REVAMPING ETHYLENE PLANT PYROLISIS HEATERS FOR REDUCED CARBON DIOXIDE EMISSIONS

BACKGROUND

[0001]Pyrolysis of hydrocarbon mixtures, such as whole crudes or other hydrocarbon mixtures, to produce olefins and other chemicals is a heat intensive process. One of the modes of supplying heat of reaction is an air preheater. Presently, plants primarily use fuel fired air preheaters that lead to emissions associated with firing.

SUMMARY

[0002]This summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter.

[0003]In one aspect, embodiments disclosed herein relate to a pyrolysis process for converting a hydrocarbon mixture to produce olefins by preheating a hydrocarbon feed in a first preheat zone of a convection section to recover a pre-heated hydrocarbon stream. The pre-heated hydrocarbon stream is mixed with a steam feed to recover a mixed hydrocarbon steam stream. The mixed hydrocarbon steam stream is fed to a second preheat zone of the convection section to vaporize the portion of the heated hydrocarbon stream and recover a cracking feed stream. An air feed electric heater heats air and produces a combustion air feed. The combustion air feed is combusted with a fuel to produce a combustion mixture in one or more burners in a radiant section. The cracking feed stream is cracked in the coils in the radiant section, producing a cracked hydrocarbon product that is quenched in a transfer line exchanger downstream of the radiant coils to recover a cooled hydrocarbon product stream.

[0004]In another aspect, embodiments disclosed relate to a pyrolysis system for converting a hydrocarbon mixture to produce olefins including a pyrolysis heater with a convection heating zone and a radiant heating zone. The convection heating zone contains a first preheat zone for preheating the hydrocarbon feed and recovering a pre-heated hydrocarbon stream. The system includes an electric heater for heating a dilution steam and producing a steam feed. The convection heating zone includes a second preheat zone for vaporizing a portion of the mixture of the pre-heated hydrocarbon stream and the steam feed, recovering a cracking feed stream. The system includes a heat transfer fluid heating coil within the convection heating zone of the pyrolysis heater. An air feed electric heater heats air to produce a combustion air feed. The radiant heating zone has one or more burner nozzles to combust a fuel with the combustion air feed to produce a flue gas fed to the convection heating zone. The system includes one or more coils in the radiant heating zone for cracking hydrocarbons in the cracking feed stream and recovering a cracked hydrocarbon product. A feed line directs the cracked hydrocarbon product to a transferline exchanger for quenching, to recover a cooled hydrocarbon product stream.

[0005]In another aspect, embodiments disclosed herein relate to a process for retrofitting an existing pyrolysis system including a pyrolysis heater with a convection heating zone and a radiant heating zone. The convection heating zone has a first preheat zone and a second preheat zone. An electric heater heats a dilution stream to produce a steam feed. A heat transfer fluid heating coil is in the convection heating zone of the pyrolysis heater. An air feed electric heater heats air producing a combustion air feed. The radiant heating zone includes one or more burner nozzles and one or more coils. A feedline directs a cracked hydrocarbon product to a transfer line exchanger for quenching. The process of retrofitting this system includes removing a secondary transfer line exchanging, adding an electric heater to pre-heat a dilution steam to produce a steam feed, and adding a flow line to feed to feed the steam feed to a pre-heated hydrocarbon feed stream. The one or more coils are replaced or repurposed in the convection heating zone with an air pre-heat system. An air feed electric heater is added for further heating the pre-heated air producing a combustion air feed.

[0006]Other aspects and advantages of the claimed subject matter will be apparent from the following description and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

[0007]FIGS. 1 and 1A are simplified process flow diagrams of systems for thermal cracking of hydrocarbon mixtures in accordance with one or more embodiments.

[0008]FIG. 2 is a process flow diagram of an air pre-heater in accordance with one or more embodiments.

[0009]FIGS. 3A-3E are process flow diagrams of air pre-heater configurations in accordance with one or more embodiments.

DETAILED DESCRIPTION

[0010]Embodiments disclosed herein relate generally to the pyrolysis of hydrocarbon mixtures, such as whole crudes or other hydrocarbon mixtures, to produce olefins, such as ethylene. Embodiments disclosed herein also relate to retrofitting an existing system for the pyrolysis of hydrocarbon mixtures to produce olefins.

