field trip report - final

8/13/2019 Field Trip Report - Final

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Field Trip Report

Ammonia Production at Gibson Island

University of Queensland

Declan Scott 423391

James Crook 423711

Josh Hampson 426775

Field Trip ReportAmmonia Production at Incitec Pivot - Gibson Island


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Field Trip Report


Table of Contents:

1. Introduction ......................................................................................................................................1

2. Ammonia Production Process ......................................................................................................1

3. Ammonia Synthesis Reaction .......................................................................................................2

3.1. Reaction Conditions ...............................................................................................................2

3.2. Temperature Effects ...............................................................................................................3

3.3. Reaction Kinetics .....................................................................................................................4

4. Reactor Equipment ..........................................................................................................................5

4.1. Reactor Type .............................................................................................................................5

4.2. Catalyst Type: ...........................................................................................................................5

5. Nitrogen Production .......................................................................................................................5

6. Safety & Environmental Concerns ..............................................................................................6

7. Energy Savings & Emission Reduction ......................................................................................6

8. References ..........................................................................................................................................7

9. Appendix A: Reaction Mechanism ...............................................................................................7

List of Figures:

Figure 1: Equilibrium Constant Temperature Dependence (Eggeman, 2000).................................3Figure 2: Ammonia Synthesis Heat of Reaction Temperature Dependence (Bohnet, 2003)...........3

Figure 3: Temperature Dependence of Equilibrium Conversion for Ammonia Synthesis (Modak,

2002).................................................................................................................................................4

Figure 4: Ammonia Synthesis Reaction Rate...................................................................................4


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1. IntroductionAmmonia (NH3) is a toxic, colourless gas possessing a distinct, penetrating odour. The gas exists

naturally in the atmosphere, produced by certain species of nitrogen-fixing micro-organisms.

However, since 1913, ammonia has been commercially produced using the Haber-Bosch process,

whereby hydrogen and atmospheric nitrogen are catalytically reacted to synthesize ammonia. In

todays increasingly industrialized world, ammonia forms a critical global resource.

Ammonia is the second most commonly manufactured chemical with applications across a wide

range of industries. Of the worlds total ammonia production, 85% is consumed in the production of

fertilizers with the remainder primarily being used in explosives or as industrial solvents (Appl,

1999). Given its primary application in the fertilizer industry, production of ammonia has closely

mirrored global population growth. It is believed that 40% of the worlds protein requirements can

be directly attributed to ammonia production (Eggeman, 2000). Furthermore, the mining and

resource sector is critically reliant on the gas for gaining access to underground deposits.

Incitec Pivot is a global manufacturer of a wide range of fertilizers, explosives and industrial

chemicals (Incitec Pivot, 2013). The company directly owns and operates 20 facilities around the

world, with seven of these being ammonia production plants located throughout Australia. This

investigation focuses on ammonia production at Incitec Pivots Gibson Island plant in South-East

Queensland, which operates at an ammonia production capacity of 300,000 tonnes per annum.

2. Ammonia Production ProcessAmmonia is predominantly produced by reacting nitrogen and hydrogen over an iron catalyst

(Bohnet, 2003). In order to do this, a mixture of nitrogen and hydrogen must first be obtained.Hydrogen can be produced from water through the process of steam reforming. The first step in this

process is steam methane reforming in the primary reformer, which takes place according to the

reaction below:

CH4+H2O CO + 3H2 H298 =206kJ/ml

This is a highly endothermic reaction and as such is carried out at high temperatures through the

application of external heating. At Gibson Island, the primary reformers operate at approximately

1400F and 400 psig (Incitec Pivot, 2013). Simultaneously to the steam reforming reaction, the

water gas shift reaction shown below also takes place in the primary reformers (Eggeman, 2000).

CO+H2O CO2+H2 H298 =-41.17kJ/ml

Product gases from the primary reforming process still contain approximately 5 to 8% unreacted

methane and are therefore fed to a secondary reformer to complete the reforming process

(Eggeman, 2000). The heat required to drive the endothermic reforming reaction is obtained by

combusting a portion of the feed gases in an oxygen-starved environment. This combustion takes

place according to the two reactions shown on the following page.


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H2+0.5O2 H2O H298 =-241.83kJ/ml

CH+1.5O2 CO+2H2O H298 =-519.31kJ/ml

This combustion is achieved through the addition of air and, as oxygen is consumed, nitrogen

remains in the gases to be used later in the ammonia synthesis step. The remaining gases are heatedby this combustion and then passed over reforming catalysts to undergo the same steam methane

reforming and water gas shift reactions which take place in the primary reformer. Following this the

methane content of the syngas has now been reduced to around 0.2 to 0.3%.

In order to convert any remaining carbon monoxide to carbon dioxide, the syngas mixture is then

fed to two shift converters, which are usually adiabatic, packed-bed reactors. This is usually done in

a high and low temperature step and involves the familiar water-gas shift reaction below.

