Abstract:
A system and method for separating a fluid mixture is provided which employ vacuum distillation apparatus, and optionally, gas-handling apparatus operable to introduce a gas into the liquid mixture prior to being dispersed within the vacuum distillation apparatus. The liquid mixture is dispersed within the vacuum distillation apparatus as micro-sized droplets. When used, the gas that has been introduced into the liquid mixture is rapidly liberated from the micro-sized droplets thereby causing the droplets to break into still smaller droplets thereby maximizing the vaporization of the more volatile components in the liquid mixture.

Description:
RELATED APPLICATION 
     The present application claims the benefit of U.S. Provisional Patent Application No. 61/170,303, filed Apr. 17, 2009, which is incorporated by reference herein in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The apparatus and methods described herein are useful for separating fluid mixtures. In one embodiment, the apparatus and method can be used to separate a fluid stream comprising water and alcohol into at least one stream comprising primarily alcohol and one stream comprising primarily water. The separation is accomplished through the use of one or more vacuum distillation steps. 
     2. Description of the Prior Art 
     Water and alcohol mixtures form hydrogen bonds in the liquid and on the surface. The surface bonding force is surface tension and in normal distillation processes the escaping alcohol vapors and water vapors need to have enough energy to overcome the surface tension forces (including the chamber pressure). The vapor force needed to overcome the surface tension force is also referred to as vapor pressure. Vapor can escape the liquid when vapor pressure inside the liquid is greater than the surface tension forces. As one would expect, increasing the temperature of a liquid increase the liquids vapor pressure. Boiling begins when the vapor pressure exceeds the surface tension forces. 
     In standard distillation strippers, the trays used to separate the alcohol from the beer are at different temperatures and concentrations of alcohol. The beer in the tray has a depth and a flat surface. The flat surface indicates the surface tension is fixed and constant. The deeper the liquid in the tray the higher the vapor pressure has to be to form a bubble, or there needs to be movement of the fluid in the tray to place an alcohol molecule nearer the surface, reducing the required vapor pressure. The vaporizing process on each tray is in equilibrium and the standard alcohol water vapor-liquid-equilibrium (V-L-E) curve describes the process well. In order to break the alcohol-water azeotrope and recover a highly enriched alcohol distillate, the distillation environment must be changed so as to cause the beer not to follow V-L-E curve behavior. 
     SUMMARY OF THE INVENTION 
     In one embodiment of the present invention there is provided a system for separating a mixture of at least two liquids having different boiling points. The system comprises a supply conduit operably connected with a source of the liquid mixture. The conduit carries the liquid mixture into primary distillation apparatus where it is dispersed. The primary distillation apparatus includes a primary vacuum chamber, a primary rectifier column, and a primary condenser. The system further comprises bottoms distillation apparatus operably connected with the primary distillation apparatus. The bottoms distillation apparatus includes a bottoms vacuum chamber, a bottoms rectifier column, and a bottoms condenser. The primary vacuum chamber is operably connected with the bottoms vacuum chamber by a bottoms conduit carrying a bottoms stream from the primary vacuum chamber for dispersion within the bottoms vacuum chamber. The bottoms condenser is operably connected with the primary lower rectifier column to deliver a vapor stream from the bottoms distillation apparatus to the primary distillation apparatus. Vacuum apparatus is connected with the primary and bottoms distillation apparatus for generating vacuum conditions therein. 
     In another embodiment of the present invention there is provided a system for separating a mixture of at least two liquids having different boiling points. The system comprises a supply conduit operably connected with a supply of the liquid mixture. Gas handling apparatus is connected with the supply conduit and operable to introduce a gas into the liquid mixture. The system further comprises primary distillation apparatus into which the liquid mixture from the gas handling apparatus is dispersed. The primary distillation apparatus includes a primary vacuum chamber, a primary rectifier column, a secondary rectifier column, a primary condenser, and a secondary condenser. The primary condenser is operably connected with the secondary rectifier column to deliver a vapor stream from the primary condenser to the secondary rectifier column. Vacuum apparatus is operably connected with the primary distillation apparatus for generating a vacuum therein. 
