Kok Keong Lau

Process simulation and optimal design of membrane separation system for CO2 capture from natural gas

Abstract Membrane process, a relatively new technology among other available techniques, can be used for the purpose of CO 2 capture from natural gas. Over the decades, membrane performance has been described by different mathematical models, but there is limited work done in the field of process simulation where membrane models can be incorporated with other unit operations using commercially available simulator. In this paper, a two dimensional cross flow mathematical model for membrane separation has been incorporated with Aspen HYSYS as a user defined unit operation in order to optimize and design the membrane system for CO 2 capture from natural gas. Parameter sensitivities, along with process economics, have been studied for different design configurations (including recycle streams and multiple stages). It has been observed that double stage with permeate recycle system gives the optimum design configuration due to minimum process gas cost involved with it.

Removal of high concentration CO2 from natural gas at elevated pressure via absorption process in packed column

Abstract Carbon dioxide (CO2) removal is an essential step in natural gas (NG) processing to provide high quality gas stream products and minimize operational difficulties. This preliminary study aims to investigate the removal of CO2 at high concentration level from the mixture of CO2-NG gas stream at elevated pressure via absorption process. This is to explore the possibility of exploring high CO2 content natural gas reserves by treatment at offshore platform. A mixed amine solvent, Stonvent-II, was used for the absorption of approximately 75 vol% CO2 in CO2-NG stream at a pressure of 10 barg. The initial solvent temperature was varied in order to study the impact of initial temperature on the absorption performance. Preliminary study at temperatures of 35°C and 45°C indicates that Stonvent-II was able to perform almost 100% removal of CO2 under both conditions. However, the CO2 absorption effect took place faster when the initial liquid temperature was lower. This is because when the initial liquid temperature is high, the temperature increase in the packing bed caused by the reaction heat is high which impacts the efficiency of absorption negatively.

Current Development and Challenges of Mixed Matrix Membranes for CO2/CH4 Separation

In the early stage of membrane technology development in gas separation, utilization of polymeric membranes has gained attention due to their robustness and ease of fabrication. However, the performance of polymeric membranes is limited by the trade-off between permeability and selectivity. Meanwhile, inorganic membrane is capable to exhibit great enhancement in separation performance but unfortunately its fabrication process is hard and costly. Thus, development of mixed matrix membranes (MMMs) by incorporating inorganic fillers into the polymer matrix has become a potential alternative to overcome the limitations of polymeric and inorganic membranes in gas separation. Nevertheless, fabrication of defect-free MMMs with improved separation performance and without compromising the mechanical and thermal stability is extremely difficult and challenging. In the current review paper, various types of inorganic fillers for MMMs fabrication and recent reported efforts to tailor the underlying problems on MMMs fabrication were discussed. The future outlook to advance the performance of MMMs in gas separation especially for CO2/CH4 separation was highlighted.

Rapid-synthesis of zeolite T via sonochemical-assisted hydrothermal growth method

Sonochemical-assisted method has been identified as one of the potential pre-treatment methods which could reduce the formation duration of zeolite as well as other microporous and mesoporous materials. In the present work, zeolite T was synthesized via sonochemical-assisted pre-treatment prior to hydrothermal growth. The durations for sonochemical-assisted pre-treatment were varied from 30min to 90min. Meanwhile, the hydrothermal growth durations were ranged from 0.5 to 3days. The physicochemical properties of the resulting samples were characterized using XRD, FESEM, FTIR and BET. As verified by XRD, the samples synthesized via hydrothermal growth durations of 1, 2 and 3days and sonochemical-assisted pre-treatment durations of 60min and 90min demonstrated zeolite T structure. The samples which underwent sonochemical-assisted pre-treatment duration of 60min yielded higher crystallinity with negligible change of zeolite T morphology. Overall, the lengthy synthesis duration of zeolite T has been successfully reduced from 7days to 1day by applying sonochemical-assisted pre-treatment of 60min, while synthesis duration of 0.5days via sonochemical-assisted pre-treatment of 60min was not sufficient to produce zeolite T structure.

Issues and Challenges in the Development of Deca-Dodecasil 3 Rhombohedral Membrane in CO2 Capture from Natural Gas

Removal of CO2 from natural gas using a membrane-based process has been adopted on an industrial scale, and about 200 membrane separation plants have been installed all over the world. In industry, inorganic membranes are preferred over the polymeric and mixed matrix membranes in CO2 separation from natural gas because of their good chemical stability and high separation performance. Among all inorganic membranes, deca-dodecasil 3 rhombohedral zeolite (DDR) membrane exhibits the highest selectivity in CO2 separation from natural gas. However, synthesis of DDR membranes requires a long duration, with a minimum of 25 days. Therefore, reduction of duration in the synthesis of the DDR membrane remains a challenging issue. In this review, separation performance of polymeric, mixed matrix and inorganic membranes are compared. Synthesis methods of DDR membranes and their performance in CO2 separation reported by various researchers are discussed. The challenges and issues related to the formation of DDR membranes are also included. In conclusion, the future direction and perspective on the development of DDR membranes for CO2 separation are summarized.

Zeolitic Imidazolate Frameworks (ZIF): A Potential Membrane for CO2/CH4 Separation

Removal of carbon dioxide (CO2) from natural gas is of importance because the existence of CO2 in natural gas increases the cost of the sweetening process. In recent years, membrane technology has emerged as an attractive alternative in separating CO2 from CH4 due to its economical, efficient, and environmentally-friendly process. Here, we review the different types of membranes used in CO2/CH4 gas separation. Zeolitic imidazolate frameworks (ZIFs) membranes are emphasized and ZIF-8 membranes are selected for further discussion due to their remarkable properties, including high chemical and thermal stability, facile and controllable pore apertures, and high CO2 permeance. Different types of methods used for the synthesis of ZIF membranes and the challenges encountered in the growth of the membrane are summarized. Potential use of microwave technology in fabricating a continuous and low-defects ZIF membrane within a short period of time are discussed and highlighted. In conclusion, future direction and perspectives are indicated.

