Dimethyl Ethers from Formaldehyde and Methanol - ACS Publications

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Conceptual design of a novel process for the production of poly(oxymethylene) dimethyl ethers from formaldehyde and methanol Niklas Schmitz, Eckhard Ströfer, Jakob Burger, and Hans Hasse Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02314 • Publication Date (Web): 14 Sep 2017 Downloaded from http://pubs.acs.org on September 19, 2017

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Industrial & Engineering Chemistry Research

Conceptual design of a novel process for the production of poly(oxymethylene) dimethyl ethers from formaldehyde and methanol Niklas Schmitz, Eckhard Ströfer, Jakob Burger,∗ and Hans Hasse Laboratory of Engineering Thermodynamics, University of Kaiserslautern, Germany OME Technologies GmbH, Kaiserslautern, Germany E-mail: [email protected]

Abstract Poly(oxymethylene) dimethyl ethers (OME) are environmentally benign alternative fuels. This work presents the conceptual design of a novel OME process which employs aqueous solutions of formaldehyde and methanol as feedstock. In this process, OME of the desired chain lengths n = 3 − 5, and water are separated from the reactive mixture (formaldehyde+water+methanol+methylal+OME). Thermodynamic limits are identified by studying distillation boundaries and chemical equilibria. By that it is shown that OME of chain lengths n = 3 − 5 can be separated from the reactor outlet by distillation. The separation of water is either carried out using an adsorption or a membrane process. Adsorption isotherms of water on Zeolite 3A are determined experimentally. The OME process is simulated and optimized using a reduced process model accounting for the mass balances and the thermodynamic limits. Favorable operating points of the process are identified using multi-objective optimization. * To

whom correspondence should be addressed

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Introduction Poly(oxymethylene) dimethyl ethers (OME) are oligomers of the general chemical structure H3 C−O−(CH2 O)n −CH3 with n ≥ 2. OME are alternative fuels derived from the C1-value-added chain. OME reduce the soot and indirectly also the NOx formation during the combustion process in engines. 1–3 Thus, OME have the potential to significantly reduce engine emissions, which recently undergo a heavy public debate. In addition, OME are also considered as physical solvents for the absorption of CO2 from natural gas, 4 as safe fuels for direct oxidation fuel cells, 5,6 and as green solvents for the chemical industry. 7 Synthesis routes for the production of OME are based on synthesis gas via the methanol route which enables the flexible use of both conventional (coal, gas) and renewable raw materials (biomass, CO2 ). The production of OME can thus be perfectly adapted to a changing raw materials landscape. Generally, for the synthesis of OME, a source of formaldehyde (e.g. aqueous/methanolic formaldehyde solution, paraformaldehyde, trioxane) and a source of CH3 -end groups (e.g. methanol, dimethyl ether, methylal) are required. An overview over the literature reporting on the OME synthesis from different reactants is given in refs. 8,9 Presently, processes for producing OME of chain lengths n = 3 − 5, which is the desired chain length when using OME as fuels, 1,3 require the use of expensive intermediates as reactants. One example is the OME synthesis from trioxane and methylal, for which several authors (e.g. see 10,11 ) investigated the chemical equilibrium and the reaction kinetics, and Burger et al. 12,13 conceptually designed a production process. Trioxane and methylal are produced from formaldehyde and methanol in additional process steps. 14–16 The advantage of using trioxane and methylal is an entirely water-free and methanol-free OME process, which is favorable for the single-pass yields of OME in the reactor 11 and the work up of the reaction mixture. The system exhibits zeotropic behavior, such that separating the OME of the desired chain lengths from the reaction mixture is straightforwardly solved by distillation. The OME production costs via that synthesis route are in the range of the costs of classical diesel fuel production. 17 A flowsheet of this synthesis route is given in Figure S1 in the Supporting Information. This disadvantage of using trioxane and methylal 2 ACS Paragon Plus Environment

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as reactants, is the tedious production of trioxane and methylal in intermediate process steps. In other approaches trioxane is replaced by paraformaldehyde. They are described in the patent literature (e.g. see ref. 18 ). Because the production of intermediates like methylal, trioxane or paraformaldehyde generally requires additional process steps, a process that directly employs aqueous solutions of formaldehyde and methanol, is highly desired. Such processes have been described in patent literature. Ströfer et al. 19 describe a process with several distillation columns and a liquid-liquid phase separator. Moulton and Naegeli 20 and Arnold et al. 21 describe a process in which OME are separated from the reaction mixture using extraction processes. In a preferred variant of their process, the OME are extracted with diesel fuel to produce a mixture of OME and diesel. The information that was disclosed on these processes is, however, patchy and no reports on them are available in the scientific literature. This work presents the conceptual design of a novel process which employs aqueous solutions of formaldehyde and methanol for producing a pure OME mixture of chain lengths n = 3 − 5. In this process, OME of chain lengths n = 3 − 5, and water are separated from the reactive multicomponent mixture (formaldehyde+water+methanol+methylal+OME) which shows a liquid phase split 22 and several reactive and non-reactive azeotropes. Water is present in these mixtures because conventional formaldehyde solutions usually contain water and because water is a coupled product of the OME synthesis from formaldehyde and methanol. In these mixtures, chemical reactions of formaldehyde with water and methanol to poly(oxymethylene) glycols and poly(oxymethylene) hemiformals have to be considered not only as part of the complex reaction network in the acidcatalyzed reactor, 23 but also in all distillation steps. 24 It is shown here that OME of chain lengths n = 3 − 5 can be separated from the reactor outlet by reactive distillation without any additional components such as entrainers or extraction agents. The separation of water is carried out using either an adsorption or a membrane process. The process is simulated and optimized using a reduced process model, in which material balances and fundamental thermodynamic limitations are considered. In a first case study, it is assumed that chemical equilibrium is reached in the reactor outlet and that all separations are se-



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lective and sharp. In a second case study, the operating points of the process are further optimized. Instead of assuming chemical equilibrium in the reactor outlet, an isothermal fixed-bed reactor is designed using a reaction kinetic model. 23 For the design of an adsorption process, experiments for determining adsorption isotherms of water on Zeolite 3A are carried out in the present work. Favorable operating points of the novel process are identified using multi-objective optimization. This paper is organized as follows. After presentation of the physico-chemical properties, it is described how OME of chain lengths n = 3 − 5 can be separated. Then the separation of water is discussed along with adsorption experiments, before finally the process is simulated and optimized.

Physico-chemical properties Chemical reactions Formaldehyde (FA, CH2 O) reacts with water (H2 O) to poly(oxymethylene) glycols (MGn , HO−(CH2 O)n −H) and with methanol (MeOH, H3 C−OH) to poly(oxymethylene) hemiformals (HFn , HO−(CH2 O)n −CH3 ). 25,26 The reactions of formaldehyde with water are described by reactions (1) and (2). − ⇀ FA + H2 O − ↽ − − MG1 −− ⇀ FA + MGn−1 ↽ − − MGn ; n ≥ 2

(1) (2)

The reactions of formaldehyde with methanol are described by reactions (3) and (4). −− ⇀ FA + MeOH ↽ − − HF1

(3)

− ⇀ FA + HFn−1 − ↽ − − HFn ; n ≥ 2

(4)

Reactions (1) to (4) occur at all pH-levels without adding a catalyst. The chemical equilibrium of reactions (1) to (4) is far on the side of the products, such that the amount of monomeric formalde-


