Simulation Of Pressure Swing Adsorption Research Articles

Carbon nanofibers with gas selective layer containing rich and accessible ultramicropores for methane/nitrogen separation

The adsorption separation and purification of coalbed methane (CH4) has attracted global interest as an important technique for the utilization of unconventional natural gas. The success for this method relies on the development of adsorbent materials that can selectively enrich CH4. Toward this goal, we report a facile in-situ growth method to prepare carbon nanofibers with ultramicroporous gas selective layer as adsorbent for CH4/N2 separation. The thickness of gas selective layer can be precisely tuned from 90 nm to 525 nm. The optimal carbon nanofibers exhibited a high CH4/N2 selectivity of 6.8 with the CH4 uptake reaching 0.97 mmol g−1 as well as two orders of magnitude faster CH4 diffusion kinetics than the commercial activated carbon adsorbent. The high accessibility to the adsorption sites and short diffusion path of the nanofibers facilitate the rapid mass transfer and intensify the dynamic CH4/N2 separation. Pressure swing adsorption simulations indicated the recovery and productivity over the carbon nanofibers with 90 nm-thick gas selective layer increased by 40 % and 20 %, respectively, as compared to the 525 nm-thick counterparts. This work offers a new insight for designing high-performance CH4/N2 adsorbents through suitable combination between rich ultramicropores and short diffusion path.

Efficient N2/CH4 separation in a stable metal–organic framework with high density of open Cr sites

Natural gas purification by economic and low-energy adsorptive separation processes is important. However, as a difficult global problem, N2 removal from N2/CH4 mixtures is challenging because of their similar physical properties. In this study, the separation of N2/CH4 by Cr-metal–organic frameworks (MOFs) (MIL-100Cr, MIL-101Cr and TYUT-96Cr) with various open Cr site densities was studied. Gas sorption isotherms showed that TYUT-96Cr exhibited the highest N2/CH4 uptake ratio (13.67 at 0.01 bar and 298 K) of the selected Cr-MOFs, which was consistent with the highest density of Cr (III) sites. This material provided the highest CH4 productivity (0.716 mmol/cm3) per cycle for N2 removal from a 5:95 N2/CH4 mixture to produce 99.99% pure CH4, as shown by the breakthrough experiments. Pressure-swing adsorption simulations indicated that, by a two-bed six-step method, pristine 95% methane was enriched to 98.9% by TYUT-96Cr. The largest CH4 productivity, high breakthrough selectivity and thermal stability of TYUT-96Cr among the Cr-MOFs, made it a promising adsorbent in trace N2 removal for practical industrial application.

Ultramicroporous carbon granules with narrow pore size distribution for efficient CH4 separation from coal‐bed gases

AbstractThe adsorptive separation of CH4 from low‐grade coal‐bed gas can be performed at decentralized and remote coal mines, and it uses more energy‐ and is cost‐efficient than the traditional cryogenic distillation process. Herein, we present a facile method to prepare ultramicroporous carbon granules with a narrow pore‐size distribution at 0.5–0.6 nm. To our knowledge, such centered and uniform pore‐size distribution in carbon granules has never been reported. The carbon granules can be directly utilized in adsorption columns without a granulation or pelletization process. The granular oil‐tea‐shell‐derived porous carbon (GOC‐2) exhibited a record‐high CH4 uptake of 1.82 mmol/g and CH4/N2 selectivity of 5.8 at 1.0 bar and 298 K among carbon granules. The excellent CH4/N2 separation performances were confirmed from the results of dynamic breakthrough experiments and pressure swing adsorption simulations. This work provides a novel strategy for developing ultramicroporous carbon granules and guides the future design of efficient CH4/N2 separation adsorbents.

A simple and practical process modeling methodology for pressure swing adsorption

Although many dynamic models exist for the design and simulation of pressure swing adsorption (PSA) processes, these models involve the solution of a complex system of coupled partial differential equations. Process engineers need a simple, practical, and yet robust short-cut model that helps decide whether to implement a PSA system in a process flowsheet. This work presents a “virtual” moving bed modeling methodology that considers only mass and energy balances and adsorption isotherms to describe the cyclic steady state behavior of PSA systems. Similar to tray efficiencies in distillation calculations, adsorption efficiencies are further introduced to account for system “non-ideality.” A lab-scale air separation system is used to illustrate the application of this modeling methodology.Topical Heading: Process Systems Engineering

