Patent Publication Number: US-10775080-B2

Title: LNG gasification systems and methods

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Nonprovisional application Ser. No. 15/285,965, filed Oct. 5, 2016, which claims priority to U.S. Provisional Application Ser. No. 62/237,223, filed on Oct. 5, 2015, the content of which are hereby incorporated herein by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to liquefied natural gas and, more particularly, to systems and methods for capturing refrigeration from gasification or vaporization of liquefied natural gas. 
     BACKGROUND 
     Liquefied Natural Gas (LNG) is a clean, viable source of heat and power. When natural gas is cooled and liquefied, the density increases about 600 times. In other words, LNG occupies about 1/600 th  the volume or space than the same volume of natural gas vapor. LNG is, accordingly, easier to transport, and far more energy dense than natural gas in a vapor form. Liquefying natural gas into LNG, however, requires a significant amount of energy and special cryogenic tanks are needed to prevent rapid heat gain after the natural gas is liquefied. 
     When natural gas is liquefied into LNG, the energy content becomes more comparable to other liquid fuels. Nonetheless, as illustrated below, LNG generally provides less energy per gallon as compared to other fuels (measured in British Thermal Units (Btu)): 
     One gallon diesel=135,000 Btu 
     One gallon gasoline=125,000 Btu 
     One gallon Liquefied Petroleum Gas (LPG)=91,000 Btu 
     One gallon Liquefied Natural Gas (LNG)=83,000 Btu 
     These other fuels, including, for example, diesel, gasoline, kerosene, and propane, may be used in applications similar to LNG. However, as described below with respect to embodiments of the disclosed invention, unlike LNG, these other fuels do not provide significant “cold energy.” 
     As a fuel, LNG is versatile and can be used in just about any way other fuels are used. These uses may be, for example, in boilers, dryers, power generation, marine fuel, locomotives, drilling rigs, asphalt plants, port operations, and over-the-road trucks. Once LNG arrives at an industrial site, it is vaporized or gasified into a gas for consumption. Prior to vaporization or gasification, LNG is not flammable and will not combust unless it is in a mixture with air where the natural gas is between 5% to 15% mixture with air. Vaporizing or gasifying LNG theoretically requires the same amount of energy from the environment as compared to the energy required to liquefy natural gas. 
     The amount of energy required to liquefy natural gas tends to vary with, for example, gas composition, gas pressure, product purity, product pressure, and refrigeration technology. With respect to liquefaction, for example, small scale liquefiers of less than about 500,000 gallons per day may require about 0.7 to 0.9 kWh/gallon of LNG, not including energy associated with inlet gas compressions, pretreatment, and other processes. 
     Conventional vaporization or gasification LNG systems and methods use various heat exchangers. The NFPA-59A Standard for Production, Storage, and Handling of Liquefied Natural Gas lists five options: (1) Ambient vaporizer, (2) Heated Vaporizer, (3) Integral Heated Vaporizer, (4) Remote Heated Vaporizer, and (5) Process Vaporizer. However, with the conventional vaporization or gasification LNG systems and methods, the cooled process fluid used to vaporize or gasify the LNG is discharged to the environment and/or the “cold energy” is not otherwise utilized. 
     SUMMARY 
     According to an embodiment, a method for capturing refrigeration is disclosed comprising providing a predetermined flowrate of liquefied natural gas to a first heat exchanger; providing a process fluid to the first heat exchanger, wherein the process fluid is warmer than the liquefied natural gas; heating at least a portion of the liquefied natural gas with at least a portion of the process fluid; calculating an amount of energy transferred in the first heat exchanger from the process fluid to the liquefied natural gas; and utilizing the process fluid for cooling. 
     According to another embodiment, a skid for capturing refrigeration from liquefied natural gas vaporization is disclosed comprising: a first heat exchanger mounted on the skid, the first heat exchanger having a natural gas inlet, a natural gas outlet, a process fluid inlet, and a process fluid outlet; wherein process fluid is configured to flow from the process fluid inlet through the first heat exchanger to the process fluid outlet and then to the process fluid inlet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features and advantages of the invention will be apparent from the following drawings wherein like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. 
         FIG. 1  is an embodiment of a vaporization or gasification system; 
         FIG. 2  is an embodiment of a vaporization or gasification system; 
         FIG. 3  is an embodiment of a heat exchanger and tank; 
         FIG. 4  is a an illustrative depiction of phase and state changes of natural gas; 
         FIG. 5  is an embodiment of a display; 
         FIG. 6  is an embodiment of a controller; 
         FIG. 7  is an embodiment of a vaporization or gasification system; 
         FIG. 8  is an embodiment of a vaporization or gasification system; and 
         FIG. 9  is an embodiment of an industrial processing including a vaporization or gasification system. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the invention are discussed in detail below. In describing embodiments, specific terminology is employed for the sake of clarity. However, the invention is not intended to be limited to the specific terminology so selected. A person skilled in the relevant art will recognize that other equivalent parts can be employed and other methods developed without departing from the spirit and scope of the invention. 
