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Experimental Evaluation of the Efficiency of Membrane Cascades Type of “Continuous Membrane Column” in the Carbon Dioxide Capture Applications

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Abstract

The performance of a three-module single-compressor membrane cascade of the “continuous membrane column” type in separation of a ternary gas mixture close in composition to power plant flue gas (N2/O2/CO2 = 84/9.6/6.4 vol %) has been experimentally evaluated. Within the scope of the study, the operation of each of the sections of the device, stripping and enrichment, has been analyzed and the relations between the compositions of gas stream taken out of these sections and the ratio of flow rate of these streams to the feed flow rate of the membrane cascade have been determined. In addition, the effectiveness of carbon dioxide capture has been assessed. The CO2 purity achieved was as high as 91 vol %. The prospects of using the device under study for capturing carbon dioxide from power plant flue gas have been demonstrated.

INTRODUCTION

Carbon dioxide (CO2) is a greenhouse gas, and an increase in its concentration in the atmosphere disturbs the thermoregulation of the planet and is the main cause of global warming. It is important to note that the increase in carbon dioxide in the atmosphere is primarily due to anthropogenic emissions and the main source of carbon dioxide emissions is the energy industry, in particular, the combustion of fossil fuels (for example, coal- and natural gas-fired cogeneration (CHP) plants). During the past century, the concentration of CO2 in the atmosphere increased from 275 to 387 ppm. Such an increase in CO2 concentration has already led to a noticeable increase in temperature on the planet. The developed climate models show that the established trend will dramatically change the global climate by 2100 [1].

One approach to reducing CO2 emissions into the atmosphere is carbon sequestration or carbon capture and storage (CCS), the process consisting of CO2 capture from an emission source, transportation, and long-term disposal in geological formations. This method involves the capture of carbon dioxide in places of its production—directly in the CHP process flow diagram—and its subsequent long-term storage. To date, three main approaches have been developed to capture CO2 generated as a result of CHP operation:

(1) postcombustion CO2 capture from CHP flue gas;

(2) precombustion CO2 capture; and

(3) oxy-fuel combustion, which involves the preliminary recovery of oxygen from air before combustion, to ensure a high concentration of carbon dioxide in flue gas and thereby make its practically ready for sequestration.

At present, the main traditionally applied approach to the capture of carbon dioxide is the postcombustion capture of CO2 from flue gas of thermal power plants by chemical absorption using amines (amine scrubbing) [2]. However, despite the fact that this approach has demonstrated its effectiveness and is used in the processes of carbon dioxide removal from gas systems with a low content of the target component [3], this technology is characterized by a number of serious drawbacks [4] including high energy costs, corrosion of pipelines and equipment, high investment costs, loss of absorbent solution due to its degradation, and potential environmental hazards. In addition, as noted in a number of works, the amine scrubbing system used to capture 90% CO2 in flue gas would require about 30% of the energy produced by the CHP plant so that the cost of CO2 capture would be $ 40–100/t CO2, thereby leading to a significant increase in the cost of electricity generated by this plant by 50–90% [5, 6]. In connection with this, a tendency has been formed to search for and develop new technologies characterized by energy efficiency and consistent with the basic “green chemistry” principles.

The most promising alternative to the chemical absorption method are technological solutions based on the membrane gas separation method, since these processes are characterized by energy efficiency [79], ease of scaling, environmental friendliness, and the possibility of “tuning” the process by selecting the most efficient membranes and varying the configurations of membrane devices. Attracted by the economic and environmental advantages of membrane technology, many researchers [1013] focused on two areas of investigation, the search and creation of new materials (membranes) [1419] and the designing of new membrane processes and devices [2024].

