https://doi.org/10.1016/j.seppur.2021.118959
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Advanced configurations that simplify the PFD with respect to the reference CAP configuration aim at reducing the investment costs associated with NH3-based capture processes. This objective is pursued by decreasing the number of units of the capture process without affecting negatively its energetic performance and the fulfilment of the process specifications and constraints. Therefore, the different process sections and unit operations of the reference CAP configuration shown in Fig. 1 have been analysed in terms of goals, operating conditions and feed streams in order to identify pieces of equipment that: (i) either can be integrated within another unit operation, or (ii) can be removed from the capture process; in addition, both cases might require the adaptation of the boundary conditions and/or the redefinition of the constraints of certain process sections. As a result, advanced configurations of the reference CAP that do not include the CO2-WW section or the appendix stripper, or that integrate the appendix stripper with the NH3 desorber have been identified. Further details for each of them are provided below.
Configuration B1 without the CO2-WW column, where the CO2 stream that leaves the top of the CO2 desorber is directly cooled down before being sent to compression; the condensate stream produced in the cooler of process section B is sent to the solvent recovery section as in the reference configuration shown in Fig. 1. Consequently, the operating conditions of the CO2 desorber need to be adapted in order to still meet the constraints downstream in the process, i.e. yNH3CO2comp,in below 50 ppmv and no solid formation in the condensate of the CO2 stream before the CO2 compression section, which will affect the specific reboiler duty of the CO2 desorber thus the energetic performance of the capture process [14]. Therefore, the configuration without the CO2-WW column is assessed by means of multi-variable sensitivity analyses whereby the influence of the CO2 desorber decision variables on the process performance is studied. As a result, the optimal set of operating conditions of the CO2 desorber, i.e. PCO2des and fcr, that minimize ω while meeting the process specifications and constraints provided in Table 2 are obtained for each cˆNH3 case.
Configuration C1 without the appendix stripper, whose PFD is shown in Fig. 6. In addition to the removal of the appendix stripper, Configuration C1 includes an additional packing section at the top of the column in the FG pre-conditioning section that further cools down the FG to values below ambient temperature, referred to as Direct Contact Chiller (DCCh) hereinafter. The DCCh uses chilled water for cooling purposes, which can reach values as low as 1.5 °C, instead of cooling water at 21.2 °C. The purpose of the DCCh is to reach temperatures of the FG entering the CO2 absorber as low as those of the CO2– and NH3-depleted FG exiting the FG-WW column in order to minimize water accumulating in the solvent circulating between the CO2 absorber and the CO2 desorber. As a consequence, the purge of solvent from the CO2 absorber-CO2 desorber loop is avoided, which is the source of most of the NH3 and CO2 recuperated in the appendix stripper. Therefore, the appendix stripper is removed and other inlet streams with minor content of NH3 (and CO2), i.e. the purge stream from the CO2-WW column and the condensate of the CO2 stream before compression, are fed to the CO2 desorber instead. Since the flowrates of the latter streams are two and three orders of magnitude smaller than the flowrate of the cold-rich bypass, respectively, their feed stage to the CO2 desorber has a minor/negligible effect on both the energy consumption of the process and the control of NH3 emissions within the CO2 stream; in order to minimize the number of packing sections in the CO2 desorber and considering their temperature, we have chosen to feed both streams together with the hot CO2-rich stream. The temperature of the FG entering the CO2 absorber can be controlled by means of the liquid-to-gas flowrate ratio for the chilling water and the inlet FG to the capture process, (L∕G)FG−DCCh [kg kg−1], in combination with the temperature of the water entering the DCCh, TwaterFG−DCCh [°C]. Increasing values of (L∕G)FG−DCCh make the temperature of the FG entering the CO2 absorber to approach asymptotically TwaterFG−DCCh. Therefore, we have set the value of (L∕G)FG−DCCh to 2.0 kg kg−1 so that further increments do not have any effect on the FG temperature. Subsequently, TwaterFG−DCCh is set to the maximum value that avoids the accumulation of water in the solvent circulating between the CO2 absorber and the CO2 desorber, thus leading to a flowrate of purged solvent, Fpurge [kgtCO2captured−1], that is equal to zero. As for the chilling considered in the FG-WW column, the minimum temperature achievable has been set to 1.5 °C. Then, the performance of the configuration without appendix stripper is assessed by analysing the impact on the energy needs of the process and on the consumption of aqueous NH3 solution.
Fig. 6. Flow diagram of Configuration C1 without the appendix stripper and including the DCCh at the top of the DCC in the FG pre-conditioning section for the chilling of the FG before entering the CO2 absorber. The remaining process sections, i.e. (A) the FG post-conditioning section, (C) the solvent recovery section with the NH3 desorber, and (G) the CO2 compression section, are omitted for the sake of visuality but are those included in the reference CAP configuration shown in Fig. 1. The variables that govern the performance of the configuration, i.e. the process parameters that are varied in the sensitivity analysis, and the process constraints have been colour-coded in
and
, respectively. (For interpretation of the references to colour in this figure caption, the reader is referred to the web version of this article.)
