Process description of advanced configurations to improve the energetic performance

https://doi.org/10.1016/j.seppur.2021.118959

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As mentioned above, the main contribution to the overall energy consumption of solvent-based CO₂ capture processes is the thermal energy required in the CO₂ desorber, where CO₂ is stripped off with high purity and the CO₂-lean solvent is regenerated for re-use in the CO₂ absorber. In the case of NH₃-based capture processes applied to cement plants, steam has been reported to account approximately for 30% of the cost of CO₂ capture [40]. Consequently, the advanced configurations developed in this work aiming at improving the energetic performance of NH₃-based capture processes have focused on the reduction of the reboiler duty in the CO₂ desorber. One advanced configuration of the CO₂ desorber and one advanced configuration of the solvent recovery section specific of NH₃-based capture processes have been studied in detail in this work, namely the RecVC and the VIS. These have been combined with the best performing advanced configurations of the CO₂ desorber available in literature for solvent-based capture processes, as long as they are compatible with NH₃-based capture processes; namely, the LVC and the MPD have been identified as the most promising advanced configurations of the CO₂ desorber for NH₃-based capture processes. As a result of the combination of different advanced concepts of the CO₂ desorber and of the solvent recovery section, ten different combinations of advanced configurations aiming at improving the energetic performance of NH₃-based CO₂ capture processes have been studied in detail. All of them are able to decrease the steam requirements for solvent regeneration in the CO₂ desorber, at the cost of increasing the electricity consumption and/or the requirements of low temperature excess heat that might be available in the CO₂ point source. All advanced configurations in this section have been implemented upon the process configuration shown in Fig. 7 using the IS (Configuration C2), since its performance has been proven to be superior to that of the separated NH₃ desorber and appendix stripper, as it has been shown in Section 4.2.2. Nevertheless, the advanced configurations presented in this section could also be implemented on processes using the separated NH₃ desorber and appendix stripper where the vapour distillate streams from both columns are mixed. Similarly, the different advanced configurations of the FG post-conditioning section described in Section 4.1.1 are also compatible. The different types of advanced configurations that aim at improving the energetic performance of NH₃-based capture are illustrated in Fig. 12, where only the relevant process sections, i.e. CO₂ desorber section and solvent recovery section, are shown. Each of them are described below.

Configuration E1 with the RecVC, where the vapour distillate stream obtained in the IS is compressed and recycled to the bottom of the CO₂ desorber, instead of being sent to the bottom of the CO₂ absorber. In terms of capital costs, recycling the distillate stream obtained in the solvent recovery section to the bottom of the CO₂ absorber is the most efficient solution as it does not require additional equipment. Nevertheless, the vapour distillate obtained in the IS is a concentrated stream in CO₂ and NH₃, i.e. around 30 and 25 wt%, respectively, hence the utilization of these high concentrations in the CO₂ desorption section would seem more appropriate than a dilution in the absorption section. Accordingly, Sutter et al. [13] proposed to mix the vapour distillate stream obtained in the NH₃ desorber with the CO₂-rich liquid stream leaving the CO₂ absorber, before it is pumped to the pressure of the CO₂ desorber, assuming that the energy demand of a direct compression of the vapour stream would be too high. However, the option of mixing a vapour with a liquid stream would require a vessel providing enough residence time to completely dissolve the vapour distillate stream into the CO₂-rich liquid stream, in order to avoid cavitation in the pump positioned downstream. Additionally, the positive impact on the energy performance of NH₃-based capture processes when mixing the vapour distillate of the IS with the CO₂-rich stream is expected to be negligible due to the fact that the mass flowrate of the former only represents up to 1.5% of the mass flowrate of the latter. On the contrary, the mass flowrate of the IS vapour distillate may represent up to 50% of the steam mass flowrate required for solvent regeneration in the reboiler of the CO₂ desorber. Considering that the compression of the vapour distillate will increase its temperature above the solvent regeneration temperature, it will deliver heat of enough quality to be used in the reboiler of the CO₂ desorber, thus allowing for useful heat integration and leading to a decrease of the steam requirements for solvent regeneration. Therefore, the vapour distillate of the IS is compressed up to the pressure of the CO₂ desorber in the case of the RecVC. A multi-stage compressor is used with isentropic efficiency of 85% and driver efficiency of 95%, as defined in literature [33], [39]; four compression stages, each of them with the same compression ratio, are selected in order to make the equipment versatile for a broad range of outlet pressures, namely between 7.5 and 67.5 bar. Such multi-stage compressor configuration allows for stage-wise compression ratios ranging between 1.6 and 2.9. Inter-cooling of the vapour in between compression stages is carried out by means of the CO₂-rich stream leaving the rich/lean heat exchanger with the aim of maximizing heat integration before entering the CO₂ desorber. The temperature of the vapour has been given a lower limit in each inter-cooling stage to avoid condensation, using as a threshold the boiling point of pure water at the corresponding pressure.

