Systematic Investigation of the Effects of Operating Conditions on the

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Energy & Fuels 2008, 22, 3587–3593

3587

Systematic Investigation of the Effects of Operating Conditions on the Liquid-Phase Dimethyl Ether (LPDME) Process Gholamreza Moradi,*,† Javad Ahmadpour,† and Fereydon Yaripour‡ Catalysis Research Center, Department of Chemical Engineering, Faculty of Engineering, Razi UniVersity, Kermanshah 67149-67346, Iran, and Catalysis Research Group, Petrochemical Research and Technology Company, NPC, Post Office Box 14965-115, Tehran, Iran ReceiVed June 4, 2008. ReVised Manuscript ReceiVed August 13, 2008

The effects of various reaction conditions including temperature (200-240 °C), pressure (20-50 bar), and H2/CO molar feed ratio (1-2) on the performance of the liquid-phase direct dimethyl ether (DME) synthesis from syngas, over a bifunctional catalyst (CuO/ZnO/Al2O3 + H-ZSM-5) have been studied. The experiments have been designed by general full factorial design (GFFD), and the effects of reaction conditions as well as their interactional effects on the yield of DME as the response variable have been determined. The investigation of the analysis of variation (ANOVA) table showed that all of the three variables and their interaction significantly affected the response. The relative order of importance of the factors on DME yield was found to be temperature > pressure > H2/CO molar feed ratio. In addition, the optimum operating conditions for the maximum yield of DME were 240 °C, 50 bar, and H2/CO ) 2. Under these conditions, the yield of DME reached 51.0%.

1. Introduction The increasing demand of dimethyl ether (DME) explains the renewed interest in studying processes for its synthesis, which references as the syngas-to-dimethyl ether (STD) process originally proposed by Haldor-Topsoe for obtaining DME from syngas on bifunctional catalysts.1,2 Nowadays, because of both environmental protection and the increased price of crude oil, DME is considered as an important clean fuel for the 21st century, because it has low NOx emission and near-zero smoke amount, can be applied in diesel trucks, for power generation, and in fuel cells, and can replace liquefied petroleum gas (LPG) as cooking gas. Also, to replace chlorofluorocarbons (CFCs), which destroy the ozone layer of the atmosphere, DME has been used as an aerosol propellant. Furthermore, DME is a useful chemical intermediate for the production of many important chemicals, such as dimethyl sulfate, methyl acetate, and light olefins.3-6 There are two ways for the preparation of DME from syngas: (a) in two reaction steps, first, synthesis of methanol on a metallic catalyst and, subsequently, dehydration of methanol to DME on an acidic catalyst and (b) in a one step, over a metal-acid bifunctional catalyst. The latter approach is thermodynamically more favorable, also being an opportunity for * To whom correspondence should be addressed. Telephone: +989123895988. Fax: +988314274542. E-mail: [email protected]. † Razi University. ‡ Petrochemical Research and Technology Company. (1) Topp-Jorgensen, J. U.S. Patent 4,536,485, Haldor Topsoe A/S, Denmark, 1985. (2) Hansen, J. B.; Joensen, F. H.; Topsoe, H. F. A. U.S. Patent 5,189,203, Haldor Topsoe A/S, Denmark, 1993. (3) Xu, M.; Lunsford, J. H.; Goodman, D. W. Appl. Catal., A 1997, 149, 289–301. (4) Yaripour, F.; Baghaei, F.; Schmidt, I.; Perregaard, J. Catal. Commun. 2005, 6, 147–152. (5) Jia, M.; Li, W.; Xu, H.; Hou, S.; Ge, Q. Appl. Catal., A 2002, 233, 7–12. (6) Cai, G.; Liu, Z.; Shi, R.; He, C.; Yang, L.; Sun, C.; Chang, Y. Appl. Catal., A 1995, 125, 29–38.

development of a new process.3,7 The key steps in the STD process are supposed to be methanol synthesis, methanol dehydration, and the water-gas shift (WGS) reactions.6,8,9 Methanol synthesis (CO hydrogenation): CO + 2H2 T CH3OH ∆H 0 ) -90.6 kJ/mol

(1)

Methanol dehydration: 2CH3OH T CH3OCH3 + H2O ∆H 0 ) -23.4 kJ/mol (2) Water-gas shift: CO + H2O T CO2 + H2 ∆H 0 ) -41.2 kJ/mol

(3)

The combination of reactions 1-3 gives the overall reaction. Overall reaction: 3CO + 3H2 T CH3OCH3 + CO2 ∆H 0 ) -245.8 kJ/mol

