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KINETICS OF LIQUID PHASE CATALYTIC DEHYDRATION OFMETHANOL TO DIMETHYL ETHERMakarand R. Gogate a , Byung Gwon Lee a , Sunggyu Lee a & Conrad J. Kulik ba Department of Chemical Engineering , The University of Akron , Akron, OH, 44325b Electric Power Research Institute , 3412 Hillview Avenue, Palo Alto, CA, 94304Published online: 12 Mar 2007.

To cite this article: Makarand R. Gogate , Byung Gwon Lee , Sunggyu Lee & Conrad J. Kulik (1990) KINETICS OF LIQUID PHASECATALYTIC DEHYDRATION OF METHANOL TO DIMETHYL ETHER, Fuel Science and Technology International, 8:6, 637-671, DOI:10.1080/08843759008915948

To link to this article: http://dx.doi.org/10.1080/08843759008915948

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FUEL SCIENCE AND TECHNOLOGY INT'L., 8(6), 637-671 (1990)

KINETICS OF LIQUID PHASE CATALYTIC DEHYDRATION

OF METHANOL TO DIMETHYL ETHER

Makarand R . Gogate, Byung Gwon Lee, Sunggyu ~ e e # and Conrad J. ~ulik*

Department of Chemical Engineering The University of Akron

Akron, OH 44325

*~lectric Power Research Institute 3412 Hillview Avenue Palo Alto, CA 94304

ABSTRACT

The kinetics of the liquid phase catalytic dehydration of methanol to dimethyl ether were investigated. The experiments were carried out under low concentrations of feed in a 1-L stirred autoclave, according to a statistical experimental design. The inert liquid phase used for this investigation was a 7 8 : 2 2 blend of paraffinic and naphthenic mineral oils. A complete thermodynamic analysis was carried out in order to determine the liquid phase concentrations of the dissolved species. ~-globil kinetic model was developed for the rate of dimethyl ether synthesis in terms of the liquid phase concentration of methanol. The activation energy of the reaction was found to be 18,830 cal/gmol. Based on a step-wise linear regression analysis of the kinetic data, the order of the reaction which gave the best fit was 0.28 with respect to methanol. ~ffects of the solid to liquid and the gas to liquid mass transfer resistances on the kinetic rate have also been investigated.

# To whom all correspondence should be addressed.

copyright O 1990 by Marcel Dekker, Inc.

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GOGATE ET A L .

.~ INTRODUCTION

The .p~odu&' ih 'o'f '+5bl?he from c0a l~base .d 'syngas,

and inz'thanbi s6urces w i i i l h e i j r become a nece s s i t y i n , . t h e nedr f u t u r e '(chanq e t al-. ' ( i978,)) . Un t i l now, on ly

'Cwo pfocesses halie achieved .any medsure of comiiercial

sPgnif;l'cance. h o n g 'ttiese-, t h e Be.rq.ius Process was . . active1.y operated .in 'wori'd war 11 i n 'Germany wi th an

annual capa=i ty 'bf '6"er fo"? mi.li.i.on t o n s of o i l . The

~ i s c h e r - ~ t o ~ s d h 'p-roc3sS 'i's a n i -nd i rec t s y n t h e s i s r o u t e

f o r 'hyddrbcarbonS 'from 'syiitlieiik g a s . his process 1s

;c"rier;+--y ib ropekatisn in &isbi.bei-g., : s o ~ g h P;frica..

'Mtal.yti'c dehydrat'i'dn 'o'f methanol ' to 'd'imethyl 'ether

is a h o a i pb in t 'in <he -Fedct'ion sequcinde 'df ,me'thanol t o

'+~ihe hang .tit ai.. ((.i975),, kha'ng 'et .i1.. '(:1977),).. I n

kacrt;, ii't wak '<lie 'f'irs't :?+act:ion ' to 'be discovered and

&cu-merited :in ' t h i s two-step :rea&'ib" :sciQuenck '(woodhouse

( ~ 4 ~ ) ) . : :iiydr?icarbon !foTXa't:idn :from metihano1 wa-s more

r e btes= r a ' n .d=>'ii]e-"f& -:is~ti"** .in '$he .*ens& 'fha't i i t :was , ifliirst -repor'ted ,dui;ing idirie'&yl 'ethr :.eyn<tiebis -from

1methan5.l rover ,a : ~ a - x i e0 l - i t e '(Mat'tvx !('1962)').. !Since ~ ' then:, numerous stirdyes ;have ]lieen 'mn'dertaken :to .study

ttie.se ' react ' ions (( zahne~ ,6137'1) .. ,~;j:im6t3 .(19-8'.~):)-.

