Catalytic and kinetic study of methanol dehydration to ...courses.engr.uky.edu/CME/cme456-001/cme 456 2018/2018 AiCHE problem... · In the past deeade, the use of dimethyl ether (DME)

CHEMieAL ENGINEERING RESEARCH AND DESIGN 9 O (2 0 1 2 ) 8 2 5 - 8 3 3

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Chemical Engineering Research and Design

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Catalytic and kinetic study of methanol dehydration todimethyl ether

S. Hosseininejad, A. Afacan, R.E. Hayes*The Department 0/Chemical and Materials Engineering, University 0/Alberta, Edmonton, Alberta, Canada T6G 2G6

A B S T R A C T

Dimethyl ether (DME), as a solution to environmental pollution and diminishing energy supplies, can be synthesizedmore efficiently, compared to conventional methods, using a catalytic distillation column for methanol dehydrationto DME over an active and selective catalyst. In this work, using an autoclave batch reaetor, a variety of eommereialcatalysts are investigated to find a proper catalyst for this reaction at moderate temperature and pressure (110-135 Cand 900 kPa). Among the -y-alumina, zeolites (HY, HZSM-5 and HM) and ion exehange resins (Amberlyst 15, Amberlyst35, Amberlyst 36 and Amberlyst 70), Amberlysts 35 and 36 demonstrate good activity for the studied reaction atthe desired temperature and pressure. Then, the kinetics of the reaction over Amberlyst 35 is determined. Theexpetimental data are deseribed well by Langmuir-Hinshelwood kinetic expression, for which the surface reaetionis the rate determining step. The calculated apparent activation energy for this study is 98 kj/mol.

© 2011 The Institution of Chemieal Engineers. Published by Elsevier B.V. AU rights reserved.

Keyiuords: Dimethyl ether; Kinetics; Rate equation; Zeolites; Amberlyst

1. Introduction

T\vo major challenges faeed today are elimate ehange indueedby global warming and the threat of the diminution of eheapand readily available energy supplies, espeeially liquid trans-portation fuels. Both of these challenges are driving the searchtowards alternative fuels. With a more varied fuel mix, atten-tion must be paid to the reduetion of emissions, espeeiallyparticulate matter (PM) and NOx. Although these emissionsare not an urgent issue for stoiehiometrie gasoline fuelledengines equipped with three way eatalytie eonverters, withlean burn eompression ignition direet injeetion (CIDI) enginesthat use diesel fuel, both of these emissions are signifieantproblems. As Europe and North America move to a dieselbased eeonomy as an aid to improving fuel eeonomy andhenee a reduetion in greenhouse gas (GHG) emissions, theseissues are rising to the top of the priority list. Current regula-tions have led to a major review of engine design with strictfortheoming limits on emissions. Reformulated diesel fuel willlikely play a key role in this future. Reformulation ineludes,for example, bio-diesel, redueed sulphur eontent, adding oxy-genates and using alternate fuels.

In the past deeade, the use of dimethyl ether (DME) as analternative fuel has been researched, and many ear eompanieshave been developing DME engines and related teehnology.DME has a eetane number and ignition temperature eloseto that of diesel fuel and gives low NOx, low smoke andlow engine noise, eompared to eonventional diesel engines(Semelsberger et al., 2006). It ranks near the top in well-to-wheel (WTW) effieieney among alternative fuels, regardlessof vehicle teehnology, and when eoupled to hybrid enginesean have higher WTW effieieney eompared to fuel eells withsimilar overall levels of GHG emissions.

DME ean also be used as a ehemieal feedstoek to makemanyproduets, sueh as short olefins (ethylene and propylene),gasoline, hydrogen, acetic acid and dimethyl sulfate. DME canbe easily transported to the areas far from oil and gas sources.

DME ean be produeed by the dehydration of methanol,whieh in turn is made from synthesis gas. Synthesis gas (syn-gas) ean be produeed from, for example, natural gas, eoaland biomass. Traditionally, DME has been produced from syn-gas in a two step process, in whieh methanol is producedfrom syngas, purified, and then converted to DME in anotherreaetor.

