Abstract:
In another embodiment of the invention, a method for recovering boil-off gas from a head space of a liquid storage tank having a fluid contained therein is provided. In one embodiment, the method can include the steps of flowing the boil-off gas from the liquid storage tank to a heat exchanger by opening a valve; creating a vacuum within the heat exchanger by cooling and condensing the boil-off gas in the heat exchanger by using cold energy from vaporization of liquid nitrogen from a liquid nitrogen storage tank to form a cooled fluid; and introducing the cooled fluid to the liquid storage tank, thereby reducing the temperature within the liquid storage tank.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Application 61/935,913, filed Feb. 5, 2014; U.S. Provisional Application 62/040,010, filed Aug. 21, 2014; U.S. Provisional Application 62/042,277, filed Aug. 27, 2014; and U.S. Provisional Application 62/042,280, filed Aug. 27, 2014, all of which are hereby incorporated by reference in their entireties. 
     
    
     TECHNICAL FIELD OF THE INVENTION 
       [0002]    The present invention relates to an apparatus and method for recovering condensable gases originating from a liquid storage tank. More specifically, embodiments of the present invention are related to recovering the condensable gases by introducing said volatile gases into a heat exchanger and cooling them against a vaporizing liquid nitrogen stream. The cooled condensable fluid is then reintroduced into the liquid storage tank, which provides further cooling to the liquid within the liquid storage tank. 
       BACKGROUND OF THE INVENTION 
       [0003]    As liquefied natural gas (LNG) becomes more readily available and the overall price declines, it becomes more economically practical to use LNG as a fuel for automotive purposes, particularly larger vehicles such as trucks and busses. However, before large scale use of LNG can occur, the appropriate infrastructure must be in place to service said vehicles. 
         [0004]    As part of the infrastructure, fueling stations having liquid storage tanks can be used to provide the LNG. An inherent problem with storage tanks is that there is an inevitable loss of a certain amount of liquid product that fills the vapor space of the storage tank. This evaporated product is known as boil-off gas. 
         [0005]    Under certain conditions, the amount of boil-off gas within the head space of the storage tank will increase, which will then lead to an increase in pressure within the storage tank. This could lead to an unsafe condition, and therefore, it has been common practice to include a venting mechanism with the storage tank, such as a pressure relief valve, that vents the boil-off gas to the atmosphere until the pressure within the storage tank is below a given threshold. 
         [0006]    While this method of pressure control is cost effective, it has several drawbacks. Depending on the type of liquid contained within the storage tank, releasing the associated gas could be hazardous to the environment, increase fire hazards, and/or create noxious odors. 
         [0007]    There have been proposed methods for preventing the need for venting, which can include condensing the boil-off gas, either internally or externally of the storage tank. However, many of these systems are overly complicated, include large pieces of equipment, and require that the boil-off gas be condensed before returning it to the storage tank. 
         [0008]    Therefore, it would be desirable to have an improved process for recovering boil-off gas that was simple and efficient. Preferably, it would be desirable to have a process that did not require the use of complicated systems or very large pieces of equipment. 
       SUMMARY OF THE INVENTION 
       [0009]    The present invention is directed to a process that satisfies at least one of these needs. In one embodiment, the process for recovering boil-off gas from a head space of a liquid storage tank having a fluid contained therein is provided. In one embodiment, the method can include the steps of: 
         [0010]    (a) measuring a condition selected from the group consisting of outside temperature, temperature within the liquid storage tank, pressure within the liquid storage tank, liquid level within the liquid storage tank, heat absorption by the liquid storage tank, and combinations thereof; 
         [0011]    (b) flowing boil-off gas from the liquid storage tank to a heat exchanger by opening a valve, wherein the valve is configured to adjust the flow rate of the boil-off gas to the heat exchanger based on the condition measured in step (a); 
         [0012]    (c) creating a vacuum within the heat exchanger by cooling and condensing the boil-off gas in the heat exchanger by using cold energy from a flow of nitrogen to form a cooled fluid, wherein the heat exchanger is in fluid communication with an outlet of a liquid nitrogen storage tank, such that the heat exchanger is configured to receive a flow of nitrogen from the liquid nitrogen storage tank; and 
         [0013]    (d) introducing the cooled fluid to the liquid storage tank, thereby reducing the temperature within the liquid storage tank, 
         [0014]    wherein the flow rate of the boil-off gas pumped from the liquid storage tank to the heat exchanger and/or the flow rate of the nitrogen from the liquid nitrogen storage tank is adjusted, such that during the cooling step (c), BTUs of heat are removed from the boil-off gas faster than BTUs are gained by the liquid storage tank, 
         [0015]    wherein the heat exchanger is disposed at a point above the top of the liquid storage tank, such that the cooled fluid has a static head pressure exceeding that of the fluid within the liquid storage tank. 