[0011]Hydrocarbon mixtures useful in embodiments disclosed herein may include various hydrocarbon mixtures having a boiling point range, where the end boiling point of the mixture may be greater than 450° C. or greater than 500° C., such as greater than 525° C., 550° C., or 575° C. The amount of high boiling hydrocarbons, such as hydrocarbons boiling over 550° C., may be as little as 0.1 wt %, 1 wt % or 2 wt %, but can be as high as 10 wt %, 25 wt %, 50 wt % or greater. Processes disclosed herein can be applied to crudes, condensates and hydrocarbons with a wide boiling curve and end points higher than 500° C. Such hydrocarbon mixtures may include whole crudes, virgin crudes, hydroprocessed crudes, gas oils, vacuum gas oils, heating oils, jet fuels, diesels, kerosenes, gasolines, synthetic naphthas, raffinate reformates, Fischer-Tropsch liquids, Fischer-Tropsch gases, natural gasolines, distillates, virgin naphthas, natural gas condensates, atmospheric pipestill bottoms, vacuum pipestill streams including bottoms, wide boiling range naphtha to gas oil condensates, heavy non-virgin hydrocarbon streams from refineries, vacuum gas oils, heavy gas oils, atmospheric residuum, hydrocracker wax, and Fischer-Tropsch wax, among others. In some embodiments, the hydrocarbon mixture may include hydrocarbons boiling from the naphtha range or lighter to the vacuum gas oil range or heavier. If desired, these feeds may be pre-processed to remove a portion of the sulfur, nitrogen, metals, and Conradson Carbon upstream of processes disclosed herein. Lighter hydrocarbon feeds, such as ethane, propane, butanes, etc., and mixtures of multiple of these various lighter hydrocarbons may also be used as feedstocks to cracking heaters herein.

[0012]The thermal cracking reaction proceeds via a free radical mechanism. Hence, high ethylene yield can be achieved when hydrocarbons are cracked at high temperatures. Lighter feeds, like butanes and pentanes, require a high reactor temperature to obtain high olefin yields. Heavy feeds, like gas oil and vacuum gas oil (VGO), require lower temperatures. Crude contains a distribution of compounds from butanes to VGO and residue (material having a normal boiling point over 520° C., for example).

[0013]Many countries require a reduction in CO2 emission. When fossil fuels are burned to supply energy, CO2 production is often high. Embodiments disclosed herein aim to reduce the fuel consumption for the same process duty by efficiently designing the heaters, preheating steam with an electric heater, preheating air with an electric heater, or a combination thereof. In conventional processes, excess enthalpy in the flue gas is used to generate high pressure steam. It may be possible to reduce the steam production and utilize the heat energy available in the fuel only for process heat duty. In doing so, the heater may reduce, or eliminate, CO2 production and H2 import.

[0014]Current heater designs are based on producing as much steam as possible to meet the olefin plant energy requirements to drive turbines. This results in firing more fuel in the cracking heater. Embodiments disclosed herein aim to reduce the fuel consumption by redesigning the heater to be more fuel efficient while producing less steam. This reduces CO2 emissions in the heater, which is a major source of CO2 emissions in the ethylene plant. In some embodiments, ethane cracking also produces a significant amount of hydrogen, but the amount is not sufficient to meet the firing demand.

[0015]To reduce the CO2 emissions in the heaters, one method proposed in the prior art is to use air preheat or to preheat the fuel. When air preheat is used with traditional heater design, super high pressure (SHP) steam production is still relatively high and hence the reduction in fuel consumption is small due to the increased heat duty required. Alternate heater designs are proposed to quench the hot effluents using an exchanger heating the process fluid first and then the residual energy to generate SHP steam at lower overall rates.

[0016]Embodiments disclosed herein use the convection section of a pyrolysis reactor to preheat the feed hydrocarbon mixture. Steam may be injected at appropriate locations to increase the vaporization of the hydrocarbon mixture and to control the heating. The vaporization of the hydrocarbons occurs at relatively low temperatures and/or adiabatically, so that coking in the convection section will be suppressed.

[0017]Multiple heating steps may be used to heat the hydrocarbons to the desired temperature. This will permit cracking optimally, such that the throughput, steam to oil ratios, heater inlet and outlet temperatures and other variables may be controlled at a desirable level to achieve the desired reaction results, such as to a desired product profile while limiting coking in the radiant coils and associated downstream equipment.