' Corresponding author.E-mail address: [email protected] (R.E. Hayes).Reeeived 10 March 2011; Received in revised form 26 July 2011; Aeeepted 8 October 2011

0263-8762/$ - see front matter © 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.doi:10.1016/j.eherd.2011.10.007

826 CHEMICAL ENGINEERING RESEARCH A N D DESIGN 9 0 ( 2 0 I 2 ) 8 2 5 - 8 3 3

Nomenclature

CE-AH

fesKQR

(rDME)

T

concentration (molm"^)activation energy (J mol"^)enthalpy change of reaction (Jmol"^)rate constantadsorption equilibrium constantenthalpy of adsorptiongas constant (Jmol"-' K"̂ )rate of formation of dimethyl ether

temperature (K)

SubscriptsDMW0

dimethyl ethermethanolwaterinitial value

In the conventional method, DME synthesized in a fixed-bed reactor is purified using at least two distillation columns.To reduce both capital and operating costs, and to increaseenergy efficiency, process integration can be considered. Cat-alytic distillation (CD) is an integrated process where thereactor and distillation column are combined into a singleunit. The advantages of using CD for methanol dehydrationinclude a higher selectivity of products to DME synthesis,higher conversion compared to a single reactor and loweroperational cost. However, the CD requires operation at mod-erate temperature and pressure (40-180 C and 800-1200 kPa).Most of the catalysts previously studied for this reaction aresolid-acid catalysts (e.g. zeolites) which tend to be active athigh temperature (250 C), and less research has been done atthe milder conditions required for CD.

DME is produced by the conventional bimolecular catalyticdehydration of methanol using solid acids (Spivey, 1991). BothBr0nsted and Lewis acid sites can catalyze the methanol DMEreaction. Silica alumina, 7-alumina and different kinds of zeo-lites, namely, Mordenite, ZSM-5 and Y show good methanolconversion and selectivity to DME at high temperature andpressure. Ion exchange resins have shown activity at lowertemperature, and cannot be used at high temperature. A goodcatalyst for methanol dehydration reaction should work atas low a temperature as possible to avoid subsequent dehy-dration of DME to olefins or hydrocarbons. Because catalyticdistillation of DME takes place at relatively low pressure(800-1200kPa) and temperatures in the range of 50-180 =C (DiStanislao et al, 2007), it is not clear from the literature as tothe best choice of catalyst. The final choice will have a combi-nation of strongest acidic strength and the highest number ofactive sites and resistance to water inhibition and side productformation. Although the acidity of the catalyst plays a cru-cial role in its performance, other factors such as thermal andmechanical stability, pore size and distribution as well as costwill determine the final choice.

The purpose of the present study was to find a suit-able commercial catalyst for methanol dehydration to DMEreaction at moderate temperature (110-135 =C) and pres-sure (900 kPa). The activity of commercial solid-acid catalystsincluding 7-alumina, HY, HZSM-5, HM zeolites and ionexchange resins (Amberlyst 15, Amberlyst 35, Amberlyst 36,Amberlyst 70) were investigated using an autoclave batch

To temperaturecontroller

Liquidsampling

Fig. 1 - Diagram of the batch reactor used in this study.

reactor. Reaction kinetics over the selected commercial cata-lyst was then measured to determine a reaction kinetic model.

2. Experimental

The reactions were carried out in a 480 cm^ stainless steelbatch autoclave equipped with a variable speed stirrer (fourblade glass impeller) and a heating jacket. A diagram of thereactor is given in Fig. 1. Reactor temperature was controlledusing a Parr 4841 proportional controller. The temperature wasmeasured with a J-type thermocouple. The liquid was sam-pled through a 1.6 mm diameter tube fitted with a sintered316 stainless steel filter with pore size of 300 mesh to preventit from being plugged by catalyst. A Swagelok needle valvewas used to control the liquid flow rate. Vapour samples werecollected through the gas vent, which was also fitted with aSwagelok needle valve. A condenser was connected to the gasvent to prevent methanol and water vapour escaping throughthe gas vent during gas sampling.

The liquid samples were analyzed using a Hewlett-Packard5710A series gas Chromatograph (CC) equipped with a thermalconductivity detector (TCD) and a 3m long, 1.6mm diameterSupelco Co. stainless steel HayeSep D column with mesh size80/100. The carrier gas was UHP helium with the flow rate of35 cm^/min. The detector and injection port temperatures areset to 200 C and 250=C, respectively. The oven temperaturewas 165 C.

