Abstract:
A method and system for removing gaseous contaminants from a gas stream by contacting the gas stream with a wash solution and regenerating the wash solution in a regeneration system for future use in removing gaseous contaminants from the gas stream.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application Ser. No. 61/430,280, filed Jan. 6, 2011, the disclosure of which, to the extent not inconsistent herewith, is incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The disclosed subject matter relates to methods and systems for removal of gaseous contaminants from gas streams. 
     2. Description of Related Art 
     In conventional industrial technologies for gas purification, impurities, such as H 2 S, CO 2  and/or COS are removed from a gas stream such as flue gas, natural gas, syngas or other gas streams by absorption in a liquid wash solution, e.g., in a liquid solution comprising an amine compound. 
     Used wash solution is subsequently regenerated in a regenerator column (also referred to as a “regenerator”) to release the impurities present in the solution, typically by countercurrent contact with steam. The steam needed for regeneration is typically produced by boiling the regenerated wash solution in a reboiler that is, located near the bottom portion of the regenerator column. In addition, the reboiling of the regenerated wash solution may provide further release of impurities present in the wash solution. 
     In conventional absorption-regeneration processes as described above, regenerated and reboiled wash solution is typically re-used in another absorption cycle. However, the reboiled solution may have a temperature as high as 100-150° C. To enable efficient absorption, wash solutions based on amine compounds are typically cooled before being passed to another round of absorption. Cooling has conventionally been accomplished by heat-exchange with used wash solution from the absorption process. 
     The energy produced by the reboiler is not only used for regeneration, but also at other locations in an absorption-regeneration process. In general, the energy requirements of a conventional gas purification process are of three types: binding energy, stripping energy and sensible heat. Binding energy is required for breaking the chemical bond formed between the impurities and the wash solution, whereas stripping energy is required for production of the steam needed for releasing the impurities from the wash solution. Sensible heat is in turn needed for heating of the wash solution prior to regeneration. In conventional systems and processes, part of the produced energy may be lost for example, in the system coolers, which reduce the temperature at specified locations in the system, e.g., the cooler located near the absorber inlet for cooling return wash solution before feeding it to the absorber. Moreover, energy may be lost in condensers located at the top of the absorber, regenerator etc., and in the form of water vapor exiting the process, mostly at the top of the regenerator where water vapor is present in the purified CO 2  gas. 
     Thus, contaminant removal from gas streams, and in particular the regeneration of wash solutions, is an energy intensive process. Reduction of energy requirements at different parts of a gas purification process could potentially reduce the total energy required by the system. 
     SUMMARY 
     According to aspects illustrated herein, there is provided a system for regenerating a wash solution utilized to remove gaseous contaminants from a gas stream, the system comprising: a first heat exchanger for heat transfer between a hot regenerated wash solution and a used wash solution to form a first heated used wash solution; a second heat exchanger for heat transfer between the hot regenerated wash solution and at least a portion of the used wash solution from the first heat exchanger to form a second heated used wash solution; and a regenerator arranged to receive the used wash solution, the first heated used wash solution and the second heated used wash solution, wherein the second heated used wash solution has a temperature greater than the first heated used wash solution and the first heated used wash solution has a temperature greater than the used wash solution. 
     According to another aspect illustrated herein, there is provided a process for regenerating a wash solution utilized in removing gaseous contaminants from a gas stream, the process comprising: providing a first portion of a used wash solution to a regenerator; providing a second portion of the used wash solution to a first heat exchanger to transfer heat between a hot regenerated wash solution and the second portion of the used wash solution to form a first heated used wash solution; providing a first portion of the first heated used wash solution to a regenerator; providing a second portion of the first heated used wash solution to a second heat exchanger for heat transfer between the hot regenerated wash solution and the first heated used wash solution to form a second heated used wash solution; and providing the second heated used wash solution to the regenerator, wherein the second heated used wash solution provided to the regenerator has a temperature greater than a temperature of the first heated used wash solution provided to the regenerator and the first heated used wash solution provided to the regenerator has a temperature greater than a temperature of the used wash solution provided to the regenerator. 
