Response Surface Modeling and Optimization of CO Hydrogenation

Article pubs.acs.org/IECR

Response Surface Modeling and Optimization of CO Hydrogenation for Higher Liquid Hydrocarbon Using Cu−Co−Cr + ZSM-5 Bifunctional Catalyst Pravakar Mohanty,† Sachchit Majhi,† J.N. Sahu,‡ and K. K. Pant*,† †

Department of Chemical Engineering, Indian Institute of Technology, Delhi, India-110016 Department of Chemical Engineering, Faculty of Engineering, University of Malaya, Kuala Lumpur 50603, Malaysia

‡

S Supporting Information *

ABSTRACT: This paper represents an extensive statistical analysis of the combined effects of operating variables (temperature, pressure, reaction time, and H2/CO flow rate) toward CO-hydrogenation for liquid hydrocarbon which was performed in a fixed bed benchtop reactor system, by means of response surface methodology (RSM). The application of RSM in conjunction with a central composite rotatable design (CCRD) was used for modeling and optimizing the performance of a multivariable FTsynthesis process using bifunctional CuO−CoO−Cr2O3 + ZSM-5 catalyst. The CuO−CoO−Cr2O3 catalyst was synthesized by a coprecipitation method, and its physiochemical characterization was done by using Brunauer−Emmett−Teller, temperatureprogrammed reduction, thermogravimetric analysis, X-ray diffraction, and transmission electron mocroscopy techniques. Through this work a 50 full factorial (CCRD) experimental design was employed. Maximum CO conversion was predicted and experimentally validated to determine optimum conditions that allow improvement of the performance of the catalyst for a long run time of 120 h. The optimum values of CO conversion, temperature, pressure, and (H2/CO) molar ratio were found to be 64.3%, 310 ± 4 °C, 33−36 bar, and 1.0, respectively.

■

INTRODUCTION Rapid consumption of petroleum resources has aroused critical challenges that may lead to an acute energy crisis. To mitigate the burden, the usage of both natural gas and coal is considered an effective method to reduce substantial dependency on crude oil consumption. There are many issues related to global warming because of the increasing use of conventional fossil fuels.1 Biomass derived syngas (mixture of H2, CO, and CO2) is a virtuous source to solve these acute problems in the future. Gasification of biomass or a mixture of biomass with coal can be well synchronized as an alternative source for ensuring the continuous demand of fuel and energy markets.2 Production of syngas utilized through Fischer−Tropsch (FT) synthesis can offer the possibility of converting syngas to clean liquid hydrocarbons free from sulfur. Several metals such as Ni, Co, Ru, and Fe have good potential for FT synthesis. Currently more than 90% of the H2 production is covered by fossil fuels, like naphtha and thermo chemical reforming of CH4 (approx. ∼50%).1−3 However, due to uncertainty in the supply of natural gas and its global price fluctuations, the systems based on biomass gasification may be a better option as an alternative. Gasification is going to attract many industries as well as energy utilization sectors as a core technology to produce syngas, where production of H2 can be controlled by the water−gas shift reaction (WGS).4 FTS hydrocarbons contain high n-paraffin content, high cetane, sulfur-free, aroma-free, and nitrogen-free diesel range hydrocarbons, especially on cobalt-based catalysts. Zeolite is a special material with its unique pores and channels, which delivers optimum results at a reaction temperature of 240 ± 10 °C, where cobalt, copper, and chromium mixtures present in the catalyst shift the product distribution toward the formation © 2012 American Chemical Society

of high cetane diesel range isoparaffins, alcohols, and aromatics through the transformation of the primary FTS products on acidic sites.4 These kind of mixed catalysts have varying molecular diffusion rates in their pores with shape selectivity, as well as acidic properties, which makes them widely acceptable.2,5 Statistical tools like experimental design methods and response surface methodology (RSM) is applied in this research work for modeling the process parameters using a high-pressure syngas conversion (FT synthesis) process.1,6,7 Many independent variables can be controlled by applying RSM with few experimental runs. The design of experiments through implication of RSM motivates the researcher to evaluate rigorous models with full factorial design to establish the functional relationships between many independent variables for targeted response. The analysis of variance (ANOVA) provides a statistical platform upon which diagnostic tests can be assured to evaluate the suitability of the model and its process parameters.1,8 The catalytic activities are not only dependent on the alkali ions added, but also related to the counteranion of the salt used and the nature of the supports. The acidic supports such as SiO2, Cr2O3, and Al2O3 require in different proportions to be used as a promoter to obtain the desired selectivity of higher hydrocarbons9,10 The objectives of the present study are (i) an assessment of combined effects of different operating variables (temperature, pressure, reaction time, and H2/CO flow rates) on CO conversion (%) and (ii) to define optimum values, arranged for each Received: Revised: Accepted: Published: 4843

