System and method for unloading compressed natural gas

Abstract

A system and method for unloading highly pressurized compressed natural gas from transport vessels by depressurizing the gas through flow lines linking a series of automated flow control valves that lower the gas pressure to a predetermined level, the valves being linked in series with and separated by heat exchangers in which the lower pressure gas flowing through the system is also reheated to a predetermined temperature by a heat exchange medium recirculation system in which the heat exchange medium is reheated by a heat source that can be internal to the system. The use of a minor portion of the depressurized and reheated gas as fuel gas to reheat the heat exchange medium is also disclosed. The subject system can be skid-mounted if desired.

Claims

1. A compressed natural gas unloading system for use in unloading highly pressurized compressed natural gas from a transport vessel, the system comprising gas flow lines enabling fluid communication between at least two automated flow control valves that are spaced apart in a flow direction and cooperate to lower the gas pressure to a predetermined level, at least one heat exchanger and a heat exchange medium recirculation system, wherein at least one heat exchanger is disposed between the at least two spaced-apart automated flow control valves and cooperates with the heat exchange medium recirculation system to heat gas flowing through the compressed natural gas unloading system to a predetermined temperature, wherein a gas flow line disposed between the at least two spaced-apart automated flow control valves is divided into two different flow paths that each pass through a different heat exchanger and are then recombined, and wherein a surge vessel is configured to be selectively placed in fluid communication with the recombined different flow paths.

2. A compressed natural gas unloading system for use in unloading highly pressurized compressed natural gas from a transport vessel, the system comprising gas flow lines enabling fluid communication between at least two automated flow control valves that are spaced apart in a flow direction and cooperate to lower the gas pressure to a predetermined level, at least one heat exchanger and a heat exchange medium recirculation system, wherein at least one heat exchanger is disposed between the at least two spaced-apart automated flow control valves and cooperates with the heat exchange medium recirculation system to heat gas flowing through the compressed natural gas unloading system to a predetermined temperature, wherein the heat exchange medium recirculation system comprises a contained heat exchange medium, at least one heater for the heat exchange medium, at least one pump, and a fluid recirculation loop configured to recirculate the heat exchange medium through the heater, pump and heat exchangers; and wherein the at least one heater is configured to be fueled by a minor portion of lower-pressure gas that is diverted to the at least one heater downstream of the last at least one heat exchanger.

3. The compressed natural gas unloading system of claim 2 wherein the minor portion of lower pressure gas that is diverted to the at least one heater downstream of the last at least one heat exchanger has a flow rate that is controlled by an automated flow control valve installed in a gas flow line to the heater.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The system and method of the invention are further described and explained in relation to the following drawings wherein:

(2) FIG. 1 is a simplified diagrammatic view of one embodiment of the apparatus of the subject system and is also illustrates an embodiment of the inventive method of the invention that is practiced when using the subject apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(3) System 10 and the method of the invention are further disclosed in relation to FIG. 1 of the drawings. Compressed natural gas (“CNG”) is received into system 10 through inlet valve 12 in flow line 14 at a transport pressure of about 3000 to about 4200 PSI. Pressure relief valve 88 is disposed between inlet valve 12 and heat exchanger 16 to protect system 10 from any gas source in which the gas pressure exceeds the maximum operational pressure for which system 10 is designed, such as, for example, about 4500 PSI. Heat exchanger 16 is desirably a shell and tube heat exchanger designed for use at pressures exceeding the operational pressures disclosed above.