(4)

Reactions 1 and 3 are catalyzed by a standard methanol synthesis catalyst, and reaction 2 is catalyzed by a dehydration catalyst. Because methanol formed by reaction 1 is consumed for the formation of DME and water in reaction 2. The water generated in reaction 2 is shifted by the WGS reaction 3, forming carbon dioxide and hydrogen; the latter is a reactant for the methanol synthesis in reaction 1. Thus, one of the products of each step is a reactant for another. This creates a strong driving force for the overall reaction, achieving a higher DME yield in a single pass.9,10 One-step DME synthesis is a strong exothermic reaction process because it consists of methanol formation, methanol dehydration, and water-gas shift reactions, which are all highly exothermic. The traditional fixed bed reactor is not appropriate (7) Adachia, Y.; Komotob, M.; Watanabec, I.; Ohnoc, Y. Fujimotoa K. Fuel 2000, 79, 229–234. (8) Takeguchi, T.; Yanagisawa, K.; Inui, T.; Inoue, M. Appl. Catal., A 2000, 192, 201–209. (9) Mao, D.; Yang, W.; Xia, J.; Zhang, B. J. Catal. 2005, 230, 140– 149. (10) Li, J. L.; Zhang, X. G.; Inui, T. Appl. Catal., A 1996, 147, 23–33.

10.1021/ef8004338 CCC: $40.75  2008 American Chemical Society Published on Web 11/04/2008

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for this process because of its limited heat-removal capacity. The slurry phase reactor has many merits compared to a fixed bed reactor. The existence of a liquid medium makes it easier to remove reaction heat, to achieve almost isothermal conditions.11 The effects of operating conditions, such as temperature, pressure, feed composition, and weight ratio of the methanol catalyst component to the dehydration catalyst on the direct synthesis of DME, were studied by some researchers.8,12-15 The main experimental approach for investigation the effects of operational conditions and obtaining the optimal conditions is the classical method of varying one parameter at a time while keeping the other factors constant, thus measuring the influence of each parameter separately. The major disadvantage of the one factor at a time strategy is that it fails to consider any possible interaction between the factors and, thus, might miss the real optimum. However, in this work, the effects of operational conditions on the performance of the liquid-phase dimethyl ether (LPDME) process have been investigated in a systematic manner by an experimental design method. Such a study about the LPDME process, however, has not been found in the literature, except in our previous works. In our previous study,16 which was based on the Taguchi method with a L9 orthogonal array design, a large number of STD process conditions have been considered with as few experiments (data) as possible, omitting interaction and error estimation, for determination the factors that were critical for the STD process. In another work, the effects of two main reaction variables (temperature and H2/CO molar feed ratio) on stability and durability of a bifunctional catalyst, made up of CuO/ZnO/Al2O3 and H-ZSM-5 zeolite, were studied by a factorial experimental design method. This bifunctional catalyst (CuO/ZnO/Al2O3 + HZSM-5), which was prepared by physical mixing of two commercial catalysts, has shown very good stability.17 Therefore, we decided to investigate the effect of different operating conditions on the activity and selectivity more completely. Because the specific changes of pressure were not included in our previous studies and its statistical significance was not evaluated, also, some interactions between factors were left out. In this work, the main purpose is to perform statistically designed experiments with a larger array, using a three-factor (H2/ CO molar ratio of feed, temperature, and pressure) three-level general full factorial design of experiment (DOE) to investigate simultaneously effects of individual and interactional parameters on the performance of our bifunctional catalyst and to find the optimum operating conditions of the LPDME synthesis process from synthesis gas. To the best of the knowledge of the authors of this work, this is the first complete report on possible interactions or main effects between the specified factors. 2. Experimental Section 2.1. Catalyst Preparation. The bifunctional catalyst (BFC) was prepared by admixing of the two catalysts, commercial methanol (11) Tan, Y.; Xie, H.; Cui, H.; Han, Y.; Zhong, B. Catal. Today 2005, 104, 25–29. (12) Lewnard, J. J.; Hsfung, T. H.; Whitel, J. F.; Brown, D. M. Chem. Eng. Sci. 1990, 45, 2735–2741. (13) Lee, S.; Gogate, M. R.; Kulik, C. J. Chem. Eng. Sci. 1992, 47 (13/ 14), 3769–3776. (14) Brown, D. M.; Bhatt, B. L.; Hsiung, T. H.; Lewnard, J. J.; Waller, F. J. Catal. Today 1991, 8, 279–304. (15) Erena, J.; Garona, R.; Arandes, J. M. Catal. Today 2005, 107/108, 467–473. (16) Moradi, G. R.; Ghanei, R.; Yaripour, F. Int. J. Chem. React. Eng. 2007, 5, Article A14. (17) Moradi, G. R.; Nazari, M.; Yaripour, F. Chem. Eng. J. 2008, 140, 255–263.