In the field of designing of new membrane processes and devices, studies have been conducted to evaluate the efficiency of carbon dioxide separation using spiral-wound and hollow-fiber membrane modules [25]. It was shown that unlike hollow-fiber membrane modules, which are widely used in natural gas treatment [2629] and characterized by low cost and large membrane area, spiral-wound membrane modules exhibit higher performance in carbon dioxide removal from flue gas because of smaller pressure loss along the membrane in modules of the spiral-wound configuration [30]. Zhao et al. [31] theoretically considered the process of carbon dioxide capture from power plant flue gases for a single-membrane process. They showed that the use of a single membrane module does not ensure sufficient efficiency of the separation process to concentrate carbon dioxide to ≥95 vol %. The same research group [32] made a theoretical assessment of various two-module membrane cascades using one and two compressors. It was shown that a single-compressor, two-module membrane cascade provides an economic advantage over chemical absorption using amines in the range of carbon dioxide concentrations in the target gas stream from 50 to 70 vol %. In the case of concentrating carbon dioxide over 90 vol %, no economic benefits were observed.

In this study, we propose a new configuration of the membrane device for carbon dioxide capture from CHP flue gas—a three-module membrane cascade of the “continuous membrane column” type. Earlier, the authors, both theoretically and experimentally, demonstrated the performance of such a device in deep purification of a gas with a low concentration (<2 vol %) of highly permeable impurities and performed a comprehensive experimental study of the operation of the device in the process of separation of gas mixtures [3335]. The configuration considered is a single-compressor three-module membrane cascade, with one module in the stripping section and two modules in the enrichment section. The efficiency of carbon dioxide removal from CHP flue gas was evaluated by using as an example a model ternary gas mixture (N2/O2/CO2), similar in composition to the flue gas of a cogeneration plant. The main goal of the study was an experimental evaluation of the efficiency of concentrating carbon dioxide, which is taken out from the enrichment section, as well as an assessment of the efficiency of cleaning air components (N2 and O2) taken off from the stripping section of the device. A commercially available MDK-3 gas separation membrane with low CO2/N2 and CO2/O2 selectivity values of 8 and 4, respectively, was used. The considered membrane device has proved effective in the tasks of concentration of carbon dioxide from CHP flue gas (up to 91.6 vol % CO2 content in the permeate stream) at a low relative recovery of the product.

MATERIALS AND METHODS

To determine the efficiency of separation of gas mixtures in the steady-state mode of operation of the three-module configuration of the membrane column apparatus, a gas mixture was prepared that was similar in composition to CHP flue gas (N2/O2/CO2 in the ratio of 84/9.6/6.4 by volume). The mixture was obtained using the volumetric method in a 40-L cylinder under a pressure of 60 atm. To prepare model gas mixtures and perform gas chromatographic analysis, a number of individual pure gases He (99.9999 vol %), N2 (99.9995 vol %), O2 (99.999 vol %), and CO2 (99.99 vol %) were purchased from the Research Institute NII KM (Moscow, Russia).

The gas separation membranes used in the study were a composite membrane MDK-3 with a selective layer of a 5 μm thickness, purchased from Vladipor (Russia), and a synthesized 10-μm symmetric membrane based on quaternized chitosan. The MDK membrane is a film of an organosilicon block copolymer deposited on an ultrafiltration membrane UVFK reinforced with a polyester fiber nonwoven providing it with the necessary mechanical strength. The organosilicon block copolymer Carbosil is a block copolymer of oligocarbonate with a-oligobischloroformatesiloxane. The quaternized derivatives were prepared using crab shell chitosan (Bioprogress, Russia) with a molecular weight of 8.7 × 104–2.5 × 105 and a degree of deacetylation of 80%, without additional purification and potato starch (Verkhovochi starch plant, Belarus), GOST 7699-78, with a molecular weight of 1.5 × 105. Exhaustive alkylation (quaternization) of chitosan (starch) was carried out in aqueous CH3COOH (NaOH) solutions containing glycidyltrimethylammonium chloride (GTMAC) for 8 h at 85°C in an inert atmosphere. The chitosan (starch)/GTMAC molar ratio varied in the range from 1 : 1 to 1 : 3. The products of the syntheses were isolated by precipitation in cold acetone, purified to remove unreacted GTMAC by washing three times with methanol, and dried to constant weight. The product yield determined gravimetrically reached 40%.

