Patent Publication Number: US-2021187436-A1

Title: System for flare gas recovery using gas sweetening process

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of and claims priority to U.S. patent application Ser. No. 15/951,432, filed on Apr. 12, 2018, the entire contents of which are incorporated by reference herein. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to systems and methods that integrate a flare gas recovery process with a gas sweetening process used in oil and gas refining. 
     BACKGROUND 
     Many industrial plants around the world utilize gas flares primarily to burn off waste gas that is released by safety valves. The safety valves can open during planned events, such as plant startup and shutdown, or during an unplanned event during processing, for example, to prevent over-pressuring in industrial plant equipment. By burning the waste gas, the flare breaks down waste gas into compounds that are more environmentally friendly when released into the atmosphere as well as prevents large volumes of flammable gas to be blown by wind to areas that can potentially cause safety issues. 
     In a flare, a continuous flow of waste gas is provided to the gas flare to maintain a constant flame. If the flare tip loses its flame, the flare will fail to burn the waste gas and the waste gas will simply discharge into the atmosphere. Because the flare discharges combusted gases to the atmosphere, an associated piping system called a flare header, which routes fluids to the flare, normally operates a little above atmospheric pressure. The waste gas that enters the flare header has a pressure that is too low to be of practical use in an oil and gas refining plant. 
     SUMMARY 
     This document relates to systems and methods that integrate a flare gas recovery process with a gas sweetening process used in oil and gas refining. In particular, this specification describes a system and method of utilizing liquid amine solvent from a gas sweetening unit as motive fluid for an ejector for application in flare gas recovery. 
     The present disclosure includes one or more of the following units of measure with their corresponding abbreviations, as shown in Table 1: 
     
       
         
           
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Unit of Measure 
                 Abbreviation 
               
               
                   
               
             
            
               
                 Degrees Fahrenheit 
                 ° F. 
               
               
                 Parts per million 
                 ppm 
               
               
                 Pounds per square inch (pressure) 
                 psi 
               
               
                 Pounds per square inch gauge (pressure) 
                 psig 
               
               
                 One million 
                 MM 
               
               
                 Standard cubic feet per day 
                 SCFD 
               
               
                 gallons per minute 
                 gpm (U.S. measure) 
               
               
                 Mole 
                 mol 
               
               
                   
               
            
           
         
       
     
     Oil refineries and gas processing facilities across the world can produce large amounts of waste gas. Flare gas recovery systems can be installed to recover this waste gas. A flare gas recovery process is a process that reutilizes a waste gas as a fuel gas when typically, the waste gas would be sent to a gas flare for disposal. Recovering waste gas can save operation costs associated with purchased fuel because some or all of the recovered flare gas can be used as fuel. Furthermore, flare gas recovery systems can reduce emissions and increase the life of the flare tip. If the recovered flare gas is further processed and cleaned, the flare gas can even be acceptable for venting. Flare gas recovery systems can include equipment to compress the waste gas so that the gas can be recycled back to the plant. However, instead of using multi-stage compressors, which are typically associated with high capital costs due to associated equipment, installation, and high operation costs, the systems and method described in this document use another option for compressing flare gas—which is to employ ejectors. 
     Ejectors rely on a Venturi effect to pressurize flare gas by utilizing available pressure from a fluid called a motive fluid. Ejectors are considered static equipment and are generally associated with low capital and operating costs in comparison to compressors. The ejector converts the pressure energy available in the motive fluid to velocity energy, brings in the low pressure suction fluid, mixes the two fluids, and discharges the mixture at an intermediate pressure without the use of rotating or moving parts. 
     In some embodiments, a liquid-driven ejector can be integrated with a flare gas recovery system. Such systems are more complicated than systems that use vapor-driven ejectors because of the need to separate the liquid and vapor phases downstream of the ejector. The advantage of a liquid-driven ejector, however, is that the liquid can be pumped and recycled as motive fluid, resulting in a net discharge of only the recovered flare gas and therefore, having significantly less impact on downstream units. Water is a viable option for motive fluid, but utilizing water introduces additional issues, such as water treatment and special materials to handle sour water, corrosion issues, and additional filtration needs. The integration and utilization of available solvent from the gas sweetening unit therefore provides the advantages of avoiding the issues associated with water-driven ejectors, while also adding the capability of cleaning the flare gas before recycling it back to the facility. 
     In an example implementation, a flare gas recovery system includes a primary gas sweetening unit; and a liquid-driven ejector in continuous fluid communication with the primary gas sweetening unit. The ejector includes an inlet configured to receive a motive fluid including a regenerable amine solvent in a rich state from the primary gas sweetening unit; a gas inlet configured to receive a suction fluid including a gas; and a fluid outlet configured to either directly or indirectly discharge to the primary gas sweetening unit a two-phase fluid including a mixture of the suction fluid and the amine solvent in a rich state. 
     In an aspect combinable with the example implementation, the amine solvent interacts with one or more components of the suction fluid in the ejector, the one or more components include hydrogen sulfide, carbon dioxide, or both. 
     In another aspect combinable with any one of the previous aspects, the amine solvent interacts with one or more components of gas by chemical binding, physical binding, or both, to produce the amine solvent in the rich state from the motive fluid and a gas configured for gas sweetening feed, combustion, venting, or flaring from the suction fluid. 
     Another aspect combinable with any one of the previous aspects further includes a filtration package to remove impurities from the solvent, wherein the impurities include corrosion particles or salts that form in the system during operation. 
     Another aspect combinable with any one of the previous aspects further includes a circulation pump to supply flow of the motive fluid from the primary gas sweetening unit to the ejector. 
     Another aspect combinable with any one of the previous aspects further includes a separator to separate the two-phase fluid into a rich solvent liquid phase and a sweetened gas vapor phase. 
     Another aspect combinable with any one of the previous aspects further includes a secondary gas sweetening unit operating at a lower pressure than the primary gas sweetening unit, wherein the rich solvent liquid phase from the separator is cycled back to the primary gas sweetening unit, and the sweetened gas vapor phase from the separator is delivered as feed to the secondary gas sweetening unit. 
     Another aspect combinable with any one of the previous aspects further includes a booster pump to pressurize the motive fluid to the ejector to meet operating conditions of the secondary gas sweetening unit. 
     In another aspect combinable with any one of the previous aspects, the suction fluid includes a flare gas from a source including a main flare header, upstream of a flashback protection device. 
     In another aspect combinable with any one of the previous aspects, the suction fluid includes a flare gas from a source including one or more of emergency valves in the primary gas sweetening unit or a main flare header, upstream of a flashback protection device. 
     In another example implementation, a method of supplying flare gas for a flare gas recovery system includes supplying a flow of flare gas to an ejector of the flare gas recovery system; supplying a continuous flow of regenerable amine solvent in a rich state to the ejector from a primary gas sweetening unit that is in fluid communication with the flare gas recovery system; and combining the flare gas and solvent together in the ejector to form a two-phase fluid, where the continuous flow of the solvent is configured to increase pressure of the flare gas to allow for delivery of the two-phase fluid either directly or indirectly back to the primary gas sweetening unit. 
     In an aspect combinable with the example implementation, combining of the flare gas and solvent causes removal of a portion of one or more components from the gas, the one or more components including hydrogen sulfide or carbon dioxide, by chemical binding, physical binding, or both, thereby resulting in the two-phase fluid including of the solvent in a rich state and the gas suitable for one or more of gas sweetening feed, combustion, venting, and flaring. 
     Another aspect combinable with any one of the previous aspects further includes filtering of the solvent to remove impurities, the impurities including corrosion particles or salts. 
     In another aspect combinable with any one of the previous aspects, supplying the solvent in rich state is provided by a pressure source, the pressure source including booster pumps designated for the flare gas recovery system, to meet operating conditions of a secondary gas sweetening unit. 
     In another aspect combinable with any one of the previous aspects, supplying the solvent in rich state is provided by a pressure source, the pressure source including circulation pumps in the primary gas sweetening unit or additional circulation pumps designated for the flare gas recovery system. 