Moradi et al. synthesis catalyst (manufactured by KMT Co.) and methanol dehydration catalyst (supplied by Su¨d-Chemie Co., sample 304 H/06), namely, H-ZSM-5. Two commercial catalysts were finely milled and sieved to sizes less than 90 µm and well-mixed at a mass ratio of 3:1.16 Then, several times, the mixture was molded under pressure into tablets and then crushed for obtaining a homogeneous powder, which were then sieved to 90-120 mesh size particles to avoid mass-transfer limitation. 2.2. Characterization of Catalyst. The Brunauer-Emmett-Teller (BET) surface area, pore volume, and pore diameter of catalysts were measured by a N2 adsorption-desorption isotherm at liquid nitrogen temperature (77 K) using NOVA 2000 Series instrument (Quantachrome, Boynton Beach, FL). Temperature-programmed reduction (TPR) of the catalysts were carried out in a stream of 5.1 vol % H2 balanced with Ar at a flow rate of 50 mL/min using a Pulse Chemisorb 2705 instrument (micromeritics). Chemical composition of the catalysts was determined by X-ray fluorescence (XRF) techniques. PW-1800 Philips X-ray fluorescence has been used for elemental analyzing. A PW-1800 Philips X-ray diffractometer with monochromatized Cu KR radiation (λ ) 1.5406 Å) was used for X-ray measurement. The crystal sizes of CuO of the catalysts were evaluated from the full width at half-maximum of the CuO (111) X-ray diffraction (XRD) peaks by using the Scherrer equation with a correction for the instrumental broadening. Acidity measurement was performed by temperature-programmed desorption of ammonia (NH3-TPD) with a conventional flow apparatus, which included an online thermal conductivity detector (TCD). In a typical analysis, NH3-TPD was performed using 0.35 g of the catalyst, which was degassed at 600 °C in a helium flow, cooled to 150 °C, and then saturated with NH3 for 15 min. After saturation, the sample was purged with He for 30 min to remove weakly adsorbed NH3 on the surface of the catalyst. During this time, a constant TCD level was attained. The temperature of the sample was then raised at a heating rate of 5 °C/min from 150 to 700 °C, and the amount of ammonia in the effluent was measured via a TCD and recorded as a function of the temperature. The specific surface area of metallic copper was measured by the decomposition N2O on the metallic copper surface as follows:

2Cu + N2O w N2 + (Cu-O-Cu)S

(5)

The pulse titration technique was employed in the test. Ar was used as the carrier gas, and the amount of the consumed N2O was determined by a TCD. The specific surface area of the metallic copper was calculated assuming a reaction stoichiometry of two Cu atoms per oxygen atoms with a Cu surface density of 1.46 × 1019 Cu atoms/m2.18 2.3. Experimental Setup and Catalytic Tests. Figure 1 shows a schematic view of the laboratory scale setup.17 In brief, the reactants CO and H2 and nitrogen as the internal standard were fed through a set of mass flow controllers and blended in a mixer. The mixture was preheated to the reaction temperature before entering into the reactor. The STD reaction was carried out in a 1 L slurry phase autoclave reactor, in which 10.5 g of bifunctional catalyst was suspended in 350 g of pure liquid paraffin (C16H34). At a 3 wt % slurry (corresponding to 10.5 g of catalyst per 350 g of solvent) in a mechanically agitated slurry reactor, the gas-solid mass transfer was not limit for the overall rate. The impeller speed in the autoclave reactor was set to 1600 rpm. In a preliminary experiment, it was checked for the intraparticle mass-transfer limitation: above 1500 rpm of impeller speed, no mass-transfer resistances were found. To ensure that gas-liquid mass-transfer limitations were absent, the experiment was carried out at 1600 rpm. Space velocity of syngas was set at 1100 mLn (g of catalyst)-1 h-1. The selection of 1100 mLn (g of catalyst)-1 h-1 for space velocity is based on the experimental results from independent tests, under which the reaction system is far from the thermodynamic equilibrium region. Before each activity test, the bifunctional (18) Chinchen, G. C.; Hay, C. M.; Vandervell, H. D.; Waugh, K. C. J. Catal. 1987, 103, 79.

