(567a) Development of Large Scale PSA Processes for CO2 Capture with O2 Removal Capability

Extremely large gas flow rates, like that associated with flue gas produced from a 550 MW coal fired power plant, can exceed 25,000,000 SLPM. Even larger flue gas flow rates can be produced from steel mills. The flue gas produced from a 550 MW coal fired power plant consists primarily of 15 vol% CO₂, 65 vol% N₂, 8 vol% O₂, 12 vol% H₂O and ppm levels of SO₂ and NO_x at a pressure slightly above about 100 kPa and a temperature between ambient and 125 ^oC. According to the Department of Energy (DOE), the goal is to capture and concentrate the CO₂ and produce it in a stream containing at least 95 vol% CO₂ with at least 90 % recovery and with less than 10 ppm of O₂. Pressure and vacuum swing adsorption processes are being touted for this application. However, an issue arises with these low pressure flue gas streams when fed to a pressure swing adsorption (PSA) process.

The issue is that the feed cannot be compressed very much because of the cost of compression. Hence, the feed must be compressed only slightly with a blower to about 120 kPa, thereby limiting the axial pressure drop along the bed to be less than about 20 kPa atm differential. Moreover, because the feed pressure is so low, to effectively regenerate the beds a vacuum must be applied, typically down to absolute pressures of around 5 kPa to 10 kPa. This is commonly referred to as a vacuum swing adsorption (VSA) process. To make matters even worse, the regeneration steps, typically consisting of countercurrent depressurization (CnD) and light reflux (LR) steps, also have a limit on the magnitude of the axial pressure drop, for if it is too large, then the beds are not effectively regenerated because the light end of the bed never experiences the lowest vacuum pressure experienced by the heavy or feed end of the bed. These limits on the axial pressure drop during the feed and regeneration steps of a VSA cycle are dictated by the interstitial velocity in the bed which varies with time and position.

A unique scaling procedure was developed to meet these constraints while minimizing the total number of beds required in the VSA system by operating one or more identical VSA units in parallel. Each of these VSA units contains a minimum number of identical beds. Many of these multibed VSA units meet the required process performance, including the pressure drop constraints and now even the 10 ppm limit on the O₂ concentration in the CO₂ product. A brief overview of the scaling procedure will be given, along with the latest results from this study on designing a PSA process that can meet all the required process performance constraints including the 10 ppm limit on the O₂ concentration.