Jurnal Dme Synthesis

Journal of Natural Gas Chemistry 12(2003)219227

Direct Dimethyl Ether Synthesis

Takashi Ogawa, Norio Inoue, Tutomu Shikada, Yotaro OhnoDME Development Co., Ltd, Shoro-koku Shiranuka-ch, Hokkaido, 088-0563 Japan

[Manuscript received September 1, 2003; revised October 20, 2003]

Abstract: Dimethyl ether (DME) is a clean and economical alternative fuel which can be produced fromnatural gas through synthesis gas. The properties of DME are very similar to those of LP gas. DME canbe used for various fields as a fuel such as power generation, transportation, home heating and cooking,

etc. It contains no sulfur or nitrogen. It is not corrosive to any metal and not harmful to human body. Aninnovative process of direct synthesis of DME from synthesis gas has been developed. Newly developedcatalyst in a slurry phase reactor gave a high conversion and high selectivity of DME production. Oneand half year pilot scale plant (5 tons per day) testing, which was supported by METI, had successfullyfinished with about 400 tons DME production.

Key words: dimethyl ether, DME, slurry reactor, natural gas, syn-gas, coal, clean fuel, diesel engine

1. Introduction

Future energy demand especially in the pacific

and Asian regions is forecasted to be huge. Therefore

limited energy supply as well as environmental issue

caused by consumption of fuel would be substantial

obstacles to realize constant economic growth in these

regions. Dimethyl ether (DME), which is recently rec-

ognized as a new clean fuel and is synthesized from

natural gas, will give a solution of secure energy sup-

ply and environmental conservation.

DME is a colorless gas at an ambient condition

and easily liquefied under light pressure. Since its

physical and chemical characteristics are very similar

to those of LP gas, it is an easy substitute for LP gas.

Almost half of household in Japan use LP Gas for

cooking and home heating. DME is not only an easy

substitute for LP gas, but also a very clean substitute

for diesel fuel because a DME fueled diesel car emits

neither soot nor particle matters (PM) [1].

DME can be distributed and stored by using LP

gas handling technology, which means DME does not

need costly LNG tankers or LNG terminals [2]. Once

natural gas is converted to DME, it will provide a new

competitive route to transport natural gas compared

with the LNG chain.

DME is now manufactured with two processes,

which are methanol synthesis and methanol dehydra-

tion process. In order to use DME as a fuel, it must

be produced at low cost in large quantities. DME

synthesis from synthesis gas (syn-gas: H2+CO gas)

developed for these years [35]. JFE (formally NKK)

Corporation has been making remarkable progress in

a development of direct DME synthesis from syn-gas

with a bubble column slurry reactor since 1989 [69].

This JFE Direct DME Synthesis will open a new way

to economical DME mass production.

JFE Co., collaborating with Taiheiyo Coal Mining

Co., Sumitomo Metal Industry, Ltd. and CCUJ (Cen-

ter for Coal Utilization, Japan), finished a project of

5 ton-DME/day (5TPD) pilot plant in 2001 very suc-

cessfully [10]. This project was funded by Ministry of

Economy, Trade and Industry (METI).

Based on this achievement, DME Development

Co., Ltd started 100 ton-DME/day (100TPD) demo-

plant project at Hokkaido Japan in 2002.

Corresponding author. Tel: +81-1547-5-9245; Fax: +81-1547-5-9247

E-mail: [email protected]

220 Takashi Ogawa et al./ Journal of Natural Gas Chemistry Vol. 12 No. 4 2003

2. Whats DME?

DME is the simplest ether having the chemical

formula of CH3OCH3. DME is now used almost only

for a spray propellant (paints, agricultural chemicals,

cosmetics, etc.). Approximately 10,000 tons/year

are produced in Japan, and about 150,000 tons/year

worldwide. Table 1 shows physical properties and

combustion characteristics of DME and other relat-

ing fuels. DME is a colorless gas at an ambient con-

dition. As its vapor pressure is about 0.6 MPa at

25

, DME is easily liquefied under light pressure.

Its physical properties are similar to those of propane

and butane, which are the principal constituents of

LP gas.