[0018]The process of cracking hydrocarbons in a pyrolysis reactor may be divided into three parts, namely a convection section, a radiant section, and a quench section, such as in a transfer line exchanger (TLE). In the convection section, the feed is pre-heated, partially vaporized, and mixed with steam. In the radiant section, the feed is cracked (where the main cracking reaction takes place). One or more radiant process coils are disposed within the radiant section, the process coils being heated by the radiant heat from the burners, resulting in the heating and/or conversion of the fluids being processed. The combustion gas passes from the radiant section to the convective heating section prior to being expelled as a flue gas to the atmosphere. In the TLE, the reacting fluid being cracked is quickly quenched to stop the reaction and control the product mixture. Instead of indirect quenching via heat exchange, direct quenching with oil is also acceptable.

[0019]Embodiments herein efficiently utilize the convection section to enhance the cracking process. Heating may be performed in a convection section of a single pyrolysis reactor in some embodiments. Additional heating may be provided by one or more external electric heaters. In some embodiments, the electric heaters use available renewable sources of energy. In other embodiments, separate heaters may be used for the respective fractions. The heating coils may be used for preheating a feed (feed preheat coils), heating a boiler feed water, superheating steam, or heating or superheating a feed stream prior to the feed being fed to the radiant coil. In some embodiments, the hydrocarbon feed enters the top row of the convection bank and is pre-heated with hot flue gas generated in the radiant section of the heater, at the operating pressure to medium temperatures without adding any steam. The waste heat in the combustion gas may also be used to heat a heat transfer fluid, which may be used to pre-heat combustion air or for other purposes within the plant. The outlet temperatures may be in the range from 150° C. to 400° C., depending upon the hydrocarbon feed and throughput. At these conditions, 5% to 70% (volume) of a crude may be vaporized. For example, the outlet temperature of this first heating step may be such that naphtha (having a normal boiling point of up to about 200° C.) is vaporized. Other cut (end) points may also be used, such as 350° C. (gas oil), among others. Because the hydrocarbon mixture may be pre-heated with hot flue gas generated in the radiant section of the heater, limited temperature variations and flexibility in the outlet temperature can be expected. Embodiments herein allow the heat extraction from the combustion flue gas into the heat exchange medium (heat transfer fluid) to be at any section of the heater, as needed. Some embodiments extract heat from multiple sections. In contrast, in the prior art, the heat extraction for air pre-heat is typically limited to the coldest part of the convection section. When the heat is extracted into a heat transfer fluid from multiple sections according to embodiments herein, the sections can be in series or in parallel or a combination of series and parallel.

[0020]Heat extraction may be performed according to embodiments herein using a heat transfer fluid. The heat transfer fluid can be one or different types of fluid to aim at different service temperatures. The heat extracted from the heater using the heat transfer fluid can then be used to warm up the combustion oxidant or other process fluid(s).

[0021]Embodiments herein may use the heat extraction with the heat transfer fluid to also regulate the flue gas temperature profile in the convection section. The ability to regulate the flue gas temperature profile may improve the heater process performance and emission reduction, such as by controlling a flue gas temperature at a selective catalytic reduction (SCR) catalyst bed for optimum NOx reduction.

[0022]The heat extracted from the selected heater, such as to heat combustion air or other fluids, can be indirectly applied back to the same heater or to other heat transfer equipment. Further, embodiments herein may provide options to dilute the pre-heated oxidant with flue gas (e.g., external flue gas recirculation to the burners to lower the NOx).

[0023]Following cracking in the radiant coils, a transfer line exchanger (TLE) may be used to cool the products very quickly and generate steam. One or more coils may be combined and connected to the TLE. The TLE can be double pipe or shell and tube exchanger(s). Embodiments disclosed herein are directed toward a TLE that reduces SHP steam production, and thus reduces CO2 generation and H2 import requirements.

[0024]In one or more embodiments, maximum fuel energy may be transferred to heating the reaction mixture and to initiate the reaction. Olefin selectivity may be high only when the effluent mixture is quickly quenched after the reaction. One way to quickly quench the reaction, stopping the production of olefins, is to directly quench the effluent with cold fluid. Water, oil, or steam can be used as the cold fluid. Since coil outlet pressure is low, low pressure steam or medium pressure steam can also be used. When indirect quench is used, a small TLE may be used, and a minimum amount of steam may be needed. In such embodiments, the temperature may be reduced sufficiently so that reaction rate is reduced quickly, and, at the same time, the effluent mixture is still hot enough to pre-heat the reaction mixture using one or more downstream exchangers. As the TLE may be small, SHP steam production may be low. Since SHP steam production is reduced, the convection section may be modified to be flexible for different feeds and operating modes. The same convection section may also work during decoke and high steam conditions.

