Importance of Incorporating a Vacuum Pump Performance Curve in Dynamic Adsorption Process Simulation

Abstract

The importance of incorporating a vacuum pump performance curve into a mathematical model that describes the regeneration steps in a vacuum swing adsorption (VSA) cycle was assessed. This was accomplished by evaluating the relationship between the pressure and flow rate at the exit of a 13X zeolite bed, initially and uniformly saturated with 70 vol % CO2 in N2 at 333.15 K and 1 atm, which then underwent only countercurrent depressurization and light reflux (LR) steps, that is, the regeneration steps in a VSA cycle. Overall, this study revealed that it is imperative to use a realistic vacuum pump model in cyclic adsorption process simulation, especially for estimating the ideal isentropic energy consumed during the regeneration steps in a VSA cycle, and it revealed the possible inaccurate performance predictions that might result when using a valve equation alone to describe these steps. The study on the effect of valve size (c v) without the vacuum pump present to mimic the behavior of the vacuum pump performance curve showed that ideal isentropic energies from the valve equation can be overestimated significantly, in this case ranging from 1 to 6 kJ/mol CO2 removed. The study on the effect of LR flow rate only with the valve or only with the vacuum pump present showed marked differences in performance when using a valve equation to mimic a vacuum pump performance curve during the LR step. As secondary outcomes, the study on the effect of bed size when operating with a Pfeiffer ACP 40 vacuum pump alone showed that it was easier for it to regenerate beds of a smaller size; it showed a power consumption benefit during depressurization, a feature of VSA processes when the vacuum pump is not restricted by a valve; and it showed how to determine an approximate bed size. The study on the effect of valve size (c v) when placed before the suction side of the vacuum pump showed that a valve should never be placed there to avoid choking of the vacuum pump and expending more energy; it showed that if a valve is required for flow rate control, then its c v should be chosen carefully; and it showed how to determine an approximate valve size.

Optimization of pressure-vacuum swing adsorption processes for nitrogen rejection from natural gas streams using a nitrogen selective metal organic framework

“…These results are consistent with what is expected for type-I isotherms, the optimal operation is when the column is completely regenerated, or tends towards complete regeneration [28]. While it is common in PSA process optimization studies to consider an evacuation pressure of P L = 3 kPa, many commercial vacuum pumps become less efficient as the vacuum pressure decreases [34]. As the adsorbent bed size increases, the loss in efficiency increases the time, and energy, necessary to achieve a desired vacuum level [34].…”

“…While it is common in PSA process optimization studies to consider an evacuation pressure of P L = 3 kPa, many commercial vacuum pumps become less efficient as the vacuum pressure decreases [34]. As the adsorbent bed size increases, the loss in efficiency increases the time, and energy, necessary to achieve a desired vacuum level [34]. Therefore, the last parametric optimization performed was to determine the effect of evacuation pressure on the process purities and recoveries.…”

Guidelines for Techno-Economic Analysis of Adsorption Processes

“…Care should be taken to the specified units of volume, as standard (Sm 3 , 15°C and 1 atm) or normal (Nm 3 , 0°C and 1 bar) volume will need to be converted to actual volume. The influence of vacuum pump curves in dynamic process simulation was investigated by Jiang et al (2019), and impacts on attainable evacuation pressure and calculated energy consumption were significant. For TEAs, a vacuum pump should selected and its attainable pressure and flow rate considered during the process design.…”

Optimization of pressure‐vacuum swing adsorption processes for nitrogen rejection from natural gas streams using a nitrogen selective metal organic framework

“…[ 28 ] While it is common in PSA process optimization studies to consider an evacuation pressure of P L = 3 kPa, many commercial vacuum pumps become less efficient as the vacuum pressure decreases. [ 34 ] As the adsorbent bed size increases, the loss in efficiency increases the time, and energy, necessary to achieve the desired vacuum level. [ 34 ] Therefore, the last parametric optimization performed was to determine the effect of evacuation pressure on the process purities and recoveries.…”