Acetalization of Formaldehyde with Methanol in Batch and Continuous

Ind. Eng. Chem. Res. 1996, 35, 3707-3720

3707

Acetalization of Formaldehyde with Methanol in Batch and Continuous Reactive Distillation Columns Aspi K. Kolah, Sanjay M. Mahajani, and Man Mohan Sharma* Department of Chemical Technology, University of Bombay, Matunga, Bombay 400 019, India

Methylal, an important raw material and a solvent, is produced by acetalization of aqueous formaldehyde with methanol. This acetalization reaction was carried out in a closed system in the presence of a cation-exchange resin Indion 130 as catalyst and was found to be equilibrium limited. In order to increase the conversion for this reaction, reactive distillation was carried. Batch reactive distillation was performed in the presence of the cation-exchange resin Indion 130 as catalyst. Continuous reactive distillation was performed in a reactive distillation column (RDC) using three different types of catalyst packing. The first type of catalyst packing was coarse size macroporous cation-exchange resin Indion 130, which was directly packed along with Raschig rings. The second type of catalyst packing was Indion 130 tied in cloth bags. The third type of catalyst packing used was a silica-supported organic catalyst. Up to 99% conversion of formaldehyde was achieved by reactive distillation. Vapor-liquid equilibrium data for the quaternary system formaldehyde-methanol-methylal-water were experimentally obtained and correlated by the UNIFAC method. On the basis of the experimental results of the single-feed continuous reactive distillation column, preliminary modeling has been performed for the calculations of the minimum reflux ratio and the number of reactive equilibrium stages in the column used for synthesis. Introduction Methylal is oxidized to produce high-purity formaldehyde (70%) compared to 55% formaldehyde which can be obtained from methanol oxidation. This formaldehyde can then be used for the production of acetal resin which has a very promising future because of its favorable properties like hardness, toughness, rigidity, spring elasticity, resistance to fuels, etc. It is widely used in automotive parts, domestic household appliances, machine construction, medical technology, toys, cosmetics, etc. Methylal is also a very good industrial solvent. The removal of formaldehyde, at the 1-3% level, from aqueous solutions of 2-butyne-1,4-diol, pentaerythritol, etc., by acetalization with methanol has been reported [(Kolah and Sharma (1995), Merger and Horler (1988)] where methylal is obtained, which is a value added product. The technology for the commercial production of methylal by the reactive distillation method has been described by Masamoto and Matsuzaki (1993). The reactive distillation was performed using a distillation tower and multireaction units. The middle portion of the distillation tower was furnished with stages from which the liquid components were withdrawn and pumped to the reactor units, containing solid acid catalyst. The reactive solutions containing the resulting methylal was fed to the distillation tower, where methylal was distilled as the overhead product. Reactive distillation is a unit operation that combines simultaneous chemical reaction and multicomponent distillation in the same vessel. The advantages of reactive distillation include the attractive potential in exploiting the exothermicity of reactions for supplying the heat required for distillation, a lower mole ratio of reactants which yet overcome the thermodynamic limitations encountered in simple reactions, lower capital costs over conventional process schemes that utilize chemical reactors followed by distillation, and improved product selectivities. * Author to whom correspondence should be addressed.

S0888-5885(95)00563-X CCC: $12.00

Excellent surveys have been published on reactive distillation by Doherty and Buzad (1992), Sharma (1995), and Gaikar and Sharma (1989). Reactive distillation processes are now commercially exploited for the manufacture of methyl tert-butyl ether (MTBE), which is used as an octane number enhancer and is reputed to be the world’s fastest growing chemical (DeGarmo et al. (1992)), and ethyl tert-butyl ether and tert-amyl methyl ether, which are also used as octane number enhancers. Reactive distillation is also used for the manufacture of nylon 6,6, esterification of acetic acid with alcohols like methanol and ethanol, use of hydrolysis reactions of esters like methyl acetate, and the manufacture of isooctane alkylate. Reactive distillation is also used for the separation of close boiling mixtures such as m-xylene and p-xylene. Reactive polymers as catalysts, for their use in reactive distillation systems must fulfill some essential requirements. The polymer should have the necessary porosity and pore structure and satisfactory activity and selectivity. For satisfactory use in the distillation process, the outer physical surface of the polymer must be sufficiently large to permit a low-pressure drop across the bed, and the external wetting efficiency should tend to unity. The conventional polystyrene based resins (0.5-1.2 mm diameter), while satisfying the former requirements for a good heterogeneous catalyst, do not satisfy the latter requirement necessary for the distillation process. Many different approaches to overcome this problem have been tried. Yuxiang and Xien (1992) have used Amberlyst-15 tied in cloth bags, twisted in a helical form with a corrugated wire screen in between for production of MTBE. For the hydrolysis of methyl acetate, Fuchigami (1990) has utilized catalytic packing molded from resin particles and thermoplastic powder. Another approach for making molded packing is mixing the inactive polymer components, e.g., styrene and divinylbenzene, with a thermoplastic material and molding by extrusion into the desired shape with heating to the melting point of the thermoplastic material, followed by sulfonation of the molded element © 1996 American Chemical Society

3708 Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996

Figure 1. Reaction mechanism of acetalization of hemiformal with methanol in the presence of cation-exchange resins as catalysts.

[Chaplits et al. (1976)]. Flato and Hoffmann (1992) have successfully used a 6 × 6 mm Raschig ring-shaped macroreticular cation-exchange resin for producing MTBE by reactive distillation. Silica gel grafted with poly(phenylsilsequioxane) and sulfonated has been reported by Carlier et al. (1993), but this is likely to be expensive. Carlier et al. (1991/ 1992b) have described the radical polymerization of vinylbenzyl chloride (VBC), using a precursor which is either a methacrylate monomer from the grafting of trimethoxypropylsilane (methacrylate silane), a thiol transfer agent (propylthiol)trimethoxysilane, or an azo initiator from the reaction of azobis(cyanovaleric acid) with grafted aminopropylsilane. In another study carried out by Carlier et al. (1991/1992a), silica gel has been grafted with trialkoxysilane coupling agents carrying either a γ-propylthiol or a γ-propylamine group. Suzuki et al. (1986) have increased the capacity of silica-based catalyst by using a sol-gel process using Si(OEt)2 and a functional silane, to form a silica network. Challa (1983) used the initiator route (with an azothioaryl compound) to fix vinylpyridine copolymers that are able to act as ligands for copper catalysts for the oxidative coupling of phenols. Boven et al. (1990, 1991) described grafting of poly(methyl methacrylate) onto silica and other substrates, giving elongated conformations with high graft densities. There is considerable literature on the modeling and simulation of reactive distillation columns (RDCs). A detailed review on the same has been published by Doherty and Buzad (1992). Correlation of experimental data with the theoretical aspects has been done previously by Yuxiang and Xien (1992), Sundmacher and Hoffmann (1993), Bravo et al. (1993), etc. In this work a detailed study of the acetalization of formaldehyde with methanol to produce methylal has been carried out. The reaction was carried out in a closed system for determination of the equilibrium conversion. Batch reactive distillation and continuous reactive distillation were performed to overcome the equilibrium limitations of the reaction. For the continuous reactive distillation an attempt has been made to synthesize a low-cost cation-exchange resin catalyst supported on silica. There is very little literature on the use of silica-supported catalysts in RDCs. A preliminary attempt at modeling of the single-feed continuous reactive distillation has been performed. A

