Here the calculation of the technological scheme of a membrane cascade type of «Continuous membrane column» for carbon dioxide capture from flue gases of combined heat and power plants was performed. A parametric analysis of the proposed scheme was carried out in order to determine the selectivity values of the membrane used and its area providing the optimum ratio between the purity of captured CO
2, the recovery rate and the CO
2 content in the residual stream. The goal of the process is to achieve a purity of the captured carbon dioxide ≥ 95 mol.%, the recovery rate ≥ 90 % and the concentration of CO
2 in the residual stream ≤ 2 mol.%. The flue gas parameters are listed in
Table 1. It is important to note that all further calculations were performed for the feed mixture pressure of 0.15 MPa and the pressure in the permeate side of 0.02 MPa. These values were determined based on a number of literature sources addressing the issue of CO
2 capture from CHP plant flue gases by membrane gas separation [
10,
31]. In addition, the pressure of 0.02 MPa is the minimum achievable in practice [
10], and, given the pressure of the separated mixture at 0.1 MPa, compression to a value of 0.15 MPa seems economically justified.
Figure 3 shows the process flow diagram designed in the Aspen™ Plus flowsheet screen. Here, in addition to the three membrane units (one (M1) in the stripping section and two (M2 and M3) in the enrichment section), three compressors are used: C1 to compress the feed gas mixture flow to increase the partial pressure gradient of CO
2, C2 to compress the captured CO
2 to prepare it for storage or transportation and VC - vacuum compressor to evacuate the permeate side of membrane units (M1, M2) and further compress permeated mixture prior to feed the membrane unit M3.
The key characteristics determining the appropriateness of a particular process design are the carbon dioxide content in the product and residual streams, and also the recovery rate of carbon dioxide. Therefore, it is necessary to determine the influence of the process parameters of the proposed technological scheme and their ranges available for optimization on the above-mentioned characteristics. A parametric analysis of the proposed process flow diagram was therefore carried out.
3.1. Influence of membrane selectivity on CO2 capture efficiency
The membrane CO
2/N
2 selectivity value was determined for further calculations required to achieve the key process characteristics. Carbon dioxide permeability was set at 1 000 GPU based on the parameters used in [
10] and commercial availability of membrane with similar permeability - MTR Polaris™. The calculation was performed at the ultimate values of the considered membrane area range in the stripping and enrichment sections of 83 000 and 4 500 m
2, respectively.
Figure 4 shows the influence of membrane selectivity on the carbon dioxide content in the product and residual streams withdrawn from the enrichment and stripping sections of membrane cascade, respectively. The graphs clearly demonstrate that the selectivity of the membrane has a significant influence on the process efficiency. Therefore, a membrane with a selectivity lower than 12 cannot achieve CO
2 content ≤ 2 mol. % in the residual stream and moreover, a membrane’s selectivity higher than 32 is necessary to produce a CO
2 with purity of 95 mol. % and higher. The calculation results are in good agreement with previous experiments in which a deep purification of a low permeable component and CO
2 capture was investigated [
32,
33]. With these experiments it was shown that even when using a membrane with a low selectivity (2.5) it is possible to achieve high product purity (99.997 vol.%), which corresponds to a low content of a high permeable impurity. In the case of experimental study of membrane cascade during CO
2 capture it was shown that the use of a membrane with selectivity of 8 does not allow to achieve the required product purity, namely, in the limiting ratio of the withdrawn streams from the membrane cascade sections a CO
2 purity of 91.23 vol.% was achieved. Obtained dependence for the withdrawn gas streams from the membrane cascade is explained by the fact that separation of that mixture even at the low selective membrane provides permeation of most part of carbon dioxide to the permeate side performing the process at a high stage-cut value (> 0.6). At the same time permeate of unit M1 is formed mainly by nitrogen that in its turn does not allow achieving sufficient CO
2 content to create high partial pressure drop in the enrichment section. As a result, the low driving force in the enrichment section prevents CO
2 separation with the required product purity. Increasing the selectivity of the membrane (α(CO
2/N
2) ≥ 32) solves this issue. The results obtained, firstly, are in good agreement with the results presented in [
10]; secondly, they demonstrate the possibility to use the gas transport characteristics of the MTR Polaris™ membrane for further calculations of the membrane cascade.