Operating Conditions on the LPDME Process

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Figure 1. Schematic view of the experimental setup. Table 1. Factors and Levels Used in the GFFD factor

type

levels

values

H2/CO T (°C) P (bar)

fixed fixed fixed

3 3 3

1, 1.5, 2 200, 220, 240 20, 35, 50

Table 2. Specifications of Designed Experiments by GFFD factors and levels run number

H2/CO ratio

T (°C)

P (bar)

yield of DMEa (mol %)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

1 1 1 1 1 1 1 1 1 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 2 2 2 2 2 2 2 2 2

200 200 200 220 220 220 240 240 240 200 200 200 220 220 220 240 240 240 200 200 200 220 220 220 240 240 240

20 35 50 20 35 50 20 35 50 20 35 50 20 35 50 20 35 50 20 35 50 20 35 50 20 35 50

4.25 7.86 7.81 16.85 22.33 30.50 22.57 36.91 39.48 4.85 7.38 8.14 17.90 18.20 35.30 30.73 47.14 49.49 7.94 14.35 16.95 21.75 30.31 39.49 36.93 50.71 51.04

a

Average of two replications.

catalyst was reduced with pure hydrogen at the normal pressure according the following heating program: heated from room temperature to 250 °C with a heating rate of 1 °C/min and was kept for 6 h at this temperature. Then, the catalysts were cooled to room temperature in the presence of hydrogen flow. After this pretreatment, the feed (H2/CO/N2) was introduced into the slurry reactor. The outlet stream of the reactor passed through the back-

pressure regulator, where its pressure was reduced to atmospheric pressure, and then a small portion of the reactor effluent was sent to the gas chromatograph (GC) for online analysis. A Varian CP3800 gas chromatograph was equipped with two packed columns: HaySep Q (80-100 mesh, 2 m × 1/8 in. × 2.0 mm, SS) and Chrompach Molecular Sieve 13X (80-100 mesh, 2 m × 1/8 in. × 2.0 mm, SS) for separating CO2, H2, N2, CO, MeOH, DME, CH4, and C3H8 and then detecting by a thermal conductivity detector (TCD). For each experiment, the carbon balance over the reactor was calculated. The average absolute deviation was less than 3%. 2.4. Design of the Experimental Method. The technique of statistical design for experiments can be used for process characterization, optimization, and modeling. It has been widely accepted in the manufacturing industry for improving product performance and reliability, process capability, and yield. In the statistical design experiments, the factors involved in an experiment at their respective levels were simultaneously varied. Thus, a lot of information can be taken with a minimum number of experimental trials. The experiments in which the effects of more than one factor on response are investigated are known as full factorial experiments. The most important advantages are that not only the effects of individual parameters but also their relative importance in a given process are obtained and that the interactional effects of two or more variables can also be known. This is not possible in a classical experiment.19 To determine the effects of the reaction parameters and their interactions on the LPDME process over a bifunctional catalyst (Cu-ZnO-Al2O3/H-ZSM-5) and to find the optimum operating conditions of this process, a general full factorial design (GFFD) with three factors in three levels for each factor has been used. To determine the statistical significance of the effects, duplicate determinations were made for each of these experiments to evaluate experimental error. Table 1 summarizes these factors (temperature, pressure, and H2/CO molar feed ratio) and their respective levels. The levels of the factors were chosen based on information from the literature and our previous results. Table 2 shows the experimental matrix and the results obtained for the analyzed response. The response was the yield of DME and was defined and calculated as follows:

YDME )

2Fout yDME,out × 100 Fin yCO,in

(6)

(19) Montgomery, D. C. Design and Analysis of Experiments; John Wiley and Sons: New York, 1996.

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Table 3. Characterization Results of KMT, H-ZSM-5, and BFC catalyst

surface area (m2/g)

total pore volume (mL/g)

average pore radius (Å)

dispersion (%)

Cu area (m2/g)

crystal size (nm)

H2 consumptiom (cm3 g-1)

KMT H-ZSM-5 BFC

91 391 133

0.255 0.389 0.192

55.9 19.9 28.7

8.4

28.5

29.0

34.67

11.6

39.1

21.1

47.53

where Fin and Fout are the molar flow rates of feed and product (mol/h), respectively, and yi is the mole fraction of component i in the feed stream, yi,in, or in the product stream, yi,out. In the tables corresponding to experimental results, the data points are average ones that have been measured during steady-state conditions. Analysis of variance (ANOVA) based on the linear statistical model was used to analyze the results. Various statistical data (standard error of estimate, sum of squares of the errors, F statistics, and p value) were examined.