CO+H2O CO2+H2 H298 =-41.17kJ/ml

Following shift conversion, carbon dioxide is then removed from the process gases by a packed bedabsorber column using aMDEA as the chemical solvent (Incitec Pivot, 2013). The removed CO2 is

then either fed to the Urea plant or the Liquid CO2plant, which are also located on site. As the

absorber does not achieve 100% CO2 removal, process gases are then fed to the methanator to

reduce carbon dioxide content down to the levels required to prevent ammonia synthesis catalyst

poisoning (Eggeman, 2000). These reactions are simply the reverse of the reforming reactions:

CO + 3H CH4+H2O H298 =-206kJ/ml

CO2+ H2 CO+H2O H298 =41.17kJ/ml

Process gases, which are now predominantly a 3:1 hydrogen/nitrogen mixture, are then compressed

and fed to the ammonia converter where the following synthesis reaction occurs:

+ 3 2 H298 =-92.44kJ/ml

Produced ammonia is then fed to either a storage tank or the Urea plant. Temperature has a

significant effect on the conversion achieved for this reaction and therefore the ammonia synthesis

reaction conditions are of considerable interest.

3. Ammonia Synthesis Reaction3.1.Reaction Conditions

Ammonia synthesis at Gibson Island is carried out at a pressure of 2600-2650 psig and an inlet

temperature of 750F (399C). These conditions typically give a single-pass conversion of 19%

(Incitec Pivot, 2013). This synthesis reaction is carried out at this elevated pressure in order to

increase the equilibrium concentration of NH3.


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3.2.Temperature EffectsTemperature has a significant effect on the ammonia synthesis process. Lowering temperature

increases the equilibrium constant, K, and therefore shifts the reaction towards the right. However,

kinetics of the reaction limit the lowest useful temperature to around 350C (Eggeman, 2000).

Figure 1 below shows how equilibrium constant varies with temperature for the ammonia synthesisreaction as well as a number of other earlier reactions discussed above.

Figure 1: Equilibrium ConstantTemperature Dependence (Eggeman, 2000)

As can be seen above, increasing reactor temperature significantly decreases the equilibrium

constant in what appears to be a linear trend. This is due to the exothermic nature of the reaction,

which make it more favourable at lower temperatures.

As an exothermic reaction, the ammonia synthesis reaction will always have a negative heat of

reaction however the magnitude of this is temperature dependent. Figure 2 shows the negative heat

of reaction for the reaction 0.5N2 +1.5H2 NH3.

Figure 2: Ammonia Synthesis Heat of Reaction Temperature Dependence (Bohnet, 2003)

50

55

60

65

70

0 100 200 300 400 500 600 700

-HeatofReaction(kJ/mol)

Temperature (C)

Figure 1: Equilibrium Constant Temperature Dependence (Eggeman, 2000)


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As shown in Figure 2 on the previous page, increasing temperature initially decreases H until a

minimum is reached and H then begins increasing slowly with temperature (becoming more

negative).

Equilibrium conversion for the ammonia synthesis reaction decreases with increasing temperature

as it is an exothermic reaction and therefore favoured by lower temperatures. Figure 3 below shows

the effect of temperature on conversion at different reaction rates of 0-20 kg/m3.

hr. Equilibrium

conversion is indicated by the curve with a reaction rate of 0, indicating a system at equilibrium.

Figure 3: Temperature Dependence of Equilibrium Conversion for Ammonia Synthesis (Modak, 2002)

If the reaction were operated without a catalyst, very high temperatures would be required in order

to overcome the activation energy to dissociate nitrogen. However, at these temperatures the

equilibrium conversion would be very low and hence ammonia production would be minimal.

Introducing a catalyst solves this problem by reducing the activation energy for the reaction and

allowing it to take place at a much lower temperature. Although equilibrium conversion is higher at

this temperature a recycle stream is still required to increase overall conversion further.

3.3.Reaction KineticsThe kinetics of ammonia production is strongly influenced by

operating temperature and pressure. Increasing pressure increases

the reaction rate as well as increasing the equilibrium constant,

thus high pressures are preferable for ammonia synthesis.

Increasing temperature increases the rate of formation of ammonia

but also reduces the equilibrium ammonia concentration (Bohnet,

2003). Therefore, as shown in Figure 4, rate of formation can be

seen to initially increase with temperature, reach a maximum and

then decrease as the system approaches equilibrium:

Figure 4: Ammonia Synthesis Reaction Rate

Temp & Pressure Dependence (Bohnet, 2003)


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Ammonia synthesis is a heterogeneous catalytic reaction and a proposed mechanism for this

reaction can be seen in Appendix A. Studies have confirmed that the nitrogen adsorption and

dissociation can be regarded as the rate-controlling step for this reaction (Bohnet, 2003).

4. Reactor Equipment4.1.Reactor Type

Gibson Island utilizes a plug flow reactor for ammonia production. This is used over other reactor

types as the reaction occurs in the gas phase at a very high pressure (Incitec Pivot, 2013). As it is an

exothermic reaction, heat must be removed throughout the reaction in order to control the

temperature and maximize the conversion. This can be done through three main reactor

configurations; tube-cooled converters, quench converters or indirectly cooled multibed converters

(Bohnet, 2003). Tube-cooled converters are internally cooled with the main cooling medium being

the feed gas, which can flow counter or cocurrently to the reacting gas. In quench converters the

catalyst volume is divided into several beds with heat being removed by injecting colder synthesis

gas. Indirectly cooled multibed converters use the same configuration with boiler feed water usually

providing the cooling mechanism.