(4) The CNG is desirably heated to about 105° F. in heat exchanger 16 and, upon exiting the heat exchanger, passes through flow lines 18, 24 and valves 20, 26 to be throttled from the inlet pressure to an intermediate pressure of 300 PSI in one or more automated flow control valves 22, 28. The number of throttle valves is determined by the maximum required flow rate measured in standard cubic feet per minute or hour and the lowest operational pressure of the outlet or delivery pressure. At a very high initial transport pressure and at low or “idle” flow rate the throttle valve trim is very small. As the transport gas pressure drops, the flowing gas density drops proportionally and the valve trim or effective orifice diameter must increase to allow for greater flow. As the transport pressure drops, the trim or orifice of first automated flow control valve 22 reaches a maximum open position and the first throttle valve is unable to flow additional volume at the upstream and downstream pressure differential. At this point, automated flow control valve 28 throttle valve begins to open and adds to the max flow rate through automated flow control valve 22. The percent of opening of the orifice in automated flow control valve 28 is adjusted automatically by sensing the downstream pressure and moving the diaphragm and stem of flow control valve 28, thereby either increasing or decreasing the effective temperature due to the Joule Thompson cooling of an expanding gas.

(5) Flow control valves 22, 28 are desirably configured (such as by the use of different orifices and stems, or by the use of other similarly effective components) with different trim levels that are inversely proportional to the preset cracking pressures. When so configured, a flow control valve 22 with a higher preset cracking pressure will desirably have a finer trim (crack open to a lesser extent) than a flow control valve 28 having a lower preset cracking pressure. For example, where the trim level of flow control valve 22 is ⅛ inch and the preset cracking pressure is set at 350 PSI, a representative trim level of flow control valve 28 is ½ inch and the preset cracking pressure of flow control valve 28 is at a lower value, such as about 325 PSI. The trim level and preset cracking pressure of a flow control valve to be used in a particular service within the system of the invention can be specified, reconfigured or adjusted as needed in relation to the inlet pressure and flow rate of the CNG.

(6) The gas flow as apportioned by automated flow control valves 22, 28, is then directed past pressure safety valve 90 and conventional temperature and pressure sensors by flow lines 30, 32 and enters a second set of shell and tube heat exchangers 34, 36. The length and diameter of heat exchangers 34, 36 is desirably determined by the residence time required to reheat the flowing gas based on the maximum gas flow capacity measured in pounds of flowing gas per hour.

(7) The flowing gas is heated to about 105° F. in heat exchangers 34, 36, and the gas exiting heat exchangers 34, 36 through flow lines 38 requires at least one more throttle to further drop the pressure of the flowing gas to its design outlet or discharge pressure (the pressure at which the gas can typically be delivered to an industrial user). Because the first throttling valve, automated flow control valve 22, senses the downstream pressure to adjust its percent of orifice opening, the next throttle valve, automated flow control valve 40, will be adversely affected by changes in the upstream pressure as a result of the pressure drop across the upstream valves 22, 28. This can cause potential feedback or “hunt” oscillations as the control system seeks to determine an effective percent of orifice opening. A large volume buffer such as buffer vessel 46 can be used to compensate for “throttle slams” and reduce and slow the changes in downstream pressure in automated flow control valve 40, allowing the valves trim time to adjust the percent of open in the trim orifice and the control valve. The volume of buffer vessel 46 is desirably sufficient to provide from about a 15 second to about a 60 second flow of gas to maintain the gas flow rate as flow control valves 22, 28 respond to sensed line pressures. Valve 42 and pressure relief valve 92 are desirably provided in flow line 44 to facilitate control and protect the system.

(8) Although only one automated flow control valve 40 is depicted in FIG. 1 for use between heat exchangers 34, 36 and 48, it should be appreciated by those of ordinary skill in the art upon reading this disclosure one or more valves operating similarly to automated flow control valves 22, 28 may be required to drop the pressure of the flowing gas to a level that is consistent with the predetermined delivery pressure and the maximum flow rate.