As a preliminary study, the gas transport characteristics were determined and the ideal and apparent (effective) selectivities of the MDK-3 membrane and the symmetric membrane based on quaternized chitosan were calculated. The permeability of individual gases was measured by the Daynes–Barrer time lag method [36, 37] detailed in previous papers of the authors [3840]. To determine the permeability of the components of the gas mixture and calculate the effective selectivity of the MDK-3 membrane, the gas chromatographic method was used as described in detail in [41, 42]. The experimentally determined gas transport characteristics of the membrane are given in Tables 1 and 2.

Table 1.   Permeability of the MDK-3 membrane and the symmetric membrane based on quaternized chitosan (2% q-CTS), measured for individual gases and gas mixture components
Table 2.   Ideal and effective selectivity of the MDK-3 membrane and the symmetric membrane based on quaternized chitosan (2% q-CTS)

Based on the results of a preliminary study of the gas transport properties of these membranes, we found that the symmetric membrane based on quaternized chitosan is not suitable for further evaluating the efficiency of separation of gas mixtures in a cascade of the continuous membrane column type because it showed poor gas transport properties (both permeability and selectivity) for the gas systems under consideration.

The gas mixture separation efficiency of the “membrane column” apparatus was evaluated on an experimental setup, a schematic diagram of which is shown in Fig. 1; the process flow sheet is shown in Fig. 2. The separation system consists of three countercurrent radial membrane modules, one in the stripping section and two in the enrichment section. The system represents a closed volume of connected pipelines, made of stainless steel 12Kh18N10T, and F-4 fluoroplastic seals. As shutoff valves, Swagelok nonlubricated stainless steel valves with fluoroplastic seal were used. The unit is equipped with a KNF N145STE vacuum pump (Germany). Flow rate measurement and control were carried out using Bronkhorst EL-FLOW Prestige FG-201CV gas flow meters (the Netherlands). Both stripping and enrichment sections are connected to a Chromos GC-1000 gas chromatograph (Russia) equipped with a thermal conductivity detector for analyzing the composition of the mixture taken off from these sections of the membrane unit. In addition, the unit is equipped with a sampling port to monitor changes in the composition of the gas mixture in the recycle gas stream.

Fig. 1.
figure1

Schematic diagram of the experimental setup for evaluating the gas mixture separation efficiency in a steady-state mode of operation of the membrane column: (1) membrane module of the stripping section, (2, 3) membrane modules of the enrichment section, (4) pressure reducing valve, (5) pressure gauges, (6) gas flow controllers, and (7) recycle stream sampling port.

Fig. 2.
figure2

Process flow chart.

The main elements of the presented scheme are countercurrent radial membrane modules (Fig. 3). A nonporous polymer membrane is installed between two clamping flanges made of stainless steel 12Kh18N10T. The membrane divides the cell into a high-pressure (HPC) and a low-pressure compartment (LPC). To ensure mechanical strength, the membrane on the LPC side is supported by a porous stainless steel substrate. Particular attention was paid to sealing the edges of the tested polymer films. During the experiments, concentric seals made of fluoroplastic were used. The sealing of the edge of the film is achieved by preloading the sample using concentric protrusions and grooves made in the corresponding places of the module. The effective diameter of the membrane is 25.8 cm, which corresponds to an area of 522.8 cm2. The effective working volume of the high-pressure compartment is in the 1-mm gap between the distribution disk and the membrane, making approximately 50 cm3. It is important to note that the separation element 3 in the enrichment section has a number of differences from modules 1 and 2, including a smaller active membrane area (14 cm2) and the absence of a distribution disk in the high-pressure compartment.

Fig. 3.
figure3

Schematic of the radial membrane module.

EXPERIMENTAL

Prior to experiments, the membrane was prepared by purging with an inert gas to remove traces of solvent and other substances adsorbed on the membrane and was pumped to a vacuum of 1 × 10−2 Pa. Helium (≥99.9999 vol %) was used as the purge gas.