     In yet another embodiment of the present invention there is provided a method for separating a mixture of at least two liquids having different boiling points. The method comprises introducing a gas into a feed stream of the mixture and subsequently directing the gas-containing mixture into primary distillation apparatus operating under vacuum conditions. Within the primary distillation apparatus, the mixture is separated into a primary vapor stream predominantly comprising the vapor of the liquid having the lower boiling point and a primary bottoms stream predominantly comprising the liquid having the higher boiling point. The bottoms stream is removed from the primary distillation apparatus, and a bottoms gas is introduced into the bottoms stream. The gas-containing bottoms stream is directed into bottoms distillation apparatus operating under vacuum conditions. Within the bottoms distillation apparatus, the gas-containing bottoms stream is separated into a secondary vapor stream predominantly comprising the vapor of the liquid having the lower boiling point and a secondary bottoms stream predominantly comprising the liquid having the higher boiling point. The secondary vapor stream from the bottoms distillation apparatus is combined with the primary vapor stream. At least a portion of the combined vapor stream is condensed to form an enriched condensate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  schematically depicts a vacuum distillation apparatus in accordance with one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Systems and methods according to the present invention are directed toward separating mixtures of two or more liquids into a low-boiling point liquid stream, predominantly comprising the most volatile liquid of the mixture, and a high-boiling point liquid stream, predominantly comprising the least volatile liquid of the mixture. The apparatus and methods described herein are particularly suited for separating alcohol and water mixtures in an energy efficient manner when compared to traditional methods. The energy savings afforded by the present invention allow for the recovery of relatively pure alcohol streams for a fraction of the cost as other traditional methods. In one embodiment according to the present invention, the recovered alcohol stream comprises at least 90% alcohol, or at least 95% alcohol, or even at least 99% alcohol. 
     Although the methods and apparatus described herein are not limited to alcohol and water mixtures, in certain embodiments the alcohol to be separated from the aqueous mixture is selected from the group consisting of C1-C12 alcohols, diols, polyols and combinations thereof. In particular embodiments, the alcohol selected from the group consisting of methanol, ethanol, propanol, butanol, and combinations thereof. While the method and apparatus can be configured to be used with any relative concentration of alcohol in the water mixture, in certain embodiments the mixture comprises less than about 25%, or less than about 20%, or even less than about 13% alcohol (weight basis) with the balance comprising substantially water. In other embodiments, such as when the mixture comprises ethanol, the alcohol and water mixture comprises between about 0.5-50%, or between about 1-25%, or even between about 1.5-13% by weight alcohol. 
     The methods and apparatus may also be employed in the separation of other materials from fluid mixtures such as volatile organic compounds and finely divided and dispersed solid materials, as would occur during water remediation operations. 
     As shown in  FIG. 1 , a liquid mixture, such as a beer ethanol mixture obtained from a fermentation process, is delivered from a source to a separation system  10  by a primary main pump  14 . In certain embodiments, the mixture is optionally directed through a primary heater  16  where its temperature is raised. As explained below, pre-heating the liquid mixture can help facilitate separation of the more volatile component in the liquid mixture. 
     In certain embodiments according to the present invention, the liquid mixture is then passed through gas-handling apparatus, depicted in  FIG. 1  as a gas diffuser  18  that is operably connected to a supply of gas  20 . Gas diffuser  18  operates to introduce a gas into the liquid, which, as explained in greater detail below, also helps facilitate separation of the more volatile component in the liquid mixture. In particular embodiments, the gas introduced into the liquid mixture by diffuser  18  is carbon dioxide; however it is within the scope of the present invention to utilize other gases as well. In other embodiments, the gas used is capable of dissolving into the liquid mixture. The quantity of gas to be introduced into the liquid mixture can vary depending upon the composition of the liquid mixture and other operating conditions within separation system  10 . In certain embodiments, the gas can be present in the liquid mixture at levels well below saturation or the liquid mixture can be saturated with the gas. 
     After passing through diffuser  18 , the gas-containing liquid mixture is directed toward primary distillation apparatus  22  via conduit  23 . Primary distillation apparatus generally comprises a primary vacuum chamber  24 , a primary rectifier column  26 , and a primary condenser  28 . The liquid mixture is dispersed within vacuum chamber by a nozzle assembly  30  comprising one or more nozzles operable to generate a fine spray of the liquid. In certain embodiments, spraying the liquid mixture results in the generation of an aerosol of micro-sized droplets. The spray forms droplets that have a distribution of sizes. The small droplet projectile in primary vacuum chamber  24  has no gravitational mass placing pressure on it as does the depth of liquid in a traditional distillation tray. The droplet reduces the amount of vapor pressure required to form bubbles compared to tray liquids. Surface tension forces cause the droplets to be spherical. The surface tension on a large droplet is essentially the same as the surface tension on the liquid surface of a tray. However, as droplets get smaller surface tension starts to decrease, due to curvature of the droplet surface. This effect generally takes place at very small droplet sizes. 
