Mass and Energy Balance Essay

Abstract The objective is to produce a proposal for a chemical process plant which will be able to produce 550,000 tonne/year ammonia using LPG as the raw material. Different processes where researched and then finally one was picked, steam reforming. This was decided to be the most viable and cost effective process using the raw materials we had available. The report explains in detail how the process works and all aspects of how the plant will work including the mass and energy balance across the plant. What is Ammonia Ammonia (NH3) is a stable compound and is used as a starting material for the manufacture of many important nitrogen compounds and can also be directly used as fertilisers. It is produced by reacting hydrogen and nitrogen. It is a colourless gas with a sharp odour. The boiling point is -33.35oC and its freezing point is -77.7oC.1 Care must be taken when handling ammonia as can cause deep burns in the skin; irritation in the eyes and nose and when inhaled can cause coughing, sore throat and headache.2 There are different methods for the manufacture of ammonia. The three main methods are steam reforming, partial oxidation and electrolysis. Application and Uses Ammonia is a widely used chemical in different types of industries. One of the main user of ammonia is the agricultural industries for fertilisers. Around 80% of ammonia produced is for fertilisers such as urea, ammonium sulphate and ammonium nitrate.3 It is also used as a building block for nitrogen containing compounds like nitric acid (HNO3). It is also used in the fibres and plastics industry for the production of acrylonitrile, melamine etc., and manufacture of explosives. Ammonia is also used in water treatment such as pH control and also in combination with chlorine to purify industrial and municipal water supplies. Less commonly uses include as a refrigerant in compression and absorption systems, manufacture of household ammonia, in the food and beverage industry 4. Figure 1: Pie chart showing the uses of Ammonia. Market Trends Globally ammonia prices have been headed up due the large demand of fertilisers that are needed in the crop production to obtain high yield6. The current selling price of ammonia in Europe goes up to $600 per tonne7. Figure 2: Shows the global demand for Ammonia (D.a.NH3- Direct application of Ammonia) As we can see from the chart the trend of ammonia demand globally is upward. It is said that the global ammonia market is to generate revenues of approx. US$102 billion in 2019. As there is continous growth in population in the developing countries the likely to cause demand for foodstuffs are to increase even further. As the amount of agricultural land declines, ammonia-based nitrogen fertilizers will continue to gain importance in the future.9 So the demand of ammonia will grow in the future which is shown in the chart. Processes There are many different processes involved in the ammonia production. The most common processes for ammonia are partial oxidation, steam reforming and electrolysis. From these 3 processes the best process route is then selected and that process would be most economical and that meetes the design brief. Partial Oxidation Partial oxidation involves the reaction of oxygen with fuel to produce hydrogen. The following equations represent the partial oxidation of ethane, propane, butane and pentane. 10 C2H6 + O2 2CO + 3H2, C3H8 + 1.5O2 3CO + 4H2, C4H10 + 2O2 4CO + 5H2, C5H12 + 2.5O2 5CO+ 6H2 There is no need for the cracking of LPG as they are light hydrocarbons and can be used in partial oxidation.11 See Partial Oxidation flow sheet (Reference 1: Partial Oxidation Flow Sheet) Hazards and Environmental Impact The main emission is carbon dioxide which is a greenhouse gas and Partial Oxidation process emits more carbon dioxide compared to Steam Methane Reforming. Carbon dioxide emissions can be reduced by recycling it and selling it to urea and nitro-phosphate plants.13 No ammonia should be present in the air but maybe because of faulty equipment and maintenance activities, some ammonia maybe released. Ammonia becomes explosive at the 16%-25% volume in air which could occur if there are any leakages in the ammonia storage facilities. It is also toxic by inhalation and pulmonary oedema can occur up to 48 hours after exposure and could be fatal.12 Nitrogen dioxide that is released is a toxic gas can be harmful when inhaled but can be avoided as can be detected because of the smell. The large amount of waste water from this process is another problem but there is a river near the Milford Haven site. Also water pollution is a concern which may occur because of the suspended and dissolved impurities. It could also affect the aquatic life. Therefore the water must be treated in a full three stage water treatment plant before disposing it. 13 The disadvantage of partial oxidation is that the capital costs are higher for partial oxidation compared to any other process. It is estimated to be  £100-120 million for an annual production of 7.7 million GJ while for SR it will only be  £70 million. 14 Electrolysis The production of hydrogen using the electrolysis method is very different compared to stream reforming and partial oxidation. Electrolysis produces hydrogen by splitting water into hydrogen and oxygen using volts of current to separate the hydrogen to one electrode and oxygen at the other in a cell. Oxygen is the by-product in the process of producing ammonia which is valuable because it can be used in other chemical processes or sold to other companies for profit. In electrolysis there is no CO2 produced therefore there is no pollution. Standard electrolytic ammonia production energy consumption historically has been about 12 megawatt-hour. The fuel cost alone of making ammonia is $600 metric ton, and including capital and operating expenses that metric ton of ammonia cost about $800 to make. Compare electrolytic and using uses of natural gas as raw material the economically, for the past 100 years the cost of natural gas has not been higher than $1 and the fuel cost for a metric ton of ammonia from natural gas has been $30-$40. Figure [ 3 ]: Ammonia Manufacturing Process Figure 3: Ammonia Manufacturing Process Steam Reforming Gas purification