     According to other aspects illustrated herein, there is provided a method of reducing an amount of energy consumed by a regenerator, the method comprising: separating a used wash solution into a plurality of portions; providing a first portion of the used wash solution to a regenerator, the used wash solution having a first temperature (T 1 ); heating a second portion of the used wash solution to form a first heated used wash solution having a second temperature (T 2 ); heating a third portion of the used wash solution to form a second heated used wash solution having a third temperature (T 3 ); and providing the first and second heated used wash solutions to the regenerator, wherein a temperature distribution of T 1 &lt;T 2 &lt;T 3  is maintained, thereby reducing an amount of energy consumed by the regenerator. 
     The above described and other features are exemplified by the following figures and detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring now to the figures, which are exemplary embodiments, and wherein the like elements are numbered alike: 
         FIG. 1  is a schematic representation of a system for removing gaseous contaminants from a gas stream; and 
         FIG. 2  is a schematic of a regenerator; and 
         FIG. 3  is a graph presenting data related to a simulated test of the system described herein. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a system  100  for removing gaseous contaminants from a gas stream  120 . Gas stream  120  may be any stream of gas that includes gaseous contaminants, and includes, but is not limited to a flue gas stream from a combustion source, a natural gas stream, a syngas, and the like. The gaseous contaminants present in the gas stream  120  include, but are not limited to, acid gas impurities such as CO 2 , H 2 S, and the like. 
     Gas stream  120  is introduced to an absorber  130 , which is arranged to allow contact between the gas stream and a wash solution. In one embodiment, the absorber  130  is a packed bed column. The packed bed column may have multiple sections of the same, or different packing material. As shown in  FIG. 1 , the absorber  130  includes two (2) absorption sections, an upper absorption section  132  and a bottom absorption section  134 . The absorber  130  is not limited in this regard as more or less absorption sections may be present in the absorber. 
     Gas stream  120  containing gaseous contaminants enters the absorber  130  at an entry point  131  and travels up a length L of the absorber. As shown in  FIG. 1 , the entry point  131  is located at a bottom portion  133  of the absorber  130 . As the gas stream  120  travels up the length L of the absorber  130  it is contacted with a wash solution in the absorption sections  132 ,  134 . The wash solution typically travels down the length L of the absorber  130  such that it is in countercurrent contact with the gas stream  120 . 
     In one embodiment, the wash solution is an amine-based wash solution. Examples of amine-based wash solutions include, but are not limited to, amine compounds such as monoethanolamine (MEA), diethanolamine (DEA), methyldiethanolamine (MDEA), diisopropylamine (DIPA) and aminoethoxyethanol (diglycolamine). The most commonly used amine compound in industrial plants are the alkanolamines MEA, DEA, MDEA and some blends of conventional amines with promoters, inhibitors, and the like. The amine-based wash solution may also include a promoter to enhance the chemical reaction kinetics involved in the capture of CO2 by the ammoniated solution. For example, the promoter may include an amine (e.g. piperazine) or an enzyme (e.g., carbonic anhydrase or its analogs), which may be in the form of a solution or immobilized on a solid or semi-solid surface. However, it is understood that the systems and processes as disclosed herein may be applied to any solution involved in a process with an absorption/regeneration scheme. 
     At least a portion of the wash solution is introduced to the absorber  130  at a top portion  135  of the absorber via a line  136  and travels down the length L of the absorber where it contacts the gas stream  120  in the absorption sections  132 ,  134 . 
     In the absorber  130 , gaseous contaminants, such as carbon dioxide (CO 2 ), present in the gas stream  120  are absorbed by the wash solution, thereby forming a used wash solution  138  and a reduced contaminant gas stream  140 . The used wash solution  138  is rich in contaminants absorbed from the gas stream  120 . 
     As shown in  FIG. 1 , the reduced contaminant gas stream  140  is released from the top portion  135  of the absorber  130 . The reduced contaminant gas stream  140  may undergo further processing (not shown) prior to being sent to a stack for release to an environment. Further processing, of the reduced contaminant gas stream  140  may include, e.g., particulate removal. 
