Natural Gas Processing Solutions Using the 700XA Gas Chromatograph

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1 Application Note Oil & Gas Natural Gas Processing Solutions Using the 700XA Gas Chromatograph Optimizing the efficiency and profitability of a natural gas processing plant requires responding to the changing composition of the inlet gas and adjusting the extraction of the various processes efficiently to account for market conditions. The natural gas processing applications for the 700XA gas chromatograph are designed to as preengineered solutions for the common applications in a fieldmounted analyzer to provide realtime feedback to the control system and the operators to enable tight control of product specifications and maximize the economic return of the facility. Application Overview Natural gas processing involves removing acidic gases such as H 2 S and CO 2 and water from the raw inlet gas, extracting the condensates to produce natural gas (commonly referred to as residue or sales gas ) and fractionating the condensate into various liquid products. The level of fractionation depends on the design of the plant and may only remove enough ethane for the condensate to be stable at ambient conditions ( stabilized condensate, NGL product, or YGrade ) or may fractionate to produce individual component products. Inlet Gas and Acid Gas Removal The composition of the inlet from the separator can vary considerably and depends on which wells are flowing, the effect of ambient conditions on the primary and inlet separator(s), and the operating state of the gathering network. Knowing the inlet gas composition will provide the basis for advanced process control models and also allow for the calculation of massbalance and recovery rates across the process. A significant issue for gas processing plants is the risk of twophase flow into the plant. Inlet gas flow measurement devices are designed for singlephase vapor flow only, and will generate large errors when measuring twophase flow that will result in incorrect massbalance and recovery calculations, and limit the implementation of advanced feedforward control models. Additionally, if Amine systems are used to remove acidic gases, liquid hydrocarbons entering the amine contactor will cause foaming that will result in decreased throughput and offspecification products. A Typical 700XA Gas Chromatograph Configured for a Natural Gas Processing Application For this specific application, refer to the Avoiding Amine Foaming Issues Utilizing a Gas Chromatograph application note for additional information. A 700XA C9+ with the Hydrocarbon Dew Point (HCDP) calculation features gas chromatograph can help mitigate Amine foaming by determining the HCDP of the inlet gas. As the inlet gas becomes richer, the HCDP will increase, and when the HCDP is equal to the flowing gas temperature liquid hydrocarbons will begin to form. By using the 700XA C9+ HCDP gas chromatograph to calculate the HCDP at the process pressure, an alarm can be generated before the HCDP reaches the process temperature and before liquids form (Figure 1).

2 This provides an early warning of twophase flow that operators and control systems can use to make control adjustments (such as slow down the flow through the primary separators to increase retention time and remove more condensate) and lower the risk of amine foaming and increase the accuracy of massbalance and advanced control algorithms. Figure 1 Calculating the Hydrocarbon Dew Point of the Inlet Gas to Provide an Early Warning of Liquid Hydrocarbon Formation. Ethane Recovery Most natural gas processing plants are designed for two modes of operation: ethane recovery or ethane rejection. The mode of operation is selected depending on the economics of the ethane and natural gas products markets. In ethane recovery mode, the goal is to remove as much ethane from the feed as possible to be sold as ethane product. The residue stream will have very little ethane, and the demethanizer bottoms will have relatively high levels of ethane. Temperature Process Temperature 2Phase Early Warning Alarm Twophase flow condition When the natural gas prices are high enough to make the ethane more valuable as a part of the sales gas, or when there is low market demand for ethane, the process will be operated in ethane rejection mode, where the residue stream mode, the residue stream will have high ethane levels controlled to the gas quality limits specified in the sales gas tariffs and demethanizer bottoms will have relatively low ethane levels. Process Pressure Trigger to take action to avoid HC liquids entering the contactor In both modes of operation, the DeEthanizer must still recover most of the ethane that is left in the DeEthanizer inlet, as any ethane that is remaining in the deethanizer bottoms will appear in the propane product or exported NGL as a contaminant. DeMethanizer The DeMethanizer is the first stage of fractionation that separates the natural gas ( sales gas, or residue ) from the condensates (NGL). For small plants, this is the only fractionation that occurs and a single gas chromatograph that measures the three streams (inlet, residue gas, and NGL) may be appropriate. In these cases, the IRN (InletResidueNGL) standard application may be appropriate (refer to the InletResidueNGL Solutions for Natural Gas Processing Applications application note for more details). On larger plants there is sufficient justification for dedicated analyzers to provide a faster feedback of composition to provide tighter process control. The variation of ethane in the residue gas is constrained by the sales gas tariff limits and can be accurately measured using the standard and C9+ applications used throughout the natural gas transmission segments. However, the demethanizer bottom composition can vary considerably from one plant to another and between rejection and recovery modes, so a customized application tuned to the expected composition is the most appropriate solution when a dedicated GC is required. The DeEthanizer Tower application (DET) is designed to analyze the inlet, ethane product, and deethanizer bottoms in a single GC to provide accurate online feedback to the operators and the control system. The Inlet stream analysis is configured to measure the wide variations in ethane concentration found in a plant that can switch between recovery and rejection modes. The ethane overhead is configured to measure the ethane and contaminants at the concentrations typically found in ethane purity specifications, and the deethanizer bottoms stream is configured for a wide range of NGL compositions. To ensure the ethane product meets the contractual specifications, the Ethane or EPS application (shown on Figure 2), is configured for a dedicated fast analysis of the ethane and the commonly specified contaminants. A typical ethane specification is > 95 % ethane, < 1 % Ethylene, and < 3.5 % propane or heavier hydrocarbons. As ethylene levels in raw natural gas are rarely high enough to result in off specification concentrations in natural gas processing, the EPS application measures the methane, ethane, propane and butanes and heavier hydrocarbons as well as carbon dioxide. Page 2

