Chemical Engineering and Processing 44 (2005) 687–694

Distillation of formaldehyde containing mixtures: laboratoryexperiments, equilibrium stage modeling and simulation

Michael Otta, Hartmut Schoenmakersb, Hans Hassea,∗a Institute of Thermodynamics and Thermal Process Engineering, University of Stuttgart, D-70550 Stuttgart, Germany

b BASF AG, Global Process Engineering, Ludwigshafen, Germany

Received 28 March 2003; received in revised form 3 September 2003; accepted 3 September 2003Available online 11 November 2004

Abstract

Formaldehyde is one of the most important chemical intermediates. In technical processes, it is used in aqueous solutions, which alsocontain methanol. These solutions are complex reacting mixtures, in which formaldehyde is bound in a variety of oligomerization products.Distillation experiments with mixtures of formaldehyde, water and methanol were carried out in a 50 mm diameter column with 3.5 m ofstructured packing. Two types of separations were studied: (a) removal of methanol, for which the bottom product is formaldehyde–water, and( tematicallyv simulationw ple modelt e of reactionk thanol arep©

K

1

tpDositfltph

nolicountcon-

0d

b) removal of formaldehyde, for which the bottom product is pure water. Reflux ratio, pressure, and the feed composition were sysaried. A physico-chemical model of vapor–liquid equilibria of formaldehyde containing mixtures was used as a basis for a processith an equilibrium stage model. It is assumed that chemical equilibrium is established on each stage. Predictions with that sim

ogether with stage efficiencies from the internals supplier show good agreement only in some cases and indicate that the influencinetics has to be taken explicitly into account. Reactive distillation line diagrams for the ternary system formaldehyde–water–meresented for the first time.2004 Elsevier B.V. All rights reserved.

eywords:Distillation; Formaldehyde; Water; Methanol; Reactive distillation

. Introduction

Formaldehyde (CH2O, FA) is one of the most impor-ant chemical intermediates. It is processed to a variety ofroducts including technical polymers, resins and foams[1].ue to its high reactivity, it is commonly used in aque-us solutions. These solutions also contain methanol, whichtems from the formaldehyde production process and its used as stabilizer to avoid solid precipitation. Typicalechnical formaldehyde solutions contain 0.35–0.45 g g−1

ormaldehyde and less than 0.02 g g−1 methanol. In these so-utions, formaldehyde is almost entirely chemically boundo the solvents in methylene glycol (HOCH2OH, MG),oly(oxymethylene) glycols (H(OCH2)nOH, MGn, n> 1),emiformal (HOCH2OCH3, HF), and poly(oxymethylene)

∗ Corresponding author. Tel.: +49 711 685 6105; fax: +49 711 685 6140.E-mail address:hasse@itt.uni-stuttgart.de (H. Hasse).

hemiformals (H(OCH2)nOCH3, HFn, n> 1), which areformed in the following reactions:

CH2O + H2O � HO(CH2O)H (I)

HO(CH2O)n−1H + HO(CH2O)H � HO(CH2O)nH + H2O

(n ≥ 2) (II)

CH2O + CH3OH � CH3O(CH2O)H (III)

CH3O(CH2O)n−1H + CH3O(CH2O)H

� CH3O(CH2O)nH + CH3OH (n ≥ 2) (IV)

These reactions always occur in aqueous and methaformaldehyde solutions and are explicitly taken into accin the model used in the present work. Under certain

255-2701/$ – see front matter © 2004 Elsevier B.V. All rights reserved.oi:10.1016/j.cep.2003.09.011

688 M. Ott et al. / Chemical Engineering and Processing 44 (2005) 687–694

ditions additional reactions may occur, like the Cannizzaroreaction yielding formic acid or the formation of acetals likemethylal. The formation of formic acid or acetals occurs onlyat high temperatures in alkaline or acidic solutions, respec-tively. For more information, see[2]. In formaldehyde dis-tillations, these conditions are usually not given. The modelpresented here can be extended straightforwardly to includesuch reactions also.

Whereas the formation of new species by reactions(I)–(IV) is explicitly taken into account in the process model,in the presentation of its results the overall composition isused in this work. This allows a direct comparison with re-sults from standard analysis (titration for formaldehyde andwater, gas chromatography for methanol). The formation ofthe oligomers can only be monitored by NMR-spectroscopy[3–6].