Syngas of a mixture of hydrogen, carbon monoxide, carbon dioxide and water can be broken down in to individual components and further cleansed through purification. The syngas will enter a shift reformer, which breaks down the carbon monoxide in to hydrogen and carbon dioxide using steam (H2O). Carbon dioxide is much more environmentally friendly than CO and can either be released in to the atmosphere or used in other steam reforming processes in the future. Desulphurisation Sulphur is a problem when carrying out steam reforming as it acts as a poison for the catalysts involved. It is important that this is removed prior to the syngas entering the system. The process is carried out in the presence of a catalyst, which is usually nickel. This nickel acts as an absorber for the sulphur, and so several catalyst-filled tubes within the system with a large internal surface area will allow the sulphur to collect to be disposed of suitably. The Process Hydrocarbons usually contain sulphur which needs to be removed. The purification section is the first bed of the whole steam reforming process. Feed is passed through tubes containing zinc oxide. The sulphur in the feed reacts with the zinc oxide to produce zinc sulphide. This is to ensure that the feed travelling to the steam reformer does not poison the catalysts in this section in any way. The catalysts used in the steam reforming process are nickel-based. These are easily poisoned by sulphur species. The purified feed is mixed with steam and then is passed to the primary reformer, which involves a nickel-based catalyst where the steam reforming process is carried out. Once the hydrocarbon is cleansed of sulphur, the reforming process can begin. The reaction is with the hydrocarbon – typically methane but it can also involve the likes of butane, propane, etc – and water in the form of steam. The reaction for methane (CH4) is shown below. CH4 + H2O 3H2 + COΔH = +251kJmol-1 C3H8 + 6H2O 3CO2 + 10H2 C2H6 + 4H2O 2CO2 + 7H2 C4H10 + 8H2O 4CO2 + 7H2 C5H12 + 10H2O 5CO2 + 16H2 Reactions for other hydrocarbons, such as ethane (C2H6), propane (C3H8), butane (C4H10) and pentane (C5H12) are also shown, with their respective steam amounts required and the products gained. Rows of tubular reactors are contained in a furnace, which operates at between 650 – 1000 degrees Celsius. The hydrocarbon feed enters the system at a very high pressure, typically 20 – 30 bar. The process is carried out in the presence of a nickel-based catalyst which is packed into cylindrical tubes through which the steam/hydrocarbon gas mixture is passed. The catalysts act as surface for which the hydrocarbon will absorb and the steam. (Reference 2: Steam Reforming Flow Sheet) Justification Steam reforming is the most viable proposition as we have all of the raw materials available within easy access, whereas if we were to use other processes, then we would have to source other materials e.g. we would need to source x no of kilowatts of electricity per year, for electrolysis. Mass Balance Cp Values Energy Balance Material Costs Simple Plant Cost Using a base of around  £410 per ton of ammonia, and output at 550,000 tonnes, it would be assumed that the plant would produce  £225,500,000 a year of ammonia. The Burrup plant in Australia was built at a cost of  £457 million and produces roughly 800,000 tonnes a year of ammonia. Using the 2/3 power rule, as follows, will allow the costs of the new 550k p/a plant to be shown. C = Cref(S/Sref)2/3 C = 457000000 * (550,000/800,000)2/3 C =  £355,984,702 The output of the new plant is  £225,500,000, but the plant costs  £355,984,702 to build, so it would take around a year and seven months for the plant to be profitable, based on an estimation without including the costs of the raw materials. Taylor Method Pay Back Time Sustainability The environment is constantly changing, whether by nature or by human led processes. Sustainability is about trying to manage this change through balancing social, economic and environmental needs, both locally and globally for present and future generations. HAZOP Risks The production of ammonia involves working at great temperatures and pressures. As such, it is vital that the equipment used in the plant is designed to withstand these conditions to function properly. The high temperatures and pressures involved in the production of ammonia can potentially put tremendous amounts of strain on the pipes and vessels used. The risks associated with this are: * Explosions from sudden release of pressurised gases from ruptured vessels * Fragmentation from rupture of the pipes * Fire * Poisoning from exposure to leaked materials * Chemical or thermal burns, again from exposure to leaked materials Not only are these hazards life-threatening, they would also be very expensive to put right for the production company. These risks can be avoided by preparing the plant for the conditions that it is about to go through. It is more economically viable to run the steam reformer at as high a temperature and pressure as possible. Magnesium oxide-lined furnaces, MgO, has a melting point of around 2800 degrees Fahrenheit, making it ideal for lining the furnaces used in the production of hydrogen. Hydrogen itself will cause some materials to become brittle and eventually break. Hydrogen features an active electron and thus will behave like a halogen, causing erosion in the metals that it comes into contact with. This can be avoided by using high-purity stainless steel in the sections of the plant which will come into contact with the hydrogen. This steel must have a maximum hardness of 80 HRB on the Rockwell Scale. Ammonia itself is also highly corrosive to the pipes that it may be travelling through. For this reason, it is recommended that stainless steel is also used here, at a similar hardness of that shown above. Most ammonia plants use centrifugally cast high-alloy tubing to hold the nickel-base catalyst in the primary reformer furnace. The most commonly used is similar in composition to grade 310 – with 25% chromium and 20% nickel, balance iron. This has a carbon content in the range of 0.35 – 0.45% for improved high-temperature creep and rupture stress. Thermal protection of piping involves fire brick owing to the high temperatures involved.