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hyde in these solutions is very small. By contrast, all reactions involving the formation of the acetals methylal and OME have to be catalyzed by acids. The formation of methylal (MAL, H3 C−O−(CH2 O)−CH3 ) is described by reaction (5). 27 This reaction is an acetalization in which the coupled product water is formed. Methylal can formally be seen as an OME of chain length n = 1. H+

− ⇀ HF1 + MeOH − ↽ − − MAL + H2 O

(5)

Long chain OME with n ≥ 2 are formed by two different mechanisms. On the one hand, OME of chain lengths n ≥ 2 are formed by an acetalization of poly(oxymethylene) hemiformals (n ≥ 2) and methanol according to reaction (6). 23,28 Again, water is formed as coupled product. H+

−− ⇀ HFn + MeOH ↽ − − OMEn + H2 O ; n ≥ 2

(6)

On the other hand, OME of chain lengths n ≥ 2 are also formed by a growth mechanism from methyal to OME2 and OMEn by insertion of monomeric formaldehyde 23,28 as described by reactions (7) and (8). H+

−− ⇀ FA + MAL ↽ − − OME2 H+

−− ⇀ FA + OMEn−1 ↽ − − OMEn ; n ≥ 3

(7) (8)

All of the reactions (1) to (8) are reversible. Thus, it is possible to recycle methylal and OME of undesired chain lengths back to the reactor where the OME synthesis takes place.

Reaction model A model describing the coupled system of reactions (1) to (8) was previously developed by our group. 9,23 The chemical equilibrium is described using chemical equilibrium constants based on mole fractions. 9 The reaction model was extended to a pseudo-homogeneous model of the reaction 5 ACS Paragon Plus Environment


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kinetics based on kinetic experiments using the heterogeneous ion-exchange catalyst Amberlyst 46. 23 The reaction model explicitly accounts for the formation of the poly(oxymethylene) glycols and poly(oxymethylene) hemiformals and thus includes all of the reactions (1) to (8). In this work, the chain lengths of the poly(oxymethylene) glycols, poly(oxymethylene) hemiformals, and OME is limited to n = 10. More details and model parameters are given in our previous work. 9,23

Vapor-liquid-liquid equilibrium A scheme of the model for calculating the vapor-liquid-liquid equilibrium in the system (formaldehyde+water+methanol+methylal+OME) is shown in Figure 1. The vapor-liquid equilibrium is calculated from the extended Raoult’s law and the liquid-liquid equilibrium is calculated from the isoactivity criterion. In the absence of an acidic catalyst, only reactions (1) to (4) have to be considered. Chemical reactions take place in all phases. As all poly(oxymethylene) glycols and poly(oxymethylene) hemiformals with n ≥ 2 are treated as non-volatile, reactions (2) and (4) are, however, not considered in the gas phase which is assumed to be a mixture of ideal gases. The non-ideality of the liquid phase is considered using an UNIFAC-based activity coefficient model. The model is based on the work of Maurer 29 and has been continuously extended by our group, e.g. see refs. 22,30–36 Its extension to include OME is based on measurements of the liquidliquid equilibrium in OME-containing subsystems. 22 All information for calculating the activity coefficients and also correlations for activity-based chemical equilibrium constants of reactions (1) to (4) are adopted from our previous work. 22 The vapor-pressure correlations of the OME were taken from Burger et al., 13 the vapor-pressure correlations of all other components were taken from Kuhnert et al. 36

True composition and overall composition In reactive systems containing formaldehyde, methanol, and water, two different ways of describing the composition are used. Besides the true composition of all components, also overall concentrations are given. The latter are found when the unstable poly(oxymethylene) gly6 ACS Paragon Plus Environment

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cols and poly(oxymethylene) hemiformals would completely decompose into formaldehyde, water and methanol. The true species concentrations quantify all poly(oxymethylene) glycols and poly(oxymethylene) hemiformals. In this work in most cases overall concentrations are depicted when results are presented. These are the concentrations that are obtained by conventional analysis, see below, and are thus more intuitive and comprehensive. The calculations using the reaction and the vapor-liquid-liquid equilibrium model, however, explicitly account for the poly(oxymethylene) glycols and poly(oxy-methylene) hemiformals.

Attainable splits using distillation Consider a continuous reactor in which OME are formed from formaldehyde and methanol. Figure 2 shows a typical true composition of the reactor outlet stream in chemical equilibrium at T = 343.15 K, which is calculated by the reaction model. The reactor outlet comprises formaldehyde, methanol, water, poly(oxymethylene) glycols, poly(oxymethylene) hemiformals, methylal, and OME. The OME of desired chain lengths and water have to be separated from the reactor outlet. The separations are challenging because of the large number of components, the similar chemical structure of OME compared to the poly(oxymethylene) glycols and poly(oxymethylene) hemiformals, and the reactive nature of the mixtures. There are a large number of azeotropes and distillation boundaries, which are partly identified and discussed in the following. Reactive distillation boundaries are shown in Figure 3 for the ternary systems (formaldehyde +water+OMEn ) and (formaldehyde+methanol+OMEn ) for n = 2 and n = 3 at p = 1.013 bar. They are reactive in the way that reactions (1) to (4) have been taken explicitly into account when determining the distillation boundaries. The solid lines mark the distillation boundaries with the arrow pointing toward the light-boiler. The liquid-liquid miscibility gap at T = 293.15 K is shown as light gray-shaded area and the region of formaldehyde precipitation is qualitatively shown as dark gray-shaded area. For the system (formaldehyde+water+OMEn ), the topology (i. e. number of azeotropes, dis-



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tillation regions) is identical for n = 2 and n = 3. The ternary map is split into three distillation regions. OMEn is the heavy-boiling node in region I, water the heavy-boiling node in region III. Light-boiling node of all regions is a ternary azeotrope. For the system (formaldehyde+methanol+OMEn ), the topology is different for n = 2 and n = 3. The presence of a ternary saddle azeotrope in the system (formaldehyde+methanol+OME2 ) leads to four distillation regions in this system. OME2 is the heavy-boiling node in regions I and IV. Methanol is the light-boiling node in regions III and IV. By contrast, the system (formaldehyde+methanol+OME3 ) exhibits only two distillation regions. OME3 is the heavy-boiling node in both distillation regions of the system (formaldehyde+methanol+OME3 ) and methanol is the light-boiling node in region II. To get an impression where a possible reactor outlet of an OME process is located in these systems, the reactor outlet in chemical equilibrium is calculated at T = 343.15 K for varying ratios of formaldehyde to methanol and three overall mass fractions of water in the reactor feed: 0 g/g, 0.1 g/g and 0.2 g/g. The compositions in chemical equilibrium are projected from the whole component space to the respective subsystem by a hypothetical removal of all other components from the equilibrium mixture. The projected compositions are shown as dashed lines in Figure 3. Figure 3 (top) shows that for both systems (formaldehyde+water+OME2 ) and (formaldehyde+methanol+OME2 ) the projected concentrations in chemical equilibrium are located in distillation regions in which OME2 cannot be obtained as a pure product. Figure 3 (bottom) shows that for the systems (formaldehyde+water+OME3 ) and (formaldehyde+methanol+OME3 ) the distillation regions in which OME3 can be obtained as a pure bottom product are considerably bigger compared to the corresponding systems with OME2 . Even more, from the ternary mixtures (formaldehyde+methanol+OME3 ), OME3 can always be separated quantitatively as pure bottom product. For the system (formaldehyde+water+OME3 ), the region in which OME3 can be separated quantitatively as pure bottom product also increases with increasing pressure. This is shown in Figure 4, where the distillation boundaries in this system are given at p = 1.013 bar and additionally at p = 3 bar.


