Natural gas liquids recovery from gas turbine fuel

Transcription

1 Natural gas liquids recovery from gas turbine fuel By Simone Amidei, Francesca Monti, Riccardo Valorosi / GE Oil & Gas GE imagination at work

2 Natural gas liquids recovery from gas turbine fuel By Simone Amidei, Francesca Monti, Riccardo Valorosi / GE Oil & Gas Abstract With the increase in gas and oil fuel prices of recent years, there has been a growing interest in making existing equipment and processes more efficient and reliable, as well as in reducing wastes associated with many aspects of the oil and gas industry. GE Oil & Gas is continuously improving equipment performance and processes to optimize overall plant production, and rich gas fuel treatment is a good example of an increase in production associated with a reduction in emissions Introduction For plants in which gas turbines are used to drive centrifugal compressors with raw gas as the working fluid, the gas turbine fuel is generally supplied from the same gas that is processed by the centrifugal compressors. Raw natural gas typically consists primarily of methane (CH 4, the shortest and lightest hydrocarbon molecule. However, it also contains varying amounts of heavier gaseous hydrocarbons: Ethane (C2H 6 Propane (C3H 8 Normal butane (n-c4h 10 Isobutane (i-c4h 10 Pentanes and higher molecular weight hydrocarbons in traces When processed and purified into finished by-products, all of these heavier components are collectively referred to as NGL (Natural Gas Liquids. There are some parameters that must be taken into consideration when raw gas is used as a fuel for a gas turbine: notably, the Wobbe index, dew point, and heating value. The combination of these values influences the performance and the maintenance of the gas turbine, as well as the amount of CO 2 released to the atmosphere for a given shaft power. In transportation applications, the hydrocarbon dew point temperature in the gas might cause condensation at the pipeline pressure, forming liquids, which could damage the pipeline and influence the gas turbine performance and life. In fact, the gas in a pipeline quite often contains quantities of NGL that can be economically recovered with a simple system both to provide fuel at a given specification and to avoid burning this valuable NGL product in the gas turbine. There are various methods for recovery of NGL on a large scale. The most common is a low temperature separation process. In this process, the raw gas is chilled down to the required temperature where the heavy hydrocarbons are condensed. Then the liquids are separated from the lean gas in a knock-out drum. The application of this process normally requires external refrigeration and therefore energy, but thanks to the pressure ratio available between the pipeline and the fuel gas network, by installing a turboexpander-compressor unit 1

3 it is possible to achieve a high level of liquid recovery without the need for external power. Moreover, since the NGL can be re-injected into the main gas stream transported by the pipeline, an additional fractionation unit (demethanizer is not required. GE Oil & Gas has developed an auxiliary skid, referred to as FuelEx, for small scale NGL recovery based on the above process. This system allows combustion to be optimized while recovering NGL from the natural gas stream used as the fuel gas for the gas turbine, thereby also reducing the amount of CO 2 produced during combustion. How it works: The FuelEx skid recovers NGL by means of dew point control, separating C2+ components and returning them to the pipeline at the compressor discharge. The residual gas from the NGL recovery section is the final, purified sales gas, which is pipelined to the end-user markets or to a larger recovery plant for a final separation to provide NGL to the market. RECOVERY SKID Lean fuel gas NLG Raw gas Gas turbine figure 1 Concept block diagram Raw gas Compressor Raw gas The NGL recovery is accomplished with a simple Low Temperature Separation system (LTS that uses a GE Turboexpander equipped with Active Magnetic Bearings (AMB to avoid oil contamination of the turbine fuel gas. A dehydration system, based on molecular sieve technology, is included in the package if water is present. GE Oil & Gas has developed a process for regeneration of the driers without requiring the installation of additional compression and avoiding the loss of product (natural gas or liquid effluents in the plant. In addition, this approach makes it possible to selectively capture and return to the original raw gas stream other components, such as CO 2 or heavy hydrocarbons, that low temperature processes may not be suitable for. Finally, with this unit in service, it is possible to deliver a dry stream for the main compressor dry gas seal panel without any further treatment. The dehydration unit and the selection of expander/compressor technology with magnetic bearings allows the gas turbine to be constantly fed with the ideal fuel gas composition. The main functional characteristics of the FuelEx skid are: Assures the recovery of the NGL from the gas turbine fuel gas Assures the proper raw gas pretreatment (filtration and dehydration Supports remote operation (except for start-up Achieves maximum operating flexibility and simplified operation Avoids emissions and product losses (gas and liquids during normal operation, startup and normal stop System description for pipeline applications The system for this application is divided into two main sections: Dehydration section This is an option that depends on the pipeline gas quality. The gas from the compression station discharge header is dehydrated with molecular sieve absorption technology upstream of the chilling train. The regeneration gas is taken from the dry gas leaving the dehydrator, heated and then sent directly to the compression station suction header, where it is mixed with the main stream. Due to the small amount of 2