detailed simulation study would involve a knowledge of hydrodynamic and transport properties, which would have to be obtained experimentally and is a subject beyond the scope of this work. Hence, we have restricted ourselves to the determination of the number of reactive equilibrium stages that exist in the column based on the assumption of fast reaction accompanied by rapid mass transfer, using the approach suggested by Barbosa and Doherty (1988b) for the design of RDC. Their work for single reaction is extended here for multiple reactions with appropriate assumptions.

Mechanism of Reaction Formaldehyde reacts with methanol in the presence of an acid catalyst to give methylal. This is an equilibrium-limited chemical reaction.

HCHO + 2CH3OH a CH2(OCH3)2 + H2O

(1)

The reaction mechanism for the acetalization of formaldehyde with methanol involves formation of an intermediate compound called hemiacetal, i.e., hemiformal which is formed by the addition of the nucleophilic alcohol molecule to the carbonyl group. In the presence of acid catalyst, the hemiacetal reacts with methanol to form the acetal, i.e., methylal. The mechanism of this reaction of acetalization of hemiformal with methanol in the presence of cation-exchange resin as catalyst is shown in Figure 1.

Experimental Section Experimental work of this study can be divided into three categories: (1) experiments in a batch mode, carried out in an autoclave, to determine equilibrium conversion and study the effect of different parameters, (2) batch reactive distillation with simultaneous removal of methylal as the overhead product, (3) continuous reactive distillation in a RDC (see Figure 2) and preparation of a silica-coated catalyst for its use in RDC. A detailed description of the experimental procedure and analysis is given in the appendix.

Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996 3709

Chemical Equilibria:

(A) Gas Phase yMGPφ yWyFAP

(10)

yHFPφ K2(T) ) yMyFAP

(11)

K1(T) )

(B) Liquid Phase psMGpφ xMG γMG ps ps xWxFA γWγFA

(12)

psHFpφ xHF γHF K2(T) ) s s p p xMxFA γMγFA

(13)

K1(T) )

W FA

M FA

(C) Material Balance

∑xi ) 1 ∑yi ) 1 Figure 2. Schematic diagram of the experimental setup of the reactive distillation column.

Model for Vapor-Liquid Equilibrium Formaldehyde reacts with water to form methylene glycol. Methylene glycol can further undergo polymerization reaction to form poly(oxymethylenes). Formaldehyde also reacts with methanol to form hemiformal, which can undergo further polymerization to form poly(hemiformals). For the case of simplicity, the formation of poly(oxomethylenes) and poly(hemiformals) has not been considered in this work. Since no catalyst was added to the Othmer still, the reaction of hemiformal with methanol to form methylal does not take place; hence, this reaction has not been considered for correlation of the vapor-liquid equilibrium data.

HCHO + H2O S OHCH2OH

(2)

HCHO + CH3OH S CH3OCH2OH

(3)

The thermodynamic equilibrium within the quaternary system formaldehyde-water-methanol-methylal is briefly described here, however, more details on this subject may be obtained from Maurer (1986). Physical Equilibria: s pFA xFAγFA ) PyFA

formaldehyde

psWxWγW ) PyW

water

(4) (5)

methanol

psMxMγM ) PyM

(6)

methylene glycol

psMGxMGγMG ) PyMG

(7)

hemiformal methylal

psHFxHFγHF ) PyHF s pMET xMETγMET ) PyMET

(8) (9)

(14) (15)

The evaluation of these equations requires the following parameters: (1) The vapor pressure of formaldehyde, water, methanol, methylene glycol, hemiformal, and methylal. The vapor pressures of molecular formaldehyde, water, methanol, and methylal are taken from Reid et al. (1988). The vapor pressures of methylene glycol and hemiformal are taken from Maurer (1986). (2) The chemical equilibrium constants K1 and K2. The equilibrium constant K1 for the formation of methylene glycol was taken from Hall and Piret (1949) as

ln K1 ) -22.57 + 7368/(T in K)

(16)

and K2 for the formation of hemiformal was taken from Hall and Piret (1949) as

ln K2 ) -16.2707 + 6462.14/(T in K)

(17)

(3) The activity coefficients, which are determined by the UNIFAC group contribution method. The splitting of the components into groups is shown in Table 1. Formaldehyde, water, methanol, methylal, and methylene glycol are taken as autonomous groups. In the present case the UNIFAC model reduces to the UNIQUAC model where hemiformal is split into two groups. This method was adopted to use the literaturereported parameters of the UNIFAC model as far as possible. This approach is useful when the polymerization reactions of methylene glycol and hemiformal are considered. When methylal was split into CH2 and OCH3 groups and VLE was determined using parameters determined by Maurer (1986), large deviations were obtained from experimental data. This necessitated changing of the interaction parameters of Maurer (1986) or taking methylal as an autonomous group. All the interaction parameters except those for interactions between methylal as one group and the remaining components as the other were taken from Maurer (1986). The interaction parameters between methylal as one group and H2O, CH3OH, CH2O, CH3O, CH2OH, and CH2(OH)2 as the other were determined.

3710 Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996 Table 1. UNIFAC Volume and Area Parameters group no.

group name

size param r

surface param q

1 2 3 4 5 6 7

CH2O H2O CH2(OH)2 CH3OH CH3O -CH2OH CH2(OCH3)2

0.9183 0.92 2.6744 1.4311 1.145 1.2044 2.9644

0.78 1.40 2.94 1.432 1.088 1.124 2.716

Since no experimental vapor-liquid equilibrium data were available in the literature for the quaternary system formaldehyde-water-methanol-methylal, experimental data were obtained using an Othmer still. The experimental vapor-liquid equilibrium data are given in Table 5. An unconstrained simplex search technique described by Nelder and Mead (1965) has been used for determination of the interaction parameters. Table 6 gives the complete set of UNIFAC interaction parameters for the interaction of all the groups. Table 5 shows the calculated data presented together with the experimental data. It is seen that the predictions are rather poor at lower methylal concentrations. Modeling of RDC with Multiple Reactions Assumptions. 1. Only three reactions, i.e., formation of methylene glycol (eq 2), hemiformal (eq 3) and methylal (eq 18), take place in the reactive system investigated. The equilibrium constant for the reaction given by eq 18 is

the compositions becomes difficult for reactive systems involving four or more components. In order to reduce the degrees of freedom of the system, the concept of transformed variables has been introduced. The major advantage of transformed variables is that reactive systems can be analyzed in the same way as nonreactive distillation systems (Barbosa and Doherty (1988a), Ung and Doherty (1995)). The transformed compositions Xi are defined as follows:

Xi )

(

xi - νiTΨ-1xRef

T 1 - νTOT Ψ-1xRef

)

,

i ) 1, ..., C - R

(20)

where Ψ denotes the matrix of stoichiometric coefficients for the R reference components as shown below. The number of reference components should be equal to the number of independent reactions.

