Abstract:
This invention relates to a method and apparatus for drying organic material using the energy contained in the organic material to be dried to drive the drying process. The organic material could be sewage sludge, food scraps, manure, wood, bagasse etc. The latent heat of evaporation is recovered through the use of a heat pump mechanism. This allows for the retention of a majority of the heat within the system, allowing optimal drying conditions to be maintained throughout the drying process.

Description:
[0001]    The drying of waste organic materials has long been a topic of interest among scientists and engineers, due to the benefits associated with it, among which are decreased transportation costs and potential for combustion or other thermal processes. However, the energy cost associated with thermally drying waste organic materials often outweighs the potential benefits. 
         [0002]    The biodrying process offers a unique drying solution for applications in which the liquid being removed is water, and the material being dried is biodegradable. In biodrying, the latent best of evaporation required for the drying is provided by biological activity in the substrate. This process is sped up by constant controlled aeration, which also provides connective moisture removal. However, vapor removal cools off the reacting mixture, slowing down the biological process and limiting the drying rate and pathogen inactivation. Also, as the material dries beyond 50% solids biological activity is dramatically reduced and further gains in solids content are very slow and costly. As a result, drying times in Biodrying reactors are long, increasing capital and operational costs. 
         [0003]    Existing biodrying technologies make use of a number of configurations to minimize these downsides while still meeting design specifications. However, these solutions require a sacrifice in the form of energy, residence time, or pathogen removal. Open, windrow-type reactors such as those employed by Herhof since the mid 1990&#39;s (1) are easy to retrofit onto existing composting operations and can handle very large quantities of substrate at the cost of lower efficiency values and non-uniform drying. Closed-type hatch reactors with carefully-controlled forced aeration and no mechanical agitation such as those designed by Zawadska and Frei (2) (3) require little energy investment, hut are limited by the amount of drying they can achieve, reaching a maximum of about 50% moisture removal without significantly increasing energy demand. Closed, batch systems with mechanical agitation such as the rotating drum used by Future Fuels (1) or the auger-mixed reactor designed by Choi et al (4) require an energy investment in the form of amendments to reach efficient drying temperatures. Continuous systems, such as the one designed by Navaee-Ardeh et al (5) (6) and Frei et al (3) achieve a good balance between product uniformity and moisture content, but fail to maintain high enough temperatures for pathogen removal making them less useful in processes for which this is necessary. 
         [0004]    Despite the number of biodrying reactor configurations developed to this day, there has been little to no regard to improving the thermodynamics of the process in their design. The proposed invention seeks to fix this issue through the recovery and return of the latent heat of evaporation of the liquid being removed to the reacting mass. The heat of vaporization is the largest contributor to the total enthalpy of the moist gas removed from a Biodrying process, see  FIG. 7 , It is observed that for temperatures between 35 and 60 degrees centigrade 70 to 90% of the total enthalpy is in the form of latent heat of vaporization. Recovering and recycling this heat enables greater ventilation rates without cooling the mass, significantly reducing the drying time, and obtaining a drier product. Furthermore, due to the heat recovery leas of the organic matter energy is necessary for drying and consequently there is a conservation of the energy in the dried product, producing a more energetic biofuel.  FIG. 8  illustrates the impact of the present invention, in (he amount of energy required to dry the product as function of different initial moisture contents. It is observed that this invention dramatically reduces the amount of biological decomposition needed for drying. 
         [0005]    1. Biodrying for mechanical biological treatment of wastes A review of process science and engineering. Veils et al, 2009, Bioresource Technology, pp 2747 
         [0006]    2. Biodrying of Organic Fraction of Municipal Solid Waste. Zawadska et al, 2010, Drying Technology, pp 1220 
         [0007]    3. Novel Drying Process Using Forced Aeration through a Porous Biomass Matrix, Frei et al., 2004, Vol 22, 1191 
         [0008]    4. Composting of High Moisture Materials, Biodrying Poultry Manure in a Sequentially Fed Reactor, Choi el al., 2001, Compost Science and Utilization, 303 
         [0009]    5. Key Variables Analysis of a Novel Continuous Biodrying Process for Drying Mixed Sledge. Navaee-Ardeh et al, 2009, Bioresource Technology 3379 
         [0010]    6. Emerging Biodrying Technology for the Drying of Pulp and Paper Mixed Sludges. Navaee-Ardeh et al., 2006, Drying Technology, 737 
       SUMMARY 
       [0011]    The present invention overcomes the limitations of composting for drying of organic material by recovering the heat of condensation usually lost in the moist air released to the atmosphere. This is accomplished by condensing the air moisture prior to release, recovering the heat, and returning the recovered heat back to the reacting mix. The temperature necessary for practical and efficient drying of the material is therefore maintained, in spite of very low biological rates of reaction. The practice described not only makes the process energy efficient, but minimizes the loss of energy present in the organic material and speeds up the reaction rates, thus minimizing the retention time required for the drying process. 
