Improving Process Control During Ammonia Production Using The
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Improving Process Control During Application Note Ammonia Production Using the Thermo Scientific Prima PRO Process Mass Spectrometer Graham Lewis, Thermo Fisher Scientific, Winsford, Cheshire, United Kingdom Key Words • Steam-to-carbon ratio • H/N ratio • Air requirement • Rapid multi-stream sampler (RMS) • Methane slippage • Magnetic sector analyzer • Shift reaction Introduction as heating fuels, prices tend to be very volatile. This price Ammonia is one of the most important industrial chemicals volatility means plants must maximize the efficiency in the world, with demand forecasted to grow 3% of the ammonia production unit to maintain annually over the next five years. Close to 80% of the a strong bottom line. ammonia produced globally is used for the production of The Ammonia Process fertilizers. Ammonia is also an important component of The manufacture of ammonia can be described simply explosives, fibers, plastics, and intermediates for dyes and by the classic Haber process formula: pharmaceuticals. N + 3H → 2NH The first step in ammonia production is the conversion 2 2 3 of natural gas, liquefied petroleum gas or naphtha However, the production of the two reactants from into hydrogen. Because these raw materials are in high raw materials (i.e. fuel, steam and air) requires a number demand for many other applications including use of process steps. All of these steps occur during the gas phase and require fast, accurate gas analysis to optimize the process. PRIMARY SECONDARY CO SHIFT WATER REFORMER REFORMER CONVERTER SEPARATER Figure 1 is a schematic of a typical ammonia plant HIGH based on natural gas feedstock. The streams DESULPHURIZATION TEMP. that require analysis are: NATURAL GAS STEAM • Natural gas • CO absorber outlet LOW 2 AIR TEMP. FUEL • Primary reformer feed • Raw synthesis gas • Primary reformer effluent • Converter inlet WATER • Secondary reformer effluent • Converter outlet PURGE HEAT COOL • High temperature shift outlet • High pressure purge • Low temperature shift outlet • Hydrogen recovery. CO2 AMMONIA CO2 ABSORBER METHANATOR NH3 CONVERTER SEPARATOR PURIFICATION Figure 1. Typical ammonia process based on natural gas feedstock. 2 Process Analytical Requirements to form hydrogen and carbon monoxide: The analysis of these 12 streams requires a wide CH4 + H2O → 3H2 + CO. Some of the CO will react range of compounds to be analyzed, including: with the steam to form even more hydrogen: CO + H2O → CO + H . The steam-to-carbon gas ratio requires • Organics: C to C 2 2 1 5 tight control because energy is wasted when excess steam • Inorganics: N , H , NH , CO, CO , O , Ar, H S. 2 2 3 2 2 2 is produced unnecessarily. In addition, excess methane Concentrations range from tens of percent down to requires more energy for compression and causes parts per million (ppm). In theory, a set of discrete inefficient catalyst activity. analyzers can be used to measure some of the required components. However, installation Air Requirement and maintenance costs will be In addition to being the source of hydrocarbon for the extremely high and vital analysis process, natural gas is also used as fuel. The physical information will be missing. properties of the fuel gas must be measured accurately. Alternatively, a process gas Doing so ensures the air requirement for combustion chromatograph (GC) can be used, control optimizes the energy consumption of the unit. but long analysis cycle times and Methane Slippage frequent calibration and maintenance Only 30 to 40% of the hydrocarbon is converted in the intervals limit the usefulness of this primary reformer. The exhaust is therefore fed into a technique. This application note will secondary reformer where the conversion to CO and H discuss the advantages gained by 2 continues. Air (the source of N ) is introduced at this stage using a multi-point, multi-component 2 and combustion takes place at around 1250°C (2282°F). solution: the Thermo Scientific™ Prima™ PRO Process Mass It is important to minimize the amount of unreacted Spectrometer. This field-proven methane, or methane slippage, from the secondary reformer. analyzer provides fast, accurate and If it is not minimized, this methane builds up in the reliable data that can be ammonia converter loop. Because this process takes place Thermo Scientific used as part of a dynamic at high pressure, the methane needs more compression Prima PRO Process process control model. energy which ultimately reduces ammonia yield. Mass Spectrometer Shift Reaction Advantages of Mass Spectrometry It is important to remove all CO from the process before The Prima PRO process mass spectrometer offers it enters the ammonia converter, or catalyst poisoning analysis times measured in seconds rather than minutes will occur. This removal takes place by ‘shifting’ the and the ability to measure both inorganic and organic CO to CO2 after which the CO2 is absorbed. This shift species over a wide dynamic range. One Prima PRO mass occurs in two steps during high and low temperature spectrometer can therefore monitor all sample points, shifts. If the CO is not properly removed, it can shift from feedstock to final product, in the ammonia process. back to methane, creating a highly exothermic reaction that can damage the next process stage, the methanator. Key Control Parameters Steam-to-Carbon Ratio H/N Ratio Steam and natural gas (or another source of The methanator is designed to remove any residual hydrocarbon) react together in the primary reformer CO and CO2. The syn-gas output from the methanator should be ideally comprised of 75% H2 and 25% N2. In practice, there will also be some residual CH4 and argon from the air. The production of ammonia takes place over an iron oxide catalyst in the converter. During this process, it is vital to maintain the ratio of H2 and N2 as close as possible to the stoichiometric ratio of 3-to-1. Analyzer Requirements While the ammonia process presents a series of challenges to a typical process mass spectrometer, the Prima PRO mass spectrometer has been designed to overcome these challenges and offers a unique set of capabilities. Rapid Multi-Stream Sampling To effectively monitor all process streams, the mass spectrometer requires a fast, reliable means of switching between streams. Other technologies use solenoid valve manifolds that have too much dead volume or rotary Figure 2. Rapid multi-stream sampler within the Prima PRO Process Mass valves that suffer from poor reliability. Figure 2 is a Spectrometer. schematic of the rapid multi-stream sampler (RMS) of the Prima PRO mass spectrometer. which offers an unmatched combination of sampling 3 speed and reliability and allows sample selection from TYPICAL LEVEL PRECISION one of 32 or one of 64 streams. Stream settling times are %mol %mol application-dependent and completely user-configurable. CH4 80 - 95 0.02 The RMS includes digital sample flow recording for C2H6 1 - 5 0.005 every selected stream which can be used to trigger an C3H8 1 - 3 0.005 alarm if the sample flow drops or if a filter in the sample N-C4H10 1 - 2 0.005 conditioning system becomes blocked. CO2 1 - 3 0.003 N 2 - 5 0.01 The RMS within the Prima PRO mass spectrometer 2 Ar 0.5 0.001 can be heated to 120°C (248°F) and the stream selector H 0 - 1 0.002 position is optically encoded for reliable, software- 2 controlled stream selection. Temperature and position Natural Gas Stream (Primary Reformer Feed) control signals are communicated via the internal network. The RMS has a unique standard three year TYPICAL LEVEL PRECISION %mol %mol warranty. There are no other multi-stream sampling CH 0.5 - 2 0.002 devices that offer the same level of guaranteed reliability. 4 CO 10 - 14 0.03 Precision of Analysis CO2 8 - 10 0.01 The mass spectrometer is required to monitor a wide N2 23 - 24 0.02 range of component concentrations. To be used as part Ar 0.1 - 0.5 0.001 of a dynamic plant control strategy, the data must H2 55 - 65 0.03 be reliable and available. Secondary Reformer Effluent The heart of the Prima PRO mass spectrometer is a magnetic sector analyzer which offers unrivalled TYPICAL LEVEL PRECISION %mol %mol precision and accuracy compared with other mass CH 3 - 10 0.01 spectrometers. Key advantages of magnetic sector 4 N 20 - 26 0.01 analyzers include improved precision, accuracy, long 2 Ar 0.1 - 5 0.002 intervals between calibrations, and resistance to H 65 - 75 0.02 contamination. Typically, analytical precision is between 2 He 0.5 - 1 0.002 two and 10 times better than a quadrupole analyzer, depending on the gases analyzed and the complexity Synthesis Gas of the mixture. A unique feature of the Prima PRO mass TYPICAL LEVEL PRECISION spectrometer magnet is that it is laminated. It scans %mol %mol as fast as a quadrupole analyzer, offering the unique CH4 5 - 10 0.01 combination of rapid analysis and high stability. This N2 15 - 22 0.01 capability allows for the rapid and extremely stable Ar 1 - 5 0.002 analysis of an unlimited number of user-defined gases. H2 55 - 60 0.02 The scanning magnetic sector is controlled with 24-bit NH3 10 - 15 0.01 precision using a magnetic flux measuring device for Convertor Outlet extremely stable mass alignment. The Prima PRO mass spectrometer features an enclosed Figure 3. Typical stream performance specifications ion source that provides high sensitivity, minimum for the Prima PRO Process Mass Spectrometer. background interference and maximum contamination resistance. The high-energy (1000 eV) analyzer offers extremely rugged performance in the presence of gases A guaranteed performance specification for the and vapors that have the potential for contaminating the Prima PRO mass spectrometer is issued based on an analyzer.