'Every +kne 'a '.ihemicd :ka&ion c i s r i tudieh. :the 'goal . .

and/ :or ' the ,per~@ltive .m+ be '-dSf:fei%it. The f c a t a l + t i c

,dehydrat ion :of .&%mX tto dirnefhyl :iti~br :is ,a rdlassic

example lin 'egis ,megar*. : 3 h ~ .%&tion may :h 'carried . o u t

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METHANOL TO DIMETHYL ETHER 6,39591

with a number of different objectives. Firstly, it can

be carried out as a key intermediate step in the MTG

process. As such, it can be used to produce pure

dimethyl ether which itself is a very important chemical

commodity (Brake (1984)), in the areas of aerosols,

refrigerants, and solvents. Thirdly, the dehydration

reaction may be carried out in a single stage along with

the methanol synthesis reaction to reduce the chemical

equilibrium limitations and simultaneously improve the

syngas conversions (Lee et al. (1988). Lee (1989)).

This original idea of improved syngas conversions due to

reduced chemical equilibrium limitations as applied to

the liquid phase methanol synthesis reaction was

introduced. The dimethyl ether synthesis reaction can

also be carried out inside the compression ignition

engine as an ignition promoter for t h e methanol fuel

(Brook et al.. (1984) ) . Although the objective a d the end-use may be

different, the intrinsic ctlaracter~i'cs such as the

reaction kinetics may be. studiied separately and

independently utiliizinq a! prnpea reactor configuration,

catalyst system'.. andl process: varfables so as to tie the

information. with the ultimate ob?j:ective. Documented

kinetic: stud*es: of methanoiL d~iztiygration to dimethyl

ether are very. scarce (Adamska! et al. (.1984), Lin et

a1. (1981),, ~ub$oi et al..(,m8m)))) and the! reaction has

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640 GOGATE ET AL.

always been carried out in the vapor phase. In this

investigation, a liquid phase reaction kinetic study is

carried out on this reaction with an ultimate objective

of utilizing this kinetic rate data to tie with the

methanol synthesis reaction. No other kinetic study

like this has previously been reported in literature.

-EXPERIMENTAL SECTION

Exuerimental desiqn and Formulation

The dehydration of methanol to dimethyl ether can

be represented by the stoichiometric equation:

The reaction is carried out in the liquid phase with the

dehydration catalyst slurried in the liquid. The

catalyst employed for this study was gamma-alumina

supplied by ~arshaw-~ilterol ~irtnershi~, Inc. The

inert liquid phase was Whi$e mineral oil (Witco-40)

supplied by Witco Corporation. The properties of the

catalyst have been summarized in Table I. The particle

size distribution of the catalyst was determined using a

Coulter Multisizer (Coulter Scientific Instruments) and

is shown in Figure 1. Properties of Witco-40 oil have

been given elsewhere (Lee (1989)).

The dehydration of methanol could be studied in

syngas environment or alternately, an inert such as

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METHANOL TO DIMETHYL ETHER

Table I

Properties of Gamma-Alumina Catalyst

Catalyst Identiiication : Gamma Alumina Manufacturer: Harshaw-Filter01 Partnership S~ecific Surface Area: 198 m2/aram IN-,-BETI . > . ,. ~ b r e Volume: 0.43 ~m3/~ram Particle Size: < 80M : 90

< 45M : 55 < 30M : 25 .........................................................

nitrogen could be used. Although the reaction kinetics

of this reaction is best determined in the practical

synqas conditions, preliminary experiments performed to

investigate the role of gamma-alumina in catalyzing the

syngas components for methanol synthesis reaction

revealed an important fact that over only gamma-alumina

catalyst slurried in Witco-40 oil, syngas components

are not catalyzed for any reaction at the temperature

and pressure ranges of practical interest. Thus, if the

methanol dehydration to dimethyl ether reaction is

studied separately over alumina catalyst, syngas acts as

an inert. Therefore, it was decided to replace syngas

by nitrogen in the reaction rate measurements. Similar

conclusion was reached by Marsden et al. (1980) and Lin

et al. (1981), even though the work was done in the

vapor phase.

For systematic analysis and interpretation of the

reaction rate data, a full factorial design was

considered with four variables, viz., the temperature,

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6 U ,..- GOGATE ET AL. -.