December 7, 2011 February 28, 2012 March 13, 2012 March 13, 2012 dx.doi.org/10.1021/ie202866q | Ind. Eng. Chem. Res. 2012, 51, 4843−4853

Industrial & Engineering Chemistry Research

Article

operating variables for maximization of the response variables depending on the percent of CO conversion.

■

Table 1. Physiochemical Properties of Different Catalyst

MATERIALS AND METHODS

Reagents. All synthetic chemicals and analytical reagents used in the catalyst preparation are of high purity (>99.9% pure). Ingredients used for synthesis of catalyst are chromium nitrate (Cr (NO3)3·6H2O), copper nitrate, (Cu(NO3)2·6H2O), cobalt nitrate, (Co(NO3)2·6H2O) and KOH flakes. ZSM-5 (SAR = 35) was procured from Süd-Chemie India (P), Ltd. Preparation of Cu−Co−Cr + ZSM-5 Bifunctional Catalyst. Catalyst (CAT-A) CuO−CoO−Cr2O3 (32:36:32 wt %, Cu/Co/Cr) was prepared by mixing chromium nitrate (Cr(NO3)3·6H2O), copper nitrate, (Cu(NO3)2·6H2O), and cobalt nitrate, (Co(NO3)2·6H2O) in aqueous KOH solution. The details of the catalyst preparation are discussed elsewhere.2 Finely grounded powder of CuO−CoO−Cr2O3 was physically mixed with ZSM-5 powder. Thus the obtained catalyst was a mixture of CAT-A and ZSM-5 in equal ratio, pelletized, and is designated as CAT-B. The performance of the CAT-B was tested in a fixed-bed reactor. The analysis of tail gaseous products and hydrocarbon was carried out with a thermal conductivity detector (TCD) and a flame ionization detector (FID). Periodically liquid products were collected for GC−MS analysis; weight balance and >95% carbon balance were considered from FT data-analysis. Multivariate Experimental Design. Different empirical models can be built using the following successive steps: (a) A previously explained suitable experimental design setup has been used in a fixed-bed benchtop reactor setup (BTRS), and the response variables were evaluated on it. (b) Calculation of the appropriate statistical analysis was carried out to check the fittings of models with respect to severe experimental results as discussed in our previous paper.2 Out of many statistical methods, response surface methodology (RSM) is used to quantify the relationship between the controllable input parameters and obtained response surfaces.11,12 In this work a standard RSM design named as the central composite rotatable design (CCRD) is used to obtain optimum reaction variables for maximizing the CO-conversion (%) during FTS to obtain sulfur-free liquid hydrocarbons in the desired range of hydrocarbons (C5+ to C15+). CCRD has been widely used for fitting a second-order model and to come out with a minimum number of experiments for a complex experimental process, as the CCRD method has wide potential and acceptance.13−15 In the present work, Design Expert Software, version 6.0.6 (StatEase Inc., Silicon Valley, CA, USA) is implemented for the design of experiments and regression. Five numerical variables are used: temperature (X1), pressure (X2), reaction time on stream (X3), CO flow rate (X4) and H2 flow rate (X5) for the evaluation of the bifunctional catalyst meant for the COhydrogenation process. The different ranges of the independent variables selected for CO hydrogenation and the experimental design analysis are pronounced repeatedly through this study as a prior requirement for the desired response.3 The optimization is conducted for a single bifunctional catalyst in high pressure BTRS to find a good response. Hence the total number of experimental tests required for CCRD, with five variables is 50. Table 1 in the Supporting Information indicates the complete design matrix required for actual catalytic conversion of syngas

sample

CAT-A

CAT-B

used CAT-B

specific surface area (m2 g−1) Vpore (cm3 g−1) Dpore (Å) Ddisp (%) Cu size (nm) Cu/Co/Cr loading (wt %)

