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 vapor or supercritical product stream ( 20 ), and energy is recovered by expanding said vapor 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.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 13/388,793, filed Feb. 3, 2012, which is a national phase of PCT/EP2010/056750, filed May 17, 2010, which claims priority to European Patent Application No. 09169330.9, filed Sep. 3, 2009. The entire contents of these applications is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to waste heat recovery in a chemical process and plant. The invention is applicable for example to process and plants for the synthesis of ammonia. 
     PRIOR ART 
     Waste heat recovery is known in the art. In a chemical process, various waste heat sources can be available, e.g. from the cooling of intermediate product streams or flue gas of any combustion process. Heat available at a temperature of at least 400-500° C. is usually recovered by producing hot steam, which can be used in the process itself, or expanded in a turbine to produce energy. Heat available at a lower temperature is generally not suitable to produce energy and can be recovered e.g. by pre-heating some process stream(s). In some cases, however, the recovery of low-temperature heat may not be economically convenient. 
     In the rest of the description, reference will be made to heat recovery in a process and plant for the synthesis of ammonia, that is a preferred but not exclusive application of the invention. 
     A process for the production of ammonia and for the production of the corresponding make-up syngas is disclosed in EP-A-2022754. Basically, ammonia is produced by catalytic reaction of a make-up synthesis gas, in a synthesis loop operating at 80-300 bar. The make-up syngas is produced in a front-end section by reforming a hydrocarbon such as natural gas. More in detail, a desulphurized hydrocarbon is steam-reformed in a primary reformer, obtaining a first gas product containing CO, CO 2  and H 2  at a temperature around 800° C. Said first gas product is further reacted with air or oxygen in a secondary reformer or auto-thermal reformer (ATR), obtaining a second gas product at around 1000° C. Said second gas product is then treated in a series of equipments to remove carbon oxides and obtain a gas composition suitable for ammonia synthesis i.e. having a H 2 /N 2  molar ratio close to 3:1. These equipments include CO shift converters where CO is converted to carbon dioxide and hydrogen; a CO 2 -removal unit such as a CO 2  washing column where carbon dioxide is removed by scrubbing the gas with an appropriate solvent; a methanator reactor where the residual carbon oxides are catalytically converted to methane. The shift conversion usually takes place in a high-temperature shift reactor (HTS) at around 500° C., and then in a low-temperature shift reactor (LTS) operating around 200° C. 
     The heat released by the cooling of the hot gas effluent from the secondary reformer and from the combustion side of primary reformer is usually recovered by producing high-pressure superheated steam and preheating process streams. Said steam is made available for other process use, or converted into energy with a steam turbine. The low-temperature heat of the syngas effluent for example from the low-temperature shift reactor, however, is more difficult to recover. 
     The LTS reactor effluent has a relatively low temperature of around 200-220° C., and conversion into energy with a steam turbine is not attractive at such a low temperature. In the known art, the heat recovered from the cooling of the LTS reactor effluent is used for re-generating the solution (e.g. amine-based) of the CO 2 -washing column; however this use is not efficient in terms of energy, and recent CO 2 -removal processes such as physical washing or passing the syngas through a PSA device, no longer require a significant heat input. 
     Hence, in modern ammonia plants there is little or no use for the heat that may be recovered from the LTS reactor effluent. There are other potential sources of recoverable heat, e.g. the effluent from the methanator or the synthesis loop downstream the hot gas/gas exchanger, or the flue gas from the steam reformer which is generally sent to the stack at 150 to 300° C. However the prior art does not provide an effective way for full exploitation of these potential heat sources. 
     The same problem can be faced in other processes similar to that of synthesis of ammonia, 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 operating at a pressure higher than said front-end section, obtaining said product in a high pressure liquid state. 
     SUMMARY OF THE INVENTION 
     The aim of the invention is to provide efficient recover of the low-temperature heat which is potentially available in a chemical process as above, especially in a process for the synthesis of ammonia. 