SWLE: ,001 256 C H A N N FULL RANG! COULTER X - A X ' I ~ ( A C C : L A W ) ~ ~ i n . 0 i a . M U ~ T I S I Z E R Y i A X I S ( 0 1 S P L A V ) : Number !iff. 07/93/88' BLANK SUBSTRACT: No . , ...

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METHANOL TO DIMETHYL ETHER

the flow rate of methanol, the reaction pressure, and

the impeller speed (Table 11). The flow rate of

nitrogen was set at 1 SLPM for all the design runs. The

two levels of these four variables were chosen so as to

cover the range of practical interest. The temperature

and the pressure were chosen accordingly. To ascertain

whether the liquid to solid mass transfer limits the

Table II

Two-Level Factorial Design in Standard Order

Choice of Upper and Lower Levels

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644 GOGATE ET AL.

kinetic rate at these reaction conditions, the impeller

speed was varied from 1250 to 1800 rpm. The range of

methanol flow rate was chosen so as to simulate the

typical values of the liquid phase concentrations of

methanol in the methanol synthesis process. The liquid

phase concentration is linked to the multicomponent

phase and chemical reaction equilibria of the system and

hence cannot be related to the flow rate apriori. The

range of the methanol flow rate in the experiment was

also partially governed by the maximum output of the

Simplex Minipump used to deliver methanol to the slurry

reactor system. Based on these, the methanol flow rate

range was chosen to be between 4 and 20 mL/h. The

experiments were conducted at different catalyst

loadings in oil.

The approach used for modeling the reaction rate

data was that of a global kinetic rate expressed as a

power law function of the concentration driving force.

For the dimethyl ether synthesis reaction, this

concentration driving force is the difference between

the liquid phase concentration of methanol at the phase

equilibrium conditions and the methanol concentration in

the isolated liquid phase which is in equilibrium as

determined by the chemical equilibrium constant relation

(Lee (1988) j .

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METHANOL TO DIMETHYL ETHER 645

EXPERIMENTAL APPARATUS

A schematic for the experimental system used to

carry out the reaction rate measurements is given

elsewhere (Parameswaran et al. (1989)). The heart of

the system is a l-L stirred autoclave reactor

(manufactured by Autoclave Engineers, Inc.). Methanol

stream from the outlet of the Simplex Minipump was

combined with the inert nitrogen stream in an

electrically heated Tee-connection. The mixture was then

introduced into the reactor by means of a tube that

extended all the way to the bottom of the reactor. It

was then discharged near the eye of the impeller. The

impeller shaft had an internal bore which enabled the

circulation of gas/vapor inside the reactor. This also

ensured effective dissipation of gas bubbles and

suspension of solid particles in liquid.

A proportional temperature controller was used to

control the reactor temperature. The flow rate of

nitrogen was maintained at a setpoint by an electronic

mass flow controller. The flow rate of methanol was

controlled by adjusting the plunger displacement of the

pump. The product vapors, dimethyl ether, water,

unreacted methanol, and nitrogen were passed through a

twin-pipe heat exchanger. Methanol and water were

condensed in a high pressure sample holder. The flow

rate of the non-condensables was measured using a wet

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646. GOGATE ET A,L:

test meter: All the product gas. and liquid analyses

were performed on a Hewlett-Packard gas chromatograp? . .

(HP 5 8 9 0 - A ) . A Carbosieve S (Supelco, Inc.) packed . . ,

column was used along with a thermal conductivity

detector. The gas chromatograph was also equipped w i t h 5 . , . .. .

sixypqrt Valco injection valve for on~line injection of

samples: The materjal balance of the experiment was

checked to ensure the accuracy of the experimental

reaction rate measdrements:

dev

RESULTS AND DISCUSSION

For the purpose of a global kinetic model,

elopment for the liquid phase catalytic dehydra , . kion

of methanol to dimethyl ether, it is first necessary ~~ . to

compute the liquid phase concentfations a f the dissolved . .

species corresponding to the vapor phase c?mpositi?ns:

Op importance are the phase equilibrjum and the isdated

llquld phase chemical eguilibyium concentration . . .of

methanol because a difPerence in these two provides the

driving force for the reversible . . . dimethyl ether . synthesis reaction: The kinetic driying foEes and the

reaction rate . data . . . ,are then analyzed using a ;ine.ar . .

regr,ession analysis: The values of the order and . - the .

activation energy which give the best - .. fit . . (ire. the least errpr) are cho5en: The reqr,essi_on analysis is

applied to each ofthe two cata_lyst ;oadi,ngs:

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HETHMOL TO DIMETHYL ETHER

Estimation of Dhase equilibrium concentrations

These were determined using the standard phase

equilibrium relations. The number of components in the

system are four. The Lewis-Randall standard state was

used for the vapors, methanol, water, and dimethyl

ether. The Henry's Law standard state was used for the

permanent gas, nitrogen. Redlich-Kwong equation of

state was used for the calculation of the saturation

fugacity coefficients of these three species. The

theoretically calculated pure liquid fugacities for

methanol, water, and dimethyl ether are given in

Table 111.