3. Results and Discussion 3.1. Characterization of the Catalyst. The details of characterization results of the methanol synthesis catalyst, dehydration catalyst, and bifunctional catalyst have been presented in our previous works.16,17 In this section, a summary of the previous characterization results and new obtained characterization results has been given for a better understanding of the relation of characteristics of the bifunctional catalyst and its activity. Table 3 shows some characterization results. The prepared bifunctional catalyst has a specific surface area and pore volumes that are higher than the methanol synthesis catalyst, equal to 133 m2/g and 0.192 mL/g, with the average pore diameter equal to 28.7 Å, which is the main reason of a higher dispersion of Cu atoms in the bifunctional catalyst (11.6%) in comparison to the methanol synthesis catalyst (8.4%). The XRD patterns of the methanol synthesis catalyst, dehydration catalyst, and bifunctional catalyst showed that the major peaks of the methanol synthesis catalyst pertain to CuO [Joint Committee on Powder Diffraction Standards (JCPDS) 041-0254] and major peaks of the dehydration catalyst are related to an alumina-silica compound (JCPDS 42-0305). Also, from XRD results and the Scherrer equation, the sizes of CuO in the methanol synthesis catalyst and bifunctional catalyst have been estimated as shown in Table 3, because the higher dispersion of the size of the CuO crystal decreased from 29 nm in KMT to 21 nm in the bifunctional catalyst. Therefore, it can be seen that physical mixing of two catalysts provided better dispersion of the Cu catalyst and resulted in a higher surface area for Cu in the bifunctional catalyst in comparison to the methanol synthesis catalyst (Table 3). Both of the H2-TPR patterns of methanol synthesis (KMT) and bifunctional catalyst (BFC) showed only one reduction peak with a maxima about 200 °C, which is known as the reduction of CuO. The amounts of hydrogen uptake during TPR experiments have been shown in the last column of Table 3. It shows that BFC consumes more

Figure 2. NH3-TPD profile of the solid-acid catalyst (H-ZSM5).

hydrogen for reduction than the methanol synthesis catalyst, which can be contributed to a higher dispersion of CuO in the bifunctional catalyst. From the above results, it can be concluded that physical mixing of KMT and H-ZSM-5 provided a higher dispersion for Cu, which in turn increased the activity of the methanol synthesis catalyst. XRF results showed that the molar ratios of Cu/Zn in the methanol synthesis catalyst and Si/Al in the dehydration catalyst are 2 and 54, respectively. A high molar ratio of Si/Al in H-ZSM-5 provided suitable acid strength for dehydration function. NH3-TPD was performed to monitor the acid strength and the amount of acid sites on H-ZSM-5. As shown in Figure 2, two major NH3 desorption peaks as well as a shoulder peak can be observed at 222 °C (weak acid sites) and 450 °C (strong acid sites) and 685 °C, respectively. The amount of NH3 desorption based on the area under the first peak is 9.39 × 10-2 cm3 (NH3)/g of catalyst, from the second peak is 5.21 × 10-2 cm3 (NH3)/g of catalyst, and for the shoulder peak is 8.76 × 10-3 cm3 (NH3)/g of catalyst. Thus, the total acid site density is 0.1 cm3 (NH3)/g of catalyst. The too strong acid sites promote the formation of the larger amount CO2 and hydrocarbons, resulting in lower selectivity for DME.16 3.2. Catalytic Activity and Stability. The stability of the bifunctional catalyst and methanol synthesis catalyst were evaluated under the same reaction conditions over a 60 h period, during which the reactor was operated continuously under the test conditions. The changes in the yield of DME and the yield of methanol as a function of the reaction time are depicted in Figure 3. It clearly shows that, after the steady state was established (after about 15 h), both the yield of DME and the yield of methanol have remained essentially constant with the reaction time on stream, which indicates that no noticeable deactivations of the catalysts were occurring. Furthermore, comparing the yield of DME (YDME) from the LPDME process to the yield from methanol (only) synthesis with time on stream (Figure 3) demonstrates that the overall methanol yield can be increased in principle by combining the methanol synthesis with the methanol dehydration reaction. Synergy in total methanol equivalent production is obtained by effective removal of the products from the methanol synthesis reaction. A very similar phenomenon was also reported previously by other researchers.20

Figure 3. Long-term test of activity and yield of the bifunctional and methanol synthesis catalysts. P ) 5 MPa, T ) 240 °C, H2/CO ) 2, and SV ) 1100 mLn (g of catalyst)-1 h-1.