     Another aspect combinable with any one of the previous aspects further includes separating the two-phase fluid into a rich solvent liquid phase and a sweetened gas vapor phase. 
     Another aspect combinable with any one of the previous aspects further includes cycling the liquid phase back to the primary gas sweetening unit, and delivering the vapor phase to a secondary gas sweetening unit. 
     In another aspect combinable with any one of the previous aspects, supplying the flow of flare gas to the ejector includes supplying gas from a main flare header, upstream of a flashback protection device. 
     In another aspect combinable with any one of the previous aspects, supplying the flow of flare gas to the ejector includes supplying gas from one or more of emergency valves in the primary gas sweetening unit or a main flare header, upstream of a flashback protection device. 
     In another example implementation, a flare gas recovery system includes a primary gas sweetening unit; and a liquid-driven ejector in continuous fluid communication with the primary gas sweetening unit. The ejector includes an inlet configured to receive a motive fluid including a regenerable amine solvent in a lean state from the primary gas sweetening unit; a gas inlet configured to receive a suction fluid including a gas; and a fluid outlet configured to either directly or indirectly discharge to the primary gas sweetening unit a two-phase fluid including a mixture of the suction fluid and the amine solvent in a rich state. 
     In an aspect combinable with the example implementation, the liquid-driven ejector includes a first liquid-driven ejector. 
     Another aspect combinable with any one of the previous aspects further includes a second liquid-driven ejector in continuous fluid communication with the primary gas sweetening unit. 
     In another aspect combinable with any one of the previous aspects, the second liquid-driven ejector includes an inlet configured to receive a motive fluid including a lean or sour gas stream; a gas inlet configured to receive a suction fluid including a flare gas; and a fluid outlet configured to either directly or indirectly discharge to the first liquid-driven ejector a two-phase fluid including a mixture of the suction fluid and motive fluid. 
     In another aspect combinable with any one of the previous aspects, the two-phase fluid includes primarily flare gas. 
     In another aspect combinable with any one of the previous aspects, the amine solvent interacts with one or more components of the suction fluid in the ejector, the one or more components include hydrogen sulfide, carbon dioxide, or both. 
     In another aspect combinable with any one of the previous aspects, the amine solvent interacts with one or more components of gas by chemical binding, physical binding, or both, to produce the amine solvent in the rich state from the motive fluid and a gas configured for gas sweetening feed, combustion, venting, or flaring from the suction fluid. 
     Another aspect combinable with any one of the previous aspects further includes a filtration package to remove impurities from the solvent, wherein the impurities include corrosion particles or salts that form in the system during operation. 
     Another aspect combinable with any one of the previous aspects further includes a circulation pump to supply flow of the motive fluid from the primary gas sweetening unit to the ejector; and a separator to separate the two-phase fluid into a rich solvent liquid phase and a sweetened gas vapor phase. 
     Another aspect combinable with any one of the previous aspects further includes a secondary gas sweetening unit operating at a lower pressure than the primary gas sweetening unit, wherein the rich solvent liquid phase from the separator is cycled back to the primary gas sweetening unit, and the sweetened gas vapor phase from the separator is delivered as feed to the secondary gas sweetening unit. 
     Another aspect combinable with any one of the previous aspects further includes a booster pump to provide adequate pressure to the motive fluid to the ejector, to meet operating conditions of the secondary gas sweetening unit. 
     In another aspect combinable with any one of the previous aspects, the suction fluid includes a flare gas from a source including a main flare header, upstream of a flashback protection device. 
     In another aspect combinable with any one of the previous aspects, the suction fluid includes a flare gas from a source including one or more of emergency valves in the primary gas sweetening unit or a main flare header, upstream of a flashback protection device. 
     In another example implementation, a method of supplying flare gas for a flare gas recovery system includes supplying a flow of flare gas to a flare gas ejector of the flare gas recovery system; supplying a continuous flow of a lean or sour gas stream to the ejector; combining the flare gas and lean or sour gas stream together in the ejector to form a mixed gas fluid, supplying a flow of the mixed-gas fluid to an amine ejector of a gas sweetening unit; supplying a continuous flow of regenerable amine solvent in a lean state to the amine ejector from the primary gas sweetening unit that is in fluid communication with the flare gas recovery system; and combining the mixed-gas fluid and solvent together in the amine ejector to form a two-phase fluid, where the continuous flow of the solvent is configured to increase pressure of the mixed-gas fluid to allow for delivery of the two-phase fluid either directly or indirectly back to the primary gas sweetening unit. 
     In an aspect combinable with the example implementation, combining of the mixed-gas fluid and solvent causes removal of a portion of one or more components from the mixed-gas, the one or more components including hydrogen sulfide or carbon dioxide, by chemical binding, physical binding, or both, thereby resulting in the two-phase fluid including of the solvent in a rich state and the gas suitable for one or more of gas sweetening feed, combustion, venting, and flaring. 
     Another aspect combinable with any one of the previous aspects further includes filtering of the solvent to remove impurities, the impurities including corrosion particles or salts. 
     In another aspect combinable with any one of the previous aspects, supplying the solvent in lean state is provided by a pressure source, the pressure source including circulation pumps in the primary gas sweetening unit or additional circulation pumps designated for the flare gas recovery system. 
     Another aspect combinable with any one of the previous aspects further includes separating the two-phase fluid into a rich solvent liquid phase and a sweetened gas vapor phase. 
     Another aspect combinable with any one of the previous aspects further includes cycling the liquid phase back to the primary gas sweetening unit, and delivering the vapor phase to a secondary gas sweetening unit. 
     In another aspect combinable with any one of the previous aspects, supplying the lean solvent is further assisted by an additional pressure source, the pressure source including booster pumps designated for the flare gas recovery system, to meet operating conditions of the secondary gas sweetening unit. 
     In another aspect combinable with any one of the previous aspects, supplying the flow of flare gas to the flare gas ejector includes supplying gas from a main flare header, upstream of a flashback protection device. 
     In another aspect combinable with any one of the previous aspects, supplying the flow of flare gas to the flare gas ejector includes supplying gas from one or more of emergency valves in the primary gas sweetening unit or a main flare header, upstream of a flashback protection device. 
     The subject matter described in this specification can be implemented in particular implementations, so as to realize one or more of the following advantages. The integrated processes and systems described in this document can provide an alternative to using a gas flare system, or another waste gas disposal system, which allows a gas refinery company to meet certain quality and regulatory emissions standards. The integrated systems and methods described in this document can reduce capital and operating costs by reducing the need for additional power and equipment, such as a knockout vessel and a cooler, in comparison to existing systems that recover waste gases. The integrated systems and methods described in this document can reduce capital and operating costs by reducing the need for additional processing, such as cooling or removing acid gas. The integrated systems and processes described in this document can require less area in comparison to existing systems used for disposing waste gases. Although the gas could be treated when passing through the ejector and subsequently sent to an end user, certain embodiments of the integrated systems and processes described in this document recycle waste gas back to the process to reduce net production of waste gas. For example, in some embodiments, the gas from an ejector outlet can be routed back to an amine unit in the system. The integrated systems and methods described in this document provide additional capability to clean recovered flare gas by nature of the chosen motive fluid. Other advantages will be apparent to those of ordinary skill in the art. 
     The details of one or more implementations of the subject matter of this specification are set forth in the accompanying drawings and the description. Other features, aspects, and advantages of the subject matter will become apparent from the description, the drawings, and the claims. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram of an example system that includes a flare gas recovery unit and two gas sweetening units. 
         FIG. 2  is a schematic diagram of another example system that integrates a flare gas recovery unit with a gas sweetening unit, where the flare gas is recovered from a main flare header. 
         FIG. 3  is a schematic diagram of another example system that integrates a flare gas recovery unit with a gas sweetening unit, where the flare gas is recovered from the gas sweetening unit. 
         FIG. 4  is a schematic diagram of an example system that includes a flare gas recovery unit and a gas sweetening unit. 
         FIG. 5  is a schematic diagram of another example system that integrates a flare gas recovery unit with a gas sweetening unit, where the flare gas is recovered from a main flare header. 
         FIG. 6  is a schematic diagram of another example system that integrates a flare gas recovery unit with a gas sweetening unit, where the flare gas is recovered from a main flare header. 
     