     As used herein, the term “about” or “approximately” when referring to a measurable value such as an amount, a temporal duration, and the like, is meant to encompass variations of ±20% or ±10%, more preferably ±5%, even more preferably ±1%, and still more preferably ±0.1% from the specified value, as such variations are appropriate to perform the disclosed methods. 
     Referring now to  FIGS. 1 and 2 , an embodiment of an LNG vaporization or gasification system  101  is illustrated. The LNG vaporization or gasification system  101  generally comprises a heat exchanger or vaporizer  103  having an LNG inlet  105 , a natural gas outlet  107 , a warm process fluid inlet  109 , and a cold process fluid outlet  111 . According to an embodiment, the heat exchanger or vaporizer  103  may be mounted on a platform or skid  121  along with connecting piping and equipment, which are further described below. 
     According to an embodiment, the heat exchanger or vaporizer  103  may be a remote heated vaporizer. For example, in remote heated vaporizers, the primary heat source (e.g., heat exchangers, industrial equipment, heaters, etc.) is separated from the actual vaporizing heat exchanger, and an intermediate or process fluid is used as the heat transport medium from the primary heat source or sources to the heat exchanger or vaporizer. According to various embodiments, the process fluid may be, for example, but not limited to water, steam, isopentane, glycol, mixtures thereof or other fluids known to one of ordinary skill in the art. 
     As illustrated in  FIG. 2 , according to an embodiment, for example, the heat exchanger or vaporizer  103  may be a shell and tube heat exchanger. As known to one of ordinary skill the art, a shell and tube heat exchanger may generally comprises a shell  133  and a tube or tubes  135 , which fluidly segregates fluid flowing in the shell  133  from fluid flowing through the tube or tubes  135 . Such a configuration allows heat transfer between fluid flowing in the shell  133  and fluid flowing through the tubes  135 . According to an embodiment, the process fluid enters the warm process fluid inlet  109  of the heat exchanger or vaporizer  103  and flows on the shell-side to the cold process fluid outlet  111  of the heat exchanger or vaporizer  103 , and the LNG enters the LNG inlet  105  of the heat exchanger or vaporizer  103  and flows through the tube-side to the natural gas outlet  107  of the heat exchanger or vaporizer  103 . According to other embodiments, the process fluid may flow through the tube-side and the LNG may flow through the shell-side of the heat exchanger or vaporizer  103 . 
     According to still another embodiment, the heat exchanger or vaporizer  103  may comprise a counter-flow shell and tube heat exchanger where flow in the shell-side is counter to, or opposite, the flow in the tube-side. Such a flow configuration provides efficient heat transfer between, for example, LNG flowing through the tube-side and process fluid flowing through the shell-side of the heat exchanger. 
     As illustrated at  FIG. 2 , the heat exchanger or vaporizer  103  may provide a tortuous path from the LNG inlet  105  to the natural gas outlet  107  in the tubes  135  through the shell  133  having process fluid flowing therethrough. As further explained by the heat transfer equations below, heat transfer between the LNG in the tube or tubes  135  and process fluid in the shell  133  changes the phase of the natural gas from liquid to vapor. According to an embodiment, the heat exchanger or vaporizer  103  of the LNG vaporization or gasification system  101  may remove energy from a continuous or intermittent water stream flowing, for example, counter to the direction of flow of LNG. 
     The connecting piping may generally comprise piping  123  for fluidly connecting the LNG inlet  105  of the heat exchanger or vaporizer  103  to a position near or adjacent to an edge of the platform or skid  121 , piping  125  for fluidly connecting the natural gas outlet  107  of the heat exchanger or vaporizer  103  to another position near or adjacent to an edge of the platform or skid  121 , piping  127  for fluidly connecting the warm process fluid inlet  109  of the heat exchanger or vaporizer  103  to another position near or adjacent to an edge of the platform or skid  121 , and piping  129  for fluidly connecting the cold process fluid outlet  111  of the heat exchanger or vaporizer  103  to another position near or adjacent to an edge of the platform or skid  121 . For example, the connecting piping  123 ,  125 ,  127 ,  129  may be provided at positions near or adjacent to edges of the platform or skids  121  so that the platform or skid  121  may be easily placed on a site for installation with on-site or existing piping and equipment. Such a configuration may avoid damage to both on-site piping and equipment and piping and equipment located on the platform or skid  121 . According to other embodiments, the connecting piping  123 ,  125 ,  127 ,  129  may extend past the edges of the skid or platform  121 . According to still other embodiments, the connecting piping  123 ,  125 ,  127 ,  129  may not extend past the edges of the skid or platform  121 . 
     The connecting piping  123 ,  125 ,  127 ,  129  may include various sizes of piping, tubing, flanges or threaded connections, valves, regulators, vents or pressure relief valves, and other types of pipes, valves, and fittings as known to one of ordinary skill in the art. Pipe and equipment supports  131  may be further included on the skid or platform  121  to support the connecting piping  123 ,  125 ,  127 ,  129  and other equipment located on the skid or platform (including the heat exchanger or vaporizer  103 ). The pipe and equipment supports  131  may raise the connecting piping  123 ,  125 ,  127 ,  129  and other equipment located on the skid or platform (including the heat exchanger or vaporizer  103 ) above the surface of the skid or platform  121  for operator access to the equipment. 