It should be noted that the critical temperature of

dimethyl ether is 27OC and it should be classified as a

gas at the experimental temperature and be handled by

the Henry's Law approach. However, it is very difficult

to measure the solubility of dimethyl ether in Witco-40

oil and particularly so, because it is shipped at its I

vapor pressure of 62.4 ~ ~ f / c m ~ at room temperature. To

circumvent this problem, it was decided to classify

dimethyl ether as a vapor. Then, it could be handled

using the hypothetical pure liquid fugacity standard

state. Prausnitz et al. (1980) have treated this

approach valid uptil a ratio of operating temperature to

critical temperature of 2. In this case, this ratio was

on an average 1.3 and thus well below 2:

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Table I11

GOCATE ET AL.

Pure Liquid Fugacities of Vapors

Methanol

........................................................ T (OK) pSat (MPa) vSat (m3/~mol) F~~ (MPa) ........................................................ 503.15 6.409 0.395 4.634

water

Dimethvl ether

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METHANOL TO DIMETHYL ETHER 649

since the saturation vapor pressure of dimethyl

ether at the operating temperature of about 250°C is

much greater than that of water or methanol, it is

expected that the pure liquid fugacity should also

follow the same trend. This can be easily seen from

Table 111. Since the Henry's law constant or the pure

liquid fugacity is inversely proportional to the

solubility, it can be postulated that the mol fraction

of dimethyl ether in the liquid phase will be much less

than that of either methanol or water. This becomes

more obvious from the examination of the reaction rate

data given in Table IV. This provides an ideal

situation for this reaction since the product dimethyl

ether will tend to escape from the liquid phase as soon

as it is formed, thereby decreasing the extent of the

reverse reaction.

Since the Henry's law standard state was used for

nitrogen, it was necessary to perform equilibrium

solubility measurements for nitrogen in Witco-40 oil.

These experiments were conducted at various pressures at

each of the four different temperatures, viz., J O ~ C ,

175OC. 200°C, and 2 4 0 ~ ~ . From the experimental

solubility data, Henry's law constants for nitrogen were

computed at each of these four different temperatures.

These values are listed in Table V. From this table, it

is seen that the solubility of nitrogen in Witco-40 oil

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Oil' 2). 64V4: 2.5298; 2 .6493' 2L.5299)

Libuid' Phase Com~ositions: at ~uuilibrium. f ~ n a l 7 / & &

N2 0 . 3 7 3 1 0.3774: 0'.3792' 0 3 5 9 1

Oil 2'. 6494' 2:. 5298 2.6493 2 .5299

Reaction' Rate tmol/ku.cat-h)

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METHANOL TO DIMETHYL ETHER 651

Table IV (continued)

Summary of Slurry Reactor Experimental Data

Run no.

5 6 7 8

T (OK) 5 0 3 . 1 5 5 4 3 . 1 5 5 0 3 . 1 5 5 4 3 . 1 5

N (RPM) 1250 1250 1250 1250

Liauid Phase Com~ositions 1mol/dm2l

N2 0 . 6 2 0 6 0 . 6 4 2 6 0 . 6 4 7 7

( O H ~ O M 10.'0229 0 . 0 1 5 1 (0.. 0934

CH,$OCH3 '0.'0660 0 .0262

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GOGATE ET AL.

Table IV (continued)

Summary of Slurry Reactor Experimental Data

9 10 11 12 ......................................................... T (OK) 503.15 543.15 503.15 543.15

F (mL/h) 4 4 2 0 2 0

P (MPa) 5.52 5.52 5.52 5.52

N (RPM) 1800 1800 1800 1800 ......................................................... Liquid Phase Com~ositions (mol/dmll

Oil 2.6499 2.5298 2.6494 2.5298

Liquid Phase Compositions at Equilibrium fmol/dmll

N2 0.3731 0.3774 0.3791 0.3590

Oil 2.6494 2.5298 2.6494 2.5298

Reaction Rate (mol/ka.cat-hl

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METHANOL TO DIMETHYL ETHER 653

Table IV (continued)

Summary of Slurry Reactor Experimental Data

......................................................... Run no. .........................................................

