Development of an Integrated ReactionâDistillation Process for the

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Development of an integrated reactiondistillation process for the production of methylal Jan-Oliver Weidert, Jakob Burger, Mario Renner, Sergej Blagov, and Hans Hasse Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03847 • Publication Date (Web): 21 Dec 2016 Downloaded from http://pubs.acs.org on December 27, 2016

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Industrial & Engineering Chemistry Research

Development of an integrated reaction-distillation process for the production of methylal Jan-Oliver Weidert,†,¶ Jakob Burger,∗,† Mario Renner,‡ Sergej Blagov,‡,¶ and Hans Hasse† †University of Kaiserslautern, Laboratory of Engineering Thermodynamics, Erwin-Schroedinger-Str. 44, 67663 Kaiserslautern Germany ‡INEOS Paraform GmbH & Co KG, Mainz, Hauptstrasse 30, Mainz, Germany ¶Present address: BASF SE, 67063 Ludwigshafen, Germany E-mail: [email protected]

Abstract An integrated reaction-distillation process for the production of methylal from aqueous, methanolic formaldehyde solutions is developed. The process consists of a serial connection of a reactor in which the feed is converted to chemical equilibrium by a heterogeneously catalysed reaction followed by a pressure-swing distillation sequence, in which a heterogeneously catalysed reactive section as well as a vapour side draw are used. The catalyst is an acidic ion exchange resin. The process yields methylal in purities over 0.999 g/g, methanol can be recovered in purities over 0.94 g/g and water is withdrawn in a purity of 0.99 g/g. The conversion of formaldehyde is above 99.9%. The process development is carried out based on steady-state simulations with a model, which explicitly accounts for the oligomerisation reactions of formaldehyde in

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aqueous, methanolic formaldehyde solutions. The model is validated by a comparison to experiments that are carried out in a laboratory glass distillation column.

Introduction Methylal (formaldehyde dimethyl acetal, also known as dimethoxy methane or DMM) is an important chemical that is used as a solvent, in pharmaceutical industries or in the production of highly concentrated formaldehyde-solutions. 1 As shown in recent studies, it could become an important intermediate in the production of fuel components 2,3 which would significantly increase its global demand. Thus, the development of economic production processes for methylal is necessary. Methylal production in industrial scale is extensively described in patent and academic literature. A survey is given by Drunsel. 4 It is commonly produced from methanolic formaldehyde solutions which typically also contain water from the formaldehyde-production process. The common process of methylal synthesis consists of a serial arrangement of a reactor in which the feed is converted using a heterogeneous acidic catalyst and a downstream separation sequence which is needed to separate the product from the unreacted educts and eventually from byproducts (see e.g. 5,6 ). Since the lowest boiling node in the distillation line diagram of the reactor product is an azeotropic mixture formed by methylal and methanol, pure methylal can not be obtained by ordinary distillation in a single column. As there are many ways to overcome that limitation, the methylal production processes described in literature vary. Some authors use extractive distillation with water, 7 alcohols 8 or other compounds 9,10 as entrainer. However, these entrainer components must be separated from the products and recovered, what puts the economic efficiency of these processes into question. In contrast, Goering et al. 5 and Yu et al. 11 describe a pressure swing distillation process in which no further entrainer is necessary. However, one major disadvantage of that serial connection of a reactor with a separation unit results from the limited formaldehyde-conversion in the reactor. Realizing chemical reaction and separation


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within one apparatus by using reactive distillation can overcome this limitation. Patent DE 1177126 12 describes a process that basically also consists of a serial connection of reaction and separation, but in which a heterogeneously catalysed reactive section is additionally integrated into the distillation column in order to increase the overall formaldehyde conversion. However, methylal can only be obtained in azeotropic purity from that process if methanol is used in excess in the feedstock. A semi-integrated process where exterior reactors are placed along the distillation column and connected via side draw streams are described in Patents US 4967014 13 and US 6379507. 14 Producing methylal by direct reactive distillation has been focus of research of different groups over the last years. Zhang et al. 15 and Liu et al. 7 investigate the production of methylal by reactive distillation in a pilot scale distillation column and experimentally study the influence of different operation parameters of the column on the process, but no simulations of the experiments were carried out. Modeling of methylal process is quite demanding. Methanolic, aqueous formaldehyde solutions are reactive mixtures in which oligomerisation reactions occur. 16 These reactions have to be taken into account to ensure a reliable, predictive process design. 17 Both reaction kinetics and vapour-liquid equilibrium must be described by activity-based models 18 in order to keep thermodynamic consistency. To our knowledge, the only previously published model of methylal production is that of Kolah et al. 19 Even though these authors also describe experimental studies in the design of a reactive distillation process for methylal production, in their paper no comparison between the model and the experiments is given so that the quality of the model remains unclear. In the present work, a new process for the production of methylal is developed. The process consists of a serial connection of a reactor and a distillation sequence. As feed, an intermediate stream from a formaldehyde production process which consists of 0.8 g/g methanol and each 0.1 g/g of formaldehyde and water is used. The design and optimization of the process is carried out by steady-state simulations based on models developed in former work of our group. This model explicitly accounts for the oligomerisation reactions of aqueous,