The most important operation for separating formalde-hyde containing mixtures is distillation. Due to the reactions(I)–(IV) , formaldehyde distillations are always reactive dis-tillations. It should be kept in mind that distilling an aqueousmethanolic formaldehyde solution is in fact a separation of areactive system with about 20 reacting components present inappreciable amounts. Despite its technical importance, thereare only very few reports in the literature on distillation offormaldehyde containing mixtures, most of which are over 20years old[7–11]. Both experimental data and reports on mod-e hede ns.T out-d tions( hd Thep ex-t ydec sim-u Thee andc ions( asp

as the vapor pressures of the higher oligomers are low. Thegas phase is treated as a mixture of ideal gases, whereas inthe liquid phase non-idealities are taken into account usingthe UNIFAC method. For more details, see[13,16,18].

Distillation process models rely on the accurate descrip-tion of the thermophysical properties of the mixtures whichare separated. Modern thermophysical models of formalde-hyde containing mixtures explicitly take into account boththe reactions(I)–(IV) as well as non-idealities in the liquidphase[13–19]. In the present work, the latest model devel-oped in the group Maurer[18] is used, which is based onexperience and extensive experimental data from 15 years ofresearch. The quality of the representation of vapor–liquidequilibria with that model is good and compares favorably tothat typically obtained for well-studied non-reacting systems.

The process model used in the present work is an equi-librium stage model. It is based on the assumption that bothchemical and physical equilibrium are reached on each stage.As values for NTSM are taken from the internals supplier’sdata sheets, the process model is completely predictive. It isknown that the kinetics of reactions(I)–(IV) may influencedistillation processes[20]. It is, however, not in the scope ofthe present work to include reaction kinetic in the processmodel. The equilibrium model presented here is a prereq-uisite to such an extension. For that extension a thermody-namically consistent reaction kinetic model is needed, whichh stedfT hatp dis-t ns( hata siblei

werec .5 mM ionsw tomp lde-h

lly impo

ling and simulation are scarce. The previously publisxperimental work only covers distillation in tray columhe process models from the literature are meanwhileated as they do not properly take into account the reacI)–(IV) . A very recent work[12] is a modeling study, whicoes not include comparisons with experimental data.resent work closes the gap by presenting results from

ensive experimental work on distillation of formaldehontaining mixtures and a study on their modeling andlation with a state-of-the-art equilibrium stage model.quilibrium model is based on extensive vapor–liquidhemical equilibrium data. It takes the chemical reactI)–(IV) explicitly into account in the liquid phase. In the ghase, it is sufficient to consider only reactions(I) and(III)

Fig. 1. Two types of technica

as become available only very recently but is not yet teor the ternary mixtures formaldehyde–water–methanol[21].his will be the aim of future work. It should be noted treviously published process models for formaldehyde

illation [8–11] either neglect all oligomerization reactioII) and(IV) or consider only short-chain oligomers, so tn extension to include reaction kinetic effects is not pos

n a meaningful way.To be able to test the model, laboratory experiments

arried out in a 50 mm diameter column equipped with 3ontz A3-500 structured packing. Two types of separatere studied: (a) removal of methanol, for which the botroduct is formaldehyde–water, and (b) removal of formayde, for which the bottom product is pure water (cf.Fig. 1).

rtant formaldehyde distillations.

M. Ott et al. / Chemical Engineering and Processing 44 (2005) 687–694 689

Fig. 2. Vapor–liquid equilibrium in the system formaldehyde–water–methanol.

Reflux ratio, pressure, and the feed composition were sys-tematically varied.