     Vacuum chamber  24 , which can be insulated and/or heated, operates under vacuum conditions, which in certain embodiments, is between about 27-28 in Hg of vacuum. With reference to these conditions, the liquid, gas-infused mixture is generally at a superheated condition. With many polar liquids, especially water and alcohol mixtures, the normal making and breaking of hydrogen bonds gives a liquid the characteristics of a fluid. Lowering the temperature of water causes the water molecules vibration to slow down and results in fewer hydrogen bonds being broken. At the opposite end of the spectrum, where the mixture is superheated, the molecular vibrations are high and hydrogen bonds between the alcohol and water are broken. Care must be taken when selecting the conditions within which spraying of the particles occurs because at too low of a temperature, not all of the alcohol vaporizes, but at too high a temperature all the alcohol vaporizes but so does an unnecessary amount of additional water. 
     The positive pump pressure on the mixture holds the diffused gases, which also includes some alcohol and some water in vapor form, in the liquid mixture. Thus, the superheated temperature for a particular process is a function of the pressure within primary vacuum chamber  24  and the concentration levels of the various components making up the liquid mixture. 
     Upon entering primary vacuum chamber  24 , the pressure acting on the liquid droplets falls considerably and there is no longer enough force to hold the gases suspended in the liquid and tiny bubbles start to form inside the droplet. As the liquid and droplets transition from high pressure (pump) to low pressure (chamber) the bubbles (from the diffused gases, alcohol vapors and some water vapors) grow. As the pressure continues to decreases the bubbles grow larger until the bubbles start to fill the droplet. The size of the droplet starts to grow as the displaced liquid has no other place to go but to the surface. As the vapor bubbles and droplet expand fewer hydrogen bonds are holding the water surface molecules to the other water molecules just under the surface. As the droplet continues to expand fewer internal water molecules are available to bond and a point is reached where the expanding bubble pressures overwhelm the surface tension forces and the droplet literally explodes. This releases the diffused gases, all the alcohol vapors and some water vapors. This explosion process releases all the vapors at one time and is an entirely different mechanism for releasing vapors than the classical V-L-E curve mechanism. 
     After the explosion the liquid left form extremely small droplets. These extremely small droplets have a smaller surface tension as discussed earlier. If there is any leftover liquid alcohol its vapor pressure in the extremely small droplets is greater than the reduced surface tension forces and it escapes the liquid using the classical V-L-E method (albeit, with reduced surface tension). These two steps release all the alcohol and some of the water vapor within 250 milliseconds of being sprayed. The vapors concentrations released follow liquid concentrations levels on the V-L-E curve at the process temperature. 
     In certain embodiments according to the present invention, only small amounts of diffused gas, CO 2  for example, are used where the alcohol concentrations are higher (approximately 15% for a water-ethanol mixture) as the droplets are assured to have enough ethanol (or other VOCs) in the droplet for the bubble count to be high enough to overcome the surface tension and explode. Theoretically, there is a homogeneous mixture of alcohol and water, where alcohol is found in every droplet, at the concentration level of the mixture; however, there may actually be a distribution of droplets where some have smaller concentration levels and others higher levels. By adding diffused gas a second distribution of droplets containing the diffused gas is established. The overlap between these two distributions will even out the number of gas molecules (diffused gas, alcohol and water vapor) in the droplet in order for it to explode. When small alcohol concentrations are being distilled, more diffused gases are used to assure there are enough bubbles to break the surface tension and release the vapors. 
     As explained below, liquid reflux from primary rectifier column  26  flows downwardly into the primary vacuum chamber  24 . In certain embodiments, a majority, if not all, of the more volatile components in the reflux vaporize prior to re-entering primary vacuum chamber  24 . However, small to trace amounts of the more volatile components (i.e., the alcohol in the case of an alcohol-water feed) may not vaporize and be collected in the bottom of primary vacuum chamber  24  along with an portions of the sprayed initial liquid mixture not vaporized in the primary vacuum chamber. This bottoms liquid is collected and discharged from primary vacuum chamber  24  using a bottoms pump  32 . While the bottoms from primary vacuum chamber  24  generally contains only a relatively minor amount of the more volatile components present in the initial liquid mixture feed (in some embodiments less than about 2 or 3% by weight based on the total weight of the bottoms stream in the case of an alcohol-water mixture), it may be desirable to recover as much of these volatile components as possible. Therefore, in certain embodiments the bottoms stream is fed through a bottoms heater  34  and/or bottoms gas-handling apparatus which comprises bottoms gas diffuser  36  and gas source  38 . Diffuser  36  may operate in essentially the same manner as gas diffuser  18 , and in particular embodiments operates to deliver CO 2  into the bottoms stream. 
     The bottoms stream is then delivered, via conduit  39 , to bottoms distillation apparatus  40  which comprises a bottoms vacuum chamber  42 , a bottoms rectifier column  44 , and a bottoms condenser  45 . Bottoms vacuum chamber  42  may be configured similar to primary vacuum chamber  24 , in that it too may be insulated and/or heated and comprise a nozzle assembly  46  comprising one or more nozzles operable to generate a fine spray of the bottoms liquid. In bottoms vacuum chamber  42  substantially all of the more volatile components, alcohol for example, is vaporized. The bottom stream  48  of the bottoms vacuum chamber  42  predominantly comprises the less volatile components (i.e., those having a higher boiling point), mostly water and only trace amounts of alcohol, for example, ethanol. Bottoms stream  48  is discharged from system  10  using a discharge pump  50 . 
