A method and an apparatus for treating a gas stream containing an acid gas.
In a typical chemical absorption reaction, an acid gas is separated from a gas stream by an absorption liquid which contains one or more reactant chemicals. The reaction is then reversed to release the acid gas, so that the reactant chemicals can be reused. One example of a chemical absorption reaction is the reaction of CO2 gas with an organic solvent such as an aqueous amine. The treatment of CO2 gas emissions has recently been a focus of attention, in view of global concerns regarding harm to the environment being caused by greenhouse gas emissions.
The present invention is a method of treating a gas stream containing an acid gas, comprising providing an absorption liquid containing a reactant chemical and passing gas bubbles of the gas stream through the absorption liquid so that the acid gas in the gas stream reacts with the reactant chemical in the absorption liquid, thereby separating the acid gas from the gas stream.
In one particular method aspect, the invention is a method of treating a gas stream containing an acid gas, comprising: immersing a permeable membrane having a plurality of pores in an absorption liquid containing a reactant chemical, and passing the gas stream through the pores in the permeable membrane so that the gas stream forms gas bubbles which float up through the absorption liquid and so that the acid gas in the gas stream reacts with the reactant chemical in the absorption liquid, thereby separating the acid gas from the gas stream.
In another particular method aspect, the invention is a method of treating a gas stream containing an acid gas, comprising: immersing a permeable membrane module in an absorption liquid containing an inorganic solvent as a reactant chemical, wherein the permeable membrane module is comprised of a plurality of hollow membrane loops each defining a permeable conduit, wherein each of the hollow membrane loops has a plurality of pores, filling the hollow membrane loops with the gas stream so that the gas stream passes through the pores to form gas bubbles which float up through the absorption liquid and so that the acid gas in the gas stream reacts with the reactant chemical in the absorption liquid, thereby separating the acid gas from the gas stream, and regenerating the reactant chemical.
In another particular method aspect, the invention is a method of treating a gas stream containing carbon dioxide, comprising: immersing a permeable membrane module in an absorption liquid containing a reactant chemical comprising an inorganic solvent capable of reacting in a reversible reaction with carbon dioxide, wherein the permeable membrane module is comprised of a plurality of hollow membrane loops each defining a permeable conduit, wherein each of the hollow membrane loops has a plurality of pores, filling the hollow membrane loops with the gas stream so that the gas stream passes through the pores to form gas bubbles which float up through the absorption liquid and so that the carbon dioxide in the gas stream reacts with the reactant chemical in the absorption liquid, thereby separating the carbon dioxide from the gas stream, and regenerating the reactant chemical using a steam regeneration process.
The present invention is also a gas absorption apparatus for use in treating a gas stream containing an acid gas.
In one particular apparatus aspect the invention is a gas absorption apparatus comprised of a housing adapted to hold an absorption liquid containing a reactant chemical. The housing has a gas inlet and a gas outlet. A permeable membrane having a plurality of pores is interposed between the gas inlet and the gas outlet. A gas stream containing an acid gas entering the housing through the gas inlet must pass through the pores in the permeable membrane in order to exit the housing via the gas outlet. The gas stream passes through the pores as gas bubbles which float up through the absorption liquid in order to reach the gas outlet while a reaction occurs between the acid gas in the gas stream and the reactant chemical in the absorption liquid, thereby separating the acid gas from the gas stream.
In another particular apparatus aspect, the invention is a gas absorption apparatus, comprising:
As will hereinafter be further described, inorganic solvents, such as potassium carbonate, have an inherent disadvantage when used in a chemical absorption process in that they provide a slow reaction rate. However, the slow reaction rate can be accommodated by the use of the permeable membrane. Firstly, the gas bubbles produced by passing the gas stream through the pores in the permeable membrane provide a gas-liquid contact area which increases as the size of the pores and the gas bubbles decreases. Secondly, when a permeable membrane is used to pass the gas stream through the absorption liquid, there is greater control over gas and liquid phase pressures and flow rates, which may compensate somewhat for the slower reaction rates associated with inorganic solvents.