     According to an embodiment, the LNG vaporization or gasification system  101  may be configured to vaporize or gasify approximately 40,000 to 100,000 standard cubic feet per hour (scfh) of natural gas in approximately 10,000 scfh increments. According to other embodiments, the LNG vaporization or gasification system  101  may be configured to vaporize or gasify in approximately up to 40,000 scfh, up to 60,000 scfh, up to 80,000 scfh, up to 100,000 scfh flow rates. For example, for such embodiments, such natural gas flow rates allow for the heat exchanger  103  and other piping and equipment, further explained below, to be placed on one skid or platform  121 , or a skid or platform  121  having multiple pieces. For example, the skid or platform  121  including the heat exchanger  103  and the other piping and equipment may be factory assembled so that the assembly can be quality tested in a controlled environment and then delivered to and installed on a site. 
     Referring still to  FIGS. 1 and 2 , the connecting piping  123 ,  125 ,  127 ,  129  may be configured to include various sensors, transmitters, and safety systems. For example, the connecting piping  123  and  125  may include pressure relief valves  141  to protect the system from over-pressure. According to an embodiment, a vent manifold  142  fluidly connected to the pressure relief valves  141  may be located on the skid or platform  121 . The vent manifold  142  may provide a central and local location on the skid or platform  121  to vent natural gas or LNG from one or more pressure relief valves  141  during a pressure relief event. 
     According to an embodiment, the connecting piping  123 ,  125 ,  127 ,  129  may include temperature sensors and/or transmitters  143 , pressure sensors and/or transmitters  145 , and flow meters and/or transmitters  147 . As explained in more detail below, the various sensors and transmitters may be used to control the LNG gasification or vaporization system  101  and optimize captured refrigeration capacity. 
     According to an embodiment, the connecting piping  125  may also include a regulator  149  to reduce the pressure of the natural gas to a predetermined pressure after it exits the heat exchanger or vaporizer  103 . For example, the regulator may reduce the pressure to less than a maximum allowable operating pressure of boilers, dryers, power generation equipment, marine fuel equipment, locomotives, drilling rigs, asphalt plants, port operation equipment, over-the-road trucks, or other equipment requiring natural gas. 
     Referring now to  FIG. 3 , a simplified schematic is illustrated of a heat exchanger or vaporizer  103  with an LNG storage tank  151  fluidly connected thereto. According to an embodiment, more than one LNG storage tank  151  may be similarly fluidly connected to the heat exchanger or vaporizer  103 . For example, the LNG storage tank  151  may be a vertical on-site cryogenic storage tank. According to an embodiment, the LNG storage tank  151  may be a vertical double-walled cryogenic tank which operates between, for example, 60 and 90 psig. The LNG storage tank  151  may be provided with pressure relief valves to vent natural gas if the operating pressure exceeds a predetermined maximum allowable working pressure of the LNG storage tank. According to an embodiment, the LNG storage tank  151  may be an annular tank which includes an inner tank and an outer tank having a vacuum space therebetween, along with insulation, in order to provide insulation and minimize the amount of heat that may enter the tank. The vacuum space may include molecular sieves to avoid matter entering the vacuum space and eliminate convective heat transfer. According to an embodiment, no metal-to-metal contacts are located between the inner and outer tanks in order to minimize conduction of heat. 
     As illustrated in  FIG. 3 , the LNG storage tank  151  may be fluidly connected to the LNG inlet  105  of the heat exchanger or vaporizer  103  such that LNG can flow from the LNG storage tank  151  to the LNG inlet  105  of the heat exchanger or vaporizer  103 . For example, no pumping equipment may be needed to transfer LNG from the LNG storage tank  151  to the heat exchanger or vaporizer  103 . According to an embodiment, the LNG storage tank  151  is located away from the platform or skid  121  and may store saturated liquid natural gas in a range of approximately 60 psig to 90 psig at a temperature of about −210° F. or conditions where the natural gas is in a liquid state. 
     As described above, the natural gas changes phase, from liquid to vapor, in the heat exchanger or vaporizer  103  by heat transfer with the process fluid. As the liquid natural gas changes to vapor, the natural gas undergoes a temperature increase, which is characterized herein as providing “Cold Energy,” which is based on the change in enthalpy of the natural gas between the liquid phase and the vapor phase. According to embodiments, this Cold Energy can be captured and used in other processes. 
     According to embodiments illustrated at  FIG. 4 , the total enthalpy change is independent of the path from one state to another. For example,  FIG. 4  illustrates that LNG may take different paths to ultimately achieve the same state, where the total enthalpy change of both paths is the same. 
     From a thermodynamic perspective, enthalpy is calculated as follows in equation (1):
 