[

ν(C-R+1)1 .. .. .. ν(C-R+1)R . . . νiR . Ψ) . . . . ν C1 .. .. .. νCR

]

(21)

νiT is the row vector of stoichiometric coefficients of component i for each reaction and is given by

νiT ) (νi1, νi2, ..., νiR)

(22)

CH3OCH2OH + CH3OH S CH2(OCH3)2 + H2O (18)

νTTOT is the row vector of the sum of the stoichiometric coefficients for each reaction and is given by

defined below and is assumed to be independent of temperature as reported by Masamoto and Matsuzaki (1994).

T ) (νT1, νT2, ..., νTR) νTOT

xMETxW γMETγW ) 3.2 K3(T) ) xHFxM γHFγM

(19)

The formation of higher polymerization products, viz., poly(oxymethylenes) and poly(hemiformals), is neglected. 2. It is assumed that all the reactions taking place in the present reactive system are sufficiently fast and that the RDC can be considered to be equivalent to “the continuously stirred tank reactor cells connected in series”. 3. The vapor and liquid streams leaving each cell (stage) are assumed to be in reactive phase equilibrium. 4. The heats of the reactions involved are negligible. Further, it is assumed that the molar heat of vaporization/condensation of the mixture is constant throughout the column irrespective of its composition. Heat of mixing is negligible. The increase in the sensible heat with an increase in the temperature through the column and heat losses from the column are also neglected, and hence energy balance for the individual stages of the column has not been considered while formulating the model equations. The internal reflux ratio remains constant throughout the column. 5. The column has a large number of reactive stages. This assumption facilitates reduction of the finite difference equations describing the material balance for each stage to first-order ordinary differential equations, which in turn offer simplicity in the solution procedure. Transformed Composition Variables for Multiple Reaction Systems. Graphical presentation of

(23)

νTTOT ) (νT1, νT2, ..., νTR) xREF is the column vector of mole fraction of the R reference components in liquid phase. A detailed derivation for the transformed variables is given by Ung and Doherty (1995). Note that the sum of the transform compositions equals 1.0, and the compositions have the characteristics that they do not change in value before and after reaction and that the R reference components should be chosen from the C components of the reactive mixture in such a way that it should be possible to find the inverse of the Ψ matrix. The present system consists of six components, viz., formaldehyde (1)-water (2)-methanol (3)-methylene glycol (4)-hemiformal (5)-methylal (6), with three independent reactions (R ) 3) indicated by eqs 2, 3, and 18. The three components 4, 5, and 6 have been chosen as the reference components. The three transformed composition variables X1, X2, and X3 are found to be

X1 )

x1 + x4 + x5 + x6 1 + x4 + x5 + x6

(24)

X2 )

x2 + x4 - x6 1 + x4 + x5 + x6

(25)

X3 )

x3 + x5 + 2x6 1 + x4 + x5 + x6

(26)

X1, X2, and X3 sum to unity, and hence only two are independent. Since the system has 2 degrees of freedom


figure to maintain its clarity. It should be noted here that the surface Y1-Y3-T lies above the surface X1X3-T and does not touch it anywhere except the four stable corners represented by the pure components. This observation proves the absence of any possible reactive azeotropes (X ) Y) over the entire range of compositions. Model Equations for Single Feed RDC. The equations are formulated on the basis of the work done by Barbosa and Doherty (1988b) on the design of a single-feed RDC for a system involving a single reaction. Their approach has been extended for multiple reactions in the present work. Finite-Difference Equations for the Stripping Section. Writing the component material balance for components i and reference components REF around envelope A of Figure 4 gives

Ln+1xi,n+1 ) Vyi,n + Bxi,B n d i νiT i ) 1, 2, 3, ..., C - R - 1 (27) j)1 dt

∑

Figure 3. Vapor-liquid equilibrium diagram for the reactive system expressed in the form of transformed variables. (- ‚ -) Outline of the plane of transformed vapor compositions.

Ln+1xREF,n+1 ) VyREF,n + BxREF,B n d i T νREF REF ) C - R, ..., C (28) i)1 dt

∑

where

n

di

) ∑ i)1 dt

[] n

dζ1

n

dζr

∑ i)1 dt . .

∑ i)1 dt

where ζi is the extent of the ith reaction. From the set of eqs 28 n

di

) Ψ-1[VyREF,n - Ln+1xREF,n+1 + BxREF,B] ∑ i)1 dt

(29)

where Ψ is given by eq 21. n (di/dt) in From eq 29, substituting the value of ∑i)1 eq 27 results in

Ln+1[xi,n+1 - νiTΨ-1xREF,n+1] ) V[yi,n - νiTΨ-1yREF,n] + B[xi,B - νiTΨ-1xREF,B] (30) Introducing the transformed variables into eq 30 results in Figure 4. Schematic representation of a single-feed reactive distillation column.

(C - R - 1 ) 6 - 3 - 1 ) 2), the vapor-liquid equilibrium data can be expressed on a three-dimensional figure with either X1 and X2 or X1 and X3 as two of its axes and temperature as the third dimension. Figure 3 shows the behavior of this reactive system of six components expressed in X1, X3, and temperature coordinates. The corresponding transformed vapor phase compositions (Y) were also determined. However, only the outline of the Y1-Y3-T surface is shown in the

T Ψ-1xREF,n+1]Xi,n+1 ) Ln+1[1 - νTOT T V[1 - νTOT Ψ-1yREF,n+1]Yi,n + T B[1 - νTOT Ψ-1xREF,B]Xi,B (31)

The overall material balance across the column can be written as n

T Ln+1 ) V + B - νTOT

di

∑ i)1 dt

(32)


Equations 31 and 32 can be combined and written as follows:

Xi,n+1 )

s n* 1 Y + X sn* + 1 i,n sn* + 1 i,B i ) 1, 2, ..., C - R - 1 (33)

where sn* is the modified reboil ratio for plate n defined as

sn* )

T Ψ-1yREF,n) V(1 - νTOT T B(1 - νTOT Ψ-1xREF,B)

(34)

Finite-Difference Equations for the Enriching Section. The equation for the operating line for the enriching section can be derived in the same manner as described earlier for the stripping section to give

