Abstract:
A method for removing an unwanted chemical from a working fluid of a system includes: 1) pumping into a tank a working fluid that includes an unwanted, first chemical; 2) injecting into the tank a second chemical in an amount that provides or substantially provides a stoichiometric ratio relative to the amount of the unwanted, first chemical in the working fluid; 3) mixing the second chemical with the working fluid in a first portion of the tank to generate a molecule that precipitates out of the working fluid and that includes the unwanted, first chemical; 4) holding the working fluid in a second portion of the tank to separate the molecule from the working fluid; and 5) ejecting the working fluid from the second portion. The flow rate of the working fluid and the size of the first and second portions of the separation tank are coordinated to make the working fluid take at least 4 minutes to flow through the tank.

Description:
CROSS REFERENCE TO RELATED APPLICATION AND CLAIM OF PRIORITY 
     This application claims priority from commonly owned U.S. Provisional Patent Application 61/436,928 filed 27 Jan. 2011, and titled “Equipment &amp; Control Methods for a Zero Liquid Discharge System Applied to Evaporative Cooling Towers”, which is incorporated by reference. 
    
    
     BACKGROUND 
     There are many different types of working fluids—fluids that are used primarily to reduce and/or convey energy such as heat and/or to convey matter such as a chemical, molecule and/or an arrangement of molecules from one place to another in a system. The type of fluid used as a working fluid in a system often depends on the specific requirements of the system. For example, oil in an engine is desired because it can withstand a moderate amount of heat and lubricates many moving parts within the engine to reduce the amount of unwanted heat generated by the engine. 
     Water is another type of working fluid and is often used in systems to convey heat and/or matter within the system. For example, water is often used to remove from a system excess heat generated in the system. In such uses, the water flows through a heat exchanger of the system where it absorbs heat from the system. The heated water then flows to another heat exchanger—often a cooling tower—to release its heat to the ambient environment. A cooling tower works by directing ambient air through a flow of water droplets. As the air contacts the water droplets, some of the water&#39;s heat is removed from the water via convection, and some of the water&#39;s heat is removed from the water via evaporation. Consequently, water must be routinely added to the system to replace the water lost from evaporation. 
     Unfortunately, the water of such systems must also be routinely treated to maintain a desired hardness. Hardness is a measure of the amount of cations—positively charged atoms or molecules—in the water, which are usually caused by dissolved metals. The larger the number of cations or amount of dissolved metals in the water, the harder the water. The two most common metals that contribute to water&#39;s hardness are calcium (Ca) and Magnesium (Mg). These metals typically exist as carbonate molecules CaCO 3  and MgCO 3  that when dissolved in water yield Ca +2 , Mg +2 , and CO 3   −2 . Because water that evaporates does not include the metals, and thus the cations, and because water that is added to the system typically does include such metals, the hardness of the water or working fluid of the system increases over time. This increase can lead to excessive hardness in the water which can cause scaling in the system which can foul components of the system like pipes, valves and pressure sensors. Excessive hardness in the water or working fluid can also promote corrosion in the system, causing premature and possibly catastrophic failure. 
     One method for reducing the hardness of the water in such systems is to simply dump a large portion of the water out of the system and replace the dumped water with water whose hardness levels are less than the levels of the dumped water. Unfortunately, this consumes much water because the amount of water needed to replace the dumped water can be large, and the frequency at which the dump and replacement should occur can be high. Also, the water that is dumped is too contaminated to be readily used for something else. To reduce the amount of water dumped and the frequency at which the dump occurs, many chemically treat the water to keep the minerals in a less destructive form. Unfortunately though, chemically treating the water doesn&#39;t allow one to avoid having to periodically dump the water because the water and the chemical program have limits to the amount of minerals and chemicals that they can hold. 
     Thus, there is a need for a system that more efficiently removes unwanted minerals and other unwanted chemicals from a working fluid such as water in a cooling tower system. 
     SUMMARY 
     In an aspect of the invention, a method for removing an unwanted chemical from a working fluid of a system includes: 1) pumping into a tank a working fluid that includes an unwanted, first chemical; 2) injecting into the tank a second chemical in an amount that provides or substantially provides a stoichiometric ratio relative to the amount of the unwanted, first chemical in the working fluid; 3) mixing the second chemical with the working fluid in a first portion of the tank to generate a molecule that precipitates out of the working fluid and that includes the unwanted, first chemical; 4) holding the working fluid in a second portion of the tank to separate the molecule from the working fluid; and 5) ejecting the working fluid from the second portion. The flow rate of the working fluid and the size of the first and second portions of the separation tank are coordinated to make the working fluid take at least 4 minutes to flow through the tank. 