A Hewlett-Packard 5970 series CC/MS equipped with aDB-5MS capillary column with 30 m in length and 0.25 mmin diameter was used to determine if the liquid samplescontained any product other than that DME. This CC wasable to detect product with molecular weight of 15-550. The

CHEMICAL E N G I N E E R I N G RESEARCH A N D DESIGN 9 0 ( 2 0 I 2 ) 8 2 5 - 8 3 3 827

Table 1 - Properties of three different zeolite catalysts.Zeolyst product Zeolite SÍO2/AI2O3 (mole ratio) Surface area (m /̂g)

CBV 28014CBV 8014CBV 21

ZSM-5ZSM-5Mordenite

2808020

400425500

injector and detector temperatures were set to 280 C. Theoven temperature was kept constant at 35 C for 5 min andthen increased to 280 =C at a rate of 10 = C/min. One microlitreof liquid sample was injected with a split ratio is 100:1 andgas sample injection is splitless 10 [ÍL of sample. The resultof the GC-MS analysis showed that there was no detectableby-product produced at this operating condition.

The three liquids used as starting materials were methanol,water and tetrahydrofuran, the latter being used as a diluent.Research grade (99.9%) methanol was obtained from FisherScientific. The water was obtained from a reverse osmosissystem. Tetrahydrofuran (THF) was obtained from Fisher Sci-entific.

The catalysts were 7-alumina, zeolites (Y, ZSM-5 and Mor-denite) and ion exchange resins (Amberlyst 15, Amberlyst 35,Amberlyst 36, Amberlyst 70 and Amberlite IR-120).

2.1. Zeolites

ZSM-5 and Mordenite, namely, CBV21, CBV8014 and CBV28014were obtained from the Zeolyst International Company(USA). These catalysts have different SÍO2/AI2O3 ratios, whichindicate different acidity strengths. Increase in SÍO2/AI2O3decreases the acidity strength but the amount of acidityremains almost the same (Khandan et al., 2008). Table 1shows the zeolites' properties, as provided by the Zeolyst Com-pany. Zeolites were received in NH4* form, which is inactivefor methanol dehydration. They were calcined in a Ther-molyne 79400 tube furnace to convert the ammonium cationsto hydrogen by removing the ammonia. The catalysts wereheated at 6= C/min from 25 to 500-C and then held at 500=Cfor 4h. After removal from the furnace, the calcined zeolitewere moved to a vacuum chamber to prevent adsorption ofwater from the air.

2.2. Amberlyst

Amberlyst 15, 35, 36 and 70, and Amberlite IR-120 wereobtained from the Rohm and Haas Company (USA). They werereceived in wet form, and were dried prior to use using a vac-uum dryer. Amberlyst series catalysts' properties are shownin Table 2, which were provided by the company. The acid-ity of the catalyst was measured according to the proceduresuggested by Rohm and Haas Co. We used one tenth of the

values suggested by the procedure. For instance, 1.5 g of eachAmberlyst is ion exchanged with 100 cm-' of sodium nitrateand 100 cm^ of HCl in regeneration. The procedure includespassing sodium nitrate through the catalyst bed where thecation exchange happens. After exchanging the hydrogen ionsin catalyst with sodium, the catalysts were washed and regen-erated by HCl to ion exchange the sodium ions by hydrogen.The regenerated catalyst is again ion exchanged by sodiumnitrate, and exactiy 100 cm^ of solution is collected, andtitrated by standard NaOH solution.

3. Catalyst screening tests

A set of experiments was conducted using all of the com-mercial solid-acid catalysts. The reaction was conducted at110 ±1C. For each run, 4±0.005 g catalyst and 120 ±0.2 gsolution were charged into the reactor. The reactor was runfor 3.5 h, and the DME synthesis rate and methanol conversionof the reaction were compared. It was found that 7-alumina,zeolites (Y) and Amberlite IR-120 did not have any detectibleconversion of methanol. ZSM-5 and Mordenite on the otherhand had small but quantifiable conversions of less than 3%methanol conversion at temperatures up to 130 C.

ZSM-5, HM and Amberlyst 70 were also tested at 150 Cand 1.7 MPa. The DME moles produced per gram catalyst andmethanol conversion as a function of reaction time is shownin Figs. 2 and 3. It can be seen that Mordenite has abouthalf of activity of the Amberlyst 70. Although both catalystsshowed some methanol conversion, the reaction temperatureand pressure were higher than those desired.

Amberlyst 15, Amberlyst 35, Amberlyst 36, Amberlyst 70catalyst performance was studied at 110 C and 900 kPa for 8 husing 6 ± 0.005 g catalyst and 120 ± 0.2 g solution. Fig. 4 showsthe DME produced per gram of catalyst, while Fig. 5 shows themethanol conversion as a function of reaction time. Both fig-ures show that the DME production and methanol conversionfor Amberlysts 35 and 36 are higher than Amberlysts 15 and70. This was expected because both Amberlysts 35 and 36 havehigher acidity than that Amberlysts 15 and 70.

