Article

Cycle Development and Design for CO 2 Capture from Flue Gas by Vacuum Swing Adsorption

Cooperative Research Centre for Greenhouse Gas Technologies, Department of Chemical Engineering, Monash University, Victoria 3800, Australia.
Environmental Science and Technology (Impact Factor: 5.33). 02/2008; 42(2):563-9. DOI: 10.1021/es0706854
Source: PubMed
ABSTRACT
CO2 capture and storage is an important component in the development of clean power generation processes. One CO2 capture technology is gas-phase adsorption, specifically pressure (or vacuum) swing adsorption. The complexity of these processes makes evaluation and assessment of new adsorbents difficult and time-consuming. In this study, we have developed a simple model specifically targeted at CO2 capture by pressure swing adsorption and validated our model by comparison with data from a fully instrumented pilot-scale pressure swing adsorption process. The model captures nonisothermal effects as well as nonlinear adsorption and nitrogen coadsorption. Using the model and our apparatus, we have designed and studied a large number of cycles for CO2 capture. We demonstrate that by careful management of adsorption fronts and assembly of cycles based on understanding of the roles of individual steps, we are able to quickly assess the effect of adsorbents and process parameters on capture performance and identify optimal operating regimes and cycles. We recommend this approach in contrast to exhaustive parametric studies which tend to depend on specifics of the chosen cycle and adsorbent. We show that appropriate combinations of process steps can yield excellent process performance and demonstrate how the pressure drop, and heat loss, etc. affect process performance through their effect on adsorption fronts and profiles. Finally, cyclic temperature profiles along the adsorption column can be readily used to infer concentration profiles-this has proved to be a very useful tool in cyclic function definition. Our research reveals excellent promise for the application of pressure/vacuum swing adsorption technology in the arena of CO2 capture from flue gases.

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Cycle Development and Design for
CO
2
Capture from Flue Gas by
Vacuum Swing Adsorption
JUN ZHANG AND PAUL A. WEBLEY*
Cooperative Research Centre for Greenhouse Gas Technologies,
Department of Chemical Engineering, Monash University,
Victoria 3800 Australia
Received March 19, 2007. Revised manuscript received July
31, 2007. Accepted September 27, 2007.
CO
2
capture and storage is an important component in the
development of clean power generation processes. One CO
2
capture technology is gas-phase adsorption, specifically pressure
(or vacuum) swing adsorption. The complexity of these
processes makes evaluation and assessment of new adsorbents
difficult and time-consuming. In this study, we have developed
a simple model specifically targeted at CO
2
capture by
pressure swing adsorption and validated our model by
comparison with data from a fully instrumented pilot-scale
pressure swing adsorption process. The model captures non-
isothermal effects as well as nonlinear adsorption and
nitrogen coadsorption. Using the model and our apparatus,
we have designed and studied a large number of cycles for CO
2
capture. We demonstrate that by careful management of
adsorption fronts and assembly of cycles based on understanding
of the roles of individual steps, we are able to quickly
assess the effect of adsorbents and process parameters on
capture performance and identify optimal operating regimes and
cycles. We recommend this approach in contrast to exhaustive
parametric studies which tend to depend on specifics of
the chosen cycle and adsorbent. We show that appropriate
combinations of process steps can yield excellent process
performance and demonstrate how the pressure drop, and heat
loss, etc. affect process performance through their effect on
adsorption fronts and profiles. Finally, cyclic temperature profiles
along the adsorption column can be readily used to infer
concentration profiles-this has proved to be a very useful
tool in cyclic function definition. Our research reveals excellent
promise for the application of pressure/vacuum swing
adsorption technology in the arena of CO
2
capture from flue
gases.
Introduction
With the increasing focus on global warming caused by
greenhouse gas emissions, Carbon Capture and Storage (CCS)
is seen as a promising mitigation initiative and has attracted
considerable research development and demonstration
efforts over the last two decades (1–3). In particular, since
the capture cost is the major component of the overall cost
of CCS, many capture technology solutions have been
proposed and investigated (3). Among the capture technology
candidates, pressure/vacuum swing adsorption technology
(referred to as PSA or VSA) has been frequently investigated
because of its low energy requirements and relative simplicity.