3 Figure 2 Typical process flow diagram of natural gas processing applications showing gas chromatograph installation locations Acid Gas Removal DeHydration DeMethanizer DeEthanizer DePropanizer Residue/ SalesGas Ethane Propane From Inlet Separator NGL Table 1 Specifications for the 700XA gas processing recommended applications. Applications C9+ HCDP IRN DET EPS PPS PPX Stream Description Inlet Sales Gas/ Residue Inlet Residue NGL DeEthanizer Overheads DeEthanizer Bottoms Ethane Standard Propane Standard Propane Extended Sample Phase Vapor Vapor Vapor Vapor Liquid Liquid (1) Liquid Liquid (1) Liquid Liquid Analysis Time 5 minutes 3 minutes < 10 minutes < 6 minutes < 5 minutes < 5 minutes < 5 minutes Nitrogen 0 10 % % 0 10 % Methane % % % 0 5 % % 0 10 % 0 1 % 0 1 % Carbon dioxide 0 10 % % 0 10 % 0 1 % (2) 0 1 % 0 5 % Ethane 0 20 % % 0 20 % 0 60 % % 0 5 % % % % Propane 0 10 % % 0 10 % 0 70 % 0 5 % 0 80 % 0 10 % % % Propylene 0 10 % Isobutane 0 5 % % 0 5 % 0 20 % 0 2 % 0 20 % 0 10 % 0 10 % nbutane 0 5 % % 0 5 % 0 20 % 0 2 % 0 20 % 0 5 % 0 5 % Isopentane 0 1 % % 0 1 % 0 10 % npentane 0 1 % % 0 1 % 0 10 % Hexanes % 0 1 % 0 10 % C % Heptanes Octanes Nonanes % % 0 10 % Notes: (1) DeEthanizer Overheads and Ethane are sampled as a liquid, vaporized at the sample point, and transported to the gas chromatograph as a vapor through heated lines. The sample conditioning system is for vapor samples. (2) The carbon dioxide is not included in the calibration gas and will be calibrated using a relative response factor. C % C % C % C % Page 3

4 Propane Recovery Many small plants will export the NGL to a fractionation facility after the ethane removal as stabilized NGL that is best analyzed with a customized gas chromatograph application tailored to the specifics of the feed gas and the process. Larger plants may recover the propane and must meet propane product specifications. Figure 4 Typical Liquid Phase Sample Conditioning System. A common industry specification is the A 2140 HD5 propane specification of > 90 % propane, < 5 % propylene, and < 2.5 % butanes and heavier hydrocarbons. Most processors do not need to continuously analyze the propylene levels, and to ensure a fast analysis, the standard Propane Purity application (PPS) measures methane, ethane, propane, ibutane, nbutane, and pentanes and heavier hydrocarbons. The extended propane purity application (PPX) adds the propylene measurement for locations that do need to continuously measure the propylene concentration. System Design The sample system design is critical to the performance of any analyzer system. For the vapor systems, particulate filters, liquid filters, checkvalves, and bypass flow rotameters are included as standard (Figure 3). Flow switches in the bypass flow can be included as an option. To ensure there is no contamination between the sample and the calibration streams, doubleblockandbleed solenoids are used. Figure 3 Typical Vapor Phase Sample Conditioning System. All sample injection valves vent through dedicated atmospheric referencing valves to enable the purging samples to be routed to a safe sample vent (such as to the flare, vapor recovery system, or back into the process) while enabling the sample loop to be referenced to atmospheric pressure prior to injection into the analytical flow path to ensure accurate and repeatable measurements. All of the vapor state sample conditioning components are mounted inside a stainless steel heated cabinet that can optionally include a viewing window for easy system checks. The sample systems can be mounted integral to the 700XA gas chromatograph, or mounted remotely. Liquid samples are initially conditioned through a liquid particulate filter with fast bypass loop and armored rotameter mounted externally to the main SCS enclosure (Figure 4). A very low volume side stream is passed into the vaporizing regulator mounted inside the heated SCS enclosure. The vaporized sample passes through a manual ball valve that allows for the manual calibration of the liquid analysis and then is injected into the GC oven through a dedicated sample injection valve in the analytical oven. The 700XA Recommended Solutions The 700XA recommended applications are designed with the heritage of remote and unattended operation that places the emphasis on reliability and ease of use over complexity. The hardware is designed to maximize reliability that results in several years between maintenance (and thus, downtime) on the analytical oven. The entire system is factory tested in an environmental chamber that ramps the temperature from 18 C to 55 C (0 F to 130 F) over 24 hours. This final test not only ensures that your gas chromatograph leaves the factory meeting the promised specifications; it also ensures peak performance onsite whether the 700XA is installed close to the process exposed to the ambient conditions, or installed in an enclosed shelter. Page 4

