Haber-Bosch Process Alternatives

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“Should resources be invested in searching for an alternative to the Haber-Bosch process”.

Introduction:

Ammonia’s ongoing consumption in the world is startling. The production of ammonia (NH3) was the turn of the 20th century, a startling breakthrough in organic chemistry for the world to stand in astonishment. Two scientist; Fritz Haber and Carl Bosch, made this breakthrough through the chemical procedure called the: ‘Haber-Bosch Process,’ (N2+ 3 H2→ 2 NH3.) However they weren’t the first to try the synthesis of ammonia from its elements (www.nobelprize.org, 1920.) prior to the discovery of synthetic ammonia and long before the commercial application of it, early farmers knew that certain properties of carbon based by-products which led to: human waste being scattered in Chinese farmlands, grinding of skeletons in Europe and the exploitation of Peru’s readily-available guano, due to its natural nitrogenous compounds, and the discovery of Nitrogen Fixation Processes (www.firt.org, 2012.) The need for Ammonia was directly related to the world’s survival, the fixed nitrogen from the air is an incredible and needed ingredient for fertilizing. Many principles of chemical and high—pressure processes were discovered and expended for the optimisation of the known nitrogen fixation process. Industries in the 19th and 20th century saw the ongoing need for nitrogen and turned to their already in use factories for producing coal to use the by-product of coking, ammonia sulphate. This along with the previously mentioned methods was how ammonia was produced pre-Haber-Bosch Process. From these early discoveries evidence can be seen as to why the Haber – Bosch process is the best way of producing ammonia for the growing world. Two more scientists by the name of Priestly and Cavendish used electrical sparks in the air to produce nitrates, done by dissolving the oxides of nitrogen to form alkalis. Nitrogen Fixation, fixating Nitration as calcium cynamide proved evasive for commercial use, but later proved useful for the production of chemicals requiring the cynamide configuration. There were numerous other process ect. Thermal Processing, cyanide formation, aluminium nitride formation and the slow process of decomposing to ammonia were deemed to elusive for sustainability due to the scarce amount of chemical components for the organic production to be made, resulting in to high of cost. With Habers-Bosch large-scale catalytic synthesis of ammonia from elemental hydrogen and nitrogen gas which had reactants that were inexpensive (Hydrogen, Nitrogen and Iron as a catalyst.) Using high pressure (~5000c) alongside high pressure (~150-200 atm), the process involved forcing almost completely unreactive gaseous nitrogen and hydrogen to have the product of Ammonia. This high-energy process has undergone extensive modifications in the 21st century which goes on to prove that resources should not be devoted in the search for an alternative to the currently used Haber-Bosch process because the structure of the process is the most balanced of the mentioned processes of making ammonia even in the 21st century with a need of 150 million metric tonnes of ammonia (Chemical and Engineering News, 1996); 80% of which is used in agriculture where 48% of the resulting produce is responsible for the world’s ongoing consumption, however there are a few methods that give an idea for very good alternatives that could replace Haber-Bosch, but would not be as efficient.

Discussion on Methods:

What has made the Haber – Bosch process so great is its low cost and readily available materials as seen in this method, this however has been as previously mentioned modified many times since Haber’s work:

Ammonia synthesis from nitrogen and hydrogen, a reversible reaction is as follows:

(1)

And the equilibrium constant is

(2)

(www.ias.ac.in, 2012)

From our understanding of organic chemistry we can see that production of ammonia is a exothermic reaction with a exponential amount of heat released. As previously mentioned the reaction is reversible, the forward reaction being ammonia synthesis, and the reverse reaction being ammonia decomposition. The decrease of the volume derives from the decrease in the number of moles of gas from the equations, two and one. With the use of Le Chatelier’s Principle (See appendix one), from this it can be seen that by increasing the pressure in the reaction causes the equilibrium to shift to the right resulting in a higher yield of ammonia produce since there is a pressure drop accompanying the transformation; the decrease in the temperature then also causes the equilibrium position to move to the right again resulting in a higher yield of ammonia since the reaction is exothermic as previously mentioned. Figures 1A and B show the effect of temperature and pressure on the equilibrium mole fraction of ammonia. It can be seen that the ammonia mole fraction decreases as the temperature increased just as the pressure increases.

