Analysis of CO 2 Pressure Swing Adsorption Simulation by Considering the Transport Phenomena in the Adsorber

This study focused on CO 2 separation technology with adsorption. This paper describes the analysis carried out by a CO 2 pressure swing adsorption simulation to scale up the absorber. An unsteady one-dimensional balance model was constructed by considering the material, energy, and momentum. In the CO 2 breakthrough test, the beginning time and CO 2 concentration at outlet of CO 2 breakthrough in the calculation were almost equivalent to that of experiment results. The correlation consistency of the calculation results with the analysis model and the experimental results obtained by a bench scale experiment was evaluated. The transport phenomena in the adsorber were investigated at the adsorption, rinse, and desorption steps according to the calculation results. The starting time of CO 2 breakthrough obtained by the analysis is equal to that obtained by the adsorption breakthrough experiment. This con-firms that the CO 2 adsorption, and the temperature and velocity distribution in the adsorber, change as a function of the adsorption, rinse, and desorption steps, respectively. Additionally, the CO 2 concentration of the captured gas and the amount of CO 2 quantity were 93.4% per day and 2.9 ton/day, respectively. These values are equal to those obtained by the bench scale experiment.


Introduction
Currently, global warming is considered as a global problem, and various global warming prevention measures are being developed. Carbon dioxide (CO 2 ) is How to cite this paper: Esaki, T., Kuronuma, H. and Kobayashi, N. (2021) Analysis of CO 2 Pressure Swing Adsorption Simulation by Considering the Transport Phenomena in the Adsorber. Journal of Materials Science and Chemical Engineering, 9, 39-54.
The CO 2 percentage in the exhausted gas volume has been reported to be approximately 10% -20%. Hence, it is necessary to separate CO 2 using the CCS process and collect pure CO 2 in the high state [1]. In the CCS process, the energy cost for CO 2 separation is large. Amine absorption, physical absorption, pressure swing adsorption (PSA), film separation, and low-temperature processing are considered in separation technology. Particularly, PSA is a separation technology that uses an adsorbent whereby CO 2 is attached or removed by introducing an exhaust into the adsorption tower to prioritize the absorption of CO 2 . The pressure inside the tower is decompressed in a pulsometer after adhesion balance and the CO 2 is collected.
With regard to the CO 2 adhesion pill, substantial development and proposals have been contributed by previous studies on CO 2 separation and collection technology using the PSA method (for example, Kodama et al., 2013) [2]. Additionally, the improvement of the CO 2 separation performance by PSA driving has been reported. Li et al. (2009) removed the water vapor from activated alumina, selected CO 2 as a route for separating the adhesion in zeolite 13X, and ensured the promoting effect of the CO 2 adhesion separation.
The CO 2 dissociation has practical use at large-scale facilities (1000 -10,000 ton/day) and takes place inside the shaft furnace used in steel ironwork and recovery plants. Therefore, it is assumed that the scale of the adsorption tower, wherein the adhesion pill is filled, is also large. When driving for scale-up to expand the adsorption tower, it is necessary to understand the migration phenomenon in the adsorption tower and consider it in the design to achieve a high CO 2 collection rate. In this study, the behavior inside the adsorption tower was considered in the modal PSA using numerical analysis to scale up the CO 2 adsorption tower. Choi et al. (2003) [3] investigated absorption and washing, and obtained the optimal value of the driving condition by experiment and analysis with consideration to laboratory-scale PSA, which assumes the selection of an exhausting process and considers the temperature change in the adsorption tower and the consistency between the experimental and analysis results. Anshul (2010) [4] performed a check in the tower while driving, and used numerical analysis to achieve a high CO 2 collection density.
Many PSA analyses have been reported, but only few analyses have focused on the momentum of the introduced gas. When using the CO 2 separation and recovery technique, the CO 2 amount is high at the gas stream entrance. Particularly, depending on the adhesion operation at the exit, the CO 2 gas volume de-creases. The pressure fluctuation caused by the change in the adsorption and desorption is large because it becomes a batch wise process later in the modal PSA. Additionally, the momentum of the CO 2 gas depends on the phenomena of adsorption and desorption, and disappears and appears accordingly. Therefore, it is assumed to influence the adsorption and desorption. In this study, a non-regular simultaneous equation that considers the mass, energy, and momentum was established, and an analysis model was developed. To establish each equation, the required physical properties were obtained from the literature, while unclear values were obtained by experiment.
We confirmed that the verification of analysis model by comparing to the experimental CO 2 breakthrough result. The beginning time of breakthrough to the adsorber and CO 2 concentration at outlet adsorber time variation was evaluated in the experiment and calculation. The calculation result obtained by the analysis model was larger than the experimental result for the adsorption tower obtained in the bench scale experiment; therefore, the consistency of the model was confirmed. The CO 2 in the tower and the temperature and flow distribution were considered by the numerical value more than the calculation result. Additionally, the phenomena inside the PSA adsorption tower were elucidated and the PSA driving was estimated.