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In the process of interest, the reactor outlet is a multicomponent mixture and thus distillation boundaries in the multicomponent space have to be considered. To check whether OME with n ≥ 3 can be separated from the reactor outlet, multicomponent distillation lines have to be calculated. The proposed split is feasible if a distillation line is found that connects the desired overhead and bottom products. The results show that OME with n ≥ 3 can be separated with good yield into the bottom product. For example at a column pressure of p = 1.5 bar and a reactor outlet as given in Table 4 (stream 3), the bottom product yields of the OME of various chain lengths are visualized in Figure S2 in the Supporting Information. For this distillation line formaldehyde, methanol, water, methylal and OME2 are not found in the bottom product, whereas 85% of OME3 and 100% of OME with n ≥ 4 are found in the bottom product. This results in a total bottom product yield of 95% for OME with n ≥ 3. The reactive distillation boundaries in Figure 3 also show that a sharp separation of water from the reactor outlet is not possible. Considering the system (formaldehyde+water+OMEn ), separation of pure water is only possible in distillation region III, where water is the heavy-boiling node. However, a large amount of water would remain in the overhead product in distillation region III, which would have to be recycled back to the reactor. This would shift the chemical equilibrium in the reactor to lower OME concentrations, as water is a coupled product of the OME synthesis. Distillation is thus not the desired unit operation for the separation of water.

Separation of water Water is the smallest and the most polar molecule of the system. Both of these physical properties can be exploited in an adsorption or a membrane process using zeolites as adsorber or membrane material. If a membrane process is preferred also hydrophilic polymer membranes are conceivable. The long-term chemical stability against formaldehyde would have to be investigated both for zeolites and polymers. In the patent literature, Dams et al. 37 investigated a membrane process for separating water from solutions of formaldehyde, water and methanol using a using a hydrophilic



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polymer membrane. This work investigates an adsorption process from the liquid-phase. The feasibility of adsorbing water from formaldehyde-containing mixtures using zeolites was indicated by Hasse. 38 The structure of the zeolites allows water to diffuse into the pores, whereas all other components are too big (molecular sieve). In addition, due to it’s high polarity, water has the highest chemical affinity of all components in the system to the zeolites, which have an ionic crystalline structure.

Adsorption experiments Chemicals Paraformaldehyde (> 0.95 g/g) and Zeolite 3A (pore size 3 Å) were purchased from Carl Roth. Methanol (> 0.999 g/g) and methylal (> 0.99 g/g) were purchased from Sigma-Aldrich. Ultrapure water was produced with a Milli-Q water purification system from Merck. Methanolic formaldehyde solutions were prepared by dissolving paraformaldehyde in methanol, which is described in previous work. 9 . OME2 and OME4 (both > 0.98 g/g) were provided by BASF SE. Before usage, Zeolite 3A was dried at a temperature of 363 K and a pressure of 100 mbar in a vacuum oven.

Analysis The overall mass fractions of methanol, methylal, and all OME were determined by gas chromatography and 1,4-dioxane as internal standard. The details of the gas chromatographic method are given in previous work. 9 The relative errors for the overall mass fractions are 5% for methanol, and 2% for methylal and all OME. 9 The overall mass fraction of formaldehyde was determined using the sodium-sulfite titration method with hydrochloric acid as titer. 39 The overall mass fraction of water was determined with Karl Fischer titration. The relative error for both the overall mass fraction of formaldehyde and water is 2%. 9 The sum of all overall mass fractions was between 0.97 and 1.03 g/g in all samples. To provide consistent data sets, all overall mass fractions were normalized to a sum of 1.00 g/g by proportional weighing. 10 ACS Paragon Plus Environment

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Experimental procedure Water adsorption isotherms were obtained from single-stage batch experiments. Liquid mixtures comprising formaldehyde, water, methanol, methylal, and OME were prepared. The composition of these mixtures are chosen to be similar to the process reactor outlet stream after removal OME with n ≥ 3. Around 20 g of the mixtures and 10 g of dried Zeolite 3A were added to a flask and equilibrated over night at isothermal compositions (achieved by an oil bath thermostatization) under gentle shaking. It was checked in preliminary experiments that the equilibrium is established. Samples of the liquid mixtures after adsorption were taken with a syringe. The temperature was measured with a Pt 100 resistance thermometer with an accuracy of ± 0.1 K. The loading on the zeolite with water was calculated from a mass balance. The mass balance is based on the assumption that water is the only adsorbing component and given by Eq. (9), (m)

(m)

mInitial x˜H2 O,Initial − x˜H2 O,Eq qH2 O = · (m) mZeolite 1 − x˜

(9)

H2 O,Eq

where mInitial is the total mass of the initial liquid mixture, mZeolite is the mass of dried zeolite, and (m)

(m)

x˜H2 O,Initial , x˜H2 O,Eq are the overall mass fractions of water in the initial mixture, and the liquid bulk phase in adsorption equilibrium, respectively. The relative uncertainty of the loading of water in the adsorbed phase qH2 O is smaller than 10%. This was calculated by Gaussian error propagation accounting for the uncertainty of the relative error of the measured overall mass fraction of water in equilibrium. In total, 12 experiments at 298.15 K and 311.85 K were carried out. Overall mass fractions were varied: formaldehyde between 0.05 and 0.20 g/g, water between 0.01 and 0.18 g/g, methanol between 0.05 and 0.10 g/g, methylal between 0.35 and 0.70 g/g, OME2 between 0.15 and 0.20 g/g, and OME4 was approximately 0.02 g/g. OME4 was added as a standard, as it is too big to diffuse into the pores.



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Experimental results & modeling Numerical data for the compositions of the mixtures prior- and after adsorption, the total mass of the initial liquid mixtures, and the mass of zeolite for each experiment are given in the Supporting Information. The water adsorption isotherms are shown in Figure 5. Despite variation of the composition in the experiments, clear trends are observed for the two temperatures. An increased temperature leads to decreased loadings of water in the adsorbed phase. This is expected, as adsorption of water on zeolites is an exothermic process. To check whether the assumption of exclusive adsorption of water is justified, the composition of all other components in the liquid bulk phase in equilibrium are calculated under the said as(m)

sumption from the measured x˜H2 O,Eq . These calculated overall mass fractions are compared to the measured ones in a parity plot in Figure 6. The good agreement of both data for all components indicates that water is adsorbed with high selectivity. The two experimental isotherms were correlated by a Langmuir isotherm with parameters qH2 O,max and KL , c.f. Eq. (10). (m)

qH2 O =

qH2 O,max · KL · x˜H2 O,Eq (m)

1 + KL · x˜H2 O,Eq

(10)

The parameters are given in Table 1 for both investigated temperatures. Figure 5 shows that the experimental data are well correlated by the Langmuir isotherm. The parameter qH2 O,max is 0.166 g/g at 298.15 K. This maximum loading of water is in good agreement with data from Teo and Ruthven 40 who investigated adsorption of water on Zeolite 3A from mixtures of ethanol and water.


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Process design Process concept A flowsheet of the novel OME process is depicted in Figure 7. The feed stream 1 comprises formaldehyde and methanol and eventually water. The feed stream 1 is mixed with the two recylce streams 7 and 9 and fed into the heterogeneously-catalyzed reactor to obtain a stream comprising formaldehyde, water, methanol, methylal, and OME of various chain lengths (stream 3). The separation of the desired OME product fraction n = 3 − 5 is carried out in two distillation columns C1 and C2. The first distillation column C1 separates OME of chain lengths n ≥ 3 as bottom product from the reactor outlet (stream 5). The second distillation column C2 separates the product fraction n = 3 − 5 as overhead product (stream 6) from OME of longer chain lengths n ≥ 6 (stream 7) which are recycled back to the reactor. The feasibility of the separation in column C2 was shown in the work of Burger et al. 12 The overhead product of column C1 (stream 4) is fed into the adsorption unit. It could also be replaced by a membrane unit. However, a fixed-bed adsorption unit with at least two fixed-beds is proposed here, such that one fixed-bed can be regenerated by temperature and/or pressure shift while the other fixed-bed is on stream. Stream 8 is obtained by regeneration of the fixed-bed adsorption unit. The water-depleted stream 9 is recycled back to the reactor.

