Abstract:
Apparatus and process for distilling a fluid mixture using low temperature glide heat are disclosed. A substantial portion of the glide heat is at a temperature lower than the peak distillation temperature. The disclosure achieves a maximal amount of distillative effect from a given heat source. Applications include absorption refrigeration and absorption power cycles. Referring to FIG.  1,  column  104  and desorber  105  distill fluid in conduit  101  using low temperature glide heat. Divider  108  proportions fluid between them.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     One portion of this disclosure was disclosed in U.S. patent application Ser. No. 10/041,819, filed Dec. 31, 2001. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     One portion of this disclosure was conceptualized under federal contract DEFG36-GO011045. 
    
    
     REFERENCE TO A MICROFICHE APPENDIX 
     Not applicable 
     BACKGROUND OF THE INVENTION 
     There is an increasing need to extract beneficial effect (e.g., power and/or refrigeration) from low temperature heat, which is frequently regarded as waste heat. Heat-activated absorption cycles provide exceptional promise in this regard. 
     Many low temperature heat sources have a temperature glide—the more heat that is extracted, the lower the temperature becomes. All sensible heat sources have that characteristic, e.g., exhaust combustion gas from an engine or furnace, hot process liquids, or geothermal liquid. Pure component latent heat (e.g., saturated steam) is isothermal, but mixed component condensation has a glide. Solar radiation may be regarded as isothermal, but once it is captured in a heat transfer fluid it has a glide. 
     In order to extract maximum useful effect from a glide heat source, the application process needs to accomplish two equally important objectives: the heat acceptance should have a glide comparable to that of the source, and it should reduce the source temperature a maximal amount. These two considerations respectively denote the quality and the quantity of heat input. 
     In principle absorption cycles excel at those two objectives, since they are thermodynamically capable of accepting heat input with temperature glide, and also of being powered by heat down to very low temperatures, e.g., less than 100° C. 
     However, the prior art practice in heat-activated absorption cycles does not make full use of this capability. Flame powering or steam powering has no need for it—all the heat is input at the hottest end of the hottest component of the cycle—the generator. In volatile absorbent cycles, that component is also known as the reboiler. 
     Prior art examples of inputting heat to absorption cycles are found in U.S. Pat. Nos. 3,690,121; 4,307,572; 6,357,255; and International Publication WO 01/94757. 
     With a volatile absorbent, the process of desorbing the sorbate vapor from the sorbent liquid is effectively distillation. When heat is referred to as “low temperature,” it signifies that at least part of it is at a temperature below the peak distillation or de sorption temperature. 
     In its broadest aspect, this invention discloses the distillation of a fluid mixture using low temperature glide heat. Distillation is generically useful in many applications, and this disclosure allows it to be accomplished with lower temperature heat than heretofore possible. The particular focus of this disclosure is on distillation incorporated in an absorption cycle with a volatile absorbent, and with useful effect either chilling or power. 
     Thus included among the objects of this invention is to achieve more beneficial distillation effect from a given low temperature glide heat source than has heretofore been possible, and in the context of an absorption cycle, to achieve more refrigeration and/or power from that source than heretofore possible. 
     BRIEF SUMMARY OF THE INVENTION 
     The above and additional useful objects are obtained by an apparatus for distilling a fluid mixture using low temperature heat comprised of: 
     a) a distillation column; 
     b) a heat recovery vapor generator (HRVG); 
     c) a splitter for the fluid mixture, which directs a minor fraction to the upper reflux portion of the distillation column, and the remainder to the heat recovery vapor generator; 
     d) a flowpath for two-phase fluid from the HRVG to the lower portion of the distillation column; and 
     e) a heat exchanger in the central portion of said column which is supplied column bottom liquid. 
     Related variants of this disclosure extend to various sensible heat recuperation schemes between column fluids, and to the use of more than one heat recovery vapor generator in series, as described further below. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     FIG. 1 illustrates a simplified flowsheet of an absorption refrigeration cycle which achieves high utilization of a low temperature glide heat source in a fin-tube desorber. 
     FIG. 2 illustrates an absorption power cycle which derives high power output from a glide heat source which heats desorber and superheater in parallel. 
     FIG. 3 illustrates an absorption refrigeration cycle heated by engine waste heat, where at least part of the refrigeration cools the engine inlet air. 
     FIG. 4 illustrates a three-pressure absorption refrigeration cycle powered by low temperature glide heat from an engine. 
     FIG. 5 illustrates a desorption/distillation of a fluid mixture using low temperature glide heat wherein there are two heat recovery vapor generators in series, and accordingly two reboils to the column from them, plus also a generator-absorber heat exchanger (GAX) reboil. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, inlet fluid mixture in supply line  101  is distilled to vapor in conduit  102  and bottom liquid in conduit  103  by distillation column  104 , desorber (heat recovery vapor generator)  105 , and solution heat exchanger  106 . After preheating in solution-cooled rectifier  107  and solution heat exchanger (SHX)  106 , the fluid mixture is split by valve  108  into two streams, one as feed/reflux to the column, and the other to co-current downflow desorption in desorber  105 , where it exchanges heat counter-currently with low temperature glide heat. By keeping desorber  105  separate from column  104 , efficient fin tubes  109  can be used to optimize heat transfer from a combustion gas. The two-phase desorbed mixture from the bottom of desorber  105  is phase separated, preferably inside column  104 , where the vapor reboils the column, and the separated liquid joins column liquid and is withdrawn through generator heat exchanger (GHX)  110 , where its heat causes additional reboil. After further cooling in SHX  106 , the bottom liquid (strong absorbent, i.e., strong in absorbing power) is reduced in pressure by valve  111  and enters absorber  112 . Vapor in conduit  102  is condensed in condenser  113 , precooled in precooler  114 , letdown in pressure by means for pressure reduction  115 , and evaporated in evaporator  116 . The mostly evaporated mixture is heated in precooler  114  with further evaporation, and the vapor is absorbed in absorber  112 . Useful chilling is produced by evaporator  116 , and useful heating can be obtained from absorber  112 , plus to some extent from condenser  113  as well. 
