic, 10−20 wt %; hydrophobic, 85−95 wt %). Figure 2 represents the current state of the art of membrane material. The majority of the hydrophilic membranes have a water permeance less than 30 kg/(h·m2·bar) and a selectivity less than 1000, i.e., a water permeance of 1−10 kg/(h·m2·bar) and a selectivity between 2 and 100. However, these single data give no information on the technical feasibility of and economic interest in a hybrid process due to the large number of parameters that have to be taken into account, including purification and productivity specifications, the structure of the process, operating parameters, membrane performances, and membrane life span. Process

Figure 2. Water permeance versus water selectivity for different membranes whose performances have been published in the literature.

simulation is the most used tool to determine the economic interest in processes. Interest is often determined by economic comparison of the hybrid and conventional processes25,26 or the calculation of profitability,27 which requires experimental data on the system membrane/compounds studied. Ji et al.28 studied the effect of three different membranes on the removal of volatile organic compounds from water. Hömmerich et al.29 studied the combination of pervaporation and distillation for the production of MTBE. Nevertheless, every economic study depends on many factors (utilities and energy costs, plant location, etc.) that differ in each case and are hardly usable for other processes. General design methodologies have been recently proposed. Bausa et al.30 developed a shortcut method to simulate a hybrid process. Ayotte-Sauvé et al.31 used a thermodynamic approach with the concept of power of separation for a coupling of distillation and membrane separation. Pettersen et al.32 proposed a comparison between different hybrid configurations by studying the effect of the main design variables. For the specific case of acetic acid dehydration, different studies have been proposed. Huang et al.33 suggested a new hybrid process using a perfluoro membrane to remove water from the distillate stream of a distillation column. In this case, the membrane performances are known. Verhoef et al.34 used the parameter of the fraction of acetic acid permeating through the membrane to simulate different membrane performances. However, in their study, only three different performances were studied. From an industrial point of view, the problem is different. If a process already exists, process engineers have to answer the following questions: what is the potential for the use of the membrane separation process in this specific process and what could the decrease in energy consumption be with the use of a membrane? To our knowledge, there is no study that considers both economic comparison and membrane development aspects. Moreover, most of the published studies consider membranes that have been already developed. In this work, a methodology to compare processes involving membrane separation units is developed to define the minimum membrane performances necessary to reach industrial specifications with the aim of saving energy of an existing process. 7769

dx.doi.org/10.1021/ie500467k | Ind. Eng. Chem. Res. 2014, 53, 7768−7779


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models with a large number of fitting parameters induces many degrees of freedom, which leads to a decrease in the physical relevance of the parameters.39 A compromise between the number of parameters and the physical significance of these parameters must be made. For our study, a global transmembrane model based on solution−diffusion theory was chosen. The flux Ji of a component i is a function of the partial pressure on the feed side, pi,F, and on the permeate side, pi,Perm, the membrane permeability 7i , and the membrane thickness l.40

From this point of view, our study is novel. It will be applied to the dehydration of acetic acid. Our study intends to answer the following questions: 1. What are the possibilities and limitations of pervaporation in the case of the acetic acid dehydration process? 2. What is the best strategy for hybrid process design (membrane surface area, energy requirement)? 3. Are currently available materials usable with our industrial specifications?

