Abstract:
A fluidized spray vessel. A vessel design is provided for recovering heat from gaseous heat streams. The vessel utilizes a semi-fluidized bed for obtaining desirable liquid/vapor contact times. A spray section is provided in which liquid is sprayed through nozzles designed to provide a mean droplet size having a terminal velocity of from about sixty percent to about ninety five percent of the superficial upward gas velocity. These spray tower design criteria enhance spray tower performance, and thus enables more efficient heat recovery to be practiced, particularly in systems where relatively low grade heat sources are encountered.

Description:
RELATED APPLICATIONS 
   This application is based on, and claims priority from U.S. Provisional Patent Application Ser. No. 60/306,401, filed on Jul. 17, 2001 the disclosure of which is incorporated herein by this reference. 

   TECHNICAL FIELD 
   This invention relates to recovery of heat from hot gas streams, and, where appropriate, to the recovery of heat from moderate temperature combustion gas sources, such as boilers and incinerators. More specifically, the invention is directed to novel structures and methods for recovery of heat by direct contact of water with a hot gas stream. 
   BACKGROUND 
   Although various methods and structures have been provided for recovery of waste heat, in so far as is known to me, conventional counter-current spray towers heretofore have not provided for more than one transfer unit for either mass or energy transfer systems. In part, this is because in such conventional spray tower designs, droplets fall through a rising gas in which the gas superficial velocity is at only a fraction of the terminal velocity of entering droplets. 
   In contact devices, it is important to observe that as the average droplet diameter decreases, the total surface area for the liquid increases (area is proportional to 1 divided by the diameter of the average droplet). Also, an average contact period (dwell time) for a droplet entering a contact chamber depends on the terminal velocity of the droplet, its trajectory, and the path distance, as well as upon the velocity of the gas encountered. 
   Unfortunately, conventional spray tower design has not matched nozzle design developments. For the most part, conventional spray tower designs have ignored the use of any droplet diameter component, as a consequence of using design methods such as the Souder-Brown equation, in which no droplet diameter component appears. Thus, it would be desirable to provide an improved spray tower that utilizes improved spray nozzle technology to develop a narrow range of liquid droplet particle size. Also, it would be desirable to enhance spray tower performance by providing spray nozzles that maximize droplet surface area. Finally, it would be desirable to provide a spray tower in which dwell time is optimized, so as to optimize heat transfer between the droplet and the gas stream through which it flows. 
   SUMMARY 
   A novel semi-fluidized spray tower design has been developed, and is disclosed herein. The spray tower has been selected with spray nozzles with a predetermined mean droplet size and surface area. Increased droplet dwell time in the countercurrent gas stream is provided, compared to conventional spray tower design criteria. In one embodiment, a spray tower built according to this new method has three distinct sections, including, from bottom to top, (1) a fluidization section, (2) a semi-fluidization spray section, and (3) a coalescing section. 
   In one embodiment, such an innovative spray tower is provided in a single chamber design. 
   In yet another embodiment, the spray tower is provided in a two chamber design. 
   In various embodiments, the spray tower is provided in an open system, where water to be heated directly contacts the hot gas stream. 
   In other embodiments, the spray tower is provided in a closed system, where water to be heated does not directly contact the hot gas stream. 
   Various embodiments of the invention are disclosed in which the mechanical or functional features described herein are achieved in disparate physical configurations. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
     In order to enable the reader to attain a more complete appreciation of the invention, and of the novel features and the advantages thereof, attention is directed to the following detailed description when considered in connection with the accompanying drawings, wherein: 
       FIG. 1  shows a generalized system schematic that shows a process generating waste heat, a conduit for taking a hot gas stream containing the waste heat to the spray tower, and the innovative spray tower design provided herein. 
       FIG. 2  provides a detailed view of a dual chamber fluidized spray vessel design. 
       FIG. 3  provides a vertical schematic of a dual chamber fluidized spray tower, indicating certain key dimensional data. 
       FIG. 4  provides a vertical schematic of a single chamber fluidized spray tower, indicating certain key dimensional data. 