Analysis of CO<sub>2</sub> Pressure Swing Adsorption Simulation by Considering the Transport Phenomena in the Adsorber

This study focused on CO2 separation technology with adsorption. This paper describes the analysis carried out by a CO2 pressure swing adsorption simulation to scale up the absorber. An unsteady one-dimensional balance model was constructed by considering the material, energy, and momentum. In the CO2 breakthrough test, the beginning time and CO2 concentration at outlet of CO2 breakthrough in the calculation were almost equivalent to that of experiment results. The correlation consistency of the calculation results with the analysis model and the experimental results obtained by a bench scale experiment was evaluated. The transport phenomena in the adsorber were investigated at the adsorption, rinse, and desorption steps according to the calculation results. The starting time of CO2 breakthrough obtained by the analysis is equal to that obtained by the adsorption breakthrough experiment. This confirms that the CO2 adsorption, and the temperature and velocity distribution in the adsorber, change as a function of the adsorption, rinse, and desorption steps, respectively. Additionally, the CO2 concentration of the captured gas and the amount of CO2 quantity were 93.4% per day and 2.9 ton/day, respectively. These values are equal to those obtained by the bench scale experiment.

Zeolites for CO2-CO-O2 Separation to Obtain CO2-Neutral Fuels.

Carbon dioxide release has become an important global issue due to the significant and continuous rise in atmospheric CO2 concentrations and the depletion of carbon-based energy resources. Plasmolysis is a very energy-efficient process for reintroducing CO2 into energy and chemical cycles by converting CO2 into CO and O2 utilizing renewable electricity. The bottleneck of the process is that CO remains mixed with O2 and residual CO2. Therefore, efficient gas separation and recuperation are essential for obtaining pure CO, which, via water gas shift and Fischer-Tropsch reactions, can lead to the production of CO2-neutral fuels. The idea behind this work is to provide a separation mechanism based on zeolites to optimize the separation of carbon dioxide, carbon monoxide, and oxygen under mild operational conditions. To achieve this goal, we performed a thorough screening of available zeolites based on topology and adsorptive properties using molecular simulation and ideal adsorption solution theory. FAU, BRE, and MTW are identified as suitable topologies for these separation processes. FAU can be used for the separation of carbon dioxide from carbon monoxide and oxygen and BRE or MTW for the separation of carbon monoxide from oxygen. These results are reinforced by pressure swing adsorption simulations at room temperature combining adsorption columns with pure silica FAU zeolite and zeolite BRE at a Si/Al ratio of 3. These zeolites have the added advantage of being commercially available.

Equilibrium and kinetics of CO2 adsorption onto activated carbon

Knowledge of adsorption characteristics of adsorbent-adsorbate pairs is essential for designing adsorption beds for adsorption cooling and adsorptive gas capturing applications. We investigated the adsorption isotherms and the adsorption kinetics of CO2 onto microporous activated carbon powder of type Maxsorb III. Measurements were performed with gravimetric apparatus for temperatures from 30 to 70°C and pressures up to 7MPa for adsorption isotherms and up to 4MPa for adsorption kinetics. The gravimetric adsorption data obtained were consistent with previously measured isotherms with volumetric apparatus. Both absolute and excess adsorption data have been fitted precisely with Tóth and Dubinin-Astakhov isotherm equations. The classical linear driving force (LDF) model with a constant mass transfer coefficient failed to correlate the experimental adsorption kinetics data. To overcome this problem, the authors presented a modified LDF equation with a variable mass transfer coefficient which is a function of the equilibrium and instantaneous uptakes. This modified LDF equation led to a better fitting and could be implemented easily in simulation of pressure swing adsorption (PSA), temperature swing adsorption (TSA) and adsorption chiller applications.

A rigorous criterion for approach to cyclic steady-state in PSA simulations

An algorithm based on rigorous mathematical analysis is presented for determining the cyclic steady-state (CSS) in the simulation of Pressure Swing Adsorption (PSA) processes. The algorithm is founded on the observation that cyclic simulation of PSA processes is a Fixed-Point Iteration problem whose fixed point is the CSS. Therefore, instead of reducing the difference between the bed profiles of some dependent variables in two successive simulated cycles below a chosen tolerance value, the proposed algorithm reduces the difference between their current states and the CSS values to a set tolerance without requiring any a priori knowledge of the CSS. Three assumptions are made (1) convergence is linear in some neighbourhood of the CSS solution (2) the axial profiles at the end of the high pressure adsorption step moves only in one direction with respect to cycle number (3) the initial and ultimate convergence rates do not differ markedly. We demonstrate the effectiveness of the algorithm by applying it to a number of PSA/VSA/TSA processes simulated in the literature. For all the cases investigated, approach to the true CSS within a stated tolerance is achieved as indicated by the algorithm.