     By providing the second chemical in an amount that provides or substantially provides a stoichiometric ratio relative to the amount of the unwanted, first chemical in the working fluid, and allowing at least four minutes for the second chemical to react with substantially all of the unwanted, first chemical, the working fluid leaves the separation tank substantially free of the unwanted chemical and at a desired pH. Thus, one does not have to dump or blow-down any of the working fluid to remove unwanted chemicals in the fluid. Furthermore, after the system has removed much of the unwanted chemical that initially existed in the working fluid or that accrued over time in the working fluid, the system may be tuned to remove the unwanted chemical that enters the system working fluid via the addition of working fluid to replace losses from system leaks, wind drift losses, and/or evaporation. In such a steady-state situation, the amount of working fluid that flows through the separation tank may be as low as 0.5% of the total flow of the working fluid through the fluid&#39;s system. 
     In another aspect of the invention, a system to remove an unwanted chemical from a working fluid includes a tank having a first portion operable to receive a working fluid that includes an unwanted, first chemical, and a second chemical operable to react with the unwanted, first chemical to generate a molecule that includes the unwanted, first chemical and that precipitates out of the working fluid. The tank also includes a second portion operable to hold the working fluid while the molecule is separated from the working fluid. The system further includes a first valve operable to control the flow of the working fluid into the tank; a second valve operable to inject the second chemical into the tank; a controller operable to instruct the second valve to inject an amount of the second chemical that provides or substantially provides a stoichiometric ratio relative to the amount of the unwanted, first chemical in the working fluid; a third valve operable to purge the molecule from the tank; and an outlet through which the working fluid is expelled from the tank when working fluid flows through the tank. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a schematic view of a system for removing an unwanted chemical from the working fluid of a cooling tower system, according to an embodiment of the invention. 
         FIG. 2  is a partial cross-sectional view of a separation tank included in the system for removing an unwanted chemical shown in  FIG. 1 , according to an embodiment of the invention. 
         FIG. 3  is a schematic view of the separation tank in  FIG. 2  showing the flow of the working fluid through the tank, according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a schematic view of a system  20  for removing an unwanted chemical (not shown) from a working fluid (not shown) of another system  22 , according to an embodiment of the invention. The system  22  may be any system that uses a working fluid to reduce the amount of excess energy, such as heat, that the system  22  generates. The system  22  may also use a working fluid to convey energy, such as heat, and/or matter, such as a chemical, molecule and/or an arrangement of molecules, from one location in the system  22  to another location in the system. Here, the system  22  is a cooling tower system that removes heat from another system such as a power generation system and releases the heat into the atmosphere. In this cooling tower system, water is the working fluid. In operation, the heat generated from the power station is transferred to the water of the cooling tower system  22  via the heat exchanger  24 . The water then carries the heat to the cooling tower  26  where the heat is released to the atmosphere via convection and removed from the water via evaporation. The remaining water then flows back to the heat exchanger  24  via a pump  28  to pick up more heat and perform the cycle again. A make-up water source  30  periodically adds water to the system  22  to replace the water that is lost from evaporation, and/or leaks in the system, such as that caused by wind that blows a portion of the water out of the cooling tower and improperly seated seals in the system&#39;s piping components. 
     The system  20  for removing an unwanted chemical from the working fluid of the system  22  includes a separation tank  32  (discussed in greater detail in conjunction with  FIGS. 2 and 3 ) in which the working fluid is received, mixed with a second chemical (not show) that reacts with the unwanted, first chemical to form one or more molecules (also not shown), and held for a period to separate the one or more molecules from the working fluid. The amount of the second chemical injected into the separation tank is the amount that provides or substantially provides a stoichiometric ratio relative to the amount of the unwanted, first chemical in the working fluid. The stoichiometric ratio is the ratio of the amount of the second chemical to the amount of the unwanted, first chemical such that when their reaction is completed the second chemical and the unwanted, first chemical are consumed. The duration of the period that the working fluid is held in the tank  32  may be any desired duration that allows enough time for the one or more molecules to be generated and separated from the working fluid. By providing the second chemical in an amount that provides or substantially provides a stoichiometric ratio relative to the amount of the unwanted, first chemical in the working fluid, and allowing at least four minutes for the second chemical to react with substantially all of the unwanted, first chemical, the working fluid leaves the separation tank substantially free of the unwanted, first chemical. 