     At least a portion of the used wash solution  138  is withdrawn and leaves the absorber  130  at a first withdrawal level  139 .  FIG. 1  illustrates the first withdrawal level  139  in the bottom portion  133  of the absorber  130 , i.e., downstream of the bottom absorption section  134  with respect to the flow of the wash solution. However, it is contemplated that the first withdrawal level may be located at any position on the absorber  130 . 
     The used wash solution  138  that is withdrawn at the first withdrawal level  139  may be regenerated in a regenerator, where the contaminants are separated from the used wash solution in order to produce a regenerated wash solution for re-use in the absorber  130 . 
     Still referring to  FIG. 1 , after leaving the absorber  130  at the first withdrawal level  139 , the used wash solution  138  is separated into two portions, a first portion  138   a  and a second portion  138   b . In one embodiment, the used wash solution  138  is separated into two equal portions, e.g., 50% of the used wash solution  138  forms first portion  138   a , while 50% of the used wash solution  138  forms second portion  138   b . However, it is contemplated that in other embodiments the used wash solution  138  is separated into two unequal portions, e.g., 10% of the used wash solution  138  forms the first portion  138   a  while 90% of the used wash solution forms second portion  138   b . In a particular embodiment, 30-60% of the used wash solution  138  forms the first portion  138   a , with the remainder of the used wash solution forming the second portion  138   b.    
     The first portion of the used wash solution  138   a  is provided to a cooling unit  142  that is in fluid communication with the absorber  130 . In the cooling unit  142 , the temperature of used wash solution  138   a  is reduced and the used wash solution  138   a  returned to the absorber  130  as a cooled used wash solution  144 . The cooled used wash solution  144  is returned to the absorber  130  at a first reintroduction level  146 . The first reintroduction level  146  of the cooled used wash solution  144  is located upstream from the first withdrawal level  139 , with respect to the flow of the wash solution in the absorber  130 . 
     As shown in  FIG. 1 , the absorber  130  has a second withdrawal level  148  that is located at a position downstream of the upper absorption section  132  and upstream of the first reintroduction level  146  with respect to the flow of the wash solution in the absorber  130 . A used wash solution  150  that is partially saturated with contaminants absorbed from the gas stream  120  is withdrawn from the absorber  130  at the second withdrawal level  148  and is provided to the cooling unit  142 . The used wash solution  150  is combined with the used wash solution  138   a , cooled in the cooling unit  142 , and returned to the absorber as part of the cooled used wash solution  144 . 
     The temperature to which the used wash solutions  138   a ,  150  are cooled depends on several factors, including, but not limited to, the availability of cooling media, reaction kinetics of the wash solution and the contaminants present in the gas stream  120 , and the characteristics of the packing material used in the absorption sections  132 ,  134 . In one embodiment, the cooling unit  142  reduces the temperature of the used wash solutions  138   a ,  150  to a temperature of about 40 degrees Celsius (40° C.). 
     The cooled used wash solution  144  is introduced to the absorber  130  via the first introduction level  146  and flows downstream and passes through the bottom absorption section  134  where it contacts the gas stream  120 . As the cooled used wash solution  144  contacts the gas stream  120 , contaminants are absorbed from the gas stream before the wash solution is withdrawn at the first withdrawal level  139  as used wash solution  138 . This process may be repeated. 
     The second portion of the used wash solution  138   b  that is withdrawn from the absorber  130  is provided to a regeneration system  160 . The regeneration system  160  includes a regenerator  162  that is arranged to receive the used wash solution for the regeneration thereof. The used wash solution is provided to the regenerator  162  in at least three portions: used wash solution  141 , a first heated used wash solution  164   b  and a second heated wash solution  166 . The regenerator  162  may be, for example, a column, such as a packed bed column or a column containing trays. If a packed bed column, the regenerator  162  may include multiple sections having the same or different packing material. 
     The regenerator  162  removes, or absorbs, the gaseous contaminants from the used wash solution (“regenerates”) to form a regenerated wash solution  168  and a contaminant stream  170 . It is contemplated that the used wash solution  141 ,  164   b ,  166 , is regenerated by stripping the gaseous contaminants by breaking the chemical bond between the contaminants and the wash solution. 