Process model For the simulation and optimization of the OME process, a reduced process model is developed. The reactor is modeled as isothermal fixed-bed reactor described by a cascade of ten continuously stirred tank reactors, among which the amount of catalyst is equally distributed. The reaction rates are calculated using the kinetic model from our previous work. 23 For the distillation columns, infinite height and infinite reflux ratio is assumed. This enables sharp separations only constrained by the thermodynamic limits (azeotropes and distillation boundaries). 41,42 The thermodynamic limits have been discussed above. For the distillation column C1, it is assumed that OME of chain lengths n ≥ 3 are separated perfectly sharp as bottom product. 13 ACS Paragon Plus Environment


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For column C2 it is assumed that OME with n = 3 − 5 are obtained in a perfectly sharp split as top product, and OME with n ≥ 6 are obtained as bottom product. The adsorption unit is modeled as one equilibrium stage, in which adsorption equilibrium between the adsorbent phase and the liquid bulk phase is established. The adsorption equilibrium is described using the Langmuir isotherm (c.f. Eq. (10)) with parameters from Table 1. A scheme of the model is shown in Figure 8. The flow rate of zeolite m˙ Zeolite can be interpreted as follows. For a given mass of zeolite in one fixed-bed mZeolite , the ratio mZeolite /m˙ Zeolite is the time, after which the fixed-bed needs to be regenerated. Although being quite simple, the process model still provides a stream table including flows and compositions of all streams.

Process optimization Design parameters and objectives The design parameters investigated in the present study are listed in Table 2. The bounds of the ratio of formaldehyde to methanol in the feed result from the mass balance of the overall process. If the process would be operated at the lower bound, only OME3 would be produced, at the upper bound only OME5 would be produced and thus within these bounds mixtures of OME (n = 3 − 5) are produced. The upper bound of the overall mass fraction of water in the feed is limited to 0.2 g/g, as higher concentrations of water lead to considerably decreased yields of OME in the reactor, see the chemical equilibrium lines in Figure 3. The reactor temperature is fixed to TReactor = 343.15 K in order to reduce the formation of the side products trioxane and methyl formate. 9,10 The pseudo residence time in the reactor is defined as ratio of the mass of catalyst to the mass flow rate of the product stream 6 (τ = mCat /m˙ Product ). For τ = ∞ chemical equilibrium is reached. The temperature of the adsorption unit is fixed to TAdsorption = 298.15 K. The overall mass fraction of water in stream 9 (water-depleted stream after adsorption unit) is limited to 0.05 g/g. Its value determines how much zeolite is needed respectively how often it has to be regenerated. The value of the bound is simply chosen to a quite large value so that the bound is not active at all relevant solutions of the optimization. 14 ACS Paragon Plus Environment

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At this early stage of design costing functions are not available. Therefore, a multi-objective optimization problem is formulated. Several objectives are simultaneously optimized without the need for explicitly weighting them. A general view of this concept is given by Burger et al. 43 The objectives comprise technical quantities of the process having a significant impact on process costs and are given in Table 3. The overall mass fraction of water in the feed shall be maximized. A higher overall mass fraction of water in the feed lowers the cost of the feed stream as water removal from formaldehyde solutions is expensive. The flow rates of the recycle streams 7 and 9 (which are scaled here with respect to the flow rate of the product (stream 6) and the pseudo residence time shall be minimized. The size of the adsorption unit is considered through the flow rate m˙ Zeolite . It is also scaled with respect to the flow rate of the product and shall be minimized. The overall mass fraction of water in the feed and the pseudo residence time are design parameters and objectives on the same time. Multi-objective optimization problems are solved by the calculation of Pareto sets. The Pareto sets contain all solutions, in which the improvement of one objective can only be achieved by the decline in at least one other objective. Here, the sandwiching method to approximate the Pareto sets is used. 44 More details on the algorithm 45,46 is given elsewhere. Because of the large number of objectives, two cases are considered in the following.

Case study 1 In case study 1, the first three objectives in Table 3 are considered. The Pareto set is shown in Figure 9 in a level curve plot. The two objectives scaled recycle flow rate (stream 7) and scaled recycle flow rate (stream 9) are plotted on the axes. The full lines represent Pareto-optimal solutions on which the overall mass fraction of water in the feed, which is the third objective, is constant. The grey-shaded area represents the range of all Pareto-optimal solutions within the (m)

bound x˜H2 O,1 ≤ 0.2 g/g. The dashed lines are inferior solutions and thus not part of the Pareto set. Because the reactor (pseudo residence time) and the adsorption unit (scaled flow rate of zeolite) are not considered as objectives, the results yield a reactor with infinite amount of catalyst (τ = ∞), and



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an adsorption unit with infinite amount of zeolite (m˙ Zeolite = ∞) and perfectly sharp water removal (m)

(x˜H2 O,9 = 0 g/g). However, case study 1 is still interesting to understand the relation between the feed composition and the rate of recycle streams. The results in Figure 9 show that with increasing overall mass fractions of water, the recycle streams are increased. This is because the presence of water reduces the yields of OME in the reactor. It becomes apparent that even for no water in the feed and the idealized units of reactor and adsorption, there will be a recycle mass flowrate bigger than 3 times the product flowrate. For 0.1 g/g or 0.2 g/g water in the feed, the recycle mass flowrate will at least be 4 or 5 times the product flowrate, respectively. Besides the overall mass fraction of water in the feed, the results depend on the ratio of formaldehyde to methanol in the feed. For a ratio of formaldehyde to methanol equal to 1.60 g/g and 1.69 g/g , respectively, the solutions are indicated by symbols. Recycle stream 9 is rather big compared to stream 7. In addition, stream 9 occurs as overhead column product. It is thus reasonable to choose a trade-off where this stream is rather small. So a ratio of formaldehyde to methanol around 1.69 g/g is a good value independent of the overall mass fractions of water in the feed. As mentioned before, the ratio of formaldehyde to methanol determines the chain length distribution of OME in the product stream. The resulting product composition will be discussed in detail alongside the results of case study 2 in which the ratio of formaldehyde to methanol takes similar values.