2. Thermophysical properties

The fluid property model for formaldehyde containingmixtures on which the simulations of the present work arebased was developed by Albert et al.[18]. The model ofAlbert et al. [18] takes into account chemical reactions inboth phases and physical interactions between all species inthe liquid phase. The vapor phase is described as an idealmixture of formaldehyde, water, methanol, methylene glycoland hemiformal. The vapor pressures of higher oligomersare assumed to be negligible.Fig. 2depicts that model. Thevapor–liquid equilibrium is calculated using the extendedform of Raoult’s law, with activity coefficients from the UNI-FAC group contribution method. More details and model pa-rameters are given in[18]. Case studies carried out in theframe of the present work showed that it is sufficient toconsider only poly(oxymethylene) glycols and -hemiformalswith up to 10 formaldehyde segments if the overall formalde-hyde concentration does not exceed 0.45 g g−1 (the amountof the oligomers with higher chain length is small under thatcondition). Hence, for all simulations 10 reactions leading toMG–MG as well as 10 reactions leading to HF–HFwerec

ntalv odelu crip-t slyp ain-i hicha ies,w scrip-t in-c ation

of dew and bubble lines, especially with the prediction ofazeotropic points[22]. Instead of touching at the azeotropicpoint, the dew and bubble line show unphysical behavior withtwo points of intersection.

For a more detailed discussion and an example, see[22].The model used here is free from such flaws.

3. Reactive distillation lines

Distillation lines are important tools for conceptualprocess design and were up to now not available forformaldehyde containing systems. Therefore, based onthe physico-chemical vapor–liquid equilibrium model de-scribed in the previous section distillation lines for the sys-tem formaldehyde–water–methanol were calculated in thepresent work. The result for 1 bar in overall concentrationsis shown inFig. 3, which also contains information on theboiling temperatures.Fig. 3 can be considered as a reactivedistillation diagram for a complex system with more than 20components represented in overall concentrations. In calcu-lating that diagram the assumption of chemical equilibriumis used. There is a low-boiling azeotrope in the binary sys-tem formaldehyde–water and a distillation boundary runningfrom that azeotrope to the pure methanol node, which sepa-rates the diagram into two distillation fields.

ut inb anolr oseo onfi lidp quide

4

entsw wni edw ur-f edi per-a s andt prod-u wasa ippedw phonr pera-t eres r 2 h.M -stateo

-t am-p gt was

10 10onsidered.

Besides the very good prediction of experimeapor–liquid equilibria, an additional advantage of the msed here is the thermodynamic consistency of the des

ion of chemical and physical equilibria. In all previouublished models on distillation of formaldehyde cont

ng mixtures pseudo chemical equilibrium constants, wre formulated with mole fractions and not with activitere used. This procedure results in an inconsistent de

ion of chemical and vapor–liquid equilibria. With suchonsistent models, problems can arise with the calcul

The experiments from the present work were carried ooth distillation fields. The separations of the type ‘methemoval’ were carried out in the upper distillation field, thf the type ‘formaldehyde removal’ in the lower distillatield (cf. Fig. 1). At high formaldehyde concentrations, sorecipitate occurs which has to be avoided. The solid–liquilibrium line inFig. 3 is only qualitative.

. Experiments

The set-up used for the reactive distillation experimhich were carried out at BASF, Ludwigshafen, is sho

n Fig. 4. A 50 mm diameter laboratory column equippith 3.5 m structured packing (Montz A3-500, specific s

ace area 500 m2 m−3) was used. The internals were dividn seven packing sections with sampling valves and temture measurements below all sections. Concentration

emperatures were also measured in the feed, bottomct and distillate. For all experiments, the feed locationbove the fourth packing segment. The column was equith a total condenser, an electrically heated thermosy

eboiler and a protection heating to ensure adiabatic oion. The distillate flow rate and the reboiler heat duty wet. A stable steady-state was typically observed afteeasurements were made after at least 2 h of steadyperation.

The sodium sulfite method[2] was applied for the deermination of the overall formaldehyde content in the sles. The Karl–Fischer titration[23] was used for analysin

he overall water content. For that titration, the solution

690 M. Ott et al. / Chemical Engineering and Processing 44 (2005) 687–694

Fig. 3. Distillation lines in the system formaldehyde–water–methanol atp= 1 bar.

thermostated to 50◦C as elevated temperatures favor the de-composition of the poly(oxymethylene) glycols, which re-lease water. For the determination of the overall methanolconcentration gas chromatography was used with a SPB1capillary column from Supelco. The relative error for eachof the three methods is typically below 2%. The sum of theoverall mass fractions of formaldehyde, water and methanolas determined by those methods was in most cases between0.98 and 1.02 g g−1.