Operating Conditions on the LPDME Process

Energy & Fuels, Vol. 22, No. 6, 2008 3591

Table 4. ANOVA for Factors and Their Interactions from GFFD source

DF

seq SS

adj SS

adj MS

F

FR)0.05

p

percentage contribution (%)

H2/CO T P H2/CO × T H2/CO × P T×P H2/CO × T × P error total

2 2 2 4 4 4 8 27 53

742.01 9070.07 1484.41 175.55 27.49 444.58 68.18 0.74 12013.04

742.01 9070.07 1484.41 175.55 27.49 444.58 68.18 0.74

371.00 4535.04 742.20 43.89 6.87 111.15 8.52 0.03

13455.90 164480.20 26918.83 1591.75 249.29 4031.10 309.09

3.35 3.35 3.35 2.73 2.73 2.73 2.31

0.000 0.000 0.000 0.000 0.000 0.000 0.000

6.18 75.50 12.36 1.46 0.23 3.70 0.57 0.00 100

Table 5. Least-Squares Means for Yield of DME H2/CO

mean

T

mean

P

mean

1.0 1.5 2.0

20.951 ( 0.0391 24.348 ( 0.0391 29.942 ( 0.0391

200 220 240

8.837 ( 0.0391 25.848 ( 0.0391 40.555 ( 0.0391

20 35 50

18.198 ( 0.0391 26.132 ( 0.0391 30.911 ( 0.0391

H2/CO × T

mean

H2/CO × P

mean

T×P

mean

1.0 × 200 1.0 × 220 1.0 × 240 1.5 × 200 1.5 × 220 1.5 × 240 2.0 × 200 2.0 × 220 2.0 × 240

6.640 ( 0.0678 23.226 ( 0.0678 33.988 ( 0.0678 6.791 ( 0.0678 23.800 ( 0.0678 42.451 ( 0.0678 13.081 ( 0.0678 30.518 ( 0.0678 46.227 ( 0.0678

1.0 × 20 1.0 × 35 1.0 × 50 1.5 × 20 1.5 × 35 1.5 × 50 2.0 × 20 2.0 × 35 2.0 × 50

14.559 ( 0.0678 22.365 ( 0.0678 25.930 ( 0.0678 17.828 ( 0.0678 24.237 ( 0.0678 30.978 ( 0.0678 22.208 ( 0.0678 31.792 ( 0.0678 35.826 ( 0.0678

200 × 20 200 × 35 200 × 50 220 × 20 220 × 35 220 × 50 240 × 20 240 × 35 240 × 50

5.684 ( 0.0678 9.862 ( 0.0678 10.966 ( 0.0678 18.835 ( 0.0678 23.613 ( 0.0678 35.096 ( 0.0678 30.076 ( 0.0678 44.919 ( 0.0678 46.671 ( 0.0678

H2/CO × T × P

mean

H2/CO × T × P

mean

H2/CO × T × P

mean

1.0 × 200 × 20 1.0 × 200 × 35 1.0 × 200 × 50 1.0 × 220 × 20 1.0 × 220 × 35 1.0 × 220 × 50 1.0 × 240 × 20 1.0 × 240 × 35 1.0 × 240 × 50

4.255 ( 0.1174 7.859 ( 0.1174 7.805 ( 0.1174 16.849 ( 0.1174 22.330 ( 0.1174 30.500 ( 0.1174 22.573 ( 0.1174 36.908 ( 0.1174 39.483 ( 0.1174

1.5 × 200 × 20 1.5 × 200 × 35 1.5 × 200 × 50 1.5 × 220 × 20 1.5 × 220 × 35 1.5 × 220 × 50 1.5 × 240 × 20 1.5 × 240 × 35 1.5 × 240 × 50

4.855 ( 0.1174 7.377 ( 0.1174 8.143 ( 0.1174 17.903 ( 0.1174 18.198 ( 0.1174 35.300 ( 0.1174 30.725 ( 0.1174 47.138 ( 0.1174 49.490 ( 0.1174

2.0 × 200 × 20 2.0 × 200 × 35 2.0 × 200 × 50 2.0 × 220 × 20 2.0 × 220 × 35 2.0 × 220 × 50 2.0 × 240 × 20 2.0 × 240 × 35 2.0 × 240 × 50

7.942 ( 0.1174 14.352 ( 0.1174 16.951 ( 0.1174 21.754 ( 0.1174 30.311 ( 0.1174 39.489 ( 0.1174 36.929 ( 0.1174 50.712 ( 0.1174 51.039 ( 0.1174