    
    
     DETAILED DESCRIPTION 
     This document describes systems and methods that integrate a flare gas recovery unit with a gas sweetening unit, and is presented to enable any person skilled in the art to make and use the disclosed subject matter in the context of one or more particular implementations. 
     After crude oil or natural gas is extracted, it must be refined to produce commercial fuels and other products. Oil or gas that contains significant amounts of sulfur compounds like hydrogen sulfide is considered “sour,” and oil refineries and gas processing plants utilize “sweetening” processes to remove these sulfur compounds. Gas sweetening units typically utilize an aqueous solution of amine solvent to remove hydrogen sulfide and carbon dioxide from sour gas. 
       FIG. 1  depicts a general schematic of a first exemplary system  100  that includes a primary gas sweetening unit  110  and flare gas recovery system  130 . The primary gas sweetening unit  110  of  FIG. 1  comprises an amine contactor  112 , a flash drum  114 , an amine stripper  118 , an amine circulation pump  120 , and an amine cooler  122 . The amine contactor  112  is a counter-flow gas-liquid contactor that can be referred as an absorber, treater, or scrubber. The amine contactor  112  is a vessel comprising internal components, which can include trays or packing, to increase gas-liquid contact. 
     The flash drum  114  operates at a lower pressure than the contactor  112  and allows light hydrocarbons to flash (that is, evaporate) from the amine solvent. The flash drum  114  is sized for liquid surge, liquid holdup, and residence time for vapor to separate from the liquid amine solvent. In some embodiments, the flash drum  114  is equipped with a tower  116 . The flash drum tower  116  can remove acid gas such as hydrogen sulfide, which can be present in the vapor separated from the amine solvent, before the vapor is sent to another downstream process or end user. 
     Still referring to  FIG. 1 , the amine stripper  118  is a vessel, which can also be referred to as a regenerator. The amine stripper  118  comprises internal components, for example, trays or packing, and effectively serves as a distillation tower to boil off acid gas to regenerate the amine solvent. The distinction between acid gas and sour gas is that sour gas is mostly hydrocarbons with some acidic gas content, and acid gas contains little to no hydrocarbons. 
     The circulation pump  120  pressurizes the regenerated amine solvent to recycle the amine solvent back to the contactor  112 . The circulation pump  120  can comprise a single pump or multiple pumps in parallel or in series. The circulation pump  120  can be sized to accommodate upset scenarios which require much higher flow rates than is normally required by the primary gas sweetening unit  110 . The circulation pump  120 , as depicted in  FIG. 1 , employs a recycle line which routes a portion of the amine solvent back to the suction of the pump  120 . 
     The amine cooler  122  brings the temperature of the solvent down before the solvent is recycled back to the contactor  112 . The lower solvent temperature increases the efficiency of cleaning the sour gas that enters the contactor  112 . The cooler  122  can be a shell-and-tube heat exchanger, an air cooler, or a combination of multiples of both. 
     Gas sweetening units can optionally comprise auxiliary and variant equipment such as additional heat exchangers and vessels that have not been described above, but a majority of gas sweetening units across the world implement some variation or combination of the major equipment outlined. 
     Gas sweetening units can operate at a variety of operating temperatures and pressures. In some embodiments, sour gas at a temperature of between 70-130° F. via stream  111  enters the bottom of an amine contactor  112 , as amine solvent at a temperature of between 80-140° F. via stream  113  enters from the top. The amine solvent that enters the amine contactor  112  is at least approximately 10° F. hotter than the sour gas that enters the amine contactor  112 . As the amine solvent contacts the sour gas, the solvent removes (or “cleans”) the sulfur compounds, carbon dioxide, and other contaminants from the sour gas, by chemical and physical binding. Once the solvent has passed through contactor  112 , the solvent is considered to be in a “rich” state—also referred as “rich solvent”-because the solvent contains the hydrogen sulfide removed from the sour gas. The sweetened gas exits from the top of contactor  112  via stream  129 , and rich solvent exits from the bottom via stream  115 . The sweetened gas (stream  129 ) can contain approximately 5-60 ppm hydrogen sulfide and is sent downstream for sale or further processing. Rich solvent  115  is sent to a flash drum  114  operating between atmospheric pressure to 90 psig, where any flashed vapor travels up a flash drum tower  116  and exits via stream  119 , where the flashed vapor can then be utilized as fuel, vented, flared, or a combination of these. 
     Rich solvent liquid  117  from flash drum  114  is sent to an amine stripper  118  with a top operating pressure between 5-17 psig. The hydrogen sulfide and carbon dioxide is boiled off via heat input to the bottom of stripper  118  operating between 230-270° F. in order to regenerate the amine solvent. The regenerated solvent is then considered to be in a “lean” state—also referred as “lean solvent”—that is once again suitable to be used for cleaning additional sour gas. Sour gas  123 , comprising hydrogen sulfide and carbon dioxide exits the top of stripper  118 , and lean solvent  121  is pumped out of the bottom of stripper  118  by circulation pump  120 . Lean solvent  127  is cooled in heat exchanger  112  to approximately 80-140° F. before re-entering contactor  112  to be used again to clean additional sour gas. The transport of vapor and liquid within, to, and from the gas sweetening unit  110  can be achieved using various piping, pump, and valve configurations. 
     Still referring to  FIG. 1 , the exemplary system  100  includes the flare gas recovery system  130  that is integrated with the gas sweetening unit  110 , as described above. The system  100  utilizes liquid amine solvent  125  from the gas sweetening unit  110  as motive fluid for an ejector in the flare gas recovery system  130 . 
     The flare gas recovery system  130  includes an ejector  134  that comprises an inlet that continuously receives the regenerable amine solvent, which serves as a high-pressure motive fluid from the gas sweetening unit  110  via stream  125 . The ejector  134  also comprises a gas inlet configured for receiving a flare gas  133  as a low-pressure suction fluid. The motive fluid operates at a higher pressure than the suction fluid. For example, the amine solvent (motive fluid) operates at approximately 990 psig, and the flare gas (suction fluid) operates at approximately 0.5 psig. The motive and suction fluid mix within the ejector  134 , and then discharge at an intermediate pressure. Because the motive fluid is amine solvent  125  from the gas sweetening unit  110 , the motive fluid is capable of removing hydrogen sulfide and carbon dioxide from the flare gas. 
       FIG. 1  shows a certain implementation in which the suction gas of the ejector  134  is supplied by a flare header  131  via stream  133 . System  100  can include a secondary gas sweetening unit  150  (including a secondary amine contactor  152 ), which operates at a lower pressure than the primary gas sweetening unit  110 . The ejector  134  can be installed near the flare header  131  and utilize amine solvent from a nearby gas sweetening unit, such as the primary gas sweetening unit  110 , as motive fluid to mix with and pressurize flare gas. The two-phase mixture  137  can be discharged to a separator  136 , where vapor phase  143  is separated from liquid phase  141  of the mixture. The liquid phase includes rich solvent and can be recycled back to the primary gas sweetening unit  110  and returned to a flash drum  114  via stream  141 . The vapor phase  143  includes sweetened gas and can be delivered as additional feed (for example, in addition to the feed  151  to the secondary amine contactor  152 ) to the secondary gas sweetening unit  150 . The type of system shown in  FIG. 1  can be applicable when there exists at least two gas sweetening units operating at different pressures. In some implementations, a booster pump is included to provide adequate pressure to the amine solvent from the primary gas sweetening unit  110  which is being utilized as motive fluid for the ejector  134 , so that the recovered flare gas can be sent to the secondary gas sweetening unit  150 . 
     The flare gas recovery system  130  design takes into consideration the integrated operation with the flare  170 , which includes flashback prevention  132 . Flashback prevention involves preventing reverse flow of gas and potentially, the flame from the flare, as flare gas  135  is being burned at the flare  170 . Flashback prevention can comprise a liquid seal drum, a molecular seal, a fluidic seal, a flame arrestor, or any combination thereof. The source of flare gas to the ejector  134  (or analogous  234 ,  334 ) is upstream of the flashback prevention  132 . 
       FIG. 2  provides a general schematic of a second exemplary system  200  that includes a gas sweetening unit  210  and flare gas recovery system  230 . As depicted in  FIG. 2 , the sweetening unit  210  is substantially the same as the sweetening unit  110  of  FIG. 1 , but the configuration of the flare gas recovery system  230  differs from the flare gas recovery system  130  of  FIG. 1 . Like element numbers across the figures can be substantially the same; for example, the amine contactors  212  and  312  can be substantially the same as the amine contactor  112 . The ejector  234  can be installed near the flare header  231  and utilize amine solvent  225  from a nearby gas sweetening unit, such as the primary gas sweetening unit  210 , as motive fluid to mix with and pressurize flare gas  233 . The two-phase mixture from the ejector  234  can be recycled back to the primary gas sweetening unit  210  and discharged directly back to a flash drum  214  via stream  239 . The vapor phase can be separated from liquid phase in the flash drum  214  because the both the liquid and gas from the ejector  234  are recycled back to the gas sweetening unit  210 . Because flare gas is being recovered and recycled to the gas sweetening unit  210 , downstream units can require modification to accommodate the increased vapor flow—for example, flash drum tower  216 . The recovered flare gas can then be utilized as fuel, vented, flared, or a combination of these. 
       FIG. 3  provides a general schematic of a third exemplary system  300  that includes a gas sweetening unit  310  and flare gas recovery system  330 . As depicted in  FIG. 3 , the sweetening unit  310  is substantially the same as the sweetening unit  110  of  FIG. 1 , but the configuration of the flare gas recovery system  330  differs from the flare gas recovery system  130  of  FIG. 1 . The ejector  334  can be installed near a gas sweetening unit, such as the gas sweetening unit  310 , and utilize amine solvent  325  from the gas sweetening unit  310  as motive fluid to mix with and pressurize waste gas  343  from the gas sweetening unit  310 . The two-phase mixture from the ejector  334  can be recycled back to the gas sweetening unit  310  and discharged directly back to a flash drum  314  via stream  339 . For example,  FIG. 3  shows a certain implementation in which the suction gas of the ejector  334  is supplied by emergency valves from the gas sweetening unit  310  via stream  341 . Some or all of the losses from the unit can immediately be recovered before reaching a flare header  331 . Although the configuration is different, the source of flare gas to the ejector  334  is still upstream of the flashback prevention  332 . 
     In some implementations, an additional circulation pump is included to provide adequate flow of amine solvent from the primary gas sweetening unit  110  (or analogous  210 ,  310 ) which is being utilized as motive fluid for the ejector  134  (or analogous  234 ,  334 ). 
     Referring to the exemplary system  100  in  FIG. 1 , in one example of the systems described in this document, the amine contactor  112  of the primary gas sweetening unit  110  can operate at approximately 980 psig. The lean amine solvent from the amine cooler  122  can enter the contactor  112  at approximately 140° F., and the sweet gas (stream  129 ) exits the contactor  112  at approximately 120° F. The flash drum  114  can operate at approximately 80 psig and receives liquid from the bottom of the contactor  112  and the bottom of the separator  136  from the flare gas recovery system  130 . The flashed vapor can travel up the flash drum tower  116  and be sent to boilers, where the gas is burned to provide heat for another process. The liquid from the flash drum  114  can be sent to the amine stripper  118  with a bottom operating pressure of approximately 15 psig and a bottom operating temperature of approximately 265° F. The circulation pump  120  can normally circulate at approximately 9500 gpm with a discharge pressure of approximately 990 psig. Approximately 7000 gpm of the amine solvent can be circulated back to the contactor  112  through cooler  122 , 1200 gpm can be sent to the ejector  134  of the flare gas recovery system  130 , and the balance can be recycled back to the suction of circulation pump  120 . 
     In some embodiments, a portion of the flare gas from the flare header  131  can be sent to the ejector  134 , upstream of the seal drum  132 , which is utilized for flashback prevention and liquid knockout. The amine solvent and flare gas can be mixed within ejector  134  and discharged at approximately 210 psig. The vapor-liquid mixture  137  can be sent to separator  136 , where the liquid  141  at the bottom is sent back to the primary gas sweetening unit  110 , and the vapor  143  at the top is sent as additional feed to the secondary gas sweetening unit  150 , which operates at approximately 180 psig. 
     The approximate flow rates and compositions of the streams can be: 
     
       
         
           
               
               
            
               
                   
                   
               
               
                   
                 Stream Number* 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 units 
                 111 
                 113 
                 119 
                 121 
                 123 
                 125 
                 129 
                 131 
                 137 
                 143 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Vapor flow 
                 MMSCFD 
                 600 
                 — 
                 0.9 
                 — 
                 63   
                 — 
                 535    
                 2   
                 2 
                 2 
               
               
                 Liquid flow 
                 gpm 
                 — 
                 7000 
                 — 
                 8200 
                 — 
                 1200 
                 — 
                 — 
                 1200 
                 — 
               
               
                 Vapor composition 
                 mol % 
                   
                   
                   
                   
                 ** 
                   
                 ** 
               
               
                 methane 
                   
                 70.0 
                 — 
                 91.0  
                 — 
                 — 
                 — 
                 78.4  
                 85.5  
                 90.5 
                 — 
               
               
                 ethane 
                   
                 6.0 
                 — 
                 5.0 
                 — 
                 — 
                 — 
                 6.7 
                 2.0 
                 2.1 
                 — 
               
               
                 propane 
                   
                 2.5 
                 — 
                 — 
                 — 
                 — 
                 — 
                 2.8 
                 — 
                 — 
                 — 
               
               
                 i-butane 
                   
                 0.5 
                 — 
                 — 
                 — 
                 — 
                 — 
                 0.6 
                 — 
                 — 
                 — 
               
               
                 n-butane 
                   
                 0.4 
                 — 
                 — 
                 — 
                 — 
                 — 
                 0.4 
                 — 
                 — 
                 — 
               
               
                 i-pentane 
                   
                 0.4 
                 — 
                 — 
                 — 
                 — 
                 — 
                 0.4 
                 — 
                 — 
                 — 
               
               
                 n-pentane 
                   
                 0.3 
                 — 
                 — 
                 — 
                 — 
                 — 
                 0.3 
                 — 
                 — 
                 — 
               
               
                 carbon dioxide 
                   
                 5.0 
                 — 
                 — 
                 — 
                 48.0 
                 — 
                 — 
                 2.5 
                 — 
                 — 
               
               
                 hydrogen sulfide 
                   
                 5.5 
                 — 
                 — 
                 — 
                 52.0 
                 — 
                 — 
                 3.0 
                 — 
                 — 
               
               
                 nitrogen 
                   
                 9.4 
                 — 
                 4.0 
                 — 
                 — 
                 — 
                 10.5  
                 7.0 
                 7.4 
                 — 
               
               
                 water 
                   
                 0.1 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                   
               
               
                 *Refer to FIG. 1. 
               
