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Application Note

absorber outlet 2 • CO • Raw synthesis gas • Converter inlet • High purge • recovery. • Converter outlet

3 2NH →

2 + 3H 2 Low temperature shift outlet Secondary reformer effluent High temperature shift outlet • as heating fuels, prices tend to be very volatile. This price as heating fuels, prices tend to be very volatile. efficiency volatility means plants must maximize the of the production unit to maintain a strong bottom line. The Ammonia Process simply The manufacture of ammonia can be described by the classic Haber process formula: N from However, the production of the two reactants a number raw materials (i.e. fuel, steam and air) requires during the of process steps. All of these steps occur gas phase and require fast, accurate gas analysis to optimize the process. Figure 1 is a schematic of a typical ammonia plant based on feedstock. The streams that require analysis are: • Natural gas • Primary reformer feed • Primary reformer effluent • •

A PURGE AMMONI TION TER TER ARA TER WA WA SEP PURIFICA . HIGH TEMP. LOW TEMP TER TOR ARA CO SHIFT CONVER SEP Y TER COOL CONVER SECONDAR REFORMER 3 NH Rapid multi-stream sampler (RMS) Magnetic sector analyzer H/N ratio Y TOR • • • PRIMAR REFORMER METHANA T HEA FUEL STEAM over the next five years. Close to 80% of the over the next five years. Close to 80% 2 TION

CO ABSORBER

2 CO TURAL GAS AIR NA DESULPHURIZA Figure 1. Typical ammonia process based on natural gas feedstock. pharmaceuticals. the conversion The first step in is or naphtha of natural gas, liquefied petroleum gas are in high into hydrogen. Because these raw materials use demand for many other applications including Introduction chemicals Ammonia is one of the most important industrial 3%in the world, with demand forecasted to grow annually production of ammonia produced globally is used for the component of . Ammonia is also an important for dyes and , fibers, , and intermediates Graham Lewis, Thermo Fisher Scientific, Winsford, Cheshire, United Kingdom Graham Lewis, Thermo Fisher Improving Process Control During Control Process Improving Using the Production Ammonia PRO Prima Scientific Thermo Mass Spectrometer Process

Methane slippage Shift reaction Steam-to- ratio Air requirement Key Words • • • • 2 Process Analytical Requirements to form hydrogen and : 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 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 from the air. The production of ammonia takes place over an 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 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. individual customer’s specific stream details. This performance will be demonstrated during start-up by a A set of typical performance specifications for the Prima trained Thermo Fisher Scientific service engineer. PRO mass spectrometer on ammonia process streams is shown in figure 3. It indicates an 8-hour analysis Analysis of Low-Level Carbon Monoxide precision (single standard deviation) based on an analysis The gas exiting the low temperature shift reactor time of 15 seconds. Thermo Scientific™ GasWorks contains minimal levels of carbon monoxide, typically software permits analysis optimization on a per stream between 0.1% and 0.5%. The analysis of trace levels basis, enabling selection of the most appropriate of carbon monoxide in the presence of percentage levels ‘speed versus precision’ setting depending on process of and presents a problem to all control requirements. Similarly, the most efficient peak process mass spectrometers because these spectra overlap measurements for each stream and the most appropriate with one another. In all other process streams containing display units (% or ppm) can also be selected. these three species, the CO levels are high enough to allow the use of fragment peaks. While it is possible in the low temperature shift, it is not measurable with the Improving H/N Ratio necessary precision. In this case, analysis of CO in the The Prima PRO mass spectrometer measures the low temperature shift by NDIR is recommended. It is easy H/N ratio to a relative precision of 0.05% (i.e. precision Application Note to integrate the NDIR CO analysis with the Prima PRO of 0.0015 on a ratio of 3). In figure 4, the blue trend mass spectrometer data: a connector is fitted to the RMS indicates the H/N ratio immediately after the mass to select the sample for the mass spectrometer to analyze spectrometer was installed while the red trend indicates and, at the same time, to divert the fast loop to the external the H/N ratio after implementing process control based NDIR analyzer. The Prima PRO mass spectrometer on data provided by the mass spectrometer. can also be used to provide a backup alarm for CO breakthrough. Using Mass Spectrometry Data to Control the Process The unique combination of magnetic sector stability, Figure 5. fast multi-stream switching and GasWorks quantitative Prima PRO Process Mass software ensures the process data produced by the Spectrometer service kit. Prima PRO mass spectrometer is accurate and reliable. A range of industry standard protocols enable transfer of this data to process control systems to optimize the ammonia process. Steam-to-Carbon Ratio A detailed evaluation of the Prima PRO mass Summary spectrometer was conducted by an accredited fuel gas calibration laboratory. The performance was The Prima PRO mass spectrometer offers the best benchmarked against a range of gas compositions in available online measurement precision and stability combination with an ISO 17025 accredited calibration for ammonia process monitoring and control. Its fault cylinder. The Prima PRO mass spectrometer provided tolerant design combined with extended intervals exceptional analytical performance, ensuring confidence between maintenance as well as simplified maintenance in the data. In addition, GasWorks software easily procedures ensure maximum availability of the analyzer. calculates the steam-to-carbon ratio with just two analog The plug-n-play standard service kit that is shipped inputs; one for the steam flow in weight per unit time with each analyzer is shown in figure 5. The reliability and the other for natural gas flow in weight per unit of the Prima PRO mass spectrometer is reflected in its time. Alternatively, the Prima PRO analyzer can simply industry-best three year parts and labor warranty. transfer the composition data to the DCS using one of its Advantages of the Prima PRO industry standard communication protocols. Process Mass Spectrometer • Optimizes processes, including: After start-up After implementation 3.05 - Gas mixing and burner control in reformer - Steam/carbon ratio 3.00 - Optimization of H/N ratio

2.95 - 0.05% RSD - Methane slippage to ±20 ppm accuracy 2.90 • Facilitates monitoring of: H/N Ratio 2.85 - H2S in natural gas - Inert gas build-up in synthesis loop 2.80 - Catalyst activity to schedule plant outages

2.75 0.00 0.50 1.00 1.50 2.00 2.50 3.00 Time

Figure 4. Improvements achieved in the H/N ratio using the Prima PRO Process Mass Spectrometer. thermoscientific.com/primapro

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