[0025]Embodiments herein relate to alternative heat recovery methods, which do not require that the chemical energy of the fuel be converted to heat and work energy via generation of steam (with associated emissions) and can significantly reduce the fuel consumption of an existing furnace by at least 5% and as much as 30%. Such embodiments as described herein are also inexpensive and do not require significant additional structural changes to a convection section. Further, the heat recovery of embodiments herein maintains a desirable thermal profile in the convection section, with high efficiency and allow for flexibility in feed preheating. Lastly, such a heat recovery method can be applied to an existing furnace without, in most cases, significant modifications to the heating surface, structures, and foundation loading.

[0026]Referring now to FIG. 1, a simplified process diagram of the above embodiments is illustrated. A fired tubular furnace 100 is used for cracking hydrocarbons in a hydrocarbon feed stream 120 to ethylene and other olefinic compounds. The fired tubular furnace 100 has a convection section or zone 118 and a radiant section or zone 144. Convection section 118 contains a first pre-heat zone 119 and a second pre-heat zone 137. The furnace contains one or more process tubes (radiant coils) 141 through which a portion of the hydrocarbons fed through hydrocarbon feed line 120 are cracked to produce product gases upon the application of heat. In some embodiments, the hydrocarbon stream 120 is mixed with a steam stream 122 and heated in the first pre-heat zone 119 in convection section 118 and combined to form a pre-heated hydrocarbon stream. The pre-heated hydrocarbon stream is then fed to a second pre-heat zone 137 in convection section 118.

[0027]Radiant and convective heat is supplied by combustion of a heating medium introduced to the radiant section 144 of the furnace through a plurality of burner nozzles 153, such as hearth burners (floor burners), or wall burners (not shown), and exiting through an exhaust at the top of the furnace. Downstream of the radiant section 144 is a TLE 159. In some embodiments, at the top of convection section 118, air 112 is fed into an air pre-heater zone (APH) 115 to produce pre-heated air 117 that will be used in the burner nozzles 153 in the radiant section 144. In other embodiments, the air pre-heater zone 115 may be located outside the convection section 118 such that the flue gas is routed to a separate heat exchanger as illustrated in FIG. 1A. In FIG. 1A, the hot flue gas 113 is routed to the APH to provide heat. The cooled flue gas 116 exits APH 115. Air 112 is fed into the APH 115 to produce pre-heated air 117 that will be used in the burner nozzles 153 in the radiant section 144. Some embodiments may not contain an air pre-heater zone 115. Turning back to FIG. 1, in some embodiments, the pre-heated air 117 exiting the APH 115 flows to the electric heater 150 through a line 147 to further heat the pre-heated air. In some embodiments, both APH 115 and the electric heater 150 are used. In other embodiments, only the electric heater 150 is used to heat the air without the presence of the APH 115.

[0028]In some embodiments, a dilution steam stream 126 is heated in an electric heater 129, producing a steam feed 132. The pre-heated hydrocarbon stream 123 exiting the first pre-heat zone 119 in convection section 118 to mix with the steam feed 132, producing a mixed hydrocarbon steam stream 135. The mixed hydrocarbon steam stream 135 is then fed to the second pre-heat zone 137 in the convection section 118 and then to the radiant section 144 for cracking to produce olefins such as ethylene. The cracked product exiting the radiant section 156 is then fed to a primary TLE 159 for rapid quenching. The rapid quenching produces a cooled hydrocarbon product stream 162.

[0029]The radiant section 144 fuel consumption may be reduced if the reaction duty is minimized to convert only the feed to products. This may be accomplished by feeding the feedstock at high inlet temperature. After the radiant section 144, to preserve the olefins, the reaction mixture may be quenched quickly. This can be done in two ways. Directly quenching with quench fluid like water, steam, or oil. Alternatively, indirect quenching can be used. With indirect quenching, high pressure steam is generated. The reaction mixture will enter the tube side (or shell side) of a TLE 159. The other side of the TLE 159 will generate steam 73 through a boiler feed water steam generating system 165. Since generating steam has very high heat transfer coefficient, the mixture may be quenched quickly in a short distance in the TLE 159. Typically, the radiant coil outlet temperature will be 750 to 950° C. depending upon the feed and coil design. The product mixture is cooled to 300 to 450° C. at start of run and it may reach 425 to 650° C. at end of run. Most cracking reactions stop around 650° C. and hence the TLE 159 (first exchanger which is used to quench the fluid quickly) is designed to achieve high start-of run outlet temperatures (˜600° C.). This will produce only a small quantity of SHP steam. As a result, convection section 118 need not superheat a large quantity of SHP steam and thereby saves energy in the superheating of the steam. By only generating a small amount of SHP steam, the energy in the steam make is shifted to process fluid for improved cracking performance. This may reduce the heating duty significantly, and consequently fuel consumption and CO2 production are reduced.