The dehydration of methanol over Amberlyst 15, Amberlyst35, Amberlyst 36 and Amberlyst 70 at 130 C and pressure of900 kPa was also carried out to examine the effect of temper-ature on performance of these catalysts. The trend in activity

Table 2 - Amberlyst series catalysts properties.Name

Acidity (eqmv./kg)'Surface area (m^/g)Average pore diameter (A)Mean size (mm)Pore volume (ml/g)Swelling (water to dry)Operating temperature limit

^ Calculated by Rohm and Haas Co.

Amberlyst 15

4.6533000.6-0.850.437120

procedure.

Amberlyst 35

5.13SO3000.7-0.950.3540150

Amberlyst 36

5.38332400.6-0.850.254150

Amberlyst 70

2.7362200.5NANA190

828 CHEMICAL ENGINEERING RESEARCH AND DESIGN 9 O ( 2 O I 2 ) 8 2 5 - 8 3 3

0,16

? 0.14

^ 0.10<uo^ 0.08o^ 0.06

oE 0.04LU

Q 0.02

0.00

Amberlyst 70HMHZSM-5

[. i T i i

Reaction time (h)

Fig. 2 - Cumulative moles of DME produced over Anüberlyst70 and H-ZSM-5 at 150 °C and 1.7 MPa. Startingcomposition pure methanol.

f u

30

20

10

n 1

• Amberlyst 70V HM

'_ • HZSM-5

•

-

: I-: 5 î• i it, i í . i i

i 1 i i .

î

î

ii i i 1 i

' 1 ' ' ' '

• x

i-

-

1 -

-

--

î :i i i i i i ~

Reaction time (h)

Fig. 3 - Methanol conversion over Amberlyst 70 andH-ZSM-5 at 150 °C and 1.7 MPa. The same experiments asshown in Fig. 2.

was the same as observed at 110 C, with all catalysts showingan inerease in eonversion.

Water inhibits catalytic methanol dehydration to DME overeither solid-acids or ion exehange resins. Water and methanolmoleeules eompete for adsorption at eatalytie active sites on

0.06

0.05

E 0.04

0.03

.2 0.02oEluZ 0.01

0.00 O-O

9 Amberlyst 15V Amberlyst 35• Amberlyst 36O Amberlyst 70

V

4 6

Reaction time (ii)

10

Fig. 4 - Cumulative moles of DME produced over fourAmberlyst catalysts at 110 °C and 900kPa with puremethanol initial composition.

18

16

§ 12

î 10

0Ö-o

Amberlyst 15Amberlyst 35Amberlyst 36Amberlyst 70

V

O

4 6

Reaction time (h)

10

Fig. 5 - Methanol conversion over four Amberlyst catalystsat 110 °C and 900kPa using pure methanol as feed. Thesame experiments shown in Fig. 4.

U.UÖ

0.06

0.04

0.02

n nn r

•

•0

_

-

i 1 i i i

Amberlyst 15Amberlyst 35Amberlyst 36Amberlyst 70

#

0

i 1 i i i

i , i i i

•

•

o

i 1 i i i

i 1 i i i i

i i

1

i i

i

_

o

_

-

Reaction time (h)

Fig. 6 - Cumulative moles of DME produced over fourAmberlyst catalysts at 130 °C and 900kPa with 2.5 mol/Lwater in methanol solution as initial composition.

the surfaee of aeid eatalyst. Fig. 6 shows the reaetion overAmberlysts 15, 35, 36 and 70 at 130 X and pressure of 900kPafor an initial water eoncentrations of 2.5 M and 4 ±0.005 geatalyst. The same trend was observed with 3.5 M water inmethanol. The figure shows that Amberlyst 35 and Amberlyst36 have the same aetivity and mueh higher than Amberlysts15 and 70. The amount of the DME formation was observed todeerease slightly by increasing the initial water eoneentrationfrom 2. 5 M to 3.5 M.

Fig. 7 shows the initial rates of reaetion for Amberlysts 15,35, 36 and 70 for pure methanol, 2.5 M and 3.5 M water eon-eentrations in methanol. The initial rate of reaetion for eacheatalyst was obtained using nonlinear regression between theDME moles produeed and the reaction time data. The initialrate of the reaetion is equal to the value of the derivativeat time 0. This figure also shows that Amberlysts 35 and 36have higher initial rate and show more aetivity than thatAmberlysts 15 and 70 for pure methanol and for both watereoneentrations.