Xi,m )

rm* + 1 1 Yi,m-1 Y rm* rm* i,D

i ) 1, 2, ..., C - R (35)

where T Ψ-1yREF,m-1) (1 - νTOT rm* + 1 ) (rext* + 1) (36) T (1 - νTOT Ψ-1yREF,N)

and T Ψ-1xREF,D) (1 - νTOT rext rext* ) T (1 - νTOT Ψ-1yREF,D)

(37)

where rm* is the modified reflux ratio for plate m. Differential Equations. The solution of the design equations (33) and (35) generates a set of discrete points in the composition space. It is convenient to approximate this set of discrete points by continuous profiles by approximating the finite-difference equations by a set of first-order ordinary differential equations. Neglecting the stage index (m), the following equation is obtained for the stripping section

dXsi dhs

)

s* 1 Y s - Xis + X s* + 1 i s* + 1 i,B i ) 1, 2, ..., C - R - 1 (38)

For the enriching section the following equation is obtained, where integration is performed by reversing the direction and using the distillate composition as the initial condition for integration since an a priori knowledge of the composition of the feed tray

dXri dh

r

) Xir -

r* + 1 r 1 Yi + Yi,D r* r*

i ) 1, 2, ..., C - R (39)

is not known Solution Algorithm. Vapor-Liquid Equilibrium. The aim is to determine transformed vapor compositions (Y) and saturation temperature for the given transformed liquid compositions (X). (1) Fix the pressure (pressure in all the runs investigated was 101.3 kPa). (2) Assume temperature and the compositions of reference components 4, 5, and 6.

(3) Calculate compositions of components 1, 2, and 3 from eqs 10, 11, 16, 17, and 19. Assume an activity coefficient equal to unity. (4) Calculate activity coefficients at the corresponding compositions and temperature using the model developed. (5) Calculate vapor compositions using eqs 4-9. (6) Check whether eq 15 is satisfied. If yes, go to step 7 and, if not, change the temperature (Newton-Raphson method) and go to step 3. (7) Check whether eq 14 is satisfied. If yes, stop the program and, if not, go to step 4. Column Profiles. (1) Specify pressure, input (feed), and output (distillate and bottoms) compositions and reflux ratio. (2) Perform the calculations for simple vapor-liquid equilibrium (without reaction) for stage one (i.e., reboiler), in the stripping section. (3) Differential equations for both the enriching and stripping sections (eqs 38 and 39) were solved as initial value problems with the starting values of the transformed variables of the corresponding compositions of the first reactive stage in the respective section. RungeKutta method was used to solve the differential equations. (4) Transformed variables of the vapor compositions, required to solve the equations, can be obtained by solving the algorithm for vapor-liquid equilibrium of the reactive system described in the earlier part of this section. (5) The equations were solved until the operating curves obtained intersect each other. If the value of reflux ratio is more than the minimum reflux ratio, the profiles intersect. (6) Number of stages until intersection in each section were determined. Results and Discussion Silica-Coated Catalyst Preparation. Inorganic silica gel supported organic polymeric catalyst was prepared as described earlier. A catalyst of capacity 0.2 mequiv/g was obtained by this method, which involves polymerization of divinylbenzene coated onto silica gel of size 2-3 mm, and was directly packed inside the RDC along with the Raschig ring packings This low capacity of the catalyst in comparison with the conventional styrene-divinylbenzene catalyst can be explained by the fact that over 90% of the catalyst is an inactive support. This catalyst, however, suffers from the drawback of mechanical shattering of the catalyst particles on stirring in an agitated system especially in an aqueous solution and physical degradation in contact with alkali. This property of physical degradation in contact with alkali can be advantageously used for destruction of the silica gel support since it is soluble in alkali, leaving behind only the organic portion which is then to be disposed of by a suitable means. Reaction in an Autoclave. Effect of Agitation. During initial runs using Indion 130 as catalyst, it was found that the speed of agitation had no effect on the overall rate of reaction in the range of 12-33 rps. Thus, it can be concluded that in the range of stirrer speeds employed the reaction was not affected by external mass-transfer resistance. Thus, in all further experiments a stirrer speed of 16.7 rps was used. Effect of Particle Size. The overall rate of reaction was not affected by variation in the particle size of Indion 130 catalyst from 0.3 to 0.6 mm. This leads to the conclusion that the resistance due to intraparticle


Figure 5. Effect of temperature on the overall rate of reaction. Reaction conditions: formaldehyde concentration, 37%; mole ratio MeOH to HCHO, 2:1; catalyst loading, 2.5% (w/w) Indion 130. Table 2. Equilibrium Conversion of Formaldehyde in a Closed System formaldehyde concn, % (w/v)

mole ratio MeOH:HCHO

reaction temp, K

equilibrium % convn of formaldehyde

37 37 37 37 37 37 37 37 37 37 37 37 37

2.0:1 2.0:1 2.25:1 2.5:1 2.75:1 3.0:1 3.0:1 3.25:1 3.5:1 3.75:1 4.0:1 4.0:1 6.0:1

333 348 348 348 348 348 373 348 348 348 348 373 348

47 47 52 56 59 62 61 65 67 70 73 72 81

diffusion in the macropores of the ion-exchange resin was not important, and hence in all further experiments commercial resin catalyst with an average bead size of 0.5 mm was used. It can be concluded that the micropore diffusional resistance was absent because the gel region of the catalyst particle swells in the presence of the polar solvent, leading to easy accessibility of the acid groups to the reactants and free mobility of all the components. Effect of Temperature. Reactions were carried out at 333 and 346 K using Indion 130 as catalyst. The rate of reaction increased with an increase in temperature as shown in Figure 5. It can be seen that the equilibrium conversion is nearly the same, indicating that the heat of reaction is very low. Effect of Mole Ratio of Methanol to Formaldehyde. It can be observed from Table 2 that, as the mole ratio of methanol to formaldehyde is increased from 2:1 to 6:1, the equilibrium formaldehyde conversion increases from 47% to 81%. Figure 6 also shows the rate of reaction at mole ratios of 2:1 and 3:1. Batch Reactive Distillation. Figures 7-9 show the profiles of formaldehyde conversion, cumulative distillate volume per unit volume of reactants, and mole percent of methylal in distillate with time for different mole ratios of methanol to formaldehyde and catalyst loading. It can be seen from these figures that as the mole ratio of methanol to formaldehyde is increased from 2:1 to 6:1 there is an increase in the conversion of formaldehyde accompanied by a decrease in the distil-

Figure 6. Effect of the mole ratio of MeOH to HCHO on the overall rate of reaction. Reaction conditions: formaldehyde concentration, 37%; catalyst loading, 2.5% (w/w) Indion 130; temperature, 348 K.