28.8 0.04 ± 0.01 59 ± 2

136.5 0.22 ± 0.01 65 ± 2 4.4 15−33 16/18/16/50

124.4 0.21 ± 0.01 64 ± 2

21−35 32/36/32

18−40

to the higher liquid hydrocarbons further emphasized through this study. The response model is shown in eq 1 Y = bo′ +

n

n

i=1

i=1

n

n

∑ biXi + ∑ biiXi 2 + ∑ ∑ bijXiXj i=1 j>1

(1)

where Y is denoted for response prediction, bo stands for constant coefficient, bi stands for linear coefficients, bii stands for the interaction coefficients, bij stands for the quadratic coefficients, and X, Xj stands for the coded values to determine the CO conversion (%) during FTS.10 The estimated test calculated for CCRD covers up to 2n factorial. Its origin holds at the center and 2n points are fixed axially at a distance (where n = number of variables) to generate the quadratic terms and to replicate maximum tests at the center. The axial points are chosen in such a way that it can allow readability by ensuring the variance of the model prediction, which becomes constant at all points equidistant from the design center.10,15 Replicates of the test at the entry are very important as they provide an independent estimation for experimental error. In this work for the variables used, the recommended number of tests at the center is six. Hence the total number of tests (N) required for the five independent variables is tested for the desired ranges of values. Further they are coded to lie (α ± 1) at the factorial points, 0 for the center points and (±) for the axial points.11−13 N = 2n + 2n + 8 = 50

■

(where n = 5 variables) (2)

RESULTS AND DISCUSSION Physical and Chemical Characterization of the Catalyst. The BET surface area, pore volume, and the average pore diameter of the CuO−CoO−Cr2O3 (32:36:32 wt %, CAT-A), CuO−CoO−Cr2O3+ZSM-5 (16:18:16:50 wt %, CAT-B) catalysts are presented in Table 1. This final composition of the catalyst was analyzed through both UV−visible spectroscopy and energy dispersive X-ray (EDX) analyzer. The surface composition or point analysis for a catalyst is possible during SEMEDX, whereas for confirmation of the final composition of the reduced catalyst, both UV−visible spectrophotometer and EDX analyses were done to substantiate the different metal loading compositions. To investigate the desired calcination temperature, dried precalcined fine powder (CAT-A) is set to thermogravimetric analysis to understand the mass reduction with respect to temperature. TGA−DTG results/graph is presented in Figure 1. The mass loss during oxidation proclaims that between 320 and 380 °C the percent of weight variation is approximately 35%. After 380 ± 5 °C temperature the weight loss from the catalyst is trivial, which is suitable for the calcination 4844

dx.doi.org/10.1021/ie202866q | Ind. Eng. Chem. Res. 2012, 51, 4843−4853


Article

CO-hydrogenation reaction to get the maximum active sites in the bifunctional catalyst.19 This peak may occur because the lower number density and subsequent lower collision probability of “Co” and “O” species bring the formation of less oxidized phases. Figure 3 shows the XRD patterns of these two catalysts (CAT-B). XRD peaks in (CAT-B-fresh) resulting at 2θ of 23.1°

Figure 1. TGA/DTA curves of precalcined catalyst (CAT-A) and used catalyst (CAT-B).

temperature for its oxide phase formation. The 12% weight loss indicates the decomposition temperature of the Cr(NO3)3 phase at 170 ± 5 °C, while the decomposition temperature of Cu(NO3)2 starting from 225 ± 5 °C, may be due to the formation of CuCrO4 and Cr2O3, as described by Liang et al.16, although the physical mixing of CAT-A with ZSM-5 (Si/Al, SAR = 35) tends to formulate CAT-B for testing purposes. During calcination, metal oxides of Cu and Cr spinel interacted at the alloy shell level to form bimetallic oxides, as copper, cobalt, and chromium inter-associated to form alloys that are more helpful in the synthesization of liquid hydrocarbons, which is well supported by Baker et al.17 Reduced catalysts are also analyzed by using UV−visible spectrophotometry and EDX analyses to substantiate the different metal loading composition. During EDX analysis the microscope is operated in scanning transmission electron microscopy (STEM) mode, providing a nominal probe diameter of ∼0.7 nm. The metallic cobalt should be presented either pure or alloyed with copper and chromium to synchronize active sites for chain propagation.1,18 Figure 2

