Esterification of Acrylic Acid and n-Butanol in a Pilot-Scale Reactive

Article pubs.acs.org/IECR

Esterification of Acrylic Acid and n‑Butanol in a Pilot-Scale Reactive Distillation ColumnExperimental Investigation, Model Validation, and Process Analysis Alexander Niesbach, Ron Fuhrmeister, Tobias Keller, Philip Lutze,* and Andrzej Górak Department of Biochemical and Chemical Engineering, Laboratory of Fluid Separations, TU Dortmund University, Emil-Figge-Straße 70, D-44227 Dortmund, Germany S Supporting Information *

ABSTRACT: Due to a high risk of polymerization and complex thermodynamic behavior of the chemical system, the current production process for n-butyl acrylate synthesis is cost-intensive and challenging. Reactive distillation integrates chemical reactions and distillation into one unit at the same time and is one of the best-known examples of process intensification. To facilitate industrial application of this concept for the production of n-butyl acrylate, reliable experimental data are required. This article presents an experimental and theoretical investigation of the synthesis of n-butyl acrylate using reactive distillation. Experiments were conducted in a pilot-scale reactive distillation column, and the decisive operational parameters were varied. To predict the experimental results, a nonequilibrium-stage model was applied and the model was validated using the experimental data. The validated model was then used to perform a process analysis, showing trends in the conversion of acrylic acid and nbutanol and the purity of n-butyl acrylate that can be used for prospective optimization studies.

1. INTRODUCTION During the last years, acrylic acid and its derivates are increasingly demanded due to their use as building blocks in the chemical synthesis.1 From 2003 to 2006, the overall world production of acrylic acid has increased from 3.4 million1 to 4.7 million tons per year.2 Furthermore, for the period from 2010 to 2015, an increase in the global demand of acrylic acid of 4.8% per year3 was predicted. Acrylic acid can be converted to butyl acrylate (BA). This reaction accounts for about 30% of its global demand.4 Areas of application for n-butyl acrylate are for example the production of adhesives, varnishes, papers, and textiles.5 To produce n-butyl acrylate, acrylic acid and n-butanol are esterified in an equilibrium limited reaction to n-butyl acrylate and water. The resulting chemical system possesses a complex thermodynamic behavior and a high risk to polymerize, which makes the production and separation of n-butyl acrylate very challenging. The currently applied process is homogeneously catalyzed and uses two reactors.1 The separation of n-butyl acrylate and the recovery of the reactant needs three distillation columns attached after the reactors.1 Reactive distillation (RD) is one of the best known examples of process intensification. In an RD column, the chemical reaction and separation by distillation is combined in one unit operation. Hence, the required amount of equipment is decreased which also results in reduced investment and operating costs. The combination of reaction and separation by distillation helps to overcome limitations of both operations, such as azeotropes or reaction equilibria. RD is applied in various industrial processes and has been the focus of a variety of publications, which are summarized in the reviews of Hiwale et al.6 and Sundmacher et al.7,8 Thus far, RD is not applied for the production of n-butyl acrylate and only a few publications can be found on this topic. The reaction kinetics and the © 2012 American Chemical Society

chemical equilibrium of the heterogeneously catalyzed esterification of acrylic acid and n-butanol were experimentally investigated by Schwarzer and Hoffmann.9 They theoretically studied an RD column using an equilibrium-stage model and a catalytic tube reactor using their own kinetic data. Zeng et al.5 theoretically investigated the production of n-butyl acrylate with a single RD column. On the basis of kinetics measured by Schwarzer and Hoffmann,9 they studied the design and control of the column. Within their study, it was clearly shown that the use of a reactive distillation column for the synthesis of n-butyl acrylate is beneficial, as n-butyl acrylate concentrations of more than 99.8 mol-% can be achieved in a single reactive distillation column. As a result, they provided a control strategy for an industrial RD column for the production of high purity n-butyl acrylate. Keller et al.10 performed simulations with a nonequilibrium-stage model. Using the kinetic data of Schwarzer and Hoffmann,9 they simulated a pilot-scale RD column for the production of n-butyl acrylate to prove the technical feasibility of this process. Nevertheless, the production of n-butyl acrylate in an RD column was only studied on a theoretical basis by all of these publications. To date, only one experimental investigation of the inhibition of acrylic acid and n-butyl acrylate polymerization in an RD column has been published.11 As a result of the high polymerization tendency of acrylic acid and n-butyl acrylate, the effects of temperature, the atmospheric composition, and the inhibitor concentration on the minimum time until significant polymerization occurs were investigated. The polymerization is highly exothermic, and therefore, the inhibition of this polymerization is important for productionReceived: Revised: Accepted: Published: 16444

July 20, 2012 October 18, 2012 November 21, 2012 November 21, 2012 dx.doi.org/10.1021/ie301934w | Ind. Eng. Chem. Res. 2012, 51, 16444−16456

Industrial & Engineering Chemistry Research

Article

Scheme 1. Reaction Scheme of the Esterification of Acrylic Acid and n-Butanol

Table 1. Pure Component Boiling Points at p = 1.013 bar and Calculated Pure Component Boiling Points at 0.267 bar using Aspen Plus component

IUPAC name

formula

CAS number

Tb (K)

ref

Tb,sim (K)

AA BA BuOH water

2-propenoic acid 2-propenoic acid, butyl ester 1-butanol water

C3H4O2 C7H12O2 C4H10O H2O

79-10-7 141-32-2 71-36-3 7732-18-5

414 421 391 373

15 15 15 15

376 377 357 340

Table 2. Azeotropic Data of the System at p = 0.267 bar components AA BA BuOH BuOH BuOH a

BA water BA water BA

reference

water

cal. 16 16 16 16

a

type

Tb (K)

xAA

xBA

homogeneous heterogeneous homogeneous heterogeneous heterogeneous

379 335 356 335 333

0.3658

0.6342 0.1840 0.1150 0.0860

xBuOH

xwater 0.8160

0.8850 0.1930 0.1540

0.8070 0.7600

Calculated using UNIQUAC-HOC.

46 (Rohm and Haas) was selected for n-butyl acrylate synthesis.12 Amberlyst 46 consists of a styrene-co-divinylbenzene matrix with active sulfonic acid groups at the outer surface that suppress the formation of undesired byproducts, such as ethers of olefins.13,14 Amberlyst 46 shows a thermal stability up to temperatures of 393 K.14 A main challenge of this system is the tendency of acrylic acid and n-butyl acrylate to polymerize. In preliminary work,11 a concept for inhibition of polymerization in a pilot-scale RD column was developed. The influence of temperature, the inhibitor concentration, and the atmospheric composition on the inhibition period of acrylic acid and n-butyl acrylate were experimentally investigated.11 The temperature was varied from 373 to 398 K, the expected temperature range for the pilotscale RD column during the experimental investigation. Phenothiazine and hydroquinone monomethyl ether were used as inhibitors, and concentration was varied on the basis of an analysis of the expected residence time distributions in the RD column. Two different atmospheres (air and nitrogen) were investigated; inhibition in air was more efficient when using a mixture of both inhibitors. In addition to these experiments, the liquid−liquid separation was investigated, and startup and shutdown procedures for pilot-scale RD experiments were developed.11 The boiling points of the pure components and azeotropic mixtures are important properties when using distillation and RD columns. The boiling points of the pure components at atmospheric pressure and calculated boiling points using Aspen Plus at 0.267 bar are listed in Table 1. As illustrated in the table of azeotropic data (Table 2), the chemical system shows complex thermodynamic behavior. In total, five azeotropes exist in the system, two homogeneous and three heterogeneous azeotropes. The low boiler of this system is the ternary heterogeneous azeotrope consisting of water, nbutanol, and n-butyl acrylate. The heavy boiler of this system is the binary homogeneous azeotrope of acrylic acid and n-butyl acrylate. This binary azeotrope was not found by Gmehling,16

process reliability. An inhibitor dosage concept that allows an experimental investigation of the synthesis of n-butyl acrylate in a pilot-scale RD column was developed and is used in this study. To facilitate the industrial application of the RD concept for the production of n-butyl acrylate, reliable experimental data is necessary. A set of experimental data, varying the decisive operational parameters, allows for a validation of RD models that can be used in process design studies and process optimization to enable the industrial use of this technology. This article presents a set of reliable experimental composition and temperature profiles for the synthesis of n-butyl acrylate in a pilot-scale RD column. In preliminary studies,11 the polymerization behavior of acrylic acid and n-butyl acrylate was investigated and a concept was developed to allow for an experimental investigation of this system. Within this paper, an experimental design was developed, and experiments varying the decisive operational parameters, distillate-to-feed mass ratio, reflux ratio, molar feed ratio, and top pressure of the column, over a broad range are presented. A nonequilibrium-stage model developed in Aspen Custom Modeler was used to predict experimental results, and a comparison of these experimental results with simulated results is presented. Finally, the validated model allows a variation of the process parameters. On the basis of both experimental and simulated results, tendencies for conversion and purity of n-butyl acrylate, which can be used in future optimization studies, are shown.