[0030]Alternatively (not shown in the figures) the effluents can be cooled by generating low pressure steam, medium pressure steam, or high pressure steam after the TLE and a resulting hot stream is exchanged with preheat air.

[0031]FIG. 1 illustrates an embodiment of the air pre-heater zone (115, FIG. 1). The APH 115, whether within the same convection section as illustrated in FIG. 1 or within a separate heat recovery zone as illustrated in FIG. 1A, enables an increase in radiant efficiency whilst keeping overall thermal efficiency high.

[0032]Turning now to FIG. 2, FIG. 2 illustrates an embodiment of the system 100 with an air pre-heater 232 having an air inlet 270, a pre-heated air outlet 272, a heat transfer fluid inlet 274, a heat transfer fluid outlet 276, and one or more heat exchange surfaces (not illustrated) that is configured for indirectly heating air 210 received via the air inlet 270 by indirect heat exchange with a hot heat transfer fluid received via the heat transfer fluid inlet 274 to produce the pre-heated air stream 212. The pre-heated air stream 212 is recovered via the pre-heated air outlet 272, and a cooled heat transfer fluid is recovered via the heat transfer fluid outlet 276. One or more flow lines (not illustrated) fluidly connect the pre-heated air outlet with the one or more burners 153, for supplying the pre-heated air stream 212 to the one or more burners. In some embodiments, the pre-heated air stream 212 may feed an air feed electric heater 150 before supplying the air 212 to the one or more burners 153. Further, a heat transfer fluid circulation system 224 includes a flow line 228 for circulating the cooled heat transfer fluid from the heat transfer fluid outlet 276 to an inlet of the heat transfer fluid heating coil 222 disposed within the convective heating section 252, and a flow line 230 for circulating the hot heat transfer fluid from an outlet of the heat transfer fluid heating coil to the heat transfer fluid inlet 274. A pump 226 may be used to convey the heat transfer fluid within the heat transfer fluid circulation system 224. Heat transfer fluid circulation loops according to embodiments herein include piping from an expansion tank (not shown) to a pump 226, piping 228 from a pump to one or more heat transfer fluid heating coils 222, and piping 230 from the heat transfer fluid heating coils to an external heat exchanger 232.

[0033]Heat transfer fluids for use at elevated temperatures up to 400° C. can be categorized as synthetics, hot oils, and inorganics, including silicones. A suitable synthetic heat transfer fluid in processes that require elevated temperatures in the liquid state is usually a eutectic mixture of biphenyl and diphenyl oxide, commonly called biphenyl or HTF (Heat Transfer Fluid). This fluid can be found on the market, for example, under the brand names DOWTHERM-A and THERMINOL VP-1. Such fluids can be used up to 400° C. Other heat transfer fluids can be selected based on their properties and stability under the operating conditions. Silicone based heat transfer fluids offered under brand names such as SYLTHERM 800 are also suitable.

[0034]The air pre-heater can be a single heat exchanger per heater, or multiple heat exchangers per radiant heater or a single pre-heater for multiple radiant heaters, the variations depending on the size and capacity of a single unit. The air pre-heater may be a plate type or tubular type of heat exchanger and may include extended heating surfaces such as studs or fins, for example. Since the heat transfer coefficient of the lower pressure air is quite low in comparison to the heat transfer fluid, an appropriate design would be for the heat transfer fluid to flow inside tubes with the air flowing outside the tubes. An extended surface in the form of heat transfer fins can be provided on the outside of the tubes to increase the heat transfer surface area and to compensate for the much lower heat transfer coefficient on the air side. The tubes may be round or elliptical steel tubes. The fins may be applied to the tube in the form of spiral fins or helical fins which are wound continuously around each individual tube, and may be edge wound, bonded to the tube by tension, L-footed or embedded into a groove on the tube surface. Alternatively, many tubes can be incorporated into a plate-fin structure by inserting tubes into a plate fin pack and mechanically expanding the tubes into the plate pack. In either of the above designs the air will be in crossflow relative to the heat transfer fluid, and the heat transfer tube side may have multiple passes. A liquid phase or vapor phase heat transfer fluid may be used.

