Initial Bed Temperature

However, at initial bed temperatures above -120 C, the specific cooling duty will increase strongly. This is related to the decreasing amount of CO2 being captured, which will decrease exponentially above -120 C. For example, when feeding a N2/CO2 mixture containing 10 vol% CO2 to a bed cooled at -120 °C, 90% of the fed CO2 is recovered. However, when feeding the same mixture to a bed cooled at -110 C, only 12% CO2 is recovered. 

Therefore, a relatively low amount of extra cooling will result in much higher CO2 recovery rates. This effect is directly related to the exponential temperature dependency of the 

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Figure 5 gives the simulation results with the model given for the conditions used by Briggs et al. to obtain Fig. 3. Data points are shown in Fig. 5b, but not in 5a. Mass spectrometer readings were not calibrated, and only normalized data are shown in Fig. 3a. The simulation estimates the shape of the midbed temperature and the SO3 vol% variations successfully. It also reproduces the initial bed temperature lag for the first minute after introduction of the S03/S02 reactant mixture (Fig. 5b), as well as the absence of a lag when air is introduced to the catalyst bed displacing the reactant mixture (Fig. 5a). The model also gives the slow adjustment of the bed temperature after the maximum and minimum temperatures, although the rates of cooling and heating are not correct. The most serious deficiency of the model is that it overestimates the temperature rise and drop by 15 and 8°C, respectively.

It is possible (with lower initial bed temperature, higher initial loading, or higher regeneration temperature or pressure) for the transition paths to contact the saturated vapor curve in Fig. 16-22 rather than intersect beneath it. For this case, liquid benzene condenses in the bed, and the effluent vapor is saturated during part of regeneration [Friday and LeVan, AlChE J., 30,679 (1984)]. 

At the chosen initial bed temperature of -140 °C, more than 99% of CO2 is recovered. After 600 seconds, the CO2 desublimation front reaches the end of the bed and the capture cycle should be stopped. The conditions at 600 seconds are used as initial conditions for the simulation of the recovery step. Figure 2.5b and e show that during the recovery step extra CO2 will be deposited on the packing surface and that all deposited CO2 is removed after again 600 seconds. 

When feeding the gas mixture at realistic flue gas temperatures (which are generally lower than 250 C) during the capture step, insufficient heat is stored in the packing to evaporate previously condensed water again. A possibility would be to introduce extra heat into the bed in the initial period of the recovery step. However, more practical is to carry out the H2O capture step in a separate smaller bed, which can be cooled down to temperatures much higher than the initial bed temperature of the CO2 capture bed. 

Also for other inlet compositions and initial bed temperatures, the two models agree very well (results are not included here). The recovery step has also been simulated using the two models. As already explained earlier, additional CO2 will deposit on the packing in the initial phase of the recovery step. This is described by both models and again matches well, as observed in the temperature and mass deposition profiles after 10 seconds in Figure 2.8a and b, respectively. 

This section aims at giving an overview of the influences of several process parameters on the process performance, using the sharp front approach. The influences of the initial bed temperature, inlet composition, inlet temperature and packing material are analyzed on the basis of two aspects the amount of CO2 deposited per unit of bed volume and the required specific cooling duty, which is defined as follows ... 

The numerator of the equation represents the amount of energy required to cool down the bed after the recovery step from to temperature Tq, which is the initial bed temperature before starting the capture step. The denominator gives the amount of CO2 captured during... 

Figure 2.9 Specific cooling duty (a) and mass deposition (b) as a function of the initial bed temperature for different inlet CO2 fractions...

The CO2 content in the outlet stream was analyzed with an IR-analyzer (Sick-Maihak, s610,0-3 vol%). The front of sublimated CO2 was visually inspected with a camera. Axial temperature profiles have been measured for different initial bed temperatures and inlet mole CO2 and H2O fractions (see Table 2.7). 

The initial temperature profile used in the simulations is taken from the experiments. As mentioned earlier, the initial bed temperature is not totally uniform, but is increasing slightly from the inlet towards the outlet. This is caused by the heat leak into the system. To account for this heat leak in more detail, the tube was first cooled down, then cooling was stopped and the temperature rise was measured as a function of time in the radial centre and close to the tube wall. It was found that the temperature difference between these two locations was minimal and the temperature rise could be well described by an additional radiative energy infiux. Therefore, the following contribution was added to the energy balance ... 

During the capture step, the temperature at the outlet of the bed is almost stable at the initial bed temperature (approximately -130 C), but increases strongly as soon as CO2 starts to breakthrough. When the bed is at a temperature of -100 °C, the capture step is stopped. When the recovery step is started, it can be observed that the temperature increases quickly to a temperature of approximately -76 C corresponding to a saturation temperature of pure CO2 at the operating pressure (1.2 bar). 

The initial bed temperature was set at -150 °C, which resulted in more than 99.9% CO2 recovery. A breakthrough time (duration of each step) of 600 seconds was chosen. 

LNG consumption. An initial bed temperature of -160 °C results in even more elRcient use of the beds and therefore slightly lower costs and LNG consumption. However, the temperature difference between LNG (-162°C) and the refrigerated N2 becomes too small. 

Figures 9.15 and 9.16 show experimental data obtained by Basmadjiah and co-workers with the system C02-5A sieve. Equilibrium theory predicts that at temperatures below the reversal temperature (region I of Figure 9.9) changes in the initial bed temperature have no effect on the concentration break-...

FIGURE 9.15. Experimental concentration and temperature breakthrough curves for adsorption of 0.75% mole CO2 from He on 5A molecular sieve at 1 atm showing the effect of initial bed temperature (80 or 180 F). Feed temperature, 80° F. (From ref. 28 reprinted with permission from Canadian Journal of Chemical Engineering 53, 234, 1975.)... 

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