     The basic concept of the invention is to use an available low-temperature heat source to increase the enthalpy of the high-pressure liquid product stream produced by the synthesis loop, and to use the product stream as the working fluid of a thermodynamic cycle, e.g. a Rankine cycle. The product stream can be fully or partly evaporated by the recovered waste heat; in some embodiments of the invention, the product stream is at a pressure above critical pressure, and heating results in a supercritical fluid stream. 
     Accordingly, an aspect of the invention is 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, and reacting said at least one make-up reactant in a synthesis section, obtaining said product in a liquid state and at a high pressure, the process being characterized in that: a) at least a portion of the liquid product delivered by the synthesis section is heated by indirect 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; b) said expandable stream is expanded to recover energy, obtaining an expanded stream, and c) said expanded stream is condensed by heat exchange with a suitable cooling medium, obtaining a product condensate stream. 
     In accordance with different embodiments of the invention, and also in accordance with the features of the chemical product, the product after heating can be in a vapour state or in a supercritical state, if the synthesis pressure is over the critical pressure of the product, and the temperature after heating is also over the critical temperature. Below the critical point, the liquid product can be fully or partly evaporated by the heat exchange with said waste heat source. 
     Preferably, said synthesis section is operating at a pressure higher than said front-end section. 
     The term of waste heat source is used with reference to heat made available at a relatively low temperature, where conventional energy recovery by production of high-pressure steam is not possible or not convenient; heat available at less than 350° C. may be considered waste heat; preferably the invention is applied to recovery of heat in the range 50-300° C., more preferably 100 to 250° C. and even more preferably around 200-250° C. 
     The waste heat source is any process stream(s) available at the above temperature ranges or any stream or flue gas coming from a combustion process. One or more process stream(s) can form the waste heat source. 
     A possible heat source is a process stream, or at least a portion thereof, taken from the front-end section. More preferably, the waste heat source or one of the waste heat sources is the make-up reactant, or at least a portion thereof. 
     In further embodiments, a flue gas stream from a combustion process, possibly after one or more heat recoveries at high temperature, forms the waste heat source, or one of the waste heat sources, of the invention. One example of this last case is the flue gas from a steam reformer, particularly a steam reformer of the front-end of a plant for the synthesis of ammonia, that is used to supply heat to react the feedstock at high temperatures, and after various heat recoveries is sent to the stack at a temperature between 150 and 300° C., thus still containing a significant heat amount at lower temperature. 
     The expansion of above step b) may take place in any suitable expander or more expanders in series or in parallel, for example a single or multi-stage turbine. Said expander is preferably connected to an electric generator, so that the mechanical energy of the expansion, collected by the expander, is converted into electric energy. The energy can be exported or used to feed auxiliaries such as compressors or pumps present in the plant. If convenient, the turbine can be mechanically connected to drive any compressor or pump. 
     Condensation at above step c) is carried out preferably at a temperature slightly greater than ambient temperature, so that the cooling fluid can be any fluid available at ambient temperature. Ambient air for example can be the cooling fluid. Hence, in a preferred embodiment, the condensation pressure is chosen to determine a condensation temperature slightly above ambient temperature, for example 35° C. 
     According to a further aspect of the invention, after said condensation step a portion of the product condensate is pumped again at a high pressure, and preferably 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, thus obtaining an expandable stream; after said heating, said expandable stream is then expanded to recover energy; after expansion, the stream is condensed back to liquid, thus forming a closed loop. The expansion and condensation of said closed loop may take place in the same expander(s) and condenser(s) of the above steps b) and c), or in separate equipments. 
     A preferred application of the invention is to improve the energy balance of a process and plant for ammonia synthesis. In this case, the make-up reactant is ammonia make-up syngas, and the liquid product is the liquid ammonia delivered by a high-pressure synthesis loop, usually operating at 80-300 bar pressure and preferably at 100-180 bar. The waste heat source stream, or one of the waste heat sources, is preferably a stream of ammonia make-up syngas, taken downstream a low-temperature shift (LTS) section or LTS reactor. Usually, the temperature of said stream after the LTS reaction is in the range 200-250° C. 
