Abstract:
The present invention provides a method and system for generating electricity using a drive for the electrical generator that is powered by an engine (e.g., turbine). The method and system uses high pressure hot gases produced by combustion of a fuel and an oxygen-bearing gas, using at least a portion of the electricity generated to power manufacturing plant equipment. Additionally, hot waste gases from the heat engine are transported to a process dryer (e.g., rotary kiln dryers) to dry minerals, salt, pigments, sands, and clay.

Description:
RELATED APPLICATION 
       [0001]    This application claims the priority benefit of U.S. Provisional Patent Application No. 61/925,945, entitled METHOD OF DRYING SALT AND SIMILAR MATERIALS THROUGH THE USE OF HEAT ENGINE WASTE HEAT, filed Jan. 10, 2014, which is incorporated by reference herein. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention is broadly concerned with the use of hot waste gases from a heat engine used in electrical generation, to dry salt, sulfate of potash, or magnesium chloride minerals within process dryers such as rotary kiln dryers, fluidized bed dryers, or dispersion dryers. Even more preferably, at least some portion of the electrical power that is generated is used for running plants and equipment, such as those used for manufacturing those minerals. 
         [0004]    2. Description of the Prior Art 
         [0005]    Drying processes for salt (sodium chloride) and similar products, such as sulfate of potash and magnesium chloride, typically utilize either direct fired natural gas heating or indirect heating through heat exchangers, with steam or a hot oil fluid providing the heat. These dryers may be a rotary kiln dryer of either counter-current or co-current design, a fluidized bed dryer, or a dispersion dryer. While effective, direct fired heating is very inefficient (35-40%) and much energy is lost to the environment. Heating with steam or hot oil is also effective, but also has efficiency limits since the heat energy passes through at least two steps to dry the process material. There is need for a drying system for salt and similar products with an improved efficiency from multiple uses of the energy input into the system (above 75%). 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention provides a method of drying a material. The method comprises providing a source of hot waste gas, and introducing that hot waste gas into a dryer. A material to be dried is introduced into the dryer. The material to be dried is selected from the group consisting of minerals, salts, pigments, sands, and clay. The material has an initial moisture content upon being introduced into the dryer, and the hot waste gas causes the material to exit the dryer at a final moisture content that is lower than the initial moisture content (i.e., the hot waste gas dries the material). 
         [0007]    In another embodiment, the invention provides a method of drying a material selected from the group consisting of sodium chloride, potassium chloride, magnesium chloride, and potassium sulfate. In this embodiment, the method comprises providing a source of hot waste gas having a temperature of at least about 100° C., and introducing the hot waste gas into a dryer. The material to be dried is introduced into the dryer. That material has an initial moisture content upon being introduced into the dryer, and the hot waste gas causes the material to exit the dryer at a final moisture content that is about 90% or lower than the initial moisture content. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    Figure ( FIG. 1 ) is a process flow diagram of the inventive combined heat and power gas motor coupled to a rotary kiln dryer; 
           [0009]      FIG. 2  is a process flow diagram of the inventive combined heat and power gas motor coupled to a fluidized bed dryer; and 
           [0010]      FIG. 3  is a process flow diagram of the inventive combined heat and power gas motor coupled to a dispersion dryer. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0011]    Referring to  FIG. 1 , a combined heat and power gas motor system  10  is depicted. The system  10  includes a generator  12  operably connected to a dryer  14 . Generator  12  is a motor generator that includes an outside combustion air inlet  16  and a fuel (e.g., natural gas) inlet  18 . Generator  12  further includes an exhaust gas outlet  20 , coupled with a fan  22 , and an electrical transmission line  24 . 
         [0012]    The generator  12  can be any conventional generator, but is preferably a combustion-type gas turbine. In a typical gas turbine-driven electrical generating system, an oxidizing gas including air, oxygen or an oxygen rich mixture is fed to a compressor, driven by a gas turbine. The oxidizing gas is compressed and as it exits the compressor into a combustion chamber, it is mixed with a fuel and ignited, producing high pressure hot gases that pass through the gas turbine impacting blades in the turbine, causing them to rotate a shaft that drives the compressor and an electrical generator, thus generating electricity. The high pressure hot gases lose pressure as they expend work on the turbine blades and exit the gas turbine as low pressure or atmospheric pressure hot gases, usually at temperatures described below. 