Figure 3. XRD analysis of catalyst for CAT-B (fresh) and CAT-B (used).

and 43.2° refers to Cu as per JCPDS-04-0836. Peaks at 35.7° and 53.8° both in fresh and used (CAT-B) correspond to CoCr2O4, which is observed from JCPDS-80-1668 and have fcc lattice structure (4 2 2).2 Cobalt and chromium are found in the lattice form of CuCrO4 and CoCrO4 crystallites, some of which are bcc tetragonal in shape, and some other structures are end-centered with orthorhombic structure as confirmed by JCPDS-73-1045. Copper and chromium are found as CuCrO4 at (2θ) 62.9° and 63.1° matched with JCPDS-77-2474 for the used catalyst shown in Figure 3 (CAT-B).20 Analysis of pre- and post used samples depicts that Co3O4 is converted to CoO and CoO to Co. Further the dispersion of Co and Cu particles is suppressed in fresh catalyst by provoking the largest Co3O4 clusters (2θ = 36.8°) surrounded by other alloys. Some CoO particles are identified as reductions of CoO to Co2+, and these are observed at 41.4° and 62.2°, respectively, whereas reflection lines for Cr2O3 are unchanged after the reaction.21,31 It can be observed that for both catalysts a similar pattern corresponds to Cu phases, probably indicating either the higher dispersion of Cu sites or the amorphous nature of cobalt. XRD peaks are observed indicating metal particle size (d) = 7.2, 11.8, and 8.7 nm, which is in good agreement with the standard XRD patterns and TEM analysis. TEM analyses of fresh CAT-A after calcination are shown in Figure 4. Morphology of the particles, their level of dispersion, and threadlike structures indicate mesoporous framework with pentagonal array of cylindrical channels. Fairly uniform particles with an average diameter of 15 ± 3 nm in sizes are estimated from TEM analysis for CAT-A, which is in good agreement with results of XRD analysis. Many distinct and widely distributed particles are supposed to be partially separated, indicating a tendency of reduced metal Cu particles that have been correlated with their motive force or the enhanced interfacial

Figure 2. TPR analysis of fresh CAT-A after calcinations.

illustrates the reducibility condition required for CAT-A. The temperature programmed reduction (TPR) profile indicates a peak centered at 312 °C, attributed to the reduction of Co3O4 to CoO and to Co species where approximately 67% reduction took place, which is performed in situ before starting the 4845



Article

The response surface method, CCRD, is employed to study the optimization of the FT synthesis process using a bifunctional catalyst, through five process variables. This method has been approved previously as an economical means to obtain maximum information in a short period with the least number of experiments. Fair correlation coefficients have been established using these five process variables varied with wide ranges (Table 2, Supporting Information).11,13,16,24 Coded Empirical Model Equations for the CO Conversion. Process variable effects are investigated for CO conversion to gain a better understanding of the results, and three-dimensional plots are discussed further. On the basis of CCRD through response surface methodology, the results revealed that temperature, pressure, and CO/H2 flow rates have a significant effect on percent of CO conversion as summarized in Table 3 of the Supporting Information. The theoretical CO-hydrogenation process can reach a maximum up to 99% conversion; however the H2/CO molar ratio at the inlet must equal the H2/CO usage ratio, which indicates that the average H2/CO ratio throughout the reactor will be equal to the inlet value, promoting higher WGS reaction and unutilized hydrogen. A further effect of CO2 implication probability is applicable if the CO2 is formed due to a WGS reaction (CO + H2O → CO2 + H2). Equations 4 and 5 are used to calculate the percent of CO conversion and GHSV.

