Low Grade Heat Recovery from Process Streams for Power Generation

Abstract

Methods are described for generating electrical power from low grade heat sources from refining and petrochemical processes, including overhead vapors from vapor-liquid contacting apparatuses such as distillation columns, absorbers, strippers, quenching towers, scrubbers, etc. In many cases, these overhead vapors exit the apparatuses at a temperature from about 90° C. (194° F.) to about 175° C. (347° F.). Rather than rejecting the low temperature heat contained in these vapors to cooling air and/or cooling water, the vapors may instead be used to evaporate an organic working fluid. The vapors of the working fluid may then be sent to a turbine to drive a generator or other load.

1 . A method for generating electrical power from a low grade heat source, the method comprising:
 (a) passing a hydrocarbon-containing feed to a vapor-liquid contacting apparatus to provide an overhead product and a bottoms liquid;   (b) indirectly exchanging heat between the overhead product and an organic fluid to provide a cooled overhead product and a vapor-enriched fluid; and   (c) passing the vapor-enriched fluid to a turbine to generate electricity and provide a turbine exhaust.   
     
     
         2 . The method of  claim 1 , further comprising,
 (d) condensing the turbine exhaust to regenerate the organic fluid, and   (e) pumping the organic fluid to an evaporator for indirect heat exchange with the overhead vapor in step (b).   
     
     
         3 . The method of  claim 1 , wherein the hydrocarbon-containing feed stream comprises at least about 90% hydrocarbons by weight. 
     
     
         4 . The method of  claim 1 , wherein the vapor-liquid contacting apparatus is a distillation column. 
     
     
         5 . The method of  claim 4 , wherein the distillation column is a product fractionation column of a hydrocracking process. 
     
     
         6 . The method of  claim 1 , wherein, in step (b), the overhead product has a temperature from about 90° C. (194° F.) to about 150° C. (302° F.) immediately prior to exchanging heat and the cooled overhead product has a temperature from about 65° C. (149° F.) to about 100° C. (212° F.) immediately subsequent to exchanging heat. 
     
     
         7 . The method of  claim 1 , wherein the organic fluid comprises a fluorocarbon or a chlorofluorocarbon. 
     
     
         8 . The method of  claim 2 , wherein step (d) is carried out using an air cooled exchanger. 
     
     
         9 . The method of  claim 1 , wherein indirect heat exchange in step (b) comprises (i) indirectly exchanging heat between the overhead vapor and an intermediate heat transfer medium and (ii) indirectly exchanging heat between the intermediate heat transfer medium and the organic fluid. 
     
     
         10 . The method of  claim 9 , wherein the intermediate heat transfer medium is water. 
     
     
         11 . The method of  claim 9 , wherein, immediately prior to step (b)(ii), the heat transfer medium comprises heat that is transferred from the overhead vapor and from at least one additional source of low grade heat. 
     
     
         12 . The method of  claim 11 , wherein the additional source of low grade heat is a refinery or petrochemical process stream having a temperature from about 90° C. (194° F.) to about 150° C. (302° F.). 
     
     
         13 . The method of  claim 1 , wherein, a net benefit of electricity generated in step (c) is from about 0.005 to about 0.02 watts per BTU/hr of waste heat in the hydrocarbon-containing feed stream. 
     
     
         15 . A method for generating electrical power from a low grade heat source, the method comprising:
 (a) passing liquid and vapor feeds to upper and lower sections, respectively, of a vapor-liquid contacting apparatus to provide an overhead vapor and a bottoms liquid;   (b) indirectly exchanging heat between the overhead vapor and an organic fluid to provide a cooled overhead product and a vapor-enriched fluid; and   (c) passing the vapor-enriched fluid to a turbine to generate electricity.   
     
     
         16 . An apparatus for generating electrical power a from low grade heat source, the apparatus comprising:
 (a) an evaporator for indirectly exchanging heat between the low grade heat source and an organic fluid;   (b) a turbine in communication with a vapor-enriched fluid conduit from the evaporator;   (c) a condenser in communication with an exhaust conduit from the turbine; and   (d) a cooler in communication with a low grade heat source outlet from the evaporator.   
     
     
         17 . The apparatus of  claim 16 , wherein the condenser is an air cooled exchanger. 
     
     
         18 . The apparatus of  claim 16 , wherein both the evaporator and condenser are tubular heat exchangers. 
     
     
         19 . The apparatus of  claim 18 , wherein the evaporator, the condenser, or both have disposed therein a plurality of tubes having fins or recessions on their surfaces. 
     
     
         20 . The apparatus of  claim 19 , wherein the plurality of tubes of the evaporator, the condenser, or both have circumferentially extending fins having outer edges that include a plurality of notches.