         [0013]    The dryer  14  includes an exhaust gas inlet  26 , a wet material inlet  28 , an exhaust gas outlet  30 , and a dried material outlet  32 . The dryer  14  can be most any type of dryer, with a preferred type being a rotary kiln dryer. Rotary dryers are slightly inclined cylindrical shells supported by two riding rings running on a set of rollers, and are suitable for drying a wide range of materials because of their ability to process materials having considerable variation in size and composition. A rotary dryer uses lifters mounted in the shell to produce a cascade of particles falling through a hot gas stream. The mechanical lifting of the material allows rotary dryers to be used to dry materials ranging from fine to coarse. It also aids in breaking up lumps, thus promoting a more uniformly dried material. The proper design of a rotary dryer is based upon several factors. The dryer diameter determines the gas velocity. The lifter design determines how the material will fall through the gas stream. The full width of the dryer becomes a shower of material. Chains may also be used when processing very wet material to improve heat transfer. Dryers are also designed for either co-current or counter-current flow, depending on the particular process and application needs. Rotary kiln dryers meeting these requirements are commercially available, such as those sold by Metso Minerals Industries Inc. (Danville, Pa.). 
         [0014]    In use, generator  12  would be operated following “normal” procedures, using the desired fuel coming into fuel inlet  18 , etc. That is, generator  12  would be run just as it would if it were not operably connected to dryer  14 , as part of the heat and power gas motor system  10 . Additionally, the generator  12  will generate electricity can be used for local electrical demand requirements (e.g., to power the facility and/or equipment present or near the facility of the system  10 ), or sold back to a utility. The difference in the inventive system  10  is that exhaust gas outlet  20  of generator  12  is connected to a fan  22 , so that it can be metered to exhaust gas inlet  26  of dryer  14 . It will be appreciated that fan  22  will also preferably include an outlet line or vent (not shown) that allows it to vent the exhaust to the outside (similar to the venting that would occur if the generator  12  were not a part of the system  10 ), for situations where it is not desired to transport any or all of the exhaust gas to the dryer  14 . 
         [0015]    Typical gases that will be found in the waste gas emitted from generated  12  include those selected from the group consisting of carbon monoxide, carbon dioxide, oxygen, nitrogen, argon, and mixtures thereof The gas transported out of exhaust gas outlet  20 , and into exhaust gas inlet  26  should have a temperature of at least about 100° C., preferably from about 200° C. to about 600° C., and more preferably from about 250° C. to about 540° C. Additionally, the gases should be less than saturated with water. More particularly, the gases should have less than about 6% by mass water per cubic meter of gas, preferably less than about 4% by mass water per cubic meter of gas, and more preferably from about 2% by mass water per cubic meter of gas to about 4% by mass water per cubic meter of gas, as measured following a standard Fischer titration. The gases will typically have a pressure of less than about 1 psig (and preferably from about 0.1 psig to about 0.3 psig), but that pressure can be boosted with the fan  22 , as shown. The flow rate of the gas into the inlet  26  will be dependent on the flow rate of the material to be dried, but typical flow rates will fall in the range of from about 130,000 lbs/hour to about 400,000 lbs/hour. 
         [0016]    While hot exhaust gas is being directed into dryer  14  (in this case a kiln dryer), the material  34  to be dried is also metered into the dryer. Examples of material  34  that can be dried with the present inventive method include minerals, salts, salts, pigments, sands, and clay. Ideal salts for use in this method include those selected from the group consisting of sodium chloride, potassium sulfate (sulfate of potash), magnesium chloride, potassium chloride, sodium sulfate, ammonium sulfate, ammonium nitrate, and urea. 
         [0017]    In the system  10  of  FIG. 1 , the exhaust gases are fed co-currently to the material  34 , but it will be appreciated that those gases could be fed counter-currently to the material  34 , depending upon the specific dryer design. The material  34  being introduced into inlet  28  of dryer  14  will have a typical moisture content of from about 5% by weight to about 50% by weight water, preferably from about 5% by weight to about 40% by weight, and more preferably from about 3% by weight to about 30% by weight water, based upon the total weight of the wet material  34  taken as 100% by weight. The flow rate of the material  34  into the inlet  28  is material- and process-dependent, but typical flow rates are from about 0.4 tons/min to about 1.2 tons/min. 