         [0012]    At the same time, the amount of gaseous emissions from the process and their associated potential for environmental impact due to odors, dust or other volatile organic compounds are minimized. Furthermore, sub-products from the decomposition of the organic material such as ammonia and carbon dioxide are recovered. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0013]      FIG. 1  illustrates a schematic view of a process for drying wet organic materials; in accordance with an embodiment of the present invention; 
           [0014]      FIG. 2  illustrates a schematic view of a process for drying wet organic materials; in accordance with an embodiment of the present invention; 
           [0015]      FIG. 3  illustrates a schematic view of a process for drying wet organic materials: in accordance with an embodiment of the present invention; 
           [0016]      FIG. 4  illustrates a schematic view of a process for drying wet organic materials; in accordance with an embodiment of the present invention; 
           [0017]      FIG. 5  illustrates a schematic view of a process for drying wet organic materials; in accordance with an embodiment of the present invention; 
           [0018]      FIG. 6  Illustrates a schematic view of a process for drying wet organic materials; in accordance with an embodiment of the present invention. 
           [0019]      FIG. 7  is a graph showing the factors contributing to the total energy present in moist air 
           [0020]      FIG. 8  is a graph showing data that describes the impact of drying on energy recovery in the substrate 
           [0021]      FIG. 9  is a graph illustrating the results of applying this invention in a prototype to drying of food waste. 
           [0022]      FIG. 10  is a graph illustrating Temperatures during Food Waste Drying in a Prototype of this invention 
           [0023]      FIG. 11  is a graph illustrating the Relative Humidity of Circulating Air during Food Waste Drying in a Prototype of this invention 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0024]    The subject matter of embodiments of the present invention is described with specificity herein to meet statutory requirements. But the description itself is not intended to necessarily limit the scope of claims. Rather, the claimed subject matter might be embodied in other ways to include different steps or combinations of steps similar to the ones described in this document, in conjunction with other present or future technologies. Terms should not be interpreted as implying any particular order among or between various steps herein disclosed unless and except when the order of individual steps is explicitly described. 
         [0025]      FIG. 1  illustrates a schematic view of one possible embodiment  10  of a continuous drying system for wet organic material which employs the use of a heat pump to recover the heat of condensation. An organic material to be dried  12  enters a drying vessel  14 . A gas mixture  16  enters the drying vessel and is continually distributed throughout the organic material to be dried by the use of a mixing apparatus. The entering gas mixture provides oxygen to support biological activity in the material present in the drying vessel. The biological activity generates heat and the subsequent release of moisture from the material being dried. The gas mixture within the drying vessel incorporates the heat and moisture produced by the biological activity within the drying vessel, and a stream of warm wet gas  18  exits the drying vessel. This warm wet gas is transported by a blower  20  and enters a condenser  22  where precipitation occurs and moisture is expelled as condensate  24 . The dried gas exits the condenser and re-enters the drying vessel as the incoming gas stream  16 . Within the condenser line  26  evaporation occurs and the liquid within is transformed into its gaseous state and exits the condenser. This gas is passed through a compressor  28  where Its pressure and temperature are greatly raised. At this point, supplemental heating may be applied from an outside source  43 . The heated gas  30  then enters the internal portion, of the heat exchanger which is located within the drying vessel where it transfers its latent heat to the contents of said vessel. The cooled gas stream  32  then passes out of the heat exchanger and through an expansion valve  34  where its temperature Is lowered further, prior to re-entering the condenser. 