The function of the permeable membrane is to facilitate a controlled flow of the gas stream through the permeable membrane such that the gas stream will form gas bubbles as it passes through the permeable membrane. The permeable membrane may therefore be comprised of any structure and/or material which comprises pores which enable the gas stream to pass through the permeable membrane in order to form gas bubbles.
The permeable membrane may therefore be constructed of any natural or synthetic material or combination or materials. The permeable membrane may also be constructed as a solid material with pores or may be constructed of fibers. The permeable membrane may be configured in any manner which facilitates the controlled flow of the gas stream therethrough while causing the production of gas bubbles. The permeable membrane may be configured as a conduit, as a planar membrane, or in any other configuration which enables the permeable membrane to perform its intended functions.
Preferably the permeable membrane is comprised of one or more permeable conduits (i.e., hollow membranes) which include pores in their walls so that the gas stream can be directed through the conduits and pass through the walls of the conduits in order to form gas bubbles which then contact the absorption liquid.
In a non-limiting preferred embodiment, the permeable membrane is comprised of one or more permeable conduits (i.e., hollow membranes) which are constructed from fibers as hollow fiber membranes. The fibers may be comprised of any suitable material, but in the preferred embodiment are comprised of a polymer such as polysulfone or PVDF (polyvinylidene).
In alternate non-limiting embodiments, the permeable membrane may be comprised of one or more permeable conduits (i.e., hollow membranes) which are not constructed as fibers. Such permeable conduits may also be comprised of any suitable material such as, for example, a ceramic material or a metal. Examples of such permeable conduits include permeable ceramic tubes, permeable metal tubes, and sintered metal tubes.
The size of the pores in the permeable membrane is selected to provide a relatively high gas-liquid contact area between the gas bubbles and the absorption liquid while facilitating a suitable flowrate of the gas stream through the permeable membrane at a suitable gas stream pressure with acceptable energy losses.
Preferably the permeable membrane is constructed so that the pores have a “size rating” or “representative size” of between about 0.01 micrometers and about 100 micrometers. More preferably the permeable membrane is constructed so that the pores have a “size rating” or “representative size” of between about 0.1 micrometers and about 10 micrometers.
By size rating or representative size, it is meant that all of the pores in the permeable membrane do not necessarily fall within the prescribed size range, but that the prescribed range reflects an average, median or some other representative measure of the size of the pores in the permeable membrane.
The invention may be defined with reference to the size rating of the pores in the permeable membrane because the size rating of the pores is somewhat determinative of the size rating of the gas bubbles which are produced as the gas stream passes through the pores. However, the invention may also be defined with reference to the size rating of the gas bubbles which contact the absorption liquid.
In this regard, the preferred size rating of the gas bubbles which contact the absorption liquid may be described generally to be of the same order of magnitude as the size rating of the pores in the permeable membrane. For example, the size rating of the gas bubbles may preferably be between about 0.01 micrometers and about 100 micrometers, or more preferably between about 0.1 micrometers and about 10 micrometers.
Alternatively, the size rating of the gas bubbles which contact the absorption liquid may be described generally as the size of gas bubble which is produced under the operating conditions of the invention by pores having a size rating within the range of between about 0.01 micrometers and about 100 micrometers, or more preferably between about 0.1 micrometers and about 10 micrometers.
The gas stream is treated by separating the acid gas from the gas stream. The acid gas is preferably separated from the gas stream by being absorbed by the reactant chemical in the absorption liquid. The acid gas may be comprised of any substance which is an acid or which becomes an acid when placed in an aqueous environment, including but not limited to carbon dioxide (CO2), hydrogen sulphide (H2S), sulphur dioxide (SO2), nitrogen dioxide (NO2), and combinations thereof.
The reactant chemical may be comprised of any substance or combination of substances which is capable of reacting with the acid gas in order to separate the acid gas from the gas stream, so that the acid gas is effectively absorbed by the absorption liquid. Preferably the reaction between the reactant chemical and the acid gas is reversible.
For example, the reactant chemical may be comprised of an organic solvent or an inorganic solvent. Representative non-limiting examples of organic solvents include amines, and a representative amine solvent is monoethanolamine (MEA). Representative non-limiting examples of inorganic solvents include potassium carbonate, sodium carbonate and aqueous ammonia.
These and other features of the invention will become more apparent from the following description in which reference is made to the appended drawings. The drawings are for the purpose of illustration only and are not intended to in any way limit the scope of the invention to the particular embodiment or embodiments shown, wherein:
Referring to
There is a gas phase 11 and an absorption liquid phase 13 with a permeable membrane 15 which acts as a barrier to separate the two phases. A pressure difference is applied across permeable membrane 15, where the arrow 17 represents the direction of force caused by the pressure difference. As a result, the gas phase 11, which has a higher pressure, will be pushed across the permeable membrane 15 through the pores 28 in the permeable membrane 15. The resulting gas bubbles 30 will disperse into the absorption liquid phase 13. The size and size distribution of the gas bubbles 30 will depend somewhat upon the size of the pores 28 in the permeable membrane 15. The size and size distribution of the gas bubbles 30 may also depend upon other variables such as, for example, the wetability of the permeable membrane 15, the pressure of the gas stream, and the velocity of the gas stream as it passes through the pores 28.
The concept of generating gas bubbles 30 using a permeable membrane 15 is independent from the configuration of the permeable membrane 15, its manner of construction, or the materials from which it is constructed.
In other words, it doesn't matter if the permeable membrane 15 is a flat sheet or a hollow membrane defining a conduit. As long as the permeable membrane 15 has pores 28 and there is a pressure differential favoring the gas phase 11 side, the gas bubbles 30 will be generated in the absorption liquid phase 13.
In the embodiments described herein, a hollow membrane defining a conduit is used as the permeable membrane 15 as this configuration presents certain advantages which will be apparent from the discussion. However, it will be understood that a flat permeable membrane 15, or any other size and shape of permeable membrane 15, could be substituted without departing from the invention.
A preferred embodiment of a gas absorption apparatus generally identified by reference numeral 10 will now be described with reference to
Structure and Relationship of Parts:
Referring now to
In the preferred embodiment depicted in
In the preferred embodiment, the hollow membrane 26 is preferably constructed of polysulfone fibers, and has a plurality of pores 28. The pores 28 are preferably rated at a size of between about 0.01 micrometers and about 100 micrometers. In the preferred embodiment the pores 28 are more preferably rated at a size of between about 0.1 micrometers and about 10 micrometers.
The sparger 24 may, however, be constructed of any material, in any manner of construction, and may have any configuration which enables the sparger 24 to provide a controlled flow rate of the gas stream through the pores 28, thereby producing the gas bubbles 30.
For example, the sparger 24 may be comprised of a permeable conduit constructed from metal, a ceramic material or some other material or combination of materials. Such a sparger 24 may be constructed from fibers or from a solid material and may be configured in a similar manner as depicted in
Sparger 24 is connected to gas inlet 20, such that a gas stream passing through gas inlet 20 enters sparger 24 and then exits the pores 28 as gas bubbles 30 which float up through absorption liquid 14 in order to reach gas outlet 22, while a reaction occurs between the acid gas contained in the gas stream and the reactant chemical in the absorption liquid 14.
Referring now to
Referring again to
Referring now to
Referring now to
Operation:
The operation of the preferred embodiment will now be discussed with reference to FIGS. 2 to 5. Referring to
More particularly, the acid gas in the gas stream reacts with the reactant chemical in the absorption liquid 14, thereby separating the acid gas from the gas stream.
As shown in
The acid gas contained in the gas stream may be any substance which is an acid or which becomes an acid when placed in an aqueous environment, including but not limited to carbon dioxide (CO2), hydrogen sulphide (H2S), sulphur dioxide (SO2), nitrogen dioxide (NO2), and combinations thereof.
The reactant chemical may be comprised of any substance or combination of substances which is capable of reacting with the acid gas in order to separate the acid gas from the gas stream, so that the acid gas is effectively absorbed by the absorption liquid 14. Preferably the reaction between the reactant chemical and the acid gas is reversible.
As non-limiting examples, the reactant chemical may be comprised of an organic solvent compound or an inorganic solvent compound. Non-limiting examples of organic solvents include amines such as monoethanolamine (MEA) Non-limiting examples of inorganic solvents include potassium carbonate (K2CO3), sodium carbonate (Na2CO3) and aqueous ammonia.
Referring to
Advantages:
The use of permeable membranes to distribute the gas stream significantly increases the gas-liquid contact area between the gas stream and the absorption liquid, and thus between the acid gas and the reactant chemical.
Capital cost savings may also potentially be realized by using a hollow fiber membrane as the permeable membrane. A hollow fiber membrane is lightweight, compact and flexible and does not corrode.
A permeable membrane can be used to improve the absorption efficiency of existing aqueous amine processes. However, use of a permeable membrane with an inorganic solvent such as potassium carbonate has been found to provide a number of advantages, as compared to aqueous amine processes. The cost of the reactant chemical is lower, lower steam usage is required during regeneration, there is little or no oxidation and degradation of the reactant chemical, and there is relatively lower hydrocarbon solubility.
Information Regarding Properties and Selection of the Hollow Fiber Membrane:
It has been found that an amine based reactant chemical tends to attack a PVDF (polyvinylidene fluoride) fiber in a short period of time, even at ambient temperature and atmospheric pressure.
Therefore, the stability of two polymers, PVDF and polysulfone, in inorganic based solutions was evaluated. The stability tests were conducted by soaking the fibers in a test solution in a glass jar under ambient temperature and elevated temperature. The basic testing solutions were potassium carbonate solution with different concentrations. Piperazine (PZ) was also mixed with potassium carbonate solution, mainly to function as catalyst.
Stability Tests of PVDF Fiber:
The results of the stability tests for PVDF hollow fiber membranes in different potassium carbonate solutions are given in Table 1 below.
For comparison, commercial PVDF flat fiber membranes were also tested under the same conditions. The results indicate that piperazine attacks PVDF fiber at the elevated temperature with or without the potassium carbonate. Potassium carbonate also adds some degree of coloration to the PVDF fiber at elevated temperatures. At room temperature, the PVDF fiber survived. Another observation was that PVDF fiber started changing color where it contacts with air. Oxygen from air has been considered to cause the PVDF fiber to change the color. Therefore, another test was conducted by bubbling the soaking solution with nitrogen to remove oxygen, then soaking the membrane in an oxygen free solution. The results still show that piperazine attacks the PVDF fiber. The commercial PVDF flat fiber membranes also show some coloured spots at elevated temperature.
Based on these tests, it seems feasible to use potassium carbonate solution (2M) and PVDF fiber membranes for absorbing acid gas at room temperature.
Stability Tests of Polysulfone Fiber:
There is no confirmed literature about the stability of polysulfone fiber in potassium carbonate based solutions. The stability tests for polysulfone fibers were also conducted by soaking the polysulfone fibers in four different absorbing solutions in glass vials at room temperatures and at elevated temperatures. Visual observations were recorded at different times.
The results of the stability tests for polysulfone fibers in different potassium carbonate solutions are given in Table 2 below. The note “OK” in Table 2 refers to: no visible color change, and no opacity change of the soaked fiber.
In general, the color change indicates some chemical reaction occurring on the polymer surface. The opacity change is an indication of a wetability change. The summary from these tests is that polysulfone fiber does not change significantly in a potassium-based solution at room temperature and at 50 degrees C.
Flow Rate Test Results From PVDF and Polysulfone Fiber:
A group of PVDF hollow fiber loops was set up and soaked in 2M potassium carbonate solutions, in a sealed glass cylinder. The inlet gas stream was pressurized through the pores from the fiber wall and was bubbled through the testing solution. The outlet gas stream was connected to a soap bubble flow meter and the off gas flow rates were recorded. The initial inlet gas pressure was 20 psi, and the results are plotted in
The similar test of the off gas flow rate from polysulfone fiber loops were also given in
Based upon the above testing, it was determined that a polysulfone fiber bundle is an acceptable option for use as a permeable membrane in the practice of the invention. Issues relating to the design of a permeable membrane comprising a polysulfone fiber bundle include the following:
The use of polysulfone hollow fiber membranes as the sparger by pressurizing the gas stream through the fiber wall into the liquid phase was chosen for implementation.
Configuration of Permeable Membranes:
A schematic diagram of a sparger unit utilizing polysulfone hollow fiber membranes as the permeable membrane 15 is given in
The mounting plate 34 is mounted in a plastic cylinder as the housing 12. The cylinder can hold a certain amount of an absorption liquid 14 containing a reactant chemical. The absorption liquid 14 can also be pumped through the cylinder in a controlled flow rate. Because the fiber loops 40 are sealed onto the mounting plate 34, the gas stream can only pass through the pores 28 in the walls of the hollow fiber membranes 40 in order to pass through the absorption liquid 14 in the cylinder toward the gas outlet 22. The off gas obtained at the gas outlet 22 is sent to a gas chromatograph for analysis. The size of the gas bubbles generated from the hollow fiber membranes 40 is directly related to the size of the pores 28 in the walls of the hollow fiber membranes 40.
Experiments consisting of three runs were performed using the parameters described in Table 3.
Referring to Table 3, it is noted that Run #2 was performed using the same set of fiber loops 40 as Run #1. However, the mounting plate 34 was removed from the housing 12 after Run #1. The hollow fiber loops 40 were rinsed with water, air dried and then remounted in the mounting plate 34 for Run #2.
The measured factors for Run #1, Run #2 and Run #3 were as follows:
1. total fiber length/membrane 15 surface area
2. total running time.
3. pressure of the gas stream at the gas inlet 20
4. CO2 content in the gas stream at the gas outlet 22
5. off gas flow rate at the gas outlet 22
6. conversion rate of the reactant chemical in the absorption liquid 14
Summarized Results From Run #1:
The input gas stream was 15% CO2 and 85% N2. The absorption liquid 14 contained in the housing 12 contained 2M K.2CO3 as the reactant chemical. The pressure of the gas stream at the gas inlet 20 was set at 20 psi (1.36 atm). The CO2 concentration in the gas stream at the gas outlet 22 was monitored and recorded by a gas chromatograph during the running time. The total running time was 213 hours or about 9 days. Some key operation parameters for Run #1 are provided in Table 4.
The off gas CO2 concentration detected by the gas chromatograph during Run #1 is plotted in
During the running time of Run #1, the off gas stream flow rate was also measured manually using a soap bubble flow meter. The results are plotted in
After Run #1 was completed, the mounting plate 34 was removed from the cylinder and the fiber loops 40 were rinsed with water and dried overnight. The same set of fiber loops 40 were then re-mounted in the mounting plate for Run #2.
Results and Comparison Between Run #1 and Run #2:
Since Run #1 and Run #2 used the same set of hollow fiber loops 40, a comparison of results for Run #1 and Run #2 was performed in order to evaluate the performance of the hollow fiber loops 40 in Run #2 after washing and drying. The off gas stream CO2 content and flow rate for Run #1 and Run #2 are plotted in
Run #1 and Run #2 both continued for longer than 200 hours. For Run #2, the pressure of the gas stream at the gas inlet 20 was 30 psi, as compared with 20 psi in Run #1. With respect to CO2 absorbing efficiency for Run #1 and Run #2, the first 100 hours were very similar and appear to be independent of the pressure of the gas stream at the gas inlet 20. After 100 hours, the CO2 contents of the off gas streams for Run #1 and Run #2 began to be distinguishable. Run #2 had a higher feed gas stream pressure and a higher off gas stream flow rate, with the result that the effectiveness of the reactant chemical in the absorption liquid 14 diminished relatively more quickly than for Run #1. As a consequence, the equilibrium of the reaction between the CO2 may shift and cause a decrease in efficiency of the absorbing reaction. This reaction equilibrium shift could be controlled by circulating the absorption liquid 14 in the cylinder.
It appears from a comparison of the results for Run #1 and Run #2 that the off gas stream flow rate is related to the performance capacity of the hollow fiber loops 40 (i.e., the efficiency of the membrane surface). The observation from both Run #1 and Run #2 is that the off gas flow rate dropped gradually, but that a higher gas stream pressure at the gas inlet 20 provides a higher off gas stream flow rate.
As mentioned, there was no circulation of the absorption liquid 14 in the cylinder during either Run #1 or Run #2. When the running times reached about 150 hours for both Run #1 and Run #2, clear crystals began to appear in the absorption liquid 14. The analysis of these crystals using Raman spectroscopy indicated that the crystals are potassium bicarbonate. The solubility of potassium bicarbonate is much smaller than that of potassium carbonate at room temperature. Therefore the potassium bicarbonate precipitates out for the liquid phase. The growth of these crystals appears to be detrimental to the sparging operation, and may be a cause of the drop in the off gas stream flow rates in both Run #1 and Run #2.
Results and Observation From Run #3:
Run #3 was performed for two different reasons: 1) the total hollow fiber loop 40 length was increased in order to increase the operation capacity; and 2) the absorption liquid 14 in the cylinder was circulated in order to avoid the precipitation of potassium bicarbonate.
The total hollow fiber loop 40 length in Run #3 was about 600 cm. According to fiber dimensions measured from an electronic scanning microscope image the actual total fiber surface area was 23.5 cm2. By comparison, the total surface area of the hollow fiber loop 40 set for Run #1 and Run #2 was 8.5 cm2. The pressure of the gas stream at the gas inlet 20 was 30 psi during most of Run #3. Both the off gas stream flow rate and the off gas stream CO2 content were recorded during Run #3. The total running time for Run #3 was 550 hours (23 days).
During Run #3, the absorption liquid 14 in the cylinder was changed three times at hours 143, 243, and 377 respectively. Meanwhile, the absorption liquid 14 drained from the cylinder was collecting to test the potassium carbonate conversion rate using Raman spectroscopic analysis.
The variation in the amount of CO2 contained in the off gas stream during Run #3 is depicted in
The time and time intervals between changes of the absorption liquid 14 in the cylinder are provided in Table 5. The CO2 content in the off gas stream at the absorption liquid 14 changing time and the potassium carbonate conversion rate of the drained absorption liquids 14 are also included in Table 5.
*Note the fourth change of absorption liquid 14 at 508 hours; this point was selected to provide the same CO2 content in the off gas stream in order to provide a comparison of the number of hours in the time interval for the third change and the fourth change of the absorption liquid 14.
**The first change of absorption liquid 14 at 143 hours was performed by pumping fresh absorption liquid 14 into the cylinder instead of by draining the cylinder.
During Run #3, each time interval between changes of the absorption liquid 14 in the cylinder can be considered as a cycle. Each cycle (except for the second cycle following the first change of the absorption liquid 14) exhibited a similar running time and a similar absorbing efficiency. The shorter running time and slightly lower absorbing efficiency for the second cycle may be attributed to the different manner in which the second change of the absorption liquid 14 was performed. Although there is no potassium carbonate conversion rate data for the first cycle and the fourth cycle, the combination of the CO2 content in the off gas stream and the off gas stream flow rate could be used to provide an estimate of the potassium carbonate conversion rate.
As mentioned previously, one other purpose for the Run #3 was to try to increase the performance capacity of the apparatus by increasing the total fiber length or the available membrane surface area. The optimization of absorbing efficiency and the off gas stream flow rate can be used as the criteria for overall performance. Therefore, the off gas stream flow rate is one of the most important factors to determine.
Quick drops in the off gas stream flow rate were observed at the beginning of each of Run #1, Run #2 and Run #3. These quick drops of the off gas stream flow rate may be due to the quick plugging of the very small pores 28 on the fiber wall. Although Run #2 and Run #3 have different lengths of hollow fiber loop 40, the initial drop in off gas stream flow rate are similar for these Runs, possibly due to a similarity in pore size distribution in the fiber wall.
The drops in off gas stream flow rate are much smaller after the initial stage for each of Run #1, Run #2 and Run #3. The total length of hollow fiber loop 40 for Run #3 is almost two times longer than that for Run #2, but the off gas stream flow rate for Run #3 is not doubled. If the hollow fiber loop 40 is too long, pressure drop along the fiber wall may cause additional pore 28 plugging. Therefore, it may advisable to increase the total membrane surface area by increasing the number of hollow fiber loops 40 instead of by increasing the length of the hollow fiber loops 40.
The above described testing of polysulfone hollow fiber loops 40 as a sparger in a gas absorption apparatus 10 provided the following conclusions:
Typically, acid gas absorption processes employ an aqueous solution of a salt containing sodium or potassium as the cation with an anion selected so that the resulting solution is buffered at a pH about 9-11. Such a solution, being alkaline in nature, will absorb CO2 and other acid gases. Salts, which have been proposed for processes of this type, include sodium and potassium carbonates, phosphate, borate, arsenite and phenolate, as well as salts of weak organic acids. Sodium and potassium carbonate solutions have been used extensively for the absorption of CO2 from gas streams because of their relative low cost and ready availability.
The success of the absorption and desorption of carbon dioxide in a solution of alkali carbonate depends upon the reversibility of the reaction. The reaction equilibrium tends to go towards the right at low temperature and towards the left at higher temperature. Other factors such as high operating pressure, high partial pressure of CO2, or concentration of the alkali carbonate solution, could also shift the reaction equilibrium to the right.
Very general comparisons are given in Table 6 for an overall evaluation of reactant chemicals. MEA is selected as a representative of amine-based reactant chemicals to compare with other reactant chemicals. In Table 6, the comment of fast and slow, high and low are relative to each other. Overall, potassium carbonate with additives such as a promoter appear to be the best choice. However, other amine-based reactant chemicals such as MEA with a promoter would delivery even better results. The attraction of using aqueous ammonia as a reactant chemical is the by-product, ammonium bicarbonate, which can potentially be used as a fertilizer. For the application of producing CO2 from flue gas, the regeneration of CO2 has to incorporate a waterwashing tower to reabsorb the NH3 in the regeneration cycle.
The major drawback in a conventional potassium carbonate gas absorbing process is the slow reaction rate. The consequences of the slow reaction rate are the lower carbonate to bicarbonate conversion rate and the higher cost of steam for CO2 regeneration.
Two advantages of using permeable membranes include: 1) a potentially much higher gas-liquid contact surface area; and 2) more controllable gas phase pressure, liquid phase pressure, and flow rate. These advantages could compensate for the slow absorbing reaction rate associated with potassium carbonate as a reactant chemical. If a higher carbonate to bicarbonate conversion rate can be obtained using permeable membranes, the regeneration energy could also potentially be reduced significantly. The advantages of having little or no solvent degradation and/or oxidation of potassium carbonate will also potentially reduce the operating cost.
In this patent document, the word “comprising” is used in its non-limiting sense to mean that items following the word are included, but items not specifically mentioned are not excluded. A reference to an element by the indefinite article “a” does not exclude the possibility that more than one of the element is present, unless the context clearly requires that there be one and only one of the elements.
It will be apparent to one skilled in the art that modifications may be made to the illustrated embodiment without departing from the spirit and scope of the invention as hereinafter defined in the Claims.
Number | Date | Country | Kind |
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2,470,807 | May 2005 | CA | national |
Number | Date | Country | |
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Parent | 11140468 | May 2005 | US |
Child | 11604467 | Nov 2006 | US |