               
                 ** Composition is in dry basis. 
               
            
           
         
       
     
     Referring to the exemplary system  200  in  FIG. 2 , in one example of the systems described in this document, the amine contactor  212  of the gas sweetening unit  210  can operate at approximately 980 psig. The lean amine solvent from the amine cooler  222  can enter the contactor  212  at approximately 140° F., and the sweet gas (stream  229 ) can exit the contactor  212  at approximately 120° F. The flash drum  214  can operate at approximately 80 psig and receive liquid from the bottom of the contactor  212  and a vapor-liquid mixture from the ejector  234  from the flare gas recovery system  230 . The flashed vapor can travel up the flash drum tower  216  and be sent to boilers, where the gas is burned to provide heat for another process. The liquid from the flash drum  214  can be sent to the amine stripper  218  with a bottom operating pressure of approximately 15 psig and a bottom operating temperature of approximately 265° F. The circulation pump  220  can normally circulate at approximately 9500 gpm with a discharge pressure of approximately 990 psig. Approximately 7000 gpm of the amine solvent can be circulated back to the contactor  212  through cooler  222  such that 1200 gpm is sent to the ejector  234  of the flare gas recovery system  230  and the balance is recycled back to the suction of circulation pump  220 . 
     In some embodiments, portion of the flare gas from the flare header  231  can be sent to the ejector  234 , upstream of the seal drum  232  for flashback prevention and liquid knockout. The amine solvent and flare gas can be mixed within ejector  234  and discharged a s avapor-liquid mixture back to the flash drum  214  of the gas sweetening unit  210 . 
     The approximate flow rates and compositions of the streams can be: 
     
       
         
           
               
               
            
               
                   
                   
               
               
                   
                 Stream Number* 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 units 
                 211 
                 213 
                 219 
                 221 
                 223 
                 225 
                 229 
                 231 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Vapor flow 
                 MMSCFD 
                 600 
                 — 
                  2.79 
                 — 
                 63   
                 — 
                 535    
                 2   
               
               
                 Liquid flow 
                 gpm 
                 — 
                 7000 
                 — 
                 8200 
                 — 
                 1200 
                 — 
                 — 
               
               
                 Vapor composition 
                 mol % 
                   
                   
                   
                   
                 ** 
                   
                 ** 
               
               
                 methane 
                   
                 70.0 
                 — 
                 90.6  
                 — 
                 — 
                 — 
                 78.4  
                 85.5  
               
               
                 ethane 
                   
                 6.0 
                 — 
                 3.1 
                 — 
                 — 
                 — 
                 6.7 
                 2.0 
               
               
                 propane 
                   
                 2.5 
                 — 
                 0.0 
                 — 
                 — 
                 — 
                 2.8 
                 — 
               
               
                 i-butane 
                   
                 0.5 
                 — 
                 — 
                 — 
                 — 
                 — 
                 0.6 
                 — 
               
               
                 n-butane 
                   
                 0.4 
                 — 
                 — 
                 — 
                 — 
                 — 
                 0.4 
                 — 
               
               
                 i-pentane 
                   
                 0.4 
                 — 
                 — 
                 — 
                 — 
                 — 
                 0.4 
                 — 
               
               
                 n-pentane 
                   
                 0.3 
                 — 
                 — 
                 — 
                 — 
                 — 
                 0.3 
                 — 
               
               
                 carbon dioxide 
                   
                 5.0 
                 — 
                 — 
                 — 
                 48.0 
                 — 
                 — 
                 2.5 
               
               
                 hydrogen sulfide 
                   
                 5.5 
                 — 
                 — 
                 — 
                 52.0 
                 — 
                 — 
                 3.0 
               
               
                 nitrogen 
                   
                 9.4 
                 — 
                 6.3 
                 — 
                 — 
                 — 
                 10.5  
                 7.0 
               
               
                 water 
                   
                 0.1 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                   
               
               
                 *Refer to FIG. 2. 
               
               
                 ** Composition is in dry basis. 
               
            
           
         
       
     
     Referring to the exemplary system  300  in  FIG. 3 , in one example of the systems described in this document, the amine contactor  312  of the gas sweetening unit  310  can operate at approximately 980 psig. The lean amine solvent from the amine cooler  322  can enter the contactor  312  at approximately 140° F., and the sweet gas (stream  329 ) can exit the contactor  312  at approximately 120° F. The flash drum  314  can operate at approximately 80 psig and receive liquid from the bottom of the contactor  312  and a vapor-liquid mixture from the ejector  334  from the flare gas recovery system  330 . The flashed vapor can travel up the flash drum tower  316  and be sent to boilers, where the gas is burned to provide heat for another process. The liquid from the flash drum  314  can be sent to the amine stripper  318  with a bottom operating pressure of approximately 15 psig and a bottom operating temperature of approximately 265° F. The circulation pump  320  can normally circulate approximately 9500 gpm with discharge pressure of approximately 990 psig. Approximately 7000 gpm of the amine solvent can be circulated back to the contactor  312  through cooler  322  such that 190 gpm is sent to the ejector  334  of the flare gas recovery system  330 , and the balance is recycled back to the suction of circulation pump  320 . 
     Some of the flare gas from the flare header  331  is sent to the ejector  334 , upstream of the seal drum  332 , which is utilized for flashback prevention and liquid knockout. The ejector  334  is also lined up to receive flare gas directly from the gas sweetening unit  310 , by stream  341  which is an emergency valve discharge header for the gas sweetening unit  310 . In some cases, an emergency valve in the gas sweetening unit  310  can be opened and the gas can be recovered before being sent to the flare header  331 . The amine solvent and flare gas can be mixed within ejector  334  and discharged as a vapor-liquid mixture back to the flash drum  314  of the gas sweetening unit  310 . 
     The approximate flow rates and compositions of the streams can be: 
     
       
         
           
               
               
            
               
                   
                   
               
               
                   
                 Stream Number* 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 units 
                 311 
                 313 
                 319 
                 321 
                 323 
                 325 
                 329 
                 341 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Vapor flow 
                 MMSCFD 
                 600 
                 — 
                  1.25 
                 — 
                 63   
                 — 
                 535    
                  0.35 
               
               
                 Liquid flow 
                 gpm 
                 — 
                 7000 
                 — 
                 8200 
                 — 
                 190 
                 — 
                 — 
               
               
                 Vapor composition 
                 mol % 
                   
                   
                   
                   
                 ** 
                   
                 ** 
               
               
                 methane 
                   
                 70.0 
                 — 
                 88.6  
                 — 
                 — 
                 — 
                 78.4  
                 56.6  
               
               
                 ethane 
                   
                 6.0 
                 — 
                 5.4 
                 — 
                 — 
                 — 
                 6.7 
                 4.9 
               
               
                 propane 
                   
                 2.5 
                 — 
                 0.6 
                 — 
                 — 
                 — 
                 2.8 
                 2.0 
               
               
                 i-butane 
                   
                 0.5 
                 — 
                 — 
                 — 
                 — 
                 — 
                 0.6 
                 — 
               
               
                 n-butane 
                   
                 0.4 
                 — 
                 — 
                 — 
                 — 
                 — 
                 0.4 
                 — 
               
               
                 i-pentane 
                   
                 0.4 
                 — 
                 — 
                 — 
                 — 
                 — 
                 0.4 
                 — 
               
               
                 n-pentane 
                   
                 0.3 
                 — 
                 — 
                 — 
                 — 
                 — 
                 0.3 
                 — 
               
               
                 carbon dioxide 
                   
                 5.0 
                 — 
                 — 
                 — 
                 48.0 
                 — 
                 — 
                 13.9  
               
               
                 hydrogen sulfide 
                   
                 5.5 
                 — 
                 — 
                 — 
                 52.0 
                 — 
                 — 
                 15.1  
               
               
                 nitrogen 
                   
                 9.4 
                 — 
                 5.4 
                 — 
                 — 
                 — 
                 10.5  
                 7.6 
               
               
                 water 
                   
                 0.1 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                   
               
               
                 *Refer to FIG. 3. 
               