 

Figure one. (A is the mole fraction of ammonia in the state of equilibrium at varying temperatures to result in a given value of pressure. B is difference pressures at fixed values of temperature in Kelvin, data supplied by www.ChemWiki.org

1.0

       

1.0

0.9

       

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500

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0

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Temperature (K)

Pressure (Atm)

The conclusion then that ammonia synthesis according to the first equation is an equilibrium reaction that is favoured by low temperature and high pressure which. The reaction does not proceed at ambient temperature because nitrogen requires a lot of activation energy for the dissociation to happen (www.Topsoe.com). In the gas phase of the reaction, the dissociation occurs only at around 3000°C (ChemWiki.com). Looking at the hydrogen molecule in the reaction, which has a weaker molecular bond, only dissociates markedly happened at temperatures above ~1000°C (ChemWiki.com, 2012). Which shows that the reaction cannot be performed at lower temperature because it needs high activation energy to happen, if there was an increase in the temperature with a enough level, the reverse reaction predominates (www. http://abacus.bates.edu/, 2010). This is where scientist decided to have the role of the iron catalyst come in. Figure two below shows the energy profiles for ammonia synthesis in the absence and presence of the catalyst. The hydrogen and nitrogen molecules lose their translational ability to be bound to the catalyst surface. This reduces the activation energy dramatically and makes the forward reaction go faster, which makes sense to us because catalyst can’t do nothing else but speed the reaction up. Other minor components of the catalyst includecalciumandaluminium oxides, which are there to support the absorbent iron catalyst and help it maintain its surface area over time, andpotassium, which increases theelectrondensity of the catalyst and so improves its activity (ChemWiki.com, 2012). This means that scientist can rid the need for extremely high temperature conditions, a problem Haber encounted while trying to find commercial success. Something for us to know it the use of lower temperature reaction conditions means there is limited reverse reaction which is energy saving as well, this reinforces the idea of the Haber-Bosch Process being the best there is. For industrial use however, there is still a need for reasonably high temperatures (250–400°C) to dissociate the N2 and H2.

(Figure two, the effect of catalyst on the activation energy). Supplied by www.Chemguide.uk.

Now that we know the advantages of the current Haber – Bosch Process, we can look into how carbon-free ammonia comes into the world. Licht wrote a paper on the two current chemical reactions that are now used most widely in ammonia synthesis:

CH4+ 2H2O→4H2+ CO2

N2+ 3H2→2NH3

These won’t be explained because they already have previously.

3CH4+ 6H2O +4 N2→ 3CO2+8NH3

Licht wrote of a proposal for the use of ammonia as a fuel for automobiles. Although Licht did not specify the products of the ammonia oxidation, it looked into the possibility that this fuel cell might prove reversible in the case where a product was nitrogen gas, which looks into the fixation of Nitrogen, an electrochemical path to ammonia. However scientist need to overcome the extreme stability of the nitrogen-nitrogen bond in N2gas, nitrogen fixation always requires the need of gas with a metal (www.theenergycollective.com, 2012), which in biological systems the metal in question is molybdenum (as well as the used iron as the catalyst), making molybdenum, along with iodine, the only elements in the periodic table that are essential to this project.Haber he speculated that there was a better catalyst, uranium which we will look at later.

A metal catalyst is required in the electrochemical process as stated, in this process the catalyst is iron, but in this case it is necessary that the iron, present as an oxide in the process, be in the form of nanoparticles suspended in molten alkali hydroxides, where future research can be done on the use of other molten oxides, notably, cesium hydroxide, which may prove superior however the variables in the process with respect to temperature, operating voltage, current and the physical nature electrodes will also need to be detailed in research before this can happen. (www.atomicinsight.com,2012). For this processto happen a eutectic melting mixture of KOH and NaOH, potassium and sodium hydroxides respectively; the authors explored also the use of other molten oxides, notably, cesium hydroxide, which may prove as a better source for the process. These hydroxides are only molten at higher temperatures, and steam and air or pure nitrogen gas are bubbled through the molten hydroxides ( in the case of air, carbon dioxide, meaning that with the removal of this gas from the air would be a side benefit of the process, a benefit to the Haber-Bosch process.)The precise stoichiometry of the reaction varies with the conditions, but one form of reaction mentioned by the authors, this done through a controlled environment, is this:

N2+ 10H2O → 2NH3+ 5O2+ 7H2

Both pure oxygen and hydrogen are important to the reaction, and goes to show that the reaction offers many potential synergies for the benefit of the scientist. The gases on the right side are not produced as an explosive mixture, because ammonia and hydrogen are formed on one side of the cell, at the cathode, whereas the oxygen is formed at the anode (www.atomicinsight.com).The mixture of cathodic gases, ammonia and hydrogen in the reaction, are easily separated by compression as Lecht found. The overall electrochemical efficiency is quite high compared to other attempts at the electrochemical reduction of nitrogen to ammonia gas, which is around 46%, an efficiency that may well be competitive with Haber-Bosch process ammonia synthesis, however this does not include the heat penalty associated with melting the alkali metal hydroxides and keeping them molten which is the reason nitrogen fixation was proved a commercial disaster when Haber was working, where 38% of the land would be needed and the cost being severely higher, Haber’s process uses only 14% of the land to produce ammonia.

