The present invention relates generally to delivery systems for cryogenic liquids and, more particularly, to a system for dispensing and accurately metering a cryogenic liquid, such as liquid natural gas, to a use device.
Liquid natural gas (LNG) is a plentiful, environmentally friendly and domestically available energy source and, therefore, is an attractive alternative to oil. As a result, LNG is increasingly being used as a fuel for vehicles. This is especially true for fleet and heavy duty vehicles.
A key issue in the commercialization of LNG is the ability to accurately meter and dispense it. The National Institute of Standards and Technology of the United States Department of Commerce has developed guidelines for federal Weights and Measures certification whereby dispensed LNG must be metered on a mass flow basis with an accuracy of plus or minus 1.5% for the quantity of product dispensed. Given the potential for widespread use of LNG, a great need exists for a LNG dispensing system that is capable of Weights and Measures certification.
The mass flow of a liquid can be determined by measuring its volumetric flow and then applying a density factor for the liquid. Such an approach is utilized by the cryogenic liquid dispenser system disclosed in U.S. Pat. No. 5,616,838 to Preston et al. The dispenser disclosed in the Preston et al. ""838 patent features a meter and a temperature sensor submerged within LNG that partially fills a sump. LNG from the sump is dispensed through the meter and the sump is supplied from a bulk supply tank so that the LNG in the sump is maintained at the proper level. The volumetric flow rate from the meter and the temperature from the sensor are supplied to a microprocessor whereby the mass flow rate of the dispensed LNG may be calculated.
The accuracy of the system of the Preston et al. ""838 patent is limited, however, due to the fact that LNG is made up of many chemical components. More specifically, while the methane content of LNG is typically well above 90%, the balance includes substances such as ethane, propane, butane, nitrogen, hydrogen, carbon monoxide, oxygen and sulfur. As a result, the density of LNG cannot be determined with a high degree of accuracy simply by the conventional temperature correlations, which are based upon an approximation of the LNG composition.
The Preston et al. ""838 patent also discloses that a pair of submerged pressure sensors may be substituted for the sump temperature sensor and that the pressure differential measured thereby may be used by the microprocessor in combination with the volumetric flow rate to determine density. Such an arrangement, however, presents stability issues in that the signals provided to the microprocessor by the pressure sensors have proven to be erratic.
The use of capacitors for measuring the dielectric of a cryogenic liquid, and the use of this data for calculating the density of the liquid, is also known. Systems employing this approach are disclosed in U.S. Pat. Nos. 3,933,030 to Forster et al. and U. S. Pat. No. 4,835,456 to Lu et al. The system of the Forster et al. ""030 patent requires a redundancy of capacitance probes and fails to indicate the purity of the dispensed product. Furthermore, the system determines the density of the product being dispensed from the measured dielectrics based solely upon an approximation of the Clausius-Mosotti constant for the product. As such, the system of the Forster et al. ""030 patent fails to provide density measurements that are adjusted to compensate for variations in the purity of the product being dispensed. This disadvantage adversely impacts the accuracy that is obtainable with the system.
The system of the Liu et al. ""456 patent uses a number of complex calculations to obtain the density of the product being dispensed from measured dielectrics for the product. More specifically, the system of the Liu et al. ""456 patent implements a rigorous application of molecular dielectric theory using a dielectric susceptibility function in the application of the Clausius-Mosotti formula and the quantitation of a susceptibility parameter. The approach of the system, however, requires sophisticated, complex and expensive measurement and computational equipment.
Accordingly, it is an object of the present invention to provide a cryogenic liquid dispensing system that meters the amount of product dispensed on a mass flow basis.
It is another object of the present invention to provide a cryogenic liquid dispensing system that can accurately meter mixtures of cryogenic liquids.
It is another object of the present invention to provide a cryogenic liquid dispensing system that meters the amount of product dispensed with high enough accuracy that the system may be federal Weights and Measures certified.
It is still another object of the present invention to provide a cryogenic liquid dispensing system that provides an indication when the purity of the cryogenic liquid that is to be dispensed falls below a predetermined level.
It is still another object of the invention to provide a cryogenic liquid dispensing system that may be constructed with low complexity and cost.
The present invention is directed to a cryogenic liquid dispensing system for use, for example, in dispensing LNG to a vehicle. The system includes a bulk storage tank containing a supply of LNG. A sump is in communication with the bulk storage tank and contains a volumetric flow meter submerged in LNG to avoid two-phased flow through the meter. A temperature probe and a capacitor are also submerged in the LNG in the sump. The meter communicates with a dispensing hose via a dispensing line that includes a dispensing valve. A drain line bypasses the dispensing valve and features a check valve so that LNG trapped in the hose after dispensing is forced back into the sump due to an increase in pressure as a result of ambient heating.
A microprocessor communicates with the meter, dispensing valve, temperature probe and capacitor and contains dielectric data for pure methane over a range of LNG temperatures. The microprocessor uses the temperature from the temperature probe to select a corresponding dielectric for pure methane. This dielectric is compared with the dielectric measured by the capacitor and, if the difference is outside of a predetermined range, the LNG is considered too impure and is not dispensed. In one embodiment of the system, the microprocessor is also programmed with an approximate linear relationship between density and dielectric for LNG. The dielectric measured by the capacitor is used with the relationship to determine the density of the LNG.
Alternatively, the microprocessor may contain density data for pure methane over a range of LNG temperatures and an algorithm for computing a density compensation factor that is a function of the dielectric measured by the capacitor and the dielectric for pure methane. The microprocessor uses the temperature from the temperature probe to obtain a density for pure methane. The density compensation factor is calculated and applied to the density for pure methane to arrive at the density for the LNG.
In a further embodiment of the system, a compensating meter is substituted for the capacitor. The resulting two meters have different equations for relating mass flow to density. As a result, the equations may be solved to determine the mass flow and density of the LNG. The density of pure methane at the temperature of the LNG and the measured density of the LNG may be compared to determine the purity of the LNG.