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Fig. 12. Flow diagrams combining advanced configurations of the solvent recovery section and of the CO₂ desorber section. The operating variables that govern the performance of each configuration, i.e. the decision variables of the optimization problem, have been colour-coded in

for the solvent recovery section and in

for the CO₂ desorber operating conditions. Other important process variables whose values have been fixed for our simulations or depend on some of the decision variables have been introduced in lighter colours for each process section, i.e.

and

, respectively. The pump of the purge solvent stream and the heat exchanger of the solvent recovery section are represented in

 in those configurations where the piece of equipment might be required depending on the process operating conditions. (For interpretation of the references to colour in this figure caption, the reader is referred to the web version of this article.)

In addition to the influence of the operating conditions of the CO₂ absorber section, which affect the performance of all process sections, the energetic assessment of the RecVC depends on the operating conditions of the CO₂ desorber section and of the solvent recovery section. Namely, the pressure of the CO₂ desorber, PCO2des [bar], and the specific mass flowrate of vapour distillate recycled from the IS to the bottom of the CO₂ desorber, FRecVC [kgtCO2captured−1], determine both the energy savings in the reboiler of the CO₂ desorber and the additional electric power required by the new multi-stage compressor. The mass flowrate of vapour distillate exiting the top of the IS can be modified by changing the number of equilibrium stages of the column and/or the feeding stage of the inlet streams; the stripper reboiler duty is thus determined to meet the maximum NH₃ concentration allowable in the bottoms stream. In addition, the optimal value of the cold-rich split fraction of the RSS, fcr [-], that minimizes the energy performance of the capture process depends on the pressure of the CO₂ desorber, as pointed out elsewhere [14].

Configuration E2 with the LVC, where the hot CO₂-lean stream leaving the reboiler of the CO₂ desorber is expanded to lower pressure. The resulting vapour phase is re-compressed up to the CO₂ desorber pressure and introduced again into the bottom of the CO₂ desorber, while the liquid CO₂-lean stream exiting the L-V separator is sent to the rich/lean heat exchanger. In order to take advantage of synergies with the RecVC, the use of the same multi-stage compressor is proposed. Therefore, different alternatives of the LVC are possible depending on the pressure to which the hot CO₂-lean stream is expanded. In this work, we have considered the following variations of the LVC: (i) Configuration E2a (LVC(4)) where the hot CO₂-lean stream is expanded to the inlet pressure of the last stage of the multi-stage compressor; (ii) Configuration E2b (LVC(3)) where it is flashed to the inlet pressure of the second-to-last stage of the compressor; (iii) Configuration E2c (LVC(4,3)) where the hot CO₂-lean stream is expanded in two steps, first to the inlet pressure of the last stage of the multi-stage compressor, and then to the inlet pressure of the second-to-last compression stage; and (iv) Configuration E2d (LVC(4,3,2)) where it is flashed in three steps down to the inlet pressure to the second stage of the multi-stage compressor. The vapour streams resulting from the saturated liquid expansion are mixed with the vapour distillate from the IS being compressed in the multi-stage compressor (the same concept would apply in the case of using independent NH₃ desorber and appendix stripper). Due to the lower temperature level of the vapour resulting from the expansion of the lean stream, no additional cooling is needed. Depending on the pressure of the liquid stream exiting the last L-V separator, the split stream purged from the solvent cycle might have to be compressed up to 7 bar in order to avoid the partial vaporization of the cold stream entering the heat exchanger of the solvent recovery section, thus the possibility of a temperature crossover. For the sake of brevity, Fig. 12 only shows configurations E2a and E2d; then, configurations E2b and E2c can be easily drawn following the description provided above. Other derivations of the LVC such as one-step expansion to the inlet pressure of the second stage of the compressor or below, or four-step expansion down to atmospheric pressure have not been included in this study due to low performance; the additional demand of electrical work required for compression is far from being compensated by the energy savings in the reboiler of the CO₂ desorber.