         [0026]    The process described continues until the material, in the drying vessel reaches a required moisture content at which point the dried material  36  exits the drying vessel. A condenser bypass line  38  is provided to transport the gas leaving the drying vessel via the blower  20  directly back to the drying vessel. This bypass of the condensation step is necessary to allow the accumulation of heat and moisture in the gas mixture needed for the startup of the process. The oxygen content in the gas entering the drying vessel is monitored, and a waste gas stream  40  and a makeup gas stream  42  are used to maintain the oxygen concentration required for optimal biological activity within the drying vessel 
         [0027]      FIG. 2  illustrates another embodiment of the process  10   a  described in  FIG. 1 , wherein the dried gas that exits the condenser  22   a  is pre-heated in an additional heat exchanger  44   a  prior to being returned to the drying vessel  14   a.  Pre-heated, dried gas  46   a  exits the additional heat exchanger to return to the drying vessel as the incoming gas stream  16   a.  Further heat transfer occurs within the drying vessel by means of the heat exchanger within the drying vessel as described in  FIG. 1 . Each component of the process described in  FIG. 2  ( 12   a  through  42   a ) is identical in function to its counterpart in  FIG. 1  ( 12  through  42 ), although it may be different in design. Supplemental heat  47   a  may be added to the fluid after exiting the heat exchanger  44   a.    
         [0028]    The purpose of the adaptation of the process  10   a  is to further optimize the heat transfer that will occur between the gas mixture  16   a  entering the drying vessel and the organic material to be dried. The moisture content within the drying vessel impedes the efficiency of heat transfer due to the loss of heat associated with evaporation of this moisture. This is particularly the case at the onset of the process. The additional heat transfer unit, therefore, provides a means of more efficiently recovering the heat gained in the compressor and making it available for use in the drying process which occurs within the drying vessel. The gas mixture  16   a  which enters the drying vessel after being discharged from the heat exchanger  44   a  is also at a higher temperature than the gas  16  introduced into the drying vessel in the embodiment of the process  10 . The overall increase in heat provided to the material within the drying vessel serves to accelerate the process in embodiment  10   a  as compared to the process  10 . 
         [0029]      FIG. 3  illustrates a third possible embodiment  10   b  of the invention in which the warm wet gas leaving the drying vessel  14   b  is pre-cooled within a heat exchange vessel  48   b  prior to entering the condenser  22   b.  The cooling is accomplished by exposing the incoming warm wet gas stream  50   b  to the cooled dried gas stream exiting the condenser  52   b  as these streams pass simultaneously through the pre-cooling vessel  48   b.  This pre-cooling of the wet gas entering the condenser serves to optimize the removal of moisture in the condenser since less energy is needed to lower the temperature of the gas within the condenser in order for condensation to occur. With the exception of the addition of the pre-cooling heat exchange vessel  48   b  this embodiment of the invention  10   b  is identical to  10   a . Each component of the process described in  FIG. 3  ( 12   b  through  46   b ) is identical in function to its counterpart in  FIG. 2  ( 12   a  through  46   a ), although it may be different in design. Supplemental heat  53   b  may be added to the fluid after exiting the heat exchanger  44   b.    
         [0030]    A fourth possible embodiment  10   c  is illustrated in  FIG. 4 , which is identical to embodiment  10   a  illustrated in  FIG. 2 , except that there is no heat exchanger within the drying vessel  54   c.  Furthermore, a supplemental heat source  47   c  may be incorporated tor increased heat transfer. In this embodiment, heat transfer occurs by direct contact of the gas mixture  16   c  with the material within the drying vessel. Each of the components described in  FIG. 4  ( 12   c  through  46   c ), is identical in function to its counterpart in  FIG. 2  ( 12   a  through  46   a ), although it may be different in design. The process configuration  10   c  would simplify the design of the drying vessel at the expense of efficiency of heat transfer as compared to the configuration  10   a  illustrated in  FIG. 2 . 