       FIG. 5A  provides a key to understanding the configurations illustrated in  FIGS. 5B ,  5 C,  5 D, and  5 E. 
       FIG. 5B  shows a brief process diagram illustrating the use of a dual chamber, open or direct contact type system incorporating a fluidized spray tower. 
       FIG. 5C  shows a brief process diagram illustrating the use of a dual chamber, closed or indirect contact type system incorporating a fluidized spray tower. 
       FIG. 5D  shows a brief process diagram illustrating the use of a single chamber, open or direct contact type system incorporating a fluidized spray tower. 
       FIG. 5E  shows a brief process diagram illustrating the use of single chamber, closed or indirect contact type system incorporating a fluidized spray tower. 
   

   In the various figures, a prime mark (′) has been utilized to denote similar features or structures amongst the various embodiments, where appropriate, without further mention thereof. In such cases, the reader is referred to the discussion of the feature or structure with respect to other embodiments where similar features or structures were earlier introduced or explained, and a prime mark was not utilized in the referenced figure. 
   The foregoing figures, being exemplary, contain various elements that may be present or omitted from actual implementations depending upon the circumstances. An attempt has been made to draw the figures in a way that illustrates at least those elements that are significant for an understanding of the various embodiments and aspects of the invention. However, various other elements of the exemplary spray tower and a method of using the same to recover waste heat are also shown and briefly described to enable the reader to understand how various optional features may be utilized, in order to provide an efficient, reliable, semi-fluidized bed spray tower system. 
   DETAILED DESCRIPTION 
   In  FIG. 1 , an overall system configuration is depicted for a typical application for an innovative fluidized spray tower.  FIGS. 1 and 2  depict the operation of a basic, two chamber type open spray tower design. In an open type design, there is direct contact between the hot gas stream and the liquid medium, normally water, which is to be heated. 
   Process equipment  10  such as a boiler generates hot exhaust gas  12 . Hot exhaust gas may also be advantageously provided from an engine, such as a gas turbine engine. Or, the hot exhaust gas may be provided from a process gas stream in an industrial process plant such as a paper mill. Such hot gas  12  may include as primary constituents, water vapor, carbon dioxide, nitrogen, and a little oxygen, for example, in a typical boiler stack application. The hot gas  12  is provided to spray tower  20  through a hot gas conduit  22 . Spray tower  20  structures may be fabricated using conventional fabrication techniques in a vertically standing substantially tubular cylindrical shell design. However, other convenient shapes may be utilized, and any of such equivalent structures may be utilized according to the teachings herein in a method of achieving heat recovery in a semi-fluidized direct contact heat transfer apparatus. 
   As better seen in  FIG. 2 , the hot gas  12  enters the spray tower  20  through a hot gas inlet  24 , located in the lower portion  26  of the spray tower  20 . The hot gas  12  is substantially prevented from downward escape by a waste condensate pool  32 . Waste condensate  35  travels to sewer  36  through waste condensate drain  34 . 
   After entry into spray tower vessel  20 , the hot gas  12  gas enters the fluidization section  30  at the bottom portion of the spray tower  20 . In the fluidization section  30  of tower  20 , the upward gas velocity as represented by reference arrows  37  is designed for 200 percent or more of the terminal velocity of the mean droplet size of the liquid medium (usually water) preselected for the spray nozzles in the device, as further described herein below. In this section, it is desirable to prevent the downward flow and escape of liquid droplets. 
   A liquid medium such as cold water stream  41  is provided through cold water inlet  42 . Water droplets  43  of a pre-determined mean droplet size are generated by one or more sets of spray nozzles  40  that are provided in fluid communication with water inlet  42 . The cold water stream  41  emerges through spray nozzles  40 , which sprays droplets  43  downward, thus opposing the up flowing internal gas stream indicated by reference arrows G. 