In summary, this plot proved that the bifunctional catalyst with HZSM-5 as a dehydration component exhibits very high activity and stability for the direct synthesis of DME from syngas. 3.3. Analysis of Experimental Data. After gathering the experimental data based on the general factorial design method (Table 2), the general linear model for three-factor fixed effects was used to conduct an ANOVA. In this model, the response (yield of DME) may be described by the linear statistical model Yijkl ) µ + τi + βj + γk + (τβ)ij + (τγ)ik + (βγ)jk + (τβγ)ijk + εijkl where µ is the overall mean effect, τi is the effect of the ith level (i ) 1, 2, or 3) of the temperature factor, βj is the jth level (j ) 1, 2, or 3) effect of the pressure factor, γk is the

Figure 4. Main effect plot (data means) for the yield of DME.

effect of the kth level (k ) 1, 2, or 3) of the H2/CO factor, (τβ)ij is the effect of the interaction between τi and βj, (τγ)ik is the effect of the interaction between τi and γk, (βγ)jk is the effect of the interaction between βj and γk, (τβγ)ijk is the effect of the interaction between τi, βj, and γk, and εijkl is the error component for i, j, and k levels for the lth replicate (l ) 1 or 2). Statistical ANOVA was performed to see whether or not the process parameters are statistically significant. In the ANOVA table, the quantities, such as degrees of freedom (DF), sequential sums of squares (seq SS), adjusted sums of squares (adj SS), adjusted mean of squares (adj MS), F value, p value (p) and relative percentage contribution among the factors, have been computed. If the calculated F value is higher than the FR,V1,V2 value of the confidence table, where R is risk and V1 and V2 are degrees of freedom associated with the numerator and denominator, an effect is considered statistically relevant. Usually, the significant level (R) is set to 0.05.19,21,22 The results of ANOVA analysis for the yield of DME are shown in Table 4. As can be seen, the F value of all factors and interactions is considerably greater than the extracted F value of tables with 95% confidence. This means that the variance of each factor and their interactions are significant compared to the variance of error and that all of them have an important effect on the response. Another important statistic parameter in the ANOVA table is the p value (20) Ng, K. L.; Chadwick, D.; Toseland, B. A. Chem. Eng. Sci. 1999, 54, 3587. (21) Dieter, G. E. Engineering Design: A Material and Processing Approach; McGraw-Hill, Inc.: New York, 1991. (22) Phadke, M. S. Quality Engineering Using Robust Design; PrecticeHall: Englewood Cliffs, NJ, 1989.

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Moradi et al.

Figure 5. Interaction plot (data means) for the yield of DME.

(p). In Table 4, the p value for each term has been computed. If the p value is less than or equal to the R level, then the effect for the term is significant. All factors and their interactions have p values less than 0.05 (Table 4), which means that the effects of these factors as well as two- and three-factor interaction effects on YDME as the response parameters are significant. In the other words, the mean YDME values are different for different temperature, pressure, and H2/CO levels (Table 5). Meanwhile, the relative contribution percentage of each factor and interactions on YDME are mentioned in the last column of Table 4, which demonstrates that the temperature of the process, with relative percent of contribution equal to 76% of the total effect, is the most significant factor on the yield of DME as compared to pressure, H2/CO molar feed ratio, and interaction terms. Because, in this region, the reaction rates are almost kinetically controlled, the yield of DME increases with the temperature evaluation. Pressure is the next important significant factor in the yield of DME. As seen in Table 2, when the pressure is increased in the range from 20 to 50 bar, the yield of DME increases, which is the logical consequence, whereby methanol synthesis is the limiting step of the overall reaction. Also, methanol synthesis is a mole-number-reducing reaction; therefore, the increase of the reaction pressure accelerates the reaction of methanol especially and the whole reaction as well. The H2/ CO molar ratio showed the least impact among the factors studied with the assigned variance of values. Under specified temperature (240 °C) and pressure (50 bar), with the increase of the H2/CO molar feed ratio from 1 to 1.5, the yield of DME increases from 39.48 to 49.49 mol % but further dilution of reactant feed with H2 (H2/CO ) 2) has not significant effect on the yield of DME (YDME ) 51.04 mol %). The synergy shows that, with an increase of the H2/CO ratio from 1 to 1.5, the methanol synthesis reaction is accelerated, which leads to a higher DME yield; however, a further increase in H2/CO to 2 results in the WGS reaction taking place to shift the equilibrium composition to the CO + H2O side, and then the former water suppresses the DME synthesis. Thus, the effect of pressure on the response is more significant than that of feed composition. Moreover, the very small relative percentage contribution of interaction terms indicates that the effect of these terms on the response variable (YDME) is not as large as the effect of the temperature, pressure, and H2/CO molar ratio of feed. The main effect and an interaction plot confirm these results. These plots are shown in Figures 4 and 5, respectively. The term “main effect” is the average (mean) of all of the responses produced by changing the level of a factor, and it is used to determine which factors influence the response and to compare the relative