The gas mixture was leaked into the pre-evacuated system from a cylinder at the feed inlet point at a constant pressure, maintained with a pressure-reducing valve (4) at the levels of 0.15 and 0.3 MPa, and mixed with the gas stream leaving the element in the enrichment section (2). The pressure in the low-pressure compartments of membrane elements 1 and 2 was maintained at 0.02 and 0.04 MPa (abs), which corresponds to a pressure differential of 0.13 and 0.26 MPa, respectively. The gas passed through the membranes in elements 1 and 2 is removed using a constant-speed vacuum pump and sent to element 3. Pressure in both the compartments is monitored using pressure gauges (5). Both streams, from the stripping and enrichment sections, are regulated with a gas low controller (6) and pass through the mixing chamber before they enter the flow-through metering valve of the gas chromatograph.

The composition of the sample was determined using gas chromatography. The separation of the components was carried out under isothermal conditions on two chromatographic columns packed with molecular sieves (for the separation of nitrogen and oxygen) and Porapak Q (for the separation of carbon dioxide and air components). The concentration of the components was determined with the thermal conductivity detector. The gas chromatographic analysis procedure is detailed in [43]. The carrier gas was helium of 99.99999+ (vol %) purity. Detailed information on the conditions of the gas chromatographic analysis is presented in Table 3.

Table 3.   Chromatographic analysis conditions

RESULTS AND DISCUSSION

Performance Analysis of the Stripping Section

In the framework of this work, the gas mixture separation efficiency in a three-module configuration of the “membrane column” device operated in the steady-state mode was experimentally assessed. The experimental study was carried out on the example of carbon dioxide capture from a mixture similar in composition to thermal power plant flue gas. The efficiency was determined as the concentration of the more permeable component (carbon dioxide) in the take-off streams of the stripping and enrichment sections. It is noteworthy that the separation of the gas mixture in the membrane column occurs in such a way that the gas stream taken off from the stripping section mainly consists of the less permeable component (in this case, nitrogen), i.e., is depleted in the easily permeable component. In turn, the gas stream taken off from the enrichment section contains a concentrate of the easily permeable component.

To evaluate the performance of both sections, relative quantities (\(F_{{{\text{str}}}}^{{{{{\text{N}}}_{{\text{2}}}}}},\)\(F_{{{\text{str}}}}^{{{{{\text{O}}}_{2}}}}\), and \(F_{{{\text{str}}}}^{{{\text{C}}{{{\text{O}}}_{{\text{2}}}}}}\)) were used. For the extraction section, the ratio of the concentration of the less permeable component in the take-off stream of the stripping section \(C_{{{\text{str}}}}^{{{{{\text{N}}}_{{\text{2}}}}}}\) to its concentration in the feed stream \(C_{{{\text{feed}}}}^{{{{{\text{N}}}_{{\text{2}}}}}}\) (Eq. (1)) and the ratio of the concentration of more permeable components in the feed stream (\(C_{{{\text{feed}}}}^{{{{{\text{O}}}_{{\text{2}}}}}}\) and \(C_{{{\text{feed}}}}^{{{\text{C}}{{{\text{O}}}_{{\text{2}}}}}}\)) to their concentration in the stream taken off from the stripping section (\(C_{{{\text{str}}}}^{{{{{\text{O}}}_{{\text{2}}}}}}\) and \(C_{{{\text{str}}}}^{{{\text{C}}{{{\text{O}}}_{{\text{2}}}}}}\)) (Eqs. (2), (3)) depending on rstr, which is the ratio of flow rate from the stripping section lstr to feed flow rate lfeed (Eq. (4)), were used. In the case of determining the performance of the enrichment section (\(F_{{{\text{enr}}}}^{{{{{\text{N}}}_{{\text{2}}}}}},\)\(F_{{{\text{enr}}}}^{{{{{\text{O}}}_{2}}}}\), and \(F_{{{\text{enr}}}}^{{{\text{C}}{{{\text{O}}}_{2}}}}\)), we used the ratio of the concentration of more permeable components in the take-off stream of the enrichment section \(C_{{{\text{enr}}}}^{{{{{\text{O}}}_{2}}}}\) and \(C_{{{\text{enr}}}}^{{{\text{C}}{{{\text{O}}}_{2}}}}\) to their concentration in the feed stream \(C_{{{\text{feed}}}}^{{{{{\text{O}}}_{2}}}}\) and \(C_{{{\text{feed}}}}^{{{\text{C}}{{{\text{O}}}_{2}}}}\) (Eqs. (5), (6)) and the ratio of the concentration of the less permeable component in the feed stream \(C_{{{\text{feed}}}}^{{{{{\text{N}}}_{{\text{2}}}}}}\) to its concentration in the take-off stream of the enrichment section \(C_{{{\text{enr}}}}^{{{{{\text{N}}}_{{\text{2}}}}}}\) (Eq. (7)) depending on the ratio of the flow rate from the enrichment section to the feed flow rate (8).