2. METHODOLOGY 2.1. Thermodynamic Aspect. The nonrandom two-liquid (NRTL) model35 is used to represent the vapor/liquid equilibrium (VLE) of the mixture. Details are provided in Appendix A. 2.2. Modeling of Mass Transfer in Pervaporation. Commercial process simulator software does not have ready-touse pervaporation module packages; they have to be added.34 The equations for the flux calculation have to be chosen and implemented in simulation tools. Aspen Custom Modeler is used for that purpose. 2.2.1. Choice of the Flux Calculation Model. Mass transfer modeling in pervaporation is an important aspect for the understanding and development of this process. Although the global mechanism of pervaporation is accepted by the majority of the research community, the choice of mass transfer equations has not been settled yet. In 2001, Lipnizki and Trägårdh36 quoted more than 150 references in a review dedicated to pervaporation modeling. The focus here will be on composite membranes that represent the majority of the commercial membranes. They are composed of an active layer deposited on one or two support layers. Transport across the membrane is a five-step mechanism called the “solution−diffusion” model as follows:37 (i) transport from the bulk feed to the membrane surface (ii) sorption into the membrane top layer (iii) diffusion through the top layer (iv) desorption from the top layer to the support layer (v) transport from the pore of the support layer to the bulk vapor The large number of models can be explained by the fact that each step can be defined by different theories. Moreover, some authors developed an overall transmembrane mass transfer model, and some authors developed resistance-in-series models that take into account some or all of the previous steps. There are numerous difficulties in choosing the most appropriate model. Depending on the conditions, some steps can be assumed to be negligible. For example, in the case of organic recovery in dilute water solution, concentration polarization may be significant, whereas in pervaporation with hydrophilic membranes, it may occur less frequently.38 Steps (iv) and (v) can be combined into one single step. The major difference between models is the number of fitted parameters they require. To fit these parameters with experimental data, the type of experiments performed is a very important criterion. Sorption experiments could be very difficult due to the small thickness of the active layer of commercial membranes, and it could be very difficult to determine the composition of the active layer. It can also be difficult to calculate the diffusion across the membrane. In fact, only flux and selectivity can be easily measured in pervaporation experiments. To conclude this topic, the use of

Ji =

7i (p − pi ,Perm ) l i ,F

(1)

The permeability includes the sorption at the interface of the membrane and the diffusion across the membrane. The partial pressure on the feed side is defined in eq 2. pi ,F = xi ,Fγi ,FPisat

(2)

where xi is the mole fraction of component i in the feed, γi is the activity coefficient of component i, and Psat i is the saturated vapor pressure at the temperature of the feed. The saturated vapor pressure is calculated by the Antoine law, and the activity coefficients are calculated by the NRTL model. Following the Dalton law, the partial pressure on the permeate side is defined in eq 3. pi ,Perm = yi ,Perm PPerm

(3)

where yi is the permeate mole fraction of component i, and PPerm is the permeate pressure. The permeance Qi of component i is the permeability divided by the membrane thickness, and the selectivity α is defined as the permeance ratio. Qi =

7i l

α=

Qi

(4)

Qj

(5)

Equation 1 then becomes Ji = Q i(xi ,Fγi ,FPisat − yi ,Perm PPerm)

(6)

Because permeance includes diffusion and sorption, this term is temperature and composition dependent. To take these effects into account, some authors added the effect of plasticization to the composition dependence41,42 or added an Arrhenius equation for the temperature dependence.43 The plasticization effect is used to represent the coupling that can occur when one component is entrained by the diffusion or the sorption of another one. This phenomenon is common in pervaporation, especially for organic membranes.44 Nevertheless, the aim of this study is to simulate different types of membranes (organic or inorganic), in which there may not necessarily be coupling. Therefore, in this study, the coupling effect will be neglected. An Arrhenius-type equation is used to take into account the temperature effect as follows: ⎛E ⎛ 1 1 ⎞⎞ − ⎟⎟⎟ Q i(T ) = Q iref exp⎜⎜ Ai ⎜ T ⎠⎠ ⎝ R ⎝ Tref 7770

(7)



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where EAi is the activation energy of the component i, and Tref is the reference temperature at which Qref i is calculated. The activation energies have a substantial impact on the flux calculation; their values are an important parameter. These activation energies have been chosen based on the literature (58 kJ/mol for acetic acid and 47 kJ/mol for water).19 This publication has been selected because the authors used industrial membranes and worked with operating conditions close to those of our case study (80 °C for the temperature and acetic acid mass composition range of less than 20%). All the previous equations can be summarized in a global equation for the transmembrane flux calculation as follows: ⎛E ⎛ 1 1 ⎞⎞ − ⎟⎟⎟(xi ,Fγi ,FPisat − yi ,Perm PPerm) Ji = Q iref exp⎜⎜ A ⎜ T ⎠⎠ ⎝ R ⎝ Tref