13 14 15 16 ......................................................... T (OK) 503.15 543.15 503.15 543.15

F W / h ) 4 4 20 2 0

P (MPa) 8.27 8.27 8.27 8.27

N (RPM) 1800 1800 1800 1800 ......................................................... Liquid Phase Com~ositions (mol/dmJ~

CHjOCH3 0.0070 0.0136 0.0157 0.0292

Oil 2.6499 2.5306 2.6499 2.5306

Liquid Phase Com~ositions at Equilibrium ~mol/dmzl

Oil 2.6499 2.5306 2.6499 2.5306

Reaction Rate (mol/kq.cat-hl

CH,0CH3 1.6600 5.1100 3.5100 11.3100 ......................................................... Note: The value for run 7 marked by * is not available. The value can be expected to be similar to run 15.

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654 GOGATE ET AL.

Table V

Henry's Law Constants for Nitrogen

increases as the temperature is increased and

correspondingly, the Henry's law constant decreases.

This behavior is in accordance with a rule of thumb

proposed by Gerrard (1976) based on the critical

temperature of the dissolving gas in the solvent.

Although the solubility of nitrogen increases with

temperature, this increase is very moderate and the plot

of equilibrium fugacity vs. the mol fraction in the

liquid phase is nearly linear (175°~-2400~). Thus the

behavior of nitrogen in the liquid phase could be nearly

treated to be ideal (Figure 2).

Estimation of Chemical E~uilibrium Concentrations

To determine the kinetic driving force for the

synthesi's reaction in terms of the reactive species, it

was necessary to determine, apart from the physical

equilibrium concentrations, the isolated chemical

equilibrium concentrations. These isolated liquid phase

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METHANOL TO DIMETHYL ETHER

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656 GOGATE ET AL.

concentrations will conform to the chemical equilibrium

constant for the dehydration reaction.

The chemical reaction equilibrium constants for the

liquid phase cannot be calculated directly for this

dehydration reaction because the standard heats and the

free energies of formation are unavailable for the

liquid species. Hence the vapor phase reaction

equilibrium constants were calculated first. Based on

the heats and free energies of formation, an equation

relating the vapor phase reaction equilibrium constant

to the temperature was obtained. The relationship,

between the vapor phase equilibrium constant and the

temperature can be given as:

Kc = exp (-14.46 + (2712.5/T) + 2.156 in T - 0.003656 T + 7.42*10-~ T ~ ) ... ( 2 )

The liquid phase chemical reaction equilibrium constant

is calculated from the above vapor phase constant using

the following relation:

In the above relation, K's are the pure liquid

fuqacities of the individual vapors and K1 and Kc are

the liquid and vapor phase chemical reaction equilibrium

constants respectively. Ideally, the activity

coefficients of the dissolved species are required to be

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METHANOL TO DIMETHYL ETHER 657

included in the right hand side of the above equation.

However, as the liquid phase concentrations of the

dissolved chemical species are low, the activity

coefficients can be treated as being equal to 1. In

other words, the liquid phase can be treated as being

ideal. Thus,

In the above relation, e is the equilibrium extent of

the reaction. The superscript o indicates the moles of

the dissolved species at phase equilibrium. The number

of moles of each species in an isolated liquid phase at

chemical equilibrium can be found out from these, once

the equilibrium extent of the reaction as governed by

the above equation is found out.

The values of concentrations of the various

reacting species at both the physical and chemical

equilibrium are listed in Table IV. An analysis of the

physical and chemical equilibrium concentration of

dimethyl ether yields an interesting result. The data

show that the chemical equilibrium concentration of

dimethyl ether is always greater than its physical

equilibrium concentration. This means that the product

dimethyl ether would never limit the kinetic rate since

the saturation of the liquid phase with respect to

dimethyl ether is still far away from the chemical

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658 GOGATE ET AL.

eqyilibrium concentration. A similar result was also

obtained in the previous section from a preliminary

analysis of the liquid phase fugacity data. Thus, at

the reaction conditions, the product, dimethyl ether

would never limit the,kinetic rate.

statistical Analvsis of the Reaction Rate Data

Based on the results of preliminary experiments,

the variables which might have a significant effect on

the synthesis rate of dimethyl ether were sorted out and

the experimental design took into account these

variables. It was-necessary to quantify the effects of

these variables by a statistical technique:

The influence of the four design variables on the

rate of dimethyl ether synthesis and the levels of their

effect were computed using Yates algorithm (Box et al.