[0035]The heat transfer fluid circulation loop may include other supplementary waste heat sources in addition to heat recovery from the convection section. For example, other sources of waste heat may be used to pre-heat or further heat the heat transfer fluid within the circulation loop.

[0036]Embodiments herein are further directed toward the arrangement of the heat transfer fluid heating coils (illustrated in FIGS. 3A-3E and described further below) disposed within the convective heating section. Embodiments herein include: a heat transfer fluid heating coil 322 disposed within the convective heating section (118, FIG. 1) of the furnace; one or more steam heating coils (see FIGS. 3A-3E); and one or more hydrocarbon feed preheat coils (see FIGS. 3A-3E) configured for receiving and pre-heating a hydrocarbon feed to produce a pre-heated hydrocarbon feed. The radiant process coil disposed within the radiant heating section is configured for receiving the pre-heated hydrocarbon feed and producing a cracked hydrocarbon product.

[0037]An air preheater 232 having an air inlet 270, a preheated air outlet 272, a heat transfer fluid inlet 274, a heat transfer fluid outlet 276, and one or more heat exchange surfaces (not illustrated) is configured for indirectly heating air 210 received via the air inlet 270 by indirect heat exchange with a hot heat transfer fluid received via the heat transfer fluid inlet 274 to produce the pre-heated air stream 212. The preheated air stream 212 is recovered via the preheated air outlet 272, and a cooled heat transfer fluid is recovered via the heat transfer fluid outlet 276. One or more flow lines (not illustrated) fluidly connect the preheated air outlet with the one or more burners, for supplying the pre-heated air stream 212 to the one or more burners. Further, a heat transfer fluid circulation system 224 includes a flow line 228 for circulating the cooled heat transfer fluid from the heat transfer fluid outlet 276 to an inlet of the heat transfer fluid heating coil 322 disposed within the convective heating section (118, FIG. 1) of the furnace, and a flow line 230 for circulating the hot heat transfer fluid from an outlet of the heat transfer fluid heating coil to the heat transfer fluid inlet 274. A pump 226 may be used to convey the heat transfer fluid within the heat transfer fluid circulation system 224.

[0038]In one or more embodiments, the burner fuel may be preheated before being fed to the fuel fired burners. The fuel preheater may be an integrated part of the convection section or a separate heat exchange unit, such as an electric heater. In an electric heater, the fuel may be preheated up to temperatures below onset of cracking or other undesired reactions, such as to temperature of less than 500° C., for example. In some embodiments, a hot oil recirculation system uses hot oil in a recirculation loop to heat the fuel. Such systems would heat the fuel used for combustion in the burners. In one or more embodiments, the fuel may be pre-heated in the convection section before being additionally preheated in an external heater prior to being fed to one or more burners.

[0039]FIGS. 3A-E show systems for preheating combustion air with intermediate heat transfer fluid heat recovery according to embodiments herein. Common features of these embodiments include: (i) a circulating heat transfer fluid, (ii) a heat recovery section or sections 322 (heat transfer fluid heating coil(s)) inside the convection section, (iii) a heat transfer fluid circulation loop 324 for circulating the heat transfer fluid, and (iv) an air preheater APH.

[0040]In addition to the heat transfer fluid heating coils 322, one or more additional heating coils may be disposed within the convective heating section and arranged as illustrated in FIGS. 3A-E. Such additional heating coils may include steam superheating coils (SSH). Other heating coils may include feed preheat coils (FP) used for heating a hydrocarbon feed, mixed feed preheat coils (MP) used for mixing hydrocarbon and steam mixtures, as well as boiler feed water (BFW) heating coils. The relative position of a particular type of coil within the convective section is denoted by upper (U), middle (M), or lower (L), USSH being the upper steam superheating coil, for example.

[0041]FIG. 3A illustrates one embodiment for recovering heat from a flue gas according to embodiments herein. The coils included within the convective heating section of the furnace are arranged in the following order from lowest (highest flue gas temperature, closest to the combustion box) to highest (lowest flue gas temperature, furthest from the outlet of the combustion box): a mixed preheat coil (MP); a lower steam superheating coil (LSSH); an upper steam superheating coil (USSH), a heat transfer fluid heating coil 322, a boiler feed water heating coil (BFW), and a feed preheat coil (FP). As described above, heat transfer fluid heating coil 322 receives cool heat transfer fluid from circulation pump 226, heats the heat transfer fluid, and the heated heat transfer fluid circulates via flow line 230 to the air preheat coils APH, which may include one or more finned tubes or other heat transfer surfaces as described above.