Figure 3. Schema of the membrane module in ACM. (8)

Evp = nPermRTPerm

2.2.2. Specific Case of Vapor Permeation. In the case of vapor permeation, the feed phase is vapor. Consequently, there is no vaporization of the feed, and there is no temperature decrease in the module. Therefore, the process can be considered isothermal, and the driving force of the process is adapted to the vapor phase. The equations presented in section 2.2.1 are adapted to the vapor phase of the feed stream as follows: Ji = Q i(yi ,F fi ,F PF − yi ,Perm fi ,Perm PPerm)

⎡⎛ ⎤ ⎞(γgas− 1)/ γgas ⎢⎜ Patm ⎟ − 1⎥ ⎥ 1 ⎢⎣⎝ PPerm ⎠ ⎦ (12)

where Evp is the power needed in the vacuum pump (kW), FPerm is the permeate flow rate (mol/s), R is the universal gas constant, TPerm is the permeate temperature (K), γgas is the adiabatic gas expansion coefficient, η is the vacuum pump efficiency, Patm is the atmospheric pressure (bar), and PPerm is the permeate pressure (bar). It should be noted that the energy used to run the vacuum pump is electricity. A conversion factor is used to transform electric energy into heat energy with the objective of process comparison as follows:

(9)

where f i represents the fugacity of component i. 2.2.3. Implementation of the Flux Calculation Equations in Process Simulation Software. The software Aspen Custom Modeler (ACM) is used to simulate the pervaporation process. The module is divided into 100 submodules to take into account the decrease of the temperature along the membrane as well as the evolution of the composition. The number of 100 is a compromise between the time of simulation and a good and proper accounting of the evolutions of temperature and composition in the membrane module. For each submodule, mass and energy balances are solved. ACM software-specific procedures calculate feed, retentate, and permeate enthalpies. The retentate temperature is calculated from the energy balance

1 electric kW = 2 thermal kW

(13)

2.3. Hybrid Process Simulation. 2.3.1. Implementation of the Membrane Process in Aspen Plus. The ACM model is then exported to Aspen Plus Software. This action allowed the integration of the pervaporation process into global processes to take into account the influence of the recycling streams. In our study, the final retentate composition is fixed by the program user. A VBA program coded in Excel is used to calculate the number of pervaporation stages needed to reach the final retentate composition depending on the strategy used. The partial vaporization of the permeate results in a decrease of the retentate temperature. This leads to a decrease of the driving force due to the decrease in the saturated vapor pressure of the component. To limit its impact on membrane productivity, a maximum temperature difference between feed and retentate is fixed by the module. When this difference is reached, the pervaporation module is completed. A heat exchanger is placed between two pervaporation modules to heat the next feed stream. Therefore, the utilization of the pervaporation process consists of a series of pervaporation modules and heat exchangers as shown in Figure 4. 2.3.2. Energy Analysis of the Hybrid Process. The energetic gain is calculated with the following equation:

N−1 N−1 N−1 N N N N N N nRet hlRet (TRet ) = nRet hlRet (TRet ) + nPerm hv Perm (TPerm )

(10) N−1 N TRet = TPerm

1 γgas η γgas −

(11)

A cross-flow pattern is assumed, which means that the permeate stream is well mixed. This hypothesis is often proposed for vacuum conditions. The following hypotheses conditions are assumed: (i) Ideal mixing is considered at the permeate side. (ii) For each submodule, the permeate temperature is equal to the feed temperature. (iii) Concentration and temperature polarization are not taken into account. (iv) No pressure drop is considered.

energetic gain (%)= ∑ E vp + ∑nbPV E HE duty + Eb,dist column ⎞ ⎛ ⎟ 100⎜1 − nbPV Eb,conv process ⎝ ⎠

(14)

where Evp is the energy required for the vacuum pump (in thermal equivalents), EHE duty is the energy required by the heat exchanger, Eb,dist column is the energy required by the boiler of the distillation column in the hybrid process, and Eb,conv process is

The module schema is presented in Figure 3. The energy needed for the vacuum pump is calculated by the following equation:45 7771



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Figure 4. Schema of a series of membrane modules and heat exchangers.