(1976)). Results in Table VI show that the reaction

temperature and the methanol flow rate have significant

positive effects on the reaction rate. The effects of

the reaction pressure and.the impeller speed, on the

other hand, were found to be almost negligible. As the

synthesis rate is unaffected by the impeller speed in

the speed-range investigated (1250 RPM-1800 RPM), it can

be concluded that the solid to liquid mass transfer

(actually, from liquid to the active sites of the

catalyst), does not limit the kinetic rate at these

conditions. On the other hand, the reaction rate data

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METHANOL TO DIMETHYL ETHER

Table VI

Statistical Analysis of the Reaction Rate Data (Yates Algorithm)

AV.

T

F

TF

P

TP

FP

TFP

N

TN

FN

TFN

PN

TPN

F PN

TFPN

a) T : Temperature (OK) F : Methanol Flow Rate (mL/hour) P : Reaction pressure (MPa) N : Impeller speed (RPM)

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660 GOGATE ET AL.

indicate that with an increase of five times in the

methanol flow rate, the dimethyl ether synthesis rate

increases by only about twice. This becomes very clear

from the datapoints 6 and 8. This can be construed as

being due to the extremely low solubility of methanol in

the liquid phase.

The results of the statistical analysis are

summarized in Table VI. It can be seen that all the

interactions of the second order and above are quite

negligible. This is the case with most practical

engineering designs. There are significant positive

effects of the temperature and the methanol flow rate on

the rate of dimethyl ether synthesis. Since both these

effects are significant, their interaction is also

significant. Thus; the effects of the temperature and

the'methanol flow rate are significantly interlinked.

DeveloDment of Global Kinetic Rate Ex~ression

The kinetics of the dimethyl ether synthesis

reaction by the liquid phase dehydration of methanol has

been analyzed by fitting a global rate expression

between the dimethyl ether synthesis rate and the

concentration driving force for methanol. This

concentration driving force, as has been mentioned

previously, is a difference between the physical

equilibrium concentration and the chemical equilibrium

concentration of methanol. This approach is

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METHANOL TO DIMETHYL ETHER 66 1

thermodynamically consistent when an elementary reaction

of the type of A = B takes place. For this reaction,

this approach is very close to being so since the

reaction rate data suggest that the reaction takes place

exactly as described by the stoichiometry. In other

words, for all of the reaction runs, reaction rates of

dimethyl ether and water synthesis were found to be

exactly half of that of the reacfion rate for methanol

disappearance. Thus two moles of methanol undergo

dehydration reaction to give one mole of dimethyl ether

and one mole of water. The kinetic driving forces for

this case are summarized in Table VII.

The rate of dimethyl ether synthesis from methanol

over gamma-alumina catalyst in the liquid phase can now

be expressed by:

Using a stepwise linear regression analysis, the

values of the order of the reaction, the frequency

factor, and the activation energy of the reaction were

computed. The error term in the regression analysis was

the least when the value of n is 0 . 2 8 . Thus, the order

of the reaction with respect to methanol was taken as

0 . 2 8 . For this reaction order, the frequency factor and

the activation energy of the reaction were found to be:

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GOGATE ET AL.

Table VII

Kinetic Data for Dimethyl Ether Synthesis Reaction (5 grams of Gamma-Alumina in 550 mL of Witco-40 Oil)

a: not available. Values similar to run 15. ,

The results of the regression analysis are given in

Table VIII. Using this model, the comparison of the

observed and the predicted rates is given in Figure 3.

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METHANOL TO DIMETHYL ETHER 663

Table VIII

stepwise Linear Regression Analysis (5 grams of Gamma-Alumina in 550 mL of Witco-40 Oil)

Order Frequency Factor Act. Energy Error (%) ......................................................... 0.20 1.164 + 10' 17220 5.36

0.22 1.552 10% 17470 5.07

0.24 2.066 * 10' 17720 4.85

0.26 2.746 ' 10' 17970 4.72

0.28 3.662 10' 18830 4.70

0.30 4.886 10' 18480 4.79

0.32 6.508 10' 18740 4.99

0.34 8.655 10' 18990 5.30

0.36 1.150 10'' 19240 5.70 .........................................................