 H=U+pV   (1)
 
Where
 
     H=enthalpy 
     U=internal energy 
     p=pressure exerted by the natural gas 
     V=volume occupied by the natural gas 
     The Cold Energy (in Btu) captured by the process fluid is expressed in equation (2):
 
Cold Energy= ρV *( ĥ   sat   −ĥ   vapor )  (2)
 
Where
 
     ĥ sat =enthalpy of saturated liquid 
     ĥ vapor =enthalpy of vapor 
     V=volume occupied by the saturated liquid 
     ρ=density of saturated liquid 
     As illustrated by equation (2), the total difference in enthalpy is the total Cold Energy generated by the phase change of the natural gas, which depends on saturation conditions (i.e. temperature and pressure). 
     Refrigeration can be further calculated based on the change in enthalpy and the mass flowrate of LNG, according to equation (3):
 
1 ton of refrigeration=12,000 Btu/hr  (3)
 
     According to embodiments, approximately half of the energy required to liquefy natural gas may be recovered upon vaporization or gasification, such as at a customer site. For example, Table 1 illustrates the Cold Energy recovered by embodiments of the present invention: 
     
       
         
           
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Power at the Liquifier: 
                 Free Cold Energy: 
               
            
           
           
               
               
               
               
               
            
               
                 Energy In: 
                 1 kw = 
                   
                 Energy Out: 
                 Free Cold 
               
               
                 kw-hr/LNG gal 
                 Btu/hr 
                 Btu/gal LNG 
                 Btu/LNG gal 
                 % eff 
               
               
                   
               
               
                 0.7 
                 3412 
                 2388 
                 1229 
                 51% 
               
               
                 0.9 
                 3412 
                 3071 
                 1229 
                 40% 
               
               
                   
               
            
           
         
       
     