5 700XA Specifications and Drawings Please consult Rosemount Analytical if your requirements are outside the specifications listed below. Improved performance, other products and material offerings may be available depending on the application. Construction Environmental temperature: 20 to 60 C (4 to 140 F) Environmental temperature without safety certification: 40 to 60 C (40 to 140 F) Enclosure Protection Rating: IP66 Dimensions (without sample system): Wallmount: 711 mm H x 445 mm W x 498 mm D (28'' H x 17.5'' W x 19.6'' D) Pipemount: 711 mm H x 445 mm W x 671 mm D (28'' H x 17.5'' W x 26.4'' D) Floormount: 1532 mm H x 445 mm W x 612 mm D (60.3'' H x 17.5'' W x 24.1'' D) Corrosion Protection: GC Enclosure Material: Copper free aluminum coated with industrial grade powder coat suitable for high humidity and saltladen environments. Process Wetted Materials: Stainless steel. Where the function of an item excludes the use of stainless steel (e.g. glass rotameter tubes), materials that are resistant to corrosion are used. Electronics: All electronic circuit boards are tropicalized with a clear conformal coating. Mounting: Floorstanding (standard), wall or pipemount (optional) Approximate Weight (without sample system): 50 kg (110 lbs.) Area Safety Certification Options:* CSA: USA and Canada Class I, Division 1, Groups B, C, and D ATEX / IECEx Ex II 2G Ex d IIC Gb T6 (Ta = 20 C to 60 C) *Stated Tratings can vary based on applications. Performance Capabilities Oven: Airless, maximum 150 C (302 F) Valves: Sixport and tenport diaphragm chromatograph valves. Other types of valves, such as liquid injection or rotary valves, may be used depending on the application Carrier Gas: Applicationdependent. Typically zerograde helium, nitrogen, or hydrogen Sample & Calibration Gas Input Pressure Range: bar: bar (recommended) or 15 psig Carrier Gas Input Pressure Range (recommended): bar ( psig) Detector: Thermal conductivity detector (TCD), flame ionization detector (FID), TCD/TCD or TCD/FID dual detector configurations possible; flame photometric detector (FPD) available. Gating Options: Fixedtime, slope sensing gating of peaks Streams: Up to 20 externally controlled streams or up to 8 internal (includes calibration stream) Chromatograms stored/archived internally: Stores over 80 days of analysis report data and up to 2500 individual chromatograms. Electronics Power: Standard: 24V DC (21 30V DC) Optional: V AC, Hz Typical Power Consumption at 22 C (72 F): Startup: 105 Watts DC (125 Watts AC) Steady State: 35 Watts DC (40 Watts AC) Steady State with LOI: 50.5 Watts DC (58 Watts AC) Communications (Standard) Ethernet: Two ports one RJ45 and one fourwire with 10/100 mbps Analog inputs: Two standard isolated inputs filtered with transient protection, 4 20 ma (user scalable and assignable) Analog outputs: Six isolated outputs, 4 20 ma Digital inputs: Five inputs, user assignable, optically isolated, rated to 30V 0.5 A Digital outputs: Five userassignable outputs, Form C and electromechanically isolated, 24V DC Serial: Three termination blocks, configurable as RS232, RS422 or RS485 and one RS232 Dsub (9pin) Modbus/PC Connection Communications (Optional) Two expansion slots available for additional communications. Each slot has the capacity to add one of the following: Four analog inputs (isolated) card Four analog outputs (isolated) card Eight digital inputs (isolated) card Five digital outputs (isolated) card One RS232, RS422 or RS485 serial connection card One modem card, k baud Additionally, a FOUNDATION fieldbus module is available. Memory Capacity: 1 GB of flash memory for data storage; 128 MB of SDRAM system memory with 2 MB static RAM (batterybacked) Touch Key Local Operator Interface (Optional) The 700XA local operator interface (LOI) allows for maintenance and operation of a 700XA without a laptop or PC. The LOI is a stateoftheart high resolution color display that is touch key infrared activated and supports all core GC operations. Page 5