Fig. 8 shows the eorrelation between the initial rate ofthe reaetion at 110 °C and the aeidity eapaeity of Amberlysteatalysts. It ean be seen that there is a direet relation-ship between the initial reaetion rate and the acidity ofthe eatalyst. As the acidity of the catalyst increases, therate of reaetion inereases. These preliminary investigations

CHEMICAL ENGINEERING RESEARCH AND DESIGN 9 O ( 2 O I 2 ) 8 2 5 - 8 3 3 829

0.05

t; 0.04 -

0.03 -=,

S 0.02 -

0.01 -

0.000 1 2 3

Initial water concentration (moi/L)

Fig. 7 - Initial reaction rate as a function of initial waterconcentration over four Amberlyst catalysts at 130 °C and900 kPa.

1 ' '

î

•I

-

1 , ,

. . { 1 , 1

, 1 , , .

1 1 1 . .

îi

î

1 , , ,

, 1 , , , , _

• Amberlyst 15o Amberlyst 35» Amberlyst 36 ~^ Ambertyst 70

i :

5 :5 "-

• Amberlyst 15T Amberlyst 35

: • Amberlyst 36• Amberlyst 70

'-

y

y

. . 1

yy

y

ly

y

. 1 .

' ' ' .

l •

-j

•

0.010

0.009 -

3 0.008Eai

^ 0.007 -

= 0.006 -

0.005 -

0.0042 3 4 5

Amberlyst Acidity (Eq/kg)

Fig. 8 - The initial reaction rate of DME production as afunction of catalyst acidity for four Amberlyst catalysts at110 °C using pure methanol as initial composition.

show that Amberlysts 35 and 36 have higher DME pro-duction and consequently higher initial rate of reactionat lower reaction temperatures. Although, both Amberlysts35 and 36 shown very similar acti'vity Amberlyst 35 hasmore crosslinks and less swelling than that Amberlyst 36.Amberlyst 35 has better catalytic properties and physicalstability. Thus, we chose Amberlyst 35 for further kineticsstudies.

4. Kinetic study of Amberlyst 35

Many investigations on the kinetics of the synthesis of DMEby dehydration of methanol on solid-acid catalysts have beenpublished. The majority agree that the mechanism followseither Langmuir-Hinshelwood (Gates and Johanson, 1971) orEley-Rideal kinetic models (Kiviranta-Paakkonen et al., 1998),with water and DME both acting as reaction inhibitors. A sum-mary of some of published kinetic models for DME synthesisby catalytic dehydration of methanol is given in Table 3. Somestudies have proposed a mechanism for this reaction. Lu et al.(2004) developed a detailed intrinsic mechanism containingseven elementary reactions. This mechanism was used byMollavali et al. (2008) to derive kinetic global reaction equa-tions, as shown in Table 3. The two groups used different ratedetermining steps and arrived at different form rate equa-tions. In the mechanism introduced by Gates and Johanson(1969), shown in Fig. 9, it is assumed that two methanolmolecules occupy two adjacent acid sites. On the otherhand, in the Eley-Rideal (ER) model proposed by Kiviranta-Paakkonen et al. (1998), only one methanol molecule adsorbson the acid site which reacts with a second molecule from theliquid bulk phase (see Fig. 10). The models developed by thesegroups can be represented by the generic equation:

(1 + KMCM -I- (KwCw)(1)

fes is the surface reaction rate constant, and KM, KW, andKD, and CM, CW, and CD are the adsorption equilibriumconstants and concentration of methanol, water and DME,respectively. The power m takes the value of two for theLangmuir-Hinschelwood model and a value of one for theEley-Rideal model. The value of n is 0.5,1 or 2.

Most models presented in the literature show that waterformed during the reaction inhibits the reaction, and thatthe inhibition by dimethyl ether is very small compared towater. In addition, Gogate et al. (1990) indicates that becauseof higher vapour pressure of DME compared to methanol andwater, it can be assumed that the mol fraction of DME in theliquid-phase will be much less than that of water or methanol.Hence, the extent of the reverse reaction is decreased and theequilibrium conversion is close to 100%. The generic equationrepresented by Eq. (1) can be simplified to:

»"DME =(1 -I- (K

(2)

Table 3 - Kinetic models studied for methanol dehydration to DME.Reaction kinetic equation Catalyst used Reference

1.

2.

3.

4.

S.

6.