Note: Frequency Factor [=] dm3/kg.cat-h

Activation Energy [ = I Cal/gmol

An interesting observation can be made about the

activation energy of this reaction. The value of E/RT

for the reaction is about 19 at a temperature of 5 0 3 ~ ~

while the same value at a temperature of 5 4 3 O ~ is about

17. For a heterogeneous catalytic reaction which

operates in a mass transfer free zone, this value as a

rule of thumb is about 20. Thus, at these reaction

conditions, this number is consistent with our apriori

assumption of the experiment in that the reaction

Operates nearly in the mass transfer free zone.

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0 f I I I I I I 0 2 4 6 8 10 12 14

I

Observed R a t e (mol/kg .ca t -hour)

Figure 3. Comparison between Observed and Pred ic ted Model Rates ( 5 grams o f Gamna-Alumina i n 550 mL o f Witco-40 O i l )

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METHANOL TO DlHETHYL ETHER

Experiments at Hiqher Catalyst Loadinqs

In conducting experiments for this case, the

impeller speed and the reaction pressure were held

constant at their centerpoint values at 1500 RPM and 7

MPa respectively, since it was shown in the previous

analysis that these two variables have statistically

insignificant effects on the reaction rate. The kinetic

data for this case are presented in Table IX. The data

were analyzed using a stepwise linear regression

technique. The results of the regression analysis are

given in Table X. The error became the least when the

reaction order was 0.18. Then, the best fitting kinetic

model for the high catalyst loading case was written as:

~ D M E = 0.126 lo6 e-10840/RT (C-C eq)0.18 .. . (6)

Using this model, the predicted rates of the

dimethyl ether synthesis reaction were determined and a

comparison between the observed and the predicted rates

is given in Figure 4.

For this case, the value of E/RT is about 11 at a

temperature of 503OK, while the same value at a

temperature of 543OK is 10. Both of these values are

considerably less than the empirical value of 20.

Although it was postulated that the reaction rate might

be limited by gas-to-liquid mass transfer at this high

catalyst loading, an actual decrease in the reaction

order and activation energy indicate otherwise. Had the

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GOGATE ET AL.

Table IX

Kinetic Data for Dimethyl Ether Synthesis Reaction (15 grams of Gamma-Alumina in 550 mL of Witco-40 Oil)

Note: T: Temperature (OK)

F: Methanol Flow Rate (mL/hour)

Table X

Stepwise Linear Regression Analysis (15 grams of Gamma-Alumina in 550 mL of Witco-40 Oil)

Order Freq. Fact. Act. Energy Error (P) ........................................................ 0.08 54170 10210 1.25

Note: Frequency Factor [.=I dm3/kg.cat-h

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METHANOL TO DIMETHYL ETHER 667

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6 68 COGATE ET AL.

reaction rate been limited by the gas-to-liquid mass

transfer, the reaction drder should have increased from

0.28 and approached a value of unity since mass transfer

is a first order rate process. The decrease in the order

of the reaction was finally attributed to the resistance

of the solid film.

CONCLUSIONS

A complete thermodynamic and kinetic study was

carried out on the liquid phase catalytic dehydration of

methanol to dimethyl ether. Based on the statistical

analysis of the reaction rate data, only the temperature

and the flow rate of methanol have significant effects

on the dimethyl synthesis rate. The solid-to-liquid and

the gas-to-liquid mass transfb; do not limit the

kinetic rate at a catalyst loading of 5 grams in 550 mL

of Witco-40 oil. At higher catalyst loadings (15 grams

in 550 mL of Witco-40 oil), .there is a drop in both the

apparent reaction order and the activation energy. This

was attributed to the resistance of the solid film. The

gas-to-liquid mass transfer do not limit the kinetic rate

at this high catalyst loading.

ACKNOWLEDGEMENT

This work was fully supported by the Electric Power

Research Institute by its Research contract RP-317-06.