((1))
((2))
((3))
$${{r}_{{{\text{str}}}}} = \frac{{{{l}_{{{\text{str}}}}}}}{{{{l}_{{{\text{feed}}}}}}},$$
((4))
((5))
((6))
((7))

The results of the experimental evaluation of the gas mixture separation efficiency of the membrane column system are presented in Figs. 4–6. As noted earlier, the stream taken from the stripping section mainly consists of the less permeable component, since depletion in the more permeable component occurs in this part of the system. The plots in Fig. 4 show that the highest nitrogen concentration in the stream from the stripping section is achieved with a minimum value (from the considered range) of the ratio of the flow from this section to the feed flow. An increase in the ratio of these flows leads to a decrease in the concentration of nitrogen in the stream from the stripping section. This dependence is due to several factors. First, an increase in the flow rate taken from the stripping section at the flow rate from the enrichment section kept unchanged has to lead to an increase in the feed flow (as follows from the mass balance equation), which in turn leads to an increase in the amount of impurity substance introduced into the device. Second, as was shown earlier [33], the highest concentration of the hardly permeable component in the take-off stream from the stripping section is achieved under the following conditions: at the maximum flow from the enrichment section (to withdraw most of the easily permeable component from the system) or at the ratio of flow from the stripping section to feed flow close to zero. Thus, it is logical that with an increase in the amount of gas withdrawn from the stripping section, the concentration of the hardly permeable component in this stream decreases.

Fig. 4.
figure4

Dependence of the ratio of nitrogen concentration in the retentate stream of the stripping section to the concentration in the feed stream on the ratio of the flow rate from the stripping section to the feed flow rate at a pressure differential across the membrane of 0.26 and 0.13 MPa.

Fig. 5.
figure5

Dependence of the ratio of oxygen concentration in the feed stream to the concentration in the retentate stream of the stripping section on the ratio of the flow rate from the stripping section to the feed flow rate with a pressure differential across the membrane of 0.26 and 0.13 MPa.

Fig. 6.
figure6

Dependence of the ratio of carbon dioxide concentration in the feed stream to the concentration in the permeate stream takeof the stripping section on the ratio of the flow rate from the stripping section to the feed flow rate at a pressure differential across the membrane of 0.26 and 0.13 MPa.

The plots in Figs. 5 and 6 show that the dependence of the concentration ratios of the more permeable components (relative to nitrogen) upon the ratio of the take-off flow to the feed flow is opposite in character. With an increase in the flow rate of the take-off stream from the striping section at the flow rate from the enrichment section kept unchanged, the concentration of both easily permeable components (oxygen and carbon dioxide) in the stripping section increases. As noted above, with an increase in the take-off flow rate from the stripping section, the feed flow rate also increases, leading to an increase in the amount of substance of the easily permeable components in the system. In order to achieve a minimum concentration of these components in the gas stream from the stripping section, it is necessary to ensure the maximum removal of the easily permeable gases from the stripping section or, in other words, to find a compromise between the two take-off streams. Otherwise, a negative effect is observed, consisting in the return of the more permeable components to the recycle stream (where their concentration exceeds their content in the feed stream) of the membrane unit and mixing with the feed stream, which ultimately leads to excessive “contamination” of the gas mixture entering the stripping section. Thus, it can be concluded that in order to achieve the lowest concentration of the more permeable components in the take-off stream from the striping section, it is necessary to ensure the withdrawal of these components from the system through the enrichment section and to prevent these components from concentration at the point of entry of the feed mixture due to mixing of the streams.