Table 1. Physical properties of DME and other fuel

Properties Chemical Boiling Liquid Specific Heat of Vapor Ignition Explosion Cetane Net Net

formula point density gravity vaporization pressure temperature limit numberb calorific calorific

(K) (g/cm3)a (vs. air) (kJ/kg) (atm)a (K) value value

(106J/Nm3) (106J/kg)

DME CH3OCH3 247.9 0.67 1.59 467 6.1 623 3.417 5560 59.44 28.90

Propane C3H8 231 0.49 1.52 426 9.3 777 2.19.4 (5)b 91.25 46.46

Methane CH4 111.5 0.55 510 905 515 0 36.0 50.23

Methanol CH3OH 337.6 0.79 1,097 743 5.536 5 21.10

Diesel 180370 0.84 0.66.5 4055 41.86

a: at 293 K; b: estimated value

Synthesized DME contains neither sulfur nor ni-

trogen. A toxicity study of DME has confirmed that

its toxicity is very low, similar to LP gas, far much

lower than Methanol. DME does not corrode any

metals. Some artificial rubbers swell in liquid DME,

but, for example, NBR (Nitric rubber) is durable and

can be used in liquid and gas conditions. Since DME

is decomposed in a troposphere in dozens of hours, it

does not cause ozone layer depletion [11].

DMEs calorific value of 28.90 106 J/kg is 1.37

times higher than that of methanol. Its calorific value

of 59.44 106 J/Nm3 as a gas is 1.65 times higher

than that of methane. Although DME has about 65%

of propanes calorific value, a same size of DME tank

can store or carry 85% energy of propane because

DMEs liquid density is 1.37 times heavier than that

of propane.

DME flame is visible blue like that of natural gas.

A cooking oven for natural gas can be used for DME

with very small modification. Cetane number of DME

is very high so that it can be used as diesel substitute,

and DME combustion exhaust gas is far much cleaner

than that of diesel [1].

3. Direct DME synthesis process

3.1. Direct DME synthesis from hydrogen and

carbon monoxide

Table 2 shows reactions concerning with direct

DME synthesis and their reaction heats. There are

mainly two overall reaction routes that synthesize

DME from synthesis gas (syn-gas: H2+CO gas), re-

action (a) and (b). The reaction (a) synthesizes DME

in three steps, which are methanol synthesis reaction

(c), dehydration reaction (d) and water-gas shift reac-

tion (e). When the shift reaction does not take place,

reaction (c) and (d) are combined to the reaction (b),

which is the other DME synthesis route. Haldor Top-

soe A/S and some other direct DME syntheses follow

reaction (b) [3]. JFE direct DME synthesis follows

reaction (a).

Table 2. Reaction formulas concerning

DME synthesis

Reaction heatReaction

(kJ/mol)

(a) 3CO+3H2 CH3OCH3+CO2 -246

(b) 2CO+4H2 CH3OCH3+H2O -205

(c) 2CO+4H2 2CH3OH -182

(d) 2CH3OH CH3OCH3+H2O -23

(e) CO+H2O CO2+H2 -41

Since both reaction (a) and (b) generate two

molecules of products from six molecules of syn-gas,

the higher reaction pressure gives higher syn-gas con-

version. In consideration of process design, JFE Di-

rect DME Synthesis reaction pressure is around 3 to

7 MPa, and its standard pressure is 5 MPa. Table 3

shows reaction conditions. A catalyst loading ratio,

Journal of Natural Gas Chemistry Vol. 12 No. 4 2003 221

W/F, is defined as a ratio of catalyst weight (kg) to reactant gas flow rate ((kgmol)/h).

Table 3. Reaction conditions of direct DME synthesis

Reaction condition Temperature/ Pressure/MPa Fed syn-gas H2/CO ratio W/F/((kgh)/kg)

Experimental 240280 3.07.0 0.52.0 3.08.0

Standard 260 5.0 1.0 4.0

Methanol synthesis is an equilibrium-restricted

reaction. However when the dehydration reaction

(d) takes place simultaneously, the syn-gas conver-

sion rises dramatically. Figure 1 shows stoichiometric

equilibrium syn-gas conversion of DME synthesis (a)

and (b) under 5 MPa, and methanol synthesis (c) un-

der 5 and 9 MPa.