               
                 ** Composition is in dry basis. 
               
            
           
         
       
     
     In some implementations, a filtration package can be included to remove impurities like salts or corroded materials that accumulate in the solvent used for the gas sweetening process. The filtration package can comprise a filter housing, a filter element or cartridge, an additional circulation pump, or a combination of multiples of these. Impurities collect on the filter element or cartridge as a fluid passes through the filter. The filter element or cartridge can be cleaned or replaced periodically. 
       FIG. 4  depicts a general schematic of a fourth exemplary system  400  that includes a primary gas sweetening unit  410  and flare gas recovery system  430 . The primary gas sweetening unit  410  of  FIG. 4  comprises an amine contactor  412 , a flash drum  414 , an amine stripper  418 , an amine circulation pump  420 , and an amine cooler  422 . The amine contactor  412  is a counter-flow gas-liquid contactor that can be referred as an absorber, treater, or scrubber. The amine contactor  412  is a vessel comprising internal components, which can include trays or packing, to increase gas-liquid contact. 
     The flash drum  414  operates at a lower pressure than the contactor  412  and allows light hydrocarbons to flash (that is, evaporate) from the amine solvent. The flash drum  414  is sized for liquid surge, liquid holdup, and residence time for vapor to separate from the liquid amine solvent. In some embodiments, the flash drum  414  is equipped with a tower  416 . The flash drum tower  416  can remove acid gas such as hydrogen sulfide, which can be present in the vapor separated from the amine solvent, before the vapor is sent to another downstream process or end user. 
     Still referring to  FIG. 4 , the amine stripper  418  is a vessel, which can also be referred to as a regenerator. The amine stripper  418  comprises internal components, for example, trays or packing, and effectively serves as a distillation tower to boil off acid gas to regenerate the amine solvent. The distinction between acid gas and sour gas is that sour gas is mostly hydrocarbons with some acidic gas content, and acid gas contains little to no hydrocarbons. 
     The circulation pump  420  pressurizes the regenerated amine solvent to recycle the amine solvent back to the contactor  412 . The circulation pump  420  can comprise a single pump or multiple pumps in parallel or in series. The circulation pump  420  can be sized to accommodate upset scenarios which require much higher flow rates than is normally required by the primary gas sweetening unit  410 . The circulation pump  420 , in some aspects, can employ a recycle line which routes a portion of the amine solvent back to the suction of the pump  420 . Further, a booster pump (or pumps) may be positioned to pressurize the rich amine solvent  415  to the ejector  434 . 
     The amine cooler  422  brings the temperature of the solvent down before the solvent is recycled back to the contactor  412 . The lower solvent temperature increases the efficiency of cleaning the sour gas that enters the contactor  412 . The cooler  422  can be a shell-and-tube heat exchanger, an air cooler, or a combination of multiples of both. 
     Gas sweetening units can optionally comprise auxiliary and variant equipment such as additional heat exchangers and vessels that have not been described above, but a majority of gas sweetening units across the world implement some variation or combination of the major equipment outlined. 
     Gas sweetening units can operate at a variety of operating temperatures and pressures. In some embodiments, sour gas at a temperature of between 70-130° F. via stream  411  enters the bottom of an amine contactor  412 , as amine solvent at a temperature of between 80-140° F. via stream  413  enters from the top. The amine solvent that enters the amine contactor  412  is at least approximately 10° F. hotter than the sour gas that enters the amine contactor  412 . As the amine solvent contacts the sour gas, the solvent removes (or “cleans”) the sulfur compounds, carbon dioxide, and other contaminants from the sour gas, by chemical and physical binding. Once the solvent has passed through contactor  412 , the solvent is considered to be in a “rich” state—also referred as “rich solvent”-because the solvent contains the hydrogen sulfide removed from the sour gas. The sweetened gas exits from the top of contactor  412  via stream  429 , and rich solvent exits from the bottom via stream  415 . The sweetened gas (stream  429 ) can contain approximately 5-60 ppm hydrogen sulfide and is sent downstream for sale or further processing. 
     As shown, in this example implementation, rich solvent  415  can be sent to an ejector  434  and used as a motive fluid (discussed later) for the ejector  434  prior to (or in place of) being sent to a flash drum  414  operating between atmospheric pressure to 90 psig, where any flashed vapor travels up a flash drum tower  416  and exits via stream  419 , where the flashed vapor can then be utilized as fuel, vented, flared, or a combination of these. 
     Rich solvent liquid  417  from flash drum  414  is sent to an amine stripper  418  with a top operating pressure between 5-17 psig. The hydrogen sulfide and carbon dioxide is boiled off via heat input to the bottom of stripper  418  operating between 230-270° F. in order to regenerate the amine solvent. The regenerated solvent is then considered to be in a “lean” state—also referred as “lean solvent”—that is once again suitable to be used for cleaning additional sour gas. Sour gas  423 , comprising hydrogen sulfide and carbon dioxide exits the top of stripper  418 , and lean solvent  421  is pumped out of the bottom of stripper  418  by circulation pump  420 . Lean solvent  427  is cooled in heat exchanger  412  to approximately 80-140° F. before re-entering contactor  412  to be used again to clean additional sour gas. The transport of vapor and liquid within, to, and from the gas sweetening unit  410  can be achieved using various piping, pump, and valve configurations. 
     Still referring to  FIG. 4 , the exemplary system  400  includes the flare gas recovery system  430  that is integrated with the gas sweetening unit  410 , as described above. The system  400  utilizes liquid rich amine solvent  415  from the gas sweetening unit  410  (for example, circulated from the bottom of the contactor  412 ) as motive fluid for the ejector  434  in the flare gas recovery system  430 . 
     The flare gas recovery system  430  includes the ejector  434  that comprises an inlet that continuously receives the rich amine solvent  415 , which serves as a high-pressure motive fluid from the gas sweetening unit  410  via stream  415 . The ejector  434  also comprises a gas inlet configured for receiving a flare gas  433  as a low-pressure suction fluid. The motive fluid operates at a higher pressure than the suction fluid. For example, the rich amine solvent (motive fluid) operates at approximately 990 psig, and the flare gas (suction fluid) operates at approximately 0.5 psig. The motive and suction fluid mix within the ejector  434 , and then discharge at an intermediate pressure. Because the motive fluid is rich amine solvent  415  from the gas sweetening unit  410 , the motive fluid is capable of removing hydrogen sulfide and carbon dioxide from the flare gas. 
       FIG. 4  shows a certain implementation in which the suction gas of the ejector  434  is supplied by a flare header  431  via stream  433 . In some aspects, system  400  can include a secondary gas sweetening unit (not shown, but similar to unit  150  in  FIG. 1 , which includes a secondary amine contactor  152 ), which operates at a lower pressure than the primary gas sweetening unit  410 . The ejector  434  can be installed near the flare header  431  and utilize rich amine solvent from a nearby gas sweetening unit, such as the primary gas sweetening unit  410 , as motive fluid to mix with and pressurize flare gas. The two-phase mixture  437  can be discharged back to the flash drum  414 , where the flashed vapor phase  419  is separated from the rich liquid phase  417  of the mixture. As shown in this example, a pressure reducing device  439 , such as a valve or orifice, is positioned in the conduit for stream  415  between the take-off to, and the return from, the liquid-driven ejector  434 . In some aspects, the pressure reducing device  439  may equalize (or help equalize) the pressure of the rich solvent  415  and the two-phase mixture  437  from the ejector  434  returning to the flash drum  414 . 
     In some aspects, the type of system shown in  FIG. 4  can be applicable when there exists at least two gas sweetening units operating at different pressures. In some implementations, a booster pump is included to provide adequate pressure to the rich amine solvent from the primary gas sweetening unit  410  which is being utilized as motive fluid for the ejector  434 , so that the recovered flare gas can be sent to a secondary gas sweetening unit. 
     The flare gas recovery system  430  design takes into consideration the integrated operation with the flare  470 , which includes flashback prevention  432 . Flashback prevention involves preventing reverse flow of gas and potentially, the flame from the flare, as flare gas  435  is being burned at the flare  470 . Flashback prevention can comprise a liquid seal drum, a molecular seal, a fluidic seal, a flame arrestor, or any combination thereof. The source of flare gas to the ejector  434  (or analogous  234 ,  334 ) is upstream of the flashback prevention  432 . 
     Referring to the exemplary system  400  in  FIG. 4 , in one example of the systems described in this document, the amine contactor  412  of the primary gas sweetening unit  410  can operate at approximately 980 psig. The lean amine solvent from the amine cooler  422  can enter the contactor  412  at approximately 140° F., and the sweet gas (stream  429 ) exits the contactor  412  at approximately 120° F. The flash drum  414  can operate at approximately 80 psig and receives liquid from the bottom of the contactor  412  and the bottom of the separator  436  from the flare gas recovery system  430 . The flashed vapor can travel up the flash drum tower  416  and be sent to boilers, where the gas is burned to provide heat for another process. The liquid from the flash drum  414  can be sent to the amine stripper  418  with a bottom operating pressure of approximately 15 psig and a bottom operating temperature of approximately 265° F. The circulation pump  420  can normally circulate at approximately 9500 gpm with a discharge pressure of approximately 990 psig. Approximately 7000 gpm of the amine solvent can be circulated back to the contactor  412  through cooler  422 , 1200 gpm can be sent to the ejector  434  of the flare gas recovery system  430 , and the balance can be recycled back to the suction of circulation pump  420 . 
     In some embodiments, a portion of the flare gas from the flare header  431  can be sent to the ejector  434 , upstream of the seal drum  432 , which is utilized for flashback prevention and liquid knockout. The rich amine solvent and flare gas can be mixed within ejector  434  and discharged at approximately 210 psig. The vapor-liquid mixture  437  can be sent to separator  436 , where the liquid  441  at the bottom is sent back to the primary gas sweetening unit  410 , and the vapor  443  at the top can be sent as additional feed to a secondary gas sweetening unit, which can operate at approximately 180 psig. 
     The approximate flow rates and compositions of the streams can be: 
     
       
         
           
               
               
            
               
                   
                   
               
               
                   
                 Stream Number* 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 units 
                 411 
                 413 
                 419 
                 421 
                 423 
                 425 
                 429 
                 431 
                 437 
                 443 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Vapor flow 
                 MMSCFD 
                 600 
                 — 
                 0.9 
                 — 
                 63   
                 — 
                 535    
                 2   
                 2 
                 2 
               
               
                 Liquid flow 
                 gpm 
                 — 
                 7000 
                 — 
                 8200 
                 — 
                 1200 
                 — 
                 — 
                 1200 
                 — 
               
               
                 Vapor composition 
                 mol % 
                   
                   
                   
                   
                 ** 
                   
                 ** 
               
               
                 methane 
                   
                 70.0 
                 — 
                 91.0  
                 — 
                 — 
                 — 
                 78.5  
                 85.0  
                 90.5 
                 — 
               