6 Recommended Installation The drawings below represent the minimum recommended installation guidelines for the 700XA Gas Chromatograph. Please consult Rosemount Analytical for detailed installation recommendations for your application. Figure 5 IRN OD Local Figure 6 IRN OD Remote Page 6

7 Option Selection Use the matrix below to choose the options for your application and build up the model string to order. 700XA SCS XA Level 1 Level 2 Level 3 Level 4 Level 5 Level 6 Level 7 Level 8 Level 9 700XA Process Gas Chromatograph Hazardous Area Approval A ATEX/IECEx C CSA rated Mounting F Floormount P Pipemount W Wallmount Power AC Universal 85 to 240V AC DC 24V DC Front Panel EN Extended switch panel 18 streams switches and FID ignite button (ignitor not used inless option XXYY are chosen on level 12) SK Local Operator Interface (LCD) SN Standard switch panel eight stream switches not available when an FID option is selected on Level 12 Additional* I/O and Serial/Modem Communications Slot A 5A Additional I/O Port Four Additional Channels of Analog Inputs 5D Additional I/O Port Eight Additional Channels of Digital Inputs 5M Modem 5N None 5P Additional I/O Port Four Additional Channels of Analog Outputs 5R Additional Serial Port (1) RS422 or 485 (Factory/Field configurable) 5S Additional Serial Port (1) RS232 5T Additional I/O Port Five Additional Channels of Digital Outputs Additional* I/O and Serial/Modem Communications Slot B 6A Additional I/O Port Four Additional Channels of Analog Inputs 6D Additional I/O Port Eight Additional Channels of Digital Inputs 6M Modem 6N None 6P Additional I/O Port Four Additional Channels of Analog Outputs 6R Additional Serial Port (1) RS422 or 485 (Factory/Field configurable) 6S Additional Serial Port (1) RS232 6T Additional I/O Port Five Additional Channels of Digital Outputs FOUNDATION Fieldbus Interface 1F Interface Module Sample System Mounting Location L Local Sample System Sample System mounted under GC, with tubing and signaling connections by RAI R Remote Sample Handling System Includes an external junction box for SHS signal wiring, attached to the 700XA. Signal Wiring and tubing connections are others. Gas Processing Application IRN Gas Processing Inlet Residue NGL (three streams) DET DeEthanizer Tower Overheads and Bottoms (two streams) EPS Ethane Purity PPS Propane Purity PPX Propane Purity Extended (with Propylene) SCS Level 1 Level 2 Level 3 Level 4 Level 5 Level 6 Level 7 Level 8 Sample Conditioning System for 700XA Gas Processing Applications Gas Processing Application SHS type IRN Inlet/Residue/NGL DET DeEthanizer Tower EPS Ethan Purity PPS Propane Purity PPX Propane Purity Extended (with Propylene) Hazardous Area A Local Sample System C CSA rated components Class I, Division 1, Groups B, C, D Mounting LS1 Local Sample System Sample System mounted under GC, with tubing and signaling connections by RAI RS1 Remote Sample Handling Sysem Includes external junction box, with signal wiring and tubing connection by others. Power A1 110V AC A2 220V AC Window PL Plexiglass Lexan Vapor Flow Switches V1 Single Vapor Flow Switch V2 Two Vapor Flow Switches V3 Three Vapor Flow Switches V4 Four Vapor Flow Switches Vapor Sample Entry H1 Single Vapor Heat Trace Boot H2 Two Vapor Heat Trace Boots H3 Three Vapor Heat Boots H4 Four Vapor Heat Trace Boots B ⅛ Bulkhead Fittings Liquid Flow Switches L1 Single Liquid Flow Switch L2 Two Liquid Flow Switch L3 Three Liquid Flow Switches L4 Four Liquid Flow Switches (*) Standard Communications 5 Di/ 5 DO/2 AI/ 6 AO (All Isolated and (3) RS422, RS422 or RS485 (Factory/Field configurable) serial ports. Vapor Sample Probe Vapor Sample Probe/Regulator Vaporizing Regulator Heating Sample Probe Liquid Sample Probe Heat Traced Sample Line Optional Accessories Provide line size, process connection details, process pressure and temperature. Provide line size, process connection details, process pressure and temperature. For remote mount at the sample point For vapor samples. To be mounted at the sample point. Provide line size and process connection details. ⅛ th Stainless steel heat traced sample tubing including termination kit. Specify length required. Page 7

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