         [0016]    In another embodiment of the invention, a method for recovering boil-off gas from a head space of a liquid storage tank having a fluid contained therein is provided. In one embodiment, the method can include the steps of flowing the boil-off gas from the liquid storage tank to a heat exchanger by opening a valve; creating a vacuum within the heat exchanger by cooling and condensing the boil-off gas in the heat exchanger by using cold energy from vaporization of liquid nitrogen from a liquid nitrogen storage tank to form a cooled fluid; and introducing the cooled fluid to the liquid storage tank, thereby reducing the temperature within the liquid storage tank. 
         [0017]    In optional embodiments: 
         [0018]    the heat exchanger is disposed at a point above the top of the liquid storage tank, such that the cooled fluid has a static head pressure exceeding that of the fluid within the liquid storage tank; 
         [0019]    the heat exchanger is disposed at or below the top of the liquid storage tank; 
         [0020]    the created vacuum results in an increase of boil-off gas flowing from the liquid storage tank; 
         [0021]    the boil-off gas is fully condensed during the step of cooling the boil-off gas in the heat exchanger; 
         [0022]    the method can also include the step of varying the flow rate of the boil-off gas to the heat exchanger based on a measured value; 
         [0023]    the measured value is selected from the group consisting of outside temperature, temperature within the liquid storage tank, pressure within the liquid storage tank, liquid level within the liquid storage tank, heat absorption by the liquid storage tank, and combinations thereof; 
         [0024]    the step of cooling the boil-off gas in the heat exchanger further comprises the step of removing BTUs of heat from the boil-off gas faster than BTUs gained by the liquid storage tank, such that the pressure within the liquid storage tank is reduced; 
         [0025]    the liquid storage tank is vacuum jacketed; 
         [0026]    the fluid is a gas at atmospheric pressure and ambient temperatures; 
         [0027]    the fluid is selected from the group consisting of liquid natural gas, argon, and ethylene; and/or 
         [0028]    the method can also include the step of pressurizing the liquid nitrogen to a pressure such that the boiling point of the liquid nitrogen is warmer than the freezing point of the fluid; 
         [0029]    the method can also include the steps of providing a check valve downstream the first valve and upstream the heat exchanger, the check valve in fluid communication with the liquid storage tank and the heat exchanger; providing a second valve in fluid communication with the liquid storage tank and the heat exchanger, the second valve disposed downstream the heat exchanger, the second valve configured to open and close based on the pressure within a line between the liquid storage tank and the first valve 
         [0030]    In another embodiment of the invention, an apparatus for recovering boil-off gas is provided. In one embodiment, the apparatus can include a liquid storage tank configured to contain a fluid in its liquid state disposed therein, wherein the fluid is a gas at atmospheric pressure and ambient temperatures; a liquid nitrogen storage tank configured to contain liquid nitrogen therein; a heat exchanger in fluid communication with a head space of the liquid storage tank and an outlet of the liquid nitrogen storage tank, the heat exchanger configured to transfer heat from the boil-off gas received from the head space of the liquid storage tank to the nitrogen received from the liquid nitrogen storage tank, thereby cooling the boil-off gas to produce a cooled fluid; a measuring device configured to measure a condition selected from the group consisting of outside temperature, temperature within the liquid storage tank, pressure within the liquid storage tank, liquid level within the liquid storage tank, heat absorption by the liquid storage tank, and combinations thereof; and a first valve in fluid communication with the heat exchanger and the head space of the liquid storage tank, the first valve configured to adjust the flow rate of the boil-off gas sent to the heat exchanger based on the condition measured by the measuring device. 