Configuration C2 with the IS, whose PFD is shown in Fig. 7, integrates the NH3 desorber and the appendix stripper in one single column. Such integration is possible as long as the NH3 desorber and the appendix stripper have the same specifications, i.e. both columns produce: (i) a gaseous distillate at the top of the column that recovers most of the NH3 and (almost all the) CO2 contained in the column’s feed stream, and (ii) an almost pure water liquid stream at the bottom of the column. While the former is always fulfilled, the latter feature is common to both columns if the apparent NH3 concentration in the NH3-lean stream is up to 150 ppmwt (equivalent to 0.0088 molNH3kgH2O−1). Although the suitability of the IS depends on the operating conditions of the FG-WW column, it is independent of the configuration of the FG-WW column and the FG post-conditioning section. Therefore, the FG post-conditioning section of the reference configuration considered in the full PFD shown in Fig. 7 can be exchanged for any of the advanced configurations of the FG post-conditioning section described in Section 4.1.1. On the one hand, the high-pressure stream resulting from the mixing of the solvent purged from the CO2 absorber-CO2 desorber loop, the water stream purged from the CO2-WW section and the condensate of the CO2 gas stream are fed to the top of the IS after exchanging heat with the bottom stream of the column, as for the appendix stripper. On the other hand, the NH3-rich stream enters the IS at the top half of the column, as in the case of the NH3 desorber. The process improvement introduced by the IS is based not only on the decrease of the investment costs due to the elimination of one piece of equipment with respect to the reference configuration, but also on a competitive energy performance. The latter is highly affected by the heat integration implemented upstream. The high-pressure stream fed to the top of the column flashes when entering the IS, which by default operates at atmospheric pressure, thus cooling down the top of the column and serving the purpose of the condenser of the NH3 desorber. The heat of the bottom stream, which leaves the IS approximately at 100 °C (almost pure water at atmospheric pressure), is used to heat up the high-pressure stream fed to the top of the IS. The pinch point temperature in the counter-current heat exchanger is reached between the inlet hot stream (bottom stream of the IS) and the outlet cold stream (high-pressure stream fed to the top of the IS): The high pressure of the latter stream, whose flowrate is considerably lower than that of the IS bottom stream, prevents its vaporization within the heat exchanger. Then, the NH3-lean hot outlet stream, still approximately at 90 °C, is able to heat up the NH3-rich stream before entering the top of the FG-WW column. Aiming at minimizing the refrigeration duty associated with the chilling of the NH3-lean stream, a fraction of the hot NH3-lean stream is purged before entering the NH3-rich/NH3-lean heat exchanger. In order to also minimize the reboiler duty of the IS, the value of the aforementioned split fraction is set to equalize the heat capacity of both inlet streams to the NH3-rich/NH3-lean heat exchanger.
Fig. 7. PFD of Configuration C2 with the IS. Notice that the NH3 desorber and the appendix stripper of the solvent recovery sections have been integrated in one single column, i.e. the IS. The implementation of the IS is shown in combination with the FG post-conditioning section of the reference configuration. However, the block of the latter can be exchanged for any of the advanced configurations of the FG post-conditioning section shown in Fig. 2.
The advanced Configuration C2 with the IS is assessed by analysing its impact on the energy requirements of the process and on the consumption of water and aqueous NH3 solution, with respect to the reference configuration with the appendix stripper and the NH3 desorber. As far as the energy consumption associated with the IS is concerned, the column configuration is of paramount importance, i.e. the number of stages and the feed stages. On the one hand, increasing number of equilibrium stages decreases the reboiler duty of the IS towards an asymptotic minimum value. On the other hand, while the reboiler duty of the IS is always minimized when the high-pressure stream is fed to the top of the column, as mentioned above, the optimal feed stage of the NH3-rich stream depends on the total number of equilibrium stages. Aiming at providing a fair assessment of the IS with respect to the reference configuration, the same number of equilibrium stages as for the NH3 desorber of the reference configuration, i.e. nine including the condenser and the reboiler of the column, has been selected. Furthermore, no significant energetic improvements are found for a larger number of equilibrium stages. Therefore, the appendix stripper is effectively removed in terms of investment costs with respect to the reference configuration. For such column configuration, the optimal feed stage of the NH3-rich stream to the IS that minimizes its reboiler duty is equilibrium stage 2. Additionally, the energetic performance of the IS is also affected by the operating conditions of the CO2 absorber, of the CO2 desorber and of the FG-WW column. The effect of the CO2 absorber decision variables and of the CO2 desorber decision variables on the reboiler duty of the IS is similar to their effect on the reboiler duty of the appendix stripper for the reference configuration. On the contrary, the value of the FG-WW column parameters might have to be modified with respect to the reference configuration with the NH3 desorber if the apparent NH3 concentration in the NH3-lean stream is above the maximum allowable value, i.e. 150 ppmwt for this work, since a split has to be purged. Therefore, the performance of the IS has been compared with the performance of the configuration that includes both the appendix stripper and the NH3 desorber for different configurations of the FG post-conditioning section. The most promising advanced configurations of the FG post-conditioning section, according to the results shown in Section 4.1.2, have been considered, namely Configuration A1 with the SNA and Configuration A3 with the ANA, as well as the reference configuration of the FG post-conditioning section with the SNA and the acid-wash column included in Fig. 1. The sets of operating conditions of the FG-WW column parameters that are used for the assessment of Configuration C2 with the IS are provided in Table 6 for each configuration of the FG post-conditioning section and cˆNH3 case, i.e. low, mid and high.