[0042]For embodiments retrofitting a heat recovery system, to arrive at the heat recovery system as illustrated in FIG. 3A the heat recovery system is modified such that a portion of the tubes previously used to preheat BFW are re-purposed to instead heat a heat transfer fluid. In some embodiments, the upper steam superheat coil is also modified to account for the smaller amount of steam generated. Reducing steam generation is an objective of some embodiments herein.

[0043]FIG. 3B illustrates one embodiment for recovering heat from a flue gas according to embodiments herein. The coils included within the convective heating section of the furnace are arranged in the following order from lowest (highest flue gas temperature, closest to the combustion box) to highest (lowest flue gas temperature, furthest from the outlet of the combustion box): a mixed preheat coil (MP); a lower steam superheating coil (LSSH); an upper steam superheating coil (USSH); a heat transfer fluid heating coil 322, including a lower heat transfer fluid heating coil and an upper heat transfer fluid heating coil, intermediate of which is a selective catalytic reduction catalyst bed (SCR); a boiler feed water heating coil (BFW); and a feed preheat coil (FP). As described above, heat transfer fluid heating coil 322 receives cool heat transfer fluid from circulation pump 226, heats the heat transfer fluid, and the heated heat transfer fluid circulates via flow line 230 to the air preheat coils APH, which may include one or more finned tubes or other heat transfer surfaces as described above.

[0044]In FIG. 3B the heat recovery section is similarly modified to FIG. 3A, however, the BFW coils above and below a catalyst bed for NOx removal are both re-purposed for heating the heat transfer fluid. In this case, the heat transfer fluid is heated in two sections in series.

[0045]FIG. 3C illustrates one embodiment for recovering heat from a flue gas according to embodiments herein. The coils included within the convective heating section of the furnace are arranged in the following order from lowest (highest flue gas temperature, closest to the combustion box) to highest (lowest flue gas temperature, furthest from the outlet of the combustion box): a mixed preheat coil (MP); a lower steam superheating coil (LSSH); a middle steam superheating coil (MSSH); an upper steam superheating coil (USSH); a heat transfer fluid heating coil 322; a boiler feed water heating coil (BFW); and a feed preheat coil (FP).

[0046]FIG. 3C is a similar arrangement as FIG. 3A, except that instead of eliminating some of the tubes in the USSH coils, the fin density is modified to account for the reduced steam generation.

[0047]FIG. 3D illustrates one embodiment for recovering heat from a flue gas according to embodiments herein. The coils included within the convective heating section of the furnace are arranged in the following order from lowest (highest flue gas temperature, closest to the combustion box) to highest (lowest flue gas temperature, furthest from the outlet of the combustion box): a mixed feed preheat coil; a lower steam superheating coil; a middle steam superheating coil; an upper steam superheating coil; the heat transfer fluid heating coil; and a boiler feed water heating coil. The furnace system of FIG. 3D further includes a stack 340 connected to a gas turbine exhaust (not illustrated). Stack 340 includes a feed preheat coil (FP) and a second mixed feed preheat coil (SMP), the second mixed feed preheat coil being fluidly connected to the mixed feed preheat coil (MP).

[0048]In retrofitting to arrive at the arrangement of FIG. 3D, a feed preheat (FP) coil and a portion of the USSH and LSSH coils are bypassed and part of BFW coil is used for the heat transfer fluid heating section. A gas turbine exhaust (GTE) stream is additionally used to preheat feed (FP) and partially preheat mixed feed (MP) which was the duty previously carried out in the convection section. By moving the feed preheat and mixed feed preheat to a new section with GTE heating it allows maximum heat recovery from the GTE and more duty available for the heat transfer fluid recovery section and consequently the air preheat section.

[0049]FIG. 3E illustrates another embodiment for recovering heat from a flue gas according to embodiments herein. The coils included within the convective heating section of the furnace are arranged in the following order from lowest (highest flue gas temperature, closest to the combustion box) to highest (lowest flue gas temperature, furthest from the outlet of the combustion box): a lower mixed preheat coil; a lower steam superheating coil; a middle steam superheating coil; an upper steam superheating coil; the heat transfer fluid heating coil, including a lower heat transfer fluid heating coil and an upper heat transfer fluid heating coil, intermediate of which is, from lowest to highest, an upper mixed feed preheat coil, a dilution steam superheating coil, and a lower feed preheat coil; and the feed preheat coil.