Configuration E3 with the MPD, where the CO₂ desorber is divided in two different sections with the upper section operating at higher pressure than the lower section (and the reboiler). The vapour stream exiting the top of the lower section of the CO₂ desorber is compressed to the pressure of the upper section. A pressure ratio of 2 between the upper and the lower section of the MPD, thus in the compressor placed between the two sections of the CO₂ desorber, rMPD [-], has been selected in this work, following the guidelines provided in literature for solvent-based capture processes [29]. Before entering the bottom of the upper CO₂ desorber section, the compressed vapour stream transfers heat to the CO₂-rich stream in a counter-current heat exchanger. The latter stream enters then the top of the lower section of the CO₂ desorber, together with the condensate of the compressed vapour stream obtained in the L-V separator. The cold stream of this heat exchanger, i.e. the CO₂-rich stream, which enters as a boiling liquid, has considerably greater flowrate than the hot stream, i.e. the compressed vapour stream, so that it experiences a minor increase in temperature. Therefore, the pinch point temperature of this heat exchanger is reached at the cold-side. The upper section of the CO₂ desorber contains 6 equilibrium stages, while the lower section consists of 3 stages plus the reboiler; the number of equilibrium stages of the MPD and the feed stages are thus kept as in the CO₂ desorber of the benchmark state-of-the-art configuration.

As in the case of the RecVC, both the LVC and the MPD require finding the trade-off between additional electrical work required for vapour compression and the associated energy savings achieved in the CO₂ desorber. In this work, both advanced configurations have been assessed in combination with the RecVC, so that the operating conditions of the solvent recovery section will affect the performance of both the LVC and of the MPD. The MPD has not been tested in combination with the LVC because the lower pressure levels of the lower section of the MPD are expected to hinder the effect of the LVC [29].

The RecVC, the LVC and the MPD are expected to be promising advanced configurations for NH₃-based processes due to the broad range of solvent compositions and pressures that are allowed in the CO₂ desorber. Other advanced configurations of the CO₂ desorber available in literature that have been reported to improve significantly the energetic performance of solvent-based capture processes [29], [58], such as the RVC, the IHD, the Stripper Overhead Compression (SOC) or the Multi-effect Desorber (MED), have not been considered in this study for NH₃-based capture processes due to four reasons: (i) worse performance when applied to NH₃-based capture processes; (ii) incompatibilities with the heat integration network provided by the rich/lean heat exchanger and the RSS included in the benchmark CAP configuration; (iii) incompatibilities with the control of the NH₃ concentration in the CO₂ stream exiting the top of the CO₂ desorber; and/or, (iv) increasing drastically the process complexity without improving significantly the energy performance.

Configuration C3 with the VIS, where the IS operates at sub-ambient pressures in order to decrease its reboiler temperature to values below 100 °C, which might enable the use of increasing amounts of excess heat available at the CO₂ point source. The heavy and the light key components in the separation carried out in the IS are H₂O and NH₃, respectively, instead of being NH₃ and CO₂, as in the case of the separation in the CO₂ desorber. Therefore, while the minimum specific equivalent work associated with the solvent regeneration in the CO₂ desorber has been found at pressures around 20 bar and above [14], the stripping of NH₃ (and CO₂) from water has been reported to be favoured energetically under vacuum [59]. The VIS requires a vacuum pump that withdraws the vapour distillate from the stripper and guarantees pressures below atmospheric within the column, which increases the electrical work required by the capture process. Aiming at maximizing its impact on the overall energetic performance of the capture process, the VIS is implemented in combination with the RecVC, which allows for a trade-off between: (i) the high temperature steam required in the CO₂ desorber for solvent regeneration, (ii) the low temperature thermal energy required in the VIS for the recovery of NH₃ (and CO₂), and (iii) the electricity required in the multi-stage compressor of the RecVC and in the vacuum pump of the VIS. Contrary to operating the CO₂ desorber at vacuum conditions, the VIS allows to operate the CO₂ desorber at the optimal pressure level that minimizes the steam requirements for solvent regeneration as well as the CO₂ compression work.

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