3.2. The effect of the membrane area
Figure 5 demonstrates the influence of the membrane area in the stripping section on the carbon dioxide recovery rate. The graphs in the figure show that the membrane area has a significant effect on the carbon dioxide recovery rate. The graphs show the mutual influence of the stripping and enrichment section membrane areas on the characteristics of the process. Thus, using a membrane area of 4 500 m
2 in the enrichment section and ~53 000 m
2 of membrane area in the stripping section is required to achieve the target CO
2 recovery rate. At the same time, by reducing the membrane area in the enrichment section, the required area in the stripping section area increases by 42.5 % to ~75 400 m
2. Furthermore, it can be seen that using the 1 500 m
2 of membrane in the enrichment section does not allow to reach the required CO
2 recovery rate in the considered range of membrane area in the stripping section. The resulting dependencies are explained by the operating principle of the membrane cascade. The stripping section membrane area determines the permeate flow and the carbon dioxide content. The low partial pressure ratio across the membrane due to a relatively low CO
2 content (17 mol. %) in the feed stream requires the processing at a high stage-cut value in the unit M1, which can be provided only by usage of the large area of the membrane at a set feed pressure of 0.15 MPa. As the stream enriched with carbon dioxide (up to 63 mol. %) enters the enrichment section of the membrane cascade, a considerably smaller membrane area is required for its capture and consequently the separation can be performed at a lower stage-cut values. However, even a small reduction of the membrane area in the enrichment section (by 1 000 m
2) leads to inefficient capture process of CO
2 and return of its substantial share to the stripping section of the cascade and increase of the required membrane area. Based on the results obtained, it is reasonable to optimise the process flow diagram by changing the membrane area in the enrichment section as a relatively small increase in its area provides significant savings of membrane material area in the stripping section.
In order to determine the range of membrane area values in the enrichment section available for membrane cascade optimisation, the effect of the membrane area in this cascade section was determined at various fixed membrane area values in the stripping section. The results are shown in
Figure 6. As can be seen from the graphs, and as noted earlier, increasing the membrane area in the enrichment section provides a significant increase in CO
2 recovery, but this approach is only effective with more than 52 780 m
2 of membrane area in the stripping section. For smaller membrane areas in the stripping section, increasing the membrane area in the enrichment section does not help to achieve a ≥90% CO
2 recovery rate. This is because the smaller membrane area in the stripping section does not ensure sufficient carbon dioxide enrichment of the stream entering the enrichment section. This in turn does not allow the creation of the necessary partial pressure ratio to recover more than 90% CO
2 in the enrichment section of the membrane cascade, even at high values of the stage-cut provided by increasing the membrane area. Thus, in terms of capital costs, further optimisation is advisable with a membrane area of 52 780 m
2 in the stripping section.
In order to verify the previously obtained results against another criteria - residual carbon dioxide content in the stripping section retentate stream of ≤ 2 mol %, an analysis of the effect of the membrane area used on this characteristic was performed.
Figure 7 shows the dependence of the carbon dioxide content in the residual stream of the stripping section on the membrane area in the stripping section. As in the previous case, it can be seen that both sections of the membrane cascade have an effect on achieving the separation process target characteristic of ≤ 2 mol % CO
2 in the residual stream. In contrast to the previously discussed relationship, here the entire considered range of membrane area in the enrichment section achieves the target value. Again, a small increase (across the entire membrane cascade) of the membrane area in the enrichment section allows the separation process to be performed with a significantly smaller membrane area in the stripping section, namely, implementing the process using 4 500 m
2 of membrane area in the enrichment section, the required membrane area in the stripping section is ~53 000 m
2, while reducing the enrichment section to 3 000 m
2 leads to an increase in the required area in the stripping section to 68 000 m
2. The explanation of these dependencies boils down to a discussion of earlier results. Here, the CO
2 content of the residual stream from the stripping section is determined by the amount of CO
2 withdrawn as permeate in the unit M1, i.e. the process stage-cut in this unit and the ability to capture the majority of the carbon dioxide in the enrichment section. Thus, it was found that the previously established minimum membrane area in the stripping section fully meets the requirement for the residual CO
2 content in the stripping section retentate stream. In addition, the combined results suggest that the minimum required membrane area in the enrichment section is 4 500 m
2.