               
                 ethane 
                   
                 6.0 
                 — 
                 5.0 
                 — 
                 — 
                 — 
                 6.7 
                 2.0 
                 2.1 
                 — 
               
               
                 propane 
                   
                 2.5 
                 — 
                 — 
                 — 
                 — 
                 — 
                 2.8 
                 — 
                 — 
                 — 
               
               
                 i-butane 
                   
                 0.5 
                 — 
                 — 
                 — 
                 — 
                 — 
                 0.6 
                 — 
                 — 
                 — 
               
               
                 n-butane 
                   
                 0.4 
                 — 
                 — 
                 — 
                 — 
                 — 
                 0.4 
                 — 
                 — 
                 — 
               
               
                 i-pentane 
                   
                 0.4 
                 — 
                 — 
                 — 
                 — 
                 — 
                 0.4 
                 — 
                 — 
                 — 
               
               
                 n-pentane 
                   
                 0.3 
                 — 
                 — 
                 — 
                 — 
                 — 
                 0.3 
                 — 
                 — 
                 — 
               
               
                 carbon dioxide 
                   
                 5.0 
                 — 
                 — 
                 — 
                 48.0 
                 — 
                 — 
                 2.5 
                 — 
                 — 
               
               
                 hydrogen sulfide 
                   
                 5.5 
                 — 
                 — 
                 — 
                 52.0 
                 — 
                 — 
                 3.0 
                 — 
                 — 
               
               
                 nitrogen 
                   
                 9.4 
                 — 
                 4.0 
                 — 
                 — 
                 — 
                 10.5  
                 7.0 
                 7.4 
                 — 
               
               
                 water 
                   
                 0.1 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 1.0 
                 — 
                 — 
               
               
                   
               
               
                 *Refer to FIG. 4. 
               
               
                 ** Composition is in dry basis. 
               
            
           
         
       
     
       FIG. 5  depicts a general schematic of a fifth exemplary system  500  that includes a primary gas sweetening unit  510  and flare gas recovery system  530 . The primary gas sweetening unit  510  of  FIG. 5  comprises an amine contactor  512 , a flash drum  514 , an amine stripper  518 , an amine circulation pump  520 , an amine cooler  522 , and an amine ejector  534 . The amine contactor  512  is a counter-flow gas-liquid contactor that can be referred as an absorber, treater, or scrubber. The amine contactor  512  is a vessel comprising internal components, which can include trays or packing, to increase gas-liquid contact. 
     The flash drum  514  operates at a lower pressure than the contactor  512  and allows light hydrocarbons to flash (that is, evaporate) from the amine solvent. The flash drum  514  is sized for liquid surge, liquid holdup, and residence time for vapor to separate from the liquid amine solvent. In some embodiments, the flash drum  514  is equipped with a tower  516 . The flash drum tower  516  can remove acid gas such as hydrogen sulfide, which can be present in the vapor separated from the amine solvent, before the vapor is sent to another downstream process or end user. 
     Still referring to  FIG. 5 , the amine stripper  518  is a vessel, which can also be referred to as a regenerator. The amine stripper  518  comprises internal components, for example, trays or packing, and effectively serves as a distillation tower to boil off acid gas to regenerate the amine solvent. The distinction between acid gas and sour gas is that sour gas is mostly hydrocarbons with some acidic gas content, and acid gas contains little to no hydrocarbons. 
     The circulation pump  520  pressurizes the regenerated amine solvent to recycle the amine solvent back to the contactor  512 . The circulation pump  520  can comprise a single pump or multiple pumps in parallel or in series. The circulation pump  520  can be sized to accommodate upset scenarios which require much higher flow rates than is normally required by the primary gas sweetening unit  510 . The circulation pump  520 , in some aspects, can employ a recycle line which routes a portion of the amine solvent back to the suction of the pump  520 . 
     The amine cooler  522  brings the temperature of the solvent down before the solvent is recycled back to the contactor  512 . The lower solvent temperature increases the efficiency of cleaning the sour gas that enters the contactor  512 . The cooler  522  can be a shell-and-tube heat exchanger, an air cooler, or a combination of multiples of both. 
     Gas sweetening units can optionally comprise auxiliary and variant equipment such as additional heat exchangers and vessels that have not been described above, but a majority of gas sweetening units across the world implement some variation or combination of the major equipment outlined. 
     Gas sweetening units can operate at a variety of operating temperatures and pressures. In some embodiments, sour gas at a temperature of between 70-130° F. via stream  511  enters the bottom of an amine contactor  512 , as amine solvent at a temperature of between 80-140° F. via stream  513  enters from the top. The amine solvent that enters the amine contactor  512  is at least approximately 10° F. hotter than the sour gas that enters the amine contactor  512 . As the amine solvent contacts the sour gas, the solvent removes (or “cleans”) the sulfur compounds, carbon dioxide, and other contaminants from the sour gas, by chemical and physical binding. Once the solvent has passed through contactor  512 , the solvent is considered to be in a “rich” state—also referred as “rich solvent”-because the solvent contains the hydrogen sulfide removed from the sour gas. The sweetened gas exits from the top of contactor  512  via stream  529 , and rich solvent exits from the bottom via stream  515 . The sweetened gas (stream  529 ) can contain approximately 5-60 ppm hydrogen sulfide and is sent downstream for sale or further processing. Rich solvent  515  is sent to a flash drum  514  operating between atmospheric pressure to 90 psig, where any flashed vapor travels up a flash drum tower  516  and exits via stream  519 , where the flashed vapor can then be utilized as fuel, vented, flared, or a combination of these. 
     Rich solvent liquid  517  from flash drum  514  is sent to an amine stripper  518  with a top operating pressure between 5-17 psig. The hydrogen sulfide and carbon dioxide is boiled off via heat input to the bottom of stripper  518  operating between 230-270° F. in order to regenerate the amine solvent. The regenerated solvent is then considered to be in a “lean” state—also referred as “lean solvent”—that is once again suitable to be used for cleaning additional sour gas. Sour gas  523 , comprising hydrogen sulfide and carbon dioxide exits the top of stripper  518 , and lean solvent  521  is pumped out of the bottom of stripper  518  by circulation pump  520 . Lean solvent  527  is cooled in heat exchanger  512  to approximately 80-140° F. before re-entering contactor  512  to be used again to clean additional sour gas. The transport of vapor and liquid within, to, and from the gas sweetening unit  510  can be achieved using various piping, pump, and valve configurations. 
     Still referring to  FIG. 5 , the exemplary system  500  includes the flare gas recovery system  530  that is integrated with the gas sweetening unit  510 , as described above. The system  500  utilizes liquid amine solvent  525  from the gas sweetening unit  510  as motive fluid for the amine ejector  534  in the flare gas recovery system  530 . The amine ejector  534  is positioned, in this example, within the gas sweetening unit  510 . 
     As illustrated in  FIG. 5 , a flare gas ejector  540  is also included in the flare gas system  530  and comprises an inlet that continuously receives a lean or sour gas stream  542  (“gas stream  542 ”), which serves as a high-pressure motive fluid. The gas ejector  540  also comprises a gas inlet configured for receiving a flare gas  533  as a low-pressure suction fluid. The motive fluid operates at a higher pressure than the suction fluid. For example, the gas stream  542  (motive fluid) operates at approximately 990 psig, and the flare gas (suction fluid) operates at approximately 0.5 psig. The motive and suction fluid mix within the flare gas ejector  534 , and then discharge at an intermediate pressure to the suction of the amine ejector  534  as flare gas stream  541 . 
       FIG. 5  shows a certain implementation in which the discharge of the flare gas ejector  540  is supplied to the suction of the amine ejector  534 . The amine ejector  534  comprises an inlet that continuously receives liquid amine solvent  525  from the gas sweetening unit  510 , which serves as a high-pressure motive fluid. The motive fluid operates at a higher pressure than the suction fluid (flare gas stream  541 ). For example, the lean amine liquid solvent  525  (motive fluid) operates at approximately 990 psig, and the flare gas stream  541  (suction fluid) operates at approximately 5-10 psig. The motive and suction fluid mix within the amine ejector  534 , and then discharge at an intermediate pressure to the flash drum  514 . 
     The type of system shown in  FIG. 5  can also be applicable when there exists at least two gas sweetening units operating at different pressures. In some implementations, a booster pump is included to provide adequate pressure to the amine solvent from the primary gas sweetening unit  510  which is being utilized as motive fluid for the amine ejector  534 , so that the recovered flare gas can be sent to a secondary gas sweetening unit. 
     System  500  can include a secondary gas sweetening unit (not shown, but similar to unit  150  in  FIG. 1 , which includes a secondary amine contactor  152 ), which operates at a lower pressure than the primary gas sweetening unit  510 . The amine ejector  534  can be installed near the flare header  531  and utilize lean amine solvent from a nearby gas sweetening unit, such as the primary gas sweetening unit  510 , as motive fluid to mix with and pressurize flare gas. 
     The flare gas recovery system  530  design takes into consideration the integrated operation with the flare  570 , which includes flashback prevention  532 . Flashback prevention involves preventing reverse flow of gas and potentially, the flame from the flare, as flare gas  535  is being burned at the flare  570 . Flashback prevention can comprise a liquid seal drum, a molecular seal, a fluidic seal, a flame arrestor, or any combination thereof. The source of flare gas to the ejector  534  (or analogous  234 ,  334 ) is upstream of the flashback prevention  532 . 
       FIG. 5  provides a general schematic of a fifth exemplary system  500  that includes a gas sweetening unit  510  and flare gas recovery system  530 . As depicted in  FIG. 5 , the sweetening unit  510  is substantially the same as the sweetening unit  110  of  FIG. 1 , but the configuration of the flare gas recovery system  530  differs from the flare gas recovery system  130  of  FIG. 1 . Like element numbers across the figures can be substantially the same; for example, the amine contactor  512  can be substantially the same as the amine contactor  112 . The amine ejector  534  can be installed near the flare header  531  and utilize amine solvent  525  from a nearby gas sweetening unit, such as the primary gas sweetening unit  510 , as motive fluid to mix with and pressurize flare gas  533 . The two-phase mixture from the ejector  534  can be recycled back to the primary gas sweetening unit  510  and discharged directly back to a flash drum  514  via stream  537 . The vapor phase can be separated from liquid phase in the flash drum  514 , because the both the liquid and gas from the ejector  534  are recycled back to the gas sweetening unit  510 . Because flare gas is being recovered and recycled to the gas sweetening unit  510 , downstream units can require modification to accommodate the increased vapor flow—for example, flash drum tower  516 . The recovered flare gas can then be utilized as fuel, vented, flared, or a combination of these. 
     In some implementations, an additional circulation pump is included to provide adequate flow of amine solvent from the gas sweetening unit  510 , which is being utilized as motive fluid for the ejector  534 . 
     Referring to the exemplary system  500  in  FIG. 5 , in one example of the systems described in this document, the amine contactor  512  of the gas sweetening unit  510  can operate at approximately 980 psig. The lean amine solvent from the amine cooler  522  can enter the contactor  512  at approximately 140° F., and the sweet gas (stream  529 ) can exit the contactor  512  at approximately 120° F. The flash drum  514  can operate at approximately 80 psig and receive liquid from the bottom of the contactor  512  and a vapor-liquid mixture from the ejector  534  from the flare gas recovery system  530 . The flashed vapor can travel up the flash drum tower  516  and be sent to boilers, where the gas is burned to provide heat for another process. The liquid from the flash drum  514  can be sent to the amine stripper  518  with a bottom operating pressure of approximately 15 psig and a bottom operating temperature of approximately 265° F. The circulation pump  520  can normally circulate at approximately 9500 gpm with a discharge pressure of approximately 990 psig. Approximately 7000 gpm of the amine solvent can be circulated back to the contactor  512  through cooler  522  such that 1200 gpm is sent to the ejector  534  of the flare gas recovery system  530  and the balance is recycled back to the suction of circulation pump  520 . 
     In some embodiments, a portion of the flare gas from the flare header  531  can be sent to the flare gas ejector  540 , upstream of the seal drum  532  for flashback prevention and liquid knockout. The lean amine solvent and flare gas discharge  541  can be mixed within amine ejector  534  and discharged as a vapor-liquid mixture back to the flash drum  514  of the gas sweetening unit  510 . 
     The approximate flow rates and compositions of the streams can be: 
     