3. RESULTS AND DISCUSSION 3.1. Baseline Study: Conventional Process. The methodology detailed above is applied to the dehydration of acetic acid. The process specifications are presented in Table 1.

the energy required by the boiler of the distillation column in the conventional process. The pressure of the permeate stream is an important parameter. Indeed, the flux across the membrane is proportional to the partial pressure difference on both sides of the membrane. Minimizing the permeate pressure is recommended to obtain maximum flux. However, in practice, the vapor has to be condensed. Condensation can be very expensive if the condensing temperature is low. Generally, a condensation temperature of 20 °C is easy to obtain using cold water, which is generally abundant and inexpensive. The pressure of the permeate is fixed at 30 mbar. The choice of pressure is described in Appendix B. 2.4. Conventional and Hybrid Process Comparison. The aim of this method is to find the minimum membrane performances that have to be achieved for industrial application of the pervaporation process in a hybrid process. Our comparison method has been performed in relation to industrial requirements. After the thermodynamic model has been chosen, it is necessary to define process specifications. These specifications will be the basis for the comparison between the hybrid and conventional processes. The compositions and flow rates of the input streams of the process and the composition to be achieved at the output streams of the process must be defined. The maximum allowed loss rate for each component must be fixed. Conventional process simulation is then performed, taking these specifications into account. Afterward, the minimum energy savings for the industrialization of the process must be determined. It should be noted here that the comparison process cannot be limited to the comparison of energy costs (OPEX). Investment costs (CAPEX) must also be taken into account, including the membrane surface necessary to achieve separation. This is a key parameter in selecting the type of process because the capital cost of the membrane is not negligible. However, technical and economic studies require data that are difficult to quantify: the real cost of the membrane, the utilities cost that depend on the factory implantation region, and, especially, the difference between process development and economy or debottlenecking an existing process. The presented method will focus on hybrid processes coupling membrane separation and distillation. The minimum energy savings for the industrial application of the pervaporation process is fixed. Once the configuration of the hybrid process is chosen, the energy consumption of the distillation column is decreased. A membrane separation module using pervaporation or vapor permeation, depending on the phase nature, equipped with a hydrophilic or hydrophobic membrane, depending on the removed component, is added to remove one compound of the supply stream to achieve the specifications.

Table 1. Process Specifications for the Study feed throughput (kg/h) water mass composition of feed stream (%) acetic acid mass composition of feed stream (%) acetic acid mass composition of output stream (%) acetic acid loss rate (%)

10 000 30 70 99, 99.2, 99.4, 99.6, 99.8 100 °C) because of the high composition of acetic acid. A heat exchanger is used before the first module to recover heat. The temperature difference in each module is fixed at a value of 20 °C. A heat exchanger placed between each module reheats streams to 90 °C. The required heat duty of each heat exchanger is calculated by Aspen Plus. The pressure of the permeate is fixed at 30 mbar. 3.3.3. Results and Discussion. For each permeance/ selectivity pair, the method calculates the total membrane surface needed to achieve the objectives, the total energy needed for the pervaporation process (the heat duty for each heat exchanger and the energy required for a vacuum at the permeate side), and the total acetic acid rate loss, which can be larger than the limited value. The comparison between the conventional process (i.e., a distillation column) and a hybrid process with different values of membrane performances can be found in the Supporting Information. In all simulation cases, the energy consumption of the distillation column of the hybrid process is below the energy consumption of the conventional process. If the energy consumption of the membrane process is not taken into account, the use of such a hybrid process could be very interesting regardless of the membrane performances. The results of the simulation campaign show that the operating cost of the pervaporation process represents approximately 10% of the energy cost of the hybrid process. In addition, the electrical consumption of the pervaporation process represents approximately 50% (thermal equivalent) of the total energy consumption of the membrane process. This shows the strong impact of vacuum prorating on the energy requirement.