The final step in determining the influence of the membrane area on the process characteristics was an analysis of the effect of this parameter on the purity of the captured carbon dioxide. The results in the form of dependence of purity of the captured CO
2 on the membrane area in the enrichment section are presented in
Figure 8. The graphs show that at all combinations of the membrane areas in stripping and enrichment sections the target purity of a product (≥ 95 mol.%) is reached. Such dependencies are explained by the fact that all values of membrane area (from the considered range) allow to concentrate the CO
2 in the stripping section enough for subsequent capture of this component with its content more than 95 mol.% in product stream. In addition, it was found that increasing the membrane area in the enrichment section leads to a decrease in the purity of the captured CO
2. This is related to an increase in the value of the stage-cut value in the M3 module. As the stage-cut value increases, and taking into account a CO
2 recovery rate of >90%, a small amount of nitrogen begins to permeate. On the other hand, as the membrane area in the enrichment section decreases, the purity of the captured CO
2 increases significantly up to 99.82 mol.% for a membrane area of 75 400 m
2 and up to 99.7 mol.% for a membrane area of 52 780 m
2 (optimum value) in the stripping section. Such results suggest the possibility of CO
2 capture of the highest grade. Taking into consideration the previously obtained results establishing the values of membrane areas in stripping and enrichment sections equal to 52 780 and 4 500 m
2 correspondingly based on the product recovery rate, it can be concluded that these parameters are optimal for the process of carbon dioxide capture from CHPP flue gases. Within the scope of the paper, the calculation was performed for the hollow fiber membrane modules.
3.3. Feasibility study for a membrane cascade type of «Continuous Membrane Column» for carbon dioxide capture from CHPP flue gases
As a result of the parametric analysis of the proposed technological scheme of the three-module membrane cascade configuration, the main technological parameters of the process were established (
Table 2).
The following formula [
10] was used to calculate the cost of CO
2 extraction per ton:
where, Cc is the cost of capture per ton of CO2, $/ton CO2; P is the power required for CO2 capture equipment, kW; T is the CHPP capacity factor (operating time per year), h/year; E is the electricity cost, $/kWh; C is the capital cost of equipment, $; is the mass flow of captured CO2, ton/h.
The capital costs of equipment is calculated from the following simplified formula:
where
Astr and
Aenr are the area of membrane used in the stripping and enrichment sections, respectively, m
2;
SM is the cost of 1 m
2 of membrane,
$/m
2 (~ 50
$/m
2 based on the MTR Polaris™ membrane manufacturer [
10]);
SC1 - cost of compressor unit C1,
$;
SC2 - cost of compressor unit C2,
$;
SVC - cost of vacuum compressor VC,
$.
Compression work is calculated according to the formula:
where
PC1,
PC2 and
PVC are the compression work of compressor C1, C2 and vacuum compressor respectively.
PC1,
PC2 and
PVC are each calculated separately according to the formula:
where Pi is compression work, kW; Lin is compressor inlet flow, mol/s; γ is adiabatic expansion coefficient of the gas mixture; R is the universal gas constant; Tin is inlet gas temperature, K; nv is compressor efficiency; Pout and Pin are compressor inlet and outlet pressures.
The adiabatic expansion coefficient of the gas mixture was calculated as follows:
where Cp and Cv are the heat capacities of the pure components at constant pressure and temperature, respectively; J/(mole K); and are the molar fractions of CO2 and N2 in the inlet stream, respectively. The heat capacity values of the pure components were obtained from the Aspen™ Properties database.
The efficiencies of the vacuum and compression parts are generally dissimilar. Therefore, a formula has been applied to calculate the efficiency, establishing a correspondence between the efficiency and the pressure ratio at the inlet and outlet of the apparatus stage:
As a result of the calculation of the compression work, it was found that PC1 = 360, PC2 = 900 and PVC = 1 270 kW. Thus, the total compression work P is 2 530 kW.
The cost of compressor equipment providing a capacity of ~100 m
3 min-1 varies over a fairly wide range, so it is reasonable to calculate the cost of this equipment through linking it to its capacity and assuming that 1 kW =
$500 [
34]. Hence, the cost of each compressor unit is: ~
$178 000,
$635 700 and
$450 600 for
C1,
VC and
C2 respectively. Taking the cost of 1 m
2 of membrane (including housing costs) to be
$50, the capital cost is ~
$4 129 350. Thus, assuming a CHPP capacity factor of 7 446 hr year
-1, and an electricity cost of
$0.04/kW, the cost of capturing a ton of CO
2 is
$31.