Case study 2 In case study 2, all objectives in Table 3 are considered. However, instead of accounting for the two recycle streams separately, they are now summed up and considered together to reduce the objective space to four dimensions. The calculated Pareto set and its image in the design space is shown in Figure 10 in different projections. In one row, the objective overall mass fraction of (m)

water in the feed is constant: x˜H2 O,1 = 0.0 g/g (top), 0.1 g/g (middle) and 0.2 g/g (bottom). In each row, the Pareto set in the objective space is shown in a level curve plot in the left diagram. The objectives sum of the scaled recycle streams 7 and 9 and the scaled flow rate of zeolite are plotted


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on the axes. The full lines represent Pareto-optimal solutions on which the pseudo residence time, which is the remaining objective, is constant. In each row, the image of the Pareto set to the design space is shown in the right diagram. The Pareto set in the objective space is linked to the design space by a color code. Within one row of diagrams, points with the same curve parameter and the same color belong to the same solution. When increasing the overall mass fraction of water in the feed from the top row to the bottom row, both the sum of the flow rates of the recycle streams 7 and 9 and the mass flow of zeolite increase. This is a combined effect of the reduced yields of OME in the reactor (as discussed above) and the larger amount of water that has to be separated in the adsorber, as more water is fed into the process. With increasing overall mass fractions of water in the feed, the optimal ratio of formaldehyde to methanol slightly decreases. The smaller the pseudo residence time, the smaller are the yields of OME in the reactor, and the sum of the flow rates of the recycle streams 7 and 9 and the mass flow of zeolite increase. For τ = 1000 s the reactor is still in the kinetic regime, but chemical equilibrium (τ = ∞) is almost reached in the reactor outlet. With increasing pseudo residence time, the optimal ratio of formaldehyde to methanol slightly increases. On the entire Pareto set, the ratio of formaldehyde to methanol remains in a quite small range of 1.66 g/g to 1.72 g/g. This range confirms the value of 1.69 g/g that was selected earlier on the basis of case study 1. Pick any curve in the objective space (left column of diagrams) and observe the trade-off between recycle flow rates and flow rate of zeolite. Via the color code, the overall mass fraction of (m)

water in the water-depleted stream 9 after the adsorption unit x˜H2 O,9 can be read from the design space (right column of diagrams). High flow rates of zeolite (blue) lead to low recycles and low (m)

(m)

values of x˜H2 O,9 . Small flow rates of zeolite (red) lead to large recycles and large values of x˜H2 O,9 . The reason is that larger efforts during adsorption lead to a more effective removal of water, and as a consequence, also to better OME yields in the reactor.



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Operating points The Pareto set in Figure 10 is now used to identify two promising operating points of the novel (m)

(m)

OME process, one for x˜H2 O,1 = 0.0 g/g and one for x˜H2 O,1 = 0.1 g/g. In both cases, the pseudo residence time is chosen to be τ = 1000 s, at which the reactor operates close to chemical equilibrium with reasonable reactor size. This corresponds to a space-time-yield of (1/τ ) = 3.6 kg/(kgCat ·h). The remaining two degrees of freedom are chosen to represent a good compromise between the conflicting objectives sum of the scaled recycle streams 7 and 9 and the scaled flow rate of zeolite. The chosen operating points OP1 and OP2 are indicated in Figure 10. Overall concentration profiles in the reactor are shown in Figure 11 for operating point OP2. Figure 11 also indicates that adding more catalyst to the reactor would not result in substantially increased yields of the OME as the chemical equilibrium is almost reached. In addition, the overall mass fractions of methylal and OME of undesired chain lengths (OME2 and OMEn ≥ 6 ) are almost constant. For the OME of undesired chain lengths, this is in agreement with findings from Burger et al. 13 for the OME production process from methylal and trioxane. Caused by the large recycle streams, the concentrations of methylal and OME of undesired chain lengths in the reactor inlet are already close to their equilibrium concentrations. The stream tables of the operating points are given in Table 4 (OP1) and Table 5 (OP2). For the calculation of the stream tables, the flow rate of the product (stream 6) is chosen to 1000 kg/h. It is remarkable that when increasing the overall mass fraction of water in the feed from 0.0 g/g (OP1) to 0.1 g/g (OP2), the flow rate of zeolite is almost doubled clearly indicating that low water concentrations in the feed are desirable. This might be achieved by integration of the novel OME process with present 39 or innovative formaldehyde production technology, e.g. non-oxidative dehydrogenation of methanol 47 or hydrogenation of CO in the liquid phase. 48 The presented operating points of novel OME process are both characterized by high ratios of formaldehyde to methanol, which turned out to be optimal in terms of the multi-objective optimization study. It will have to be identified during practical operation of the novel OME process whether this leads to formaldehyde precipitation in the process. Highly concentrated formaldehyde 18 ACS Paragon Plus Environment

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solutions have, however, been shown to be manageable in chemical processes. 49 When accepting higher recycle flow rates, the process might also be operated at lower ratios of formaldehyde to methanol to avoid handling of concentrated formaldehyde solutions. An outcome of the multi-objective optimization study is the chain length distribution of the OME in the product stream. It is shown for the two operating points OP1 and OP2 in Figure 12. As a comparison, the chain length distribution of OME produced from trioxane and methylal according to the process from Burger et al. 13 is also shown in Figure 12. For the OME production from formaldehyde and methanol, the chain length distribution is slightly shifted to lower chain lengths when compared to OME production from methylal and trioxane.

Conclusion In the present work, a novel process for the production of OME from formaldehyde and methanol is presented. In this process, OME of the desired chain lengths n = 3 − 5 and the coupled product water are separated from the reactive mixture (formaldehyde+water+methanol+methylal+OME). By studying chemical equilibria and distillation boundaries, it was shown that OME of chain lengths n ≥ 3 can be separated as bottom product in a reactive distillation column with good yield. The separation of water in the novel OME process is carried out using either an adsorption or a membrane process with zeolites as adsorber or membrane material, respectively. For the design of an adsorption process, water adsorption isotherms from process-relevant mixtures have been determined experimentally. Regarding the membranes, further research in aqueous OME systems is needed. The novel OME process was designed using a reduced process model. Favorable operating points of the novel OME process have been identified using multi-objective optimization. Stream tables of the novel OME process are given for two promising operating points. The chain length distribution of the produced OME mixture (n = 3 − 5) is: OME3 : 0.477 g/g, OME4 : 0.315 g/g, OME5 : 0.208 g/g for operating point OP1 (no water in feed), and OME3 : 0.498 g/g, OME4 :



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0.307 g/g, OME5 : 0.194 g/g for OP2 (0.1 g/g water in feed). The novel OME process significantly contributes to the market integration of OME as its production technology enables integration of OME into the C1-value-added chain without producing expensive intermediates. It produces OME from the readily available C1-feedstocks formaldehyde and methanol containing also water. Thus, compared to known OME production technology in which prior to OME synthesis different intermediates have to be produced from formaldehyde and methanol (c.f the flowsheet in Figure S1 in the Supporting Information), the novel OME process (c.f. the flowhseet in Figure 7) is beneficial in both CAPEX and OPEX. In the main equipment, the novel OME process saves two reactors and five distillation columns, whereas one additional unit for the separation of water is necessary (either adsorption or membrane process).

Nomenclature Abbreviations CAPEX

Capital expenditure

FA

Formaldehyde

HF

Poly(oxymethylene) hemiformal

MAL

Methylal

MeOH

Methanol

MG

Poly(oxymethylene) glycol

OME

Poly(oxymethylene) dimethyl ether

OP

Operating point

OPEX

Operational expenditure

UNIFAC

Universal Quasichemical Functional Group Activity Coefficients


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Symbols and indices KL

Parameter for Langmuir isotherm

m

Mass

m˜ i

Overall mass of component i

m˙

Mass flow

n

Oligomer chain length of component

qH2 O

Loading of water in the adsorbed phase

qH2 O,max

Parameter for Langmuir isotherm

T

Temperature

τ

Pseudo residence time (m)

Overall mass fraction of component i

(m)

Overall mass fraction of component i in adsorption equilibrium

x˜i

x˜i,Eq

Acknowledgement This work was funded by the German Ministry of Food and Agriculture (BMEL) through grant 22403914.

