Abstract:
A method and a system for supplying pressurized gas from a liquefied-gas (LG) storage tank, the method including the steps of: (a) providing a system including: (i) a storage tank for storing liquefied gas, the tank having a lower liquid region and a vapor region thereover; (ii) a heat exchanger external to the storage tank; (iii) pumping means driven by a liquid; (iv) a line for directly transferring the vaporized gas from the heat exchanger to the consumer; (b) pumping at least a portion of the liquefied gas from the lower region to the heat exchanger using the pumping means; (c) heating the liquefied gas in the heat exchanger to produce the vaporized gas; and (d) supplying the vaporized gas directly to the consumer, according to consumer demand.

Description:
This is a continuation-in-part (CIP) of U.S. patent application Ser. No. 09/674,700, filed Nov. 6, 2000 now U.S. Pat. No. 6,470,690. 
    
    
     FIELD AND BACKGROUND OF THE INVENTION 
     The present invention relates to a method and a system for supplying pressurized gas from a liquefied-gas (LG) storage tank. 
     LG systems are widely used in residential, agricultural, and industrial settings and they are expected to be a reliable source of energy, to operate safely, continuously, and to constantly supply guaranteed output. One critical performance criterion of LG systems is the delivery of a constant, stable and reliable flow of vaporized gas to the burners. 
     Some commonly used systems and methods of vaporization employ over-capacity storage tanks and vaporizers. The over-capacity storage tanks are expensive and provide inconsistent vapor pressure, especially during extreme ambient conditions. They also waste space, call for surplus gas stock and unnecessarily large and expensive storage area. 
     In one known system, the heat of vaporization is supplied by convection with respect to the ambient heat. However, this requires large heat-convection surfaces, according to the demand for the vaporized gas. Moreover, such systems are incapable of delivering gas at pressures exceeding that of the tank pressure. 
     In another known system, an external vaporizer is used to heat and vaporize the liquefied gas, with the vaporized gas being recirculated to the storage tank. The vaporized gas supplied to the consumer is delivered via a separate line connected to the storage tank, according to consumer demand. The recirculation of vaporized gas requires a large installation, and correspondingly high investment cost and maintenance expenses. Alternatively, the vaporized gas can be delivered directly to the consumer. However, various mechanical and control-related failures, as well as reduced consumer demand, can cause liquid-phase liquefied gas to be introduced to or to be condensed in the consumer delivery line. Liquid-phase liquefied gas in the vaporized gas is a major problem for many consumers. Consequently, such a process scheme generally requires the addition of vapor/liquid separation equipment for providing the consumer with solely vaporized gas. This equipment increases system complexity, size and cost, and introduces additional reliability and safety problems. 
     In my pending U.S. patent application Ser. No. 09/674,700, which is hereby incorporated for all purposes as if fully set forth herein, a system and method are disclosed in which the liquefied gas is circulated through an external heat exchanger and returned to the storage tank as a heated liquid. The sensible heat of the heated liquefied gas provides all of the requisite heat for vaporizing the gas (or at least a substantial portion thereof), which occurs within the storage tank. The vaporized gas is delivered via a separate vapor line connected to the storage tank, according to consumer demand. 
     The above-described system has numerous advantages with respect to other known systems. However, the storage tank operates under equilibrium or near-equilibrium conditions, such that the vapor produced is substantially saturated vapor. Consequently, any cooling that occurs in the supply line results in condensation of a portion of the vaporized gas. The system is not particularly suitable for high-pressure applications, because the entire system, including the storage tank and lines, must be pressurized according to the demand pressure. This means that the entire system operates under high pressure, a safety issue that preferably should be avoided. 
     There is therefore a recognized need for, and it would be highly advantageous to have a method and a system for supplying vaporized gas on consumer demand by producing superheated gas, thereby guaranteeing a reliable flow in any weather conditions and allowing for an improved fire safety than in systems known heretofore. It would be of further advantage if such a system would be simple and energy-efficient. 
     SUMMARY OF THE INVENTION 
     The present invention is a method and a system for supplying pressurized gas from a liquefied-gas (LG) storage tank. According to one aspect of the invention, the method includes the steps of: (a) providing a system including: (i) a storage tank for storing liquefied gas, the tank having a lower liquid region and a vapor region thereover; (ii) a heat exchanger external to the storage tank; (iii) pumping means driven by a liquid; and (iv) a line for directly transferring the vaporized gas from the heat exchanger to the consumer; (b) pumping at least a portion of the liquefied gas from the lower region to the heat exchanger using the pumping means; (c) heating the liquefied gas in the heat exchanger to produce the vaporized gas; and (d) supplying the vaporized gas directly to the consumer, according to consumer demand. 