What else could aid in the Haber-Bosches super process? A better catalyst. Haber sought for a better catalyst in the 20th century with investigations into uranium. With an understanding of the activity of the key component of the Haber-Bosch process which is the catalyst, could help to better the industrial nitrogen fixation still further from what we previously discussed and remove the need for high temperatures and pressures.

http://courses.chem.psu.edu/chem210/mol-gallery/oxygen/n2-lewis.gifRuthenium, osmium, uranium and cobalt-molybdenum can all catalyse the Haber-Bosch process (ChemWiki.com, 2012), but iron catalysts are cheap. It’s the most commonly used catalyst which was developed more than a century ago and is a potassium-doped iron catalyst. A soluble version of such a catalyst might be even more efficient because it could overcome the rate-limiting step of nitrogen dissociation from a solid catalyst surface, which was demonstrated before. Scientist have found that soluble iron catalysts have proven ineffective for the process in its need to be reducing the N-N triple bond as seen in figure three and also cannot produce large amounts of ammonia for commercial use. Germany has developed a molecular iron complex that can react with

(Figure three, The N-N Triple bond, a problem that persist in the formation of an improved haber-bosch process).

nitrogen gas in the presence of a potassium reducing agent to generate a complex containing two nitrides bound to the iron and potassium cations which contain a mixed iron (2+/3+) nitride.Germany suggests that the formation of this developed core structure has three iron atoms working together to break the dinitrogen triple bond through a six-electron reduction (NewScientist.com, 2014). The resulting nitride of the reaction then reacts with hydrogen gas to generate a high yield of the product ammonia. Unfortunately, this process leads to the use of the iron and so is not catalytic, a problem for the process as it reduces the yield on ammonia because with an absence of a catalyst the reaction is so slow that virtually no reaction happens in a reasonable time (ChemWiki.com, 2012). The catalyst ensures that the reaction is fast enough for a dynamic equilibrium to be set up within the very short time that the gases are actually in the reactor. Germany’s work on the core provides important clues as to precisely how nitrogen cleavage and N-H bond formation occurs, which might allow them to build a complex that does work catalytically in solution, something that can be further investigated in the future. This is great because it means the Haber-Bosch Process can be simplified and eventually bring a greater yield of product, seeing the growth of ammonia grow in the world.

Conclusion:

Through economic situations, the Haber-Bosch Process is the most essential to the world, however it’s also the most relevant in terms of the future. Nitrogen Fixation as we looked at can potentially be a competitor to the Haber-Boch Process, due to it’s overall electrochemical efficiency which as stated is quite high in comparison to other attempts at the electrochemical reduction of nitrogen to ammonia gas, it is around 46%, an efficiency that may well be competitive with Haber-Bosch process ammonia synthesis. Unfortunately there is no process being studied in thermodynamics to reduce the heat loss from the process, ChemGuide.uk states that “A 46% increase is substantial for the future of Ammonia production, but heat loss is an issue for it that so far, can’t be reduced with what we know today”.

The next thing we looked at was the catalyst in the current Haber-Bosch Process. Haber dwelled into Uranium but backed out of the idea because of the money, however with the advance we discussed in the study on catalytic effects it’s clear and hypothesises that there is a high possibility that in the future Ruthenium, osmium, uranium and cobalt-molybdenum can all be used in the synthesis of ammonia with a high yield in produce, which is being investigated currently. Advances in the Haber-Bosch Process are our best approach for the future, scientifically and economically, that is until thermodynamics are better understood.

 

 

 

Appendix

Summary of Le Chatelier’s Principle by www.ChemWiki.ucdavis.edu

(1) If the concentration of a reactant is increased, the equilibrium position shifts to use up the added reactants by producing more products.

(2) For gaseous reactions, gas pressure is related to the number of gas particles in the system; more gas particles means more gas pressure. Consider a reaction which is accompanied by decrease in number of moles, such as, ammonia synthesis (equation one). Increasing the pressure on this equilibrium system will result in the equilibrium position shifting to reduce the pressure, that is, to the side that has the least number of gas particles.

(3) In an endothermic reaction, energy can be considered as a reactant of the reaction while in an exothermic reaction, energy can be considered as a product of the reaction. Consider an exothermic reaction which is accompanied by release of heat, such as ammonia synthesis (equation one). Reducing the temperature of this equilibrium system (which result in taking the heat away) will result in the equilibrium position shifting to increase the temperature (producing more heat), that is, to shift the equilibrium position to the right.

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