Introduction
The conversion of gas into liquid fuels is an innovative technology that is well-known to many people. This technique is so promising and enhancing new options for environmentally friendly manufacturing of fuels and chemicals as well as other natural gases. Looking into the large natural gas and coal reserves and deteriorating petroleum reserves all over the world, it is anticipated to play an important role in the future. Cobalt-based chemical agents are the ideal catalysts for hydrogen synthesis since they have a high FTS functionality, discernment for long-chain paraffin as well as a low process for the water-gas change reaction (He, Zhang & Fan 276). Usually, Iron and Cobalt-based catalysts have small quantities of potassium and other metals like calcium, manganese, copper, zinc, and magnesium which as a promoter to boost their selectivity and activity in general. Since potassium has a stronger basicity, it has a great impact on adsorptions of other reacting substances (CO and H2) on the dynamic sites, and results in an improvement FTS activity, development in the choosiness to olefins, overpowering the formation of methane and selectivity change to advanced molecular weight outcomes.
Consequently, the kinetics of FTS on cobalt agent has received an overwhelming attention from all quarters; in fact, various studies show the kinetic rate and data expressions. The reaction demands for both hydrogen and carbon monoxide indicate a range between 0.5 to 2 and -1.0 t0 0.65, correspondingly. The activation momentums from these reports show a scale between 98 and 103 kJ/mol. The mechanical kinetic ratio expressions for cobalt agent are founded on the creation of the monomer varieties which is the rate that determines every phase in the utilization of synthesis gas. Various kinetic equations have been proposed in various studies for many cobalt catalysts and this can be either achieved through empirical methods (applying a power-law rate expression) or even fitting the proposed mechanisms.
The main objective of this experiment was to come up with a fundamental rate expression for conversion of CO to Fischer-Tropsch products above an impregnated cobalt catalyst on the intrinsic realistic mechanism. FT reaction’s kinetics was researched and its rate equations tested in contrast to experimental data obtained from the chosen catalysts. Later a model was effectively designed, and the kinetics variables were ascertained. At the same time, the equation of the power law kinetic for carbon monoxide was achieved.
2. Experiment
Preparation of the Catalyst
An ideal quantity of 15wt.%Co/10wt.%K/Al2O3 was made by supplying the system with an aqueous mixtures of Co (NO3)2.6H2O with KNO3 to emerge with some dampness of γ-Al2O3, which was at the first desiccate at a temperature of 400ºC for a period of 8 hours so as to detach the surface that absorbed pollutions BET an area of 217 m2/g, pore capacity of about 0.7 cm2/g. The infused sample was left to dry at 110ºC for 2 hours and calcined in air at temperatures of 400ºC for 8 hours (at a warming rate of 10ºC and 110ºC as well as 400ºC). The oxidised agent was then reduced in situ and pure H2 at temperatures of 400ºC for 16 hours (rate of heating was 10ºC between 25 to 400ºC).
FTS Setup
Fitscher-Tropsch Synthesis Process can be applied in the conversation of synthesis gas, a combination of hydrogen and carbon monoxide, into long-chain referred to as hydrocarbons5. A chemical process known as XTL is responsible for the change of natural gas, coal, and biomass to synthetic liquid fuels. This chemical reaction was discovered at the start of the last century and industrialized into a commercial process by Hans Tropsch and Franz Fischer. FTS changes synthesis gas which is a mixture of hydrogen and carbon monoxide to several long chain hydrocarbons, which is later processed to get the anticipated product, for instance, synthetic diesel. Since these products are similar to those gotten from crude oil, the feasibility of the method relies on the unit cost of the product in contrast to the prices of oil. To achieve this target, several advancements must be made to this process like the designing of better reactors and boosting the catalytic properties, like enhancing the selectivity of iron-based FTS catalysts thus averting the likelihoods of poisoning cobalt FTS catalysts (Mansouri et al. 1)
Conceivably, Iron based FTS compounds are less expensive and can produce some products. This catalyst changes a bit of carbon monoxide into carbon dioxide via the water gas shift reaction, better abbreviated as (WGS) at a higher temperature. This relies mostly on the operating temperature as well as the ratio of H2/CO. These catalysts can lead to the generation of a light hydrocarbon stream, a high absorption of the branched products and these are crucial for the chemical industry and fuel, and other heavier hydrocarbons. Also, it is worthy to note that Iron catalysts are poison-tolerant in comparison with cobalt catalysts. Conversely, the main disadvantage of Iron catalyst is that it deactivates quickly. Also, since some the products form through Fe-catalyzed FTS, modifying the selectivity to achieve desired product is a big challenge and wide-ranging steps involved in separating them to get a pure product (Yan et al. 297).