     Referring to FIG. 2, weak absorbent solution in conduit  201  is separated into volatile component vapor of at least 95% purity in conduit  202  and strong absorbent liquid in conduit  203  by fractionating apparatus comprised of fractionating column  204  and co-current desorber  205 . Preheater  217  (an “absorption heat exchanger” (AHX) in this example) heats the solution to near saturation temperature before it is divided by divider  208  into a reflux stream for column  204  and a feed stream for desorber  205 . Desorbed mixture from desorber  205  is separated and fractionated in column  204  to bottom liquid and overhead vapor, and the bottom liquid causes additional reboil via heat exchange from GHX  210 . Distilled vapor in conduit  202  is superheated in superheater  218  and work-expanded in expander  219 . The superheating is done over the same approximate temperature range as desorption, i.e., desorber  205  and superheater  218  are heated in parallel, thus maximizing the temperature glide linearity. The expanded vapor is absorbed into the strong absorbent after pressure letdown by valve  211 , in absorber  212 , cooled both by external fluid in the colder section, and by absorbent in AHX  217 . Pump  220  completes the absorbent cycle. The low temperature glide heat can be geothermal liquid, solar heated liquid, combustion exhaust gases, etc. Thus, a simple, economical, and highly efficient absorption power cycle is realized. 
     Referring to FIG. 3, an absorption refrigeration cycle is integrated with a combustion engine such that engine waste heat powers the absorption cycle at HRVG  309 , and the chilling from the absorption cycle cools engine inlet air at evaporator  316 . The engine is comprised of compressor  331 , combustor  333 , and work expander (turbine)  334 . The absorption cycle illustrates one variant from those of FIGS.  1  and  2 —there is an external GHX  335  which supplies additional reboil to column  304  from an additional portion of the feed sorbent liquid, controlled by valve  336 . 
     Referring to FIG. 4, another variant of the absorption refrigeration unit (ARU) integrated with a combustion engine is illustrated. In this example, heat recovery steam generator  439  consumes much of the waste heat from turbine  434 . Hence, in order to obtain sufficient chilling from the ARU, it is necessary to incorporate a second HRVG  440  which is at a lower temperature and pressure than primary HRVG  409 . The remaining components necessary to utilize this even lower temperature portion of glide heat include vapor-liquid separator  442 , intermediate pressure absorber  443 , pump  444 , flow divider  441 , and IP letdown valve  445 . 
     Referring to FIG. 5, a modification of the disclosed desorption/distillation system is presented which allows for a larger temperature glide. In the preceding figures, the desorption (generation heat recovery) step is always a partial evaporation, not total evaporation. Thus the temperature glide possible is always less than the dew point-bubble point difference. FIG. 5 illustrates one method for increasing that glide, to beyond even the dew point-bubble point difference, while still avoiding the problematic total evaporation. Much of the apparatus is common with earlier figures, i.e., column  504 , solution-cooled rectifier (SCR)  507 , GHX  510 , HRVG  505  containing fin tubes  509 , and solution letdown  511 . The new features are the vapor-liquid separator  525  plus a second higher temperature HRVG  526 . Separator  525  sends intermediate reboil to a mid height of column  504 , and only the liquid continues to higher temperature desorption. Also illustrated in FIG. 5 is another means of obtaining additional reboil from the bottom liquid—GAX  527 . 
     The desorption in the heat recovery vapor generator should be either cocurrent or crosscurrent mass exchange, to ensure higher transfer coefficients. The heat transfer should be either fully countercurrent or a hybrid of crosscurrent and countercurrent. The desorption is preferably downflow, to allow the desorber to automatically drain when the solution pump is off, thus avoiding need for a bypass damper. 
     As illustrated in the several figures, there are various options within the basic disclosure for sensibly heating and/or cooling the column liquids so as to achieve higher utilization of the low temperature glide heat. The GHX can be internal to the column or external. Preheating can be done by any of AHX, SCR, and SHX. The vapor-liquid separations can be in separate vessels, or in the sump of the column. Higher temperature heat input can be via a second HRVG as illustrated, or by other prior art methods, e.g. thermosyphons or integrated heating loops. 
     One preferred working pair for the cycle embodiment of this invention is ammonia-water. Other combinations of interest include CO 2  as sorbate and methanol, amine, or other known CO 2  sorbent; light hydrocarbon sorbate (C 4  or smaller) with heavy hydrocarbon sorbent (C 8 +, e.g., alkylate or naphtha); and halocarbons as sorbate with known sorbents such as the glymes. In the distillation embodiment, any fluid mixture is contemplated.