It is important to note that the range of concentration ratios for oxygen and carbon dioxide varies by several orders of magnitude, while their concentrations in the gas mixture are commensurable (9.6 and 6.4 vol %, respectively). This is due to the CO2/N2 and CO2/O2 selectivity values of the membranes used, which are 8 and 4, respectively. Since the separation of the gas mixture occurs in three membrane modules, and the recycle flow is many times the take-off flows from both sections, a multiplicative separation is realized in this device. Therefore, it is the difference in the membrane selectivity that is responsible for the significant range of oxygen and carbon dioxide concentration ratios. Thus, when the ratio of the flow rate from the stripping section to the feed flow rate is 0.55, the nitrogen, oxygen, and carbon dioxide concentrations achieved are 98.6, 1.398, and 0.0013 vol %, respectively, at a pressure difference of 0.26 MPa. In the case of operation of the column at a differential pressure of 0.13 MPa across the membrane, the achieved concentrations of these components are 97.77 vol % for nitrogen, 2.22 vol % for oxygen, and 0.0015 vol % for carbon dioxide.

Performance Analysis of the Enrichment Section

Figures 7–9 present plots of the ratios of concentrations of the gas mixture components in the take-off stream from the enrichment section to their concentrations in the feed stream. As noted above, the membrane column system operates in such a way that the more permeable components of the gas mixture concentrate in the enrichment section. In the case of separation of a mixture consisting of nitrogen, oxygen, and carbon dioxide in the enrichment section, oxygen and carbon dioxide will accumulate and nitrogen depletion will occur.

Fig. 7.
figure7

Dependence of the ratio of nitrogen concentration in the feed stream to the concentration in the stream taken from the stripping section on the ratio of the flow rate from the stripping section to the feed flow rate at a pressure differential across the membrane of 0.26 and 0.13 MPa.

Fig. 8.
figure8

Dependence of the ratio of oxygen concentration in the stream taken from the enrichment section to the concentration in the feed stream on the ratio of the flow rate from the stripping section to the feed flow rate at a pressure differential across the membrane of 0.26 and 0.13 MPa.

Fig. 9.
figure9

Dependence of the ratio of carbon dioxide concentration in the stream taken from the enrichment section to the concentration in the feed stream on the ratio of the flow rate from the stripping section to the feed flow rate at a pressure differential across the membrane of 0.26 and 0.13 MPa.

Figure 7 shows the plot of the ratio of nitrogen concentration in the feed stream to the concentration in the take-off stream from the stripping section versus the ratio of the take-off flow rate from the stripping section to the feed flow rate. It can be seen from the plot that with an increase in the flow from the stripping section, the amount of nitrogen in the stream taken from the enrichment section decreases. This change is due to the fact that an increase in the flow rate from the stripping section at the flow rate from the enrichment section kept unchanged increases the flow rate of the gas mixture fed to the system. Thus, the amount of substance of the more permeable components increases, leading to an increase in the concentration of these components both in the recycle stream and in the enrichment section. An increase in concentration entails an increase in the concentration gradient through the membrane in membrane module 3 from which the product of the enrichment section is withdrawn, leading, in turn, to the preferential transport of the more permeable penetrating components across the membrane. Nitrogen, which is the least permeable gas for the given membrane, predominantly does not penetrate into the low-pressure compartment of this module and is taken out back to the system in the recycle stream. As can be seen from the graph, for the ratio of the flow rate from the stripping section to the feed flow rate in the range from 0.55 to 0.82, the nitrogen content in the stream from the stripping section remains high (≤~45 vol %) and decreases significantly, starting from the value of 0.87 for this ratio. This is due to the fact that in the indicated range of flow rate ratios, there is no excessive concentration of carbon dioxide in the recycle stream because of the effective removal of this component in the enrichment section, so that the mixing of the stream having an increased concentration of carbon dioxide from the enrichment section with the feed stream at the gas mixture injection point can be precluded.