late purity. Up to 99% conversion of formaldehyde can be achieved by using a 6:1 mole ratio of methanol to formaldehyde. An increase in the initial catalyst loading from 2.5% to 5% (w/w) for 4:1 mole ratio methanol to formaldehyde increases the rate of conversion of formaldehyde as shown in Figure 8. Continuous Reactive Distillation Using Indion 130 Packed along with Raschig Rings. Commercially available coarse-sized cation-exchange resin Indion 130 of average size 1 mm (0.045 kg) was packed into the column along with Raschig rings. In a typical run formaldehyde was fed into the column at position I1 and methanol at position I6, a mole ratio of methanol to formaldehyde was 2.5:1, and a maximum flow rate of formaldehyde that could be fed into the column without the column flooding was only 6.85 × 10-5 mol/ s. A total of 97% conversion of formaldehyde was obtained, with the distillate containing 79 mol % methylal. Due to the small size of the macroporous resin, a very high pressure drop is exerted since the resin blocks the gas riser space of the Raschig rings. Continuous Reactive Distillation Using Indion 130 Tied in Cloth Bags. Commercially available coarse-sized cation-exchange resin Indion 130 of average size 1 mm was tied in cloth bags of diameter 0.0070.01 m and 0.025 m in height, which were packed inside the catalytic section of the column. Experimental results are tabulated in Table 3. It can be seen from this table that much higher flow rates of formaldehyde and methanol can be handled by this combination. Continuous Reactive Distillation Using a SilicaCoated Catalyst. Operating variables for the process are (a) mole ratio of methanol to formaldehyde, (b) feed position of methanol and formaldehyde, (c) reflux ratio, (d) feed flow rate and catalyst loading in the column, and (e) catalyst life. The experimental results are summarized in Table 4. Effect of Mole Ratio of Methanol to Formaldehyde. It can be seen from runs 1-5 of Table 4 that as the mole ratio of methanol to formaldehyde is increased from 2.0:1 to 3.5:1 the conversion of formaldehyde increases, concurrent with significantly lower methylal purity of the distillate. This is because methanol as a reactant increases the conversion within the reactor,


Figure 7. Batch reactive distillation of methylal.


while methanol, being a low boiling compound, lowers the distillate purity of methylal in the distillation column.

Effect of the Feed Positions of Methanol and Formaldehyde. The feed positions of methanol and formaldehyde are also very important parameters in the



operation of the RDC, as can be seen from Table 4. For the system to operate optimally, provision should be made for maximum contact area between the reactants so that more of the column is used as a reactor and not as a distillation column only. Aqueous formaldehyde solution, being the high boiler, should enter the column from the top, while the reverse holds good for methanol. It can be seen from Table 4 that, when formaldehyde enters the column at the top of the reactive section and methanol at the bottom of the reactive section (runs 1-5), the highest conversion and distillate purity are obtained compared to that obtained for other intermediate entry points for methanol and formaldehyde, as shown in runs 7-11. Effect of Reflux Ratio. The change of reflux ratio has a significant effect on the operation of the column. It can be seen from runs 2 and 3 of Table 4 that the higher the reflux ratio, the higher was the distillate purity and the lower is the conversion of formaldehyde, and vice versa. Effect of the Feed Flow Rate and Catalyst Loading in the Column. The effect of feed flow rate can be seen from Table 4. From runs 3 and 6 it can be observed that as the flow rate increases a decrease in conversion was observed, which is due to the low catalytic activity of the catalyst. A flow rate of 1.37 × 10-4 mol/s for formaldehyde was found to be an optimum flow rate for 100 g of catalyst loading in the column. Due to the low catalytic activity of the catalyst, it can be seen from Table 4 that, when 50 g of the catalyst was loaded into the column, the conversion of formaldehyde is lower than that obtained when 100 g of catalyst were loaded into the column (runs 3 and 12). Catalyst loading above 100 g did not have any significant effect on the conversion of formaldehyde.

Figure 10. Composition profiles in a single-feed RDC in terms of transformed variables.

Catalyst Life. The catalyst was found to perform satisfactorily. For 15, reaction cycles of 3 h duration were performed. A drop of about 25% in the catalytic activity was noticed after 15 cycles. Modeling of Single-Feed RDC. The model equations were solved as described earlier. For a typical run (run 11 of Table 4) with a reflux ratio of 1.0 and the given input and output conditions, the intersection of the profiles of the transformed variables is shown in Figure 10. The intersection was observed after 3.3 stages in the rectifying section and 4.0 stages in the stripping section. Neumann and Sasson (1984) have reported that around 7-8 nonreactive stages can be realized in a column of 900 mm with the same diameter and similar column internals operated under otherwise identical hydrodynamic conditions. The profiles for the temperature and the compositions of the different components involved along the length


Figure 11. Predicted temperature profile in a single-feed RDC.

Figure 12. Predicted concentration profile for formaldehyde, methylene glycol, and hemiformal in a single-feed RDC.

Figure 13. Predicted concentration profile for methanol in a single-feed RDC.

Figure 14. Predicted concentration profile for methylal in a single-feed RDC.

Figure 15. Predicted concentration profile for water in a singlefeed RDC.

of the column are as shown in Figures 11-15. The methanol and hemiformal concentrations pass through a maximum in moving from top to bottom. It can be seen from Figure 12 that the concentration of free formaldehyde in the column is very low and much of the formaldehyde exists in the form of either methylene glycol or hemiformal. Calculations were also done to evaluate the value of the minimum reflux ratio for the same input and output conditions. Equations for operating curves, when solved for the infinite (very large) number of stages, yield pinch points for both of the sections, and there is no further change in the composition of any of the components. At minimum reflux ratio, the point of intersection happens to be either of these two pinch points. The value of minimum reflux was found to lie between 0.5 and 0.6, at which the pinch point of the stripping section profile just touches the operating curve of the enriching section. Assumption (5) of the procedure for solving the reactive distillation problem using differential equations may yield slightly inaccurate concentration profiles for a low number of stages obtained in the present exercise. However, it is expected here that the trends predicted

25 25 25

1 2 3

I1 I1 I1

molar feed ratio MeOH:HCHO 2.5:1 3.0:1 3.0:1

feed rate HCHO, mol/s

2.74 × 10-4 2.40 × 10-4 1.37 × 10-4

feed position MeOH

I6 I6 I6

1.0 1.5 1.0

reflux ratio

100 100 100 100 100 100 100 100 100 100 100 50

1 2 3 4 5 6 7 8 9 10 11 12

I1 I1 I1 I1 I1 I1 I1 I1 I2 I2 I1 I1

molar feed ratio MeOH:HCHO 2.0:1 2.5:1 2.5:1 3.0:1 3.5:1 2.5:1 2.5:1 3.0:1 2.5:1 3.0:1 2.5:1 2.5:1

feed rate HCHO, mol/s

1.37 × 10-4 1.37 × 10-4 1.37 × 10-4 1.37 × 10-4 1.37 × 10-4 2.06 × 10-4 1.37 × 10-4 1.37 × 10-4 1.37 × 10-4 1.37 × 10-4 1.37 × 10-4 1.37 × 10-4

feed position MeOH

I6 I6 I6 I6 I6 I6 I5 I5 I5 I5 I4 I6

1.0 2.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

reflux ratio 90 95 96 97 99 91 90 92 74 76 79 84

convn % (HCHO)