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methanolic formaldehyde solutions. The methylal formation in the process is catalysed by acidic ion exchange resins. Amberlyst 15 of Rohm & Haas was used in the present work. Reaction kinetics of methylal formation from aqueous, methanolic formaldehyde solutions using Amberlyst 15 as catalyst have recently been studied in detail by our group. 20 The reaction kinetic model developed in 20 takes the oligomerisation reactions into account. It is the basis of the simulations carried out in the present work. Based on the model, the process from which both methylal and all further components are obtained in high purities is designed. For the proof of feasibility of the process and the validation of the models used for the process design, laboratory distillation experiments in a glass column equipped with gauze packings Sulzer CY and reactive structured packings Sulzer Katapak-SP 11 are carried. The results agree well with the model predictions and pove that the new process is feasible.

Chemical Reactions Formaldehyde is commonly used in liquid aqueous solutions that sometimes also contain methanol. These mixtures are highly reactive multicomponent mixtures. 21 In aqueous solutions, poly(oxymethylene)glycols MGn s are formed:

(I)

H2 O + CH2 O HO(CH2 O)H {z } | MG1

HO(CH2 O)n−1 H + CH2 O HO(CH2 O)n H {z } |

(n ≥ 2)

(II)

MGn

With methanol, formaldehyde forms poly(oxymethylene)hemiformals HFn s:

CH3 OH + CH2 O HO(CH2 O)CH3 | {z } HF1


(III)

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HO(CH2 O)n−1 CH3 + CH2 O HO(CH2 O)n CH3 {z } |

(n ≥ 2)

(IV)

HFn

Reactions (I) - (IV) occur also in the absence of catalysts, but are accelerated both by acids and bases. Since formaldehyde is one of the most important intermediates in chemical industry, experimental investigation and modeling of the equilibria and kinetics of these reactions have been focus of extensive research, see e.g.,

16,22,23

and references therein. The

equilibrium distribution in such solutions mainly depends on the initial concentration as well as the temperature. 24 Under the conditions studied in the present process development, formaldehyde is predominantly bound in HF1 and MG1 , thus, the concentration of monomeric formaldehyde is always very low. In presence of an acidic catalyst, the hemiacetals HFn react with methanol forming acetals: H+

HO(CH2 O)n CH3 + CH3 OH H3 CO(CH2 O)n CH3 + H2 O

The first acetal (n

=

(V)

1) is methylal. This reaction has recently been studied in our

group. 20 The results show that the chemical equilibrium of the methylal formation is nearly independent of temperature and that the formation of longer-chain acetals (n ≥ 2) is not important for the conditions studied in the present work. Also, no indications for the formation of side products such as formic acid or methyl formate were found. Hence, these components were not explicitly accounted for the present process development. Furthermore, the methanol/formaldehyde or water/formaldehyde ratios in the process are always very high so that the short-chain oligomers of formaldehyde with methanol and water are predominant. As strong acidic catalysts are used to accelerate reaction (V), it is appropriate to assume that chemical equilibrium of the formation of the poly(oxymethylene)glycols and poly(oxymethylene)hemiformals (Reactions (I) - (IV)) is attained instantaneously. The methylal formation is kinetically controlled and, hence, the rate-determining step in the reactive system.



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Process model The process development was carried out computer-based by steady-state simulations. The columns were modeled by an equilibrium stage model and the reboiler of each column was assumed to be an additional equilibrium stage. The model which is used to describe the vapour-liquid equilibrium including all chemical reactions as given above is illustrated in Figure 1. It is based on a work by Maurer 25 that originally only considered formaldehyde, methanol and water. This model was continuously improved (e.g. 26,27 ) and successfully applied to simulations of formaldehyde distillation processes. 28,29 For the process development which is presented here, an extension of the model published by Albert et al. 30 and Kuhnert et al., 31 who added methylal as a further substance, is used. Note that in 30,31 methylal is treated as an inert substance as no acidic catalysts were used. In the liquid phase, the formation of poly(oxymethylene)glycols and poly(oxymethylene)hemiformals are taken into account. Additionally, if acidic catalysts are present, e.g. in a reactor or the heterogeneously catalysed section of a reactive distillation column, the methylal formation is considered. The vapour phase is assumed to be a reactive mixture of ideal gases. However, in the gas phase, only the formation of MG1 and HF1 has to be taken into account. All higher MGn s and HFn s only appear in the liquid phase due to their low vapour pressure. The physical equilibrium is described using the extended version of Raoult’s law in which activity coefficients are calculated by the UNIFAC group contribution method, in which a segmentation method that was especially adapted to such formaldehyde containing mixtures is used. Chemical reaction rates and equilibrium constants are also described using activities to ensure thermodynamic consistency. More details on the parameters of the UNIFAC model and the chemical equilibrium constants of the POM-formation reactions are described in Kuhnert et al, 31 the parameters of the reaction kinetic model of the methylal formation are given in Drunsel. 20 For the process development presented in the present paper, the maximum chain length of the poly(oxymethylene)glycols and poly(oxymethylene)hemiformals si set to n = 5 as longer chains are only present in negligible amounts. Enthalpies of the formaldehyde-containing 6 ACS Paragon Plus Environment