     Illustrative Example 1 
     As a first illustrative example of an embodiment, the LNG storage tank  151  is configured to hold 14,000 gallons of LNG at 60 psig and natural gas vapor exits the heat exchanger or vaporizer  103  at 60 psig and 60° F., where the following conditions are met: 
     ĥ sat =380.60 Btu/lbm 
     ĥ vapor =31.443 Btu/lbm 
     ρ=24.391 lbm/ft 3    
     V=1871.53 ft 3    
     Accordingly, the Cold Energy generated by Example 1 is 15.9*10 6  Btu or 15.9 million Btu (mmBtu). In other words, 15.9 mmBtu is required to vaporize 14,000 gallons of LNG. 
     Illustrative Example 2 
     As a second illustrative example of an embodiment, LNG is vaporized at the same conditions as Example 1 and the mass flowrate of LNG is 15 gallons per minute (gpm). According to Equations (2) and (3), the refrigeration capacity generated by vaporizing 15 gallons per minute of LNG is about 94 tons. 
     As described above, the Cold Energy can be extracted during the gasification or vaporization process and captured by the process fluid in the heat exchanger or vaporizer  103 . In other words, the process fluid releases heat to the LNG. For example, based on an energy balance of the heat exchanger, the required flow rate to achieve a desired temperature of the process fluid at the cold process fluid outlet  111  of the heat exchanger or vaporizer  103  can be calculated by the following equations (4) and (5):
 
Heat transfer of process fluid= q   w   ={dot over (m)}   w   *c   p *( T   w out   −T   w in )  (4)
 
Heat transfer of LNG= q   LNG   ={dot over (m)}   LNG *( h   LNG out   −h   LNG in )  (5)
 
Where:
 
     {dot over (m)} w =mass flowrate of process fluid 
     c p =heat capacity of process fluid 
     T w out =temperature of process fluid exiting the heat exchanger 
     T w in =temperature of process fluid entering the heat exchanger 
     h LNG out =enthalpy of natural gas exiting the heat exchanger 
     h LNG in =enthalpy of natural gas entering the heat exchanger 
     Assuming an adiabatic process (i.e. no heat loss or gain from the environment), the required flow rate of the process fluid can be calculated as follows by the following equations (6) and (7):
 
 q   w   =q   LNG   ={dot over (m)}   LNG *( T   w out   −T   w in )= {dot over (m)}   w   c   p *( h   LNG out   −h   LNG in )  (6)
 
 {dot over (m)}   w   =q   LNG /( c   p *( T   w out   −T   w   in ))  (7)
 