     The vapor rich in the more volatile components generated in bottoms vacuum chamber  42  is drawn up into bottoms rectifier column  44 . Bottoms rectifier column  44  may contain a packing material or distillation trays which provide a surface area for further mass transfer and separation of the more volatile components (e.g., alcohol) from the less volatile components (e.g., water). The volatiles-rich vapor stream continues through bottoms rectifier column and enters bottoms condenser  45  in which a portion of the volatiles-rich vapor is condensed. The liquid condensate from bottoms condenser  45  may be stored in bottoms reflux tank  52  until it is refluxed to bottoms rectifier column  44  and/or the primary rectifier column  26  at an appropriate entry point within the respective rectifier column. 
     The vapor rich in volatiles from primary vacuum chamber  24  is drawn by vacuum up into the primary rectifier column  26 . Primary rectifier column may also contain a packing material or distillation trays to assist in separation of the more volatile components in the vapor. Upon entering the primary rectifier column  26 , the volatiles-rich vapor from primary vacuum chamber  24  is combined with the volatiles-rich vapor exiting bottoms condenser  45 . The merged volatiles-rich vapor stream is drawn through primary rectifier column  26  in which the vapor stream contacts liquid reflux fed to primary rectifier column  26  by one or more reflux streams. Upon exiting primary rectifier column  26 , the volatiles-rich vapor enters primary condenser  28  wherein a portion of the volatiles-rich vapor is condensed. This condensate may be stored in reflux tank  54  until it is refluxed to primary rectifier column  26 . 
     The vapor exiting primary condenser  28  is directed toward a secondary rectifier column  56  that may be configured similar to primary rectifier column  26 . In secondary rectifier column  56  further separation of the more volatile components from less volatile components occurs. The highly enriched vapor exiting secondary rectifier column  56  is directed to a secondary condenser  58  wherein the vapor stream, now comprising essentially all of the more volatile components (e.g., at least about 99% by weight alcohol in certain embodiments) is substantially entirely condensed. A portion of the condensate from condenser  58  is stored in secondary reflux tank  60  until it is refluxed back to secondary rectifier column  56 . Any refluxed liquid reaching the bottom of secondary rectifier column  56  may be collected and refluxed to primary rectifier column  26  by a rectifier pump  62 . A chiller  64  may be used to cool the refluxed bottoms from secondary rectifier column  56 . 
     Any uncondensed materials (primarily air) exiting secondary condenser  58  are discharged to the atmosphere after passing through vacuum apparatus  66 , which may comprise one or more vacuum pumps. One or more check valves (not shown) may be installed on either the upstream or downstream sides of the vacuum apparatus  66  to prevent leakage into the system while the vacuum pump is shut down. Vacuum apparatus  66  is employed to generate vacuum conditions favorable to effect separation of the volatile components in both primary distillation apparatus  22 , bottoms distillation apparatus  40 , and secondary rectifier column  56 . In certain embodiments, such vacuum conditions are generally between about 27-28 in. Hg of vacuum, which permits the various processes occurring within separation system  10  to be carried out at temperatures ranging from about 32° F. to about 105° F., more preferably from about 60° F. to about 105° F. 
     These relatively low operating temperatures permit a heat pump  68  to be used to supply the necessary heating or refrigeration used in the various heat exchangers of system  10  (e.g., heaters  16 ,  34 , condensers  28 ,  45 ,  58 , and chiller  64 ) thereby resulting in further energy savings. 
     In certain applications, it may be discovered that some portions of system  10  are not essential to recovering a distillate product having the desired purity. For example, in some embodiments, it may be discovered that bottoms distillation apparatus  40  may be omitted or taken off-line. In such instance, the bottoms stream from primary vacuum chamber  24  can simply be discarded. In other embodiments, it may be discovered that secondary rectifier column  56  is not required to achieve a distillate product having the desired purity. In that instance, a distillate may be recovered from primary condenser  28 . 
     It is recognized that various pieces of equipment may be redundantly installed to allow for servicing of the equipment while keeping the overall system operational. For example, multiple vacuum pumps could be installed in parallel. The plurality of pumps may all be utilized during start up of the system, and once a steady state is achieved, one or more pumps could be switched off to reduce operating costs and allow for equipment servicing. Further, additional heat pumps may be utilized to supply the necessary heating and refrigeration. Electric heaters may also be used in place of or to supplement the energy from heat pump  68 .