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mixtures are calculated by a model developed by Hasse 32 and Albert 33 which was extended to methylal in the present work. For details, see Drunsel. 4

Basic Process Design The feed-stock used for the process has an excess of methanol. As determined in the experiments by Drunsel et al., 20 an almost complete formaldehyde conversion is therefore achieved in an equilibrium reactor which allows the use of a serial connection of a reactor and a downstream distillation sequence without any recycle stream of educts. Moreover, a basic conceptual process design of the distillation sequence can be carried out neglecting formaldehyde, which reduces the reactive multicomponent mixture to a nonreactive ternary system as qualitatively shown in Figure 2. The lowest boiling node in the distillation line diagram of that system is a binary azeotropic mixture consisting of about 0.94 g/g methylal and 0.06 g/g methanol at 1 bar. With increasing pressure, the methanol concentration in the azeotrope increases. Moreover, there is a liquid-liquid miscibility gap in the binary system methylal/water and in part of the ternary system. Note that some authors (e.g. 34,35 ) report a second, homogeneous binary azeotrope in the binary system methylal/water. That azeotrope would occur in the small homogeneous region on the water-rich side of the liquidliquid equilibrium region. The model used in the present work does not predict its existence, which is hard to proof of falsify experimentally. Neither that azeotrope nor the liquid-liquid equilibrium are important for the present process design as the concentrations are never in a region where they may occur. For obtaining methylal in a higher purity than in the azeotrope, pressure swing distillation is used in the process developed here. Figure 3 illustrates the idealized separation steps of a mixture obtained from an equilibrium reactor (FC−1 ) by pressure swing distillation for the ternary system. In the first column, a near-azeotropic mixture of methylal and methanol is obtained as a distillate. Moreover, the column has a side draw which allows the recovery of



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methanol from the process throughout the column C-1. It is operated at 1 bar to enable the use of cooling water in the condenser. The distillate is subsequently separated in a second distillation column C-2, which is operated at a higher pressure and from which methylal is obtained as the sump product whereas the top product is recycled to the first column.

Distillation experiments and model validation For the validation of the model used for the process design as well as for the experimental investigation of the behavior of formaldehyde in the system, distillation experiments were carried out in a continuously operated laboratory glass column. As the maximum operation pressure of the column is limited to ambient pressure, only the first column of the pressure swing system could be investigated. Moreover, the column only consists of a single feed and thus, the recycled distillate stream from the second column was not considered in the experiments. In a first set of experiments, the column was operated with a vapour side draw. Based on the results of these experiments and the simulations, for a second set of experiments the column was equipped with a heterogeneously catalysed reactive section. However, caused by the lower separation efficiency of the reactive packing and the therefore lower total separation efficiency of the column, the vapour side draw could not be used in these experiments. In this section, one typical experiment of each set is discussed, the full set is published in. 4

Experimental Setup and Procedure The column used for the experiments was manufactured by Iludest (Waldb¨ uttelbronn, Germany) and consists of 6 sections with a length of about 70 cm each and an inner diameter of 0.05 m. It is equipped with an electrically heated reboiler. Each section has a sample port that allows taking of liquid samples during the experiments. The sections are isolated with a vacuum jacket and additionally heated for the minimization of heat losses. Temper-


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atures are measured at the bottom of each section as well as in the reboiler and at to top of the column by calibrated Pt100 resistance thermometers (accuracy 0.1 K). The feed is preheated and its temperature is controlled. Mass flow of both feed and sump as well as the distillate stream are measured gravimetrically with scales (manufacturer of all scales used: Sartorius, Göttingen, Germany), the mass flow of the reflux is detected by a coriolis flow meter (manufacturer: Rheonik, Odelzhausen, Germany). For the vapour side draw, the jet pump principle is applied. As a working fluid for the jet pump which is fed by a piston pump (manufacturer of all pumps: Ismatec Laboratoriumstechnik, GmbH, Wertheim, Germany), the condensed side draw stream itself is used. After being withdrawn from the column, the stream is condensed and collected together with the working fluid in a drum. One fraction of the collected liquid is then used as a working fluid of the jet pump again, the other fraction is withdrawn from the drum by a gear pump and detected gravimetrically. For the steady-state operation of the column, the reboiler heat duty and the reflux mass flow were set. After the start-up at total reflux, the operating parameters were set to the desired values. Establishment of steady-state took up to 8 hours and was determined by monitoring the temperature profile of the column and by taking samples from the product streams. After steady-state was reached, the column was left undisturbed for at least 90 minutes before samples were taken from the sample ports of each section and from all product streams.