     Further waste heat can be recovered from the flue gas stream of the reformer, the effluent of the methanator of the front-end section, or the liquid ammonia itself. Hence, the waste heat source may comprise 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 
     According to a preferred embodiment, liquid ammonia delivered by the synthesis loop at said pressure of 80-300 bar is heated to around 250° C., by heat exchange with said waste heat source(s), obtaining an expandable ammonia stream. Said expandable ammonia stream is expanded in a suitable expander such as ammonia turbine, and the ammonia effluent from said expander is condensed in a suitable air- or water-cooled condenser. 
     In more preferred embodiments, the pressure of the liquid ammonia stream at the outlet of the synthesis loop is in the range 100 to 180 bar, and the temperature is around 0° C., preferably in the range −30 (30 below zero) to +10° C. The heating at around 250° C. results in a supercritical fluid, the critical point of ammonia being 113 bar, 132° C. Said supercritical stream of ammonia is expanded through said expander or turbine, to recover energy; after expansion the ammonia stream is then condensed to liquid state. The condensation pressure, namely the expander outlet pressure, is preferably in the range 10-25 bar and more preferably around 14 to 20 bar, so that the condensation temperature is around ambient temperature, preferably slightly above the ambient temperature and more preferably around 35° C. The heat source for heating the liquid ammonia is preferably the make-up syngas effluent from a low-temperature shift reactor or the methanator or the synthesis loop downstream the hot gas-gas exchanger, or the flue gas from the reformer. All the make-up syngas effluent, or a part thereof, can form the heat source, according to embodiments of the invention, as well as the flue gas from the reformer. 
     In a particularly preferred embodiment, liquid ammonia at around 150 bar and 0° C. is heated to 200° C. by heat exchange with the make-up syngas exiting a LTS reactor at about 220° C.; a supercritical ammonia stream at 150 bar and 200° C. is obtained, and said stream is expanded in at least one turbine or another suitable expander, exiting at around 14 bar pressure. The ammonia effluent from said expander is then condensed in a water-cooled or air-cooled condenser, obtaining ammonia condensate at around ambient temperature, such as 30° C. More preferably, this condensate is further cooled by heat exchange with the cold liquid ammonia output of the synthesis loop. 
     According to one of the embodiments of the invention, a portion of the ammonia condensate can be pumped again to the high pressure of the loop, then evaporated in a suitable heat exchanger recovering further waste heat, expanded in the ammonia turbine or expander, and condensed in said condenser. Then, a portion of the ammonia evolves in a closed-loop Rankine cycle to produce energy. 
     The main advantage of the invention is that the waste heat is recovered in an efficient way, obtaining a valuable energy output. As a consequence, the overall efficiency of the process is improved. The use of ammonia as working fluid allows useful exploitation of the low-temperature heat, that would be unsuited, as stated above, for power generation via a steam turbine. 
     In fact, a low temperature heat source would only allow to produce saturated steam at a few bars pressure, which is not suited to efficient power generation via a steam turbine. In order to achieve a significant expansion ratio across the turbine, there would be the need of a low pressure output, resulting in a large flow rate and then in a large and expensive turbine stage. Moreover, due to poor inlet steam conditions, a steam turbine would also suffer the formation of condensate at the outlet, which is highly aggressive on the turbine blades. All the above drawbacks are overcome by use of the high-pressure ammonia as a working fluid. 
     Moreover, the invention also makes use of the significant pressure of the liquid product. Referring to the application to ammonia plants, in the prior art the pressure of the liquid ammonia stream is lowered through an expansion valve, which means that the pressure energy is lost. The invention provides efficient recovery of this pressure energy. 
     Another aspect of the invention is a plant adapted to carry out the above process. A plant according to the invention comprises a front-end section adapted to provide at least one make-up reactant, and a high-pressure synthesis section for reacting said at least one make-up reactant and obtaining a chemical product in a liquid state, the plant being characterized by 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 and adapted to deliver mechanical energy produced by expansion of said stream, and a condenser downstream said expander, and disposed to condense the effluent of said expander. 