""DME =

Ion exchange resin

"/-AI2O3

7-AI2O3

7-AI2O3

Ion exchange resin

7-AI2O3

Gates and Johanson (1971)

Bercic and Levee (1992)

Bercic and Levee (1992)

Lu et al. (2004)

An et al. (2004)

Mollavali et al. (2008)

830 CHEMICAL ENGINEERING RESEARCH A N D DESIGN 9 0 ( 2 0 I 2 ) 8 2 5 - 8 3 3

IO — H —O — H---0

= 0 0 = S-\ /

-S/

0---H —0---H —O

O — H---0 — H---0/ H^; " \

S = 0 ^ C —H 0=SW H '. . /

O---H — 0---H—O

0-H

S = 0

O.

o

0 = S

Fig. 9 - Gates and Johanson (1969) mechanism for methanol dehydration reaction.

H

H '

1

H1

\ l" 0

/H

0-H'

0

)

H

, 0, ' \H-

L j

"^H

-0W

o=s-/

H—O

H /,0

d — H ' H- -O

/ w—s=o o=s—

\ . /0 H —0

\ i

/ /N0-- -H H--0

/ W- s = o o=s-

o H —o

Fig. 10 - Kiviranta-Paakkonen et al. (1998) mechanism for methanol dehydration reaction.

Ail of the calculations performed for the kinetie modellingstudy were based on the ealeulation of the initial rate of reac-tion, as discussed earlier.

To test the importance of external diffusion on the reae-tion kineties, the effect of stirring speed was examined. It wasfound that for 750 rpm and 650 rpm the initial reaetion rateswere 0.047 and 0.046mol/gcath, thus the external diffusion isnot a limiting factor.

The effeet of internal diffusion limitation on overall kinet-ics of dehydration of methanol to DME was investigatedby eomparing two catalyst sizes, 0.2-0.6 mm. For the twotests, the reaetor was eharged with 4 g Amberlyst 35 and120 g methanol, pressurized to 900 kPa and heated to 130'C.The stirrer speed was set to 750 rpm. There was no ehangeobserved in the rate of produetion of DME, therefore, we eaneonelude that internal diffusion was not signifieant.

4.1.rate

The effect of methanol concentration on reaction

The first set of tests was performed to examine the effeet ofmethanol eoneentration on the rate. The methanol eoneentra-tion was varied between 5 M and 24.6 M using tetrahydrofuran(THF) as an inert diluent. For temperatures of 110, 120, and130 =C, the reaetor was eharged with 10, 6 or 4 g Amberlyst35, respeetively, and 120g of methanol/THF solution. Fig. 11shows the effeet of methanol eoneentration on the initialreaetion rate for three temperatures. The initial rate stayedrelatively eonstant in the range of methanol concentrationsinvestigated.

The observed effect of methanol concentration can be usedto discriminate the models. In the absenee of water, Eq. (2) eanbe written to express the initial reaetion rate as:

Model 1 (LH) (rDME)o =(1

Model 2

(3)

(4)

We ean linearize Eqs. (3) and (4) with respeet to methanoleoneentration:

Model 1 (LH)('•DME)O

0.5

(5)

(6)

To determine whieh model fits the experimental data inthe absenee of water, the left hand side of Eqs. (5) and (6)was plotted versus methanol eoneentration. CM, as shown inFigs. 12 and 13. Comparison of these figures indieates thatthe experimental data fits better vwth Langmuir-Hinshelwood(model 1) for the temperature range studied. Hence, theLangmuir-Hinshelwood was selected as the best model for ourstudy.

Using a linear regression for the data shown in Fig. 13, thesurfaee reaetion rate eonstant, tes and the adsorption equi-librium eonstants. KM were determined. For higher methanoleoneentrations, the value obtained for KMCM is significantly

0.05

u 0.04

0.03

g 0.02

= 0.01

0.00

' ' I ' ' '

Î Í

n 5

• 110°C

o 120 °C

T 130 »C

10 15 20

Methanol concentration (mol/L)

25 30

Fig. 11 - Effect of methanol concentration on initial reactionrate at different temperatures and 900 kPa for differentconcentrations of methanol/THF solutions.

CHEMICAL E N G I N E E R I N G RESEARCH A N D DESIGN 9 O ( 2 0 I 2 ) 8 2 5 - 8 3 3 831

2.5e+8

2.08+8 -

1.5e+8 -

1.0e+8 -

5.0e+7 -

0.05 10 16 20

Methanol concentration (C„) (moi/L)

Fig. 12 - Left hand side of Eq. (6) versus methanolconcentration.

higher than 1 (25 » 1); thus, 1 / y ^ term in Eq. (5) is negligi-ble compared to KMCM/\/fes term. If we ignore the 1 in Eq. (5),then the model can be simplified further:

Model 1 (LH) (7)