2. CHEMICAL SYSTEM The chemical system investigated in this study is the heterogeneously catalyzed esterification of acrylic acid (AA) and n-butanol (BuOH) into n-butyl acrylate (BA) and water, as shown in Scheme 1. In the course of our experimental investigation, at the Laboratory of Technical Chemistry B at TU Dortmund University, a catalyst screening was first performed. During this screening, the strongly acidic ion-exchange resin Amberlyst 16445

dx.doi.org/10.1021/ie301934w | Ind. Eng. Chem. Res. 2012, 51, 16444−16456


Article

4. EXPERIMENTAL INVESTIGATIONS 4.1. Column Setup. The experimental investigation of the synthesis of n-butyl acrylate and water from acrylic acid and nbutanol was performed in a pilot-scale RD column. A scheme of the used column is shown in Figure 1. The column has a

who observed a zeotropic behavior at 0.267 bar. In contrast, the simulations by Zeng5 using NRTL-HOC and by us using UNIQUAC-HOC estimate a homogeneous azeotrope, which is shown in Table 2. Due to the high boiling point and the composition, this azeotrope causes a more complex process design. Ignoring it during the simulation and an upcoming optimization of the process, although it was estimated using two different gE-models, would lead to insufficient n-butyl acrylate purity in a real process at the bottom of the column, if it exists. By taking this azeotrope into account, the necessary purity of n-butyl acrylate is guaranteed either way. Therefore, it was considered during the process simulations.

3. MATERIAL AND METHODS 3.1. Analytical Methods. Concentrations of n-butanol, nbutyl acrylate, and acrylic acid were analyzed using a Shimadzu GC-14B gas chromatograph (GC). An autosampler (AOC 20i) and a flame-ionization detector (FID) were attached to the GC. An FFAP capillary column (Innopeg) with a length of 25 m and an inner column diameter of 0.32 mm was used. The inner surface consisted of a polyethylene glycol film, which was fractional coated with 2-nitro-terephthalate. Helium was used as a carrier gas, with a velocity of 27 cm s−1. The analysis was performed using a temperature program, beginning at 403 K, which was maintained for 6 s. At a rate of 30 K min−1, the temperature was increased to 483 K and maintained for another 2 min and 40 s. Single-component calibration curves were developed prior to analyzing the samples and used to determine the mass fractions of each component. The internal standard used for the analysis was acetonitrile. Test mixtures of known compositions were continuously analyzed to verify the accuracy of the calibration curves. To demonstrate the high reliability of the analysis, the absolute deviations and the corresponding standard deviations of the GC analysis of the calibration curves are illustrated in Table 3. Table 3. Average Relative Deviation Δrel of Weight Fractions and Corresponding Standard Deviation of GC Analysis of Calibration Curves for Three Organic Components component

BuOH

BA

AA

Δrel (%) s(Δrel) (%)

0.10 0.08

0.08 0.07

0.13 0.08

Acetonitrile had a retention time of 2 min and 30 s, followed by n-butanol with 3 min and 5 s, n-butyl acrylate with 3 min and 20 s, and acrylic acid with 6 min. Each sample was analyzed three times, and the average value was used in further investigations. The Karl Fischer titration method was used for the analysis of the water concentration with a Mettler-Toledo V30 titrator. 3.2. Chemicals. Acrylic acid and n-butanol were supplied from BASF. The purity was ≥99.5 wt % for acrylic acid and ≥99.0 wt % for n-butanol. n-Butyl acrylate was obtained from Merck with a guaranteed purity of ≥99.0 wt %. Acrylic acid and n-butyl acrylate contained 200 ppm of the inhibitor hydroquinone monomethyl ether. The inhibitors phenothiazine, with a purity of ≥99.0 wt %, and hydroquinone monomethyl ether, with a purity of ≥98.0 wt %, were provided by Merck. Acetonitrile, used as internal standard for the GC analysis, was purchased from VWR International with a guaranteed purity of ≥99.9 wt %.

Figure 1. Scheme of pilot-scale RD column used for experimental investigation of n-butyl acrylate synthesis from acrylic acid and nbutanol.

nominal diameter of 50 mm and is made of glass. The process control was done using a Siemens PCS 7 (Siemens AG) process control system. The total height of the column is 12 m, 5.7 m of which is used for structured packings, 6.3 m for liquid distributors, the total condenser, the reboiler, and other peripherals. The column was partitioned into six sections (Figure 1). Sulzer BX separation packings were used for the two sections at the top and at the bottom of the column. The two central sections contained the reactive packing Sulzer Katapak SP-11 with the immobilized catalyst Amberlyst 46. The sections were plugged using liquid distributors, which were 16446



Article

used to minimize nonideal flow conditions, such as wall effects and poor liquid distribution. The distributors allow connection of a feed pipe, gathering of a side stream, to measure the temperature in the gas phase using PT100 thermocouples (TST 42, Endress + Hauser Ltd.) and to take samples of the liquid phase. Samples were taken from all distributors except the two feed stages, because accurate sampling was not possible at these stages because of configurational issues. To maintain adiabatic operation of the column during the experimental investigation, the column is isolated with three layers: one layer of mineral wool, an electric heating device, and a second layer of mineral wool. The set point of each electric heating device was determined using the mean value of the temperatures obtained from the liquid distributors below and above each section. A pressure transducer (DMU01, Afriso-Euro-Index Ltd.) was used to measure the pressure at the top of the column. The pressure drop along the column was measured using an U-tube manometer. A natural circulation evaporator with a volume of 1.3 l and a maximum electric heat duty of 6.6 kW was applied at the bottom of the column. A heat exchanger with MARLOTHERM heat transfer fluid was used to heat the evaporator. The collection of the bottom product was done by an overflow. A total condenser was attached at the top of the column, where water was applied as a coolant. After subcooling the condensate in a second heat exchanger, it was split into a distillate stream and a reflux stream. The reflux ratio and the mass flow rate of the distillate were adjusted by the use of two separate pumps. Mass balances (Sartorius Industry MC1, Sartorius AG) were used to monitor the mass flow rate of the distillate via the time derivative of the change in mass. A Coriolis-type flow meter (RHM 015, Rheonik Ltd.) was applied to measure the reflux flow rate. The reflux stream was preheated to a temperature of 10 K below its boiling point by a thermostat (SE 6, Julabo Ltd.) before it was fed to the column. A peristaltic pump was installed above the condenser to feed the inhibitor solution into the column. Two cooling traps with liquid nitrogen at 77 K as a coolant and a second gas phase condenser were installed above the total condenser for safety. To operate the column under reduced pressure a vacuum pump (Ilmvac LVS910Z-ef) was attached at the top of the column. To accurately adjust the column pressure, the speed frequency of the pump was continuously controlled. Membrane pumps (Prominent Gamma g/4b) were used to feed the reactants from receiver tanks. The mass flow rate was controlled using mass balances (Sartorius Industry MC1, Sartorius AG) by monitoring the time derivative of the change in mass. n-Butanol is the light boiling feed and was fed into the column at a height of 1.3 m, below the reactive section, while the acrylic acid, the heavy-boiling component, was fed into the column at a height of 3.5 m, above the reactive section. Both feeds were fed at temperatures 10 K below their boiling points. They were heated up using thermostats (Haake N6, Thermo Fischer Scientific Ltd.). All relevant characteristics of the pilot-scale RD column are summarized in Table 4. 4.2. Polymerization Inhibition. During experiments in the RD column with acrylic acid and n-butyl acrylate, inhibitors must be well distributed over the whole column height and acrylic acid and n-butyl acrylate must be in contact with inhibitors at all times.11 On the basis previous considerations,11

