7.2.1
Base Case Conditions
The base case for the analysis of the standalone regenerator is a counter-current regenerator with supported amine adsorbent. Pure steam is used as the regen- erator gas at the inlet. 96% of adsorbed CO2 is removed in the base case regenerator. A diagram of the single regenerator is given in Figure 7.1 and values of the operating and simulation parameters that were used are given in Table 7.1.
The same vessel size as Fisher et al. (2005) for the regenerator in the amine absorption process was used for the standalone regenerator of the adsorbent. Fisher et al. (2005) used one absorber for one regenerator and a similar config- uration is used here where one regenerator is used to regenerate the adsorbent coming from an adsorber. The flowrate of steam used as the regeneration gas was chosen such that 96% of CO2 is removed from the adsorbent. In this mov- ing bed regenerator, steam is fed directly into the column whereas in amine absorption, steam is supplied to a reboiler which heats the amine solution.
Although the same packing is assumed in the regenerator as for the adsorber, the hold-up of solid in the regenerator is higher than in the base case adsorber as hold-up was found to increase for higher adsorbent superficial velocities, according to Equation 3.49.
The pore concentrations of CO2, Cp,COin 2, water, C in
p,H2O , and N2, C in p,N2 are the same as those at the outlet of the base case adsorber (Chapter 6). The
CHAPTER 7. ANALYSIS OF A REGENERATOR 160
Figure 7.1: Single Regenerator (Counter-current) incoming adsorbent temperature, Tin
ads, has been chosen to be high enough so that steam in the regeneration gas does not condense on the adsorbent.
The same LDF constants as for the base case adsorber have been assumed for the base case regenerator. Due to insufficient experimental data available for calculating ki values, the same value as for the base case adsorber has been assumed and the effect of a variation in this mass transfer coefficient was investigated.
Finally, the solver choices for the number of discretisations and the weight factor, α, are the same as for the base case adsorber in Chapter 6.
7.2.2
Profiles for the Regenerator Base Case
The CO2 loading increases slightly as adsorbent enters the regenerator at x = 10 m (cf. Figure 7.2) because the gas contains the highest concentration of CO2 at the gas outlet and the temperature of the adsorbent is lowest as it enters the regenerator (cf. Figure 7.3). However, as the temperature of the adsorbent in the regenerator increases and the concentration of CO2 in the gas drops as the adsorbent leaves the regenerator (at the gas inlet in Figure 7.4), the
Table 7.1: Parameters used for the base case regenerator
Parameter Unit Value
L m 10 DC m 6 εa - 0.092 εp - 0.4 εpk - 0.01 Pin Pa 101325 Tin g ◦C 100 ˙ Min mol.s−1 1000 ˙ min kg.s−1 375 yin CO2 - 0 yin H2O - 1 yin N2 - 0 kCO2 s −1 10 kH2O s−1 10 kN2 s−1 10 Aext m2 0 Cin p,CO2 mol.m −3 2.06 Cin p,H2O mol.m −3 4.36 Cin p,N2 mol.m −3 30.46 qin CO2 mol.kg −1 0.45 qin H2O mol.kg −1 0.95 qin N2 mol.kg −1 0 Tin ads ◦C 89.0 Number of discretisations - 100 α - 0.75
CO2 loading drops as the adsorbent reaches the bottom of the counter-current adsorber, as shown in Figure 7.2. Steam is used as the regeneration gas and it is adsorbed therefore the loading of water increases. The amount of adsorbed N2remains at zero because it is assumed that this component is non-adsorbable but it is present within the pores of the incoming adsorbent. Profiles of the mole fraction of CO2 water and N2 in the base-case regenerator are shown in Figure 7.4. Because CO2 is desorbed, its mole fraction in the gas increases. On the other hand, water is adsorbed so its mole fraction drops in the bulk gas.
Temperature profiles of the adsorbent and gas are shown in Figure 7.3. The solid temperature increases as it passes through the regenerator. As ad- sorbent enters the column at a lower temperature than the regeneration gas, its temperature increases to meet the gas temperature. In addition, there is a release of heat in the regenerator because the overall amount of CO2 and
CHAPTER 7. ANALYSIS OF A REGENERATOR 162
Figure 7.2: Loading profiles for a counter-current regenerator with amine sup- ported adsorbent
Figure 7.3: Temperature profiles for a counter-current regenerator with amine supported adsorbent
water adsorbed increases as shown by Figure 7.5. This also contributes to the overall rise in temperature of the solid and gas to above the steam and solid inlet temperatures. A similar trend in the adsorbent temperature profile was found for a counter-current regenerator of zeolite 13X (Kim et al. (2013a)). The temperature of the adsorbent used increased as it was indirectly heated with steam.
The pressure drop profile for the regenerator is shown in Figure 7.6. A lower pressure drop is found in the regenerator than for the adsorber consid- ered in Chapter 6 (Figure 6.6) because the velocity of the gas is higher in the
Figure 7.4: Mole fraction profiles for a counter-current regenerator with amine supported adsorbent
Figure 7.5: Profiles of flue gas molar flowrate for a counter-current regenerator with amine supported adsorbent
CHAPTER 7. ANALYSIS OF A REGENERATOR 164
Figure 7.6: Pressure profile for a counter-current regenerator with amine sup- ported adsorbent