Pressure Swing Adsorption: An Alternate Carbon Capture Process ...
Pressure Swing Adsorption: An Alternate Carbon Capture Process ...
Pressure Swing Adsorption: An Alternate Carbon Capture Process ...
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<strong>Process</strong> Systems Engineering<br />
<strong>Pressure</strong> <strong>Swing</strong> <strong>Adsorption</strong>:<br />
<strong>An</strong> <strong>Alternate</strong> <strong>Carbon</strong> <strong>Capture</strong><br />
<strong>Process</strong> for IGCC Power Plants<br />
Alexander W. Dowling<br />
Advisor: Lorenz T. Biegler<br />
March 11 th , 2012
Research Objectives<br />
• Demonstrate methods for optimal <strong>Pressure</strong><br />
<strong>Swing</strong> <strong>Adsorption</strong> (PSA) process synthesis<br />
• Design cost effective PSA cycle for H 2 -CO 2<br />
separation in IGCC power plant<br />
Simplified IGCC<br />
Flowsheet<br />
2
Optimal Cycle Synthesis<br />
? <br />
Discrete variables make<br />
this too computationally<br />
expensive to solve<br />
Step 1 Step 2 Step 3 Step 4 … Step n <br />
“Parts Box” of Steps<br />
<strong>Adsorption</strong><br />
<strong>Pressure</strong><br />
Equalization Desorption Heavy Product Purge<br />
<strong>An</strong>d many<br />
more…<br />
3
PSA “Superstructure”<br />
H 2 to<br />
Turbine<br />
Only use continuous variables to<br />
model generic PSA cycle<br />
α Bottom Reflux Fraction<br />
β Top Reflux Fraction<br />
φ Feed Fraction<br />
P ads <strong>Adsorption</strong> <strong>Pressure</strong><br />
P des Desorption <strong>Pressure</strong><br />
Feed from<br />
WSR, φ(t)<br />
CO 2 to<br />
Pipeline<br />
4
PSA Model: Transport Equations<br />
5
PSA Model: <strong>Adsorption</strong><br />
6
Optimization Formulation<br />
3 methods for<br />
accommodating CSS constraint<br />
7
1. Periodic Boundary Conditions<br />
• Add bed initial condition variables and<br />
periodic boundary condition constraint to<br />
optimization formulation<br />
• Simultaneous optimize cycle and find steadystate<br />
state<br />
• Calculate first derivatives from direct<br />
sensitivity equations<br />
8
2. Direct Substitution<br />
• Repeat cycle from initial bed until converged<br />
to cyclic steady-state<br />
• Suitable for derivative free optimization<br />
• Quadratic penalty function for inequality<br />
constraints<br />
9
3. Fixed Horizon<br />
• Simulate bed for a sufficiently large number<br />
of fixed cycles to near cyclic-steady state<br />
• Calculate first derivatives from adjoint<br />
sensitivity equations<br />
• Suitable for large number of states<br />
10
Solver Details<br />
• IPOPT for derivative based formulations<br />
– First derivatives from sensitivity equations<br />
– Second derivatives approximated with LBFGS<br />
• BOBYQA for direct substitution formulation<br />
– DFO code based on quadratic approximation to<br />
objective function<br />
– Accommodates variable bounds<br />
11
Case Study 1<br />
• Common poor starting<br />
• DFO approach terminates at a much poorer solution<br />
– Local minimia?<br />
• Derivative based approaches don’t always converge to<br />
satisfied KKT conditions<br />
– Terminate due to resource limits or integrator failure<br />
– Noisy first derivatives, approximate second derivatives<br />
12
Case Study 2<br />
• Common good starting point<br />
• DFO approach terminates at an infeasible solution<br />
• PBC sensitivity equation system too large for 5<br />
component case<br />
13
Designed Cycle<br />
H 2 H 2 <br />
CO 2 CO CO 2 CO 2 2 <br />
Step 1 Step 2 Step 3 Step 4 Step 5 <br />
Switch Beds and Repeat<br />
Best 5 Component<br />
Solution<br />
Legend: CO 2 Sorbent Loading<br />
Low<br />
High<br />
14
87.2 kWh/tonne CO 2 captured<br />
3.2 MPa<br />
H 2 to<br />
Turbine<br />
17.2<br />
kWh/tonne<br />
< 2.2 MPa<br />
α Bottom Reflux Fraction<br />
β Top Reflux Fraction<br />
φ Feed Fraction<br />
P ads <strong>Adsorption</strong> <strong>Pressure</strong><br />
P des Desorption <strong>Pressure</strong><br />
5.1 MPa<br />
Feed from<br />
WSR, φ(t)<br />
11.3<br />
kWh/tonne<br />
-39.5 kWh/tonne -1.7 kWh/tonne<br />
< 0.2 MPa<br />
100.0 kWh/tonne<br />
CO 2 to<br />
Pipeline<br />
15 MPa<br />
15
Technology Comparison<br />
Economic Metric: Cost of Electricity<br />
IGCC without<br />
<strong>Carbon</strong> <strong>Capture</strong>*<br />
IGCC with Selexol<br />
<strong>Carbon</strong> <strong>Capture</strong>*<br />
IGCC with PSA<br />
<strong>Carbon</strong> <strong>Capture</strong><br />
$ 76 / MWh $ 106 / MWh $ 103 / MWh<br />
Goal: $ 83 / MWh<br />
Majority of cost increases due to<br />
Water Gas Shift Reactor.<br />
*Cost and Performance Baseline for Fossil Energy Plants Vol 1: Bit. Coal and Nat.<br />
Gas to Elec., NETL<br />
16
Conclusions<br />
• Compared PSA optimal process synthesis<br />
formulations<br />
• Demonstrated novel application of adjoint<br />
sensitivity equations to PSA optimization<br />
• Demonstrated cost competitiveness of PSA for<br />
H 2 -CO 2 separation in IGCC power plant<br />
"This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government<br />
nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the<br />
accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe<br />
privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or<br />
otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency<br />
thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any<br />
agency thereof."<br />
17
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