86 96 99

convn % (HCHO) 27.46 32.26 28.69

1.52 1.83 1.10

0.01 0.00 0.00

82.82 84.92 74.15 70.61 55.05 60.43 69.21 61.58 60.41 51.13 67.49 58.80

16.78 14.86 25.25 27.96 42.29 38.31 28.37 36.02 36.93 46.03 30.10 38.65

0.40 0.22 0.60 1.42 2.61 1.25 1.40 2.38 2.63 2.80 2.40 2.51

0.00 0.00 0.00 0.01 0.05 0.03 0.02 0.02 0.03 0.04 0.01 0.04

distillate composition, mol % methylal methanol water HCHO

71.02 65.91 70.21

distillate composition, mol % methylal methanol water HCHO 11.78 15.53 15.47

exptl pressure, kPa

101.3 101.3 101.3 101.3 101.3 101.3 101.3 101.3 101.3 101.3 101.3 101.3 101.3 101.3 101.3 101.3 101.3 101.3 101.3 101.3 101.3 101.3

exptl temp, K

316 317 318 318 319 319 321 322 324 326 327 328 328 329 329 330 330 335 343 348 358 362

78.80 70.14 62.96 64.69 61.13 58.00 54.53 53.20 35.68 35.08 29.73 22.55 21.91 15.52 14.59 12.73 11.83 8.17 4.51 2.29 0.45 0.39

20.05 28.75 32.29 33.28 30.59 30.12 29.55 31.69 28.27 30.75 38.00 32.26 33.55 29.51 32.46 30.54 31.39 28.68 21.59 17.01 8.98 7.16

1.15 1.11 4.76 2.03 8.28 10.23 15.40 14.61 30.83 29.13 27.96 41.66 41.25 52.41 50.35 51.63 51.51 58.86 70.50 78.00 86.94 89.51 0.55 0.52 0.50 5.22 5.05 4.31 3.53 3.29 2.56 2.59 5.10 5.27 4.29 3.40 2.70 3.63 2.94

exptl liquid composition, mol % methylal methanol water HCHO 85.33 84.26 83.91 83.16 82.25 81.39 79.78 77.98 76.18 73.71 71.55 68.02 68.33 64.46 66.15 65.42 63.73 60.71 60.14 51.06 38.08 37.89

13.98 14.40 14.71 15.47 15.53 16.02 16.83 18.71 17.83 21.99 23.96 23.95 24.63 25.82 27.28 28.08 29.17 28.05 30.17 34.25 27.37 35.01

0.69 1.44 1.38 1.37 2.22 2.86 3.37 3.29 5.93 4.23 4.43 7.91 6.92 9.54 6.39 6.26 6.92 10.93 9.46 14.42 33.43 26.12

0.02 0.02 0.02 0.06 0.06 0.06 0.12 0.12 0.18 0.18 0.24 0.18 0.31 0.23 0.27 1.12 0.98

exptl vapor composition, mol % methylal methanol water HCHO

84.33 82.38 84.18

3.66 1.00 0.24

0.06 0.05 0.06 0.07 0.11 0.15 0.16 0.20 0.23 0.27 0.10 0.44

0.56 11.36 7.03 16.56 18.77 3.23 9.08 15.74 14.56 19.37 13.10 7.65

2.93 1.31 1.09 0.74 0.22 2.57 2.68 2.01 6.61 5.76 6.50 4.41

317 316 318 318 322 320 321 321 321 323 321 320

average distillate temp, K

318 319 318

average distillate temp, K

86.70 82.46 80.67 80.56 79.89 78.75 78.07 76.76 75.80 73.87 68.35 68.72 67.66 66.61 64.88 62.86 61.18 56.19 49.39 40.24 27.34 26.50

12.86 17.16 17.84 18.79 17.55 18.06 17.41 18.94 15.79 17.88 23.84 20.95 22.11 21.33 23.37 23.71 25.12 27.30 27.85 30.10 28.60 30.02

0.44 0.38 1.49 0.65 2.56 3.16 4.49 4.27 8.07 7.89 7.49 10.04 9.96 11.82 11.52 12.85 13.07 15.86 21.93 28.68 42.96 42.42

0.03 0.03 0.03 0.35 0.36 0.33 0.29 0.27 0.24 0.24 0.58 0.63 0.65 0.82 0.97 1.10 1.06

calcd vapor composition, mol % methylal methanol water HCHO

96.45 87.28 91.82 82.34 80.90 94.05 88.08 82.05 78.60 74.60 80.30 87.50

bottoms composition, mol % methylal methanol water HCHO

0.23 0.08 0.10

bottoms composition, mol % methylal methanol water HCHO

Table 5. Experimental and Calculated Vapor-Liquid Equilibrium Data for the System Formaldehyde-Water-Methanol-Methylal

cat., g

run

feed position HCHO

Table 4. Methylal Synthesis in a RDC Using Silica-Coated Catalyst

cat., g

run

feed position HCHO

Table 3. Methylal Synthesis in a RDC Using Indion 130 Tied in Cloth Bags

315.0 315.9 317.1 317.3 318.6 319.3 321.0 321.9 323.8 327.1 327.5 328.0 328.8 329.1 328.9 329.1 329.0 335.1 346.1 350.1 361.5 368.2

calcd temp, K

371 365 366 361 358 368 365 361 363 358 364 366

average bottoms temp, K

365 360 361

average bottoms temp, K


3718 Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996 Table 6. UNIFAC Interaction Parameters Aij/(T in K) 1 1 2 3 4 5 6 7

2

3

240.0a -149.0a 149.0a -180.6a 0.0a -180.6a 24.0c

0.0a -181.0b 240.0a -181.0a 303.1c

240.0a 0.0a -181.0a 240.0a -181.0a 20.8c

4

5

6

7

339.7a 289.6b 289.6a

0.0a -149.0a -149.0a -180.6b

339.7b 0.0a 135.9c

339.7a 289.6a 289.6a 0.0a 339.7a

-180.6a -2.4c

-36.3c 64.53c 17.1c 38.53c -1.1c -34.4c

-21.3c

a From Maurer (1986). b From Ghemling et al. (1982). c Determined in this work.