Supporting Information Available The following files are available free of charge. • SI.pdf - Numerical data on the adsorption experiments: Compositions of the mixtures priorand after adsorption, total mass of the initial liquid mixtures, and the mass of zeolite for each experiment. Flowsheet of the route leading to OME via the intermediates trioxane and methylal. Visualization of the yields of one multicomponent distillation line. This material is available free of charge via the Internet at http://pubs.acs.org/.



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9. Schmitz, N.; Homberg, F.; Berje, J.; Burger, J.; Hasse, H. Chemical equilibrium of the synthesis of poly(oxymethylene) dimethyl ethers from formaldehyde and methanol in aqueous solutions. Ind. Eng. Chem. Res. 2015, 54, 6409–6417. 10. Burger, J.; Ströfer, E.; Hasse, H. Chemical equilibrium and reaction kinetics of the heterogeneously catalyzed formation of poly(oxymethylene) dimethyl ethers from methylal and trioxane. Ind. Eng. Chem. Res. 2012, 51, 12751–12761. 11. Lautenschütz, L.; Oestreich, D.; Haltenort, P.; Arnold, U.; Dinjus, E.; Sauer, J. Efficient synthesis of oxymethylene dimethyl ethers (OME) from dimethoxymethane and trioxane over zeolites. Fuel Process. Technol. 2017, 165, 27–33. 12. Burger, J.; Hasse, H. Multi-objective optimization using reduced models in conceptual design of a fuel additive production process. Chem. Eng. Sci. 2013, 99, 118–126. 13. 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. 14. Masamoto, J.; Matsuzaki, K. Development of methylal synthesis by reactive distillation. J. Chem. Eng. Jpn. 1994, 27, 1–5. 15. Grützner, 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. 16. Weidert, J.-O.; Burger, J.; Renner, M.; Blagov, S.; Hasse, H. Development of an integrated reaction–distillation process for the production of methylal. Ind. Eng. Chem. Res. 2017, 56, 575–582. 17. Schmitz, N.; Burger, J.; Ströfer, E.; Hasse, H. From methanol to the oxygenated diesel fuel poly(oxymethylene) dimethyl ether: An assessment of the production costs. Fuel 2016, 185, 67–72. 23 ACS Paragon Plus Environment


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18. Chen, J.; Song, H.; Xia, C.; Kang, M. Reaction system and process for preparing polymethoxy dimethyl ether (US 2015/0094497). 2014. 19. Ströfer, E.; Hasse, H.; Blagov, S. Method for producing polyoxymethylene dimethylethers from methanol and formaldehyde (WO 2006134088). 2006. 20. Moulton, D. S.; Naegeli, D. W. Diesel fuel having improved qualities and method of forming (US 5746785). 1998. 21. Arnold, U.; Lautenschütz, L.; Oestreich, D.; Sauer, J. Verfahren zur Herstellung von Oxymethylenethern und deren Verwendung (EP 2987781). 2016. 22. Schmitz, N.; Friebel, A.; von Harbou, E.; Burger, J.; Hasse, H. Liquid-liquid equilibrium in binary and ternary mixtures containing formaldehyde, water, methanol, methylal, and poly(oxymethylene) dimethyl ethers. Fluid Phase Equilib. 2016, 425, 127–135. 23. Schmitz, N.; Burger, J.; Hasse, H. Reaction kinetics of the formation of poly(oxymethylene) dimethyl Ethers from formaldehyde and methanol in aqueous solutions. Ind. Eng. Chem. Res. 2015, 54, 12553–12560. 24. Ott, M.; Schoenmakers, H.; Hasse, H. Distillation of formaldehyde containing mixtures: laboratory experiments, equilibrium stage modeling and simulation. Chem. Eng. Process 2005, 44, 687–694. 25. 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. 26. Hahnenstein, I.; Albert, M.; Hasse, H.; Kreiter, C. G.; Maurer, G. NMR spectroscopic and densimetric study of reaction kinetics of formaldehyde polymer formation in water, deuterium oxide, and methanol. Ind. Eng. Chem. Res. 1995, 34, 440–450.


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27. 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. 28. Oestreich, D.; Lautenschütz, L.; Arnold, U.; Sauer, J. Reaction kinetics and equilibrium parameters for the production of oxymethylene dimethyl ethers (OME) from methanol and formaldehyde. Chem. Eng. Sci. 2017, 163, 92–104. 29. Maurer, G. Vapor-liquid equilibrium of formaldehyde-and water-containing multicomponent mixtures. AIChE J. 1986, 32, 932–948. 30. Hasse, H.; Hahnenstein, I.; Maurer, G. Revised vapor-liquid equilibrium model for multicomponent formaldehyde mixtures. AIChE J. 1990, 36, 1807–1814. 31. Hasse, H.; Maurer, G. Vapor-liquid equilibrium of formaldehyde-containing mixtures at temperatures below 320 K. Fluid Phase Equilib. 1991, 64, 185–199. 32. 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. 33. Albert, M.; Coto Garcia, B.; Kreiter, C.; Maurer, G. Vapor-liquid and chemical equilibria of formaldehyde-water mixtures. AIChE J. 1999, 45, 2024–2033. 34. Albert, M.; Coto Garcia, B.; Kuhnert, C.; Peschla, R.; Maurer, G. Vapor–liquid equilibrium of aqueous solutions of formaldehyde and methanol. AIChE J. 2000, 46, 1676–1687. 35. 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. 36. 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



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+ methanol + methylal) and comparison with predictions. Ind. Eng. Chem. Res. 2006, 45, 5155–5164. 37. Dams, A.; Fried, A.; Hammann, F. Process for the production of concentrated aqueous formaldehyde solutions by pervaporation (EP 0652201). 1995. 38. Hasse, H. Dampf-Flüssigkeits-Gleichgewichte, Enthalpien und Reaktionskinetik in formaldehydhaltigen Mischungen. Ph.D. thesis, TU Kaiserslautern, Kaiserslautern, 1990. 39. Fiedler, E.; Grossmann, G.; Kersebohm, D. B.; Weiss, G.; Witte, C. Formaldehyde, Ullmann’s Encyclopedia of Industrial Chemistry; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2000. 40. Teo, W. K.; Ruthven, D. M. Adsorption of water from aqueous ethanol using 3-Å molecular sieves. Ind. Eng. Chem. Proc. Des. Dev. 1986, 25, 17–21. 41. Petlyuk, F.; Avet’yan, V. Investigation of the rectification of three-component mixtures with infinite reflux. Theor. Found. Chem. Eng. 1971, 5, 499–507. 42. Ryll, O.; Blagov, S.; Hasse, H. ∞/∞-Analysis of homogeneous distillation processes. Chem. Eng. Sci. 2012, 84, 315–332. 43. Burger, J.; Ströfer, E.; Hasse, H. Process design in World 3.0 - Challenges and strategies to master the raw material change. Chem. Eng. Technol. 2016, 39, 219–224. 44. Serna Hérnandez, J. I. Multi-objective optimization in mixed integer problems: with application to the Beam Selection Optimization problem in IMRT. Ph.D. Thesis, TU Kaiserslautern, Kaiserslautern, 2012. 45. Bortz, M.; Burger, J.; Asprion, N.; Blagov, S.; Böttcher, R.; Nowak, U.; Scheithauer, A.; Welke, R.; Küfer, K.-H.; Hasse, H. Multi-criteria optimization in chemical process design and decision support by navigation on Pareto sets. Comput. Chem. Eng. 2014, 60, 354–363.