   In the mid-tower semi-fluidized spray section  48 , spray nozzles  40  (see  FIG. 3 , for example) are oriented to distribute droplets evenly downward over a cross-sectional area, in one embodiment, oriented perpendicular to the spray tower  20  vertical axis. Spray nozzles  40  are designed and provided to develop a pre-determined mean droplet size having a terminal velocity from about sixty (60) percent to about ninety five (95) percent of the local superficial upward gas velocity, the flow of which is indicated by reference arrows  50 . Thus, in the upward flowing gas stream, the droplets fall relative to a fixed reference point along the vertical axis (indicated along centerline  52 ) at a rate from about five (5) percent to about forty (40) percent of their terminal velocity. Of course, in any spray nozzle system, some droplets are generated in a spectrum of droplet sizes that includes droplets larger and smaller than the mean preselected size. However, very small droplets entrain in the upward flowing gas stream and leave the semi-fluidized section  48 . If such droplets do not impinge on the containment vessel interior walls  54  or other droplets  43 , they are carried upward into the coalescing section  56  above the spray nozzles  40 . However, large droplets, and those that become large droplets, fall, growing as they combine with other droplets, and eventually pass out of the semi-fluidized section and into the fluidized section. Other droplets  58  impinge on the tower walls and then flow down into the contact water reservoir  74 . Initially, substantially all small water droplets  43  of preselected size are suspended at the top of the fluidized section  30 , and do not fall down through the section until they agglomerate with other particles by increasing their size (droplet  43 ′) and terminal velocity to ultimately become larger particles  44 , which particles fall downward into waste condensate pool  32 . 
   At the top of the tower, above spray from nozzle(s)  40 , coalescing section  56  is provided in which a coalescing device  68  acts as a target to impinge and /or to intercept entrained droplets  67 . The entrained droplets  67  are thus mostly captured by coalescing into larger droplets, and then the larger droplets  69  fall back from the coalescing section  56  into the semi-fluidized section  48 . 
   A cooled gas stream  70  leaves the spray tower  20  at a cooled gas outlet  72 . The heat removed from the entering hot gas stream  12  is thus captured in contact water contained in the contact water reservoir  74 , supported by reservoir bottom plate  76 . In the embodiment shown in  FIG. 2 , the reservoir bottom plate  76  is located intermediate the hot gas inlet  24  and the cooled gas outlet  72 . A hot water stream  80  exits the reservoir  74  space outward via contact water reservoir outlet  82 . Pump  83  can be provided to recirculate the water exit stream  80  for reuse in the semi-fluidized portion of spray tower  20 , with makeup cold water stream  41  provided as necessary. 
   With the operation of the basic two chamber type, open system spray tower  20  design having been described, as particularly set forth in  FIG. 2  and more generally in  FIG. 5B , it is appropriate to describe alternate embodiments and additional structural details. First, with respect to  FIG. 2 , in the mid-portion  100  of tower  20 , the contact water reservoir bottom plate  76  supports not only the contact water  101  captured, but also provides support for, and is sealingly affixed to, an upward oriented first gas passageway  102 , tubular in nature, and in the embodiment shown in  FIG. 2 , a cylindrical tube that is located along the centerline  52  of the spray tower  20 . At the lower end  104  of first gas passageway one or more baffle(s)  106  and endplate  108  provide for a desirable change in direction of entering gas, to help deflect droplets. At the upper end  110  of first gas passageway, one or more baffle(s)  112  and endplate or hat portion  115  provide for deflection of downwardly oriented spray of droplets, and provide a tortuous gas path having desirable change in direction for the upwardly direct gas  116  exiting the first gas passageway  102 . 
   At the upper portion  120  of the spray tower  20 , a second gas passageway  122  is provided. As shown in the embodiment depicted in  FIG. 2 , the second gas passageway  122  is also of a cylindrical tubular shape. At the lower end  123  of the second gas passageway  122 , one or more baffle(s)  124  are provided as well as end plate or target  126  (circular, as depicted affixed to baffles  124 ), to assist in impinging and/or intercepting droplets, by providing a tortuous gas pathway through which the exiting gas must flow, in order to minimize droplets lost via entrainment. 