Many CO
2
-selective adsorbents have been developed and
tested and various pressure swing cycles have been designed
and investigated (4, 5). Most of the reported studies rely on
numerical modeling to reach conclusions; these models are
reliant on a variety of simplifying assumptions and boundary
conditions (6–11). However, justifications for the use of
specific adsorption cycles are generally not given and the
detailed analysis of the role of specific cyclic component
steps in the context of CO
2
capture, such as feed, pressure
equalization, evacuation, product purge, and repressurization
(co- and counter-current), have not been conducted.
Although many detailed pressure/vacuum swing adsorp-
tion models have been developed these models are often
difficult to use, run slowly, and rely on an expert user to
determine appropriate operating parameters. As a result,
there is a very wide range of reported performance data from
pressure swing adsorption simulations with the same ad-
sorbent, ranging from recoveries of less than 20% and purities
of 30% up to recoveries of 80% and purities in excess of 95%.
It is difficult for the inexpert PSA engineer to make sense of
these reports especially if a new adsorbent has been
developed and the engineer wishes to evaluate this adsorbent.
While simple equilibrium analysis can yield useful data (e.g.,
the analytical solution to PSA by Chang and Knaebel and Hill
with the assumptions of isothermal operation and linear
isotherms (12, 13), Tezel’s approximate isothermal adsorbent
screening (14), and the more elaborate equilibrium analysis
of Pigorini and LeVan (15)), there is a need for a simple but
reliable method for designing a PSA cycle for CO
2
capture
and assessing a new adsorbent quickly using this cycle. In
particular, it is of great interest to understand the power
requirements for a CO
2
capture process and hence a rapid
indicator would be extremely useful.
It is the goal of the current study to develop and use a simple,
fast, yet reliable model for selection and evaluation of a cycle
and operating conditions for CO
2
capture from process streams
using pressure/vacuum swing adsorption. A similar effort for
more general PSA cycles was made by Serbezov and Sotirchos
(16) who developed semianalytical solutions for 4-step PSA
cycles assuming linear isotherms and isothermal operation.
Our model is also a local-equilibrium-based one but includes
nonisothermal operation-this is an essential component since
the heat of adsorption during CO
2
capture by PSA can be
significant. In addition, we do not restrict ourselves to linear
isotherms since CO
2
adsorption on virtually all adsorbents
is strongly nonlinear and indeed it is this nonlinearity which
determines the optimal operating conditions. Finally, the
role of coadsorption of nitrogen, a major constituent of flue
gas, on CO
2
recovery must be included. After development
of this model, we illustrate the role of each of the various
process steps in PSA cycles for CO
2
capture and introduce
the concept of composition front management. We also show
a comparison of the model predictions with experimental
data from our pilot-scale pressure swing adsorption plant.
The results from our models can be readily linked to economic
models allowing rapid estimation of capture costs and the
relative benefits of new adsorbents and process conditions
on capture cost.
Experimental Section
To validate our model and to generate data for the capture
of CO
2
from flue gas by VSA, a three-column pilot-scale
apparatus was constructed (17). From our experience, it is
difficult to scale up benchtop PSA data reliably because the
* Corresponding author phone: (+61 3) 9905 3628; fax: (+61 3)
9905 9602; e-mail: paul.webley@eng.monash.edu.au.
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    • "This included absorption, adsorption and membrane separation technologies, which are the leading candidates [17,18]. In all these methods, the adsorption technology has always been so much attractive for its simplicity of operation, the reusable nature of adsorbents, the low energy consumption and low investment cost considering the specific dry flue gas systems [19][20][21][22][23]. Strategies like PSA (pressure swing adsorption) and TSA (temperature swing adsorption) have been proposed and investigated for adsorption [24][25][26][27][28][29][30][31][32][33][34][35][36]. "
    [Show abstract] [Hide abstract] ABSTRACT: A dual-reflux pressure swing adsorption (DR PSA) process with two adsorption beds and six steps was employed for recovering two products simultaneously with a high purity and high recovery efficiency. With silica gel as adsorbent, the feed gas (15% CO2/85%N2) was separated at 299.65 K and 200 kPa by the DR PSA process. Adsorption isotherms of CO2 and N2 on silica gel were measured and fitted by the extended Langmuir model. The process was then simulated with a commercial software, Aspen Adsorption. The simulation results revealed that feed position, method of equalizing-pressure and light component reflux flow rate would affect the performance in terms of purity and recovery significantly. The efficiency and feasibility of the DR PSA process were evaluated by the experiment and results showed that 99.62% of CO2 could be recovered with purity of 99.18% via DR PSA process meanwhile light product N2 could be enriched to 99.64% with recovery of 99.56% under optimal operating conditions. Overall, the DR PSA process is promising for separating a N2/CO2 mixture and producing gases with a high purity and recovery efficiency.