Figure 1. Stoichiometric equilibrium conversion of

DME and methanol synthesis

Compared DME synthesis reaction (a) with reac-

tion (b), reaction (a) gives much higher syn-gas con-

version in all temperature conditions.

Figure 2 also shows syn-gas conversions of the two

overall DME syntheses, (a) and (b), and the methanol

synthesis (c), as a function of H2/CO ratio of syn-gas.

In each reaction, the equilibrium conversion reaches

its maximum point when the H2/CO ratio is equal to

its stoichiometric number, which are 1.0 for (a) and

2.0 for both (b) and (c). The maximum equilibrium

conversion of (a) is higher than (b) by more than 10

points.

Figure 2. Equilibrium conversion of synthesis gas

vs. H2/CO ratio

Reaction (a) has some other process advantages

compared with reaction (b). In overall reaction (a),

reaction (e) simultaneously converts by-product wa-

ter, therefore the water does not accumulate near the

synthesis catalyst. Water leads the catalyst to degra-

dation. To bring longer catalyst life, its important to

avoid water accumulation over time.

By-product of reaction (a) is CO2. Separation

or distillation of DME from CO2 is much efficient and

not energy consuming compared with separation from

water (in the case of reaction (b)). Because of these

reasons, JFE direct DME synthesis focused the devel-

opment of the catalyst system and its reaction process

on the reaction (a).

In the process of direct DME synthesis from nat-

ural gas, the natural gas will be converted to syn-gas

of H2/CO=1 at first with O2 and by-product CO2 of

the reaction (a) in an auto-thermal reformer (ATR)

by the following overall reaction (f). Then the total

reaction follows overall reaction (g), and eventually

natural gas (methane) is converted DME and water

via reaction (g), just like via natural gas steam re-

forming plus DME synthesis reaction (b).

Table 4. DME synthesis from natural gas in the direct DME synthesis plant

Unit Reaction

(ATR) 2CH4+O2+CO2 3CO+3H2+H2O (f)

(DME synthesis reactor) 3CO+3H2 CH3OCH3 (DME)+CO2 (a)

(DME plant total) 2CH4+O2 CH3OCH3 (DME)+H2O (g)


3.2. A liquid phase slurry reactor

The reaction of DME synthesis is highly exother-

mic, and catalyst is gradually deactivated at high tem-

perature (over 300

). Then it is very crucial to re-

move the reaction heat and to control the reactor tem-

perature even. The direct DME synthesis reactor is

specialized with a liquid phase reactor (DME slurry

reactor) and newly developed catalyst system that re-

alizes the reaction (a) efficiently. Figure 3 shows an

image of the DME slurry reactor.

Figure 3. Concept of slurry reactor for direct DME

synthesis

Catalyst is present as a fine powder slurried in in-

ert high-boiling-point oil. Syn-gas is fed from the bot-

tom of the reactor and forms small and homogeneous

bubbles that react while rising in the catalyst slurry.

Thanks to its homogeneous liquid phase mixing, tem-

perature in the reactor is homogenized so that it is

easily controlled at a high syn-gas conversion (very

exothermic condition). The catalyst was modified to

promote the three reactions (c), (d) and (e) simulta-

neously in the slurry reactor. There are much fewer

restrictions to the catalyst shape and strength in the

slurry reactor than in a fixed bed reactor.

3.3. JFE direct DME synthesis process

Figure 4 shows a schematic process flow diagram

of the direct DME synthesis process from natural gas.

The process consists of mainly three sections, syn-gas

preparation (auto-thermal reformer), DME synthe-

sis (slurry reactor), and separation/purification (CO2,

DME, methanol distillation columns).

Natural gas is converted to syn-gas with O2(steam) and by-product CO2 in an auto-thermal re-

former (ATR). The syn-gas is compressed and fed to

the DME slurry reactor. The euent from the reactor

is DME, by-product CO2, small amount of methanol

and unreacted syn-gas. DME and other by-products

are chilled and separated as a liquid from unreacted

gas. In this separation DME works as a solvent to re-

move CO2 from unreacted syn-gas, which is recycled

to the reactor. DME and other by-products are fed

to the distillation columns. DME and methanol are

purified. CO2 is recycled to the ATR and converted

to syn-gas. Methanol is also recycled to the DME

reactor and finally converted to DME.