4.2.Catalyst Type:In order to reduce heat requirements and achieve commercially viable reaction rates, various

catalysts are used throughout the ammonia production process. In the steam reforming and

methanation stages of synthesis gas production, a nickel-based catalyst is typically used (Bohnet,

2003). Regarding the shift conversion reactions, metal oxides are typically used with different

specific catalysts used for the high and low temperature steps (Bohnet, 2003).

Since the invention of the Haber-Bosch process, iron-based catalysts have been almost exclusively

used for the ammonia synthesis reaction. The effectiveness of the iron catalyst is improved by

adding reduction-resistant metal oxides such as aluminium and potassium oxides (Bohnet, 2003).

IPLs Gibson Island plant employs iron-based catalysts, predominantly using Amomax-10, which is

mainly composed of the iron oxide containing material wustite (Bohnet, 2003).

5. Nitrogen ProductionThe nitrogen used by Incitec Pivot in its ammonia production process is essentially a free input

(Incitec Pivot, 2013). Nitrogen is obtained as a byproduct from the combustion of natural gas with

oxygen in air and therefore has no directly attributable cost.

The heat required to facilitate the secondary reforming reactions is supplied by injecting and

combusting a portion of feed gas (Eggeman, 2000). Air is supplied to provide the oxygen required

for this combustion. As the oxygen is consumed, the inert nitrogen content of the air remains in the

product syngas. Following the methanantion and removal of carbon dioxide from the syngas (as

outlined in Section 2), the resultant process gas consists of the required 3:1 ratio of nitrogen to

hydrogen.


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6. Safety & Environmental ConcernsSince its commercialization, ammonia production has demonstrated itself to be a relatively safe

undertaking, however, a number of potentially hazardous factors must be considered. A report by

Appl, 1999 identifies four major areas for potential harm; ignition or explosion of the process

syngas or hydrocarbon feed supply, the operation of vessels at elevated temperatures and pressures,and the toxic release of ammonia during the synthesis loop.

Due to their respective compositions, the natural gas feed system and the syngas (comprised of

approximately 75% hydrogen) are inherently flammable. Ignition of these streams could lead to

catastrophic explosions, resulting in significant damage to surrounding equipment and potential

fatalities of personnel. Additionally, temperature and pressure control systems are particularly

critical for the methanation reactors. These pressure vessels are prone to deviations from safe

operating conditions, which can cause a dangerous runaway reaction (Eggeman, 2000).

Whilst the ignition of ammonia is not a significant risk due to its high ignition temperature (651

0

C),it is a highly toxic compound and even a short exposure to concentrations as low as 2500ppm has

caused fatalities (Appl, 1999). Under certain conditions, air-ammonia mixtures can be denser than

air, meaning a toxic cloud produced by an accidental atmospheric release can flood the surrounding

operating plant rather than dispersing. Effective design, fabrication, operation and inspections are

critical to mitigating the above safety concerns.

From an environmental perspective, ammonia production is generally regarded as a relatively clean

operation. Atmospheric emissions and contamination of surrounding water systems are the major

environmental concerns. Most harmful is the emission of NO2in the flue gas of the fired primary

reformer and the contamination of water, primarily by the condensation of process steam ahead ofthe carbon dioxide removal as well as accidental spillages (Appl, 1999). Additionally, with

increasing urbanization a growing awareness has developed for mitigating noise generation and

emission from ammonia plants.

7. Energy Savings & Emission ReductionEnergy consumption is a function of design, operation and maintenance of the ammonia plant

(Appl, 1999). Ammonia production is a highly energy intensive operation although, regarding the

Incitec Pivot plant, a number of optimisation opportunities exist. An important energy saving

strategy is the conservation of process heat, primarily through the use of heat exchangers andinsulation. It was observed that a number of high temperature piping and process units are exposed

directly to surroundings and therefore a more thorough insulation system would be beneficial.

Another significant energy sink at the Incitec Pivot plant is the pumps. It would be beneficial to

reduce this workload by using gravitational forces. For example, the plug flow reactors can be

orientated vertically such that the feed enters at the top of the reactors and the pressure drop is

supplied by gravitational energy rather than pump work.


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8. ReferencesBOHNET, M. 2003. Ammonia - Production Processes. Ullmann's Encyclopedia of Industrial Chemistry.

Wiley.

EGGEMAN, T. 2000. Ammonia. Kirk-Othmer Encyclopedia of Chemical Technology.John Wiley & Sons,Inc.

INCITEC PIVOT. 25/09/2013 2013.RE: CHEE3005 Field Trip.

MODAK, J. 2002. Haber process for ammonia synthesis.Resonance,7,69-77.

9. Appendix A: Reaction MechanismBelow is a proposed reaction mechanism for the heterogeneous catalytic reaction for ammonia

synthesis based on experimental results.

Source: Bohnet, 2003

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