     According to embodiments of the invention, one or more heat exchanger(s), expander(s) and condenser(s) may be provided. According to the above disclosed preferred embodiments of the invention, the plant is preferably an ammonia plant, 
     Another aspect of the invention is a method for revamping a plant for the synthesis of a chemical product, especially ammonia, by recovering waste heat from the front-end section of the plant in accordance with the above process. A plant for producing ammonia, comprising a front-end reforming section adapted to provide a make-up ammonia or syngas, and a high-pressure synthesis loop delivering liquid ammonia, is revamped by the following: 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; the provision of at least an expander and preferably of a generator connected to said expander, for the expansion of said ammonia and the production of energy from said waste heat; the provision of a condenser adapted to condense the ammonia effluent at the outlet of said expander. 
     In a preferred embodiment, the waste heat source is the syngas effluent from the LTS reactor of the front-end. Hence, the method comprises the steps of providing at least one heat exchanger, for example a plate or tube heat exchanger, feeding at least a portion of the liquid ammonia produced in the synthesis loop to one side of said heat exchanger, and feeding at least a portion of the LTS reactor effluent to the other side of said exchanger. All the above-disclosed waste heat sources can also be used in the revamping process. 
     Still another aspect of the invention is 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 being characterized in that: said waste heat is used 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 stream in a vapour or supercritical state, and energy is recovered by expanding said expandable stream across at least one suitable expander. 
     The following is a description of preferred and non-limiting embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a plant for the synthesis of ammonia, featuring the recovery of the low-temperature heat content of the make-up syngas in accordance with one embodiment of the invention. 
         FIG. 2  is a variant of  FIG. 1 , where a portion of ammonia is used as working fluid in a closed-loop. 
         FIG. 3  is a scheme of a conventional plant for the synthesis of ammonia, modified according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to  FIG. 1 , a synthesis loop  10  delivers liquid ammonia  11  at a pressure of 80-300 bar and temperature around −30 to 10° C. The synthesis loop  10  is fed with a make-up synthesis gas which is produced in a front-end of the ammonia plant for example by steam reforming of natural gas or another suitable hydrocarbon. 
     The front-end may comprise a primary and a secondary reformer followed by a high-temperature and a low-temperature shift reactor. The LTS reactor is shown in  FIG. 1  as  19 . Downstream said LTS reactor, the make-up syngas passes in a CO 2  removal unit  25 ; the CO 2 -free syngas  26  is further treated according to the needs, e.g. in a methanator, and then is fed to a main syngas compressor  27 . The compressed syngas  28  is fed to the high-pressure synthesis loop  10 , e.g. at a pressure of 150 bar. 
     According to one of embodiments of the invention, the plant of  FIG. 1  comprises an energy recovering section  50  operating with ammonia as working fluid, and recovering heat from the LTS reactor effluent  18 . The liquid ammonia  11 , or at least a part thereof, is heated with waste heat recovered from said effluent  18 , obtaining an expandable stream  20  which is the working fluid of a suitable expander, for example an ammonia turbine  13  connected to a generator  14 . 
     Referring more in detail to  FIG. 1 , the liquid ammonia  11  is pre-heated in indirect preheater  15 , by heat exchange with condensate ammonia  23  from the condenser  22  downstream the ammonia turbine  13 . The pre-heated liquid ammonia  16  is fed to one side of a main indirect heat exchanger  17 . The other side of said heat exchanger  17  receives the make-up syngas effluent  18  from the LTS reactor  19 . The heat content of the effluent  18 , usually between 200 and 250° C., is then used to heat the ammonia stream  16  and increase its enthalpy. The cooled syngas  24 , after passage through said heat exchanger  17 , is sent to the CO 2  removal unit  25 . 
     The main heat exchanger  17  delivers the expandable ammonia stream  20 , which is expanded across the ammonia turbine  13 . The effluent  21  of said ammonia turbine  13  is condensed in a water-cooled or air-cooled condenser  22 . The condensate ammonia  23  obtained in said condenser  22  is further cooled in the pre-heater  15 , by heat exchange with the pre-heating liquid ammonia  11 , leading to ammonia output  29 . It should be noted that the ammonia liquid output  29 , at a low pressure and temperature, is obtained after a useful exploitation of the energy pressure of the stream  11  as well as heat content of the effluent  18 . 