Table 4. Characteristics of Pilot-Scale RD Column column diameter position of acrylic acid feed position of n-butanol feed inhibitor solution feed height of rectifying section height of reactive section height of stripping section mass of dry catalyst per meter reactive packing condenser type reboiler type operating top pressure range

50 mm 3.5 m 1.3 m above condenser 2.2 m 2.2 m 1.3 m 0.205 kg m−1 total naturally circulating evaporator 0.3−0.4 bar

the inhibitor was fed at two different positions: at the top of the column and with the acrylic acid feed. For the feed at the top of the column, inhibitors were dissolved in n-butanol and fed via a customized distribution element above the condenser. A concentrated n-butanol inhibitor solution was prepared and introduced using a peristaltic pump. The necessary concentration of inhibitor to prevent polymerization during the RD experiment was found to be 1000 ppm for the entire column.11 The inhibitor solution at the top of the column contained 2.0 wt % phenothiazine and 2.0 wt % hydroquinone monomethyl ether. Because the feed of acrylic acid was preheated before it entered the column, there was a risk that polymerization might occur, blocking the feed pipe. To minimize the amount of acrylic acid polymer formed, 1000 ppm phenothiazine was added to the acrylic acid feed tank.11 4.3. Procedure. The startup and shutdown of an RD column is challenging, especially when working with polymerizing components. Many papers in recent years have introduced strategies for column startup, especially for RD columns with heterogeneous catalystsso-called “catalytic distillation” columns.17−21 In past work at the Laboratory of Fluid Separations, a start-up strategy for the pilot-scale catalytic distillation column used in this study was developed by Keller et al.22 for the synthesis of n-propyl propionate. However, because of the unique requirements of a system with a high risk of polymerization, a modified startup and shutdown procedure was developed and introduced in an earlier publication.11 Each experiment was performed until a steady state was reached (in average 11 h). The steady state was defined by checking several conditions, according to Keller et al.:22 • The temperature deviation at each measured location is below ±0.5 K. • The reaction stoichiometry and overall mass and component balances in the RD column are fulfilled. When a steady-state column operation was presumed, it was checked using data reconciliation, as described in the Supporting Information. After confirming the steady-state condition, samples were taken from the liquid distributors (except the feed distributors) and from the distillate and bottom streams. The samples were analyzed using gas chromatography and Karl Fischer titration (see section 3.1) to gain a detailed analysis of the composition. Three profiles were taken in 60-min intervals during the steady-state operation to generate reliable experimental data with high accuracy. 16447



Article

5. MODELING AND SIMULATION First attempts of tray to tray modeling of distillation processes were made by Berman et al.23 and were carried out by hand. Since then, much progress has been made in model design and computer technology. The development of models describing conventional distillation processes has been summarized in several textbooks.24,25 The modeling of RD processes has evolved from classical distillation models and was described by Taylor26 and Sundmacher.7 Two basic categories of RD models are equilibrium-stage (EQ) and nonequilibrium-stage (NEQ) approaches. In EQ modeling approaches, it is assumed that exiting vapor and liquid streams of each stage are in thermodynamic equilibrium, often in contrast to the real behavior of the system. Nonequilibrium-stage models take mass and energy transfer in account in a more detailed way, considering the actual transport rates. Furthermore, they account for the hydrodynamics of the column internals,26 to increase the accuracy of the simulation results. In this work, we use a nonequilibrium-stage model to describe the RD column.27−29 The hydrodynamic behavior of the column interior was determined using packing-specific calculations developed by Rocha30 and Bravo31 for Sulzer BX separation packing and by Brunazzi32 for Sulzer Katapak SP-11 catalytic packing. Mass and heat transfer was described using the two-film theory,33 where the thermodynamic equilibrium is assumed to appear only at the interface (in contrast to the EQ approach). The mass transfer rate between phases was calculated using effective diffusion coefficients,34,35 while the heat transfer was determined using the Chilton−Colburn analogy.36 The UNIQUAC model37 was used to account for nonideal behavior in the liquid phase, and the Hayden O’Connell equation of state was applied to account for nonidealties in the gas phase (see section 5.1). The heterogeneously catalyzed reaction was described using a Langmuir−Hinshelwood− Hougen−Watson (LHHW)38,39 approach. Further details are given in section 5.2. An axial discretization was conducted, and the height of each discrete was based on the height equivalent to a theoretical plate (HETP) value of the packing used (Table 5). In preliminary simulation studies, it was found that the use

The activity coefficients of all components in the liquid phase were calculated using the UNIQUAC model.37 Nonidealties in the gas-phase, such as the self-association of acrylic acid, were taken into account using the Hayden O’Connell equation of state40 to calculate vapor-phase fugacity coefficients. The Hayden O’Connell equation of state was chosen as it is used for carboxylic acids with the tendency of dimerization at moderate pressure conditions.40 The set of binary interaction parameters used in calculations is summarized in Table 6. Table 6. UNIQUAC Binary Interaction Parameters component 1

n-butanol

water

acrylic acid

n-butyl acrylate

n-butanol

n-butanol

acrylic acid

water

n-butyl acrylate

n-butyl acrylate

acrylic acid

i

j 1 2 1 2 1 2 1 2 1 2 1 2

aij (−) 2 1 2 1 2 1 2 1 2 1 2 1

−3.5 2.5 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

bij (K) 1070.7 −1007.1 −425.9 240.8 59.5 −144.6 −336.6 297.0 −199.2 −289.9 −219.8 130.3

The binary interaction parameters water/n-butanol,41 water/ acrylic acid,42 n-butanol/acrylic acid,42 and n-butyl acrylate/nbutanol43 were taken from the Aspen Plus database and were checked against literature data. No data were available in the literature for the water/n-butyl acrylate and n-butyl acrylate/ acrylic acid systems. Therefore, the parameters of these binary systems were predicted using the UNIFAC approach.44 5.2. Reaction Kinetics. Several studies of the reaction equilibrium (e.g., refs 45 and 46) and the reaction kinetics (e.g., refs 9 and 47−50) of n-butyl acrylate synthesis from acrylic acid and n-butanol are available. However, none of these articles include kinetic data for the ion-exchange resin Amberlyst 46. Experiments to determine the equilibrium constant and reaction rate were performed at the Laboratory of Technical Chemistry B12 and are summarized in this study. For the mathematical description of the reaction, an activity-based Langmuir−Hinshelwood−Hougen−Watson (LHHW) approach was used. The reaction rate of component i is described as follows:38,39

Table 5. Details on Axial Discretization in the NEQ Model height of one discrete HETP of Sulzer BX HETP of Sulzer Katapak SP-11

component 2

water

HETP/4 0.14 m 0.5 m

ri =

dni = νimcat,dry cact dt ⎛ −E k 0 exp RT(Ka ) ⎜aAA aBuOH − ⎝

(

of four discretes per stage is sufficient. The use of more discretes significantly increases the computational effort, whereas the results remain nearly constant. A detailed description of the model can be found in the paper of Klöker et al.27 The commercially available simulation environment Aspen Custom Modeler was used for all calculations. 5.1. Thermodynamic and Physical Properties. The accuracy of simulation results depends heavily on reliable thermodynamic and physical data. The thermodynamic data in this work were calculated using Aspen Plus. Aspen Custom Modeler uses an exported Aspen Plus file to access the calculated property data. The models used to calculate physical properties within Aspen Plus are summarized in the Supporting Information.