(9) After the pressure of the flowing CNG has been further reduced by one or more automated flow control valves 40, the flowing CNG is then desirably reheated to the predetermined final gas temperature. System 10 of the invention preferably comprises a heat exchange medium recirculation system comprising at least one heater such as gas-fired heater 62 and an optional supplemental heater 70, preferably including an inline electrical heater, at least one fluid recirculation pump 72, valves 64, 75, 76, flow lines 60, 74, 78, 80, 82, 84, controller 86, and other instrumentation as may be desired by those of ordinary skill in the art. It will be noted, for example, that temperature and pressure gauges are noted at various positions in FIG. 1 that are not specifically identified by reference numerals but are well known to those of skill in the art, who will be familiar with seeing them on piping and instrumentation drawings of this type. On the shell side of each heat exchanger 16, 34, 36 and 48, flow lines 78, 80, 82, 84, 60 are provided (conventional connections not shown) for use in recirculating a contained heated heat exchange medium through each of the heat exchangers in a direction that is countercurrent to the direction of the CNG flow.

(10) A thermocouple disposed near the tube-side outlet of heat exchanger 48 measures the gas temperature as it exits heat exchanger 48. The thermocouple desirably reports the exit temperature of the low pressure gas downstream of heat exchanger 48 to controller 86 that is desirably linked, together with automated flow control valves 22, 28, 40 and the various other temperature and pressure sensors shown in FIG. 1, to a programmable logic controller (“PLC”) 96 or other similarly effective controller that is either resident in or communicatively linked to other control elements of system 10 and is preprogrammed to monitor and adjust the settings of all automated devices in system 10 as discussed herein to achieve the intermediate pressures, temperatures and flow rates required to reduce the pressure of the inlet CNG to the predetermined gas delivery pressure and to reheat the pressurized gas to a desirable predetermined temperature.

(11) A preferred heat exchange medium for use in the shell and tube heat exchangers disclosed herein is a glycol water mix, typically a 50/50 mix of ethylene glycol and water. The heated glycol water mix is desirably circulated in a closed loop by an electrically driven centrifugal pump 72. An inline natural gas fired glycol heater is used to supply the majority if not all the heat to the glycol water mix. The natural gas fuel for the in line glycol heater is sourced from the natural gas stream exiting the system through gas flow line 50. A minor portion of the gas flow is desirably diverted through line 54, valve 56 and automated flow control valve 58 to supply fuel gas at an even lower pressure such as about 3 to 10 inches of water column (0.25-0.36 PSI) to heater 62, thereby using a very small percentage of the delivered gas to reheat the offloaded gas as the gas pressure is reduced during the unloading process.

(12) In the embodiment depicted in FIG. 1, the heat output of secondary heater 70, preferably electric powered, can be adjusted by controller 86 or by PLC 96 to increase or decrease the wattage of electricity or BTU's of gas burned in heaters 70, 62, respectively, to heat the heat exchange medium contained in the closed loop recirculation system sufficiently to reheat the recirculating heat exchange medium to a desired temperature.

(13) The in line electrical heater is controlled by a thermostat sensing the final gas temperature. The thermostat varies the wattage in the electrical heating element, thereby increasing or decreasing the final gas temperature to achieve the desired or selected flowing gas temperature. Where very fine control of the final gas temperature is required, such as plus or minus 1° F., the fast response time of the inline electrical heater is used. In some applications where final gas temperature has a broad range, the electrical heater can be eliminated.

(14) The thermocouple is desirably communicatively linked to a PLC 96 or a process control thermostat that pulses or varies the wattage powering the inline electric heater. (In a conventional process control the heater would measure the temperature of the glycol water mix and vary the wattage to maintain a preset fluid temperature similar to a hot tub thermostat control circuit. The distinction here is the thermostat is reading the flowing gas temperature and then varying the wattage to control the gas temperature not the glycol water mix temperature. There is a thermocouple thermostat that interlocks with the power supply to the inline electrical heater that prevents the inline heater from overheating the glycol mix. Should the temperature interlock reach a preset maximum safe glycol temperature, regardless of the final gas flowing temperature, the power is removed from the inline electrical heater.

(15) Other alterations and modifications of the invention will likewise become apparent to those of ordinary skill in the art upon reading this specification in view of the accompanying drawings, and it is intended that the scope of the invention disclosed herein be limited only by the broadest interpretation of the appended claims to which the inventor and/or Applicant are legally entitled.