Evaluation of Catalyst at Various Conditions
The current practical were carried out with reagents of H2, Carbon monoxide, and N2 in a temperature at temperatures varying between 205 to 245°C, H2/CO supply proportions of 1/1-3/1 (mol/mol) and pressure of 8 bar. The appointments of the variables and the related levels were done carefully to yield ideal results. During the entire experiment, the velocities of the experiment space were maintained at 2700 to 5200 h-1 as recommended by Fazlollahi et al. (1).
Determining the effect of pressure on the action of the catalyst at various barometers was part of the objectives of the experiment. While maintaining the same reaction ratio of CO/H2 and pressure for different sessions, observations were made and recorded. It was noted that when the temperatures were adjusted upwards by 20oC, that is, from 220oC to 240oC, there was a significant rise in the activity of the catalyst (C1+). However, when IRE and CRE were used in turn, the productivity of C1+ was reduced substantially unlike C5+ that showed a significant rise in the yields. Furthermore, at 220oC when the molar percentage of Ce is increased, the catalytic capacity of C5+ gets substantially decreased. Therefore, it can be deduced that Ce minimalizes the action of the catalyst at 220oC as witnessed by a reduction in the amount of C1-C4 produced. With an up-regulation of the Molarity of the Ce, there is a corresponding down-regulation of the amount of C5+ products. This is mainly attributed to the partial reduction of CeO2 to CeO 2-x as well as the integration of CO following the formation of additional active sites during FTS. It can be observed that with the rising temperature so does the rise in the productivity of C5+ with all the catalysts sites being put to task.
Results
Kinetic Models
Kinetic expressions are important in deriving rate expressions that are in turn used to be to design Kinetics models. An example is the Langmuir–Hinshelwood–Houngen–Watson expression used to arrive at kinetic models. This theory propagates that a reaction mechanism had to be applied when designing kinetic models. There are two primary assumptions on which this theory is anchored: attraction heats remain unchanged and essential reaction rates are proportionate to surface cases of catalyst and the reacting substances.
Application of the Polymath software is also regarded important in the kinematic models. Here, the least square approach and non-linear regression determinations are obtained from summaries of values to come up with the power-law equation and the parameters needed for the experimental data to make the Polymath® software usable. The software applies the Levenberg–Marquardt algorithm to carry out estimates and the constancy values for the model.
However, finding the most suitable model is requires one to meet certain conditions. To begin with, one must be in possession of the constants that are positive that will be aplied in optimizing the model or equation by giving a reliable R2. Secondly, the constants of the equation have to be in tandem with the Arrhenius and Vanthouff guidelines and thirdly, the equation has to be elaborate enough to predict the performance of a dissimilar reactor. It has been estalished that good equations are elaborate enough to encompass all the afore-listed parameters (Fazlollahi 1230).