     The pre-heater  15 , the main exchanger  17  and the condenser  22  are heat exchangers known in the art, such as plate or tube heat exchangers, and are no further discussed. 
     The cooled syngas  24  at the output of the heat exchanger  17  is further treated in the CO 2 -removal unit  25  and in other equipments, shown as block  40 , to obtain the syngas  26  that feeds the synthesis loop  10 . 
     Example: liquid ammonia  11  is available at 150 bar pressure and 0° C. Said ammonia  11  is pre-heated to 30° C. through the pre-heater  15 , and further heated to 200° C. into the main exchanger  17 , by the make-up syngas  18  entering the same exchanger  17  at a temperature of 220° C. A supercritical ammonia stream at about 150 bar and 200° C. enters the inlet of turbine  13 . The turbine outlet stream  21  is at 14 bar and around 35° C. This level of pressure and temperature is chosen so that condenser  22  can be cooled with ambient air, i.e. it does not require refrigeration. In an ammonia plant capable of 2050 MTD (metric tons per day) of ammonia, the gross output of said turbine  13 , at the above turbine inlet and outlet conditions, is about 5.4 MW. 
     In a variant of the above embodiment ( FIG. 2 ), a portion  30  of the condensed ammonia  23  is pumped in a pump  31  to the nominal pressure of the loop  10 , i.e. the pressure of liquid ammonia  11 . The resulting stream  32  is evaporated or heated in a further heat exchanger  33 , obtaining a stream  34  which is expanded in the turbine  13 . Hence, a portion of the ammonia evolves as the working fluid of a Rankine cycle  35 . This variant is useful when another source of waste heat is available to provide the heat input of the exchanger  33 . 
     According to further embodiments, the stream  34  may be fed at the inlet of the turbine  13  together with the stream  20 , or to an intermediate stage. The heat exchanger  33  may receive heat by the same source of heat exchanger  17 , or any other waste heat source available in the process. The stream  32  may also be heated again in the same exchanger  17 . 
     Another example is given in  FIG. 3 . A mixture of natural gas feed  107  and steam  108  are pre-heated in a preheater  110  and reacted in a primary reformer  101  and a secondary reformer  102 , which receives a further oxidizer such as air supply  114 . 
     Downstream the secondary reformer  102 , the plant basically comprises a (series of) shift converter(s)  103 , a CO 2  washing column  104 , a methanator  105 . The gas exiting the methanator is cooled in a heat exchanger, condensate is separated in a separator  128  and the make-up syngas is fed to ammonia synthesis loop  106  via a suitable syngas compressor. The ammonia so obtained is discharged from the synthesis loop  106  through the flow line  32 . 
     The secondary reformer effluent at around 1000° C. and 60 bar is cooled in a heat exchanger  116  to around 350° C. and fed to the shift converter(s)  103  where the carbon monoxide content of the reformed gas is catalytically converted with unreacted steam to carbon dioxide plus and additional volume of hydrogen. The effluent of the shift converter, or last of shift converters in series, has a temperature around 220° C. and needs to be cooled near ambient temperature before feeding to a separator  121  upstream the washing column  104 . The outlet gas flow from top of the column  104  is reheated to around 300° C. and fed to the methanator  105 . 
     According to the invention, the plant is revamped by the provision of the main items such as ammonia expander  13 , generator  14  and ammonia condenser  22 , and by feeding the high-pressure ammonia stream  32 , or at least a part thereof, to the gas cooler  119 , to recover the heat released by the gas effluent of the shift converter(s)  103 , and obtain a supercritical ammonia stream which is the input for the expander  13 . The effluent of the condenser may be treated as in  FIGS. 1 and 2 . The gas cooler  119  may be provided as a new unit, during the revamping, if appropriate. Further and auxiliary items such as pumps, valves, etc. are not shown in the simplified scheme of  FIG. 3 .