Effect of Synthesis Parameters on the Formation of Zeolitic Imidazolate Framework 8 (ZIF-8) Nanoparticles for CO2 Adsorption

Zeolitic imidazole frameworks-8 (ZIF-8) is a subclass of metal-organic frameworks (MOFs) with the transition metal cations (Zn2+) linked by imidazolate anions forming tetrahedral frameworks in zeolite-like topologies. This article reports on the synthesis of ZIF-8 nanoparticles by varying the synthesis parameters at room temperature. The crystallization duration, molar ratios, and pH of the mixture solution were varied in order to study the effects of these parameters on the formation of ZIF-8 nanoparticles. The structural and morphology transformation of the resultant particles were characterized using x-ray diffraction, field emission scanning electron microscopy, and Brunauer–Emmett–Teller (BET) surface analysis. The CO2 adsorption characteristics of ZIF-8 nanoparticles were tested using CO2 physisorption analysis. Mature structural evolution was observed for ZIF-8 synthesized at 60 and 1440 min, but insufficient crystallization was found for ZIF-8 synthesized at 5 min. Meanwhile, ZIF-8 nanoparticles synthesized under lower amount of methanol resulted in larger particle size and higher crystallinity. Poorly intergrown ZIF-8 nanoparticles were observed for samples synthesized using a mixture solution with pH 8.2. Although different particle sizes and relative crystallinities were obtained for the ZIF-8 samples, synthesis using different molar ratios of the mixture solution, insignificant variations of BET surface areas, and CO2 adsorption capacities were found.

CO2 and CH4 permeation through zeolitic imidazolate framework (ZIF)-8 membrane synthesized via in situ layer-by-layer growth: an experimental and modeling study

In this work, a general model representing the permeation of CO2 and CH4 through Zeolitic Imidazole Framework-8 (ZIF-8) membrane synthesized via in situ layer-by-layer growth under microwave irradiation is developed. The model is formed based on the pressure drop concept in order to predict the intercrystalline properties of the ZIF-8 membrane according to the experimental permeation data of CO2 and CH4. The model combines Knudsen diffusion, viscous flow and generalized Maxwell–Stefan models, which considered the support resistance, gas diffusivity and intercrystalline pores of the membrane layer. The simulated data are fitted well with the experimental gas permeation results and consistent with the physical characterizations, including X-ray diffraction (XRD) and scanning electron microscopy (SEM). Results showed that, layer-by-layer growth managed to reduce the intercrystalline pores present in the ZIF-8 membrane layer, with the approximate pore radius of 2.1 × 10−7 m and porosity of 1.15 × 10−4. However, the presence of the small pores can significantly affect the performance of the ZIF-8 membrane which resulted in CO2/CH4 ideal selectivity of ∼1.

High frequency ultrasonic-assisted CO2 absorption in a high pressure water batch system

Physical absorption process is always nullified by the presence of cavitation under low frequency ultrasonic irradiation. In the present study, high frequency ultrasonic of 1.7MHz was used for the physical absorption of CO2 in a water batch system under elevated pressure. The parameters including ultrasonic power and initial feed pressure for the system have been varied from 0 to 18W and 6 to 41bar, respectively. The mass transfer coefficient has been determined via the dynamic pressure-step method. Besides, the actual ultrasonic power that transmitted to the liquid was measured based on calorimetric method prior to the absorption study. Subsequently, desorption study was conducted as a comparison with the absorption process. The mechanism for the ultrasonic assisted absorption has also been discussed. Based on the results, the mass transfer coefficient has increased with the increasing of ultrasonic power. It means that, the presence of streaming effect and the formation of liquid fountain is more favorable under high frequency ultrasonic irradiation for the absorption process. Therefore, high frequency ultrasonic irradiation is suggested to be one of the potential alternatives for the gas separation process with its promising absorption enhancement and compact design.

Prediction of the bubble nucleation rate in a quasi-stable cavitating nozzle using 2D computational fluid dynamics and enhanced classical nucleation theory

Most of the numerical studies of the cavitation process available were simulated based on the Rayleigh-Plesset equation, which has a presumed value for the number of bubble nuclei in the domain. However, in the computational domain for most cases, the bubble nuclei density varies, depending on the hydrodynamics and different bubble nucleation rates. Therefore, the modeling of the bubble nucleation rate based on the number of molecules required to form a critical cluster, which is affected by the hydrodynamics in the domain, is one of the challenges that must be overcome for bubble nuclei density prediction. In this work, an enhanced Classical Nucleation Theory (CNT) model was written as in-house code for integration in Computational Fluid Dynamics (CFD) to estimate the water vapor bubble nucleation rate across a quasi-stable cavitating flow nozzle. The enhanced CNT model included microscopic factors that specifically address water vapor bubble nucleation. Because these factors vary in the radial direction, this work was extended into the 2D domain to estimate the bubble nucleation rate in both the axial and the radial directions. The bubble nucleation rate was estimated through various numbers of molecules (depending on the hydrodynamics in the domain) required to form a critical cluster. The Population Balance Model was used to estimate the bubble nuclei density based on the bubble nucleation rate. The simulated results indicated that the bubble nucleation rate was highest at the throat of the nozzle, especially near the wall of the nozzle. Thus, a higher bubble nuclei density was found near the wall after the throat of the nozzle.