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known as Haber-Bosch process which was developed and patented by Fritz Haber and Carl Bosch in 1916. Since then more work on ammonia production was ...
Development of Ammonia Synthesis Poppy Puspitasari

Noorhana Yahya

Electrical and Electronic Engineering Department University Technology PETRONAS Bandar Seri Iskandar, 31750 Tronoh Perak Malaysia e-mail : [email protected]

Fundamental Applied Sciences Deparment University Technology PETRONAS Bandar Seri Iskandar, 31750 Tronoh, Perak, Malaysia e-mail: [email protected]

Abstract— Ammonia production

is a high energy and capital-intensive industry as it obliges high temperature (400-500oC) and also high pressure (150-300 bar) for its daily processes. Two moles of ammonia are obtained by reacting one mole of nitrogen and three moles of hydrogen gases in the existence of conventional catalyst which are magnetite (Fe3O4). The process to produce ammonia is known as Haber-Bosch process which was developed and patented by Fritz Haber and Carl Bosch in 1916. Since then more work on ammonia production was carried out with the goal to achieve higher ammonia yield. Keywords- Ammonia Synthesis, Reactor, Yield

I.

INTRODUCTION

Ammonia has been ranked number two on the list of chemicals requiring the most energy to be produced. Identifying better way to synthesize ammonia could have a major impact on the production of ammonia. Using Haber process, only 15% of nitrogen and hydrogen were converted to ammonia. By repetitive recycling of the unreacted nitrogen and hydrogen, the overall conversion is about 98% [1]. 2H2 (g) + N2 (g)  2NH3 (g)

(1)

Based on Le Chatelier Principle, reactions conducted at higher pressure will favor the forward reaction producing more ammonia. Catalysts are materials that can be reacted to enhance the rate of ammonia production. The presence of catalyst precursors is very essential for ammonia production. Good catalyst support can increase the rate of ammonia because of active chemical reaction. Brookhaven et.al. [2] revealed the metal ruthenium could be more efficient in ammonia synthesis. It was reported that ruthenium catalyst displayed five times higher activity under same temperature and at half of the pressure [3]. A new approach by using nanomaterials as catalyst can achieve very high rate of ammonia production [4]. Reducing catalyst size to nanometers will greatly increase the surface area which in turn increases the level of catalytic activity.

Magnetic induction is a new discovery in ammonia production. By applying magnetic field to the catalyst, it will reduce the cost of production. Instead of high pressure and temperature, the electromagnetic field can result high yield of ammonia at room temperature and ambient pressure [4]. Ferromagnetism is a phenomenon in which the electrical uncharged material strongly attracts to each other. This ferromagnetic material aligns the electrons parallel to each other against the force of thermal agitation [5]. Because of good alignment, the catalytic actvity can easily improve the reaction rate for ammonia production. Currently, magnetite (Fe3O4) is used as the catalyst in ammonia synthesis. It is incorporated with aluminum, potassium, calcium and irreducible oxides [5]. Due to the easy cation substitution for Al3+ and Fe3+, the uniform distribution of aluminum in solid can be obtained [6]. A new finding suggested that wustite as a new catalyst in ammonia synthesis [7] has an increase of 30% compared to those obtained from magnetite. Wustite is favourable compared to magnetite due to its ability to be reduced while maintaining its mechanical strength and thermoresistency. Also ruthenium based catalyst has potential due to its long term stability and activity also low pressure and temperature condition for ammonia productions [8]. Ruthenium with potassium metal is suggested as a very effective catalyst which performed greatly under atmospheric pressure [9]. High ammonia yield approximately 40-50% was produced using Ru/C catalyst. The temperature and pressure conditions are 370-400°C and 50-100 atm respectively [10]. The major disadvantage of this catalyst is its extremely high cost [11-12]. Arrhenius kinetic energy was first proposed by a Dutch chemist J.H. Van’t Hoff in 1884 and was justified by Swedish chemist Svante Arrhenius in 1889, five years later [13]. It is remarkably accurate for the dependency of temperature with the constant reaction rate. It can be seen as the best empirical relationship in modeling temperature variance coefficient. It was mentioned that an increase of 10 degree Celsius doubles the reaction rate which is supported by Arrhenius. The amount of energy required to ensure reaction happens is known as activation energy. Upon collision, this energy can be used to bend and break bonds leading to chemical reactions.