         [0031]    In optional embodiments: 
         [0032]    the heat exchanger is disposed above the top of the liquid storage tank; 
         [0033]    the heat exchanger is disposed at or below the top of the liquid storage tank; 
         [0034]    the liquid storage tank and the liquid nitrogen storage tank are vacuum jacketed; 
         [0035]    the liquid nitrogen storage tank is configured to pressurize the liquid nitrogen to a pressure such that the boiling point of the liquid nitrogen is warmer than the freezing point of the fluid; 
         [0036]    the apparatus can also include a pressure increasing device in fluid communication with the liquid nitrogen storage tank and the heat exchanger, the pressure increasing device configured to pressurize the liquid nitrogen to a pressure such that the boiling point of the liquid nitrogen is warmer than the freezing point of the fluid; and/or 
         [0037]    the apparatus can also include a check valve downstream the first valve and upstream the heat exchanger, the check valve in fluid communication with the liquid storage tank and the heat exchanger; a second valve in fluid communication with the liquid storage tank and the heat exchanger, the second valve disposed downstream the heat exchanger, the second valve configured to open and close based on the pressure within a line between the liquid storage tank and the first valve. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0038]    These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, claims, and accompanying drawings. It is to be noted, however, that the drawings illustrate only several embodiments of the invention and are therefore not to be considered limiting of the invention&#39;s scope as it can admit to other equally effective embodiments. 
           [0039]      FIG. 1  shows an embodiment of the present invention. 
           [0040]      FIG. 2  shows an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0041]    While the invention will be described in connection with several embodiments, it will be understood that it is not intended to limit the invention to those embodiments. On the contrary, it is intended to cover all the alternatives, modifications and equivalence as may be included within the spirit and scope of the invention defined by the appended claims. 
         [0042]    In certain embodiments of the invention, the system can include a liquid storage tank, a heat exchanger in fluid communication with the head space of the liquid storage tank and mounted at a point above the liquid storage tank such that liquid condensing within the heat exchanger possesses sufficient static head to reintroduce itself back into the liquid storage tank without the use of an external pump. 
         [0043]    In certain embodiments, a vacuum can be created when the boil-off gas condenses, which enhances the flow of boil-off gas from the liquid storage tank. 
         [0044]    In one embodiment, the heat exchanger could be configured to be able to remove BTUs of heat from the boil-off gas faster than the steady state gain by the liquid storage tank, thereby reducing the overall temperature within the liquid storage tank. 
         [0045]    Other typical off-gas recovery systems also use large heat exchangers that are sized in order to accommodate large events (e.g., loading and unloading of the liquid storage tank); however, these large events do not occur very often and therefore, the heat exchanger is typically oversized for a majority of its use. However, certain embodiments of the present invention allow for improved flow rates by enhancing the flow of gas via the created vacuum, which in turn helps to prevent large fluctuations in internal pressures of the liquid storage tank, which in turn allows for the heat exchanger to be appropriately sized since it is not having to accommodate such large variations in flows. 
         [0046]    Now turning to  FIG. 1 , liquid storage tank  10  is filled with a liquid that would typically be gaseous under atmospheric pressure and ambient temperatures. As such, liquid storage tank  10  is operated at increased pressures and temperatures that are lower than ambient conditions. During the course of normal operation, a certain amount of the liquid within liquid storage tank  10  will vaporize and enter the head space within liquid storage tank  10  until an equilibrium is established. The amount of gaseous molecules within the head space is dependent on at least the volume of liquid storage tank  10 , the volume of liquid within liquid storage tank  10 , the pressure and temperature within liquid storage tank  10 . As the temperature rises, the pressure within liquid storage tank  10  will also increase. 