)

⎞ 1 ⎟ a a K eq BA water ⎠

(1 + K1aAA + K 2aBuOH + K3aBA + K4a water)2

⎛ −1888.66 ⎞ Keq(T ) = exp⎜ + 8.17⎟ ⎝ T (K ) ⎠

(1)

(2)

The kinetic expression includes the stoichiometric coefficient vi, the dry catalyst mass mcat,dry, and the concentration of active sites cact. The dry catalyst mass for the Sulzer Katapak-SP11 packing with Amberlyst 46 is mcat,dry = 0.205 kg/m, as determined in earlier work by Buchaly.51 The concentration of active sites, measured according to the German industry standard DIN 54403,52 was cact = 0.79 equiv/kg. The temperature dependency of the equilibrium constant was 16448



Article

formation of two liquid phases that cause a high polymerization risk due to the distribution coefficients of acrylic acid, n-butyl acrylate, and the inhibitors11 and the hydrodynamic behavior of the column internals.30−32 The distillate-to-feed mass ratio was varied between 0.30 and 0.40, the reflux ratio was varied between 0.75 and 2.00, the top pressure was adjusted in the range of 0.3−0.4 bar, and the molar feed ratio was adjusted between 1 and 3 mol BuOH to 1 mol AA. For all experiments except experiment E11, which was used to enlarge the experimental database, the molar feed ratio, χBuoH,AA, was set to either 2 or 3 to increase the acrylic acid conversion and thereby decrease the risk of polymerization. Except for experiment E9, which was performed with a total feed flow rate of 2.9 kg/h, all experiments were performed with a total feed flow rate of 3.8 kg/h. Experiments E2, E7, and E12 used the same operating conditions and were performed to test whether the operating conditions in the RD column were stable and the experiments were reproducible, as well as whether catalyst deactivation occurred during the experimental investigation. Catalyst deactivation was additionally investigated in preliminary batch experiments for which the stability of the catalyst was determined using long-term capacity analysis. As after a first deactivation of rejuvenated catalyst a steady-state was reached for the catalyst capacity and the experiments were performed with catalyst in steady-state condition; a constant concentration of active sites was assumed. The experimental results of all experiments are summarized in the Supporting Information. The experimental operating conditions are summarized in Table 8. 6.2. Results of the Experimental Investigation. Within the experimental investigation, 12 pilot-scale RD experiments were performed that satisfied all criteria needed for data reconciliation. A description of the applied data reconciliation and detailed data reconciliation results for one experiment are shown in the Supporting Information. After a steady state was reached, three column profiles were taken in 60-min intervals. A typical composition and temperature profile for an RD experiment is shown in Figure 3. In the concentration profile shown in Figure 3, the experimentally determined mole fractions of n-butanol, acrylic acid, n-butyl acrylate, and water are shown. Excess n-butanol led to high concentrations observed over the entire column. As already mentioned in section 5.1, the system shows azeotropic behavior with a light boiling heteroazeotrope consisting of nbutanol, water, and n-butyl acrylate. This azeotrope accounts for the high amount of n-butyl acrylate in the distillate stream and the lower concentrations of BA at the bottom of the column in this experiment. The temperature profile of the vapor phase on the right side of Figure 3 shows a maximum above the reboiler and a second maximum at the feed stage of acrylic acid, which was fed 10 K below its boiling point. 6.3. Reproducibility. To determine stable operating conditions for the RD column and whether catalyst deactivation occurred in the reactive section, experiments E2, E7, and E12 were performed under the same operating conditions. The experiments were performed at the beginning, middle, and end of the experimental investigation. The profiles of all three experiments are comparable. Small deviations mostly result from slightly different operating conditions (see Table 8). The deviations of the mole fractions of the components at a single sample point are within the analytical error of the gas chromatography in most cases.

taken into account using the van’t Hoff equation, and the temperature dependency of the reaction rate was described using an Arrhenius approach. The other constants were determined as summarized in Table 7 using the experimental data. Table 7. Constants for the Calculation of Reaction Rate for n-Butyl Acrylate Synthesis Determined by the Laboratory of Technical Chemistry B at TU Dortmund University12 constant

value

unit

k0 Ea K1 K2 K3 K4

3.41 × 1010 81260 0.244 0.748 2.105 2.288

mol/(eq(H+) s) J/mol

6. EXPERIMENTAL RESULTS 6.1. Experimental Design and Catalyst Deactivation. To validate the nonequilibrium-stage model introduced in section 5, a series of RD experiments was carried out in the RD column described in section 4.1. To provide reliable experimental data, 12 RD experiments based on a factorial experimental design were performed. For the RD column shown in Figure 1, five operational parameters were varied: mass-based distillate-to-feed ratio, DF; reflux ratio, RR; molar feed ratio, χBuOH,AA; total feed mass flow, ṁ Feed,Total; and column top pressure, p. The inhibitor solution, a third feed in the system, was always kept in a fixed 1:18 ratio with the total mass flow of the two other feed streams. As this is the first experimental investigation of a heterogeneously catalyzed RD column for the synthesis of n-butyl acrylate, a factorial design of experiments was performed,53 and all four operational parameters were varied. A scheme of the experimental design is shown in Figure 2. The ranges of operational parameters were selected on the basis of the results of simulation studies using the nonequilibrium-stage model described in section 5. The choice was based on several constraints including the maximum operating temperature of the ion-exchange resin Amberlyst 46,14 the

Figure 2. Factorial experimental design varying RR 0.75−2.00; top pressure p 0.3−0.4 (bar); distillate-to-feed mass ratio DF 0.4−0.5; and molar feed ratio χBuOH,AA: 1.0−3.0. 16449



Article

Table 8. Reconciled Molar Flow Rates and Operating Conditions of All Experiments Carried out in the Pilot-Scale RD Column for Synthesis of n-Butyl Acrylate BuOH feeda AA feed ṁ F,BuOH + Inh (kg/h) ṁ F,AA (kg/h) exp 1 exp 2 exp 3 exp 4 exp 5 exp 6 exp 7 exp 8 exp 9 exp 10 exp 11 exp 12 a

molar feed ratio χBuOH,AA (mol/mol)

top pressure p (bar)

reflux ratio RR (−)

distillate-to-feed ratio DF (kg/h)/(kg/h)

distillate ṁ D (kg/h)

bottom ṁ B (kg/h)

objective function

2.908 2.909 2.896 2.905 2.609 2.599 2.921 2.911 1.950 2.580

0.901 0.904 0.903 0.888 1.189 1.203 0.870 0.904 0.904 1.220

3.14 3.13 3.12 3.18 2.13 2.10 3.26 3.13 2.10 2.06

0.400 0.352 0.302 0.400 0.302 0.402 0.352 0.301 0.400 0.300

2.006 1.517 0.955 1.014 0.762 0.744 1.458 1.988 1.996 0.988

0.392 0.460 0.402 0.494 0.494 0.403 0.463 0.504 0.500 0.509

1.495 1.754 1.528 1.873 1.878 1.532 1.755 1.921 1.428 1.933

2.314 2.059 2.271 1.920 1.920 2.270 2.036 1.894 1.426 1.867

0.48 1.51 0.90 2.91 2.78 1.75 9.68 2.21 0.76 9.38

2.010

1.783

1.10

0.351

1.493

0.454

1.721

2.072

4.26

2.899

0.910

3.10

0.351

1.450

0.462

1.759

2.049

2.75

Values include n-butanol fed with the inhibitor solution.

Figure 4. Comparison of the experimental and simulated results of nbutyl acrylate synthesis in a pilot-scale RD column. Symbols represent the experimentally obtained results of experiment E7 and lines represent the simulated results of experiment E7: (left) molar compositions of liquid phase; (right) temperature profile of vapor phase.

Figure 3. Results of pilot-scale RD experiment E2: (left) molar composition of liquid phase. (right) temperature profile of vapor phase.