       
         
           
               
               
            
               
                   
                   
               
               
                   
                 Stream Number* 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 units 
                 511 
                 513 
                 519 
                 521 
                 523 
                 525 
                 529 
                 531 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Vapor flow 
                 MMSCFD 
                 600 
                 — 
                  2.79 
                 — 
                 63   
                 — 
                 535    
                 2   
               
               
                 Liquid flow 
                 gpm 
                 — 
                 7000 
                 — 
                 8200 
                 — 
                 1200 
                 — 
                 — 
               
               
                 Vapor composition 
                 mol % 
                   
                   
                   
                   
                 ** 
                   
                 ** 
               
               
                 methane 
                   
                 70.0 
                 — 
                 90.6  
                 — 
                 — 
                 — 
                 78.4  
                 85.5  
               
               
                 ethane 
                   
                 6.0 
                 — 
                 3.1 
                 — 
                 — 
                 — 
                 6.7 
                 2.0 
               
               
                 propane 
                   
                 2.5 
                 — 
                 0.0 
                 — 
                 — 
                 — 
                 2.8 
                 — 
               
               
                 i-butane 
                   
                 0.5 
                 — 
                 — 
                 — 
                 — 
                 — 
                 0.6 
                 — 
               
               
                 n-butane 
                   
                 0.4 
                 — 
                 — 
                 — 
                 — 
                 — 
                 0.4 
                 — 
               
               
                 i-pentane 
                   
                 0.4 
                 — 
                 — 
                 — 
                 — 
                 — 
                 0.4 
                 — 
               
               
                 n-pentane 
                   
                 0.3 
                 — 
                 — 
                 — 
                 — 
                 — 
                 0.3 
                 — 
               
               
                 carbon dioxide 
                   
                 5.0 
                 — 
                 — 
                 — 
                 48.0 
                 — 
                 — 
                 2.5 
               
               
                 hydrogen sulfide 
                   
                 5.5 
                 — 
                 — 
                 — 
                 52.0 
                 — 
                 — 
                 3.0 
               
               
                 nitrogen 
                   
                 9.4 
                 — 
                 6.3 
                 — 
                 — 
                 — 
                 10.5  
                 7.0 
               
               
                 water 
                   
                 0.1 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                   
               
               
                 *Refer to FIG. 5. 
               
               
                 ** Composition is in dry basis. 
               
            
           
         
       