     Although the system  20  may be used to remove an unwanted chemical from any working fluid, the embodiments of the system  20  shown in  FIGS. 2 and 3  and discussed in conjunction with these FIGS. are designed for a system  22  that uses water as its working fluid, such as the cooling tower system  22 . 
     Still referring the  FIG. 1 , the system  20  includes a sensor  34  to obtain information about the water flowing toward the separation tank  32 , and another sensor  36  (discussed in greater detail in conjunction with  FIGS. 2 and 3 ) to obtain information about the water flowing through the tank. For example, in this and other embodiments of the system  20 , the sensor  34  includes four sensors  38 ,  40 ,  42 , and  44 . The sensor  38  includes a temperature transducer to sense the temperature of the water flowing toward the separation tank  32 . The sensor  40  includes a pH transducer to sense the pH of the water flowing toward the separation tank  32 . The sensor  42  includes a conductivity transducer to sense the electrical conductivity of the water flowing toward the separation tank  32 . And, the sensor  44  includes an oxidation reduction potential transducer to sense the amount of dissolved oxygen in the water flowing toward the separation tank  32 . The sensor  36  includes a pressure transducer to sense the pressure of the water flowing inside the separation tank  32 . 
     Other embodiments are possible. For example, the system  20  may include fewer or more sensors depending on the information needed to decide whether or not to modify the operation of one or more of the system components. This information substantially depends on the type of working fluid being cleaned and the type of unwanted chemical being removed from the fluid. 
     The system  20  also includes a second chemical source  46  from which the second chemical is injected into the separation tank  32  through a valve  47 , a valve  48  to control the amount of water flowing through the system  20 , and a chelant source  50  from which a chelants—chelating agent or sequestering agent—may be added to the water before the water returns to the cooling tower system  22 . The second chemical is discussed in greater detail in conjunction with  FIGS. 2 and 3 , and the chelant may be any desired chelant capable of binding with an unwanted, first chemical that exists in the water flowing through the cooling tower system  22  to hinder the unwanted, first chemical&#39;s ability to react with other chemicals in the system  22 . For example, in this and other embodiments, the valve  48  may be a conventional flow control valve that allows infinite adjustment to the amount of water flowing through the system  20 . The chelant may be any natural or man-made carboxylated, or polycarboxylated agent or dispersant capable of stabilizing hardness and other matter in the system, and releasing hardness and suspended matter in the separation tank, when reacting with the second chemical. Additionally, the chelant may include corrosion inhibitors to protect system components from damage. 
     Other embodiments are possible. For example, the system may include fewer or more components, such as a pump to increase the pressure in the flow, depending on the type of working fluid being cleaned and the type of unwanted chemical being removed from the fluid. 
     Still referring to  FIG. 1 , the system  20  may also include a controller  52  to monitor information from a sensor in the system  20 , such as the one or more sensors  36 - 44 , and/or a sensor in the cooling tower system  22 . The system  22  may include a flow meter  54  that provides the controller  52  information about the amount of water flowing through the system  22 , a temperature transducer  56  that provides the controller  52  information about the temperature of the water flowing through the system  22 , and a flow meter  58  that provides the controller  52  information about the amount of water being added to the system  22  from the make-up water source  30 . The controller  52  may then, in response to the information received, instruct one or more of the other components  46 - 50  of the system  20  to perform a function. 
     For example, in this and other embodiments, the controller is a conventional proportional-integral-derivative (PID) programmable logic controller (PLC) that uses Ladder Logic as its programming language to process the information received from one or more of the sensors  36 - 44  and  54 - 58 . In response to an increase in the amount of water added to the cooling tower system  22 , and thus an increase in the amount of cations in the system&#39;s water, the controller  52  may determine whether or not the flow rate through the separation tank  32  is less than the maximum flow rate allowed to keep the water in the separation tank  32  for at last four minutes. If the flow rate is below this maximum, then the controller  52  may open the valve  48  to increase the flow through the system  20 , and increase the amount of second chemical injected into the separation tank to maintain the stoichiometric ratio. By doing this, the system  20  can continue to efficiently remove unwanted chemicals from the cooling tower system&#39;s water. If the flow rate is at the maximum, then the controller  52  may alert a technician of the condition, who can then address the situation. 