         [0047]    In one embodiment, whenever the pressure exceeds a lower set point, valve  11  is opened and boil-off gas  12  exits from the head space of liquid storage tank  10  and passes thru valve  11  on its way to a warm end of heat exchanger  30 , wherein boil-off gas  12  is cooled and condensed against a working fluid to form cooled fluid  32 . Since cooled fluid  32  is at a height above the liquid level within liquid storage tank  10 , cooled fluid  32  posses a positive static head, which allows it to be reintroduced to liquid storage tank  10  without the use of an external pressure increasing device, thereby effectively providing refrigeration to liquid storage tank  10  and recovering valuable boil-off gas. 
         [0048]    In the embodiment shown, the working fluid is nitrogen. Liquid nitrogen storage tank  20  contains liquid nitrogen, which is fed to a cold end of heat exchanger  30  via line  22 . The liquid nitrogen absorbs heat from boil-off gas  12 , vaporizes and is then vented  34  to the atmosphere. Valve  24  can be used to help control the flow rate of the liquid nitrogen. 
         [0049]      FIG. 2  displays another embodiment of the invention. In this embodiment, the apparatus can include check valve  11  and control valve  33 . Under normal operations, valve  11  is open and boil-off gas  12  flows through check valve  13 . As before, boil-off gas  12  enters heat exchanger, wherein boil-off gas  12  is cooled and condensed against a working fluid to form cooled fluid  32 . In one embodiment, valve  33  is in the open position, thereby allowing cooled fluid  32  to flow back into liquid storage tank  10 . 
         [0050]    In another embodiment, in the event that gravity and head pressure are insufficient to cause cooled fluid  32  to flow into liquid storage tank  10 , valve  33  can be closed, which allows for cooled fluid  32  to build up in the line. Additionally, as more boil-off gas  12  passes through check valve  11 , the pressure will further increase. At a predetermined pressure, valve  33  can be opened, thereby introducing cooled fluid  32  to liquid storage tank  10 . In certain embodiments, some of the cooled fluid  32  (which can be condensed) will evaporate; thereby further increasing the pressure between check valve  13  and valve  33 . This advantageously allows for the heat exchanger to be at a lower height than in the embodiment shown in  FIG. 1 . In one embodiment, valve  33  is preferably a shutoff valve. In another embodiment, valve  33  is a control valve. 
         [0051]    Additional embodiments can include monitoring of certain conditions. For example, the following conditions can all be monitored: outside temperature, temperature within the liquid storage tank, pressure within the liquid storage tank, liquid level within the liquid storage tank, and/or heat absorption by the liquid storage tank. Additionally, each of these conditions can then be used to control the flow rates of the boil-off gas and/or liquid nitrogen fed to the heat exchanger. In one embodiment, the flow rates can be varied in order to ensure that the amount of refrigeration introduced back to the liquid storage tank exceed the steady state heat gain by the liquid storage tank due to external forces (e.g., ambient air temperatures, loading/unloading of vessel). 
         [0052]    In an additional embodiment, the method can also include adjusting the storage and/or operating pressure of the liquid nitrogen, such that the liquid nitrogen is warmer than the freezing point of the boil-off gas, thereby reducing the risk of solids forming within the heat exchanger and/or lines. As an example, argon becomes a solid at about −308° F. and nitrogen has a boiling point of about −321° F. at 1 atm. However, by maintaining liquid nitrogen within a pressure range of 20-30 psi, the boiling point of the liquid nitrogen rises to about −300° F. to −305° F., thereby eliminating the opportunity of creating solid argon. 
         [0053]    While 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 in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, language referring to order, such as first and second, should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps or devices can be combined into a single step/device. 
         [0054]    The singular forms “a”, “an”, and “the” include plural referents, unless the context clearly dictates otherwise. 
         [0055]    Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur. 
         [0056]    Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range.