Figure 4. TEM images fresh CAT-A after calcination.

stresses developed in those regions, which tend to separate them from the Co−Cr phases during reaction.1−2 The TEM image shown in Figure 4 elucidates that the metal oxide phases fitted with different bonding force and metal grain formation took place in the range of 15−20 nm which also defines the peculiar physiochemical properties of the (Cu−Co−Cr) bimetallic catalyst, but it is different from those on the outer surface of the catalyst. The samples are spastic and not singlecrystalline structures in nature.22,23 Regression of Model Equations for FT Synthesis. CCRD is used to develop a correlation between temperatures (°C), pressure (bar), reaction time on stream (TOS), H2, and CO flow rate (mL/min) based on response surface methodology (RSM). For more precision and minimization of experimental error using the least number of experiments, statistical methods of factorial (DOE) with runs 43−50 at the center point are used through this study. Predicted model fittings and their regressions are estimated from the software after proceeding through a sequential model sum of squares. Experiments are planned to obtain a quadratic model consisting of 2n (n = 5) trials plus a star configuration (α ± 2), and there replicates at the center point. Selected independent variables in this study included five factors A, B, C, D, and E, which stands for temperature (°C), pressure (bar), reaction time on stream (h), CO flow rate (mL/min), and H2 flow rate (mL/min), respectively, for the evaluation of a bifunctional catalyst. The final empirical model in terms of coded factors for optimum conversion of CO (Y) are given in eq 3:

CO conversion (%) =

(4)

GHSV =

(g of feed × minute−1) (g of catalyst loaded)

(5)

Figure 6a represents the three-dimensional response surfaces, for the combined effect of temperature in the range of 250 to 350 °C, and pressure in the range of 22 to 39 bar, on CO conversion (%) during the CO-hydrogenation process. The combined effect of temperature, pressure, and reaction time on CO conversion has been depicted in Figure 6a, whereas the effect of run time and temperature on CO conversion has been presented in Figure 6b. Brief experimental conditions carried out are as follows: pressure of 22−39 bar, molar ratio of H2/CO, 1−2. For CAT-B the hydrocarbon distribution is influenced by the change in temperature, and detailed product distribution with respect to temperature has been discussed elsewhere.2 As the pressure increases from 22 to 33 bar, conversion increased from 54% to 61%, which is most favorable for the temperature range of 290 to 315 °C.25 With an increase in the temperature from 275 to 315 °C, the conversion attends the maximum up to 63 (Figure 6b). This might be due to the fact that either the activation energy for condensation and propagation steps is smaller than those for hydrogenation of light hydrocarbons or there might be hydrocracking reactions at higher temperatures. It is also evidenced from Figure 7 that the CO conversion increases from 54 to 63, as pressure increases from 22 to 35 bar and finally reaches to a steady state value. From the 3D-surface and contour plots the dependency of these variables can be explained and are shown in Figure 7a,b. Figure 7b personifies the conversion, which increases with respect to flow rate of hydrogen and favors the rate of reaction, in the rage of 300 to 315 °C. Almost steady conversion is observed at 35 bar and at 310 °C.

Y = 63.25 + 0.06A + 1.38B − 0.11C − 4.34D + 5.48E − 1.13AB − 0.34AC + 0.64AD − 0.21AE − 0.55BC − 0.12BD + 0.55BE − 0.69CD + 0.54CE + 0.95DE − 2.32A2 − 1.45B2 − 0.08C 2 − 2.10E2

(moles of CO)in − (moles of CO)out 100 (moles of CO)in

(3)

Statistical Analysis for the CO Conversion. Equation 3 has been used to visualize the effects of experimental factors on CO conversion (%) and is presented briefly in Figures 5−9. Figure 4 depicts the comparative analysis of actual and predicted conversion of CO. These results indicate that the predicted values are in good agreement with the experimental results. The values of R2 and Radj2 shown in Figure 5 are found to be 0.96 and 0.93, respectively. 4846



Article

Figure 5. Graphical comparison between experimental and simulated CO conversion (%) value.