978-1-4577-1884-7/11/$26.00 ©2011 IEEE

Activation energy can be regarded as the height of the potential barrier or energy barrier separating between the potential energy of reactants and the product of reactions. There should be a correct number of molecules with energy equal or greater than the activation energy. Le Chatelier Principle shows that rates of reaction decreases with increasing temperature. This can be related with barrier less reactions, in which the reaction proceeding relies on the capture of molecules in a potential well. Increasing temperature leads to a reduction of collision is expressed as a reaction cross section that decreases with increasing temperature. Also, the kinetic energy can be related to chemical thermodynamics study of the interrelation of heat and with chemical reactions. The thermodynamics potential are the quantitative measures of energy stored as they evolve from an initial state to the final state. The fundamental laws of the four equations or ‘fundamental Gibbs free energy, are typically used in order to predict the energy exchanges. Arrhenius equation demonstrated that, −𝐸𝑎

𝑘 = 𝐴𝑒 ( 𝑅𝑇 )

(2)

where, Ea= the activation energy, R = 8.314 x 10-3 kJ mole-1 K-1 T = temperature (K), A = proportionality constant Taking the natural logarithm of Arrhenius equation gives, 𝐸𝑎

1

𝑅

𝑇

ln(𝑘) = − ( ) 𝑥 ( ) + ln(𝐴) Y= mx + b

(3) (4)

1

Plot ln(k) vs and the data will give straight line and by 𝑇 determining the slope, the value of activation energy can be calculated. II.

AMMONIA SYNTHESIS

In early 1970, British Petroleum Company, cooperated with M.W. Kellog Company was created new type of catalyst, which is Ruthenium with carbon support (Ru/C). The yield of ammonia was successfully increased to around 40-50% by using this Ru/C catalyst, [14]. The temperature and pressure required also reduced to 370-400oC and 50-100 atm [14]. Nevertheless, this temperature and pressure are still considered high and the price of Ru is very expensive compared to iron. It has been reported that carbon have a tendencies to react with H2 to produce methane, CH4 during the catalytic activity [15]. In the meantime in 1996, a group of researchers reported and patented a new high-activity ammonia synthesis catalyst based on wustite which was increased the reaction rate by 30% compared to the traditional catalyst. However, this catalyst is less active than those based on magnetite and much less resistant to thermal deactivation

[16]. It is believed that nanotechnology can give an enormous impact in terms of profits to the industry and environment. Efficiency of the catalytic reaction could be increased by applying nanocatalyst and the operation condition (temperature and pressure) required for the reaction will be lower. The less rigorous reaction condition, high cost savings can be created in terms of materials used, electricity, labor used etc. The industry will also experience the condition of environmental friendly because nanocatalyst can result in 100% catalyst selectivity for a desired product [17]. Traditionally and conventionally magnetite (Fe 3O4) was used as catalyst for ammonia synthesis. Typical promoter for magnetite are aluminium, potassium and calcium [18]. Therefore, magnetite is known as a precursor due to the easy cation substitution of Al3+ for Fe3+, thus homogeneous distribution of aluminium in solid can be gained [19]. Nowadays, wustite was proposed as a new precursor for ammonia synthesis catalyst [20]. 30% yield of ammonia was obtained by using wustite as catalyst, compared to magnetite. It was also reported that wustite appears to be encouraging to magnetite due to its ability to be reduced while thermoresistancy and mechanical strength are fully maintained [20]. Several research groups in the past decades have been studied that ruthenium-based catalyst was a promising candidate to replace magnetite for ammonia synthesis. The rutheniumbased catalyst is well known to its long term stability and activity. It is also capable to produce high ammonia yield in low pressure and low temperature conditions [21]. Ru/MgO catalyst synthesized by sol-gel method for ammonia synthesis was also studied [22]. It was found that specific surface area and metal dispersion were increased when increasing Ru carbonyl complex concentration. The highest ammonia formation rate was studied on magnesia supported ruthenium Ru/MgO (Ru: 7.1 wt. %) with high surface (290 m2/g) [49]. Ruthenium promoted by potassium metal has been projected as a very active catalyst for non conventional ammonia synthesis which may be carried out under atmospheric pressure [23]. Ruthenium supported on -Al2O3 which was varied with KOH was tested as ammonia synthesis catalyst under atmospheric pressure. It was found that the highest hourly yield of ammonia was achieved with 8% Ru/Al2O3KOH at 623K and atmospheric pressure [24]. By using Ru/C catalyst, high ammonia yield approximately 40-50% was successfully produced at 370-400oC and 50-100 atm respectively. The disadvantage was high cost of ruthenium [25]. Regarding the price of ruthenium, it was reported that carbon have a tendencies to react with H2 to produce methane, (CH4) during the catalytic activity [26]. It is well known that iron and ruthenium are good catalysts for ammonia synthesis at different temperatures. Mutual influence in various scopes on these two active catalysts was reported. It was studied that although ruthenium is very active at optimum temperature (573-623K), the existence of iron had resulted in very low activity, signifying negative synergism of iron and ruthenium [27]. Ammonia synthesis by using Ru/C catalysts with specific carbon supports namely activated carbon fiber (ACF),

