Separation of CO2 and N2 By Rapid PSA Using 13X Zeolite and a Dual Reflux Cycle

Rapid pressure swing adsorption (PSA) or rapid cycle PSA is identical to traditional PSA except that the cycle times and thus cycle step times are typically much shorter. Traditional PSA systems utilize cycle times on the order of minutes to tens of minutes, with individual step times no shorter than 0.5 min (30 s) or so. However, since the mid 80’s there has been a great deal of interest in trying to reduce the cycle time to make the columns smaller and thus increase the feed throughput. In fact, any reduction in cycle time results in a proportional decrease in column size, a gas separation process feature that is absolutely unique to PSA. In this study, a stream of 15 vol% CO₂ in N₂ was separated with a single bed rapid PSA system that approximated a 3-bed PSA process utilizing both heavy reflux (HR) and light reflux (LR) streams (i.e., a dual reflux (DR) cycle). The HR and LR streams were approximated with flows from cylinders containing 70 vol% CO₂ in N₂ and pure N₂, respectively. The 16.35 cm long by 3.18 cm inside diameter bed was filled with 0.3 mm diameter beads of 13X zeolite (18.88 g). The effects of cycle time and temperature on the recovery of CO₂ in the heavy product were examined for cycle times of 1 and 2 min at room temperature and at 70°C, with a high and low absolute pressures of 108 kPa and 5 kPa. For the two different cycle times, the amount of gas fed into the bed per cycle was constant. The corresponding feed throughputs were 2,100 L(STP)/h kg) for the 2 min cycle and 4,200 L(STP)/(h kg) for the 1 min cycle. The overall recovery of CO₂ in the heavy product ranged from 81.1 to 90.3%, with the most CO₂ recovered at 70°C and the 2 min cycle time. The CO₂ in the heavy product ranged from 72.5 to 79.1 vol%, with the highest occurring at room temperature with a 1 min cycle time. Overall, this preliminary study showed, possibly for the first time, that a HR step in a rapid PSA cycle is effective at concentrating the heavy component (CO₂) just like in a traditional PSA cycle.