Therefore, the pilot-scale RD experiments are reproducible, and no catalyst deactivation was observed during experimental investigation.

of Figure 4. However, for the reflux ratios 0.75 and 1.00, a larger deviation was observed. An explanation for the deviation at these reflux ratios is given in section 7.1. The absolute deviation of the acrylic acid conversion, taking all experiments into account, was 1.07%, and the absolute deviation of the nbutanol conversion was found to be 1.82%, illustrating very good agreement of simulated and experimental results. 7.1. Nonlinear Wave Propagation. At lower reflux ratios of 0.75 and 1.00, agreement between simulated and experimental composition and temperatures of the distillate and bottom product was still high, but the profiles show large deviations, especially in the rectifying section. Figure 5 compares experimental and simulation results for experiment E4. The temperature profile and the composition of the distillate and final product agree, with high accuracy. A conversion of 40.0% for acrylic acid and a conversion of 11.7% for n-butanol were determined in the experiment, while the calculated conversions were 38.6% and 12.3% for acrylic acid and n-butanol, respectively. For the other experiments with low reflux ratios, comparable deviations were obtained with respect to the agreement of experimental and simulation data. Large

7. MODEL VALIDATION The simulation of RD processes requires detailed modeling of multicomponent mass transfer, chemical reaction kinetics, and vapor−liquid-equilibria. The nonequilibrium model described in section 5 was used. During the experimental investigation of the RD column, no side products such as di-n-butyl ether or soluble polymers of acrylic acid and n-butyl acrylate were detected. Side product formation was therefore neglected during the simulation with Amberlyst 46 in the pilot-scale RD column. Figure 4 compares the simulated and experimental composition and temperature profiles for experiment E7. Symbols represent the experimentally determined results, and the lines represent the simulated results. Excellent agreement was found between experimental and simulated composition and temperature profiles over the entire column height. These results demonstrate the suitability of a nonequilibrium-stage model for the description of the RD of this chemical system. For reflux ratios above 1.00, the simulations were found to be highly accurate in comparison to the experimental results, showing accuracies comparable to results 16450



Article

Figure 5. Comparison of experimental and simulated n-butyl acrylate synthesis in a pilot-scale RD column. Symbols represent the experimentally obtained results of experiment E4 and the lines represent the simulated results of experiment E4: (left) molar composition of liquid phase; (right) temperature profile of vapor phase.

Figure 6. Comparison of experimental temperature measurements of experiments E2 and E3, close to steady-state conditions.

temperature measurements in experiment E2, comparable to the results of experiments with reflux ratios of 1.50 and 2.00, showed very little variability. In experiment E3, it was observed that while the operating conditions were moving closer to the expected steady-state conditions, at one point the concentration profile of n-butyl acrylatethe wave of n-butyl acrylatewhich should have its maximum in the middle of the rectifying section, shifted toward the top of the column. Together with the wave of n-butyl acrylate, the acrylic acid wave positioned at the lower boundary of the rectifying section, moved upward, increasing the acrylic acid concentration in the rectifying section. These effects are shown in Figure 5. The movement of the waves was experimentally observed due to changes in the temperature profiles and the mass flows of the distillate stream and bottom product. The agreement of the experimental and simulated results for the lower reflux ratios showed deviations for the reflux ratios 0.75 and 1.00, while still showing good accuracy in the calculation of distillate and bottom product concentration and conversions.

differences between the experimental and simulation results were obtained in the rectifying section of the column. Deviations between the experimental and simulated results can be explained by the theory of nonlinear wave propagation. Although much research has been conducted to fully understand RD columns, nonlinear dynamics and control of RD processes have only been addressed marginally.7 Research into modeling and control of RD columns that takes nonlinear dynamic behavior into account was mostly performed in recent years.54−56 One type of nonlinear dynamic behavior appearing in distillation and RD processes is the formation and movement of sharp concentration and temperature profiles along the height of the distillation column, referred to as “nonlinear waves” or “nonlinear wave propagation”. These “constantpattern waves” have been investigated in steady-state and dynamic processes.7,57 The simulations for nonreactive systems were performed using equilibrium or nonequilibrium-stage modeling approaches.58−60 Nonlinear waves in RD columns were investigated by several authors, who have developed models and control strategies for continuous and batch reactive distillation columns.61−64 Because of the position of the three waves in the column and the interaction of these waves, sharp concentration profiles (large variation in concentration over short sections) were calculated for the rectifying section of the column. This should have been the steady-state composition within the experimental investigation. However, these concentration profiles are often very sensitive to small variations in operating parameters, as described by Sundmacher.7 Therefore, complex control structures are necessary to stabilize such waves in pilot-scale lab columns. In the case of an RD column, experimental investigation is even more complex, as equivocality is possible, leading to different column profiles with the same operating parameters. The troubled operation during the experiment close to the steady-state conditions is illustrated in Figure 6, showing the temperature profile of experiments E2 and E3, measured in the highest liquid redistributor directly below the condenser. The temperature measured during experiment E3 showed high variability, although the operating conditions were close to steady-state conditions (steady-state temperature determined with nonequilibrium-stage model is 358 K), whereas the

8. PROCESS ANALYSIS In this section, the validated model is used to determine the effect of varying decisive process parameters on the resulting composition and temperature profiles. This investigation illustrates the influence of the operating parameters on conversion and product purity and indicates a suitable operating window to produce high purity n-butyl acrylate. The investigation was performed with one validated experiment (simulation and experimental results) and compared to a simulation in which one operational parameter is changed but still located within the lower and upper boundaries of the process parameters chosen for the experimental investigation. Within this investigation, the reflux ratio, the distillate-to-feed mass ratio, the molar feed ratio, the top pressure of the column, and the total feed mass flow were varied. 8.1. Reflux Ratio. In the given operation window, the effect of changing the reflux ratio is large compared to other operating parameters. The effect of changing the reflux ratio of experiment E2 from 1.52 to 2.00 is shown in Figure 7. Experiment E2 was performed at a reflux ratio of 1.52, a distillate-to-feed mass ratio of 0.46, a molar feed ratio of nbutanol to acrylic acid of 3.1, a total feed mass flow of 3.81 kg/h, and a column top pressure of 0.35 bar. The simulation (E2*) was conducted at a reflux ratio of 2.0, while all 16451



Article

Figure 7. Comparison of the experimental and simulated results of nbutyl acrylate synthesis in a pilot-scale RD column. Symbols represent the experimentally obtained results of experiment E2 and continuous lines represent the simulated results of experiment E2. The dashed lines represent the simulation results of E2* with a reflux ratio of 2.0: (left) molar compositions of liquid phase; (right) temperature profile of vapor phase.

Figure 8. Comparison of the experimental and simulated n-butyl acrylate synthesis in a pilot-scale RD column. Symbols represent the experimentally obtained results of experiment E8 and continuous lines represent the simulated results of experiment E8. Dashed lines represent the simulation results of E8* with a distillate-to-feed ratio of 0.4: (left) molar compositions of liquid phase; (right) temperature profile of vapor phase.

other operating conditions were unchanged. The concentration profiles show the largest deviations in the reactive section. Because of the high molar feed ratio of 3.1, the mole fraction of n-butanol is relatively high over the entire column. Below the reactive section, only minor changes were observed, with smaller molar fractions of n-butyl acrylate and an increase in the mole fraction of acrylic acid. In the reactive section and rectifying section of the column, the changed reflux ratio results in lower n-butyl acrylate and acrylic acid concentrations and an increase in the concentration of n-butanol. These low concentrations result from the increased liquid load on the column that results from the higher reflux ratio. The recycled distillate stream contains a high amount of n-butanol, which dilutes both acrylic acid and n-butyl acrylate. The change in the composition of the reactive section causes a decrease in temperature. The temperature and the composition change results in a decrease of the reaction rate. Therefore, the conversion of acrylic acid decreases from 26.6% in experiment E2 to 21.7% in simulation E2*, with a reflux ratio of 2.0. Due to lower conversion of acrylic acid, the total product flow of nbutyl acrylate also decreased by 18% from 0.43 kg/h for experiment E2 to 0.35 kg/h for simulation E2*. For further optimization studies, it can be concluded that an increase in the reflux ratio results in a decrease in reactant conversions and, because of the reflux of the light-boiling components, in a reduction in n-butyl acrylate purity at the bottom of the RD column. 8.2. Distillate-to-Feed Mass Ratio. Experiment E8 was used to investigate the effect of the distillate-to-feed mass ratio. A comparison of experiment E8 with simulation E8* is shown in Figure 8. Experiment E8 was performed at a reflux ratio of 1.99, a distillate-to-feed mass ratio of 0.50, a molar feed ratio (nbutanol to acrylic acid) of 3.1 mol/mol, a total feed mass flow of 3.82 kg/h, and a top pressure of the column of 0.30 bar. The simulated steady state was obtained under the same operating conditions, except with a distillate-to-feed ratio of 0.40 (kg h)/ (kg h). The change in the distillate-to-feed ratio results in changes in the axial composition profile over the whole column height. Due to the lower distillate-to-feed ratio and therefore the lower heat duty in the reboiler (lowered by ∼20%), the total amount of vapor is reduced. The higher distillate-to-feed