Chemicals and Analysis The feed used for the experiments was prepared by dissolving solid para-formaldehyde in a mixture of methylal, methanol, and water. Gas chromatography with the internal standard method was applied to determine the overall concentrations of methanol, water and methylal. The concentration of formaldehyde in the samples was so low that the influence on the results of the gas chromatography of methylal, methanol, and water is not important. The sodium sulfite method was used to determine the concentration of formaldehyde in the mixtures. 21 Upon the analysis, the formaldehyde oligomers are split up, hence, overall concentrations 9 ACS Paragon Plus Environment


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of methanol, water, methylal and formaldehyde are measured. Each sample was analyzed several times, the maximum relative deviations were 5% for methanol, 6% for water, 4% for formaldehyde and 5% for methylal. More details on the analysis and the purities of the used chemicals are given in. 20

Experiments with vapour side draw For the set of experiments with a vapour side draw, each of the 6 sections of the column was equipped with 0.48 m of structured wire gauze packing Sulzer CY. As feed, a mixture consisting of 0.21 g/g methylal, 0.644 g/g methanol, 0.142 g/g water and 0.004 g/g formaldehyde according to the composition as expected from the equilibrium reactor of the process at an operating temperature of 60 ◦ C was used. The optimal arrangement of feed- and side draw stream was determined by rigorous simulations prior to the experiments. Feed is located at a packing height of 1.92 m (from bottom), the side draw is placed at 0.96 m. The measured overall concentration profile in the liquid phase, the composition of the vapour side draw stream and the temperature profile of one of the experiments is shown together with the results of the associated rigorous simulation in Figure 4. As distillate, the near-azeotropic mixture of methylal and methanol with some minor traces of water and formaldehyde was obtained with a mass flow of 10.6 g/min. Methanol was recovered from the column via the side draw stream (mass flow 32.1 g/min) in a purity of 0.95 g/g with a rest of water and formaldehyde. Formaldehyde is concentrated in the stripping section of the column between the bottom and the side draw. As a sump product, a binary mixture of 0.98 g/g water and 0.02 g/g formaldehyde is obtained. The column was operated with a feed mass-flow of 47.9 g/min and at a pressure of 0.973 bar, the reboiler duty was set to 1.4 kW and the reflux-ratio was 6.0. For the simulations with the equilibrium stage model, the separation efficiency of the CYpacking was taken from the Sulzer product data sheet 36 with ten theoretical stages per meter of packing leading to a total separation efficiency of thirty stages for the packing. Moreover, 10 ACS Paragon Plus Environment

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the reboiler was assumed to be an additional theoretical stage. The mass flow of the distillate, vapour side draw and reflux stream were taken from the experiments to fix the three degrees of freedom in the simulation. The comparison between the simulation and experimental results is shown in Figure 4. The simulations with the equilibrium stage model yield discrete results. In the figures, these discrete data points are joined by linear line segments as a guide for the eye. Note further that in the simulation the formation of POM-oligomers was taken into account, but the results are given in overall concentrations. The agreement between simulation and experimental data for methanol, methylal and water is very good. At the sample port placed below the feed, deviations occur due to incomplete mixing of the feed stream and the liquid stream inside the column in the experiment. For formaldehyde however, there are deviations up to 100% in the stripping section. These deviations might be caused by the fact that a steady-state profile was not reached regarding formaldehyde which is only fed in very low concentrations. Nevertheless, the agreement of the concentration in the product streams is still good. Similar results were obtained for the other experiments which are discussed in. 4 Both the experiments and the simulations show that formaldehyde concentrates in the sump of the column. These formaldehyde losses in the water stream may be undesired for economic reasons and also regarding the waste-water treatment. These losses can be minimized by installing a reactive section in the column C-1.

Experiments with heterogeneously catalysed reactive section For the reactive distillation experiments, two sections of the column were equipped with the structured reactive packing Sulzer Katapak-SP 11 (each section with three elements of a height of 0.1 m). The catalyst bags of each element were filled with 14.7 g of the acidic ion exchange resin Amberlyst 15. For an improved distribution of the liquid phase in the reactive packings, one element (height 0.16 m) of the CY-Packing was placed above the reactive elements on the top of each section. Since the column was operated without a vapour 11 ACS Paragon Plus Environment


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side draw, the feed composition for the experiments was changed to 0.211 g/g methylal, 0.643 g/g methanol, 0.126 g/g water, and 0.02 g/g formaldehyde so that the concentrations in the reactive sections in the experiments were in a range according to the concentrations in the reactive section of the column C-1 operated with the reactive section and the vapour side draw in the pressure swing system. The experiments were carried out at different distillate/feed and reflux ratios with a liquid load in the reactive section of approximately 2.8 m3 /(m2 h). Figure 5 shows the profiles obtained from a typical experiment along with the simulated profiles. The column was operated with a feed mass-flow of 57.0 g/min and at a pressure of 0.973 bar, the reboiler duty was set to 700 W and the reflux-ratio was 4.0. As distillate, the near-azeotropic mixture was obtained with a mass flow of 15.8 g/min. In the reactive section, as expected, formaldehyde reacts with methanol forming methylal and water. Thus, concentration of formaldehyde in the reactive section decreases. For the simulation of the experiments, the equilibrium stage model as explained in Section was used. The separation efficiency of the CY packing was again assumed to be ten stages per meter. Although most authors report an NTSM-value of 2 for the reactive packing Katapak-SP 11 (e.g. 37,38 ), for the simulations carried out here, the NTSM was set to 3. This separation efficiency was found a suitable value for the packing in previous simulations of reactive distillation experiments accomplished using the same column and the same packings. 39 Hence, both reactive sections consist of one equilibrium separation stage filled with 44.1 g of catalyst. Like in the simulations of the experiments with a vapour side draw, the reflux and the distillate mass flow were taken from the experiments to fix the degrees of freedom in the simulation. The comparison between simulated and experimental profiles are presented in Figure 5 and show a good agreement. Deviations in the compositions are in the order of the expected uncertainty of the analysis. The conversion of formaldehyde determined in the experiment was 86.1%, a conversion of 86.6% was found in the simulation.