Table 3. Limits for the Industrial Application of the Pervaporation Process max loss rate of acetic acid (%) max total surface (m2) min energy savings (%)

0.5 5000 20

The concept of response surfaces is proposed for the first time, to our knowledge, to represent membranes that can be used on an industrial scale. Hybrid processes equipped with membranes whose performances respond to fixed limits (cf. Table 3) are interesting and could be an alternative to conventional processes. Response surfaces consist of these “interesting” membrane performances. Depending on the acetic acid composition objective, response surfaces are represented in Figure 7 by zones of different colors. Membrane performances that can be used in industrial applications and performances that have been published in the literature are shown in Figure 7. The objective output composition of acetic acid has a great influence on the performances required to use pervaporation on an industrial scale. Indeed, acetic acid mass purity higher than 99.2% cannot be achieved by this configuration with an energy economy of 20%. The selectivity of the membrane is the principal limitation of the use of this technology. In the case of an energy savings of 10%, the response surfaces are larger, which correspond to membrane performances that are more achievable. In this specific case, some membranes that 7774



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can be compared with a conventional process with exactly the same separation. 3.4.2. Conditions of the Simulation. The water composition of the distillate stream is decreased to 90%; this action will decrease the energy consumption of the column. A vapor permeation module equipped with a hydrophobic membrane is placed on the top distillation stream. The aim of the membrane separation system is to recover the acetic acid in this stream and thus to reach the organic loss rate specification. The permeate stream is recycled to the distillation column. The retentate stream becomes the water outlet of the hybrid process. In the column, only the reflux is condensed, and the top stream phase is gaseous. The advantage of this configuration is a decrease in both the heat duty and the condensation duty of the column. The simulation is performed with Aspen Plus software for the hybrid process. A design specification is applied to the distillation column to reach the acetic acid composition at the bottom stream. The column feed stage of each stream is optimized to minimize the energy consumption of the column. Because the process is isothermal, only one module is required to achieve the process specifications. 3.4.3. Results and Discussion. Acetic acid permeance is varied from 1 to 23 kg/(h·m2·bar), membrane selectivity is varied from 10 to 650, and surface membrane size is varied from 1000 to 5000 m2. The comparison between the conventional process and the hybrid process with different values of membrane performances can be found in the Supporting Information. The results of the simulation show that the operating cost of the vapor permeation module is negligible compared with that of the distillation column. Moreover, energy savings are dependent on the acetic acid loss rate because the higher the acetic acid loss rate is, the lower the energy used in the conventional process is. Simulations showed that the criteria for industrial application of the vapor permeation process can be reached. Because the permeate stream is recycled to the column, the quantity of acetic acid that remains in the retentate is a crucial factor for the acetic acid loss rate. Thus, the selectivity and surface of the membrane are important factors. Nevertheless, because the purity of the acetic acid outlet is fixed by the column, the hybrid process easily reaches this specification. The limit is the acetic loss rate at the retentate (water outlet of the process). In all cases, there is a limit of the selectivity value depending on the surface membrane. Table 4 shows these limits.

Figure 7. Comparison of membrane performances that can be used on an industrial scale depending on the acetic acid outlet composition and membrane performances of published material in the case of pervaporation with a hydrophilic membrane.