     
     In some implementations, a filtration package can be included to remove impurities like salts or corroded materials that accumulate in the solvent used for the gas sweetening process. The filtration package can comprise a filter housing, a filter element or cartridge, an additional circulation pump, or a combination of multiples of these. Impurities collect on the filter element or cartridge as a fluid passes through the filter. The filter element or cartridge can be cleaned or replaced periodically. 
       FIG. 6  depicts a general schematic of a sixth exemplary system  600  that includes a primary gas sweetening unit  610  and flare gas recovery system  630 . The primary gas sweetening unit  610  of  FIG. 6  comprises an amine contactor  612 , a flash drum  614 , an amine stripper  618 , an amine circulation pump  620 , an amine cooler  622 , and an amine injector  634 . The amine contactor  612  is a counter-flow gas-liquid contactor that can be referred as an absorber, treater, or scrubber. The amine contactor  612  is a vessel comprising internal components, which can include trays or packing, to increase gas-liquid contact. 
     The flash drum  614  operates at a lower pressure than the contactor  612  and allows light hydrocarbons to flash (that is, evaporate) from the amine solvent. The flash drum  614  is sized for liquid surge, liquid holdup, and residence time for vapor to separate from the liquid amine solvent. In some embodiments, the flash drum  614  is equipped with a tower  616 . The flash drum tower  616  can remove acid gas such as hydrogen sulfide, which can be present in the vapor separated from the amine solvent, before the vapor is sent to another downstream process or end user. 
     Still referring to  FIG. 6 , the amine stripper  618  is a vessel, which can also be referred to as a regenerator. The amine stripper  618  comprises internal components, for example, trays or packing, and effectively serves as a distillation tower to boil off acid gas to regenerate the amine solvent. The distinction between acid gas and sour gas is that sour gas is mostly hydrocarbons with some acidic gas content, and acid gas contains little to no hydrocarbons. 
     The circulation pump  620  pressurizes the regenerated amine solvent to recycle the amine solvent back to the contactor  612 . The circulation pump  620  can comprise a single pump or multiple pumps in parallel or in series. The circulation pump  620  can be sized to accommodate upset scenarios which require much higher flow rates than is normally required by the primary gas sweetening unit  610 . The circulation pump  620 , in some aspects, can employ a recycle line which routes a portion of the amine solvent back to the suction of the pump  620 . 
     The amine cooler  622  brings the temperature of the solvent down before the solvent is recycled back to the contactor  612 . The lower solvent temperature increases the efficiency of cleaning the sour gas that enters the contactor  612 . The cooler  622  can be a shell-and-tube heat exchanger, an air cooler, or a combination of multiples of both. 
     Gas sweetening units can optionally comprise auxiliary and variant equipment such as additional heat exchangers and vessels that have not been described above, but a majority of gas sweetening units across the world implement some variation or combination of the major equipment outlined. 
     Gas sweetening units can operate at a variety of operating temperatures and pressures. In some embodiments, sour gas at a temperature of between 70-130° F. via stream  611  enters the bottom of an amine contactor  612 , as amine solvent at a temperature of between 80-140° F. via stream  613  enters from the top. The amine solvent that enters the amine contactor  612  is at least approximately 10° F. hotter than the sour gas that enters the amine contactor  612 . As the amine solvent contacts the sour gas, the solvent removes (or “cleans”) the sulfur compounds, carbon dioxide, and other contaminants from the sour gas, by chemical and physical binding. Once the solvent has passed through contactor  612 , the solvent is considered to be in a “rich” state—also referred as “rich solvent”-because the solvent contains the hydrogen sulfide removed from the sour gas. The sweetened gas exits from the top of contactor  612  via stream  629 , and rich solvent exits from the bottom via stream  615 . The sweetened gas (stream  629 ) can contain approximately 5-60 ppm hydrogen sulfide and is sent downstream for sale or further processing. Rich solvent  615  is sent to a flash drum  614  operating between atmospheric pressure to 90 psig, where any flashed vapor travels up a flash drum tower  616  and exits via stream  619 , where the flashed vapor can then be utilized as fuel, vented, flared, or a combination of these. 
     As shown, in this example implementation, rich solvent  615  can be sent to the amine ejector  634  and used as a motive fluid (discussed later) for the ejector  634  prior to (or in place of) being sent to the flash drum  614  operating between atmospheric pressure to 90 psig, where any flashed vapor travels up a flash drum tower  616  and exits via stream  619 , where the flashed vapor can then be utilized as fuel, vented, flared, or a combination of these. 
     Rich solvent liquid  617  from flash drum  614  is sent to an amine stripper  618  with a top operating pressure between 5-17 psig. The hydrogen sulfide and carbon dioxide is boiled off via heat input to the bottom of stripper  618  operating between 230-270° F. in order to regenerate the amine solvent. The regenerated solvent is then considered to be in a “lean” state—also referred as “lean solvent”—that is once again suitable to be used for cleaning additional sour gas. Sour gas  623 , comprising hydrogen sulfide and carbon dioxide exits the top of stripper  618 , and lean solvent  621  is pumped out of the bottom of stripper  618  by circulation pump  620 . Lean solvent  627  is cooled in heat exchanger  612  to approximately 80-140° F. before re-entering contactor  612  to be used again to clean additional sour gas. The transport of vapor and liquid within, to, and from the gas sweetening unit  610  can be achieved using various piping, pump, and valve configurations. 
     Still referring to  FIG. 6 , the exemplary system  600  includes the flare gas recovery system  630  that is integrated with the gas sweetening unit  610 , as described above. The system  600  utilizes liquid rich amine solvent  615  from the gas sweetening unit  610  as motive fluid for the amine ejector  634 . The amine ejector  634  is positioned, in this example, within the gas sweetening unit  610 . The amine ejector  634  comprises an inlet that continuously receives the rich amine solvent  615 , which serves as a high-pressure motive fluid from the gas sweetening unit  610  via stream  615 . The ejector  634  also comprises a gas inlet configured for receiving a stream  641  from the flare gas system  610  (as described later). 
     As illustrated in  FIG. 6 , a flare gas ejector  640  is also included in the flare gas system  630  and comprises an inlet that continuously receives a lean or sour gas stream  642  (“gas stream  642 ”), which serves as a high-pressure motive fluid. The gas ejector  640  also comprises a gas inlet configured for receiving a flare gas  633  as a low-pressure suction fluid. The motive fluid operates at a higher pressure than the suction fluid. For example, the gas stream  642  (motive fluid) operates at approximately 990 psig, and the flare gas (suction fluid) operates at approximately 0.5 psig. The motive and suction fluid mix within the flare gas ejector  634 , and then discharge at an intermediate pressure to the suction of the amine ejector  634  as flare gas stream  641 . 
       FIG. 6  shows a certain implementation in which the discharge of the flare gas ejector  640  is supplied to the suction of the amine ejector  634 . The amine ejector  634  comprises an inlet that continuously receives liquid rich amine solvent  615  from the gas sweetening unit  610 , which serves as a high-pressure motive fluid. The motive fluid operates at a higher pressure than the suction fluid (flare gas stream  641 ). For example, the rich amine liquid solvent  615  (motive fluid) operates at approximately 990 psig, and the flare gas stream  641  (suction fluid) operates at approximately 5-10 psig. The motive and suction fluid mix within the amine ejector  634 , and then discharge at an intermediate pressure to the flash drum  614 . 
     The type of system shown in  FIG. 6  can also be applicable when there exists at least two gas sweetening units operating at different pressures. In some implementations, a booster pump is included to provide adequate pressure to the amine solvent from the primary gas sweetening unit  610  which is being utilized as motive fluid for the amine ejector  634 , so that the recovered flare gas can be sent to a secondary gas sweetening unit. 
     System  600  can include a secondary gas sweetening unit (not shown, but similar to unit  150  in  FIG. 1 , which includes a secondary amine contactor  152 ), which operates at a lower pressure than the primary gas sweetening unit  610 . The amine ejector  634  can be installed near the flare header  631  and utilize rich amine solvent from a nearby gas sweetening unit, such as the primary gas sweetening unit  610 , as motive fluid to mix with and pressurize flare gas. 
     The flare gas recovery system  630  design takes into consideration the integrated operation with the flare  670 , which includes flashback prevention  632 . Flashback prevention involves preventing reverse flow of gas and potentially, the flame from the flare, as flare gas  635  is being burned at the flare  670 . Flashback prevention can comprise a liquid seal drum, a molecular seal, a fluidic seal, a flame arrestor, or any combination thereof. The source of flare gas to the ejector  634  (or analogous  234 ,  334 ) is upstream of the flashback prevention  632 . 
       FIG. 6  provides a general schematic of a sixth exemplary system  600  that includes a gas sweetening unit  610  and flare gas recovery system  630 . As depicted in  FIG. 6 , the sweetening unit  610  is substantially the same as the sweetening unit  110  of  FIG. 1 , but the configuration of the flare gas recovery system  630  differs from the flare gas recovery system  130  of  FIG. 1 . Like element numbers across the figures can be substantially the same; for example, the amine contactor  612  can be substantially the same as the amine contactor  112 . The amine ejector  634  can be installed near the flare header  631  and utilize rich amine solvent  615  from a nearby gas sweetening unit, such as the primary gas sweetening unit  610 , as motive fluid to mix with and pressurize flare gas  641 . The two-phase mixture from the ejector  634  can be recycled back to the primary gas sweetening unit  610  and discharged directly back to a flash drum  614  via stream  637 . The vapor phase can be separated from liquid phase in the flash drum  614 , because the both the liquid and gas from the ejector  634  are recycled back to the gas sweetening unit  610 . Because flare gas is being recovered and recycled to the gas sweetening unit  610 , downstream units can require modification to accommodate the increased vapor flow—for example, flash drum tower  616 . The recovered flare gas can then be utilized as fuel, vented, flared, or a combination of these. 
     As shown in this example, a pressure reducing device  643 , such as a valve or orifice, is positioned in the conduit for stream  615  between the take-off to, and the return from, the liquid-driven amine ejector  634 . In some aspects, the pressure reducing device  643  may equalize (or help equalize) the pressure of the rich solvent  615  and the two-phase mixture  637  from the ejector  634  returning to the flash drum  614 . 
     In some implementations, an additional circulation pump is included to provide adequate flow of amine solvent from the gas sweetening unit  610 , which is being utilized as motive fluid for the ejector  634 . 
     Referring to the exemplary system  600  in  FIG. 6 , in one example of the systems described in this document, the amine contactor  612  of the gas sweetening unit  610  can operate at approximately 980 psig. The lean amine solvent from the amine cooler  622  can enter the contactor  612  at approximately 140° F., and the sweet gas (stream  629 ) can exit the contactor  612  at approximately 120° F. The flash drum  614  can operate at approximately 80 psig and receive liquid from the bottom of the contactor  612  and a vapor-liquid mixture from the ejector  634  from the flare gas recovery system  630 . The flashed vapor can travel up the flash drum tower  616  and be sent to boilers, where the gas is burned to provide heat for another process. The liquid from the flash drum  614  can be sent to the amine stripper  618  with a bottom operating pressure of approximately 15 psig and a bottom operating temperature of approximately 265° F. The circulation pump  620  can normally circulate at approximately 9500 gpm with a discharge pressure of approximately 990 psig. Approximately 7000 gpm of the amine solvent can be circulated back to the contactor  612  through cooler  622  such that 1200 gpm is sent to the ejector  634  of the flare gas recovery system  630  and the balance is recycled back to the suction of circulation pump  620 . 
     In some embodiments, a portion of the flare gas from the flare header  631  can be sent to the flare gas ejector  640 , upstream of the seal drum  632  for flashback prevention and liquid knockout. The rich amine solvent  615  and flare gas discharge  641  can be mixed within amine ejector  634  and discharged as a vapor-liquid mixture back to the flash drum  614  of the gas sweetening unit  610 . 
     The approximate flow rates and compositions of the streams can be: 
     
       
         
           
               
               
            
               
                   
                   
               
               
                   
                 Stream Number* 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 units 
                 611 
                 613 
                 619 
                 621 
                 623 
                 627 
                 629 
                 631 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Vapor flow 
                 MMSCFD 
                 600 
                 — 
                  2.79 
                 — 
                 63 
                 — 
                 535    
                 2   
               
               
                 Liquid flow 
                 gpm 
                 — 
                 7000 
                 — 
                 8200 
                 — 
                 1200 
                 — 
                 — 
               
               
                 Vapor composition 
                 mol % 
                   
                   
                   
                   
                 ** 
                   
                 ** 
               
               
                 methane 
                   
                 70.0 
                 — 
                 90.6  
                 — 
                 — 
                 — 
                 78.4  
                 85.5  
               
               
                 ethane 
                   
                 6.0 
                 — 
                 3.1 
                 — 
                 — 
                 — 
                 6.7 
                 2.0 
               
               
                 propane 
                   
                 2.5 
                 — 
                 0.0 
                 — 
                 — 
                 — 
                 2.8 
                 — 
               
               
                 i-butane 
                   
                 0.5 
                 — 
                 — 
                 — 
                 — 
                 — 
                 0.6 
                 — 
               
               
                 n-butane 
                   
                 0.4 
                 — 
                 — 
                 — 
                 — 
                 — 
                 0.4 
                 — 
               
               
                 i-pentane 
                   
                 0.4 
                 — 
                 — 
                 — 
                 — 
                 — 
                 0.4 
                 — 
               
               
                 n-pentane 
                   
                 0.3 
                 — 
                 — 
                 — 
                 — 
                 — 
                 0.3 
                 — 
               
               
                 carbon dioxide 
                   
                 5.0 
                 — 
                 — 
                 — 
                 48.0 
                 — 
                 — 
                 2.5 
               
               
                 hydrogen sulfide 
                   
                 5.5 
                 — 
                 — 
                 — 
                 52.0 
                 — 
                 — 
                 3.0 
               
               
                 nitrogen 
                   
                 9.4 
                 — 
                 6.3 
                 — 
                 — 
                 — 
                 10.5  
                 7.0 
               
               
                 water 
                   
                 0.1 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                   
               
               
                 *Refer to FIG. 6. 
               
               
                 ** Composition is in dry basis. 
               
            
           
         
       
     
     In some implementations, a filtration package can be included to remove impurities like salts or corroded materials that accumulate in the solvent used for the gas sweetening process. The filtration package can comprise a filter housing, a filter element or cartridge, an additional circulation pump, or a combination of multiples of these. Impurities collect on the filter element or cartridge as a fluid passes through the filter. The filter element or cartridge can be cleaned or replaced periodically. 
     Various modifications, alterations, and permutations of the disclosed implementations can be made and will be readily apparent to those or ordinary skill in the art, and the general principles defined can be applied to other implementations and applications, without departing from scope of the disclosure. In some instances, details unnecessary to obtain an understanding of the described subject matter can be omitted so as to not obscure one or more described implementations with unnecessary detail and inasmuch as such details are within the skill of one of ordinary skill in the art. The present disclosure is not intended to be limited to the described or illustrated implementations, but to be accorded the widest scope consistent with the described principles and features. 
     Certain implementations of the subject matter have been described in this document. Other implementations are, however, within the scope of the following claims.