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Process design The detailed design of the pressure swing system was carried out based on the validated model. For the final design, a hierarchical approach was used. In the first step, a simplified configuration of the process with only the pressure swing system was designed while both the vapour side draw and the reactive section were neglected. Even after these simplifications, the system has numerous parameters that have to be fixed in the optimization procedure namely the operating pressures, reflux ratios, numbers of stages, feed positions and the recirculation flows. For an optimal design both capital and operating expenditures need to be taken into account, and a detailed knowledge of cost functions is needed. 40 The design presented hereafter is not explicitly based on theses functions but considers the most important factors influencing both capital and operating costs. A more detailed design will anyhow depend on the conditions of the specific site where the project is to be realized. The operating pressure of the first column was fixed to 1 bar which allows using standard cooling water in the condenser. The operating pressure of the second column was set to 4 bar. The concentration of methylal in the azeotropic mixture is thereby shifted from 0.94 g/g at 1 bar to 0.88 g/g at 4 bar. A further increase of the operating pressure would lead to a higher concentration difference and, hence, decrease the recirculating stream of the process. However, higher pressures also go along with an increasing investment for the column C-2. For the design of the columns ˙ at the set operating pressures, the N, Q-curve strategy described by Zeck 41 which yields the total number of equilibrium stages and the optimal feed position was used. This method was applied here choosing Q˙ = 1.4 Q˙ min for the operating points of both columns. For applying ˙ the N, Q-curve strategy, specifications of both the top and bottom stream of both columns were set. The specified concentration of the near-azeotropic mixtures in the top products of both columns was systematically varied in the process design. Note that for given top specs the recycle stream and all other parameters like the reboiler heat duties are known so that an optimization can be carried out. For the final operating point, the top concentrations were chosen so as to minimize the total reboiler duty. The entire procedure described above 13 ACS Paragon Plus Environment


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must be carried out in closed loop simulations of the entire process which requires iterations as the recirculated stream has to be taken into account in a consistent manner. In a further step, the optimal position of the vapour side draw stream was determined leaving the total number of stages of the first column unchanged. In the final step, the reactive section was integrated. It has to be located between the position of the feed from the reactor and the side draw to ensure that methylal, which is formed in the reactive section, is withdrawn from the column by the distillate stream. For the determination of the size of the reactive section in which as in the laboratory reactive-distillation experiments the catalytic packing Katapak-SP 11 with Amberlyst 15 as a catalyst is used, the minimal mass of catalyst was determined that allows fulfilling the specification of organic load in the sump product of column C-1 by simulations. For the final design, the minimum required catalyst mass was doubled to compensate model uncertainties. The column diameter was fixed so that a reasonable liquid load in the reactive packings is achieved. 42 The design of the height of the reactive zone was subsequently calculated from the determined mass of catalyst assuming a catalyst volume fraction of K−SP

11

= 0.4 for the packing. 38 With the

assumption of NTSM = 2 for Katapak-SP 11 packing in industrial scale, 38 the height of the reactive section was determined to be four theoretical reactive stages or, respectively, 2 meters. These four stages were integrated into the column and two additional theoretical stages were added below and above the reactive section to ensure the separation of methylal from the liquid phase above the reactive section and the vapour side draw. Figure 6 gives an overview of the final process. The concentrations and mass-flows resulting from the process simulation at the design operating point are given in Table 1. The given composition at the reactor outlet was directly measured by the experiments described in. 20 In the design operating point given here, the reactor is operated at t = 60 ◦ C and at a pressure of 2 bar in order to avoid partial evaporation of the reacting mixture. From the downstream pressure swing sequence, methylal is obtained in a purity of 0.999 g/g. Methanol is recovered via the vapour side draw stream of the first column with a purity of 0.94 g/g which is sufficient


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for the use as a feed source in the formaldehyde-production process. Due to the reactive section, the concentration of organic components in the sump of the first column is decreased to 0.004 g/g. Temperatures in the reactive section are between 60 ◦ C and 70 ◦ C which is in the range where the reaction kinetic experiments 20 and the laboratory reactive distillation experiments were carried out. The first column C-1 is operated at a reflux ratio of 2.0. Reflux ratio of the second column is 2.2. The weight-based formaldehyde-conversion attained in the equilibrium reactor is 92.4%, by the use of the reactive section in the column it is increased to 99.9% for the entire process.

