Abstract:
In the method of using ambient air to vaporize liquefied gas, the steps include transferring heat from a stream of ambient air to a stream of liquefied gas, thereby cooling the air stream, and vaporizing the liquid; transferring heat from a source into the cooled air stream; and then discharging the heated air stream to atmosphere, sufficient heat being transferred to obviate objectionable fog production resulting from step c).

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to overcoming the problem of fog production during or as a result of vaporization of liquefied gases, as for example liquefied natural gas (LNG), nitrogen, oxygen, and ethylene. 
     Liquefied gases (for example those listed above) frequently require heating to convert the liquid back into gas, for use. Conventionally, this heating process is referred to as vaporization and the devices employed as vaporizers. 
     One of the most common and least expensive sources of heat is ambient air. Many of the liquefied gases are stored at temperatures below the freezing point of water. Thus heat exchanges with ambient air can produce large amounts of cold air and, in certain atmospheric conditions, a ground fog. Most applications are small enough that the fog can be readily dissipated, but some are so large that the fog forms a hazard or nuisance. A good example is a receiving and re-gasification terminal for LNG (Liquefied Natural Gas). These facilities can have heat requirements in excess of 500,000,000 BTU/hr. Traditional methods of vaporization take a portion of the product stream, and burn it to produce the required heat. This can consume up to about 3% of the vaporized product and represents a substantial operating cost. 
     Additionally, new restrictions on NOx reduction have made combustion vaporization more difficult to live within the air pollution requirements at the re-gasification sites. The use of ambient air in conjunction with conventional heating systems has enormous appeal from both an economic and air pollution standpoint, if the fog issues can be overcome. 
     SUMMARY OF THE INVENTION 
     It is a major object of the invention to provide an efficient, low-cost solution to the above described problem. Basically, the invention provides a method, and apparatus, of using ambient air to vaporize liquefied gas, without objectionable resulting fog production. Steps of the basic method include: 
     a) transferring heat from a stream of ambient air to a stream of liquefied gas, thereby cooling the air stream, and vaporizing the liquid, 
     b) transferring heat from a source into the cooled air stream, 
     c) and then discharging the heated air stream to atmosphere, 
     d) sufficient heat being transferred in step b) to obviate objectionable fog production resulting from step c). 
     Additional steps may be provided and include one of more of the following:
         i) removing water from the cooled air stream,   ii) providing and operating a vaporizer in which step a) is effected; providing and operating a re-heater in which said step b) is effected; and providing ducting to conduct flow of cooled air from the vaporizer to the re-heater,   iii) providing and operating a back-up vaporizer for use while the first mentioned vaporizer is operated in thaw mode, for de-icing,   iv) supplying liquefied gas to the vaporizer, and flowing vaporized gas from the vaporizer, the gas consisting of at least one of the following: LNG, nitrogen, oxygen, methane, ethylene, and mixtures thereof,   v) providing a heat source, for the re-heater, to comprise a fuel fired back-up unit, or a fuel fired duct heater; or a source of stored heat; or waste heat from an electrical power plant; or waste heat from a cogeneration installation.       

     These and other objects and advantages of the invention, as well as the details of an illustrative embodiment, will be more fully understood from the following specification and drawings, in which: 
    
    
     
       DRAWING DESCRIPTION 
         FIG. 1  is a psychometric chart; 
         FIG. 2  is a schematic view of liquefied gas vaporizer unit to which ambient air is supplied; 
         FIG. 3  is a schematic view of preferred system or apparatus incorporating the invention; 
         FIG. 4  is a view like  FIG. 3 , but showing operation in thaw mode; and 
         FIG. 5  is a block diagram showing operation of vaporizers in tandem, or selective operation of one vaporizer unit to allow thawing of a second unit, and water rejection. 
     
