Waste Heat Recovery in a Chemical Process and Plant, Particularly for the Synthesis of Ammonia

Abstract

A method for recovering waste heat in a process for the synthesis of a chemical product, particularly ammonia, where the product is used as the working fluid of a thermodynamic cycle; the waste heat is used to increase the enthalpy content of a high-pressure liquid stream of said product ( 11 ), delivered by a synthesis section ( 10 ), thus obtaining a vapour or supercritical product stream ( 20 ), and energy is recovered by expanding said vapour or supercritical stream across at least one suitable ex-pander ( 13 ); the method is particularly suited to recover the heat content of the syngas effluent after low-temperature shift.

1 . A process for the synthesis of a chemical product, comprising the steps of:
 obtaining at least one make-up reactant in a front-end section;   reacting said least one make-up reactant in a synthesis section, obtaining said product in a high-pressure liquid state;
 heating at least a portion of the liquid product delivered by the synthesis section by heat exchange with a waste heat source made available by said process, obtaining an expandable stream of said product in a vapour state or supercritical state; 
 expanding said expandable product stream to recover energy, obtaining an expanded product stream, and 
 condensing said expanded product stream by heat exchange with a suitable cooling medium, obtaining a product condensate stream. 
   
     
     
         2 . The process according to  claim 1 , said waste heat source being one or more process stream(s) at a temperature below 350° C. 
     
     
         3 . The process according to  claim 2 , said waste heat source comprising one or more of the following: a process stream taken from said front-end section; a process stream taken from said synthesis loop; a flue gas from a combustion process; a flue gas from a reforming process of said front-end section. 
     
     
         4 . The process according to  claim 1 , wherein a portion of the product condensate is pumped substantially at the same pressure of said synthesis section; said portion of the product condensate is then re-heated by heat exchange with said waste heat source or a further waste heat source; after said heating, said portion of product stream is then expanded to recover energy and condensed back to liquid, thus forming a closed loop. 
     
     
         5 . The process according to  claim 1 , said chemical product being ammonia, said liquid product being liquid ammonia at a pressure in the range 80-300 bar. 
     
     
         6 . The process according to  claim 5 , wherein:
 said liquid ammonia at a pressure of 100-180 bar and a temperature in the range −30 to 10° C. is heated to around 250° C. by heat exchange with said waste heat source, obtaining a supercritical ammonia stream;   the supercritical ammonia stream is then expanded to a condensation pressure between 10 and 20 bar; and   the ammonia stream is then condensed at a condensation temperature corresponding to said condensation pressure, obtaining said low pressure ammonia condensate.   
     
     
         7 . The process according to  claim 6 , the condensation pressure being around 14-16 bar and the condensation temperature being around 35° C., the condensation being effected by heat exchange of the condensing ammonia stream with cooling air or water at ambient temperature. 
     
     
         8 . The process according to  claim 5 , said waste heat source comprising one or more of the following:
 the ammonia make-up syngas taken from a low-temperature shift reactor of a front-end reforming section;   the ammonia make-up syngas effluent from a methanator of the front-end reforming section;   the flue gas from a steam reformer of the front-end section;   the hot product stream from the synthesis loop.   
     
     
         9 . A plant for the synthesis of a chemical product, the plant comprising a front-end section adapted to provide at least one make-up reactant, and a high-pressure synthesis section for reacting said least one make-up reactant and obtaining said chemical product in a liquid state, the plant being characterized by comprising an energy recovery section, said energy recovery section comprising at least:
 a heat exchanger disposed to exchange heat between at least a portion of the liquid product delivered by the synthesis section, and a waste-heat source stream, obtaining an expandable stream of said product in a vapour state or supercritical state;   an expander receiving said expandable stream in vapour state or supercritical state, and delivering mechanical energy produced by expansion of said stream, and   a condenser downstream said expander, and disposed to condense the effluent of said expander.   
     
     
         10 . A method for revamping a plant for producing ammonia, the plant comprising a front-end reforming section adapted to provide a make-up ammonia syngas, and a high-pressure synthesis loop, the method comprising the steps of:
 arranging a heat exchange for heating at least a portion of the liquid ammonia product, by means of heat exchange with at least one source of waste heat, so obtaining a stream of heated, high-pressure ammonia stream in a vapour or supercritical state;   providing the provision of at least an expander and preferably of a generator connected to said expander, for the expansion of said ammonia stream and the production of energy from said waste heat; and   providing a condenser adapted to condense the ammonia effluent at the outlet of said expander.   
     
     
         11 . The method according to  claim 10 , said source of waste heat being one or more of the following: the ammonia make-up syngas taken from a low-temperature shift reactor of the front-end reforming section; the ammonia make-up syngas effluent from a methanator of the front-end reforming section; the flue gas from a steam reformer of the front-end section; the hot product stream from the synthesis loop. 
     
     
         12 . A method for recovering waste heat in a process for the synthesis of a chemical product, particularly ammonia, where at least one make-up reactant is obtained in a front-end section, and reacted in a synthesis section operating at a pressure higher than said front-end section, obtaining said product in a liquid state and at a high pressure, the method comprising the steps of:
 using said waste heat to increase the enthalpy content of at least a portion of the liquid product delivered by the synthesis section, by indirect heat exchange, thus obtaining an expandable product stream in a vapour or supercritical state, and   recovering energy by expanding said vapour or supercritical stream across at least one suitable expander.   
     
     
         13 . The process according to  claim 1 , said waste heat source being one or more process stream(s) at a temperature in the range 50-300° C.