by this model are indicative. Moreover, the value of minimum reflux ratio remains unchanged when the equations are solved either as differential equations or as finite-difference equations (Julka and Doherty (1990)). It should also be noted here that the calculations for vapor-liquid equilibria may yield multiple composition values at a particular temperature which satisfy all the equations of vapor-liquid equilibria. The solution of appropriate significance (i.e., when all the x and y values lie between 0 and 1) should be selected. Conclusions Acetalization reaction of formaldehyde with methanol was investigated in this work. It was found that this is an equilibrium-limited reaction. In order to increase the conversion of formaldehyde, it was necessary to use reactive distillation. Up to 99% conversion of formaldehyde could be obtained by batch and continuous reactive distillation. Lower mole ratios of methanol to formaldehyde are required for continuous reactive distillation than for batch reactive distillation when obtaining a 99% conversion of formaldehyde. An increase in the mole ratio of methanol to formaldehyde and the catalyst loading resulted in a favorable effect on the conversion of formaldehyde. The best results were obtained when the column was operated with two feeds; methanol from the bottom and formaldehyde from the top of the catalytic section. A new inorganic silica gel supported organic catalyst was used in the continuous reactive distillation column. The size of this catalyst can be conveniently changed, and hence it can be directly used in the RDC, without leading to serious pressure drop problems. Efforts to model the behavior of the single-feed continuous reactive distillation column proved successful; 7-8 reactive equilibrium stages were realized in a column of 900 mm operated under conditions employed in this study. The calculations for minimum reflux indicate that the operating reflux ratio was approximately 2 times the minimum reflux ratio. Acknowledgment A.K.K. and S.M.M. are thankful to the University Grants Commission, New Delhi, India, for awarding a Senior Research Fellowship during the tenure of this work. Nomenclature B ) bottoms product molar flow rate C ) total number of reacting components D ) distillate molar flow rate K ) chemical equilibrium constant L ) liquid molar flow rate P ) pressure, atm Pφ ) standard pressure, 1 atm pis ) saturated vapor pressure of component i rm* ) modified reflux ratio for plate m

R ) total number of independent reactions sn* ) modified reboil ratio for plate n T ) temperature, K t ) time V ) vapor flow rate xREF ) column vector of mole fractions of the R reference components in the liquid phase xi ) mole fraction of component i in the liquid phase Xi ) transformed composition variables in the liquid phase yi ) mole fraction of component i in the vapor phase yREF ) column vector of mole fractions of the R reference components in the vapor phase Yi ) transformed composition variables in the vapor phase Greek Letters γi ) liquid phase activity coefficient of component i νir ) stoichiometric coefficient of component i in reaction r νiT ) row vector of the stoichiometric coefficients of component i for each reaction νTr ) sum of stoichiometric coefficients for reaction r νTTOT ) row vector of the sum of the stoichiometric coefficients for each reaction Ψ ) square matrix of stoichiometric coefficients for the R reference components in the R reactions Subscripts B ) bottoms product D ) distillate F ) feed FA ) formaldehyde HF ) hemiformal i ) components m ) generic tray of the rectifying section M ) methanol MET ) methylal MG ) methylene glycol n ) generic tray of the stripping section r ) reactions REF ) reference components T, TOT ) total W ) water 1, 2, 3 ) reactions Superscripts 0 ) initial value r ) rectifying section s ) stripping section -1 ) inverse of matrix * ) modified variable

Appendix Materials. Methanol, formaldehyde, thyomylphthalein, sodium sulfite, and sodium hydroxide were obtained from s.d. Fine Chemicals, India. Divinylbenzene was obtained from Ion Exchange (India) Ltd. Silica gel was obtained from Drier Chemicals, India. Coarse-sized Indion 130 was obtained from Ion Exchange (India) Ltd. The physical properties of Indion 130 are as follows: internal surface area, 55 m2/g; cross-linking density, 20%; weight capacity, 4.8 meqiuv/g; porosity, 40%; temperature stability, 393 K. Experimental Procedure. Reaction in an Autoclave. In order to measure the equilibrium conversion of formaldehyde, the reaction was carried out in a stainless steel autoclave of capacity 1 × 10-4 m3 with an inside diameter of 0.05 m, manufactured by Parr Instruments Co. A four-bladed pitch turbine impeller was used for agitation. Temperature was maintained within (0.5 K of the desired value using a PID controller.


Batch Reactive Distillation. The experiments on batch reactive distillation were carried out in the presence of cation-exchange resin Indion 130 as a catalyst. The experiments were carried out in a fully baffled agitated glass reactor of capacity 2.5 × 10-4 m3 , with an i.d. of 0.06 m. The reactor was equipped with a stuffing box and a glass column of inner diameter 0.0254 m and height 0.3 m, packed with glass beads of diameter 5 × 10-3 m. A condenser was attached to the glass column through which chilled water was circulated. The glass reactor was kept in a constanttemperature bath maintained at (1 K of the desired temperature. In the glass reactor a methanol and formaldehyde solution was first heated to the desired temperature, and then the catalyst was added. Complete addition of the catalyst was considered as the starting time of the reaction. All the reactions were carried out using Indion 130 as catalyst. Distillation Column Reactor. The experiments were performed in a packed distillation column, shown in Figure 1. A glass tube of 25 mm i.d. and 600 mm height was employed as the reaction zone, and another glass tube of 25 mm i.d. and 300 mm height was employed as the reactive enriching section. The column was heated by an electric blanket and insulated with asbestos tape. The reboiler was heated by a mantle heater. The condenser was cooled using chilled water at 276 K. The input flow of aqueous formaldehyde and methanol was through valves I1-I6 as shown in Figure 1. The inputs were through plastic tubes, and the flow rates were measured volumetrically. The column was filled with a combination of Raschig ring type packing of height 4.5 mm and diameter 3 mm and the desired weight of catalyst. Three different types of catalyst packings were used. The first type of catalyst packing used was commercially available coarse-sized macroporous cation-exchange resin Indion 130 of average size 1 mm, which was directly packed into the catalytic section of the column along with Raschig rings. The second type of catalyst packing used was commercially available coarse-sized macroporous cation-exchange resin Indion 130 tied in cloth bags of diameter 0.007-0.01 m and 0.025 m in height, which were packed inside the catalytic section of the column along with Raschig rings. The third type of catalyst packing used was a silica-supported organic catalyst, the preparation of which is given below. Silica-Coated Catalyst Preparation. For reactive distillation in an RDC, an inorganic-supported organic catalyst was prepared (Malshe (1996)). The inorganic support used was commercially available silica gel particles of size 2-3 mm. These particles were treated with 1% potassium hydroxide to reduce the surface activity, i.e., to reduce the surface area and increase the pore volume. After the potassium hydroxide treatment, the silica gel was heated in a furnace at 673 K for 3 h, cooled, washed with water, and dried in an oven at 393 K. The required quantity of the organic monomer, i.e., divinylbenzene (30% of pore volume), along with 1% azobis(isobutyronitrile) as a polymerization initiator was dissolved in carbon tetrachloride as solvent, and uniform deposition of the organic material was performed onto the silica gel. The solvent was subsequently evaporated by using a vacuum pump. This organic-coated silica gel was then polymerized under a nitrogen blanket and then sulfonated using chlorosulfonic acid. The excess acid was then washed away using deionized water. Analysis. The concentration of formaldehyde was determined by the sodium sulfite method (Walker (1964)); i.e., after the solution is neutralized, it was