have been published in the literature could be used to achieve an acetic acid composition of 99% as the objective. In Figure 7, commercially available membranes whose performances have been published in the literature have been plotted with filled symbols.19 The water permeances of these membranes are large enough to be used on an industrial scale, but their selectivities are too small. For industrial use of pervaporation with these study process specifications, commercial membrane performances must be improved in terms of selectivity. In Figure 7, membranes that have been produced at the lab scale and whose performances have been published in the literature (cf. Figure 2) are represented by open symbols. Some membrane performances9,18 have response surfaces corresponding to an acetic acid outlet composition of 0.99 and a net energy gain (NEG) of 10%. Efforts must be made to develop these types of membranes and improve their stabilities for development on an industrial scale. 3.4. Hydrophobic Pervaporation Membrane. 3.4.1. Potential of the Hybrid Process. This case study is a hybrid process coupling distillation and vapor permeation with a hydrophobic membrane. The scheme of this hybrid process is shown in column a, first row of Table 2. The permeate stream must be recompressed to be recycled to the column, and it could be interesting to compress the distillate stream to increase the driving force transfer and thus reduce the surface of the membrane as well as to overcome the pressure loss of the membrane module. Neither pressure modification has been taken into account. The temperature of the feed will depend on the pressure, and it must be superheated to prevent partial condensation. A heat exchanger is placed on the distillate stream to overheat the vapor and to avoid condensation in the membrane module, which could lead to a decrease of the separation performance. The following three parameters are varied: membrane surface, acetic acid permeance, and acid selectivity. For each pair of performances, Aspen Plus is used to calculate the acetic acid loss in the retentate, the energy of the column, and the energy of the conventional process (distillation column) with the same acetic acid rate loss. As was done in section 3.3, these data are used in the process specification of a conventional process (distillation column). In this way, the hybrid process

Table 4. Minimum Values of Selectivity and Acetic Acid Permeance by Membrane Surface membr surf. (m2)

min selectivity

min acetic acid permeance

3000 4000 5000

160 160 160

21 17 13

A comparison of the membrane performances that can be used in industrial applications and performances that have been published in the literature is shown in Figure 8. The comparison of minimum membrane performances with published material shows that membranes that have been developed have performances far too small to be used on an industrial scale. 7775



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APPENDIX A Because water and acetic acid are polar molecules, the liquid phase of the mixture is highly nonideal. A thermodynamic model is required to calculate the liquid activity coefficients required to calculate the driving forces of the membrane separation processes. The nonrandom two-liquid (NRTL) model is used.35 The equation for the NRTL model is as follows: ln γi =

∑j xjτjiGji ∑k xkGki

+

∑ j

⎛ ∑ x τ G ⎞ ⎜⎜τij − m m mj mi ⎟⎟ ∑k xkGkj ⎠ ∑k xkGkj ⎝ xjGij

(15)

For Tlower < T < Tupper Gij = exp( −αijτij) Figure 8. Comparison of membrane performances that can be used on an industrial scale depending on acetic acid outlet composition and membrane performances of published material using vapor permeation with a hydrophilic membrane.

τij = aij +

bij T

(16)

+ cij ln T + fij T

(17)

αij = cij + dij ln(T − 273.15)

(18)

In addition, acetic acid is a carboxylic acid that can dimerize in the vapor phase, which results in the strong nonideality of the vapor phase. This phenomenon must be taken into account. A predictive model for acid vapor phase dimerization is chosen. The Hayden−O’Connell (HOC) correlation is used to predict the behavior of carboxylic acids in the vapor phase.46 Model parameters taken from the Aspen Properties databank are used. They are summarized in Tables 5 and 6.

4. CONCLUSION In this work, a method used to calculate minimum membrane performances for industrial application of the pervaporation or vapor permeation process has been developed. The determination of the minimum membrane performances is done by rigorously comparing hybrid and conventional processes. The modeling of the hybrid process is performed with Aspen software by incorporating a custom model for the membrane module. Using this method, the distillation column can be simulated, the model for the flux calculation can be selected, and the energy required of reheaters and vacuum pumps can be taken into account. The method has been applied to the dehydration of acetic acid. The results showed the following: (i) Pervaporation can improve distillation for acetic acid/ water separation. Energy gain up to 20% is achievable in principle. (ii) Two different process design strategies have been investigated: • hydrophilic membrane in a pervaporation module placed on the bottom stream of a distillation column • hydrophobic membrane in a vapor permeation module placed on the top stream of a distillation column (iii) Current materials offer promising performances for hydrophilic membranes, but no acidophilic material reaches the target. The modeling methodology presented in this work requires further clever improvements to better take into account the influences of temperature and feed composition on membrane performances. The flux coupling effect could also be taken into account. Moreover, the simulation strategy of the pervaporation process could be improved by considering different types of membrane and an adapted reheating strategy for each stage. The next step is a technical and economic study based on this work to better understand the impact of CAPEX on the choice of process type.