Because it was found that the methanol concentration hadnegligible effect on initial reaction rate, two more testswere conducted at temperatures of 115 and 135 C. Linearregression of the Arrhenius plot (see Fig. 14) was used todetermine parameters feo and Eg in Arrhenius equation (Eq.(8)) were determined. The calculated values for feo and Egare 6.12 x 10''kmol/skgcat and 98kJ/mol, respectively. Thevalue of apparent activation energy is similar to the activa-tion energy calculated by Kiviranta-Paakkonen et al. (1998) andDi Stanislao et al. (2007) which are 95kJ/mol and 98kJ/mol,respectively.

fes = feo exp í — j (8)

4.2. The effect of initial water concentration onreaction rate

In this set of tests, the initial water concentration in the reac-tor was varied from 0 to 3.5 M to determine the effect of waterconcentration on the initial reaction rate. The reactor was

5 10 15 20

Methanol concentration (C„) (moi/L)

Fig. 13 - Left hand side of Eq. (5) versus methanolconcentration.

-11.0 -

-11.5 -

-12.0 -

-12.5 -

-13.00.00246 0.00249 0.00252 0.00255

1/T (K"')

0.00258 0.00261

Fig. 14 - Arrhenius plot for methanol dehydration reactionat temperature range of 110-135 °C and 900 kPa using puremethanol as initial composition.

0.045

3 0.040 -

0.035 -

.2 0.030 -

0.025 -

0.0200 1 2 3 4

Initial water concentration (mol/L)

Fig. 15 - Effect of water concentration on initial reactionrate at 130 °C and 900 kPa using different concentrations ofmethanol/^vater solutions as initial composition.

charged with 4g Amberlyst 35 catalyst and 120g of 1.5, 2.5and 3.5 M water/methanol solutions. Then the reactor waspressurized to 900kPa and heated to 130-C. Fig. 15 showsthat water concentration has significant effect on the initialreaction rate, consistent with previous work (An et al., 2004;Kiviranta-Paakkonen et al., 1998). In the presence of water, Eq.(2) becomes:

(9)

Eq. (9) can be rearranged to the linear form:

(10)

To determine the best value of n in Eq. (10) from the suggestedvalues (i.e. 0.5,1 and 2), the left hand side of Eq. (10) was plottedversus (C^VCM) and (CW/CM) as shown in Figs. 16 and 17. Itcan be seen from these two figures, when n = 1.0, the linearregression line fits the experimental data better. Thus Eq. (10)can be written as:

(11)fes

/TDME KM CM

The value for the surface reaction rate constant fes calculatedby linear regression of Eq. (12) is 1.19 x lO"^ mol/kgcats at

832 CHEMICAL ENGINEERING RESEARCH AND DESIGN g 0 ( 2 0 I 2 ) 8 2 5 - 8 3 3

0.00 0.08

Fig. 16 - Left hand side of Eq. (11) versus

130 'C, which is in good agreement with the value we obtainedfrom the initial rates for the set of experiments conducted inthe absence of water (i.e. 1.21 x 10~^mol/kgcats)

(12)

In Eq. (11), Kw and KM are temperature dependence adsorp-tion equilibrium constants of water and methanol and can bedefined using Van't Hoff relationship

exp

The ratio of KW/KM can be written as

(13)

(14)

(15)

where KWO/KMO = K and Q= (AHM - AHw). To determine K and Q

in Eq. (15), set of experiments was conducted using a constantwater concentration (i.e. 3.5 mol/L) with reactor tempera-tures of 110, 115, 120, 130 and 135 C. Using linear regressionbetween ln(Kw/KM) versus 1/T shown in Fig. 18, K and Q val-ues found to be 1.57 x 10"^ and 24.6kJ/mol, respectively. Thetemperature dependence of the ratio of adsorption equilib-

1.51. I ' ' ' ' I

0.9

I 1 I 1 I 1 1 I r 1-, I I

I , l i l i l í I

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16

Fig. 17 - Left hand side of Eq. (11) versus (CW/CM).

0.00246 0.00249 0.00252 0.00255

1/T0.00258 0.00261

Fig. 18 - Plot of ln(Kw/KM) versus 1/T in temperature range110-135 °C and 900kPa using 3.5 mol/L water/methanolsolution as initial composition.

30

20

10

0^

. ' ' ' ' 1

-

_

A//

1 1 1 1 1

ooo

1 '

110 C -pureMeOH130 C-pure MeOH130 C 3.5 mol/L water

O

1 ,

. - - ^

1 '.

