|Sutrasno Kartohardjono||Department of Chemical Engineering, Faculty of Engineering, Universitas Indonesia, Kampus UI Depok, Depok 16424, Indonesia|
|Mohamad Sofwan Rizky||Department of Chemical Engineering, Faculty of Engineering, Universitas Indonesia, Kampus UI Depok, Depok 16424, Indonesia|
|Eva Fathul Karamah||Department of Chemical Engineering, Faculty of Engineering, Universitas Indonesia, Kampus UI Depok, Depok 16424, Indonesia|
|Woei-Jye Lau||Advanced Membrane Technology Research Centre, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia|
As a type of gas that contributes to air pollution, nitrogen oxide (NOx) has harmful effects on humans and the environment. Among several types of NOx, nitric oxide (NO) and nitrogen dioxide (NO2) are most commonly found in air. The utilization of membranes as reactors is a system that combines chemical reactions with the separation process through membranes to increase the conversion of the reaction. This study investigated the absorption process by utilizing a hollow fiber membrane module (polysulfone) as a bubble reactor with H2O2 (0.5 wt.%) and HNO3 (0.5M) as the absorbent. NOx feed gas was flown into the tube side of the membrane; the shell side was filled with static H2O2 and HNO3 and the shell input and the tube output flow were closed to create gas bubbles. The experimental results showed that the absorption efficiency increased, but the mass transfer coefficient and flux decreased as the number of fibers in the membrane module increased at the same feed gas flow rate. The NOx loading is relatively constant as the amount of fiber in the membrane module increased at the same feed gas flow rate. The experimental results also showed that the mass transfer coefficient, flux, and NOx loading increased with increasing feed gas flow rate, but the absorption efficiency decreased when using the same number of fibers in the membrane module. The maximum NOx absorption efficiency achieved in this study was 94.6% at the feed gas flow rate of 0.1 L/min, using a membrane module with 48 fibers.
Absorption efficiency; Hollow fiber; Mass transfer coefficient; NOx loading
Currently, atmospheric pollution is a significant problem across the globe; apart from SO2, the most dangerous and toxic gas is NOx (nitrogen oxide). NOx is produced from the reaction between nitrogen (N) and oxygen (O) during the combustion process at high temperatures. Among several types of NOx, such as N2O, nitrogen monoxide (NO), N2O3, nitrogen dioxide (NO2), N2O4, and N2O5, NO and NO2 are the ones most often found in atmospheric air, with NO comprising >90% of the total amount of gas (Kumar et al., 2015). Furthermore, 49% of NOx comes from motor vehicles, 27% comes from the activities of the electricity generation industry, and 19% comes from household activities. NOx has also been reported to have the ability to cause acid rain, form fog fumes, decrease water quality, destroy ecosystems, and contribute to global warming (Choi et al., 2014; Gao et al., 2018; Kartohardjono et al., 2019a; Sun et al., 2019). In addition, NOx is also having harmful effect on the human being (Kartohardjono et al., 2019b).
The reduction of NOx in exhaust gases, such as those from boilers and the nitric acid (HNO3) industry, is currently attracting much attention due to increasingly stringent environmental regulations; for example, the annual average quality standard for NO2 in ambient air is set at 0.053 ppm (Indonesia, 1999). Several technologies have been pioneered for this purpose, including Selective Catalytic Reduction, Selective Non-Catalytic Reduction, adsorption, and absorption using column contactors (Kartohardjono et al., 2019b). Similarly, the absorption of NOx using membrane technology has also been developed; in this type of system, the contactors offer several advantages: a continuous process that is easy to operate, low energy consumption, easy scale-up, low separation costs, and the ability to produce high-quality products (Kartohardjono et al., 2017)
Hollow fiber membrane contactors have been widely used as gas-liquid contactors due to the large ratio between their surface area for contact and the equipment volume (Lipnizki and Field, 2001). Furthermore, gas absorption through the membrane contactor also the integrates separation and absorption processes to exploit the benefits of both. Moreover, the hollow fiber membrane module has two different spaces for each fluid, shell, and tube. The in-flowing liquid provides selective absorption to certain gas species, while the porous membrane acts as the contacting interface between the liquid and gas phases, allowing the unidirectional transport of gas into the liquid. For example, the gas component to be removed is absorbed into the solution when the gas stream contacts with the liquid (Wang and Yu, 2017). The hollow fiber membrane module also provides a large ratio of surface area to volume, which is very beneficial for gas-liquid contact (Anggraini et al., 2019). Additionally, the mass transfer between phases that occurs in membrane modules is driven by differences in the concentration of the inter-phase components and pressure drops.
In contrast, the use of membranes as reactors involves the combination of chemical reactions with the separation process through membranes with the usual intention of increasing the conversion of the reaction. A bubble reactor is a type of reactor that is in two phases of gas and liquid; it is a cylindrical vessel with a gas distributor (sparger) at the bottom. The fluid in the gas phase flows into the vessel and, in the process, gas bubbles are formed and move through the liquid inside the vessel. The advantages of this type of reactor include high mass transfer rates, high density, and low operating and maintenance costs (Shaikh and Al-Dahhan, 2013). Bubble reactors are widely applied in chemical, petrochemical, biochemical, metallurgical, and materials industries.
A number of studies have investigated the use of membranes as additional units in processes involving bubble reactors or columns. Xia et al. (2013) immersed a hollow fiber membrane module in a liquid cylindrical bubble column reactor to test the hydrodynamic effect induced by an installed sparger. Moreover, Adewuyi et al. (2014) investigated if a membrane could remove the remaining moisture from the results of NOx gas absorption using Na2S2O8 solvents from a bubble reactor made from glass. The present study was conducted to examine the utilization of hollow fiber membrane modules to reduce NOx from gas mixtures using a mixture of hydrogen peroxide (H2O2) and HNO3 as the absorbent. It was expected that the fibers in the membrane module would increase the contact area between NOx and the absorbent solution to enhance the NOx absorption.
This study utilized hollow fiber membrane modules to absorb NOx using a mixture of HNO3 0.5M and H2O2 0.5% w/t as the absorbent. The feed gas flow rate and the number of membrane fibers were the main variables used in this study. The aim was to understand their effects on the overall mass transfer coefficient, flux, absorption efficiency, and NOx loading. The experimental results showed that the mass transfer coefficient, flux, and NOx loading increased by increasing the feed gas flow rate, while the absorption efficiency decreased. The highest NOx absorption efficiency achieved in this study was 94.6% at the feed gas flow rate of 0.2 L/min with the membrane module with 48 fibers in the contactor. Moreover, the mass transfer coefficient and flux decreased while the absorption efficiency increased as the number of fibers in the contactor increased at the same feed gas flow rate. The NOx loading was relatively constant as the number of fibers in the contactor increased at the same feed gas flow rate. In this study, the increase in the efficiency of NOx absorption was insignificant when the amount of fibers in the membrane module increased from 16 to 42 at the feed gas flow rate of 0.1 L/min. However, the NOx absorption efficiency decrease was insignificant if the gas flow rate increased from 0.1 to 0.2 L/min on a membrane module with 48 fibers. The NOx absorption efficiency achieved in this study was already high (about 93.8%), even for the membrane module with the lowest number of fibers (16 fibers) and at the highest feed gas flow rate (0.2 L/min).
The authors wish to acknowledge that they received financial support for this study
from the PDUPT Project via Directorate of Research and Services Universitas Indonesia
through Contract No. NKB-1665/UN2.R3.1/HKP.05.00/ 2019.
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