         [0031]      FIG. 5  illustrates yet another possible embodiment of the invention  10   d  in which a pre-drying vessel  60   d  is implemented for optimization of the drying process. Partially dried organic material  62   d  Is passed from the pre-drying vessel, to a final drying vessel  54   d.  The embodiment of the invention  10   d  maximizes heat transfer between the heated dried gas and the material within the drying vessel due to the decreased moisture content within the final drying vessel  54   d.  Each of the components described in  FIG. 5  ( 12   d  through  46   d ) is identical in function to its counterpart in  FIG. 4  ( 12   c  through  46   c ), although it may be different in design. Supplemental heat  63   d  may be added to the fluid after exiting the heat exchanger  44   d.    
         [0032]    In this configuration, the condenser bypass line  38   d  enters an additional heat exchanger  56   d  where the temperature of the wet gas mixture is increased without the removal of moisture prior to its re-entering the pre-drying vessel  60   d.  The bypass line is used during initial startup and when a new hatch of material is added to the pre-drying vessel while pre-dried material is being processed concurrently in the final drying vessel, In normal operation, the wet gas mixture leaves the pre-drying vessel and passes through the condenser  22   d  and a heat exchanger  44   d.  The use of separate heat exchangers  56   d  and  44   d  for the two streams of gas leaving the pre-drying vessel allows for optimal retention of moisture in the case of the condenser bypass line  38   d,  and optimal drying in the case of the gas which passes through the condenser. The dried heated gas  46   d  which exits the heat exchanger  44   d  becomes the inlet gas mixture  16   d  for the pre-drying vessel  60   d  and the final drying vessel  54   d.  The heal and water from the moist material In the final drying vessel is combined with that from the pre-drying vessel  18   d  and follows the process previously described. 
         [0033]    In another embodiment of the invention  10   e , illustrated in  FIG. 6 , a drying vessel with an internal heat exchanger, as in embodiments  10 ,  10   a  and  10   b  (See  FIG. 1 .  FIG. 2  and  FIG. 3 ), is used as the final drying vessel. The flow of the gas mixture through the pre-drying vessel  60   e  in this embodiment of the invention is identical to that of the drying vessel  54   c  described for embodiment  10   c  in  FIG. 4 . Each of the components described in  FIG. 6  ( 12   e  through  46   e ) is identical in function to its counterpart in  FIG. 4  ( 12   c  through  46   c ), although it may be different in design. Supplemental hear  65   e  may be added to the fluid after exiting the heat exchanger  56   e.    
         [0034]    This embodiment is identical to embodiment  10   d  illustrated in  FIG. 5  in that it makes use of a pre-drying vessel  60   e  from which partially dried material  62   e  is passed to the final drying vessel  14   e  in order to optimize the removal of moisture from the material to he dried. Heat for the drying process in the final drying vessel  14   e  is provided by heat transfer by the heat exchange unit within said drying vessel 
         [0035]      FIG. 9  Illustrates data obtained in testing the present invention in a pilot scale prototype. The prototype consisted of a rotating drum biodrying reactor connected to a compression heat pump that recirculated air in an out of the dryer unit. The Biodryer was fed food waste that was previously macerated to size of ¼ of an inch. Samples of the food waste mixture were taken, from the dryer over time and analyzed in the laboratory to obtain moisture content. The results are presented in  FIG. 9 . It is clear that the drying process took place significantly faster than reported for other Biodrying experiences. It takes usually 5 to 10 days to reduce moisture from 75% to 30%. Here the process occurred in a matter of hours, 
         [0036]      FIG. 10  illustrates the on-line measurements of temperatures in the prototype during the same run presented in  FIG. 9 . The temperature in the mixture rose to 30 C and equilibrium was reached between the reacting mixture and the surrounding air. Heat of condensation was transferred from the dry air returning front the heat pump at a temperature close to 45 C. The water evaporation during the drying process cools the mixture and the air somewhat. 
         [0037]      FIG. 11  illustrates the relative humidity of the air circulating in and out of the Biodryer during the same prototype run as in  FIGS. 9 and 10 . The warm, air coming from the heat pump was dry with a low, 10%, relative humidity, while the air out of the unit was moist with a relative humidity of 60%. This illustrates the correct functioning of the unit. Moisture is removed from the moist organic reacting material and conducted to the cold side of the heat pump for condensation. Air is dried and heated and returned to the unit to recover the heat of condensation.