     By monitoring the conditions of the water flow through the cooling tower system  22 , and modifying the flow of the water through the system  20 , and the amounts of the second chemical and the chelant injected into the water, the controller  52  may tune the system  20  to efficiently remove an unwanted chemical from the water, and continually tune the system  20  in response to changes in the water conditions in the cooling tower system  22  and/or flow conditions in the system  20 . This may be beneficial when the system  20  is initially used to remove an unwanted chemical from the water because the system may be initially tuned to quickly remove a large amount of the unwanted chemical from the water, and then re-tuned as the concentration of the unwanted chemical reduces. This may also be beneficial when the system  20  is used to maintain a consistent removal of an unwanted chemical from the water because the system can fine tune itself in response to small changes in the flow conditions in the cooling tower system  22  and/or flow conditions in the system  20 . In such a steady-state situation, the amount of working fluid that flows through the separation tank may be as low as 0.5% of the total flow of the working fluid through the fluid&#39;s system. 
     Other embodiments are possible. For example, the controller  52  may include a computer such as a conventional desktop or laptop computer having a Windows, or OSX operating system, and executing a programmable language other than Ladder Logic. Or, the system may not include a controller  52 , but instead be manually tuned by a technician. 
       FIG. 2  is a partial cross-sectional view of the separation tank  32  shown in  FIG. 1 , according to an embodiment of the invention. The tank  32  includes an inlet  60  through which the water and the second chemical enter the tank  32 , a first portion  62  where the water and the second chemical are received and substantially mix to generate the molecule ( 64  in  FIG. 3 ), a second portion  66  that holds the water while the molecule  64  is separated from it, and an outlet  68  through which clarified water is expelled from the tank  32 . The flow rate of the water through the tank  32  and the size of the first and second portions  62  and  66 , respectively, are coordinated to make the water take at least four minutes to flow through the tank  32 . 
     The size and shape of the first and second portions  62  and  66 , respectively, may be any desired size and shape that together retain the water inside the tank for at least four minutes. For example, in this and other embodiments, the first portion  62  of the tank  32  includes an inverted, truncated cone, the second portion  66  includes a cylinder, and the water takes about ten minutes to flow through the tank  32 . The base of the inverted, truncated cone matches an end of the cylinder such that the cone and cylinder together form the tank  32  having a longitudinal axis  70  that is aligned with the cone&#39;s longitudinal axis and the cylinder&#39;s longitudinal axis. Furthermore, in this and other embodiments, the tank  32  is disposed such that the inverted, truncated cone is below the cylinder relative to the direction of gravity&#39;s pull. By orienting the tank  32  in this way, the water and the second chemical enter at the tank&#39;s bottom and percolate through the first portion  62  of the tank  32 . The water then flows up toward the outlet  68 , while gravity pulls any of the molecules  64  that remain suspended in the water to separate the suspended molecules from the water before the water leaves the tank  32  through the outlet  68 . 
     Still referring to  FIG. 2 , the inlet  60  through which the water and the second chemical enter the tank  32 , may be configured as desired to promote mixing of the water and the second chemical. For example, in this and other embodiments, the inlet  60  is located at the bottom of the tank&#39;s first portion  62  and is oriented such that the water and the second chemical flow toward the bottom of the tank  32  as they enter the tank. The water flows through the tube  72  whose exit forms the inlet  60 , and the second chemical flows through the tube  74  whose exit is concentric and coplanar with the exit of the tube  72 . In addition, the tube  74  is moveable relative to the tube  72  along the tank&#39;s longitudinal axis  70  in the direction of the arrows  76  to allow the location of the exit of the tube  74  to be adjusted relative to the location of the exit of the tube  72 . In this manner, the location of the second chemical&#39;s entrance into the tank  32  can be modified relative to the location of the water&#39;s entrance into the tank  32  and within the first portion of the tank  32 , to promote the mixing of the water and the second chemical. 