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46. Burger, J.; Asprion, N.; Blagov, S.; Böttcher, R.; Nowak, U.; Bortz, M.; Welke, R.; Küfer, K.H.; Hasse, H. Multi-objective optimization and decision support in process engineering - implementation and application. Chem. Ing. Tech. 2014, 86, 1065–1072. 47. Su, S.; Zaza, P.; Renken, A. Catalytic dehydrogenation of methanol to water-free formaldehyde. Chem. Eng. Technol. 1994, 17, 34–40. 48. Bahmanpour, A. M.; Hoadley, A.; Tanksale, A. Formaldehyde production via hydrogenation of carbon monoxide in the aqueous phase. Green Chem. 2015, 17, 3500–3507. 49. Ströfer, E.; Sohn, M.; Hasse, H.; Schilling, K. Highly concentrated formaldehyde solution, production and reaction thereof (US 7193115). 2007.



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Table 1: Parameters for the Langmuir isotherm according to Eq. (10) for the adsorption of water. T / (K)

KL

qH2 O,max / (g/g)

298.15 311.85

419 30

0.166 0.134

Table 2: Design parameters of the process. design parameter

symbol

lower bound

ratio of formaldehyde to methanol in feed (stream 1) overall mass fraction of water in feed (stream 1) reactor temperature pseudo residence time in reactor adsorber unit temperature overall mass fraction of water in stream 9

e FA,1 /m e MeOH,1 m (m) x˜H2 O,1 TReactor τ TAdsorption (m) x˜H2 O,9

upper bound

1.41 2.34 0 0.2 343.15 K (fixed) 0 ∞ 298.15 K (fixed) 0 0.05

Table 3: Objectives used for the multi-objective optimization of the process. Two case studies are investigated, for which the considered objectives are marked. objective

symbol

overall mass fraction of water in feed (stream 1) scaled recycle flow rate (stream 7) scaled recycle flow rate (stream 9) pseudo residence time scaled flow rate of zeolite

x˜H2 O,1 m˙ 7 /m˙ Product m˙ 9 /m˙ Product τ m˙ Zeolite /m˙ Product

(m)


goal maximize minimize minimize minimize minimize

case 1

case 2

× × ×

× × × × ×

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Table 4: Stream table of the OME process for operating point OP1. The stream names are given in Figure 7. Required mass flow of zeolite: m˙ Zeolite = 862 kg/h. stream mass flow / (kg/h)

1

2

3

4

5

6

7

8

9

1116

5351

5351

3965

1386

1000

386

116

3849

overall mass fraction / (g/g) Formaldehyde Methanol Water Methylal OME2 OME3 OME4 OME5 OME6 OME7 OME8 OME9 OMEn ≥10

0.629 0.371 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000

0.434 0.303 0.409 0.000 0.000 0.000 0.000 0.421 0.193 0.116 0.156 0.000 0.000 0.000 0.000 0.161 0.007 0.029 0.039 0.000 0.000 0.000 1.000 0.010 0.165 0.165 0.223 0.000 0.000 0.000 0.000 0.229 0.128 0.128 0.173 0.000 0.000 0.000 0.000 0.178 0.000 0.089 0.000 0.344 0.477 0.000 0.000 0.000 0.000 0.059 0.000 0.227 0.315 0.000 0.000 0.000 0.000 0.039 0.000 0.150 0.208 0.000 0.000 0.000 0.027 0.027 0.000 0.103 0.000 0.369 0.000 0.000 0.019 0.019 0.000 0.071 0.000 0.257 0.000 0.000 0.013 0.013 0.000 0.049 0.000 0.177 0.000 0.000 0.009 0.009 0.000 0.033 0.000 0.120 0.000 0.000 0.006 0.006 0.000 0.022 0.000 0.078 0.000 0.000

Table 5: Stream table of the OME process for operating point OP2. The stream names are given in Figure 7. Required mass flow of zeolite: m˙ Zeolite = 1756 kg/h. stream mass flow / (kg/h)

1

2

3

4

5

6

7

8

9

1241

6539

6539

5200

1339

1000

339

241

4959

overall mass fraction / (g/g) Formaldehyde Methanol Water Methylal OME2 OME3 OME4 OME5 OME6 OME7 OME8 OME9 OMEn ≥10

0.564 0.336 0.100 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000

0.445 0.338 0.425 0.000 0.000 0.000 0.000 0.446 0.198 0.134 0.168 0.000 0.000 0.000 0.000 0.176 0.028 0.046 0.057 0.000 0.000 0.000 1.000 0.011 0.160 0.160 0.201 0.000 0.000 0.000 0.000 0.211 0.117 0.117 0.148 0.000 0.000 0.000 0.000 0.155 0.000 0.076 0.000 0.372 0.498 0.000 0.000 0.000 0.000 0.047 0.000 0.230 0.307 0.000 0.000 0.000 0.000 0.030 0.000 0.145 0.194 0.000 0.000 0.000 0.020 0.020 0.000 0.097 0.000 0.383 0.000 0.000 0.014 0.014 0.000 0.066 0.000 0.261 0.000 0.000 0.009 0.009 0.000 0.044 0.000 0.174 0.000 0.000 0.006 0.006 0.000 0.028 0.000 0.112 0.000 0.000 0.004 0.004 0.000 0.018 0.000 0.070 0.000 0.000



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

FA + FA +

H2O MeOH

ļ ļ

MG1 HF1

Vapor

FA

H2O

MeOH

MG1

HF1

MAL

OMEn

FA

H2O

MeOH

MG1

HF1

MAL

OMEn

FA FA FA FA

+ + + +

H2O MGn-1 MeOH HFn-1

ļ ļ ļ ļ

Page 30 of 40

MG1 MGn ; n 2 HF1 HFn ; n 2

Liquid 1

FA

H2O

MeOH

MG1

MGn

HF1

HFn

MAL

OMEn

FA

H2O

MeOH

MG1

MGn

HF1

HFn

MAL

OMEn

FA FA FA FA

+ + + +

H2O MGn-1 MeOH HFn-1

ļ ļ ļ ļ

MG1 MGn ; n 2 HF1 HFn ; n 2

Liquid 2

Figure 1: Scheme of the reactive vapor-liquid-liquid equilibrium in the system (formaldehyde+water+methanol+methylal+OME).


Page 31 of 40

0.15

true mass fraction / (g/g)

0.10

0.05

9

10

8

OME

OME

7

OME

6

OME

5

4

OME

OME

3

OME

OME

2

MAL OME

9

10

MG

8

MG

7

6

MG

MG

5

MG

4

MG

3

2

MG

MG

1

MG

MG

9

10

8

HF

HF

7

HF

6

HF

5

4

HF

HF

3

HF

2

1

HF

2

HF

HF

FA

0.00 MeOH H O

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 2: True mass fractions of all species in chemical equilibrium at T = 343.15 K. Overall mass fractions of reactor feed: 0.60 g/g formaldehyde, 0.40 g/g methanol. Corresponding overall mass fractions in chemical equilibrium: 0.358 g/g formaldehyde, 0.147 g/g methanol, 0.071 g/g water. Overall mass fractions of methylal and OME are identical to their true mass fractions.