     According to another aspect of the present invention there is provided a system for supplying vaporized gas to a consumer, including: (a) a storage tank for storing liquefied gas, the tank having a lower liquid region and a vapor region thereover; (b) a heat exchanger external to and fluidly communicating with the storage tank, within which heat exchanger, a heat exchange liquid and the liquefied gas come into indirect heat-exchange relation; (c) pumping means, driven by a liquid, for pumping at least a portion of the liquefied gas from the storage tank through the heat exchanger; (d) a gas supply line, operatively connected to the heat exchanger, for directly transferring the vaporized gas from the heat exchanger to the consumer upon demand; and (e) a control system associated with the heat exchanger and configured to control a rate of heat supply to the liquefied gas so as to vaporize the liquefied gas. 
     According to further features in the described preferred embodiments, the vaporized gas is a superheated gas. 
     According to still further features in the described preferred embodiments, the vaporized gas produced in step (c) is supplied directly to the consumer. 
     According to still further features in the described preferred embodiments, the liquid for driving the pumping means is used for heating the liquefied gas in step (c). 
     According to still further features in the described preferred embodiments, the liquid is heated externally to the system of step (a). 
     According to still further features in the described preferred embodiments, the method further includes heating the line to prevent condensation of the vaporized gas. 
     According to still further features in the described preferred embodiments, the heat for heating the line is provided by the liquid from the pumping means. 
     According to still further features in the described preferred embodiments, the liquid includes water. 
     According to still further features in the described preferred embodiments, the liquid consists of water. 
     According to still further features in the described preferred embodiments, the method further includes the step of (e) controlling a flow of the liquid so as to reduce an amount of liquid-phase gas in the line. 
     According to still further features in the described preferred embodiments, the controlling includes reversing the flow of the heat-exchange liquid. 
     According to still further features in the described preferred embodiments, the controlling includes increasing a temperature of the heat-exchange liquid. 
     According to still further features in the described preferred embodiments, the control system is configured to control a rate of heat supply to the liquefied gas so as to superheat the liquefied gas. 
     According to still further features in the described preferred embodiments, the system is designed and configured such that the heat exchange liquid is used as the liquid for driving the pumping means. 
     According to still further features in the described preferred embodiments, the system further includes: (f) heating means for heating the gas supply line so as to prevent condensation of the vaporized gas. 
     According to still further features in the described preferred embodiments, the heating means includes a line of the liquid for driving the pumping means, and the gas supply line and the heat-exchange liquid line are configured in heat-exchange relation. 
     According to still further features in the described preferred embodiments, the pumping means includes a hydraulic motor, connected to the pumping means, and driven by the liquid. 
     According to still further features in the described preferred embodiments, the pumping means includes a pump selected from the group consisting of a vane pump, an impeller pump, and a gear pump. 
     According to still further features in the described preferred embodiments, the heat exchanger and the pumping means are solely mechanical, such that any electrical components are disposed remotely from the storage tank. 
     According to still further features in the described preferred embodiments, the line of the liquid for driving the pumping means is disposed in side-by-side heat-exchange relation with the gas supply line. 
     According to still further features in the described preferred embodiments, the line of the liquid for driving the pumping means is disposed in coaxial heat-exchange relation with the gas supply line. 
     According to still further features in the described preferred embodiments, the system further includes (f) an auxiliary heat exchanger, operatively connected to the gas supply line, and disposed in series and downstream with respect to the heat exchanger, for ensuring full vaporization of the liquefied gas. 
     According to still further features in the described preferred embodiments, the auxiliary heat exchanger is heated by the heat exchange liquid. 
     According to still further features in the described preferred embodiments, the control system is further designed and configured to change a flow direction of the heat exchange liquid. 
     According to still further features in the described preferred embodiments, the control system is further designed and configured to change a flow direction of the heat exchange liquid in response to an indication from at least one indicator, such that fully vaporized gas is delivered to the consumer. 
     According to still further features in the described preferred embodiments, the control system is designed and configured to increase a temperature of the heat exchange liquid in response to an indication from at least one indicator, such that fully vaporized gas is delivered to the consumer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is herein described, by way of example only, with reference to the accompanying drawings. With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice. 
     In the drawings: 
     FIG. 1 is a block diagram illustrating a system for supplying vaporized gas according to the parent application, U.S. patent application Ser. No. 09/674,700, and 
     FIG. 2 is a block diagram illustrating one embodiment of a system for supplying vaporized gas according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The principles and operation of the system in the invention according to the present invention may be better understood with reference to the drawings and the accompanying description. 
     Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawing. The invention is capable of other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting. 
     As used herein in the specification and in the claims section that follows, the term “liquid” refers to a non-explosive and, preferably, a non-inflammable liquid. 
     Referring now to the drawings, FIG. 1 is a block diagram illustrating a vaporization system  10  for vaporizing gas according to my parent application, U.S. patent application Ser. No. 09/674,700. A storage tank  20  contains gas, such as propane or butane, in liquefied form, in a lower region  22 . Upper region  23 , which is in fluid contact with lower region  22 , contains gas in vapor form. Liquefied gas from lower region  22  is circulated, preferably by a water-driven turbine pump  40 , through a heat exchanger  50 . The liquefied gas is heated in heat exchanger  50 , the heating being controlled such that the liquefied gas is returned in liquid form to storage tank  20 . The sensible heat of the heated liquefied gas provides all of the requisite heat for vaporizing the gas, or at least a substantial portion thereof. The vaporized gas, produced in storage tank  20 , is delivered to a consumer  100 , according to demand, via a separate vapor line  70  connected to storage tank  20 . 
     Heat exchanger  50  is a surface heat exchanger typically using water as the heat exchange liquid. The water, which is heated and pressurized externally to vaporization system  10 , is introduced to heat exchanger  50  via line  36 . The water exiting heat exchanger  50  can also serve to drive water-driven turbine pump  40 , before leaving system  10  via line  32 . 
     Although, in many respects, the above-described system is advantageous relative to the art known heretofore, there exist several distinct disadvantages, including: 
     1) the storage tank operates under equilibrium or near-equilibrium conditions, such that the vapor produced is substantially saturated vapor. Consequently, any cooling that occurs in the supply line results in condensation of a portion of the vaporized gas, a known problem in the art; 
     2) the entire system, including the storage tank and lines, is pressurized according to the demand pressure. In high-pressure gas delivery applications, this means that the entire system must operate under high pressure, a safety issue that preferably should be avoided. 
     By sharp contrast, the inventive system and method produce superheated gas. Moreover, the storage tank can operate at low/ambient pressure, as desired, even when pressure vaporized gas is demanded by the consumer. 
     FIG. 2 is a block diagram illustrating a system for supplying vaporized gas, according to the present invention. System  110  includes a storage tank  112  containing gas, such as propane or butane, in liquefied form within a lower region  114  and in a vapor form within an upper region  116 . Liquefied gas from lower region  114  is pumped, preferably by a water-driven turbine pump  140 , via line  145 , through a heat exchanger  150 . The heating in heat exchanger  150  is controlled so as to completely vaporize and, preferably, to superheat the gas. The superheated gas exits heat exchanger  150  through line  149 , for delivery to a consumer  100 , according to demand. 
     Heat exchanger  150  is a surface heat exchanger that typically uses water as the heat exchange liquid. The water, which is heated and pressurized externally to vaporization system  110 , is introduced to heat exchanger  150  via line  130 . The water exiting heat exchanger  150  via line  163  can also serve to drive water-driven turbine pump  140 , before leaving system  110  via line  172 . 
     Pump  140  is driven via a mechanical linkage by a hydraulic motor or turbine, which recover mechanical power from the water flowing through line  163 . Pump  140  may be of various types known to those skilled in the art, including but not limited to vane pumps, impeller pumps, and gear pumps. 
     Although in the description, the water pumped by pump  132  is introduced to heat exchanger  150  prior to water-driven turbine pump  140 , it is possible to pass the water through pump  140  before introducing the water to heat exchanger  150 . 
     The vaporized gas exits heat exchanger  150  through line  149 . The vaporized gas discharged from heat exchanger  150  is superheated so as to substantially prevent condensation of the gas en route to consumer  100 . Preferably, line  149  is insulated. Optionally, line  149  is heated to further inhibit condensate formation in the vaporized gas being delivered to consumer  100 . 
     In a presently-preferred embodiment, line  172 , containing hot, spent water, is disposed in heat-exchange relation with line  149  (e.g., side by side piping, coaxial piping, etc.), such that heat is transferred to line  149  and condensation is prevented. Consequently, inventive system  110  does not require additional equipment such as condensate traps or separators, and condensate return lines. Moreover, a single hot water supply, delivered by pump  132 , serves three functions: powering of water-driven turbine pump  140 , supplying heat to heat exchanger  150 , and heating line  149 , which delivers vaporized gas to consumer  100 . In addition to eliminating or at least drastically reducing the presence of liquid-phase gas in the gas delivered to the consumer, this design and configuration allow all electrical components in system  110  to be set at a distance from high-pressure, gas-containing lines and equipment. 