     The contaminant stream  170  may be subjected to further processing, such as condensation, or may be provided directly to a storage facility. In one embodiment, the used wash solution  141 ,  164   b ,  166  contains CO 2 , which is removed from the wash solution in the regenerator  162  as a CO 2  gas stream  170 , which is subsequently condensed and stored for later use. 
     As shown in  FIG. 1 , the used wash solution  138   b  from the absorber  130 , is separated into two portions, with the first portion  141  provided directly to the regenerator  162  without heating or cooling the wash solution, while a second portion  143  is provided to a heat exchanger  172 . Examples of heat-exchangers include, but are not limited to, shell-and-tube heat exchangers, and plate and frame heat exchangers. 
     In one embodiment, the used wash solution  138   b  is separated into two equal portions, e.g., 50% of the used wash solution  138   b  forms the first portion  141  provided to the regenerator  162 , while 50% of the used wash solution  138   b  forms the second portion  143  provided to the heat exchanger  172 . However, it is contemplated that in other embodiments the used wash solution  138   b  is separated into two unequal portions, e.g. 10% of the used wash solution  138   b  forms the first portion  141  provided to the regenerator  162  while 90% of the used wash solution forms the second portion  143  provided to the heat exchanger  172 . In a particular embodiment, 1-10% of the used wash solution  138   b  forms the first portion  141  provided to the regenerator  162 , with the remainder of the used wash solution  138   b  forming the second portion  143  that is provided to the heat exchanger  172 . 
     The first portion  141  of the used wash solution provided to the regenerator  162  has a temperature (T 1 ) that is the same or less than the temperature of the used wash solution  138  at the time it was withdrawn from the absorber  130 . Typically, the temperature of the used wash solution  141  provided to the regenerator  162  is between 40 and 60 degrees Celsius. 
     The second portion  143  of the used wash solution provided to the heat exchanger  172  gains thermal energy and increases in temperature to form the first heated used wash solution  164 . In one embodiment, as shown in  FIG. 1 , the first heated used wash solution  164  is separated into two portions  164   a  and  164   b , with the first portion  164   a  provided to the regenerator  162  at an entry point downstream with respect to the flow of the used wash solution in the regenerator of where the used wash solution enters the regenerator  162 . The second portion of the used wash solution  164   b  is provided to a separator  174 . 
     In one embodiment, the first heated used wash solution  164  is separated into two equal portions, e.g., 50% of the first heated used wash solution  164  forms the first portion  164   a  provided to the regenerator  162 , while 50% of the first heated used wash solution  164  forms the second portion  164   b  provided to the separator  174 . However, it is contemplated that in other embodiments, the first heated used wash solution  164  is separated into two unequal portions, e.g., 10% of the first heated used wash solution  164  forms the first portion  164   a  provided to the regenerator  162  while 90% of the first heated used wash solution  164  the second portion  164   b  provided to the separator  174 . 
     In a particular embodiment, 30-60% of the first heated used wash solution  164  forms the first portion  164   a  provided to the regenerator  162 , with the remainder forming the second portion  164   b  that is provided to the separator  174 . 
     The first heated used wash solution  164   a  provided to the regenerator  162  has a temperature (T 2 ) that is greater than the temperature of the used wash solution  141  provided to the regenerator. Typically, the temperature (T 2 ) of the first heated used wash solution  164   a  provided to the regenerator  162  is between 80 and 100 degrees Celsius. 
     The separator  174  removes the gaseous components (vapors)  176  from the second portion of the used wash solution  164   b  and provides the used wash solution to a heat exchanger  178 . The gaseous components  176  are provided to the regenerator  162  at an entry point downstream with respect to where the first heated used wash solution  164   a  is provided to the regenerator. The used wash solution  164   b  provided to the heat exchanger  178  is heated therein to form a second heated used wash solution  166 . 