     Illustrative Example 3 
     According to an embodiment, the process fluid may be, for example, but not limited to, water, glycol, or a water/glycol mixture. For purposes of a third illustrative example of an embodiment, LNG is vaporized at the same conditions as Example 1 and the mass flowrate of LNG is 15 gallons per minute and the process fluid is water with an inlet temperature of 60° F. with a desired outlet temperature of 32° F. Assuming an adiabatic process, the required flowrate of water at 1 atm can be calculated by Equation (7) as 73.1 gallons per minute where: c p =heat capacity of water=1 Btu/lbm° F. 
     Referring now to  FIG. 5 , an embodiment of a display  201  is illustrated. The display  201  reflects equipment, piping, and sensors located on an embodiment of the platform or skid  121  of an LNG vaporization or gasification system  101 . The display  201  shows a heat exchanger or vaporizer  103  having an LNG inlet  105 , natural gas outlet  107 , warm process fluid inlet  109 , and cold process fluid outlet  111 . The display  201  also indicates equipment that may be located off or away from the platform or skid  121 , such as, for example, ambient vaporizers  161 . 
     The display  201  indicates readings, such as real-time transmissions, from various sensors and/or transmitters located throughout the illustrated embodiment. On the process fluid piping, a temperature sensor/transmitter  143  and flow meter/transmitter  147  is located on the process fluid inlet piping to the heat exchanger or vaporizer  103 . Another temperature sensor/transmitter  143  is located on the process fluid outlet piping from the heat exchanger or vaporizer  103 , and may additionally include a flow meter/transmitter. A differential pressure sensor/transmitter  145  is located between the process fluid inlet and outlet piping. This differential pressure sensor/transmitter  145  may indicate, for example, freezing process fluid or other fouling in the heat exchanger or vaporizer  103 . On the natural gas piping, other temperature and pressure sensors/transmitters  143 ,  147  may be located on the piping. 
     The display  201  indicates different flow paths of the LNG. From the LNG storage tank  151 , the LNG may flow to either the heat exchanger or vaporizer  103 , to the ambient vaporizers  161 , or to a sump used, for example, for blowdown or recycle to the LNG storage tank  151 . The flow of LNG may be controlled by various valves  171  to control LNG flow to either the heat exchanger or vaporizer  103 , one or more ambient vaporizers  161 , sump, or any combination thereof. 
     Referring now to  FIG. 6 , the various sensors/transmitter readings from sensors/transmitters  143 ,  145 ,  147  reflected in the display  201  may be used in the above described equations and algorithms by a controller or programmable logic controller (PLC)  203  to calculate the total refrigeration generated and captured by the LNG vaporization or gasification system  101  and the efficiency of the vaporizer compared to an adiabatic process, explained above. As explained above, the controller or PLC  203  may calculate the Cold Energy expected to be generated by an adiabatic process of the LNG vaporization or gasification system  101  and compare it to the Cold Energy actually generated by the LNG vaporization or gasification system  101  in order to determine efficiency of the system. Further, the controller or PLC  203  may further calculate the Cold Energy actually generated by the LNG vaporization or gasification system  101  and calculate the total energy provided by the LNG to display the net energy resulting from energy savings (Cold Energy) and energy generated by the LNG. 
     According to an embodiment, the controller or PLC  203  may also track use of the ambient vaporizers  161  according to a predetermined length of time for using the ambient vaporizers  161 . For example, at a predetermined length of flowing LNG through the ambient vaporizers  161 , the controller  203  may divert LNG flow from one ambient vaporizer  161  to another in order to prevent freezing in the ambient vaporizers  161 . 
     According to an embodiment, the controller or PLC  203  may further control the valves  171  in order to adjust the flow of LNG to prevent freezing in one or more of the ambient vaporizers  161 . Further, the controller or PLC  203  may control the valves  171  to divert LNG flow to or away from the heat exchanger or vaporizer  103  according to refrigeration requirements of other equipment. 
     According to an embodiment, the controller or PLC  203  may control the flow of LNG to the heat exchanger or vaporizer  103  to be a predetermined flow rate. The controller or PLC  203  may adjust the predetermined flow rate of LNG to the heat exchanger or vaporizer  103  based on, for example, temperature of process fluid entering or exiting the heat exchanger or vaporizer  103 . Further, the controller or PLC  203  may adjust the predetermined flow rate of LNG to the heat exchanger or vaporizer  103  based on, for example, flow rate of process fluid through the heat exchanger or vaporizer  103 . 
     Referring to  FIGS. 7 and 8 , for example, different embodiments of a process are illustrated which depict process fluid flow.  FIG. 7  illustrates, for example, a load  211  requiring refrigeration or cooling water. The load  211  utilizes the refrigeration captured by the process fluid and returns relatively warmer water back to the heat exchanger or vaporizer  103 .  FIG. 8  illustrates, for example, a load  211  requiring refrigeration or cooling water where the load  211  similarly utilizes the refrigeration captured by the process fluid but discharges otherwise utilizes the cooling water elsewhere. A heat source  213  provides the relatively warmer water to the heat exchanger or vaporizer  103 . 
     Referring now to  FIG. 9 , another embodiment is illustrated of an LNG vaporization or gasification system  101 . For example, the LNG vaporization or gasification system  101  may include the system as depicted in  FIG. 1  and the display and controls of  FIGS. 5 and 6 . According to an embodiment, the process fluid, such as glycol or water or glycol-water mixture, may be pumped through the heat exchanger or vaporizer  103  by at least one or more pumps  221 . After the process fluid captures the Cold Energy through the heat exchanger or vaporizer  103 , the process fluid may flow through a process heat exchanger  223  to provide the Cold Energy to an industrial process such as, for example, an NH 3  or ammonia system. According to an embodiment, an ammonia compressor  231  may provide ammonia to the process heat exchanger  223  (via control valve  231 ) or to one or more ammonia condensers  233 . The control valve  231  may adjust the flow from the ammonia compressor  231  to the process heat exchanger  223  by based on a predetermined temperature set point of ammonia measured by temperature sensor  235  downstream of the process heat exchanger  223 . For example, such a configuration may reduce the load and energy requirements of the condensers  233 . 
     The embodiments illustrated and discussed in this specification are intended only to teach those skilled in the art the best way known to the inventors to make and use the invention. Nothing in this specification should be considered as limiting the scope of the present invention. All examples presented are representative and non-limiting. The above-described embodiments of the invention may be modified or varied, without departing from the invention, as appreciated by those skilled in the art in light of the above teachings. It is therefore to be understood that, within the scope of the claims and their equivalents, the invention may be practiced otherwise than as specifically described.