(70b) Development of Asymmetric Membrane Gas Permeation Simulation Modules for Generalized Plant-Wide Optimization of Power Plants with CCS: Effect of Detailed Porous Support Transport Sub-Model

Development of asymmetric membrane gas
permeation simulation modules for generalized plant-wide optimization of power plants
with CCS: Effect of detailed porous support transport sub-model

NETL is currently
developing a modular framework for the analysis and optimization of power
generation systems, particularly coal-fired power plants with carbon capture
and sequestration (CCS) [1]. The framework consists of a collection of
simulation modules for various parts of a power plant and a structure that
allows their integration into a specified configuration. Unlike most assessment
studies for CCS technologies in literature, the integration of CCS modules to
the modular framework provides plant-wide performance indicators and a common
basis for the evaluation of new and existing CCS technologies. The modular
approach is particularly advantageous given that beyond capital investment and
the obvious compromises in operating cost and plant efficiency, CCS processes
usually involve parasitic demands and modifications to other parts of the power
generation system. This approach also permits the easy integration of new
processes and the capability to carry out constrained multi-objective
optimization routines by varying key system parameters for a given modular
plant design. Rigorous models for gas permeation modules were developed in
order to evaluate membrane carbon capture systems under the modular framework.

Gas permeation is a
well established technology for many industrial applications [2]. As a carbon
capture technology, a membrane system is advantageous because it does not
require large amounts of water or solvents, and its non-moving, modular
components are easy to operate and maintain. Recent studies conclude that multi-staged gas
permeation processes can compete with amine absorption as a viable carbon
capture technology [3-6]. Developments in membrane properties and process
design are required in order to achieve the desired separation while minimizing
energy and membrane area demands. Merkel et al. recently proposed a promising two-stage
system configuration that incorporates different strategies to maximize the
separation driving force [5]. The most prominent feature of the design is a
counter-current sweep module that uses a portion of the air feed to the boiler
as a sweep. The recirculation of carbon dioxide through the boiler increases
its partial pressure in the flue gas stream to be treated, while the sweep flow
decreases its permeate side partial pressure. Both effects increase the
trans-membrane carbon dioxide partial pressure difference, which in turn reduce
the process energy and area demands. A rigorous numerical model that captures
the complexity of these effects is necessary when evaluating this type of
processes.

There are several
numerical models in the literature that describe the behavior of gas permeation
in asymmetric membranes. Kaldis et al. provide a good
summary of these efforts [7]. The asymmetric membrane architecture was
developed in order to address conflicting requirements. In order to achieve
high gas permeances the membrane must be sufficiently thin yet mechanically stable in order to endure the imposed
pressure gradient. By coating a thin (0.5-1 μm) selective layer on a porous support
two to three orders of magnitude thicker both requirements are fulfilled. The
transport across the thin selective layer is usually modeled according to the
solution-diffusion model, while simplifying assumptions about the gas transport
in the porous support are usually made. Experimental results [8, 9] suggest
that the simplifying assumptions about the porous support are not capable of predicting
observed behavior for gas permeation systems with sweep streams.

One-dimensional, multicomponent, hollow-fiber gas permeation models were
developed according to the shell and fiber lumen flow equations of Pan et al.
[10] and the detailed model for the porous support gas transport similar to
Chan et al. [11]. Equivalent models, according to the limiting assumptions about
the porous support were also developed. The models were used to simulate the
performance of a module with and without sweep for the carbon capture from a
flue gas stream. The predicted performance for no-sweep operation was almost
identical regardless of the porous support gas transport sub-model. On the
other hand, significant discrepancies were predicted for modules with sweep
streams. Since sweep operated modules are a vital feature of novel processes, simulation
models must be developed with enough detail to predict their performance
accurately. The analysis of the different models under the modular framework
will determine the sensitivity of the overall performance to the assumed
transport mechanism for the porous support. This will in turn justify or reject
the inclusion of the detailed sub-model, and potentially enable the
optimization of porous support properties.

References

1.      Miller, DC, Eslick, JC, Lee,
A, Morinelly, JE. A modular framework for the analysis and optimization of
power generation systems with CCS. Energy Procedia,
2010; In Press

2.      Coker, DT, Freeman, BD, Fleming, GK. Modeling multicomponent gas separation using hollow-fiber membrane
contactors. AIChE J., 1998; 44:1289-1302

3.      Favre, E. Carbon dioxide recovery from post-combustion
processes: Can gas permeation membranes compete with absorption? J. Membr. Sci., 2007;294:50-59

4.      Hussain, A, Hägg, M-B. A feasibility study of CO₂
capture from flue gas by a facilitated transport membrane. J. Membr. Sci., 2010;359:140-148

5.      Merkel, TC, Lin,
H, Wei, X, Baker, R. Power plant post-combustion carbon dioxide capture: An
opportunity for membranes. J. Membr. Sci., 2010;359:126-139

6.      Zhao, L, Riensche, E, Blum,
L, Stolten, D. Multi-stage gas separation membrane
processes used in post-combustion capture: Energetic and economic analyses. J. Membr. Sci., 2010;359:160-172

7.      Kaldis, SP, Kapantaidakis, GC, Sakellaropoulos, GP. Simulation of multicomponent
gas separation in a hollow fiber membrane by orthogonal collocation ? hydrogen
recovery from refinery gases. J. Membr. Sci., 2000;173:61-71

8.      Sandru, M, Haukebø,
SV, Hägg, M-B.
Composite hollow fiber membranes for CO₂ capture. J. Membr. Sci., 2010;346:172-186

9.      Dittmeyer, R, Höllein, V, Daub, K.
Membrane Reactors for hydrogenation and dehydrogenation processes based on
supported palladium. J. Mol. Catal. A: Chem., 2001;173:135-184

10.  Pan, CY. Gas separation by high-flux, asymmetric
hollow-fiber membrane. AIChE J. 1986;32:2020-2027

11.  Chan, CH, Khor, KA, Xia, ZT.
A complete polarization model of a solid oxide fuel cell and its sensitivity to
the change of cell component thickness. J Power Sources, 2001;93:130-140