Figure 4. Schematic process flow diagram of JFE direct DME synthesis process

4. Process development

4.1. 5TPD project

JFE started the DME synthesis process devel-

opment with a 50 kg/d bench plant in 1994 [8].

5TPD pilot plant project started in 1997 at Kushiro-

city in Hokkaido Japan, collaborating with Taiheiyo

Coal Mining Co., Sumitomo Metal Industry LTD and

CCUJ (Center for Coal Utilization, Japan). This

project was funded by METI.

After finishing the construction of the plant, its

operation started in September 1999 and very suc-

cessfully finished in December 2000. During the one

and half years, six plant operation runs were con-

ducted. Total plant operation time reached 4,300

hours (syn-gas production: about 4,100 h, DME pro-

duction: about 3,000 h). Total amount of produced

DME was about 400 tons.


4.2. Process flow of 5TPD pilot plant

Figure 5 shows the process flow of 5TPD pilot

plant. Picture 1 shows the plant. Since natural

gas was not available at Kushiro-city (the plant site)

during the operation period, LP gas and Coal Bed

Methane were used as feedstock. Basic process flow

of the plant was almost same as those of Figure 4.

The dimensions of the slurry phase reactor were of

0.55 meter-diameter and 15 meter-height.

Figure 5. Process flow of the 5TPD DME pilot plant

Picture 1. The 5TPD DME pilot plant

4.3. Experimental results

(1) Effect of reaction temperature and pressure

The dependence of once-through conversion on

the reaction pressure and temperature is shown in

Figures 6 and 7.

In the Figure 7, by-product CO2 was eliminated

from the selectivity calculation. Figure 6 shows that

the CO conversion increased with pressure, corre-

sponding to the higher equilibrium conversion at the

higher pressure.

The once-through CO conversion reached higher

than 50% at 260

and 5 MPa. The conver-

sion increased kinematically with temperature but has


its maximum because the equilibrium conversion re-

stricts it. Figure 7 shows the DME catalyst did not

synthesize undesirable heavier by-products such as

wax or higher alcohols.

Figure 6. CO conversion vs. reaction conditions

Figure 7. Product selectivity vs. reaction condi-

tions

(2) The catalyst life

Continuous experiment for 1,000 h was conducted

to clarify the change of catalyst activity. Sulfur con-

tent in the syn-gas was controlled under 1.0106

to avoid poisoning the catalyst. The degradation of

the CO conversion was confirmed to be lower than

10 points of the initial value after 1,000 h operation.

The selectivity was almost constant. This pattern of

degradation of the activity was almost same as that

of conventional methanol synthesis catalyst or fixed

bed DME synthesis catalyst [3]. As a result of the life

test, this catalyst system has commercial durability.

Figure 8. The catalyst life test

Reaction conditions: 260 , 5 MPa, W/F=4 (gh)/mol, molar

ratio of H2 to CO is 1.0

(3) Process material balance

Figure 9 shows a typical example of the mate-

rial balance of 5TPD pilot plant. Steam and recycled

CO2 were both fed to the ATR to control the reform-

ing reaction and the composition of syn-gas. H2/CO

ratio of this syn-gas was controlled to 1.04, which was

the best ratio to maximize DME productivity in this

case. DME yield achieved 5.7 tons per day and total

CO conversion reached almost 95% in this material

balance.

Figure 9. Typical material balance of 5TPD pilot plant


(4) The catalyst activity in terms of W/F

Figure 10 shows once-through CO conversion of

the 50 kg/d bench plant and the 5TPD pilot plant,

as a function of W/F. Once-through CO conversion

deeply depends on W/F. Since the catalyst slurry is

solely stirred by gas flow in the reactor, the range of

W/F is depends on the reactor dimensions and cata-

lyst concentration.

Figure 10. Effect of W/F on once-through CO con-

version

Figure 11 shows CO conversion as a function of

recycle ratio. Recycle ratio was a ratio of recycled

unreacted gas flow rate to make-up syn-gas flow rate.

While recycle ratio was changed, W/F was kept at

3.8, then once-through CO conversion was almost con-

stant.