activated carbon (AC), and carbon molecular sieve (CMS) was observed. The ammonia synthesis was obtained in 350-450oC and 3.0 MPa in a microreactor. The result was found that Ru – Ba/ACF gave the highest turnover-frequency (TOF) value (0.089 s-1) due to high purity and electronic conductivity of ACF [28]. Structure sensitivity of ruthenium catalysts supported on the graphitised carbon was also studied. Ammonia synthesis studies have found that the reaction rates (400oC, 63 bar, 8.5% NH3 or 400oC, 90 bar, 11.5% NH3) showed in terms of TOF had increased versus particle size in spite of of the type of promoter [28]. Nowadays, Yahya et al was proposed a new type of nanocatalyst, namely Mn0.8Zn0.2Fe2O4 [29]. The catalyst was synthesis using sol-gel method and was reacted under electromagnetic (EM) induction. It was obtain high yield ammonia due to the synergism of Mn, and Fe metal (which was reduced in hydrogen gas) and the EM induction. There are two other factors besides catalysts that could assist in the ammonia yield which are promoter and support. It is well known that metal crystallites particularly in nano scale range have high surface area and surface energy. Therefore, the agglomeration is favored to overcome these side effects which finally lead to the formation of bigger crystallite size. The catalytic activity would take place occasionally due to less surface area available for the reactants to be adsorbed a process. In order to make sure that the catalyst works efficiently, small amount of chemical additive or promoter is often added. There are two categories of promoter which are textural or carrier and electronic. One of the best solutions to avoid agglomeration problem is by adding metal crystallites on the carrier or supporter. Carrier which acquire great features such as outstanding thermal stability and high surface area is highly needed. Alumina,γAl2O3 is well known as typical carrier for ammonia synthesis catalyst. It has high surface area (100-300 m2/g) and also able to endure at higher temperature environment. Furthermore, higher degree of dispersion of metal crystallites on carrier is also needed which could stimulate the catalytic activity. This can be achieved through the synthesis approach [30]. Electronic promoter is another crucial component for catalyst and usually doped in relatively small amount. Nevertheless, disproportionate doping may reduce the catalytic activity as it can largely cover the surface of metal crystallites. Example of promoter for ammonia synthesis catalyst is potash or potassium hydroxide (KOH). Potassium, K+ performs as electron donor which donates it electrons directly into dorbital of iron. The continuous donation will generates a high electron density region of iron. regrettably, this occurrence will lead to the destabilization of iron. Therefore, the surplus electrons will be transferred to the π antibonding orbitals (π*) in nitrogen molecules. The occupation of antibonding orbital makes the N≡N elongate and weaker. As a consequence, the N≡N bond cleavage is increased [31, 32].

III.

MICROREACTOR

Microreactor channels has become important since they offer several advantages over conventional analytical techniques including small volume requirements, portability, fast sampling times, ability to multiplex and compatible with other technique [33]. Mostly microreactor has multiple parallel channels with diameter between 10 to several hundred micrometers where the chemical reaction occur [34]. Microreactor as miniaturized chemical reactors has been used in many application such as hydrogen reforming process [35], biochemical analysis [36], catalyst and material screening [34]. There has been reported the microreactor system of gassolid-liquid channels, with the solid-catalyst in most cases [33]. The most difficult part for that system is about the pressure drops, the achievement of large catalyst surface areas, and the maximization of heat transfer in exothermic systems. Some researcher has been reported for green chemistry by using microreactors. The review of this research was explores that miniaturization reactor may revolutionise chemical synthesis, highlighting in particular the environmental benefits of this new technology which include in situ reagent generation, solvent free mixing, and integrated separation techniques [37]. The applications of microstructured reactors for heterogeneous catalysed gas phase reactions has been investigated. Many examples of partial oxidations were described including the process of ethylene oxide synthesis [38]. There has been reported that by using microdevice, carbon monoxide concentration was able to reduce in a hydrogen-rich methanol reformate gas from 0.5% to 10% ppm. This device integrates two heat exchanger and a reactorheat exchanger in a single device with a volume and mass of only 60cm3 and 150g and reaches a heat recovery of 90% [39]. IV.

CONCLUSION

There are many ways to achieve the high yield of ammonia. Choosing the best condition for the optimum yield is the important thing. High temperature and high pressure condition are not suitable for green environment. The new method to synthesis ammonia is by using the magnetic field in the ambient condition. The reactor can be develop more smaller and efficient by using microreactor operated in ambient condition. REFERENCES [1] [2]

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