     To keep the exit of the tube  74  from fouling, a check valve  80  covers the exit of the tube  74 . The valve  80  may be any desired check valve that seals the exit of tube  74  when the second chemical is not injected into the tank  32 . For example, in this and other embodiments, the valve  80  includes two opposing flaps  82  and  84  that are very similar to a duck&#39;s bill. The pressure of the water passing through the exit of the tube  72  and the pressure of the water inside the tank  32  urge the two flaps  82  and  84  together. When together in this manner, the two flaps  82  and  84  seal the exit of the tube  74  to prevent water from entering the tube  74 . To open the check valve  80  the second chemical inside the tube is urged to flow out of the tube  74  through the tube&#39;s exit. When the second chemical reaches the two flaps  82  and  84 , the second chemical urges the flaps  82  and  84  to spread apart, and thus allow the second chemical to enter the tank  32 . 
     Still referring to  FIG. 2 , the second chemical may be any desired caustic or alkaline donor that will react with the unwanted chemical in the water to form a molecule that will precipitate out of the water. For example, in this and other embodiments, the second chemical includes Sodium or Potassium Hydroxide. This reacts well with most scale inhibitors. In other embodiments, the second chemical may include a mixture of caustics or alkaline donors that best suit the water being treated. 
     To control the flow of the second chemical through the tube  74  and into the tank  32 , the system  20  may include any desired valve capable of performing this task. For example, in this and other embodiments, the valve that controls the flow of the second chemical into the tank is the valve  47  (in  FIG. 1 ), which is an electric flow control valve. To control the flow of water through the tube  72  and into the tank  32 , the system  20  may include any desired valve capable of performing this task. For example, in this and other embodiments, the valve that controls the flow of the water into the tank is the valve  48  (in  FIG. 1 ), which is also an electric flow control valve. 
     Still referring to  FIG. 2 , the system  20  includes a valve  86  to purge from the tank  32  the one or more molecules that precipitate out of and are separated from the water in the tank  32 . The valve  86  may be any desired valve that when opened allows the one or more molecules to flow out of the tank  32 , and when closed does not allow water to leave the tank  32  through it. For example, in this and other embodiments, the valve  86  includes five valves  86   a ,  86   b ,  86   c ,  86   d  and  86   e , each controlling the flow of one or more molecules from a respective one of five regions within the tank&#39;s first portion  62 . As discussed in greater detail in conjunction with  FIG. 3 , each of these valves  86   a - 86   e  may be independently opened and closed to control the accumulation of one or more molecules in their corresponding regions within the first portion  62 . After the one or more molecules are purged from the tank  32 , the molecules are directed to an accumulation tank  88  where the volume of the accumulation tank is recorded each time it is filled and emptied for disposal. 
     Still referring to  FIG. 2 , the system  20  also includes a pressure transducer  36  to sense the water pressure in the tank  32 . The pressure transducer  36  may be any desired transducer capable of sensing the water pressure. For example, in this and other embodiments, the pressure transducer  36  includes six transducers  36   a ,  36   b ,  36   c ,  36   d ,  36   e , and  36   f  (discussed in greater detail in conjunction with  FIG. 3 ), each sensing the water pressure at a respective one of six regions within the tank  32 . Each of these transducers  36   a - 36   f  is coupled to a respective one of six tubes  90   a - 90   f  that extend into the water flowing through the tank  32 . Each of the tubes  90   a - 90   f  are disposed at a unique location along the tank&#39;s wall, and each are moveable toward and away from the longitudinal axis  70  to allow the respective pressure transducer to sense the water pressure at any desired location along the radius of the tank  32 . To keep the tubes  90   a - 90   f  clear and thus allow each of the transducers  36   a - 36   f  to accurately sense the water pressure in the tank  32 , clean water is periodically urged through each of the pipes  90   a - 90   f  from the pipe  91  into the tank  32 . 
     Still referring to  FIG. 2 , the system  20  also includes a flow diverter  92  to direct the combined flow of the second chemical and the water away from the longitudinal axis  70 . By directing the combined flow away from the axis  70  as the flow passes through the first portion  62  of the tank  32  and enters the second portion  66 , the combined flow through each of the portions  62  and  66  becomes more uniform and more consistent, and thus less likely to concentrate into a path within each portion that provides the least resistance. The flow diverter  92  may be any desired device capable of directing the flow in this manner and withstanding the corrosive environment inside the tank  32 . For example, in this and other embodiments, the flow diverter  92  includes an inverted, truncated cone whose surface  94  is smooth and parallel with the inside surface of the tank&#39;s first portion  62 . Because the surface  94  is parallel to the inside surface of the first portion  62 , the—flow as it proceeds through the first portion  62  curls upward in a constant cross-sectional manner, and thus the flow through the second portion  66  remains substantially uniform and consistent. 