entities under constant total pressure of the fixed bed catalytic system. Higher CO partial pressure invites the formation of high molecular weight hydrocarbons. An increase in CO/H2 molar ratio enhances the CO insertion, C−C chain growth, and moderate product termination, and thereby results in higher liquid hydrocarbons.26−28 As space velocity increases the CO conversion decreases. The water−gas shift reaction over this catalyst has low activity due to the presence of bimetallic copper−cobalt and copper−chromium phases, which are prominent as confirmed from XRD peaks. From this study and published literature, it is being revealed that the main ratecontrolling step is not only the diffusion of syngas through the catalyst pore, but also other parameters like molar ratio, individual component partial pressure, space velocity, and active metallic phases available with the catalyst.26 The normal hydrocarbon can diffuse into the zeolite pores easily, and once it enters zeolite channels, cracks and isomerizes by acidic sites inside the zeolite channels. The different acid sites densities can be evaluated by temperature programmed desorption (TPD) using a mixture of 3.0 vol % NH3 in He preadsorbed at 35 °C. Prior to NH3 adsorption, catalyst samples (300 mg) were reduced in H2/Ar (1.5 L·h−1) with a ramp of 5 °C per minute. When samples are ammonia-saturated at room temperature for 2 h and the quartz chamber is evacuated and flushed with helium at 50 °C for 30 min, NH3 desorbed using 5 °C min−1

Figure 8a,b shows three-dimensional response surfaces of the combined effect of temperature and CO flow rates (mL/min) on % CO conversion, at a constant pressure of 32 bar. Gradually conversion increases with the increase in temperature and CO flow rate (mL/min). Figure 8a shows the maximum conversion of 65%, at a temperature of 300 °C, and H2 flow rate at 85 (mL/min). Higher hydrocarbon molecules are heat sensitive, undergoing variety of cracking and dealkylation reactions that become increasingly important with increase in temperature. Selectivity toward aromatics and alcohols increases up to 300 °C and thereafter decreases due to thermodynamic constraints. As syngas passed through the CAT-B (mixture of ZSM-5) capsule, having dissimilar molecular diffusivity values of hydrogen and carbon monoxide indicates that they would pass through the zeolite shell at different diffusion rates which is either temperature or pressure dependent. The optimum synthesis gas ratio is another crucial parameter for the process of CO-hydrogenation, and the response surface plot is shown in Figure 9a. The importance of the CO/H2 molar ratio in forming higher liquid hydrocarbons is depicted in Figure 9b. Conventional syngas from coal gasification has a higher CO to H2 ratio as compared to CH4 synthesized syngas. To eliminate coke formation and higher activity, higher H2 partial pressure is always desirable. However a significant change in the CO/H2 molar ratio affects the partial pressure of both gas 4847



Article

Figure 6. The combined effect of different parameters on CO conversion (%), (a) effect of temperature (°C) and pressure (bar); (b) effect of run time (h) and temperature (°C).

ramp rate from 50 to 700 °C. From NH3-TPD it is revealed that two desorption peaks are clearly observed on its desorbed profile (not shown in the figure); the peak before 120 °C is attributed to the physically adsorbed NH3, and the other at 370 ± 15 °C is related to the strong acidic sites on the zeolite surface after calcination of CAT-B. The TPD spectra of both ZSM-5 and CAT-B depicts that acidic properties of zeolite (ZSM-5) are similar, regardless of the varied size of the CoO and Cr2O3 cluster formed after mixing the catalysts together for active phase formation, as the use of bifunctional catalyst (CAT-B) allows alcohol synthesis from syngas over the metallic

function and further transformation of alcohol into hydrocarbons over the acidic sites. Fulfillment of either step in the reaction synthesis time span promotes the displacement of thermodynamic equilibrium toward methanol or higher alcohol synthesis. CAT-B pellets have a significantly large external surface area, with proportionate zeolite quantity per unit reactor volume. Long-chain hydrocarbons have larger molecular sizes, and they occupy longer times inside the zeolite capsule. Therefore, it is sufficient for these molecules to be cracked and isomerize as reported by Yang et al.26 Simultaneously carbon deposition is 4848



Article

Figure 7. Percent of CO conversion at different reaction conditions: (a) effect of pressure (bar) and reaction time (h); (b) combined effect of temperature (°C) and hydrogen flow rate at constant pressure of 32 bar and 1 molar ratio.