strength of the effects. The strength of the interaction is calculated in the same way for levels of combined factors defined by the product of the pseudo-levels. An interaction plot, in other words, is to determine if two factors interact with their effect on the response. Each data point in Figure 4 represents the mean of the response variable (YDME) for each factor level. Also, the dotted red line represents a reference line at the overall (grand) mean, which is 25.08% YDME. This plot indicates that the temperature, pressure, and H2/CO molar ratio of feed have a positive effect on the yield of DME. A comparison of the slopes of the lines can be used for determination of the relative magnitude of the factor effects. We can compare that the difference between levels in each graph in the mean effect plot (Figure 4) shows which factor is more significant. Hence, the relative order of importance of the factors based on the ∆ of the means of each level of three factors is temperature > pressure > H2/CO molar feed ratio. Beside that, the maximum point in each graph marks the optimum value of the particular factor. Therefore, according to Figure 4, the factor levels indicate the optimum conditions are as follow: temperature ) 240 °C, pressure ) 50 bar, and H2/CO ) 2. This result is consistent with the p value and the relative percentage contribution of temperature and H2/CO molar ratio of feed in the ANOVA table. Understanding the interaction between two factors gives a better insight into the overall process analysis. Any individual factor may interact with any or all of the other factors creating the possibility of the presence of a large number of interactions. As we know, an interaction is present when the change in the response mean from the low to high level of a factor depends upon the level of a second factor. It is clear from Figure 5 that, except T × P (in the lower middle) and H2/CO × T in the 220-240 °C (in the upper middle), the other two-factor interactions are not noticeable on the yield of DME, because the lines on the plots are approximately parallel to each other for different levels of two factors, which indicates a lack of interaction between these factors. Furthermore, no significant three-factor interaction (H2/CO × T × P) is identified (Table 4). From the standpoint of statistical analysis, because the temperature and pressure together have more than 88% of the total effect and show the highest interaction (3.7%), the major change of the DME yield can be obtained by simply adjusting these two factors when a certain value of H2/CO molar ratio has been provided in the reaction medium. 4. Conclusion A general factorial design was used to investigate simultaneous effects of individual and interactional parameters on the

Operating Conditions on the LPDME Process

performance of our bifunctional catalyst. The following conclusions were driven from the analysis of the results obtained with the factorial design: (1) This study indicated that all three variables (temperature, pressure, and H2/CO molar feed ratio) significantly affected the yield of DME as response. The relative order of importance of these factors based on the ANOVA table and ∆ of the means of each level for three factors is temperature > pressure > H2/CO molar feed ratio. (2) From the standpoint of statistical analysis, because the temperature and pressure together had more than 88% of the total, effects as well as the interactions among these two factors had the highest percentage (3.7%) on the response; therefore, the major change for the DME yield can be obtained by simply adjusting these two factors when a certain value of the H2/CO molar ratio was provided in the reaction medium. (3) Finally, according to the experiment data analysis results, the factor levels that indicated the optimum operational conditions for having a maximum yield of DME are 240 °C reaction temperature, 50 bar pressure, and H2/CO molar feed ratio equal to 2 in the studied ranges of operational conditions. Under these conditions, the yield of DME reached 51.0%.

Energy & Fuels, Vol. 22, No. 6, 2008 3593 Acknowledgment. The authors acknowledge NPC (R&T) for their financial support of this study. The authors express their sincere gratitude to those who contributed to this research.

Nomenclature T ) reaction temperature P ) reaction pressure H2/CO ) hydrogen/carbon monoxide molar ratio Fin ) molar flow rates of feed (mol/h) Fout ) molar flow rates of product (mol/h) yi,in ) mole fraction of component i in the feed stream yi,out ) mole fraction of component i in the product stream YDME ) yield of DME SV ) space velocity T × P ) interaction effect between T and P on response (DME yield) H2/CO × T ) interaction effect between H2/CO and T on response H2/CO × P ) interaction effect between H2/CO and P on response H2/CO × T × P ) interaction effect between H2/CO, T, and P on response EF8004338