Twenty-one distillation experiments were carried out-grouped into two series: experiments of the type ‘methanolremoval’ in the upper distillation field and experiments of thetype ‘formaldehyde removal’ in the lower distillation field (cf.alsoFigs. 1 and 3). In both series, the experimental work pro-gram was structured around a base case. Only one parametercompared to that base case was varied. The varied parame-ters were reflux, overall flow rate, the pressure and the feedcomposition. The feed compositions cover a wide range ineach distillation field. An overview is given inTables 1 and 2.

5. Comparison model-experiment

Only some typical results from the experimental study arepresented here together with their prediction by the equilib-r tions,t refluxa per-i thei ts

Table 1Overview of the distillation experiments of the type ‘methanol removal’

Number Varied parameter Change compared to base case

1 Base case2 Feed flow rate Decrease3 Feed flow rate Increase4 Reflux Decrease5 Reflux Increase6 Pressure Decrease7 Pressure Further decrease8 Feed concentration methanol Increase9 Feed concentration methanol Further increase

10 Feed concentration methanol Further increase

Table 2Overview of the distillation experiments of the type ‘formaldehyde removal’

Number Varied parameter Change compared to base case

11 Base case12 Feed flow rate Decrease13 Feed flow rate Increase14 Reflux Decrease15 Reflux Increase16 Pressure Decrease17 Pressure Further decrease18 Feed concentration methanol Increase19 Feed concentration methanol Further increase20 Feed concentration methanol Further increase21 Feed concentration methanol Further increase

are in the range in which a NTSM value of 6 m−1 can beassumed. It should be noted that the results for the concen-tration and temperature profiles from these simulations areentirely predictive.

ium stage process model described above. In the simulahe feed flow rate and the column pressure as well as thend distillate flow rate were specified according to the ex

mental numbers. The values for NTSM were taken fromnternals supplier Montz. TheF-factors in the experimen

M. Ott et al. / Chemical Engineering and Processing 44 (2005) 687–694 691

Fig. 4. Laboratory distillation column.

5.1. Methanol removal

Fig. 5shows the results for the base case experiment 1. Thepredicted profiles of experiment 1 are in qualitative agree-ment with the experimental data. Quantitative agreement isfair at a first glance. Deviations, however, occur in the pre-dictions of the small number for the methanol concentrationin the bottom product, for which the simulation gives resultswhich are too low. The relative error is about 60%. Further-more, the experimental value for the formaldehyde concen-tration in the top product is larger than the predicted value.These findings are typical for all simulations carried out forthe separations of the type ‘methanol removal’. The devia-tions can be interpreted as a result of the neglection of reactionkinetic effects. Due to the comparatively slow oligomeriza-

tion reactions, methanol bound in the poly(oxymethylene)hemiformals is not readily released in the stripping section.This explains why the experimental methanol concentrationin the bottom product is higher than the predicted one. Alsothe difficulty of removing formaldehyde in the rectifying sec-tion can be understood as resulting from a chemically limitedprocess, in this case an absorption. The reactions in the liq-uid phase are too slow for binding formaldehyde from thegas phase in the oligomers up to the equilibrium concentra-tion. Therefore, formaldehyde is only poorly absorbed andthe measured formaldehyde concentration in the rectifyingsection is larger than that predicted by the equilibrium model.

Fig. 6 shows the predicted profiles for experiment 10 to-gether with the experimental data. In this experiment, themethanol concentration in the feed was very large. The feed

692 M. Ott et al. / Chemical Engineering and Processing 44 (2005) 687–694

Fig. 5. Experimental and predicted concentration profiles of experiment 1 (base case and methanol removal). Symbols: experimental data (open: column, filled:feed), lines: prediction.

location was not optimal, as can be seen fromFig. 6. Theagreement of the prediction with the experimental data isvery good for that experiment. This is probably due to thefact that the separation capacity installed in the column wasvery large compared to the fairly simple separation task inthat experiment (cf. profiles inFig. 6).