Multi-component adsorptive separation: use of lumping in PSA process simulation

Being a discrete-continuous process, approach to a cyclic steady state in computer simulation of Pressure Swing Adsorption is through iterative procedures and simulation itself is quite computation-intensive. Considering the fact that simulation based design itself is an iterative process, it is imperative that simulation be computationally very efficient and phenomenologically as close to the physics of adsorption-desorption as possible. Utility of lumping the components of a gas mixture into fewer pseudo-components was computationally examined in the simulation of a representative multi-step cycle of a pressure swing based adsorptive separation process applied to natural gas treatment. The actual feed had six components competing for adsorbent sites. Five different lumping alternatives were studied and compared with the simulation results for a full six-component simulation under identical equipment dimensions and operating conditions. Lumping could reduce the number of equations to be solved by more than half and the corresponding reduction in CPU time was about 90%. The six component mixture of Natural Gas was found to be sufficiently represented by two pseudo-components. The predicted recovery (in terms of Methane and Ethane) and quality (in terms of content of higher hydrocarbons) of the raffinate differed by not more than 0.8% and 0.02% respectively. The paper discusses possible heuristics for decision-making regarding appropriate lumping as verified by extensive simulation studies.

Simulation of pressure swing adsorption in fuel ethanol production process

Fermentation derived ethanol is gaining wide popularity as a car fuel additive. A major challenge in the production of ethanol is the high energy cost associated with the separation of ethanol from the large excess of water. Distillation is usually the method of choice; however, water cannot be completely removed due to the presence of the azeotrope. The pressure swing adsorption (PSA) process is attractive for the final separation since it requires little energy input and is capable of producing a very pure product. The goal of this work was to perform a thorough analysis of the PSA process and find process improvements with the aid of mathematical modeling. A general purpose package for the simulation of a cyclic PSA process was developed. The system of partial differential equations was solved via method of lines using a stiff equation integration package. Parameters for the model are based on the data from an operating plant as well as data from the literature. For the ethanol production technology our model provides a fundamental understanding of the dynamics of the cyclic process and effects of some operating parameters.

Clinoptilolites for nitrogen/methane separation

Clinoptilolites, naturally occurring zeolites, currently used by nuclear industry for ion exchange and drying, were evaluated for the nitrogen/methane separation. Ion exchange of purified clinoptilolite was carried out individually with the cations Na +, Mg 2+, Ca 2+, K + and H +, and the adsorption isotherms and diffusion rates were measured. Purified and Mg-clinoptilolite show potential for nitrogen/methane separation and high-pressure adsorption isotherms were measured in a differential absorption bed. Pressure swing adsorption simulations were performed for purified clinoptilolite, Mg-clinoptilolite and the commercial sorbent ETS-4. Purified clinoptilolite shows slightly higher recovery and lower productivity than ETS-4 for similar product purity (∼95%). Mixed ion-exchanged clinoptilolites of Mg/Ca cations, K/Na cations and Mg/Na cations were also prepared and studied for the nitrogen/methane separation. Mg/Na (50/50) mixed ion-exchanged clinoptilolite exhibits very good equilibrium and kinetic selectivity in the low-pressure range, better than purified clinoptilolite. Mixed-exchange clinoptilolites were also found to show a wide range of adsorption characteristics with varying ratios of cation exchange, thereby making them suitable for further improvements in nitrogen/methane separation.

Analog circuit for simulation of pressure swing adsorption: kinetic model

Analog circuit for simulation of pressure swing adsorption: kinetic model

Analog circuit for simulation of pressure swing adsorption

An electric circuit was used to simulate gas resistance, inductance, mass transfer resistance, and accumulation in the adsorbent and the void of the packed bed for the cyclic pressure swing adsorption process in the form of a boundary value problem consisting of an ordinary differential equation set. One-column rapid pressure swing adsorption for producing oxyen-enriched air was simulated numerically with a simplified algorithm. Under the chosen operating conditions, the changing pressure in the product storage tank, the axial distribution of pressure in the column, relationships between product purity and product flowrate, between exhaust flowrate and product flowrate, and between oxygen recovery and product purity, were obtained. The theoretical results appear to be consistent with the experimental ones.