The same values of the FG-WW column parameters are considered for the corresponding processes using the same advanced configuration of the FG post-conditioning section but with both the appendix stripper and the NH3 desorber. These sets correspond to the optimal operating conditions determined qualitatively from the results shown in Fig. 5 (see Section 4.1.2). As far as the reference configuration of the FG post-conditioning section is concerned, it has been combined with the IS at two different sets of operating conditions of the FG-WW column decision variables: (i) At the optimal operating conditions determined qualitatively from the results shown in Fig. 3 (see Section 4.1.2); and, (ii) at the operating conditions that allow for the minimum specific energy consumption of the process, thus at low FG-WW column productivity values. For the latter, the set of operating conditions of the FG-WW column reported in Table 3 have been used for the reference configuration with the appendix stripper and the NH3 desorber; on the contrary, a new set that minimizes the energy consumption for the same productivity value has been found instead for the advanced configuration with the IS in order to constrain the apparent NH3 concentration of the NH3-lean stream to the maximum allowable content specified in Table 2 for liquid purge streams.
Table 6. Operating conditions of the FG-WW column selected for the assessment of the IS in combination with different configurations of the FG post-conditioning section. The set of operating conditions selected for each configuration of the FG post-conditioning section corresponds to the pareto points labelled as “Opt” in Fig. 3, Fig. 5 (see Section 4.1.2). A new “Base” set of operating conditions of the FG-WW column of the reference FG post-conditioning section has been determined for its combination with the IS in order to meet the apparent NH3 concentration constraint in the purge stream (see Table 2) while minimizing the energy requirements for the same FG-WW column productivity reported in Table 3 for each cˆNH3 case. As for the remaining process operating conditions, i.e. CO2 absorber parameters and CO2 desorber parameters, the values selected for the assessment of Configuration C2 with the IS are those reported in Table 3 for each cˆNH3 case.
| Variable | Units | Configuration of the FG post-conditioning section and cˆNH3 case | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Empty Cell | Empty Cell | Ref | A1 (SNA) | A3 (ANA) | |||||||||
| Empty Cell | Empty Cell | Base | Opt | Opt | Opt | ||||||||
| Empty Cell | Empty Cell | low | mid | high | low | mid | high | low | mid | high | low | mid | high |
| cˆNH3FG−WW×103 | [molNH3kgH2O−1] | 8.81 | 8.82 | 8.80 | 8.80 | 8.83 | 8.82 | 5.67 | 7.18 | 8.69 | 8.82 | 8.82 | 8.81 |
| (L∕G)FG−WW | [kgkg−1] | 0.100 | 0.100 | 0.100 | 0.104 | 0.113 | 0.103 | 0.128 | 0.125 | 0.113 | 0.100 | 0.105 | 0.110 |
| TleanFG−WW | [°C ] | 15.7 | 16.6 | 5.7 | 7.6 | 7.9 | 8.1 | 1.6 | 1.5 | 3.9 | 7.6 | 8.1 | 1.5 |
| hleanFG−WW | [mm−1] | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 | 0.81 | 0.82 | 0.82 |
| (Lbot∕G)FG−WW | [kgkg−1] | 0.195 | 0.197 | 0.126 | 2.08 | 2.25 | 2.05 | 2.57 | 2.50 | 2.26 | 2.00 | 2.10 | 2.19 |
| TpaFG−WW | [°C ] | 12.5 | 13.6 | 1.5 | 1.5 | 1.5 | 1.5 | 1.5 | 1.6 | 1.5 | 1.5 | 1.5 | 1.5 |
| hpaFG−WW | [mm−1] | 0.53 | 0.46 | 0.58 | 0.50 | 0.53 | 0.50 | 0.20 | 0.27 | 0.25 | 0.29 | 0.28 | 0.29 |
| (L∕G)WW | [kgkg−1] | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0.163 | 0.163 | 0.163 |
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