Pressure Swing Adsorption
Step Figure 1 and Table 1 present the process of the PSA method considered in this study. The exhaust including CO 2 circulates gas inside the adsorption tower during the adsorption in the elevated pressure process from an art process, and CO 2 adheres to the adhesion pill. After the adhesion balance, the fluid is changed and the process shifts to the washing process. The purge process converts impurities into a dense fog and supplies CO 2 gas to the adsorption tower. The supplied CO 2 has a high degree of purity, owing to the washing process, and remains in the tower where it is decompressed during the decompression attachment and removal process. Finally, CO 2 with high attached or removed purity is collected. The adsorption in the elevated pressure, washing, and decompression attachment and removal processes assumes that the driving is successively switched over for one of the adsorption towers in the PSA method.
The phenomena in the adsorption tower caused by each process occurring in the tower are considered. The adsorption tower specifications were established by an analysis model as presented in Table 2. The equipment shape of the adsorption tower imitates the ASCOA-3 adsorption tower of JFE Engineering Corp. Considering the corrosive nature of the dense fog, and the condensation by which the gas inflowing into the CO 2 adsorption tower is SO x and NO x , we assume that gaseous H 2 O is removed. The adhesion pill uses ZEORAM F-9 obtained from Sumitomo Seika Chemicals Co., Ltd., which is a NaX-type zeolite and can be uniformly filled into the adsorption tower. Journal of Materials Science and Chemical Engineering

Analysis Model
The phenomenon of substance migration in the adsorption tower, the energy, and momentum were modeled for each ingredient, and an income and expenditure system was built. The model target is a pillar-like adsorption tower. The analysis model is a one-dimensional non-stationary model that solves the income and expenditure problem in a coalesced manner. The adsorption tower is divided and its minute volume is set to one cell. Additionally, we assumed that the movement is formed by the gradient between the cell and it neighbors. The diffusion, advective, and adhesion items are considered, and the heat conduction of the adhesion pill, advective term, and adhesion heat are also considered by the heat energy income and expenditure in the material balance. The viscosity, advective term, adhesion, and pressure loss are considered in the momentum balance. The CO 2 performs advection by the gas stream between the adsorbent particles. When only CO 2 spreads in the pore, advection is performed for the ad- The income and expenditure process and the subjunctive condition of the analysis model are discussed below.
The material balance system in the adsorber is expressed as follows: The heat energy balance system in the adsorber is expressed as follows: The momentum of gas balance in the adsorber is expressed as follows: The material balance system in the adsorbent particle is expressed as follows: Initial conditions in calculation.
The gas density and concentration in the adsorber are expressed as follows: The gas velocity and temperature in the adsorber are expressed as follows: The amount of CO 2 in the adsorbent particle is expressed as follows: Boundary conditions. 1) Inlet of adsorber.
The gas density and concentration in the adsorber are expressed as follows: 2) Outlet of adsorber.
a) Adsorption and Rinse step.
The gas density and concentration in the adsorber are expressed as follows:

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The gas velocity and temperature in the adsorber are expressed as follows: The gas density and concentration in the adsorber are expressed as follows: The gas velocity and temperature in the adsorber are expressed as follows: 3) Surface of adsorbent particle.
The amount of CO 2 adsorption on the surface of the adsorbent particle is expressed as follows: 4) Center of adsorbent particle.
The amount of CO 2 adsorption at the center of the adsorbent is expressed as follows: ( ) Subjunctive conditions. • The effect of gravity is ignored.
• The interaction of each component in the adsorbent pore is ignored.
• The density, heat capacity, and viscosity of each component are constant values.
• The momentum change is not affected by the adsorbent.
The coefficient for each transport phenomenon considered in the analysis model was obtained from previous studies [5] [6] [7]. The amount of the equilibrium CO 2 adsorption, adsorption heat, heat capacity, adsorbent density, and thermal conductivity of the adsorbent packed beds were experimentally determined. The coefficient values obtained from the numerical analysis are summarized in Table 3.

Consistency of Experimental and Simulation Results
The consistency of the experimental result obtained for the adsorption tower of ASCOA-3 was investigated with consideration to the developed analysis model described in the previous section. The analysis conditions are listed in Table 4.
The actual gas exhausted by a steel process was introduced through an experiment, and the CO 2 density of the gas exiting the adsorption tower was measured.

T. Esaki et al. Journal of Materials Science and Chemical Engineering
When the adsorption tower reached the adhesion balance, the exit density reached the entrance density. To confirm the consistency of the analysis model, the breakthrough time of the CO 2 gas was estimated. The ratio of CO 2 breakthrough calculated the density of the CO 2 gas concentration at the exit as 1.0, which is equivalent to the CO 2 gas concentration at the entrance. The changes of the exit CO 2 density in the experimental and analysis results are shown in Figure   2. The CO 2 was mixed into the supply gas at 0 s. The adhesion of CO 2 progressed throughout the experiment, and breakthrough began at 830 s. It was found thatthe ratio of CO 2 breakthrough of 90% was reached in 1000 s. The breakthrough began at 810 s according to the calculation result obtained by the analysis model, and the ratio of CO 2 breakthrough exceeded 90% at 900 s. The CO 2 density changes in the adsorption tower and the aging of the absorbed amount are shown in Figure 3 and Therefore, the actual phenomenon was accurately expressed by the analysis model.

Transport Phenomena in Adsorber
The adsorption under elevated pressure obtained by the analysis model, the washing, and the phenomena in the tower caused by the decompression attachment and removal process were considered. The driving conditions in each process are listed in Table 4. The pressure was boosted and 111.3 kPa were supplied to the gas washing process after adsorption under elevated pressure was provided by the decompression and desorption process. Therefore, the partial CO 2 pressure caused by the decompression attachment and removal process was equal to that of the washing process, and the flow rate was determined for the introduced gas differential-pressure at the tower entrance. The CO 2 pressure change and pick-up rate of 0.01 -0.99 for each process at the Ainai location are presented in Figure 5 and Figure 6. The time of 0 s was set for the decompression attachment and removal processes to complete 10 kPa. The dependence of CO 2 on the tower inflow by the adsorption in the elevated pressure process was higher than that in the analysis result, and the pressure increased. Subsequently, the pick-up rate reached the adhesion balance in 10 s at the tower entrance, and the CO 2 that was not absorbed performed advection at the lower part of the tower. From this point, there was almost no CO 2 inflow pressure and adhesion at the lower part of the tower (0.8 -1.0) at 150 s during the adsorption at the end of the elevated pressure process, and it was found that CO 2 outflow to the exterior of the tower had not occurred. In the washing process, the pressure exhibited large fluctuations in the early stage under the influence of adhesion. At the end of the washing process, the adhesion advanced to the central part of the tower at 0.5 in 200 s. The pressure rapidly decreased in the decompression attachment and removal process, and the attachment and removal progressed. The pick-up rate also provided a good indication for the progress of the attachment and removal process. However, during the decompression attachment and removal process, the absorbed amount increased at the lower part of the tower. The reasons for this are as follows: CO 2 gas was introduced by the adsorption under elevated pressure; advection from the washing process occurred at the lower part of the tower; CO 2 adhered to the lower part of the tower. Thus, the CO 2 migration phenomena induced by each process are elucidated. The aging changes for the N 2 density in the adsorption tower are shown in Figure 7. The adsorption under elevated pressure shows that the N 2 density increased over time at the adsorption tower entrance by the introduction of pressurized gas. Moreover, it was found that, during the washing process, the N 2 density at the exit, which is the lower part of the tower, was higher than that at the entrance. The N 2 that entered the tower with the introduction of CO 2 is a washing gas that spreads the advection throughout the lower part of the tower and then flows out through the tower exit. The N 2 amount that remains in the tower after the washing process is sufficiently smaller compared with the N 2 amount that remains after the process of adsorption under elevated pressure. Therefore, the N 2 amount left by the process of decompression attachment and removal is small, and CO 2 gas with high purity can be collected.    The filling layer temperature in the adsorption tower was considered. The temperature at each filling layer location is indicated by the aging changes in Figure  8. In this process, the larger temperature difference began to change, but this influenced the adsorption and desorption heat. Compared with the tower entrance, the temperature in the tower increased more gradually with the progress of the adhesion caused by the adsorption in the elevated pressure process. Additionally, the adhesion progressed by the introduction of CO 2 gas in the washing process, and the temperature increased. At this time, the CO 2 adhesion caused by the high pressure was completed at location 0.15 in the tower. The temperature decreased to the adhesion heat caused by the heat transfer and advection to the lower part of the tower. Therefore, the temperature decreased at the end of the washing process. Subsequently, attachment and removal were performed in the decompression attachment and removal process, and the temperature decreased simultaneously. The time interval required for the temperature to change by 12 K at most, even if sufficient time is provided, was investigated. The adsorption and desorption heat calculated from the absorption isotherm obtained in this analysis was sufficiently small, compared with the adhesion heat induced by other adhesion pills and adhesion matter, because the adhesion pill considered in this study was combined with CO 2 . Based on the absorption isotherm, the dependence of the absorbed amount is also small, and it is assumed that each fixed number does not depend on temperature. In this analysis, the temperature change did not influence the driving of the adsorption tower, and it was assumed that it is possible to drive while maintaining satisfactory performance.
The flow distribution at each location of the adsorption tower is shown in Figure 9. The flow rate that existed throughout is shown in Figure 10. Compared with the flow distribution, the adsorption exhibited significant fluctuations in the elevated pressure, washing, and decompression attachment and removal processes at the adsorption tower entrance. Notably, the adsorption under the elevated pressure process was 0.12 m/s in 100 s and 0.12 m/s by 0.03 m/s; the decompression attachment and removal process occurred at 300 s; the washing process occurred at 200 s. For the process to change, more flow distribution is caused by the advection of gas, and the phenomena of adsorption, desorption, and decompression occurs. Because it was put in the flow rate, it decompressed to the adsorption under the elevated pressure in the attachment and removal process. The absolute value was small. Additionally, N 2 existed because it accompanied a substantial amount of CO 2 in the gas collected by the decompression attachment and removal process and with adsorption under elevated pressure. The momentum that formed in the adsorption under the elevated pressure process spread throughout the adsorption at the lower part of the tower during the washing process, and CO 2 and N 2 flowed out. In the decompression attachment and removal process, the current velocity was reversed around the entrance to allow the current to be pulled in by the pulsometer through the adsorption tower entrance. However, the momentum that remained after the washing process performed advection at the lower part of the tower. Therefore, Journal of Materials Science and Chemical Engineering    in the decompression attachment and removal process, CO 2 was pulled in by the pulsometer and collected at the entrance of the upper part of the tower. At the lower part of the tower, N 2 and CO 2 fluid flow occurred at the exit.

CO2 Concentration and Recovery Ratio
With regard to the CO 2 density and CO 2 collection rate, the electric power of the pulsometer yielded twice as much and the CO 2 with the driving of the adsorption tower compared with that considered in the analysis. This result was compared with the experimental result obtained for the adsorption tower by the bench scale experiment. The analysis results and experimental results are presented in Table 5. The CO 2 density is the average CO 2 density of the gas obtained during the decompression attachment and removal process. The CO 2 collection rate is the percentage of CO 2 collected in the decompression attachment and removal process to the CO 2 amount introduced in the adsorption process under elevated pressure. The pulsometer power can be used to calculate the electric power of the pump from the suction pressure and flow rate used in the decompression attachment and removal process. The amount of collected CO 2 , which is later introduced to the driving through a set of adsorption towers, was standardized in one day. Value equal to the experimental results were mostly obtained in the analysis results. However, the CO 2 density was approximately 90% higher than the analysis result. In the adsorption under elevated pressure and washing processes, the CO 2 did not flow through the tower exit because the remaining N 2 was collected in the pulsometer along with CO 2 during the decompression attachment and removal process. It was found that 24% of the CO 2 that was introduced by adsorption under elevated pressure while driving in the decompression attachment and removal process of 10 kPa could not be collected at the current rate of collection. Compared with the analysis result, this is more depended on the outflow to adhesion and outside of the tower caused by the lower part of the tower in the decompression attachment and removal process. The pulsometer power and C, which will later be present in industrial applications, need improvement of the O twice yield. It is necessary to consider that the quantity of the introduced gas flow, and the diameter and height of the adsorption tower, influence the time of each process in the driving of the PSA cycle. With the model used in the analysis, the collected amount increased because 24% of CO 2 was not collected. From this point onward, the design factor for scaling up, by which we assume that it is possible to increase the throughput, and the sensitivity analysis results were obtained.

Conclusions
This study carried out CO 2 separation with consideration to scaling up and developed a PSA method for CO 2 collection. A model was built for the income and expenditure system of the substance in the adsorption tower, and the heat, momentum, and migration in the tower were estimated by numerical analysis. The physical properties of the analysis model were obtained by experiment, and the consistency between the experimental result, which was obtained using a bench-scale adsorption tower, and the developed analysis model was investigated. The following conclusions were drawn from this study: 1) The migration phenomenon of the substance in the tower, the energy, and the momentum were expressed in the C programming language, and the analysis model of a possible PSA route was established. The CO 2 density at the adsorption tower, absorbed amount, temperature, and flow distribution in the PSA driving process were investigated, and the influence on the CO 2 collection was confirmed to be higher compared with that in the analysis result.
2) The breakthrough opening time in the CO 2 adhesion breakthrough test was 830 s, the CO 2 density in the tower was balanced, and breakthrough was initiated. This result is identical to the opening time in the bench scale experiment.
3) With a CO 2 concentration of 93.4% in the captured gas and a CO 2 quantity of 2.9 ton/day, twice as much CO 2 was obtained from the density of CO 2 collected by PSA driving. This value is approximately equal to the value of the adsorption tower in the bench scale experiment. Additionally, this model can be useful as an analysis tool in future scale-up studies.