     In another embodiment, as shown in  FIG. 2 , the first heated used wash solution  164  is separated into two portions  164   a  and  164   b  after leaving the heat exchanger  172 . The first portion  164   a  is provided to the regenerator  162  at an entry point downstream of where the used wash solution  141  enters the regenerator and the second portion  164   b  is provided directly to the heat exchanger  178  where it is heated to form the second heated used wash solution  166 . The arrangement in  FIG. 2  does not include a separator  174  for removing gaseous components from the first heated used wash solution  164 . In this embodiment, for ease of operation and for efficient thermal energy utilization, the outlet of the heat exchanger  178  may be heated to the point of vaporization, e.g., very close to the bubble point of the solution. 
     In  FIG. 2 , the first heated used wash solution  164  may be separated into two equal portions, e.g., 50% of the first heated used wash solution  164  forms the first portion  164   a  provided to the regenerator  162 , while 50% of the first heated used wash solution  164  forms the second portion  164   b  provided to the heat exchanger  178 . However, it is contemplated that in other embodiments the first heated used wash solution  164  is separated into two unequal portions, e.g., 10% of the first heated used wash solution  164  forms the first portion  164   a  provided to the regenerator  162  while 90% of the first heated used wash solution  164  the second portion  164   b  provided to the heat exchanger  178 . In a particular embodiment, 30-60% of the first heated used solution  164  forms the first portion  164   a  provided to the regenerator  162 , with the remainder forming the second portion  164   b  that is provided to the heat exchanger  178 . 
     Referring now to both  FIGS. 1 and 2 , the temperature of the first heated used wash solution  164   b  is increased in the heat exchanger  178 , thereby forming the second heated used wash solution  166 . The second heated used wash solution  166  is provided to the regenerator  162 . The temperature (T 3 ) of the second heated wash solution  166  that is provided to the regenerator  162  is greater than the temperature (T 2 ) of the first heated used wash solution  164   a  provided to the regenerator. The temperature of the second heated used wash solution  166  (T 3 ) is also greater than the temperature of the used wash solution  141  (T 1 ) provided to the regenerator  162 . In one embodiment, the temperature (T 3 ) of the second heated used wash solution  166  is between 110 and 150 degrees Celsius. The temperature distribution of the used wash solutions  141 ,  164   a ,  166  that are provided to the regenerator  162  is T 1 &lt;T 2 &lt;T 3 . 
     The second heated used wash solution  166  is provided to the regenerator  162  at an entry point downstream of where the first heated used wash solution  164   a  and the used wash solution  141  are introduced. 
     As shown in  FIGS. 1 and 2 , the temperature (T 3 ) of the second heated used wash solution  166  provided to the regenerator  162  is greater than the temperature (T 2 ) of the first heated used wash solution  164   a  provided to the regenerator, and the temperature of the first heated used wash solution provided to the regenerator is greater than the temperature (T 1 ) of the used wash solution  141  provided to the regenerator (T 1 &lt;T 2 &lt;T 3 ). Maintaining the temperature distribution of the used wash solutions  141 ,  164   a ,  166  enables maximum utilization of thermal energy in the regenerator  162  by simultaneously minimizing the loss of energy required to strip the contaminants away from the wash solution (the “stripping energy”). Maximum utilization of the thermal energy thus reduces the energy consumption of the regenerator  162 . 
     The regeneration energy required to run a conventional solvent-based gaseous contaminant capture process is distributed in different forms: (1) the energy required heating the wash solution to initiate a regeneration reaction (“sensible heat”); and (2) the steam energy required to remove the contaminants from the wash solution, i.e., solvent (“stripping energy”). 
     The theoretical minimum amount of energy needed to remove the contaminants from the solvent is set to the binding energy of the solvent. However, the energy spent on stripping the contaminants can be minimized by effective thermal energy utilization. The temperature distribution along the regenerator  162  is such that the temperature is the highest at the bottom, where regeneration is enhanced. Having a temperature distribution that satisfies the following formula: used wash solution (T 1 )&lt;first heated used wash solution (T 2 )&lt;second heated used wash solution (T 3 ), facilitates the minimization of stripping energy. 