During hydrocracking, the simple splitting of hydrocarbon skeletons result in a product of rich iso-paraffins.25 The direct production of iso-paraffin from syngas can be accomplished by the conventional FTS reaction and subsequent hydrocracking or hydroisomerization of the products, either in one or consequent stages. The addition of a small amount of physically mixed ZSM-5 to Cu−Co−Cr2O3 bifunctional catalyst changes the hydrocarbon distribution drastically by the selective cracking of higher hydrocarbons (wax). It is due to the space-confined effect and cobalt particle size related to the basic behavior of the catalyst. Thus these results suggested a direct proportionality between surface basicity and shape-selective effect; further in order to get high octane number hydrocarbons, hydrocracking

also consumed via the boudouard reaction, which might be a favorable condition for the formation of higher hydrocarbons. The low diffusion efficiency of CO and H2 is governed by the low CO/H2 ratio in the interior part of the catalyst pellet. This might increase methane selectivity, as H2 diffuses more quickly than CO, especially inside small pores or channels, whereas the CO2 in the FTS reaction is mainly from the water−gas shift (WGS) reaction, and its selectivity is hardly changed.5 Isomerization is favored for the ZSM-5 + Cu−Co−Cr2O3 (CAT-B) mixture, while smaller Co−Cu−Cr2O3 pellets with a large cobalt particle size increases reducibility, and as a consequence, enhanced the catalytic activity. External higher surfaces of ZSM-5 are favorable for the growth of paraffin synthesis. 4849



Article

Figure 8. The combined effect of process variables on CO conversion (%): (a) effect of temperature (°C) and CO flow rate (mL/min) (P = 32 bar, molar ratio = 1); (b) effect of pressure (bar) and CO flow rate (mL/min) (T = 300 °C, molar ratio = 1).

sensitive to the change of syngas pressure. In CAT-B, the cobalt ions in combination with metallic copper sites individually or collectively dominated oxygenate synthesis, which is important to induce reducibility with H2 at 330 ± 10 °C, before syngas interaction inside Cu−Co channels, which are the active sites developed for reaction. Further addition of alkali metals to both low-temperature (e.g., CuO/ZnO, CuO/ZnO/Al2O3) and high temperature alcohol (methanol) synthesis catalysts (e.g., ZnO/ Cr2O3) could steer the behavior of such alcohol synthesis.29 The liquid product contains mainly normal paraffin and normal αolefins, which might be due to its easy entrance into ZSM-5 channels; then by acidic sites of zeolite, long chain hydrocarbons

and isomerization are helpful by acidic sites of zeolites. The combined effects are more helpful to convert the heavier FTS linear hydrocarbons into isoparaffins and alcohol synthesis.27−30 The identification of different key components in liquid hydrocarbon, during the CO hydrogenation reaction is analyzed by GC−MS (Perkin-Elmer clarus-600). Different mass spectra hits distinct peaks were identified as different hydrocarbons like benzene, ethyl benzene, methyl phenols, benzyl alcohols, n-propylbenzene, cyclobenzene, cyclohexane, decane, n-undecane, n-tridecane, and polyaromatic hydrocarbons. Therefore, increasing syngas partial pressure results in an increased productivity of almost all products except CO2, which is not very 4850



Article

Figure 9. (a) Effect of pressure (bar) and H2 flow rate (mL/min) on CO conversion (%) (CO/H2 = 1:1 mL/min, T = 300 °C); (b) effect of H2 flow rate (mL/min) and CO flow rate (mL/min) on CO conversion (P = 32 bar, T = 300 °C).

due to a rapid and extensive deactivation of the small Co particles.31

can be converted into lighter ones by the effect of isomerization and cracking. A different study by Tskcshita et al.30 revealed that H2 has high solubility in transition metallic sites like cobalt, whose crystal structure is an important factor in the H2 absorption. Finally the overall partial pressures of H2 and CO in the core part of the capsule catalyst are differnet than the original pressure value of the syngas. There is not much effect of the WGS activity, as CAT-B shows very low selectivity toward CO2 (

Response Surface Modeling and Optimization of CO Hydrogenation

Recommend Documents