Figure 8 shows the dependence of the ratio of oxygen concentration in the stream taken from the enrichment section to the concentration in the feed stream on the ratio of the take-off flow rate from the stripping section to the feed flow rate. It was found that the dependence is nonmonotonic in nature. In other words, when the ratio of the flow rate from the stripping section to the feed flow rate is in the range from 0.55 to 0.82, oxygen accumulates in the take-off stream from the enrichment section and the concentration of this component reaches its maximum. With a further increase in the flow rate ratio, the oxygen concentration in the stream withdrawn from the enrichment section decreases. Moreover, the oxygen concentration in this stream exceeds its content in the gas mixture in the entire considered range of flow rate ratios, indicating that it concentrates in this section of the apparatus, in contrast to nitrogen, which showed a monotonic decrease in concentration. The initial increase and the subsequent decrease in oxygen concentration is explained by the ability of the membrane system in question to effectively concentrate carbon dioxide in the enrichment section, which leads to the gradual displacement of other components of the feed gas mixture from the enrichment section of the device. Thus, it is the accumulation of the most permeable component (carbon dioxide) in the recycle stream and in the high-pressure compartment of the modules of the enrichment section that determines the possibility of concentrating the less permeable component.

Figure 9 presents a plot of the ratio of carbon dioxide concentration in the stream taken from the enrichment section to the concentration in the feed stream versus the ratio of the take-off flow rate from the stripping section to the feed flow rate. In contrast to the plot of the dependence of changes in oxygen concentration in the enrichment section, the change in carbon dioxide content in this part of the membrane system is monotonic and is accompanied by an increase in concentration. In the 0.55–0.82 range of the ratio of the flow taken from the stripping section to the feed flow, there is a smooth increase in the carbon dioxide concentration, which is explained by the effective removal of this component from the enrichment section and the absence of the effect of excessive concentration in the recycle stream of the device. With an increase in the ratio of the flow from the stripping section to the feed flow and going beyond the upper bound of the previously considered range of values, the rate of change in the CO2 concentration is characterized by a sharp increase. This behavior of the dependence is explained by an increase in the amount of substance introduced into the system due to an increase in the feed flow rate, which leads to an increase in the concentration of carbon dioxide in both the recycle stream and the enrichment section of the device. Thus, carbon dioxide excessively concentrates in the recycle stream of the system, thereby entailing the emergence of two interrelated effects: first, a sharp increase in the carbon dioxide concentration in the stream taken from the enrichment section, which leads to the conclusion that the device can effectively concentrate the more permeable component in this part of the system; second, because of the small membrane area in module 3, from which the permeate stream of the enrichment section is taken out, it is not possible to withdraw all the carbon dioxide concentrate, which leads to mixing of the carbon dioxide-rich stream leaving the enrichment section with the feed stream at point I of introducing the mixture into the system. The enrichment of the feed mixture in the carbon dioxide that has not been removed from the system results in the “contamination” of the take-off stream from the stripping section, as has been shown above in the discussion the performance of the stripping section.

Evaluation of Performance of the Three-Module Membrane Column in Carbon Dioxide Capture from CHP Flue Gas