Figure 11. Total CO conversion as a function of re-

cycle ratio(W/F=3.8 (gh)/mol, 260 , 5 MPa)

Total CO conversion reached more than 95%

with the recycle ratio of 1.8. Since by-products of

this DME reaction were CO2 and small amount of

methanol, these by-products were easily removed from

unreacted syn-gas, so that high total CO conversion

was achieved.

(5) Product selectivity and DME quality

Table 5 shows typical product selectivity of 5TPD

pilot plant (CO2 was eliminated from the product se-

lectivity calculation).

Table 5. Selectivity of 5TPD pilot plant

(Product C-mol ratio) DME/(DME+Methanol)=0.91

(Product H-mol ratio) H2O/(DME+Methanol+H2O)=0.013

DME yield was as much as 90%. Very small

amount of H2O was produced, which means that over-

all reaction well followed the DME synthesis reaction

(a). Table 6 shows typical product DME quality of

the plant. This quality was sufficient for IEAs rec-

ommended standards for DME as vehicle fuel, which

states that acceptable concentration of methanol is

lower than 0.01% and water is lower than 0.05%.

Table 6. Product DME quality of 5 ton/day Plant

Product content

DME 99.9%

Methanol + H2O < 0.01%

5. Engineering of the DME slurry reactor

When the DME slurry reactor is scaled-up, both

catalyst activities and hydrodynamics of the cata-

lyst slurry must be well considered. JFE started the

study of high pressure DME slurry reactor with a (9

cm-diameter2 meter-height) reactor and a (4 cm-

diameter4 meter-height) reactor. Effects of reac-

tion conditions and products yields were studied with

these reactors.

The size of the reactor of the 5TPD pilot plant was

0.55 meter-diameter15 meter-height. Axial slurry

mixing, gas-hold-up as well as the temperature con-

trollability were tested with this reactor. The reactor

has 30% to 50% of gas-hold-up, which is very typi-

cal feature of a high-pressure slurry reactor. Liquid

(catalyst+oil) height itself was about 7 m, but slurry

height was about 10 to 14 m under operation condi-

tions. Figure 12 shows stability and homogeneousness


of the reactor inner temperature. In Figure 12, each

circle indicates temperatures of every 15 minutes at

each TI (Thermocouple) position for 8 h.

Since the temperature of the fed syn-gas to the

reactor was around 200220

, temperature of the

lower part of the reactor was slightly lower than 260

. As shown in Figure 12, most of reactor tempera-

ture in the slurry phase was even. The temperature

difference of the reactor higher than 1.8 m was almost

within 2

, which means that the slurry mixing was

very homogeneous. Also the sharp temperature drop

at the 10.1 meter TI position meant the slurry phase

did not form and had clear upper level.

Figure 12. Temperature stability of 5TPD DME re-

actor

DME yield deeply depends on W/F (Figure 9),

but gas residence time (RTg) in the slurry phase must

be considered because necessary time for reactants

phase transfer in the slurry phase is crucial in a large

reactor.

Figure 13 shows an effect of RTg on once-through

CO conversion. RTg was defined as the ratio of actual

fed gas flow rate to the slurry volume.

RTg=[slurry volume]/[actual gas flow rate] (sec)

As shown in Figure 13, when RTg was more than

around 100 s, CO conversion reached almost constant.

This result means that the time for reactants transfer

in the slurry phase should be sufficient. The result

of Figure 13 also means that, as far as the reactor

diameter can be scaled-up, scale-up of the reactor is

scale-up of the reactor diameter because the reactor

does not need extra slurry height for excess RTg.

Figure 13. Effect of gas residence time RTg on CO

conversion

6. Conclusions

JFE direct DME synthesis technology successfully

finished 5TPD pilot plant project with achieving suf-

ficient DME synthesis results and very stable DME

slurry reactor operation. Based on these results, DME

Development Co., Ltd is now conducting a 100TPD

DME demo-plant project in Hokkaido Japan in order

to establish commercial DME production technology.

This project is also supported by METI. The produc-

tion of 100 tons of DME per day will start in 2004.

Acknowledgements

The authors would like to express sincere appre-

ciation for Agency of Natural Resources and Energy,

Coal Division (METI) for their long term financial

support to the DME research Project.

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