Conclusion In the present work, a continuous highly integrated and efficient reaction-distillation process for the production of methylal was developed. The process consists of a serial arrangement of a reactor and a downstream pressure swing distillation system, in which both a vapour side draw and a heterogeneously catalysed reactive section is used. The process was designed especially for the use of a methanolic, aqueous formaldehyde-solution as a feed that contains of an excess of methanol. However, the process concept can easily be adapted to any other feed composition. The process design and optimization was carried out based on steady-state rigorous simulations with an equilibrium stage model in which the chemistry of aqueous, methanolic formaldehyde solutions is accounted for. The model was validated by the comparison to distillation experiments in laboratory scale with a vapour side draw. Moreover, reactive distillation experiments with a heterogeneously catalysed reactive zone equipped with Sulzer Katapak-SP 11 were carried out. The reaction kinetics were taken from previous work of our group. 20 Separation efficiencies of the packings were taken directly as given by the manufacturer, hence, the simulations are completely predictive. The integrated pressure swing distillation system was subsequently designed by a hierarchic strategy, in which the basic arrangement was designed first, followed by the integration of the side draw and the reactive section. The process yields methylal in a purity of 0.999 g/g along with 15 ACS Paragon Plus Environment


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the recovery of methanol in a purity of 0.94 g/g and the withdrawal of water in a purity of 0.996 g/g. The conversion of formaldehyde achieved in the process is 99.9%.


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Nomenclature Abbreviations: Fa

formaldehyde

HF

hemiformal

Mal

methylal

MeOH

methanol

MG

methylene glycol

K-SP 11

Katapak-SP 11

NTSM

number of theoretical stages per meter

POM

polyoxymethylene

Symbols and indices: B

bottom (sump product)

D

distillate

catalyst volume fraction

F

feed

N

number of theoretical equilibrium stages

n

number of formaldehyde segments in formaldehyde oligomer

p

pressure

Q˙

reboiler heat duty

Q˙ min

minimum reboiler heat duty demand

SD

side draw

t

temperature

W

water

(m)

overall liquid mass fraction of component i

(m)

overall vapour mass fraction of component i

x˜i y˜i



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Acknowledgement We thank Sulzer Chemtech AG (Winterthur, Switzerland) for the supply of Katapak-SP 11 and Rohm & Haas (Frankfurt, Germany) for the supply of Amberlyst 15.

References (1) Lojewska, J.; Wasilewski, J.; Terelak, K.; Lojewski, T.; Kolodziej, A. Selective oxidation of methylal as a new catalytic route to concentrated formaldehyde: Reaction kinetic profile in gradientless flow reactor. Catal. Commun. 2008, 9, 1833–1837. (2) Burger, J.; Siegert, M.; Ströfer, E.; Hasse, H. Poly(oxymethylene) dimethyl ethers as components of tailored diesel fuel: Properties, synthesis and purification concepts. Fuel 2010, 89, 3315 – 3319. (3) Burger, J.; Ströfer, E.; Hasse, H. Production process for diesel fuel components poly (oxymethylene) dimethyl ethers from methane-based products by hierarchical optimization with varying model depth. Chem. Eng. Res. Des. 2013, 91, 2648–2662. (4) Drunsel, J.-O. Entwicklung von Verfahren zur Herstellung von Methylal und Ethylal. Ph.D. thesis, University of Kaiserslautern, 2011. (5) Göring, M.; Hoffmockel, M.; Lingnau, J.; Mueck, K.-F. Verfahren zur Herstellung von Acetalen. Patent 2004, EP 1824807 . (6) Kaufhold, M.; M¨ uller, W. Process for the recovery of pure Methylal from methanolmethylal mixtures. Patent 1982, US 4385965 .


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(7) Liu, H.; Gao, H.; Ma, Y.; Gao, Z.; Eli, W. Synthesis of High-Purity Methylal via Extractive Catalytic Distillation. Chem. Eng. Technol. 2012, 35, 841–846. (8) Enk, E.; Spes, H. Verfahren zur Gewinnung von alkohol- und wasserfreien Acetalen durch extraktive Destillation. Patent DE 1127339, 1962. (9) Wang, Q.; Yu, B.; Xu, C. Design and Control of Distillation System for Methylal/Methanol Separation. Part 1: Extractive Distillation Using DMF as an Entrainer. Ind. Eng. Chem. Res. 2012, 51, 1281–1292. (10) Tanaka, Y.; Yamamoto, S. Process for purification of methylal. Patent US 6160185, 1996. (11) Yu, B.; Wang, Q.; Xu, C. Design and Control of Distillation System for Methylal/Methanol Separation. Part 2: Pressure Swing Distillation with Full Heat Integration. Ind. Eng. Chem. Res. 2012, 51, 1293–1310. (12) K¨ unstle, G.; Knörr, F. Verfahren zur kontinuierlichen Herstellung von Methylal. Patent DE 1177126, 1964. (13) Masamoto, J.; Ohtake, J. Process for producing formaldehyde and derivatives thereof. Patent US 4967014, 1990. (14) Satoh, S.; Tanigawa, Y. Process for producing methylal. Patent US 6379507, 2002. (15) Zhang, X.; Zhang, S.; Jian, C. Synthesis of methylal by catalytic distillation. Chem. Eng. Res. Des. 2011, 89, 573–580. (16) Hahnenstein, I.; Hasse, H.; Kreiter, C. G.; Maurer, G. 1H- and 13C-NMR-Spectroscopic Study of Chemical Equilibria in Solutions of Formaldehyde in Water, Deuterium Oxide, and Methanol. Ind. Eng. Chem. Res. 1994, 33, 1022–1029. (17) Maiwald, M.; Gr¨ utzner, T.; Ströfer, E.; Hasse, H. Quantitative NMR spectroscopy of complex technical mixtures using a virtual reference: chemical equilibria and reaction 19 ACS Paragon Plus Environment