ratio of experiment E8 leads to higher concentrations of nbutanol, the light-boiling component, in the reactive section, as n-butanol is stripped out of the lower part of the column. Decreasing the distillate-to-feed ratio leads to lower n-butanol concentrations in the reactive section and higher concentrations of n-butyl acrylate and acrylic acid. Comparable to the effect caused by a change in the reflux ratio, the higher concentration of acrylic acid leads to a faster reaction rate, and acrylic acid conversion is therefore increased by 3%. Because of the higher concentrations of the heavier boiling components acrylic acid and n-butyl acrylate, the temperature profile shows a small increase in the reactive section. The slight increase in nbutanol in the bottom product results in a reduction of the temperature above the reboiler. For further optimization studies aimed at high conversion and high purity of n-butyl acrylate, it can be concluded that for the investigated operating range, a decrease in the distillate-tofeed ratio results in higher concentrations of acrylic acid in the reactive section and therefore in higher conversion. The overall concentration of n-butyl acrylate in the column will also increase with increasing conversion. 8.3. Molar Feed Ratio. An illustration of changes to the molar feed ratio is shown in Figure 9. Experiment E7 was performed with a reflux ratio of 1.46, a distillate-to-feed mass ratio of 0.46, a molar feed ratio of n-butanol to acrylic acid of 3.3, a total feed mass flow of 3.79 kg h−1, and a top pressure of 0.35 bar. The simulated steady-state results were obtained at a molar feed ratio of 2.0, while the other operating conditions were kept constant. Due to the decreased molar feed ratio, the molar fraction of n-butanol is lowered in the entire column, and the acrylic acid concentration in the reactive section is increased. As the reaction rate depends on the concentration of all components, it remained nearly constant. The lower ratio of n-butanol to acrylic acid leads to increased n-butanol conversion. In experiment E7, the conversion of n-butanol was 9.1% and increased to 13.7%, with a decrease in the molar feed ratio from 3.3 to 2.0. In contrast, the conversion of acrylic acid is slightly diminished, from 27.3% to 26.7%, by the decrease of the molar feed ratio. The purity of n-butyl acrylate in the bottom product is increased with decreasing molar feed ratio because of the significant decrease in n-butanol in the bottom product. The 16452



Article

Pressure in the column directly influences the conversion of acrylic acid and n-butanol. With decreasing pressure in the column, the temperature decreases significantly as a result of the decreasing boiling point of the mixture. With a reduction of 0.1 bar, from 0.40 to 0.30 bar, the temperature profile in the pilot-scale RD column decreases by up to 8 K. This reduction has a large influence on reaction rates, which are decreased, leading to a 25% drop in the conversion of both components. The conversion of acrylic acid is 27.0% at 0.40 bar but drops to 20.2% at 0.30 bar; the conversion of n-butanol drops from 9.3% at 0.40 bar to 6.9% at 0.30 bar. Because of the reduced conversion of both reactants, the n-butyl acrylate concentration in the bottom product is reduced from 13.1 to 7.4 wt %. To summarize, an increase in column pressure directly increases the conversion of both reactants and, as a result, the purity of n-butyl acrylate in the bottom product. Because of the maximum operating temperature of the catalyst14 and the polymerization risk of the components11 an arbitrary increase in pressure is not possible, and the operating window of an RD column for an optimization study must be limited. 8.5. Total Liquid Load. The effect of changing the total liquid load by increasing the total feed flow from 2.85 to 3.60 kg/h in the RD column is shown in Figure 11.

Figure 9. Comparison of the experimental and simulated n-butyl acrylate synthesis in a pilot-scale RD column. Symbols represent the experimentally obtained results of experiment E7 and continuous lines represent the simulated results of experiment E7. The dashed lines represent the simulation results of E7* with a molar feed ratio of 2.0: (left) molar composition of liquid phase; (right) temperature profile of vapor phase.

composition of the distillate stream remains nearly constant. As a result of less n-butanol in the entire column, the temperature profile shifts to higher temperatures while maintaining the same shape. An increase in the molar feed ratio results in a dilution of the other components in the entire column. n-Butanol is not completely converted to n-butyl acrylate and water and therefore reduces the purity of n-butyl acrylate in the bottom product. The positive effect of higher feed ratios on the conversion of acrylic acid is small. Therefore, lower molar feed ratios should be used to achieve high n-butanol conversion and increase the purity of the n-butyl acrylate product. 8.4. Top Pressure. Figure 10 shows the result of changing the top pressure in the column. Experiment E1 was conducted

Figure 11. Comparison of experimental and simulated n-butyl acrylate synthesis in a pilot-scale RD column. Symbols represent the experimentally obtained results of experiment E9 and the continuous lines represent the simulated results of experiment E9. Dashed lines represent the simulation results of E9* with a total feed flow of 3.60 kg/h: (left) molar composition of liquid phase; (right) temperature profile of vapor phase.

Experiment E9 was performed at a reflux ratio of 2.00, a distillate-to-feed mass ratio of 0.50, a molar feed ratio of nbutanol and acrylic acid of 2.1, a top pressure of the column of 0.40 bar, and a total feed flow of 2.85 kg/h. Simulation E9* was performed under the same operating conditions, but with a total feed flow of 3.60 kg/h. For experiment E9, the resulting liquid load varied from 2.27 to 5.26 m3/(m2 h) and the F-factor varied from 0.65 to 0.94 Pa0.5. For the simulation E9*, the resulting liquid load varied from 3.01 to 6.87 m3/(m2 h), and the F-factor varied from 0.86 to 1.23 Pa0.5. Apart from the concentration profile of n-butyl acrylate, no significant effect was observed, with a 21% increase in total feed flow. Both packing types still operated within the limits of the allowable gas and liquid loads (see section 5). The deviation of the nbutyl acrylate profile results from lower residence times (lowered by 12% for the entire column) in the reactive section which causes a reduction of the acrylic acid conversion by 4.3%.

Figure 10. Comparison of the experimental and simulated n-butyl acrylate synthesis in a pilot-scale RD column. Symbols represent the experimentally obtained results of experiment E1 and the continuous lines represent the simulated results of experiment E1. Dashed lines represent the simulation results of E1* with a top pressure of 0.30 bar: (left) molar composition of liquid phase; (right) temperature profile of vapor phase.

with a reflux ratio of 2.01, a distillate-to-feed mass ratio of 0.39, a molar feed ratio of n-butanol to acrylic acid of 3.1, a total feed mass flow of 3.81 kg/h, and a top pressure of 0.40 bar. The simulation E1* was performed at the same operating conditions, but with a top pressure of 0.30 bar. 16453



■

The reduction of the conversion shows, that the RD column for the synthesis of n-butyl acrylate is a kinetically controlled process. Consequently, the total feed flow affects the residence time of the liquid in the reactive section and thereby the conversion of the reactants. Longer residence times within an optimization study increase the yield and purity of n-butyl acrylate in the bottom product.

Article

ASSOCIATED CONTENT

S Supporting Information *

Experimental results of all pilot-scale reactive distillation experiments, including the operating conditions, obtained conversions, and liquid phase molar composition profiles as well as temperature profiles of the vapor phase. Furthermore, a description of the data reconciliation and data reconciliation results of one RD experiment are shown and the models used for the calculation of thermodynamic and physical properties are summarized. This information is available free of charge via the Internet at http://pubs.acs.org/.