     The used wash solution  141 , the first heated used wash solution  164   a  and the second heated used wash solution  166  are regenerated in the regenerator, as discussed above, to form the regenerated wash solution  168 . The regenerated wash solution  168  is withdrawn from a bottom portion  169  of the regenerator  162  and provided to a reboiler  180 , which is positioned downstream of the regenerator (with respect to the flow of the wash solution) and arranged to receive the regenerated wash solution. 
     The reboiler  180  boils the regenerated wash solution  168  to form a steam  182  and a hot regenerated wash solution  184 . The steam  182  is provided to the regenerator  162  to facilitate the removal of contaminants from the used wash solution  141 ,  164   a ,  166  present in the regenerator. The hot regenerated wash solution  184 , also referred to as “hot lean solution,” is provided to the absorber  130  for removal of gaseous contaminants from the gas stream  120 . 
     The hot regenerated wash solution  184  may be provided directly to the absorber  130  for re-use. However, to take advantage of the thermal energy present in the hot regenerated wash solution  184 , as shown in  FIGS. 1 and 2 , the hot regenerated wash solution is provided to the heat exchanger  178 , where it exchanges heat with the used wash solution  164   b . Accordingly, after passing through the heat exchanger  178 , the hot regenerated wash solution  184  has a decreased temperature as compared to the temperature after leaving the reboiler  180 . 
     In one embodiment, the hot regenerated wash solution  184  has a temperature between about 100 and 140 degrees Celsius after passing through the heat exchanger  178 . Heating the used wash solution  164   b  with the hot regenerated wash solution  184  eliminates the need for a separate heating medium provided to the heat exchanger  178 , thereby reducing costs and energy consumption of the system  100 . 
     After passing through the heat exchanger  178 , the hot regenerated wash solution  184  is provided to the heat exchanger  172 , where it exchanges heat with the used wash solution  143  to form the first heated used wash solution  164 . Accordingly, after passing through the heat exchanger  172 , the hot regenerated wash solution  184  has a decreased temperature as compared to the temperature after leaving the heat exchanger  178 . In one embodiment, the hot regenerated wash solution  184  has a temperature between about 80 and 120 degrees Celsius after passing through the heat exchanger  172 . 
     Heating the used wash solution  143  by exchanging heat with the hot regenerated wash solution  184  eliminates a separate heating medium for the heat exchanger  172 , thereby reducing cost and energy consumption of the system  100 . This enables utilizing thermal energy from the hot regenerated wash solution  184  as sensible heat in the regeneration process. 
     After passing through the heat exchanger  172 , the hot regenerated wash solution  184  is provided to a cooling unit  186 . The cooling unit  186  is disposed between the heat exchanger  172  and the absorber  130  and is arranged to receive the hot regenerated wash solution  184  and cool the temperature of the same to form a cooled regenerated wash solution  188 . The cooled regenerated wash solution  188  has a temperature of between about 25 and 50 degrees Celsius. 
     The cooled regenerated wash solution  188  is provided to the absorber  130  at an entry point via line  136 . The entry point of the cooled regenerated wash solution  188  is located at the top portion  135  of the absorber  130 . The cooled regenerated wash solution  188  is contacted with the gas stream  120  to remove gaseous contaminants therefrom, thereby repeating the cycle of absorption and regeneration. 
     By utilizing the thermal energy in the hot regenerated wash solution  184  and maintaining the above-mentioned temperature distribution in the regenerator, the overall energy consumption of the system  100  may be decreased as compared to conventional systems. 
     EXAMPLES 
     Example 1 
     To determine the energy consumption of a system according to the description herein, a system simulating the schematic illustrated in  FIG. 1  was employed. The simulation had a 90% CO 2  removal from a flue gas operating with about 13-14 mole. % inlet CO 2 . As compared to a conventional system, the system described herein utilizes 30-40% less energy. The results of the simulation are shown in  FIG. 3 . 
     Unless otherwise specified, all ranges disclosed herein are inclusive and combinable at the end points and all intermediate points therein. The terms “first,” “second,” and the like, herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. The terms “a” and “an” herein do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item. All numerals modified by “about” are inclusive of the precise numeric value unless otherwise specified. 
     While the invention has been described with reference to various exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.