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

Formaldehyde

Formaldehyde 0.0

0.0

1.0

0.2

0.8

0.0 0.4

0.6

0.8

1.0

IV

1.0

Water OME2

0.0

0.0

0.2

0.4

0.0

1.0

0.2

0.6

0.6

0.4

0.4

I

II

0.8

0.2

0.2

II

1.0 0.0

0.0 0.2

0.4

0.6

MeOH

0.6

III OME3

1.0

0.8

0.4

0.6

I

0.8

1.0

0.2

0.8

0.4

0.8

0.6

Formaldehyde

Formaldehyde 0.0

0.2

III

I

I 0.2

0.4

II

0.8

0.2

III

0.0

0.6

0.6

0.4

II

0.8

0.4

0.6

0.6

1.0

1.0

0.2

0.8

0.4

OME2

Page 32 of 40

0.8

1.0

1.0

Water OME3

0.0

0.0 0.2

0.4

0.6

0.8

1.0

MeOH

Figure 3: Reactive distillation boundaries in the systems (formaldehyde+water+OMEn ) and (formaldehyde+methanol+OMEn ) for n = 2 (top) and n = 3 (bottom) at p = 1.013 bar. (•): Azeotropes. Solid lines: distillation boundaries. Light gray-shaded area: liquid-liquid miscibility gap at T = 293.15 K. Dark gray-shaded area: formaldehyde precipitation (qualitatively). Dashed lines: projected compositions in chemical equilibrium at T = 343.15 K for three different overall mass fractions of water in the reactor inlet: 0 g/g, 0.1 g/g and 0.2 g/g. Overall concentrations in g/g.


Page 33 of 40

F o r m a ld e h y d e 0 .0 0

1 .0 0

0 .2 5

0 .7 5

0 .5 0

0 .5 0

3 b a r 0 .7 5

1 .0 1 3 b a r

0 .2 5

1 .0 0

O M E 3

0 .0 0

0 .0 0

0 .2 5

0 .5 0

0 .7 5

1 .0 0

W

a te r

Figure 4: Reactive distillation boundaries in the system (formaldehyde+water+OME3 ) at p = 1.013 bar and p = 3 bar. (•): Azeotropes. Solid lines: distillation boundaries at p = 1.013 bar. Dotted lines: distillation boundaries at p = 3 bar. Light gray-shaded area: liquid-liquid miscibility gap at T = 293.15 K. Dark gray-shaded area: formaldehyde precipitation (qualitatively). Dashed lines: projected compositions in chemical equilibrium at T = 343.15 K for three different overall mass fractions of water in the reactor inlet: 0 g/g, 0.1 g/g and 0.2 g/g. Overall concentrations in g/g.

/ (g/g)

0.20

0.10

2

H O

0.15

q

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


T = 298.15 K T = 311.85 K

0.05

0.00

0.05

0.10

xm

/ (g/g)

(

)

H O, Eq

0.15

2

Figure 5: Water adsorption isotherms on Zeolite 3A from mixtures of formaldehyde, methanol, methylal, OME2 , OME4 , and water. The composition of these mixtures was chosen to represent process-relevant mixtures, c.f. Supporting Information and Figure 6. (◦) Exp. data at 298.15 K. () Exp. data at 311.85 K. Lines: Correlation with Langmuir isotherm.


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

calc. overall mass fraction / (g/g)


Page 34 of 40

0.8

0.6

0.4

0.2

0.0 0.0

0.2

0.4

0.6

0.8

meas. overall mass fraction / (g/g)

Figure 6: Measured- and calculated overall mass fractions of all components but water of the liquid bulk phase equilibrium. For the calculated overall mass fractions, it was assumed that water is the only adsorbing component, which is proven here by the good parity of calculated- and measured data. Formaldehyde (N), methanol (H), methylal (), OME2 (△), OME4 (▽).


Page 35 of 40

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


6 9 4

1

2

3

5

C1

C2

Ads

R

8

7 Figure 7: Flowsheet of the novel OME process. R: Reactor; C1, C2: Distillation columns; Ads: Adsorption unit.

4 m

H2O

Liquid

9 m

Zeolite m 8 m

Zeolite m

H2O

Zeolite (1 qH O ) m 2

Ads

Figure 8: Scheme of the equilibrium stage model used for the design of the adsorption unit.



7 P ro d u c t

6 0 .2 g /g

/m

5

9

0 .1 g /g

4

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

3 0 .0

x

0 .1

H

(m ) 2

O ,1

= 0 .0 g /g

0 .2

m 7

0 .3 /m

0 .4

0 .5

P ro d u c t (m)

Figure 9: Pareto set of case study 1 (τ = ∞, m˙ Zeolite = ∞, x˜H2 O,9 = 0 g/g) in a level curve plot. The full lines represent Pareto optimal solutions at which the objective overall mass fraction of (m) water in the feed x˜H2 O,1 is constant. The dashed lines are inferior solutions and thus not part of the Pareto set. On each curve, solutions with a ratio of formaldehyde to methanol equal to 1.60 g/g (•) and 1.69 g/g (), respectively, are indicated. The grey-shaded area represents all Pareto optimal (m) solutions for x˜H2 O,1 ≤ 0.2 g/g.


Page 36 of 40

Page 37 of 40

Image in design space 0.05

/ (g/g) )

H O, 9

(

2

1

2

3

m

1000 s

0.02 750 s

0.01

4

m

/

5

6

0.00 1.66

7

1.68

m m /

FA,1

Product

1.70

1.72

MeOH,1

0.05 : OP2

: OP2

/ (g/g)

0.04

) (

2

xm

H O, 9

750 s

1000 s

0.03

1000 s

0.02

750 s

0.01

1

2

3

m

4

m

/

5

6

0.00 1.66

7

1.68

m m /

FA,1

Product

1.70

1.72

MeOH,1

0.05 0.04

/ (g/g)

Product 7

+

9

)/

m m m (

)

750 s

1000 s

0.03

2

(

Product 7

+

9

)/

m m m

0.03

1000 s

Zeolite

10 9 8 7 6 5 4 3

0.04

xm

750 s

Zeolite

10 9 8 7 6 5 4 3

: OP1

: OP1

H O, 9

10 9 8 7 6 5 4 3

xm

(

7

+

9

)/

m m m

Product

Pareto set in objective space

(

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


1000 s

750 s

0.02 0.01

1

2

3

m

4

m

/

Zeolite

5

6

0.00 1.66

7

1.68

m m /

FA,1

Product

1.70

1.72

MeOH,1

Figure 10: Projections of the pareto set (left) and its image in the design space (right) for case study (m) 2. In one row, the objective overall mass fraction of water in the feed is constant: x˜H2 O,1 = 0.0 g/g (top), 0.1 g/g (middle) and 0.2 g/g (bottom). The lines represent Pareto-optimal solutions on which the objective pseudo residence time is constant. The Pareto set in the objective space is linked to the design space by a color code. OP1 and OP2: Two chosen operating points of the novel OME process.


overall mass fraction / (g/g)

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

overall mass fraction / (g/g)


Page 38 of 40

0.50 0.40 0.30 0.20 0.10 0.00 0.0

0.2

0.4

0.6

0.8

1.0

dimensionless reactor length / (-) 0.10 0.08 0.06 0.04 0.02 0.00 0.0

0.2

0.4

0.6

0.8

1.0

dimensionless reactor length / (-)

Figure 11: Overall concentration profiles of the fixed-bed reactor of the OME process for operating point OP2 (TReactor = 343.15 K). Formaldehyde (N), water (•), methanol (H), methylal (), OME2 (△); (b) OME3 (◦), OME4 (▽), OME5 (◃), OMEn ≥ 6 (⋄). The total amount of catalyst is equally distributed along the reactor.


Page 39 of 40

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


OME5

OME5

OME5 OME3

OME3

OME3 OME4

OME4

OP1

OME4

OP2

Burger et al.

Figure 12: OME chain length distributions of different OME processes. OP1, OP2: Operating points of the process of this work. Burger et al.: 13 OME production from methylal and trioxane.



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

C2

C1

For Table of Contents only


Page 40 of 40

Dimethyl Ethers from Formaldehyde and Methanol - ACS Publications

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