5.2. Formaldehyde removal

Fig. 7shows results for the base case experiment of the se-ries in which separations of the type ‘formaldehyde removal’were studied (experiment 11). It can be seen that only the

F iment1 filled:f

concentration profile for methanol and the composition ofthe top product are correctly predicted. The concentrationsof formaldehyde observed in the experiments are higher thanthe predictions. Accordingly, the predicted water concentra-tions are too high. The results for the other experiments of thetype ‘formaldehyde removal’ are similar. Also these devia-tions can be understood as a consequence of reaction kineticeffects. The slow oligomerization reactions keep formalde-hyde bound in heavy boiling products in reality, whereas theequilibrium model predicts an easy release.

The variation of the reflux has an important influence onthe column performance. By decreasing the reflux ratio, the

F nt 11( (open:c

ig. 6. Experimental and predicted concentration profiles of exper0 (methanol removal). Symbols: experimental data (open: column,

eed), lines: prediction.

ig. 7. Experimental and predicted concentration profiles of experimebase case and formaldehyde removal). Symbols: experimental dataolumn, filled: feed), lines: prediction.

M. Ott et al. / Chemical Engineering and Processing 44 (2005) 687–694 693

Fig. 8. Experimental and predicted concentration profiles of experiment 14(formaldehyde removal). Symbols: experimental data (open: column, filled:feed), lines: prediction.

Fig. 9. Experimental concentration profiles of experiment 14 (formaldehyderemoval) compared to a simulation with fitted NTSM numbers. Symbols:experimental data (open: column, filled: feed), lines: prediction.

internal flows in the column are reduced, the residence timeon the stages increases, so that reaction kinetic effects inthe column become less important.Fig. 8 shows results ofexperiment 14, which was carried out with a decreased refluxratio. The predicted column profile is in better agreement withthe experimental data than in the base case experiment 11.

Of course, the simulation results can be improved by ad-justing the NTSM value. The drawback of this procedure isthat the simulations are no longer predictive.Fig. 9 showsthe simulated and measured concentration profiles for exper-iment 14 which result when the NTSM value is fitted to theexperimental data. Different numbers of NTSM for the rec-tifying and stripping section were used. As the mixture instripping section is predominantly aqueous, whereas in therectifying section it contains large amounts of methanol, thiscould be justified by the experience that in many cases dif-

ferent NTSM values are observed for aqueous and organicsystems, respectively. The result of this fit is a NTSM valuein the rectifying section of 3.33 m−1 and in the stripping sec-tion of only 1 m−1, which is much lower than the supplier’svalue of 6 m−1. This shows that fitting NTSM values is not themethod of choice to obtain reliable process simulations forformaldehyde distillations. It is only an empirical means tocompensate reaction kinetic effects, which, however, shouldbe taken into account explicitly. The flaws of this empiricalprocedure become evident when it is applied to the simu-lations of the type ‘methanol removal’, where even fittingNTSM values does not give satisfactory results.

6. Conclusion

Laboratory-scale formaldehyde distillations were carriedout in a 50 mm column equipped with 3.5 m of Montz A3-500. The influence of different operating parameters (feedconcentration, pressure, overall flow rates and reflux) werestudied. The results are compared to predictions based onan equilibrium stage model. For describing the properties ofthe complex reacting multicomponent mixtures studied inthe present work, a thermodynamically consistent physico-chemical model is used, which was taken from the literature.With NTSM values taken from the internals supplier, the dis-t isonw riums dis-t d-j m.R t int f thea

A

B ofS

A

FHH

MMMM

N

illation process model is entirely predictive. The comparith the new experimental data shows that such equilibtage models work only in some cases for formaldehydeillations. Deviations result from reaction kinetic effects. Austing NTSM values is no way to cope with that probleeaction kinetics have to be explicitly taken into accoun

he process simulation. This is the aim of current work outhors.

cknowledgements

The authors would like to thank Harald Bader and Jurgenrauch from BASF and David Oberle from Universitytuttgart for their contributions.

ppendix A. Nomenclature

A formaldehydeF hemiformalFn poly(oxymethylene) hemiformals withn formalde-

hyde segmentsE index for methanolEOH methanolG methylene glycolGn poly(oxymethylene) glycol withn formaldehyde

segmentsTSM number of theoretical stages per meter

694 M. Ott et al. / Chemical Engineering and Processing 44 (2005) 687–694

W waterxi overall mass fraction of component i

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