4 Lean fuel gas Rich fuel gas Dehydration unit (optional Dry gas Heat exchanger TEC NGL Vessel figure 2 Simplified diagram Pump (optional 5 14,000 NGL skids Heat exchanger Pressure kpa 12,000 10,000 8,000 6,000 4,000 2, ,000-94,000-92,000-90,000-88,000-86,000-84,000-82,000-80,000-78,000-76,000 Enthalphy kj/mol DEW Bubb Cooling Expansion Vap Liq Compressor Pump out NGL SIDE figure 3 Thermodynamic concept. Colors refer to the simplified diagram (Figure 2 regeneration gas that is used, the effect on the main stream is negligible. The selection of molecular sieves is influenced by the characteristics of the raw gas and contaminants to be treated. Low temperature separation section The dry gas at high pressure is chilled and expanded to a lower pressure where NGL liquids condense. After separation, the gas is recompressed to the desired pressure by the compressor directly coupled to the expander (load compressor. The system will balance the operating pressure downstream of the expander accordingly. The NGL produced is pumped back to the pipeline or to a designated receiving point, passing through a set of heat exchangers designed to chill the high pressure gas to the desired level. The same recovery is performed in a similar gas-to-gas exchanger where the clean gas is reheated before compression. The size of the exchangers has been defined to maintain the clean gas temperature and the gas turbine fuel temperature at suitable levels. This section can be adjusted on a case-by-case basis to meet other process requirements, if any. Dehydration unit (optional 1 Heat exchanger 2 Turboexpander figure 4 Whole skid configuration NGL reciprocating pump (optional Vessel 3 Raw gas (with NGL Lean fuel gas (without NGL NGL The core of the system is the GE EC series turboexpander unit. It is derived from a mechanical redesign of the casing based on the GE barrel compressor series. This design is more flexible for accommodating different types of rotors without replacing the casing and therefore, without impacting the skid layout. The new GE turboexpander line is designed and manuractured in the GE Oil & Gas Nuovo Pignone plant in Florence Italy and benefits from the experience gained from more than 2000 GE Oil & Gas turboexpander installations throughout the world. The expander-compressor has new features including: new Active Magnetic Bearing sizes for high speed application (65000 rpm new maintenance concept with plug in connectors and removable bundle new variable nozzle system with electric actuator flexibility to replace/upgrade the rotors in case of a future change in the main process 4 3

5 Fuel composition without NGL recovery Gas kg/h figure 5 FuelEx innovative turboexpander Nitrogen 45 CO 2 75 Methane 3,250 Ethane 435 Propane 270 n-butane 75 i-butane 60 n-pentane 11 i-pentane 12 n-hexane 11 Fuel composition with NGL recovery Gas kg/h Nitrogen 45 CO 2 75 Methane 3,900 Ethane 160 Propane 19 n-butane 1 i-butane 1 n-pentane 0 i-pentane 0 figure 6 Rotor details Application of FuelEx rich gas treatment The diagram above shows the diverse opportunities provided by rich gas treatment in pipeline, upstream and downstream applications to recover liquids and avoid burning these valuable products as part of the gas turbine fuel. In Pipeline applications, the NGL recovery skid can be used to: Treat fuel gas in the turbocompressor or process the natural gas in a distribution pipeline, recovering the NGLs and returning them to the main pipeline instead of consuming them as gas turbine fuel Mini-NGL recovery plant. A scaled up FuelEx solution (200MMscfd can be used to treat the entire pipeline gas stream, recovering the C2+ portion in a larger volume. It employs the same principles as n-hexane 0 Gas Total NGL recovery kg/h Ethane 275 Propane 251 n-butane 74 i-butane 59 n-pentane 11 i-pentane 12 n-hexane 11 figure 7 Example of NGL recovery in pipeline application FuelEx, but using a larger turboexpander, the natural gas can be treated to separate the C2+ portion in a larger volume Fractionate NGL into its individual components using non-proprietary or patented processes to optimize recovery 4

6 Main applications Transportation pipeline / gas gathering Onshore / offshore upstream plants Reinjection plants Gas-oil separation plants figure 8 Examples of FuelEx applications In upstream or downstream applications, the FuelEx concept can be used to recover NGLs that can be separated and mixed with other hydrocarbon streams or sent to another part of the process, saving them from wasteful burning. For more information Global Headquarters Via Felice Matteucci, Florence, Italy T F [email protected] Nuovo Pignone S.p.A. Americas Regional Headquarters 4424 West Sam Houston Parkway North Houston, Texas P.O. Box 2291 Houston, Texas T F ge.com/oilandgas The information contained herein is general in nature and is not intended for specific construction, installation or application purposes. GE reserves the right to make changes in specifications or add improvements at any time without notice or obligation General Electric Company All Rights Reserved In all these applications, a central purpose of FuelEx is to produce a fuel gas with the proper specifications. In the event that the gas is too rich to be burned as a gas turbine fuel, it is possible to separate C2+ to reach an almost pure methane fuel, which will also reduce CO 2 emissions. This application is very interesting above all for DLN/DLE gas turbines, which are very sensitive to C2+ components. This auxiliary system delivers increased benefits in going from a single gas turbine installation with a FuelEx skid to scaled up solutions for process optimization and entire plants. Solutions for fuel gas in pipeline applications have already been designed and are scheduled to be installed in Future studies involve scaled up solutions to treat distribution pipeline gas. 5