9. CONCLUSION AND OUTLOOK

■

The demand for n-butyl acrylate and acrylic acid is increasing rapidly. n-Butyl acrylate is currently produced in a very challenging and cost intensive process, because of the high complex thermodynamic behavior and a high polymerization risk within the system. Process intensification can decrease the amount of equipment needed, thereby potentially reducing the investment and operating costs. Extensive theoretical and experimental investigations must be conducted to determine the potential applicability of the RD concept for this reaction system. Up to the authors knowledge, for the first time, an experimental pilot-scale study has been performed for the heterogeneously catalyzed RD of acrylic acid with n-butanol to synthesize n-butyl acrylate and water in a glass column with a nominal diameter of 50 mm. The separation sections were equipped with Sulzer BX, and the reactive section was equipped with the reactive packing Sulzer Katapak SP-11. The stability of the catalyst Amberlyst 46 was investigated; no significant deactivation occurred during approximately 300 h of experimental investigation in the pilot-scale RD column. All experiments shown in this paper underwent data reconciliation, and therefore, the data can be used as a reliable set of temperature and composition profiles for model validation. In preparation for the model validation, a set of property data was generated using Aspen Plus, and the reliability of the data was checked by comparison to experimental data from the literature. The composition and temperature profiles of the experiments with reflux ratios of 1.50 and 2.00 were in agreement with the modeling results of the nonequilibriumstage model. It was found that the composition profiles for the experiments with lower reflux ratios of 0.75 and 1.00 showed deviations, especially in the reactive and rectifying sections. These deviations are results of nonlinear wave propagation, resulting in sharp concentration profiles at the top of the pilotscale RD column.7 Nevertheless, the conversions of both reactants, and therefore the composition of the distillate stream and the bottom product stream, show very good agreement across the whole experimental study. Finally, the validated model was used to present a process analysis that varied the decisive operational parameters: reflux ratio, distillate-to-feed ratio, pressure at the top of the column, total feed mass flow, and molar feed ratio. This study provides information that can be used to identify a suitable operation window to produce high-purity n-butyl acrylate in a future optimization study. In future work, the nonequilibrium-stage RD model validated here will be used for process optimization to identify optimal industrial RD processes that are capable of producing highpurity n-butyl acrylate.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The research leading to these results has received funding from the European Community’s Seventh Framework Programme (FP7-2007-2013) under grant agreement no. 228867, F3 Factory. The support of “Fonds der chemischen Industrie” with some of the required chemicals is highly appreciated.

■

NOMENCLATURE

Latin Letters

a = specific surface area (m2/m3) ai = activity of component i (mol/mol) aij = UNIQUAC binary interaction parameter (−) bij = UNIQUAC binary interaction parameter (K) Cact = concentration of active sites (mol/kg3) Ea = activation energy (J/mol) Ka = activity-based kinetic constant ṁ i = mass flow rate of flow i (kg/h) mcat,dry = mass of dry catalyst per meter packing height (kg/ m) nc = number of components ni̇ = mole flow rate of flow i (mol/h) p = top pressure of reactive distillation column (bar) R = ideal gas constant (J/(mol K)) ri = reaction rate of component i (mol/s) si = standard deviation for variable i T = temperature (K) wi = weight fraction of component i in the liquid phase (g/g) xi = molar fraction of component i in the liquid phase (mol/ mol) zi,exp = experimental value for data reconciliation zi,rec = reconciled value used for data reconciliation Greek Letters

Φi = objective function for data reconciliation σi = experimental error of measurement i χi,j = molar feed ratio of component i to j (moli/molj)

Subscripts

exp = experimental rec = reconciled eq = at chemical equilibrium Abbreviations

AA = acrylic acid BA = n-butyl acrylate BuOH = n-butanol 16454



Article

(19) de La Fuente, R. L.-N.; Tlacuahuac, A. F. Optimal Start-Up and Product Transition Policies of a Reactive Distillation Column. Ind. Eng. Chem. Res. 2007, 46, 2092. (20) Lai, I.-K.; Liu, Y.-C.; Yu, C.-C.; et al. Production of High-Purity Ethyl Acetate Using Reactive Distillation: Experimental and Start-Up Procedure. Chem. Eng. Process. 2008, 47, 1831. (21) Forner, F.; Döker, M.; Gmehling, J.; et al. Anfahrstrategien für die Reaktivrektifikation in Boden- und Packungskolonnen. Chem. Ing. Tech. 2007, 79, 367. (22) Keller, T.; Muendges, J.; Jantharasuk, A.; et al. Experimental Model Validation for n-Propyl Propionate Synthesis in a Reactive Distillation Column Coupled with a Liquid−Liquid Phase Separator. Chem. Eng. Sci. 2011, 66, 4889. (23) Berman, S.; Isbenjian, H.; Sedoff, A.; et al. Esterification Continuous Production of Dibutyl Phthalate in a Distillation Column. Ind. Eng. Chem. Res. 1948, 40, 2139. (24) Seader, J. D.; Henley, E. J. Separation Process Principles; WileyVCH: New York, 1998. (25) Holland, C. Multicomponent Distillation; Prentice-Hall Inc.: NJ, 1963. (26) Taylor, R.; Krishna, R. Modelling Reactive Distillation. Chem. Eng. Sci. 2000, 55, 5183. (27) Klöker, M.; Kenig, E. Y.; Hoffmann, A.; et al. Rate-Based Modelling and Simulation of Reactive Separations in Gas/VapourLiquid Systems. Chem. Eng. Process. 2005, 44, 617. (28) Górak, A.; Hoffmann, A. Catalytic Distillation in Structured Packings: Methyl Acetate Synthesis. AIChE J. 2001, 47, 1067. (29) Schneider, R.; Noeres, C.; Kreul, L. U.; et al. Dynamic Modeling and Simulation of Reactive Batch Distillation. Comput. Chem. Eng. 2001, 25, 169. (30) Rocha, J. A.; Bravo, J. L.; Fair, J. R. Distillation Columns Containing Structured Packings: A Comprehensive Model for Their Performance. 1. Hydraulic Models. Ind. Eng. Chem. Res. 1993, 32, 641. (31) Bravo, J. L.; Rocha, J. A.; Fair, J. R. Mass Transfer in Gauze Packings. Hydrocarbon Process. 1985, 64, 91. (32) Brunazzi, E.; Viva, A. Experimental investigation of reactive distillation packing Katapak-SP11 Hydrodynamic aspects and size effects. Proceedings of Distillation and Absorption; London, September 2006. (33) Lewis, L. S.; Whitman, W. G. Principles of Gas Absorption. Ind. Eng. Chem 1924, 16, 1215. (34) Hougen, O. A. Chemical Process Principles; Wiley-VCH: New York, 1954. (35) Taylor, R.; Krishna, R. Multicomponent Mass Transfer; WileyVCH: New York, 1993. (36) Chilton, T. H.; Colburn, A. P. Distillation and Absorption in Packed Columns. A Convenient Design and Correlation Method. J. Ind. Eng. Chem. 1935, 27, 255. (37) Abrams, D. S.; Prausnitz, J. M. Statistical Thermodynamics of Liquid Mixtures: a New Expression for the Excess Energy of Partly or Completely Miscible Systems. AIChE J. 1975, 21, 116. (38) Hougen, O. A.; Watson, K. M. Chemical Process Principles Part III - Kinetics and Catalysis; Wiley VCH: New York, 1947. (39) Hougen, O.; Watson, K. Solid Catalysts and Reaction Rates General Principles. Ind. Eng. Chem. 1943, 35, 529. (40) Hayden, J. G.; O’Connell, J. P. A Generalized Method for Predicting Second Virial Coefficients. Ind. Eng. Chem. Process Des. Dev. 1975, 14, 209. (41) Iwakabe, K.; Kosuge, H. Isobaric Vapor−Liquid−Liquid Equilibria with a Newly Developed Still. Fluid Phase Equilib. 2001, 192, 171. (42) Chubarov, G. A.; Danov, S. M.; Brovkina, G. V. Liquid-Vapor Equilibrium in the Systems Butyl Alcohol - Acrylic Acid, Propyl Alcohol - Acrylic Acid, Water - Acrylic Acid, and Acetic Acid - Acrylic Acid. Zh. Prikl. Khim. 1976, 49, 1447. (43) Chubarov, G. A.; Danov, S. M.; Brovkina, G. V. Liquid-Vapor Equilibrium in the Methyl Alcohol - Methyl Acrylate, Methyl Acrylate - Butyl Acrylate Systems. Zh. Prikl. Khim. 1976, Vol 49, 1648.