   At the upper water level limit  150  of the reservoir  74  for contact water or other liquid medium, a downwardly extending reservoir drain pipe  152  is provided, extending from upper end  151  downward through bottom plate  76  and on downward toward the lower portion  26  of the vessel  20 , to a lower end  153 , in fluid communication with drain  34 , and thus allowing condensate  154  to join waste condensate  35  to drain out of vessel  20  through the waste condensate drain  34 . 
   In other embodiments, a closed process system design can be provided as indicate in  FIGS. 5C and 5E . First, in  FIG. 5C , water  80  leaving the contact water reservoir  74  is sent to a pump  200 , which provides motive force for sending the water through a heat exchanger  202 . Heat exchanger  202  is provided with a cold water supply stream  204 , which cold water supply stream is heated in the heat exchanger  202  to provide a hot, non-contact water stream  210  exiting the heat exchanger  202 . The cooled contact water stream  206  enters vessel as the inlet cold water stream at spray nozzles  220 . 
   A single chamber embodiments is illustrated in  FIGS. 5D and 5E . Like in the case of a dual chamber design, the single chamber design can be provided in either (1) a direct contact design, or (2) a closed system, non-contact design. Note that in the single chamber design depicted in these figures, the bottom portion  30  as shown in vessel  20  of  FIG. 2  is dispensed with, and the hot gas enters directly under baffling  300  and shortly encounters spray from nozzles  302  and/or  304 . Note that both an outside, cold water inlet stream  310  is provided, as well as a recycle stream  312 , sent through pump  314 , to further warm the process water recirculating in the unit. Pump  314  also serves as a hot contact process water  316  outlet. Overflow is sent outward through internal reservoir outlet or drain  152 ′ and is then sent to sewer  36  or other appropriate end use or disposal point. If the configuration is for a closed system design, as set forth in  FIG. 5E , then a heat exchanger system as earlier explained in relation to  FIG. 5C  is utilized. 
   Turning now to  FIG. 3 , some exemplary dimensional data for one desirable embodiment of spray vessel  20 ′ are illustrated. As shown, the spray nozzles  40 ′ are located a distance S apart, vertically. From the upper row of nozzles  40 ′ 1  to the top of the vessel  20 ′, a distance 3.5S is provided. From the lower nozzle  40 ′ 3  a distance of 2S is provided above the outlet end  115  of the first gas passageway  102 . Also, first gas passageway  102  is shown in a 48 inch height, which may be desirable in many cases, but that distance should be considered merely exemplary for this one embodiment. Various other dimensions are detailed, including a lower portion  30  (reference  FIG. 2 ) dimension of 3.5 times the diameter “d” of the gas outlet  72 . A sloping bottom sump  400  is provided in a height of 0.5 times the overall vessel  20 ′ diameter D. 
   Similar dimensions are indicated in  FIG. 4  for a single vessel chamber design of the type schematically illustrated in  FIGS. 5D and 5E . 
   It is to be appreciated that the various aspects and embodiments of the fluidized spray tower designs described herein are an important improvement in the state of the art, especially for recovery of heat from low grade heat sources. Although only a few exemplary embodiments have been described in detail, various details are sufficiently set forth in the drawings and in the specification provided herein to enable one of ordinary skill in the art to make and use the invention(s), which need not be further described by additional writing in this detailed description. Importantly, the aspects and embodiments described and claimed herein may be modified from those shown without materially departing from the novel teachings and advantages provided by this invention, and may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Therefore, the embodiments presented herein are to be considered in all respects as illustrative and not restrictive. As such, this disclosure is intended to cover the structures described herein and not only structural equivalents thereof, but also equivalent structures. Numerous modifications and variations are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention(s) may be practiced otherwise than as specifically described herein. Thus, the scope of the invention(s), as set forth in the appended claims, and as indicated by the drawing and by the foregoing description, is intended to include variations from the embodiments provided which are nevertheless described by the broad interpretation and range properly afforded to the plain meaning of the claims set forth below.