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kinetics of formaldehyde - water - 1,3,5-trioxane. Anal. Bioanal. Chem. 2006, 385, 910–917. (18) Hasse, H. In Reactive Distillation: Status and Future Directions; Sundmacher, K., Kienle, A., Eds.; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, 2003; Chapter 4, pp 63–96. (19) Kolah, A. K.; Mahajani, S. M.; Sharma, M. M. Acetalization of Formaldehyde with Methanol in Batch and Continuous Reactive Distillation Columns. Ind. Eng. Chem. Res. 1996, 35, 3707–3720. (20) Drunsel, J.-O.; Renner, M.; Hasse, H. Experimental Study and Model of Reaction Kinetics of Heterogeneously Catalyzed Methylal Synthesis. Chem. Eng. Res. Des. 2012, 90, 696–703. (21) Walker, F. Formaldehyde; Chapman & Hall: London. Third edition, 1963. (22) Maiwald, M.; Fischer, H. H.; Ott, M.; Peschla, R.; Kuhnert, C.; Kreiter, C. G.; Maurer, G.; Hasse, H. Quantitative NMR Spectroscopy of Complex Liquid Mixtures: Methods and Results for Chemical Equilibria in Formaldehyde - Water - Methanol at Temperatures up to 383 K. Ind. Eng. Chem. Res. 2003, 42, 259–266. (23) Ott, M.; Fischer, H. H.; Maiwald, M.; Albert, K.; Hasse, H. Kinetics of oligomerization reactions in formaldehyde solutions: NMR experiments up to 373 K and thermodynamically consistent model. Chem. Eng. Process. 2005, 44, 653 – 660. (24) Gr¨ utzner, T.; Hasse, H. Solubility of Formaldehyde and Trioxane in Aqueous Solutions. J. Chem. Eng. Data 2004, 49, 642–646. (25) Maurer, G. Vapor-Liquid Equilibrium of Formaldehyde and Water-Containing Multicomponent. AlChE J. 1986, 32, 932–948.


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(26) Hasse, H.; Hahnenstein, I.; Maurer, G. Revised vapor-liquid equilibrium model for multicomponent formaldehyde mixtures. AIChE J. 1990, 36, 1807–1814. (27) Albert, M.; Hahnenstein, I.; Hasse, H.; Maurer, G. Vapor-liquid equilibrium of formaldehyde mixtures: New data and model revision. AIChE J. 1996, 42, 1741–1752. (28) Ott, M.; Schoenmakers, H.; Hasse, H. Distillation of formaldehyde containing mixtures: laboratory experiments and equilibrium stage based modeling and simulation. Chem. Eng. Proc. 2005, 44, 687–694. (29) Gr¨ utzner, T.; Hasse, H.; Lang, N.; Siegert, M.; Ströfer, E. Development of a new industrial process for trioxane production. Chem. Eng. Sci. 2007, 62, 5613 – 5620. (30) Albert, M.; Hahnenstein, I.; Hasse, H.; Maurer, G. Vapor-Liquid and Liquid-Liquid Equilibria in Binary and Ternary Mixtures of Water, Methanol, and Methylal. J. Chem. Eng. Data 2001, 46, 897–903. (31) Kuhnert, C.; Albert, M.; Breyer, S.; Hahnenstein, I.; Hasse, H.; Maurer, G. Phase Equilibrium in Formaldehyde Containing Multicomponent Mixtures: Experimental Results for Fluid Phase Equilibria of (Formaldehyde + (Water or Methanol) + Methylal)) and (Formaldehyde + Water + Methanol + Methylal) and Comparison with Predictions. Ind. Eng. Chem. Res. 2006, 45, 5155–5164. (32) Hasse, H. Dampf-Fl¨ ussigkeitsgleichgewichte, Enthalpien und Reaktionskinetik in formaldehydhaltigen Mischungen. Ph.D. thesis, University of Kaiserslautern, 1990. (33) Albert, M. Thermodynamische Eigenschaften formaldehydhaltiger Mischungen. Ph.D. thesis, University of Kaiserslautern, 1999. (34) Lesteva, T. M.; Kachalova, R. V.; Morozova, A. I.; Ogorodnikov, S. K.; Trenke, K. M. Study of Azeotropy in Binary and Ternary Systems. Zh. Prikl. Khim. 1967, 40(8), 1808–1814. 21 ACS Paragon Plus Environment