    Full-text · Article · Mar 2016
    • "An alternative to stripping PSA is enriching PSA, which aims to produce a pure heavy product (concentrated in the more strongly-adsorbed component) with some of the heavy product used as a reflux or purge stream to 'regenerate' a column containing an unsaturated adsorbent bed. Various models and experiments investigating enriching PSA can be found in the literature including, for example, those by Ebner and Ritter [3], Yoshida et al. [4], Reynolds et al. [5] and Zhang and Webley [6] . A disadvantage of either stripping PSA or enriching PSA is that only one of the separation's product streams is of high-purity while the purity of the waste stream is limited [4,7]. "
    [Show abstract] [Hide abstract] ABSTRACT: A non-isothermal model of dual reflux pressure swing adsorption (DR-PSA) was developed using a commercially available software package to numerically solve the dynamics of the unit operation. Importantly the model includes a full energy balance, which is a feature not reported previously in the literature for simulations of DR-PSA cycles even though bed temperature swings of 10 to 20 K have been observed in experimental studies. The simulation allowed solution of the pressure-flow network for cycles in the PL-A configuration, where feed gas enters the middle of the low pressure column and where pressure inversion is conducted by transferring gas rich in the more adsorbed component between beds. At cyclic steady state the simulations contained material balance errors comparable in magnitude to those reported previously for isothermal DR-PSA simulations; however, these were accounted for using a robust correction scheme. Predictions of the pressure, flow and temperature profiles within the beds for a range N2 + CH4 mixtures being separated using activated carbon were in good agreement with the corresponding results of 24 DR-PSA experiments recently reported by Saleman et al. [Chem. Eng. J. 2015, DOI: 10.1016/j.cej.2015.07.001]. The root mean square deviation of the predicted methane mole fractions from the experimental values were 0.003 and 0.024 for the light (N2-rich) and heavy (CH4-rich) product streams, respectively. Parametric studies conducted with the model show how the cycle design can be optimised with respect to reflux flow and the feed-purge step duration, and also illustrate the need for reliable values of the sorption mass transfer coefficient.
    No preview · Article · Feb 2016 · The Chemical Engineering Journal
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    • " The first paper in this theme is titled, " Multi-objective optimisation of a hybrid vacuum swing adsorption and low-temperature post-combustion CO 2 capture, " by Li Yuen Fong et al. (2016). Adsorption processes have been used for air separation, gas dehydration and CO 2 removal from synthesis gas for hydrogen produc- tion. Zhang and Webley (2008) stated that the pressure/vacuum swing adsorption technology has been frequently investigated for the carbon captures process due to its relative simplicity and low energy requirements. However, one disadvantage of adsorption is that the gaseous feed needs to be treated before passing it through the adsorber and like the membrane process"
    [Show abstract] [Hide abstract] ABSTRACT: Energy supply and its efficient use in production are key to ensuring the healthy functioning of the world economies. Based on that, to ensure sustainability, the supply and use of energy have to apply the principle of minimising negative environmental impacts and even improving the environment through net-regenerative development. In this context, ensuring cleaner energy is the cornerstone for cleaner production, especially for reducing the emissions of greenhouse gases and other pollutants, which are directly related to the types and loads of the energy sources used. This introductory article presents a review of the main lessons recently learned in the area of more efficient energy use, cleaner fuels and biofuels, cleaner production, CO2 capture, optimisation and waste management. This article provides the readers with ideas and technologies that can be incorporated into real world solutions and can serve as the foundations for future research. With firm realisation of the importance of these issues, the 17th conference "Process Integration, Modelling and Optimisation for Energy Saving and Pollution Reduction - PRES" was held in Prague, Czech Republic, to provide a platform for discussing ideas and devising solutions for cleaner energy supplies that are used more effectively and efficiently. This was followed by comprehensive research that resulted in several high quality scientific contributions published in the Journal of Cleaner Production. The wide topical coverage and the high quality provided excellent directions for future collaborative research of the PRES family - including process level emission minimisation, self-sufficient regions, and industrial symbiosis for optimizing usage of waste heat and waste material flows.
    Full-text · Article · Oct 2015 · Journal of Cleaner Production
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