     The flow diverter  92  also includes a spherical end  96  to help reduce eddys that might occur as the water flows past the diverter  92 . Such eddys can cause a portion of the water flow to stagnate and not exit the tank  32 . In addition, the flow diverter  92  is moveable along the longitudinal axis  70  in the direction indicated by the arrows  98 . This allows one to increase or decrease the cross-sectional area of the flow entering the second portion  66 , improving molecule settling as the—water enters and flows through the second portion  66 . Moving the flow diverter  92  toward the tank&#39;s outlet  68  increases the cross-sectional area of the flow entering the second portion  66 , and moving the flow diverter  92  toward the inlet  60  decreases the cross-sectional area of the flow entering the second portion  66 . 
       FIG. 3  is a schematic view of the separation tank  32  in  FIG. 2  showing the flow of the water through the tank  32 , according to an embodiment of the invention. As the molecules  64  precipitate out of the water in the tank  32 , the molecules  64  aggregate in the first portion  62  and form a bed  100  of molecules  64  that hinders the flow of subsequent water through the tank  32 . By hindering the flow in this manner, the bed  100  acts like a filter that promotes mixing of the second chemical with the water as they flow through the first portion  62 , and traps molecules  64  formed and forming in the water as the water flows through the first portion  62 . Between the bed  100  and the water in the second portion  66  of the tank lies an interface  102  between the two. If the interface  102  is too far into the second portion  66  of the tank  32 , the bed  100  of molecules  64  may plug the tank  32  and stop the flow of water through the tank  32 , or may escape the tank  32  through the outlet  68 . If the interface  102  is not far enough into the first portion  62  of the tank  32 , the water may flow too quickly through the tank  32  and allow some of the molecules  64  and/or unwanted chemical to escape the tank  32  through the outlet  68 . 
     To control the location of the interface  102  between the bed  100  of molecules  64  and the water flowing through the second portion  66  of the tank  32 , the controller  52  (in  FIG. 1 ) monitors the pressure sensed by each of the pressure transducers  36   a - 36   f  (in  FIG. 2 ), and in response to the pressures sensed, opens one or more of the valves  86   a - 86   e  to purge one or more molecules  64  from one or more locations in the bed  100 . Because the water continually flows through the tank  32 , and because the flow of water through the bed  100  is hindered, the water pressure of the flow in the bed  100  is greater than the water pressure of the flow in the second portion. Thus, as shown in  FIG. 3  the water pressure sensed by the pressure transducers  36   e  and  36   f  ( FIG. 2 ) are less than the water pressure sensed by the pressure transducers  36   a - 36   d  ( FIG. 2 ). The change in the water pressure between the transducers  36   d  and  36   e  indicates that the interface  102  lies between the location of the respective tubes  90   d  and  90   e . Because this location of the interface  102  is desired for this tank  32  and this working fluid (water), the controller  52  keeps each of the valves  86   a - 86   e  ( FIG. 2 ) closed. If, however, the controller  52  sees that the pressures sensed by each of the pressure transducers  36   a - 36   f  are the same or substantially the same, then the controller  52  determines that the interface  102  is too far into the second portion  66  of the tank  32 . Because this location of the interface  102  is not desired for this tank  32  and this working fluid (water), the controller  52  opens the valve  86   e  and purges one or more of the molecules from this location in the bed  100  until the controller sees that the water pressure sensed by the pressure transducers  36   e  and  36   f  are the same and less than the water pressure sensed by the pressure transducers  36   a - 36   d . To purge molecules from the bed quicker, the controller  52  may open one or more of the valves  86   a - 86   d.    
     Other embodiments are possible. For example, the controller  52  may monitor, over time, the pressure sensed by one or more of the pressure transducers  36   a - 36   f . Then, in response to a change in the pressure sensed over time by one of the pressure transducers, the controller  52  may open one or more of the valves  86   a - 86   e  to purge one or more molecules  64  from one or more locations in the bed  100 . 
     The preceding discussion is presented to enable a person skilled in the art to make and use the invention. Various modifications to the embodiments will be readily apparent to those skilled in the art, and the generic principles herein may be applied to other embodiments and applications. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein.