CLIAMS:1. A process for capturing carbon-dioxide from a gas stream comprising carbon-dioxide, said process comprising the following steps:
a) activating an adsorbent comprising an alkali metal carbonate impregnated support to form an activated adsorbent with active adsorption sites having reduced energies;
b) passing said gas stream through an adsorber containing said activated adsorbent to adsorb the carbon-dioxide on said activated adsorbent at a first pre-determined temperature and at a first pre-determined pressure to form a carbon-dioxide laden adsorbent;
c) transferring said carbon-dioxide laden adsorbent to a desorber; and
d) passing a fluid through said desorber for partially desorbing the carbon-dioxide from said carbon-dioxide laden adsorbent at a second pre-determined temperature and at a second pre-determined pressure to obtain a partially regenerated adsorbent; and
e) returning said partially regenerated adsorbent to said adsorber.
2. The process as claimed in claim 1, wherein said support is one of alumina and silica-alumina.
3. The process as claimed in claim 1, wherein the process step (a) of activating the adsorbent is carried out for a time period ranging from 1 minute to 20 minutes.
4. The process as claimed in claim 1, wherein the alkali metal carbonate is one of K2CO3 and Na2CO3.
5. The process as claimed in claim 1, wherein the activated adsorbent is hydrated K2CO3.
6. The process as claimed in claim 1, wherein the amount of the alkali metal carbonate ranges from 5 wt% to 60 wt%.
7. The process as claimed in claim 1 or claim 2, wherein the support is characterized by a surface area in the range of 170 to 550 m2/g; pore volume in the range of 0.18 cm3/g to 0.95 cm3/g; and pore size of said support is in the range of 100Å to 300Å.
8. The process as claimed in claim 1, wherein the process step (b) of adsorption is carried out in said adsorber and the process step (c) of desorption is carried out in said desorber in a circulating bubbling flow regime.
9. The process as claimed in claim 1, wherein
a) said first pre-determined temperature ranges from 40ºC to 90ºC;
b) said second pre-determined temperature ranges from 110 ºC to 200ºC; and
c) said first pre-determined pressure and said second pre-determined pressure ranges from 1 bar to 2 bar.
10. The process as claimed in claim 1, wherein the differential temperature between the process step (b) of adsorption and the process step (d) of desorption ranges from 20ºC to 110ºC.
11. The process as claimed in claim 1, wherein the process step (a) of activation is carried out by passing one of water-vapor and the gas stream comprising a mixture of water-vapor and carbon-dioxide.
12. The process as claimed in claim 1, wherein the fluid is at least one selected from the group consisting of nitrogen, carbon-dioxide and water-vapor.
13. The process as claimed in claim 1, wherein
a) the residence time of the activated adsorbent in said adsorber ranges from 1 minute to 10 minutes; and
b) the residence time of the carbon-dioxide laden adsorbent in said desorber ranges from 1 minute to 5 minutes.
14. The process as claimed in claim 1, wherein the efficiency of carbon-dioxide removal ranges from 40% to 90%. ,TagSPECI:FIELD
The present disclosure relates to a process for capturing carbon-dioxide from a gas stream.
BACKGROUND
The atmospheric carbon-dioxide (CO2) levels are increasing continuously due to rapid industrial growth. Major industrial sites like thermal power plants, oil refineries and other processing plants such as cement, steel, aluminum and the like cause most of the carbon-dioxide (CO2) emission into the environment. The increased level of atmospheric carbon-dioxide (CO2) is considered to be one of the main causes for global warming. In order to combat global warming, several precautionary measures are required. The precautionary measures include the use of low carbon or carbon free energy sources like nuclear and wind, and other alternative methods such as capture and sequestration of carbon-dioxide (CO2) have been encouraged in recent years. Carbon-dioxide (CO2) capture combined with sequestration is a promising means for regulating carbon-dioxide (CO2) emissions.
Among the various conventional post combustion carbon-dioxide (CO2) capture methods such as absorption, adsorption, membrane separation cryogenic separation and the like, the adsorption route is advantageous due to its enhanced carbon-dioxide (CO2) capture capacity, lower regeneration energy and low operational cost which provide easy retrofit to existing systems.
Use of solid dry regenerable adsorbents offer a huge potential for selective uptake and release of carbon-dioxide (CO2).
However, some disadvantages linked to the conventional method/s using adsorption techniques are as follows:
poor multi-cycle adsorption capacity;
requires high regeneration temperature for decomposing the stable intermediate species (for example KAl(CO3)2(OH)2) formed on the surface of the adsorbent during the adsorption of carbon-dioxide (CO2); the overall energy demand of the process is increased due to higher regeneration temperature of the adsorbent; and
the efficiency of removal/capture of carbon-dioxide (CO2) from the gas stream decreases with every cycle of regeneration of the adsorbent.
In order to overcome the drawbacks associated with the conventional method/s, there is a need for an advantageous process for capturing carbon-dioxide (CO2) from gas stream effectively. Further, there is a need for a process that requires lower temperature for regeneration of adsorbents, thereby minimizing the overall energy demand.
OBJECTS
Some of the objects of the present disclosure, which at least one embodiment herein satisfies, are as follows:
An object of the present disclosure is to provide a process for capturing carbon-dioxide (CO2) from a gas stream.
Another object of the present disclosure is to provide a process for capturing carbon-dioxide (CO2) that offers a lower differential temperature between adsorption and desorption, thereby minimizing the overall energy demand.
Yet another object of the present disclosure is to provide a process for capturing carbon-dioxide (CO2) that requires lower temperature for the regeneration of adsorbents.
Still another object of the present disclosure is to provide a process for capturing carbon-dioxide (CO2) that regenerates the adsorbents partially.
Yet another object of the present disclosure is to provide a process for capturing carbon-dioxide (CO2) that increases the life of the adsorbents.
Still another object of the present disclosure is to provide a process for capturing carbon-dioxide (CO2) from a gas stream, which is simple and cost-effective.
A further object of the present disclosure is to ameliorate one or more problems associated with the conventional methods or at least provide a useful alternative.
Other objects and advantages of the present disclosure will be more apparent from the following descriptions which are not intended to limit the scope of the present disclosure.
SUMMARY
The present disclosure envisages a process for capturing carbon-dioxide from a gas stream. The process includes forming an activated adsorbent with active adsorption sites having reduced energies. The gas stream is then passed through an adsorber containing the activated adsorbent to adsorb carbon-dioxide on the activated adsorbent sites of the activated adsorbent to obtain a carbon-dioxide laden adsorbent. The carbon-dioxide laden adsorbent is transferred to a desorber. The carbon-dioxide laden adsorbent is partially desorbed from the carbon-dioxide laden adsorbent by passing a fluid through the desorber to obtain a partially regenerated adsorbent. The partially regenerated adsorbent is returned to the adsorber for use in adsorption of the carbon-dioxide. Due to partial regeneration of the adsorbent, the overall energy demand is reduced.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The present disclosure will now be described with reference to the accompanying non-limiting drawings:
Figures 1A and 1B illustrate a system for capturing the carbon-dioxide (CO2) in a gas stream in accordance with the present disclosure;
Figure 2 is a graph of variation in differential pressure between an adsorber and a desorber versus time and variation in temperature of the adsorber and the desorber versus time in accordance with the present disclosure;
Figure 3 is a graph of variation in weight% of a partially regenerated adsorbent and a carbon-dioxide (CO2) laden adsorbent versus variation in temperature in accordance with the present disclosure;
Figure 4 illustrates the X-Ray Diffraction (XRD) pattern of an activated adsorbent and a partially regenerated adsorbent in accordance with the present disclosure;
Figure 5 is a graph of variation in carbon-dioxide (CO2) removal in percentage as a function of gas stream velocity in accordance with the present disclosure;
Figure 6 is a graph of variation in carbon-dioxide (CO2) removal in percentage as a function of desorption temperature in accordance with the present disclosure;
Figure 7 is a graph of variation in efficiency of carbon-dioxide (CO2) removal as a function of time and variation in percentage of carbon-dioxide (CO2) in treated gas as a function of time in accordance with the present disclosure;
Figure 8 is a graph of variation in carbon-dioxide (CO2) removal in percentage as a function of concentration of water-vapor (H2O) in the gas stream in accordance with the present disclosure; and
Figure 9 is a graph of variation in percentage of carbon-dioxide (CO2) in treated gas as a function of time in accordance with the present disclosure.
DETAILED DESCRIPTION
The disclosure will now be described with reference to the accompanying embodiments, which do not limit the scope and ambit of the disclosure. The description provided is purely by way of example and illustration.
The embodiments herein, the various features, and advantageous details thereof are explained with reference to the non-limiting embodiments in the following description. Descriptions of well-known components and processing techniques are omitted so as to not unnecessarily obscure the embodiments herein. The examples used herein are intended merely to facilitate an understanding of ways in which the embodiments herein may be practiced and to further enable those of skill in the art to practice the embodiments herein. Accordingly, the examples should not be construed as limiting the scope of the embodiments herein.
The description of the specific embodiments will so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein.
As discussed earlier, the problems associated with the conventional method/s using adsorption techniques are poor regeneration characteristics of the adsorbents such as high regeneration temperature and poor multi-cycle adsorption capacity, requirement of higher regeneration temperature for decomposing the stable intermediate species (for example KAl(CO3)2(OH)2) formed on the surface of the adsorbent during the adsorption of carbon-dioxide (CO2) on to the adsorbent, increase in energy demand due to the requirement of higher regeneration temperature, and lower efficiency of removal/capture of the carbon-dioxide (CO2) from the gas stream.
In order to forestall the drawbacks associated with the conventional method/s using adsorption techniques, the present disclosure envisages a process for capturing carbon-dioxide (CO2) from the gas stream using solid adsorbents. The solid adsorbents used in the process of the present disclosure can be regenerated.
In accordance with the present disclosure, an adsorbent comprising an alkali metal carbonate impregnated support is first activated to form an activated adsorbent with active adsorption sites having reduced energies. The gas stream is passed through an adsorber containing the activated adsorbent to adsorb the carbon-dioxide (CO2) contained in the gas stream on the activated adsorbent at a first pre-determined temperature and at a first-predetermined pressure and form a carbon-dioxide laden. The carbon-dioxide (CO2) laden adsorbent is then introduced led into a desorber. A fluid is passed through the desorber to partially desorb the carbon-dioxide (CO2) from the carbon-dioxide (CO2) laden adsorbent at a second pre-determined temperature and at a second pre-determined pressure to form a partially regenerated adsorbent. The partially regenerated adsorbent is returned to the adsorber for further adsorption of carbon-dioxide (CO2) in the gas stream.
In accordance with the present disclosure, the adsorber and the desorber are the fluidized bed reactors.
In accordance with one embodiment of the present disclosure, the support is at least one of alumina and silica-alumina having a surface area in the range of 170 to 550 m2/g; pore volume in the range of 0.18 cm3/g to 0.95 cm3/g; and pore size in the range of 100Å to 300Å.
In accordance with another embodiment of the present disclosure, the amount of the alkali metal carbonate ranges from 5 wt% to 60 wt%, the weights being expressed with respect to the total weight of the alumina or the silica-alumina support.
In accordance with the present disclosure, the alkali metal carbonate can be at least one of K2CO3 and Na2CO3, K2CO3 being preferred, since, equilibrium temperature of K2CO3 is much higher than Na2CO3. Due to this, K2CO3 have higher potential than Na2CO3 to adsorb carbon-dioxide (CO2) at higher temperatures.
In accordance with the present disclosure, the activated adsorbent is hydrated K2CO3.
During activation of the adsorbent, K2CO3 impregnated on alumina or silica alumina support is hydrated to form hydrated K2CO3. The step of activating the adsorbent is depicted by the following reaction:
K2CO3 + H2O ? hydrated K2CO3
Due to the formation of hydrated K2CO3, water molecules are adsorbed on some of the active energetic sites available for adsorption of the carbon-dioxide (CO2) on the adsorbent. Due to this, the overall active energetic sites present on the adsorbent for adsorption of carbon-dioxide (CO2) get reduced.
Typically, the second pre-determined temperature is greater than the first pre-determined temperature.
In accordance with one embodiment of the present disclosure, the first pre-determined temperature ranges from 40ºC to 90ºC.
In accordance with another embodiment of the present disclosure, the second pre-determined temperature ranges from 110ºC to 200ºC.
In accordance with yet another embodiment, the first pre-determined pressure and the second pre-determined pressure ranges from 1 bar to 2 bar.
In accordance with still another embodiment of the present disclosure, the pressure difference between the first pre-determined pressure and the second pre-determined pressure ranges from 80 mm H2O to 150 mm H2O, typically 100 mm H2O.
In accordance with the present disclosure, the differential temperature between the step of adsorption and the step of desorption ranges from 20ºC to 110ºC.
In accordance with the present disclosure, the gas stream is flue gas.
In accordance with the present disclosure, there is provided a system 100 for capturing carbon-dioxide (CO2) in a gas stream.
Figure 1A illustrates the system 100 for capturing carbon-dioxide (CO2) in the gas stream in accordance with the present disclosure. The system 100 includes an adsorber 102, a first cyclone separator 102a, a first standpipe 104, a riser 106, a desorber 108, a second cyclone separator 108a, a second standpipe 110, a conduit vessel 112, a first cylinder C1, a plurality of second cylinder C2, a bubbler B, a dryer D, an analyzer (IR analyzer) AN and a computer PC.
In accordance with the present disclosure, the temperature inside the adsorber 102 and the desorber 108 is stabilized by circulating the adsorbent without injecting the carbon-dioxide (CO2) and steam (H2O) in the adsorber 102 and the desorber 108. After stabilization of the temperature in the adsorber 102 and the desorber 108, the gas stream comprising carbon-dioxide (CO2) is introduced into the adsorber 102 comprising 35 wt% K2CO3.
In order to facilitate the adsorption of carbon-dioxide (CO2) from the gas stream on the adsorbent, it is necessary to activate the adsorbent. If the adsorbent is not activated, the alkali metal carbonate (K2CO3) supported on the alumina support will not react with carbon-dioxide (CO2) to form alkali metal hydrogen carbonate (KHCO3).
In accordance with one embodiment of the present disclosure, the adsorbent can be activated by water-vapor (H2O) present in the gas stream.
In accordance with another embodiment of the present disclosure, the adsorbent can be activated by directly injecting steam (H2O) in the adsorber 102.
In accordance with the present disclosure, the steam is generated by a steam generator ‘SG’ (as shown in Figure 1B).
In accordance with the present disclosure, the ratio of water-vapor and carbon-dioxide in the gas stream ranges from 0.5 to 2.
The step of activation of the adsorbent is carried out for a time period ranging from 1 minute to 20 minutes to form an activated adsorbent, typically 10 minutes to 20 minutes.
The residence time of the activated adsorbent in the adsorber 102 ranges from 1 minute to 10 minutes.
The activated adsorbent formed is shown by the following reaction:
K2CO3 + H2O ? hydrated K2CO3.
The reaction mentioned herein-above being exothermic, due to which the amount of energy released on the formation of hydrated K2CO3 is approximately 103 kJ/mol. Further, the activated adsorbent reacts with carbon-dioxide (CO2) in the gas stream to facilitate adsorption of the carbon-dioxide (CO2) on the activated adsorbent. The adsorption of carbon-dioxide (CO2) on the adsorbent is shown by the following reaction:
Hydrated K2CO3. + CO2 ? 2 KHCO3 + x H2O
(where, x ranges from 0.1 to 1)
On adsorption of the carbon-dioxide (CO2) on the activated adsorbent, metal hydrogen carbonate (KHCO3) is formed. The reaction mentioned herein-above is exothermic, due to which the amount of energy released on the formation of KHCO3 is approximately 38 kJ/mol. On adsorption of carbon-dioxide (CO2) on the activated adsorbent, a carbon-dioxide (CO2) laden adsorbent is formed. The gas stream leaving the adsorber 102 is fed to the analyzer AN via the dryer D. The results obtained from the analyzer AN can be seen on the computer PC.
In accordance with one embodiment of the present disclosure, the gas stream leaving the adsorber 102 is treated gas.
In accordance with one embodiment of the present disclosure, the flow of the gas stream leaving the adsorber (i.e., treated gas) can be controlled by a valve V11.
The gas stream leaving the adsorber 102 is passed through a first cyclone separator 102a to remove the adsorbents entrained in the treated gas. The carbon-dioxide (CO2) laden adsorbent is then fed to the desorber 108 via the first standpipe 104 and the riser 106.
The residence time of the carbon-dioxide (CO2) laden adsorbent in the desorber 108 can range from 1 minute to 5 minutes.
The flow of the carbon-dioxide (CO2) laden adsorbent is controlled by a valve V1.
A fluid such as nitrogen (N2), carbon-dioxide (CO2) and water-vapor (H2O) is introduced into the desorber 108 through the operative bottom end of the desorber 108. The adsorbed carbon-dioxide (CO2) from the carbon-dioxide (CO2) laden adsorbent is partially desorbed by the fluid to form a partially regenerated adsorbent. The flow of desorbed carbon-dioxide (CO2) leaving the desorber 108 can be controlled by a valve V12.
In accordance with the present disclosure, the fluid comprising nitrogen (N2) and carbon-dioxide (CO2), can be stored in the cylinders C1 and C2.
In accordance with the present disclosure, the circulation of the fluid in the system 100 can be controlled by valves V3, V4, V5, V6, V7, V8 and V9.
In accordance with the present disclosure, partial regeneration of the adsorbent is preferred because less temperature is required for partial regeneration compared to complete regeneration of the adsorbent. Further, partial regeneration of the adsorbent retains water of crystallization (hydrated species), thereby reducing the activation energy required for adsorption of carbon-dioxide (CO2) from the gas stream. Due to reduction in the activation energy, higher temperature is not required for the adsorption of carbon-dioxide (CO2) from the gas stream. Furthermore, due to the retention of water of crystallization, active adsorption sites having reduced energies are retained on the adsorbent. Due to the retention of water of crystallization, lower ratio of water-vapor (H2O) and carbon-dioxide (CO2) in the gas stream is required for the activating the adsorbent.
In accordance with the present disclosure, due to the retention of active adsorption sites having reduced energies during partial regeneration of the adsorbent, activation energy required for adsorption of carbon-dioxide (CO2) is reduced. Hence, the adsorption of carbon-dioxide (CO2) can be carried out at a lower temperature (in the range of 40ºC to 90ºC).
Thus, due to the partial regeneration and the reduced activation energy, the overall energy demand required for adsorption and desorption of carbon-dioxide (CO2) is reduced.
In accordance with one embodiment of the present disclosure, the desorbed carbon-dioxide (CO2) leaving the desorber 108 is passed through the second cyclone separator 108a to remove the partially regenerated adsorbent.
In accordance with the present disclosure, the carbon-dioxide (CO2) can be passed through the desorber 108 to partially desorb the carbon-dioxide (CO2) from the carbon-dioxide laden adsorbent to obtain the partially regenerated adsorbent. Due to this, need of nitrogen (N2) for regenerating the adsorbent may be obviated. Therefore, the operating cost can be reduced.
Further, the partially regenerated adsorbent is fed to the adsorber 102 via the second standpipe 110 and the conduit vessel 112. The flow of the partially regenerated adsorbent to the conduit vessel 112 can be controlled by a valve V2.
In accordance with one embodiment of the present disclosure, the flow of carbon-dioxide (CO2) laden adsorbent in the desorber 108 and the flow of partially regenerated adsorbent in the adsorber 102 can be controlled by,
the valves V1 and V2 respectively; and
maintaining a stable pressure equivalence between the adsorber 102 and the desorber 108 (i.e., the pressure difference between the adsorber and the desorber typically is maintained at 100 mm H2O).
By controlling the flow of carbon-dioxide (CO2) laden adsorbent in the desorber 108 and partially regenerated adsorbent in the adsorber 102, a constant fluidized bed height can be maintained in the adsorber 102 and the desorber 108.
In accordance with the present disclosure, the adsorbent inside the adsorbent 102 and carbon-dioxide (CO2) laden adsorbent in the desorber 108 is fluidized, typically in a circulating bubbling flow regime.
In accordance with the present disclosure, it is necessary to maintain the pressure difference between the adsorption and the desorption in the range of 80 mm H2O to 150 mm H2O, typically 100 mm H2O. Diffusion coefficient being pressure dependent, it is necessary to maintain the mentioned pressure difference. If the mentioned pressure difference is not maintained, the diffusion coefficient of water-vapor (H2O) in the adsorbent is reduced, because of which the adsorption of carbon-dioxide (CO2) on the adsorbent is reduced.
In accordance with the present disclosure, treating the gas stream by the process described herein-above, the efficiency of carbon-dioxide removal ranges from 40% to 90%.
The present disclosure is further illustrated herein-below with the help of the following examples. The examples used herein are intended merely to facilitate an understanding of the ways in which the embodiments herein may be practiced and to further enable those of skilled in the art to practice the embodiments herein. Accordingly, the examples should not be construed as limiting the scope of the embodiments herein.
Example 1- Activity testing of solid adsorbent in circulating mode.
25 kg of adsorbent comprising K2CO3 impregnated on alumina support was charged into an adsorber which was similar to that depicted in Figure 1A. A gas stream, containing water-vapor (H2O) and carbon-dioxide (CO2) in the ratio equal to 1.91, was introduced into the adsorber with a velocity of 0.11 m/s. The flow of the gas stream in the adsorber was co-current to the flow of adsorbent in the adsorber. Due to water-vapor (H2O), the adsorbent was activated to form an activated adsorbent. After activation of the adsorbent, the activated adsorbent reacted with the carbon-dioxide (CO2) at a temperature of 70ºC, at a pressure of approximately 1.75 bar and for a time period of 7.5 minutes to obtain a carbon-dioxide (CO2) laden adsorbent. The gas stream leaving the adsorber (i.e., treated gas) was passed through a first cyclone separator to separate/remove carbon-dioxide (CO2) laden adsorbent from the treated gas. The carbon-dioxide (CO2) laden adsorbent was then fed to desorber to partially desorb the carbon-dioxide (CO2) from the carbon-dioxide (CO2) laden adsorbent by passing a fluid (for a time period of 3.5 minutes) through the desorber comprising nitrogen (N2) or carbon-dioxide (CO2) to form a partially regenerated adsorbent. The velocity at which the fluid was introduced into the desorber was 0.7 m/s. The partially regenerated adsorbent was formed at a temperature of 150ºC and at a pressure of approximately 1.80 bar. During the partial regeneration, losses due to decomposition of hydrated K2CO3 species and KHCO3 were observed by thermo gravimetric analysis (TGA). The results obtained by TGA are depicted in Figure 3.
Figure 3 is a graph of variation in weight% of a partially regenerated adsorbent and a carbon-dioxide (CO2) laden adsorbent as a function of temperature in accordance with the present disclosure. The graph depicts that, weight loss of the partially regenerated adsorbent (represented by curve A) was approximately 2.15 weight % with respect to the carbon-dioxide (CO2) laden adsorbent (represented by curve B). It was observed that, the adsorption capacity of the activated adsorbent was 0.53 mmol CO2 per gram of the activated adsorbent (i.e., approximately 25% capacity of the adsorbent was utilized in the adsorber).
The carbon-dioxide (CO2) leaving the desorber was passed through a second cyclone separator to remove/separate the partially regenerated adsorbent. The partially regenerated adsorbent was then re-circulated to the adsorber for the adsorption of the carbon-dioxide (CO2). In order to have continuous recirculation of the adsorbent between the adsorber and the desorber, it is necessary to maintain a stable differential pressure of 100 mm H2O between the adsorber and the desorber stable. During the adsorption, temperature ranges from 68ºC to 85ºC (represented by curve A in Figure 2) and during the desorption, temperature remains constant (i.e., 150ºC, which is represented by curve B in Figure 2).
Further, the partially regenerated adsorbent retained the hydrated species (water of crystallization) and maintains lesser energetic active adsorption sites for the adsorption of carbon-dioxide (CO2), thereby enabling the adsorption process to be carried out at a higher temperature ranging from 80ºC to 100ºC. Furthermore, due to retention of hydrated species, the reaction kinetics for activating the adsorbent was enhanced. Samples of the carbon-dioxide (CO2) laden adsorbent and the partially regenerated adsorbent were collected for analysis. The results of the analysis are depicted in Figure 4.
Figure 4 illustrates the X-Ray Diffraction (XRD) pattern of an activated adsorbent and a partially regenerated adsorbent in accordance with the present disclosure. Figure 4 indicates the presence of K2CO3. 1.5 H2O and KHCO3 in the carbon-dioxide (CO2) laden adsorbent (represented by curve B) and the partially regenerated adsorbent (represented by curve A).
After each run, analysis of the gas stream leaving the adsorber was done by IR analyzer to identify the composition of the gas stream. Further, for every run, efficiency of desorption (carbon-dioxide (CO2) removal) in percentage can be calculated by the following equation:
?"CO" ?_"2" " removal efficiency = " ("C" _(??CO?_2?_"in" ) "-" "C" _(??CO?_2?_"out" ) )" × 100" /"C" _(??CO?_2?_"in" ) " "
Wherein,
CCO2in - inlet concentration of carbon-dioxide (CO2) in (vol%); and
CCO2out - outlet concentration of carbon-dioxide (CO2) in vol%.
Further, residence time of the gas stream (tgas in seconds) and the adsorbent (tsolid in minutes) in the adsorber and the desorber can be calculated by the following equation:
t_gas=V_(Adsorber or Desorber)/?_(Adsorber or Desorber) ×3600
t_solid=W_solid/CMCR×60
Wherein,
VAdsorber or Desorber - volume of the adsorber and the desorber in m3;
?Adsorber or Desorber - volumetric flow-rate of the gas stream and the fluid in (m3/h) in the adsorber and the desorber respectively;
Wsolid - weight of the adsorbent in kg; and
CMCR - circulation rate of the adsorbent in kg/h.
Example 2 – Effect of different velocities of the gas stream.
Figure 5 is a graph of variation in carbon-dioxide (CO2) removal in percentage as a function of gas stream velocity in accordance with the present disclosure. The process as described herein-above was carried out with different gas velocities such as 0.06 m/s, 0.08 m/s and 0.11 m/s having constant water-vapor (H2O) / carbon-dioxide (CO2) ratio (equal to 1.6).
It was found that, the carbon-dioxide (CO2) removal from the gas stream having velocity equal to 0.06 m/s was 76.6%, while it reduced to 66.7% at 0.11 m/s.
Thus, with increase in velocity of the gas stream entering the adsorber, the carbon-dioxide (CO2) removal from the gas stream decreased due to decrease in gas-adsorbent contact time.
Example 3 – Effect of different desorption temperatures.
Figure 6 is a graph of variation in carbon-dioxide (CO2) removal in percentage as a function of desorption temperature in accordance with the present disclosure. The process as described herein-above was carried out with different desorption temperatures viz. 130ºC, 150ºC and 200ºC.
It was found that, at a desorption temperature of 130ºC, carbon-dioxide (CO2) removal was 71.87%, while it increased to 81.33% for 200ºC.
Thus, with increase in desorption temperature, carbon-dioxide (CO2) removal increased.
Further, it was found that, at a desorption temperature of 130ºC, carbon-dioxide (CO2) removal was 71.87%, while it increased from 80.06% to 81.33% with increase in desorption temperature from 150ºC to 200ºC. It was observed that, with increase in desorption temperature from 150ºC to 200ºC; carbon-dioxide (CO2) removal did not change significantly. Hence, 150ºC can be considered as an optimum temperature for desorption.
Furthermore, carbon-dioxide (CO2) removal at 130ºC can be increased by increasing the residence time of the carbon-dioxide (CO2) laden adsorbent in the desorber.
Example 4 – Effect of different fluids on the efficiency of removal of carbon-dioxide (CO2) from the carbon-dioxide (CO2) laden adsorbent.
Figure 7 is a graph of variation in efficiency of carbon-dioxide (CO2) removal as a function of time (represented by B) and variation in percentage of carbon-dioxide (CO2) in treated gas (represented by A) as a function of time in accordance with the present disclosure.
The process as described herein-above was conducted at an adsorption temperature of 75ºC, a desorption temperature of 150ºC and for a time period of 6 hours. The composition of the gas stream entering the adsorber was 8.47 vol% carbon-dioxide (CO2), 13.06 vol% water-vapor (H2O) and rest vol % nitrogen (N2). The velocity of the gas stream entering the adsorber and the velocity of the fluid comprising carbon-dioxide (CO2) and nitrogen (N2) entering the desorber were maintained constant.
It was found that, the efficiency of carbon-dioxide (CO2) removal from the carbon-dioxide (CO2) laden adsorbent using nitrogen (N2) was 76.25%, whereas, using carbon-dioxide (CO2) (represented by B), the efficiency was 50.7%.
The efficiency of removal of carbon-dioxide (CO2) from the carbon-dioxide (CO2) laden adsorbent using carbon-dioxide (CO2) decreased due to low concentration gradient of carbon-dioxide (CO2) at the solid-gas interface.
From Figure 7, it is observed that, by changing the fluid from nitrogen (N2) to carbon-dioxide (CO2), the percentage of carbon-dioxide (CO2) in treated gas increased from 2 vol% to 4.2 vol% (represented by A).
Hence, it can be concluded that, the efficiency of removal of carbon-dioxide (CO2) using nitrogen (N2) is more than that obtained using carbon-dioxide (CO2).
Example 5- Removal of carbon-dioxide (CO2) using gas saturator.
The temperature controlled gas saturator was introduced to facilitate better control of supply of water-vapor (H2O) for the adsorption of carbon-dioxide (CO2) in the adsorber. The gas stream comprising water-vapor (H2O), carbon-dioxide (CO2) and nitrogen (N2) was passed through the gas saturator to saturate the gas stream with water-vapor (H2O). The water-vapor (H2O) content in the gas stream was varied by changing the temperature of the gas saturator.
It was found that, the concentration of the water-vapor (H2O) in the gas stream increases with increase in the temperature of the gas saturator. The process as described herein-above was carried out at an adsorption temperature of 80ºC and a desorption temperature of 150ºC. The circulation rate of the activated adsorbent and the partially regenerated adsorbent in the desorber and the adsorber respectively, was maintained constant. The effect of water-vapor (H2O) in the gas stream on the efficiency of removal of carbon-dioxide (CO2) is illustrated in Figure 8.
