Patent Application
20090282863
The present invention relates to a method of producing NGL's at gas Pressure Reduction Stations when the pressure is letdown from gas main transmission lines to local gas distribution lines.
In gas Pressure Reduction Stations, the gas is pre-heated before the pressure is dropped to prevent the formation of hydrates which can cause damage to the pipeline and associated equipment. The typical pressure reduction varies between 400 to 900 PSIG (pounds per square inch gage) for main transmission gas lines to local distribution lines and from 50 to 95 PSIG from local distribution lines to consumers. When gas is depressurised the temperature drops. The rule of thumb is that for every 100 pounds of pressure drop across a pressure reducing valve the gas temperature will drop by 7 F. When the pressure is reduced by the use of an expander, the temperature drop is greater because it produces work. The heat required to prevent formation of hydrates is normally provided by hot water boilers, gas fired line heaters or waste heat from; gas turbines, gas engines or fuel cells. In some stations, due to its large volumetric flows and pressure drops, energy can be and is recovered, by a combination of gas expander and boiler. For a more efficient recovery, combinations of gas expanders with CHP processes (Combined Heat and Power) or CCHP (Combined Cooling Heat and Power) processes are possible. The limitation in these applications are the economics which are driven by flow volumes, pressure delta, seasonal volumetric flows and 24 hour volumetric flows. Because of so many variables that impact on the economics of adding a gas expander be it with: a boiler, CHP or CCHP the current gas pipeline operators choose to pre-heat the gas by the use of boilers and or heaters. In all of the above practices, there is no attempt made to recover NGL's present in the natural gas stream at Metering and Pressure Reduction Stations. The typical practice is to have large facilities upstream in the transmission line known as Straddle Plants which recover a percentage of the NGL's for feedstock to the petrochemical industry.
According to the present invention there is provided a method to remove water present in the gas stream, produce NGL's and then pre-heat the gas to meet pipeline specifications. This method recovers NGL's, removes water and eliminates the present practice of using natural gas as a fuel for; boilers, heaters, gas turbines, gas engines or fuel cells to pre-heat the natural gas before pressure reduction. Moreover, the present invention provides the ability to recover most of the energy available for recovery at pressure reduction stations. A first step has at least one heat exchanger, with a first flow path for passage of incoming high pressure gas that indirectly exchanges heat with a counter current lower pressure cold gas stream. The low pressure cold gas stream flow can be controlled to meet desired temperatures in the high pressure gas stream through the use of a by-pass around the heat exchanger. The now cold high pressure gas enters a vessel separator, where water is removed. A second step involves passing the high pressure cold and water free gas stream through a gas expander, dropping the pressure to local distribution pipeline spec generating shaft work and a further drop in temperature. The shaft rotates a power generator producing electricity and the lower pressure colder gas enters a separator where NGL's are recovered. The objective being to control the temperature upstream of the gas expander to meet the desired NGL's recovery. The third step involves the use of the generated electricity as an heat source to the heat exchanger that controls the gas supply temperature to the local distribution pipeline. This eliminates the existing practice of combusting natural gas to pre-heat the gas to prevent the formation of hydrates. The fourth step involves the use of air exchangers to release part or all of the cold energy to the surroundings, this provides the ability to export electricity at warm atmospheric conditions.
These and other features of the invention will become more apparent from the following description in which reference is made to the appended drawings, the drawings are for the purpose of illustration only and are not intended to in any way limit the scope of the invention to the particular embodiment or embodiments shown, wherein:
The typical method that presently is used to pre-heat natural gas at Pressure Reduction Stations will now be described with reference to
In this typical gas pre-heating process, gas enters a station via gas supply line 1. The gas stream enters filter 20 to remove any debris in the stream. The filtered gas exits the filter through line 2 and enters heat exchanger 21 for pre-heating. The heated gas exits through line 3 and the pressure is reduced at Pressure Reducing Valve (PRV) 22. A by-pass with PRV 23 is provided for service reliability, for scheduled and unscheduled maintenance. The PRV pressure is controlled by Pressure Transmitter (PT) 27 at a pre-set pressure. The low pressure controlled gas stream 4 feeds a gas slipstream 5 for combustion in a heater/boiler 24. The gas slipstream flow 5 is controlled by Temperature Controller (TC) 26 at a pre-set temperature. The gas stream 6 is metered at Flow Meter (FM) 25 and delivered to consumers.
The preferred embodiment will now be described with reference to
The low pressure cold gas in line 10 flows into separator 56 where NGL's are separated and recovered. The NGL's exit through line 11. The lean cold gas exits the separator through line 12 and can be routed through line 13 and line 15 to meet desired operations temperatures. The lean gas stream in line 13 enters an air exchanger 57 where the cold energy is dissipated into the atmosphere by natural draft, the amount of cold energy dissipated to the atmosphere is dependent on the choice and objectives of the local plant. The lean stream exits air exchanger 57 through line 14 at near atmospheric temperatures. The warmer lean gas stream 14 can be blended through line 16 or line 18 to meet desired operations temperatures. The lean and cold gas stream in line 15 can be sent directly or blended with stream 16 and sent to heat exchanger 51 to cool in a counter current flow the incoming high pressure rich gas stream. The lean depressurized gas exits heat exchanger 51 through line 19 and blends with stream 18 into stream 20. The blended stream 20 enters line 4 and is routed to heater 58 to increase the lean gas temperature to local distribution pipeline specifications. The heat is supplied by the power generator 55 and transmitted through electrical wires 23 to the heating elements in heater 58. The heated lean gas in line 21 is measured in meter 59. A temperature controller 60 controls the heat supplied to heater 58. A pressure controller 61 controls the pressure to the local distribution pipeline 22.
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The preferred embodiment in