    
    
     DETAILED DESCRIPTION 
     The process can best be understood by referring to a conventional psychometric chart (see  FIG. 1 ). If the ambient air is saturated (100% relative humidity) and cooled, the result will be a saturated stream at a lower temperature. Any time two saturated streams of different temperatures are mixed (as is the case when the cold stream is reintroduced to the environment), the result is a precipitate, usually in the form of fog. However, if the cold air is slightly reheated, the mixing in any portion escapes the precipitation and hence the condition to form fog. 
     As shown in  FIG. 2 , a first heat transfer apparatus, such as vaporizer  10 , receives a liquid or liquefied gas stream  11 , at inlet  12 , and discharges a stream  11   a  of vaporized gas via outlet  13 . Vaporization occurs by virtue of heat transfer from ambient (or near ambient temperature) air  14  entering the vaporizer at inlet  15  and exhausted from the vaporizer via outlet  16 . The cooled air exhaust is indicated at  17 . Typically, water is condensed from the air stream in the vaporizer, and may be separated as via a separator  19 , for commercial utilization, or other use, as indicated at  21 . 
     As shown in  FIG. 3 , the cooled air exhaust  17  enters a duct  22 , and flows at  23  to a re-heater  24  (a heat exchanger) at which, or in which, heat is transferred into the cooled air stream. Sufficient heat is received by the air stream  23  to raise the air temperature to a level T 2 , which is less than the temperature T 1  of the supplied ambient air  14 , as to obviate objectionable fog production that would otherwise be produced by discharging the cooled air stream to atmosphere. A heat source for the re-heater is indicated at  25 , and a control  26  may be used to control the heat supply, to achieve the selected temperature level T 2  of the air stream discharged from the re-heater. Water in the air flow  23  may be removed as indicated by collection zone  27 , and discharged at  28 , below the duct. See also  FIG. 1 . The control  26  may be governed, as for example from a set point, in response to variations in the temperature and humidity of the ambient air  14  supply, and in relation to the amount of ambient air supplied (pumped as at  30 ) per time interval, to adjust the amount of heat supplied to the re-heater, to optimize the level of heating of the cooled air to prevent fogging. Such controlling is indicated by ambient air parameter sensor or sensors at  35 , and their connection at  36  to control  26 . 
     The amount of energy required to reheat varies depending on the ambient temperature and relative humidity. At an ambient temperature of 70° F. and 100% relative humidity, and a 20° F. air exhaust temperature, the energy requirement is only 30% of the energy required for full vaporization. As the relative humidity decreases, the reheat requirement diminishes until at about 50% relative humidity; no reheating is required. As the ambient temperature decreases, the relative amount of reheat increases. 
     The energy for reheat can come from a variety of sources. Most vaporizer installations will have a certain number of operating hours below 32° F. (freezing point of water), which may preclude the ambient air for cycle de-icing of the vaporizer heat exchanger surfaces. As a result, a large installation is likely to have a fuel-fired backup vaporization system for those conditions. Partial use of this heating system is a likely source of the energy for reheat. Other possibilities include direct fuel-fired duct heaters, stored heat or the use of waste heat from electrical power plants or cogenerations installations. It is possible to use the ambient air itself to provide the reheat, but then the cold exhaust from it may require reheat. Theoretically, it is possible to provide a unit in which all the heat comes from ambient air. 
       FIG. 4  shows the vaporizer of  FIG. 3  operating in thaw (de-icing) mode, in which ambient air is passed through the vaporizer, but liquid or liquefied gas flow into the vaporizer is eliminated, whereby the un-cooled ambient air flow melts accumulated ice. See ice and ice water removal at  40  and  41 . 
     In  FIG. 5  first and second vaporizers  10  and  10   a  are employed, each discharging cooled air via duct  22  to the re-heater  24 , as referred to. The two vaporizers may be operated in tandem, as shown. Either one may be operated in thaw mode, as by shutting off of valve  60  in a liquefied gas supply line  61 , and the other vaporizer may then be operated as a back-up vaporizer to vaporize the liquefied gas supply. See also ambient air discharge control valves  67 .