         [0018]    The exhaust gas preferably retains essentially the same temperatures described above as it enters the dryer  14 . As the material  34  passes through dryer  14 , the heat from the exhaust gas evaporates the moisture from material  34 , thus drying it. The water vapor and other gases exit the dryer  14  from exhaust gas outlet  30 , where they can be treated, as necessary, using conventional treatment methods. The dried material  34   a  exits the dryer  14  through dried material outlet  32 , where it is ready to be packaged, further processed, transported, etc., depending upon the particular material and its desired final use. Advantageously, the dried material  34   a  exits outlet  32  with a reduction in moisture content from the initial moisture content of material  34  upon entering inlet  28 . That is, the final moisture content of dried material  34   a  is less than about 50% of the initial moisture content of material  34 , preferably less than about 40%, more preferably less than about 30%, and even more preferably from about 10% to about 20% of the initial moisture content of material  34 . For example, if material  34  entered the dryer  14  with a moisture content of 50%, dried material  34   a  would exit with a moisture content of about 25% or less, preferably about 20% or less, more preferably about 15% or less, and even more preferably from about 5% to about 10%. Thus, the dried material  34   a  will have a typical moisture content of from about 1% by weight to about 15% by weight, preferably from about 1% by weight to about 10% by weight, and more preferably from about 1% by weight to about 5% by weight, based upon the total weight of the dried material  34   a  taken as 100% by weight. 
         [0019]      FIG. 2  depicts an alternative embodiment to that shown in  FIG. 1 , with like numbering representing similar parts. The sole difference between the embodiments is that dryer  14 , which was previously a rotary kiln dryer, has been replaced with a fluidized bed dryer  14   a.  A “fluidized bed” refers to a bed of finely divided solids through which a gas is passed, and which is in a state between that of a static bed and one where all the solids are suspended in the gas stream, as in pneumatic conveying. The introduction of an appropriate gas flow into the material bed brings about the onset of fluidization. Bubbles of gas pass through the bed of material, creating a condition of rapid mixing. The bed has the appearance of a vigorously boiling liquid, and the bed of material takes on many of the properties of a fluid. It exerts a hydrostatic head, and the material will flow through a hole in the vessel, or over and under a weir within the bed. The boiling action in a fluidized bed brings particles into contact with each other, removing dust that is carried off in the gas stream. The same boiling action ensures very thorough mixing, giving uniform temperature conditions and enabling complete drying to take place without overheating the material. Suitable fluidized bed dryers  14   a  can be obtained from Ventilex USA, Inc. (Middletown, Ohio). 
         [0020]    A further embodiment is depicted in  FIG. 3 , where a dispersion dryer  14   b  now replaces dryer  14 / 14   a.  This dryer  14   b  differs the previous embodiments in that wet material  34  enters dryer  14   b  at the top  36  via a mechanical feeder  38 , such as a belt conveyor or screw conveyor. The hot exhaust gas enters the dryer  14   b  via a tangential inlet  40  at the bottom  42  of the dryer  14   b,  creating a circular flow of gas with the dryer  14   b.  This flow, coupled with a specially designed perforated plate (not shown) to address specific wet process material characteristics, picks up the moist process particles, conveys them up through the dryer  14   b,  dries them, and carries them through a combined gas and exhaust outlet  44  on the top  36  of dryer  14   b.  The gas and dried process material are separated downstream, typically via a device such as a cyclone separator  46 . Suitable dispersion dryers  14   b  can be obtained from Allgaier Process Technology (Uhingen, Germany). 
         [0021]    It will be appreciated that modifications can be made to the above to accommodate certain situations. For example, should that temperature be too high for the material to he dried, or for the dryer being utilized, heat exchanger cooling loops could be included on the gas motor to provide heat to a heat transfer fluid (e.g., Therminol manufactured by Eastman, Kingsport Tenn.), which in turn can heat an air stream feeding the dryer. A third option would be to dilute the hot exhaust gases from the gas motor with ambient air to the point that temperature is not an issue for either process material or equipment. This option requires additional auxiliary equipment for air movement and balancing. 
         [0022]    Finally, one of ordinary skill in the art would understand that the gas motor is sized based on the amount of heat needed to dry the wet process material completely, with the amount of electricity generated being dependent on the equipment sizing.