     It must be emphasized that various designs and configurations for achieving the requisite heat-exchange relation will be apparent to those skilled in the art. 
     It is known that various mechanical and control-related failures, as well as reduced consumer demand, can cause liquid-phase liquefied gas to be introduced to or to be condensed in consumer delivery line  149 . Liquid-phase liquefied gas in the vaporized gas is a major problem for many consumers, and is also associated with various safety problems. Consequently, such a process scheme generally requires the addition of vapor/liquid separation equipment for providing the consumer with solely vaporized gas. In a preferred embodiment, shown in FIG. 2, system  110  includes an auxiliary heat exchanger  155 , the main purpose of which is to further improve system safety and reliability, as will be explained hereinbelow. Auxiliary heat exchanger  155  is disposed in series and downstream with respect to heat exchanger  150 , towards consumer  100 . Auxiliary heat exchanger  155  is preferably heated by a hot water supply delivered by pump  132 . 
     In the event that liquid-phase liquefied gas accumulates in consumer delivery line  149  between heat exchanger  150  and auxiliary heat exchanger  155 , the liquid-phase gas is evaporated in auxiliary heat exchanger  155  using the enthalpy supplied by the hot water supply (delivered by pump  132 ). 
     Preferably, the design and sizing of heat exchanger  150  and the control of system  110  are such that heat exchanger  150  vaporizes all of the gas feed during normal working conditions. Hence, auxiliary heat exchanger  155  transfers a relatively small portion of heat to the gas that is being heated for supply to the consumer. During severe weather conditions, and more importantly, in the event of a malfunction in heat exchanger  150  (e.g., a sticking float control that allows liquid-phase liquefied gas to enter and pass through heat exchanger  150  to consumer delivery line  149 ), auxiliary heat exchanger  155 , which is disposed on-line and is in a hot, working state, automatically delivers the requisite heat for vaporizing and superheating any and all liquid-phase liquefied gas passing through heat exchanger  150  and/or condensing in line  149  between heat exchangers  150 ,  155 . 
     The control of system  110  is by no means trivial. The design criteria include: 
     Delivery of exclusively vapor-phase gas to the consumer 
     Substantially 100% reliability 
     Minimization of safety risks associated with electrically-powered components in the vicinity of a system containing inflammable or explosive gases 
     Quick response to consumer demand 
     Ability to cope with periods in which there is no consumer demand 
     In one embodiment, controller  200  receives measured data from temperature indicator  202  and pressure indicator  204 , both of which are preferably disposed within line  149 . Controller  200  is preferably pre-programmed with saturation curve data, i.e., saturation temperature as a function of pressure. The required degree of superheating (e.g., a pre-determined number of ° C. above the saturation temperature) may be input as a set-point into controller  200 . Alternatively or additionally, a condensate indicator  206  in line  149  enables feedback control, in which the degree of superheating is set and/or corrected based on the presence of condensate in line  149 . This may be important if exact T-P saturation curves are not available for the gas, or as a backup to temperature indicator  202  and pressure indicator  204 . Controller  200  preferably receives input on demand from consumer  100 . Controller  200  is preferably designed to control at least one of the following: temperature of hot water line  130 , flowrate of hot water line  130 , and total heat supplied to heat exchanger  134 . The control of the temperature of hot water line  130  is particularly important in that it enables partial decoupling of the recirculation rate of the heat exchange liquid from the rate of liquefied gas introduced to pump  140 . 
     In another preferred embodiment, pump  132  and hot water line  130  are designed and configured so as to allow a reversal of the hot water flow. Controller  200 , upon receiving input from at least one of the indicators, or a manual command from a system operator, reverses the direction of the flow in hot water line  130 , such that the hot water heats the gas in counter-current fashion. This enables auxiliary heat exchanger  155  to deliver additional heat to the gas. In the event that a large quantity of liquid-phase gas is present in line  149  after heat exchanger  150 , the counter-current flow of hot water, coupled with a larger ΔT across the heat exchanger and an improved heat transfer coefficient, combine to significantly increase the amount of liquid-phase gas that is vaporized and superheated in auxiliary heat exchanger  155 . The upper bound for the temperature of the gas delivered to the consumer is easily controlled by limiting the temperature of the hot water discharged from heat exchanger  134 . 
     Although the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims. All publications, patents and patent applications mentioned in this specification are herein incorporated in their entirety by reference into the specification, to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated herein by reference. In addition, no citation or identification of any reference in this application shall be construed as an admission that such reference is available as prior art to the present invention.