Figure 10 shows the dependence of carbon dioxide concentration in the stream withdrawn from the enrichment section upon the ratio of the take-off flow rate from the stripping section to the feed flow rate. In the case of separation of a gas mixture that is similar in composition to CHP flue gas, it is interesting from an applied point of view to study the possibility of using such an apparatus in the task of carbon dioxide capture from flue gases. It is known that in order to solve this task, it is necessary to obtain carbon dioxide with a purity of at least 95 vol % at the product to feed flow ratio of ~0.01, which means at rstr = 0.99 in the case of the membrane column and the values selected previously for analysis. From Fig. 10 it is seen that the dependence curve monotonically increases with increasing rstr. Moreover, as already noted, a smooth increase in the carbon dioxide concentration in the range of rstr values from 0.55 to 0.82 is replaced by a sharp increase, which continues up to rstr = 0.95, which corresponds to a carbon dioxide concentration of 77 vol %. At the next point in the plot, corresponding to an rstr value of 0.98 and a carbon dioxide concentration of 91 vol %, there is a “bend” of the curve, which corresponds to the maximum possible concentration of carbon dioxide in the apparatus at a pressure differential of 0.26 MPa. Figure 11 shows the dependence of the concentration of carbon dioxide in the stream withdrawn from the stripping section upon the ratio of the flow rate from the stripping section to the feed flow rate. The plot shows that in the same range of rstr values from 0.55 to 0.95, the concentration of carbon dioxide does not exceed 0.5 vol %, indicating that the stripping section provides effective depletion of the stream in the more permeable component. At rstr of 0.98, there is a sharp increase in the concentration of carbon dioxide in the take-off stream from the stripping section, corresponding to 4.6 vol %. It is noteworthy that at a pressure differential of 0.13 MPa across the membranes, similar trends are observed in both sections of the membrane system with a lower carbon dioxide content in both take-off streams. This indicates that at such a ratio of flow rates, the stripping section of the device does not provide sufficient depletion of the stream in the easily permeable component, thereby leading to loss of the target component carbon dioxide and a decrease in the relative recovery of the product (up to 20%). This problem can be solved in several ways: by organizing a recycle of the stream taken from the stripping section and returning to the feed inlet point, which will require the use of an additional compression unit (due to the pressure loss of the mixture along the membrane element); an increase in the membrane area in the stripping section, which will cause an increase in the gas flow through the membrane and lead to a loss of vacuum in the space downstream of the membrane and a corresponding increase in the power of the vacuum pump to provide the necessary vacuum level for maintaining the separation efficiency in this membrane module; and using membrane materials with significantly higher selectivity (more than 50 for the CO2/N2 system and more than 25 for the CO2/O2 system), which are currently being developed in the world exactly for the task of carbon dioxide capture.

Fig. 10.
figure10

Dependence of carbon dioxide concentration in the stream taken from the enrichment section on the ratio of the flow rate from the stripping section to the feed flow rate at a pressure differential across the membrane of 0.26 and 0.13 MPa.

Fig. 11.
figure11

Dependence of carbon dioxide concentration in the stream taken from the stripping section on the ratio of the flow rate from the stripping section to the feed flow rate at a pressure differential across the membrane of 0.26 and 0.13 MPa.

CONCLUSIONS

Thus, in the framework of the study, an experimental assessment has been made of the efficiency of the separation process in a three-module membrane cascade of the “continuous membrane column” type for the N2/O2/CO2 ternary mixture, which is close in composition to flue gases, in a steady-state mode of operation of the column with continuous withdrawal of the separated components. The operation of the stripping and enrichment sections depending on the ratio of flows in the column and at various pressure differentials has been analyzed. The dependences of the composition of the take-off gas streams from the membrane system on their ratio to the feed stream have been determined. The possibility of concentrating carbon dioxide from a mixture up to 90 vol % in the enrichment section of the column using a commercially available MDK-3 organosilicon membrane with an effective CO2/N2 selectivity of 8 has been shown. The recommendations are given for optimizing the given configuration of the membrane device to achieve a high relative recovery of carbon dioxide.

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Funding

The study was supported by the Russian Science Foundation, grant no. 18-19-00453.

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Correspondence to I. V. Vorotyntsev.

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Translated by S. Zatonsky

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Atlaskin, A.A., Trubyanov, M.M., Yanbikov, N.R. et al. Experimental Evaluation of the Efficiency of Membrane Cascades Type of “Continuous Membrane Column” in the Carbon Dioxide Capture Applications. Membr. Membr. Technol. 2, 35–44 (2020). https://doi.org/10.1134/S2517751620010023

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Keywords:

  • membrane gas separation
  • membrane cascade
  • single-compressor apparatus
  • carbon dioxide