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(35) Qui, Z.; Luo, Z.; Hu, Y. Vapor-Liquid Equilibria of Two Binary Systems Containing Methylal. Gaoxiao huaxue gongcheng xuebao 1994, 8(2), 106–110. (36) Sulzer Chemtech LTD, Structured Packings for Distillation, Absorption and Reactive Distillation. Winterthur, Switzerland, 2011. (37) Steinigeweg, S.; Gmehling, J. Esterification of a Fatty Acid by Reactive Distillation. Ind. Eng. Chem. Res. 2003, 42, 3612–3619. (38) Götze, L.; Bailer, O.; Moritz, P.; von Scala, C. Reactive distillation with Katapak. Catal. Today 2001, 69, 201 – 208. (39) Parada, S. Nebenreaktionen bei der heterogen katalysierten Reaktivdestillation am Beispiel der Herstellung von Butylacetat. Ph.D. thesis, University of Stuttgart, 2009. (40) Bonet, J.; Galan, M.; Costa, J.; Thery, R.; Meyer, X.; Meyer, M.; Reneaume, J. Pressure optimisation of an original system of pressure swing with a reactive column. Proceedings of Distillation and Absorption 2006 2006, (41) Zeck, S. Einfluss von thermophysikalischen Stoffdaten auf die Auslegung und den Betrieb von Destillationskolonnen. Chem. Ing. Tech. 1990, 62, 707–717. (42) Aferka, S.; Viva, A.; Brunazzi, E.; Marchot, P.; Crine, M.; Toye, D. Liquid load point determination in a reactive distillation packing by X-ray tomography. Can. J. Chem. Eng. 2010, 88, 611–617.


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Table 1: Stream table of the methylal production process (Numbers of streams according to Figure 6). Stream m ˙ / kg h−1 t / ◦C p / bar (m) x˜Fa / g g−1 (m) x˜MeOH / g g−1 (m) x˜W / g g−1 (m) x˜Mal / g g−1

1 1000 60.0 2.00 0.100 0.800 0.100 0.000

2 1000 60.0 2.00 0.008 0.616 0.149 0.227

3 115.8 99.6 1.00 0.004 0.000 0.996 0.000

4 639.5 67.7 0.98 0.000 0.940 0.059 0.001

5 628.8 40.3 0.96 0.000 0.068 0.000 0.931


6 628.8 40.3 4.00 0.000 0.068 0.000 0.931

7 384 85.2 4.00 0.000 0.111 0.000 0.888

8 244.8 88.3 4.04 0.000 0.000 0.001 0.999


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1: Vapour-liquid equilibrium in the system methylal/methanol/water/formaldehyde.


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MeOH

64.5 °C

Azeotrope 41.7 °C

Mal

W

42.0 °C

100 °C

Figure 2: Scheme of distillation line diagram of the system methylal/methanol/water at p = 101.35 kPa: (−) Distillation lines (qualitatively), (· · · ) liquid-liquid miscibility gap (qualitatively). In the quantitative diagram which can be obtained from the vapour-liquid equilibrium model of, 31 the topology is difficult to discern.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

MeOH

SDC-1

FC-1 F*C-1 DC-2 BC-1

DC-1 BC-2

W

Mal DC-2

DC-1

FC-1

C-2

C-1

SDC-1 BC-1

BC-2

Figure 3: Separation in the pressure swing distillation process in the (reduced) ternary system methylal/methanol/water. F∗C−1 : overall feed of column C-1.


Page 26 of 29

3

2.5

2.5

2.5

2

2

2

1.5 1

hPack / m

3

hPack / m

3

1.5 1

1.5 1

0.5

0.5

0.5

0

0

0

0.25 0.5 0.75 (m) x ˜i / g g−1

1

0

0.05 0.1 0.15 (m) x ˜Fa / g g−1

3

3

2.5

2.5

2

2 hPack / m

0

hPack / m

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


hPack / m

Page 27 of 29

1.5 1

0

0 1

100

1 0.5

0.25 0.5 0.75 (m) y˜i / g g−1

50 75 t / oC

1.5

0.5

0

25

0

0.05 0.1 0.15 (m) y˜Fa / g g−1

Figure 4: Comparison between experimental (symbols) and calculated (lines) profiles of experiment with vapour side draw. Overall mass fractions: (4) methylal, () methanol, (×) water, and (◦) formaldehyde; (∗) temperature; filled symbols: vapour side draw.


3

3

2.5

2.5

2.5

2

2

2

1.5 1

hPack / m

3

hPack / m

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

hPack / m


1.5 1

1.5 1

0.5

0.5

0.5

0

0

0

0

0.25 0.5 0.75 (m) x ˜i / g g−1

1

0

0.01 0.02 (m) x ˜Fa / g g−1

Page 28 of 29

40

50 60 t / oC

70

Figure 5: Comparison between experimental (symbols) and calculated (lines) reactive profile. Overall liquid phase mass fractions: (4) methylal, () methanol, (×) water, and (◦) formaldehyde; (∗) temperature. Grey: heterogeneously catalysed reactive sections.

7 5

1

R

2

6

4

3

8

Figure 6: Flow sheet of the developed methylal production process. Grey: heterogeneously catalysed reactive section.


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TOC GRAPHIC

Figure 7: TOC GRAPHIC.


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