[0050]In retrofitting to arrive at the embodiment of FIG. 3E, the BFW heating coil is replaced with a heat transfer fluid section. Part of the USSH section is also replaced by a heat transfer fluid heating coil.

[0051]For each of the embodiments described above, a thermal calculation was performed based on re-arranging the pre-existing convection layout with a convection section employing a heat transfer fluid according to embodiments herein. The results are summarized in Table 1. Improvements in radiant efficiency ranged from 3 to 10% as compared to arrangements without the heat transfer fluid heating coil and the external heat transfer fluid air preheater. The resulting reductions in CO2 emissions were in the range 8-20%.

[0052]Embodiments herein will have minimal impact to the heater structure steel, foundation loading and require minimum plot space. By contrast, adding an air pre-heater to the convection section involves significant structural alterations.

[0053]In addition to the advantages identified above, embodiments herein save plot space and cause less congestions to the heater area that may interfere with the heater operation and maintenance.

[0054]Further, embodiments herein may be used to retrofit existing convection section heat recovery systems. In some embodiments, an existing system may be retrofitted with the electric heater (FIG. 1, 129). These existing systems may have an existing secondary TLE that may be removed or taken out of service and replaced with the electric heater (FIG. 1, 129). When an existing system is retrofitted with an electric heater, the size of the electric heater may be based on available space and existing system component size. The existing system may also be fitted with an APH circulating a heat transfer fluid such as synthetics, hot oils, and inorganics, including silicones. In some embodiments, an existing system may be retrofitted with the air feed electric heater 150. When an existing system is retrofitted with an electric heater, the size of the electric heater may be based on available space and existing system component size. For example, when retrofitting a cracking furnace having a steam superheating coil, a boiler feedwater heating coil, and an APH coil, each coil comprising multiple tubes connected to a feed header and an effluent header, the method may include: disconnecting one or more tubes of a boiler feedwater heating coil from a boiler feedwater circulation system; fluidly connecting the one or more tubes to a heat transfer fluid circulation system; and fluidly connecting an APH to the heat transfer fluid circulation system and to burners of the furnace. Retrofitting may also include taking one or more tubes of a steam superheating coil out of service, and/or modifying a density of tubes of a steam superheating coil. Retrofitting may allow heat transfer surface optimization or meet the new process design requirement, including material upgrade, extended surface adjustments (fin or stud etc.), tube and fin tip circle diameter changes, process flow direction changes and number of flow streams or parallel passes. Further, as the existing heater may have spare tube rows in the convection section, the spare rows may be used to heat the heat transfer fluid, or the spare rows combined with part of the existing heating surface may form a new coil to heat the heat transfer fluid. The APH may be placed at the top of the convection section provided the heater structure is adequate. In other embodiments, the APH may be placed near ground level. When a heat transfer fluid is used, the APH and pumps may be placed near ground level. When using a heat transfer fluid, some BFW and some SHP coil services may be used to preheat the heat transfer fluid instead of the BFW or SHP. An electric heater may be located at ground level for air preheating, and an additional optional electric heater may be present at ground level for dilution steam.

[0055]As used herein, coils are referred to (named) based on the materials being processed, and specific coils referred to in the specification and claims herein are defined based on a coil function (“boiler feed water heating” coil, for example). Systems defined by and including such coils necessarily include fluid connections from a feed supply system (e.g., water tank) and to an effluent processing system (e.g., boiler), along with associated tanks, pumps, valves, controls, etc. of the feed supply and effluent processing systems.

Example

[0056]Table 2 shows example calculations for a liquid naphtha cracking system. The calculations of the “Revamp” case are based off conditions including a high hydrocarbon feed inlet temperature, low excess air, high cross over temperature, high fuel temperature, and thus, lower boiler feed water (BFW) duty. Additional duty available in flue gas is used for air preheating. In Addition, an electric heater is used to preheat the air to very high temperatures.

[0057]Embodiments of the present disclosure may provide at least one of the following advantages. The system efficiently pre-heats air and a hydrocarbon stream, allowing for a reduction in CO2 emissions from fired tubular furnaces during cracking processes. The system modifications relative to a conventional system reduce process duty while reducing auxiliary (non-process) duties to match the reduced overall fuel firing. The electrical preheating minimizes fossil fuel consumption. The system results in less firing, less CO2 emissions, and less stream production while maintaining the process objectives of the heater.

[0058]Although only a few example embodiments have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the example embodiments without materially departing from this invention. Accordingly, all such modifications are intended to be included within the scope of this disclosure as defined in the following claims.