-

_

O -

;

1 1

Reaction time (h)

Fig. 19 - Comparison of model predictions for typicalexperiments used to obtain the rate parameters. Note thatthe initial rates were used in the analysis. The lines are themodel predictions.

rium constants of water and methanol can be calculated withfollowing equation.

_ = exp ( - 6 . 4 6 + — ) (16)

Finally, the predictive ability of the model is tested againstthe experimental data. The initial rates being used to calculatethe rate constants, the resulting model was used to predict theresults from the full experiments. Fig. 19 shows three exper-iments with the predictions. Two are for pure methanol asa starting composition, with two temperatures selected. Thethird shows the experiment with added water. It is seen fromthe figure that the fits are reasonable.

5. Conclusions

The activity of a series commercial solid-acid catalysts forthe dehydration of methanol to dimethyl ether was tested.The catalysts were 7-alumina, HY, HZSM-5, HM zeolites andion exchange resins (Amberlyst 15, Amberlyst 35, Amberlyst36, and Amberlyst 70). It was found that 7-alumina and HY,HZSM-5, HM zeolites did not have a promising activity in thetemperature range of 110-135 C. Ion exchange catalysts had

CHEMICAL E N G I N E E R I N G RESEARCH A N D DESIGN 9 O ( 2 0 I 2 ) 8 2 5 - 8 3 3 833

signifieant activity, with Amberlysts 35 and 36 having similaractivities.

The kinetics of dehydration of methanol to DME overAmberlyst 35 was studied in the absenee of mass transfer lim-itations to determine a reaetion kinetic model. The methanoleoneentration did not have any effeet on the reaetion rate,whieh is in aeeordanee with the meehanism proposed by Gatesand Johanson (1971). In this meehanism, the two moleeules ofmethanol, occupy two adjacent aeid sites, and the reaetionhappens between those molecules. It was also found that thepresenee of water had inhibiting effeet on the reaetion rateby eompeting with methanol moleeules over acid sites. It wasfound that the Langmuir-Hinshelwood model is the best fit fordata.

Acknowledgements

This work was sponsored by a grant from Alberta AgriculturalReseareh Institute. The authors would also like to aeknowl-edge helpful diseussions with Dr. D. Bressler and Dr. K.T.Chuang.

References

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Bercic, C, Levee, J., 1992. Intrinsie and global reaction rate ofmethanol dehydration over gamma-alumina pellets.Industrial and Engineering Chemistry Research 31, 1035-1040.

Di Stanislao, M., Malandrino, A., Patrini, R., Viva, A., Brunazzi, E.,2007. Creen Fuel Synthesis via Reactive Distillation, RécentsProgrès en Cénie des Procédés, Numéro 94. SPFC, Paris,Franee, pp. 1-8.

Gates, B., Johanson, L., 1969. The dehydration of methanol andethanol eatalyzed by polystyrene sulfonate resins. Journal ofCatalysis 14 (1), 69-76.

Gates, B., Johanson, L., 1971. Langmuir-Hinshelwood kinetics ofthe dehydration of methanol catalyzed by cation exchangeresins. AIChE Journal 17 (4), 981-983.

Gogate, M.R., Lee, B.G., Lee, S., Kulik, C.J., 1990. Kinetics of liquidphase catalytic dehydration of methanol to dimethyl ether.Petroleum Science and Teehnology 8 (6), 637-671.

Khandan, N., Kazemeini, M., Aghaziarati, M., 2008. Determiningan optimum catalyst for liquid phase dehydration of methanolto dimethyl ether. Applied Catalysis A: General 349, 6-12.

Kiviranta-Paakkonen, P.K., Struekmann, L.K., Linnekoski, J.A.,Krause, A.O.I., 1998. Dehydration of the aleohol in theetherifieation of isoamylenes with methanol and ethanol.Industrial and Engineering Chemistry Research 37,18-24.

Lu, W., Teng, L., Xiao, W., 2004. Simulation and experimentalstudy of dimethyl ether synthesis from syngas in a fluidizedbed reaetor. Chemieal Engineering Science 59 (22-23),5455-5464.

MoUavali, M., Yaripour, F., Atashi, H., Sahebdelfar, S., 2008.Intrinsic kinetics study of dimethyl ether synthesis on(-AI2O3). Industrial and Engineering Chemistry Research 47,3265-3273.

Semelsberger, T.A., Borup, R.L., Greene, H.L., 2006. Dimethyl ether(DME) as an alternative fuel. Journal of Power Sources 156,497-511.

Spivey, J., 1991. Review: Dehydration catalysts for themethanol/dimethyl ether reaetion. Chemical EngineeringCommunications 110,123-142.

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