Figure 8 is a graph of variation in carbon-dioxide (CO2) removal in percentage as a function of concentration of water-vapor (H2O) in the gas stream in accordance with the present disclosure.
It was found that, with 6.36 vol% of water-vapor (H2O) in the gas stream, carbon-dioxide (CO2) removal was 51.7 %, whereas, with 14.77 vol% of water-vapor (H2O), carbon-dioxide (CO2) removal was 80.6 %.
Thus, with increase in the concentration of water-vapor (H2O) in the gas stream, carbon-dioxide (CO2) removal increases.
Figure 9 is a graph of variation in percentage of carbon-dioxide (CO2) in treated gas as a function of time in accordance with the present disclosure. The graph depicts that, with increase in the concentration of water-vapor (H2O) in the gas stream, the concentration of carbon-dioxide (CO2) in the treated gas leaving the adsorber decreases (curve A represents higher concentration of water-vapor (H2O) in the gas stream than curve B).
Example 6 - Effect of direct steam injection on the efficiency of adsorption of carbon-dioxide (CO2).
In order to activate the adsorbent, the steam, generated from the steam generator, was introduced in the adsorber with high precision metering pump. After activating the adsorbent, the gas stream comprising 8-10 vol% carbon-dioxide (CO2), 10-23 vol% water-vapor (H2O) and nitrogen (N2) was introduced into the adsorber. The process as described herein-above was carried out at an adsorption temperature of 80ºC and a desorption temperature of 150ºC. The residence time of the activated adsorbent and the carbon-dioxide (CO2) laden adsorbent in the adsorber and the desorber was 7.3 minutes and 3.6 minutes respectively. The circulation rate of the activated adsorbent and the partially regenerated adsorbent in the desorber and the adsorber respectively, was maintained constant.
Initially, steam (H2O) was injected at a temperature of 130ºC and at a pressure of 1.7 bar in the adsorber for activating the adsorbent,
After activating the adsorbent, the process as described herein-above was carried out using the gas stream having the ratio of water-vapor (H2O) and carbon-dioxide (CO2) as 1.67 and 1.14.It was observed that, for the gas stream having the ratio of water-vapor (H2O) and carbon-dioxide (CO2) as 1.67, the efficiency of adsorption of carbon-dioxide (CO2) was 58.3%.
Whereas, for the gas stream having the ratio of water-vapor (H2O) and carbon-dioxide (CO2) as 1.14, the efficiency of adsorption of carbon-dioxide (CO2) was 31.3%.
It was found that, due to the condensation of the steam in the adsorber and the hygroscopic nature of the adsorbent, the percentage of removal of carbon-dioxide (CO2) in both the cases was less.
Further, it was also found that, the condensation of the steam in the adsorber and the hygroscopic nature of the adsorbent cumulatively leads to difficulty which is critical for continuous circulation of the adsorbent.
TECHNICAL ADVANCEMENTS
The process for capturing carbon-dioxide from the gas stream comprising carbon-dioxide of the present disclosure described herein above has several technical advantages including but not limited to the realization of:
the adsorbent comprising potassium carbonate impregnated alumina support that is partially regenerated;
the formation of the activated adsorbent with active adsorption sites having reduced energies;
the partially regenerated adsorbent that retains water of crystallization during desorption;
the adsorption process that can be carried out at a higher temperature;
the lower temperature differential between adsorption and desorption;
a process that reduces the overall energy demand; and
a process that requires lower H2O/CO2 ratio.
Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.
The use of the expression “at least” or “at least one” suggests the use of one or more elements or mixture or quantities, as the use may be in the embodiment of the disclosure to achieve one or more of the desired objects or results.
Any discussion of documents, acts, materials, devices, articles or the like that has been included in this specification is solely for the purpose of providing a context for the disclosure. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the disclosure, as it existed anywhere before the priority date of this application.
Wherever a range of values is specified, a value up to 10% below and above the lowest and highest numerical value respectively, of the specified range, is included in the scope of the disclosure.
The numerical values mentioned for the various physical parameters, dimensions or quantities are only approximations and it is envisaged that the values higher/lower than the numerical values assigned to the parameters, dimensions or quantities fall within the scope of the disclosure, unless there is a statement in the specification specific to the contrary.