This technique takes into account that the kinetic restrictions may rely on coverage and temperature. Hence, it is from the onset that TPD which is a high coverage and low temperature it is used. There was a debate that during the start of the desorption process a less amount of species desorbs, which implies that the coverage can be regarded as constant. Arrhenius plot linearization (in (rate) vs. 1/T) of this short period produces a straight line plus a slant Ead θ /) (R and an intercept n *ln ( ) ln v t), (θ + d θ, where θ) (− Ead which is the coverage dependent adsorption energy whereas n is desorption order and v t), (d θ – the coverage dependent pre-exponential element). The complete analysis employs complete TPD family curves with dissimilar preliminary coverage, in the place of one TPD curve. The first coverage is picked from which the kinetic factors are anticipated. The rate of desorption and resultant temperature at the coverage are obtained from each of the TPD curves.
The slopes and the intercepts they produce the energy which activates the pre-exponential factor, for the selected coverage. This process is done again and again for the different coverages. Irrespective of the analysis employed, one is required to exercise a lot of care so as to interpret the results obtained. Diverse analysis methods can produce different values for the factors. Furthermore, the validity of the obtained information must be confirmed against other explanations if possible, as the transition state geometries as well as the molecule adsorption state. This is great more especially when examining the pre-exponential parameter obtained from TPD analysis. Niemantsverdriet and de Jong have given a general argument of the TPD analysis method. Other explanations that can be gotten from TPD experiment is the impact of lateral connections on the adsorption energies. Lateral interactions are ostensible in TPD tests as a dependent coverage change in the temperature of adsorption.
Conclusion
The experiment offers an insight into mechanisms that can be deployed to yield considerable commercial benefits. Synthesis of fuels is beneficial that just having crude-oil in storage. Derived gasoline as is cleaner and produce as attested by emission less residues. Through the application of the FTS protocol, the processing of straight chain hydrocarbons, including paraffin, waxes, olefins and oxygenated products from synthesis gas is made a fruitful undertaking. Production of synthesized products is made possible from a carbon-rich sources including coal, natural gas. Taking into regard the diversity of sources that can be utlized to yieled synthesis gas, the complete process beginning with carbon containing rich materilas to liquid synthetic fuels or chemicals now known as XTL (X to Liquids); X stand for the hydrocarbon source.
XTL opens with gasification and purification of the hydrocarbon material, the modification by modifications product through cleaning, altering the hydrogen/CO ratio, storage; all geared to ensuring a successful FTS undertaking. When the gas contains all the required properties, it is called synthesis gas. It is a blend of CO and H2 that is ready to be catalytically changed into products through the FTS process. Apart from the primary FTS products; there some by-products including water and carbon dioxide that can be put to good use. FTS most preferred catalysts include iron, cobalt, ruthenium and nickel; however the high cost of ruthenium and the increasing selectivity of nickel to methane makes cobalt be used more often for industrial FTS catalytic processing. At the industrial level, the operating temperatures for the FTS processes can range between 200 – 250oC, under 10-50 bar pressure in which slurry is used, fixed bed or fluidized bed reactors with minor variations depending on the manufacturer’s choice.
Work Cited
Fazlollahi, Farhad, et al. "Development of a kinetic model for Fischer–Tropsch synthesis over Co/Ni/Al 2 O 3 catalyst." Journal of Industrial and Engineering Chemistry 18.4 (2012): 1223-1232.
He, Leilei, Yulong Zhang, and Maohong Fan. "Development of composited rare-earth promoted cobalt-based Fischer–Tropsch synthesis catalysts with high activity and selectivity." Applied Catalysis A: General 505 (2015): 276-283.
Kizilkaya, A. C. Effect of adsorbate interactions on catalytic reactivity: elementary surface reactions on rhodium and cobalt. Diss. Technische Universiteit Eindhoven, 2014.
Yan, Zhen, et al. "Fischer–Tropsch synthesis on a model Co/SiO 2 catalyst." Journal of Catalysis 268.2 (2009): 196-200.
Mansouri, Mohsen, et al. "Hydrogenation of CO on Cobalt Catalyst in Fischer–Tropsch Synthesis." Journal of Thermodynamics & Catalysis2012 (2012).