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METHANOL TO DIMETHYL ETHER 669

NOMENCLATURE

frequency factor (dm3/kg.cat. hour)

concentration of the dissolved species at phase equilibrium (mol/dm3)

concentration of the dissolved species at chemical equilibrium (mol/dm3)

equilibrium extent of the reaction, mols

activation energy (cal/gmol)

methanol flow rate (mL/h)

pure liquid fugacity of vapor (MPa)

Henry's law constant (MPa.Kmo1 solvent/Kmol solute)

chemical reaction equilibrium constant in the vapor phase, dimensionless

chemical reaction equilibrium constant in the liquid phase, dimensionless

order of the reaction

impeller speed (RPM)

reaction pressure (MPa)

reaction rate of the reactive species (mol/dm3. kg cat)

gas law constant (cal/gmol.K)

saturation volume of the vapor (m3/kmol)

reaction temperature (K)

REFERENCES

Adamska, R. D., Cybulski, A . , Stankievicz, A . , and Leszczynski, Z., "Kinetics and Mathematical Simulation of Methanol to Dimethyl ether Dehydration", Przem. Chem, 63, 190-193, 1984.

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670 GOGATE ET AL.

Box, G. E. P., Hunter, W. G., and Hunter, J. S., Statistics for Experimenters, John Wiley and Sons, New YOrk, NY, 1976.

Brake, L. 0.. "Preparation of Dimethyl Ether by Catalytic Dehydration of Methanol", U. S. Patent 4,595,785, 1986.

Brooks, D. L., Rallis, C. J., Lane, N. W., Cipolat, D., "Methanol with Dimethvl Ether Iqnition Promoter as Fuel for Compression 1gnitIon ~ngine" Pmc. Intersoc. Energy convers. Enq. Conf., 19(2), 654-658, 1984.

Chanq, C. D., Silvestri, A. J., and Smith, R. L., "Production of Gasoline Hydrocarbons", U. S. Patent 3,928,483, 1975.

Chanq, C. D., Kuo, J. C. W., Lanq, W. Hi, Jacob, S. M., Wise, J. J., and Silvestri, A. J., "Process Studies on the conversion of Methanol to Gasoline", Ind. Eng. Chem. Proc. Des. Dev., 17, 255-260. 1978.

Chang, C. D., and Grover, S. S., "Conversion of Methanol to Gasoline Components", U. S. Patent 4,058,576, 1977.

Fujimoto, K., Asmi, K., Shikada, T., and Tominaqa, H. O., "Selective Synthesis of Dimethyl ether from Synthesis Gas", Chem. Lett., 12, 2051-2054, 1984.

Gerrard, W., Solubility of Gases in Liquids, Plenum Press, New York, 1976.

Lee, B. G., "Post-treatment and Regeneration of Industrial Coprecipitated Methanol Synthesis Catalyst", Ph. D. Dissertation. The University of Akron, 1989.

Lee, S., Parmeswaran, V., Lee, B., and Goqate, M., "Novel Developments and Enhancements in Methanol Synthesis", Paper Presented at U. S. DOE Indirect Liquefaction Contractors Review Meeting, Pittsburgh, PA, NOv. 15, 1988.

Lee, S., Methanol Synthesis ~echnoloq~, CRC Press, Inc., 1989.

Lin, Y. M., and Wang, I., "A Kinetic Study of Dehydration of Methanol to Dimethyl ether over Solid Acid-Base Catalysts", J. of the Chinese Inst. of Chem. Enqrs., 12, 173-182, 1981.

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Marsden, W. L., Wainwright, M. S., and Friedrich, J. B., "Zinc Promoted Raney Copper Catalysts For Methanol Synthesis", Ind. Eng. Chem. Prod. Res. Dev., 19, 551-556, 1980.

Mattox, W. J., "Method for Converting Alcohol to Ethers", U. S. Patent 3,036,134, 1962.

Parameswaran, V. R., Lee, S., and Wender, I., "The Role of Water in Methanol Synthesis", Fuel Sci. Tech. Int'l, 7 (7), 899-918, 1989.

Prausnitz, J. M., Anderson, T. F., Grens, E. A . , Eckert, C. A., Hsieh. R., and O'Connell, O.P., Computer Calculations for Multicomponent Vapor-Liquid and Liquid-Liquid Equilibria, Prentice-Hall, Inc., Englewood Cliffs, N. J., 1980.

Rubio, F., Delgado, D. S., Del Castillo, D., and Arvelo, A . R., "Catalytic Dehydration of Methanol in Vapor Phase", Ing. Quim, 12, 113-119, 1980.

Woodhouse, J. C., "Catalytic Dehydration of Methanol to Dimethyl ether", U. S. Patent, 1,949,344, 1934.

Zahner, J. C., "Conversion of Modified Synthesis Gas to Oxygenated Organic Chemicals", U. S. Patent 4,011,275,

RECEIVED: September lh, 1989 ACCEFTED: October 2 , 1989

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