DF = distillate-to-feed mass ratio EQ = equilibrium-stage FID = flame-ionization detector GC = gas chromatography HETP = height equivalent of a theoretical plate HOC = Hayden O’Connell equation-of-state IP = inhibition period LHHW = Langmuir−Hinshelwood−Hougen−Watson kinetic approach NEQ = nonequilibrium-stage RR = reflux ratio UNIFAC = universal quasichemical functional group activity coefficients UNIQUAC = universal quasichemical RD = reactive distillation

■

REFERENCES

(1) Bell, S. L. Acrylic Acids and Esters; Process Economics Program 6D, SRI Consulting, CA, 2003. (2) Glauser, J.; Blagoev, M.; Fujita, K. CEH Marketing Research Report: Acrylic Acid and Esters. Chemical Economics Handbook; SRI Consulting, CA, 2007. (3) Acrylic Acid, Acrylate Esters and Superabsorbent Polymers; IHS Inc.: Colorado, 2011. (4) Nexant Inc. PERP Program - Acrylic Acid; Nexant ChemSystems: New York, 2006. (5) Zeng, K.-L.; Kuo, C.-L.; Chien, I.-L. Design and Control of Butyl Acrylate Reactive Distillation Column System. Chem. Eng. Sci. 2006, 61, 4417. (6) Hiwale, R. S.; Bhate, N. V.; Mahajan, Y. S.; et al. Industrial Applications of Reactive Distillation: Recent Trends. Int. J. Chem. Reactor Eng. 2004, 2, 1. (7) Sundmacher, K.; Kienle, A. Reactive Distillation; Wiley-VCH: Weinheim, 2003. (8) Górak, A.; Stankiewicz, A. I. Intensified Reaction and Separation Systems. Annu. Rev. Chem. Biomol. Eng. 2011, 2, 431. (9) Schwarzer, S.; Hoffmann, U. Experimental Reaction Equilibrium and Kinetics of the Liquid-Phase Butyl Acrylate Synthesis Applied to Reactive Distillation Simulations. Chem. Eng. Technol. 2002, 25, 975. (10) Keller, T.; Tretjak, S.; Lacroix, C. et al. Process Intensification of n-Butyl Acrylate Synthesis Using Catalytic Distillation - A Theoretical Study. Proceedings of the 19th International Congress of Chemical and Process Engineering, Prague, Aug 28−Sept 1, 2010. (11) Niesbach, A.; Daniels, J.; Schroeter, B.; et al. The Inhibition of Acrylic Acid and Acrylate Ester Polymerisation in a Heterogeneously Catalysed Pilot-Scale Reactive Distillation Column. Chem. Eng. Sci. 2012, DOI: 10.1016/j.ces.2012.10.029. (12) Laboratory of Technical Chemistry B. Kinetic Data of the Esterification of Acrylic Acid and n-Butanol; Oral Communication, Dortmund, 2012. (13) Duarte, C.; Buchaly, C.; Kreis, P.; et al. Esterification of Propionic Acid with n-Propanol Catalytic and Non-Catalytic Kinetic Study. Inz. Chem. Procesowa 2006, 27, 273. (14) AMBERLYST Polymeric Catalysts; Rohm and Haas Company: Paris, 2005. (15) Yaws, C. L. Handbook of Transport Property Data; Gulf Publishing Company: Houston, 1995. (16) Gmehling, J.; Menke, J.; Krafczyk, J. et al. Azeotropic data; Wiley-VCH: Weinheim, 2004. (17) Reepmeyer, F.; Repke, J. U.; Wozny, G. Time Optimal Start-Up Strategies for Reactive Distillation Columns. Chem. Eng. Sci. 2004, 59, 4339. (18) Wu, H.-X.; Tang, Z.-G.; Hu, H.; et al. Predictions for Start-Up Processes of Reactive Distillation Column via Artificial Neural Network. Chem. Eng. Technol. 2006, 29, 744. 16455



Article

(44) Fredenslund, A.; Jones, R. L.; Prausnitz, J. M. GroupContribution Estimation of Activity Coefficients in Nonideal Liquid Mixtures. AIChE J. 1975, 21, 1086. (45) Zaleskova, N. I.; Fomin, V. A.; Selyakova, V. A.; et al. Catalytic Activity of Sulfonated Cation-Exchange Resins in the Esterification of Acrylic Acid with n-Butanol. Zh. Prikl. Khim. 1983, 56, 1792. (46) Zheleznaya, L. L.; Karakhanov, R. A.; Lunin, A. F.; et al. Continuous Process of Preparation of n-Butyl(Meth)Acrylate by Esterification of (Meth)Acrylic Acid by Butanol on Thermostable Sulfo-Cation Exchanger. Zh. Prikl. Khim. 1987, 60, 1181. (47) Darge, O.; Thyrion, F. C. Kinetics of the Liquid Phase Esterification of Acrylic Acid with Butanol Catalysed by Cation Exchange Resin. J. Chem. Technol. Biotechnol. 1993, 58, 351. (48) Dupont, P.; Védrine, J. C.; Paumard, E.; et al. Heteropolyacids Supported on Activated Carbon as Catalysts for the Esterification of Acrylic Acid by Butanol. Appl. Catal., A 1995, 129, 217. (49) Chen, X.; Xu, Z.; Okuhara, T. Liquid Phase Esterification of Acrylic Acid with 1-Butanol Catalyzed by Solid Acid Catalysts. Appl. Catal., A 1999, 180, 261. (50) Maytán, E. A.; Valenca, G. P. Esterification of Acrylic Acid with 1-Butanol in Liquid Phase Catalyzed with AL-MCM-41. Av. Cienc. Ing. 2010, 1, 35. (51) Buchaly, C.; Kreis, P.; Górak, A. Hybrid Separation Processes Combination of Reactive Distillation with Membrane Separation. Chem. Eng. Process. 2007, 46, 790. (52) Prüfung von Ionenaustauschern - Bestimmung der Totalen Kapazität von Kationenaustauschern. DIN Deutsches Institut für Normung e.V.: Berlin, 2009; DIN 54403. (53) Fisher, R. A.; Prance, G. T. Design of Experiments; Macmillan Publishing Company: New York, 1974. (54) Marquardt, W. Traveling Waves in Chemical Processes. Int. Chem. Eng. 1990, 30, 585. (55) Luyben, W. L.; Yu, C.-C. Reactive Distillation Design and Control; Wiley-VCH: Hoboken, 2008. (56) Paramasivan, G.; Kienle, A. A Reactive Distillation Case Study for Decentralized Control System Design Using Mixed Integer Optimization. Proceedings of 20th European Symposium on Computer Aided Process Engineering, Naples, June 6−9, 2010. (57) Hwang, Y.-L. Nonlinear Wave Theory for Dynamics of Binary Distillation Columns. AIChE J. 1991, 37, 705. (58) Hankins, N. P. A Non-Linear Wave Model with Variable Molar Flows for Dynamic Behaviour and Disturbance Propagation in Distillation Columns. Chem. Eng. Res. Des. 2007, 85, 65. (59) Marquardt, W. Nichtlineare Wellenausbreitung - Ein Weg zu Reduzierten Dynamischen Modellen von Stofftrennprozessen. Dissertation. University of Stuttgart, Stuttgart, 1988. (60) Kienle, A. Low-Order Dynamic Models for Ideal Multicomponent Distillation Processes Using Nonlinear Wave Propagation Theory. Chem. Eng. Sci. 2000, 55, 1817. (61) Balasubramhanya, L. S.; Doyle, F. J., III Nonlinear Model-Based Control of a Batch Reactive Distillation Column. J. Process Control 2000, 10, 209. (62) Grüner, S.; Mohl, K.-D.; Kienle, A.; et al. Nonlinear Control of a Reactive Distillation Column. Control Eng. Pract. 2003, 11, 915. (63) Kim, B.-K.; Hwang, H.; Woo, D.; et al. Design and Control of a Reactive Distillation Column Based on a Nonlinear Wave Propagation Theory: Production of Terephthalic Acid. Ind. Eng. Chem. Res. 2010, 49, 4297. (64) Reder, C.; Gehrke, V.; Marquardt, W. Steady State Multiplicity in Esterification Distillation Columns. Comput. Chem. Eng. 1999, 23, S407.

16456


Esterification of Acrylic Acid and n-Butanol in a Pilot-Scale Reactive

Recommend Documents