treated with a sodium sulfite solution. This resulted in the liberation of sodium hydroxide which was titrated with hydrochloric acid using thymolphthalein as an indicator. Methanol, methylal, and water concentrations were determined by using gas chromatographic techniques [Chemito 8510, Toshniwal Brothers Pvt. Ltd., India, fitted with a thermal conductivity detector (TCD)]. A 2 m × 3.175 × 10-3 m stainless steel column packed with Porapak Q was used with hydrogen as the carrier gas at a flow rate of 4.17 × 10-7 m3/s. The oven temperature was maintained at 363 K and the TCD block temperature at 423 K. Literature Cited Barbosa, D.; Doherty, M. F. The Simple Distillation of Homogeneous Reactive Mixtures. Chem. Eng. Sci. 1988a, 43, 541. Barbosa, D.; Doherty, M. F. Design and Minimum-Reflux Calculations for Single-Feed Multicomponent Reactive Distillation Columns. Chem. Eng. Sci. 1988b, 43, 1523. Boven, G.; Oosterling, M.; Challa, G.; JamSchouten, A. Radical Grafting of Poly(methylmethacrylate) onto Silica Wafers, Glass Slides and Glass Beads. Polymer 1990, 31, 2377. Boven, G.; Folkersma, R.; Challa, G.; JamSchouten, A. Grafting Kinetics of Poly(methyl mecrylate) on Microparticulate Silica. Polym. Commun. 1991, 32, 50. Bravo, J. L.; Pyhalahti, A.; Jarvelin H. Investigations in a Catalytic Distillation Pilot Plant: Vapor/Liquid Equilibrium, Kinetics, and Mass-Transfer Issues. Ind. Eng. Chem. Res. 1993, 32, 2220. Carlier, E.; Guyot, A.; Revillon, A.; Darricades, M. R. L, and Petiaud, R. Functional Polymers Supported on Porous Silica: I. Grafting Active Precursors onto Porous Silica. React. Polym. 1991/1992a, 16, 41. Carlier, E.; Guyot, A.; Revillon, A. Functional Polymers supported on Porous Silica: II. Radical Polymerisation of Vinylbenzyl Chloride from Grafted Precursors. React. Polym. 1991/1992b, 16, 115. Carlier, E.; Revillon, A.; Guyot, A., Baumgartner, P. Functional Silica Supported Polymers: IV. Synthesis and Catalytic Activity of Silica Grafted Sulfonated Polyphenylsilsesquioxane. React. Polym. 1993, 21, 15. Challa, G. Polymeric Chain Effects in Polymeric Catalysts. J. Mol. Catal. 1983, 21, 1. Chaplits, D. N.; Kazakov, V. P.; Lazariants, E. G.; Chabotaev, V. F.; Balashov, M. I.; Serafimov, L. A. Ion-Exchange Molded Catalyst and Method of its Preparation. U.S. Patent 3,965,039, 1976; Chem. Abstr. 1977, 86, 91241. DeGarmo, J. L.; Parulekar, V. N.; Pinjala, V. Consider Reactive Distillation. Chem. Eng. Prog. 1992, 88, 43. Doherty, M. F.; Buzad, G. Reactive Distillation by Design. Trans. Inst. Chem. Eng. 1992, 70A, 448. Flato, J.; Hoffmann, U. Development and Startup of a Fixed Bed Reaction Column for Manufacturing Antiknock Enhancer MTBE. Chem. Eng. Technol. 1992, 15, 193. Fuchigami, Y. Hydrolysis of Methyl Acetate in Distillation Column Packed with Reactive Packing of Ion Exchange Resin. J. Chem. Eng. Jpn. 1990, 23, 354. Gaikar, V. G.; Sharma, M. M. Separations Through Reactions and other Novel Strategies (State-of-the-art-Review). Sep. Purif. Meth. 1989, 18, 111. Hall, M. W.; Piret, E. L. Distillation Principles of Formaldehyde SolutionssState of Formaldehyde in Vapor Phase. Ind. Eng. Chem. 1949, 41, 1277. Julka, V.; Doherty, M. F. Geometric behaviour and minimum flows for nonideal multicomponent distillation, Chem. Eng. Sci. 1990, 45, 1801. Kolah, A. K.; Sharma, M. M. Formaldehyde Removal from Aqueous Solutions. Sep. Technol. 1995, 5, 13. Malshe, V. C. Inorganic Supported Polymeric Catalyst. Unpublished work, 1996. Masamoto, J.; Matsuzaki, K. Development of Methylal Synthesis by Reactive Distillation. J. Chem. Eng. Jpn. 1994, 27, 1. Maurer, G. Vapor-Liquid Equilibrium of Formaldehyde and Water-Containing Multicomponent Mixtures. AIChEJ. 1986, 32, 932. Merger, F.; Horler, H. Process for Removing Formaldehyde from Aqueous Solutions of 2-Butyne-1,4-diol. Eur. Pat. Appl. EP. 309915, 1988; Chem. Abstr. 1989, 111, 194103.

3720 Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996 Nelder, J. A.; Mead, R. A Simplex Method for Function Minimization. Comput. J. 1965, 7, 308. Neumann, R.; Sasson, Y. Recovery of Dilute Acetic Acid in a Packed Chemorectification Column. Ind. Eng. Chem. Process Des. Dev. 1984, 23, 654. Reid, R. C.; Prausnitz, J. M.; Poling, B. E. The Properties of Gases and Liquids; McGraw-Hill Book Co.: New York, 1989. Sharma, M. M. Some Novel Aspects of Cationic Ion Exchange Resins as Catalysts. React. Funct. Polym. 1995, 26, 3. Sundmacher, K.; Hoffmann, U. Activity Evaluation of a Catalytic Distillation Packing for MTBE Production. Chem. Eng. Technol. 1993, 16, 279. Suzuki, S.; Tohmori, K.; Ono, Y. Vapor-phase Nitration of Benzene over Polyorganosiloxanes bearing Sulfo groups. Chem. Lett. 1986, 747. Ung, S.; Doherty, M. F. Vapor-Liquid Phase Equilibrium in Systems with Multiple Chemical Reactions. Chem. Eng. Sci. 1995, 50, 23.

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Received for review September 8, 1995 Revised manuscript received April 10, 1996 Accepted April 11, 1996X IE950563X

X Abstract published in Advance ACS Abstracts, July 1, 1996.

Acetalization of Formaldehyde with Methanol in Batch and Continuous

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