Comparison of several packings for CO2 chemical absorption in a packed column
Highlights
► CO2 absorption in a packed column. ► Comparison of packing types for absorption. ► High efficiency random packing from CO2 absorption.
Introduction
In the battle against global warming, CO2 capture and storage has gained widespread attention as an option for reducing greenhouse gas emissions. Chemical absorption and stripping of CO2 with aqueous solvents including alkanolamines and hot potassium carbonate (K2CO3) solutions are well-known and effective processes for removing CO2 from power plant flue gases (Jassim and Rochelle, 2006). Compared with alkanolamines, potassium carbonate is less toxic, less prone to oxidative solvent degradation and has a low heat of regeneration (Cullinane and Rochelle, 2004, Uyanga and Idem, 2007). The reaction of K2CO3 with CO2 has been studied extensively and occurs via the following overall chemical reaction (Savage et al., 1980):The main disadvantage with using potassium carbonate is that it has a low CO2 absorption efficiency. Various techniques can be used to improve the absorption efficiency of this process including adding promoters to the K2CO3 solvent system and/or enhancing the efficiency of the packed column. A variety of promoters including piperazine (PZ) (Cullinane and Rochelle, 2004), diethanolamine (Rahimpour and Kashkooli, 2004) and arsenic trioxide (Epp et al., 2007) have been studied with K2CO3 solvent. However equipment efficiency has not received as much attention for this application. Packed columns are the most commonly used equipment for capture of CO2 from power plant flue gases using solvent absorption. One way of improving equipment efficiency with a packed column is to replace traditional packings with newer types of packings (random or structured) which have been specially designed to improve performance. Newer structured packings can be advantageous due to lower pressure drops and improved efficiencies however they are much more expensive than random packings. With both operating and capital costs being important considerations when implementing carbon capture and storage on a large scale it is imperative to improve the efficiency of the CO2 absorption process while keeping the cost of the equipment to a minimum. Therefore a randomly packed column with novel internals has the potential to achieve improved performance with lower operating costs while minimizing capital costs.
Randomly packed columns have been widely used in industry, including applications for gas absorption with high liquid loads, solvent extraction and high pressure distillation (Kister et al., 1994). Significant efforts have been dedicated to developing new, high-efficiency random packings for some time (Brierley, 1994, Cao, 2000, Fei, 1996, Li and Liu, 2000). It has been shown that the Super Mini Ring (SMR), a novel random packing, has enhanced performance due to the elaborate design of the twisting-inwards arc units (Fei, 1989) (refer to Fig. 1). The SMR packing has been widely used in carbon dioxide absorption from synthetic ammonia plants and in LPG purification with significant economic benefits (Fei and Wen, 1995, Ma and Fei, 2000).
The objectives of the present study were to compare the efficiency of CO2 absorption into potassium carbonate solutions using a laboratory sized column filled with Super Mini Rings (SMRs, a novel random packing), Pall Rings (a commonly used random packing) and Mellapak (a structured packing). Hydrodynamic and mass transfer correlations for predicting the performance of these different packings have also been investigated.
Section snippets
Materials and methods
Experiments were performed in a glass column of 0.074 m internal diameter with CO2 absorption into potassium carbonate solution at atmospheric pressure. The height of the packed bed was 0.9 m. A schematic diagram showing the experimental setup is shown in Fig. 2. A mixture of CO2 (14 or 85 mol%) and N2 was used for the gas phase and 30 wt% K2CO3 solution was used as the liquid solvent. Both phases flowed through the column counter currently via distributors, and flow rates were measured with
Results and discussion
It is desirable to have low pressure drop and high mass transfer rates when operating a packed column. When designing a column for CO2 capture it is also particularly important to ensure that the capital and operating costs of the equipment is kept to a minimum, as the cost of capture is one of the barriers to implementing carbon capture and storage technologies on a large scale. This study has therefore looked at optimizing the hydrodynamic and mass transfer performance of three different
Conclusions
Ø 13 mm SMR packing has been shown to have a 20% and 30% higher mass transfer coefficient when compared to Mellapak 700Y and Ø 13 mm Pall Ring, respectively. Thus the height of a packed column with SMR would be substantially lower than that with Pall Rings or Mellapak. Meanwhile, compared to other packings the flooding gas velocity for SMR packing increased when the liquid loads were above 25 kg m−2 s−1. Thus Ø 13 mm SMR has a higher gas flux at higher liquid loads when compared to Ø 13 mm Pall Ring
Acknowledgments
The authors acknowledge the Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC) and the Particulate Fluid Processing ARC Special Research Centre (PFPC) for financial support and vacation student Moshe Ross for assistance in completing the experimental work for this study.
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