Table 5. Temperature-Dependent NRTL Interaction Parameters for the Binary Water/Acetic Acid Mixture component i component j databank source aij aji bij [K] bji [K] cij dij dji τii Gii Tlower (°C) Tupper (°C)

water acetic acid APV72 VLE-HOC 3.3293 −1.9763 −723.888 609.8886 0.3 0 0 0 1 20 230

Table 6. Hayden−O’Connell Binary Parameters for the Water/Acetic Acid System water acetic acid

water

acetic acid

1.7 2.5

2.5 4.5

The model was validated by comparing the vapor/liquid equilibrium (VLE), bubble temperatures, and dew point temperatures simulated with Aspen Properties software and experimental data published in the Dortmund Data Bank (DDB). The results are presented in Figures 9 and 10. In these figures, the NRTL-HOC model properly represents the VLE, bubble temperature, and dew point of the binary water/acetic 7776



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APPENDIX B

A study using Aspen Properties software on the vapor/liquid equilibrium pressure is performed to determine the minimum temperature of condensation. In Figure 11, the evolution of the bubble and dew temperatures for the binary water/acetic acid system at 30 mbar is shown.

Figure 9. Isobaric VLE for the binary water/acetic acid system at 1 atm: (×) experimental data from the DDB and () the NRTL-HOC model.

Figure 11. Bubble and dew temperatures for the binary acetic acid/ water system at 30 mbar.

The minimum condensation temperature is 24 °C. At this temperature, it is possible to use cold water to condense the vapors. This pressure corresponds to a primary vacuum, which is easily obtainable in industry. The pressure of the permeate is fixed at 30 mbar.

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ASSOCIATED CONTENT

S Supporting Information *

Tables for the study of the effect of membrane performance on the cost of the hybrid process for a composition of acetic acid at the output of the hybrid process of 99.4% by weight. This material is available free of charge via the Internet at http:// pubs.acs.org.

Figure 10. Isobaric bubble temperature and dew point for the binary water/acetic acid system at 1 atm: (×) experimental data from the DDB and () the NRTL-HOC model.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

acid system. The model parameters can be used to calculate the liquid activity. The Antoine equation was used to calculate the vapor pressure Psat i of each component i: ln Pisat = c1i +

c 2i + c4iT + c5i ln T + c6iT c7i T + c 3i

Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS The authors want to thank Solvay for financial and technical support.

(19)

The coefficients c1i, c2i, c3i, c4i, c5i, c6i, c7i, c8i, and c9i were taken from the Aspen Properties software databank, and values for c1i−c7i are presented in Table 7.

NOMENCLATURE E = power (kW) EA = activation energy (kJ/mol) Eb = power needed in distillation boiler (kW)

Table 7. Antoine Coefficients for Each Pure Component (K) component

c1i

c2i

c3i

c4i

c5i

c6i

c7i

Tlower

Tupper

water acetic acid

73.65 53.27

−7258.2 −6304.5

0 0

0 0

−7.3037 −4.2985

4.1653 × 10−6 0

2 0

273.2 289.8

647.1 592.0

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f = fugacity hl = liquid enthalpy (kJ/kmol) J = flux (kg/h·m2) l = membrane thickness (m) n = molar flow rate (kmol/h) p = partial pressure (bar) Psat = saturated vapor pressure 7 = permeability (kg·m/(h·m2·bar)) Q = permeance (kg/(h·m2·bar)) R = universal gas constant (SI units) T = temperature (°C or K) x = liquid mole fraction y = vapor mole fraction w = mass fraction Greek Symbols

γ = liquid activity coefficient γgas = adiabatic gas expansion coefficient η = vacuum pump efficiency Subscripts and Superscripts

atm = atmosphere (unit of pressure) conv = conventional F = feed stream HE = heat exchanger i = component i of the mixture N = submodule number N NEG = net energy gain ref = temperature reference Ret = retentate stream Perm = permeate stream vp = vacuum pump

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